Three-dimensional (3D) nanowire (NW) networks are promising architectures for effectively translating the extraordinary properties of one-dimensional objects into a 3D space. However, to uniformly grow NWs in a 3D confined space is a serious challenge due to the coupling between crystal growth and precursor concentration that is often dictated by the mass flow characteristic of vapor or liquid phase reactants within the high-aspect ratio submicrometer channels in current strategies. We report a pulsed chemical vapor deposition (CVD) process that successfully addressed this issue and grew TiO2 nanorods uniformly covering the entire inner surface of highly confined nanochannels. We propose a mechanism for the anisotropic growth of anatase TiO2 based on the surface-reaction-limited CVD process. This strategy would lead to the realization of NW-based 3D nanoarchitectures from various functional materials for the applications of sensors, solar cells, catalysts, energy storage systems, and so forth.
In summary, single-crystalline anatase TiO2 NR arrays were grown uniformly covering the entire inner surface of highly confined nanochannels by a surface-reaction-limited pulsed CVD approach. This technique applied separated exposures of gaseous TiCl4 and H2O precursors. The anisotropic growth of TiO2 crystal is believed to be the result of the combined effects of the surface-related precursor molecule absorption and reaction. The (001) surface of TiO2 crystal has also been found to be essential for the formation of NR morphology. Further in-depth understanding of the nucleation step is greatly desired for fully revealing the NR formation process. Understandings established on this surface reaction-limited CVD of TiO2 NRs would eventually allow us to realize a NW-based 3D nanoarchitecture from a variety of functional materials for the applications of sensors, solar cells, catalysts, energy storage systems, and so forth.
TiO2 NRs grown in nanochannels. (a) Schematic presentation of the pulsed CVD growth inside an AAO template, where TiO2 NRs can be uniformly grown along the entire inner channel walls. (b) Overview of a cross section of the AAO template after 660 growth cycles. (c-e) Highermagnification SEM images showing the uniform and dense coating of TiO2 NRs within the AAO channels at the top, middle, and bottom sections, respectively, as indicated by the dashed yellow boxes in (b). (f) TiO2 NRs rooted on the walls of AAO channels showing a squarelike cross section and well-faceted shape.
Growth rate and evolution of TiO2 NR structures. (a) SEM images of typical NR morphology after different growth cycles. From left to right are 85, 170, 330, 660, 900, and 1200 cycle results. (b) Plots of NR length (square symbol) and thickness (diamond symbol) vs growth cycles. The data were collected from several hundred NR samples. (c) Plots of aspect ratio of NRs (square symbol) and roughness factor (diamond symbol) vs growth cycles. The highest aspect ratio was ∼7 at 900 cycles. The maximum value of roughness factor reached ∼3000 at 660 cycles.
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