Using Temperature-Programmed Photoelectron Emission (TPPE) to Analyze Electron Transfer on Metallic Copper and Its Relation to the Essential Role of the Surface Hydroxyl Radical

Abstract

Surface processes such as coatings, corrosion, photocatalysis, and tribology are greatly diversified by acid–base interactions at the surface overlayer. This study focuses on the action of a metallic copper surface as an electron donor/acceptor related to the inactivation of viruses. It was found that regarding Cu2O or Cu materials, electrostatic interaction plays a major role in virus inactivation. We applied the TPPE method to clarify the mechanism of electron transfer (ET) occurring at light-irradiated copper surfaces. The TPPE characteristics were strongly influenced by the environments, which correspond to the temperature and environment dependence of the total count of emitted electrons in the incident light wavelength scan (PE total count, NT), the photothreshold, and further the activation energy (ΔE) analyzed from the Arrhenius plot of NT values obtained in the temperature increase and subsequent temperature decrease processes. In this study, we re-examined the dependence of the TPPE data from two types of Cu metal surfaces: sample A, which was mechanically abraded in alcohols, water, and air, and sample C, which was only ultrasonically cleaned in these liquids. The NT for both samples slowly increased with increasing temperature, reached a maximum (NTmax) at 250 °C (maximum temperature, Tmax), and after that, decreased. For sample A, the NTmax value decreased in the order H2O > CH3OH > C2H5OH > (CH3)2CHOH > C3H7OH, although the last alcohol gave Tmax = 100 °C, while with sample C, the NTmax value decreased in the order C3H7OH > (CH3)2CHOH > C2H5OH > CH3OH > H2O. Interestingly, both orders of the liquids were completely opposite; this means that a Cu surface can possess a two-way character. The NT intensity was found to be strongly associated with the change from the hydroxyl group (–Cu–OH) to the oxide oxygen (O2−) in the O1s spectra in the XPS measurement. The difference between the above orders was explained by the acid–base interaction mode of the –Cu–OH group with the adsorbed molecule on the surfaces. The H2O adsorbed on sample A produces the electric dipole –CuOδ−Hδ+ ⋅⋅⋅ :OH2 (⋅⋅⋅ hydrogen bond), while the C3H7OH and (CH3)2CHOH adsorbed on sample C produce RO−δHδ+ ⋅⋅⋅ :O(H)–Cu− (R = alkyl groups). Gutmann’s acceptor number (AN) representing the basicity of the liquid molecules was found to be related to the TPPE characteristics: (CH3)2CHOH (33.5), C2H5OH (37.1), CH3OH (41.3), and H2O (54.8) (the AN of C3H7OH could not be confirmed). With sample A, the values of NTmaxa and ΔEaUp1 both increased with increasing AN (Up1 means the first temperature increase process). On the other hand, with sample C, the values of NTmaxc and ΔEcUp1 both decreased with increasing AN. These findings suggest that sample A acts as an acid, while sample C functions as a base. However, in the case of both types of samples, A and C, the NTmax values were found to increase with increasing ΔEUp1. It was explained that the ΔEUp1 values, depending on the liquids, originate from the difference in the energy level of the hydroxyl group radical at the surface denoted. This is able to attract electrons in the neighborhood of the Fermi level of the base metal through tunnelling. After that, Auger emission electrons are released, contributing to the ET in the overlayer. These electrons are considered to have a strong ability of reducibility.