Investigating Nanostructured Materials by Multi-modal in situ X-ray Methods

Understanding the structural transformations affecting nanomaterials is essential to rationally improve their properties and functionalities. Despite recent advances in several X-ray investigation routines, the transformation phenomena affecting nanomaterials entail structural modifications on several length scales that cannot be covered by a single analytical method. In this thesis, we establish a multi-modal X-ray scattering setup to collect structural information spanning from fractions of Å to hundreds of nanometers via simultaneous acquisition of small-angle X-ray scattering (SAXS) and X-ray total scattering (TS). We then apply the method to investigate the model syntheses of Pd, Cu, and CuPd nanocrystals in solution, and to further elucidate the degradation of photo- electrochemical (PEC) CuBi2O4 thin films during operation. We reveal that a Pd(II)-amino initial complex directly converts into Pd nanocrystals without formation of intermediates, in agreement with the classical nucleation model. We employ the pair-distribution function (PDF) analysis of the in situ TS signal and complementary in situ X-ray absorption spectroscopy (XAS) during the synthesis of Cu nanocrystals to reveal a multi-step conversion of an initial Cu(II)-amino complex into a partially reduced Cu(I) organometallic intermediate prior to the nucleation of polycrystalline Cu2O seeds. We further demonstrate via PDF and SAXS that individual Cu2O seeds fuse together to form larger Cu2O nanocrystals, which slowly convert into metallic Cu. We thus show that the synthesis of Cu nanocrystals follows a complex non-classical crystallization pathway. Both in situ XAS and PDF suggest that alloyed CuPd nanocrystals form via preliminary formation of Pd-rich seeds, which promote the direct reduction of the Cu(I)-amino intermediate into the metallic form without formation of the Cu2O phase. PDF analysis further indicates that the CuPd nanocrystals nucleate and grow as multi-twinned icosahedra. Furthermore, SAXS data reveals that the as-formed CuPd icosahedra assemble within the reaction medium into large face-centered cubic (fcc) supercrystals due to the entropic forces arising by the ordering of the solvent molecules into lamellae. We reveal by both PDF and SAXS that the fast decrease in the performances of CuBi2O4 electrodes correlates directly with the formation of a metallic Bi phase. We also identify additional degradation phenomena, such as the emergence of metallic Cu and the dissolution of the electrode in contact with the electrolyte, which further affect the CuBi2O4 thin film activity and morphology. Overall, our study shows that the multi- modal acquisition of PDF, SAXS, and XAS significantly enhances our understanding on the transformation mechanisms affecting nanomaterials and poses the methodological basis to investigate the fabrication and operation of a wide range of nanostructured materials.