Electrochemical and photoelectrochemical characterization of photoanodes based on titania nanotubes modified by a BiVO4 thin film and gold nanoparticles

The new method of ZnIn2S4 synthesis on the titania nanotubes substrate with enhanced stability and photoelectrochemical performance

In this work, ZnIn2S4 layers were obtained on fluorine doped tin oxide (FTO) glass and TiO2 nanotubes (TiO2NT) using a hydrothermal process as photoanodes for photoelectrochemical (PEC) water splitting. Then, samples were annealed and the effect of the annealing temperature was investigated. Optimization of the deposition process and annealing of ZnIn2S4 layers made it possible to obtain an FTO-based material generating a photocurrent of 1.2 mA cm-2 at 1.62 V vs. RHE in a neutral medium. In contrast, the highest photocurrent in the neutral electrolyte obtained for the TiO2NT-based photoanode reached 0.5 mA cm-2 at 1.62 V vs. RHE. In addition, the use of a strongly acidic electrolyte allowed the generated photocurrent by the TiO2NT-based photoanode to increase to 3.02 mA cm-2 at 0.31 V vs. RHE. Despite a weaker photoresponse in neutral electrolyte than the optimized FTO-based photoanode, the use of TiO2NT as a substrate allowed for a significant increase in the photoanode's operating time. After 2 h of illumination, the photocurrent response of the TiO2NT-based photoanode was 0.21 mA cm-2, which was 42% of the initial value. In contrast, the FTO-based photoanode after the same time generated a photocurrent of 0.02 mA cm-2 which was only 1% of the initial value. The results indicated that the use of TiO2 nanotubes as a substrate for ZnIn2S4 deposition increases the photoanode's long-term stability in photoelectrochemical water splitting. The proposed charge transfer mechanism suggested that the heterojunction between ZnIn2S4 and TiO2 played an important role in improving the stability of the material by supporting charge separation.

Modification of TiO2 nanotubes by graphene-strontium and cobalt molybdate perovskite for efficient hydrogen evolution reaction in acidic medium

Herein, we demonstrate that modification of TiO₂ nanotubes with graphene-strontium and cobalt molybdate perovskite can turn them into active electrocatalysts for hydrogen evolution reaction (HER). For this purpose, a simple method of hydrothermal synthesis of perovskites was developed directly on the TiO₂ nanotubes substrate. Moreover, the obtained hybrids were also decorated with graphene oxide (GO) during one-step hydrothermal synthesis. The obtained materials were characterized by scanning electron microscopy with energy dispersive X-ray analysis, Raman spectroscopy, and X-ray diffraction analysis. Catalytic properties were verified by electrochemical methods (linear voltammetry, chronopotentiometry). The obtained hybrids were characterized by much better catalytic properties towards hydrogen evolution reaction compared to TiO₂ and slightly worse than platinum. The optimized hybrid catalyst (decorated by GO) can drive a cathodic current density of 10 mA cm^-2 at an overpotential of 121 mV for HER with a small Tafel slope of 90 mV dec^-1 in 0.2 M H₂SO₄.

Scaling Up the Process of Titanium Dioxide Nanotube Synthesis and Its Effect on Photoelectrochemical Properties

In this work, for the first time, the influence of scaling up the process of titanium dioxide nanotube (TiO₂NT) synthesis on the photoelectrochemical properties of TiO₂ nanotubes is presented. Titanium dioxide nanotubes were obtained on substrates of various sizes: 2 × 2, 4 × 4, 5 × 5, 6 × 6, and 8 × 8 cm². The electrode material was characterized using scanning electron microscopy as well as Raman and UV–vis spectroscopy in order to investigate their morphology, crystallinity, and absorbance ability, respectively. The obtained electrodes were used as photoanodes for the photoelectrochemical water splitting. The surface analysis was performed, and photocurrent values were determined depending on their place on the sample. Interestingly, the values of the obtained photocurrent densities in the center of each sample were similar and were about 80 µA·cm². The results of our work show evidence of a significant contribution to wider applications of materials based on TiO₂ nanotubes not only in photoelectrochemistry but also in medicine, supercapacitors, and sensors.

Graphite-bridged indirect Z-scheme system TiO2-C-BiVO4 film with enhanced photoelectrocatalytic activity towards serial bisphenols

Due to the increase in the occurrence of bisphenols (BPs) in the environments, it is urgent to develop efficient and ecofriendly methods for their removal. A novel, indirect Z-scheme TiO₂-C-BiVO₄ film was prepared by a sol-gel method combined with hydrothermal carbonization. The doped graphite carbon was generated in situ from glucose, which acted as an electron-transfer bridge for the Z-scheme system to enhance the heterojunction tightness between TiO₂ and BiVO₄. This resulted in an increasing separation efficiency of photogenerated electrons and holes and a stronger redox ability of the TiO₂-C-BiVO₄ film for the degradation and detoxification of BPs. The degradation efficiency of BPs was over 95% in 240 min, except for that of 4,4'-sulphonyldiphenol (BPS) due to the presence of the OSO group, and all of the BPs were nearly completely mineralized when the reaction time reached 360 min. Consequently, the inhibition ratio towards Vibrio fischeri decreased significantly along with the loss and mineralization of aromatic intermediates during photoelectrocatalytic degradation. 2,2-bis(4-Hydroxyphenyl) butane (BPB), 4,4'-(1-phenylethylidene)-bisphenol (BPAP), and (4,4'-hexafluoroisopropylidene) diphenol (BPAF), with relatively high toxicity levels and lipophilicity and as toxic product precursors, require attention in terms of environmental safety. Overall, this work provides a promising and environmentally friendly way to remove BPs from water.

Current progress in developing metal oxide nanoarrays-based photoanodes for photoelectrochemical water splitting

Solar energy driven photoelectrochemical (PEC) water splitting is a clean and powerful approach for renewable hydrogen production. The design and construction of metal oxide based nanoarray photoanodes is one of the promising strategies to make the continuous breakthroughs in solar to hydrogen conversion efficiency of PEC cells owing to their owned several advantages including enhanced reactive surface at the electrode/electrolyte interface, improved light absorption capability, increased charge separation efficiency and direct electron transport pathways. In this Review, we first introduce the structure, work principle and their relevant efficiency calculations of a PEC cell. We then give a summary of the state-of the-art research in the preparation strategies and growth mechanism for the metal oxide based nanoarrays, and some details about the performances of metal oxide based nanoarray photoanodes for PEC water splitting. Finally, we discuss key aspects which should be addressed in continued work on realizing high-efficiency metal oxide based nanoarray photoanodes for PEC solar water splitting systems.

Ultrathin TiO2/BiVO4 nanosheet heterojunction arrays modified with NiFe-LDH nanoparticles for enhanced photoelectrochemical oxidation of water

Herein, we have constructed NiFe-LDH nanoparticles modified ultrathin TiO₂/BiVO₄ nanosheet heterojunction arrays and explored the effects of different NiFe-LDH loading amount on photoelectrochemical water oxidation performance. The photocurrent of as-prepared TiO₂/BiVO₄/NiFe-LDH photoanode is about 2.5 times than that of TiO₂/BiVO₄, which is ascribed to the synergistic effect of heterojunction and co-catalyst. The heterojunction between TiO₂ and BiVO₄ suppresses the recombination of photogenerated electron-hole pairs effectively and the co-catalyst of NiFe-LDH accelerates the surface water oxidation reaction kinetics.

Widening of the electroactivity potential range by composite formation - capacitive properties of TiO2/BiVO4/PEDOT:PSS electrodes in contact with an aqueous electrolyte

Composites based on the titania nanotubes were tested in aqueous electrolyte as a potential electrode material for energy storage devices. The nanotubular morphology of TiO2 was obtained by Ti anodization. TiO2 nanotubes were covered by a thin layer of bismuth vanadate using pulsed laser deposition. The formation of the TiO2/BiVO4 junction leads to enhancement of pseudocapacitance in the cathodic potential range. The third component, the conjugated polymer PEDOT:PSS, was electrodeposited from an electrolyte containing the monomer EDOT and NaPSS as a source of counter ions. Each stage of modification and deposition affected the overall capacitance and allowed for an expansion of the potential range of electroactivity. Multiple charge/discharge cycles were performed to characterize the electrochemical stability of the inorganic-organic hybrid electrode. Capacitance values higher than 10 mF·cm-2 were maintained even after 10000 galvanostatic cycles (i c = i a = 0.5 mA·cm-2).