Surface and Nanoscience Division, Materials Science Group, IGCAR, HBNI, Kalpakkam, India.
Phys Chem Chem Phys. 2019 Mar 13;21(11):6198-6206. doi: 10.1039/c8cp06811a.
The present work focuses on synthesis and X-ray absorption studies of single phase oxygen deficient anatase TiO2 thin films. These films are prepared in a two-step method viz. the synthesis of near stoichiometric anatase TiO2 films using an open atmospheric spray pyrolysis method followed by vacuum annealing at their corresponding synthesis temperatures (Ts = 450 °C, 500 °C) for different time durations (t = 2, 4, 6, 8 hours). XRD and Raman studies of these films ascertained the formation and retention of the anatase phase post annealing, indicating that there was no phase change due to prolonged annealing. Extended X-ray absorption spectra (EXAFS) and X-ray absorption near edge spectra (XANES) revealed the presence of an oxygen vacancy and its effect on the local co-ordination. The co-ordination number obtained from EXAFS analysis revealed that the number density of the oxygen vacancy is higher in the case of thin films synthesized at 450 °C than in the case of the films synthesized at 500 °C. As the oxygen vacancy leads to changes in local co-ordination, which in turn have a profound effect on the pre-edge features of the X-ray absorption spectra (XAS), theoretically simulated XAS spectra of pure anatase TiO2 and oxygen deficient anatase TiO2 were generated using FEFF and were found to match with the experimentally observed spectra. In addition, the ambiguities in whether a change in the metal-oxygen bond length has any effect on the pre-edge features or not were delineated in the present study by comparing the pre-edge peak positions of the oxygen deficient TiO2 films. Our results matched with those of some of the researchers who have studied the rutile phase TiO2, wherein it was concluded that in the case of the titanium-oxygen system, the mean Ti-O bond length does have an effect on the pre-edge peak position. It was observed that as the Ti-O mean bond length increases, the pre-edge peak positions shift towards lower energy, which is in concurrence with the literature available for other Ti-O systems with similar geometry. The second pre-edge peak intensity, which is a measure of disorder, is higher for TiO2450 °C-2 h and TiO2500 °C-2 h thin films. This is attributed to the annealing effect, which suggests that TiO6 octahedrons are the most disordered for thin films annealed for 2 h and become more ordered upon annealing for longer times.
本工作重点研究单相缺氧锐钛矿 TiO2 薄膜的合成和 X 射线吸收研究。这些薄膜采用两步法制备，即使用开式大气喷雾热解法制备近化学计量比的锐钛矿 TiO2 薄膜，然后在相应的合成温度 (Ts = 450°C、500°C) 下进行真空退火，时间分别为 2、4、6、8 小时。这些薄膜的 XRD 和拉曼研究确定了退火后锐钛矿相的形成和保留，表明由于长时间退火没有发生相转变。扩展 X 射线吸收谱 (EXAFS) 和 X 射线吸收近边谱 (XANES) 表明存在氧空位及其对局部配位的影响。从 EXAFS 分析得到的配位数表明，在 450°C 下合成的薄膜中氧空位的数密度高于在 500°C 下合成的薄膜。由于氧空位导致局部配位的变化，进而对 X 射线吸收谱 (XAS) 的预边特征产生深远影响，因此使用 FEFF 生成了纯锐钛矿 TiO2 和缺氧锐钛矿 TiO2 的理论模拟 XAS 谱，并与实验观察到的谱进行了比较。此外，通过比较缺氧 TiO2 薄膜的预边峰位置，本研究还阐明了金属-氧键长的变化是否对预边特征有影响的不确定性。我们的结果与研究金红石相 TiO2 的一些研究人员的结果相匹配，他们得出的结论是，在钛-氧系统中，平均 Ti-O 键长确实对预边峰位置有影响。观察到随着 Ti-O 平均键长的增加，预边峰位置向更低的能量移动，这与其他具有相似几何形状的 Ti-O 系统的文献一致。第二个预边峰强度是无序的度量，对于 TiO2450°C-2 h 和 TiO2500°C-2 h 薄膜更高。这归因于退火效应，表明 TiO6 八面体在 2 h 退火的薄膜中最无序，在更长时间退火后变得更加有序。
