In situ growth of α-Fe2O3@Co3O4 core-shell wormlike nanoarrays for a highly efficient photoelectrochemical water oxidation reaction

Nano-bismuth vanadate supported on fibrous silica reduces the intrinsic charge impedance for superior photoelectrochemical water-splitting performance

Bismuth vanadate (BiVO4) is one of the top-notch materials used in photoelectrochemical (PEC) water-splitting studies owing to its promising properties. However, its practical application is significantly hindered by its inherent limitations, which reduce its efficiency in water-splitting processes. In this study, a novel approach involving size transformation and improved dispersion of BiVO4 was achieved via a microemulsion method, with fibrous silica serving as a support matrix. The fabricated catalyst, fibrous silica bismuth vanadate (FSBVO), was comprehensively characterized using XRD, FTIR, FESEM, TEM, UV-Vis/DRS, Mott-Schottky analysis, EIS, and PL spectroscopy and compared with commercial BiVO4. The PEC analysis demonstrated that the FSBVO photoanode delivered a remarkable performance, attaining a photocurrent density of 19.8 mA cm-2 at 1.23 VRHE and a solar-to-hydrogen conversion efficiency of 24.35%, which correspond to enhancements of approximately 27.5 and 27.4 times, respectively, compared with those obtained using the pristine BiVO4 photoanode. Further in-depth studies revealed that the improvement in the PEC water-splitting performance was mainly attributed to the transformation of BiVO4 into nanoparticles and the distinctive Si-Bi interaction, which increased the carrier density and facilitated efficient electron transport, thereby accelerating the oxygen evolution kinetics. This study highlights the potential of FSBVO photoanodes and provides valuable insights for designing advanced materials to enhance the PEC water-splitting efficiency.

Enhanced directional transfer of charge carriers and optimized electronic structure in fluorine doped polymeric carbon nitride nanosheets for efficient photocatalytic water splitting

The high photogenerated charge carrier recombination and sluggish oxygen evolution reaction (OER) kinetics of polymeric carbon nitride (PCN) photocatalysts limit their application in photocatalytic water splitting. Herein, fluorine (F) doped PCN (PCNF-x) nanosheets with high crystallinity were prepared using dicyandiamide (C2H4N4) and ammonium hydrogen fluoride (NH4HF2) through high temperature thermal polymerization. This process not only resulted in PCNF-x nanosheets with a large number of pores, but also improved the crystallinity of PCNF-x nanosheets. Under illumination, the PCNF-0.5 nanosheets exhibited an excellent photocatalytic water splitting activity with a comparable H2 evolution rate of 135.30 μmol h-1 g-1 and O2 evolution rate of 63.75 μmol h-1 g-1, which were 2.3-fold, 3.3-fold, and 25-fold as compared to those of PCNF-1, PCNF-0.2, and pristine PCN nanosheets, respectively. Photoluminescence (PL) spectra and density functional theory (DFT) calculations indicate that F doping of PCN nanosheets brings two changes in PCNF-x nanosheets, one is the increase in crystallinity after F doping effectively weakens the bulk defects of PCNF nanosheets, which is conducive to the directional transfer of charge carriers; the other is the modulation of the electronic structure after F doping, which optimizes the reaction mechanism of the OER in PCNF-x nanosheets. Both the enhancement in charge carrier transfer and the optimization of the reaction mechanism significantly contribute to the improved photocatalytic performance of water splitting in the fluorine doped PCN nanosheets.

Dependence of copper(I) stability on long-range electromagnetic effects of Au under reducing atmospheres: the size effect of Au cores

It has been widely recognized that adjusting the size of Au particles has emerged as a significant approach in catalyst design, catalyst screening, and comprehension of reaction mechanisms. However, the essential factors of Au nanoparticles used only as an additive to enhance the activity of traditional multicomponent thermocatalysts have not been fully revealed. In this study, a series of Au@Cu2O core-shell nanocatalysts were synthesized through a controllable method, featuring core sizes ranging from 11 to 33 nm and an average shell thickness of approximately 55 nm. It was revealed that the size effect of Au cores plays a very vital role in the stability of the active Cu+ species under reducing atmospheres (H2, acetylene and formaldehyde) as well as the catalytic performance of the catalysts in the ethynylation of formaldehyde. The experimental findings revealed that Au@Cu2O core-shell catalysts with Au core sizes ranging from 11 to 16 nm exhibited a higher abundance of electron-deficient Cu+ species in the shell, which is attributed to the strong long-range electromagnetic effects of the Au core in the absence of photoexcitation or an applied electric field. Additionally, the active Cu+ species demonstrated remarkable stability under reducing atmospheres. Although the stability of Cu+ decreased slightly when the Au core size exceeded 16 nm, the Cu+ content remained above 80%. Notably, the Au@Cu2O catalysts with Au core sizes ranging from 11 to 16 nm exhibited excellent catalytic activity in the ethynylation of formaldehyde.

Cocatalysts-Photoanode Interface in Photoelectrochemical Water Splitting: Understanding and Insights

Sluggish oxygen evolution reactions on photoanode surfaces severely limit the application of photoelectrochemical (PEC) water splitting. The loading of cocatalysts on photoanodes has been recognized as the simplest and most efficient optimization scheme, which can reduce the surface barrier, provide more active sites, and accelerate the surface catalytic reaction kinetics. Nevertheless, the introduction of cocatalysts inevitably generates interfaces between photoanodes and oxygen evolution cocatalysts (Ph/OEC), which causes severe interfacial recombination and hinders the carrier transfer. Recently, many researchers have focused on cocatalyst engineering, while few have investigated the effect of the Ph/OEC interface. Hence, to maximize the advantages of cocatalysts, interfacial problems for designing efficient cocatalysts are systematically introduced. In this review, the interrelationship between the Ph/OEC and PEC performance is classified and some methods for characterizing Ph/OEC interfaces are investigated. Additionally, common interfacial optimization strategies are summarized. This review details cocatalyst-design-based interfacial problems, provides ideas for designing efficient cocatalysts, and offers references for solving interfacial problems.

Three-dimensional Ni foam supported NiCoO2@Co3O4 nanowire-on-nanosheet arrays with rich oxygen vacancies as superior bifunctional catalytic electrodes for overall water splitting

Earth abundant transition metal oxide (EATMO)-based bifunctional catalysts for overall water splitting are highly desirable, but their performance is far from satisfactory due to low intrinsic activities of EATMOs toward electrocatalysis of both oxygen and hydrogen evolution reactions and poor electron transfer and transport capabilities. A three-dimensional (3-D) Ni-foam-supported NiCoO2@Co3O4 nanowire-on-nanosheet heterostructured array with rich oxygen vacancies has been synthesized, showing OER activity superior to most reported catalysts and even much higher than Ru and Ir-based ones and HER activity among the highest reported for non-noble-metal-based catalysts. The excellent activities are ascribed to the highly dense, ultrathin nanowire arrays epitaxially grown on an interconnected layered nanosheet array greatly facilitating electron transfer and providing numerous electrochemically accessible active sites and the high content of oxygen vacancies on nanowires greatly promoting OER and HER. When adopted as bifunctional electrodes for overall water splitting, this heterostructure shows an overvoltage (at 10 mA cm-2) lower than most reported electrolyzers and high stability. This work not only creates a 3-D EATMO-based integrated heterostructure as a low-cost, highly efficient bifunctional catalytic electrode for water splitting, but also provides a novel strategy to use unique heteronanostructures with rich surface defects for synergistically enhancing electrocatalytic activities.

Dual co-catalysts activated hematite nanorods with low turn-on potential and enhanced charge collection for efficient solar water oxidation

Hematite (α-Fe₂O₃) photoanode suffers from significant photocarrier recombination and sluggish water oxidation kinetics for photoelectrochemical water splitting. To address these challenges, this work demonstrates the construction of dual co-catalysts modified Fe₂O₃nanorods photoanode by strategically incorporating CoPi and Co(OH)_xfor photoelectrochemical water oxidation. The Fe₂O₃/CoPi/Co(OH)_xnanorods photoanode exhibits the lowest ever turn-on potential of 0.4V_RHE(versus reversible hydrogen electrode) and a photocurrent density of 0.55 mA cm^-2at 1.23V_RHE, 358% higher than that of pristine Fe₂O₃nanorods. The dual co-catalysts modification enhances the light-harvesting efficiency, surface photovoltage and hole transfer kinetics of the hybrid photoanode. The dual co-catalyst coupling also increases the carrier density and significantly reduces the depletion width (1.9 nm), resulting in improved conductivity and favorable band bending, boosting photogenerated hole transfer efficiency for water oxidation.

Tailored Co3O4-Based Nanosystems: Toward Photocatalysts for Air Purification

The adverse effects of NO_x (NO + NO₂) gases on the environment and human health have triggered the development of sustainable photocatalysts for their efficient removal (De-NO_x). In this regard, the present work focuses on supported Co₃O₄-based nanomaterials fabricated via chemical vapor deposition (CVD), assessed for the first time as photocatalysts for sunlight-activated NO oxidation. A proof-of-principle investigation on the possibility of tailoring material performances by heterostructure formation is explored through deposition of SnO₂ or Fe₂O₃ onto Co₃O₄ by radio frequency (RF) sputtering. A comprehensive characterization by complementary analytical tools evidences the formation of high-purity columnar Co₃O₄ arrays with faceted pyramidal tips, conformally covered by very thin SnO₂ and Fe₂O₃ overlayers. Photocatalytic functional tests highlight an appreciable activity for bare Co₃O₄ systems, accompanied by a high selectivity in NO_x conversion to harmless nitrate species. A preliminary evaluation of De-NO_x performances for functionalized systems revealed a direct dependence of the system behavior on the chemical composition, SnO₂/Fe₂O₃ overlayer morphology, and charge transfer events between the single oxide constituents. Taken together, the present results can provide valuable guidelines for the eventual implementation of improved photocatalysts for air purification.

3D-printed Cu2O photoelectrodes for photoelectrochemical water splitting

Photoelectrochemical (PEC) water splitting is an alternative to fossil fuel combustion involving the generation of renewable hydrogen without environmental pollution or greenhouse gas emissions. Cuprous oxide (Cu2O) is a promising semiconducting material for the simple reduction of hydrogen from water, in which the conduction band edge is slightly negative compared to the water reduction potential. However, the solar-to-hydrogen conversion efficiency of Cu2O is lower than the theoretical value due to a short carrier-diffusion length under the effective light absorption depth. Thus, increasing light absorption in the electrode-electrolyte interfacial layer of a Cu2O photoelectrode can enhance PEC performance. In this study, a Cu2O 3D photoelectrode comprised of pyramid arrays was fabricated using a two-step method involving direct-ink-writing of graphene structures. This was followed by the electrodeposition of a Cu current-collecting layer and a p-n homojunction Cu2O photocatalyst layer onto the printed structures. The performance for PEC water splitting was enhanced by increasing the total light absorption area (A a) of the photoelectrode via controlling the electrode topography. The 3D photoelectrode (A a = 3.2 cm2) printed on the substrate area of 1.0 cm2 exhibited a photocurrent (I ph) of -3.01 mA at 0.02 V (vs. RHE), which is approximately three times higher than that of a planar photoelectrode with an A a = 1.0 cm2 (I ph = -0.91 mA). Our 3D printing strategy provides a flexible approach for the design and the fabrication of highly efficient PEC photoelectrodes.

Fabrication of a TiO2/Fe2O3 Core/Shell Nanostructure by Pulse Laser Deposition toward Stable and Visible Light Photoelectrochemical Water Splitting

Here, we report the fabrication of TiO₂/Fe₂O₃ core/shell heterojunction nanorod arrays by a pulsed laser deposition (PLD) process and their further use as photoelectrodes toward high-performance visible light photoelectrochemical (PEC) water splitting. The morphology, phase, and carrier conduction mechanism of plain TiO₂ and TiO₂/Fe₂O₃ core/shell nanostructure were systematically investigated. PEC measurements show that the TiO₂/Fe₂O₃ core/shell nanostructure enhances photocurrent density by nearly 2 times than the plain ones, increases visible light absorption from 400 to 550 nm, raises the on/off separation rate, and delivers high stability with only a 3% decrease of current density for tests of even more than 14 days. This work provides a method to design an efficient nanostructure by combination of a facile hydrothermal process and high-quality PLD process to fabricate a clean surface and excellent crystallinity for charge separation, transfer, and collection toward enhanced PEC properties.

α-Fe2O3 hollow meso-microspheres grown on graphene sheets function as a promising counter electrode in dye-sensitized solar cells

Although nanoparticles, nanorods, and nanosheets of α-Fe₂O₃ on graphene sheets have been synthesized, it remains a challenge to grow 3D α-Fe₂O₃ nanomaterials with more sophisticated compositions and structures on the graphene sheets. Herein, we demonstrate a facile solvothermal route under controlled conditions to successfully fabricate 3D α-Fe₂O₃ hollow meso–microspheres on the graphene sheets (α-Fe₂O₃/RGO HMM). Attributed to the combination of the catalytic features of α-Fe₂O₃ hollow meso–microspheres and the high conductivity of graphene, α-Fe₂O₃/RGO HMM exhibited promising electrocatalytic performance as a counter electrode in dye-sensitized solar cells (DSSCs). The DSSCs fabricated with α-Fe₂O₃ HMM displayed high power conversion efficiency of 7.28%, which is comparable with that of Pt (7.71%).

Although nanoparticles, nanorods, and nanosheets of α-Fe₂O₃ on graphene sheets have been synthesized, it remains a challenge to grow 3D α-Fe₂O₃ nanomaterials with more sophisticated compositions and structures on the graphene sheets.