Growth of Rutile Titanium Dioxide Nanowires by Pulsed Chemical Vapor Deposition

T dioxide (TiO2), due to its excellent solid-state physical-chemical properties, has demonstrated a wide range of applications in hydrogen production, lithium-ion batteries, fuel cells, gas sensors, detoxification, photovoltaics, photocatalysts, and supercapacitors. The one-dimensional (1D) morphology, such as a TiO2 nanowire (NW), is considered as a superior candidate for achieving higher performance in those applications compared to the bulk form. For example, a TiO2 NW-based electrode can provide a large surface area for effectively collecting photons and/or electrons. The high crystal quality of the NWs is essential to reduce the scattering effect and hence improve the electron mobility compared to the porous TiO2 films composed of nanosized particles. In addition, using TiO2 NWs as electrodes could be beneficial to the mechanical stability of the device. Typically, TiO2 exhibits three different polymorphs (anatase, brookite, and rutile), which have different properties and result in different performance. Therefore, to synthesize TiO2 nanostructures with defined phase, shape, dimension, and high quality crystallinity is of fundamental importance for achieving desired functionality and performance. Nonetheless, a well-controlled growth of TiO2 NW is rather challenging due to the existence of multiple polymorphs and the thermodynamically unfavorable crystallography for anisotropic crystal growth, particularly with large aspect ratio. The templated sol-gel method, hydrothermal synthesis, and electrospinning process have been demonstrated for creating the NW morphology. However, phase-purity/selectivity, crystallinity, and impurity involvement are typical concerns of these methods. Post-heat-treatment is always needed to improve the crystallinity of NWs. On the other hand, high-temperature deposition can achieve high crystal quality. TiO2 NWs have also been grown by a chemical vapor deposition (CVD) process. However, large-scale, controlled synthesis of TiO2 NWs via vapor deposition is still a challenge due to the extremely low vapor pressure (10 Torr at 1577 C) and high melting point of Ti, that result in a very small and sensitive deposition condition window for the formation of the TiO2 NW morphology. 21 Recently, our group demonstrated a surface reaction-limited pulsed CVD technique that can grow highly uniform anatase TiO2 nanorods (NRs) over a large area, even inside highly confined submicrometer-sized spaces. This technique has the potential to achieve a large-scale synthesis of TiO2 1D nanostructure arrays with controlled dimensions and phases. In this paper, we report the growth of rutile TiO2 NWs through this technique in a clean and controllable manner. A bifurcated growth of TiO2 NWs was observed. The resulting TiO2 NWs exhibited curvy and high-index surfaces. Control experiments further illustrated the effect of purging time, Au coating, and temperature on the product phase and morphology. This research enriched our knowledge on the recently developed surface