Design of efficient Mn-doped α-Fe2O3/Ti-doped α-Fe2O3 homojunction for catalyzing photoelectrochemical water splitting

To produce clean chemical fuel of hydrogen efficiently, applying photocatalysts for conducting photoelectrochemical water splitting is indispensable. Hematite (α-Fe₂O₃) has been considered as one of the most effective photocatalysts for water oxidation due to excellent visible-light responses, high stability and source abundance properties, but low electrical conductivity and slow oxidation evolution kinetics limit its application. In this study, a novel α-Fe₂O₃ homojunction is constructed via doping Ti and Mn in two layers using two-step hydrothermal synthesis followed by one-step annealing process. Co-doping effect of Ti and Mn in α-Fe₂O₃ and growing sequence of Mn doped α-Fe₂O₃ (Mn:Fe₂O₃) and Ti doped α-Fe₂O₃ (Ti:Fe₂O₃) are also investigated to illustrate the efficient design of Mn:Fe₂O₃/Ti:Fe₂O₃ homojunction. The optimized Mn:Fe₂O₃/Ti:Fe₂O₃ electrode shows the highest photocurrent density of 2.10 mA/cm² at 1.60 V_RHE respectively comparing to those of 0.10, 1.20 and 0.22 mA/cm² for Ti:FeOOH, Ti:Fe₂O₃ and α-Fe₂O₃ electrodes. The outstanding performance of Mn:Fe₂O₃/Ti:Fe₂O₃ homojunction is attributed to the smaller charge-transfer resistance, higher carrier density, and less charge recombination. This work gives a rational design for hematite-based photocatalysts and successfully attains greatly improved photocatalytic ability for water oxidation. Development of homojunction using heteroatom doping in thus verified to be highly applicable on synthesizing promising photocatalysts.     

Electrodeposited zirconium-doped α-Fe2O3 thin film for photoelectrochemical water splitting

Nanostructured hematite thin films were doped with zirconium successfully using electrodeposition method for their implementation as photoanode in photoelectrochemical (PEC) cell for hydrogen generation. XRD, Raman, XPS, SEM and UV-visible spectroscopy techniques were used to characterize the thin films. Highest photocurrent density of 2.1 mA/cm² at 0.6 V/SCE was observed for 2.0 at.% Zr⁴⁺ doped α-Fe₂O₃ sample with solar to hydrogen conversion efficiency of 1.43%. Flatband potential (−0.74 V/SCE) and donor density (2.6 × 10²¹ cm⁻³) were found to be maximum for the same sample. These results suggest substantial potential of hematite thin films with controlled doping of zirconium in PEC water splitting applications.     

Nanostructured bilayered thin films in photoelectrochemical water splitting – A review

In the quest for achieving the desired efficiency, balanced economics and prolonged durability of the photoelectrochemical (PEC) system for hydrogen generation, heterostructures consisting of two or more semiconductors are being looked upon as favourite material alternatives. This communication describes the basic principles involved and summarizes most of the work done in this domain. Band gap, electronic band edge alignment of the materials with each other and with the redox potential of water, lattice mismatch of the materials and optimization of thickness of each layer at the junction in the PEC devices appear to be crucial for attaining enhanced photoresponse and efficiency. Based on the studies reported in the literature and from our own studies, heterojunction systems are considered as effective tool towards extending the spectrum to the visible range and for effective separation of charge carriers leading to development of efficient solar hydrogen production system.     

Nanomaterials for photoelectrochemical water splitting – review

Photoelectrochemical (PEC) water splitting using nanomaterials is one of the promising techniques to generate hydrogen in an easier, cheaper and sustainable way. By modifying a photocatalyst with a suitable band width material can improve the overall solar-to-hydrogen (STH) energy conversion efficiency. Nanomaterials can tune their band width by controlling its size and morphology. In many studies, the importance of nanostructured materials, their morphological and crystalline effects in water splitting is highlighted. Charge separation and transportation is the major concern in PEC water splitting. Nanomaterials are having high surface to volume ratio which facilitates charge separation and suppress electron-hole pair recombination. This review focuses on the recent developments in water splitting techniques using PEC based nanomaterials as well as different strategies to improve hydrogen evolution.     

Atom doping in α-Fe2O3 thin films to prevent hydrogen permeation

Prevention of hydrogen (H) penetration into passive films and steels plays a vital role in lowering hydrogen damage. This work reports effects of atom (Al, Cr, or Ni) doping on hydrogen adsorption on the α-Fe₂O₃ (001) thin films and permeation into the films based on density functional theory. We found that the H₂ molecule prefers to dissociate on the surface of pure α-Fe₂O₃ thin film with adsorption energy of −1.18 eV. Doping Al or Cr atoms in the subsurface of α-Fe₂O₃ (001) films can reduce the adsorption energy by 0.03 eV (Al) or 0.09 eV (Cr) for H surface adsorption. In contrast, Ni doping substantially enhances the H adsorption energy by 1.08 eV. As H permeates into the subsurface of the film, H occupies the octahedral interstitial site and forms chemical bond with an O atom. Comparing with H subsurface absorption in the pure film, the absorption energy decreases by 0.01–0.22 eV for the Al- and Cr-doped films, whereas increases by 0.82–0.96 eV for the Ni-doped film. These results suggest that doping Al or Cr prevents H adsorption on the surface or permeation into the passive film, which effectively reduces the possibility of hydrogen embrittlement of the underlying steel.     

Photoactivity of MWCNTs modified α-Fe2O3 photoelectrode towards efficient solar water splitting

MWCNTs (Multiwalled Carbon Nanotubes) modified α-Fe₂O₃ (hematite) photoelectrodes have been investigated for their possible application in hydrogen generation via photoelectrochemical (PEC) splitting of water. Enhanced photoresponse seen in comparison to the pristine α-Fe₂O₃ films is credited to the effective charge facilitation and charge separation provided by MWCNT conducting support. 0.2 wt% MWCNTs modified α-Fe₂O₃ thin film exhibited the maximum photocurrent density of 2.8 mA/cm² at 0.75 V/SCE. Measured values of flat band potential, donor density, resistance, Applied bias photon-to-current efficiency (ABPE) and Incident-photon-to current-conversion efficiency (IPCE) support the observed enhancement in photocurrent.     

α-Fe2O3 nanoarrays photoanodes decorated with Ni-MOFs for enhancing photoelectrochemical water oxidation

Owing to severe recombination of photogenerated charges, sluggish kinetics of oxygen evolution reaction (OER) and high overpotential, the efficiency of photoelectrocatalytic (PEC) water splitting is severely restricted currently. Herein, a metal-organic framework (Ni-MOF) as cocatalyst has been introduced onto Fe₂O₃ nanoarrays for PEC water oxidation. The new Ni-MOFs/Fe₂O₃ photoanode obviously improves the PEC water oxidation performance with respect to the Fe₂O₃. Specifically, a high photocurrent density is achieved on the Ni-MOF/Fe₂O₃ film, which corresponds to two-fold over the pristine Fe₂O₃ film at 1.23 V vs. RHE. Moreover, the photoanode also exhibits a significant cathodic shift of the onset potential (~240 mV) relative to the bare Fe₂O₃. The enhanced PEC performance is attributed to effective utilization the surface-reaching holes and reduction of the surface charge recombination, which are confirmed by electrochemical impedance spectroscopy and the derived Bode analysis. This study brings new insight into the development of MOF-based materials in the field of PEC water splitting.     

Enhanced photocurrent by MOFs layer on Ti-doped α-Fe2O3 for PEC water oxidation

Low photocurrent density of hematite (α-Fe₂O₃) originating from the inherent defects usually hinders its application in photoelectrochemical (PEC) water oxidation. In this paper, the synergetic effect of increase of oxygen vacancies and in-situ constructing heterojunction by coating MOFs on the α-Fe₂O₃ nanoarrays gives rise to the boosted photocurrent of α-Fe₂O₃ from 0.25 mA/cm² to 2.1 mA/cm² at 1.23 V (vs. RHE). The results showed that the appropriate energy band structure engineered by the presence of MOFs layer not only facilitated the PEC water oxidation, but also enhanced the light absorption performance. With inducing oxygen vacancies in further, the intrinsic conductivity of photoanode can be well ameliorated. The value of carrier density is improved one order higher to promote charge transfer between the interfaces and raise the carrier separation efficiency as a result.     

Application of CuS–ZnS PN junction for photoelectrochemical water splitting

A CuS thin film was prepared by the sulfurization of the electrodeposited copper layer on the FTO substrate using sulfur powder at 400 °C. Surface morphology and structure of the CuS thin film were investigated by scanning electron microscopy and X-ray diffraction. The surface morphology of the CuS thin film was worm-like with the diameter of 70 nm and its crystal structure was hexagonal. Band gap energy of the CuS thin film was obtained as 1.5 eV using absorption spectra. Photoelectrochemical response of the CuS thin film was analyzed under chopped illumination at negative and positive potentials. It showed photoelectrochemical response at negative potentials (ca. 2.6 μA cm^?2 at ?0.4 V vs. Ag/AgCl), but not at positive potentials, which confirmed its p-type semiconductivity. A ZnS thin film was synthesized by spray pyrolysis method and characterized using field emission scanning electron microscopy, X-ray diffraction and UV–vis spectrometer. It was shown that the surface morphology was smooth with the grain size of about 50–150 nm. Also, its crystal structure and band gap energy were hexagonal and 3.72 eV, respectively. In order to obtain PN (positive–negative) junction and increase photoelectrochemical response, the ZnS (n-type semiconductor) thin film was deposited on CuS (p-type semiconductor). Linear scan of elemental composition confirmed the presence of FTO, CuS and ZnS layers. Photoelectrochemical characterization showed more photoresponse than the CuS thin film at negative potentials (13.6 μA cm^?2 at ?0.4 V vs. Ag/AgCl) and no photoresponse at positive potentials. The results confirmed the synthesizing of PN junction at the interface of CuS and ZnS.     

Recent trends in development of hematite (α-Fe2O3) as an efficient photoanode for enhancement of photoelectrochemical hydrogen production by solar water splitting

Solar-assisted water splitting using photoelectrochemical (PEC) cell is an environmentally benign technology for the generation of hydrogen fuel. However, several limitations of the materials used in fabrication of PEC cell have considerably hindered its efficiency. Extensive efforts have been made to enhance the efficiency and reduce the hydrogen generation cost using PEC cells. Photoelectrodes that are stable, efficient and made of cost-effective materials with simple synthesizing methods are essential for commercially viable solar water splitting through PEC technology. To this end, hematite (α-Fe₂O₃) has been explored as an excellent photoanode material to be used in the application of PEC water oxidation owing to its suitable bandgap of 2.1 eV that can utilize almost 40% of the visible light. In this study, we have summarized the recent progress of α-Fe₂O₃ nanostructured thin films for improving the water oxidation. Strategic modifications of α-Fe₂O₃ photoanodes comprising nanostructuring, heterojunctions, surface treatment, elemental doping, and nanocomposites are highlighted and discussed. Some prospects related to the challenges and research in this innovative research area are also provided as a guiding layout in building design principles for the improvement of α-Fe₂O₃ photoanodes in photoelectrochemical water oxidation to solve the increasing environmental issues and energy crises.     

Catalytic activity of electrodeposited ternary Co–Ni–Rh thin films for water splitting process

The influence of the deposition parameters on the composition and structure of Co–Ni–Rh ternary alloys was studied. The catalytic activity of the coatings for the hydrogen evolution process was investigated in 6 M KOH electrolyte. The thin films were deposited from baths containing a mixture of Co²⁺, Ni²⁺, and Rh³⁺ chloride complexes. A wide range of alloy compositions were achieved by applying different deposition potentials from −0.5 to – 0.9 V vs SCE. The obtained coatings were examined by energy dispersive X-ray spectroscopy (EDX) and X-ray diffraction (XRD) techniques. The surface morphology and chemical composition were also characterized with scanning electron microscopy (SEM) combined with EDX. The hydrogen evolution activity of some selected electrodes were examined in 6 M KOH using current-potential curve and electrochemical impedance spectroscopy (EIS) techniques. The SEM results showed that the surface morphology of the electrodes can be tailored by modification of the deposition potential. The higher exchange current densities were observed in catalytic measurements for the ternary alloys, which confirms their better catalytic activity in the water-splitting process.     

Enhanced photoelectrochemical performance of anatase TiO2 by metal-assisted S–O coupling for water splitting

In this study, hybrid density functional theory calculations have been used to investigate the electronic structures of (Mg, S), (2Al, S), (Ca, S), and (2Ga, S) codoped anatase TiO₂, aiming at improving their photoelectochemical performance for water splitting. It is found that the acceptor metals (Mg, Al, Ca, and Ga), assisting the coupling of the incorporated S with the neighboring O in TiO₂, lead to the fully occupied energy levels in the forbidden band of TiO₂, which is driven by the antibonding state π* of the S–O bond. It is also found that the metal-assisted S–O coupling can prevent the recombination of the photo-generated electron–hole pairs and effectively reduce the band gap of TiO₂. Among these systems, the (Mg, S) codoped anatase TiO₂ has the narrowest band gap of 2.206 eV, and its band edges match well with the redox potentials of water. We propose that this metal-assisted S–O coupling could improve the visible light photoelectrochemical activity of anatase TiO₂.     

High-throughput characterization of Ag–V–O nanostructured thin-film materials libraries for photoelectrochemical solar water splitting

Ag–V–O thin-film materials libraries, with both composition (Ag_22-77V_23-78O_x) and thickness (123–714 nm) gradients were fabricated using combinatorial reactive magnetron co-sputtering aiming on establishing relations between composition, structure, and functional properties. As-deposited libraries were annealed in air at 300 °C for 10 h. High-throughput characterization methods of composition, structure and functional properties were used to identify photoelectrochemically active regions. The phases AgV₆O₁₅, Ag₂V₄O₁₁, AgVO₃, and Ag₄V₂O₇ were observed throughout the composition gradient. The photoelectrochemical properties of Ag–V–O films are dependent on composition and morphology. An enhanced photocurrent density (~300–554 μA/cm²) was obtained at 30 to 45 at.% Ag along the thickness gradient. Thin films of these compositions show a nanowire morphology, which is an important factor for the enhancement of photoelectrochemical performance. The photoelectrochemically active regions were further investigated by high-throughput synchrotron-X-ray diffraction and transmission electron microscopy (Ag₃₂V₆₈O_x) which confirmed the presence of Ag₂V₄O₁₁ as the dominating phase along with the minor phases AgV₆O₁₅ and AgVO₃. This enhanced photoactive region shows bandgap values of ~2.30 eV for the direct and ~1.87 eV for the indirect bandgap energies. The porous nanostructured films improve charge transport and are hence of interest for photoelectrochemical water splitting.     

High throughput preparation of Ni–Mo alloy thin films as efficient bifunctional electrocatalysts for water splitting

The synthesis of cost-effective and high-performance electrocatalysts for water splitting is the main challenge in electrochemical hydrogen production. In this study, we adopted a high throughput method to prepare bi-metallic catalysts for oxygen/hydrogen evolution reactions (OER/HER). A series of Ni–Mo alloy electrocatalysts with tunable compositions were prepared by a simple co-sputtering method. Due to the synergistic effect between Ni and Mo, the intrinsic electrocatalytic activity of the Ni–Mo alloy electrocatalysts is improved, resulting in excellent HER and OER performances. The Ni₉₀Mo₁₀ electrocatalyst shows the best HER performance, with an extremely low overpotential of 58 mV at 10 mA cm^?2, while the Ni₄₀Mo₆₀ electrocatalyst shows an overpotential of 258 mV at 10 mA cm^?2 in OER. More significantly, the assembled Ni₄₀Mo₆₀//Ni₉₀Mo₁₀ electrolyzer only needs a cell voltage of 1.57 V to reach 10 mA cm^?2 for overall water splitting.     

Deep surface reconstruction of Co-based cocatalyst on Ti-doped α-Fe2O3 photoanode for highly efficient water oxidation

Transition metal-based cocatalysts play important roles in promoting the surface kinetics of hematite (α-Fe₂O₃) photoanode. However, their performances are restricted by the shallow reconstruction process for generating highly efficient metal oxyhydroxides, where the oxygen evolution reaction (OER) occurs. Therefore, a Brnsted base-regulated strategy is developed to promote the in situ surface reconstruction of cocatalysts on Ti-doped α-Fe₂O₃ (Ti–Fe₂O₃) under photoelectrochemical conditions. After deep surface reconstruction by electrochemical activation, the CoWO₄ cocatalyst decorated Ti–Fe₂O₃ photoanode (a-CoWO₄/Ti–Fe₂O₃) delivers a photocurrent density of 0.88 mA cm^?2 at 1.23 V_RHE, which is about 3.0 times of activated Ti–Fe₂O₃ (a-Ti-Fe₂O₃) and 1.5 times of activated CoO_x/Ti–Fe₂O₃. Tungstate promotes the surface reconstruction of cobalt-based cocatalyst, resulting in a significant increase in bulk charge separation efficiency (η_sep) and surface charge injection efficiency (η_inj). Moreover, the type-II heterojunction between CoWO₄-derived CoOOH and a-Ti-Fe₂O₃ drives the rapid separation and transfer of photogenerated electron-hole pairs, and enhances the performance of Ti–Fe₂O₃ photoanode.     

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.     

High-efficiency photoelectrochemical water splitting with heterojunction photoanode of In2O3-x nanorods/black Ti–Si–O nanotubes

Constructing heterojunction was an efficient way to promote photoelectrochemical (PEC) water splitting performance of TiO₂-based nano-photoanode. In this work, we demonstrated the feasible preparation of oxygen vacancies-induced In₂O₃ (In₂O_3-x) nanorods/black Si-doped TiO₂ (Ti–Si–O) nanotubes heterojunction photoanode for enhanced PEC water splitting. Black Ti–Si–O nanotubes were fabricated through Zn reduction of the as-annealed Ti–Si–O nanotubes, followed by In₂O_3-x nanorods coupling by a facile electrodepositing and Ar heat treatment. Solar to hydrogen conversion efficiency of the heterojunction photoanode reached as high as 1.96%, which was almost 10 times that of undoped TiO₂. The improved PEC properties were mainly attributed to co-doping effects of Si and Ti³⁺/oxygen vacancy as well as In₂O_3-x decoration, which resulted in enhanced optical absorption and facilitated separation-transport process of photogenerated charge carriers. Charge transfer process in the composite system and hydrogen production mechanism were proposed. This work will facilitate designing TiO₂-based nano-photoanodes for promoting water splitting by integrating with elements doping, oxygen vacancies self-doping and semiconductors coupling.     

Efficient photoelectrochemical water splitting and impedance analysis of WO3−x nanoflake electrodes

Solar-powered water splitting with photoelectrochemical (PEC) devices is considered to be a promising method to simultaneously harvest and store solar energy at a large scale. Nanostructured semiconductors offer potential advantages in PEC application due to their large surface area and size-dependent properties, such as increased absorption coefficient, increased band-gap energy and reduced carrier-scattering rate. In this contribution, self-doped tungsten trioxide (WO_3?x) nanoflake arrays were synthesized via a new route which involves the dealloying of Fe–W amorphous alloy, thermal treatment in air and properly cathodic polarization. The effects of different cathodic polarization current leading to different x value in WO_3?x on the morphology, phase, and photoelectrochemical performance of the resultant samples were investigated. It was found that WO_3?x with the appropriate x value presents a dramatic photoelectrochemical current density of 8.7 mA cm^?2 in the presence of methanol as a hole scavenger, five folds larger than that of pristine WO₃ nanoflakes. UV–vis reflection spectra suggest that the light absorption spectrum range of WO_3?x extends from UV to visible light region. Electrochemical impedance spectroscopy disclosed that the unique nanoflake architecture and the surface defects offer improved light harvesting as well as efficient charge transportation.     

Enhanced photoelectrochemical performance of an α-Fe2O3 nanorods photoanode with embedded nanocavities formed by helium ions implantation

Hematite is a prospective semiconductor in photoelectrochemical (PEC) water oxidation field due to its suitable bandgap for the solar spectrum absorption. Nevertheless, the low transfer and separation efficiency of the charge carriers are restricted by its diffusion length of hole which is 2–4 nm and further reduce the PEC performance. Here, we report an innovative method, by introducing nanocavities into the α-Fe₂O₃ nanorod arrays photoanodes through helium ions implantation, to improve the charge carriers' transfer and separation efficiency and further to enhance water oxidation performance. The result indicates that, the photocurrent density of nanocavities embedded α-Fe₂O₃ photoanode (S2-A sample) reaches 1.270 mA/cm² at 1.6 V vs. RHE which is 1-fold higher than that of the pristine α-Fe₂O₃ (0.688 mA/cm²) and the photocurrent density of S2-A sample reaches 0.652 mA/cm² at 1.23 V vs. RHE. In this work, the ion implantation combined with post annealing method is found to be a valid method to improve the photoelectrochemical performance, and it also can be further used to modify the other semiconductor photoelectrodes materials.     

Electrodeposition of Ir–Co thin films on copper foam as high-performance electrocatalysts for efficient water splitting in alkaline medium

Iridium-based bimetallic alloy system with unique performance is of great interest for high-temperature corrosive environment as a barrier layer or for water splitting of hydrogen/oxygen evolution reactions as a highly efficient and stable electrocatalyst. In this work, iridium-cobalt (Ir–Co) thin films were galvanostatically electrodeposited on a copper (Cu) foam electrode as an electrocatalyst for water splitting in 1.0 M KOH alkaline medium. The effects of loading and solution temperature on hydrogen evolution performance of Ir–Co deposits were investigated. The results show that Ir–Co deposits were adhered to substrates, with porous structure and hollow topography. The concentrations of Ir in the deposits with the loadings of 4.6, 3.2 and 0.8 mg·cm^?2 were 88, 88 and 75 wt%, respectively. Ir–Co deposit with the loading of 3.2 mg·cm^?2 required an overpotential of 108 mV for hydrogen evolution reaction to reach a current density of 30 mA cm^?2, having a low Tafel slope value of 36 mV·dec^?1. The changes in the solution temperature and catalyst loading had a significant effect on hydrogen evolution performance of Ir–Co/Ir–Co–O electrocatalysts. With the increasing of catalyst loading, the electrocatalytic activity increased firstly and then decreased. As the solution temperature was increased from 20 to 40 °C, the electrocatalytic activity of Ir–Co–O electrocatalyst increased, and then decreased with the rising of temperature. The apparent thermal activation energy obtained from Arrhenius plot was ~13.9 kJ mol^?1. Ir–Co/Ir–Co–O deposits exhibited relatively good electrocatalytic stability and durability. The present work demonstrates a possible pathway to develop a highly active and durable substitute for thin film electrocatalysts for water splitting of hydrogen evolution reaction.