Understanding and controlling the formation of nanoparticles at the surface of functional oxide supports is critical for tuning activity and stability for catalytic and energy conversion applications. Here we use a latest generation environmental transmission electron microscope to follow the exsolution of individual nanoparticles at the surface of perovskite oxides, with ultra-high spatial and temporal resolution. Qualitative and quantitative analysis of the data reveals the atomic scale processes that underpin the formation of the socketed, strain-inducing interface that confers exsolved particles their exceptional stability and reactivity. This insight also enabled us to discover that the shape of exsolved particles can be controlled by changing the atmosphere in which exsolution is carried out and additionally, this could also produce intriguing heterostructures consisting of metal-metal oxide coupled nanoparticles. Our results not only provide insight into the in situ formation of nanoparticles, but also demonstrate the tailoring of nanostructures and nano-interfaces.

Distinct photocatalytic performance was observed when Ta 3 N 5 was synthesized from commercially available Ta 2 O 5 or from Ta 2 O 5 prepared from TaCl 5 via the sol−gel route. With respect to photocatalytic O 2 evolution with Ag + as a sacrificial reagent, the Ta 3 N 5 produced from commercial Ta 2 O 5 exhibited higher activity than the Ta 3 N 5 produced via the sol−gel route. When the Ta 3 N 5 photocatalysts were decorated with Pt nanoparticles in a similar manner, the Ta 3 N 5 from the sol−gel route exhibited higher photocatalytic hydrogen evolution activity from a 10% aqueous methanol solution than Ta 3 N 5 prepared from commercial Ta 2 O 5 where no hydrogen can be detected. Detailed surface and bulk characterizations were conducted to obtain fundamental insight into the resulting photocatalytic activities. The characterization techniques, including XRD, elemental analysis, Raman spectroscopy, UV−vis spectroscopy, and surface-area measurements, revealed only negligible differences between these two photocatalysts. Our thorough characterization of the surface properties demonstrated that the very thin outermost layer of Ta 3 N 5 , with a thickness of a few nanometers, consists of either the reduced state of tantalum (TaN) or an amorphous phase. The extent of this surface layer was likely dependent on the nature of precursor oxide surfaces. DFT calculations based on partially oxidized Ta 3 N 4.83 O 0.17 and N deficient Ta 3 N 4.83 consisting of reduced Ta species well described the optoelectrochemical properties obtained from the experiments. Electrochemical and Mott−Schottky analyses demonstrated that the surface layer drastically affects the energetic picture at the semiconductor−electrolyte interface, which can consequently affect the photocatalytic performance. Chemical etching of the surface of Ta 3 N 5 particles to remove this surface layer unites the photocatalytic properties with the photocatalytic performance of these two materials. Mott−Schottky plots of these chemically etched Ta 3 N 5 materials exhibited similar characteristics. This result suggests that the surface layer (1−2 nm) determines the electrochemical interface, which explains the different photocatalytic performances of these two materials.