Defects and Surface Chemistry of Novel PH-Tunable NiO-Mn3O4 ± MnxNi1-xO Heterostructured Nanocrystals as Determined Using X-ray Photoemission Spectroscopy

We have developed a novel set of (Mn3O4 and/or MnxNi1-xO)/NiO heterostructured nanocrystals (HNCs) that show promise for multifunctionality. A two-step procedure was used to synthesize our HNC samples. Thermal decomposition was used first to synthesize NiO core nanoparticles, while hydrothermal synthesis under varying pH conditions was then used to produce overgrowth of a MnxNi1-xO shell and/or Mn3O4 islands on the NiO core. In this work, we report on the investigation of the defects and surface/interface chemistry of our HNC samples using x-ray photoelectron spectroscopy (XPS). The results from a detailed analysis of the Mn 2p XPS spectra show that the Mn3+:Mn2+ ratio increases with the increasing pH value used in synthesis of the samples. This is consistent with a trend toward a predominance of Mn3O4 islands at higher pH values, a predominance of MnxNi1-xO shell at the lowest pH values, and a mixture of both for intermediate pH values. A reduction of the satellite features of the Ni 2p XPS with increasing pH of the synthesis medium is attributed predominantly to surface/interface defects of the HNCs. Fitting of the O 1 s XPS spectra shows that Ni-OH and Mn-OH are likely the dominant contributions to the lateral peak, whereas defects such as oxygen in an oxygen-deficient environment and/or oxygen vacancies comprise a smaller contribution. Analysis of both the Ni 2p and O 1 s XPS measured from our samples only shows evidence for a Ni2+ chemical environment (i.e., negligible Ni3+) in octahedral coordination, consistent with a rocksalt structure and well-ordered NiO or MnxNi1-xO nanomaterial. The presence of point defects and the nature of the surface/interface chemistry as determined using XPS suggests that our HNC samples may also be suitable for heterogeneous catalysis applications.