Atomic Layer Deposition to Improve the Attachment of Molecular Chromophores and Catalysts to Mesoporous Inorganic Photoelectrodes for Solar Fuels Production

Oral, Mark Losego, presentation 014
Publication date: 31st March 2013

This talk will describe ongoing collaborations between North Carolina State University and the University of North Carolina to use atomic layer deposition (ALD) to improve the attachment of molecular modifiers, like organic dyes and catalysts, to inorganic mesoporous photoelectrochemical electrodes used in aqueous applications, including solar water splitting.  Commercialization of dye-sensitized solar cells is possible because challenges to long-term stability have been overcome.  However, using molecular modifiers in aqueous environments poses an even greater challenge because of the anchor group chemistry’s susceptibility to hydrolysis and desorption.  Illumination, pH, and electrical current flow necessary for device operation further enhance rates of molecular detachment.  Recently [1], we have shown that ultrathin (< 1 nm) ALD coatings of inorganic oxides improve molecular attachment by orders of magnitude in photoelectrochemical devices under operating conditions.  Fig. 1A summarize our general approach to using ALD to improve the durability of these molecular modifiers.  In one examples, we used ALD to protect phosphonate anchor chemistries for a dye molecule and demonstrated an order of magnitude decrease in desorption rate at pH = 7 (3.2 x 10^-5 s^-1 for ALD protected dyes versus >30 x 10^-5 s^-1 for unprotected dyes).  Similar improvements are also shown for dyes with carboxylate binding chemistry.  We use in-situ FTIR measurements during ALD deposition to understand how these vapor-phase treatments affect molecular chemistry and surface binding (Fig. 1B).  Ultrafast spectroscopies evaluate changes in electron transfer kinetics (Fig. 1C), and electrochemical measurements are used to evaluate device performance and longevity.  This talk will summarize our findings for Al₂O₃, TiO₂, and bilayer ALD structures used in stabilizing various Ru complexes, including molecular dyes for photoelectrochemical cells and molecular catalysts for water electrolyzers.  In general, we observe Al₂O₃ ALD layers to deleteriously impede electron transfer kinetics but provide excellent stability over a large range of conditions. TiO₂ ALD layers improve electron transfer kinetics but provide less stability.  Ultimately, ALD treatments appear very promising for resolving the fundamental challenge of attachment in molecularly modified, aqueous-based photoelectrochemical systems, and this talk will outline our current state-of-the-art scientific understanding of this process as well as describe pathways for future technology development.

Fig. 1: (A) Schematic diagram showing how ALD layers are used to encapsulate molecular anchor chemistries and improve attachment durability; (B) In-situ FTIR characterization of Al2O3 ALD on carboxylate bound dyes suggesting increased bidentate binding of anchor chemistry; (C) Transient absorption data from excited state dye molecules adsorbed to mesoporous TiO2 with 0, 1, 2, 3, 5, and 10 cycles of Al2O3 ALD illustrating a reduction in back electron transfer rate.
[1] Hanson, K.; Losego, M. D.; Kalanyan, B.; Ashford, D. L.; Parsons, G. N.; Meyer, T. J. Stabilization of [Ru(bpy)2(4,4’-(PO3H2)bpy)]2+ on Mesoporous TiO2 with Atomic Layer Deposition of Al2O3. Chem. Mater. 2013, 25, 3-5.

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