Spin Polarized Electron Dynamics Enhance Water Splitting Efficiency by Yttrium Iron Garnet Photoanodes: A New Platform for Spin Selective Photocatalysis

This work presents a spectroscopic and photocatalytic comparison of water splitting using yttrium iron garnet (Y3Fe5O12, YIG) and hematite (α-Fe2O3) photoanodes. Despite similar electronic structures, YIG significantly outperforms widely studied hematite, displaying more than an order of magnitude increase in photocurrent density and a factor of two increase in Faradaic efficiency. Probing the charge and spin dynamics by ultrafast, surface-sensitive XUV spectroscopy reveals that the enhanced performance arises from 1) reduced polaron formation in YIG compared to hematite and 2) an intrinsic spin polarization of catalytic photocurrents in YIG. Ultrafast XUV measurements show a reduction in the formation of surface electron polarons compared to hematite due to site-dependent electronphonon coupling. This leads to spin polarized photocurrents in YIG where efficient charge separation occurs on the Td sub-lattice compared to fast trapping and electron/hole pair recombination on the Oh sub-lattice. These lattice-dependent dynamics result in a long-lived spin aligned hole population at the YIG surface, which is directly observed using XUV magnetic circular dichroism. Comparison of the Fe M2,3 and O L1-edges show that spin aligned holes are hybridized between O 2p and Fe 3d valence band states, and these holes are responsible for highly efficient, spin selective water oxidation by YIG. Together, these results point to YIG as a new platform for highly efficient, spin selective photocatalysis.