(Invited) Driving Metal Oxide Water Oxidation Catalyst By Visible Light Absorber Separated By an Ultrathin Proton Conducting Silica Membrane with Embedded Molecular Wires
Authors: Frei Heinz 
Published: January 2022

We are developing nanoscale photosynthetic assemblies for coupling the catalysis for CO2 reduction with the H2O oxidation half reaction, taking an inorganic molecular approach that utilizes heterobinuclear units such as ZrOCoII as light absorber coupled to metal oxide catalyst. Artificial nanoscale photosystems are inspired by natural photosynthesis, the only existing technology for producing energy-dense chemicals on the terawatt scale, which has as key design feature the closing of the cycle of water oxidation and formation of primary reduction intermediates on the shortest possible length scale, the nanometer scale, while separating the incompatible redox environments by an ultrathin membrane. To accomplish the photocatalytic cycle of CO2 reduction by H2O under membrane separation, we are exploring a Co oxide – silica core-shell nanotube geometry.1-3 The inside surface of the Co oxide nanotube core provides the catalytic sites for H2O oxidation, which are separated from light absorber and sites of CO2 reduction on the outside by an ultrathin (2 nm) dense phase silica layer. The latter functions as proton conducting, O2 impermeable membrane.4 Tight, molecular-level control of charge transfer between light absorber and Co oxide nanotube catalyst is accomplished by oligo-para(phenylene vinylene) molecules with 3 aryl units embedded in the silica shell.5 This approach addresses the requirements of robustness and tunability of the electronic properties of the photosystem components with the objective of converting the maximum fraction of the solar photon energy into chemical energy of the fuel.6 The core-shell nanotube geometry offers the opportunity of assembling macroscale arrays of enormous numbers of nanotubes, each operating as independent photosynthetic unit while at the same time providing separation of evolving O2 and reduced CO2 products on all length scales from nano to centimeters.3

Detailed characterization of charge transfer between light absorber and Co oxide catalyst across the silica membrane was investigated by transient optical absorption spectroscopy and photoelectro-chemical methods using nanotube, spherical, or planar morphology depending on the type of experiment. Ultrafast optical monitoring allowed us to detect transient positive charge (hole) on the silica embedded molecular wire and revealed very efficient, 255 ps transfer to the Co oxide catalyst.7 Short circuit current measurements upon visible light sensitized hole injection using sensitizers with different redox potentials showed that charge transfer is controlled by the HOMO and LUMO energetics of the silica embedded wire molecules.4,5 Synthetic methods were developed for the accurate selection of embedded molecular wire loading and tuning of the concentration. Electron transfer processes of visible light excited ZrOCo or TiOCo light absorbers coupled to silica embedded wires and Co oxide catalyst are being explored by transient optical spectroscopy and photoelectrochemical methods, and will be the main topic of the presentation.8

Time permitting, recent application of ultrathin silica membrane with embedded molecular wires for separating incompatible catalytic environments of electronically coupled inorganic and microbial components will also be discussed.9

[1] Kim, W.; Edri, E.; Frei, H. Acc. Chem. Res.
49, 1634 (2016)

[2] Kim, W.; McClure, B. A.; Edri, E.; Frei, H. Chem. Soc. Rev. 45, 3221 (2016)

[3] Edri, E.; Aloni, S.; Frei, H., ACS Nano, submitted

[4] Yuan, G.; Agiral, A.; Pellet, N.; Kim, W.; Frei, H. Faraday Discuss.
176, 233 (2014)

[5] Edri, E.; Frei, H. J. Phys. Chem. C
119, 28326 (2015)

[6] Frei, H. Curr.
Opin. Electrochem.
2, 128 (2017)

[7] Edri, E.; Cooper, J. K.; Sharp, I. D.; Guldi, D. M.; Frei, H. J. Am. Chem. Soc.
139, 5458 (2017)

[8] Katsoukis, G.; Frei, H., to be submitted

[9] Cornejo, J. A.; Sheng, H.; Edri, E.; Ajo-Franklin, C.; Frei, H., submitted