Quantum dot based heterostructures for unassisted photoelectrochemical hydrogen generation

Abstract

TiO 2 hollow nanowires (HNWs) and nanoparticles (NPs) constitute prom- ising architectures for QDs sensitized photoanodes for H 2 generation. We sensitize these structures with CdS/CdSe quantum dots by two different methods (chemical bath deposition, CBD and succesive ionic layer adsorp- tion and reaction, SILAR) and evaluate the performance of these photoelec- trodes. Remarkable photocurrents of 4 mA · cm and 8 mA · cm − 2 and hydrogen generation rates of 40 ml · cm − 2 · day − 1 and 80 ml · cm − 2 · day − 1 have been obtained in a three electrode confi guration with sacrifi cial hole scavengers (Na 2 S and Na 2 SO 3 ), for HNWs and NPs respectively, which is confi rmed through gas analysis. More importantly, autonomous generation of H 2 (20 ml · cm − 2 · day − 1 corresponding to 2 mA · cm − 2 photocurrent) is obtained in a two electrode confi guration at short circuit under 100 mW · cm − 2 illumina- tion, clearly showing that these photoanodes can produce hydrogen without the assistance of any external bias. To the best of the authors? knowledge, this is the highest unbiased solar H 2 generation rate reported for these of QDs based heterostructures. Impedance spectroscopy measurements show similar electron density of trap states below the TiO 2 conduction band while the recombination resistance was higher for HNWs, consistently with the much lower surface area compared to NPs. However, the conductivity of both structures is similar, in spite of the one dimensional character of HNWs, which leaves some room for improvement of these nanowired structures. The effect of the QDs deposition method is also evaluated. Both structures show remarkable stability without any appreciable photocurrent loss after 0.5 hour of operation. The fi ndings of this study constitute a relevant step towards the feasibility of hydrogen generation with wide bandgap semiconductors/ quantum dots based heterostructures.