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Chemical modification of semiconductor surfaces with molecular electrocatalysts provides a strategy for developing integrated homogeneous-heterogeneous materials capable of converting sunlight to fuels and other value-added products, but their development is hampered by an incomplete understanding of the factors limiting their performance. Although kinetic models have been separately developed to describe photoelectrochemical or homogeneous electrocatalytic reactions, related modeling for molecular-modified hybrid photoelectrodes has not been as extensively elaborated. This presentation addresses the interplay between light absorption, charge transfer, and catalytic activity during photoelectrosynthetic transformations at a molecular-modified semiconductor surface. The analysis provides opportunities to better understand the principles governing these hierarchal constructs and develop improved photocatalytic assemblies. 

Applications of polymeric coatings have emerged as a promising direction for preparing multilayered assemblies and controlling surface properties. In addition to providing a foundation for interfacing soft materials onto solid supports, polymers afford opportunities to develop hybrid constructs with properties difficult to achieve using monolayer-based chemical modification methods. In particular, the microenvironments of polymers are proposed to facilitate charge transfer to redox-active sites, manage delivery of chemical substrates, improve product specificity during catalytic transformations, and lend chemical protection to underpinning solid-state supports as well as embedded components. In this article, we highlight selected examples of polymeric materials utilized in electrocatalytic and photoelectrosynthetic fuel production. 

Human-engineered systems capable of generating fuels from sustainable energy sources provide an approach to satiating modern societies' energy demands, with minimal environmental impact. Strategies to address this challenge for science and the imagination often draw inspiration from the biological process of photosynthesis that powers our biosphere and supplied the fossil fuels global economies rely on. In this context, the active sites of enzymes have inspired researchers to develop molecular complexes that capture key structural and functional principles of nature's catalysts. However, not all aspects of biological energy transducing systems are or should be targets of chemical mimicry in designing an artificial photosynthesis, and some of the more favorable properties associated with solid-state heterogeneous catalysts have motivated molecular based surface-modification strategies. In this presentation, I will discuss efforts from our research group to develop heterogeneous–homogeneous architectures that combine the form factors of their underpinning solid-state supports with molecular coatings, enabling cooperative control and tunability of physical properties. 

Chemical modification of semiconductor surfaces with electrocatalysts provides a strategy for developing integrated materials capable of converting sunlight to fuels and other value-added products, but their development is hampered by an incomplete understanding of the factors limiting their performance. Although kinetic models have been separately developed to describe photoelectrochemical or homogeneous electrocatalytic reactions, related modeling for molecular-modified photoelectrodes has not been as extensively elaborated. This work addresses kinetic parameters pertinent to heterogeneous-homogeneous catalysis at molecular-modified semiconductors. Photoelectrosynthetic hydrogen evolution using a cobalt porphyrin-modified gallium phosphide cathode is analyzed under variable scan rates, pH values, and light intensity, yielding information on the relationship between the external quantum efficiency, illumination conditions, and turnover frequency. 

Rational design of soft-to-hard material interfaces offers new opportunities to control matter and energy across the nano- and meso-scales, thus providing a chemical strategy to tailor the structural and physical properties of surfaces with molecular level precision. In the context of energy transduction, interfacing molecular catalysts with solid-state substrates is a promising approach to developing hybrid materials for generating solar fuels. However, effective integration of the requisite components, while controlling their redox properties and stability, remains a major challenge. Taking inspiration from nature, where specific amino acid residues and soft-material coordination environments control the redox properties of metal centers in proteins during enzymatic catalysis, we show that thin-film polymer surface coatings provide a novel strategy for assembling human-engineered catalysts onto solid supports. This presentation describes recent results from our laboratory aimed at better understanding the electrochemical and optical properties of hydrogen production catalysts assembled onto polymer-modified electrode surfaces. The polymer immobilization method results in unique electronic and vibrational spectroscopic signals associated with the immobilized molecular species. In addition, the use of discrete polymer architectures, coupled with rational synthetic modifications to the catalyst’s ligand environment, affords control over the chemical stability and redox potentials of surface immobilized molecular complexes, spanning a ~250 mV range. 

Controlling matter and information across the nano-, meso-, and macro-scales is a challenge for science and the imagination. In this presentation, we highlight recent advances in our research efforts to develop synthetic methodologies for constructing an integrated photocathode for light activating chemical transformations that include capturing, converting, and storing solar energy as fuel. A recent example involves development of a direct one-step method to chemically graft porphyrin catalysts that chemically transform water to hydrogen as well as carbon dioxide to carbon monoxide onto a visible-light-absorbing gallium phosphide (GaP) semiconductor. The porphyrin complexes are prepared using a synthetic strategy that yields a tetrapyrrole macrocycle with a pendant 4-vinylphenyl attachment group. This structural modification allows use of the UV-induced immobilization chemistry of olefins to attach intact metalloporphyrin complexes to the semiconductor surface. Solar hydrogen production is demonstrated via photoelectrochemical testing in pH neutral aqueous solutions under simulated solar illumination. Key features of the constructs presented here include use of metalloporphyrins with built-in chemical sites for direct grafting to a GaP semiconductor, creating novel hybrid photoactive assemblies capable of converting photonic energy to fuel. 

The molecular modification of semiconductors has applications in energy conversion and storage, including solar fuels production. We have developed a synthetic methodology using surface-grafted polymers with discrete chemical recognition sites for assembling human-engineered catalysts in three-dimensional environments, providing additional control over the redox properties and stability of the composite material. This presentation will highlight the versatility of polymeric coatings to interface cobalt-containing catalysts with semiconductors for solar fuel production. Spectroscopic techniques, including ellipsometry, grazing angle attenuated total reflection Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy, provide detailed information on the structure and composition of the assemblies at the nano and meso scales. Photoelectrochemical measurements confirm the hybrid photocathode uses solar energy to power reductive fuel-forming transformations in aqueous solutions without the use of organic acids, sacrificial chemical reductants, or electrochemical forward biasing. 

Catalysts are central to energy conversion in biology and technology; they provide low-energy pathways for steering chemical transformations and are used in applications ranging from manufacturing fuels and fine chemicals to controlling the bioenergetic reactions essential to all living organisms. Accordingly, the study of homogeneous molecular catalysts, including porphyrins, has provided researchers significant insights regarding the mechanisms and structure−function relationships governing myriad catalytic processes, as well as design principles for further improving the performance of human-engineered catalysts. Our research group has recently reported on the favorable catalytic properties of piextended porphyrins for hydrogen evolution, demonstrating the promise of extended macrocycles as a design element and structural motif for preparing electrocatalysts. The pi-extended architecture provides an alternative strategy, compared to using electron-withdrawing or electron-donating functional groups, for adjusting the redox properties of a molecular catalyst and thus a promising avenue for catalyst design warranting further analysis.