Synthesis of hexagonal WO3 nanocrystals with various morphologies and their enhanced electrocatalytic activities toward hydrogen evolution

Abstract The hydrogen evolution reaction (HER) achieved via electrochemical and photoelectrochemical measurements in an acid electrolyte (0.5 M H 2 SO 4 ) was examined on hexagonal WO 3 photoanodes of different morphological structures comprised of WO 3 nanotubes (NT), nanorods (NR) and nanospheres (NS). The WO 3 synthesized using free (NR) and inorganic templates (NS and NT) thru a new sonication/hydrothermal route was comprehensively characterized utilizing XRD, TEM-SAED, photoluminescence, UV–Vis diffuse reflectance, Raman, FTIR and N 2 adsorption techniques. Herein, the WO 3 NT with an average diameter of 2.9 nm and energy gap of 2.3 eV shows the best HER activity with an overpotential of −500 mV to offer a current density of 8.0 mA cm −2 that strikingly enhanced into 14 mA cm −2 ; at −410 mV, under visible light illumination (λ > 420). The electrochemical properties determined by cyclic voltammetry, EIS and Tafel plots argued that the activity of WO 3 NT is accelerated due to small size, enhanced wettability (WO 3 .H 2 O); so as to facilitating the reaction with the electrolyte, together with the 1D assisted electron transfer. It was also emphasized that the HER activity is mainly controlled by the high oxygen vacancies; emphasized via XPS technique, decreased resistances and crystal orientations when using WO 3 NTs rather than the high surface texturing properties devoted for the WO 3 NRs (S BET  = 54.3 m 2  g −1 , V p  = 0.084 cm 3  g −1 , pore radius = 6.5 nm). This wok provides a new approach for synthesizing a free Pt WO 3 NTs photo-electrocatalyst with excellent stability; towards a remarkable HER, as examined via chronopotentiometry technique for 100 h.