Electrostatic surface charge at aqueous/α-Al2O3 single-crystal interfaces as probed by optical second-harmonic generation
Electrostatic surface charge at aqueous/α-Al2O3 single-crystal interfaces as probed by optical second-harmonic generation
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DOI：
10.1021/jp040297d
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发表时间：
2005-04-28
影响因子：
3.3
通讯作者：
Eisenthal, KB
中科院分区：
文献类型：
--
作者：
Fitts, JP;Shang, XM;Eisenthal, KB
利用二次谐波产生（SHG）光谱表征了刚玉（α - al2o3）单晶衬底（0001）和（1102）晶体表面的ph依赖性静电充电行为。每个表面的零电荷点的pH值（pH(pzc)）是通过监测连续三次pH滴定对1、10和100 mM无碳酸盐的NaNO3水溶液的SH响应来确定的。三个滴定曲线的交叉点对应于pHpzc，（0001）表面的pH值为4.1 +/- 0.4,（1 (1)/ bar 02）表面的pH值为5.2 +/- 0.4。在固定pH值下记录的SHG测量值作为NaNO3浓度的函数被发现证实了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，表面水化引起的表面弛豫和成键变化不能解释实验观测和模型预测之间的差异。α - al2o3单晶表面的pH值（pzc）明显比公布的氧化铝颗粒的pH值（通常在pH 8到10之间）更酸。这种差异表明，假设表面酸度的差异反映了配位环境和决定电位的表面基团的局部结构的差异，则铝（氢）氧化物颗粒的充电行为是由与缺陷相关的表面位置决定的。
Second harmonic generation (SHG) spectroscopy was used to characterize the pH-dependent electrostatic charging behavior of (0001) and (1102) 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 pHpzc, occurs at pH 4.1 +/- 0.4 for the (0001) surface and pH 5.2 +/- 0.4 for the (1 (1) over bar 02) 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.