Publication date: 31st March 2013
Developing visible light-driven systems using inexpensive semiconductor materials open new pathway to solar fuel research. In this context, semiconductor quantum dots (QDs) have emerged as an important class of energy harnessing material that offers great promise in photoelectrochemical hydrogen generation, without the assistance of any external bias[1]. The biggest advantage of quantum dots over molecular photosensitizers that comes into perspective is their tunable optical properties and surface chemistries. In this work we explore the the photoelectrocatalytic activity of QDs sensitizers at different TiO2 nanostructures such as hollow nanowires (HNWs) and nanoparticles (NPs). The hydrogen evolution capability of these systems has been evaluated and discussed. We sensitize TiO2 nanostructures with CdS/CdSe quantum dots by two different methods (chemical bath deposition, CBD and successive ionic layer adsorption and reaction, SILAR). Remarkable photocurrents of 4 mA·cm−2 and 8 mA·cm−2 and hydrogen generation rates of 40 ml·cm-2·day-1 and 80 ml·cm-2·day- have been obtained in a three electrode configuration with sacrificial hole scavengers (Na2S and Na2SO3), for HNWs and NPs respectively, which is confirmed through gas analysis. More importantly, autonomous generation of H2 (20 ml·cm-2·day-1 corresponding to 2 mA·cm−2 photocurrent) is obtained in a two electrode configuration at short circuit under 100 mW·cm-2 illumination, clearly showing that these photoanodes can produce hydrogen without the assistance of any external bias. Impedance spectroscopy measurements show similar electron density of trap states below the TiO2 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. Autonomous generation of H2 (20.1 ml·cm-2·day-1 corresponding to 2 mA·cm−2 photocurrent) is obtained for CdSe sensitized TiO2 heterostructure in a two electrode configuration at short circuit under illumination. This result clearly shows that TiO2/QDs heterostructured photoanodes can autonomously produce hydrogen without the assistance of any external bias[1].
[1] Rodenas, P.; Song, T.; Sudhagar, P.; Marzari, G.; Han, H.; Badia-Bou, L.; Gimenez, S.; Fabregat-Santiago, F.; Mora-Sero, I.; Bisquert, J.; Paik, U.; Kang, Y. S., Quantum Dot Based Heterostructures for Unassisted Photoelectrochemical Hydrogen Generation. Advanced Energy Materials 2013, 3, (2), 176-182.
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