Temperature Programmed Photoelectron Emission (TPPE) to Analyze Electron Transfer on Metallic Copper and Its Relation to the Essential Role of the Surface Hydroxyl Radical
Authors: Momose Yoshihiro 
Published: December 2023

Surface processes such as coatings, corrosion, photocatalysis, and tribology are greatly diversified by acid-base interaction at the surface overlayer. This study focuses on the action of metallic copper surface as an electron donor/acceptor related to inactivation of viruses; it was found that regarding Cu2O or Cu materials electrostatic interaction plays a major role in virus inactivation. We applied TPPE method to clarify the mechanism of electron transfer (ET) occurring at light irradiated copper surfaces. The TPPE characteristics was 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 (E) analyzed from Arrhenius plot of NT values obtained in the temperature-increase and subsequent temperature-decrease processes. In this study we reexamined 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 &gt; CH3OH &gt; C2H5OH &gt; (CH3)2CHOH &gt; C3H7OH, although the last alcohol gave Tmax = 100C, while with Sample C the NTmax value decreased in the order: C3H7OH &gt; (CH3)2CHOH &gt; C2H5OH &gt; CH3OH &gt; H2O. Interestingly, both orders of the liquids were completely opposite: this means that Cu surface can possess two-way character. The NT intensity was found to be strongly associated with the change of 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 Cu-OH+---:OH2 (--- hydrogen bond), while the C3H7OH and (CH3)2CHOH adsorbed on Sample C produce ROH+---: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), H2O (54.8) (the AN of C3H7OH could not be confirmed). With Sample A the values of NTmaxa and EaUp1 both increased with increasing AN (Up1 means the 1st temperature-increase process). On the other hand, with Sample C the values of NTmaxc and EcUp1 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 EUp1. It was explained that the EUp1 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 by tunnelling. After that due to Auger emission electrons are released, leading to the ET in the overlayer. These electrons are considered to have a strong ability of reducibility.