Mechanism Study of ZnO Nanorod-Bundle Sensors for H2S Gas

Mar 24, 2011 - primarily by forming a depletion region owing to adsorption of ionized oxygen species on the surface of sensing materials. The changing...
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Mechanism Study of ZnO Nanorod-Bundle Sensors for H2S Gas Sensing Jaehyun Kim and Kijung Yong* Surface Chemistry Laboratory of Electronic Materials, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea

bS Supporting Information ABSTRACT: This work reports the H2S gas-sensing properties of ZnO nanorod bundles and an investigation of their gas sensing mechanism. A one-dimensional ZnO nanostructure was synthesized using the hydrothermal method; scanning electron microscopy (SEM); and X-ray diffraction (XRD) spectra confirmed that the structures were crystalline ZnO of hexagonal structure. A furnace-type gas sensing system was used to characterize the nanorod bundles’ sensing properties in air containing dilute H2S gas (50 ppm) at sensing temperatures Ts e 500 °C. The response of ZnO nanorod-bundle sensors to H2S gas increased with Ts; this trend may be due to chemical reaction of nanorods with gas molecules. X-ray photoelectron spectroscopy results indicated that the sensing mechanism of the ZnO nanorod-bundle sensor was explained by both the wellknown surface reaction between H2S and adsorbed oxygen on ZnO, and the formation of zincsulfur bonding in ZnO nanorods, which becomes a dominant sensing mechanism at high Ts above 300 °C.

1. INTRODUCTION Metal oxide (MO) sensors are widely used for the detection of toxic and explosive gases in air. MO sensors detect gases primarily by forming a depletion region owing to adsorption of ionized oxygen species on the surface of sensing materials. The changing thickness of the depletion layer occurs due to two factors: reaction between adsorbed oxygen ions and the target gas molecule; and displacement of the ions by the molecule. Types of oxygen adsorption at the MO surface depend on sensing temperature, Ts.1 Oxygen species detection at different Ts can be described using eq 1.2 β O2 ðgÞ þ R 3 e þ  S OR β 2

ð1Þ

where OR β* is a chemisorbed oxygen species, and * is an unoccupied chemisorption site; R and β are integers (1 or 2) for explanation of ionized state and of atomicmolecular state each other. The temperature dependency of the dominant oxygen species is expressed, and dominant species are changed from molecular ions to atomic ions at Ts > 175 °C.2 One-dimensional (1D) nanostructured chemoresist materials, such as ZnO, SnO2, WO3 and V2O5, have good sensing properties including speedy response and recovery.37 Various sensing mechanisms have been proposed for these sensors. The surfacedepletion-controlled model has been applied to one-dimensional metal oxide nanosensor.4 ZnO, a wide band gap material (3.37 eV) with a wurzite crystal structure, has been studied as a gas sensing material, and various r 2011 American Chemical Society

synthesis methods have been applied for ZnO nanomaterials growth including thermal evaporation,8 chemical vapor deposition (CVD),911 laser ablation,12 arc plasma reaction13 and solution methods.14,15 Among these methods, hydrothermal growth of ZnO has the advantages of low temperature processing, large scale synthesis, and low cost.14 Various types of ZnO nanostructures have been studied for gas sensing applications of diverse gases such as ethanol, methane, nitric oxide, chlorobenzene, acetone, ammonia and hydrogen.7,1621 As a well-known toxic gas in air pollution, H2S is an important target molecule not only for ZnO gas sensors but also for most of other metal oxide sensors. In several mechanism studies, the surface reaction of H2S has been explained using eq 2,22,23 which describes surface reaction with adsorbed oxygen species:  2H2 SðgÞ þ 3OR ð2Þ 2 ðadÞ S 2H2 OðgÞ þ 2SO2 ðgÞ þ 3R 3 e

In this reaction, the thickness of the depletion layer is controlled by surface reaction of H2S with chemisorbed oxygen species on the sensing material’s surface. In addition to surface reaction with adsorbed oxygen species, chemical conversion of ZnO with H2S is another possible reaction mechanism affecting conductivity of nanowires sensor. The chemical conversion of gas sensing materials through the direct reaction with target gas is known to be an important mechanism that changes the conductivity and produces a high Received: October 22, 2010 Revised: March 1, 2011 Published: March 24, 2011 7218

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The Journal of Physical Chemistry C Scheme 1. Illustration of Nanorods-Bundle Sensora

a

ZnO nanorod bundles were deposited on Au/Ti electrodes. Ag paste was used to connect electrodes and Au wires transferring electric signals to IV measurement system.

sensitivity in several sensing materials of metal or metal oxides. For example, the chemical conversion of CuO nanoparticle to metallic CuS has been reported in H2S sensing by Xue et al.,24 and formation of AuS by reaction of H2S on the gold nanoparticles was also reported by Mahendra et al.25 Moreover, various types of complex reactions between ZnO and sulfur compounds (SOx, H2S, (NH4)2S, and so on.) have been reported from surface chemistry studies on ZnO.2629 Not only the surface chemistry of various gases on ZnO,30 but also the fabrications of ZnS 1-dimensional nanostructures through the chemical conversion of ZnO have been reported.3133 Although sensing results of H2S gas using ZnO nanomaterials have been reported by several researchers,23,34 the basic surface reaction mechanism underlying sensing H2S gas is not fully understood. In this work, we report the mechanism study of ZnO nanorod bundles sensor for H2S gas sensing. The sensing mechanism of ZnO nanorod bundles sensor was investigated using XPS analysis at various Ts to determine the mechanism by which ZnO nanorod-bundle sensors detect H2S gas. The dependence of sensor response on Ts that we observed is consistent with an activated process. From these results, we propose a model in which diffused of S (forming ZnS bond) is the dominant sensing mechanism, especially at high Ts.

2. EXPERIMENTAL SECTION 2.1. Synthesis of ZnO Nanorod Bundle Powder. ZnO nanorod bundle powder for gas sensors was synthesized using a hydrothermal reaction. The precursor solution was prepared using zinc chloride (ZnCl2, 98þ%, Acros organics), ammonium hydroxide (NH4OH, 28 wt % NH3 in H2O, SIGMA-Aldrich), and deionized water (D.I. water). The D.I. water was prepared in a three-stage Millipore-Q Plus purification system and had a resistivity >18.2 MΩ 3 cm. ZnCl2 (1.1 g) was dissolved in 80 mL warm D.I. water using a sonicator, then 4 mL NH4OH was added to the solution; white ZnO precipitates formed; these were dispersed by sonicating the solution for >1 h, then the dispersed solution was heated to 150 °C with constant stirring (∼300 rpm) in a 120-mL Teflon autoclave. After 4 h reaction at 150 °C, the reactor was cooled to room temperature (RT). White precipitates were separated from products by centrifugation and then the ZnO nanorod bundle powder was washed two or three times in D.I. water and acetone alternately. Between washings, the product was centrifuged to increase yield. After washing, the ZnO powder was kept in D.I. water. 2.2. Fabrication of Nanorod Bundle Gas Sensor. ZnO nanorod bundle gas sensors were fabricated on quartz using a simple fabrication procedure (Scheme 1). Two Ti/Au electrodes (0.7  0.4 cm) were deposited 1 mm apart on a 2  1 cm quartz substrate. First, to strengthen adhesion between Au electrode

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and quartz, a Ti buffer film of 75-nm thickness was deposited onto the quartz substrate by thermal evaporation. Then Au electrodes (∼300 nm thick) were deposited by same method on the Ti thin film. After electrode deposition, to maintain the concentration of ZnO nanorod bundles during deposition, 0.015 g of dried ZnO powder was sonicated in 5 mL D.I. water for 20 min. A droplet of well-dispersed ZnO nanorod bundles in D.I. water was dropped between electrodes and dried at 95 °C for 1 day; then annealing at 500 °C was performed to make good contacts with electrodes. The electrodes were connected with 0.3 mm diameters Au wires using Ag paste. The Ag paste on the sensor was dried at 95 °C for 1 day to maintain good adhesion between electrodes and the tips of Au-wires, then the other tips of the Au wires were linked with Pt wires which were connected to an electrical measuring instrument. 2.3. Analysis Techniques. The sample morphology and crystal structure of the ZnO nanorod bundles were observed using field-emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). The H2S gas sensing properties of the fabricated sensors were tested in a furnace-type sensing system (Scheme 2). The horizontal tubular furnace system was used to measure the gas sensing properties. An outer diameter and a thickness of quartz tube are 45 and 2.5 mm, respectively. The thermocouple (TC) was positioned at the center of the quartz tube. Diluted H2S (50 ppm) was passed over the sensor at 400 sccm using a mass flow controller (MFC, MKS Inc.). The chemical composition of the samples was analyzed using XPS with Al KR radiation. Argon ion sputtering (3 keV, 36 μAcm2, ∼1 nm/min) was used to acquire XPS composition depth profiles.

3. RESULTS AND DISCUSSION The hydrothermal method synthesized a uniformly high density of nanorod bundles (Figure 1a). The average length of ZnO nanorods was about 1 μm, with an aspect ratio of ∼10. Hexagonally faceted products indicate that the grown nanorods were highly crystalline. The ZnO nanorod bundles had the typical wurtzite hexagonal ZnO crystal structure (Figure 1b) with lattice constants a = 3.253 Å and c = 5.209 Å (JCPDS card 800075). No diffraction peaks from impurities were detected. The nanorods had a flower-like bundle structure, which should provide multijunctions in sensor devices and has the advantage of controlling contacts during sensing.7 H2S sensing properties of the ZnO nanorod sensors were measured at various Ts. Diluted H2S gas (50 ppm) was injected every 20 min; the sample was purged with air between successive injections. At 300 e Ts e 500 °C, the sensing results (Figure 2a) showed stable responses and quick recovery after H2S gas exposure and air purge. The response time is defined as the time to reach 90% of the maximum sensing response and the recovery time is defined as a time to fall to 10% of maximum sensing response upon air purging. The response clearly increased upon H2S gas exposure and returned to the original value upon air purging. As Ts increased, the recovery time decreased but the response time was unaffected (Figure 2b). At lower temperature than 300 °C, the response was stable upon H2S gas exposure but after air purging, the response did not return to within 10% of the difference between the sensed value and the original value without UV treatment or heat treatment in our sensing system. These data are shown in the Supporting Information (Figure S1). 7219

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Scheme 2. Illustration of Gas Sensing Systema

Hot wall furnace was used to control the temperature of the sensor. Mass flow controllers were used to control gas flows and the system was connected to mechanical pump for purging gas. a

Figure 1. (a) SEM images of the ZnO nanorod bundles synthesized at 150 °C over 4 h; (b) XRD spectrum of ZnO nanorod bundles.

An Arrhenius plot of gas response vs Ts is shown in Figure 2c. For a reducing gas such as H2S, response, R can be expressed as R = (Ig  Ia)/Ia, where Ig and Ia indicate current generated during gas and air flow, respectively. The value of R increased to ∼35 at 500 °C. The exponential dependence of response on temperature indicates that the H2S sensing mechanism includes an activated process, such as H2S chemisorption or chemical reaction of ZnO with H2S. Regarding response/recovery, our experiments showed reproducible trends on temperature dependence. Recovery time monotonically decreases with increasing Ts but response time does not show any dependence on Ts as shown in Figure 2b. From the definition of response time, it depends on the maximum current value (or resistivity) and the initial shape of response vs time graph. The maximum current value after enough exposure time for saturated adsorption is closely related to temperature sensing through the reaction mechanism but the initial response time does not show strong dependence on Ts. It is thought that this is because the initial response time fluctuates depending on several factors such as mass transfer of sensing gas

through the sensor system, injection rate of gases, exhausting rate fluctuation during sensing, etc. Before reaching the saturated coverage of reacting gas adsorption, these mass transfer factors can affect the initial response time. However, in the case of recovery, the sensor material has already saturated amounts of adsorbates, and thus the activated desorption/diffusion rate is enhanced with increasing temperature, which explains the decreasing recovery time on increasing Ts . XPS was used to study the H2S sensing mechanism. The depth region near surface of Zn 2p XPS data of ZnO nanorod bundles were taken before (Figure 3a) and after (Figure 3b) H2S sensing at 500 °C. Figure 3c showed the depth region of Zn 2p XPS of ZnO nanorod bundle taken after H2S sensing at 500 °C. To remove surface contaminants, samples were sputtered with Ar for 3 min before XPS measurements. Before H2S sensing, the binding energy of each bulk level was 1021.9 eV (Figure 3a),29 which corresponds to the binding energy of the Zn 2p peak of bulk ZnO. After H2S sensing, the binding energy increased to 1022.2 eV, which indicates that Zn 2p binding energy increased, 7220

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Figure 2. H2S gas sensing properties of ZnO nanorod bundles sensor at various temperatures. (a) Response and recovery of the sensor. (b) Response time and recovery time of the sensor at various temperature. (c) Arrhenius-style plot of gas response. R is the value of gas response.

Figure 3. Zn 2p XPS depth profiles of ZnO nanorod bundles after (a) annealing at 500 °C in air and (b) exposure to H2S gas at 500 °C. Red line: spectrum taken on surface of samples; blue line: bulk level taken after Argon sputtering. Dotted lines: peak values of each bulk state Zn2p spectrum (blue line) (a) 1021.9 eV; (b) 1022.2 eV. (c) The depth region of Zn 2p XPS spectrum of ZnO nanorod bundle after exposure to H2S gas at 500 °C.

probably due to chemical reaction of ZnO with H2S.29 The magnitude of the binding energy shift that we observe is somewhat smaller than that reported by others.29 The difference is likely due to the small amount of S in our sample ( 300 °C, a strong S peak occurred with binding energy ∼162 eV; this peak corresponds to the dominant ZnS bond formation. At Ts < 200 °C, only a broad feature at 164171 eV was observed on the ZnO nanorod bundle surface. The chemical binding states corresponding to this binding energy can be assigned to sulfur oxygen compounds, such as chemisorbed SO2 (163165.5 eV), sulfite (SO3) (∼166.4 eV), and sulfate (SO42-) (168 170 eV).26,28 Even at RT exposure to H2S, a very weak SO4 signal was detected (Figure 5f). The origin of sulfur oxide compounds (Figure 5c) can be explained by surface reaction of H2S with chemisorbed oxygen on ZnO (eq 2). Various types of sulfur oxides, may be formed as a result of the reaction of SO2 with oxygen adatoms (eq 3) or by disproportionation (eq 4) of chemisorbed SO2.28 SO2 ðgÞ þ nOðadÞ S SO2 þ n ðadÞ

n ¼ 1 or 2

ð3Þ

Figure 8. EDS mapping images of ZnO nanorod taken after exposure to H2S gas at 500 °C.

mSO2 ðadÞ S ðm  1ÞSOðgÞ þ SO1 þ m ðadÞ m ¼ 2 or 3

ð4Þ

These SO2 reactions formed sulfur oxide compounds, which caused the broad peak feature at 164171 eV (Figure 5). When ZnO nanorod bundles are exposed to H2S at Ts < 300 °C, H2S molecules react with preadsorbed oxygen species on the ZnO surface. The H2S molecules can displace adsorbed oxygen and also react with oxygen to produce sulfur oxides. This oxygen consumption generates free electrons and increases the current density of ZnO nanorods at moderate Ts. This explanation can be supported the analysis of O 1s XPS spectra at various temperatures in Figure 6. All of the O1s spectra are deconvoluted into two peaks at ∼530 and ∼531.5 eV, similar to as-grown ZnO nanorods bundle. The peak position at ∼530 eV is assigned to lattice oxide in ZnO, and the binding energy feature at ∼531.5 eV is attributed to the formation of hydroxide.35 However, the XPS peak at 200 °C indicates that additional peak at ∼532 eV is 7222

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appeared beside those two peaks. The peak near 532 eV can be assigned to SOx compound.26,28 By contrast, at Ts > 300 °C, the increase in current could be caused by activation of shallow donors due to the formation of a sub-band by S doping in the ZnO bulk level.36 The formation of ZnS chemical bonds is an activated process, which is consistent with the exponential dependence of response on Ts (Figure 2c) Moreover, some of these ZnS are converted to ZnS microcrystallites. Although it is an activated process, conversion of metal-oxide to metal-sulfide is a thermochemically spontaneous process.37,38 Transformation of highly resistive ZnO to conductive ZnS has been reported previously.34 However, in the current study, the ZnS peak observed in XRD data were very weak (Figure 7). After H2S gas treatment, small and broad peak positioned at 28.5° appeared in XRD spectra. The low intensity peak corresponding to ZnS(002)29 indicates that some ZnS bonds converted to ZnS microcrystallites with a low crystallinity. We suspect that local formation of ZnS bonding can form shallow donors in the ZnO lattice. EDS mapping images and the corresponding elemental composition are shown in Figure 8 and Table 1. Elemental Composition Corresponded to EDS Mapping of ZnO Nanorod in Figure 8aa

a

element

(keV)

mass%

atom

O

0.525

22.07

53.54

S

2.307

0.32

0.38

Zn

8.630

77.61

46.08

The fitting coefficient is 0.2810.

Table 1, respectively. The EDS mapping through an analysis of transmission electron microscopy confirmed the incorporation of S in bulk ZnO nanorods. The atomic percent of sulfur element in ZnO was lower than 0.5% in Table 1. This shows one of the evidence for explanation of sulfur incorporation in ZnO nanorods. We propose an H2S gas sensing mechanism of ZnO nanorod bundles (Scheme 3). On bare ZnO nanorods, oxygen species adsorb to the ZnO surface, forming a rather thick depletion region (Scheme 3a). Then, when the nanorods are exposed to H2S, it reacts with adsorbed oxygen (eqs 24). This reduction of oxygen coverage thins the depletion layer and increases the conductivity (Scheme 3b). The change of band bending and the depletion region caused by the change in surface oxygen concentration is the dominant mechanism under this condition. However, at Ts > 300 °C, beside this surface reaction with oxygen, H2S also decomposes to form ZnS bonds in ZnO; this change causes the formation of a shallow donor level and causes a drastic increase in conductivity (Scheme 3c). ZnO is an n-type ionic semiconductor in which the carriers originate from oxygen vacancies; the chemisorbed S generated by surface reactions tend to be captured by the oxygen vacancies.39An impurity atom in a semiconductor can form either a shallow donor in a substitutional site or a deep-level donor in a lattice. In the current case, the following mechanism seems reasonable: S dopants form shallow donors in ZnO, and oxygen vacancies are changed from deep-level donors to shallow donors when S fills substitutional sites. When the S diffuses into ZnO at high temperature, the impurities supply excess carriers to the conduction band, and

Scheme 3. Band Diagrams and Schematic Image of Electric Properties of (a) Oxygen Ionosorption Surface before Sensing, (b) H2S Gas Adsorption and Surface Reaction with Surface Oxygen and (c) Donor Level Formation of ZnO Nanorod Bundles with Sulfur Chemisorption at Ts > 300°Ca

a

The graphics at the bottom of show the conduction between ZnO nanorods to describe the flow of current in our stacked ZnO nanorods sensor. 7223

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The Journal of Physical Chemistry C these increase the electrical conductivity of ZnO. This action of doped S in ZnO has also been previously reported.40 The formation of zincsulfur bonding in ZnO nanorods becomes a dominant sensing mechanism at high Ts above 300 °C.

4. CONCLUSIONS ZnO nanorod bundles were synthesized using the hydrothermal method, and the H2S gas sensing properties of a simply fabricated ZnO sensor were evaluated in a furnace-type sensing system. The presence of crystallized ZnO was confirmed by FESEM and XRD. The gas response of ZnO nanorod bundles sensor increased exponentially with sensing temperature above 300 °C. The sulfur diffusion was found in the bulk ZnO during high temperature sensing. The ZnO sensor detects H2S gas by both surface depletion caused by change of surface oxygen concentration, and by the formation of a shallow donor level caused by activities of diffused S at high temperature. ’ ASSOCIATED CONTENT

bS

Supporting Information. The H2S gas response and recovery of gas sensor at Ts < 300 °C, C1s-XPS depth profile of ZnO nanorod bundles. This information is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by grants from the National Research Foundation (NRF2010-0009545, NRF20100015975), and by the Korean Research Foundation Grants funded by the Korean Government (MOEHRD) (KRF-2008005-J00501). ’ REFERENCES (1) Yamazoe, N.; Sakai, G.; Shimanoe, K. Catal. Surv. Asia 2003, 7, 63. (2) Barsan, N.; Weimar, U. J. Electroceram. 2001, 7, 143. (3) Wan, Q.; Li, Q. H.; Chen, Y. J.; Wang, T. H.; He, X. L.; Li, J. P.; Lin, C. L. Appl. Phys. Lett. 2004, 84, 3654. (4) Li, C. C.; Du, Z. F.; Li, L. M.; Yu, H. C.; Wan, Q.; Wang, T. H. Appl. Phys. Lett. 2007, 91, 032101. (5) Kim, Y. S.; Ha, S. C.; Kim, K. W.; Yang, H. S.; Choi, S. Y.; Kim, Y. T.; Park, J. T.; Lee, C. H.; Choi, J. Y.; Park, J. S.; Lee, K. Y. Appl. Phys. Lett. 2005, 86, 213205. (6) Yu, H. Y.; Kang, B. H.; Pi, U. H.; Park, C. W.; Choi, S. Y.; Kim, G. T. Appl. Phys. Lett. 2007, 86, 253102. (7) Feng, P.; Wan, Q.; Wang, T. H. Appl. Phys. Lett. 2007, 87, 213111. (8) Yao, B. D.; Chan, Y. F.; Wang, N. Appl. Phys. Lett. 2007, 81, 757. (9) Chang, P.; Fan, Z.; Wang, D.; Tseng, W.; Chiou, W.; Hong, J.; Lu, J. G. Chem. Mater. 2004, 16, 5133. (10) Haga, K.; Kamidaira, M.; Kashiwaba, Y.; Sekiguchi, T.; Watanabe, H. J. Cryst. Growth 2000, 214/215, 77. (11) Park, D.; Tak, Y.; Yong, K. J. Nanosci. Nanotechnol. 2008, 8, 623. (12) Cao, H.; Wu, J. Y.; Ong, H. C.; Dai, J. Y.; Chang, R. P. H. Appl. Phys. Lett. 1998, 73, 572. (13) Dong, L. F.; Cui, Z. L.; Zhang, Z. K. Nanostruct. Mater. 1997, 8, 815.

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