Recent progress in TiO2-based photocatalysts for hydrogen evolution reaction: A review

https://doi.org/10.1016/j.arabjc.2019.12.012 1878-5352 2020 The Authors. Published by Elsevier B.V. on behalf of King Saud University. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Ha Huu Do , Dang Le Tri Nguyen , Xuan Cuong Nguyen , Thu-Ha Le , Thang Phan Nguyen , Quang Thang Trinh , Sang Hyun Ahn , Dai-Viet N. Vo *, Soo Young Kim *, Quyet Van Le b,*


Introduction
Currently, the world faces unprecedented challenges concerning energy and environmental topics. The use of nonrenewable fossil fuel to meet the daily energy demands has released a huge amount of CO 2 gas compares with renewable energies (Fig. 1), contributing to global warming as well as air pollution (Pao and Tsai, 2010). Hydrogen is considered as an ideal solution to tackle this problem, owing to its sustainable, energydense, and eco-friendly properties (Baykara, 2018, Catapan et al., 2018, Im et al., 2009. It is utilized in a wide variety of applications, including petroleum refining, electric production, gas welding, automobile fuel, and rocket fuel for space programs (Staffell et al., 2019;Tusek and Suban, 2000). Several countries such as China, Britain, and the United States have used hydrogen as an alternative fuel for transportation to reduce greenhouse gas emissions in large cities . Thus, finding a simple and efficient method to manufacture hydrogen is necessary. Commercial hydrogen can be manufactured using various approaches such as gasification of coal, steam reforming of natural gas, cryogenic distillation process, and water splitting (Haryanto et al., 2005). Among these methods, water splitting has attained economic and environmental efficiency, owing to several reasons. The first two methods can generate a large amount of hydrogen. However, these methods required high energy consumption (temperature > 1000°C) to conduct reactions, demanding a robust and safe system. The third method is based on the various boiling points of gaseous which require extremely low temperature. Whereas, the separation of water can be implemented at ambient temperature and pressure, thus, lowering the production costs. However, utilizing large amounts of electric energy to produce hydrogen from water leads to the limitation of electrocatalytic water splitting. Therefore, the use of photocatalyst to create hydrogen from water is a reasonable solution to reduce the cost of manufacture in particular utilization, owing to solar energy. Accordingly, numerous studies on potential photocatalysts have focused on the hydrogen evolution reaction (HER), such as metal-organic frameworks (MOFs) Silva et al., 2010;He et al., 2013;Jayaramulu et al., 2016Santaclara et al., 2017;Wen et al., 2016;Zhang et al., 2015a,b;Zhen et al., 2016), carbon nitrides Cao and Yu, 2014;Chen et al., 2018;Dong et al., 2013;Fang et al., 2019;Guo et al., 2018;Hou et al., 2013;Li et al., 2015a,b,c;Wang et al., 2018;Wang et al., 2012), and graphene (Iwase et al., 2011;Min and Lu, 2012;Zhang et al., 2012). Amongst the promising photocatalysts, TiO 2 is considered as a typical material for potential applications in photocatalysis, owing to its high stability, nontoxicity, reasonable cost, and Fig. 1 A represented diagram for generating CO 2 from fossil fuels and renewable energies (Avtar et al., 2019). eco-friendliness (Chen et al., 2016a, b;Rossetti et al., 2015;Hong et al., 2018). Traditionally, anatase and rutile have gained a great deal of interest in the photocatalytic field. The two phases of TiO 2 most commonly used in photocatalysis are anatase and rutile. Because it has good optical absorption capacity and low cost, several studies have indicated that anatase is more efficient than rutile for catalyzing reaction under solar light, although its experimental bandgap (E g = 3.2 eV) is broader than that of rutile (E g = 3.0 eV) (Alkaim et al., 2013). The enhanced catalytic activity is accounted for the high-density hydroxyl species on the surface as well as the large surface area. Hydroxyl groups are crucial to prohibit the reconsolidation of electron-hole pairs, and the great specific surface area creates conducive conditions for the absorption of the reactant on the surface of catalyst, playing a pivotal role in photocatalytic reactions (Chen et al., 2015;Pang et al., 2014). Some studies scrutinized the domination of internal factors on the yield of TiO 2 in the photocatalytic area (Yamazaki et al., 2018). For instance, Yamazaki et al. investigated the importance of a morphological change of TiO 2 from nanorods (NRs) to nanoparticles (NPs) on the photocatalytic performance for the oxygen evolution reaction (Cheng et al., 2014;Liu et al., 2014a,b;Suhaimy et al., 2018;Yamazaki et al., 2018). Cheng's study indicated that the specific surface area of TiO 2 is crucial in photocatalytic applications (Cheng et al., 2014). Generally, the catalytic activity of TiO 2 is governed by various parameter including the specific surface area, crystallinity, and morphology. However, the large bandgap width and low conductivity cause difficulty in the exciton dissociation and electron transfer, which restricts the potential applications of TiO 2 . Thus, some strategies were employed to modify TiO 2 to reduce its bandgap and increase its conductivity. For example, the bandgap of TiO 2 was effectively reduced through replacing titanium or oxygen atoms in TiO 2 lattice with metal and nonmetal dopants (Bakar and Ribeiro, 2016;Hou et al., 2014;Kocˇı´et al., 2010;McManamon et al., 2015;Sood et al., 2015;Wu et al., 2013). Another method is to modify the surface of TiO 2 by an inorganic acid such as H 2 SO 4 to generate hydroxyl groups on the surface of TiO 2 , vacancies, and intensify the light absorption efficiency (Li et al., 2015a, b,c). Moreover, the combination of TiO 2 with advanced materials such as carbon based materials, transitional metal dichalcogenides (TMDs), metal oxides, and metal-organic frameworks (MOFs) were also investigated to promote the photocatalytic activity of TiO 2 toward the HER (Majeed et al., 2017;Xiang et al., 2012). For example, graphene has unique electronic and optical properties, with zero bandgap Park et al., 2017;Zheng et al., 2018). Therefore, the incorporation of graphene into TiO 2 not only increases the electrical conductivity but also reduces titania's bandgap, thus improving the overall performance of the catalysts. Apart from graphene, other 2D materials such as TMDs, and transition metal carbides are also expected to be excellent in combination with TiO 2 . Moreover, MOFs, which are known as porous crystalline materials with outstanding features including large specific surface area, high thermal stability, adjustable structural components, have also been rationally investigated by means of forming composite catalysts with TiO 2 for the HER (Antwi-Baah and Farha et al., 2012;Xia et al., 2015).
Herein, we highlight the recent finding in the utilization of TiO 2 for solar water splitting. Strategies for modulating the key features toward the high performance of TiO 2 for the HER, including structural engineering, compositional engineering, and doping are highlighted and discussed. The advantages and limitations of each modification approach are reviewed. Finally, the existing approaches and possibilities for the improvement of TiO 2 as photocatalysts are provided.

Fundamental of photocatalytic HER
The separation of water takes place under UV, visible, or UV-Vis irradiation, including several principal steps (Fig. 2) (Jafari et al., 2016). First, a photocatalyst absorbs solar energy that is equal to or greater than the bandgap of TiO 2 to form excitons. Second, as a result, the electron moves from the valence band (VB) to the conduction band (CB) while the hole remains in the VB. Third, protons that adsorbed on the surface of TiO 2 received the new generated electron to produce hydrogen (Eq. (1)) and water is oxidized by the holes to form oxygen (Eq. (2)).
2H þ + 2e À !H 2 E 0 = 0.00 eV ð1Þ The initial condition for a reaction under the light resource is that the catalyst has a value of CB, which is smaller than the reduction potential of H + /H 2 (0 eV) and its VB level larger than the reduction potential of O 2 /H 2 O (1.23 eV). Therefore, TiO 2 is a promising candidate for a photocatalytic HER with a CB level of À0.3 eV and a VB level of 2.9 eV. TiO 2 as a catalyst for solar water splitting was first reported by Fujishima and Honda (Fujishima and Honda, 1972). The photocatalytic experiments were evaluated under UV light in a photoelectrochemical system with TiO 2 as an anode and platinum as a cathode. Oxygen was released on the facade of TiO 2 and hydrogen was exhausted from the Pt electrode. This result paved the way for the photocatalytic field. However, with a large original bandgap, TiO 2 is only photoactive under the UV light, which accounts for 4% of solar energy. Moreover, a subprocess usually appears in the photoreduction reaction, which is a reconsolidation of charge pairs, leading to go down catalytic efficiency. To overcome these challenges, several campaigns have been implemented to promote the photocatalytic activity of TiO 2 for the HER.
2.2. TiO 2 photocatalyst for HER 2.2.1. Structural engineering 2.2.1.1. TiO 2 nanostructure engineering. TiO 2 is a well-known n-type inorganic semiconductor. Therefore, it can pair with p-type semiconductor such as p-Si to form a p-n heterojunction structure which expedites the electron moving from p-Si through TiO 2 to active sites. For example, Andoshe et al. fabricated TiO 2 nanorods on a p-type silicon plate as a cathode material for photocatalytic water splitting (Andoshe et al., 2016). TiO 2 NRs/p-Si were created through a hydrothermal process with tetrabutoxytitanium as a precursor. The use of TiO 2 substantially increased the optical absorption of p-Si sample. The evidence is that the reflectance values measured at 550 nm are 37.5% and 1.4% for p-Si and TiO 2 NRs/p-Si pattern, respectively. Besides, TiO 2 NRs/p-Si also has better light absorption than TiO 2 seed layer/p-Si. As a result, when Pt nanoparticles were deposited on these samples, the photoelectrochemical (PEC) performance was remarkably enhanced. A saturation current density as high as 40 mA cm À2 , an onset potential approximately 440 mV, given by Pt-TiO 2 NRs/p-Si sample. Furthermore, heterojunction devices are durable over 52-hours test. Another finding based on TiO 2 NRs was reported by Yoon et al. (2019). In short, MIL(125)-NH 2 was deposited on the surface of TiO 2 NRs, which flourished on FTO/glass pattern. This device operated as an anode for separating of water under solar energy. Catalytic performance was evaluated in alkaline solution (pOH = 0.4) under AM 1.5 G irradiation. The outcome reveals that a photocurrent density of 1.63 mA/cm 2 at 1.23 V vs RHE was observed. This value is about 3-time higher than that of the bare TiO 2 NRs. There are some factors contribute for this finding, including large specific surface area and high crystallinity of TiO 2 NRs and an appropriate bandgap width of MOF (MIL(125)-NH 2 ). Specially, the combination of MIL(125)-NH 2 and TiO 2 NRs generated a type (II) heterojunction structure, which facilities the electron transport to the active area for HER.
Besides, mesoporous TiO 2 materials have attracted increasing attention in photocatalytic applications owing to their outstanding properties such as low cost, high stability, good electronic structure, and optical features. Many studies have been conducted to enhance the photocatalytic activity of mesoporous TiO 2 materials. Two structures, involving rutile mesoporous single-crystal NRs (R-MSC) and anatase mesoporous single crystal nanosheets (A-MSC) were synthesized and investigated for their photocatalytic activity in water splitting ( Fig. 3a)  . The morphology and size of mesoporous TiO 2 were mainly affected by three factors, including the seeding concentration, hydrohalic acid condition, and temperature. The study results revealed that rutile mesoporous single-crystal NRs with a seeding concentration of 0.3 mM (R-MSC-0.3) showed the best photocatalytic performance for hydrogen generation from water, which is much higher than of rutile single-crystal (R-SC). The same tendency occurred for A-MSC-0.3 and A-SC (Fig. 3b). The promoted catalytic activity was attributed to the increased specific surface area and the monocrystalline solid. In addition, the facets of R-MSC are active centers, having an important role in photocatalytic reactions (Fig. 3c). With the tunability of the structure as well as the model, mesoporous TiO 2 has many prospective applications in the field of photocatalysis.
2.2.1.2. Black TiO 2 . Initially, Chen et al. synthesized black TiO 2 from white TiO 2 through a hydrogenation process for hydrogen production (Chen et al., 2011). The result indicated that the optical absorption range of black TiO 2 is wider than that of white TiO 2 . As a result, black TiO 2 has a higher photocatalytic yield than that of white TiO 2 . This is a breakthrough in the use of TiO 2 for photocatalytic water splitting. Therefore, many research groups have followed this direction to promote the catalytic activity of TiO 2 in potential applications. For example, Wang et al. fabricated a core-shell black TiO 2 structure by a hydrogen plasma method for catalyzing the HER . This material showed a hydrogen production rate per mass unit of 10 mmol h À1 g À1 . This rate is much higher than the value of pristine TiO 2 . However, the black TiO 2 has a low porosity with a surface area of less than 50 m 2 g À1 . To tackle this problem, Zhou et al. found a new technique to fabricate an ordered mesoporous TiO 2 (OMT) with an ethylenediamine encircling step before hydrogenation was performed to achieve an ordered mesoporous black TiO 2 (OMBT) (Fig. 4a) . The morphology of OMBT can be observed by transmission electron microscopy (TEM) in Fig. 4(b,c). The obtained OMBT exhibited a high specific surface area of 124 m 2 g À1 , a high crystallinity, and a wide range of light from the UV-Vis to the infrared region. Besides, the separation and lifetime of photoinduced charges were substantially increased, leading to improved catalytic activity of OMBT structures. As a consequence, OMBT materials showed a hydrogen production rate of 136.2 lmol h À1 which is nearly 2-fold higher than that of original OMT (76.6 lmol h À1 ) under a standard light source (AM 1.5) (see Fig. 4d). In addition, catalytic stability of OMBT was maintained over 30 h test.   2.2.2. Compositional engineering 2.2.2.1. Graphene/TiO 2 composites. The invention of graphene opened a new chapter in the scientific field. Graphene is the first two-dimensional material with outstanding characteristics such as high specific surface area and good electron transferability, which minimize the reconsolidation of photogenerated charges to enhance the photocatalytic yield of the material. Graphene does not have any bandgap, whereas the bandgap of TiO 2 is large. Therefore, the incorporation of graphene and TiO 2 is considered as a perfect couple for photocatalytic applications. This was first reported for photocatalytic hydrogen production by Zhang et al. (2010). Graphene sheets (GSs)/ TiO 2 composites with the different ratios were created via a simple sol-gel process. A series of GSs/TiO 2 structures were investigated for the splitting of water under UV-Vis light in the mixture including sulfide/sulfite ions. A GSs/TiO 2 composite with 5 wt% graphene oxide (GO) showed a hydrogen generation rate of 8.6 lmol h À1 which is much higher than that of stand-alone TiO 2 crystal (4.5 lmol h À1 ). The improved catalytic efficiency of GSs/TiO 2 was attributed to the good conductivity of GSs, which facilitates the motion of electrons to the surface of the photocatalyst. Moreover, the decreased performance of the composite with 10 wt% GO can be accounted for by the concurrence of electrons and holes, causing their recombination, which diminished the photocatalytic activity for water splitting. To innovate the material types, Kim et al. incorporated nanographene oxides (NGOs) with TiO 2 to create a core-shell structure of NGO/TiO 2 before it was photoreduced to form r-NGOT for photocatalytic water splitting (Kim et al., 2011). Besides, TiO 2 deposited on the lm-size r-GO (r-LGOT) was also fabricated to compare the catalytic efficiency (Fig. 5). Photocatalytic reactions were measured under UV irradiation with methanol as an electron donor. The results indicated that the r-NGOT core-shell showed the highest yield of hydrogen evolution with the amount of NGO of 0.7 wt%. The hydrogen production rate of the r-NGOT composite with a core-shell structure is faster than that of r-LGOT and bare TiO 2 . This outcome can be attributed to the presence of r-GO, which impeded electron-hole pair reconsolidation and improved the moving of electron on the surface of r-NGOT. Moreover, the addition of platinum onto r-NGOT (Pt/r-NGOT) remarkably reinforced the catalytic activity for hydrogen evolution. Hydrogen was generated at a rate of 50 lmol h À1 for Pt/r-NGOT-0.7 and 27 lmol h À1 for Pt/TiO 2 . The enhanced performance of hydrogen evolution was attributed to the good conductance of r-NGOT, which can boost the charge separation as well as the mobility of electron. In addition, r-NGOT created a convenient path for the motion of electrons from the TiO 2 CB to Pt sites.
To date, scientists have focused on the improvement of visible-light-driven photocatalysts to maximize the exploration of solar energy, because visible light is account for 53% of solar energy, whereas this percentage is only 4% for UV rays.
As an attempt to improve the visible light absorption, Agegnehu et al. fabricated vanadium-doped TiO 2 NR on reduced graphene oxide through a facial hydrothermal method and the performance of obtained materials was investigated (Agegnehu et al., 2016). For intensitive understanding, Vdoped TiO 2 NPs with various proportions of vanadium, including 5, 10, and 15 wt% were coated on RGO to form nanocomposites. High-resolution TEM images of 10% Vdoped TiO 2 and 10% V-doped TiO 2 /RGO are shown in Fig. 6(a,b). The photocatalytic activity was evaluated under a 300 W xenon arc lamp as a light source for irradiation in a 20% aqueous methanol solution. The obtained result revealed that 10% V-doped TiO 2 /RGO exhibited the highest performance, with a hydrogen generation rate of 120 lmol h À1 . In comparison, the speed of hydrogen formation of 10% Vdoped TiO 2 /RGO is four times quicker than that of 5% V-TiO 2 , 1.6 times quicker than that of 10% V-TiO 2 , and twice as fast as that of 10% V-TiO 2 (Fig. 6c). The highest hydrogen generation rate of 10% V-TiO 2 /RGO in compare to the rest of the studied samples is attributed to two factors. First, the doping of vanadium onto TiO 2 reduced its bandgap width from 3.1 eV for bare TiO 2 to 2.51 eV, hence improving the light absorption. Second, RGO acts as a cocatalyst, which can trap the excited electrons, leading to suppress the charge recombination (Liu et al., 2010;Perera et al., 2012). Beside, RGO provides active sites for photoreduction reaction of water (Xiang et al., 2012). In the suggested reaction mechanism (Fig. 6d), the photocatalyst absorbs a photon from visible light illumination to separate the excitons. As a result, the negative electrons are promoted to the CB and the positive holes are located in the VB. These electrons can be moved to the surface of V-TiO 2 and the reduced graphene oxide sheets to facilitate the proton reduction reaction. This motion significant limits the recombination of charged particles, leading to improved photocatalytic performance for hydrogen evolution.
Recently, the simultaneous doping of metal ions and graphene for TiO 2 was a subject of continued research to enhance photocatalytic hydrogen production activity. Lang et al. fabricated an Ag-rGO-TiO 2 composite by depositing Ag nanocubes and TiO 2 nanolayers on the surface of reduced graphene oxides (Lang et al., 2018). Moreover, the authors also created Ag-TiO 2 for the comparison. Their structures are shown in Fig. 7(a,b). Catalytically, under visible light with methanol/water (20 vol% methanol), Ag-rGO-TiO 2 gave a hydrogen for-mation rate per mass unit of 0.53 lmol g À1 h À1 , whereas TiO 2 and Ag-TiO 2 were not found to be photocatalytically active. The enhanced activity was accredited to the performance of rGO as a conductive bridge for the transportation of electrons from Ag to TiO 2 . In addition, the formation of a Schottky barrier on the surface of rGO-TiO 2 reinforces the hot electron pervasion from rGO to TiO 2 , as shown in Fig. 7d. For the Ag-TiO 2 structure, the hot electrons recombine with holes owing to the lack of a formed barrier (Fig. 7c).

Transition metal dichalcogenides/TiO 2 composites.
Another class of 2D materials is the TMDs, which are also considered as potential candidates for incorporation with graphene to boost the efficiency of TiO 2 materials in photocatalytic applications. For example, MoS 2 /TiO 2 catalysts with various contents of MoS 2 were fabricated by Zhu's group through a facial mechanochemistry method . These catalysts were evaluated in terms of their photoactive yield under UV irradiation. The results revealed that 4%-MoS 2 /TiO 2 gave the greatest performance, with a reaction rate of 150.7 lmol h À1 , whereas pure TiO 2 only had a hydrogen generation rate of 3.1 lmol h À1 . The authors explained that MoS 2 acts as an electron container, which prevents the remix of electron-hole pairs. Moreover, the good conductivity of MoS 2 facilitates photo-induced charge separation, leading to improved catalytic performance. Ma and coworkers performed an interesting study by using a MOF as a reactant to fabricate a flower-like MoS 2 /TiO 2 nanohybrid system through a facial hydrothermal process ). An SEM image of MoS 2 /TiO 2 is shown in Fig. 8a. Photocatalytic experiments were conducted in the visible light condition with fluorescein as a photosensitizer. A remarkable enhancement in the photocatalytic activity was recorded, showing a yield of hydrogen evolution rate per mass unit of 10046 lmol h À1 g À1 (Fig. 8b). This performance was ascribed to the formation of active centers and uniform dispersion of the MoS 2 and TiO 2 phases, facilitating the motion of electrons to reduce protons. In the proposed mechanism, excited electrons from fluorescein move to the CB of TiO 2 . Later, these electrons transfer to the surface of MoS 2 before facilitating the reduction of water (Fig. 8c). Jing et al. deposited WS 2 onto mesoporous TiO 2 (m-TiO 2 ) for the photocatalytic HER (Jing and Guo, 2007). The coating of WS 2 improved the light absorption ability of TiO 2 , contributing to the enhancement in the yield of TiO 2 . A hydrogen formation rate per mass unit of 2.2 lmol h À1 g À1 was observed for WS 2 /m-TiO 2 with Pt as a cocatalyst under visible light irradiation. Xiang et al. investigated the influence of MoS 2 and graphene as cocatalysts for the photocatalytic hydrogen generation of TiO 2 NPs (Xiang et al., 2012). TiO 2 /MoS 2 /graphene with the different amounts of MoS 2 and graphene were prepared by a hydrothermal way. The structural properties were analyzed by TEM, as can be seen in (Fig. 9(a,b)). Photocatalytic experiments on these composites were carried out under UV irradiation with ethanol as a scavenger. A TiO 2 /MoS 2 /graphene composite with 0.5 wt% MoS 2 /graphene (95/5 wt%) showed the highest activity, with a hydrogen generation rate of 165.3 lmol h À1 . Compared to the others, the hydrogen generation rate of TiO 2 /MoS 2 /graphene is four and five times higher than that of TiO 2 /MoS 2 and TiO 2 /graphene, respectively (Fig. 9c). This yield can be explained by the fact that the MoS 2 /graphene mixture plays a pivotal key in the prevention of charge recombination, thanks to a synergetic effect, and supplies numerous active sites for the HER. The mechanism is illustrated in Fig. 9d. 2.2.2.3. Metal oxides/TiO 2 composite. Zinc oxide (ZnO) is widely studied in photocatalytic applications, owing to its high photochemical stability, photosensitivity, large bandgap, and nontoxicity. Thus, the incorporation of ZnO and TiO 2 has been studied for the field of photocatalysis. For example, Hussein et al. successfully synthesized ZnO/TiO 2 nanocomposites for the photocatalytic HER . Catalytically, the activity of the catalysts was tested in a methanol solution under UV-Vis irradiation. This nanocomposite demonstrated increased HER performance compared to that of the stand-alone TiO 2 . The observed increased activity of this nanocomposite was ascribed to the larger surface area and total pore volume, and smaller interface resistance. Another study on ZnO/TiO 2 was reported by Xie and coworkers . The authors mixed TiO 2 and ZnO with different ratios before depositing Pt as a cocatalyst for hydrogen evolution. The results revealed that 0.5 wt% Pt/TiO 2 -ZnO gave the best performance, with a hydrogen generation rate per mass unit of 2150 lmol h À1 g À1 under visible light irradiation. Considering the homogeneous catalysts, the hydrogen generation rates per mass unit are 68 and 3.0 lmol h À1 g À1 for TiO 2 and ZnO, respectively. Furthermore, the durability of the catalyst was also tested for H 2 production. The productivity reduced by only 12% and 23% compared with the first test time after 7 and 14 days, respectively.
One promising approach to fabricating metal oxide/TiO 2 composites is to use MOFs as precursors. The advent of MOFs opened a new chapter in the field of materials science and technology (Low et al., 2014). MOFs are known as the first member in the family of cage-like porous materials, which are constructed by the combination of metal clusters and organic compounds (Lu et al., 2014). Traditionally, MOFs were used in gas separation, gas storage, catalysis, sensors, and drug delivery applications, thanks to their outstanding properties such as large surface area, high porosity, and adjustable chemical structures (Chaemchuen et al., 2013). Furthermore, MOFs were used as sacrificial templates to create metal oxide/TiO 2 hybrids for the photocatalytic HER. Several illustrative metal oxide/TiO 2 systems based on MOF materials are discussed herein. Bala and coworkers utilized a Co-based MOF as a precursor to synthesize Co 3 O 4 /TiO 2 nanocomposites for photocatalytic splitting water (Bala et al., 2015). This material with 2 wt% displayed a reaction rate of 7 mmol g À1 h À1 under UV illumination. The intensified catalytic performance was attributed to the formation of a homogeneous catalyst and the presence of Co 3 O 4 as a co-catalyst to boost the electron transfer as well as electron-hole pair separation. In a similar example, Mondal and Pal used a Cu-based MOF as a sacrificial template to fabricate composites (Mondal and Pal, 2016). The optimized Cu/CuO/TiO 2 hybrid nanocomposite gave a yield of 286 mmol g À1 h À1 under solar illumination, which was much better than that of a conventional CuO/ TiO 2 hybrid system, owing to the formation of a small heterojunction and Cu loading into the TiO 2 matrix. Dekrafft et al. utilized an Fe-based MOF to fabricate an Fe 2 O 3 @TiO 2 core-shell structure before depositing Pt on the surface for the photocatalytic HER, as shown in Fig. 10a (Dekrafft et al., 2012). The catalyst exhibited a much faster hydrogen formation rate than those of Fe 2 O 3 , TiO 2 , and their mixture. The plot of hydrogen evolution with respect to time is illustrated in the inset of Fig. 10b. 2.2.2.4. Transition metal carbide/TiO 2 composites. Transition metal carbides are an important member of the young family of MXenes. They have gained much attention for potential applications, owing to their outstanding thermal, optical,   mechanical, and electronic features (Anasori et al., 2017;Hantanasirisakul and Gogotsi, 2018;Pang et al., 2019). Therefore, the combination of metal carbides and TiO 2 has been explored for catalytic utilization in energy conversion. In 2016, a mixture of metal carbide/TiO 2 was first reported by the Wang group for the separation of water into hydrogen under visible light (Wang et al., 2016). TiO 2 /Ti 3 C 2 T x composites were investigated with respect to their photocatalytic activity with different ratios. The optimal sample is TiO 2 /Ti 3 C 2 T x -5%, exhibiting a hydrogen generation rate per mass unit of 17.8 lmol h À1 g À1 upon visible light irradiation, which is superior to that for the homogeneous catalysts under the same condition. The reason for this is that the Ti 3 C 2 T x can facilitate the electron-hole separation and charge transportation, thus, promoting the overall performance of photocatalytic HER. Recently, Peng et al. provided a new approach to fabricate metal carbide composites for photocatalytic applications (Peng et al., 2018). Briefly, TiO 2 nanolayers were grown on Ti 3 C 2 T x by a hydrothermal method before undergoing photodecomposition of copper metal to form Cu y /TiO 2 @Ti 3 C 2 T x . Catalytic reactions proceeded under simulated solar light in methanol solution. Hydrogen was generated at a hydrogen production rate of 860 lmol h À1 for one gram Cu/TiO 2 @Ti 3 -C 2 T x catalyst. In comparison, 1 g TiO 2 @Ti 3 C 2 T x catalyst had a slow hydrogen production rate of only 65 lmol h À1 . This result was ascribed the presence of Cu species as a cocatalyst and Ti 3 C 2 T x acting as an assistant of TiO 2 for removing holes, thereby facilitating detaching the excitons and electron transfer. From the initial results, metal carbide/TiO 2 composites have opened a promising horizon for scientists to find the ideal catalysts for hydrogen production from water splitting.
2.2.3. Doping 2.2.3.1. Metal-doped TiO 2 . Modifying TiO 2 with other heteroatoms can change its bandgap width as well as extend the optical adsorption range, leading to enhanced photocatalytic activity. Noble metals such as Pt, Pd, Ru, Rh, Au, and Ag are considered the most effective materials for the field of catalysis in general and the photocatalytic HER in particular (Banerjee et al., 2015;Huang et al., 2018;Ouyang et al., 2018;Wu et al., 2013a,b;Wu et al., 2016a,b;Zhang et al., 2016). However, the important issue is that their price is high. The use of a small amount of precious metals to improve the catalytic activity of TiO 2 is an alternative approach. For instance, Zhu et al. deposited Pt onto a circular template of C-HS-TiO 2 created from D-glucose as a precursor (as shown in Fig. 11a) . The results indicated that 1 wt% Pt/C-HS-TiO 2 gave the highest hydrogen production rate of 2856.8 lmol h À1 compared with economic TiO 2 and C-TiO 2 hollow sphere (Fig. 11b). The reason for this that the addition of Pt onto C-HS-TiO 2 with the hollow spherical morphology improved the optical absorption ability in the visible light region. Moreover, a stability test of the photocatalysts was conducted after 22 days. The yield only declined by a small percentage of 8% for 1 wt% Pt/C-HS-TiO 2 (Fig. 11c). Another platinized TiO 2 was reported by Li et al. (2015a,b). The authors fabricated sub-10-nm rutile TiO 2 NPs by a facile hydrolysis method before doping 1 wt % Pt into these particles as a cocatalyst. The photocatalytic system was tested under the different light sources in 10 ml methyl alcohol (sacrificial reagent). A reaction rate per mass unit of 1954 lmol h À1 g À1 was observed under simulated solar light, whereas the hydrogen rate per mass unit observed with visible light proving energy for the photocatalytic reaction was 932 lmol h À1 g À1 . These results were confirmed by the appearance of OH-groups on the surface of TiO 2 , which contribute to decreasing its bandgap. Additionally, TiO 2 with small particle sizes increases the proportion of surface/sub-surface deficiencies to overcome the adverse effects of the bulk phase, leading to improved electron-hole separation. To date, the highest quantum efficiency is attributed to Pt-doped TiO 2 , which was reported by Guayaquil-Sosa et al. (2017). Mesoporous TiO 2 with special properties as mentioned above is modified with various amounts of Pt, including 1.00, 2.50, and 5.00 wt%. Among these, 2.5 wt% Pt-TiO 2 exhibited the highest quantum yield of  . 22.6% due to the effective reduction of the optical bandgap from 2.99 eV (bare TiO 2 ) to 2.34 eV.
Apart from Pt, Au has also drawn significant attention as a dopant for improving the performance of TiO 2 as a photocatalyst for the HER. Au is commonly fabricated as nanoscale structures, such as NPs, NRs, nanowires to incorporate the other materials for the catalytic reaction. Several studies indicated that Au with a particle size of less than 5 nm exhibits high catalytic performance (Haruta 1997;Valden et al., 1998). For example, Fang et al. generated a mesoporous Au-TiO 2 nanocomposite through a copolymer-assisted sol-gel method for solar hydrogen evolution (Fang et al., 2012). The results revealed that in the presence of ascorbic acid, water is reduced to hydrogen at a rate per mass unit of 7 lmol h À1 g À1 under visible light, which is 3-fold higher than that for Pt-TiO 2 under similar experimental conditions. Three reasons were suggested to explain these results. The catalytic activity of Au-TiO 2 was attributed to the contribution of defect/impurity states and the poorly visible light absorption of the TiO 2 matrix was improved by the Au surface plasmons. Moreover, the Au NP plays an important role in providing electrons to TiO 2 for photoreduction. Another special structure of Au investigated for its photocatalytic activity for the HER is Au NRs (AuNRs). Wu et al. created AuNR/TiO 2 nanodumbells as a potential candidate for the photocatalytic HER by coating TiO 2 on the two ends of gold NRs through a wet-chemical method (Fig. 12a) (Wu et al., 2016a,b). The photocatalytic performance of these catalysts is displayed in Fig. 12b. Specifically, AuNR/TiO 2 nanodumbells exhibited a yield of 11.6 lmol g À1 h À1 upon visible light irradiation, whereas the AuNR@TiO 2 core-shell structure did not show any catalytic activity under similar experimental conditions. These results can be justified by the fact that the transfer of hot electrons takes place in the photoreduction of water for AuNR/TiO 2 nanodumbells (Fig. 12c). The AuNR@TiO 2 core-shell structure only exhibits plasmon-induced resonance energy transfer (Fig. 12d). The third noble metal that has been applied for catalytic utilization is silver (Ag). Liu et al. synthesized a TiO 2 nanosheet film (TiO 2 -NSF) by a simple hydrothermal process before depositing Ag NPs on its surface (Liu et al., 2014a,b). Under the irradiation of UV-Vis light, the Ag/TiO 2 -NSF was found to be 8.5 times more effective than the homogeneous TiO 2 catalyst. The reason for this is that the Ag NPs support the prevention of the remix of electron-hole pairs, leading to enhanced electron transfer in the reduction of water. The second reason is the synergetic effect between the electron transfer and surface plasmon resonance absorption. The codoping of noble metals on TiO 2 has also been researched for photocatalytic H 2 production from water. Rahul et al. incorporated Au-Pt NPs and Ti 3+ to generate an Au-Pt/ Ti 3+ nc-TiO 2 catalyst before it was reconstituted into titania inverse opal (Au-Pt/Ti 3+ io-TiO 2 ) for solar water splitting (Rahul et al., 2018). Hydrogen was detected at a rate per mass unit of 181.77 mmol h À1 g À1 for the Au-Pt/Ti 3+ io-TiO 2 photocatalyst, which is higher than any other catalyst, including Au-Pt/Ti 3+ nc-TiO 2 , Au/Ti 3+ nc-TiO 2 , and Pt/Ti 3+ nc-TiO 2 . This activity is attributed to the appropriate photonic effects on TiO 2 electronic absorption properties. Another pair of noble metals was used to produce Ag-Au bimetallic clusterdoped TiO 2 . This incorporation enhanced the light adsorption region from 400 to 650 nm (Patra and Gopinath, 2016). As a result, the Ag-Au/TiO 2 composite displayed a hydrogen production rate per mass unit of 718 mmol h À1 g À1 , whereas the rate was much lower for the Ag-TiO 2 and Au-TiO 2 mixtures. The improved catalytic activity is mainly attributed to the formation of a Schottky junction. Moreover, the generation of hot electrons also has a significant function in boosting the reaction rate. A fascinating study utilizing MOFs as a precursor to synthesize TiO 2 was reported by Yan et al. (2017). NH 2 -MIL-125(Ti) was pyrolyzed at 400°C to form hierarchical TiO 2 before different amounts of Pd were deposited on it for the photocatalytic HER (Fig. 13a). Pd/TiO 2 with 1.5 wt% showed excellent performance of 979.7 lmol h À1 under UV-Vis irradiation, whereas this value was 112.7 lmol h À1 under simulated solar light (Fig. 13b,c). These yields were attributed to the positive synergetic effect, constituting convenient conditions for the splitting and conveyance of electron-hole pairs.
Non-noble metals are also utilized to intensify the photocatalytic activity of TiO 2 (Dholam et al., 2009;Sadanandam et al., 2013). Many studies have demonstrated that copper metal has special property to catalyze certain reactions in the area of energy conversion, such as the CO 2 reduction reaction (Maina et al., 2017). Hence, the combination of copper and TiO 2 materials can provide promising candidates for hydrogen evolution with a high yield. For example, Cu-deposited TiO 2 for the photocatalytic HER was first reported by Wu et al. (Wu and Lee, 2004). The optimal sample with 1.2 wt% Cu showed a reaction rate 10 times faster than that with bare titania. The oxidation of Cu was observed during the photoreduction process. This work represented an initial step in using Cu to boost the catalytic activity of TiO 2 . In 2017, Rather et al. generated a Cu +1 /Cu 0 -TiO 2 mesoporous nanocomposite (Cu-mpTiO 2 ) for solar hydrogen production (Rather et al., 2017). Cu-mpTiO 2 photocatalysts were created from mesoporous TiO 2 with high surface area and Cu NPs via incipient Fig. 12 (a) SEM image of the as-prepared AuNR/TiO 2 nanodumbells. (b) Photocatalytic hydrogen production rate of various catalysts. (c,d) Schematic representation of the photocatalytic reaction mechanism for (c) AuNR/TiO 2 dumbbell and (d) coreshell AuNR@TiO 2 under visible light . wetness impregnation. The photoreactions were tested under direct sunlight in 20% vol. methanol. Water was photoreduced to hydrogen with an amount of 1000 lmol and an apparent quantum yield of 11.39%. In comparison, the apparent quantum efficiency of commercial Cu-TiO 2 is 4.1%, whereas the bare mesoporous TiO 2 did not show any performance. Recently, Montoya et al. synthesized various catalysts by depositing transition metals, including Ni, Co, and Cu, on the surface of TiO 2 to realize an improved photocatalytic hydrogen generation by a photoreduction method (Montoya and Gillan, 2018). All the transition metals used to modify TiO 2 resulted in higher performance than that of unmodified TiO 2 . The best yield was observed for Cu(1%):TiO 2 , with a hydrogen generation rate per mass unit of 8500 lmol h À1 g À1 . This was attributed to the function of the 3d transition metal, restraining the reformation of excitons and facilitating the transportation of electrons for the reduction of water.
Similar to noble metal-doped TiO 2 , the codoping of TiO 2 with non-noble metals has also been studied for the HER (Luna et al., 2017;Sun et al., 2012Sun et al., , 2015. An example is Fe-Ni-codoped TiO 2 to achieve boosted visible-light-driven photocatalytic activity. Sun et al. investigated the catalytic activity of bare TiO 2 , Ni-doped TiO 2 , Fe-doped TiO 2 , and Fe-Nicodoped TiO 2 for water splitting under visible light (Sun et al., 2012). These catalysts were fabricated by an alcoholthermal method. Based on the Brunauer-Emmett-Teller (BET) measurement, a relatively high surface area of 98.35 m 2 g À1 was estimated for Fe-Ni/TiO 2 (5 wt% Fe and 4.0 wt% Ni), which is approximately 2-fold larger than that of bare TiO 2 . As a result, this catalyst displayed a hydrogen evolution rate of 361.64 lmol g À1 h À1 in the ethanol solution, whereas Fe-TiO 2 and Ni-TiO 2 had a lower rate, and TiO 2 alone was not active under similar conditions. The reason for this is that the co-doping of Fe and Ni improved the visible light absorption capacity of TiO 2 , which was verified by UV-Vis spectroscopy. Also, the enhanced separation of electronhole pairs was proved through photoluminescence spectroscopy. The reaction mechanism is shown in Fig. 14 A similar example is Ni-Pd/TiO 2 , which was reported by Luna et al. (Luna et al., 2017). Ni-Pd/TiO 2 was created in a solution of ammonium hydroxide via a radiolysis method. Single metaldoped TiO 2 was fabricated for comparison. Photocatalytic experiments were performed under a 400 W mercury arc as a UV-Vis illumination source. In methanol solution (50% vol.), hydrogen was generated with a production rate of 200 lmol h À1 . The bimetallic doping created a synergetic effect in facilitating electron transfer. Moreover, Ni-Pd NPs play a vital role as catalytic centers for the formation of hydrogen.   (Sun et al., 2012).
Additionally, co-metal doping has also been utilized to boost the catalytic activity of TiO 2 . For example, Tanigawa et al. synthesized Cr/Ta co-doped anatase TiO 2 (Cr,Ta-TiO 2 (A)) and rutile TiO 2 (Cr,Ta-TiO 2 (R)) for hydrogen production by a facile hydrothermal method (Tanigawa and Irie, 2016). Platinum was then deposited onto these materials for water splitting applications. The photocatalytic experiments were implemented under visible light illumination. The HER takes place in the presence of I À for Cr,Ta-TiO 2 (A) with a hydrogen evolution rate per mass unit of 11.7 lmol h À1 g À1 , whereas oxygen evolution only occurs in the presence of IO 3 À for (Cr,Ta-TiO 2 (R)). This result could be explained by the fact that the Cr 3d orbital in Cr,Ta-TiO 2 (A) was accounted for the oxidation iodide ion, and was not concerned in O 2 evolution, whereas the electrons in the CB of Cr,Ta-TiO 2 (A) were accounted for the HER.

Nonmetal-doped TiO 2 .
Many studies have demonstrated that N-doped TiO 2 not only decreases the bandgap width of TiO 2 , but also improves the photocatalytic performance for the HER (Hou et al., 2017;Khore et al., 2017;Liu et al., 2016;Preethi et al., 2016;Reddy et al., 2017;Taherinia et al., 2019). For instance, the Sonawane group researched the yield of N-doped TiO 2 for the photocatalytic HER (Khore et al., 2017). They found that N-TiO 2 had a high hydrogen generation rate per mass unit of 7990 lmol h À1 g À1 under natural sunlight. Moreover, the catalytic activity of N-TiO 2 was tested under a xenon lamp, and the hydrogen generation was detected with a rate per mass unit of 4740 lmol h À1 g À1 . These results were explained by the fact that natural sunlight includes both UV and visible light, and narrowed bandgap width is due to the appearance of nitrogen in the TiO 2 lattice via the Fourier transform infrared spectrum. Liu and coworkers prepared ultrafine N-doped TiO 2 through a simple solvothermal route with polyvinylpyrrolidone (PVP) as a nitrogen source . They observed that the photocatalyst with 0.1 g exhibited a significantly increased hydrogen generation rate per mass unit of 323 lmol h À1 g À1 , which is much quicker than that of TiO 2 . The authors suspected that the boosted performance was related to the ultrafine particle size, the hydroxyl groups, and the addition of nitrogen on TiO 2 . Hou and coworkers synthesized N-doped TiO 2 mesoporous nanofibers for the HER (Hou et al., 2017). They found that the nitrogenation of TiO 2 could improve the performance for the splitting of H 2 O into hydrogen compared to the bare TiO 2 . N-doped TiO 2 mesoporous nanofibers gave a hydrogen production rate per mass unit of 39.5 lmol h À1 g À1 , whereas the rate for the bare TiO 2 was 1.5 lmol h À1 g À1 . This high hydrogen production yield can be accounted for the narrow bandgap of N-doped TiO 2 . The second nonmetal used as a dopant for TiO 2 is sulfur. Yang et al. successfully synthesized S-doped core-shell black TiO 2 for improved H 2 generation . This catalyst yielded a hydrogen production rate per mass unit of 0.258 mmol h À1 g À1 . The excellent performance was ascribed to the high amounts of Ti 3+ and S in the shell layer, which broaden the optical absorption ability from UV light into the visible and near-infrared regions. Sun et al. mixed sulfur powder and TiO 2 nanotubes with an equal ratio to prepare Si-doped TiO 2 nanotubes (Sun et al., 2017). The photocatalytic performance was estimated under visible light in a methanol solution (20% vol.). The results indicated that an excellent yield of hydrogen production rate per mass unit of 9610 lmol h À1 g À1 was attained, which was ascribed the presence of S 2 2À anions in the TiO 2 nanotubes. Moreover, the third group in used for improving the photocatalytic activity of TiO 2 is that of halogen elements such as Cl, Br, and F. Several works have revealed that these elements could broaden the optical absorption, leading to improvements in the photocatalytic HER (Gao et al., 2019, Luo et al., 2004. Recently, Andoshe et al. conducted a fascinating study by co-doing of nonmetal N and S to the TiO 2 NRs structure with the various contents for application as photoanodes for photoelectrocatalytic water oxidation (Andoshe et al., 2018). In this work, they increased the photocurrent density from 0.7 mA cm À2 (bare TiO 2 NRs) to 2.82 mA cm À2 ((N, S) co-doped TiO 2 NRs) at a potential of 1.23 V vs. RHE. The improvement in device performance was postulated to come from the newly created defect energy level near the valance band edge, which reduced the bandgap of TiO 2 from 3.1 eV to 2.88 eV. Notably, (N, S) co-doped TiO 2 NRs exhibited an external quantum efficiency of 97%, which is substantially higher than that of pristine TiO 2 NRs (19.1%) under UV light.
Another approach for increasing the catalytic activity of TiO 2 is simultaneous metal and nonmetal doping. For example, a systematic study of the modification of TiO 2 with cations and anions was conducted by Lin and Shih (2016). The authors fabricated a series of M/N-TiO 2 (M: Cr, Cu, Ni, Nb) structures by a microwave-supported hydrothermal process for solar hydrogen evolution. The photoreduction of water takes place in the presence of methanol as an electron donor under various light sources. The results revealed that Cu/N-TiO 2 exhibited the highest hydrogen production rate per mass unit of 27.4 lmol h À1 g À1 under UV light and 283 lmol h À1 g À1 under visible light. The synergistic effect created by copper and nitrogen doping can account for these outcomes. Zhang et al. investigated Cu/S-TiO 2 for catalyzing of the water-splitting process under visible light (Zhang et al., 2015a,b). Cu/S-TiO 2 showed enhanced catalytic activity in a methanol solution as an electron donor. Hydrogen was produced at a rate per mass unit of 7.5 mmol h À1 g À1 . In comparison, this quantity of S-TiO 2 was only 0.7 mmol h À1 g À1 , whereas hydrogen was not detected for bare TiO 2 . The enhanced performance was attributed to the good dispersion of Cu species in S-TiO 2 , creating a convenient condition for the separation and transportation of electrons in the photoreduction of water. In 2018, Mandari et al. used the rare earth metal Ga incorporated with N for doping TiO 2 in solar hydrogen production (Mandari et al., 2018). Ga/N-TiO 2 with 2% Ga displayed a reaction rate of 5.32 lmol h À1 which was attributed to the improvement in the optical absorption and efficient electron-hole pair separation.
Another study using the four elements Cu, Ga, In, and S to improve the photocatalytic ability of TiO 2 for the HER was reported by Kandiel and Takanabe (2016). Cu-Ga-In-S/TiO2 was fabricated by a solvent-induced deposition method. Copper-gallium-indium-sulfide (CGIS) with different weight percentages, including 1, 5, 10, 25, 50, and 75 wt%, was coated onto the surface of TiO 2 in toluene. Photocatalytic reactions were carried out under visible light with a mixture including catalyst and Na 2 S/Na 2 SO 3 . Ru was used as a cocatalyst in a photoreactor system. The catalyst that showed the highest yield was CuGa 2 In 3 S 8 (50 wt%)/TiO 2 , which gave a hydrogen evolution rate of 50.6 lmol h À1 . The intensified activity was attributed to the extended visible light adsorption range of TiO 2 by CGIS. TiO 2 was not active in photocatalytic hydrogen evolution under visible light. Moreover, a small amount of CGIS also improved the photocatalytic activity of TiO 2 . This could enable increased economic efficiency for large-scale applications.

Challenges and perspectives
TiO 2 photocatalysts are considered as promising materials for hydrogen production from photocatalytic HER. Many works have demonstrated that TiO 2 -based photocatalysts exhibited excellent performance. Nevertheless, the main challenge of TiO 2 is its wide bandgap, which leads to rapid quenching of photo-induced excitons. This reduces the photocatalytic activity of TiO 2 . Moreover, the effect of the synthetic methods on catalytic activity obviously needs to be investigated, and the electron transfer is also not clear. Although the reported TiO 2 materials showed high activity, the recycling of TiO 2based photocatalysts has not been studied to save the cost of manufacturing in industrial applications. Furthermore, the investigation on mesoporous TiO 2 with high porosity and well-defined pore channels that facilitate the transportation of protons to catalytic centers needs more attention. In addition, the incorporation of two-dimensional materials (e.g., graphene, TMDs), MOF materials, and TiO 2 can also create promising candidates for solar hydrogen production. Finally, owing to the increasing development of computational methods, the properties of TiO 2 -based catalysts such as the electronic density of states, band structures, and active sites can be thoroughly studied and designed toward highperformance photocatalytic HER. Thus, to achieve a better understanding and improved development of TiO 2 photocatalysis for the HER, a combination of empirical and theoretical study is required

Conclusion
In this review, the application of TiO 2 -based catalysts in the reduction of water through photocatalysis has been discussed. Compared to other catalysts, TiO 2 has outstanding properties like low cost, high firmness, nontoxicity, and environmental friendliness. However, TiO 2 also has several disadvantages in photocatalytic applications, including a large bandgap, high hydrogen overpotential, and the remix of exciton. To tackle these problems, the different strategies involving metal deposition, nonmetal doping, graphene/TiO 2 hybrids, and MOF/ TiO 2 composites have been applied to boost the catalytic activity of TiO 2 . Moreover, the morphology and particle size also have a vital role in the extent of their photocatalytic characteristics. Therefore, a combination of compositional and structural engineering of TiO 2 -based catalysts is expected to give better device performance. However, it needs more investigation in the near future.

Declaration of Competing Interest
The authors declare no conflicts of interest.