Preparation of hybrid WO3–TiO2 nanotube photoelectrodes using anodization and wet impregnation: Improved water-splitting hydrogen generation performance

Hybrid tungsten trioxide-titanium dioxide (WO₃–TiO₂) nanotube photoelectrodes were prepared using simple anodization and wet impregnation. These hybrid nanotube photoelectrodes significantly enhanced their photoelectrochemical (PEC) water-splitting performances compared with pure TiO₂ nanotube photoelectrodes. This study aims to determine the optimum soaking time in ammonium paratungstate used as the tungsten precursor for incorporating WO₃ species into TiO₂ nanotube photoelectrodes. A low content of WO₃ species successfully diffused into the TiO₂ lattice and formed W–O–Ti bonds, which significantly promoted effective charge separation by trapping photo-induced electrons from TiO₂. Thus, the photocurrent density, photoconversion efficiency, STH efficiency, and H₂ generation of the resultant hybrid nanotubes were increased. However, excess WO₃ species in the TiO₂ nanotubes resulted in poor PEC water-splitting performance. This behavior was attributed to the large agglomerates of WO₃ species were covered on the surface nanotubes that formed undesirable layers. Consequently, these undesirable layers would act as recombination sites for photo-induced electrons and holes.     

Photo-electrochemical properties of oxide semiconductors on porous titanium metal electrodes

For the purpose of producing hydrogen using solar energy, we investigated the potential of porous titanium metal sheet (PTMS) with high surface area for use as the basal plates for various types of oxide semiconductor photo-electrodes. The TiO₂ photoelectrodes were prepared by oxidation of PTMS and flat titanium metal sheet (FTMS). The photocurrents of the TiO₂/PTMS electrodes were always higher than TiO₂/FTMS under the same oxidation conditions. The reflectance of PTMS was lower than FTMS over the entire wavelength spectrum, suggesting that the scattered light was absorbed more effectively on the former. A nanocrystalline WO₃ layer-loaded PTMS electrode (WO₃/PTMS) showed a high photocurrent compared to WO₃/FTMS, suggesting that PTMS is highly suitable as basal plates for semiconductor photoelectrodes.     

Hydrogen production from photo-driven electrolysis of biomass-derived oxygenates: A case study on methanol using Pt-modified WO3 thin film electrodes

In this paper, we demonstrate the feasibility of H₂ production from biomass-derived oxygenates with photoelectrochemical cells (PECs) based on the tandem cell hybrid photoelectrode configuration. As a proof of concept, we have studied the simplest oxygenate, methanol, which is photoelectrochemically oxidized at thin film tungsten oxide (WO₃) photoelectrodes. When the methanol oxidation reaction (MOR) is coupled with the hydrogen evolution reaction (HER), this process is known as methanol electrolysis. We demonstrate that catalytic modification of the WO₃ surface by the electrodeposition of Pt particles can greatly increase MOR activity at the photoanode, resulting in a significant increase in H₂ production rates from methanol electrolysis. This improvement is greatest at low overpotentials and high Pt loadings, with the demonstrated MOR current density of Pt-WO₃ being nearly four times that of the oxygen evolution reaction (OER) on WO₃ at a potential of 0.8 V vs. the Reversible Hydrogen Electrode. We also illustrate how the increase in WO₃ photocurrent and the decrease in the oxidation onset potential, compared to the OER, make it possible to use WO₃-based photoelectrodes in a simple tandem cell configuration whereby a common PV component such as a-Si can provide the remaining voltage to achieve unassisted methanol electrolysis. Results from methanol electrolysis reveal the potential to utilize a similar approach for larger biomass-derived oxygenates, which could be a promising pathway to H₂ production from renewable feedstock using photo-driven electrolysis.     

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.     

Dimensionally stable anodes for the oxygen evolution reaction: Ruthenium dioxide on a nickel metal substrate

Dimensionally stable anodes (DSA) for the oxygen evolution reaction (OER) gradually passivate under operating conditions due to the formation of insulating TiO₂ at the interface of the Ti-metal substrate and the catalytically active metal oxide layer (typically a mixture of ruthenium and iridium oxides). The incorporation of a catalytically active, yet stable, interfacial buffer layer or substrate is a potential means of promoting the overall performance. Here, we prepared DSA-like RuO₂ on Ni films by the thermal decomposition method and contrast their OER activity with an analogous RuO₂ on Ti-metal film. The fabrication process resulted in the presence of NiO and Ni oxides/(oxy)hydroxides at the surface of the rutile RuO₂ layer. Along with RuO₂, the former species were active in the water splitting reaction under the testing conditions. Ni–RuO₂ films showed lower OER overpotentials relative to Ti–RuO₂ suggesting a synergistic effect between Ni- and Ru-oxides and the Ni layer between Ti and RuO₂ to improve overall performance. Further studies should optimize the NiO–RuO₂ loading levels, understand the NiO–RuO₂ synergy toward catalytic performance and determine the stability under accelerated testing conditions.     

Photocatalytic hydrogen generation by WO3 in synergism with hematite-anatase heterojunction

The present study reports about exploration of a multi-component photocatalytic system comprising of WO₃, TiO₂ and Fe₂O₃ with tandem n-n heterojunctions. The ternary WO₃/TiO₂/Fe₂O₃ nanocomposite with WO₃ nanoparticles over the interfaces of Fe₂O₃ and TiO₂ is synthesized by wet precipitation followed by thermal decomposition. The WO₃/TiO₂/Fe₂O₃ nanocomposite has an enhanced photocatalytic performance towards hydrogen generation by water splitting reaction under visible light irradiation, when compared to the Fe₂O₃/TiO₂ system. A band gap of 2.10 eV, favouring visible light absorption was achieved with the distribution of WO₃ nanopartcles over the interfaces of Fe₂O₃ and TiO_2. The as prepared WTF heterojunction exhibited a maximum hydrogen production rate of 10.2 mL h⁻¹ for a catalyst loading of 0.025 g mL⁻¹. The enhanced photocatalytic performance is tested in presence of various sacrificial agents and proton source. In both cases, the higher photocatalytic efficiency is attributed to the more visible light harnessing ability and pronounced charge separation owing to the tandem n-n heterojunctions generated between TiO₂ with WO₃ and TiO₂ with Fe₂O₃ semiconductors and enhancing the lifetime of the photogenerated electron-hole pairs.     

A composite visible-light photocatalyst for hydrogen production

A composite photocatalyst, Pd–TiO_2−xN_x–WO₃, was synthesized by the template method and characterized by energy dispersive X-ray microanalysis (EDX), X-ray diffraction (XRD), UV–vis spectrometer, and scanning electron microscope (SEM). The results of EDX analysis reveals that the molecular formula of the composite photocatalyst can be expressed as Pd–TiO_1.72N_0.28–WO₃. The UV–vis absorption spectrum indicates that the absorption edge of the catalyst red-shifts to around 600 nm. Under the irradiation with ultraviolet and visible light, the catalyst showed good performance for photocatalytic hydrogen production with a Na₂S/Na₂SO₃ system as the sacrificial agent.     

Controlled synthesis of MnO2@TiO2 hybrid nanotube arrays with enhanced oxygen evolution reaction performance

A novel tube-in-tube nanostructure of MnO₂@ TiO₂ hybrid arrays has been obtained by a facile and controllable chemical bath deposition method. Scrutiny on the hybrid arrays indicates that the chemical bath deposition method favors the growth of the MnO₂ nanotubes with different diameter which can modulate the oxygen evolution reaction (OER) activity as well as bandgap width of the hybrid. In terms of OER activity, onset potential (E_s) shifts negatively from 0.698 V (vs.Ag/AgCl) of pristine titania nanotube arrays (TNAs) to 0.501 V of the hybrid loaded with 26.6%wt MnO₂, and the current density on the hybrid electrode can be significantly enhanced up to 20.87 mA/cm², almost 97 times higher than that on TNAs electrode (0.214 mA/cm²). Optical absorption measurement suggests that the bandgap width (E_g) can be tuned by loading MnO₂ onto the TNAs implying interaction between the MnO₂ and TNAs. The MnO₂@TiO₂ hybrid nanotube arrays may find promising potential in electrochemical water splitting, photocatalysis, thermocatalysis and other sustainable energy applications.     

Pulsed sonoelectrodeposition of leaf-like Bi2WO6 film with exposed [001] facet as a photocathode with a remarkable high photovoltage in water splitting

Bi₂WO₆ is one of the promising triplet bismuth compound that has a layered structure with a high photocatalytic activity for photo-electrochemical (PEC) water splitting systems. Here, Bi₂WO₆ synthesizes by the sonoelectrochemical method in pulse mode of ultrasound. Unexpectedly, synthetic samples show photocathode rather than photoanode activity in PEC systems. Applying of this method creates a leaf-like morphology with exposed [001] crystal facet and controllable amount of surface defects. The co-existence of oxygen and metal vacancies play a significant role in suppressing charge recombination and enhancing charge transporting in photoelectrodes. The creation of high surface vacancies leads to change the conduction and valence band positions and cause hydrogen evolution by Bi₂WO₆ photoelectrodes. Another surprising result for the synthesized film by pulse mode is the creation of high photovoltage about 1.25 V that has a remarkable effect in suppressing charge recombination rate and proposed driving force for water reduction. Furthermore, the onset potential of the photoelectrodes improves and records high efficiencies (ABPE = 2.46% at −0.8 V and IPCE≈28% at 450 nm). The obtaining results introduces the sonoelectrochemical method as a promising method for the synthesis of highly efficient photoelectrodes.     

Evidencing enhanced oxygen and hydrogen evolution reactions using In–Zn–Co ternary transition metal oxide nanostructures: A novel bifunctional electrocatalyst

Ternary transition metal oxides are gaining popularity for cost effective bifunctional electrocatalytic activities and to realization of novel water splitting devices. In this regard, In₂O₃/ZnO/Co₃O₄ based ternary oxide nanostructures were investigated in detail for their oxygen/hydrogen evolution reaction (OER/HER) in alkaline environment. The ternary oxides were at first processed through a simple chemical route involving hydrothermal treatment. The prepared nanostructures were then investigated by using high-resolution transmission electron microscopy (TEM/HRTEM) to ascertain their morphological traits. X-ray diffraction, Raman signals and photoluminescence data demonstrated the In₂O₃ phase to be prevalent in the ternary mixture on par with that of ZnO and Co₃O₄. The valence state of various metal ions and the In–O, Zn–O and Co–O bonding was verified using XPS. The ternary oxide coated electrodes exhibited excellent overall water splitting activity. Overpotential values of 398 and 510 mV were registered for OER and HER experiments under a current density of ±10 mA cm⁻², demonstrating the material to be an ideal OER/HER electrocatalyst at room temperature. The exceptional long-term stability in ternary oxides and their Tafel slope (88 mV/dec for OER and 60 mV/dec for HER) further affirmed their unique anodic/cathodic characteristics for water splitting applications.     

Role of sputtered WO3 underlayer and NiFeCr-LDH co-catalyst in WO3–BiVO4 heterojunction for enhanced photoelectrochemical water oxidation

WO₃–BiVO₄ (WO-60s/BVO) heterojunction was synthesized by radio-frequency sputtered WO₃ onto FTO substrate, followed by spin-coating of BiVO₄ layer. Furthermore, Cr incorporated NiFe-LDH (NiFeCr-LDH) oxygen evolution reaction (OER) co-catalyst was electrodeposited onto WO-60s/BVO. The sputtered WO₃ underlayer in the WO-60s/BVO facilitated enhanced electron-hole separation and less transient time for the electron to arrive at back contact than conventional spin-coated WO₃ layers. Incorporating Cr into NiFe-LDH increased the electrical conductivity of LDH, which resulted in an enhanced transfer of photogenerated charge-carrier and significant promotion of the OER kinetics. The heterojunction with sputtered WO₃ underlayer and NiFeCr-LDH co-catalyst attained photocurrent density of 4.9 mA cm⁻² at 1.23 V vs. RHE with an IPCE value greater than 56% in the 350–470 nm wavelength range. Moreover, the WO-60s/BVO-NiFeCr photoanode showed only 7% decay in photocurrent after 6 h with H_2, and O₂ evolution of 98 and 47 μmol cm⁻² h, respectively, suggesting high stability for OER.     

Preparation of direct Z-scheme Bi2WO6/TiO2 heterojunction by one-step solvothermal method and enhancement mechanism of photocatalytic H2 production

Direct Z-scheme Bi₂WO₆/TiO₂ heterojunction photocatalyst was prepared by one-step solvothermal method. The catalyst was characterized by XRD, TEM, XPS, UV–Vis DRS, photoluminescence spectroscopy and photoelectrochemical studies. The photocatalytic hydrogen production experiments show that Bi₂WO₆ did not generate H₂ and the H₂-production rate of TiO₂ is only 0.1 mmol⋅g⁻¹h⁻¹. The hydrogen production rate of the Bi₂WO₆/TiO₂ heterojunction photocatalyst reaches 12.9 mmol⋅g⁻¹h⁻¹, which is 129 times that of TiO₂. Compared with TiO₂, the enhanced H₂-production activity of the heterojunction catalyst can be attributed to the wider light absorption range and the efficient separation and migration of carriers at the close contact interface between Bi₂WO₆ and TiO₂. Based on the work functions of Bi₂WO₆, TiO₂ and their heterojunctions, combined with the results of electron paramagnetic resonance spectroscopy and Mott-Schottky measurements, the photocatalytic H₂ production mechanism of Z-scheme heterojunction Bi₂WO₆/TiO₂ was proposed. This work provides an easy and simple way to design a binary Z-scheme photocatalyst with efficient catalytic H₂-production activity without electron mediators.     

Photodeposited CoOx as highly active phases to boost water oxidation on BiVO4/WO3 photoanode

Surface decoration of photoanodes with oxygen evolution cocatalysts is an efficient approach to improve the photoelectrochemical water splitting performance. Herein, ultrafine CoO_x was selectively immobilized on the surface of BiVO₄/WO₃ photoanode by using the photogenerated holes to in-situ oxidize Co₄O₄ cubane. The composited photoanode (CoO_x/BiVO₄/WO₃) displayed an enhanced photoelectrochemical (PEC) water oxidation performance, with a photocurrent density of 2.3 mA/cm² at 1.23 V_RHE under the simulated sunlight irradiation, which was 2 times higher than that of bare BiVO₄/WO₃. The characterization results for the morphological, optical and electrochemical properties of the photoelectrodes revealed that, the enhanced PEC performances could be attributed to the improved charge carrier separation/transport behaviors and the promoted water oxidation kinetics when the photoelectrodes were loaded with CoO_x.     

Hydrogen generation by the reaction of Al with water using oxides as catalysts

Hydrogen generation by the reaction of pure Al powder in water with the addition of Al(OH)₃, γ‐Al₂O₃, α‐Al₂O₃, or TiO₂ at mild temperatures was investigated. It was found that the reaction of Al with water is promoted and the reaction induction time decreases greatly by the above hydroxide and oxides. X‐ray diffraction analyses revealed that the hydroxide and oxide phases have no any change during the Al–water reaction, indicating that they are just as catalysts to assist the reaction of Al with water. A possible mechanism was proposed, which shows that hydroxide and oxides could dissociate water molecules and promote the hydration of the passive oxide film on Al particle surfaces. Copyright © 2013 John Wiley & Sons, Ltd.     

Vanadium based zinc spinel oxides: Potential materials as photoanode for water oxidation and optoelectronic devices

In the recent past, layered zinc-based vanadium spinel oxides (ZnVOs) have shown an intriguing way to accomplish the challenges of energy conversion, storage, and utilization issues. Here, through first-principles calculations, a comprehensive study has been carried out to investigate the AV₂M (where A = Zn, Zn₂, Zn₃, Zn_4, and M = O₄, O₆, O₇, O₈, O₉ respectively) electronic, photocatalytic, and optical properties. Formation energies with a negative sign express that the final compounds from the pure elements are possible and cohesive energies revealed that compounds are energetically stable. Spin-polarized calculations are also taken into account for better approximation of the electronic properties (band structure and density of states). All layered structures show indirect bandgap for spin-up calculations in range 0.3 eV–2.4 eV, while spin-down calculations show mix trends in range 2.3 eV–3.50 eV. An appropriate band edge with large enough kinetic over-potentials of the oxygen evolution reaction (ΔE_V ≥ 1.244 eV) makes them potential candidates as photoanode for water splitting. ZnV₂O₄ is more suitable for OER as it has small kinetic overpotential as compared to the oxidation potential of water. Interestingly, all ZnVOs display a dramatically large coefficient (~10⁵ cm⁻¹) for optical absorption. Photogenerated electrons and holes on the layered zinc-based vanadium spinel oxide surfaces could make these spinel oxides promising materials for photocatalytic water splitting and solar energy conversion.     

Layered WO3/TiO2 nanostructures with enhanced photocurrent densities

Layered WO₃/TiO₂ nanostructures, fabricated by magnetron sputtering, demonstrate significantly enhanced photocurrent densities compared to individual TiO₂ and WO₃ layers. First, a large quantity of compositions having different microstructures and thicknesses were fabricated by a combinatorial approach: diverse WO₃ microstructures were obtained by adjusting sputtering pressures and depositing the films in form of wedges; later layers of TiO₂ nanocolumns were fabricated thereon by the oblique angle deposition. The obtained photocurrent densities of individual WO₃ and TiO₂ films show thickness and microstructure dependence. Among individual WO₃ layers, porous films exhibit increased photocurrent densities as compared to the dense layer. TiO₂ nanocolumns show length-dependent characteristics, where the photocurrent increases with increasing film thickness. However, by combining a WO₃-wedge type layer with a layer of TiO₂ nanocolumns, PEC properties strikingly improve, by about two orders of magnitude as compared to individual WO₃ layers. The highest photocurrent that is measured in the combinatorial library of porous WO₃/TiO₂ films is as high as 0.11 mA/cm². Efficient charge-separation and charge carrier transfer processes increase the photoconversion efficiency for such films.     

Metal-organic framework derived Co3O4/TiO2 heterostructure nanoarrays for promote photoelectrochemical water splitting

In this work, we proposed a simple and new method to fabricate Metal-organic frameworks (MOFs) derived Co₃O₄ modified TiO₂ nanorods (NRs) photoelectrode by immersion and anneal treatment. The positively charged Co-MOF (ZIF-67) was adsorbed on the negatively charged TiO₂ NRs by electrostatic interaction, and then annealed in air to obtain the Co₃O₄/TiO₂ photoelectrodes. The photoelectrochemical (PEC) performance of the Co₃O₄/TiO₂ photoelectrodes has been significantly improved compared with the pure TiO₂, the best photocurrent density of Co₃O₄/TiO₂ photoelectrode could reach 1.04 mA/cm² (1.23 V vs RHE) which was almost 1.65 times than that of pure TiO₂. On the Co₃O₄/TiO₂ photoelectrodes, the significant improvement in PEC performance could be attributed to the constructed p-n heterostructure, which can promote charge transfer within the system and improve the efficiency of electron/hole separation. Meanwhile, under the action of the MOFs-derived Co₃O₄, the number of active sites increases significantly and visibly improve the photoresponse performance.     

Photoelectrochemical application of mesoporous TiO2/WO3 nanohoneycomb prepared by sol–gel method

A mesoporous TiO₂/WO₃ nanohoneycomb at a molar ratio of 3:1 was prepared by sol–gel method for photoelectrochemical splitting of water. In order to create a highly porous structure, the composite TiO₂/WO₃ with a block copolymer internal template was deposited on the substrate covered with polystyrene (PS) nanospheres. A mesoporous TiO₂/WO₃ composite nanohoneycomb was obtained after removing the PS spheres and copolymer by thermal treatment. It exhibited a lower band gap energy than TiO₂ so that the optical absorption edge was shifted toward the visible light region. It also showed a better photoelectrochemical efficiency of water splitting and higher production of hydrogen due to lower energy gap, higher reactive surface area, and better charge separation efficiency.     

Electrocatalyst of RuO2 decorating TiO2 nanowire arrays for acidic oxygen evolution

Oxygen evolution reaction (OER) at the anode limits the efficiency of hydrogen production from water electrolysis substantially. A novel electrocatalyst of RuO₂ decorating TiO₂ nanowire arrays for OER was successfully prepared using a cyclic voltammetric method with electrodeposition of RuO₂ nanoparticles on the TiO₂ nanowire (TNW) arrays synthesized hydrothermally. Even though the electrodes with the composite electrocatalyst have a lower loading of RuO₂, they have higher electrocatalytic activity and stability for acidic oxygen evolution than the Ti/RuO₂ electrode prepared by conventional thermal decomposition method. The core-shell structure of the TNW@RuO₂ electrocatalyst not only increases the specific surface area of the electrodes, but also inhibits the adverse effect of the poor conductivity of TiO₂. This novel OER electrocatalyst can improve the efficiency and reduce the cost of hydrogen production from electrolytic water splitting.     

Cycle durability of metal oxide supports for PEFC electrocatalysts

In order to develop durable electrocatalysts for polymer electrolyte fuel cells, six different metal oxides, namely MoO₃, SnO₂, Nb₂O₅, Ta₂O₅, TiO₂, and WO₃, are selected as thermochemically stable, carbon-free platinum electrocatalyst support materials. The stability of Pt on these alternative oxide support materials is systematically analyzed, for the first time, using common experimental protocols simulating realistic fuel cell vehicle operation. Pt/SnO₂ shows the best performance in terms of both electrochemical activity, and stability against dissolution. Pt dissolution rates in Pt/SnO₂ are comparable to those of conventional Pt/C electrocatalysts. These results suggest that SnO₂ is a promising candidate as an alternative electrocatalyst support.