Synergistic Combination of a Novel Metal-Free Mesoporous Band

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Kinetics, Catalysis, and Reaction Engineering

Synergistic Combination of a Novel Metal-Free Mesoporous Bandgap-modified Carbon Nitride Grafted Polyaniline Nanocomposite for Decontamination of Refractory Pollutant Balakumar Vellaichamy, and Prakash Periakaruppan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01098 • Publication Date (Web): 16 Apr 2018 Downloaded from http://pubs.acs.org on April 16, 2018

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Industrial & Engineering Chemistry Research

Synergistic Combination of a Novel Metal-Free Mesoporous Bandgap-modified Carbon Nitride Grafted Polyaniline Nanocomposite for Decontamination of Refractory Pollutant Balakumar Vellaichamy and Prakash Periakaruppan* Department of Chemistry, Thiagarajar College, Madurai – 625 009, Tamil Nadu, India.

ABSTRACT A synthesis of novel sulfur and phosphoruos co-doped graphitic carbon nitride (SP-g-C3N4) covalently grafted polyaniline (PANI) metal-free nanocomposite via an in-situ oxidative polymerization is reported here. The nanocomposite was characterized by various instrumentation techniques such as UV-DRS, FT-IR, XRD, SEM, HR-TEM, EDX, and SEM mapping analysis. The as-prepared SP-g-C3N4−PANI metal-free nanocomposite shows an outstanding photocatalytic activity for the dedyeing of methylene blue (MB) under visible light irradiation. The nanocomposite catalyst greatly promotes the charge separation efficiency of photogenerated electrons and holes compared to its single counterpart viz. PANI, S-g-C3N4, P-gC3N4, and SP-g-C3N4 for catalytic degradation. The present novel approach provides a new insight into the design and development of nanocomposite materials for the removal of toxic contaminants for public safety and security. Also, study of this kind will undoubtedly expedite new researches on solar-driven water splitting. KEYWORDS: Nanocomposite; Photocatalytic dedyeing; Methylene blue; Visible light, Durability

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1. INTRODUCTION Organic dyes are commonly used in various industries such as tannery, textile, paper, paints, plastic, pharmaceutical, food, cosmetic, and medicine.1-2 They are easily discharged and accumulated in waters. Due to their high solubility, their contamination in the environment poses risk to human and animals.3 Therefore, the removal of organic dyes has attracted increasing attention of global concerns. Generally, methylene blue (MB) is a common component of industrial wastewater contaminant and it causes permanent burns to the eyes of human and animals, nausea, vomiting, profuse sweating, mental confusion and methemoglobineamia.4 Up to now, a variety of technologies have been exploited to degrade MB including adsorption, membrane separation, electrochemical treatment, filtration, ion exchange, reverse osmosis, and catalytic reduction.5-8 Unfortunately, most of these methods suffer from limitations such as wastewater composition, operation costs, time consuming, and expensive which impede their widespread applications. In recent years, photocatalytic methods have gained much interest, because of their simplicity, low cost, and easy operation and after the completion of reaction, the catalyst can be easily separated and reused.9,10 In photocatalysis field, the construction of intimate heterojunction between two appropriate semiconductors is an effective strategy to enhance the photocatalytic performance. Recently, several kinds of polymeric graphitic carbon nitride (g-C3N4) based nanocomposites have been developed by coupling g-C3N4 with other types of inorganic photocatalysts.11-15 Nevertheless, most of these modification methods possess further some disadvantages such as tedious synthetic steps and introduction of metal and metal oxides and toxic solvents, which may increase the cost and pollute environment.16, 17 Hence, there is an urgent need to construct a new metal-free photocatalytic material that is very fast and easily recoverable for the degradation of organic pollutants for public safety and security.


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Generally, polymeric g-C3N4, a π-conjugated material has good visible-light response because of its narrow band gap of 2.7 eV and it has become the focus of attention worldwide because of its potential applications in various fields such as sensor, drug delivery, bio imaging and especially photocatalysis.18-21 However, the homogeneous g-C3N4 is inefficient for practical applications due to low conductivity of electrons and high recombination of photoexcited electrons and holes as well as low surface area.21,

22

To increase the efficiency, the band gap of g-C3N4 can be

narrowed by sulfur (S) or phosphorous (P) doping. The S or P doped pristine g-C3N4 changes the band structure by stacking its 2p orbitals on the valence band.23, 24 Nevertheless, doping of gC3N4, thus far, still suffers from the general limitations of incapable utilization of sunlight beyond 460 nm and low separation efficiency. Therefore, various strategies have been proposed to solve this problem by preparing novel nanostructures, which can provide a large scaffold for anchoring various substrates.

25-27

As one of the important conducting polymers, polyaniline

(PANI) is an excellent candidate material because of its π-conjugated electron systems, unique electron and hole transporting properties, good chemical stability which is beneficial for improving the utilization efficiency of visible-light for photocatalysis.28,

29

Therefore, it is

expected that the covalently grafted of sulfur-phosphorous-g-C3N4 with PANI seems to be ideal for improving visible light photocatalytic activity. The introduction of a secondary PANI with inherently strong photocatalytic properties to form bipolymeric SP-g-C3N4-PANI catalysts is anticipated to produce synergistic effects that can further boost the photocatalytic performance. The SP-g-C3N4 covalently grafted to PANI plays an essential role in tuning the band gap structure, extending the light absorption, increasing the charge transfer mobility and creating more active sites of photogenerated carriers at their interfaces, which is usually deemed responsible for the enhanced photocatalytic activity. Until now, there is no report on the



photocatalytic degradation of MB using a covalently grafted SP-g-C3N4 with PANI. The aim is to effectively exploit the synergistic benefits and the highly accessible surface area of metal-free bipolymeric platform to boost the photocatalytic activity and durability. In-situ polymerization has been successfully used for the synthesis of nanomaterials in recent years. 30-35 In comparison to conventional methods, in-situ polymerization is efficient and facile for rapid preparation of nanostructured materials without addition of any other oxidant and reductant. Herein we demonstrate the fabrication of a novel metal-free synergistic SP-g-C3N4 grafted PANI nanohybrid photocatalyst via an in-situ chemical oxidative polymerization of aniline in the presence of SP-g-C3N4. The as-prepared covalently grafted metal-free SP-g-C3N4−PANI nanohybrid shows an excellent photocatalytic performance in the dedyeing of MB over single counterparts of S-g-C3N4, P-g-C3N4, SP-g-C3N4 and PANI. The photocatalytic dedyeing of MB has been investigated in detail vis-à-vis various reaction conditions. 2. EXPERIMENTAL SECTION 2.1 Chemicals Aniline, thiourea and (NH4)2HPO4 were purchased from Sigma–Aldrich and used without any further purification. Aniline was distilled under reduced pressure and all other chemicals used in the present study were commercially available high purity Analar grade (Merck, India). 2.2 Synthesis of SP-g-C3N4 In a typical experiment, 3 g of thiourea was dissolved in 15 ml of deionized water under stirring. Then, 0.06 g of (NH4)2HPO4 was added. The obtained solution was heated to 100 °C to remove the water. The solid product was dried at 100 °C in an oven followed by milling and annealing at


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520 °C for 2 h. The prepared catalyst is referred to as SP-g-C3N4. The S-g-C3N4 catalyst was prepared according to the same procedure mentioned above in the absence of (NH4)2HPO4. 2.3 Synthesis of SP-g-C3N4-PANI nanocomposite 0.1 mg of SP-g-C3N4 was dissolved in 20 ml of water. 0.5 ml of aniline was added into the above solution by continuous stirring for 10 min, followed by the addition of 1:1.25 ratio of aniline and ammonium persulphate drop wise to the reaction system. Polymerization of aniline starts taking place within 1 min, and the color of the solution changes from pale yellow to greenish black. The reaction was allowed by stirring for another 6h. The greenish black color indicates the complete formation of SP-g-C3N4-PANI nanocomposite. Finally, the SP-g-C3N4-PANI (1) was filtered and washed with acetone, then by double distilled water to remove any other unreacted monomer and dried in an air oven at 60 ⁰C for 12 h. Similar procedure was adopted for the synthesis of SP-gC3N4-PANI (2-4) by changing the weight percentage (0.2, 0.3 and 0.4 mg) of SP-g-C3N4. The overall nanocomposite synthesis process and application has been schematically represented in Scheme 1. 2.4 Instrumental characterization The UV-Vis diffuse reflectance spectroscopy was recorded on an UV-2450 spectrophotometer (Shimadzu Corporation, Japan) using BaSO4 as the reference. Chemical transformation on the catalyst surface was detected by Fourier transform-infrared spectra (FT-IR) using a model 460 Plus FT-IR spectrometer (JASCO). The crystal structure of the prepared samples was recorded in X-ray diffraction unit, Cu Kα radiation (λ = 1.5418º A) on JEOL JDX 8030 X- ray diffractometer. High-resolution transmission electron microscopy and corresponding selected area electron diffraction (HR-TEM/SAED) were carried out on a transmission electron 5 ACS Paragon Plus Environment


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microscope (TEM, FEI TECNAI T20 G2). The samples for TEM were prepared by placing a drop of the as-prepared solution on carbon-coated copper grids followed by drying. The surface mapping analysis was measured by SEM measurements done at VEGA3 TESCAN, USA. Photoelectrochemical and electrochemical measurements were performed in 0.1 M Na2SO4 electrolyte solution in a three-electrode quartz cell. Pt wire was used as a counter electrode and Ag/AgCl in saturated KCl was used as a reference electrode. The SP-g-C3N4-PANI nanocomposite modified on indium-tin oxide (ITO) was used as the working electrode for investigation. The photoluminescence spectra of the photocatalyst were recorded using a fluorescence spectrophotometer (JASCO – FP- 6200). Finally the photodegradation experiments were performed in a HEBER immersion type photoreactor (HIPR-MP125). 2.5 Photocatalytic activity The photocatalytic activity of SP-g-C3N4-PANI (1) naocomposite was evaluated with its catalytic dedyeing of MB under visible light irradiation. A 150 W Xe arc lamp with an ultra violet (λ>400 nm) cut off filter was used as the visible light irradiation source. For each test, 50 mg catalyst was added into 100 mL of 10 mg/L MB solution and irradiated with the visible light. During the irradiation process at regular time intervals, a 5 ml aliquot of the reaction mixture was taken every 10 min and centrifuged at 2000 rpm. Then the supernatant liquid was separated and analyzed by UV-visible absorbance spectra at 663 nm to evaluate the dedyeing of MB at different time intervals. The degradation efficiency of dye was calculated by the following equation, Photodegradation (%) = C0-C/C0 x 100


(1)

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Where, C0 and C are the initial concentration of MB before and after visible light irradiation respectively. After completing the measurements, the photocatalyst was separated from the solution by centrifugation and decantation. The separated catalyst was washed with deionized water many times and dried. The dried catalyst was then used for the next cycle of the photocatalytic degradation of fresh MB. The same experimental procedure was repeated five times, and the FTIR, XRD and TEM for photocatalyst were recorded after the experiments. 3. RESULTS AND DISCUSSION 3.1 UV-DRS The optical absorption of S-g-C3N4, SP-g-C3N4, PANI, SP-g-C3N4-PANI (1-4) was investigated by UV-vis diffuse reflectance spectra (DRS) as shown in Figure. 1. The optical band gap energy can be determined by Tauc equation, 36

Where, Eg, α, h, and ν are the band gap, optical absorption coefficient, Plank constant, and photonic frequency respectively, and c is a proportionality constant. It can be seen in Figure. 1A (curve a), whereas the S-g-C3N4 displays an absorption edge at about 470 nm, the absorption edge of SP-g-C3N4 (Figure. 1A (curve b)) shows a red shift. The typical absorption edges of the S-g-C3N4 and SP-g-C3N4 samples appearing at about 470 and 480 nm respectively correspond to the band gap of approximately 2.64 and 2.52 eV respectively. The pure PANI (Figure. 1A (curve



c)) sample not only absorbs UV light, but also has strong absorption in visible light and near infrared regions, which can be ascribed to n-π* transitions in the PANI molecules.37 The nanocomposites, SP-g-C3N4-PANI (1-4) (Figure. 1B (curve a-d)) exhibit a stronger absorption edge in the visible region at wavelength longer than 450 nm, and the red shifts are also observed. These results can be attributed to covalent grafting of SP-g-C3N4 to the PANI in the nanocomposite. As compared with SP-g-C3N4, the absorption intensities of SP-g-C3N4-PANI (1-4) in visible-light region are significantly improved. Among these photocatalysts, the optical absorption band edge of SP-g-C3N4-PANI (3) is shifted to lower band gap energies, implying a band gap narrowing and covalently grafting of the SP-g-C3N4 to the PANI in the crystal lattice.1214

This modification accompanied by the unique nanostructures can be used to generate more

electrons and holes pairs, mass transfer, and charge separation, which contributes to enhance the photocatalytic performance compared to other counterparts. 3.2 FT-IR The FT-IR spectrum of pure PANI (Figure. 2A (curve a)) shows the peak at 3482 cm−1 which can be attributed to hydrogen bonded N–H bond between amine and imine sites. The peaks at 1588 cm−1 is ascribed to the C=N stretching of quinoid ring and 1503 cm−1 is ascribed to the C=C stretching of benzenoid ring. The peaks at1300 and 825 cm−1 can be assigned to N–H bending mode and out-of plane deformation of C–H in the benzenoid ring of PANI.25, 37 For pure S-gC3N4 (Figure. 2A (curve b)), the characteristic peaks appearing at 1200 and 1600 cm−1 are ascribed to the typical stretching modes of C-N heterocycles. S-related vibrations are not observed for S-g-C3N4, which is consistent with previously reported results.23 Further, the FT−IR spectrum of SP-g-C3N4 (Figure. 2A (curve c)) shows a characteristic peak appearing at 950 cm−1


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which is assigned to the P–N stretching mode.24 The FT-IR spectra of SP-g-C3N4-PANI (1-4) (Figure. 2B (curve a-d)) show the characteristic FT-IR peaks for both SP-g-C3N4 and PANI. The spectra clearly show a decreasing PANI peak intensity when the mole ratio of SP-g-C3N4 is increased from 3 to 4. This confirms that SP-g-C3N4-PANI (3) nanocomposite is the right form of SP-g-C3N4 covalently grafted to PANI. 3.3 XRD The crystal structure and phase composition of S-g-C3N4, SP-g-C3N4, PANI, SP-g-C3N4-PANI (1-4) was investigated by X-ray diffraction spectra as shown in Figure. 2 (C and D). It can be seen from Figure. 2C (curve a) that the XRD patterns of S-g-C3N4 with a peak at 13.1 can be assigned to the in-plane structural packing motif of tris-triazine units and is indexed as the (1 0 0) plane. Another high-angle peak at 27.4 is characteristic of an interlayer stacking of conjugated aromatic systems, which is indexed to the (0 0 2) plane corresponding to the average interlayer distance of d = 0.326 nm.22, 23 Additionally, the additional weak diffractions situated at about 17.6 and 22.2 can be attributed to (6 0 0) and (6 5 0) planes of graphitic carbon nitride, respectively.23 The pure PANI (Figure. 2C (curve b)) shows a diffraction peaks of 2θ = 18.15º which can be attributed to the periodicity parallel and perpendicular to polymer chain. The peak at 2θ = 20.07º is an evidence of the characteristic distance between the ring planes of benzene rings in adjacent chains or close contact interchains.38 The peak centered at 2θ = 24.96º can be assigned to the scattering of PANI chains at interplanar spacing which indicates that pure PANI has some degree of crystallinity.37, 39 Furthermore, the XRD patterns of SP-g-C3N4 (Figure. 2C (curve c)) show two characteristic peaks appearing at 13.1 and 26.7 which can be assigned to the in-plane structural packing motif of tris-triazine units and interlayer stacking of conjugated aromatic systems of g-C3N4. After doping of P, the overall peak intensity significantly decreases 9 ACS Paragon Plus Environment


and the peak at 27.1 (S-g-C3N4) is shifted to 27.5 (SP-g-C3N4) which is due to the doping of P into the S-g-C3N4.40 Figure. 2D (curve a-d) shows the XRD patterns of different weight ratios of synthesized SP-g-C3N4-PANI (1-4) nanocomposites. It can be seen from Figure. 2D (curve a & b) that as the quantity of SP-g-C3N4 increases from 1-2, the diffraction pattern of the nanocomposite materials is quite similar to that of pure PANI. When the quantity of SP-g-C3N4 is increased to (3), the diffraction patterns of the nanocomposite materials and PANI merge. But, further increasing the quantity of SP-g-C3N4 increases the amount of nanocomposite sample and the diffraction pattern becomes almost similar to that of pure SP-g-C3N4. Based on the above results, it is observed that SP-g-C3N4-PANI (3) is the right composite with high photocatalytic activity and stability. 3.4 Surface Morphology and Mapping Analysis The morphologies and compositions of the synthesized photocatalyst were determined by SEM and TEM. As can be seen, the pure PANI possesses a flaky lamellar structure with the particles of indefinite shape as shown in Figure. S1A. The S-g-C3N4 (Figure. S1B) clearly shows the layered like morphologies and after doping of P, the layered like morphologies contain many smaller crystals as shown in Figure. S1C. The morphology of SP-g-C3N4-PANI (1-4) nanocomposite (Figure. S1D-G) shows a deposition of small PANI particles on to the surface of SP-g-C3N4. Figure. S1H shows the EDX spectral analysis of SP-g-C3N4-PANI (3), which confirms the existence of C, N, O, P and S. Further, Figure. 3A-F shows the SEM images of selection area of the SP-g-C3N4-PANI (3) and the elements mapping analysis. The elements mapping of SP-g-C3N4-PANI (3) also confirms that the nanocomposite is composed of C, N, O, P and S. Furthermore the nanocomposite of SP-g-C3N4-PANI (3) was examined by TEM. Figure 4A-C shows the different magnification HR-TEM images of mesoporous structured SP-g-C3N410 ACS Paragon Plus Environment

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PANI (3). The crystallinity of SP-g-C3N4-PANI (3) can be observed by selected area emission diffraction (SAED) pattern as shown in Figure. 4D. 3.5 TGA Figure. 5 displays the TGA curves of PANI, S-g-C3N4, SP-g-C3N4 and SP-g-C3N4-PANI nanocomposite. It can be seen that for pure PANI, the gradual weight loss appearing at 80-300 °C is attributed to the release of absorbed water and dopant molecules.41 Then PANI starts to degrade quickly when the temperature is increased above 300 °C, which is due to the complete degradation of PANI with different polymerization degree.42 The S-g-C3N4 is fairly stable when the heat temperature is below 600 °C, and total weight loss is nearly 90% when the temperature is up to 750 °C, which implies that S-g-C3N4 undergoes decomposition.43 Compared to S-gC3N4, SP-g-C3N4 shows higher stability up to 600 °C and also only 70% complete weight loss is observed at 800 °C. TGA thermogram of SP-g-C3N4-PANI (3) indicates that this nanocomposite is more stable compared to other counterparts suggesting that the degradation mechanism of SPg-C3N4-PANI (3) nanocomposite is mainly controlled by composition of SP-g-C3N4. From TGA results, it can be inferred that the enhanced thermal stability is due to the interaction between covalently grafting of SP-g-C3N4 with PANI. 3.6 BET analysis The specific surface areas of pure PANI, S- g-C3N4, SP- g-C3N4 and SP- g-C3N4-PANI (3) were examined by using isothermal N2 adsorption-desorption BET analysis as shown in Figure. 6. All the isotherms possess the characteristics of type IV,44 which show a quick rise between two platforms in isotherms at relative pressure of 0.6-1.0 according to the International Union of Pure and Applied Chemistry.45 This result indicates that all the materials are found to have a 11 ACS Paragon Plus Environment


concentrated distribution of mesopores. The corresponding surface area, pore volume and pore size were calculated using the Brunauer–Emmett–Teller method.46 The SP- g-C3N4-PANI (3) nanocomposite is found to exhibit a largest surface area, 126.47 m2 g−1 as compared to other synthesized counterparts as tabulated in Table 1. The increase in the surface area is related to the covalently grafting of SP- g-C3N4 and PANI. The N2 isotherm and pore-size distribution of SP- gC3N4-PANI (3) nanocomposite indicate that a lot of mesopores are generated during the in-situ polymerization of PANI covalently grafted to SP-g-C3N4. The detailed surface area and pore structure parameters of the materials are tabulated in Table 1. The high mesoporous nature, surface area, adsorption capacity of the SP- g-C3N4-PANI (3) makes the nanocomposite significantly improve its photocatalytic activity. 3.7 Photocatalytic Activity MB is relatively a stable chemical molecule in aqueous solutions, which is selected as a probe molecule to assess the reactivity of the catalyst. To compare the photocatalytic activities of PANI, S-g-C3N4, P-g-C3N4 SP-g-C3N4 and SP-g-C3N4-PANI, a series of photocatalytic dedyeing experiments were performed using MB as a model pollutant under visible light irradiation. The characteristic absorption of MB at 663 nm was employed to monitor the photocatalytic degradation process.2 In the absence of the catalyst, the peak intensity at 663 nm corresponding to the MB is unaltered with time under visible light irradiation, indicating that the photocatalytic degradation does not proceed. Figure. 7A shows the photocatalytic dedyeing of MB using pure PANI under visible light irradiation. The catalytic degradation efficiency of S-g-C3N4 and SP-gC3N4 is illustrated in Figure. 7B and C respectively. In the presence of the SP-g-C3N4-PANI (3) photocatalyst, 99% degradation of MB is able to be achieved in 30 min as shown in Figure. 7D. The intense blue color of the MB gradually fades during the process of photodegradation as the 12 ACS Paragon Plus Environment

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function of irradiation time and finally disappears. For a comparison, the photocatalytic degradation of MB using PANI, S-g-C3N4, SP-g-C3N4, SP-g-C3N4 and SP-g-C3N4-PANI (1-4) was also performed under the identical conditions as shown in Figure. 8A. The C/C0 vs time plot is displayed in Figure. 8B. A good linear correlation of ln(C/C0) vs. reaction time t, up to 99.9% is obtained (Figure. 8C). The rate constant, k, of MB photodegradation was derived from the following equation,25 -ln C/C0 = kt

(2)

where k is the rate constant and t is the reaction time. C0 and C are the initial MB concentration and concentration at time t, respectively. Pure PANI, S-g-C3N4, P-g-C3N4, SP-g-C3N4, SP-g-C3N4-PANI (1, 2 & 4) shows a lower rate constants compared to SP-g-C3N4-PANI (3) nanocomposite as shown in Figure. 8D. The significant enhancement of photocatalytic performance can be attributed to synergistic effect between PANI and SP-g-C3N4. The band gap of SP-g-C3N4 is determined to be 2.56 eV and PANI has a band gap of 2.8 eV, which indicate that both SP-g-C3N4 and PANI can be excited by visible light. This SP-g-C3N4 covalently grafted PANI plays an important role in the charge separation efficiency and photogeneration of electron–hole pairs. The SP-g-C3N4-PANI (3) photocatalyst shows an excellent photocatalytic activity compared to other counterparts. Also the SP-g-C3N4-PANI (3) exhibits higher photocatalytic activity compared to other earlier reported results which are summarized in Table 2.25, 37, 39, 47-54 3.8 Influence of reaction conditions on photocatalytic efficiency Effect of pH The pH of the solution plays an important role in the degradation of MB. In order to study the effect of pH, the degradation experiments were carried out using fixed concentration of MB (10 13 ACS Paragon Plus Environment


mg/L) and photocatalyst (50 mg/100 ml) in the pH range of 3.0 to 11.0 by addition of either HCl or NaOH (0.1 M) as shown in Figure. S2A. When the pH of the solution is increased from 3.0 to 7.0, the degradation percentage of MB increases from 38. 2 % to 99.9 %. When the pH is further increased from 7.0 to 11.0, the dedyeing percentage decreases, which is due to the strong adsorption of the MB onto the catalyst surface at high pH. This is blocking light from arriving and reduces the formation of hydroxyl radicals.55 The maximum MB degradation of 99.9 % is achieved at pH 7.0. Effect of catalyst loading The influence of catalyst amount on decolorization of MB under visible light using SP-g-C3N4PANI (3) nanocomposite photocatalyst was studied by varying the amount of catalyst between 10 mg to 70 mg at a fixed concentration of 100ml of MB (10mg/L) and the degradation time is fixed as 60 min at pH 7. The results of catalyst loading on photodegradation are shown in the Figure. S2B. The amount of dye dedyeing increases when the loading of the catalyst increases from 10 mg to 50 mg and then decreases gradually. Therefore, optimum loading amount of the catalyst was considered as 50 mg of SP-g-C3N4-PANI (3) nanocomposite. This is due to increase in the availability of the active surface area or the active sites of the catalyst which increases the adsorption of photons as well as dye molecules onto the catalyst.39, 56 Further, the decrease in the degradation efficiency on increasing the catalyst loading of above 50 mg can be attributed to deactivation of activated molecules by collision with ground state molecules of the catalyst producing a shielding effect on the penetration of light.57 Therefore, 50 mg catalyst was selected as the optimal loading of photocatalyst for the subsequent photocatalytic degradation experiments for obtaining maximum dedyeing efficiency.


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Effect of initial dye concentration The effect of initial dye concentration on the photodegradation phenomenon of MB was investigated by varying the concentration of dye molecules from 10 mg/L to 50 mg/L at a fixed concentration of the photocatalyst of 50 mg/100 ml at pH 7 as shown in Figure. S2C. It is observed that increasing of dye concentration decreases with degradation rate, which is due to complete screening of the dye molecules resulting in the reduction of active photocatalyst surface.58 Once the MB concentration is increased, most of light is absorbed by the MB molecules and photons do not reach the surface of photocatalyst to activate it to generate hydroxyl radicals for the degradation of MB solutions.59 Hence the optimum concentration of MB for better photocatalytic degradation is found to be 10mg/L. 3.9 Possible photocatalytic mechanism Based on all the above results and discussion, a possible mechanism of degradation of MB by visible light irradiation using SP-g-C3N4-PANI (3) nanocomposite for an improved photocatalytic activity is illustrated in Figure. 9. The enhancement of the photocatalytic acitivity is mainly due to the charge separation induced by the synergistic effect of covalently grafted SPg-C3N4 and PANI nanocomposite. As shown in Figure. 9, the polymeric semiconductor SP-gC3N4 in the composite photocatalyst absorbs photons and excites electrons and hole pairs when the system is irradiated with visible light. The HOMO of PANI absorbs the photons to excite electrons and jumps to LUMO of PANI through π-π* transitions. These transitions in PANI are responsible for the generation of electrons and holes in the conduction band (LUMO) and valence band (HOMO) of PANI molecules, respectively.40, 48, 56 These excited-state electrons are readily injected into the conduction band (CB) of SP-g-C3N4, and subsequently, the



photogenerated holes in valence band (VB) of SP-g-C3N4 are able to move freely to the composite’s surface through HOMO of PANI because of the enjoined electric fields of the two materials. The present SP-g-C3N4-PANI (3) nanocomposite reacts with adsorbed water and oxygen to yield active species of hydroxyl and superoxide radicals.53 The band gap of PANI is 2.8 eV; the π*-orbital and π-orbital edge potentials are determined at -2.14 eV and +0.62 eV. The CB and VB potentials of g-C3N4 are at -1.13 eV and +1.57 eV, respectively. On the basis of relative energy levels of PANI (π-orbital and π*-orbital) and g-C3N4 (VB and CB), the π-orbital and π*-orbital edge potentials of PANI are more negative than VB and CB potentials of gC3N4.60-62 These reactions prevent the electron-hole pairs from recombining which would otherwise reduce the efficiency and durability of the photocatalyst. The possible photocatalytic decomposition reactions are proposed as follows: SP-g-C3N4-PANI

SP-g-C3N4 (h+)-PANI(eCB-)

hγ

eCB- + O2 ͦ

O2- + 2eCB- + 2H+ PANI(h+) + H2O MB + ͦ OH

ͦ

ͦ

O2-

OH + OH-

H+ + ͦ OH degraded products

3.10 Photoelectrochemical and electrochemical studies The photocurrent and electrochemical impedance spectroscopy are the common electrochemical methods which are widely used in evaluating the interface charge transfer efficiency and separation of photogenerated electron−hole pairs. Figure. S3A shows the transient photocurrent responses of S-g-C3N4, SP-g-C3N4 and SP-g-C3N4-PANI (3) electrodes recorded under visible 16 ACS Paragon Plus Environment

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light irradiation. It is observed that the photocurrent densities rapidly decrease to zero as soon as the lamp is turned off, and the photocurrents maintain stable values when the lamp is turned on, which is reproducible as well.51, 63 Figure. S3A indicates that when visible light source is turned on or off, the instantaneous photocurrent of S-g-C3N4 and SP-g-C3N4, are in a small degree. Significantly, the photocurrent of the SP-g-C3N4-PANI (3) Figure. S3A electrode is higher than that of S-g-C3N4 and SP-g-C3N4 electrode under same condition, indicating a smaller recombination and a more efficient charge separation of photo-generated electron-hole pairs occurring at the interface between SP-g-C3N4 and PANI in nanocomposite which is due to the strong synergistic effect.64 Electrochemical impedance spectroscopy (EIS) was also used to further investigate the charge transfer and recombination processes of the photocatalyst as shown in Figure. S3B. It can be seen in Figure. S3B (curve a-d) that the radius of the arc on the EIS Nynquist plot of nanocomposite of covalently grafted SP-g-C3N4-PANI (3) electrode is much smaller than those of pure PANI, S-g-C3N4 and SP-g-C3N4 electrodes under visible light irradiation,60 indicating a more effective charge separation of photo-generated electron-hole pairs and fast interface charge transfer taking place in the SP-g-C3N4-PANI (3) nanocomposite. 3.11 Photoluminescence spectra Photoluminescence emission spectra of various photocatalyst samples can be used to investigate the photogenerated electrons and holes and to understand why introduction of PANI leads to enhanced visible light activity for MB dedyeing. It can be seen in Figure. S4, that for S-g-C3N4, SP-g-C3N4 and SP-g-C3N4-PANI (3), the emission peak appears at about 470 nm, which is attributed to the recombination of electron–hole pairs. After the formation of nanocomposite, the intensities of the photocatalysts decreases significantly.55 This result indicates that the



nanocomposite of covalently grafted SP-g-C3N4 and PANI (3) has a lower recombination rate of electrons and holes under visible-light irradiation. 3.12 Stability and reusability of the SP-g-C3N4-PANI (3) nanocomposite The stability and reusability are other important factors to determine the sustainability of the photocatalyst. The recyclability test was performed to evaluate the efficiency of the photocatalyst for MB degradation using the selected SP-g-C3N4-PANI (3) catalyst which was used repeatedly for the degradation of MB. The catalyst shows high photocatalytic activity for the degradation, even after five cycles as shown in Figure. S5. It is found that 96.6 % of photocatalytic activity is able to be retained even after using it for five times, revealing a good stability. This also reveals that the SP-g-C3N4-PANI (3) nanocomposite is not deactivated or poisoned significantly. A very slight decrease of catalytic activity is due to the loss of the SP-g-C3N4-PANI (3) nanocomposite during the washing/centrifugation process because a very low catalyst concentration is used. To evaluate the structural stability, the crystalline structure of SP-g-C3N4-PANI (3) nanocomposite before and after five reaction cycles was investigated and no obvious changes are observed as demonstrated by the FT-IR, XRD and HR-TEM analysis as shown in Figure. 10. As can be seen in Figure 10A, FT-IR spectra taken before and after five reaction cycles show that there is no change in the composition of the photocatalyst. Figure. 10B shows the X-ray diffraction patterns of SP-g-C3N4-PANI (3) nanocomposite obtained before and after the dedyeing of MB. There is no significant crystalline structural change observed which can be seen from Figure. 10B. The morphology also remains unaltered which is confirmed by TEM (Figure. 10C) of the nanocomposite after five reaction cycles. These results suggest that the SP-g-C3N4-PANI (3) photocatalyst still shows excellent photocatalytic activity and durability.


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4. Conclusion In summary, we have demonstrated a synthesis of novel sulfur and phosphoruos codoped graphitic carbon nitride covalently grafted polyaniline metal-free nanocomposite via an in-situ chemical oxidative polymerization. The SP-g-C3N4-PANI (3) nanocomposite shows an enhanced photocatalytic activity in the dedyeing of MB under visible-light irradiation. The maximum degradation (99.9 %) of MB is able to be achieved by SP-g-C3N4-PANI (3) photocatalyst and the rate constant is found to be 0.113 min-1 which is dominant over its counterparts. The significant enhancement of photocatalytic performance can be attributed to synergistic effect of SP-g-C3N4 and covalently grafted PANI. These results are very well supported by the electrochemical impedance, photoelectrochemical, and photoluminescence spectral analysis. The present study provides a new strategy to prepare novel SP-g-C3N4-PANI nanocomposite for various potential applications in environmental and engineering fields. In addition, this study will certainly expose new researches on solar-driven water splitting in generation of H2. SUPPORTING INFORMATION The SEM images of PANI, S-g-C3N4, SP- g-C3N4, SP- g-C3N4-PANI (1), SP- g-C3N4-PANI (2), SP- g-C3N4-PANI (3), SP- g-C3N4-PANI (4), EDX spectrum of SP- g-C3N4-PANI (3), optimization graphs, effect of pH, catalyst concentration and MB concentration, the electrochemical impedance, photoelectrochemical and photoluminescence spectral analysis and repeated photocatalytic reduction experiments are presented in supplementary information.



AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] *Tel: +91 9842993931; Fax: +91 4522312375

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Ajayan, P. M. Exfoliated Graphitic Carbon Nitride Nanosheets as Efficient Catalysts for Hydrogen Evolution Under Visible Light. Adv. Mater. 2013, 25, 2452–2456.


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Scheme, Figure and Table captions Scheme 1 A schematic representation of the formation of SP-g-C3N4-PANI nanocomposite and photocatalytic degradation of MB using SP-g-C3N4-PANI Figure. 1A UV-vis spectra of S-g-C3N4 (curve a), SP- g-C3N4 (curve b) and PANI (curve c) and Figure.1B UV-vis spectra of SP- g-C3N4-PANI (1) (curve a), SP- g-C3N4-PANI (2) (curve b), SP- g-C3N4-PANI (3) (curve c) and SP- g-C3N4-PANI (4) (curve d) Figure. 2A FT-IR spectra of PANI (curve a), S-g-C3N4 (curve b) and SP- g-C3N4 (curve c), Figure. 2B FT-IR spectra of SP- g-C3N4-PANI (1) (curve a), SP- g-C3N4-PANI (2) (curve b), SPg-C3N4-PANI (3) (curve c) and SP- g-C3N4-PANI (4) (curve d), Figure. 2C XRD patterns of Sg-C3N4 (curve a), PANI (curve b) and SP- g-C3N4 (curve c) and Figure. 2D XRD patterns of SPg-C3N4-PANI (1) (curve a), SP- g-C3N4-PANI (2) (curve b), SP- g-C3N4-PANI (3) (curve c) and SP- g-C3N4-PANI (4) (curve d) Figure. 3 SEM images of selection areas of the SP- g-C3N4-PANI (3) for elements mapping of C, N, O, S and P (A-F) Figure. 4 HR-TEM images of SP- g-C3N4-PANI (3) (A-C) with different magnifications and SAED pattern of SP- g-C3N4-PANI (3) (D) Figure. 5 TGA curve of PANI, S- g-C3N4, SP- g-C3N4 and SP- g-C3N4-PANI (3) Figure. 6 N2 adsorption and desorption isotherm of PANI, S- g-C3N4, SP- g-C3N4 and SP- gC3N4-PANI (3) Figure. 7 Photocatalytic reduction of MB in the presence of PANI (A), S- g-C3N4 (B), SP- gC3N4 (C) and SP- g-C3N4-PANI (3) (D) 31 ACS Paragon Plus Environment


Figure. 8 (A) Photocatalytic degradation percentage of MB by (a) without catalyst (b) PANI, (c) S- g-C3N4, (d) P- g-C3N4, (e) SP- g-C3N4 (f) SP- g-C3N4-PANI (1), (g) SP- g-C3N4-PANI (2), (h) SP- g-C3N4-PANI (3), and (i) SP- g-C3N4-PANI (4), (B) Photocatalytic degradation curves of C/C0 vs time, (C) Plot of ln(C0/C) against the reaction time for reduction kinetics of MB in the presence of SP- g-C3N4-PANI (3) and (D) The rate constants, k, of the photocatalytic degradation of MB with various catalysts (a) PANI, (b) S- g-C3N4, (c) P- g-C3N4, (d) SP- g-C3N4, (e) SP- gC3N4-PANI (1), (f) SP- g-C3N4-PANI (2), (g) SP- g-C3N4-PANI (3), and (h) SP- g-C3N4-PANI (4) Figure. 9 Schematic illustration of the mechanism for the high photocatalytic performance of SP- g-C3N4-PANI (3) nanocomposite Figure. 10 (A) FT-IR spectra of SP- g-C3N4-PANI (3) – pre catalysis (curve a) and post catalysis (curve b). (B) XRD patterns of SP- g-C3N4-PANI (3) – pre catalysis (curve a) and post catalysis (curve b) and (C) HR-TEM image of SP- g-C3N4-PANI (3) post catalysis Table 1 N2 adsorption/desorption measurement results of synthesized photocatalytic materials Table 2 Photocatalytic activity of different nanomaterials catalysts for the degradation of MB


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Scheme 1



Figure. 1


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Figure. 2



Figure. 3


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Figure. 4



Figure. 5


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Figure. 6



Figure. 7


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Figure. 8



Figure. 9


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Figure. 10



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Table 1 Materials PANI S-g-C3N4 SP-g-C3N4 SP-g-C3N4-PANI (3)

Surface area (m2g-1) 42.58 55.73 82.32 126.47

Pore Volume (cm3g-1) 0.172

0.236 0.368 0.473


Average pore size (nm) 6.46 6.17 5.27 3.84

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Table 2 Materials

PAN/LPT PANI–g-C3N4 PANI/CdO PANI/TiO2 Ag/TiO2@PANI film PANI-CdS g-C3N4/N-TiO2 g-C3N4/ Zn2Ge4 bentonite/gC3N4 g-C3N4/TiO2 TiO2/PAn g-C3N4–CdS SP– g-C3N4PANI

Catalyst Weight (%)

MB concentration

Irradiation time (min)

Degradation (%)

30 mg/100ml 100mg/50ml 0.4 mg/mL 50mg/100ml 2mg/20ml

1.0×10−5 M 10 mg L-1 1.5×10−5 M 10 mg L-1 10 mg L-1

360 60 60 60 8h

100mg/200ml 7.5mg/50ml 50mg/100ml

20mg L-1 5mg L-1 10 mg L-1

100mg/100ml 100mg/100ml 100mg/50ml 80mg/200ml 50mg/100ml

Ref.

92.8 99.0 99.6 -

Rate constant (k) (min-1) 0.02082 0.017 0.091 0.2695/h

5h 80 150

92 73 -

0.491/h 0.015 0.35/ h

49 50 51

20 mg L-1

3h

90

0.74/ h

52

10 mg L-1 10 mg L-1 25 mg L-1 10 mg L-1

360 90 180 30

91 99.9

0.0121 0.133

44 53 54 This work


25 37 39 47 48


TOC Graphic


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Synergistic Combination of a Novel Metal-Free Mesoporous Band

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