Adsorption and Thin-Film Adhesion on Single-Crystalline Surfaces: Enthalpies, Entropies, and Kinetic Prefactors for Surface Reactions

Abstract

Chemical bonding at solid surfaces and interfaces is influential in a wide range of important technological applications including catalysis, fuel cells, batteries, chemical sensors, and device fabrication for microelectronics, computers, solar cells, and all variety of coatings. Adsorption and adhesion energetics are key elements in understanding interfacial properties, and these properties can be used to develop functional industrial materials. First, the properties of single-crystalline oxide surfaces are reviewed in detail, particularly in regards to the adsorption energetics of these surfaces. This includes the largest collection of experimental adsorption data on single-crystalline oxide surfaces ever presented, from which trends in the thermodynamic properties of adsorbates are revealed which greatly expand our understanding of the physical processes occurring on these surfaces. Among these trends is the discovery that the entropy of adsorbed molecules tracks their gas-phase entropy, retaining ~2/3 of that entropy upon adsorption. This allows for a method of predicting not only entropies of adsorption, but also the kinetic prefactors associated with many classes of elementary surface reactions. These estimations of desorption prefactors are then used to improve calculations of adsorption energies from temperature programmed desorption (TPD) measurements for many systems. Metal adsorption on oxide surfaces and the strength of the binding at metal / oxide interfaces are then discussed. The motivation here is to understand oxide-supported transition metal nanoparticles such as those used in industrial heterogeneous catalysis. For metal atom adsorption, adsorption energetics and adhesion energies are directly related to the energy of the adsorbed atoms, which define their stability, sintering rates, and reactivity, and which are found to vary with both the size of the nanoparticle and the nature of the oxide support. The experimental techniques necessary for obtaining these values, as well as the data analysis involved, is explained, and in several cases improved upon. In particular, a new single crystal adsorption calorimeter capable of making the first direct measurements of adsorption energies for metals with high bulk cohesive energies has recently been completed. These studies greatly expand upon the understanding of and ability to measure the thermodynamic properties associated with adsorption on single-crystalline surfaces.