LETTERSPUBLISHED ONLINE: 16 MARCH 2015 | DOI: 10.1038/NMAT4220

Conducting polymer nanostructures forphotocatalysis under visible lightSrabanti 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 harvestenergy fromsolar light, o�eringadesirableway to solveenergyand environment issues1. Here, we show that one-dimensionalpoly(diphenylbutadiyne) nanostructures synthesized by pho-topolymerization using a soft templating approach have highphotocatalytic activity under visible light without the assis-tance of sacrificial reagents or precious metal co-catalysts.These polymer nanostructures are very stable even af-ter repeated cycling. Transmission electron microscopy andnanoscale infrared characterizations reveal that the morphol-ogy and structure of the polymer nanostructures remain un-changed after many photocatalytic cycles. These stable andcheap polymer nanofibres are easy to process and can bereused without appreciable loss of activity. Our findings mayhelp the development of semiconducting-based polymers forapplications in self-cleaning surfaces, hydrogen generationand photovoltaics.

Increasing attention is being drawn towards nanomaterials forphotocatalytic solar-energy conversion to identify robust newmeth-ods for water purification and environmental protection at lowercost and energy consumption1. For an optimized use of solar energy,efficient and stable photocatalysts that are capable of harvestingvisible light are required. Over the past decades, oxide-based semi-conductors, in particular TiO2, have been recognized as efficientphotocatalysts for the degradation of organic pollutants in wastew-ater2,3. The limitation of TiO2 (like most other photocatalysts)4 forapplications is essentially due to the low quantum yield that resultsfrom charge-carrier (e−/h+) recombination and the necessity to useultraviolet irradiation because of the large value of its bandgap5. Toovercome this limitation and produce photocatalysts responsive tovisible light, surface-tuning strategies and modification of oxideson the nanometre scale have been developed through doping orsurface modification6. Indeed, TiO2 doping with N, C or S, or itsmodification with metal nanoparticles (Ag, Au, Pt), has extendedits activity towards the visible region6,7. However, the photocatalyticactivity of the modified materials in visible light is still not sufficientfor commercial applications. Alternative materials have been pro-posed for photocatalysis and solar-energy conversion. For example,polymeric carbon nitride was found to be an efficient photocatalystthat produces hydrogen from water under visible light irradiation,but a sacrificial donor is required8. Hence, large-scale productionof stable visible-light-active photocatalysts remains a challenge for

industrial applications9. The central issue of current research isto consider the extension of the light absorption spectrum to thevisible region, thus facilitating the use of sunlight as an inexpensive,renewable energy source for photocatalytic processes.

Conjugated polymer nanostructures are emerging as new energymaterials for applications in solar cells, fuel cells and rechargeablelithium batteries, owing to their advantages of low-cost facilesynthesis, excellent electrical and electrochemical activity, highcarrier mobility and mechanical properties10. Significant attemptshave been focused on understanding the polymer nanostructuresfor energy conversion and storage applications11. Although oxide-based semiconductors have been widely explored, photocatalyticactivity studies of conjugated polymers are still scarce12,13.Semiconductor nanostructures modified with conducting polymershave been studied for photocatalytic applications12–14. Polypyrrolefilms containing TiO2 nanoparticles exhibit higher photocatalyticactivity than bare TiO2 for the degradation of dyes under ultravioletradiation12. On the other hand, TiO2 modified by conjugatedderivatives of polyisoprene exhibit photocatalytic activity undervisible 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 varioustextile dyes under ultraviolet irradiation, but not under visiblelight13. A potential use of conjugated polymer photocatalysts insynthetic applications is illustrated by the visible-light-drivenpinacol coupling of benzaldehyde using poly(p-phenylene) as aphotoredox catalyst14.

Here, we demonstrate that nanofibres of a conjugated polymer,poly(diphenylbutadiyne) (PDPB), are highly efficient under visiblelight for the degradation of pollutants (phenol and methyl orangehave been used as model pollutants). Furthermore, we show thatthese photocatalysts are very stable even after repeated cycling.Interestingly, we find that PDPB nanofibres exhibit a muchhigher photocatalytic activity than bulk PDPB. To the best of ourknowledge, our findings constitute the first experimental evidenceof the photocatalytic activity of conjugated polymer nanostructuresunder visible light for water decontamination.

PDPB belongs to the family of poly(diacetylenes), whichare one of the interesting research targets among π-conjugatedpolymers, having fascinating physicochemical properties withtailorable pendant side groups and terminal functionalities15,16.We have developed a controlled soft-template-mediated synthesis

1Laboratoire de Chimie Physique, UMR 8000-CNRS, Bât. 349, Université Paris-Sud, 91405 Orsay, France. 2Laboratoire Charles Coulomb (L2C) UMR 5221CNRS-Université de Montpellier, 34095 Montpellier, France. 3Départment CASER, Ecole SITI, Conservatoire National des Arts et Métiers, CNAM, 292 rueSaint-Martin, 75141 Paris Cedex 03, France. 4Sorbonne Universités, UPMC Univ. Paris 06, UMR 7197-CNRS, Laboratoire de Réactivité de Surface,F-75005 Paris, France. 5CNRS, UMR 7197, Laboratoire de Réactivité de Surface, F-75005 Paris, France. 6Laboratoire de Physicochimie des Polymères et desInterfaces (LPPI), Université de Cergy-Pontoise, 95031 Cergy-Pontoise Cedex, France. 7CNRS, Laboratoire de Chimie Physique, UMR 8000, 91405 Orsay,France. *e-mail: hynd.remita@u-psud.fr

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LETTERS NATUREMATERIALS DOI: 10.1038/NMAT4220

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Figure 1 | Synthesis and characterization of PDPB nanofibres. a, Photograph of swollen hexagonal phases before polymerization (transparent gel) and afterpolymerization (yellow gel) by ultraviolet irradiation. b, Schematic of polymerization of diphenylbutadiyne (DPB) by ultraviolet irradiation. c, Small-angleX-ray scattering spectra of swollen hexagonal phases before (black squares) and after polymerization of 1,4-diphenylbutadiyne by ultraviolet irradiation(red cycles). The di�raction patterns are characteristic of a hexagonal phase, as demonstrated by four Bragg peaks (black arrows) whose positions are inthe ratio 1:

√3:2:√

7. From the peak position and sample composition, one evaluates that the diameter of the oil-swollen surfactant-stabilized tubes is16 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 nanostructuressynthesized by ultraviolet light irradiation. g–i, AFM–infrared spectra recorded in three di�erent spectral regions for PDPB. g, Inset: Atomic forcemicrograph 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 oil-swollen surfactant tubes arranged on a triangular lattice in waterunder ultraviolet irradiation17,18. The hydrophobic domain ofthe mesophases can accommodate high concentrations (up to20wt.%) of 1,4-diphenylbutadiyne (DPB) monomer, which candirectly polymerize by photoirradiation in the presence of afree-radical initiator (benzoin methyl ether, BME, 1%)19. Thedoped mesophases become yellow after polymerization but remaintranslucent (Fig. 1a). The DPB monomers undergo polymerizationvia 1,4-addition reaction to form alternating ene–yne polymerchains on irradiation with ultraviolet light (Fig. 1b).

The polymer-doped hexagonal phases were translucent andbirefringent owing to their anisotropic structure. Small-angle X-rayscattering (SAXS) experiments (experimental set-up describedin Supplementary Information) demonstrate that the hexagonalstructure is preserved on polymerization (Fig. 1c). The as-prepared

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

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NATUREMATERIALS DOI: 10.1038/NMAT4220 LETTERSb MO−Vis

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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 structureswere characterized by attenuated total reflection–Fourier transforminfrared (ATR–FTIR) spectroscopy (Fig. 1f). The band at 2,146 cm−1corresponding to the antisymmetric acetylene infrared active bandof DPB monomer, was not detected after irradiation, provingcomplete polymerization of DPB. The nanofibres were alsocharacterized by the combination of a nanoscale probe from anatomic force microscope with a tunable infrared source, producinga nanoscale infrared (nanoIR) instrument that can survey variousregions of the polymer via AFM topography imaging and acquirehigh-resolution local chemical spectra at selected regions on thesample (Fig. 1g–i)21. The intense signal obtained at 1,490 cm−1 isassociated with the π-conjugated enyne unit as well as aromaticring stretching and bending vibration of the benzene ring inthe PDPB polymer (Fig. 1g). Figure 1h shows the large intensitychange of the C–H stretchingmodes of the polymer nanostructures,which is associated with the frequency band at 3,054 cm−1. In thespectral region 2,300–2,100 cm−1 (alkyne band of monomer) nosignal was detected (Fig. 1i) (as a result of the conformation of thepolymer the alkyne stretching vibration is not active). The nanoPDPB has also been characterized by NMR and the same chemicalstructures were obtained for bulk PDPB and nano PDPB (owingto 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 majordecomposition occurring at approximately 540 ◦C (SupplementaryFig. 3). The molar mass of polymer has been found to be1,625 gmol−1, as determined by gel permeation chromatography(GPC), and corresponds to oligomers of degree of polymerization8. The polymers are relatively short chains, presumably becausethey are induced by polymerization in the oil-confined domains.The PDPB nanofibres are probably formed by π-stacking of theseoligomers in the oil tubes of the mesophases.

The disposal of coloured wastes such as dyes from industry intowater is toxic to aquatic life and creates serious environmentalpollution. Methyl orange (an azo dye) is fairly stable under visiblelight irradiation without a photocatalyst and almost no photolysiswas observed under the present experimental conditions. Here, westudied the degradation of methyl orange (MO) and phenol (chosenas model pollutants) in water under ultraviolet and visible lightirradiation in the presence of bulk PDPB and nano PDPB. Beforeany photocatalytic tests, dark absorption tests of bulk PDPB andnano PDPB were carried out, by putting the polymer samples intocontact with the model pollutants in the absence of irradiation. Inthe case of bulk PDPB, a decrease in the concentration of phenoland MO with time was measured, suggesting adsorption of phenolandMOon the bulk polymer surface. In contrast, no adsorptionwasobserved on PDPB nanofibre networks (Supplementary Fig. 4).

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a b−1

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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 innano 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 PDPBwas evaluated by measuring the decomposition rate of the modelpollutants in water under both ultraviolet–visible (xenon lamp)and visible light (using a filter at λ > 450 nm) (Fig. 2). Thephotocatalytic activities of bulk PDPB and nano PDPB werecompared with that of commercial TiO2 (P25, which is very activeunder ultraviolet light) and silver-modified P25 TiO2 (Ag–TiO2,which is active under visible light irradiation). (Synthesis details aregiven in Supplementary Fig. 5)7. Nano PDPB demonstrates goodphotocatalytic activity for the degradation ofMOunder ultraviolet–visible light: a 45% degradation rate is reached after 15min whereaswith TiO2 and Ag–TiO2, 75% and 95% degradation are achievedrespectively (Fig. 2a). Most interestingly, under visible light, nanoPDPB shows a high photocatalytic activity, even higher than thatof plasmonic Ag-modified TiO2: we found 75% photodegradationof MO after a 240min irradiation with nano PDPB, whereas only17% degradation is obtained with Ag–TiO2 (Fig. 2b). Total organiccarbon (TOC) measurements indicate ∼50% of mineralization ofMO after 270min irradiation under visible light. In contrast, bulkPDPB shows a weak photocatalytic activity under both ultravioletand visible light. Similar photocatalytic degradation trends wereobtained with phenol: nano PDPB exhibits good photocatalyticactivity under ultraviolet light, with 82% degradation of phenolafter 270min irradiation, but this activity is lower than that ofbare or modified TiO2 (Fig. 2c). However, under visible lightillumination nano PDPB exhibits a much higher photocatalyticactivity than Ag–TiO2. As shown in Fig. 2d, 70% of phenol isdegradedwith nano PDPB after 270min irradiation, comparedwithonly 18% with Ag–TiO2. With nano PDPB, 60% of mineralizationof phenol is reached after 270min irradiation under visible light.Again, the photocatalytic activity of bulk PDPB was found to beweak for phenol degradation both under ultraviolet and visiblelight (Fig. 2c,d).

To understand the origin of the photocatalytic activity ofPDPB nanofibres under visible light, it is necessary to investigatetheir electronic properties. For this, cyclic voltammetry (CV)measurements and density functional theory (DFT) calculationshave been carried out. The CVmeasurements were used to evaluatethe experimental HOMO and LUMO energy levels—from theionization potential and the electronic affinity, respectively—anddetermine the bandgap for bulk and nano PDPB (SupplementaryFig. 6a and Supplementary Table 1). For both PDPB structures, wefound that themain p-doping (oxidation) and n-doping (reduction)processes are irreversible, and the values of the peak potentialsversus standard hydrogen electrode (SHE) are very similar:+2.08V

(oxidation) and −1.98V (reduction) for nano PDPB, and +2.08V(oxidation) and −1.96V (reduction) for bulk PDPB, yielding anenergy gap of ∼4.06 eV as a first approximation. This numericalvalue is surprising given that PDPB absorbs visible light. Indeed,a careful analysis of the CV of both polymer structures (zoom atlower current, Supplementary Fig. 6b) reveals onsets of oxidationand reduction processes occurring at lower potentials, affordingestimation of much lower energy gaps of approximately 1.81 eVand 1.66 eV, for nano PDPB and bulk PDPB respectively. Notethat these results are consistent with the calculated value of thePDPB bandgap, which is 1.95 eV on the basis of DFT (consideringoligomeric PDPB structures comprising various numbers of unitsfrom one to eight, as detailed in Fig. 3a, Supplementary Table 2 andSupplementary Fig. 7)22,23. Hence, our experimental and theoreticalresults demonstrate a possible photocatalytic activity under visiblelight of both bulk PDPB and nano PDPB. When illuminated withphotons of energy exceeding (or equal to) the bandgap (E≥1.81 eVor λ ≤ 685 nm), excess electrons and holes are formed in theconjugated polymer chains.

Organic pollutants can be degraded and mineralized byphotocatalysts. Photocatalysis involves the generation of chargecarriers—that is, excess electrons (e−) and holes (h+), and thecatalytic 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 conductionband. Consequently, the electrons and holes migrate, when theyescape recombination, to the semiconductor surface and generatehighly oxidative radicals (in the presence of oxygen and water),which can degrade and mineralize organic pollutants24. Underirradiation, electrons are injected from the conducting polymer andreact with oxygen to form the oxidizing O2

·− superoxide radical(E0, O2/O2

·−=−0.33VSHE)25:

O2+e−→O2·− (1)

Indeed, the calculated energy level of the conduction band of PDPBmakes this reaction thermodynamically favourable. Therefore, theeffects of scavengers on the photocatalytic degradation of MOand phenol were examined to investigate the role of excess chargecarriers and the contribution of the reactive oxygen species (ROS)in this process.

To investigate the role of the excess electrons in the photocatalyticprocess, experiments were conducted in the presence of Cu2+

scavengers (it reacts with electron to yield Cu+; ref. 26). Thisreaction competes with reaction (1) and the presence of Cu2+ causes

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NATUREMATERIALS DOI: 10.1038/NMAT4220 LETTERSa

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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 visiblelight 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) afterfive 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 afterdegradation. d,e, NanoIR spectra recorded in two di�erent spectral regions of the PDPB nanofibres before and after catalysis. The large intensity change forthe 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 isassociated with the π-conjugated enyne unit as well as aromatic ring stretching and bending vibration of the benzene ring in the PDPB polymer. f, TEMimage 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 (SupplementaryFigs 8 and 9). The photodegradation efficiency of MO is reducedfrom 78% to 22% in the presence of Cu2+ after 4 h under visiblelight irradiation.

Furthermore, MO photodegradation tests under argonatmosphere have been performed to address the crucial roleof oxygen. Under deaerated conditions, a suppression of O2

·−

radical production occurs, inhibiting the degradation of the modelpollutant by PDPB: After 4 h irradiation only 1% of the MO wasdegraded (see Supplementary Fig. 10). This confirms the crucialrole of O2

·− in the photodegradation process.According to the valence band energy level, HO− (or H2O)

cannot react with the holes to yield oxidative HO· (E0, HO·/HO−=+2.80 VNHE) (ref. 25). However, a small quantity of HO· radicalswas detected using Tris(hydroxymethyl)aminomethane (Tris) as a

probe27 (Supplementary Table 3) and these radicals can be formedby the following reactions equations (2)–(5):

O2·−+H+→HO2

· (2)

2HO2·→H2O2+O2 (3)

H2O2+O2·−→HO·

+O2+HO− (4)

H2O2+hν→2HO· (5)

All these results suggest that under ultraviolet and visible lightexcitation, charge carriers are generated in the PDPB polymers,inducing the formation of ROS, which are responsible for thedegradation process with the main contribution from O2

·−. In

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LETTERS NATUREMATERIALS DOI: 10.1038/NMAT4220

parallel, the holes (h+) also diffuse to the surface and, according tothe calculated energy level of the valence band, they may drive theoxygen-evolution half-reaction at pH 7 (ref. 28):

2H2O+4h+→O2+4H+

The holes may also directly oxidize the pollutant molecules duringthe catalytic degradation reaction.

The electron–hole recombination can be decreased by the addi-tion of a scavenger. To investigate the electron–hole recombinationprocess, a photocatalytic degradation study has been conducted inthe presence of isopropanol, which is known as a hole scavenger29.The decomposition kinetics by PDPB nanofibres is noticeablyaccelerated in the presence of isopropanol (0.1M; Supplemen-tary Fig. 10). This indicates that the competitive reaction of ex-cess holes with isopropanol decreases the recombination rate,leading to more available excess electrons for oxidative radicalO2

·−-mediated degradation.On the basis of the bandgap structure of the polymer and

the experiments with scavengers, a possible mechanism for thedegradation of organic pollutant has been proposed. Figure 3bsummarizes the generation of charge carriers under irradiation andthe formation of oxidative radicals responsible for the oxidationand mineralization of the pollutants. It should be noted that thecalculated energy levels of the valence band and the conductionband also make these PDPB nanostructures thermodynamicallyfavourable for the water-splitting reaction.

However, the following question remains: why do PDPBnanofibres exhibit a much higher activity than bulk PDPB? X-raydiffraction patterns of bulk PDPB and nano PDPB are very close(Supplementary Fig. 11), with similar crystallinity. The difference inthe photocatalytic activity between nano and bulk PDPB might bedue to the larger size and presence of more defects in bulk PDPBfavouring higher e−–h+ recombination. This dependence of thephotocatalytic activity on the size and morphology of the structurehas 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 thephotocatalysts. We found that the photocatalytic activity does notdecrease even after several cycles (Fig. 4a), suggesting that PDPBnanofibres can be efficiently recycled and reused for repeatedcycles without appreciable loss of activity. This feature is crucialfor industrial applications, where stable and good recyclabilityof photocatalysts is highly desirable. No differences have beenfound, either in the morphology of the PDPB nanostructure orin their chemical structure after photocatalytic degradation, asattested by nanoIR, NMR and FTIR characterizations (Fig. 4b–e andSupplementary Figs 2 and 12). Furthermore, TEM images show thatthe morphology of the polymer nanostructures remained the sameafter repeated photocatalytic cycling (Fig. 4f and SupplementaryFig. 13). These results attest that the PDPB nanofibres are verystable photocatalysts. The concept of using polymer nanostructuresas a visible light active photocatalyst could be extended to othersemiconducting polymers.

In conclusion, PDPB nanofibres obtained by π-staking ofoligomers present a high photocatalytic activity under ultravioletand visible light. These photocatalysts are very stable with cycling.Our results demonstrate that conducting polymer nanofibres offerthe prospective development of a new generation of efficient andcheap visible light driven photocatalysts for environmental protec-tion. The emerging understanding of the mechanism as well asnovel metal-free materials should bring the promise to fulfilmentof actual applications in the near future. Moreover, the applicationof conjugated nanostructures in the field of photocatalysis can begeneralized to longer chain polymers.

MethodsSynthesis of polymer nanostructures in mesophases. The swollen hexagonalmesophases were prepared following a previously published method with somemodifications. Typically, 1 g of the surfactant (sodium dodecyl sulphate) wasdissolved in 2ml of 0.3mol l−1 NaCl in glass tubes. After vigorous agitation at30 ◦C, the surfactant had completely dissolved to give a transparent and viscousmicellar solution. The subsequent addition of cyclohexane containing monomer1,4-diphenylbutadiyne (DPB) (10% of mass) and initiator benzoin methyl ether(BME) (1%) in the micellar solution under stirring leads to a white unstableemulsion. 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 perfectlycolourless, translucent, birefringent and stable gel: a hexagonal mesophase. Thedoped mesophases with the monomer and the initiator for polymerization wereused as soft templates to synthesize polymer nanostructures induced byirradiation using ultraviolet light with an Oriel 300W Xenon ultraviolet–visiblelamp at a distance of 5 cm for 12 h. After reaction, the materials were extracted ina water–ethanol mixture, centrifuged, and then washed several times to eliminatethe surfactant, co-surfactant and salt.

Photocatalytic activity measurements. The photocatalytic activity of the PDPBnanofibres has been evaluated for photodecomposition of phenol (C6H5OH) andmethyl orange (MO) in water. The photodegradation reactions were carried outin quartz cell reactor with a 10mm optical path containing 3.5ml of3.7×10−3mol l−1 of phenol and 6×10−5 mol l−1 of MO in the presence of 1 g l−1of PDPB nanofibres. The suspension containing PDPB with pollutants was stirredin the dark for 2 h to establish adsorption–desorption equilibrium beforeirradiation. Then the solutions were irradiated with an Oriel 300 W xenon lampthrough an infrared water filter and an ultraviolet cutoff filter (λ>450 nm; forthe experiments under visible light) and bubbled with O2 at a fixed flow ratealong with magnetic stirring. Next, 0.5ml of aliquot was collected from thereactor at different time points. In the case of P25 and Ag-modified P25, after thedegradation reaction, the solution was centrifuged to separate the catalyst andobtain 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 330photodiode array detector (D2 lamp). MO solutions were characterized using aHP Agilent diode array 8453 ultraviolet–visible spectrophotometer by followingthe signal at wavelength 464 nm that corresponds to the maximal absorption.Multiple photocatalysis experiments were performed under identical reactionconditions to determine reproducibility. After completion of the degradation ofMO and phenol, PDPB nanofibres were recovered by filtration, and then dried at30 ◦C overnight. This used catalyst was re-employed in the next cycle underidentical conditions.

The total organic carbon (TOC) technique was used to study themineralization of phenol and MO for different irradiation times using aShimadzu TOC-LCSH.

For the detection of the hydroxyl radical, Tris was used as a probe. Duringthe reaction between Tris and hydrogen-abstracting species, such as hydroxylradicals, formaldehyde is produced. The concentration of formaldehyde wasquantified 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 andthe catalyst) before and after polymerization.

The morphology of the PDPB was examined by TEM using a JEOL JEM 100CXII transmission electron microscope at an accelerating voltage of 100 kV andhigh-resolution scanning electron microscopy using an IDFix SAMx systeminstalled on a Zeiss Supra 55 FEG-SEM.

ATR–FTIR spectra of PDPB nanofibres were obtained using a Brüker Vertex70 FTIR spectrometer. The morphology, eventually combined with the localspectrum of PDPB on a solid substrate, was determined by combining aconventional atomic force microscope (AFM) with a tunable pulsed laser as theinfrared source—a commercial set-up, nanoIR (3,600 cm−1 to 1,000 cm−1, AnasysInstrument). A gel permeation chromatography (GPC) machine (‘AGILENT2000’) was used for molecular weight determination. The ultraviolet–visibleabsorption spectra of the solid polymers were recorded with a Carry 5000AGILENT UV-Vis-NIR spectrophotometer. 1H and 13C NMR spectra wererecorded using a Bruker Avance 250 MHz spectrometer. Cyclic voltammetry(CV) was used to estimate the energetic levels EHOMO and ELUMO of bulk PDPBand nano PDPB using a Princeton Applied Research potentiostat/galvanostatmodel 273A (EG&G).

Computational methodology. DFT calculations employing the B3LYPfunctional were carried out on monomer and short PDPB oligomers (comprisingn=2, 4, 6 and 8 monomers) using the GAMESS-US Quantum Chemistrysoftware. The calculations were performed in the gas phase. The highest occupied

6 NATUREMATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials

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NATUREMATERIALS DOI: 10.1038/NMAT4220 LETTERSmolecular orbital energy level (EHOMO) and the lowest unoccupied molecularorbital energy level (ELUMO) were obtained.

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

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AcknowledgementsS.G. acknowledges Marie Curie COFUND, RBUCE-UP (Research Based UniversityChairs 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 contributionsS.G. carried out fabrication of the polymer nanostructure, performed the experiment onphotocatalytic activity and also contributed to writing of the manuscript. N.A.K.conducted the photocatalysis experiments. L.R. characterized the doped mesophases bySAXS 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 polymernanostructures with cycling. P.B. characterized the polymer nanostructures by TEM. F.G.and P-H.A. provided NMR characterizations, theoretical calculations, bandgapmeasurements and electrochemical investigations. H.R. supervised the entire project andalso wrote the manuscript.

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

Competing financial interestsThe authors declare no competing financial interests.

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