Water Oxidation with Cobalt‐Loaded Linear Conjugated Polymer Photocatalysts

Abstract The first examples of linear conjugated organic polymer photocatalysts that produce oxygen from water after loading with cobalt and in the presence of an electron scavenger are reported. The oxygen evolution rates, which are higher than for related organic materials, can be rationalized by a combination of the thermodynamic driving force for water oxidation, the light absorption of the polymer, and the aqueous dispersibility of the relatively hydrophilic polymer particles. We also used transient absorption spectroscopy to study the best performing system and we found that fast oxidative quenching of the exciton occurs (picoseconds) in the presence of an electron scavenger, minimizing recombination.


Equation 1
Transmittance of the polymer photocatalysts in water dispersion was measured on a Formulaction S.A.S. Turbiscan AGS system with an 880 nm NIR diode and a detector at 180° (relative to the light source) in a cylindrical glass cell. Samples were prepared by dispersing the photocatalysts in 5 mL water and then diluting with water up to 30 mL total volume. All samples were sonicated before each measurement.
Co-catalyst loading. Each sample of photocatalyst (100 mg) was dispersed in a mixture of water (100 mL) and methanol (10 mL) by ultrasonication for 10 minutes. Co(NO3)2 was added and the mixture was then illuminated with a 300 W Xe light source (full arc) for 2 hours while keeping the reaction at 12 °C using a chiller unit circulating water through a jacket around the reactor. The suspension was then filtered, and the solids were washed with methanol. After this the sample was dried at 60 °C under reduced pressure.
Photocatalysis experiments. Photocatalytic O2 production was measured in a Pyrex top-irradiation reaction vessel connected to a glass closed gas circulation system. For each experiment photocatalyst (50 mg), water (100 mL) containing AgNO3 (0.01 M), and La2O3 (200 mg) were used. The photocatalyst was dispersed using ultrasonication for 10 minutes and the solution was evacuated several times to completely remove air. The reaction was then illuminated with a 300 W Xe light source for the time specified at a fixed distance under reduced pressure. The Xe light source was cooled by water circulating through a metal jacket and the reaction was kept at 12 °C using a chiller unit circulating water through a jacket around the reactor. The head space was analyzed by gas chromatography equipped with a thermal conductivity detector, referencing against standard gases with known concentrations of oxygen.
Transient absorption spectroscopy. The details of the transient absorption spectrometer are reported elsewhere. [8] Briefly, samples were excited with a 400 nm pump pulse at 5 kHz generated using a Pharos-SP-10W (Light Conversion, 1030 nm) operating at 10 kHz coupled to an Orpheus optical parametric amplifier (Light Conversion) in tandem with a Lyra harmonic generator (Light Conversion). The pump power was measured with a power meter (Thor labs) before each measurement and kept at 750 w. White light generation was achieved by focusing a portion of the Pharos-SP-10W output (10 kHz) onto sapphire within a Harpia-TA spectrometer (Light Conversion). Pump (ca. 600 m diameter) and probe (ca. 400 m) beams were overlapped on the sample which was placed in a quartz cuvette (2 mm). Data were recorded using the Harpia application software and analysed using Carpetview (Light Conversion). TA samples were prepared at the concentrations indicated in the manuscript. Experiments of P10/Co in water with and without degassing using Ar showed no difference (up to 3.4 ns). Attempts to degas P10 and P10/Co in the presence of Ag + lead to rapid aggregation and precipitation of the photocatalyst. Therefore, reported experiments in the presence of Ag + are carried out in the presence of air without Ar degassing.            1).

SUPPORTING INFORMATION
[c] Relative surface area calculated from the total particle surface area divided by total particle weight assuming a density of 1 g cm -3 .
Figure S10. Distribution of particle size of the 10 polymer photocatalysts in water, as determined using static light scattering.                  Figure S29. Photoluminescence spectra of P10 measured in the solid-state loaded with cobalt and the material after 6 hours of photocatalytic oxygen production. Figure S30. UV-Vis spectra of P10 measured in the solid-state loaded with cobalt and the material after 6 hours of photocatalytic oxygen production. Figure S31. FT-IR spectra of P10 measured as KBr pallets loaded with cobalt and the material after 6 hours of photocatalytic oxygen production. Figure S32. Correlation of the ζ-potentials of P1, P10, P28 and P35 measured in water with the observed oxygen evolution rate under broadband illumination.