Article pubs.acs.org/IECR
Hydrogen-Plasma-Assisted Growth of Anatase TiO2 Nanoneedles on Ti Plates Ruey-Chi Wang,†,* Chia-Chi Hsu,† and Shu-Jen Chen‡ †
Department of Chemical and Materials Engineering, National University of Kaohsiung, Kaohsiung 81148, Taiwan Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 80778, Taiwan
‡
ABSTRACT: Pure anatase TiO2 nanoneedles are prepared on substrates via a dry process that involves hydrogen plasma treatments followed by the oxidation of Ti plates at a low temperature of 500 °C. Scanning electron microscopy and highresolution transmission electron microscopy images show that the nanoneedles have diameters and lengths in ranges of 20−50 nm and 100−200 nm, respectively, and are single-crystalline anatase structures that grow along the [11̅6] direction. The TiO2 nanoneedles nucleate on microgrooves that result from the oxidation of microcracks induced by the hydrogen plasma. The diameters, lengths, and area densities of the TiO2 nanoneedles can be varied by adjusting the power of the hydrogen plasma. A stress-assisted vapor−solid mechanism for the formation of the nanoneedles is proposed. The room-temperature photoluminescence spectrum of the TiO2 nanoneedles exhibits a relatively strong ultraviolet emission centered at 375 nm and broad visible emission at around 640 nm. This study proposes a route for growing anatase-phase TiO2 one-dimensional nanostructues on substrates that have potential use in photocatalytic, photoelectrochemical, and sensing applications.
1. INTRODUCTION Nanocrystalline TiO2 materials have attracted considerable research attention due to their use in applications such as gas sensors,1 photocatalysts,2 optical devices,3 and solar cells.4 TiO2 materials mainly exist in one of three crystallographic structures: anatase, rutile, or brookite. The anatase phase has received the most attention due to it having the highest photoelectrochemical performance.5 Recently, some wet techniques have been developed for preparing TiO2 onedimensional (1D) nanomaterials, including sol−gel,6,7 hydrothermal,8,9 and electrochemical routes.10 However, these wetchemical methods usually contaminate the products and require that the crystallinity be improved by postannealing.11,12 In addition, it is difficult to grow TiO2 1D nanostructures on substrates via wet processes. Routes that directly heat Ti substrates have been widely employed for fabricating TiO2 1D nanostructures. The TiO2 nanostructures synthesized via dry routes usually exhibit high crystalline quality. However, due to the high melting temperature of Ti, the process temperature is usually above 850 °C and the synthesized TiO2 is generally the rutile phase.13−16 Recently, Cheung et al.17 developed a route for fabricating anatase TiO2 nanowires at a low temperature of 450 °C by dipping Ti foil in KF solution before heating. However, the synthesized TiO2 nanowires contain a high concentration of K impurities. Therefore, preparing pure anatase TiO2 1D nanostructures on substrates via dry processes remains a challenge. In the present study, single-crystalline pure anatase TiO2 nanoneedles were directly grown on substrates via a dry process that involves hydrogen plasma treatments followed by the oxidation of Ti plates at a low temperature of 500 °C in air. The morphology and density of the nanoneedles can be adjusted by varying the power of the plasma. The growth direction and related growth mechanisms of the nanoneedles are unique. This © 2012 American Chemical Society
study presents a method for fabricating single-crystalline anatase TiO2 nanoneedles on substrates at low temperature. The obtained materials have potential in photocatalytic and photoelectrochemical applications.
2. EXPERIMENTAL PROCEDURE TiO2 nanoneedles were fabricated by heating plasma-pretreated Ti plates in air. Commercial Ti plates (Nilaco, purity: 99.5%) with a thickness of 0.1 mm were first etched in 37 wt % HCl aqueous solution (Sigma-Aldrich), washed with distilled water, and finally dried at 60 °C to remove the oxidation layer on the surface. The titanium plates were then pretreated with hydrogen plasma at a power of 150−300 W for 5 min using an inductively coupled plasma (ICP)-etcher (Cirie-200, AST, Taiwan). The oxidation was carried out in a horizontal furnace tube, as shown in Figure 1. Prior to heating, the pretreated Ti plates were placed in the tube, and the system was purged using argon at a flow rate of 100 standard cubic centimeters per minute (sccm) for 1 h. Oxygen was then introduced into the tube at a flow rate of 1 sccm and the furnace was heated to 500 °C at a rate of 20 °C/min. The growth temperature of 500 °C was maintained for 1.5 h, and the working pressure was maintained at 10 Torr. The system was then cooled to room temperature. The morphology, crystallography, microstructure, composition, and chemical bonding of the as-prepared materials were characterized using field-emission scanning electron microscopy (FE-SEM, 6700F, JEOL, Japan), X-ray diffraction (XRD, D/MAX-2500, Rigaku, Japan), field-emission transmission electron microscopy (FE-TEM, Tecnai G2 F20, FEI, USA) Received: Revised: Accepted: Published: 3677
November 8, 2011 January 23, 2012 February 9, 2012 February 9, 2012 dx.doi.org/10.1021/ie202558s | Ind. Eng. Chem. Res. 2012, 51, 3677−3681
Industrial & Engineering Chemistry Research
Article
The area density of the nanoneedles in Figure 2f is larger than that of those in Figure 2d. Notably, the nanoneedles grown in these shallow grooves exhibit a longer morphology and higher area density compared to those of nanoneedles grown outside the grooves. Figure 3 panels a and b show XRD patterns of untreated Ti plates and Ti plates treated with H2 plasma at 300 W for 5 min,
Figure 1. Schematic diagram of the experimental setup.
with energy dispersive spectrometry (EDS), and X-ray photoelectron spectrometry (XPS, PHI 5000 VersaProbe, PHI, USA), respectively. Photoluminescence (PL) properties were measured using fluorescence spectrophotometry (F-7000, Hitachi, Japan) at an excitation wavelength of 300 nm.
3. RESULTS AND DISCUSSION Figure 2 panels a and b show low- and high-magnification SEM images of a TiO2 film formed on the surface of a Ti plate after being heated at 500 °C in air. The surface is rough and no nanostructures formed. Figure 2 panels c and d show low- and high-magnification SEM images of a TiO2 film formed on the surface of a Ti plate pretreated with H2 plasma at 150 W for 5 min followed by oxidation at 500 °C. Some nanorods can be observed on the surface, with diameters and lengths in ranges of 10−30 nm and 80−100 nm, respectively. Figure 2 panels e and f show low- and high-magnification SEM images of a TiO2 film formed on the surface of a Ti plate pretreated with H2 plasma at 300 W for 5 min followed by oxidation at 500 °C. Some nanoneedles grew at the rough sites, especially in the shallow grooves spread over the surface. The nanoneedles have diameters and lengths in ranges of 20−50 nm and 100−200 nm, respectively, making them larger than those in Figure 2d.
Figure 3. XRD patterns of (a) untreated Ti plates, (b) Ti plates treated with H2 plasma at 300 W for 5 min, and (c) Ti plates treated with H2 plasma at 300 W for 5 min followed by oxidation at 500 °C for 1.5 h.
respectively. All the peaks in the two patterns correspond to those for the hexagonal structure of Ti (JCPDS card 89-2762). Figure 3c shows an XRD pattern of Ti plates treated with H2 plasma followed by oxidation at 500 °C for 1.5 h. In addition to the peaks of Ti, peaks corresponding to the anatase and rutile phases of TiO2 appear (JCPDS cards 89-4203 and 87-0920). The microstructures of the synthesized nanoneedles were characterizated by high-resolution TEM (HRTEM). Figure 4a shows a bright-field image of a TiO2 nanoneedle, which has a
Figure 2. Low- and high-magnification SEM images of TiO2 films formed on the surface of Ti plates after (a,b) oxidation at a temperature of 500 °C for 1.5 h, (c,d) plasma treatment at 150 W for 5 min followed by oxidation at a temperature of 500 °C for 1.5 h, and (e,f) plasma treatment at 300 W for 5 min followed by oxidation at a temperature of 500 °C for 1.5 h. 3678
dx.doi.org/10.1021/ie202558s | Ind. Eng. Chem. Res. 2012, 51, 3677−3681
Industrial & Engineering Chemistry Research
Article
Figure 4. TEM characterization of an as-synthesized TiO2 nanoneedle: (a) bright-field image, (b) high-resolution image, (c) selected area electron diffraction pattern, and (d) EDS spectrum.
tapered morphology and diameters of 5 nm at the top and 40 nm at the root. Figure 4 panels b and c show a high-resolution image and select area electron diffraction (SAED) pattern taken from the region marked by the box in Figure 4a, respectively. The nanoneedle is a single-crystalline anatase structure that grew along the ⟨11̅6⟩ direction. Many studies have reported anatase TiO2 nanorods growing along the [110], [210], or [101] directions,15,17 but growth along [11̅6] has rarely been reported. To confirm the characterization, more nanoneedles were examined. The results verify that the nanoneedles have single-crystalline features, a ⟨11̅6⟩ growth direction, and an anatase structure. Although the XRD pattern in Figure 3c exhibits several peaks corresponding to different planes of anatase and rutile phases, the signals arise from the whole layer on the surface of the sample exposed to X-rays, including the TiO2 nanoneedles, TiO2 film, and part of the Ti substrate. Only the results from TEM electron diffraction patterns of individual nanoneedles represent the microstructure of the TiO2 nanoneedles. Therefore, the rutile phase shown in the XRD patterns may be attributed to the oxide layer underneath the nanoneedles. Figure 4d shows a TEM-EDS spectrum of a nanoneedle, which contains obvious signals of Ti and O in addition to the copper and carbon signals from the carboncoated TEM copper grid. The atomic ratio of Ti to O is around 1:2, which is consistent with the stoichiometry of TiO2. The growth of TiO2 nanoneedles may be initiated via a vapor-solid (VS)18 rather than a vapor−liquid−solid (VLS) mechanism, since no catalysts were used or found. To determine whether the composition of the Ti plates changed after the plasma treatment, XPS characterization was performed. The results are shown in Figure 5. Figure 5 curves a and b show photoelectron spectra of the Ti plate before and after hydrogen plasma treatment, respectively. Both spectra
Figure 5. XPS spectra of the surfaces of (a) untreated Ti plates and (b) Ti plates treated with H2 plasma at 300 W for 5 min.
show obvious O 1s peaks of Ti−O bonding at Eb ≈ 530.6 eV, which are attributed to the oxidation of Ti on the surface of the Ti plates. The results show that slight oxidation on the surface of the Ti plates occurred before heat treatment. In addition, the two spectra exhibit O 1s peaks of O−H bonding at Eb ≈ 532.5 eV.19 The plasma treatment did not obviously change the composition of the Ti plates, and thus the nucleation of the TiO2 nanoneedles was not initiated by the compositional variation of the plates. Figures 6 panels a and b show SEM images of Ti plates before and after hydrogen plasma treatment at 300 W for 5 3679
dx.doi.org/10.1021/ie202558s | Ind. Eng. Chem. Res. 2012, 51, 3677−3681
Industrial & Engineering Chemistry Research
Article
Figure 6. SEM images of the surfaces of (a) untreated Ti plates and (b) Ti plates treated with H2 plasma at 300 W for 5 min. (c) Schematic diagram of the proposed growth mechanisms of the TiO2 nanoneedles.
nucleated and grew gradually via a stress-assisted VS mechanism. The growth direction of the nanoneedles is related to the relative surface energy magnitude of distinct crystallographic planes from a thermodynamic viewpoint when the growth occurs at a high temperature with a low deposition velocity. Although the surface energies of anatase TiO2 have been previously calculated, only some planes have been reported. The magnitude of the surface energy has been reported to follow the sequence (101) < (100) < (001) < (110).20 The surface energy of the {116} planes and the ⟨116⟩ growth directions of anatase TiO2 nanowires have never been reported. However, TiO2 spheres consisting of active {116} plane-oriented nanocrystallites have recently been reported.21 The photocatalytic measurements suggest that the {116} planes should have similar surface energy as the {110} planes. Therefore, the {116} planes should be high-energy surfaces that could facilitate the deposition of TiO2 with a high crystal growth velocity. The TiO2 nanoneedles reported in this work have a growth direction of [11̅6], which has not been previously reported but is significant to crystallographic studies. Figure 2f shows that the nanoneedles preferentially nucleate at the microgrooves spread over the surface. These microgrooves are assumed to form via the oxidation and blunting of the original sharp cracks initiated by the hydrogen plasma. Figure 6c shows a schematic diagram of the proposed growth mechanisms of the TiO2 nanoneedles. Figure 7 shows a room-temperature PL spectrum of the TiO2 nanoneedles. The spectrum exhibits a relatively strong ultraviolet emission centered at 375 nm and broad visible emission at around 640 nm. The ultraviolet emission is attributed to the near-band-edge excitonic emission of TiO2, and the visible emission is ascribed to the oxygen vacancies of TiO2.22 The as-synthesized TiO2 nanoneedles are promising
min, respectively. Both images show a rough morphology, but the surface of the Ti plates after plasma treatment exhibits additional sharp microcracks and nanocracks with widths in a range of 100−400 nm spread over the surface. The initiation of cracks is attributed to the bombardment or etching of plasma at some weak sites of the Ti plates, such as grain boundaries or some regions with residual stress that formed during the fabrication processes. There are two possible mechanisms for the formation of TiO2 on the surface of the Ti plate during the oxidation process. First, the TiO2 layer could be formed via the oxidation of Ti plates by the downward diffused oxygen from the air. Second, the TiO2 on the surface could be obtained via the condensation of TiO2 vapor formed by the reaction of evaporated Ti atoms and oxygen atoms in the air. Since the working temperature is lower than the melting point of TiO2, the TiO2 vapor directly condensed to a solid phase and became deposited on the surface. When the Ti on the surface was fully oxidized, the subsequent Ti on the surface was supplied by the Ti atoms diffused upward from the deeper part of the Ti plates. The atoms diffused through the TiO2 layer to the surface, and then evaporated and reacted with oxygen to form TiO2 vapor. The two mechanisms both contribute to the formation of TiO2. The first mechanism only generates a TiO2 layer by the direct oxidation of Ti on the surface of the substrate; the second mechanism involves the condensation of TiO2 vapor, which could lead to the growth of TiO2 nanoneedles by a VS growth mode. Since the Ti atoms have the highest energy in the cracks, it is there that they react with oxygen and the TiO2 layer thickens most quickly. Accordingly, the stress induced by the lattice mismatch between the Ti and TiO2 layers accumulates most in the cracks. Cracks thus facilitate the nucleation and growth of TiO2 nanoneedles. Consequently, the TiO2 nanoneedles 3680
dx.doi.org/10.1021/ie202558s | Ind. Eng. Chem. Res. 2012, 51, 3677−3681
Industrial & Engineering Chemistry Research
Article
(4) Liu, B.; Aydil, E. S. Growth of Oriented Single-Crystalline Rutile TiO2 Nanorods on Transparent Conducting Substrates for DyeSensitized Solar Cells. J. Am. Chem. Soc. 2009, 131, 3985. (5) Maeda, M.; Watanabe, T. Evaluation of Photocatalytic Properties of Titanium Oxide Films Prepared by Plasma-Enhanced Chemical Vapor Deposition. Thin Solid Films 2005, 489, 320. (6) Miao, Z.; Xu, D.; Ouyang, J.; Guo, G.; Zhao, X.; Tang, Y. Electrochemically Induced Sol-Gel Preparation of Single-Crystalline TiO2 Nanowires. Nano Lett. 2002, 2, 717. (7) Attar, A. S.; Ghamsari, M. S.; Hajiesmaeilbaigi, F.; Mirdamadi, S.; Katagiri, K.; Koumoto, K. Study on the Effects of Complex Ligands in the Synthesis of TiO2 Nanorod Arrays Using the Sol-Gel Template Method. J. Phys. D: Appl. Phys. 2008, 41, 155318. (8) Wen, B.-M.; Liu, C.-Y.; Liu, Y. Solvothermal Synthesis of Ultralong Single-Crystalline TiO2 Nanowires. New J. Chem. 2005, 29, 969. (9) Kakiuchi, K.; Hosono, E.; Imai, H.; Kimura, T.; Fujihara, S. {111}-Faceting of Low-Temperature Processed Rutile TiO2 Rods. J. Cryst. Growth 2006, 293, 541. (10) Liu, S.; Huang, K. Straightforward Fabrication of Highly Ordered TiO2 Nanowire Arrays in AAM on Aluminum Substrate. Sol. Energy Mater. Sol. Cells 2005, 85, 125. (11) Qiu, J.; Yu, W.; Gao, X.; Li, X. Sol-Gel Assisted ZnO Nanorod Array Template To Synthesize TiO2 Nanotube Arrays. Nanotechnology 2006, 17, 4695. (12) Liu, B.; Boercker, J. E.; Aydil, E. S. Oriented Single Crystalline Titanium Dioxide Nanowires. Nanotechnology 2008, 19, 505604. (13) Peng, X.; Chen, A. Aligned TiO2 Nanorod Arrays Synthesized by Oxidizing Titanium with Acetone. J. Mater. Chem. 2004, 14, 2542. (14) Peng, X.; Wang, J.; Thomas, D. F.; Chen, A. Tunable Growth of TiO2 Nanostructures on Ti Substrates. Nanotechnology 2005, 16, 2389. (15) Huo, K.; Zhang, X.; Hu, L.; Sun, X.; Fu, J.; Chu, P. K. One-Step Growth and Field Emission Properties of Quasialigned TiO 2 Nanowire/Carbon Nanocone Core-Shell Nanostructure Arrays on Ti Substrates. Appl. Phys. Lett. 2008, 93, 013105. (16) Xiang, B.; Zhang, Y.; Wang, Z.; Luo, X. H.; Zhu, Y. W.; Zhang, H. Z.; Yu, D. P. Field-Emission Properties of TiO2 Nanowire Arrays. J. Phys. D: Appl. Phys. 2005, 38, 1152. (17) Cheung, K. Y.; Yip, C. T.; Djurišić, A. B.; Leung, Y. H.; Chan, W. K. Long K-Doped Titania and Titanate Nanowires on Ti Foil and Fluorine-Doped Tin Oxide/Quartz Substrates for Solar-Cell Applications. Adv. Funct. Mater. 2007, 17, 555. (18) Wang, R.-C.; Li, C.-H. Improved Morphologies and Enhanced Field Emissions of CuO Nanoneedle Arrays by Heating ZnO Coated Copper Foils. Cryst. Growth Des. 2009, 9, 2229. (19) da Cruz, N. C.; Rangel, E. C.; Wang, J.; Trasferetti, B. C.; Davanzo, C. U.; Castro, S. G. C.; de Moraes, M. A. B. Properties of Titanium Oxide Films Obtained by PECVD. Surf. Coat. Technol. 2000, 126, 123. (20) Lazzeri, M.; Vittadini, A.; Selloni, A. Structure and Energetics of Stoichiometric TiO2 Anatase Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 119901. (21) Jiao, Y.; Peng, C.; Guo, F.; Bao, Z.; Yang, J.; Schmidt-Mende, L.; Dunbar, R.; Qin, Y.; Deng, Z. Facile Synthesis and Photocatalysis of Size-Distributed TiO2 Hollow Spheres Consisting of {116} PlaneOriented Nanocrystallites. J. Phys. Chem. C 2011, 115, 6405. (22) Sekiya, T.; Kamei, S.; Kurita., S. Luminescence of Anatase TiO2 Single Crystals Annealed in Oxygen Atmosphere. J. Lumin. 2000, 87− 89, 1140.
Figure 7. Room-temperature PL spectrum of the as-synthesized TiO2 nanoneedles.
materials for photocatalytic and photoelectrochemical applications.
4. CONCLUSION Pure anatase TiO2 nanoneedles were directly grown on Ti plates by employing a hydrogen plasma treatment followed by oxidation at 500 °C for 1.5 h. SEM and HRTEM images show that the nanoneedles had diameters and lengths in the ranges of 20−50 nm and 100−200 nm, respectively, and are singlecrystalline anatase structures that grew along the ⟨11̅6⟩ direction. The diameter, length, and growth density of the nanoneedles can be varied by adjusting the power of the plasma employed. The nanoneedles nucleated and grew from the microgrooves that resulted from the cracks induced by the hydrogen plasma via a stress-assisted vapor−solid mechanism. The PL spectrum of the TiO2 nanoneedles exhibits a relatively strong ultraviolet emission centered at 375 nm and broad visible emission at around 640 nm. This study presented a route for growing anatase TiO2 1D nanostructures on substrates, which have potential in photocatalytic and photoelectrochemical applications.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work was financially supported by the National Science Council of Taiwan under Grant NSC 100-2221-E-390-009MY3. The authors would like to thank the Center for Micro/ Nano Technology Research, National Cheng Kung University, and the Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Taiwan for the provision of HRTEM and FE-SEM, respectively.
■
REFERENCES
(1) Ponce, M. A.; Parra, R.; Savu, R.; Joanni, E.; Bueno, P. R.; Cilense, M.; Varela, J. A.; Castro, M. S. Impedance Spectroscopy Analysis of TiO2 Thin Film Gas Sensors Obtained from Water-Based Anatase Colloids. Sens. Actuators B: Chem 2009, 139, 447. (2) Peng, T.; Zhao, D.; Dai, K.; Shi, W.; Hirao, K. Synthesis of Titanium Dioxide Nanoparticles with Mesoporous Anatase Wall and High Photocatalytic Activity. J. Phys. Chem. B 2005, 109, 4947. (3) Gao, S. A.; Xian, A. P.; Cao, L. H.; Xie, R. C.; Shang., J. K. Influence of Calcining Temperature on Photoresponse of TiO2 Film under Nitrogen and Oxygen in Room Temperature. Sens. Actuators B: Chem. 2008, 134, 718. 3681
dx.doi.org/10.1021/ie202558s | Ind. Eng. Chem. Res. 2012, 51, 3677−3681
