Non-conventional bulk heterojunction nanoparticle photocatalysts for sacrificial hydrogen evolution from water

Photocatalyst systems combining donor polymers with acceptor molecules have shown the highest evolution rates for sacrificial hydrogen production from water for organic systems to date. Here, new donor molecules have been designed and synthesised taking inspiration from the structure–performance relationships which have been established in the development of non-fullerene acceptors. While a conventional bulk heterojunction (BHJ) pairing consists of a donor polymer and acceptor small molecule, here we have successfully reversed this approach by using new p-type small molecules in combination with a n-type conjugated polymer to produce non-conventional BHJ (ncBHJ) nanoparticles. We have applied these ncBHJs as photocatalysts in the sacrificial hydrogen evolution from water, and the best performing heterojunction displayed high activity for sacrificial hydrogen production from water with a hydrogen evolution rate of 22 321 μmol h−1 g−1 which compares well with the state-of-the-art for conventional BHJ photocatalyst systems.


General Experimental
Synthesis and characterisation.All reactants and reagents were purchased from commercial suppliers and used without further purification unless otherwise stated.N-Bromosuccinimide was recrystallised from water prior to use S1 PNF222 was purchased from Ossila (batch M2052A2).All reactions were performed under an inert atmosphere in oven dried glassware unless otherwise stated.
NMR spectra were recorded on Jeol ECS 400 MHz and Jeol ECZ 500 MHz spectrometers.
Chemical shifts are reported in ppm downfield of tetramethylsilane (TMS) using TMS or the residual solvent as an internal reference.NMR spectra were processed using MestReNova.
IR spectra were collected on a Thermo Scientific Nicolet FTIR spectrometer.
High resolution mass spectrometry (HRMS) was measured on a Thermo-Finnigan LTQ FT mass spectrometer.Samples were prepared in MeCN or a 9:1 MeCN:CH2Cl2 mixture.
UV-vis absorbance spectra were measured using a UV-1800 spectrophotometer (Shimadzu)   and UVProbe version 2.33 software.
Emission spectra were recorded on a Horiba Fluoromax ® 4 luminescence spectrometer using FluorEssence TM software.Photo-luminescent quantum yields (PLQY) measurements were carried out relative to that of Rhodamine-6G (0.94 in ethanol S2 ).
Elemental analyses were obtained on an Exeter Analytical CE440 Elemental Analyser.
Melting points were determined in open-ended capillaries using a Stuart Scientific SMP10 melting point apparatus at a ramping rate of 1 °C/min.They are recorded to the nearest 1 °C and are uncorrected.
TGA were performed using a TA ST Q600 instrument.Two alumina crucibles with a small amount of sample (1−5 mg) and an equal (±0.01 mg) amount of alumina reference powder were used for the TGA and the temperature was increased at a rate of 10 °C/min from 25 °C to 1000 °C.Data obtained was analysed using TA Instruments Universal Analysis 2000 (Version 4.5A, Build 4.5.0.5) software.
Cyclic voltammetry was recorded in CH2Cl2 solution using a Autolab Potentiostat (PGSTAT128) and NOVA 2.1 software with internal resistance compensation applied.A glassy carbon disk working electrode, Pt wire, and Ag/Ag + (0.1 M AgNO3 in acetonitrile) were used as the working, counter, and reference electrodes, respectively.An analyte molarity of ca. 10 −3 M was employed in the presence of 10 −1 M (TBAPF6) as a supporting electrolyte.
Solutions were degassed with Ar and experiments run under a blanket of Ar.The working electrode was polished between each experiment.Measurements were corrected to the halfwave potential of the ferrocene/ferrocenium (Fc/Fc + ) redox couple as an internal standard.
Ionisation potential (IP) and electron affinity (EA) values were approximated using the voltammetry results and are presented on the absolute electrochemical scale (0 V vs. standard hydrogen electrode (SHE) = −4.44V vs. vacuum).S3 The IP and EA were obtained by considering both the oxidation potential of Fc/Fc + in CH2Cl2 (+0.46 V vs. SCE) S4 and the fact that SCE has a potential of +0.24 V vs. SHE.S5 The oxidation potential of Fc/Fc + can therefore be estimated as +0.70 V vs. SHE.According to the Nernst equation, a further adjustment of +0.12 V is required due to the catalysis occurring in a pH 2 solution.The IP and EA were then approximated using the onset of the oxidation (Eonset ox ) or reduction (Eonset red ) wave respectively Nanoparticle dispersions were diluted by placing 50 µL of 0.5 mg/mL dispersion in 3 mL of DI water (pre filtered through 0.45 µ PTFE) prior to measurements.Measurements were performed in triplicate and averaged.Density functional theory calculations were performed with Orca v 5 S6 using the B3LYP hybrid functional S7 and def2-SVP basis set.S8,S9 Preparation of photocatalyst nanoparticles.To prepare the nanoparticles, individual stock solutions of DTSRh, DTS13 and PNF222 in chloroform solvent at a concentration of 0.50 mg mL -1 were prepared.These solutions were heated overnight at 80 °C to ensure complete dissolution then allowed to cool before being filtered using a 0.45-µm PTFE filter.The nanoparticle precursor solutions were created by mixing the stock solutions in the desired nanoparticle composition ratio with total volume of 10 mL.Subsequently, the nanoparticle precursor solution was added to a pre-prepared 0.5 wt.% solution of sodium dodecyl sulfate (SDS) surfactant in deionised water (10 mL) resulting in a bilayer.This mixture was vigorously stirred for 15 min at 40 °C to create a pre-emulsion, which was further subjected to sonication for 20 min using an ultrasonic processor (Misonix Sonicator Ultrasonic Processor XL fitted with a 3 mm microtip) to form a mini emulsion.The mini-emulsion was heated at 85 °C under a stream of nitrogen with gentle stirring to remove the chloroform, leaving a surfactant-stabilised nanoparticle dispersion in water.Finally, the dispersion was filtered (0.45-μm PTFE) to remove any large aggregates before being stored in an amber vial to avoid over exposure to light.
Hydrogen evolution measurements.Hydrogen evolution from DTSRh, DTS13 and PNF222 nanoparticles was measured using ascorbic acid as the sacrificial hole scavenger and platinum as the co-catalyst.DTSRh, DTS13 and PNF222 nanoparticles with varying ratios of each component (2 mg from a 0.5 mg mL -1 dispersions) loaded with 10% platinum (1.25 mL from a 0.4 mg mL -1 aqueous potassium hexachloroplatinate solution) were made up to 25 mL total volume using an ascorbic acid solution (0.2 M) in a quartz flask with stirrer.The flask was degassed with nitrogen and irradiated with a 300 W Xe Oriel light source fitted with am AM1.5G filter.The hydrogen evolution rates were determined by taking gas from the head space at regular intervals and running these on a gas chromatograph (Thermo Scientific Trace 1300) equipped with a molecular sieve column.The amounts of hydrogen being produced were then calculated by using an external calibration.
CryoTEM imaging was done on a FEI Tecnai F20 transmission electron microscope operating at 200 kV, equipped with a Gatan cryoholder at −170 °C.Images were recorded using a Gatan CMOS Rio camera.S10 which began with a fourfold bromination of 2,2'-bithiophene 1 to produce tetrabromide 2 in good yield.Protection of the terminal 5,5'-positions of 2 was achieved via lithium halogen exchange followed by addition of trimethylsilyl chloride to give 3 in moderate yield (68%).A second lithium halogen exchange, followed by addition of dichlorodi-(n-hexyl)silane provided the desired tercyclic core 4 (65%).
Removal of the TMS groups and concomitant bromination of 4 was achieved using Nbromosuccinimide (NBS) in tetrahydrofuran (THF) in the absence of light to produce 5 in 68% yield.Compound 5 was then immediately subject to a Suzuki-Miyaura cross-coupling reaction with 4-formylbenzene boronic acid to afford the key intermediate dialdehyde 6 (60%).
Compound 6 was subject to Knoevenagel condensation with N-ethylrhodanine 7 to yield DTSRh in 76% yield while 1,3-indandione was introduced by using modified reactions conditions proposed by Zhang et al.S11 yielding DTS13 in 45%.Both compounds were purified using flash chromatography followed by slow reprecipitation in hexane at -18 °C.

3,3′-Dibromo-5,5′-bis(trimethylsilyl)-2,2′-bithiophene (3)
Compound 2 (5.00 g, 10.4 mmol) was dissolved in dry Et2O (50 mL) and the resulting solution was cooled to −78 °C using an acetone/dry ice bath.n-BuLi (2.5 M in hexanes, 8.7 mL, 21.8 mmol) was added dropwise over 5 minutes and the reaction was left for 2 hours at −78 °C.Chlorotrimethylsilane (2.37 mL, 21.77 mmol) was then added dropwise, and the reaction was left to stir at −78 °C for a further 15 minutes before the reaction mixture was allowed to warm to room temperature and stirred overnight.The reaction mixture was then poured on to cold water (100 mL) and extracted into Et2O (3 × 30 mL).The combined organic layers were washed with brine (50 mL) and dried over MgSO4 before the removal of the solvent under reduced pressure to yield the crude product as a purple oil.Purification was achieved using flash column chromatography (SiO2; 100% n-hexane; Rf = 0.49) to give 3 as a light yellow waxy solid (3.30 g, 68%).Mp. 86-87 °C.(Lit 87-88 °C).S14 The characterisation obtained was in good in agreement with literature.n-BuLi (1.90 mL, 2.5 M in hexanes 4.69 mmol) was added dropwise to a solution of 3 (1.00g, 2.13 mmol) in anhydrous THF (20 mL) under argon at −78 °C via an acetone/dry ice bath.The resulting mixture was stirred for a further 30 minutes at this temperature before dichlorodihexylsilane (0.67 mL, 2.55 mmol) was added in one portion and the mixture was warmed to room temperature and left to stir 2 hours.After this period the mixture was quenched with water and extracted into Et2O (3 × 20 mL).The combined organics were collected, dried over MgSO4 and solvent was removed under reduced pressure.The crude solid was purified with flash column chromatography (SiO2; 100% n-hexane; Rf = 0.77) to yield 4 as a colourless oil (0.74 g, 65%).

2,2'-(((4,4-dihexyl-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(4,1phenylene))bis(methanylylidene))bis(1H-indene-1,3(2H)-dione) (DTS13)
To a solution of dialdehyde (6) (0.10 g, 0.18 mmol,) and 1,3-indandione (0.10 g, 0.70 mmol) in CHCl3 (5 mL) in a foil wrapped round bottom flask, one drop of pyridine was added.The resulting solution was heated under reflux for 18 hours.After cooling to room temperature, MeOH (50 mL) was added, and the resulting precipitate was collected and was purified using flash column chromatography (SiO2; 100% CHCl3; Rf = 0.50) to yield a deep purple solid which was further purified by dissolving in a minimum amount of CH2Cl2 then triturating in MeOH,     After the first run, the photocatalytic reaction was degassed then the second run was commenced with the light source.Following the second run the photocatalytic reaction was degassed again and 5 mmol AA were added.Experimental conditions: 4 mL of a 0.5 mg mL −1 nanoparticle dispersion to obtain 2 mg of nanoparticles, 10 wt.% Pt (1.25 mL from a 0.4 mg mL −1 aqueous K2PtCl6 solution), 19.75 mL of a 0.2 M AA solution added to make the total volume up to 25 mL, degassed by nitrogen bubbling, irradiated with a 300 W Xe light source equipped with AM 1.5G filter).a In some literature examples the pH is not explicitly stated but presumed to be unaltered which is pH 2 for a solution of ascorbic acid.b HER rounded to the nearest hundred.c Either reduced pressure is used or pressure is not explicitly stated in the photocatalytic reaction conditions.

Figure S1 :
Figure S1: Molecular structures of Y6 and ITIC

Figure S8 :
Figure S8: Dynamic light scattering results for DTS13:PNF222 (1:1) nanoparticles in the presence of either ascorbic acid (AA, black and red trace) or triethylamine (TEA, green and blue trace) before and after photoirradiation (solution degassed with N2, 1.5 AG filter and 300 W Xe light source used for photoirradiation).

Figure S10 :
Figure S10: Mass normalised hydrogen evolution rate of DTS13:PNF222 (1:1) ncBHJ nanoparticles over longer timescales.The first run started with two hours in the dark then the light source was initiated.

Figure S12 :
Figure S12: Photocatalytic hydrogen evolution experiment of the single component nanoparticles used in this study.Experimental conditions: 2 mg nanoparticles from 0.5 mg mL −1 dispersions, 10 wt% Pt loading (1.25 mL from a 0.4 mg mL −1 aqueous K2PtCl6 solution), 0.2 M ascorbic acid solution added to make the total volume up to 25 mL and degassing with N2, atmospheric pressure, irradiated with a 300 W Xe light source equipped with an AM1.5G filter.

Table S2 :
UV/vis and cyclic voltammetry results for DTSRh and DTS13.
a Eg opt = 1240.68/onset.b Eg calc obtained from DFT. c Reversible reductions are quoted as Ehalf otherwise peak potentials are provided.irr Irreversible peak

Table S3 :
Dynamic light scattering results for nanoparticles of DTSRh, DTS13, PNF222 and combinations thereof.

Table S4 :
Dynamic light scattering results for DTS13:PNF222 (1:1) nanoparticles and also in the presence of either ascorbic acid (AA, black and red trace) or triethylamine (TEA, green and blue trace) before and after photoirradiation (solution degassed with N2, 1.5 AG filter and 300 W Xe light source used for photoirradiation).

Table S5 .
Literature examples of photocatalytic hydrogen evolution performance of BHJ nanoparticles.