DOI: 10.1021/cg901210c
Room-Temperature Synthesis of Single-Crystalline Anatase TiO2 Nanowires
2010, Vol. 10 1646–1651
Wan-Yu Wu,† Yu-Ming Chang,‡ and Jyh-Ming Ting*,‡,# †
Department of Materials Science National University of Tainan, Tainan, Taiwan, ‡Department of Materials Science and Engineering National Cheng Kung University, Tainan, Taiwan, and #Institute of Nanotechnology and Microsystems Engineering National Cheng Kung University, Tainan, Taiwan Received October 2, 2009; Revised Manuscript Received November 23, 2009
ABSTRACT: Single-crystalline anatase TiO2 nanowires have been synthesized using a novel, room-temperature method involving the use of seed particles and a radio frequency magnetron sputter deposition technique. Also, the growth time required was less than 60 min. Transmission electron microscopy and X-ray absorption near-edge structure analyses show that the obtained nanowires are single-crystalline anatase TiO2. Cathodoluminescence spectroscopy spectrum shows that the nanowires’ band-to-band emission exhibits a blue shift from to 3.20 (387) to 3.46 eV (358 nm). Conventional vapor-liquid-solid and vapor-solid-solid growth modes are not applicable to explain the growth of the obtained single-crystalline anatase TiO2 nanowires at room-temperature. A folded-growth mode is therefore suggested.
1. Introduction Nanostructured titanium dioxide (TiO2), such as nanoparticles, nanowires, nanorods, and nanotubes, have received considerable attention due to their various potential applications. They are used, for example, in dye-sensitized solar cells (DSSCs),1-3 photocatalyzation,4-6 gas sensors,7 cosmetics,8 and energy storage.9 Among the various types of TiO2 nanostructures, there is a particular interest in single-crystalline TiO2 nanowires, especially those exhibiting the anatase phase. The electron transport in single-crystalline nanowires is expected to be several orders of magnitude faster than the percolation through a random polycrystalline nanowires network.10 For example, DSSCs having single-crystal-like anatase TiO2 nanowire electrodes were shown to exhibit a high full-sun efficiency of 9.3% due to rapid electron transfer in the nanowires.2 However, there are only limited reports on the synthesis of single-crytsalline TiO2 nanowires having either anatase or rutile phase. Anatase phase TiO2 nanowire arrays were prepared in a prolonged process (tens of hours) involving the use of anodic alumina membranes, a sol-gel method, and final thermal heat treatment at 650 °C.11 TiO2 nanowires were obtained by annealing hydrothermally synthesized TiO2 nanopowders at 500 °C for 10 h.12 Physical vapor deposition methods were used to synthesize single-crystalline TiO2 nanowires 850 °C13 and 1100 °C.14,15 The phase in the former is unknown and the latter is rutile. For the fabrication of single nanowire field-effect transistor, rutile phase single-crystalline TiO2 nanowires were grown by atmospheric-pressure chemical vapor deposition at 850-950 °C in air for 2 h.16 Rutile phase wire-like singlecrystalline TiO2 was obtained by thermal annealing of titanium foils at 750-950 °C for 0.5-6 h.17 Various hydrothermal processes were also used for the fabrication of anatase18,19 or rutile20 phase single-crystalline TiO2 nanowires with19 or without19,20 a final heat treatment step. The hydrothermal treatments were all more than or much more than 24 h. It therefore appears that not only the studies on the synthesis of single-crystalline TiO2 nanowires are limited, but also *Corresponding author. E-mail:
[email protected]. pubs.acs.org/crystal
Published on Web 03/12/2010
the synthesis methods involve elevated temperature and/or prolonged times. To meet the requirements for many current and future technologies, for example, flexible electronics, lowtemperature, fast-rate synthesis of single-crystalline TiO2 nanowires is thus called for. Here we show a brand new approach for room-temperature, fast-rate growth of singlecrystalline anatase TiO2 nanowires. The approach involves the use of seeded-substrates and a conventional sputter deposition method. The resulting nanowires are single-crystalline anatase TiO2 having an average diameter of 75 nm. 2. Experimental Section The growth of TiO2 nanowires began with seeding glass substrates with nanoparticles. The seeding was performed first by dip coating indium-tin oxide (ITO) glass substrate in an aqueous solution of SnCl2 3 2H2O þ NaOH. The solution’s pH value was 13.1. After the dip coating, the substrate was dried in air to allow the precipitation of the seed particles. Seeded substrate was then placed in an RF magnetron sputter deposition chamber for the growth of TiO2 nanowires at room temperature. The target used was a 3-in. TiO2 target (purity: 99.99%). The growth of TiO2 was carried out at an RF power of 150 W and a working distance of 50 mm, while the working pressure, atmosphere, and deposition time were varied. No substrate heating or bias was applied during the sputter deposition. The resulting samples were characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry (XRD), selected area diffraction (SAD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge structure (XANES), and cathodoluminescence (CL) spectroscopy.
3. Results and Discussion In general, the precipitates were found to be spherical particles having an average diameter of 130 ( 8 nm. Occasionally, cubic precipitates were observed concurrently. The phase of the cubes was identified to be single-crystalline Na2O. XRD analysis gives little information on the phase of the spherical particles due to their small amount. Moreover, SAD analysis also failed to give any information of the spherical precipitates as they were easily destroyed under the electron beam. The surface chemical composition and state of the spherical precipitates were then examined using XPS analysis. r 2010 American Chemical Society
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Figure 1. (A) XPS survey spectrum of spherical precipitates. The pertinent high resolution XPS spectra showing (B) Sn 3d5/2 and (C) O 1s peaks.
The XPS survey spectrum is shown in Figure 1A. Elements of Sn, O, Na, and Cl were found in the precipitates. The detected element C was a contaminant. Quantitative analysis gives atomic percentages of 21.7%, 63.4%, 13.5%, and 1.4% for Sn, O, Na, and Cl, respectively. The large amount of oxygen is ascribed to the contribution from the ITO substrate. Element Na originated from the added NaOH. Element Cl is a residue from the SnCl2 3 2H2O. The high-resolution XPS spectrum of Sn 3d5/2 is shown in Figure 1B. There are two peaks at 485.8 and 486.4 eV. The former is assigned to Sn2þ, while the latter to Sn4þ.21 The peak at 486.4 eV is from the ITO substrate but not the precipitates as TEM analysis to be given below further shows that the precipitates are SnO. Figure 1C shows a highresolution XPS spectrum of O 1s. The O 1s spectrum was
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fitted with three peaks at 528.9, 529.8, and 531.0 eV. The peak at 528.9 eV is assigned to SnO.21,22 The peaks at 529.8 and 531.0 eV are assigned to the lattice oxygen and the oxygendeficient region in the crystalline ITO substrate, respectively.23,24 As a result, it is believed that the spherical precipitates are tetragonal SnO. Using the obtained seeded-substrates, nanowires were obtained after the sputter deposition. An insignificant difference was found among the nanowires obtained under different working pressures (5 10-1, 1 10-1, 5 10-2, and 1 10-2 Torr). Figure 2A shows nanowires obtained at 1 10-1 Torr. The coverage of nanowires on the substrate does not appear to be high. At a higher magnification (Figure 2B), it is seen that the nanowires grow out of the seed particles. The diameter of the nanowires ranges from 50 to 103 nm, averaging at 75 ( 15 nm. It is noted that a very thin TiO2 layer also codeposits on the substrate surface. One of the indications from Figure 2A,B is that area density of the nanowires is governed by the seed coverage on the substrate. Although the nanowire yield is not high, the major breakthrough is that single crystalline anatase TiO2 nanowires can be grown from the seed particles using a room temperature sputter deposition process. Figure 2C gives a TEM image of the nanowires and the corresponding SAD pattern that shows the nanowires obtained are single crystalline anatase TiO2. Figure 2D further shows the lattice spacing between the (101) planes. The electronic structure of TiO2 nanowires was also examined using XANES spectroscopy. Figure 3 shows the Ti L2,3-edge XANES spectrum of sputterdeposited TiO2 nanowires obtained at pure Ar or 0% O2. The Ti L2,3-edge XANES spectrum was recorded in the total electron yield mode (surface-sensitive) for the Ti 2p absorption. As shown in Figure 3A, the Ti L2,3-edge XANES spectrum splits into two sets of peaks between 450 and 472 eV, labeled as L3 and L2 edges, respectively, due to the spin-orbit coupling of Ti 2p core electrons. They correspond to the electronic transitions from the 2p3/2 (L3-edge) and 2p1/2 (L2edge) core levels to a 3d excited state of Ti atoms.25 For both L3 and L2 edges, the crystal field splits the 3d state of Ti into t2g (formed by dxy, dxz and dyz orbitals) and eg (formed by dx2-y2 and dz2 orbitals) subands in the octahedral symmetry, each L feature reveals two contributions, that is, C3 and D3 for L3, and C2 and D2 for L2. The L2-edge features are broader than the L3-edge due to a shorter lifetime of the 2p1/2 core holes as a result of a radiationless electron transition from the 2p3/2 to the 2p1/2 levels, accompanying by the promotion of a valence electron into the conduction band, while an additional or an asymmetry splitting in the eg peak (E3 and E2) is attributed to the distortions from octahedral symmetry.26 More importantly, the relative intensity of the D3 and E3 peaks dictates the different distortions of the local coordination environment, which can be use to distinguish between anatase and rutile TiO2 polymorphs. For the anatase phase, the intensity of D3 is greater than that of E3 and vice versa is true for the rutile phase. In other words, the Ti L2,3-edge XANES spectrum further confirms the phase of the TiO2 nanowires to be anatase. The leading edge of the Ti L2,3-edge XANES spectrum located between 455.5 and 457.0 eV is shown in Figure 3B. We also observed that as the oxygen content increases from 0% to 100%, the shoulder of eg band in the L3 edge, that is, E3 peak, becomes resolved. This indicates that oxygen enhances long-range ordering in the anatase TiO2 structure or the TiO2 nanowires obtained at pure Ar have more oxygen vacancies.23,27 Figure 3C gives the O K-edge XANES
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Figure 2. (A) An SEM image of sputter-deposited TiO2 nanowires on glass substrate. (B) The nanowires grow out of particles. (C) A TEM image of sputter-deposited TiO2 nanowires and the corresponding SAD pattern. (D) A HRTEM image showing the lattice spacing between (101) planes. The RF power was 150 W and the pressure was 5 10-1 Torr, except that the pressure was 1 10-1 Torr for (A).
spectrum of the sputter-deposited TiO2 nanowires. The O K-edge XANES spectra were also recorded in the total electron yield mode for oxygen 1s absorption. The O Kedge XANES spectrum splits into two sets of peaks between 525 and 565 eV. The first two main features are located at 531 and 533 eV, labeled as A and B peaks, respectively. They correspond to electron transitions from the O 1s orbital to a covalently mixed state derived from the O 2p states hybridized with Ti 3d states. The peaks at higher energy region are due to the transitions to the O (2p) antibonding state and O (2p)-Ti (4sp) hybridized levels, labeled as C and D peaks, respectively.28 A remarkable difference between anatase and rutile TiO2 phase is the spectral feature in the higher energy region. Anatase TiO2 exhibits two peaks, while rutile TiO2 exhibits three peaks.27 Again, the O K-edge XANES spectroscopy analysis shows that the TiO2 nanowires are anatase TiO2. The leading edge of the O K-edge XANES spectrum located between 528.5 and 532.5 eV is shown in Figure 3D. As the oxygen contents increases from 0% to 100%, the peak generally shifts to the lower absorption energy. This also indicates that the TiO2 nanowires obtained at pure Ar have more oxygen vacancies. The existence of oxygen vacancies was also detected by XPS analysis. The high-resolution XPS Ti 2p spectrum consists of four fitted peaks of Ti4þ 2p3/2,
Ti4þ 2p1/2, Ti3þ 2p3/2, and Ti3þ 2p1/2. The presence of titanium suboxide Ti2O3 (Ti3þ peaks) indicates the existence of the defected state of oxygen vacancies and the nonstoichiometric nature of sputter-deposited TiO2 nanowires. These defects contribute to emission peaks found in CL spectra as seen below. Figure 4 shows a CL spectrum of sputter-deposited TiO2 nanowires. The solid curve is the detected CL spectral line, while the dotted curve is the fitted curve to the CL spectral line. As shown in Figure 4, the CL spectrum can be deconvoluted into five peaks after Gaussian fitting. These peaks are at 358 nm (3.46 eV), 416 nm (2.97 eV), 460 nm (2.69 eV), 495 (2.50 eV), and 527 nm (2.35 eV). The dominant emission peak at 358 nm is contributed by the band-to-band emission of the TiO2 nanowires in the ultraviolet region. The band gap energy of anatase TiO2 phase is 3.20 eV (387 nm). Therefore, it appears that the TiO2 nanowires exhibit a blue shift due to the quantum confinement.29 The peak at 416 nm (2.97 eV) is attributed to the self-trapped excitons localized on TiO6 octahedral.30 The peaks at 460 and 527 nm originated from self-trapped excitons, F (an oxygen-ion vacancy occupied by two electron) and F þ (an oxygen-ion vacancy occupied by one electron) centers, respectively.31,32 As for the emission peak at 495 nm, surface defects states associated with oxygen vacancies are believed to be the origin.33
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Figure 3. XANES spectra of (A) Ti L2,3-edge, (B) Ti L2,3-leading edge, (C) O K-edge, and (D) O K-leading edge of sputter- deposited TiO2 nanowires obtained at pure Ar.
Figure 4. CL spectrum of sputter-deposited TiO2 nanowires obtained at an RF power of 150 W, a pressure of 5 10-2 Torr, and a deposition time of 30 min. The solid curve is the detected CL spectral lines, the dotted curve is the fitted curve to the CL spectral line, and the dashed peaks represent the Gaussian fitting results.
Finally, the growth mode is briefly discussed. The growth of nanowires has been widely explained using the vaporliquid-solid (VLS) mechanism.34-39 This mechanism generally requires a growth temperature above or near the eutectic point of the seed and the nanowire materials. The VLS model has therefore been used to explain the growth at high temperatures. At lower temperatures ranging from 200 to 450 °C, the growth of nanowires has been explained by the vaporsolid-solid (VSS) model.40,41 In the current study, the VLS
model is not applicable due to the low growth temperature used. The substrate temperature is at room temperature and far below the melting points of any materials involved in the growth process. SnO has a high melting point (>1000 °C), so do any other TiSnOx phases or SnO-TiO2 solid solutions that may form. On the other hand, the VSS model requires the vapor and the seed materials to form a compound or solidsolution phase. This was not observed either at the present time. In this study, TEM analysis suggests that the growth of TiO2 nanowires follows an unconventional growth model. Figure 5 shows TEM images of a sample obtained at an early stage of the growth. The growth time was less than 5 min. Figure 5A is a general view of the sample. The dark particles are the seed particles, while the rest are deposited TiO2 film. In the beginning of the growth, the TiO2 arriving at the areas where there is no seed simply leads to the growth of a film, as pointed by the arrows. On the other hand, the TiO2 impinging on the seeds diffuses on the surface of the seeds, resulting in folding of TiO2 layers on the seed particles, as shown in Figure 5B. This figure also shows the (200) spacing of the SnO seed and the (101) spacing of the TiO2. Some of the (101) planes are indistinguishable due to the existence of defects, as indicated above during the discussion of Figures 3 and 4. During or after the folding, some TiO2 grows into infant nanowires as shown in Figure 5C. Continuous growth then gives the formation of TiO2 nanowires. Although the detailed growth mechanism requires further studies, we have demonstrated a novel growth method for fabricating single crystal anatase TiO2 nanowires at room temperature and have shown the interesting CL spectrum of the obtained nanowires.
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growth time (
