A review on H2 production through photocatalytic reactions using TiO2

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A review on H2 production through photocatalytic reactions using TiO2/ TiO2-assisted catalysts Article  in  Fuel · May 2018 DOI: 10.1016/j.fuel.2018.02.068

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Indian Institute of Technology (ISM) Dhanbad

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Fuel 220 (2018) 607–620

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Review article

A review on H2 production through photocatalytic reactions using TiO2/ TiO2-assisted catalysts Rohini Singh, Suman Dutta

T

⁎

Department of Chemical Engineering, Indian Institute of Technology (ISM), Dhanbad 826004, India

A R T I C L E I N F O

A B S T R A C T

Keywords: Hydrogen generation TiO2 nanomaterials Semiconductor photocatalysts Sol-gel method Dye sensitization Hydrogen fuel

Photocatalytic water-splitting technology by using nano-sized TiO2 can produce low cost and environmentfriendly hydrogen using renewable resource such as solar energy, which can fulfil the future requirements of energy. Nano-sized TiO2 is highly efficient as a semiconductor photocatalyst possessing high surface area. The significant drawbacks of utilising TiO2 as photocatalysts are reduced absorption capacity of visible radiation and fast recombination of photoexcited electron/hole (e−/h+) pair. Its activity is restricted to UV light, which is only ∼3–5% of the solar spectrum. This present review elucidates various aspects and the recent researches related to TiO2 nano photocatalysis for the effective solar hydrogen generation via photocatalytic water splitting technology.

1. Introduction The increased growth in population and industrial development has tremendously increased the generation of waste products and consumption of energy worldwide. This situation creates the need for clean and sustainable alternative source of energy. Since the late 1960s, A. Fujishima [1] have been involved in unfolding the fascinating characteristics of titanium dioxide (TiO2) as semiconductor photocatalyst. Hydrogen is considered as an ideal energy carrier for the future having a high energy capacity. It is an environmental friendly fuel since, it never produces any air pollutants or greenhouse gases and thus reduces our concern from global warming issue. It is considered as the cleanest source of energy for transportation [2–4]. Semiconductor photocatalysed water splitting under sunlight is an effective way to generate environmental friendly hydrogen. Research in the area of solar hydrogen generation started from 1972 by the well-known Honda-Fujishima effect. Hydrogen was produced by photoelectrochemical (PEC) water splitting using TiO2 as photoanode and Pt as cathode. Whilst light is irradiated with energy larger than the bandgap of TiO2, electron/hole pairs are formed in the conduction band (CB) and valence band (VB), respectively. The electrons (e−) then migrate towards Pt cathode on the application of an anodic potential to an external circuit. Therefore, electrons (e−) participates in the reduction reaction and produces H2 whereas, holes (h+) carry out oxidation reaction and produces O2. The overall reaction mechanism is illustrated in Fig. 1. Innumerable photoelectrochemical cells have been designed by the researchers across the globe for the effective utilization of solar energy. However, ⁎

Corresponding author. E-mail address: [email protected] (S. Dutta).

https://doi.org/10.1016/j.fuel.2018.02.068 Received 9 July 2017; Received in revised form 17 January 2018; Accepted 8 February 2018 0016-2361/ © 2018 Elsevier Ltd. All rights reserved.

development of suitable photoelectrode with high stability and thermodynamic properties such as band gap energy is a great challenge [5] (See Fig. 2). Solar hydrogen generation methods can be broadly classified as- (1) photoelectrochemical (PEC) water splitting and (2) photocatalytic or photochemical water splitting [6]. Tremendous research has been done to achieve photocatalytic water splitting to generate hydrogen considering sunlight as the most powerful source of energy that delivers the power of ∼1.2 × 1017 W, continuously [7]. In recent researches, sincere efforts have been made to combine heterogeneous catalysis and solar energy harvesting technologies. Heterogeneous photocatalysis efficiently utilize the ultraviolet (UV) radiation from the solar spectrum as renewable source for the execution of the low cost photocatalytic reactions [8]. In order to solve the energy and environment related issues, researchers across the globe have focused on finding methods to produce H2 from water in the presence of sunlight, such as solar cells assisted water electrolysis, and photocatalytic water splitting. Badawy et al. [9] synthesised mesoporous simonkolleite–TiO2 nanostructured composite for simultaneous photocatalytic hydrogen production and dye decontamination so that two noble cause i.e. clean energy generation and water treatment could be accomplished via single effort. Photocatalytic water splitting technology has great capability for economical and clean hydrogen generation as it utilises renewable sources of energy such as sunlight and water in the presence of a suitable photocatalyst. Titania doped with W and Mo were developed by Evaporation Induced Self Assembly (EISA) that are active at the visible light range [10]. Gupta & Melvin [11]

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R. Singh, S. Dutta

Fig. 1. (A) PEC water splitting using a TiO2 photoanode and Pt cathode, and (B) Photocatalytic water splitting using Pt loaded TiO2.

Fig. 2. Different hydrogen production methods.

and efforts to modify semiconductors for visible light utilization to explore the scope of photocatalysis. It this period industries emerged, and dye-sensitized photocatalysts have been extensively evaluated for the destruction of dyes in waste streams. In the third generation period since 2000 till date, focussed on nanotechnology in photocatalysis for energy conversion and controlling air/water pollution [13]. Different UV active semiconductor photocatalyst have been developed and can be classified into four groups based on their electronic configuration: (1) d0 metal (Ti4+, Zr4+, Nb5+, Ta5+, W6+, andMo6+) oxide photocatalysts, (2) d10 metal (In3+, Ga3+, Ge4+, Sn4+, and Sb5+) oxide photocatalysts, (3) f0 metal (Ce4+) oxide photocatalysts, and (4) a small group of non-oxide photocatalysts [14]. Numerous n-type semiconductor metal sulphides (CdS) and oxides (TiO2, ZnO, CdS, SnO2 and WO3) are regarded as the promising and most efficient materials for the photocatalytic degradation of organic wastes and photo assisted solar hydrogen generation on the basis of their band positions w.r.t. the normal hydrogen electrode (NHE). However, most of the previously mentioned catalyst have some drawbacks such as (a) wider band gap causing inability to utilize visible radiations of the solar spectrum (b) lower stability in an aqueous medium leading to the agglomeration and hence reduction of reaction sites, and (c) rapid bulk and surface e−/h+ recombination rates [15]. TiO2 is one of the most effective semiconductor photocatalysts having high chemical stability, photocatalytic activity, non-toxicity and low cost. There are seven polymorphs of TiO2. However, only rutile, anatase, and brookite exist in nature [16]. TiO2 has attracted considerable interest due to its photochemical,

mentioned that TiO2/reduced graphene oxide (TiO2/RGO) or TiO2/ graphene (TiO2/GR) can be considered as the next generation photocatalyst for hydrogen production. Graphene has the capability to trap excited electrons of TiO2 for a longer time due to its high conductivity and surface area. Lucchetti et al. [12] carried out simultaneous hydrogen generation and removal of nitrate via photocatalytic reforming of glycerol over zero-valent nano copper/P25. This paper aims to review the various aspects of TiO2/TiO2 assisted nano photocatalyst such as its properties, characterization techniques, applications, mechanism of solar hydrogen production, different methods of synthesis and modification to enhance the hydrogen production.

2. Development of TiO2 based catalysts for hydrogen generation The catalyst used in photocatalytic reactions are invariably semiconductor materials. The overall progress of realization of different photocatalyst can be reported in three generations. The first generation (1975–1985) is focused primarily on the studies that provided the understanding of the semiconductor/solution interface under irradiation from studies of monocrystalline semiconductor surfaces and the exploration of different types of semiconductors. At the time it was confirmed that polycrystalline materials would be more appropriate for most of the applications and the first generation of photocatalysis work focused on suspensions of fine particles. The second generation (1986–2000) focussed on polycrystalline thin films 608

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R. Singh, S. Dutta

approaches such as crystal phase, electronic structure and textural modification of semiconductor photocatalysts have been investigated to enhance overall activity of the photocatalytic processes [23]. Advanced ion-beam techniques i.e., RF-magnetron sputtering (RF-MS) deposition, ionized cluster beam (ICB) deposition, and metal ion-implantation were found capable of developing visible light active TiO2 photocatalyst [24]. In addition, detailed characterization of the photocatalyst is compulsory in order to confirm the optical, structural, functional, compositional, morphological and surface properties. 3. Properties of TiO2 nanoparticles as photocatalyst Since photocatalysis is a surface phenomenon; therefore, the properties of TiO2 nanoparticles including crystal size, morphology, crystalline phase, specific surface area, pore size and pore volume influence its photocatalytic performance [25,26]. TiO2 has become most widely used photocatalyst due to its high photocatalytic efficiency, enhanced biological and chemical stability, environmental friendliness, nontoxicity, economical and corrosion resistance [27]. The properties of a material significantly depend on the experimental conditions and their method of synthesis [28].

Fig. 3. Statistical distribution of scientific publications focusing on nanomaterials for photocatalytic H2 production [19].

photocatalytic and photovoltaic properties [17]. The researches on the development of noble photocatalysts that can be used in the slurry and immobilized system have great importance. However, immobilized photocatalyst eliminates the necessity of agitation and minimises agglomeration. In addition, the recycling becomes comparatively easy [18]. Numerous research efforts have proved that nano-TiO2 is more effective as a photocatalyst than its bulk counterpart. Compared to other photocatalysts for H2 production, TiO2 has received more attention. Fig. 3 [19] shows the statistical distribution of scientific publications based on nanomaterials for photocatalytic H2 generation. TiO2 semiconducting nanoparticles of less than 10 nm exhibits significant improvement in the photocatalytic performance due to the quantum size effect [19]. Various new and improved physical and chemical properties emerge in the nanoscale particles and they differ with the shape and size variation. The quantum confinement effect controls the electron and hole mobility as the charge carriers in the semiconductor nanomaterials. The transport characteristics of photons are significantly influenced by the size and geometry of the particles. As the size of material decreases, the surface-to-volume ratio and specific surface area increases significantly. The large surface area is always desirable for most of the TiO2-based applications, which mainly occurs on the surface. The phase transitions in TiO2 under different pressure and temperature is also size dependent. Different methods have been adopted by the researchers to synthesise TiO2 nanoparticles such as sol-gel, vapour deposition, solvo/hydrothermal and mechanochemical methods. The sol-gel route is the most commonly used method due to its capability of synthesizing high purity TiO2 at low processing temperature with stability and versatility of processing [20]. Mohammadi et al. [21] had synthesised TiO2 nanoparticles in a spinning disc reactor (SDR) and investigated wide range of parameters in order to optimise particle yield and concluded that high flow rates, high water/TTIP ratio and high rotational speed had a significant effect in the achievement of the desired results. Abbas et al. [22] explored a process based on the hydrolysis of TiCl4 to synthesise TiO2 nanoparticles and demonstrated that controlled dialysis, storage time and temperature can result into the narrow particle size distribution and stable dispersions. Despite being environmental friendly and cheap, photocatalytic water splitting technology is in the experimental stage because of the low light energy conversion efficiency, which is not economical for the application in the large-scale industry production. Presently, solar to hydrogen conversion efficiency of TiO2 semiconductor photocatalyst is still low, due to the (a) rapid recombination of photo-excited e−/h+ pairs, (b) fast backward reaction of H2 and O2 evolved during the reaction, and (c) inability to harness visible light of the solar spectrum. The recombination (electron/hole and hydrogen/ oxygen) rate of the TiO2 nano photocatalyst must be reduced to enhance the visible light activity and hydrogen generation rate. Different

3.1. Structural and morphological properties TiO2 is wide band gap transitional metal oxide. TiO2 crystallizes in a large number of polymorphs: the high pressure columbite-like, baddeleyite-like, cotunnite-like and fluorite-like structure [29]. Besides these, TiO2 exists in three well-known crystalline polymorphs in order of abundance as Rutile, Anatase and Brookite with band gaps (Eg) of 3.02, 3.2, and 2.96 eV, respectively. However, only anatase and rutile can be synthesised at low temperature in pure form. Many significant researches have been done on the synthesis and photocatalytic application of both anatase and rutile phases whereas, very few studies have been reported regarding the synthesis of brookite phase. It has been examined that it is very difficult to prepare brookite phase with high purity and large surface area leading to its limited application as compared to the anatase and rutile phases [30]. The structure of brookite form of TiO2 was found by Pauling and Sturdivant in 1928 having the highest oxidation potential (−0.46 V) as compared to anatase (−0.45 V) and rutile (−0.37 V) phases [31]. Rutile is considered as the most stable phase of TiO2, whereas anatase and brookite are metastable and can be transformed to rutile phase by providing proper heat treatment. The sol–gel derived TiO2 typically remains in the anatase phase, but brookite is often observed at low temperature in an acidic medium as a by-product [32]. Heat treatment in terms of calcination time and temperature have a major contribution in the synthesis of photocatalysts by significantly influencing their surface area, morphology, crystallinity, porosity, surface hydroxyl groups and phase transformations [33,34]. Literature review reveals that the nanomaterials give better H2 production compared to micromaterials. However, all the nanosized photocatalysts are not suitable for effective hydrogen generation due to their insufficient redox potential required for the same. Various significant crystalline properties of TiO2 are shown in Table 1 Crystalline properties of TiO2 [32].

609

Properties

Rutile

Anatase

Brookite

Crystal structure Lattice constant (Å)

Tetragonal a = 4.5936 c = 2.9587

Tetragonal a = 3.784 c = 9.515

Space group Molecule (cell) Volume/molecule (Å3) Density (g cm−3)

P42/mnm 2 31.216 4.13

I41/amd 2 34.061 3.79

Orthorhombic a = 9.184 b = 5.447 c = 5.154 Pbca 4 32.172 3.99

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Fig. 4. Crystal structures of the (a) anatase (b) brookite and (c) rutile phase of TiO2 (Worldwide web).

4. Mechanism of semiconductor photocatalytic hydrogen generation

Table 1. As mentioned earlier, numerous investigations have proved that TiO2 photocatalyst is more effective as a in the form of nanoparticles than in bulk form. Fig. 4 shows the crystal structure of rutile, anatase and phase of TiO2 (See Fig. 5). TiO2 nanomaterial with different morphologies such as nanowires, nanospheres, nanorods, nanotubes, nanoflowers and nanosheet can be synthesised by various techniques [35]. It has been reported that TiO2 nanotubes arrays (TNAs) have large specific surface area and can be used more effectively as compared to the TiO2 nanoparticles [36]. Table 2 listed TiO2 and its composites with different morphology and photocatalytic performance.

The electronic and optical behaviour of the semiconductor have a prime contribution in the overall mechanism of photocatalytic water splitting. The semiconductor consists of valence band, full of electrons with low-energy, and an empty high-energy conduction band. The energy difference between the two bands is known as the band gap energy (Eg ). The reduction and oxidation reactions that occur on the catalyst surface are responsible for the solar hydrogen generation and photocatalytic water/air treatment, respectively. Sufficient energy is needed to break the H–O bond to decompose H2O. For H2 production to take place, the CB bottom edge must be more negative than the reduction potential of H+ to H2 (EH+/H2 = 0 V vs. NHE at pH = 0). While for O2 formation from water, the VB top-edge should be more positive than the oxidation potential of H2O to O2 (EO2/H2O = 1.23 V vs. NHE at pH = 0) [38]. In general, two parameters affect the photoactivity of a catalyst under visible light irradiation-(a) absorption performance and (b) separation capability of photogenerated e−/h+ pair [39]. Fig. 6 illustrates the principle of photocatalytic water splitting reaction. The fundamental mechanism of photocatalytic hydrogen evolution can be described in four different steps namely adsorption, photoexcitation, charge diffusion, and reduction/oxidation reaction.

3.2. Optical and electronic properties The electronic arrangement, light absorption capability, and charge transport phenomena have a major role in the application of TiO2 as a photocatalyst. The valence band (VB) potential must be more positive than the oxidation potential of water (1.23 V w. r. t. normal hydrogen electrode (NHE)) and conduction band (CB) potential must be more negative than the reduction potential of water (0.0 V w. r. t. NHE) for the occurrence of the photocatalytic reactions [37].

4.1. Adsorption Adsorption capacity of the photocatalysts is one of most significant factor that influences the overall hydrogen evolution via photocatalytic water splitting. Initially, the water molecules are adsorbed on the surface of the selected photocatalyst as shown in Eq. (1). TiO2

H2 ⟶OH2 O(ads)

(1)

4.2. Photoexcitation Prior to irradiation, both electrons and holes in a semiconductor resides in the valence band. As soon as, the catalyst is exposed to the radiation of greater or equal energy than the band gap (Eg) of semiconductors, the electrons get transited from the low energy band i.e. valence band (VB) to the high energy conduction band (CB) leaving holes behind in the VB. For each photoexcited electron in the conduction band, a hole is created in the valence band forming electron/hole pairs as shown in Eq. (2).

hυ → e−CB + h+VB

Fig. 5. Schematic presentation of the different steps involved in a photocatalytic process.

610

(2)

Commercial TiO2 Sol-gel

Commercial TiO2

Solid state reaction and proton exchange Electrospinning

nanoparticle

nanoparticle

nanoparticle

Nanosheets

611

TiO2-ZnO (0.1 g/200 mL) TiO2,TiO2/ZnO, TiO2/ZnO/ CuO CuO/TiO2 (1g/l)

Cu & Ni co-modified TiO2 (80 mg/80 mL) Cu/S-TiO2 (0.2 g/300 mL) Cu/TiO2 (0.1 g/100 mL) N-doped TiO2 CuO/CF/TiO2 (CF:Carbon fibre) (50 mg/50 mL) CuO/TiO2 (0.1 g/10 mL) TiO2/CuO (100 mg/100 mL) CuO/TiO2 (6.5 mg/100 mL) CuOx/TiO2

Ag deposited and Fe3+ ions doped anatase TiO2 (0.4 g/600 mL) Pt/TiO2 (1 g/L) Ag-Ce/TiO2

electrochemical anodization

Nanotube arrays

electrochemical anodic oxidation of pure titanium Commercial TiO2

nanotubes

Sol-gel

hydrothermal Commercial Commercial TiO2 Commercial TiO2 – Electrospinning

nanorod

nanoparticle

nanoparticle

nanoparticle

nanoparticle

Nanofibers, nanorods, nanoparticle nanoparticle Commercial TiO2

impregnation

Metallic plate of Ti Commercial TiO2

Hydrothermal, photodeposition Wet impregnation

ethanol

Complex precipitation

methanol

methanol

ethanol

methanol

methanol

methanol

methanol ethanol

glycerol

methanol

methanol

ethanol

methanol

ethanol

formaldehyde methanol, glycerol ethanol

methanol

ethanol

methanol

Simple mixing

Impregnation

Sol-gel associated hydrothermal method RF Magnetron sputtering Wet impregnation and calcination

– nanoparticle

nanoparticle

electroless plating method

Solution based synthesis via Titanium isopropylate & thiourea –

microwave-assisted chemical reduction hydrothermal

photodeposition

sputtering Reduction of cupric ions Electrochemical anodic oxidation solvothermal

Electrospinning

Photo-deposition

hydrothermal

methanol

glycerol

Impregnation Impregnation and calcination

ethylene glycol

Remazole Dye methanol

Sacrificial agent

Chemical bath deposition

Sol-gel In-situ photodeposition

Method of modification

nanoparticle

nanoparticle

Sol-gel and Commercial

solvothermal

nanoparticle

nanoparticle

anodization Commercial TiO2 Ti sheet(> 99% purity)

Sol-gel solvothermal

nanoparticle nanosheet

Simonkolleite-TiO2 Rutile TiO2 (10 mg/60 mL) Cu(OH)2/TiO2 (4 cm2/80 mL) Cobalt doped TiO2 (100 mg/50 mL) N/TiO2, PdO & Pt loaded NTiO2, PdO/Pt/N-TiO2 (0.5 g/225 mL) M/TiO2/RGO :M = Au or Pt (50 mg/200 mL) Au/Ti0.91O2 (40 mg/300 mL) TiO2/WO3/Au (0.05 g/70 mL) Au or Pt/TiO2 Cu/TiO2 Ag-Fe/TiO2

Nanofiber, nanoparticle Nanotubes nanoparticle nanotubes

Method of preparation

Morphology

Photocatalyst (Dosage)

Table 2 TiO2 and TiO2 based catalysts with different morphology and photocatalytic H2 generation performance.

400 W high pressure Hg lamp

High pressure Hg pen-lamp (254 nm, 2.2 mW/cm2) 400 W high pressure Hg lamp

300 W Xe lamp

150 W of 2 metal halide light bulbs 500 W Xe lamp (1.5 mW/cm2) UV lamp (100 W, 365 nm)

500 W halogen Lamp (400–650 nm) Xe lamp 300 W Xenon lamp

7.5 mmol/h/g

500 W Xe lamp (5 mW/cm2, 400–500 nm)

18,500 µmol/h/g

0–180 µmol (3 h) TiO2-190 µmol/h TiO2-ZnO-1300 µmol/h 18,24.3,192.2 µmol/min

0–20.3 µmol/h/g

8.23 mmol/h/g

[110]

[109]

[108]

[105]

[104]

[102]

[101]

[92] [100]

[86]

[85]

[84]

[83]

[76]

(continued on next page)

35.37–139.03 µmol/h/cm2

760–4500 µmol/h/m 0–9000 µmol/g (5 h)

2

10,571 µmol (300 min)

0–3200 µmol/cm (6h)

2

13.5 mmol/h/g & 4.62 mmol/h/g

1846 µmol/h(1wt%Pt)

[73]

[69] [70] [72]

0.06 µmol/cm2/h 1–7 µmol/min 0.47–1.35 μmol/(cm2h) 515.45 µmol/h/g

[68]

[67]

[66]

[63]

269.63 µmol/h

6753 µmol/h/g

670 µmol/h

55,544,772,2460 µmol/h/g respectively

[61]

[36]

0–7 µmol/h/cm2 11,021 µmol/h/g

[9] [25]

References

2.1 mmol & 3.3 mmol in 240 min. 22.1 mmol/g/h (5 h)

Hydrogen evolution

400 W medium pressure lamp (365 nm maximum wavelength) Visible light Xenon lamp and a high pressure Hg lamp as UV light source (254) 300 W xenon lamp (λ < 420 nm)

16 W Hg lamp (UV light source), 500 W Xenon lamp (visible light source)

high pressure Xe/Hg lamp of 350 W 125 W high pressure mercury vapour lamp Xenon lamp(400 nm) & Hg lamp(254 nm)

300 W Xe arc lamp

300 W Xe lamp

300 W Xenon lamp(λ > 300 nm)

400 W mercury lamp

solar and UV light (400 W Hg vapour lamp)

PLS-SXE300UV Xe lamp

medium pressure mercury lamp (150 W) A 300 W xenon lamp(λ < 420 nm)

Irradiation source

R. Singh, S. Dutta

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R. Singh, S. Dutta

[112]

[113]

[127]

15–60 µmol/h

10.24 mL/h/cm2

2061 µmol/h/g(1.3 wt% CuO)

As the photocatalytic water splitting is a surface phenomenon, the charge carriers (e− and h+) migrates from bulk to the surface of the semiconductor photocatalyst for the redox reactions to take place. Particle size and crystallinity of the semiconductor controls the rate of migration of the charge carrier. The probability of the charge carriers (e−/h+) to reach the photocatalyst surface without getting captured by the crystal defects increases with improved crystallinity. The electrons and holes have to travel a short distance to reach the catalyst surface if the particle size is small. 4.4. Reduction/oxidation reaction The final step involves the surface redox reaction of electron reacting with H+ to generate hydrogen (reduction) and hole reacting with H2O to form oxygen (oxidation). These reactions are enhanced by the number of active reaction sites on the photocatalyst surface, and thus higher surface area of the selected photocatalyst is an important parameter for this process. By utilising semiconductor nanomaterials as photocatalyst, adsorption and photo reactions can be improved due to their large reactive surface area. Photocatalytic splitting of H2O into H2 and O2 consist of half reactions as shown in Eqs. (3) and (4):

Four low-power UV-LED (3 W, 365 nm)

[111] 20–200 µmol/h

400 W high pressure Hg lamp (250 < λ < 600 nm) 400 W high pressure Hg lamp (250λ < 600 nm) AM1.5, 100 mW/cm2

References Irradiation source

Hydrogen evolution

4.3. Charge diffusion

2H+ + 2e− → H2

→ O2 +

(3)

4H+

Wet impregnation and calcination

H2 O(l) → H2 (g) + 1/2O2 (g)

Factors that influence the hydrogen generation via photocatalytic reaction are given below

Commercial TiO2

–

Two-step anodization

Commercial TiO2

nanofibers

nanofibers

Nanotube array

nanoparticle

TiO2/SnO2 (0.5 g/L) TiO2/CuO (0.4 g/L) CdSe/CdS/TiO2

•

CuO-TiO2 (80 mg/80 mL)

Method of preparation Morphology Photocatalyst (Dosage)

(5)

5. Major factors that influence photocatalyst activity for hydrogen production

• Surface area and particle size: TiO

Table 2 (continued)

(4)

The overall reaction: – Thermodynamically, water splitting into H2 and O2 is an uphill reaction, with a large positive change in the Gibbs free energy (ΔG0 = +238 kJ/mol) as shown in Eq. (5). For an effective H2 production utilising a visible light active semiconductor, the band gap should be in the range of 1.23–3 eV [40].

ethylene glycol,Na2S glycerol SILAR method

methanol electrospinning

methanol electrospinning

Method of modification

Sacrificial agent

2H2 O+

4h+

• 612

2 photoactivity relies on surface properties of the catalyst. Reduction in the surface area resulted from the proper heat treatment at higher temperature. However, during heat treatment, the particle size also increases with decrease in the surface area of the photocatalyst. In addition, numerous synthesis parameters such as precursor concentration, operating temperature and pressure, pH, calcination time and temperature influence a lot upon the grain size of nano-TiO2 powders [41]. Shape of the photocatalyst can be controlled by the absorption of shape controllers on the surface of the particles. For the shape control of TiO2 photocatalyst, both ammonia and triethanolamine can be utilised as a stabilizer of Ti (IV) ion against hydrolysis and performs as a shape controller to synthesise ellipsoidal anatase particles [42]. Band gap energy: The electronic structure has a major contribution in the semiconductor photocatalysis. Narrowing of band gap energy (Eg) of the semiconductor photocatalyst, for eg. TiO2 is always desired to harvest the solar radiation under visible region. For this purpose, various band gap engineering techniques are developed by the researchers across the globe mentioned in Section 7.2. Usually, for H2 production using visible light active modified TiO2 nanoparticles, Eg should be in the range of 1.23–3 eV. Corrosion resistance: Metals are mostly used as photocatalysts due to their electronic structure as well as their thermal and mechanical stability. However, the metallic elements can be easily corroded on

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R. Singh, S. Dutta

Fig. 6. List of various techniques influencing H2 production.

•

•

•

within the range of 2.9–3.2 eV as mentioned earlier, it can only utilize UV light for hydrogen production. Hence, the incapability to utilize visible light hampers the effectiveness of solar photocatalytic hydrogen generation. For the semiconductors to be visible light active, the band gap has to be in the range of 1.23–3 eV. In order to eliminate the above circumstances, various efforts are made in terms of band gap tuning and reduction of e−/h+ recombination, to synthesise an efficient visible light active semiconductor photocatalyst.

being suspended in the aqueous solution for a certain period. Therefore, it is essential to develop an effective corrosion barrier on the metal, such as surface coating or doping with other corrosion resistant elements. Sacrificial agent/electron donor in the water solution: On addition of hole scavenger or electron donor, hydrogen evolution rate is enhanced significantly because the scavenger have the capability of reducing the recombination of photo generated electron and hole pair. However, in the photocatalysis process the hole scavenger is consumed gradually and thereby reduces the efficiency of hydrogen generation. Yan et al. [43] compared the performance of a single bed and dual-bed system for hydrogen evolution using KI as sacrificial agent and inferred that since KI can be regenerated and recycled, the dual-bed system can be used for continuous and a steady hydrogen production through photocatalytic water splitting. pH: The effect of pH on the particle size and morphology of synthesised photocatalyst has been reported by many researcher. In addition, pH of the aqueous slurry system used for photocatalytic splitting influences the hydrogen generation rate significantly. Mahshid et al. [44] had concluded that only powder synthesised from an acidic medium has small particle size with spherical morphology. Calcination of photocatalyst: In addition to calcination time and temperature, calcination atmosphere also influence the hydrogen production capabilities of TiO2 as photocatalyst. The catalysts exhibited activities towards hydrogen production from water/methanol solution, according to calcination atmosphere of Ar > air > N2 > vacuum ∼H2 [45].

7. Synthesis and modification of TiO2 nanoparticles to enhance their visible light activity The main aim of the modification is to enhance their visible light activity. In this section, we discuss various methods for the preparation and modification of TiO2 nanoparticles. 7.1. Methods for the synthesis of TiO2 nanoparticles Numerous methods for the synthesis of TiO2 nanomaterials have been reported including (1) sol-gel method, (2) hydrothermal/solvothermal method, (3) physical/chemical vapour deposition (4) oxidation, sonochemical and microwave assisted methods. 7.1.1. Sol-gel method The sol-gel method is an economical, simple, and low-temperature preparation procedure. It has mostly preferred in catalyst synthesis due to its ability to prepare high purity catalysts with controlled morphology. Innumerable photocatalysts have been successfully synthesised by the sol–gel method, including ZnO, TiO2, SrTiO3, WO3, and ZrO2. The hydrolysis and polycondensation processes are involved in the sol–gel method, during which M−OH−M or M−O−M bonds are formed between the metallic atoms M of the precursor molecules that finally results in hydroxides or oxides formation. The hydrolysis reaction of butyl titanate based on Eq. (6) leads to the nucleation of the basic units of titanium dioxide whereas the condensation reaction based on Eq. (7) leads to the growth of the original basic units for the sol formation.

6. Limitations of TiO2 based solar hydrogen production Photocatalytic water splitting is a clean technology for the production of hydrogen. Many limitations have to be overcome before this technology achieves the industrial acceptance, explained in the later section (see Section 8). However, one of the major issues in the establishment of TiO2 based solar hydrogen production is the development of highly stable and effective visible-light-active photocatalyst [46]. Currently, TiO2 based solar to hydrogen conversion efficiency by photocatalytic water-splitting is low due to the following reasons: Rapid recombination of photo-excited e−/h+ pairs: The photo-excited electrons and holes can recombine in bulk or on the surface of the semiconductor photocatalyst within a very short time and release energy in the form of heat. Fast backward reaction: Rapid recombination of H2 and O2 into water reduces the hydrogen evolution rate. Therefore, effective separation of H2 and O2 is required to control the backward reaction. Inability to utilize visible light of solar spectrum: Since the band gap of different crystalline forms of TiO2, i.e. anatase, rutile, and brookite lie

Ti(OC4 H9)4 + 4H2 O→ Ti(OH)4 + 4C4 H9 OH

(6)

≡ Ti−OH + HO−Ti ≡ →≡Ti−O−Ti ≡ +H2 O

(7)

However, due to the high moisture sensitivity of titanium alkoxides, white precipitation is immediately formed on exposure with air which makes the hydrolytic process difficult to control at room temperature. Thus, the TiO2 obtained is generally found to be in the amorphous phase [47]. TiO2 synthesised by sol-gel route are usually in anatase, amorphous 613

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significantly influence the morphology of the prepared films.

or rutile phase based on the preparation method and calcination temperature. The advantages of TiO2 synthesis by sol-gel route include the preparation of crystalline nanopowder with high purity and homogeneity at a relatively low temperature as mentioned earlier in this section [48]. Ivanova et al. [49] prepared titanium and titanium/ manganese oxide catalysts by the sol-gel method and observed that the refractive index of TiO2-MnO is increasing as compared to the pure oxide as well as the sol-gel derived thin film exhibited enhanced optical properties. Masjedi et al. [50] demonstrated that Schiff-base ligand applied in the appropriate amount could be effective in particle size control. Hussain et al. [51] successfully synthesised sol-gel derived novel TiO2 nanoparticles with high surface area in a vortex reactor with optimized operating parameters. The sol-gel process has been commonly used in the synthesis of anatase TiO2 nanopowder. In this process, amorphous TiO2 powders or films were formed at low temperatures or at the room temperature. Further, calcination or hydrothermal treatments of the amorphous TiO2 specimens at higher temperatures leads to the formation of anatase phase [52]. Tian et al. [53] prepared visible light active nanocrystal anatase TiO2 by doping Ce through solgel method.

7.1.4. Mechanochemical methods A mechanochemical method offers a mechanical mixing process, followed by the chemical reaction at higher temperature [57]. Nowadays, this process is being adopted by many researchers for the synthesis of TiO2 nanoparticles. This method involves high-energy collisions of the grinding media and reactant powders, resulting in pulverizations and chemical reactions during intensive mixing. Under these processes precursors such as metal sulphates, chlorides and acetates undergo chemical reactions with carbonates, oxides, sulphides or hydroxides because of high-energy milling. The particle size can be controlled by varying the annealing temperature and soluble salt matrix as a byproduct is then washed out [58]. The mechanochemical processes are more beneficial as compared to the conventional methods due to the low-cost raw materials and simplicity of the process [59]. 7.1.5. Oxidation, sonochemical and microwave assisted methods TiO2 nanomaterials can be achieved by oxidation of titanium metal using oxidants or under anodization. Electrochemical oxidation of titanium foil has been extensively studied by the researchers for the preparation of TiO2 nanotubes. The sonochemical method arises from acoustic cavitation as well as the formation, growth, and implosive collapse of bubbles in a liquid. TiO2 nanoparticle with anatase and brookite crystalline phases can be synthesised using the hydrolysis of titanium tetra isopropoxide in pure water under ultrasonic radiation. Microwave radiation is utilised to synthesise several TiO2 nanomaterials and the principal frequencies of microwave heating lies between 900 and 2450 MHz. Conductive current flows within the material due to the movement of ionic constituents at lower microwave frequencies and can transfer energy from the microwave field to the material. However, at higher frequencies, the absorption of energy is mainly due to molecules with a permanent dipole, which leads to the reorientation under the effect of a microwave electric field. The major advantages of using microwaves for industrial processing are rapid heat transfer and selective heating.

7.1.2. Hydrothermal/Solvothermal method Hydrothermal method is an essential technique for the processing of advanced nano-sized material in the area of electronics, catalysis, and ceramics. The hydrothermal method involves crystal growth and transformation which leads to the formation of ultra-fine crystals. Hydrothermal preparation is executed in the steel autoclaves under controlled temperature (T < 200 °C) and pressure (P < 10 Mpa) with the reactions in aqueous solutions. The internal pressure in the autoclave is determined by the temperature and the amount of solution added. Zhu et al. [54] proposed an organic free preparation of TiO2 nanostructures with controlled phase and morphology through batch supercritical hydrothermal treatment (400 °C) of titanate nanotubes (TNTs) with H2O2 in NaOH aqueous solution. Tri-phase (rutile, anatase, and brookite), bi-phase (rutile and anatase), and mono-phase (rutile) TiO2 nanomaterials with distinct morphologies were successively synthesised by some researchers [55] using a hydrothermal-hydrolysis technique and adjusting the molar ratio of Ti4+/Ti3+ in a precursor solution. The solvothermal method is almost similar to hydrothermal method. However, the solvent utilised in solvothermal synthesis is nonaqueous and the temperature can be incremented much higher as compared to the hydrothermal method. The solvothermal technique for the preparation of TiO2 nanoparticles exhibits much better control on the morphology, particle size distribution, and the degree of crystallinity as compared to the hydrothermal methods. Various surfactants can be used for achieving tuned morphology of the resulting nanostructure. The solvothermal technique has gathered focus in preparation of ceramic materials (ZrO2, CeO2, Fe2O3). The temperature and pressure can be increased to a higher level as compared to that in a hydrothermal method. The solvothermal technique has higher crystallization capacity than the hydrothermal method with controlled size distribution and low agglomeration tendency [56].

7.2. Modification of TiO2 nanoparticles to enhance hydrogen production The major drawbacks of TiO2 as photocatalyst are very low absorption of visible light and rapid recombination of photogenerated e−/ h+ pair. Its activity is limited to UV radiation, which is ∼3–5% of solar spectrum whereas visible light is ∼40%. Various techniques enhancing H2 generation have been broadly classified as (1) chemical additives and (2) photocatalyst modification techniques (see Fig. 6). Several strategies adopted to optimize photocatalyst include crystal growth, morphology control, surface sensitization, heterostructuring and metal/ non-metal doping for visible light absorption [60]. Various modification and band gap engineering of UV-active photocatalyst are done, in order to develop visible light active semiconductor photocatalysts with improved stability. The photocatalyst modification techniques are (1) noble metal loading, (2) ion doping and (3) dye sensitization.

7.1.3. Physical/Chemical vapour deposition Vapour deposition refers the process in which material in a vapour phase gets condensed to the solid form. Chemical vapour deposition (CVD) involves chemical reaction unlike physical vapour deposition (PVD). In PVD, materials are vaporized from a solid/liquid phase and transferred through a vacuum or low pressure gaseous/plasma, where it is condensed. Different PVD methods are ion plating, thermal deposition, sputtering, ion implantation, laser surface alloying and laser vaporization. In chemical vapour deposition (CVD), a thin solid film is deposited onto a support through chemical reactions to modify their mechanical, thermal, electrical, optical, corrosion resistance and wear resistance properties. The CVD method is rather complicated and requires better control on the reaction conditions since any variation may

7.2.1. Addition of electron donors Due to fast recombination of photo-excited e−/h+ pair, it is difficult to accomplish photocatalytic water splitting for hydrogen generation using TiO2 as semiconductor photocatalyst. Electron donors also known as hole-scavengers reacts with the holes in valence band and reduces the photocatalytic electron/hole recombination resulting in enhanced rate of hydrogen evolution. Since electron donors gradually get consumed in the photocatalytic reaction, continuous addition of electron donors is required for sustainable hydrogen generation. Organic compounds are generally utilised as electron donors for photocatalytic hydrogen generation as they can easily get oxidized by the holes present in the semiconductor photocatalysts. Meanwhile, electrons in the CB 614

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the TiO2 semiconductor photocatalyst from UV to visible region and restricts the electron-hole recombination as a cocatalyst. Noble metals are generally used for bimetallic cocatalyst. According to various reports, Pt is the most effectively used monometallic cocatalysts. It has been observed that Pt is used in the majority of the efficient bimetallic cocatalysts, such as Pt–Pd, Pt–Ag and Pt–Cu bimetallic nanoparticles [84]. Although noble metal (Pt, Pd, Au, Ag) modified TiO2 is particularly stable and effective for H2 production under solar irradiation, however, due to their high cost, alternative low cost cocatalysts has to be researched [85]. Among numerous transition metals, Cu has been reported as a suitable dopant for TiO2 due to their low cost as compared to the expensive noble metals [86,87]. Zhang et al. [88] synthesised multi-heterojunction nanofibers for the higher interfacial electron transfer due to the improved charge carriers transport channel.

carry out the reduction of H+ to H2 molecules. Ethanol, methanol, lactic acid, EDTA, and formaldehyde have been utilised and reported as effective electron donors (sacrificial reagents) to enhance the rate of hydrogen production. Some researchers [61] have utilised glycerol as sacrificial agent for a stable and efficient hydrogen generation. 7.2.2. Photocatalyst modification techniques 7.2.2.1. Noble metal loading. Noble metals such as Pt [62–64], Pd [65], Au [66–69], Cu [70], Rh, Ru [71] and Ag [72,73] have been considered to be very efficient for enhanced visible light response of TiO2 [74,75]. As the Fermi levels of TiO2 is higher than the noble metals, photogenerated electrons gets transferred from conduction band (CB) of TiO2 to the CB of noble metal particles cocatalyst deposited on the surface of TiO2, whereas photo-generated holes reside in the valence band (VB) of TiO2. These activities significantly reduce the e−/h+ recombination, resulting in the enhanced hydrogen evolution rate. Deposition of Pt provokes the presence of defect states, Ti3+, in the band gap allowing the electrons to excite from the VB to the formed defect states or from these states to the CB with less energy. Pt is also one of the most active metals as it gives rise to the highest Schottky barrier. Therefore, platinum captures the electrons and lowers the overpotential of H+/H2 [76]. As illustrated by Xu et al. [77] controlling the size and distribution of noble metal nanoparticles on the photocatalyst support is considered to be an effective solution to increase the catalyst activity. Supporting materials also improve the long-term stability of the photocatalysts and provide high specific surface area with enhanced catalyst active reaction sites. Suitability of the metals for H2 production depends on the work function i.e. 4.26, 4.65, 4.98, 5.1 and 5.12 eV for Ag, Cu, Rh, Au and Pd respectively. Au nanoparticles are considered as a potential candidate and photosensitizers for the formation of visible light active TiO2 due to its surface plasmonic resonance (SPR) effect in which the photoexcited electrons of Au gets injected to TiO2 under solar irradiation as shown in Fig. 7. According to the recent researches in photocatalytic heterojunction system, the synergistic effect of schottky barrier and SPR significantly contributes in promoting charge separation and further improvement in photocatalytic H2 generation [78,79]. Aysin et al. [80] have listed different techniques such as hydrothermal method, sol–gel process, photochemical deposition, chemical reduction method, and laser induction, to load silver onto TiO2. Liu et al. [81] illustrated that the improvement in the photoactivity of Ni/ TiO2 is due to the increased photo-excited electrons and holes separation efficiency and the significantly improved absorption of light attributed to the surface plasmon effect of Ni nanoparticles. Researchers [82] have successfully synthesised rod-like N-dopedTiO2/ Ag composites by a modified sol–gel method. Visible light active anatase Ag–Ce/TiO2 NTs was prepared by Fan et al. [83] on doping Ce and Ag nanoparticles to exhibit enhanced photocatalytic performance. The bimetallic component shifts the photo response of

7.2.2.2. Ion doping. One of the most challenging techniques for the photocatalyst modification is by doping with different ions. Metal elements (Nb, Ce, Mn, Zn, Mg, Sn, Cu and Zr) as well as non-metals (B, N, F, Ca, S) have been reported to be effectively dopes in TiO2 for achieving improved light absorption capability [89]. Doping of TiO2 has been proved as a significant way in band gap engineering in order to change the optical properties of semiconductor photocatalysts. The insertion of a metal ions dopant in the TiO2 lattice may affect the band edge positions or introduce impurity energy level or defects or additional energy levels in the band gap of the photocatalyst. Electron transfer from these levels to the conduction band (CB) of semiconductor needs lower photon energy as compared to an unmodified or pure semiconductor. However, the impurity energy levels formed can also act as recombination centre for the photoexcited electrons and holes in the case of doping at a high concentration. Therefore, it is desired to modify the photocatalyst with an improved trapping-to-recombination rate ratio. Photocatalytic reactions can occur only if the trapped electron reaches to the photocatalyst surface as photocatalytic water splitting is a surface reaction. Transitional metal ion and rare earth metal ion doping have been thoroughly studied for increasing the TiO2 photocatalytic activities. It was reported that the photo-response of TiO2 could be shifted into visible region by the successful doping of metal ions [90]. As per the reported investigations, application of anion (N, F, C, S etc.) doping in TiO2 to enhance hydrogen evolution rate under visible radiations and have immense capability to expand its photocatalytic response towards the visible light of the solar spectrum. Many techniques have been adopted to synthesise N-doped TiO2, including sol–gel, hydrothermal method, oxidation of titanium nitride, pulsed laser deposition and sputtering [91]. Wang et al. [92] investigated Ndoped TiO2 film having (211) orientation, deposited by RF magnetron sputtering for the hydrogen generation by the photocatalytic water splitting. Carbon doping was studied for the reduction in the band gap of n-TiO2 by Xu et al. [93]. Anions have less probability to form

Fig. 7. Schematic illustration for the water photosplitting process in the presence of co-catalyst.

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recombination centres as compared to the cations or metal ions. 7.2.2.3. Dye sensitization. Dye sensitization is the technique adopted to harness visible solar spectrum for the effective conversion of energy [94]. Some dyes (safranine, O/EDTA and T/EDTA), possess enhanced redox property and visible light sensitivity, thus they can be successfully utilised in solar cells as well as photocatalytic systems. Under visible light irradiation, the dyes are excited and transfer electrons to CB of semiconductors in order to increase the performance of photocatalytic water splitting reactions for the hydrogen generation. The visible light active dyes are sufficient to produce hydrogen even without semiconductors. However, the hydrogen evolution rate by the dyes alone is very less. High hydrogen generation rate can be achieved by effective absorption of visible radiations and efficient injection of electrons from excited dyes to the CB of semiconductor photocatalyst. The CB of TiO2 lies slightly below the excited state energy level of many dyes mentioned above, and this is one of the reasons behind the effective electron transfer. Dye-sensitized TiO2 can fully utilize visible light to accelerate photocatalytic reactions and have significant photovoltaic applications, for example, dyesensitized cells and photolysis of water for hydrogen evolution. TiO2 is an appropriate semiconductor for dye-sensitized solar cells because of its stability even under extreme conditions. Recently, sensitization of TiO2 with the visible light active dyes has been perceived as an effective route to increase the quantum yield (QE) for hydrogen generation assisted by visible light of the solar spectrum. Jin et al. [95] proposed that eosin-sensitized CuO/TiO2 led to a high quantum efficiency for photocatalytic water splitting to generate hydrogen. Sensitization of the TiO2 particles for harvesting visible light has been investigated for solar hydrogen generation and photo-decomposition of polluting materials for water purification. In the sensitization process, it is essential that the sensitized dye molecule gets effectively adsorbed on the TiO2 surface [96]. There are numerous adsorption modes such as electrostatic interaction, hydrogen bonding, covalent attachment, and many more. Researchers have reported that the electrostatic interactions are more effective than the covalent attachment in the improved photosensitization process [97]. In order to investigate different techniques to enhance the properties of the dye coating, the surface functionalization is desired to link the dye molecule effectively. Several approaches have been developed to functionalize the electrode surface that involves chemical or electrochemical adsorption of the organic molecule on the oxide surface [98]. Malekshoar and Ray [99] loaded Molybdenum sulphide over Eosin Y sensitized TiO2 through in-situ solar photo-deposition method to achieve enhanced hydrogen generation rate. Fig. 8 shows the schematic description of the water photosplitting process in the presence of dye sensitizer.

Fig. 9. Schematic illustration for the water photosplitting process in the presence of composite semiconductors.

another significant technique to harness visible solar radiations for enhancing hydrogen evolution. When the semiconductor photocatalyst with a wider band gap is coupled with the semiconductor having the narrow band gap and a more negative CB level, electrons from CB are transferred from the small band gap to the large band gap semiconductor photocatalyst. Therefore, an effective accumulation of electron is observed in the CB of wide band gap semiconductor photocatalyst as shown in Fig. 9. The method is almost identical to that of dye sensitization; however, the only difference is that electrons are injected from one semiconductor to another instead of from excited dye as in the case of dye sensitization. Following conditions should be met to synthesise a highly active composite semiconductor photocatalyst (i) semiconductors should be corrosion resistant, (ii) the narrow band gap semiconductor should have the excitation capability by visible light and (iii) transfer of electrons should be fast and effective. Cheap and efficient photocatalysts were fabricated by simply mixing TiO2 nanoparticles (NPs) and CuO NPs. The synergistic effect of the semiconductors could effectively reduce charge (e−/h+) recombination, enhance interfacial charge transfer, improves visiblelight adsorption capability and offers increased active photocatalytic reaction sites [100,101]. The two NPs combine with each other such as TiO2/CuO mixture in an aqueous solution due to their opposite surface charge [102]. Copper compounds facilitate the separation of charge (e−/h+) and provides increased reduction reaction sites for hydrogen evolution [103,104]. As the reduction potential of CuO on the TiO2 surface is more than that of pure TiO2, CuO is favourably reduced to Cu2O or Cu0 by trapping the photogenerated electrons and reducing e−/h+ recombination [105]. Wider bandgap semiconductors exhibit better stability as compared to those with narrow band gaps but show a reduced H2 evolution ability under solar light. Hence, semiconductors coupling is an effective process adopted for preparing photocatalysts with high photoactivity and stability [106]. Shifu et al. [107] prepared the coupled photocatalyst WO3/TiO2 by wet ball milling whereas PérezLarios et al. [108] synthesised TiO2-ZnO mixed oxide photocatalysts via sol-gel method for photocatalytic hydrogen generation. Bai et al. [109] designed a novel “forest-like” TiO2/ZnO/CuO photocatalyst via three steps methods – electrospinning, hydrothermal treatment and photodeposition with enhanced hydrogen generation rate. Xu et al. [110] observed that TiO2 or CuO alone hardly produces hydrogen in 1 h’s reaction, which proposed that the coupling of TiO2 and CuO is essential in achieving high catalytic activity. In addition to that, Lee et al. successfully fabricated electrospun TiO2/SnO2 [111] and TiO2/ CuO [112] novel structured composites for highly efficient photocatalytic hydrogen generation. Wang et al. [113] fabricated and characterized CdSe/CdS/TiO2 nanotube composite photocatalyst for the photocatalytic hydrogen generation and pollutant degradation.

7.2.2.4. Composite semiconductors. Coupling of semiconductors is

Fig. 8. Schematic illustration for the water photosplitting process in the presence of dye sensitizer.

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8. Worldwide Research scenario of solar hydrogen production Hydrogen generation from solar photocatalytic water splitting has been explored since the Honda-Fujishima effect was first proposed in 1972. The “Ministry of Science and Technology of China” initiated a project of “National Basic Research Program of China (973 Program) – the Basic Research of Mass Hydrogen Production using Solar Energy” in 2003 for the effective research and development in the field hydrogen production under solar radiation. In various countries such as-Japan, Australia, United States, Germany, researchers as well as their government have considered the possibility of a hydrogen economy. Numerous programs related to hydrogen energy, such as the “Agreement on the production and utilization of hydrogen” by “International Energy Association (IEA)”, the “Multiyear Plan for the Hydrogen R&D Program” by United States, and the “World Energy Network” by Japan, have been introduced [114].

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9. Future aspects In future R & D on the photocatalytic water splitting to produce hydrogen, it is expected to deal with the following limitations:-

• Enhancement the visible light response of the photocatalyst: Band gap

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energy (Eg) engineering is a basic parameter for the synthesis of visible light active semiconductor photocatalysts. As far as, optical absorption capability is concerned, narrow and direct band gap semiconductors are best suited to achieve maximum absorbance and effective harvesting of low energy photons. Stability and efficiency of photocatalysts are essential in realizing the practical applications of them for photocatalytic hydrogen production. Preparation of the new photocatalyst and cocatalyst by cost effective methods: In order to achieve the target of 10% solar energy conversion efficiency, significant R & D efforts are needed in the design of effective photocatalysts with the visible light response. It is advantageous to develop low cost noble technique that allows the direct preparation of TiO2 nanostructures in crystalline form at ambient or low temperatures since additional heat treatment step at elevated temperatures cause unnecessary increment in the particle size with a significant reduction in surface area and porosity [33]. Blackening of TiO2 could be a promising strategy for improving the solar hydrogen production efficiency via (a) oxidation of low valence Ti compounds, (b) reduction of TiO2 and various other methods [115]. Design of low cost and highly effective photoreactor: The hydrogen generation rate from photocatalytic methods not only depends on the catalyst photoactivity but also on the design of a low cost efficient photoreactor. An ideal photoreactor should have the capability of absorbing the incident radiations and expected to accelerate photocatalysed reactions in an efficient manner with minimum photonic losses. Various technical and economic challenges are reported by many researchers during design and fabrication of a large scale photocatalytic reactor for hydrogen generation. For example, continuous stirring within a photoreactor is not possible in largescale photocatalyst slurry system due to economic reasons. However, the design could allow self-mixing within the photoreactor [116]. Photoreactor design and configuration plays an important role, especially in the semiconductor assisted photochemical water splitting as it greatly influence the critical conversion efficiency of light energy. It is defined as the conversion efficiency of light energy when the hydrogen energy output is equal to an equivalent energy input of the system, which is marked as ƞcr. Thus, if the hydrogen energy output of the system will be equal to the equivalent input energy, the conversion efficiency of light energy of a certain system is equal to ƞcr in order to compensate the cost of the system. The profit in terms of energy can be achieved only if the conversion efficiency of light energy exceeds the critical conversion

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efficiency in a system [117]. Priya et al. [118] had developed two solar slurry photocatalytic reactors i.e., (1) batch reactor (BR) and (2) batch recycle reactor with continuous supply of inert gas and compared their performance. It was concluded that the application of photoreactors with a large surface area per unit volume, capability to handle concentrated catalyst suspensions, rapid desorption of products and enhanced residence time could be a solution to improve the process of photocatalysis. Besides, it can be combined with the technology of membrane separations making the so-called photocatalytic membrane reactors (PMRs) where the chemical reaction, the photocatalyst recovery and the separation of products can be achieved simultaneously [119]. Photocatalytic water splitting with separate H2 and O2 generation is essential because it minimises the explosion risk and cost of hydrogen purification. A novel twin photoreactor was designed by Lo et al. [120] with a modified Nafion membrane for separate evolution of H2 and O2 in photocatalytic water splitting under visible light irradiation. Low cost and highly effective solar concentrators: One of the key issues in photocatalytic hydrogen generation is the maximum conversion of the solar energy. However, two major limitations of solar energy are: (1) the irregular way in which it reaches at the earth’s surface and (2) inefficient collection of solar energy. The first limitation can be solved by converting solar energy into storable hydrogen energy whereas to overcome the second problem, highly efficient solar concentrator must be used. Compound Parabolic Concentrators (CPCs) is a type of low-concentration collector [121]. Li et al. [122] mentioned that the efficiency of TiO2 based solar-driven photocatalytic processes is much lower than the acceptable solar-to-hydrogen efficiency (10%) for benchmark applications. Wei et al. [123] successfully designed a new compound parabolic concentrator (CPC) for solar photocatalytic hydrogen generation and investigated various parameters such as orientation, acceptance angle, absorber tube diameter and concluded with the daily hydrogen production rate of 160.34 NL. Reaction mechanisms: Several significant factors govern the kinetics of the photocatalytic H2 generation process which is required for the photocatalytic reactors scale-up. The feasibility studies were conducted by Ruban et al. [124] in a novel bench-scale tubular photocatalytic reactor for the optimization of operating variables such as concentration of sulphide and sulfite ion, catalyst concentration, pH, wastewater volume, lamp power, and recycle flow rates the evolution of H2 from aqueous sodium sulphide utilising CdS-ZnS/ TiO2 nanoparticles. Study of interfacial properties of the photocatalyst and cocatalysts: Another major factor affecting the photocatalytic performance of a semiconductor is its surface and interfacial properties. The electron and energy transfer at the interface depends on the surface energy and chemisorption behaviour of the semiconductor photocatalyst. Usually, large surface energy should exhibit enhanced catalytic activity. Recent research has been focused on the synthesis of semiconductor crystals with highly reactive facets. Storage of hydrogen: Hydrogen storage is a considerable issue in realizing hydrogen economy. Proper safety measures should be followed during the production, storage, transport and usage of hydrogen fuel due to the high inflammability [125]. Hydrogen can be stored in an appropriate solid-state material such as carbon materials, metal hydrides and metal–organic frame- works, cryogenic liquid or as pressurized gas [126]. Sacrificial reagents to improve the activity of overall reaction system: For an effective photocatalytic hydrogen generation system, there is an immense need of sacrificial reagents or electron donors to improve the overall activity of the reaction system. If sacrificial reagents are costly than that of hydrogen generation such as methanol or ethanol, their application in hydrogen evolution is meaningless. However, the usage of organic pollutants or industrial wastes as sacrificial reagents, followed by their conversion into

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