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Abstract
The photoelectrochemical and/or electrochemical reduction of CO2 has historically faced many challenges associated with large reduction potentials, competing proton reduction in water as well as many possible coupling reactions that can occur on the electrode surfaces. To date? the vast majority of work on CO2 photoreduction has focused on proton-coupled electron transfer (PCET) reactions, which usually require the participation of multiple electrons and protons as well as long-lived intermediates and are therefore inefficient. A much more direct and more efficient pathway would be the one-electron reduction of CO2 to CO2•-. This pathway, however, has a standard redox potential (Eo) of -1.9 V vs the standard hydrogen electrode (SHE), rendering it not accessible photochemically with most semiconductors.
This work shows how diamond can be used for photochemical and electrochemical reduction of CO2. The first part of the work utilizes the negative electron affinity (NEA) property of diamond to achieve CO2 photoreduction. The photo-excited conduction-band electrons of diamond can emit into the solution to become solvated electrons, and readily access the one electron activation pathway for CO2 reduction. CO2 photoreduction via this pathway with boron-doped diamond electrode shows high selectivity (>90%) for CO product and minimum H2 formation (<5%) in water. Cost-effective diamond nanoparticle samples are also investigated. Detonation nanodiamond (4-6 nm) shows degradation upon illumination, but better-quality 125 nm diamond nanoparticles are able to selectively reduce CO2 to CO photochemically.
The second part investigates the possibility of extending the photo-emissive property of diamond into the visible light range. Diamond can only be excited with ultraviolet light due to the large bandgap (5.5 eV). To achieve visible light excitation of electrons to the conduction band of diamond, one method is to develop plasmonic nanoparticle/diamond heterostructures where the plasmon-induced “hot” electrons may then transfer to the conduction band of diamond. Ag/diamond and Au/diamond nanocomposite are successfully synthesized for this purpose and tested for CO2 photoreduction. Ag-implanted boron-doped diamond samples are also fabricated and tested.
The third part of the work takes advantage of the excellent electrochemical properties diamond has to offer. A method is presented on developing a molecular catalyst-diamond interface for electrochemical reduction of CO2. A CO2 molecular catalyst cobalt porphyrin is covalently attached to the diamond surface, and the heterogeneous catalysis system reduces CO2 to CO in acetonitrile. Finally, the work also presents a method to fabricate diamond field emission tips. The field emission tips may be used to produce solvated electrons electrically.
