Photocatalytic Overall Water Splitting Under Visible Light Enabled by a Particulate Conjugated Polymer Loaded with Palladium and Iridium

Abstract Polymer photocatalysts have received growing attention in recent years for photocatalytic hydrogen production from water. Most studies report hydrogen production with sacrificial electron donors, which is unsuitable for large‐scale hydrogen energy production. Here we show that the palladium/iridium oxide‐loaded homopolymer of dibenzo[b,d]thiophene sulfone (P10) facilitates overall water splitting to produce stoichiometric amounts of H2 and O2 for an extended period (>60 hours) after the system stabilized. These results demonstrate that conjugated polymers can act as single component photocatalytic systems for overall water splitting when loaded with suitable co‐catalysts, albeit currently with low activities. Transient spectroscopy shows that the IrO2 co‐catalyst plays an important role in the generation of the charge separated state required for water splitting, with evidence for fast hole transfer to the co‐catalyst.


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Reagents and solvents were purchased from commercial suppliers (Manchester Organics, Sigma-Aldrich, Kanto Chemical, Wako Pure Chemical Industries, and Tanaka Kikinzoku) and used without further purification. P10 was synthesized using a previously reported method. [1,2] FT-IR spectra were recorded on a Bruker Tensor 27 with an ATR attachment at room temperature. UV-Visible spectra of the polymers were recorded on a Shimadzu UV-2550 UV-Vis spectrometer as powders in the solid state.
Photoluminescence spectra of the polymer powders were measured with a Shimadzu RF-5301PC fluorescence spectrometer at room temperature. Time-correlated single photon counting (TCSPC) experiments were performed on an Edinburgh Instruments LS980-D2S2-STM spectrometer equipped with picosecond pulsed LED excitation sources and a R928 detector, with a stop count rate below 5%.    Intensity / a.u.

Loading of Ir on P10
Binding energy / eV            Photocatalytic water splitting of P10 (triangles, squares, and circles are using polymer photocatalyst from separate batches, red symbols relate to hydrogen production while blue symbols relate to oxygen production) in distilled water (120 mL) in gas-closed circulation system. Light source: Xe light source (PerkinElmer CERMAX PE300BF 300 W Xe, full arc) in a top-irradiation cell with a Pyrex window, irradiation area: 33 cm 2 . Activities were measured after an initial stabilization period.          [a] Fluorescence life-times for polymers in ethanol solution suspension obtained from fitting time-correlated single photon counting decays to a sum of three exponentials, which yield τ1, τ2, and τ3 according to τAVG is the weighted average lifetime calculated as ∑ =1 .  Ar bubbling for 20 minutes. Samples were not stirred during ultrafast TAS measurements as they were found to be suitably stable during the TA experiment (ca. 30 minutes).

Global analysis:
Initially global lifetime analysis (GLA) was carried out within the carpetview software.
The principles of GLA and global target analysis (GTA) of TA data have been discussed in detail elsewhere. [3,4] The aim of GLA is to provide a way to visualise complex sets of time-resolved spectra by decomposing them into a small number of compartment populations and to examine their time dependence. Here we use a parallel evolution GLA approach where the TA data is fitted to a small number of compartments which are all initially populated and the population of each compartment is modelled by a single exponential decay function, which will be convoluted (⨂) by an instrument Singular value decomposition (SVD) analysis was carried out on both data sets, and this indicated that four compartments (labelled 0,1,2,3) were required in the GLA fitting of the P10 and P10-IrO2 data.
The complex nature of the system under study, which likely contains multiple excited state decay pathways means that although the data is well fitted to 4 main decay spectra these should not be considered to be spectra of individual photochemical species. Global analysis and the associated decay associated spectra (DAS) provide a way to represent the complex TA spectra of P10 into compartments with common decay lifetimes.
S-28  spectrum. An additional broad feature centred around 630 nm in the 0.6 ps DAS for P10 suggests some electron or polaron pair formation on the ultrafast timescale in-line with our recent report of ultrafast polaron formation from hot excitonic states with P10. [5] The DAS for 6 ps is similar to the 0.6 ps except that the contribution from the higher energy emissive state is decreased, indicating that this is significantly shorter lived. Interestingly the 6 ps DAS at wavelengths > 700 nm appears to consist of