# 6 H Hydrothermal Synthesis of W-Doped VO2(M) for Smart Windows in Tropical Climates

## TL;DR

A fast 6-hour method to make thermochromic VO2 material is developed, suitable for smart windows in tropical climates.

## Contribution

A rapid hydrothermal synthesis of W-doped VO2(M) without annealing is introduced, lowering transition temperatures.

## Key findings

- VO2(M) was synthesized in 6 hours at 270°C with pH ≈ 8.5, avoiding long reaction times.
- Tungsten doping reduced the transition temperature by ~17°C per weight percent.
- The resulting powders are suitable for dispersion in polymer films for smart window applications.

## Abstract

What are the main findings?
Rapid 6 h hydrothermal synthesis of VO2(M)pH ≈ 8.5 enables phase stabilizationW-doping lowers Tc by ~17 °C per wt.%Crystallites below 35 nm without annealingPowders suitable for dispersion processing

Rapid 6 h hydrothermal synthesis of VO2(M)

pH ≈ 8.5 enables phase stabilization

W-doping lowers Tc by ~17 °C per wt.%

Crystallites below 35 nm without annealing

Powders suitable for dispersion processing

What are the implications of the main findings?
Enables low-cost thermochromic coatingsScalable route for smart-window materialsFacilitates integration into polymer films

Enables low-cost thermochromic coatings

Scalable route for smart-window materials

Facilitates integration into polymer films

Thermochromic smart windows are a promising technology to reduce energy consumption in buildings, particularly in tropical regions where cooling demands are high. Vanadium dioxide (VO2) is the most studied thermochromic material due to its reversible semiconductor-to-metal transition near 68 °C. Conventional synthesis routes require long reaction times and post-annealing steps. In this work, we report a rapid hydrothermal synthesis of monoclinic VO2(M) and tungsten-doped VO2(M) powders obtained within only 6 h at 270 °C, using vanadyl sulfate as precursor and controlled precipitation at pH ≈ 8.5. Differential scanning calorimetry confirmed the reversible transition at 59 °C for the undoped VO2, with a hysteresis of 18 °C, while tungsten doping reduced the transition temperature by ~17 °C per wt.% of W. X-ray diffraction verified the monoclinic phase with minor traces of VO2(B), a non-thermochromic polymorph of VO2, and microstructural analysis revealed crystallite sizes below 35 nm. Electron microscopy and dynamic light scattering confirmed particle sizes suitable for dispersion in polymeric matrices. This approach significantly reduces synthesis time compared to typical hydrothermal methods requiring 20–48 h and avoids further annealing. The resulting powders provide a low-cost and scalable route for fabricating thermochromic coatings with transition temperatures closer to ambient conditions, making them relevant for smart-window applications in tropical climates, where lower transition temperatures are generally regarded as beneficial.