Polypyrrole nanostructures modified with mono- and bimetallic nanoparticles for photocatalytic H₂ generation

Abstract

Green hydrogen production by photocatalytic water splitting offers a promising way to solve environment and energy issues. Conjugated polymer nanostructures (CPNs), with highly π-conjugated structures, have been demonstrated as a new class of photocatalysts. However, pristine CPNs show poor photocatalytic activity for hydrogen generation owing to fast charge carrier recombination and sluggish kinetics. The investigation of application of polypyrrole in photocatalytic hydrogen generation is also rare. Here, we report the surface modification of polypyrrole nanostructures (NSs) with Pt- and Ni-based nanoparticles for photocatalytic hydrogen evolution. PPy nanostructures (NSs) were synthesized using lamellar mesophases as soft templates. The PPy-NSs were modified with mono- and bimetallic (Pt, Ni, Pt–Ni) co-catalyst nanoparticles induced by radiolysis. The prepared Pt-PPy, Ni-PPy and PtNi-PPy exhibit excellent photocatalytic activity for H₂ generation and a synergistic effect is obtained with co-modification with Pt and Ni. The effects of the nature of the metal precursors and the loading ratio were studied. We show that the loading rate of co-catalysts is crucial for H₂ generation, and an excess of co-catalyst can drastically decrease the activity. The composite photocatalyst Pt-Ni-PPy nanostructures are very active and stable with cycling. Our results indicate that modified CPNs with mono- and bimetallic NPs are promising photocatalysts for hydrogen evolution.

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1. Introduction

Hydrogen is a clean, promising and environmentally friendly energy source to solve the energy crisis and environmental pollution. The production of hydrogen through a photocatalytic water-splitting process (WSP) has attracted increasing attention.^1–3 Many studies have focused on inorganic metal oxides (e.g., TiO₂,^4,5 BiVO₄,⁶ and ZnO⁷), (oxy)sulfides (e.g., CdS⁸ and SnS₂ (ref. 9)), and (oxy)nitrides (such as InN¹⁰ and GaN¹¹), which are often based on metal cations possessing d⁰ and d¹⁰ electronic configurations, but generally confined by the relatively limited properties such as large band gap, photocorrosion and self-oxidation.^12,13

Recently, conjugated polymers (CPs) have been attracting increasing attention for photocatalysis application owing to their unique delocalized conjugated system, high carrier mobility, narrow and tunable band gaps, and efficient absorption of UV-Vis and/or near-infrared light. We have demonstrated that conjugated polymer nanostructures exhibit high photocatalytic activity under ultraviolet and visible light irradiation, and that nanostructuration is a key factor for their photocatalytic application.^14–18 These conjugated polymer nanostructures were found to be more active than plasmonic TiO₂ under visible light for water treatment and emerge as a new class of photocatalysts.¹⁹ However, the photocatalytic hydrogen evolution activity of CPs is weakened by their fast photogenerated electron–hole pair recombination and low catalysis kinetics. On the other hand, CPs are also excellent conductive supports for stabilizing co-catalysts to enhance their properties or extend their functions.^20–24 Incorporation of metals into semiconductors has been demonstrated to be a promising way to promote charge carrier separation and increase the active sites as well.²⁵

Noble metals with a low Fermi level can trap electrons from the conduction band (CB) of a semiconductor. Among these noble metals, Pt is a very efficient electron scavenger and is the best candidate as a co-catalyst for hydrogen production due to its largest work function.²⁶ Ni is also a very promising candidate as a co-catalyst for H₂ production owing to its low-cost and high activity.^27–29 Ni nanoparticles not only provide a large surface area and active sites, but also decrease the electron–hole recombination.^27,28

Because of the high cost and scarcity of platinum, its replacement with low cost and abundant catalysts (or co-catalysts) is desirable, but remains a great challenge.³⁰ Many studies focus on decreasing the loading of this precious metal by alloying it with another metal of lower cost to increase the photocatalytic activity of the modified photocatalyst for hydrogen evolution. Indeed formation of alloys can reduce the metal–metal interatomic distance, induce electronic effects, greatly increase the number of active sites and accelerate the mass transfer between the different active sites.³¹ For example, strategies using Pd–Pt,^32,33 Ni–Pt,^34,35 Ni–Pd,²⁷ Ni–Au²⁸ or multi-metallic³⁰ heterojunctions have been investigated.

Here we report for the first time that modified conjugated polymer polypyrrole nanostructures with mono (Pt, Ni) and bimetallic (Pt–Ni) nanoparticles (NPs) are very active for hydrogen generation, and that a synergistic effect is obtained by alloying Pt with Ni. Polypyrrole (PPy) nanostructures were synthesized in soft templates formed by lamellar mesophases. Metal nanoparticles (Pt, Ni, and Pt–Ni nanoalloys) of homogeneous size and distribution induced by radiolysis were supported on PPy NPs. Radiolysis is an efficient technique to synthesize metal nanoparticles (NPs) and especially bimetallic NPs of controlled size, distribution and structure in solutions, heterogeneous media or on supports.^36,37 The photocatalytic activity of the composite nanomaterials with various metal loadings was investigated for hydrogen generation under UV-visible light irradiation. The obtained composite NSs show excellent photocatalytic performance for hydrogen generation due to the extended absorption region of PPy NSs, efficient electron–hole pair separation due to nanosized Pt, Ni and Pt–Ni particles and a synergistic effect obtained by alloying Pt with Ni.

2. Experimental section

2.1. Chemical reagents

Pyrrole (Py, C₄H₅N, 98%), as a monomer, iron chloride (FeCl₃) as an oxidizing agent, sodium dodecyl sulfate (SDS) as a surfactant, sodium chloride (NaCl), cyclohexane (C₆H₁₂, 99.7%), and n-pentanol (C₅H₁₂O, ≥99%) as a co-surfactant were used to synthesize PPy NSs. Platinum(II)acetylacetonate (Pt (C₅H₇O₂)₂, ≥99.98%) and nickel(II)acetylacetonate (C₁₀H₁₄NiO₄, 95%) were used as platinum and nickel precursors, respectively. All the reagents and methanol (CH₃OH, 99.9%) were purchased from Sigma-Aldrich, and ethanol (CH₃CH₃OH, ≥99%) was purchased from VWR International. Deionized water (Milli-Q, 18.6 MΩ) was used throughout all experiments. All the reagents were pure and used without further purification.

2.2. Materials preparation

2.2.1. Synthesis of PPy NSs. The conjugated polymer nanostructures, polypyrrole (PPy NSs), were prepared using the methodology described in previous publications, but with small modifications.¹⁴ Briefly, lamellar mesophases were used as a soft template. These mesophases were formed using SDS (0.8 mg) as a surfactant, aqueous solution (2 mL) containing NaCl (0.1 M), pyrrole (100 μL) in cyclohexane (6 mL) as an oil phase, n-pentanol (1.2 mL) as a co-surfactant, and FeCl₃ (64.8 mg) (for monomer oxidation) in the water phase. One mesophase containing Py and one mesophase containing FeCl₃ were mixed (as indicated in Fig. 1a and b). After 5 minutes, the mesophase obtained after mixing turned from transparent to black indicating the oxidation of Py and its polymerization. The PPy-NSs were extracted by simple washing with ethanol and centrifugation. A black powder was obtained after washing with ethanol and drying in an oven at 60 °C.

Fig. 1 (a) Synthesis process of PPy-NSs using a soft template (a lamellar mesophase); (b) doped mesophases with (i) FeCl₃ and (ii) Py; (iii) PPy-NSs in the mesophase; (c) radiolytic synthesis of metal nanoparticles on PPy NSs (the composite nanomaterials are labeled M-PPy-NSs).

2.2.2. Pt-PPy-NS, Ni-PPy-NS and PtNi-PPy-NS synthesis. The metal ions were reduced by solvated electrons and alcohol radicals induced by solvent radiolysis.³⁸ For each sample, 20 mg of PPy-NSs were added to 25 mL of an aqueous solution containing 0.1 M ethanol (added as an HO˙ radical scavenger)³⁶ and the metallic precursors. N₂ was bubbled for 15 minutes to remove oxygen. Then, the samples were exposed to a ⁶⁰Co panoramic γ-source at a dose rate of 4 kGy h⁻¹ for 30 minutes (dose 2.0 kGy). After irradiation, the suspensions were centrifuged and washed with water (10 mL) 3 times. Then, the samples were dried at 50 °C for 2 h (Fig. 1c). Samples with different metal loadings were labeled x%M-PPy-NSs, where x% indicates the percentage mass of the metal with respect to the mass of PPy. The materials were collected by centrifugation. The supernatant was completely transparent indicating that all the metallic species were deposited on the PPy nanostructures.

2.3. Characterization

Small Angle X-ray Scattering (SAXS) was used to characterize the mesophases before and after polymerization. The mesophases doped with only Py, only FeCl₃ and containing PPy (obtained after oxidation of Py with FeCl₃) were put in glass capillaries (diameter = 1.5 mm), and a high brightness X-ray tube with low power and an aspheric multilayer optic (GeniX 3D from Xenocs) were employed to deliver an ultralow divergent beam (0.5 mrad). A two-dimensional Schneider 2D image plate detector prototype was used to collect the scattering intensity.

Mott–Schottky (MS) plot measurement was carried out by using a typical three electrode setup (Pt wire and Ag/AgCl as counter and reference electrodes, respectively). A PPy film on FTO was used as the working electrode. The MS spectra were measured in the voltage window of 0.2 V–1.0 V in the dark (increment: 20 mV, frequency: 1 kHz). An aqueous solution of 1 M NaSO₄ was used as the electrolyte.

UV-Vis absorption spectra were recorded with an HP 8453 spectrophotometer

The size, dispersion and morphology of the conducting polymer nanostructures and metal nanoparticles were examined by scanning electron microscopy (SEM, ZEISS Supra 55 V P FEG-SEM) and transmission electron microscopy (TEM, JEOL JEM 2010 UHR operating at 200 kV). The chemical analyses were obtained using an Energy-Dispersive X-ray Spectroscopy (EDS) microanalyzer (PGT-IMIX PC) mounted onto the microscope. High angle annular dark field images were obtained using scanning transmission electron microscopy with a Cs corrected JEOL-ARM-200F at 200 kV.

XPS measurements were performed on a K alpha spectrometer from ThermoFisher, equipped with a monochromatic X-ray source (Al K_α, 1486.6 eV) with a spot size of 400 μm. The hemispherical analyzer was operated in CAE (Constant Analyser Energy) mode, with a pass energy of 200 eV and a step of 1 eV for the acquisition of survey spectra, while for the acquisition of narrow scans, pass energies of 50 eV and 100 eV and a step of 0.1 eV were used. The charge build-up was neutralized by means of a “dual beam” flood gun. The obtained spectra were treated by means of Avantage software provided by the manufacturer. A Shirley type background subtraction was used and the peak areas were normalized using the Scofield sensitivity factors. The binding energies were calibrated against the C 1s binding energy set at 284.8 eV. The peaks were analyzed using mixed Gaussian–Lorentzian curves (70% Gaussian character).

2.3.2. Photocatalytic production of hydrogen. We used methanol as a hole scavenger. Some studies have reported that different sacrificial donors without H (such as N₂S or I⁻) could enhance the yield of hydrogen during the photocatalytic process.³⁹ However, their photocatalytic activities are not as efficient as that of methanol as a hole scavenger.

H₂ production from a methanol–water mixture solution was assessed in a closed quartz reactor under a N₂ atmosphere with vigorous stirring. For the experiments, 20 mg of the photocatalyst was dispersed in 20 mL of a degassed aqueous solution with 25 vol% methanol, as a hole scavenger. The samples were irradiated using an Oriel 300 W xenon lamp with an infrared water filter for 5 h under stirring. Every 1 h, 0.2 mL of the gas sample was taken using a syringe from the quartz reactor. The amount of H₂ was determined by gas chromatography (GC) using a Shimadzu GC-14B instrument.

3. Results and discussion

3.1. Characterization

The mesophases doped with Py, with FeCl₃ and the mesophase resulting from the mixing of the two previous ones were characterized by small angle X-ray scattering (SAXS) (see Fig. 2a). All the three samples are lamellar phases, characterized by two peaks located at scattering vectors q₀ and 2q₀; the increase of the scattering intensity at small q is also typical of lamellar phases. The spacing (periodicity of the lamellar phase) d = 2π/q₀ = 6.3 nm is the same for the three samples. We note that the lamellar phase containing the monomer is more ordered (sharper second order peak) than the other samples.

Fig. 2 (a) SAXS of the mesophases doped with Py, FeCl₃ and the mesophase resulting from the mixing of the other two mesophases and containing PPy (induced by oxidation of Py by Fe³⁺); (b) Mott–Schottky plot of PPy and PtNi-PPy; (c) UV-Vis spectrum of PPy and (d) the corresponding Kubelka–Munk plot.

The electrochemical Mott–Schottky measurement was performed to obtain the flat band potential which can be given by the intercepts of the extrapolated lines (slope) (Fig. 2b). The carrier concentration (N_A) is determined using the equation:⁴⁰ where e is the electron charge, ε_r is the dielectric constant (∼10 for PPy), and ε₀ is the free space (8.85 × 10⁻¹⁴ Fcm⁻¹). The N_A of PtNi-PPy (17.7 × 10¹⁹ m⁻³) is five times higher than that of PPy (3.5 × 10¹⁹ m⁻³), indicating enhanced electronic conductivity, which will lead to a decrease of charge carrier recombination and better electron transfer.^41–43

The UV-Vis spectrum of PPy exhibits an absorption in the visible region (400 nm–700 nm) and near-IR region (700 nm–1000 nm) (Fig. 2c). The peak at 470 nm is due to the π–π* transitions. The corresponding band gap (E_g) of PPy was determined from the Kubelka–Munk function (Fig. 2d): the value of E_g of PPy was found to be 2.2 eV, which is similar to that of previous reports.¹⁴

SEM images show uniform PPy NSs with an average size of 40 nm (Fig. S1†). TEM was used to characterize the morphology, size, and distribution of the metal nanoparticles (NPs) synthesized by radiolysis and the composite nanomaterials (Fig. 3). Aggregates of PPy nanoplates were observed (see Fig. 3a, c and e). For platinum modified polymer nanostructures, well dispersed Pt NPs with a diameter of about 2–3 nm were observed on the surface of PPy NSs (Fig. 3a). Fig. 3b shows the (111) facets of Pt with an interplanar spacing of 0.23 nm indicating the formation of crystalline Pt NPs. For nickel-modified polymer nanostructures, Ni-based NPs of 5 nm diameter which were homogeneously dispersed on the surface of PPy-NSs were observed as shown in Fig. 3c and d. Fig. 3d presents the interplanar spacing of the Ni-based nanoparticles. The lattice spacings of 0.20 and 0.24 nm correspond to the (111) plane of Ni and the (111) plane of NiO, respectively, which proves the presence of Ni⁰–NiO-based nanoparticles on PPy NSs. In the case of co-modification with Pt and Ni, metal NPs of a homogeneous size (2 nm) were observed in Fig. 3e showing a smaller crystallite size than monometallic NPs, which supports the formation of Pt–Ni nanoalloys.⁴⁴ High-resolution STEM images (Fig. 3f) present the lattice spacings of 0.22 nm and 0.19 nm, which are indexed to the (111) and (200) planes of the fcc PtNi alloy, respectively. EDS analysis of the metal nanoparticles shows that the nanoparticles contain both Pt and Ni (Fig. S2†) indicating the bimetallic nature of the NPs.

Fig. 3 TEM images of 0.2%Pt-PPy-NSs (a and b), 5%Ni-PPy-NSs (c and d), 0.05%Pt0.05%Ni-PPy-NSs (e) and HAADF-STEM image of 0.05%Pt0.05%Ni-PPy (f).

The surface composition and the oxidation states of the metal NPs in the modified PPy NSs were analyzed by X-ray photoelectron spectroscopy (XPS). XPS patterns of Pt-PPy-NSs, Ni-PPy-NSs and PtNi-PPy-NSs are shown in Fig. 4.

Fig. 4 XPS spectra of (a) C 1s, (b) N 1s in PtNi-PPy-NSs, (c) Pt 4f in Pt-PPy-NSs, (d) narrow scan spectrum of Ni 2p in Ni-PPy-NSs, (e) narrow scan spectrum of nickel states in PtNi-PPy-NSs and (f) narrow scan spectrum of Ni 2p in PtNi-PPy-NSs.

The wide region of spectroscopy of PtNi-PPy-NSs is shown in Fig. S3.† The signals at 284.1 eV, 402.5 eV, 533.2 eV and 201.2 eV correspond to C 1s, N 1s, O 1s and Cl 2p, respectively, which are due to PPy.^31,45 Other signals at 70.5 eV and 53.3 eV correspond to Pt 4f and Ni 3p, respectively. The narrow range spectra of C 1s and N 1s are depicted in Fig. 4a and b, which also prove the presence of PPy.

The Pt 4f core-level spectrum shows an intense doublet at 71.7 eV and 75.0 eV corresponding to Pt 4f_7/2 and Pt 4f_5/2 of Pt⁰ which indicates that Pt²⁺ was reduced into Pt metal in the Pt-PPy-NS sample (Fig. 4c).

The Ni 2p core-level spectrum shows the presence of metallic nickel, the component Ni 2p_3/2 at 852.5 eV, alongside with its oxide and hydroxide, which are expected as nickel is very sensitive to oxygen (Fig. 4d).^27,28,46

Given the low loading levels of both Ni and Pt, especially in the case of PtNi-PPy-NSs (Fig. 4e and f), the acquisition conditions were adjusted in order to have the best possible signal/noise ratio in detriment of spectral resolution. Fig. 4e and f show that Pt and Ni nanoparticles are partly oxidized. The Pt 4f signals represent an intense doublet at 71.4 eV and 74.7 eV, corresponding to the core level energies of Pt 4f_2/7 and Pt 4f_5/2 of the Pt metal. Other doublets (71.4 eV and 75.9 eV) and (75.0 eV and 77.9 eV) can be ascribed to the +2 and +4 oxidation of Pt, respectively. The peaks at 68.0 eV and 72.4 eV correspond to the Ni 3p_3/2 and Ni 3p_1/2 corresponding to NiO (Fig. 4e).^31,47Fig. 4f shows the presence of the Ni metal, NiO and Ni(OH)₂. The presence of the oxidized Pt and Ni species is probably caused by the air during the XPS analysis. It is noteworthy that in the bimetallic materials (PtNi), the binding energy of Pt is slightly lower by 0.3 eV compared to that in the monometallic Pt-PPy, which indicates the strong interaction between the metals and PPy supports.⁴⁸

3.2. Photocatalytic H₂ generation

The addition of co-catalyst (such as Pt, Ni–NiO, and PtNi) NPs for photocatalytic hydrogen generation can reduce the activation energy of the reaction, provide active sites for proton reduction and H–H bond formation and decrease the recombination of charge carriers, resulting in an improvement of photocatalytic activity for hydrogen production.^27,28 In addition, the amount of loading of the co-catalyst is also a crucial factor for hydrogen generation.⁴⁹

3.2.1. Pt-PPy-NSs. The photocatalytic H₂ generation efficiency of the as-prepared samples was investigated in 25 vol% methanol aqueous solution under UV-Vis light. As shown in Fig. 5a, with an increase of time, 0.2%Pt-PPy-NSs show a remarkable enhancement in the photocatalytic activity. Interestingly, 0.05%Pt-PPy-NSs and 0.1%Pt-PPy-NSs present higher photocatalytic performance than nanostructures comprising higher loadings of Pt (0.3%, 0.5% and 1% loading rate). This indicates that higher photocatalytic activity is obtained with a very small amount of co-catalyst. The H₂ production rate with 0.2%Pt-PPy-NSs is 1400 μmol h⁻¹ g⁻¹, and the H₂ production rates with 0.05%Pt-PPy-NSs and 0.1%Pt-PPy-NSs are 312 and 341 μmol h⁻¹ g⁻¹, respectively. For loadings larger than 0.2%, the H₂ production rate decreases dramatically, showing that an excess metal loading can induce a dramatic decrease of the photocatalytic activity. At high loading, metal nanoparticles can play the role of recombination centers, inducing a decrease in the photocatalytic activity.⁵⁰ Higher metal loading can also induce hindering of the absorption of light by the semiconductor.

Fig. 5 (a) A comparison of the photocatalytic H₂ generation rates for different loading rates of Pt in PPy-NSs as indicated in the legend; (b) hydrogen production with 0.2%Pt-PPy-NSs after 4 cycles, with each cycle for 5 h; (c) a comparison of the photocatalytic H₂ generation rates for different loading rates of Ni on PPy-NSs as indicated in the legend; (d) hydrogen production with cycling in the presence of 5%Ni-PPy-NSs.

The stability of the photocatalytic activity was investigated with 0.2%Pt-PPy-NSs under UV-visible light (see Fig. 5b). Modified PPy nanostructures with Pt NPs show very good stability with cycling.

3.2.2. Ni-PPy-NSs. As shown in Fig. 5c, the photocatalytic activity of Ni-PPy-NSs increases with the Ni loading until 5% loading, then a decrease in the photocatalytic activity is observed. Interestingly, 5%Ni-PPy-NSs exhibit excellent photocatalytic activity for hydrogen production (289 μmol h⁻¹ g⁻¹), despite the fact that this rate is much lower than that obtained with 0.2%Pt-PPy-NSs (1400 μmol h⁻¹ g⁻¹). XPS signals show the concomitant presence of NiO and Ni(OH)₂ (Fig. 4b). The Fermi level of NiO is lower than that of Ni⁰, and therefore, the photogenerated electrons in the NiO/Ni structure migrate from the conduction band of the semiconductor to the Ni layer, and then transfer to the NiO layer, which leads to efficient charge carrier separation. Ni(OH)₂ as a co-catalyst can also efficiently transport photogenerated electrons from the conduction band of the semiconductor to Ni(OH)₂/Ni clusters leading to high photocatalytic activity for hydrogen generation.⁵¹

It is worth mentioning that the nature of the metal precursors is one of the factors that may strongly affect the size, shape and/or the surface state of the metal nanoparticles, and therefore their activity as co-catalysts. Our results show that PPy-NSs modified with metal nanoparticles prepared with platinum (II)acetylacetonate (Pt(C₅H₇O₂)₂) and nickel (II)acetylacetonate (Ni(C₅H₇O₂)₂), as metallic precursors, exhibit higher photocatalytic properties than modified PPy-NSs prepared with potassium tetrachloroplatinate(II) (K₂PtCl₄) and nickel formate (C₂H₂NiO₄), respectively (Table S1†). Cui and co-workers investigated the influence of metal precursor ligands on the selectivity of alloy particle shape and structures.⁵² Therefore, the choice of the metal precursor is very important for the co-catalyst activity for hydrogen production.

The stability of Ni-PPy-NSs with cycling was investigated. As shown in Fig. 5d, after 5 cycles, the amount of H₂ dropped by 33%. The small decrease of the photoactivity can be due to the leaching of Ni nanoparticles during the photocatalytic cycles. Indeed, Ni is a very efficient catalyst for many reactions, but it is known to leach in water.⁵³

3.2.3. PtNi-PPy-NSs. A strategy to avoid Ni leaching with cycling is to alloy Ni with another noble metal such as Au or Pt. Ni from a Pt–Ni alloy does not dissolve in an electrolyte owing to the Ni(OH)₂ passivated surface and the enhanced stability of Ni in the Pt lattice.^47,54

To investigate the photocatalytic hydrogen production and stability of the bimetallic (PtNi) doping of PPy-NSs, experiments were conducted with the same total metal loading rate (0.1%) under the same conditions (see Fig. 6a–c). Interestingly, we found that a 50/50 mixture of Ni and Pt shows an enhanced photocatalytic activity (664 μmol h⁻¹ g⁻¹) compared with a pure Pt loading (341 μmol h⁻¹ g⁻¹) and a pure Ni loading (52 μmol h⁻¹ g⁻¹) (Fig. 6a and S4†). Some related studies on photocatalytic H₂ generation with different photocatalysts have been listed in Table S2† for comparison. The enhanced photocatalytic activity of PtNi-PPy-NSs may be due to the increased photoefficiency by the Schottky barrier leading to a longer lifetime of charge carriers and strong UV-Vis absorption due to the plasmon resonance of Pt in the UV range.⁵⁵ Synergistic effects are expected (such as electronic and geometry effects) to induce enhancement of photocatalytic activity. Such a synergistic effect for hydrogen production with bimetallic nanoparticles was also recently observed for Ni–Au/TiO₂ and Ni–Pd/TiO₂: the association of Ni with another metal (such as Pd or Au) induces an increase of the photocatalytic activity of semiconductors for hydrogen production.^27,28,56 In the case of Ni–Pd/TiO₂, the study of light absorption, charge-carrier dynamics, and photocatalytic activity revealed that the main role of the metal NPs is to act as catalytic sites for recombination of atomic hydrogen. Platinum is very efficient in electron scavenging and proton reduction, while Ni is a very good co-catalyst for H–H bond formation.^27,28,35,49

Fig. 6 (a) Photocatalytic H₂ generation rate with the same loading rate for different samples. (b) The photocatalytic H₂ generation rate for different loading rates of Ni–Pt on PPy-NSs. (c) Hydrogen production with cycling in the presence of 0.1%PtNi-PPy-NSs. (d) Proposed photocatalytic mechanism for hydrogen generation. Metal NPs act as electron traps, and proton reduction catalysts.

Fig. 6b and S5† show the effect of the total metal loading on photocatalytic hydrogen production for samples with a fixed Pt : Ni ratio of 1 : 1 and the catalytic activity strongly depends on the Ni/Pt ratio (Fig. S6†). The 0.1%PtNi-PPy-NSs exhibit the highest photocatalytic activity yield rate of hydrogen compared with the PPy modified with other loading rates. Fig. 6c shows their very high stability with cycling, which indicates that PtNi-PPy-NSs are much more stable than Ni-PPy-NSs during the photocatalytic cycle tests.

The mechanism of hydrogen formation is similar to that proposed for Pd–Ni/TiO₂ and Au–Ni/TiO₂.^27,28 The proposed photocatalytic mechanism for hydrogen generation is shown in Fig. 6d. The generation of the electron–hole pairs takes place in the organic semiconductor PPy NSs. When the energy of the incident light is higher or equal to the band gap of PPy-NSs, the photogenerated electrons can be promoted from the HOMO to the LUMO of PPy nanostructures, leaving holes in the HOMO of PPy-NSs (PPy-NSs → h⁺ + e⁻). Water molecules are reduced by the electrons to produce H₂ (H₂O + e⁻ → H₂ + OH⁻) in the presence of co-catalysts. Methanol was used as a hole scavenger: CH₃OH reacts with the holes to give CO₂ and H₂. This hole scavenging results in better charge carrier separation and leads to enhancement of the light conversion quantum yield.⁵⁷ Platinum is known to be an efficient sink for electrons.⁵⁸ The presence of Pt-based nanoparticles decreases the charge carrier recombination, and therefore induces the increase of the quantum yield. The excited electrons migrate from PPy-NSs to the metal NPs, and then H⁺ species are reduced on the metal surface to promote proton reduction to generate atomic hydrogen, while these hydrogen radicals recombine to produce molecular hydrogen (Fig. 6d).⁵⁹ The metal nanoparticles (such as Pt, Ni and Pt–Ni) not only serve as electron sinks, but also provide effective proton reduction sites due to their relatively low over-potential.²⁷ Ni-based nanoparticles are very active co-catalysts in H˙ recombination to form H₂.^27,28 Finally, H˙ recombination appears on the surface of the metal NPs producing molecular hydrogen H₂, which is facilitated by the presence of Ni-based cocatalysts. The enhancement of hydrogen generation compared with that of the monometallic samples is due to a synergistic effect between Ni and Pt: the presence of Pt induces better electron scavenging and higher H˙ production, while the association with Ni promotes H˙ recombination leading to higher H₂ production. Furthermore, the modification with bimetallic Pt–Ni nanoparticles leads to photocatalysts with high stability during cycling.

4. Conclusion

In conclusion, polypyrrole nanostructures (NSs) modified with mono- and bimetallic nanoparticles present high photocatalytic activity for hydrogen evolution. PPy NSs were successfully synthesized in soft templates formed by lamellar mesophases. Monometallic Pt, Ni–NiO and bimetallic Pt–Ni nanoparticles of homogeneous size and dispersion on the surface of PPy NSs were obtained by radiolytic reduction. Our work also demonstrates that the photocatalytic activity is very sensitive to the metal loading. 0.2%Pt-PPy-NSs present the best activity compared with the other loading rate percentages. Modification with nickel-based nanoparticles also gives promising results for green hydrogen production. 5%Ni-PPy-NSs show enhanced photocatalytic performance due to the formation of a heterojunction between PPy and NiO–Ni nanoparticles. However, the photocatalytic activity decreases slightly with time, probably because of Ni leaching.

Pt-Ni-PPy exhibits much higher activity than its monometallic counterparts, and a synergistic effect is obtained. The presence of Pt leads to better electron scavenging and higher H˙ formation, while the association with Ni promotes H˙ recombination leading to higher H₂ generation. These Pt-Ni-PPy nanostructures are also very stable with cycling.

Further studies will focus on the development of composite materials based on conjugated polymer nanostructures modified with co-catalysts made of abundant elements for solar fuel production.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

X. Y. gratefully acknowledges the financial support from the China Scholarship Council (CSC). This work was supported by the University Paris-Saclay and IRS MOMENTOM.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ta11088g

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