Systematic Exploration of the Synthetic Parameters for the Production of Dynamic VO2(M1).

Giulia Bragaggia,Claudia Capone,Silvia Gross,Federico Zorzi,Andrea Cacciatore,Valerio Causin,Elisa Poffe

doi:10.3390/molecules26154513

Abstract

Thermochromic dynamic cool materials present a reversible change of their properties wherein by increasing the temperature, the reflectance, conductivity, and transmittance change due to a reversible crystalline phase transition. In particular, vanadium (IV) dioxide shows a reversible phase transition, accompanied by a change in optical properties, from monoclinic VO2(M1) to tetragonal VO2(R). In this paper, we report on a systematic exploration of the parameters for the synthesis of vanadium dioxide VO2(M1) via an easy, sustainable, reproducible, fast, scalable, and low-cost hydrothermal route without hazardous chemicals, followed by an annealing treatment. The metastable phase VO2(B), obtained via a hydrothermal route, was converted into the stable VO2(M1), which shows a metal–insulator transition (MIT) at 68 °C that is useful for different applications, from energy-efficient smart windows to dynamic concrete. Within this scenario, a further functionalization of the oxide nanostructures with tetraethyl orthosilicate (TEOS), characterized by an extreme alkaline environment, was carried out to ensure compatibility with the concrete matrix. Structural properties of the synthesized vanadium dioxides were investigated using temperature-dependent X-ray Diffraction analysis (XRD), while compositional and morphological properties were assessed using Scanning Electron Microscopy, Energy Dispersive X-ray Analysis (SEM-EDX), and Transmission Electron Microscopy (TEM). Differential Scanning Calorimetry (DSC) analysis was used to investigate the thermal behavior.

Highlights

Thermochromic materials represent a wide and extensive class of materials that falls within the field of the chromogenic device technology [1]
The samples were prepared via hydrothermal synthesis, since this route is wellestablished [23,24] and widely used for its flexibility in tuning different parameters such as concentration, additives, temperature, and reaction time
A pure crystalline phase of monoclinic VO2(B) was produced via a straightforward, reproducible, and scalable subcritical hydrothermal route, followed by an annealing process under nitrogen to induce the formation of the VO2(M1) polymorph

Summary

Introduction

Thermochromic materials represent a wide and extensive class of materials that falls within the field of the chromogenic device technology [1] They are used to vary the throughput of visible light for several applications, e.g., smart windows [2,3]. The atmospheric content of carbon dioxide has risen from 315 ppm at the end of the 1950s to 419 ppm in 2021, with a range of uncertainty of 0.1 ppm [7], mostly due to the burning of fossil fuels and with relevant consequences for life on Earth [8] Because of their high energy uptake, buildings represent one of the most crucial issues in this area and a large number of technologies has been explored, with particular focus on smart windows, which have variable throughput of solar energy and visible light that can lower energy consumption. The VO2(B) polymorph, space group C2/m, is an interesting cathode material for Li-ion batteries and shows a reversible structure switch from amorphous to crystalline phase under high pressure [15,16]; VO2(D), space group P2/c, is a new VO2 phase observed by Qu et al [17] that can transform to VO2(R) at about 593 K [18]

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Conclusion