In this study, hematite nanoparticles (α-Fe2O3 NPs) were synthesized by hydrothermal method, with morphologies (e.g., nanorhombohedra, nanobars, and nanospheres) facilely tuned by changing the concentrations of glycol in the hydrothermal solution. Then a low-cost and scalable electrophoretic deposition method was used to fabricate nanostructured α-Fe2O3 films as photoanodes for solar hydrogen generation. It was found that the film of α-Fe2O3 nanobars showed the highest photoelectrochemical (PEC) performance compared to those films of α-Fe2O3 nanorhombohedra and nanospheres, with photocurrent density reaching 0.7 mA/cm2 at 0.6 V versus Ag/AgCl. This PEC improvement may be related to the smaller diameters of nanobars shortening the carrier migration distance, reducing the recombination rate of photo-generated carriers. Moreover, all the α-Fe2O3 films showed much higher PEC performances with surface modified by Sn4+, mainly due to the reduced surface charge recombination, as the Sn4+ doped overlayer passivated surface defects. For the film of α-Fe2O3 nanobars, the photocurrent density was increased by 100%, reaching 1.4 mA/cm2 at 0.6 V versus Ag/AgCl.

The photocathodic current density of ZnO/hematite hetero nanoarchitectures electrode has been reported in the current investigation. The electrode was obtained with a cheap and two-step hydrothermal functionalization of pristine silicon doped hematite film. The optical, structural, and morphological properties of the electrodes have been studied in detail and it is found that the ZnO functionalization of hematite changes its crystallographic properties by decreasing the Bragg peak intensity ratio for (104) planes. The morphology obtained in this case is unique in the sense that it does not cover the original hematite film and is formed in an isolated manner. Finally, employing a qualitative energy band gap model for mixed metal oxides energy levels has identified the photocathodic properties of the electrode. Here it is found that the photocathodic properties of the electrode are much higher when an electron transfer takes place from the conduction band of ZnO into the electrolyte while hole generated in ZnO is transferred back to hematite but it also degrades the structures after running photoanodic current density sweep. That points to the decrease of water oxidation behavior of hematite in conjunction with ZnO.

Hematite (α-Fe2O3) photoanodes are widely studied as candidates for water splitting photoelectrochemical (PEC) cells. To speed up the development of high efficiency hematite photoanodes, systematic investigations of the effect of material properties such as dopants and microstructure on PEC properties that determine the photoanode performance are crucial. Toward this end, this work presents a route for reproducible fabrication of thin film hematite photoanodes with reproducible microstructure and PEC properties. Hematite thin (50 nm) films are deposited by pulsed laser deposition from a Ti-doped (1 cation%) Fe2O3 target onto cleaned transparent conducting substrates (fluorinated tin oxide, FTO, coated glass substrates). Special attention is paid to rigorous cleaning of the substrates prior to the hematite deposition, which is found to be crucial for achieving highly reproducible results. Specimens prepared by this route display homogenous conformal coating with very little spread in PEC properties between different specimens, meeting the necessary prerequisite for systematic investigation of hematite photoanodes.

The mesoporous network within photocatalytically and photoelectrochemically active LaTiO2N (LTON) single crystals was investigated by electron microscopy techniques including electron diffraction and scanning transmission electron microscopy imaging. The perovskite-related oxynitride particles were obtained by thermal ammonolysis from monocrystalline micrometer-sized La2Ti2O7 (LTO) particles grown by flux-assisted solid state synthesis. Special attention was paid to the crystal transformation from the monoclinic layered LTO to the orthorhombic perovskite-related LTON within the monocrystalline particles. A detailed analysis of pore directions and pore sizes with respect to the LTON particle shape was performed. The pore formation mechanism taking place during thermal ammonolysis was discussed. Based on the mechanistic understanding of the transformation from the oxide to the oxynitride, a further extension of the mesoporous network toward higher surface areas was proposed for improved photoelectrochemical activity of oxynitride particles, while high crystallinity and particle sizes in the micrometer range continue to enable efficient charge transport.

In searching for a suitable semiconductor material for hydrogen production via photoelectrochemical water splitting, α-Fe2O3 received significant attention as a promising photoanode due to its band gap (∼2.1 eV), good stability, low cost, and natural occurrence. α-Fe2O3 thin films were prepared by economic and facile dip coating method and subsequently subjected to an anodic potential of 700 mV versus Ag/AgCl in 1M KOH for different anodization times (1, 10, and 900 min) under illumination. X-ray diffractometry revealed increase in crystallites size from ∼31 nm for nanoparticles in pristine state to ∼38 and 44 nm after anodization for 1 and 900 min, respectively. A clear positive correlation between anodization time and grain (particle) size was observed from field emission gun scanning electron microscopy and atomic force microscopy (AFM); longer exposure time to anodizing conditions resulted in larger grains. Grain size increased from ∼57.9 nm in pristine state to ∼153.5 nm after anodization for 900 min. A significant smoothening of the surface with increase in anodization time was evident from AFM analysis.

Titania nanotube arrays were synthesized electrochemically by anodization of titanium foils, and the synthesized titania nanotubes were then implanted with indium ions. The effect of In-ions implantation on crystallization and phase transformation of titania was investigated using in-situ high-temperature X-ray diffraction and synchrotron radiation diffraction from room temperature to 1000 °C. Diffraction results show that crystalline anatase first appeared at 400 °C in both the non-implanted and the In-implanted materials. The temperature at which crystalline rutile temperature appeared was 600 °C for non-implanted materials and 700 °C for In-implanted materials, and the indium implantation inhibited the anatase-to-rutile transformation. Although In3+ is expected to increase oxygen vacancy concentration and then the rate of titania transformation, the observations are consistent with implanted In-ions occupying the Ti sublattice substitutionally and then inhibiting the transformation. The relatively difficult anatase-to-rutile transformation in the In-implanted material appears to result from the relatively large In3+ radius (0.080 nm). The In3+ partly replaces the Ti4+ (0.061 nm), which provides a greater structural rigidity and prevents relaxation in the Ti bonding environment.

The synthesis technique that can be used to accelerate the discovery of materials for various energy conversion and storage applications is presented. Specifically, this technique allows a rapid and controlled synthesis of mixed metal oxide particles using plasma oxidation of liquid droplets containing mixed metal precursors. The conventional wet chemical methods for synthesis of multimetal oxide solid solutions often require time-consuming high pressure and temperature processes, and so the challenge is to develop rapid and scalable techniques with precise compositional control. The concept is demonstrated by synthesizing binary and ternary transition metal oxide solid solutions with control over entire composition range using metal precursor solution droplets oxidized using atmospheric oxygen plasma. The results show the selective formation of metastable spinel and the rocksalt solid solution phases with compositions over the entire range by tuning the metal precursor composition. The synthesized manganese doped nickel ferrite nanoparticles, NiMn z Fe2−z O4 (0 ≤ z ≤ 1), exhibits considerable electrocatalytic activity toward oxygen evolution reaction, achieving an overpotential of 0.39 V at a benchmarking current density of 10 mA/cm2 for a low manganese content of z = 0.20.

Gold nanoparticles have been deposited on the surface of hematite nanorod array photoanode to improve the photoelectrochemical water splitting performance. The Au nanoparticles induce the Fermi level equilibration, the surface catalysis, and the plasmonic enhancement effects in the Au/hematite photoanode. The Fermi level equilibration effect promotes the extraction of photo-generated charge carriers, suppressing the charge recombination. Surface catalysis effect reduces the overpotential for photoelectrochemical water oxidation. In the Au/hematite sample, the Fermi level equilibration and the surface catalysis effect make major contribution to photocurrent enhancement while the plasmonic effect makes a little contribution. In addition, the Au@SiO2 particle has been immobilized on hematite nanorod array surface that has been passivated. In the Au@SiO2/hematite sample, the photocurrent enhancement originating from plasmonic effects is negligible. Both the Femi level equilibration and the surface catalysis effects were excluded due to the isolated Au and hematite while surface passivation is mainly responsible for the photocurrent enhancement.

A novel CuWO4/Cu1−x Zn x WO4/ZnWO4 hybrid photocatalyst with sandwiched heterojunction structure was prepared by a one-port synthesis with Zn doping into CuWO4. The crystalline structure, optical, and morphological properties as well as photocatalytic performance of the as-prepared hybrid photocatalyst were studied. By adjusting the amount of Zn doped, the optimal doping level was determined to be 0.1 wt% Zn2+. More than 80% photocataytic degradation of rhodamine B was achieved within 20 min over 0.1 wt% Zn2+ doped CuWO4, while only 20% was achieved for the pure CuWO4. The enhancement was proposed to be due to the formation of a CuWO4/Cu1−x Zn x WO4/ZnWO4 sandwiched heterojunction. Such tandem type heterojunction was found to be efficient for charge separation compared to traditional single heterojunction, which, in turn, resulted in a significantly enhanced photocatalytic activity. Our finding is also expected to be valuable for the exploration of CuWO4-material as a new group of efficient photocatalysts.

In semiconductor system for solar-energy utilization by photoelectrochemical (PEC) water splitting, the effective absorption of visible light and the efficient separation and transfer of photogenerated charge carriers are still of key importance. In this manuscript, composite photoanodes of PbO sensitized ZnO nanorod arrays were prepared by a two-step hydrothermal process and used as anodes for PEC test under visible-light irradiation. The photocurrent achieved the highest value of 94 μA cm−2 at 0.8 V (versus Ag/AgCl electrode) when the amount of Pb source was optimized to form only a thin layer (a few nanometers) of PbO nanoparticles on the surfaces of ZnO nanorods. Such a nanostructure enabled the visible-light absorption, and also ensured the sufficient contact of PbO with ZnO to form junction with a type II band alignment and the sufficient contact with aqueous solution to form interfaces, thus facilitating the excitation, separation, and transfer of charge carriers to generate photocurrent and finally enhancing the PEC activity.