Charge transfer in nanocrystalline-Au/ ZnO nanorods investigated by x-ray
spectroscopy and scanning photoelectron microscopy
J. W. Chiou, S. C. Ray, H. M. Tsai, C. W. Pao, F. Z. Chien, and W. F. Ponga兲 Department of Physics, Tamkang University, Tamsui 251, Taiwan
M.-H. Tsai
Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan J. J. Wu and C. H. Tseng
Department of Chemical Engineering, National Cheng Kung University, Tainan 701, Taiwan C.-H. Chen and J. F. Lee
National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan J.-H. Guo
Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720 共Received 22 March 2007; accepted 17 April 2007; published online 9 May 2007兲
O K- and Zn and Au L3-edge x-ray absorption near-edge structure 共XANES兲, x-ray emission
spectroscopy共XES兲, and scanning photoelectron microscopy 共SPEM兲 are performed to investigate the electronic structure of ZnO nanorods with nanocrystalline 共nc兲-Au particles grown on the surfaces. The XANES spectra of nc-Au/ ZnO nanorods reveal the decrease of the number of both O 2p and Zn 4s / 3d unoccupied states with the increase of the nc-Au particle size. The number of Au 6s / 5d unoccupied states increases when the size of nc-Au particle decreases, indicating that the deposition of nc-Au particles on the surface of ZnO nanorods promotes charge transfer from the ZnO nanorods to nc-Au particles. Excitation energy dependent XES and SPEM spectra show that the number of electrons in the valence band of O 2p-Zn 4sp hybridized states decreases as the nc-Au particle size increases, revealing that more electrons are excited from the valence band to the conduction band of ZnO nanorods and the storage of electrons in nc-Au particles. © 2007 American Institute of Physics. 关DOI:10.1063/1.2738369兴
Among semiconductors, ZnO and TiO2 have been
rec-ognized as preferable photocatalysis materials because of their high photosensitivity, nontoxicity, large band gap, and chemical stability.1The photocatalytic activities were found to be enhanced when noble metals such as Au, Ag, Cu, and Pt nanoparticles were deposited on these semiconductors be-cause the metal nanoparticles store electrons within them.2,3 Wood et al. observed that the Fermi level共Ef兲 shifted toward conduction-band energy level in the metal-ZnO quantum dots, which increased the efficiency of photocatalytic reactions.3 In principle, the noble metal acts as a sink, im-proving interfacial charge transfer associated with the photo-induced electron-hole separation in the photocatalytic pro-cess. The photocatalytic properties of nanocrystalline 共nc兲-Au/ZnO nanorods for various UV irradiation periods have been studied by Wu and Tseng.4This work focuses on how the conduction and valence bands of ZnO nanorods are changed and the correlation between charge transfer and the size of the nc-Au particles grown on the surface of ZnO nanorods using x-ray absorption near-edge structure 共XANES兲, x-ray emission spectroscopy 共XES兲, and scanning photoelectron microscopy 共SPEM兲. The present study will further provide specific information of electronic states on the O and Zn sites and the role played by nc-Au particles on the charge separation in nc-Au/ ZnO nanorods, which has not been well understood.
Zn, Au L3-edge XANES, and SPEM were performed at the National Synchrotron Radiation Research Center in
Hsin-chu, Taiwan. XES and corresponding XANES measurements of the O K edge were carried out at beamline-7.0.1 at the Advanced Light Source, Lawrence Berkeley National Labo-ratory. The nc-Au particles grown on the vertically aligned ZnO nanorods were photosynthesized in various concentra-tions of HAuCl4/ethanol and irradiated with 365 nm UV. Three samples denoted by S-3-10, S-4-10, and S-5-30 corre-spond to the ZnO nanorods that were irradiated under UV in 1⫻10−3, 1⫻10−4, and 1⫻10−5M HAuCl
4/ethanol solutions
for 10, 10, and 30 min, respectively. The sizes of nc-Au par-ticles共dAu兲 are ⬃30, 15, and 5 nm, respectively, for samples
S-3-10, S-4-10, and S-5-30. Pure ZnO nanorods deposited on the Si substrate were also used for comparison.
Figure1presents the x-ray diffraction共XRD兲 spectra of nc-Au/ ZnO nanorods with various sizes of nc-Au particles and pure ZnO nanorods. The XRD spectrum of sample S-3-10 has a strong nc-Au characteristic feature at ⬇38°. In contrast, this feature is very weak in the spectrum of sample S-4-10 and is absent in that of sample S-5-30. This is because nanocrystalline Au particles grown in very dilute HAuCl4/ethanol solutions were expected to be small and
di-lute and were less likely to be detected by XRD. The sec-ondary electron image共SEI兲 and corresponding backscatter-ing electron images 共BEI兲 of sample S-4-10 shown in the insets共a兲 and 共b兲 of Fig.1, clearly reveal that nc-Au particles are distributed all over the surfaces of ZnO nanorods. The inset 共c兲 of Fig. 1 shows the high-resolution transmission electron microscopy 共HRTEM兲 image of sample S-5-30, which contains images of nc-Au particles on the surface of ZnO nanorods. Details of sample preparations and their pho-tocatalytic behaviors can be found elsewhere.4
a兲Author to whom correspondence should be addressed; electronic mail:
APPLIED PHYSICS LETTERS 90, 192112共2007兲
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Figure2displays normalized O K-edge XANES spectra of the various nc-Au/ ZnO nanorods and pure ZnO nanorods. Features in the energy range of 530– 545 eV were attributed to electron transitions from O 1s to 2p 共along the bilayer兲 and O 2p 共along the c axis兲 states.5–8 The intensities of these features for nc-Au/ ZnO nanorods are reduced relative to those of pure ZnO nanorods and the amount of reduction increases with the size of the nc-Au particles, which indi-cates that the number of O 2p unoccupied states of the nc-Au/ ZnO nanorods is less than that of pure ZnO nanorods and decreases with the increase of the size of the nc-Au particles. The lower inset in Fig. 2 displays normalized Zn L3-edge XANES spectra of nc-Au/ ZnO nanorods and pure
ZnO nanorods. The features between 1012 and 1027 eV are associated with the transition of Zn 2p electrons to 4s / 3d states.5,6 The intensities of these nc-Au/ ZnO nanorod fea-tures also decrease like those in the O K-edge XANES spec-tra as the nc-Au particle size increases and are smaller than those of pure ZnO nanorods. In contrast, as presented in the upper inset of Fig.2, the intensities of the various features in
the Au L3-edge XANES spectra of nc-Au/ ZnO nanorods are
larger than those of the reference Au foil and the intensities increase as the particle size of nc-Au decreases. In pure Au, according to the dipole-transition selection rules, these fea-tures correspond mainly to the Au 2p3/2 to 6s / 5d
transitions.9,10Although Au 5d orbitals are full for a free Au atom, Au 5d states can be observed at the edge because of s-p-d rehybridization.10 Note that the Au L3-edge XANES
spectrum could not be obtained for sample S-5-30 similar to the XRD spectrum because of small coverage. The results of the Au L3-edge XANES for nc-Au/ ZnO nanorods can be
interpreted as an increase of electron transfer for larger nc-Au particles. The reduction of the number of Au 6s / 5d unoccupied states indicates that the larger nc-Au particles gain more electrons from the ZnO nanorods. Hence, the charge separation capacity of the nc-Au particles depends strongly on their sizes, leading to the XANES results shown in Fig.2.
Figure 3共a兲 displays the O K␣ emission spectra of nc-Au/ ZnO nanorods and pure ZnO nanorods obtained at various excitation energies Ex, which are denoted by a1, a2,
a3, and a4 in Fig. 2 of the O K-edge XANES spectra, to
study the predominant contribution of specific admixture of occupied O 2p and Zn 3d / 4sp states. All XES spectra show three similar distinct main features, but with different inten-sities. The three features at ⬃526, 523, and 520 eV were attributable mainly to O 2p-Zn 4sp and O 2p-Zn 3d hybrid-ized states.7,8,11The feature with the largest intensity is cen-tered at⬃526 eV and shift by ⬃0.15 eV upward in spectra a1and a4relative to those in spectra a2and a3, which
indi-cate that oxygen atoms in ZnO nanorods are loindi-cated at slightly nonequivalent sites.12 The intensity of each feature decreases as the size of the nc-Au particles increases, sug-gesting that the number of electrons in the valence band of O 2p-derived states decreases as the size of the nc-Au particles increases. This trend implies that more electrons are trans-ferred from the valence band to the conduction band of the nc-Au/ ZnO nanorods. Consequently, the storage of electrons in larger nc-Au particles in S-3-10 exceeds those in S-4-10 and S-5-30 with smaller particles. This observation is con-sistent with the decrease in the intensities of the O K- and Zn, and Au L3-edge XANES spectral features as the size of
FIG. 1. 共Color online兲 XRD spectra of the nc-Au/ZnO nanorods and pure ZnO nanorods. The upper left insets are共a兲 SEI and 共b兲 BEI of sample S-4-10 and the upper right inset共c兲 is the HRTEM image of sample S-5-30 with the corresponding nc-Au particles shown.
FIG. 2. 共Color online兲 O K-edge XANES spectra of nc-Au/ZnO nanorods and pure ZnO nanorods. The upper inset displays Au L3-edge XANES spec-tra of nc-Au/ ZnO nanorods and the lower inset displays Zn L3-edge
XANES spectra of nc-Au/ ZnO nanorods and pure ZnO nanorods.
FIG. 3.共Color online兲 共a兲 Comparison of the XES spectra of nc-Au/ZnO nanorods with that of pure ZnO nanorods at selected excitation energies Ex;
共b兲 XES and corresponding XANES spectra of O 2p states of nc-Au/ZnO nanorods and pure ZnO nanorods.
192112-2 Chiou et al. Appl. Phys. Lett. 90, 192112共2007兲
the nc-Au particles increases. Figure3共b兲 presents XES and corresponding XANES spectra of O 2p states of nc-Au/ ZnO nanorods and pure ZnO nanorods. A well-defined band gap
between the valence-band maximum 共VBM兲 and
conduction-band minimum can be obtained for the various nc-Au/ ZnO nanorods. This band gap,⬃3.3 eV, is the same as that obtained by Dong et al. for nanostructured ZnO materials,7 suggesting that the band gap of nc-Au/ ZnO na-norods is independent of the size of the nc-Au particles, al-though the nc-Au particles in this nanocomposited system acts crucially as a sink.
Figure4displays spatially resolved valence-band photo-emission spectra of nc-Au/ ZnO nanorods and pure ZnO na-norods. The Zn 3d SPEM images in the insets show cross-sectional views of nanorods, in which the bright areas have maximum Zn 3d intensities. The SPEM spectra show photo-electron yield from the selected areas in the sidewall regions of pure ZnO nanorods and nc-Au/ ZnO nanorods indicated by S1– S4 in the images. The energy of valence-band
photo-emission spectra has been calibrated by the Ef of a clean gold metal. The zero energy refers to VBM, which is the threshold of the emission spectrum and is also referred to as Ef. The two prominent features at ⬃4.5 and 7.5 eV in the spectra are dominated by the occupied O 2p states and the O 2p and Zn 4sp hybridized states of ZnO nanorods.13 The figure reveals that the intensities of the two main features decrease as the size of the nc-Au particles increases, so as the number of electrons in the valence band of O 2p-Zn 4sp hybridized states, consistent with the XES result presented in Fig.3共a兲. In addition, a shoulder共indicated by an arrow兲 in the 0 – 4 eV region near/at Ef is present for larger nc-Au particles in the S-3-10 and S-4-10 samples. It is absent for the smaller nc-Au particles in the S-5-30 sample and pure ZnO nanorods. The electronic states close to Efare generally known to be dominated by the transition metal d band in transition metal compounds.14 So, the density of states 共DOS兲 near/at Ef can be attributed to Au 5d and O 2p hy-bridized states. The bottom of Fig. 4 shows the difference between the valence-band spectra of nc-Au/ ZnO nanorods and pure ZnO nanorods, which is attributable to the Au 5d and Zn 4sp DOSs of the nc-Au/ ZnO nanorods. The intensity
of the shoulder increases with the size of the nc-Au particles, as shown in the difference spectra of S-3-10 and S-4-10, providing evidence of the existence of Au 5d states near/at the Ef of the nc-Au/ ZnO nanorods. The increase of the DOSs of Au 5d states with the nc-Au particle size agrees with Au L3-edge XANES and XRD results stated previously. Figure4 demonstrates an important finding that the contact of nc-Au particles with ZnO nanorods promotes interfacial charge transfer and increases the DOSs of Au 5d states near/at the Ef, although the energy position of Ef in
nc-Au/ ZnO nanorods remains the same as that of pure ZnO nanorods.
In the previous study,4 the enhancement of the photo-catalytic activity for the degradation of methyl orange under 365 nm irradiation is achieved by loading nc-Au particles with sizes smaller than 15 nm and is more pronounced as the size of the nc-Au particles is reduced to 5 nm, whereas the photocatalytic activity of nc-Au/ ZnO nanorods is much lower than that of the ZnO nanorods when the size of the nc-Au particles is increased to 30 nm. In the present work, the XANES, XES, and SPEM results consistently show elec-tron transfer from ZnO nanorods to nc-Au particles and the storage of electrons in nc-Au particles, which increase with the size of the nc-Au particles. It suggests that the enhance-ment of the photocatalytic activity of the ZnO nanorods is ascribed to the enrichment of the photoinduced charge sepa-ration by loading nc-Au particles. However, the dependence of charge separation ability on the size of the nc-Au particles demonstrated in this work is not fully consistent with the photocatalytic activity of the nc-Au/ ZnO nanorods found by Wu and Tseng,4suggesting that other factors such as scatter-ing of the incident UV irradiation by the nc-Au particles may influence the photocatalytic activity of the nc-Au/ ZnO nano-rods as well.
This work was supported by the National Science Coun-cil of Taiwan under Contract No. NSC 95-2112-M032-014. The Advanced Light Source is supported by the U.S. Depart-ment of Energy under Contract No. DE-AC02-05CH11231.
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FIG. 4. 共Color online兲 Valence-band photoemission spectra obtained from selected positions S1– S4shown in the upper inset, which presents the Zn 3d
SPEM cross-sectional images of pure ZnO nanorods and nc-Au/ ZnO nano-rods. The lower inset shows the different valence-band spectra of nc-Au/ ZnO nanorods and pure ZnO nanorods.
192112-3 Chiou et al. Appl. Phys. Lett. 90, 192112共2007兲
