Understanding Crystallization of Anatase into Binary SiO2/TiO2 Sol-Gel Optical Thin Films: An in Situ Thermal Ellipsometry Analysis
Understanding Crystallization of Anatase into Binary SiO2/TiO2 Sol-Gel Optical Thin Films: An in Situ Thermal Ellipsometry Analysis
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DOI：
10.1021/jp109653p
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发表时间：
2011-02-24
影响因子：
3.7
通讯作者：
Grosso, David
中科院分区：
文献类型：
--
作者：
Louis, Benjamin;Krins, Natacha;Grosso, David
以不同摩尔组成的TiCl4和正硅酸乙酯为原料，采用浸渍法制备了折射率在1.45~2.20之间可调的SiO_2/TiO_2干凝胶超薄保护膜。用原位热椭偏分析跟踪了这些二元氧化物体系在820℃以下热处理后光学性质的变化。关于Ti/Si原子比例和处理条件，所涉及的脱水、冷凝、粘性烧结、晶化和扩散烧结等现象已经被清楚地识别和研究。由于纳米二氧化钛的晶粒度对其半导体性能的影响，对其结晶动力学进行了专门的研究。我们发现，根据二氧化硅和二氧化钛的比例和升温速度，二氧化钛的结晶过程在300到600摄氏度之间发生。二氧化硅含量超过40%的薄膜在820℃时仍然是非晶态的，二氧化硅含量低往往与较低的晶化温度(T-c)和较大的晶体尺寸有关。最小的锐钛矿型纳米粒子的尺寸接近4 nm，而最大的锐钛矿型纳米粒子受薄膜厚度的限制在35 nm左右。有趣的是，我们观察到，较快的升温速度导致了纯二氧化钛的高T-c，而二元氧化物薄膜的T-c则相反，这是由于二氧化硅网络的逐渐凝聚导致原子扩散的减弱。电子显微镜研究表明，在二氧化硅存在的情况下，可以得到更光滑的表面拓扑结构和更均匀的薄膜结构。
SiO2/TiO2 xerogel ultrathin protective coatings, with tunable refractive index between 1.45 and 2.20, have been prepared by dip-coating sol-gel solutions of TiCl4 and tetraethyl orthosilicate with various molar compositions. In situ thermal ellipsometry analysis is used to follow the modifications of optical properties of these binary oxide systems upon thermal treatment up to 820 degrees C. Involved phenomena, such as dehydration, condensation, viscous sintering, crystallization, and diffusive sintering have been clearly identified and studied with respect to the Ti/Si atomic proportion and the treatment conditions. Due to the influence of the crystal size on the semiconducting properties of titania nanoparticles, its crystallization kinetic has been specifically investigated. We show that the crystallization of TiO2 is globally taking place between 300 and 600 degrees C depending on the silica to titania ratio and on the heating rate. Films containing more than 40% of silica remain amorphous at 820 degrees C. Poor silica contents tend to be associated to lower crystallization temperature (T-c) and larger crystal sizes. The size of the smallest detected anatase nanoparticles was close to 4 nm, while those of the largest were limited by the film thickness around 35 nm. Interestingly, we observed that, while faster heating rates induce high T-c for pure TiO2, the inverse tendency is observed for binary oxide films, which is explained by the diminution of the atomic diffusion due to the progressive condensation of the silica network. Electronic microscopy investigations revealed that smoother surface topologies and more homogeneously structured films are obtained in the presence of silica.