Thursday, 17 May 2018: 13:55
Room 612 (Washington State Convention Center)
Literature on the photoelectrochemical (PEC) activity of TiO2 electrodes modified by CdS and or CdSe depositions are mostly performed using electrolytes with sacrificial reagents. The overall performance of a PEC system can vary significantly simply due to the use of different electrolytes, especially those with and without sacrificial reagents. In this paper, we have used a well-known TiO2 nanotube structure modified by CdS/CdSe nanoparticles deposited by successive ionic layer adsorption and reaction (SILAR) method to highlight the difference in PEC activities arising from sulfite oxidation and water oxidation. Linear Sweep Voltammograms measured under a standard AM1.5G simulated spectra, 1 sun (100 mWcm-2) exhibited a photocurrent density of 0.92 mAcm-2 at 1.23 V vs. RHE for the TiO2/CdS/CdSe photoactive electrodes using an aqueous alkaline electrolyte (1 M NaOH) for which water oxidation occurs. However, for the similar photoanode an 8-fold higher photocurrent of 7.4 mAcm-2 (at 1.23 V vs. RHE) was observed using an Na2S/Na2SO3 aqueous electrolyte where sulfite oxidation was the dominant mechanism for the enhanced photocurrent. A maximum STH efficiency of 0.71% and water splitting efficiency of 12.4% was determined for water oxidation. The STH efficiency parameter is more relevant to water splitting systems and to report such device efficiencies 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 were found to improve under acidic conditions showing a slightly increased PEC activity and faster reaction kinetics.