Exploration of anion effects in solvothermal synthesis using in situ X-ray diffraction.

Solvothermal synthesis presents a facile and highly flexible approach to chemical processing and it is widely used for preparation of micro- and nanosized inorganic materials. The large number of synthesis parameters in combination with the richness of inorganic chemistry means that it is difficult to predict or design synthesis outcomes, and it is demanding to uncover the effect of different parameters due to the sealed and complex nature of solvothermal reactors along with the time demands related to reactor cleaning, sample purification, and characterization. This study explores the effect on formation of crystalline products of six common anions in solvothermal treatment of aqueous and ethanolic precursors. Three different cations are included in the study (Mn2+, Co2+, Cu2+) representing chemical affinities towards different regions of the periodic table with respect to the hard soft acid base (HSAB) classification and the Goldschmidt classification. They additionally belong to the commonly used 3d transition metals and display a suitable variety in solvothermal chemistry to highlight anion effects. The results of the solvothermal in situ experiments demonstrate a clear effect of the precursor anions, with respect to whether crystallization occurs or not and the characteristics of the formed phases. Additionally, some of the anions are shown to be redox active and to influence the formation temperature of certain phases which in turn relates to the observed average crystallite sizes.

Unveiling the formation mechanism of PbxPdy intermetallic phases in solvothermal synthesis using in situ X-ray total scattering.

Pd possesses attractive catalytic properties and nano-structuring is an obvious way to enhance catalytic activity. Alloying Pd with Pb has been shown to enhance the catalytic effect of alcohol oxidation. Further optimization of the catalytic effect can be accomplished by controlling the particle size and key to this is understanding the formation mechanism. By monitoring solvothermal syntheses using in situ X-ray total scattering, this study unveils the formation mechanism of PbxPdy intermetallic nanoparticles. The formation occurs through a multi-step mechanism. Initially, Pd nanoparticles are formed, followed by incorporation of Pb into the Pd-structure, thus forming PbxPdy intermetallic nanoparticles. By varying the reaction time and temperature, the incorporation of Pb can be controlled, thereby tailoring the phase outcome. Based on the in situ solvothermal syntheses, ex situ autoclave syntheses were performed, resulting in the synthesis of Pb3Pd5 and Pb9Pd13 with a purity above 93%. The catalytic effect of these intermetallic phases towards the hydrogen evolution reaction (HER) is assessed. It is found that Pd, Pb3Pd5, and Pb9Pd13 have comparable stabilities, however, the overpotential increases with increasing amounts of Pb.

Composition space of PtIrPdRhRu high entropy alloy nanoparticles synthesized by solvothermal reactions.

High entropy alloy (HEA) nanoparticles hold promise in heterogeneous catalysis, and recently, simple and benign solvothermal synthesis was achieved for the equimolar PtIrPdRhRu. Here we experimentally explore the available composition space in this system, and we find that single-phase products can be obtained at significant deviations from the equimolar case.

Mapping the redox chemistry of common solvents in solvothermal synthesis through in situ X-ray diffraction.

Solvothermal technology shows great promise in "green" materials synthesis, processing, and recycling. The outcome of a specific solvothermal reaction depends strongly on the solvent properties, and the versatility of solvothermal synthesis hinges on the very large changes in solvent properties as a function of temperature and pressure. Here, six simple 3d transition metal nitrate salts (Cu(ii), Ni(ii), Co(ii), Fe(iii), Mn(ii), Cr(iii)) were dissolved in five common solvents (water, ethanol, ethylene glycol, glycerol, and 10% hydrogen peroxide solution) and heated stepwise up to 450 °C at a pressure of 250 bar using an in situ reactor while X-ray scattering data was recorded. A range of crystalline phases were observed in the form of metallic phases, metal oxides, and other ionic compounds. These data by themselves provide simple recipes for synthesis of many technologically important 3d transition metal nanomaterials. However, more generally the oxidation states of the metals in the synthesized materials can be used to map the solvent redox properties under solvothermal conditions. It is found that glycerol and ethylene glycol are strongly reducing, ethanol is moderately reducing, while water is weakly oxidizing. The behavior of the hydrogen peroxide solution is more complex including both oxidization and reduction. Furthermore, it is observed that the reducing powers of ethanol, ethylene glycol, and glycerol are enhanced with increasing temperature. The mapping of the redox properties of these common solvents provides a method for tailoring a given reaction through choice of solvent and reaction temperature. Solvothermal processes represent an environmentally benign alternative to the use of toxic reducing agents in chemical reactions, and quantification of the redox chemistry is a first step in rational materials design.

Time-resolved grazing-incidence pair distribution functions during deposition by radio-frequency magnetron sputtering.

Characterization of local order in thin films is challenging with pair distribution function (PDF) analysis because of the minute mass of the scattering material. Here, it is demonstrated that reliable high-energy grazing-incidence total X-ray scattering data can be obtained in situ during thin-film deposition by radio-frequency magnetron sputtering. A benchmark system of Pt was investigated in a novel sputtering chamber mounted on beamline P07-EH2 at the PETRA III synchrotron. Robust and high-quality PDFs can be obtained from films as thin as 3 nm and atomistic modelling of the PDFs with a time resolution of 0.5 s is possible. In this way, it was found that a polycrystalline Pt thin film deposits with random orientation at 8 W and 2 × 10-2 mbar at room temperature. From the PDF it was found that the coherent-scattering domains grow with time. While the first layers are formed with a small tensile strain this relaxes towards the bulk value with increasing film thickness.

The Chemistry of Nucleation: In Situ Pair Distribution Function Analysis of Secondary Building Units During UiO-66 MOF Formation.

The concept of secondary building units (SBUs) is central to all science on metal-organic frameworks (MOFs), and they are widely used to design new MOF materials. However, the presence of SBUs during MOF formation remains controversial, and the formation mechanism of MOFs remains unclear, due to limited information about the evolution of prenucleation cluster structures. Here in situ pair distribution function (PDF) analysis was used to probe UiO-66 formation under solvothermal conditions. The expected SBU-a hexanuclear zirconium cluster-is present in the metal salt precursor solution. Addition of organic ligands results in a disordered structure with correlations up to 23 Å, resembling crystalline UiO-66. Heating leads to fast cluster aggregation, and further growth and ordering results in the crystalline product. Thus, SBUs are present already at room temperature and act as building blocks for MOF formation. The proposed formation steps provide insight for further development of MOF synthesis.