University of Paderborn, Technical and Macromolecular Chemistry, Warburger Str. 100, 33098 Paderborn, Germany.
Phys Chem Chem Phys. 2011 Jul 28;13(28):12959-67. doi: 10.1039/c1cp20913b. Epub 2011 Jun 20.
The adsorption of single polyacrylic acid (PAAc) molecules was investigated on stepped hydroxide-stabilized polar ZnO(0001)-Zn surfaces using atomic force microscope (AFM) topography and force distance spectroscopy. Stepped surfaces of ZnO(0001)-Zn were prepared by a wet chemical etching procedure and PAAc molecules were adsorbed from aqueous NaClO(4) solutions. AFM single molecule topography studies could be utilized to show that polyacrylic acid molecules specifically adsorb on the non-polar (10-10) step edge faces at low ionic strengths. The radius of gyration of the dissolved PAAc in aqueous solution was measured by means of static light scattering experiments yielding a radius of gyration of R(g)=136 nm at pH 7.4 in 50 mM NaClO(4)/NaOH solution, which is in good agreement with the size of the adsorbed PAAc molecules as measured using AFM. The obtained results could be rationalized in terms of binding-site configurations at step edges and the effect of the chemical environment on both local electric double layer charge and molecular conformation of the PAAc molecules. The point of zero charge of the ZnO(10-10) surface was measured with chemical force microscopy to be pH(PZC)=10.2 ± 0.2. The specific adsorption of polyacrylic acid at non-polar ZnO step-edges can be explained by coordinative bonds formed between the carboxylic acid group and the Zn-surface atoms. On the hydroxide stabilized polar surface only weak hydrogen bonds can be formed in addition to van-der-Waals forces. Thus a "diffusion and trapping" mechanism keeps the adsorbed PAAc molecules mobile on the ZnO(0001)-Zn surface terraces due to small interaction forces until they are trapped at the (10-10) step faces by stronger coordinative bonds from the carboxylic groups to zinc atoms located in the first atomic layer of the crystal structure.
使用原子力显微镜（AFM）形貌和力距离谱研究了在稳定的氢氧化锌（0001）-Zn 极性 ZnO 表面上单聚丙烯酸（PAAc）分子的吸附。通过湿法化学蚀刻程序制备 ZnO（0001）-Zn 的阶梯表面，并从 NaClO4 水溶液中吸附 PAAc 分子。AFM 单分子形貌研究可用于表明，在低离子强度下，聚丙烯酸分子特异性地吸附在非极性（10-10）台阶边缘面上。通过静态光散射实验测量溶解在水溶液中的 PAAc 的旋转半径，在 pH 7.4 时，在 50 mM NaClO4/NaOH 溶液中，旋转半径 R（g）=136nm，这与使用 AFM 测量的吸附 PAAc 分子的大小非常吻合。所得结果可以根据台阶边缘的结合位点构型以及化学环境对局部双电层电荷和 PAAc 分子的分子构象的影响来合理化。用化学力显微镜测量 ZnO（10-10）表面的零电荷点，pH（PZC）=10.2±0.2。非极性 ZnO 台阶边缘上的聚丙烯酸的特异性吸附可以通过羧酸基团与 Zn 表面原子之间形成的配位键来解释。在稳定的极性表面上，除了范德华力之外，只能形成较弱的氢键。因此，由于相互作用力较小，“扩散和捕获”机制使吸附的 PAAc 分子在 ZnO（0001）-Zn 表面台地上保持移动状态，直到它们被位于晶体结构第一层中的羧酸基团与锌原子之间形成的更强配位键捕获在（10-10）台阶面上。
