The presence of superoxide ions and related dioxygen species in zinc oxide—A structural characterization by in situ Raman spectroscopy

Zinc oxide exhibits unique properties that are reflected in a wide variety of applications, particularly in the field of transparent, conductive films. However, less attention has been paid to their color. Here, we present the synthesis of yellow‐gray ZnO films at room temperature by femtosecond pulsed laser deposition. In situ Raman investigations of these polycrystalline ZnO films reveal the existence of superoxide ions, O2−, in zinc oxide, which are responsible for the yellow color, and are also detected in ZnO powder and single crystals. In addition, further dioxygen species are identified in the samples, including the O2‐molecule. The negative charge excess caused by the dioxygen species creates metallic zinc as a byproduct. Structural analysis reveals an unforced realization of the dioxygen species in the ZnO lattice. Density functional theory (DFT) calculations support the assumed structural displacements as well as the observed, unexpected Raman bands. These results open up completely new insights into the behavior of ZnO. The Raman spectra of zinc oxide powder and single crystals as well as of ZnO films support the presence of the yellow superoxide ion O2− by the band at ~1140 cm−1. Additionally, the band at ~1560 cm−1 identify the O2‐molecule inside the deposited films and in pressed powders. The corresponding structural process in ZnO proceeds by filling a face‐connected, normally unoccupied, OZn4 tetrahedron (yellow) with subsequent formation of a dioxygen species (red bond, distortions are disregarded).