Aziz Genç

Polymer-Enhanced Stability of Inorganic Perovskite Nanocrystals and Their Application in Color Conversion LEDs.

Cesium lead halide (CsPbX3, X = Cl, Br, I) nanocrystals (NCs) offer exceptional optical properties for several potential applications but their implementation is hindered by a low chemical and structural stability and limited processability. In the present work, we developed a new method to efficiently coat CsPbX3 NCs, which resulted in their increased chemical and optical stability as well as processability. The method is based on the incorporation of poly(maleic anhydride-alt-1-octadecene) (PMA) into the synthesis of the perovskite NCs. The presence of PMA in the ligand shell stabilizes the NCs by tightening the ligand binding, limiting in this way the NC surface interaction with the surrounding media. We further show that these NCs can be embedded in self-standing silicone/glass plates as down-conversion filters for the fabrication of monochromatic green and white light emitting diodes (LEDs) with narrow bandwidths and appealing color characteristics.

High-performance thermoelectric nanocomposites from nanocrystal building blocks

The efficient conversion between thermal and electrical energy by means of durable, silent and scalable solid-state thermoelectric devices has been a long standing goal. While nanocrystalline materials have already led to substantially higher thermoelectric efficiencies, further improvements are expected to arise from precise chemical engineering of nanoscale building blocks and interfaces. Here we present a simple and versatile bottom–up strategy based on the assembly of colloidal nanocrystals to produce consolidated yet nanostructured thermoelectric materials. In the case study on the PbS–Ag system, Ag nanodomains not only contribute to block phonon propagation, but also provide electrons to the PbS host semiconductor and reduce the PbS intergrain energy barriers for charge transport. Thus, PbS–Ag nanocomposites exhibit reduced thermal conductivities and higher charge carrier concentrations and mobilities than PbS nanomaterial. Such improvements of the material transport properties provide thermoelectric figures of merit up to 1.7 at 850 K.

What do you do, titanium? Insight into the role of titanium oxide as a water oxidation promoter in hematite-based photoanodes

Hematite (α-Fe2O3) is a promising photoanode in solar water splitting devices with a set of intrinsic limitations that lessen its maximum performance; among the methods used for improving its photoactivity, titanium doping has witnessed an intensive research during recent years. However, the origin of the Ti-induced enhancement remains elusive to date with the lack of a systematic mechanistic study. In this contribution, we prepared mesoporous hematite (host)–titania (guest) composite films by mixing the respective preformed nanoparticles obtained by a non-aqueous sol–gel route in a wide range of loading levels (0–20 mol%) up to the solid state solubility limits of both components. Voltammetric and impedance measurements were performed observing an optimum 10% doping, with a 15-fold photocurrent increase (up to 1.3 mA cm−2 at 1.23 VRHE) and a 100-fold decrease in the charge transfer resistance. The roles of surface states and charge donor (dopant) densities were also assessed, assuming a charge transfer mechanism through hole trapping at surface states and its isoenergetic transfer to water; an optimum 10–15% doping range was obtained similarly to photocurrent, where the maximum overlapping between surface and water states is prevalent. Finally, HR-TEM and EELS measurements were employed to detect the presence of pseudobrookite and titania phases (20% doping), evincing that hematite–pseudobrookite heterojunctions have a beneficial cascade of charge transfers but titania-pseudobrookite heterojunctions depict a deleterious “hole mirror” mechanism that prevents water photooxidation. Tailoring the combined effect of donor states (conductivity), phase coexistence (solubility), heterojunctions (energetics) and surface states (kinetics) in the composite paves the way for understanding the mechanism of other dopant-induced changes and could be extended to further photoactive materials.

Solvothermal, Chloroalkoxide-based Synthesis of Monoclinic WO3 Quantum Dots and Gas-Sensing Enhancement by Surface Oxygen Vacancies

We report for the first time the synthesis of monoclinic WO3 quantum dots. A solvothermal processing at 250 °C in oleic acid of W chloroalkoxide solutions was employed. It was shown that the bulk monoclinic crystallographic phase is the stable one even for the nanosized regime (mean size 4 nm). The nanocrystals were characterized by X-ray diffraction, High resolution transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis, Fourier transform infrared and Raman spectroscopy. It was concluded that they were constituted by a core of monoclinic WO3, surface covered by unstable W(V) species, slowly oxidized upon standing in room conditions. The WO3 nanocrystals could be easily processed to prepare gas-sensing devices, without any phase transition up to at least 500 °C. The devices displayed remarkable response to both oxidizing (nitrogen dioxide) and reducing (ethanol) gases in concentrations ranging from 1 to 5 ppm and from 100 to 500 ppm, at low operating temperatures of 100 and 200 °C, respectively. The analysis of the electrical data showed that the nanocrystals were characterized by reduced surfaces, which enhanced both nitrogen dioxide adsorption and oxygen ionosorption, the latter resulting in enhanced ethanol decomposition kinetics.

Cu2ZnSnS4–Ag2S Nanoscale p–n Heterostructures as Sensitizers for Photoelectrochemical Water Splitting

A cation exchange-based route was used to produce Cu2ZnSnS4 (CZTS)-Ag2S nanoparticles with controlled composition. We report a detailed study of the formation of such CZTS-Ag2S nanoheterostructures and of their photocatalytic properties. When compared to pure CZTS, the use of nanoscale p-n heterostructures as light absorbers for photocatalytic water splitting provides superior photocurrents. We associate this experimental fact to a higher separation efficiency of the photogenerated electron-hole pairs. We believe this and other type-II nanoheterostructures will open the door to the use of CZTS, with excellent light absorption properties and made of abundant and environmental friendly elements, to the field of photocatalysis.

Co–Cu Nanoparticles: Synthesis by Galvanic Replacement and Phase Rearrangement during Catalytic Activation

The control of the phase distribution in multicomponent nanomaterials is critical to optimize their catalytic performance. In this direction, while impressive advances have been achieved in the past decade in the synthesis of multicomponent nanoparticles and nanocomposites, element rearrangement during catalyst activation has been frequently overseen. Here, we present a facile galvanic replacement-based procedure to synthesize Co@Cu nanoparticles with narrow size and composition distributions. We further characterize their phase arrangement before and after catalytic activation. When oxidized at 350 °C in air to remove organics, Co@Cu core-shell nanostructures oxidize to polycrystalline CuO-Co3O4 nanoparticles with randomly distributed CuO and Co3O4 crystallites. During a posterior reduction treatment in H2 atmosphere, Cu precipitates in a metallic core and Co migrates to the nanoparticle surface to form Cu@Co core-shell nanostructures. The catalytic behavior of such Cu@Co nanoparticles supported on mesoporous silica was further analyzed toward CO2 hydrogenation in real working conditions.

Mn3O4@CoMn2O4-CoxOy Nanoparticles: Partial Cation Exchange Synthesis and Electrocatalytic Properties toward the Oxygen Reduction and Evolution Reactions.

Mn3O4@CoMn2O4 nanoparticles (NPs) were produced at low temperature and ambient atmosphere using a one-pot two-step synthesis protocol involving the cation exchange of Mn by Co in preformed Mn3O4 NPs. Selecting the proper cobalt precursor, the nucleation of CoxOy crystallites at the Mn3O4@CoMn2O4 surface could be simultaneously promoted to form Mn3O4@CoMn2O4-CoxOy NPs. Such heterostructured NPs were investigated for oxygen reduction and evolution reactions (ORR, OER) in alkaline solution. Mn3O4@CoMn2O4-CoxOy NPs with [Co]/[Mn] = 1 showed low overpotentials of 0.31 V at -3 mA·cm(-2) and a small Tafel slope of 52 mV·dec(-1) for ORR, and overpotentials of 0.31 V at 10 mA·cm(-2) and a Tafel slope of 81 mV·dec(-1) for OER, thus outperforming commercial Pt-, IrO2-based and previously reported transition metal oxides. This cation-exchange-based synthesis protocol opens up a new approach to design novel heterostructured NPs as efficient nonprecious metal bifunctional oxygen catalysts.

Size and aspect ratio control of Pd2Sn nanorods and their water denitration properties

Monodisperse Pd2Sn nanorods with tuned size and aspect ratio were prepared by co-reduction of metal salts in the presence of trioctylphosphine, amine, and chloride ions. Asymmetric Pd2Sn nanostructures were achieved by the selective desorption of a surfactant mediated by chlorine ions. A preliminary evaluation of the geometry influence on catalytic properties evidenced Pd2Sn nanorods to have improved catalytic performance. In view of these results, Pd2Sn nanorods were also evaluated for water denitration.

Probing the surface reactivity of nanocrystals by the catalytic degradation of organic dyes: the effect of size, surface chemistry and composition

We herein present a comprehensive study on how the catalytic performance and reusability of Au nanocrystals (NCs) are affected by systematic variations of crystal size, surface coating and composition. The reductions of different organic dyes (4-nitrophenol, rhodamine B and methylene blue) by borohydride ions were used as model catalytic reactions. The catalytic performance of the Au NCs ranged between 3.6 to 110 nm was found to be dependent on crystal size, indicating that Au surface atoms have a distinct size-dependent reactivity in this reaction. Similarly, the catalytic performance was found to be strongly dependent on the nature of the coating molecule, obtaining lower catalytic activities for molecules strongly bound to the Au surface. Finally, the catalytic performance was found to be dependent on the chemical composition of the NC (Au, Ag, Pt) and the model dye used as a testing system, with the highest degradation rate found for methylene blue, followed by 4-nitrophenol and rhodamine B. We believe that this study provides a better understanding of the catalytic performance of Au NCs upon controlled modifications of the structural and morphological parameters, and a working environment that can be used to facilitate the selection of the optimum NC size, coating molecule and evaluation system for a particular study of interest.

Surface Modification of TiO2 Nanocrystals by WOx Coating or Wrapping: Solvothermal Synthesis and Enhanced Surface Chemistry

TiO2 anatase nanocrystals were prepared by solvothermal processing of Ti chloroalkoxide in oleic acid, in the presence of W chloroalkoxide, with W/Ti nominal atomic concentration (R(w)) ranging from 0.16 to 0.64. The as-prepared materials were heat-treated up to 500 °C for thermal stabilization and sensing device processing. For R(0.16), the as-prepared materials were constituted by an anatase core surface-modified by WO(x) monolayers. This structure persisted up to 500 °C, without any WO3 phase segregation. For R(w) up to R(0.64), the anatase core was initially wrapped by an amorphous WO(x) gel. Upon heat treatment, the WO(x) phase underwent structural reorganization, remaining amorphous up to 400 °C and forming tiny WO3 nanocrystals dispersed into the TiO2 host after heating at 500 °C, when part of tungsten also migrated into the TiO2 structure, resulting in structural and electrical modification of the anatase host. The ethanol sensing properties of the various materials were tested and compared with pure TiO2 and WO3 analogously prepared. They showed that even the simple surface modification of the TiO2 host resulted in a 3 orders of magnitude response improvement with respect to pure TiO2.