Synthesis, Characterization, and Gas Sensing Properties of Porous

ARTICLE pubs.acs.org/JPCC

Synthesis, Characterization, and Gas Sensing Properties of Porous Nickel Oxide Nanotubes Xuefeng Song,† Lian Gao,‡ and Sanjay Mathur*,† † ‡

Institute of Inorganic Chemistry, University of Cologne, Cologne 50939, Germany State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P. R. China

bS Supporting Information ABSTRACT: A novel approach was employed to synthesize porous NiO nanotubes with controllable interior voids based on an effective interplay of Kirkendall effect and volume change upon phase transformation. For this purpose, nickel nanowires were chemically converted into Ni3S2/Ni core
shell structures, followed by a controlled oxidation, whereby the associated volume change (Ni f NiO conversion) resulted in 1D porous structure with voids. The voids between the Ni core and Ni3S2 shell could be controlled by adjusting the oxidation conditions that enabled fabrication of hollow and double-walled morphologies. Phase composition, morphological evolution, and porosity of double-walled NiO nanotubes were analyzed by X-ray diffraction, scanning and transmission electron microscopy, and N2 adsorption
desorption studies. Gaseous sulfur oxides formed during the oxidation of Ni3S2/Ni structures resulted in a perforated structure with multiple voids with pores ranging between 1 and 14 nm. The unique complex structure with the interpenetrating voids and the surface porosity resulted in a high specific surface area of 161.6 m2 3 g
1. The gas sensing property of such double-walled structure was found to vary as a function of the concentric void between the core and the shell. Gas-sensing measurements in hollow porous core
shell NiO nanotubes exhibited excellent sensitivity toward ethanol, originating from efficient adsorption of target molecules in the interior voids and their rapid diffusion and transport through the porous structures.

’ INTRODUCTION Tuning the architecture in nanostructured materials is a commonly employed strategy to optimize their performance due to their size and structure-dependent properties.1 Extensive efforts have been devoted to fabricate hollow structures of semiconductors and metals following various driving mechanisms (such as Kirkendall effect,2 Ostwald ripening,3 and chemically induced transformation4). One-dimensional nanostructures (such as nanowires, nanobelts, and nanotubes) are receiving significant attention because of their potential applications in energy conversion,5 separation science,6 environmental protection,7 and gas sensors.8 Metal oxide nanotubes are promising structures for chemical sensors because of their hollow interior space and lower tendency to agglomerate when compared with nanoparticles. Conventional synthetic strategies of nanotubes generally employ various positive and negative templates. Positive templates utilize 1D scaffolds on which oxide materials are coated on the outer surfaces of the template, whereas the porous materials with monodisperse cylindrical pores (anodic aluminum oxides and polycarbonate membranes) are used as negative templates, where oxide materials are deposited inside the pores.9 However, the necessity to remove the templates and a selective etching make it tedious to obtain pure product. Moreover, the above-mentioned methods are not available to change interior size of metal oxide nanotube by templates with fixed size. Recently hollow structures manifesting high internal voids have shown to have superior sensor properties.10 r 2011 American Chemical Society

Nevertheless, developing an efficient approach for large-scale production of porous nanotube with controllable internal voids is still a challenge mainly due to the complex structure features. Ethanol sensors have potential application in the field of screening the intoxicated drivers, evaluating the wine quality, as well as monitoring the food and biomedical safety.11 Onedimensional semiconductor metal oxides including SnO2 nanowires,12 ZnO nanotubes,13 porous α-Fe2O3 nanorods,14 α-Fe2O3/SnO2 nanorods,15 and V2O5 nanobelts,16 have been elucidated as effective materials for ethanol detection. As a p-type wide-bandgap semiconductor, NiO is a very promising material and has attracted increasing attention due to its excellent chemical stability and pronounced electrical properties. In addition, the gas sensors based on NiO film have been shown to possess significant sensing properties.17 Above-mentioned prior researches inspired us to synthesize an ethanol sensor costeffectively based on porous NiO nanotube with controllable interior void between the core and the shell, which would provide affluent active sites and space for the adsorption of detected gas to improve gas sensing property. In this work, a chemical approach based on a dipolar selfassembly and nonequilibrium interdiffusion (Kirkendall effect) Received: August 22, 2011 Revised: October 2, 2011 Published: October 05, 2011 21730

dx.doi.org/10.1021/jp208093s | J. Phys. Chem. C 2011, 115, 21730–21735

The Journal of Physical Chemistry C

ARTICLE

mechanisms was used to prepare hollow Ni3S2/Ni core
shell structure, which were thermally oxidized to obtain hollow porous core
shell NiO nanotube. The porous structures consisted of nanoplates and nanopores in the tube wall resulting from the release of nascent gaseous sulfur oxides formed in the oxidation process and volume change associated with the thermal transformation of Ni3S2 into NiO. Gas sensors fabricated using assynthesized hollow porous core
shell NiO nanotube showed high sensitivity toward the detection of ethanol molecules, which was comparatively analyzed against other gases.

’ EXPERIMENTAL SECTION Preparation of Hollow Ni3S2/Ni Core
Shell Structures. Ni nanowires were prepared using hydrazine hydrate (N2H4 3 H2O) as a reducing agent for the nickel precursor. In a typical synthesis, 0.7 mmol of Ni(OOCCH3)2 3 4H2O was dissolved in 60 mL of ethylene glycol (EG) in a 150 mL flask, and the mixture was heated under continuous stirring to 110 °C, followed by refluxing for 10 min. Following this, 2 mL of hydrazine hydrate (85%) was added dropwise to the above mixture under continuous stirring for 30 min. The black product formed in the solution was treated with 2.5 mmol of thioacetamide (CH3CSNH2) dissolved in 8 mL of EG and stirred at 110 °C for 0.5 h. The resulting product was recovered using an external magnetic field and redispersed in ethanol and water several times, respectively, followed by drying in an oven at 60 °C. Preparation of Hollow and Porous Core
Shell NiO Nanotubes. Hollow core
shell NiO nanotubes were prepared by thermal oxidation of as-prepared product in air. The oxidation process was carried out inside a quartz tube heated at a rate of 10 °C min
1 in a horizontal tube furnace, whereby the temperature was maintained at 500 °C for 2 h. Characterization. The phase composition of as-prepared samples was characterized on a STOE-STADI MP X-ray diffractometer operating in a transmission mode using Cu Kα (λ = 1.5406 Å) radiation. Transmission electron microscopy (TEM) images were collected using a JEOL-2100F electron microscope. Energy-dispersive X-ray spectroscopy (EDX) analysis was recorded on an OXFORD ISIS spectroscope, which was attached to the JEOL-2100F electron microscope. Scanning electron microscopy (SEM) images were collected on a FEI Nova NanoSEM 430 microscope. Thermogravimetric analysis (TGA) of the samples was carried out in the range 40 to 600 °C with a heating rate of 10 °C/min on NETZSCH STA 409C/CD equipment. The nitrogen adsorption/desorption isotherms were obtained at 77 K with a Micromeritics ASAP 2010 micropore analysis system. Specific surface areas (SSAs) were determined using the Barrett
Emmett
Teller (BET) method, and average pore diameters were calculated using the Barrett
Joyner
Halenda (BJH) method. Room temperature I
V characteristics of as-obtained sensor were studied using a Keithley 2400 source meter. Gas Sensing Measurements. The gas sensors were fabricated by coating slurry of the obtained NiO nanotubes onto an alumina plate with prefabricated interdigitated Au electrodes and Au wires attached on the both side of the plate. A ceramic heater placed underneath the alumina plate controlled the operating temperature. The gas-sensing test was performed on homemade equipment connecting with the computer controlling system.8 The relative humidity of the environment during the experiments was in the range of 30
40%. Ethanol gas was introduced into the

Figure 1. XRD patterns of as-prepared samples (a) Ni nanowires, (b) hollow Ni3S2/Ni core
shell structures, (c) Ni3S2 tubes, (d) hollow porous core
shell NiO nanotube, and (e) simple porous NiO tubes.

testing chamber with a flow rate of 20 sccm. The sensor sensitivity to ethanol gas is defined as S = Rg/Ra, where Rg and Ra are the resistance in test gas and in air, respectively.

’ RESULTS AND DISCUSSION The XRD pattern (curve a in Figure 1) of Ni nanowires exhibited diffraction peaks due to pure face-centered cubic nickel (JCPDS no.03-051). When thioacetamide (CH3CSNH2) solution was added as a source of sulfide (S2
) ions, the XRD pattern (curve b in Figure 1) showed, besides the Ni peaks, several new peaks that were indexed as the Ni3S2 composition (JCPDS no. 44-1418), indicating the formation of a Ni3S2 shell, as the oxidation initially occurred on the surface of the nickel nanowires. After 50 min, all Ni was converted to the Ni3S2 phase (curve c in Figure 1), indicating a quantitative transformation of Ni nanowires into Ni3S2 tubes. As-obtained hollow Ni3S2/Ni core
shell structures were converted to cubic NiO (JCPDS no.78-0643) upon calcination at 500 °C for 2 h, as evident in curve d (Figure 1). The absence of characteristic peaks due to Ni3S2 and Ni phases and the absence of sulfur (EDX analysis) corroborated the complete oxidation of the Ni3S2 shell and Ni core into NiO phase. Under similar calcination conditions, the porous tubes obtained from calcination of Ni3S2 tube possessed the similar crystalline structure, in which all of peaks could be assigned to cubic NiO phase (curve e in Figure 1). To investigate the composition of as-prepared hollow Ni3S2/ Ni core
shell structures, TG-DTA measurement was performed (Figure 2). The presence of an endothermic peak in the DTA curve indicated the conversion of Ni3S2 into NiO because the oxidation of Ni to NiO would be an exothermic reaction.18 TG curve showed a total mass loss of ∼8.7%. The steep weight loss (5.4%) exhibited in the temperature range of 370
470 °C was ascribed to the transformation of Ni3S2 shell to NiO. The content of Ni3S2 in hollow Ni3S2/Ni core
shell structures was ca. 80 wt % by calculation, indicating sufficient interior voids in hollow core
shell NiO nanotubes. At temperatures >470 °C, the weight of the samples was constant, suggesting the formation of a stable compound with definite composition (NiO phase). Figure 3a shows a representative image of hollow and porous NiO nanotube of ca. 350 nm diameter and length up to 10 μm. SEM image (Figure 3b) showed the porous morphology to be composed of several NiO nanoplatelets and pores on the surface as well. Evidently, there were some hollow spaces formed 21731

dx.doi.org/10.1021/jp208093s |J. Phys. Chem. C 2011, 115, 21730–21735

The Journal of Physical Chemistry C between the core and shell producing a hollow core
shell structure (Figure 3c and inset). The representative high-resolution TEM (HR-TEM) image of the NiO shell (Figure 3d) exhibited lattice fringes with an interplanar distance of 0.241 nm corresponding to the spacing of the (111) planes in NiO crystals. The average void between adjacent nanoparticles was ca. 4 nm (marked by red circles). The selected area electron diffraction (SAED) pattern confirmed the polycrystalline nature of NiO shell (inset of Figure 3d). To investigate the transformation of hollow Ni3S2/Ni core
shell structure into porous core
shell NiO nanotube, we performed TEM and SEM analyses of samples prepared in different stages of the reaction. The wire-like architecture of Ni nanowires was found to be constituted by an assembly of interconnected Ni

Figure 2. TG
DTA curves of as-prepared hollow Ni3S2/Ni core
shell structures.

ARTICLE

nanospheres with a diameter of ca. 200 nm, which produced structures with lengths up to several tens of micrometers. Figure 4b,c revealed that Ni3S2/Ni core
shell structure maintained the 1D morphology observed in Ni nanowires. EDX spectra (Figure S1, Supporting Information) confirmed the aggregative architecture to consist of Ni and S only. A careful comparison of the region-selective EDX mapping showed higher content of S in the outermost shell, further corroborating the core
shell structure. The size of core could be adjusted to control the interior void between the core and the shell by tuning the reaction time. The core finally disappeared after 50 min of reaction to form a Ni3S2 nanotube (Figure 4d
g). After the calcination at 500 °C, the Ni3S2 tube was oxidatively transformed into porous NiO tube with no significant changes in the morphology (Figure S2, Supporting Information). On the basis of the above results, a plausible growth mechanism of the unique hollow and porous NiO nanotube could be schematically described (Scheme 1) as an interplay of selfassembly of Ni nanoparticles (I) into nanowires (II), nonequilibrium interdiffusion due to Kirkendall effect (III), and thermal oxidation process (IV). Hydrazine hydrate used in the synthesis served as both ligand and reducing agent to form spherical Ni nanoparticles, which then self-assembled into Ni nanowires via magnetic dipole interactions among the Ni nanospheres.19 When CH3CSNH2 was introduced to the Ni nanowires suspension, it reacted with water at the reaction temperature to produce H2S, which reacted with the Ni nanowires to form a thin Ni3S2 film on the nanowire surface. Because the thickness of the film depended on the time of the chemical reaction, the core
shell composition in Ni3S2/Ni structure could be tuned by adjusting the reaction time. The void inside the nanotube was created due to an outward diffusion of nickel

Figure 3. SEM (a,b), TEM (c), and HRTEM image (d) of the hollow porous core
shell NiO nanotube. The inset shows magnified SEM image of partial core
shell NiO nanotube and the SAED pattern. 21732

dx.doi.org/10.1021/jp208093s |J. Phys. Chem. C 2011, 115, 21730–21735

The Journal of Physical Chemistry C

ARTICLE

Figure 4. (a) SEM image of as-obtained Ni nanowires and (b,c) SEM images of hollow Ni3S2/Ni core
shell structures. TEM images of as-obtained samples at different time intervals (d) 0, (e) 10, (f) 30, and (g) 50 min.

Scheme 1. Synthesis Strategy to Obtain Hollow Porous Core-Shell NiO Nanotube

leading to a hollow core
shell structure. In this case, the Kirkendall effect involved different transport rates for Ni and S atoms through the incipient Ni3S2 barrier layer that was formed promptly at the surface of the Ni nanowires in the presence of H2S. Interior void was formed because the diffusion rate of atomic Ni was much faster than that of atomic S, which resulted in a net outward directional flow of matter. Similar phenomena were reported on the formation of hollow Co3S4 and Co9S8 spheres.2a If the transport of S would be faster, then solid Ni3S2 nanowire would be expected as the final product. Adjusting the reaction kinetics to stop the reaction in the appropriate stage allowed formation of a unique hollow Ni3S2/Ni core
shell structure. When the reaction was pushed to completion, pure Ni3S2 tubes were formed at the expense of the Ni core. The reaction steps could be depicted as follows Ni2þ þ N2 H4 f ½NiðN2 H4 Þ3 2þ

ð1Þ

½NiðN2 H4 Þ3 2þ þ N2 H4 f Ni V þ 4NH3 v þ 2N2 v þ H2 v þ 2Hþ

ð2Þ

CH3 CSNH2 þ H2 O f H2 SðaqÞ þ CH3 CONH2

ð3Þ

3Ni þ 2H2 S f Ni3 S2 V þ 2H2 v

ð4Þ

The formation of a porous surface structure is possibly due to the volume change and release of gaseous sulfur oxides during the conversion of Ni3S2 to NiO. The volume change upon phase transformation has been utilized to synthesize porous materials in the previous reports.20 When Ni3S2 reacted with O2 to generate sulfur oxides at high temperature, the internal pressure possibly ruptured the side walls to release excess pressure evident in the formation of small pores throughout the observed structures. The SSA and porosity of as-prepared nanotube and core
shell structures were determined by nitrogen adsorption
desorption analysis and BJH method, which revealed a porosity of type IV for both systems with a distinct hysteresis loop ranging from 0.4 to 1.0 P/P0. The SSA of hollow porous core
shell NiO nanotube and porous NiO tubes using the BET method was 21733

dx.doi.org/10.1021/jp208093s |J. Phys. Chem. C 2011, 115, 21730–21735

The Journal of Physical Chemistry C

ARTICLE

Figure 5. N2 adsorption
desorption isotherm of samples (black line f hollow porous core
shell NiO nanotube; red line f porous NiO tubes). The inset is BJH pore-size distribution curve of hollow porous core
shell NiO nanotube.

161.6 and 97.3 m2 3 g
1, respectively. The improved SSA of hollow porous core
shell NiO nanotube was ascribed to the additional voids and surface offered by the internal core. The inset in Figure 5 shows the pore size distribution plot calculated according to the BJH method from the desorption branch of the isotherm, with a broad pore size distribution from 1 to 14 nm and centered at 3.7 nm, which is supported by the TEM data. Figure 6a shows gas-sensing measurements performed using NiO nanotubes obtained after different reaction time periods. The linear slope of current
voltage (I
V) curves for core
shell NiO tube obtained at 30 min was found to be higher than that for other NiO nanotubes. Because the change in conductivity in conductometric sensors is related to the overall surface area and the concentration of negatively charged chemical species, such as O2
, O2
, and O
, adsorbed on the surface, it is likely that the coverage of oxygen is higher in porous core
shell NiO tube. The ethanol-sensing properties of as-prepared NiO nanotubes (Figure 6b) showed sensitivity (S = Rg/Ra) of the hollow core
shell NiO tube (4.2) to be higher than that of the NiO tube (1.4) for ethanol concentration of 50 ppm. The sensitivity of core
shell nanotube obtained after 10 min was poor, possibly because of the narrow interior void, which suggested that appropriate interior void corresponding to the size of the analyte molecules is indispensable for excellent gas-sensing property. The sensitivities of hollow core
shell NiO tube were found to increase drastically with increasing ethanol concentration (S = 22.6 for 200 ppm ethanol), as shown in Figure S3 of the Supporting Information. However, the sensitivity was found to be only 4.7 in NiO nanotubes. Previous reports have demonstrated that the surface structures (particle size and porosity) associated with the material architecture are the key factors to influence the amount of adsorbed oxygen species and the gassensing properties.10 When the sensor was exposed to ethanol gas, sensitivity (S = Rg/Ra) increased because of the decreased electrical conductivity of NiO, whereby ethanol reacted with adsorbed oxygen to form water vapor releasing the electrons into the NiO nanotubes (upper inset of Figure 6a). Because tubes possessed similar crystalline structure (XRD evidence), we believe

Figure 6. (a) I
V curves and (b) dynamic response-recovery curves of ethanol (50 ppm) sensing on different nanostructures at 250 °C. The inset of (a): schematic diagram of ethanol sensing process (upper) and the I
V measurement (lower). The inset of (b): schematic illustration of the effective adsorption of ethanol molecules within the interior cavity.

that the hollow core
shell NiO structures obtained after 30 min of reaction time allowed more efficient adsorption of ethanol as schematically illustrated in the inset of Figure 6b. Additionally, the porosity of hollow core
shell NiO tube enabled rapid diffusion of ethanol molecules and mass transportation into inner parts of sensor, which further augmented the sensitivity. To examine the selectivity of the porous core
shell NiO nanotubesbased sensor, we measured the responses of the sensor to various gases at 250 °C (Figure S4 of the Supporting Information). Among ethanol, acetone, CH4, NH3, and CO, used as the analyte gases, significantly higher selectivity of the porous core
shell NiO nanotubes was observed toward ethanol, which could be attributed to the strong interaction between the ethanol and the surface-adsorbed oxygen species (O2
, O2
, and O
), as known for other reducing gases (e.g., CO), resulting in a facile release of trapped electrons by oxygen species when compared with the same concentration of other gases interacting with the sensor at 250 °C.21

’ CONCLUSIONS A facile experimental approach was developed to synthesize porous NiO nanotubes with controllable interior volume based 21734

dx.doi.org/10.1021/jp208093s |J. Phys. Chem. C 2011, 115, 21730–21735

The Journal of Physical Chemistry C on a sacrificial-template strategy, where Ni and Ni3S2/Ni nanowires were used as precursors. The formation of unique core
shell NiO structure was attributed to an interplay of Kirkendall effect and associated volume change upon phase transformation, which led to a larger specific area (161.6 m2 3 g
1) in comparison with that of NiO tubes (97.3 m2 3 g
1). The porous core
shell nanotubes showed improved gas-sensing performance toward ethanol possibly due to efficient adsorption of target gas and their rapid diffusion through the porous structure. Given the versatility of the chemical transformation demonstrated here, the current strategy is extendable to other transition-metal oxide and will open-up new avenues for designing sensor materials based on nanoscaled architectures.

’ ASSOCIATED CONTENT

bS

Supporting Information. TEM images of porous NiO tubes, EDX spectra of single hollow Ni3S2/Ni core
shell structure, and the comparison of gas concentration-dependent sensitivities. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: (+49) 2214704899.

’ ACKNOWLEDGMENT We gratefully acknowledge the financial support provided by the University of Cologne and the Federal Ministry of Education and Research (BMBF: Koliwin project-grant agreement no. 55102006). ’ REFERENCES (1) (a) Yang, M.; Ma, J.; Zhang, C. L.; Yang, Z. Z.; Lu, Y. F. Angew. Chem., Int. Ed. 2005, 44, 6727. (b) Gao, C. L.; Liang, Y. Y.; Han, M.; Xu, Z.; Zhu, J. M. J. Phys. Chem. C 2008, 112, 9272. (c) Chen, C. H.; Jin, L.; Espinal, A. E.; Firliet, B. T.; Xu, L. P.; Aindow, M.; Joesten, R.; Suib, S. L. Small 2010, 6, 988. (d) Liang, X. D.; Gao, L.; Yang, S. W.; Sun, J. Adv. Mater. 2009, 21, 2068 . (2) (a) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S. G.; Somorjai, A.; Alivisatos, A. P. Science 2004, 304, 711. (b) Chen, M. H.; Gao, L. Inorg. Chem. 2006, 45, 5145. (3) (a) Song, X. F.; Gao, L. J. Phys. Chem. C 2008, 112, 15299. (b) Chang, Y.; Teo, J. J.; Zeng, H. C. Langmuir 2005, 21, 1074. (c) Teo, J. J.; Chang, Y.; Zeng, H. C. Langmuir 2006, 22, 7369. (4) (a) Li, H. X.; Bian, Z. F.; Zhu, J.; Zhang, D. Q.; Li, G. S.; Huo, Y. N.; Li, H.; Lu, Y. F. J. Am. Chem. Soc. 2007, 129, 8406. (b) Zhang, H. G.; Zhu, Q. S.; Zhang, Y.; Wang, Y.; Zhao, L.; Yu, B. Adv. Funct. Mater. 2007, 17, 2766. (5) (a) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. D. Nat. Mater. 2005, 4, 455. (b) Hochbaum, A. I.; Yang, P. D. Chem. Rev. 2010, 110, 527. (6) Lee, S. B.; Mitchell, D. T.; Trofin, L.; Nevanen, T. K.; S€oderlund, H.; Martin, C. R. Science 2002, 296, 2198. (7) Kim, T. H.; Lee, J. Y.; Hong, S. H. J. Phys. Chem. C 2009, 113, 19393. (8) (a) Mathur, S.; Barth, S.; Shen, H.; Pyun, J. C.; Werner, U. Small 2005, 1, 713. (b) Pan, J.; Ganesan, R.; Shen, H.; Mathur, S. J. Phys. Chem. C 2010, 114, 8245. (c) Ramirez, F. H.; Prades, J. D.; Tarancon, A.; Barth, S.; Casals, O.; Diaz, R. J.; Pellicer, E.; Rodriguez, J.; Morante, J. R.; Juli, M. A.; Mathur, S.; Rodriguez, A. R. Adv. Funct. Mater. 2008, 18, 2990.

ARTICLE

(d) Xiao, L. S.; Shen, H.; Hagen, V. R.; Pan, J.; Belkoura, L.; Mathur, S. Chem. Commun. 2010, 46, 6509. (9) (a) Hoyer, P. Adv. Mater. 1996, 8, 857. (b) Du, N.; Zhang, H.; Chen, B. D.; Ma, X. Y.; Liu, Z. H.; Wu, J. B.; Yang, D. R. Adv. Mater. 2007, 19, 1641. (c) Chen, J.; Xu, L. N.; Li, W. Y.; Gou, X. L. Adv. Mater. 2005, 17, 582. (d) Qu., X.; Kobayashi, N.; Komatsu, T. ACS Nano 2010, 4, 1732. (e) Hou, S. F.; Harrell, C. C.; Trofin, L.; Kohli, P.; Martin, C. R. J. Am. Chem. Soc. 2004, 126, 5674. (10) Lai, X. Y.; Li, J.; Korgel, B. A.; Dong, Z. H.; Li, Z. M.; Su, F. B.; Du, J.; Wang, D. Angew. Chem., Int. Ed. 2011, 50, 2738. (11) Chen, D. L.; Hou, X. X.; Wen, H. J.; Wang, Y.; Wang, H. L.; Li, X. J.; Zhang, R.; Lu, H. X.; Xu, H. L.; Guan, S. K.; Sun, J.; Gao, L. Nanotechnology 2010, 21, 035501. (12) Chen, Y. J.; Nie, L.; Xue, X. Y.; Wang, Y. G.; Wang, T. H. Appl. Phys. Lett. 2006, 88, 083105. (13) Chen, Y. J.; Zhu, C. L.; Xiao, G. Sens. Actuators, B 2008, 129, 639. (14) Wang, Y.; Cao, J. L.; Wang, S. R.; Guo, X. Z.; Zhang, J.; Xia, H. J.; Zhang, S. M.; Wu, S. H. J. Phys. Chem. C 2008, 112, 17804. (15) Chen, Y. J.; Zhu, C. L.; Wang, L. J.; Gao, P.; Cao, M. S.; Shi, X. L. Nanotechnology 2009, 20, 045502. (16) Liu, J.; Wang, X.; Peng, Q.; Li, Y. Adv. Mater. 2005, 17, 764. (17) (a) Hotovy, I.; Huran, J.; Siciliano, P.; Capone, S.; Spiess, L.; Rehacek, V. Sens. Actuators, B 2004, 103, 300. (b) Hotovy, I.; Rehacek, V.; Siciliano, P.; Capone, S.; Spiess, L. Thin Solid Films 2002, 418, 9. (18) (a) Grau, J.; Akinc, M. J. Am. Ceram. Soc. 1996, 79, 1073. (b) Schlemmer, S.; Luca, A.; Gerlich, D. Int. J. Mass Spectrom. 2003, 223
224, 291. (19) (a) Zhang, Y. H.; Guo, L.; He, L.; Liu, K.; Chen, C. P.; Zhang, Q.; Wu, Z. Y. Nanotechnology 2007, 18, 485609. (b) Liu, C. M.; Guo, L.; Wang, R. M.; Deng, Y.; Xu, H. B.; Yang, S. H. Chem. Commun. 2004, 40, 2726. (20) (a) Toberer, E. S.; Schladt, T. D.; Seshadri, R. J. Am. Chem. Soc. 2006, 128, 1462. (b) Liu, J.; Xue, D. F. Adv. Mater. 2008, 20, 2622. (c) Railsback, J. G.; Johnston-Peck, A. C.; Wang, J.; Tracy, J. B. ACS Nano 2010, 4, 1913. (d) Zhang, J.; Wang, S. R.; Xu, M. J.; Wang, Y.; Zhu, B. L.; Zhang, S. M.; Huang, W. P.; Wu, S. H. Cryst. Growth Des. 2009, 9, 3532. (e) Liu, X. H.; Zhang, J.; Wang, L. W.; Yang, T. L.; Guo, X. Z.; Wu, S. H.; Wang, S. R. J. Mater. Chem. 2011, 21, 349. (21) (a) Zhang, J.; Wang, S. R.; Wang, Y. M.; Wang, Y.; Zhu, B. L.; Xia, H. J.; Guo, X. Z.; Zhang, S. M.; Huang, W. P.; Wu, S. H. Sens. Actuators, B 2009, 135, 610. (b) Zhang, J.; Wang, S. R.; Wang, Y.; Xu, M. J.; Xia, H. J.; Zhang, S. M.; Huang, W. P.; Guo, X. Z.; Wu, S. H. Sens. Actuators, B 2009, 139, 411. (c) Chiu, H.; Yeh, C. S. J. Phys. Chem. C 2007, 111, 7256.

21735

dx.doi.org/10.1021/jp208093s |J. Phys. Chem. C 2011, 115, 21730–21735