Electrocatalytic and Optical Properties of Cobaloxime Catalysts Immobilized at a Surface-Grafted Polymer Interface

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) of 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 nonmodified nanoITO electrodes in buffered aqueous solutions indicate the immobilized cobaloxime units remain catalytically active. The relatively large surface area of the nanostructured support, coupled with its visual transparency, also permits optical characterization of the modified electrodes. In general, the cobaloxime–polymer assembly possesses optical and electronic properties similar to those of 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–function relationships governing the optoelectronic and catalytic properties of surface-immobilized catalyst–polymer assemblies.

Hybrid Photoelectrochemical Water Splitting Systems: From Interface Design to System Assembly

“…Although PEC performance was not evaluated for the hybrid electrodes, the results highlighted a novel and effective strategy for covalently binding of a molecular complex onto the surfaces of semiconductor photoelectrodes for hybrid PEC system assemblies. With similar strategies, many molecular complexes on metal phosphide photoelectrodes have been described with result in high‐performance PEC water reduction 36–40. Recently, Rose and co‐workers devised a surface PNP linker, for surface binding to p‐Si with a nickel phosphine complex ( Figure 5 ).…”

“…The cobaloxime complex was stable on the GaP surface after the PEC reaction had occurred. Based on this binding strategy, other hybrid systems have used p‐GaP as the light harvester and cobalt or iron complexes immobilized on GaP as catalysts for efficient PEC water reduction 37–40. InP, with its narrow bandgap, is also an excellent photosensitizer and can be used for water splitting with added catalysts 41,42.…”

“…For nonoxide photoelectrodes (e.g., p‐GaP, Si, etc. ), presurface‐treatment is useful for effective interface design by introducing additional surface active sites for molecular complex coordination and by depositing metal oxide overlayers to provide easy surface anchoring 37–39. Although the stability of binding groups in aqueous solutions may be insufficient, surface anchoring groups can be protected for PEC stabilization by various techniques, such as TiO 2 or Al 2 O 3 overlayer deposition, polymer overlayer modification and electropolymerization.…”

“…In addition, the semiconductor electrode should possess long‐term stability under light irradiation conditions for water splitting reactions. To date, numerous semiconductor photoelectrodes have been developed for PEC water splitting, including p‐Si,25–35 p‐GaP,36–40 InP,41,42 Cu 2 O,43,44 CdSe quantum dots (QDs),45–49 CdS QDs50, p‐GaInP51, and La 5 Ti 2 Cu 0.9 Ag 0.1 S 5 O 7 52 for water reduction and α‐Fe 2 O 3 ,53–66 WO 3 ,67–75 BiVO 4 75–84 and n‐Si85 for water oxidation. For more information about the properties of the semiconductors, including the conduction (CB) and valence band (VB), stabilities as a function of pH values, the light absorption behavior, and advanced preparation methods for different morphologies, readers are invited to refer to related review papers 86–89…”

A robust ALD-protected silicon-based hybrid photoelectrode for hydrogen evolution under aqueous conditions

“…Silicon is highly abundant and can be cost-effectively processed for the industrial construction of photovoltaic panels into crystalline silicon, which has a conduction band (CB) energy level of ∼–0.5 V vs. NHE (pH = 0) and a narrow band-gap of 1.12 eV 5. These features make silicon the material of choice among other semiconductors6–9 for the fabrication of H 2 -evolving photoelectrodes able to harvest solar irradiation over the whole visible spectrum 10,11. However, the major drawback of using silicon as a photoelectrode material is its inherent instability under aqueous or aerobic conditions.…”