Subsistema Edificado

3.2.1.

Synthesis

3.2.1.1. Alpha- and beta-Ga2O3

A reference sample of -Ga2O3 was synthesised in two stages, using a procedure similar to that used by Lavalley et al.47 and Hou et al..52 Gallium nitrate hydrate (3.0 g, Aldrich, 99.9% metals basis) was dissolved in distilled water (50 ml), and concentrated aqueous ammonia, diluted 50% v/v with distilled water, was added until no further precipitation was observed. The solution was left to stand overnight, then a fine white precipitate was collected by vacuum filtration, washed with water and acetone and dried at 70 °C overnight. The product was identified by powder XRD as gallium oxyhydroxide,

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GaOOH.1 The GaOOH was heated at 500 °C for 4 hours at which point powder XRD showed a single phase of -Ga2O3.10

A reference sample of -Ga2O3 was obtained by heating gallium nitrate hydrate at 220 °C in air for 18 hours, followed by re-grinding and heating the product at 800 °C in air for 22 hours. The final product was identified by powder XRD as a single phase of

-Ga2O3.6

3.2.1.2. Gamma-Ga2O3

Samples of -Ga2O3 were synthesised by two methods.

The first was the solvothermal oxidation of metallic gallium, based on the method of Kim et al.,74 but optimised to give the most crystalline products at lower temperatures. All reactions between 12 and 72 hours in length and in either 2-aminoethanol (MEA) or diethanolamine (DEA) were found to produce -Ga2O3, as confirmed by laboratory powder XRD (see Section 3.3.2.1). Two samples, one highly crystalline and one nanocrystalline, were chosen for neutron scattering studies and were prepared as follows. Sample one: 0.3 g of Ga (Aldrich 99.99%) and 5 ml of 2-aminoethanol (Aldrich ≥ 99.0%) were placed in a Teflon-lined stainless steel autoclave which was sealed and transferred to a pre-heated fan oven at 240 °C for 72 hours. The solid product was dispersed into 10 ml of hot methanol, recovered by suction filtration, washed with further methanol and dried at 70 °C overnight. Sample two: 0.3 g of Ga (Aldrich 99.99%) and approximately 5 ml of diethanolamine (Aldrich ≥ 98.5%) were placed in a Teflon-lined stainless steel autoclave which was sealed and transferred to a pre-heated fan oven at 240 °C for 11 hours. The reaction mixture was diluted with 15 ml of

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methanol, filtered to remove unreacted gallium and the white solid product was isolated and washed via centrifugation, before being dried at 70 °C overnight.

The second method used to prepare -Ga2O3 was that of Arean et al..75 Gallium nitrate hydrate (3 g, Aldrich 99.9% metals basis) was dissolved in ethanol (50 ml). Concentrated aqueous ammonia, diluted 50% v/v with ethanol, was added to the gallium nitrate solution to achieve a pH of 9.0. The resulting precipitate was immediately filtered, washed with ethanol and dried at 70 °C for 12 hours before being calcined at 500 °C for 1 hour.

3.2.1.3. A novel gallium oxyhydroxide

Similar solvothermal reactions to those used for the preparation of -Ga2O3, but performed for longer periods of time, resulted in the formation of a novel crystalline phase, Ga5O7(OH), and optimal conditions for the preparation of this phase were found to be the use of 0.1 g Ga in a 1:7 water:diethanolamine mixture and a reaction time of 6 days at 240 °C.

3.2.1.4. Gallium nitrate nonahydrate

The as-purchased gallium nitrate hydrate (Aldrich, 99.9% metals basis) was examined by powder XRD and found to contain a secondary phase in addition to the expected Ga(NO3)3.9H2O whose structure was recently determined by Hendsbee et al..76 Several batches were purchased for testing and were always found to be the same.

An attempt to prepare phase pure Ga(NO3)3.9H2O using the method mentioned by Roy

et al.1 was made. Metallic gallium (0.21 g, Aldrich 99.99%) was added to distilled water (15 ml) and concentrated nitric acid (25 ml) and allowed to dissolve. The solution was heated to boiling until the volume had reduced by approximately 75% and was then re-

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diluted with distilled water. The heating and re-dilution procedure was repeated thrice more and then the solution was left to slowly evaporate. On standing for several weeks, large (1 – 3 mm), colourless, rhomboidal crystals were observed. These were examined using single crystal X-ray diffraction and were found to be isostructural with gallium “nanoclusters” of formula [Ga13(3-OH)6(2-OH)18(H2O)24](NO3)15 as described by Rather et al..77 Examination of powder XRD revealed that the nanoclusters were the secondary phase found in commercial gallium nitrate hydrate.

An in situ powder XRD examination of the thermal decomposition of commercial gallium nitrate hydrate confirmed the existence of these two phases by their different decomposition behaviours (Figure 3.3).

Figure 3.3 In black: thermodiffraction of the decomposition of commercial gallium nitrate hydrate. The red trace is the simulated pattern for Ga(NO3)3.9H2O and the blue trace is the simulated

pattern for the [Ga13(3-OH)6(2-OH)18(H2O)24](NO3)15 “nanoclusters”.

A phase pure sample of Ga(NO3)3.9H2O was obtained via recrystallisation. Gallium nitrate hydrate (18 g, Aldrich, 99.9% metals basis) was dissolved in distilled water (50

10 12 14 16 18 20 22 24 26 28 30 8.85 7.38 6.33 5.54 4.93 4.44 4.04 3.71 3.43 3.19 2.98 d-spacing / Å 50 °C D iffr a cte d in te n sity / a .u . Diffraction angle, 2/ ° 150 °C

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ml) followed by addition of 1 ml concentrated HNO3. The volume of the solution was reduced to approximately 3 ml on a rotary evaporator. The addition of water and HNO3 and subsequent evaporation was repeated twice and then crystallisation was induced via

agitation with a glass rod. For materials studied using neutron diffraction this proved to be a convenient method for preparing samples free of protons (thus reducing the problem of strong incoherent scattering from H in the measured data), by use of D2O (Aldrich, 99.9 atom % D) in place of H2O and inducing crystallisation within a nitrogen-filled glove-bag (Aldrich “AtmosBag”).

3.2.1.5. Delta- and epsilon-Ga2O3

Heating Ga(NO3)3.9H2O (or Ga(NO3)3.9D2O) at 220 °C for 12 hours in air produced a poorly crystalline material which is herein designated “-Ga2O3” as it would match the proposed phase of Roy et al.1 (see Figure 3.2). Prolonged heating at 400 °C produced a more crystalline phase which is herein designated -Ga2O3, in line with literature precedent.1

The synthesis of -Ga2O3 via the decomposition of nanostructured GaOOH formed in a microwave reaction in the presence of urea as a templating agent was recently reported by Ge et al..78 Attempts to replicate this synthesis under standard hydrothermal conditions were unsuccessful.

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