Chemical and Engineering Materials Division, Neutron Scattering Directorate, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA.
J Phys Chem B. 2012 Jun 21;116(24):7291-5. doi: 10.1021/jp302155a. Epub 2012 Jun 8.
We used quasielastic neutron scattering (QENS) to study the dynamics of phenanthrenequinone (PQ) on the surface of onion-like carbon (OLC), or so-called carbon onions, as a function of surface coverage and temperature. For both the high- and low-coverage samples, we observed two diffusion processes; a faster process and nearly an order of magnitude slower process. On the high-coverage surface, the slow diffusion process is of long-range translational character, whereas the fast diffusion process is spatially localized on the length scale of ∼4.7 Å. On the low-coverage surface, both diffusion processes are spatially localized; on the same length scale of ∼4.7 Å for the fast diffusion and a somewhat larger length scale for the slow diffusion. Arrhenius temperature dependence is observed except for the long-range diffusion on the high-coverage surface. We attribute the fast diffusion process to the generic localized in-cage dynamics of PQ molecules, and the slow diffusion process to the long-range translational dynamics of PQ molecules, which, depending on the coverage, may be either spatially restricted or long-range. On the low-coverage surface, uniform surface coverage is not attained, and the PQ molecules experience the effect of spatial constraints on their long-range translational dynamics. Unexpectedly, the dynamics of PQ molecules on OLC as a function of temperature and surface coverage bears qualitative resemblance to the dynamics of water molecules on oxide surfaces, including practically temperature-independent residence times for the low-coverage surface. The dynamics features that we observed may be universal across different classes of surface adsorbates.
我们使用准弹性中子散射（QENS）研究了苯醌（PQ）在洋葱状碳（OLC）表面的动力学，即所谓的碳洋葱，作为表面覆盖率和温度的函数。对于高覆盖率和低覆盖率的样品，我们都观察到了两个扩散过程：一个较快的过程和近一个数量级慢的过程。在高覆盖率的表面上，慢扩散过程具有长程平移特征，而快扩散过程在约 4.7 Å 的空间尺度上是局部化的。在低覆盖率的表面上，两种扩散过程都是局部化的；对于快扩散过程，空间尺度约为 4.7 Å，对于慢扩散过程，空间尺度稍大。除了高覆盖率表面上的长程扩散外，还观察到了 Arrhenius 温度依赖性。我们将快扩散过程归因于 PQ 分子的通用局域笼内动力学，将慢扩散过程归因于 PQ 分子的长程平移动力学，这取决于覆盖率，可能是空间受限的或长程的。在低覆盖率的表面上，没有达到均匀的表面覆盖率，并且 PQ 分子经历了其长程平移动力学的空间限制的影响。出乎意料的是，PQ 分子在 OLC 上的动力学作为温度和表面覆盖率的函数，与氧化物表面上水分子的动力学具有定性的相似性，包括低覆盖率表面的实际温度独立停留时间。我们观察到的动力学特征可能在不同类别的表面吸附剂中具有普遍性。
