Towards optimizing cobalt based metal oxide nanocrystals for hydrogen generation via NaBH4 hydrolysis

Here, we evaluate H-2 generation, via NaBH4 hydrolysis, for a rationally designed library of cobalt based metal oxide nanocrystals. Specifically, we developed highly surface active wusite cobalt oxide nanorods (wz-CoO-NRs) with varied dimensions through the thermal decomposition of cobalt oleate (Co-OL) and cobalt stearate (Co-ST) - yielding multiple 'nail' structures grown along (002) direction. The length of wz-CoO-NRs (from 30 to 200 nm) was controlled by the ratio of Co-OL to Co-ST, temperature ramp rate, and reaction time. Further, by adjusting oleic acid concentration (as a surfactant) and/or (other) metal co-precursors (e.g. iron oleate (Fe-OL) and nickel oleate (Ni-OL)), shape (e.g. nanorod to urchin-like, irregular cube, flower, or sphere), crystal structure(s), and composition (from wz-CoO to cubic type CoO (cu-CoO) or ferrite metal oxides (Fe3O4, CoFe2O4, and Co0.5Ni0.5Fe2O4)) were serially tuned. The library of cobalt based metal oxide nanocrystals was compared, as catalysts, for H-2 generation via NaBH4 decomposition, and performance was observed to be a function of crystal length for wz-CoO-NRs. A maximum hydrogen production rate of 10,367 mL min(-1) g(-1) at 293 K with an activation energy of 27.4 kJ mol(-1) was observed for 200 nm wz-CoO-NRs, which is among the lowest reported activation energies for this reaction. Enhanced reactivity is likely due to increased favorable surface chemistries (multiple catalytically active (002) sites on the edge of multiple nails) and lowered charge transfer energy for longer (e.g. 200 nm) wz-CoO-NRs.