Molten Salts Derived Copper Tungstate Nanoparticles as Bifunctional Electro‐Catalysts for Electrolysis of Water and Supercapacitor Applications

Silver nanoparticle-decorated NiFe2O4/CuWO4 heterostructure electrocatalyst for oxygen evolution reactions

In this work, Ag nanoparticles decorated with NiFe2O4/CuWO4 heterostructure were synthesized using the step-wise precipitation method. The influence of varying Ag loading on the NiFe2O4/CuWO4 heterostructure and its electrochemical OER performance was extensively studied in 1 M KOH electrolyte. The obtained LSV profile was analyzed to determine the overpotential, Tafel slope, and onset potential. The heterostructure with an optimal Ag loading of 5 wt% required the least overpotential (1.60 V vs. RHE) for generating a current density of 10 mA cm-2 with a lower Tafel slope of 44.5 mV dec-1, indicating its faster OER kinetics. Furthermore, the composite remained stable over a period of 24 hours with a minimum rise in the overpotential after the stability test. The enhanced OER performance of the as-prepared catalyst can be attributed to the presence of multiple metallic elements in the Ag-loaded NiFe2O4/CuWO4 composite, which created a diverse array of oxygen-vacant sites with varying reactivity, enhancing the charge-transfer kinetics; and thus contributing to the overall efficiency of OER. Therefore, optimizing the Ag concentration and engineering a microstructure represents an encouraging strategy for developing cost-effective catalysts for next-generation energy-conversion applications.

Neodymium-Doped Novel Barium Tungstate Nanospindles for the Enhanced Oxygen Evolution Reaction

In this work, pristine, 0.02, 0.04, and 0.06 M neodymium (Nd)-doped barium tungstate nanostructures were synthesized via a simple co-precipitation method for the water oxidation process. The obtained X-ray diffraction high-intensity peak at a 2θ value of 26.4° corresponding to the (112) lattice plane confirmed the formation of a tetragonal structure of BaWO₄. Moreover, the BaWO₄ morphology was examined by scanning electron microscopy, which showed the existence of nanospindles. An energy-dispersive X-ray spectrum confirmed the subsistence of the produced materials, for example, barium (Ba), tungsten (W), oxide (O), and neodymium (Nd), with weight percentages of 28.58, 46.63, 16.64, and 8.16%, respectively. The 0.04 M Nd-doped BaWO₄ product was explored to attain a high surface area of 18.18 m²/g, a pore volume of 0.079 cm³/g, and a pore diameter of 2.215 nm. Compared to the other prepared electrodes, the 0.04 M Nd-doped BaWO₄ product exhibited low overpotential values of 330 mV and 450 mV to deliver current densities of 10 mA/cm² and 50 mA/cm², respectively. In addition, the optimized electrode achieved a small Tafel slope value of 158 mV dec^-1 and followed the Volmer-Heyrovsky mechanism. Moreover, the electrical conductivity of BaWO₄ was tuned due to the addition of a rare-earth metal dopant, and it exhibited the charge-transfer resistance and solution resistance values of 0.98 and 1.01 Ω, respectively. The prepared electrocatalyst was further studied by using cyclic voltammetry, and it exhibited a high double-layer capacitance value of 29.3 mF/cm² and high electrochemically active surface areas of 1.465 cm². The electrochemical performance was greatly improved depending on the concentration of the doping agent, and it was well consistent with the obtained results. The best electrocatalyst was subjected to a chronoamperometry test, which exhibited excellent stability even after 20 h. Hence, this work suggests that alkaline metal tungstates have a cost-effective, efficient, and promising electrocatalyst, and it is a new approach for the water oxidation process.

Bismuth-Based Multi-Component Heterostructured Nanocatalysts for Hydrogen Generation

Developing a unique catalytic system with enhanced activity is the topmost priority in the science of H2 energy to reduce costs in large-scale applications, such as automobiles and domestic sectors. Researchers are striving to design an effective catalytic system capable of significantly accelerating H2 production efficiency through green pathways, such as photochemical, electrochemical, and photoelectrochemical routes. Bi-based nanocatalysts are relatively cost-effective and environmentally benign materials which possess advanced optoelectronic properties. However, these nanocatalysts suffer back recombination reactions during photochemical and photoelectrochemical operations which impede their catalytic efficiency. However, heterojunction formation allows the separation of electron–hole pairs to avoid recombination via interfacial charge transfer. Thus, synergetic effects between the Bi-based heterostructured nanocatalysts largely improves the course of H2 generation. Here, we propose the systematic review of Bi-based heterostructured nanocatalysts, highlighting an in-depth discussion of various exceptional heterostructures, such as TiO2/BiWO6, BiWO6/Bi2S3, Bi2WO6/BiVO4, Bi2O3/Bi2WO6, ZnIn2S4/BiVO4, Bi2O3/Bi2MoO6, etc. The reviewed heterostructures exhibit excellent H2 evolution efficiency, ascribed to their higher stability, more exposed active sites, controlled morphology, and remarkable band-gap tunability. We adopted a slightly different approach for reviewing Bi-based heterostructures, compiling them according to their applicability in H2 energy and discussing challenges, prospects, and guidance to develop better and more efficient nanocatalytic systems.

Tin Oxide Based Hybrid Nanostructures for Efficient Gas Sensing

Tin oxide as a semiconductor metal oxide has revealed great potential in the field of gas sensing due to its porous structure and reduced size. Especially for tin oxide and its composites, inherent properties such as high surface areas and their unique semiconducting properties with tunable band gaps make them compelling for sensing applications. In combination with the general benefits of metal oxide nanomaterials, the incorporation of metal oxides into metal oxide nanoparticles is a new approach that has dramatically improved the sensing performance of these materials due to the synergistic effects. This review aims to comprehend the sensing mechanisms and the synergistic effects of tin oxide and its composites in achieving high selectivity, high sensitivity and rapid response speed which will be addressed with a full summary. The review further vehemently highlights the advances in tin oxide and its composites in the gas sensing field. Further, the structural components, structural features and surface chemistry involved in the gas sensing are also explained. In addition, this review discusses the SnO2 metal oxide and its composites and unravels the complications in achieving high selectivity, high sensitivity and rapid response speed. The review begins with the gas sensing mechanisms, which are followed by the synthesis methods. Further key results and discussions of previous studies on tin metal oxide and its composites are also discussed. Moreover, achievements in recent research on tin oxide and its composites for sensor applications are then comprehensively compiled. Finally, the challenges and scope for future developments are discussed.

Superlative photoelectrochemical properties of 3D MgCr-LDH nanoparticles influencing towards photoinduced water splitting reactions

In the present work, we report the synthesis of single system three-dimensional (3D) open porous structure of MgCr-LDH nanoparticles in a substrate-free path by using one-step formamide assisted hydrothermal reaction followed by visible light irradiation for significant photoelectrochemical (PEC) properties that manifest towards photocatalytic H₂ and O₂ production. The as-prepared nanostructured materials were characterized by various physico-chemical characterization techniques. Moreover, this unique synthetic approach produces 3D open porous network structure of MgCr-LDH nanoparticles, which were formed by stacking of numerous 2D nanosheets, for effective light harvestation, easy electronic channelization and unveil superlative PEC properties, including high current density (6.9 mA/cm²), small Tafel slope of 82 mV/decade, smallest arc of the Nyquist plot (59.1 Ω cm⁻²) and photostability of 6000 s for boosting water splitting activity. In addition, such perfectly self-stacked 2D nanosheets in 3D MgCr-LDH possess more surface active defect sites as enriched 50% oxygen vacancy resulting a good contact surface within the structure for effective light absorption along with easy electron and hole separation, which facilitates the adsorption of protons and intermediate for water oxidation. Additionally, the Cr³⁺ as dopant pull up the electrons from water oxidation intermediates, thereby displaying superior photocatalytic H₂ and O₂ production activity of 1315 μmol/h and 579 μmol/h, respectively. Therefore, the open 3D morphological aspects of MgCr-LDH nanoparticles with porous network structure and high surface area possess more surface defect sites for electron channelization and identified as distinct novel features of this kind of materials for triggering significant PEC properties, along with robustly enhance the photocatalytic water splitting performances.

Raspberry-like CuWO4 hollow spheres anchored on sulfur-doped g-C3N4 composite: An efficient electrocatalyst for selective electrochemical detection of antibiotic drug nitrofurazone

We report a highly selective and sensitive electrochemical sensor for the determination of nitrofurazone (NZ) based on sulfur-doped graphitic carbon nitride with copper tungstate hollow spheres (Sg-C₃N₄/CuWO₄). Here, a Sg-C₃N₄/CuWO₄ composite was synthesized by a facile ultrasonic method. The physicochemical properties of the composite were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Then, the surface morphology of the composite material was investigated by field emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM). Moreover, the electrochemical activity of the as-synthesized composite material was initially tested using electrochemical impedance spectroscopy (EIS). The electroanalytical techniques namely cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were carried out for the electrochemical studies. The proposed sensor exhibits lower LOD and good sensitivity of about 3 nM and 1.24 μAμM^-1 cm^-2 to NZ detection. In addition, the Sg-C₃N₄/CuWO₄ modified electrode showed excellent repeatability, reproducibility, long-term storage stability and excellent selectivity. The developed sensor was successfully applied for the determination of NZ in human urine and serum samples and achieved good recovery results.

Self-Assembled Interwoven Nanohierarchitectures of NaNbO3 and NaNb1-x Ta x O3 (0.05 ≤ x ≤ 0.20): Synthesis, Structural Characterization, Photocatalytic Applications, and Dielectric Properties

Dependence on fossil fuels for energy purposes leads to the global energy crises due to the nonrenewable nature and high CO₂ production for environmental pollution. Therefore, new ways of nanocatalysis for environmental remediation and sustainable energy resources are being explored. Herein, we report a facile surfactant free, low temperature, and environmentally benign hydrothermal route for development of pure and (5, 10, 15, and 20 mol %) Ta-doped horizontally and vertically interwoven NaNbO₃ nanohierarchitecture photocatalysts. To the best of our knowledge, such a type of hierarchical structure of NaNbO₃ has never been reported before, and changes in the microstructure of these nanoarchitectures on Ta-doping has also been examined for the first time. As-synthesized nanostructures were characterized by different techniques including X-ray diffraction analysis, electron microscopic studies, X-ray photoelectron spectroscopic studies, etc. Ta-doping considerably affects the microstructure of the nanohierarchitectures of NaNbO₃, which was analyzed by FESEM analysis. The UV-visible diffused reflectance spectroscopy study shows considerable change in the band gap of as-synthesized nanostructures and was found to be ranging from 2.8 to 3.5 eV in pure and different mole % Ta-doped NaNbO₃. With an increase in dopant concentration, the surface area increases and was equal to 5.8, 6.8, 7.0, 9.2, and 9.7 m²/g for pure and 5, 10, 15, and 20 mol % Ta-doped NaNbO₃, respectively. Photocatalytic activity toward the degradation of methylene blue dye and H₂ evolution reaction shows the highest activity (89% dye removal and 21.4 mmol g^-1 catalyst H₂ evolution) for the 10 mol % NaNbO₃ nanostructure which was attributed to a change in the conduction band maximum of the material. At 100 °C and 500 kHz, the dielectric constants of pure and 5, 10, 15, and 20 mol % Ta-doped NaNbO₃ were found to be 111, 510, 491, 488, and 187, respectively. The current study provides the rational insight into the design of nanohierarchitectures and how microstructure affects different properties of the material upon doping.

Modified, Solvothermally Derived Cr-doped SnO2 Nanostructures for Enhanced Photocatalytic and Electrochemical Water-Splitting Applications

Cr-doped SnO₂ nanostructures with a dopant concentration ranging from 1 to 5% have been successfully prepared using low-temperature modified solvothermal synthesis. The as-prepared nanoparticles showed a rutile tetragonal structure with a rough undefined morphology having no other elemental impurities. The particle shape and size, band gap, and specific surface area of the samples were investigated by scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, UV-visible diffused reflectance spectroscopy, and Brunauer-Emmett-Teller surface area studies. The optical band gap was found in the range of 3.23-3.67 eV and the specific surface area was in the range of 108-225 m²/g, which contributes to the significantly enhanced photocatalytic and electrochemical performance. Photocatalytic H₂ generation of as-prepared Cr-doped SnO₂ nanostructures showed improved effect of the increasing dopant concentration with narrowing of the band gap. Electrochemical water-splitting studies also stressed upon the superiority of Cr-doped SnO₂ nanostructures over pristine SnO₂ toward hydrogen evolution reaction and oxygen evolution reaction responses.

Morphology-oxygen evolution activity relationship of iridium(iv) oxide nanomaterials

This work demonstrated shape tuning of IrO2 nanoparticles to nanocube and nanorods in molten salt and demonstrated the exemplary performance of IrO2 nanorods as an electrocatalyst for oxygen evolution reaction even surpassing commercial IrO2.

Investigation of enhanced electro-catalytic HER/OER performances of copper tungsten oxide@reduced graphene oxide nanocomposites in alkaline and acidic media

In this paper, we investigate the electro-catalytic hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) of synthesized copper tungsten oxide@reduced graphene oxide (CuWO4@rGO) nanocomposites.

Synthesis, Characterization and Enhanced Visible Light Photocatalytic Performance of ZnWO4-NPs@rGO Nanocomposites

ZnWO4 nanoparticles on reduced graphene oxide (ZnWO4-NPs@rGO) nanocomposites were synthesized using the hydrothermal method. Structural, morphological, optical, and photocatalytic studies of the ZnWO4-NPs@rGO nanocomposites were successfully investigated. Photo-catalytic performances of the ZnWO4-NPs@rGO nanocomposites were examined for the degradation of hazardous methylene blue dye (HMBD) in a neutral medium. ZnWO4-NPs@rGO nanocomposites show superior photo-catalytic performances over pure ZnWO4 nanoparticles. ZnWO4-NPs@rGO nanocomposites degrade ~98% dye while pure ZnWO4 nanoparticles degrade ~53% dye in 120 min. The prepared nanocomposites also show excellent recycled photo-catalytic efficiencies over multiple cycles.

Bottom-up Approach for Designing Cobalt Tungstate Nanospheres through Sulfur Amendment for High-Performance Hybrid Supercapacitors

Nanofabrication of heteroatom-doped metal oxides into a well-defined architecture via a "bottom-up" approach is crucial to overcome the boundaries of the metal oxides for energy storage systems. In the present work, this issue was addressed by developing sulfur-doped bimetallic cobalt tungstate (CoWO₄ ) porous nanospheres for efficient hybrid supercapacitors via a single-step, ascendable bottom-up approach. The combined experimental and kinetics studies revealed enhanced electrical conductivity, porosity, and openness for ion migration after amendments of the CoWO₄ via sulfur doping. As a result, the sulfur-doped CoWO₄ nanospheres exhibited a specific capacity of 248.5 mA h g^-1 with outstanding rate capability and cycling stability. The assembled hybrid supercapacitor cell with sulfur-doped CoWO₄ nanospheres and activated carbon electrodes could be driven reversibly in a voltage of 1.6 V and exhibited a specific capacitance of 177.25 F g^-1 calculated at 1.33 A g^-1 with a specific energy of 63.41 Wh kg^-1 at 1000 W kg^-1 specific power. In addition, the hybrid supercapacitor delivered 94.85 % initial capacitance over 10000 charge-discharge cycles. The excellent supercapacitive performance of sulfur-doped CoWO₄ nanospheres may be credited to the sulfur doping and bottom-up fabrication of the electrode materials.

Efficient electrocatalyst of α-Fe2O3 nanorings for oxygen evolution reaction in acidic conditions

Large-scale application of sustainable energy devices urgently requires cost-effective electrocatalysts to overcome the sluggish kinetics related to the oxygen evolution reaction (OER) under acidic conditions. Here, we first report the highly efficient electrocatalytic characteristics of α-Fe₂O₃ nanorings (NRs), which exhibits prominent OER electrocatalytic activity with lower overpotential of 1.43 V at 10 mA cm⁻² and great stability in 1 M HCl, surpassing the start-of-the art Ir/C electrocatalyst. The significantly optimized OER activity of the α-Fe₂O₃ NRs mainly attributes to the synergistic effect of the excellent electrical conductivity and a large effective active surface because of their unique nanoring structure, disordered surface, and the dynamic stability of α-Fe₂O₃ NRs in acidic conditions.

α-Fe₂O₃ NRs is obtained for OER with lower small overpotential and great stability in 1 M HCl, surpassing Ir/C electrocatalyst.

High-Surface-Area Sodium Tantalate Nanoparticles with Enhanced Photocatalytic and Electrical Properties Prepared through Polymeric Citrate Precursor Route

NaTaO₃ nanoparticles with a high surface area of 46.2 m²/g have been successfully synthesized using a polymeric citrate precursor route. As-prepared nanoparticles were extensively characterized by X-ray diffraction, Fourier transform infrared, transmission emission microscopy, and scanning electron microscopy studies for the analysis of phase purity, crystallinity, and morphology. NaTaO₃ nanoparticles show efficient photo-induced removal of the methylene blue dye under sunlight, which were confirmed by using liquid chromatography-mass spectroscopy. 86% dye has been degraded in 200 min at neutral pH, whereas the same amount of dye was decolorized in only 80 min at alkaline pH. Also, the dielectric behavior of the as-prepared nanoparticles at different annealing temperatures was explained by the Maxwell-Wagner polarization mechanism. At 500, 600, and 700 °C annealing temperatures, the dielectric constant and dielectric loss at 500 kHz for the samples were found to be 21.5, 18, and 16 and 0.012, 0.022, and 0.029, respectively. The unsaturated hysteresis loop shows weak ferroelectric behavior of NaTaO₃ nanoparticles with remanent and saturation polarizations of 0.0013 and 0.21 μC/cm², respectively, and S-E hysteresis shows a bipolar strain of 0.10%.

Facile Synthesis of Mesoporous α-Fe2O3@g-C3N4-NCs for Efficient Bifunctional Electro-catalytic Activity (OER/ORR)

Mesoporous α-iron oxide@graphitized-carbon nitride nanocomposites (α-Fe2O3@g-C3N4-NCs) were synthesized using urea-formaldehyde (UF) resins at 400 °C/2 h. The mesoporous nature of the prepared nanocomposites was observed from electron microscopy and surface area measurements. The electrochemical measurements show the bifunctional nature of mesoporous α-Fe2O3@g-C3N4-NCs in electrolysis of water for oxygen evolution and oxygen reduction reactions (OER/ORR) using 0.5 M KOH. Higher current density of mesoporous α-Fe2O3@g-C3N4-NCs reveals the enhanced electrochemical performance compared to pure Fe2O3 nanoparticles (NPs). The onset potential, over-potential and Tafel slopes of mesoporous α-Fe2O3@g-C3N4-NCs were found lower than that of pure α-Fe2O3-NPs. Rotating disc electrode experiments followed by the K-L equation were used to investigate 4e- redox system. Therefore, the mesoporous α-Fe2O3@g-C3N4-NCs bifunctional electro-catalysts can be considered as potential future low-cost alternatives for Pt/C catalysts, which are currently used in fuel cells.

Bi2WO6 and FeWO4 Nanocatalysts for the Electrochemical Water Oxidation Process

Polyvinylpyrrolidone (PVP)-assisted nanocatalyst preparation was succeeded by employing a controlled solvothermal route to produce efficient electrodes for electrochemical water-splitting applications. Bi₂WO₆ and FeWO₄ nanocatalysts have been confirmed through the strong signature of (113) and (111) crystal planes, respectively. The binding natures of Bi-W-O and Fe-W-O have been thoroughly discussed by employing X-ray photoelectron spectroscopy which confirmed the formation of Bi₂WO₆ and FeWO₄. The freestanding nanoplate array morphology of Bi₂WO₆ and the fine nanosphere particle morphology of FeWO₄ nanocatalysts were revealed by scanning electron microscopy images. With these confirmations, the fabrication of durable, long-term electrodes for electrochemical water splitting has been subjected to efficient oxidation of water, confirmed by obtaining 2.79 and 1.96 mA/g for 0.5 g PVP-assisted Bi₂WO₆ and FeWO₄ nanocatalysts, respectively. The water oxidation mechanism of both nanocatalysts has been revealed with the support of 24 h stability test over continuous water oxidation and faster charge transfer achieved by the smaller Tafel slope values of 75 and 78 mV/dec, respectively. Generally, these nanocatalysts are utilized for photocatalytic applications. The present study revealed the PVP-assisted synthesis to produce electrocatalytically active nanocatalysts and their electrochemical water-splitting mechanism which will offer a pathway for research interests with regard to the production of multifunctional nanocatalysts for both electro- and photocatalytic applications in the near future.

Tuning the NIR photoabsorption of CuWO4−x nanodots with oxygen vacancies for CT imaging guided photothermal therapy of tumors

NIR photoabsorption of CuWO_4−x can be tuned, and the resulting CuWO_4−x nanodots can act as efficient all-in-one nanoagent for simultaneous CT/IR imaging and photothermal therapy of tumors.