The Structural, Photocatalytic and Photoelectric Properties of Oxide-Based Heterojunction Nanostructures

In order to improve the efficiency of water splitting in photocatalysis, a series of photoelectrodes based on TiO2 nanostructures were proposed in this study. The optical and photoelectric properties of these photocatalysts influenced by ionic defects and semiconductor-composite heterojunctions were investigated.
The dopant-free oxygen-deficient TiO2 nanotube arrays were prepared by electrochemical anodization in the aqueous and organic electrolytes, respectively yielding TiO2(aq) and TiO2(EG) nanotube arrays, followed by long-time annealing at four temperatures – 450, 550, 650, and 750 °C. The evolution of architectures (i.e., anatase nanotubes and rutile film) in TiO2 nanotube arrays is confirmed by XRD patterns and SEM micrographs. The depth profiles of these annealed TiO2 samples are obtained from XPS analysis, and the elemental-concentration stable zones within the TiO2 nanostructures show the approximate O/Ti atomic ratios, revealing the extent of oxygen deficiency. The TiO2(aq) samples annealed at high temperatures (i.e., 650 and 750 °C) have O/Ti atomic ratios significantly less than 2 compared to the low-temperature-annealed TiO2(aq) samples, and the TiO2(EG) samples annealed at these four temperatures show extreme O/Ti atomic ratios around 1.5, revealing that the oxygen vacancy concentration in TiO2 nanotube arrays is governed by the annealing temperature and the experimental conditions in the anodization procedure. The optical absorption spectra demonstrate quite different behavior between these two kinds of TiO2 nanotube arrays: a blue shift in absorption edge along with a notable increase in the long-wavelength absorption due to the presence of oxygen vacancies is observed in TiO2(aq) samples; on the other hand, a red shift in absorption edge and an increase in absorbance within the wavelength region of 400-600 nm both result from the carbon doping effect, and are examined in TiO2(EG) samples. For the photocurrent density measurement under controlled light irradiation, the low-temperature-annealed TiO2 samples exhibit large photocurrent responses under light sources containing UV because the high specific surface area provides a large number of active sites for chemical reactions. A strong photocurrent response is found for high-temperature-annealed TiO2 samples under filtered white light (visible light range, λ > 500 nm), which is attributed to the presence of a high concentration of oxygen vacancies.
Nanostructured composites composed of TiO2 nanotube arrays and SrTiO3 or CeO2 nanoparticles were fabricated, forming an array of TiO2(EG) nanotubes coated with SrTiO3 or CeO2 nanoparticles. The UV-Vis and UPS spectra were adopted to identify the band structures of the TiO2-SrTiO3 and TiO2-CeO2 heterojunctions. The oxygen vacancy concentration, which can be modified by adjusted the experimental parameters, in composites strongly influenced the band structure of the heterojunction and the photoelectric properties of the composite samples. Compared to the TiO2(EG) nanotube arrays, the photocurrent densities and the capability of photocatalytic water splitting for these composite samples under irradiation are enhanced because the semiconductor heterojunctions in the composites promote the separation of the photo-induced e-/h+ pairs.