Highly Stable and Reversible Coloration of Oxide Nanoparticles Film by Ultrafast‐CW Laser Induced Band Engineering

Abstract Coloration in oxides meets the challenge in achieving high stability and repeatability simultaneously. In this work, reversible coloration of TiO₂ nanoparticles (NPs) film is demonstrated by alternate ultrafast (UF) laser and continuous wave (CW) laser irradiation. Under UF laser irradiation, oxygen vacancies are introduced in the TiO₂ lattice, which lead to the bandgap reduction (2.73 to 1.50 eV) and surface blackening. The blackened area can achieve high resolution with size down to ≈1.1 µm. This coloration in TiO₂ also shows excellent thermal stability that color fading is negligible even after thermal annealing at 700 K for 2 h in air. The generated high temperature by CW irradiation is known to be beneficial for the O₂ dissociation on defected TiO₂, thus promoting the internal oxygen diffusion to eliminate the vacancies. The energy band structure of TiO₂ can be recovered accordingly, and decoloration is achieved. TiO₂‐NPs film shows stable microstructures with barely changed optical properties after 12 cycles of coloration–decoloration process. This band engineering‐induced reversible coloration can also be applicable for broad oxides, e.g., ZnO, HfO₂, and Ga₂O₃. Such highly stable and reversible coloration is promising in high‐performance micro/nano‐devices development for color display and optical encryption.