Hierarchically Engineered Titanium Suboxide Films for High-Efficiency Solar Thermal Conversion

We report the development of broadband solar absorber coatings based on titanium suboxide composite thin films on aluminium substrates. The films are fabricated via scalable DC magnetron sputtering using a Ti target, followed by post-annealing in a fixed O₂ partial pressure of 0.45 mbar. By tuning deposition time and annealing temperature, a composite phase of Ti₂O₃ and TiO₂ was achieved. The Raman mapping of the films substantiates the distribution and coexistence of the two phases. The optimized sample, deposited for 10 min and annealed at 500 ᵒC, exhibited a superior solar absorptance (αₛ = 0.913) and optimally low thermal emittance (εₜ = 0.11). Nevertheless, the 15- and 20-min deposited films also showed a promising absorptance (>0.85) and emittance values (<0.13). Morphological studies revealed island-type nanostructures, leading to enhanced photothermal performance via electric field confinement, which is validated by optical simulations. This work provides a promising route toward efficient, scalable, and cost-effective spectrally selective solar absorbers for solar thermal applications.