电化学沉积
Materials Science in Semiconductor Processing 10(2007)241–245
Synthesis of sulfur-doped ZnO nanowires
by electrochemical deposition
X.H. Wang, S. Liu Ã, P. Chang, Y. Tang
School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, PR China
Available online 9May 2008
Abstract
Sulfur-doped zinc oxide (ZnO)nanowires have been successfully synthesized by an electric field-assistedelectrochemical deposition in porous anodized aluminum oxide template at room temperature. X-ray diffraction and the selected area electron diffraction results show that the as-synthesized nanowires are single crystalline and have a highly preferential orientation. Transmission electron microscopy observations indicate that the nanowires are uniform with an average diameter of 70nm and length up to several tens of micrometers. X-ray photoelectron spectroscopy further reveals the presence of S in the ZnO nanowires. Room-temperature photoluminescence is observed in the doped ZnO nanowires, which exhibits a violet emission and blue emissions besides the typical photoluminescence spectrum of a single crystal ZnO. r 2008Elsevier Ltd. All rights reserved.
PACS:61.46.Hk; 78.67. Àn; 81.16.Be
Keyword:ZnO nanowires; Electrochemical deposition; Photoluminescence
1. Introduction
In the late 1990s, interest in the II–VIsemicon-ductor zinc oxide (ZnO)was renewed when room-temperature, optically pumped lasing was demon-strated for ZnO thin films[1–5]. Since that time the optical properties of ZnO bulk single crystal, thin films,and nanostructures have been studied exten-sively. In order to obtain different ZnO nanostruc-tured materials, varieties of methods have been developed. For example, by the high-temperature physical evaporation [6], the microemulsion hydro-thermal process [7], the template-induced method [8–10], or the reduction and oxidation of ZnS [11].
Corresponding author.
E-mail address:[email protected](S.Liu).
1369-8001/$-see front matter r 2008Elsevier Ltd. All rights reserved. doi:10.1016/j.mssp.2008.02.003
Although controlling the size of the nanowires can bring about changes in their optical and electronic properties, doping of the nanowires either through in-situ or postprocessing techniques will provide a far more favorable approach to modulate their properties. Efficientimpurity incorporation includ-ing Mg and transition metals in ZnO has been carried out in order to obtain novel properties and broaden possible applications [12–15]. However, little attention has been paid to S doping in ZnO thin filmsand nanostructures because of the difference between the low stability of sulfur and the high growth temperature of ZnO. Sulfur has a much smaller electronegativity (2.58)than that of
) oxygen (3.44),and the atom radius of sulfur (1.09A ). The is much larger than that of oxygen (0.65A
difference between S and O makes it possible to
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X.H. Wang et al. /Materials Science in Semiconductor Processing 10(2007)241–245
obtain some novel properties of ZnO via S doping. Yoo et al. [16]have reported that the band gap of ZnO could be tuned via S dopant. S impurities are also expected to increase the electrical conductivity of ZnO by supplying excess carriers [17]. In this article, we report the synthesis and luminescence of sulfur-doped ZnO nanowires. Herein, anodized aluminum oxide (AAO)template was used because AAO template synthesis (byelectrochemical deposi-tion) is a versatile and particularly simple approach, especially in the preparation of ionic doped compound nanowires. Generally, photolumines-cence (PL)spectrum of a single crystal ZnO consists mainly of two bands [18]. The as-synthesized nanowires exhibit broad visible emission. It has been suggested that sulfur substitutes for oxygen within the ZnO lattice, and the enhanced visible emission is due to the commensurate increase in oxygen vacancies [19–21]. This special characteristic suggests that the S-doped ZnO nanomaterial may be used for visible light emitter. 2. Experiment
The AAO templates with ordered nanopore were prepared by using a two-step anodization process [22,23].
ZnO:Snanowires were synthesized by an electric field-assistedelectrochemical deposition in porous AAO template at room temperature. The device used in the experiment was reported recently by the group, i.e., G. H. Yue et al. [24]. Na 2S aqueous solution prepared was 0.0125mol/L.The consis-tency Zn(NO3) 2aqueous solution was 0.00125mol/
L. The cathode connects the aqueous solution of Na 2S, and the anode connects the aqueous solution of Zn(NO3) 2. The transverse electric fieldwas added with a voltage of 12.5V AC and a frequency of 50Hz. In the experiment, the doped ZnO nanowires were obtained at a constant voltage of 20V DC for 20min. The current density was about 10mA/cm2at first.It decreased gradually to about 10À3mA/cm2after 20min, and then it nearly did not change. For a reference purpose, a sample of undoped ZnO nanowires was prepared under almost the same conditions but with 0.0125mol/LNaNO 3aqueous solution substituting for Na 2S.
Structural characterization was performed by means of X-ray diffraction (XRD)using a Riga-ku/Max-2400diffractometer with Cu K a radiation
). The bright-fieldimage of (wavelength1.54056A
nanowires and selected area electron diffraction pattern (SAED)were taken by an EM-400T transmission electron microscope (TEM).The X-ray photoelectron spectrum (XPS)data were collected on a PHI-5702X-ray photoelectron spectrometer using a monochromatic Al K a X-ray sources. PL spectrum excited with 325nm UV light at 295K were collected with an FLS920T combined fluorescencelifetime and a steady-state spectro-photometer.
3. Results and discussion
Fig. 1(a)and (b)show the TEM images of dispersed nanowires with an average diameter of 70nm obtained by electrochemical deposition in the anodic aluminum oxide templates. The dispersed
Fig. 1. TEM image of the ZnO:Snanowires:(a)and (b)images of the ZnO:Snanowires. The inset is the SAED pattern of as-synthesized products.
nanowires were obtained by dissolving AAO templates in 1mol/LH 2SO 4solution for 24h at 301C. The detailed information of the ZnO:Snanowires was studied by SAED. The SAED pattern shown in the inset of Fig. 1(b)suggests that the as-synthesized nanowires are single crystalline with a wurtzite structure.
Fig. 2illustrates the XRD patterns of doped and undoped ZnO nanowires, which revealed that the lower Bragg angle peaks are located at 36.091and 35.851, respectively. Except for ZnO (101) orienta-tions, no extra diffraction peaks from S-related secondary phases or impurities were observed. This indicates that both the as-grown products have single phase and (101) preferred orientation. It is
known that the ionic radii of the substitute S 2À(184pm) is larger than that of O 2À(124pm). Thus, doping with S ions causes a slight shift of about 0.241of XRD peaks toward the lower diffraction angle. The slight variation of the doped sample as compared to the undoped sample is an indication of incorporation of S ions in the ZnO lattice. The ultra strong intensities relative to the background signal indicate high purity of the resulting products.
Fig. 3(a)–(c)show Zn 2p , O 1s , and S2p XPS spectra of ZnO:Snanowires, respectively. The binding energies in all the XPS spectra have been calibrated using C 1s at 284.6eV as shown in Fig. 3(d).The binding energy of Zn 2p 3/2is located at 1022.2and 1021.8eV, showing that the Zn 2p 3/2peak consists of ZnO and ZnS, respectively [25,26]. Figs. 3(b)shows O 1s XPS spectrum, the stronger O 1s peak at 530.2eV may be attributed to O 2Àions in the Zn–Obond [27]. While another at 531.3eV is usually associated with the loosely bound oxygen (e.g.,adsorbed O 2, –OH)[28]. Fig. 3(c)shows the S 2p core level peak. The 2p 3/2component is located at about 162.0eV. This binding energy is smaller than those of sulfur and related compounds, namely, elemental sulfur (164.0eV), chemisorbed SO 2(163–165.5eV), sulfite(–SO3) (166.4eV), and sulfate (ÀSO 4) (168–170eV) [29]. This implies that the peak can be assigned to that of ZnS [30]. The atomic concentrations of the chemical elements were calculated from the peak areas and indicated 8.4atomic %S, 37.9atomic %O. The EDX spectrum was shown in Fig. 4to further reveal the presence of S in the ZnO nanowires. EDX analysis
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Fig. 4. Room-temperature photoluminescence spectrum for the ZnO:Snanowires.
confirmsthat these wire-like nanomaterials are indeed composed of Zn, S, and O. The S content of individual nanowire is identifiedto be in the range of 5–10%by EDX. These results indicate that pure ZnO:Snanocrystals formed.
The optical properties of the synthesized samples have been studied by using PL measurements that were performed at room temperature. From Fig. 4, we observed that the near-band-edge (NBE)ultra-violet (UV)peaks at 378nm (3.2eV) and 392nm (3.37eV). From Fig. 4we also observed the spectra of visible emissions, showing a violet emission at 456nm (2.74eV) and green emissions at 533nm (2.35eV) and 507nm (2.43eV). It is generally accepted that the visible emissions are attributed to the single ionized oxygen vacancy in the ZnO, and the emission results from the radiative recom-bination of a photogenerated hole with an electron occupying the oxygen vacancy, while the UV emission can be explained by the NBE emission of the wide band gap ZnO [31,32]. Blue emissions at around 406, 420, and 434nm were also observed in the PL spectrum for the as-synthesized ZnO:Snanowires. 4. Conclusion
To conclude, we have reported the synthesis of ZnO:Snanowires via electric field-assistedelectro-chemical approach in AAO template at room temperature. TEM, SAED, XRD, and XPS ana-lyses show that the ZnO:Snanowires are single crystals with a wurtzite structure. The as-grown products have an average diameter of 70nm. Broad-band luminescence in the region of 350–650nm is observed by PL spectra of the as-grown products. Visible emission in the sulfur doped ZnO nanowires can be attributed to the increase in oxygen vacancies caused by the sulfur substitutes for oxygen within the ZnO lattice. In the PL spectrum for the as-synthesized ZnO:Snanowires, blue emissions at around 406, 420, and 434nm were also observed. Acknowledgment
This work was supported by the Nature Science Foundation of Gansu Province (Grantno. 3ZS051-A25-034).
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