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Electrocatalytic and Optical Properties of Cobaloxime Catalysts Immobilized at a Surface-Grafted Polymer Interface Brian Lawrence Wadsworth, Anna Mary Beiler, Diana Khusnutdinova, Samuel I. Jacob, and Gary F. Moore ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02194 • Publication Date (Web): 14 Oct 2016 Downloaded from http://pubs.acs.org on October 17, 2016
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ACS Catalysis
Electrocatalytic and Optical Properties of Cobaloxime Catalysts Immobilized at a Surface-Grafted Polymer Interface Brian L. Wadsworth, Anna M. Beiler, Diana Khusnutdinova, Samuel I. Jacob and Gary F. Moore* School of Molecular Sciences and the Biodesign Institute Center for Applied Structural Discovery (CASD), Arizona State University, Tempe, AZ 85287-1604, United States ABSTRACT: We report on the electrocatalytic and optical properties of cobaloxime hydrogen-production catalysts assembled on a polymer-modified nanostructured indium tin oxide (nanoITO) electrode. The hybrid construct is assembled using built-in ligand sites (pyridyl groups) on the surface-attached polymer to direct, template, and assemble cobaloxime units. The conductive nature of the nanoITO substrate allows direct electrochemical measurements of the CoIII/CoII and CoII/CoI redox couples of the cobaloxime-polyvinylpyridine assembly recorded in organic electrolyte solutions, confirming the polymer interface used in this work does not preclude formation of reduced cobalt species. Electrochemical measurements using modified and non-modified nanoITO electrodes in buffered aqueous solutions indicate the immobilized cobaloxime units remain catalytically active. The relatively high surface area of the nanostructured support, coupled with its visual transparency, also permits optical characterization of the modified electrodes. In general, the cobaloximepolymer assembly possesses optical and electronic properties similar to the non-surface-attached counterpart, albeit with enhanced chemical reversibility. We propose that the unique encapsulating environments of surface-grafted polymeric architectures can provide a molecular strategy for improving the chemical stability of surface-immobilized catalysts. The modular nature of the attachment chemistry used in this work should allow application to a range of catalysts, polymers, and transparent conducting oxide surfaces. Thus the construct sets the stage for an improved understanding of structure and function relationships governing the optoelectronic and catalytic properties of surface-immobilized catalyst-polymer assemblies.
KEYWORDS: catalysis, hydrogen fuel, polymer interfaces, chemical reversibility, transparent conducting oxides
INTRODUCTION
M
olecular electrocatalysts that mimic the active sites of metalloenzymes are often not as efficient as their bioinorganic counterpart, indicating the extended coordination sphere of the surrounding protein likely plays an important 1 role in catalysis. Expanding the ligation environment around the active site of human-engineered catalysts could provide benefits in terms of selectivity, specificity, efficiency and du2 rability in technological applications. We hypothesize that surface−grafted polymer chains can yield protective surface coatings that offer appropriate functional groups to direct, template, and assemble molecular catalysts, as well as chemically stabilizing environments for catalysts encapsulated within polymers. Unlike the active sites of traditional surface electrocatalysts, which are integral to the electrode and contribute to the Fermi level, molecular catalysts are distinct entities with tunable electronic and chemical properties. In this vein, both fully integrated architectures and discrete electrocatalytic assemblies powered by a separated photovoltaic (PV) module can be pursued as approaches to using solar energy to power chemical transformations, including the production of fuels and other value-added products. Hard−to−soft matter interfaces are also important in medical, 3 electronic, and energy applications. However, an incomplete understanding of their chemistry hinders the ability
to synthetically fine−tune desired properties. To address this challenge, further analysis and understanding of interfacial composition and structure is required along with develop4 ment of new synthetic methodologies. In photoelectrocatalytic applications, the coupling of one−photon/one−electron photochemical charge separation to multi−electron catalysis is also a critical issue. For example, the efficiency of the initial light capture and charge separation steps in photosynthetic organisms is near unity; but losses due to inherent metabolic processes and use of intermediate energy carriers reduce the overall energy conversion efficiency to less than a few per5 cent. In principle, human engineered systems could bypass such efficiency constraints by having improved optical properties, electronic interfaces and fewer energy transduction 6,7 steps. Synthetic methodologies for directly coupling hydrogen8 producing molecular cobaloxime catalysts to visible-lightabsorbing gallium phosphide (GaP) semiconductors via a polymeric interface have been previously reported by our 9 research group. The two-step assembly method involves initial photoinduced grafting of an appropriate vinyl monomer, yielding a surface-attached polymer brush containing built-in ligand sites (imidazole or pyridyl groups) for assembling cobaloxime catalysts following a second step involving wet chemical treatment of the polymer-modified surface
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using a solution of the precursor complex, Co(dmgH2)(dmgH)Cl2. More recently, we have reported the ability to export this attachment chemistry across distinct crystal face orientations of GaP including the (111)A and (111)B surfaces which consist of predominantly atop Ga or atop P, 9e respectively. We speculate the molecular attachment to these GaP surfaces occurs primarily through bridging oxygen atoms, as evidenced by analysis of surface oxygen content 10 performed prior to and following surface functionalization. Likewise, we note that UV-induced attachment of olefins has been successfully deployed on other hydroxyl and oxygen terminated surfaces, including TiO2 and ZnO, where it is postulated that these reactions take place via the selfinitiated photografting and photopolymerization (SIPGP) 11 mechanism. These findings prompted us to investigate if the attachment chemistry initially developed to functionalize GaP with grafted cobaloxime-polymers could be exported to other oxide-terminated surfaces. The use of transparent conducting oxides (TCOs) provides a platform for studying molecular components immobilized at surfaces, and nanostructured indium tin oxide (ITO) has been utilized as an underpinning electrode for both water oxidation and proton reduction ca12 talysis. In addition to providing increased surface area compared to analogous planar substrates, the high porosity al13 lows for internal diffusion of solvent and electrolytes. The spectroscopic transparency of a TCO such as ITO ranges from the near-IR through the UV region, making it an exceptional substrate for probing the optical properties of surfaceimmobilized components by directly monitoring spectroscopic changes associated with specific redox transitions, allowing measurements that can be prohibitive using semiconducting substrates. By extending the polyvinylpyridine (PVP) attachment strategy to a TCO, we now show for the first time 1) spectroscopic evidence that the cobaloxime-polymer attachment chemistry initially developed for use on GaP surfaces can also be used to functionalize ITO substrates, 2) direct electrochemical measurement of redox features of polymer immobilized cobaloximes at potentials that are insulating using GaP semiconducting substrates, 3) a comparison of the cobaloxime-polymer redox features observed in organic versus aqueous solvents, 4) an estimate of the potentials required to achieve a similar per cobalt turnover frequency using the CoPVP-nanoITO or previously reported Co-PVP-GaP constructs, 5) potential extension of cobaloxime-polymer constructs to PV-electrolysis approaches for producing solar fuels, 6) a comparison of the electrochemically active cobaloxime loading versus total cobalt loading on the polymer, and 7) direct spectroscopic evidence of reduced cobaloximes within the confines of the surface-grafted polymer.
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ITO SiO2
ITO nanoparticles Anneal at 350o C
nanoITO
ITO
nanoITO
SiO2
4-vinylpyridine UV light
nanoITO
ITO
PVP-nanoITO
SiO2
Co(dmgH2)(dmgH)Cl2 Triethylamine
ITO
nanoITO
Co-PVP-nanoITO
SiO2
Scheme 1. A schematic (not to scale) depicting the strategy used to assemble the cobaloxime-polymer modified nanoITO electrodes. GaP surfaces (Scheme 1). Sample preparation starts with placing cleaned ITO or nanoITO slides into an argon sparged quartz flask containing the 4-vinylpyridine monomer. The flask is then illuminated with UV light (254 nm) for 2 hours before removing and cleaning the polymer-functionalized slides with successive solvent washes followed by drying under a stream of nitrogen then under vacuum. The polymermodified slides are then placed in a sealed flask containing an argon-sparged methanolic solution of triethylamine and cobaloxime precursor, Co(dmgH2)(dmgH)Cl2. After 12 hours, the cobaloxime-polymer-modified slides are removed from the flask and cleaned with successive solvent washes followed by drying under a stream of nitrogen then under vacuum (see Methods for details).
RESULTS AND DISCUSSION NanoITO electrodes were fabricated by spin-coating a suspension of ITO nanoparticles (
