One of the challenges facing the world is the need to fulfill the growing demand for energy via renewable resources while minimizing CO2 emissions. Photoelectrochemical water splitting into hydrogen and oxygen is one such system capable of renewable solar energy conversion. Previously, a photoanode composed of nanostructured SnO2 coated with the perylene diimide dye N,N′-bis(phosphonomethyl)-3,4,9,10-perylenediimide (PMPDI) plus photoelectrochemically deposited cobalt oxide (CoOy) was shown to photoelectrochemically oxidize water with 31 ± 7% Faradaic efficiency. A non-ideal part of that prior system is that the addition of the known CoOy water oxidation catalyst (WOC) resulted an increase in charge-carrier and thus in a reduction of the total photocurrent, rather than the anticipated increase. Herein we show deposition of an ultrathin alumina overlayer applied by atomic layer deposition (ALD) on the SnO2/PMPDI photoanode can improve the photoactivity and catalytic activity of the system; the addition a ca. 1-nm-thick AlOx layer deposited on a 4000 nm (i.e., 4 micron) thick mesoporous anode system can and does have a positive, 2.5-fold improvement in the steady-state photocurrent with 29 ± 9% Faradaic efficiency vs the control anode without alumina passivation by reducing charge-carrier recombination. Moreover, ALD-deposited AlOx layer does help support the understanding of the “anti-catalysis” of co-depositing a CoOy WOC on the SnO2/PMPDI DS-PECs—specifically the picture of direct CoOy–SnO2 contact-mediated recombination—but was unable to improve the photocurrent in a net SnO2/PMPDI/AlOx(/CoOy) system. We attribute the lack of a performance enhancement by CoOy WOC to incomplete coverage of each SnO2 nanoparticle by the AlOx. Overall, we find the addition of an optimized ultrathin AlOx layer (0.6 nm thick; deposited at 85 °C) improves the SnO2/PMPDI/AlOx system’s photoactivity by a factor of up to ca. 3-fold with reduced recombination. These results document that metal-oxide passivation by low-temperature ALD can be an effective strategy for improving the water oxidation performance of even nanostructured dye sensitized-photoelectrochemical cell. These results indicate that further optimizing the ALD protocol by tailoring it towards mesoporous structures merits experimental testing.

Ultrathin alumina passivation for improved photoelectrochemical water oxidation catalysis of tin oxide sensitized by a phosphonate-functionalized perylene diimide first without, and then with, CoOy

ACS Fall 2021

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