Synthesis and optical properties of vanadium dioxide nanoparticles in nanoporous glasses

A method for the synthesis of vanadium dioxide (VO₂) nanoparticles in nanoporous silicate glass matrices with a pore size of 17 and 7 nm has been developed. According to this, vanadium pentoxide (V₂O₅) nanoparticles are initially grown in the pores, and then V₂O₅ is reduced to VO₂ in hydrogen. The optical transmission spectra of 1-mm-thick VO₂-modified glass samples have been measured. The temperature dependence of the transmission coefficient has been studied in the course of the semiconductor-metal phase transition in VO₂ nanoparticles.     

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.     

Recent progress in VO2 smart coatings: Strategies to improve the thermochromic properties

Vanadium dioxide (VO₂) has attracted a great interest for smart coating applications because of its promising thermochromic properties. Thermochromic performance of VO₂ is closely related to the phase composition and the microstructure, which are largely dependent on the synthesis method and growth control. This review summarizes the recent progress in fabrication of VO₂ by gas deposition. Representative deposition techniques, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), sol–gel and chemical solution methods and their relative merits are discussed. To be practically applicable, high-performance thermochromic VO₂ films are desired, often featured with a suitable phase transition temperature (T_c), high luminous transmittance (T_lum) and good modulation capability of solar energy (ΔT_sol). Focused on the strategies used to improve thermochromic properties, this review also covers topics such as multilayer construction, elemental doping, substrate selection, and structure modification. Some theoretical progresses in understanding thermochromic coatings, including phase transition mechanism and energy modeling are also provided. Although significant progress has been made in improving the thermochromic performance of VO₂ films, challenges are still present, particularly in commercial applications. Discussions on future trend and perspectives, as well as some important issues, of VO₂ films used as smart coatings will be given finally.     

Chemical Modulation of Metal–Insulator Transition toward Multifunctional Applications in Vanadium Dioxide Nanostructures

The metal–insulator transition (MIT) of vanadium dioxide (VO₂) has been of great interest in materials science for both fundamental understanding of strongly correlated physics and a wide range of applications in optics, thermotics, spintronics, and electronics. Due to the merits of chemical interaction with accessibility, versatility, and tunability, chemical modification provides a new perspective to regulate the MIT of VO₂, endowing VO₂ with exciting properties and improved functionalities. In the past few years, plenty of efforts have been devoted to exploring innovative chemical approaches for the synthesis and MIT modulation of VO₂ nanostructures, greatly contributing to the understanding of electronic correlations and development of MIT-driven functionalities. Here, this comprehensive review summarizes the recent achievements in chemical synthesis of VO₂ and its MIT modulation involving hydrogen incorporation, composition engineering, surface modification, and electrochemical gating. The newly appearing phenomena, mechanism of electronic correlation, and structural instability are discussed. Furthermore, progresses related to MIT-driven applications are presented, such as the smart window, optoelectronic detector, thermal microactuator, thermal radiation coating, spintronic device, memristive, and neuromorphic device. Finally, the challenges and prospects in future research of chemical modulation and functional applications of VO₂ MIT are also provided.     

Porous nano-structured VO2 films with different surfactants: synthesis mechanisms, characterization, and applications

Porous nano-structured vanadium dioxide (VO₂) thin films have been prepared on mica substrates via sol–gel process using surfactant cetyltrimethyl ammonium bromide, nonionic surfactant polyethylene glycol, and anionic surfactant sodium dodecyl sulfate as nano-structure directing agents. Models concerning the structure forming were proposed to explain the synthesis mechanisms between V₂O₅ colloid and different surfactants. Porous nano-structured VO₂ films with sphere-shaped, island-shaped and strip-shaped nanocrystals are synthesized in the experiments, and the optical properties and thermochromic properties of these films are compared. The porous nano-structured VO₂ films showed excellent infrared transmittance (nearly 70 %), low transition temperature (59.7 °C without doping), wide hysteresis width (37.8 °C), and different optical transmittance difference before and after the phase transition (39–67 %). The results suggest that these porous nano-structured VO₂ films have significant importance in practical application in VO₂-based optical and electronic devices.     

Phase transition of single crystal VO2(R) nanorods in solution revealed by reversible change in surface charge state and structure

The VO₂ single crystal nanorods were synthesized by hydrothermal method and investigated by XRD, SEM, SPM and TEM. Phase transition revealed by a reversible surface charge transfer for VO₂ nanorods during heating-cooling circle in solution was observed by zeta potential measurement. The monoclinic VO₂ nanorods were negatively charged because the exposed crystal surface planes are mostly composed of oxygen atoms as obtained by structure simulation, while the corresponding crystal planes change to a mostly positively charged state in VO₂(R) during phase transition since the exposed planes change into V-atom dominated ones. The study provided a novel method to characterize the phase transition in solution for VO₂ nanocrystals, and the phenomenon may have important potential applications.     

Preparation of nanocrystalline VMg(OH)<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> and VO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> · MgO from organometallic precursors

A process is proposed for the synthesis of nanocrystalline VMg(OH) _x and VO _x · MgO with specific surface areas of up to 1200 and 470 m²/g, respectively. The synthesized VO _x · MgO oxides consist of nanocrystals (2′–5 nm), which form platelike agglomerates. As distinct from conventional impregnation of magnesium oxide, the aerogel process for VO _x · MgO synthesis ensures a uniform vanadium distribution in MgO.     

Low temperature synthesis and photoluminescence of Ca<Subscript>3</Subscript>(VO<Subscript>4</Subscript>)<Subscript>2</Subscript>:Eu<Superscript>3+</Superscript> nanocrystals with Li<Superscript>+</Superscript> addition

Different crystalline Ca₃(VO₄)₂ nanocrystals have been synthesized successfully via a facile low temperature method with lithium addition. After different ration of Li⁺ doping into the Ca₃(VO₄)₂: Eu³⁺ host, the crystallinity of the sample becomes different, resulting in different of luminescence intensity of the characteristic emission of Eu³⁺ ions. This approach provides economically viable route for large-scale synthesis of this kind of nanomaterials.     

Combustion synthesis of Eu2?+? and Dy3?+? activated Sr3(VO4)2 phosphor for LEDs

Combustion synthesis and photoluminescence (PL) characterization of Sr₃(VO₄)₂:Eu,Dy phosphors are presented in this paper. PL emission of Sr₃(VO₄)₂:Eu phosphor shows green broad emission band centring at 511 nm and a red sharp band at 614 nm by excitation wavelength of 342 nm. The PL emission spectrum of Sr₃(VO₄)₂:Dy phosphor exhibits an intense blue emission peak at 479 nm, yellow broad band centring at 573 nm and red band at 644 nm by the excitation wavelength of 426 nm in near visible blue region. The excitation wavelength of Eu (342 nm) and Dy (426 nm) activated Sr₃(VO₄)₂ phosphor are well matched with the excitation of near UV excited solid state lighting and blue chip excitation of light emitting diodes, respectively. The effect of Eu^2 +and Eu^3 +ions concentration on the emission intensity of Sr₃(VO₄)₂ was also investigated. The Sr₃(VO₄)₂:Eu is a potential green and red emitting phosphor as well as Sr₃(VO₄)₂:Dy is blue and yellow emitting phosphor for solid state lighting i.e. white LEDs. The XRD and SEM characteristics of Sr₃(VO₄)₂ materials was also reported in this paper.     

Thermal and luminescent properties of M2Zn(VO3)4 (M = Rb, Cs)

We have developed processes for the synthesis of the Rb₂Zn(VO₃)₄ and Cs₂Zn(VO₃)₄ tetrametavanadates. Rb₂Zn(VO₃)₄ has been prepared by solid-state reaction (350°C) between presynthesized RbVO₃ and ZnV₂O₆ powders, and Cs₂Zn(VO₃)₄ has been prepared by the Pechini method (sol-gel process). Both metavanadates crystallize in monoclinic symmetry (sp. gr. P2₁/m). Thermochemical characterization results demonstrate that the vanadates undergo complex transformations during heating to 450°C and subsequent cooling. As a result, the materials are in a nonequilibrium state at room temperature and consist of both the parent double metavanadates and their peritectic decomposition products. We believe that the formation of the structure of the M₂Zn(VO₃)₄ compounds from their melts is a kinetically hindered process. These compounds are structurally stable only at temperatures below 369 (Rb₂Zn(VO₃)₄) or 420°C (Cs₂Zn(VO₃)₄). We have measured for the first time the diffuse reflectance and photoluminescence excitation spectra of the two tetrametavanadates in their emission range and their photoluminescence spectra at various excitation wavelengths and determined their chromaticity coordinates. Their X-ray luminescence and scintillation decay characteristics have been determined for the first time under pulsed electron beam excitation. The electron excitation dissipation processes in the cesium and rubidium compounds are shown to be similar. We discuss the origin of the emission bands in the mixed vanadates and their potential application areas.     

Fabrication, structural and electrical characterization of VO2 nanowires

The structural and electrical properties of VO₂ nanowires synthesized on Si₃N₄/Si substrates or molybdenum grids by a catalyst-free vapour transport method were investigated. The grown VO₂ nanowires are single crystalline and rectangular-shaped with a preferential axial growth direction of [1 0 0], as examined with various structural analyses such as transmission electron microscopy, electron diffraction, X-ray diffraction, and X-ray photoelectron spectroscopy. In particular, it was found that growing VO₂ nanowires directly on Si₃N₄ deposited molybdenum transmission electron microscopy grids is advantageous for direct transmission electron microscopy and electron diffraction characterizations, because it does not involve a nanowire-detachment step from the substrates that may cause chemical residue contamination. In addition to structural analyses, VO₂ nanowires were also fabricated into field effect transistor devices to characterize their electrical properties. The transistor characteristics and metal-insulator transition effects of VO₂ nanowires were investigated.     

Three‐Layered Hollow Nanospheres Based Coatings with Ultrahigh‐Performance of Energy‐Saving,Antireflection, and Self‐Cleaning for Smart Windows

In this study, a well‐controlled interfacial engineering method for the synthesis of SiO₂/TiO₂/VO₂ three‐layered hollow nanospheres (TLHNs) and TLHNs‐based multifunctional coatings is reported. The as‐prepared coatings allow for an outstanding integration of thermochromism from the outer VO₂(M) layer, photocatalytic self‐cleaning capability from the middle TiO₂(A) layer, and antireflective property from internal SiO₂ HNs. The TLHNs coatings exhibit excellent optical performance with ultrahigh luminous transmittance (T_lum‐l = 74%) and an improved solar modulation ability (ΔT_sol = 12%). To the best knowledge, this integrated optical performance is the highest ever reported for TiO₂/VO₂‐based thermochromic coatings. An ingenious computation model is proposed, which allows the n_eff of nanostructured coatings to be rapidly obtained. The experimental and calculated results reveal that the unique three‐layered structure significantly reduces the refractive index (from 2.25 to 1.33 at 600 nm) and reflectance (Rave, from 22.3 to 5.3%) in the visible region as compared with dense coatings. Infrared thermal imaging characterization and self‐cleaning tests provide valid evidence of SiO₂/TiO₂/VO₂ TLHNs coatings' potential for energy‐saving and self‐cleaning smart windows. The exciting inexpensive and universal fabrication process for well‐defined structures may inspire various developments in processable and multifunctional devices.     

Study on optical and electrical switching properties and phase transition mechanism of Mo6+-doped vanadium dioxide thin films

In the present work using V₂O₅ and MoO₃ powders as precursors, a novel method, the inorganic sol-gel method, was developed to synthesize Mo⁶⁺ doped vanadium dioxide (VO₂) thin films. The structure, valence state, phase transition temperature, magnitude of resistivity change and change in optical transmittance below and above the phase transition of these films are determined by XRD, XPS, four-point probe equipment and spectrophotometer. The results showed that the main chemical composition of the films was VO₂, the structure of MoO₃ in the films didn't change, and the phase transition temperature of the VO₂ was obviously lowered with increasing MoO₃ doped concentration. The magnitude of resistivity change and change in optical transmittance below and above phase transition were also decreased, of which the magnitude of resistivity change was more distinct. However, when the MoO₃ concentration was 5 wt%, the magnitude of resistivity change of doped thin films still reached more than 2 orders, and the change in optical transmittance below and above phase transition was maintained. Analysis showed that the VO₂ doped films formed local energy level, and then reduced the forbidden band gap of VO₂ as the donor defect changing its optical and electrical properties and lowering the phase transition temperature.     

Hydrothermal synthesis of single-crystal VO2(B) nanobelts

Single crystalline VO₂(B) nanobelts with a metastable structure were obtained through a simple hydrothermal synthetic method. The VO₂(B) nanobelts were characterized by means of X-ray diffraction, transmission electron microscopy, selected area electronic diffraction, field-emission scanning electron microscopy and X-ray energy-dispersive spectroscopy techniques. The as-obtained VO₂(B) nanobelts are 400–600 nm long, typically 100–150 nm wide and 20–30 nm thick. The belt-like VO₂(B) with a high surface area may be beneficial to lithium insertion between the VO₆ layers for application in batteries.     

VO2/TiN Plasmonic Thermochromic Smart Coatings for Room‐Temperature Applications

Vanadium dioxide/titanium nitride (VO₂/TiN) smart coatings are prepared by hybridizing thermochromic VO₂ with plasmonic TiN nanoparticles. The VO₂/TiN coatings can control infrared (IR) radiation dynamically in accordance with the ambient temperature and illumination intensity. It blocks IR light under strong illumination at 28 °C but is IR transparent under weak irradiation conditions or at a low temperature of 20 °C. The VO₂/TiN coatings exhibit a good integral visible transmittance of up to 51% and excellent IR switching efficiency of 48% at 2000 nm. These unique advantages make VO₂/TiN promising as smart energy‐saving windows.     

A polymeric complex method to nanocrystalline BiCu2VO6 with visible light photocatalytic activity

Nanocrystalline BiCu₂VO₆ was successfully synthesized by a facile polymeric citrate complex method. The samples were characterized by X-ray diffraction (XRD) analyses, scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), UV-vis diffuse reflectance spectra (DRS), thermogravimetric analyses (TG) and differential thermal analyses (DTA). BiCu₂VO₆ showed photocatalytic activity for the degradation of RhB under visible light irradiations with the coexistence of H₂O₂. As compared to BiCu₂VO₆ prepared via a solid state reaction, BiCu₂VO₆ prepared via a polymeric citrate complex method showed higher photocatalytic activity. The visible light photocatalytic activity of BiCu₂VO₆ was also proposed.     

A low-temperature solution route to hollow NH4VO3 microspheres with controllable shells

NH₄VO₃ hollow microspheres with controllable shells have been synthesised by the interaction between AgNO₃ and NH₄VO₃ via a facile one-step low-temperature solution-based method. The diameters and surface roughness of NH₄VO₃ hollow spheres can be readily controlled by altering the molar ratios of NH₄VO₃ to AgNO₃. When the molar ratios of NH₄VO₃ to AgNO₃ increase from 6?:?1 to 8?:?1, the diameters of NH₄VO₃ hollow spheres increase from 500–600 to 800–900?nm and the building blocks of the shells are assembled by nanoparticles and nanorods. The introduction of AgNO₃ and H₂O₂ plays an important role in the formation of NH₄VO₃ hollow microspheres.     

Thermochromic VO2 nanorods and other vanadium oxides nanostructures

Thermochromic VO₂ nanorods were prepared via thermal conversion of the metastable VO₂–B phase synthesized by hydrothermal methods. We observe an increased thermochromic transition temperature to ∼75–80 °C by variable-temperature infrared spectroscopy. Nano- and sub-micron structures of other vanadium oxides (V₃O₇, (NH₄)_0.5V₂O₅, and V₂O₅) were obtained simply by varying the starting materials in the hydrothermal synthesis. We also obtained nanostructures of the high temperature tetragonal rutile phase of VO₂ by thermolysis of single-source vanadium (IV) precursors.     

Enhanced Intercalation Dynamics and Stability of Engineered Micro/Nano‐Structured Electrode Materials: Vanadium Oxide Mesocrystals

An additive and template free process is developed for the facile synthesis of VO₂(B) mesocrystals via the solvothermal reaction of oxalic acid and vanadium pentoxide. The six‐armed star architectures are composed of stacked nanosheets homoepitaxially oriented along the [100] crystallographic register with respect to one another, as confirmed by means of selected area electron diffraction and electron microscopy. It is proposed that the mesocrystal formation mechanism proceeds through classical as well as non‐classical crystallization processes, and is possibly facilitated or promoted by the presence of a reducing/chelating agent. The synthesized VO₂(B) mesocrystals are tested as a cathodic electrode material for lithium‐ion batteries, and show good capacity at discharge rates ranging from 150–1500 mA g^?1 and a cyclic stability of 195 mA h g^?1 over fifty cycles. The superb electrochemical performance of the VO₂(B) mesocrystals is attributed to the porous and oriented superstructure that ensures large surface area for redox reaction and short diffusion distances. The mesocrystalline structure ensures that all the surfaces are in intimate contact with the electrolyte, and that lithium‐ion intercalation occurs uniformly throughout the entire electrode. The exposed (100) facets also lead to fast lithium intercalation, and the homoepitaxial stacking of nanosheets offers a strong inner‐sheet binding force that leads to better accommodation of the strain induced during cycling, thus circumventing the capacity fading issues typically associated with VO₂(B) electrodes.     

Preparation and electrochemical properties of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> by a rheological-phase-assisted microwave synthesis method

By using LiCO₃ and MnO₂, a rheological-phase-assisted microwave synthesis method has been applied in the fast preparation of spinel LiMn₂O₄ in order to reduce the cost of cathode materials. Comparing with the pristine LiMn₂O₄ obtained by the traditional solid-state reaction method, the structure and surface morphology of the samples synthesized by the rheological-phase-assisted microwave synthesis method have been investigated. The powders were used as positive materials for lithium-ion battery, whose charge/discharge properties and cycle performance have been examined in detail. As a result, the powders prepared by the rheologicalphase-assisted microwave synthesis method at 750°C are pure spinel LiMn₂O₄ with regular shapes and uniform distribution, which exhibit higher capacity and much better reversibility than the sample prepared by the traditional solid-state reaction. The text was submitted by the authors in English.