Photoresponse of n-TiO2 thin film and nanowire electrodes

Thin film and nanowire electrodes of n-type titanium oxide (n-TiO₂) were fabricated and their photoresponses towards water-splitting reaction were studied. A more than twofold increase in maximum photoconversion efficiency was observed when a single-layer thin film of n-TiO₂ was replaced by nanowires. Highest photoconversion efficiency was observed at an applied potential of 0.61 V vs. E_aoc where the electrode potential at open circuit, E_aoc was found to be −1.0 V SCE⁻¹ at illumination intensity of 40 mW cm⁻² in 5.0 M KOH solution. The maximum photoconversion efficiency was approximately of another twofold increase for water splitting at nanowire electrodes when methanol was used as a sacrifacial hole scavenger (depolarizing agent) in the electrolyte. Also, this maximum photoconversion efficiency was found at a lower applied potential of 0.41 V vs. E_aoc in presence of methanol. The band-gap energy of the n-TiO₂ films and nanowires annealed at 750°C was found to be 2.98 eV, which indicates their rutile structure.     

Spray pyrolytically deposited nanoporous Ti doped hematite thin films for efficient photoelectrochemical splitting of water

Nanoporous hematite (α-Fe₂O₃) thin films doped with Ti⁴⁺ deposited by spray-pyrolysis were successfully used in photoelectrochemical splitting of water for solar hydrogen production. X-ray diffraction, field emission scanning electron microscopy, UV–visible absorption and photoelectrochemical studies have been performed on the undoped and Ti⁴⁺ doped hematite thin films. Morphology of α-Fe₂O₃ thin films was observed to be nanoporous, with increased porosity (pore size ∼12 to 20 nm) on increasing doping concentration. A significant decrease in the bandgap energy from 1.95 to 1.27 eV was found due to doping. α-Fe₂O₃ film doped with 0.02 M Ti⁴⁺ ions exhibited best solar to hydrogen conversion efficiency (photoconversion efficiency) of 1.38% at 0.5 V/SCE. Highest photocurrent densities of 0.34 mA/cm² at zero bias and 1.98 mA/cm² at 0.5 V/SCE were obtained by incorporating 0.02 M Ti⁴⁺ in α-Fe₂O₃, which are significantly larger than earlier reported values. Donor density (30.8 × 10²⁰ cm⁻³) and flatband potential (−1.01 V/SCE) obtained were also maximum for this sample. Hydrogen collected in 1 hr at Pt electrode with the best photoelectrode was 2.44 mL with 150 mW/cm² visible light source.     

Photoresponse and stability improvement of ZnO nanorod array thin film as a single layer of photoelectrode for photoelectrochemical water splitting

ZnO nanorod array thin film with Al-doping and hydrogen treatment was developed as a photoelectrode combining the functions of transparent conducting oxide thin film and photoactive 1-dimensional nanostructured semiconductor into a single layer for photoelectrochemical water splitting. It was demonstrated that hydrogen treatment and Al-doping enhanced the dark currents, photocurrents, and hydrogen generation efficiencies largely and the enhancement by hydrogen treatment was more significant. The maximum photoinduced hydrogen generation efficiency was about 0.020%. Furthermore, hydrogen treatment also improved the photosensitivity and the stability under illumination significantly. The minimum decay time constant and rise time constant were 1.71 and 1.22 s, respectively. And after current-voltage scanning upon illumination for 50 cycles, the 1-dimensional morphology still remained unchanged but those without Al-doping and/or hydrogen treatment were altered seriously. The good photoresponse and stability made the Al-doped ZnO nanorod array thin film with hydrogen treatment have wide applications in the photoelectrochemical field.     

Gallium-doped tungsten trioxide thin film photoelectrodes for photoelectrochemical water splitting

A sol–gel method was used to synthesise different compositions of Ga-doped tungsten trioxide thin films using tungstic acid and gallium(III) nitrate as starting materials. The precursor solutions were drop-casted on FTO glass and annealed at 500 °C for 30 min, and the resulting materials were characterised with SEM, XRD, UV/Vis spectrophotometry and photoelectrochemical analysis. The Ga-doped WO₃ samples exhibited a greater grain than undoped WO₃, and a monoclinic structure was observed for all WO₃ samples. The band gap reduction of WO₃ from 2.74 to 2.60 eV indicated the red-shift of light absorption towards the visible light region in the solar spectrum. The donor carrier density for the doped WO₃ increased, and the conduction band edge position exhibited a positive shift. The photoactivity of WO₃ increased threefold when the photoanode was 20% doped with gallium.     

Improved 3D porous structures of Ni electrodes prepared by high-pressure cold spray and post annealing for water splitting

For improving the performance of water splitting, high pressure cold spray was applied to prepare Ni electrodes with controlled 3D porous structures. Al was added as pore former to endow the Ni electrodes with porous structures. Heat treatment was performed to improve the adhesion of the prepared coatings and thus boost the durability of the obtained Ni electrodes. The electrochemical results and characterizations revealed that porous Ni electrodes were successfully obtained. The porosity of obtained electrodes was increased with the addition of Al. Alloy phases such as NiAl and Ni₃Al were detected after heat treatment. An oxide layer was found surrounding Ni layers after chemical leaching. The thickness of oxide layer was found increased with heating temperature and Al addition. NiO, Ni(OH)₂ and NiOOH were found in the leached samples. With porous coatings, the electrodes possessed an improved current density of HER/OER. The Tafel slops of HER/OER were about 120mv/dec which indicated that the rate of water splitting was controlled by Volmer step. Due to the existence of oxide layers and unconnected pores, some samples exhibited Warburg diffusion behavior. Benefitting from the loose and porous microstructure, N30A sintered at 400 °C performed the best catalytic activity for HER/OER, and is considered the best sample for water splitting in alkaline condition.     

Hierarchically SrTiO3@TiO2@Fe2O3 nanorod heterostructures for enhanced photoelectrochemical water splitting

In this particular work, the fabrication of SrTiO₃@TiO₂@ Fe₂O₃ nanorod heterostructure has been demonstrated via hydrothermal growth of SrTiO₃ cubic on the rutile TiO₂ nanorod as a template and later sensitized with Fe₂O₃ for photocatalytic solar hydrogen production in a tandem photoelectrochemical cell and dye-sensitized solar cell (DSSC) module. The photocatalytic solar hydrogen production of this heterostructure was optimized by controlling the amount of Sr and Fe on the surface of photocatalyst. The details of the influencing parameters on the physicochemical and photoelectrochemical properties are discussed. It was found that the morphology and quality of the fabricated materials were greatly manipulated by the concentration of Sr and Fe. The optimized 0.025 M SrTiO₃@TiO₂@ Fe₂O₃ heterostructure exhibited a higher photoconversion efficiency with a long electron lifetime, low charge transfer resistance and large donor density at the electrode and electrolyte interface. This composite has significantly improved the photocatalytic hydrogen production, yielding 716 μmol/cm² of maximum accumulative hydrogen. These results show that morphology rendering and manipulation of energy band alignment is crucial in creating efficient heterojunctions for excellent contributions in photocatalytic applications.     

Physically and chemically synthesized TiO2 composite thin films for hydrogen production by photocatalytic water splitting

TiO₂ thin films have been synthesized by radio-frequency magnetron sputtering and sol–gel method to study the hydrogen generation by photocatalytic water splitting under visible light irradiation. Photoelectrochemical cell with chemical bias, involving photo-anode in form of TiO₂ film deposited on conducting indium tin oxide (ITO) film and Pt as cathode, is developed. The effect of conducting ITO layer on photo-voltage is studied by varying the thickness of ITO films. Constant H₂ generation rate is obtained for long period of time by both the TiO₂ films because of the separated evolution of H₂ and O₂ gas, thus eliminating the back-reaction effect. Sputter-deposited film as compared to sol–gel-synthesized film showed better H₂ generation rate, mainly explained in terms of the higher visible light absorption achieved by oxygen vacancies created in the TiO₂ film by the energetic target ions during deposition in pure Ar gas pressure.     

Efficiency of solar water splitting using semiconductor electrodes

Reliable measurement of the photoconversion efficiency for semiconductor electrodes is essential to the assessment of electrode performance. In this paper, the influence of the choice of light source on measured photoconversion efficiencies for semiconductor photoelectrodes is examined. Measurements of efficiency performed under xenon lamp and solar illumination are compared with efficiencies calculated by integrating the incident photon conversion efficiency (IPCE) over the lamp and solar spectra. It is shown that use of a xenon lamp as the light source can lead to a large overestimate of the photoconversion efficiency, relative to that obtained under standard AM1.5 solar illumination. The overestimate is greater when a water filter is fitted to the xenon lamp, and when a wide-band gap semiconductor such as TiO₂ is used as the photoelectrode. Achievable photoconversion efficiencies using rutile TiO₂ are calculated taking into account the losses due to imperfect absorption, reflection and charge-carrier recombination; these calculated efficiencies agree with the measurements to within experimental uncertainties. It is demonstrated that many photoconversion efficiencies presented in the literature are overestimated. It is concluded that reliable estimation of efficiency under standard conditions is best obtained by measuring the IPCE as a function of wavelength, and integrating over the AM1.5 solar spectrum, or by measuring under sunlight with a similar zenith angle to that of the AM1.5 spectrum.     

ZnBi38O60/Bi2O3 photocathode for hydrogen production from water splitting

Hydrogen production from water splitting into photoelectrochemical cells is a promising alternative for reducing the use of fossil fuels. Here, we synthesize by spray pyrolysis a porous ZnBi₃₈O₆₀/γ-Bi₂O₃ film with a surface area of 744 m² g⁻¹ for use as a photocathode in water-splitting cells. The film of ZnBi₃₈O₆₀ with 3 wt% Bi₂O₃ has 2.3 eV bandgap energy and a conduction band energy of −2.14 V vs. RHE at pH 6.99, which is thermodynamically suitable for reducing H⁺ to H₂. Under illumination, the film produces a current density of −1.55 mA cm⁻² at 0 V vs. RHE with an onset potential of 0.84 V vs. RHE. HC-STH efficiency is 0.09% at 0.17 V vs. RHE and IPCE at 0 V vs. RHE is 3.8% at 480 nm. Under continuous operation, the ZnBi₃₈O₆₀/γ-Bi₂O₃ film shows a stable photocurrent of −0.4 mA cm⁻² at 0 V vs. RHE for 1800 s with 100% Faradaic efficiency.     

Design of laser-induced graphene electrodes for water splitting

Efficient energy storage from intermittent renewables can rely on the conversion of temporary energy excess by alkaline electrolysis, yielding oxygen and green hydrogen, which can be stored and used on demand. Electrodes made of laser-induced graphene (LIG) materials offer many advantages over the traditional graphene processing routes, due to inherent simplicity and low cost-benefit. Despite poorly studied, LIG electrodes are promising for water splitting when properly doped/modified with metals. However, proper design and processing optimization should be considered. The present study is devoted to the laser processing effects on the LIG electrode performance towards water splitting in alkaline media. Promising guidelines were obtained for hydrogen production, showing high electrochemical activity, while the microstructural degradation can be minimised by selecting suitable laser processing conditions, such as 3.6 W of laser power, 100 mm/s of laser scan rate, 36 mJ/mm of energy density and 2 laser scans.     

Catalyst loading for Pt-nanowire thin film electrodes in PEFCs

Among electrocatalysts with novel nanostructures in low temperature polymer electrolyte fuel cells (PEFCs), Pt nanowires (Pt-NWs), as one-dimensional (1-D) nanomaterials, are recognized as promising candidates. It has also been reported that the excellent catalytic performance of the nanostructure benefited from their unique 1-D features, but also bring unusual shapes and bulky specific volumes, which make Pt-NWs difficult to fabricate into fuel cell electrodes by any conventional procedures. To understand the effect of catalyst loading on the Pt-NW electrode structure, Pt-NW thin film electrodes of various catalyst loadings were examined towards the oxygen reduction reaction (ORR) ability at the cathode side in low temperature PEFCs. SEM, XRD and electrochemical impedance spectroscopy (EIS) measurements were performed to help understanding and elucidating the electrode role under ‘real’ conditions. The results showed a similar optimal catalyst loading as compared with conventional GDEs with spherical electrocatalysts, but exhibiting a different electrode structure with increasing Pt-NW loading, although a similar larger mass transfer resistance was observed at high Pt-NW loading. The mechanism is further discussed in this paper.     

Hydrogen generation from photocatalytic water splitting over TiO2 thin film prepared by electron beam-induced deposition

Photocatalytic TiO₂ thin films were prepared via an electron beam-induced deposition (EBID) method. The effects of post-calcination treatment on the properties of the prepared TiO₂ thin films were studied. X-ray diffraction (XRD), scanning electron microscope-energy dispersive spectrometry (SEM-EDS), and UV–V is absorption spectrometry were performed to reveal the crystallinity, surface morphology, chemical composition, and light absorbance of the prepared TiO₂ thin films. The photoelectrochemical characteristics of the TiO₂ thin films were investigated with a potentiostat. Under UV irradiation, a photocurrent of ˜2.1 mA was observed for the TiO₂ thin film with post-calcination at 500 °C. A water-splitting reaction was conducted over the TiO₂ thin film with the best photoelectrochemical performance. The yields of hydrogen and oxygen were 59.8 and 30.6 μmole, respectively, after 8 h of reaction under UV irradiation.     

Combinatorial development of nanoporous WO3 thin film photoelectrodes for solar water splitting by dealloying of binary alloys

A combinatorial materials approach is suggested for the development of nanoporous thin film oxides for photoelectrochemical solar water splitting. As a precursor for nanoporous WO₃ films, metallic nanoporous W films were synthesized by dealloying sputtered W_1−xAl_x and W_1−xFe_x (0.06 < x < 0.67) thin film materials libraries in aqueous HNO₃ solutions with different concentrations for 24 h under open circuit conditions. The variation of the etchant concentration provided different film nanostructures. The films were then transformed into nanoporous WO₃ by controlled thermal oxidation at 500 °C in air. Screening of the photoelectrochemical properties of nanoporous WO₃ films shows a strong porosity- and thickness-dependence of the photocurrent. At the same time the photocurrent density does not depend on precursor composition, because dealloying in acid solutions of certain concentration leads to formation of identical nanostructures in a broad range of precursor compositions.     

Designed synthesis of hierarchical Mn3O4@SnO2/Co3O4 core-shell nanocomposite for efficient electrocatalytic water splitting

The hierarchical Mn₃O₄@SnO₂/Co₃O₄ core-shell nanocomposite has been successfully synthesized via a facile structural construction strategy. The SnO₂/Co₃O₄ nanosheets (SnO₂/Co₃O₄ NSs) were enclosed on the surface of Mn₃O₄ nanorods (Mn₃O₄ NRs). The prepared materials were investigated as the catalyst toward oxygen evolution reaction (OER) performance in alkaline. By contrast, the design of core-shell hierarchical nanocomposite of Mn₃O₄@SnO₂/Co₃O₄ possesses the obvious electrocatalytic OER performance than others, showing the overpotential of approximately 420 mV at a current density of 10 mA cm⁻² with a low Tafel slope of 70.1 mV dec⁻¹, in which the interesting structure can provide the interfacial and cooperative effect between core (hierarchical SnO₂/Co₃O₄ NSs) and shell (1D Mn₃O₄ NRs) that 1D Mn₃O₄ NRs can act as an electron acceptor and accelerates electron transfer, and that hierarchical SnO₂/Co₃O₄ NSs provide a large specific surface area and multiple exposed surface active sites between electrode material and electrolyte.     

Conductivity controlling of Cu2O film photoelectrode for water splitting by a novel electrochemical approach - Differential potentiostatic deposition

Cuprous oxide (Cu₂O) is a kind of low-cost and promising material for water splitting to produce hydrogen (p-type Cu₂O) and oxygen (n-type Cu₂O). However, the reason of conductivity transforming from p-type to n-type for Cu₂O films during potentiostatic deposition is waiting to be revealed. In this work, a novel electrochemical technology, differential potentiostatic deposition (DPD), is developed by coupling a 3-electrode setup with a resistor connected in series with the counter electrode circuit through a potentiostat. By this approach, deposition current density is adjusted in a short period to simulate different stages in a traditional potentiostatic deposition (TPD). The result shows that semiconducting conductivity of Cu₂O film changes from p-type to n-type with time during a long-term TPD in basic CuSO₄ solution. Employing the DPD method, conductivity of Cu₂O film transforms from p-type to n-type with current density decreasing. Through characterizing thickness, composition and photoelectrochemical performance of Cu₂O films, the mechanism of semiconducting conductivity transformation for Cu₂O films is proposed. Besides, the results indicate that the DPD is an effective method to tune the conductivity of metal oxide photoelectrodes for water splitting.     

Highly efficient Bi2O3/MoS2 p-n heterojunction photocatalyst for H2 evolution from water splitting

The design of p-n heterojunction photocatalysts to overcome the drawbacks of low photocatalytic activity that results from the recombination of charge carriers and narrow photo-response range is promising technique for future energy. Here, we demonstrate the facile hydrothermal synthesis for the preparation of Bi₂O₃/MoS₂ p-n heterojunction photocatalysts with tunable loading amount of Bi₂O₃ (0–15 wt%). The structure, surface morphology, composition and optical properties of heterostructures were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), UV–visible absorption spectroscopy, Brunauer-Emmett-Teller (BET) surface area, photoluminescence (PL), electrochemical impedance spectroscopy (EIS). Compare to pure Bi₂O₃ and MoS_2, the Bi₂O₃/MoS₂ heterostructures displayed significantly superior performance for photocatalytic hydrogen (H₂) production using visible photo-irradiation. The maximum performance for hydrogen evolution was achieved over Bi₂O₃/MoS₂ photocatalyst (10 μmol h⁻¹g⁻¹) with Bi₂O₃ content of 11 wt%, which was approximately ten times higher than pure Bi₂O₃ (1.1 μmol h⁻¹g⁻¹) and MoS₂ (1.2 μmol h⁻¹g⁻¹) photocatalyst. The superior performance was attributed to the robust light harvesting ability, enhanced charge carrier separation via gradual charge transferred pathway. Moreover, the increased efficiency of Bi₂O₃/MoS₂ heterostructure photocatalyst is discussed through proposed mechanism based on observed performance, band gap and band position calculations, PL and EIS data.     

In situ modification by graphidyne as interlayer in titanium dioxide thin film / platinum for water splitting photocatalysis

Modifying titanium dioxide (TiO₂) with graphdiyne (Gry) makes that the absorption range of UV-vis-nIR spectrum expands in visible light area. However, in traditional methods, the two materials are combined by hydrothermal process. In this paper, in-situ synthesis, which monomer directly polymerizes on the surface of TiO₂ instead that polymer and TiO₂ combined by hydrothermal method, is employed to get hybrid materials. In the process, TiO₂ thin film was deposited on the glass slide followed by being enfolded in copper (Cu) foil to initiate polymerization and synthesize Gry on the surface of TiO₂ directly. It turns out that with co-catalyst Platinum (Pt) the efficiency of TiO₂-Gry with introduction of only 1.9 wt% Gry is 12.6% higher than the efficiency of TiO₂ under same test conditions. The improvement is attributed to band structure of the combined material and efficiency of charge transfer. In this system, Gry acts as an excellent medium for charge transfer, which benefits from its intrinsic property and effective contact with TiO₂ by this synthesis. The work points out a new way to get well-combined platelike photocatalyst on transparent substrate. It gives a promising prospect for practical photocatalysis in the future that the quantity and mass ratio of combined materials can be recorded and modified.     

Integrating Co3O4 nanoparticles with MnO2 nanosheets as bifunctional electrocatalysts for water splitting

Developing high-efficiency and low-cost electrocatalyst is significant for the application of water splitting technology. Herein, Co₃O₄ nanoparticles and MnO₂ nanosheets are separately synthesized and subsequently assembled into a unique 0/2-dimensional heterostructure via van der Waals interactions. The consequent composites expose abundant accessible active sites and expedite the reaction kinetics, which can be testified by the superiorities in Tafel slope, exchange current density and double-layer capacitance, only requiring overpotentials of 355 and 129 mV for oxygen and hydrogen evolution reactions in 1.0 M KOH at 10 mA cm^?2, respectively. Moreover, a cell voltage of 1.660 V can drive the electrolyzer at 10 mA cm^?2. Benefitted from robust integration, the original aggregation and restacking of individual materials have been overcome, thereby leading to superior elelctrocatalysis durability. This facile and universal strategy may inspire the researchers on the design and construction of advanced functional composites.     

CeO2 modified Fe2O3 for the chemical hydrogen storage and production via cyclic water splitting

The chemical hydrogen storage (hydrogen reduction) and production (water splitting) behaviour of Ce-modified Fe₂O₃ mixed oxides were investigated. Fe_1−xCe_xO_2−δ (x = 0, 0.05, 0.1, 0.2, 0.3, 0.4 and 1) oxides prepared by chemical precipitation were characterized by XRD (X-ray diffraction), H₂-TPR (hydrogen temperature-programmed reduction) and H₂O-TPO (steam temperature-programmed oxidation) tests. XRD results showed that two kinds of Fe–Ce–O solid solutions (Ce-based and Fe-based) coexisted in Fe–Ce mixed oxides. H₂-TPR experiment suggested that Ce addition could reduce hydrogen reduction temperature while H₂O-TPO experiments over reduced oxides showed that Fe–Ce mixed oxides could split water to produce hydrogen at a lower temperature and complete in a shorter time. Both redox reactions (hydrogen reduction and water splitting) were sensitive to the temperature and active at a high temperature. The successive redox cycles were carried out over the Fe_0.7Ce_0.3O_2−δ mixed oxide at 750 °C. It kept a stable production of hydrogen in the successive redox process at the condition of serious agglomeration of the materials. The highest hydrogen storage amount was up to 1.51 wt% for the Fe–Ce sample with a 30% substitution of Ce for Fe.     

Effect of Mo-doping in SnO2 thin film photoanodes for water oxidation

New semiconducting metal oxides of various compositions are of great interest for efficient solar water oxidation. In this report, Mo-doped SnO₂ (Mo:SnO₂) thin films deposited by reactive magnetron co-sputtering in the Ar and O₂ gas environment are studied. The Sn to Mo ratio in the films can be controlled by changing the O₂ partial pressure and the deposition power of the Sn and Mo targets. Increasing the Mo concentration in the film leads to the increase in the oxygen vacancy density, which limits the maximum achievable photocurrent density. The thin films exhibit a direct band gap of 2.7 eV, the maximum achievable photocurrent density of 0.6 mA cm⁻² at 0 V_RHE and the onset potential of 0.14 V_RHE. The incident photon to current transfer (IPCE) efficiency of 22% is shown at a 450 nm wavelength. The initial performance of the Mo:SnO₂ thin films is evaluated for solar water oxidation.