Plasmonic enhancement of semiconducting double cable polythiophene–perylene diimide polymer performance in artificial photosynthesis of H₂O₂

Abstract

Organic semiconducting double cable polymers (DCPs) have emerged as promising single-component donor–acceptor (D–A) photoactive materials. However, their light harvesting and energy conversion efficiencies are limited by their intrinsic chemical nature. Herein, we demonstrate that interfacing DCPs with rationally selected functional additives is an effective avenue to enhance their performance in light-driven processes without altering their chemical nature. Capitalizing on the ability of polythiophenes (PTh) to transfer as-prepared plasmonic gold nanoparticles (AuNPs) from aqueous suspensions to the organic phase without the need for ligand exchange, a DCP comprising PTh covalently conjugated to a perylene diimide (PDI) derivative (PTh-PDI-DCP) was employed to produce PTh-PDI-DCP/AuNPs hybrids containing different wt% of AuNPs. Monitoring changes in the optical and electronic properties of the resulting hybrids as a function of varying wt% of AuNPs enabled identification of the optimum AuNPs content, which was found to be 13 ± 1 wt% for PTh-PDI-DCP/AuNPs and 10 ± 1 wt% for P3HT/AuNPs prepared for comparison. In photoelectrochemical (PEC) characterizations and application as a photocatalyst in artificial photosynthesis of hydrogen peroxide (H₂O₂), the PTh-PDI-DCP/AuNPs hybrid outperformed all other systems. Comparing the photocatalytic cycles with the highest production rates in 10 photocatalytic cycles of artificial photosynthesis of H₂O₂, PTh-PDI-DCP/AuNPs achieves an H₂O₂ production rate of 233 ± 12 µM mg⁻¹ h⁻¹, which corresponds to about 3.8, 2.3 and 1.4-fold enhancements relative to AuNPs, P3HT and P3HT/AuNPs, respectively, and 1.4-fold over PTh-PDI-DCP. In summary, this study provides a viable pathway for expanding the design space of organic photocatalysts via the rational integration of DCPs with complementary functional additives.

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Introduction

The application of organic semiconducting polymers (OSPs) as photocatalysts in artificial photosynthesis of solar fuels has attracted significant attention over the past decade.¹ Since it directly converts solar energy into chemical energy, photocatalysis is an attractive way of harnessing solar energy, addressing global energy and environmental challenges.² The widespread adoption of OSPs in diverse applications,³ including their recent emergence as photocatalysts, is attributed to their ease of solution processability, light weight, and relatively higher transparency and flexibility compared to inorganic semiconductors.⁴ In addition, the molecular design and chemical nature of OSPs and their corresponding optoelectronic properties have evolved significantly,⁵ resulting in increasingly commendable control over structure–property relationships for customization against the requirements of target applications.⁶

In the context of the light harvesting and energy conversion efficiencies of OSPs as photocatalysts, managing the lifetime of excitons generated upon photoexcitation of OSPs is of paramount significance.⁷ OSPs typically display high binding energies (0.3–1.0 eV) manifesting in short exciton lifetimes and rapid recombination, which limit the efficiencies of photo-driven processes.⁸ Exciton management strategies include interface-driven concepts as well as chemical and architectural molecular design level interventions.⁹ One of the key interface-driven strategies involves enhancement of charge separation and migration of excitons by physically interfacing the photoelectron donor (D) material with an electron acceptor (A) displaying higher electron affinity.¹⁰ This bulk heterojunction (BHJ) arrangement has been exploited to interface small molecule and polymeric organic semiconductors with fullerenes as well as non-fullerene acceptors (NFAs) in a nanoparticle setting for achieving enhanced photocatalytic efficiencies.^11–14 Building on the advancements in BHJ arrangements,¹⁵ a natural evolution has been the development of single component materials where D–A components are covalently conjugated.¹⁶ The D–A covalent conjugation spatially locks the D–A molecules together, which improves the D–A electronic communication as well as simplifies the photoactive layer fabrication process. The D–A covalent conjugation strategy has led to the development of the following molecular designs of organic semiconducting materials: (a) covalent conjugates of small molecule-based D–A components,¹⁷ (b) OSPs with D–A components as part of the π-conjugated system,¹⁸ and (c) double cable polymers (DCPs) consisting of linear donor OSPs covalently conjugated to acceptor molecules.¹⁹ Interestingly, OSPs have also evolved into a range of molecular architectures in terms of dimensionality. These include linear OSPs, conjugated microporous polymers (CMPs), and covalent organic frameworks (COFs).²⁰ All these molecular architectures of OSPs are being extensively researched for developing advanced photocatalysts.²¹ Recent progress in developing advanced more efficient photocatalysts is primarily focused on refinements of OSPs for achieving suitable bandgaps, absorption ranges and energies of frontier orbitals.^22,23 The intrinsic photocatalytic efficiencies of OSP systems can, however, be enhanced by interfacing them with rationally selected functional additives without the need for changing their chemical structures.²⁴ This heterostructure strategy, although holding enormous potential, has been relatively less explored for augmenting the photocatalytic performance of OSPs.

Among linear OSPs, DCPs have emerged as promising single component D–A photoactive materials.^25,26 We have previously contributed to the development of new DCPs and evaluated their photocatalytic activities.^27–29 Recently, we have started exploring the possibilities of interfacing DCPs with rationally selected functional additives to enhance their efficiencies as photocatalysts. In this context, apart from the addition of NFAs³⁰ and perovskites³¹ to DCPs to improve their photoactivity, the heterostructure strategy interfacing DCPs with functional additives has not been explored much. Filling in this knowledge gap and for the proof of concept, this study demonstrates an enhancement of the photocatalytic performance of a DCP in artificial photosynthesis of H₂O₂ as a result of interfacing it with plasmonic AuNPs. The integration of OSPs with plasmonic nanostructures enables strong plasmon–exciton coupling that leads to increased light absorption via localized electromagnetic field amplification and facilitates charge generation and separation,³² which can ultimately improve the photocatalytic performance of OSPs. In this study, we employed a polythiophene (PTh) based DCP “PTh-PDI-DCP” comprising PTh covalently conjugated to a perylene diimide (PDI) derivative. The interest in integrating PDI in the DCP photocatalytic system is driven by its exceptional electron accepting character and high photochemical stability.³³ PDI derivatives have been employed in diverse photocatalytic reactions,³⁴ including H₂O₂ production.³⁵ The molecular engineering strategies applied to PDI-based materials have resulted in precise tuning of optical absorption profiles, charge transport characteristics and redox potentials.³⁶ The charge carrier recombination in PDI derivatives, however, remains a persistent limitation.³⁷ Recent investigations have focused on the construction of heterojunction photocatalysts, including Z-scheme³⁸ and S-scheme configurations,³⁹ achieved by integration with inorganic semiconductors,⁴⁰ and OSPs.⁴¹ Our PTh-PDI-DCP is a contribution of these evolutions. Such hybrid systems have shown significantly improved photocatalytic performance. Learnings from these collective advancements are laying the foundation for the development of novel molecular architectures and heterostructures designed to precisely control D–A interactions, leading toward next generation photocatalytic systems. For preparing DCP/AuNPs hybrids, we capitalized on the unique ability of polythiophenes to associate with metal surfaces through a combination of sulfur–metal coordination⁴² and π–metal interactions.⁴³ This association of PThs with metal surfaces enabled us to transfer the as-prepared AuNPs from aqueous suspensions to the organic phase without the need for ligand exchange. The resulting PTh-PDI-DCP/AuNPs showed enhanced photocatalytic performance in the artificial photosynthesis of H₂O₂. For comparison, P3HT was used to produce a P3HT/AuNPs hybrid. Due to the favourable alignment of bandgaps and frontier orbital energies between polythiophene, PDI and the work function of AuNPs, the unique PTh-PDI-DCP/AuNPs hybrid system outperformed all other systems as a photoactive material in photoelectrochemical (PEC) characterizations and artificial photosynthesis of H₂O₂. Overall, this study provides a viable pathway for expanding the design space of photoactive materials via the rational integration of DCPs with complementary functional additives.

Experimental section

Materials and methods

Chloroform (CHCl₃) (≥99.5%), methanol (≥99%), n-hexane (≥99%), isopropanol (IPA, ≥98–99%), acetone (≥98%), toluene (≥99%), acetonitrile (97–99%, ACS reagent), tetrahydrofuran (THF) (≥99.5%), sodium perchlorate monohydrate (NaClO₄·H₂O, >98%), 2,5-dibromo-2-hexylthiophene (DB3HT), tert-butylmagnesium chloride (t-BuMgCl), (3-bis(diphenylphosphino)propane)dichloronickel(II) [Ni(dppp)Cl₂], cesium carbonate (Cs₂CO₃), tetrakis(triphenylphosphine)palladium(0) [Pd(PPh₃)₄], fluorine-doped tin oxide-coated glass (FTO, 10–12 Ω sq⁻¹), hydrogen peroxide (H₂O₂, 35 wt% aqueous solution), para-benzoquinone (p-BQ, 98–99%), tert-butanol (tert-BuOH, 98–99%), silver nitrate (AgNO₃, 99.5%), potassium iodide (KI, >99%), potassium hydrogen phthalate (KHP), sodium citrate tribasic dihydrate (≥99%), hydrogen tetrachloroaurate(III) (HAuCl₄, 99.9%), Nafion solution (20 wt% in lower aliphatic alcohols and water, containing 34% water content) and Durapore® hydrophobic polyvinylidene fluoride (PVDF) membranes (pore size 0.45 µm, diameter 47 mm) were purchased from Sigma-Aldrich and used as received. Deionized water (DI H₂O) was obtained from a Millipore Milli-Q purification system. Gold nanoparticles (AuNPs) were synthesized using a modified Frens method as reported in the literature.^44,45

Synthesis of P3HT and PTh-PDI-DCP

P3HT and DCP consisting of PTh conjugated to N-(4-bromophenyl)-N′-hexylheptyl-3,4,9,10-perylene diimide (PDI) denoted as PTh-PDI-DCP were synthesized following previously reported protocols.²⁷ The schemes for the synthesis of P3HT and PTh-PDI-DCP (Scheme S1) and the corresponding characterization data (Fig. S1) are provided in the supplementary information (SI). P3HT was synthesized using Grignard metathesis (GRIM) polymerization. Briefly, in a 250 mL three-neck round-bottom flask, 3.2 g (10.0 mmol) of DB3HT was dissolved in 100 mL of THF under an inert atmosphere. To this solution, 5.2 mL (10.5 mmol) of t-BuMgCl was added dropwise at room temperature. The reaction mixture was then refluxed for 1 h. After that, the mixture was allowed to cool to room temperature and Ni(dppp)Cl₂ was added to the reaction mixture as a catalyst and the polymerization was carried out for 10 minutes. The reaction was then quenched by pouring the reaction mixture into cold methanol, followed by purification using Soxhlet extraction by sequential extraction with methanol, acetone and CHCl₃. P3HT was obtained after the rotary evaporation of the CHCl₃ extract. For the synthesis of PTh-PDI-DCP, a polythiophene derivative with pendant boronic ester groups (PTh-BE) was synthesized using a combination of GRIM polymerization and side chain engineering as reported previously.²⁷ PTh-BE was then conjugated to a perylene diimide (PDI) derivative via a Suzuki coupling reaction. For Suzuki coupling, 50 mg of PTh-BE was dissolved in 20 mL of toluene in a 50 mL Schlenk flask. 24 mg of Cs₂CO₃, 6 mg of Pd(PPh₃)₄ and 28 mg of PDI derivative were added to this solution. The reaction mixture was heated at 110 °C for 48 hours under an inert atmosphere. After completion of the reaction, the mixture was precipitated in cold methanol, followed by purification by Soxhlet extraction with methanol, acetone, n-hexane and finally CHCl₃ to extract the polymer. PTh-PDI-DCP was obtained after removing CHCl₃ using the rotary evaporator, followed by drying under vacuum. The chemical structures of all the materials are provided in Fig. 1a and Scheme S1.

Fig. 1 (a) Chemical structures of the materials used in this study. (b) HOMO–LUMO energy levels and band-gap alignment of PTh and PDI, along with the work function of AuNPs. (c) Particle size distribution of the synthesized AuNPs; inset: STEM image of AuNPs and digital photograph of AuNPs suspension in DI H₂O. (d) Illustration of the biphasic system used to transfer AuNPs from the aqueous to the organic phase. (e) ICP-OES estimation of the wt% of AuNPs extracted using P3HT and PTh-PDI-DCP in a biphasic DI H₂O/CHCl₃ system using varying volumes of aqueous suspension of AuNPs. (f) Average current densities obtained from CV and LSV measurements of P3HT and PTh-PDI-DCP as a function of AuNPs wt% in the resulting hybrids in the dark and under illumination. (g) UV/Vis absorption spectra of AuNPs, P3HT, P3HT/AuNPs, PTh-PDI-DCP and PTh-PDI-DCP/AuNPs. (h) Emission spectra of P3HT, P3HT/AuNPs, PTh-Opy, PTh-PDI-DCP and PTh-PDI-DCP/AuNPs (λ_ex = 440 nm). (i) TRPL spectra of P3HT, P3HT/AuNPs, PTh-PDI-DCP and PTh-PDI-DCP/AuNPs recorded at λ_ex = 440 nm and detection wavelength of 565 nm.

Instrumentation

Gel permeation chromatography (GPC) analyses were carried out using a PSS SECcurity GPC system (Polymer Standards Service GmbH) equipped with an autosampler, a PSS SDV 5 μm guard column and two PSS SDV 5 μm linear M columns connected in series along with a refractive index detector. THF was used as the eluent at a flow rate of 1.0 mL min⁻¹ at 40 °C. Calibration was performed using linear polystyrene (PS) standards (266 to 8.51 × 10⁵ g mol⁻¹) and data were analyzed using WinGPC software. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra were recorded on a Bruker Alpha ATR-FTIR spectrometer. Nuclear magnetic resonance (NMR) spectroscopic analyses were carried out using a Bruker Ascend™ 600 MHz spectrometer. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) images were acquired using an FEI Nova 450 NanoSEM. Dynamic light scattering (DLS) measurements were performed using a Malvern Zetasizer Nano ZSP. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was conducted on an Agilent Technologies 5110 VDV ICP-OES system. UV/Vis absorption spectra were recorded using an Agilent Cary 60 UV/Vis spectrophotometer on solutions prepared at a concentration of 0.05 mg mL⁻¹ in CHCl₃. Fluorescence emission spectra were obtained for the same solutions using an Agilent Cary Eclipse fluorescence spectrophotometer employing an excitation wavelength of 440 nm. Time-resolved photoluminescence (TRPL) spectroscopic analyses were performed using a Horiba Fluorolog-3 FL3-221 spectrofluorometer. All TRPL spectra were recorded at room temperature. Excitation was provided by an N-450 nanoLED source (λ_ex = 440 nm) with a pulse duration of ∼1.3 ns. The instrument offers a measurable lifetime range from 200 ps to 0.1 ms. Emission was monitored at 565 nm. Sample solutions were prepared in CHCl₃ at a concentration of 0.006 mg mL⁻¹ and spectra were recorded under identical conditions. The photoluminescence decay profiles were analyzed by fitting the decay curves with a multi-exponential function (EXPDECAY3) to extract charge carrier lifetimes. Atomic Force Microscopy (AFM) measurements were performed using a Park NX7 system in non-contact (tapping) mode. Samples were freshly prepared by spin-coating solutions of polymers and their hybrids (18 mg mL⁻¹ in toluene). For AuNPs thin films, 100 µL of the synthesized AuNPs suspension in DI water was spin-coated onto Si wafer substrates. Spin-coating was performed at 2000 rpm for 30 s. The resulting films were subsequently subjected to thermal annealing at 110 °C for 60 s to remove residual solvent and improve film uniformity. All samples were subsequently scanned over a 1 × 1 µm² area at a defined scan rate of 0.2 Hz under a relative humidity of 48%. Images were acquired at a resolution of 512 × 512 pixels with a cantilever drive amplitude of 1.1%. Post-acquisition image processing was conducted using Park Systems software and the open-source Gwyddion software. X-ray diffraction (XRD) analysis of the thin films was performed using a Bruker D2 Phaser diffractometer equipped with Cu Kα radiation (λ = 1.54 Å). The instrument was operated at an accelerating voltage of 40 kV and a current of 45 mA. Grazing-incidence X-ray diffraction (GI-XRD) measurements were conducted over a 2θ range of 2–90° with a scanning rate of 4° min⁻¹. Raman spectra were collected using a Fergie BRX 1024 Raman spectrometer equipped with a 785 nm excitation laser. The spectra were acquired using a 40× objective lens on an optical microscope with a laser power of 50 mW and an acquisition time of 20s. Samples for GIXRD and Raman spectroscopy were prepared by depositing 1 mg of the material (polymers and their hybrids) dissolved in 200 µL of CHCl₃ onto Si wafer substrates. For AuNPs, 100 µL of the AuNPs suspension in DI H₂O was deposited onto Si wafer substrates. The coated substrates were thermally annealed at 110 °C and subsequently dried overnight at 70 °C prior to analysis.

Preparation of hybrids containing plasmonic additives through intrinsic interactions of polymers P3HT and PTh-PDI-DCP with AuNPs. 10 mL solutions containing 1 mg of P3HT in CHCl₃ were added to five separate glass vials. Similarly, 10 mL solutions containing 1 mg of PTh-PDI-DCP in CHCl₃ were added to 5 separate glass vials. Five different volumes (5, 7.5, 10, 12.5 and 15 mL) of the aqueous suspensions of AuNPs were added to the vials containing 10 mL CHCl₃ solutions of P3HT and PTh-PDI-DCP. The resulting five biphasic systems were stirred overnight at room temperature. The concentration of Au in the aqueous solution of as prepared AuNPs was determined by ICP-OES and was found to be 0.45 mg mL⁻¹. The amounts of AuNPs transferred to the organic phases were quantified using ICP-OES analysis performed on the polymer containing CHCl₃ phases after separating them from the aqueous AuNPs phases. The ICP-OES analysis was also performed on polymer solutions before bringing them into contact with the aqueous AuNPs suspensions.

PEC characterization. FTO coated glass substrates were thoroughly cleaned using a diluted Triton X-100 solution, followed by sequential ultrasonication in IPA, acetone and a final rinse with methanol. The cleaned substrates were dried under a N₂(g) stream and subsequently placed in a vacuum oven at 120 °C for 1 h to ensure complete removal of the residual solvents. Polymer solutions were prepared in CHCl₃ at a concentration of 1 mg per 300 µL. For the AuNPs, 300 µL of the aqueous AuNPs dispersion was mixed with (1 vol%) Nafion solution (20 wt%) to make a stable film. All the solutions were subsequently filtered through 0.25 µm syringe filters before use. To minimize photoinduced degradation, the filtered solutions were stored in amber glass vials. Thin films were deposited by drop-casting the solutions onto the dried FTO substrates (1 × 1 cm²), followed by thermal drying at 60 °C for 3 h. The film thicknesses were determined through cross-sectional analysis using SEM. PEC measurements were performed using a Gamry Instruments Reference 600 potentiostat. The polymer-coated FTO substrates (active area 1 × 1 cm²) were electrically connected to a stainless-steel working electrode and immersed in a 0.2 M NaClO₄ solution in acetonitrile. A platinum (Pt) wire served as the counter electrode, and an Ag/AgCl electrode immersed in a saturated KCl solution served as the quasi-reference electrode. Cyclic voltammetry (CV) was carried out in the potential window of −1.2 to +1.2 V (vs. Ag/AgCl) at a scan rate of 10 mV s⁻¹ with a step size of 2 mV to investigate the electrochemical doping behaviour of the photocatalyst films. Tafel slopes were extracted from linear sweep voltammograms (LSVs) to evaluate charge-transfer kinetics. Electrochemical impedance spectroscopy (EIS) measurements were performed over a frequency range of 0.1 Hz to 100 kHz to determine the solution resistance (R_s) and charge-transfer resistance (R_ct). Chronoamperometry (CA) measurements were conducted at a constant potential of +1.0 V (vs. Ag/AgCl) to assess the operational stability of the photocatalyst films. All PEC measurements were performed under illumination using a Xe lamp (H11 xenon-simulated, high-performance halogen filament bulb, 75 W, 12 V), delivering a light intensity of 60 mW cm⁻² as measured with a fluxmeter, operated at full intensity.

Artificial photosynthesis of H₂O₂. For artificial photosynthesis of H₂O₂, photocatalyst solutions of P3HT, P3HT/AuNPs, PTh-PDI-DCP and PTh-PDI-DCP/AuNPs were prepared in anhydrous CHCl₃ at a concentration of 0.5 mg per 300 µL. For AuNPs, 200 µL of the aqueous AuNPs dispersion was used. The polymer-only, AuNPs-only and polymer/AuNPs hybrids were separately loaded onto the PVDF membranes via capillary action by immersing the PVDF membranes in the respective photocatalyst solutions. The catalyst loaded membranes were subsequently dried in an oven at 70 °C overnight. The photocatalyst loading on each membrane was quantified gravimetrically by recording the membrane mass before and after catalyst deposition using a Mettler Toledo METT-XPR36 microbalance. The mass difference was attributed to the amount of photocatalyst immobilized onto the porous PVDF membrane.

For photocatalytic H₂O₂ generation, each catalyst loaded membrane was affixed to a Teflon coated magnetic stir bar and placed in a 25 mL single-neck round-bottom flask (RBF). DI water (5 mL) was added, and the flask was sealed with a rubber septum. The reaction mixture was purged with oxygen for 30 minutes and subsequently irradiated under continuous stirring for 1 h using a Xe lamp. After irradiation, the membrane was retrieved for reuse. Prior to reuse, the recovered membranes were thoroughly washed with DI water and dried at 70 °C overnight.

The amount of H₂O₂ produced during photocatalysis was quantified using iodometric titration. After removal of the photocatalyst-loaded membrane, 1 mL of 0.1 M KHP and 1 mL of 0.4 M KI aqueous solution were added to the irradiated reaction mixture. The solution was stirred at room temperature for 30 min to allow the oxidation of iodide ions (I⁻) by H₂O₂, forming triiodide ions (I₃⁻). The concentration of I₃⁻ was determined by measuring the absorbance at 352 nm using UV/Vis spectroscopy. A calibration curve was constructed using standard H₂O₂ solutions of known concentrations to facilitate accurate quantification. Based on the 1 : 1 reaction stoichiometry between H₂O₂ and I₃⁻, the measured I₃⁻ concentration directly corresponds to the amount of H₂O₂ generated.

To elucidate the photocatalytic reaction mechanism, four identical photocatalyst-loaded membranes were separately attached to Teflon coated magnetic stir bars and placed in individual 25 mL RBFs containing 5 mL of Milli-Q water. Three different scavengers were added to three of the flasks: (i) 1 mL of 1 mM AgNO₃ as an electron (e⁻) scavenger,⁴⁶ (ii) 1 mL of 0.1 mM p-benzoquinone (p-BQ) to trap superoxide radicals (˙O₂⁻), and (iii) tert-butanol added at 10 vol% relative to the total water volume to scavenge hydroxyl radicals (˙OH).⁴⁷ The fourth flask containing no scavenger served as the control.

Results and discussion

The citrate stabilized AuNPs used in this study were synthesized using the Frens method,⁴⁵ and P3HT was synthesized using GRIM polymerization (Scheme S1).⁴⁸ The ¹H NMR spectrum of P3HT showed all the resonance signals corresponding to the protons existing in different chemical environments in P3HT (Fig. S1). The number average molecular weight (M_n) of P3HT was determined using GPC and was found to be around 5100 Da (Đ = 1.08) (Fig. S1). PTh-PDI-DCP was synthesized by using a synthesis strategy that relied on a combination of GRIM polymerization, followed by controlled side chain engineering to synthesize polythiophene bearing boronic ester side chains (PTh-BE). The boronic ester side chains were used as functional handles to conjugate PDI moieties to polythiophene via the Suzuki coupling reaction to yield PTh-PDI-DCP (Scheme S1).²⁷ The M_n of PTh-BE was 7 900 Da (Đ = 1.05), which was found to increase to 10 900 Da (Đ = 1.07) for PTh-PDI-DCP (Fig. S1). The ¹H NMR spectrum (Fig. S1) of PTh-PDI-DCP exhibited two distinct resonance signals in the thiophene proton region corresponding to protons in different chemical environments: signal h (δ ≈ 7.0 ppm) arises from thiophene rings bearing 3-hexyl substituents, while signal g (δ ≈ 7.1 ppm) corresponds to thiophene rings incorporated into the main chain via the comonomer derived from 2-((2,5-dibromothiophen-3-yl)methoxy)tetrahydro-2H-pyran (DBT-THP). A comparison of the integrals of h and g protons indicates that ∼31% of the repeat units in PTh-PDI-DCP contain thiophene units derived from DBT-THP and ∼69% of the thiophene units derived from 2,5-dibromo-3-hexylthiophene (DB3HT). The methylene protons flanked by hydroxyl and thiophene groups were assigned to signal i (δ = 4.6 ppm), whereas those adjacent to ester and thiophene groups were assigned to signal i′ (δ = 5.5 ppm). Integrals of the g thiophene protons relative to the i methylene protons suggest that ∼12% of the repeat units contain hydroxyl functionalized side chains in PTh-PDI-DCP. A comparison of the i and i′ proton integrals indicates that ∼19% of the repeat units in PTh-PDI-DCP possess ester linkages. A comparison of the i′ proton signal with the protons of PDI (signal r for the methine proton at δ = 5.2 ppm) shows that about 13% of the repeat units in PTh-PDI-DCP are conjugated to PDI moieties (Fig. S1). Collectively, the ¹H NMR analysis reveals that side-chain functionalities other than 3-hexyl groups account for about 31% of the repeat units in PTh-PDI-DCP, with about 12% carrying hydroxyl groups, about 6% boronic ester groups and about 13% PDI units. It is worth highlighting here that by controlling the reaction time during the Suzuki coupling reaction we can reproducibly synthesize PTh-PDI-DCP with decent control over the degree of substitution. The ATR-FTIR spectra (Fig. S1) of all the materials and the related discussion are included in the SI.

For constructing a DCP/plasmonic NPs hybrid system, the selection of AuNPs as plasmonic additives was driven by their favourable work function relative to the D and A components of PTh-PDI-DCP. The chemical structures of P3HT and PTh-PDI-DCP and a depiction of citrate stabilized AuNPs are presented in Fig. 1a. We determined the band gaps and frontier orbital (HOMO–LUMO) positions of the D and A components from UV/Vis absorption spectra and cyclic voltammograms taking ferrocene/ferrocenium (Fc/Fc⁺) as a reference redox couple; the details are provided in the SI (Fig. S2).²⁸ The energy-level alignment as summarized in Fig. 1b suggests the possibility of favourable electronic communication and charge-carrier transfer between PTh, PDI and AuNPs. In addition, the ability of AuNPs to extend the absorption spectrum and generate hot electrons via localized surface plasmon resonance (LSPR) can also potentially contribute to the enhancement of the light harvesting and energy conversion abilities of PTh-PDI-DCP.⁴⁹

The DLS analysis of the aqueous suspension of AuNPs revealed an average hydrodynamic diameter of 28 nm (Fig. 1c), which was corroborated by the STEM imaging as well (Fig. 1c inset) that revealed an average diameter of 24 ± 4 nm. The DLS and STEM sizes were further supported by the characteristic optical appearance (ruby red colour) of the aqueous AuNPs suspension (Fig. 1c inset). To incorporate the AuNPs into the OSPs (P3HT and PTh-PDI-DCP) to produce OSP/AuNPs hybrid systems, the CHCl₃ solutions (1 mg mL⁻¹) of both the polymers were separately brought into contact with different volumes of the aqueous suspension of AuNPs containing 0.45 ± 0.04 mg mL⁻¹ of AuNPs. The biphasic systems were stirred overnight at room temperature and, by relying on the intrinsic interactions of polymers P3HT and PTh-PDI-DCP with AuNPs, the OSPs were allowed to extract AuNPs from the aqueous phase to the organic phase. The illustration of the biphasic system is presented in Fig. 1d. The amount of AuNPs transferred to the organic phase was quantified using ICP-OES performed on the polymer solutions in CHCl₃ after bringing them into contact with the aqueous AuNPs suspensions. The ICP-OES data helped us in determining the amount of AuNPs that the OSPs P3HT and PTh-PDI-DCP, as wt% of their own total mass (1 mg) in the solution, were able to extract from the different volumes of aqueous AuNPs suspensions at a concentration of 0.45 ± 0.04 mg mL⁻¹. These experiments were performed in triplicate; hence, the wt% values reported are averages of three independent measurements. Interestingly, the amounts of AuNPs extracted using 1 mg of both the polymers kept on increasing while increasing the volumes of AuNPs aqueous suspensions brought into contact with the CHCl₃ solutions of OSPs. By quantifying the amounts of Au in polymer solutions in CHCl₃, it was found that 1 mg of P3HT dissolved in 10 mL of CHCl₃ was able to extract the amounts of AuNPs that were equivalent to 3 ± 1, 6 ± 1.5, 12 ± 1, 19 ± 4 and 24 ± 3 wt% of its mass when its solution was interfaced with 5, 7.5, 10, 12.5 and 15 mL of AuNPs aqueous suspension, respectively. Similarly, 1 mg of PTh-PDI-DCP dissolved in 10 mL of CHCl₃ was able to extract the amounts of AuNPs that were equivalent to 10 ± 1, 12 ± 1, 15 ± 1, 23 ± 2.5 and 33 ± 3 wt% of its mass when its solution was interfaced with 5, 7.5, 10, 12.5 and 15 mL of AuNPs aqueous suspension, respectively. While gradually increasing the volume of AuNPs, the aqueous phase containing AuNPs started turning bluish at 12.5 mL and 15 mL. This suggests aggregation of AuNPs; therefore, higher volumes of AuNPs suspension were not tested. Consequently, this process led to five different hybrids of each of the OSPs: P3HT/AuNPs hybrids containing 3 ± 1, 6 ± 1.5, 10 ± 1, 16 ± 3 and 19 ± 2.5 wt% of AuNPs and PTh-PDI-DCP/AuNPs hybrids containing 9 ± 1, 11 ± 1, 13 ± 1, 19 ± 2.5 and 25 ± 3 wt% of AuNPs (Fig. 1e and Table S1).

After successfully demonstrating the ability of organic semiconducting P3HT and PTh-PDI-DCP to extract and transfer AuNPs from the aqueous to the organic phase, we set out to determine the optimum amount of AuNPs that should be incorporated into P3HT and PTh-PDI-DCP for maximizing the photocatalytic performance of the resulting hybrids. To identify the optimal AuNPs loading, OSP/AuNPs hybrid systems containing different wt% of AuNPs were evaluated using CV and LSV. We used the changes in the average current densities, from CV and LSV, of P3HT and PTh-PDI-DCP in the dark and under illumination as a function of varying wt% of AuNPs incorporated to identify the optimum wt% of AuNPs (Fig. 1f and S3). Both CV and LSV analyses revealed a gradual increase in current densities with increasing wt% of AuNPs for both the hybrid systems, i.e., P3HT/AuNPs and PTh-PDI-DCP/AuNPs. The current densities were found to reach maximum values at an AuNPs content of 10 ± 1 wt% for P3HT/AuNPs hybrids and 13 ± 1 wt% for PTh-PDI-DCP/AuNPs hybrids. A further increase in the wt% of AuNPs incorporated in the hybrids resulted in a decrease in the average current density.

Pristine P3HT exhibited an average current density of 45 ± 4 µA mg⁻¹ cm⁻² in the dark, which increased to a maximum value of 192 ± 11 µA mg⁻¹ cm⁻² for the P3HT/AuNPs hybrid containing 10 ± 1 wt% of AuNPs. Under illumination, the current density of pristine P3HT was recorded to be 78 ± 11 µA mg⁻¹ cm⁻², whereas in the case of P3HT/AuNPs hybrids the maximum current density of 317 ± 16 µA mg⁻¹ cm⁻² was recorded for the P3HT/AuNPs hybrid containing 10 ± 1 wt% of AuNPs. Similarly, pristine PTh-PDI-DCP exhibited an average current density of 349 ± 12 µA mg⁻¹ cm⁻² in the dark, which increased to a maximum value of 572 ± 15 µA mg⁻¹ cm⁻² for the PTh-PDI-DCP/AuNPs hybrid containing 13 ± 1 wt% of AuNPs. Under illumination, PTh-PDI-DCP exhibited a current density of 521 ± 23 µA mg⁻¹ cm⁻², which increased to a maximum value of 722 ± 17 µA mg⁻¹ cm⁻² for the PTh-PDI-DCP/AuNPs hybrid containing 13 ± 1 wt% of AuNPs (Fig. 1f). The observed enhancement in current densities produced by P3HT and PTh-PDI-DCP upon incorporation of AuNPs serves as an indication of facile electronic communication and charge-carrier transport among the components constituting the hybrids, i.e., PTh, PDI, and AuNPs. The lower current densities observed at higher wt% of AuNPs incorporated in the hybrids could be because of the suboptimal dispersion of nanoparticles within the polymer matrix. Such overloading likely results in partial aggregation of AuNPs, reduced electrical connectivity and a diminished fraction of photogenerated electrons reaching the electrodes.^50,51

In addition to PEC characterization, UV/Vis absorption and fluorescence spectroscopic analyses were performed on the hybrids to assess changes in their optical properties. The UV/Vis absorption spectrum of the aqueous AuNPs suspension exhibited a characteristic LSPR band at around 520 nm (Fig. 1g),⁵² which is characteristic for the size of the AuNPs determined from DLS and STEM analyses (Fig. 1c). Pristine P3HT displayed an absorption band spanning over 350–550 nm. Upon incorporation of AuNPs, the UV/Vis absorbance spectrum of the P3HT/AuNPs hybrid showed a bathochromic shift of the absorption maximum (λ_max). The bathochromic shift gradually increased with increasing wt% of AuNPs in the hybrid, reaching a maximum shift of 9 nm from λ_max = 425 for P3HT to λ_max = 434 nm for the P3HT/AuNPs hybrid containing 10 ± 1 wt% of AuNPs (Fig. 1g). Similarly, addition of AuNPs in PTh-PDI-DCP resulted in an approximately 8 nm red shift from λ_max = 454 nm for PTh-PDI-DCP to λ_max = 462 nm for the PTh-PDI-DCP/AuNPs hybrid containing 13 ± 1 wt% of AuNPs (Fig. 1g).

These bathochromic shifts indicate electronic coupling between the OSPs and plasmonic AuNPs.^53,54 It is interesting to note that increasing the AuNPs wt% in P3HT/AuNPs hybrids beyond 10 ± 1 wt% and in PTh-PDI-DCP/AuNPs hybrids beyond 13 ± 1 wt% resulted in a gradual decrease in the bathochromic shift in λ_max values (Fig. S4). After recording the UV/Vis absorbance spectra, fluorescence spectra were recorded for P3HT, P3HT/AuNPs, PTh-Opy, PTh-PDI-DCP and PTh-PDI-DCP/AuNPs at an excitation wavelength of 440 nm (Fig. 1h). PTh-Opy is a polythiophene derivative bearing tetrahydropyranyl ether (Opy) side chains (Scheme S1) that was used to synthesize PTh-PDI-DCP. The emission profile of PTh-Opy was used as a reference to quantify the change in fluorescence behaviour of PTh-PDI-DCP. The incorporation of AuNPs led to a decrease in fluorescence intensities of the hybrids. The maximum fluorescence quenching was observed for the P3HT/AuNPs hybrid containing 10 ± 1 wt% AuNPs and for PTh-PDI-DCP/AuNPs containing 13 ± 1 wt% of AuNPs (Fig. S5). A further increase in the AuNPs wt% in the hybrid systems did not follow the decreasing trend, which may be because of the aggregation of AuNPs that led to inefficient electronic interaction between the organic semiconducting polymer and AuNPs. This observation is in line with the changes in the UV/Vis absorbance spectra as discussed earlier. In addition, the trends in the change in the emission as a function of the wt% of AuNPs incorporated were fully consistent with the trends in the current density changes as a function of the wt% of AuNPs incorporated (Fig. 1f and S5). To quantify fluorescence quenching, relative fluorescence quenching coefficients (χ, %) were calculated by comparing the emission intensities of the pristine polymers P3HT, PTh-Opy and PTh-PDI-DCP and hybrids P3HT/AuNPs containing 10 ± 1 wt% of AuNPs and PTh-PDI-DCP/AuNPs containing 13 ± 1 wt% of AuNPs that displayed maximum fluorescence quenching (Fig. 1h). Relative to pristine P3HT, the P3HT/AuNPs hybrid exhibited a maximum χ value of about 40%. Because of its DCP nature, PTh-PDI-DCP has a low intrinsic emission intensity with a χ value of about 60% relative to P3HT. The fluorescence intensity of PTh-Opy was comparable to that of P3HT. A χ value of about 57% was recorded when comparing the emission of PTh-PDI-DCP with that of PTh-Opy (Fig. 1h). The low intrinsic fluorescence of PTh-PDI-DCP can be attributed to intramolecular electronic communication between the covalently conjugated PTh (D) and PDI (A). The PTh-PDI-DCP/AuNPs hybrid exhibited an additional χ value of about 42% relative to pristine PTh-PDI-DCP, revealing a further fluorescence quenching arising from the interfacial interactions and charge transfer involving AuNPs. When compared with the emission of PTh-Opy, an overall χ value of about 75% was observed for the PTh-PDI-DCP/AuNPs hybrid system, underscoring the synergistic contribution of intramolecular D–A coupling and charge transfer to AuNPs. These effects are enabled by the favourable alignment of the frontier orbital energy levels of the constituent components, as illustrated in Fig. 1b. In addition, TRPL spectroscopy was employed to investigate the charge transfer dynamics (Fig. 1i). All samples were excited at a fixed wavelength of 440 nm, and the average exciton lifetimes (τ_av) were determined by fitting the decay profiles using a multi-exponential function (EXPDECAY3). The extracted values are summarized in Table S2. The τ_av values of 2.3 ns for P3HT, 1.7 ns for the P3HT/AuNPs hybrid, 1.1 ns for PTh-PDI-DCP, and 0.8 ns for the PTh-PDI-DCP/AuNPs hybrid reveal a systematic decrease in exciton lifetime upon incorporation of AuNPs. This indicates more efficient charge separation in the hybrid systems compared to their pristine counterparts. These findings are consistent with the steady-state fluorescence results (Fig. 1h) and further support the enhanced contribution of non-radiative pathways in the hybrid materials. It can be inferred that interfacing PTh-PDI-DCP with AuNPs facilitates directional electron transfer from the PTh donor to the PDI acceptor and subsequently to the AuNPs. The suppression of radiative recombination leads to the observed shortening of exciton lifetimes in PTh-PDI-DCP, P3HT/AuNPs, and PTh-PDI-DCP/AuNPs compared to pristine P3HT. Based on the combined insights from electrochemical measurements and UV/Vis, fluorescence and TRPL spectroscopic analyses, AuNPs loadings of 10 ± 1 wt% for P3HT and 13 ± 1 wt% for PTh-PDI-DCP were identified as optimal for maximizing the charge separation and potential photocatalytic performance enhancement. Accordingly, these OSP/AuNPs hybrid systems were selected for further characterization studies.

The successful incorporation of AuNPs into the polymer matrices is confirmed by the STEM micrographs in Fig. 2a–d. The pristine P3HT film (Fig. 2a) exhibits a smooth and continuous morphology with a uniform texture, which is characteristic of well-formed polymer thin films. Upon incorporation of AuNPs (Fig. 2b), electron-dense AuNPs are clearly observed uniformly dispersed within the P3HT matrix. Similarly, the pristine PTh-PDI-DCP film (Fig. 2c) displays a uniform layer morphology. In the STEM image of PTh-PDI-DCP/AuNPs (Fig. 2d), the AuNPs appear as well-distributed dark contrast features distributed throughout the polymer matrix, indicating effective spatial dispersion. Overall, the STEM micrographs confirm the successful incorporation of AuNPs into both polymer systems. The thin films of the P3HT/AuNPs and PTh-PDI-DCP/AuNPs hybrids were also characterized using AFM. The 2D/3D topographical representations and corresponding 2D/3D phase images of bare AuNPs (Fig. 2e–h), pristine P3HT (Fig. 2i–l) and pristine PTh-PDI-DCP (Fig. 2m–p), together with their respective hybrids, P3HT/AuNPs (Fig. 2q–t) and PTh-PDI-DCP/AuNPs (Fig. 2u–x), provided detailed insight into surface morphological evolution upon NP incorporation. The pristine polymer films exhibit relatively smooth and homogeneous surface textures, indicative of uniform film formation. In contrast, the composite films display a noticeable increase in surface roughness, clearly evidencing the presence of embedded AuNPs within the polymer matrices. The emergence of pronounced nanoscale protrusions and granular features in the hybrid films is attributed to the incorporation of AuNPs, which resulted in enhanced vertical height variations. The AFM analysis demonstrated that AuNPs incorporation significantly altered the nanoscale surface architecture, increasing roughness and inducing a granular morphology (Fig. 2e–x). To further investigate the structural organization of the polymer/AuNPs hybrid systems, GI-XRD measurements were performed on a neat AuNPs film, pristine polymer films (P3HT and PTh-PDI-DCP) and their corresponding (P3HT/AuNPs and PTh-PDI-DCP/AuNPs) hybrid films deposited on Si wafer substrates (Fig. S6). The pristine P3HT film exhibits a distinct diffraction peak at 2θ = 5.1°, which is assigned to the (100) reflection corresponding to the lamellar stacking of the alkyl side chains.⁵⁵ This peak indicates a well-defined semicrystalline organization and reflects the relatively high crystallinity of P3HT. In contrast, the PTh-PDI-DCP film shows a significantly weaker and less defined low-angle reflection consistent with its reduced crystallinity and more random chain orientation arising from its D–A conjugated backbone structure and random copolymer nature.²⁹ Both polymer films display a broad diffraction feature centred around 2θ ≈ 16–20°, which can be attributed to amorphous halo scattering associated with disordered polymer regions (Fig. S6).⁵⁶ The neat AuNPs film exhibits characteristic diffraction peaks at 2θ ≈ 38°, 46° and 65°, respectively, corresponding to the (111), (200) and (220) lattice planes of face-centered cubic (fcc) gold.⁵⁷

Fig. 2 STEM micrographs: (a) pristine P3HT; (b) P3HT/AuNPs hybrid; (c) pristine PTh-PDI-DCP; and (d) PTh-PDI-DCP/AuNPs hybrid. AFM images (e–x) show the topography and phase images of films of the materials used in this study. (e and f) Topography and (g and h) phase images of AuNPs. (i and j) Topography and (k and l) phase images of P3HT. (m and n) Topography and (o and p) phase images of P3HT/AuNPs. (q and r) Topography and (s and t) phase images of PTh-PDI-DCP. (u and v) Topography and (w and x) phase images of PTh-PDI-DCP/AuNPs.

These reflections confirm the formation of crystalline metallic AuNPs. The same characteristic Au reflections are observed in both P3HT/AuNPs and PTh-PDI-DCP/AuNPs composite films, revealing the successful incorporation of AuNPs within the polymer matrices without altering their metallic phase.⁵⁸ Notably, a slight shift of the diffraction peaks toward lower 2θ values is observed in the hybrid films compared to the pristine materials (Fig. S6). This shift suggests a marginal increase in the interplanar spacing, which may arise from the interfacial interactions between the AuNPs and polymer chains. Such interactions can perturb polymer chain packing, influence backbone planarity and modify the local structural ordering within the hybrid system. This behaviour is further supported by Raman spectroscopic analysis of the same films (i.e., AuNPs, P3HT, P3HT/AuNPs, PTh-PDI-DCP and PTh-PDI-DCP/AuNPs) (Fig. S7). The composite films exhibit a noticeable shift of the polymer vibrational bands (∼1480 cm⁻¹ and ∼1400 cm⁻¹),⁵⁹ toward lower wavenumbers compared to the pristine polymers, indicating modifications in molecular packing and backbone planarity upon incorporation of AuNPs.⁶⁰ In addition, a significant enhancement in Raman intensity is observed for the polymer/AuNPs hybrid films (Fig. S7). This enhancement can be attributed to the LSPR effect of the AuNPs, which amplifies the local electromagnetic field and results in surface-enhanced Raman scattering (SERS)-like behaviour within the composite films.⁶⁰ A distinct Raman band at ∼300 cm⁻¹ observed in the AuNPs-containing samples can be attributed to Au–O–C interfacial interactions.⁶¹

After establishing the optimum wt% AuNPs, the resulting P3HT/AuNPs and PTh-PDI-DCP/AuNPs hybrids were subjected to detailed PEC investigation (Fig. 3). A schematic illustration of the PEC measurement configuration, together with the proposed mechanism underlying the enhanced performance of the PTh-PDI-DCP/AuNPs hybrid, is presented in Fig. 3a. FTO-supported thin films of the photoactive materials were fabricated by drop-casting 1 mg of material from CHCl₃ solutions defining an active area of 1 × 1 cm². AuNPs mixed with Nafion were deposited from an aqueous suspension. The cross-sectional SEM analysis of the films revealed film thicknesses in the range of 15–22 µm (Fig. S8). All PEC measurements were conducted in triplicate using three independently fabricated films per material and the reported data represent averages of the three independent measurements. CV measurements were performed under darkness and illumination on AuNP, P3HT, P3HT/AuNPs, PTh-PDI-DCP and PTh-PDI-DCP/AuNPs films (Fig. 3b). For the P3HT/AuNPs and PTh-PDI-DCP/AuNPs films, changes in the CV and LSV as a function of varying wt% of AuNPs were recorded and discussed earlier. Here, we compare the current densities of the hybrids that showed the highest current densities, which were P3HT/AuNPs with 10 ± 1 wt% of AuNPs and PTh-PDI-DCP/AuNPs with 13 ± 1 wt% of AuNPs (Fig. 1f and Fig. S3). In CV, the films of AuNPs exhibited a current density of 48 ± 3 µA mg⁻¹ cm⁻² in the dark, which increased to 64 ± 5 µA mg⁻¹ cm⁻² under illumination. Pristine P3HT films displayed a current density that increased from 42 ± 4 µA mg⁻¹ cm⁻² in the dark to 76 ± 9 µA mg⁻¹ cm⁻² upon illumination. In contrast, pristine PTh-PDI-DCP films generated substantially higher current densities of 341 ± 11 µA mg⁻¹ cm⁻² in the dark and 513 ± 18 µA mg⁻¹ cm⁻² under illumination. Incorporation of AuNPs led to a pronounced enhancement in photocurrent responses. The P3HT/AuNPs hybrid exhibited current densities of 185 ± 9 µA mg⁻¹ cm⁻² in the dark and 322 ± 13 µA mg⁻¹ cm⁻² under illumination, significantly exceeding those of pristine P3HT (>4 times enhancement compared to P3HT). Among all investigated systems, the PTh-PDI-DCP/AuNPs hybrid displayed the highest response, reaching 552 ± 12 µA mg⁻¹ cm⁻² in the dark and 737 ± 19 µA mg⁻¹ cm⁻² under illumination (>1.4 times enhancement compared to PTh-PDI-DCP) (Table 1). The LSV measurements corroborated the current density trends observed in the CV analyses (Fig. 3c). A comparison of photocurrent densities under illumination revealed that P3HT/AuNPs exhibited about a 4-fold enhancement relative to pristine P3HT, whereas PTh-PDI-DCP/AuNPs showed about a 1.35-fold enhancement compared to pristine PTh-PDI-DCP. Notably, PTh-PDI-DCP/AuNPs generated approximately a 9-fold higher photocurrent density compared to the widely used model P3HT. The comparatively lower performance of pristine P3HT is attributed to its higher charge-carrier recombination rate. The superior photocurrent response of PTh-PDI-DCP/AuNPs reflects enhanced charge-carrier mobility arising from favourable energy-level alignment and efficient electronic communication among the constituting components (PTh, PDI and AuNPs), as illustrated in Fig. 1b. To evaluate charge-transfer kinetics, Tafel slopes were extracted from the LSV measurements of the individual photocatalysts (Fig. 3d). The films consisting of AuNPs alone exhibited Tafel slopes of 557 mV dec⁻¹ in the dark and 461 mV dec⁻¹ under illumination, accompanied by a reduction in overpotential from 0.63 to 0.59 V. Pristine P3HT showed Tafel slopes of 586 mV dec⁻¹ (in the dark) and 489 mV dec⁻¹ (under illumination), with a corresponding overpotential decrease from 0.62 to 0.59 V. In comparison, the P3HT/AuNPs hybrid (10 ± 1 wt% AuNPs) displayed lower Tafel slopes of 414 mV dec⁻¹ in the dark and 357 mV dec⁻¹ under illumination. PTh-PDI-DCP films exhibited faster charge-transfer kinetics, with Tafel slopes of 331 mV dec⁻¹ in the dark and 261 mV dec⁻¹ under illumination, alongside an overpotential decrease from 0.51 to 0.46 V. Notably, the PTh-PDI-DCP/AuNPs hybrid (13 ± 1 wt% AuNPs) demonstrated the most favourable kinetics, with Tafel slopes decreasing from 238 mV dec⁻¹ in the dark to 203 mV dec⁻¹ under illumination and a notable reduction in overpotential from 0.48 to 0.43 V (Table 1). These results are fully consistent with the CV and LSV analyses and confirm superior interfacial charge-transfer behaviour in the hybrid systems. EIS was employed to quantify R_ct using Nyquist plots (Fig. 3e and f) as a function of varying wt% of AuNPs. AuNP-only films exhibited an R_ct of 1096 ± 15 Ω mg⁻¹ cm⁻² in the dark, which decreased to 714 ± 24 Ω mg⁻¹ cm⁻² under illumination. Pristine P3HT films showed higher R_ct values of 1403 ± 54 Ω mg⁻¹ cm⁻² (in the dark) and 877 ± 31 Ω mg⁻¹ cm⁻² (under illumination). Incorporation of optimum wt% of AuNPs significantly reduced R_ct, with P3HT/AuNPs (10 ± 1 wt%), exhibiting values of 559 ± 22 Ω mg⁻¹ cm⁻² in the dark and 311 ± 14 Ω mg⁻¹ cm⁻² under illumination. Pristine PTh-PDI-DCP displayed lower R_ct values of 328 ± 11 Ω mg⁻¹ cm⁻² (in the dark) and 229 ± 13 Ω mg⁻¹ cm⁻² (under illumination). The PTh-PDI-DCP/AuNPs hybrid containing an optimum wt% of AuNPs (13 ± 1 wt%) exhibited the lowest R_ct values of 151 ± 14 Ω mg⁻¹ cm⁻² in the dark and 107 ± 6 Ω mg⁻¹ cm⁻² under illumination (Fig. 3f and Table 1), which are consistent with its superior photocurrent response. The R_ct values for both the hybrids decreased with the increase in the wt% of AuNPs in the hybrids, reaching the lowest values at the optimum wt% of AuNPs. The R_ct values start increasing with a further increase in the wt% of AuNPs beyond the optimum wt%. Stable electrode–electrolyte contact was confirmed by comparable R_s values in the range of 20–50 Ω mg⁻¹ cm⁻², across all fabricated photocatalyst devices (Fig. 3e and f). The equivalent circuit model used to fit the impedance data is illustrated in Fig. S9.

Fig. 3 (a) Schematic illustration of the three-electrode configuration used for PEC studies along with the proposed mechanism for enhancement of current generation in the PTh-PDI-DCP/AuNPs hybrid, (b) cyclic voltammograms recorded at a scan rate of 10 mV s⁻¹ with a step size of 2 mV over a potential range of −1.2 to +1.2 V (vs. Ag/AgCl) reference electrode, using 0.2 M NaClO₄ in acetonitrile as the electrolyte, (c) linear sweep voltammograms, (d) Tafel plots, (e and f) Nyquist plots recorded at +1.0 V (vs. Ag/AgCl), and (g) chronoamperometric responses recorded at +1.0 V (vs. Ag/AgCl) in the dark and under illumination.

Table 1 Summary of the (photo)electrochemical performance of the photocatalysts

Photocatalyst	J av form CVs (µA mg^−1 cm^−2) dark/illumination	Tafel slopes from LSVs (mV dec^−1) dark/illumination	EIS (Z) (Ω mg^−1 cm^−2) dark/illumination	CA response (µA mg^−1 cm^−2) dark/illumination
AuNPs	48 ± 3/64 ± 5	557/461	1096 ± 15/714 ± 24	40/80
P3HT	42 ± 4/76 ± 9	586/498	1403 ± 54/877 ± 31	30/70
P3HT/AuNPs	185 ± 9/322 ± 13	414/357	559 ± 22/311 ± 14	170/275
PTh-PDI-DCP	341 ± 11/513 ± 18	313/261	328 ± 11/229 ± 13	200/330
PTh-PDI-DCP/AuNPs	552 ± 12/737 ± 19	238/203	151 ± 14/107 ± 6	410/660

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Chronoamperometric (CA) measurements were conducted at an applied potential of +1.0 V (vs. Ag/AgCl) for 60 min in the dark and under illumination to evaluate operational stability (Fig. 3g). The AuNPs film exhibited a steady current of about 40 µA mg⁻¹ cm⁻² in the dark and about 80 µA mg⁻¹ cm⁻² under illumination. Pristine P3HT showed a stable current of about 30 µA mg⁻¹ cm⁻² in the dark and around 70 µA mg⁻¹ cm⁻² under illumination. In contrast, the P3HT/AuNPs hybrid (10 ± 1 wt%) maintained a significantly higher current of about 170 µA mg⁻¹ cm⁻² in the dark and about 275 µA mg⁻¹ cm⁻² under illumination. Pristine PTh-PDI-DCP generated a stable current of nearly 200 µA mg⁻¹ cm⁻² in the dark and around 330 µA mg⁻¹ cm⁻² under illumination. Most notably, the PTh-PDI-DCP/AuNPs (13 ± 1 wt%) hybrid exhibited the highest and most stable photocurrent response, maintaining a current of about 410 µA mg⁻¹ cm⁻² in the dark and 660 µA mg⁻¹ cm⁻² under illumination over the 1 h measurement period (Fig. 3g and Table 1). The operational stability of PTh-PDI-DCP/AuNPs was further assessed by performing 50 consecutive CV cycles at a scan rate of 100 mV s⁻¹ in the dark and under illumination (Fig. S10). Collectively, these results demonstrate that the incorporation of AuNPs into P3HT and PTh-PDI-DCP polymer matrices markedly enhances PEC performance and operational stability, underscoring the synergistic role of plasmonic NPs in promoting charge separation and transport, and interfacial charge-transfer kinetics.⁶² For convenience, a comprehensive summary of the electrochemical performance of AuNPs, P3HT, P3HT/AuNPs, PTh-PDI-DCP and PTh-PDI-DCP/AuNPs in the dark and under illumination is presented in Table 1.

Building on the PEC characterization studies, all materials were subsequently evaluated as photocatalysts for the artificial photosynthesis of H₂O₂. The photocatalytic performance of AuNPs, P3HT, P3HT/AuNPs (10 ± 1 wt% of AuNPs), PTh-PDI-DCP and PTh-PDI-DCP/AuNPs (13 ± 1 wt% of AuNPs) was evaluated in the artificial photosynthesis of H₂O₂. Photocatalysts were immobilized onto porous hydrophobic PVDF membranes via capillary action and successful deposition was confirmed by ATR-FTIR spectroscopy (Fig. S11). The bare PVDF spectrum exhibited characteristic symmetric and asymmetric C-CF₂ stretching bands at 1166 and 1233 cm⁻¹, respectively. Upon photocatalyst loading, these bands decreased in intensity, accompanied by new features corresponding to the chemical functionalities of the photocatalysts. Digital photographs and SEM images of the coated membranes, before and after the photocatalytic experiments, confirmed the retention of the porous morphology even after ten consecutive cycles (Fig. 4a–v).

Fig. 4 Digital photograph (a) and SEM image (l) of the pristine PVDF membrane. (b–k) Digital photographs of PVDF membranes loaded with AuNPs (b and g), P3HT (c and h), P3HT/AuNPs (d and i), PTh-PDI-DCP (e and j), and PTh-PDI-DCP/AuNPs (f and k) before (left) and after (right) use in the artificial photosynthesis of H₂O₂. (m–v) SEM images of PVDF membranes with photocatalyst loading, acquired before and after use in the artificial photosynthesis of H₂O₂.

Quantification of H₂O₂ was performed via iodometric titration in which I⁻ ions are oxidized to I₃⁻ by the generated H₂O₂. The resulting I₃⁻ species exhibit a UV/Vis absorbance peak at 352 nm, with the concentration directly proportional to the amount of H₂O₂ produced (eqn (1)). Calibration curves (0–100 μM) were prepared from 35% w/w aqueous H₂O₂ to enable accurate quantification (Fig. S12).

H₂O₂ + 3I⁻ + 2H⁺ → I₃⁻ + 2H₂O (1)

All photocatalytic cycles for H₂O₂ generation were conducted in the absence of sacrificial agents. An illustration of the proposed reaction mechanism for the artificial photosynthesis of H₂O₂ is shown in Fig. 5a. Control experiments performed without light, without O₂ purging and in the absence of a catalyst under illumination produced negligible H₂O₂, confirming that light, oxygen and the photocatalyst are all essential for driving the reaction.⁶³

Fig. 5 (a) Proposed mechanism for the artificial photosynthesis of H₂O₂, (b) photocatalytic H₂O₂ production rates for 10 photocatalytic cycles, (c) comparison of the maximum H₂O₂ production rates displayed in an individual cycle by the photocatalysts investigated in this study, (d) H₂O₂ production by all photocatalysts cumulated over 10 photocatalytic cycles, and (e) comparison of H₂O₂ production rates for all photocatalysts in the absence and presence of three scavengers, highlighting the contributions of different reaction pathways to the artificial photosynthesis of H₂O₂.

To probe the contribution of AuNPs, both the pristine polymers P3HT and PTh-PDI-DCP as well as their AuNPs hybrids P3HT/AuNPs and PTh-PDI-DCP/AuNPs were systematically evaluated over 10 consecutive photocatalytic cycles, each cycle performed in triplicate (Fig. 5b). For AuNPs alone, the highest H₂O₂ production rate (61 ± 6 μM mg⁻¹ h⁻¹) was observed during the first cycle. Pristine P3HT and P3HT/AuNPs reached the maximum production rates of 102 ± 4 μM mg⁻¹ h⁻¹ and 165 ± 6 μM mg⁻¹ h⁻¹, respectively, in the third cycle. This suggests that P3HT/AuNPs displayed about a 1.6-fold higher production rate compared to pristine P3HT. In comparison, pristine PTh-PDI-DCP exhibited a peak H₂O₂ production rate of 170 ± 4 μM mg⁻¹ h⁻¹, while PTh-PDI-DCP/AuNPs reached a maximum H₂O₂ production rate of 233 ± 12 μM mg⁻¹ h⁻¹, both in their third cycles. This corresponds to about 1.4- and 2.3-fold enhancements in H₂O₂ production rates for PTh-PDI-DCP/AuNPs relative to PTh-PDI-DCP and pristine P3HT, respectively. The enhancement in the photocatalytic performance upon AuNPs incorporation is attributed to efficient charge transfer because of the favourable band-gap alignment. This is consistent with the observations from the UV/Vis, fluorescence, TRPL spectroscopic and PEC studies. The variations in activity across cycles likely arise from morphological rearrangements of the photocatalyst coatings on PVDF membranes. After ten cycles, the H₂O₂ production declined for all the photocatalysts (37 ± 4 μM mg⁻¹ h⁻¹ for AuNPs, 37 ± 5 μM mg⁻¹ h⁻¹ for P3HT, 67 ± 2 μM mg⁻¹ h⁻¹ for P3HT/AuNPs, 55 ± 2 μM mg⁻¹ h⁻¹ for PTh-PDI-DCP and 72 ± 4 μM mg⁻¹ h⁻¹ for PTh-PDI-DCP/AuNPs) (Fig. 5b). Comparing the H₂O₂ production rates in the 10th photocatalytic cycle reveals that P3HT/AuNPs and PTh-PDI-DCP/AuNPs maintained H₂O₂ production rates 1.8 and 1.3-fold higher than their respective pristine polymers, with PTh-PDI-DCP/AuNPs exceeding P3HT by ∼1.95-fold. The maximum photocatalytic H₂O₂ production rates displayed by each catalyst in the highest performing cycle from Fig. 5b are compared in Fig. 5c.

The amount of the H₂O₂ produced in each of the 10 photocatalytic cycles was used to determine the cumulative H₂O₂ produced by each photocatalyst over a period of 10 h (Fig. 5d). The H₂O₂ productions cumulated over the 10 consecutive photocatalytic cycles were found to be 416 ± 41 μM mg⁻¹ for AuNPs, 675 ± 40 μM mg⁻¹ for P3HT, 966 ± 40 μM mg⁻¹ for P3HT/AuNPs, 1084 ± 62 μM mg⁻¹ for PTh-PDI-DCP and PTh-PDI and 1343 ± 73 μM mg⁻¹ for DCP/AuNPs. These results clearly indicate the enhanced photocatalytic activity of the hybrid systems relative to their individual counterparts (Table 2). These data also reveal the superior photocatalytic performance of PTh-PDI-DCP/AuNPs, which achieved 1.25, 1.4, 2.0 and 3.2-fold higher H₂O₂ production compared to pristine PTh-PDI-DCP, P3HT/AuNPs, pristine P3HT and AuNPs, respectively. This pronounced enhancement is attributed to improved electronic communication and efficient charge separation enabled by the favourable band alignment among the PTh, PDI and AuNPs components. Detailed H₂O₂ production across all cycles are summarized in Table S3.

Table 2 Summary of the results obtained from the artificial photosynthesis of H₂O₂ experiments

Photocatalyst	A	B	C
			AgNO3	p-BQ	tert-BuOH
Average maximum H2O2 production rate (μM mg^−1 h^−1) in the photocatalytic cycle exhibiting the highest H2O2 yield (A), cumulative H2O2 production (μM mg^−1) by all photocatalysts over ten consecutive cycles (B), and average percentage decrease (%) in H2O2 production in the presence of scavengers evaluated over five photocatalytic cycles (C).
AuNPs	61 ± 6	416 ± 41	61 ± 5	43 ± 6	11 ± 6
P3HT	102 ± 4	675 ± 40	63 ± 7	20 ± 5	13 ± 6
P3HT/AuNPs	165 ± 6	966 ± 40	65 ± 4	42 ± 7	17 ± 5
PTh-PDI-DCP	170 ± 4	1084 ± 62	55 ± 7	36 ± 5	39 ± 7
PTh-PDI-DCP/AuNPs	233 ± 12	1343 ± 73	68 ± 5	50 ± 5	44 ± 3

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In addition to evaluating photocatalytic H₂O₂ production efficiencies, the underlying reaction pathways operative in each photocatalyst system (AuNPs, P3HT, P3HT/AuNPs, PTh-PDI-DCP and PTh-PDI-DCP/AuNPs) were systematically investigated using scavenger studies (Fig. 5e and Table S4). To elucidate the dominant charge-carrier and reactive-species contributions, three selective scavengers were employed: AgNO₃ was used as an electron (e⁻) scavenger to probe oxygen reduction reactions (ORRs), p-BQ was used to quench superoxide radical anions (˙O₂⁻) associated with the one-electron two-step ORR pathway and tert-BuOH was employed to scavenge hydroxyl radicals (˙OH) generated during the water oxidation reaction (WOR).⁶⁴ To unveil the underlying mechanism of photocatalysis and the contributions of different reaction pathways, photocatalytic H₂O₂ production was quantified over five cycles in the presence of three scavengers. A schematic representation of the mechanism for the artificial photosynthesis of H₂O₂ is presented in Fig. 6. For AuNPs, the addition of AgNO₃, p-BQ and tert-BuOH resulted in H₂O₂ production decreases of 61 ± 5%, 43 ± 6% and 11 ± 6%, respectively. These results reveal the central role of photogenerated electrons and indicate contributions from both ORR (one-electron two-step and two-electron one-step) pathways, with a significant involvement of the ˙O₂⁻ mediated one-electron two-step pathway. The strong suppression of H₂O₂ production in the presence of p-BQ indicates the involvement of ˙O₂⁻ intermediates, which are likely stabilized by citrate species adsorbed on the AuNPs surface, thereby favouring the stepwise oxygen reduction pathway. For pristine P3HT, AgNO₃, p-BQ and tert-BuOH induced reductions of 63 ± 7%, 20 ± 5% and 13 ± 6%, respectively (Table 2), indicating that the H₂O₂ generation predominantly proceeds via the direct two-electron one-step ORR pathway. In contrast, P3HT/AuNPs exhibited relatively stronger suppression by p-BQ (42 ± 7%), alongside reductions of 65 ± 4% and 17 ± 5% upon addition of AgNO₃ and tert-BuOH, respectively. This shift highlights the role of AuNPs incorporation in stabilizing superoxide intermediates, thereby introducing a substantial contribution from the one-electron two-step ORR pathway.⁶⁵ Pristine PTh-PDI-DCP displayed percentage decreases of 55 ± 7%, 36 ± 5% and 39 ± 7% upon addition of AgNO₃, p-BQ and tert-BuOH, respectively (Table 2), indicating a dominant contribution from the two-electron ORR pathway alongside a notable involvement of the WOR. This behaviour is attributed to the presence of covalently conjugated PDI in PTh-PDI-DCP, which facilitates electron stabilization and enables ˙O₂⁻ formation. The contribution of the one-electron two-step pathway is further supported by the presence of electron-rich heteroatoms within the DCP architecture that stabilize ˙O₂⁻ intermediates.⁶⁶ Upon incorporation of AuNPs into PTh-PDI-DCP, the resulting hybrid exhibited markedly enhanced scavenger sensitivity, with H₂O₂ production decreases of 68 ± 5%, 50 ± 5% and 44 ± 3% upon addition of AgNO₃, p-BQ and tert-BuOH, respectively (Table 2). These results reveal a highly synergistic interplay between the ORR and WOR pathways. The percentage (%) decrease in H₂O₂ concentration determined for mechanistic studies is summarized in Table S5. Upon photoexcitation, electrons generated along the PTh backbone are efficiently stabilized by the covalently conjugated PDI units and subsequently transferred to AuNPs, where the plasmonic effect enables the reduction of O₂ to H₂O₂. Concurrently, the lower-lying energy levels of PDI facilitate water oxidation,⁶⁷ with oxidative equivalents relayed back to the polymer backbone, thereby sustaining charge balance and continuous catalytic turnover. Consistent with the mechanistic insights, artificial photosynthesis experiments (Table 2) demonstrate that PTh-PDI-DCP/AuNPs achieves the highest H₂O₂ production among all tested systems. This superior performance arises from favourable band-gap alignment among the PTh, PDI and AuNP components, which promotes efficient electronic coupling, charge separation and the cooperative activation of the ORR and WOR pathways. A comprehensive comparative summary of the performance of the photocatalytic systems reported in the literature for the artificial photosynthesis of H₂O₂ is provided in Table S6. For readers’ convenience, we have converted the production data for the listed literature in common units µM mg⁻¹ h⁻¹. The PTh-PDI-DCP/AuNPs system reported here was evaluated in the artificial photosynthesis of H₂O₂ under visible light irradiation using a porous hydrophobic PVDF membrane as a support with a minimal catalyst loading. The system achieved an H₂O₂ production rate of 233 ± 12 μM mg⁻¹ h⁻¹ in the highest performing photocatalytic cycle and an H₂O₂ production cumulated over 10 h of 1343 ± 73 μM mg⁻¹, which represents competitive performance relative to the performance of previously reported OSP-based photocatalysts (Table S6).

Fig. 6 Mechanism of the artificial photosynthesis of H₂O₂.⁶⁸

Conclusions

In conclusion, we have developed a PTh-PDI-DCP/AuNPs hybrid photocatalyst that exhibits markedly enhanced performance arising from a combination of LSPR and efficient electronic communication between the semiconducting PTh-PDI-DCP and plasmonic AuNPs. The hybrid was prepared by relying on the inherent ability of heteroatom containing OSPs to associate with metal surfaces via π–metal interactions and surface coordination, which enabled intimate contact and effective interfacial interactions between OSPs and AuNPs. The optimal AuNPs loading in PTh-PDI-DCP was determined to be 13 ± 1 wt%, as established through a combination of ICP-OES, fluorescence spectroscopy, cyclic voltammetry, linear sweep voltammetry and electrochemical impedance (Nyquist) analyses. Spectroscopic and electrochemical studies reveal favourable band-gap alignment among PTh, PDI and AuNPs, together with strong interfacial charge-transfer interactions. Consequently, the PTh-PDI-DCP/AuNPs hybrid outperforms all control systems (AuNPs, P3HT, P3HT/AuNPs and PTh-PDI-DCP) in PEC evaluations, demonstrating significantly enhanced photocurrent generation, accelerated reaction kinetics, reduced charge-transfer resistance (R_ct) and superior operational stability. Beyond PEC performance, the PTh-PDI-DCP/AuNPs hybrid exhibits exceptional activity in the artificial photosynthesis of H₂O₂, achieving a maximum production rate of 233 ± 12 μM mg⁻¹ h⁻¹ in the most productive photocatalytic cycle. This corresponds to ∼1.4 and ∼2.3-fold enhancements relative to pristine PTh-PDI-DCP and pristine P3HT, respectively. Mechanistic investigations indicate that this superior performance originates from the cooperative coupling of the WOR and ORR pathways. This synergy is enabled by the ability of AuNPs to promote directional charge transport. In the PTh-PDI-DCP/AuNPs architecture, PTh functions as the primary electron donor, PDI serves as an electron acceptor and AuNPs act as plasmonic nano-interfaces that generate plasmon-induced energetic charge carriers (hot electrons) and serve as efficient sinks for photogenerated charge carriers from the polymer components. Collectively, these findings establish PTh-PDI-DCP/AuNPs as a highly efficient and robust photocatalyst for artificial photosynthesis and represent a significant advance in the rational design of photoactive hybrid materials. Overall, this work highlights the potential of strategically interfacing DCPs with functional additives to expand the design space of next generation photocatalysts for solar to chemical energy conversion.

Author contributions

The manuscript was written through contributions from all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

All data supporting this study are included within the article and its supplementary information (SI). Supplementary information: Synthesis scheme, characterization and related data of some compounds and films, photocatalysis data, photocatalytic activities of reported photocatalysts. See DOI: https://doi.org/10.1039/d6py00158k.

Acknowledgements

This work was financially supported by the Higher Education Commission (HEC) of Pakistan (NRPU Project No. 20-15989) and the Faculty Initiative Fund (FIF), LUMS. B. Y. acknowledges support from the HFSP (RGY0074/2016) and the HEC for NRPU (Project No. 20-1740/R&D/10/3368, 20-1799/R&D/10-5302 and 5922), TDF-033 grants, and the start-up fund from LUMS. The research reported here was partially funded (PKCN-2023-150) by the Commonwealth Scholarship Commission and the Foreign, Commonwealth and Development Office in the UK. SI is grateful for their support. All views expressed here are those of the authors not the funding body. Support from Prof. Raja Shahid Ashraf (GCU, Lahore, Pakistan) for fluorescence spectroscopy, and Prof. Hatice Duran and Sümeyye Sönmez (TOBB University of Economics and Technology,  UNAM Institute of Materials Science and Nanotechnology, Bilkent University, Ankara, Türkiye) for TRPL spectroscopy is highly appreciated. L. F. and I. A. would like to acknowledge funding from EPSRC Prosperity partnership grant (UKRI606).

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