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Viable water splitting systems by solar energy hold a catalogue of requirements, among others stability in aqueous solutions, high absorbance and energy conversion efficiency in the visible spectra. In this project, the theoretically suggested high potential of semiconducting transition metal dichalcogenides, such as just 1 nm thin membranes of MoS2, for solar water splitting will be explored. The expertise of the TU Munich group in advanced nanofabrication methods and optoelectronic as well as Raman experiments will be combined with the expertise of Dr. Joel Agers group at JCAP/LBNL in electrochemical and spectroscopy techniques. The goal of the project is to pave the way for on-chip solar hydrogen production in nanodevices made from 2D layered materials.
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Final report:
In the scope of this project, µ-Raman spectroscopy was successfully applied on mono- and few-layer MoS2 to study the impact of environment, e.g. on the charge carrier density [1] and to explore the photocatalytic stability of the atomistic thin membranes [2]. Such studies are of great interest, since it is predicted that transition metal dichalcogenides such as MoS2 feature high potential for application in solar hydrogen production.
In the course of the project it could be demonstrated that physisorbed ambient molecules act as molecular gates via charge transfer across the interface from MoS2 to the molecule, predominantly H2O and O2. This charge transfer effectively depletes the charge carrier density of the exfoliated MoS2 flakes that are intrinsically n-type doped. The steady state between laser induced desorption and physisorption of molecules is tunable by the power of the incoming light. The charge carrier density can be adjusted over more than two-orders of magnitude by such a photogating process without the need of electrical contacts.
The modification of the phonon modes energies in MoS2 for measurements in DI-water compared to vacuum could be explained in the same way by a change in the charge carrier density due to charge transfer from MoS2 to water molecules. In a comprehensive series of measurements we could establish that the basal planes of defect free MoS2 crystals completely immersed in DI-water can be treated as photocatalytical stable even for illumination by light with an energy larger than the fundamental band gap of MoS2 and also for extremely high irradiation dose (P ~ 10mW/µm2 equivalent to 107 suns). The catalytically active edges of MoS2 sheets, however, are degraded in the above mentioned photocatalytic conditions only for illumination with photon energies larger than the bandgap. The photostability was significantly enhanced for the edge sites by degassing the water end replacing the dissolved oxygen by inert nitrogen gas.
Interestingly, the oxidation time at the edges is about 1 min for bi- and multilayer and significantly longer for the monolayer with approx. 45 min using Di-water with O2 as electrolyte. In conclusion, MoS2 monolayer can be treated as photocatalytically stable in aqueous solutions under moderate irradiation intensities as typically used for solar driven water splitting. This study further emphasizes the great potential of MoS2 and other semiconducting transition metal dichalcogenides as materials for solar energy harvesting. The great success of the project substantiated by two publications in high impact journals is the result of intense interaction between the project partners together with an on-site visit of Dr. Ager at WSI and a research stay of several weeks of MSc Eric Parzinger (PhD candidate) at JCAP.
Publications:
[1] B. Miller, E. Parzinger, A. Vernickel, A. Holleitner, and U. Wurstbauer, Photogating of mono- and few-layer MoS2, Appl. Phys. Lett. 106, 122103 (2015).
[2] E. Parzinger, B. Miller, B. Blaschke, J. A. Garrido, J. W. Ager, A. Holleitner, and U. Wurstbauer, Photocatalytic Stability of Single- and Few-Layer MoS2, ACS Nano, 9(11), 11302 - 11309 (2015).
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