Linear sweep voltammetry recorded in 0.5 M H2SO4 with 5 mV s− 1 scan rate, showing the effect of varying (a) O2/Ar ratio with 30 mTorr pressure and 200 W power, (b) sputter pressure with 0.90 O2/Ar ratio and 200 W power, and (c, d) Tafel plots of (a, b) for electrocatalytic HER of WO3 thin films on Ni foam.