TiO₂-Based Photocatalytic Coatings on Glass Substrates for Environmental Applications

Abstract

To address environmental pollution and energy shortage issues, titanium dioxide (TiO₂)-based photocatalysts, as an efficient pollution removal and fuel production technology, have been widely used in the field of photocatalysis. In practical applications, TiO₂-based photocatalysts are usually prepared on various substrates to realize the separation of the catalyst from water and improve photocatalytic stability. Herein, the research progress of TiO₂-based heterogeneous photocatalytic coatings deposited on glass substrates with various deposition techniques is reviewed. Such TiO₂-based composite coatings obtained using different techniques showed excellent self-cleaning, pollution removal, air purification, and antibiosis performance. The various deposition techniques used for the preparation of TiO₂ coatings, such as wet chemical deposition (WCD), electrodeposition, physical vapor deposition (PVD), and chemical vapor deposition (CVD) were discussed together with photocatalytic applications by highlighting the typical literature. Finally, the challenges and prospects of developing TiO₂-based heterogeneous coatings were put forward.

1. Introduction

Environmental pollution and energy shortage have been persistent problems faced by researchers committed to finding solutions [1,2,3]. Semiconductor-based photocatalysis is a green and sustainable advanced oxidation process (AOP) that can directly convert green solar energy into chemical energy [4,5,6]. Therefore, semiconductor photocatalysis has been widely used in the environmental and energy fields [7,8,9,10,11,12,13]. Various semiconductor materials have been developed as photocatalysts, such as TiO₂, g-C₃N₄, ZnO, Bi-based, and Ag-based materials [14,15,16,17,18,19,20,21,22,23]. As shown in Figure 1, TiO₂ was the most researched photocatalyst among many semiconductor photocatalysts due to its good photostability, low cost, and environmental friendliness [24,25,26,27]. However, the low photocatalytic efficiency of TiO₂ under visible light limits its practical applications due to the low absorption of visible light and high rate of photogenerated carrier recombination [28,29,30]. Metal and non-metal loading or doping and heterojunction building could enlarge the light response range and improve charge separation to address these issues [31,32,33,34,35,36,37]. In addition, nanostructured TiO₂-based composites are difficult to separate and recover in a flow system or aqueous suspension, resulting in low efficiency and stability during environmental applications [38,39,40,41]. It is worth noting that TiO₂ nanoparticles (NPs) will inevitably remain in the environment and migrate in water and soil, which poses a risk to human and environmental health [42,43]. Photocatalytic coatings could realize the immobilization of nanosized photocatalysts and have emerged as an efficient strategy for solving these problems [44]. The obtained photocatalytic coating system eliminates the steps of filtering and recovering the catalyst after use, which allows the coating to be directly reused and run continuously for a long time without a large amount of catalyst loss and inactivation [45,46,47].

Photocatalytic coatings have been applied from the laboratory to the industrial fields in our daily life [48,49,50,51]. Researchers have deposited TiO₂ nanocomposites onto various substrates to explore their photocatalytic performance [52]. Due to its high-temperature resistance and chemically inert properties, glass has shown great potential in depositing TiO₂-based photocatalytic heterogeneous coatings as a substrate using various techniques [53,54,55]. Although some studies have comprehensively summarized TiO₂-based photocatalytic coatings, including their preparation methods and applications [56], it is necessary to summarize and discuss the deposition strategies of TiO₂-based photocatalytic coatings on glass substrates with different photocatalytic applications. Thus, we intend to provide a review of TiO₂-based photocatalytic heterogeneous coating deposition technology on glass substrates and their corresponding applications in the environmental field.

2. The Basic Principle of TiO₂ Photocatalysis

It was observed by Fujishima and Honda [57] in 1972 that using a TiO₂ electrode in an electrochemical cell under UV irradiation led to water splitting even when no external voltage was applied, indicating the photocatalytic activity of TiO₂ and thus opening up a new field of applications. TiO₂ has become one of the most promising materials in the field of photocatalysis [58]. In the photocatalytic process, there are three main crystal phases of TiO₂: rutile, anatase, and brookite [59]. In all these three forms, Ti atoms are coordinated to O atoms, forming TiO₆²⁻ octahedral units. The formation of cells for the three TiO₂ phases depends on how the TiO₆²⁻ octahedral units contact the neighboring octahedrons. It is shown in Figure 2 that adjacent octahedra units in anatase, rutile, and brookite share four, two, and three edges, respectively [59]. All of these crystalline structures could act as photocatalysts. The effect of the crystal phase on the photocatalytic performance of TiO₂ has attracted extensive attention. At present, pure- and mixed-phase TiO₂ photocatalysts have been synthesized and applied in photocatalysis.

The photocatalytic process for TiO₂ involving the formation of advanced oxidants is illustrated in Figure 3. Bandgaps (E_g) of the rutile and anatase phases are about 3.0 and 3.2 eV, respectively [60]. When the light (hv ≥ E_g) irradiates on TiO₂, its photon absorption causes electrons to be excited from the valence band (VB) to the corresponding conduction band (CB), resulting in photogenerated electrons (e⁻) and positively charged holes (h⁺). Photogenerated e⁻ and h⁺ are reducing agents and solid oxidants, respectively. When they migrate to the surface of TiO₂, they will undergo redox reactions with the adsorbed oxygen or water molecules to form superoxide anion radicals (O₂^•−) and hydroxyl radicals (•OH), respectively. Other reactive oxygen species, such as hydroperoxide radical (•OOH) and H₂O₂, can also be formed by further redox processes, dimerization, or disproportionation during the photocatalytic process [61,62].

3. Preparations and Applications of TiO₂-Based Photocatalytic Coatings

The selection of the substrate is an essential step in photocatalytic coatings. The substrate materials should have good properties, such as good adhesion, high-temperature stability and strong adsorption of pollutants of TiO₂ coatings. Previous studies have shown that common substrates for fixing heterogeneous photocatalysts based on TiO₂ nanostructures include glass [38,63], steel [64], metal or metal oxide [65], polymers [54,66], ceramics [67], cement mortar [68], zeolite [69], non-woven fabric [70], and latex [71], etc. Although there are many options for choosing a substrate for photocatalytic coatings, glass substrates are preferred for photocatalytic research. Glass is low-cost, easily fabricated, highly chemically/physically/thermally stable, and optically attractive, which makes it an ideal material for photocatalytic coatings [72]. Glass substrates have the desirable property of light transparency, which guarantees better photocatalytic performance [73]. The photocatalytic activities of photocatalytic glass-based coatings are determined by fixing methods. The standard deposition methods for depositing TiO₂ nanostructured heterogeneous photocatalytic coatings onto glass materials as substrates are given in the following chapters, such as wet chemical deposition methods (dip coating, spin coating, spray coating), electrodeposition, physical vapor deposition (including magnetron sputtering and pulsed laser deposition), and chemical vapor deposition and other techniques.

3.1. Wet Chemical Deposition

3.1.1. Dip Coating

The dip-coating method is a low-cost and easy-to-operate process that is one of the most commonly employed techniques for depositing TiO₂ nanoparticles (NPs) on glass materials [74]. The dip-coating method involves growing a thin film on the glass substrate by immersing it in a precursor solution containing the photocatalytic material to be deposited. The sol-gel method is often used to prepare TiO₂ precursor solutions in a typical dip-coating process [75]. In addition, before depositing the photocatalytic coating, it is necessary to clean the selected glass material with plasma or wash it with a specific solution, such as acids [76], organic solvents, or neutral detergents [77]. After the precursor solution and glass substrate are prepared, the dip-coating process is started (as shown in Figure 4). The glass substrate is immersed in the pre-solution for a period of time by means of dip coating equipment, and then the substrate is extracted at a certain speed. The immersion time and dipping times can determine the coatings’ thickness. After the deposition, the obtained films are dried and annealed to promote the crystallization of TiO₂. It should be emphasized that two or more precursor solutions may be used in the preparation of TiO₂-based heterogeneous photocatalytic coatings.

According to the literature [78], the amount and crystalline phases of TiO₂ can significantly affect the photocatalytic performances of the obtained coatings. The increase in dip-coating times only increases the thickness of the coating, but dose not change the crystal phase. The crystalline phases are determined by the annealing temperatures in the post-thermal process. Moreover, glass materials often contain many sodium ions. If the photocatalytic coating is too thin, the sodium diffusion from the glass substrate could adversely affect the photocatalytic activity. Overall, when using dip-coating technology to obtain photocatalytic coating, we should carefully control the number of dip-coating cycles and proper annealing procedures. Appropriate parameters will produce the desired TiO₂ coating with suitable coating thickness and crystal phase. Rajendran et al. [79] deposited activated carbon-doped TiO₂ heterogeneous film onto a glass substrate with titanium tetraisopropoxide as the starting material using a sol-gel-assisted dip-coating technique. The coatings were dried at 200 °C for 30 min and then annealed at 400 °C or 600 °C for 1 h. The coating crystal structures obtained were anatase and rutile, respectively. When annealed at 600 °C, the crystal structure changed from the anatase type to the rutile type. The intensity of XRD patterns increased with film thickness and annealing temperature. In addition, the authors found that the doping of activated carbon affected the photocatalytic performance of TiO₂ coatings. Similarly, Rapsomanikis et al. [80] synthesized Ce-TiO₂ coatings via the sol-gel method and deposited them on the borosilicate glass using dip coating. The coatings were dried at room temperature for 1 h and calcined at 500 °C at a heating rate of 5 °C/min. The introduction of Ce effectively expanded the response range to visible light and reduced the recombination rate of e⁻/h⁺, thus enhancing the photocatalytic decolorization of Basic Blue 41 (BB-41) in visible light.

Moreover, Manna et al. [77] reported a TiO₂-ZrO₂ photocatalytic heterogeneous coating with efficient photocatalytic removal of methylene blue (MB) dye from water. TiO₂-ZrO₂ composite photocatalysts were fabricated using a sol-gel process using zirconium (IV) oxychloride octahydrate powder (ZOC), titanium (IV) isopropoxide (TTIP), and block co-polymer (surfactant). Subsequently, TiO₂-ZrO₂ was dip-coated on the float glass substrate. Then, the coatings were thermally treated at 400 °C with a 1.5 °C/min heating rate after drying at 80 °C. After successfully introducing ZrO₂, the composite film exhibited a spherical and clumped distribution morphology, as shown in Figure 5a,b. The TiO₂-ZrO₂ composite coating also had a regular texture and extremely root-mean-square (RMS) roughness surface, thus exhibiting superhydrophobicity and self-cleaning. As shown in Figure 5c, in the presence of TiO₂-ZrO₂ coating, the characteristic peak intensity of MB was significantly reduced after 20 min of ultraviolet irradiation. The obtained mesoporous TiO₂-ZrO₂-coated float glass had a self-cleaning capability, photocatalytic performance, and antireflective properties (Figure 5d), which could effectively replace the empty texture solar cover glass in the market. However, the performances of the TiO₂-ZrO₂-coated float glass on a large scale need to be evaluated in actual applications. Additionally, Bagheri et al. [81] synthesized an optimized CNT-modified TiO₂/WO₃/CdS ternary nanocomposite and deposited it on fluorine-doped tin oxide (FTO) glass using the sol-gel and dip-coating methods. As shown in Figure 5e, CNT-modified TiO₂/WO₃/CdS particles aggregated on the FTO glass surface and formed a photocatalytic coating with a thickness of about 781.50 nm. TiO₂ was composited with CdS (2.25 eV), WO₃ as a light-storing material, and CNTs with a large electron storage capacity in this heterojunction. Consequently, heterojunction formation significantly reduced electronic bandgap value and increased visible light absorption. The CNT/TiO₂/WO₃/CdS coatings on FTO glass could be used as a photoanode in the photocatalytic fuel cell to degrade Reactive blue 19 (RB19) (Figure 5g). In the optimal light intensity (890 lm), 99.9% and 70% removal efficiency for RB19 and COD were obtained, respectively (Figure 5f).

3.1.2. Spin Coating

Spin coating technology is an advanced, cheap, easy-to-control uniform coating deposition technology that can be used to deposit NPs onto different substrate surfaces [82,83]. A typical spin-coating process is present in Figure 6, and the process can be divided into three stages after preparing the material and substrate. Firstly, the substrate is placed onto the cage of the spin coater, and the prepared solution is dropped onto the upper surface of the glass substrate. Then, a wet film is produced by centrifugal force during rotation. As the spinning proceeds, the excess liquid flows away from the substrate. Finally, solvent evaporation leads to the coating’s formation. Further calcination may be required to crystallize the deposited coatings for the desired application. The spinning speed and spin-coating recycling times can determine the coatings’ thickness. Thus, the thickness of the coating can be controlled.

Peeters et al. [84] reported a self-cleaning borosilicate coating; a TiO₂ film embedded with gold (Au) NPs was deposited on a glass surface by spin coating (Figure 7a). After being calcined at 550 °C for 3 h with a 1 °C/min heating rate, uniform, smooth, highly transparent, and photoactive Au NPs with anatase TiO₂ thin coatings were obtained. Au NPs with an average diameter of 16 ± 4 nm distributed on TiO₂ coatings at different depths (Figure 7b,c). Compared with commercial Pilkington Activ^TM, embedding Au NPs leads to better photocatalytic self-cleaning activity under dark and irradiation conditions (Figure 7d). The plasmonic effect of Au NPs was fully utilized to alleviate the problem of poor visible light response of TiO₂ coating. Therefore, the self-cleaning glass with Au-TiO₂ composite coatings has great application potential under actual sunlight. Further study is needed to verify the self-cleaning application of the developed photocatalytic coatings in real life. Komaraiah et al. [85] covered a glass substrate with high-quality nanocrystalline Fe-doped TiO₂ using a simple spin-coating technique, producing a coating for MB decomposition under the irradiation of visible light. Compared with pure TiO₂, the bandgap decreased with the increase in Fe concentration, and the 7% Fe-doped TiO₂ (FTO7) film exhibited a high decolorization rate (96.7%) for MB with excellent stability in 10 cycles (see Figure 7e–g). The mechanism of the FTO7 coating is shown in Figure 7h. Incorporating Fe³⁺ into the TiO₂ lattice substituted Ti⁴⁺, resulting in a charge imbalance. Then, Fe³⁺ trapped the photogenerated electron and hole to form Fe²⁺ and Fe⁴⁺, respectively. The formation of Fe²⁺ and Fe⁴⁺ decreased the distance between CB and VB, which can enhance the charge carrier separation. As a result, the doping of appropriate Fe³⁺ ions was beneficial to the improvement of photocatalytic activity.

Romero-Morán et al. [86] deposited TiO₂ NPs on soda-lime glasses using a water-diluted spin-on-glass (SOG) solution to fabricate TiO₂/SiO₂ composite coatings using the spin-coating method (Figure 8a). The TiO₂/SiO₂ coatings prepared with TiO₂ NPs suspensions with different crystalline phases showed significant differences in surface roughness, antibacterial activity, and the generation of •OH (Figure 8b–d). The antibacterial activity was consistent with the trend of •OH production by photocatalysis: Degussa (P25)-TiO₂/SiO₂ > anatase-TiO₂/SiO₂ > rutile-TiO₂/SiO₂. In addition, pNDA photo-bleaching experiments in the presence of scavengers were conducted to further study the embedding of amorphous SiO₂ and the embedded TiO₂ NP suspensions with three crystalline phases during photocatalysis. The relationship between the generation of •OH and the charge carriers transfer mechanism of different TiO₂/SiO₂ composite coatings was also mainly explained in Figure 8d. Hernández-Del Castillo et al. [87] compared TiO₂ and In-Ni co-doped TiO₂ (T-InNi) coatings on glass substrates using the spin-coating method. TiO₂ and T-InNi coatings had high roughness and porous texture, with an average thickness of 3.59 and 3.37 μm, respectively (Figure 9a). The co-doping of In and Ni caused oxygen vacancy defects in the composite films, significantly inhibiting the recombination of e⁻/h⁺ pairs and boosting light absorption from the narrowed bandgap (Figure 9b,c). The obtained coatings were conducted to remove eosin yellow (EY) from drinking water, as shown in Figure 9d–f. The T-InNi coatings exhibited a 97.5% degradation rate after 5 h by UV irradiation treatment and a 75% under natural solar light, which was higher than that of TiO₂ coatings. The detailed photocatalytic mechanism of T-InNi coatings is present in Figure 9g. When exposed to UV and solar light, T-InNi with reduced bandgap was more easily excited to produce e⁻ and h⁺. The e⁻ and h⁺ reacted with oxygen dissolved in water and water to form O₂^•− and •OH, which can be used to degrade EY.

3.1.3. Spray Coating

Compared with dip coating and spin coating, the coating obtained by the spraying process is rougher, which can obtain a larger surface area and improve its photocatalytic activity [88,89]. The classic spray-coating technique is efficient for obtaining high-purity coatings due to its relatively low cost and simple equipment [89,90]. After preparing a certain suspension and pretreating the substrate, the suspension was sprayed on the glass substrate from a nozzle, and the coating obtained on the glass is dried and calcined at a controlled time and temperature (Figure 10).

Zong et al. [91] deposited fluorosilicone/SiO₂-TiO₂ (FSi/SiO₂-TiO₂) on a glass substrate by spray coating. The obtained coatings had obvious self-cleaning performance and photocatalytic activity, which effectively prevents many natural pollutants from depositing on the outdoor coated glasses due to ultraviolet radiation. However, fluorosilane used for surface modification to obtain low surface energy is expensive and may damage the environment. Therefore, using environmentally friendly chemicals to produce superhydrophobic surfaces would be desirable. For example, Wang et al. [92] deposited PDMS-modified SiO₂-TiO₂ ternary composite onto the surface of a glass surface using spray coating to obtain a SiO₂-TiO₂@PDMS coating with excellent self-cleaning performance and photocatalytic activity. Compared with fluorosilane, PDMS is a cost-effective material with many advantages in environmental protection and safety. The SiO₂-TiO₂@PDMS coating was subjected to a thermal treatment at different temperatures for 2 h. The coatings’ wettability and adhesion to water droplets can be adjusted by changing the temperature of thermal treatment. The water contact angles of the SiO₂-TiO₂@PDMS coating with a “lotus effect”-like self-cleaning capability after heat treatment at 25 °C and 400 °C were 147.8° and 152.2°, respectively (Figure 11a,b). This suggested that high-temperature treatment enhanced the coating hydrophobicity. Excitingly, the outstanding hydrophobicity of the SiO₂-TiO₂@PDMS coating still had a specific degradation performance on methyl orange compared to the blank sample (Figure 11c). In addition, it was found that the coating exhibited liquid repellency under acid (pH = 2), alkaline (pH = 11), and organic dye droplets, which proved the excellent mechanical stability of the coating even when the surface was mechanically scraped and exposed to ultraviolet light, as shown in Figure 11d.

For excellent hydrophobic coatings, the durability of the surface properties is critical for practical applications. When coatings are damaged by mechanical wear and/or tear, their superhydrophobic properties will be reduced or even lost. Therefore, epoxy resin (EP) has been progressively used to prepare composite coatings because of its good adhesion and corrosion resistance [93]. Li et al. reported a hydrophobically modified TiO₂ with fluorine-containing hydrogen-containing polysiloxane (F-PMHS) [94]. They prepared an M-TiO₂@EP superhydrophobic composite coating on the glass substrate using EP as the adhesion, which achieved a static contact angle of over 153° for water droplets in a wide pH range (Figure 11e,f). Moreover, the coating had a specific self-cleaning capability and strong hydrophobicities in a simple simulation experiment and in 15-day outdoor experiments (Figure 11f,g). Therefore, these excellent properties allow M-TiO₂@EP coating to be a candidate for promising outdoor and harsh conditions applications. Unlike the previous preparation steps, the composite coating was prepared by one-step spraying. The preparation method was simple, which makes the coating applicable in large-scale applications.

Figure 11. (a) Water droplets rolling off to remove dust from the surface of the SiO₂-TiO₂@PDMS coating; (b) wettability and adhesion of SiO₂-TiO₂@PDMS-25 (up) and SiO₂-TiO₂@PDMS-400 (down) coatings to water droplets and the wetting states of water droplets on their surfaces; (c) photocatalytic degradation of methyl orange with different samples; (d) wettability of water, NaOH solution (stained with rhodamine B), HCl solution (stained with methylene blue), and methyl orange solution on the surface of the SiO₂-TiO₂@PDMS-400 coating, the UV-irradiated and mechanically destroyed SiO₂-TiO₂@PDMS-400 coating, reproduced with permission from [92]. Copyright 2021, Elsevier B.V.; (e) the preparation process of M-TiO₂, superhydrophobic M-TiO₂ coating, and M-TiO₂@EP coating; (f) the static contact angle for water droplets on the M-TiO₂@EP coating surface under different pH and self-cleaning ability test of M-TiO₂@EP coating with graphite powder as impurity; (g) outdoor test sample A (g), reproduced with permission from [94]. Copyright 2022, Elsevier B.V.

Figure 11. (a) Water droplets rolling off to remove dust from the surface of the SiO₂-TiO₂@PDMS coating; (b) wettability and adhesion of SiO₂-TiO₂@PDMS-25 (up) and SiO₂-TiO₂@PDMS-400 (down) coatings to water droplets and the wetting states of water droplets on their surfaces; (c) photocatalytic degradation of methyl orange with different samples; (d) wettability of water, NaOH solution (stained with rhodamine B), HCl solution (stained with methylene blue), and methyl orange solution on the surface of the SiO₂-TiO₂@PDMS-400 coating, the UV-irradiated and mechanically destroyed SiO₂-TiO₂@PDMS-400 coating, reproduced with permission from [92]. Copyright 2021, Elsevier B.V.; (e) the preparation process of M-TiO₂, superhydrophobic M-TiO₂ coating, and M-TiO₂@EP coating; (f) the static contact angle for water droplets on the M-TiO₂@EP coating surface under different pH and self-cleaning ability test of M-TiO₂@EP coating with graphite powder as impurity; (g) outdoor test sample A (g), reproduced with permission from [94]. Copyright 2022, Elsevier B.V.

3.2. Electrodeposition

With the continuous development and diversification of technology, the electrodeposition of metal particles on conductive glass substrate has become a mature technology [95,96]. Compared with other deposition methods, electrodeposition has the advantages of low cost, short reaction time, low temperature, easy control, and environmental friendliness [97,98]. The deposition rate and time can be well controlled to obtain a uniform film with controllable composition. Specifically, a suspension for electrodeposition and a conductive glass substrate (mainly indium-doped tin oxide (ITO) and FTO glass) is prepared first. After that, the conductive glass is used as the working electrode, the Pt wire/foil is a counter electrode, and a standard calomel electrode or Ag/AgCl is used as the reference electrode to start the electrodeposition process at a certain deposition voltage and a specific temperature [99]. The coating is formed and may need annealing (Figure 12).

Zhou et al. [100] prepared uniform TiO₂/SiO₂ composite films on ITO glass substrates using electrodeposition. ITO glass served as the cathode and Pt foil served as the anode with a 2 V constant bias. The deposited film was washed with distilled water, and then it was dried and calcined at 450 °C for 2 h. The obtained material was used to evaluate electrochemically assisted photocatalytic degradation of 4-chlorophenol. The photoelectrocatalytic (PEC) activity of the TiO₂/SiO₂ composite film was affected by the deposition time, calcination temperature, and the initial pH value. Furthermore, the electrode potential of PEC experiments was kept at 0.8 V. The results indicated that the PEC activity of the TiO₂/SiO₂ composite film electrode was significantly improved, which was about 14 times higher than that of the pure TiO₂ film electrode. This result was attributed to the incorporation of SiO₂ enhancing the charge separation of photogenerated e⁻/h⁺ pairs. Moreover, Lu et al. [101] successfully fabricated TiO₂/CdSe heterostructured films using a two-step electrodeposition method for the first time. Firstly, the anatase TiO₂ films were electrodeposited on FTO conductive glass using TiOSO₄ as the precursor, and then CdSe was electrodeposited on the obtained TiO₂ glass substrate with CdCl₂ and SeO₂ as precursors (Figure 13a). The two-step deposition process was completed under different electrodeposition parameters. The deposition voltages of TiO₂ and CdSe were −1.2 V and −0.5 V, respectively. The heterogeneous structure formation expanded the sunlight absorption range, which led to a more remarkable photocatalytic activity of the TiO₂/CdSe coatings than that of pure TiO₂ single-layer coatings (Figure 13b,c).

Vasile et al. [102] deposited TiO_x thin film from an acidic aqueous solution comprising TiOSO₄ on an FTO glass substrate by cathodic electrodeposition at −1.05 V. Then, FTO/TiO_x was used as the anode, and a platinum foil was used as the cathode to deposit FeOOH at 1.2 V. After the deposition, FTO/TiO_x/FeOOH was annealed to obtain FTO/TiO₂/Fe₂O₃ structure. TiO₂/Fe₂O₃ had a compact cauliflower morphology with a deposition thickness of about 100 nm (Figure 13d). It could be observed from the optical absorption spectra of the sample that the TiO₂/Fe₂O₃ composite structure improved the light absorption in the ultraviolet and visible regions, as shown in Figure 13e. The formation of heterostructures also promoted the separation and charge carrier transfer in the hematite–titania composite (see Figure 13f). Consequently, FTO/TiO₂/Fe₂O₃, a photoanode for solar water oxidation, had good PEC performance.

3.3. Physical Vapor Deposition (PVD)

3.3.1. Magnetron Sputtering

With the rapid progressive applications of photocatalytic TiO₂-based coatings, magnetron sputtering has become an exciting method for preparing high-purity uniform coatings on glass substrates [103,104]. The basic magnetron sputtering process occurs at reduced pressure (typically 0.1 to 0.5 Pa) [105]. As shown in Figure 14, gas atoms are ionized by electric and magnetic fields to produce gas⁺ and electrons, which bombard the target (or cathode) with high energy, causing the metal on the target surface to be sputtered in atomic or ionic states and condensed into a thin film on the substrate [106]. Oxygen or nitrogen as reactive gases can be introduced with inlet gas to form composite films of oxides or nitrides. According to the current mode, magnetron sputtering can be divided into direct current (DC) magnetron sputtering (suitable for conductive targets) and radio frequency (RF) magnetron sputtering (suitable for any target). In addition, magnetron sputtering deposition methods can be classified as single-target and multi-target sputtering depending on the number of targets.

Recently, Mazur et al. [107] deposited TiO₂ and SiO₂ thin films on standard microscope slide glasses, using microwave-assisted reactive magnetron sputtering of high-purity Ti and Si targets. The prepared antireflection multilayer coatings were amorphous, which might be due to the low temperature of the sputtering process (not exceeding 100 °C). Compared with the bare glass substrates, the deposited TiO₂/SiO₂ coated glass had a series of excellent properties, such as antireflectivity, photocatalytic activity, higher hardness, and transmittance. Therefore, the antireflection multilayer coatings could be used as self-cleaning and protective surfaces. Similarly, Villamayor et al. [108] deposited TiO₂ and NiO/TiO₂ coatings using DC and pulsed DC reactive magnetron sputtering on glass slides and monocrystalline silicon wafers as substrates. After deposition, annealing of the coatings at 450 °C for 2 h was performed to obtain anatase TiO₂. The TiO₂ coating had a columnar porous structure with a thickness of about 740 nm (Figure 15a). The introduction of NiO could change the surface morphology of the TiO₂ coating. NiO/TiO₂ coatings with NiO thicknesses of 10 nm and 20 nm were obtained at sputtering powers of 25 W and 50 W, respectively. When the deposited NiO thickness was increased up to 20 nm from 10 nm, the small cluster-like structure became a cauliflower-like structure, obviously changing surface morphology (see Figure 15b,c). The increased sputtering power made the film more compact and reduced the surface roughness (Figure 15d,e). The formation of a p-n heterojunction also improved the reduction of bandgap value and the mobility of charge carriers, resulting in a higher MB degradation efficiency on NiO/TiO₂ coatings (Figure 15f,g). Interestingly, the test samples had higher photocatalytic efficiency than the commercial Pilkington Activ glass sample. In addition, the results suggested that controlling the deposition thickness and morphology of the NiO layer on TiO₂ films was necessary. The decrease in photocatalytic activity of the thicker coating might be due to the increase in p-n heterojunctions, which counteracts the influence of the decrease in surface area.

Ratova et al. [109] used pulsed DC magnetron sputtering of one target (Ti) in reactive mode for an undoped TiO₂ coating in an Ar/O₂ mixture atmosphere or N-doped TiO₂ coatings in an Ar/dry air atmosphere. It can be observed from Figure 16a that the film deposited in O₂ was uniform with a smooth surface. After the deposition on soda lime glass slides, no additional substrate heating was provided, and the deposition process temperature was below 150 °C. At this time, the study found that the crystallization of the TiO₂ coatings depended on the process pressure, pulse repetition rate, and duty cycle. In addition, although the N-doping led to a narrower band gap, it caused faster recombination of the active photoproduction species. Therefore, N doping did not cause an improvement in photocatalytic activity. The results showed that the TiO₂ coatings had the highest photocatalytic activity for methylene blue and stearic acid (SA) under the conditions of working pressure of 4 Pa, pulse repetition frequency of 100 kHz, and duty cycle of 50% in Ar/O₂ atmosphere (Figure 16b,c). Similarly, Ji et al. [110] combined a thermochromic VO₂ with a photocatalytic TiO₂ layer by reactive DC magnetron sputtering to obtain multifunctional TiO₂/VO₂ bilayer coatings on heated glass. The TiO₂/VO₂ bilayer coating and TiO₂ coating had similar surface morphology, but the TiO₂/VO₂ bilayer film was less smooth than the TiO₂ film due to the presence of VO₂ (Figure 16d,e). Incorporating VO₂ synergistically enhanced thermochromic, luminous, and photocatalytic performances. As shown in Figure 16f, the bilayer TiO₂/VO₂ coatings lead to glazing that can purify indoor air and lower solar energy inflow, which are essential for energy-efficient buildings with reduced needs for ventilation and air conditioning. The photocatalytic properties of the samples were evaluated based on the SA decomposition reaction. The results showed that the degradation rate of TiO₂/VO₂ bilayer coatings was much faster than that of TiO₂ films under Xenon lamp illumination with or without an AM1.5 filter (Figure 16g,h). The TiO₂/VO₂ bilayer coatings had better performances in terms of light transmittance, indoor air purification, and solar energy modulation.

3.3.2. Pulsed Laser Deposition

Pulsed laser deposition (PLD) is a physical vapor deposition technique. The technique uses high-energy laser pulses to vaporize the target material, generating plasma plumes from the target, and then transporting and condensing atoms onto the glass substrate to obtain a coating with optical properties [111,112]. Such a deposition process is carried out in the presence of a vacuum, reactive gas (GPLD), or liquid (LPLD) [113]. As shown in Figure 17, this technique allows one to tune the porosity and density of TiO₂-layered films by carefully controlling the process parameters. The obtained specific vertically oriented morphologies can facilitate the transport of photogenerated charge carriers and produce effective visible light scattering, making it an excellent method for synthesizing photocatalytic coatings with complex structures and composites [114,115].

Linnik et al. [116] prepared ZrO₂-doped TiO₂ targets with different weight percent concentrations and then used PLD technology to deposit ZrO₂/TiO₂ composite coatings on glass substrates. The increase in ZrO₂ content could narrow the bandgap and enhance the light absorption capability. The coatings of ZrO₂/TiO₂ composite were also obtained by depositing in pure O₂ or mixed N₂:CH₄ atmospheres (5:1 or 9:1) to investigate the effects of active atmospheres on photocatalytic reduction of toxic dichromate (Cr(VI)) ions. The results showed that ZrO₂ (10%)/TiO₂ deposited under N₂:CH₄ (5:1) mixture atmosphere had the highest conversion rate of photo-reduced Cr(VI) ions under UV and visible light irradiation. Interestingly, Olvera-Rodríguez et al. [117] deposited multilayer-type TiO₂/Au/TiO₂ thin films on a conductive glass substrate using PLD. The film had about 212 nm total thickness and consisted of six layers of TiO₂ and five layers of Au alternately (Figure 18a). By coupling with the Au layer, the optical properties of the composite coating were improved mainly from the improved light absorption and reduced recombination rate of photogenerated charges (Figure 18b,c). The TiO₂/Au/TiO₂ coating with excellent optical properties was used as a photocatalytic photoanode to degrade paracetamol in electro-oxidation (EO) and PEC, respectively. It was found that the PEC degradation rate of paracetamol was faster under the assistance of UVA light irradiation than that of EO (Figure 18d,e). Also, the added Cl⁻ from NaCl was oxidized at the photoanode to form active chlorine under UVA light irradiation, which could also degrade paracetamol. Thus, the degradation rate was faster in the presence of Cl⁻ (Figure 18f).

3.4. Chemical Vapor Deposition (CVD)

CVD has unique advantages, such as simple equipment and diverse precursors that allow the fabrication of continuous, compositionally controlled, homogeneous films [118]. Therefore, CVD is a powerful tool for synthesizing high-quality TiO₂-based photocatalytic coatings on various substrate surfaces [119,120]. In a typical CVD process, there are two possible scenarios where the substrate is exposed to volatile precursors. Firstly, the vaporized precursor A reacts with the treated glass substrate and the final product is produced in situ at the glass substrate, or the precursor A is directly deposited on the glass after a chemical reaction (Figure 19a). Alternatively, the vaporized precursor A reacts with the vaporized precursor B, forming thin films on the glass substrate (Figure 19b). Volatile by-products generated during the process are generally removed with the gas stream. Depending on the different precursors, CVD techniques can be defined as aerosol-assisted CVD (AA-CVD), atmospheric pressure CVD (AP-CVD), plasma-enhanced CVD (PE-CVD), or aerosol-assisted metal–organic CVD (AA-MO-CVD).

Alotaibi et al. [121] deposited ZrO₂-TiO₂ composite films on glass substrates by AACVD. The deposition was conducted using zirconium acetylacetonate and titanium isopropoxide as precursors under N₂ atmosphere at 450 °C. After deposition, the glass was cooled down to room temperature in N₂, and then annealed at 600 °C for 1 h. Finally, ZrO₂-TiO₂ composite film showed a dome with cracks, and the thickness of the film was close to 500 nm (Figure 20a,b). Compared with TiO₂ film, ZrO₂-TiO₂ composite film showed better photocatalytic activity in the degradation of resazurin, which was attributed to the improvement of the surface acidity of the composite film (Figure 20c). In addition, there is a high hydroxyl concentration on the surface of ZrO₂-TiO₂ film, which can be used as hole traps to inhibit the recombination of the carrier.

Quesada-González et al. [122] deposited interstitial boron (B)-doped TiO₂ thin films on a float glass substrate using APCVD, starting from boron isopropoxide (B[(CH₃)₂CHO]₃) and titanium tetrachloride (TiCl₄) as precursors (Figure 20d). The interstitial doping of B caused a change in the morphology of the TiO₂ films, and significantly increased the crystallinity and the average crystallite size. The doping of B did not narrow the band gap, but the recombination rate of e⁻/h⁺ reduced due to the disordered structure. Thus, the B-TiO₂ films exhibited superior photocatalytic properties for the degradation of SA. Similarly, Sotelo-Vazquez et al. [123] synthesized P-doped TiO₂ thin films deposited on glass by APCVD, using TiCl₄ and triethyl phosphate ((EtO)₃PO) as precursors. P groups (P⁵⁺ species and P³⁻ species) affected the photocatalytic activity differently. In the P-TiO₂ composite coatings containing only P⁵⁺ species, charge carrier concentrations increased by several orders of magnitude, while incorporating P³⁻ species seriously reduced the photogenerated carrier lifetimes. Additionally, the photocatalytic performance of the P ions-TiO₂ composite coatings was also reduced, which was proven in the SA mineralization results.

Lang et al. [124] deposited pristine TiO₂ and Ag NPs-TiO₂ composite films on the quartz glass substrate using PE-CVD, originating from AgNO₃ and TTIP as precursors. Then, the obtained coatings were annealed at 600 °C for 12 h to achieve a typical anatase structure under N₂ gas flow at 200 standard cubic centimeters per minute (sccm). The addition of Ag NPs changed the morphology of the films with the appearance of particle aggregation (Figure 20e,f). In addition, doping a moderate amount of Ag NPs could enhance visible light absorption and reduce the bandgap, thus exhibiting significantly higher photocatalytic activity compared with pure TiO₂. However, when excessive Ag NPs were loaded, an e⁻/h⁺ recombination occurred on the Ag NPs, thus deactivating the carriers and reducing the photocatalytic activity in turn (Figure 20g,h).

Similarly, de Oliveira et al. [125] used AA-MO-CVD to obtain a coating containing TiO₂ and Cu₂O with Ti(IV) oxide bisacetylacetonate and Cu(II) acetylacetonate as precursors at a substrate temperature of 550 °C. The coating was deposited on a (100)-oriented silicon single crystal and alkaline earth boroaluminosilicate glass substrates. The photocatalytic TiCuO composite coatings could be used as a self-cleaning surface to induce an antifouling activity. The coated glass was immersed in seawater to evaluate its antifouling effect. Marine biofouling field tests proved they had seawater resistant properties and showed a reduced colonization on TiO₂ and 16TiCuO (16 was the atomic percentage of copper measured by EDS) coatings after 38 days (Figure 20i). TiCuO composite coatings are promising, non-toxic fouling release films for marine and industrial applications. However, an effective antifouling surface in practical applications should resist various fouling trophic scales, which requires further research. The photocatalytic experimental results showed that although the optical bandgap decreased with the Cu content increasing, the content of Cu had an optimal value equivalent to that of pure TiO₂ with micro-flowers in degrading Orange G dye (Figure 20j). Under the same experimental conditions, more Cu content was detrimental to the photocatalytic activity (Figure 20k).

Figure 20. (a) SEM top view of ZrO₂-TiO₂ composite films; (b) ZrO₂-TiO₂ composite showing the film thickness; (c) the degradation of resazurin redox dye on the ZrO₂-TiO₂ composite thin films under UVA irradiation, reproduced with permission from [121]. Copyright 2015, Royal Society of Chemistry; (d) scheme of the significant effect of boron on TiO₂ coatings grown by APCVD, reproduced with permission from [122]. Copyright 2016, American Chemical Society; SEM images of the surface and cross-section of (e) pristine TiO₂ film and (f) Ag (1 wt.%)-TiO₂; (g,h) the degradation of MB over different films [124]; (i) the changes of surface morphology shown by SEM images indicated coatings’ colonizations over time of immersion in the Atlantic Ocean for different samples; (j) the (αhv)^1/2 change of the three coatings with energy; (k) the degradation kinetics of Orange G by the TiO₂ and various Ti-Cu-O coatings under irradiation of UV (371 nm) [125].

Figure 20. (a) SEM top view of ZrO₂-TiO₂ composite films; (b) ZrO₂-TiO₂ composite showing the film thickness; (c) the degradation of resazurin redox dye on the ZrO₂-TiO₂ composite thin films under UVA irradiation, reproduced with permission from [121]. Copyright 2015, Royal Society of Chemistry; (d) scheme of the significant effect of boron on TiO₂ coatings grown by APCVD, reproduced with permission from [122]. Copyright 2016, American Chemical Society; SEM images of the surface and cross-section of (e) pristine TiO₂ film and (f) Ag (1 wt.%)-TiO₂; (g,h) the degradation of MB over different films [124]; (i) the changes of surface morphology shown by SEM images indicated coatings’ colonizations over time of immersion in the Atlantic Ocean for different samples; (j) the (αhv)^1/2 change of the three coatings with energy; (k) the degradation kinetics of Orange G by the TiO₂ and various Ti-Cu-O coatings under irradiation of UV (371 nm) [125].

3.5. Other Techniques

The advantages and disadvantages of the four deposition techniques in practical applications are shown in

Table 1. Advantages and disadvantages of the deposition techniques discussed in Section 3.

Method	Advantages	Disadvantages	Reference
WCD	Simplicity Cost-effectiveness Versatility Larger-scale production	Difficulty in controlling the uniformity of the film layer Low material density and poor adhesion Possible severe cracking Requires post-deposition thermal annealing	[75,83,89]
Electro- deposition	Low cost Environmentally friendly Controllable operation Low-temperature synthesis	Slow dispersion of surface charges and particles in the electrolyte Conductive substrate required Inability to deposit above 80 °C Requires post-deposition thermal annealing	[97,99]
PVD	Impurity-free High adhesion or durability No harsh chemicals and limited wastes Does not require complex chemical reactions	High temperature and pressure required Often need post-deposition thermal annealing	[108,115]
CVD	Simple equipment Diversified precursor Controllable operation	High temperature (400–900 °C) Disposal and storage of highly corrosive precursors or by-products High cost of high-purity chemicals High toxicity of produced waste gases Often need post-deposition thermal annealing	[119,120]

. Each different deposition technique has its own unique characteristics, and even dip coating was able to yield coatings with different properties and applications due to different process parameters. Based on the techniques and applications of TiO₂-based photocatalytic coating deposition on glass substrates mentioned in Section 3, we could couple them to prepare some designed coatings for specific applications. For composite coatings, some research has been carried out using coupling of techniques. Assaker et al. [126] combined spin-coating and electrodeposition techniques to deposit Znln₂S₄/TiO₂ composite films on ITO glass. The coupling process is environmentally friendly and avoids high temperatures and pressure. First, the TiO₂ film was deposited on the ITO glass substrate using the spin-coating method. After deposition, the films were dried at 100 °C for 15 min and annealed in O₂ at 450 °C for 1 h. After that, the TiO₂ coating with ITO glass served as the working electrode, and the platinum sheet served as the counter electrode. The nanocrystalline Znln₂S₄ films were deposited on the ITO-coated glass substrate using ZnCl₂, InCl₃, and Na₂S₂O₃·5H₂O as the precursors using electrodeposition at room temperature. Due to the homogeneous nucleation process, uniform and dense surface morphology of the three samples was observed (Figure 21a,b), and Znln₂S₄ grains with slightly reduced size and round shape were observed on the Znln₂S₄/TiO₂ composite film (Figure 21c). TiO₂ had no photocatalytic activity under irradiation of visible light (λ > 420 nm) due to its wide band gap (3.6 eV). The Znln₂S₄/TiO₂ composite coating exhibited higher photocatalytic activity than Znln₂S₄ with a narrow bandgap (2.4 eV) (Figure 21d,e). This was mainly due to heterojunction formation between Znln₂S₄ and TiO₂, which caused the electron transfer between two conduction bands (Figure 21f). In addition, there are some studies that combine two or more deposition methods to prepare new photocatalytic coatings, and we have not discussed this topic in depth.

4. Conclusions and Perspectives

In summary, the research progress of TiO₂-based heterogeneous photocatalytic coatings deposited on glass substrates is reviewed in this paper. The photocatalytic properties and applications of the heterogeneous coatings are discussed, from the deposition techniques to photocatalytic applications. The deposition techniques presented in this review involve wet chemical deposition, electrodeposition, physical vapor deposition, and chemical vapor deposition. No ideal method exists, and each technique has specific specifications and characteristics. It is necessary to analyze the final properties and applications of the required photocatalytic coatings and determine the most suitable technology to address specific challenges. Coupling different techniques to develop composite coatings using the advantages of each deposition technique is a good option, but there are few related studies.

There is no denying that TiO₂-based photocatalytic coatings have excellent application potential to overcome the current environmental and energy challenges. Large-scale photocatalytic coatings for practical applications must be durable and sustainable. Therefore, we must fully explore the long-term performance of larger-scale stable TiO₂-based heterogeneous coatings with efficient photocatalytic performances. Coupling different deposition techniques to take advantage of each technique would be a solution for improving photocatalytic deposition performance. Unfortunately, the use of combined methods is often incompatible with industrial processes. At present, it is still the best prospect to improve the photocatalytic performance of TiO₂-based coatings by using the plasma characteristics and metal–semiconductor junction effect of metal NPs. And the incorporation of larger-scale stable TiO₂-based heterogeneous coatings is mainly achieved using the following aspects to improve the photocatalytic properties of TiO₂ coatings: (1) narrowing the bandgap of TiO₂; (2) reducing the recombination rate of e⁻/h⁺; (3) enhancing the light absorption capacity; and (4) controlling the surface morphology. However, it should be noted that the concentration level of the dopant will affect the improvement of the performance of TiO₂-based coatings. Excessive doping or loading may result in e⁻/h⁺ pair recombination, which may cause a decrease in photocatalytic performance. In addition to excellent photocatalytic properties, the composite coating achieves considerable self-cleaning, antireflective, mechanical and antibacterial properties. Therefore, TiO₂-based heterogeneous photocatalytic coatings have a bright future, which could be applied in many applications for practical environmental control.