Hf doping effect of tungsten bronze Cs_0.32WO₃ on tuning transparent thermal insulation performance for energy saving

Cesium tungsten bronze (Cs_0.32WO₃) is a unique energy-saving material known for its exceptional visible transmittance and near-infrared (NIR) absorption. It has been demonstrated that ion-doping modification is a successful strategy for enhancing performance. However, the modification mechanism of the ion substitution at the W site remains unclear. In this study, we conducted Hf doping modification to investigate its effect and understand the modification mechanism. A series of Hf-doped Cs_0.32WO₃ (Hf-Cs_0.32WO₃) nanopowders were synthesized using the solid-state method. The doping of 1 % Hf elements improves both integral visible transmittance (T_lum, 380–780 nm) and near-infrared shielding efficiency (Ψ_NIR, 780–2500 nm) of Cs_0.32WO₃, achieving 47.9 % of T_lum and 72.0 % of Ψ_NIR. The improved T_lum can be attributed to the broadened bandgap (E_g) induced by Hf doping, and the higher oxygen vacancy content is responsible for the enhanced Ψ_NIR. The additional oxygen vacancies contribute free electrons and introduce defect energy levels, enhancing NIR absorption and compensating for the negative effect of the reduced concentrations of W⁵⁺ ions and free electrons caused by Hf doping. This study shows that the transparent thermal insulation performance of Cs_0.32WO₃ can be improved by suitable low-valence ion doping, thereby widening the research perspective for designing new cesium tungsten bronze NIR shielding materials.