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.

In situ X-ray diffraction study of the solvothermal formation mechanism of gallium oxide nanoparticles.

Gallium oxides are of broad interest due to their wide band gaps and attractive photoelectric properties. Typically, the synthesis of gallium oxide nanoparticles is based on a combination of solvent-based methods and subsequent calcination, but detailed information about solvent based formation processes is lacking, and this limits the tailoring of materials. Here we have examined the formation mechanisms and crystal structure transformations of gallium oxides during solvothermal synthesis using in situ X-ray diffraction. γ-Ga2O3 readily forms over a wide range of conditions. In contrast, ß-Ga2O3 only forms at high temperatures (T > 300 °C), and it is always preceded by γ-Ga2O3, indicating that γ-Ga2O3 is a crucial part of the formation mechanism of ß-Ga2O3. The activation energy for formation of ß-Ga2O3 from γ-Ga2O3 is determined to be 90-100 kJ mol-1 in ethanol, water and aqueous NaOH based on kinetic modelling of phase fractions obtained from multi-temperature in situ X-ray diffraction data. At low temperatures GaOOH and Ga5O7OH form in aqueous solvent, but these phases are also obtained from γ-Ga2O3. Systematic exploration of synthesis parameters such as temperature, heating rate, solvent and reaction time reveal that they all affect the resulting product. In general, the solvent based reaction paths are different from reports on solid state calcination studies. This underlines that the solvent is an active part of the solvothermal reactions and to a high degree determines different formation mechanisms.

Zr4+ solution structures from pair distribution function analysis.

The structures of metal ions in solution constitute essential information for obtaining chemical insight spanning from catalytic reaction mechanisms to formation of functional nanomaterials. Here, we explore Zr4+ solution structures using X-ray pair distribution function (PDF) analysis across pH (0-14), concentrations (0.1-1.5 M), solvents (water, methanol, ethanol, acetonitrile) and metal sources (ZrCl4, ZrOCl2·8H2O, ZrO(NO3)2·xH2O). In water, [Zr4(OH)8(OH2)16]8+-tetramers are predominant, while non-aqueous solvents contain monomeric complexes. The PDF analysis also reveals second sphere coordination of chloride counter ions to the aqueous tetramers. The results are reproducible across data measured at three different beamlines at the PETRA-III and MAX IV synchrotron light sources.

Pair distribution function and 71Ga NMR study of aqueous Ga3+ complexes.

The atomic structures, and thereby the coordination chemistry, of metal ions in aqueous solution represent a cornerstone of chemistry, since they provide first steps in rationalizing generally observed chemical information. However, accurate structural information about metal ion solution species is often surprisingly scarce. Here, the atomic structures of Ga3+ ion complexes were determined directly in aqueous solutions across a wide range of pH, counter anions and concentrations by X-ray pair distribution function analysis and 71Ga NMR. At low pH (<2) octahedrally coordinated gallium dominates as either monomers with a high degree of solvent ordering or as Ga-dimers. At slightly higher pH (pH ≈ 2-3) a polyoxogallate structure is identified as either Ga30 or Ga32 in contradiction with the previously proposed Ga13 Keggin structures. At neutral and slightly higher pH nanosized GaOOH particles form, whereas for pH > 12 tetrahedrally coordinated gallium ions surrounded by ordered solvent are observed. The effects of varying either the concentration or counter anion were minimal. The present study provides the first comprehensive structural exploration of the aqueous chemistry of Ga3+ ions with atomic resolution, which is relevant for both semiconductor fabrication and medical applications.

Phase control for indium oxide nanoparticles.

The indium oxides, c-In2O3, h-In2O3, InOOH and In(OH)3, constitute an important class of wide band gap semiconductors. Synthesis of any indium oxide phase involves manoeuvring in a complex matrix of process parameters, and some phases are only obtained through controlled phase transformations. Considering the widespread use of indium oxide semiconductors it is restrictive that no coherent picture exists of the formation mechanisms of individual phases and phase transformations between them. Here we access the indium oxide system through solvothermal synthesis and/or powder calcinations, and we use in situ X-ray scattering in combination with thermal analysis to investigate the complex phase relations. This allows us to unravel synthesis pathways for the different indium oxide phases, and the insight is used to develop procedures for scalable continuous flow solvothermal synthesis. Direct formation of crystalline nanomaterials from precursor solutions was observed for In(OH)3, InOOH and cubic c-In2O3, while formation of hexagonal h-In2O3 requires thermal decomposition of InOOH. The in situ X-ray scattering data reveal new phase transformations from In(OH)3 to InOOH, and from InOOH to c-In2O3. Interestingly, solvothermal synthesis conditions facilitate different reactions mechanisms than dry powder calcinations, and both In(OH)3 and InOOH have different transformations under dry and wet conditions.

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.