Production
of hydrogen at large scale requires development of non-noble,
inexpensive, and high-performing catalysts for constructing water-splitting
devices. Herein, we report the synthesis of Zn-doped NiO heterostructure
(ZnNiO) catalysts at room temperature via a coprecipitation method
followed by drying (at 80 °C, 6 h) and calcination at an elevated
temperature of 400 °C for 5 h under three distinct conditions,
namely, air, N2, and vacuum. The vacuum-synthesized catalyst
demonstrates a low overpotential of 88 mV at −10 mA cm–2 and a small Tafel slope of 73 mV dec–1 suggesting relatively higher charge transfer kinetics for hydrogen
evolution reactions (HER) compared with the specimens synthesized
under N2 or O2 atmosphere. It also demonstrates
an oxygen evolution (OER) overpotential of 260 mV at 10 mA cm–2 with a low Tafel slope of 63 mV dec–1. In a full-cell water-splitting device, the vacuum-synthesized ZnNiO
heterostructure demonstrates a cell voltage of 1.94 V at 50 mA cm–2 and shows remarkable stability over 24 h at a high
current density of 100 mA cm–2. It is also demonstrated
in this study that Zn-doping, surface, and interface engineering in
transition-metal oxides play a crucial role in efficient electrocatalytic
water splitting. Also, the results obtained from density functional
theory (DFT + U = 0–8 eV), where U is the on-site Coulomb repulsion parameter also known as Hubbard U, based electronic structure calculations confirm that
Zn doping constructively modifies the electronic structure, in both
the valence band and the conduction band, and found to be suitable
in tailoring the carrier’s effective masses of electrons and
holes. The decrease in electron’s effective masses together
with large differences between the effective masses of electrons and
holes is noticed, which is found to be mainly responsible for achieving
the best water-splitting performance from a 9% Zn-doped NiO sample
prepared under vacuum.