Browsing by Subject "Artificial photosynthesis"

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Open Access
Design of multifunctional prussian blue analogues for solar driven water oxidation
(2021-07) Ghobadi, Türkan Gamze Ulusoy
The development of earth-abundant, robust, and low-cost photoanodes for water oxidation is one of the most critical steps in ‘artificial leaf’. A promising approach in this field is to build dye-sensitized photoanodes by coupling a molecular photosensitizer (PS) with a water oxidation catalyst (WOC) on a proper semiconductor (SC) for efficient charge separation. All dye-sensitized photoanodes reported in the literature consist of either a ruthenium photosensitizer, a ruthenium water oxidation catalyst, or both. We aim to overcome this critical challenge by developing a new family of organic- or iron-based donor-acceptor chromophores incorporated in a Prussian blue (PB) structure, which are coated on proper semiconductors. Our studies within this context could be divided into three sections: (i) PB based photocatalytic water oxidation: In this work-package, an entirely precious metal-free chromophore-donor-water oxidation catalyst triad system is developed. The synthesis involves the coordination of a porphyrin derivative to a bridging Fe(CN)5 group, which is then reacted with cobalt ions to prepare a covalently linked chromophore-Prussian blue analogue (CoFe(CN)5–Ligand) assembly. Light-driven water oxidation studies in the presence of an electron scavenger indicate that the triad is active and maintains a steady activity for at least 3 hours. Transient absorption experiments and computational studies reveal that the Fe(CN)5 group is more than just a linker. It takes part in electron donation and co-operates with porphyrin in the charge separation process. (ii) PB based photoelectrochemical water oxidation: Here, we move one step forward and design a ruthenium-free water oxidation photoanode by the sensitization of titanium dioxide (TiO2) nanowires with a PB-organic chromophore structure. A phenazine-based organic group, Janus Green B (JG), is chosen as the chromophore since it has a broad absorption response in the visible and near-infrared ranges. The resulting multifunctional PB modified TiO2 electrode demonstrates a low-cost and easy-to-construct photoanode, which exhibits a remarkable excited-state lifetime in the order of nanoseconds and an extended light absorption capacity of up to 500 nm. Moreover, the photoanode retains its structural integrity and photoelectrochemical activity for at least 2 hours. Despite all the above-mentioned improvements, the performance of the cell, [CoFe–JG]/TiO2, is relatively poor due to improper band energy alignment between the chromophore and the semiconductor. In a follow-up study, we tune the chromophore and the semiconductor to achieve a proper band energy alignment, and thus, to improve the performance. Another phenazine-based molecule, Safranin O (SF), is utilized as the organic photosensitizer. Moreover, a visible-light absorbing semiconductor, WO3, is used to utilize the solar spectrum completely. [CoFe–SF]/WO3 exhibits a record photocurrent density of 1.3 mA/cm2 at 1.23 VRHE, demonstrating that proper modification of components in PB based dye-sensitized photoanodes could pave the way for the development of high-performance water splitting cells. (iii) Iron chromophore based photoelectrochemical water oxidation: In this section, the iron site that has been previously utilized as a relay is promoted to an iron chromophore. Five cyanide ligands are coordinated to the iron site to destabilize the metal-centered states. At the same time, an electron-deficient cationic pyridinium group occupies the remaining coordination sphere of the octahedral iron site to facilitate the metal-to-ligand charge transfer (MLCT) process. This iron complex is coated initially on TiO2 nanowires and then reacted with cobalt ions to produce a CoFe PB (CoFe(CN)5-L) layer on the electrode surface. In this photoanode, the excited-state lifetime of the iron chromophore exceeds 1 ns, which demonstrates the first example of an iron-sensitized water oxidation cell in the literature. Overall, this thesis presents an alternative perspective to realize high performance, low-cost, stable, and robust dye-sensitized water oxidation systems. The impact of the acquired knowledge in this thesis is also discussed to define the current status, challenges, and future of PB based water oxidation systems.
Open Access
Investigation of cyanide-based catalysts and hybrid assemblies for artificial photosynthesis
(2022-09) Akbari, Sina Sadigh
The storage and conversion of solar energy appear to be a highly promising solution to the global energy dilemma due to the sustainability and abundance of sunlight. Among various strategies, artificial photosynthesis, in which solar energy is directly converted to chemical bonds, has gained a great deal of attraction over the past decades. Such a design requires a catalyst coupled with a semiconductor and/or a photosensitizer as light-harvesting components with proper band levels for efficient charge separation. Therefore, the development of an earth-abundant, robust, and visible-light absorbing photocatalytic assembly has been one of the bottlenecks for the advancement of scalable cells. This work aims to overcome this critical challenge by developing new cyanide-based hybrid assemblies for light-driven water splitting and CO2 reduction. CoFe Prussian blue analogues (CoFe-PBAs) have recently emerged as active water oxidation catalysts with excellent long-term stabilities. In the first study, a ZnCr layered double hydroxide (ZnCr-LDH) with a two-dimensional (2D) morphology and a CoFe-PBA are combined to afford a precious metal-free photocatalytic assembly involving a visible light-absorbing semiconductor (SC) and a water oxidation catalyst (WOC). The SC−WOC hybrid materials exhibit a threefold enhancement in activity compared to bare ZnCr-LDH, which is maintained for 6 h under photocatalytic conditions. The band energy diagram was extracted from optical and electrochemical studies to clarify the origin of the improved photocatalytic performance. The assembly is observed to have an appropriate band energy alignment that facilitates charge transfer from the valence band of ZnCr-LDH to the HOMO level of CoFe-PBA for the water oxidation process. In a follow-up study, we move one step forward by coupling exfoliated Dion−Jacobson type niobate nanosheets as a 2D semiconductor with the CoFe-PBA to construct an SC−WOC hybrid structure, which produces a p−n junction. The assembly exhibits a promoted activity (89.5 μmol g−1 h−1) with a proper band energy alignment for the photocatalytic water oxidation process, and it is stable throughout a 12 h photocatalytic study. These studies mark a straightforward pathway to developing low-cost and precious metal-free assemblies for light-driven water oxidation. Metal dicyanamides are another well-known cyanide-based materials, which can be employed as a catalyst for hydrogen evolution reaction (HER) and CO2 reduction due to the partial electron delocalization and the proper coordination environment of metal ions. Herein, we promote a cobalt dicyanamide coordination polymer, Co-dca, for the first time, as a selective catalyst to reduce CO2 to CO in the presence of a ruthenium photosensitizer (Ru PS) under visible light irradiation. A series of photocatalytic experiments under various reaction conditions were performed to reveal the role of the PS, the scavenger, and the solvent in the selectivity and the activity of the photocatalytic process. We find that Co-dca exhibits an activity of 254 µmol h−1 g−1 and a CO selectivity as high as 93%. Furthermore, cobalt dicyanamide also displayed enhanced H2 evolution activity (25000 µmol h−1 g−1), which is maintained for at least 12 h in a mixed aqueous solution containing a Ru-based photosensitizer and a sacrificial electron donor. The effect of various reaction parameters on the photocatalytic activity of the system was investigated. Overall, this thesis presents various low-cost and stable SC-WOC hybrid assemblies based on CoFe-PBA for the light-driven water oxidation process. Moreover, we suggest the use of Co-dca, for the first time, as catalysts for HER and CO2 reduction reactions.