Raut U, Famá M, Teolis B D, Baragiola R A
Laboratory of Atomic and Surface Physics, University of Virginia, Charlottesville, VA 22904, USA.
J Chem Phys. 2007 Nov 28;127(20):204713. doi: 10.1063/1.2796166.
We have characterized the porosity of vapor-deposited amorphous solid water (ice) films deposited at 30-40 K using several complementary techniques such as quartz crystal microgravimetry, UV-visible interferometry, and infrared reflectance spectrometry in tandem with methane adsorption. The results, inferred from the gas adsorption isotherms, reveal the existence of microporosity in all vapor-deposited films condensed from both diffuse and collimated water vapor sources. Films deposited from a diffuse source show a step in the isotherms and much less adsorption at low pressures than films deposited from a collimated source with the difference increasing with film thickness. Ice films deposited from a collimated vapor source at 77 degrees incidence are mesoporous, in addition to having micropores. Remarkably, mesoporosity is retained upon warming to temperatures as high as 140 K where the ice crystallized. The binding energy distribution for methane adsorption in the micropores of ice films deposited from a collimated source peaks at approximately 0.083 eV for deposition at normal incidence and at approximately 0.077 eV for deposition at >45 degrees incidence. For microporous ice, the intensity of the infrared bands due to methane molecules on dangling OH bonds on pore surfaces increases linearly with methane uptake, up to saturation adsorption. This shows that the multilayer condensation of methane does not occur inside the micropores. Rather, filling of the core volume results from coating the pore walls with the first layer of methane, indicating pore widths below a few molecular diameters. For ice deposited at 77 degrees incidence, the increase in intensity of the dangling bond absorptions modified by methane adsorption departs from linearity at large uptakes.
我们使用了几种互补技术，如石英晶体微重力测量法、紫外可见干涉测量法和红外反射光谱法，并结合甲烷吸附，对在30 - 40 K下气相沉积的非晶态固体水（冰）膜的孔隙率进行了表征。从气体吸附等温线推断出的结果表明，在所有由扩散和准直水蒸气源冷凝而成的气相沉积膜中都存在微孔。从扩散源沉积的膜在等温线上显示出一个台阶，并且在低压下的吸附量比从准直源沉积的膜少得多，这种差异随着膜厚度的增加而增大。以77度入射角从准直蒸气源沉积的冰膜除了具有微孔外，还具有介孔。值得注意的是，在升温至高达140 K冰结晶的温度时，介孔仍然保留。对于从准直源沉积的冰膜微孔中甲烷吸附的结合能分布，垂直入射沉积时在约0.083 eV处出现峰值，入射角大于45度沉积时在约0.077 eV处出现峰值。对于微孔冰，由于孔表面悬挂的OH键上的甲烷分子导致的红外波段强度随甲烷吸收量线性增加，直至饱和吸附。这表明甲烷在微孔内不会发生多层凝聚。相反，核心体积的填充是由于用第一层甲烷覆盖孔壁，这表明孔宽度小于几个分子直径。对于以77度入射角沉积的冰，在大量吸收时，由甲烷吸附改性的悬挂键吸收强度的增加偏离线性。
