Abstract
High energy X-ray diffraction is used to follow in situ the crystallization of a cobalt gallate from metallic gallium under solvothermal conditions in an aminoalcohol, revealing the formation and decay of transient metastable layered double hydroxides. Photocatalytic studies show the spinel product has activity as a water oxidation catalyst for oxygen evolution.
Highlights
Spinel oxides are an important family of materials studied for their electronic and magnetic properties and find use in many functional applications,[1] in particular catalysis[2] and photocatalysis.[3,4] The metal deficient spinel γ-Ga2O3 has been applied for the photocatalytic degradation of volatile aromatics[5] and mixed-metal gallium oxide spinels, such as ZnGa2O4, have been employed for the photocatalytic degradation of organic pollutants.[6]
Moorhouse et al recently reported a time-resolved in situ powder X-ray diffraction (XRD) study of Bi5Ti3Fe0.5Cr0.5O15 crystallization from a molten salt reaction using high energy X-rays.[7]
The material Co0.973(8)Ga1.767(8)O3.752(8) produced by this method was shown to be a largely inverse spinel with a composition and inversion parameter different to cobalt gallium oxides synthesized at high temperature, typically having a similar composition of CoGa2O4.21,22 To understand the formation mechanism of this spinel, time-resolved XRD experiments were carried out using the Oxford-Diamond In Situ Cell (ODISC)[23] on beamline I12 at the Diamond Light Source, UK,[24] that makes use of a large image plate detector to allow powder diffraction patterns to be taken in a few seconds with excellent d-spacing resolution
Summary
A similar reaction conducted using solely MEA as the solvent formed a cobalt gallium oxide spinel with onset of crystallization after around 150−175 min. In this case there was no formation of any crystalline intermediate (Figure 1c). The refined parameters are very similar to those reported for Co−Ga LDHs in the literature.[26,27] A further, shorter offline experiment aimed at isolating the first transient phase gave an LDH (present among other unidentified material(s)) that has a larger unit cell than the second LDH (see Figure S4), in agreement with the shift in d-spacing of the diffraction data observed in situ.
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