Periodic Reporting for period 1 - PIERCAT (Gap-plasmon electrochemistry coupled with photo-induced enhanced Raman spectroscopy to probe oxygen vacancy dynamics (in-situ) and hot charge carrier kinetics for photoelectrochemical CO2 reduction)
Reporting period: 2023-06-01 to 2025-05-31
Increasing energy demands and depleting fossil fuel reserves galvanized the efforts to search for alternative renewable energy sources. Photo-electrochemical (PEC) water splitting, carbon dioxide photo-catalytic reduction (CO2 RR), and alcohol oxidation reaction (AOR) for fuel cells are considered alternative pathways to produce high-value-added chemicals and fuels. Such approaches require robust electrode materials and catalysts to drive the redox reactions efficiently. Metal oxides (MOs) have been widely used in this context and have shown great potential for the advancement of these technologies. Many physical and chemical properties of MOs can be controlled by tuning the imperfections in the crystal lattice, of particular interest, oxygen vacancies (VOs).
Therefore, the overall aim of the project is to probe the metal oxide (catalyst) interfaces, i.e. to understand the critical role of defect states (for instance, VOs) and their impact on photocatalytic CO2 reduction (CO2 RR). In this context, CeO2 is used as a model metal oxide catalyst; relevant defect dynamics and their influence on catalytic activity have been evaluated by in situ X-ray photoelectron spectroscopy, surface-enhanced Raman spectroscopy and wavelength-dependent photocatalytic CO2 RR measurements.
The following specific research objectives (SROs) were set for the project and have been successfully implemented.
SRO1: To fabricate the MO thin films with specific design configurations: This objective has been achieved by fabricating the pristine, Au NP, Au/SiO2 NP embedded CeO2 thin films.
SRO2: To quantitatively elucidate the defect dynamics in CeO2 and associated charge transfer kinetics: This objective has been implemented fully with the help of state-of-the-art in situ XPS and SERS measurements.
SRO3: To reveal the photocatalytic CO2 RR mechanism over pristine, and Au, Au/SiO2 NP embedded CeO2 thin films: The objective resulted a new and measurable findings on how the selectivity of CO2 RR can be impacted by the metal cation reduction under illumination.
Therefore, the overall aim of the project is to probe the metal oxide (catalyst) interfaces, i.e. to understand the critical role of defect states (for instance, VOs) and their impact on photocatalytic CO2 reduction (CO2 RR). In this context, CeO2 is used as a model metal oxide catalyst; relevant defect dynamics and their influence on catalytic activity have been evaluated by in situ X-ray photoelectron spectroscopy, surface-enhanced Raman spectroscopy and wavelength-dependent photocatalytic CO2 RR measurements.
The following specific research objectives (SROs) were set for the project and have been successfully implemented.
SRO1: To fabricate the MO thin films with specific design configurations: This objective has been achieved by fabricating the pristine, Au NP, Au/SiO2 NP embedded CeO2 thin films.
SRO2: To quantitatively elucidate the defect dynamics in CeO2 and associated charge transfer kinetics: This objective has been implemented fully with the help of state-of-the-art in situ XPS and SERS measurements.
SRO3: To reveal the photocatalytic CO2 RR mechanism over pristine, and Au, Au/SiO2 NP embedded CeO2 thin films: The objective resulted a new and measurable findings on how the selectivity of CO2 RR can be impacted by the metal cation reduction under illumination.
The main achievements (results) of the project are divided into two sub-sections.
1. Results on pristine CeO2 thin films: In the first part of the project, we studied the light-induced valence cation reduction (Ce4+ to Ce3+) and exciton generation across mid-gap states in pristine CeO2 thin films. XPS measurements under illumination (in situ) enabled me to track the correlation between Ce3+(%) and laser power. SERS measurements further confirmed the key role of photo-induced metal cation reduction concerning the charge transfer processes across the semiconductor-molecule interface. Finally, we showed the impact of these results on photocatalytic CO2 RR. The results shed light on defect dynamics in metal oxide semiconductors and their impact on the selectivity of photocatalytic CO2 RR reactions. An illustrative figure is attached.
2. Results on Au, Au/SiO2 nanoparticle (NP) embedded CeO2 thin films: In the second part of the project, Au nanoparticles (Au NP), and SiO2-coated Au nanoparticles (Au/SiO2 NP) are embedded in CeO2 thin films. These catalysts are investigated for photocatalytic CO2 RR measurements. The results show an enhanced CH4 yield for Au-CeO2, which is attributed to the hot electron injection from Au NP to CeO2. Whereas, the CO yield is more prominent in Au-SiO2/CeO2 thin films. The discrepancy is attributed to the shift in valence band position facilitated by the built-in electric field across the metal/insulator junction leveraged by plasmonic effects.
The above results were achieved as per the specific research objectives (SROs) set for the project. As an outcome, the results were published in a prestigious journal, Advanced Functional Materials.
1. Results on pristine CeO2 thin films: In the first part of the project, we studied the light-induced valence cation reduction (Ce4+ to Ce3+) and exciton generation across mid-gap states in pristine CeO2 thin films. XPS measurements under illumination (in situ) enabled me to track the correlation between Ce3+(%) and laser power. SERS measurements further confirmed the key role of photo-induced metal cation reduction concerning the charge transfer processes across the semiconductor-molecule interface. Finally, we showed the impact of these results on photocatalytic CO2 RR. The results shed light on defect dynamics in metal oxide semiconductors and their impact on the selectivity of photocatalytic CO2 RR reactions. An illustrative figure is attached.
2. Results on Au, Au/SiO2 nanoparticle (NP) embedded CeO2 thin films: In the second part of the project, Au nanoparticles (Au NP), and SiO2-coated Au nanoparticles (Au/SiO2 NP) are embedded in CeO2 thin films. These catalysts are investigated for photocatalytic CO2 RR measurements. The results show an enhanced CH4 yield for Au-CeO2, which is attributed to the hot electron injection from Au NP to CeO2. Whereas, the CO yield is more prominent in Au-SiO2/CeO2 thin films. The discrepancy is attributed to the shift in valence band position facilitated by the built-in electric field across the metal/insulator junction leveraged by plasmonic effects.
The above results were achieved as per the specific research objectives (SROs) set for the project. As an outcome, the results were published in a prestigious journal, Advanced Functional Materials.
In a scientific point of view, the project results shed light on defect dynamics in metal oxide semiconductors and their impact on the selectivity of photocatalytic reactions. This often-overlooked phenomenon appears to extend across a range of cation centers, and may hold particular promise for photocatalysis, where precise control over defect states is essential for enhancing efficiency and product selectivity. Considering the societal aspects, photo-catalytic CO2 reduction (CO2 RR) can offer a viable solution for the recycling of excess atmospheric CO2 to produce high-value-added chemical feedstocks and fuels, thus leveraging the simultaneous transition to a sustainable CO2-neutral society. Further, creating awareness regarding the environmental issues concerned with the energy crisis is an essential aspect of the project. We believe that our project results made this possible (at least in the scientific community) with the help of publications in top-tier journals. As such, our project results are in line with the European Green Deal.