Efficient Generation of Hydrogen Peroxide and Formate by an Organic Polymer Dots Photocatalyst in Alkaline Conditions

Abstract A photocatalyst comprising binary organic polymer dots (Pdots) was prepared. The Pdots were constructed from poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole), as an electron donor, and 1‐[3‐(methoxycarbonyl)propyl]‐1‐phenyl‐[6.6]C61, as an electron acceptor. The photocatalyst produces H2O2 in alkaline conditions (1 M KOH) with a production rate of up to 188 mmol h−1 g−1. The external quantum efficiencies were 30 % (5 min) and 14 % (75 min) at 450 nm. Furthermore, photo‐oxidation of methanol by Pdots, followed by a disproportionation reaction and an oxidation reaction, produced the high‐value chemical formate. On the basis of various spectroscopic and electrochemical measurements, the photophysical processes of the system were studied in detail and a reaction mechanism was proposed.


Detection of H2
Hydrogen in solution was detected by using a Unisense microsensor in the same condition of photocatalytic H2O2 production test.

Dynamic Light Scattering (DLS) Measurements
The hydrodynamic diameter of samples was measured by a Zetasizer Nano-S from Malvern Instruments Nordic AB.

Steady-State Absorption and Fluorescence Measurements.
Steady-state UV−vis measurements were analyzed by using a PerkinElmer Lambda 750 UV−vis spectrophotometer. Steady-state fluorescence spectra were analyzed by using a Fluorolog 3-222 emission spectrophotometer (Horiba Jobin-Yvon) together with the FluorEssence software.

Time-resolved fluorescence lifetime measurement
The fluorescence decays of all samples were recorded using a streak camera/spectrograph combination (Hamamatsu, ≤ 5 ps fwhm) and 470 nm, 100 fs pulse was used for excitation.

Photo-electrochemical measurements
TiO2 firstly grown onto FTO glasses followed by the calcination at 573 K for 1h and then cooled down to room temperature. Afterwards, a film of PFBT was spin-coated onto TiO2 film with PFBT solution in CH2Cl2. The as-prepared FTO was then employed as the working electrode for PEC methanol oxidation test, Ag/AgCl and Pt wire worked as reference and counter electrode respectively.
For test in solutions of pH 14, 7 and 0, 1 M KOH, 1 M KCl and 1 M HCl solution were respectively employed as the electrolytes, linear sweep voltammetry (LSV) method was used to scan from 0 to 1 V vs. NHE at scan rate of 50 mV s -1 . In Chopped LSV measurement, the scan rate was 1 mV s -1 .

NMR measurements
Proton nuclear magnetic resonance spectra (1 H NMR) were recorded on a Varian FT-NMR spectrometer (400 MHz for 1 H NMR). Pdots prepared in D2O were used. Typically, the reaction conditions were the same as photocatalytic activity test except that KOH aqueous solution and MeOH were replaced by KOH D2O solution and CH3OD respectively. After reaction, 0.6 mL reaction solution was extracted from the cuvette for NMR measurement.
For the experiments of Cannizzaro reaction, the same system as above was used except the existence of 2 mg mL -1 HCHO (typically in the form of low polymer where the n is from 2 to 8). [3] Samples were maintained for 24 h for completed reaction because the depolymerisation of HCHO is slow without heating.

Quantum Yield Measurement
External quantum efficiency (EQE) of PFBT-PCBM dots was measured under the same condition as for photocatalytic H2O2 generation except that the concentration of Pdots was 100 µg mL -1 . The light source was replaced by Xenon lamp with light filter (QD 450 nm) and the irradiation density was 2.9 Mw cm -2 . EQE was calculated by the equation below:

X-ray photoelectron spectroscopy (XPS) measurement
X-ray photoelectron spectra (XPS) were recorded by using the instrument PHI Quantera II from Physical Electronics. For sample preparation, both samples before and after reaction was dried by being exposed in fume hood for 48 h. was used to measure (list of Elements) and any potential stable interferences.
Liquid samples were injected into the plasma at a flowrate of 400 μL min -1 using a sample introduction system composed of a high efficiency concentric Micromist nebulizer (SCP Science, Canada) attached to a double-pass spray chamber maintained at 2℃. The optimized parameters are given in Table S1.       washed-PFBT-PCBM Pdots (color represents photon counts where red represents high and blue represents low); (c) fluorescence decay at 540 nm and relative mono-exponential fits.
Streak camera was conducted to deeply investigate the potential effect of Pd to charge transfer. As shown in Figure S7, fluorescence lifetime of washed-PFBT dots and washed-PFBT-PCBM Pdots were measured to be 453 ± 4 and 91 ± 1 ps, respectively, which was similar to PFBT dots and PFBT-PCBM dots.
Considering the hydrodynamic sizes of these two binary dots were also similar ( Figure S9), we can have the conclusion that Pd residuals did not have stark        Considering that depolymerization of HCHO is sluggish without heating in alkaline, both irradiated and un-irradiated samples were kept in dark for 24 h before conducting NMR measurement. This sluggish property of HCHO depolymerization is expected not to have effect during photocatalysis reaction since the Cannizzaro reaction could proceed once two HCHO molecules are generated and the self-polymerization of HCHO can be avoided. As shown in Figure S18, the signal appealed at chemical shift of 8.25 ppm is attributed to the H of HCOOwhile signal for the H of HCHO at 9 ppm is not observed, which means the added HCHO was completely converted. and H2O2 is calculated to be around 12:5. Therefore, the ratio of generated H2O2 and formate which is around 1:1 can be contributed by both Cannnizzaro reaction and the reaction between HCHO and H2O2.