Hetero-nanostructured metal oxide-based hybrid photocatalysts for enhanced photoelectrochemical water splitting – A review

This review is mainly focused on nanostructured metal oxide-based efficient photocatalysts for photoelectrochemical (PEC) water splitting applications. Owing to their distinctive physical and chemical properties, metal-oxide nanostructures have attracted a wide research interest for solar power-stimulated water splitting applications. Hydrogen generation by solar energy-assisted water splitting is a clean and eco-friendly route that can solve the energy crisis and play a significant role in efforts to save the environment. In this review, synthesis strategies, control of morphology, band-gap properties, and photocatalytic application of solar water splitting using hierarchical hetero-nanostructured metal oxide-based photocatalysts, such as titanium dioxide (TiO₂), zinc oxide (ZnO), and tungsten/wolfram trioxide (WO₃), are discussed.     

Highly efficient visible-light-assisted photocatalytic hydrogen generation from water splitting catalyzed by Zn0.5Cd0.5S/Ni2P heterostructures

Development of heterostructured photocatalysts which can facilitate spatial separation of photo-generated charge carriers is crucial for achieving improved photocatalytic H₂ production. Consequently, herein, we report the synthesis of Zn_0.5Cd_0.5S/Ni₂P heterojunction photocatalysts with varying amount of Ni₂P, 0.5 (S1), 1 (S2), 3 (S3), 5 (S4) and 10wt% (S5) for the efficient visible-light-assisted H₂ generation by water splitting. The heterostructures were characterized thoroughly by PXRD, FE-SEM, EDS, HR-TEM and XPS studies. FE-SEM and HR-TEM analyses of the samples unveiled the presence of Zn_0.5Cd_0.5S microspheres composed of smaller nanocrystals with the surface of the microspheres covered with Ni₂P nanosheets and the intimate contact between the Zn_0.5Cd_0.5S and the Ni₂P. Further, visible-light-assisted photocatalytic investigation of the samples showed excellent water splitting activity of the heterostructure, Zn_0.5Cd_0.5S/1wt%Ni₂P (S2) with very high H₂ generation rate of 21.19 mmol h^?1g^?1 and the AQY of 21.16% at 450 nm with turnover number (TON) and turnover frequency (TOF) of 251,516 and 62,879 h^?1 respectively. Interestingly, H₂ generation activity of S2 was found to be about four times higher than that of pure Zn_0.5Cd_0.5S (5.0 mmol h^?1g^?1) and about 240 times higher than that of CdS/1wt%Ni₂P. The enhanced H₂ generation activity of S2 has been attributed to efficient spatial separation of photogenerated charge carriers and the presence of highly reactive Ni₂P sites on the surface of Zn_0.5Cd_0.5S microspheres. A possible mechanism for the enhanced photocatalytic H₂ generation activity of Zn_0.5Cd_0.5S/1wt%Ni₂P (S2) has been proposed and is further supported by photoluminescence and photocurrent measurements. Furthermore, the catalyst, S2 can be recycled for several cycles without significant loss of catalytic activity and photostability. Remarkably, the H₂ generation activity of S2 was found to be even higher than the reported examples of Zn_xCd_1-xS doped with noble metal cocatalysts. Hence, the present study highlights the importance of Zn_0.5Cd_0.5S/Ni₂P heterostructures based on non-noble metal co-catalyst for efficient visible-light-driven H₂ production from water splitting.     

Photocatalytic H2 production from water splitting under visible light irradiation using Eosin Y-sensitized mesoporous-assembled Pt/TiO2 nanocrystal photocatalyst

Sensitized photocatalytic production of hydrogen from water splitting is investigated under visible light irradiation over mesoporous-assembled titanium dioxide (TiO₂) nanocrystal photocatalysts, without and with Pt loading. The photocatalysts are synthesized by a sol–gel process with the aid of a structure-directing surfactant and are characterized by N₂ adsorption–desorption analysis, X-ray diffraction, UV–vis spectroscopy, scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray analysis. The dependence of hydrogen production on the type of TiO₂ photocatalyst (synthesized mesoporous-assembled and commercial non-mesoporous-assembled TiO₂ without and with Pt loading), the calcination temperature of the synthesized photocatalyst, the sensitizer (Eosin Y) concentration, the electron donor (diethanolamine) concentration, the photocatalyst dosage and the initial solution pH is systematically studied. The results show that in the presence of the Eosin Y sensitizer, the Pt-loaded mesoporous-assembled TiO₂ synthesized by a single-step sol–gel process and calcined at 500 °C exhibits the highest photocatalytic activity for hydrogen production from a 30 vol.% diethanolamine aqueous solution with dissolved 2 mM Eosin Y. Moreover, the optimum photocatalyst dosage and initial solution pH for the maximum photocatalytic activity for hydrogen production are 3.33 g dm⁻³ and 11.5, respectively.     

Hydrogen production from water splitting under UV light irradiation over Ag-loaded mesoporous-assembled TiO2-ZrO2 mixed oxide nanocrystal photocatalysts

Hydrogen production from the photocatalytic water splitting reaction is very attractive because it is an environmentally friendly process, where hydrogen is produced from two abundantly renewable sources, i.e. water and solar energy, with the aid of photocatalysts. TiO₂ is the most widely investigated photocatalyst; however, it alone still exhibits low performance to photocatalytically produce hydrogen. Hence, the aim of this work focused on the enhanced photocatalytic hydrogen production over Ag-loaded mesoporous-assembled TiO₂-ZrO₂ mixed oxide nanocrystal photocatalysts under UV light irradiation. The TiO₂-ZrO₂ mixed oxides with various TiO₂-to-ZrO₂ molar ratios were synthesized by a sol-gel process with the aid of a structure-directing surfactant, followed by Ag loading via a photochemical deposition method. The influences of photocatalyst preparation parameters, i.e. calcination temperature, phase composition, and Ag loading, were studied. The results revealed that the mesoporous-assembled TiO₂-ZrO₂ mixed oxide nanocrystal photocatalyst with a TiO₂-to-ZrO₂ molar ratio of 93:7 calcined at 500 °C exhibited the highest photocatalytic hydrogen production activity, and the Ag loading of 0.5 wt.% further greatly enhanced the photocatalytic activity of such TiO₂-ZrO₂ mixed oxide photocatalyst.     

Photocatalytic splitting of seawater and degradation of methylene blue on CuO/nano TiO2

The main objective of this study was to prepare effective photocatalysts for splitting of seawater for solar fuel – H₂ and degradation of seawater organic pollutants such as dyes. To enhance photocatalytic activities, CuO is supported on nano TiO₂ (CuO/nano TiO₂). By X-ray absorption near edge structure (XANES) spectroscopy, CuO clusters are found on nano TiO₂. The 2.5% CuO/nano TiO₂ has greater activities in photocatalytic splitting of water and seawater than nano TiO₂ by 9.9 and 7.8 times, respectively. Interestingly, the 2.5% CuO/nano TiO₂ is also very active for photocatalytic splitting of water and seawater contaminated with dyes such as methylene blue (MB) (10 ppm). Under a 5-h irradiation of the UV–Vis light, about 99% of MB is degraded while 3.1 μmol/h g cat of H₂ are generated from seawater in the photocatalysis process.     

Hydrothermally stabilized Fe(III) doped titania active under visible light for water splitting reaction

Fe³⁺ doped TiO₂ photocatalysts were prepared by hydrothermal treatment for the photocatalytic water splitting to produce stoichiometric hydrogen and oxygen under visible light irradiation. It was found that hydrothermal treatment at 110 °C for 10 h was essential for the synthesis of highly stabilized Fe³⁺ doped TiO₂ photocatalysts. The synthesized photocatalysts were characterized by field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) and BET surface area techniques. The doping of highly stabilized Fe³⁺ in the titania matrix leads to significant red shift of optical response towards visible light owing to the reduced band gap energy. Optimum amount of Fe³⁺ doped TiO₂, 1.0 wt% Fe/TiO₂, showed drastically improved hydrogen production performance of 12.5 μmol-H₂/h in aqueous methanol and 1.8 μmol-H₂/h in pure water, respectively. This Fe/TiO₂ photocatalyst was stable for 36 h without significant deactivation in the water splitting reaction.     

The size and valence state effect of Pt on photocatalytic H2 evolution over platinized TiO2 photocatalyst

Photocatalytic hydrogen production from water or organic compounds is a promising way to resolve our energy crisis and environmental problems in the near future. Over the past decades, many photocatalysts have been developed for solar water splitting. However, most of these photocatalysts require cocatalyst to facilitate H₂ evolution reaction and noble metals as key cocatalysts are widely used. Consequently, the condition of noble metal cocatalyst including the size and valence state etc plays the key role in such photocatalytic system. Here, the size and valence state effect of Pt on photocatalytic H₂ evolution over platinized TiO₂ photocatalyst were studied for the first time. Surprisingly, it was found that Pt particle size does not affect the photoreaction rate with the size range of several nanometers in this work, while it is mainly depended on the valence state of Pt particles. Typically, TOFs of TiO₂ photodeposited with 0.1–0.2 wt% Pt can exceed 3000 h⁻¹.     

High photocatalytic hydrogen production from methanol aqueous solution using the photocatalysts CuS/TiO2

Photocatalysts CuS/TiO₂ for hydrogen production were synthesized by hydrothermal method at high temperature and characterized by XRD, UV–visible DRS, XPS, EDX, SEM and TEM. When TiO₂ was loaded with CuS, it showed photocatalytic activities for water decomposition to hydrogen in methanol aqueous solution under 500 W Xe lamp. Among the photocatalysts with various compositions, the one with 1 wt% CuS-loaded TiO₂ showed the maximum photocatalytic activity for water splitting, which indicated CuS could improve the separation ratio of photoexcited electrons and holes. What's more, the amounts of the produced hydrogen was about 570 μmol h⁻¹, which had exceeded pure titania (P25) 32 times. In the present paper, it is proven that CuS can act as an effective co-catalyst to enhance the photocatalytic H₂ production activity of TiO₂.     

Photocatalytic hydrogen production from aqueous methanol solution with CuO/Al2O3/TiO2 nanocomposite

The photocatalytic hydrogen production from aqueous methanol solution was investigated with ZnO/TiO₂, SnO/TiO₂, CuO/TiO₂, Al₂O₃/TiO₂ and CuO/Al₂O₃/TiO₂ nanocomposites. A mechanical mixing method, followed by the solid-state reaction at elevated temperature, was used for the preparation of nanocomposite photocatalyst. Among these nanocomposite photocatalysts, the maximal photocatalytic hydrogen production was observed with CuO/Al₂O₃/TiO₂ nanocomposites. A variety of components of CuO/Al₂O₃/TiO₂ photocatalysts were tested for the enhancement of H₂ formation. The optimal component was 0.2 wt% CuO/0.3 wt% Al₂O₃/TiO₂. The activity exhibited approximately tenfold enhancement at the optimum loading, compared with that with pure P-25 TiO₂. Nano-sized TiO₂ photocatalytic hydrogen technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy.     

Solar photocatalytic degradation of azo dye: comparison of photocatalytic efficiency of ZnO and TiO2

The photocatalytic activity of commercial ZnO powder has been investigated and compared with that of Degussa P25 TiO₂. Laboratory experiments with acid brown 14 as the model pollutant have been carried out to evaluate the performance of both ZnO and TiO₂ catalysts. Solar light was used as the energy source for the photocatalytic experiments. These catalysts were examined for surface area, particle size and crystallinity. The effect of initial dye concentration, catalyst loading, irradiation time, pH, adsorption of acid brown 14 on ZnO and TiO₂, intensity of light and comparison of photocatalytic activity with different commercial catalysts were studied. The progress of photocatalytic degradation of the acid brown 14 has been observed by monitoring the change in substrate concentration of the model compound employing HPLC and measuring the absorbance in UV–Visible spectrophotometer for decolourisation. The photodegradation rate was determined for each experiment and the highest values were observed for ZnO suggesting that it absorbs large fraction of the solar spectrum and absorption of more light quanta than TiO₂. The complete mineralisation was confirmed by total organic carbon (TOC) analysis, COD measurement and estimation of the formation of inorganic ions such as NH₄⁺, NO₃⁻, Cl⁻ and SO₄²⁻.     

Enhanced hydrogen production from water splitting by Sn-doped ZnO/BiOCl photocatalysts and Eosin Y sensitization

The construction of semiconductor heterojunction for photocatalytic H₂ production from water splitting is an efficient and environment-friendly technology. In this work, ZnO/BiOCl (ZBC) and Sn-doped ZnO/BiOCl (ZBC-S) photocatalysts with Z-scheme heterojunction were successfully prepared by simple hydrothermal method. The photocatalytic H₂ evolution from water splitting by the as-prepared photocatalysts was investigated. The formation of ZnO/BiOCl heterojunction reduces the recombination probability of the photogenerated carriers. The impurity levels originated from Sn doping reduce the band gap width of ZnO and BiOCl to some extent, thereby enhancing the light absorption ability. The ZBC-S composite exhibits the best photocatalytic activity. In addition, the photocatalytic efficiency of H₂ production was improved by sensitization with Eosin Y (EY) dye. The H₂ production rate under simulated sunlight reaches 4146.77 μmol g^?1 h^?1, which is 27 times higher than that of pure ZnO. Finally, the Z-scheme electron transfer route in ZnO/BiOCl heterojunction was determined, and the photocatalytic H₂ production mechanism of EY sensitized ZBC-S was proposed.     

Construction of ternary hybrid layered reduced graphene oxide supported g-C3N4-TiO2 nanocomposite and its photocatalytic hydrogen production activity

Reduced graphene oxide (rGO) supported g-C₃N₄-TiO₂ ternary hybrid layered photocatalyst was prepared via ultrasound assisted simple wet impregnation method with different mass ratios of g-C₃N₄ to TiO₂. The synthesized composite was investigated by various characterization techniques, such as XRD, FTIR, Raman Spectra, FE-SEM, HR-TEM, UV vis DRS Spectra, XPS Spectra and PL Spectra. The optical band gap of g-C₃N₄-TiO₂/rGO nanocomposite was found to be red shifted to 2.56 eV from 2.70 eV for bare g-C₃N₄. It was found that g-C₃N₄ and TiO₂ in a mass ratio of 70:30 in the g-C₃N₄-TiO₂/rGO nanocomposite, exhibits the highest hydrogen production activity of 23,143 μmol g^?1h^?1 through photocatalytic water splitting. The observed hydrogen production rate from glycerol-water mixture using g-C₃N₄-TiO₂/rGO was found to be 78 and 2.5 times higher than g-C₃N₄ (296 μmol g^?1 h^?1) and TiO₂ (11,954 μmol g^?1 h^?1), respectively. A direct contact between TiO₂ and rGO in the g-C₃N₄-TiO₂/rGO nanocomposite produces an additional 10,500 μmol g^?1h^?1 of hydrogen in 4 h of photocatalytic reaction than the direct contact between g-C₃N₄ and rGO. The enhanced photocatalytic hydrogen production activity of the resultant nanocomposite can be ascribed to the increased visible light absorption and an effective separation of photogenerated electron-hole pairs at the interface of g-C₃N₄-TiO₂/rGO nanocomposite. The effective separation and transportation of photogenerated charge carriers in the presence of rGO sheet was further confirmed by a significant quenching of photoluminescence intensity of the g-C₃N₄-TiO₂/rGO nanocomposite. The photocatalytic hydrogen production rate reported in this work is significantly higher than the previously reported work on g-C₃N₄ and TiO₂ based photocatalysts.     

CuS,NiS as co-catalyst for enhanced photocatalytic hydrogen evolution over TiO2

TiO₂ photocatalysts loaded CuS and NiS as co-catalyst were prepared by hydrothermal approach and characterized by XRD, UV–visible DRS, BET, XPS, SEM and TEM. When TiO₂ was loaded MS as co-catalyst, it showed higher photocatalytic activities for splitting water into hydrogen in methanol aqueous solution under 500 W Xe lamp. Among the photocatalysts with various compositions, the maximum evolution of H₂ obtained from 5 wt% CuS–5 wt% NiS–TiO₂ sample was about 800 μmol h⁻¹, which was increased up to about twenty-eight times than that of TiO₂ alone. It was proven that CuS, NiS can act as effective dual co-catalysts to enhance the photocatalytic H₂ production activity of TiO₂.     

Laser-assisted synthesis of ZnO/ZnSe hybrid nanostructured films for enhanced solar-light induced water splitting and water decontamination

In this work, a novel method was developed to synthesize visible light-responsive ZnO/ZnSe photocatalysts, using rapid, simple Pulsed Laser Ablation of Solids in Liquids (PLASL) method. Using this method, we synthesized high-purity ZnO/ZnSe hybrid nanostructures with an optimum weight ratio of 10:4, which exhibited better photocatalytic activity. The addition of ZnSe on the ZnO semiconductor surface improved the light absorption of the composites and simultaneously decreased the photoluminescence emission intensity, which is an indicator of the inhibition of the electron-hole recombination rate. The nanostructured composites synthesized using the PLASL method were applied for photo-electrochemical water splitting and photo-degradation studies. The synthesized ZnO/ZnSe hybrid nanocomposites exhibited excellent visible-light-driven photocatalytic activity, which is highly desirable for water decontamination and photo-electrochemical water oxidation. This work, therefore, empirically demonstrates how this simple, cost-effective laser-based technique can be employed to rapidly synthesize high-purity, visible-light responsive photocatalysts expanding the potential for future solar energy conversion technologies.     

Fabrication of ZnO/SrTiO3 nanoarrays and its photoelectrochemical performances

The ZnO/SrTiO₃ nanomaterials were fabricated by a chemical conversion hydrothermal method in order to utilize the high electron transfer rate of one-dimensional ZnO nanorods and photocatalytic activity of SrTiO₃. The technological parameters, such as TiO₂ sol concentration, TiO₂ sol dipping cycle, Sr(NO₃)₂ concentration and reaction temperature, were investigated in the synthetic process and the reaction mechanism of the ZnO/SrTiO₃ nanomaterials was proposed. A photocurrent density of 7.53 mA/cm² was obtained for the as-prepared ZnO/SrTiO₃ photocatalyst, attributed to its improved absorption spectrum and appropriate nanostructure, which indicates a potential application in photoelectrochemical water splitting.     

Growth of anatase and rutile TiO2@Sb:SnO2 heterostructures and their application in photoelectrochemical water splitting

We report three-dimensional (3D) nanostructures based on shape- and phase-controlled TiO₂ coated transparent conducting oxide (TCO) nanowire array. Core-shell and branched nanostructures were obtained using an aqueous chemical bath deposition (CBD) method at room temperature. Adjusting the pH of a TiCl₄ solution is a key factor that determines the morphology of the nanostructure. Spherical TiO₂ anatase covered a Sb-doped SnO₂ (ATO) nanowire when pH was maintained at a high level. In contrast, branched nanostructures with TiO₂ rutile nanorods were synthesized by keeping a TiCl₄ solution going down to a low pH. Nanorods were grown epitaxially along the [001] direction on ATO nanowires. Morphological and structural analysis indicates that phases and shapes of the 3D hybrid nanostructure are determined by the pH of the solution and the reaction time. A two-fold higher photoconversion efficiency of rutile TiO₂ rod@ATO was obtained under simulated solar illumination compared to that of the anatase TiO₂ nanoshell@ATO. These 3D hybrid nanostructures can offer (i) a large surface area and efficient charge transport in the TiO₂ nanostructure, and (ii) an effective charge collection path through one-dimensional TCO, which is promising for various areas, including photoelectrochemical water splitting, as well as for application in electronic and photonic nanodevices.     

Light-assisted synthesis of Au/TiO2 nanoparticles for H2 production by photocatalytic water splitting

A series of Au/TiO₂ photocatalysts was synthesized via the light assistance through the photo-deposition for H₂ production by photocatalytic water splitting using ethanol as the hole scavenger. Effect of solution pH in the range of 3.2–10.0 on the morphology and photocatalytic activity for H₂ production of the obtained Au/TiO₂ photocatalysts was explored. It was found that all Au/TiO₂ photocatalysts prepared in different solution pH exhibited comparable anatase fraction (～0.84–0.85) and crystallite size of TiO₂ (21–22 nm), but showed different quantity of deposited Au nanoparticles (NPs) and other properties, particularly the particle size of the Au NPs. Among all prepared Au/TiO₂ photocatalysts, the Au/TiO₂ (10.0) photocatalyst exhibited the highest photocatalytic activity for H₂ production, owning to its high metallic state and small size of Au NPs. Via this photocatalyst, the maximum H₂ production of 296 μmol (～360 μmol/g?h) was gained at 240 min using the 30 vol% ethanol as the hole scavenger at the photocatalyst loading of 1.33 g/L under the UV light intensity of 0.24 mW/cm² with the quantum efficiency of 61.2% at 254 nm. The loss of the photocatalytic activity of around 20% was observed after the 5th use.     

Fabrication of nano N-doped In2Ga2ZnO7 for photocatalytic hydrogen production under visible light

N-doped In₂Ga₂ZnO₇ photocatalysts were fabricated by solid state reaction route. All the prepared photocatalysts were successfully characterised by PXRD, optical absorption spectra, SEM, TEM, XPS, BET surface area and photoresponse studies. The formation of In₂Ga₂ZnO₇ was confirmed by the PXRD pattern. Optical absorption spectra showed that the visible light absorption of all the photocatalysts were enhanced by nitrogen doping. Among all the prepared photocatalysts, 1 wt% Pt loaded N-GaInZn-500 showed enhanced photocatalytic activity towards hydrogen evolution under visible light irradiation in presence of 10 vol% methanol solution as sacrificial agent. The excellent photocatalytic activity of N-GaInZn-500 is in agreement with N-content, bandgap energy, PL intensity and Surface area.     

Visible light photocatalytic water splitting for hydrogen production from N-TiO2 rice grain shaped electrospun nanostructures

We report on the visible light-driven hydrogen production from splitting of water molecules by nitrogen-doped TiO₂ (N-TiO₂) with a rice grain-like nanostructure morphology. The N-TiO₂ nanostructures are prepared using sol-gel and electrospinning methods followed by post-annealing of the composite nanofibers. The nanostructures are characterized by microscopy and spectroscopy. First order rate constants for the visible light-assisted photocatalysis in the degradation of methylene blue (MB) dye are found to be 0.2 × 10⁻³ and 1.8 × 10⁻³ min⁻¹ for TiO₂ and N-TiO₂ (5 wt% of nitrogen), respectively. The N-TiO₂ utilized in water splitting experiments and evaluated hydrogen (H₂) of 28 and 2 μmol/h for N-TiO₂ and TiO₂, respectively. The improvement may be attributed due to the N-doping and higher surface area as ∼70 m²/g.     

Recent advances in non-metals-doped TiO2 nanostructured photocatalysts for visible-light driven hydrogen production,CO2 reduction and air purification

The generation of hydrogen and oxygen from the photocatalytic water splitting reaction under visible light is a promisingly renewable and clean source for H₂ fuel. The transition metal oxide semiconductors (e.g. TiO₂, WO_3, ZnO, and ZrO₂) are have been widely used as photocatalysts for the hydrogen generation. Because of safety, low cost, chemical inertness, photostability and other characteristics (bandgap, corrosion resistance, thermal and environmental stability), TiO₂ is considered as a most potential catalyst of the semiconductors being investigated and developed. However, the extensive applications of TiO₂ are hampered by its inability to exploit the solar energy of visible region. Other demerits are lesser absorbance under visible light, and recombination of photogenerated electron-hole pairs. In this review, we focus on the all the possible reactions taking place at the catalyst during photo-induced H₂ from water splitting reaction, which is green and promising technology. Various parameter affecting the photocatalytic water splitting reactions are also studied. Predominantly, this review is focussed on bandgap engineering of TiO₂ such as the upward shift of valence band and downward shift of conduction bands by doping process to extend its light absorption property into the visible region. Furthermore, the recent advances in this direction including various new strategies of synthesis, multiple doping, hetero-junction, functionalization, perspective and future opportunities of non-metals-doped TiO₂-based nanostructured photocatalysts for various photocatalytic applications such as efficient hydrogen production, air purification and CO₂ reduction to valuable chemicals have been discussed.