Highly Efficient Organic Photocatalyst with Full Visible LightSpectrum through π−π Stacking of TCNQ−PTCDIZijian Zhang,† Jun Wang,† Di Liu,‡ Wenjiao Luo,† Mo Zhang,§ Wenjun Jiang,† and Yongfa Zhu*,†

†Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China‡School of Chemical & Environmental Engineering, China University of Mining &Technology, Beijing 100083, P. R. China§Department of Biophysics and Structural Biology, Institute of Basic Medical Sciences, Chinese Academy of Medical Sciences &School of Basic Medicine, Peking Union Medical College, Beijing 100005, P. R. China

*S Supporting Information

ABSTRACT: Self-assembled TCNQ−PTCDI composite photo-catalysts can not only degrade phenol at a rate of 0.154 h−1, which is10.4 times higher than that of pure PTCDI, but also produce oxygenat a rate of ∼14 μmol·g−1·h−1 without cocatalysts. The π−πinteractions between TCNQ and PTCDI result in fast transfer ofcarriers and reduced carrier recombination. The interaction lowersthe valence band and narrows the band gap, thus leading to astronger oxidizability and a broad spectral response (∼730 nm).Moreover, the presence of TCNQ stabilizes the composite todecrease the accumulation of negative charge, which results in anexcellent stability of the composite. The high catalytic activity can potentially be utilized in the fields of environmental and energyapplications.

KEYWORDS: photocatalysis, water oxidation, pollutants degradation, visible light, self-assembly

■ INTRODUCTION

Over the past few decades, research into photocatalysis inenvironmental treatment and clean energy production hasaroused wide attention. In particular, inorganic photocatalyticmaterials, represented by titanium dioxide and zinc oxide,1 havereached commercial production. However, compared withtraditional inorganic photocatalysts, organic photocatalyticmaterials, with rich chemical structures, flexible designs, lowcosts, and other advantages, still represent an unknown world.Depending on a number of organic chemistry reactions, theenergy levels of molecule [e.g., the highest occupied molecularorbital (HOMO) and lowest unoccupied molecular orbital(LUMO)] can be regulated. In this way, the energy bands,namely, the valence band (VB) and conduction band (CB), oforganic semiconductor photocatalyst also are designable andcontrollable, whose positions determine the oxidizing (VB)/reducing (CB) abilities and the response to the light spectrum(band gap).With research ongoing, most current studies of organic

photocatalysts focus on metal complexes and polymers.Organometallic complexes are constructed from ions andorganic ligands by coordination bonds, where ions function asthe main catalytic centers and organic ligands sensitize ions to awide spectrum. Strictly, in terms of mechanism, organometalliccomplexes cannot be considered as completely organicphotocatalytic materials. In addition, these complexes oftentake expensive and toxic ruthenium2 and iridium3 as metalcenters, the poor stabilities of which allow the ions to dissolve

out, leading to a heavier pollution load. As a result, applicationof organometallic complexes at a large scale is limited by thedicussed defects. The other type of organic photocatalystsconsists of covalent polymers, such as carbon nitride,4

poly(diphenylbutadiyne),5 poly(ursol),6 and carbazolic frame-works.7 With these materials, organic compounds play acatalytic role, and toxicity and high cost are avoided, but theability to conduct charges is poor. In addition, in the catalyticprocess, vast recombination of photogenerated electron−holepairs and depolymerization severely restrain the furtherimprovement of catalytic activities. Moreover, cumbersomesyntheses also increase the difficulties for the widespreadapplications of these materials.On the basis of these concerns, our groups are trying to find

a new accessible class of ecofriendly organic photocatalystmaterials that have high catalytic activity, proven effectivenessin charge transport, and excellent structural stability. Perylenetetracarboxylic diimide (PTCDI), a widely used n-typesemiconductor, has attracted much attention because of itsoutstanding stability, excellent electron affinity, and charge-carrier migration.8 On the strength of these properties, manyapplications have been investigated, such as organic field-effecttransistors (OFETs),9 solar cells,10 photon harvesting,11

sensors,12 and photoswitches.13 Another material, 7,7,8,8-

Received: August 14, 2016Accepted: October 12, 2016Published: October 12, 2016

Research Article

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© 2016 American Chemical Society 30225 DOI: 10.1021/acsami.6b10186ACS Appl. Mater. Interfaces 2016, 8, 30225−30231

tetracyanoquinodimethane (TCNQ), is regarded as one of themost powerful electron acceptors14 and is widely used incharge-transfer (CT) complexes.15 In addition, the electrical,16

magnetic,17 and electrochemical18 properties arising from itsπ−π stacking have attracted a huge amount of scientists topresent numerous works every year. With its conjugatedsystem, TCNQ, interacting with other conjugated materials,such as nanotubes,19 graphene,20 and C3N4,

21 suggests otherproperties as well.Unfortunately, because of the strong π−π stacking,

commercial nonsubstituted PTCDI shows an intrinsic insol-ubility in most solvents,22 resulting in significant difficulties inself-assembly and modification. Recently, we achieved somebreakthroughs with PTCDI: We presented an economical andrapid method in concentrated sulfuric acid to form a self-assembled nanophotocatalyst for the degradation of organicpollutants under visible light and explained that π channel andinternal electrical fields enhanced the separation and migrationof photogenerated electron−hole pairs.23 On this basis, weproduced a novel organic TCNQ−PTCDI composite photo-catalyst. Both PTCDI and TCNQ have large conjugated πstructures; as a result, they can combine ideally through strongπ−π stacking interactions. It is gratifying that TCNQ can alsobe solvated by concentrated sulfuric acid, which means that arapid assembly method might be suitable.

■ MATERIALS AND METHODSMaterials. Perylene tetracarboxylic diimide (PTCDI) and 7,7,8,8-

tetracyanoquinodimethane (TCNQ) were purchased from SinopharmChemical Reagent Corp., Beijing, P. R. China. Sulfuric acid andsodium hydroxide were purchased from Beijing Chemical Works. All

reagents used in this research were commercially available and usedwithout further purification.

Synthesis of the TCNQ−PTCDI Composites. Keeping the totalmass of PTCDI and TCNQ at 200 mg, the appropriate amount ofPTCDI was put into a polyethylene tube according to the differentmass ratio of TCNQ and dissolved with 1 mL of concentrated sulfuricacid under ultrasonication for 1 h. The same processing was performedfor TCNQ. After that, dark violet and yellow solutions were obtainedfor PTCDI and TCNQ, respectively. Then, the two solutions weremixed in a tube under ultrasonication for 1 h to obtain a homogeneoussolution. The well-mixed solution was carefully added dropwise andwith stirring to 30 mL of water, and the pH was adjusted to neutralwith saturated NaOH solution. Dark violet products were precipitatedand allowed to stand overnight; the product was then filtered anddried at 65 °C in a vacuum drying oven. In this way, compositephotocatalysts with different TCNQ mass ratios ranging from 10% to80% were synthesized. As shown in the optical photograph presentedin Figure S1, the final products were dark brown with a bit of metalluster before grinding.

Characterization. The crystallinity of the composites wasdetermined on a Bruker D8 ADVANCE diffractometer under CuKα radiation. The structures and morphologies were examined byfield-emission scanning electron microscopy (SEM; LEO-1530). High-resolution transmission electron microscopy (HRTEM) images wereobtained on a JEM 2010F field-emission gun transmission electronmicroscope at an accelerating voltage of 200 kV. Atomic forcemicroscopy (AFM) measurements were carried out on an SPM-9700scanning probe microscope (Shimadzu Corporation). Fourier trans-form infrared (FTIR) spectra were obtained using a Bruker V70FTIRspectrometer. UV−vis diffuse reflectance spectroscopy (UV-DRS) wasperformed on a Hitachi U-3010 UV−vis spectrophotometer withBaSO4 as the reference. Raman spectra were obtained with a Horiba JYHR800 confocal microscope Raman spectrometer under an Ar-ionlaser (514 nm). The photocurrent and electrochemical impedance

Figure 1. Photocatalytic activities of TCNQ−PTCDI composite materials: (a) Apparent rate constants k of TCNQ−PTCDI compositephotocatalysts for the degradation of 5 ppm phenol with different TCNQ mass ratios under visible light (λ > 420 nm); (b) C/C0 curves for thedegradation processes of PTCDI, 50%-TCNQ−PTCDI, g-C3N4, and Bi2WO6; (c) amount of evolved oxygen in photocatalytic water oxidation with50%-TCNQ−PTCDI under visible light (λ > 420 nm); (d) C/C0 curves for the degradation processes of 2,4-dichlorophenol and bisphenol A with50%-TCNQ−PTCDI under visible light (λ > 420 nm); (e) apparent rate constants k of 50%-TCNQ−PTCDI composite photocatalyst for thedegradations of 2,4-dichlorophenol and bisphenol A under visible light (λ > 420 nm); (f) action spectrum of 50%-TCNQ−PTCDI, wavelength-dependent phenol degradation with 50%-TCNQ−PTCDI (band-pass filter, 450 ± 15, 500 ± 15, 550 ± 15, 600 ± 15, 650 ± 15 nm).

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spectroscopy (EIS) were measured on an electrochemical system(CHI-660B). Electron spin resonance (ESR) spectra were recordedfrom the sample mixture, containing spin-trapping probes such as 5,5-dimethyl-1-pirroline-N-oxide (DMPO), 2,2,6,6-tetramethyl-4-piperi-done (TEMP), or 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)and products, after exposure to visible light for selected times usingan electron−nuclear double resonance (ENDOR) spectrometer(JEOL ES-ED3X) at room temperature. X-ray photoelectron spec-troscopy (XPS) was performed to estimate the VB positions of PTCDIand 50%-TCNQ−PTCDI using a PHI 5300 ESCA system.Photoelectrochemical Measurements. To investigate the

photoelectrochemical performance of the composites, a standardthree-electrode cell was employed, with a composite as the workingelectrode, a saturated calomel electrode (SCE) as the referenceelectrode, and a platinum wire as the counter electrode. Na2SO4 wastaken as the electrolyte solution. The working electrodes wereprepared as follows: A composite (5 mg) was suspended in 1 mL ofpure/deionized water under grinding and ultrasonication. A darkpurple slurry was obtained and dip-coated onto an indium tin oxide(ITO) glass electrode.Photocatalytic Degradation Experiments. The photocatalytic

degradation of phenol was conducted under visible light (>420 nm).The light source was a 500-W Xe lamp with a 420-nm cutoff filter,produced by Institute for Electric Light Sources, whose average lightintensity was 35 mW cm−2. In the photocatalytic experiments, 25 mgof powder composite photocatalyst was dispersed in an aqueoussolution of phenol (50 mL, 5 ppm). Before the light irradiation, thesuspensions were stirred in the dark for 1 h to ensure absorption−desorption equilibrium. At intervals of 1 h, aliquots of 2 mL werewithdrawn and centrifuged. The concentration of phenol was analyzedby high-performance liquid chromatography (Shimadzu LC-20AT)with UV detection (270 nm). A Venusil XBP-C18 (AgelaTechnologies Inc.) column was used, and the mobile phase consistedof methanol and pure water (55:45 for phenol, 75:25 for 2,4-dichlorophenol, and 70:30 for bisphenol A, v/v) at a flow rate of 1 mLmin−1.Photocatalytic Water Oxidation Experiments. The photo-

catalytic water oxidation reaction was carried out with a Labsolar-IIIAG system (PerfectLight, Beijing) in the presence of 0.01 mol·L−1

silver nitrate as an electron acceptor. The photocatalyst powders (50mg) were added to 100 mL AgNO3(aq) in the reaction cell with amagnetic stirrer. The light source was a 500-W xenon lamp with a 420-nm cutoff filter. The amount of evolved oxygen was determined usinga gas chromatograph (GC7920, thermal conductivity detector, Arcarrier).

■ RESULTS AND DISCUSSION

Figure 1 presents the photocatalytic activities of the photo-catalysts under visible light (>420 nm) in the degradation oforganic pollutants and the oxidation of water for oxygenevolution. Dramatically, the apparent rate constant ofdegradation for 5 ppm phenol was obviously enhanced withincreasing TCNQ content (see Figure 1a). When the massratio reached 50%, the photocatalyst showed the highestactivity of 0.154 h−1, nearly 10.4 times that of pure PTCDI(0.0148 h−1), which could degrade 90% of the phenol (see theC/C0 curve in Figure 1b). Compared with the state-of-the-artinorganic photocatalysts graphitic carbon nitride (g-C3N4)

21

and bismuth tungstate (Bi2WO6),24 50%-TCNQ−PTCDI also

showed the highest photocatalytic activity. However, when theratio was over 50%, the phenol-degradation rate constantdecreased notably. Furthermore, the highly efficient 50%-TCNQ−PTCDI catalyst could produce oxygen from waterwithout cocatalysts under visible light, as shown in Figure 1c. Inthe first 1 h, there was no oxygen without visible lightirradiation, whereas after the light was turned on, the amount ofoxygen increased rapidly at a rate of ∼14 μmol·g−1·h−1. These

results confirm the strong photocatalytic oxidizing ability of the50%-TCNQ−PTCDI photocatalyst. Moreover, the 50%-TCNQ−PTCDI photocatalyst was found to have an extensiveapplicability for the degradation of organic pollutants with fullvisible-range activity, which was representatively tested forbisphenol A, giving a rate of 0.085 h−1, and for 2,4-dichlorophenol, giving a rate of 0.081 h−1 (see Figure 1d,e.)Meanwhile, the action spectrum, wavelength-dependent photo-degradation with band-pass-filter results, presented in Figure 1f,suggests a wide response at different wavelengths covering thewhole visible range. We were interested in why the 50%-TCNQ−PTCDI composite could gain the strongest oxidationability and why the activity declined when the mass ratio ofTCNQ less than or greater than 50%.From the information on gas adsorption and SEM, these

composite materials are originally nonporous and accumulateto form a mesoporous structure. Also, according to thestatistical results in Figure S2, most of the pores are distributedin the range of less than 25 nm. At the same time, the contentof TCNQ results in no obvious and regular differences insurface area, pore volume, and pore size (see Table S1). Themorphology of the 50%-TCNQ−PTCDI composite looks likelayer-by-layer shale (see Figure 2a). With increasing TCNQ

content, the morphology of composites became irregular in theSEM images (Figure S3). By AFM, the thickness of each layerwas detected in the range of 0.36−3 nm (Figure S4). In the X-ray diffraction (XRD) image of 50%-TCNQ−PTCDI (Figure2b), compared to those of the pure TCNQ and PTCDIcounterparts in Figures S5 and S6, respectively, the 120, 220,320, and 200 faces of TCNQ have a strong intensity, with 2θdecreasing by ∼0.2°, which means an increase of the d spacing.In the presence of TCNQ, PTCDI could not crystallize well toform a long-range-ordered structure; as a result, the XRD peaksare widened , mainly including 11.82° (0.74 nm), 25.07° (0.35nm), and 27.02° (0.33 nm). In the HRTEM image of 50%-TCNQ−PTCDI (Figure 2c), there are many nanocrystallinePTCDI areas with a d spacing of ∼0.36 nm and a diameter of<10 nm, and the directions of the lattice fringes are different

Figure 2. Structure-related results for TCNQ−PTCDI compositematerials: (a) scanning electron microscopy image of 50%-TCNQ−PTCDI composite, (b) X-ray diffraction image of 50%-TCNQ−PTCDI, (c) high-resolution transmission electron microscopy imageof 50%-TCNQ−PTCDI, (d) magnification of the dashed region inpanel c.

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depending on the region. In the magnification of the dashedregion indicated Figure 2c, shown in Figure 2d, the d spacingranges from 0.34 to 0.38 nm, which is normally observed forπ−π stacking.25 These structure-related results confirm that themain effort between TCNQ and PTCDI is a π−π interaction,and with the synergy of hydrogen bonding, PTCDI and TCNQcan assemble into composite semiconductor photocatalysts.The process is modeled in Scheme 1. On the other hand, the

mean free path of photogenerated carriers is about 10 nm; thus,a photocatalyst size of less than 10 nm is good forphotogenerated carriers transferring to the surface andsubstrate. This is also one of the reasons that nanoscalephotocatalysts can gain high efficiency, which makes photo-generated carriers transfer faster and more easily with lesschance of recombination, compared with micro- or macro-photocatalysts.Although TCNQ is an excellent electron-transferring ma-

terial and beneficial for charge separation in the TCNQ−PTCDI photocatalyst, too much TCNQ will also shadePTCDI, leading to less photon absorption and a lowerquantum yield. Thus, there is a balance between chargeseparation and light absorption, where 50% of TCNQ by massmight represent the balance point. At less than 50%, TCNQenhances the electron transport and induces the formation ofnanocrystals of PTCDI, resulting in an increased catalyticactivity; on the contrary, more than 50%, TCNQ shadesPTCDI, and reduces the number of photogenerated electronsand holes, resulting in a decrease of catalytic activity. As for thenature of the mass ratio of 50%, the molecular weight ofPTCDI is 392.32, whereas that of TCNQ is 204.19. Therefore,when the mass ratio in 1:1, the mole ratio is 1:1.92, nearly 1:2.In other words, a PTCDI molecule can combine with twoTCNQ molecules through π−π interactions (see Scheme 1).Certainly, because of defects in crystallization, the compositedoes not show a strict 1:2 relationships.With the outstanding photocatalytic ability of this material,

the stability of its π−π stacking self-assembled structure wastested by repeated cycling experiments. As shown in Figure 3,the photocatalyst exhibited excellent stability. Figure 3apresents the results for the cyclic photodegradation of 5 ppmphenol. The decrease in photocatalytic activity during the firstcycle was mainly caused by the loss of photocatalyst. After eachcycle, we centrifuged the solution to separate out thephotocatalyst; however, because of its nanoscale size andexcellent dispersibility in water, there was always a large amountof photocatalyst that did not completely settle, resulting in aloss of photocatalyst close to 45%. To ensure the rigor andreliability of the data, we did not add any photocatalyst in thesubsequent cycles. As a result, there was a significant decreasein photocatalytic activity after the first cycle. However, the

subsequent three cycles exhibited similar reaction rates, thusconfirming the cycling performance and stability of thephotocatalyst for application in photodegradation. Figure 3bpresents the results for three cycles of water oxidation; again,the decrease of the reaction rate was due to the loss ofphotocatalyst. After the cyclic experiments, all of the photo-catalyst powders were collected and analyzed by UV-DRS andXRD (see Figures S7 and S8). No significant changes wereobserved in either the UV-DRS and XRD spectra after therepeated reactions, which confirms the excellent stability of thecomposite photocatalyst. In addition, the increasing widths ofthe peaks in the XRD spectrum indicates a decrease of thecatalyst grain size caused by stirring and flowing.At the same time, the spectroscopic and electrochemical

properties were investigated. The principal vibration modes ofTCNQ in the Raman spectrum (Figure 4a) of 50%-TCNQ−PTCDI at 1208 cm−1 (CCH bending) and 1603 cm−1 (CC ring stretching) downshifted by about 5 and 10 cm−1 (cf. theRaman spectrum of pure TCNQ in Figure S9), which might bethe result of an increase in the conjugation length.21 The sameresult was confirmed by FTIR spectroscopy (Figure S10): Apeak of conjugate −CN at 2220 cm−1 showed an obvious redshift of about 20 cm−1. This shift indicates a decrease of thebond strength because of the π−π interaction and the enlargedconjugated system. Moreover, in the Raman spectrum, above2250 cm−1, there is a very strong fluorescence peak,demonstrating the recombination of electron−hole pairs inPTCDI,8 which leads to a low quantum efficiency. A directresult is that the catalytic activity under 550-nm irradiation issignificantly lower than at other wavelengths (see Figure 1d).However, the intensity of fluorescence decreased obviouslywith increasing amount of TCNQ, confirming that TCNQcould facilitate the separation of photoinduced electron−hole

Scheme 1. Model Diagram of TCNQ−PTCDI π−π-Stacking-Assembled Structure

Figure 3. Cyclic experiments using 50%-TCNQ−PTCDI for (a)phenol (5 ppm) degradation and (b) water oxidation.

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pairs through fast electron transport. In the UV-DRS spectra(Figure 4b), with increasing TCNQ content, the absorption ofTCNQ−PTCDI in the visible range was extended by about 30nm, and the band structure for charge migration and separationcould be optimized,26 indicating band narrowing of ∼0.07 eV.The band-edge shift is significant because, in this case, electronscan be stimulated under lower-energy irradiation, making fulluse of the whole spectrum, in comparison to UV photo-catalysts. Meanwhile, the lower the excitation energy, the lessphotocorrosion occurs. To confirm this model, electrochemicalimpedance spectroscopy (EIS) was performed. A smaller arcradius on the EIS Nyquist plot of TCNQ−PTCDI withincreasing TCNQ content under visible light irradiation (Figure5a) can be observed (the same trend without irradiation isshown in Figure S11), suggesting a more effective separationefficiency of photoinduced electron−hole pairs and a fastercharge transfer. In addition, the photocurrent (Figure 5b),whose intensity is related to the density of photogeneratedcharges, was obviously enhanced, from ∼0.3 to ∼3 mA. Thus, alower probability of recombination of photogenerated carriersin the TCNQ−PTCDI composites can be confirmed.Further, we investigated the mechanism of photocatalytic

oxidation. The processes of phenol degradation and inter-mediate formation were investigated by high-performanceliquid chromatography (HPLC) (Figure 6a). The peak at3.78 min represents phenol, whose intensity decreased after 8 hof irradiation as a result of the photocatalytic reaction. Inaddition, several new peaks appeared at short retention time,implying that phenol was oxidized to other intermediates, suchas maleic anhydride, dihydroxybenzene, and 4,4-dihydroxybi-phenyl.27 Figure 6b shows the photodegradation of phenol inthe presence of hole scavengers (formic acid, ammonium

oxalate), a hydroxyl-radical scavenger (t-BuOH), and anelectron scavenger (AgNO3) under visible light. The photo-catalytic activity of 50%-TCNQ−PTCDI decreased to differentextents in the different cases, which implies that holes andsuperoxide radicals contribute to the photocatalytic reaction.Further, the ESR detection of in situ active species (Figure6c,d) indicated that both 1O2 and

•O−2, which also contribute

to the photocatalytic oxidation, could be detected after 10 minof irradiation. In short, the photogenerated holes, 1O2, and•O−

2 are the main active species and govern the photocatalyticoxidation process. Moreover, photogenerated electrons aretransferred from PTCDI to dissolved oxygen through TCNQ,and then the dissolved oxygen gains electrons, changing into•O−

2 (with holes)28 to oxidize the phenol. The activity of thecomposite also decreased when an electron scavenger, AgNO3,was added, which resulted from the decrease of superoxideradical.In addition, the transfer of photoinduced electrons from the

HOMO of PTCDI to the LUMO of TCNQ is promotedoutstandingly by charge transfer between the PTCDI donorand the TCNQ acceptor. Our previous work demonstrated thatH-type π−π stacking of PTCDI can cause a deepening of thevalence band (HOMO) and narrowing of the band gap,23

which results in a one-dimensional fast electron-transferchannel and internal electric field (see the diagram of themechanism in Figure 6e). To investigate the electronicstructure of the composite, X-ray photoelectron spectroscopy(Figure S12) was performed. The valence-band electronicstructures of PTCDI and 50%-TCNQ−PTCDI are different,with the VB position of the composite being much deeper, at

Figure 4. Spectroscopy of TCNQ−PTCDI composites: (a) Ramanspectra of PTCDI, 50%-TCNQ−PTCDI, and 50%-TCNQ−PTCDIand (b) UV-DRS spectra of PTCDI, TCNQ, and 50%-TCNQ−PTCDI.

Figure 5. Electrochemistry properties of TCNQ−PTCDI composite:(a) photocurrent spectra of pure PTCDI and 50%-TCNQ−PTCDIand (b) electrochemical impedance spectroscopy (EIS) Nyquist plotof TCNQ−PTCDI composites upon irradiation (λ > 420 nm),showing a smaller arc radius on the EIS Nyquist plot of TCNQ−PTCDI with increasing TCNQ content.

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+2.86 eV, than that of PTCDI, at +2.15 eV. The deeper VBposition contributes to a stronger oxidation ability of holes forthe oxidation of organic pollutants and splitting of water.The progress of the water oxidation process can be explained

by the following three reactions

+ → + ++ +Ag H O Ag H O2 2 (1)

+ → + +− +TCNQ H O TCNQ H O2 2 (2)

+ → + +− +PTCDI H O PTCDI H O2 2 (3)

Equation 1 presents the overall reaction in the photocatalyticprocess. The rate declines gradually after long-time irradiation,as a result of the consumption of silver nitrate. In the presenceof TCNQ, photogenerated charge carriers are separated andtransferred rapidly: Photoinduced electrons are transferred toAg+ by TCNQ, and more holes are left in PTCDI to oxidizewater. Under these conditions, with a decrease in Ag+, thephotoinduced electrons cannot be depleted. As the reactionproceeds, because of their high electron affinities, TCNQ andPTCDI are transformed into TCNQ− and PTCDI− anions (eqs2 and 3), which gradually accumulate, leading to a collapse ofthe π−π stacking. As a result, the rate of oxygen evolutiondecreases.

■ CONCLUSIONSA highly efficient TCNQ−PTCDI composite photocatalyst wassynthesized by a solution-based self-assembly route in water.π−π interactions play a significant role in accelerating charge-carrier transfer, decreasing the recombination of photo-generated electron−hole pairs, and deepening the VB position.The composite photocatalyst with PTCDI and TCNQ in a 1:2molar ratio (1:1 mass ratio) was found to have the highest

activity, which was 10.4 times that of pure PTCDI for thedegradation of organic pollutants, and to generate oxygen at arate of ∼14 μmol·g−1·h−1 without cocatalysts. The compositematerial has good stability and recyclability. With its potentialin environmental treatment and clean energy production, 50%-TCNQ−PTCDI might be a promising photocatalyst in thenear future.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b10186.

Optical photograph; SEM, AFM, XRD, Raman spectros-copy, FTIR spectroscopy, EIS, and XPS data; and tableof structural characteristics (PDF)

■ AUTHOR INFORMATION

Corresponding Author*E-mail: zhuyf@mail.tsinghua.edu.cn.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This work was partially supported by the National BasicResearch Program of China (973 Program) (2013CB632403),the Chinese National Science Foundation (21437003,21673126, 21621003), and the Collaborative InnovationCenter for Regional Environmental Quality.

Figure 6. Investigation of mechanism: (a) HPLC map of initial 5 ppm phenol (red) and 5 ppm phenol after 8 h of photocatalytic degradation (blue),(b) photodegradation of phenol in the presence of scavengers, (c) ESR detection of 1O2, (d) ESR detection of •O−

2, (e) schematic of the mechanismof photocatalytic oxidation.

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ACS Applied Materials & Interfaces Research Article

DOI: 10.1021/acsami.6b10186ACS Appl. Mater. Interfaces 2016, 8, 30225−30231

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