Structural Investigation of Cobalt Oxide Clusters Derived from Molecular Cobalt Cubane, Trimer, and Dimer Oligomers in a Phosphate Electrolyte Xiaobo Li, † Edwin B. Clatworthy, † Stuart Bartlett, † Anthony F. Masters, † and Thomas Maschmeyer* ,†,‡ † Laboratory of Advanced Catalysis for Sustainability, School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia ‡ Australian Institute of Nanoscale Science and Technology, The University of Sydney, Sydney, NSW 2006, Australia * S Supporting Information ABSTRACT: Cobalt oxide clusters were formed from three molecular cobalt oligomers, the Co-cubane, [Co 4 (μ 3 -O) 4 (μ-OAc) 4 (py) 4 ], Co-trimer, [Co 3 (μ 3 - O)(μ-OAc) 6 (py) 3 ][PF 6 ], and Co-dimer, [Co 2 (μ-OH) 2 (μ-OAc)(OAc) 2 (py) 4 ]- [PF 6 ], in phosphate buffer electrolyte after aging. Phosphate is essential for the formation of these cobalt oxide clusters. XAS characterization shows the cobalt oxide clusters are Co II/III O x clusters with Co II (O) 4 and Co III (O) 6 subunits. The cobalt oxide clusters formed have a structure similar to that of aged “CoPi” oxygen evolution catalysts prepared by electrodeposition. ■ INTRODUCTION Of all the proposed renewable energy sources, hydrogen obtained from photocatalytic water splitting is potentially the most sustainable. Water is the cheapest and most abundant hydrogen feedstock acting as both the fuel precursor and combustion product. 1,2 Water oxidation is arguably the most difficult half-reaction in water splitting as it requires the transfer of four oxidative equivalents to generate four protons and electrons and the formation of the molecular oxygen bond. A water oxidation catalyst (WOC) with a low overpotential is required to enable this reaction to proceed as efficiently as possible. Additionally, to be applicable at a large scale the WOC must avoid noble metal elements, such as iridium and ruthenium. Among the limited set of candidates, cobalt-based WOCs have attracted considerable attention. Nocera et al. demonstrated a low-overpotential heterogeneous cobalt oxide “CoPi” WOC under neutral conditions. 3,4 Molecular cobalt compounds with cubane structures analogous to the Mn 3 Ca(μ- O) 4 cluster of photosystem II have also been examined widely. 5-9 However, there is evidence showing that, in some cases, the real active WOC species are probably amorphous species generated by the transformation of the cobalt compounds. 10 In our previous study three cobalt molecular clusters, the Co-cubane, [Co 4 (μ 3 -O) 4 (μ-OAc) 4 (py) 4 ], Co- trimer, [Co 3 (μ 3 -O)(μ-OAc) 6 (py) 3 ][PF 6 ], and Co-dimer, [Co 2 (μ-OH) 2 (μ-OAc)(OAc) 2 (py) 4 ][PF 6 ], were investigated as water oxidation reaction (WOR) catalysts using electro- chemical, photochemical, and photoelectrochemical method- ologies in a phosphate buffer electrolyte. It was found that the species responsible for the water oxidation activity observed are derived from the transformation of these cobalt clusters into amorphous active clusters. 11 It is of fundamental interest to investigate the structural influence of the initial cobalt cluster size/nuclearity on the nature of the active amorphous species obtained, so as to better understand their structure and functionality. Although the structures of the cobalt WOCs have previously been studied, those studies concentrated on the cobalt WOCs from Co(II) precursors. 12,13 Herein we report that the stability of the initial molecular cobalt oligomers in phosphate buffer is highly dependent on their structure. The structure of the resulting amorphous cobalt oxide clusters has been established as cobalt oxide Co II/III O x clusters comprising Co II (O) 4 and Co III (O) 6 subunits. They have a structure similar to that of the CoPi with a reduced cobalt valence state after storage. The larger the initial molecular cobalt oligomer size, the larger the amorphous cobalt clusters. ■ RESULTS AND DISCUSSION The Co-cubane is soluble in aqueous solution, whereas the Co- trimer and Co-dimer are not. To investigate their electro- chemical behavior in water and the role of phosphate buffer, four different electrolytes were prepared: electrolyte I, CH 3 CN/H 2 O with 0.167 M Na 2 SO 4 (CH 3 CN/H 2 O = 1:2 (v/v)), pH 7.0; electrolyte II-Pi, CH 3 CN/H 2 O with 0.167 M Na 2 SO 4 /0.033 M KPi (K 2 HPO 4 + KH 2 PO 4 ) (CH 3 CN/H 2 O= Received: November 17, 2016 Revised: February 1, 2017 Published: May 3, 2017 Article pubs.acs.org/JPCC © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.6b11607 J. Phys. Chem. C XXXX, XXX, XXX-XXX