Enhanced photoelectrochemical charge transfer on Mn-doped CdS/TiO2 nanotube arrays: The roles of organic substrates

The application of photoelectrochemical (PEC) systems to harness solar energy while treating wastewater or biomass-laden streams is an emerging area of sustainability. On this basis, we investigate the Mn-doped CdS quantum dots supported on TiO2 nanotube arrays (TNT) as the visible light-responsive photoanode. The introduction of Mn-dopant (optimum Mn:Cd = 0.15) enhanced the photoelectron lifetime of the composite photoanode. This made available more photoholes for surface transfer, as reflected by the increase in photocurrent. The addition of organic compounds drastically enhanced the photocurrent, given their faster oxidation rates compared to water. Depending on the type of organic compounds, whether they are direct hole scavengers (formic acid, diethanolamine, triethanolamine) or hydroxyl radical scavengers (methanol, glucose, urea), tend to impart different extent of photocurrents enhancement. While the measured photocurrent was in most cases higher for Mn-doped CdS/TNT than the undoped composite, we found that direct hole scavengers were up to an order of magnitude more efficient than the hydroxyl radical scavengers in promoting surface holes transfer. Interestingly, the Mn-dopant was highly beneficial in improving the oxidation of hydroxyl radical scavengers, presumably due to the enhanced charge carrier lifetime that was necessary for more efficient production of hydroxyl radicals. For the first time, the current doubling-like effects were verified for compounds with alcohol functional groups through the measurements of incident photon-to-current conversion efficiencies (IPCE) above unity.