Yijie Zhou

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.

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.

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 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.

CsxWO3 nanoparticle-based organic polymer transparent foils: low haze, high near infrared-shielding ability and excellent photochromic stability

Well-dispersed cesium-doped WO3 (CWO) nanocrystals are an attractive candidate as hosts that exhibit excellent near infrared-shielding ability owing to two kinds of absorptions: localized surface plasmon resonance absorption around 900 nm and small polaron absorption near 1800 nm. In this work, a simple thermal reduction method was introduced to synthesize a CWO powder, which was further dispersed by bead milling to obtain a CWO nanodispersion liquid. The CWO nanodispersion liquid can be employed to make a unique nonlinear optical foil with 70% visible transmittance and a near infrared transmittance less than 10%, while maintaining a high transparency with less than 1.0% haze. However, the main issue concerning CWO nanoparticle-based polymer foils is the detrimental photochromic effect under strong ultraviolet (UV) irradiation, which hampers their extensive application in energy-saving windows, such as in automobiles and architecture. Herein, the photochromic effect is largely relieved by embedding CWO nanoparticles into inert polyethylene terephthalate (PET) instead of mixing with resins like polyurethane or acrylic resin, and the final organic–inorganic nonlinear optical foils exhibit almost no coloration under strong UV illumination. Therefore, this study provides a new insight into making hybrid foils with high stability, transparency and multifunctionality in the optical and magnetic fields.

Core-regenerated vapor–solid growth of hierarchical stem-like VOx nanocrystals on VO2@TiO2 core–shell nanorods: microstructure and mechanism

This paper presents a novel vapor–solid growth approach of stem-like 1D vanadium oxide nanostructures in a core-regenerated fashion on VO2@TiO2 core–shell rods under a very low temperature (400 °C). The composition and microstructure of the as-grown nanostructures were analyzed by HRTEM, EDS and XPS, confirming the vapor–solid mechanism. The confinement effect of the shell was analyzed by varying experimental parameters (coating thickness and heating rate), and the shell was found to play a critical role in generating and accumulating vapor with supersaturated concentration for the nanocrystal nucleation. Additionally, PVP was added as a surface modifier to favor the growth, probably by forming carbonaceous/nitrogen gaseous species as carrier media, as deduced by FTIR analysis. By exploring the impact of different heating rates, we found that the mass transfer process was facilitated only using a relatively high heating rate (>1.33 °C s−1). However, such a growth was still limited only within the early stages (~15 min), because the confined vacuum interface will disappear with the progress of the crystallization of the nanoporous anatase shell. A unique 1D rods to 2D nano-flakes evolution can be achieved by increasing the heating rate from 1.3 °C s−1 to over 8 °C s−1. These findings provide a valuable understanding on the thermodynamics and kinetics during an unique VS based nanocrystal growing process, and also showed a new method to obtain hierarchical heterogeneous nanostructures.

Organic–inorganic hybrid optical foils with strong visible reflection, excellent near infrared-shielding ability and high transparency

Research on functional flexible films has recently been attracting widespread attention especially with regards to foils, which can be designed artificially on the basis of the practical requirements. In this work, a foil with high visible reflection and a strong near infrared shielding efficiency was prepared by a simple wet chemical method. In the process of making this kind of optical foil, emulsion polymerization was first introduced to synthesize polymer opals, which were further compressed between two pieces of polyethylene terephthalate (PET) foil under polymer melting temperature to obtain a photonic crystal film with a strong reflection in the visible region to block blue rays. The following step was to coat a layer of the inorganic nano paint, which was synthesized by dispersing Cs-doped WO3 (CWO) nanoparticles homogenously into organic resin on the surface of the PET to achieve a high near infrared shielding ability. The final composite foil exhibited unique optical properties such as high visible reflectance (23.9%) to block blue rays, and excellent near infrared shielding efficiency (98.0%), meanwhile it still maintained a high transparency meaning that this foil could potentially be applied in energy-saving window films. To sum up, this study provides new insight into devising flexible hybrid films with novel optical properties, which could be further extended to prepare other optical films for potential use in automobile, architectural and other decorative fields.

Scalable Preparation of Photochromic Composite Foils with Excellent Reversibility for Light Printing

Photochromic inks for repeatable light-printed media have attracted increasing attention owing to the fact that they may be widely applied to reduce the consumption of papers and plastics and conserve the environment. Therefore, it is of practical significance to develop convenient photochromic inks with a low cost and on a large scale. In this study, a simple one-step hydrothermal route was used to prepare tungsten trioxide (WO3 ) nanoparticles, which were further used to make photochromic inks and transparent photochromic films. The obtained transparent photochromic film could rapidly respond to UV light within tens of seconds, then return to its initial state, with different recovery times at different temperatures, and also exhibit good reversible coloration-bleaching effect. A typical polyethylene terephthalate (PET) foil coated with the photochromic ink could also be repeatedly printed with various patterns and displayed excellent rewritable performance over tens of cycles. This study proposes a simple method for widespread applications of WO3 -based photochromic inks.