Calculation Evidence of Staged Mott and Peierls Transitions in VO₂ Revealed by Mapping Reduced-Dimension Potential Energy Surface

Unraveling the metal–insulator transition (MIT) mechanism of VO₂ becomes tremendously important for understanding strongly correlated character and developing switching applications of VO₂. First-principles calculations were employed in this work to map the reduced-dimension potential energy surface of the MIT of VO₂. In the beginning stage of MIT, a significant orbital switching between σ-type d_<i>z</i>² and π-type d_{<i>x</i>²–<i>y</i>²}/d_<i>yz</i> accompanied by a large V–V dimerization and a slight twisting angle change opens a band gap of ∼0.2 eV, which can be attributed to the electron-correlation-driven Mott transition. After that, the twisting angle of one chain quickly increases, which is accompanied by the appearance of a larger change in band gap from 0.2 to 0.8 eV, even though orbital occupancy is maintained. This finding can be ascribed to the structure-driven Peierls transition. The present study reveals that a staged electron-correlation-driven Mott transition and structure-driven Peierls transition are involved in MIT of VO₂

Modification of Mott Phase Transition Characteristics in VO₂@TiO₂ Core/Shell Nanostructures by Misfit-Strained Heteroepitaxy

Vanadium dioxide (VO₂) is a key material for thermochromic smart windows that can respond to environmental temperature and modulate near-infrared irradiation by changing from a transparent state at low temperature to a more reflective state at high temperature, while maintaining visible transmittance. Here, we demonstrate for the first time that the Mott phase transition characteristics in VO₂ nanoparticles can be remarkably modified by misfit strains occurring at the epitaxial interface between VO₂ and the anatase TiO₂ of VO₂/TiO₂ core–shell particles. The heteroepitaxial growth of the as-synthesized particles followed an unprecedented orientation relationship, and an epitaxial growth mechanism is proposed to explain this behavior. A relatively small theoretical coherent misfit (3–11%) and a moderate heating rate (20 °C·min^–1) in the preparation of the core–shell structure were critically important from the thermodynamic and kinetic perspectives, respectively. The misfit-induced interfacial strain along the uniaxial <i>c</i>_R axis increased the transition temperatures, especially on the cooling portion of the heating–cooling cycle, leading to a notably reduced transition hysteresis loop width (from 23.5 to 12.0 °C). Moreover, the optical band gap was also engineered by the interfacial effect. Such a reduced hysteresis showed a benefit for enhancing a rapid response for energy saving thermochromic smart windows

Preparation and Characterization of Self-Supporting Thermochromic Films Composed of VO₂(M)@SiO₂ Nanofibers

Nanofibers of VO₂(A) with the diameter and length averagely at 100 nm and 10–20 μm were prepared via a facile one-step hydrothermal method by reducing NH₄VO₃ with 1,3-propylene glycol in an acidic solution. The obtained VO₂(A) was coated by SiO₂ to form VO₂(A)@SiO₂ core–shell nanocomposites, which were then transformed into VO₂(M)@SiO₂ by annealing under nitrogen atmosphere. The resulted composites maintained the original fibrous morphology, particularly with a large amount of pores emerging inside the fiber due to the volume shrinkage during the phase transition, which may improve its thermal insulation ability in real applications. The VO₂(M)@SiO₂ nanofibers were arranged into a self-supporting film by filtration, which shows excellent thermochromic properties

Functional Fiber Mats with Tunable Diffuse Reflectance Composed of Electrospun VO₂/PVP Composite Fibers

Thermochromic VO₂ nanoparticles have been dispersed into polyvinyl pyrrolidone (PVP) fibers by electrospinning of a VO₂–PVP blend solution. The structure and optical properties of the obtained composite fiber mat were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet–visible (UV–Vis) spectrophotometry, and Fourier transform infrared (FT-IR) spectroscopy. The fiber mat revealed two diffuse reflectance states in infrared spectral region at temperatures under and above the phase transition temperature of VO₂ and its IR reflectance is smaller in high temperature. The difference of diffuse reflectance between the two states (Δ<i>R</i>_dif) was obvious to be more than 25% in the wavelengths from 1.5 μm to 6 μm. The diffuse reflectance of the fiber mat could be controlled by adjusting the diameter of the fiber or the content of VO₂ in the fibers and this particular optical property was explained by a multiple scattering-absorbing process

Facile and Low-Temperature Fabrication of Thermochromic Cr₂O₃/VO₂ Smart Coatings: Enhanced Solar Modulation Ability, High Luminous Transmittance and UV-Shielding Function

In the pursuit of energy efficient materials, vanadium dioxide (VO₂) based smart coatings have gained much attention in recent years. For smart window applications, VO₂ thin films should be fabricated at low temperature to reduce the cost in commercial fabrication and solve compatibility problems. Meanwhile, thermochromic performance with high luminous transmittance and solar modulation ability, as well as effective UV shielding function has become the most important developing strategy for ideal smart windows. In this work, facile Cr₂O₃/VO₂ bilayer coatings on quartz glasses were designed and fabricated by magnetron sputtering at low temperatures ranging from 250 to 350 °C as compared with typical high growth temperatures (>450 °C). The bottom Cr₂O₃ layer not only provides a structural template for the growth of VO₂ (R), but also serves as an antireflection layer for improving the luminous transmittance. It was found that the deposition of Cr₂O₃ layer resulted in a dramatic enhancement of the solar modulation ability (56.4%) and improvement of luminous transmittance (26.4%) when compared to single-layer VO₂ coating. According to optical measurements, the Cr₂O₃/VO₂ bilayer structure exhibits excellent optical performances with an enhanced solar modulation ability (Δ<i>T</i>_sol = 12.2%) and a high luminous transmittance (<i>T</i>_lum,lt = 46.0%), which makes a good balance between Δ<i>T</i>_sol and <i>T</i>_lum for smart windows applications. As for UV-shielding properties, more than 95.8% UV radiation (250–400 nm) can be blocked out by the Cr₂O₃/VO₂ structure. In addition, the visualized energy-efficient effect was modeled by heating a beaker of water using infrared imaging method with/without a Cr₂O₃/VO₂ coating glass

Additional file 1 of Correlation study on firing temperature and color of plain pottery excavated from the Tang Dynasty tomb of Liu Jing in Shaanxi, China

Additional file 1: Table S1. Chroma values on the surfaces of thirty-two plain pottery fragments. Table S2. Chemical compositions obtained by XRF on the surfaces of thirty-two plain pottery fragments (wt%). Table S3. Color saturation values on the surfaces of replicas. Table S4. The standard deviation and error of the estimated firing temperatures of the samples fired by the loess from Yaozhou kiln site. Table S5. The standard deviation and error of the estimated firing temperatures of the samples fired by the eastern mausoleum of Qin Dynasty in Shaanxi Province. Fig S1. Porosity and average pore size of MS-07, TMS-08 and, TMS-09. Fig S2. Thermal expansion curves and the first order derivative curves of replicas fired at 600 °C with 3, 5, and 10 °C/min, respectively. Fig S3. The C*−T correlation curve of the samples fired by the loess near the eastern mausoleum of Qin Dynasty in Shaanxi Province

Ultrathin Nanoribbons of in Situ Carbon-Coated V₃O₇·H₂O for High-Energy and Long-Life Li-Ion Batteries: Synthesis, Electrochemical Performance, and Charge–Discharge Behavior

The ever-growing demands of Li-ion batteries (LIBs) for high-energy and long-life applications, such as electrical vehicles, have prompted great research interest. Herein, by applying an interesting one-step high-temperature mixing method under hydrothermal conditions, ultrathin V₃O₇·H₂O@C nanoribbons with good crystallinity and robust configuration are in situ synthesized as promising cathode materials of high-energy, high-power, and long-life LIBs. Their capacity is up to 319 mA h/g at a current density of 100 mA/g. Moreover, the capacity of 262 mA h/g can be delivered at 500 mA/g, and 94% of capacity can be retained after 100 cycles. Even at a large current density of 3000 mA/g, they can still deliver a high capacity of 165 mA h/g, and 119% of the initial capacity can be kept after 600 cycles. Importantly, their energy density is up to 800 Wh/kg, which is 48–60% higher than those of conventional cathode materials (such as LiCoO₂, LiMn₂O₄, and LiFePO₄), and they can maintain an energy density of 355 Wh/kg at a high power density of 8000 W/kg. Furthermore, based on ex situ X-ray diffraction and X-ray photoelectron spectroscopy technology, their exact charge–discharge behavior is reasonably described for the first time. Excitingly, it is found for the first time that the as-synthesized V₃O₇·H₂O@C nanoribbons are also great promising cathode materials for Na-ion batteries