Usami Akira, Ozaki Hajime
Central Research Institute of Electric Power Industry, 2-11-1 Iwatokita, Komae-shi, Tokyo 201-8511, Japan.
J Phys Chem B. 2005 Feb 24;109(7):2591-6. doi: 10.1021/jp040178y.
The light transmittance, T, in nanocrystalline TiO2 films was studied as a function of the light wavelength, lambda, the nanocrystal radius, a, and the film thickness, d. Two types of TiO2 nanoparticles were employed: a commercial powder (P25) and synthesized particles from titanium isopropoxide (SP). The X-ray diffraction measurements revealed that both P25 and SP are mainly anatase and the average crystal sizes, 2a, of P25 and SP are 50.3 and 23.7 nm, respectively. Despite the visual difference between micron-order thin films of P25 and SP, the light hemispherical transmittance corrected with the surface specular reflectance has a clear dependence of ln(T) = -0.5beta lambda(-4)a(3)d, with beta = 1.5 x 10(3) from visible to near-infrared wavelengths. The dependence and beta value were successfully explained by the simplest model on the basis of the Rayleigh scattering theory. This indicates that the nanocrystalline TiO2 thin films are a typical medium where the simplest scattering model is a good approximation. However, the model was inapplicable to light scattering in relatively thick P25 films of 1.5-3.0 microm because of nonnegligible internal multiple scattering. For the moderate thickness films, ln(T) proportional to lambda(gamma), where gamma increases from -4 in proportion to the film thickness is an alternative approximation. With these light scattering models, the light absorption rate of the TiO2 crystal was successfully evaluated from experimental extinction rates.
研究了纳米晶TiO₂薄膜的透光率T与光波长λ、纳米晶体半径a和薄膜厚度d之间的函数关系。使用了两种类型的TiO₂纳米颗粒：一种是商业粉末（P25），另一种是由异丙醇钛合成的颗粒（SP）。X射线衍射测量表明，P25和SP主要都是锐钛矿型，P25和SP的平均晶体尺寸2a分别为50.3 nm和23.7 nm。尽管P25和SP的微米级薄膜在视觉上存在差异，但用表面镜面反射率校正后的光半球透过率具有明确的依赖关系ln(T) = -0.5βλ⁻⁴a³d，在可见光到近红外波长范围内，β = 1.5×10³。基于瑞利散射理论，用最简单的模型成功解释了这种依赖关系和β值。这表明纳米晶TiO₂薄膜是一种典型的介质，在其中最简单的散射模型是一个很好的近似。然而，由于不可忽略的内部多次散射，该模型不适用于1.5 - 3.0微米相对较厚的P25薄膜中的光散射。对于中等厚度的薄膜，ln(T) ∝ λ⁽γ⁾，其中γ从 -4开始随着薄膜厚度的增加而增加，这是一种替代近似。利用这些光散射模型，从实验消光率成功评估了TiO₂晶体的光吸收率。
