VOx@ graphene electro-catalysts for water splitting and dopamine sensing

Vanadium oxide@graphene (VOG) composites were synthesized using facile hydrothermal method by varying the synthesis time from 6 to 24 h. The samples with best catalytic characteristics were reprepared by replacing graphene oxide with N-doped graphene in the initial hydrothermal reaction. The role of graphene/N-doped graphene in determining the catalytic properties of vanadium oxide was effectively studied using various characterizations viz. X-ray diffraction, Fourier Transform Infrared spectroscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. Maximum hydrogen evolution reaction (HER) activity with high current density of –31.07 mA/cm2 (Tafel slope = 162 mVdec−1) has been achieved for VONG-24 sample, indicating Volmer reaction as rate determining step. The high electrochemical surface area (ECSA) of VOG-24 sample in 1 M KOH corroborated the enhanced current density of 144.90 mA/cm2 (Tafel slope = 83 mVdec−1) for maximum oxygen evolution reaction (OER) activity. The VOG12/GCE sample with a 0.29 μM limit of detection (LOD) proved to be the best electrochemical sensor for dopamine (DA) in a wide linear detection range i.e., 1–150 μM. Both catalytic as well as sensor responses of VOG samples were deeply influenced by the amount of O-species along with relative ratios of the V3+, V4+ and V5+. The nature of the graphene in the initial mixture emerged as a crucial parameter in tailoring the vanadium valence state content in the composites.