One-dimensional metal oxide nanostructures for heterogeneous catalysis

Growth of zinc oxide nanowires by a hot water deposition method

Recently, various methods have been developed for synthesizing zinc oxide (ZnO) nanostructures, including physical and chemical vapor deposition, as well as wet chemistry. These common methods require either high temperature, high vacuum, or toxic chemicals. In this study, we report the growth of zinc oxide ZnO nanowires by a new hot water deposition (HWD) method on various types of substrates, including copper plates, foams, and meshes, as well as on indium tin oxide (ITO)-coated glasses (ITO/glass). HWD is derived from the hot water treatment (HWT) method, which involves immersing piece(s) of metal and substrate(s) in hot deionized water and does not require any additives or catalysts. Metal acts as the source of metal oxide molecules that migrate in water and deposit on the substrate surface to form metal oxide nanostructures (MONSTRs). The morphological and crystallographic analyses of the source-metals and substrates revealed the presence of uniformly crystalline ZnO nanorods after the HWD. In addition, the growth mechanism of ZnO nanowires using HWD is discussed. This process is simple, inexpensive, low temperature, scalable, and eco-friendly. Moreover, HWD can be used to deposit a large variety of MONSTRs on almost any type of substrate material or geometry.

Recent progress in polyaniline-based composites as electrode materials for pliable supercapacitors

Significant contributions have been made towards the development of flexible energy storage devices to meet the ever-growing energy demand. Flexibility, mechanical stability, and electrical conductivity are three critical qualities that distinguish conducting polymers from other materials. Polyaniline (PANI) has drawn considerable attention among the various conducting polymers for use in flexible supercapacitors. PANI offers several desirable properties including high porosity, a large surface area, and high conductivity. Despite its merits, it also suffers from poor cyclic stability, low mechanical strength, and notable discrepancy between theoretical and actual capacitance. These shortcomings have been addressed by creating composites of PANI with structurally sturdy elements such as graphene, carbon nanotubes (CNTs), metal-organic framework (MOFs), MXenes, etc., thus enhancing the performance of supercapacitors. This review outlines the several schemes adopted to prepare diverse binary and ternary composites of PANI as the electrode material for flexible supercapacitors and the significant impact of composite formation on the flexibility and electrochemical performance of the fabricated pliable supercapacitors.

Optimizing the synergy between alloy and alloy-oxide interface for CO oxidation in bimetallic catalysts

Creating synergetic metal-oxide interfaces is a promising strategy to promote the catalytic performance of heterogeneous catalysts. However, this strategy has been mainly applied to monometallic catalysts, while scarcely applied to alloy catalysts. In this work, we present a comprehensive study on the synergetic alloy-oxide interfaces in the bimetallic Pt-Co/Al₂O₃ catalysts for CO oxidation. A series of Pt₁Co_x/Al₂O₃ catalysts with various Co/Pt molar ratios with x ranging from 0.5 to 3.8 was synthesized via a facile wet-chemistry strategy. Among them, the Pt₁Co_0.5/Al₂O₃ catalyst exhibits the best catalytic performance for CO oxidation, with the lowest CO complete conversion temperature of -10 °C and the highest mass specific rate of 2.61 (mol CO) h^-1 (g Pt)^-1. From in situ X-ray absorption fine structure and diffuse reflectance infrared Fourier-transform spectroscopy studies, the superior catalytic performance of Pt₁Co_0.5/Al₂O₃ originates from the optimal length of the three-dimensional alloy-oxide perimeter sites. We further extended this strategy to other bimetallic systems of Pt-Fe and Pt-Ni, which also show similar structural properties and remarkable promotional effects on the catalytic activity.

Soft-template-assisted synthesis: a promising approach for the fabrication of transition metal oxides

The past few decades have witnessed transition metal oxides (TMOs) as promising candidates for a plethora of applications in numerous fields. The exceptional properties retained by these materials have rendered them of paramount emphasis as functional materials. Thus, the controlled and scalable synthesis of transition metal oxides with desired properties has received enormous attention. Out of different top-down and bottom-up approaches, template-assisted synthesis predominates as an adept approach for the facile synthesis of transition metal oxides, owing to its phenomenal ability for morphological and physicochemical tuning. This review presents a comprehensive examination of the recent advances in the soft-template-assisted synthesis of TMOs, focusing on the morphological and physicochemical tuning aided by different soft-templates. The promising applications of TMOs are explained in detail, emphasizing those with excellent performances.

Controlling the morphology of metal–organic frameworks and porous carbon materials: metal oxides as primary architecture-directing agents

We give a comprehensive overview of how the morphology control is an effective and versatile way to control the physicochemical properties of metal oxides that can be transferred to metal–organic frameworks and porous carbon materials.

Evidence of oxygen defects mediated enhanced photocatalytic and antibacterial performance of ZnO nanorods

Defect engineered one-dimensional (1D) ZnO nanostructures have found great interest in diverse fields, including water detoxification and environmental remediation. In this article, we report a facile, low-temperature hydrothermal synthesis of defect enriched ZnO nanorods at different pH conditions. The dimension of all the synthesized ZnO nanostructures was restricted to 1D with changes only in their aspect ratios, unlike previous reports where change in morphology accompanies the effect of pH. With an increment in the pH value of the reaction mixture, oxygen defect concentration was controlled and confirmed using XPS and Raman spectroscopy. Considerable increase in optical light absorption and reduction in the bandgap, as inferred from the UV-vis study, corroborating the pH-dependent enrichment of defect states in 1D ZnO. Superior photosensitivity of oxygen defect rich ZnO nanorods was utilized to study their sunlight-induced photocatalytic and bactericidal activity towards its application in wastewater treatment. Within 4 h and 30 min of sunlight exposure (900 W/cm²), a 100% bacterial population (S.aureus, 10⁶ cells/m) killing and complete degradation of methylene blue dye (10μM) were achieved. Enhanced reactive oxidative species (ROS) formation due to the presence of additional oxygen defect states is ascribed to be the prime factor facilitating improved degradation efficiency. Additionally, during the optimization study, ZnO nanorods were found to be active against bacterial cells even in the absence of light opening avenues in antimicrobial food packaging and protective surface coatings.

Heterogeneously Catalyzed Pechmann Condensation Employing the Tailored Zn0.925Ti0.075O NPs: Synthesis of Coumarin

A novel heterogeneous catalytic method was developed for the synthesis of coumarin and its derivatives using the Ti(IV)-doped ZnO matrix forming catalyst Zn_0.925Ti_0.075O having a high surface area and good Lewis acidity. The catalyst shows high activity toward a broad spectrum of the substituted phenols with β-ketoesters such as ethyl acetoacetate, ethyl butyryl acetate, ethyl benzoyl acetate, and so forth in good yields over short reaction times during the synthesis of coumarins. The methodology was further extended for the synthesis of ayapin molecules. The catalyst also shows recycle activity up to seven cycles with very good stability.

Synthesis of Heart/Dumbbell-Like CuO Functional Nanostructures for the Development of Uric Acid Biosensor

It is always demanded to prepare a nanostructured material with prominent functional properties for the development of a new generation of devices. This study is focused on the synthesis of heart/dumbbell-like CuO nanostructures using a low-temperature aqueous chemical growth method with vitamin B₁₂ as a soft template and growth directing agent. CuO nanostructures are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) techniques. CuO nanostructures are heart/dumbbell like in shape, exhibit high crystalline quality as demonstrated by XRD, and have no impurity as confirmed by XPS. Apparently, CuO material seems to be porous in structure, which can easily carry large amount of enzyme molecules, thus enhanced performance is shown for the determination of uric acid. The working linear range of the biosensor is 0.001 mM to 10 mM with a detection limit of 0.0005 mM and a sensitivity of 61.88 mV/decade. The presented uric acid biosensor is highly stable, repeatable, and reproducible. The analytical practicality of the proposed uric acid biosensor is also monitored. The fabrication methodology is inexpensive, simple, and scalable, which ensures the capitalization of the developed uric acid biosensor for commercialization. Also, CuO material can be used for various applications such as solar cells, lithium ion batteries, and supercapacitors.

The synergy between atomically dispersed Pd and cerium oxide for enhanced catalytic properties

We report a photochemical synthesis of Pd/CeO₂ catalysts with atomically dispersed Pd. Compared to atomically dispersed Pd/CeO₂ with a cubic CeO₂ support (Pd/CeO₂-CP), atomically dispersed Pd/CeO₂ with a truncated octahedral CeO₂ support (Pd/CeO₂-TOP) exhibited higher activity and selectivity, owing to the synergy between Pd atoms and the (111) surface of CeO₂. When compared to Pd/CeO₂ with Pd clusters and nanoparticles via chemical reduction, Pd/CeO₂-TOP showed excellent activity with an enhancement factor of 324 in CO oxidation, as well as an activity enhancement by a factor of 344 in selective oxidation of benzyl alcohol.

Gate- and Light-Tunable pn Heterojunction Microwire Arrays Fabricated via Evaporative Assembly

One-dimensional (1D) nano/microwires have attracted considerable attention as versatile building blocks for use in diverse electronic, optoelectronic, and magnetic device applications. The large-area assembly of nano/microwires at desired positions presents a significant challenge for developing high-density electronic devices. Here, we demonstrated the fabrication of cross-stacked pn heterojunction diode arrays by integrating well-aligned inorganic and organic microwires fabricated via evaporative assembly. We utilized solution-processed n-type inorganic indium-gallium-zinc-oxide (IGZO) microwires and p-type organic 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-PEN) microwires. The formation of organic TIPS-PEN semiconductor microwire and their electrical properties were optimized by controlling both the amounts of added insulating polymer and the widths of the microwires. The resulting cross-stacked IGZO/TIPS-PEN microwire pn heterojunction devices exhibited rectifying behavior with a forward-to-reverse bias current ratio exceeding 10². The ultrathin nature of the underlying n-type IGZO microwires yielded gate tunability in the charge transport behaviors, ranging from insulating to rectifying. The rectifying behaviors of the heterojunction devices could be modulated by controlling the optical power of the irradiated light. The fabrication of semiconducting microwires through evaporative assembly provides a facile and reliable approach to patterning or positioning 1D microwires for the fabrication of future flexible large-area electronics.

Fabrication of a silica/titania hollow nanorod and its electroresponsive activity

In this study, a 1D oriented hollow SiO₂/TiO₂ (HST) rod-like material was successfully fabricated via a sequential combination of sol–gel use, TiO₂ incorporation, and a sonication-mediated etching and redeposition method.

Wafer-Scale Microwire Transistor Array Fabricated via Evaporative Assembly

One-dimensional (1D) nano/microwires have attracted significant attention as promising building blocks for various electronic and optical device applications. The integration of these elements into functional device networks with controlled alignment and density presents a significant challenge for practical device applications. Here, we demonstrated the fabrication of wafer-scale microwire field-effect transistor (FET) arrays based on well-aligned inorganic semiconductor microwires (indium-gallium-zinc-oxide (IGZO)) and organic polymeric insulator microwires fabricated via a simple and large-area evaporative assembly technique. This microwire fabrication method offers a facile approach to precisely manipulating the channel dimensions of the FETs. The resulting solution-processed monolithic IGZO microwire FETs exhibited a maximum electron mobility of 1.02 cm(2) V(-1) s(-1) and an on/off current ratio of 1 × 10(6). The appropriate choice of the polymeric microwires used to define the channel lengths enabled fine control over the threshold voltages of the devices, which were employed to fabricate high-performance depletion-load inverters. Low-voltage-operated microwire FETs were successfully fabricated on a plastic substrate using a high-capacitance ion gel gate dielectric. The microwire fabrication technique involving evaporative assembly provided a facile, effective, and reliable method for preparing flexible large-area electronics.

Room-temperature synthesis of mesoporous CuO and its catalytic activity for cyclohexene oxidation

CuO nanoleaves with a mesoporous structure have been synthesized in the presence of triethylamine at room temperature. The mesoporous CuO nanoleaves exhibit excellent catalytic activity for solvent-free cyclohexene oxidation with oxygen.

Study of the photocatalytic properties of bio-mimicked hollow SnO2 microstructures synthesized with Ceiba pentandra (L.) Gaertn. (kapok) as a natural template

Hollow SnO₂ microstructures have been simply fabricated via a template impregnation technique using Ceiba pentandra (L.) Gaertn. as a natural template.

Reactive-template fabrication of porous NiO nanowires for electrocatalytic O2 evolution reaction

Macro–mesoporous NiO nanowire was demonstrated as an active electrocatalyst for O₂ evolution reaction in alkaline media.

Calcination system-induced nanocasting synthesis of uniform Co3O4 nanoparticles with high surface area and enhanced catalytic performance

Nanoscale Co₃O₄ synthesized by open-system nanocasting with uniform size, high surface area, large pore-distribution and abundant active-sites exhibited improved catalysis.

Surface oxygen-vacancy induced photocatalytic activity of La(OH)3 nanorods prepared by a fast and scalable method

Uniform one-dimensional La(OH)₃ nanorods with surface oxygen-vacancy induced photocatalytic activity were synthesized by a facile, fast and scalable method.

Use of urchin-like Ni(x)Co(3-x)O4 hierarchical nanostructures based on non-precious metals as bifunctional electrocatalysts for anion-exchange membrane alkaline alcohol fuel cells

Bifunctional electrocatalysts based on non-precious metals were developed for the dioxygen reduction and methanol oxidation reactions. These electrocatalysts can be considered as candidate cathode and anode materials for anion-exchange membrane (AEM) alkaline alcohol fuel cells. A series of Ni-doped cobalt oxide (NixCo3-xO4) hierarchical nanostructures composed of one-dimensional nanorods was prepared by an inexpensive hydrothermal method. X-ray diffraction patterns showed that the NixCo3-xO4 crystallized in a cubic spinel phase. The electrochemical performance of the catalysts was investigated using a conventional cyclic voltammetry technique. The electrocatalytic behaviour of the NixCo3-xO4 hierarchical nanostructures was compared with the behaviour of Co3O4 and Co0.33Ni0.67O. The synergistic behaviour of the Ni in the NixCo3-xO4 nanostructures was established with respect to the Ni content. NixCo3-xO4 hierarchical nanostructures show a better catalytic behaviour than Co3O4 and Co0.33Ni0.67O. Although the NixCo3-xO4 compositions all showed good catalytic behaviour, Ni1Co2O4 was identified as a superior bifunctional electrocatalyst for the oxygen reduction and methanol oxidation reactions in alkaline media. The effect of the Ni content on the electrocatalytic properties of the NixCo3-xO4 hierarchical nanostructures was clearly shown. The use of these electrocatalysts based on non-precious metals could have a commercial impact on the development of non-platinum electrocatalysts for application in AEM alkaline alcohol fuel cells.