Observation of Optical Band-Gap Narrowing and Enhanced Magnetic Moment in Co-Doped Sol-Gel-Derived Anatase TiO2 Nanocrystals
Observation of Optical Band-Gap Narrowing and Enhanced Magnetic Moment in Co-Doped Sol-Gel-Derived Anatase TiO2 Nanocrystals
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DOI：
10.1021/acs.jpcc.8b06646
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发表时间：
2018-11-22
影响因子：
3.7
通讯作者：
Vasundhara, M.
中科院分区：
文献类型：
--
作者：
Akshay, V. R.;Arun, B.;Vasundhara, M.
由于氧化物半导体中存在缺陷的明确性质，二氧化钛和掺杂二氧化钛纳米晶体的磁性行为一直是一个挑战。本研究开发了一种简单、低温的溶胶-凝胶法制备低维、高效、稳定的锐钛矿型TiO2纳米晶。粉末x射线衍射图和拉曼光谱证实TiO2形成了单相锐钛矿结构。高分辨率透射电子显微镜研究揭示了溶胶凝胶衍生纳米晶体的结晶性质。共掺杂TiO2纳米晶的微拉曼光谱中晶格参数的增加以及最强烈的E-g(1)模式的移位和展宽表明Co在TiO2中掺入。在紫外可见光谱中，吸收边向可见区移动表明Co掺入TiO2使带隙缩小。x射线光电子能谱证实了共掺杂TiO2样品中Co2+和Co3+的存在。在室温下，空气退火3%共掺杂TiO2的氧空位缺陷导致了束缚极化子的形成，导致了弱铁磁行为。观察到，无论掺杂离子是磁性的还是非磁性的，仅结合的磁性极化子的重叠就能诱导铁磁性，而磁性杂质在Co浓度较高时产生增强的顺磁矩。通过估计束缚磁极化子的浓度，详细地了解了这些磁性的变化，这与光致发光的研究结果是一致的。在共掺杂TiO2纳米结构中观察到的带隙缩小以及与磁性杂质浓度相关的磁相互作用机制从应用的角度来看是有利的，特别是在自旋电子和磁光器件领域。
The magnetic behavior of TiO2 and doped TiO2 nanocrystals has been a challenge due to the unambiguous nature of defects present in oxide semiconductors. Here, a simple, low-temperature sol gel method is developed for the synthesis of low-dimensional and highly efficient stable anatase TiO2 nanocrystals. The X-ray powder diffraction pattern and Raman spectra confirm the formation of a single-phase anatase structure of TiO2. High-resolution transmission electron microscopy studies reveal the crystalline nature of the sol gel-derived nanocrystals. The increase in lattice parameters together with the shifting and broadening of the most intense E-g(1) mode in micro-Raman spectra of Co-doped TiO2 nanocrystals indicate the incorporation of Co in TiO2. Shifting of the absorption edge to the visible region in UV-visible spectra indicates narrowing of the band gap due to Co incorporation in TiO2. X-ray photoelectron spectra confirm the presence of Co2+ and Co3+ in Co-doped TiO2 samples. Oxygen vacancy defects lead to the formation of bound magnetic polarons which induces a weak ferromagnetic behavior in air-annealed 3% Co-doped TiO2 at room temperature. It is observed that irrespective of the dopant ion, whether magnetic or nonmagnetic, the overlapping of bound magnetic polarons alone can induce ferromagnetism, while the magnetic impurities give rise to an enhanced paramagnetic moment for higher Co concentrations. A detailed understanding on the variation of these magnetic properties by estimating the concentration of bound magnetic polarons is presented, which is in corroboration with the photoluminescence studies. The observed band-gap narrowing in Co-doped TiO2 nanostructures and the mechanism underlying the magnetic interactions associated with the magnetic impurity concentration are advantageous from an applied perspective, especially in the field of spintronic and magneto-optic devices.