Colloidal Nanocrystals as Heterogeneous Catalysts for Electrochemical CO2 Conversions

In the past decade, atomically engineered nanomaterials with different sizes and exposed facets have been proven to be excellent model systems to advance catalytic studies. Colloidal chemistry is one of the most powerful wet-chemistry techniques to tailor-make nanomaterials, thus making colloidal nanocrystals an ideal playground to investigate structural and compositional reaction sensitivities but also to study degradation pathways. In this Perspective, we focus on colloidally synthesized nanocrystals as heterogeneous catalysts for the electrochemical CO2 reduction reaction. We discuss very recent studies from us and from others, which encourage the scientific community to explore the tunability offered by colloidal chemistry even further. For example, synergistic interactions of the metallic nanocrystal catalyst with domains of different chemical nature could be exploited with the aim of revealing new catalytic motifs that promote the activity, selectivity, and stability of electrocatalysts for CO2 conversion

Size, Shape, and Internal Atomic Ordering of Nanocrystals by Atomic Pair Distribution Functions: A Comparative Study of γ-Fe2O3 Nanosized Spheres and Tetrapods

Due to their limited length of structural coherence nanocrystalline materials show very diffuse powder X-ray diffraction patterns that are difficult to interpret unambiguously. We demonstrate that a combination of high-energy X-ray powder diffraction and atomic pair distribution function analysis can be used to both assess the geometry (i.e., size and shape) and determine the internal atomic ordering of nanocrystalline materials in a straightforward way. As an example we consider cubic γ-Fe2O3 nanosized crystals shaped as spheres and tetrapods

Formation and microscopic investigation of iron oxide aligned nanowires into polymeric nanocomposite films

We present a microscopic investigation of nanocomposite films of iron oxide (g-Fe2O3) magnetic nanowires (NWs) aligned into polymers, formed upon evaporation of solutions of acrylate polymer/magnetic nanoparticles under magnetic field (MF). The field causes the assembly of the g-Fe2O3 nanoparticles along the direction of the MF lines, resulting in magnetic NWs embedded throughout the entire volume of the polymer film. The scanning electron microscopy and the trans- mission electron microscopy studies show that the cylindrical-shaped NWs have � 15-lm average length and are isotropically distributed throughout the film. The study with the MF microscopy tech- nique not only proves that the composed NWs are magnetic but also makes possible the magnetic study of each individual NW in a nondestructive way. In this way it becomes possible for the localized study of the magnetic properties alteration after the binding of various molecules onto individual NWs, opening up the way of using these films in sensor devices applied in various fields ranging from biology to environmental purposes. Microsc. Res. Tech. 73:952-958, 2010. V C 2010 Wiley-Liss, Inc

Theory-guided enhancement of CO2 reduction to ethanol on Ag-Cu tandem catalysts via particle-size effects

In the CO2 reduction reaction, the design of electrocatalysts that selectively promote alcohols over hydrocarbons (e.g., ethanol over ethylene) hinges on the understanding of the pathways and specific sites that forms alcohols. Herein, theoretical considerations guide state-of-the-art synthesis of well-defined catalysts to show that higher selectivity toward ethanol is achieved on Cu(110) edge sites compared to Cu(100) terraces. Specifically, we study the catalytic behavior of Cu nano-cubes (Cucub) of different sizes in the framework of tandem catalysis with CO-producing Ag nanospheres. We predict and experimentally find that the smaller Cucub possess higher selectivity for ethanol in view of their larger edge-to-faces ratio and of the fact that ethylene is produced at terraces while ethanol is selectively produced at step edges. These results call for synthetic developments toward Cu nanostructures exposing only edge sites, such as hollow cubic nanocages, to further increase ethanol selectivity. More generally, this study encourages the application of well-defined nano catalysts as a bridge between theory and experiments in electrocatalysis.This work was financially supported by Gaznat S.A. J.R.P. acknowledges the H2020 Marie Curie Individual Fellowship grant SURFCAT with Agreement No. 837378. This publication was created as part of NCCR Catalysis, a National Centre of Competence in Research funded by the Swiss National Science Foundation. The theoretical effort was supported by Spanish MICIUN’s RTI2018-095460–B-I00, Ramón y Cajal RYC-2015-18996, and María de Maeztu MDM-2017-0767 Grants, and partly by Generalitat de Catalunya via 2017SGR13. M.J.K. and F.C.V. are thankful to Red Española de Supercomputación (RES) for supercomputing time at SCAYLE (Projects QS-2019-3-0018, QS-2019-2-0023, and QCM-2019-1-0034). The use of supercomputing facilities at SURFsara was sponsored by NWO Physical Sciences

Nonhydrolytic synthesis of high-quality anisotropically shaped brookite TiO2 nanocrystals.

A surfactant-assisted nonaqueous strategy, relying on high-temperature aminolysis of titanium carboxylate complexes, has been developed to access anisotropically shaped TiO2 nanocrystals selectively trapped in the metastable brookite phase. Judicious temporal manipulation of precursor supply to the reaction mixture enables systematic tuning of the nanostructure geometric features over an exceptionally wide dimensional range (30-200 nm). Such degree of control is rationalized within the frame of a self-regulated phase-changing seed-catalyzed mechanism, in which homogeneous nucleation, on one side, and heterogeneous nucleation/growth processes, on the other side, are properly balanced while switching from the anatase to the brookite structures, respectively, in a continuous unidirectional crystal development regime. The time variation of the chemical potential for the monomer species in the solution, the size dependence of thermodynamic structural stability of the involved titania polymorphs, and the reduced activation barrier for brookite nucleation onto initially formed anatase seeds play decisive roles in the crystal-phase- and shape-tailored growth of titania nanostructures by the present approach

Determination of surface properties of various substrates using TiO2 nanorod coatings with tunable characteristics

We present a novel approach to cover different substrates with thin light-sensitive layers that consist of organic-capped TiO2 nanorods (NRs). Such NR-based coatings exhibit an increasing initial hydrophobicity with increasing NR length, and they demonstrate a surface transition from this highly hydrophobic state to a highly hydrophilic one under selective UV–laser irradiation. This behaviour is reversed under long dark storage. Infrared spectroscopy measurements reveal that light-driven wettability changes are accompanied by a progressive hydroxylation of the TiO2 surface. The surfactant molecules that cover the NRs do not appear to suffer for any significant photocatalytic degradation

Dynamical formation of spatially localized arrays of aligned nanowires in plastic films with magnetic anisotropy.

We present a simple technique for magnetic-field-induced formation, assembling, and positioning of magnetic nanowires in a polymer film. Starting from a polymer/iron oxide nanoparticle casted solution that is allowed to dry along with the application of a weak magnetic field, nanocomposite films incorporating aligned nanocrystal-built nanowire arrays are obtained. The control of the dimensions of the nanowires and of their localization across the polymer matrix is achieved by varying the duration of the applied magnetic field, in combination with the evaporation dynamics. These multifunctional anisotropic free-standing nanocomposite films, which demonstrate high magnetic anisotropy, can be used in a wide field of technological applications, ranging from sensors to microfluidics and magnetic devices

Long-Range Exciton Diffusion in Two-Dimensional Assemblies of Cesium Lead Bromide Perovskite Nanocrystals

F\"orster Resonant Energy Transfer (FRET)-mediated exciton diffusion through artificial nanoscale building block assemblies could be used as a new optoelectronic design element to transport energy. However, so far nanocrystal (NC) systems supported only diffusion length of 30 nm, which are too small to be useful in devices. Here, we demonstrate a FRET-mediated exciton diffusion length of 200 nm with 0.5 cm2/s diffusivity through an ordered, two-dimensional assembly of cesium lead bromide perovskite nanocrystals (PNC). Exciton diffusion was directly measured via steady-state and time-resolved photoluminescence (PL) microscopy, with physical modeling providing deeper insight into the transport process. This exceptionally efficient exciton transport is facilitated by PNCs high PL quantum yield, large absorption cross-section, and high polarizability, together with minimal energetic and geometric disorder of the assembly. This FRET-mediated exciton diffusion length matches perovskites optical absorption depth, opening the possibility to design new optoelectronic device architectures with improved performances, and providing insight into the high conversion efficiencies of PNC-based optoelectronic devices

Architectural Control of Seeded-Grown Magnetic−Semicondutor Iron Oxide−TiO2 Nanorod Heterostructures: The Role of Seeds in Topology Selection

A colloidal nonaqueous approach to semiconductor−magnetic hybrid nanocrystals (HNCs) with selectable heterodimer topologies and tunable geometric parameters is demonstrated. Brookite TiO2 nanorods, distinguished by a curved shape-tapered profile with richly faceted terminations, are exploited as substrate seeds onto which a single spherical domain of inverse spinel iron oxide can be epitaxially grown at either one apex or any location along their longitudinal sidewalls in a hot surfactant environment. The topologically controlled arrangement of the component material lattices, the crystallographic relationships holding between them, and strain distribution across individual heterostructures have been studied by combining X-ray diffraction and absorption techniques with high-resolution transmission electron microscopy investigations. Supported by such structural knowledge, the synthetic achievements are interpreted within the frame of various mechanistic models offering complementary views of HNC formation..