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Correlation between Electronic Structures and Photocatalytic Activities of Nanocrystalline-(Au, Ag, and Pt) Particles on the Surface of ZnO Nanorods J. W. Chiou,† S. C. Ray,*,‡,§ H. M. Tsai,‡ C. W. Pao,‡,|| F. Z. Chien,‡ W. F. Pong,*,‡ C. H. Tseng,^ J. J. Wu,^ M.-H. Tsai,# C.-H. Chen,|| H. J. Lin,|| J. F. Lee,|| and J.-H. Guo3 †
Department of Applied Physics, National University of Kaohsiung, Kaohsiung 811, Taiwan Department of Physics, Tamkang University, Tamsui 251, Taiwan § School of Physics, University of the Witwatersrand, Private Bag 3, WITS 2050, Johannesburg, South Africa National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan ^ Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan # Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan 3 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States
)
‡
ABSTRACT: O K-, Au, Ag, and Pt L3-edge X-ray absorption near-edge structure (XANES), X-ray emission spectroscopy, and scanning photoelectron microscopy (SPEM) measurements have been performed to study the correlation between the electronic structures and photocatalytic activities of nanocrystalline (nc)-(Au, Ag, and Pt) particles on the surface of ZnO nanorods (ZnO-NRs). The O K-edge XANES spectra reveal greater occupation of the O 2p orbitals, i.e., a greater negative effective charge of the O ions, in nc-Pt/ZnO-NRs than of nc-(Au, Ag)/ZnO-NRs. This result suggests that nc-Pt particles have weaker photocatalytic activities than those of nc-(Au, Ag) particles on the surface of ZnO-NRs. Well-defined bandgaps of nanoparticle-coated ZnO-NRs increase in the order Au (3.3 eV) f Pt (3.5 eV) f Ag (3.6 eV), which can be correlated with an decreasing Pauling’s electronegativity and a reduction of the screening effect. The valence-band SPEM measurement of nc-(Au, Ag, and Pt)/ZnO-NRs does not support the general argument that the Fermi levels of the (Au, Ag)/semiconductor composites are shifted toward the conduction-band edge relative to that of the Pt/semiconductor composite.
1. INTRODUCTION Photocatalytic activities of semiconducting ZnO have been found to be enhanced by coating noble nanocrystalline (nc)metal particles such as Au, Ag, or Pt on the surface of ZnO nanofibers/quantum dots (QDs)/nanorods (NRs).1-5 In principle, the noble metal acts as a sink, promoting interfacial charge transfer associated with the photoinduced electron-hole separation in the photocatalytic process. Therefore, the kinetics of electron transfer between photoexcited ZnO-NRs and nc-metal particles are importantly responsible for photocatalytic performance. In earlier optical studies, Wood et al.2 and Subramanian et al.4,5 reported that Au and Ag metal islands can scavenge photogenerated electrons and that the Fermi level (Ef) shifts toward the conduction-band edge in metal-ZnO QDs. The lining up of chemical potentials in both the metal islands and ZnO QDs gives rise to electron transfer at the metal-ZnO interface that shifts the conduction-band edge. The ohmic interaction between Pt metal islands and the semiconductor was demonstrated by Wood et al. to efficiently improve photocatalytic reactions in several composite systems.2 Decorating the semiconductor surface with either (Au, Ag) or Pt metal particles has usually been argued to r 2011 American Chemical Society
exhibit either an accumulation of electrons or rapid electron transfer at the interface, which shifts or pins the Ef in (Au, Ag)/ ZnO and Pt/ZnO composites.2,4,5 In an earlier report, we studied how the occupation of conduction- and valence-band states was altered and established a correlation between charge transfer and the size of the nc-Au particles on the surface of ZnO-NRs.6 The electron-hole separation and electron transfer are principally related to the electronic structures and the nature of the interface between the semiconductor and metal. Thus, the electronic structures of nc-(Au, Ag)/ZnO-NRs and nc-Pt/ZnO-NRs composites have been obtained by using O K-, Au, Ag, and Pt L3-edge X-ray absorption near-edge structure (XANES), X-ray emission spectroscopy (XES), and scanning photoelectron microscopy (SPEM) experimental techniques. The present work provides a systematic analysis of how the electronic structures of nc-(Au, Ag, and Pt)/ZnO-NRs vary and an understanding of the roles played Received: October 21, 2010 Revised: December 21, 2010 Published: January 20, 2011 2650
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Figure 1. (a) XRD spectra of nc-(Au, Ag, and Pt)/ZnO-NRs and pure ZnO-NRs; (b), (c), and (d) SEI of Au-3-10, Ag-3-30, and Pt-30-30 samples, respectively; (e) HR-TEM image of the Au-3-10 sample.
by nc-(Au, Ag, and Pt) particles, which affect photocatalytic activities on the surface of ZnO-NRs.
2. EXPERIMENTAL SECTION O K-, Au, Ag, and Pt L3-edge XANES and SPEM measurements were performed at the National Synchrotron Radiation Research Center in Hsinchu, Taiwan. XES and corresponding O K-edge XANES measurements were carried out at beamline7.0.1 at the Advanced Light Source, Lawrence Berkeley National Laboratory. The energy resolutions of XES and XANES measurements were ∼0.35 and 0.1 eV, respectively. The Au-3-10/ Au-5-30 and Ag-3-30/Ag-5-30 samples were obtained by submerging ZnO-NRs in 1 10-3 M/1 10-5 M HAuCl4/ ethanol and AgNO3/ethanol solutions for 10/30 and 30/30 min, respectively, and were irradiated under UV with a wavelength of 365 nm. Since ZnO-NRs can be significantly eroded in H2PtCl6/ ethanol solution under 365 nm irradiation, the nc-Pt particles were grown on the vertically aligned ZnO-NRs using an rf sputtering system with a Pt target. Pt-10-120 and Pt-30-30 samples were grown with currents of 10 mA (with a 120 s sputtering time) and 30 mA (with a 30 s sputtering time), respectively. The sizes of nc-Au-5-30 and Ag-5-30 particles were ∼5 nm and the sizes of nc-Au-3-10 and Ag-3-30 particles were ∼30 nm. The sizes of nc-Pt-10-120 and Pt-30-30 particles were estimated to be 3-4 nm. The sizes of nc-(Au, Ag, and Pt) particles were determined using high-resolution transmission electron microscopy (HR-TEM).
3. RESULTS AND DISCUSSION Figure 1(a) presents X-ray diffraction (XRD) spectra of nc-(Au, Ag, and Pt)/ZnO-NRs and pure ZnO-NRs. Apparently, the XRD spectrum of the Au-3-10 sample has a strong nc-Au characteristic feature at ∼38, corresponding to the (111) orientation. The corresponding feature of the Ag-3-30 sample is relatively weak. It was undetectable for other nc-(Au, Ag) and nc-Pt samples, because the nc-(Au, Ag) particles were grown in very dilute HAuCl4/ethanol and AgNO3/ethanol solutions and the nc-Pt particles were grown by a short period of rf sputtering time. Figure 1(b)-(d) displays the secondary electron images (SEI) of Au-3-10, Ag-3-30, and Pt-30-30 samples. These images reveal that nc-metal particles were well-covered over the surfaces of ZnO-NRs. Figure 1(e) presents the HR-TEM image of the Au-3-10 sample, which shows images of nc-Au particles on the surface of ZnO-NRs. Details of the preparations and photocatalytic behaviors of these samples can be found elsewhere.3 The electronic structure and photocatalytic activity of a transition metal nanoparticle are expected to depend on its size, especially when its size is small enough, so that the quantum confinement effect becomes significant. There has been evidence of the quantum size effect for a size of 5 nm for transition metal nanoparticles. Thus, to be rigorous, Pt, Au, and Ag nanoparticles with the same or comparable sizes should be considered in order to compare their electronic structures and photocatalytic activities. However, this was a difficult task due to the differing preparation techniques and growth characteristics of these nanoparticles. With this limitation, 2651
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Figure 2. O K-edge XANES spectra of nc-(Au, Ag, and Pt)/ZnO-NRs and pure ZnO-NRs. The lower inset displays the difference between the O K-edge XANES spectra of nc-(Au, Ag, and Pt)/ZnO-NRs and pure ZnO-NRs. The upper inset displays Au, Ag, and Pt L3-edge XANES spectra of nc-(Au, Ag, and Pt)/ZnO-NRs and corresponding Au, Ag, and Pt foils. Zero energy refers to Au, Ag, and Pt L3-edge are 11919, 3351, and 11564 eV, respectively.
the electronic structures and photocatalytic activities reported and discussed in this work pertain to the particular nanoparticle sizes described above. Figure 2 displays normalized O K-edge XANES spectra of nc-(Au, Ag, and Pt)/ZnO-NRs and pure ZnO-NRs. Features in the energy range 530-546 eV (A1-E1) have been attributed to electron transitions from O 1s to O 2pσ (along the bilayer) and O 2pπ (along the c-axis) states.6-9 The intensities of the features of all nc-(Au, Ag, and Pt)/ZnO-NRs are lower than those of pure ZnO-NRs, and they decrease overall as the size of the nc-(Au, Ag, and Pt) particles increases. After subtraction of the spectra of nc-(Au, Ag, and Pt)/ZnO-NRs by that of pure ZnO-NRs, the lower panel of Figure 2 shows that the nc-metal particles reduce O K-edge XANES intensity and the reduction of the intensity increases in the order Au f Ag f Pt. This result reveals that the occupation of the O 2p orbitals and the negative effective charge of the O ion in nc-Pt/ZnO-NRs are enhanced relative to that of nc-(Au, Ag)/ZnO-NRs and the enhancement increases in the order Au f Ag f Pt. For Ag, Au, and Pt binary oxides, one would expect that the occupancy of the oxygen 2p orbitals follows the trend of their ionicities or the electronegativities of Ag, Au, and Pt atoms. However, the present physical systems are ternary systems. There is a delicate competition between Zn and Ag, Au, or Pt cations. The occupancy of oxygen 2p orbitals will depend on the relative energies of Zn and noble metal valence sp orbitals, which hybridize with O 2p orbitals. It is well-known that the work function of a particular metal depends on the orientation of the surface or surface atomic arrangement and the size of the particle
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or the thickness of the film. Thus, the trend of the occupation of O 2p orbitals also does not follow that of the work functions of bulk Au, Ag, and Pt. The upper inset in Figure 2 presents normalized Au, Ag, and Pt L3-edge XANES spectra of nc-(Au, Ag, and Pt)/ ZnO-NRs and corresponding foils. In these spectra, the zero energy refers to the absorption edge of all spectra. According to the dipole-transition selection rules, these features correspond mainly to the Au and Pt (Ag) 2p3/2 to 6s/5d (5s/4d) transitions for pure Au and Pt (Ag) foils.10,11 Consistent with an earlier study,6 the intensities of the various features in the Au, Ag, and Pt L3-edge XANES spectra of nc-(Au, Ag, and Pt)/ZnO-NRs are larger than those of the corresponding Au, Ag, and Pt foils. Photocatalytic reduction of the metal ion has been exploited to improve the performance of nc-Au/ZnO-NRs composites under UV-irradiation.3 The various features in the O K-edge XANES spectra shown in Figure 2 consistently reveal that the amount of charge transfer between ZnO-NRs and nc-(Au, Ag, and Pt) particles depends strongly on not only the particle size but also the metal element. Although the electronic structure and phtotocatalytic activity are dependent on the size of the nanoparticle and Figure 2 shows that the intensities of XANES spectra of all three transition metals have some degrees of variation with respect to their sizes, the threshold positions of the XANES features differ insignificantly. Figure 3(a) displays XES and corresponding XANES spectra of the O 2p states of nc-(Au, Ag, and Pt)/ZnO-NRs. The maximum intensities of the features in the XES and XANES spectra were arbitrarily normalized to unity. The XES spectra obtained at excitation energy Eex = 570 eV show three similar distinct main features at ∼526 (A2), 523 (B2), and 520 eV (C2), which are attributable to O 2p-Zn 4sp and O 2p-Zn 3d hybridized states.6,9 A well-defined bandgap, Eg, indicated by the dotted lines is obtained by extrapolating the leading edges in the XANES and XES spectra to the baselines, corresponding to the conductionband maximum (ECBM)/conduction-band edge and valence-band maximum (EVBM)/ valence-band edge,9,12 respectively. The combined emission and absorption spectra yield an Eg of ∼3.3 eV for pure ZnO-NRs and nc-Au/ZnO-NRs. The Eg’s are 3.5 and 3.6 eV, respectively, for nc-Pt/ZnO-NRs and nc-Ag/ZnO-NRs composites. The details of EVBM, ECBM, and Eg values of nc-(Au, Ag, and Pt)/ZnO-NRs composites are tabulated in Table 1. Since the energy resolutions for XES and XANES measurements were ∼0.35 and 0.1 eV, respectively, the combined experimental energy resolution was ∼0.45 eV, which is larger than the energy differences between these Eg’s. Thus, strictly speaking, one can only say that Eg’s for pure ZnO-NRs, nc-Au/ZnO-NRs, nc-Pt/ZnONRs, and nc-Ag/ZnO-NRs are comparable. However, if XES and XANES measurements had systematic uncertainties or errors from the mean values, then one can speculate that the Eg increases slightly in the order of Au f Pt f Ag nanoparticles coated ZnO-NRs, which seems to correlate with the O-Au, O-Pt, and O-Ag Pauling’s electronegativity differences, or ionicities, of 0.90, 1.16, and 1.51, respectively.13,14 The correlation may be related to the screening effect, which is reduced for the increase of the ionic character. Since the Eg’s do not differ much, which also indicates that their Ef’s are essentially the same. Note that Ef of the metal/semiconductor system is usually pinned at the interface state, which usually lies at the middle of the bandgap of the semiconductor,15 so that Ef’s are essentially the same. On the other hand, enhancement of photocatalytic activity by noble metal nanoparticles depends not only on the couplings between O 2p states and metallic states but also the conductivity of near-Ef metal states, through which photoexcited 2652
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Figure 3. (a) XES and corresponding XANES spectra of O 2p states of nc-(Au, Ag, and Pt)/ZnO-NRs and pure ZnO-NRs. (b) Valence-band photoemission spectra obtained from selected positions p, q, r, and s shown in the upper inset, which presents Zn 3d SPEM cross-sectional images of pure ZnO-NRs and nc-(Au, Ag, and Pt)/ZnO-NRs. The lower inset shows the different valence-band spectra between nc-(Au, Ag, and Pt)/ZnO-NRs and ZnO-NRs.
Table 1. EVBM, ECBM, and Eg values of nc-(Au, Ag, and Pt)/ ZnO-NRs composites EVBM (eV)
ECBM (eV)
Eg = ECBM - EVBM (eV)
pure ZnO
527.5
530.8
3.3
Au-3-10
527.5
530.8
3.3
Au-5-30 Ag-3-30
527.5 527.5
530.8 531.1
3.3 3.6
Ag-5-30
527.5
531.1
3.6
Pt-10-120
527.5
531.0
3.5
Pt-30-30
527.5
531.0
3.5
electrons move away from the interface into the metal, so that they may be different for nc-(Au, Ag, and Pt)/ZnO-NRs. Previous theoretical calculations suggested that the widths of both O 2p and Zn 3d bands decrease as the concentration of Mg(x) in Zn1-xMgxO increases, suggesting Mg-induced enhancement of the localization of O 2p and Zn 3d states and the ionic character of Zn1-xMgxO.16 Figure 3(b) displays spatially resolved valence-band photoemission spectra of nc-(Au, Ag, and Pt)/ZnO-NRs and pure ZnO-NRs. The Zn 3d SPEM images in the insets show crosssectional views of ZnO-NRs, in which the bright areas have maximum Zn 3d intensities. The SPEM spectra show photoelectron yield from the selected areas in the sidewall regions of pure ZnO-NRs and nc-(Au, Ag, and Pt) particles on the surface of ZnO-NRs, indicated by p, q, r, and s, respectively, in the images. The energies of the valence-band photoemission spectra were calibrated using the Ef of clean gold. The zero energy is the threshold of the emission spectrum and is also referred to as Ef. The two prominent features at ∼4.6 eV (B3) and 7.5 eV (C3) in
the spectra are dominated by occupied O 2p states and O 2p and Zn 4sp hybridized states of ZnO-NRs.6,9 The figure reveals that the overall intensities of the two main features (B3 and C3) decrease from nc-(Au, Ag) to nc-Pt particles on the surface of ZnO-NRs. Additionally, the intensity of the shoulder (indicated by A3) in the 0-3.8 eV region near/at Ef is higher in nc-Pt/ZnONRs than those of nc-(Au, Ag)/ZnO-NRs. The intensity of A3 of nc-Pt/ZnO-NRs is significantly enhanced, providing clear evidence of the presence of a larger partial density of nc-Pt 5d states near/at Ef in nc-Pt/ZnO-NRs than those of nc-(Au 5d, Ag 4d) states in nc-(Au, Ag)/ZnO-NRs, suggesting that more electrons occupied the states near/at Ef in nc-Pt/ZnO-NRs than in nc-(Au, Ag)/ZnO-NRs. However, Ef’s of nc-(Au, Ag, and Pt)/ZnO-NRs are almost the same as shown at the bottom of Figure 3(b), which presents the difference between the valence-band spectra of nc-(Au, Ag, and Pt)/ZnO-NRs and pure ZnO-NRs. Previous optical studies based on Fermi level equilibration2,4,5 of transition metal nanoparticles in the solvent concluded that the shift of Ef toward the conduction-band edge of the (Au, Ag)/semiconductor composites relative to that of the Pt/semiconductor composite was due to accumulation of electrons at the interface associated with the lining up of Ef’s in both (Au, Ag) metal particles and semiconductors. The valence-band SPEM spectra shown in Figure 3(b) do not have a similar trend. The discrepancy might be due to the fact that SPEM measurements were performed for nanoparticles on the surfaces of ZnO nanorods, not in the solvent. The 4f states of Pt and Au are highly localized, so that their orbital energies can be used as an indicator of the on-site electrostatic potentials associated with the chemical or charge states, of these atoms. Thus, measurements of Pt and Au 4f spectra will be useful to gain information about the chemical or charge states of photoabsorbing Pt and Au atoms. Since the 2653
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the density of near-Ef Pt d states has a high feature, which is absent for nc-(Au, Ag) decorated ZnO-NRs. This high feature suggests that the near-Ef states of the Pt nanoparticle on the ZnO nanorod surface is dominated by highly localized d states with a large effective mass and low conductivity, which may explain why nc-Pt did not enhance the photocatalytic activity as nc-(Au, Ag) did.
Figure 4. Kinetics of MO photodegradation in the presence of nc-(Au, Ag, and Pt)/ZnO-NWs and ZnO-NWs. C0 and C represent initial MO concentration and evolution of MO concentration during photodegradation, respectively.
charge states of surface/interface and bulk atoms are expected to be different, the 4f spectra can be used to identify whether the photoabsorbing Pt and Au atoms are located on the surface/ interface or in the interior of the particle. But, these states are very deep below the Ef and are beyond the energy range of present SPEM measurements. However, progressive measurements of 4f states and data analysis are under consideration. The kinetics of methyl orange (MO) photodegradation show that the photocatalytic activities for MO degradation are enhanced in nc-(Au, Ag)/ZnO-NRs as shown in Figure 4. This figure shows kinetics of MO photodegradation of nc-(Au, Ag, and Pt)/ ZnO-NWs and pure ZnO-NWs under 365 nm irradiation to elucidate photocatalytic activities of nc-(Au, Ag, and Pt)/ZnONWs composites. In comparison with pure ZnO-NWs, photocatalytic activities for MO degradation are enhanced in nc-Au-5-30 and nc-Ag-5-30 samples. However, enhancement of the photocatalytic activity is not observed in nc-Au-3-10 and nc-Ag-3-30 samples and photocatalytic activities in nc-Pt-10-120 and nc-Pt-30-30 samples are significantly less active than those of pure ZnO-NWs and nc-(Au, Ag) samples. For MO photodegradation, photocatalytic activities shown in Figure 4 demonstrated that nc-(Au, Ag)/ZnO-NWs is overall more active than nc-Pt/ZnO-NWs, indicating that nc-Pt particles had weaker photocatalytic activities than nc-(Au, Ag) nanoparticles on the surface of ZnO-NWs. However, the photocatalytic-activity measurements for nc-(Au, Ag, Pt) decorated ZnO-NRs found that nc-(Au, Ag) transfers electrons more rapidly and better dissipates the accumulated charge than nc-Pt. Thus, the effect of noble metal nanoparticles on the photocatalytic activity follows the trend nc-(Au, Ag)> nc-Pt. It should be noted that the removal of MO is not only correlated directly to the electron transport on the surface decorated with nc-metal, but also depends on other factors, such as surface area, nanoparticle size, radiation source, etc.17-19 However, in the present work, the bottom panel of Figure 2 shows that the depletion of unoccupied O 2p states follows the trend Pt > Ag > Au. The alteration of the O 2p band by noble metal nanoparticles is associated with hybridization between orbitals of noble metal atoms and O atoms at the interface. The holes due to photo excitation of these hybridized states may not be mobile, which will attract photoexcited electrons and hinder them from moving into the noble metal nanoparticles. The bottom panel of Figure 3(b) also shows that
4. CONCLUSIONS In summary, lower photocatalytic activities were observed in nc-Pt/ZnO-NRs than in nc-(Au, Ag)/ZnO-NR composites. This trend correlates with a greater negative effective charge of the O ions in nc-Pt/ZnO-NRs than those of the O ions in nc-(Au, Ag)/ ZnO-NRs. The higher intensity of feature A3 in the SPEM spectra of nc-Pt/ZnO-NRs indicates that nc-Pt particles greatly enhance the density of occupied states near/at Ef relative to those of nc-(Au, Ag)/ZnO-NRs. The present results do not support the general argument that the accumulation of electrons at the interface, caused by the lining up of Ef’s in both (Au, Ag) nanoparticles and semiconductors, shifts the Ef toward the conduction-band edge in the (Au, Ag)/semiconductor composite relative to that of the Pt/semiconductor composite. ’ AUTHOR INFORMATION Corresponding Author
*E-mail: raysekhar@rediffmail.com (S.C.R.); wfpong@ mail.tku.edu.tw (W.F.P.).
’ ACKNOWLEDGMENT The authors (J.W.C. and W.F.P.) acknowledge the National Science Council of Taiwan for financial support under Contract Nos. NSC 97-2112-M390-002-MY2 and NSC96-2112-M032012-MY3. ’ REFERENCES (1) Lin, D.; Wu, H.; Qin, X.; Pan, W. Electrical Behavior of Electrospun Heterostructured Ag-ZnO Nanofibers. Appl. Phys. Lett. 2009, 95, 112104:1–112104:3. (2) Wood, A.; Giersig, M.; Mulvaney, P. Fermi Level Equilibration in Quantum Dot-Metal Nanojunctions. J. Phys. Chem. B 2001, 105, 8810– 8815. (3) Wu, J.-J.; Tseng, C.-H. Photocatalytic Properties of nc-Au/ZnO Nanorod Composites. Appl. Catal. B: Environ. 2006, 66, 51–57. (4) Subramanian, V.; Wolf, E. E.; Kamat, P. K.V. Green Emission to Probe Photoinduced Charging Events in ZnO-Au Nanoparticles. Charge Distribution and Fermi-Level Equilibration. J. Phys. Chem. B 2003, 107, 7479–7485. (5) Subramanian, V.; Wolf, E. E.; Kamat, P. K. V. Catalysis with TiO2/Gold Nanocomposites. Effect of Metal Particle Size on the Fermi Level Equilibration. J. Am. Chem. Soc. 2004, 126, 4943–4950. (6) Chiou, J. W.; Ray, S. C.; Tsai, H. M.; Pao, C. W.; Chien, F. Z.; Pong, W. F.; Tsai, M.-H.; Wu, J. J.; Tseng, C. H.; Chen, C.-H.; Lee, J. F.; Guo, J.-H. Charge Transfer in Nanocrystalline-Au/ZnO Nanorods Investigated by X-ray Spectroscopy and Scanning Photoelectron Microscopy. Appl. Phys. Lett. 2007, 90, 192112:1–192112:3. (7) Chiou, J. W.; Jan, J. C.; Tsai, H. M.; Pao, C. W.; Pong, W. F.; Tsai, M.-H.; Hong, I.-H.; Klauser, R.; Lee, J. F.; Wu, J. J.; Liu, S. C. Electronic Structure of ZnO Nanorods Studied by Angle-Dependent X-ray Absorption Spectroscopy and Scanning Photoelectron Microscopy. Appl. Phys. Lett. 2004, 84, 3462–3464. (8) Chiou, J. W.; Krishna Kumar, K. P.; Jan, J. C.; Tsai, H. M.; Bao, C. W.; Pong, W. F.; Chien, F. Z.; Tsai, M.-H.; Hong, I.-H.; Klauser, R.; 2654
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