Templating Sol-Gel Hematite Films with Sacrificial Copper Oxide: Enhancing Photoanode Performance with Nanostructure and Oxygen Vacancies

Recent progress in defect-engineered metal oxides for photocatalytic environmental remediation

Rapid industrial advancement over the last few decades has led to an alarming increase in pollution levels in the ecosystem. Among the primary pollutants, harmful organic dyes and pharmaceutical drugs are directly released by industries into the water bodies which serves as a major cause of environmental deterioration. This warns of a severe need to find some sustainable strategies to overcome these increasing levels of water pollution and eliminate the pollutants before being exposed to the environment. Photocatalysis is a well-established strategy in the field of pollutant degradation and various metal oxides have been proven to exhibit excellent physicochemical properties which makes them a potential candidate for environmental remediation. Further, with the aim of rapid industrialization of photocatalytic pollutant degradation technology, constant efforts have been made to increase the photocatalytic activity of various metal oxides. One such strategy is the introduction of defects into the lattice of the parent catalyst through doping or vacancy which plays a major role in enhancing the catalytic activity and achieving excellent degradation rates. This review provides a comprehensive analysis of defects and their role in altering the photocatalytic activity of the material. Various defect-rich metal oxides like binary oxides, perovskite oxides, and spinel oxides have been summarized for their application in pollutant degradation. Finally, a summary of existing research, followed by the existing challenges along with the potential countermeasures has been provided to pave a path for the future studies and industrialization of this promising field.

Advances in Engineered Metal Oxide Thin Films by Low-Cost, Solution-Based Techniques for Green Hydrogen Production

Functional oxide materials have become crucial in the continuous development of various fields, including those for energy applications. In this aspect, the synthesis of nanomaterials for low-cost green hydrogen production represents a huge challenge that needs to be overcome to move toward the next generation of efficient systems and devices. This perspective presents a critical assessment of hydrothermal and polymeric precursor methods as potential approaches to designing photoelectrodes for future industrial implementation. The main conditions that can affect the photoanode's physical and chemical characteristics, such as morphology, particle size, defects chemistry, dimensionality, and crystal orientation, and how they influence the photoelectrochemical performance are highlighted in this report. Strategies to tune and engineer photoelectrode and an outlook for developing efficient solar-to-hydrogen conversion using an inexpensive and stable material will also be addressed.

In Situ GISAXS Observation and Large Area Homogeneity Study of Slot-Die Printed PS-b-P4VP and PS-b-P4VP/FeCl3 Thin Films

Mesoporous hematite (α-Fe₂O₃) thin films with high surface-to-volume ratios show great potential as photoelectrodes or electrochemical electrodes in energy conversion and storage. In the present work, with the assistance of an up-scalable slot-die coating technique, locally highly ordered α-Fe₂O₃ thin films are successfully printed based on the amphiphilic diblock copolymer poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) as a structure-directing agent. Pure PS-b-P4VP films are printed under the same conditions for comparison. The micellization of the diblock copolymer in solution, the film formation process of the printed thin films, the homogeneity of the dry films in the lateral and vertical direction as well as the morphological and compositional information on the calcined hybrid PS-b-P4VP/FeCl₃ thin film are investigated. Because of convection during the solvent evaporation process, a similar dimple-type structure of vertically aligned cylindrical PS domains in a P4VP matrix developed for both printed PS-b-P4VP and hybrid PS-b-P4VP/FeCl₃ thin films. The coordination effect between the Fe³⁺ ions and the vinylpyridine groups significantly affects the attachment ability of the P4VP chains to the silicon substrate. Accordingly, distinct feature sizes and homogeneity in the lateral direction, as well as the thicknesses in the perpendicular direction, are demonstrated in the two printed films. By removing the polymer template from the hybrid PS-b-P4VP/FeCl₃ film at high temperature, a locally highly ordered mesoporous α-Fe₂O₃ film is obtained. Thus, a facile and up-scalable printing technique is presented for producing homogeneous mesoporous α-Fe₂O₃ thin films.

Solution-Processed Synthesis of Copper Oxide (Cu x O) Thin Films for Efficient Photocatalytic Solar Water Splitting

This article reports a solution-processed synthesis of copper oxide (Cu x O) to be used as a potential photocathode for solar hydrogen production in the solar water-splitting system. Cu x O thin films were synthesized through the reduction of copper iodide (CuI) thin films by sodium hydroxide (NaOH), which were deposited by the spin coating method from CuI solution in a polar aprotic solvent (acetonitrile). The phase and crystalline quality of the synthesized Cu x O thin films prepared at various annealing temperatures were investigated using various techniques. The X-ray diffraction and energy dispersive X-ray spectroscopy studies confirm the presence of Cu2O, CuO/Cu2O mixed phase, and pure CuO phase at annealing temperatures of 250, 300, and 350 °C, respectively. It is revealed from the experimental findings that the synthesized Cu x O thin films with an annealing temperature of 350 °C possess the highest crystallinity, smooth surface morphology, and higher carrier density. The highest photocurrent density of -19.12 mA/cm2 at -1 V versus RHE was achieved in the photoelectrochemical solar hydrogen production system with the use of the Cu x O photocathode annealed at a temperature of 350 °C. Therefore, it can be concluded that Cu x O synthesized by the spin coating method through the acetonitrile solvent route can be used as an efficient photocathode in the solar water-splitting system.

Performance improvement of a p-Cu2O nanocrystal photocathode with an ultra-thin silver protective layer

A Ag film with 10 nm thickness is deposited on a directionally grown Cu₂O single crystal photocathode by high power impulse magnetron sputtering (HiPIMS). The continuous Ag film not only forms a dense proton isolator, but also constructs a Schottky junction with Cu₂O to accelerate the transportation of photo-induced electrons. After optimization, the photocathode exhibits much-improved activity and stability versus the unmodified one, and over 98% stability (state-of-the-art) remains after 5 h of light-chopping chronoamperometry. The sustained photocurrent density is 4.0 mA cm^-2 at 0 V_RHE.

Flexible cupric oxide photocathode with enhanced stability for renewable hydrogen energy production from solar water splitting

CuO is a promising but unstable photocathode in solar water splitting. Herein, a flexible CuO photocathode is prepared and its degradation mechanisms and stabilization strategies have been discussed. Briefly, we find alkali environment and low light intensity are the critical factors in the stabilization of the CuO photocathode. For practical usage, a composite semiconductor layer, composed of TiO₂, La₂O₃ and NiO, is deposited on the CuO photocathode, which is proved to be effective for enhancing the stabilization of the CuO photocathode. 100% of the photocurrent density has been retained after 20 minutes of continuous illumination. The optimized stable photocurrent density is measured as 0.3 mA cm⁻² at 0.5 V_RHE.

A composite semiconductor layer, composed of TiO₂, La₂O₃ and NiO, is deposited on a CuO photocathode, and shown to be effective for enhancing the stabilization of the CuO photocathode. 100% of the photocurrent density is retained after 20 min of continuous illumination.

Highly self-diffused Sn doping in α-Fe2O3 nanorod photoanodes initiated from β-FeOOH nanorod/FTO by hydrogen treatment for solar water oxidation

In this study, we present an advanced strategy of low-temperature hydrogen annealing combined with high- temperature quenching in air for activating α-Fe₂O₃ nanorod photoanodes to boost the photoelectrochemical performance. We report that various low-temperature annealing conditions (340, 360, 380, and 400 °C) under hydrogen gas flow convert β-FeOOH into magnetite (Fe₃O₄) as well as introduce Sn⁴⁺ diffusion from FTO substrates to its surface. Furthermore, high-temperature quenching (800 °C) resulted in the phase change of magnetite (Fe₃O₄) into hematite (α-Fe₂O₃) and self Sn⁴⁺ doping into the hematite lattice. Thus, the hydrogen-assisted thermally activated hematite photoanode achieved a photocurrent density of 1.35 mA cm^-2 at 1.23 V vs. RHE and 1.91 mA cm^-2 at 1.4 V vs. RHE, which is 70% and 80% higher than that of directly quenched hematite at 800 °C. These combined two step strategies provide new insight into high Sn-self doping for α-Fe₂O₃ photoanodes and allow for further development of more efficient solar water oxidation systems.

Robust Hierarchically Structured Biphasic Ambipolar Oxide Photoelectrodes for Light-Driven Chemical Regulation and Switchable Logic Applications

Tunable ambipolar photoelectrochemical behavior emerges from microdomains of nanostructured p-type CuFeO₂ and n-type Fe₂ O₃ that arise from a single facile solution-processed thin film. The switchable operation of this system is controlled by chemical, optical, or electronic inputs with a uniquely high photocurrent response (on the order of 1 mA cm^-2 ), suitable for robust practical application as an oxygen photoregulator.

Facet-selective charge carrier transport, deactivation mechanism and stabilization of a Cu2O photo-electro-catalyst

A facet-dependent photo-deactivation mechanism of Cu₂O was verified and reported, which is caused by the facet-dependent charge carrier transport.

A nanostructured hematite film prepared by a facile “top down” method for application in photoelectrochemistry

Nanostructured hematite films on iron foil have been successfully prepared by using a facile “top down” method for application in photoelectrochemistry.