Artificial Photosynthesis

Design and characterization of BGNSCs for solar water splitting

Nanorods of GZNO (Ga(1-x)ZnxN(1-x)Ox) with a wide range of Zn contents (0 < x < 0.60) were prepared with band gaps as low as 2.5 eV. For the first time, quantitative fitting of the optical response below the band gap has been achieved, allowing us to observe a response with an E^-3 scaling attributed to the scattering of free carriers from ionized defects. The absorbance from the free carriers varies strongly with Zn content in GZNO, suggesting that either the free carrier or impurity concentration is strongly composition dependent. This optical signature can now be utilized to guide the optimization of this semiconductor system, though further spectroscopic experiments are planned to deconvolute the response from free carriers and defects. Preliminary experiments find that the low-E absorbance is suppressed by sample annealing.

Our earlier TEM experiments have shown that hexagonal wurtzite GZNO can contain abundant intergrowths with a similar composition but with a different cubic zinc-blende type structure.  Using high resolution time-of-flight neutron diffraction data, we have been able to identify signatures of these defects in the bulk of GZNO samples through the use of Fourier difference maps to find minority (defect) sites influencing the diffraction pattern. These defect sites (shown as yellow in map) occur at two sets of sites related to the original cation (pink spheres) and anion (grey spheres) site positions by vectors of (1/3, 2/3, 0) and (2/3, 1/3, 0) in precisely the manner that is expected for stacking faults. Quantification of the stacking faults as a function of Zn content is being carried out in the neutron data presently, which should eventually enable the similar quantification of defects using laboratory XRD data.  This will enable direct testing of the influence of synthesis conditions on defect concentration, allowing the impact of defects on photoactivity to be determined. The effect of these defects on the band gap of GZNO semiconductors remains to be resolved.

Inorg. Chem. 2013, 52, 8389-8398, DOI: 10.1021/ic400011n