Shedding Light on Vacancy-Doped Copper Chalcogenides: Shape-Controlled Synthesis, Optical Properties, and Modeling of Copper Telluride Nanocrystals with Near-Infrared Plasmon Resonances

Size- and shape-controlled synthesis of copper chalcogenide nanocrystals (NCs) is of paramount importance for a careful engineering and understanding of their optoelectronic properties and, thus, for their exploitation in energy- and plasmonic-related applications. From the copper chalcogenide family copper telluride NCs have remained fairly unexplored as a result of a poor size-, shape-, and monodispersity control that is achieved <i>via</i> one-step syntheses approaches. Here we show that copper telluride (namely Cu_2–<i>x</i>Te) NCs with well-defined morphologies (spheres, rods, tetrapods) can be prepared <i>via</i> cation exchange of preformed CdTe NCs while retaining their original shape. The resulting copper telluride NCs are characterized by pronounced plasmon bands in the near-infrared (NIR), in analogy to other copper-deficient chalcogenides (Cu_2–<i>x</i>S, Cu_2–<i>x</i>Se). We demonstrate that the extinction spectra of the as-prepared NCs are in agreement with theoretical calculations based on the discrete dipole approximation and an empirical dielectric function for Cu_2–<i>x</i>Te. Additionally we show that the Drude model does not appropriately describe the complete set of Cu_2–<i>x</i>Te NCs with different shapes. In particular, the low-intensity longitudinal plasmon bands for nanorods and tetrapods are better described by a modified Drude model with an increased damping in the long-wavelength interval. Importantly, a Lorentz model of localized quantum oscillators describes reasonably well all three morphologies, suggesting that holes in the valence band of Cu_2–<i>x</i>Te cannot be described as fully free particles and that the effects of localization of holes are important. A similar behavior for Cu_2–<i>x</i>S and Cu_2–<i>x</i>Se NCs suggests that the effect of localization of holes can be a common property for the whole class of copper chalcogenide NCs. Taken altogether, our results represent a simple route toward copper telluride nanocrystals with well-defined shapes and optical properties and extend the understanding on vacancy-doped copper chalcogenide NCs with NIR optical resonances

Titanium Doping and Its Effect on the Morphology of Three-Dimensional Hierarchical Nb₃O₇(OH) Nanostructures for Enhanced Light-Induced Water Splitting

This study presents a simple method that allows us to modify the composition, morphological, and surface properties of three-dimensional hierarchical Nb₃O₇(OH) superstructures, resulting in strongly enhanced photocatalytic H₂ production. The superstructures consist of highly ordered nanowire networks and self-assemble under hydrothermal conditions. The presence of titanium affects the morphology of the superstructures, resulting in increased surface areas for higher doping levels. Up to 12 at. % titanium is incorporated into the Nb₃O₇(OH) crystal lattice via substitution of niobium at its octahedral lattice sites. Further titanium excess results in the formation of niobium-doped TiO₂ plates, which overgrow the surface of the Nb₃O₇(OH) superstructures. Photoluminescence spectroscopy indicates fewer charge recombination processes near the surface of the nanostructures with an increasing titanium concentration in the crystal lattice. The combination of larger surface areas with fewer quenching sites at the crystal surface yields higher H₂ evolution rates for the doped samples, with the rate being doubled by incorporation of 5.5 ± 0.7 at. % Ti

Model for Hydrothermal Growth of Rutile Wires and the Associated Development of Defect Structures

Crystal defects play a major role in determining the electrical properties of semiconductors. Hydrothermally grown TiO₂ rutile nanowire arrays are frequently used as electrodes in photovoltaic devices. However, they exhibit a characteristic defect structure that may compromise performance. A detailed scanning and transmission electron microscopy study of these defects reveals their internal structure and is suggestive at their origin. We propose an anisotropic layer-by-layer growth model, which combined with steric effects and Coulombic repulsion on high atom-density facets, can explain the observed V-shaped defect cascade in the nanowires

Unravelling Kinetic and Thermodynamic Effects on the Growth of Gold Nanoplates by Liquid Transmission Electron Microscopy

The growth of colloidal nanoparticles is simultaneously driven by kinetic and thermodynamic effects that are difficult to distinguish. We have exploited in situ scanning transmission electron microscopy in liquid to study the growth of Au nanoplates by radiolysis and unravel the mechanisms influencing their formation and shape. The electron dose provides a straightforward control of the growth rate that allows quantifying the kinetic effects on the planar nanoparticles formation. Indeed, we demonstrate that the surface-reaction rate per unit area has the same dose-rate dependent behavior than the concentration of reducing agents in the liquid cell. Interestingly, we also determine a critical supply rate of gold monomers for nanoparticle faceting, corresponding to three layers per second, above which the formation of nanoplates is not possible because the growth is then dominated by kinetic effects. At lower electron dose, the growth is driven by thermodynamic and the formation and shape of nanoplates are directly related to the twin-planes formed during the growth