Nanocrystals as Precursors in Solid-State Reactions for Size- and Shape-Controlled Polyelemental Nanomaterials

Solid-state reactions between micrometer-size powders are among the oldest, simplest, and still widely used methods for the fabrication of inorganic solids. These reactions are intrinsically slow because, although the precursorsare "well mixed" at the macroscale, they are highly inhomogeneous at the atomic level. Furthermore, their products are bulk powders that are not suitable for device integration. Herein, we substitute micrometersize particles with nanocrystals. Scaling down the size of the precursors reduces the reaction time and temperature. More importantly, the final products are nanocrystals with controlled size and shape that can be used as active materials in various applications, including electro- and photocatalysis. The assembly of the nanocrystal precursors as ordered close-packed superlattices enables microscopy studies that deepen the understanding of the solid-state reaction mechanism. We learn that having only one of the two nanocrystal precursors dissolving and diffusing toward the other is crucial to obtain a final nanocrystalline product with homogeneous size and shape. The latter are regulated by the nanocrystal precursor that is the most stable at the reaction temperature. Considering the variety of controlled nanocrystals available, our findings open a new avenue for the synthesis of functional and tunable polyelemental nanomaterials.

Synthesis of metal oxide photoanodes for water splitting using nanocrystals as precursors

Chethana Janardhana Gadiyar

Photoelectrochemical (PEC) water splitting devices are envisioned to play a key role in the societal transition towards sustainable energy sources. In this context, multinary metal oxide semiconductors are the most promising candidates for the role of photoanodes to drive the water oxidation reaction. Nevertheless, the ideal material has yet to be found. Furthermore, when aiming at maximizing PEC performance, the ability to tailor-make material platforms with tunable morphological characteristics in an unrestricted compositional range is critical for achieving optimal design specifications, as well as for understanding the sensitivities of performance parameters to different compositional and structural parameters. This thesis focuses on the mechanistic studies and development of synthesis routes for ternary metal oxides as photoanodes for the water splitting reaction. A nanocrystal (NC) seeded-growth approach is introduced and demonstrated to attain a material tunability not always accessible via other synthesis techniques. In the first example, size-controlled NCs were reacted with a molecular precursor to synthesize ternary metal oxides. Specifically, surface oxidized Cu NC seeds with diameter changing from 6 nm to 70 nm were combined with vanadium acetylacetonate to form Cu2V2O7 . The seed size was found to regulate the grain size of the obtained ternary oxide, which was indeed obtained with tunable grain dimensions ranging from 29 nm to 63 nm. In-situ X-Ray diffraction studies explained this result as they evidenced the occurance of a solid state reaction between the NCs and the reacted molecular precursor upon annealing. This achieved tunability allowed us to correlate the PEC performance to the grain dimensions in a size regime close to the charge carrier diffusion length of Cu2V2O7 (20 - 40 nm). Moving forward, the nanocrystal seeded growth approach was further extended to Cu2V2O7/WO3 heterostructured nanocomposites. Here, Cu and WO3 NCs were combined in different ratio and simultaneously reacted with vanadium acetylacetonate. The result was a material platform with tunable composition and mesostructure which helped us to identify the optimal conditions to achieve better charge extraction at the interfacial region, which led to improved PEC performance. Finally, the molecular precursor was substituted with a second NC reactant and solid state chemistry between NCs was explored in the general framework of copper based ternary metal oxides. Scaling down the size of the reactants compared to traditional solid state chemistry among powders reduces the reaction time and temperature. More importantly, the ternary metal oxide products were NCs with tunable size and shape, a control not straightforward via other approaches. Binary superlattices of Cu and Fe3O4 NCs were used as a platform to monitor the reaction mechanism by the combination of various electron microscopy techniques. This study showed that having only one of the two NC precursor dissolving and diffusing toward the other is crucial to obtain a final nanocrystalline product with homogeneous size and shape. The latter are regulated by the NC precursor which is the most stable at the reaction temperature. Considering the variety of controlled NCs available, our findings open up a new avenue for the synthesis of functional and tunable polyelemental nanomaterials.

EPFL2020

Colloidal Nanocrystals as Precursors and Intermediates in Solid State Reactions for Multinary Oxide Nanomaterials

Raffaella Buonsanti, Anna Loiudice, Valeria Mantella

Polyelemental compounds with dimensions in the nanosized regime are desirable in a large variety of applications, yet their synthesis remains a general challenge in chemistry. One of the major bottlenecks to obtaining multinary systems is the complexity of the synthesis itself. As the number of elements to include in one single nano-object increases, different chemical interactions arise during nudeation and growth, thus challenging the formation of the targeted product. Choosing the reaction conditions and identifying the parameters which ensure the desired reaction pathway are of the uttermost importance. When, in addition to composition, the simultaneous control of size and shape is sought after, the development of new synthetic strategies guided by the fundamental understanding of the formation mechanisms becomes crucial. In this Account we discuss the use of colloidal chemistry to target multinary oxide nanomaterials, with focus on light absorbers which can drive chemical reactions. We propose the combination of soft and solid-state chemistries as one successful strategy to target this family of polyelemental compounds with control on composition and morphological features. To start with, we highlight studies where in situ forming nanoparticles act as reaction intermediates, which we found in both oxide (i.e., Bi-V-O) and sulfide (Cu-M-S, with M = V, Cr, Mn) nanocrystals (NCs). Examples of ternary sulfides are mentioned only with the purpose of showing that similar mechanisms can apply to different families of multinary nanomaterials. Using this new knowledge, we demonstrate that reacting pre-synthesized NCs with well-defined composition and size with molecular precursors allows significant control of these same property-dictating features (i.e., composition and grain size) in the resulting ternary and quaternary compounds. For example, nanostructured BiV1-xSbxO4 thin films with tunable composition and nanostructured beta-Cu2V2O7 with tunable grain size were accessed from colloidally synthesized Bi1-xSbxNCs (0 < x < 1) and sizecontrolled Cu NCs reacted with a vanadium molecular precursor, respectively. The analysis of reaction aliquots revealed that the formation of these materials occurs via a solid-state reaction between the NC precursors and V-containing amorphous nanoparticles, which form in situ from the molecular precursors. With the aim to achieve better control on the reaction product, we finally propose the use of colloidally synthesized NCs as reactants in solid state reactions. As the first proof of concept, ternary metal oxide NCs, including CuFe2O4, CuMn2O4, and CuGa2O4 with defined size and shape regulated by the NC precursors were obtained. Considering the huge library of single component and binary NCs accessible by colloidal chemistry, the extension of this synthetic concept, which combines soft and solid-state chemistries, to a larger variety of polyelemental nanomaterials is foreseen. Such an approach will contribute to facilitate a more rapid translation of design principles to materials with the desired composition and structural features.

AMER CHEMICAL SOC2021

Metal Clusters on Surfaces

Michael Hugentobler

The morphology, stability and reactivity of nanometre-sized particles (Au, Pt) is investigated on three different supports. Gold nanoparticles with a diameter comprised between 4 and 6nm are stabilized in nanosized pits of well defined depth in highly oriented pyrolytic graphite (HOPG). These pits are produced by creation of artificial defects, followed by etching under a controlled oxygen atmosphere. At low Au coverage, clusters are found on the edges of the hexagonal pits maximizing the contact to dangling bonds on graphite multisteps. Larger coverage results in Au beads of well defined shape and with a constant bead density per unit length. Most remarkable is the stability of these nanostructures under ambient conditions. Temperatures as high as 650 K do not alter the morphology of the gold clusters. Above 700 K, the gold clusters catalyze the etching of graphite layers when heated under atmospheric conditions. The depth of the etching channel is defined by the depth of the pit and the channel width can be controlled by the cluster size. Hydrogen, oxygen and water are dissociated on the Au clusters at low temperatures (> 400 K). CO and CO2 is produced above 700 K which confirms the etching of the multilayer graphene sheets. The so-called water-gas shift reaction has been observed where oxygen and water dissociate on the Au clusters at low temperatures. Thermal desorption spectroscopy (TDS) measurements on the Pt=YSZ system have been performed and the desorption peaks of CO and O2 have been evidenced. Carbon monoxide desorbs from the surface at a temperature of 535 K and oxygen at 705 K, in good agreement with values reported in literature. Catalytic measurements have confirmed the known strong catalytic activity of Pt. The CO oxidation takes place at temperatures between 450 K and 700 K. The adsorption and desorption temperature of the two educts have been confirmed during the catalysis measurements. Size-selected Aun clusters (n = 5, 7) are deposited from the gas phase at room temperature with well-controlled kinetic energy on rutile TiO2 . Two different surface reconstructions have been used as support: TiO2(110) – (1 × 1) and TiO2(110) – (1 × 2). Systematic studies of morphology, stability and adsorption sites have been performed using scanning tunnelling microscopy (STM). The mean size of the clusters is maintained during deposition at a kinetic energy of 7.1eV per atom, indicating that there is no fragmentation. Au clusters are not stable neither on the TiO2(110) – (1 × 1) nor on the TiO2(110) – (1 × 2) reconstruction during annealing of the substrate to 800 K. A progressive transition into a pronounced 3-d formation is observed. The size distribution of the particles is small and the growth mode (Ostwald ripening) independent of the surface reconstruction. The higher corrugation of the TiO2(110) – (1 × 2) reconstruction results in a pronounced stabilization of the clusters on terraces, in contrast to the TiO2(110) – (1 × 1) reconstruction where 90% of the clusters are found on step edges at elevated temperatures. The catalytic activity of the Au clusters sets in during the 2d - 3d transition. These measurements have to be confirmed and a full understanding of the process should be found.

EPFL2010