Influence of energy band alignment in mixed crystalline TiO₂nanotube arrays: good for photocatalysis, bad for electron transfer

Abstract

Despite the wide application ranges of TiO 2 , 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 TiO 2 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 TiO 2 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.

Morphology dependent electrical conduction and breakdown in single TiO2 nanotubes

“…It should be mentioned that the obtained values of E 1 and E 2 for both EGTNT and DMTNT are in decent agreement with the localized trap state of 133 meV below the CB state and activation energy of 28 meV as reported previously by Mohammadpour et al 45 and Chen et al , 46 respectively. The magnitudes of the activation energy correspond to shallow impurities, for example, oxygen vacancies, which are introduced during the growth or annealing process of the TiO 2 nanotubes.…”

“…The values of activation energies derived from the slope of ln J vs. 45 and Chen et al 46 respectively. The magnitudes of the activation energy are corresponding to shallow impurities, for example, oxygen vacancies, which are introduced during the growth or annealing process of the 47 , and Rothschild et al 48 28 there is a difference in donor density in two types of NTs.…”

Low‐Temperature Processing Methods for Tin Oxide as Electron Transporting Layer in Scalable Perovskite Solar Cells

“…[30] Notably, the density of surface/bulk energy traps in TiO 2 and SnO 2 nanostructures depends on its morphology and crystal phase and can be in the range of 10 16 to 10 19 cm À3 . [31][32][33] In contrast, SnO 2 is also a pioneering candidate as ETL in PSCs, which represents higher electron mobility than that of TiO 2 along with superior optical properties, wide bandgap, high transparency, and higher stability. [34] In practice, PSCs based on low-temperature TiO 2 (LT-TiO 2 ) ETL demonstrate incompetent efficiencies due to poor crystallinity, low electrical conductivity, and high trap sites densities.…”