Fitts Jeffrey P, Shang Xiaoming, Flynn George W, Heinz Tony F, Eisenthal Kenneth B
Department of Chemistry, Columbia University, New York, New York 10027, USA.
J Phys Chem B. 2005 Apr 28;109(16):7981-6. doi: 10.1021/jp040297d.
Second harmonic generation (SHG) spectroscopy was used to characterize the pH-dependent electrostatic charging behavior of (0001) and (102) crystallographic surfaces of corundum (alpha-Al2O3) single-crystal substrates. The pH value of the point of zero charge (pH(pzc)) for each surface was determined by monitoring the SH response during three consecutive pH titrations conducted with 1, 10, and 100 mM NaNO3 carbonate-free aqueous solutions. The crossing point of the three titration curves, which corresponds to the pH(pzc), occurs at pH 4.1 +/- 0.4 for the (0001) surface and pH 5.2 +/- 0.4 for the (102) surface. SHG measurements that were recorded as a function of NaNO3 concentration at fixed pH values were found to corroborate the pH(pzc) values identified in the pH titrations. A comparison of the SHG results with surface protonation constants calculated using a simple electrostatic model suggests that surface relaxation and bonding changes resulting from surface hydration do not account for differences between experimental observations and model predictions. The measured pH(pzc) values for the alpha-Al2O3 single-crystal surfaces are significantly more acidic than published values for Al-(hydr)oxide particles which typically range from pH 8 to 10. This discrepancy suggests that the charging behavior of Al-(hydr)oxide particles is determined by surface sites associated with defects assuming that differences in surface acidity reflect differences in the coordination environment and local structure of the potential-determining surface groups.
二次谐波产生（SHG）光谱用于表征刚玉（α-Al2O3）单晶衬底（0001）和（102）晶面的pH依赖性静电充电行为。通过监测在使用1 mM、10 mM和100 mM无碳酸盐硝酸钠水溶液进行的三次连续pH滴定过程中的SH响应，确定每个表面的零电荷点pH值（pH(pzc)）。三条滴定曲线的交点对应于pH(pzc)，（0001）表面出现在pH 4.1±0.4，（102）表面出现在pH 5.2±0.4。在固定pH值下记录的作为硝酸钠浓度函数的SHG测量结果证实了pH滴定中确定的pH(pzc)值。将SHG结果与使用简单静电模型计算的表面质子化常数进行比较表明，表面水合作用导致的表面弛豫和键合变化不能解释实验观察结果与模型预测之间的差异。α-Al2O3单晶表面测量的pH(pzc)值比通常在pH 8至10范围内的铝（氢）氧化物颗粒的已发表值明显更酸性。这种差异表明，假设表面酸度的差异反映了潜在决定表面基团的配位环境和局部结构的差异，铝（氢）氧化物颗粒的充电行为由与缺陷相关的表面位点决定。
