PNNL

Abstract

The impact of solution exposure on the charging properties of oxide coatings on Fe metal-core oxide-shells has been examined by sample biasing during XPS measurements. The Fe nanoparticles were suspended in relatively unreactive acetone and were analyzed after particle containing solutions were deposited on SiO2/Si substrates, and/or Au substrates. The particle and substrate combinations were subjected to ± 10V d.c. biasing in the form of square waves (SQW) pulses with 5V amplitude. The samples experienced variable degrees of charging for which low energy electrons at ~1 eV, 20µA and low energy Ar+ ions were used to minimize. Application of d.c. bias and/or square wave pulses drastically influences the extent of charging, which is utilized to gather additional analytical information about the sample under investigation. This approach allows separation of otherwise overlapping peaks. Accordingly, the O1s peaks of the silicon oxide substrate, the iron oxide nanoparticles, and that of the casting solvent can be separated from each other. Similarly the C1s peak belonging to the solvent can be separated from that of the adventitious carbon. The charging shifts of the iron nanoparticles are strongly influenced by the surrounding solvent. Hence, acetone exhibits the largest shift, water the smallest, and methanol in between. Dynamical measurements performed by application of the voltage stress in the form of SQW pulses gives information about the time constants of the processes involved, which led us postulate that these charging properties we probe in these systems, stem mainly from ionic movement(s).