NiFe-LDH was synthesized by hydrothermal method and colloidal Au clusters were prepared in DMF as reported previously. Carbonate ions in NiFe-LDH were exchanged to perchlorate ions, and the perchlorate ion–intercalated sample was dispersed in formamide, resulting in formation of colloidal NiFe-NS solution. Au-cluster-loaded NiFe-NS (Au/NiFe-NS) was prepared by mixing colloidal solutions of the NiFe-NS and Au clusters. Scanning TEM measurements revealed that small Au clusters around 1.2 nm in diameter are homogeneously dispersed on LDH.
Fig. 1 shows iR-corrected polarization curves of pristine NiFe-NS, Au clusters and Au/NiFe-NS electrocatalysts in 1 M KOH aqueous solution. The Au/NiFe-NS exhibited much larger oxidation current at a lower potential than the pristine NiFe-NS whereas Au clusters showed negligible activity, indicating that catalytic activity of NiFe-NS was significantly enhanced through the Au-cluster loading. XPS and XAFS measurements elucidated that Fe sites of Au/NiFe-NS are in more reduced states compared to those in the pristine NiFe-NS, resulting from charge transfer from Au clusters to Fe sites. Since bond breaking between the Fe site and intermediates is a rate determining step, we then conclude that reduction of Fe ions contributes to weaken binding energy and enhance catalytic activities. We achieved enhancement of catalytic activities of NiFe-NS via modification of electronic states of Fe sites using charge transfer interaction induced by Au cluster loading.
Fig. 1 IR-corrected polarization curves for OER on pristine NiFe-NS, Au clusters and Au/NiFe-NS in 1 M KOH aqueous solution.