A Review on Metal Ions Modified TiO₂ for Photocatalytic Degradation of Organic Pollutants

Abstract

TiO₂ is a semiconductor material with high chemical stability and low toxicity. It is widely used in the fields of catalysis, sensing, hydrogen production, optics and optoelectronics. However, TiO₂ photocatalyst is sensitive to ultraviolet (UV) light; this is why its photocatalytic activity and quantum efficiency are reduced. To enhance the photocatalytic efficiency in the visible light range as well as to increase the number of the active sites on the crystal surface or inhibit the recombination rate of photogenerated electron–hole pairs electrons, various metal ions were used to modify TiO₂. This review paper comprehensively summarizes the latest progress on the modification of TiO₂ photocatalyst by a variety of metal ions. Lastly, the future prospects of the modification of TiO₂ as a photocatalyst are proposed.

1. Introduction

In the past, industrial development has been accompanied by the discharge of a large number of organic pollutants, which has caused great harm to the environment and living beings in general [1,2]. Research on the purification of wastewater is currently available and ongoing. Various traditional methods including physical adsorption, biodegradation, catalytic oxidation, and high-temperature incineration have been proposed [3].

Table 1. Traditional photodegradation methods and limitations.

Approach	Limitations
Biological treatment	Chlorinated phenols are resistant to biodegradation and can accumulate in sediments. They transfer the contaminants from one medium to another and require disposal or further treatment The biodegradation of organic pollutants such as 4-CP is slow and incomplete, and its byproducts are more toxic compared to other pollutants. Biological processes usually require considerable processing time to decompose 4-CP. By use of the biological treatment, it cannot be degraded due to a large number of aromatic structures in the dye molecules and the stability of modern dyes. Azo bond is reduced to form a colorless but toxic and potentially carcinogenic aromatic amine.
Adsorption technology and activated carbon adsorption method	The post-treatment of wastewater and regeneration of adsorbent materials requires an expensive operation. Activated carbon adsorption requires the safe disposal of carbon. During the adsorption process, the system cannot tolerate the suspended solids in the influent water as a result of blockage. The operating cost is high due to the requirements of the carbon system. The treatment may be problematic if the polluted carbon is not regenerated.
Chemical precipitation	Requirements for a large amount of chemicals and a large amount of sludge produced. Requires further treatment or disposal. Due to the large amount of sludge needing to be treated, the method is not feasible economically.
Air stripping	It is susceptible to pollution. Aesthetic limitations due to tower height. Challenges involving mechanical reliability.
Membrane adsorption	The purchasing cost of membranes and residues (very concentrated filtrate) is high and must be collected or may require further processing. The physical method is not destructive, but only transfers pollutants to other media, causing secondary pollution.

presents the traditional treatment methods and their limitations. These treatment methods have some shortcomings, and the treatment effect on organic wastewater is not very ideal. Recent studies have been devoted to a promising approach, the advanced oxidation processes (AOP) for the degradation of organic pollutants from wastewaters [3,4,5,6,7,8,9,10], due to its ability to completely mineralize the targeted pollutants [11]. There are various types of AOPs including ozone (photo-ozonation, ozonation, O₃+Fe²⁺/Fe³⁺, and ozonation + catalyst and O₃+H₂O₂), photolysis (VUV or UV), hydrogen peroxide (photo-Fenton: H₂O₂+Fe²⁺/Fe³⁺+UV, Fenton-like reagents: H₂O₂+Fe²⁺-solid/Fe³⁺-solid, Fenton: H₂O₂+Fe²⁺/Fe³⁺, and H₂O₂+UV), and photocatalysis [12,13].

The ordinary chemical oxidation process has limitations in degrading some harmful substances because of its poor oxidation ability. The intermediate products in the advanced oxidation process can react with hydroxyl radicals to oxidize harmful substances into carbon dioxide and water. AOPs are a chemical purification treatment used to remove inorganic/organic materials in wastewater and water via oxidation through reactions with hydroxyl radicals (·OH). AOPs can be categorized as heterogeneous systems (semiconductor photocatalytic process) and homogeneous systems (Fenton and photo-Fenton systems, ozone, UV/H₂O₂) [14]. Photocatalytic degradation is a part of AOP for the degradation of organic pollutants which has proven as an effective technology [15,16]. It is more effective in comparison with the AOP as it can easily mineralize various organic components and the semiconductors used are not expensive.

2. TiO₂ Photocatalysis

Photocatalysis is a general term for a photoinduced reaction that is accelerated in the presence of a catalyst. Heterogeneous photocatalysis is the most widely used and effective process for the degradation of organic pollutants and does not produce harmful intermediates at ambient temperature and pressure [17,18]. This process starts with electron excitation, which is transferred from the valence band (VB) to the empty conduction band (CB). In the presence of light, the catalyst absorbs adequate energy to become excited from the light which is equal to its energy bandgap. The excited electrons move toward the conduction band via the holes acting as a positive charge in the valence band, owing to which photogenerated species, including e⁻/h⁺ pair, are created in the photocatalytic system. The generated species react with -OH groups or oxygen molecules to further produce reactive oxygen species such as superoxide anion radicals (O₂•⁻) and hydroxyl radicals (•OH). Thereafter, these reactive oxygen species attack the organic molecules, decomposing them via oxidation. Finally, the e-/h⁺ pairs are formed on the photoexcited catalyst’s surface [19,20].

These materials are generally N-type semiconductors with discontinuous band structures different from metals, usually composed of low-valence bands full of electrons. Common semiconductor photocatalysts include ZnO, TiO₂, Bi₂₀Ti₂₀, Bi₂WO₆, Nb₂O₅, Fe₂O₃, SrTiO₃, BiTiO₃, CuS/ZnS, ZnWO₄, WO₃, ZnS, Ag₂CO₃, and SnO₂, etc., but most photocatalysts have an impact on the photocatalytic performance due to the photocorrosion phenomenon. Among the aforementioned photocatalysts, Titanium dioxide (TiO₂) has been widely studied by researchers due to its low cost, high efficiency, and stability. The disadvantage of TiO₂ is that it cannot be activated by visible irradiation, but by UV. Its advantages over other semiconductors include being biologically and chemically inert, relatively easy to use and produce, photo-catalytically stable, relatively cheap, and able to efficiently catalyze reactions, with no risk to humans or the environment [11]. TiO₂ exists in three polymorphs such as rutile, anatase, and brookite. Both rutile and anatase have tetragonal crystal lattices while brookite appears as orthorhombic crystal lattices [21]. Rutile form is more stable than the other polymorphs among these three phases. TiO₂ has applications in various products including sunscreen lotions, capacitors, paint pigments, toothpaste, food coloring agents, solar cells, and electrochemical electrodes [22].

In 1967, Kenichi Honda and Akira Fujishima jointly discovered that light irradiating a titanium dioxide electrode can carry out the electrolysis of water. The ionized water or oxygen was converted into photo-living groups with oxidizing ability. Their energy was equivalent to a high temperature of 3600 K and is highly oxidizing. In 1972, Fujishima et al. first proposed the theory of photocatalytic water splitting on TiO₂ electrodes, and then in 1973, they proposed the use of titanium dioxide photocatalyst for environmental remediation, thus promoting the development of photocatalytic technology [23]. In 1995, Fujishima et al. [24] discovered that a film containing a certain amount of TiO₂ was super-hydrophilic under ultraviolet light, which promoted the derivation of the field of photocatalyst and film binding. This technology utilizes some special photocatalyst powder into sewage, which could decompose toxic metal substances in the water to obtain pure water under the irradiation with ultraviolet rays. This technology was also used to remediate polluted rivers and found harmless to the environment. Since then, researchers have found ways to modify the TiO₂ photocatalyst in a large amount. By 2004, Sonawane reported that Fe-TiO₂ film could degrade methyl orange solution within 4 h under sunlight, with a degradation efficiency (95%) [25].

The bandgap of pure nano-titanium dioxide powder is about 3.2 eV, which can only absorb ultraviolet light below 400 nm. In the natural environment, ultraviolet light is less than 10%, so pure nano-titanium dioxide does not have the function of a photocatalyst. This greatly affects the utilization rate of the catalyst to sunlight. Other key factors that limit the practical application of photocatalysts are the low quantum yield of the catalyst and the faster recombination of electrons and photogenerated holes. Generally, the recombination efficiency of photogenerated carriers determines the quantum quality of the photocatalyst, which affects carrier recombination. The main factors of the surface charge transfer process are the surface morphology, grain size, crystal phase structure, and surface lattice defects of the catalyst, as well as the intensity of light radiation.

To make the TiO₂ photocatalyst absorb visible light and far-infrared light and improve the photocatalytic efficiency, the photocatalyst needs to be modified. At present, researchers have proposed two ways to enhance the photocatalytic performance of TiO₂: one is to prevent photogenerated electrons from recombining with photogenerated holes so that they can effectively participate in the catalytic degradation reaction process; the other is to introduce other elements into TiO₂ lattice, thereby reducing the bandgap energy of the catalyst and expanding its photoresponse range. Generally, the activated CB electrons and VB holes recombine into a neutral body to generate energy, which is lost in the form of light energy or heat. Therefore, the photogenerated electrons can be captured by the catalyst or the mobility of the surface charge of the catalyst can be improved. To slow down the recombination of hole–electron pairs, modification of TiO₂ lattice with metal ions is an effective way to improve photocatalytic performance. In the subsequent sections, recent studies on metal ion modified TiO₂ (with morphologies such as nanoparticles, microspheres, nanofibers, nanocrystals, nanosheets, nanotubes, nanopowders) were summarized and reported.

3. Mechanisms of TiO₂ Photocatalysts for Organic Pollutants

The mechanism of the photocatalytic reaction in the presence of TiO₂ photocatalysts consists of a free radical reaction initiated by light irradiation (photons) [26]. When the energy of solar radiation exceeds the bandgap of TiO₂ (i.e., photon energy reaches or exceeds its bandgap energy), the surface of the photocatalyst becomes excited, and the electrons transit from the valence band (VB) to the conduction band (CB). In the CB, corresponding electron holes are derived in the VB at the same time, forming electron–hole pairs (i.e., generating electron (${\mathrm{e}}^{-}$) and hole (${\mathrm{H}}^{+}$) pairs). VB holes have strong oxidation reaction activity (1.0~3.5 V) because they lose electrons and act as reducing agents, and electrons in the conduction band have good reducibility 0.5~1.5 V when they undergo reduction. Under light irradiation, positive holes and electrons are generated in the VB (${\mathrm{hv}}_{\mathrm{vb}}^{+}$) and CB (${\mathrm{e}}_{\mathrm{cb}}^{-}$) of TiO₂ as presented in Equation (1) [27]. These holes can either form hydroxyl radicals (Equation (3)) or react directly with organic molecules (Equation (5)) which subsequently oxidize the organic molecules (Equation (6)) [28]. The electrons can also react with organic compounds to produce reduction products (Equations (1)–(7)). The role of oxygen is important as it reacts with the photogenerated electrons. Organic compounds can then undergo oxidative degradation through their reactions with hydroxyl and peroxide radicals, VB holes, as well as reductive cleavage via reactions with electrons yielding various byproducts and finally mineral end-products [29].

$$\mathrm{hv}+{ \mathrm{TiO}}_2 \to {\mathrm{hv}}_{\mathrm{vb}}^{+}+{ \mathrm{e}}_{\mathrm{cb}}^{-}$$

(1)

$${\mathrm{OH}}_{\mathrm{surface}}^{-}+{ \mathrm{hv}}_{\mathrm{vb}}^{+} \to { \mathrm{OH}}^{•}$$

(2)

$${\mathrm{H}}_2{\mathrm{O}}_{\mathrm{absorbed}}+{ \mathrm{hv}}_{\mathrm{vb}}^{+} \to { \mathrm{H}}^{+}+{ \mathrm{OH}}^{•}$$

(3)

$${\mathrm{O}}_{\mathrm{absorbed}}+{ \mathrm{e}}_{\mathrm{cb}}^{-} \to { \mathrm{OH}}_2^{-•}$$

(4)

$$\mathrm{Organic}+{ \mathrm{hv}}_{\mathrm{vb}}^{+} \to \mathrm{Oxidation} \mathrm{products}$$

(5)

$$\mathrm{Organic}+{ \mathrm{OH}}^{•} \to \mathrm{Degradation} \mathrm{products}$$

(6)

$$\mathrm{Organic}+{ \mathrm{e}}_{\mathrm{cb}}^{-} \to \mathrm{Reduction} \mathrm{products}$$

(7)

Additionally, VB holes (${\mathrm{h}}_{\mathrm{vb}}^{+}$) react with water (${\mathrm{H}}_2\mathrm{O}$) and the hydroxyl ion (${\mathrm{OH}}^{-}$) to form hydroxyl radicals (${\mathrm{HO}}^{•}$), while electrons (${\mathrm{e}}_{\mathrm{cb}}^{-}$) react with adsorbed oxygen molecule (O₂), thus reducing it to superoxide radical anion (${\mathrm{O}}_2^{•-}$) which reacts with protons (${\mathrm{H}}^{+}$) to form peroxide radicals (${\mathrm{HO}}_2^{•}$) as shown in Equations (8)–(11) [30].

$${\mathrm{h}}_{\mathrm{vb}}^{+}+{ \mathrm{H}}_2\mathrm{O} \to { \mathrm{HO}}^{•}+{ \mathrm{H}}^{+}$$

(8)

$${\mathrm{h}}_{\mathrm{vb}}^{+}+{ \mathrm{OH}}^{-} \to { \mathrm{HO}}^{•}$$

(9)

$${\mathrm{e}}_{\mathrm{cb}}^{-}+{ \mathrm{O}}_{2 }\to { \mathrm{O}}_2^{•-}$$

(10)

$${\mathrm{O}}_2^{•-}+{ \mathrm{H}}^{+} \to { \mathrm{HO}}_2^{•}$$

(11)

In the presence of light irradiation using metal ion modified TiO₂ and because the metal ions create intermediate states in the TiO₂ structure, the surface electrons in the intermediate states become excited, having sufficient energy to access more light absorption and transfer electrons to the TiO₂ surface that stimulate more electrons under light, which as a consequence advances the redox reactions [31,32,33]. These properties being displayed by the metal ions help to improve the photocatalytic reactions. The transfer of electrons takes place at the VB to the CB via the creation of holes in the former and these holes subsequently react with ${\mathrm{H}}_2\mathrm{O}$ molecules present in the pollutant as well as helping to form the OH radicals [31,32,33,34]. The formed OH radicals are supportive and found significant success in degrading various dyes under light irradiation [35,36,37]. The photocatalytic mechanism of metal-ion modified TiO₂ for organic pollutants is illustrated in Figure 1.

4. Modification with Metal Ions

Modification with metal-ion can act as electron/hole traps and alter e⁻/h⁺ recombination rate, according to the below mechanisms [39]:

$${\mathrm{M}}^{\mathrm{n}+}+{ \mathrm{e}}_{\mathrm{cb}}^{-} \to { \mathrm{M}}^{\left(\mathrm{n}-1\right)+} \left(\mathrm{electron} \mathrm{trap}\right)$$

(12)

$${\mathrm{M}}^{\mathrm{n}+}+{ \mathrm{h}}_{\mathrm{vb}}^{+} \to { \mathrm{M}}^{\left(\mathrm{n}+1\right)+} \left(\mathrm{hole} \mathrm{trap}\right)$$

(13)

where energy level for ${\mathrm{M}}^{\mathrm{n}+}/{\mathrm{M}}^{\left(\mathrm{n}-1\right)+}$ lies below the CB edge (${\mathrm{e}}_{\mathrm{cb}}$) and the energy level for ${\mathrm{M}}^{\mathrm{n}+}/{\mathrm{M}}^{\left(\mathrm{n}+1\right)+}$ above the VB edge (${\mathrm{e}}_{\mathrm{vb}}$).

The presence of metal ions allows visible light absorption and introduces trap/recombination sites within the TiO₂ bands which may as a result increase the life span of photoinduced charge carriers as well as the reduction in quantum efficiency. Modifications with metal ions can influence TiO₂ photocatalysis according to these principles: (i) improve the electron–hole separation (strength) by selective trapping [40,41]; (ii) due to their ability to act as recombination centers, they reduce carriers’ lifespan (weakness) [42,43], and (iii) enhance optical absorption in visible light range. The commonly used metal ions are the metals, transition metal, rare earth metal, or noble metal [44]. Modification of TiO₂ by metal ions is generally carried out via sputtering [45], ion implantation [46], or via chemical processes (for example sol–gel) [47,48,49,50]. Figure 2 presents a table of commonly used metallic ions to enhance the photocatalytic degradation of organic pollutants.

In general, various types of metals (Transition, rare earth, or other metals) have been used to modify TiO₂ photocatalysts.

Table 2. General properties and principal applications of the metals.

Classes of Metals	Features/Properties	Principal Applications
Transition metals	They have incompletely filled d orbitals. The transition metals are more electronegative than the other metals and form stable compounds with neutral molecules (such as water or ammonia). The main advantages of these metals are malleability and ductility. Examples of transition metals used to modify TiO2 photocatalysts include vanadium (V), nickel (Ni), copper (Cu), manganese (Mn), zirconium (Zr), iron (Fe), chromium (Cr), molybdenum (Mo), cobalt (Co), niobium (Nb), tungsten (W), and zinc (Zn). The density of V, Ni, Cu, Mn, Zr, Fe, Cr, Mo, Co, Nb, W, and Zn are 6.0, 8.90, 8.96, 7.3, 6.52, 7.87, 7.15, 10.2, 8.86, 8.57, 19.3, and 7.134 g cm−3, respectively.	Luminescence, electronic device, and water pipes applications.
Noble metals	These metals are known as iron lovers due to their ability to dissolve in iron either as solid solutions or in the molten state. They have outstanding resistance to chemical attacks even at high temperatures. Furthermore, they are well known for their catalytic properties and associated capacity to facilitate or control the rates of chemical reactions. Examples of noble metals used to modify TiO2 photocatalysts include ruthenium (Ru), palladium (Pd), platinum (Pt), gold (Au), and silver (Ag). The density of Ru, Pd, Pt, Au, and Ag are 12.1, 12.0, 21.5, 19.3, and 10.5 g cm−3, respectively.	Hydrogenation, total oxidation and, more recently, partial oxidation heterogeneous catalysts.
Rare-earth metals	Rare earth metals possess good magnetic and luminescent properties. The common rare earth metals used to modify TiO2 photocatalysts include cerium (Ce), erbium (Er), holmium (Ho), gadolinium (Gd), terbium (Tb), neodymium (Nd), ytterbium (Yb), samarium (Sm), lanthanum (La), europium (Eu), and yttrium (Y). The density of Ce, Er, Ho, Gd, Tb, Nd, Yb, Sm, La, Eu, and Y is 6.77, 9.07, 8.80, 7.90, 8.23, 7.01, 6.90, 7.52, 6.15, 5.24 and 4.47 g cm−3, respectively.	Cellphones, electrical and electronic components, lasers, glass, magnetic materials, fluorescent lights, defense, clean energy.
Metals	Metals are malleable, ductile, good conductors of heat and electricity. Metals have a luster with high tensile strength. Typical metals used to modify TiO2 photocatalysts include gallium (Ga), indium (In), aluminum (Al), and tin (Sn). The density of Ga, In, Al, and Sn is 5.91, 7.31, 2.70, and 7.287 g cm−3, respectively.	Automobiles, electronic devices.

presents the general properties and principal applications of the metals. In the subsequent sections, the modifications using metal ions TiO₂ photocatalysts are described and discussed. The following section starts with the modification using transition metal, which is subsequently followed by lanthanides or rare earth metal modifications. At the end of this section, the modifications of other metals were presented.

4.1. Modification with Transition Metals

Modifying TiO₂ with transition metals has shown great promise through the extension of spectral response and the ability to achieve visible light-activated photocatalysis [51]. Modifying with transition metal including Cu, Cr, Mn, Fe, V, and Zn can reduce the e⁻/h⁺ recombination and decrease the bandgap by creating an intra-bandgap state, which shifts the light absorption into the visible light region [52,53]. The formation of additional energy levels in the bandgap of TiO₂ can be described below [54]:

$$\left(1\right){ \mathrm{M}}^{+}+\mathrm{hv} \to { \mathrm{M}}^{\left(\mathrm{n}+1\right)+}+{ \mathrm{e}}^{-}$$

(14)

$$\left(2\right) {\mathrm{M}}^{+}+\mathrm{hv} \to { \mathrm{M}}^{\left(\mathrm{n}-1\right)+}+{ \mathrm{h}}^{+}$$

(15)

where M is the metal.

The energy necessary for reducing the metal ion should be less negative than the conduction band edge of titania while its oxidation energy should be less positive than the valence band edge of TiO₂. Additionally, photocatalytic reactions can occur only if the trapped holes and electrons are transferred to the catalyst’s surface. Therefore, metal ions should be situated near the surface of TiO₂ crystallites for a better charge transfer [55]. If the amount of metal ions is more than optimal, the metal ions tend to behave as recombination centers and the transfer of h⁺/e⁻ pairs to the interface is more complex. Generally, the metal ions are precipitated on the surface as oxides forming a MO_x/TiO₂ nanocluster and not incorporated in the structure of TiO₂. If the energetic positions of CB and VB of components forming the composite are suitable (i.e., a type II heterojunction), beneficial charge transfer occurs within the photocatalyst where ${\mathrm{e}}^{-}$ is accumulated in the CB edge of one component and ${\mathrm{h}}^{+}$ is accumulated in the VB edge of the other component. Therefore, the charge carriers are separated efficiently, leading to improved photocatalytic property [56,57].

4.1.1. Modification with Vanadium (V)

Vanadium exists in several oxidation states (V²⁺ to V⁵⁺) and a variety of species. The types of species and oxidation state are a function of the redox potential, pH, concentration, and other factors [58]. The radius of the V ion is nearly identical to that of titanium (Ti) and can be conveniently introduced into titania [59]. V ions with different valence states (V³⁺ to V⁵⁺) can transfer between V³⁺ to V⁵⁺ under oxidizing and reducing conditions. For example, the tetragonal crystal structure of VO₂ is similar to that of titania, which is responsible for an increase in visible light absorption and photogenerated holes and electrons [60]. The photogenerated holes and electrons can be migrated, trapped, and released on the TiO₂ surface by V⁴⁺ ions, and play a role in charge transfer species. It is very difficult for V⁵⁺ ions to enter the lattice of TiO₂, making it appropriate to form V₂O₅ on the surface and thus responsible for h⁺ and e⁻ separation [61]. Subsequently, this photogenerated e⁻ and h⁺ can be accepted by the adsorbed O₂ and surface hydroxyl group (OH⁻), and therefore transform OH⁻ and O₂ into hydroxyl radical (•OH) and superoxide radicals (•O₂⁻) active species, respectively.

Vanadium-TiO₂ (V-TiO₂) at low concentrations has been found to improve TiO₂ photocatalytic behavior via the existence of the photogenerated charge and the more efficient expansion of absorption spectrum as compared to other metals [62,63]. V-TiO₂ has an enhanced photocatalytic activity due to the following reasons: (i) improving the absorption in the visible light range, (ii) improved quantum efficiency due to effective h⁺ to e⁻ pair separation, and (iii) presence of both V⁵⁺ and V⁴⁺ species in the V-TiO₂. Additionally, the second reason can contribute to the increased electron transfer and visible light absorption, whereas the latter enhances e⁻–h⁺ separation and is a potential electron acceptor [64,65,66]. V-TiO₂ have been extensively studied in experiments and theory due to evidence of ferromagnetism above room temperature or at room temperature [67,68]. Vanadium with a narrow bandgap can be coupled with TiO₂ by various approaches such as metal-ion implantation, sol–gel, hydrothermal, and coprecipitation approach. Generally, the sol–gel method is frequently used for depositing the V-TiO₂ films while the coprecipitation approach is used to prepare V-TiO₂ powders. Among these approaches, (compared with the hydrothermal approach), the sol–gel and coprecipitation approach is quite harsh and complicated; however, the hydrothermal approach is easy to execute and involves low cost [69,70].

Table 3. Summary of recent progress on V-TiO₂ photocatalysts for organic pollutants degradation.

Material (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutant (Volume)	Light Source	Unmodified	Modified	Ref.
V-TNT nanosheet (mixed crystal of anatase and rutile phases)	1.0% V-TNT	0.1 g	Hydrothermal method	20 mg/L RhB (100 mL)	UV/Visible light	70% within 70 min for TNT	The reaction rate was 3.26-fold and 9.27-fold as compared to unmodified TNT under visible light and UV–vis irradiation, respectively.	[71]
V2O5/TiO2 nanoparticles (anatase phase)	3 wt% V2O5/TiO2	0.05 g	Incipient wet impregnation method	0.2 mg/L Eosin Y (200 mL)	Visible light	8% within 180 min for V2O5	93% within 180 min, the stability only decreased by 20% for the first run	[72]
V2O5/P25 nanoparticles (anatase phase)	3 wt% V2O5/P25	0.05 g	Incipient wet impregnation method	0.2 mg/L Eosin Y (200 mL)	Visible light	8% within 180 min for V2O5	81% within 180 min	[72]
V2O5/TiO2 nanoparticles (anatase phase)	3 wt% V2O5/TiO2	0.05 g	Incipient wet impregnation method	0.2 mg/L Eosin Y (200 mL)	Visible light	100% within 180 min for TiO2 and P25, 33% within 180 min for V2O5	40% within 180 min	[72]
V2O5/TiO2 coatings (anatase phase)	2.0 wt% V2O5/TiO2	15 mm × 10 mm	Sol–gel method	8 mg/L MO (10 mL)	Sunlight	35% within 8 h	53% within 8 h	[73]
V-TiO2 nanoparticles (rutile to stable anatase phase)	1.0% V-TiO2	0.05 g	Sol–gel method	10 mg/L RhB (100 mL)	Xenon Lamp	21.56% within 300 min	53.74% within 300 min	[74]
V-TiO2 nanopowders (anatase phase)	50 wt% V2O5/TiO2	0.1 g	Solid-state dispersion method	25 mg/L 2,4-CP (50 mL)	UV-B	72% within 30 min for pure V2O5, 66% within 30 min for pure TiO2	85% within 30 min	[75]
V-TiO2 coupons (rutile to stable anatase phase)	6.0 wt% V-TiO2	15 mm × 10 mm	PEOx	10 mg/L MB (100 mL)	Tungsten-halogen	55% within 180 min for pure TiO2	85% within 180 min	[76]
V-TiO2 nanoparticles (anatase to brookite and rutile phases)	0.125 mol%-V-TiO2	0.1 g/L	MWASG	85 mg/L MB and SMX (100 mL)	UV	0.031 min−1 for MB within 60 min	0.035 and 0.0262 min−1 for MB and SMX, respectively within 60 min	[77]

MWASG: Microwave-assisted sol–gel method; PEOx: plasma electrolytic oxidation; RhB: rhodamine B; 2,4-CP: 2,4-dichlorophenol; MB: methylene blue; SMX: sulfamethazine.

presents the summary of recent progress on V-TiO₂ photocatalysts for organic pollutants degradation.

4.1.2. Modification with Nickel (Ni)

Nickel (Ni) has good activity and it is less expensive than noble metals [78]. This element is used in many applications due to its physicochemical properties. Ni is a good candidate for substituting Ti atoms in the TiO₂ structure [79]. The incorporation of Ni ions into the TiO₂ lattice can actively modify the TiO₂ physical properties via the creation of an impurity energy level. Upon modifying TiO₂ with Ni, the recombination of photogenerated hole–electron pairs is suppressed effectively, thus resulting in an enhanced photocatalytic activity [79,80,81]. The structure of the Ni²⁺ valence band is 3d8. When the Ni ions trap the photogenerated hole–electron pairs, the valence layer (d) is converted from a high to low spin state, and thus results in a significant spin energy loss. Based on the crystal field theory, charge carriers that are being trapped by Ni ions tend to migrate to the H₂O molecules (adsorbed on the surface) to restore their energy, and as a consequence will prevent the recombination of hole–electron pairs [82]. Furthermore, Ni²⁺ plays an important role in the improvement of thermal stability as well as controlling the morphology of TiO₂ photocatalysts.

Table 4. Summary of recent progress on Ni-TiO₂ photocatalysts for organic pollutants degradation.

Material (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutant (Volume)	Light Source	Unmodified	Modified	Ref.
Ni-TiO2 nanotubes (mixed crystal of anatase and rutile phases)	10% Ni-TNT	0.1 g	Hydrothermal method	5 mg/L MB (50 mL)	Sunlight	71.4% within 90 min for pure TiO2	~65% within 90 min	[83]
Ni-TiO2 nanoparticles (mixed crystal of anatase and rutile phases)	0.9 wt% Ni- TiO2	1.5 g/L	Sol–gel method	10 mg/L MB (100 mL)	Hg vapor lamp	62% within 120 min for pure TiO2	50% within 120 min	[84]
Ni-TiO2 nanoparticles (anatase phases)	0.01% Ni-TiO2	0.1 g	Hydrothermal method	10 mg/L MB (100 mL)	Tungsten-halogen lamp	-	75% within 300 min	[85]
Ni-TiO2 inverse opal photonic microarray (IOPM) (anatase phases)	3.0 wt% Ni-TiO2	-	Sol–gel method	10 mg/L MB (100 mL)	Sunlight	60% within 90 min for TiO2 IOPM	95% within 90 min (1.58 times larger compared to that over TiO2 IOPM)	[86]
Ni-TiO2 nanoparticles (anatase phases)	1.0 wt% Ni-TiO2	1 g/L	MWASG	10 mg/L BPA (200 mL)	Phillip lamp	60.1% (0.0098 min−1) within 120 min	93% (0.0255 min−1) within 120 min	[87]
Ni-TiO2 nanoparticles (anatase phases)	0.50 wt% Ni-TiO2	2 g/L	Sol–gel method	25 mg/L IBP (50 mL)	Solar light	76% within 6 h for pure TiO2	78% (0.0046 min−1) within 6 h	[88]
Ni-TiO2 nanoparticles (anatase to rutile phase)	10 wt% Ni-TiO2	2 g/L	Coprecipitation method	100 mg/L ECR (200 mL)	Visible light	20.9% within 120 min for pure TiO2	37.4% within 120 min	[89]
NiO/TiO2 nanopowders (mixed phase of rutile and anatase)	0.5 wt% NiO/TiO2	0.05 g	Modified combustion-based method	15 mg/L MB (50 mL)	Daylight emission	85% within 210 min	90% within 210 min	[90]
Ni-TiO2 nanopowders (anatase phase)	0.05 mol% Ni	1 g L−1	Sol–gel method (non-aqueous)	10 mg L−1 MB (500 mL)	Xenon Lamp	88.0% within 180 min for pure TiO2	98.9% within 180 min	[91]
Ni-TiO2 nanoparticles (anatase phase)	1.0% Ni-TiO2	0.025 g	Sol–gel method	10−5 mol/L MO (50 mL)	Visible light	0.00075 and 0.002631 min−1 for visible and UV irradiation, respectively. 16.3 and 95.6% for visible and 95.6 for visible and UV light, irradiation within 120 min	13.8% for visible light irradiation within 120 min. 0.00063 min−1 and 0.00390 min−1 for visible and UV light irradiation, respectively	[92]
Ni-TiO2 nanoparticles (anatase phase)	1.0% Ni- TiO2	0.05 g/0.5 g	Sol–gel method	10 mg/L 4-CP, NPX (250 mL)	UV	68.9 and 84.9% for 4-CP and NPX, respectively within 6 h	89.5 and 84% for 4-CP and NPX, respectively within 6 h	[93]
Ni-TiO2 nanoparticles (anatase phase)	1 wt% Ni-TiO2	0.1 g	Sol–gel method	5 mg/L MO, MB (100 mL)	Visible light	44.18 and 26.80% degradation rates for MB and MO, respectively for unmodified TiO2 within 5 h. 45% of MB for Degussa P25 within 5 h.	71.18 and 39.57% for MB and MO, respectively within 5 h	[94]

MWASG: Microwave-assisted sol–gel method; MB: methylene blue; MO: methyl orange; NPX: naproxen; ECR: Eriochrome cyanine red; IBP: ibuprofen; BPA: bisphenol A; MG: malachite green; 4-CP: 4-chlorophenol.

presents the summary of recent progress on Ni-TiO₂ photocatalysts for organic pollutants degradation.

4.1.3. Modification with Copper (Cu)

The introduction of Cu ion can directly trap generated electrons by excitation of light, and thus prevents the rapid recombination of hole–electron pairs [95]. It can increase the surface diffusivity of the TiO₂ [96], and thereby enables the interaction of holes and electrons with other compounds (e.g., H₂O) more quickly [97], generating feasible reactive species, and subsequently bypassing the recombination of the hole–electron pair [98]. Cu with redox potentials of 0.16 V (Cu²⁺/Cu⁺) and 0.52 V (Cu²⁺/Cu) has been used as a suitable modifier for various visible light responsive photocatalysts. Ti⁴⁺ and Cu²⁺ have similar ionic radii and therefore Cu²⁺ can easily penetrate into TiO₂ matrixes as a deep acceptor in conjunction with neighboring oxygen vacancies or substitute the positions of Ti⁴⁺ [99]. In addition, modification with Cu shifts the absorption edges of both the photocatalysts towards the visible region [100].

The behavior involved in modifying TiO₂ with Cu is strongly linked to parameters of synthesis and diverse approaches used for fabrication of materials as well as analysis, and has led to various experimental findings in the past. Studies have observed the diverse speciation of Cu-TiO₂ with the majority of studies reporting the presence of Cu in a Cu²⁺ valence. These species are frequently reported to substitute for Ti⁴⁺ to give a solution phase with the composition Cu_xO₂Ti_1-x and an increased lattice density of oxygen vacancies [101], and reportedly may also occupy interstices in anatase [102]. In addition to solid solutions, Cu²⁺ has been found to exist in amorphous or crystalline CuO nanoclusters as well as surface localized Cu(OH)₂ in titania [103,104,105]. The presence of monovalent ${\mathrm{Cu}}^{+}$ (less common as compared to ${\mathrm{Cu}}^{2+}$) has also been reported, both in Cu₂O nanoclusters and in substitutional positions [102,106]. The CB edges of CuO₂ and CuO are suitable for the enhancement of TiO₂ photocatalytical effect [107,108,109]. Generally, an efficient photocatalytic activity is understood to be driven by a reduced charge carrier recombination in Cu-TiO₂ photocatalyst. This was found to arise as a result of photogenerated electrons which facilitates the reduction of ${\mathrm{Cu}}^{2+}+{ \mathrm{e}}^{-} \to { \mathrm{Cu}}^{+}$, and thus extends the valence band hole lifetimes at the surfaces, which are able to react with adsorbed species to form active radical species [103,104]. Alternatively, the presence of Cu₂O or CuO phase in TiO₂ may improve exciton lifetime through electron capture in the secondary phase [110] or improve the activity via the increment in surface area [111].

Table 5. Summary of recent progress on Cu-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutants (Volume)	Light Source	Unmodified	Modified	Ref.
Cu-TiO2 nanoparticles (anatase phase)	1.0% Cu-TiO2	0.5 g/0.05 g	Sol–gel method	10 mg/L 4-CP (250 mL)	UV	79% (0.1210 L g−1 min−1) within 6 h.	90% (0.1827 L/g−1 min−1) within 6 h	[93]
Hybrid Cu-TiO2/polythiophene nanorods (rutile phase)	-	0.1 g	Sol–gel method	5 mg/L RhB (150 mL)	Visible light	6.9% within 75 min	70.5% within 75 min. Furthermore, there was no significant decrease in photocatalytic activity after three continuous cycles.	[112]
Cu-TiO2 nanoparticles (anatase phase)	1.0% Cu-TiO2	0.05 g/0.5 g	Sol–gel method	10 mg/L NPX (250 mL)	UV	84.9% (0.0124 L g−1 min−1) within 6 h.	87.4% (0.0259 L g−1 min−1) within 6 h.	[93]
Hybrid Cu-TiO2/polythiophene nanorods (rutile phase)	-	0.1 g	Sol–gel method	5 mg/L OG (150 mL)	Visible light	6.9% within 75 min	98% within 75 min. Furthermore, there was no significant decrease in photocatalytic activity after three continuous cycles.	[112]
Cu-TiO2 nanoparticles (anatase phase)	0.2 wt% Cu-TiO2	-	Sol–gel method	50 mg/L MO (50 mL)	UV	27% (0.45 min−1) within 30 min	73% (2.83 min−1) within 30 min	[113]
Cu-TiO2 nanocrystals (mixed anatase and brookite phases)	0.1% Cu-TiO2	1 g/L	Sol–gel method	10 mg/L PNP (10 mL)	Visible irradiation (halogen lamp)	12% for P25 within 24 h	65% (5 times more efficient than P25) within 8 h	[114]
Cu-TiO2 nanoparticles (stable anatase phase)	2% Cu-TiO2	0.002 g	Sol–gel method	10 mg/L MB (50 mL)	150 W Xenon lamp	0.0046 min−1 within 2 h	0.0082 min−1 within 2 h	[115]
Cu-TiO2 nanoparticles (stable anatase phase)	2% Cu-TiO2	0.002 g	Sol–gel method	10 mg/L PNP (50 mL)	150 W Xenon lamp	18% (0.0016 min−1) within 2 h	29% (0.0027 min−1) within 2 h	[115]
Cu-TiO2 nanoparticles (anatase to rutile phase)	1.2% Cu-TiO2	0.08 g	Sol–gel method	35 mg/L E131 VF (100 mL)	UV	0.083 min−1 within 30 min	0.0044 min−1 within 100 min	[116]
Cu-TiO2/GO nanoparticles (anatase phase)	1 wt% Cu -TiO2/GO	0.05 g	Impregnation method	20 mg/L TC (50 mL)	300 W mercury lamp	23.1% within 90 min for pure TiO2	98% within 90 min (reaction rate constant was about 1.4 times than that of TiO2/GO), the removal ratio of Cu-TiO2/GO exceeded 98% after five cycles.	[117]
Cu-TiO2 nanopowders (anatase to rutile phase)	Cu: TiO2 both in powder (42.35%)	0.3 g	Sol–gel method	10 mg/L MB (50 mL)	Visible light	2.6% (0.00008 min−1) within 6 h	42.4% (0.00140 min−1) within 6 h	[98]
Cu-TiO2 film (anatase to rutile phase)	Cu: TiO2 both in film (25.10%)	0.3 g	Sol–gel method	10 mg/L MB (50 mL)	Visible light	2.6% (0.00008 min−1) within 6 h	25.1% (0.00082 min−1) within 6 h	[98]
Cu-TiO2 nanopowders (Anatase phase)	2.0% Cu-TiO2	1 g/L	Sol–gel method	20 mg/L DFC (100 mL)	Visible light	25% within 7 h	33.26% within 7 h	[118]
Cu-TiO2 nanopowders (Anatase to rutile phase)	3% Cu-TiO2	0.125 mol L−1	Sol–gel method	20 mg/L MB (50 mL)	Xenon Lamp	0.08124 min−1 within 6 h	0.00575 min−1 within 6 h	[119]
Cu-TiO2 nanoparticles (anatase phase)	-	0.1 g	MWASG	30 mg/L MO (100 mL)	Visible light	1.4 × 10−3 min−1 within 6 h	7.0 × 10−3 min−1 within 6 h	[120]
Cu-TiO2 nanoparticles (anatase phase)	-	0.1 g	MWASG	30 mg/L MB (100 mL)	Visible light	7.0 × 10−4 min−1 within 6 h	5.6 × 10−3 min−1 within 6 h	[120]
Cu-TiO2 nanoparticles (anatase phase)	-	0.1 g	MWASG	30 mg/L MO (100 mL)	UV	5 × 10−3 min−1 within 6 h	1.2 × 10−2 min−1 within 6 h	[120]
Cu-TiO2 nanoparticles (anatase phase)	-	0.1 g	MWASG	30 mg/L MB (100 mL)	UV	2.5 × 10−3 min−1 within 6 h	8.6 × 10−3 min−1 within 6 h	[120]
Cu-TiO2 nanoparticles (anatase phase)	2 wt% Cu-TiO2	0.38 g/L	Sol–gel method	10 mg/L Phenol (100 mL)	UV	82% within 60 min for P25	98% within 60 min	[121]
Cu-TiO2 nanoparticles (anatase phase)	2 wt% Cu-TiO2	0.38 g/L	Sol–gel method	10 mg/L Phenol (100 mL)	Visible light	35 and 75% after 60 min and 180 min, respectively for P25	22 and 37% after 60 min and 180 min, respectively	[121]
Cu-TiO2 nanopowders (anatase phase)	0.21 mol% Cu-TiO2	3 g·dm−3	Sol–gel method	20 mg/L 2-CP (50 mL)	UV	-	98.92% within 6 h	[122]
Cu/TiO2/bentonite composite nanoparticles (anatase phase)	-	0.02 g	Thermal decomposition and reduction method	10 mg/L Deltamethrin insecticide (50 mL)	Sunlight	87.01% within 120 min for TiO2/bentonite	97% within 120 min	[123]
Cu-TiO2 nanoparticles (anatase phase)	3% Cu-TiO2	0.1 g	Sol–gel method	15 mg/L MO (100 mL)	Xenon Lamp	0.0011 min−1 within 60 min	61% (0.0166, min−1) within 60 min	[124]
Cu-TiO2 films (anatase phase)	4% Cu-TiO2	0.1 g	Sol–gel method	25 mg/L MB (100 mL)	UV	92% ((0.015 min−1) within 180 min	16% (0.001 min−1) within 180 min	[125]
Cu/TiO2 nanoparticles (anatase to rutile phase)	10% Cu-TiO2	0.5 g	Coprecipitation method	100 mg/L ECR (200 mL)	Visible light	20.9% within 120 min	60.6% within 120 min	[89]
Cu-TiO2 nanoparticles (anatase phase)	1% Cu-TiO2	2 g/L	Sol–gel method	100 mg/L MO, MB (100 mL)	Visible light	45% for MB within 5 h for Degussa P25	81.22 and 44.05% for MB and MO, respectively within 5 h.	[94]
Cu-TiO2 nanoparticles (anatase phase)	5% Cu-TiO2	0.5 g/L	Sol–gel method	60 mg/L Orange II (35 mL)	UV	95% for P25 within 180 min	82% within 180 min	[126]
Cu-TiO2 nanoparticles (anatase phase)	5% Cu-TiO2	0.5 g/L	Impregnation method	60 mg/L Orange II (35 mL)	UV	99% for P25 within 180 min	90% within 180 min	[126]
Cu2+/TiO2 nanoparticles (mixed anatase and brookite phases)	0.5 mol% Cu2+/TiO2	1 g/L	Sol–gel method	10 mg/L PNP (100 mL)	UV/Visible light	20 and 50% and under visible and UV/Visible light, respectively for Degussa P25 within 24 h	42 and 55% under visible and UV/visible light, respectively within 24 h	[127]
Cu-TiO2 nanoparticles (mixed anatase and rutile phases)	-	0.1 g	Sol–gel method	20 mg/L MB (50 mL)	Visible light	89.69% within 60 min	27.5% within 60 min	[128]

MB: methylene blue; PNP: p-nitrophenol; MO: methyl orange; 2-CP: 2-chlorophenol; ECR: Eriochrome cyanine red; DFC: diclofenac; TC: tetracycline; OG: orange G; E131 VF: food colorant; MWASG: microwave-assisted sol–gel method; NPX: naproxen; RhB: rhodamine B; 4-CP: 4-chlorophenol.

presents the summary of recent progress on Cu-TiO₂ photocatalysts for organic pollutants degradation.

4.1.4. Modification with Manganese (Mn)

Manganese (Mn) has a great potential to permit significant optical absorption in infrared solar light or visible light, via the introduction of intermediate bands within the forbidden gap as well as the combined effects of narrowed bandgap [129,130]. The intermediate bands provide adequate carrier mobility and hence significant curvature [131]. Mn ions can be easily incorporated into the lattice of TiO₂ to obtain a deformed structure. It has been reported that Mn in 3d states has some contributions to the TiO₂ conduction band, which will further impact the bandgap of TiO₂. Modifying with Mn cations has been reported to favor the charge separation acting like electrons trap as Mn⁴⁺ or Mn³⁺ [129] and holes trap as Mn²⁺ [132,133], thus prolonging the separation of photoinduced carriers and increasing the photocatalytic activity [134].

In Mn-photocatalytic materials with a mixture of oxidation states could be seen as a limitation because it is impossible to easily control the amount of one specific oxidation state [135]. However, this can be approached as a benefit since the oxidation state of Mn can act as a charge separator, attracting photogenerated holes in the Mn²⁺/Mn³⁺ state or as a photogenerated electron in the Mn⁴⁺/Mn³⁺ state, the oxidation states mixture can improve the photocatalytic activity [133]. When TiO₂ semiconductors are modified with Mn and applied in the photocatalytic degradation of organic dyes, the reduction in bandgap can be achieved, thereby improving the performance [136,137,138]. This is due to the following reasons: when Mn is introduced in the TiO₂ network at different oxidation states (2+, 3+, or 4+), there is a deformation of the TiO₂ crystalline structure and optimization of its optical properties [139,140,141]. Ref. [142] confirmed a high stability feature when TiO₂ was modified with Mn¹⁺ to Mn⁶⁺. Mn²⁺ can enter the lattice of TiO₂, replacing the position of the original titanium atom while forming a new chemical bond. In addition, modifying with Mn²⁺ can also increase the number of hydroxyl groups on the surface of TiO₂ in the aqueous solution, forming an active center for the adsorption of water [143].

Mn-TiO₂ nanoparticles can be prepared by (i) sol–gel technique [131,144], (ii) hydrothermal treatment [145], (iii) ultrasonic synthesis [146], (iv) liquid flame aerosol synthesis technique [32], (v) impregnation method [147], (vi) calcination [148], and (vii) thermal treatment [149].

Table 6. Summary of recent progress on Mn-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutants (Volume)	Light Source	Unmodified	Modified	Ref.
Mn-TiO2 nanoparticles (anatase phase)	10% Mn-TiO2	0.025 g	Sol–gel method	10−5 mol/L MO (50 mL)	UV/Visible light	0.00075 min−1 and 0.002631 min−1 for visible light and UV light, respectively within 120 min. 95.6 and 16.3% under UV light and visible light, respectively	0.00070 min−1 and 0.00074 min−1 for visible light and UV light, respectively within 120 min. Modifying with Mn presents a negative effect on the photocatalytic degradation of MO. 12.8% under UV light	[92]
Mn2+/TiO2 nanoparticles (mixed crystal of anatase and rutile phases)	1.90 wt% Mn2+/TiO2	0.05 g	Hydrothermal method	50 mg/L MG (50 mL)	Visible light	48% within 180 min for pure TiO2	81% within 180 min	[150]
Mn-TiO2 nanopowders (anatase phase)	0.6% Mn-TiO2	0.05 g/L	Sol–gel method	10 mg/L DCF (100 mL)	UV	51% within 240 min	0.0033 min−1 within 240 min	[151]
Mn-TiO2 nanoparticles (mixture of anatase and rutile phases)	0.1% Mn-TiO2	0.1 g	Modified sol–gel method	Benzene, Xylene, toluene (100 mL)	UV/Visible light	60, 60 and 55% for benzene, xylene and toluene, respectively under UV light.	60, 60, and 40% for benzene, xylene and toluene, respectively under UV light. 34 and 22% for toluene and xylene, respectively under visible irradiation within 60 min	[152]
Mn-TiO2 nanopowders	4 wt% Mn-TiO2	0.1 g/L	Hydrothermal route	10 mg/L Phenol (50 mL)	UV	30% within 7 h	90% within 7 h	[153]
Mn-TiO2 nanoparticles (anatase phase)	1 wt% Mn-TiO2	1 g	Sol–gel method	30 mg/L Toluene (100 mL)	VUV	70% without photocatalyst	89.8% within 120 min	[154]
Mn-TiO2 nanoparticles (anatase phase)	0.3 wt% Mn-TiO2	0.1 g	Sol–gel method	5 mg/L MO, MB (100 mL)	Visible light	26.80 and 44.18% degradation rates for MO and MB, respectively for unmodified TiO2 within 5 h. 45% of MB for Degussa P25 within 5 h.	15.48 and 16.41% for MO and MB, respectively within 5 h	[94]
Mn-TiO2 films (anatase phase)	5 wt% Mn-TiO2	0.5 g/L	Sol–gel method	6 mg/L Orange II (50 mL)	UV	95% for P25 within 180 min	97% within 180 min	[126]
Mn-TiO2 nanoparticles (anatase-brookite phase)	5 wt% Mn-TiO2	0.5 g/L	Impregnation method	6 mg/L Orange II (100 mL)	UV	99% for P25 within 180 min	97% within 180 min	[126]
Mn-TiO2 nanoparticles (mixed anatase and brookite phases)	0.5 mol% Mn-TiO2	1 g/L	Sol–gel method	10 mg/L PNP (100 mL)	UV/Visible light	50 and 20% under UV/Visible and visible light irradiation for Degussa P2 within 8 h.	19 and 5% under UV/Visible and visible light irradiation, respectively within 8 h.	[127]
Mn-TiO2 nanoparticles (anatase phase)	5 wt% Mn-TiO2	0.1 g	Sol–gel method	5 mg/L MB, MO (100 mL)	UV/Visible light	65.3 and 72.4% under UV irradiation for MO and MB, respectively within 60 min. 4.1 and 6.2% for MO and MB, respectively under visible irradiation within 240 min	85.2.1 and 93.6% under UV irradiation for MO and MB, respectively within 60 min. 65.2 and 73.2% for MO and MB, respectively under visible irradiation within 240 min	[38]

MB: methylene blue; PNP: p-nitrophenol; MO: methyl orange; DCF: diclofenac.

presents the summary of recent progress on Mn-TiO₂ photocatalysts for organic pollutants degradation.

4.1.5. Modification with Zirconium (Zr)

Zirconium (Zr) has a wide bandgap with a more positive (4.0 V vs NHE) VB and a more negative (−1.0 V vs NHE) CB than titania [155]. The tetravalent cations of both Ti and Zr have a comparable atomic radius, and their oxides have similar physicochemical properties. It has been previously reported that the modification of TiO₂ with Zr can improve physicochemical characteristics including an increase in the specific surface area [156], decrease in crystallite size as a result of dissimilar coordination geometry and nuclei [157]; enhance surface acidity [158], the transition temperature between anatase and rutile [156,158], and the adsorption and the hydrophilic properties [159], and enhance thermal stability and imbalance of charge resulting from the formation of capture traps as well as the formation of Ti-O-Zr bonds. These features favor a higher efficiency while separating photogenerated carriers, thus improving the photocatalytic efficiency [160,161]. Moreover, the hydroxyl groups that are trapped on the catalyst surface by holes improve the quantum yield and reduce the recombination reactions [159,162]. Modifying with Zr conquers charge recombination by electron trapping [163].

Liu et al. [164] prepared mesoporous Zr-TiO₂ NPs via the solvothermal approach and observed a higher photocatalytic activity in comparison with the commercialized P25 titania. Li et al. [165] reported a macro–mesoporous Zr-TiO₂ material via the facile surfactant self-assembly approach and observed a remarkable photocatalytic activity towards RhB decomposition under UV light radiation as compared to pure ZrO₂ and unmodified TiO₂ materials. A study by [166] found that modifying with Zr⁴⁺ can increase the surface area, and suppress crystal growth.

Table 7. Summary of recent progress on Zr-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutant (Volume)	Light Source	Unmodified	Modified	Ref.
Zr-TiO2 nanoparticles (mixed anatase and rutile phases)	0.01% Zr-TiO2	0.1 g	Sol–gel method	20 mg/L MB (50 mL)	Visible light	89.69% within 60 min	81.9% within 60 min	[128]
Zr-TiO2 films (stable anatase phase)	0.05% Zr/TiO2	0.005 g	EISA via dip-coating	0.001 mM herbicide chloridazon, henol, 4-CP (50 mL)	Xenon lamp	42% within 4 h for phenol, 58% within 4 h for 4-CP	The Zr-TiO2 film showed the superior photocatalytic activity for all pollutants but showed lowest rate using phenol	[167]
Zr-TiO2 nanoparticles (anatase phase)	0.2% Zr-TiO2	0.4 g	Sol–gel method	1.0±0.5 mg/m3 formaldehyde	UV	10% within 48 h for P25	95.14% within 48 h. Furthermore, 94.38% removal efficiency was achieved even after seven cycles	[168]
Zr-TiO2 nanoparticles (anatase phase)	Zr:TiO2 = 0.08:0.92	0.005 g	EISA method	0.001 mM formaldehyde	Xenon lamp	45% within 48 h for TiO2	92% after 48 h	[166]
Zr-TiO2 nanoparticles (anatase phase)	5% Zr-TiO2	0.04 g/L	A combined sol–gel and CVD method	10 ppm IBP	UV	-	80% after 40 min	[33]
Zr-TiO2 nanoparticles (mixed anatase and brookite phase)	2.0% Zr-TiO2	1 g/L	Sol–gel method	10 mg/L PNP (100 mL)	UVA/Visible light	20 and 84% for pure TiO2 and commercial Evonik P25, respectively UV-A light. 25 and 12% for pure TiO2 and Evonik P25, respectively under visible irradiation within 17 h	96% under UV-A light, 38% under visible irradiation within 17 h	[169]
Zr-TiO2 nanoparticles (anatase phase)	0.5% Zr-TiO2	0.25 g	Sol–gel method	10 mg/L antipyrine (phenazone) (10 mL)	Visible light	-	90% within 360 min. Furthermore, 8% lower degradation rate was achieved, even after six hours of irradiation	[170]
Zr-TiO2 hollow microspheres (anatase phase)	12.6 wt% Zr-TiO2	0.02 g	Facile solvothermal method	20 mg/L RhB (50 mL)	UV	14.6% within 60 min	96.3% within 60 min	[171]
Zr-TiO2 nanoparticles (mixed anatase and brookite phase)	2.0% Zr-TiO2	1 g/L	Sol–gel method	10 mg/L PNP (100 mL)	UV/Visible light	20 and 84% for pure TiO2 and commercial Evonik P25, respectively within 24 h under UV–Visible light. 25 and 12% for pure TiO2 and commercial Evonik P25, respectively within 24 h under visible light.	96% within 24 h under UV-Visible light. 38% within 24 h under visible light	[169]
Zr-TiO2/reduced Graphene Oxide nanoparticles (mixed anatase and brookite phase)	3% ZrO2–TiO2	0.025 g	Sol–gel method	20 mg/L EB (100 mL)	Visible light	-	75.75% within 90 min	[172]
ZrO2/TiO2 nanofibers (anatase phase)	40 wt% ZrO2/TiO2 (Zr:Ti = 1:3)	0.02 g	Sol–gel method	10 mg/L MB (50 mL)	Mercury lamp	43.3% within 180 min.	82.7% within 180 min	[173]
ZrO2/TiO2 nanoparticles (anatase phase)	6.9% ZrO2–TiO2	0.02 g	Facile surfactant self-assembly	5 mg/L RhB (100 mL)	UV	52.5% within 3 h.	86.9% within 3 h. Rate constant was 3.0 times higher than unmodified TiO2 within 3 h	[165]
ZrO2/TiO2 nanoparticles (anatase phase)	5% ZrO2/TiO2	0.1 g	Sol–gel method	5 × 10−5 mol/L MB, MO (100 mL)	UV/Visible light	65.3 and 72.4% for MO and MB, respectively within 60 min under UV light. 4.1 and 6.2% for MO and MB, respectively within 240 min under visible light.	70.1 and 77.3% for MO and MB, respectively within 60 min under UV light. 22.6 and 38.2% for MO and MB, respectively within 240 min under visible light.	[38]
Zr-TiO2 nanoparticles (anatase phase)	6 mol% Zr-TiO2	1 g/L	Sol–gel method	5 mg/L Ponceau BS (180 mL)	UV	24.45% (0.0107 min−1) within 30 min	99.3% (0.1810 min−1) within 30 min	[155]

PNP: p-nitrophenol; MO: methyl orange; RhB: rhodamine B; MB: methylene blue; EB: eosin blue; 4-CP: 4-chlorophenol; IBP: ibuprofen.

presents the summary of recent progress on Zr-TiO₂ photocatalysts for organic pollutants degradation.

4.1.6. Modification with Iron (Fe)

Iron (Fe) has a half-filled (3d5) electronic configuration, similar ionic radii of Fe³⁺ to that of Ti⁴⁺ and can be easily incorporated into the crystal lattice of TiO₂ [174,175], and it has been documented that modifying the crystal lattice of TiO₂ with Fe³⁺ ions weakens the surrounding oxygen atoms bonding. Consequently, the oxygen atoms are readily released from the lattice causing an oxygen vacancy, hence more oxygen/H₂O/OH- can be adsorbed onto the TiO₂ surface resulting in the decrease in CB electrons. In addition, Fe³⁺ ions can perform as a shallow charge trap in the TiO₂ lattice, because the energy level (redox potential) of Fe²⁺/Fe³⁺ lies close to that of Ti³⁺/Ti⁴⁺, which favors the development of charge carrier separation, further causing a decrease in the e⁻/h⁺ pair recombination and expanding the photoresponse of TiO₂ into the visible light range [175,176].

Ali et al. [174] used the sol–gel method for the successful synthesis of Fe-TiO₂ NPs which could be activated in the presence of visible light. Naghibi et al. [177] also studied the properties of TiO₂ modified with four metals including Cu, Fe, Cd, and Ce, and observed that the Fe-TiO₂ presents the smallest crystal size with forbidden bandwidth among these four metals-modified TiO₂. [178] found that the Fe³⁺-TiO₂ powder showed strong absorption characteristics in the visible region and [179] observed that introducing Fe³⁺ into the TiO₂ lattice facilitates the formation of a redox site, enabling absorption of the visible portion. The semi-filled electronic structure of Fe³⁺ ions not only facilitates the separation of photoelectrons and holes but also reduces the bandgap of TiO₂ [180]. In addition, modifying with Fe³⁺ can introduce more oxygen vacancies in TiO₂ surface and lattice [181], which is beneficial for increasing the number of surface OH groups and improvement in photocatalytic degradation.

Table 8. Summary of recent progress on Fe-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutants (Volume)	Light Source	Unmodified	Modified	Ref.
Fe-TiO2 nanopowders (mixture of anatase and rutile phases)	2wt% Fe-TiO2	0.01 g	Conventional solid-state reaction	20 mg/L MB (50 mL)	Xenon arc lamp	90.80% within 210 min	94.93% within 210 min	[182]
Fe-TiO2 nanoparticles (anatase phase)	1 wt% Fe-TiO2	0.1 g	Sol–gel method	5 mg/L MO, MB (100 mL)	Visible light	44.18 and 26.80% degradation rates for MB and MO, respectively for unmodified TiO2 within 5 h; 45% for MB using Degussa P25 within 5 h	1.41 and 7.94% for MB and MO, respectively within 5 h	[94]
Fe-TiO2 nanoparticles (anatase phase)	0.1 mol% Fe-TiO2	0.03 g/L	Precipitation method	MO, 4-CP (100 mL)	Xenon arc lamp	30 and 45% of 4-CP and MO, respectively within 4 h	65 and 95% for 4-CP and MO, respectively within 4 h	[183]
Fe-TiO2 nanoparticles (anatase phase)	0.0017 mol% Fe-TiO2	3 g/L	Sol–gel method	10 mg/L AO7 (100 mL)	Visible light	10% within 60 min	80% within 60 min	[184]
Fe-TiO2 nanoparticles (anatase phase)	1.0% Fe-TiO2	0.05 g/0.5 g	Sol–gel method	10 mg/L NPX, 4-CP (100 mL)	UV	68.9% (0.1069 L g−1 min−1) and 84.9% (0.0124 L g−1 min−1) for 4-CP and NPX, respectively for unmodified TiO2 within 6 h	97.7% (0.1111 L g−1 min−1) and 37% (0.0053 L g−1 min−1) for NPX and 4-CP, respectively within 6 h	[93]
Fe-TiO2 nanoparticles (anatase phase)	0.5% Fe-TiO2	0.5 g/L	Sol–gel method	10 mg/L Phenol (200 mL)	Visible light	33% within 90 min	57% (about 73% increase) within 90 min. that the activity of pristine TiO2 was higher than that of the modified catalysts with high content	[185]
Fe-TiO2 nanoparticles (anatase phase)	0.1 wt% Fe-TiO2	0.5 g	Sol–gel method	20 mg/L AO7 (500 mL)	Visible light	25% within 300 min	54% within 300 min	[186]
Fe-TiO2 nanoparticles (mixture of anatase and rutile phases)	0.05 mol% Fe-TiO2	0.5 g/L	Facile ultrasonic assisted hydrothermal method	10 mg/L PNP (50 mL)	Visible light	42% within 5 h for pure TiO2	92% within 5 h	[187]
Fe-TiO2 nanoparticles (mixture of anatase and rutile phases)	0.075 mol% Fe-TiO2 NPs	0.5 g/L	Facile ultrasonic assisted hydrothermal method	10 mg/L MB (50 mL)	Visible light	50% within 180 min for pure TiO2	93% within 180 min	[187]
Fe3+/TiO2 nanoparticles (anatase phase)	3% Fe3+/TiO2	25 cm × 25 cm	Sol–gel spin coating technique	5 mg/L MO (50 mL)	Visible light	34% within 360 min.	95.25% within 360 min.	[188]
Fe3+/TiO2 nanoparticles (anatase phase)	3% Fe3+/TiO2	25 cm × 25 cm	Sol–gel spin coating technique	5 mg/L MB (50 mL)	Visible light	34% within 360 min.	Fe-TiO2 (3% Fe3+) film exhibits high efficiency of about 97.8% after 10 cycling runs of MB degradation.	[188]
Fe-TiO2 nanoparticles (anatase phase)	0.5%Fe-TiO2	0.20 g/L	Wet-impregnation method	5 ppm MO (200 mL)	UV/Visible light	85 and 11% under UV light and visible light irradiation, respectively within 2 h.	82.8 and 74.4% under UV light and visible light irradiation, respectively. within 2 h	[189]
Fe-TiO2 thin films (anatase phase)	0.5% Fe-TiO2	-	Sol–gel method	2.51 × 10−4 M NB (-)	UV	70.52% within 240 min	88.45% within 240 min	[190]
Fe-TiO2 nanoparticles (mixture of anatase and rutile phases)	2.5 wt% Fe-TiO2	0.1 g	Sol–gel method	10 mg/L MB (50 mL)	UV	~17% within 240 min. 54% within 90 min	40% within 240 min. 78% within 90 min. Furthermore, the photocatalytic activity only decreased 7.7% after three runs.	[191]
Fe-TiO2 nanoparticles (anatase phase)	0.5 mol% Fe-TiO2	0.5 g/L	Ultrasonic dispersion method	10 mg/L RhB (50 mL)	UV	60% within 150 min	91.11% within 150 min	[192]
Fe-TiO2 microsized powder (anatase phase)	-	0.2 g	sol–gel and hydrothermal treatment	50 mg/L BO2 (3 L)	Solar	25% within 180 min.	≥90% within 180 min.	[193]
Fe-TiO2 nanoparticles (anatase phase)	6 wt% Fe-TiO2	-	Sol–gel method	10 mg/L MB (100 mL)	Visible light	36% without irradiation for Fe-TiO2	99.5% within 3 h	[194]
Fe-TiO2 nanoparticles (anatase phase)	0.4 wt% Fe-TiO2	0.2 g/L	Sol–gel method	10 mg/L Acid Blue 80 (50 mL)	UV	27.86% (1.6 × 10−3 min−1) within 120 min	31.13% (2.0 × 10−3 min−1) within 120 min	[195]
Fe-TiO2 nanoparticles (anatase phase)	0.4 wt% Fe-TiO2	0.2 g/L	Ultrasound assisted approach	10 mg/L Acid Blue 80 (50 mL)	UV	27.86% (1.6 × 10−3 min−1) within 120 min	38.01% (2.4 × 10−3 min−1) within 120 min	[195]
Fe-TiO2 nanoparticles (anatase phase)	1.5 wt% Fe-TiO2	0.4 g/L	Hydrothermal	20 mg/L Diazinon (100 mL)	UV	~20% within 100 min	75% within 100 min	[196]
Fe-TiO2 nanoparticles (anatase phase)	10% Fe-TiO2	0.005 g	Sol–gel method	20 mg/L MB (250 mL)	UV	76.09% within 150 min	96.66% within 150 min	[197]
Fe-TiO2 nanoparticles (anatase phase)	2% Fe-TiO2	0.5 g/L	Sol–gel method	50 mg/L AO7 (2.5 L)	UV/Visible/Solar light	9.2% within 6 h	UV (100%), visible (100%) and solar light (90%) within 6 h. Furthermore, the Fe-TiO2 photocatalysts were stable and can maintain performance up to 6 recycle use.	[198]
Fe-TiO2 nanotubes (anatase phase)	Hydrothermal temperature of 150 °C for 3 h	-	Hydrothermal	5 mg/L CR (35 mL)	Visible light	26.32% within 180 min	92.5% within 180 min	[199]
Fe-TiO2 thin films (anatase phase)	0.02% Fe/TiO2	-	Sol–gel method	20 mg/L MB (100 mL)	Visible light	~84 within 200 min	No enhancement in photocatalytic activity of Fe-TiO2 thin films was achieved. This was be attributed to sodium diffusion from the substrate used.	[200]
Fe3+/TiO2 nanoparticles (anatase phase)	7% Fe3+/TiO2	-	Simple spin coating technique	3 mg/L MB (50 mL)	Xenon arc lamp	80% within 4 h	96.7% within 4 h. Furthermore, the degradation rate of MB was ~83.8% after 10 cyclic runs.	[201]
Fe-TiO2 nanoparticles (anatase phase)	0.5 wt% Fe-TiO2	-	Sol–gel method	20 mg/L MO (200 mL)	Visible light	24% within 60 min	98% within 60 min	[202]
Fe-TiO2 nanoparticles (anatase phase)	3% Fe-TiO2	0.2 g	Sol–gel method	20 mg/L RhB (100 mL)	Solar	34% within 120 min	64% within 120 min	[203]
Fe-TiO2 nanoparticles (anatase-brookite phase)	5 wt% Fe-TiO2	0.5 g/L	Impregnation method	6 mg/L Orange II (100 mL)	UV	99% for P25 within min	69% within 180 min	[126]
Fe-TiO2 nanoparticles (anatase-brookite phase)	5 wt% Fe-TiO2	0.5 g/L	Sol–gel method	6 mg/L Orange II (100 mL)	UV	95% for P25 within min	95% within 180 min	[126]
Fe-TiO2 nanopowders (anatase phase)	0.5% Fe-TiO2	0.1 g/L	Sol–gel method	2.45 × 104 M NB (-)	Mercury lamp	32.14% within 240 min for pure TiO2	84.91% within 240 min	[204]
Fe-TiO2 nanopowders (anatase phase)	1 wt% Fe-TiO2	0.1 g	Sol–gel method	10 mg/L MO, 4-CP (100 mL)	Visible light	31 and 28% for MO and 4-CP, respectively within 180 min	54 and 49% for MO and 4-CP, respectively within 180 min	[205]
Fe3+/TiO2 nanoparticles (mixed anatase and brookite phases)	0.5 mol% Fe3+/TiO2	1 g/L	Sol–gel method	10 mg/L PNP (100 mL)	UV/Visible light	50 and 20% under UV/Visible light and visible light, respectively using Degussa P25 within 8 h	56 and 44% under UV/Visible light and visible light, respectively within 8 h	[127]
Fe-TiO2 nanoparticles (mixture of anatase and rutile phases)	2 wt% Fe-TiO2	0.25 g/L	Surface impregnation method	0.0755 mg/L Carbendazim and 0.0096 mg/L for propiconazole(-)	Sunlight	-	98.5 and 92% of carbendazim and propiconazole, respectively within 60 min	[206]
Fe-TiO2 nanotube (anatase phase)	1% Fe-TNAs	0.001 g/L	solvothermal method	20 mg/L MB (100 mL)	Solar	-	98.79% within 120 min	[207]
Fe-TiO2 nanotube (anatase phase)	0.1 mol% Fe-TiO2	1 g	Sol–gel method	10 mg/L AR-27 (1000 mL)	Visible light	78% within 2 h.	99% within 2 h. The removal efficiency was kept at 89% after catalyst used for 4 times.	[208]
Fe-TiO2 thin films (anatase phase)	25 wt% Fe-TiO2	-	Sol–gel method	4 mg/L MB (200 mL)	Visible light	0.018/h within 8 h	0.038/h within 8 h	[209]
Fe-TiO2 nanoparticles (anatase phase)	1 wt% Fe-TiO2	0.03 g	Sol–gel method	10 mg/L Orange II (500 mL)	UV/Visible light	30% within 60 min	24% within 60 min	[210]
Fe3+/TiO2 nanoparticles (anatase phase)	0.1% Fe3+/TiO2	0.2 g	Sol–gel method	10 mg/L MB (100 mL)	Xenon arc lamp	90% within 6 h	0.716 min−1 within 6 h	[211]
Fe3+/TiO2 thin films (anatase phase)	25 wt% Fe3+/TiO2	-	Sol–gel method	10 mg/L MB (100 mL)	Visible light	13.4% within 8 h	24.9% within 8 h	[212]
Fe-TiO2 nanoparticles (anatase phase)	0.5 wt% Fe-TiO2	0.25 g/L	Sol–gel method	(0.37–8.45) × 10−4 M NB (-)	Mercury lamp	48% within 240 min	99% within 240 min	[213]
Fe-TiO2 nanotube (anatase phase)	5 wt% Fe/TNAs	0.5 mol·L−1	Electrochemical anodization and subsequent dip-coating	10 mg/L BPA (200 mL)	Xenon lamp	-	18.3% within 240 min	[214]
Fe-TiO2 nanoparticles (anatase phase)	0.15% Fe-TiO2	0.3 g/L	Sol–gel method	20 mg/L RB5 (250 mL)	Xenon lamp	0.0180 min−1 within 60 min	0.0875 min−1 within 60 min	[215]
Fe-TiO2 nanoparticles (anatase phase)	0.5 wt.% Fe-TiO2	0.25 g/L	Sol–gel method	(0.37–8.45) × 10−4 M NB (-)	Medium-pressure mercury lamp	48% within 240 min	99% within 240 min	[213]
Fe-TiO2 nanoparticles (anatase phase)	1.0% Fe-TiO2	0.05 g/0.5 g	Sol–gel method	10 mg/L NPX,4-CP (100 mL)	UV	79% (0.1210 L g−1 min−1) within 6 h	97% (0.1111 L g−1 min−1) within 6 h	[93]
Fe-TiO2 nanoparticles (anatase phase)	0.05 wt% Fe-TiO2	0.001 g/cm2	Sol–gel method	25 mg/L DB15 (100 mL)	UV	3.3% within 1 h	31% within 1 h	[216]
Fe-TiO2 nanoparticles (anatase phase)	1.75 wt% Fe-TiO2	0.03 g/L	Nanosol/dip-coating method.	30 mg/L MB (50 mL)	UV	-	76% within 2 h	[217]

A07: acid orange 7; RB5: reactive black 5; 4-CP: 4-chlorophenol; NPX: naproxen; MB: methylene blue; MO: methyl orange; BO2: basic orange 2; RR 198: reactive red 198; PNP: p-nitrophenol; AR-27: acid red 27; NB: nitrobenzene; BPA: bisphenol A; RhB: rhodamine B; NB: nitrobenzene; CR: Congo red; PNP: 4-nitrophenol.

presents the summary of recent progress on Fe-TiO₂ photocatalysts for organic pollutants degradation.

4.1.7. Modification with Chromium (Cr)

Chromium (Cr) has received many considerations owing to its partially filled d shell, and optically active nature. Cr³⁺ is suitable with an abundant electron shell structure [78]. Cr³⁺ ions have fewer valence electrons in comparison with Ti⁴⁺. TiO₂ crystalline lattice deformation is evoked by substitution of Ti⁴⁺ by Cr³⁺, creating an extra energy level between the VB and the CB of TiO₂, which permits those photons which have lower energy to accomplish the photocatalytic activity in the visible light range [218]. Because of the narrowing of the bandgap between valence and acceptor, the electron movement is better even at a lower temperature. In addition, such substitution restrains the crystal growth of TiO₂ as well as reducing TiO₂ crystallization, which may improve the absorption of light derived from the size effect [219]. Peng et al. [220] studied the influence of modification with Cr on the photocatalytic activity of TiO₂ and observed a shift in absorption edges of TiO₂ towards high wavelength region with the increasing Cr content. Various approaches are being employed to fabricate Cr-TiO₂ such as the spray pyrolysis [221], sol–gel method [220], and sputtering [45].

Table 9. Summary of recent progress on Cr-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutants	Light Source	Unmodified	Modified	Ref.
Cr-TiO2 nanoparticles (anatase phases)	0.5 wt% Cr-TiO2	1.5 g/L	Sol–gel method	10 mg/L MB (100 mL)	Hg vapor lamp	62% within 120 min for pure TiO2	58% within 120 min	[84]
Cr-TiO2 nanoparticles (anatase phase)	1 mol% Cr	0.1 g	Sol–gel method	15 mg/L MB (50 mL)	UV	33.43% for pure anatase within 24 h	50.01% within 24 h	[222]
Cr-TiO2 nanoparticles (anatase phase)	4 mol% Cr	0.1 g	Sol–gel method	100 ppm CR (50 mL)	UV	0.73% for pure anatase within 24 h	17.78% within 24 h	[222]
Cr-TiO2 nanoparticles (anatase phase)	10% Cr-TiO2	0.025 g	Sol–gel method	10−5 mol/L MO (50 mL)	UV/Visible light	95.6 and 16.35% under UV and visible light irradiation, respectively within 120 min	0.00276 min−1 (3.7 larger than that of the unmodified TiO2 (0.00075 min−1) within 120 min	[92]
Cr-TiO2 nanoparticles (anatase phase)	1 wt% Cr-TiO2	0.1 g	Sol–gel method	5 mg/L MO, MB (100 mL)	Visible light	44.18 and 26.80% for MB and MO, respectively for unmodified TiO2. 45% for MB within 5 h for Degussa P25.	11.47 and 15.48% for MB and MO, respectively within 5 h.	[94]
Cr-TiO2 nanoparticles (anatase phase)	0.5% Cr-TiO2	0.008 g	Sol–gel method	10 mg/L 4-CP (100 mL)	Visible light	76.5% within 390 min	90.7% within 390 min	[223]
Cr-TiO2 nanoparticles (anatase phase)	-	0.4 g	FSP, coprecipitation, and sol–gel synthesis techniques	50 μM 4-CP (50 mL)	Visible light	2% within 4 h	61% within 4 h.	[224]
Cr-TiO2 nanoparticles (mixed anatase and brookite phases)	0.5 mol% Cr-TiO2	1 g/L	Sol–gel method	10 mg/L PNP (100 mL)	UV/Visible light	50 and 20% under UV/Visible and visible light, respectively for Degussa P25 within 8 h.	38 and 27% under UV/Visible and visible light, respectively within 8 h.	[127]

4-CP: 4-chlorophenol; FSP: flame spray pyrolysis; MO: methyl orange; PNP: p-nitrophenol; MB: Methyl blue; CR: Congo red.

presents the summary of recent progress on Cr-TiO₂ photocatalysts for organic pollutants degradation.

4.1.8. Modification with Molybdenum (Mo)

Modification of TiO₂ using metals with a higher oxidation state (e.g., Mo⁶⁺ and Mo⁵⁺) increases the photocatalytic activity due to improved transfer and separation of photogenerated holes and electrons [225]. Moreover, Mo⁶⁺ is also a suitable transition metal to be introduced into the lattice of TiO₂ due to similarity in the ionic radii of Ti⁴⁺ and Mo⁶⁺ [226], so Mo can easily be incorporated into TiO₂ crystal structure, producing the impurity levels, which as a consequence narrows the bandgap of TiO₂ [77]. In addition, Mo⁶⁺ has no electron in the d orbital and can also be reduced to lower oxidation states (Mo⁴⁺, Mo⁵⁺), implying its several oxidation states in the TiO₂ matrix can act as a superficial potential trap for the photoinduced e⁻ to h⁺ pairs, thus lengthening the lifetime of carriers and increasing the photocatalytic activity. Furthermore, the redox potential of Mo⁶⁺/Mo⁵⁺ (vs. NHE) is 0.4 V, allowing Mo⁶⁺ to capture photoinduced electrons, inhibiting the recombination of charge carriers [227]. Mo-TiO₂ photocatalysts can be synthesized using evaporation-induced self-assembly, hydrothermal method, and sol–gel process [228,229,230,231,232,233].

Table 10. Summary of recent progress on Mo-TiO₂ photocatalysts for organic pollutants degradation

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutants (Volume)	Light Source	Unmodified	Modified	Ref.
Mo-TiO2 nanocrystals (anatase phase)	5% Mo-TiO2	0.03 g	Facile hydrothermal method	25 ppm Benzene (-)	UV	13.6% within 4 h	79.1% (0.0065 min−1, which was 4.81 times higher than the unmodified TiO2) within 4 h. Furthermore, the degradation ratio of benzene at the sixth run was 76.4%.	[234]
Mo-TiO2 layer (rutile to stable anatase phase)	6.0% Mo-Ti O2	15 mm × 10 mm	PEOx	10 mg/L MB (100 mL)	660-nm Tungsten-halogen	55% within 180 min for pure TiO2	86% within 180 min	[76]
Mo-TiO2 nanoparticles (anatase phase)	1 wt% Mo-TiO2	0.2 g/L	EISA method	0.156 mmol L−1 4-CP (1.5 L)	UV	32% within 100 min	95% (three times faster than Degussa P25) within 100 min	[233]
Mo-TiO2 nanoparticles (anatase to brookite and rutile phases)	0.125 mol% Mo-TiO2	0.1 g/L	MWASG	85 mg/L MB, SMX (100 mL)	UV	0.031 min−1 for MB within 64 min	0.04 and 0.0318 min−1 for MB and SMX, respectively within 64 min	[77]
MoO3/P25 nanoparticles (anatase phase)	0.25% MoO3/P25	0.1 g	Impregnation method	15 mg/L MB (100 mL)	High pressure sodium lamp	8% within 150 min	38% within 150 min	[235]

MB: Methylene blue; SMX: sulfamethoxazole; PEOx: plasma electrolytic oxidation; EISA: evaporation induced self-assembly; MWASG: microwave-assisted sol–gel method; 4-CP: 4-chlorophenol.

presents the summary of recent progress on Mo-TiO₂ photocatalysts for organic pollutants degradation.

4.1.9. Modification with Cobalt (Co)

Co²⁺ has a similar radius as Ti⁴⁺, which easily enters into lattice interstitial sites or replaces Ti⁴⁺ to broaden its photocatalytic activity [236,237]. Co-TiO₂ is a capable candidate owing to its excellent properties such as stability, optically active nature, transparency, low cost and high n-type carrier mobility [238,239]. Furthermore, the presence of cobalt (Co²⁺ or Co³⁺) enhances crystallization due to the increased amounts of oxygen vacancies in the lattice of TiO₂, created by replacing the Ti⁴⁺ sites of the TiO₂ lattice [182,240,241]. Ebrahimian et al. [242] prepared Co-TiO₂ NPs with a great absorption range in the visible light range. Iwasaki et al. [243] synthesized Co-TiO₂ and observed that Co²⁺ could shift the edge of light absorption of TiO₂ to the visible light range and enhance photocatalytic activity in both visible and UV regions. Additionally, [92,240] reported that modifying TiO₂ lattice with Co enhances the photocatalytic activity of MO and carbamazepine under both visible and UV-A irradiations in comparison with unmodified TiO₂.

Some studies have greatly emphasized the contradictory influence of cobalt: some authors have reported its incorporation as detrimental for photocatalytic activity, while some have observed a slight improvement in photodegradation or selectively for some organic compounds. Studies by [244,245] show that that the photocatalytic activity of Co-TiO₂ is lower than unmodified TiO₂ in the presence of UV light. Meanwhile, 0.5% Co-TiO₂ presents the highest activity in the presence of visible light. Generally, most authors have concluded on the following facts (i) Co is present as the divalent form; (ii) Co (II) states are located within the bandgap; (iii) cobalt can be found in interstitial positions of TiO₂ [246].

Table 11. Summary of recent progress on Co-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutants (Volume)	Light Source	Unmodified	Modified	Ref.
Co-TiO2 nanosheets (anatase phase)	2.81% Co-TiO2	100 mg	Hydrothermal route	30 mg/L TC (100 mL)	Visible light	51.56% within min	50.19% within 140 min.	[247]
Co-TiO2 nanoparticles (anatase phase)	1 wt% Co-TiO2	0.5 g/L	Sol–gel and precipitation methods	10 mg/L MO (250 mL)	White compact fluorescent lamp	16.3% within 360 min.	34.7% within 360 min	[248]
Cobalt phthalocyanine complex sensitized TiO2 nanoparticles (anatase phase)	-	0.3 g/L	Hydrothermal methods	40 mg/L 4-CP (200 mL)	Visible light	25% within 90 min.	50% within 90 min.	[249]
Co-TiO2 film (anatase phase)	8% Co-TiO2	-	MBE methods	20 mg/L MB, Azo (100 mL)	Visible light	38 and ~37% for MB and AZO dyes, respectively within 70 min.	91 and 88% for MB and Azo dye, respectively within 70 min	[239]
Co-TiO2 nanosheets (Co-TNT) (anatase phase)	0.152% Co- TNT/rGO	0.152 g	Hydrothermal methods	30 mg/L TC (100 mL)	Visible light	48.57% within 180 min.	~60% within 180 min. Reusing the optimal photocatalyst after five successive cycles showed ~7% decline in its activity for degrading of TC.	[250]
Co-TiO2 nanoparticles (anatase phase)	0.06% Co/TiO2	0.1 g	Sol–gel using cobalt resinate as both template and cobalt source.	10 mg/L MB, RhB (100 mL)	Visible light	Unmodified TiO2 and unmodified Degussa P25 titania (P25) was 55 and 4.7%, respectively within 4 h.	82 and 55% for MB and RhB, respectively within 4 h	[251]
Co-TiO2 nanoparticles (anatase phase)	0.060% Co/TiO2	0.1 g	Sol–gel using resin acids as both template and cobalt source.	10 mg/L MB, RhB (100 mL)	Visible light	Unmodified TiO2 and unmodified Degussa P25 titania (P25) were 5.2 and 4.7%, respectively within 4 h.	86 and 94% for MB and RhB, respectively within 4 h	[251]
Co-TiO2 nanoparticles (anatase phase)	1 wt% Co-TiO2 NPs	0.5 g	Sol gel method	40 mg/L 2, 4-DCP (100 mL)	Visible light	43.65% within 120 min	68.03% within 120 min	[252]
Co-TiO2 flim (anatase phase)	1 wt% Co-TiO2	0.5 g	Phase inversion technique	40 mg/L 2, 4-DCP (100 mL)	UV/Visible light	-	61.6 and 63.74% in the presence of UV and visible light, respectively within 120 min. Furthermore, the membrane flux was increased by 53%.	[252]
Co-TiO2 nanoparticles (anatase phase)	2 wt% Co-TiO2 NPs	0.38 g/L	Photochemical deposition-assisted sol–gel technique.	10 mg/L Phenol (120 mL)	UV/Visible light	~95 and ~58% in the presence of UV and visible light, respectively for neat TiO2 within 180 min. Moreover, the neat P25 shows ~85 and ~80% in the presence of UV and visible light, respectively.	~98 and ~67% in the presence of UV and visible light, respectively within 180 min	[121]
Co-TiO2 nanopowders (anatase phase)	1 wt% Co	0.1 g/L	Sol–gel method	25 mg/L NB (-)	Medium-pressure mercury lamp	-	~81% within 180 min	[253]
Co-TiO2 nanopowders (anatase phase)	2 wt% Co- TiO2	0.01 g	Conventional solid-state reaction	20 mg/L MB (100 mL)	Xenon arc lamp	90.80% within 210 min.	90.92% within 210 min	[182]
Co-TiO2 nanoparticles (anatase phase)	1% Co-TiO2	0.1 g	Sol–gel method	5 mg/L MO, MB (100 mL)	Visible light	44.18 and 26.80% degradation rates for MB and MO, respectively for unmodified TiO2 within 5 h. 45% for MB for Degussa P25	66.17 and 38.14% for MB and MO, respectively within 5 h.	[94]
Co-TiO2 nanoparticles (anatase phase)	0.05% Co-TiO2	2 g/L	Sol–gel method	75 μM Crystal violet (100 mL)	UV	60% within 120 min	~100% within 120 min	[254]
CoO-RGO-TiO2 nanoparticles (anatase phase)	0.5% TiO2–RGO–CoO	0.05 g	Sol–gel method	10 mg/L 2-CP (100 mL)	Visible light	35.7% within 12 h.	58.9% within 12 h	[255]
Co-TiO2 nanoparticles (anatase phase) c	1% Co-TiO2	0.01 g	Sol–gel method	10 mg/L RhB (10 mL)	Visible light	~86% within 40 min	99% within 40 min	[237]
Co-TiO2 nanopowders (anatase phase)	1% Co-TiO2	0.1 g/L	Sol–gel method	2.5 × 10−4 M NB (-)	UV	57.13% within 2 h	81.03% within 2 h	[256]{FormattingCitation}
Co-TiO2 nanopowders (anatase phase)	2% Co-TiO2	0.1 g/L	Sol–gel method	2.5 × 10−4 M NB (-)	UV	57.13% within 2 h	67.17% within 2 h	[256]{FormattingCitation}
Co-TiO2/(GO) inverse opal photonic crystal (IO PC) (anatase phase)	-	0.1 g	Simple casting and calcination process using polystyrene opal as the template	0.013 M 4-CP (100 mL)	LT50 lamp	60% within 120 min.	75% within 120 min	[257]
Co-TiO2 nanopowders (anatase phase)	0.05 mol% Co-TiO2	1 g L−1	Sol–gel method (non-aqueous)	10 mg L−1 MB (500 mL)	UV	88.0% within 120 min	97.7% within 120 min	[91]
Co-TiO2 nanoparticles (anatase phase)	1.75% Co-TiO2	0.03 g/L	Nanosol/dip-coating method.	30 mg/L MB (50 mL)	UV	-	69% within 2 h	[217]
Co-TiO2 nanoparticles (anatase phase)	1% Co-TiO2	0.025 g	Sol–gel method	10−5 mol/L MO (50 mL)	Visible light	0.00075 and 0.002631 min−1 for visible and UV irradiation, respectively. 16.3 and 95.6% for visible and 95.6 for visible and UV light, irradiation within 120 min	0.00149 and 0.00280 min−1 (2 times larger than that of the unmodified TiO2) for visible and UV irradiation, respectively.	[92]
Co-TiO2 nanoparticles (anatase phase)	5% Co-TiO2	0.1 g	Sol–gel method	5×10−5 mol/L MB, MO (100 mL)	UV/Visible light	65.3 and 72.4% for MO and MB, respectively within 60 min under UV irradiation. 4.1 and 6.2% for MO and MB, respectively within 240 min under visible irradiation.	78.2 and 86.4% for MO and MB, respectively within 60 min under UV irradiation. 53.2 and 64.3% for MO and MB, respectively within 240 min under visible irradiation.	[38]
Co-TiO2 nanoparticles (mixed anatase and brookite phases)	0.5 mol% Co-TiO2	1 g/L	Sol–gel method	10 mg/L PNP (100 mL)	UV/Visible light	50 and 20% under UV/Visible and visible light, respectively for Degussa P25 within 8 h.	32 and 15% under UV/Visible and visible light, respectively within 8 h.	[127]

2,4-DCP: 2,4-dichlorophenol; Co-TiO₂: cobalt modified TiO₂; PNP: p-nitrophenol; MO: methyl orange; RhB: rhodamine B; MB: methyl blue; NB: nitrobenzene; 4-CP: 4-chlorophenol; 2-CP: 2-chlorophenol; TC: tetracycline.

presents the summary of recent progress on Co-TiO₂ photocatalysts for organic pollutants degradation.

4.1.10. Modification with Niobium (Nb)

The incorporation of niobium (Nb) can act as electrons sink as well as minimize the recombination of e⁻ to h⁺ pairs, and thus make more carriers available for reduction or oxidation processes on TiO₂ surface [258]. Modifying with Nb⁵⁺ enables the release of one electron for every Nb introduced: the Fermi level of TiO₂ shifts upward into the CB and results in a typical n-type metallic characteristic in the electronic structure [259]. Furubayashi et al. [260] prepared an Nb⁵⁺-TiO₂ film and their results show a room temperature resistivity of 2 to 3 × 10⁻⁴ Ω cm when the Nb concentration was >3%. Modifying of Nb5+ into TiO₂ lattice can extend the absorption that spans from UV to visible region as well as to the mid-infrared region. This allows TiO₂ to be active in the presence of both visible and UV light [261]. Some studies have also shown that charge transfer can be improved upon implantation with small Nb⁵⁺ content. Upon modifying with Nb, Nb⁵⁺ replaces Ti⁴⁺ and the donor is formed at the CB of TiO₂, providing electron for Ti⁴⁺ as well as obtaining a high concentration of carrier [260]. Su et al. [262] have showed that modifying with Nb increases the electrical conductivity, decreases bandgap, and increases optoelectronic property. Khan et al. [263] also demonstrated that charge-compensating (NbTi-VTi)³⁻ complexes serve as the dominant defect in Nb-TiO₂ composite enabling the improvement in the photocatalytic activity via the formation of shallow defect level.

Table 12. Summary of recent progress on Nb-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutants (Volume)	Light Source	Unmodified	Modified	Ref.
Nb-TiO2 Nanoparticles (Anatase phase)	0.5 mol%Nb-TiO2	1 g/L	Sol–gel method	100 uM 4-CP (-)	Visible light	8% within 120 min	The degradation of Nb-TiO2 was only slightly enhanced from the bare TiO2 case.39% within120 min	[264]
Nb-TiO2 nanoparticles (anatase phase)	0.5 at% Nb-TiO2	0.5 g/L	Sol–gel method	100 uM 4-CP (30 mL)	Visible light	~90% within 4 h	~100% within 4 h	[265]
Nb2O5/TiO2 hexagonal (anatase phase)	60 wt% or 0.6 of Nb2O5	1 g/L	Sol–gel method	50 mg/L MB (100 mL)	Visible light	~14% within 150 in	~23% within 150 in	[266]
Nb-TiO2 microspheres microspheres (anatase phase)	-	0.1 g	Ultrasonic spray pyrolysis method combined with impregnation method	10 mg/L MB (100 mL)	Visible light irradiation	5% within 60 min	Nb-TiO2 microspheres show higher activity as compared to unmodified TiO2, 26% within 60 min	[267]
Nb2O5/TiO2 nanoparticles (anatase phase)	0.5 mol% Nb2O5/TiO2	50 mg	Coprecipitation	10 μmol/L MB (300 mL)	UV	P25 showed the highest photocatalytic activity within 1 h.	Degradation rate for 0.5 mol% Nb2O5-TiO2 was slightly higher than the unmodified TiO2 within 1 h	[268]
Nb-TiO2 nanoparticles (anatase phase)	2 mol% Nb-TiO2	-	Hydrothermal method	10 ppm MB (-)	Solar light	~40% within 100 min	~40% within 100 min	[262]
Nb–TiO2 films (mixture of anatase and rutile phases)	0.58 mol% Nb–TiO2	-	Spin coating technology	10 ppm MB (-)	UV	~10% (1.07 × 10−3 min−1) within 120 min	~60% (4.97 × 10−3 min−1) within 120 min	[269]

XC: carbon xerogel; 4-CP: 4-chlorophenol; MB: methyl blue.

presents the summary of recent progress on Nb-TiO₂ photocatalysts for organic pollutants degradation.

4.1.11. Modification with Tungsten (W)

Tungsten (W) can easily replace Ti in the TiO₂ matrix due to the similarities in ionic radii of Ti⁴⁺ and W⁶⁺. W in its high oxidation state (+6) can act as an electron trap center and the substitutional modification of Ti⁴⁺ with W⁶⁺ can impart an almost small change in the TiO₂ matrix [270]. Modifying with W can only reduce the recombination in the TiO₂ bulk, whereas the defects present on the TiO₂ surface can act as recombination centers [271]. Furthermore, modifying with W can lead to the placement of electrons in a state below the CB or donor state [272]. These electrons can receive energy from visible light and be transferred to the CB. Finally, these excited electrons can generate O²•⁻, OH•, and OH• which are responsible for the heterogeneous photocatalytic treatment of organic pollutants [273]. When the anatase TiO₂ is modified with WO₃, separation of charge is improved as a result of the coupling of both materials [274]. Moreover, the presence of WO₃ can increase the TiO₂ acidity, modifying the substrate’s affinity for TiO₂ surface and thus affects the adsorption equilibrium and photooxidation activity of the catalyst [275,276]. Three major advantages of WO₃ include the thermodynamically favorable position of VB edge for the oxidation of H₂O (3 V vs NHE), visible light response with a bandgap energy of 2.6–2.8 eV, and photochemical stability in acidic media [277]. Superior photocorrosion resistance and chemical stability are the other advantages provided when TiO₂ is modified with WO₃. Various techniques including sol–gel, chemical vapor deposition, impregnation, mechanical blending, and liquid phase plasma (LPP) method have been also applied to the modification of host photocatalyst with transition metals [278,279].

Table 13. Summary of recent progress on W-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutant	Light Source	Unmodified	Modified	Ref.
W-TiO2 nanoparticles (anatase phase)	5% W-TiO2	0.5 g	Liquid phase plasma	10 mg/L DEP (600 mL)	UV/Blue light	1.22 × 10−2 min−1 and 4.72 × 10−2 min−1 for UV light and blue light, respectively within 180 min	0.92 × 10−2 min−1 and 29.37 × 10−2 min−1 for UV light and blue light, respectively within 180 min	[280]
W-TiO2 nanoparticles (anatase phase)	2.5 wt% W-TiO2	2 g/L	Sol–gel method	600 mg/L COD (150 mL)	Fluorescent light	-	50% within 34 h	[281]
W-TiO2/reduced graphene oxide nanoparticles (anatase phase)	1% W-TiO2	0.01 g	Sol–gel method	10 mg/L PNP (50 mL)	UV	30% within 180 min	62% within 180 min	[271]
W-TiO2 nanoparticles (anatase phase to stable rutile phase)	TiO2/W50 ppm	0.1 g	Hydrothermal method	20 mg/L RhB, MB, MO (100 mL)	Visible light	~81% of MO within 15 min. 0.0031 min−1, 0.01612 min−1 and 0.1441 min−1 for RhB, MB and MO, respectively	Degradation rate is 10 times the unmodified TiO2. 0.03181 min−1, 0.04148 min−1, 0.2730 min−1 for RhB, MB and MO, respectively. ~99.4% of MO within 15 min. Furthermore, 93.1 and 87.1% of MB and RhB were degraded, respectively, within 60 min, while 99.61% of MO within 20 min.	[282]
W-TiO2 nanoparticles (anatase phase)	0.5 mol% W-TiO2	0.5 g	Sol–gel method	50 mg/L CR (250 mL)	Visible light	1.25 min−1 for Degussa P25 within 180 min	2.62 min−1 within 180 min	[283]
WO3/TiO2 nanotube (anatase phase)	-	1 cm × 2 cm	Electrochemical approach	100 ppm Toluene (-)	Xenon lamp	-	65% within 60 min	[284]
Colloidal W- TiO2 nanocrystals (anatase phase)	2% W–TiO2	1 g/L	Hydrothermal method	10 mg/L Phenol (-)	UV	48.9% (0.1092 h−1) within 6 h	80.0% (0.2694 h−1) within 6 h	[285]
W-TiO2 layer (rutile to stable anatase phase)	6 wt% W-TiO2	15 mm×10 mm	PEOx method	10 mg/L MB (100 mL)	660-nm Tungsten-halogen	55% within 180 min for pure TiO2	95% within 180 min	[76]
W-TiO2 nanoparticles (anatase phase)	1 wt% W-TiO2	0.2 g/L	EISA method	0.156 mol L−1 4-CP (70 mL)	UV	32% within 100 min	88% within 100 min	[233]
WO3/TiO2 nanoparticles (anatase phase)	10% WO3/TiO2	0.05 g	combination of hydrothermal and calcination method	10 mg/L MB (50 mL)	Visible light	40.7 and 55.3% using pure TiO2 and WO3, respectively for MB within 2 h. 41.6 and 54.8% using pure TiO2 and WO3, respectively for MET within 2 h.	87.8 and 67.1% for MB and MET, respectively within 2 h. After three cycles, 10% WO3/TiO2 samples could still remove 63.2% of MET and 79.8% of MB.	[286]
W-TiO2 nanoparticles (anatase phase)	-	0.5 g/L	Sol–gel method	30 mg/L SMZ (200 mL)	Metal-halide lamps	80% within 30 min	The extended reuse of the photocatalysts for five consecutive runs obtained an SMZ degradation of 97.7, 97.6, 96.2, 95.1, and 90.34% in the same order.	[287]
W-TiO2 nanoparticles (anatase phase)	1.5 wt% W-TiO2	-	Sol–gel method	10 mg/L Thymol (100 mL)	Solar light	9.65% (0.085 × 10−2 min−1) within 120 min	48.76% (0.582 × 10−2 min−1) within 120 min	[288]
W-TiO2 nanosheets (anatase phase)	-	64 m2	Spray pyrolysis method	1 mM oxalic acid 2.5 × 10−4 M NB (-)	Sunlight	42% within 320 min	~83% within 180 min	[289]
W-TiO2 coupons (rutile to stable anatase phase)	6 wt% W-TiO2	15 mm × 10 mm	PEOx method	10 mg/L MB (100 mL)	660-nm Tungsten-halogen	55% within 180 min for pure TiO2	95% within 180 min	[76]

PNP: p-nitrophenol; SMZ: sulfamethazine; MB: methyl blue; PEOx: plasma electrolytic oxidation; 4-CP: 4-chlorophenol; RhB: rhodamine B; CR: Congo red; MO: methyl orange; COD: chemical oxygen demand; DEP: diethyl phthalate.

presents the summary of recent progress on W-TiO₂ photocatalysts for organic pollutants degradation.

4.1.12. Modification with Zinc (Zn)

Zn²⁺ ion can easily substitute Ti⁴⁺ ion in TiO₂ lattice without destroying the crystal structure, thereby stabilizing the anatase phase. Zn-TiO₂ powder can be synthesized using various preparative methods such as sol-hydrothermal, homogeneous hydrolysis, sol–gel and solid-phase reaction, electrospinning, controlled hydrolysis of titanium (IV) butoxide, assembly process based on a Ligand exchange reaction; micro-emulsion and sol–gel method.

Table 14. Summary of recent progress on Zn-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutants (Volume)	Light Source	Unmodified	Modified	Ref.
Zn-TiO2 nanoparticles (anatase phase)	1 wt% Zn-TiO2	0.1 g	Single step of sonochemical method	10 mg/L RhB (100 mL)	UV	18% within 90 min	35% within 90 min	[290]
Zn-TiO2 nanoparticles (mixed anatase and rutile phases)	0.01% Zn-TiO2	0.1 g	Sol–gel method	20 mg/L MB (50 mL)	Visible light	89.69% within 60 min	99.64% within 60 min	[128]
Zn-TiO2 nanoparticles (mixed anatase and brookite phases)	0.5 mol% Zn-TiO2	1 g/L	Sol–gel method	10 mg/L PNP (100 mL)	UV/Visible light	50 and 20% under UV/Visible and visible light, respectively for Degussa P25 within 8 h.	75 and 32% for UV/Visible and visible light, respectively within 8 h	[127]
Zn-TiO2 nanoparticles (anatase phase)	5 wt% Zn-TiO2	0.007 g	Sol–gel method	5 mg/L MB (50 mL)	Visible light	50% within 150 min	28.75 and 50% within 35 min and 47 min, respectively	[291]
Zn-TiO2 nanoparticles (anatase phase)	0.2 wt% Zn-TiO2	-	Sol–gel method	50 mg/L MO (50 mL)	Visible light	27% (0.45 min−1) and 3% (0.40 min−1) under UV irradiation and visible irradiation, respectively within 30 min	62% (1.54 min−1) and 40% (0.92 min−1) under UV irradiation and visible irradiation, respectively within 30 min	[113]
Zn-TiO2 nanoparticles (anatase phase)	5% Zn-TiO2	0.1 g	Oil-in-water microemulsions, (O/W) microemulsion method	30 mg/L Phenol (200 mL)	UV	84% within 5 h	93% within 5 h	[292]

MB: methylene blue; MO: methyl orange; PNP: p-nitrophenol; RhB: rhodamine B.

presents the summary of recent progress on Zn-TiO₂ photocatalysts for organic pollutants degradation.

4.2. Modification with Noble Metals

4.2.1. Modification with Gold (Au)

Gold (Au) can act as an electron acceptor since the energy gap (Eg) of Au (5.1–5.47 eV) is greater than TiO₂ (~4.7 eV) [293]. Owing to its plasmon resonance effect, modifying TiO₂ with Au will allow the achievement of visible light-driven photocatalyst [294,295]. Au-TiO₂ can also enhance the functional abilities of the material, causing alteration of the chemical or photocatalytic properties. Thus, the interaction between the photogenerated active species and the substrate of the Au-TiO₂ interface is increased and easily distinguished [296,297,298]. The collective oscillating motion of CB electrons on the Au nanoparticles (NPs) surface is mainly responsible for the reductive properties of the NPs and originates from a well-known surface plasmon resonance (SPR) [299]. Furthermore, modifying with Au promotes the gradual injection of hot electrons into TiO₂, and as a consequence increases the life span of the hole–electron pair and improve the photocatalytic activity [300]. The presence of Au in TiO₂ lattice structure may cause the switching of Ti⁴⁺/Ti³⁺ in defects, which effectively minimizes the wide bandgap of TiO₂. The presence of Ti⁴⁺/Ti³⁺ states inside the nanostructure enhances the electron-transfer mechanism inside the crystal lattice of TiO₂, increasing the photocatalysis efficiency [301]. This property can overcome the limitations of using unmodified TiO₂ photocatalyst due to its low recycling capability and complications of recovery from H2O.

The key parameters for the Au-TiO₂ activity are the Au particle size and the crystal phase of TiO₂. Additionally, Au-TiO₂ possesses a high performance coupled with a longer life span [302]. On the other hand, the dependence Au-TiO₂ activity is also influenced by the particle size of Au in the action mechanisms, which can be classified into the electron–hole transfer mechanism [303]. Deposition of Au occurred without changing the TiO₂ crystal structure and also achieved effective separation of photogenerated electron–hole in the UV–visible (UV-vis) light conditions, due to the smaller Au particle size of 15 nm.

Table 15. Summary of recent progress on Au-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutant	Light Source	Unmodified	Modified	Ref.
Au-TiO2 nanoparticles (anatase phase)	1% Au-TiO2	0.005 g	Photodeposition method	10 mg/L MB, CBN, MNZ (10 mL)	UV	73.9 and 88.7% for CBN and MNZ, respectively	35.7 and 46.8% for CBN and MNZ, respectively	[304]
Au-P25 nanoparticles (mixed anatase rutile phase)	1% Au-P25	0.005 g	Precipitation–deposition method	10 mg/L BPA (10 mL)	UV	35% within 24 h	76% within 24 h	[305]
Au-TiO2 nanoparticles (mixed anatase rutile phase)	1% Au-TiO2	0.005 g	Sol–gel method	10 mg/L BPA (10 mL)	UV	35% within 24 h	~57% within 24h	[305]
Au-TiO2 nanoparticles (mixed anatase rutile phase)	1% Au-TiO2 NPs	0.005 g	Sol–gel method	10 mg/L BPA (10 mL)	UV	35% within 24 h	~56% within 24 h	[305]
Au-meso-TiO2 nanoparticles (mixed anatase rutile phase)	1% Au-meso-TiO2	0.005 g	Sol–gel method	10 mg/L BPA (10 mL)	UV	35% within 24 h	37% within 24 h	[305]
TiO2 hollow microspheres impregnated with biogenic Au nanoparticles (stable anatase phase)	5% Au-TiO2 NPs	0.005 g	Sol–gel method	2.5 × 10−4 mol/L Phenol (10 mL)	Visible light	21% within 1 h	95% within 1 h	[306]
Au-TiO2 nanoparticles (anatase phase)	0.154 wt% Au-TiO2	0.004 g	Simple synthesis route	3.125 × 10−5 mol/L MB (-)	Visible LED light	25% within 150 min	97% within 150 min	[307]
Au-TiO2 yolk-shell (anatase phase)	0.14 at% Au-TiO2	0.005 g	Seed-growth method	300 ppm gaseous toluene (-)	Visible light	20% within 3 h	57.3% within 3 h	[308]
Au-TiO2 nanoparticles (anatase phase)	-	-	Sol–gel, wet chemical synthesis, hydrothermal, and plasma oxidative pyrolysis	20 mg/L CR, MO (100 mL)	UV	-	40.1 and 19.7 for MO and CR, respectively within 210 min.	[309]
Ag-TiO2 nanoparticles (anatase phase)	0.005 wt% Ag-TiO2	1.005 g	Sol–gel, wet chemical synthesis, hydrothermal, and plasma oxidative pyrolysis	20 mg/L CR, MO (100 mL)	UV	-	52.3 and 34.4% for MO and CR, respectively within 3.5 h	[309]
Au-TiO2 nanoparticles (anatase phase)	0.1% Au-TiO2	0.5 g	Hydrothermal method	10 mg/L Resorcinol (500 mL)	UVA	72.36% (0.0044 min−1) within 5 h	95.34% (0.0102 min−1) within 5 h (1.5 times of magnitude higher than pure TiO2 NPs while the rate constant was 2.5 times greater than that for pure TiO2). Furthermore, it shows 95% after five repeating experiments	[310]
Au-TiO2 films (anatase phase)	0.02 wt% Au/TiO2 meso-porous thin films	-	Sol–gel method	5 mg/L TC (100 mL)	UVA	27% within2 h	After 6 cycles, the percentage removal of TC is decreased from 58.51% to 57.89% only (i.e., a 0.62% decrease)	[311]
Au-TiO2 plasmonic nanohybrids (anatase phase)	-	0.005 g	Wet chemical method	10 mg/L MB, MO (100 mL)	Sunlight	25% within 20 min	94% for MB, 85% for MO, and 87% for the mixture of MO and MB within 20 min (3.3 times greater than that of the unmodified TiO2)	[312]
Au-TiO2 nanoparticles (Mixed anatase rutile phase)	2 wt% Au-TiO2	1.3 g/L	Deposition–precipitation method	50 mg/L CFS (100 mL)	UV/Visible light	<5% within 2 h under visible light irradiation	95 and 65% within 2 h under UV–visible and visible light irradiation, respectively	[313]
Au-TiO2 photoanode on carbon cloth (anatase phase)	0.1 wt% Au-TiO2	2 cm2	Sol–gel method	78.5 mg/L paracetamol (100 mL)	PEC/Sunlight	-	62 and 66% under PEC process and sunlight radiation in solar PEC, respectively within 180 min	[314]
Au-TiO2 film (anatase phase)	3 at% Au-TiO2 film	10 cm × 10 cm	Chemical spray pyrolysis technique	1 mmol/L benzoic acid (-)	UV	37% within 400 min	49% within 400 min	[29]

CBN: carbofuran; MNZ: metronidazole; MO: methyl orange; CFS: ceftiofur sodium; MB: methylene blue; CR: Congo red; TC: tetracycline; BPA: bisphenol A.

presents the summary of recent progress on Au-TiO₂ photocatalysts for organic pollutants degradation.

4.2.2. Modification with Silver (Ag)

Compared to gold and platinum, silver (Ag) is the cheapest coinage metal having a high charge carrier density [315]. It also has other advantages such as higher antibacterial activity, lower cost, and toxicity [316]. Modifying with Ag ions can create a new impurity band about 0.7 eV below the CB of titania which will thus narrow the energy bandgap of titania [317]; subsequently, Ag enhances the electron–hole separation by acting as an electron trapper to capture the electrons while reacting with the contaminants [318]. Hence, Ag assists in charge separation via the formation of a Schottky barrier between the metal and TiO₂ [319,320]. The incorporation of silver ion also extends the absorption of TiO₂ under visible light due to SPR effect, excited by visible light [321,322,323,324]. The presence of Ag ion can also contribute in two ways: (i) reinforcement of the photoinduced charge carriers as well as enhancing the electromagnetic field at the interfaces, (ii) promotion of interfacial charge-transfer process which limits the recombination rates. The electronic band-alignment at the Ag-clusters-TiO₂ interface favors the migrations of photogenerated electrons to metallic particles.

This process increases the lifespan of holes leading to redox reactions required for the photodegradation of the organic pollutants [119]. There are many approaches to synthesize Ag modified TiO₂, including solvothermal [325,326], reduction using UV irradiation [327], hydrothermal [328], electrochemical [329], microwave [330], deposition–precipitation [331,332], sonochemical [333], and sol–gel methods [334,335].

Zhou et al. [336] reported Ag decorated Ti³⁺ self-doped porous black TiO₂ pillar using a combined oil bath and wet impregnation approach. They claimed that the synergistic effect between Ti³⁺ self-doping and Ag decoration enhances photocatalytic activity. Liu et al. [337] presented the formation of Ag impurity band above the VB and observed the presence of AgO on the TiO₂ surface as well as the presence of Ag in the bulk. They noted that this Ag system will fail to show the energy level matching concept as the presence of Ag-ions is required on the TiO₂ surface. Contrary to various conventional reports, some reports have also confirmed the formation of impurity bands starting from 0.7 eV below CB [338]. Xiong et al. [339] also introduced 0.25 wt% Ag NPs into mesoporous TiO₂ and observed enhanced degradation of RhB in the presence of UV light irradiation. In addition, the Ag-TiO₂ catalysts present a higher photocatalytic efficiency for indigo carmine in the presence of visible light which was higher than the commercial P25 titania or unmodified TiO₂ [339].

Table 16. Summary of recent progress on Ag-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutant (Volume)	Light Source	Unmodified	Modified	Ref.
Ag-TiO2 nanoparticles (anatase phase)	5% Ag-TiO2	0.0113 mg	Sol–gel method	8 mg/L 2-CP (300 mL)	UV	16% within 150 min	30.4% within 150 min	[340]
Ag-TiO2 thin films (anatase phase)	5%Ag-TiO2	2.5 cm × 2.5 cm	Sol–gel method	5 mg/L MO, MB (25 mL)	Visible light	40.10% for MB within 240 min	98.86% for MB within 240 min, 96.34% for MO within 180 min. After ten consecutive cycles, approximately 98.85% MB dye were removed.	[341]
Ag-TiO2 Films (anatase phase)	Ag-TiO2 films	-	Sol–gel method	10 mg/L RB (-)	Solar light/Visible light/UV	45% within 70 min	98, 78, and 50% under solar light, visible and UV within 70 min. After four consecutive cycles, approximately 96.5% RB dye was removed.	[342]
Ag-TiO2 nanopowders (anatase phase)	10% Ag-TiO2	0.05 g	Sol–gel method	0.03 mg/L MB (50 mL)	UV	96% within 2 h, 97% within 35 min	97% within 35 min, 96% within 2 h	[343]
Ag-TiO2 nanoparticles (anatase phase)	10 wt% Ag-TiO2	0.3 g/L	Polyol method	1000 mg/L DCA (100 mL)	UV	65.4% within 480 min	79.3% within 480 min	[344]
Ag-TiO2 nanoparticles (anatase phase)	10 wt% Ag-TiO2	0.05 g	Sol–gel method	0.01 mmol/L MB (-)	Visible light	~30% within 50 h	Almost 100% within 50 h	[317]
Ag-TiO2 nanoparticles (anatase phase)	1.4 wt% Ag-TiO2	0.05 g	Photoreduction method	10 mg/L 4-CP (50 mL)	UV	-	After 8 times of reaction, the degradation effect of 4-CP decreased to 65%.	[345]
Ag-TiO2 nanoparticles (anatase phase)	4.0 mol% Ag-TiO2	0.18 g	Sol–gel method	10 mg/L MB (180 mL)	Visible light irradiation	30% within 60 min	96% within 60 min. After five consecutive cycles, approximately 89% MB dye was removed.	[318]
Ag TiO2 nanopowders (anatase phase)	Ag 3% TiO2	0.125 mol/L	Sol–gel method	2.10−5 mol./L MB (-)	UV	0.08124 min−1 within 120 min	0.11319 min−1 within 120 min	[119]
Ag/TiO2 photoanode (anatase phase)	-	-	Photoreduction method	30 mg/L RhB (250 mL)	UV	97.3% (0.0301 min−1) within 120 min	99.5% (0.0451 min−1) within 120 min	[346]
Ag/TiO2 nanoparticles (mixture of anatase and rutile phases)	1.06 wt% Ag-TiO2	1.5 g/L	Photodeposition method	5 mg/L DXM (600 mL)	UV/VIS	2% within 240 min	77.6 and 63.8% for UV and visible light irradiation, respectively	[347]
Ag/TiO2 nanoparticles (mixture of anatase and rutile phases)	0.03 wt% Ag-TiO2	1.0 g/L	LP followed by wet impregnation and reduction methods	125 µM MO (130 mL)	UV	6.54 × 10−3 min−1 and 0.19 × 10−3 min−1 for UV and visible light irradiation within 120 min, respectively	28.74 × 10−3 min−1 and 16.78 × 10−3 min−1 for UV and visible light irradiation within 120 min, respectively	[348]
Ag/TiO2 nanoparticles (anatase phase)	0.25 wt% Ag-TiO2	1 g/dm3	MWASG	20 mg/L MO (100 mL)	UV	57% within 70 min	99.5% within 70 min	[349]
Ag-TiO2 nanoparticles (mixture of anatase and rutile phases)	0.15 wt% Ag-TiO2	0.5 g	Sol–gel method	8 mg/L MB (100 mL)	Visible light	40 and 42% for unmodified TiO2 and P25, respectively within 7 h	~68% within 7 h	[350]
Ag2+/TiO2 nanoparticles (anatase-brookite TiO2 nanoparticles with a spherical shape)	2 mol% Ag2+/TiO2	1 g/L	Sol–gel method	10 mg/L PNP (100 mL)	UV/Visible light	50 and 20% for Degussa P25 under UV/Visible light and visible light, respectively within 8 h	60 and 31% under UV/Visible light and visible light, respectively within 8 h	[127]
Ag-TiO2 nanopowders (anatase phase)	3% Ag-TiO2	0.125 molL−1	Sol–gel method	20 mg/L MB (100 mL)	UV	0.08124 min−1 within 6 h	0.11319 min−1 within 6 h	[119]
Ag-TiO2 nanoparticles (anatase phase)	10% Ag-TiO2	0.5 g	Sol–gel, mechanothermal decomposition method	5 × 10−5 M MO (500 mL)	UV/Solar	69% within 60 min under UV, 8% within 80 min under solar irradiation	98.9% within 60 min under UV, 99.3% within 80 min under solar irradiation	[351]
Ag-TiO2 Nanoparticles (anatase phase)	1.75% Ag-TiO2	0.03 g/L	Nanosol/dip-coating method	30 mg/L MB (50 mL)	UV	-	74% within 2 h	[217]

MWASG: microwave-assisted sol–gel method; RB: rose Bengal; DXM: dexamethasone; 4-CP: p-chlorophenol; DCA: dichloroacetic acid; 2CP: 2-chlorophenol; PNP: p-nitrophenol; LP: laser pyrolysis; MO: methyl orange; RhB: rhodamine B; MB: methylene blue; 4-CP: 4-chlorophenol.

presents the summary of recent progress on Ag-TiO₂ photocatalysts for organic pollutants degradation.

4.2.3. Modification with Platinum (Pt)

Platinum (Pt) is a shiny silvery metal with low chemical activity. It is stable in air and a humid environment. The oxidation state of platinum is +2 to +6, which easily forms coordination compounds and has good ductility, thermal conductivity, and electrical conductivity. Pt is a noble metal that can increase the TiO₂ photoefficiency [352]. The energy level of Pt is lower than the one of the TiO₂ CB, so the energy required for the electron transition is lower and may be induced by the light with higher wavelengths. Therefore, the absorption spectrum can be broadened from the UV towards the VIS region. In the presence of UV light, the photogenerated electrons quickly transfer from the TiO₂ surface to the Pt particles, leading to effective separation of electron–hole, and as a consequence improving the photocatalytic activity [353]. The deposition of Pt nanoclusters onto TiO₂ surface serves as an electron sink, consequently slowing down charge–pair recombination as well as changing reaction pathways by providing catalytic sites where active intermediates can be stabilized [121].

Table 17. Summary of recent progress on Pt-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutant (Volume)	Light Source	Unmodified	Modified	Ref.
Pt-TiO2 nanoparticles (mixed rutile and anatase phase)	5 wt% Pt-TiO2	1 g/L	hydrothermal method	10 mg/L RhB (100mL)	Visible light	Amount of RhB degraded in 90 min reaction time 0.326 ± 0.011 × 10−9moles/min/mg.	99.5% within 90 min (4.5-fold better than pristine TiO2)	[354]
Pt-TiO2 nanocrystals (mixed anatase and brookite phases)	0.1wt% Pt-TiO2	1g/L	Sol–gel method	10 mg/L PNP (10mL)	Halogen lamp	20 and 12% for TiO2 and P25, respectively within 24 h under visible light. 47 and 84% for TiO2 and P25, respectively within 24 h under UV/Visible light.	~46% within 24 h under visible light. ~78% within 24 h under UV/Visible light.	[114]
Pt-TiO2 film (mixed anatase and brookite phases)	0.1% Pt-TiO2	size of 100 mm × 120 mm	Immersion and reduction method	100 ppb/500 ppb Ethenzamide (-)	V-UV	50 and 42% for 500 ppb and 100 ppb, respectively.	94.52 and 100% for 500 ppb and 100 ppb, respectively. The rate constant (0.180 min−1) was 1.86 times as compared to unmodified nanoporous TiO2 film (0.097 min−1)	[355]
Pt-TiO2 nanoparticles (mixed rutile and anatase phase)	10 wt% Pt-TiO2	0.3 g/L	Polyol method	1000 mg/L DCA (100 mL)	UV	34.6% within 420 min	Almost 100% within 420 min	[344]
Pt-P25 nanoparticles (mixed rutile and anatase phase)	/	0.5 g L−1	Sol–gel method	20 mg/L MG (250 mL)	UV	70% within 30 min for P25	100% within 30 min	[356]
Pt-MTiO2 nanoparticles (mixed rutile and anatase phase)	1.5 wt% Pt-MTiO2	1.5 g/L	facile synthesis procedure	CIP (-)	Visible light	~5% within 120 min	100% within 120 min. After five cycles, approximately 100% was still maintained.	[357]
Pt/P25 and Pt/TiO2 nanoparticles (mix ed rutile and anatase phase)	0.14 wt% Pt/TiO2 and Pt/P25	1.0 g/L	laser pyrolysis (LP) followed by wet impregnation and reduction methods;	125 µM MO (-)	UV/Visible light	6.54 × 10−3 and 9.6 × 10−3 min−1 for pure P25 and TiO2 (LP), respectively in the presence of UV light. 0.19 × 10−3 and 1.8 × 10−3 min−1 for pure P25 and TiO2 (LP), respectively in the presence of VIS lamp	In the presence of UV lamp, the rate constant of Pt/P25 and Pt/TiO2 was 9.78 × 10−3 min−1 and 10.16 × 10−3 min−1. In the presence of VIS lamp, the rate constant of Pt/P25 and Pt/TiO2 were 5.47 × 10−3 min−1 and 4.1 × 10−3 min−1.	[348]
Pt-TiO2 nanoparticles (mixed anatase and brookite phases)	0.8 wt% Pt-TiO2	0.2 g/L	Sol–gel method	6 mg/L MB(50 mL)	UVC	-	57% within 60 min	[358]
Pt-TiO2 nanoparticles (mixed anatase and brookite phases)	0.5 mol% Pt-TiO2	1 g/L	Sol–gel method	10 mg/L PNP (100 mL)	UV/Visible light	50 and 20% for UV/Visible light and visible light, respectively for Degussa P25 within 8 h	80 and 45% for UV/Visible light and visible light, respectively within 8 h	[127]
Pt-TiO2 nanoparticles (mixed anatase and brookite phases)	3 wt% Pt-HPT	0.3 g/L	Hydrothermal method	10 mg/L RhB (100 mL)	UV/Visible light	0.0013 min−1 for core(metal)-shell (TiO2) within 180 min	63% (0.0053 min−1) within 180 min	[359]
Pt-TiO2 nanoparticles (mixed anatase and brookite phases)	10 wt% Pt-TiO2	0.3 g/L	Polyol method	1000 mg/L DCA (100 mL)	UV	34.6% within 420 min	100% within 420 min	[344]

CIP: ciprofloxacin; LP: laser pyrolysis; HPT: highly porous TiO₂; DCA: dichloroacetic acid; MG: malachite green; MO: methyl orange; MB: p-nitrophenol; methylene blue; PNP: RhB: rhodamine B.

presents the summary of recent progress on Pt-TiO₂ photocatalysts for organic pollutants degradation.

4.2.4. Modification with Ruthenium (Ru)

Modifying with ruthenium (Ru) can reduce the Egap of TiO₂, allowing the visible light-driven photocatalysts, and acts as an electron acceptor/donor, which efficiently minimizes the recombination due to acceleration of electron transfer [360,361]. Ru-TiO₂ nanotubes have been widely used for the photocatalytic degradation of organic pollutants under visible or UV light [362,363].

Table 18. Summary of recent progress on Ru-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutants (Volume)	Light Source	Unmodified	Modified	Ref.
Ru-TiO2 nanotube arrays (anatase phase)	0.15 wt%Ru-TiO2	-	Anodic oxidation	0.11 M TB (20 mL)	UV/Visible light	84 and 26% under UV and visible light, respectively within 120 min	82 and 28% under UV and visible light, respectively within 120 min.	[364]
[Ru(4,4′-dicarboxy-2,2′-bipyridine)3]Cl2 (RuC)-TiO2 (anatase phase)	-	0.2 g	Hydrothermal method	20 mg/L Bromophenol blue (500 mL)	UV	0.0018 min−1 within 120 min	0.0038 min−1 within 120 min	[365]
Ru-TiO2 nanoparticles (mixed anatase and brookite phases)	0.2 wt% Ru-TiO2	0.002/L	Incipient wet impregnation method	100 mg/L 2-CP (500 mL)	UV/Visible light	45 and 40% for UV and visible light, respectively within 60 min for pure TiO2.	61 and 53% for UV and visible light, respectively within 60 min.	[366]
RuO2/TiO2 composite nanotube arrays (anatase phase)	0.0030 mol/L Ru-TiO2	-	Anodic oxidation method combined with dipping	8 mg/L MB (12 mL)	Fluorescent lamp	37.8% within 2 h	69% within 2 h	[362]
Ru-TiO2 nanotube arrays (mixed anatase and brookite phases)	0.16 wt% Ru-TiO2	0.01 mol L−1	Electrochemical anodization	4 ppm TB (-)	UV	0.0150 min−1; 81.4% color removal within 120 min	1.33 higher activity than the unmodified TiO2	[363]

2-CP: 2-chlorophenol; PNP: p-nitrophenol; MB: Methyl blue; TB: Terasil blue.

presents the summary of recent progress on Ru-TiO₂ photocatalysts for organic pollutants degradation.

4.2.5. Modification with Palladium (Pd)

Modification of TiO₂ with Palladium (Pd) has also received some attention due to their good stability coupled with high effectivity [367]. Pd is one of the most active elements for interacting with the surface of oxides as support. It has been revealed that the absorbability in the visible light region notably increases with the Pd incorporation; moreover, this metal can act as trapping sites for electrons and hence increase the photocatalytic activity of TiO₂.

Table 19. Summary of recent progress on Pd-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutants (Volume)	Light Source	Unmodified	Modified	Ref.
Pd-TiO2 nanoparticles (anatase phase)	0.5 wt% Pd-TiO2	0.01 g	Sol–gel methods	20 mg/L MB, MO (1 L)	UV	96.7% (0.024 min−1) and 82.5% (0.012 min−1) for MB and MO, respectively within 120 min.	92.6% (0.018 min−1) and 99.4% (0.044 min−1) for MO and MB, respectively within 120 min. The catalytic performance remained nearly unchanged and the degradation of MB was maintained at 95.9% after 10 continuous cycles.	[368]
Pd-TiO2 flower-like structures (anatase phase)	2 wt% Pd-TiO2	0.01 g	UV light-induced method	10 mg/L BPA (500 mL)	UV/Visible light	100% within 45 min.	100% within 45 min, 50% within 10 min under UV light; 100% within 240 min under visible light. The rate constants were 2.92, and 3.88 times higher than the P25 TiO2. Furthermore, they were 1.65 and 1.91 times higher than those of unmodified TiO2 under visible and UV lights, respectively.	[369]
Pd-TiO2 nanoparticles (mixed anatase and brookite phases)	1.3 wt% Pd-TiO2	0.01 g/L	Photodeposition	20 mg/L MB (100 m)	Xenon Lamp	0.008 min−1 within 45 min.	0.023 min−1 within 45 min.	[370]
Pd-TiO2 nanoparticles (anatase phase)	1 wt% Pd-TiO2	1 g/L	Hydrothermal	5 mM Oxalic acid (100 mL)	Fluorescent lamps	50, 54, and 87% for bare Aldrich rutile, bare P25 and P25 (cubical), respectively within 2 h,	96, 70, 72% for AA-Pd(spherical), AA-Pd(cubical), P25-Pd(spherical), respectively within 120 min. ~100 for AR-Pd(cubical) within 100 min	[371]
Pd-TiO2 nanoparticles (anatase phase)	1 wt% Pd-TiO2	1 g/L	Hydrothermal	0.5 mM Phenol	Fluorescent lamps	87, 63, 41% for P25, AA and bare Aldrich rutile, respectively within 2 h	72, 66, 98, 90, 75, and 71% for P25-Pd (spherical), P25-Pd(cubical), AA-Pd(spherical), AA-Pd(cubical), AR-Pd(spherical)), and AR-Pd(cubical), respectively within 2 h.	[371]

AA: Aldrich anatase; AR: Aldrich rutile; P25: Evonik AEROXIDE P25; BPA: bisphenol A; MB: methyl blue; MO: methyl orange.

presents the summary of recent progress on Pd-TiO₂ photocatalysts for organic pollutants degradation.

4.3. Modification with Rare Earth Metals

Modifying with yttrium (Y) was found to be successful for improving the photocatalytic response of TiO₂ under visible light irradiations [372,373]. Y-TiO₂ can be synthesized using different techniques such as deposition–precipitation with urea, sol–gel, impregnation and chemical coprecipitation method. It has been reported that Y-TiO₂ gives improved photocatalytic response attributed to the visible light absorption, electron–hole pairs separation, higher interfacial charge transfer, lower crystallite size, and high specific surface area. Niu et al. [372] found that introducing yttrium shifted the absorption edge of TiO₂ towards visible light, reduced the crystallite size, inhibited anatase to rutile phase transformation and decreased the photogenerated electron–hole pair recombination.

Table 20. Summary of recent progress on Y-TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutants (Volume)	Light Source	Unmodified	Modified	Ref.
Y-TiO2 nanoparticles (Anatase phase)	-	4 cm2	Electrochemical method	0.21 mM Toluene (200 μL)	Visible light	9% within 60 min	21% within 60 min	[374]
Y3+/TiO2 nanoparticles (Anatase phase)	0.25 mol% Y3+/TiO2	0.125 g	Hydrothermal methods	0.21 mM Phenol (100 mL)	UV/Visible light	2.40 h−1 within 60 min	3.85 h−1 within 60 min	[375]
Y3+/TiO2 nanoparticles (Anatase phase)	0.25 mol% Y3+/TiO2	0.125 g	Sol–gel methods	0.21 mM Phenol (-)	UV/Visible light	2.40 h−1 within 60 min	1.43 h−1 within 60 min	[375]

presents the summary of recent progress on Y-TiO₂ photocatalysts for organic pollutants degradation.

Modification with lanthanide ions can form complexes with various Lewis bases such as thiols amines, alcohols due to the interaction of their f-orbitals with different functional groups. Adding lanthanides in TiO₂ lattice suppresses e⁻ to h⁺ recombination and also increases the adsorptive capacity of the model pollutants [376]. It also stabilizes the mesoporous structure, prevents agglomeration, and increases thermal stability [53,377]. The common lanthanides include lanthanum (La), erbium (Er), neodymium (Nd), gadolinium (Gd), thulium (Tm), ytterbium (Yb), holmium (Ho), terbium (Tb), praseodymium (Pr), samarium (Sm), and europium (Eu).

Among the lanthanide ions, modifying TiO₂ with cerium (Ce) has received considerable attention due to the following: (1) it forms labile oxygen vacancies easily with the relatively high mobility of bulk oxygen species (2) the redox couple Ce³+/Ce⁴⁺ with the ability of ceria to shift between Ce₂O₃ and CeO₂ under reducing and oxidizing conditions [378,379,380]. This enables Ce to reduce e- to h⁺ recombination within TiO₂ via the trapping of an electron. The cerium ions (Ce³⁺/Ce⁴⁺) have variable valence states with multi-electron energy levels [381]. CeO₂ has a strong absorption ability to UV light with a small bandgap [382], its electrons are easy to jump and the 5d and 4f orbitals of Ce⁴⁺ are without electrons, which leads to large deformation and strong polarization which allows for effective separation of photoinduced carriers and the improvement of the photocatalytic activity of TiO₂ [383]. Ce-TiO₂ materials can be prepared by hydrothermal and sol–gel methods [380,384,385,386].

Xie et al. [387] prepared the monodisperse Ce-TiO₂ microspheres via a facile solvothermal process. Their results showed that the photocatalytic activity of TiO₂ was improved activity for MB under visible light irradiation upon introduction with Ce. Aman et al. [388] presented a 5% Ce-TiO₂ and their results show an improved activity for MB (50 ppm) with photodegradation of MB of ~100% within 60 min in the presence of visible light.

Table 21. Summary of recent progress on lanthanides modified TiO₂ photocatalysts for organic pollutants degradation.

Material (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutants (Volume)	Light source	Unmodified	Modified	Ref.
Er-TiO2 mesoporous spheres (anatase phase)	1 mM of Er-TiO2	1 g	Solvothermal method	10 ppm RhB (50 mL)	Visible light	84.27% (0.0428 min−1) within 60 min	98.78% (0.1544 min−1) within 60 min	[389]
Ce-TiO2 nanoparticles (mixture of anatase and rutile phases)	1% Ce-TiO2	0.5 g/L	Facile EDTA-citrate method	10 mg/L CIP, NOR (200 mL)	Sunlight	69.29 and 75% for CIP and NOR, respectively within 180 min	90–93% for both CIP and NOR within180 min	[390]
Ce-TiO2 nanoparticles (anatase phase)	0.5% Ce-TiO2	0.5 g	Facile hydrothermal method	MEK (200 mL)	Visible light	-	76% for 0.5% Ce-TiO2	[391]
Ce-TiO2 nanoparticles (anatase phase)	0.5% Ce-TiO2	0.5 g	Facile hydrothermal method	MEK (200 mL)	UV	-	62% for 0.5% Ce-TiO2	[391]
Ce-TiO2 nanoparticles (anatase phase)	0.41% Ce-TiO2	10 mg/cm2	Sol–gel method	25 mg/L DB15	Visible light	3.3% within 1 h	33% within 1 h	[216]
Ce-TiO2 mesoporous (mixture of anatase and rutile phases)	2.5% Ce-TiO2	0.001 g	EISA approach	20 mg/L Phenol (100 mL)	UV	-	95–99% within 4 h	[392]
Ce-TiO2 nanocrystalline films (anatase phase)	2.5% Ce-TiO2	-	EISA approach	20 mg/L Phenol (100 mL)	Solar light	99% within 4 h for the mesoporous TiO2	55-68% within 4 h	[392]
Ce-TiO2 nanocrystalline films (anatase phase)	10% Ce-TiO2	0.05 g	Sol–gel method	2.5 × 10−5 M BB-41 (80 mL)	UV	16.8 × 10−3 min−1 within 180 min	15.9 × 10−3 min−1 within 180 min	[393]
Ce-TiO2 nanocrystalline films (anatase phase)	12% Ce-TiO2	0.05 g	Sol–gel method	2.5 × 10−5 M BB-41 (80 mL)	Visible light	11.2 × 10−3 min−1 within 180 min	16.1 × 10−3 min−1 within 180 min	[393]
CeO2/TiO2 core-shell nanoparticles (anatase phase)	-	0.04 g	Hydrothermal route assisted with the Stöber method	1 × 10−5 M RhB (40 mL)	Xe arc-lamp	0.004 min−1 for CeO2 nanocubes and 0.003 min−1 for unmodified TiO2 within 120 min	0.012 min−1 within 120 min	[394]
CeO2/TiO2 nanoparticles (mixture of anatase and rutile phases)	0.25% CeO2/TiO2	0.1 g	Sol–gel method	40 ppm MB (100 mL)	Visible light	96.43% within 150 min	97.86% within 150 min	[395]
Ho-TiO2 nanotubes (anatase phase)	-	4 cm2	Electrochemical method	0.21 mM Toluene (200 μL)	Visible light	9% within 60 min	30% within 60 min	[374]
Er-TiO2 nanotubes (anatase phase)	-	4 cm2	Electrochemical method	0.21 mM Toluene (200 μL)	Visible light	9% within 60 min	22% within 60 min	[374]
Gd-TiO2 nanotubes (anatase phase)	-/	4 cm2	Electrochemical method	0.21 mM Toluene (200 μL)	Visible light	9% within 60 min	28% within 60 min	[374]
Tb-TiO2 nanotubes (anatase phase)	-	4 cm2	Electrochemical method	0.21 mM Toluene (200 μL)	Visible light	9% within 60 min	28% within 60 min	[374]
Nd3+/TiO2 nanosphere (anatase phase)	1.0 mol% Nd3+/TiO2	0.05 g	template-free method (recombined coprecipitation with hydrothermal method	20 mg/L MB (50 mL)	Visible light	91.83% (0.46 h−1) within 120 min	99.14% (2.3 h−1) within 120 min	[396]
Nd3+/TiO2 nanosphere (anatase phase)	1.0 mol% Nd3+/TiO2	0.05 g	template-free method (recombined coprecipitation with hydrothermal method	20 mg/L MB (50 mL)	Sunlight	60.09% of dyes degraded without catalyst	almost completely degradation within 80 min.	[396]
Nd-TiO2 nanoparticles (anatase phase)	2.0 mM Nd-TiO2	0.02 g	Sol–gel method	40 mg/L CR (100 mL)	UV	64% within 30 min	86% within 30 min, the photocatalytic efficiency after utilizing five times was more than 92%.	[397]
Nd-TiO2 nanoparticles (anatase phase)	2.0 mM Nd- TiO2	0.02 g	Sol–gel method	40 mg/L MB (100 mL)	UV	74% within 45 min	92% within 45 min, the photocatalytic efficiency after utilizing five times was more than 92%.	[397]
Yb3+/TiO2 nanoparticles (anatase phase)	1% Yb3+/TiO2	0.05 g	Sol–gel method	0.21 mM phenol (5 mL)	Visible light	45% within 180 min	89% within 180 min	[398]
Er3+/TiO2 nanoparticles (anatase phase)	0.5% Er3+/TiO2	0.05 g	Sol–gel method	0.21 mM Phenol (5 mL)	Visible light	-	75% within 180 min	[398]
Er3+/TiO2 nanoparticles (anatase phase)	2 mol% Er3+/TiO2	0.025 g	Electrospinning	10 mg/L MO (25 mL)	Solar light	78% within 60 min	43% within 60 min	[399]
Er3+/TiO2 nanoparticles (anatase phase)	0.5 mo% Er3+/TiO2	0.05 g	Electrospinning	10−5 mol/L RhB (25 mL)	Solar light	71.3% within 10 h	91.3% within 10 h	[399]
Er3+/TiO2 nanoparticles (anatase phase)	0.5 mol% Er3+/TiO2	0.05 g	Electrospinning	20 mg/L Phenol (25 mL)	Solar light	23.9% within 72 h	46.1% within 72 h	[399]
Yb-TiO2 nanoparticles (anatase phase)	10% Yb-TiO2	1 g/L	Hydrothermal process	10 ppm Phenol (100 mL)	Solar light	~Initial Rate with 31.9 mol/L/h	~Initial Rate with 54 mol/L/h	[400]
Er-TiO2 nanoparticles (anatase phase)	2% Er-TiO2	1 g/L	Hydrothermal process	10 ppm Phenol (100 mL)	Solar light	~Initial Rate with 31.9 mol/L/h	~Initial Rate with 144 mol/L/h	[400]
Gd-TiO2 nanoparticles (anatase phase)	1.8 at% Gd- TiO2	0.1 g	Sol–gel method sintering at 550 °C	20 mg/L CR (100 mL)	UV	42% within 1 h	76% within 1 h	[401]
Gd-TiO2 nanoparticles (anatase phase)	1.8 at% Gd- TiO2	0.1 g	Sol–gel method sintering at 700 °C	20 mg/L CR (100 mL)	UV	35% within 1 h	84% within 1 h	[401]
Sm-TiO2 nanoparticles (anatase phase)	1.8 at% Sm-TiO2	0.1 g	Sol–gel method sintering at 700 °C	20 mg/L CR (100 mL)	UV	35% within 1 h	74% within 1 h	[401]
Sm-TiO2 nanoparticles (anatase phase)	1.8 at% Sm-TiO2	0.1 g	Sol–gel method sintering at 550 °C	20 mg/L CR (100 mL)	UV	42% within 1 h	70% within 1 h	[401]
Er-TiO2 nanotube arrays (anatase phase)	10 wt% Er-TiO2	-	Electrochemical anodization	0.5 mM Toluene (8 mL)	Visible light	-	2.85 × 10−3 μmol·min−1 within 60 min	[402]
Ho-TiO2 nanotube arrays (anatase phase)	10 wt% Ho-TiO2	-	Electrochemical anodization	0.5 mM Toluene (8 mL)	Visible light	-	2.87 × 10−3 μmol·min−1 within 60 min	[402]
Gd-TiO2 nanoparticles (anatase phase)	0.3% Gd-TiO2	0.01 g	Sol–gel method	10 mg/L RhB (10 mL)	Visible light	75% within 240 min	93% within 240 min	[403]
La-TiO2 nanoparticles (mixture of anatase and rutile phases)	0.05 La-TiO2	1 g/L	Ultrasound-assisted wet impregnation method	10 mg/L MB (1 L)	UV	0.1372 ± 0.0038 min−1 and 0.1332 ± 0.0051 min−1 for pristine TiO2 and P25-TiO2, respectively within 30 min	0.1528 ± 0.0017 min−1 within 30 min	[404]
Gd-TiO2 nanoparticles (anatase phase)	3% Gd3+/TiO2	0.05 g	Impregnation method	20 mg/L RhB (60 mL)	Mercury lamp	75% within 50 min	81–96% within 50 min	[405]
Ce3+/TiO2 nanoparticles (mixture of anatase and rutile phases)	1 mol% Ce3+/TiO2	0.2 g/L	Combustion synthesis method	20 mg/L MB (100 mL)	Visible light	41% within 120 min	72% within 120 min	[406]
La-TiO2 nanoparticles (anatase phase)	1.0% La–TiO2	0.05 g	Sol–gel method	10 mg/L RhB (100 mL)	562-nm Xenon Lamp	21.56% within 300 min	11.09% within 300 min	[74]
Ce-TiO2 nanoparticles (anatase phase)	1.0% Ce–TiO2	0.05 g	Sol–gel method	10 mg/L RhB (100 mL)	562-nm Xenon Lamp	21.56% within 300 min	83.43% within 300 min	[74]
Eu-TiO2 nanopowders (anatase phase)	3% Eu-TiO2	0.125 molL−1	Sol–gel method	20 mg/L MB (250 mL)	Xenon Lamp	0.08124 min−1 within 6 h	0.03719 min−1 within 6 h	[119]
Gd-TiO2 nanopowders (anatase phase)	1 wt% Gd-TiO2	0.1 g	Sol–gel method	10 mg/L MO (100 mL)	Visible light	31% within 180 min	78% within 180 min	[205]
Gd-TiO2 nanopowders (anatase phase)	1 wt% Gd-TiO2	0.1 g	Sol–gel method	10 mg/L 4-CP (100 mL)	Visible light	28% within 180 min	69% within 180 min	[205]
Eu-TiO2 nanoparticles (anatase phase)	10% Eu-TiO2	0.3 g	LPP process	50 mg/L ASA (250 mL)	Blue light	0.685 × 10−3 min−1 within 24 h	1.475 × 10−3 min−1 within 24 h	[407]
Eu-TiO2 nanoparticles (anatase phase)	10% Eu-TiO2	0.3 g	LPP process	50 ppm ASA (250 mL)	UV	9.37 × 10−3 min−1 within 24 h	10.65 × 10−3 min−1 within 24 h	[407]

at. %: atomic %; NOR: norfloxacin; MEK: methyl ethyl ketone; EE2: 17-α-ethinylestradiol; DB15; direct blue 15; BB-41: basic blue 41; RhB: rhodamine B; LPP: liquid phase plasma; MB: methylene blue; ASA: acetylsalicylic acid; RB: reactive blue; Cr: Congo red; MO: methyl orange; CIP: ciprofloxacin; 4-CP: 4-chlorophenol; EISA: evaporation-induced self-assembly.

presents the summary of recent progress on lanthanides modified TiO₂ photocatalysts for organic pollutants degradation.

4.4. Modification with Other Metal Ions

The other metal ions are also introduced in TiO₂ crystal lattices to replace Ti ions, thus improving the transferring rate of interfacial electrons, lowering the recombination between e⁻ and h⁺, and adjusting the energy band structure [408]. Simultaneously, the energy level is introduced into the bandgap of TiO₂ to broaden the absorption band edge to the visible light range, and finally, improve the activity of TiO₂ under visible light [408,409].

Indium (In)-TiO₂ has been observed to increase the photoelectron chemical responses and photocatalytic activity of TiO₂ by enhancing the transfer of electrons and reduction in recombination rate of photoinduced charge carriers due to its CB/VB potential difference from TiO₂ [410,411]. Indium has lower toxicity, is relatively cheap, and has multiple oxidation states (In°, In⁺¹, In⁺³), which can help to improve charge mobility and electron trapping over the surface of TiO₂ [412]. The result by [413] shows an enhanced photocatalytic activity for 2,4-dichlorophenoxyacetic acid in the presence of UV light irradiation. Pozynak et al. [414] reported an efficient separation of photogenerated charges in nanocrystalline In₂O₃/TiO₂ photocatalyst during the degradation of 2-CP.

The presence of transition metal such as Gallium ion (Ga³⁺) at the Ti⁴⁺ sites can induce oxygen vacancies gaps [415,416] and build deficiencies near the CB in TiO₂, which function as electron traps and increase the isolation of e- to h⁺ pairs [417,418]. Simultaneously, modifying with Ga shift the absorption edge towards visible regime as well as improving the separation between photoexcited charge carriers [419]. The synthesis of Ga–TiO₂ can be achieved by laser pyrolysis [420], sol–gel method [417], traditional solid-state reaction [421,422], and hydrothermal method [419,423,424,425]. However, these methods often require either a high experimental temperature to promote the reactions [420] or post heat treatment for crystallization [417,423]. Generally, a higher heating temperature always leads to grain growth and agglomeration, which decreases the specific surface area and is detrimental to photocatalytic activity.

Table 22. Summary of recent progress on other metals modified TiO₂ photocatalysts for organic pollutants degradation.

Materials (TiO2 Phase Transition)	Optimum	Catalyst Dosage	Synthetic Methods	Pollutants (Volume)	Light Source	Unmodified	Modified	Ref.
Ga3+-TiO2 nanoparticles (anatase phase)	0.5 wt% Ga3+-TiO2	0.1 g	Ultrasonic irradiation	20 mg/L MB (100 mL)	Solar light	70.6% (0.0096 min−1) in 150 min	86.4% (0.021 min−1) in 150 min	[418]
In2O3-TiO2 nanopowders (anatase phase)	5 mol% In2O3/TiO2	0.5 g	Sol–gel method	200 mg/L CR (-)	UV	0.15 h−1 in 4 h	0.86 h−1 in 4 h	[426]
In2O3-TiO2 nanorods (anatase phase)	0.4 wt% In2O3/TiO2	1 × 1.5 cm2	Hydrothermal method	10 uM MO (-)	Sunlight	86% in 6 h	94% in 6 h	[427]
In2O3-TiO2 nanorods (anatase phase)	0.4 wt% In2O3/TiO2	1 × 1.5 cm2	Hydrothermal method	2.5 uM BPA (-)	Sunlight	65% in 6 h	68% in 6 h	[427]
In2O3-TiO2 nanoparticles (mixture of anatase and rutile phases)	0.15 mol% In2O3/TiO2	0.5 g	Sol–gel method	8 mg/L MB (100 mL)	high-voltage Mercury lamp	40 and 42% in 7 h for pure TiO2 and P25, respectively.	~80% in 7 h	[350]
TiO2-In2O3 porous structure and spherical morphology (anatase phase)	1 mol% In2O3/TiO2	0.02 g	UAS assisted method	10 ppm MO (20 mL)	Visible light	26% in 5 h for pure In2O3	~98% within 5 h	[411]
TiO2-In2O3 porous structure and spherical morphology (anatase phase)	1 mol% In2O3/TiO2	0.02 g	UAS assisted method	10 ppm RhB (20 mL)	Visible light	18% in 3 h for pure In2O3	95% within 3 h	[411]
Ga3+-TiO2 nanoparticles (anatase phase)	0.5 wt% Ga3+-TiO2	-	Ultrasonic irradiation	20 ppm Phenol (100 mL)	Solar	0.0052 min−1 in 180 min	0.021 min−1 and ~4 times higher activity than unmodified TiO2	[418]
Ga-TiO2/rGO nanoparticles (mixed anatase and rutile phases)	1.0 wt% Ga-TiO2/rGO	0.01 g	Simple synthesis process	10 mg/L RB dye (-)	Visible light	0.0015 min−1 in 180 min	0.0029 min−1 in 180 min	[428]
Ga2O3-TiO2 nanoparticles (anatase phase)	0.1% Ga2O3-TiO2	0.05 g	Sol–gel method	0.08 mmol/L Imazapyr (100 mL)	UV	19% in 180 min for mesoporous Ga2O3.	98% within 180 min (10 times higher than the mesoporous Ga2O3). The photodegradation efficiency of imazapyr continues to maintain over 95% after five cycles.	[429]
Al-TiO2 nanoparticles (anatase to rutile phase)	0.25% Al-TiO2	2 g/L	Impregnation method	10−6 M MB (100 mL)	LED lamp	75% within 60 min for P-25	-	[430]
Al-TiO2 nanoparticles (anatase to rutile)	0.25% Al-TiO2	0.5 g/L	Impregnation method	10−6 M MB (100 mL)	LED lamp	-	80% in 60 min	[430]
Al-TiO2 nanoparticles (anatase to rutile)	0.25% Al-TiO2	1 g/L	Impregnation method	MB (100 mL)	LED lamp	-	85% in 60 min	[430]
Sn-TiO2 nanoparticles (anatase to rutile)	1% Sn-TiO2	0.2 g	Sol–gel method	600 ppm DES (-)	Solar light	38% in 90 min,48% in 180 min	~100% in 90 min, 75% in 150 min	[431]
Sn-TiO2 nanoparticles (anatase to rutile phase)	5 mol% Sn-TiO2	0.005 g	Sol–gel method	10 mg/L MB (25 mL)	Solar lamp	98% within 60 min for TiO2-NTs, 94% within 60 min for TiO2–P25	98% (reaction rate of 0.1215 min−1 which is 1.8 and 2.7 times as much as those of TiO2-NTs and TiO2−P, respectively) within 30 min	[432]
Sn-TiO2 nanoparticles (anatase to rutile phase)	1 mol% Sn-TiO2	0.3 g	Sol−gel method	10 mg/L RhB (300 mL)	UV	46.2% within 180 min	99.5% (0.02732 min−1) within 180 min	[433]
Sn-TiO2 Hollow Spheres	1 mol% Sn-TiO2	0.025 g	Sol−gel method templated by polystyrene spheres	0.00385 g/L MO (50 mL)	UV	~55% within 240 min	~72% within 240 min	[434]
SnO2-TiO2 nanoparticles (anatase to rutile phase)	1% Sn-TiO2	1 g/L	Sonication–impregnation method	10 mg/L TC (-)	Visible light	-	95% within 15 min, 81–95% within 20 min	[435]
Sn-TiO2 nanoparticles (anatase phase)	0.20 ± 0.03% Sn-TiO2	-	Washcoating method	ERY (1 L)	UV-A	~18% within 240 min	67% within 240 min, 50% within 2280 min	[436]
Sn-TiO2 nanoparticles	5% Sn-TiO2	0.8 g/L	Hydrothermal method	10 mg/L DCF (-)	UV	~62% within 300 min	89% within 300 min 53% after 4 cycles (1200 min)	[437]
Sn-TiO2 nanoparticles	1 mol% Sn-TiO2	0.2 g	Sol−gel method	40 ppm 2,4-DCA (-)	UV	77% within 180 min	93% (16.8 × 10−3 min−1) within 180 min	[438]
Al3+/TiO2 nanoparticles (mixed anatase and brookite phases)	0.5 mol% Al3+/TiO2	1 g/L	Sol−gel method	10−4 M PNP (100 mL)	UV/Visible light	50 and 20% under UV/Visible and visible light, respectively for Degussa P25 within 8 h,	64 and 24% under UV/Visible and visible light, respectively within 8 h	[127]

PNP: p-nitrophenol; CR: Congo red; UAS: Ultrasonic aerosol spray; MB: methylene blue; RhB: rhodamine B; DCA: dichloroacetic acid; DCF: diclofenac; ERY: erythromycin; TC: tetracycline; MO: methyl orange; BPA: bisphenol A; RB: reactive blue.

presents the summary of recent progress on other metals modified TiO₂ photocatalysts for organic pollutants degradation.

5. Conclusions and Future Outlooks

TiO₂ is the most widely used photocatalyst for the photodegradation of organic pollutants. This paper presents the recent development of metal ion modified TiO₂ for the photodegradation of organic pollutants. Among the metal ions, metal such as Ni, Fe, Co, Cu, Au, Ag, Zr, W, and Mn have been widely explored and found to show beneficial influence on photocatalytic activity. However, the application of metal ion modified TiO₂ photocatalyst for organic pollutants continues to be limited due to several obstacles (preparation method, security, cost, commercial use, and efficiency). Some of the recommendations drawn from this review are listed as follows:

The activity of TiO₂ photocatalyst has a lot to do with its preparation method. The preparation methods are different, and the shape and size, surface, and structural properties of the catalyst are different. The main methods for preparing metal ion modified TiO₂ include sol–gel, precipitation, immersion, and hydrothermal method. It was found that the samples prepared by sol–gel and hydrothermal methods produce better results. Its photocatalytic performance is much higher than that of photocatalysts prepared by other methods. This is mainly because the reaction process of these two methods is simple, the operation is controllable, and the prepared powder has a relatively small particle size, high purity, and good chemical uniformity. However, there is a limitation in the long preparation cycle. Therefore, future research should be devoted to new ways to find simple and effective ways to improve some defects of metal ions modified TiO₂.

There is a need for continuous in-depth study on the use of sunlight irradiation, and the economical applicability of this approach for removing organic pollutants. To date, various metal ion modified TiO₂ photocatalysts have been developed and reported. However, their preparations consume considerable chemicals which are expensive, time-consuming and complex. Traditional TiO₂ is generally a massive particle, and the control of the morphology of TiO₂ is an effective way to increase their contact with pollutants. For example, the preparation of hollow, porous, or larger specific surface area TiO₂ nanoscale particles can effectively solve the traditional small contact surface of TiO₂ and pollutants. In addition, TiO₂ itself can be modified to add its hydrophilic functional groups. It can increase the compatibility of TiO₂ with H₂O, thereby promoting the working process of TiO₂. Therefore, it is a necessity to develop a relatively scalable, inexpensive, environmentally friendly, and easy synthesis route.

There are also many defects in the application of TiO₂ in daily life. The limiting factor for its development is the photocatalytic performance problem. In addition, if the traditional photocatalyst is unmodified, it cannot adapt to the change to other light sources (e.g., sunlight). The water body purified by TiO₂ in daily life will take away part of the TiO₂, and at the same time, the photocatalytic degradation of organic pollutants often produces some intermediate products. However, in most cases, the toxicity of the intermediate products tends to be stronger than the initial organic pollutants, and as a consequence poses greater harm to the environment and human beings. Therefore, there is a need for an extensive toxicological study of these intermediate products. Therefore, in terms of these issues, how to improve the flexibility of particles in practical applications and how to improve the safety of TiO₂ also needs to be considered in future research.

The applicability of this method requires additional engineering amplification testing. However, it is hoped that through rapid and continuous evaluation of the pilot plant configuration, a large-scale solar-driven photocatalytic activity treatment process with low site area requirements and high efficacy can be realized in near future. Furthermore, the majority of the studies use artificial organic pollutants and studying the performance of TiO₂ photocatalysts under real situations should be the focus of future work.

Finally, we hope that this paper can assist researchers to better understand the recent trend in the removal of organic pollutants using metal ion modified TiO₂ photocatalysts and also hope that this field can flourish in the future.