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

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.

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

Préparation et caractérisation de nanoparticules bimétalliques (Pt-Au) sur un substrat en diamant dopé au bore

Bahaa El Roustom

Using petrol as a main source of energy, during the 20th century, has caused considerable amounts of damage among which are atmospheric pollution and global warming. At the same time, many geologists and economists expect that the discovery rate of new petrol sources will not follow the market's demand causing a serious shortage of petrol in the near future. These alarming perspectives have motivated scientists around the world to develop new clean and renewable energy sources. It is in that objective that electrochemists intensified their research in order to develop the fuel cell technology. This latter consists in directly converting chemical energy into electricity. The main advantage of this technology over traditional energy production is that the fuel cell energy efficiency is Carnot cycle independent. The theoretical efficiency of such an energy source is nearly 90%. Direct Alcohol Fuel Cell (DAFC), fed by ethanol on one hand and by air (oxygen) in the other hand, is a very interesting option for such kind of energy production. Ethanol is a renewable energy source of vegetal origin whose CO2 production is consumed by plants during the cycle. Even though Pt has long been known to be a perfect catalyst for DAFC, it has some principal limitations, the most important ones being poisoning by intermediate products (OH on the cathodic side and CO on the anodic side) and a relatively high cost compared to other materials. These problems have pushed the experts in the field to focalize their research on the development of more suitable and better performing materials. Gold nanoparticles have shown a surprising catalytic activity, very different from the bulk, making them a preferential candidate to replace Pt. A system based on using two or more metals as catalysts (example Au-Pt) dispersed on an appropriate substrate seems to be also an interesting candidate to enhance the cell's efficiency. The substrate should be chemically inert, non-oxydable and should not present any interference with the electrochemical and electrocatalytic behaviour of the nanoparticles. In the first part, the employed substrate, which is boron doped diamond (BDD) is characterized. The substrate's main characteristics are its chemical inertness, its weak residual current and its resistance to corrosion. These characteristics contribute to making the substrate an ideal support for nanoparticles. In this work, the substrate is a diamond film, but for fuel cell applications diamond should be used as powder or dispersed particles. In chapters III and IV, we studied the effect of boron incorporation on the morphology and the behaviour of the diamond film. For this purpose, samples with different ratios of boron/carbon (B/C) in the vapour phase during preparation were characterized (B/C = 300, 1000, 2500 and 6000 ppm). Results showed that the grain size decreases with increasing boron content in the film. This is accompanied by a transition towards a quasi-metallic character. It was also found that graphitic impurities in the diamond film increase when the B/C ratio increases. We also established some graphitic impurities increase in the diamond film when the ratio of B/C increases. The identity of these impurities was established in chapter III as being surface redox couples (semi-quinone/semi-hydroquinone). Using cyclic voltametry we were able to observe a total elimination (B/C=300 ppm) or partial elimination (B/C=6000 ppm) of these surface redox couples. We also found in chapter IV that these surface redox couples are responsible of the electrochemical activity of the BDD electrode. BDD electrodes with different doping level B/C=300, 1000 and 6000 ppm) in the vapour phase are modified with IrO2 nanoparticles using the method of thermal decomposition. Our results showed that, at low potentials, the behaviour of the composite electrode IrO2/BDD with respect to the reaction of oxygen evolution is independent of the doping level. On the other hand, at high potentials, the weakly doped electrode (B/C=300 ppm) presents a supplementary overpotential due to the interface BDD-IrO2. An approach based on the double junction is developed in this work describing two distinct behaviours depending on the boron concentration in the diamond film. A new method for Au nanoparticles deposition on highly doped BDD electrodes (B/C= 2500 ppm) is presented in chapter VI. This method involves two steps: the first step consists on sputtering the equivalent of few nanolayers of Au on the diamond surface; the second step is a heat treatment in air at high temperatures. Several parameters that could influence the dispersion, the stability as well as the diameter of the Au nanoparticles are studied. In chapter VII, the behaviour of the composite electrode Au/BDD is studied in a solution containing redox couples in acid media. This electrode was simulated as an arrangement of active Au microelectrodes dispersed on a less active BDD substrate. The overall behaviour of the composite electrode (Au/BDD) has been related to the electrochemical rate constants on each material. In chapter VIII, another new synthesis method for a bimetallic system formed by Au and Pt nanoparticles dispersed on a BDD substrate is developed. This method consists in electrodepositing a small amount of Pt on a composite electrode Au/BDD. Our results showed that Pt was preferentially deposited on Au covering its whole surface. In fact, Au nanoparticles serve as germ of nucleation for Pt. In the following step, the bimetallic PtbAu/BDD is heated to 400°C in air to enhance the interaction between Au and Pt. The experiments showed that Au reappeared on the surface. The electrochemical behaviour of the bi-metallic electrode is characterized by a negative shift in the peak potential of reduction of Pt oxides. This is not the case of heterogeneous alloys obtained by the fusion of two metals at high temperatures. The composite electrode Au/BDD as well as the bi-metallic electrode Pt-Au/BDD are used for electrocatalytic applications. The studied reaction in chapter IX is the oxygen reduction reaction (ORR) in an acid medium. We found that the number of exchanged electrons increases with increasing the amount of Pt at the surface. Oxygen is reduced into H2O2 on the Au/BDD electrode and into H2O on the Pt-Au/BDD electrode. In chapter X we studied the reaction of oxidation of oxalic acid. The behaviour of both Au and Pt electrodes seemed very similar for this reaction. The bi-metallic electrode containing both Pt and Au on its surface has shown an intermediate behaviour between the behaviour of the two metals.

EPFL2006

Development of remote C-H bond functionalization of redox-active alcohol derivatives and oxidative transformations under photoredox catalysis

Alexandre Julien Florent Leclair

The development of radical-based remote C(sp3)-H functionalizations of redox-active alcohol derivatives, oxidative cycloaddition and rearrangement transformations under visible light photocatalysis constitute the basis of this thesis.The first chapter describes the exploration of several strategies to effect the Î²- and Î³-C(sp3)-H functionalization of alcohols under reductive conditions through the 1,5-hydrogen atom transfer of reactive radical intermediates. Two types of N-phthalimidoyl precursors were synthesized and used under metal and photoredox catalysis to effect a formal Î³-C(sp3)-H functionalization. The fragmentation of primary N-phthalimidoyl oxalates could be smoothly realized under photoredox catalysis. However, the generated alkoxycarbonyl radicals did not allow for a distal functionalization and instead could only be trapped by direct addition to electron-poor alkenes. More potent 1-alkoxy vinyl radical intermediates proved to be too unstable and no control of the fragmentation could be achieved. Two approaches were designed to pursue the Î²-C(sp3)-H functionalization of redox-active alcohols. An iron-catalyzed C-H azidation of N-acyloxy imidates was discovered and optimized to enable, after simple hydrolysis, the synthesis of valuable Î²-azido alcohols. N-phthalimidoyl carbonates were also investigated to this end, however the exceptional hydrogen abstracting properties of these electrophilic (alkoxycarbonyl)oxyl radicals could not be tempered and only decomposition to the parent alcohol was detected.The study of two photoredox catalyzed oxidative transformations is discussed in the second chapter. Building on the propensity of indoles to produce radical cation intermediates under photoinduced single electron oxidation, an innovative oxidative ring-expansion of 1-(indol-2-yl)cyclobutan-1-ols was developed. Under mild aerobic visible-light conditions, a series of 2,3,4,9-tetrahydro-1H-carbazol-1-ones were accessed with complete regioselectivity in the presence of a catalytic amount of mesityl acridinium salt. Gratifyingly, this reaction could be extended to benzo[b]thiophenes analogues. The synthetic potential of this transformation was demonstrated by achieving a total synthesis of (Â±)-uleine, using our methodology to furnish one of the key fragment. A series of experiments were performed to gain insights in the mechanism of this transformation. A complex cascade reaction was suggested, involving two successive oxidative 1,2-alkyl shifts and initiated by the formation of a radical cation. Next were started synthetic studies toward the total synthesis of (Â±)-decursivine, a moschamine-related indole alkaloid, with the aim of developing an intramolecular oxidative [3+2]-cycloaddition under photoredox catalysis. Several precursors were synthesized and attempted under a large variety of reaction conditions to effect the cycloaddition, unfortunately without success. At best, the dimerization of a transient phenolic radical was observed. Efforts to promote an intramolecular reaction remained unsuccessful.

EPFL2021