Hyperbranched Conjugated Polymer Dots: The Enhanced

2 days ago - recorded on a Nicolet 5700 spectrometer, and the samples were ... Optical absorption spectra were recorded in employing Evolution 201 UV...
1 downloads 0 Views 4MB Size
Download PDF
Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

pubs.acs.org/Macromolecules

Hyperbranched Conjugated Polymer Dots: The Enhanced Photocatalytic Activity for Visible Light-Driven Hydrogen Production Peng Zhao,† Lijie Wang,‡ Yusen Wu,† Tao Yang,† Yun Ding,† Hua Gui Yang,‡ and Aiguo Hu*,† †

Shanghai Key Laboratory of Advanced Polymeric Materials, School of Materials Science and Engineering, and ‡Key Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, P. R. China

Downloaded by BUFFALO STATE at 20:39:40:374 on May 29, 2019 from https://pubs.acs.org/doi/10.1021/acs.macromol.9b00551.

S Supporting Information *

ABSTRACT: Conjugated polymer (CP) aggregates with linear/cross-linked structures are widely reported as photocatalysts for hydrogen evolution reaction. For full disclosure of the relationship between the photocatalytic performance and structural features of the CP photocatalysts, homogeneous dispersion of these CPs in aqueous medium is necessary, which however is difficult to be achieved. Herein, we report several coassembled polymeric dots (Pdots) consisting of PEG45-b-PMMA103 and CPs with various structural features. We found that the Pdots of hyperbranched soluble CP nanoparticles (SCPNs) exhibit a high H2 evolution rate up to 840 μmol h−1 g−1 with no platinum or rhodium as a cocatalyst, superior to their analogues with the linear or cross-linked structure. A possible charge-transfer mechanism suggests that the photoelectrons directly mobilize to the surface of these single-particulate Pdots over three-dimensional skeleton and successfully avoid the ineffective intermolecular charge transfer, leading to the shortened diffusion path of photoelectrons and enhanced photolysis efficacy. We believe that the high dispersion stability (2 months), solution processability, and structural tunability of these Pdots with hyperbranched SCPNs would inspire further research on designing multicomponent photocatalysts for highly efficient visible light-driven hydrogen evolution reaction.

A

systems free of platinum/rhodium or organic solvent is of essential importance. Fortunately, these issues can be partially resolved by converting CP aggregates into water-dispersed polymer dots (Pdots).20,30,31 The encapsulation of linear CPs using amphiphilic block copolymer worked well to significantly improve the photocatalytic activities in the absence of organic solvent as sacrificial agent. Thus, considering the 3D photoelectron mobility and solution processability, we are more interested in Pdots based on the hyperbranched SCPNs. Herein, we report a new type of Pdots based on singleparticulate SCPNs that photocatalyze the hydrogen evolution reaction in the visible light region with no need of platinum or rhodium as cocatalyst. We prove that the aggregation of hyperbranched SCPNs show more significant impact on the photocatalytic activity, compared to that of linear CPs because of the relatively inefficient intermolecular photoelectron transfer. However, single-particulate dispersion of the SCPNtype photocatalysts ensures the photoelectrons to be directly transported to the surface of a SCPN over 3D skeletons, avoiding the inefficient intermolecular transfer and enhancing the photolysis efficacy. Hyperbranched SCPNs have been synthesized through a variety of strategies including miniemulsion system and

s a renewable and green energy, solar energy has presented great potential in artificial photosynthesis fields, among which light-driven hydrogen evolution reaction is of great concern.1−4 In recent years, conjugated polymer (CP) aggregates including linear/cross-linked structures, are of particular interest considering their numerous merits, such as chemical versatility,5−10 tunable band levels,11−13 and controllable microstructures.14−16 However, as far as we know, no report has focused on the photocatalytic ability of the more interesting soluble CP nanoparticles (SCPNs) for hydrogen production. The three-dimensional (3D) photoelectron mobility and single-particulate dispersion of these SCPNs as a photocatalytic entity would be more suitable for revealing the relationship between the photocatalytic performance and structural features of CP photocatalysts.17−21 In addition, in most of these work, noble metal cocatalysts (platinum/ rhodium) or organic solvent (methanol/alkylamine) were usually introduced to drastically increase the H2 evolution rate (HER) of CPs.22−29 While, the introduction of these noble metals leads to the severe rising of the cost, and the exact role of CPs in these complex systems either as organic photocatalysts or as photosensitizers is ambiguous, which offers limited scope for understanding the photolysis process and impedes structure optimization of the photocatalysts. In addition, the presence of organic solvent could also bring challenges in terms of practical H2 production and environmental protection.30 Therefore, exploring CP photocatalytic © XXXX American Chemical Society

Received: March 19, 2019 Revised: May 15, 2019

A

DOI: 10.1021/acs.macromol.9b00551 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules confined polymerization in nanoreactors.32−39 Herein, a novel double-monomer (A2 + B4) methodology40,41 was utilized to synthesize two series of donor−acceptor (D−A) type hyperbranched SCPNs (Scheme 1) through direct (thiophene)

intermediates followed by polymerization in a hyperbranching manner. For example, gel permeation chromatography (GPC) analysis showed that a combination of 4,7-bis(3,4-dimethoxythiophen-2-yl)benzothiadiazole (A2, TBT) and 2,2′,7,7′tetrabromo-9,9′-spirobifluorene (B4) led to the complete consumption of the tetrabromoarene monomer and the rapid formation of AB3 intermediate in 5 min (Figure S1). The hyperbranched SCPNs were then obtained through the coupling reaction of the AB3 intermediate, where the residual TBT monomer also served as a cross-linker. In other words, the polymerization degree/size of the hyperbranched SCPNs could be controlled by adjusting the feeding ratio of A2 and B4 monomers or by simply stopping the reaction at a given time. Following this line, two series of hyperbranched SCPNs were obtained (namely, HF-CPx and HE-CPx, where x represents the reaction time in hours, while F and E, respectively, stand for 2,2′,7,7′-tetrabromo-9,9′-spirobifluorene and 1,1,2,2tetrakis(4-bromophenyl)ethene). Their counterparts with cross-linked network structure (CF-CPs and CE-CPs) were prepared when the reaction time was extended to over 20 h. As a comparison, the linear CPs with a similar structure (LF-CPs and LE-CPs) were synthesized by replacing the B4 monomers with 2,7-dibromo-9,9′-spiro-bifluorene or 1,2-bis (4-bromophenyl)-1, 2-diphenylethene. More structural details are shown in Figure S2. 1 H NMR spectrum (Figure S3) of one SCPN (HF-CP10) shows the signals of spirobifluorene units at 6.90−7.90 ppm

Scheme 1. Schematic Illustration of the Synthesis of Hyperbranched SCPNs and Linear CPs, as Well as the Preparation of the Corresponding Pdots in Water

arylation polycondensation reaction. The polycondensation reaction was well controlled by the in situ generation of AB3

Figure 1. TEM images of hyperbranched HF-CP5 (A,B) and HF-CP10 (C,D) nanoparticles; PEG45-b-PMMA103 micelles (E,F), LF-CPs-Dots (G,H), and HF-CP10-Pdots (I,J) assemblies on a copper grid. SEM−energy-dispersive X-ray spectroscopy (EDS) element maps of HF-CP10-Pdots (K−P). Hydrodynamic diameter change measured by DLS analysis (Q) and schematic diagram of the core−shell structure of hyperbranched SCPNs (R). Images (A), (C), (E), (G) ,and (I) have a scale bar of 200 nm, the scale bar is 20 nm for other TEM images. B

DOI: 10.1021/acs.macromol.9b00551 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 2. (A) Photograph of HF-CP10 powder. (B) Photograph of HF-CP10 and HE-CP10 solutions in DMAc (0.1 mg/mL). (C) Luminescence image of HF-CP10 and HE-CP10 in DMAc irradiated with a 365 nm UV lamp. (D) Smooth film of HF-CP10 casted on a glass slide. (E) UV−vis spectra of F-CP series in solid state. (F) UV−vis spectra of E-CP series in the solid state.

Figure 3. (A) HER when using different CPs as photocatalysts under visible light irradiation (λ > 420 nm) with ascorbic acid as sacrificial agent. (B) Stability and reusability test using HE-CP10 as photocatalysts. (C) HER vs irradiation wavelength when using HE-CP10 as photocatalysts and HE-CP10 in suspension (inset). (D) Dispersion stability of different Pdots series.

region and the signals of TBT units at 8.38 ppm and 3.66−3.90 ppm. The terminal dimethoxythiophene group shows a characteristic peak at 6.40 ppm, similar to that in the model compound (M). Interestingly, the integration of this peak in HF-CP10 is much higher than that in LF-CPs (0.54 vs 0.02, calibrated with the peaks at 8.38 ppm), suggesting the presence of a large amount of terminal groups in the highly branched structure of HF-CP10. The calculated degree of branching (DB) of HF-CP10 is 0.70 according to the equation (DB = 2T/ (2T + L)) proposed by Hölter and Frey.42 The 13C NMR spectra of the hyperbranched SCPNs are not well resolved because of the complicated chemical environment around

those aromatic carbon atoms. Nevertheless, the peak of the quaternary carbon in the spirobifluorene unit (66.0 ppm) and the peaks (60.1 and 50.2 ppm) corresponding to the methoxy groups in the TBT unit confirm the presence of these two building units (Figure S4). The dimethoxythiophene at the terminal position shows two characteristic peaks at 98.1 ppm (thiophene) and 57.2 ppm (methoxy group at the outside position) in the model compound (M). These signals in the 13 C NMR spectrum of HF-CP10 are much stronger than those in the 13C NMR spectrum of LF-CPs, further corroborating the highly branched structure of the hyperbranched SCPNs. A similar trend is observed in the HE-CP10 and the LE-CP C

DOI: 10.1021/acs.macromol.9b00551 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

of their poor dispersibility (Figure S15).49−51 Interestingly, higher HERs from linear CP aggregates (LF-CP-A, 239 μmol h−1 g−1 and LE-CP-A, 295 μmol h−1 g−1) are probably ascribed to a favorable electron−hole pair isolation and charge transfer (a weaker fluorescence intensity, Figure S13), in good agreement with the results reported by Wang et al.43 In their reports, the twisted structures of the branched CPs decrease the π-conjugated system and directly lead to a poor photocatalytic activity. However, the possible photocatalytic activity loss from aggregation effects and the 3D photoelectron mobility of the hyperbranched CPs were not well considered.17,18,52 To eliminate the aggregation-induced photocatalytic activity loss, we fabricated homogenous water-dispersible Pdot systems, where a diblock copolymer, poly(ethylene oxide)-bpoly(methyl methacrylate) (PEG45-b-PMMA103) was introduced as the dispersant.30 In brief, HF-CP10 and the PEG45-bPMMA103 were first dissolved in THF, and then slowly dropped into distilled water in an ultrasonic bath. The mixture was further sonicated and placed in the fume hood to slowly evaporate the organic solvent and half of the water. The ratio of HF-CP10 and the copolymer dispersant was optimized to yield homogenous HF-CP10-Dots (Figure S16). Other Pdots (denoted as HE-CP10-Dots, LF-CP-Dots and LE-CP-Dots) were prepared following the same procedure. Unlike the loose micellar structure of PEG45-b-PMMA103 (Figure 1) or the densely particulate structure of HF-CP10, the HF-CP10-Dots show a well-defined core−shell structure. In TEM image (Figure 1J), the inner part of the encapsulated HF-CP10 nanoparticle is darker than the outer part because of a high electron density of the π-conjugated inner core.53 Each HFCP10 nanoparticle is well settled in the core of one polymer dot. The poly(methyl methacrylate) (PMMA) layer close to the HF-CP10 nanoparticle is invisible in TEM because of its amorphous structure (irregularly fixed into the interfacial lacuna), while the outer PMMA layer might form well-packed structure and is dimly visible. To further reflect the structural information of HF-CP10-Dots, field emission SEM and EDS element mapping of HF-CP10-Dots were investigated. The C, N, O, and S elements appear to be homogeneously dispersed throughout the Pdot samples, which is an inevitable result of the core−shell structure. Yet, once we zoom in one single Pdot in the nanometer scale, a low spectral resolution for these light elements leads to the difficulties in excluding the S or N element distribution in shells (10 nm thick) from a ∼70 nm Pdot.54 Alternatively, we used the parameter of Rg/Rh to further prove the shape information of the assembly. Rg and Rh, respectively, mean root-mean square z-average radius and hydrodynamic radius measured from static and DLS (Table S4). Rg reflects the density distribution of an assembly in real physical space and Rh is an average hydrodynamic radius. For example, the calculated Rg/Rh of HF-CP10-Dots is 0.58, lower than the value of 0.77 for a uniform hard sphere (SCPN has a Rg/Rh of 0.79), indicating that HF-CP10-Dots have a densely packed core and a relatively loose shell in aqueous media in accordance with the aforementioned core−shell structure.55−58 This core−shell and single-particulate characteristics of the assembly are also supported by the DLS results (Figure 1Q), where the diameter of the HF-CP10-Dots (72.8 nm) is only slightly larger than that of HF-CP10 (58.3 nm). In a sharp contrast, the LF-CPs-Dots show ill-defined and irregular structure as observed with TEM (Figure 1G,H), which is

polymers as well (Figures S5 and S6). Fourier transform infrared spectra of the hyperbranched SCPNs show typical signals of C−H stretching vibration at 2920 and 2840 cm−1, skeleton vibration of the aromatic rings at 1450 and 1490 cm−1, and the CN and N−S stretching modes at 1390 and 1570 cm−1, respectively (Figure S7).43 Powder X-ray diffraction profile indicates the noncrystalline character of these SCPNs (Figure S8). Multidetection GPC apparatus with online multiangle laser light scattering detectors was used to evaluate the absolute molecular weight. HF-CP10 and HE-CP10 present a molecular weight of 8261 and 8510 kD, respectively, close to that of a dendritic analogue of generation 8.0−8.5 (Tables S1 and S2). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) images indicate that the prepared hyperbranched SCPNs have regular spherical shapes (Figures 1 and S9). For example, HF-CP10 and HE-CP10 have a center diameter of 48.1 and 51.9 nm, respectively, which are consistent with the hydrodynamic size of 58.3 and 62.9 nm measured with dynamic light scattering (DLS) analysis (Figures S10 and S11). While for HF-CP5 and HE-CP5, the center diameters were measured as 19.2 and 21.1 nm, respectively (TEM analysis). The cross-linked CPs are not soluble in any solvent and show amorphous structures, and the linear CPs tend to form irregular aggregates once the organic solvent was removed (Figure S12). While the hyperbranched SCPNs are obtained as black powders with metallic clusters, they are readily dissolved in N,N-dimethylacetamide (DMAc) and tetrahydrofuran (THF), and present strong red fluorescence emission and excellent film processability (Figure 2), which are keys for further fabrication of the homogeneous Pdot system and multicomponent photocatalyst complex.19,20 The UV−vis spectra of these CPs show broad absorption band in the visible region (up to 620 nm) and narrow band gaps (∼2.0 eV), which benefits a strong photon harvest in the visible light region.44 The similar absorption spectra and band gaps indicate that the optical properties are not the underlying reasons accounted for different catalytic activities of these CPs (Table S3).45,46 To reveal the relationship between the photocatalytic performance and structural features of CP photocatalysts, a simple photocatalytic system consisting of pristine CP aggregates was first measured using ascorbic acid as sacrificial agent (pH = 4). As shown in Figure 3, the hyperbranched CP10 aggregates (HF-CP10-A and HE-CP10-A), respectively, showed a HER of 125 and 140 μmol h−1 g−1 under visible light irradiation (λ > 420 nm), superior to hyperbranched CP5 aggregates (Table S3). The larger size of hyperbranched CP10 with a weak fluorescence intensity (Figure S13) probably indicated a more favored electron−hole pair isolation or intramolecular charge mobility, which makes the pair recombination of CP10 slower compared with that of CP5.13,47,48 We further evaluated the photocatalytic stability and reusability of these SCPN aggregates. HE-CP10-A (aggregates) presents a slight decline of photocatalytic activity after 7 repeat circles (21 h). Even under the irradiation of 620 nm light, H2 was also successfully detected, indicating the broad wavelength range for photocatalytic H2 evolution in the presence of these SCPNs as photocatalysts. The HF-CP10 aggregates show the same features as depicted in Figure S14. As a comparison, we also evaluated the photocatalytic performance of the cross-linked and linear CP aggregates. The cross-linked CPs have a rather low HER possibly because D

DOI: 10.1021/acs.macromol.9b00551 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

Figure 4. (A) HOMO and LUMO band levels of the CPs determined by cyclic voltammetry and optical absorption. (B) Photocurrent measurement comparison of LE-CPs-A and HE-CP10-A film. (C) Photoluminescence (PL) spectra of the as-prepared Pdots system in water (10 μg/mL, CPs), excited at 520 nm with the same irradiation conditions. (D) Time-resolved fluorescence decay of CPs in aqueous solution/ suspension excited with 440 nm laser and detected at the maximum wavelength of fluorescence emission.

emission intensity and a shorter PL lifetime (0.65 vs 0.82 ns) probably indicated a superior charge mobility within the 3D skeletons of hyperbranched CPs.18,37,64 Inspired from the aforementioned change in HER and optical properties, a possible charge-transfer mechanism was explained in Figure 5.

ascribed to the random entanglements among linear CPs and PEG45-b-PMMA103 chains. We then evaluated the photocatalytic activity of these homogeneous Pdots in the same setup. Surprisingly, different from the aforementioned results, two series of Pdots generated from hyperbranched SCPNs present a superior photocatalytic activity to that of Pdots obtained from linear CPs. For example, the HF-CP10-Dots show a significantly higher HER (684 μmol h−1 g−1) compared to HF-CP10-A (125 μmol h−1 g−1), even higher than the value of LF-CPs-Dots. Also, the E-CP series shared a similar trend. Notably, the highest HER of 840 μmol h−1 g−1 was achieved with HE-CP10-Dots, higher than one of the most efficient Pdot catalysts, PFODTBT-Dots30 (742 μmol h−1 g−1 in the same setup) and other types of photocatalysts (Table S5). This change in photocatalytic activities could be preliminary explained in views of disparity in energy levels. As shown in Figure 4, the reduction potentials of the hyperbranched CPs (−0.80 or −0.82 eV) are more negative than those of linear CPs (∼−0.60 eV), indicating hyperbranched CPs exhibit a stronger thermodynamic driving force for H+ reduction in comparison with linear CPs (0.56 vs 0.40 eV).59 In addition, charge diffusion pathways, including intramolecular charge mobility and intermolecular charge transfer, could influence the HER of photocatalysts.52,60−62 A strong photocurrent response and weak emission intensity from linear CPs indicated a superior separation of electron− hole pairs and accelerated charge transfer among CPs (Figures 4B and S13).63 However, the charge diffusion pathways may change once the CP photocatalysts are converted into a homogeneous Pdots system. To reveal more details on the charge diffusion of the CPs in Pdots systems, time-resolved transient PL decay spectra of HE-CP10-A/LE-CP-A and the corresponding Pdots systems were recorded in aqueous solutions. The fluorescence of CPs in Pdots decay faster than that of CP aggregates, indicating an improved efficacy of charge transfer from the photocatalysts to water in accordance with the change of HER.7,8 In addition, considering the singleparticulate dispersion of hyperbranched Pdots, the lower

Figure 5. Possible charge mobility/transfer mechanism of hyperbranched and linear CPs in forms of aggregates or Pdots.

The hydrophobic polymer chains buried in the CP aggregates have an inferior chance to contact with aqueous solution (where sacrificial agent is present).13 An extra intermolecular charge-transfer process from the internal to the external is thus required. Compared with linear CP aggregates, hyperbranched CPs exhibit weaker intermolecular interaction and inefficient intermolecular charge transfer,65−67 which limit the photocatalytic activity when the polymers were simply dispersed in aqueous media (aggregates). However, once the CP aggregates were converted into homogeneous Pdots in water, the charge diffusion path is greatly shortened. The facilitation of this E

DOI: 10.1021/acs.macromol.9b00551 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

solid (50 mg, 44%). The similar procedure was conducted to obtain HF-CP10 (70 mg, 61%), except DMAc was used as the diluent and ethyl acetate as antisolvent. For cross-linked CF-CPs, the reaction time was set to over 20 h. Deionized water and ethyl acetate were used to remove K2CO3 and oligomeric residues to give dark black solid (80 mg, 69%). HE-CP5, HE-CP10, and CE-CPs were prepared in a similar manner (see the Supporting Information). General Procedure for the Synthesis of Linear CPs. 2,7Dibromo-9,9′-spiro-bifluorene (95 mg, 0.2 mmol), TBT (84 mg, 0.2 mmol), K2CO3 (111 mg, 0.8 mmol), and Pd(OAc)2 (1 mg, 0.004 mmol) were, respectively, added into a 10 mL sealed tube, then pivalic acid (8 μL) and DMAc (4 mL) were injected. After three freeze−pump−thaw circles, the tube was sealed under N2 atmosphere and stirred at 100 °C for 10 h. After dilution with dichloromethane, the solution was washed with brine and dried over MgSO4. The concentrated organic solution was reprecipitated in stirred diethyl ether to give a black solid (LF-CPs, 53 mg, 36%). The molecular weight is measured as 8518 Da (PDI = 1.42) by GPC equipped with a series of PS gel columns. LE-CPs were prepared in a similar manner, please see Supporting Information. Synthesis of PFODTBT. 4,7-Bis(2-bromo-5-thienyl)-2,1,3-benzothiadiazole (92 mg, 0.2 mmol), 9,9-dioctylfluorene-2,7-bis(boronic acid pinacol ester) (128 mg, 0.2 mmol), and Pd(PPh3)4 (9 mg, 0.008 mmol) were dissolved in a mixture of toluene (4 mL) and aqueous Na2CO3 (1 mL, 2 mol/L).68 After two freeze−pump−thaw circles, the tube was sealed under N2 atmosphere and stirred at 110 °C for 48 h. After dilution with chloroform, the mixture was washed with brine and dried over anhydrous MgSO4. The concentrated organic solution was reprecipitated in stirred acetone to a give black solid (100 mg, 63%). The molecular weight is measured as 9407 Da (PDI = 2.12) by GPC equipped with a series of PS gel columns. Preparation of Pdots. A series of Pdots were prepared by using a modified coassembly method.20 Briefly, stock solutions of CPs (1.5 mL, 1 mg/mL in THF) and PEG45-b-PMMA103 (1.5 mL, 1 mg/mL in THF) were mixed. After addition of extra THF (45 mL), the mixture was sonicated for 30 min to maintain homogeneity and then added dropwise to deionized water (100 mL) in a 250 mL beaker under sonication. After 10 h sonication treatment, the THF and half of water spontaneously evaporated at a temperature of 30 °C in the fume hood to give the homogeneous Pdots solution (∼50 mL). In addition, the concentration of CPs was determined using UV−vis analysis. Photocatalytic Hydrogen Evolution Activity Test. Photocatalytic reactions were operated in a Pyrex top-irradiation reaction vessel connected to a glass closed gas system. Hydrogen evolution reaction of CP aggregates as photocatalysts was carried out by dispersing 20 mg catalyst powders in an aqueous solution (55 mL). Before irradiation, the aqueous suspension was sonicated for 30 min and violently stirred overnight. Finally, ascorbic acid (1.76 g) was added as sacrificial agent and NaOH was used to adjust pH to 4.0 in order to eliminate the proton source coming from ascorbic acid.30 Then the reaction solution was evacuated several times to remove air completely prior to irradiation with a 300 W xenon lamp (CELHXUV300) and a current of 15 A. More details on the spectral output of the lamp, please see Figure S18. For visible light irradiation, 420 nm long-pass cutoff filters were used. The temperature of the reaction solution was maintained at room temperature by a flow of cooling water during the reaction. The head space of the reactor was periodically sampled (per 30 min) and the components were quantified by gas chromatography equipped with thermal conductivity detector (TCD) using argon as the carrier gas. For Pdot series, 50 mL Pdot aqueous solution was used and followed similar procedures as mentioned above, and an average value of 2 h was adopted to minimize the errors.

process is more significant in HF(E)-CP-dots as every hyperbranched polymeric nanoparticle is isolated in a single Pdot. The photoelectrons could directly migrate to the watertouched interface over the 3D skeletons, avoiding the inefficient intermolecular photoelectron transfer, and then participated in the process of water photolysis. Consequently, a combination of the shortened diffusion pathway, efficient charge transfer (three-dimensional transfer), and a more negative conduction band position (−0.82 eV) provide HECP10-Dots with the highest HER (840 μmol h−1 g−1). However, for the Pdots of linear CPs, the aggregation of linear CPs due to their strong π−π interaction30 would drive the photoelectron to go through a mixed intramolecular mobility and intermolecular transfer pathway, and the HER improvement is therefore limited. Finally, the apparent quantum yield (AQY) of HE-CP10Dots in aqueous solution (19 μg/mL) was measured at 500 nm (ascorbic acid as the sacrificial agent). The relatively low AQY of 0.9% could be further improved by increasing the concentration of the photocatalysts.20 In addition, the Pdots system retained the core−shell structure even after photoreaction for 5 h (Figure S17). The dispersion stability of the Pdots was also evaluated, and encouragingly, the Pdots of hyperbranched SCPNs present a superior stability in water than Pdots of linear CPs. They remain homogenous and transparent even after 2 month storage in ambient environment, while the Pdots of linear CPs tend to precipitate in water (Figure 3D). The well-defined core−shell structure of hyperbranched CP10-Dots is surely beneficial for their micellar stability. We believe such a kind of stable and robust photocatalysts would avoid the redispersion operation before the setup for hydrogen production and show wide potential in industrial production. In summary, we reported hyperbranched SCPNs as photocatalysts for the hydrogen evolution reaction with no need of platinum or rhodium as the cocatalyst. Singleparticulate dispersion of hyperbranched SCPNs in the Pdot assemblies ensures photoelectrons to be directly transported to the surface of these single-particulate polymeric dots over three-dimensional skeletons and avoids the ineffective intermolecular photoelectronic transfer, leading to the shortened electron diffusion path and enhanced photolysis efficacy. Considering the film processibility and the 3D photoelectron transfer process of hyperbranched SCPNs, multicomponent monolayer photocatalysts would be designed and developed for overall water splitting, which is also underway in our laboratory.

■

EXPERIMENTAL SECTION

General Procedure for the Preparation of Hyperbranched SCPNs and Cross-linked CPs. Standard direct (thiophene) arylation polycondensations were employed for the synthesis of FCP series. Typically, 2,2′,7,7′-tetrabromo-9,9′-spirobifluorene (63.2 mg, 0.1 mmol), TBT (86.8 mg, 0.2 mmol), Pd(OAc)2 (1.5 mg, 0.006 mmol), and K2CO3 (66 mg, 0.48 mmol) were, respectively, added into a 10 mL sealed tube, then pivalic acid (8 μL) and DMAc (3 mL) was injected. After three freeze−pump−thaw circles, the tube was sealed under N2 atmosphere and stirred at 100 °C for 5 or 10 h to give HF-CP5 and HF-CP10, respectively. For HF-CP5, after dilution of the reaction mixture with THF (4 mL) and removal of the K2CO3 by centrifugation, the concentrated supernatant was added dropwise into stirred diethyl ether (50 mL). The formed precipitate was isolated through centrifugation and rinsed with diethyl ether for three times, and then dried under vacuum at 25 °C overnight to obtain a dark red F

DOI: 10.1021/acs.macromol.9b00551 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules

■

(10) Wang, Y.; Wang, X.; Antonietti, M. Polymeric Graphitic Carbon Nitride as a Heterogeneous Organocatalyst: From Photochemistry to Multipurpose Catalysis to Sustainable Chemistry. Angew. Chem., Int. Ed. 2012, 51, 68−89. (11) Vyas, V. S.; Lau, V. W.-h.; Lotsch, B. V. Soft Photocatalysis: Organic Polymers for Solar Fuel Production. Chem. Mater. 2016, 28, 5191−5204. (12) Zhang, G.; Lan, Z.-A.; Wang, X. Conjugated Polymers: Catalysts for Photocatalytic Hydrogen Evolution. Angew. Chem., Int. Ed. 2016, 55, 15712−15727. (13) Sprick, R. S.; Bonillo, B.; Clowes, R.; Guiglion, P.; Brownbill, N. J.; Slater, B. J.; Blanc, F.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. Visible-Light-Driven Hydrogen Evolution Using Planarized Conjugated Polymer Photocatalysts. Angew. Chem., Int. Ed. 2016, 128, 1824−1828. (14) Wang, X.; Chen, L.; Chong, S. Y.; Little, M. A.; Wu, Y.; Zhu, W.-H.; Clowes, R.; Yan, Y.; Zwijnenburg, M. A.; Sprick, R. S.; Cooper, A. I. Sulfone-containing covalent organic frameworks for photocatalytic hydrogen evolution from water. Nat. Chem. 2018, 10, 1180− 1189. (15) Sprick, R. S.; Jiang, J.-X.; Bonillo, B.; Ren, S.; Ratvijitvech, T.; Guiglion, P.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. Tunable Organic Photocatalysts for Visible-Light-Driven Hydrogen Evolution. J. Am. Chem. Soc. 2015, 137, 3265−3270. (16) Wang, X.; Maeda, K.; Chen, X.; Takanabe, K.; Domen, K.; Hou, Y.; Fu, X.; Antonietti, M. Polymer Semiconductors for Artificial Photosynthesis: Hydrogen Evolution by Mesoporous Graphitic Carbon Nitride with Visible Light. J. Am. Chem. Soc. 2009, 131, 1680−1681. (17) Xu, Y.; Chen, L.; Guo, Z.; Nagai, A.; Jiang, D. Light-Emitting Conjugated Polymers with Microporous Network Architecture: Interweaving Scaffold Promotes Electronic Conjugation, Facilitates Exciton Migration, and Improves Luminescence. J. Am. Chem. Soc. 2011, 133, 17622−17625. (18) Liu, X.; Xu, Y.; Jiang, D. Conjugated Microporous Polymers as Molecular Sensing Devices: Microporous Architecture Enables Rapid Response and Enhances Sensitivity in Fluorescence-On and Fluorescence-Off Sensing. J. Am. Chem. Soc. 2012, 134, 8738−8741. (19) Woods, D. J.; Sprick, R. S.; Smith, C. L.; Cowan, A. J.; Cooper, A. I. A Solution-Processable Polymer Photocatalyst for Hydrogen Evolution from Water. Adv. Energy Mater. 2017, 7, 1700479. (20) Wang, L.; Fernández-Terán, R.; Zhang, L.; Fernandes, D. L. A.; Tian, L.; Chen, H.; Tian, H. Organic Polymer Dots as Photocatalysts for Visible Light-Driven Hydrogen Generation. Angew. Chem., Int. Ed. 2016, 55, 12306−12310. (21) Ma, H.; Ma, W.; Chen, J.-F.; Liu, X.-Y.; Peng, Y.-Y.; Yang, Z.-Y.; Tian, H.; Long, Y.-T. Quantifying Visible-Light-Induced Electron Transfer Properties of Single Dye-Sensitized ZnO Entity for Water Splitting. J. Am. Chem. Soc. 2018, 140, 5272−5279. (22) Sachs, M.; Sprick, R. S.; Pearce, D.; Hillman, S. A. J.; Monti, A.; Guilbert, A. A. Y.; Brownbill, N. J.; Dimitrov, S.; Shi, X. Y.; Blanc, F.; Zwijnenburg, M. A.; Nelson, J.; Durrant, J. R.; Cooper, A. I. Understanding structure-activity relationships in linear polymer photocatalysts for hydrogen evolution. Nat. Commun. 2018, 9, 4968. (23) Zhang, G.; Lan, Z.-A.; Wang, X. Surface engineering of graphitic carbon nitride polymers with cocatalysts for photocatalytic overall water splitting. Chem. Sci. 2017, 8, 5261−5274. (24) Pan, Z.; Zheng, Y.; Guo, F.; Niu, P.; Wang, X. Decorating CoP and Pt Nanoparticles on Graphitic Carbon Nitride Nanosheets to Promote Overall Water Splitting by Conjugated Polymers. ChemSusChem 2017, 10, 87−90. (25) Zhang, G.; Lan, Z.-A.; Lin, L.; Lin, S.; Wang, X. Overall water splitting by Pt/g-C3N4 photocatalysts without using sacrificial agents. Chem. Sci. 2016, 7, 3062−3066. (26) Li, L.; Cai, Z.; Wu, Q.; Lo, W.-Y.; Zhang, N.; Chen, L. X.; Yu, L. Rational Design of Porous Conjugated Polymers and Roles of Residual Palladium for Photocatalytic Hydrogen Production. J. Am. Chem. Soc. 2016, 138, 7681−7686.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.9b00551.

■

More characterization of CPs and measurement data of photocatalytic hydrogen evolution (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hua Gui Yang: 0000-0003-0436-8622 Aiguo Hu: 0000-0003-0456-7269 Author Contributions

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

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (21674035), the Fundamental Research Funds for the Central Universities (22221818014) and Shanghai Leading Academic Discipline Project (B502). A.H. thanks the “Eastern Scholar Professorship” support from Shanghai local government. P.Z. thanks Dr. Mingwei Wang for his kind help in light scattering analysis.

■

REFERENCES

(1) Sivula, K.; van de Krol, R. Semiconducting materials for photoelectrochemical energy conversion. Nat. Rev. Mater. 2016, 1, 15010−150020. (2) Joya, K. S.; Joya, Y. F.; Ocakoglu, K.; van de Krol, R. WaterSplitting Catalysis and Solar Fuel Devices: Artificial Leaves on the Move. Angew. Chem., Int. Ed. 2013, 52, 10426−10437. (3) Chen, F.; Huang, H.; Guo, L.; Zhang, Y.; Ma, T. The Role of Polarization in Photocatalysis. Angew. Chem., Int. Ed. 2019, 58, 2−15. (4) Li, H.; Sun, Y.; Yuan, Z.-Y.; Zhu, Y.-P.; Ma, T.-Y. Titanium Phosphonate Based Metal-Organic Frameworks with Hierarchical Porosity for Enhanced Photocatalytic Hydrogen Evolution. Angew. Chem., Int. Ed. 2018, 57, 3222−3227. (5) Zhang, G.; Zhang, M.; Ye, X.; Qiu, X.; Lin, S.; Wang, X. Iodine Modified Carbon Nitride Semiconductors as Visible Light Photocatalysts for Hydrogen Evolution. Adv. Mater. 2014, 26, 805−809. (6) Kailasam, K.; Schmidt, J.; Bildirir, H.; Zhang, G.; Blechert, S.; Wang, X.; Thomas, A. Room Temperature Synthesis of HeptazineBased Microporous Polymer Networks as Photocatalysts for Hydrogen Evolution. Macromol. Rapid. Comm. 2013, 34, 1008−1013. (7) Pachfule, P.; Acharjya, A.; Roeser, J.; Langenhahn, T.; Schwarze, M.; Schomäcker, R.; Thomas, A.; Schmidt, J. Diacetylene Functionalized Covalent Organic Framework (COF) for Photocatalytic Hydrogen Generation. J. Am. Chem. Soc. 2018, 140, 1423−1427. (8) Zhang, J.; Zhang, G.; Chen, X.; Lin, S.; Möhlmann, L.; Dołęga, G.; Lipner, G.; Antonietti, M.; Blechert, S.; Wang, X. Co-Monomer Control of Carbon Nitride Semiconductors to Optimize Hydrogen Evolution with Visible Light. Angew. Chem., Int. Ed. 2012, 51, 3183− 3187. (9) Martin, D. J.; Reardon, P. J. T.; Moniz, S. J. A.; Tang, J. Visible Light-Driven Pure Water Splitting by a Nature-Inspired Organic Semiconductor-Based System. J. Am. Chem. Soc. 2014, 136, 12568− 12571. G

DOI: 10.1021/acs.macromol.9b00551 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules (27) Han, Q.; Wang, B.; Zhao, Y.; Hu, C.; Qu, L. A Graphitic-C3N4 ″Seaweed″ Architecture for Enhanced Hydrogen Evolution. Angew. Chem., Int. Ed. 2015, 54, 11433−11437. (28) Zhang, M.; Wang, X. Two dimensional conjugated polymers with enhanced optical absorption and charge separation for photocatalytic hydrogen evolution. Energy Environ. Sci. 2014, 7, 1902−1906. (29) Ong, W.-J.; Tan, L.-L.; Ng, Y. H.; Yong, S.-T.; Chai, S.-P. Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability? Chem. Rev. 2016, 116, 7159− 7329. (30) Pati, P. B.; Damas, G.; Tian, L.; Fernandes, D. L. A.; Zhang, L.; Pehlivan, I. B.; Edvinsson, T.; Araujo, C. M.; Tian, H. An experimental and theoretical study of an efficient polymer nanophotocatalyst for hydrogen evolution. Energy Environ. Sci. 2017, 10, 1372−1376. (31) Tseng, P.-J.; Chang, C.-L.; Chan, Y.-H.; Ting, L.-Y.; Chen, P.Y.; Liao, C.-H.; Tsai, M.-L.; Chou, H.-H. Design and Synthesis of Cycloplatinated Polymer Dots as Photocatalysts for Visible-LightDriven Hydrogen Evolution. ACS Catal. 2018, 8, 7766−7772. (32) Zhao, P.; Dai, Y.; Wu, Y.; Wang, L.; Huang, B.; Ding, Y.; Hu, A. Combinatorial synthesis of soluble conjugated polymeric nanoparticles and tunable multicolour fluorescence sensing. Polym. Chem-UK. 2017, 8, 5734−5740. (33) Deng, S.; Zhi, J.; Zhang, X.; Wu, Q.; Ding, Y.; Hu, A. SizeControlled Synthesis of Conjugated Polymer Nanoparticles in Confined Nanoreactors. Angew. Chem., Int. Ed. 2014, 53, 14144− 14148. (34) Huang, B.; Zhao, P.; Dai, Y.; Deng, S.; Hu, A. Size-controlled synthesis of soluble-conjugated microporous polymer nanoparticles through sonogashira polycondensation in confined nanoreactors. J. Polym. Sci. Pol. Chem. 2016, 54, 2285−2290. (35) Wang, L.; Zhao, P.; Feng, C.; Wu, Y.; Ding, Y.; Hu, A. Controlled synthesis of water-dispersible conjugated polymeric nanoparticles for cellular imaging. Eur. Polym. J. 2018, 105, 1−6. (36) Zhao, P.; Wu, Y.; Feng, C.; Wang, L.; Ding, Y.; Hu, A. Conjugated Polymer Nanoparticles Based Fluorescent Electronic Nose for the Identification of Volatile Compounds. Anal. Chem. 2018, 90, 4815−4822. (37) Wu, X.; Li, H.; Xu, B.; Tong, H.; Wang, L. Solution-dispersed porous hyperbranched conjugated polymer nanoparticles for fluorescent sensing of TNT with enhanced sensitivity. Polym. Chem. 2014, 5, 4521−4525. (38) Ma, B. C.; Ghasimi, S.; Landfester, K.; Vilela, F.; Zhang, K. A. I. Conjugated microporous polymer nanoparticles with enhanced dispersibility and water compatibility for photocatalytic applications. J. Mater. Chem. A. 2015, 3, 16064−16071. (39) Wu, X.; Zhang, Z.; Hang, H.; Chen, Y.; Xu, Y.; Tong, H.; Wang, L. Solution-Processable Hyperbranched Conjugated Polymer Nanoparticles Based on C3h-Symmetric Benzotrithiophene for Polymer Solar Cells. Macromol. Rapid. Comm. 2017, 38, 1700001. (40) Gao, C.; Yan, D. Hyperbranched polymers: from synthesis to applications. Prog. Pol. Sci. 2004, 29, 183−275. (41) Yan, D.; Gao, C. Hyperbranched polymers made from A(2) and BB ’(2) type monomers. 1. Polyaddition of 1-(2-aminoethyl)piperazine to divinyl sulfone. Macromolecules 2000, 33, 7693−7699. (42) Hölter, D.; Burgath, A.; Frey, H. Degree of branching in hyperbranched polymers. Acta Polym. 1997, 48, 30−35. (43) Yang, C.; Ma, B. C.; Zhang, L.; Lin, S.; Ghasimi, S.; Landfester, K.; Zhang, K. A. I.; Wang, X. Molecular Engineering of Conjugated Polybenzothiadiazoles for Enhanced Hydrogen Production by Photosynthesis. Angew. Chem., Int. Ed. 2016, 55, 9202−9206. (44) Pan, C.; Takata, T.; Nakabayashi, M.; Matsumoto, T.; Shibata, N.; Ikuhara, Y.; Domen, K. A Complex Perovskite-Type Oxynitride: The First Photocatalyst for Water Splitting Operable at up to 600 nm. Angew. Chem., Int. Ed. 2015, 54, 2955−2959.

(45) Chen, X.; Liu, L.; Yu, P. Y.; Mao, S. S. Increasing Solar Absorption for Photocatalysis with Black Hydrogenated Titanium Dioxide Nanocrystals. Science 2011, 331, 746−750. (46) Maeda, K.; Wang, X.; Nishihara, Y.; Lu, D.; Antonietti, M.; Domen, K. Photocatalytic Activities of Graphitic Carbon Nitride Powder for Water Reduction and Oxidation under Visible Light. J. Phys. Chem. C 2009, 113, 4940−4947. (47) Szarko, J. M.; Rolczynski, B. S.; Guo, J.; Liang, Y.; He, F.; Mara, M. W.; Yu, L.; Chen, L. X. Electronic Processes in Conjugated Diblock Oligomers Mimicking Low Band-Gap Polymers: Experimental and Theoretical Spectral Analysis. J. Phys. Chem. B 2010, 114, 14505−14513. (48) Li, T.; Zhao, L.; He, Y.; Cai, J.; Luo, M.; Lin, J. Synthesis of gC3N4/SmVO4 composite photocatalyst with improved visible light photocatalytic activities in RhB degradation. Appl. Catal. B Environ. 2013, 129, 255−263. (49) Schwarze, M.; Stellmach, D.; Schröder, M.; Kailasam, K.; Reske, R.; Thomas, A.; Schomäcker, R. Quantification of photocatalytic hydrogen evolution. Phys. Chem. Chem. Phys. 2013, 15, 3466−3472. (50) Zhang, Z.; Wang, C.-C.; Zakaria, R.; Ying, J. Y. Role of Particle Size in Nanocrystalline TiO2-Based Photocatalysts. J. Phys. Chem. B 1998, 102, 10871−10878. (51) Fox, M. A.; Dulay, M. T. Heterogeneous photocatalysis. Chem. Rev. 1993, 93, 341−357. (52) Jassby, D.; Farner Budarz, J.; Wiesner, M. Impact of Aggregate Size and Structure on the Photocatalytic Properties of TiO2 and ZnO Nanoparticles. Environ. Sci. Technol. 2012, 46, 6934−6941. (53) Pu, K.-Y.; Li, K.; Liu, B. A Molecular Brush Approach to Enhance Quantum Yield and Suppress Nonspecific Interactions of Conjugated Polyelectrolyte for Targeted Far-Red/Near-Infrared Fluorescence Cell Imaging. Adv. Funct. Mater. 2010, 20, 2770−2777. (54) Wang, H.; Chu, P. K. Surface Characterization of Biomaterials. In Characterization of Biomaterials; Bandyopadhyay, A., Bose, S., Eds.; Academic Press: Oxford, 2013; Chapter 4, pp 105−174. (55) Hirzinger, B.; Helmstedt, M.; Stejskal, J. Light scattering studies on core-shell systems: determination of size parameters of sterically stabilized poly(methylmethacrylate) dispersions. Polymer 2000, 41, 2883−2891. (56) Duan, H.; Chen, D.; Jiang, M.; Gan, W.; Li, S.; Wang, M.; Gong, J. Self-assembly of unlike homopolymers into hollow spheres in nonselective solvent. J. Am. Chem. Soc. 2001, 123, 12097−12098. (57) Douglas, J. F.; Roovers, J.; Freed, K. F. Characterization of branching architecture through ″universal″ ratios of polymer solution properties. Macromolecules 1990, 23, 4168−4180. (58) Vagberg, L. J. M.; Cogan, K. A.; Gast, A. P. Light-scattering study of starlike polymeric micelles. Macromolecules 1991, 24, 1670− 1677. (59) Vyas, V. S.; Haase, F.; Stegbauer, L.; Savasci, G.; Podjaski, F.; Ochsenfeld, C.; Lotsch, B. V. A tunable azine covalent organic framework platform for visible light-induced hydrogen generation. Nat. Commun. 2015, 6, 8508. (60) Hotze, E. M.; Bottero, J.-Y.; Wiesner, M. R. Theoretical Framework for Nanoparticle Reactivity as a Function of Aggregation State. Langmuir 2010, 26, 11170−11175. (61) Lu, Q.; Yu, Y.; Ma, Q.; Chen, B.; Zhang, H. 2D TransitionMetal-Dichalcogenide-Nanosheet-Based Composites for Photocatalytic and Electrocatalytic Hydrogen Evolution Reactions. Adv. Mater. 2016, 28, 1917−1933. (62) Chen, F.; Huang, H.; Ye, L.; Zhang, T.; Zhang, Y.; Han, X.; Ma, T. Thickness-Dependent Facet Junction Control of Layered BiOIO3 Single Crystals for Highly Efficient CO2 Photoreduction. Adv. Funct. Mater. 2018, 28, 1804284. (63) Yu, F.; Wang, Z.; Zhang, S.; Ye, H.; Kong, K.; Gong, X.; Hua, J.; Tian, H. Molecular Engineering of Donor−Acceptor Conjugated Polymer/g-C3N4 Heterostructures for Significantly Enhanced Hydrogen Evolution Under Visible-Light Irradiation. Adv. Funct. Mater. 2018, 28, 1804512. (64) Li, T.; Zhao, L.; He, Y.; Cai, J.; Luo, M.; Lin, J. Synthesis of gC3N4/SmVO4 composite photocatalyst with improved visible light H

DOI: 10.1021/acs.macromol.9b00551 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules photocatalytic activities in RhB degradation. Appl. Catal. B-Environ. 2013, 129, 255−263. (65) Zhu, X.; Zhou, Y.; Yan, D. Influence of branching architecture on polymer properties. J. Polym. Sci. Pol. Phys. 2011, 49, 1277−1286. (66) Grayson, S. M.; Fréchet, J. M. J. Convergent dendrons and dendrimers: from synthesis to applications. Chem. Rev. 2001, 101, 3819−3868. (67) Kim, Y. H.; Webster, O. W. Hyperbranched polyphenylenes. Macromolecules 1992, 25, 5561−5572. (68) Hai, J.; Zhao, B.; Zhang, F.; Sheng, C.-X.; Yin, L.; Li, Y.; Zhu, E.; Bian, L.; Wu, H.; Tang, W. Synthesis and photovoltaic performance of novel thiophenyl-methylene-9H-fluorene-based low bandgap polymers. Polymer 2013, 54, 4930−4939.

I

DOI: 10.1021/acs.macromol.9b00551 Macromolecules XXXX, XXX, XXX−XXX