- Email: [email protected]
Highly efficient photocatalytic hydrogen evolution from water-soluble conjugated polyelectrolytes
Nano Energy 60 (2019) 775–783
Contents lists available at ScienceDirect
Nano Energy journal homepage: www.elsevier.com/locate/nanoen
Full paper
Highly efficient photocatalytic hydrogen evolution from water-soluble conjugated polyelectrolytes
T
Zhicheng Hua,b, Xi Zhanga, Qingwu Yina, Xiaocheng Liua, Xiao-fang Jianga, Zhiming Chena, Xiye Yanga, Fei Huanga,b,∗, Yong Caoa a
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, PR China b South China Institute of Collaborative Innovation, Dongguan, 523808, PR China
ARTICLE INFO
ABSTRACT
Keywords: Conjugated polyelectrolytes Photocatalytic hydrogen evolution Organic photocatalysts Charge transfer
Organic semiconductors for photocatalytic application have received wide attention recently. However, developing organic photocatalysts for highly efficient photocatalytic hydrogen evolution is a great challenge. Herein, we show highly efficient photocatalytic hydrogen evolution from a series of novel conjugated polyelectrolytes (CPEs) by modulating their interaction with co-catalysts. The water-soluble CPEs greatly enhance the photocatalytic performance with 50-fold improvement over that of their non-dissolved precursor conjugated polymer. More importantly, the photocatalytic activity of these CPEs can be optimized by regulating their interaction with Pt co-catalysts through molecular engineering on the side chain species and counterions of CPEs. The cationic CPEs perform better than anionic CPEs due to the robust interaction between quaternary ammonium side chains in cationic CPEs and Pt co-catalysts. It is found that the optimized interface contact between cationic CPEs and Pt co-catalysts can result in more efficient charge transfer and higher photocatalytic activity.
1. Introduction Hydrogen is regarded as a clean energy source that can replace fossil fuels to address rising global environmental concerns [1]. The rational design of high-performance photocatalytic materials for hydrogen evolution remains an important challenge [2–7]. Organic semiconductors for photocatalysis have received significant recent attention due to their easily tunable features via facile molecular engineering [8–10]. The light-harvesting capability and electronic band structures of organic semiconductors can be well-regulated to enable versatile photo-redox applications (including water reduction/oxidation and CO2 reduction) and provide a good template to closely mimic natural photosynthesis [11–17]. Recently, organic materials, including graphitic carbon nitrides (gC3N4) [18–26], covalent organic frameworks [27] and porous/linear conjugated polymers [28–45] have emerged as efficient photocatalysts to drive hydrogen evolution. The properties of organic materials, including solubility, energy level, absorption, porosity, doping etc., can be fine-tuned by chemical structure evolution, which have led to the widespread study of these materials for photocatalytic hydrogen evolution [8–10,46,47]. However, several challenges remain in the
development of organic photocatalysts, which must be addressed to further increase their photocatalytic activity. First, the exact structure of most polymers (such as g-C3N4-based materials) remains unclear, which is a barrier to the precise understanding of chemical structurephotocatalytic performance relationships. In addition, organic conjugated polymers are suffering from short exciton diffusion length and low carrier mobility, resulting in high recombination rate of charge carriers and hinder the improvement of their photocatalytic activity [8]. Consequently, it is highly desirable to enable organic photocatalysts with good water dispersity/solubility to shorten the path of photo-induced excitons emigrating to the solid-liquid interface and high mobility/conductivity to reduce the exciton recombination inside polymers. Moreover, an important criterion for high photocatalytic activity is rapid charge transfer from organic photocatalysts to metal co-catalysts [8,19,44]. However, precise modulation of the interaction between the organic photocatalysts and metal co-catalysts has rarely been attempted. Conjugated polyelectrolytes (CPEs) are a class of conjugated polymers with ionic functionalities attached to their backbones that have been widely applied in chemical biosensors, imaging, and optoelectronic devices [48–52]. The optoelectronic properties of CPEs can be
∗ Corresponding author. Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, PR China. E-mail address: [email protected] (F. Huang).
https://doi.org/10.1016/j.nanoen.2019.04.027 Received 24 November 2018; Received in revised form 2 April 2019; Accepted 5 April 2019 Available online 09 April 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
Nano Energy 60 (2019) 775–783
Z. Hu, et al.
precisely regulated via chemical synthesis and structural evolution. The ionic side chains also endow CPEs with good dispersibility/solubility [51,52] which are highly desirable characteristics for organic photocatalysts [35,53–55]. More importantly, polar side chains in CPEs can interact with metal electrodes, resulting in improved charge transport/ transfer from conjugated polymers to metal electrodes [56–59]. CPEs with ionic side chains can bind with oppositely charged molecules/ metal nanoparticles through electrostatic interaction [60–63]. Consequently, the interaction between CPEs and metal co-catalysts might be well-regulated, which is rarely reported but potentially boosting the photocatalytic activity of tailor-made CPEs-metal co-catalysts for hydrogen evolution. Herein, we demonstrate a series of novel CPE-based organic photocatalysts, capable of highly efficient hydrogen evolution by optimizing their interaction with co-catalysts. Compared to their non-dissolved precursor conjugated polymer, the ionic side chains functionalized CPEs greatly boost the photocatalytic performance with 50-fold improvement. Moreover, the interaction between CPEs and Pt co-catalysts was well-regulated by altering their side chain species and counterions. Our study indicates that both side chain species and counterions of CPEs can largely determine their photocatalytic activity. The cationic CPEs perform better than anionic CPEs and oversized cations/anions in these CPEs can impede the charge transfer from CPEs to Pt co-catalysts and decrease the photocatalytic activity. Further study indicates that the quaternary ammonium-like side chains in cationic CPEs act as binding centers to interact with Pt co-catalysts, resulting in more efficient charge transfer from CPEs and Pt co-catalysts, which was evidenced by the observation from fs-TAS spectroscopy. The demonstrated knowledge of CPEs and their interaction with co-catalysts allows us to formulate novel design guidelines to engineer organic photocatalysts system to achieve high-performance hydrogen evolution.
Table S1), implying the gradually reduced thermodynamical driving force for water reduction. To fully explore the potential of these CPEs, we selected two widely adopted sacrificial agents: triethanolamine (TEOA) and ascorbic acid (AA) for photocatalytic hydrogen evolution. The photocatalytic activities of these polymers for hydrogen evolution from water were investigated using Pt co-catalysts (3 wt% of CPE, Pt co-catalysts were prepared by dissolving H2PtCl4 into the CPEs reaction with irradiation for 1 h to enable the formation of Pt cocatalysts [36]) in TEOA/H2O and AA/H2O solution. The neutral amine-functionalized PFN can only be dispersed, not dissolved, in TEOA/H2O and forms an opaque colloidal solution, whilst the cationic CPEs are readily dissolved in TEOA/H2O and forms a clear micelle solution (Fig. S4). The photocatalytic hydrogen evolution rates (HERs) of these CPEs as a function of time are presented in Fig. 2a–c. The photocatalytic experiments showed that PFN delivered a HER of 0.23 μmol h−1, similar to that of previously reported conjugated polymers with non-polar side chains and similar absorption spectra [37]. Surprisingly, PFN-Br, which has ionic side chains, achieved substantial 50-fold higher HER of 11.50 μmol h−1. The effect of Pt co-catalysts weight ratio on the photocatalytic performance was also investigated (Fig. S7). An optimized 3.0 wt% of Pt co-catalysts can enable the highest HER (11.50 μmol h−1) of PFN-Br. It should be noted that PFN-Br without Pt co-catalysts exhibited an apparently lower HER of 1.70 μmol h−1. The much-enhanced photocatalytic activity of PFN-Br indicates that the introduction of ionic side groups and Pt cocatalysts plays an important role in the improvement of the photocatalytic process. Because PFN and PFN-Br share similar electron affinities (Table S1), light-harvesting capabilities (Figs. S5–S6) and even comparable lengths of side chains, we attribute the enhanced photocatalytic activities of PFN-Br over that of PFN to the much better dispersibility/solubility of the former. It has been widely accepted that the interface between organic photocatalysts and water showed large impact on the photocatalytic activity [9]. Since the exciton diffusion length of conjugated polymers is limited (< 30 nm) [64] large interface area between polymers and water will provide more active sites for cocatalysts loading [9,65,66] and shorten migration distance of charge carriers. In our case, the CPEs possess high solubility in water and formed a clear micelle solution, thus it can be regarded that interface area between CPEs and water are greatly enlarged in comparison with that between other non-dissolved conjugated polymers (such as PFN) and water. The better solubility of CPEs in water must have greatly shortened the path for excitons in CPEs to migrate to the interface between polymers and water, resulting in decreased recombination inside the CPEs and increased photocatalytic activity [67]. The photocatalytic HERs of other CPEs with broadened absorption were also tested in TEOA/H2O. Unexpectedly, PFNBT-Br, PFNDTBT-Br and PFNDPP-Br showed clearly lower photocatalytic hydrogen evolution activity in TEOA/H2O than PFN-Br. The HERs of PFNBT-Br, PFNDTBT-Br and PFNDPP-Br were reduced to 3.90 μmol h−1, 0.20 μmol h−1 and 0.138 μmol h−1, espectively (Fig. 2a and c). The extremely lower photocatalytic activity of PFNDTBT-Br and PFNDPP-Br in TEOA/H2O can be easily understood from their lower LUMO energy levels, which are very close to the reduction potential of water (Fig. 2d), providing limited driving force for water reduction [68]. However, with AA as the sacrificial agent, it can be observed that photocatalytic HERs of PFNDTBT-Br and PFNDPP-Br were much enhanced. The pH values of TEOA/H2O and AA/H2O solution were estimated to be 10.87 and 4.0, respectively. Using the Nernst equation, E(H+/H2) = -0.059 pH (V vs NHE), the reduction potentials of TEOA/H2O and AA/H2O were thus calculated to be -0.641 V and -0.236 V (vs NHE), respectively. The water reduction of AA/H2O is 0.4 V more positive than that of TEOA/ H2O solution, suggesting a larger driving force between the CPEs and the water reduction potential (Fig. 2d) (especially for PFNDTBT-Br and PFNDPP-Br with lower LUMO energy levels) [69,70]. Expectedly, the HERs of PFNDTBT-Br and PFNDPP-Br in AA/H2O were appreciably enhanced. PFNDTBT-Br showed an initial photocatalytic HER of
2. Results and discussion 2.1. Photocatalytic activity of cationic CPEs CPEs possess cationic or anionic side chains attached to their backbones, which endow them with good solubility in polar solvents. Here, we first prepared a series of polyfluorene-based cationic CPEs with a quaternary ammonium bromide salt as templates to explore the potential application of CPEs in photocatalytic hydrogen evolution. (The experimental procedure and characterization are shown in Support Information, Figs. S1–S6). Fluorene monomer and three electron-withdrawing moieties, benzo[c] [1,2,5]thiadiazole (BT), 4,7-di (thiophen-2-yl)benzo[c] [1,2,5]thiadiazole (DTBT) and 2,5-bis(2- butyloctyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4dione (DPP), were used to construct D-A conjugated polymers in combination with a fluorene moiety. For comparison, PFN (Fig. 1a) with neutral amino groups was also prepared. After quaternarization procedures, CPEs (PFN-Br, PFNBT-Br, PFNDTBT-Br and PFNDPP-Br, Fig. 1a) with successively extended absorption spectra and varied energy levels were obtained. Compared with PFN, which is soluble only in non-polar or low-polar solvents (such as toluene and chloroform), cationic CPEs tend to be highly soluble in water/alcohol-like polar solvents. With increasing electron-withdrawing capability of the copolymerizing moieties, PFNBT-Br, PFNDTBT-Br and PFNDPP-Br showed increasingly strong intramolecular charge transfer bands, with gradually red-shifted absorption edges to 500 nm, 645 nm and 715 nm in methanol, respectively, indicating the enhanced light-harvesting capability and potential photocatalytic activity of PFNBT-Br, PFNDTBT-Br and PFNDPP-Br in the visible range (Fig. 1b). The optical band gaps of these CPEs were determined to be 2.33 eV, 1.93 eV and 1.72 eV, respectively, calculated from their thin-film absorption edges (Fig. S6). The lowest unoccupied molecular orbital (LUMO) energy levels of these CPEs (PFN-Br, PFNBT-Br, PFNDTBT-Br and PFNDPP-Br) were estimated to be -2.41 eV, -3.16 eV, -3.53 eV and -3.83 eV (vs Evacuum, Fig. 1c and 776
Nano Energy 60 (2019) 775–783
Z. Hu, et al.
Fig. 1. Chemical structures (a), UV-vis absorption spectra (in methanol) (b) and energy level diagram (c) of PFN-Br, PFNBT-Br, PFNDTBT-Br and PFNDPP-Br. Fig. 2. (a) Photocatalytic HERs of CPEs as a function of time (Reaction condition: 2.5 mg CPEs dissolved in 50 mL TEOA/H2O (1/9, v/v) solution); (b) Photocatalytic HERs of CPEs as a function of time (Reaction condition: 2.5 mg CPEs dissolved in 50 mL H2O with 0.2 M AA as the sacrificial agent, pH = 4.0); (c) Summary of HERs of CPEs. (d) Schematic energy diagram of PFNDTBT-Br and PFNDPP-Br and water reduction potentials.
777
Nano Energy 60 (2019) 775–783
Z. Hu, et al.
Fig. 3. (a) Summary of electron mobilities of CPEs. (b) Absorption spectrum of PFNDPP-Br and AQY values of PFNDPP-Br as a function of light wavelength.
Scheme 1. Chemical structures of cationic and anionic CPEs with different counter cations/anions.
44.6 μmol h−1 and an average HER of 30.58 μmol h−1 in 5 h (Fig. 2b–c). For PFNDPP-Br, with DPP as the copolymerizing unit, the photocatalytic stability was improved. PFNDPP-Br delivered an efficient average HER of 27.90 μmol h−1, with almost no decline in photocatalytic activity. Another observation in these CPEs is that PFN-Br showed low HER of 0.85 μmol h−1 in AA/H2O. The lower highest occupied molecular orbital (HOMO) energy level of PFN-Br in comparison with PFNDTBT-Br and PFNDPP-Br indicates an increase in the oxidizing capability of the hole might be achieved for PFN-Br [27]. In addition to the above-discussed factors of the sacrificial agent and energy levels, the charge transporting properties of CPEs might be another important determinant of their photocatalytic activity. In the water reduction process, the generated electrons in the CPEs must quickly migrate from polymers to Pt co-catalysts, thus requiring the CPEs to possess high electron mobilities to achieve high photocatalytic activity. However, the electron mobilities of PFN-Br and PFNBT-Br (6.33 × 10−7 cm2 V−1 s−1 and 3.62 × 10−6 cm2 V−1 s−1) are much lower than those of PFNDTBT-Br and PFNDPP-Br (1.17 × 10−4 cm2 V−1 s−1 and 8.13 × 10−5 cm2 V−1 s−1), respectively (Fig. 3a and Fig. S8). The much-enhanced electron mobilities in PFNDTBTBr and PFNDPP-Br must have reduced the recombination rate inside the CPEs and improved the photocatalytic activity [71]. Furthermore, it has been reported that the more stable radical anion in conjugated polymers might enhance the charge separation, thereby increasing the probability of electron migration to a nearby Pt co-catalyst [27]. Indeed, radical anions can be easily introduced into CPEs and tuned via molecular engineering of the side chains and backbones [72,73]. In our case, from the electron spin resonance (ESR) results (Fig. S9) of the CPEs, it can be deduced that PFNDPP-Br and PFNDTBT-Br can form more stable radical anions than those of PFN-Br and PFNBT-Br, in accordance with their photocatalytic performance. These results indicate that CPEs are not only promising
photocatalysts for hydrogen evolution but also a good model to explore the design principles of organic photocatalysts. The apparent quantum yield (AQY) values of the photocatalytic performance of PFNDPP-Br as a function of wavelength were also collected (Fig. 3b). At 550, 600, 650 and 700 nm, PFNDPP-Br rendered AQY values of 0.12%, 0.40%, 0.44% and 0.19%, respectively, indicating a broader photoresponse for PFNDPP-Br. These results validate our rational design strategy to realize high-performance photocatalytic evolution in the visible range. The results of longer-term photocatalytic experiments (Fig. S10) showed that an average HER of around 17.5 μmol h−1 was maintained after a 20-h photocatalytic reaction. Moreover, the absorption spectra of PFNDPP-Br before and after a 20-h photocatalysis (Fig. S11) showed no difference, indicating that PFNDPP-Br is a promising organic photocatalyst for long-term use. 2.2. Side chains and counterions effect As a crucial component of CPEs, the counterions strongly determine their aggregation, doping behavior and charge transport properties, which are central to their application in organic electronics [74,75]. Modulating the side chains and ionic species can effectively alter the interaction between conjugated polymers and metal co-catalysts, thus affecting the charge transfer between conjugated polymers to metal cocatalysts [19,76]. Herein, cationic CPEs (PFN-BIm4 and PFN-BF4, Scheme 1) balanced by tetra(1H-imidazol-1-yl) borate anions (BIm4- ) and tetrafluoroborate anions (BF4−) were prepared via ion-exchange. X-ray photoelectric spectroscopy (XPS) results of PFN-BIm4 and PFNBF4 powders (Fig. S1) confirmed that all the bromide anions have been exchanged into BIm4- and BF4−. PFN-BIm4 and PFN-BF4 showed similar light-harvesting capabilities to their precursor, PFN-Br, as indicated by their UV-vis absorption spectra (Figs. S5–S6). Likewise, we prepared 778
Nano Energy 60 (2019) 775–783
Z. Hu, et al.
Fig. 4. (a) Photocatalytic HERs of PFN-Br, PFN-BF4 and PFN-BIm4 as a function of time. (b) Photocatalytic HERs of PFS-Na, PFS-NMe and PFS-NBu as a function of time; (Reaction condition: 2.5 mg CPEs dissolved in 50 mL TEOA/H2O (1/9) solution) (c) Summary of HERs of CPEs with different cations/anions; (d) Zeta potentials of PFN-Br solution, PFS-Na solution, Pt cocatalysts in TEOA/H2O solution.
anionic CPEs (PFS-Na, PFS-NMe and PFS-NBu, Scheme 1) balanced by sodium cations, tetramethylammonium cations and tetrabutylammonium cations, which presented similar optical properties and energy levels (Figs. S5–S6 and Table S1). The 1H-NMR spectra of PFS-NMe and PFS-NBu indicated that almost all Na+ have been exchanged into N+Me4 and N+Bu4. Photocatalytic tests of these CPEs (Fig. 4a–c) in TEOA/H2O showed that PFN-BF4 delivered similar photocatalytic activity to that of PFN-Br, reaching a HER of 12.05 μmol h−1. Unexpectedly, PFN-BIm4 showed a much lower HER of 5.65 μmol h−1. The molar mass of the BIm4- counter anion is larger than that of PFN-Br; thus, the low HER of PFN-BIm4 can be partly attributed to the larger molar mass of the BIm4- counter anions, which decrease the weight ratio of the conjugated backbones per gram. The HERs of CPEs with different copolymer units were analyzed in greater detail to exclude the effect of molecular weight per unit (Table S2). It was found that the photocatalytic activity of PFN-BIm4 was much lower than that of PFN-Br and PFN-BF4, implying that the oversized BIm4- counter anions [74] severely hindered the photocatalytic performance of the CPEs. Dynamic light scattering (DLS) was conducted to investigate the aggregation state of the CPEs in solution. In fact, the aggregation degree of PFN-BIm4 in solution was between those of PFN-Br and PFN-BF4, implying that the aggregation state of these CPEs was not the main determinant of their photocatalytic activity (Fig. S12). Turning to the anionic CPEs, PFS-Na showed a HER of 0.59 μmol h−1, which is higher than that of PFN, but much lower than that of PFN-Br. PFS-NMe showed a HER of 0.3375 μmol h−1, lower than that of PFS-Na, whilst PFS-NBu showed even further reduced photocatalytic activity with a HER of 0.017 μmol h−1 (Fig. 4b–c). These results suggest that the photocatalytic activity of PFS-based CPEs gradually decreases with increasing size of the counter cations, a trend that is consistent with that observed for the PFN-series of CPEs. In particular, PFS-NBu with oversized counter anions showed poor photocatalytic activity, an analogous situation to PFN-BIm4, suggesting that oversized counterions impede the charge transfer from CPEs to Pt co-catalysts and reduce the photocatalytic activity. These results indicate that the
rational design of counterions is essential to the photocatalytic activity of CPEs. 2.3. Interaction between CPEs and Pt co-catalysts Although both cationic and anionic CPEs showed better photocatalytic activity than neutral PFN, the much lower photocatalytic activity of PFS-based than PFN-based CPEs is worthy of further exploration. Cationic CPEs and anionic CPEs are balanced by counterions with different charges, thus exhibiting varied interaction with metal cocatalysts with opposite charges. Besides, considering that the photocatalytic conditions and backbones of these CPEs were the same, the difference in their HERs suggests that the different side chains attached to their backbones play an important role in their photocatalytic hydrogen generation. It has been reported that the positively charged surfaces of semiconductors could absorb metal-based anions (such as PdCl42−), enhancing the stability and photocatalytic performance of dispersed metal catalysts [77]. Here, the cationic PFN-Br might likewise have absorbed PtCl42− on the surface in the early stage of formation of Pt nanoparticles, resulting in close contact between the CPEs and Pt cocatalysts (Fig. S13 and Fig. S14). The interaction between PFN-Br and PtCl42− was also evidenced by the finding from titration experiments that PtCl42− caused severe aggregation and precipitation of PFN-Br in concentrated solution, whilst no aggregation and precipitation were observed in PFS-Na solution (Fig. S13). To study the interaction between the CPEs and Pt co-catalysts, the zeta potentials of PFN-Br and PFS-Na and the Pt nanoparticles were tested (Fig. 4d and Fig. S15). The zeta potentials of PFN-Br and PFS-Na solution were estimated to be 35.7 mV and -69.4 mV (Fig. 4d), respectively, indicating that the net electrical charge of PFN-Br was positive, whilst that of PFS-Na was negative. The zeta potential of the Pt co-catalysts in TEOA/H2O solution was estimated to be -42.7 mV, suggesting a net negative charge. Thus it can be deduced that the Pt cocatalysts were preferentially adsorbed onto PFN-Br via electrostatic interaction, allowing closer interface contact and more efficient charge 779
Nano Energy 60 (2019) 775–783
Z. Hu, et al.
transfer from the backbones of the latter to the former [78]. The close intimate contact between PFN-Br and Pt cocatalysts was also evidenced by FT-IR results, of which the bending vibration of quaternary ammonium groups in PFN-Br shifted obviously (Fig. S16). It should be noted that similar zeta potential can be also observed in Pt co-catalysts in AA/ H2O (Fig. S15), indicating similar preferentially adsorption occurred in PFN-Br and Pt co-catalysts. Efficient charge transfer from photocatalysts to co-catalysts must have enhanced the electronic communication between the two, thus boosting the photocatalytic activity; such behavior has been observed in a CdS/Pt-based inorganic photocatalytic system [79]. In contrast, the repulsion between the Pt cocatalysts and PFS-Na would lead to poor interface contact (Fig. S17), leading to less efficient charge transfer between the two and thus poor photocatalytic activity. The more efficient charge transfer between PFN-Br and Pt co-catalysts was also confirmed by the results of timeresolved photoluminescence kinetics, showing that PFN-Br with Pt cocatalysts delivered a shorter lifetime than that of PFS-Na with Pt cocatalysts, indicating the stronger and faster photoluminescence quenching in the former case (Fig. S18). These results demonstrate that rational design of polar side chains can enable intimate contact between organic photocatalysts and metal co-catalysts. Femtosecond transient absorption spectroscopy (fs-TAS) was used to further study the interaction between the CPEs and Pt co-catalysts. As shown in Fig. 5a, both PFN-Br and PFS-Na solution exhibit broad positive induced absorption features from 500 nm to NIR region, which can be attributed to the overlapping electron and hole absorption of polyfluorene [80,81]. To elucidate the nature of this excited state, fsTAS measurements of PFN-Br and PFS-Na solution with hole scavenger TEOA were performed. fs-TAS experiments showed an initial increase in the photoinduced positive absorption feature above 650 nm, attributing primarily to photogenerated electrons [81] in the polyfluorene backbones. The corresponding TAS kinetics, probed at peak of 720 nm, of PFN-Br and PFS-Na solutions with TEOA (Fig. 5b) showed a longer lifetime compared with those in pristine PFN-Br and PFS-Na solutions (Fig. S19), suggesting a much-suppressed charge recombination in the former due to the hole scavenging by TEOA [81]. Moreover, it was also observed that PFS-Na with TEOA showed a longer lifetime than PFN-Br with TEOA, indicating faster hole scavenging in PFS-Na. This can be attributed to the better capability of PFS-Na, compared with PFN-Br, to achieve intimate contact with both water and TEOA (Fig. S20). In the TAS kinetics probed from 1000 to 1050 nm, a signal attributed to electron transfer appeared [82]. Moreover, PFN-Br solution with Pt cocatalysts showed faster quenching and shorter lifetime than PFS-Na solution with Pt co-catalysts (Fig. 5c), indicating that the former achieved a more efficient electron transfer to Pt co-catalysts, which coincides well with the above observations of the photocatalytic performances. Comparing the photocatalytic performance of PFN-Br and PFS-Na, it is obvious that the charge transfer from organic photocatalysts to Pt co-catalysts is vital to the overall photocatalytic
performance, and that it is possible to regulate the polar side chains of CPEs to achieve high photocatalytic activity. 3. Conclusion In summary, we have demonstrated a series of novel high-performance organic photocatalysts for hydrogen evolution. The introduction of ionic side chains into conjugated polymers greatly improves the dispersibility/solubility of CPEs in water and their photocatalytic activity with 50-fold improvement. By molecular engineering of the backbone structures of the CPEs, highly efficient photocatalytic hydrogen evolution in the visible range was achieved. It was shown via side chain engineering that quaternary ammonium side chains interact robustly with Pt co-catalysts, leading to more efficient charge transfer and higher photocatalytic activity. The size of the counterions is a key concern when designing novel organic photocatalysts. Overall, CPEs provide a good template for methodical investigation of organic photocatalysts due to their well-defined and easily tunable chemical structures, good dispersibility and interface wettability with water, strong interaction with metal co-catalysts via binding center and high photocatalytic activity. Our intensive photocatalytic study on these novel conjugated polyelectrolytes and their photocatalytic performance provides novel design guidelines for high-performance organic photocatalysts for hydrogen evolution. 4. Methods 4.1. Synthesis and characterization of polymers Synthetic procedures of the CPEs are described in Support Information. The 1H-NMR data of the monomers and polymers were recorded on a Bruker AVANCE Digital NMR workstation operating at 500 MHz. Electrochemical cyclic voltammetry (CV) was conducted on a CHI-660E electrochemical workstation equipped with a glassy carboncoated electrode. A saturated calomel electrode (SCE) was used as the reference electrode, and a Pt sheet was used as the counter electrode. The CV was conducted in acetonitrile solution (containing 0.1 M Bu4NPF6) at a scan rate of 0.05 V s−1 at room temperature under the protection of nitrogen. The potential of ferrocene/ferrocenium (Fc/ Fc+) reference was measured to be 0.32 V versus SCE. Thus, the LUMO and HOMO energy levels of the CPEs were calculated with the following equations: ELUMO = -e(Ere + 4.48) (eV), EHOMO = -e(Eox + 4.48) (eV), respectively. The molecular weights of the neutral polymers were determined by a Waters GPC 2410, where CHCl3 was used as the mobile phase. The molecular weights of the CPEs were calculated from their neutral precursor polymers. UV-vis absorption spectra were collected from an HP 8453 spectrophotometer. The fluorescence decay curves were recorded on an Edinburgh Instruments FLS920 combined with steady-state and time-resolved fluorescence spectrometers. The pH Fig. 5. (a) fs-TAS spectra of PFN-Br and PFS-Na solution in water (50 μg/mL, with/ without 10% TEOA and 3% Pt co-catalysts) taken at a delay time of 2 ps following pulsed 400 nm excitation (20 μW); (b) quencher-dependent TAS kinetics probed at 720 nm for PFN-Br and PFS-Na solution with TEOA; (c) quencher-dependent TAS kinetics probed at 1000 to 1050 nm for PFN-Br and PFS-Na solution with TEOA and Pt co-catalysts.
780
Nano Energy 60 (2019) 775–783
Z. Hu, et al.
values of the reaction solution were determined by a pH-meter (PHS-25, Rex, China). The zeta-potentials were measured on a Nano-ZS zetasizer (HORIBA Instruments, Japan). PFN-Br and PFS-Na were dissolved in deionized H2O (50 μg/mL) and stored for 24 h before testing. Solutions of Pt nanoparticles in TEOA/H2O and AA were prepared by dissolving H2PtCl4 and irradiated for 1 h to enable the formation of Pt nanoparticles. Transmission electron microscopy was performed on a TF20 Joel 2100F (FEI) transmission electron microscope. The samples were prepared by dropping the reaction solution onto 300-mesh carboncoated copper grids and evaporating the solvent immediately. Electrononly devices with the architecture of ITO/Al/CPEs/Al were prepared and the electron mobilities of the CPEs were extracted by fitting the data using the space-charge-limited current (SCLC) model. Electron spin resonance spectroscopy of the CPEs (in the solid state) was conducted on a JEOL JES-FA200 ESR spectrometer (300 K, 9.063 GHz, Xband). FT-IR spectra were carried out with a NEXUS 670 instrument.
Acknowledgment
4.2. Hydrogen evolution experiments
[1] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Chem. Rev. 110 (2010) 6446. [2] S. Chen, T. Takata, K. Domen, Nat. Rev. Mater. 2 (2017) 17050. [3] C. Ding, J. Shi, Z. Wang, C. Li, ACS Catal. 7 (2017) 675. [4] Z. Yu, F. Li, L. Sun, Energy Environ. Sci. 8 (2015) 760. [5] Z. Li, W. Luo, M. Zhang, J. Feng, Z. Zou, Energy Environ. Sci. 6 (2013) 347. [6] D. Wang, R. Yu, Y. Chen, N. Kumad, N. Kinomur, M. Takano, Solid State Ionics 172 (2004) 101. [7] X. Lai, J.E. Halperta, D. Wang, Energy Environ. Sci. 5 (2012) 5604. [8] W.-J. Ong, L.-L. Tan, Y.H. Ng, S.T. Yong, C.-P. Chai, Chem. Rev. 116 (2016) 7159. [9] V.S. Vyas, V.W. Lau, B.V. Lotsch, Chem. Mater. 28 (2016) 5191. [10] G. Zhang, Z.-A. Lan, X. Wang, Angew. Chem. Int. Ed. 55 (2016) 15712. [11] P.D. Frischmann, K. Mahata, F. Würthner, Chem. Soc. Rev. 42 (2013) 1847. [12] S. Wang, L. Yi, J.E. Halpert, X. Lai, Y. Liu, H. Cao, R. Yu, D. Wang, Y. Li, Small 8 (2012) 265. [13] N. Yang, Y. Liu, H. Wen, Z. Tang, H. Zhao, Y. Li, D. Wang, ACS Nano 7 (2013) 1504. [14] S.J.A. Moniz, S.A. Shevlin, D.J. Martin, Z.X. Guo, J. Tang, Energy Environ. Sci. 8 (2015) 731. [15] D.L. Ashford, M.K. Gish, A.K. Vannucci, M.K. Brennaman, J.L. Templeton, Chem. Rev. 115 (2015) 13006. [16] A. Iwase, S. Yoshino, T. Takayama, Y.H. Ng, R. Amal, A. Kudo, J. Am. Chem. Soc. 138 (2016) 10260. [17] Y. Zhao, H. Tang, N. Yang, D. Wang, Adv. Sci. 5 (2018) 1800959. [18] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76. [19] V.W. Lau, I. Moudrakovski, T. Botari, S. Weinberger, M.B. Mesch, V. Duppel, J. Senker, V. Blum, B.V. Lotsch, Nat. Commun. 7 (2016) 12165. [20] H. Wang, B. Wang, Y. Bian, L. Dai, ACS Appl. Mater. Interfaces 9 (2017) 21730. [21] C. Liu, H. Huang, L. Ye, S. Yu, N. Tian, X. Du, T. Zhang, Y. Zhang, Nano Energy 41 (2017) 738. [22] S. Yu, J. Li, Y. Zhang, M. Li, F. Dong, T. Zhang, H. Huang, Nano Energy 50 (2018) 383. [23] K.C. Christoforidis, Z. Syrgiannis, V.L. Parola, T. Montini, C. Petit, E. Stathatos, R. Godin, J.R. Durrant, M. Prato, P. Fornasier, Nano Energy 50 (2018) 468. [24] H. Wang, Y. Bian, J. Hu, L. Dai, Appl. Catal. B Environ. 238 (2018) 592. [25] W. Lei, Y. Mi, R. Feng, P. Liu, S. Hu, J. Yue, X. Liu, J.A. Rodriguez, J.-o. Wang, L. Zheng, K. Tang, S. Zhu, G. Liu, M. Liu, Nano Energy 50 (2018) 552. [26] H. Kasap, R. Godin, C. Jeay-Bizot, D.S. Achilleos, X. Fang, J.R. Durrant, E. Reisner, ACS Catal. 8 (2018) 6914. [27] V.S. Vyas, F. Haase, L. Stegbauer, G. Savasci, F. Podjaski, C. Ochsenfeld, B.V. Lotsch, Nat. Commun. 6 (2015) 8508. [28] R.S. Sprick, J.-X. Jiang, B. Bonillo, S. Ren, T. Ratvijitvech, P. Guiglion, M.A. Zwijnenburg, D.J. Adams, A.I. Cooper, J. Am. Chem. Soc. 137 (2015) 3265. [29] L. Li, Z. Cai, Q. Wu, W.-Y. Lo, N. Zhang, L.X. Chen, L. Yu, J. Am. Chem. Soc. 138 (2016) 7681. [30] C. Yang, B.C. Ma, L. Zhang, S. Lin, S. Ghasimi, K. Landfester, K.A.I. Zhang, X. Wang, Angew. Chem. Int. Ed. 55 (2016) 9202. [31] R.S. Sprick, B. Bonillo, R. Clowes, P. Guiglion, N.J. Brownbill, B.J. Slater, F. Blanc, M.A. Zwijnenburg, D.J. Adams, A.I. Cooper, Angew. Chem. Int. Ed. 28 (2016) 1824. [32] L. Li, R.G. Hadt, S. Yao, W.-Y. Lo, Z. Cai, Q. Wu, B. Pandit, L.X. Chen, L. Yu, Chem. Mater. 28 (2016) 5394. [33] L. Wang, Y. Wan, Y. Ding, S. Wu, Y. Zhang, X. Zhang, G. Zhang, Y. Xiong, X. Wu, J. Yang, H. Xu, Adv. Mater. 29 (2017) 1702428. [34] P.B. Pati, G. Damas, L. Tian, D.L.A. Fernandes, L. Zhang, I.B. Pehlivan, T. Edvinsson, M. Araujo, H. Tian, Energy Environ. Sci. 10 (2017) 1372. [35] H. Lu, R. Hu, H. Bai, H. Chen, F. Lv, L. Liu, S. Wang, H. Tian, ACS Appl. Mater. Interfaces 9 (2017) 10355. [36] J. Chen, C.-L. Dong, D. Zhao, Y.-C. Huang, X. Wang, L. Samad, L. Dang, M. Shearer, S. Shen, L. Guo, Adv. Mater. 29 (2017) 1606198. [37] D.J. Woods, R.S. Sprick, C.L. Smith, A.J. Cowan, A.I. Cooper, Adv. Energy Mater. 7 (2017) 1700479. [38] X. Zong, X. Miao, S. Hua, L. An, X. Gao, W. Jiang, D. Qu, Z. Zhou, X. Liu, Z. Sun, Appl. Catal. B Environ. 211 (2017) 98.
This work was financially supported by the Natural Science Foundation of China (No. 21634004, 21490573) and the Foundation of Guangzhou Science and Technology Project (No. 201707020019). Z. Hu thanks the support from China Postdoctoral Science Foundation (No. 2017M622684) and the Fundamental Research Funds for the Central Universities, South China University of Technology (No. D2180460). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.04.027. References
The hydrogen evolution measurements were performed on a Labsolar-IIIAG photocatalytic system (PerfectLight) equipped with a 50-mL reactor. For the photocatalytic experiment using TEOA as the sacrificial agent, CPEs (2.5 mg) were firstly dissolved in 250 μL (for PFNDPP-Br, 1 mL of methanol was used) methanol and then dispersed into 50 mL of TEOA/H2O (1:9, v/v) to obtain a clear solution. For the photocatalytic experiment using AA as the sacrificial agent, CPEs (2.5 mg) were firstly dissolved in 250 μL of methanol (for PFNDPP-Br, 1 mL of methanol was used) and then dispersed into a mixture of 50 mL of AA solution (0.2 M, pH = 4 adjusted by NaOH aq.; for PFNDPP-Br, 30 mL of AA solution and 20 mL of glycol were used to aid dissolution). PFN was dissolved in 250 μL of THF and dispersed into TEOA/H2O to form a colloidal solution. The total concentration of the polymers and CPEs for photocatalytic measurement in the reaction solution was 50 μg/mL. Pt co-catalysts (3 wt% of CPE) were prepared by dissolving H2PtCl4 aqueous solution into the CPEs reaction and irradiated for 1 h to enable the formation of Pt nanoparticles before testing. The reaction was held in a vacuum for 30 min before testing to remove the dissolved oxygen and methanol. The photocatalytic reactions were illuminated with a solar simulator (300-W Xe light source, λ > 300 nm). The luminous power reaching the surface of the reaction solution was calibrated to be 162.4 mW cm−2 by a power meter. The produced gas was analyzed by a GC7900 gas chromatograph. Hydrogen was detected with a thermal conductivity detector, referencing against standard gas with a known concentration of hydrogen. The photocatalyticHERs of all the CPEs as a function of time are presented in Figs. S21–S36. The AQY was measured at selected wavelengths enabled by different band pass filters (CEL-QD, 380, 450, 550, 600, 650 and 700 nm). The AQY at a given wavelength was calculated from the following equations: No=(EλT%)/ (hc); N=(V × 6.02 × 1023)/(22.4t); AQY = 2N/No. No represents the number of incident photons, N represents the number of collected H2 molecules, E represents the energy of incident light at a given wavelength, determined by a calibrated power meter, λ represents the wavelength of incident light, V represents the volume of H2 molecules detected in a fixed time (t), determined by GC, T% represents the transmittance of the quartz cell. 4.3. Fs-TAS measurements The fs-TAS measurements were carried out with a commercial transient absorption spectrometer (MS2004i, Transpec-SP) equipped with a 500-Hz Solstice (Newport Corp.) Ti:sapphire regenerative amplifier outputting 800-nm, 150-fs pulses. This laser light was split into two parts to generate the pump and the probe pulses. The tuneable pump pulse was generated in a TOPAS-Prime (Spectra-Physics) optical parametric amplifier and used to excite the sample with energies of 20 μW at 400 nm. 781
Nano Energy 60 (2019) 775–783
Z. Hu, et al.
Zhicheng Hu received his B.S. degree in polymer materials and engineering in 2012 from South China University of Technology (SCUT), and gained his Ph.D. degree in materials science in 2017 under the supervision of Prof. Fei Huang. He is currently working as a postdoc in Prof. Fei Huang's group at State Key Laboratory of Luminescent Material and Devices, SCUT. His current research focuses on the design and synthesis of water/alcohol-soluble conjugated polymers for polymer solar cells and photocatalytic water splitting.
[39] F. Yu, Z. Wang, S. Zhang, H. Ye, K. Kong, X. Gong, J. Hua, H. Tian, Adv. Funct. Mater. 28 (2018) 1804512. [40] P.-J. Tseng, C.-L. Chang, Y.-H. Chan, L.-Y. Ting, P.-Y. Chen, C.-H. Liao, M.-L. Tsai, H.-H. Chou, ACS Catal. 8 (2018) 7766. [41] Z. Wang, X. Yang, T. Yang, Y. Zhao, F. Wang, Y. Chen, J.H. Zeng, C. Yan, F. Huang, J.-X. Jiang, ACS Catal. 8 (2018) 8590. [42] J. Kosco, M. Sachs, R. Godin, M. Kirkus, L. Francas, M. Bidwell, M. Qureshi, D. Anjum, J.R. Durrant, I. McCulloch, Adv. Energy Mater. 8 (2018) 1802181. [43] B. Tian, B. Tian, B. Smith, M.C. Scott, R. Hua, Q. Lei, Y. Tian, Nat. Commun. 9 (2018) 1397. [44] M. Sachs, R.S. Sprick, D. Pearce, S.A.J. Hillman, A. Monti, A.A.Y. Guilbert, N.J. Brownbill, S. Dimitrov, X. Shi, F. Blanc, M.A. Zwijnenburg, J. Nelson, J.R. Durrant, A.I. Cooper, Nat. Commun. 9 (2018) 4968. [45] X. Zhang, F. Shen, Z. Hu, Y. Wu, H. Tang, J. Jia, X. Wang, F. Huang, Y. Cao, ACS Sustain. Chem. Eng. 7 (2019) 4128. [46] M. Waqas, Y. Wei, D. Mao, J. Qi, Y. Yang, B. Wang, D. Wang, Nano Research 10 (2017) 3920. [47] Y. Wei, J. Wang, R. Yu, J. Wan, D. Wang, Angew. Chem. Int. Ed. 58 (2019) 1422. [48] H. Jiang, P. Taranekar, J.R. Reynolds, K.S. Schanze, Angew. Chem. Int. Ed. 48 (2009) 4300. [49] C. Zhu, L. Liu, Q. Yang, F. Lv, S. Wang, Chem. Rev. 112 (2012) 4687. [50] C. Duan, K. Zhang, C. Zhong, F. Huang, Y. Cao, Chem. Soc. Rev. 42 (2013) 9071. [51] C.-C. Chueh, C.-Z. Li, A.K.-Y. Jen, Energy Environ. Sci. 8 (2015) 1160. [52] Z. Hu, F. Huang, Y. Cao, Small Methods 1 (2017) 1700264. [53] D.L. Jiang, C.K. Choi, K. Honda, W.S. Li, T. Yuzawa, Takuzov Aida, J. Am. Chem. Soc. 126 (2004) 12084. [54] X. Yang, Z. Hu, Q. Yin, C. Shu, X.-F. Jiang, J. Zhang, X. Wang, J.-X. Jiang, F. Huang, Y. Cao, Adv. Funct. Mater. (2019) 1808156. [55] W.-W. Yong, H. Lu, H. Li, S. Wang, M.-T. Zhang, ACS Appl. Mater. Interfaces 10 (2018) 10828. [56] Z. Hu, Z. Wang, X. Zhang, H. Tang, X. Liu, F. Huang, Y. Cao, iScience 13 (2019) 33. [57] Y. Liu, V.V. Duzhko, Z.A. Page, T. Emrick, T.P. Russell, Chem. Res. 49 (2016) 2478. [58] W. Wu, G.C. Bazan, B. Liu, Chemistry 2 (2017) 760. [59] Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y. Cao, Nat. Photon. 6 (2012) 591. [60] J. Borges, J.F. Mano, Chem. Rev. 114 (2014) 8883. [61] F.-X. Xiao, M. Pagliaro, Y.-J. Xu, B. Liu, Chem. Soc. Rev. 45 (2016) 3088. [62] X. Liu, Z. Chen, R. Xu, R. Zhang, Z. Hu, F. Huang, Y. Cao, Small Methods 2 (2018) 1700407. [63] D. Mao, J. Wan, J. Wang, D. Wang, Adv. Mater. (2018) 1802874. [64] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2004) 3693. [65] P. Hu, C. Chen, R. Zeng, J. Xiang, Y. Huang, D. Hou, Q. Li, Y. Huang, Nano Energy 50 (2018) 376. [66] X.-Y. Lai, C.-R. Wang, Q. Jin, R.-B. Yu, D. Wang, Sci. China Mater. 58 (2015) 192. [67] S. Cao, J. Yu, J. Phys. Chem. Lett. 5 (2014) 2101. [68] C. Ye, J.-X. Li, Z.-J. Li, X.-B. Li, X.-B. Fan, L.-P. Zhang, B. Chen, C.-H. Tung, L.-Z. Wu, ACS Catal. 5 (2015) 6973. [69] L.-Z. Wu, B. Chen, Z.-J. Li, C.-H. Tung, Acc. Chem. Res. 47 (2014) 2177. [70] X. Zhang, B. Peng, S. Zhang, T. Peng, ACS Sustain. Chem. Eng. 3 (2015) 1501. [71] S. Guo, Z. Deng, M. Li, B. Jiang, C. Tian, Q. Pan, H. Fu, Angew. Chem. Int. Ed. 128 (2016) 1862. [72] Z. Wu, C. Sun, S. Dong, X.-F. Jiang, S. Wu, H. Wu, H.-L. Yip, F. Huang, Y. Cao, J. Am. Chem. Soc. 138 (2016) 2004. [73] Z. Hu, Z. Chen, K. Zhang, K. Zheng, R. Xie, X. Liu, X. Yang, F. Huang, Y. Cao, Sol. RRL 1 (2017) 1700055. [74] R. Yang, A. Garcia, D. Korystov, A. Mikhailovsky, G.C. Bazan, T.-Q. Nguyen, J. Am. Chem. Soc. 128 (2006) 16532. [75] Z. Hu, R. Xu, S. Dong, K. Lin J. Liu, F. Huang, Y. Cao, Mater. Horiz. 4 (2017) 88. [76] J. Huang, K.L. Mulfort, P. Du, L.X. Chen, J. Am. Chem. Soc. 134 (2012) 16472. [77] P. Liu, Y. Zhao, R. Qin, S. Mo, G. Chen, L. Gu, D.M. Chevrier, P. Zhang, Q. Guo, D. Zang, B.-H. Wu, G. Fu, N. Zheng, Science 352 (2016) 797. [78] J. Qi, X. Lai, J. Wang, H. Tang, H. Ren, Y. Yang, Q. Jin, L. Zhang, R. Yu, G. Ma, Z. Su, H. Zhao, D. Wang, Chem. Soc. Rev. 44 (2015) 6749. [79] X.-B. Li, Y.-J. Gao, Y. Wang, F. Zhan, X.-Y. Zhang, Q.-Y. Kong, N.-J. Zhao, Q. Guo, H.-L. Wu, Z.-J. Li, Y. Tao, J.-P. Zhang, B. Chen, C.-H. Tung, L.-Z. Wu, J. Am. Chem. Soc. 139 (2017) 4789. [80] G. Leem, Z.A. Morseth, E. Puodziukynaite, J. Jiang, Z. Fang, A.T. Gilligan, J.R. Reynolds, J.M. Papanikolas, K.S. Schanze, J. Phys. Chem. C 118 (2014) 28535. [81] R. Godin, Y. Wang, M.A. Zwijnenburg, J. Tang, J.R. Durrant, J. Am. Chem. Soc. 139 (2017) 5216. [82] K.L. Corp, C.W. Schlenker, J. Am. Chem. Soc. 139 (2017) 7904.
Xi Zhang received his B. S. degree in materials in 2017 from South China University of Technology. He is currently working as a Ph.D candidate in Prof. Fei Huang's group at State Key Laboratory of Luminescent Material and Devices, SCUT. His current research focuses on the design and synthesis of conjugated polymers for photocatalytic water splitting.
Qingwu Yin received his BS degree in 2015. He is a Ph.D candidate now in the State Key Laboratory of Luminescent Materials and Devices, SCUT. His research focuses on the device and physics of high efficiency polymer solar cells.
Xiaocheng Liu received his BS degree from Lanzhou University in 2016. He is a master student in the State Key Laboratory of Luminescent Materials and Devices, SCUT. His research interests focus on novel organic functional materials for polymer solar cells.
Dr. Xiao-Fang Jiang obtained her PhD degree from National University of Singapore in 2015. After that, she worked as a senior postdoc. fellow in State Key Laboratory of Luminescent Materials and Devices from SCUT. Currently, she is associate professor in School of Physics and Telecommunication Engineering from South China Normal University. Her research interests focus on developing ultrafast laser techniques such as transient absorption spectroscopy to characterize and understand the photophysics in molecular and nanocomposites, nonlinear optical properties of novel plasmonic nanostructures for applications in bio-sensing, bio-imaging and optoelectronic devices.
782
Nano Energy 60 (2019) 775–783
Z. Hu, et al. Zhiming Chen received his BS degree in 2014. He is a Ph.D. candidate now in the State Key Laboratory of Luminescent Materials and Devices from SCUT since 2015. His research focuses on the device and physics of high efficiency polymer solar cells.
Fei Huang received his B.S. degree in chemistry from Peking University in 2000 and gained his Ph.D. degree in materials science from the SCUT in 2005 under the supervision of Prof. Yong Cao. After postdoctoral work at the University of Washington with Prof. Alex K.-Y. Jen, he began his academic career in 2009 as a professor at the SCUT. His main interests are in the fields of organic functional materials and devices for optoelectronics.
Xiye Yang received his BS degree in Mechanical Engineering from South China University of Technology in 2014 and gained his PhD degree in Materials Science from the SCUT in 2018 under the supervision of Prof. Fei Huang. He is currently working as a postdoc fellow in Prof. Xiaohui Wang's group in State Key Laboratory of Pulp & Paper Engineering from SCUT. His current research focuses design and synthesis of supramolecular based novel functional materials towards optoelectronic applications.
Yong Cao was appointed as Professor of Institute of Polymer Optoelectronic Materials and Devices at SCUT in 1999. He obtained his bachelor's degree in chemistry at the former Leningrad University (Russia) in 1965 and his Ph.D. degree from Tokyo University at 1987. He was previously professor at Institute of Chemistry, Chinese Academy of Science, 1986–1988; visiting scientist at Institute of Polymer and Organic Solids, UCSB, 1988–1990, and senior scientist at the Uniax Corporation, 1990–1999. He has been a member of the Chinese Academy of Science since 2001. His research interests are organic/polymer optoelectronic materials and devices.
783
Contents lists available at ScienceDirect
Nano Energy journal homepage: www.elsevier.com/locate/nanoen
Full paper
Highly efficient photocatalytic hydrogen evolution from water-soluble conjugated polyelectrolytes
T
Zhicheng Hua,b, Xi Zhanga, Qingwu Yina, Xiaocheng Liua, Xiao-fang Jianga, Zhiming Chena, Xiye Yanga, Fei Huanga,b,∗, Yong Caoa a
Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, PR China b South China Institute of Collaborative Innovation, Dongguan, 523808, PR China
ARTICLE INFO
ABSTRACT
Keywords: Conjugated polyelectrolytes Photocatalytic hydrogen evolution Organic photocatalysts Charge transfer
Organic semiconductors for photocatalytic application have received wide attention recently. However, developing organic photocatalysts for highly efficient photocatalytic hydrogen evolution is a great challenge. Herein, we show highly efficient photocatalytic hydrogen evolution from a series of novel conjugated polyelectrolytes (CPEs) by modulating their interaction with co-catalysts. The water-soluble CPEs greatly enhance the photocatalytic performance with 50-fold improvement over that of their non-dissolved precursor conjugated polymer. More importantly, the photocatalytic activity of these CPEs can be optimized by regulating their interaction with Pt co-catalysts through molecular engineering on the side chain species and counterions of CPEs. The cationic CPEs perform better than anionic CPEs due to the robust interaction between quaternary ammonium side chains in cationic CPEs and Pt co-catalysts. It is found that the optimized interface contact between cationic CPEs and Pt co-catalysts can result in more efficient charge transfer and higher photocatalytic activity.
1. Introduction Hydrogen is regarded as a clean energy source that can replace fossil fuels to address rising global environmental concerns [1]. The rational design of high-performance photocatalytic materials for hydrogen evolution remains an important challenge [2–7]. Organic semiconductors for photocatalysis have received significant recent attention due to their easily tunable features via facile molecular engineering [8–10]. The light-harvesting capability and electronic band structures of organic semiconductors can be well-regulated to enable versatile photo-redox applications (including water reduction/oxidation and CO2 reduction) and provide a good template to closely mimic natural photosynthesis [11–17]. Recently, organic materials, including graphitic carbon nitrides (gC3N4) [18–26], covalent organic frameworks [27] and porous/linear conjugated polymers [28–45] have emerged as efficient photocatalysts to drive hydrogen evolution. The properties of organic materials, including solubility, energy level, absorption, porosity, doping etc., can be fine-tuned by chemical structure evolution, which have led to the widespread study of these materials for photocatalytic hydrogen evolution [8–10,46,47]. However, several challenges remain in the
development of organic photocatalysts, which must be addressed to further increase their photocatalytic activity. First, the exact structure of most polymers (such as g-C3N4-based materials) remains unclear, which is a barrier to the precise understanding of chemical structurephotocatalytic performance relationships. In addition, organic conjugated polymers are suffering from short exciton diffusion length and low carrier mobility, resulting in high recombination rate of charge carriers and hinder the improvement of their photocatalytic activity [8]. Consequently, it is highly desirable to enable organic photocatalysts with good water dispersity/solubility to shorten the path of photo-induced excitons emigrating to the solid-liquid interface and high mobility/conductivity to reduce the exciton recombination inside polymers. Moreover, an important criterion for high photocatalytic activity is rapid charge transfer from organic photocatalysts to metal co-catalysts [8,19,44]. However, precise modulation of the interaction between the organic photocatalysts and metal co-catalysts has rarely been attempted. Conjugated polyelectrolytes (CPEs) are a class of conjugated polymers with ionic functionalities attached to their backbones that have been widely applied in chemical biosensors, imaging, and optoelectronic devices [48–52]. The optoelectronic properties of CPEs can be
∗ Corresponding author. Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou, 510640, PR China. E-mail address: [email protected] (F. Huang).
https://doi.org/10.1016/j.nanoen.2019.04.027 Received 24 November 2018; Received in revised form 2 April 2019; Accepted 5 April 2019 Available online 09 April 2019 2211-2855/ © 2019 Elsevier Ltd. All rights reserved.
Nano Energy 60 (2019) 775–783
Z. Hu, et al.
precisely regulated via chemical synthesis and structural evolution. The ionic side chains also endow CPEs with good dispersibility/solubility [51,52] which are highly desirable characteristics for organic photocatalysts [35,53–55]. More importantly, polar side chains in CPEs can interact with metal electrodes, resulting in improved charge transport/ transfer from conjugated polymers to metal electrodes [56–59]. CPEs with ionic side chains can bind with oppositely charged molecules/ metal nanoparticles through electrostatic interaction [60–63]. Consequently, the interaction between CPEs and metal co-catalysts might be well-regulated, which is rarely reported but potentially boosting the photocatalytic activity of tailor-made CPEs-metal co-catalysts for hydrogen evolution. Herein, we demonstrate a series of novel CPE-based organic photocatalysts, capable of highly efficient hydrogen evolution by optimizing their interaction with co-catalysts. Compared to their non-dissolved precursor conjugated polymer, the ionic side chains functionalized CPEs greatly boost the photocatalytic performance with 50-fold improvement. Moreover, the interaction between CPEs and Pt co-catalysts was well-regulated by altering their side chain species and counterions. Our study indicates that both side chain species and counterions of CPEs can largely determine their photocatalytic activity. The cationic CPEs perform better than anionic CPEs and oversized cations/anions in these CPEs can impede the charge transfer from CPEs to Pt co-catalysts and decrease the photocatalytic activity. Further study indicates that the quaternary ammonium-like side chains in cationic CPEs act as binding centers to interact with Pt co-catalysts, resulting in more efficient charge transfer from CPEs and Pt co-catalysts, which was evidenced by the observation from fs-TAS spectroscopy. The demonstrated knowledge of CPEs and their interaction with co-catalysts allows us to formulate novel design guidelines to engineer organic photocatalysts system to achieve high-performance hydrogen evolution.
Table S1), implying the gradually reduced thermodynamical driving force for water reduction. To fully explore the potential of these CPEs, we selected two widely adopted sacrificial agents: triethanolamine (TEOA) and ascorbic acid (AA) for photocatalytic hydrogen evolution. The photocatalytic activities of these polymers for hydrogen evolution from water were investigated using Pt co-catalysts (3 wt% of CPE, Pt co-catalysts were prepared by dissolving H2PtCl4 into the CPEs reaction with irradiation for 1 h to enable the formation of Pt cocatalysts [36]) in TEOA/H2O and AA/H2O solution. The neutral amine-functionalized PFN can only be dispersed, not dissolved, in TEOA/H2O and forms an opaque colloidal solution, whilst the cationic CPEs are readily dissolved in TEOA/H2O and forms a clear micelle solution (Fig. S4). The photocatalytic hydrogen evolution rates (HERs) of these CPEs as a function of time are presented in Fig. 2a–c. The photocatalytic experiments showed that PFN delivered a HER of 0.23 μmol h−1, similar to that of previously reported conjugated polymers with non-polar side chains and similar absorption spectra [37]. Surprisingly, PFN-Br, which has ionic side chains, achieved substantial 50-fold higher HER of 11.50 μmol h−1. The effect of Pt co-catalysts weight ratio on the photocatalytic performance was also investigated (Fig. S7). An optimized 3.0 wt% of Pt co-catalysts can enable the highest HER (11.50 μmol h−1) of PFN-Br. It should be noted that PFN-Br without Pt co-catalysts exhibited an apparently lower HER of 1.70 μmol h−1. The much-enhanced photocatalytic activity of PFN-Br indicates that the introduction of ionic side groups and Pt cocatalysts plays an important role in the improvement of the photocatalytic process. Because PFN and PFN-Br share similar electron affinities (Table S1), light-harvesting capabilities (Figs. S5–S6) and even comparable lengths of side chains, we attribute the enhanced photocatalytic activities of PFN-Br over that of PFN to the much better dispersibility/solubility of the former. It has been widely accepted that the interface between organic photocatalysts and water showed large impact on the photocatalytic activity [9]. Since the exciton diffusion length of conjugated polymers is limited (< 30 nm) [64] large interface area between polymers and water will provide more active sites for cocatalysts loading [9,65,66] and shorten migration distance of charge carriers. In our case, the CPEs possess high solubility in water and formed a clear micelle solution, thus it can be regarded that interface area between CPEs and water are greatly enlarged in comparison with that between other non-dissolved conjugated polymers (such as PFN) and water. The better solubility of CPEs in water must have greatly shortened the path for excitons in CPEs to migrate to the interface between polymers and water, resulting in decreased recombination inside the CPEs and increased photocatalytic activity [67]. The photocatalytic HERs of other CPEs with broadened absorption were also tested in TEOA/H2O. Unexpectedly, PFNBT-Br, PFNDTBT-Br and PFNDPP-Br showed clearly lower photocatalytic hydrogen evolution activity in TEOA/H2O than PFN-Br. The HERs of PFNBT-Br, PFNDTBT-Br and PFNDPP-Br were reduced to 3.90 μmol h−1, 0.20 μmol h−1 and 0.138 μmol h−1, espectively (Fig. 2a and c). The extremely lower photocatalytic activity of PFNDTBT-Br and PFNDPP-Br in TEOA/H2O can be easily understood from their lower LUMO energy levels, which are very close to the reduction potential of water (Fig. 2d), providing limited driving force for water reduction [68]. However, with AA as the sacrificial agent, it can be observed that photocatalytic HERs of PFNDTBT-Br and PFNDPP-Br were much enhanced. The pH values of TEOA/H2O and AA/H2O solution were estimated to be 10.87 and 4.0, respectively. Using the Nernst equation, E(H+/H2) = -0.059 pH (V vs NHE), the reduction potentials of TEOA/H2O and AA/H2O were thus calculated to be -0.641 V and -0.236 V (vs NHE), respectively. The water reduction of AA/H2O is 0.4 V more positive than that of TEOA/ H2O solution, suggesting a larger driving force between the CPEs and the water reduction potential (Fig. 2d) (especially for PFNDTBT-Br and PFNDPP-Br with lower LUMO energy levels) [69,70]. Expectedly, the HERs of PFNDTBT-Br and PFNDPP-Br in AA/H2O were appreciably enhanced. PFNDTBT-Br showed an initial photocatalytic HER of
2. Results and discussion 2.1. Photocatalytic activity of cationic CPEs CPEs possess cationic or anionic side chains attached to their backbones, which endow them with good solubility in polar solvents. Here, we first prepared a series of polyfluorene-based cationic CPEs with a quaternary ammonium bromide salt as templates to explore the potential application of CPEs in photocatalytic hydrogen evolution. (The experimental procedure and characterization are shown in Support Information, Figs. S1–S6). Fluorene monomer and three electron-withdrawing moieties, benzo[c] [1,2,5]thiadiazole (BT), 4,7-di (thiophen-2-yl)benzo[c] [1,2,5]thiadiazole (DTBT) and 2,5-bis(2- butyloctyl)-3,6-di(thiophen-2-yl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4dione (DPP), were used to construct D-A conjugated polymers in combination with a fluorene moiety. For comparison, PFN (Fig. 1a) with neutral amino groups was also prepared. After quaternarization procedures, CPEs (PFN-Br, PFNBT-Br, PFNDTBT-Br and PFNDPP-Br, Fig. 1a) with successively extended absorption spectra and varied energy levels were obtained. Compared with PFN, which is soluble only in non-polar or low-polar solvents (such as toluene and chloroform), cationic CPEs tend to be highly soluble in water/alcohol-like polar solvents. With increasing electron-withdrawing capability of the copolymerizing moieties, PFNBT-Br, PFNDTBT-Br and PFNDPP-Br showed increasingly strong intramolecular charge transfer bands, with gradually red-shifted absorption edges to 500 nm, 645 nm and 715 nm in methanol, respectively, indicating the enhanced light-harvesting capability and potential photocatalytic activity of PFNBT-Br, PFNDTBT-Br and PFNDPP-Br in the visible range (Fig. 1b). The optical band gaps of these CPEs were determined to be 2.33 eV, 1.93 eV and 1.72 eV, respectively, calculated from their thin-film absorption edges (Fig. S6). The lowest unoccupied molecular orbital (LUMO) energy levels of these CPEs (PFN-Br, PFNBT-Br, PFNDTBT-Br and PFNDPP-Br) were estimated to be -2.41 eV, -3.16 eV, -3.53 eV and -3.83 eV (vs Evacuum, Fig. 1c and 776
Nano Energy 60 (2019) 775–783
Z. Hu, et al.
Fig. 1. Chemical structures (a), UV-vis absorption spectra (in methanol) (b) and energy level diagram (c) of PFN-Br, PFNBT-Br, PFNDTBT-Br and PFNDPP-Br. Fig. 2. (a) Photocatalytic HERs of CPEs as a function of time (Reaction condition: 2.5 mg CPEs dissolved in 50 mL TEOA/H2O (1/9, v/v) solution); (b) Photocatalytic HERs of CPEs as a function of time (Reaction condition: 2.5 mg CPEs dissolved in 50 mL H2O with 0.2 M AA as the sacrificial agent, pH = 4.0); (c) Summary of HERs of CPEs. (d) Schematic energy diagram of PFNDTBT-Br and PFNDPP-Br and water reduction potentials.
777
Nano Energy 60 (2019) 775–783
Z. Hu, et al.
Fig. 3. (a) Summary of electron mobilities of CPEs. (b) Absorption spectrum of PFNDPP-Br and AQY values of PFNDPP-Br as a function of light wavelength.
Scheme 1. Chemical structures of cationic and anionic CPEs with different counter cations/anions.
44.6 μmol h−1 and an average HER of 30.58 μmol h−1 in 5 h (Fig. 2b–c). For PFNDPP-Br, with DPP as the copolymerizing unit, the photocatalytic stability was improved. PFNDPP-Br delivered an efficient average HER of 27.90 μmol h−1, with almost no decline in photocatalytic activity. Another observation in these CPEs is that PFN-Br showed low HER of 0.85 μmol h−1 in AA/H2O. The lower highest occupied molecular orbital (HOMO) energy level of PFN-Br in comparison with PFNDTBT-Br and PFNDPP-Br indicates an increase in the oxidizing capability of the hole might be achieved for PFN-Br [27]. In addition to the above-discussed factors of the sacrificial agent and energy levels, the charge transporting properties of CPEs might be another important determinant of their photocatalytic activity. In the water reduction process, the generated electrons in the CPEs must quickly migrate from polymers to Pt co-catalysts, thus requiring the CPEs to possess high electron mobilities to achieve high photocatalytic activity. However, the electron mobilities of PFN-Br and PFNBT-Br (6.33 × 10−7 cm2 V−1 s−1 and 3.62 × 10−6 cm2 V−1 s−1) are much lower than those of PFNDTBT-Br and PFNDPP-Br (1.17 × 10−4 cm2 V−1 s−1 and 8.13 × 10−5 cm2 V−1 s−1), respectively (Fig. 3a and Fig. S8). The much-enhanced electron mobilities in PFNDTBTBr and PFNDPP-Br must have reduced the recombination rate inside the CPEs and improved the photocatalytic activity [71]. Furthermore, it has been reported that the more stable radical anion in conjugated polymers might enhance the charge separation, thereby increasing the probability of electron migration to a nearby Pt co-catalyst [27]. Indeed, radical anions can be easily introduced into CPEs and tuned via molecular engineering of the side chains and backbones [72,73]. In our case, from the electron spin resonance (ESR) results (Fig. S9) of the CPEs, it can be deduced that PFNDPP-Br and PFNDTBT-Br can form more stable radical anions than those of PFN-Br and PFNBT-Br, in accordance with their photocatalytic performance. These results indicate that CPEs are not only promising
photocatalysts for hydrogen evolution but also a good model to explore the design principles of organic photocatalysts. The apparent quantum yield (AQY) values of the photocatalytic performance of PFNDPP-Br as a function of wavelength were also collected (Fig. 3b). At 550, 600, 650 and 700 nm, PFNDPP-Br rendered AQY values of 0.12%, 0.40%, 0.44% and 0.19%, respectively, indicating a broader photoresponse for PFNDPP-Br. These results validate our rational design strategy to realize high-performance photocatalytic evolution in the visible range. The results of longer-term photocatalytic experiments (Fig. S10) showed that an average HER of around 17.5 μmol h−1 was maintained after a 20-h photocatalytic reaction. Moreover, the absorption spectra of PFNDPP-Br before and after a 20-h photocatalysis (Fig. S11) showed no difference, indicating that PFNDPP-Br is a promising organic photocatalyst for long-term use. 2.2. Side chains and counterions effect As a crucial component of CPEs, the counterions strongly determine their aggregation, doping behavior and charge transport properties, which are central to their application in organic electronics [74,75]. Modulating the side chains and ionic species can effectively alter the interaction between conjugated polymers and metal co-catalysts, thus affecting the charge transfer between conjugated polymers to metal cocatalysts [19,76]. Herein, cationic CPEs (PFN-BIm4 and PFN-BF4, Scheme 1) balanced by tetra(1H-imidazol-1-yl) borate anions (BIm4- ) and tetrafluoroborate anions (BF4−) were prepared via ion-exchange. X-ray photoelectric spectroscopy (XPS) results of PFN-BIm4 and PFNBF4 powders (Fig. S1) confirmed that all the bromide anions have been exchanged into BIm4- and BF4−. PFN-BIm4 and PFN-BF4 showed similar light-harvesting capabilities to their precursor, PFN-Br, as indicated by their UV-vis absorption spectra (Figs. S5–S6). Likewise, we prepared 778
Nano Energy 60 (2019) 775–783
Z. Hu, et al.
Fig. 4. (a) Photocatalytic HERs of PFN-Br, PFN-BF4 and PFN-BIm4 as a function of time. (b) Photocatalytic HERs of PFS-Na, PFS-NMe and PFS-NBu as a function of time; (Reaction condition: 2.5 mg CPEs dissolved in 50 mL TEOA/H2O (1/9) solution) (c) Summary of HERs of CPEs with different cations/anions; (d) Zeta potentials of PFN-Br solution, PFS-Na solution, Pt cocatalysts in TEOA/H2O solution.
anionic CPEs (PFS-Na, PFS-NMe and PFS-NBu, Scheme 1) balanced by sodium cations, tetramethylammonium cations and tetrabutylammonium cations, which presented similar optical properties and energy levels (Figs. S5–S6 and Table S1). The 1H-NMR spectra of PFS-NMe and PFS-NBu indicated that almost all Na+ have been exchanged into N+Me4 and N+Bu4. Photocatalytic tests of these CPEs (Fig. 4a–c) in TEOA/H2O showed that PFN-BF4 delivered similar photocatalytic activity to that of PFN-Br, reaching a HER of 12.05 μmol h−1. Unexpectedly, PFN-BIm4 showed a much lower HER of 5.65 μmol h−1. The molar mass of the BIm4- counter anion is larger than that of PFN-Br; thus, the low HER of PFN-BIm4 can be partly attributed to the larger molar mass of the BIm4- counter anions, which decrease the weight ratio of the conjugated backbones per gram. The HERs of CPEs with different copolymer units were analyzed in greater detail to exclude the effect of molecular weight per unit (Table S2). It was found that the photocatalytic activity of PFN-BIm4 was much lower than that of PFN-Br and PFN-BF4, implying that the oversized BIm4- counter anions [74] severely hindered the photocatalytic performance of the CPEs. Dynamic light scattering (DLS) was conducted to investigate the aggregation state of the CPEs in solution. In fact, the aggregation degree of PFN-BIm4 in solution was between those of PFN-Br and PFN-BF4, implying that the aggregation state of these CPEs was not the main determinant of their photocatalytic activity (Fig. S12). Turning to the anionic CPEs, PFS-Na showed a HER of 0.59 μmol h−1, which is higher than that of PFN, but much lower than that of PFN-Br. PFS-NMe showed a HER of 0.3375 μmol h−1, lower than that of PFS-Na, whilst PFS-NBu showed even further reduced photocatalytic activity with a HER of 0.017 μmol h−1 (Fig. 4b–c). These results suggest that the photocatalytic activity of PFS-based CPEs gradually decreases with increasing size of the counter cations, a trend that is consistent with that observed for the PFN-series of CPEs. In particular, PFS-NBu with oversized counter anions showed poor photocatalytic activity, an analogous situation to PFN-BIm4, suggesting that oversized counterions impede the charge transfer from CPEs to Pt co-catalysts and reduce the photocatalytic activity. These results indicate that the
rational design of counterions is essential to the photocatalytic activity of CPEs. 2.3. Interaction between CPEs and Pt co-catalysts Although both cationic and anionic CPEs showed better photocatalytic activity than neutral PFN, the much lower photocatalytic activity of PFS-based than PFN-based CPEs is worthy of further exploration. Cationic CPEs and anionic CPEs are balanced by counterions with different charges, thus exhibiting varied interaction with metal cocatalysts with opposite charges. Besides, considering that the photocatalytic conditions and backbones of these CPEs were the same, the difference in their HERs suggests that the different side chains attached to their backbones play an important role in their photocatalytic hydrogen generation. It has been reported that the positively charged surfaces of semiconductors could absorb metal-based anions (such as PdCl42−), enhancing the stability and photocatalytic performance of dispersed metal catalysts [77]. Here, the cationic PFN-Br might likewise have absorbed PtCl42− on the surface in the early stage of formation of Pt nanoparticles, resulting in close contact between the CPEs and Pt cocatalysts (Fig. S13 and Fig. S14). The interaction between PFN-Br and PtCl42− was also evidenced by the finding from titration experiments that PtCl42− caused severe aggregation and precipitation of PFN-Br in concentrated solution, whilst no aggregation and precipitation were observed in PFS-Na solution (Fig. S13). To study the interaction between the CPEs and Pt co-catalysts, the zeta potentials of PFN-Br and PFS-Na and the Pt nanoparticles were tested (Fig. 4d and Fig. S15). The zeta potentials of PFN-Br and PFS-Na solution were estimated to be 35.7 mV and -69.4 mV (Fig. 4d), respectively, indicating that the net electrical charge of PFN-Br was positive, whilst that of PFS-Na was negative. The zeta potential of the Pt co-catalysts in TEOA/H2O solution was estimated to be -42.7 mV, suggesting a net negative charge. Thus it can be deduced that the Pt cocatalysts were preferentially adsorbed onto PFN-Br via electrostatic interaction, allowing closer interface contact and more efficient charge 779
Nano Energy 60 (2019) 775–783
Z. Hu, et al.
transfer from the backbones of the latter to the former [78]. The close intimate contact between PFN-Br and Pt cocatalysts was also evidenced by FT-IR results, of which the bending vibration of quaternary ammonium groups in PFN-Br shifted obviously (Fig. S16). It should be noted that similar zeta potential can be also observed in Pt co-catalysts in AA/ H2O (Fig. S15), indicating similar preferentially adsorption occurred in PFN-Br and Pt co-catalysts. Efficient charge transfer from photocatalysts to co-catalysts must have enhanced the electronic communication between the two, thus boosting the photocatalytic activity; such behavior has been observed in a CdS/Pt-based inorganic photocatalytic system [79]. In contrast, the repulsion between the Pt cocatalysts and PFS-Na would lead to poor interface contact (Fig. S17), leading to less efficient charge transfer between the two and thus poor photocatalytic activity. The more efficient charge transfer between PFN-Br and Pt co-catalysts was also confirmed by the results of timeresolved photoluminescence kinetics, showing that PFN-Br with Pt cocatalysts delivered a shorter lifetime than that of PFS-Na with Pt cocatalysts, indicating the stronger and faster photoluminescence quenching in the former case (Fig. S18). These results demonstrate that rational design of polar side chains can enable intimate contact between organic photocatalysts and metal co-catalysts. Femtosecond transient absorption spectroscopy (fs-TAS) was used to further study the interaction between the CPEs and Pt co-catalysts. As shown in Fig. 5a, both PFN-Br and PFS-Na solution exhibit broad positive induced absorption features from 500 nm to NIR region, which can be attributed to the overlapping electron and hole absorption of polyfluorene [80,81]. To elucidate the nature of this excited state, fsTAS measurements of PFN-Br and PFS-Na solution with hole scavenger TEOA were performed. fs-TAS experiments showed an initial increase in the photoinduced positive absorption feature above 650 nm, attributing primarily to photogenerated electrons [81] in the polyfluorene backbones. The corresponding TAS kinetics, probed at peak of 720 nm, of PFN-Br and PFS-Na solutions with TEOA (Fig. 5b) showed a longer lifetime compared with those in pristine PFN-Br and PFS-Na solutions (Fig. S19), suggesting a much-suppressed charge recombination in the former due to the hole scavenging by TEOA [81]. Moreover, it was also observed that PFS-Na with TEOA showed a longer lifetime than PFN-Br with TEOA, indicating faster hole scavenging in PFS-Na. This can be attributed to the better capability of PFS-Na, compared with PFN-Br, to achieve intimate contact with both water and TEOA (Fig. S20). In the TAS kinetics probed from 1000 to 1050 nm, a signal attributed to electron transfer appeared [82]. Moreover, PFN-Br solution with Pt cocatalysts showed faster quenching and shorter lifetime than PFS-Na solution with Pt co-catalysts (Fig. 5c), indicating that the former achieved a more efficient electron transfer to Pt co-catalysts, which coincides well with the above observations of the photocatalytic performances. Comparing the photocatalytic performance of PFN-Br and PFS-Na, it is obvious that the charge transfer from organic photocatalysts to Pt co-catalysts is vital to the overall photocatalytic
performance, and that it is possible to regulate the polar side chains of CPEs to achieve high photocatalytic activity. 3. Conclusion In summary, we have demonstrated a series of novel high-performance organic photocatalysts for hydrogen evolution. The introduction of ionic side chains into conjugated polymers greatly improves the dispersibility/solubility of CPEs in water and their photocatalytic activity with 50-fold improvement. By molecular engineering of the backbone structures of the CPEs, highly efficient photocatalytic hydrogen evolution in the visible range was achieved. It was shown via side chain engineering that quaternary ammonium side chains interact robustly with Pt co-catalysts, leading to more efficient charge transfer and higher photocatalytic activity. The size of the counterions is a key concern when designing novel organic photocatalysts. Overall, CPEs provide a good template for methodical investigation of organic photocatalysts due to their well-defined and easily tunable chemical structures, good dispersibility and interface wettability with water, strong interaction with metal co-catalysts via binding center and high photocatalytic activity. Our intensive photocatalytic study on these novel conjugated polyelectrolytes and their photocatalytic performance provides novel design guidelines for high-performance organic photocatalysts for hydrogen evolution. 4. Methods 4.1. Synthesis and characterization of polymers Synthetic procedures of the CPEs are described in Support Information. The 1H-NMR data of the monomers and polymers were recorded on a Bruker AVANCE Digital NMR workstation operating at 500 MHz. Electrochemical cyclic voltammetry (CV) was conducted on a CHI-660E electrochemical workstation equipped with a glassy carboncoated electrode. A saturated calomel electrode (SCE) was used as the reference electrode, and a Pt sheet was used as the counter electrode. The CV was conducted in acetonitrile solution (containing 0.1 M Bu4NPF6) at a scan rate of 0.05 V s−1 at room temperature under the protection of nitrogen. The potential of ferrocene/ferrocenium (Fc/ Fc+) reference was measured to be 0.32 V versus SCE. Thus, the LUMO and HOMO energy levels of the CPEs were calculated with the following equations: ELUMO = -e(Ere + 4.48) (eV), EHOMO = -e(Eox + 4.48) (eV), respectively. The molecular weights of the neutral polymers were determined by a Waters GPC 2410, where CHCl3 was used as the mobile phase. The molecular weights of the CPEs were calculated from their neutral precursor polymers. UV-vis absorption spectra were collected from an HP 8453 spectrophotometer. The fluorescence decay curves were recorded on an Edinburgh Instruments FLS920 combined with steady-state and time-resolved fluorescence spectrometers. The pH Fig. 5. (a) fs-TAS spectra of PFN-Br and PFS-Na solution in water (50 μg/mL, with/ without 10% TEOA and 3% Pt co-catalysts) taken at a delay time of 2 ps following pulsed 400 nm excitation (20 μW); (b) quencher-dependent TAS kinetics probed at 720 nm for PFN-Br and PFS-Na solution with TEOA; (c) quencher-dependent TAS kinetics probed at 1000 to 1050 nm for PFN-Br and PFS-Na solution with TEOA and Pt co-catalysts.
780
Nano Energy 60 (2019) 775–783
Z. Hu, et al.
values of the reaction solution were determined by a pH-meter (PHS-25, Rex, China). The zeta-potentials were measured on a Nano-ZS zetasizer (HORIBA Instruments, Japan). PFN-Br and PFS-Na were dissolved in deionized H2O (50 μg/mL) and stored for 24 h before testing. Solutions of Pt nanoparticles in TEOA/H2O and AA were prepared by dissolving H2PtCl4 and irradiated for 1 h to enable the formation of Pt nanoparticles. Transmission electron microscopy was performed on a TF20 Joel 2100F (FEI) transmission electron microscope. The samples were prepared by dropping the reaction solution onto 300-mesh carboncoated copper grids and evaporating the solvent immediately. Electrononly devices with the architecture of ITO/Al/CPEs/Al were prepared and the electron mobilities of the CPEs were extracted by fitting the data using the space-charge-limited current (SCLC) model. Electron spin resonance spectroscopy of the CPEs (in the solid state) was conducted on a JEOL JES-FA200 ESR spectrometer (300 K, 9.063 GHz, Xband). FT-IR spectra were carried out with a NEXUS 670 instrument.
Acknowledgment
4.2. Hydrogen evolution experiments
[1] M.G. Walter, E.L. Warren, J.R. McKone, S.W. Boettcher, Q. Mi, E.A. Santori, N.S. Lewis, Chem. Rev. 110 (2010) 6446. [2] S. Chen, T. Takata, K. Domen, Nat. Rev. Mater. 2 (2017) 17050. [3] C. Ding, J. Shi, Z. Wang, C. Li, ACS Catal. 7 (2017) 675. [4] Z. Yu, F. Li, L. Sun, Energy Environ. Sci. 8 (2015) 760. [5] Z. Li, W. Luo, M. Zhang, J. Feng, Z. Zou, Energy Environ. Sci. 6 (2013) 347. [6] D. Wang, R. Yu, Y. Chen, N. Kumad, N. Kinomur, M. Takano, Solid State Ionics 172 (2004) 101. [7] X. Lai, J.E. Halperta, D. Wang, Energy Environ. Sci. 5 (2012) 5604. [8] W.-J. Ong, L.-L. Tan, Y.H. Ng, S.T. Yong, C.-P. Chai, Chem. Rev. 116 (2016) 7159. [9] V.S. Vyas, V.W. Lau, B.V. Lotsch, Chem. Mater. 28 (2016) 5191. [10] G. Zhang, Z.-A. Lan, X. Wang, Angew. Chem. Int. Ed. 55 (2016) 15712. [11] P.D. Frischmann, K. Mahata, F. Würthner, Chem. Soc. Rev. 42 (2013) 1847. [12] S. Wang, L. Yi, J.E. Halpert, X. Lai, Y. Liu, H. Cao, R. Yu, D. Wang, Y. Li, Small 8 (2012) 265. [13] N. Yang, Y. Liu, H. Wen, Z. Tang, H. Zhao, Y. Li, D. Wang, ACS Nano 7 (2013) 1504. [14] S.J.A. Moniz, S.A. Shevlin, D.J. Martin, Z.X. Guo, J. Tang, Energy Environ. Sci. 8 (2015) 731. [15] D.L. Ashford, M.K. Gish, A.K. Vannucci, M.K. Brennaman, J.L. Templeton, Chem. Rev. 115 (2015) 13006. [16] A. Iwase, S. Yoshino, T. Takayama, Y.H. Ng, R. Amal, A. Kudo, J. Am. Chem. Soc. 138 (2016) 10260. [17] Y. Zhao, H. Tang, N. Yang, D. Wang, Adv. Sci. 5 (2018) 1800959. [18] X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J.M. Carlsson, K. Domen, M. Antonietti, Nat. Mater. 8 (2009) 76. [19] V.W. Lau, I. Moudrakovski, T. Botari, S. Weinberger, M.B. Mesch, V. Duppel, J. Senker, V. Blum, B.V. Lotsch, Nat. Commun. 7 (2016) 12165. [20] H. Wang, B. Wang, Y. Bian, L. Dai, ACS Appl. Mater. Interfaces 9 (2017) 21730. [21] C. Liu, H. Huang, L. Ye, S. Yu, N. Tian, X. Du, T. Zhang, Y. Zhang, Nano Energy 41 (2017) 738. [22] S. Yu, J. Li, Y. Zhang, M. Li, F. Dong, T. Zhang, H. Huang, Nano Energy 50 (2018) 383. [23] K.C. Christoforidis, Z. Syrgiannis, V.L. Parola, T. Montini, C. Petit, E. Stathatos, R. Godin, J.R. Durrant, M. Prato, P. Fornasier, Nano Energy 50 (2018) 468. [24] H. Wang, Y. Bian, J. Hu, L. Dai, Appl. Catal. B Environ. 238 (2018) 592. [25] W. Lei, Y. Mi, R. Feng, P. Liu, S. Hu, J. Yue, X. Liu, J.A. Rodriguez, J.-o. Wang, L. Zheng, K. Tang, S. Zhu, G. Liu, M. Liu, Nano Energy 50 (2018) 552. [26] H. Kasap, R. Godin, C. Jeay-Bizot, D.S. Achilleos, X. Fang, J.R. Durrant, E. Reisner, ACS Catal. 8 (2018) 6914. [27] V.S. Vyas, F. Haase, L. Stegbauer, G. Savasci, F. Podjaski, C. Ochsenfeld, B.V. Lotsch, Nat. Commun. 6 (2015) 8508. [28] R.S. Sprick, J.-X. Jiang, B. Bonillo, S. Ren, T. Ratvijitvech, P. Guiglion, M.A. Zwijnenburg, D.J. Adams, A.I. Cooper, J. Am. Chem. Soc. 137 (2015) 3265. [29] L. Li, Z. Cai, Q. Wu, W.-Y. Lo, N. Zhang, L.X. Chen, L. Yu, J. Am. Chem. Soc. 138 (2016) 7681. [30] C. Yang, B.C. Ma, L. Zhang, S. Lin, S. Ghasimi, K. Landfester, K.A.I. Zhang, X. Wang, Angew. Chem. Int. Ed. 55 (2016) 9202. [31] R.S. Sprick, B. Bonillo, R. Clowes, P. Guiglion, N.J. Brownbill, B.J. Slater, F. Blanc, M.A. Zwijnenburg, D.J. Adams, A.I. Cooper, Angew. Chem. Int. Ed. 28 (2016) 1824. [32] L. Li, R.G. Hadt, S. Yao, W.-Y. Lo, Z. Cai, Q. Wu, B. Pandit, L.X. Chen, L. Yu, Chem. Mater. 28 (2016) 5394. [33] L. Wang, Y. Wan, Y. Ding, S. Wu, Y. Zhang, X. Zhang, G. Zhang, Y. Xiong, X. Wu, J. Yang, H. Xu, Adv. Mater. 29 (2017) 1702428. [34] P.B. Pati, G. Damas, L. Tian, D.L.A. Fernandes, L. Zhang, I.B. Pehlivan, T. Edvinsson, M. Araujo, H. Tian, Energy Environ. Sci. 10 (2017) 1372. [35] H. Lu, R. Hu, H. Bai, H. Chen, F. Lv, L. Liu, S. Wang, H. Tian, ACS Appl. Mater. Interfaces 9 (2017) 10355. [36] J. Chen, C.-L. Dong, D. Zhao, Y.-C. Huang, X. Wang, L. Samad, L. Dang, M. Shearer, S. Shen, L. Guo, Adv. Mater. 29 (2017) 1606198. [37] D.J. Woods, R.S. Sprick, C.L. Smith, A.J. Cowan, A.I. Cooper, Adv. Energy Mater. 7 (2017) 1700479. [38] X. Zong, X. Miao, S. Hua, L. An, X. Gao, W. Jiang, D. Qu, Z. Zhou, X. Liu, Z. Sun, Appl. Catal. B Environ. 211 (2017) 98.
This work was financially supported by the Natural Science Foundation of China (No. 21634004, 21490573) and the Foundation of Guangzhou Science and Technology Project (No. 201707020019). Z. Hu thanks the support from China Postdoctoral Science Foundation (No. 2017M622684) and the Fundamental Research Funds for the Central Universities, South China University of Technology (No. D2180460). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.nanoen.2019.04.027. References
The hydrogen evolution measurements were performed on a Labsolar-IIIAG photocatalytic system (PerfectLight) equipped with a 50-mL reactor. For the photocatalytic experiment using TEOA as the sacrificial agent, CPEs (2.5 mg) were firstly dissolved in 250 μL (for PFNDPP-Br, 1 mL of methanol was used) methanol and then dispersed into 50 mL of TEOA/H2O (1:9, v/v) to obtain a clear solution. For the photocatalytic experiment using AA as the sacrificial agent, CPEs (2.5 mg) were firstly dissolved in 250 μL of methanol (for PFNDPP-Br, 1 mL of methanol was used) and then dispersed into a mixture of 50 mL of AA solution (0.2 M, pH = 4 adjusted by NaOH aq.; for PFNDPP-Br, 30 mL of AA solution and 20 mL of glycol were used to aid dissolution). PFN was dissolved in 250 μL of THF and dispersed into TEOA/H2O to form a colloidal solution. The total concentration of the polymers and CPEs for photocatalytic measurement in the reaction solution was 50 μg/mL. Pt co-catalysts (3 wt% of CPE) were prepared by dissolving H2PtCl4 aqueous solution into the CPEs reaction and irradiated for 1 h to enable the formation of Pt nanoparticles before testing. The reaction was held in a vacuum for 30 min before testing to remove the dissolved oxygen and methanol. The photocatalytic reactions were illuminated with a solar simulator (300-W Xe light source, λ > 300 nm). The luminous power reaching the surface of the reaction solution was calibrated to be 162.4 mW cm−2 by a power meter. The produced gas was analyzed by a GC7900 gas chromatograph. Hydrogen was detected with a thermal conductivity detector, referencing against standard gas with a known concentration of hydrogen. The photocatalyticHERs of all the CPEs as a function of time are presented in Figs. S21–S36. The AQY was measured at selected wavelengths enabled by different band pass filters (CEL-QD, 380, 450, 550, 600, 650 and 700 nm). The AQY at a given wavelength was calculated from the following equations: No=(EλT%)/ (hc); N=(V × 6.02 × 1023)/(22.4t); AQY = 2N/No. No represents the number of incident photons, N represents the number of collected H2 molecules, E represents the energy of incident light at a given wavelength, determined by a calibrated power meter, λ represents the wavelength of incident light, V represents the volume of H2 molecules detected in a fixed time (t), determined by GC, T% represents the transmittance of the quartz cell. 4.3. Fs-TAS measurements The fs-TAS measurements were carried out with a commercial transient absorption spectrometer (MS2004i, Transpec-SP) equipped with a 500-Hz Solstice (Newport Corp.) Ti:sapphire regenerative amplifier outputting 800-nm, 150-fs pulses. This laser light was split into two parts to generate the pump and the probe pulses. The tuneable pump pulse was generated in a TOPAS-Prime (Spectra-Physics) optical parametric amplifier and used to excite the sample with energies of 20 μW at 400 nm. 781
Nano Energy 60 (2019) 775–783
Z. Hu, et al.
Zhicheng Hu received his B.S. degree in polymer materials and engineering in 2012 from South China University of Technology (SCUT), and gained his Ph.D. degree in materials science in 2017 under the supervision of Prof. Fei Huang. He is currently working as a postdoc in Prof. Fei Huang's group at State Key Laboratory of Luminescent Material and Devices, SCUT. His current research focuses on the design and synthesis of water/alcohol-soluble conjugated polymers for polymer solar cells and photocatalytic water splitting.
[39] F. Yu, Z. Wang, S. Zhang, H. Ye, K. Kong, X. Gong, J. Hua, H. Tian, Adv. Funct. Mater. 28 (2018) 1804512. [40] P.-J. Tseng, C.-L. Chang, Y.-H. Chan, L.-Y. Ting, P.-Y. Chen, C.-H. Liao, M.-L. Tsai, H.-H. Chou, ACS Catal. 8 (2018) 7766. [41] Z. Wang, X. Yang, T. Yang, Y. Zhao, F. Wang, Y. Chen, J.H. Zeng, C. Yan, F. Huang, J.-X. Jiang, ACS Catal. 8 (2018) 8590. [42] J. Kosco, M. Sachs, R. Godin, M. Kirkus, L. Francas, M. Bidwell, M. Qureshi, D. Anjum, J.R. Durrant, I. McCulloch, Adv. Energy Mater. 8 (2018) 1802181. [43] B. Tian, B. Tian, B. Smith, M.C. Scott, R. Hua, Q. Lei, Y. Tian, Nat. Commun. 9 (2018) 1397. [44] M. Sachs, R.S. Sprick, D. Pearce, S.A.J. Hillman, A. Monti, A.A.Y. Guilbert, N.J. Brownbill, S. Dimitrov, X. Shi, F. Blanc, M.A. Zwijnenburg, J. Nelson, J.R. Durrant, A.I. Cooper, Nat. Commun. 9 (2018) 4968. [45] X. Zhang, F. Shen, Z. Hu, Y. Wu, H. Tang, J. Jia, X. Wang, F. Huang, Y. Cao, ACS Sustain. Chem. Eng. 7 (2019) 4128. [46] M. Waqas, Y. Wei, D. Mao, J. Qi, Y. Yang, B. Wang, D. Wang, Nano Research 10 (2017) 3920. [47] Y. Wei, J. Wang, R. Yu, J. Wan, D. Wang, Angew. Chem. Int. Ed. 58 (2019) 1422. [48] H. Jiang, P. Taranekar, J.R. Reynolds, K.S. Schanze, Angew. Chem. Int. Ed. 48 (2009) 4300. [49] C. Zhu, L. Liu, Q. Yang, F. Lv, S. Wang, Chem. Rev. 112 (2012) 4687. [50] C. Duan, K. Zhang, C. Zhong, F. Huang, Y. Cao, Chem. Soc. Rev. 42 (2013) 9071. [51] C.-C. Chueh, C.-Z. Li, A.K.-Y. Jen, Energy Environ. Sci. 8 (2015) 1160. [52] Z. Hu, F. Huang, Y. Cao, Small Methods 1 (2017) 1700264. [53] D.L. Jiang, C.K. Choi, K. Honda, W.S. Li, T. Yuzawa, Takuzov Aida, J. Am. Chem. Soc. 126 (2004) 12084. [54] X. Yang, Z. Hu, Q. Yin, C. Shu, X.-F. Jiang, J. Zhang, X. Wang, J.-X. Jiang, F. Huang, Y. Cao, Adv. Funct. Mater. (2019) 1808156. [55] W.-W. Yong, H. Lu, H. Li, S. Wang, M.-T. Zhang, ACS Appl. Mater. Interfaces 10 (2018) 10828. [56] Z. Hu, Z. Wang, X. Zhang, H. Tang, X. Liu, F. Huang, Y. Cao, iScience 13 (2019) 33. [57] Y. Liu, V.V. Duzhko, Z.A. Page, T. Emrick, T.P. Russell, Chem. Res. 49 (2016) 2478. [58] W. Wu, G.C. Bazan, B. Liu, Chemistry 2 (2017) 760. [59] Z. He, C. Zhong, S. Su, M. Xu, H. Wu, Y. Cao, Nat. Photon. 6 (2012) 591. [60] J. Borges, J.F. Mano, Chem. Rev. 114 (2014) 8883. [61] F.-X. Xiao, M. Pagliaro, Y.-J. Xu, B. Liu, Chem. Soc. Rev. 45 (2016) 3088. [62] X. Liu, Z. Chen, R. Xu, R. Zhang, Z. Hu, F. Huang, Y. Cao, Small Methods 2 (2018) 1700407. [63] D. Mao, J. Wan, J. Wang, D. Wang, Adv. Mater. (2018) 1802874. [64] P. Peumans, A. Yakimov, S.R. Forrest, J. Appl. Phys. 93 (2004) 3693. [65] P. Hu, C. Chen, R. Zeng, J. Xiang, Y. Huang, D. Hou, Q. Li, Y. Huang, Nano Energy 50 (2018) 376. [66] X.-Y. Lai, C.-R. Wang, Q. Jin, R.-B. Yu, D. Wang, Sci. China Mater. 58 (2015) 192. [67] S. Cao, J. Yu, J. Phys. Chem. Lett. 5 (2014) 2101. [68] C. Ye, J.-X. Li, Z.-J. Li, X.-B. Li, X.-B. Fan, L.-P. Zhang, B. Chen, C.-H. Tung, L.-Z. Wu, ACS Catal. 5 (2015) 6973. [69] L.-Z. Wu, B. Chen, Z.-J. Li, C.-H. Tung, Acc. Chem. Res. 47 (2014) 2177. [70] X. Zhang, B. Peng, S. Zhang, T. Peng, ACS Sustain. Chem. Eng. 3 (2015) 1501. [71] S. Guo, Z. Deng, M. Li, B. Jiang, C. Tian, Q. Pan, H. Fu, Angew. Chem. Int. Ed. 128 (2016) 1862. [72] Z. Wu, C. Sun, S. Dong, X.-F. Jiang, S. Wu, H. Wu, H.-L. Yip, F. Huang, Y. Cao, J. Am. Chem. Soc. 138 (2016) 2004. [73] Z. Hu, Z. Chen, K. Zhang, K. Zheng, R. Xie, X. Liu, X. Yang, F. Huang, Y. Cao, Sol. RRL 1 (2017) 1700055. [74] R. Yang, A. Garcia, D. Korystov, A. Mikhailovsky, G.C. Bazan, T.-Q. Nguyen, J. Am. Chem. Soc. 128 (2006) 16532. [75] Z. Hu, R. Xu, S. Dong, K. Lin J. Liu, F. Huang, Y. Cao, Mater. Horiz. 4 (2017) 88. [76] J. Huang, K.L. Mulfort, P. Du, L.X. Chen, J. Am. Chem. Soc. 134 (2012) 16472. [77] P. Liu, Y. Zhao, R. Qin, S. Mo, G. Chen, L. Gu, D.M. Chevrier, P. Zhang, Q. Guo, D. Zang, B.-H. Wu, G. Fu, N. Zheng, Science 352 (2016) 797. [78] J. Qi, X. Lai, J. Wang, H. Tang, H. Ren, Y. Yang, Q. Jin, L. Zhang, R. Yu, G. Ma, Z. Su, H. Zhao, D. Wang, Chem. Soc. Rev. 44 (2015) 6749. [79] X.-B. Li, Y.-J. Gao, Y. Wang, F. Zhan, X.-Y. Zhang, Q.-Y. Kong, N.-J. Zhao, Q. Guo, H.-L. Wu, Z.-J. Li, Y. Tao, J.-P. Zhang, B. Chen, C.-H. Tung, L.-Z. Wu, J. Am. Chem. Soc. 139 (2017) 4789. [80] G. Leem, Z.A. Morseth, E. Puodziukynaite, J. Jiang, Z. Fang, A.T. Gilligan, J.R. Reynolds, J.M. Papanikolas, K.S. Schanze, J. Phys. Chem. C 118 (2014) 28535. [81] R. Godin, Y. Wang, M.A. Zwijnenburg, J. Tang, J.R. Durrant, J. Am. Chem. Soc. 139 (2017) 5216. [82] K.L. Corp, C.W. Schlenker, J. Am. Chem. Soc. 139 (2017) 7904.
Xi Zhang received his B. S. degree in materials in 2017 from South China University of Technology. He is currently working as a Ph.D candidate in Prof. Fei Huang's group at State Key Laboratory of Luminescent Material and Devices, SCUT. His current research focuses on the design and synthesis of conjugated polymers for photocatalytic water splitting.
Qingwu Yin received his BS degree in 2015. He is a Ph.D candidate now in the State Key Laboratory of Luminescent Materials and Devices, SCUT. His research focuses on the device and physics of high efficiency polymer solar cells.
Xiaocheng Liu received his BS degree from Lanzhou University in 2016. He is a master student in the State Key Laboratory of Luminescent Materials and Devices, SCUT. His research interests focus on novel organic functional materials for polymer solar cells.
Dr. Xiao-Fang Jiang obtained her PhD degree from National University of Singapore in 2015. After that, she worked as a senior postdoc. fellow in State Key Laboratory of Luminescent Materials and Devices from SCUT. Currently, she is associate professor in School of Physics and Telecommunication Engineering from South China Normal University. Her research interests focus on developing ultrafast laser techniques such as transient absorption spectroscopy to characterize and understand the photophysics in molecular and nanocomposites, nonlinear optical properties of novel plasmonic nanostructures for applications in bio-sensing, bio-imaging and optoelectronic devices.
782
Nano Energy 60 (2019) 775–783
Z. Hu, et al. Zhiming Chen received his BS degree in 2014. He is a Ph.D. candidate now in the State Key Laboratory of Luminescent Materials and Devices from SCUT since 2015. His research focuses on the device and physics of high efficiency polymer solar cells.
Fei Huang received his B.S. degree in chemistry from Peking University in 2000 and gained his Ph.D. degree in materials science from the SCUT in 2005 under the supervision of Prof. Yong Cao. After postdoctoral work at the University of Washington with Prof. Alex K.-Y. Jen, he began his academic career in 2009 as a professor at the SCUT. His main interests are in the fields of organic functional materials and devices for optoelectronics.
Xiye Yang received his BS degree in Mechanical Engineering from South China University of Technology in 2014 and gained his PhD degree in Materials Science from the SCUT in 2018 under the supervision of Prof. Fei Huang. He is currently working as a postdoc fellow in Prof. Xiaohui Wang's group in State Key Laboratory of Pulp & Paper Engineering from SCUT. His current research focuses design and synthesis of supramolecular based novel functional materials towards optoelectronic applications.
Yong Cao was appointed as Professor of Institute of Polymer Optoelectronic Materials and Devices at SCUT in 1999. He obtained his bachelor's degree in chemistry at the former Leningrad University (Russia) in 1965 and his Ph.D. degree from Tokyo University at 1987. He was previously professor at Institute of Chemistry, Chinese Academy of Science, 1986–1988; visiting scientist at Institute of Polymer and Organic Solids, UCSB, 1988–1990, and senior scientist at the Uniax Corporation, 1990–1999. He has been a member of the Chinese Academy of Science since 2001. His research interests are organic/polymer optoelectronic materials and devices.
783