Growth Mechanism of Highly Branched Titanium Dioxide Nanowires

Communication pubs.acs.org/crystal

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 TiO2 rutile nanostructures were formed via (101) twins. On the basis of detailed, highresolution 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.

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effectively reduces the amount of sensitizer that can be absorbed onto TiO2. Producing branched nanowires that afford a higher surface area would potentially improve sensitized solar cell efficiency. TiO2 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 TiO2 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. TiO2 branched nanowires were synthesized by a hydrosolvothermal 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) tetrabutoxide was dissolved in a mixture of 0.5 mL of water and 7 mL

he utilization of semiconducting oxide-based nanomaterials 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 nanomaterials 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 (TiO2), 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 TiO2-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 TiO2 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 © 2013 American Chemical Society

Received: September 21, 2012 Revised: January 14, 2013 Published: January 15, 2013 422

dx.doi.org/10.1021/cg301388e | Cryst. Growth Des. 2013, 13, 422−428

Crystal Growth & Design

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Table 1. Experimental Reaction Conditionsa reaction code

titanium tetrabutoxide (mol/Lb)

HCl (37 wt %) (mL)

toluene (mL)

DI water (mL)

thioureac (mol/L)

A B C

0.05 0.1 0.2

2 2 2

7 7 7

0.5 0.5 0.5

0.1 0.1 0.1

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RESULTS AND DISCUSSION

Phase and Morphology. There were no morphological differences between TiO2 structures grown on Si wafers and those on amorphous glass. With the exception of the 1 h duration reactions (which yielded little product), XRD (Figure 1) clearly demonstrates that samples are composed of rutile TiO2, increasing in crystallinity with longer reaction duration (as observed by the decrease in the full width at half-maximum, fwhm, of the (110) rutile reflection). The intensity ratio of the (002) reflection to the (110) peak dramatically decreases with time (see for example, samples C1.5−C12 in Figure 1). This indicates that at early growth stages, the wires grow predominately along the [001] direction. With increasing reaction times, there is increased branching (see Figure S1) and the wire growth rate along the [001] direction decreases and mainly grows along other directions (e.g., the ⟨110⟩ direction). This result is consistent with TEM analysis in the following discussion. SEM micrographs of corresponding nanostructures (Figure 1) demonstrate the morphological features of TiO2 nanostructures synthesized for 12 h under low, intermediate, and high precursor concentrations. At low Ti concentrations, 8−10 μm elongated TiO2 nanostructures are formed, consisting of a core nanowire with thin, lathe-like primary branches growing from the core nanowire facets. With increasing Ti concentration, there is a significant increase in the density of these branches with a concurrent lengthening from the core nanowire. At high Ti concentrations, the large branch density leads to dense spherulitic structures. Branched Structure Analysis. SEM analysis of branched TiO2 nanostructures (Figure 2a, sample A3, and Figure 2b, sample A6) reveal the core nanowire running the entire length of the structure with primary branches growing at approximately 70° from its surface. The cores of the main wires, which are 250−300 nm wide, are composed of multiple single crystal nanowires (ca. 10−20 nm diameter, as shown in the SEM and

a

Samples are listed based on reaction code (e.g., A, B, C) followed by reaction duration. For example, A6 stands for experiment conducted at reaction condition A for 6 h. All synthesis reactions were performed at 180 °C. bThe titanium tetrabutoxide concentration is calculated as moles of Ti ions in the mixed solvent of toluene and DI water. cThe thiourea was used in the reaction solution; however, we found no effect of thiourea on the branch formation mechanism.

of toluene with 2 mL of a 37% solution of HCl. A cleaned Si wafer was placed at the bottom of the reaction solution with the polished side facing up before heating the solution to collect nanowires products for SEM characterization after reaction. Amorphous glass slides (5 mm × 10 mm) were also placed in the reaction solution as a control to differentiate the effect of a crystalline surface as a substrate for TiO2 nanowire growth. Syntheses were conducted at 180 °C for 0.5−12 h durations. Nanowires were collected from the top surfaces of Si wafers in the reaction vessels, washed three times with DI water, and dried in an oven at 40 °C overnight. Phase analysis was performed using X-ray diffraction (XPERT MPD powder diffractometer (PANalytical B.V., Netherlands), using Cu Kα radiation (λ = 1.5405 Å). Structural analysis was performed using transmission electron microscopy (TEM, FEI Titan 300 kV FEG TEM/STEM System w/EDS & EELS, Netherlands) and scanning electron microscopy (SEM, Carl Zeiss 1540XB, Germany).

Figure 1. SEM micrographs and XRD of TiO2 nanostructures synthesized at 180 °C under reaction conditions of A, B, and C. Left and middle columns are high and low magnification SEM micrographs, respectively, for sample A12 (a and b), B12 (c and d), and C12 (e and f). Right column: XRD patterns of TiO2 nanocrystals synthesized at 180 °C under conditions of A, B, C for 1.5 h, 3 h, 6 h, and 12 h. 423



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nanowire highlighting one branch that corresponds to the SADP. Electron diffraction analysis reveals that the branch and the main wire (both growing along the [001] direction) are related by twinning on the (101) plane. These branches grow in the (010) plane of the main wire, and the angle between the primary branch and the main wire is 65.57°, which is similar to our SEM observations. Growth Mechanism. The mechanism of the branching of TiO2 nanowires was uncovered through a series of reaction time studies (i.e., 1 h, 1.5 h, 3 h, and 6 h for reaction conditions A, B, and C). SEM micrographs provided in Figures S1 and S2 (in the Supporting Information) present the growth sequence of the branched nanowires and clearly show an increase in branch density as a function of time and concentration of Ti precursor. SEM and TEM analyses (Figure 3) of reactions carried out for short durations (i.e., 1 h) at 180 °C for low, intermediate, and high Ti precursor concentrations uncover fine, nanoparticulate-based whiskers growing on the sides of nanowires. SEM analysis (Figure 3a−f) of these structures clearly demonstrates an increase in branching with increasing concentration of Ti precursor. HRTEM analysis (Figure 3g,h) indicates that these fine particles are anatase TiO2. With increasing reaction duration, these anatase TiO2 nanoparticles coincide with the tips of the nanowires, and they likely undergo a solid-state transformation to rutile, resulting in both rutile nanowire growth and the formation of branches. Sample A6 (Figure 2e) clearly demonstrates that the main wires of the TiO2 structure are thicker in the center and become narrower toward both ends of the wire. The branches located in the middle of the main wire are thicker and longer, and they become narrower and shorter toward the ends of the main wire, indicating that growth of the main wire and branching occurred simultaneously. SEM imaging of wires grown at different times and concentrations show that the density of wires increases (Figure S1) with increasing Ti precursor concentration. On the basis of the measurements of the average distance between primary branches for samples reacted for 3 h, an increase in Ti concentration yields structures with a decreased distance between branches (i.e., 0.55 μm, 0.38 μm, and 0.27 μm for samples A3, B3, and C3, respectively). At higher concentrations of Ti precursor, more nuclei consisting of anatase particles were produced, increasing the probability of a branching event. HRTEM imaging (Figure 3g,h) of sample B1 indicates that the nanowire whisker tips consist of {101} planes, and the crystal facets (103) of the anatase nanoparticle are aligned with the (101) face of the rutile crystal. A selected area electron diffraction (SAED) pattern of sample A1 shows very weak intensity reflections from anatase (shown in Figure S2 in Supporting Information). Estimation of the surface energies29 suggests that this spherical-like anatase structure has {100}, {221}, and {103} exposed planes,24 although we do not observe anatase particles with {221} facets. To gain insight into the interaction between the surfaces of rutile and anatase nanoparticles, we used the Matsui-Akaogi (MA) force field to estimate the potential energies of (10̅ 1) and (101) facets of rutile with (100), (221), (103), and (1̅03) facets of anatase at low and high concentrations of the titanium precursor. The MA force field was developed for titanium oxide in a vacuum. We assume that this force field is also applicable to the interactions between titanium and oxygen atoms in an aqueous environment, provided that we account for the ionic screening and the solvent effects on electrostatic and van der Waals interactions.

Figure 2. (a and b) Top view and side view SEM micrographs of samples A3 and A6, respectively; c and d are SEM and cross sectional TEM micrographs of a main wire of sample A12, respectively; e is a HRTEM image of a single wire; f is a selected area diffraction pattern of sample A1.5. The inset in b is a magnified region in the SEM micrograph; insets of d and e are fast Fourier transform (FFT) patterns; the inset of f is the corresponding TEM micrograph to the diffraction pattern.

HRTEM cross-sectional micrographs; Figure 2c,d, sample A12, respectively). Additional TEM analysis reveals that these TiO2 nanowires grow along the [001] direction (Figure 2e, sample A12 and Figure 2f, sample A1.5). The sidewalls of the nanowires belong to the {110} planes, while the corners exhibit the ⟨100⟩ direction (as shown in SEM and HRTEM crosssectional images). Nanowire growth only occurs along the highenergy [001] direction.32 SEM imaging perpendicular to the long axis of the branched nanowires of sample A3 (Figure 2a) and parallel to the long axis of the branched nanowires of sample A6 (Figure 2b) confirms that primary branches grow predominantly in the planes of {100} of the main wire structure with an angle of about 70° with the ⟨001⟩ direction of the main wire structure (as shown in a high magnification SEM image, inset of Figure 2b) with secondary branches growing from these primary branches. The primary branches form an angle of approximately 70° to the long axis of the main wire (estimated from measurements of SEM micrographs in Figure 2a,b). TEM selected area diffraction patterns (SADP, Figure 2f) were used to provide an accurate value of this angle (ca. 65.57°). The inset in Figure 2f is a bright field TEM micrograph of a TiO2 424



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(i.e., (103)) surface to realize the phase transformation to rutile. The {103} of anatase surfaces become {101} surfaces of rutile. This preferential binding explains the formation of branched nanowires with higher concentrations of binding nanoparticles leading to spherulites. Figure 4 depicts a schematic of the transformation from anatase nanoparticles to rutile structures observed in this work. On the basis of our analysis of HRTEM images and the previous study of the transformation from anatase to rutile,24 we propose that nanoparticles (e.g., less than 10 nm) of anatase formed initially are more stable than rutile.33 Within a short period of time, these anatase particles coarsen and transform into rutile phase since rutile is a more stable phase at larger crystallite sizes. When the (103) crystal face of the anatase particle is aligned with the (101) crystal face of rutile, there are two possibilities of anatase particle alignment with the rutile nanowire tip: (i) the [001] direction of anatase is parallel to the [001] rutile direction, resulting in rutile nanowire growth shown in Figure 4, step A.1; (ii) the [001] direction of anatase is aligned at an angle of ∼65° with the [001] rutile direction (Figure 4, Step B.1). In this case, the transformation of anatase particles to rutile nanowires leads to the formation of branches (Figure 4, Step B.2 and B.3). The interface where branching occurs shares the same crystal plane (i.e., (101)) as the original rutile nanowire, resulting in a (101) twin structure. The branch grows along the [001] direction, but out of the corner of the main rutile wire at an angle of 65.57° between the branch and the c-axis ([001] direction) of the rutile wire (Figure 4, Step B.3). After the formation of the branch, the [001] growth front of the core nanowire is effectively blocked by the base of the newly formed primary branch (shown in Figure 4, step B.3 with arrows). A SEM image (Figure S4-a) depicts the incomplete growth of core nanorods at the corners of the larger wires. The branching occurs at the attachment of the anatase particles on the tip of the rutile nanowire at the early growth stage before {101} faces of the tip transform into the (001) face. Therefore, although there are many anatase particle attachments on the rutile nanowire (Figure 3a−f), the branch density is not comparable to the density of the anatase particle attached on the rutile nanowire. After longer reaction durations (e.g., 12 h), anatase particles have been completely consumed and the nanowire growth along [001] direction decreases or stops, and the wire tips smoothen, forming (001) facets (Figure 2c). The anatase particles are absorbed on the side {110} facets of core rutile nanowires and transform into new, parallel rutile nanowires. After short reaction durations (i.e., sample C 0.5), it is observed that the main wire and the branches are composed of smaller core wires (seen in the TEM image provided in Figure S4-b). With increasing reaction duration, growth of the outside region of the rutile nanowires results in {110} surfaces (Figure 2b). The crystallite size of the outside region of the main wires is larger than the core crystallite wires (Figure 2c). These smaller diameter core wires have formed at early stages of growth, when the precursor concentration is high, leading to a high nucleation rate (therefore, smaller diameters). The wires likely grow via particle attachment as well as ion-by-ion growth. This has been observed previously in oxide particle growth.6,34 As the reaction progresses, less precursor is available, leading to a reduced nucleation rate (i.e., fewer nanoparticles available for oriented attachment) and thus, growth on already nucleated surfaces, which leads to larger crystallite diameters in the outer regions of the wires. This result agrees with the XRD analysis discussed previously.

Figure 3. SEM (left) and TEM (right) micrographs of samples A1 (a and b), B1 (c and d), and C1 (e and f). g and h are HRTEM micrographs of sample B1. Inset of h is a fast Fourier transform (FFT) pattern of the outlined area in h. A: anatase; R: rutile.

The computational details are described in Supporting Information. Tables S1 and S2 present the maximum values of the attraction energy per unit area between different pairs of rutile and anatase facets in parallel configuration. Here the maximum energy of attraction is obtained by scanning the surface−surface distance and relative surface alignments. We find that, regardless of the precursor concentration, the (10̅ 3) facet of the anatase always has the lowest binding energy per unit area with (101) and (1̅01) facets of rutile. Among different anatase facets, the strengths of attraction with both (101) and (10̅ 1) facets of rutile are in the order of (100) < (103) < (221) < (1̅03). The simulation results indicate a preferential binding of the (1̅03) facet of an anatase particle to the (1̅01) facet of rutile (Table S1). The anatase crystal deforms its higher energy 425



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Figure 4. (a) Illustration of the preferred attachment of anatase crystals to rutile rods depicting the mechanism of transformation from anatase to rutile, nanowire growth along the [001] direction, and the branch (101) twin formation; (b) graphic illustrations of configurations between rutile (1̅01) and anatase facets: (a) anatase (103), (b) anatase (1̅03).

that increase in density with increasing Ti precursor concentration and time. The core nanowires consist of multiple single crystal nanowires (ca. 10−20 nm diameter), which initially form at high concentrations of precursor. The nanowires exhibit a [001] growth direction along its main axis and primary branches, formed at (101) twins. The main rutile wires and branches likely grow via particle oriented attachment as well as ion by ion growth. At early growth stages, oriented attachment dominates due to the higher concentration

Because of the higher surface energies of the {101} and {001} planes (vs the {110} plane32), the growth velocity along the [001] direction is faster than the sides of the {110} surface, leading the rutile nanowire formation.

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CONCLUSIONS In conclusion, branched rutile-TiO2 nanostructures were synthesized by a hydro-solvothermal reaction. These nanostructures consist of a core nanowire with multiple branches 426



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(10) Park, K. W.; Han, S. B.; Lee, J. M. Photo(UV)-enhanced performance of Pt-TiO2 nanostructure electrode for methanol oxidation. Electrochem. Commun. 2007, 9, 1578−1581. (11) Huang, S. Y.; Ganesan, P.; Park, S.; Popov, B. N. Development of a Titanium Dioxide-Supported Platinum Catalyst with Ultrahigh Stability for Polymer Electrolyte Membrane Fuel Cell Applications. J. Am. Chem. Soc. 2009, 131, 13898−13899. (12) Wagemaker, M.; Kentgens, A. P. M.; Mulder, F. M. Equilibrium lithium transport between nanocrystalline phases in intercalated TiO2 anatase. Nature 2002, 418, 397−399. (13) Sun, C. H.; et al. Higher charge/discharge rates of lithium-ions across engineered TiO2 surfaces leads to enhanced battery performance. Chem. Commun. 2010, 46, 6129−6131. (14) Beuvier, T.; et al. TiO2(B) Nanoribbons As Negative Electrode Material for Lithium Ion Batteries with High Rate Performance. Inorg. Chem. 2010, 49, 8457−8464. (15) Armstrong, A. R.; et al. Lithium Coordination Sites in LixTiO2(B): A Structural and Computational Study. Chem. Mater. 2010, 22, 6426−6432. (16) Oh, J. K.; Lee, J. K.; Kim, H. S.; Han, S. B.; Park, K. W. TiO2 Branched Nanostructure Electrodes Synthesized by Seeding Method for Dye-Sensitized Solar Cells. Chem. Mater. 2010, 22, 1114−1118. (17) Feng, X. J.; et al. Vertically Aligned Single Crystal TiO2 Nanowire Arrays Grown Directly on Transparent Conducting Oxide Coated Glass: Synthesis Details and Applications. Nano Lett. 2008, 8, 3781−3786. (18) Chen, C. M.; Hsu, Y. C.; Cherng, S. J. Effects of annealing conditions on the properties of TiO2/ITO-based photoanode and the photovoltaic performance of dye-sensitized solar cells. J. Alloys Compd. 2011, 509, 872−877. (19) Wang, H. H. Preparation of Nanoporous TiO2 Electrodes for Dye-Sensitized Solar Cells. J. Nanomater. 2011, No. 547103. (20) Prabakar, K.; Son, M.; Kim, W. Y.; Kim, H. TiO2 thin film encapsulated ZnO nanorod and nanoflower dye sensitized solar cells. Mater. Chem. Phys. 2011, 125, 12−14. (21) Du, J.; Qi, J.; Wang, D.; Tang, Z. Facile synthesis of Au@TiO2 core-shell hollow spheres for dye-sensitized solar cells with remarkably improved efficiency. Energy Environ. Sci. 2012, 5, 6914−6914. (22) Wang, S. A Novel and Highly Efficient Photocatalyst Based on P25-Graphdiyne Nanocomposite. Small 2012, 8, 265−271. (23) Shin, B.; Won, J.; Son, T.; Kang, Y. S.; Kim, C. K. Barrier effect of dendrons on TiO2 particles in dye sensitized solar cells. Chem. Commun. 2011, 47, 1734−1736. (24) Wu, J. H.; et al. Crystal morphology of anatase titania nanocrystals used in dye-sensitized solar cells. Cryst. Growth Des. 2008, 8, 247−252. (25) Jang, S. H.; Kim, Y. J.; Kim, H. J.; Lee, W. I. Low-temperature formation of efficient dye-sensitized electrodes employing nanoporous TiO2 spheres. Electrochem. Commun. 2010, 12, 1283−1286. (26) Wang, G.; Li, G. Titania from nanoclusters to nanowires and nanoforks. Eur. Phys. J. D 2003, 24, 355−360. (27) Guang-Lai, L.; Guang-Hou, W.; Jian-Ming, H. Morphologies of rutile form TiO2 twins crystals. J. Mater. Sci. Lett. 1999, 18, 1243− 1246. (28) Kinsinger, N. M.; Wong, A.; Li, D. S.; Villalobos, F.; Kisailus, D. Nucleation and Crystal Growth of Nanocrystalline Anatase and Rutile Phase TiO2 from a Water-Soluble Precursor. Cryst. Growth Des. 2010, 10, 5254−5261. (29) Penn, R. L.; Banfield, J. F. Formation of rutile nuclei at anatase {112} twin interfaces and the phase transformation mechanism in nanocrystalline titania. Am. Mineral. 1999, 84, 871−876. (30) Penn, R. L.; Banfield, J. F. Morphology development and crystal growth in nanocrystalline aggregates under hydrothermal conditions: Insights from titania. Geochim. Cosmochim. Acta 1999, 63, 1549−1557. (31) Penn, R. L.; Banfield, J. F. Oriented attachment and growth, twinning, polytypism, and formation of metastable phases: Insights from nanocrystalline TiO2. Am. Mineral. 1998, 83, 1077−1082.

of precursor producing more nuclei and therefore more anatase particles. At later growth stages, the reduced concentration of precursor produces fewer nuclei (including anatase particles), and growth via ion-by-ion attachment becomes dominant. Branched growth occurs via an oriented absorption of anatase nanocrystals with subsequent transformation to the rutile phase. {103} crystal facets of anatase particles are aligned with {101} facets found on the tips of rutile wires via two different pathways, both leading to wire growth along the [001] direction with subsequent (101) twin formation. These observations were confirmed by Matsui-Akaogi force field calculations, which demonstrated that regardless of the precursor concentration, the (10̅ 3) facet of anatase always has the lowest binding energy to both (101) and (1̅01) facets of rutile. Through an understanding of the mechanism of this growth, we enable greater control over nanostructure morphology, which can potentially enhance the performance of energy-based devices requiring crystalline, high surface area materials.

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ASSOCIATED CONTENT

S Supporting Information *

Additional experimental and modeling data of the branching mechanism are provided in Figures S1−S8 and Tables S1−S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Cui, Y.; Lieber, C. M. Functional nanoscale electronic devices assembled using silicon nanowire building blocks. Science 2001, 291, 851−853. (2) Duan, X. F.; Huang, Y.; Cui, Y.; Wang, J. F.; Lieber, C. M. Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices. Nature 2001, 409, 66−69. (3) Yi, C.; Zhaohui, Z.; Deli, W.; Wang, W. U.; Lieber, C. M. High performance silicon nanowire field effect transistors. Nano Lett. 2003, 3, 149−152. (4) Li, Z. Direct hydrothermal synthesis of single-crystalline hematite nanorods assisted by 1,2-propanediamine. Nanotechnology 2009, 20, No. 245603. (5) Kwon, S.; Fan, M.; Cooper, A. T.; Yang, H. Q. Photocatalytic applications of micro- and Nano-TiO2 in environmental engineering. Crit. Rev. Environ. Sci. Technol. 2008, 38, 197−226. (6) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746−750. (7) Navarro, R. M.; Alvarez-Galvan, M. C.; de la Mano, J. A. V.; AlZahrani, S. M.; Fierro, J. L. G. A framework for visible-light water splitting. Energy Environ. Sci. 2010, 3, 1865−1882. (8) Puangpetch, T.; Sommakettarin, P.; Chavadej, S.; Sreethawong, T. Hydrogen production from water splitting over Eosin Y-sensitized mesoporous-assembled perovskite titanate nanocrystal photocatalysts under visible light irradiation. Int. J. Hydrogen Energy 2010, 35, 12428− 12442. (9) Jeng, K. T.; et al. Membrane electrode assembly-based photoelectrochemical cell for hydrogen generation. Int. J. Hydrogen Energy 2010, 35, 10890−10897. 427



Communication

(32) Oliver, P. M.; Watson, G. W.; Kelsey, E. T.; Parker, S. C. Atomistic simulation of the surface structure of the TiO2 polymorphs rutile and anatase. J. Mater. Chem. 1997, 7, 563−568. (33) Barnard, A. S.; Zapol, P.; Curtiss, L. A. Modeling the morphology and phase stability of TiO2 nanocrystals in water. J. Chem. Theory Comput. 2005, 1, 107−116. (34) Zheng, H. Observation of single colloidal platinum nanocrystal growth trajectories. Science 2009, 324, 1309−1312.

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Growth Mechanism of Highly Branched Titanium Dioxide Nanowires

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