CO2 Photoelectrochemical Reduction Selectivity via Molecular Additives on Gold Nanoparticle-Modified Gallium Nitride

Photoelectrochemical CO₂ reduction is an appealing avenue for the sustainable conversion of carbon dioxide, a potent greenhouse gas, into valuable chemicals and fuels, using solar energy. However, the efficacy and commercial viability of these processes are often hindered by low selectivity and efficiency, particularly in aqueous environments where the hydrogen evolution reaction (HER) dominates. The study published in Nano Letters, led by Professor Harry Atwater and a distinguished team of researchers at California Institute of Technology including Dr. Aisulu Aitbekova, Dr. Nicholas Watkins, Dr. Matthias Richter, Dr. Phillip Jahelka, Dr. Jonas Peters, and Dr. Theodor Agapie from the California Institute of Technology presents a significant advance in the field of photoelectrochemical (PEC) CO₂ reduction using gold nanoparticles on p-type gallium nitride (p-GaN) substrates. The research demonstrates the potential of molecular additives to significantly improve the selectivity of CO₂ reduction over the competing HER , which is a common challenge in the PEC CO₂ reduction process.

The team’s innovative approach involved the functionalization of gold/p-GaN photocathodes with a thin film derived from diphenyliodonium triflate. This modification led to a dramatic reduction in hydrogen generation from 90% to 18% thereby enhancing the selectivity of the system towards carbon monoxide production. The Faradaic efficiency for CO increased by 50%, and the partial current density saw a threefold increase. This marked improvement underscores the significance of surface chemistry and microenvironment manipulation in enhancing the performance of semiconductor-based photocatalytic systems. One of the notable aspects of this study is the use of gold nanoparticles, which are known for their plasmonic properties, to facilitate electron–hole separation at the metal-semiconductor interface. This interaction is critical in PEC systems as it enhances the charge separation efficiency, crucial for increasing the reaction rates of desired products. The research team demonstrated that even without altering the photocathode’s light absorption properties, the molecular film could effectively suppress undesired reactions such as HER.

The research investigated the structural and property relationships within the catalytic system. By employing optical absorption measurements, the study confirmed that the molecular film, irrespective of its thickness, does not detrimentally impact the photocathode’s ability to absorb light. This finding is particularly important as any decrease in light absorption could negate the benefits brought by enhanced catalytic selectivity. The study further leveraged advanced characterization techniques such as scanning electron microscopy, X-ray photoelectron spectroscopy, and electrochemical impedance spectroscopy, to gain insights into the morphological and electronic properties of the modified photocathodes. These techniques helped confirm the formation of a Schottky contact between the gold nanoparticles and the p-GaN substrate, a key feature for effective charge carrier dynamics. Moreover, the work extends beyond the mere fabrication and testing of these devices. It delves into the mechanism of action by which the diphenyliodonium triflate film modifies the interface and microenvironment around the gold nanoparticles. The evidence suggests that the film acts by reducing the accessibility of protons, thereby suppressing the HER and shifting the reaction selectivity towards CO₂ reduction. This mechanistic insight opens new avenues for designing tailored interfaces in semiconductor-based photocatalytic systems for improved performance. The implications of these findings are broad and significant. By demonstrating a way to control the chemical environment at the catalyst surface, the study provides a blueprint for the design of more effective PEC systems that can operate under practical conditions with higher selectivity and efficiency. This could potentially lead to the development of commercially viable processes for CO₂ conversion, aligning with global sustainability goals.

One of the primary challenges in PEC CO₂ reduction is the competing HER, which often dominates and reduces the efficiency of carbon-containing product formation. By significantly suppressing the HER (from 90% to 18%) and improving the Faradaic efficiency for CO production, the study provides a pathway to more economically viable and efficient processes for converting CO₂ into useful compounds like CO, which is a valuable feedstock for further chemical synthesis. The authors demonstrated how the surface modification of semiconductor materials with molecular films can manipulate the electrochemical environment to favor specific reactions. This insight is crucial for the design of next-generation semiconductor-based catalysts that are more active, selective, and stable. Converting CO₂ into higher-value chemicals using solar energy directly tackles the problem of excessive atmospheric CO₂, a major contributor to climate change. By improving the efficiency and selectivity of this conversion process, the technology contributes to a circular carbon economy, potentially reducing reliance on fossil fuels and decreasing carbon footprint. Moreover, the improvements in CO₂ reduction processes have direct implications for industries focusing on sustainable production methods. The ability to efficiently convert CO₂ into commercially valuable products can lead to new opportunities in synthetic fuels, chemical feedstocks, and reduction of industrial carbon emissions.  Furthermore, the findings contribute to the fundamental understanding of the interaction between light, semiconductors, and catalytic processes. This understanding is essential for the development of technologies such as artificial photosynthesis and solar fuel generation, pushing the boundaries of what is possible in renewable energy technologies. Overall, the Caltech scientists addressed a specific scientific challenge—improving the selectivity of CO₂ reduction over HER—but also sets a foundational platform for future research and development in PEC systems, potentially leading to significant advancements in renewable energy technologies and their applications.