Recent advances and strategies to tailor the energy levels, active sites and electron mobility in titania and its doped/composite analogues for hydrogen evolution in sunlight

Abstract

Hydrogen energy is realized to be promising as a potential replacement for fossil fuels; it is expected to become a prominent source of energy in the foreseeable future. Therefore, straightforward and cost-effective methods of hydrogen production are urgently required and are considered the current state-of-the-art in the field. Accordingly, the strategy of hydrogen production is very important; it can be achieved through photocatalytic water splitting under irradiation of suitable light sources, where materials are key factors to efficiently conduct this process. In this direction, titanium dioxide/titania (TiO₂) has been demonstrated to be an excellent photocatalytic semiconductor towards hydrogen production through water splitting. However, the wide band gap structure, faster carrier recombination and inadequate charge transfer characteristics of TiO₂ photocatalysts greatly limit their efficiency and restrain their practical applications in the field. In this context, the size, morphology and chemical properties of TiO₂ have been tailored through a range of modification strategies. Closer insights into these processes reveal that the key factors of these modifications essentially involve tuning the crystal structures and surface characteristics and, thereby, the energy structures of titania. Therefore, in this review, we focus on energy structure modifications of TiO₂ through various strategies, such as (i) tuning of size and morphology, (ii) creating defect structures, (iii) elemental doping, (iv) composites and (v) surface decoration with metal nanoparticles and co-catalysts. The content of this review provides insight into understanding the energy structure of TiO₂ that supports photocatalytic hydrogen generation and therefore endows it with prospects for real-time application in the field to achieve sustainable hydrogen production.

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1. Introduction

The current massive worldwide consumption of energy threatens the availability of energy sources in the future. Next to fossil fuels, hydrogen has become the most important and promising source of energy; this importance is not exaggerated, as hydrogen is believed to have potential to even replace fossil fuels. Hydrogen is a potential clean energy because its consumption does not produce pollution or toxic gases. Currently, hydrogen is being produced through various techniques, as listed in Table S1 in the ESI;† it is mainly produced from petroleum and natural gas steam reforming processes. Although the consumption of hydrogen is green, its production through existing techniques results in release of CO₂ and other greenhouse gases into the environment; this has become a major factor in the increase of global warming. Therefore, a green methodology is crucially needed to produce hydrogen. On account of this, the ability of producing hydrogen through water splitting using semiconductors, especially under irradiation of light (photocatalysis), represents a significant leap in the field of hydrogen production. In general, water splitting by semiconductors is analogous to the photosynthetic process in several ways; this process has been under constant investigation since 1980 (ref. 1 and 2) and has extensively been studied by many researchers worldwide.^3–13 Although photon-to-energy (PTE) conversion through a range of photocatalysts has been found to have potential, it has not yet reached the stage of implementation for practical applications. However, massive research in this direction is rapidly progressing.

Because the photocatalytic phenomenon has potential for water splitting, the key challenge lies in finding and constructing suitable materials to accomplish this process by the most efficient means. Understanding photocatalytic water splitting provides insight into the construction of efficient materials. Fig. 1 shows an energy (E) vs. pH plot, also known as a Pourbaix diagram, which demonstrates the thermodynamic stability of water with increasing pH from 0 to 14; also, Table 1 provides the redox potentials of water under acidic (0), neutral (7) and basic (14) pH conditions. From these insights, in a photocatalytic semiconductor, the valence band (VB) edge potential should be more positive than the oxidation potential (+1.23 eV) and the conduction band (CB) edge potential should be more negative than the reduction potential (0.0 eV), as shown in Fig. 2. It should be noted that ideal positioning of the band edges at these potentials will lead to overall splitting of water molecules. Accordingly, the positioning of the CB edge potential is crucial to design hydrogen evolution photocatalysts, as shown in Fig. 3.

Fig. 1 Reduction–oxidation potential of water with increasing pH (reprinted from the web: https://chem.libretexts.org/Textbook_Maps/General_Chemistry/Book%3A_Chem1_(Lower)/16%3A_Electrochemistry/24.04%3A_The_Nernst_Equation).

Table 1 Redox potentials of water in acidic, neutral and basic solutions

Reaction	Acidic (pH 0)	Neutral (pH 7)	Basic (pH 14)
O2 evolution	O2 + 4H^+ + 4e^− → 2H2O E° = +1.23 V	O2 + 4H^+ + 4e^− → 2H2O E° = +0.81 V	O2 + 2H2O + 4e^− → 4OH^−  = +0.40 V
H2 evolution	2H^+ + 2e^− → H2 E° = 0 V	2H^+ + 2e^− → H2 E° = −0.41 V	2H2O + 2e^− → H2 + 4OH^−  = −0.83 V

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Fig. 2 Ideal positioning of band edge positions in a photocatalyst with respect to the redox potential of water.

Fig. 3 Band edge positioning of O₂ and H₂ evolution photocatalysts.¹⁴ Reprinted with permission from ref. 14. Copyright 2014 RSC.

From a materials perspective, photochemical reactions of TiO₂ open new possibilities in employing TiO₂ for photocatalytic processes; it has gradually become a pioneering material in the field. In addition to the abovementioned band edge requirements; a photocatalyst should also possess properties such as decreased recombination rates and enhanced charge transportation towards achieving enhanced efficiency. On the other hand, current limitations of TiO₂ include their wide band gap energy, which requires UV light to activate. Considering the photocatalytic process, because photons are driving forces to activate the photocatalyst, it is more convenient if the photocatalyst can be activated under visible light rather than UV light.¹⁵ In future, it will be more economical if sunlight can be employed to activate photocatalysts because the earth receives 5%, 45% and 50% UV, visible and NIR energies; these are called “full-sunlight”-driven photocatalytic materials. These mechanics of an ideal photocatalyst essentially suggest that TiO₂ must be equipped with additional features and functions to overcome its present limitations. In this direction, the physical and chemical structures of TiO₂ have been modified, which eventually alters the energy structure of TiO₂ and improves its quantum efficiency and PTE conversion efficiency towards photocatalytic hydrogen production.

1.1 Scope of the review

This section provides a brief account of the present status of various materials for various photocatalytic applications and, thereby, the scope of this review.

Since the demonstration of photocatalytic (PC) hydrogen production, numerous reports have appeared in the literature.^14–20 In this direction, in 2009, Kudo and Miseki comprehensively reviewed various heterogeneous catalysts^16,17,21 for water splitting, where the PC water splitting abilities of elements are discussed groupwise as oxides, (oxy)nitrides and (oxy)sulphides. Similarly, in 2010, Maeida et al.¹⁸ reviewed developments in the water splitting process. In 2014, Ma et al. broadly reviewed fuel generation on TiO₂-based photocatalysts, especially for production of hydrogen by biomass conversion and water splitting, as well as generation of carbon-based chemical fuels.¹⁹ Visible light water splitting is considered to be the holy grail of chemistry,²⁰ and it has been widely explored through various strategies.²¹ In recent years, an increasing number of research papers have been published that explain the photocatalytic pathways for achieving enhanced solar-to-hydrogen (STH) conversion efficiency. More importantly, these studies have rapidly and vastly increased; more advances have been achieved since 2011,^22–24 especially in the construction of visible-light active titania-based materials for hydrogen evolution under solar light. It is realized that TiO₂ is a promising semiconductor due to its characteristic features, such as strong oxidizing power, chemical stability, optoelectronic properties, cost-effectiveness, environmental benignancy, and, importantly, its band edge potential with respect to the water oxidation potential for water splitting, as shown in Fig. 4.^25–28 Consequently, interesting results have also been obtained in band gap engineering through the creation of defects and off-stoichiometry oxygen to essentially tune the band edge potential of TiO₂ with respect to the redox potential of water.³² In addition, studies on the modification of TiO₂ for visible light activity have been largely explored for various photocatalytic applications.^33–41

Fig. 4 Band edge positions of TiO₂ with respect to water redox potential towards H₂ production.

The present review describes the strategies used and the developments achieved since 2011, with a neat focus on two main aspects of titania modification for hydrogen evolution in visible light. One is tailoring the energy structure of titania and the other is tweaking its crystal structure and surface character. Band gap reduction and band alignments induced by hydrogen doping, self-doping, codoping of cations, codoping of N-TiO₂ with carbon materials, 2D materials and the formation of their composites are described. New strategies developed to form titania composites of specific oxides, such as Cu, Zn, Zr and heterojunctions, are reviewed in depth; in addition to engineering the energy structure, these strategies induce lattice and surface defects, create surface traps, and cause charge recombination/separation/migration, thus refining the electron pathways. Noble metal loading on titania has been used for many decades to enhance its water splitting activity; the mechanism by which it influences the energy structure and the SPR effect of Au and Ag loading are discussed. Finally, reports wherein well-established visible light-active sulphides are formed into composites with titania to overcome the limitation of photocorrosion are described. With regard to tweaking the crystal structure and surface character of titania, new and promising strategies used to obtain desirable shapes, size, surface areas, numbers of active sites and porosities as well as their effects on the energy structure and reactivity of titania are discussed.

1.2 Evaluation parameters of a photocatalyst towards hydrogen generation

The photocatalytic efficiency of a photocatalyst can be evaluated by various parameters that are essentially required for quantitative analysis of its efficiency. These parameters and their physical significance are discussed below.

(i) Turnover number (TON). The TON is defined as the ratio of the number of reacted electrons to the number of atoms in the photocatalyst (eqn (1)) or on the surface of the photocatalyst (eqn (2)). The number of reacted electrons can be calculated from the amount of H₂ evolved.

TON = No. of reacted electrons/No. of atoms in the photocatalyst (1)

TON = No. of reacted electrons/No. of atoms on the surface of the photocatalyst (2)

(ii) Quantum yield (QY). The QY can be generally defined as the number of times a specific “event” occurs per photon absorbed by the system. Accordingly, in a photocatalytic system, the QY can be calculated by determining the ratio of the number of reacted electrons to the number of incident photons. In general, the measurement of the real amount of photons absorbed by a photocatalyst in a dispersed system is difficult because of scattering effects. Therefore, the obtained quantum yield is always an apparent quantum yield (AQY) and is anticipated to be less than the real quantum yield because the absorbed photons are usually fewer than those of the incident light.²¹

AQY = [No. of reacted electrons/No. of incident photons] × 100

(iii) Solar-to-hydrogen (STH) conversion efficiency. This is an essential parameter in evaluating the ability of photocatalysts towards their implementation in commercial solar photocatalytic water-splitting systems; it can be calculated as follows.

STH (%) = [Output energy as H₂/Energy of incident solar light] × 100

STH (%) = [H₂(mmol⁻¹) × 237 (kJ mol⁻¹)/Pm (MW cm⁻²) × Area (cm²)] × 100

2. Crystallinity and crystal structure

Improving the crystalline properties of a photocatalyst plays an important role in enhancing its photocatalytic activity. Increasing the crystallinity essentially decreases the density of defects, the number of recombination centers, and the diffusion length of the excited charge carriers and increases the catalytic sites in the materials, leading to enhanced photo-activity of the catalyst. The active catalytic sites are the key to enhance the rate of water-splitting reactions. Further, improving the crystalline properties of a photocatalyst leads to fine dispersion with minimum agglomeration of particles and increases the porosity and surface area, which are very influential parameters toward achieving enhanced photocatalytic activity. As discussed, in addition to appropriate positioning of the band edge positions, structural and surface characteristics such as crystallinity, porosity, surface area, and particle shape and size are value-added parameters for enhanced performance of photocatalysts. In this context, this section provides insight into the effects of the crystal structures and morphologies of TiO₂ and TiO₂-based materials on their energy structures towards enhancing their photocatalytic hydrogen evolution properties. The catalytic sites directly favor light absorption (irrespective of the light source) and increase the number of electrons produced and transported for efficient photocatalytic activity. A larger surface area and smaller particle size considerably shorten the distance for the migration of excitons to the surface, thus decreasing the possibility of recombination.²⁹

Among the three crystal phases of titania, the rutile and anatase phases are highly photoactive and have been largely explored for their photocatalytic activities.³⁰ The discrepancy in the band gaps of rutile (3.0 eV) and anatase (3.2 eV) is due to the higher CB edge position; eventually they show significant differences in their photocatalytic activity.³¹ Many studies have demonstrated that anatase-TiO₂ shows better activity compared to rutile phase.³² However, it has been reported that a mixture of these phases synergistically enhances their photocatalytic properties due to their modified energy structures and efficient electron transport and mobility.³³ Melián et al.³⁴ reported efficient hydrogen evolution due to the synergetic effects of an anatase : rutile (87 : 13) mixture of TiO₂. In their study, the materials were prepared through a sol–gel process, followed by either (i) hydrothermal treatment (150 °C/24 h) or (ii) heat treatment between 400 °C and 900 °C; these materials showed enhanced activity. It was also found that balanced amounts of rutile and anatase phase (with a smaller percentage of rutile phase) with a polyhedral particle shape (truncated bipyramid obtained by calcination), along with high surface area and small crystal size, yielded maximum hydrogen production.³⁴ A green preparation method involving solid–solid transformation of NH₄TiOF₃ nanoplates to TiO₂ nanoplates with exposed (001) facets was evaluated for H₂ generation; 10 μmol h⁻¹ g⁻¹ of catalyst could be used due to the comprehensive impact of the specific surface area, the size of the photocatalyst, the degree of crystallisation, and the exposed (001) facets.³⁵ High-pressure columbite phase stabilized by incorporating a plastic strain of TiO₂ with enormous amounts of oxygen vacancies showed absorption in the visible light range with a band gap of ∼0.7 eV, resulting in high photocatalytic hydrogen evolution.^36,37 However, there are some contradictory reports; for example, Lin et al. reported that N-TiO₂ with a smaller anatase crystallite size (∼8 nm) showed lower hydrogen production activity, which was attributed to high electron–hole recombination due to the poor crystalline quality of TiO₂.^38,39 Interestingly, in the photocatalytic Z-scheme water splitting process,⁴⁰ two semiconducting materials with different band gaps are conventionally used (i) to absorb a larger portion of solar light and (ii) to drive the proton reduction reaction (hydrogen evolution) and the oxygen evolution reaction at different sites. In these approaches, hydrogen and oxygen separation can be easily achieved, resulting in overall lower hydrogen production costs. Further, in this system, the shuttle electron mediator leads to efficient utilization of visible light compared to the conventional single-step photocatalysis process because energy is needed to activate each photocatalyst, as depicted in Fig. 5.⁴¹

Fig. 5 Schematic of a Z-scheme photosystem.⁴⁵

Xu et al. reported Z-scheme water splitting using anatase–rutile TiO₂ nanofibers, which showed enhanced transportation of photo-exited electrons from anatase to rutile phase. Increasing the rutile-to-anatase ratio resulted in the formation of more Z-scheme heterojunctions; thus, higher photocatalytic H₂-production activity was achieved.⁴² In another study, the effects of nano- and micro-rutile/anatase phases of TiO₂ on hydrogen generation were studied; it was observed that the rate of hydrogen generation increases in the presence of nano-anatase/rutile phase compared to the micro-sized systems due to the better synergistic effects between the rutile and anatase phases in the nano-systems and their increased surface areas.^43,255 In a study on overall water splitting into H₂ and O₂ using rutile TiO₂, it was highlighted that rutile phase decreases the density of surface traps and the exposed crystal facets separate the redox active sites. Mixed phases of TiO₂ were also studied for simultaneous photocatalytic hydrogen evolution and degradation of organic pollutants (by consuming photogenerated holes); it was found that the mixed phase facilitated better inhibition of recombination. It has been reported that the dominant (001) facets of mesoporous nanosheets of mixed phase TiO₂ and nanorods converted into mesoporous nanosheets prominently enhance their light absorption in addition to improving their adsorption and diffusion of reactant molecules.⁴⁴ Table S2† shows a list of different crystal phases/structures of TiO₂ photocatalysts and their hydrogen evolution abilities.

3. Tweaking of surface characteristics

3.1 Porosity

High porosity provides a higher surface-to-volume ratio, which may result in augmentation of the surface reactivity and increase hydrogen production. Parida et al. demonstrated excellent photocatalytic activity of hierarchical and fibrous meso–macroporous N-doped TiO₂ (as shown in Fig. 6), which they attributed to the interior macroporous channels interconnected with mesoporous networks in the catalyst. These provided an easy path for electrons for effectual surface charge transfer in the system. The macrochannels were also found to act as light transfer paths to transport the incident photon flux into the internal surface of TiO₂ and allow photoabsorption, reflection and scattering in the system.⁴⁵

Fig. 6 Meso–macroporous N-TiO₂ calcined at (A and B) 400 °C (C and D) 800 °C. Reprinted with permission from ref. 23. Copyright 2013 Elsevier.

Niphadkar et al. obtained the best results for a mesoporous composite containing 60% TiO₂ with siliceous MCM-41 (MTi60) to show enhanced hydrogen production (0.08805 mmol g⁻¹ h⁻¹) compared to other composite photocatalysts.⁴⁶ The mesostructural feature of siliceous MCM-41 was found to be connected without pore blockages even with high TiO₂ loading. The sheet-like morphology of TiO₂ in MTi60 was reported to be favorable to maximize hydrogen generation. Wang et al. prepared three-dimensionally ordered macroporous (3DOM) N-doped anatase TiO₂ (N-TiO₂) that showed increased activity under visible light. The 3DOM structure demonstrated enhanced pore structure, periodicity, large pore volume and multiple scattering for better visible light harvesting and hydrogen evolution.⁴⁷ Han et al. used graphitic-C₃N₄ nanosheet (g-C₃N₄ NSs) hybridized N-doped TiO₂ nanofibers and demonstrated better visible light absorption due to the enhanced surface area and electron transport ability, resulting in increased photo-conversion and a better rate of hydrogen evolution.⁴⁸ Sreethawong et al. synthesized nano-crystallite TiO₂ with a high surface area of 80 m² g⁻¹ and a total pore volume of 0.12 cm³ g⁻¹; it displayed enhanced hydrogen generation of 437.5 μmol h⁻¹.⁴⁹ The mesoporous crystalline anatase-TiO₂ was prepared by utilizing mesoporous silica KIT-6 as the precursor; interestingly, this led to the production of mesoporous anatase TiO₂ instead of rutile phase, even at a calcination temperature of 750 °C, which was essentially due to the template confinement effect of the mesoporous silica. Similarly, TiO₂ with a surface area of 207 m² g⁻¹ and loading of 0.5% Pt showed enhanced H₂ production of 2000 μmol h⁻¹ g⁻¹, which was found to be around 1.7 times greater than that of Degussa P25 TiO₂.⁵⁰ Table S3† shows various porous-structured TiO₂ photocatalysts and their hydrogen evolution.

3.2 Particle shape

Researchers have demonstrated various morphologies of TiO₂, such as sheets, rods, and hierarchical structures, and achieved increased hydrogen evolution compared to that of spherical particles. Teng et al. obtained sheet-based CuO/TiO₂ composites and compared their hydrogen evolution capacity and stability with those of bare-TiO₂ rods. It was found that the interface of CuO and TiO₂ facilitated stable p–n heterojunctions due to the strong interaction of the facets of the TiO₂ sheets with CuO, which eventually led to decreased recombination of the excited charge carriers.⁵¹

A butterfly wing architecture of TiO₂ was synthesized,^52,53 as shown in Fig. 7, using the wings of the butterfly Papilio nephelus boisduva as a template. Interestingly, this wing architecture of TiO₂ decreased UV light reflection by about 40%. The structure was found to act as an optical trap wherein light was repeatedly reflected and absorbed within the system. As a result, remarkable improvements in the light absorption and harvesting properties were observed, and significantly enhanced water-splitting was achieved. Kiatkittipong et al. prepared sodium titanate and hydrogen titanate tunnel-like structures; they showed enhanced separation of charge carriers compared to TiO₂, which was essentially due to their tunnel-like structures.⁵⁴ Guo et al. prepared mesoporous titanium phosphates and tuned their morphologies from fibrous shapes to near-hexagonal shapes, as shown in Fig. 8, by optimizing the concentration of phosphoric acid. As a result, the presence of abundant hydroxyl groups, 2D layered structures, large specific surface areas and enormous amounts of exchangeable protons resulted in enhanced hydrogen evolution.⁵⁵

Fig. 7 Image of WA-TiO₂: (a and b) WA-TiO₂ FESEM image; (c) TEM image of WA-TiO₂. Reprinted with permission from ref. 63. Copyright 2013 Elsevier.

Fig. 8 TEM image and molecular structure of layered α-titanium phosphate. Reprinted with permission from ref. 65. Copyright 2014 Elsevier.

1D/3D hybrid hierarchical TiO₂ composite (HHC) and 3D hierarchical TiO₂ (3D-HTiO₂) structures were prepared by Haider et al. and produced 661 and 1310 μmol h⁻¹ g⁻¹ of hydrogen, respectively, which are around 1.72 and 3.4 times greater than the amount produced by Cryst-TiO₂-Memb (titania nanotube membrane); this was essentially due to their enhanced specific areas. Similarly, lower and higher concentrations of NaOH (a morphology directing agent) in the reaction led to the formation of self-assembled loosely and densely packed hierarchical spheres, respectively, as shown in Fig. 9;⁵⁶ they showed hydrogen evolution of up to 1996 μmol h⁻¹ g⁻¹, which was 5.18 times greater than that of Cryst-TiO₂-Membrane. Table S4† shows a list of different particle shapes of TiO₂ photocatalysts towards H₂ evolution.

Fig. 9 a) SEM, b), c) TEM, and d) HRTEM images of 3D hierarchical protonated titanate microspheres. Reprinted with permission from ref. 66. Copyright 2014 ACS.

3.3 Core–shell structured titania

Core/shell structures are generally biphasic materials that consist of an inner core and an outer shell material, as depicted in Fig. 10. These core/shell structures include a wide range of combinations of organic and inorganic materials.⁵⁷ The ‘core’ architecture supports the ‘shell’ material in terms of surface area, charge and porosity, where synergistic interactions between the core and shell materials enhances electron transfer and trapping in the system and increases its efficiency.⁵⁸ Im et al. reported that a CuS–TiO₂ core/shell photocatalyst liberated 10-fold more H₂ than CuS; this was mainly because of the slower charge carrier recombination rates due to the core/shell architecture. TiO₂ shells covered the core CuS completely, and the photoexcited electrons from the CB of TiO₂ moved to the valence band of CuS because its reduction potential is higher than that of TiO₂; also, the core CuS acted as a good electron collector.⁵⁹

Fig. 10 Schematic of the core–shell structure.⁵⁶

Fang et al. prepared Au@TiO₂ core–shell structures, as shown in Fig. 11, and decorated them with CdS nanoparticles to demonstrate the absorption of longer wavelength light.⁶⁰ This unique ternary design displayed improved photocatalytic activity, which was attributed to the facile electron transfer path from CdS to Au via TiO₂ (CdS → TiO₂ → Au); it eliminated the need for a co-catalyst by acting as an interior active catalyst for H⁺ reduction for H₂ generation.⁶¹

Fig. 11 Schematic of H₂ generation in Au@TiO₂–CdS nanostructures. Reprinted with permission from ref. 48. Copyright 2013 ACS.

Yang et al. synthesized core–shell structured rutile TiO₂-sulfide nanoparticles. It was found that the formation of Ti³⁺ and sulphide enhanced the visible light absorption (from 3.0 eV to 1.7 eV) of the system.⁶² The TiO₂-coated iron-oxide core shell particles showed high photocatalytic activity, with release of 2700 μmol h⁻¹ of H₂ with a quantum yield of 3.86% at 550 nm due to efficient charge transfer from TiO₂ to Fe₂O₃ and the suppression of e⁻/h⁺ pair recombination.^63,64 Liu et al. synthesized TiO₂ microspheres containing B dopant,⁶⁵ and they created a gradient by migrating boron from the core to the shell of anatase TiO₂ to modulate the hydrogen and oxygen generation reactions. As a result, 4 times higher H₂ generation was observed in the boron-free shell (50 μmol h⁻¹) than in the shell with boron because of the stronger reducing power of the boron-free shell, whereas the holes in the latter showed stronger oxidative power than those in the former, as depicted in Fig. 12. This is essentially because extra electrons caused downward band bending in the system;⁶⁶ also, the Ti³⁺ formed by inducing interstitial boron into TiO₂ contributed electrons to the neighboring Ti⁴⁺ ions and enhanced the photocatalytic properties of the system.^67,68

Fig. 12 Boron-free and boron-rich shells of TiO₂. Reprinted with permission from ref. 46. Copyright 2012 Wiley.

Kim et al. prepared core–shell structured NiFe₂O–TiO₂, as shown in Fig. 13, that exhibited remarkably higher H₂ evolution than pristine TiO₂. Upon UV-light irradiation of the system, the photoexcited electrons from the CB of the TiO₂ shell migrated to the surface of the shell and were finally injected into the CB of the NiFe₂O₄ magnetic core. Fe ions in NiFe₂O₄ were found to be reduced after the photoreaction, which indicates that the core attracted electrons during the photocatalytic reaction and enhanced the hydrogen evolution efficiency.⁶⁹ Lin et al. reported that a highly porous, three-dimensional well-connected thorough-pore structured TiO₂ aerogel was a superior material for photocatalytic hydrogen generation (328 μmol g⁻¹ h⁻¹); it showed about a 9.63 times higher rate than commercial P25-TiO₂ nanoparticles. The small arc-radius, highly porous, three-dimensional, well-connected thorough-pore structure of the aerogel provided highly effective charge carrier separation and transfer in the system.⁷⁰ Further, Parayil et al. synthesized a TiO₂ aerogel that produced 390 μmol h⁻¹ g⁻¹ of H₂ in 4 h of irradiation. The enhancement was attributed to the decreased particle sizes, large pore diameters, high surface area and low density of the TiO₂ aerogel foam.^71,72

Fig. 13 Core–shell structure of NiFe₂O₄@TiO₂. Reprinted with permission from ref. 46. Copyright 2014 Elsevier.

4. Tailoring of energy structure

It is known that the conduction band (CB) of TiO₂ is composed of an empty orbital of the metal cation with a d⁰ configuration. The crystal structure of titania and the bonding character between the metal and oxygen strongly influence its valence band (VB) because it consists of an O 2p orbital and is placed at 3.0 eV.^73,74 Therefore, doping of titania with different elements is a well-recognized method to modify its crystal structure, and in most cases, the host ion will be replaced by the dopant.⁷⁵ The transition metal or non-metal cation dopant has a partially filled d orbital, so a donor or acceptor level will be formed in its forbidden energy gap; this essentially tends to decrease the band gap energy of TiO₂ and enables visible light absorption.⁷⁶ Therefore, it is crucial to have a new electron donor level (DL) other than O 2p to lower the CB, through which the band gap energy of TiO₂ can be decreased.^77–80 The process of doping in TiO₂ essentially induces impurity levels in the semiconductor and modulates its electrical and optical properties,^81,82 thereby increasing the charge carrier separation and efficiency to absorb 43% of visible light. These doped impurity-like atoms function as either acceptors or donors to the TiO₂. The donors have more valence electrons and therefore donate their extra electrons to the CB of TiO₂; this increases the electron carrier concentration in TiO₂ and makes it n-type.⁸³ More importantly, the nearest energy band to the CB and VB is generally referred to as the dopant d-band; on irradiation, it excites electrons from the dopant band to the CB or from the VB to the dopant band and thereby requires less photon energy, which facilitates visible light absorption of the photocatalyst.⁸⁴

4.1 Photoexcited charge carrier trapping

The process of doping may lead to crystal losses by generating off-stoichiometricity through creating vacancies in TiO₂ that can essentially act as traps or recombination centers. Therefore, the concentration of dopants is crucial when using the doping strategy to enhance the absorption range of titania, especially in the visible region. The recombination of the photoexcited e⁻/h⁺ pair must be delayed for a proficient charge transfer process on the photocatalyst surface. Accordingly, charge carrier trapping inhibits recombination and augments the lifetime of the excited e⁻/h⁺ pairs to more than a fraction of nanosecond. Further, the surface states created by the dopants are localized; therefore, the surface trap population depends on the energy dissimilarity between the traps and the bottom of the CB and there is a decrease in entropy when the electrons undergo the trapping process.⁸⁵ Gratzel et al. reported electron paramagnetic resonance (EPR) spectroscopy of illuminated colloidal TiO₂ at 4.2 K and showed the presence of trapped photoexcited electrons (to form Ti³⁺ defect sites) and holes in the bulk TiO₂ semiconductor.⁸⁶ Similarly, picosecond and nanosecond laser photolysis was employed to examine the charge carrier recombination and trapping in ∼6 nm colloidal particles of TiO₂. It was observed that the CB electron trapping was very fast, occurring within 30 ns, and the hole trapping required an average time of 250 ns. It should be noted that recombination of trapped electrons with free or trapped holes occurs in 10⁻¹¹ to 10⁻⁶ s, and the mean lifetime of a single electron–hole at a lower charge carrier concentration is around 10⁻¹⁰ s; meanwhile, at higher concentrations, recombination occurs in a fraction of a nanosecond. Holes are comparatively unreactive towards electrons in the trapped state.⁸⁷ Thus, the carrier trapping should be rapid to achieve successful photochemical conversion. Charge carrier de-trapping is also important because the photocatalytic reaction will only be able to occur if migration of trapped electrons/holes to the surface occurs in the system. It has been realized that surface doping of TiO₂ particles is better for charge transfer because deep-doped species can behave as recombination centers due to the difficulty of electron/hole transfer to the interface. This essentially signifies that it is pertinent to maintain the dopant concentration if TiO₂ must behave as an extrinsic semiconductor; hence, optimum concentration plays a key role. As a result, co-doping and mixed metal oxide doping have been found to overcome the negative effects of doping.⁸⁸

4.2 Metal ion/cation doping

The electronegativity, spatial distribution, chemical state and ionic radius of hetero-atoms are the basic parameters of this method. To date, numerous and encouraging advances have inspired the development of hetero-atom doping in semiconductor photocatalysts towards proficient utilization of visible (solar) light for photocatalytic applications.^65,89–91

4.2.1 Lanthanides as dopants. Doping of lanthanide ions into TiO₂ potentially leads to active materials for photocatalytic reactions. Among the previously studied lanthanides, La, Ce, Pr, Nd, Sm, Er and Gd show promising activity because of their ability to transport charge carriers to the interface. Initially, in 2004, Zhang et al. reported that the exceptional spectral characteristics and electronic configuration of lanthanides can modify the crystal structure of TiO₂,⁹² which was later validated by Zhou et al. in 2006.⁹³ However, Zongyan et al. showed in 2008 that the 4f/5d states of lanthanides considerably contribute to induce impurity energy levels (IELs) and shift the absorption edges to the visible region while maintaining strong redox potentials in the system. The position of the IELs depends on the atomic number, and it moves from the CB to the VB with increasing atomic number of the lanthanide dopant. It has been proposed that a continuum in the VB or CB formation in TiO₂ with the atomic orbital of the dopant atoms is more desirable than discrete energy levels.⁹⁴ Further, it was also found that lanthanide doping can cause significant development in the photoresponse of TiO₂, and it is sensitive to the electronic and atomic configurations and ionic radius of the dopant. Pristine TiO₂ consists of VB O 2p and CB Ti 3d states, and the Ti 3d states can be split into t_2g and e_g states in an octahedral ligand field with O_h symmetry; hence, the CB is divided into upper and lower parts, and these effects lead to band gap narrowing in TiO₂. Properly positioned IELs permit low energy photons to excite the electrons from the VB to the CB; they can also trap and detrap electrons, therefore extending the lifetime of charge carriers.⁹⁵ Additionally, lanthanides tend to fill oxygen into the lattices of TiO₂ and create surface oxygen vacancies for electron harvesting⁹⁶ because the lanthanides have higher bond strengths and affinity for oxygen atoms than the Ti atoms. Kuvarega et al. also proved that doping of Sm, La, Eu, Yb, Nd, etc., is more suitable for TiO₂ to act as a photocatalyst, which also aids the separation of photo-generated excitons and prolongs their lifetime for better photocatalytic performance.^97,98

Peng et al. have particularly studied (amongst lanthanides) the effects of rare earth (RE)-doped (La³⁺, Eu³⁺, Er³⁺, Gd³⁺) Pt/RE/TiO₂–CdS (with 1.0 wt% deposited Pt) systems. They found that photocatalytic hydrogen evolution was enhanced by 62.0%, 40.4%, 34.7% and 30.0%, respectively, compared to bare Pt/TiO₂–CdS. RE ions tend to enter the TiO₂ lattice; this causes crystal lattice stress, escalating the Fermi level and causing a negative shift of the flat band potential in TiO₂. The negative shift in the CB position of TiO₂, as shown in Fig. 14, allows it to attain higher reduction power or increases the driving force for electron transport to the H⁺/H₂O redox level, competently contributing to hydrogen generation. Accordingly, the order of enhancement was found to be La³⁺ > Eu³⁺ > Er³⁺ > Gd³⁺, which was opposite to the order of the shift in flat band potential (Gd³⁺ > Er³⁺ > La³⁺ > Eu³⁺); this indicates the existence of a necessary exchange between the reduction power and electron injection. However, the energy difference in the CB of CdS and the CB of TiO₂ decreases with increasing negative shift, which was found to be a disadvantage in this case.⁹⁹ Yttrium-doped TiO₂ nanoparticles were reported to show high photocatalytic activity due to oxo-bridge formation, which favors the photocatalytic process by re-designing the charge separation and transfer processes.^100,101

Fig. 14 Rare earth material-doped TiO₂ band structure. Reprinted with permission from ref. 73. Copyright 2013 Elsevier.

4.2.2 Cations as co-dopants.
4.2.2.1 Fe and Ni as co-dopants. Enormous numbers of studies have been performed on doping of almost all possible metal ions to date.^102–107 The most extensively studied dopant is iron; since its first study in 1983 as a mono-dopant of TiO₂, it has been proven to exhibit visible light activity, which has been reported to decrease the work function and induce good redox capability and cause oxidization and reduction at the interface. Co-doping is an up-and-coming research area; therefore, we examined the effects of co-doping Fe, Ni and Rh into TiO₂.¹⁰⁸ Fan et al. reported an enhanced visible light hydrogen generation of 515.45 μmol h⁻¹ g⁻¹ when Ag nanoparticles (Ag NPs) were doped into Fe 3d-doped TiO₂ nanoparticles (Fe/TiO₂ NTs) arrays.¹⁰⁹ In addition to obtaining a Fe impurity level (which can maintain and prolong the lifetimes of excited electrons and allow transfer again to the CB of TiO₂), the coating of Ag⁰ metal particles (with strong electron-accepting ability) on the surface acted as electron traps because the Fermi level of Ag is lower than that of TiO₂. This causes the excited electrons to transfer themselves easily to Ag⁰ (and, subsequently, to the 3d level of Fe³⁺), leading to electron hopping and decreased recombination of excitons.

Fe, Ni co-doped¹¹⁰ TiO₂ nanoparticles prepared by Sun et al. using a thermal method showed enhanced charge carrier separation and also extended the absorption of visible light to 514 nm.¹¹¹ The observed visible light absorption indicated that the decreased band gap of TiO₂ can be attributed to the impurity level and decreased charge carrier recombination rates, which resulted from the creation of defects that trapped and detrapped electrons for efficient charge separation and transfer during the photocatalytic process.¹¹² Overall, appropriate co-doping largely facilitates the favorable alignment of energy levels and allows efficient electron transfer, visible light absorption and charge separation for photocatalytic hydrogen evolution. Ni(OH)₂ decorated on TiO₂ nanotubes (TNT) in aqueous glycerol solution resulted in high H₂ generation of 4719 μmol h⁻¹ gcat⁻¹, which is 12 times higher than that of pristine TNT. The improved activity is due to the high electronegativity of the Ni(OH)₂ quantum dots present on the surface of TNT, which facilitates efficient shuttling of photoexcitons, resulting in decreased electron–hole recombination in TiO₂ during photocatalysis.^113,114

4.2.2.2 Multi-cations as co-dopants. It has been found that through co-doping of multiple cations, one can tune the optical, electronic and photocatalytic properties of TiO₂ with enhanced thermo-dynamical stability. However, achieving good crystallinity is an enormous challenge in multi-cation co-doped systems. Allured et al. studied a pyrochlore A₂B₂O₇-type material that was synthesized using a simple fluorite-type structure (AO₂ and its multiples)¹¹⁵ and its photoactive counterpart.^116,117 Upon the introduction of Fe in the Bi₂Ti₂O₇ (BTO) lattice, they observed increased absorption of visible light in the system. Further, density functional theory (DFT) demonstrated that the introduction of Fe enhances the visible light absorption more than other elements in the 3d group^118,119 because the inter-band created with Fe doping is closer to the VB.¹¹⁵ However, the Fe 3d band is probably more prominent with increasing concentration of Fe; it may substitute Bi and Ti atomic sites in BTO, which can lead to electron photoexcitation between the 3d bands of Fe (valence band) and Ti (conduction band) and enhance the hole effective mass due to the increased Fe 3d-band presence.

Multi-metal co-doped titania films were prepared by Li et al. using dopants such as Mn and W. They reported that the entry of Mn and W into the TiO₂ lattice occurs by partial substitution of Ti. Mn and W induce new electronic states above the VB edge of TiO₂ and lead to enhanced solar light absorption. The red shift with increasing concentration of dopants is ascribed to the charge-transfer transition between the d electrons of Mn/W and the CB of titania. Accordingly, their XPS studies revealed the existence of greater oxygen content than the required stoichiometry. This further revealed that this oxygen content consists of interstitial oxygen atoms that are present due to the titanium vacancies which originated from the valence difference in W⁶⁺ and Ti⁴⁺. This allowed efficient detrapping of charge carriers to the surface; as a result, hydrogen evolution up to 900 μmol h⁻¹g⁻¹ (from 300 μmol h⁻¹ g⁻¹) was achieved for the optimal dopant concentration.¹²⁰ Huang et al. reported that Rh, Nb-codoped TiO₂ nanorods (with Ti_0.996Nb_0.002Rh_0.002O₂ as an example) exhibited both absorption of visible light and efficient charge carrier separation. Montoya studied the UV photodeposition of Co, Ni, or Cu onto TiO₂, resulting in visible light absorption with improved H₂ evolution of 8500 μmol h⁻¹ g⁻¹ (7% QY) for Cu (1%) : TiO₂versus 500 μmol h⁻¹ g⁻¹ (0.4% QY) for P25-TiO₂. TiO₂–Ni(OH)₂/CNT/CdS hybrid showed H₂ generation of 12 mmol g⁻¹ h⁻¹, which was attributed to the combined effects of Ni(OH)₂ and CNTs.¹²¹

The effects of Rh co-modification on Sn-grafted TiO₂ (aggregated Sn species were reported¹²² to induce a downshift of the LUMO, decreasing the reducing power of the photogenerated electrons for H₂ evolution, unlike isolated species) were studied by Sheng et al. The photon energy required to photoexcite the isolated oxide is higher than that for the extended oxide of the same metal; therefore, the oxidation and reduction potentials of the holes and electrons are also much higher for these systems.¹²³ Therefore, no obvious change in the absorption was observed, which was also in agreement with theoretical calculations. However, the enhancement of hydrogen evolution from 0.05 mmol to 1.72 mmol was observed. Their XPS findings revealed the presence of Rh⁰ and Rh³⁺ species in the co-modified Sn-grafted TiO₂,¹²⁴ where it was proposed that the isolated Sn species acted as mediators for the electron transfer from TiO₂ to Rh (Fig. 15) and also aided in decreasing the charge recombination, thus enhancing the photocatalytic activity.

Fig. 15 Photocatalytic hydrogen evolution (inset shows a schematic of H₂ evolution) over Rh-loaded Sn/TiO₂. Reprinted with permission from ref. 88. Copyright 2014 Elsevier.

This design was further proposed to have enormous value if it could be extended to other metal ions (to replace Sn) that could also extend the visible light activity of TiO₂.¹²² Accordingly, many combinations of these co-dopants can be found, and we have selectively reported the most recent and promising examples. Extensive studies can also be found on the usage of various metals for co-doping with Pt-doped TiO₂ to enhance its visible light activity; hawse have also reviewed these in a separate section under noble metal modifications of TiO₂.

4.2.2.3 Ti³⁺ as dopant. The visible-light photocatalytic activity of Ti³⁺ self-doped anatase is generally attributed to two main factors.¹²⁵ The presence of Ti³⁺ in TiO₂ generates anion vacancies in the lattice, which induces additional anion vacancies below the CB. Meanwhile, the increment in these vacancies leads to a decreased band gap in TiO₂. Thus, visible light can induce the electron transition from the VB of TiO₂ to the Ti³⁺ states and O_V states.¹²⁶ Crystalline anatase phases of nitrogen-doped TiO₂ (N-TiO₂), boron–nitrogen co-doped TiO₂ (B–N-TiO₂), and undoped TiO₂ (TiO₂) were synthesized by Sayed et al. using a facile xerogel process followed by reduction in a glycol medium; this process forcibly created Ti³⁺ ions in the phases (R-N-TiO₂, R-B–N-TiO₂, and R-TiO₂, where R stands for reduced) and enhanced their visible light activity, as observed in Fig. 6, where the extension of the absorption range appears to reach a maximum in self-doped TiO₂.¹²⁷ Accordingly, they found that self-doped TiO₂ showed hydrogen evolution of 2.5 μmol/50 mg, in contrast with the nil activity of bare TiO₂. As observed in Fig. 16, the extension of the absorption range appears to reach a maximum in self-doped TiO₂.¹²⁷

Fig. 16 Diffuse reflectance spectra of TiO₂, N-TiO₂, B–N-TiO₂ and pristine TiO₂. Reprinted with permission from ref. 91. Copyright 2012 ACS.

Furthermore, the same group worked on combining self-doped TiO₂ with graphene. They synthesized reduced self-doped TiO₂–graphene nanoplatelets (TiO₂R–G) that exhibited a decrease in their band gap energy, resulting in enhanced visible light absorption; this was largely promoted by the Ti³⁺ dopant. It was also found that the enhanced electron transport properties of graphene increased the lifetime of the excited charge carriers.¹²⁸ Further, their findings were well supported by the theoretical calculations performed by Mozos et al.¹²⁹ Both the theoretical and experimental results showed that band gap decrease was possible due to the existence of oxygen vacancies owing to the formation of Ti³⁺. Silva et al. prepared Ti³⁺/Ti²⁺ reduced states in TiO₂ which showed visible light activity, with a solar-to-hydrogen energy conversion efficiency of 0.89% and a quantum yield of 43% at 655 nm.¹³⁰ Kong et al. induced defects such as oxygen vacancies and Ti³⁺ through plasma treatment, which decreased the band gap from 3.13 eV to 2.88 eV.¹³¹ Wang et al. also confirmed enhanced visible light response due to the significant red shift and band gap decrease in these systems.⁴⁴ These observations were mainly attributed to the presence of oxygen vacancies and Ti³⁺; this was also confirmed by EPR signals and Raman spectra. O-Vacancies/carbon layer coated single crystal TiO₂ with absorption at 415 nm showed 69.7 μmol h⁻¹ g⁻¹ H₂ evolution (quantum efficiency ∼2.3%); this enhancement can be attributed to efficient separation of charge carriers by O-vacancies/carbon coating.¹³²

4.2.2.4 Hydrogen-doped TiO₂. Hydrogen doping in titania essentially leads to black coloration of TiO₂, which is called black titania (B-TiO₂). Structurally, B-TiO₂ possesses oxygen vacancies (Ti³⁺),^133–135 surface disorders and surface hydrogenations,^136,137 which play roles in its visible light absorption and photocatalytic activity. The first report on black titania (TiO₂ nanocrystals hydrogenated at 20 bar H₂ atmosphere) by Chen et al. in 2011 successfully demonstrated large solar light absorption (∼30%).⁹¹ Zhou et al. reported a new method using hydrogen plasma to produce black titania (TiO₂@TiO_2−xH_x) that showed ∼83% solar light absorption; this was attributed to localized surface plasmon resonance, similar to Ag.¹³⁸ Elbanna et al. confirmed that reduced or black TiO₂ shows higher photocatalytic activity compared to white TiO₂ due to the presence of Ti³⁺, oxygen vacancies and disorder layers, which improves light absorption.¹³⁹ Further, the treatment was also found to convert surface Ti⁴⁺ (3d⁰) into Ti³⁺ (3d¹) by introducing H doping (TiO_2−xH_x) and/or oxygen vacancies (TiO_2−x) and to amplify the concentration of carriers (electron) in the system. The Raman spectral peak-shifting and broadening of TiO_2−xH_x indicated the breaking of the original symmetry of the TiO₂ lattice, which is essentially due to the disordered surface of B-TiO₂. The decreased intensity of the PL signal revealed the decreased possibility of recombination in the black TiO_2−xH_x system. They further claimed that the H₂ treatment reduced the localized Ti³⁺ states. As a result, the black TiO_2−xH_x sample showed remarkable hydrogen evolution (≈10 mmol h⁻¹ g⁻¹) and is categorized among the best catalysts.

4.3 Anions as co-dopants

The doping of titania using non-metals, such as N, S, P, F and C, was also found to enhance the photocatalytic properties of TiO₂. Among non-metals, carbon, nitrogen and sulfur have been found to be highly promising for hydrogen evolution. For example, the system of C-TiO₂, N-TiO₂ and S-TiO₂ with triethanolamine as a sacrificial agent produced significantly increased hydrogen evolution amounts of 2947 μmol h⁻¹ g⁻¹, 1008 μmol h⁻¹ g⁻¹ and 1269 μmol h⁻¹ g⁻¹, respectively; this is ascribed to their enhanced visible light absorption due to the formation of electronic states above the VB edge of TiO₂.¹⁴⁰ Doping of various single anions into TiO₂ has been greatly studied; herein, we have examined recent and promising strategies such as combining various carbon forms into N-doped TiO₂ and co-doping of anions (S, Ce and P) into TiO₂.

4.3.1 Co-doping of N-TiO₂. In the system of nitrogen-doped TiO₂, the interstitial N or NO_x species intercalate into the TiO₂ structure and largely facilitate photocatalytic hydrogen generation. Further, this activity was found to be dependent on the amount of nitrogen species loaded into the TiO₂.¹⁹ It has been claimed that N doping replaces oxygen atoms in TiO₂ and produces localized energy states above the O 2p VB at 0.73 eV, which considerably reduces the band gap energy of the host titania.¹⁴¹ To account for this, Mountjoy et al. performed XANES analysis; they reported that these impurity states located in the band structure of TiO₂ lead to narrowing of its band gap.¹⁴² In another study, it was observed that the increase in the exposed high surface energy facets upheld the formation energy of oxygen vacancies, which can function as reaction centers to enhance the photocatalytic activity.¹⁴³ Hence, the effects of co-doping in N-doped TiO₂ can be versatile and result in overall positive or negative effects depending upon the doping strategies and dynamics.

4.3.2 P, Ce, S as co-dopants for N-doped TiO₂. Nitrogen doping was also found to have negative effects on TiO₂ by inducing long-range distortion in the TiO₂ crystal structure and maximizing the carrier recombination. Oxygen vacancies in N-TiO₂ act as recombination centers that further decrease the photocatalytic activity.⁸⁵ However, the mechanics of overall hydrogen generation by N-doped TiO₂ have not yet been realized to date.^21,144,145 Interestingly, Zheng et al. found that P–N co-doping into TiO₂ efficiently decreases the long-range defects and recombination induced by N doping.¹⁴⁶ Notably, the N-TiO₂ and P–N-TiO₂ microstructures were systematically examined by TEM, as shown in Fig. 17 and 18. Fig. 17 shows long-range defects in the N-TiO₂ sample, while the features of P–N-TiO₂ include very negligible amounts of local distortions. Appropriate mole ratios of P to N can drastically minimize these long-range defects.¹⁴⁶ Accordingly, the hydrogen and oxygen generation rates for P–N-TiO₂ were found to be 11.2 μmol g⁻¹ h⁻¹, in contrast to zero μmol g⁻¹ h⁻¹ for N-TiO₂. However, it was further found that their photocatalytic hydrogen evolution depended on the concentration of dopants and the reaction temperatures during the synthesis of P–N doped TiO₂. These observations provided new strategies to repair the structural damage in semiconductors that arises from the hybrid-heteroatom-doping process.

Fig. 17 (a–c) N-TiO₂ microstructures: (a) long-range defects, (b) anti-phase boundary in the lattice image and (c) lattice image depicting [110] twins. Reprinted with permission from ref. 121. Copyright 2014 Elsevier.

Fig. 18 (a–d) P–N-TiO₂ microstructures: (a) TEM image of a grain with an inset SAED pattern; (b) lattice image with an inset FFT image; (c) lattice image depicting the inhomogeneous crystal structure with an inset FFT image; and (d) IFFT image from two sets of [111] reflections. Reprinted with permission from ref. 121. Copyright 2014 Elsevier.

Sun et al. reported cerium co-doped N-TiO₂ and showed that Ce doping hinders the growth of crystallites and also prevents the phase transition. Further, the controlled effects of Ce and N largely contributed to the red shift; as a result, the hydrogen generation was found to be best for 0.6% (mole ratio) Ce co-doping. A precise amount of Ce ions was found to act as hole traps, whereas a surplus amount of Ce considerably decreased the light absorption by covering the surface of TiO₂ and decreased the photocatalytic activity by behaving as recombination centers.¹⁴⁷

2D nanosheets of N–S-TiO₂ with exposed (001) facets were synthesized via a hydrothermal process by Xiang et al. using thiourea as a precursor.¹⁴⁹ Their XPS analysis confirmed the incorporation of S and N into TiO₂ through cationic substitution-type S-doping and interstitial N-doping, respectively. The absence of anionic N (O–Ti–N (BE = 397.0 eV)) was observed; this indicated the presence of interstitial nitrogen in the lattice of TiO₂, which generally occurs in O atmosphere, unlike substitutional nitrogen, which is likely to occur in the absence of oxygen.¹⁵⁰ Most of the S species are in the S⁶⁺ state, and doping of anionic sulfur is difficult due to the larger ionic radius of S²⁻ (1.7 Å) than of O²⁻ (1.22 Å) and the higher required activation energy. In contrast, the replacement of Ti⁴⁺ by S⁶⁺ was found to be more favorable through chemical modification. The observed visible light activity was ascribed to the synergetic effect of doping and exposure of (001) facets of the 2D nanosheets, which enhanced the adsorption of water molecules. However, the application of this photocatalyst for hydrogen generation did not appear to be significant. Moreover, the anatase S-doped TiO₂ with a high surface area showed better activity than N-TiO₂;¹⁵¹ the red shift shown in Fig. 19 was mainly responsible for the enhanced visible light activity.¹⁵²

Fig. 19 UV-visible spectra of TiO₂, S-TiO₂ and N-TiO₂. Reprinted with permission from ref. 140. Copyright 2008 Hindawi.

Bai et al. reported the co-doping of C and S into hierarchical meso/nanoporous TiO₂ and achieved enhanced H₂ generation efficiency.¹⁵³ The calcination temperature was found to promote the crystallization of TiO₂ and enhance its light absorption ability. Co-doping of C and S on TiO₂ were also found to accelerate the electron transfer and create active sites to trap charge carriers. Further, the hierarchical meso/nanoporous structure increased the light absorption and reflection, which increased the accessibility for the reactants to reach their desired reaction sites.

4.3.3 Carbon as co-dopant for N-doped TiO₂. It is observed from literature reports that similar to N-doping, C-doping can also involve either substitution^{148,154–156} or interstitial^157,158 species in TiO₂ systems. Chen et al. prepared C–N-TiO₂ through a sol–gel method and observed enhanced activity. Carbon doping tends to form layers of a complex mixture of active carbon and carbonate species, which retards the crystal growth and enhances the light absorption of TiO₂. The carbonaceous species act as a photosensitizer and enhance the electron transport rate. Nitrogen doping induces visible light absorption and shifts the flat band potential to a higher level, which eventually enhances the driving force for electron transport from the carbonaceous species and accelerates interfacial electron transport. Many groups have explored different methods of preparing these co-doped species; however, their application in hydrogen evolution did not yield satisfactory results.¹⁵⁹

Liu et al. reported the codoping of carbon and nitrogen into mesoporous TiO₂; a high surface area of 100 m² g⁻¹ from 54 m² g⁻¹ was achieved. The mesoporous structure of TiO₂ linked by carbon species enabled uniform and enhanced light distribution inside the pore channels and increased the light absorption. Further, it was found that the synthesized photocatalyst was 37 times more efficient than commercial P25-TiO₂. It was proposed that moderate C, N-codoping of 0.75% was favorable for the photocatalytic evolution of H₂ (81.8 μmol g⁻¹ h⁻¹).¹⁶⁰ Jia et al. synthesized carbon-coated N-doped TiO₂ nanotube films; they showed increased carrier mobility and lower recombination rates because of the good electronic properties of graphitized carbon, which acted as a photosensitizer for the TiO₂ system.¹⁶¹

4.3.4 Graphene with N-doped TiO₂. It is known that the graphene is a 2D carbon nanomaterial with a zero band gap that possesses excellent electrical conduction in two dimensions.¹⁶² It was found that incorporation of graphene in TiO₂ increases the photocatalytic activity and H₂ evolution^163–166 because of its excellent electron transport nature and suppression of the recombination possibilities in TiO₂.^167,168 Graphene oxide (GO) with N-doped TiO₂ showed higher hydrogen evolution of 112.0 μmol h⁻¹ g⁻¹ under a xenon lamp, which was about 13.6 times higher than that of P25 photocatalyst. This was ascribed to the synergetic effects of N-doping and GO incorporation, which enabled fast electron transport; also, the electron collection by GO inhibited the recombination of charge carriers.¹⁶⁹ However, the oxygen-containing groups on the rGO sheets acted as recombination centres, which affected the PC activity. To overcome this and to obtain good interfacial contact, Mou et al. deposited TiO₂ onto N-doped graphene (NGR) and achieved enhanced visible light photocatalytic hydrogen generation.¹⁷⁰ The rGO/g-C₃N₄-TiO₂ ternary hybrid layered photocatalyst showed an increased H₂ yield of 23 143 μmol g⁻¹ h⁻¹, which was attributed to the visible light activity and decreased recombination of charge carriers by rGO.¹⁷¹ The doped nitrogen provided positive sites for the incorporation of TiO₂ nanocrystals on the NGR sheets and created close interfacial contacts between the TiO₂ nanoparticles and NGR. Further, NGR was found to have superior electrical conductivity compared to that of reduced graphene oxide (rGO), which was attributed to the recovery of the sp² graphite network and decreased defects,¹⁷² leading to effectual charge separation and charge transfer in NGR/TiO₂. The strong coupling between NGR and TiO₂ increased the interfacial connection of the two moieties for better electron separation and transfer, as shown in Fig. 20.

Fig. 20 Schematic of the increased charge transfer in TiO₂ and NGR sheets. Reprinted with permission from ref. 144. Copyright 2014 ACS.

4.4 Effect of both cations and anions as dopants

4.4.1 Ga, In and La as Co-dopants for N-doped TiO₂. Co-doping of TiO₂ with both cationic and anionic species appropriately results in a significant extension of the absorption range of TiO₂ into the visible region. Li et al. used Ga and N as co-dopants to passivate the carrier recombination centers (inside TiO₂) generated by mono-doping.^173–175 Similarly, they enhanced the doping concentrations by the columbic attraction between the oppositely charged dopants.^176–178 In these systems, in addition to the introduction of oxygen vacancies, the new N 2p band of the N dopant formed over the O 2p states, where the N impurities were found to associate with Ti³⁺ and decrease the band gap energy of N-doped TiO₂. Gallium was introduced as Ga³⁺ to enhance the oxygen vacancies introduced by the hole doping of N, which was found to be p-type doping.^{90,174,175,179–187} Therefore, the TiO₂ co-doping led to synergetic band gap realignment and good response to visible light for efficient hydrogen evolution.

Sasikala et al. compared the effects of In-N co-doped-TiO₂ to those of In or N-doped TiO₂. The In-N co-doped TiO₂ showed decreased band gap energy, which was attributed to the addition of the 5s, 5p orbitals of In to the CB and N 2p orbitals in the VB. As a result, the hydrogen evolution was found to increase to 14 μmol/12 h for the co-doped sample and 07 μmol/12 h for the mono-doped TiO₂ sample.¹⁸⁸ Fanke Meng et al. reported the co-doping of nitrogen into La₂Ti₂O₇ (LTO) nanosheets synthesized by a hydrothermal process; they showed visible-light absorption up to 560 nm and enhanced solar light-mediated photocatalytic hydrogen generation.¹⁸⁹ Table S5 in the ESI† presents a list of recently reported doped photocatalysts for hydrogen production.

5. Composite systems

5.1 Carbon-based TiO₂ composites

Parayil et al. synthesized TiO₂-carbon hybrids using 2,3-dihydroxynaphthalene and TiO₂ by a ligand to-metal charge transfer complexation process. It was observed that the carbonization under N₂ pyrolysis modified the physical properties of TiO₂ by increasing the surface area from 6.5 m² g⁻¹ to 12.5 m² g⁻¹ and resulted in fluorescence quenching of the TiO₂ emission. A noticeable improvement in hydrogen evolution of 2.6 μmol g⁻¹ h⁻¹ was observed; this was attributed to the C on the TiO₂ surface, which efficiently transported electrons¹⁹⁰ and improved the photocatalytic proton reduction to produce hydrogen.¹⁹¹ Further, it was reported that carbon-modified TiO₂ composite materials showed an improvement from no H₂ evolution to 0.21 mL g⁻¹ h⁻¹ with a carbon loading of 3.62%. Parayil et al. suggested that the interaction of carbon with rutile TiO₂ facilitated the superficial trapping of electrons in the oxygen vacancies and minimized the charge carrier recombination.¹⁹² A similar study reported that decoration of a thin carbon layer (0.3%) on TiO₂ acted as an electron trapping center and thereby enhanced charge carrier separation and achieved four-fold improvement compared to the pristine TiO₂ nanoparticles.¹⁹³ Similarly, brookite TiO₂ (br-TiO₂) integrated with g-C₃N₄ exhibited enhanced photocatalytic activity due to the more negative CB edge position of g-C₃N₄ with respect to that of br-TiO₂. This chemical bonding of the br-TiO₂/g-C₃N₄ hybrid allowed the rapid movement of electrons from the CB of g-C₃N₄ to the CB of br-TiO₂.¹⁹⁴

TiO₂ with carbon fiber (CF) and CuO as a co-catalyst was found to show enhanced activity due to the strong electronic conductivity of CF, which significantly decreased the charge transfer resistance in the material. Higher activity was found in the 1 wt% CuO/CF/TiO₂ system, where CuO enhanced the electron conductivity and largely decreased the recombination possibilities in the system.¹⁹⁵ Similarly, a TiO₂ photocatalyst loaded with 5 wt% CuS–5 wt% NiS as co-catalysts showed high photocatalytic H₂ evolution of 800 μmol h⁻¹ g⁻¹, which was around twenty-eight times higher than that of TiO₂. The dual NiS–CuS co-catalysts rapidly accepted the excited electrons from the conduction band of TiO₂, thereby creating more surface active sites for H₂ evolution to take place.¹⁹⁶

In another study, graphene and Cu were used as co-catalysts to synergistically enhance the photocatalytic activity of TiO₂; the system showed 5 times more hydrogen generation than the pristine graphene. In this system, Cu loading shifted the absorption peak towards longer wavelengths, which is attributed to the formation of new energy levels in the band structure of TiO₂. However, it was found that at increased concentrations, the co-catalysts began to function as recombination centers; hence, the optimum concentration of the cocatalyst was proposed for higher photocatalytic activity.¹⁹⁷

Graphene based-TiO₂ composites. Reduced graphene oxide-TiO₂ hybrids show substantial increments in H₂ evolution due to the enhanced mobility of electrons and decreased recombination. The photogenerated electrons can efficiently migrate from the CB of TiO₂ to graphene because the CB minimum of TiO₂ is higher than the Fermi level of graphene; this facilitates enhanced photocatalytic activity.¹⁹⁸ Cheng et al. claimed that graphene acts as a electron acceptor due to its two-dimensional π-conjugated structure;¹⁹⁹ their nanocomposite showed hydrogen evolution up to 668 μmol h⁻¹ g⁻¹, which was increased by 68% compared to that of P25-TiO₂ (397 μmol h⁻¹ g⁻¹) loaded with 0.5 wt% Pt.

Interestingly, Ag nanoparticles were assembled onto the surface of TiO₂–graphene nanocomposite, as shown in Fig. 21; this system showed increased light absorption and more competent separation of charge carriers compared to the bare TiO₂–graphene nanocomposite. The enhanced performance was mainly attributed to the surface plasmon resonance of the Ag NPs, which led to enhanced visible light absorption, longer lifetimes of the excited electron–hole pairs and rapid charge transport in the system; this resulted in the improved photoactivity of the nanocomposite.²⁰⁰

Fig. 21 (A) TEM images of TiO₂ and graphene; (B) Ag–TiO₂–graphene. Reprinted with permission from ref. 171. Copyright 2014 Elsevier.

CNT-TiO₂ composites. Multi-walled carbon nanotubes (MWCNTs) show higher surface areas when they are combined with nanostructured metal oxides such as TiO₂ and ZnO; they significantly improve the charge transportation and optical and electrical properties of metal oxides.^201,202 Eventually, the MWCNTs-TiO₂ composite nanostructures create numerous active sites for the photocatalytic reactions. For example, the functionalized MWCNT-TiO₂ nanocomposites showed improved surface electric charge and led to stronger electronic transitions in the composite. Further, the MWCNTs acted as photosensitizers and more actively absorbed visible light and effectively transported charge carriers to the surface of TiO₂; this significantly improved the H₂ generation in the system, as shown in Fig. 22, and electron microscopy images of the prepared nanocomposites are shown in Fig. 23. This MWCNTs–TiO₂ nanocomposite showed enhanced hydrogen production of 8 mmol g⁻¹ h⁻¹ due to the synergistic interaction between MWCNTs and TiO₂.²⁰³

Fig. 22 Schematic of the photocatalytic mechanism of MWNT–TiO₂. Reprinted with permission from ref. 174. Copyright 2009 IOP.

Fig. 23 SEM (a) and HRTEM (b and c) images of the MWNT–TiO₂ nanocomposite. Reprinted with permission from ref. 174. Copyright 2009 IOP.

Similarly, Zhang et al. produced a significant amount of hydrogen (37.6 mmol h⁻¹ g⁻¹) and achieved a high apparent quantum efficiency (over 95%) by constructing carbon/TiO₂/carbon nanotube (CTCNT) hybrid structures,²⁰⁴ and similar CTCNT structures showed red shifts in their optical properties;^205,206 this was attributed to the effects of non-stoichiometricity in TiO₂ due to the formation of oxygen vacancies and Ti³⁺ species. The nanocomposite possessed decreased band gap energy, as shown in Fig. 24, and the smallest gap between the band tail ends was anticipated to be about 0.88 eV.

Fig. 24 DOS of pure TNT and CTCNT-700. Reprinted with permission from ref. 175. Copyright 2014 RSC.

On the other hand, semiconducting SWCNT (s-SWCNT), as shown in Fig. 25, displayed better photocatalytic activity compared to metallic single-walled carbon nanotubes (m-SWCNTs) when combined with a TiO₂ photocatalyst. The higher permissible transitions from the van Hove singularities of s-SWCNTs are realized to be more beneficial for charge transfer in s-SWCNTs compared to SWCNTs. Also, the van Hove singularities were found to act as mid-gap bands and cause the generation of multi-excitons under UV light.²⁰⁷

Fig. 25 HRTEM images of SWCNTs. (a) Pristine SWCNTs synthesized in pure He, C₀ and (b) the pristine SWCNTs synthesized in 25 vol% CO₂, C₂₅. Reprinted with permission from ref. 207. Copyright 2011 Elsevier.

5.2 Metal oxide-based composites

Copper oxide-TiO₂ composites. Due to the high cost of noble metals, researchers are focusing more on non-noble metals, such as Cu and Ni, either as dopants or oxides to form composite materials with enhanced properties.^208–210 In this direction, composites of Cu₂O and CuO (with narrow band gap energies of 1.7 eV and 2.0 eV, respectively) with TiO₂ afforded visible light-active photocatalytic systems for hydrogen evolution.²⁰⁸ Accordingly, the CuO/TiO₂ composite showed enhanced performance towards hydrogen generation under visible light^211–213 because of its significant advantages in photon harvesting effectiveness due to the effective charge carrier separation through interfacial charge transfer.^214–217 It was found that the synthesis methods play important roles in the formation of these nanomaterials. Accordingly, wet chemical methods mostly favor formation of interfacial copper oxide–TiO₂ composites, while techniques such as sputtering create Cu/TiO₂ systems.²¹⁸ However, in all these cases, the absorption spectra display significant red shifts (up to 700 nm) and extended absorption (400 and 800 nm) of visible light due to the presence of Cu species. It should be noted that different Cu species, such as metallic copper [225 to 510 nm], 3p of Cu²⁺ clusters [44 to 500 nm], and CuO excite electrons from the VB to the exciton level [<330 nm], and the d-d transitions of Cu²⁺ [600 to 800 nm] display different optical activities in different regions of the UV-visible window.^219,220

Interestingly, the Cu doping obtained by Wang et al. using RF reactive magnetron sputtering was found to modify the orientation of TiO₂ from 101 to 001, where the 001 facets were found to be more active because of their higher surface energy;²¹⁸ water molecules can easily dissociate on the 001 facets, which is beneficial for hydrogen generation. They further proposed that the presence of Cu¹⁺ can trap photoexcited electrons for effective electron–hole separation. Yu et al. reported the preparation of Cu²⁺-doped mesoporous TiO₂ by wet impregnation, where they revealed that the doped Cu²⁺ ions facilitated the creation of oxygen vacancies in TiO₂. Accordingly, Besenbacher's group reported the dissociation of adsorbed H₂O by photon transport into closer oxygen atoms through surface oxygen vacancies in a Cu/TiO₂ system.²²¹ CuS nanoflower/TiO₂ nanospindle heterostructured nanocomposites showed H₂ generation of 1262 μmol h⁻¹ g⁻¹.²²² Copper-supported TiO₂ and zirconium titanate were evaluated for H₂ generation, and the presence of additional metallic copper during H₂ evolution was detected through EPR analysis.²²³ Fluorinated TiO₂ powder with Cu (NO₃)₂·H₂O forms crystalline and mesoporous materials where copper exists in the form of CuO, resulting in four times greater activity than TiO₂ and a fluorinated TiO₂ support.²²⁴ Xu et al. incorporated CuO particles by impregnation/adsorption/calcination methods on TiO₂ nanotubes (TNTs) prepared by a hydrothermal process.²¹⁵ The obtained 1D tubular structures of TiO₂ were found to exhibit superior electron transfer compared to that of Pt/Ni-incorporated TNTs.²²⁵ In addition, the larger surface area of the CuO–TNT nanocomposites provided active sites for reduction of protons to obtain hydrogen evolution of up to 20 mmol h⁻¹ g⁻¹. The calcination method was found to be superior to wet chemical impregnation and adsorption to generate a highly dispersed active CuO–TNTs photocatalyst.

Yu et al. reported hydrogen evolution of 3418 μmol h⁻¹ g⁻¹ with a quantum efficiency of 13.9% in a Cu(OH)₂/TiO₂ system, which was attributed to its enhanced charge separation and transportation, as depicted in Fig. 26.²²⁶ Similarly, Zhang et al. used TiO₂ nanotube arrays (TNAs)²¹⁷ instead of titania particles to form a similar interface and studied its hydrogen evolution properties.

Fig. 26 (a) and (b) Representations of charge carrier separation and transfer in a Cu(OH)₂–TiO₂ system. Reprinted with permission from ref. 191. Copyright 2011 RSC.

The common mechanism^199–226 of water splitting that occurs on these interfaces is depicted in Fig. 27.²¹³ The photoexcited electrons migrate to the CB of CuO through the CB of TiO₂, which is lower than the CB of TiO₂ (−0.26 eV), in contrast with O₂ electron transfer. Subsequently, the transferred electrons cause partial reduction of Cu²⁺ to Cu⁰ and finally form Cu clusters that act as co-catalysts to trap electrons from TiO₂, promote separation, and enable H₂ production. The accumulated electrons on CuO lead to a shift of the CB potential towards a more negative value, which supports inter-particle electron transfer from CuO to the trapped states of TiO₂ and results in Fermi level equilibration. The potential of CuO is −0.224 eV; hence, it can reduce protons efficiently (E⁰H⁺/H₂ = 0 vs. SHE pH = 0), as suggested by Lee et al.²¹³ in 2010 through the decreased PL intensity, indicating greatly decreased recombination possibilities. Rawool et al. studied lab scale optimization of the photocatalyst Cu_0.02Ti_0.98O_2−δ with an apparent quantum efficiency of 7.5%. The relative standard deviation was studied, with higher deviation of the H₂ yield on cloudy days (±3.82%) compared to sunny days (±4.53%).²²⁷

Fig. 27 Mechanism of H₂ generation from TiO₂/CuO. Reprinted with permission from ref. 189. Copyright 2013 Elsevier.

In another case, Xiong et al.²²⁸ reported that Cu₂O produces charge carriers under visible light irradiation and that when it interfaces with titania, the Ti⁴⁺ ions capture electrons and are reduced to Ti³⁺; these Ti³⁺ cations can sufficiently improve photocatalytic activity under visible light, as discussed earlier in this review. It was observed that the concentration of CuO is crucial for these interfacial reactions, and monolayer deposition of CuO was reported to be optimal for improved interfacial reactions with TiO₂. It was reported¹⁷⁶ that increasing the mole ratio of Cu(OH)₂ to TiO₂ from 1 to 7 significantly reduces the hydrogen evolution from 3718 to 87.1; this was attributed to the screening effect of Cu(OH)₂ or electron transfer to the valence band of TiO₂, which induces recombination along with CuO reduction to Cu⁺¹ and Cu⁰. It was also reported that the excess CuO can scavenge electrons²²⁹ to form Cu⁺¹ and Cu⁰, as given in eqn (3) and (4).

Cu²⁺ + e⁻ → Cu⁺,  E₀ = 0.153 eV (3)

Cu²⁺ + 2e⁻ → Cu,  E₀ = 0.341 eV (4)

ZnO–TiO₂ composites. ZnO–TiO₂ composites are being used for hydrogen generation; they also show enhanced hydrogen generation abilities under UV and visible light irradiation.^230–232 Guo et al.²³⁰ reported a ZnO–TiO₂ microporous composite with a crystalline/amorphous heterostructure (with a large surface area of 311.9 m² g⁻¹) and a tunable atomic arrangement, which led to facile adsorption and rapid surface reactions in the system. Additionally, the different CB positions of ZnO and TiO₂ were found to drive the charge carriers from ZnO to TiO₂. This was also verified by Wang et al.,¹⁸³ who explained the charge transport and separation as shown in Fig. 28.²³³ The CBs of TiO₂ and ZnO are at −4.15 and −4.21 eV, respectively, with respect to E_vac. On the other hand, the stability of Zn nanorod arrays was improved by intercalation with the (001) facets of titania sheets, and better charge separation and transport were also achieved in this system. Similarly, a mixed oxide of TiO₂–ZnO was prepared by Pérez-Larios et al.; they observed six-fold improved hydrogen evolution (1300 μmol h⁻¹ g⁻¹) compared than that of the pristine TiO₂ (190 μmol h⁻¹ g⁻¹).²³¹

Fig. 28 Energy levels of the heterojunction of ZnO–TiO₂. Reprinted with permission from ref. 216. Copyright 2014 RSC.

Barakat et al. decreased the band gap energy of titanium oxide nanofibers to 1.89 eV by incorporating Fe₂O₃ and ZnO and observed effective hydrogen generation (an exceptional value of 0.12 ml min⁻¹ mg⁻¹ cat) under visible light radiation.²³² Hematite (Fe₂O₃) possesses a narrow band gap energy (2.2 15 eV);^234,235 however, it shows poor absorptivity,²³⁶ very short exciton lifetimes (∼10 to 12 s),²³⁷ decreased oxygen evolution,²³⁸ and a smaller hole diffusion length (2 to 4 nm),²³⁹ and thus is generally coupled with other metal oxides. Accordingly, ZnO and Fe₂O₃ were dispersed in the matrix of TiO₂ fibers, where it was found that p–d repulsion (e.g., O 2p–Zn 3d) shifted the VB maximum uphill in the semiconductor without altering the CB minimum, which led to a band gap decrease, as shown in Fig. 29.²⁴⁰ The charge separation, transport and enhanced visible light absorption of the system can be understood from Fig. 30(a). Further, the IV characterization, as shown in Fig. 30(b), indicated a linear increase with the applied voltage, indicating high electron transfer of the nanofiber composites.

Fig. 29 TiO₂, ZnO and Fe₂O₃ band gap positions. Reprinted with permission from ref. 203. Copyright 2005 ACS.

Fig. 30 (a) UV-absorption spectra of TiO₂/ZnO/Fe₂O₃; (b) IV characterization of a thin film of TiZn_0.05Fe_0.06O_2.14 nanofibers. Reprinted with permission from ACS ref. 203.

ZrO₂–TiO₂ composites. ZrO₂ has been found to be a potential semiconductor; while its large band gap of 5.0 eV largely limits its application, it can be used in a mixed oxide with TiO₂.¹⁹¹ Accordingly, Hussein et al. synthesized mesoporous ZrO₂ on ZnO/TiO₂ nanocomposites (prepared by a surfactant-assisted sol–gel method) and showed improved H₂ generation. Specifically, it was found that 0.5 wt% ZrO₂ produced 30.78 μmol h⁻¹ g⁻¹, while 3.55 μmol h⁻¹ g⁻¹ was obtained for a ZnO/TiO₂ composite.²⁴¹ They claimed that TiO₂, ZnO, and ZrO₂ possess suitable band structures and satisfy the thermodynamic requirements to overcome the reductive and oxidative potentials for water splitting. As per studies by Navío et al.,²⁴² the electrons in the CB of ZrO₂ are more negative and the holes in the VB are more positive compared to the reduction and oxidation potentials of H₂/H₂O and O₂/H₂O, respectively, resulting in a higher driving force for H₂ production. Further, the observed enhanced H₂ production by the system was attributed to the multiple absorbance thresholds evidenced at 435 nm and 417 nm, corresponding to band gap energies of 2.85 eV and 2.97 eV, together with its high surface area, large pore volumes, uniform crystallite sizes, enhanced light harvesting and charge separation. Surakerk Onsuratooma et al. loaded Ag onto TiO₂–ZrO₂, which showed enhanced photocatalytic H₂ evolution in the presence of UV light. The TiO₂–ZrO₂ (93 : 7) mixed oxide nanocrystal photocatalyst showed enhanced H₂ generation that increased further with 0.5 wt% Ag loading. Hydrogen generation rates of 1.18 cm³ h⁻¹ cat and 0.51 cm³ h⁻¹ cat were obtained for the 0.93 TiO₂–0.07 ZrO₂ mixed and pure TiO₂ oxides, respectively.²⁴³ This study claimed that the surface properties of the ZrO₂–TiO₂ composite are responsible for its enhanced activity rather than their electronic structures.

5.3 Heterojunction formation with TiO₂

Semiconductor junctions of various metals and oxides with TiO₂ promote electron mobility, charge separation and visible light absorption if their energy levels are favorably aligned. Accordingly, Shwetharani et al. reported an Fe₂O₃-coupled TiO₂ system and achieved well-matched band alignment for effective transfer of energy.²⁴⁴ In this study, it was found that the Fe in Fe₂O₃ acted as an electron acceptor due to its potential of 2.4 eV and the Fe³⁺ ions served as shallow traps in TiO₂; this resulted in decreased recombination, and improved H₂ evolution up to 850 μmoles/40 min was thus obtained. It was also proposed that the mid-bands created by dopant Fe³⁺/defects/oxygen vacancies in titania led to alignment of the band edges in the synthesized heterojunctions.

Vinothkumar et al. formed a ternary heterojunctioned titanium dioxide–indium oxide–copper oxide catalyst by co-precipitation; they achieved enhanced hydrogen evolution of 1829 μmol g⁻¹ h⁻¹ for the binary catalyst and 2149 μmol g⁻¹ h⁻¹ for the ternary catalyst.²⁴⁵ The visible light absorption and red shift in the ternary oxide was due to copper ions,^246,247 and the band gap for the ternary oxide was measured to be 2.01 eV. The more negative CB potentials of Cu₂O and CuO (ref. 248 and 249) allowed mobility of electrons from Cu₂O/CuO to In₂O₃ followed by TiO₂, as shown in Fig. 31.

Fig. 31 Band structure of Cu₂O/In₂O₃/TiO₂. Reprinted with permission from ref. 208. Copyright 2014 Elsevier.

Tantalum oxide is completely inactive for hydrogen generation; however, heterojunctions of Ta₂O₅–TiO₂ with mesoporous structures and relatively high surface areas (∼130 m² g⁻¹) showed a much lower bandgap than pristine²⁵⁰ Ta₂O₅ (bandgap ∼4.0 eV) and showed good activity for H₂ evolution.²⁵¹ Wu et al. prepared TiO₂–SiO₂ composite materials via a sol–gel method; this led to the emergence of different amounts of Ti⁴⁺ ions and different textural properties. Among the series of composites, the hydrothermally synthesized system was found to produce the most hydrogen (1.35 mmol h⁻¹ g⁻¹). The observed higher activity of the hydrothermally synthesized system was attributed to (i) the increased number of Ti⁴⁺ ions in tetrahedral coordination, which shifts the CB towards a more negative potential and facilitates effective production of H₂, (ii) decreased particle sizes and (iii) non-radiative energy transfer.²⁵² Further, Rungjaroentawon et al. sensitized a TiO₂–SiO₂ mixed oxide with eosin-y on a mesoporous-assembled structure and observed enhanced activity.²⁵³

Hu et al. reported that a Ni/NiO/N-TiO_2−x heterojunction exhibits improved H₂ production (1210 μmol h⁻¹ g⁻¹) with an apparent quantum yield of 7.5%, which is 250 times higher than that of the pristine TiO₂. The Ni in the system decreased the surface free energy of the nanocomposites, thereby decreasing the thermodynamic driving force for particle growth and leading to the formation of smaller particles. NH₃ plasma treatment was found to reduce partial NiO to metallic Ni, which led to the formation of higher concentrations of oxygen vacancies; thus, improved charge separation was observed. Because the potential of Ni²⁺/Ni (Ni²⁺ + 2e⁻ = Ni, −0.23 V) is lower than the CB (−0.26 V) of TiO₂ and higher than the reduction potential of H⁺/H₂ (2H⁺ + 2e⁻ = H₂, 0.00 V),²⁴ the photoinduced electrons in the CB of TiO₂ transferred to the Ni/NiO clusters; thus, metallic Ni acted as a cocatalyst. Furthermore, the valence band electrons of the R-Ni(x)N(y)Ti series catalysts can be excited to the oxygen vacancies; the electrons are then transported from the oxygen vacancies to the CB by absorbing other photons and finally to the Ni/NiO clusters, leading to more effective visible light utilization.²⁵⁴ The Bi₂O₃–TiO₂ composite photocatalysts showed an H₂ evolution rate of 920 μmol h⁻¹ g⁻¹, exceeding that of pure TiO₂ by 73 times, with an apparent quantum efficiency (QE) of 3.7%. The improved activity is due to the shift of the CB towards more negative potential, which is attributed to energy level splitting.²⁵⁵

Yan et al. reported the formation of p–n heterojunction Pt–Sr(Zr_1−xY_x)O_3−δ–TiO₂, which showed increased photocatalytic activity of H₂ evolution. When the composite was irradiated, effective charge separation and transportation occurred due to the heterojunction of the p-type Sr(Zr_1−xY_x)O_3−δ and n-type TiO₂ semiconductors and due to the presence of Pt in the system.²⁵⁶ Li et al. formed composites of SiC (1 wt%)–NiO_x/TiO₂ (three-component) and four-component IrO₂/SiC (1 wt%)–NiO_x/TiO₂ materials that evolved 30% and 100% more H₂, respectively, than the NiO_x/TiO₂ catalysts. NiO_x/TiO₂ was not found to be active under visible light irradiation, while SiC (1 wt%)–NiO_x/TiO₂ was active because of the narrow bandgap energy of SiC. NiO_x acted as electron collectors and as active sites for hydrogen evolution, as shown in Fig. 32.²⁵⁷

Fig. 32 Schematic of band energy alignment in IrO₂/SiC-NiO_x/TiO₂. Reprinted with permission from ref. 218. Copyright 2013 Elsevier.

Fan et al. synthesized mesoporous TiO₂-pillared titanoniobate (TiO₂/HTiNbO₅) with a surface area of 171 m² g⁻¹; it exhibited enhanced hydrogen evolution of 219 μmol h⁻¹, which was about 12 times that of HTiNbO₅ (protonic titanoniobate) (18 μmol h⁻¹ g⁻¹) and 24 times that of TiO₂ (9 μmol h⁻¹ g⁻¹). The mechanism of decreased recombination in the heterojunction and the band gaps and band edges of this heterojunction are displayed in Fig. 33. It was proposed that band bending by the internal electric field and effective separation of charge carriers resulted in the higher photocatalytic activity. Further, 5 wt% loading of Pt was found to increase the hydrogen evolution to 4735 μmol h⁻¹ g⁻¹.²⁵⁸

Fig. 33 Schematic of photoexcitation in a TiO₂/HTiNbO₅ semiconductor–semiconductor pillared photocatalyst. Reprinted with permission from ref. 219. Copyright 2013 Elsevier.

5.4 Sulfide-TiO₂ composites

Water splitting by pristine sulphide-based semiconductors is not possible because their VBs contain S 2p energy levels; sulphide-based photocatalysts also suffer from photocorrosion during the photocatalytic process. During the reaction, instead of H₂O, the S²⁻ in CdS tends be to oxidized by the photogenerated holes with elution of Cd²⁺, as represented in CdS + 2h⁺ → Cd²⁺ + S,^208,209 which is the major disadvantage of metal sulfide semiconductors. Therefore, sacrificial reagents (such as Na₂S and Na₂SO₃) are required to attain photocatalytic hydrogen generation, which is not an ideal solution for enhancement of stability in sulphides. In this context, the incorporation of metal sulfides into titania has been studied extensively, which has resulted in enhanced photocatalytic activities and efficient resistance to photocorrosion. Consequently, metal sulphides such as CuS-loaded TiO₂ (ref. 259) and CdS/TiO₂ (ref. 260) have been developed and have achieved enhanced photocatalytic hydrogen evolution properties. Because sulphide semiconductors consist of d and sp orbitals, where the S 3p orbital is more negative than the O 2p orbitals, their CB potential is sufficient to reduce H₂O to H₂.²⁹ For example, Jia et al.³⁰¹ reported that TiO_2−xS_x nanotube-films coupled with ZnS–In₂S₃–Ag₂S solid solution showed improved visible light absorption up to 800 nm; the sulfur doping substitutes O in the TiO₂ lattice, which creates a mid-band because of the S 3p orbital. This leads to a decrease in the band gap energy and shifting of the flat band potential to a higher level; this results in improved electron transfer in the system and, therefore, improved photocatalytic activity.²¹³ CdS-Mn/MoS₂/CdTe/TiO₂ quaternary photocatalyst showed an evolution of 10.165 mL cm⁻² h⁻¹. The higher photocatalytic activity is due to synergetic effects of the CdS–Mn, MoS₂, CdTe and TiO₂ nanophase structures.²⁶¹

5.5 Phosphate-TiO₂ composites

Based on a series of previous reports on phosphate groups,^262,263 Luan et al. functionally modified polyhydric phosphates on the surface of nanocrystalline α-Fe₂O₃ through a wet-chemical process and then combined it with TiO₂. The phosphate groups acted as “electronic bridges” between TiO₂ and α-Fe₂O₃ which were found to be a medium to facilitate charge transportation, as shown in Fig. 34.²⁶⁴ They demonstrated that the lifetimes and separation of excitons in the TiO₂–Fe₂O₃ nanocomposite were greatly enhanced, and hydrogen evolution doubled upon the introduction of phosphate bridges.²⁶⁵

Fig. 34 Charge carrier transfer and separation in phosphate-bridged TiO₂–Fe₂O₃ composite. Reprinted with permission from ref. 264. Copyright 2015 RSC.

Similarly, Xie et al. reported that a phosphate-bridged TiO₂–BiVO₄ nanocomposite showed enhanced photocatalytic activities for water oxidation and H₂ evolution which were found to be about three times higher than those of the un-bridged TiO₂–BiVO₄.²⁶⁶ Table S6 in the ESI† shows a list of TiO₂-based composites for H₂ generation.

6. Co-catalyst/plasmonic metal supports

Metals such as Rh, Pt, Pd, Ag, Cu and Au have been extensively utilized as supporting carrier materials for TiO₂ towards photocatalytic water splitting. Kudo et al. reported that if the necessary active sites are not present on the surface for redox reactions, the probability of electron–hole recombination is strong.^267–271 Therefore, the introduction of a co-catalyst largely creates and/or facilitates active sites on the TiO₂. As a result, due to the CB level of TiO₂ and the work functions of these metals, back reactions are largely suppressed and hydrogen generation occurs efficiently, as shown in Fig. 35.²²³

Fig. 35 Mechanism of photosensitization of TiO₂ with Au nanoparticles. Reprinted with permission from ref. 223. Copyright 2013 RSC.

6.1 Pt-Supported TiO₂

Honda et al. demonstrated in 1983 that Pt can serve as an active site for hydrogen production on Pt–TiO₂.²⁷² In these systems, the accumulation of electrons in the metal particles shifts the Fermi level closer to the CB level of TiO₂, resulting in higher negative energy levels. It was found that smaller noble metal particles display more negative shifts in the Fermi level. In general, noble metals possess lower Fermi levels compared to titania; therefore, electron transport is greatly facilitated from titania to the noble metal. This process subsequently adsorbs protons on the surface and reduces them to hydrogen, as depicted in Fig. 36.^225,273 Because the work function of Pt is highest, the electron trapping ability of Pt is the strongest. Zielińska-Jurek et al. reported the order of Pt, Au and Ag-supported photocatalytic activity; it was found to be Pt > Au > Ag.²⁷⁴ Pt also possesses the lowest activation energy for proton reduction; hence, it has been used as an excellent co-catalyst for a range of photocatalysts. The electron structure of Pt-modified TiO₂ surfaces was mapped^66,275 and the dynamics of charge transfer from Pt to TiO₂ were also studied using UPS measurements.²⁷⁶ Hainer et al. investigated the effects of sacrificial agents on true water splitting; they reported higher H₂ production in the presence of sacrificial agents such as methanol and formic acid. Additionally, they claimed that earth-abundant Cu shows competitive catalytic activity with noble metal ions, such as Pt, Pd and Au.²⁷⁷ Pd and WO₃ supported on bare TiO₂ nanotubes showed a maximum quantum efficiency of 7.7% using an aqueous solution of methanol (50 vol%).²⁷⁸ Pt and CoO_x dual cocatalyst-coated TiO₂ showed hydrogen generation of 275.9 μmol h⁻¹ g⁻¹ compared to 56.5 μmol h⁻¹ g⁻¹ for TiO₂; this was attributed to precise charge carrier separation using Pt for electron separation and CoO_x for hole separation.²⁷⁹

Fig. 36 Transmission electron microscopic images of (a and b) Pt–TiO₂, (c and d) Au–TiO₂ and (e and f) Au/Pt–TiO₂. Reprinted with permission from ref. 232. Copyright 2016 Nature.

It was reported that Pt loading can enhance the separation of photoexcited charge carriers through the formation of Schottky energy barriers.²⁸⁰ Serrano et al. experimented with various amounts of metallic Pt deposition using different impregnation methods and found that the material prepared by the “citrate method”, based on reduction and encapsulation with sodium citrate, provided very good dispersion of Pt onto the TiO₂. The Pt nanoparticles were deposited and reduced preferentially within the pores of m-TiO₂, leading to stronger interactions and better dispersion than the other two explored dispersion routes (wet impregnation and photodeposition). Although it involved a lower concentration of Pt (0.78%) than the other two dispersion routes (2%), the citrate method produced close to twice the amount of hydrogen. Furthermore, the production capacity was also found to be higher when the amount of Pt loading was increased, with the optimal concentration being determined as 2% (w/w). When platinum was deposited either by incipient-wet impregnation or photodeposition, the delay to reach the stationary regime was longer compared to when the photocatalyst was prepared by the citrate method.²⁸¹

Wei et al. reported that Pt-modified TiO₂ microspheres synthesized by an impregnation–reduction process showed remarkable photocatalytic activity by loading an optimal amount (1.2 wt%) of Pt; they exhibited about 125 times higher H₂ production than unmodified TiO₂ microspheres.²⁸² Fang et al. reported that Pt rapidly traps the photoexcited electrons and reduces H⁺ to generate H₂, whereas the holes can be scavenged by electron donors, namely sacrificial agents; this results in effective inhibition of recombination of charge carriers and increased photocatalytic hydrogen production. In addition, it was found that the charge recombination was suppressed depending upon on the concentration of Pt onto the TiO₂.²⁸³ However, higher metal dispersion led to screening effects, which significantly decreased the photocatalytic activity due to the limited access of TiO₂ to light. Nguyen et al. reported that TiO₂ nanotubes with a mesh of TiO₂ nanofibrils along with Pt showed enhanced hydrogen evolution compared to the conventional Pt decoration technique. This is essentially because the Pt nanoparticles were able to efficiently trap the conduction band electrons and mediate their transfer to the liquid phase.²⁸⁴ Conversely, Pt modification on rutile TiO₂ showed lower performance in acidic and basic conditions due to photo-corrosion effects.²⁸⁵ Wu et al. formulated a Pt/N-TiO₂ system and observed twice the amount of hydrogen production compared to Pd/N-TiO₂,²⁸⁶ mainly due to rapid electron transfer from Pt to TiO₂.

6.2 Au/Ag-Supported TiO₂

It has been demonstrated that the electrons transferred from the CB of TiO₂ can locate themselves on Au NPs; this creates a Schottky barrier, which significantly decreases the charge recombination and leads to stabilization of the charge carriers.^287,288 A variety of Au/TiO₂ systems²⁸⁹ have shown enhanced photocatalytic effects.^241–290 For example, Au/TiO₂ showed enhanced H₂ generation of 32.4 μmol h⁻¹ g⁻¹ under visible light irradiation and displayed improved plasmonic effects. The impinging photons on gold nanoparticles produce a surface plasmon resonance (SPR) effect that largely augments the production of e⁻/h⁺ pairs by enhancing²⁹¹ the visible light absorption and effectively transfers them to the surface of photocatalyst for efficient photocatalytic reactions.²⁹² Tanaka et al.³⁰² compared various metal loadings (Pt, Ag or Cu) onto Au/TiO₂ and pristine TiO₂ photocatalysts. An optimal concentration of 1 wt% Au loading displayed improved H₂ generation due to the unique metal/oxide heterojunctions and dispersion of the gold particles.²⁴³ A hierarchical porous Au-decorated-N-doped TiO₂ photocatalyst showed visible light activity, which was attributed to the synergetic effects of N doping and mesoporosity for increased specific surface area and effective charge transfer, resulting in improved H₂ evolution.²⁹³ TiO₂ photocatalysts with high Au loading and large Au nanoparticle sizes (≈50 nm) produced 5.3 mmol g⁻¹ cat h⁻¹ of H₂ from the gas phase reaction of water and methanol.²⁹⁴

Similarly, Ag nanoparticles²⁹⁵ also exhibit surface plasmon resonance effects. Yang et al. deposited Ag NPs onto a TiO₂ film, as shown in Fig. 37; they observed that the Ag nanoparticles exhibited strong SPR effects, leading to stronger visible light absorption, which facilitated the communal oscillation of conduction/surface electrons and exhibited enhanced H₂ production of 7.2 μmol·cm⁻². This was ascribed to the synergistic effects between surface plasmon resonance and charge transfer in the Ag–TiO₂ system.²⁹⁶

Fig. 37 SEM images of a Ag/TiO₂ nanowire film. (a) Ag/TiO₂ nanowire film and (b) the enlarged image of (a). Reprinted with permission from ref. 245. Copyright 2014 Trans Tech.

Wu et al. deposited Ag NPs on TiO₂ nanotube arrays (TiO₂ NTs), as shown in Fig. 38; they achieved enhanced surface plasmon resonance effects and restrained recombination in the TiO₂ photocatalyst.²⁹⁷ Mixed metal loading or loading of alloys has also been performed, but by very few groups.^298,299 For example, loading of Au–Pt nanoalloy onto TiO₂ was reported by Montes-Navajas et al.; they observed that the system was more proficient in enhancing the activity of TiO₂ than loading of the individual metals onto the TiO₂.³⁰⁰ The observed efficiency was attributed to the added effects of the bimetals, such as more active centers and better electron separation and transfer. Table S7† shows a list of noble metal-loaded TiO₂ photocatalysts.

Fig. 38 Ag–TiO₂ nanotubes. Reprinted with permission from ref. 246. Copyright 2013 springer.

7. Conclusions and future perspectives

TiO₂ is being widely explored for its functional properties; this research is still in progress in various fields. Especially in the field of energy, TiO₂ has been largely studied for applications in photocatalysis, solar cells, super capacitors, batteries, etc. In this direction, TiO₂ is a truly versatile material for potential application in the field of photocatalysis, where it has been demonstrated for use in photocatalytic degradation of pollutants, hydrogen production, fuel conversion, etc. However, the properties of TiO₂ for individual applications remain to be improved to obtain enhanced efficiency. Because energy is a crucial requirement for current and future generations, photocatalytic hydrogen production has received significant attention in the field of energy research. Accordingly, TiO₂ has been found to be a potential material for photocatalytic hydrogen production applications. However, limitations such as (i) wide band gap energy, (ii) inadequate charge separation efficiency and (iii) limited charge transfer properties have largely hindered the real-time application of TiO₂. In this context, the physical and chemical properties of TiO₂ have been modified, which essentially simulates the energy structures in the material to improve its properties. Accordingly, this report reviewed physically and chemically modified TiO₂ through processes such as defect creation, doping, compositing and metal loading onto TiO₂ materials. These modifications essentially reveal that TiO₂ should possess the following properties to achieve enhanced photocatalytic hydrogen production: (i) increased crystalline nature, (ii) defect structures within the threshold limit, (iii) more negative CB potential, (iv) a support for hole transportation, (v) narrow band gap energy to absorb visible light or equipping with plasmonic materials, and (vi) a dispersed CB for effective diffusion of electrons to the surface. Further, it is also revealed that these modifications in TiO₂ substantially alter its energy structures and equip it with features and functions that increase its suitability for photocatalytic hydrogen production applications.

Future developments in TiO₂ will involve customized designs to achieve higher quantum yields and enhanced solar-to-hydrogen conversion efficiencies. Especially, materials such as black-TiO₂ are being intrinsically modified to absorb visible light. Similarly, TiO₂ can also be intrinsically modified, such as red-TiO₂, blue-TiO₂ and yellow-TiO₂, to specifically absorb certain visible wavelengths; these species have also demonstrated good photocatalytic activities. Therefore, these substantial developments and techniques for the customization of TiO₂ show promise for their real-time application towards solar light-driven photocatalytic hydrogen production for sustainable energy and a clean environment, which will be important energy requirements in the near future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the financial support from Jain University and DST Nanomission (SR/NM/NS-20/2014).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8cy01395k

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