Enhanced UV Response of Single Anodic TiO2Nanotube: Effect of Water-Modified Microstructures

Abstract

Water-modified ultraviolet (UV) light response of single anodic TiO2 nanotube (NT) is reported in this article. After keeping in deionized water at room temperature, the original smooth surface of amorphous TiO2 NTs is roughened and even changes into sponge-like structure by stacking of further oxidized and crystallized nanodots (NDs) through water-driven surface dissolution process. Two-dimensional variable range hopping (2D-VRH) model is recognized to interpret low-temperature electrical transport properties for the original NT, and the water-modified NT shows deviation from this mechanism owing to the additional electronic tunneling through the surface-crystallized NDs assemblies. The resistance in dark is increased about one order after water-modification due to surface carrier depletion by nanocontacts between crystalline NDs and the original amorphous core. The UV sensitivity S (I-UV/I-dark) of as-prepared single NT is similar to 1.6, and response and recovery times are about 0.39 and 0.45 s, respectively. S is increased to similar to 21.7 after surface water-modification, whereas response and recovery times are increased to 7.1 and 7.5 s, respectively. Surface water-modification will reduce the density of recombination centers and increase the trap state density of carriers in TiO2 NT due to surface charge-separation effect, both leading to the larger UV sensitivity and response time. Single anodic TiO2 NT shows no persistent photocurrent, and it can be applied as high-speed, high-sensitivity UV optoelectronic nanodevices.