Nitrogen modified metal oxide surfaces
Nanoscale surface modification of a number of metal oxides was performed by bombarding them with N2+ of 1–5 keV energy. Compositional and chemical state alterations, the valence state of the metals, the chemical states of the built-in nitrogen, were monitored by quantitative XPS analysis, performed in situ, without exposing the modified surface to the environment. It is demonstrated that N2+ bombardment created oxygen deficient surface with significantly lower oxygen to metal (O/Me) ratio than that produced by the purely ballistic effect of Ar+ bombardment of similar energy and ion-dose. This “excess” oxygen-loss, detected at N2+ bombardment, is substituted by an equal number of nitrogen atoms in the lattice of the reduced oxide. The implanted nitrogen established chemical bonds predominantly with the metal as manifested by its XPS chemical shift, creating by this a metal oxinitride on the surface of the parent oxide. The observed alterations are interpreted in the light of ion–solid interaction, involving the thermodynamic stability and the valence state of the oxides. A mechanism is proposed to elucidate the observed, thermodynamically unfavoured oxide-to-nitride transformation. The presented results offer a straightforward way for altering the electronic structure of the surface of oxides which may be beneficial for different, especially for photocatalytic applications of the modified oxides.
Graphical abstractN 1s XP lines recorded on the surface of Al2O3, SiO2, TiO2 and V2O5 after N2+ bombardment of 2.5 keV energy.Figure optionsDownload full-size imageDownload high-quality image (91 K)Download as PowerPoint slideHighlights► Metal oxide surfaces modified by N2+ ion bombardment of 1–5 keV energy. ► Implanted nitrogen accommodates into the matrix of partially reduced oxide. ► Treatment creates few nanometre thick oxinitride layers. ► 5–30% of lattice oxygen replaced, 0.1–0.6 N/metal atomic ratio produced. ► Mechanism and extent of O to N replacement clarified.
Journal: Catalysis Today - Volume 181, Issue 1, 12 February 2012, Pages 95–101