VO2 Nanocrystals Research Articles

Electrons-Donating Derived Dual-Resistant Crust of VO2 Nano-Particles via Ascorbic Acid Treatment for Highly Stable Smart Windows Applications.

Traditional vanadium dioxide (VO2) material faced severe challenges of low stability in acid, humid, and oxygenic environments, which hinder its real applications. Here, we report a facile improving process which can enhanced the stability of VO2 nanocrystals in the environments above. Ascorbic acid (AA), as an important antioxidant for organism in medicine and biology, was ingeniously used for enhancing the antioxidation abilities of inorganic material. At the same time, the AA could generate the hydrogen doping occurred on the surface of VO2 nanocrystals, which enhanced their Antiacid abilities simultaneously. The AA treated VO2 nanocrystals retain stable in H2SO4 and H2O2 solution and exhibit high durability in hyperthermal (60 °C) and humid (90%) environment. Characterizations and first-principles theoretical calculations confirmed that the coordination of ascorbic acid molecules on VO2 nanocrystals induced charge-carrier density reorganization and protons transferring electrostatically. Then the formed HxVO2 provides an enhancing formation energy for oxygen vacancy and protects the particles from corrosion. This work is beneficial to the VO2 nanoparticles coated and decorated processes and exhibit good potential for practical application of VO2-based smart windows.

Surface modification-assisted solvent annealing to prepare high quality M-phase VO2 nanocrystals for flexible thermochromic films

Well-crystalline and stoichiometric M-phase vanadium dioxide (VO2 (M)) ultrafine nanoparticles are desired in fabricating high-efficient VO2-based flexible thermochromic films. In this study, a novel solvent annealing technique is developed to avoid growth and agglomeration of VO2 (M) nanoparticles in the post-annealing process. Oxygen-deficient VO2 (D) nanoparticles (~36 nm) are firstly prepared by a facial hydrothermal method. A double-layered reactor is designed to separate the surfactant-modified VO2 (D) nanoparticles physically from H2O2 which serves as the oxidant. The unique reactor enables the formation of a moderate oxygenation environment in the solvent annealing process, making the transformation of oxygen-deficient VO2 to VO2 (M) more controllable. The growth and agglomeration of VO2 nanoparticles are effectively suppressed due to the surface modification by polyvinyl pyrrolidone, liquid environment and low temperature. The obtained VO2 (M) nanoparticles have an average particle size of ~41 nm. The flexible film prepared with the VO2 (M) nanoparticles exhibit excellent thermochromic properties with ΔTsol reaching 18.03% and Tlum up to 58.33%. This amazing performance is attributed to the high quality of VO2 (M) nanoparticles and ultrafine particle size that makes the surface plasmon absorption take effect in the solar light region. This work demonstrates the proposed solvent annealing technique is an effective approach to prepare high-quality ultrafine VO2 (M) nanoparticles for achieving efficient thermochromic performance of the flexible smart windows.

Tuning the optical bandgap of V2O5 nanoparticles by doping transition metal ions

Abstract In this study the influence of transition metal ions doping on optical properties of V2O5 nanoparticles is investigated. The shift in absorption edge caused by the dopant is analyzed from the UV-visible absorption spectra. The absorption edge of Ti doped V2O5 nanocrystals is shifted to longer wavelength region, while that of Zr doped V2O5 showed a blue shift. Ti doped V2O5 nanocrystal in spite of its large crystallite size showed improved optical properties and its optical bandgap (1.96 eV) is less than un-doped V2O5 (2.2 eV). This is attributed to lattice expansion by Ti ion and formation of oxygen vacancies within the bandgap. The oxygen vacancies acts as intermediate energy states and reduce the optical bandgap. However, the PL spectra of Ti-doped V2O5 did not exhibited emission peak corresponding to oxygen vacancies due to high crystallinity. The life time of excitons in excited state is investigated from time resolved PL decay. The excitons in doped sample showed tri-exponential decay whereas in pure sample they exhibited bi-exponential decay. The average lifetime of excitons in doped V2O5 is higher than pure V2O5 sample. This study reveals that by optimizing the concentration of Ti and Zr dopant ions in V2O5 lattice, its energy band structure can be tailored for photovoltaic, catalytic and photoluminescence applications. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Nanotechnology: Ideas, Innovations & Initiatives-2017 (ICN:3i-2017).

Tungsten doped M-phase VO2 mesoporous nanocrystals with enhanced comprehensive thermochromic properties for smart windows

The phase transition temperature (~68 °C) of M-VO2 film can be lowered significantly by tungsten (W) doping into the crystal lattice of VO2 due to the reduction of the strength of V-V pair interaction. However, W doping was always coupled with a serious weakening of luminous transmittance and solar modulation efficiency because W dopants can increase the electron concentration of VO2 film. Herein, the simultaneous introduction of W dopants and mesopores into M-VO2 nanocrystals was employed to prepare VO2 film. Interestingly, the obtained 0.4 at% W-doped mesoporous VO2 nanocrystals based composite films exhibited enhanced comprehensive thermochromic performance with excellent solar modulation efficiency (ΔTsol = 11.4%), suitable luminous transmittance (Tlum = 61.6%) and low phase transition temperature around 43 °C, much lower than 65.3 °C of undoped VO2. It was demonstrated that the lower phase transition temperature of VO2 can be primarily attributed to abundant lattice distortion after W doping, whereas the mesoporous structure can facilitate the uniform distribution of W dopants in VO2 nanocrystals, enhance the luminous transmittance and guarantee enough VO2 nanocrystals in the composite film to keep relatively high solar modulation efficiency. Therefore, this work can provide a new way to balance the three important parameters for the thermochromic performance of VO2 film (ΔTsol, Tlum and Tc) and probably promote the application of VO2 nanocrystals in the energy efficient windows.

Elucidating the Crystallite Size Dependence of the Thermochromic Properties of Nanocomposite VO2 Thin Films.

Fenestration elements that enable spectrally selective dynamic modulation of the near-infrared region of the electromagnetic spectrum are of great interest as a means of decreasing the energy consumption of buildings by adjusting solar heat gain in response to external temperature. The binary vanadium oxide VO2 exhibits a near-room-temperature insulator–metal electronic transition accompanied by a dramatic modulation of the near-infrared transmittance. The low-temperature insulating phase is infrared transparent but blocks infrared transmission upon metallization. There is considerable interest in harnessing the thermochromic modulation afforded by VO2 in nanocomposite thin films. However, to prepare a viable thermochromic film, the visible-light transmittance must be maintained as high as possible while maximizing thermochromic modulation in the near-infrared region of the electromagnetic spectrum, which necessitates the development of high-crystalline-quality VO2 nanocrystals of the optimal particle size embedded within the appropriate host matrix and refractive index matched to the host medium. Here, we demonstrate the preparation of acrylate-based nanocomposite thin films with varying sizes of embedded VO2 nanoparticles. The observed strong size dependence of visible-light transmittance and near-infrared modulation is explicable on the basis of optical simulations. In this article, we elucidate multiple scattering and absorption mechanisms, including Mie scattering, temperature-/phase-variant refractive-index mismatch between VO2 nanocrystals and the encapsulating matrix, and the appearance of a surface plasmon resonance using temperature-variant absorptance and diffuse transmittance spectroscopy measurements performed as a function of particle loading for the different sizes of VO2 nanocrystals. Nanocrystals with dimensions of 44 ± 30 nm show up to >32% near-infrared energy modulation across the near-infrared region of the electromagnetic spectrum while maintaining high visible-light transmission. The results presented here, providing mechanistic elucidation of the size dependence of the different scattering mechanisms, underscore the importance of nanocrystallite dimensions, refractive-index matching, and individualized dispersion of particles within the host matrix for the preparation of viable thermochromic thin films mitigating Mie scattering and differential refractive-index scattering.

Selective IR response of highly textured phase change VO2 nanostructures obtained via oxidation of electron beam deposited metallic V films

We demonstrate the growth of highly textured VO2 nanocrystals via annealing of e-beam deposited amorphous metallic V. Temperature dependent ellipsometry results reveal the pronounced reflection near the IR spectrum above the transition and an almost temperature independent weak reflection in the visible spectrum. The IR reflection displays a strong hysteresis during heating and cooling near the transition temperature at 68°C, indicating a first order transition and a strain-free structure. Our work demonstrates the feasibility to obtain high quality phase change nanostructures that transmit the visible spectrum but reflect IR and is suitable for large scale fabrication.

New insights into oxygen defects, Lewis acidity and catalytic activity of vanadia hybrid nanomaterials

Structural defects in both MCM–41 (Mobil Composition of Matter No. 41) framework and V2O5 nanocrystals and surface acid sites of the V2O5/MCM–41 hybrid nanomaterials were quantitatively determined. Oxygen defects were found to be related to Lewis acid sites in the materials. In the oxidative desulfurization of a model diesel, dibenzothiophene conversion was almost proportional to the number of Lewis acidity and generally correlated with the oxygen defects concentration in V2O5. This work confirmed that increasing surface Lewis acidity and oxygen defect concentration is an effective route to improve the catalytic activity in the oxidation of sulfur compounds.

Facile synthesis of mesoporous VO2 nanocrystals by a cotton-template method and their enhanced thermochromic properties

As a very promising thermochromic material, VO2 (M/R) (Monoclinic/Rutile) has not been widely applied in smart windows due to its intrinsic low solar modulation (ΔTsol) and low luminous transmission (Tlum). To address this issue, porous structures have been introduced into the VO2 film. Herein, mesoporous VO2 powders with pore size of about 2–10nm were synthesized using cotton as template by hydrothermal methods. The pore and crystal size of the synthesized VO2 powders can be reliably controlled by the hydrothermal temperature. The mesoporous VO2 powders were mixed with PVP to prepare the VO2-based nanocomposite films by spin coating. The VO2-based films show a better performance between ΔTsol and Tlum than that appeared in previous reports. Especially, a larger pore size could lead to a higher visible transmittance and a larger crystal size would facilitate the enhancement in the solar modulation. In this sense, the VO2-based film obtained at the hydrothermal temperature of 180°C exhibits an outstanding thermochromic performance with ΔTsol of 12.9% and Tlum up to 56.0% due to a larger crystal size and pore size. Therefore, this synthetic route shows a potential method for the application of mesoporous VO2 powders for solar control coatings.

Hybrid Nanocomposite Films Comprising Dispersed VO2 Nanocrystals: A Scalable Aqueous-Phase Route to Thermochromic Fenestration.

Buildings consume an inordinate amount of energy, accounting for 30-40% of worldwide energy consumption. A major portion of solar radiation is transmitted directly to building interiors through windows, skylights, and glazed doors where the resulting solar heat gain necessitates increased use of air conditioning. Current technologies aimed at addressing this problem suffer from major drawbacks, including a reduction in the transmission of visible light, thereby resulting in increased use of artificial lighting. Since currently used coatings are temperature-invariant in terms of their solar heat gain modulation, they are unable to offset cold-weather heating costs that would otherwise have resulted from solar heat gain. There is considerable interest in the development of plastic fenestration elements that can dynamically modulate solar heat gain based on the external climate and are retrofittable onto existing structures. The metal-insulator transition of VO2 is accompanied by a pronounced modulation of near-infrared transmittance as a function of temperature and can potentially be harnessed for this purpose. Here, we demonstrate that a nanocomposite thin film embedded with well dispersed sub-100-nm diameter VO2 nanocrystals exhibits a combination of high visible light transmittance, effective near-infrared suppression, and onset of NIR modulation at wavelengths <800 nm. In our approach, hydrothermally grown VO2 nanocrystals with <100 nm diameters are dispersed within a methacrylic acid/ethyl acrylate copolymer after either (i) grafting of silanes to constitute an amorphous SiO2 shell or (ii) surface functionalization with perfluorinated silanes and the use of a perfluorooctanesulfonate surfactant. Homogeneous and high optical quality thin films are cast from aqueous dispersions of the pH-sensitive nanocomposites onto glass. An entirely aqueous-phase process for preparation of nanocrystals and their effective dispersion within polymeric nanocomposites allows for realization of scalable and viable plastic fenestration elements.

Interconnected three-dimensionally hierarchical heterostructures with homogeneously-dispersed V2O5 nanocrystals and carbon for high performance supercapacitor electrodes

In this article, a simple “liquid phase impregnation template” strategy has been developed to synthesize hierarchically porous V2O5/C (HPVC) nanocomposites by using commercial V2O5 and phenol-formaldehyde (PF) resol as precursors and polystyrene opal microspheres as template. The amount of vanadium sol in the vanadium-PF precursor has a significant effect on the morphology and microstructure of the resulting HPVC nanocomposites. Due to the intact interconnected hierarchical network framework and the homogeneous dispersion of constitutive V2O5 nanocrystals and carbon phases, the optimal sample HPVC2 with higher surface area (645m2g−1) and mesopore surface area (394m2g−1) and an appropriate content of V2O5 (38.7wt.%), is found to be the most efficient one as supercapacitor electrode with a high specific capacitance up to 492.1F g−1 at a scan rate of 5mVs−1 in the three-electrode system. When used as symmetric electrode, the HPVC2//HPVC2 supercapacitor exhibits a high energy density up to 87.6 Wh kg−1 at a power density of 497Wkg−1, which greatly surpasses those of pure single-component counterparts and others reported V2O5/C composites. The present work conceptually provides a facile way for integrating transition metal oxide with carbon in hierarchically porous nanostructure to synthesize hybrid electrode materials for advanced energy storage/conversion devices.

Electrochemically Induced Transformations of Vanadium Dioxide Nanocrystals.

Vanadium dioxide (VO2) undergoes significant optical, electronic, and structural changes as it transforms between the low-temperature monoclinic and high-temperature rutile phases. Recently, alternative stimuli have been utilized to trigger insulator-to-metal transformations in VO2, including electrochemical gating. Here, we prepare and electrochemically reduce mesoporous films of VO2 nanocrystals, prepared from colloidally synthesized V2O3 nanocrystals that have been oxidatively annealed, in a three-electrode electrochemical cell. We observe a reversible transition between infrared transparent insulating phases and a darkened metallic phase by in situ visible-near-infrared spectroelectrochemistry and correlate these observations with structural and electronic changes monitored by X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, and conductivity measurements. An unexpected reversible transition from conductive, reduced monoclinic VO2 to an infrared-transparent insulating phase upon progressive electrochemical reduction is observed. This insulator-metal-insulator transition has not been reported in previous studies of electrochemically gated epitaxial VO2 films and is attributed to improved oxygen vacancy formation kinetics and diffusion due to the mesoporous nanocrystal film structure.

Thermochromic modulation of surface plasmon polaritons in vanadium dioxide nanocomposites.

We propose and implement a new concept for thermochromic plasmonic elements. It is based on vanadium dioxide (VO2) nanocrystals located in the near field of surface plasmon polaritons supported by an otherwise unstructured gold thin film. When the VO2 undergoes the metal-insulator phase transition, the coupling conditions for conversion of light into propagating surface plasmon polaritons change markedly. In particular, we realize thermochromic plasmonic grating couplers with substantial switching contrast as well as tunable plasmonic couplers in a Kretschmann configuration. The use of VO2 nanocrystals permits highly repetitive switching and room temperature operation. Simulations based on the actual dielectric function of our VO2 nanocrystals agree well with the experiment.

Hydrothermal synthesis of vanadium oxide nanorods and their electrochromic performance

Nanorod-like V2O5 was fabricated using a simple one-step hydrothermal method using NH4VO3 and oxalic acid as precursors. The morphology and structure of the as-prepared V2O5 nanocrystals were characterized by different techniques. Furthermore, based on the analyses of cyclic voltammetric curves and electrochromism test, the thin V2O5 nanorod film have been found to exhibit promising electrochromic performance, which can be attributed to their nanorod architecture. The as-prepared V2O5 nanocrystals are promising electrochromic candidate for potential application in smart window.

Self-template processed hierarchical V2O5 nanobelts as cathode for high performance lithium ion battery

Hierarchically nanostructured materials have proven their superiority in high performance lithium ion batteries. Herein, we successfully fabricate hierarchical vanadium pentoxide (V2O5) nanobelts by an economical and facile hydrothermal strategy followed by annealing treatment. A self-template growth of hierarchical V2O5 nanobelts was achieved from precursor VO2 (B) nanobelts, resulting in open structure of hierarchical V2O5 nanobelts composed of plate-like V2O5 nanocrystals and tiny haircuts. The hierarchical nanobelts deliver a high specific surface area of 137.5m2g−1. When used as the cathode material for lithium ion battery, the hierarchical V2O5 nanobelts exhibit high discharge capacity of 288mAhg−1, which maintain 246mAhg−1 at the end of 50th cycle with capacity retention of 85.4 %.The present electrochemical results demonstrate the high discharge capacity, superior cycling stability and rate capability of the hierarchical V2O5 nanobelts, which could be attributed to the advantages of unique hierarchical nanobelt structure.

Hydrothermal synthesis and electrochemical properties of V2O5 nanomaterials with different dimensions

In the work, four types of typical morphologies of differently dimensional pure V2O5 nanostructures including nanoflowers, nanoballs, nanowires and nanorods were synthesized through a simple hydrothermal method. The morphology of the product depends on the types of solvent and acid. Hierarchical nanoflowers and zero-dimensional (0D) nanoballs of V2O5 nanocrystals were obtained when using C2H5OH as solvent. One dimensional (1D) nanowires and nanorods were obtained if H2O was used to participate in the reaction. The morphology and structure of the samples were characterized by XRD and SEM, respectively. The electrochemical behaviors of nanoflowers, nanoballs, nanowires and nanorods as electrodes in a supercapacitor device were tested using cyclic voltammetry (CV) curves and galvanostatic charge–discharge (GCD) curves. The electrochemical test results indicate that the rod-like structure leads to a significant improvement of storage capacity, electrochemical kinetics and rate capability. And 1D V2O5 nanorods show the largest specific capacitance of 235Fg−1 at the current density of 1Ag−1 when used as supercapacitor electrode in 1mol/L Na2SO4 electrolyte. It turns out that 1D V2O5 nanorod is an ideal material compared with other morphologies for supercapacitor electrode.

TiO2 colloidal nanocrystals surface modification by V2O5 species: Investigation by 47,49Ti MAS-NMR and H2, CO and NO2 sensing properties

TiO2 and TiO2–V2O5 nanocrystals were prepared by coupling sol–gel and solvothermal methods, followed by heat-treatment at 400°C, after which the mean nanocrystal size was about 5nm. The materials were characterized by X-ray diffraction, transmission electron microscopy and solid state nuclear magnetic resonance spectroscopy. It was shown that while the TiO2 phase was always anatase even after heat-treatment at 500°C, the presence of the vanadium oxide species enhanced the surface re-configuration of the Ti ions. Hence the coordination environment of surface Ti atoms was drastically changed, by formation of further bonds and imposition of a given local geometry. The final hypothesis was that in pure titania surface rearrangement occurs, leading to the new NMR signal, but this modification was favored in the TiO2–V2O5 sample, where the Ti surface atoms were forced into the final configurations by the bonding with V atoms through oxygen. The materials heat-treated at 400°C were used to process chemoresistive sensors, which were tested to hydrogen, CO and NO2, as examples of gases with peculiar sensing mechanisms. The results evidenced that the surface deposition of V2O5 onto the anatase TiO2 nanocrystals was effective in modifying the adsorption properties of the anatase nanocrystals.

Evidence of catalytic activation of anatase nanocrystals by vanadium oxide surface layer: Acetone and ethanol sensing properties

TiO2 and TiO2–V2O5 nanocrystals were prepared by coupling sol–gel and solvothermal methods, followed by heat treatment at 400°C, after which the nanocrystal mean size was still about 5nm. The materials were used to process chemoresistive sensors, which were tested to ethanol and acetone, with concentrations ranging from 100 to 500ppm and from 25 to 100ppm, respectively. The sensing data evidenced that the surface deposition of V2O5 onto the anatase TiO2 nanocrystals enhanced the sensor response up to almost two orders of magnitude for both gases. Moreover, the sensors behavior was completely inverted: with TiO2–V2O5, the highest responses were obtained at the lowest operating temperatures, contrarily to pure TiO2, which required very high operating temperatures. The comparison of the sensing data allowed concluding that the V2O5 deposition effect could be interpreted as a catalytic contribution, in terms of lowered activation energies of the involved reactions and more favored gas adsorption at lower operating temperatures with respect to pure TiO2.

The Structural Evolution of V2O5 Nanocystals during Electrochemical Cycling Studied Using In operando Synchrotron Techniques

The structural evolution of vanadium oxide (V2O5) nanocrystals was studied during Li+ ion intercalation and deintercalation (i.e. electrochemical discharge/charge) processes using in operando high-energy x-ray diffraction (HEXRD) and in operando x-ray adsorption near edge spectroscopy (XANES). The HEXRD results clearly show that V2O5 undergoes phase transformations during the first Li+ ion intercalation (i.e. discharge) process. The analysis of the XANES data suggests that the average oxidation state of vanadium in fully charged V2O5 nanocrystals decreases to less than +5 after the first four cycles. The combined results of the unchanged crystal structure (HEXRD) and the decreased oxidation state (XANES) lead to the conclusion that some of the Li+ ions are trapped within the V2O5 framework and the V2O5 exists as Li0.18V2O5 instead of pure V2O5 after the first four cycles, while the trapped Li+ ion may increase the stability of V2O5 framework.

Hydrothermal synthesis of Mo-doped VO2/TiO2 composite nanocrystals with enhanced thermochromic performance.

This paper reports a one-step TiO2 seed-assistant hydrothermal synthesis of Mo-doped VO2(M)/TiO2 composite nanocrystals. It was found that excess Mo doping can promote formation of the VO2(M) phase, and rutile TiO2 seed is beneficial to morphology control, size reduction, and infrared modulation of Mo-doped VO2(M) nanocrystals. The Mo-doped VO2 nanocrystals epitaxially grow on TiO2 seeds and have a quasi-spherical shape with size down to 20 nm and a nearly 35% infrared modulation near room temperature. The findings of this work demonstrate important progress in the near-room-temperature thermochromic performance of VO2(M) nanomaterials, which will find potential application in constructing VO2(M) nanocrystal-based smart window coatings.

Hierarchical orthorhombic V2O5 hollow nanospheres as high performance cathode materials for sodium-ion batteries

Vanadyl ethylene glycolate hollow hierarchical nanospheres were synthesized via a template-free polyol-induced solvothermal process. After sintering, vanadium pentoxide (V2O5) with well-preserved spherical structures was obtained. Refined X-ray diffraction and transmission electron microscopy analyses identified that the V2O5 hollow nanospheres were constructed from hierarchical nanocrystals with predominantly exposed {110} crystal planes. When applied as cathode materials for sodium-ion batteries (Na-ion batteries), the V2O5 hollow nanospheres delivered a specific discharge capacity of ∼150 mA h g−1, which is equal to one Na+ ion insertion per V2O5 formula unit. Theoretical modelling on the volume expansion and stress evolution on Na+ ion insertion revealed that the prolonged cycling stability could be ascribed to the porous hollow spherical architecture. Furthermore, the exposed {110} facets of V2O5 nanocrystals with two-dimensional diffusion paths for Na+ ion intercalation also contribute to high rate capacity and excellent cycling performance.