Optical and electronic properties of native zinc oxide films on polycrystalline Zn

Abstract

The oxide layer spontaneously formed on zinc and an ''electrochemically reduced'' oxide has been characterised by a combination of X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry (SE). The onset of the main electronic absorption, which is directly related to the bandgap, is extracted from the SE measurements. The SE results are compared with simulations on the basis of zinc and bulk zinc oxide optical constant data. Measurements in the ultraviolet and visible (UV-vis) spectral range show the presence of an absorption at B1.8 eV (680 nm) which is unaccounted for from the bulk data, and is likely to originate from intragap energy levels, implicating the presence of surface defects in the layers. Analysis of the Zn LMM Auger peaks in XPS data show the presence of Zn different from bulk zinc and bulk ZnO, attributed to excess Zn in the oxide films. Mid-infrared (IR) ellipsometry shows two peaks around 0.12 and 0.15 eV (1000 and 1200 cm À1 ), which strengthen the assumption of the presence of a locally distorted structure in the oxide layers. Electrochemically reduced samples show a much thinner oxide layer and higher Zn-doping concentration films than samples purely dipped in NaOH solution. Using a self-contained multiple sample SE analysis, estimates of the refractive index and absorption coefficient (i.e., the optical constants) of the oxide films are presented from 1.5-4.4 eV (280 to 810 nm).

Understanding the Heterogeneous Electrocatalytic Reduction of Carbon Dioxide on Oxide‐Derived Catalysts

“…Early reports of OD‐catalysts indicated that using a thick Au oxide precursor could substantially improve its performance towards CO 2 RR to CO . Interestingly, based on analysis of the Pourbaix diagrams, Au and Ag are not anticipated to have native oxides, which is confirmed by XPS measurements of the bulk materials, while Zn is confirmed to natively oxidize ,. Thus, we note that the reaction site on native gold and silver electrodes are likely Au 0 and Ag 0 sites.…”

“…[11,12,114] Early reports of OD-catalysts indicated that using a thick Au oxide precursor could substantially improve its performance towards CO 2 RR to CO. [25] Interestingly, based on analysis of the Pourbaix diagrams, Au and Ag are not anticipated to have native oxides, which is confirmed by XPS measurements of the bulk materials, while Zn is confirmed to natively oxidize. [57,[115][116][117] Thus, we note that the reaction site on native gold and silver electrodes are likely Au 0 and Ag 0 sites. In situ XAS analysis also suggests that ZnO cathodes are fully reduced to Zn 0 at potentials more negative than À0.7 V vs. RHE, indicating that Zn 0 is also likely the active site for CO 2 RR.…”

Benchmarking the Oxygen Reduction Electroactivity of First‐Row Transition‐Metal Oxide Clusters on Carbon Nanotubes

“…The XPS metal 2p spectra are shown in Figure 2b. [23][24][25][26][27][28][29][30][31] It is noted that the Mn, Fe, Co and Ni spectra possess satellite features with respect to their characteristic peaks, whereas the spectra is free of satellites for Cu and Zn. The main characteristic 2p 3/2 peaks for the metals are located at 641.8 eV (Mn), 711.8 eV (Fe), 781.9 eV (Co), 856.3 eV (Ni), 933.2 eV (Cu) and 1022.5 eV (Zn), which respectively corresponds to Mn III , Fe III , Co III (a DE 2p = 15.0 eV also provides the trivalent state of Co), [23] Ni III , Cu II , Zn II .…”

“…The main characteristic 2p 3/2 peaks for the metals are located at 641.8 eV (Mn), 711.8 eV (Fe), 781.9 eV (Co), 856.3 eV (Ni), 933.2 eV (Cu) and 1022.5 eV (Zn), which respectively corresponds to Mn III , Fe III , Co III (a DE 2p = 15.0 eV also provides the trivalent state of Co), [23] Ni III , Cu II , Zn II . [23][24][25][26][27][28][29][30][31] It is noted that the Mn, Fe, Co and Ni spectra possess satellite features with respect to their characteristic peaks, whereas the spectra is free of satellites for Cu and Zn. This is relevant to the number of unpaired electrons in the hybridized orbital and the spin state.…”

Fabrication of an Electrochemical Sensor for Glucose Detection using ZnO Nanorods

“…The stability of ZnO in air is high [4]. The oxide layers naturally formed on zinc do not form a passivating film, which prevents its corrosion [5]. In the recent years, one dimensional ZnO nanorods are synthesized using the hydrothermal growth technique [6].…”