Fabrication And Photoelectrochemical Performance Of Titanium Dioxide Phase Junction

Titanium dioxide（TiO₂） has found wide applications in photo（electro）catalytic water splitting, solar cells and organic pollutant treatment, due to its physical and chemical stability, environmental friendly and low cost. In recent years, accumulated evidence demonstrate that TiO₂ surface junctions are beneficial for increasing the photoconversion efficiency. Various junctions have been introduced and explored, such as schottky junctions, heterojunctions, phase junctions, and the surface disordered layers. Although surface junctions are supposed to enhance charge separation, fabrication of devices for mechanism study and performance evaluation have still been largely unexplored.TiO₂ nanorod arrays were prepared as the buildblocks for fabrication of rutile/anatase junctions and crystalline rutile/disordered surface layer junctions. These junctions were further used to fabricate devices for photovoltaic devices and UV detectors.A protype photovoltaic（PV） device was fabricated based on TiO₂ rutile/anatase phase junctions. The anatase@rutile quasi-coaxial nanorod arrays were prepared with a hydrothermal method for rutile nanorod arrays on FTO substrate, and followed with an anatase layers through magnetron sputtering deposition. The PV devices were finally assembled with a layer of ITO deposited over anatase layer. The FTO and ITO act as the positive and negative electrode respectively. The PV tests showed the PV effects for the TiO₂ phase junction device whereas the single phase device performed as photoresistors without any PV effect. The opencircuit voltage of the phase junction device was 0.18 V and the short circuit current density was 114 μm/cm². Tests to the light illumination direction susceptablility and the positive or negative electrodes showed that the charge transfer direction between the anatase and rutile phase junction remained unchanged, providing compelling evidence that the photogenerated electrons always transfered from rutile to anatase, and at the same time the holes accumulated in the rutile side.We modified the rutile TiO₂ nanorod arrays（NRAs） with electrochemical reduction, and found that a special junction formed in the form of surface disordered layer over the bulk crystalline rutile phase when reduced under-1.6 ^-3 V with a three electrode system. After reduction, the NRAs were found to turn from white into dark gray and the nanorods of reduced NRAs（R-NRAs） were capped with a disordered shell, and the thickness of the shell increased with decreasing voltage. The photocurrent of the R-NRAs（reduced under-1.8 V for 0.5 s） was already higher than that of the pristine NRAs; when reduced under-1.8 V for 2 s, it was 2.2 times of the pristine NRAs and remained constant under continuous illumination for 5 h. Photoelectrochemical and electrochemical characterization showed that the surface disordered layer not only enhanced the charge separation but also promoted surface water oxidation.The TiO₂ surface junctions for the self-powered UV photodetectors were explored. The short circuit current of the rutile/anatase phase junction device was as high as 1.7 m A/cm², indicating the junction-promoted separation of photogenerated charge carriers without applied bias. The responsivity of the phase junction device was 0.085 A/W（385 nm UV illumination）, which was higher than that of most of the UV detectors reported. Tests under different wavelengths and modulated illumination showed outstanding visible blind characteristics and ultrafast response of 4 μs.Moreover, the R-NRAs used as the photoelectrochemical UV photodetectors were studied. The NRAs, Pt and I-/I3- were set as the photoanode, counter electrode and electrolyte respectively. The results indicated that the surface disordered layer enhanced the charge separation and the lifetime of the photogenerated electrons, thus reduced the response time and enhanced the photo conversion efficiency. The IPCE of the UV detector prepared with R-NRAs increased to 23% compared with 10% of the NRAs device.