Electron Probe Bridging Solid-State Chemistry and Surface Chemistry: Example of the TiO₂-O₂ System

Abstract

This work reports the capacity of a high-temperature electron probe to bridge solid-state chemistry and surface chemistry. This goal could be accomplished by performing the characterization of the surface layer in equilibrium with both the gas phase and bulk phase, thus dealing with a system governed by thermodynamic laws, which is free of unknown kinetic terms. In this work, we contemplate the use of the electron probe in surface defect engineering of the next generation of energy materials, that are expected to enhance the production of clean energy. The concept of this kind of engineering is considered for the TiO 2 -O 2 model system. Such example demonstrates the role of surface segregation in the formation of a quasi-isolated surface structure, that has a profound impact on the reactivity of solids and their performance in energy conversion devices. Consequently, it is essential that studies of the effect of surface defect structure on the reactivity of solids are conducted in situ and in operando, under conditions of real application. This is expected to aid directly the rational design of surface properties in the processing of high-performance energy materials for fuel cells, solar cells, batteries, catalysts and photo-catalysts, as well as sensors.

In situ surface monitoring of energy materials during processing: impact of defect disorder on surface versus bulk semiconducting properties of photocatalytic hematite (Fe2O3)

“…The related charge neutrality condition requires that the charge of chemisorbed (molecular and atomic) oxygen species is compensated by the space charge within the boundary layer. At elevated temperatures, chemisorbed oxygen species are quickly incorporated into the surface layer in the form of doubly ionized oxygen ions, 7,15 then these species migrate through the electric fields within the space charge layer. It has been shown that these fields have a substantial impact on surface segregation of defects, which is rate controlled by diffusion.…”

“…The prolonged drift of WF changes suggests the progression of a slow surface reaction that is associated with segregation of defects, as it has been observed during prolonged annealing of TiO 2 . 15 The full gas/solid equilibrium in the Fe 2 O 3 -O 2 system is established after 300 h at 1140 K.…”

“…The reactivity of metal oxides with oxygen may be considered in terms of oxygen chemisorption, oxygen incorporation into the surface layer and the transport through this layer, and oxygen transport within the bulk phase. 7,15 Progression of oxygen chemisorption is determined by the charge transfer between the physically adsorbed oxygen species and the surface layer, leading to the formation of differently ionized oxygen species. The related charge neutrality condition requires that the charge of chemisorbed (molecular and atomic) oxygen species is compensated by the space charge within the boundary layer.…”

“…While it has been commonly assumed that the gas/solid equilibration of the metal oxide/oxygen system is rate determined by bulk diffusion and the surface reaction is fast, 2–4,30 the use of WF measurements in surface monitoring of the charge transfer during oxidation of metal oxides indicates that the equilibration is rate controlled by surface segregation of defects derived from the bulk phase. 7,15 The objective of this work is to examine the gas/solid equilibration for the Fe 2 O 3 –O 2 system using the HTEP.…”

“…Efforts to build such a tool, led to the development of the high-temperature electron probe (HTEP), that is based on work function (WF) measurements. 7–15…”