Abstract
A major portion of the existing literature on TiO2 nanotubes modified by CdS and CdSe report an enhanced photoelectrochemical (PEC) performance using electrolytes with inorganic sacrificial electron donors where a dominant sulfite oxidation occurs. In this study, we distinguish the PEC performance arising entirely from water splitting to that originated by sulfite oxidation in the TiO2 nanotube system modified by CdS/CdSe nanoparticle depositions. The photocurrent density measured under the AM1.5G simulated spectra, 1 sun (100 mWcm−2) intensity for the TiO2 nanotubes modified by CdS/CdSe nanoparticles and using an aqueous alkaline electrolyte (1 M NaOH) was observed to be 0.92 mAcm−2 at 1.23 V vs. RHE which was 8-fold lower than in the case for sulfite oxidation (Jph/SO = 7.4 mAcm−2 at 1.23 V vs. RHE) using an Na2S/Na2SO3 aqueous electrolyte. A maximum STH efficiency of 0.71% and water splitting efficiency of 12.4% was determined for water oxidation using an aqueous alkaline electrolyte. To rationalize such device efficiency, we propose that the ‘water splitting efficiency’ parameter is much more meaningful since it correlates the PEC activity specifically to water splitting and not to other pathways that produce an artificially enhanced photocurrent through the use of sacrificial reagents. The incident photon-to-current conversion efficiency spectrum measured at 1.23 V (vs. RHE) bias for the TiO2 photoanodes modified by CdS/CdSe nanoparticles revealed that the current conversion efficiency is lower (∼17% or less) when absorption occurs solely from the external CdSe layers indicating higher recombination during charge transport. The increase of dark current observed in the linear sweep voltammetry plots for the voltage range of −0.25 V to +0.15 V vs. RHE was attributed to anodic dissolution of the photoanodes in aqueous electrolytes containing no sacrificial reagents. The photocurrents and onset potentials for the CdS/CdSe modified TiO2 photoanodes improved under acidic conditions showing a slightly increased PEC activity and faster reaction kinetics.
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