LETTERS PUBLISHED ONLINE: 16 MARCH 2015 | DOI: 10.1038/NMAT4220

Conducting polymer nanostructures for photocatalysis under visible light Srabanti Ghosh1, Natalie A. Kouamé1, Laurence Ramos2, Samy Remita1,3, Alexandre Dazzi1, Ariane Deniset-Besseau1, Patricia Beaunier4,5, Fabrice Goubard6, Pierre-Henri Aubert6 and Hynd Remita1,7* Visible-light-responsive photocatalysts can directly harvest energy from solar light, offering a desirable way to solve energy and environment issues1 . Here, we show that one-dimensional poly(diphenylbutadiyne) nanostructures synthesized by photopolymerization using a soft templating approach have high photocatalytic activity under visible light without the assistance of sacrificial reagents or precious metal co-catalysts. These polymer nanostructures are very stable even after repeated cycling. Transmission electron microscopy and nanoscale infrared characterizations reveal that the morphology and structure of the polymer nanostructures remain unchanged after many photocatalytic cycles. These stable and cheap polymer nanofibres are easy to process and can be reused without appreciable loss of activity. Our findings may help the development of semiconducting-based polymers for applications in self-cleaning surfaces, hydrogen generation and photovoltaics. Increasing attention is being drawn towards nanomaterials for photocatalytic solar-energy conversion to identify robust new methods for water purification and environmental protection at lower cost and energy consumption1 . For an optimized use of solar energy, efficient and stable photocatalysts that are capable of harvesting visible light are required. Over the past decades, oxide-based semiconductors, in particular TiO2 , have been recognized as efficient photocatalysts for the degradation of organic pollutants in wastewater2,3 . The limitation of TiO2 (like most other photocatalysts)4 for applications is essentially due to the low quantum yield that results from charge-carrier (e− /h+ ) recombination and the necessity to use ultraviolet irradiation because of the large value of its bandgap5 . To overcome this limitation and produce photocatalysts responsive to visible light, surface-tuning strategies and modification of oxides on the nanometre scale have been developed through doping or surface modification6 . Indeed, TiO2 doping with N, C or S, or its modification with metal nanoparticles (Ag, Au, Pt), has extended its activity towards the visible region6,7 . However, the photocatalytic activity of the modified materials in visible light is still not sufficient for commercial applications. Alternative materials have been proposed for photocatalysis and solar-energy conversion. For example, polymeric carbon nitride was found to be an efficient photocatalyst that produces hydrogen from water under visible light irradiation, but a sacrificial donor is required8 . Hence, large-scale production of stable visible-light-active photocatalysts remains a challenge for

industrial applications9 . The central issue of current research is to consider the extension of the light absorption spectrum to the visible region, thus facilitating the use of sunlight as an inexpensive, renewable energy source for photocatalytic processes. Conjugated polymer nanostructures are emerging as new energy materials for applications in solar cells, fuel cells and rechargeable lithium batteries, owing to their advantages of low-cost facile synthesis, excellent electrical and electrochemical activity, high carrier mobility and mechanical properties10 . Significant attempts have been focused on understanding the polymer nanostructures for energy conversion and storage applications11 . Although oxidebased semiconductors have been widely explored, photocatalytic activity studies of conjugated polymers are still scarce12,13 . Semiconductor nanostructures modified with conducting polymers have been studied for photocatalytic applications12–14 . Polypyrrole films containing TiO2 nanoparticles exhibit higher photocatalytic activity than bare TiO2 for the degradation of dyes under ultraviolet radiation12 . On the other hand, TiO2 modified by conjugated derivatives of polyisoprene exhibit photocatalytic activity under visible light. Two conjugated polymers, poly(3-hexylthiophene) and poly(2-methoxy-5-)2-ethylhexyloxy-1,4-phenylene vinylene, were found to be very efficient in the degradation of various textile dyes under ultraviolet irradiation, but not under visible light13 . A potential use of conjugated polymer photocatalysts in synthetic applications is illustrated by the visible-light-driven pinacol coupling of benzaldehyde using poly(p-phenylene) as a photoredox catalyst14 . Here, we demonstrate that nanofibres of a conjugated polymer, poly(diphenylbutadiyne) (PDPB), are highly efficient under visible light for the degradation of pollutants (phenol and methyl orange have been used as model pollutants). Furthermore, we show that these photocatalysts are very stable even after repeated cycling. Interestingly, we find that PDPB nanofibres exhibit a much higher photocatalytic activity than bulk PDPB. To the best of our knowledge, our findings constitute the first experimental evidence of the photocatalytic activity of conjugated polymer nanostructures under visible light for water decontamination. PDPB belongs to the family of poly(diacetylenes), which are one of the interesting research targets among π-conjugated polymers, having fascinating physicochemical properties with tailorable pendant side groups and terminal functionalities15,16 . We have developed a controlled soft-template-mediated synthesis

1 Laboratoire

de Chimie Physique, UMR 8000-CNRS, Bât. 349, Université Paris-Sud, 91405 Orsay, France. 2 Laboratoire Charles Coulomb (L2C) UMR 5221 CNRS-Université de Montpellier, 34095 Montpellier, France. 3 Départment CASER, Ecole SITI, Conservatoire National des Arts et Métiers, CNAM, 292 rue Saint-Martin, 75141 Paris Cedex 03, France. 4 Sorbonne Universités, UPMC Univ. Paris 06, UMR 7197-CNRS, Laboratoire de Réactivité de Surface, F-75005 Paris, France. 5 CNRS, UMR 7197, Laboratoire de Réactivité de Surface, F-75005 Paris, France. 6 Laboratoire de Physicochimie des Polymères et des Interfaces (LPPI), Université de Cergy-Pontoise, 95031 Cergy-Pontoise Cedex, France. 7 CNRS, Laboratoire de Chimie Physique, UMR 8000, 91405 Orsay, France. *e-mail: [email protected] NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials

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NATURE MATERIALS DOI: 10.1038/NMAT4220

LETTERS fore

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Polymerization n Ar Ar Ar Ar 1,4-diphenylbutadiyne Poly(diphenylbutadiyne) Ar = Phenyl group (PDPB)



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Figure 1 | Synthesis and characterization of PDPB nanofibres. a, Photograph of swollen hexagonal phases before polymerization (transparent gel) and after polymerization (yellow gel) by ultraviolet irradiation. b, Schematic of polymerization of diphenylbutadiyne (DPB) by ultraviolet irradiation. c, Small-angle X-ray scattering spectra of swollen hexagonal phases before (black squares) and after polymerization of 1,4-diphenylbutadiyne by ultraviolet irradiation (red cycles). The diffraction patterns are characteristic of a hexagonal phase, as demonstrated by four Bragg peaks (black arrows) whose positions are in √ √ the ratio 1: 3:2: 7. From the peak position and sample composition, one evaluates that the diameter of the oil-swollen surfactant-stabilized tubes is 16 nm. Inset: scheme of an oil-swollen hexagonal phase. d, Absorption spectra of solid PDPB nanostructures. Inset: Solid powder of PDPB nanostructures. e, Transmission electron micrograph of PDPB nanostructures prepared by the soft templating approach. f, ATR–FTIR spectra of PDPB nanostructures synthesized by ultraviolet light irradiation. g–i, AFM–infrared spectra recorded in three different spectral regions for PDPB. g, Inset: Atomic force micrograph of the PDPB nanostructure, with the local region of the polymer nanostructure used for the nanoIR spectra marked with a green star.

of PDPB nanofibres in hexagonal mesophases composed of oilswollen surfactant tubes arranged on a triangular lattice in water under ultraviolet irradiation17,18 . The hydrophobic domain of the mesophases can accommodate high concentrations (up to 20wt.%) of 1,4-diphenylbutadiyne (DPB) monomer, which can directly polymerize by photoirradiation in the presence of a free-radical initiator (benzoin methyl ether, BME, 1%)19 . The doped mesophases become yellow after polymerization but remain translucent (Fig. 1a). The DPB monomers undergo polymerization via 1,4-addition reaction to form alternating ene–yne polymer chains on irradiation with ultraviolet light (Fig. 1b). The polymer-doped hexagonal phases were translucent and birefringent owing to their anisotropic structure. Small-angle X-ray scattering (SAXS) experiments (experimental set-up described in Supplementary Information) demonstrate that the hexagonal structure is preserved on polymerization (Fig. 1c). The as-prepared 2

PDPB structures can be easily extracted from the mesophases simply by washing with ethanol, followed by centrifugation: after drying, a solid yellow powder of PDPB is obtained (Fig. 1d: inset). The PDPB structures show broad absorption in the visible range (Fig. 1d). Figure 1e shows a transmission electron microscopy (TEM) image of PDPB nanofibres (denoted as nano PDPB) of uniform diameter ∼19 nm with lengths of a few micrometres. Note that this onedimensional structure reflects the geometry of the hydrophobic domains of the hexagonal mesophases (which comprise oil tubes of diameter 16 nm, as determined by SAXS). In contrast, micrometresized spherical particles (denoted as bulk PDPB) were obtained by photopolymerization of DPB (in the presence of BME) in cyclohexane (without surfactant; Supplementary Fig. 1a). Hence, no fibrous network was obtained in the absence of mesophases, clearly demonstrating the templating effect of the mesophase for the generation of the one-dimensional nanostructures (Fig. 1e

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NATURE MATERIALS DOI: 10.1038/NMAT4220 a

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Figure 2 | Comparative photocatalytic activity of PDPB nanofibres, TiO2 and Ag–TiO2 . a–d, Photocatalytic degradation of methyl orange (a,b) and phenol (c,d) in the presence of commercial P25 TiO2 , Ag–TiO2 and the synthesized bulk PDPB and nano PDPB. a,c, Degradation carried out under visible light (>450 nm). b,d, Degradation carried out under ultraviolet light. The concentrations of nano PDPB, bulk PDPB, Ag–TiO2 and TiO2 in water were 1 mg ml−1 . Initial concentrations C0 were 6 × 10−5 mol l−1 for MO and 3.7 × 10−3 mol l−1 for phenol. The legend in a applies to all panels.

and Supplementary Fig. 1b)20 . The synthesized polymer structures were characterized by attenuated total reflection–Fourier transform infrared (ATR–FTIR) spectroscopy (Fig. 1f). The band at 2,146 cm−1 corresponding to the antisymmetric acetylene infrared active band of DPB monomer, was not detected after irradiation, proving complete polymerization of DPB. The nanofibres were also characterized by the combination of a nanoscale probe from an atomic force microscope with a tunable infrared source, producing a nanoscale infrared (nanoIR) instrument that can survey various regions of the polymer via AFM topography imaging and acquire high-resolution local chemical spectra at selected regions on the sample (Fig. 1g–i)21 . The intense signal obtained at 1,490 cm−1 is associated with the π-conjugated enyne unit as well as aromatic ring stretching and bending vibration of the benzene ring in the PDPB polymer (Fig. 1g). Figure 1h shows the large intensity change of the C–H stretching modes of the polymer nanostructures, which is associated with the frequency band at 3,054 cm−1 . In the spectral region 2,300–2,100 cm−1 (alkyne band of monomer) no signal was detected (Fig. 1i) (as a result of the conformation of the polymer the alkyne stretching vibration is not active). The nano PDPB has also been characterized by NMR and the same chemical structures were obtained for bulk PDPB and nano PDPB (owing to their similar photopolymerization routes; Supplementary Fig. 2). Thermogravimetric analysis of the PDPB polymer powder showed

the onset of decomposition at approximately 200 ◦ C, with a major decomposition occurring at approximately 540 ◦ C (Supplementary Fig. 3). The molar mass of polymer has been found to be 1,625 g mol−1 , as determined by gel permeation chromatography (GPC), and corresponds to oligomers of degree of polymerization 8. The polymers are relatively short chains, presumably because they are induced by polymerization in the oil-confined domains. The PDPB nanofibres are probably formed by π-stacking of these oligomers in the oil tubes of the mesophases. The disposal of coloured wastes such as dyes from industry into water is toxic to aquatic life and creates serious environmental pollution. Methyl orange (an azo dye) is fairly stable under visible light irradiation without a photocatalyst and almost no photolysis was observed under the present experimental conditions. Here, we studied the degradation of methyl orange (MO) and phenol (chosen as model pollutants) in water under ultraviolet and visible light irradiation in the presence of bulk PDPB and nano PDPB. Before any photocatalytic tests, dark absorption tests of bulk PDPB and nano PDPB were carried out, by putting the polymer samples into contact with the model pollutants in the absence of irradiation. In the case of bulk PDPB, a decrease in the concentration of phenol and MO with time was measured, suggesting adsorption of phenol and MO on the bulk polymer surface. In contrast, no adsorption was observed on PDPB nanofibre networks (Supplementary Fig. 4).

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NATURE MATERIALS DOI: 10.1038/NMAT4220

LETTERS a

b Ar

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Figure 3 | Schematic representation of the photocatalytic mechanism and energy level calculation of polymer structures by density functional theory. a, Energy diagram representing the evaluated HOMO and LUMO levels of PDPB polymer. b, Possible photocatalysis mechanism with charge separation in nano PDPB, with electron reducing oxygen and hole oxidizing water; the holes and generated oxidative radicals can oxidize organic pollutants (noted as M), V.B. and C.B. represent the valence band and the conduction band of PDPB polymer, respectively.

The photocatalytic activity of bulk PDPB and nano PDPB was evaluated by measuring the decomposition rate of the model pollutants in water under both ultraviolet–visible (xenon lamp) and visible light (using a filter at λ > 450 nm) (Fig. 2). The photocatalytic activities of bulk PDPB and nano PDPB were compared with that of commercial TiO2 (P25, which is very active under ultraviolet light) and silver-modified P25 TiO2 (Ag–TiO2 , which is active under visible light irradiation). (Synthesis details are given in Supplementary Fig. 5)7 . Nano PDPB demonstrates good photocatalytic activity for the degradation of MO under ultraviolet– visible light: a 45% degradation rate is reached after 15 min whereas with TiO2 and Ag–TiO2 , 75% and 95% degradation are achieved respectively (Fig. 2a). Most interestingly, under visible light, nano PDPB shows a high photocatalytic activity, even higher than that of plasmonic Ag-modified TiO2 : we found 75% photodegradation of MO after a 240 min irradiation with nano PDPB, whereas only 17% degradation is obtained with Ag–TiO2 (Fig. 2b). Total organic carbon (TOC) measurements indicate ∼50% of mineralization of MO after 270 min irradiation under visible light. In contrast, bulk PDPB shows a weak photocatalytic activity under both ultraviolet and visible light. Similar photocatalytic degradation trends were obtained with phenol: nano PDPB exhibits good photocatalytic activity under ultraviolet light, with 82% degradation of phenol after 270 min irradiation, but this activity is lower than that of bare or modified TiO2 (Fig. 2c). However, under visible light illumination nano PDPB exhibits a much higher photocatalytic activity than Ag–TiO2 . As shown in Fig. 2d, 70% of phenol is degraded with nano PDPB after 270 min irradiation, compared with only 18% with Ag–TiO2 . With nano PDPB, 60% of mineralization of phenol is reached after 270 min irradiation under visible light. Again, the photocatalytic activity of bulk PDPB was found to be weak for phenol degradation both under ultraviolet and visible light (Fig. 2c,d). To understand the origin of the photocatalytic activity of PDPB nanofibres under visible light, it is necessary to investigate their electronic properties. For this, cyclic voltammetry (CV) measurements and density functional theory (DFT) calculations have been carried out. The CV measurements were used to evaluate the experimental HOMO and LUMO energy levels—from the ionization potential and the electronic affinity, respectively—and determine the bandgap for bulk and nano PDPB (Supplementary Fig. 6a and Supplementary Table 1). For both PDPB structures, we found that the main p-doping (oxidation) and n-doping (reduction) processes are irreversible, and the values of the peak potentials versus standard hydrogen electrode (SHE) are very similar: +2.08 V 4

(oxidation) and −1.98 V (reduction) for nano PDPB, and +2.08 V (oxidation) and −1.96 V (reduction) for bulk PDPB, yielding an energy gap of ∼4.06 eV as a first approximation. This numerical value is surprising given that PDPB absorbs visible light. Indeed, a careful analysis of the CV of both polymer structures (zoom at lower current, Supplementary Fig. 6b) reveals onsets of oxidation and reduction processes occurring at lower potentials, affording estimation of much lower energy gaps of approximately 1.81 eV and 1.66 eV, for nano PDPB and bulk PDPB respectively. Note that these results are consistent with the calculated value of the PDPB bandgap, which is 1.95 eV on the basis of DFT (considering oligomeric PDPB structures comprising various numbers of units from one to eight, as detailed in Fig. 3a, Supplementary Table 2 and Supplementary Fig. 7)22,23 . Hence, our experimental and theoretical results demonstrate a possible photocatalytic activity under visible light of both bulk PDPB and nano PDPB. When illuminated with photons of energy exceeding (or equal to) the bandgap (E ≥ 1.81 eV or λ ≤ 685 nm), excess electrons and holes are formed in the conjugated polymer chains. Organic pollutants can be degraded and mineralized by photocatalysts. Photocatalysis involves the generation of charge carriers—that is, excess electrons (e− ) and holes (h+ ), and the catalytic reactions induced by these species. When a semiconductor (such as TiO2 ) is excited with photons with energy higher than (or equal to) the bandgap, electrons are injected in the conduction band. Consequently, the electrons and holes migrate, when they escape recombination, to the semiconductor surface and generate highly oxidative radicals (in the presence of oxygen and water), which can degrade and mineralize organic pollutants24 . Under irradiation, electrons are injected from the conducting polymer and react with oxygen to form the oxidizing O2 ·− superoxide radical (E 0 , O2 /O2 ·− = −0.33VSHE )25 : O2 + e− → O2 ·−

(1)

Indeed, the calculated energy level of the conduction band of PDPB makes this reaction thermodynamically favourable. Therefore, the effects of scavengers on the photocatalytic degradation of MO and phenol were examined to investigate the role of excess charge carriers and the contribution of the reactive oxygen species (ROS) in this process. To investigate the role of the excess electrons in the photocatalytic process, experiments were conducted in the presence of Cu2+ scavengers (it reacts with electron to yield Cu+ ; ref. 26). This reaction competes with reaction (1) and the presence of Cu2+ causes

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NATURE MATERIALS DOI: 10.1038/NMAT4220 100 90 80 70 60 50 40 30 20 10 0

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Figure 4 | Recycling and stability of the PDPB nanofibres. a, Photocatalytic degradation of MO and phenol after up to five cycles measured after visible light irradiation of duration 240 min. b, Topographic image of photosynthesized PDPB nanofibres by conventional AFM after degradation of methyl orange. c, NanoIR mappings of the PDPB nanostructures after photocatalytic degradation of methyl orange as measured at a fixed wavenumber (3,054 cm−1 ) after five cycles. The signal obtained at 3,054 cm−1 in chemical mapping, which originates from the benzene ring in the PDPB polymer, does not change after degradation. d,e, NanoIR spectra recorded in two different spectral regions of the PDPB nanofibres before and after catalysis. The large intensity change for the C–H stretching modes of the polymer nanostructure is associated with the frequency band at 3,054 cm−1 . The intense band at 1,490 cm−1 is associated with the π-conjugated enyne unit as well as aromatic ring stretching and bending vibration of the benzene ring in the PDPB polymer. f, TEM image of the PDPB nanostructure after degradation of methyl orange under visible light irradiation.

a decrease in the production of O2 ·− in the photocatalytic system, which in turn slows down the degradation kinetics (Supplementary Figs 8 and 9). The photodegradation efficiency of MO is reduced from 78% to 22% in the presence of Cu2+ after 4 h under visible light irradiation. Furthermore, MO photodegradation tests under argon atmosphere have been performed to address the crucial role of oxygen. Under deaerated conditions, a suppression of O2 ·− radical production occurs, inhibiting the degradation of the model pollutant by PDPB: After 4 h irradiation only 1% of the MO was degraded (see Supplementary Fig. 10). This confirms the crucial role of O2 ·− in the photodegradation process. According to the valence band energy level, HO− (or H2 O) cannot react with the holes to yield oxidative HO· (E0 , HO· /HO− = +2.80 VNHE ) (ref. 25). However, a small quantity of HO· radicals was detected using Tris(hydroxymethyl)aminomethane (Tris) as a

probe27 (Supplementary Table 3) and these radicals can be formed by the following reactions equations (2)–(5): O2 ·− + H+ → HO2 ·

(2)

2HO2 · → H2 O2 + O2

(3)

H2 O2 + O2 ·− → HO· + O2 + HO−

(4)

H2 O2 + hν → 2HO·

(5)

All these results suggest that under ultraviolet and visible light excitation, charge carriers are generated in the PDPB polymers, inducing the formation of ROS, which are responsible for the degradation process with the main contribution from O2 ·− . In

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NATURE MATERIALS DOI: 10.1038/NMAT4220

LETTERS parallel, the holes (h+ ) also diffuse to the surface and, according to the calculated energy level of the valence band, they may drive the oxygen-evolution half-reaction at pH 7 (ref. 28): 2H2 O + 4h+ → O2 + 4H+ The holes may also directly oxidize the pollutant molecules during the catalytic degradation reaction. The electron–hole recombination can be decreased by the addition of a scavenger. To investigate the electron–hole recombination process, a photocatalytic degradation study has been conducted in the presence of isopropanol, which is known as a hole scavenger29 . The decomposition kinetics by PDPB nanofibres is noticeably accelerated in the presence of isopropanol (0.1 M; Supplementary Fig. 10). This indicates that the competitive reaction of excess holes with isopropanol decreases the recombination rate, leading to more available excess electrons for oxidative radical O2 ·− -mediated degradation. On the basis of the bandgap structure of the polymer and the experiments with scavengers, a possible mechanism for the degradation of organic pollutant has been proposed. Figure 3b summarizes the generation of charge carriers under irradiation and the formation of oxidative radicals responsible for the oxidation and mineralization of the pollutants. It should be noted that the calculated energy levels of the valence band and the conduction band also make these PDPB nanostructures thermodynamically favourable for the water-splitting reaction. However, the following question remains: why do PDPB nanofibres exhibit a much higher activity than bulk PDPB? X-ray diffraction patterns of bulk PDPB and nano PDPB are very close (Supplementary Fig. 11), with similar crystallinity. The difference in the photocatalytic activity between nano and bulk PDPB might be due to the larger size and presence of more defects in bulk PDPB favouring higher e− –h+ recombination. This dependence of the photocatalytic activity on the size and morphology of the structure has also been observed in the case of semiconductors such as TiO2 , and is still a matter of investigation30,31 . Tests have been conducted to study the stability of the photocatalysts. We found that the photocatalytic activity does not decrease even after several cycles (Fig. 4a), suggesting that PDPB nanofibres can be efficiently recycled and reused for repeated cycles without appreciable loss of activity. This feature is crucial for industrial applications, where stable and good recyclability of photocatalysts is highly desirable. No differences have been found, either in the morphology of the PDPB nanostructure or in their chemical structure after photocatalytic degradation, as attested by nanoIR, NMR and FTIR characterizations (Fig. 4b–e and Supplementary Figs 2 and 12). Furthermore, TEM images show that the morphology of the polymer nanostructures remained the same after repeated photocatalytic cycling (Fig. 4f and Supplementary Fig. 13). These results attest that the PDPB nanofibres are very stable photocatalysts. The concept of using polymer nanostructures as a visible light active photocatalyst could be extended to other semiconducting polymers. In conclusion, PDPB nanofibres obtained by π-staking of oligomers present a high photocatalytic activity under ultraviolet and visible light. These photocatalysts are very stable with cycling. Our results demonstrate that conducting polymer nanofibres offer the prospective development of a new generation of efficient and cheap visible light driven photocatalysts for environmental protection. The emerging understanding of the mechanism as well as novel metal-free materials should bring the promise to fulfilment of actual applications in the near future. Moreover, the application of conjugated nanostructures in the field of photocatalysis can be generalized to longer chain polymers. 6

Methods Synthesis of polymer nanostructures in mesophases. The swollen hexagonal mesophases were prepared following a previously published method with some modifications. Typically, 1 g of the surfactant (sodium dodecyl sulphate) was dissolved in 2 ml of 0.3 mol l−1 NaCl in glass tubes. After vigorous agitation at 30 ◦ C, the surfactant had completely dissolved to give a transparent and viscous micellar solution. The subsequent addition of cyclohexane containing monomer 1,4-diphenylbutadiyne (DPB) (10% of mass) and initiator benzoin methyl ether (BME) (1%) in the micellar solution under stirring leads to a white unstable emulsion. A co-surfactant, pentanol-1 (420 µl), was then added to the mixture, which was then strongly vortexed for a few minutes. This led to a perfectly colourless, translucent, birefringent and stable gel: a hexagonal mesophase. The doped mesophases with the monomer and the initiator for polymerization were used as soft templates to synthesize polymer nanostructures induced by irradiation using ultraviolet light with an Oriel 300 W Xenon ultraviolet–visible lamp at a distance of 5 cm for 12 h. After reaction, the materials were extracted in a water–ethanol mixture, centrifuged, and then washed several times to eliminate the surfactant, co-surfactant and salt. Photocatalytic activity measurements. The photocatalytic activity of the PDPB nanofibres has been evaluated for photodecomposition of phenol (C6 H5 OH) and methyl orange (MO) in water. The photodegradation reactions were carried out in quartz cell reactor with a 10 mm optical path containing 3.5 ml of 3.7 × 10−3 mol l−1 of phenol and 6 × 10−5 mol l−1 of MO in the presence of 1 g l−1 of PDPB nanofibres. The suspension containing PDPB with pollutants was stirred in the dark for 2 h to establish adsorption–desorption equilibrium before irradiation. Then the solutions were irradiated with an Oriel 300 W xenon lamp through an infrared water filter and an ultraviolet cutoff filter (λ > 450 nm; for the experiments under visible light) and bubbled with O2 at a fixed flow rate along with magnetic stirring. Next, 0.5 ml of aliquot was collected from the reactor at different time points. In the case of P25 and Ag-modified P25, after the degradation reaction, the solution was centrifuged to separate the catalyst and obtain a transparent solution. High-performance liquid chromatography (HPLC) was used to determine the concentration of phenol and to study its degradation. A Varian Prostar 230 ternary gradient pump was combined with a Prostar 330 photodiode array detector (D2 lamp). MO solutions were characterized using a HP Agilent diode array 8453 ultraviolet–visible spectrophotometer by following the signal at wavelength 464 nm that corresponds to the maximal absorption. Multiple photocatalysis experiments were performed under identical reaction conditions to determine reproducibility. After completion of the degradation of MO and phenol, PDPB nanofibres were recovered by filtration, and then dried at 30 ◦ C overnight. This used catalyst was re-employed in the next cycle under identical conditions. The total organic carbon (TOC) technique was used to study the mineralization of phenol and MO for different irradiation times using a Shimadzu TOC-LCSH. For the detection of the hydroxyl radical, Tris was used as a probe. During the reaction between Tris and hydrogen-abstracting species, such as hydroxyl radicals, formaldehyde is produced. The concentration of formaldehyde was quantified by means of the Hantzsch method27 . Material characterization and measurement. Small-angle X-ray scattering (SAXS) was used to characterize the mesophases (doped with the monomer and the catalyst) before and after polymerization. The morphology of the PDPB was examined by TEM using a JEOL JEM 100 CXII transmission electron microscope at an accelerating voltage of 100 kV and high-resolution scanning electron microscopy using an IDFix SAMx system installed on a Zeiss Supra 55 FEG-SEM. ATR–FTIR spectra of PDPB nanofibres were obtained using a Brüker Vertex 70 FTIR spectrometer. The morphology, eventually combined with the local spectrum of PDPB on a solid substrate, was determined by combining a conventional atomic force microscope (AFM) with a tunable pulsed laser as the infrared source—a commercial set-up, nanoIR (3,600 cm−1 to 1,000 cm−1 , Anasys Instrument). A gel permeation chromatography (GPC) machine (‘AGILENT 2000’) was used for molecular weight determination. The ultraviolet–visible absorption spectra of the solid polymers were recorded with a Carry 5000 AGILENT UV-Vis-NIR spectrophotometer. 1 H and 13 C NMR spectra were recorded using a Bruker Avance 250 MHz spectrometer. Cyclic voltammetry (CV) was used to estimate the energetic levels EHOMO and ELUMO of bulk PDPB and nano PDPB using a Princeton Applied Research potentiostat/galvanostat model 273A (EG&G). Computational methodology. DFT calculations employing the B3LYP functional were carried out on monomer and short PDPB oligomers (comprising n = 2, 4, 6 and 8 monomers) using the GAMESS-US Quantum Chemistry software. The calculations were performed in the gas phase. The highest occupied

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NATURE MATERIALS DOI: 10.1038/NMAT4220 molecular orbital energy level (EHOMO ) and the lowest unoccupied molecular orbital energy level (ELUMO ) were obtained.

Received 20 May 2014; accepted 6 January 2015; published online 16 March 2015

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Acknowledgements S.G. acknowledges Marie Curie COFUND, RBUCE-UP (Research Based University Chairs of Excellence of Paris) and PRES UniverSud Paris for a postdoctoral fellowship. The authors gratefully acknowledge C’Nano Ile de France and Université Paris-Sud (ERM project) for financial support for the Cobalt-60 panoramic gamma source.

Author contributions S.G. carried out fabrication of the polymer nanostructure, performed the experiment on photocatalytic activity and also contributed to writing of the manuscript. N.A.K. conducted the photocatalysis experiments. L.R. characterized the doped mesophases by SAXS and the polymer by XRD. S.R. provided information about conducting polymers. A.D. and A.D-B. ran the nanoIR system for characterization and stability of the polymer nanostructures with cycling. P.B. characterized the polymer nanostructures by TEM. F.G. and P-H.A. provided NMR characterizations, theoretical calculations, bandgap measurements and electrochemical investigations. H.R. supervised the entire project and also wrote the manuscript.

Additional information Supplementary information is available in the online version of the paper. Reprints and permissions information is available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to H.R.

Competing financial interests The authors declare no competing financial interests.

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Conducting polymer nanostructures for photocatalysis under visible light.

Visible-light-responsive photocatalysts can directly harvest energy from solar light, offering a desirable way to solve energy and environment issues...
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