Photoredox-Mediated ATRP: A Facile Method for Modification of

Nov 17, 2016 - Graphite fluoride (GiF) and graphene fluoride (GeF) showed interesting ... work shows a great potential in the preparation of polymer-d...
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Photoredox-Mediated ATRP: A Facile Method for Modification of Graphite Fluoride and Graphene Fluoride without Deoxygenation Yurong Que,† Zhong Huang,† Chun Feng,* Yang Yang, and Xiaoyu Huang* Key Laboratory of Synthetic and Self-Assembly Chemistry for Organic Functional Molecules, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, People’s Republic of China S Supporting Information *

ABSTRACT: Graphite fluoride (GiF) and graphene fluoride (GeF) showed interesting electrochemical, electronic, and mechanical properties in comparison with their derivatives of graphite and graphene, respectively. Due to the chemical inertness of GiF and GeF, as far as we are aware, no report can be found on the modification of GiF and GeF with polymeric chains. Herein, we reported that photoredox-mediated atom transfer radical polymerization (ATRP) is able to directly introduce methacrylate-based polymers, including poly(oligo(ethylene glycol) methyl ether methacrylate) (POEGMA), poly(methyl methacrylate) (PMMA), poly(pentafluorophenyl methacrylate) (PPFMA), and poly(methacrylic acid) (PMAA), onto the surface of GiF and GeF by utilizing C−F bonds of GiF and GeF as initiating sites and Ir(ppy)3 as a photoredox catalyst under low intensity blue LED light strips (10 W, 460−470 nm) in DMF for graft polymerization without a tedious deoxygenation procedure. Owing to the attractive properties of GiF and GeF, along with the capacity of spatial control over the formation of polymeric chains on the surface of GiF and GeF endowed by the inherent nature of photoredox-mediated ATRP, there is no doubt that the strategy developed in the current work shows a great potential in the preparation of polymer-decorated GiF and GeF and the corresponding functional materials. ompared to graphite, graphite fluoride (GiF) is endowed with interesting electrochemical and electronic properties because of its special structure, that is, a puckered cyclohexane ring with each carbon bearing a fluorine alternatively above and then below the ring.1−4 Therefore, GiF has been extensively used in thermostable lubricant and hydrogen storage.1−4 Furthermore, analogous to graphene oxide (GO), graphene fluoride (GeF) is a special graphene derivative, and it can be recognized as the two-dimensional analogue of Teflon.5−12 GeF not only inherits the mechanical strength of graphene but also with ultraviolet luminescence and tunable bandgap energy from 0 to 3 eV.5−12 Thus, GeF is regarded as one of the important building blocks for preparation of highly stable lubrication, high-performance nanocomposites, and optoelectronic and photonic devices.5−12 In order to deepen the understanding of a structure−property relationship of this kind of twodimensional carbon material and extend windows of both property and application scopes of hierarchical structures with GiF and GeF, the functionalization of GiF and GeF is of paramount importance. One strategy that is gaining broad recognition for functionalization of graphite, graphene, and their derivatives is to introduce polymeric chains onto their surfaces.13−18 Graphite and graphene can be modified with polymeric chains via noncovalent interaction, and GO can be easily functionalized by employing a number of OH, COOH, and epoxy C−O−C groups on the surface of GO. On the contrary, there is only a few aromatic systems remaining for

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© XXXX American Chemical Society

GiF and GeF, especially for GiF and GeF with a high ratio of F/C and just relatively inert C−H and C−F bonds on the surface of GiF and GeF.1−4 Thus, it should be not surprising that there is no report on the modification of GiF and GeF with polymeric chains until now, as far as we are aware. Atom transfer radical polymerization (ATRP) is a widely used approach for surface functionalization of graphene and GO with polymeric chains by graft polymerization.15,19 Although C−F group is not a good initiating group for ATRP due to its high bond dissociation energy, C−F groups still could be used as initiating groups for the functionalization of poly(vinylidene difluoride) (PVDF) and perfluorosulfonic acid polymer by ATRP.20−22 In 2012, Hawker et al. reported a photochemically mediated ATRP of methacrylate-based monomers by using C−Br groups as initiating sites and ppm level of Ir(ppy)3 as photoredox catalyst.23 Compared to normal ATRP, this method has advantages of temporal and spatial controllability, low catalyst usage, and compatibility with acidic monomer.23,24 Our recent work showed that this strategy can be employed for direct modification of poly(vinyl chloride) (PVC) by using inactivated C−Cl groups of PVC as initiating sites for graft polymerization without a deoxygenation Received: September 26, 2016 Accepted: November 14, 2016

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ACS Macro Letters procedure.25 Inspired by these results, we hypothesized that it is possible to employ this photomediated ATRP to directly functionalize GiF and GeF by using C−F bonds of GiF and GeF as initiating sites for graft polymerization without a tedious deoxygenation procedure (Scheme 1). Scheme 1. Functionalization of GiF and GeF by PhotoredoxMediated ATRP

Figure 1. (A) TG and DTG curves of pristine GiF, GiF-g-POEGMA, and GiF functionalized by polymerization using AIBN as initiator instead of Ir(ppy)3. (B) FT-IR, (C) XPS, and (D) Raman spectra of pristine GiF and GiF-g-POEGMA.

To test our hypothesis, we began by examining the polymerization of oligo(ethylene glycol) methyl ether methacrylate (OEGMA) (Mn = 500 g/mol, 3.00 g, 6 mmol) by using PVDF (150 mg, Mn = 22 000 g/mol) as initiator and Ir(ppy)3 (1.1 mg, 1.7 × 10−3 mmol) as photoredox catalyst with a blue LED light source (460−470 nm, 10 W, Figure S1) in DMF (12 mL). After 24 h, the obtained product was precipitated in methanol, a good solvent for POEGMA three times for purification. From 1H NMR spectrum of purified product (Figure S2), characteristic signals attributed to PVDF and POEGMA segments are visible. This observation indicated the formation of POEGMA polymeric chains grafted from PVDF backbone; that is, part of C−F bonds of PVDF can be homolyzed to serve as initiating sites for graft polymerization of OEGMA. On the basis of this result, we then attempted to functionalize gray GiF (element analysis: F%, 61%) with POEGMA via this graft polymerization strategy. After light irradiation for 24 h, the solution became very viscous, and fluorinated graphite was well dispersed in the solution, indicating the occurrence of polymerization. After washing with THF until no OEGMA or POEGMA was found in the filtrate, black powders with excellent dispersity in DMF were obtained as shown in Figures S3 and S4. Compared to pristine GiF, C and H contents of functionalized GiF after polymerization increased from 32.75% to 36.20% and 0 to 0.85%, respectively. Pristine GiF just showed one mass loss peak located at 626 °C and started to lose mass upon heating up to 340 °C (Figure 1A), while functionalized GiF showed two mass loss peaks located at 297 and 565 °C, respectively. The first weight loss (15.9%) in the range from 150 to 400 °C was attributed to the pyrolysis of POEGMA segment based on TG curve of POEGMA homopolymer (Figure S5). Thus, the weight content of polymer grafted on GiF (Mwt%,polymer) could be estimated on the basis of the weight loss in this range (Mwt%,polymer = WL400 − WL150, Mwt%,polymer is the weight content of polymer grafted on GiF; WL400 and WL150 are the weight loss at 400 and 150 °C in the TG curve, respectively). In FT-IR spectra of pristine and functionalized GiF (Figure 1B), we can clearly notice the typical bands at 1730 cm−1 (υCO) and 2900 cm−1 (υC−H) and C−F functionalities at 1250 cm−1. From X-ray photoelectron spectroscopy (XPS) spectra of pristine and functionalized GiF (Figure 1C), an intense peak at

532.5 eV attributed to O 1s appeared with the retaining of F 1s at 688.5 eV after graft polymerization. It should be pointed out that the peak of F 1s in XPS spectra before and after polymerization are almost the same (Figure S6), which might suggest that only a few C−F bonds of GiF were homolyzed to serve as initiating sites for polymerization, similar to the case of PVC functionalization utilizing the same strategy.25 Both pristine and functionalized GiF displays two prominent peaks in Raman spectrum (Figure 1D), that is well-documented G and D bands. After graft polymerization, G and D bands of functionalized GiF shifted from 1327 to 1332 cm−1 and from 1580 to 1596 cm−1, respectively, and the intensity ratio of D and G bands increased from 1.03 to 1.14. These observations demonstrated the introduction of more disorders after grafting POEGMA polymeric chains, which is consistent with previous results of functionalization of GO with polymeric chains.26,27 Additionally, the water contact angle of GiF film after polymerization decreased from 138° to 103° (Figures S7A and S7B). All aforementioned evidence clearly confirmed that POEGMA chains were attached onto the surface of GiF. Moreover, we conducted kinetics study on this system by monitoring the conversion of OEGMA and weight content of POEGMA grafted on GiF (Figure S8). It was found that the conversion of OEGMA and weight content of POEGMA grafted on GiF increased with the extending of polymerization time, and a linear relationship between the conversion of OEGMA and the weight content of POEGMA grafted on GiF was observed. These results might indicate the living nature of photoredox-mediated ATRP. To make clear the mechanism for the introduction of polymeric chains via photomediated ATRP by using Ir(ppy)3 as photocatalyst under light irradiation, a control experiment in the absence of light was conducted. The solution did not become viscous, and the obtained sediment after purification did not show any weight loss attributed to POEGMA (Figure S9). Besides, the color of product was still gray, similar to that of pristine GiF. These observations exhibited the importance of light irradiation in the polymerization. A previous report showed that the photoredox catalyst of Ir(ppy)3 could act as a radical initiator during the polymerization upon the irradiation 1340

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ACS Macro Letters of blue light,23 which might initiate the polymerization of OEGMA monomer, and thus, polymeric radicals of POEGMA might form during the polymerization. Gao et al. reported that polymeric radicals could add to part of the double bonds of GO, which could initiate the polymerization of the remaining monomers for further formation of polymeric chains on the surface of GO.28 One might argue the incorporation of POEGMA chains onto the surface of fluorinated graphite via the similar mechanism, where Ir(ppy)3 acted as radical initiator, similar as 2,2′-azobis(isobutyronitrile) (AIBN). To address this question, another control experiment was performed, where AIBN was used as radical initiator and the solution was heated at 70 °C for 48 h according to Gao’s recipe.28 Although the solution became viscous, the obtained fluorinated graphite was still gray, and there was just less than 1% of weight loss stage attributed to POEGMA in TG curve (Figure 1A). This observation illustrated that there were just traces of POEGMA chains attached on fluorinated graphene, probably because of the lack of double bonds in GiF. We also conducted the polymerization using graphite instead of GiF under the similar conditions, where no polymer was attached onto graphite (Figure S10). This result suggested the importance of C−F bonds for the introduction of polymeric chains. Thus, all these results showed that the polymerization was indeed a photomediated process, and POEGMA segments were covalently incorporated onto GiF via graft polymerization using C−F bonds as initiating sites. In addition, the thermal stability of GiF after graft polymerization using Ir(ppy)3 as photoredox catalyst and GiF treated with AIBN were both improved obviously (Figure 1A). We speculated that the disappearance of “active” C−F bonds after graft polymerization using Ir(ppy)3 as catalyst and the curing of possible defects by radicals formed over the polymerization might lead to the improvement of thermal stability based on previous results (Figure S11).29,30 Subsequently, we attempted to functionalize GiF with different fluorine weight contents (45% and 55%) via this approach under similar conditions to demonstrate the versatility of this strategy. Obvious weight loss stages were observed for these GiF after graft polymerization, indicative of the covalent incorporation of POEGMA chains onto GiF, and the weight contents of POEGMA were estimated to be 7.8% and 12.6% for GiF with different fluorine weight contents of 45% and 55%, respectively (Figure S12). We also tried to modify GiF with poly(methyl methacrylate) (PMMA), an extensively used thermoplastic polymer, and poly(pentafluorophenyl methacrylate) (PPFMA), a popular and efficient precursor for further functionalization, by graft polymerization of MMA and PFMA monomer under similar conditions, respectively. The introduction of PMMA and PPFMA chains was confirmed by TGA, FT-IR, XPS, and Raman measurements (Figure S13). The weight contents of PMMA and PPFMA were estimated to be 15.2% and 17.2%, respectively (Figure S13A). Carboxylic acid containing polymers have been broadly used as polymeric ligands to stabilize a variety of inorganic nanoparticles such as Ag, TiO2, PbS, Fe3O4, and so on.31 The introduction of poly(methacrylic acid) (PMAA) onto GiF would not only endow GiF with excellent stability and dispersity but also extend potential application of GiF through the incorporation of inorganic nanoparticles by utilizing carboxyls as anchoring groups. One advantage of photomediated ATRP using Ir(ppy)3 as photocatalyst over traditional ATRP is its capability to polymerize an acidic monomer, like

MAA.19 Therefore, we attempted to introduce PMAA chains onto GiF under similar conditions. Satisfyingly, a typical mass loss originating from the PMAA segment appeared in TG and DTG curves after polymerization (Figure S14A). A characteristic band at 1695 cm−1 attributed to carbonyls and a broad peak at 2968 cm−1 originating from −COOH moieties in the FT-IR spectrum of the product after graft polymerization (Figure S14B), along with the appearance of the O 1s signal in the XPS spectrum (Figure S14C) and the decrease of water contact angle from 138° to 108° (Figure S7C), exhibited the introduction of PMAA chains. The weight content of the PMAA segment was estimated to be 24.5% via TGA measurement. In order to further prove the presence of PMAA segments and demonstrate the facility in the incorporation of an inorganic nanoparticle into PMAA-modified GiF, AgNO3 was added into an aqueous solution of GiF-g-PMAA followed by aging for 24 h. Then, free Ag+ cations were removed by three cycles of centrifugation and redispersion. Subsequently, sodium borohydride (NaBH4) was added to reduce Ag+ to Ag0. One can notice numerous dark dots with a diameter in the range from 6 to 20 nm located on GiF-g-PMAA (Figure 2A and 2B).

Figure 2. TEM images of GiF-g-PMAA/AgNP (A and B); STEM image (C); and TEM elemental mappings of Ag (D) and F (E) of GiF-g-PMAA/AgNP.

The overlapping of dark dots in a scanning transmission electron microscopy (STEM) image (Figure 2C) with mapping of F and Ag in energy-dispersive X-ray spectroscopy analysis (Figure 2D and 2E) clearly indicated that the dark dots located on GiF were silver nanoparticles. The obtained Ag-decorated GiF-g-PMAA (GiF-g-PMAA/ AgNP) was employed to catalyze the reduction of 4nitrophenol (4-NP) to 4-aminophenol (4-AP) in aqueous media. After 1 mL of GiF-g-PMAA/AgNP solution (2.0 mg/ mL) was added into 2.0 mL of aqueous solution containing 1.2 mmol of NaBH4 and 0.015 mmol of 4-NP, the conversion of 4NP at different intervals was monitored by UV/vis spectroscopy (Figure S15A). The absorbance at 400 nm originating from 4-NP decreased with the extending of reaction time so as to almost disappear after 50 min, along with the increase of absorbance at 295 nm originating from 4-AP. This observation exhibited that 4-NP was quantitatively reduced to 4-AP, indicative of the high catalytic activity of GiF-g-PMAA/AgNP. Since AgNP was located on the surface of GiF-g-PMAA, it was able to be easily removed from the reaction solution by centrifugation or filtration, compared to free AgNP. In order to 1341

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However, these dots attributed to POEGMA domains were not densely distributed on the surface of GeF, which might indicate a low grafting density. In summary, we have demonstrated a robust and versatile strategy of photomediated ATRP to directly modify GiF and GeF by graft polymerization without any functional group transformation for introducing ATRP initiating group, in which C−F groups of GiF and GeF can be activated by the photoredox catalyst of Ir(ppy)3 under blue light (10 W, 460−470 nm) to serve as initiating sites for ATRP of methacrylate-based monomers, including OEGMA, MMA, PFMA, and MAA. Due to the excellent coordination of PMAA with inorganic nanoparticles, Ag nanoparticles were able to be introduced onto the surface of GiF-g-PMAA, and the obtained GiF-g-PMAA/AgNP showed excellent catalytic activity, stability, and recyclability in the reduction of 4nitrophenol to 4-aminophenol. Furthermore, in analogy with the pervasive nature of photopolymerization, the current approach displays potential application in the spatial control over the formation of polymer brushes on the surface of GiF and GeF. Given the broad application of GiF and GeF, the attractive features of this strategy will make this technique a practical and efficient tool for the fabrication of GiF- and GeFbased functional materials.

test the stability and recyclability of GiF-g-PMAA/AgNP, GiFg-PMAA/AgNP was recycled by centrifugation and reused in another cycle of reaction under similar conditions. 4-NP was quantitatively reduced to 4-AP in the first four cycles, and the conversion of 4-NP to 4-AP decreased to 96% until the fifth cycle (Figure S15B). Finally, we employed this strategy to functionalize the surface of graphene fluoride (GeF), which was prepared by exfoliating graphite fluoride (GiF) with Na2O2 and HSO3Cl as exfoliating agents according to our previous report.32 The attachment of POEGMA segment was confirmed by the appearance of typical mass loss in the range from 150 to 400 °C for POEGMA (Figure 3A), along with an obvious increase in the binding

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00732. Experimental details for preparation of PVDF-g-POEGMA; graft polymerization for the modification of GiF and GeF; 1H NMR spectrum of PVDF-g-POEGMA; digital photos of solutions of GiF, GiF-g-POEGMA, GiF-gPMMA, GiF-g-PPFMA, and GiF-g-PMAA; digital photos of solids of GiF and GiF-g-POEGMA; TG curves of POEGMA, PMMA, and PPFMA; XPS spectra of pristine GiF and Gif-g-POEGMA; micrographs of water droplets on the surface of pristine GiF, GiF-g-POEGMA, and GiF-g-PMAA; 1H NMR spectra of polymerization solution; TG curves of POEGMA-modified GiF taken at different intervals; dependence of conversion of OEGMA and content of POEGMA grafted on GiF on polymerization time; evolution of content of POEGMA grafted on GiF with conversion of OEGMA; TG and DTG curves of GiF functionalized by polymerization using Ir(ppy)3 as photocatalyst without light irradiation; TG curve of graphite after graft polymerization of OEGMA using Ir(ppy)3 as photocatalyst under light irradiation; TG and DTG curves of pristine GiF and GiF treated with Ir(ppy)3/light without the addition of monomer; TG and DTG curves of GiF (F content: 45% and 55%) functionalized by polymerization of OEGMA using Ir(ppy)3 as photocatalyst under light irradiation; TG and DTG curves, FT-IR, XPS, and Raman spectra of pristine GiF, GiF-g-PMMA, and GiF-gPPFMA; TG and DTG curves, FT-IR, XPS, and Raman spectra of pristine GiF and GiF-g-PMAA; UV−vis absorbance spectra of 4-NP solutions containing GiF-gPMAA/AgNP along with time and conversion of 4-NP in five successive cycles of reduction catalyzed by GiF-gPMAA/AgNP (PDF)

Figure 3. (A) TG and DTG curves and (B) XPS spectra of GeF and GeF-g-POEGMA; TEM images of GeF (C) and GeF-g-POEGMA (D); (E) AFM image of GeF-g-POEGMA; and (F) thickness profile along the black line in the AFM image.

energy at 289.6 eV attributed to C 1s of CO (Figure 3B). The content of POEGMA was estimated to be 12.1 wt % on the basis of TGA results. After polymerization, we can not notice any obvious change for the peak of F 1s in XPS spectrum (Figure 3B), which might indicate that only a few C−F were activated to serve as initiating groups, consistent with our previous work on PVC functionalization.25 In addition, we also used TEM to investigate the structure of GeF before and after graft polymerization (Figure 3C and 3D). Typical corrugated morphology observed in TEM indicated the few-layer structure of GeF after exfoliating and the preservation of its intact fewlayer structure during the polymerization. The surface of GeF on a micasubstrate is flat, which demonstrated the homogeneous surface.30 Some protuberances were observed on GeF after graft polymerization (Figure 3E), and these dots with heights less than 10 nm (Figure 3F) located on the surface of GeF should be tufts of POEGMA chains. Thus, all these results indicated that POEGMA segments were incorporated into GeF. 1342

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Tel.: +86-21-54925520; Fax: +86-21-64166128). *E-mail: [email protected] (Tel.: +86-21-54925310; Fax: +86-21-64166128). ORCID

Xiaoyu Huang: 0000-0002-9781-972X Author Contributions †

Both authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial support from National Basic Research Program of China (2015CB931900), National Key Research & Development Program of China (2016YFA0202900), National Natural Science Foundation of China (21632009, 51373196, and 21504102), Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20020000), Youth Innovation Promotion Association (2016233), and Shanghai Scientific and Technological Innovation Project (14JC1493400, 16JC1402500, 16520710300, and 14520720100).

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DOI: 10.1021/acsmacrolett.6b00732 ACS Macro Lett. 2016, 5, 1339−1343