Solid-state chemistry | EPFL Graph Search

Solid-state chemistry, also sometimes referred as materials chemistry, is the study of the synthesis, structure, and properties of solid phase materials. It therefore has a strong overlap with solid-state physics, mineralogy, crystallography, ceramics, metallurgy, thermodynamics, materials science and electronics with a focus on the synthesis of novel materials and their characterization. A diverse range of synthetic techniques, such as the ceramic method and chemical vapour depostion, make solid-state materials. Solids can be classified as crystalline or amorphous on basis of the nature of order present in the arrangement of their constituent particles. Their elemental compositions, microstructures, and physical properties can be characterized through a variety of analytical methods. History Because of its direct relevance to products of commerce, solid state inorganic chemistry has been strongly driven by technology. Progress in the field has often been fueled by the deman

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

A Material Approach to Dye-Sensitized Solar Cells

Philippe Pierre Labouchère

With growing concerns on future energy supplies, solar energy appears as an energy source whose potential remains to be tapped at a large scale. In the last two decades, dye-sensitized solar cells (DSCs) have been considered as a competitive means to convert solar energy into electricity thanks to their low manufacturing and environmental costs, simple fabrication and high efficiency. Indeed, record efficiencies as high as 13% have been obtained for liquid DSCs and recently, solid-state DSCs using solid hole transport materials as well as perovskite materials as sensitizers have showed efficiencies above 17%. From a physical point of view, charge carrier generation in a DSC takes place in a photoactive light harvester chemisorbed on a mesoscopic semiconductor oxide. Due to the excellent electronic overlap, the exciton separation occurs by the injection of the excited electrons and holes from the dye into the conduction band of the semiconductor and redox electrolyte, respectively. A high surface area mesoscopic photoanode is necessary to ensure a high-density bulk heterojunction and efficient light harvesting in the visible part of the solar spectrum due to effective dye loading. This implies that the DSC has a large, heterogeneous interface through which photogenerated electrons may be intercepted as a result of the slow electron transport. The latter is governed by an ambipolar diffusion mechanism controlled by trap-limited hopping through a long path to the transparent conductive electrode. It is however hindered by the low electron mobility in anatase TiO2 nanoparticles and the multiple grain boundaries in the mesoporous film. The work conducted during this thesis aimed at developing new architectures for the electrodes of DSCs using notably atomic layer deposition (ALD) and inverse opal host-guest structures with different semiconducting metal oxides. First, using a self-assembled polymeric template and ALD, we fabricated a porous ZnO inverse opal backbone. Advantages are numerous: a high electronic mobility of ZnO, a precise control of the metal oxide deposition using ALD and in situ growth of ZnO nanowires at low temperatures. Besides increasing the surface area available for photoactive dye molecules, single-crystal nanowires offer a defect-free path for photoinjected electrons. However, because of the relative instability of ZnO in acidic environments, a TiO2 overlayer was deposited by ALD. This proved to increase the recombination resistance of photogenerated electrons and increased their lifetime in the porous film. This host-guest approach was further transposed to three related fields in photo-electrochemistry using different ALD precursors. For water splitting, a niobium-doped tin oxide inverse opal backbone was constructed, which served as a host for hematite deposited by atmospheric pressure chemical vapour deposition or ALD. In the field of plasmonics, a TiO2 inverse opal host was made to act as a scaffold for silver nanoparticles. For solid-state DSCs, we strived to achieve an ultra-thin ZnO inverse opal, which acted as a host for perovskite crystals. Finally, we studied the effect of very thin TiO2 and ZnO overlayers deposited by ALD on top of mesoporous ZnO and TiO2, respectively. We were able to accurately monitor the thickness of overlayers and study the effects on photovoltaic parameters such as transport and recombination rates of photogenerated electrons, photocurrent, photovoltage, fill factor and efficiency.

EPFL2014

Assessing the persistence of chalcogen bonds in solution with neural network potentials

Frédéric Célerse, Veronika Juraskova, Rubén Laplaza Solanas

Non-covalent bonding patterns are commonly harvested as a design principle in the field of catalysis, supramolecular chemistry, and functional materials to name a few. Yet, their computational description generally neglects finite temperature and environment effects, which promote competing interactions and alter their static gas-phase properties. Recently, neural network potentials (NNPs) trained on density functional theory (DFT) data have become increasingly popular to simulate molecular phenomena in condensed phase with an accuracy comparable to ab initio methods. To date, most applications have centered on solid-state materials or fairly simple molecules made of a limited number of elements. Herein, we focus on the persistence and strength of chalcogen bonds involving a benzotelluradiazole in condensed phase. While the tellurium-containing heteroaromatic molecules are known to exhibit pronounced interactions with anions and lone pairs of different atoms, the relevance of competing intermolecular interactions, notably with the solvent, is complicated to monitor experimentally but also challenging to model at an accurate electronic structure level. Here, we train direct and baselined NNPs to reproduce hybrid DFT energies and forces in order to identify what the most prevalent non-covalent interactions occurring in a solute-Cl−–THF mixture are. The simulations in explicit solvent highlight the clear competition with chalcogen bonds formed with the solvent and the short-range directionality of the interaction with direct consequences for the molecular properties in the solution. The comparison with other potentials (e.g., AMOEBA, direct NNP, and continuum solvent model) also demonstrates that baselined NNPs offer a reliable picture of the non-covalent interaction interplay occurring in solution.

2022