Yamei Li

Core-shell VO2@TiO2 nanorods that combine thermochromic and photocatalytic properties for application as energy-saving smart coatings

Vanadium dioxide (VO2) is a Mott phase transition compound that can be applied as a thermochromic smart material for energy saving and comfort, and titanium dioxide (TiO2) is a well-known photocatalyst for self-cleaning coatings. In this paper, we report a VO2@TiO2 core-shell structure, in which the VO2 nanorod core exhibits a remarkable modulation ability for solar infrared light, and the TiO2 anatase shell exhibits significant photocatalytic degradation of organic dye. In addition, the TiO2 overcoating not only increased the luminous transmittance of VO2 based on an antireflection effect, but also modified the intrinsic colour of VO2 films from yellow to light blue. The TiO2 also enhanced the chemical stability of VO2 against oxidation. This is the first report of such a single nanoparticle structure with both thermochromic and photocatalytic properties that offer significant potential for creating a multifunctional smart coating.

Surface plasmon resonance induced excellent solar control for VO2@SiO2 nanorods-based thermochromic foils

Transition-metal oxide nanocrystals are novel candidates for being used as the hosts of localized surface plasmon resonance because they exhibit fascinating properties arising from the unique characteristics of their outer-d valence electrons. VO₂(M) nanocrystal is well-known due to its reversible metal-insulator transition (MIT) temperature near room temperature (∼68 °C) corresponding to the appearance/disappearance of localized surface plasmon resonance across the MIT. In this study, a microemulsion-based method was introduced to synthesize VO₂(M)@SiO₂ nanoparticles which were applied to prepare VO₂-based thermochromic foils owing to a strong and tunable surface plasmon resonance in the metallic state. The optical transmittance spectra demonstrates that the employment of surface plasmon resonance in VO₂-based thermochromic foils greatly improves their solar regulating efficiency up to 18.54%, and provides an unprecedented insight in optimizing VO₂-based thermochromic windows for solar control.

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 cR 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.

Synthesis of VO2 nanoparticles by a hydrothermal-assisted homogeneous precipitation approach for thermochromic applications

Thermochromic VO2 particles, which have potential applications in “smart windows” for energy saving, have been successfully prepared by a convenient route combining homogeneous precipitation and hydrothermal processes. As a result, the particle size can be easily tuned from several tens to hundreds of nanometers by controlling the initial vanadium source concentration. Lower concentration yielded large rod-like crystals, while high concentration resulted in small near-spherical nanocrystals. The decrease of the size of VO2 particles leads to an improvement in thermochromic properties, along with a wider hysteresis of the phase transition temperature. In addition, the W-doping can effectively tune the phase transition temperature (Tc) down to ambient temperature with the efficiency of about −21.3 °C per at% W in the doping range from 0 to 2.0 at% W.

Microemulsion-based synthesis of V1−xWxO2@SiO2 core–shell structures for smart window applications

Microemulsion technology was introduced to prepare V1−xWxO2@SiO2 core–shell nanostructures with various morphologies (nanorod, nanosphere, and their combination) by controlling the pH of the microemulsion. Flexible foils coated with the core–shell nanoparticles exhibited high optical performance with solar regulation efficiencies up to 12.55, 14.17, and 12.90% and fairly high visible transmittance of 53.20, 45.26 and 39.41 for 0 at%, 1 at%, and 2 at% of W-doped VO2 particles, respectively. The results suggested that the current foil was very suitable for application in smart windows. Interestingly, W doping did not deteriorate the solar regulation ability, which has not been reported before. The SiO2 shell played multifunctional roles because it can not only depress the aggregation and secondary growth of the nanoparticles during the process of annealing but also obviously enhance the thermal stability of V1−xWxO2. The amazing results provide significant progress in VO2-based thermochromic coating with a Mott phase transition temperature near room temperature and pave the way for practical application to 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.

Preparation of VxW1−xO2(M)@SiO2 ultrathin nanostructures with high optical performance and optimization for smart windows by etching

A fast, low-temperature hydrothermal method was introduced to prepare a VxW1−xO2(B) ultrathin nanostructure which can be easily transformed into VxW1−xO2(M) via a fast annealing process in an inert atmosphere. Thermochromic foils coated with ultrathin V0.98W0.02O2(M) nanopowders exhibited unsatisfactory optical properties with a weak solar regulation efficiency (ΔTsol, 4.6%) and a low luminous transmittance (Tlum-L, 15.73%) in a low-temperature state. Coating the VxW1−xO2(M) nanostructure with a thin shell of SiO2 can improve the optical performance of the thermochromic foils resulting in ΔTsol of 7.15% and Tlum-L of 25.74%. Furthermore, etching the V0.98W0.02O2(M)@SiO2 core–shell nanostructure with diluted hydrochloric acid (HCl) can optimize the optical properties of the thermochromic foils well, resulting in ΔTsol and Tlum-L of up to 10.18% and 37.37% owing to the decreased size. In summary, we employed a simple method to synthesize V1−xWxO2@SiO2 ultrathin nanostructures and provided new insight into optimizing VO2-based thermochromic windows.

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 (ΔRdif) 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.

Room temperature optical constants and band gap evolution of phase pure M 1 -VO 2 thin films deposited at different oxygen partial pressures by reactive magnetron sputtering

Spectroscopic ellipsometry study was employed for phase pure VO2(M1) thin films grown at different oxygen partial pressures by reactive magnetron sputtering. The optical constants of the VO2(M1) thin films have been determined in a photon energy range between 0.73 and 5.05 eV. The near-infrared extinction coefficient and optical conductivity of VO2(M1) thin films rapidly increase with decreasing O2-Ar ratios. Moreover, two electronic transitions can be uniquely assigned. The energy gaps correlated with absorption edge (E1) at varied O2-Ar ratios are almost the same (∼2.0 eV); consequently, the absorption edge is not significantly changed. However, the optical band gap corresponding to semiconductor-to-metal phase transition (E2) decreases from 0.53 to 0.18 eV with decreasing O2-Ar ratios.

Synthesis of NaCl single crystals with defined morphologies as templates for fabricating hollow nano/micro-structures

Hollow nanostructures have a wide range of applications in nanotechnology. To accurately fabricate such nanostructures, the first and common key step is to synthesize high quality templates with controlled symmetry and geometry, ideally through a green, efficient, and economical approach. However, this has always been a challenge up to now. In the present work, we report a conceptually new simple approach, i.e., directly synthesizing templates from NaCl solutions. The properties of the templates can be tailored by selecting appropriate organic solvents and varying the crystal growth conditions. These naturally abundant NaCl single crystal templates are water-soluble, environmentally-friendly and uniform in both geometry and size, and hence are ideal for preparing high quality hollow nano/micro structures. The new approach may have the potential to replace the conventional hard or soft template approaches. Furthermore, this work has revealed the formation mechanism of nano/micron NaCl crystals with different sizes and geometries.