Influence of energy band alignment in mixed crystalline TiO2 nanotube arrays: good for photocatalysis, bad for electron transfer

Abstract

Despite the wide application ranges of TiO2, the precise explanation of the charge transport dynamic through a mixed crystal phase of this semiconductor has remained elusive. Here, in this research, mixed-phase TiO2 nanotube arrays (TNTAs) consisting of anatase and 0–15% rutile phases has been formed through various annealing processes and employed as a photoelectrode of a photovoltaic cell. Wide ranges of optoelectronic experiments have been employed to explore the band alignment position, as well as the depth and density of trap states in TNTAs. Short circuit potential, as well as open circuit potential measurements specified that the band alignment of more than 0.2 eV exists between the anatase and rutile phase Fermi levels, with a higher electron affinity for anatase; this can result in a potential barrier in crystallite interfaces and the deterioration of electron mobility through mixed phase structures. Moreover, a higher density of shallow localized trap states below the conduction band with more depth (133 meV in anatase to 247 meV in 15% rutile phase) and also deep oxygen vacancy traps have been explored upon introducing the rutile phase. Based on our results, employing TiO2 nanotubes as just the electron transport medium in mixed crystalline phases can deteriorate the charge transport mechanism, however, in photocatalytic applications when both electrons and holes are present, a robust charge separation in crystalline anatase/rutile interphases will result in better performances.