| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Energy Harvesting & Storage | Tuesday Sessions |
| Session EH-TuM |
| Session: | Surfaces & Interfaces for Solar Cells and Solar Fuels |
| Presenter: | Johanna Eichhorn, Lawrence Berkeley Lab, USA |
| Authors: | J. Eichhorn, Lawrence Berkeley Lab, USA J.K. Cooper, Lawrence Berkeley Lab, USA L.H. Hess, Lawrence Berkeley Lab, USA D. Ziegler, Scuba Probe Technologies LLC, USA D.M. Larson, Lawrence Berkeley Lab, USA M.K. Gilles, Lawrence Berkeley Lab, USA I.D. Sharp, Lawrence Berkeley Lab, USA F.M. Toma, Lawrence Berkeley Lab, USA |
| Correspondent: | Click to Email |
Photoelectrochemical water splitting is a promising route for efficient conversion of solar energy to chemical fuel. Among different photoelectrode materials, bismuth vanadate (BiVO4) is one of the most actively investigated oxide semiconductors due to its moderate bandgap, favorable conduction band position, and relatively long photocarrier lifetimes.[1] However, under relevant operating conditions, pristine BiVO4 thin films are subjected to degradation at the exposed surface facets. The degradation process in solution is accelerated by photoexcitation, which causes trapping of photogenerated holes at localized surface sites.[2] Therefore, developing approaches to stabilize these efficient semiconductor nanostructures requires a detailed understanding and control of charge separation, transport, and recombination mechanisms at their relevant length scales.
Here, we use photoconductive atomic force microscopy in combination with Kelvin probe force microscopy to correlate local surface morphology with generated photocurrent and contact potential difference maps at individual grain facets in polycrystalline BiVO4 films. Furthermore, we employ scanning transmission X-ray microscopy to trace the changes in local chemical structure and composition in pristine and photodegraded BiVO4.
The photocurrent and contact potential difference maps reveal the impact of different working conditions, such as bias voltage, excitation energy or excitation power, on the local charge carrier dynamics. Both for excitation above the bandgap (405 nm) and sub-bandgap illumination (532 nm), the photocurrent maps resolve the contributions from individual grains with nanometer spatial resolution. The photoelectrochemical performance of BiVO4 can be significantly enhanced by varying the oxygen vacancy defects and hydrogen impurities, through hydrogen annealing. Therefore, we compare the photocurrent generation of pristine and hydrogen annealed BiVO4. This careful analysis allows us to identify locally the charge transfer and loss mechanisms in these materials which ultimately contribute to desired photocurrent generation or undesired photocorrosion.[3]
[1] J. K. Cooper et al., The role of hydrogen in defining the n-type character of BiVO4 photoanodes, Chem. Mater. 2016.
[2] F. M. Toma et al., Mechanistic insights into chemical and photochemical transformations of bismuth vanadate photoanodes, Nat. Commun. 2016.
[3] J. Eichhorn et al., Nanoscale photocurrent imaging of monoclinic BiVO4 films via atomic force microscopy, 2016 (in preparation).