Distorted Carbon Nitride Structure with Substituted Benzene Moieties

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Distorted Carbon Nitride Structure with Substituted Benzene Moieties for Enhanced Visible Light Photocatalytic Activities Hyejin Kim, Suji Gim, Tae Hwa Jeon, Hyungjun Kim, and Wonyong Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14191 • Publication Date (Web): 27 Oct 2017 Downloaded from http://pubs.acs.org on October 29, 2017

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ACS Applied Materials & Interfaces

Distorted Carbon Nitride Structure with Substituted Benzene Moieties for Enhanced Visible Light Photocatalytic Activities Hyejin Kim1, Suji Gim2, Tae Hwa Jeon1, Hyungjun Kim2, and Wonyong Choi1* 1

Division of Environmental Science and Engineering, Pohang University of Science and

Technology (POSTECH), Pohang 37673, Republic of Korea. 2

Graduate School of Energy, Environment, Water, and Sustainability (EEWS), Korea

Advanced Institute of Science and Technology, 291 Daehak-Ro, Yuseong-Gu, Daejeon 305-701, Republic of Korea

* Corresponding author. E-mail: [email protected] (W.C.); phone: +82-54-279-2283 (W.C.)

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ABSTRACT

Carbon nitride (CN) is being intensively investigated as a low-cost visible light active photocatalyst but its practical applications are limited because of the fast charge pair recombination and low visible light absorption. Here, we introduce a new strategy for enhancing its visible light photocatalytic activity by designing the CN structure in which the nitrogen of tertiary amine is substituted with a benzene molecule connected by three heptazine rings. The intramolecular benzene doping induced the structural changes from planar symmetric structure to distorted geometry, which could be predicted by density functional theory (DFT) calculation. This structural distortion facilitated the spatial separation of photogenerated charge pairs and retarded charge recombination via exciton dissociation. Such unique properties of the benzeneincorporated CN were confirmed by the photoluminescence (PL) and photoelectrochemical analyses. The optimal loading of benzene doping reduced the PL of the conjugated ring system (π → π* transition) but enhanced the PL of the forbidden n → π* transition at the nitrogen atoms with lone pair electrons due to the distortion from the planar geometry. The photoelectrode of benzene-doped CN exhibited higher photocurrent and lower charge transfer resistance than bare CN electrode, which indicates that the photogenerated charge pairs are more efficiently separated. As a result, the benzene-doped CN markedly increased the photocatalytic activity for the degradation of various organic pollutants and that for H2O2 production (via O2 reduction). This study proposes a simple strategy for chemical structural modification of carbon nitride to boost up the visible light photocatalytic activity.

KEYWORDS. carbon nitride, photocatalysis, solar light utilization, charge separation, molecular doping 2 ACS Paragon Plus Environment

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1. INTRODUCTION Utilization of solar light for various environmental and energy applications has been investigated using semiconductor photocatalytic materials and the materials and methods for extending the light absorption into the visible spectrum region have attracted much attention.1,2 Recently, conjugated organic polymers as visible light active photocatalyst are being actively studied, owing to the their advantages such as high carrier mobility, tunable bandgap and low material cost compared with traditional inorganic photocatalysts.3-5 Researchers are actively searching for rational and practically viable synthetic methods to design stable and efficient polymer nanostructures for solar conversion. In recent years, graphitic carbon nitride (g-C3N4, or simply noted as CN) as metal-free organic polymer semiconductor has been extensively studied because of its superior physicochemical stability and simple fabrication method. The bandgap and band edge positions of carbon nitride (Ecb = - 1.3 VNHE and Evb = 1.4 VNHE at pH 7) are suitable for various applications, which include organic pollutants degradation, H2 production and H2O2 production under visible light.6,7 However, practical applications are restricted by the intrinsic drawbacks of carbon nitride, such as fast charge recombination and limited visible light absorption.8,9 Several attempts such as metal doping, heterojunction with other semiconductors, and co-polymerization have been tried to promote the charge separation efficiency and visible light absorption of CN. The structural design by co-polymerization requires the optimal selection of the precursors with suitable functional groups like cyano groups and amino groups, which readily react with carbon nitride precursors.10 Compared with other precursors, CN synthesized from urea exhibited the best photocatalytic performance as a high degree of polymerization can be obtained.11 Various co-monomers such as barbituric acid and 2-aminobenzonirile were selected 3 ACS Paragon Plus Environment


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for the synthesis of CN along with urea.12 Anchoring aromatic rings on the surfaces of CN enhanced the charge pair separation. In this work, we developed a simple method of synthesizing benzene-doped carbon nitride by substituting a fraction of the tertiary N atoms with a benzene ring. The incorporating of benzene ring moieties into the bare CN network induced a structure distortion and this irregularly twisted portion generated the order-disorder interfaces, which facilitated efficient charge separation and enhanced the visible light activities. By optimizing the content of benzene moiety in CN structure, markedly enhanced visible light activities were obtained.

2. EXPERIMENTAL SECTION 2.1. Material Synthesis.

Trimesic acid (TMA) was used as a dopant that introduces a

benzene moiety into the CN structure. Benzene-incorporated carbon nitride was synthesized from the reaction of the mixture of 3 g urea (99%, Aldrich) and a varied amount (7, 14, 43, 86, 143 mmol% or 2.8 mol%) of TMA (95%, Aldrich). The reagent mixture was put into an alumina crucible with a cover and calcined at 550 °C for 2 h with a heating and cooling rate of 5 °C/min. The resulting products are labeled as TMA_CN_x where x indicates the mol% value of TMA. To synthesize bare_CN, 3 g urea was calcined by following the same synthesis method of TMA_CN. 2.2. Photocatalytic Activity Test. The prepared sample (0.5 g/L) was dispersed in distilled water by ultra-sonication. An aliquot of substrate stock solution (4-cholorophenol (4-CP), cimetidine, ranitidine and trimethoprim) was added to a pyrex reactor to prepare a desired concentration (volume 30 mL). The pH of the suspension was adjusted by HClO4 or KOH. Before the light irradiation, the suspension was equilibrated for 30 min in the dark. The 4 ACS Paragon Plus Environment

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photocatalytic reactions were carried out using a 300-W Xe arc lamp (Oriel) as a light source. Light was guided through a 10-cm IR water filter and a longpass cut-off filter (λ > 420 nm). Sample aliquots of 1 mL were withdrawn at predetermined time intervals from the photoirradiated reactor by using a syringe, filtered by a 0.45 µm PTFE filter (Millipore), and injected into a 2-mL amber glass vial. The concentration of organic substrates (4-CP and pharmaceutical compounds) was analyzed using a high performance liquid chromatograph (HPLC, Agilent 1100 series) equipped with a diode array detector. The HPLC analysis was carried out using an eluent of 85% (v/v) aqueous phosphoric acid solution and acetonitrile (90 : 10 by volume) for 4-CP, while pharmaceutical compounds were analyzed using a mobile phase of phosphoric acid and methanol (85 : 15 by volume) for ranitidine and cimetidine and 25 mM ammonium acetate and acetonitrile (80 : 20 by volume) for trimethoprim. The concentration of chloride ion (Cl-) was analyzed using an ion chromatograph (IC, Dionex DX-120) that was equipped with a conductivity detector and an AS-14 (4 mm X 250 mm) column. The removal of total organic carbon (TOC) by photocatalytic reaction was monitored using a TOC analyzer (TOC-VCSH, Shimadzu). CN has been recently proposed as an ideal photocatalyst for the photochemical production of hydrogen peroxide as a solar fuel.13,14 For H2O2 production, the catalyst sample was suspended in an aqueous ethanol solution ([catalyst] = 0.5 g/L, [ethanol] = 10 vol% as an electron donor, pHi 3.0, O2-purged). The concentration of photo-generated H2O2 was measured by a colorimetric method using N,N-diethyl-1,4-phenylene-diamine sulfate (DPD).15 2.3. Characterizations of Photocatalysts. Powder X-ray diffraction (XRD) patterns were measured using an X-ray diffractometer (RIGAKU D MAX 2500) with Cu Ka radiation. The functional groups of catalyst samples were analyzed by attenuated total reflectance Fourier 5 ACS Paragon Plus Environment


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transform infrared spectroscopy (ATR-FTIR, Thermo Scientific iS50) using ZnSe crystal and Fourier transform Raman spectroscopy (FT-Raman, Bruker, Germany). X-ray photoelectron spectroscopy (XPS, VG Escalab 250) analysis was performed using a monochromatic Al Kα line (1486.6 eV) as an excitation source. The solid-state 13C NMR spectra were acquired on 400 MHz solid state NMR spectrometer (AVANCE III HD, Bruker, Germany). Elemental analysis was performed by using an elemental analyzer (Elementar Analysensysteme GmbH, Germany). High-resolution transmission electron microscopy (HR-TEM) and energy dispersive X-ray spectroscopy (EDS) analysis were carried out using a JEOL JEM-2200FS with image Cscorrector. Diffuse reﬂectance UV-visible absorption spectra (DRUVS) were collected using a spectrophotometer (Shimadzu UV-2401PC) equipped with an integrating sphere attachment. BaSO4 was used as the reference. Photoluminescence (PL) emission spectra were obtained using a fluorescence spectrophotometer (Shimadzu RF-5301). Fluorescence lifetime decays were measured by an inverted-type scanning confocal microscope (MicroTime-200, Picoquant, Germany) with a 20x objective. A single-mode pulsed diode laser (379 nm) was used as an excitation source and fluorescence decay curves were obtained using band-pass filter (450 nm) to collect emissions from the samples. 2.4. Photoelectrochemical Measurements. Photoelectrochemical (PEC) measurements were carried out using a potenstiostat (Gamry, Reference 600) connected to a conventional threeelectrode system. For slurry-type photocurrent collection measurement, a coiled Pt wire, a graphite rod and a Ag/AgCl electrode were used as a working, a counter and a reference electrode, respectively. The photocurrent was collected using the phosphotungstate redox couple (PW12O303-/4-) acting as an electron shuttle.16 The catalyst suspension was continuously purged with Ar under visible light (λ > 420 nm) irradiation and the photoexcited electrons were 6 ACS Paragon Plus Environment

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collected from the photocatalyst surface via the electron shuttle using a Pt wire electrode applied at a potential of +0.7 V (vs Ag/AgCl). The PEC performances were also measured with photoelectrodes on which photocatalysts were coated. The photoelectrodes were fabricated by applying a portion of photocatalyst suspension (5 mg in 1 mL dimethylformamide) onto a cleaned FTO glass using a spin-coating method and then calcined at 350 °C for 10 minutes. The process was repeated 10 times. The photoelectrode, a coiled Pt wire, and a Ag/AgCl electrode were utilized as a working, a counter, and a reference electrode, respectively. The photocurrent test was carried out in 0.2 M Na2SO4 solution (pH = 3, controlled by HClO4) biased with a potential of +0.5 V (vs Ag/AgCl) under Ar-purged system. Mott-Schottky analysis was done with a potential range from -0.5 V to 0 V (vs Ag/AgCl) at the selected frequency of 1 kHz and AC voltage amplitude of 30 mV. The electrochemical impedance spectroscopy (EIS) was performed in the frequency range from 10-3-105 Hz with an AC voltage amplitude of 50 mV. 2.5. Density Functional Theory (DFT) Calculations. To understand the role of incorporated benzene structural units in the photocatalytic performance of modified CN, DFT calculations were carried out by using the Vienna Ab-initio Software Package (VASP) program17 with the choice of Perdew-Burke-Emzerhof (PBE) exchange-correlation functional.18 In order to describe the electron-ion interactions, the projector-augmented wave (PAW) method was selected. The bare CN and TMA_CNs in which 25% and 50% of the nitrogen of tertiary amine sites is substituted by benzene ring were employed for the DFT calculation. For all CN and TMA_CNs structures, an orthorhombic simulation cell was adopted. The horizontal lattice parameters were obtained to be 14.28Å, 14.00 Å and 13.98 Å for the bare CN, 25% TMA_CN and 50% TMA_CN, respectively. To prevent the artificial interaction between the repeated slabs along the z-direction, ~20 Å vacuum was introduced with correction of the dipole moment. An energy 7 ACS Paragon Plus Environment


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cutoff of 450 eV was used, and Γ centered (4×4×1) k-point mesh was used to sample the reciprocal space.

3. RESULTS AND DISCUSSION Carbon nitride of which tertiary amine N sites were partially substituted with benzene ring moieties was synthesized using urea as a main precursor and TMA (trimesic acid) as a comonomer of benzene source. A plausible mechanism is illustrated in Figure 1. The amino group of urea makes nucleophilic attack on the carbon center of carboxylic group of TMA. Then, a triazine is formed via thermal condensation and the release of H2O molecule under gaseous ammonia condition generated from the pyrolysis of urea.19 After further reactions, benzene ring units can be partially incorporated in the tri-s-triazine based carbon nitride, altering the polymeric network structure. The resulting samples are noted as TMA_CN_0.007, 0.014, 0.043, 0.086 and 0.143 according to the added TMA amount (x mol%). When the benzene ring is incorporated in the internal tertiary amine site, 2D planar carbon nitride sheet can be distorted because the benzene molecule size is much bigger than a nitrogen atom. When we optimized the structures of a (2 × 2) supercell of pristine CN and a modified structural system with the same dimension but replacing one nitrogen atom of the internal tertiary amine site with a benzene ring, the latter one exhibited a distorted structure as shown in Figure 2. The XRD patterns of the original and modified CN samples are shown in Figure 3a. Two peaks at 27.5o and 13.1o are ascribed to the interlayer stacking as in graphite and the in-planar structural packing of repeated units, respectively.20 In the case of TMA_CN_0.014, these two peaks are weakened suggesting the destruction of the long-range order in the atomic arrangements because the distortion caused by the benzene doping can reduce the crystallinity 8 ACS Paragon Plus Environment

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and deteriorate the periodicity of CN network.21,22 The FT-IR spectra of bare_CN and TMA_CN_0.014 show the distinctive stretch modes of heptazine repeating units at 1200 - 1600 cm-1 and the out-of-plane bending mode of heptazine rings at 810 cm-1 in Figure 3b.23 With increasing the benzene doping concentration, XRD and FT-IR peak position did not change (Figure S1). It indicates that the main chemical skeleton seems to be retained. However, when the dopant content increased up to 2.8 mol%, XRD and FT-IR peaks became markedly wider, which indicates that the amorphous fraction is enhanced. The typical TEM images of bare_CN show the layered nanosheet morphology and TMA_CN_0.014 also clearly maintained a sheet morphology as shown in Figure S2. However, the TEM image of TMA_CN_2.8 does not show a clear appearance of the sheet morphology. It seems that an excessive distortion induced by a high level TMA doping does not make the sheet morphology stable. This phenomenon is consistent with the XRD data which indicate an enhanced amorphous fraction in TMA_CN_2.8. The atomic ratio of carbon to nitrogen (C/N) from EDS analysis increased with adding the TMA content (see Figure S2d), which is also consistent with the elemental analysis result summarized in Table S1. The results support the incorporation of the benzene unit into the carbon nitride structure. To further confirm the presence of benzene carbon species in TMA_CN samples, the solid-state C-NMR was carried out. In Figure 4a, the strong two peaks at 164.7 ppm and 156.9 ppm are ascribed to the terminal carbon (N2-C-NH2 groups) and the internal carbon (C-(N)3 groups), respectively.24 The C-NMR peaks of TMA_CN_0.014 are not different from those of pure CN because the TMA content is very small compared with the content of urea used as a carbon nitride source. However, a small peak at 136.5 ppm was observed when the TMA loading was further raised to 0.143 mol%. To see clearly whether TMA is incorporated into the CN 9 ACS Paragon Plus Environment


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structure by forming chemical bonds or simply bound onto the CN surface by physical bonding, we prepared two samples: one is the 2.8 mol% TMA-incorporated CN and the other is bare_CN physically mixed with 2.8 mol% TMA. The three peaks representing TMA at 170.65, 134.39 and 130.39 ppm remained the same in the physically mixed sample but were markedly broadened and shifted to 168.97, 136.6 and 114.09 ppm with TMA-CN-2.8 sample, which clearly indicates that the carboxylic groups of TMA reacted with urea to form a modified chemical structure. X-ray photoelectron spectroscopic (XPS) analysis was carried out to compare the chemical states of bare_CN and TMA_CN_0.014 samples. In Figure 5, the N 1s peak was deconvoluted into three components that contain the main sp2 hybridized nitrogen peak of triazine rings at 398.5 eV in both bare_CN and TMA_CN_0.014. The other two peaks are assigned to the tertiary N bonded to carbon atoms at 399.8 eV and the amino functional groups at 401.0 eV.25 The area ratio of N(sp2)/ N(sp3) was calculated as 3.33 in bare_CN and 2.86 in TMA_CN_0.014. When the structural distortion is applied to the C-N plane, N(sp2) would transform to N(sp3).26 This lower value of N(sp2)/ N(sp3) in TMA_CN_0.014 indicates that the structural distortion was caused by doping benzene units in the CN sheet. The structural distortion was also confirmed by the Raman spectroscopy comparing the relative intensity of the D band and G band (ID/IG) (Figure S3). The value of ID/IG ratio increased from 0.736 for bare_CN to 0.854 for TMA_CN_0.014, which implies that the structural distortion was enhanced by benzene doping. The BET analysis found that TMA_CN_0.014 has a higher surface area (59 m2/g) than bare_CN (44 m2/g) (see Figure S4), which might be also attributed to the structural distortion by benzene doping. The benzene doping-induced structural distortion also influenced the surface charge of CN. The zeta-potential analysis shows that the surface charge of TMA_CN_0.014 is more positive than bare_CN in a wide pH range (Figure S5). Carbon nitride can be protonated in the N 10 ACS Paragon Plus Environment

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site of amines depending on pH.27 As discussed above, the N(sp3) portion is enhanced as a result of the structural distortion, which implies that this distorted N sites are more basic to favor accepting protons. As a result, TMA_CN_0.014 shows more positive surface charge than bare_CN. The photocatalytic activities of as-prepared samples were tested by monitoring the degradation of 4-CP and three pharmaceutical compounds and the production of H2O2 through O2 reduction under visible light irradiation (see Figure 6). To find the optimal concentration of the doped benzene in the CN photocatalyst, the modified carbon nitride samples with different TMA content (0.07, 0.014, 0.043, 0.086 and 0.143) were compared. TMA_CN_0.014 exhibited the highest activity for the oxidation of 4-CP (Figure 7a) with generating quantitative amount of chlorides (see Figure 6a). The photocatalytic oxidation activity of TMA_CN_0.014 was consistently higher than bare_CN over a wide range of pH 3-9 (Figure 7b). TMA_CN_0.014 and pH 3 were chosen for further experiments. To investigate the effect of dissolved oxygen, the photocatalytic degradation of 4-CP was tested under Ar-saturated condition and both samples showed the retarded reaction rate (Figure 7c), which indicates that the presence of O2 is essentially needed to decompose 4-CP. The dioxygen molecules are required as a scavenger of conduction band (CB) electrons in CN catalysts, which should induce the production of superoxide radical anions (O2•−). The further reduction of the superoxide generates other reactive oxygen species like H2O2 and OH radical which play a dominant role in the degradation of organic compounds.28 Dioxygen molecules are needed as not only a scavenger of CB electrons, but also a reagent for the mineralization of organic compounds. It is noted that TMA_CN exhibited a higher activity not only for the removal of organic compounds but also for the mineralization of organic compounds than bare_CN in terms of TOC removal (see Figure 7d). 11 ACS Paragon Plus Environment


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TMA_CN also exhibited higher visible light activities than bare_CN for the degradation of pharmaceutical compounds (i.e., cimetidine, ranitidine and trimethoprim) which have chemical moieties such as imidazole, furan and pyrimidine. The adsorption of these organic compounds and their removal in the dark were negligible for both bare_CN and TMA_CN (see Figure S6). The production of H2O2 through the photoreduction of O2 was also tested in the presence of ethanol as an electron donor under O2 saturated condition by employing a similar method we recently reported13 (see Figure 6c). In this case, the TMA_CN_0.014 also exhibited a higher activity than bare-CN. As a result, both photooxidation and photoreduction were highly enhanced by the simple incorporation of benzene units into the CN structure. To elucidate how such a structural distortion affects the photocatalytic activity, DFT calculations were performed. In Figure 8, the real-space wavefunctions of valence band maximum (i.e., HOMO) and the conduction band minimum (i.e., LUMO) are illustrated, which shows the spatial distribution of bandgap-excited photoelectron and photohole, respectively. For the case of bare_CN, both photoelectron and photohole are homogeneously delocalized on all the heptazine rings due to a high symmetry (Figure 8a). As the real-space location of electron and hole substantially overlaps on each other, relatively fast electron-hole recombination rate is expected for bare_CN. On the contrary, when the benzene moiety is incorporated by substituting the nitrogen atom, it is observed (1) that there exists a substantial structural distortion due to the different size of the linker (N vs benzene), and (2) that photoelectron and photohole are localized at different parts of heptazine rings (Figure 8b, 8c). This prolongs the lifetime of electrons and holes by preventing their fast recombination, which consequently increases the probability of interfacial charge transfer for initiating photocatalytic reactions.29 In addition, the spatial separation of electron and hole localization sites should make the reduction and oxidation sites 12 ACS Paragon Plus Environment

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be separated with minimizing the chance of back recombination.26, 30 Incidentally, it should be noted that the doping ratio (25% and 50% of tertiary amine N substituted by benzene dopants) employed for the DFT calculation is significantly higher than the actual experimental value because of a practical choice of a small periodic simulation cell. However, it should not influence the main conclusion in a qualitative manner: the structural distortion caused by benzene incorporation breaks the local symmetry, and thereby separates the spatial locations of electron and hole near the benzene-incorporated site. The separation behavior of charge carriers in carbon nitrides was investigated to be compared with the calculation results. The TMA_CN PL spectra show two emission components (see Figure 9a), one of which is the same to the bare CN emission induced by the conjugated ring system (π → π* transitions) and the other is red-shifted. This latter emission is related to the n → π* electronic transition at the nitrogen atoms having lone pair electrons in the heptazine rings.31 In the case of perfectly symmetric and planar heptazine units, this transitions are forbidden. However, the benzene-induced distortions cause the deviation from symmetry and layer buckling, which makes the n → π* transition allowed.32 As a result, the latter PL band is markedly pronounced with TMA_CN_0.007. However, the intensity of the latter PL emission was maximal at TMA_CN_0.007 and gradually decreased with increasing the TMA content. This implies either that the benzene dopant above an optimal level might act as the charge trapping site prohibiting radiative recombination or that charge carriers can be transferred to different energy levels in the interface between the ordered heptazine-based chains and disordered chains.13,33 Figure 9b exhibits that the fluorescence life time of TMA_CN_0.014 is shorter than bare_CN with the mean radiative lifetimes of ~2.42 ns and ~1.44 ns for bare_CN and TMA_CN_0.014, respectively. This reduced PL lifetime implies that the exciton dissociation is 13 ACS Paragon Plus Environment


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facilitated in the distorted CN structure. The deviation of the PL decay curve from the single exponential decay implies that multiple processes may be involved.34 On the other hand, the bandgap of TMA_CN_0.014 which exhibited the highest photoactivity was little changed (see Figure S7), indicating that the intrinsic absorption below 450 nm (from π → π* transition) was not changed by benzene doping.35 The benzene incorporation, however, enhanced the absorption of longer wavelengths (λ > 450 nm) region with increasing the TMA content (Figure 9c).36 This extended visible light absorption contributed to the photocatalytic activity as shown in Figure S8. The photocatalytic activity of bare_CN is completely absent at λ ≥ 460 nm whereas that of TMA_CN_0.014 was extended up to 500 nm. To confirm the accelerated exciton dissociation, the photoinduced electron transfer was also investigated by collecting photocurrent using the PW12O303-/4- redox couple as an electron transfer mediator in the suspension of photocatlaysts.37 Figure 10a shows that the photocurrent obtained in the suspension of TMA_CN_0.014 was higher than that of bare_CN. This implies that the interfacial electron transfer is facilitated in the benzene-doped structure of carbon nitride, which is consistent with the higher photocatalytic activity of TMA_CN. In Figure 10b, the electrochemical impedance spectroscopy demonstrated the lower charge transfer resistance of the benzene-doped CN (showing a smaller semicircle Nyquist plot). As a result, in comparison with bare_CN, higher photocurrent was obtained with TMA_CN electrode having a lower charge transfer resistance (Figure 10c), which indicates that the photogenerated charge pairs are more efficiently separated to be directed to the external circuit.38 This is consistent with the DFT calculation result that benzene doping in carbon nitride induced the spatial separation of electron and hole, leading to more efficient charge separation. In addition, it should be noted that TMA_CN_0.014 exhibited a markedly slower decay of the photocurrent transients after turning 14 ACS Paragon Plus Environment

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off the light.39 This result implies that the trapped charge in TMA_CN is longer lived and slowly recombined. This is also fully consistent with the DFT calculation prediction that electron and hole are spatially separated in TMA_CN structure. On the other hand, Figure 10d displays the Mott-Schottky plots showing the positive slope, which demonstrates the n-type characteristics of bare_CN and TMA_CN_0.014. Furthermore, TMA_CN_0.014 should have higher charge carrier density than bare_CN, judging from its lower slope. Such a higher donor density seems to be induced by the effective electron trapping in the presence of the benzene dopants.

4. CONCLUSION A distorted carbon nitride structure is newly designed by simply doping benzene unit at the internal nitrogen site of tertiary amine in the CN structure. This structural distortion induces a change in the optimized orbital structure. The DFT calculation predicted that the benzene-doped carbon nitride has localized distribution of photoelectron and photohole in different parts of heptazine rings, which should retard their recombination. The photoelectrochemical characterization results were consistent with the DFT calculation result. As a result, the benzenedoped CN exhibited the enhanced activities for the photo-oxidation of aquatic pollutants and the photogeneration of H2O2 via O2 reduction in comparison with those of bare_CN. The distorted CN structure is more efficient in separating charge pairs and inducing exciton dissociation. The present method that was employed in the synthesis of the modified carbon nitride structure proposes a new design principle that might be applied to other 2-dimensional organic photocatalysts for achieving higher solar conversion efficiency.

ASSOCIATED CONTENT 15 ACS Paragon Plus Environment


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Supporting Information.

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Characterizations of bare_CN and TMA_CN_x samples (XRD,

FTIR, TEM, Raman, BET surface area, Zeta-Potential, Tauc plots and elemental analysis data), dark control experiments for removal of organic pollutants, and photocatalytic removal efficiency as monochromatic irradiation. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

ACKNOWLEDGMENT This research was financially supported by the Global Research Laboratory (GRL) Program (No. NRF-2014K1A1A2041044), Basic Science Research Program (NRF-2017R1A2B2008952), and KCAP (Sogang Univ.) (No. 2009-0093880), which were funded by the Korea Government (MSIP) through the National Research Foundation of Korea (NRF).


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Figure 1. Proposed reaction mechanism for the synthesis of the benzene-incorporated carbon nitride.



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Figure 2. Top and side view of geometry optimized structure of (a) bare_CN, (b) TMA_CN in which one quarter of the tertiary amine is substituted with benzene and (c) TMA_CN in which an half of the tertiary amine substituted with benzene. Red, blue and yellow ball represents carbon, nitrogen and hydrogen, respectively.


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Figure 3. (a) XRD spectra and (b) ATR FT-IR spectra of bare_CN, TMA_CN_0.014 and TMA_CN_2.8.



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Figure 4. (a) 13C solid state NMR spectra of bare_CN, TMA_CN_0.014, TMA_CN_0.143; inset shows an enlargement around 136.5 ppm peak in bare_CN and TMA_CN_0.143 spectrum. (b) 13 C NMR spectrum of TMA_CN_2.8 is compared with that of bare_CN, TMA, and their physical mixture.


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Figure 5. XPS spectra of bare_CN (a) and TMA_CN_0.014 (b) in N 1s binding energy region.



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Figure 6. (a) 4-CP degradation and the accompanying generation of chloride. (b) Pharmaceutical compounds degradation under visible light irradiation (λ > 420 nm) ([Catalyst] = 0.5g/L; [Substrates]0 = 100 µM; pH = 3.0; air-equilibrated). (c) Production of H2O2 under visible light irradiation (λ > 420 nm) ([Catalyst] = 0.5g/L; [EtOH] = 10 vol%; pH = 3.0, and continuously O2purged).


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Figure 7. (a) Time profiles of 4-CP degradation with TMA_CN with varying TMA content. (b) Effect of pH on the degradation of 4-CP for bare_CN and TMA_CN_0.014. (c) Control experiment of the photocatalytic degradation of 4-CP in the Ar-saturated condition. (d) TOC removal efficiency of bare_CN and TMA_CN_0.014 after 3 h visible light illumination.



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Figure 8. DFT optimized orbital structures for the HOMO and LUMO of bare_CN, TMA_CNs in which 25% and 50% of the nitrogen of tertiary amine sites is substituted by benzene ring are denoted as (a), (b) and (c), respectively. Isodensity contours for the bare_CN and TMA_CNs are 0.01 a.u. and 0.05 a.u., respectively.


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Figure 9. (a) PL emission spectra (λex = 380 nm) of TMA_CN_x. (b) Time-resolved PL spectra monitored at 450 nm after the excitation with 379 nm laser pulse. (c) Diffuse reflectance UVVisible spectra of bare_CN and TMA_CN_x.



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Figure 10. (a) Phosphotungstate redox couple-mediated photocurrent collected in the suspension of bare_CN and TMA_CN_0.014 under visible light. The experimental conditions were: [catalyst] = 0.5 g/L, [PW12O303-] = 2 mM, [NaClO4] = 0.1 M, pH = 3 (controlled by HClO4), Pt electrode held at +0.7 V (vs. Ag/AgCl), λ > 420 nm and continuously Ar-purged. (b) Electrochemical impedance spectroscopic Nyquist plots, (c) the photocurrent generation and decay time profiles, and (d) Mott-Schottky plots, which were obtained using the bare_CN and TMA_CN_0.014 electrode. The experimental conditions were [Na2SO4] = 0.2 M (pH = 3) with potential bias of +0.5V (vs. Ag/AgCl). The electrolyte solution was continuously Ar-purged.


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Table of Contents Graphic


Distorted Carbon Nitride Structure with Substituted Benzene Moieties

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