NiO Nanoparticles Anchored on Phosphorus-Doped α-Fe2 O3 Nanoarrays: An Efficient Hole Extraction p-n Heterojunction Photoanode for Water Oxidation

Facile synthesis of CoOx@C/Ti-Fe2O3 photoanodes for efficient photoelectrochemical water oxidation

The development of photoelectrochemical (PEC) water splitting is hindered by the slow kinetics of four-electron processes for the oxygen evolution reaction (OER) and severe charge recombination. Amorphous carbon was chosen as a carrier for the active sites due to its exceptional conductivity and strong loading capacity. In addition, this enhanced performance was attributed to the loading of oxides of cobalt. Here, amorphous carbon-covered cobalt oxides chosen as a co-catalyst loaded on α-Fe2O3 (noted as CoOx@C/Ti-Fe2O3) have been synthesized, and they show a high current density (2.86 mA cm-2 under 1.23 V vs. RHE), and a low onset potential (0.611 V vs. RHE). Experimental analysis demonstrates that the charge transfer and separation leading to accelerated OER dynamics and improved PEC performance are enhanced by CoOx@C effectively. This study provides new ideas for designing high-performance photoelectrochemical electrodes based on amorphous carbon co-catalysts.

Design of Ti-Pt Co-doped α-Fe2O3 photoanodes for enhanced performance of photoelectrochemical water splitting

This study demonstrates Ti and Pt co-doping can synergistically improve the PEC performance of the α-Fe₂O₃ photoanode. By varying the doping methods, the sample with in-situ Ti ex-situ Pt doping (Ti_i-Pt_e) exhibits the best performance. It demonstrates that Ti doping in bulk facilities charge separation and Pt doping on the surface further accelerates charge transfer. In contrast, Ti doping on the surface inhibits charge separation, and Pt doping in bulk hinders charge separation and transfer. HCl treatment is used to minimize the onset potential further, while it is favorable for the ex-situ doped α-Fe₂O₃, which is more efficient on Ti_e than the Pt_e-doped ones. On the ex-situ Ti-doped α-Fe₂O₃ after HCl treatment, anatase TiO₂ is probed, suggesting that Ti-O bonds accumulate when Fe-O bonds are partly removed, which enhances the charge transfer in surface states. Unfortunately, HCl treatment also induces lattice defects that are adverse to charge transport, inhibiting the performance of in-situ doped α-Fe₂O₃ and excessively treated ex-situ doped ones. Coupled with methanol solvothermal treatment and NiOOH/FeOOH cocatalysts loading, the optimized Ti-Pt/Fe₂O₃ photoanode exhibits an impressive photocurrent density of 2.81 mA cm^-2 at 1.23 V vs. RHE and a low onset potential of 0.60 V vs. RHE.

Hematite decorated with nanodot-like cobalt (oxy)hydroxides for boosted photoelectrochemical water oxidation

Photoelectrochemical (PEC) water splitting has been considered as an alternative process to produce green hydrogen. However, the energy conversion efficiency of PEC systems was still limited by the inefficient photoanode. Cocatalysts decoration is regarded as an efficient strategy for improving PEC performance of photoanode. In this work, nanodot-like cobalt (oxy)hydroxides was rationally decorated on hematite to fabricate CoOOH/Fe2O3 photoanode. The resulted CoOOH/Fe2O3 exhibits a high photocurrent density of 1.92 mA cm-2 at 1.23 V vs. RHE, which is 2.6 times than that of bare Fe2O3. In addition, the onset potential displays a cathodic shift of ca. 110 mV, indicating that CoOOH can efficiently accelerate water oxidation kinetics over Fe2O3. The comprehensive PEC and electrochemical characterizations reveal that CoOOH could not only provide abundant accessible Co active sites for water oxidation, but also could passivate the surface states of Fe2O3, thus increase the carrier density and decrease the interfacial resistance. As a result, the PEC water oxidation performance over Fe2O3 was significantly boosted. This work supports that the roles of CoOOH cocatalyst is generic and such CoOOH could be used for other semiconductor-based photoanodes for outstanding PEC water splitting performance.

Synergistic Effect of Redox Dual PdO x /MnO x Cocatalysts on the Enhanced H2 Production Potential of a SnS/α-Fe2O3 Heterojunction via Ethanol Photoreforming

In the quest for optimal H₂ evolution (HE) through ethanol photoreforming, a dual cocatalyst-modified heterocatalyst strategy is utilized. Tin(II) sulfide (SnS) was hybridized with α-Fe₂O₃ to form the heterocatalyst FeOSnS with a p-n heterojunction structure as confirmed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), UV-vis diffusive reflectance spectroscopy (UV-vis DRS), and Brunauer-Emmett-Teller (BET) techniques. PdO _x and PdO _x /MnO _x cocatalysts were loaded onto the FeOSnS heterocatalyst through the impregnation method, as verified by high-resolution transform electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and elemental mapping. Photocatalytic ethanol photoreforming resulted in the production of H₂ as the main product with a selectivity of 99% and some trace amounts of CH₄. The FeOSnS2-PdO _x 2%/MnO _x 1% photocatalyst achieved the highest HE rate of 1654 μmol/g, attributed to the synergistic redox contribution of the PdO _x and MnO _x species.

Constructing oxygen vacancy-enriched Fe2O3@NiO heterojunctions for highly efficient electrocatalytic alkaline water splitting

The oxygen vacancy-enriched Fe2O3@NiO heterojunctions assembled by nanoparticles and nanosheets can be used as a highly efficient and stable dual-function electrocatalyst to achieve efficient all-water splitting.

Simple electrodeposition to synthesize a NiFeSx-modified Ti-Fe2O3 photoanode: an effective strategy to improve the photoelectrochemical water oxidation reaction

Decorating a high-efficiency oxygen evolution reaction (OER) electrocatalyst as a cocatalyst on an α-Fe₂O₃ photoanode is known to be one of the most efficient methods to improve the photoelectrochemical (PEC) water oxidation activity. In our work, different from traditional methods of transition metal sulfide cocatalyst synthesis, an NiFeS_x-decorated Ti-Fe₂O₃ photoanode is synthesized through a simple one-step electrodeposition method, which benefits the interface between Ti-Fe₂O₃ and NiFeS_x. With the help of this excellent OER electrocatalyst, the photocurrent density of the NiFeS_x-Ti-Fe₂O₃ photoanode rises to 3 mA cm^-2 at 1.23 V vs. RHE, which is 2.5 times greater than the photocurrent of Ti-Fe₂O₃. Moreover, the onset potential of NiFeS_x-Ti-Fe₂O₃ shifts negatively by 170 mV compared with that of pure Ti-Fe₂O₃. Furthermore, surface photovoltage spectroscopy (SPV) and transient photovoltage (TPV) techniques and photoelectrochemical impedance spectroscopy (PEIS) were used to analyze the true effects of NiFeS_x as an efficient cocatalyst for enhancing the PEC performance of the NiFeS_x-Ti-Fe₂O₃ photoanode. This work provides a simple method for loading a low-cost and efficient cocatalyst to modify a Ti-Fe₂O₃ photoanode for the PEC water oxidation reaction.

Facile Fabrication of a Highly Crystalline and Well-Interconnected Hematite Nanoparticle Photoanode for Efficient Visible-Light-Driven Water Oxidation

Facile and scalable fabrication of α-Fe₂O₃ photoanodes using a precursor solution containing Fe^III ions and 1-ethylimidazole (EIm) in methanol was demonstrated to afford a rigidly adhered α-Fe₂O₃ film with a controllable thickness on a fluorine-doped tin oxide (FTO) substrate. EIm ligation to Fe^III ions in the precursor solution brought about high crystallinity of three-dimensionally well-interconnected nanoparticles of α-Fe₂O₃ upon sintering. This is responsible for the 13.6 times higher photocurrent density (at 1.23 V vs reference hydrogen electrode (RHE)) for photoelectrochemical (PEC) water oxidation on the α-Fe₂O₃ (w-α-Fe₂O₃) photoanode prepared with EIm compared with that (w/o-α-Fe₂O₃) prepared without EIm. The w-α-Fe₂O₃ photoanode provided the highest charge separation efficiency (η_sep) value of 27% among the state-of-the-art pristine α-Fe₂O₃ photoanodes, providing incident photon-to-current conversion efficiency (IPCE) of 13% at 420 nm and 1.23 V vs RHE. The superior η_sep for the w-α-Fe₂O₃ photoanode is attributed to the decreased recombination of the photogenerated charge carriers at the grain boundary between nanoparticles, in addition to the higher number of the catalytically active sites and the efficient bulk charge transport in the film, compared with w/o-α-Fe₂O₃.

Iron molybdenum selenide supported on reduced graphene oxide as an efficient hydrogen electrocatalyst in acidic and alkaline media

It is of great significance to develop inexpensive and high-efficiency electrocatalysts for the hydrogen evolution reaction (HER). In this work, we synthesized iron molybdenum selenide (FeSe₂-MoSe₂) loaded on reduced graphene oxide (FeSe₂-MoSe₂/rGO) by a one-step hydrothermal method. We further optimized the Fe/Mo ratio and determined the best ratio to be 1-1. In acidic (or alkaline) solution, the optimized FeSe₂-MoSe₂(1-1)/rGO has a small Tafel slope of 55 (or 80) mV dec^-1 and needs an overpotential of 101 (or 178) mV to achieve 10 mA cm^-2. These good properties are mainly due to the structure of bimetallic selenides combining rGO. Moreover, rGO enhances the electrical conductivity. Furthermore, the synergistic effect between FeSe₂-MoSe₂(1-1) and rGO results in better HER performance. Density functional theory (DFT) calculation proves that FeSe₂-MoSe₂(1-1)/rGO has a small work function. Based on our reasonable design and analysis, FeSe₂-MoSe₂(1-1)/rGO is expected to be an efficient and robust catalyst for large-scale applications.

A oxygen vacancy-modulated homojunction structural CuBi2O4 photocathodes for efficient solar water reduction

The photoelectrochemical (PEC) water reduction performance of CuBi2O4 (CBO)-based photocathodes is still far from their theoretical values due to low bulk and surface charge separation efficiencies. Herein, we propose a regrowth strategy to prepare a photocathode with CBO coating on Zn-doped CBO (CBO/Zn-CBO). Furthermore, NaBH4 treatment of CBO/Zn-CBO introduced oxygen vacancies (Ov) on CBO/Zn-CBO. It was found that Zn-doping not only increases the charge carrier concentration of CBO, but also leads to appropriate band alignment to form homojunctions. This homojunction can effectively promote the separation of electron-hole pairs, thus obtaining excellent photocurrent density (0.5 mA cm-2 at 0.3 V vs. RHE) and charge separation efficiency (1.5 times than CBO). The following surface treatment induced Ov on CBO/Zn-CBO, which significantly increased the active area of the surface catalytic reaction and further enhanced the photocurrent density (0.6 mA cm-2). In the absence of cocatalysts, the electron injection efficiency of Ov/CBO/Zn-CBO was 1.47 times improved than that of CBO. This work demonstrates a homojunction photocathode with Ov modulation, which provides a new view for future photoelectrochemical water splitting.

3D Branched Ca-Fe2 O3 /Fe2 O3 Decorated with Pt and Co-Pi: Improved Charge-Separation Dynamics and Photoelectrochemical Performance

The construction of junctions on hematite is an effective way to overcome the problems of slow charge separation and transfer kinetics, but constructing the junction is a significant challenge in photoelectrochemical (PEC) water splitting. Herein, a considerable improvement in PEC performance for α-Fe₂ O₃ was achieved following the introduction of a p-n homojunction between n-type α-Fe₂ O₃ and p-type Ca-doped α-Fe₂ O₃ through a facile hydrothermal method. The resultant 3D branched Ca-Fe₂ O₃ /Fe₂ O₃ enhanced the absorption intensity and reached a photocurrent density of 2.14 mA cm^-2 at 1.23 V vs. reversible hydrogen electrode (RHE). The merit of the desired lattice matching of the buried p-n homojunction structure built an internal electric field, which led to appropriate band alignment. These results were supported by a series of photoelectrochemical measurements, in particular, surface photovoltage (SPV) measurements. For further improvement of the charge-separation efficiency, a combination of separated cocatalysts was established on the homojunction structure, in which Pt acted as the electron collector and was deposited on the bottom, and Co-Pi as the hole-extraction cocatalyst was inserted to accelerate hole transfer on the surface of the photoanode. The resulting Co-Pi/Ca-Fe₂ O₃ /Fe₂ O₃ /Pt branched nanorods showed a significant improvement in charge-separation efficiency and photocurrent density (2.94 mA cm^-2 at 1.23 V vs. RHE). The present strategy, both the construction of the p-n homojunction and the coupling electron- and hole-transfer cocatalyst, could be expanded to many unstable or low-efficiency semiconductors for the design and fabrication of cost-effective photoanodes in PEC water splitting.

Hematite nanorod arrays top-decorated with an MIL-101 layer for photoelectrochemical water oxidation

α-Fe₂O₃ nanorod arrays were top-decorated with MIL-101 via the CVD method for constructing one intimate contact between two layers.

Heterojunction and Oxygen Vacancy Modification of ZnO Nanorod Array Photoanode for Enhanced Photoelectrochemical Water Splitting

Application of ZnO in the field of photoelectrochemical water splitting is limited because of its wide-band-gap and high recombination rate. Herein is reported the design of an efficient ZnO photoanode deposited with CoO_x nanoparticles to achieve a heterojunction and oxygen vacancies. The CoO_x nanoparticles with abundant oxygen vacancies were anchored onto the nanorod arrays by spin coating and calcination followed by a solvothermal treatment. CoO_x nanoparticles serve the dual function of forming a p-n heterojunction to facilitate the separation of photogenerated carriers, and act as a cocatalyst to decrease water oxidation barrier. Finally, oxygen vacancies increase the number of active redox sites and act as hole traps, enabling their migration to the electrode/electrolyte interface. The composite photoanode exhibits a high incident photon-to-current conversion efficiency (76.7 % at 350 nm), which is twice that of pristine ZnO, and a photoconversion efficiency of 0.68 % (0.73 V versus RHE). The current approach can be expanded to fabricate other efficient photocatalysts.

Dual-Axial Gradient Doping (Zr and Sn) on Hematite for Promoting Charge Separation in Photoelectrochemical Water Splitting

One of the crucial challenges to enhance the photoelectrochemical water-splitting performance of hematite (α-Fe₂ O₃ ) is to resolve its very fast charge recombination in bulk. Herein, we describe the design and fabrication of dual-axial gradient-doping on 1D Fe₂ O₃ nanorod arrays with Zr doping for x-axial and Sn doping for y-axial directions to promote the charge separation. This dual-axial gradient-doping structure fulfills the requirements of a greater electron-carrier concentration for increasing conductivity as well as a higher charge-separation efficiency across the dual-axial direction of Fe₂ O₃ nanorods, ultimately showing an excellent photocurrent density of 1.64 mA cm^-2 at 1.23 V vs. RHE, which is 26.3 times more than that of the bare Fe₂ O₃ . Furthermore, the remarkably improved photocurrent density, when comparing the uniform Zr-doped Fe₂ O₃ nanorod arrays (1.0 mA cm^-2 at 1.23 V vs. RHE) with dual-axial gradient-doped (Zr and Sn) Fe₂ O₃ nanorod arrays, highlights the additional charge-separation effect resulting from gradient codoping of Zr and Sn. Hence, this promising design may provide guidelines for dual-axial gradient doping into photoelectrodes to realize efficient PEC water splitting.