Shidong Ji

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.

Vanadium Dioxide Nanoparticle-based Thermochromic Smart Coating: High Luminous Transmittance, Excellent Solar Regulation Efficiency, and Near Room Temperature Phase Transition

An annealing-assisted preparation method of well-crystallized VxW1-xO2(M)@SiO2 core-shell nanoparticles for VO2-based thermochromic smart coatings (VTSC) is presented. The additional annealing process reduces the defect density of the initial hydrothermally prepared VxW1-xO2(M) nanoparticles and enhances their crystallinity so that the thermochromic film based on VxW1-xO2(M)@SiO2 nanoparticles can exhibit outstanding thermochromic performance with balanced solar regulation efficiency (ΔTsol) of 17.3%, luminous transmittance (Tlum) up to 52.2%, and critical phase transition temperature (Tc) around 40.4 °C, which is very promising for practical application. Furthermore, it makes great progress in reducing Tc of VTSC to near room temperature (25.2 °C) and simutaneously maintaining excellent optical properties (ΔTsol = 14.7% and Tlum = 50.6%). Such thermochromic performance is good enough to make VTSC applicable to practical architecture.

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.

The preparation of a high performance near-infrared shielding CsxWO3/SiO2 composite resin coating and research on its optical stability under ultraviolet illumination

Cesium-doped tungsten bronze CsxWO3 is an ideal near infrared (NIR) shielding material for solar filters. However, the CsxWO3 nanoparticle dispersed resin film exhibits obvious photochromic behavior under ultraviolet (UV) illumination, which severely hinders the commercial application of CsxWO3. In this paper, CsxWO3/SiO2 composite resin coatings were successfully prepared with excellent NIR shielding ability and outstanding optical stability under strong UV illumination by combining an ultraviolet-absorbing agent (UVA) with SiO2. The roles of UVA and SiO2 in preventing the photochromic behavior of the CsxWO3/SiO2 composite resin coating were vital, reducing the photochromic reaction rate by UVA and limiting the photochromic reaction areas by SiO2. In sum, our research provides a new solution for eliminating the photochromism of a CsxWO3-based resin film and it may have practical application in architecture and automobiles.

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.

Template-free formation of various V2O5 hierarchical structures as cathode materials for lithium-ion batteries

Various V2O5 hierarchical structures were successfully synthesized via a template-free method by annealing diverse morphological VO2 sub-microspheres which can be facilely tailored by adjusting the solvothermal reaction duration. The VO2 sub-microspheres undergo a solid → yolk–shell → hollow → yolk–shell structure process with increasing time, which is believed to result from an unusual Ostwald-ripening process. After the annealing process, multi-structural VO2 sub-microspheres changed into hierarchical structures including fist-type structures consisting of nanorods, yolk–shell and hollow sub-microspheres composed of nanorods and a yolk–shell construction made up of nanoplates. As the cathode materials for lithium-ion batteries, among them, yolk–shell sub-microspheres comprised of nanoplates exhibited high reversible capacity, excellent cycling stability at high currents and good rate capacities. Without doping and compositing, the electrode delivered reversible capacities of 119.2 and 87.3 mA h g−1 at high current densities of 2400 and 3600 mA g−1, respectively, as well as a capacity retention of 78.31% after 80 cycles at 1200 mA g−1. The excellent electrochemical performance could be attributed to the purity of the phase and synergistic effect between the yolk–shell structure and hierarchical structure of the sub-microspheres, which make the yolk–shell V2O5 hierarchical structure a promising candidate for the cathode material for lithium-ion batteries.