A Review on Oxygen-deficient Titanium Oxide for Photocatalytic Hydrogen Production

1. Introduction

The exploitation and utilization of fossil fuels, such as coal, oil, and natural gas, facilitates the development of industrialization and urbanization. However, fossil fuels are non-renewable resources, whose reserves are limited, which will certainly become scare. In addition, the use of fossil fuels has dramatically induced negative effects on ecological environment. For instance, carbon dioxide emitted during fossil fuel burning is one of the main greenhouse gases. The industrial by-products and wastes cause severe pollution to the environment. Some pollutants even harm the human health by accumulating through food chain. Hence, it is necessary and urgent to develop sustainable energy to replace fossil fuels. Up to now, some renewable energy sources, such as solar energy, wind power and geothermal power, are being greatly developed and widely used in the world. However, the replacement of fossil fuels still remains elusive due to the restriction on technique and economy [1,2,3,4,5,6].

Among the many candidates, hydrogen energy is considered as one of the most promising energy carriers. Hydrogen is one of the most abundant elements on earth, and hydrogen energy can be obtained from a variety of natural resources. Moreover, hydrogen has superb combustibility, high ignition point (585 ℃) and high heat of combustion (1.42 × 10⁵ kJ·kg^-1). Compared with most of the common fuels, it has unparalleled superiority (see Table 1). Additionally, the combustion product of hydrogen contains only water (Equation 1), while the burning of fossil fuels will produce a large quantity of carbon dioxide, sulfur oxide, nitrogen oxide and so on, which are associated with a series of severe environmental issues, including greenhouse effect, photochemical smog and acid rain [1,7,8,9]. Comparatively, hydrogen is certainly a clean, efficient and sustainable energy source with tremendous prospects for development. Nowadays, more than 95% of hydrogen in industry is produced from fossil energy, such as natural gas, petroleum and coal. However, these traditional processes for hydrogen production emit a large amount of exhaust gases like carbon dioxide, which weakens the advantage of using hydrogen as clean energy. And low efficiency and subsequent purification of the resultant hydrogen in these processes are also of great challenge [10,11,12,13]. Besides, producing hydrogen by water electrolysis is also an important method to prepare hydrogen in large scale, but it will consume a large amount of electric energy [14,15]. Fortunately, in 1972, Fujishima and Honda reported the decomposition of water to produce hydrogen over a TiO₂ electrode under ultraviolet light irradiation, thus initiating a new age of catalysis [16]. The emergence of photocatalytic technology provides a new option for hydrogen production: producing hydrogen by photocatalytic water splitting. On the one hand, there are abundant resources of raw material because over 70% of the surface of the Earth is covered by water. And the raw material water can be recycled in an ideal situation. On the other hand, the irradiation of photocatalytic reactions can be provided by sunlight, which is an inexhaustible resource for human beings. But the solar energy is an intermittent energy resource, so transforming solar energy into hydrogen is also beneficial to its effective storage and use [17,18,19].

$${\mathrm{H}}_2(\mathrm{g})+\frac{1}{2}{\mathrm{O}}_2(\mathrm{g})={\mathrm{H}}_2\mathrm{O}(\mathrm{l})\mathrm{H}=-285.8\mathrm{kJ}·{\mathrm{mol}}^{-1}$$

(1)

Photocatalyst is the key for producing hydrogen efficiently from the photolysis of water. In literature, TiO₂ was the first reported photocatalyst, which has been studied extensively and already applied in some specific area due to its high photocatalytic activity, non-toxicity, good stability, low cost and so on. Especially since 1990s, TiO₂ photocatalyst has made great progress in the fields of photodegradation of environmental pollutants and photocatalytic water splitting to produce hydrogen [20,21,22]. However, the utilization rate on solar energy by TiO₂ photocatalyst is very low due to the fact that TiO₂ could be excited only by short-wavelength ultraviolet light, which accounts for only about 5% of solar light. Consequently, scientists have paid great attention to develop the second-generation semiconductor photocatalysts of highly efficient visible-light-driven activity, including the modified TiO₂ photocatalysts and other non-TiO₂ photocatalysts.

Among the many already known methods, the construction of oxygen defects has been considered as one of most efficient ways to manipulate the band gap of titanium oxides. Literature survey indicates that oxygen-deficient titanium oxide (TiO_2-δ) can absorb more visible light than stoichiometric TiO₂ [23,24,25]. The relevant experimental studies and theoretical calculations revealed that the new intermediate energy level associated with oxygen defects could reduce the band gap of TiO₂, which would thus lead to strong absorption of visible light, and the formation of oxygen defects in titanium oxide could also enhance its electrical conductivity, thus facilitating the transfer of photogenerated electrons [23,24]. As a result, many TiO_2-δ based photocatalysts with superb performance have been developed to generate hydrogen from water splitting [26,27]. Moreover, the band gap of TiO₂ can also be effectively adjusted by some other strategies, such as ion doping and compositing with narrow band gap semiconductors, in which the former one might be also concerned with the formation of oxygen defects. However, the fast recombination of photogenerated electrons (e_CB^-) and holes (h_VB⁺) is also a key factor that results in low quantum efficiency of titanium oxide photocatalysts. Hence, other approaches including but not limited to ion doping, deposition of noble metals, and loading on supports are often adopted to enhance the photocatalytic activity of TiO₂ jointly with introducing oxygen defects [28,29,30,31,32,33,34].

Therefore, in this review, the mechanism of photocatalytic hydrogen production by water splitting over TiO₂ is firstly discussed in detail. Then the effect of introducing oxygen defects on the photocatalytic activity of TiO₂ is analyzed. And the last part of this section provides a brief overview of the research progress in photocatalytic water splitting to generate hydrogen over TiO_2-δ based photocatalysts. Afterwards, in Section II, a variety of methods to introduce oxygen defects into TiO₂ are summarized, and their merits and shortcomings are analyzed. This is of great guidance to select the proper techniques to develop TiO_2-δ based materials. In the following Section III, we will discuss the modification methods of TiO₂ photocatalysts in addition to the introducing oxygen defects, such as ion doping, deposition of noble metals, dye sensitization and so on, which are helpful for further enhancing the photocatalytic activity of TiO_2-δ. Finally, the perspectives and existing challenges of photocatalytic water splitting into hydrogen over TiO_2-δ based photocatalysts are presented in the short section of Conclusions and Outlooks.

2. Fundamentals of Producing H₂ by Photocatalytic Water Splitting over TiO₂

2.1. Mechanism of Photocatalytic Water Splitting to Generate H₂

Photocatalysis technology is based on the special energy band structure of semiconductors. In ground state, the valence band (VB) of a semiconductor is fully occupied by electrons and the conduction band (CB) is empty. There is a quantized and discontinuous band gap between the low energy VB and high energy CB. And the band gap energy (E_g) of semiconductors is narrower than that of insulators (>5 eV). Therefore, the electrons in VB of a semiconductor can be excited and leap into CB when it was stimulated by photons with certain energy (higher than E_g), leaving the same number of holes in VB (Figure 1). Photogenerated e_CB^- and h_VB⁺ possess strong reducing and oxidizing ability, respectively, and will migrate quickly to the surface of photocatalysts to participate in redox reaction [35]. The photocatalysts can directly decompose water when they are suspending in water, which does not require complex reaction system. Photocatalytic water splitting over semiconductors generally comprises the following five steps. (i) Water molecules are adsorbed on the surface of a photocatalyst. (ii) The electrons in VB leap into CB, producing e_CB^- and h_VB⁺ under the irradiation by light. (iii) The photogenerated e_CB^- and h_VB⁺ transfer to the surface of the photocatalyst. (iv) The e_CB^- reduces H⁺ into hydrogen and h_VB⁺ oxidizes H₂O to oxygen, which are commonly referred as hydrogen evolution reaction and oxygen evolution reaction. (v) The produced hydrogen and oxygen were desorbed from the surface of the photocatalyst. Among them, steps II-IV are the rate-determining steps on the photocatalytic water splitting (see Equations 2-5).

SC + 2hv → 2e^- + 2h⁺

(2)

2H⁺ + 2e^- → H₂

(3)

2H₂O + 2h⁺ → O₂ + 2H⁺

(4)

Overall reaction: H₂O + 2hv → H₂ (g) + O₂ (g)

(5)

During photocatalytic water splitting, firstly, only the photons carrying energy greater than the E_g value of a semiconductor can excite the valance electrons into CB. Because the E_g value of TiO₂ is about 3.2 eV, so only the ultraviolet light with wavelength less than 380 nm can excite its valance electrons. Next, apart from moving to the surface of the semiconductor to participate in redox reaction, those excited electrons will also recombine with the holes, releasing light and/or heat energy. The recombination of e_CB^- and h_VB⁺ is the deactivation process of the photogenerated carriers, which does not contribute to the photocatalytic water splitting, and should be avoided as much as possible (Figure 2a) [36]. Moreover, the reducing ability of e_CB^- depends on bottom of CB (CB minimum) and the oxidizing ability of h_VB⁺ relies on top of VB (VB maximum). The necessary conditions for photocatalytic water splitting are that the CB minimum is more negative than the reduction potential of H⁺/H₂ (0 V vs. NHE at pH=0) and the VB maximum is more positive than the oxidation potential of H₂O/O₂ (1.23 V vs. NHE at pH=0). This requires an E_g value no less than 1.23 eV, covering the oxidation-reduction potential of H₂O. In fact, the E_g value of photocatalysts for photocatalytic water splitting is generally required more than 1.9 eV due to the influence of mechanical and thermodynamic losses. Specifically, the CB minimum and VB maximum of TiO₂ are about -0.2 and 3 eV, respectively. Therefore, TiO₂ can split water into hydrogen and oxygen efficiently through photocatalysis (Figure 2b) [37].

2.2. Impact of Oxygen Defects on the Photocatalytic Activity of TiO₂

As mentioned above, TiO₂ can only absorb ultraviolet light because of its wide band gap. However, a large proportion (about 50%) of the solar spectrum is visible light. Thus, enhancing the capability of harvesting visible light is an effective way to improve the photocatalytic performance of TiO₂. In literature, it was reported that the original white TiO₂ would be turned black after it was thermally treated with H₂, indicating that the light absorption capability of the reduced TiO₂ (actually TiO_2-δ) was significantly enhanced. Moreover, it has been proved that the light absorption spectrum edge of TiO_2-δ will shift to long wavelength as the density of oxygen defects increased (Figure 3a), and the corresponding E_g decreased (Figure 3b) [23,38,39]. When there is an oxygen vacancy, one atom of oxygen in TiO₂ is bonded with three Ti atoms and two redundant electrons are shared by the surrounding three Ti atoms (see Figure 4a). A portion of Ti⁴⁺ will be converted into Ti³⁺ after trapping the redundant electrons. And the appearance of Ti³⁺ species in the nonstoichiometric TiO_2-δ is generally considered as the main reason that causes its absorption to visible light. Ti³⁺ species caused by oxygen defects can introduce new intermediate defect states (shallow donor) below the bottom of CB and modify the band gap structure of TiO₂ (Figure 4b–d), which means that TiO_2-δ has a narrower band gap and thus can absorb visible light [23,25,40,41].

On the other hand, the presence of oxygen vacancies enlarges the lattice spaces of TiO₂. Resultantly, the resistance for electron transfer will decrease. A low resistance for electron transfer is beneficial to the quick transfer of photogenerated electrons, thus suppressing the recombination of photogenerated e_CB^- and h_VB⁺ [42]. For example, Hao et al. [43] prepared an oxygen-deficient blue titanium oxide, reporting that the prepared TiO_2-δ electrode would present a much lower charge transfer resistance (87 Ω) compared with its TiO₂ counterpart (356 Ω) (Figure 4e,f) [43]. Additionally, the bridging oxygen vacancies are tended to cause the Ti 3d defect state in the band gap of TiO₂. The Ti interstitials in the near-surface region can provide the electronic charges that the photocatalytic reactions need [44]. As a result, TiO_2-δ will present a higher photocatalytic performance than TiO₂.

2.3. Brief Overview on Photocatalytic Water Splitting to Generate H₂ over TiO_2-δ

Since the earliest report on light-driven water splitting by Fujishima and Honda in 1972 [16], semiconductor photocatalysis has attracted great attention in the field catalysis. However, for a quite long period, semiconductor photocatalysis developed in a mild speed and many researches were focused on the photodegradation of pollutants [45,46,47,48]. After entering 21^st century, the researches on semiconductor photocatalysis have grown explosively and quite a lot of photocatalysts with excellent performance have been developed [21,29,36]. In particular, although oxygen vacancy was reported to generate a defect state in the band gap of TiO₂, leading to a narrower band gap of TiO_2-δ in 1980s, TiO_2-δ based photocatalysts are promptly developed and applied to water splitting until recent ten years [32,49].

In 2008, Sasikala et al. [50] synthesized a series of Sn- and Eu-doped TiO₂ (Ti_1-(x+0.001)Eu_0.001Sn_xO_2-δ, where 0.05<x<0.3) nanoparticles, which showed an onset of light absorption at about 450 nm and high activity for hydrogen generation. Liu et al. [32] subsequently reported an oxygen-deficient anatase TiO₂ nanosheet with dominant (001) crystalline plane, indicating that a special electron-transfer process on the reconstructed surface of TiO₂ substantially enhanced the hydrogen evolution rate from photocatalytic water splitting. TiO₂ treated by H₂ at high temperature also presented an enhanced photocatalytic activity for water oxidation and high apparent quantum efficiency for O₂ evolution (41% under light irradiation at 365 nm) [51]. An electron beam irradiated titania film shows wider range of absorbed light and higher efficiency of hydrogen production owing to the oxygen vacancies or defects enhancing mobility and separation of electrons and holes [52]. Other oxygen-deficient TiO₂ samples obtained by using the Ion Layer Gas Reaction (Spray-ILGAR) technique, microwave induced reduction, and solution plasma process also show high photocatalytic hydrogen evolution activity [53,54]. In summary, many TiO_2-δ based photocatalysts have been developed, but most of them are used to degrade pollutants and only a limited number of them are used to split water [24,26,27,31,35,55,56,57,58]. Among these limited reports, thermal treatment in hydrogen is the most widely used method of introducing oxygen defects in TiO₂ [24]. And the introduced oxygen defects in TiO₂ are generally combined with other strategies like ion doping and composing with other semiconductors to achieve high hydrogen evolution activity, which are also the recent researches that focus on [59,60,61,62,63,64,65,66].

3. Methods of Introducing Oxygen Defects in TiO₂

3.1. Reductive Treatment

Reductive treatment is the most direct way to introduce oxygen defects in TiO₂. TiO₂ can be reduced into TiO_2-δ by adding proper reducing agent. Among the many reductants, H₂ is the most widely used option because of its strong reducing ability and no impurities introduced [24,33,67,68,69]. However, H₂ treatment is usually carried out at high temperature and the explosion limit of H₂ falls in a very wide range of 4.0~75.6 vol.%. In other word, the operation of H₂ treatment on TiO₂ is quite dangerous, which needs very accurate processes. Moreover, treating TiO₂ with H₂ is usually a time-consuming work. For example, Xu et al. [70] reported black TiO₂ through H₂ treatment in a 20.0 bar of H₂ atmosphere at about 200 ℃ for 5 days. Zhang et al. [71] prepared defective TiO_2-δ hollow microspheres also by high-temperature H₂ reduction for 3 h at 550 ℃. Wierzbicka et al. [72] synthesized a reduced “grey” brookite TiO₂ photocatalyst by hydrogenating at 500 ℃, showing a remarkable noble metal free photocatalytic H₂ evolution performance, substantially higher than that of hydrogenated anatase or rutile TiO₂. The density of defects can be adjusted by tuning the H₂ treatment temperature, soaking time and H₂ concentration. For instance, Samsudin et al. [73] put TiO₂ into a continuous flow of 1 atm of pure H₂ at 500 ℃ for different times, finally obtaining TiO_2-δ with different densities of oxygen defects. They indicated that with time of H₂ treatment, the density of oxygen defects increased, the color of the products becomes deeper and deeper from white to dark gray and to bluish-gray (Figure 5a), and the light absorption ability of the resultant TiO_2-δ was significantly enhanced (Figure 5b). But more defects do not always guarantee better photocatalytic performance. Here, the photocatalytic of TiO₂ hydrogentreated for 24 h is inferior to that of the sample treated for 12 h. Thus, the control of oxygen defects density in TiO₂ is also important.

Apart from H₂, some other gases have been also used as the reductants. For example, NH₃ is also often used to reduce TiO₂. Chen et al. [56] synthesized a N-doped and oxygen-deficient TiO₂ photocatalyst by heating the commercially available pure TiO₂ in NH₃ atmosphere at 550 ℃ for 5 h. It is easy to introduce N into TiO₂ (N doping) when using NH₃ as the reducing agent. Similarly, Ihara et al. [74] prepared a N-doped oxygen-deficient titanium oxide by calcinating the hydrolytic product of Ti(SO₄)₂ with ammonia in dry air at 400 ℃ for 1 h. Additionally, some familiar reducing substances like carbon, NaBH₄ and Li can be also used to prepare oxygen-deficient TiO₂. Guan et al. [75] prepared a product of oxygen-deficient TiO₂ by a three-step process, which shows strong absorbance over the whole visible light region. In their process, a Ti coating was first pretreated in carbon powder at 1073 K for 2 h, which were then oxidized at 1073 K for 15 h in air. Next, the obtained samples were treated in carbon powder again at 973 K for 30 min, finally obtaining the product of oxygen-deficient TiO₂. Zhao et al. [76] first prepared TiO₂ anatase nanorods by a two-step hydrothermal method. Then, the obtained sample was mixed with NaBH₄ (1:1 in mole) in a mortar and thermally treated in Ar at 300 ℃ for 30 min, finally acquiring the reduced anatase nanorods. Interestingly, Martinze et al. [77] prepared a reduced blue TiO₂ by using Li foil and TiO₂ which were solved in ethylene diamine, stirring in anhydrous and dark conditions for 1440 h.

In addition, providing an anoxic environment in the treatment process of TiO₂ can also result in the same effect of adding reducing agents. For example, Pereira et al. [78] obtained oxygen-deficient TiO₂ films with enhanced visible and near-infrared optical absorption by periodically interrupting the O₂ gas supply in the process of magnetron sputtering. Dhumal et al. [79] synthesized oxygen-deficient titanium suboxide (TiO_x with x<2) nanoparticles by using a diffusion flame aerosol reactor under an oxygen lean environment in the formation zone of particles. Xiao et al. [80] reported the formation of oxygen vacancies in TiO₂ during the process of calcining TiO₂ in Ar or N₂ atmosphere. Kushwaha et al. [81] prepared a black oxygen-deficient TiO₂-graphite nanocomposite by calcining Ti-EDTA complex under hypoxic conditions. Singh et al. [41] investigated the effect of thermal treatment on TiO₂ thin films under an oxygen anoxic environment, reporting a reduction in band gap of 0.36 eV.

3.2. Pulsed Laser Irradiation

The excimer laser is a powerful tool and often used to manipulate the composition and structure of the material surfaces. Pulsed laser irradiation is a simple process for producing black oxygen-deficient TiO₂. A photochemical reduction reaction will take place during the pulsed laser absorption, thereby resulting in the evolution of oxygen deficiencies. The absorption of focused laser irradiation accompanied by fast heating cooling processes will promote the formation of porous surface [82]. As mentioned before, the dangers involved in hydrogenation operation greatly limit its application, while hydrogen plasma irradiation overcomes this shortcoming well [83,84]. For example, Wang et al. [83] synthesized a black titania with a core/shell structure (TiO₂@TiO_2-xH_x) assisted by hydrogen plasma and its photocatalytic activity for water splitting and cleaning pollutants is much better than that of TiO₂. In addition, Nd:YAG, ArF, KrF and XeCl excimer laser are also frequently used the assisted methods besides hydrogen plasma [85,86]. Nakajima et al. [86] indicated that the TiO₂ crystal surface would be successfully reduced through ArF, KrF and XeCl excimer laser irradiation, forming an oxygen-deficient TiO_2-δ layer with a thickness of 160 nm. Moreover, as shown in Figure 6a, the resistance of TiO₂ decreased after laser irradiation. Significant diffuse scattering around the (220) reflection for a wide range of Q_x (0.04~0.04) over the irradiated sample (Figure 6b) indicated a strong local lattice distortion near the surface of the sample.

3.3. Pulsed Laser Deposition

Pulsed laser deposition (PLD) is a good technique to prepare functional thin films by depositing the ablated substances on a substrate. The oxygen deficiency in the film can be adjusted by controlling the partial pressure of O₂ and laser power density. For instance, Leichtweiss et al. [57] prepared oxygen-deficient titanium oxide films with an average composition of TiO_1.6 by PLD at room temperature, which presented high efficiency for the water-splitting reaction. Kunti et al. [87] deposited TiO₂-SiO₂ composite films on amorphous quartz substrates at different partial pressures of O₂ by PLD technique, revealing the generation of oxygen defects and Ti³⁺ states in the films. Moreover, ion doped oxygen-deficient TiO₂ films can be obtained by changing the humidity of environment, atmosphere and ion implantation [88,89,90]. For instance, Socol et al. [90] fabricated N-doped crystalline TiO₂ thin films by PLD in N₂ or N₂-O₂ mixture. Nath et al. [91] synthesized TiO_1.5 nanoparticles by varying the focusing conditions of pulsed laser ablation. Rahman et al. [92] prepared TiO₂ nanostructures with different morphologies and incorporation of oxygen vacancy defects on a Si substrate by a single-step, catalyst-assisted PLD method (Figure 7). The morphology can be controlled by adjusting the deposition temperature and template.

3.4. Ion Doping

Because of the difference in electronegativity between various elements, the introduction of impurity atoms into TiO₂ will change the partial concentration of electrons in TiO₂, thus producing oxygen defects in it. For instance, Ti⁴⁺ will be converted into Ti³⁺ when the oxygen atoms in TiO₂ are replaced by highly electronegative F atoms, due to the increased electron density around Ti⁴⁺ caused by the doped F atoms [93]. As shown in Figure 8a, clear Ti³⁺ signals can be observed in the EPR spectrum of fluorine-treated anatase. And the corresponding Raman spectra also display a slight shift to higher frequency at the peak of 144 cm^-1, which is attributed to the presence of oxygen vacancies and Ti³⁺ (Figure 8b). The oxygen vacancies are spontaneously introduced during N doping [94]. Pu et al. [95] successfully prepared N-doped oxygen-deficient TiO₂ microspheres through a two-step synthesis method. Firstly, TiO₂ microspheres are synthesized by solvothermal synthesis. Then, the final oxygen-deficient titanium oxide products were obtained by electron beam irradiation using urea as the nitrogen source, and the concentration of Ti³⁺ increased with increasing dose of the electron beam irradiation. Wang et al. [30] reported a N-doped TiO₂ (TiO_2-xN_x) by a simple wet method: hydrolyzing acidic tetra-butyl titanate in ammonia solution and following by calcination at 350 ℃ for 1 h. Of course, the nitrogen source for doping is generally directly or indirectly originating from reducing NH₃, which can also promote the reduction of TiO₂. Moreover, the doping of some metal ions, such as Eu³⁺, La³⁺and Gd³⁺, can introduce oxygen defects in TiO₂ as well. Those ions with a lower valence than Ti⁴⁺ can generate anion vacancies in TiO₂ [96,97,98], thereby forming Ti³⁺ (Figure 8c). Zhang et al. [99] proved that the formation energy of a vacancy on the La-doped TiO₂ surface is lower than that formed on the pure TiO₂ surface treated in reducing conditions or oxidizing conditions by calculation (Figure 8d). Wang et al. [96] synthesized 0.4 mol% Gd and 2.0 mol% La co-doped TiO₂ microspheres via a hydrothermal method, which exhibit enhanced visible light absorption (Figure 8e). The doped La³⁺ and Gd³⁺ create abundant oxygen deficiencies and surface defects in the sample, decreasing the excitation energy of TiO₂ (Figure 8f).

3.5. Plasma-Assisted Deposition

Plasma-enhanced chemical vapor deposition (PECVD) has the features of low deposition temperature, high purity, uniform thickness and composition of films, as well as easy control of reaction parameters. It can be used to prepare various metal films, inorganic films and organic films. The structure and properties of films can be adjusted by controlling reaction conditions. For example, Hatanaka et al. [100] prepared a-TiOx:OH films using a remote PECVD technique, which show high photoconductivity. Sakai et al. [101] obtained oxygen-deficient TiO₂ anatase films by using oxygen plasma assisted reactive evaporation by increasing the supply of titanium atoms, and the resultant oxygen deficient TiO₂ films showed excellent hydrophilicity, which was conducive to thoroughly contacting with water and facilitating its splitting reaction. Li et al. [102] introduced numerous oxygen deficiencies and Ti³⁺ defects on the surface of TiO₂ nanoparticles via Ar plasma (Figure 9a,b). Similarly, Hojo et al. [103] also successfully introduced oxygen defects in a TiO₂:Nb film by annealing the sample with Ar plasma irradiation. Recently, Kawakami et al. [104] reported a kind of anatase/rutile mixed phase TiO₂ nanoparticles with many oxygen deficiencies, which were obtained by annealing the sample with low-temperature O₂ plasma.

There are excited species like ozone and OH generated during the plasma discharge in water. Thus, plasma-liquid interaction has widely been applied to prepare nanomaterials. For instance, An et al. [105] prepared gray hydrogenated TiO₂ spheres by using a plasma modified sol-gel system. Mizukoshi et al. [106] obtained a blue TiO₂ containing oxygen defects by generating discharge plasma in an aqueous ammonia solution containing TiO₂ powder. TiO₂ was reduced by the reducing species such as hydrogen radicals generated during the plasma discharge process in aqueous ammonia (Figure 9c–e).

3.6. Ultrasonic Assisted Techniques

Ultrasonic spray pyrolysis is a simple, low-cost and scalable technique [107,108]. In literature, Nakaruk et al. [108] successfully prepared fully dense TiO₂ films with oxygen deficiencies by using ultrasonic spray pyrolysis and proved that the concentration of oxygen deficiencies could be controlled by changing the annealing temperature. Besides, oxygen vacancies can also be directly induced in TiO₂ by low-frequency ultrasound (LFUS) treating. For instance, Osorio-Vargas et al. [109] prepared visible-light responsive TiO₂-based photocatalysts by dispersing P25 powder into water and exposed to LFUS environment for 6 h. Bellardita et al. [110] reported that ultrasonic treating P25 powder dispersed in water induced oxygen deficiency in TiO₂ and thus narrowed the bandgap of TiO₂ from 3.18 to 3.04 eV.

3.7. Calcination under Anoxic Condition

Thermal treatment atmosphere exerts an important influence on the formation of oxygen deficiencies [111,112,113,114]. For instance, Albetran et al. [113] revealed that the color of titania changed from the white to gray and black as the ratio of Ar/air of thermal treating atmosphere increased (Figure 10a), and the light absorption of the corresponding products was also improved (Figure 10b). Sang et al. [114] fabricated oxygen-deficient TiO₂ nanotube arrays by calcining in nitrogen, or a mixture gas of 5% hydrogen in nitrogen (Figure 10c,d), which exhibit higher photocurrent density and smaller charge transfer resistance than that of the samples calcined in air (Figure 10e,f). Qi et al. [115] prepared a defective TiO₂ sample with oxygen deficiencies by thermally treating TiO₂ at 200 ℃ under vacuum conditions. The defect concentration in the sample is positively proportional to the thermal treatment time. Li et al. [116] reported an oxygen-deficient dumbbell-shaped anatase TiO_2-x product. In detail, a TiCl₃-HAc mixed solution was solvothermally treated at 180 ℃ for 5 h and the solvothermally synthesized product was calcined under vacuum at 400 ℃ for 1 h.

3.8. Molten Salt Calcination

Du et al. [117] reported a facile strategy based on molten salt calcination to construct oxygen deficiencies in TiO₂. A flower-like TiO₂ precursor was synthesized via solvothermal method using tetrabutyl titanate and acetic acid (HAc)/N,N-dimethyl formamide (DMF) as the titanium source and solvent, respectively. The as-prepared precursor was mixed with eutectic salts of LiCl/KCl (45/55 by weight) and calcined in a muffle furnace at 400 ℃ for 2 h. The lattice oxygen of TiO₂ was consumed during the calcination because of the low partial pressure of O₂ in the molten salt, thereby introducing numerous oxygen deficiencies and Ti³⁺ in the final product.

In summary, up to now, hydrogen reduction is still the most extensively used method to prepare oxygen-deficient TiO₂ owing to the strong deoxidizing ability and purity. But it is time-consuming and has to work under high temperature conditions. Thus, some other reductants like carbon, NH₃ and Li are also used to reduce TiO₂ in literature. Synthesizing titanium oxide in an anoxic environment is widely used as well because it is easily implemented. Pulsed laser irradiation is a simple process for producing oxygen-deficient TiO₂, which is, however, more suitable for treating films because the radiation response mainly happens in the surface layer. Similarly, oxygen-deficient TiO₂ films can be easily obtained through adjusting the partial pressure of O₂ and laser power density of PLD. Introducing oxygen defects through ion doping is a natural process and the density of oxygen defects mainly depends on the doped species of ions and their concentration. Plasma discharge in water will provide reductively excited species, which can easily reduce TiO₂. But it is currently not widely applied. Oxygen-deficient TiO₂ can be prepared by ultrasonic spray pyrolysis or calcinating under anoxic condition, and the density of oxygen deficiencies can be controlled by controlling the experimental temperature. Molten salt calcination is simple and easily operated.

4. Modification Methods of TiO_2-δ Photocatalysts

TiO_2-δ has been proved to perform better than stoichiometric TiO₂ in the process of photocatalytic water splitting. Many strategies such as ion doping, constructing heterojunction and deposition of noble metals can effectively improve the photocatalytic activity of TiO₂. Thus the photocatalytic activity of TiO_2-δ should be enhanced further by these strategies.

4.1. Ion Doping

Ion doping can introduce defects into TiO₂, which could act as the capture traps of photogenerated carriers, thereby suppressing the recombination of photogenerated e_CB^- and h_VB⁺. And the lattice distortion caused by the doped atoms with different ionic size would increase the asymmetry of crystal structure, which could promote the separation of photogenerated e_CB^- and h_VB⁺ as well. Additionally, the energy band structure of TiO₂ can be effectively manipulated by ion doping. The narrowed band gap can extend the light absorption and enhance the utilization efficiency on solar energy of the resultant photocatalysts.

4.1.1. Metal Ion Doping

The doping of transition metals has been proved an effective way for regulating the band positions of TiO₂. The main principle is to insert an additional energy level between the original conduction band and valence band. For example, Sheng et al. [118] reported a Pd doped TiO₂, revealing that the photogenerated e_CB^- and h_VB⁺ were efficiently separated after Pd doping. Sasirekha et al. [119] prepared a Ru doped anatase TiO₂ supported on silica by a solid-state dispersion method, which performed well in photocatalytic reduction of carbon dioxide. Gao et al. [120] indicated that the doping of Mo, Pd, Ru and Rh could narrow the band gap of TiO₂, thus enhancing the probability of activation by visible light. Their theoretically calculating results through density functional theory revealed that the impurity states of 4d electrons would form new degenerate energy levels, thus narrowing the band gap of TiO₂ (Figure 11). Thalgaspitiya et al. [121] synthesized mesoporous composites of M doped titanium dioxide (M = Mn, Co, Ni, Mo, and W) with reduced graphene oxide (rGO), indicating that the indirect band gap of the composites could be adjusted into the range of 2.20-2.48 eV.

Rare earth ions have rich energy levels and unique features of 4f electronic transitions, providing unique opportunities for manipulating the band gap of semiconductors by elemental doping. For instance, Wang et al. [122] fabricated samples of La³⁺- or Yb³⁺-doped TiO₂ supported on r-GO, reporting that the anionic vacancies in TiO₂ lattice caused by La³⁺ and Yb³⁺ would generate Ti³⁺, thus enhancing the visible-light response of the samples. Stengl et al. [123] prepared several samples of rare earth (La, Ce, Pr, Nd, Sm,, Eu, Dy, Gd) doped TiO₂, which are all visible-light sensitive. Fang et al. [124] synthesized rare earth ions (Er³⁺ and/or Yb³⁺) doped TiO₂ photocatalysts by a hydrothermal method, indicating that the doping of Er³⁺ and/or Yb³⁺ could decrease the recombination rate of photogenerated electron-hole pairs, finally leading to the higher photocatalytic efficiency of TiO₂. In addition, the phase transition from anatase to rutile can be significantly delayed by the doping of rare earth ions [125,126].

Alkali metal and alkali earth metal ions were also used to improve the photocatalytic activity of TiO₂. Liu et al. [127] prepared a mesoporous Na doped titanium dioxide with a band gap of 3.08 eV. The doped Na ions could enter into the (004) crystalline plane of anatase TiO₂, finally leading to the dislocation defects in TiO₂. Lv et al. [128] successfully fabricated AM-TiO_2-x samples (AM = Mg, Ca, Sr, and Ba), revealing that the CB position of TiO₂ became more negative after AM-doping, thus improving the hydrogen production ability of TiO₂. Besides, the separation of carriers and transfer efficiency were also dramatically promoted (Figure 12a–c).

4.1.2. Nonmetallic Ion Doping

The doping of nonmetallic ions can expand the light absorption region of TiO₂ and suppress the recombination of photogenerated e_CB^- and h_VB⁺. Normally, the p orbital in the most outer electronic layer of the doped ions would hybridize with the 2p orbital of O in TiO₂, forming new shallow levels near the top of the valence band. For example, N doping is widely studied because the ion radius of N is closest to that of O [129,130,131,132]. Li et al. [129] prepared a N-doped TiO₂ which performs better in photocatalytic hydrogen evolution than undoped TiO₂ under the same conditions (Figure 12d). Yuan et al. [133] prepared a N-doped TiO₂ with high specific surface area by heating the mixture of urea and TiO₂. The absorption spectrum of the N-doped TiO₂ shifted to the wavelength up to 600 nm and the sample shows high photocatalytic activity on hydrogen evolution. Momeni et al. [134] prepared S-doped TiO₂ nanostructure photocatalyst films, which performed well in the removal of RhB and the hydrogen generation under visible light radiation. Carmichael et al. [135] reported B-doped titanium dioxide films with a hydrogen evolution rate of 24 µL cm^-2 h^-1, which is far exceeded the undoped TiO₂ at 2.6 µL cm^-2 h^-1. Wu et al. [136] fabricated F-doped TiO₂ particulate thin films, which could be applied in the photodegradation of organic pollutants and photoinduced splitting of water into hydrogen under the irradiation of either UV or visible light.

4.1.3. Multiple Ions Co-Doping

Different ions have different impacts on TiO₂ and thus the co-doping of multiple ions is an effective method to obtain higher photocatalytic activity. In literature, Zhu et al. [137] studied the electronic and optical properties of C-, Mo- and (Mo,C)-co-doped anatase TiO₂ using the first principles calculations. The results show that the optical absorption edges of the (Mo,C)-co-doped TiO₂ will shift towards the visible light region. Diao et al. [138] reported K, Na and Cl co-doped rutile TiO₂, exhibiting good photocatalytic degradation of gaseous formaldehyde under visible light irradiation. Li et al. [132] reported the photocatalytic activity for hydrogen production over (B,N)-co-doped TiO₂ under visible light irradiation. N doping extends the absorption edge to the visible light region and B doping acts as the shallow traps for photogenerated electrons to prolong the life of the electrons and holes_. Consequently, stronger photocurrents were observed on (B,N)-co-doped TiO₂ than those on N-doped TiO₂, B-doped TiO₂ and undoped TiO₂ (Figure 12e). Barakat et al. [139] prepared Fe_xCo_1-x-co-doped titanium oxide nanotubes, achieving distinct enhancement on the visible light absorption capacity (Figure 12f). Filippatos et al. [140] even reported a photocatalyst of H, F and Cl co-doped titanium dioxide with high hydrogen production rate.

4.2. Composite

The heterostructure formed by the recombination of two or more semiconductors with matched energy band structure can effectively improve the separation efficiency of photogenerated e_CB- and h_VB+. The built-in electric field formed along the interface will promote the transfer of electrons (Figure 13) [141,142]. Additionally, the combination with narrow band semiconductor could allow TiO₂ to respond to visible light. For instance, Smith et al. [143] synthesized a nanotubular composite of TiO₂-WO₃. This composite demonstrated an increase of 46% in water splitting efficiency compared to TiO₂ nanotubes prepared under the similar conditions. Choudhury et al. [144] prepared ultra-thin PdO-TiO₂ composite films, which could be used to photogenerate hydrogen efficiently from a methanol-water for a long period of time. Navarrete et al. [145] synthesized β-Ga₂O₃/TiO₂ composite photocatalysts for H₂ production from a water/methanol mixture (Figure 14a). The high activity is attributed to slow charge recombination of the photogenerated e_CB^- and h_VB⁺ (Figure 14b). Gholami et al. [146] confirmed that the activity of ZnO-TiO₂ composite for photodegradation of bentazon is better than that over ZnO and TiO₂ separately. Chen et al. [147] constructed a NiO/TiO₂ heterojunction on the surface of TiO₂ film. The strong inner electrical field effectively separates the photogenerated electron-hole pairs, and thus the composite exhibited a much better photocatalytic activity than the original TiO₂ film (Figure 14c,d).

Graphene-TiO₂ composite has been widely studied because its excellent mobility of charge carriers, large specific surface area, flexible structure, high transparency, and good electrical and thermal conduction [148,149,150,151,152,153,154]. Zhang et al. [152] prepared TiO₂/graphene sheets composite by a sol-gel method, exhibiting a hydrogen evolution rate of 8.6 µmol h^-1, which was nearly two times that over the commercially available Degussa P25 (4.5 µmol.h^-1). Fu et al. [154] constructed a g-C₃N₄/graphene-CNTs/TiO₂ Z-scheme photocatalytic system, in which the graphene-CNTs effectively promote the transfer of photogenerated carriers, thereby generating stronger photocurrent (Figure 14e,f).

4.3. Surface Noble Metal Deposition

The photogenerated carriers will be redistributed when the surface semiconductor in contact with metal. The electrons will transfer from n-type semiconductor to metals because the lower Fermi levels of metals. Moreover, the surface plasmon polaritons can enhance the light response of TiO₂ [155,156,157,158]. In literature, Zheng et al. [159] investigated the photocatalytic performance of TiO₂ deposited with Au, Ag and AuAg bimetallic nanoparticles. The results showed that the local surface plasmon resonance of noble metals improved the photocatalytic activity TiO₂ under visible light irradiation. Luo et al. [160] reported a visible-light-driven responsive Au/rGO/hydrogenated TiO₂ nanotube arrays ternary composite with a high hydrogen evolution rate of 45 mmol cm^-2 h^-1. The visible light harvesting was significantly improved by the Au nanoparticles due to the localized surface plasmon resonance effect. Ag and Pd have been also used to modify TiO₂ by depositing them on its surface [161,162,163,164]. For example, Ge et al. [161] decorated Ag nanoparticles onto vertically aligned TiO₂ nanotube arrays. The Ag decorated TiO₂ can efficiently drive photocatalytic water splitting under visible light irradiation owing to surface plasmon resonance of Ag (Figure 15).

4.4. Dye Sensitization

The excitation potential of some dyes is more negative than the CB potential of TiO₂. Thus the light response range of TiO₂ can be effectively expanded by dye sensitization. Dye molecules can deliver photogenerated electrons to the CB of TiO₂ and then the electrons transfer further to participate in reactions [165,166,167]. For example, Shi et al. [165] prepared Eosin Y-sensitized nanosheet-stacked hollow-sphere TiO₂ for efficient photocatalytic H₂ production under visible-light irradiation. Vallejo et al. [167] reported the enhancement on light absorption and photocatalytic activity over rGO-TiO₂ thin films after sensitized by natural dyes extracted from Bactris guineensis (Figure 16). In fact, lots of dyes have been used to sensitize TiO₂, such as complexes of Fe (II) and polypyridyl, quinacridone, hydroxoaluminum- tricarboxymonoamide phthalocyanine, and so on [168,169,170].

4.5. Loading on Supports

Loading on supports is an effective way to solve the problems of agglomeration and tough recycling of TiO₂ nanoparticles. In addition, the supporting materials of high electrical conductivity could provide channels for quick transfer of electrons, thereby decreasing the recombination rate of photogenerated e_CB^- and h_VB⁺. For example, Li et al. [171] reported a catalyst of nitrogen-doped carbon nanofiber supporting MoS₂/TiO₂, in which the photogenerated electrons could quickly transfer to the carbon fiber along the basal plane of MoS₂ . In literature, zeolite, SiO₂, and carbon materials are frequently used as the supporting materials for TiO₂ [172,173,174,175,176,177]. Najafabadi et al. [175] reported four kinds of zeolites (Na-Y, Na-mordenite, H-Y, and H-beta) supporting TiO₂, which all exhibited high hydrogen evolution rate. Especially for the Na-Y zeolite supporting TiO₂, the rate reached 250.8 µmol g^-1 h^-1, which is almost 3 times that of Degussa P25 (84.2 µmol g^-1 h^-1) under the same conditions. Kim et al. [177] prepared TiO₂ supported by SiO₂, showing much higher photocatalytic activity than pure TiO₂, which can be attributed to the large specific surface area, Ti-O-Si bonds modified narrow band gap and the local structure. Loading on supports is frequently associated with other reactions like ion doping and forming heterojunctions. Thus it can combine the advantages of varied strategies. Yin et al. [178] synthesized Bi plasmon-enhanced mesoporous Bi₂MoO₆/Ti³⁺ self-doped TiO₂ microsphere heterojunctions. The formation of heterojunction, Ti³⁺ and surface plasmon resonance (SPR) of Bi jointly achieved high catalytical activity of TiO₂ under visible light. Xing et al. [179] combined ion doping with support and synthesized a F-doped-TiO_2-x/MCF composite, which exhibited high photocatalytic activity for hydrogen evolution.

4.6. Crystal Facet Engineering

The mainly exposed facets of traditional TiO₂ photocatalysts are the thermodynamically stable (101) facets. However, the specific surface energy of (001) facets is higher than that of (101) facets, implying that the (001) facets have higher reaction activity. In addition, the uncoordinated Ti_5c atoms in the (001) facets can narrow the band gap of TiO₂. Therefore, exposing more (001) facets will help to improve the photocatalytic performance of TiO₂, which is generally realized by controlling the synthesis conditions [180,181]. For instance, Wang et al. [182] synthesized a series of (001) facets dominated TiO₂ nanosheets with high visible-light photoactivity by a simple hydrothermal method at different temperatures. Shang et al. [183] synthesized graphene-TiO₂ nanocomposites with dominantly exposed (001) facets by various dosage of graphite oxide (GO) and hydrofluoric acid (HF) during a facile solvothermal process. The well conductive and highly reactive (001) facets enhanced the photocatalytic properties and facilitated the separation of photogenerated carriers.

As a summary, Table 2 lists the hydrogen evolution efficiency from photocatalytic water splitting over typical titanium oxide based photocatalysts. Obviously, noble metal modified TiO₂ photocatalysts have incomparable advantages on hydrogen evolution over the other titanium oxide based counterparts. But the searches for alternative non-noble metals are still one of the focuses in this field because of the high cost and scarcity of noble metals. Additionally, combining multiple modification methods can achieve better results than using a single method.

5. Conclusions and Outlooks

(1) Oxygen-deficient titanium oxide (TiO_2-δ) shows higher photocatalytic activity than stoichiometric TiO₂, which can be mainly attributed to the presence of Ti³⁺ species and oxygen deficiencies. The Ti³⁺ species would lead to new intermediate defect states (shallow donor), forming below the bottom of conduction band of TiO₂, which thus narrows the band gap of TiO₂. The presence of oxygen deficiencies can decrease the transfer resistance of electrons. Resultantly, the photogenerated electrons can quickly transfer, thereby avoiding recombining with holes.

(2) Reductive treatment is the most direct and effective method to introduce oxygen defects in titanium oxides, for which H₂ is the commonest reductant, while other reductants like carbon, NaBH₄ and NH₃ can also be selected. Moreover, ion doping, pulsed laser irradiation, calcination under anoxic conditions, plasma assistance and so forth have also been proven efficient strategies for introducing oxygen defects into titanium oxides. Other modification methods for TiO₂, including ion doping, composite, surface noble metal deposition, dye sensitization and loading on supports, are also exploited to broaden the light absorption region and suppress the recombination of photogenerated e_CB^- and h_VB⁺ for TiO_2-δ. The photocatalytic activity of titanium oxides is hopefully improved further by the combination of introducing oxygen defects with these modification methods, which have reached some remarkable results.

(3) Hydrogen production by photocatalytic water splitting over TiO_2-δ based photocatalyst shows a strong development momentum. But there exist at least two major challenges at present. The first is how to control the concentration of oxygen defects in TiO_2-δ. Although the density of oxygen deficiencies can be controlled by adjusting the conditions of reduction treatment, the spontaneously introduced oxygen defects during other modification processes like ion doping and surface treatment is difficult to control and predict accurately. Thus, choosing an appropriate synthesis method is very important to prepare TiO_2-δ based photocatalysts. Secondly, the present researches on regulating energy band structure mainly concentrate on enhancing light harvesting. Actually, the positions of CB and VB are also very critical for photocatalytic water splitting, especially the position of CB. The CB position of TiO₂ is very close to the reduction potential of H⁺/H₂ (0 V vs. NHE at pH=0). The decrease of CB minimum can lead to a wider light absorption region, but the reducing ability of photogenerated electrons is also impaired at the same time. If the CB minimum is more positive than the reduction potential of H⁺/H₂, the photocatalytic hydrogen evolution activity will take a mighty blow. Thus, regulating the band gap of TiO₂ is really a challenging task, because there are numerous factors that can affect the band position of TiO_2-δ during the modifying process. Combining theoretical calculation prediction with precise control of synthesis conditions may be a solution to solve this issue. In addition, the present researches pay little attention to the adsorption of reactant (H₂O) and the desorption of products (H₂ and O₂). In fact, these two processes are also very crucial for the whole photocatalytic hydrogen evolution, which might be the next hot topic in researches. Although photocatalytic hydrogen evolution is still remained in laboratory stage, it is hopefully fruitful and prosperous with deep researches.