Journal articles made easy: Photocatalytic water oxidation
Photocatalytic water oxidation at soft interfaces
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Photocatalytic water oxidation at soft interfaces
Authors: Malte Hansen, Fei Li, Licheng Sun and Burkhard König
Why is this study important?
The search for alternative renewable energy resources is one of the biggest scientific challenges of our times.1,2 Water as an abundant resource would be the ideal source for the production of the sustainable energy carrier hydrogen. Utilizing solar radiation as an inexhaustible energy source could drive the energetically uphill water splitting reaction. To find suitable catalyst systems (further resources on catalysis here and here) for photochemical water splitting into oxygen and hydrogen the overall reaction is divided into the reductive and oxidative half reaction, respectively. The water oxidation half reaction involving four electron transfer steps and highly reactive intermediates is considered particularly difficult.3
A typical photochemical water oxidation system consists of two subunits: a light absorbing photosensitizer or dye and the water oxidation catalyst. The two subunits can be covalently connected which ensures an efficient electron transfer (electron transfer is distance dependent), but requires the synthesis of complex ligands and linkers.4-8 If dye and oxidation catalyst are prepared as separate entities different combinations can be easily realized, but the electron transfer between the subunits in homogeneous solution is diffusion controlled and depends on the concentration.9
- A review paper on 'Surface tuning for oxide-based nanomaterials as efficient photocatalysts'
- Selected further reading from RSC books:
- Molecular Solar Fuels
- Photoelectrochemical Water Splitting
- Solar Energy Conversion
- Solar Hydrogen
What is the objective?
To create a working photochemical water oxidation system which combines the advantages of covalently bound assemblies and homogeneous systems by embedding the two redox partners (further resources on redox here and here) into a phospholipid bilayer membrane. The co-embedding ensures a close proximity and high local concentrations of the subunits and thus facilitates the electron transfer while it still allows the easy variation of photosensitizer-catalyst combinations, ratios and concentrations.
What was their overall plan?
- Choose an appropriate literature known homogeneous photochemical water oxidation system.
- Modify these compounds for the introduction into a phospholipid bilayer.
- Co-embed the amphiphilic derivatives into small unilamellar vesicles and perform photochemical water oxidation.
- Compare the vesicular system with the homogeneous one under identical conditions.
- Test the versatility of the method by embedding of different water oxidation catalysts.
- Investigate the influence of the bilayer membrane on the catalytic performance.
What was their procedure?
Choose an appropriate literature known homogeneous photochemical water oxidation system
The authors decided to choose a photochemical water oxidation system consisting of 2b a derivative of the well-known photosensitizer Ru(bpy)3 and 6a a ruthenium complex bearing a ligand with two carboxylate functionalities which was originally published by the Sun group.10 The complexes were fully characterized by NMR, MS and X-Ray and the system produced oxygen under irradiation when sodium persulfate was used as a sacrificial electron acceptor. The photooxidation of Ru(bpy)3 and its derivatives with persulfate is an established and intensively investigated reaction.11 The generated oxidized photosensitizer-species has a sufficient oxidation potential to oxidize the catalyst.
Modify these compounds for the introduction into a phospholipid bilayer
The necessary amphiphilicitiy was created by the introduction of alkyl-chains. In case of the photosensitizer 2b the amphiphilic derivative was synthesized with two dodecyl amide moieties instead of the ethyl esters as described in figure 1.
The catalyst was modified by the introduction of a dodecyl chain on the 4th position of the dipicolinic acid as shown in figure 2.
Co-embed the amphiphilic derivatives into small unilamellar vesicles and perform photochemical water oxidation
Functionalized vesicles were prepared by mixing appropriate volumes of stock solutions of 2a, 6b and a phospholipid (figure 5) in organic solvents in a crimp-top vial. After evaporation of the solvent phosphate buffer containing sodium persulfate was added and the vial was sonicated to obtain small unilamellar vesicles (figure 3). The size distribution of the vesicles was determined with dynamic light scattering and the sample was degassed with argon. After irradiation with blue LEDs the amount of evolved oxygen was measured by gas chromatography (figure 4).
Figure 3: Schematic representation of the vesicle preparation.
Figure 4: Image of the LED irradiation and cooling device (left), schematic representation.
Compare the vesicular system with the homogeneous one under identical conditions
A comparison of the catalytic performance between homogeneous and vesicular systems under identical overall concentrations of photosensitizer and catalyst gave comparable turnover numbers. However, when the concentration of the catalyst was lowered the homogeneous system became less efficient, but the turnover number (TON) of the catalyst in the vesicular system increased significantly (table 1). This can be explained by the two dimensional arrangement of the two subunits in the membrane. The redox partners stay in close proximity, which facilitates the electron transfer, even at very low overall catalyst concentrations. This makes water oxidation possible at concentrations which cannot be realized in homogeneous systems.
Table 1: Evolved molecular oxygen after 20 min light irradiation and TON of photosensitizers (PS) 2 (125 µM) and water oxidations catalysts (cat) 6 in homogeneous solution (shaded in grey) and DMPC phospholipid vesicles at identical concentrations and a sodium persulfate concentration of 2.5 mM.
Test the versatility of the method by embedding of different water oxidation catalysts
To prove the versatility of embedding catalyst-photosensitizer combinations into phospholipid bilayers the authors modified two other literature known photochemical water oxidation catalysts with alkyl chains.12, 13 Vesicles functionalized with these catalysts also showed better catalytic activities than a homogeneous system under otherwise identical conditions (table 2). The literature known fact that the photosensitizer is the main limiting factor in homogeneous catalysis of these systems was also confirmed in the vesicular solutions, by the addition of new photosensitizer to used vesicles. These renewed vesicular solutions regained 60 % of the initial catalytic performance.
Table 2: Evolved molecular oxygen after 20 min light irradiation and TON of photosensitizers (PS) 2 (125 µM) and water oxidations catalysts (cat) 3 and 7 embedded in DMPC phospholipid vesicles; for comparison the performance of a homogeneous solution of catalyst 5 and photosensitizer 2c at identical concentrations is given (shaded grey). Sodium persulfate concentration: 2.5 mM.
Investigate the influence of the bilayer membrane on the catalytic performance
The physical properties of phospholipid membranes change drastically with the nature of the lipid. To investigate the effect on water oxidation activity vesicles with three different lipids were prepared. The three lipids have the same dipolar head group and differ only in the length and structure of the hydrocarbon chains (figure 5).
However, in DMPC and SMPC the co-embedded complexes produced oxygen upon irradiation, while the activity in DOPC was significantly lower. The authors explain the effect by the distinct differences in membrane fluidity. Phospholipid membranes show a significant change in the membrane fluidity at distinct temperatures. This is the transition from the relatively rigid gel phase to the liquid crystalline phase with high lateral diffusion. DMPC and SMPC have main phase transition temperatures from the gel to the liquid crystalline phase of 24 °C and 30 °C, but DOPC with a transition temperature of -21 °C is already in the liquid crystalline phase at the temperature of the experiment. Photooxidation experiments of DMPC and SMPC vesicles with embedded photosensitizers and catalyst at temperatures above their transient temperatures showed a reduced activity, while the activity remains unchanged at temperatures below the phase transition.
Table 3: Dependence of evolved molecular oxygen after 20 min light irradiation and TON of co-embedded photosensitizer 2a (125 µM) and water oxidations catalyst 6b (12.5 µM) depending on the phospholipid and the reaction temperature at a sodium persulfate concentration of 2.5 mM.
The authors conclude that phase separation and clustering of the embedded complexes, expected for the amphiphilic additives below the transition temperature of a lipid membrane, enhance the photocatalytic activity of the assembly.
What are the conclusions?
In summary, the authors have, for the first time, self-assembled photosensitizer - catalyst water oxidation systems by co-embedding two amphiphilic ruthenium complexes into the phospholipid bilayer membrane of small unilamellar vesicles. The observed oxygen production upon light irradiation is comparable to similar systems in homogeneous solutions, but superior at low concentrations of the water oxidation catalysts. Membrane embedded water photooxidation systems remain catalytically active at concentrations, where homogeneous mixtures of photosensitizer and water oxidation catalyst are inoperable. Phase separation and patch formation cluster the complexes in the membrane, which might facilitate the intermolecular electron transfer processes. The fluidity of the membrane affects the self-organization of the embedded complexes and therefore their photocatalytic performance. Highest TONs are observed in gel phase membranes, where phase separation is favoured. The method was applied to different combinations of sensitizers and oxidation catalysts and allows a rapid screening of sensitizer – catalyst combinations, ratios and concentration ranges.
What are the next steps?
Functionalized vesicles could provide a new approach for the creation of a water splitting device when combined with a hydrogen evolution electrode. This could be realized as a wired device with immobilized membranes on cathode and anode or as a Z-scheme water splitting system utilizing a redox mediator.