(In,Ga)N alloys, which have been developed for light-emitting and laser diodes, are very promising materials for solar water splitting due to their large absorption coefficient and the possibility to tune the bandgap energy across the entire solar spectrum. However, it is difficult to grow (In,Ga)N bulk layers in high structural quality because of the lack of lattice-matched substrates. In contrast, nanowires (NWs) are known to accommodate lattice mismatch by lateral elastic relaxation without the formation of defects. Moreover, the NW geometry has many other advantages for solar water splitting such as enhanced light absorption, high surface area for electrochemical reactions, and improved carrier collection efficiency.
Our NWs were prepared by plasma-assisted molecular beam epitaxy on Si substrates without any catalyst. Samples were analyzed by electrochemical mass spectroscopy (EMS), which is a combination of a photoelectrochemical cell with a quadrupole mass spectrometer. This technique allows the real-time detection of volatile products like H2, N2 and O2 generated from (In,Ga)N NW electrodes during current-voltage measurements in an electrolyte. We found that n-(In,Ga)N NWs are prone to suffer from photocorrosion, while p-(In,Ga)N NWs showed a cathodic photocurrent under illumination which was correlated with the evolution of H2 as desired. Typically, semiconductor photoelectrodes are modified by electrocatalysts to overcome kinetic limitations of the electrochemical reaction. Indeed, after photodeposition of Pt on p-type NWs the photocurrent density was significantly enhanced to 5 mA/cm2 at a potential of -0.5 V/NHE under visible light irradiation of ~40 mW/cm2. Fig. 1 shows an exemplary photograph of the formation of H2 bubbles on the NWs. In further experiments, we measured incident-photon-to-current conversion efficiencies of around 40% at -0.45 V/NHE across the entire visible spectral region. Moreover, we verified by EMS the stability of the NW photocathodes for at least 60 min. In conclusion, we demonstrated stable hydrogen evolution using p-(In,Ga)N NW photocathodes, which paves the way for the development of a new class of photoelectrodes.
This work was carried out in collaboration with the Institute for Solar Fuels at Helmholtz-Zentrum Berlin.
|1||Author||J. Kamimura , P. Bogdanoff , J. Lähnemann , C. Hauswald , L. Geelhaar , S. Fiechter , H. Riechert|
Photoelectrochemical properties of (In,Ga)N nanowires for water splitting investigated by in situ electrochemical mass spectroscopy
|Source||J. Am. Chem. Soc. , 135 , 10242 ( 2013 )|