Changes in Photoanodes during Solar Water Oxidation, the Wet Part of Artificial Photosynthesis

Abstract

Combination of animate matter with inanimate matter poses a difficult task for materials engineers, but opens up new horizons for scientists. I present the case where the iron oxide photoanode for photoelectrochemical solar water splitting is functionalized with C-phycocyanin, a protein known from blue green algae which has a light harvesting complex - one important structural motif in natural photosynthesis. The low cost and abundant mineral has a number of positive properties that make it suitable as a photoanode in photoelectrochemical cells (PEC). However, some deficiencies in its electronic structure prevent it from outperforming its costly and less abundant competitors in terms of quantum efficiency and photocurrent. Further functionalization of hematite with phycocyanin, i.e. coating and covalent cross linking, causes a doubling of the photocurrent. Detailed structural and spectroscopic analysis shows that it is the chromophore porphyrin in the phycocyanin which survives harsh pH conditions and provides also a light funnelling effect, which in the end makes an enhanced photocurrent. Our operando combination of PEC water splitting with soft x-ray spectroscopy provided new insight in the interaction of DC bias and photo-excitation with iron oxide. Anodization of hematite at bias potentials from 300 to 700 mV during illumination forms at least two new pre-edge structures in the O1s NEXAFS spectra, which can be assigned to eg spin up symmetry transitions in the O2p CTB and in the Fe3d-type UHB. We find a parallel evolution of the relative spectral weight of these transitions with the photocurrent vs. DC bias, strongly supporting that two types of electron holes are contributing to the historically suggested two types of photocurrent in hematite. The experiment was carried out entirely in situ during x-ray spectroscopy and strongly supports previous speculations that the electronic structure of the surface, and not only the short hole diffusion length in the bulk, may impede the performance of hematite photo-anodes. Meanwhile, we have made progress with probing also bio-organic motifs and hybrids with spectroscopy methods. [1] D.K. Bora, A. Braun, E.C. Constable, “In rust we trust”. Hematite - the prospective inorganic backbone for artificial photosynthesis, Energy Environ. Sci.6, 407 (2013). [2] A. Braun, K. Sivula, D. K. Bora, J. Zhu, L. Zhang, M. Grätzel, J. Guo, E. C. Constable, Direct observation of two electron holes in hematite during photo-electrochemical water splitting, J. Phys. Chem. C 116 (23) 16870 (2012).