By selecting nanoparticle size, shape, and composition during synthesis, researchers have extraordinary control over the physical and chemical properties of nanostructures, giving rise to exciting properties such as high catalytic activities, enhanced charge extraction, and improved durability. Post-synthetic modification, or chemical transformation, of these nanomaterials has been shown to greatly expand the space of tunable parameters for designer nanoparticles. We examined chemical transformations of nanoparticles, specifically ligand exchange or displacement and ion exchange, as a route not only to produce materials for renewable energy applications but also to investigate the relation of nanocrystal subsurface structure to surface energy and to measure the rates of diffusion of ions at short length scales. An important technique in recent years is the process of ion exchange, by which cations or anions in nanocrystals of a parent binary compound are replaced with ions of a different type to produce an otherwise inaccessible structure. We find that anion exchange in oxides via introduction of sulfide can be used to optimize electrocatalytic activity of cobalt oxide for the hydrogen evolution reaction and show with first-principles calculations that this arises from tuning of adsorbate binding energies to favorable values. Using x-ray diffraction and spectroscopic techniques we explain the chemically selective dissolution of constituents of cation-exchanged nanocrystals in terms of an autocatalytic surface reaction by ostensibly protective surfactants, thereby providing another route for controlling heterostructure morphology by material removal. This reaction entails the insertion and removal of ions to and from the lattice as dictated by the redox environment and local strain, Through in-situ x-ray diffraction of the cation exchange of lead sulfide to cadmium sulfide, we quantify the transport coefficients of ions through a nanoparticle shell and find that interdiffusion is accelerated by a factor of 10⁴ or more during exchange relative to expectations from high-temperature data, even though the activation barrier to diffusion is similar. These results show the need for a more careful microscopic treatment of transport in the necessarily large chemical potentials found nanoparticle transformation processes far from equilibrium. Finally, we compare some important techniques for preparing clean and reproducible nanoparticle surfaces for electrochemical investigation and demonstrate in particular the effectiveness of ligand removal by alkylation, showing that it may be a useful and general technique for future investigations of well-defined nanocrystal electrocatalysts.