Polymer and Pdots preparation and characterizations
The synthesis routes of the designed new series of PITIC-X-34 and PBTIC-X-based conjugated polymers are shown in Schemes S1 − S4. Accordingly, we synthesized the three PITIC-X and PBTIC-X-based polymers through Suzuki Miyaura and Stille coupling polymerizations, resulting in the polymers PITIC-Ph, PITIC-Th, PITIC-ThF, PBTIC-Ph, PBTIC-Th, and PBTIC-ThF respectively, and the detailed procedures are described in the Supporting Information. The as-prepared polymers were characterized by 1H nuclear magnetic resonance (NMR) spectroscopy (Figures S1 − S10), thermogravimetric analysis (TGA) (Table S1 and Figure S11), Fourier transform infrared (FTIR) spectroscopy (Figure S12a), and X-ray photoelectron spectroscopy (XPS) (Figure S12b, S13, S14, and S15).
The bulk polymer was converted to polymer nanoparticles (Pdots) using precipitation methods by dispersing the polymer in water without additional surfactants, such as PS-PEG-COOH or Triton under vigorous sonication, and the preparation details are shown in the Supplementary Information (Figure S16). The photographs of all six polymers show different colors in THF solutions, suggesting that their optical properties are easily tuned by the introduction of π-linker units on acceptor rings (Figs. 2a). The normalized ultraviolet–visible (UV–vis) absorption spectra of the six Pdots in water solution are presented in Figs. 2b and the related data are summarized in Table S2. The monomer Br-ITIC-Br shows strong absorption in the visible range of 550–750 nm, with λmax at 669 nm and a small shoulder at 620 nm (Figure S17a). The ITIC polymer prepared using the thiophene π-linker (PITIC-Th) exhibits approximately the same absorption onset of Br-ITIC-Br with a slightly blueshifted λmax (657nm). Notably, the substitution of the thiophene group with the phenyl group (PITIC-Ph) is accompanied by a large blueshift leading to λmax of 551 nm. In contrast to the difluorothiophene group (PITIC-ThF), which shows a small new shoulder at 739 nm accompanied by a redshifted absorption onset. This result demonstrates that the introduction of the difluorothiophene group can extend the absorption to the NIR region, which is beneficial for absorbing a large region of the solar spectrum. The as-prepared various comonomer π-linker units with PBTIC-X-based polymers show a redshifted absorption compared to that of PITIC-X-based polymers. This result suggests that the A-DAD-A structure presents larger conjugation than A-D-A, leading to a more highly redshifted absorption (Fig. 2b). Notably, the conversion of the bulk polymers (in THF) to Pdots (in water) is accompanied by a 20 − 50 nm redshift owing to the J aggregation of the polymer chain in aqueous solution,35, 36 as shown in Figures S18a and S18b. According to Tauc plots as shown in Figures S18c and S18d, the Eg values of these polymers are in the range of 1.45 − 2.05 eV and become lower in water solutions by approximately 0.10 eV, thereby exhibiting their narrow bandgap that facilitates absorption over a broad range of visible and NIR light. As shown in Fig. 2c, the various π-linker groups have an obvious influence on the energy level positions of the PITIC- and PBTIC-based polymers. The constructed polymers achieve deeper HOMO and LUMO values with the following arrangement of π-linkers ThF > Th > Ph. The previously discussed results indicate that the introduction of different π-linker units on acceptor rings displays a significant effect on the photophysical properties of polymers.
The prepared structure and morphology of the Pdots were determined by cryo-transmission electron microscopy (Cryo-TEM), as shown in Fig. 2d. The six polymers display spherical particles with nonuniform particle sizes ranging from 30 to 70 nm, similar to other reported.37, 38 The presence of nanometric spherical particles indicates the formation of a Pdot structure from all the polymers. In addition, the sessile drop technique was used to measure the static contact angles of the as-prepared Pdots at three different locations with water at room temperature. The water contact angles of the PITIC-X polymers are lower than those of the PBTIC-X polymers (Table S2 and Figure S20), suggesting that they possess a lower hydrophobicity. Although the as-prepared polymers are hydrophobic, their conversion to the Pdot structure enhances their water dispersion and increases the surface area of the photocatalyst. Thus, the conversion of hydrophobic polymers to Pdots without the use of any surfactants could be beneficial for achieving good water dispersibility and enhanced hydrogen production from water.
Photocatalytic Activity Under Visible And Nir Light
Next, we examined all the Pdots as photocatalysts for visible light-driven hydrogen evolution. A PAR30 light-emitting diode (LED) lamp (20 W, 6500 K, and λ > 420 nm) was used as the light source (Figure S21). The Pt cocatalyst (H2PtCl6) with optimized amount of 3% was utilized to enhance the photocatalytic activity of our constructed photocatalysts (Figure S22). Under the optimum conditions and visible light illumination, we recorded the kinetic curve of hydrogen evolution to investigate the H2 evolution efficiency of the photocatalyst, as shown in Fig. 3a. From the kinetic curves, we extracted the H2 evolution rate of the six polymers (Fig. 3b). Among them, the PITIC-ThF Pdots show the highest HER value (339.7 mmol g− 1 h− 1) followed by the PITIC-Th Pdots (168.7 mmol g− 1 h− 1) and PITIC-Ph Pdots (106.2 mmol g− 1 h− 1). Similar trends are observed for the PBTIC-X series, and the order of the HER is PBTIC-ThF Pdots (269.4 mmol g− 1 h− 1) > PBTIC-Th Pdots (121.1 mmol g− 1 h− 1) > PBTIC-Ph Pdots (69.8 mmol g− 1 h− 1). The HER value increases with increasing photocatalyst amount, in which a loading of 0.1 mg to 5 mg can increase the HER from 34 to 279 µmol/h and 27 to 178 µmol/h for the PITIC-ThF Pdots and PBTIC-ThF Pdots, respectively (Fig. 3c). This result is strong evidence to prove that the high photocatalytic activity of our materials is truly beneficial for the development of hydrogen production under visible light. The photocatalytic activity of the PITIC-ThF Pdots and PBTIC-ThF Pdots under NIR light was investigated using a Xenon lamp (AM1.5 and 3000 W m− 2) with a cutoff filter (λ > 780 nm) as a source of light. The amount of H2 generated from the PITIC-ThF Pdots and PBTIC-ThF Pdots increase over time and reach 11450 ± 800 and 1715 ± 320 µmol/g under NIR light irradiation for 4 h, respectively (Fig. 3d). Among the two polymers with the ThF π-linker, the PITIC-ThF Pdots present an HER of 4045 ± 430 µmolh− 1g− 1, which is approximately more than 5-fold that of the PBTIC-ThF Pdots (708 ± 210 µmolh− 1g− 1) (Fig. 3e). The HER of PITIC-ThF Pdot has shown a promising and unprecedented efficiency of a single polymer under NIR light. The apparent quantum yields (AQYs) of the most efficient PITIC-ThF and PBTIC-ThF Pdots were obtained under standard photocatalytic conditions using a light source with a bandpass filter (λ = 420, 500, 550, 600, and 700 nm). As shown in Fig. 3d, the AQYs of the PITIC-ThF (PBTIC-ThF) Pdots are estimated to be 3.9 (2.9), 3.2 (2.7), 3.1 (2.5), 3.9 (2.8), and 4.7% (3.1%) at 420, 500, 550, 600, and 700 nm, respectively. The AQY values are almost compatible with the absorption spectrum of the polymer photocatalyst, with a higher value at a longer wavelength of 700 nm, suggesting that these Pdots have good photoresponsivity for hydrogen production in the entire visible light region. Notably, the HER under visible and NIR light and AQY values at 700 nm of the PITIC-ThF Pdots are among the highest values ever reported in the literature (Fig. 3g, 3h, and Table S3). Although some of the reports show a high AQY at 420 nm, it later dramatically decreases when compared to irradiation with a longer wavelength of 700 nm.
Unveiling The Effect Of A-d-a Structure And Different Linkers On The Activity
We explored two important aspects to learn more about the relationship between the structure and activity of the PITIC-X- and PBTIC-X-based polymer photocatalysts: the influence of various π-linkers on polymer activity and the difference between the ITIC and BTIC moieties. The relationship between the different π-linkers and the photocatalytic activity was understood through density functional theory (DFT) and transient absorption (TA) spectroscopy. As shown in Figures S23 and S24, the HOMOs of the six polymers are delocalized to some extent over the conjugated systems, whereas the LUMOs are more localized over the IC moiety. Although the different π-linkers does not exhibit a variation in regard to the HOMO and LUMO localization on the main polymer units, the dihedral angles are variable between the different π-linker groups and the IC acceptor moieties. Figure 4a shows that the dihedral angles between the Ph π-linker and IC moiety of PITIC-Ph (34.28°) or PBTIC-Ph (34.34°) are much greater than those between the Th π-linker group and IC moiety of PITIC-Th (23.72°) or PBTIC-Th (23.47°) and those between the ThF π-linker group and IC moiety of PITIC-ThF (18.4°) or PBTIC-ThF (17.66°). This is due to the hydrogen bond formation, namely, H − F and H − S, with the ThF π-linker and only H − S for the Th π-linker, while the Ph π-linker does not show any hydrogen bond formation (Figure S25).39, 40, 41 As a result, the F − H and S − H distances for the ThF π-linker are smaller than the H − H and S − H distances of the Th π-linker, leading to a decrease in the dihedral angles when using the ThF π-linker (Figure S25). The smaller dihedral angle between the acceptor and π-linker in PITIC-ThF indicates a more planarized structure with efficient charge carrier mobility and transfer between the acceptors of different repeated moieties followed by improvement of the charge separation between the D and A for every polymer repeated moiety; thus, an enhanced exciton dissociation yield and improved photocatalytic activity are observed. Increasing the dihedral angle with Th and Ph π-linkers reduces the planarity and charge transfer between the acceptors and increase the charge recombination from D to A, resulting in less efficient photocatalytic activity (Fig. 4b). Furthermore, in Figure S26 the DFT calculations indicating that hydrogen has more affinity to adsorb and that the photocatalytic reaction occurs on the IC acceptor moiety, and compared to the Ph and Th linkers the presence of a ThF in the polymeric structure leads to stronger hydrogen adsorption, which in turn results in more favorable H2 formation energetics.
Femtosecond transient absorption spectroscopy (fs-TAS) was used to further study the excited state dynamics of the PITIC-X polymers and the effect of the different π-linkers on the excited state lifetime. Figure 5a, 5b, and 5c show the transient absorption spectra of the PITIC-ThF, PITIC-Ph, and PITIC-Th Pdots, respectively. The TA spectra of the three polymers consist of a negative signal assigned to the ground-state bleach of the polymer, which is consistent with the ground-state absorption band, and excited state absorption, which is assigned to the absorption of the polymer excitonic state.8, 42 Figs. 5d, 5e, and 5f show the bleach recovery dynamics and lifetimes of the three polymers at 564 nm (PITIC-Ph), 650 nm (PITIC-Th), and 650 nm (PITIC-ThF), respectively, which consist of two time components. The results show that the PITIC polymer with different π-linkers shows different bleach recovery dynamics and different lifetimes, where the recombination of photogenerated charge carriers decelerates more in the following order: ThF > Th > Ph. This is due to the smooth and fast charge transfer between the acceptors of different repeated units in the case of the ThF π-linker with a lower dihedral angle. Moreover, compared to the Th and Ph linkers, the ThF linker with both PITIC and PBTIC polymer series shows strong quenching emission with the steady-state photoluminescence spectra, highest photocurrent responses with the transient photocurrent response, smallest arc radii with the electrochemical impedance spectroscopy (EIS) Nyquist plots (Figure S27).
In Fig. 5g, we schematically show the relationship between the different π-linkers and photocatalytic activity according to the previous results. In the case of the Ph π-linker, after photoexcitation, the electrons transfer from D to A, but the large dihedral angle of the Ph π-linker reduces the excited electrons transfer between the acceptors of different repeated moieties, leading to the recombination of photogenerated electrons from A to D, which results in a shorter bleach recovery lifetime and ineffective charge separation. The dihedral angle decreases slightly with the Th π-linker; hence, the transfer of excited electrons between acceptors is enhanced, leading to a relatively long bleach recovery lifetime accompanied by the inhibition of charge recombination and enhancement of charge separation. On the one hand, the ThF π-linker shows effective charge separation and largely decreased charge recombination. This is due to the small dihedral angle presented by the ThF π-linker, which enhances the charge transfer between acceptors of different repeated moieties. In comparison to the PBTIC-X polymers, the PITIC-X polymers have higher photocatalytic activity in the visible and NIR regions. Despite the fact that the PBTIC-ThF absorption spectra redshifts compared to that of PITIC-ThF, the greater effective photocatalytic activity of PITIC-ThF compared to PBTIC-ThF under visible and NIR light is due to the higher crystallinity of PITIC-ThF as well as the charge distribution between A and A` of the PBTIC-based polymer (Figure S28).
Effect Of The Free-surfactant Pdot Structure On The Activity
To clarify the benefits of our presented method for preparing the Pdot structure without additional surfactants, we studied the effect of common surfactants used for Pdot preparation on the photocatalytic activity of the Pdot photocatalyst. The Pdot structure of PITIC-ThF prepared by the precipitation method in the absence and presence of common surfactants, such as PS-PEG-COOH and Triton, produces three Pdot structures: PITIC-ThF Pdots, PITIC-ThF/PS-PEG-COOH Pdots, and PITIC-ThF/Triton Pdots. Compared to the PITIC-ThF Pdots, the PITIC-ThF/PS-PEG-COOH Pdots (Fig. 6a) and PITIC-ThF/Triton Pdots (Figure S29a) inhibits the HER. This is due to the resistance arising from the surfactant that can hinder the charge mobility between polymer molecules and charge transfer between the polymer and Pt cocatalyst. Increasing the charge resistance in the presence of surfactants is confirmed by the EIS Nyquist plots, where the arc radius of the PITIC-ThF Pdots is smaller than that of the PS-PEG-COOH/PITIC-ThF Pdots (Fig. 6c), or Triton/PITIC-ThF Pdots (Figure S29b). Moreover, the transient absorption traces of the PITIC-ThF Pdots with Pt show a prominent difference in the bleach recovery dynamics and lifetime in the presence and absence of the PS-PEG-COOH surfactant, as shown in Fig. 6b. The presence of PS-PEG-COOH reduces the lifetime from 544 ± 33 ps to 126 ± 36 ps, explaining why the surfactant can hinder the charge transfer between the polymer and reactant (including the Pt cocatalyst and AA) and then accelerate charge recombination. Figure 6d shows a schematic diagram of the charge transfer from the PITIC-ThF Pdots to the Pt cocatalyst and AA. In the absence of PS-PEG-COOH, the excited electrons transfer to the Pt cocatalyst accompanied by a long bleach recovery lifetime with efficient charge separation and enhancement of the H2 evolution activity. On the other hand, the presence of PS-PEG-COOH hinders charge transfer from the PITIC-ThF Pdots to the Pt cocatalyst, and AA accelerates charge recombination with less efficient photocatalytic activity of the Pdot photocatalyst. Consequently, our Pdot preparation method without a surfactant is more efficient for achieving high photocatalytic activity for H2 evolution.