Effect of Cooperative Redox Property and Oxygen Vacancies on Bifunctional OER and HER Activities of Solvothermally Synthesized CeO₂/CuO Composites

Herein, we report the synthesis of the CeO2/CuO composite as a bifunctional oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) electrocatalyst in a basic medium. The electrocatalyst with an optimum 1:1 CeO2/CuO shows low OER and HER overpotentials of 410 and 245 mV, respectively. The Tafel slopes of 60.2 and 108.4 mV/dec are measured for OER and HER, respectively. More importantly, the 1:1 CeO2/CuO composite electrocatalyst requires only a 1.61 V cell voltage to split water to achieve 10 mA/cm2 in a two-electrode cell. The role of oxygen vacancies and the cooperative redox activity at the interface of the CeO2 and CuO phases is explained in the light of Raman and XPS studies, which play the determining factor for the enhanced bifunctional activity of the 1:1 CeO2/CuO composite. This work provides guidance for the optimization and design of a low-cost alternative electrocatalyst to replace the expensive noble-metal-based electrocatalyst for overall water splitting

Hierarchical Urchin-like Cobalt-Doped CuO for Enhanced Electrocatalytic Oxygen Evolution Reaction

Designing an efficient, low-cost, and earth-abundant electrocatalyst for water oxidation is the center of attraction in renewable energy technology, but it is still a grand challenge. Herein, we report a simple microwave-assisted solvothermal method to synthesize cobalt-doped CuO as a noble metal-free efficient electrocatalyst for oxygen evolution reaction (OER) in an alkaline medium. Optimal cobalt doping (5%) in CuO manifests an improved OER activity with an overpotential of 120 mV, which is ∼2.66 times lower than that obtained with pristine CuO to achieve a current density of 10 mA cm–2, superior to the state-of-the-art OER catalysts. A Tafel slope of 118 mV dec–1 with an electrochemical active surface area of 40 cm2 and a charge transfer resistance of 2.58 Ω were achieved with 5% cobalt-doped CuO, which validate the role of metal doping and importance of doping in transition metal oxides to enhance the electrochemical OER activity

Selective Oxidation of Styrene on Nanostructured Cerium Vanadate Catalyst

Growing demand for benzaldehyde in recent years has prompted researchers to explore an environmentally sustainable process to synthesize it from abundant materials to replace the conventional toluene chlorination method. In that direction, benzaldehyde synthesis from the catalytic oxidative cleavage of styrene is attractive, as styrene is cheap and millions of tons are produced per year. Herein, cerium vanadate (CeVO4) is demonstrated as an outstanding catalyst for the synthesis of benzaldehyde from styrene oxidation at 80 °C. The particle-shaped (CeV NP), rod-shaped (CeV NR), and bar-shaped (CeV NB) nanostructured CeVO4 (CeV) are synthesized by varying the surfactants. All the catalysts exhibit favorable properties such as high surface area, surface charge, acidity, and oxygen vacancy, which are correlated to their catalytic performance. Among the morphologies, CeV NP shows the highest styrene conversion (99.5%) along with the highest benzaldehyde selectivity (84.2%). The nonleaching of catalyst further confirms the heterogeneous reaction process, which makes it a strong candidate for industrial purposes

Morphology Controlled Solution-Based Synthesis of Cu₂O Crystals for the Facets-Dependent Catalytic Reduction of Highly Toxic Aqueous Cr(VI)

In this study, we demonstrate the systematic shape evolution of Cu₂O crystals from the octahedron, through truncated octahedron, cube, and finally to truncated cube by varying the reaction temperature with an optimum precursor concentration of 25 mM Cu­(NO₃)₂·3H₂O and 1 g of polyvinylpyrrolidone (PVP) as the shape controlling reagent. The average size of these crystals increased with temperature from ∼70 nm (at 40 °C) to ∼1 μm (at 100 °C). With a much lower (6 mM) and higher (250 mM) precursor concentration, nanoparticles and polyhedron-shaped crystals are respectively formed in the studied temperature region (40–120 °C). The role of precursor concentration, PVP quantity, reaction medium, and reaction temperature in the formation of diverse Cu₂O crystals morphologies are demonstrated and discussed. Furthermore, the catalytic activity of the as-synthesized Cu₂O crystals is tested for the reduction of Cr­(VI) at room temperature. The toxic Cr­(VI) is found to be rapidly reduced to nontoxic Cr­(III) in a short span of 4 min in the presence of Cu₂O cubes in the acidic medium. The repeat catalytic measurements of Cr­(VI) reduction for 20 cycles confirm higher stability of cube-shaped Cu₂O crystals with {100} exposed facets as compared to octahedrons with {111} exposed facets, a classic example of facets-dependent catalytic properties of crystals

Room Temperature Acid-Free Greener Synthesis of Imine Using Cobalt-Doped Manganese Tungstate

Facile synthesis of an imine compound through a greener route is still a challenging task. Industrial processes rely on the age-old Schiff reaction for the synthesis of imine, which are reversible and nongreen from an environmental viewpoint. Herein, cobalt-doped manganese tungstate with two different morphologies is synthesized and demonstrated as a recyclable catalyst for imine synthesis from the condensation of an aldehyde and an amine with 73% yield of an imine in a nonaqueous and nonacidic environment at room temperature. The high catalytic activity is attributed to cobalt doping, high surface area, strong acidic site, and the polar nature of the catalyst. The stability and recyclability test shows that the catalytic activity remains the same after several cycles, which is crucial from the industrial point of view. The formation of imine is found to follow an alternative mechanism in an irreversible manner with a polar four-membered intermediate unlike the conventional method. The demonstrated process has several advantages including irreversibility, “greener”, environmental friendly, and energy-efficient

Boosting the Photocatalytic H₂ Evolution and Benzylamine Oxidation using 2<i>D</i>/1D g‑C₃N₄/TiO₂ Nanoheterojunction

The present research aims at the elevation of solar-to-chemical energy conversion with extortionate performance and sustainability. The nanostructured materials are revolutionizing the water splitting technology into decoupled hydrogen with simultaneous value-added organic chemical production. Yet, the bottleneck in semiconductor photocatalysis is rapid charge recombination and sluggish reaction kinetics. Herein, we demonstrate an efficient and non-noble metal-based catalyst for successful redox reaction with a theoretical modeling through density functional theory (DFT) study. Implementing this robust approach on 2D/1D ultrathin g-C3N4 nanosheets and TiO2 nanowires heterojunction, we achieved H2 production of 5.1 mmol g–1 h–1 with apparent quantum efficiency of 7.8% under visible light illumination and 93% of benzylamine conversion to N-benzylidene benzylamine in situ. The interface of 2D g-C3N4 nanosheets and 1D nanowires provide ample active sites and extends the visible light absorption with requisite band edge position for the separation of photoinduced charge carriers with superior stability. The electronic properties, band structure, and stability of the heterojunction are further investigated via DFT calculations which corroborate the experimental results and in good agreement for the enhanced activity of the heterojunction

Monodispersed PtPdNi Trimetallic Nanoparticles-Integrated Reduced Graphene Oxide Hybrid Platform for Direct Alcohol Fuel Cell

The direct alcohol fuel cell has recently emerged as an important energy conversion device. In the present article, superior alcohol (ethanol, ethylene glycol, and glycerol) electrooxidation performance using trimetallic platinum–palladium–nickel (PtPdNi) alloy nanoparticles of diameters from 2–4 nm supported on a reduced graphene oxide (rGO) electrocatalyst is demonstrated. A simple and single-step solvothermal technique is adopted to fabricate the alloy/rGO hybrid electrocatalysts by simultaneous reduction of metal ions and graphene oxide. The detailed electrochemical investigation revealed that the performance of the trimetallic/rGO hybrid toward electrooxidation of different alcohols is higher than that of bimetallic alloy/rGO hybrids and the state-of-the-art Pt/C catalyst. The incorporation of Ni into the PtPd alloy is found to change the surface of the electronic structure PtPd alloy leading to higher electrochemical surface areas and improved kinetics. In addition, the hydrophilic nature of Ni not only facilitates alcohol electrooxidation but also electrooxidation of residual carbon impurities formed on the catalyst surface, thus reducing catalyst poisoning, demonstrating its role in the development of anode catalysts for the alcohol fuel cells

Controlled Synthesis of CuS/TiO₂ Heterostructured Nanocomposites for Enhanced Photocatalytic Hydrogen Generation through Water Splitting

Photocatalytic hydrogen (H2) generation through water splitting has attracted substantial attention as a clean and renewable energy generation process that has enormous potential in converting solar-to-chemical energy using suitable photocatalysts. The major bottleneck in the development of semiconductor-based photocatalysts lies in poor light absorption and fast recombination of photogenerated electron–hole pairs. Herein we report the synthesis of CuS/TiO2 heterostructured nanocomposites with varied TiO2 contents via simple hydrothermal and solution-based process. The morphology, crystal structure, composition, and optical properties of the as-synthesized CuS/TiO2 hybrids are evaluated in detail. Controlling the CuS/TiO2 ratio to an optimum value leads to the highest photocatalytic H2 production rate of 1262 μmol h–1 g–1, which is 9.7 and 9.3 times higher than that of pristine TiO2 nanospindles and CuS nanoflakes under irradiation, respectively. The enhancement in the H2 evolution rate is attributed to increased light absorption and efficient charge separation with an optimum CuS coverage on TiO2. The photoluminescence and photoelectrochemical measurements further confirm the efficient separation of charge carriers in the CuS/TiO2 hybrid. The mechanism and synergistic role of CuS and TiO2 semiconductors for enhanced photoactivity is further delineated

Giant Dielectric Constant and Superior Photovoltaic Property of the Mechanochemically Synthesized Stable CH₃NH₃PbBr₃ in a Hole Transporter-Free Solar Cell

Hybrid organic inorganic perovskites (HOIPs) have emerged as the most promising next-generation photovoltaic (PV) materials. However, syntheses of HOIPs not only require a controlled environment but also their long term stability is a major challenge. Herein, we report mechanochemical synthesis of highly stable CH3NH3PbBr3 (MAPbBr3) powder under an open atmosphere that undergoes no phase change even after 6 months. The MAPbBr3 exhibits a large dielectric constant of >104 in the frequency regime of 1 Hz to 106 Hz and in a wide temperature range of 300–475 K because of its inherent charge carrier, which is contended for the low exciton binding energy in HOIPs, leading to superior PV performance as compared to the solution-based synthesized MAPbBr3. The excitonic activation energy measured from the ac conductivity study in the same temperature range is 5 meV, which suggests facile exciton separation and generation of free charge carrier upon irradiation. The PV performance of the as-synthesized MAPbBr3 is investigated by fabricating two thin-film based device architectures without the hole-transporter layer, that is, FTO/MAPbBr3/Ag and FTO/TiO2/MAPbBr3/Ag, exhibiting a photo conversion efficiency of 4.2 and 7.31%, respectively. In contrast to the thermally evaporated metal counter electrode (e.g., Au and Ag), the spin-coated Ag film from solution is used as top contact here, thereby improving the cost-effectiveness and metal-electrode fabrication impediments

Electrochemical Pd Nanodeposits on a Au Nanoisland Template Supported on Si(100): Formation of Pd−Au Alloy and Interfacial Electronic Structures

Palladium nanoparticles have uniformly been electrodeposited on a Au nanoisland template (NIT) supported on a Si(100) substrate, which exhibits Au-rich, Pd-rich, and/or polycrystalline mixed structures upon annealing to 700 °C. Glancing-incidence X-ray diffraction (GIXRD) and energy-dispersive X-ray (EDX) elemental analysis of the as-deposited sample both show metallic Pd, while depth-profiling X-ray photoelectron spectroscopy (XPS) further reveals the presence of Pd−Au (and PdxSi) at the interfaces of the Pd nanodeposits on the Au NIT. Upon the sample being annealed to 700 °C, both Pd 3d3/2 and Au 4f7/2 XPS peaks are found to shift to lower binding energies, which further confirms Pd−Au alloy formation. The convergence of respective GIXRD features of metallic Au and Pd toward intermediate peak positions supports the formation of alloy and their crystalline nature. Depth-profiling XPS analysis of the annealed sample further shows that the Pd nanoparticles are found to consist of an ultrathin shell of PdO2, and a PdO-rich (i.e., Pd-poor) inner-core, which is consistent with the observed GIXRD patterns of PdO and Pd−Au alloy but indiscernible PdO2. We compare the above results with the experimental results for electrodeposited Pd on a bare Si(100) substrate. Our study provides new insight into the formation of Pd−Au alloy composite on Si by electrochemistry. The easy control of the Pd, Au, and Pd−Au composition in the nanodeposits as illustrated in the present method offers new flexibility for developing hybrid nanocatalysts and other applications