Zhang Chen

Transparent wood containing CsxWO3 nanoparticles for heat-shielding window applications

CsxWO3 nanoparticles were dispersed in prepolymerized methyl methacrylate (PMMA) and filled in nanopores of delignified wood to prepare transparent wood. The wood-based composite showed excellent near-infrared (NIR, ranging from 780 to 2500 nm) shielding ability and high visible light transparency. The CsxWO3/transparent wood also showed excellent mechanical properties with a fracture strength up to 59.8 MPa and modulus up to 2.72 GPa. CsxWO3/transparent wood is expected to serve as a potential material for smart-window applications.

An intermediate phase (NH4)2V4O9 and its effects on the hydrothermal synthesis of VO2 (M) nanoparticles

The hydrothermal synthesis of VO2 (M) nanoparticles is commonly considered as a result of the transformation of intermediate phase VO2 (A) or VO2 (B). Here, we found a new intermediate phase (NH4)2V4O9 that appeared in the hydrothermal synthesis and played a crucial role in the formation of VO2 (M) nanoparticles. Then, the mechanism for the transformation of intermediate (NH4)2V4O9 to VO2 (M) was described as a self-assembly–decomposition–nucleation–growth process. Pure crystalline VO2 nanoparticles were ultimately obtained and showed excellent thermochromic properties. The phase-transition temperature Tc of VO2 (M) nanoparticles is approximately 65 °C. Furthermore, VO2 flexible foils on PET at a VO2 solid content of 1.5–2.0% show excellent optical properties with luminous transmittance (Tlum) > 50% and solar energy modulation ability (ΔTsol) > 15%.

Low temperature fabrication of thermochromic VO2 thin films by low-pressure chemical vapor deposition

VO2(M) is of special interest as the material with the most potential for future application in smart windows and switching devices. However, a number of drawbacks need to be overcome, including the high processing temperature of current synthesis techniques and low thermochromic properties. This work reports the fabrication of high-performance thermochromic VO2 thin films at low temperatures below 400 °C based on a low-pressure chemical vapor deposition (LPCVD) with a vanadium(III) acetylacetonate precursor. Proper tuning of the process parameters is found to be critical in fabricating thickness-controllable highly-crystalline VO2 films. For an ∼62 nm thick VO2 film, visible transmittances of 52.3% (annealed at 400 °C) and 52.7% (annealed at 350 °C) were obtained. The corresponding solar energy modification abilities (ΔTsol) were 9.7% and 7.1%, and the transition temperatures were 45.1 °C and 50.9 °C. The underlying microscopic mechanism was studied by first-principles calculations and the results indicated that improved performances, including a low transition temperature, could be achieved by properly controlling the annealing temperature, ascribed to the combined effect of strain and oxygen vacancies. Moreover, the initial use of a pre-grown seed layer induced fast grain growth, which is favorable for further decreasing the deposition and annealing temperature to 325 °C.

Phase and morphology evolution of VO2 nanoparticles using a novel hydrothermal system for thermochromic applications: the growth mechanism and effect of ammonium (NH4+)

To reveal the formation mechanism of VO2 nanomaterials in a hydrothermal system, an experimental method was designed to study the growth and crystallization of a VO2 nanomaterial by combining the reduction of V2O5 and homogeneous precipitation method. For Route A without the addition of ammonium, VO2 (B) nanobelts were assembled by (VO2)x·yH2O thin nano-slices, and for Route B in the presence of ammonium, the VO2 (M) nanoparticles were decomposed from (NH4)2V4O9 sheets. The ammonium solution played a crucial role in the formation of the (NH4)2V4O9 intermediate phase and finally the VO2 (M) nanoparticles. Therefore, by contrasting Routes A and B, our results revealed that the ammonium (NH4+) ion changed the reaction process and significantly influenced the preparation of well-crystallized VO2 (M) nanoparticles under hydrothermal conditions. The obtained VO2 (M) nanoparticles exhibited a high phase transition enthalpy (ΔH = 32.4 J g−1). The VO2-PET composite films that were derived from these VO2 (M) nanoparticles exhibited excellent optical switching characteristics (Tlum = 33.5%, ΔTsol = 16.0%). Moreover, W-doped VO2 nanoparticles with different W doping levels were also prepared. The efficiency of W6+ dopants to lower the transition phase temperature (Tc) was determined to occur at a rate of 19.8 K per at%.

Lowered phase transition temperature and excellent solar heat shielding properties of well-crystallized VO2 by W doping

Monoclinic VO2 (M) is a key material for energy-efficient smart window applications. Fine crystalline VO2 particles with an ambient phase transition temperature are urgently required to achieve excellent properties including high luminous transmittance and solar heat shielding ability. Moreover, the anti-oxidation ability is regarded as a significant factor which determines the lifetime of VO2-based products. In this paper, well-crystallized W-doped VO2 with low phase transition temperature, excellent solar heat shielding ability and considerable anti-oxidation ability was synthesized by a solid-state reaction process. The phase transition temperature was reduced from 67.3 °C to 10.8 °C at 2.0% W doping with an efficiency of -28.1 °C per at%. Importantly, an excellent balance between the phase transition temperature and the latent heat was obtained at high doping levels (1.5-2.0%). Furthermore, W-doped VO2 particles exhibited a significantly longer exposure time (more than 5 h) at 300 °C in air than the previously reported 2 h in the literature, and the corresponding derived composite foils showed excellent luminous transmittance and solar heat shielding properties (Tlum = 49.9% and Tsol = 44.8% for 2.0% W doping).

Solid-state-reaction synthesis of VO2 nanoparticles with low phase transition temperature, enhanced chemical stability and excellent thermochromic properties

The synthesis of VO2 has been a fundamental topic in the study of VO2-based materials for energy-saving applications. Methods including hydrothermal, sol–gel and chemical vapor deposition can be used to synthesize VO2 (M1/R). However, these techniques may usually involve some problems such as expensive raw materials, complex steps, and difficulties in controlling appropriate ratios of precursor amounts. In this study, a solid-state-reaction route was developed to prepare well-crystallized VO2 (M1/R) nanoparticles with low, variable phase transition temperature, enhanced chemical stability and excellent thermochromic properties. The phase transition temperatures ranged from 43.5 °C to 59.3 °C by regulating reaction conditions, and it could be inferred from the study of the preparation process that the amorphous phases around the crystalline VO2 phases played an important role in the decrease of phase transition temperatures compared with the reported values for bulk VO2 (68 °C). Moreover, the obtained VO2 (M1/R) nanoparticles exhibited enhanced anti-oxidation and acid-resistance abilities compared with particles prepared by hydrothermal process, and the derived flexible foils on polymer from the prepared VO2 (M1/R) nanoparticles showed excellent thermochromic properties (Tlum = 54.2%, ΔTsol = 9.2%).

Novel synthesis of pure VO2@SiO2 core@shell nanoparticles to improve the optical and anti-oxidant properties of a VO2 film

Vanadium dioxide (VO2) is a promising candidate for a solar heat controlling material that can be applied as an energy-efficient thermochromic smart window. However, drawbacks, such as chemical instability and low solar modulation of VO2, have restricted its applications in this area. The synthesis of VO2@shell is the easiest method for improving these properties. Herein, a novel moderately surfactant-free strategy is proposed for the synthesis of pure VO2@SiO2 core@shell nanoparticles to improve the optical and anti-oxidant properties of VO2 particles, which involves potential interface chemistry for the synthesis of metal oxide@shell nanoparticles in a simple fashion. In addition, a MSNs/VO2@SiO2 composite film was designed, and this film exhibited better inoxidizability, higher luminous transmittance and a larger solar energy modulation ability than the pure VO2 particles.

Crystallized TiO2 (A)–VO2 (M/R) nanocomposite films with electrochromism–thermochromism dual-response properties

Thermochromic (TC) and electrochromic (EC) smart windows are the mostly studied categories of smart window, but they usually work in different wavebands, and each have their own advantages. This work offers a method to combine the advantages of TC and EC smart windows by synthesizing TC–EC dual-response TiO2–VO2 nanocomposite films. These films were prepared by dispersing VO2 nanoparticles (NPs) in TiO2 sols and then underwent annealing. The optimized film showed a luminous transmittance (Tlum) of 66.1% at 20 °C, 59.3% at 90 °C and a ΔTsol of 11.2%. By applying a voltage of −5 V, the film showed an EC transmittance modulation (ΔT) of 19.1% at 630 nm, a combined performance of Tlum = 44.7% and ΔTsol = 17.1% (increased by 52%). The color of the films could also be modified from a yellowish brown to a light blue, which is meaningful for practical usage. The current TiO2–VO2 nanocomposite film combines TC and EC performance, which may be an important breakthrough in energy saving smart windows.

An abnormal phase transition behavior in VO2 nanoparticles induced by an M1–M2–R process: two anomalous high (>68 °C) transition temperatures

Vanadium dioxide (VO2) has a reversible metal–insulator transition (MIT) at 68 °C and can be used to develop thermally and electrically sensitive devices. In this study, an abnormal phase transition behavior of VO2 nanoparticles was discovered during the comparison of pristine nanoparticles without and with high temperature thermal treatment. The single phase transition temperature at 65.1 °C for the pristine VO2 nanoparticles split into two temperatures at approximately 74 °C (T1) and 84 °C (T2) after thermal treatment at 400 °C for 6 h. Both temperatures are much larger than 68 °C. Through characterization by Raman and transmission electron microscopy (TEM), the two higher transition temperatures could be well explained by the formation of VO2 (M2). Grain boundaries were observed during the merger and fusion processes of VO2 nanoparticles at high temperatures. The grain boundaries and interfacial defects resulted in the dislocation of the lattice structure and produced stress and strain in the VO2 nanoparticles. Consequently, VO2 (M2) with a higher temperature formed in the heating process and the initial MIT (M1–R) became an M1–M2–R transition. Moreover, the thermal treatment improves the phase transition enthalpy (ΔH) of VO2, which promotes the increase in the solar modulation ability (ΔTsol) of VO2–PET composite film from 12.8% to 15.2–17.0% without loss in the luminous transmittance. These findings are of great significance to the deep understanding of MIT and the development of VO2 smart windows.

VO2(D) hollow core–shell microspheres: synthesis, methylene blue dye adsorption and their transformation into C/VOx nanoparticles

In this work, we report the fabrication of novel VO2(D) hollow core–shell microspheres (VO2(D)-HCSMs) via a facile hydrothermal method. Microscopic examination revealed that the VO2(D)-HCSMs consisted of numerous nanocrystals and had an average size of 1–10 μm and a shell thickness of 300 nm. An inside-out Ostwald ripening process was responsible for the formation of the VO2(D)-HCSMs. Meanwhile, citric acid played dual roles, acting as a reductant for reducing vanadium pentoxide and as a morphology directing agent for producing core–shell microspheres. The resulting VO2(D)-HCSMs exhibited excellent adsorption performance with a maximum adsorption capacity of 97.5 mg g−1 for methylene blue (MB). The C/VOx particles were generated by the calcination of the VO2(D)-HCSMs with adsorbed MB dye at 250 °C in air for 4 h, and the microspheres showed enhanced adsorption capacity and good reusability (over 99% MB removal after four cycles) because of the existence of amorphous carbon nanowires, making them a potential adsorbent for removing organic dyes from wastewater.