- Email: [email protected]
Novel low-temperature growth of SnO2 nanowires and their gas-sensing properties
Available online at www.sciencedirect.com
Scripta Materialia 68 (2013) 408–411 www.elsevier.com/locate/scriptamat
Novel low-temperature growth of SnO2 nanowires and their gas-sensing properties R. Rakesh Kumar,a,⇑ Mitesh Parmar,a,b K. Narasimha Rao,a K. Rajannaa and A.R. Phanic a
b
Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India School of Mechanical Systems Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea c Nano-Research for Advanced Materials and Technologies, Bangalore 560040, India Received 26 September 2012; revised 5 November 2012; accepted 7 November 2012 Available online 12 November 2012
A simple thermal evaporation method is presented for the growth of crystalline SnO2 nanowires at a low substrate temperature of 450 °C via an gold-assisted vapor–liquid–solid mechanism. The as-grown nanowires were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction, and were also tested for methanol vapor sensing. Transmission electron microscopy studies revealed the single-crystalline nature of the each nanowire. The fabricated sensor shows good response to methanol vapor at an operating temperature of 450 °C. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Physical vapor deposition (PVD); Semiconductors; SnO2 nanowires; Transmission electron microscopy (TEM); Sensors
In recent years, one-dimensional metal oxide nanostructures such as nanowires, nanotubes and nanobelts have attracted much attention due to their practical applications in various fields [1,2]. Nanowires of semiconducting oxides such as ZnO [3], SnO2 [4], In2O3 [5] and TiO2 [6] grown by different methods have been used in a variety of applications. Among them, SnO2 nanowires are particularly important because of their applications in a number of fields such as Li-ion batteries [7], gas sensors [8], UV detectors [9], field emitters [10] and superhydrophobic surfaces [11]. Therefore, it is important to grow SnO2 nanostructures. Techniques for growing SnO2 nanowires include pulse laser deposition [12], hydrothermal growth [13], thermal evaporation [7] and chemical vapor deposition [11]. The most commonly used method for producing SnO2 nanowires is tube furnace type thermal evaporation due to the simplicity and low cost of this approach. The main drawbacks of this method are the high temperatures involved (>600 °C for SnO2) and the lack of exact control of parameters such as length of the nanowires and the difficulty of aligning nanowires on the substrate [7,8,10]. To overcome the above drawbacks we have adopted the
⇑ Corresponding
author. Tel.: +91 (80) 22933190; fax: +91 (80) 23600135; e-mail: [email protected] URL: http://isu.iisc.ernet.in/~rakeshr/.
vapor–liquid–solid (VLS) mechanism using gold as a catalyst in combination with thermal evaporation (not in a tube furnace) to avoid high temperature on the substrate side (Supporting information S1). The evaporation source is located 20 cm below the substrate holder in such a way that the substrates were at room temperature even though the evaporation boat was at a higher temperature. This method enabled better control over the deposition parameters such as substrate temperature and deposition rate (length of the nanowires). In this work, we report a low-temperature process for SnO2 nanowire growth by thermal evaporation, as well as their methanol sensing properties. The experimental set-up consists of a thermal evaporation arrangement for the evaporation of tin by passing a current through evaporation boat and also has the facility to allow O2 gas through a flow meter (Supporting information S1). Initially, we have taken Si as well as SiO2 (300 nm) grown on Si substrates (1 cm 1 cm). The substrates were first cleaned with acetone by ultrasonication, followed by methanol, then DI water, and finally dried in a N2 stream. The cleaned substrates were immediately transferred to a sputter coater to deposit Au film 3 nm thick. Immediately after Au deposition, substrates were loaded into the deposition chamber. The deposition chamber was evacuated by a combination of a diffusion pump and a rotary pump with a liquid
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.11.002
R. R. Kumar et al. / Scripta Materialia 68 (2013) 408–411
N2 trap to obtain a high vacuum of 1 105 mbar. Initially, Au-coated substrates were annealed at 450 °C for 15 min prior to nanowire growth to obtain Au catalyst particles. After cooling to room temperature, the samples were again heated to 450 °C for 15 min. Before exposure to Sn vapor, oxygen is allowed into the deposition chamber from 1 105 to 4 104 mbar and maintained constant, after which the Sn vapor is admitted. Nanowire growth was performed for 15 min at 450 °C at a PO2 of 4 104 mbar. Nanowires grown on SiO2 substrate used for methanol sensing had Al electrodes deposited over the nanowire film. The grown nanowires were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM), and were tested for their methanol vapor-sensing properties with the help of temperature and concentration modulation. The temperature modulation helped to determine the optimum operating temperature of the sensor, and the concentration modulation shows how of methanol vapor concentration varies with sensor performance. Concentration modulation shows how performance of sensor varies with methanol vapor concentration. Figure 1a and b shows low- and high-magnification SEM images of the dense SnO2 nanowires grown on Au-coated Si substrates in top view, and Figure 1c shows a cross-sectional view. The nanowires were grown vertically and aligned at some angle with respect to the substrate. The length and diameter of the nanowires were of the order of 2–2.5 lm and 30–80 nm, respectively, for a deposition time of 15 min. The growth rate of the nanowires was observed to be 130 nm min1. Each nanowire was capped with Au catalyst particle at their ends, and in addition the growth temperature is well above the eutectic temperature of the Au–Sn system, confirming the VLS growth of the SnO2 nanowires. Nanowires were also grown at temperatures below 450 °C but were found to have Sn particles decorating the nanowires. For this reason, we avoided nanowire growth below 450 °C. The phase purity of the SnO2 nanowire thin film was characterized by XRD. The XRD spectrum recorded on the
409
as-grown SnO2 nanowire film on Si is shown in Figure 1d. The diffraction peaks observed at 2h = 26.5, 33.8, 51.7 indexed as (110), (101), (211) planes of tetragonal rutile SnO2, which coincides well with JCPDS Card No. 77-0448. The observation of Au peaks along with SnO2, is due to the Au catalyst used for the nanowire synthesis. The XRD result confirms the crystalline nature of the nanowires. TEM investigations of SnO2 nanowires have been carried out by dropping prepared nanowire solution onto a copper grid. Figure 2a shows a TEM image of the single nanowire with a diameter of 30 nm. A high resolution TEM (HRTEM) image recorded on the square region nanowire in Figure 2a is shown in Figure 2b. The lattice fringes in HRTEM clearly reflect the single-crystalline nature of the nanowire. The lattice spacing between the fringes was found to be 0.26 nm, corresponding to the (1 0 1) planes of SnO2. Further energy-dispersive X-ray (EDX) analysis performed on the nanowire and catalyst particle are shown in Figure 2c and d, respectively. The EDX spectrum of the wires shows Sn and O along with Cu and C. The elemental quantification performed on the nanowire indicates 68 at.% O and 31 at.% Sn, confirming the SnO2 phase (Supporting information S2). The EDX spectrum on the catalyst alloy particle consists of Au and Sn. The observation of Cu and C peaks is the result of the carbon-coated copper grid. From the above discussion, we conclude that the SnO2 nanowires grown were single crystalline in nature and pure. The as-grown SnO2 nanowires were then studied for their static-sensing properties for methanol. Being an n-type metal oxide, the resistance of SnO2 nanowire film increases in the presence of oxygen. During the sensing process, methanol undergoes dehydrogenation, resulting in the release of free electrons and hence in the variation of the film resistance [14]. As a result of this, the resistance of the sensing material decreases during sensing. Here, the sensing response was taken as: sensitivityð%Þ ¼ ½ðRair Rgas Þ=Rgas 100;
ð1Þ
Figure 1. (a and b) High- and low-magnification SEM images of the SnO2 in top view; (c) SEM image of the SnO2 nanowires in cross-section view; (d) XRD pattern of SnO2 nanowire film.
410
R. R. Kumar et al. / Scripta Materialia 68 (2013) 408–411
Figure 2. (a) TEM image of a single nanowire; (b) HRTEM recorded on the nanowires; (c and d) EDX spectrum recorded on the tip and wire.
Figure 3. (a and b) Temperature modulation and concentration modulation of the methanol-sensing studies. (c) Typical resistance variation at optimum operating temperature during methanol exposure.
where Rair is the resistance in air (base resistance) and Rgas is the saturated resistance in the presence of test gas. The methanol-sensing study for as-grown SnO2 nanowires was performed by temperature and concentration modulation. The temperature modulation helps to find an optimum operating temperature (Topt) at which the sensing response will be the maximum for a fixed concentration of analyte. In the present case, the fixed concentration of analyte was 300 ppm methanol. Hence, during the temperature modulation, the operating temperature was varied from 200 to 500 °C, and the Topt was observed to be 450 °C. As can be seen from Figure 3a, the maximum sensitivity observed at Topt was found to be 2582%. This was followed by concentration
modulation, during which the operating temperature of the sensor was fixed at Topt and the methanol concentration was varied from 100 to 1500 ppm. The concentration modulation gives an understanding of the sensing behavior with varying methanol concentration. As Topt for the present study is 450 °C, the complete concentration modulation study was carried out at that temperature. Figure 3b shows the variation of sensing response, i.e. sensitivity, with respect to change in methanol concentration. Here, the sensitivity for lower concentrations (6400 ppm) was linear and tends to saturate at higher (>600 ppm) concentrations of methanol. The sensitivity for 1500 ppm of methanol was observed to be 3268%. The typical variation in the resistance of SnO2 is shown
R. R. Kumar et al. / Scripta Materialia 68 (2013) 408–411
in Figure 3c. The sensitivity of SnO2 nanowires as well as the optimum operating temperature can be improved by selecting different electrode materials instead of Al and doping these nanowires with suitable catalyst, e.g. Au, Pd and Pt. Although Au catalyst was used in the present case, the catalyst was to control the structure of nanowires rather than enhancing the sensing response. In summary, crystalline SnO2 nanowires were grown by simple thermal evaporation at a low substrate temperature of 450 °C. Nanowire growth was achieved by using an Au-assisted VLS mechanism. Nanowires were grown uniformly throughout the substrate and were vertical with respect to the substrate, with a length and diameter of 2–2.5 lm and 30–80 nm, respectively. XRD and TEM studies confirm the crystalline nature of the nanowires. EDX measurements on the nanowires confirm the formation of SnO2 (Sn:O ratio = 31:68 at.%). SnO2 nanowire films showed good response to methanol at an operating temperature of 450 °C. The operating temperature can be further reduced by controlling the structure of nanowires by using different catalysts for the growth of SnO2 nanowires and by volumetric doping of these nanowires with catalysts such as Au, Pd and Pt. The authors thank the Advanced Facility for Microscopy and Microanalysis (AFMM), Indian Institute of Science, Bangalore, for providing the microscopy facility.
411
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.scriptamat.2012.11.002. [1] R.S. Devan, R.A. Patil, J.H. Lin, Y.R. Ma, Adv. Funct. Mater. 22 (2012) 3326. [2] J.G. Lu, P. Chang, Z. Fan, Mater. Sci. Eng. R 52 (2006) 49. [3] L.S. Mende, J.L.M. Driscoll, Mater. Today 10 (2007) 40. [4] J. Pan, H. Shen, S. Mathur, J. Nanotechnol. 2012 (2012) 917320. [5] S. Kar, S. Chakrabarti, S. Chaudhuri, Nanotechnology 17 (2006) 3058. [6] B. Liu, E.S. Aydil, J. Am. Chem. Soc. 131 (2009) 3985. [7] M.S. Park, G.X. Wang, Y.M. Kang, D. Wexler, S.X. Dou, H.K. Liu, Angew. Chem. Int. Ed. 46 (2007) 750. [8] Y. Shen, T. Yamazaki, Z. Liu, D. Meng, T. Kikuta, N. Nakatani, M. Saito, M. Mori, Sens. Actuat. B 135 (2009) 524. [9] L.Y. Jiang, X.L. Wu, Y.G. Guo, L.J. Wan, J. Phys. Chem. C 113 (2009) 14213. [10] L.H. Hu, J. Yan, M. Liao, L. Wu, X. Fang, Small 7 (2011) 1012. [11] J. Pan, X. Song, J. Zhang, H. Shen, Q. Xiong, J. Phys. Chem. C 115 (2011) 22225. [12] S. Bazargan, K.T. Leung, J. Phys. Chem. C 116 (2012) 5427. [13] L. Quin, X. Dong, Q. Pan, Z. Cheng, Q. Xiang, F. Li, Nanotechnology 19 (2008) 185705. [14] M. Parmar, K. Rajanna, Inter. J. Smart Sensing Intell. Syst. 4 (2011) 710.
Scripta Materialia 68 (2013) 408–411 www.elsevier.com/locate/scriptamat
Novel low-temperature growth of SnO2 nanowires and their gas-sensing properties R. Rakesh Kumar,a,⇑ Mitesh Parmar,a,b K. Narasimha Rao,a K. Rajannaa and A.R. Phanic a
b
Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India School of Mechanical Systems Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea c Nano-Research for Advanced Materials and Technologies, Bangalore 560040, India Received 26 September 2012; revised 5 November 2012; accepted 7 November 2012 Available online 12 November 2012
A simple thermal evaporation method is presented for the growth of crystalline SnO2 nanowires at a low substrate temperature of 450 °C via an gold-assisted vapor–liquid–solid mechanism. The as-grown nanowires were characterized by scanning electron microscopy, transmission electron microscopy and X-ray diffraction, and were also tested for methanol vapor sensing. Transmission electron microscopy studies revealed the single-crystalline nature of the each nanowire. The fabricated sensor shows good response to methanol vapor at an operating temperature of 450 °C. Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Physical vapor deposition (PVD); Semiconductors; SnO2 nanowires; Transmission electron microscopy (TEM); Sensors
In recent years, one-dimensional metal oxide nanostructures such as nanowires, nanotubes and nanobelts have attracted much attention due to their practical applications in various fields [1,2]. Nanowires of semiconducting oxides such as ZnO [3], SnO2 [4], In2O3 [5] and TiO2 [6] grown by different methods have been used in a variety of applications. Among them, SnO2 nanowires are particularly important because of their applications in a number of fields such as Li-ion batteries [7], gas sensors [8], UV detectors [9], field emitters [10] and superhydrophobic surfaces [11]. Therefore, it is important to grow SnO2 nanostructures. Techniques for growing SnO2 nanowires include pulse laser deposition [12], hydrothermal growth [13], thermal evaporation [7] and chemical vapor deposition [11]. The most commonly used method for producing SnO2 nanowires is tube furnace type thermal evaporation due to the simplicity and low cost of this approach. The main drawbacks of this method are the high temperatures involved (>600 °C for SnO2) and the lack of exact control of parameters such as length of the nanowires and the difficulty of aligning nanowires on the substrate [7,8,10]. To overcome the above drawbacks we have adopted the
⇑ Corresponding
author. Tel.: +91 (80) 22933190; fax: +91 (80) 23600135; e-mail: [email protected] URL: http://isu.iisc.ernet.in/~rakeshr/.
vapor–liquid–solid (VLS) mechanism using gold as a catalyst in combination with thermal evaporation (not in a tube furnace) to avoid high temperature on the substrate side (Supporting information S1). The evaporation source is located 20 cm below the substrate holder in such a way that the substrates were at room temperature even though the evaporation boat was at a higher temperature. This method enabled better control over the deposition parameters such as substrate temperature and deposition rate (length of the nanowires). In this work, we report a low-temperature process for SnO2 nanowire growth by thermal evaporation, as well as their methanol sensing properties. The experimental set-up consists of a thermal evaporation arrangement for the evaporation of tin by passing a current through evaporation boat and also has the facility to allow O2 gas through a flow meter (Supporting information S1). Initially, we have taken Si as well as SiO2 (300 nm) grown on Si substrates (1 cm 1 cm). The substrates were first cleaned with acetone by ultrasonication, followed by methanol, then DI water, and finally dried in a N2 stream. The cleaned substrates were immediately transferred to a sputter coater to deposit Au film 3 nm thick. Immediately after Au deposition, substrates were loaded into the deposition chamber. The deposition chamber was evacuated by a combination of a diffusion pump and a rotary pump with a liquid
1359-6462/$ - see front matter Ó 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.scriptamat.2012.11.002
R. R. Kumar et al. / Scripta Materialia 68 (2013) 408–411
N2 trap to obtain a high vacuum of 1 105 mbar. Initially, Au-coated substrates were annealed at 450 °C for 15 min prior to nanowire growth to obtain Au catalyst particles. After cooling to room temperature, the samples were again heated to 450 °C for 15 min. Before exposure to Sn vapor, oxygen is allowed into the deposition chamber from 1 105 to 4 104 mbar and maintained constant, after which the Sn vapor is admitted. Nanowire growth was performed for 15 min at 450 °C at a PO2 of 4 104 mbar. Nanowires grown on SiO2 substrate used for methanol sensing had Al electrodes deposited over the nanowire film. The grown nanowires were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM), and were tested for their methanol vapor-sensing properties with the help of temperature and concentration modulation. The temperature modulation helped to determine the optimum operating temperature of the sensor, and the concentration modulation shows how of methanol vapor concentration varies with sensor performance. Concentration modulation shows how performance of sensor varies with methanol vapor concentration. Figure 1a and b shows low- and high-magnification SEM images of the dense SnO2 nanowires grown on Au-coated Si substrates in top view, and Figure 1c shows a cross-sectional view. The nanowires were grown vertically and aligned at some angle with respect to the substrate. The length and diameter of the nanowires were of the order of 2–2.5 lm and 30–80 nm, respectively, for a deposition time of 15 min. The growth rate of the nanowires was observed to be 130 nm min1. Each nanowire was capped with Au catalyst particle at their ends, and in addition the growth temperature is well above the eutectic temperature of the Au–Sn system, confirming the VLS growth of the SnO2 nanowires. Nanowires were also grown at temperatures below 450 °C but were found to have Sn particles decorating the nanowires. For this reason, we avoided nanowire growth below 450 °C. The phase purity of the SnO2 nanowire thin film was characterized by XRD. The XRD spectrum recorded on the
409
as-grown SnO2 nanowire film on Si is shown in Figure 1d. The diffraction peaks observed at 2h = 26.5, 33.8, 51.7 indexed as (110), (101), (211) planes of tetragonal rutile SnO2, which coincides well with JCPDS Card No. 77-0448. The observation of Au peaks along with SnO2, is due to the Au catalyst used for the nanowire synthesis. The XRD result confirms the crystalline nature of the nanowires. TEM investigations of SnO2 nanowires have been carried out by dropping prepared nanowire solution onto a copper grid. Figure 2a shows a TEM image of the single nanowire with a diameter of 30 nm. A high resolution TEM (HRTEM) image recorded on the square region nanowire in Figure 2a is shown in Figure 2b. The lattice fringes in HRTEM clearly reflect the single-crystalline nature of the nanowire. The lattice spacing between the fringes was found to be 0.26 nm, corresponding to the (1 0 1) planes of SnO2. Further energy-dispersive X-ray (EDX) analysis performed on the nanowire and catalyst particle are shown in Figure 2c and d, respectively. The EDX spectrum of the wires shows Sn and O along with Cu and C. The elemental quantification performed on the nanowire indicates 68 at.% O and 31 at.% Sn, confirming the SnO2 phase (Supporting information S2). The EDX spectrum on the catalyst alloy particle consists of Au and Sn. The observation of Cu and C peaks is the result of the carbon-coated copper grid. From the above discussion, we conclude that the SnO2 nanowires grown were single crystalline in nature and pure. The as-grown SnO2 nanowires were then studied for their static-sensing properties for methanol. Being an n-type metal oxide, the resistance of SnO2 nanowire film increases in the presence of oxygen. During the sensing process, methanol undergoes dehydrogenation, resulting in the release of free electrons and hence in the variation of the film resistance [14]. As a result of this, the resistance of the sensing material decreases during sensing. Here, the sensing response was taken as: sensitivityð%Þ ¼ ½ðRair Rgas Þ=Rgas 100;
ð1Þ
Figure 1. (a and b) High- and low-magnification SEM images of the SnO2 in top view; (c) SEM image of the SnO2 nanowires in cross-section view; (d) XRD pattern of SnO2 nanowire film.
410
R. R. Kumar et al. / Scripta Materialia 68 (2013) 408–411
Figure 2. (a) TEM image of a single nanowire; (b) HRTEM recorded on the nanowires; (c and d) EDX spectrum recorded on the tip and wire.
Figure 3. (a and b) Temperature modulation and concentration modulation of the methanol-sensing studies. (c) Typical resistance variation at optimum operating temperature during methanol exposure.
where Rair is the resistance in air (base resistance) and Rgas is the saturated resistance in the presence of test gas. The methanol-sensing study for as-grown SnO2 nanowires was performed by temperature and concentration modulation. The temperature modulation helps to find an optimum operating temperature (Topt) at which the sensing response will be the maximum for a fixed concentration of analyte. In the present case, the fixed concentration of analyte was 300 ppm methanol. Hence, during the temperature modulation, the operating temperature was varied from 200 to 500 °C, and the Topt was observed to be 450 °C. As can be seen from Figure 3a, the maximum sensitivity observed at Topt was found to be 2582%. This was followed by concentration
modulation, during which the operating temperature of the sensor was fixed at Topt and the methanol concentration was varied from 100 to 1500 ppm. The concentration modulation gives an understanding of the sensing behavior with varying methanol concentration. As Topt for the present study is 450 °C, the complete concentration modulation study was carried out at that temperature. Figure 3b shows the variation of sensing response, i.e. sensitivity, with respect to change in methanol concentration. Here, the sensitivity for lower concentrations (6400 ppm) was linear and tends to saturate at higher (>600 ppm) concentrations of methanol. The sensitivity for 1500 ppm of methanol was observed to be 3268%. The typical variation in the resistance of SnO2 is shown
R. R. Kumar et al. / Scripta Materialia 68 (2013) 408–411
in Figure 3c. The sensitivity of SnO2 nanowires as well as the optimum operating temperature can be improved by selecting different electrode materials instead of Al and doping these nanowires with suitable catalyst, e.g. Au, Pd and Pt. Although Au catalyst was used in the present case, the catalyst was to control the structure of nanowires rather than enhancing the sensing response. In summary, crystalline SnO2 nanowires were grown by simple thermal evaporation at a low substrate temperature of 450 °C. Nanowire growth was achieved by using an Au-assisted VLS mechanism. Nanowires were grown uniformly throughout the substrate and were vertical with respect to the substrate, with a length and diameter of 2–2.5 lm and 30–80 nm, respectively. XRD and TEM studies confirm the crystalline nature of the nanowires. EDX measurements on the nanowires confirm the formation of SnO2 (Sn:O ratio = 31:68 at.%). SnO2 nanowire films showed good response to methanol at an operating temperature of 450 °C. The operating temperature can be further reduced by controlling the structure of nanowires by using different catalysts for the growth of SnO2 nanowires and by volumetric doping of these nanowires with catalysts such as Au, Pd and Pt. The authors thank the Advanced Facility for Microscopy and Microanalysis (AFMM), Indian Institute of Science, Bangalore, for providing the microscopy facility.
411
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/ 10.1016/j.scriptamat.2012.11.002. [1] R.S. Devan, R.A. Patil, J.H. Lin, Y.R. Ma, Adv. Funct. Mater. 22 (2012) 3326. [2] J.G. Lu, P. Chang, Z. Fan, Mater. Sci. Eng. R 52 (2006) 49. [3] L.S. Mende, J.L.M. Driscoll, Mater. Today 10 (2007) 40. [4] J. Pan, H. Shen, S. Mathur, J. Nanotechnol. 2012 (2012) 917320. [5] S. Kar, S. Chakrabarti, S. Chaudhuri, Nanotechnology 17 (2006) 3058. [6] B. Liu, E.S. Aydil, J. Am. Chem. Soc. 131 (2009) 3985. [7] M.S. Park, G.X. Wang, Y.M. Kang, D. Wexler, S.X. Dou, H.K. Liu, Angew. Chem. Int. Ed. 46 (2007) 750. [8] Y. Shen, T. Yamazaki, Z. Liu, D. Meng, T. Kikuta, N. Nakatani, M. Saito, M. Mori, Sens. Actuat. B 135 (2009) 524. [9] L.Y. Jiang, X.L. Wu, Y.G. Guo, L.J. Wan, J. Phys. Chem. C 113 (2009) 14213. [10] L.H. Hu, J. Yan, M. Liao, L. Wu, X. Fang, Small 7 (2011) 1012. [11] J. Pan, X. Song, J. Zhang, H. Shen, Q. Xiong, J. Phys. Chem. C 115 (2011) 22225. [12] S. Bazargan, K.T. Leung, J. Phys. Chem. C 116 (2012) 5427. [13] L. Quin, X. Dong, Q. Pan, Z. Cheng, Q. Xiang, F. Li, Nanotechnology 19 (2008) 185705. [14] M. Parmar, K. Rajanna, Inter. J. Smart Sensing Intell. Syst. 4 (2011) 710.