Abstract
Amorphous thin film oxygen evolving catalysts, OECs, of first-row transition metals show promise to serve as self-assembling photoanode materials in solar-driven, photoelectrochemical `artificial leaf' devices. This report demonstrates the ability to use high-energy X-ray scattering and atomic pair distribution function analysis, PDF, to resolve structure in amorphous metal oxide catalyst films. The analysis is applied here to resolve domain structure differences induced by oxyanion substitution during the electrochemical assembly of amorphous cobalt oxide catalyst films, Co-OEC. PDF patterns for Co-OEC films formed using phosphate, Pi, methylphosphate, MPi, and borate, Bi, electrolyte buffers show that the resulting domains vary in size following the sequence Pi < MPi < Bi. The increases in domain size for CoMPi and CoBi were found to be correlated with increases in the contributions from bilayer and trilayer stacked domains having structures intermediate between those of the LiCoOO and CoO(OH) mineral forms. The lattice structures and offset stacking of adjacent layers in the partially stacked CoMPi and CoBi domains were best matched to those in the LiCoOO layered structure. The results demonstrate the ability of PDF analysis to elucidate features of domain size, structure, defect content and mesoscale organization for amorphous metal oxide catalysts that are not readily accessed by other X-ray techniques. PDF structure analysis is shown to provide a way to characterize domain structures in different forms of amorphous oxide catalysts, and hence provide an opportunity to investigate correlations between domain structure and catalytic activity.
Highlights
Solar hydrogen production from water has been recognized as an attractive process to produce carbon-neutral, renewable fuel, but its development requires cheap and efficient water oxidation catalysts
Cobalt-based amorphous oxide catalytic films formed in the presence of inorganic phosphate, CoPi, show remarkable robustness that is in part due to the biomimetic oxidative regeneration of the catalyst (Kanan & Nocera, 2008; Kanan et al, 2009; Lutterman et al, 2009; Ullman & Nocera, 2013)
Resolution of the chemistries underlying Co-OEC amorphous film assembly and mechanisms for catalysis are important for developing solar fuels technologies based on artificial photosynthesis, and have possible additional relevance to the assembly and function of the CaMn4Ox water-splitting catalyst cofactor in photosynthesis (Pace et al, 2012; Nocera, 2012; Symes et al, 2011; Swiegers et al, 2011; Esswein et al, 2011; Dau et al, 2010; Mattioli et al, 2013)
Summary
Solar hydrogen production from water has been recognized as an attractive process to produce carbon-neutral, renewable fuel, but its development requires cheap and efficient water oxidation catalysts.
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