Structural characterizations of photo-catalytic titanium oxide nanoparticles made from amorphous building blocks

TiO₂ [titanium dioxide] nanoparticles (NPs) exhibit a variety of properties substantially departed from the optical and catalytic properties of the known bulk phases. A special interest in the engineering of functionality is how structures and bandgaps interact to enhance the photo-activity of TiO₂. However, fundamental understanding of bandgap-related changes is difficult at these length scales because of scarcity of methods capable of probing the bandgap and the structure of isolated small particles. In this research, we adopt a series of highly spatially resolved transmission electron microscopy (TEM) techniques to characterize the structures of amorphous TiO₂ nanoparticles (NPs) used as precursors and the TiO₂ NPs with narrowed band gap derived from these amorphous precursors. The results show: (1) The amorphous TiO₂ precursors consist of TiO₆ [titanium atom surrounded by six oxygen atoms] octahedra randomly connected with each other; (2) An unconventional phase transformation occur wherein anatase and TiO₂(B) coexist when annealing the amorphous TiO₂ NPs in a temperature range from 400°C to 900°C in oxygengas; (3) Chromium and nitrogen co-doped TiO₂ NPs have significantly narrowed band gaps originating from extra states on the top of valence band; (4) Black rutile NPs are produced by annealing amorphous TiO₂ NPs at 700 °C in argon gas, which have a (crystalline TiO₂) core-(amorphous Ti₂O₃ [titanium sesquioxide]) shell structure. Formation of Ti₂O₃ originates from diffusion of oxygen vacancies towards vacuum in the amorphous precursors; (5) The black anatase NPs are produced by annealing amorphous TiO₂ NPs at 400 °C in argon gas, whose structures are (i) core-shell, or (ii) randomly distributed phases. These two structures originate from different degrees of crystallization and diffusion rate of oxygen vacancies; (6) Cubic-TiO₂ is induced by the electron beam, which originates from the migration of interstitial Ti atoms grown on a rutile template; (7) We also discuss the origins of deviations from known phases’ EELS spectra which is the most important tool of electronic structure characterizations in this research; (8) Finally, we found a group of intense peaks in the interface between silicon and TiO₂. These peaks may originate from interface plasmon excitations.