Kershner Ryan J, Bullard Joseph W, Cima Michael J
Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room 12-011, Cambridge, Massachusetts 02139, USA.
Langmuir. 2004 May 11;20(10):4101-8. doi: 10.1021/la036268w.
The zeta potential of planar sapphire substrates for three different crystallographic orientations was measured by a streaming potential technique in the presence of KCl and (CH3)4NCl electrolytes. The streaming potential was measured for large single crystalline C-plane (0001), A-plane (1120), and R-plane (1102) wafers over a full pH range at three or more ionic strengths ranging from 1 to 100 mM. The roughness of the epi-polished wafers was verified using atomic force microscopy to be on the order of atomic scale, and X-ray photoelectron spectroscopy (XPS) was used to ensure that the samples were free of silica and other contaminants. The results reveal a shift in the isoelectric point (iep) of the three samples by as much as two pH units, with the R-plane surface exhibiting the most acidic behavior and the C-plane samples having the highest iep. The iep at all ionic strengths was tightly centered around a single pH for each wafer. These values of iep are substantially different from the range of pH 8-10 consistently reported in the literature for alpha-Al2O3 particles. Particle zeta potential measurements were performed on a model powder using phase analysis light scattering, and the iep was confirmed to occur at pH 8. Modified Auger parameters (MAP) were calculated from XPS spectra of a monolayer of iridium metal deposited on the sapphire by electron beam deposition. A shift in MAP consistent with the observed differences in iep of the surfaces confirms the effect of surface structure on the transfer of charge between the Ir and sapphire, hence accounting for the changes in acidity as a function of crystallographic orientation.
采用流动电位技术，在KCl和(CH3)4NCl电解质存在的情况下，测量了三种不同晶体取向的平面蓝宝石衬底的zeta电位。在1至100 mM的三种或更多离子强度下，在整个pH范围内测量了大尺寸单晶C面（0001）、A面（1120）和R面（1102）晶片的流动电位。使用原子力显微镜验证了外延抛光晶片的粗糙度在原子尺度量级，并且使用X射线光电子能谱（XPS）确保样品不含二氧化硅和其他污染物。结果表明，三个样品的等电点（iep）偏移多达两个pH单位，其中R面表面表现出最酸性的行为，而C面样品具有最高的iep。对于每个晶片，在所有离子强度下的iep都紧密集中在一个单一的pH值附近。这些iep值与文献中一致报道的α-Al2O3颗粒的pH 8-10范围有很大不同。使用相分析光散射对模型粉末进行颗粒zeta电位测量，并确认iep发生在pH 8。通过电子束沉积在蓝宝石上沉积的铱金属单层的XPS光谱计算了修正的俄歇参数（MAP）。与观察到的表面iep差异一致的MAP偏移证实了表面结构对Ir和蓝宝石之间电荷转移的影响，从而解释了酸度随晶体取向的变化。
