Structural and Chemical Properties of NiO_x Thin Films: Oxygen Vacancy Formation in O₂ Atmosphere

Abstract

NiOx films on Si(111) were put in contact with oxygen at elevated temperatures. During heating and cooling in oxygen atmosphere Near Ambient Pressure (NAP)‐XPS and –XAS and work function (WF) measurements, reveal the creation and replenishing of oxygen vacancies in dependence of temperature. Oxygen vacancies manifest themselves as a distinct O1s feature at 528.9 eV on the low binding energy side of the main NiO peak as well as by a distinct deviation of the Ni2p3/2 spectral features from the typical NiO spectra. DFT calculations reveal that the presence of oxygen vacancies leads to a charge redistribution and altered bond lengths of the atoms surrounding the vacancies causing the observed spectral changes. Furthermore, we observed that a broadening of the lowest energy peak in the O K‐edge spectra can be attributed to oxygen vacancies. In the presence of oxygen vacancies, the WF is lowered by 0.1 eV.

Structural and chemical properties of NiOx thin films: the role of oxygen vacancies in NiOOH formation in a H2O atmosphere

“…As reported in detail in our study of oxygen vacancy formation on NiO x thin films, such vacancies can readily form by thermal treatment in an oxygen atmosphere. 19 Increasing the sample temperature leads to the removal of oxygen atoms from the lattice, just as in the common reduction process in a vacuum. Yet, in the presence of O 2 , a balance between removal and replenishment of oxygen atoms depends on temperature.…”

“…DFT calculations of bond lengths, charge densities and BEs revealed that the NiO 1− x peak can be directly related to the formation of oxygen vacancies, which leads to a relaxation of the atoms around a vacancy influencing the charge density and, consequently, the XPS BE of these neighboring atoms. 19 Hence, the peak at 528.9 eV was labeled NiO 1− x . The other peak at 530 eV is ascribed to the presence of oxygen vacancies with chemisorbed OH, thus representing NiO 1− x –OH.…”

“…The other peak at 530 eV is ascribed to the presence of oxygen vacancies with chemisorbed OH, thus representing NiO 1− x –OH. 19 We will address the latter peak in detail below.…”

“…We observed that while the Ni2p 3/2 residuum fully vanishes, both the NiO 1− x and NiO 1− x –OH peaks retain some intensity even after cooling, which suggests a more defective surface than the as-prepared NiO x film. 19 However, the methods applied were only sensitive to the electronic environment and the bond states. As it turns out, repeated heating and cooling treatments in oxygen affect the morphology of the NiO x sample as well.…”

“…17,18 We have recently shown that NiO x films exposed to 0.5 mbar of O 2 readily form oxygen vacancies at elevated temperatures. 19 Furthermore, the obtained results point to OH chemisorption at the vacancies, which may act as a precursor to NiOOH formation, while on the other hand, Ni vacancies, usually present only at room temperature on NiO x , appear not to be involved at all. In the present study, NiO x thin films were first thermally treated in an atmosphere of 0.5 mbar O 2 to deliberately create oxygen vacancies and other structural defects such as step-edges to facilitate the dissociation of water.…”