Combining X-Ray Whole Powder Pattern Modeling, Rietveld and Pair Distribution Function Analyses as a Novel Bulk Approach to Study Interfaces in Heteronanostructures : Oxidation Front in FeO/Fe3O4 Core/Shell Nanoparticles as a Case Study

Understanding the microstructure in heterostructured nanoparticles is crucial to harnessing their properties. Although microscopy is ideal for this purpose, it allows for the analysis of only a few nanoparticles. Thus, there is a need for structural methods that take the whole sample into account. Here, a novel bulk‐approach based on the combined analysis of synchrotron X‐ray powder diffraction with whole powder pattern modeling, Rietveld and pair distribution function is presented. The microstructural temporal evolution of FeO/Fe3O4 core/shell nanocubes is studied at different time intervals. The results indicate that a two‐phase approach (FeO and Fe3O4) is not sufficient to successfully fit the data and two additional interface phases (FeO and Fe3O4) are needed to obtain satisfactory fits, i.e., an onion‐type structure. The analysis shows that the Fe3O4 phases grow to some extent (≈1 nm) at the expense of the FeO core. Moreover, the FeO core progressively changes its stoichiometry to accommodate more oxygen. The temporal evolution of the parameters indicates that the structure of the FeO/Fe3O4 nanocubes is rather stable, although the exact interface structure slightly evolves with time. This approach paves the way for average studies of interfaces in different kinds of heterostructured nanoparticles, particularly in cases where spectroscopic methods have some limitations.
R.U.I. acknowledges CAPES and CNPq (No. 206983/2014‐0) Brazilian agencies. A.G.R. and J.N. thanks the support of the Generalitat de Catalunya through the 2017‐SGR‐292 project and the Beatriu de Pinos Program (2011 BPB 00209) and the Spanish Ministerio de Economía y Competitividad (MINECO) through the MAT2016‐77391‐R project. I.P. was supported by the National Research Fund of Luxembourg (Grant No. FNR‐Inter2015/LRSF). A.L.‐O. acknowledges the MINECO through the Juan de la Cierva Program (IJCI‐2014‐21530). X.T. would like to acknowledge the financial support from the MINECO projects MAT2015‐67593‐P and BIA2014‐57658‐C2‐1‐R. The ALBA Synchrotron is acknowledged for the provision of beamtime and the MSPD beamline staff is for their help during the XRD measurements. Dr. Luis G. Martinez from IPEN/CNEN is acknowledged for useful discussions about XRD techniques. Dr. Paolo Scardi from Università degli Studi di Trento is acknowledged for the helpful discussions about WPPM and for providing PM2K v.3 software. ICN2 is funded by the CERCA Programme/Generalitat de Catalunya. ICN2 also acknowledges support from the Severo Ochoa Program (MINECO, Grant No. SEV‐2013‐0295).