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The recent discovery of “black” TiO₂ nanoparticles with visible and infrared absorption has triggered an explosion of interest in the application of TiO₂ in a diverse set of solar energy systems; however, what a black TiO₂ nanoparticle really is remains a mystery. Here we elucidate more properties and try to understand the inner workings of black TiO₂ nanoparticles with hydrogenated disorders in a surface layer surrounding a crystalline core. Contrary to traditional findings, Ti³⁺ here is not responsible for the visible and infrared absorption of black TiO₂, while there is evidence of mid-gap states above the valence band maximum due to the hydrogenated, engineered disorders. The hydrogen atoms, on the other hand, can undergo fast diffusion and exchange. The enhanced hydrogen mobility may be explained by the presence of the hydrogenated, disordered surface layer. This unique structure thus may give TiO₂ …