**1. Introduction**

At present, hydrogen is regarded as the fuel of the future. Compared to carbon fuel, hydrogen is considered a renewable and environmentally friendly source of energy. There are various methods for producing hydrogen on an industrial scale. However, all known methods are characterized by high energy consumption, which makes the process of hydrogen production on a large scale disadvantageous. The production of hydrogen by photocatalytic water splitting is technologically simple, and the outcoming gases are environmentally friendly.

TiO2 is a wide-bandgap semiconductor. In nature, TiO2 is usually found in three di fferent crystalline structures: rutile, anatase, and brookite. TiO2 in anatase form is the most widespread photocatalyst for hydrogen evolution [1]. However, it cannot be used in the spectrum of visible light, since its bandgap (Eg) for di fferent crystalline phases (anatase—3.2 eV, rutile—3.0 eV, and brookite—3.3 eV) is in the UV region. Also, the e fficiency of photocatalysis in addition to the bandgap depends on many other factors [2]. In the 1970s, Fujishima and Honda studied the photoelectrochemical splitting

of water, where a TiO2-based electrode demonstrated the ability to split water under the influence of ultraviolet radiation [3]. Photocatalysis is a complex reaction consisting of the processes starting from light absorption to generate charge carriers up to surface catalytic reactions due to which gas is formed (Figure 1). This process only requires a photocatalyst (TiO2), which is not consumed during the entire process, water, sunlight or ultraviolet radiation. During the absorption of light (≥*Eg*) by the photocatalyst, an excited electron (*e*<sup>−</sup>*CB*) transfers from the valence band into the conduction band. This transition of the electron leads to the generation of a positively charged carrier-hole (*h*+*VB*) in the valence band (Equation (1)). Also, these charge carriers can recombine among themselves (Equation (2)) [4,5].

$$TiO\_2 \stackrel{hv \gtrsim E\_{\mathcal{S}}}{\rightarrow} h\_{VB}^+ + \text{ } \mathfrak{e}\_{CB}^- \tag{1}$$

$$h\_{VB}^{+} + \ \ \bar{e\_{CB}} \stackrel{\text{reconstruction}}{\rightarrow} Energy \tag{2}$$

**Figure 1.** The mechanism of photocatalytic water splitting on TiO2 based photocatalyst.

Another important application of TiO2-based photocatalysts is based on their ability to discolor and completely decompose organic dyes contained in water. In addition, there are a number of works in which TiO2-based photocatalysts were used to neutralize harmful to the atmosphere gases [6–9]. Chemical stability, easy accessibility, nontoxicity and the ability to oxidize under the influence of radiation, allow TiO2-based photocatalysts to solve the main global problems associated with pollution of the environment and need for renewable energy [10,11]. Currently, TiO2 as a photocatalyst is commercially produced in powder form. The most common brand of TiO2 based photocatalyst are P-25 Degussa/Evonik, TiO2 nanofibers from Kertak, TiO2 Millennium PC-500, TiO2 Hombikat UV-100, Sigma Aldrich TiO2, TiO2 PK-10, and P90 Aeroxide.

This paper aims to provide a brief overview of articles published starting from 2017 on new research and developments of TiO2 based photocatalysts with significant advances. In this review, attention is also paid to the study on the mechanism of photocatalytic processes, factors affecting the activity of photocatalysts, and new techniques used to increase the activity of TiO2 based photocatalysts during the splitting of water with the evolution of hydrogen and decomposition of organic compounds utilized for water purification.

#### **2. Factors A**ff**ecting the E**ffi**ciency of Photocatalysis and Techniques Used to Improve the E**ffi**ciency of TiO2-Based Photocatalysts**

#### *2.1. Lifetime of Photogenerated Charge Carriers*

The activity of photocatalysts strongly depends on the lifetime of photogenerated electron-hole pairs. An important role is played by the rate at which charge carriers can reach the surface of the photocatalysts. The results of spectroscopic studies show that the time intervals between redox reactions or recombination involving charge carriers are extremely short, resulting in a significant reduction of the photocatalytic activity of TiO2. In the case of recombination of charge carriers in a sufficiently fast interval (<0.1 ns), the photocatalytic activity of the semiconductor is not observed. For example, the lifetime of electron-hole pairs of ~250 ns (TiO2) is considered relatively long [12]. Thus, it can be concluded that a high recombination rate and barriers, that prevent the transfer of charge carriers to the semiconductor surface, reduce photoactivity, despite the high concentration of initially photogenerated pairs. In this regard, to avoid their recombination, it becomes necessary to use cocatalysts in order to increase the lifetimes of electrons and holes.

#### *2.2. The Particle Size of Photocatalyst*

Compared to microparticles, TiO2 nanoparticles have, generally, a higher photocatalytic activity [13,14]. This is due to the small diameter of the nanoparticles, in which the charge requires minimal effort to transfer to the surface. If the particle size decreases, the distance that photogenerated electrons and holes need to travel to the surface where the reactions take place is reduced, thereby reducing the probability of recombination. For TiO2 photocatalyst microparticles, the penetration depth of UV rays is limited and amounts to about ~100 nm. This means that the inner part of the TiO2 photocatalyst microparticle remains in a passive state. [15]. Figure 2 shows the scheme of light absorption by nano- and microparticles of TiO2. This is one of the reasons for the increased interest in nanosized particles of TiO2.

**Figure 2.** Light absorption by micro- and nanoparticles of TiO2.

As shown in Figure 2, a decrease in the particle size of the photocatalyst to nanoscale facilitates the absorption of light by the entire volume of particles. However, there is a limitation regarding the minimum sizes to which it is desirable to reduce the particles of the photocatalyst, due to the onset of quantum effects. They become significant at particle sizes less than 2 nm for both anatase and rutile, and finally, this leads to a change in the bandgap. An increase in the size of the photocatalyst crystals leads to a decrease in the recombination of the electron-hole pair at the defects of the crystal lattice and to an increase in photocatalytic activity. For example, in [16], nanoparticles with a size of 25 nm were found to be more productive than nanoparticles of 15 nm. On the other hand, the bandgap is directly proportional to the size of the photocatalyst crystals. That is why it is necessary to find the optimal crystal size of TiO2 and control it in the process of its obtaining.

To increase the photoactivity of TiO2 in the visible region of the spectrum, the spectral region of its absorption should be expanded. There are several approaches for sensitizing TiO2 to visible light: doping with cationic and anionic elements or metal nanoparticles. Elements of the 3d- and 2p-groups are often used as additives to reduce the value of the bandgap of the photocatalyst [17,18].

#### *2.3. Doping with Cations*

The essence of cationic doping is the introduction of metal cations into the crystal structure of TiO2 at the position of Ti4<sup>+</sup> ions. Rare-earth, noble, and transition metal cations can be used [19]. Doping with cations significantly expands the absorption spectrum of TiO2, increases the redox potential of the formed radicals, and increases quantum efficiency by reducing the degree of recombination of electrons and holes. The nature and concentration of the dopant change the charge distribution on the TiO2 surface and affects the process of photo corrosion and photocatalytic activity [20]. However, an increase in the absorption of visible light does not always lead to an increase in the activity of the photocatalyst. As a result of doping with cations, a certain number of defects appear in the TiO2 structure, which can act as charge recombination centers, this leading to a decrease in photocatalytic activity even under the influence of UV light.

In [21], Kryzhitsky et al. show the change in the activity of photocatalytic properties of rutile and anatase forms of nanocrystalline TiO2 depending on the nature of metal-based dopants. According to the results of the study, it is found that doping does not significantly change the bandgap of rutile, while in the case of doping anatase with iron and chromium, its bandgap narrows significantly. As a result of doping with metals, the photocatalytic activity of anatase (A) increases in the following order: A < A/Co < A/Cu < A/Fe. In the case of rutile (R), its photocatalytic activity decreases in the following order: R > R/Co > R/Cu > R/Fe > R/Cr. According to the authors, the decrease in the photoactivity of TiO2 may be associated with the inhibitory effect of impurity cations.

#### *2.4. Doping with Anionic Elements*

Over the past few years, it has been shown that TiO2 samples doped with nonmetallic elements (nitrogen, carbon, sulfur, boron, phosphorus, and fluorine) in the anionic positions of TiO2, demonstrate high photoactivity in the UV and visible regions of the solar spectrum [22,23]. Among all the anions, carbon, nitrogen, and fluorine caused the most significant interest [24–26]. The substitution of oxygen atoms to carbon leads to the formation of new levels (C2p) above the ceiling of the valence band of TiO2 (O2p), which reduces the bandgap and shifts the absorption spectrum. The inclusion of carbon in TiO2 can also lead to the formation of carbon compounds on the surface of the photocatalyst, which acts as absorption centers of visible radiation [27].

Doping with nitrogen atoms is the most popular way to improve the photocatalytic performance of TiO2. Introduction of nitrogen into the TiO2 structure contributes to a significant shift of the absorption spectrum into the visible region of the solar spectrum, a change in the refractive index, an increase in hardness, electrical conductivity, elastic modulus, and photocatalytic activity in regard to visible light [28,29]. Upon substitution of anions, a new level is formed above the valence band of TiO2 [30]. As shown in Figure 3, the presence of nitrogen leads to a change of the bandgap Eg1 (TiO2) > Eg2 (N-doped TiO2), thus contributing to the absorption of photons of light with lower energy.

**Figure 3.** Changes in the band gap of TiO2 upon doping with nitrogen atoms.

Doping with nitrogen in the oxygen position is difficult since the ionic radius of nitrogen (1.71 Å) is much larger than that for oxygen (1.4 Å). Another auspicious element in the anionic positions of TiO2 is fluorine atoms [31]. Unlike nitrogen, fluorine atoms easily replace oxygen due to the close ion radius (1.33 Å for F− and 1.4 Å for <sup>O</sup>2−). The increase in photocatalytic activity is mainly associated with an improvement in the degree of crystallinity of TiO2 due to doping with fluorine [32]. It has been determined that crystallinity and the specific surface area also affect the photoactivity of TiO2 [33,34]. The crystalline modification of TiO2 in comparison with amorphous TiO2 has significantly fewer defects, which reduce the possibility of recombination processes and contributes to the efficient movement of photogenerated charge carriers in the semiconductor. Since redox reactions occur on the surface of TiO2, one of the main requirements for photocatalysts is the presence of a developed specific surface area. However, the presence of a developed specific surface area implies a large number of defects in the structure and a low degree of crystallinity and, as a result, reduces photocatalytic activity. Therefore, to increase photocatalytic activity, it is important to find a balance between the above factors.

#### *2.5. Doping*/*Loading with Metal Nanoparticles*

The application of metal nanoparticles is another alternative approach to the modification of photocatalysts. A review of the recent literature shows that metals (Co, Pt, Ag, Au, Pd, Ni, Cu, Eu, Fe, etc.) significantly increase the photocatalytic activity of TiO2 [35–37]. The low location of the Fermi level of these metals compared to TiO2 can lead to the movement of electrons from the TiO2 structure to metal particles deposited on its surface. This helps to avoid the recombination of charge carriers, since the holes remain in the valence band of TiO2. This is also beneficial for avoiding the recombination of charge carriers since the holes remain in the valence band of TiO2. A number of conducted investigations indicate that the properties of these photocatalysts depend on the dispersion of metal particles [38,39]. Enhanced photocatalytic properties of metals appear when their size decreases <2.0 nm [40]. Despite the foregoing, too high concentration of metal particles can block the surface of TiO2 and prevent the absorption of photons, leading to a decrease in the efficiency of the photocatalyst.

#### **3. The Utilization of Photocatalysts Based on TiO2**

## *3.1. Hydrogen Evolution*

Hydrogen can be produced by using nanoscale TiO2 based photocatalysts with various morphologies in the form of nanowires, nanospheres, nanorods, nanotubes, and nanosheets [41]. Table 1 lists some TiO2 nanocomposites with different structures, as well as their photocatalytic characteristics.


**Table 1.** TiO2-based photocatalysts with different structures utilized for hydrogen evolution under splitting water mixtures.

P. Melián et al. [42] demonstrated that loading of nanosized TiO2 microspheres with Au and Pt metals increases the hydrogen evolution twice. In the case of using Au, the maximum hydrogen evolution was determined at 1.5 wt.% content of Au in the sample with the yield of hydrogen 1118 μmol <sup>h</sup>−1, while for Pt its optimal content in the sample was 0.27 wt.% with the yield of hydrogen 2125 μmol h−1. The excess of dopants may result in decrease of photocatalyst activity due to possible complete coverage of TiO2 surface, thus, hindering the light to be absorbed. A literature review also showed that the di fference in the optimal ratio for each metal could be associated with the formation of recombination centers on the semiconductor surface by metal particles [51,52].

A positive e ffect on the rate of H2 production was found when using sacrificial agents acting as electron donors (hole scavengers) during photoreforming, in which the hydroxyl radical is consumed by the sacrificial agents. In general, there are two types of sacrificial reagents: organic and inorganic based electron donors. Among organic electron donors, the most e ffective are water–alcohol mixtures, in particular, methanol > ethanol > ethylene glycol > glycerol [53–59]. However, an increase in the concentration of the sacrificial agen<sup>t</sup> does not always lead to an increase in the yield of hydrogen. Y.-K. Park et al. [60] showed that the rate of hydrogen evolution also increases depending on the concentration of methanol (Figure 4a). At low concentrations, the rate of hydrogen formation in solutions is proportional to the concentration of methanol, while at higher concentrations, it approaches to a constant value [61]. Nevertheless, after adding a certain amount of methanol and ethanol, a further increase in its concentration leads to a decrease in the rate of hydrogen evolution (Figure 4b). The yield of hydrogen during photoreforming has a maximum output during 80−90 min and after it decreases. This is due to the formation of a significant amount of methane and ethane, during which photogenerated electrons (*e*<sup>−</sup>*CB*) and holes (*h*+*VB*) are consumed [54].

**Figure 4.** (**a**) Rate of hydrogen evolution from photocatalysis of aqueous methanol solution on Ag/TiO2 photocatalysts. This figure is reprinted from [60], with permission from Elsevier, 2020; (**b**) H2 production patterns for 24.47 M (100% *v*/*v*) of methanol and 17.06 M (100% *v*/*v*) of ethanol. This figure is reprinted from [54], with permission from Elsevier, 2015.

The amount of dopant also has a significant e ffect on the e fficiency of light absorption by the photocatalyst and on its photocatalytic activity. For example, Udayabhanu et al. [62] prepared Cu-TiO2/CuO nanocomposites containing di fferent amount of Cu. The color of the obtained samples, depending on the concentration of Cu-TiO2/CuO (CUT) from 1 to 4 mol% (the samples were as named CUT 1, CUT 2, CUT 3, and CUT 4) changes from light green to dark green (Figure 5). To evaluate the photocatalytic activity of hydrogen production, scientists compared the activities of obtained nanocomposites with conventional TiO2. Sunlight was used as the source of radiation, and glycerol was chosen as a sacrificial agent. According to the results, the CUT 3 sample based on Cu-TiO2/CuO possessed the highest yield of hydrogen (10.453 μmol h−<sup>1</sup> g<sup>−</sup><sup>1</sup> of H2 under sunlight and 4.714 μmol h−<sup>1</sup> g<sup>−</sup><sup>1</sup> of H2 under visible light). Nevertheless, this type of photocatalyst is e ffective only for hydrogen production, since it has shown low e fficiency in the decomposition of organic dye and metal detoxification in water.

**Figure 5.** Colour of synthesized pristine and Cu-TiO2/CuO composite nanopowders. This figure is reprinted from [62], with permission from Elsevier, 2020.

The photocatalyst e fficiency is a ffected not only by the nature of the alloying element but also by its concentration. For example, X. Xing et al. in [63] demonstrated the dependence of the yield of hydrogen on the light intensity and the concentration of the dopant. From Figure 6a, it is clear that the increase of concentrations of each photocatalyst (pure TiO2 and Au/TiO2) results in increase of the rate of hydrogen production. However, the hydrogen generation rate for Au/TiO2 photocatalyst at the same light intensity is 18–21 times higher than that for pure TiO2. The explanation to this is that Au can limit charge carriers' recombinations and, what is more, the visible light absorption of photocatalyst is enhanced by the localized surface plasmon resonance e ffect of Au nanosized particles. The influence of the light intensity (from 1 to 9 kW/m2) and the duration of exposure on the rate of hydrogen generation for Au/TiO2 nanoparticles with a concentration of 1 g/<sup>L</sup> is also shown in Figure 6b.

**Figure 6.** (**a**) Hydrogen evolution of pure TiO2 nanoparticles and Au/TiO2 nanoparticles at the same light intensity of 5 kW/m2; (**b**) Hydrogen evolution in 1–9 kW/m<sup>2</sup> light intensities of 1 g/<sup>L</sup> Au/TiO2 solutions. Both figures are reprinted from [63], with permission from Elsevier, 2020.

The photocatalytic properties of the material can be improved by creating composites based on di fferent photocatalysts. For example, E.-C. Su et al. [64] obtained a composite photocatalyst based on Pt/N-TiO2/SrTiO3-TiO2 in the form of nanotubes using a two-stage hydrothermal process (SrTiO3 is also a photocatalyst with a bandgap of 3.2 eV with a perovskite-type structure [65]). The obtained results showed that this composite photocatalyst is able to operate under sunlight with the rate of hydrogen evolution up to 3873 μmol/h/g.

In practice, photocatalytic reactions are mainly carried out at room temperature. It is found that increasing temperature has a positive e ffect on the activity of some photocatalysts, which makes it relevant to develop new photocatalysts based on the anatase form of TiO2 with a thermostable phase.
