VO2 Nanocrystals Research Articles

Solvothermal synthesis of VO2 nanoparticles with locally patched V2O5 surface layer and their morphology-dependent catalytic properties for the oxidation of alcohols.

Vanadium oxides are promising oxidation catalysts because of their rich redox chemistry. We report the synthesis of VO2 nanocrystals with VO2(B) crystal structure. By varying the mixing ratio of the components of a binary ethanol/water mixture, different VO2 nanocrystal morphologies (nanorods, -urchins, and -sheets) could be made selectively in pure form. Polydisperse VO2(B) nanorods with lengths between 150 nm and a few micrometers were formed at large water : ethanol ratios between 4 : 1 and 3 : 2. At a water : ethanol ratio of 1 : 9 VO2 nanosheets with diameters of ∼50-70 nm were formed, which aggregated to nano-urchins with diameters of ∼200 nm in pure ethanol. The catalytic activity of VO2 nanocrystals for the oxidation of alcohols was studied as a function of nanocrystal morphology. VO2 nanocrystals with all morphologies were catalytically active. The activity for the oxidation of benzyl alcohol to benzaldehyde was about 30% higher than that for the oxidation of furfuryl alcohol to furfural. This is due to the substrate structure. The oxidation activity of VO2 nanostructures decreases in the order of nanourchins > nanosheets > nanorods.

Influence of Mo/V coupled multi-electron reactions and the crystalline phase transition of VO2 on high specific capacity of lithium-ion batteries

To further improve the specific capacity and stability of amorphous vanadium-based lithium-ion battery cathode materials, MoS2 was introduced for the first time into the 70V2O5–30Li3PO4 (VP) glass cathode system. Both Mo and S participated in the redox reaction, achieving for the first time an increase in the V4+ content from 21.8% to 50.4%. This improved the electrical conductivity. Moreover, the realization of a Mo/V multi-electron coupling reaction promoted the capacity. The precipitation of LiV2O5 and MoP2O8 nanocrystals during electrocycling enhanced the cycling stability and reaction kinetics. Compared to VP, the capacity of 67V2O5–30Li3PO4–3MoS2 (VPMo3) was enhanced by more than 50% at a current density of 100 mA g-1, and the capacity retention after 50 cycles was improved by about 20%. Notably, the VO2 nanocrystals precipitated during the melt preparation underwent a metal-insulator phase transition (MIT), and their structural transformation affected conductivity.

Synthesis and elucidation of structural, optical, vibrational and magnetic properties of Mn-doped V2O5 dilute magnetic semiconductor nanocrystals

Undoped and Mn-doped (5% and 10%) V2O5 nanocrystals have been synthesized by the chemical co-precipitation process followed by heating of the chemically obtained materials at 600°C. X-ray diffractometry, transmission electron microscopy, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, micro-Raman spectroscopy, UV–visible absorption spectroscopy, photoluminescence spectroscopy, vibrating sample magnetometry and electron spin resonance spectroscopy have been used for material characterization. Williamson-Hall analyses revealed higher crystallite size but lower lattice strain, lattice stress and energy density of the Mn-doped V2O5 nanocrystals as compared to undoped ones. The vibrational properties were found to be influenced by the presence of Mn ions while photoluminescence results suggested the presence of vanadium vacancy and vanadium interstitial defects in the nanocrystals. The undoped V2O5 nanocrystals exhibited weak room-temperature ferromagnetic behaviour that got significantly stronger after Mn doping. The dilute magnetic semiconductor Mn-doped V2O5 nanocrystals are promising materials for applications in spintronics devices.

Acidity of Carboxylic Acid Ligands Influences the Formation of VO2(A) and VO2(B) Nanocrystals under Solvothermal Conditions.

Vanadium dioxide (VO2) can adopt many different crystal structures at ambient temperature and pressure, each with different, and often desirable, electronic, optical, and chemical properties. Understanding how to control which crystal phase forms under various reaction conditions is therefore crucial to developing VO2 for various applications. This paper describes the impact of ligand acidity on the formation of VO2 nanocrystals from the solvothermal reaction of vanadyl acetylacetonate (VO(acac)2) with stoichiometric amounts of water. Carboxylic acids examined herein favor the formation of the monoclinic VO2(B) phase over the tetragonal VO2(A) phase as the concentration of water in the reaction increases. However, the threshold concentration of water required to obtain phase-pure VO2(B) nanocrystals increases as the pKa of the carboxylic acid decreases. We also observe that increasing the concentration of VO(acac)2 or the concentration of acid while keeping the concentration of water constant favors the formation of VO2(A). Single-crystal electron diffraction measurements enable the identification of vanadyl carboxylate species formed in reactions that do not contain enough water to promote the formation of VO2. Increasing the length of the carbon chain on aliphatic carboxylic acids did not impact the phase of VO2 nanocrystals obtained but did result in a change from nanorod to nanoplatelet morphology. These results suggest that inhibiting the rate of hydrolysis of the VO(acac)2 precursor either by decreasing the ratio of water to VO(acac)2 or by increasing the fraction of water molecules that are protonated favors the formation of VO2(A) over VO2(B).

The Feasibility of Tungsten-Doped VO2 Films on Soda-Lime Glass with Low Thermal Budget by High Density Plasma Source for Smart Windows

Thermochromic W-doped VO2 thin films on soda-lime glass are successfully fabricated by co-sputtering technique using a high density plasma source, which is equipped by high power impulse magnetron sputtering (HIPIMS). A post-rapid thermal annealing of 500 °C is performed for the purpose of low thermal budget. The effect of doping amount of tungsten on thermochromic properties such as transmittance and transition temperature are addressed. The transition temperature can be lower down to 30 °C with a small amount of 4.5% tungsten added in VO2 nanocrystals. A solar regulation efficiency, ΔTsol = 10% is achieved at thicker TiO2 thickness. The competition of secondary phase of V2WO7.5 and oxygen-rich phase of V2O5 under different O2/Ar ratio is studied. The crystalline behavior of monoclinic phase is examined by X-ray diffraction (XRD) pattern and high resolution transmission electron microscope (HR-TEM). The good endurance property ensures the feasible use for the energy-saving applications.

Effective photodegradation of acetaminophen by V2O5 nanocrystals under simulated solar radiation

Vanadium pentoxide (V2O5) can absorb light in both ultraviolet (UV) and visible regions making it an important material for the photocatalytic degradation of environmental pollutants under solar radiation. Herein, V2O5 nanocrystals have been chemically synthesized using two different capping agents viz. cetyl trimethyl ammonium bromide (CTAB) and polyvinylpyrrolidone (PVP), and they are found to be effective photocatalysts under simulated solar radiation (SSR) for degradation of acetaminophen, which is an emerging environmental pollutant. The CTAB-capped and PVP-capped V2O5 nanocrystals degraded 65% and 71% of aqueous acetaminophen after 270 min of irradiation, respectively. The chemical oxygen demand (COD) of the acetaminophen-polluted water decreased significantly after photocatalysis. The photocatalysts were found to be usable for two consecutive cycles without significant changes in degradation efficiencies. Moreover, the solution pH influenced photocatalysis. The mechanism of the photocatalysis process has been presented considering the split nature of the V2O5 conduction band.

In-situ partial oxidation of TiVCTx derived TiO2 and V2O5 nanocrystals functionalized TiVCTx MXene as anode for lithium-ion batteries

Two-dimensional double transition metal carbides or nitrides (MXenes) have been commonly concerned in the field of energy storage due to their distinctive structure and high conductivity. MXene-based electrode materials with single metal component are difficult to achieve high specific capacity, efficient ion/electron transport and stability, which hinder its practical application. Herein, we proposed a strategy by using in situ partial oxidation of double transition metal MXene (TiVCTx) derived TiO2 and V2O5 nanocrystals blends functionalized TiVCTx MXene (for simplicity denoted as TiVC@VTO) as electrode materials to boost the lithium ion battery performance. Effect of annealing temperature on the microstructure, crystallinity, and electrochemical property of TiVC@VTO is investigated. The TiVC@VTO retains the layer structure of the original TiVCTx MXene and at the same time the composition and microstructure can be regulated by a simple thermal annealing treatment to modify the oxidation degree. Compared with pristine TiVCTx, the oxidized sample demonstrates an optimal performance with specific capacity of 331.1 mAh g−1 after 100 cycles at current density of 0.1 A g−1 and 237.6 mAh g−1 after 1000 cycles at current density of 1.0 A g−1 at 500 °C (TiVC@VTO-500). Our work offers a simple and practicable method for enhanced lithium-ion storage capability by using double transition metal MXene. Moreover, this work also provides further information about the stability of TiVCTx MXene, which will benefit energy storage in the future development of novel double transition metal MXene materials for energy storage applications.

Facile Synthesis of Island-like ZrO2-VO2 Composite Films with Enhanced Thermochromic Performance for Smart Windows

VO2-based film, as a very promising thermochromic material for smart windows, has attracted extensive attention but has not been widely applied because it is difficult to simultaneously improve in terms of both solar-modulation efficiency (ΔTsol) and visible transmittance (Tlum) when made using the magnetron-sputtering method, and it has poor durability when made using the wet chemical method. Herein, island-like ZrO2-VO2 composite films with improved thermochromic performance (ΔTsol: 12.6%, Tlum: 45.0%) were created using a simple approach combining a dual magnetron-sputtering and acid-solution procedure. The film's ΔTsol and Tlum values were increased initially and subsequently declined as the sputtering power of the ZrO2 target was raised from 30 W to 120 W. ΔTsol achieved its maximum of 12.6% at 60 W, and Tlum reached its maximum of 51.1% at 90 W. This is likely the result of the interaction of two opposing effects: Some VO2 nanocrystals in the composite film were isolated by a few ZrO2 grains, and some pores could utilize their surface-plasmon-resonance effect at high temperature to absorb some near-infrared light for an enhanced ΔTsol and Tlum. More ZrO2 grains means fewer VO2 grains in the composite film and increased film thickness, which also results in a decrease in ΔTsol and Tlum. As a result, this work may offer a facile strategy to prepare VO2-based films with high thermochromic performance and promote their application in smart windows.

Three-in-one organic-inorganic heterostructures: From scalable ball-milling synthesis to freestanding cathodes with high areal capacity for aqueous zinc-ion batteries

Aqueous Zn-ion batteries (AZIBs) have great potential in large-scale energy storage, however, developing suitable cathodes with high energy/power densities remains highly challenging. Herein, a unique organic-inorganic heterostructures (denoted as Ov-PVO/G) of poly(3,4-ethylenedioxythiophene) intercalated V2O5 nanocrystals anchored on few-layer graphene by a simple and scalable ball-milling approach are demonstrated for high areal-capacity AZIBs cathodes, in which, adjusted interlayer spacing, abundant oxygen vacancies and robust heterointerfaces can be simultaneously constructed. Moreover, the feature of being well dispersed in different polar solvents (H2O, NMP, etc.) enable facile incorporation of the Ov-PVO/G heterostructures into conductive networks of single-walled carbon nanotubes to fabricate freestanding composite cathodes. Electrochemical measurement and first-principles calculations indicate that such “Three-in-One” heterostructure can significantly enhance Zn2+ adsorption/desorption, electrical conductivity and charge transfer due to strong synergy, thereby exhibiting high-capacity and accelerated reaction kinetics during cycling. Specifically, at a practical mass loading of 12.0 mg/cm2, the freestanding Ov-PVO/G cathodes show a high areal-capacity of 6.1 mAh/cm2 at 2.4 mA/cm2 and works stable with an area-capacity of 1.63 mAh/cm2 at 120 mA/cm2 after 500 cycles. More importantly, flexible soft-packaged AZIBs based on 3×3 cm2 high mass-loading Ov-PVO/G cathodes deliver record-high and stable electric energy beyond 21.5 mAh at different bending states. This work paves a more promising way to practical application of high-performance AZIBs.

Thermochromic Fenestration Elements Based on the Dispersion of Functionalized VO2 Nanocrystals within a Polyvinyl Butyral Laminate.

The energy required to heat, cool, and illuminate buildings continues to increase with growing urbanization, engendering a substantial global carbon footprint for the built environment. Passive modulation of the solar heat gain of buildings through the design of spectrally selective thermochromic fenestration elements holds promise for substantially alleviating energy consumed for climate control and lighting. The binary vanadium(IV) oxide VO2 manifests a robust metal-insulator transition that brings about a pronounced modulation of its near-infrared transmittance in response to thermal activation. As such, VO2 nanocrystals are potentially useful as the active elements of transparent thermochromic films and coatings. Practical applications in retrofitting existing buildings requires the design of workflows to embed thermochromic fillers within industrially viable resins. Here, we describe the dispersion of VO2 nanocrystals within a polyvinyl butyral laminate commonly used in the laminated glass industry as a result of its high optical clarity, toughness, ductility, and strong adhesion to glass. To form high-optical-clarity nanocomposite films, VO2 nanocrystals are encased in a silica shell and functionalized with 3-methacryloxypropyltrimethoxysilane, enabling excellent dispersion of the nanocrystals in PVB through the formation of siloxane linkages and miscibility of the methacrylate group with the random copolymer. Encapsulation, functionalization, and dispersion of the core-shell VO2@SiO2 nanocrystals mitigates both Mie scattering and light scattering from refractive index discontinuities. The nanocomposite laminates exhibit a 22.3% modulation of NIR transmittance with the functionalizing moiety engendering a 77% increase of visible light transmittance as compared to unfunctionalized core-shell particles. The functionalization scheme and workflow demonstrated, here, illustrates a viable approach for integrating thermochromic functionality within laminated glass used for retrofitting buildings.

Reconfigurable Optical Physical Unclonable Functions Enabled by VO2 Nanocrystal Films

Optical physical unclonable function (PUF) is one of the most promising hardware security solutions, which has been proven to be resistant to machine learning attacks. However, the disordered structures of the traditional optical PUFs are usually deterministic once they are manufactured and therefore exhibit fixed challenge-response behaviors. Herein, a reconfigurable PUF (R-PUF) is proposed and demonstrated by using the reversible phase transition behavior of VO2 nanocrystals combined with TiO2 disordered nanoparticles. Both the simulation and experiment results show that the near-infrared laser speckle pattern of the R-PUF can be almost completely altered after the phase transition of VO2 nanocrystals, resulting in a reconfigurable and reproducible optical response. The similarity of the response speckles shows an obvious hysteresis loop during the rise and drop of temperature, providing a simple way to regulate and control the response behaviors of the R-PUF. More importantly, the hysteretic characteristic provides a new dimension to describe the challenge-response behavior of the R-PUF besides the laser speckle, providing an effective way to improve the security and encoding capacity of the optical PUFs. The proposed R-PUF can be employed as a promising security primitive for high robustness and high-security authentication and encryption.

Influence of water concentration on the solvothermal synthesis of VO2(B) nanocrystals

Phase and length control of VO2(B) nanocrystals afforded by manipulating the ratio of toluene to water.

A liquid approach for the synthesis of M phase VO2 nanocrystals: Facile synthesis, characterization and reaction mechanism

Vanadium dioxide is considered to be a promising material in most fields because of its special reversible metal-insulator transition (MIT) near ambient temperature. Pure VO2(M) is usually synthesized by a hydrothermal method combined with a subsequent annealing process or one-step hydrothermal synthesis. Both synthetic routes have the disadvantages of long reaction times. In this report, a new synthesis method of VO2(M) nanoparticles was established under liquid conditions. First, the V2O4·2H2O precursor was prepared successfully by using N2H4·2HCl-reduced V2O5 under thermostatic stirring without any reagent auxiliary. Second, the V2O4·2H2O suspension in water transformed into VO2(M) nanocrystals under hydrothermal synthesis. The intermediate and final products were characterized by XRD, SEM, HRTEM, XPS and DSC. The morphology of prepared VO2(M) exhibits a snowflake-like structure (approximately 1.0–4.0 μm in length, 0.5–3.0 μm in width and 50–200 nm in thickness). The phase-transition temperature is 65.49 °C. In addition, the reaction route of "V2O4·2H2O →V2O4·xH2O → VO2(B) → VO2(M) transformation" and reaction mechanism were established by testing the precipitate at different times at the hydrothermal stage.

Carbon nanotube-mediated three-dimensional vanadium oxide nanoarchitectures with tunable morphology and translatable functionality

A carbon nanotube (CNT)-mediated three-dimensional (3D) vanadium pentoxide (V2O5) nanoarchitecture with tunable morphology and translatable electrical and electrochemical functionality is developed via the step-wise chemical vapor deposition. Controlling the pressure, gas flow, and growth time, based on the collective understanding of the vapor-solid growth mechanism, enables the tailoring of V2O5 nanostructures into diverse functional structures including nanowires, nanoribbons, and nanoplatelets, where the nanoscale topography of an underlying CNT surface facilitates the conformal 3D morphing with extended scale, density, and surface area. An in-depth analysis of the V2O5-CNT interface confirms that the highly crystalline V2O5 nanocrystals are firmly connected to the CNTs with their structural and functional characteristics maintained. Here we demonstrate that the growth process and consequently emerging functionality of the 3D V2O5/CNT hybrid architecture can be translated to various practical frameworks such as porous metallic micromesh with further enhanced surface area and electrochemical conversion. The 3D V2O5/CNT hybrid architecture with application-specific structural tunability may thus be applicable to broader materials and device systems for energy conversion, sensing, photonics, and many others.

V2O5 nanocrystals: Chemical solution synthesis, hydrogen thermal treatment and enhanced rate capability as cathode materials for lithium-ion batteries

Vanadium pentoxide (V2O5) is regarded as a promising cathode material for high-performance lithium-ion batteries (LIBs). In this study, V2O5 nanocrystals with an elongated plate-like morphology are prepared via a chemical solution approach that involves the hydrolyzation of vanadyl sulfate in alkaline solution. Oxygen vacancies are intentionally created by thermal treating the V2O5 samples under hydrogen-containing gases at different temperatures. Although the annealing process does not change the shape, morphology and crystalline structure of V2O5 nanocrystals, it does bring about a small amount of oxygen vacancies, as evidently from the results of XRD patterns and Raman spectra. The presence of oxygen vacancies has positive effects on the electrochemical properties. A higher initial discharge capacity, and excellent rate capability and cycling stability are observed on the oxygen vacancy-containing V2O5 samples. Especially, the sample annealed at 350 °C is found to have an initial capacity of 284 mAh g−1and the capacity still maintains at about 153 mAh g−1 even at 10 C. The combination of nanoscale dimension and oxygen vacancies in V2O5 nanocrystals presents a simple way to improve their rate capability and cycling stability for potential high-performance LIB applications.

Size-Controllable M-Phase VO2 Nanocrystals for Flexible Thermochromic Energy-Saving Windows

Ultrafine M-phase VO2 nanocrystals with good crystallinity are desirable for highly efficient thermochromic flexible films. Herein, we report an oxygen-controlled hydrothermal method to achieve the one-step preparation of stoichiometric and size-controllable VO2(M) nanoparticles for scaled-up production. The specific ratio of oxygen intake to V ions was first determined to guarantee the acquisition of stoichiometric VO2(M). Then, the hydrothermal conditions were optimized to perform size-controllable growth of VO2(M) nanoparticles. VO2(M) nanoparticles with average particle sizes of 25, 32, 41, and 52 nm were prepared by changing the precursor solution concentration under the determined oxygen intake/V ion ratio. Smaller VO2(M) nanoparticles exhibit better thermochromic performance because a smaller size favors light transmission and promotes blue-shifting of the local surface plasmon resonance. The flexible film fabricated with 25 nm VO2(M) nanoparticles shows an excellent thermochromic performance with a solar modulation efficiency (ΔTsol) of 19.2% and a luminous transmittance (Tlum) up to 66.42%. This work proposes a simple and effective hydrothermal process producing size-controllable VO2(M) nanoparticles with low cost and scalable production and demonstrates that the size reduction of VO2(M) nanoparticles is favorable for flexible films to exhibit excellent thermochromic properties.

Structure of V-shape twinned VO2 nanocrystals epitaxially grown on sapphire

We report on the synthesis and structure of ‘V’ shape twinned vanadium dioxide (VO2) nanocrystals epitaxially grown on c-plane sapphire substrates using a vapor transport method. The (100)M twin plane played a key role in determining the morphology of VO2 nanocrystals. The growth of VO2 nanocrystals begins at the twin plane and proceeds toward two possible monoclinic [100] (aM−axis) direction resulting in ‘V’ shape twinned crystals with the angle between the sides of approximately 115.4°. At a relatively low growth temperature of 900 °C, the growth of the sides of ‘V’ was limited producing ‘coffee-bean’ shape crystals in which flat crystal facet regions are connected with rounded edges. The twinned VO2 nanocrystals were epitaxial to the c-plane sapphire substrate with the monoclinic [010] (bM−axis) normal to the substrate. In the in-plane direction, the (001)M planes of the VO2 twin crystals were aligned to the direction ±2.3° away from the sapphire (112‾0) plane. The sides of ‘V’ exhibit a rectangular cross-section truncated by the substrate. In-situ synchrotron x-ray diffraction measurement across the metal-insulator transition of the twinned nanocrystals implies that the phase transition of the coffee-bean shape nanocrystals occurs at a lower temperature with a smaller hysteresis gap than the fully grown V-shape nanocrystals.

Hydrothermal synthesis of monoclinic vanadium dioxide nanocrystals using phase-pure vanadium precursors for high-performance smart windows

The hydrothermal synthesis of monoclinic-phase vanadium dioxide (VO2(M)) has been extensively studied for applications in energy-saving smart windows. In the majority of reported hydrothermal reactions, the reaction intermediates are first synthesized using vanadium-containing agents and additives. These vanadium precursors are then added to pressurized reactors for the hydrothermal synthesis of VO2(M). However, no studies have systematically investigated the structures and compositions of vanadium precursors used for hydrothermal syntheses. In this study, we demonstrated the single-step hydrothermal synthesis of VO2(M) nanocrystals (NCs) using phase-pure vanadium precursors obtained by size-selective purification. We observed that the structures and compositions of the vanadium precursors significantly influenced the hydrothermal synthesis products. Specifically, ammonium metavanadate and hydrazine monohydrate were reacted to yield metastable vanadium precursors that were purified via size-selective precipitation and then selectively transformed into phase-pure VO2(M), VO2(B), and (NH4)2V4O9 by hydrothermal synthesis. The obtained phase-pure VO2(M) NCs exhibited enhanced luminous transmittance (55%) and solar modulation ability (18%), the value of the latter being one of the highest reported for hydrothermally synthesized VO2(M). W-doped VO2(M) NCs showed superior phase transition behaviors to those of undoped VO2(M) NCs, while the phase transition temperature was systematically reduced depending on the W-doping concentration. In addition, we experimentally demonstrated that integrating the W-doped VO2(M) into the window of a model house reduced the in-house temperature under daytime solar radiation, which exhibits the potential of our VO2(M) films for use in energy-saving window applications.

Selective MOCVD synthesis of VO2 crystals on nanosharp Si structures

High-quality single VO2 nanocrystals and ordered arrays of VO2 nanorings were selectively synthesized by chemical vapor deposition (CVD) respectively on the tip apices and on the sidewall scallops.

Near-infrared saturable and reverse saturable absorption of ion beam synthesized VO2 nanocrystals

We investigate the nonlinear optical response of a thin film of ion-implanted VO2 nanocrystals with open aperture z-scans involving femtosecond near-infrared pulses. Beyond the established nonlinearity related to the insulator-metal phase transition of VO2, the metallic state features a pronounced saturable absorption for 100 fs pulses from a modelocked Yb:fiber source at λ = 1036 nm. In contrast, we find a pronounced reverse saturable absorption for 90 fs pulses in the telecom window at λ = 1550 nm. We attribute these nonlinearities to a transient red-shift of the plasmonic resonance of the nanocrystals, in line with the temperature dependence of the linear absorption and the theoretical expectation for electronic heating. Details of the transmissivity characteristics can be tailored by the lattice temperature and/or the size of the nanocrystals. The results hold promise for the use of VO2 nanocrystals as a saturable absorber, e.g., to mode-locked near-infrared lasers.