Growth Mechanism of Highly Branched Titanium Dioxide Nanowires via Oriented Attachment Dongsheng Li, †,‡ Frank Soberanis, † Jia Fu, † Wenting Hou, † Jianzhong Wu, † and David Kisailus* ,† † Department of Chemical and Environmental Engineering, University of California, Riverside, California 92521, United States ‡ Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States * S Supporting Information ABSTRACT: Understanding fundamental crystal nucleation and growth mechanisms is critical for producing materials with controlled size and morphological features and uncovering structure-function relationships in these semiconducting oxides. Under hydro-solvothermal conditions, uniform branched and spherulitic TiO 2 rutile nanostructures were formed via (101) twins. On the basis of detailed, high- resolution scanning electron microscopy and transmission electron microscopy analyses, we propose a mechanism of branched growth and the (101) twin formation via oriented attachment and subsequent transformation from anatase to rutile. T he utilization of semiconducting oxide-based nanomateri- als has significantly expanded over the past few decades due to their broad application in various technologies, such as sensors, catalysts, photovoltaics, etc. 1-3 The controlled structure (i.e., size, shape, and orientation) of these nanoma- terials plays a key role in tailoring their resultant properties. 4 Thus, understanding their fundamental crystal nucleation and growth mechanisms is critical to uncovering structure-function relationships for improved device efficiency. One such semiconducting metal oxide nanomaterial, titanium dioxide (TiO 2 ), has been widely used in a variety of applications such as photocatalysis, 5,6 solar hydrogen generation, 7-9 methanol fuel cells, 9-11 anodes for lithium rechargeable batteries, 12-15 and photovoltaic cells. 16-22 It is a promising nanomaterial in one specific class of photovoltaics, sensitized solar cells, due to its low cost, chemical inertness, and photostability. A drawback of using TiO 2 -based sensitized solar cells is its low efficiency due to the recombination of electron-hole pairs in bulk and at interfaces (i.e., grain boundaries). For example, random nanoparticle networks with disordered pore structures are characterized by slow electron transport due to electron traps at contacts between particles. 23 Crystal surface orientation and shape can also affect solar cell efficiency. 24 Thus, in order to improve the efficiency of sensitized solar cells, considerable efforts have focused on synthesizing TiO 2 with various morphologies such as nanoparticles, 25 nanowires, 17 and branched nanowires. 16 Ordered nanostructures, for example, vertically aligned single crystal nanowires, could potentially lead to faster charge transport. Utilizing these nanowires helps to reduce this recombination and reduces the path length for electron extraction. Nanowires, however, have a lower surface area to volume ratio than fine-grained nanoparticles, which effectively reduces the amount of sensitizer that can be absorbed onto TiO 2 . Producing branched nanowires that afford a higher surface area would potentially improve sensitized solar cell efficiency. TiO 2 has three primary phases: rutile, anatase, and brookite (with rutile being the thermodynamically most stable phase). Its synthesis and phase development have been widely studied. 26,27 Generally, crystal growth and morphological evolution are related to interactions between ions, molecules or particles with crystal surfaces. 26,28-31 In this work, highly branched TiO 2 nanowires have been synthesized. We uncover the growth mechanism of these nanostructures based on interactions between particles and crystal surfaces, thus resolving a previously unexplained, yet commonly observed phenomena. TiO 2 branched nanowires were synthesized by a hydro- solvothermal method in sealed 23 mL Teflon-lined autoclaves. Si wafers (5 mm × 5 mm or 5 mm × 10 mm, Type P/⟨111⟩) or amorphous glass slides (used as the nanowire growth substrate) were cleaned first with acetone then 2-propanol, followed by a rinse in DI water under sonication. After one final rinse with DI water, the wafer was dried in air at room temperature immediately prior to using in the reaction. Titanium(IV) tetrabutoxide was used as the Ti source. Mixtures of water/ toluene were used as solvents, and HCl was added in order to slow the condensation of hydrolyzed Ti precursor. Table 1 lists the various reaction conditions. Briefly, titanium(IV) tetrabut- oxide was dissolved in a mixture of 0.5 mL of water and 7 mL Received: September 21, 2012 Revised: January 14, 2013 Published: January 15, 2013 Communication pubs.acs.org/crystal © 2013 American Chemical Society 422 dx.doi.org/10.1021/cg301388e | Cryst. Growth Des. 2013, 13, 422-428