CuGaS2 and CuGaS2–ZnS porous layers from solution-processed nanocrystals

The manufacturing of semiconducting films using solution-based approaches is considered a low cost alternative to vacuum-based thin film deposition strategies. An additional advantage of solution processing methods is the possibility to control the layer nano/microstructure. Here, we detail the production of mesoporous CuGaS2 (CGS) and ZnS layers from spin-coating and subsequent cross-linking through chalcogen-chalcogen bonds of properly functionalized nanocrystals (NCs). We further produce NC-based porous CGS/ZnS bilayers and NC-based CGS–ZnS composite layers using the same strategy. Photoelectrochemical measurements are used to demonstrate the efficacy of porous layers, and particularly the CGS/ZnS bilayers, for improved current densities and photoresponses relative to denser films deposited from as-produced NCs.Peer ReviewedPostprint (published version

Influence of colloidal Au on the growth of ZnO nanostructures

Vapor-liquid-solid processes allow growing high-quality nanowires from a catalyst. An alternative to the conventional use of catalyst thin films, colloidal nanoparticles offer advantages not only in terms of cost, but also in terms of controlling the location, size, density, and morphology of the grown nanowires. In this work, we report on the influence of different parameters of a colloidal Au nanoparticle suspension on the catalyst-assisted growth of ZnO nanostructures by a vapor-transport method. Modifying colloid parameters such as solvent and concentration, and growth parameters such as temperature, pressure, and Ar gas flow, ZnO nanowires, nanosheets, nanotubes and branched-nanowires can be grown over silica on silicon and alumina substrates. High-resolution transmission electron microscopy reveals the high-crystal quality of the ZnO nanostructures obtained. The photoluminescence results show a predominant emission in the ultraviolet range corresponding to the exciton peak, and a very broad emission band in the visible range related to different defect recombination processes. The growth parameters and mechanisms that control the shape of the ZnO nanostructures are here analyzed and discussed. The ZnO-branched nanowires were grown spontaneously through catalyst migration. Furthermore, the substrate is shown to play a significant role in determining the diameters of the ZnO nanowires by affecting the surface mobility of the metal nanoparticles

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@a 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-Cox0y NPs with [Co]/[Mn] = 1 showed low overpotentials of 0.31 Vat(-3) mA.cm(-2) and a small Tafel slope of 52 mV.dec(-1) for ORR, and overpotentials of 0.31 V at 10 mAPeer ReviewedPostprint (author's final draft

NiSn bimetallic nanoparticles as stable electrocatalysts for methanol oxidation reaction

Nickel is an excellent alternative catalyst to high cost Pt and Pt-group metals as anode material in direct methanol fuel cells. However, nickel presents a relatively low stability under operation conditions, even in alkaline media. In this work, a synthetic route to produce bimetallic NiSn nanoparticles (NPs) with tuned composition is presented. Through co-reduction of the two metals in the presence of appropriate surfactants, 3–5¿nm NiSn NPs with tuned Ni/Sn ratios were produced. Such NPs were subsequently supported on carbon black and tested for methanol electro-oxidation in alkaline media. Among the different stoichiometries tested, the most Ni-rich alloy exhibited the highest electrocatalytic activity, with mass current density of 820¿mA¿mg-1 at 0.70¿V (vs. Hg/HgO). While this activity was comparable to that of pure nickel NPs, NiSn alloys showed highly improved stabilities over periods of 10,000¿s at 0.70¿V. We hypothesize this experimental fact to be associated to the collaborative oxidation of the byproducts of methanol which poison the Ni surface or to the prevention of the tight adsorption of these species on the Ni surface by modifying its surface chemistry or electronic density of states.Peer ReviewedPostprint (author's final draft

Superior methanol electrooxidation performance of (110)-faceted nickel polyhedral nanocrystals

We present the synthesis of (110)-faceted nickel polyhedral nanocrystals (NCs) and their characterization as electrocatalysts for the methanol oxidation reaction (MOR). Ni NCs were produced at 180 °C through the reduction in solution of a Ni salt. They were combined with carbon black and Nafion and deposited over glassy carbon to study their electrocatalytic properties. Electrodes based on (110)-faceted Ni NCs displayed a first order reaction with KOH in the concentration range from 0.1 M to 1.0 M. These electrodes were characterized by higher coverages of active species, but lower diffusion coefficients of the species limiting the reaction rate when compared with electrodes prepared from spherical Ni NCs. Overall, electrodes based on faceted Ni NCs displayed excellent performance with very high current densities, up to 61 mA cm-2, and unprecedented mass activities, up to 2 A mg-1, at 0.6 V vs. Hg/HgO in 1.0 M KOH containing 1.0 M methanol. These electrodes also displayed a notable stability. While they suffered an activity loss of ca. 30% during the first 10 000 s of operation, afterward activity stabilized at very high current densities, ~35 mA cm-2, and mass activities, ~1.2 A mg-1, with only a 0.5% decrease during operation from 20 000 to 30 000 s.Peer ReviewedPostprint (author's final draft

Metal oxide aerogels with controlled crystallinity and faceting from the epoxide-driven cross-linking of colloidal nanocrystals

We present a novel method to produce crystalline oxide aerogels which is based on the cross linking of preformed colloidal nanocrystals (NCs) triggered by propylene oxide (PO). Ceria and titania were used to illustrate this new approach. Ceria and titania colloidal NCs with tuned geometry and crystal facets were produced in solution from the decomposition of a suitable salt in the presence of oleylamine (OAm). The native surface ligands were replaced by amino acids, rendering the NCs colloidally stable in polar solvents. The NC colloidal solution was then gelled by adding PO, which gradually stripped the ligands from the NC surface, triggering a slow NC aggregation. NC-based metal oxide aerogels displayed both high surface areas and excellent crystallinity associated with the crystalline nature of the constituent building blocks, even without any annealing step. Such NC-based metal oxide aerogels showed higher thermal stability compared with aerogels directly produced from ionic precursors using conventional sol-gel chemistry strategies

NH3 sensing with self-assembled ZnO-nanowire μHP sensors in isothermal and temperature-pulsed mode

Dielectrophoretic alignment is found to be a simple and efficient method to deposit the solution prepared ZnO nanowires onto micro hot plate substrates. Due to the strong surface effects, positive temperature coefficient for resistance was encountered with ZnO nanowires in the high temperature range (>250 degrees C). The response to ammonia (NH3) was evaluated in isothermal and temperature-pulsed operation mode; the relative higher response observed in the latter case demonstrates that the use of this methodology is a good strategy to improve the performance of metal oxide sensors based on nanomaterials. Here, we evaluate the response to NH3 and qualitatively describe the sensing mechanism in temperature-pulsed mode, highlighting the main differences compared to the standard isothermal methodology

Surface chemistry and nano-/microstructure engineering on photocatalytic In2S3 nanocrystals

Colloidal nanocrystals (NCs) compete with molecular catalysts in the field of homogenous catalysis, offering an easier recyclability and a number of potentially advantageous functionalities, such as tunable band gaps, plasmonic properties or a magnetic moment. Using high throughput printing technologies, colloidal NCs can be also supported onto substrates to 2 produce cost-effective electronic, optoelectronic, electrocatalytic and sensing devices. For both catalytic and technological application, NCs surface chemistry and supracrystal organization are key parameters determining final performance. Here, we study the influence of the surface ligands and the NC organization on the catalytic properties of In2S3, both in colloidal form and as a supported layer. In colloidal form, NCs stabilized in solution by inorganic ligands show the highest photocatalytic activities, which we associate with their large and more accessible surfaces. On the other hand, when NCs are supported on a substrate, their organization becomes an essential parameter determining performance. For instance, NC-based films produced through a gelation process provided five-fold higher photocurrent densities than those obtained from dense films produced by the direct printing of NC

Scalable heating-up synthesis of monodisperse Cu2ZnSnS4 nanocrystals

Monodisperse Cu2ZnSnS4 (CZTS) nanocrystals (NCs), with quasi-spherical shape, were prepared by a facile, high-yield, scalable, and high-concentration heat-up procedure. The key parameters to minimize the NC size distribution were efficient mixing and heat transfer in the reaction mixture through intensive argon bubbling and improved control of the heating ramp stability. Optimized synthetic conditions allowed the production of several grams of highly monodisperse CZTS NCs per batch, with up to 5 wt % concentration in a crude solution and a yield above 90%

Tuning branching in ceria nanocrystals

Branched nanocrystals (NCs) enable high atomic surface exposure within a crystalline network that provides avenues for charge transport. This combination of properties makes branched NCs particularly suitable for a range of applications where both interaction with the media and charge transport are involved. Herein we report on the colloidal synthesis of branched ceria NCs by means of a ligand-mediated overgrowth mechanism. In particular, the differential coverage of oleic acid as an X-type ligand at ceria facets with different atomic density, atomic coordination deficiency, and oxygen vacancy density resulted in a preferential growth in the [111] direction and thus in the formation of ceria octapods. Alcohols, through an esterification alcoholysis reaction, promoted faster growth rates that translated into nanostructures with higher geometrical complexity, increasing the branch aspect ratio and triggering the formation of side branches. On the other hand, the presence of water resulted in a significant reduction of the growth rate, decreasing the reaction yield and eliminating side branching, which we associate to a blocking of the surface reaction sites or a displacement of the alcoholysis reaction. Overall, adjusting the amounts of each chemical, well-defined branched ceria NCs with tuned number, thickness, and length of branches and with overall size ranging from 5 to 45 nm could be produced. We further demonstrate that such branched ceria NCs are able to provide higher surface areas and related oxygen storage capacities (OSC) than quasi-spherical NCs