pubs.acs.org/Langmuir © 2010 American Chemical Society
Large Scale Synthesis and Gas-Sensing Properties of Anatase TiO2 Three-Dimensional Hierarchical Nanostructures Chengxiang Wang, Longwei Yin,* Luyuan Zhang, Yongxin Qi, Ning Lun, and Ningning Liu Key Laboratory for Liquid-Solid Structural Evolution & Processing of Materials, Ministry of Education, School of Materials Scinece & Engineering, Shandong University, Jinan 250061, P.R. China Received March 4, 2010. Revised Manuscript Received June 14, 2010 Three-dimensional (3D) crystalline anatase titanium dioxide (TiO2) hierarchical nanostructures were synthesized through a facile and controlled hydrothermal and after-annealing process. The formation mechanism for the anatase TiO2 3D hierarchical nanostructures was investigated in detail. The 3D hierarchical nanostructures morphologies are formed by self-organization of several tens of radially distributed thin petals with a thickness of several nanometers with a larger surface area. The surface area of TiO2 hierarchical nanostructures determined by the Brunauer-Emmett-Teller (BET) adsorption isotherms was measured to be 64.8 m2 g-1. Gas sensing properties based on the hierarchical nanostructures were investigated. A systematic study on sensitivity as a function of temperatures and gas concentrations was carried out. It reveals an improved ethonal gas sensing response property with a sensitivity of about 6.4 at 350 °C upon exposure to 100 ppm ethanol vapor for the TiO2 hierarchical nanostructures. A gas sensing mechanism based on the adsorption-desorption of oxygen on the surface of TiO2 is discussed and analyzed. This novel gas sensor can be multifunctional and promising for practical applications. Furthermore, the hierarchical nanostructures with high surface area can find variety of potential applications such as solar cells, biosensors, catalysts, etc.
1. Introduction The synthesis of inorganic nanocrystals with controlled shape, potential materials with direction and shape dependent properties, is an important goal of advanced materials chemistry. An objective in materials science of increasing importance is to construct inorganic architectures with hierarchically complex structures and optimized properties, which have great potential as agents in optical, electronic, biological, magnetic, and photocatalytic applications.1-6 As one kind of important semiconductor with wide bandgap, titanium dioxide (TiO2) is one of the most important transition metal oxides that has been widely *Author to whom correspondence should be addressed. Phone: 86-53188396970. E-mail:
[email protected]. (1) Chan, E. M.; Mathies, R. A.; Alivisatos, A. P. Nano Lett. 2003, 3, 199–201. (2) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437, 121–124. (3) Kong, X. Y.; Wang, Z. L. Nano Lett. 2003, 3, 1625–1631. (4) Goldberger, J.; He, R.; Zhang, Y.; Lee, S.; Yan, H.; Choi, H. J.; Yang, P. Nature 2003, 422, 599–602. (5) (a) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176–2179. (b) Tian, B. Z.; Zheng, X. L.; Kempa, T. J.; Fang, Y.; Yu, N. F.; Yu, G. H.; Huang, J. L.; Lieber, C. M. Nature 2007, 449, 885–890. (c) Gao, P. X.; Ding, Y.; Mai, W. J.; Hughes, W. L.; Lao, C. S.; Wang, Z. L. Science 2005, 309, 1700–1704. (d) Huo, Z. Y.; Tsung, C. K.; Huang, W. Y.; Fardy, M.; Yan, R. X.; Zhang, X. F.; Li, Y. D.; Yang, P. D. Nano Lett. 2009, 9, 1260– 1264. (6) Yin, L. W.; Lee, S. T. Nano Lett. 2009, 9, 957–963. (7) Han, X. G.; Kuang, Q.; Jin, M. S.; Xie, Z. X.; Zheng, L. S. J. Am. Chem. Soc. 2009, 131, 3152–3153. (8) Teramura, K.; Okuoka, S.-I.; Yamazoe, S.; Kato, K.; Shishido, T.; Tanaka, T. J. Phys. Chem. C 2008, 112, 8495–8498. (9) Yamashita, H.; Harada, M.; Misaka, J.; Takeuchi, M.; Ichihashi, Y.; Goto, F.; Ishida, M.; Sasaki, T.; Anpoa, M. J. Synchrotron Rad. 2001, 8, 569–571. (10) Kim, H. G.; Hwang, D. W.; Lee, J. S. J. Am. Chem. Soc. 2004, 126, 8912– 8913. (11) Kim, I.-D.; Rothschild, A.; Lee, B. H.; Kim, D. Y.; Jo, S. M.; Tuller, H. L. Nano Lett. 2006, 6, 2009–2013. (12) Wang, Y. M.; Du, G. J.; Liu, H.; Liu, D.; Qin, S. B.; Wang, N.; Hu, C. G.; Tao, X. T.; Jiao, J.; Wang, J. Y.; Wang, Z. L. Adv. Funct. Mater. 2008, 18, 1131–1137. (13) O’Hayre, R.; Nanu, M.; Schoonman, J.; Goossens, A.; Wang, Q.; Gr€atzel, M. Adv. Funct. Mater. 2006, 16, 1566–1576. (14) Itzhaik, Y.; Niitsoo, O.; Page, M.; Hodes, G. J. Phys. Chem. C 2009, 113, 4254–4256. (15) Shankar, K.; Bandara, J.; Paulose, M.; Wietasch, H.; Varghese, O. K.; Mor, G. K.; LaTempa, T. J.; Thelakkat, M.; Grimes, C. A. Nano Lett. 2008, 8, 1654– 1659.
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investigated and used in the fields of paints, photocatalysts,7-10 gas sensors,11,12 solar cells,13-15 Li-ion battery,16 electrochromic devices,17 etc. Nanostructured titanium dioxides have attracted great interests due to their better performance than traditional structured materials. Variety of nanostructured titanium dioxides, including nanotubes,18-20 nanowires,21 nanospheres,22 threedimensional nanostructures,23 macro/mesoporous materials,24,25 have been synthesized. The synthetic routes for the nanostructures of TiO2 include hydrothermal method,22 anodic oxidation,19,20 CVD method,26 sol-gel method,27 etc. However, a direct synthesis of complex nanostructured titanium dioxides still remains a challenge. There are mainly four principal strategies for generating these complex nanostructures.28-33 Template-based (16) Aldon, L.; Kubiak, P.; Picard, A.; Jumas, J.-C.; Olivier-Fourcade, J. Chem. Mater. 2006, 18, 1401–1406. (17) Nah, Y.-C.; Ghicov, A.; Kim, D. H.; Berger., S.; Schmuki, P. J. Am. Chem. Soc. 2008, 130, 16154–16155. (18) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Adv. Mater. 1999, 11, 1307–1311. (19) Wang, J.; Lin, Z. Q. Chem. Mater. 2008, 20(4), 1257–1261. (20) Sun, W.-T.; Yu, Y.; Pan, H.-Y.; Gao, X.-F.; Chen, Q.; Peng, L.-M. J. Am. Chem. Soc. 2008, 130, 1124–1125. (21) Feng, X. J.; Shankar, K.; Varghese, O. K.; Paulose, M.; Latempa, T. J.; Grimes, C. A. Nano Lett. 2008, 8, 3781–3786. (22) Li, J.; Zeng, H. C. J. Am. Chem. Soc. 2007, 129, 15839–15847. (23) Wu, J. M.; Qi, B. J. Phys. Chem. C 2007, 111, 666–673. (24) Smarsly, B.; Grosso, D.; Brezesinski, T.; Pinna, N.; Boissiere, C.; Antonietti, M.; Sanchez, C. Chem. Mater. 2004, 16, 2948–2952. (25) Cheng, Y.-J.; Zhi, L. J.; Steffen, W.; Gutmann, J. S. Chem. Mater. 2008, 20, 6580–6852. (26) Goossens, A.; Maloney, E.-L.; Schoonman, J. Chem. Vap. Deposition 1998, 4, 109–114. (27) Chung, C.-C.; Chung, T.-W.; Yang, T. C.-K. Ind. Eng. Chem. Res. 2008, 47, 2301–2307. (28) Xie, J.; Zhang, Q.; Lee, J.; Wang, D. I. C. ACS Nano 2008, 2, 2473–2480. (29) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 16744–16746. (30) Huang, J.-Q.; Huang, Z.; Guo, W.; Wang, M.-L.; Cao, Y.-G.; Hong, M.-C. Cryst. Growth Des. 2008, 8, 2444–2446. (31) Perlich, J.; Kaune, G.; Memesa, M.; Gutmann, J. S.; Muller-Buschbaum, P Philos. Trans. R. Soc. A 2009, 367, 1783–1798. (32) Lechmann, M. C.; Kessler, D.; Gutmann, J. S. Langmuir 2009, 25, 10202– 10208. (33) Liu, B.; Zeng, H. C. J. Am. Chem. Soc. 2004, 126, 8124–8125.
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DOI: 10.1021/la100910u
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synthesis is the first approach. Hard or soft templates were used as structure-directing agents.30-32 However, the removal of the templates was a little complex. The second approach is based on phenomenon of oriented attachment of primary nanoparticles.33 The third approach is the induced anisotropic growth with use of specific capping agents26 and the last strategy is based on the Kirkendall effect.29 It is well-known that anatase, rutile, and brookite are the most common polymorphs of titania (TiO2). Anatase TiO2 with a tetragonal structure (the c-axis being 2.7 times the a-axis) is one of the most important semiconductors, playing a central role in various applications such as photovoltaic cells, photo/electrochromics, photocatalysis, photonic crystals, smart surface coatings, and sensors.34-36 Anatase is the dominant outcome of the vast majority of liquid-solid and gas-solid transformation preparation methods.36-39 It has been shown that the amorphous titania may serve as an appropriate precursor for preparing single-phase of anatase TiO2. However, brookite (a polytype of anatase) may also typically present in the synthesized products. Because the single-phase TiO2 products with anatase structure has advantages for variety of potential applications and investigations (i.e., samples free of amorphous titania, brookite, and rutile), improved synthesis routes for producing single-phase nanocrystalline titania with anatase structure are required. Singlephase titania can be synthesized by applying specific hydrolysis catalysts,40,41 by using a nonhydrolytic sol-gel method,42 and by hydrothermal processing of hydrolytic sol-gel products.43,44 The most important challenge for the preparation of TiO2 is to design a specific material for a defined function in the final materials. Despite the extensive reports on nanostrucutres of crystalline anatase titania, the organization of primary building units into three-dimensional hierarchical nanostructures with a high surface area and their applications as sensors, though, remaining a challenge. Herein, we present a facile hydrothermal and after-annealing process to synthesize three-dimensional (3D) hierarchical nanostructures of anatase TiO2 with a large surface area. The hierarchical nanostructures are formed by the self-organization of several tens of radially distributed thin nanoflakes with a thickness of several nanometers with a larger surface area and an higher surface-to-volume ratio. The morphology evolution and formation mechanism of the anatase TiO2 hierarchical nanostructures were investigated. Gas sensing properties based on the large-surface area hierarchical nanostructures are investigated. The results showed that they have a fast response to ethanol vapor. A systematic study on sensitivity as a function of temperatures and gas concentrations was carried out. This novel gas sensor can be multifunctional and promising for practical applications. Furthermore, these nanostructures with high surface area can be used as promising supporting materials in a variety of applications such as solar cells, biosensors, catalysts, etc. (34) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (35) Chen, X.; Mao, S. Chem. Rev. 2007, 107, 2891–2959. (36) Dietbold, U. Surf. Sci. Rep. 2003, 48, 53–229. (37) Zhang, H.; Bandfield, J. F. J. Mater. Chem. 1998, 8, 2073–2076. (38) Penn, R. L.; Banfield, J. F. Geochim. Cosmichim. Acta 1999, 63, 1549–1557. (39) Donnay, J. D.; Harker, D. Am. Mineral. 1937, 22, 446.13. (40) Scolan, E.; Sanchez, C. Chem. Mater. 1998, 10, 3217–3223. (41) Bokhimi; Morales, A.; Novaro, O.; Lopez, T.; Sanchez, E.; Gomez, R. J. Mater. Res. 1995, 10, 2788–2796. (42) Arnal, P.; Corriu, R. J. P.; Leclercq, D.; Mutin, P. H.; Vioux, A. J. Mater. Chem. 1996, 6, 1925–1932. (43) Wang, C. C.; Ying, J. Y. Chem. Mater. 1999, 11, 3113–3120. (44) Yanagisawa, K.; Ovenstone, J. J. Phys. Chem. B 1999, 103, 7781–7787.
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2. Experimental Section Sample Synthesis. In a typical process, 60 mg of titanium powder was added to 60 mL of 10 M NaOH solution. After sonication under stirring for 10 min, the mixture was injected into a 100 mL Teflon-lined autoclave. Hydrogen peroxide (30%, 0.5 mL) was then added into the mixture under stirring. After the mixture was stirred for 2 min, the autoclave was sealed and heated at 150 °C for different times. The system was then allowed to cool to ambient temperature. The as-obtained gray product was collected by a vacuum filter, washed with 0.2 M HCl solution five times, and then treated with deionized water to remove any possible ionic remnaints. Finally, the products were dried in a vacuum at 60 °C for 5 h, calcined at 550 °C for 2 h, and collected for further characterization. To investigate the morphology evolution and formation process of the TiO2 hierarchical nanostructures, the products synthesized at 150 °C for different times such as 10, 20, 30, 50, 1 h, and 1.5 h were prepared. To investigate the effects of H2O2 and NaOH on the formation process of the TiO2 hierarchical nanostructures, reactions of Ti þ NaOH, Ti þ H2O2 and TiO2 (P25) þ NaOH þ H2O2 were carried out, respectively. Ti foils were also used to replace Ti powders as reactants to determine whether the size and morphology of precursor reactants can affect the morphologies of final products. Materials Characterization. Samples of the as-prepared products were characterized by X-ray powder diffraction (XRD) with a Rigaku D/max-kA diffractometer with Cu Ka radiation (60 kV, 40 mA). The morphologies of the synthesized titania products were observed in a Hitachi SU-70 field emission scanning electron microscope. The selected-area electron diffraction (SAED) patterns and high-resolution transmission electron microscopy (HRTEM) analyses were carried out on a Phillips Tecnai 20U-Twin high-resolution transmission electron microscope at an acceleration voltage of 200 kV. TG analysis and DTA measurements were carried out using an under air in the temperature range from 30 to 700 °C. BET surface area was measured using a Quantachrome QuadraSorb SI surface area analyzer. Gas Sensing Properties. Gas-sensing properties were carried out using a HW-30A gas sensitivity instrument and the fabrication prccess of the TiO2 sensor was as follows. The synthesized TiO2 hierarchical nanostructures were mixed with water and then coated onto an Al2O3 tube, on which two platinum wires have been installed at each end. The operating temperatures were controlled by adjusting the heating power, using a Ni-Cr alloy coil placed through the tube. The measurement followed a static process, a given amount of tested gas was injected into a glass chamber and mixed with air. Tuning the heating voltage, we can get the change of sensitivity as a function of temperatures. Given different amounts of tested gas for different times, we can get the response curve as a function of the time and amount of gas.
3. Results and Discussion A large quantity of white color products can be finally obtained after the calcination process. The synthesized products were observed in a Hitachi SU-70 thermal field emission scanning electron microscope (FE-SEM), as shown in Figure 1. The synthesized products display three-dimensional hierarchical morphology with a uniform size distribution. The size of the 3D hierarchical particles is 1-1.5 μm in diameter. Parts c-f of Figure 1 depict magnified SEM images for some single nanostructures. The synthesized nanostructures consist of selfsupported radial nanoflakes with a larger surface area and a higher surface-to-volume ratio. The radially distributed nanoflakes are just like the petals in a natural chrysanthemum-flower structuture and have a thickness of several nanometers. The surface area of the chrysanthemum-like TiO2 nanostructures determined by the BET adsorption isotherms was measured to be 64.8 m2 g-1. Typical EDS analysis shows that the synthesized Langmuir 2010, 26(15), 12841–12848
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Figure 1. (a) (b) Low magfication FE-SEM images of anatase TiO2 three-dimensional herarchical nanostructures. (c)-(f) Magnified SEM images for some single herarchical nanostructures. The synthesized nanostructures consist of self-supported radial nanoflakes with a larger surface area.
white color products are composed of pure titanium and oxygen elements, as shown in Figure S1 (Supporting Information). The structural nature of the herarchical nanostructures was further investigated by transmission electron microscopy (TEM). Parts a and b of Figure 2a,b demonstrate typical TEM images of single herarchical nanostructure. It clearly shows that the herarchical nanostructures have a radial structure with one core section, and this coincided with that of FE-SEM images. The petal nanoflakes have a width of 10-13 nm and are very thin with a thickness of only several nanometers. The crystal structure of the synthesized products can be determined by electron diffraction pattern (shown in Figure 2c). The (101), (004), and (105) diffraction rings correspond to that of anatase TiO2. The HRTEM lattice images shown in Figure 2d suggest that the petal nanoflakes grow along the [112] direction, and no dislocations and defects can be observed in the structure. The d-spacing of 0.23 and 0.17 nm corresponds to that of (112) and (121) planes of anatase TiO2. Figure 2e shows another typical HRTEM lattice image of a petal nanoflake. There exist defects of twins and stacking faults in the nanostructure, as illustrated by arrows in Figure 2e. The inset in left upper side in Figure 2e is a corresponding FFT pattern. Defects have high energies and it means gas molecules can adsorb on defects easily. High defects densities are useful to enhance the gas sensing properties. Of course, the relationship between the defects and gas sensing property is very complicated and needs to be studied further. XRD patterns are used to describe the phase evolution of the TiO2 samples treated at different temperatures. After the hydrothermal process, gray color products were obtained. Figure 3a depicts a XRD pattern for the products obtained via the hydrothermal process but before calcinations. The products are composed of residual titanium powder (PDF#65-3362) and titanium hydrogen oxide hydrate (H2Ti4O9 3 H2O) (PDF#36-0655). Besides Ti and H2Ti4O9 3 H2O, the powder before calcinations mainly consistsed of amorphous phases containing Ti and O elements. After calcinations at 550 °C for 1 h, the color of the products changes from gray to white. The final white products are composed of anatase TiO2 with lattice constants of a = 3.7852 A˚ and c = 9.5139 A˚, as shown in Figure 3b (PDF#21-1272), with little amount of residual Ti phase remained at the core of the hierarchical nanostructures. Langmuir 2010, 26(15), 12841–12848
The phase transformation process of the hydrothermalprocess-obtained products during the heat treatment was investigated in the following TG and DTA measurements. Thermogravimetric (TG) and differential thermal analysis (DTA) curves of the as-prepared powders are displayed in Figure 4. In the TG curve, the initial mass loss of 0.5 wt % between 30 and 60 °C (endothermic DTA peak at 60 °C) corresponds to removal of water weakly adsorbed to the surface of the products. The major mass loss of about 5 wt %, occurring at temperatures between 60 and 400 °C, results from the dehydration and decomposition of H2Ti4O9 3 H2O (endothermic DTA peak at 150 °C) and the transformation to anatase TiO2. At the end of the TG curve, there is a slight increase in the mass of as-prepared powder due to oxidation of residual titanium powder (with no obvious exothermic peak, only a slight increase in ΔT). There is a broad exothermic peak on DTA curve between 150 and 600 °C. It corresponds to the amorphous-to-crystal transition taking place in a broad temperature range. Combined with the TG curve, we can estimate the contents of H2Ti4O9 3 H2O and residual titanium powder in the products before calcinations are 27 and 1.25 wt %, respectively (corresponding to 5 wt % mass loss and 1 wt % mass increase). Different experimental chemcial reactions were carried out to investigate the formation process of the 3D hierarchical nanostructures. Figure 5a shows a FE-SEM image of Ti powder precursor. The Ti precursor displays irregular shapes with several micrometers in diameter. To investigate the effects of H2O2 and NaOH on the formation of hierarchical nanostructures, the products obtained in the presnece of only H2O2 or NaOH were shown in Figure 6b,c, respectively. From Figure 5b, we can see that three-dimensional herarchical nanostructures did not form in the presence of only H2O2. It is shown from Figure 6c that although the 3D herarchical nanostructures can be formed in the presence of only NaOH, the surface of the herarchical nanostructures are disordered. The formation of the hierarchical nanostructures on the surface of Ti particle in the presence of only NaOH is considered to be related to the reaction between OH- and Ti atoms, which diffuses through the liquid-solid interface to form soluble TiO32- ions.45 The reaction between (45) Yu, X. X.; Jiaguo Yu, J. G.; Cheng, B.; Jaroniec, M. J. Phys. Chem. C 2009, 113, 17527–17535.
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Figure 2. (a) (b) Typical TEM images of hierarchical nanostructures. (c) Electron diffraction pattern from single nanostructures. (d) A HRTEM lattice image suggests that the petal nanoflake grown along the [112] direction, no dislocations and defects can be observed in the structure. The d-spacing of 0.23 and 0.17 nm corresponds to that of (112) and (121) planes of anatase TiO2. (e) Another typical HRTEM lattice image of a petal nanoflake, showing the presence of defects of twins and stacking faults in the nanostructure, as illustrated by arrows. The inset in the left upper side in Figure 2e is a corresponding fast Fourier transform (FFT) pattern.
Figure 4. TG and DTA curves of as-prepared powder. Figure 3. XRD patterns of as-prepared products (a) before and
(b) after calcinations at 550 °C for 2 h.
H2O2 and Ti powder have been investigated and Kirkendall process was used to explain the formation of hierarchical nanostructures.23,29,46 In the presence of NaOH, if Ti powder was replaced by commercial P25, no herarchical nanostructured products were obtained, as shown in Figure 5d, while as the Ti powder was replaced by Ti foils, nanobelt arrays were obtained instead of heriarchical nanostructures, as shown in Figure 5f. This indicates that Ti powder, H2O2, and NaOH are necessary for preparing TiO2 herarchical nanostructures with high surface area. Figure 6 depicts the morphology evolution of the herarchical nanostructures with synthesis time prolonging in the presence of both H2O2 and NaOH. After the reaction was kept at 150 °C for (46) DeRosa, D. M.; Zuruzi, A. S.; MacDonald, N. C. Adv. Eng. Mater. 2006, 8, 77–80.
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10 min, tiny TiOx nanoflakes grew on surfaces of the Ti particles, as shown in Figure 6a. After the reaction for 20 min, spherical hierarchical nanostructures formed, but their morphologies were not uniform, and the size including length and diameter of the nanoflakes is relatively small. Compared with the products synthesized for 10 min shown in Figure 6a, the Ti particle precursor was gradually eroded, the whole Ti particles were peeled off to form spherical hierarchical nanostructrues. With increasing reaction time, the size of the nanoflakes of the hierarchical produts increased, and the Ti at the core section of the heirarchial nanostructures was depleted gradually, and the morphologies of hierarchical nanostructures became uniform gradually, as demonstrated Figure 6b-f. It is considered that Ostwald ripening may be responsible for this phenomena.29 The growth process of hierarchical nanostructures was illustrated in Figure 7. In the first stage, H2O2 reacts with the Ti Langmuir 2010, 26(15), 12841–12848
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Figure 5. SEM images of the products obtained under different reaction conditions. (a) Ti powder precursor. (b) Ti powder þ H2O2 (30%). (c) Ti powder þ 10 M NaOH. (d) TiO2 (P25) þ H2O2 (30%) þ 10 M NaOH. (e) Cross section of a hierarchical nanostructrues under reaction conditions of Ti powder þ 10 M NaOH þ H2O2 (30%). (f) Ti foils þ 10 M NaOH þ H2O2 (30%).
Figure 6. SEM images showing the structural evolution of the obtained products kept at 150 °C for different times: (a) 10 min; (b) 20 min; (c) 30 min; (d) 50 min; (e) 1 h; (f) 1.5 h. The reactants are 60 mg of Ti powder, 60 mL of 10 M NaOH, 0.5 mL of H2O2 (30%).
atoms on the surface, leading to the formation of titanium oxide layer (TiOx), and then the interior Ti atoms will out-diffuse to the surface to react with H2O2 on the TiOx/Ti interface.29 Meanwhile, NaOH reacts with the TiOx on the surface of the Ti precursor. The reactions can be described as follows: H2 O2 þ Ti f H2 O þ TiOx TiOx þ NaOH f Na2 TiO3 þ H2 O The reaction between H2O2 and Ti will lead to a porous structure in the titanium oxide layers according to the Kirkendall effect.29 Meanwhile, NaOH solutions will corrode the titanium oxide layers on the surface, uncover the small voids and then corrode the interior structure. On the effects of the H2O2 and NaOH, the surface of titanium particles were corroded to a porous structure at the initial stage. At this stage, the concentration of TiO32- was low and there were a few TiOx flakes depositing on the surface.23 As the time increased, more TiOx oxides formed and reacted with NaOH and the concentration of TiO32- also increased, leading to the growth of TiOx nanoflakes on the surface of the Ti particle by Ti-O-Ti bonds between different TiO32ions. After the reaction of stage 2, hierarchical nanostructures Langmuir 2010, 26(15), 12841–12848
formed. It is believed that the titanium core will disappear if the reaction time is long enough. In stage 3, the as-prepared hierarchical nanostructures were washed by diluted HCl solutions and distilled water. In this step, the Na-O-Ti bonds on the surface of nanoflowers are believed to react with acid and water to form new H-O-Ti and Ti-O-Ti bonds.18 At this stage, the products consist of amorphous structure titanium oxide (TiOx), titanium hydrogen oxide hydrate (H2Ti4O9 3 H2O), and a little Ti powder at the core section. This was consistent with the XRD result of the products before calcinations. At the stage 4 of calcinations, phase transitions of the TiOx and H2Ti4O9 3 H2O to anatase TiO2 took place, and the anatase titanium dioxide (TiO2) 3D hierarchical nanostructures were synthesized. This is in good well agreement with the SEM, XRD, TEM, TG, and DTA results. As one kind of n-type semiconductor, the carriers of titanium dioxides are electrons. The concentration of electrons will increase with a temperature increase in n-type nature semiconductors. We investigated the baseline shift of the TiO2 hierarchical nanostructure gas sensor upon exposure to air as the temperature increased. It is clear that the resistance baseline shifts negatively as the temperature increases upon exposure to the air, as shown in Figure 8. Then, the current flowing through the gas sensor increases with temperature. However, the change of the resistance baselines with DOI: 10.1021/la100910u
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Figure 7. Schematic illustration of formation of TiO2 hierarchical nanostructures. In stage 1, H2O2 reacts with Ti atoms to form TiOx, then NaOH reacts with the TiOx to from Na2TiO3. On the effects of H2O2 and NaOH, the surface of the Ti particle was corroded to a porous structure. In stage 2, TiO32- will redeposit on the surface of the Ti particle as their concentration increase with time and the Ti core will be corroded further to a smaller one. In this step, hierarchical nanostructures form. In stage 3, the surface Na-O-Ti bonds are replaced by H-O-Ti and Ti-O-Ti bonds. In stage 4, as-prepared heirarchcial nanostructures were calcined at 550 °C, H2Ti4O9 3 H2O and TiOx turned into anatase TiO2.
Figure 8. Changes of resistance baseline at different operating temperatures upon exposure to the air.
temperature is very small compared to the response resistance upon exposure to ethanol vapor. This is one of the advantages of wide band gap semiconductor gas sensor. Ethanol gas sensing behavior based on the as-prepared TiO2 hierarchical nanostructures were tested at temperatures from 340 to 485 at 10 deg intervals. All the sensing experiments were carried out at a relative humidity (RH) of 54%. A typical electrical response of the as-prepared sensor to 100 ppm ethanol vapor at 390 °C was shown in Figure 9. Initially, the resistance of the sensor is about 22 MΩ. Upon exposure to ethanol vapor, the resistance of the sensor drops abruptly and reaches a steady plateau at about 12 MΩ in a relatively short period, which is about half of the former. Figure S2 (Supporting Information) shows the gas response curves of the TiO2 sample sensor at different operating temperatures upon exposure to the 100 ppm ethanol vapor. Generally, the baseline resistance decreases with increasing testing temperature. Response and recovery times are important parameters for a gas sensor. The response time is usually defined as the time needed to reach 90% of the equilibrium value after the injection of the test gas. The recovery time is defined as the time needed to return to 12846 DOI: 10.1021/la100910u
Figure 9. Typical gas response curve of the TiO2 sensor to 100 ppm ethanol vapor at 390 °C.
10% above the original response in air after stopping the flow of the test gas. To obtain a typical gas response curve, ethanol was injected onto an evaporation station at 10 s. Under the action of fans, the ethanol evaporated rapidly and mixed well with the air in the gas distribution box. After the response curve is stable, the gas distribution box was opened and the gas around the gas sensor changed back to the air. In the typical sensing curve, the response and recovery times of the as-fabricated TiO2 nanoflower sensor are about 13 and 7 s. The operating temperatures have an important effect on the response and recovery times of the the TiO2 sample gas sensor. As the operating temperature increases, the response and recovery times of the TiO2 nanoflower ethonal gas sensor become shorter. There is a small fluctuation in the range of 350-470 °C and they tend to get smaller gradually. The response and recovery times of the gas sensor become shorter as the temperature increases, as shown in Table 1. The response and recovery times of the sensors are shorter than other reports.47,48 (47) Sun, L.; Huo, L.; Zhao, H.; Gao, S.; Zhao, J. Sens. Actuators, B 2006, 114, 387–391. (48) Garzella, C.; Comini, E.; Tempesti, E.; Frigeri, C.; Sberveglieri, G. Sens. Actuators, B 2000, 68, 189–196.
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Table 1. Response and Recovery Times of the TiO2 Sample Sensor at Different Operating Temperatures 350 °C 370 °C 390 °C 410 °C 430 °C 450 °C 470 °C response time (s) recovery time (s)
12 9
13 11
13 7
10 6
9 7
7 5
7 5
This may be due to the porosity of the sensing layers fabricated by the hierarchical nanostructures TiO2. It would be fast for the gas to diffuse into and out of the sensing layers. Usually, gas sensitivity (S) is an important parameter to describe the behavior of a gas sensor, and can be expressed as the ratio of electrical resistance in air (Ra) to that in the testing gas (Rg): S = Ra/Rg. S is greatly influenced by operating temperatures and gas concentration. The relationship of sensitivity versus temperature was investigated upon exposure to 100 ppm ethanol vapor as shown in Figure 10. The sensitivity varies with different temperatures. The sensitivity first increases with temperature up to 350 °C and goes through a highest peak at 350 °C (S = 6.4); after that, it decreases gradually with minor fluctuations. The curve of the sensitivity versus temperature is similar to reported results on the metal oxide based gas sensors.11,49,50 For the gas sensing mechanism of metal oxide materials, there have been proposed different models for different gas sensing.50-52 Generally, when a metal oxide gas sensor is exposed to air, oxygen species can be adsorbed on the surface of the sensor, and then are ionized into O- (adsorption) or O2- (adsorption) by capturing free electrons from the conductance band of TiO2. Therefore, TiO2 shows a high resistance in air and this can be revealed from the weak current (∼10-7A) flow through the gas sensor. This process can be depicted as follows.53 O2 ðgasÞ T O2 ðadsorptionÞ O2 ðadsorptionÞ þ e T O2 - ðadsorptionÞ O2 - ðadsorptionÞ þ e T 2O - ðadsorptionÞ O - ðadsorptionÞ þ e T O2 - ðadsorptionÞ When the sensor is exposed to a tested gas of ethanol at a moderate temperature in our case, ethanol would react with O-, O2-, and release electrons. This process lowers the resistance of the sensor and the current rises. This can simply be described as51 C2 H5 OHðgasÞ þ O2 - ðadsorptionÞ f C2 H5 O - ðadsorptionÞ þ OH - ðadsorptionÞ C2 H5 O - ðadsorptionÞ f ðC2 H5 Þ2 OðadsorptionÞ þ O - ðadsorptionÞ þ e C2 H5 OHðgasÞ þ O2 - ðadsorptionÞ þ hole f CO2 þ H2 O þ Vo •• where Vo•• is the doubly charged oxygen vacancy. According to the mechanism, the curve in Figure 10 can be explained qualitatively. First, we assume that the reason for the current increase is just due to the reaction between ethanol and the O- (adsorption) and O2- (adsorption). When the temperature is between 340 and (49) Jing, Z. H.; Zhan, J. H. Adv. Mater. 2008, 20, 4547–4551. (50) Ruiz, A. M.; Cornet, A.; Morante, J. R. Sens. Actuators, B 2005, 111-112, 7–12. (51) Cao, M. H.; Wang, Y. D.; Chen, T.; Antonietti, M.; Niederberger, M. Chem. Mater. 2008, 20, 5781–5786. (52) Duy, N. V.; Hieu, N. V.; Huy, P. H.; Chien, N. D.; Thamilselvan, M.; Yi, J. Physica E 2008, 41, 258–263. (53) Wu, X.-H.; Wang, Y.-D.; Liu, H.-L.; Li, Y.-F.; Zhou, Z.-L. Mater. Lett. 2002, 56, 732–736.
Langmuir 2010, 26(15), 12841–12848
Figure 10. Gas response versus operating temperature of the sensor to 100 ppm ethanol vapor.
Figure 11. Response curve of the sensor to ethanol with increasing concentrations at operating temperature of 350 °C.
480 °C, chemical adsorption is the main form of adsorption for oxygen on surfaces of titanium dioxide. It can be considered that all the oxygen molecules adsorbed on surfaces of titanium dioxide exist in the form of ions at the high temperature. According to adsorption theory, the amount of oxygen molecules adsorbed on surfaces of TiO2 would increase first and then decrease as the temperature increases, so does the amount of the electrons injected into the gas senor by oxidation of ethanol molecules at a certain concentration according to the hypothesis before. This means that Rg first decreases and then increases with increasing temperature, while for Ra, it decreases slowly with increasing temperature, as shown in Figure 8 and 9. The sensitivity can be considered to be proportional to 1/Rg. As Rg changes with increasing temperature, the sensitivity first increases and then decreases. There is a critical temperature at which the sensitivity is at the maximum value. This temperature is different depending on concentration, type of materials, target gases, etc. Figure 11 depicts the resistance response upon exposure to different ethanol vapor concentrations carried out at 350 °C, at which the sensitivity has a maximum value. It is found that the resistance response was proportional to the increasing concentration of ethanol vapor from 20 to 100 ppm but slowly tended to saturation when concentrations reached higher levels. This behavior enables a conceivable discrimination of ethanol vapor in different concentrations between 20 and 100 ppm at 350 °C.
4. Conclusion In summary, anatase TiO2 three-dimensional hierarchical nanostructures were synthesized through a simple hydrothermal and after the annealing process at 550 °C. The hierarchical DOI: 10.1021/la100910u
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nanostructures are formed by the self-organization of several tens of radially distributed thin petals with a thickness of several nanometers with a larger surface area. The surface area of the TiO2 nanostructures determined by the BET adsorption isotherms was measured to be 64.8 m2 g-1. H2O2, Ti powder, and NaOH are necessary conditions to obtain uniform TiO2 nanoflowers in our experiments. At the initial time, H2O2 and NaOH corrode the surface of Ti powders. Then, the concentration of TiO32- increased as the time increased and redeposited onto the surface of Ti particles, forming nanoflakes of titanium oxides. Gas sensing properties based on the large-surface-area hierarchical nanostructures are investigated. An ethonal gas sensing response with a sensitivity of about 6.4 at 350 °C upon exposure to 100 ppm ethanol vapor is obtained. The response and recovery times of the sensors can reach about 10 s. A gas sensing mechanism based on the adsorption-desorption of oxygen on the surface of TiO2 is discussed and analyzed. This novel gas sensor is multifunctional and promising for practical applications. The
12848 DOI: 10.1021/la100910u
Wang et al.
hierarchical nanostructures with high surface area can be used as promising supporting materials in a variety of applications such as solar cells, biosensors, catalysts, etc. Acknowledgment. We acknowledge support from the National Nature Science Foundation of China (Nos. 50872071 and 50972079), the Shandong Natural Science Fund for Distinguished Young Scholars (JQ200915), Nature Science Foundation of Shandong Province (Y2007F03 and Y2008F26), Foundation of Outstanding Young Scientists in Shandong Province (No. 2006BS04030), Tai Shan Scholar Foundation of Shandong Province, and Gong Guan Foundation of Shandong Province (2008GG10003019). Supporting Information Available: Typical EDS spectrum, gas response curves, and photoluminescence spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.
Langmuir 2010, 26(15), 12841–12848
