Enhancing the photoresponse of electrodeposited WO₃ film: Structure and thickness effect

Electrodeposited WO₃ thin films were prepared on the W, Ti, and Nb metal substrates in strongly acidic solution containing a tungsten precursor of (0.025 M sodium tungstate dihydrate powder (Na₂WO₄·2H₂O)) and by varying the applied potential. The applied potential determined the thickness and crystallite size of the deposited WO₃ thin films, irrespective of the metal substrate. The thickness and crystallite size of the films, as well as the total consumed electric charge (Q), increased as the applied potential was increased from -0.27 to -0.47 V. Conversely, the photoelectrochemical (PEC) activity declined as the deposition potential increased; the optimal performance was achieved at a deposition potential of -0.27 V for all metal substrates. This potential generated a porous WO₃ film or a very thin WO₃ layer composed of small nanoparticles, both of which were favorable for electrolyte penetration leading to enhanced charge transport/transfer behavior and providing a large contact area for the electrolyte. Furthermore, the PEC performance of WO₃ on the W substrate was higher than those on the Ti and Nb substrates because of the homogenous composition of the W substrate that resulted in the least lattice disturbance. Thus, the maximum photocurrent density of 1.68 mA/cm² at 1.5 V (vs. saturated Ag/AgCl) with an IPCE of 31% at 330 nm was obtained with the electrodeposited WO₃ film grown at a deposition potential of -0.27 V on the W substrate. The charge-transport and charge-transfer behavior of the electrodeposited WO₃ film were respectively discussed based on linear sweep voltammograms and electrochemical impedance spectroscopy.