Zhang Hengzhong, Rustad James R, Banfield Jillian F
Department of Earth and Planetary Science, University of California, Berkeley, 307 McCone Hall, Berkeley, California 94720, USA.
J Phys Chem A. 2007 Jun 14;111(23):5008-14. doi: 10.1021/jp0688916. Epub 2007 May 23.
We have investigated the bonding of water molecules to the surfaces of ZnS nanoparticles (approximately 2-3 nm sphalerite) using temperature-programmed desorption (TPD). The activation energy for water desorption was derived as a function of the surface coverage through kinetic modeling of the experimental TPD curves. The binding energy of water equals the activation energy of desorption if it is assumed that the activation energy for adsorption is nearly zero. Molecular dynamics (MD) simulations of water adsorption on 3 and 5 nm sphalerite nanoparticles provided insights into the adsorption process and water binding at the atomic level. Water binds with the ZnS nanoparticle surface mainly via formation of Zn-O bonds. As compared with bulk ZnS crystals, ZnS nanoparticles can adsorb more water molecules per unit surface area due to the greatly increased curvature, which increases the distance between adjacent adsorbed molecules. Results from both TPD and MD show that the water binding energy increases with decreasing the water surface coverage. We attribute the increase in binding energy with decreasing surface water coverage to the increasing degree of surface under-coordination as removal of water molecules proceeds. MD also suggests that the water binding energy increases with decreasing particle size due to the further distance and hence lower interaction between adsorbed water molecules on highly curved smaller particle surfaces. Results also show that the binding energy, and thus the strength of interaction of water, is highest in isolated nanoparticles, lower in nanoparticle aggregates, and lowest in bulk crystals. Given that water binding is driven by surface energy reduction, we attribute the decreased binding energy for aggregated as compared to isolated particles to the decrease in surface energy that occurs as the result of inter-particle interactions.
我们使用程序升温脱附（TPD）研究了水分子与硫化锌纳米颗粒（约2 - 3纳米的闪锌矿）表面的结合情况。通过对实验TPD曲线进行动力学建模，得出了水脱附的活化能与表面覆盖率的函数关系。如果假设吸附的活化能几乎为零，那么水的结合能就等于脱附的活化能。对3纳米和5纳米闪锌矿纳米颗粒上的水吸附进行分子动力学（MD）模拟，能从原子层面深入了解吸附过程和水的结合情况。水主要通过形成Zn - O键与硫化锌纳米颗粒表面结合。与块状硫化锌晶体相比，由于曲率大幅增加，硫化锌纳米颗粒每单位表面积能吸附更多水分子，这增加了相邻吸附分子之间的距离。TPD和MD的结果都表明，水的结合能随水表面覆盖率的降低而增加。我们将结合能随表面水覆盖率降低而增加归因于随着水分子的去除，表面欠配位程度增加。MD还表明，由于距离更远，因此在高度弯曲的较小颗粒表面上吸附的水分子之间的相互作用更低，水的结合能随颗粒尺寸减小而增加。结果还表明，结合能以及水的相互作用强度在孤立的纳米颗粒中最高，在纳米颗粒聚集体中较低，在块状晶体中最低。鉴于水的结合是由表面能降低驱动的，我们将聚集体与孤立颗粒相比结合能降低归因于颗粒间相互作用导致的表面能降低。
