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

Microwave-Assisted Greener Synthesis of Defect-Rich Tungsten Oxide Nanowires with Enhanced Photocatalytic and Photoelectrochemical Performance

Exploring semiconductor materials with superior photocatalytic activity is desirable to mitigate the crisis associated with rapid depletion of fossil fuels and environmental pollutions. Herein we report the synthesis of orthorhombic stacked WO₃·H₂O square nanoplates by mixing WCl₆ (0.025 M) in ethanol at room temperature via a precipitation method. On the other hand, hierarchical urchin-like W₁₈O₄₉ nanostructures composed of nanowires were synthesized from the preceding solution within 10 min through a microwave-assisted route. The morphology evolution from nanoplates to nanowires proceeds through a dissolution and recrystallization mechanism, as demonstrated in detail by varying the reaction duration and temperature. The as-synthesized WO₃·H₂O nanoplates and W₁₈O₄₉ nanowires were employed for the photocatalytic degradation of rhodamine B and photoelectrocatalytic hydrogen generation through water splitting in a neutral medium. Furthermore, the as-synthesized W₁₈O₄₉ nanostructures are employed as electrocatalysts for hydrogen evolution reaction in both acidic and neutral electrolytes. The enhanced electrocatalytic and photocatalytic activity of W₁₈O₄₉ nanostructures are attributed to their larger surface area, oxygen vacancies, and faster charge transport properties. This work demonstrates a greener and simpler way to synthesize a promising defect-rich material (W₁₈O₄₉) in a short duration and its potential in electrocatalytic and photoelectrocatalytic hydrogen generation, and for degradation of pollutant

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

Shape-Dependent Photocatalytic Activity of Hydrothermally Synthesized Cadmium Sulfide Nanostructures

The effective surface area of the nanostructured materials is known to play a prime role in catalysis. Here we demonstrate that the shape of the nanostructured materials plays an equally important role in their catalytic activity. Hierarchical CdS microstructures with different morphologies such as microspheres assembled of nanoplates, nanorods, nanoparticles, and nanobelts are synthesized using a simple hydrothermal method by tuning the volume ratio of solvents, i.e., water or ethylenediamine (en). With an optimum solvent ratio of 3:1 water:en, the roles of other synthesis parameters such as precursor’s ratio, temperature, and precursor combinations are also explored and reported here. Four selected CdS microstructures are used as photocatalysts for the degradation of methylene blue and photoelectrochemical water splitting for hydrogen generation. In spite of smaller effective surface area of CdS nanoneedles/nanorods than that of CdS nanowires network, the former exhibits higher catalytic activity under visible light irradiation which is ascribed to the reduced charge recombination as confirmed from the photoluminescence study

Synergy of Low-Energy {101} and High-Energy {001} TiO₂ Crystal Facets for Enhanced Photocatalysis

Controlled crystal growth determines the shape, size, and exposed facets of a crystal, which usually has different surface physicochemical properties. Herein we report the size and facet control synthesis of anatase TiO₂ nanocrystals (NCs). The exposed facets are found to play a crucial role in the photocatalytic activity of TiO₂ NCs. This is due to the known preferential flow of photogenerated carriers to the specific facets. Although, in recent years, the main focus has been on increasing the surface area of high-energy exposed facets such as {001} and {100} to improve the photocatalytic activity, here we demonstrate that the presence of both the high-energy {001} oxidative and low-energy {101} reductive facets in an optimum ratio is necessary to reduce the charge recombination and thereby enhance photocatalytic activity of TiO₂ NCs

Nitrogen Doped Reduced Graphene Oxide Based Pt–TiO₂ Nanocomposites for Enhanced Hydrogen Evolution

Electrochemical hydrogen production from water is an attractive clean energy generation process that has enormous potential for sustainable development. However, noble metal catalysts are most commonly used for such electrochemical hydrogen evolution making the process cost ineffective. Thereby design of hybrid catalysts with minimal use of noble metals using a suitable support material is a prime requirement for the electrolysis of water. Herein, we demonstrate the superior hydrogen evolution reaction (HER) activity of the platinum nanoparticles (Pt NPs) supported on faceted titanium dioxide (TiO₂) nanocrystals (Pt–TiO₂) and nitrogen doped reduced graphene oxide (N-rGO) based TiO₂ nanocomposite (Pt–TiO₂–N-rGO). The ternary Pt–TiO₂–N-rGO nanocomposite exhibits a superior HER activity with a small Tafel slope (∼32 mV·dec^–1), exchange current density (∼0.22 mA·cm^–2), and excellent mass activity (∼3116 mA·mg_pt^–1) at 300 mV overpotential. These values are better/higher than that of several support materials investigated so far. The excellent HER activity of the ternary Pt–TiO₂–N-rGO nanocomposite is ascribed to the presence of Ti­(III) states and enhanced charge transportation properties of N-rGO. The present study is a step toward reliable electrochemical hydrogen production using faceted TiO₂ nanocrystals as support material

Facile Green Synthesis of WO₃·H₂O Nanoplates and WO₃ Nanowires with Enhanced Photoelectrochemical Performance

The synthesis of nanostructured materials with controlled shape without using a capping agent and/or a hazardous chemical is one of the major existing challenges. Herein, we report a facile precipitation method to synthesize stacked orthorhombic tungsten trioxide hydrate (WO₃·H₂O) nanoplates by simply mixing WCl₆ (0.025 M) in ethanol at room temperature for 1 h. On subsequent solvothermal treatment of WO₃·H₂O nanoplates at 200 °C in ethanol, formation of monoclinic tungsten trioxide (WO₃) nanowires of <20 nm diameter is demonstrated. The morphology evolution of WO₃ nanowires from WO₃·H₂O nanoplates and change in growth direction through dissolution and recrystallization process is further confirmed by varying the solvothermal duration and temperature. The as-synthesized WO₃·H₂O nanoplates and WO₃ nanowires are used as photoanodes for the hydrogen generation through photoelectrochemical (PEC) water splitting in a neutral pH. The photocurrent density of WO₃ nanowires is found ∼21 times higher than that of WO₃·H₂O nanoplates at 1.0 V vs saturated calomel electrode (SCE) and also higher than the reported WO₃ nanostructures. The superior PEC performance of WO₃ nanowires is justified on the basis of its (200) oriented one-dimensional morphology, large surface area, and small interfacial charge transfer resistance

High Performance Solid-State Asymmetric Supercapacitor using Green Synthesized Graphene–WO₃ Nanowires Nanocomposite

Development of active materials capable of delivering high specific capacitance is one of the present challenges in supercapacitor applications. Herein, we report a facile and green solvothermal approach to synthesize graphene supported tungsten oxide (WO₃) nanowires as an active electrode material. As an active electrode material, the graphene–WO₃ nanowire nanocomposite with an optimized weight ratio has shown excellent electrochemical performance with a specific capacitance of 465 F g^–1 at 1 A g^–1 and a good cycling stability of 97.7% specific capacitance retention after 2000 cycles in 0.1 M H₂SO₄ electrolyte. Furthermore, a solid-state asymmetric supercapacitor (ASC) was fabricated by pairing a graphene–WO₃ nanowire nanocomposite as a negative electrode and activated carbon as a positive electrode. The device has delivered an energy density of 26.7 W h kg^–1 at 6 kW kg^–1 power density, and it could retain 25 W h kg^–1 at 6 kW kg^–1 power density after 4000 cycles. The high energy density and excellent capacity retention obtained using a graphene–WO₃ nanowire nanocomposite demonstrate that it could be a promising material for the practical application in energy storage devices

Hybrid Dot–Disk Au-CuInS₂ Nanostructures as Active Photocathode for Efficient Evolution of Hydrogen from Water

The synthesis of hybrid 0D-2D dot–disk Au-CIS heterostructures is enabled through nucleating wurtzite ternary I–III–VI CuInS₂ (CIS) semiconductor nanostructures on cubic Au particles via thiol-activated interface reactions. Chemistry of formation of these unique hybrid metal–semiconductor nanostructures is established by correlating successive X-ray diffraction patterns and microscopic images. Furthermore, these nanostructures are explored as an efficient photocathode material for photoelectrochemical (PEC) production of hydrogen from water. Although CIS nanostructures are extensively used as PEC active materials for solar-to-hydrogen conversion, the coupled structures with Au for their exciton–plasmon coupling is observed in producing a higher photocurrent with efficient evolution of hydrogen. In the comparison of materials properties, it is observed that the cathodic photocurrent, onset potential, and the half-cell solar-to-hydrogen efficiency (HC-STH) are recorded to be superior to all CIS-based photocathodes reported up to the current time. These results suggest that designing proper heterostructured functional materials can enhance the hydrogen production in the PEC cell and would be helpful for the ongoing technological needs for a greener way of generating and storing hydrogen energy

High Performance Solid-State Asymmetric Supercapacitor using Green Synthesized Graphene–WO₃ Nanowires Nanocomposite

Development of active materials capable of delivering high specific capacitance is one of the present challenges in supercapacitor applications. Herein, we report a facile and green solvothermal approach to synthesize graphene supported tungsten oxide (WO₃) nanowires as an active electrode material. As an active electrode material, the graphene–WO₃ nanowire nanocomposite with an optimized weight ratio has shown excellent electrochemical performance with a specific capacitance of 465 F g^–1 at 1 A g^–1 and a good cycling stability of 97.7% specific capacitance retention after 2000 cycles in 0.1 M H₂SO₄ electrolyte. Furthermore, a solid-state asymmetric supercapacitor (ASC) was fabricated by pairing a graphene–WO₃ nanowire nanocomposite as a negative electrode and activated carbon as a positive electrode. The device has delivered an energy density of 26.7 W h kg^–1 at 6 kW kg^–1 power density, and it could retain 25 W h kg^–1 at 6 kW kg^–1 power density after 4000 cycles. The high energy density and excellent capacity retention obtained using a graphene–WO₃ nanowire nanocomposite demonstrate that it could be a promising material for the practical application in energy storage devices