VO₂ nanorods for efficient performance in thermal fluids and sensors

High-performance VO2/CNT@PANI with core-shell construction enable printable in-planar symmetric supercapacitors

As a progressive electronic energy storage device, the flexible supercapacitor holds tremendous promise for powering wearable/portable electronic products. Of various pseudocapacitor materials, vanadium dioxide (VO2) has garnered extensive attention due to its impressive theoretical capacitance. However, the challenges of inferior cycling life and lower energy density to be addressed. Herein, we prepare VO2 nanorods with winding carbon nanotubes (CNT) via a facile solvothermal route, followed by in situ polymerization of polyaniline (PANI) shell. Taking full advantage of the synergistic effect, the VO2/CNT@PANI composite delivers a high specific capacitance of 354.2F/g at 0.5 A/g and a long cycling life of ∼ 88.2 % over 5000 cycles resulting from the enhanced conductivity of CNT and stabilization of PANI shell. By screen printing the formulated inks with outstanding rheological behaviours, we manufacture an in-planar VO2/CNT@PANI symmetric supercapacitor (VO2/CNT@PANI SSC) device featuring an orderly arrangement structure. This device yields a remarkable areal energy density of 99.57 μWh/cm2 at a power density of 387.5 μW/cm2 while retaining approximately ∼ 87.6 % of its initial capacitance after prolonged use. Furthermore, we successfully powered a portable game machine for more than 2 min using two SSCs connected in series with ease. Therefore, this work presents a universal strategy that utilises combination and coating to boost electrochemical performance for flexible high-performance supercapacitors.

Element doping: a marvelous strategy for pioneering the smart applications of VO2

Smart materials are leading the future of materials by virtue of their autonomous response behavior to external stimuli; it is widely believed their development and application will bring a new revolution. Among them, vanadium dioxide (VO₂) is a special one showing a unique multi-stimulus responsive metal-insulator transition (MIT) accompanied by a structural phase transition (SPT) with striking changes of physical properties including optical, electrical and thermal properties, etc., making it ideal for smart windows, micro-bolometers, actuators, etc. Since the attractive performances of VO₂ are rooted in MIT behavior (coupled with SPT), element doping becomes a powerful tool in tailoring VO₂ performance. Oriented on the practical requirements, element-doped VO₂ is more promising and competitive in terms of performance, prospect, and cost. Here we focus specifically on element-doped VO₂, the recent progress and potential challenges of which are discussed. We devote attention to the crucial roles of element doping in modulating the properties and driving the practicality of VO₂, aiming to inspire current research to pioneer new applications of VO₂.

Phase B vanadium dioxide: characteristics, synthesis and applications

Starting from the numerous and unique characteristics of VO2(B), we will introduce to readers the research progress of VO2(B) in recent years, including the detailed mainstream methods for its preparation and popular fields of application.

Passive Smart Thermal Control Coatings Incorporating CaF2/VO2 Core-Shell Microsphere Structures

Existing smart radiation devices suffer from numerous disadvantages such as large thicknesses, limited dimensions, or requirements for sustained electrical power. The present study addresses these issues by proposing a smart thermal control coating based on CaF₂/VO₂ core-shell (CaF₂@VO₂) structured microspheres prepared by a solvent/hydrothermal-calcination method and distributed within an easily applied polymer matrix. Here, the dielectric-to-metallic transition property of the VO₂ shell material with increasing temperature is used to regulate the optical scattering and absorption characteristics of the CaF₂@VO₂ core-shell microspheres to realize a positive and reversible increase in the emissivity of the coating from 0.47 at 30 °C to 0.83 at 90 °C. The mechanisms behind this effect are investigated by theoretical analyses and numerical simulations. The present work can expect to promote the further research and development of new coating materials for smart thermal control applications.

Fiber optic evanescent wave humidity sensor based on SiO2/TiO2 bilayer films

A SiO₂/TiO₂ bilayer thin-film-based fiber optic humidity sensor was fabricated via a modified dip coating process with enhanced sensitivity. SiO₂ film was coated on the surface of the fiber core, followed by deposition of the TiO₂ layer on SiO₂. The relative humidity (RH) is measured by modulation in intensity of the transmitted laser at room temperature. The optical fiber humidity sensor based on SiO₂/TiO₂ film shows two-segmented linearity in measurement with sensitivities of 5.35 and 1.94 µW/% RH at 15%-50% RH and 50%-95% RH, respectively. The response time and recovery time are 25 s and 50 s, respectively. To our knowledge, the superior response time and recovery time of the sensor in our study were achieved over those fiber optic humidity sensors reported with modulation in intensity. Furthermore, this fiber optic humidity sensor has a good reproducibility and long-term stability. The sensing mechanism is attributed to effects of moisture on the refractive index and the light absorption coefficient of SiO₂ film and modulation in the transmission characteristic of evanescent waves in the optical fiber.

Synthesis of Self-Assembled Randomly Oriented VO2 Nanowires on a Glass Substrate by a Spin Coating Method

Randomly oriented vanadium dioxide (VO₂) nanowires were produced on a glass substrate by spin coating from a cosolvent. SEM studies reveal that highly dense VO₂ nanowires were grown at an annealing temperature of 400 °C. X-ray diffraction (XRD) provides evidence of the high crystallinity of the VO₂ nanowires-embedded VO₂ thin films on the glass substrate at 400 °C. Characterization by high-resolution transmission electron microscopy (HR-TEM) confirmed the formation of VO₂ nanowires. The optical band gap of the nanowires-embedded VO₂ thin films was also calculated from the transmittance data to be 2.65-2.70 eV. The growth mechanism of the solution-processed semiconducting VO₂ nanowires was proposed based on both solvent selection and annealing temperature. Finally, the solar water splitting ability of the VO₂ nanowires-embedded VO₂ thin films was demonstrated in a photoelectrochemical cell (PEC).

Natural Polyphenol-Vanadium Oxide Nanozymes for Synergistic Chemodynamic/Photothermal Therapy

The selection of suitable nanozymes with easy synthesis, tumor specificity, multifunction, and high therapeutics is meaningful for tumor therapy. Herein, a facile one-step assembly approach was employed to successfully prepare a novel kind of natural polyphenol tannic acid (TA) hybrid with mixed valence vanadium oxide nanosheets (TA@VO_x NSs). In this system, VO_x is assembled with TA through metal-phenolic coordination interaction to both introduce superior peroxidase-like activity and high near infrared (NIR) absorption owing to partial reduction of vanadium from V⁵⁺ to V⁴⁺ . The presence of mixed valence vanadium oxide in TA@VO_x NSs is proved to be the key for the catalytic reaction of hydrogen peroxide (H₂ O₂ ) to ^. OH, and the corresponding catalytic mechanism of H₂ O₂ by TA@VO_x NSs is proposed. Benefitting from such peroxidase-like activity of TA@VO_x NSs, the overproduced H₂ O₂ of the tumor microenvironment allows the realization of tumor-specific chemodynamic therapy (CDT). As a valid supplement to CDT, the NIR absorption enables TA@VO_x NSs to have NIR light-mediated conversion ability for photothermal therapy (PTT) of cancers. Furthermore, in vitro and in vivo experiments confirmed that TA@VO_x NSs can effectively inhibit the growth of tumors by synergistic CDT/PTT. These results offer a promising way to develop novel vanadium oxide-based nanozymes for enhanced synergistic tumor-specific treatment.

Nanostructured cobalt antimonate: a fast responsive and highly stable sensing material for liquefied petroleum gas detection at room temperature

Herein, cobalt antimonate (CoSb2O6) nanospheres were fabricated via the sol-gel spin-coating process and employed as a functional liquefied petroleum gas (LPG) sensor at room temperature (25 °C). The microstructure of the fabricated CoSb2O6 thin films (thickness ∼ 250 nm) was analyzed via scanning electron microscopy, which revealed the growth of nanospheres having an average diameter of ∼45 nm. The XRD analysis demonstrated the crystalline nature of CoSb2O6 with a crystallite size of ∼27 nm. Finally, the fabricated thin films were investigated as sensors for LPG and carbon dioxide (CO2) at room temperature (25 °C) and 55% R.H. (relative humidity) with different concentrations in the range of 1000-5000 ppm. The sensing results demonstrated greater variations in the electrical properties of films for the incoming LPG than that of the CO2 gas adsorption. Furthermore, to ensure the long-term stability of fabricated sensors, they were tested periodically at 10 days interval, spanning a total duration of 60 days. In summary, our fabricated LPG sensor displayed high sensitivity (1.96), repeatability, quick response time (21 s) and high long-term stability (99%). Therefore, CoSb2O6 nanospheres can be functionalized as a potential LPG-sensitive material characterized by high sensitivity, reliability and stability at room temperature.

Gas sensing mechanisms of metal oxide semiconductors: a focus review

In recent years, gas sensors have been increasingly used in industrial production and daily life. Metal oxide semiconductor gas sensing materials are favoured for their outstanding physical and chemical properties, low cost and simple preparation methods. However, the gas sensing mechanisms of metal oxide semiconductors have not been considered by researchers, resulting in omissions and errors in the interpretation of gas sensing mechanisms in many articles. This review organizes and introduces several common gas sensing mechanisms of metal oxide semiconductors in detail and classifies them into two categories. The scope and relationship of these mechanisms are clarified. In addition, this review selects four strategies for enhancing the gas sensing properties of metal oxide semiconductors and analyses the gas sensing mechanisms to highlight the importance of the gas sensing mechanism. Finally, some perspectives for future investigations on the gas sensing mechanisms of metal oxide semiconductors are discussed as well.

Ultra-smooth TiO2 thin film based optical humidity sensor with a fast response and recovery

An ultra-smooth $\text{TiO}_2$TiO₂ thin film based optical humidity sensor was fabricated via a modified dip coating process. The $\text{TiO}_2$TiO₂ film possessed a root mean square roughness of ${2.6 \pm 0.3}\;\text{nm}$2.6±0.3nm. Measurement of relative humidity (RH) was performed by modulation in the intensity of laser transmitted at room temperature. The optical humidity sensor based on $\text{TiO}_2$TiO₂ film exhibited two-segmented linearity in the whole RH range. The response time and recovery time were determined to be 27 s and 23 s, respectively. To our knowledge, the optical humidity sensor achieved the fastest recovery to date among those modulated in optical power. The fast response and recovery are attributed to the smooth surface of sensing film, which allows the rapid equilibrium between adsorption and desorption of water molecules on the film surface. In addition, this optical humidity sensor possessed an excellent reproducibility and long-term stability after aging. The sensing mechanism is based on the chemisorption of water molecules in the low RH range and formation of water droplets in the high RH range on the surface of ${\text{TiO}_2}$TiO₂ film.

VO x Quantum Dots with Multienzyme-Mimic Activities and the Application in Constructing a Three-Dimensional (3D) Coordinate System for Accurate Discrimination of the Hydrogen Peroxide over a Broad Concentration Range

The construction of efficient nanozyme with multienzyme activities in a simple way is vital for the wide biological and chemical applications. Generally, the mimic enzyme activities depend on their sizes, surface states, and materials types. Quantum dots (QDs), one type of zero-dimensional nanomaterials, are much appealing due to their abundant catalytically active surface deficiency. The vanadium oxide (VO _x) is one special transition metal oxides possessing different valence states. Inspired by these views, we synthesized VO _xQDs herein via a one-pot top-down ethanol-thermal method using bulk VO₂ as the precursor. The VO _xQDs showed not only oxidase- and peroxidase-like activities in ethanol as the main background solution (ethanol-BGS), but also exhibited additional superoxide dismutase mimetic activity in phosphate buffer solution. Furthermore, the TMB-VO _xQDs system in the ethanol-BGS produced three distinct colors in the presence of hydrogen peroxide (H₂O₂) at three different concentration gradients (10-90 μM, 0.1-10 mM, and 20-100 mM). Accordingly, we constructed a three-dimensional (3D) coordinate system (3D-CS) by using the three variables: the initial velocities, the maximum absorption values and the visual colors of the enzymatic reaction system. As a result, the rapid detection of H₂O₂ can be achieved while effectively avoiding the faked appearance due to the inhibition effects to the enzymatic system at too high H₂O₂ concentration. The applicability of the VO _xQDs based 3D-CS was further proved via the facile and accurate H₂O₂ assays in three different practical samples.

Multi-nanolayered VO2/Sapphire Thin Film via Spinodal Decomposition

Coating of VO₂-based thin film has been extensively studied for fabricating energy-saving smart windows. One of the most efficient ways for fabricating high performance films is to create multi-nanolayered structure. However, it has been highly challenge to make such layers in the VO₂-based films using conventional methods. In this work, a facile two-step approach is established to fabricate multilayered VO₂-TiO₂ thin films. We first deposited the amorphous thin films upon sputtering, and then anneal them to transform the amorphous phase into alternating Ti- and V-rich multilayered nanostructure via a spinodal decomposition mechanism. In particular, we take advantage of different sapphire substrate planes (A-plane (11-20), R-plane (1-102), C-plane (0001), and M-plane (10-10)) to achieve different decomposition modes. The new approach has made it possible to tailoring the microstructure of the thin films for optimized performances by controlling the disorder-order transition in terms of both kinetic and thermodynamic aspects. The derived thin films exhibit superior optical modulation upon phase transition, significantly reduced transition temperature and hysteresis loop width, and high degradation resistance, these improvements indicate a high potential to be used for fabricating the next generation of energy saving smart windows.

Hydrothermal Synthesis of VO2 Polymorphs: Advantages, Challenges and Prospects for the Application of Energy Efficient Smart Windows

Vanadium dioxide (VO₂ ) is a widely studied inorganic phase change material, which has a reversible phase transition from semiconducting monoclinic to metallic rutile phase at a critical temperature of τ_c ≈ 68 °C. The abrupt decrease of infrared transmittance in the metallic phase makes VO₂ a potential candidate for thermochromic energy efficient windows to cut down building energy consumption. However, there are three long-standing issues that hindered its application in energy efficient windows: high τ_c , low luminous transmittance (T_lum ), and undesirable solar modulation ability (ΔT_sol ). Many approaches, including nano-thermochromism, porous films, biomimetic surface reconstruction, gridded structures, antireflective overcoatings, etc, have been proposed to tackle these issues. The first approach-nano-thermochromism-which is to integrate VO₂ nanoparticles in a transparent matrix, outperforms the rest; while the thermochromic performance is determined by particle size, stoichiometry, and crystallinity. A hydrothermal method is the most common method to fabricate high-quality VO₂ nanoparticles, and has its own advantages of large-scale synthesis and precise phase control of VO₂ . This Review focuses on hydrothermal synthesis, physical properties of VO₂ polymorphs, and their transformation to thermochromic VO₂ (M), and discusses the advantages, challenges, and prospects of VO₂ (M) in energy-efficient smart windows application.

Considerable humidity response of a well-aligned SOMS micro-wire flexible sensor by moisture-induced releasing of trapped electrons

Sandia Octahedral Molecular Sieves micro-wires (SOMS MWs) that exhibit ultra-high response to moisture and a short response time can be produced easily in an environmentally friendly mass production process.

Multiphase TiO2nanostructures: a review of efficient synthesis, growth mechanism, probing capabilities, and applications in bio-safety and health

This review article provides an exhaustive overview of efficient synthesis, growth mechanism and research activities of multiphase TiO₂nanostructures to provide their structural, morphological, optical and biological properties co-relations.

Highly Efficient Fenton and Enzyme-Mimetic Activities of Mixed-Phase VOx Nanoflakes

Artificial enzyme mimetics is a current research area with much interest from scientific community. Some nanomaterials have been found to possess intrinsic enzyme-mimetic activity. In this study, VO_x nanoflakes with mixed-phases are synthesized via a quick and facile one-pot synthetic process and their Fenton reaction and enzyme-mimetic activities have been studied. The results show that obtained VOx is not only highly effective Fenton reagent, completely decomposing Rhodamine B (RhB) within less than 1 min, but also exhibits excellent intrinsic peroxidase-like activity as well as H₂O₂ catalase-like activity. Our results suggest that this VO_x nanomaterial can effectively mimic the enzyme cascade reaction of horseradish peroxidase (HRP). VO_x nanoflakes have excellent affinity toward 3,3',5,5'-tetramethylbenzidine (TMB) for oxidation and henceforth, it can be used for the colorimetric assay of glucose and H₂O₂. Moreover, this study indicates that VO_x nanoflakes can also be used for the efficient degradation of environmental pollutants.

Synthesis and porous h-BN 3D architectures for effective humidity and gas sensors

3D (three dimensional) architectures synthesised using an easily scalable solid state method which results in an interconnected network of porous h-BN sheets with boron trioxide are reported in this study.

Facile synthesis of defect-induced highly-luminescent pristine MgO nanostructures for promising solid-state lighting applications

Successful demonstration of the facile synthesis of defect-induced highly-luminescent pristine MgO nanostructures for white-light generation and proposed WLED application.