Photocathodic hydrogen evolution from catalysed nanoparticle films prepared from stable aqueous dispersions of P3HT and PCBM

Synthetic Metals 247 (2019) 10–17

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Photocathodic hydrogen evolution from catalysed nanoparticle films prepared from stable aqueous dispersions of P3HT and PCBM

T

Subash Rajasekara,b, Patrick Fortina, Vinay Tiwarib, Umish Srivastvab, Alok Sharmab, ⁎ Steven Holdcrofta, a b

Department of Chemistry, Simon Fraser University, Burnaby, BC, V5A 1S6, Canada Alternate Energy Division, Indian Oil Corporation R&D Center, Faridabad, 121 007, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Photoelectrochemistry Organic semiconductor Polymer nanoparticle P3HT: PCBM bulk-heterojunction Solar hydrogen generation Mini-Emulsion

Photo-assisted hydrogen evolution is achieved on photocathodes comprising of nanoparticles of poly(3-hexylthiophene) (npP3HT) and nanoparticles of phenyl-C61-butyric acid methyl ester (npPCBM) onto which ultralow loadings of Pt nanoparticles are deposited. The nanoparticles, npP3HT and npPCBM, are prepared individually via miniemulsion using surfactants of opposite head group polarity. Aqueous dispersions of npP3HT:npPCBM, devoid of organic solvent, are cast conformally onto ITO-coated glass to yield water-insoluble bulk-heterojunction films. Pt (1 μg/cm2) catalyst is deposited photoelectrochemically onto ITO/ npP3HT:npPCBM photocathodes and found to nucleate preferentially on PCBM nanoparticles. ITO/ npP3HT:npPCBM/Pt photocathodes produce 65 μA/cm2 photocurrent under 100 mW/cm2 of visible light at 0.0 VSHE and liberate H2 gas. The photocurrents observed for electrodes prepared using npP3HT:npPCBM are twice as large, and the onset potential is ∼0.4 V more positive than analogous photocathodes cast from nanoparticles each comprising an intimate blend of P3HT and PCBM. These are encouraging results for large scale synthesis of organic photoelectrochemical devices, given the simplicity of the photoelectrode, i.e., prepared from aqueous solutions and devoid of vacuum-deposited films such as charge transport layers and protective films.

1. Introduction Currently, 75% of the global energy requirement is met from nonrenewable energy sources, which are expected to deplete in several centuries [1]. It is therefore important to research renewable energy technologies. Photoelectrochemical (PEC) splitting of water into hydrogen (H2) and oxygen (O2) using semiconductor electrodes is a potential strategy to convert and store the abundant solar energy that irradiates the earth’s surface [2]. A viable semiconductor photoelectrode for PEC water splitting must possess a low bandgap, enabling it to absorb a large portion of the solar spectrum, and have its electronic valence and conduction band edges below and above the redox potentials associated with the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively. The semiconductor must also possess good chemical stability, be non-toxic and ultimately, low cost [3–5]. To date, no single semiconductor satisfactorily meeting all these requirements has been discovered or synthesized [6]. Inorganic semiconductors such as Si, TiO2, Fe2O3, WO3, Ta3N5, CdS to name just a few, have been extensively examined, as either a photoanode or ⁎

photocathode [6]. Recently GaInP/GaInAs tandem semiconductor photoelectrodes were reported to exhibit a record 16% solar-to-hydrogen (STH) efficiency [7]. However, transferring laboratory scale demonstrations to large scale implementation is challenging, and it is necessary to continue to explore new materials for PEC applications. A class of materials of growing interest for photoelectrochemistry are semiconducting organic polymers. Organic polymers offer several advantages over inorganic materials due to their low density, low cost, robust mechanical properties and ease of processing [8–11]. Organic polymers can be readily deposited conformally on various substrates by spin coating, dip coating, inkjet printing and roll-to-roll processes, in high volume [12–14]. The photophysical properties of organic semiconducting polymers are notably different from their inorganic counterparts due to their relatively low dielectric constant [15]. Thus, upon absorption of photons organic semiconductors generate Coulombicallybound electron-hole pairs (excitons) in contrast to inorganic semiconductor analogues, where “free” charge carriers are generated [16]. The bound electron-hole pair dissociates upon contact with an electron acceptor species having sufficient electron affinity, whereby the polymer, (the donor), loses an electron [17]. For effective separation of

Corresponding author. E-mail address: [email protected] (S. Holdcroft).

https://doi.org/10.1016/j.synthmet.2018.11.004 Received 30 July 2018; Received in revised form 30 October 2018; Accepted 5 November 2018 Available online 20 November 2018 0379-6779/ © 2018 Elsevier B.V. All rights reserved.

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of an electron acceptor (phenyl-C61-butyric acid methyl ester, PCBM) in 2012 that the topic gained renewed interest [21]. The sequence of events leading to hydrogen evolution in this system is shown in Fig.1 for the case where PCBM serves as the electron acceptor and Pt electrocatalyzes the hydrogen evolution reaction. The current density achieved from photoelectrochemical cells using conjugated polymer electrodes have increased significantly from microamperes to milliamperes in just a few years of research. This has been achieved by using increasingly complex device structures. For instance, large photocurrents have been obtained through the implementation of photocatalysts (Pt, MoS3) and charge transport layers (CuI, PEDOT:PSS, TiO2) [22–24]. Fumagalli et al., observed photocurrents of 3 mA/cm2 from a ITO/MoO3/P3HT:PCBM/TiO2/Pt photocathode [25]. A photocurrent of 8 mA/cm2 has been obtained by employing a multi-layered device architecture consisting of ITO/ PEDOT:PSS/P3HT:PCBM/LiF/Al/Ti-MoS3 [24]. These advances have been facilitated by advances in organic polymer photovoltaic (OPV) research, and the use of vacuum-deposited multilayer structures to reduce charge recombination and stabilize devices in the presence of electrolyte, light and heat. A commonly used technique to increase photo-conversion efficiency of organic PV devices is to increase the interfacial area of the donor/ acceptor domains by forming bulk-heterojunction photoactive layers. To achieve this, donor and acceptor materials are dissolved in a suitable (usually halogenated) solvent and cast onto a substrate. During film formation, solvent evaporates, donor and acceptor components segregate into separate domains [26–28], producing an interpenetrating nanoscale donor/acceptor blend throughout the film which increases the interfacial area [29–31]. Scale up of such a process is challenging, however, because of the employment of halogenated solvents, and relatively poor control over the phase segregation dynamics. A way to exercise control over phase segregation in large volume applications is to use nanoparticle (NP) dispersions of polymer because the domain size of segregation is dictated by the size of the nanoparticles [27,32,33]. This approach is attracting interest in OPV research [34,35]. Two widely used methods to synthesize organic polymer nanoparticles are (i) precipitation and (ii) miniemulsion [36,37]. In the precipitation method, polymer nanoparticles are formed

Fig. 1. Energy level diagram for poly(3-hexylthiophene) (P3HT) and phenylC61-butyric acid methyl ester (PCBM) illustrating photo-assisted hydrogen evolution on Pt.

electrons and holes, the exciton must diffuse from the point of generation to the donor/acceptor interface before recombination occurs. A larger donor/acceptor interfacial area favors dissociation of excitonic charge separation and consequently a high photon-to-current conversion ratio. Unfortunately, the diffusion length of an exciton in organic polymers is typically < 20 nm which means excitons generated further than this distance from a donor/acceptor interface are likely to annihilate before separating into charges [18,19]. Poly(3-hexylthiophene) (P3HT) is a quintessential conjugated polymer that has been extensively studied and is relatively well understood - it serves as a model system to explore applications of conjugated polymers, in general. P3HT has a bandgap of 1.9 eV with a conduction band edge more negative than the standard electrochemical potential of H+/H2. P3HT was demonstrated to act as a photocathode for the hydrogen evolution reaction by El-Rashiedy and Holdcroft more than two decades ago [20], but it was not until Lanzarini et al. reported on the photoelectrochemical proton reduction on P3HT in the presence

Fig. 2. Schematic representation of a) synthesis of np (P3HT:PCBM) nanoparticle dispersion via precipitation method, formed by adding P3HT and PCBM dissolved in good solvent to a poor solvent [39]. b) Synthesis of npP3HT:npPCBM nanoparticles via the miniemulsion method. P3HT nanoparticles are prepared with an anionic surfactant, SDS, and PCBM nanoparticles are prepared with a cationic surfactant, CTAB, prior to their combination to form an aqueous-based dispersion.

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without further purification. Aqueous solutions were prepared from deionized (DI) water purified using a Milli-Q water purification system (18 MΩ, EMD Millipore).

by dissolving the polymer in a suitable solvent, usually a halogenated organic solvent, and adding this to a poor solvent, typically an alcohol, so as to collapse the polymer chains, forming nanoparticles [38,39]. A schematic of this method for the case of P3HT and PCBM is shown in Fig. 2a. Each nanoparticle consists of a thoroughly mixed blend of donor and acceptor, np(P3HT:PCBM). Though this method produces nanoparticles in a few simple steps, the nanoparticles often coalesce over a short time period, leading to precipitation and loss of usable dispersed material [40]. Moreover, halogenated organic solvents, used in the initial preparation step, poses health and environmental risks for large scale industrial applications [37,41]. In the miniemulsion method, NPs are formed in water. Polymer is dissolved in a suitable solvent (which is immiscible with water) and added to water containing a surfactant to create an oil-in-water system. By applying a high shear force to the mixture, nanodroplets of polymer solution are formed, stabilized by surfactant. By allowing evaporation and capture of the organic solvent, a surfactant-stabilized, water-based dispersion of polymer nanoparticles are produced, as illustrated in Fig. 2b. The advantage of the miniemulsion method over the precipitation method is that it produces stable, water-based nanoparticle dispersions - halogenated solvents may be evaporated, captured, and recycled [42–47]. P3HT:PCBM nanoparticles synthesized by the conventional miniemulsion method, where both P3HT and PCBM are initially dissolved together, has been shown to possess a core-shell morphology, with a PCBM rich core, due to P3HT having a lower surface energy than PCBM [33,36,37,48]. The core-shell structure limits both the generation of charges from photo-generated excitons and the transportation of electronic charges through the device [27]. For this reason, and due to the presence of passive surfactants which may impede charge transfer between nanoparticles, the OPV performance of active layers prepared in this manner were reported to be lower compared to nanoparticles prepared by the precipitation method [14,33,36,37,49,50]. Satapathi et al., prepared nanoparticles of donor and acceptor material independently to prepare films for photovoltaics devices, however, the photoconversion efficiencies were very low compared to traditionally prepared BHJ OPVs [51]. In this report, we build on the approach used by Satapathi et al. and describe a method to prepare photoactive films for PEC hydrogen evolution utilizing stable, chloroform-free, water-based nanoparticle dispersions that avoid the core-shell morphology through the independent formation of P3HT (npP3HT) and PCBM (npPCBM) nanoparticles. The two independently prepared nanoparticle solutions are combined to prepare the final npP3HT:npPCBM dispersions and deposited onto ITO electrodes. Two oppositely charged surfactants are used on P3HT and PCBM nanoparticles to take advantage of the electrostatic forces that arise to ensure a well blended active layer, effectively maximizing the donor-acceptor interfacial area. Ultra-low loadings of Pt catalyst are subsequently deposited by photoelectrochemical deposition to produce photocathodes capable of evolving H2 without the need of additional multilayer deposition processes. We herein report the properties of npP3HT:npPCBM films and compare them to analogous films of np(P3HT:PCBM) prepared by the precipitation method.

2.1. Miniemulsion nanoparticle synthesis P3HT and PCBM nanoparticles were prepared by the miniemulsion method using SDS and CTAB surfactants, respectively as follows (illustrated in Fig. 2b). P3HT (10 mg) was dissolved in 1 mL CHCl3 and stirred overnight at 55 °C (solution A). SDS (2 mg) was dissolved in 1 mL distilled water (solution B) to form a solution at critical micelle concentration (CMC). Solution B was added to solution A and stirred at 1000 rpm for 60 min at RT to form a macroemulsion, which was transferred to an ultrasonic bath and sonicated (Branson 1510 ultrasonic cleaner) for 30 min at room temperature (RT) to form a miniemulsion as indicated by colour change of the emulsion from bright orange to dark green. The miniemulsion was stirred at 61 °C, for 15 min to remove CHCl3 and the resulting nanoparticle dispersion was filtered and stored (note, the emulsion is termed as “dispersion” upon evaporation of the organic solvent). PCBM nanoparticles (npPCBM) were synthesized in a similar manner except a cationic surfactant CTAB (0.3 mg), was used, at its CMC. The npPCBM dispersion was added dropwise to the npP3HT dispersion while the latter was immersed in ultrasonic bath (Branson 1510 ultrasonic cleaner) and finally filtered through cotton-tipped glass pipette to yield a water-based dispersion of npP3HT:npPCBM.

2.2. Precipitation nanoparticle synthesis Dispersions of P3HT:PCBM nanoparticles, i.e., P3HT and PCBM blended within each nanoparticle, were prepared by the precipitation method as illustrated in Fig. 2a by dissolving 10 mg of P3HT and 10 mg of PCBM separately, each in 1 mL of CHCl3, stirring overnight at 55 °C. The solutions were mixed together, and 1.5 mL of this solution added by syringe to 4 mL of isopropyl alcohol (IPA) under sonication at room temperature for 1 min (Branson 1510 ultrasonic cleaner). The obtained dispersion of np(P3HT:PCBM) was filtered through cotton-tipped glass transfer pipette before spin coating and used within 24 h. 2.3. Preparation of thin films P3HT:PCBM bulk-heterojunction thin-film electrodes were prepared by mixing 15 mg of P3HT and 15 mg of PCBM separately, each in 0.5 mL of chlorobenzene, stirring overnight at 55 °C. The solutions were mixed and filtered through 0.45 μm PTFE filter before spin coating onto a clean ITO electrode.

2.4. Fabrication of photoelectrode and Pt deposition ITO coated glass slides were cleaned sequentially under sonication for 10 min in soapy water, DI water, ethanol and IPA, followed by an O2 plasma treatment (Fischione Instruments model 1020 plasma cleaner) using 25% O2 in Ar for 10 min. The P3HT:PCBM nanoparticle dispersion was spin coated (100 μl/cycle) onto 2 cm x 1 cm ITO slides at 2000 rpm for 60 s. A film thickness of ∼200 nm was achieved with ten spin-coating cycles. Pt catalyst was photoelectrochemically deposited on top of polymer nanoparticle films by applying a cathodic potential of -0.1 VSCE to the electrode under chopped light illumination (100 mW/ cm2) in 0.1 mM K2PtCl6/1 M H2SO4. The mass of platinum deposited was calculated from the charge passed during electrodeposition [52]. In this work, 1 μg/cm2 of Pt was deposited on the nanoparticle films. At this low of a Pt loading, the Pt was deposited as nanoparticles, as demonstrated later in this report.

2. Materials and experimental methods Regioregular poly(3-hexylthiophene) (RR-P3HT, 91–94%) and 6,6phenyl-C61-butyric acid methyl ester (PCBM) (> 99.5%) was purchased from Rieke Metals Inc. and from American Dye Source Inc., respectively. Potassium hexachloroplatinate (K2PtCl6) was procured from Johnson and Matthey Inc. HPLC-grade chloroform (CHCl3, ≥99.9%), sodium dodecyl sulphate (SDS, 70%) and cetyl trimethyl ammonium bromide (CTAB, 95%) were obtained from Sigma-Aldrich. Indium tin oxide (ITO) coated glass was purchased from Colorado Concept Coatings LLC. All reagents were of analytical grade and used 12

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2.13 m Agilent J&W GC packed column in stainless steel tubing was used (inner diameter 2 mm, HayeSep N packing material, 60/80 mesh size). Argon was used as a carrier gas at a flow rate of 30 mL/min under 46.2 psi. A detailed description of this apparatus has been reported previously [52].

3. Results and discussion

Fig. 3. DLS-determined particle size of npP3HT:npPCBM prepared by the miniemulsion method measured over 30 days, depicting the stability of the nanoparticle dispersions prepared via the miniemulsion method.

Aqueous dispersions of npP3HT and npPCBM were independently synthesized via miniemulsion method using SDS and CTAB surfactants, respectively. The bright orange colour of P3HT in CHCl3 transformed to dark green upon formation of the miniemulsion and became pink after evaporation of CHCl3. Similarly, the purple colour of PCBM in CHCl3 turned white during miniemulsion formation and deep yellow after evaporation of CHCl3. The npPCBM dispersion was added dropwise to the npP3HT dispersion under sonication to form 1:1 blend of npP3HT:npPCBM dispersion in aqueous media which appeared red in colour.

2.5. Characterization techniques

3.1. Particle size measurement

The particle size distribution of the nanoparticles was measured by dynamic light scattering (DLS) using a NanoSeries Nano-ZS (Malvern Instruments, UK) equipped with a helium-neon laser source (Power = 4.0 W; λ = 633 nm). Dispersions of nanoparticles were diluted in water to avoid multiple light scattering. Typically, 10 μL of dispersion was diluted with 2 mL of water. The size of the nanoparticles was calculated from the diffusional properties of the nanoparticles based on the Stokes-Einstein relationship (see supplementary information) [53]. Each measurement was performed in triplicate. Error bars report the standard deviation of the nanoparticle size between measurements. The film thickness of nanoparticles coated on ITO glass was measured using an Alpha-step IQ profilometer (KLA-Tencor). Scanning electron micrographs were collected on a FEI Nova NanoSEM with a beam acceleration voltage of 5 kV and working distance of 5 mm. Nanoparticle-coated ITO substrates were fixed to an aluminum stub using carbon tape. UV–vis analysis was carried out on diluted dispersions of nanoparticles in quartz cuvettes using a Cary 300 Bio UV–vis spectrophotometer. Absorption spectra of nanoparticle films were measured on ∼200 nm thick nanoparticle films on ITO. For photoelectrochemical measurements, a 200 W Xe/Hg lamp was used as the light source (Uhsio America, Inc) in conjunction with a visible light band-pass filter (FSQ-KG 3, Newport Corp., λ: 300–700 nm) and neutral density filter (Thorlabs Inc.) to achieve an irradiation intensity of 100 mW/cm2, measured using a broadband power meter (841-PE, Newport Corporation) equipped with an Ophir thermal detector head (3A-P-SHV1). The cell configuration was designed to allow irradiation of the photoelectrode–electrolyte interface through the electrolyte as shown in Fig. S1. A water filter was placed in front of the electrochemical cell to minimise heating of the electrolyte. Electrochemical measurements were performed using a Pine Bipotentiostat (AFC-BP1) and data was analysed using Aftermath Scientific Data Organization software (ASTP-B01 Module). PEC measurements were performed using a 3-electrode configuration with a saturated calomel reference electrode (SCE) (+0.24 V vs SHE) and a Pt wire as counter electrode in 0.1 M H2SO4 at room temperature. The electrolyte solution was purged with nitrogen (N2) for one hour before electrochemical measurements to remove dissolved gases [52]. The experimental setup is illustrated in the SI. The headspace of the electrochemical cell was analyzed for H2 gas using a 5 mL syringe, fitted with an air tight valve (Series A-2, VICI Precision Sampling), and analyzed using an Agilent Technologies 6890 N GC system equipped with a thermal conductivity detector. A

Plots of the particle size distribution of P3HT nanoparticles and PCBM nanoparticles determined by DLS are shown in Fig. S2. The average diameters of npP3HT and npPCBM were 110 ± 5 and 412 ± 8 nm, respectively (Fig. S2 a&b). The larger size of npPCBM is due to the lower concentration of CTAB surfactant (0.3 mg/mL vs. 2 mg/mL for SDS) as particle size is inversely proportional to surfactant concentration [47]. The particle size of the npP3HT:npPCBM dispersion was measured as 151 ± 7 nm (Fig. S2 c). For comparison, nanoparticles comprising a blend of P3HT and PCBM i.e., np(P3HT:PCBM) prepared via the precipitation method possessed an average diameter of 140 ± 6 nm (Fig. S2 d). Dispersion of npP3HT:npPCBM prepared via the miniemulsion method appeared stable with no signs of agglomeration or precipitation after one month, as illustrated by the plot of average particle size with time (Fig. 3). Dispersions of np(P3HT:PCBM) prepared via the precipitation method, on the other hand, agglomerated rapidly and the onset of precipitation from solution started to occur within a few hours to 24 h depending on the concentration of the dispersion. The size of nanoparticles in the npP3HT:npPCBM dispersion differed to that of the individual nanoparticles from which they were formed, as observed in Fig. S2. However, it should be noted that the Coulombic effect between positive (npPCBM) and negative (npP3HT) charged particles that arises from cationic (CTAB) and anionic (SDS) surfactants affect the diffusion coefficient of the particles and hence the DLS-determined particle size, which is calculated from the StokesEinstein equation (see SI).

3.2. Scanning electron microscopy Nanostructured P3HT:PCBM films were prepared on ITO by spincoating the npP3HT:npPCBM or np(P3HT:PCBM) dispersions. Onto these films, Pt nanoparticles were electrodeposited. Representative SEM images of these films are shown in Fig. 4. In both examples, the nanoparticles form an interconnected network structure, but it is notable in films of npP3HT:npPCBM, npP3HT and npPCBM can be distinguished from each other. In this instance, npPCBM particles appear larger (∼350 nm) and brighter than npP3HT particles (∼90 nm). The particle diameters measured by SEM are smaller than those obtained by DLS, as DLS measures the hydrodynamic diameter of particles, resulting in diameters larger than the true particle size [39]. The much smaller bright spots are Pt particles (∼50 nm). It is observed that Pt particles preferentially deposit on PCBM, which is consistent with PCBM accepting photogenerated electrons from P3HT, as illustrated in Fig. 1. 13

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Fig. 4. (a) SEM images of npP3HT:npPCBM films under different magnification. The larger, brighter particles are npPCBM, the darker interconnected particles are npP3HT, and the smaller bright spots are Pt. (b) SEM image of np(P3HT:PCBM) film, illustrating no distinction between P3HT and PCBM domains due to the well blended nature.

the ratio of the intensity of the vibronic shoulders in the UV–vis spectrum (at 610 nm and 555 nm). The exciton bandwidth, W, of npP3HT:npPCBM films was calculated to be 157 meV, which is lower than for np(P3HT:PCBM) films (193 meV), and indicates higher structural order, (i.e., crystallinity) within nanoparticles prepared via miniemulsion method. This is due to the npP3HT:npPCBM being comprised of individual npP3HT and npPCBM nanoparticles, as it has been previously shown that the addition of PCBM disrupts the long-range order of the P3HT chains in P3HT:PCBM thin films [59,60]. Consequently, it is expected that, due to increased interchain packing, npP3HT:npPCBM films to exhibit higher charge carrier mobility than np(P3HT:PCBM) films [59,61,62].

3.3. UV–vis absorption spectroscopy The absorption spectra of nanoparticle films prepared by spin coating dispersions of npP3HT:npPCBM and np(P3HT:PCBM) are shown in Fig. 5. Three distinct peaks were observed which are characteristic of P3HT. A broad peak was observed at 517 nm, with shoulders at 555 nm and 610 nm corresponding to 0–1 and 0-0 transitions, respectively, according to the H-aggregate model [54–57]. The shape and magnitude of vibronic shoulders have been shown to provide insight into the structural order within the P3HT domains [58]. Comparing the absorption spectra of the npP3HT:npPCBM dispersions and their films to np(P3HT:PCBM) dispersions and films, it is observed that although both films show peaks at similar wavelengths, a slightly increased vibronic shoulder intensity was observed for npP3HT:npPCBM particles, indicating a higher degree of structural order within P3HT domains. As this was observed for both film and the dispersion of nanoparticles, the effect is considered inherent to the nanoparticle which could be due to the npP3HT domains in npP3HT:npPCBM nanoparticles being devoid of PCBM. According to the H-aggregate model of Spano [54,56], the ratio of the intensities of the 0-0 and 0–1 transitions is related to the free exciton bandwidth (W), Eq. (1), which provides insight into interchain order and charge carrier mobility [55,58].

3.4. Photoelectrochemical measurements Photoelectrochemcial measurements were carried out using npP3HT:npPCBM and np(P3HT:PCBM) films, onto which Pt is deposited, in 0.1 M H2SO4 under visible light (100 mW/cm2) irradiation at room temperature in a three electrode configuration with ITO/ P3HT:PCBM NPs/Pt as the working electrode, a Pt wire counter electrode and a saturated calomel electrode (SCE) reference electrode. Photopotential and photocurrent measurements were carried out to gain insight into the dynamics of the photoelectrode. The photopotential, Eph, is taken as the difference between the Fermi level of the bulk semiconductor and the redox potential of the species in solution, under illumination, and is often interpreted as the driving force of the photoelectrode [63]. For practical purposes, Eph is usually calculated from the difference in open circuit potential (OCP) under dark and

2

1-0.24W/E p ⎞ A 0-0 = ⎜⎛ ⎟ A 0-1 1 ⎝ +0.073W/E p ⎠

In Eq. (1), Ep is the energy of C]C stretching (∼170 meV),

(1) A 0-0 A 0-1

is

Fig. 5. Normalized UV–vis absorption spectrum of a) P3HT:PCBM nanoparticle dispersions prepared via miniemulsion and precipitation methods and b) corresponding nanoparticle films spin coated on ITO. 14

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irradiated conditions (Eq. (2)) [64].

E ph = |EOCP, dark - EOCP, light|

(2)

EOCP,dark is -0.05 V (after stabilizing for 100 s) for ITO/ npP3HT:npPCBM/Pt electrodes (Fig. 6) and upon illumination, EOCP shifts to a more positive potential, characteristic of a p-type photoelectrode (See Fig. S3 details), to more than +0.250 V, from which Eph is estimated to be 0.3 V. A similar value is observed for ITO/np (P3HT:PCBM)/Pt photoelectrodes. Linear sweep voltammetry curves for nanoparticle and thin film electrodes were measured between +0.6 V and -0.4 V (SCE) and subjected to 5 s intervals of dark and illumination are shown in Fig. 7a-c. Cathodic current and zero current regions are labelled as regions I and II, respectively. The photocurrents increase with increasingly negative applied potential to 100 μA/cm2, whereas in the absence of light the residual current is negligible. The onset potential (Eonset) for photocathodic current is ∼0.4 VSCE (0.64 VSHE) for the npP3HT:npPCBM electrodes, while Eonset of the np(P3HT:PCBM) electrodes is substantially more negative at ∼0.1 VSCE (0.34 VSHE). The minimum additional potential thermodynamically required to photoelectrochemically split water is thus 0.59 V and 0.94 V for npP3HT:npPCBM and np(P3HT:PCBM) electrodes, respectively. For comparison, a planar P3HT:PCBM electrode is shown in Fig. 7c.

Fig. 6. Open circuit potential (OCP) of ITO/npP3HT:npPCBM/Pt measured under light and dark conditions (I = 100 mW/cm2, λ: 300–700 nm) in 0.1 M H2SO4. A positive shift in OCP upon illumination indicates a p-type photoelectrode.

Fig. 7. (a–c) Linear sweep voltammetry and (d–f) photoelectrolysis of ITO/ npP3HT:npPCBM/Pt (pink), ITO/np (P3HT:PCBM)/Pt (blue) and ITO/P3HT:PCBM/ Pt (green) photocathodes. LSV measurements were carried out under chopped light illumination intervals, 5 s dark and 5 s light, at 5 mV/ s. Photoelectrolysis of the electrodes were tested with intermittent light illumination at -0.24 VSCE (0.0 VSHE). Tests were carried out in with 100 mW/cm2 (λ: 0.1 M H2SO4 300–700 nm) illumination intensity. Pt and SCE were used as counter and reference electrodes, respectively (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

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Fig. 8. a) Schematic depicting npP3HT(orange):npPCBM(purple) nanoparticle coated ITO substrate and Pt catalyst (grey) and b) Photograph of photocathode during photoelectrolysis showing H2 evolution (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

the excess surfactants by dialysis, the properties of the films can be controlled by manipulating the particle size and ratio of the donor-acceptor nanoparticles, and the electrocatalytic properties may be enhanced by controlling the Pt loading, it’s particle size and shape, or by using non-platinum group metal catalysts.

Photoelectrolysis of the nanoparticle and thin film electrodes were carried out at constant potential of -0.24VSCE (0 VSHE) with alternate dark and light cycles, the result of which is shown in Fig. 7d-f. Negligible current densities were observed under dark conditions, while photocurrent densities of -65 μA/cm2 and −35 μA/cm2 were measured for ITO/npP3HT:npPCBM/Pt and ITO/np(P3HT:PCBM)/Pt photoelectrodes, respectively. Photoelectrolysis of ITO/npP3HT:npPCBM/Pt prepared immediately after nanoparticle synthesis and after storing the dispersion for extended periods prior to casting onto ITO, yielded similar photocurrents, is shown in Fig. S4 indicating stability of the dispersions of synthesized nanoparticles. Photocurrents from the nanoparticle electrodes are significantly higher than the planar (ITO/ P3HT:PCBM/Pt) electrode which is less than −10 μA/cm2.

Declaration of interest None. Funding This research was financially supported by the Natural Sciences and Engineering Research Council of Canada. The authors gratefully acknowledge financial assistance to SR by way of a SFU-IOC PhD fellowship.

3.5. GC analysis A photograph of an ITO/npP3HT:npPCBM/Pt photoelectrode immersed in the photoelectrochemical cell, taken during photoelectrolysis at -0.24VSCE (0.0 VSHE) is shown in Fig. 8, where gas bubbles can be observed evolving from the electrode surface. To identify the liberated gas, GC analysis of the head-space of the photoelectrochemical cell was carried out as previously described (Fig. S5) [39]. As a control experiment, hydrogen was electrolytically generated by applying -0.35 VSCE to Pt foil for 10 s passing 37 mC of charge, with Pt wire and an SCE serving as counter and reference electrodes respectively, immersed in 0.1 M H2SO4 and the evolved hydrogen quantitatively analyzed [39]. From the elution trace of the GC chromatographs, the gas evolved from the photocathode was confirmed to be hydrogen (Fig. S5), and from the Coulombic charges passed the quantity of hydrogen liberated is calculated to be 0.63 μmol/hr/cm2 with minimum 70% Faradaic efficiency.

Acknowledgements The authors would like to thank the machine shop, electronic shop and glass blower (Bruce Harwood) at SFU for their assistance in manufacturing the PEC cell. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2018.11. 004. References [1] Ren21, Renewables 2017 Global Status Report, (2017) 33-34. [2] A. Fujishima, K. Honda, Electrochemical photolysis of water at a semiconductor electrode, Nature 238 (1972) 37–38. [3] K. Sivula, F. Le Formal, M. Gratzel, Solar water splitting: Progress using hematite αFe2O3 photoelectrodes, ChemSusChem 4 (2011) 432–449. [4] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446–6473. [5] R. Abe, Recent progress on photocatalytic and photoelectrochemical water splitting under visible light irradiation, J. Photochem. Photobiol. C Photochem. Rev. 11 (2010) 179–209. [6] T. Jafari, E. Moharreri, A.S. Amin, R. Miao, W. Song, S.L. Suib, Photocatalytic water splitting-The untamed dream: a review of recent advances, Molecules 21 (2016) 1–29. [7] J.L. Young, M.A. Steiner, H. Döscher, R.M. France, J.A. Turner, T.G. Deutsch, Direct solar-to-hydrogen conversion via inverted metamorphic multi-junction semiconductor architectures, Nat. Energy 2 (2017) 17028. [8] T.L. Benanti, D. Venkataraman, Organic solar cells: an overview focusing on active layer morphology, Photosyn. Res. 87 (2006) 73–81. [9] J.C. Brabec, S. Gowrisanker, J.M. Halls, D. Laird, S. Jia, P.S. Williams, Polymerfullerene bulk-heterojunction solar cells, Adv. Mater. 22 (2010) 3839–3856. [10] Y. Zheng, Nanostructured Thin Films for Organic Photovoltaic Cells and Organic Light Emitting Diodes, University of Florida, 2009. [11] H. Hoppe, N.S. Sariciftci, Organic solar cells: an overview, J. Mater. Res. 19 (2004)

4. Conclusions npP3HT and npPCBM nanoparticles were individually prepared by miniemulsion in aqueous solution using surfactants of opposite charge. The individual dispersions were mixed to form stable dispersions that can be cast conformally on ITO to form water-insoluble films. The nanoparticles partially coalesce to form a P3HT:PCBM bulk-heterojunction film. Irradiation of films electrocatalyzed with Pt nanoparticles were found to yield a higher photocurrent and positive onset potential compared to films prepared from dispersions of homogeneous nanoparticles of P3HT/PCBM. By separating the npP3HT and npPCBM nanoparticles, the long-range order within P3HT nanoparticles and hence electronic properties are preserved. The photoelectrochemical properties of these electrodes are unoptimized and this work therefore represents a preliminary study that provides the basis for exploring new nanoparticle systems with different photoactive polymers and greater control and variation of the dispersion formed. For example, the purity of the nanoparticle dispersions may be further improved by removing 16

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