J. Phys. Chem. C 2010, 114, 10697–10702
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TiO2 Nanowire Growth Driven by Phosphorus-Doped Nanocatalysis Myung Hwa Kim,†,§,# Jeong Min Baik,†,#,¶ Jinping Zhang,‡,⊥ Christopher Larson,† Youli Li,‡ Galen D. Stucky,‡ Martin Moskovits,*,† and Alec M. Wodtke*,† Department of Chemistry & Biochemistry, UniVersity of California at Santa Barbara, Santa Barbara, California 93106; Department of Materials, UniVersity of California at Santa Barbara, Santa Barbara, California 93106; Department of Chemistry & Nano Science, Ewha Womans UniVersity, Seoul 120-750, Korea; Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215125, China; and School of Mechanical and AdVanced Materials Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, Korea ReceiVed: January 25, 2010; ReVised Manuscript ReceiVed: April 25, 2010
Ni-catalyzed single-crystal TiO2 nanowire growth was observed to occur well below the bulk Ni melting point when a small amount of P was present. TEM, SAED, EDXS, and EELS analyses as well as consideration of the Ni/P phase diagram point to a previously unreported mechanism of nanowire growth catalyzed by a liquid Ni/P eutectic shell surrounding a solid Ni core. High-resolution elemental analysis supports the conclusion that the active catalyst is the outer liquid Ni/P layer with P present at a 3-8% level surrounding a solid Ni core. Growth proceeds by precursor adsorption onto the liquid layer followed by diffusion to the growing surface of the nanowire. This catalyst system produces rutile TiO2 nanowires efficiently. We believe that the technique is generalizable to other metal/dopant systems which could lead to the synthesis of other hard-tosynthesize nanowires. Introduction Semiconducting nanowires have attracted a great deal of attention on account of their potential utilities in optoelectronics, sensors, nanoelectronics, and electrochemical energy conversion and storage devices.1-6 Although several synthesis techniques have been developed for growing high-quality nanowires and much has been learned regarding the mechanism of growth of quasi-1-dimensional nanostructures generally, a great deal of mechanistic understanding remains to be discovered. Anisotropic quasi-1-D nanocrystals of a broad range of materials can be grown using vapor-liquid-solid (VLS) synthesis,7,8 utilizing catalytic metal nanoparticles that form low-melting point eutectic alloys with one of the reactants. The liquid catalyst nanodroplet ultimately becomes supersaturated in the desired product that precipitates as a nanocrystal and becomes the nucleus on which preferential and unidirectional crystal growth occurs. Several mechanistic accounts have been published that made excellent use of in situ transmission electron microscopy (TEM) for studying VLS growth leading to the synthesis of binary systems such as Au-Si9,10 and Au-Ge.11-13 Despite this progress, atomic-level processes taking place at the catalyst/nanowire interface are not fully understood.14 Moreover, several systems, such as those yielding metal oxide nanowires do not proceed by the conventional VLS mechanism.15-18 A recent proposal relies on solid-phase diffusion, suggesting that a solid nanoparticle can act as a catalyst by vapor-solid-solid (VSS) growth.19,20 * To whom correspondence should be addressed. E-mail:
[email protected] (M.M.),
[email protected] (A.M.W.). † Department of Chemistry & Biochemistry, UC Santa Barbara. ‡ Department of Materials, UC Santa Barbara. § Ewha Womans University. ⊥ Chinese Academy of Sciences. ¶ Ulsan National Institute of Science and Technology (UNIST). # These authors contributed equally to this work.
Figure 1. Representative SEM images of TiO2 nanowires and Raman spectrum of a single nanowire. (a) SEM images of TiO2 nanowires grown on a SiO2/Si substrate at 950 °C via atmospheric pressure chemical vapor deposition (APCVD), using TiO and Ti metal powders as precursors and Ni nanoparticles as catalyst. (b) SEM image of TiO2 nanowires with catalyst particles at the tip of the nanowires. (c) Raman spectrum of a single TiO2 nanowire transferred onto a KBr crystal. Also shown for comparison are Raman spectra of KBr as well as rutile and anatase TiO2.
In this work, we report the unexpected influence of trace P on the efficiency of growth of TiO2 nanowires from catalyst Ni nanoparticles. The analysis of this phenomenon suggests a new nanowire growth mechanism that occurs well below the expected melting point of the catalyst nanoparticle where the impurity is capable of depressing the catalyst particles’ melting point. In a manner reminiscent of “sprinkling salt on an ice cube” the P produces a liquid layer on the outer surface of the solid Ni catalyst particle that serves to accelerate reactant adsorption, diffusion, nanowire nucleation, and growth. Specifically, we
10.1021/jp1007335 2010 American Chemical Society Published on Web 05/27/2010
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J. Phys. Chem. C, Vol. 114, No. 24, 2010
Kim et al.
Figure 2. High-resolution transmission electron microscopy (HRTEM) images and selective area electron diffraction (SAED) of single nanowires. (a, b) TEM images of the head part of a single TiO2 nanowire clearly showing both the spherical catalyst particle and the nanowires formed in two growth directions (110) and (100), respectively. (c) Typical SAED of a nanowire with the growth direction along the (100) crystallographic axis of rutile TiO2. (d) Atomically resolved HRTEM image of the interfacial region between the nanowire and catalyst. (e) High angle angular dark field (HAADF) STEM image. The transition layer is about 2 nm thick and different from both the catalyst and nanowire structure.
have found that P impurities present in our reaction furnace strongly accelerate the catalytic activity of Ni nanoparticles useful for growing rutile TiO2 nanowires. When P is eliminated from the system, nanowire growth is suppressed. Intentional introduction of P also accelerates growth. These results suggest that in previous studies on nanowire growth the observed results may have been influenced by trace impurities. Moreover, the introduction of dopants destined for the catalyst particles may be a powerful method for influencing nanowire growth. These results thus demonstrate the potentially quite useful influence that trace impurities can have on catalytic growth of nanowires via a mechanism that resembles but is markedly different from the well-known VLS mechanism. Experimental Methods Single-crystalline TiO2 nanowires (NWs) were grown by atmospheric pressure chemical vapor deposition (APCVD), using TiO and Ti metal powders as precursors and Ni nano-
particles as catalyst.21 Single crystal (100) Si wafers with 200 nm thick thermally grown SiO2 layers, used as substrates, were cleaned, and a ∼50 nm Ti metal layer was deposited on them by e-beam evaporation. A ∼2 nm nickel film was vapor deposited onto the Ti layer. This nickel film broke up during nanowire growth to form the Ni catalyst nanoparticles. The NW growth was carried out in a horizontal quartz tube furnace, 2.5 cm in diameter and 122 cm long. 0.2 g of fine meshed TiO (99.95%, Aldrich) or Ti (99.99%, Aldrich) metal powders were loaded at the center of a 5 cm long quartz boat. The substrate, Ni/Ti/SiO2, was also placed in the quartz boat ∼5 mm downstream from the TiO or Ti metal powder source. The quartz boat was situated at the center of the quartz tube furnace, which was heated to 850-950 °C in air for ∼10 min and then maintained at 950 °C for a further 10 min. APPVD growth was then carried out for 2 h under flowing high-purity Ar (99.998%) at 200 sccm.
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Figure 3. HRTEM images of the interstitial region between the catalyst particle and the nanowire. (a) For the case where the growth direction of the nanowire is along the (100) axis of the rutile crystal. Here the contact region is ∼40 nm in size, while the nanowire is 87 nm wide. Both “open shoulders”, left and right, are filled with noncrystalline material which is found to be TiOx by EDXS measurements. (b) HRTEM image (zoomed in) of the triple junction having the amorphous TiOx and showing a clean side wall of the nanowire. (c) HRTEM (further zoomed in) of the contact region between the TiO2 nanowire and the catalyst particle clearly showing the noncrystalline material.
TiO2 nanowires were characterized by HRTEM (FEI Titan TEM/STEM at 300 kV) at room temperature. TiO2 nanowire samples were prepared for TEM imaging by simply touching the NW-covered substrate to the TEM grid, thereby transferring some of nanowires to the grid. Elemental analysis was carried out in the TEM using energy-dispersive X-ray spectroscopy (EDXS). In addition, scanning electron microscopy (SEM) and micro-Raman spectroscopy were used to characterize nanowires. Results and Discussion The nanowires were grown from a layered Ni/Ti/SiO2 patterned substrate as described in the Experimental Methods section. Figure 1a shows representative low-magnification SEM images of as-grown nanowires. Figure 1b clearly shows nanowires that have catalyst particles at their tips. All of the nanowires examined by SEM exhibited similar structures. The nanowires (10-30 µm long and 25-150 nm in diameter) grow out of the plane of the substrate. The apparent stripelike growth regions reflect the spatial patterning of the substrate employed to demonstrate the catalytic nature of Ni/Ti film. In other words, nanowire growth is absent where the Ni/Ti film was absent. Raman scattering was excited with a 632.8 nm laser light providing Raman spectra for single nanowires (Figure 1c). These spectra exhibited three strong Raman bands at 234, 443, and 607 cm-1, corresponding to those of bulk rutile TiO2 also shown for comparison.22 No evidence of anatase TiO2 formation which is characterized by peaks at 136, 390, 512, and 634 cm-1 was observed in the Raman spectra. Figure 2a shows a high-resolution TEM image of the head portion of a typical nanowire, showing both the catalyst particle and nanowire. All nanowires were found with a nearly spherical catalyst particle appended. Note the planar cross-section of the nanowire ends with the TiO2 surface nearly tangent to the spherical particle (Figure 2a,b). This is reminiscent of, but notably different from, material grown by a VLS mechanism, as VLS catalytic particles are typically observed to be quasihemispherical, an interface architecture that maximizes the contact area of the liquid-solid interface.
Energy-dispersive X-ray spectroscopy (EDXS) was performed in a line-scan mode across the interfacial region (red line of Figure S1). This analysis showed the catalyst particle was composed of Ni with low levels of oxygen and titanium, whereas Ti and O were the only components in the nanowire. Highresolution TEM imaging (Figure 2d) furthermore showed that the catalyst particle only deviates slightly from a spherical shape near the contact region with the TiO2 nanowire. Indeed, only very few (
