Probing environmentally significant surface radicals: Crystallographic and temperature dependent adsorption of phenol on ZnO

Abstract

Environmentally persistent free radicals (EPFRs) are toxic organic/metal oxide composite particles that have been discovered to form from substituted benzenes chemisorbed to metal oxides. Here, we perform photoelectron spectroscopy, electron energy loss spectroscopy, and low energy electron diffraction of phenol chemisorbed to ZnO(1 0 1̱ 0) and (0 0 0 1̱)-Zn to observe electronic structure changes and charge transfer as a function adsorption temperature. We show direct evidence of charge transfer from the ZnO surfaces to the phenol. This evidence can help gain a better understanding of EPFRs and be used to develop possible future remediation strategies.

Formation of environmentally persistent free radicals (EPFRs) on ZnO at room temperature: Implications for the fundamental model of EPFR generation

“…Vibrational spectroscopy suggests that at both RT and 250°C, phenol chemisorbs to these ZnO nanoparticles and forms a surface-bound phenoxy species, and DFT calculations provide a model structure for the chemisorbed species and suggest that −0.86 e is transferred to the organic from the coordinated surface Zn atoms. All of these results are consistent with previous studies of phenol adsorption on ZnO single crystals or ZnO nanoparticles exposed to phenol at elevated temperature 15,21 . The existence of an EPFR after chemisorption at RT corroborates the extraordinarily long half-life of ZnO-derived EPFRs under ambient conditions, as it suggests that the equilibrium between EPFRs and their decay products should thermodynamically favor the EPFR side.…”

“…This is consistent with our prior ultraviolet photoelectron spectroscopy results 21 , which showed that the valence band electronic structure (including the occupied molecular orbitals of the organic adsorbate) of phenol-exposed ZnO surfaces was nearly identical after RT and high temperature adsorption.…”

“…These electronic properties are accessible by both experiment and calculation, and reference to the well-developed band theory of semiconductors may prove fruitful in the interpretation of EPFR properties such as lifetime, photochemical activity, and decay modes. Moreover, such a model better captures the details of previous work regarding phenol adsorption and charge transfer in ZnO and TiO 2 single crystals, as these two systems provide excellent examples of “upward” and “downward” band bending respectively after chemisorption (in addition to more explicit signatures of electron transfer) 14,21,31 .…”

“…A previous single crystal photoelectron spectroscopy study of phenol adsorption on ZnO 21 hinted at two surprising results. First, band bending suggested that electron density was transferred from the ZnO to the adsorbate, i.e., the opposite direction of what has been proposed for EPFR formation.…”

Environmentally persistent free radicals in PM2.5: a review

“…This mechanism also applies to the formation of EPFRs on a hematite-silica surface under ambient conditions using ultraviolet (UV) light irradiation [25]. However, it can neither fully explain the formation of EPFRs from unsubstituted aromatics such as benzene [26] nor the reversed electron transfer from ZnO surfaces to phenols [27]. To solve these problems, Vejerano et al proposed a new formation mechanism of EPFRs on the surfaces of combustion or thermal treatment produced engineering nanomaterials (ENMs) not necessarily supported on micron-sized silica particles or with the presence of transition metals [28].…”