Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX
Controllable Conformation Transfer of Conjugated Polymer toward High Photoelectrical Performance: The Role of Solvent in InducedCrystallization Route Jun Zhou, Yicong Wang, Xuqiang Hao, Chenghai Ma, Ying Wang,* and Zhigang Zou* School of Chemistry and Chemical Engineering, Eco-materials and Renewable Energy Research Center (ERERC), National Laboratory of Solid State Microstructures, Kunshan Innovation Institute of Nanjing University, Jiangsu Key Laboratory for Nanotechnology, Nanjing University, Nanjing 210093, People’s Republic of China S Supporting Information *
ABSTRACT: In the present work, polyimides with different conformation (dendritic, spherulitic, and laminar) were synthesized in different solvents. The strong solvent-dependent conformation variation is found intimately related to the specific interactions between the polymer chain and solvent molecules that have a primary driving force for the chain diffusion and rearrangement. Moreover, the photoelectrical properties of polyimide were sensitively influenced by the crystal structure and polymer morphology. In laminar conformation with strong intermolecular π−π interactions, the light absorption as well as the mobility and separation of photoinduced carriers were greatly improved compared with similar values of dendritic and spherulitic ones. For photocatalytic hydrogen evolution from water splitting, polyimide with laminar conformation exhibits 16 times enhancement in activity than the dendritic. This work provides further insight into the intrinsic interacting mechanism of solvent-induced crystallization of conjugated polymer and paves an innovative way for synthesis of efficient polymer semiconductor photocatalysts.
1. INTRODUCTION Semiconducting polymers have always been of great interest in materials science because of their wide use in organic electronics and optoelectronics such as organic field-effect transistors (OFETs), organic light-emitting diodes (OLEDs), and polymer solar cells (PSCs).1−3 Despite different operation parameters in these various applications, effective charge transfer is the prerequisite accounting for the high performance of the material systems.4 Molecular packing order and intermolecular π-interactions are always critical factors determining the efficiency of charge transport in semiconducting polymers,5,6 though there is still no commonly accepted theoretical systems describing charge transport in polymer semiconductors.7 From standpoint of maximizing the transfer of charge, crystallographic packing of two-dimensional conjugated chains with close intermolecular electronic coupling presents the utmost efficient architecture for charge transport in a polymer semiconductor.6 However, facile control the structure of polymer toward this ideal structure has remained a challenge. Over the past decades, remarkable progress has been made in the exploration of new generations of polymers like polythiophene, poly(phenylenevinylene), polyimide, and their derivates.8,9 Especially, polyimide was recently reported as a photocatalyst for a solar-driven hydrogen generation from water splitting. The great potential of polymer photocatalysts for solving the urgent energy and environmental issues has inspired considerable research enthusiasm.10−13 © XXXX American Chemical Society
However, compared with the well-established organic electronics that have a widely understood structure−property relationship and are accessible from numerous research focusing on the role of semiconductors nature, microstructures, and device configurations,6 research studies of polymer semiconductors in photocatalysis are still limited to the startup stage. In this context, systemaitic evaluation of the interplay between the microstucture and photocatalytic performance of polymer at the molecular level is urgently needed to shed some light on the structure−property relationship of polymer semiconductors in photocatalysis. Herein we report a comprehensive investigation that aims to get insight into the influence of solvents used in synthesizing process on the crystal structures of polymer and microscopic morphology as well as the photocatalytic performance. The polymerization between the monomers and chains arrangement under five kinds of different solvent systems were discussed in detail, and the underlying mechanism was proposed.
2. RESULTS AND DISCUSSION To clarify the solvent effect in the crystallization of polyimide, X-ray Diffraction (XRD) was conducted for crystalline Received: September 19, 2017 Revised: December 16, 2017 Published: December 19, 2017 A
DOI: 10.1021/acs.jpcc.7b09324 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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new group of diffraction peaks (enlarged in Figure S2) represented by the peak at 2θ = 27.8° emerged and grew dominantly, implying the formation of new conformation. This new conformation stands for a configuration that polymer chains are arranged orderly in the π−π stacking direction with an interlayer spacing of 3.2 Å.14 Noteworthy, in the process of conformation transfer the relative intensity of these two peaks (I2θ = 29.5/I2θ = 27.8) monotonous decreased until the peak at 2θ = 29.5° disappeared. Therefore, these two peaks at 2θ = 29.5° and 2θ = 27.8° can be treated as the characteristic diffraction peak of branched and ordered stacking configuration, respectively. For PI- DMF and PI-DMAc, the diffraction peaks at 2θ = 29.5° symbolizing the dendritic structures disappeared completely in the XRD pattern, and the crystal structure was dominated by the well-ordered π−π stacking. Obviously, with the presence of polar solvent, the dendritic polymer chains were readily flexible and self-adjusting to ordered organization in the π-stacking direction. Electron microscope images shown in Figure 2 provided direct schematic insight into solvent-induced polymorphic transformation. Just as expected, PI-Toluene exhibited hyperbranched chains (Figure 2a,b) and bulk sheets (Figure 2c) that is similar to that of solid-state thermal condensed polyimide. This reproducibility in both XRD pattern and microstructure clearly indicated the negligible effect of toluene molecules in the process of crystallization. In the case of PI-m-Cresol, spherulites composed of bundled slim polymer chains with circular shaped periphery can be seen from Figure 2d−f. Whereas in NMP, these slim chains grew into large polymer rods (Figure 2i) that assembled into a flat disc as exhibited in Figure 2g,h. These morphological features observed in PI-mCresol and PI-NMP implied the occurrence of unidirectional growth along with low angle branching, which is the prerequisite to the formation of spherulites.15 When polymerization was carried out in DMF and DMAc, as illustrated in Figure 2j−l, both samples were found to be two-dimensional nanosheets with high aspect ratio. High-resolution transmission electron microscopy (inset of Figure 2j) disclosed well-ordered stacking configuration between conjugated polyimide chains with an interlayer spacing (3.2 Å) that is in strong accordance with the value derived from XRD. This strong dependence of crystalline structure and morphology
structure determination. As illustrated in Figure 1, as solvents changed from toluene to DMAc, an evolutionary trans-
Figure 1. Powder X-ray diffraction patterns of polyimides derived from different solvent systems.
formation in diffraction profiles can be seen. To be detailed, PI-Toluene, synthesized in nonpolar toluene, exhibited almost the same diffraction pattern with that of the hyper-branched polyimide photocatalyst PI-TC (Figure S1). PI-TC represented a dendritic polyimide that was prepared by solid-state thermal condensation at 325 °C with a characteristic diffraction peak at 2θ = 29.5°.10 The polymerization between the monomers proceeded in the melt of anhydride and then experienced crystallographic favored branched solidification in the annealing process. As for the synthesis of PI-Toluene, monomers were almost insoluble in the nonpolar toluene, so in the following polymerization and crystallization processes, toluene was presumed to be incapable of promoting the polymerization between monomers or altering the chain orientation. Consequently, the crystal structure derived from toluene was very similar to that of solid-state condensation in PI-TC and thus exhibited the same XRD pattern. When polyimide was synthesized in the polar solvent, the solvent significantly affected the organization of polyimide chains. As can be seen in XRD patterns of PI-m-Cresol and PI-NMP, a
Figure 2. Representative morphology of polyimide in the process of solvent-induced crystallization. branched chains and bulk layers of polyimide from toluene (a−c); spherulitic structure of polyimide derived from m-Cresol (d−f); rod like chains assembly from NMP (g−i); polyimide nanosheets from DMF (j,k) and DMAc (l); inset in panel j was the lattice fringe observed in PI-DMF. B
DOI: 10.1021/acs.jpcc.7b09324 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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The Journal of Physical Chemistry C Table 1. Hansen Solubility Parameters of Organic Solvents and Polyimide solvent/polymer
solvent type
δd/MPa1/2
δp/MPa1/2
δh/MPa1/2
δt/MPa1/2
Toluene C6H5CH3 m-Cresol CH3C6H4OH NMP C5H9NO DMF C3H7NO DMAc C4H9NO Polyimide
nonpolar
18.0
1.4
2.0
18.2
polar protic
18.0
5.1
12.9
22.7
polar aprotic
18.0
12.3
7.2
22.9
polar aprotic
17.4
13.7
11.3
24.8
polar aprotic
16.8
11.5
10.2
22.7
/
24.3
19.5
22.9
38.7
upon the solvents used in the synthesis process provided an opportunity for us to explore the underlying crystallizing mechanism of polyimide. Crystallization of polymers is a complicated process that requires the cooperative movement of chains of connected monomers. It is generally recognized that the shapes of polymer crystals is basically determined by three processes:16,17 that is, (1) transport of molecules to the crystal growth front by diffusion; (2) interface kinetics: the nonequilibrium process of molecules get attached to the growth front and integrated into the crystal; and (3) capillarity that dedicated to minimize the interface energy via surface tension effects and any disturbance to these processes was supposed to alter the crystal habit. With respect to solvent-induced crystallization, transport and interface kinetics were closely related to the solvent molecules because the mobility and orientation preference of polymer chains were found to be sensitively affected by the slovent−polymer interactions in polymer systems ranged from polystyrene,18,19 polyamide,20 to poly(3dodecylthiophene).21 Here, to clarify how the solvent promote the conformation transfer of polyimide, physicochemical properties of solvent molecules and the solvent−polymer interactions were briefly discussed using Hansen solubility parameter (HSP). HSP is a popular digitizing method for describing the interplay between polymers and organic solvents
via three parameters that represent dispersion interactions (δd), dipolar interactions (δp), and the hydrogen bonding interactions (δh) respectively; the total HSP (δt) was calculated by the equation of δt2 = δd2 + δp2 + δh2.22 Similar to the established basic principle “like dissolves like” in solvent liquids, the interaction between solvents and polymers is also dependent on their similarity in solubility parameters. As listed in Table 1, we summarized all the HSPs of solvent molecules and polyimide. The total HSPs of solvents exhibited a typical volcanic distribution with DMF reached the maximum, which was consistent with the findings discussed above in XRD variation. These concurrent changes in total HSPs and crystal structures clearly indicated the significant effect of solvent− polymer interactions on the resulting polymer conformation. This argument was further verified by solvent-induced crystallization performed with preprepared polyimide by solid-state condensation at 325 °C (PI-TC). The pristine structure of PI-TC gradually disappeared accompanied by formation of new conformation as observed in PI-DMF within 24 h (Figure S3). Because the temperature of 180 °C was much lower than crystallizing temperature, the polymer− solvent interaction was definitely the primary driving force for chain motion and rearrangement. In addition, we also found the different effects of protic and aprotic solvent. This is typical in the cases of PI-m-Cresol and PI-DMAc where m-Cresol is a
Figure 3. Calculated electrostatic potential (ESP) distribution of solvent molecules and building block of PI. (a) toluene, (b) m-cresol, (c) NMP, (d) DMF, (e) DMAc, and (f) PI. Carbon, nitrogen, and oxygen atoms in the structural models are shown as gray, blue, and red spheres, respectively. C
DOI: 10.1021/acs.jpcc.7b09324 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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Figure 4. (a) UV−vis absorption spectrum of PI samples. (b) Room-temperature EPR spectra of PI samples. (c) Photoluminescence spectra (PL) of PI samples. (d) Electrochemical impedance spectroscopy (EIS) of samples. (e) Photocurrent curves of PI-Toluene and PI-DMF. (f) Photocatalytic hydrogen evolution rate of PI samples under full arc light (λ>300 nm) with 3 wt % Pt as cocatalyst.
finding in turn provided a platform for the following investigations focusing on the structure−property relationship in photocatalysis. In order to explore the effect of crystal structure on optoelectronic properties during the conformation transfer, systematic characterization were performed. As shown in Figure 4a, the light absorption in the UV−vis spectrum differed much among samples with different structure. The band in ultraviolet region with absorption edge ranged from 380 to 425 nm was derived from intrinsic absorption, reflecting the energy gap between the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) in a single conjugated chain. The intrinsic absorption edge was gradually bathochromic shifted due to increasing conjugated length of polymer chains. When the crystal conformation changed from dendritic to ordered stacking, a new band at 425 nm in the visible region could be seen. This is the sign of π-delocalization via intermolecular π−π interactions, because when isolated conjugated chains are brought close, overlapping between the molecular orbitals will occur and then give rise to electronic bands.23 The new band was strongly dependent on the polymer configuration because the interchain interacting was determined by chain arrangement. In the cases where the polymer chains were dendritic branching (PI-Toluene) or radially separated (PI-m-Cresol), the orbital overlap was difficult to achieve, and therefore, the π-delocalization cannot be realized. Whereas, when polyimide was synthesized from DMF, the polymer chains were arranged closely in more ordered pattern, and intermolecular electronic couplings between the polymer chains were greatly promoted. The electron correlations between the polymer chains improved the delocalization and enhanced the photoelectronic response. For PI-NMP, even though the ordered stacking predominated over the dendritic configuration, the crystal aggregation was still spatially inconsecutive with visible boundaries between the polymer rods (Figure 2h). In this context, the intermolecular electronic couplings was necessarily limited to a low degree. After being completely transformed into ordered layer stacking, the intermolecular interactions were improved significantly in PI-DMF because of efficient orbital overlap between the closely stacking chains. This achievement was confirmed by electron paramagnetic resonance (EPR). As shown in Figure 4b, the Lorentzian line with a g value of 2.003 was the characteristic signal of delocalized electrons.24,25 The
protic solvent with polar phenol hydroxy group while the DMAc is aprotic. The total HSP values of the two solvents were equal, whereas the crystal structures of these two samples were distinctly different. Obviously, this basic difference in solvent type should lead to a different interacting manner that the total HSP could not account for. To understand the role of solvent molecule in the crystallization of polymer, distribution of electrostatic potential (ESP) was calculated to probe the details of potential interactions between polymer and solvent molecules. As illustrated in Figure 3, the most positive and negative ESP surface are colored in blue and red, respectively, and qualitative analysis can be easily made. In the ESP of PI (Figure 3f), a continuous positive surface spanned over the polymer backbone, while the negative part discretely localized in the carbonyl group. Nonuniform distribution of ESP implied the polar nature of the polymer chain and hence strongly preferred to interact with solvent molecules with comparable polarity. Screening the ESP of the solvent molecules in the order of polarity, we could find gradual spatial separation between the positive center and the negative center. As shown in the ESP of toluene in Figure 3a, the uniform distribution of electrostatic potential surface together with small magnitude clearly revealed the nonpolar character. It made toluene hardly to be involved in the crystallization process of polyimide, and this is the reason why PI-Toluene kept the same structure as that growth from solid thermal polymerization. With regard to protic m-Cresol shown in Figure 3b, the molecular polarity was raised from the terminal phenol hydroxyl but the main body remained nonpolar. This explained the significant difference in the value of δh (12.9) and δp (5.1) and further suggested that m-Cresol could interact with the polymer chain only by interfacial hydrogen bonding while the solvent main body being excluded. In comparison, however, in the cases of aprotic solvent exemplified here by DMF and DMAc (Figure 3d,e), the polar characteristic was no longer localized at certain polar groups but presented via structure asymmetry throughout the entire molecule. Therefore, the molecule itself could move freely in and out between the polymer chains to promote ordered interlayer stacking. Just for this reason, the crystal structures of PI-DMAc differed greatly with that of PI-mCresol. Here the critical role solvent molecules played in controlling the crystallization and polymorphic transformation of polyimide was clearly presented, and concurrently, this D
DOI: 10.1021/acs.jpcc.7b09324 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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As can be seen in Figure S5a, the DMF-treated PI-TC remained the pristine absorption intensity with slightly enhanced visible light response due to stacking-induced delocalization. This protocol allowed structure-transfer properties relation discussion without being confused by absorption difference. As is expected, the treated samples exhibited lower impendence in EIS (Figure S5b) when the molecular packing turned to the well-ordered stacking structure. For photocurrent analysis, as shown in Figure S5c, the treated PI-TC exhibited nearly 7 times (1.95/0.25) enhancement in pure photocurrent than that of pristine under irradiation of full arc light (λ>300 nm). To further clarify the contribution from visible absorption, we checked the photocurrent under visible light irradiation. As illustrated in Figure S 5d, although the DMFtreated samples has enhanced absorption in the visible region, the visible-light-induced photocurrent only contributed 5% (0.10/1.95) to the overall photocurrent. This result clearly indicated the advancement of ordered π-stacking in promoting charge separation. As is well-known, charge carriers can move freely in isolated conjugated chains due to the strong intramolecular electronic coupling, while the transport between adjacent chains is a rate-limiting process that requires a synergistic effect of many factors.5,6,28 The molecular packing was believed to be extremely important for charge transport because the stacking order and intermolecular interactions determined the crucial intermolecular charge transport in πstacks.29 If strong intermolecular π-interactions are attainable, charge carriers will not be confined to isolated chains but spread over the overall interactive system and thus result in delocalized transport. Otherwise, charge transport would be limited to the slow hopping between chains.30,31 Here, ordered packing in the direction of π−π stacking found in PI-DMF remarkably improved the intermolecular electronic couplings which in turn created an efficient pathway for charge carriers moving between adjacent chains.14 However, for the dendritic PI-Toluene, intermolecular interactions were so weak that charge carriers could be transported only by hopping. Lastly, to explore the overall effect of solvent-induced polymorphic transformation on photocatalytic activity, photocatalytic hydrogen production was performed under irradiation of full arc light (λ > 300 nm). As shown in Figure 4f, PI-DMF exhibited stable continuous hydrogen evolution with the highest hydrogen production rate of ∼120 μmol h−1. In contrast, the hydrogen production rate over PI-Toluene was merely ∼7 μmol h−1. The dramatic enhancement of 16 times in photocatalytic activity was attributed to the synergistic effect of enhanced light absorption and promoted charge separation derived from the structure advancement in two-dimensional π−π stacking. However, for PI-NMP and PI-m-Cresol, the hydrogen evolution performance was undesirable as the production rate decreased remarkably after 3 h. The unfavorable instability was presumed because of decomposition induced by photocorrosion as these two samples were found to be lost to the pristine crystalline after photocatalytic reaction (Figure S4). In addition, as shown in Figure S6, enhanced photocatalytic performance was also found in DMFtreated PI-TC compared with the pristine samples. The average hydrogen production rate is up to ∼40 μmol h−1, which is 1.6 times as large as than that of pristine one (∼25 μmol h−1). This result further confirm that the strong interaction between solvent molecules and polymer chain can induce the ordered reconstruction of polymer crystalline and improve the photocatalytic performance.
relative intensity has been normalized as an equal amount of samples was used for the measurement, and it represented an overall delocalization effect that included both intramolecular and intermolecular contributions. Except for PI-m-Cresol, the EPR intensity gradually increased as the conformation transferred from absolute dendritic to completely ordered π−π stacking. The increasing delocalization effect was in agreement with the enhanced light absorption in UV−vis spectrum. For PI-m-Cresol, the low degree of delocalization was caused by its unique spherulite structure in which the conjugated chains were extremely slim and radially separated. Considering that the conjugation length in single chain as well as intermolecular electronic couplings all remained at a low level, the delocalization degree was definitely far less than that of other samples. Just as expected, the packing manner between polymer chains not only affected the electronic structure, the mobility, and separation of photoinduced carriers but also relied on the specific crystal structures. The recombination rate of photoinduced carriers was measured by photoluminescence spectra (PL). As shown in Figure 4c, all samples exhibited broad emission peaks in the wavelength from 400 to 650 nm corresponding to their band edge emissions, respectively. Obviously, the excitonic emission intensity differed so much when the crystal conformation changes. According to exciton theory, for parallel-aligned H-aggregation, only the transition to the high exciton state is allowed while the decay from the optically allowed transition to the ground state (0−0 emission) is dipole-forbidden. However, for ideal linear aligned Jaggregate, the situation is the opposite.26,27 In this sense, the lowest radiative emission observed in PI-DMF was mainly due to the layer by layer π-stacking structure which was essentially H-like. This deduction can be further confirmed by study of absorption and emission behavior of DMF treated PI-TC. As shown in Figure S5a, the DMF treated PI-TC exhibited slightly blue-shifted band absorption while DMF treated PI-TC also showed quenched blue-shifted band emission (466 nm) compared with that of pristine PI-TC (474 nm) in PL (Figure S5a inset). Combining these findings together, we tend to speculate that the aggregation mode of DMF treated PI-TC is more likely H-aggregation. Given the structure similarity between PI-DMF and DMF treated PI-TC, this finding may be analogically interpreted by the H-type aggregation in PIDMF. In contrast, the other samples with either dendritic or spherulitic structure were more readily for radiative decay because of the absence of well-ordered H-type stacking and/or the possible defects-induced emission for poorly crystalline PIm-Cresol and PI-NMP. Electrochemical analysis was also performed to gain deep insight into the transport properties. As shown in Figure 4d, electronic conductivity was measured by means of electrochemical impedance spectroscopy (EIS). As can be seen from Nyquist plots, because of the optimum stacking configuration, the semidiameter of PI-DMF was much smaller, indicating that high efficiency charge transport had been obtained. The pristine drawbacks lied in the unfavorable molecular packing or poor crystallinity all increased the resistance for charge transport in other samples. On the other hand, as shown in Figure 4e, the photocurrent response of PI-DMF was also enhanced because of favorable light harvesting and charge separation. Here, to distinguish the effect of light absorption on the photocurrent, we further carried out a control experiment by using PI-TC before and after DMF treatment. E
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3. CONCLUSIONS To summarize, polyimides with conformations transformed from dendritic to regular π-stacks were synthesized via solventinduced crystallization. The polymorphic evolution was proved to be strongly solvent-dependent as chain diffusion and rearrangement were controlled by the specific polymer− solvent interactions. DMF was the optimum solvent to realize ordered packing among the five solvents adopted. The structure−property relationship was discussed on the basis of detailed photoelectrical characterizations, and we determined that the hydrogen evolution rate over PI-DMF was 16 times higher than PI-Toluene. The results presented here highlight the indispensable role ordered π−π stacking played in constructing high efficiency polymer photocatalyst.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b09324.
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Experimental section, XRD pattern of PI-TC, and PIToluene; enlarged XRD pattern in the 2θ range from 10 to 29°; XRD pattern of PI-TC before and after treatment in DMF/EG within 24 h and XRD pattern of samples after photocatalytic hydrogen evolution; optoelectronic characterizing and photocatalytic hydrogen evolution of DMF treated PI-TC (PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Ying Wang: 0000-0002-3626-0678 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.
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ACKNOWLEDGMENTS This work was financially supported by NSFC (21773113, 21273106, and 51272101), the Jiangsu Provincial Natural Science Foundation (BK20151265, BK20150396). The authors would like to thank Analysis Center of Nanjing University for the sample characterization and the High Performance Computing Center of Nanjing University for theoretical calculation of materials.
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REFERENCES
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DOI: 10.1021/acs.jpcc.7b09324 J. Phys. Chem. C XXXX, XXX, XXX−XXX
