Photoelectrochemical Study on Charge Transfer Properties of ZnO

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J. Phys. Chem. C 2009, 113, 16247–16253

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Photoelectrochemical Study on Charge Transfer Properties of ZnO Nanowires Promoted by Carbon Nanotubes Wei-De Zhang,* Liao-Chuan Jiang, and Jian-Shan Ye Nano Science Research Center, School of Chemistry and Chemical Engineering, South China UniVersity of Technology, 381 Wushan Road, Guangzhou 510640, People’s Republic of China ReceiVed: June 11, 2009; ReVised Manuscript ReceiVed: July 28, 2009

ZnO nanowires (ZnO-NWs) were adhesively grown on multiwalled carbon nanotubes (MWCNTs) arrays by a hydrothermal process. Electrochemical properties of the electrode based on the ZnO-NWs/MWCNTs nanocomposite were investigated by electrochemical impedance spectroscopy and cyclic voltammetry. The photoelectrochemical responses of the ZnO-NWs/MWCNTs nanocomposite electrode were further studied by linear sweep voltammetry and amperometry under sunlight or UV light irradiation. Compared with pure ZnO-NWs on tantalum substrate, the charge transfer rate of the ZnO-NWs/MWCNTs nanocomposite was remarkably increased because of the MWCNTs. A Mott-Schottky plot displayed a high donor density of 3.9 × 1019 cm-3, a flat band potential of -0.8 V, and a space charge layer of 7 nm. In addition, the ZnO-NWs/ MWCNTs nanocomposite yielded higher photocurrent than pure ZnO-NWs. The decay constant of the ZnONWs/MWCNTs nanocomposite was also lower than that of its pure counterpart. The recombination of photoinduced electron-hole pairs in the ZnO-NWs/MWCNTs heterojunction was hindered, thus enhancing the photoelectrical conversion efficiency. The heterojunction of ZnO-NWs/MWCNTs provides potential applications in the field of photocatalysis and photoelectrical devices. Introduction Carbon nanotubes (CNTs) have gained great attention due to their unique structure and physical properties since they were discovered. Being a new crystal form of carbon materials, CNTs have been considered as an ideal catalyst support because of their large surface area, inert chemical property, one-dimensional structure with nanopores, and unique electrical properties.1 Many researches have studied using CNTs as support to disperse metal nanoparticles to be catalysts for fuel cells or electroanalysis.2,3 Recently, efforts have also been focused on modification of CNTs with semiconductive metal oxides and sulfides such as TiO2,4-6 ZnO,7 Fe2O3,8 SnO2,9 WO3,10 In2O3,11 ZnS,12 and CdS.13 Incorporation of metal oxides/sulfides with CNTs will lead to nanocomposites possessing the properties of both components, or even with a synergistic effect, which would be useful in the field of photocatalysis. Among the above-mentioned nanocomposites, ZnO and TiO2 have been widely investigated and some researchers have reported the preparation and photocatalysis of CNTs-supported ZnO or TiO2.6,14-18 For example, Jiang et al. reported ZnO nanocrystals-coated MWCNTs prepared by noncovalent modification of MWCNTs with the dispersant of sodium dodecyl sulfate, exhibiting a higher photocatalytic activity than bulk ZnO or the mechanical mixture of ZnO and MWCNTs.15 The good interfacial combination promoted the electron transfer from ZnO nanoparticles to MWCNTs. On the other hand, charge transportation between the semiconductive oxide nanoparticles was limited due to the lack of continuous conducting pathways. This could be improved in one-dimensional nanostructures.19,20 Therefore, it is necessary to study the enhancing effect of MWCNTs on the ZnO-NWs for the charge separation and electron transfer property. * To whom correspondence should be addressed. Phone: 86-20-8711 4099. Fax: 86-20-8711 2053. E-mail: [email protected].

In the present work, ZnO-NWs on the vertically aligned MWCNTs arrays have been successfully fabricated. Wellaligned MWCNTs were selected because they provide more landing sites for the adhesive growth of nanowires and a space between each tube that would beneficially disperse nanowires compared to disordered CNTs. The three-dimensional nanostructure containing ZnO-NWs and MWCNTs heterojunction provides unique morphology with large surface area for effective conversion of photoenergy. Charge transfer properties were investigated based on photoelectrochemical (PEC) response, using the ZnO-NWs/MWCNTs nanocomposite as an electrode. The experimental results show that the recombination of photoinduced electron-hole pairs was hindered and higher photocurrent density was produced compared with that of pure ZnO-NWs, suggesting the ZnO-NWs/MWCNTs heterojunction is effective for charge separation and transportation, which is beneficial for its applications in photocatalysis and photoelectrical devices. Experimental Section Na2SO4 (99.0% purity) was purchased from Kermel Chemical Reagent (Tianjin, China). K3[Fe(CN)6] (99.5% purity) and K4[Fe(CN)6] · 3H2O (99.5% purity) were purchased from Donghua Chemical Reagent (Guangdong, China). Deionized water (>18.4 MΩ cm-1) was used for all solution preparation. Vertically aligned MWCNTs arrays on Ta foils (about 3 × 3 mm2 in area) were prepared by catalytic chemical vapor deposition.21-23 Preparation and characterization of ZnO-NWs/ MWCNTs have been reported elsewhere.24 Growth of vertically aligned ZnO nanowires on Ta foils followed the same procedure, in which bare Ta foils were used as the substrate instead of MWCNTs. In brief, a thin film of ZnO was deposited on small Ta foils by radio frequency sputtering deposition with a ZnO target at a power of 150 W for 1 h. A solution saturated with

10.1021/jp905500n CCC: $40.75  2009 American Chemical Society Published on Web 08/14/2009

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Figure 1. Top view SEM images of (A) ZnO-NWs/MWCNTs and (B) ZnO-NWs arrays on a Ta foil. The inset in panel B shows the side view of the ZnO-NWs. (C) TEM image of ZnO nanowires on a MWCNT and (D) high-resolution TEM image of a ZnO nanowire.

Zn(OH)42- for hydrothermal synthesis of ZnO nanowires was prepared by dissolving ZnO (Aldrich, 99.5% purity) in 5 M NaOH solution. Three milliliters of the obtained solution was diluted with deionized water to 50 mL in a Teflon-lined stainless steel autoclave. ZnO-coated Ta foils were attached to the bottom of another smaller Teflon beaker and placed in the autoclave. After being heated at 100 °C for 24 h, vertically aligned ZnO nanowires on the Ta foils were obtained. All other reagents were of analytical grade and were used without further purification. The electrochemical measurements were performed in 0.10 M Na2SO4 solution. Morphology of the ZnO-NWs on MWCNTs or Ta foils was observed by field-emission scanning electron microscope (JEOL JSM 6700). Transmission electron microscopy (TEM) observation of ZnO NWs-modified MWCNTs was conducted with a Philips CM 300 FEG instrument. Electrochemical measurements were performed with a CHI 660C electrochemical workstation

(Shanghai Chenhua, China). A three-electrode system was employed for measuring photocurrent with ZnO-NWs/MWCNTs as a photoanode, an Ag/AgCl (3 M KCl) electrode and a platinum wire as reference electrode and counter electrode, respectively. All potentials were referred to the Ag/AgCl (3 M KCl) electrode. The photoanode was illuminated by a 400 W ultraviolet (UV) high-pressure mercury lamp (Guangzhou Tongfangyuan Light, China) with a main wavelength of 365 nm. A metal halogen lamp (Shanghai Yaming Light, China), as a spectra physics solar simulator, was used as the sunlight source. Electrochemical impedance spectroscopy (EIS) measurements were carried out with a frequency response analyzer (PGSTAT 30, Autolab, Eco-Chemie, The Netherlands), using the above three-electrode cell. Measurements were performed under dark condition with an ac amplitude of 5 mV and frequency ranged between 100 kHz and 100 mHz. Nonlinear least-squares analysis was used to simulate the impedance plane plot.

Charge Transfer Properties of ZnO Nanowires

Figure 2. (A) Cyclic voltammograms of the (a) ZnO-NWs, (b) MWCNTs, and (c) ZnO-NWs/MWCNTs electrodes in 1.0 M KCl solution containing 5.0 mM K3[Fe(CN)6] (the dash line in (a) is the voltammogram without K3[Fe(CN)6]). (B) Plot of the separation of oxidation and reduction peak potentials vs. scan rates.

Results and Discussion ZnO nanowires have been successfully grown on the surfaces of carbon nanotubes24 or flat substrates with ZnO nanoparticles as nuclei by a hydrothermal process. Figure 1 depicts the overall morphology of ZnO nanowires on vertically aligned carbon nanotubes and Ta foils. One can see that densely packed ZnO nanowires are aligned perpendicularly (to the axis of the tubes) on the walls of the CNTs (Figure 1A), or on the flat Ta foil (Figure 1B). The inset in Figure 1B shows the side view of the ZnO nanowires, which are about 1.5 µm in length and 30-90 nm in thickness. TEM observation further confirms both the uniformity and high density of the ZnO nanowires on MWCNTs (Figure 1C). The high-resolution TEM image (Figure 1D) depicts one typical ZnO nanowire with a thickness of about 30 nm. The lattice fringes with d-spacing of 0.28 nm, which corresponds to a (1 0 -1 0) facet, are clearly observed. The inset in the upper right of Figure 1D is the selected area electron diffraction pattern of the nanowire, indicating single crystal characteristics of the nanowire. The growth direction of the nanowires is along the c-axis. The ZnO nanowire arrays on the conductive substrates provide a unique three-dimensional nanostructure for construction of electrodes.6,10 To investigate the electron transfer property of the ZnO-NWs, MWCNTs, and ZnO-NWs/MWCNTs electrodes, K3[Fe(CN)6] was used as a probe. Figure 2A shows the typical cyclic voltammetry (CV) in 1.0 M KCl solution containing 5.0 mM K3[Fe(CN)6] at ZnO-NWs, MWCNTs, and ZnO-NWs/MWCNTs electrodes, respectively. One can see that at the ZnO-NWs electrode, no response is observed without K3[Fe(CN)6], but a typical semiconductor response of unsymmetric peak appears with a large peak potential separation in the presence of K3[Fe(CN)6] (Figure 2A(a)). This indicates that the electron

J. Phys. Chem. C, Vol. 113, No. 36, 2009 16249 transfer process is difficult to occur between the redox probe and the ZnO nanowires on Ta substrate. At the bare MWCNTs electrode, a symmetric current response from the redox probe was observed. The ZnO-NWs/MWCNTs electrode exhibits larger capacitance than the bare MWCNTs electrode, signifying the modification effect of ZnO-NWs on the MWCNTs. The redox peaks of ZnO-NWs/MWCNTs indicate that electrons can transfer between Fe(CN)63- and MWCNTs electrodes via ZnONWs. The separation of peak potentials under different scan rates at the bare MWCNTs and ZnO-NWs/MWCNTs electrodes is shown in Figure 2B. At the bare MWCNTs electrode, the difference of the anodic and cathodic peak potentials (∆Ep) is 59 mV at all sweep rates (0.01-0.30 V/s), suggesting that the MWCNTs electrode is ideally reversible for Fe(CN)63-. On the other hand, ∆Ep of the ZnO-NWs/MWCNTs electrode is increasing gradually from 73 to 90 mV upon increase of scan rates from 0.02 to 0.30 V/s, indicating a quasireversible Fe(CN)63- redox reaction at the ZnO-NWs/MWCNTs electrode. This is possibly because the transportation of electrons from ZnO-NWs to MWCNTs is not fast enough to match the redox reaction rate at the MWCNTs electrode.25 Electrochemical impedance spectroscopy (EIS), which can provide the information of capacitance and resistance of the electrode materials, is an effective approach for investigating electron transfer across the electrolyte and the surface of the electrode. Figure 3 shows the typical impedance spectra of the bare MWCNTs, ZnO-NWs, and ZnO-NWs/MWCNTs electrodes in 0.10 M KCl solution containing equimolar [Fe(CN)6]3-/4at an ac frequency varying from 100 kHz to 0.1 Hz. The apparent difference in the impedance spectra is observed with the three electrodes. The Nyquist complex plane plot of the bare MWCNTs electrode exhibits an almost straight line with a slope of 45°, which is characteristic of a diffusion limiting step of the electrochemical reaction at the electrode. On the other hand, the plot of the pure ZnO-NWs electrode displays a single semicircle at the high-frequency region, an indication of a charge transfer process. This result discloses that few electroactive species can reach the ZnO surface through diffusion, which is in good agreement with the experimental result of cyclic voltammetry. Moreover, the ZnO-NWs/MWCNTs electrode displays two semicircles at the high-frequency region and a straight line at the low-frequency region, indicating the electrochemical reaction at the ZnO-NWs/MWCNTs electrode is controlled by a mixed process of charge transfer and diffusion. The equivalent circuit of the ZnO-NWs/MWCNTs electrode was also depicted in Figure 3. The ohmic serial resistance (Rs) in the high-frequency region corresponds to the resistance of the electrolyte and the metal substrate (Ta). R1 and R2 in the high-frequency region are assigned to the impedance related to charge transport at the Pt counter electrode and ZnO-NWs,26,27 while the resistances of R3 and Zw in the low-frequency region represent the resistances of MWCNTs and Warburg, respectively. The resistance of the ZnO-NWs/MWCNTs is much smaller than that of pure ZnO-NWs, showing the redox couple at the ZnO-NWs/MWCNTs electrode interface could be facile transport. Obviously, as an electron mediator with high conductivity, MWCNTs promote electron transfer during the electrochemical reaction at the ZnO-NWs/MWCNTs electrode.25 Kinetics of the electrochemical reaction at the ZnO-NWs/ MWCNTs electrode was further investigated with use of the Mott-Schottky (MS) theory, which is commonly used to determine both donor density and flat band potential at the

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Figure 3. Electrochemical impedance spectroscopy of the MWCNTs, ZnO-NWs, and ZnO-NWs/MWCNTs electrodes in 0.10 M KCl solution containing equimolar [Fe(CN)6]3-/4- (0.01 M/0.01 M) and equivalent circuit of the ZnO-NWs/MWCNTs electrode.

semiconductor/electrolyte interface. The capacitance of the semiconductor is described by the MS equation:

[

1 2 kT ) (E - EFB) 2 2 e0 Csc εε0e0NdA

]

(1)

where Csc is the charge space capacity, ε is the dielectric constant of ZnO, ε0 is the electric permittivity of vacuum, e0 is the elementary charge constant, Nd is the donor density, A is the area, k is the Boltzmann constant, T is the absolute temperature, E is the electrode applied potential, and EFB is the flat band potential. In this case, because of the contact between the Ta substrate and the electrolyte permeating through the voids of the ZnONWs network,28 the capacitance relationship can be described as:

[

]

1 2 kT 1 ) (E - EFB) + e0 Csc2 εε0e0NdA2 CH2

Figure 4. Mott-Schottky plot of the ZnO-NWs/MWCNTs electrode obtained by fitting the frequency-dependent impedance data at 10 mV with a frequency of 5000 Hz.

The donor density (Nd) can be determined from the slope of the linear region:

(2)

where CH is the Helmholtz capacitance. Although eq 2 is derived from eq 1, the slope of 1/Csc2 vs. E is unaffected. Figure 4 shows the MS plot of the ZnO-NWs/MWCNTs nanocomposite. When 1/C2 is zero, the X-intercept equals the flat band potential (EFB), which is found to be -0.8 V. The EFB value is shifted -0.3 V negatively compared to that with the ZnO nanoparticles.7

Nd ) -

( )( 2 e0ε0ε

d(1/C2) dE

)

-1

(3)

Assuming ε of the ZnO-NWs/MWCNTs as about 10.0,29 the Nd can be evaluated from the slope of the linear part, which is 3.9 × 1019 cm-3 for the ZnO-NWs/MWCNTs electrode. The calculated donor density is significantly higher than that of ZnO nanowires reported in the literatures (1017-1018 cm-3).30,31 This

Charge Transfer Properties of ZnO Nanowires

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Figure 5. Photocurrent-potential curves of the ZnO-NWs/MWCNTs electrode under (a) dark, (b) sunlight, and (c) ultraviolet light. Scan rate 50 mV/s.

is favorable for the improvement of photocatalytic activity. In addition, the slope in the linear region of the plot is positive, which serves as an indication of n-type semiconductor. The thickness of the space charge layer at the semiconductor/ electrolyte interface can also be derived from the MS plot relationship and is described by

W)

[

2εε0(E - EFB) e0Nd

]

Figure 6. Open circuit potential-time curves of the ZnO-NWs/ MWCNTs electrode under chopped sunlight and ultraviolet light illumination.

1/2

(4)

where W is the width of the depletion region. A potential of +0.50 V is chosen to calculate the space charge region because of the lack of dark current at that potential (Figure 5a). According to eq 4, the width of the space charge layer is estimated to be 7 nm. Since the space charge thickness is much smaller than the thickness of the ZnO-NWs (ca. 1.5 µm) on MWCNTs, the photocurrent as a function of space charge thickness could be increased.32 Photocurrent response of the ZnO-NWs/MWCNTs electrode is evaluated by linear sweep voltammetry (LSV) to examine charge-carrier characteristics at the semiconductor/electrolyte interface. The experiment was carried out in a 0.10 M Na2SO4 electrolyte solution under sunlight and UV illumination, respectively. LSV in a nonluminous room showed minute current in the 10-6 A cm-2 range until approximately +1.2 V, where the current was obviously increased due to water splitting,8 as depicted in Figure 5a. Figure 5b shows a paralleled increase in photocurrent under sunlight illumination. The photocurrent does not saturate completely and continues to increase to 0.87 mA cm-2 at +1.0 V. Under UV illumination, the photocurrent appeared obviously starting at -0.15 V and the current increased incessantly upon the increase of potential, but an optimized depletion layer is not fully formed. At 0 V, the photocurrent density is 0.68 mA cm-2, while it increased to 1.68 mA cm-2 at +1.0 V (Figure 5c). The increase of photocurrent discloses the effective charge separation because the recombination of photoinduced electron-hole pairs was inhibited by the increase of positive potential. In addition, the larger photocurrent at ZnONWs/MWCNTs under UV illumination than that under sunlight illumination could be attributed to the extra photogenerated excitons at the nanocomposite/electrolyte interface, which would result in the increase of the electric field produced in the depletion layer. The increased electric field would induce the photogenerated electrons transporting through the ZnO-NWs and MWCNTs to penetrate to the Ta substrate. Figure 6 shows the response of photopotential of the ZnONWs/MWCNTs electrode under sunlight and UV irradiation.

Figure 7. Short-circuit photocurrent density vs. time curves of (a) MWCNTs, (b) ZnO-NWs, (c) ZnO-NWs/MWCNTs under ultraviolet illumination, and (d) ZnO-NWs/MWCNTs under sunlight illumination in 0.10 M Na2SO4 solution.

When the sunlight was switched on, it could be seen that opencircuit potential (OCP) changed about 0.08 V. In the case of UV illumination, the OCP of the ZnO-NWs/MWCNTs was observed to change about 0.15 V. The OCP of the ZnO-NWs/ MWCNTs under UV irradiation is about 2 times as high as its counterpart under sunlight irradiation. In addition, it is also noticed that the photopotential gradually reverted when the light was switched off, but the OCP cannot reach its original level. Although recombination of photogenerated carriers occurred in the ZnO-NWs/MWCNTs heterojunction under dark condition, it was restrained effectively by the internal electrostatic field in the junction region, and the separated electrons emigrated easily to the external circuit through the interface between the MWCNTs and Ta substrate. Therefore, it is difficult for OCP to recover its initial value. PEC properties were also investigated by measuring the transient photocurrent in 0.10 M Na2SO4 electrolyte without any sacrificial reagents or cocatalysts. Figure 7 shows the photocurrent transient generated under chopped UV irradiation. The MWCNTs electrode displays very weak photoresponse (Figure 7a). The photocurrent produced at the MWCNTs electrode was probably triggered by electron excitation of MWCNTs from the valence band to the conducting band because MWCNTs can also be regarded as semiconductors.33 The ZnO-NWs shows obvious photocurrent, indicating their semiconductive characteristic. Surprisingly, photoresponse at the ZnO-NWs/MWCNTs electrode is much higher than that at the MWCNTs and ZnONWs electrodes. According to the photocurrent density of the ZnO-NWs and ZnO-NWs/MWCNTs, the short-circuit photo-

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Zhang et al. indicating that the recombination of photoinduced electron-hole pairs is more difficult at the ZnO-NWs/MWCNTs electrode than at the pure ZnO-NWs electrode. This result further proves the enhancement effect of MWCNTs on the separation of photogenerated electrons and holes. Therefore, incorporation of MWCNTs endows ZnO with excellent photoelectric properties. Conclusion

Figure 8. Schematic diagram of charge separation and transportation at the ZnO-NWs/MWCNTs heterojunction.

current density of the ZnO-NWs/MWCNTs is as high as 0.42 (Figure 7c) and 0.1 mA cm-2 (Figure 7d) under UV and sunlight irradiation, whereas it is as low as 0.04 (Figure 7b) and 0.007 mA cm-2 (figure not shown) at the ZnO-NWs electrode under the same light irradiation, respectively. The short-circuit photocurrent density of the ZnO-NWs/MWCNTs heterostructure is approximately 10 times that of the pure ZnO-NWs. Compared with the pure ZnO-NWs and MWCNTs, the photocurrent density of the ZnO-NWs/MWCNTs heterojunction is comparatively higher than the total of the photocurrent density of ZnONWs and MWCNTs, indicating a synergistic effect with heterojunction of ZnO-NWs and MWCNTs. The experimental result discloses that the MWCNTs enhanced the separation of photoinduced electron-hole pairs produced in the ZnO-NWs. This could be attributed to the unique three-dimensional structure of the ZnO-NWs/MWCNTs heterostructure. The adhesive growth of ZnO-NWs on the vertically aligned MWCNTs improves the active surface area, which enhances the effective absorption of photons, while the one-dimensional ZnO-NWs provide a continuous pathway for the transportation of photoinduced electrons. The electron transfer process in the ZnONWs/MWCNTs heterojunction is similar to that in the semiconductor-metal composite, as indicated in Figure 8. The electrons in the valence band of ZnO are excited to its conducting band, giving rise to the formation of electron and hole pairs. Meanwhile, the MWCNTs, which act as an electron acceptor with an inductive effect, promote the interfacial electron-transfer process from the ZnO-NWs to the MWCNTs. Thus, the recombination of photoinduced electrons and holes was obviously hindered. As a result, the photocurrent density of the ZnO-NWs/MWCNTs electrode is much higher than that of the pure ZnO-NWs and MWCNTs electrodes. In addition, the observed negative photocurrent (Figure 7d) with light-off may be ascribed to reaction of charge carriers and detrapping of counter charges.34 It is noticed that the transient photocurrent gradually decreased with both ZnO-NWs and ZnO-NWs/MWCNTs electrodes. This can be attributed to the recombination of photoinduced electron-hole pairs.35-37 The decay kinetics of the transient photocurrent is calculated by an exponential approach:38

Iph(t) ) (Iph(t)0) - Iph(t)∞)) • e-kt + Iph(t)∞)

(5)

where Iph(t)0) is the value of photocurrent density. Iph(t)∞) is the photocurrent density at steady state and k is the recombination rate constant. According to eq 5, the rate constant of the pure ZnO-NWs and ZnO-NWs/MWCNTs is k ) 0.105 and 0.036 s-1, respectively, under sunlight irradiation. The rate constant of the ZnO-NWs/MWCNTs is lower than that of ZnO-NWs,

In the present work, we studied the photoelectrochemical properties of the ZnO-NWs/MWCNTs heterojunction. The short-circuit photocurrent of the ZnO-NWs/MWCNTs heterojunction is 10 times that of the pure ZnO-NWs. The photopotential of the heterojunction under UV irradiation is about 2 times as high as that under sunlight irradiation. The adhesive growth of ZnO-NWs on the vertically aligned MWCNTs improves the active surface area, which enhances the effective absorption of photons. More importantly, the MWCNTs, which act as electron acceptors with inductive effect, facilitate the separation of photoinduced electron-hole pairs to thwart their recombination, thus increasing the photocurrent. The heterojunction of ZnO-NWs and MWCNTs provides potential applications in the field of photocatalysis, which are under further investigation. Acknowledgment. This work was jointly supported by the National Natural Science Foundation of China (No. 20773041), the Research Fund for the Doctoral Program of Higher Education (No. 20070561008), and the high technology research program, Ministry of Science and Technology (MOST) of China (2008AA06Z311). References and Notes (1) Baughman, R. H.; Zakhidov, A. A.; de Heer, W. A. Science 2002, 297, 787. (2) Hu, X.; Dong, S. J. Mater. Chem. 2008, 18, 1279. (3) Vairavapandian, D.; Vichchulada, P.; Lay, M. D. Anal. Chim. Acta 2008, 626, 119. (4) Yu, H. T.; Quan, X.; Chen, S.; Zhao, H. M. J. Phys. Chem. C 2007, 111, 12987. (5) Jiang, L. C.; Zhang, W. D. Electroanalysis 2009, 21, 988. (6) Wang, W.; Serp, P.; Kalck, P.; Silva, C. G.; Faria, J. L. Mater. Res. Bull. 2008, 43, 958. (7) Ahn, K. S.; Yan, Y. F.; Lee, S. H.; Deutsch, T.; Turner, J.; Tracy, C. E.; Perkins, C. L.; Al-Jassim, M. J. Electrochem. Soc. 2007, 154, B956. (8) Kay, A.; Cesar, I.; Gratzel, M. J. Am. Chem. Soc. 2006, 128, 15714. (9) Han, W. Q.; Zettl, A. Nano Lett. 2003, 3, 681. (10) Zhang, W. D.; Xu, B. Electrochem. Commnu. 2009, 11, 1038. (11) Sun, Y. P.; Murphy, C. J.; Reyes-Gil, K. R.; Reyes-Garcia, E. A.; Lilly, J. P.; Raftery, D. Int. J. Hydrogen Energy 2008, 33, 5967. (12) Ravindran, S.; Bozhilov, K. N.; Ozkan, C. S. Carbon 2004, 42, 1537. (13) Shi, J. H.; Qin, Y. J.; Wu, W.; Li, X. L.; Guo, Z. X.; Zhu, D. B. Carbon 2004, 42, 455. (14) Zhang, R. X.; Fan, L. Z.; Fang, Y. P.; Yang, S. H. J. Mater. Chem. 2008, 18, 4964. (15) Jiang, L. Q.; Gao, L. Mater. Chem. Phys. 2005, 91, 313. (16) Kim, H.; Sigmund, W. Appl. Phys. Lett. 2002, 81, 2085. (17) Bae, S. Y.; Seo, H. W.; Choi, H. C.; Park, J. J. Phys. Chem. B 2004, 108, 12318. (18) Yan, Y.; Chang, T.; Wei, P. C.; Kang, S. Z.; Mu, J. J. Dispersion Sci. Technol. 2009, 30, 198. (19) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 1165. (20) Frank, A. J.; Kopidakis, N.; Van de Lagemaat, J. Coord. Chem. ReV. 2004, 248, 1165. (21) Zhang, W. D.; Yang, F.; Gu, P. Y. Nanotechnology 2005, 16, 2442. (22) Zhang, W. D.; Thong, J. T. L.; Tjiu, W. C.; Gan, L. M. Diamond Relat. Mater. 2002, 11, 1638. (23) Zhang, W. D.; Wen, Y.; Li, J.; Xu, G. Q.; Gan, L. M. Thin Solid Films 2002, 422, 120. (24) Zhang, W. D. Nanotechnology 2006, 17, 1036. (25) Ye, J. S.; Cui, H. F.; Wen, Y.; Zhang, W. D.; Ottova, A.; Tien, H. T.; Xu, G. Q.; Sheu, F. S. Electrochem. Commun. 2005, 7, 81.

Charge Transfer Properties of ZnO Nanowires (26) Longo, C.; Nogueira, A. F.; De Paoli, M. A.; Cachet, H. J. Phys. Chem. B 2002, 106, 5925. (27) Van de Lagemaat, J.; Par, N. G.; Frank, A. J. J. Phys. Chem. B 2000, 104, 2044. (28) Wang, G.; Wang, Q.; Lu, W.; Li, J. H. J. Phys. Chem B 2006, 110, 22029. (29) Mora-Sero, I.; Fabregat-Santiago, F.; Denier, B.; Bisquert, J. Appl. Phys. Lett. 2006, 89, 203117. (30) Kim, K. K.; Kim, H. S.; Hwang, D. K.; Lim, J. H.; Park, S. J. Appl. Phys. Lett. 2003, 83, 63. (31) Li, Q. H.; Liang, Y. X.; Wang, Q.; Wang, T. H. Appl. Phys. Lett. 2004, 85, 6389. (32) Beranek, R.; Tsuchiya, H.; Sugischima, T.; Macak, J. M.; Taviera, L.; Fujimoto, S.; Kisch, H.; Schmuki, P. Appl. Phys. Lett. 2005, 87, 243114.

J. Phys. Chem. C, Vol. 113, No. 36, 2009 16253 (33) Odom, T. W.; Huang, J. L.; Kim, P.; Lieber, C. M. J. Phys. Chem. B 2000, 104, 2794. (34) Hahn, R.; Ghicov, A.; Salonen, H.; Lehto, V. P.; Schmuki, P. Nanotechnology 2007, 18, 105604. (35) Tsuchiya, H.; Macak, J. M.; Ghicov, A.; Rader, A. S.; Taveira, L.; Schmuki, P. Corros. Sci. 2007, 49, 203. (36) Ghicov, A.; Schmidt, B.; Kunze, J.; Schmuki, P. Chem. Phys. Lett. 2007, 433, 323. (37) Cui, X. L.; Ma, M.; Zhang, W.; Yang, Y. C.; Zhang, Z. J. Electrochem. Commun. 2008, 10, 367. (38) Wolcott, A.; Smith, W. A.; Kuykendall, T. R.; Zhao, Y. P.; Zhang, J. Z. Small 2009, 5, 104.

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