Ambient Pressure X-ray Photoelectron Spectroscopy Study of UV Photocatalytic Reduction of CO₂ on ZnO/Cu₂O Nanoparticle Heterojunction

Tzu-Cheng Huang¹, Sheng-Yuan Chen², Chia-Hsin Wang², Yaw-Wen Yang^1,2*

¹Department of Chemistry, National Tsing-Hua University, Hsinchu, Taiwan
²Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu, Taiwan

* Presenter:Yaw-Wen Yang, email:yang@nsrrc.org.tw

In this work, we report a UV photocatalytic reduction study of carbon dioxide with water on semiconducting heterojunctions constructed from the nanoparticles of zinc oxide (Eg = 3.2 eV) and crystalline cuprous oxide (Eg = 2.0 – 2.2 eV). Two crystalline types of cuprous oxide were selected based on the anticipated difference in photocatalytic performance: cubic structure terminated with (100) faces, and rhombic dodecahedron (r.d.) terminated with (110) faces. Ambient pressure X-ray photoelectron spectroscopy (APXPS) was used to track the change of reaction species on the photocatalyst surfaces. The ZnO/Cu₂O (r.d.) is found to be more reactive than ZnO/Cu₂O (cube) as evidenced by a larger production of carbon species such as carbonate, formate, carbonyl, and methoxy on the surface when the nano-catalysts were exposed to gaseous carbon dioxide and water of mbar pressure and illuminated with UV photons. Further, the heterojunctions composed of Cu₂O (r.d.) and different ZnO loading ranging from 7% to 40% were found to exhibit altered reaction pathway, yielding more methane instead of methanol as the ZnO loading is increased. The energy band diagrams useful in discerning the catalytic performance of photocatalysts are also constructed by means of APXPS data. The semiconductor interfaces of ZnO/Cu₂O (r.d.) and ZnO/Cu₂O (cube) all belong to type II heterojunction but with distinct band offsets between VBM of Cu₂O and CBM of ZnO, leading to different interfacial recombination rate. The speculated difference in interfacial recombination rate seems to correlate well with the changing UV photocatalytic reactivity between two types of heterojunctions

Keywords: APXPS, photocatalyst, heterojunction