Analysis of oxygen vacancy in Co-doped ZnO using the electron density distribution obtained using MEM
- Ji Hun Park†1,
- Yeong Ju Lee†1, 2,
- Jong-Seong Bae3,
- Bum-Su Kim1,
- Yong Chan Cho4,
- Chikako Moriyoshi5,
- Yoshihiro Kuroiwa5,
- Seunghun Lee6, 7Email author and
- Se-Young Jeong1Email author
© Park et al.; licensee Springer. 2015
Received: 16 December 2014
Accepted: 31 March 2015
Published: 18 April 2015
Oxygen vacancy (VO) strongly affects the properties of oxides. In this study, we used X-ray diffraction (XRD) to study changes in the VO concentration as a function of the Co-doping level of ZnO. Rietveld refinement yielded a different result from that determined via X-ray photoelectron spectroscopy (XPS), but additional maximum entropy method (MEM) analysis led it to compensate for the difference. VO tended to gradually decrease with increased Co doping, and ferromagnetic behavior was not observed regardless of the Co-doping concentration. MEM analysis demonstrated that reliable information related to the defects in the ZnO-based system can be obtained using X-ray diffraction alone.
Oxygen vacancy (VO), one of the representative native defects in oxides, has received much attention because of the important role played thereby in determining the physical properties of materials [1-3]. Various tools have been used for the qualitative and quantitative analysis of VO. These include photoluminescence , ultraviolet-visible (UV-vis) , Raman , and X-ray photoelectron spectroscopic (XPS) techniques . Quantitative analyses of VO have been successfully performed via Rutherford backscattering spectroscopy  or X-ray absorption spectroscopy using synchrotron radiation . Additionally, X-ray diffraction (XRD) is a simple and useful tool for analysis of VO because it reveals the crystal structure and the electron density distribution of periodic arrays of atoms .
Fitting of X-ray diffraction data using the Rietveld refinement has been attempted for the quantitative analyses of VO in terms of oxygen site occupancy [10,11]. However, additional corrections and the use of neutron or synchrotron X-rays were required [11-13]. Electron density profiling using the maximum entropy method (MEM) is also a suitable tool for analysis of VO because it uses the more precise Rietveld refinement that resolves summation-terminated errors and affords a better structural model [14,15]. Furthermore, MEM introduces negligible modeling errors via least-biased electronic reconstruction of X-ray diffraction patterns in real space .
We sought to confirm whether MEM analysis could be used for analysis of VO. In this study, we applied such analysis to VO that changed as a function of the Co-doping concentration in ZnO. Co-doped ZnO is a good candidate room-temperature magnetic semiconductor and has been intensively studied in terms of intrinsic ferromagnetism. However, the origin of such ferromagnetism remains controversial, and the presence thereof limits applications of the semiconductor. VO was regarded, until recently, to explain the ferromagnetism and was reported to be affected by Co-doping concentration . Herein, we analyzed the change in VO as a function of Co-doping concentration and compared the results with XPS data. A method of analysis of VO is proposed, using conventional XRD and MEM techniques.
ZnO and Co-doped ZnO (Zn1−xCoxO, x = 0.01, 0.05, 0.1) powder samples were fabricated by sol-gel methods [16,17]. Zinc acetate dihydrate (Sigma-Aldrich, St. Louis, MO, USA) and cobalt acetate tetrahydrate (Sigma-Aldrich, St. Louis, MO, USA), used as starting materials, were dissolved in 2-methoxyethanol (Sigma-Aldrich, St. Louis, MO, USA) and stabilized by monoethanolamine (Sigma-Aldrich, St. Louis, MO, USA). To exclude the possibility of external contamination, the dissolution and drying processes were performed under a pure argon gas (99.999% purity) atmosphere, and under vacuum, each for 10 h, respectively. The organic residuals in samples were completely removed via an intermediate heat treatment at 300°C and a subsequent final heat treatment at 800°C under vacuum for 10 h . The samples used in this study were characterized using XRD, and we found a high degree of crystallinity, which was comparable to that of commercially available high-quality powder samples (ZnO; CAS 1314-13-2, Sigma-Aldrich, St. Louis, MO, USA). We also characterized the samples using synchrotron radiation, and we found high sample quality. A characterization study using synchrotron radiation will be submitted to a specialized journal soon. XRD (Empyrean Series 2, PANalytical) experiments were performed to analyze the crystal structures and electron density distributions of the powder samples. The Rietveld and MEM analyses were performed using a published technique . The MEM calculation was performed using ENIGMA software  with 66 × 66 × 104 pixels. The electron density distribution was reconstructed using the VESTA visualization program . An X-ray photoelectron spectrometer (model: Theta Probe (Thermo Electron Co., Waltham, MA, USA), Korean Basic Science Institute, Busan Center) was used for atomic composition analysis. Magnetic-field-dependent magnetization was measured using a vibrating sample magnetometer (VSM) equipped with a physical property measurement system (PPMS; Model 6000, Quantum Design, San Diego, CA, USA).
Results and discussion
Reliability factors of ZnO and ZnCoO samples
R I (%)
R F (%)
Δg O (%)
An additional Rietveld refinement of the oxygen occupancies in each finalized Rietveld refinement was performed to examine the validity of this concept. Table 1 also lists the change in the oxygen occupancy [Δg O = (g O (ZnO) − g O (ZnCoO))/g O (ZnO)] for each sample, where g O refers to oxygen site occupancy. The oxygen occupancy increased for Zn0.99Co0.01O (i.e., negative Δg O was obtained) but decreased for Zn0.95Co0.05O and Zn0.9Co0.1O. Also, the oxygen vacancy was greater for Zn0.95Co0.05O than for Zn0.9Co0.1O. MEM analysis was used to further examine this trend.
Figure 3c shows the electron density line profiles along the direction of the O-Zn bond. These profiles enable precise analysis of oxygen occupancy as a function of the Co content. The lines were normalized to the electron density at the Zn(Co) atomic position to allow comparison of VO with Zn occupancy. The electron density at the O atomic position increased in the order Zn0.99Co0.01O < ZnO < Zn0.95Co0.05O < Zn0.9Co0.1O, in agreement with the Rietveld refinement results. The sample with 1% Co doping exhibited significantly lower oxygen electron density, which did not agree with the Rietveld refinement data.
The observed trend, that creation of VO was suppressed with increasing Co-doping level, is attributable to differences in the Zn-O and Co-O bond strengths; the O2− ions in the wurtzite ZnO structure are tetrahedrally coordinated and thereby form four Zn-O bonds . Doping of Co2+ ions into ZnO creates Co-O bonds, the diatomic bond dissociation energy of which is higher than that of the Zn-O bond by 84 kJ/mol (Zn-O: 284 kJ/mol, Co-O: 368 kJ/mol) . This indicates that the Co-O bonds created by Co doping enhanced the average bond strength between oxygen ions and neighboring cations, i.e., doping decreased the possibility of oxygen-cation bond dissociation during sample fabrication or post-treatment processing . The experimental results indicate that VO decreased at high-level Co doping (i.e., above 5 mol%). The supporting analyses suggest that Co doping can impede creation of VO. However, the abrupt increase of VO at 1 mol% of Co doping is not well-understood and warrants additional study.
The Rietveld refinement results of the X-ray diffraction patterns of the ZnCoO system indicated that increased Co doping of ZnO tended to decrease the VO, but the VO increased slightly upon 1 mol% of Co doping. The MEM results were in better agreement with the XPS data, which indicated that MEM analysis could be a reliable tool for the study of VO. Additional research is needed to explain the anomalous behavior at 1 mol% of Co doping. More advanced X-ray electron density studies using synchrotron radiation would provide more precise and reliable data, but nevertheless, our present work shows that MEM is a reliable technique for the analysis of defects in materials characterized by XRD, which is a readily accessible tool in the material scientist laboratory. This approach will be of particular value in early-stage studies of oxide systems.
This work was supported for 2 years by a Pusan National University Research Grant.
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