Graphitic Carbon

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A nature-mimic method to fabricate polydopamine/graphitic carbon nitride for enhancing photocatalytic degradation performance Zongxue Yu, Fei Li, Qiangbin Yang, Heng Shi, Qi Chen, and Min Xu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b01313 • Publication Date (Web): 17 Jul 2017 Downloaded from http://pubs.acs.org on July 21, 2017

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ACS Sustainable Chemistry & Engineering

A nature-mimic method to fabricate polydopamine/graphitic carbon nitride for enhancing photocatalytic degradation performance

Zongxue Yu a,b* Fei Li a,b

Qiangbin Yang a,b

Heng Shi a,b

Qi Chen a,b

Min Xu a,b

(a.College of Chemistry and Chemical Engineering, Southwest Petroleum University, Chengdu,Sichuan 610500, P R of China; b. Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province, Southwest Petroleum University, Chengdu, Sichuan 610500, PR of China) *Address correspondence to this author. Zongxue Yu: Email: [email protected], Tel: 86-15208397092 * Mailing address：School of Chemistry and Chemical Engineering, Southwest Petroleum University, 8 Xindu Avenue, Chengdu, Sichuan 610500, China

ABSTRACTS In the paper, the polydopamine/graphitic carbon nitride (PDA/g-C3N4) has been synthesized by the dopamine (DA) of polymerization modification surface of g-C3N4. In order to study the information of morphology and optical property of catalysts , the obtained PDA/g-C3N4 composites were characterized by FT-IR spectra, XRD, SEM, TEM, BET, XPS, TG, DRS spectra, photoluminescence and photocurrent generation. The polydopamine (PDA) plays multiple roles as a light absorption substance, an electron transfer acceptor, and an adhesive interface in the designing PDA/g-C3N4 photosynthetic systems. The optical results demonstrate that the PDA has an effect for PDA/g-C3N4 composite light-harvesting capacity. With the PDA ratio increasing, the photocatalysts light-harvesting ability was gradually improved. Besides, the 10%PDA/g-C3N4 composite have been shown highly efficient for degrading organic dyes methylene blue (MB), Rhodamine B (RhB), and phenol under visible-light irradiation. The degradation efficiency of MB is about 98% in 3h and the catalysts can remain higher degradation efficiency than 90% after four cycles. The polydopamine

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(PDA), as a surface modified additive with abundant of semiquinones and quinones functional ligands, was performed to improve the transfer ability of photoinduced electron and accept it from semiconductor-based photocatalysis material (g-C3N4), which can reduce electron-hole recombination of g-C3N4 and enhance the photocatalytic activity. KEYWORDS: Graphitic carbon nitride; polydopamine; photocatalytic; visible light irradiation; methylene blue

INTRODUCTION With rapidly industrial development, generated the amounts of various organic contaminants have had an influence to human habitat and are becoming serious threats to the long-term development of human society and environmental sustainable1-3. As a result, novel discoveries and green sustainable ways in materials science and engineering have been pursued to settle the crisis for environmental remediation4. Among the visible-light responsive semiconductor-based photocatalysis has attracted tremendous interest5 due to it is considered as an economic, renewable, clean, and safe technology, which only requires the inexhaustible sunlight stimulation and a suitable semiconductor materials6. And the photocatalytic process containing photon absorption, charge carrier transfer, and catalytic surface reactions can be enhanced and performed by modify and change the chemical and physical property of photocatalyst4, 7-8. Therefore, it’s of great importance to seek for advanced materials with a well visible-light response for environmental remediation. Recently, the design of visible-light responsive semiconductor photocatalysts is vastly pursued by researchers for effective utilization solar energy that comprises a large fraction of visible light9-10. For the tradition photocatalysts of TiO2 with a large band gap (~3.4eV) have only an active in the ultraviolet region and remains the bottleneck to utilize visible light5,11-12. To date, the organic and metal-free polymeric two-dimension graphitic carbon nitride (g-C3N4) photocatalyst, with a band gap about 2.7eV that can effective utilize the visible light, has attracted tremendous interesting


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on water splitting13, contaminants degradation14-15, CO2 conversion16-17 and organic synthesis18 under visible-light irradiation. Due to g-C3N4 have excellently visible light absorption, appealing electronic band structure, highs physicochemical stability and photocatalytic properties. Importantly, g-C3N4 can be simply prepared via thermal polymerization of several low-cost nitrogen-rich precursors such as melamine14, dicyandiamide19, cyanamide20, urea17, thiourea21, and ammonium thiocyanate22. Nevertheless, in practical applications the photocatalytic ability of pure g-C3N4 is still limited by several obstacles and shortcomings, owing to fast recombination of photo-generated electron-hole pairs, low electrical conductivity and the lack of absorption above 460 nm, these will weaken g-C3N4 photocatalytic activity. To address these problems, there are several modified methods for pristine g-C3N4 including semiconductor or surface modification electron acceptor coupling3, metal/nonmetal doping25-26, incorporation with carbonaceous materials7,

27-28

23-24

,

, and

noble metal deposition29, etc. Among the kind of surface modification electron acceptor coupling associated with anchoring organic groups to g-C3N4 can significantly enhance the light-harvesting ability and promote the photoexcited electron-hole separation, and then to improve the photocatalytic activity. For example, the typically surface modifiers [Ru(bpy)3]3+ has been identified as effective photosensitizers to modify semiconductor30-32, resulting in well visible-light activation. Therefore, searching a stably immobilize surfaces modifiers for g-C3N4 photocatalysts is necessary to effectively enhance light-harvesting and avoid the electron-hole recombination or accelerate its separation, and then efficiently to improve the photocatalytic activity. Polydopamine (PDA), as a mimic inspired black biopolymer of mussels and shellfish33, has displayed a significantly broad range of interesting in many different fields such as environment, energy, and biomedicine, etc. Owing to its versatile adhesion ability, high light absorption ability , and outstanding biocompatibility and hydrophilicity34. And the PDA as a versatile agent have displayed a good ability on surface functionalization, owing to it without geometric hindrance and can readily form a thin film coating on different type of materials31, 35, such as metal/non-mental



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oxides, various semiconductors, noble metals, and synthetic polymers34. Importantly, PDA possesses a good UV and visible-light absorption ability and well photoconductivity

under

visible-light

irradiation,

implying

an

increase

of

photogenerated electrons and holes30, 33. Besides, the PDA, as the catechol derivative, contain amount of catechol groups that under neutral and basic conditions it has semiquinone or quinone groups31, 36; thus, it can accept electron and proton from an electron donor. Therefore, PDA can effectively transfer and separate the photoinduced electrons and protons as an electron gate for artificial photocatalysis systems, which will effectively avoid electron-hole recombination of semiconductor photocatalysts and improve the photocatalytic activity Herein, in this work we report the metal-free polymeric g-C3N4 using the dopamine with a surface modification to form the PDA/ g-C3N4 composites, which was applied to degrade the dye contaminant water under visible irradiation. In the case of the PDA/g-C3N4 composite, the main role of PDA was taken three features: On the one hand, the PDA has been used to enhance the visible light absorption which can effectively improve the photocatalytic efficiency. On the other hand, PDA behaved as an “electron acceptor” to accelerate the g-C3N4 photogenerated electrons and protons separation and transfer, avoiding electron-hole recombination. Besides, the PDA possesses an excellently adhesion ability which can efficiently absorb the dye pollutant on the surface of photocatalysts to effectively and directly reaction. Under visible light, photocatalytic degradation methyl blue (MB) was conducted to evaluate PDA/g-C3N4 photocatalytic activity in the presence and absence of H2O2, and investigate the adding amount of PDA influence for g-C3N4 photocatalytic activity.

EXPERIMENTAL SECTION Chemicals and Materials Tri(hydroxymethyl) aminomethane(Tri), and Dopamine hydrochloride was obtained from Aladdin. CA membrane, methylene blue and melamine were obtained


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from Kelong Chemical Co. Ltd. (Chengdu, China). Deionized (DI) water from a Millipore Milli-Q system (MA, USA) was used throughout. All chemical reagents using analytical grade unless otherwise marked.

Characterizations The surface morphology and microstructures of as-prepared samples was observed through scanning electron microscopy (SEM, JSM- 7500F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM; Tecnai G2 F20 with accelerating voltage of 200 kV). Nitrogen adsorption−desorption isotherms and pore sizes of the samples were obtained at -196°C using the Brunauer-Emmett-Teller (BET). Before the measurements, the samples were outgassed under vacuum for 6 h at 150 °C. Fourier transform infrared spectra (FTIR) of the samples were obtained from FTIR Spectrometer (FTIR; WQF-520). The X-ray diffraction (XRD) analysis was conducted on an X’Pert Pro diffractometer (PANalytical, The Netherlands) with a copper K radiation source at a scan rate of 2∘ per min ranging from 5∘ to 70∘. X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific, UK) was employed to analyze the surface elemental composition. The as-prepared samples were analyzed by the Thermogravimetric Analysis (TG; STA449F3) from 30℃ to 800℃ with a heating rate of 5◦C/min under air atmosphere. The UV-vis diffuse reflectance spectra of the sample were measured on a Perkin-Elmer Lambda 850UV-vis-NIR spectrometer (USA), using BaSO4 as reference. The photoluminescence (PL) measurements were carried out on PerKinEImer LS55(USA) and the samples were excited via 330 nm leaser illumination with a scanning speed of 600 nm·min-1 at room temperature. The photoelectrochemical responses of the samples (deposited in FTO glass) were tested in a three-electrode experimental system by Electrochemical Work-station (CS310, China). The concentrations of pollutants (MB, RhB, and phenol) were detected by the UV-vis-NIR (UV-762; Shanghai precision scientific instrument co.).

Preparation of the g-C3N4 nanosheets The g-C3N4 nanosheet was synthesized by thermal treatment melamine in the



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tube furnace with 500℃ for 4h (Figure 1; step1). In detail, 10g melamine powder was transferred into a semi-closed quartz boat and was heated from room temperature to 500℃ with a heating rate of 5℃·min-1 under N2 atmosphere situation. After calcination for 4h, the sample was cooled naturally to room temperature. Collecting the creamy white sample and dispersed into the 500ml of deionize water, the soya-like suspension solution was further dispersed and peeled in a ultrasonic device (FS-600N, 400W, 20 kHz) for 30min. The sample was collected and dried with 60℃ for 24h. Thus, a pure g-C3N4 was obtained for further experiments.

Polydopamine modified g-C3N4 (g-C3N4/PDA) PDA/g-C3N4 was prepared via solution of dopamine-hydrochloride added to the aqueous dispersion of g-C3N4 sheet at 60 ℃ for 24h (Figure 1; step1). In detail, 500 mg of g-C3N4 sheet was added to 100 mL water and the soya-like suspension was dispersed by sonication for 30 min. Subsequently different content of dopamine hydrochloride were added and the mixture was stirred at room temperature for 60 min (Table S1). After a certain proportion of 100 mL Tris-HCl solution was added and the pH of mixture solution was adjusted by the 1M NaOH solution (pH=8.5), the reaction mixture was magnetically stirred vigorously at 60 ℃ for 24 h. The solution was cooled down to room temperature and centrifuged under 4000 rpm for 10 min, the collected solid samples was dispersed and thoroughly rinsed with deionized water for 3 times to remove unreacted dopamine. The finally sample was dried at 60℃for 12h and the photograph of collected material was shown in Figure S1.


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Figure 1. Schematic of synthesis the g-C3N4 (step 1) and PDA/g-C3N4 (step 2) composites

Photocatalytic experiments The photocatalytic efficiency for pure g-C3N4 and g-C3N4/PDA composites with different dopamine contents were evaluated by degradation of methylene blue (MB) under visible-light irradiation and initial concentration of MB was 20 mg·L−1. 100mg of photocatalyst was added into 100 mL of MB solution with ultrasound dispersion. Before light irradiation, the mixed suspensions were magnetically stirred in the dark condition for 30 min to ensure between MB molecules and the photocatalyst reach the adsorption equilibrium. Then a 500 Xe lamp with a cutoff filter (the reaction environmental temperature was keep about 25℃ and illumination distance is about 15cm) was used as the visible-light source. After certain time intervals, the reaction solution (~3 mL) was collected and filtrated to remove the catalysts for analysis. The same experiment was conducted on the reaction system with 2 mL(wt 30%) of H2O2 and 50mg samples powder in the MB(20 mg·L−1), RhB(10 mg·L−1), and phenol(20 mg·L−1) solution. Using the UV-vis spectroscopy to monitor the concentration of collected pollutant solutions and recording maximum absorbance at the centered characteristic peak. The degradation efficiency of pollutants was calculated and converted by the following degradation equation: Degradation efficiency (×100%) =


(1)


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Where C0, Ct, and A0, At represent the temporary concentration and absorbance of solution with reaction time at 0 and t, respectively.

DISCSSION AND RESULTS Characterization The FT-IR spectra of the PDA, pure g-C3N4 and 10%PDA/g-C3N4 samples were tested and the result was shown in the Figure 2. For the FTIR spectrum of PDA, the bands at 1508 and 1620 cm-1 is the stretching vibration of indoline and indole structure of PDA. For the g-C3N4, the typical absorption band at about 810 cm-1 is related to the stretching mode for triazine units which was shown the basic in all PDA/g-C3N4 samples and a light red shift to a lower wavenumber (Figure S2b)27, 37. The strong bands at the1200~1650 cm−1 region is characteristic of aromatic CN heterocycles, the absorption band at 1640 cm−1 is attributed to C-N stretching, and the other three bands at 1549 cm−1, 1458 cm-1and 1407 cm−1 are contributed to the typical stretching vibration modes of C-N heterocycles14. Beside the bands at about 1325 cm-1 and 1243 cm-1 is related to the stretching vibration of connected units of CNH–C (partial condensation) or N–(C)3 (full condensation)37-38. The broad weaker band at 2800~3500 cm-1 is related to the N-H stretches by uncondensed amino functional groups in the samples24. For the FTIR spectrum of PDA/g-C3N4 with different addition content of PDA comparing with pure g-C3N4 show the similar spectra and less difference, which reveal that the original structure of g-C3N4 was maintained in the PDA/g-C3N4 composites (Figure S2a). And the functional groups of lower intensity of PDA have not causing news changes to g-C3N4, but the band intensity of PDA/g-C3N4 has slightly weakened with the ratio of PDA increasing. These results suggest that the surface of g-C3N4 was modified by PDA.


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Figure 2. FTIR spectra of pure g-C3N4 and PDA/g-C3N4 composites

The X-ray Diffraction (XRD) measurement was conducted to investigate the crystal structures of pure g-C3N4 and PDA/g-C3N4 with a different PDA content, the results was presented in the Figure 3. The pure g-C3N4 was shown by two mainly diffraction peaks at 13.35° and 27.69°, which are corresponded to the (100) and (002) plane, respectively. The strong diffraction peak at 27.69° was ascribed to the interlayer stacking reflection of conjugated aromatic systems, revealing a graphitic structure with an interlayer distance of 0.322 nm6, 39. And the weak diffraction peaks at 13.35° of pure g-C3N4 represents the in-plane trigonal nitrogen linkage of tri-s-triazine units corresponding to a distance of 0.662 nm13, 40. After a PDA modification, the crystal structure of g-C3N4 substantially remains the same diffraction peak at 27.69° with minor changes and the peaks intensity gradually weaken with the increasing of PDA modification content, indicating the g-C3N4 with a non-crystalline structure in the composites and PDA surface modified. However, the (100) peak of PDA/g-C3N4 shifts to lower angle of 12.36° and the diffraction peaks gradually reduced with the increasing of PDA content, which described to the PDA adhere to g-C3N4 surface and weaken the diffraction peak of g-C3N4 especially with an amount increasing of PDA.



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It is suggested that the PDA successful modify the g-C3N4 and with relative interlayer structure to conveniently transfer the electron.

Figure 3. XRD pattern of pure g-C3N4 and PDA/g-C3N4 composite

The microscopic morphologies and microstructure of pure g-C3N4 and 10% PDA/g-C3N4 samples were revealed using SEM and TEM and the element distribution displayed by the Elemental Mapping. As shown in Figure 4a, the SEM images of pure g-C3N4 shows a typically mixed aggregated morphology with sheet and some rod-like structures and the short rods adhered to the sheet, and some of them have already mixed an integrate that the little rod-like structures was shaped under vacuum situation with suitable power41-43. In Figure 4b, comparing with the pure g-C3N4, the PDA/g-C3N4 samples was more roughly surface with many small sheet and short rods, which attributed to the PDA modified g-C3N4 with well adhesive ability make these hierarchical unites assembled. The element distribution of PDA/g-C3N4 was observed by the Elemental Mapping in Figure 4c that the element of C, N and O was symmetrical distributed on the PDA/g-C3N4 surface, and the uniform distribution of O on PDA/g-C3N4 samples surface which reveals that PDA was well adhered to g-C3N4. And Energy dispersive X-ray spectroscopy (EDS) spectra were displayed in Figure S3. The morphology of pure g-C3N4 and PDA/g-C3N4 was further investigated using TEM in Figure 4d-e. As in Figure 4d show that the pure g-C3N4 has


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a well sheet structures and some thin rob carbon nitride sheet which some of them have already fused on the fringes. In Figure 4e, TEM images of PDA/g-C3N4 sample with different magnifications shows that g-C3N4 have a sheet-like morphology typical structure with covered some PDA, and good interfacial contact between g-C3N4 and PDA. The diameter distance of white diffraction ring is well corresponding to the XRD diffraction. And the HRTEM image of the pure g-C3N4 and 10%PDA/g-C3N4 was exhibited in Figure 5. As the Figure 5a shown that the pure g-C3N4 have a clearly and uniform non-crystalline structure. However, the PDA modified g-C3N4 (10%PDA/g-C3N4) displayed a blurry surface and PDA can be observed on edge of g-C3N4 (Figure 5b), and the PDA/g-C3N4 also present a heterogeneous non-crystalline structure. It was all indicated that the g-C3N4 was successfully modified by the PDA. To investigate the specific surface area and the pore size of the pure g-C3N4 and PDA/g-C3N4, N2 physical adsorption tests were carried out and the results was depicted in Figure S4. From the surface area data reveal that the PDA modified g-C3N4 have no significant difference with the surface area of pure g-C3N4. And the pore size of PDA/g-C3N4 have a slightly decrease with the content of PDA increasing, suggesting the PDA coating on the surface of g-C3N4 and make the pore size relatively more small.



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Figure 4. SEM image of (a)prue g-C3N4 and (b) PDA/g-C3N4 composite; (c) Elemental mapping images of 10%PDA/g-C3N4; (d) TEM images of pure g-C3N4; (e) TEM image of PDA/g-C3N4 and white dotted circles indicate pores


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Figure 5. HTEM image of (a) prue g-C3N4 and (b)10%PDA/g-C3N4 composite

The information on surface stoichiometry of the element and chemical environment of pure g-C3N4 and PDA/g-C3N4 samples were obtained by X-ray photoelectron spectroscopy (XPS) analysis, these elements composition is presented in Table S2. From the Figure 6a displayed, the XPS survey spectra revealed that the pure g-C3N4 and PDA/g-C3N4 contain the same C, N and O elements. Compared with pure g-C3N4, the PDA/g-C3N4 has a higher C and O percentage which is attributed to the PDA with abundant of oxygen-containing functional group and less N content. In Figure 6b, the XPS N1s spectrum of g-C3N4 was Gaussian curve-fitted into four component peaks, corresponding with the binding energy at about 398.34, 399.05, 400.60 and 404.37 eV, attributable to the sp2-bonded aromatic N bound to C atoms (C-N=C), tertiary N bonded to carbon atoms (N-(C)3), terminal amino groups (C-N-H) and positive charge localization or charging effects, respectively

13-14, 37

. The

PDA/g-C3N4 has the same binding energies (BEs) in each corresponding peak, but the peak intensity is relative to lower, which attributed to the PDA ad-layer and surface modification the g-C3N4. Additionally, the C 1s spectrum was shown in Figure 6c, the



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pure g-C3N4 with three mainly fitted peaks at 284.73, 285.26 and 401.7 eV can be observed which correspond to C-C, C-N and N-C-(N)2, respectively7, 13. And the N-C-(N)2 is the dominant peak which due to the g-C3N4 with aromatic nitrogen heterocycle structure. However, a new peak of C-O emerged in PDA/g-C3N4 composite which is arise from PDA on the resultant samples, and the C-N peaks become more strong. This is because the PDA with aromatic nucleus structure which contain hydroxyl and amidogen. And in the Figure 6d, the O1s spectrum of the PDA/g-C3N4, the peaks at 531.2 and 532.75eV correspond to O-C=O and C-O, respectively. In the pure g-C3N4 was accredited to the adsorption of H2O or CO2, which has a general phenomenon shown in literature37, 44. It’s all indicating that the dopamine had been successfully introduced into g-C3N4. In order to investigate the content of PDA on the surface of PDA, the PDA/g-C3N4 samples with different PDA contents were tested by TG analysis (Figure S5).


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Figure 6. (a) XPS survey spectra for g-C3N4 and PDA/g-C3N4 composite (10% PDA/g-C3N4 sample). (b-d) N 1s, C 1s, O 1s spectra for g-C3N4 and PDA/g-C3N4 composite (10% PDA/g-C3N4 sample).

The optical absorbance property of pure g-C3N4 and PDA/g-C3N4 samples with different PDA content was conducted using UV-vis diffuse reflectance spectroscopy (DRS), the result is shown in Figure 7. From the Figure 7a shown that pure g-C3N4 have an absorption edge in the visible light region with the wavelength is about 460nm, corresponding to a band gap of 2.68eV. However, PDA modification has an importantly affects to the optical property of g-C3N4. With increasing PDA amount in PDA/g-C3N4 composites, the absorption spectra exhibit a slightly red shift and the enhanced absorption intensity of light in the visible-light region, suggesting the PDA/g-C3N4 composites have a well ability to harvest more visible light than that of pure g-C3N4. And the 20% PDA/g-C3N4 sample exhibit almost equally strong absorption intensity from the UV to visible light region. The bandgap energy of semiconductor was calculated by Kubelka-Munk transformation, and the plots were depicted in Figure 7b. With the increasing of PDA content, the estimated bandgaps gradually reduce, and the bandgaps of pure g-C3N4 and 20%PDA/g-C3N4 is about 2.68eV and 2.05eV, respectively. The decrease of bandgap has further supported the observation for PDA/g-C3N4 composite with a red shift at the absorption band edge which compared to the pure g-C3N4. The Kubelka-Munk function: αhν=A(hν-Eg)n/2, where α, ν, Eg, A represent the absorption coefficient, light frequency, band gap energy, and constant, respectively. The n depends on the characteristics of the transition in a semiconductor and the n value of g-C3N4 is 4 for the indirect transition14, 45-46. It is suggested that the PDA coating modification on the surface of semiconductor materials can facilitate the light absorption and fast charge separation of proton-coupled electron, and then improve the catalytic performance of photocatalysts.



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Figure 7. (a) UV−vis diffuse reflectance spectra of pure g-C3N4 and PDA/g-C3N4 samples; (b) The band gap plots of these photocatalysts

In addition, the generation, recombination of charge carriers, and transfer of electron play an vital factor in photocatalytic activity47. So the photoluminescence (PL) spectra were conducted to study the optical property of samples and the experiment results were shown in the Figure 8a. The PL spectrum of the pure g-C3N4 displays a strong PL emission at 440nm, due to the n-π∗ electronic transitions in g-C3N440, relating to the high radiative recombination of photoexcited electrons and holes. Obviously, the PDA modified g-C3N4 has a lower emission intensity comparing with the pure g-C3N4 and with the amount increasing of PDA, the emission intensity gradually weaken which shown a lower photogenerated carrier recombination rate. That is, the g-C3N4 photogenerated electrons can be effectively transferred to PAD with less charge recombination, which form more activated •OH to degrade the MB. This result can be attributed to PDA with semiquinones and quinones functional ligands which act as an electron acceptor. Hence, both the enhanced light absorption and lowered radiative electron-hole recombination will endow the PAD/g-C3N4 composite with highly photocatalytic activity. The photoresponses of g-C3N4 and 10%PAD/g-C3N4 was tested by the photocurrent experiments that attributed to the electric field and the excited photo-electrons and holes move in the opposite direction, resulting in their improved separation rate and reduced recombination possibility. The photocurrent-time (I-t)


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curves of samples were shown in Figure 8b with 60s intermittent on-off cycles under visible-light irradiation. The photocurrent value almost equal to zero under dark condition and as with a visible light irradiation the photocurrent sharply increased to certain value. However, the photocurrent value sharply decline to zero when the illumination light turned off, and then it faster returned to a steady constant value with the illumination once again, the processes was well renewable. The 10%PAD/g-C3N4 composite demonstrated about 2~3 times higher photocurrent responses than pure g-C3N4 under visible-light irradiation. This result indicated that the PAD surface modified g-C3N4 composite could prolong lifetime of photogenerated electron-hole pairs and improve its transport ability of photogenerated charge carriers. It is acknowledged that the main reasons of photocurrent generation is the photoinduced electrons diffuse to and back contact, and then the photogenerated holes was accepted by the hole acceptor48-49. The result was consistent with that of PL measurement. Furthermore, the photogenerated charge separation process also was investigated by the electrochemical impedance spectroscopy (EIS) methods (Figure S7). The semicircular Nyquist plots for pure g-C3N4 and the 10%PAD/g-C3N4 composite disclose that the arc radius 10%PAD/g-C3N4 composite is smaller than the pure g-C3N4. Suggesting the 10%PAD/g-C3N4 composite have a relatively low charge carrier transfer resistance which can improve separation and transfer efficiency of electron-hole pairs.

Figure 8. (b) Photoluminescence (PL) spectra of pure g-C3N4 and PDA/g-C3N4 samples under 330 nm leaser illumination at room temperature; (a) Transient



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photocurrent response of pure g-C3N4 and 10%PDA/g-C3N4 samples (Three electrodes systems in 0.5 mol L−1Na2SO4 aqueous solution under visible-light irradiation)

Photocatalytic evaluation The photocatalytic activities of pure g-C3N4 and PAD/g-C3N4 composite were tested by the degradation of organic dye (MB) under visible-light irradiation (λ> 420 nm). In order to reach the adsorption equilibrium, the mixed solution of MB and photocatalyst was magnetic stirred for 30min under dark situation. During the photo-degradation process, the temporal Ct/C0 changes of MB was showed in Figure 9a, that the different profiles are the pure g-C3N4, PDA/g-C3N4 with different content of PDA and the absence of catalyst to the photocatalytic degradation efficiency curves of MB under identical experimental conditions, respectively. As shown, there are about 5% degradation of MB was observed without photocatalyst after 180min of visible-light irradiation. As for pure g-C3N4 displayed a lower degradation activity and the photocatalytic efficiency for MB is about 28.56% after 180min of photo-degradation. Comparing with pure g-C3N4, the PDA modified g-C3N4 has exhibited increase photocatalytic efficiency. With the content increasing of PDA, the photocatalytic activity of PDA/g-C3N4 photocatalysts has an obviously enhanced that the degradation efficiency is about 34.54%, 45.20%, 61.06%, 96.57% and 98.84%, respectively. The 10%PDA/g-C3N4 and 20%PDA/g-C3N4 sample exhibited the highly efficient photocatalytic performance, which was about 4 times higher than the pure g-C3N4. Besides, comparing with the 5%PDA/g-C3N4 and 10%PDA/g-C3N4 sample, the 10%PDA/g-C3N4 composite have an obvious improvement for photocatalytic degradation efficiency. However, for the 10%PDA/g-C3N4 and 20%PDA/g-C3N4 composites there are no noticeable improvement on the photocatalyst efficiency with the increased amount of PDA. The phenomenon are attributed to PDA can accelerates the photoinduced electron rate through electron and proton redox-coupling methods, but superfluous PDA not only could hinder the g-C3N4 to absorb and utilize the


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stimulus light but also harvest and shield the transferred photo-electron. These all results reveal that PDA plays a vital role in improvement of photocatalytic efficiency and separation of electron-hole pairs. The absorption spectra change of MB solution of the 10%PDA/g-C3N4 photocatalysts was displayed in Figure 9b with different time. From the Figure 9b revealed that the same strongly maximum absorption peaks at 664. It is seen that the absorption curves gradually weakens as the exposure time increases and absorption peak at 664 nm nearly disappear after different time interval, indicating the MB was step-by-step degraded by PDA/g-C3N4 composite during the photocatalytic reaction processes under visible-light irradiation. The experiment data of MB degradation were fitted to study reaction kinetics by first-order kinetics equation. Figure 9c show that the fitting curves of MB photocatalytic degradation is accorded with the first-order reaction dynamics. The first-order equation follows:

ln( ) −

(2)

Where Ct and C0 are the dye concentrations in solution at each given time and t=0, respectively. And the rate constant k (min-1) is the slope of the corresponding fitting curves, t represents the reaction time. Figure 9c display the effect of PDA content on the MB photo-degradation rate with pure g-C3N4 and PDA/g-C3N4. The irradiation time (t) of fitted curves against –ln (Ct/C0) are nearly straight line that the samples with varying PDA content. The reaction rate constant (k) of pure g-C3N4 and PDA/g-C3N4 with different content of PDA is about 0.0015, 0.0020, 0.0027, 0.0040, 0.0191 and 0.0234 min-1, respectively. The reaction rate of 10%PDA/g-C3N4 and 20%PDA/g-C3N4 is about 12.6 and 15.5 times higher than pure g-C3N4 under the same experimental conditions, respectively. To improve the photocatalytic activity, the H2O2 additive was introduced to the catalytic process, that is, advanced oxidation processes (AOPs). The photocatalytic degradation results are shown in Figure 9d. Comparing with the absence of H2O2 system, although the amount of catalysts is only half of it, the MB degradation time of 10%PAD/g-C3N4 and 20%PAD/g-C3N4 samples



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is about 120min that the degradation efficiency up to 99%. And the reference sample of absence catalysts has a significantly degradation for MB and the degradation is about 30%, which is because that the H2O2 will generate the hydroxyl radical (·OH) and can directly degrade the MB. The improved photocatalytic results due to the H2O2 can faster react with photo-induced holes (h+) to produce more hydroxyl radicals (∙ OH) and avoiding the electron-hole recombination. Besides, the RhB and phenol also was chosen to investigate the photocatalytic activity of PAD/g-C3N4 (Figure S6), which also shown that the PAD/g-C3N4 have a well photocatalytic activity comparing with pure g-C3N4. From these experiments results revealed that the10%PDA/g-C3N4 and 20%PDA/g-C3N4 with less difference on photocatalytic degradation, the 10%PDA/g-C3N4 will be considered further investigation in next works.

Figure 9. (a) Photocatalytic degradation MB of pure g-C3N4 and PDA/g-C3N4 samples under visible light irradiation. (b) The UV-vis spectra changes of MB with irradiation time for 10%PDA/g-C3N4 samples; (c) Kinetic linear simulation curves of MB


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photocatalytic degradation. (d) Photocatalytic degradation MB of pure g-C3N4 and PDA/g-C3N4 samples adding H2O2 (the amount of 1%wt).

The stability of PDA/g-C3N4 catalyst was investigated via the recycle experiment for degradation the MB with 2mL H2O2 under visible light irradiation. For each recycling recycle, the collected PDA/g-C3N4 was washed by deionized (DI) water for three times and then dried at 60℃ for 12h. As shown in Figure 10, the degradation efficiency of MB remains higher than 90% after four-cycle decomposition processes, revealing the PDA/g-C3N4 excellent stability among the photocatalytic degradation process. The XRD and TG analysis of 10%PDA/g-C3N4 sample before and after the four times photocatalytic reaction was displayed in Figure S8. It was found that the diffraction peak of the recycled 10%PDA/g-C3N4 composite after four times of photocatalytic reaction had no obvious discrepancy compared with the fresh one (Figure S8a). However, the increase of diffraction peak intensity can be found through carefully observation, indicating the loss of PDA of g-C3N4 surface which is consistent with TG results (Figure S8a). It is the possible reason for the decrease of photocatalytic efficiency of photocatalysts for every recycle.



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Figure 10. Recycling photocatalytic tests of 10% PDA/g-C3N4 for degradation of MB under visible light irradiation (2mL; 30%wt H2O2)

Photocatalytic mechanism There are a lot of possible reaction pathways on the intricately photocatalytic degradation process for the eventual photo-degradation of the pollutants. However, the common reaction process can be summarized into two steps that the degradation of the organic contaminants. Firstly the organic pollutant was absorbed onto the surface of photocatalysts. And then the organic pollutant was degraded by the reactive specie such as hydroxyl radicals (•OH), holes (h+), and superoxide radicals (•O2-). To investigate the main active species with an effect for PDA/g-C3N4 photocatalysts, the different scavengers were introduced into the photocatalytic degradation reaction of MB, which to perform the relative roles of the reactive species and evaluate the photocatalytic

mechanism

of

PDA/g-C3N4.

These

scavenger

additives

of

p-benzoquinone (BQ; 10mM), disodium ethyleneiaminetetraacetate (EDTA-2Na; 10mM), tertiary butanol (t-BuOH; 10mM) were introduced into the photocatalytic reaction system for trapping the specific reactive specie superoxide radicals (•O2-), holes (h+), and hydroxyl radicals (•OH), respectively. As Figure 11 shown that the degradation efficiency has a significant decrease for the photocatalytic degradation of MB when adding the t-BuOH and EDTA for 3h visible light illumination , whereas BQ with a slightly influences. It demonstrates that the major active species are h+ and •OH among the photocatalytic degradation process. For the PDA/g-C3N4 photocatalyst, the possible photocatalytic mechanism was tentatively presented in Figure 12 under visible-light irradiation. The dopamine modified g-C3N4 contains abundant functional groups such as amine groups, catechol groups, and aromatic ring34, which can effectively absorb the cation dye MB. Besides, the polydopamine possession some active sites also can absorb organic pollutants via electrostatic interaction and hydrogen bonding. And the giant π conjugation between MB molecule (benzene ring and the pyridine ring) and g-C3N4 (hexatomic ring of C-N) can form the π-π stacking, thus these organic dye molecules would be extracted


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from solution and then concentrated on the photocatalyst surface. The high photocatalytic activity can be attributed to the PDA well light harvesting ability and fast transfer and separation ability of photo-generated electrons at the PDA modified g-C3N4 interface. Under visible-light irradiation, the g-C3N4 can induce π- π* transition and the excited-state electrons was transported from the VB to the CB which the VB by N 2p to the CB formed by C 2p orbitals of g-C3N414, 37, and then accumulating electronic(e−) in the CB and leaving holes(h+) in the VB: Semiconductor(SC) + hν → h+(VB) + e-(CB)4. For the case of PDA/g-C3N4, the CB position values for g-C3N4 is about -1.42eV vs. NHE which is beneficial to the charge transfer from g-C3N4 to PDA (-0.08 eV vs. NHE)31. And the PDA of g-C3N4 surface assemble a redox shuttle to transfer electrons and protons from donor to acceptor, owing to its abundant of catechol groups31. Resembling the natural photosystem, quinone molecules function as a two-electron gate will directly improve the electron transfered efficiency from chlorophyll by a factor of two50. And then the water molecule (H2O) or hydroxide ions (OH−) was captured by the valence band holes to produce the non-selective, extremely powerful, and oxidizing hydroxyl radical (OH•). The enriched electrons on the PDA would be trapped by the free molecular oxygen (O2) to yield superoxide radicals •O2−. Both •O2− and •OH reactive radical can directly oxidize the MB dye molecular. Subsequently, introducing the H2O2 significantly enhance the degradation rate for photocatalytic action that Fenton-like excitation of H2O2 to form more hydroxyl radicals (∙OH) comparing with H2O molecules51.



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Figure 11. Photocatalytic degradation efficiencies of MB on 10%PDA/g-C3N4 by adding the scavengers (The dosage of scavengers = 10mM, illumination time t = 3h)

Figure 12. Schematic illustration of electron transfer and possible photocatalytic degradation mechanism scheme of PDA/g-C3N4 under visible light irradiation

CONCLUSIONS ACS Paragon Plus Environment

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In summary，we have demonstrated a novel simple strategy to modified the g-C3N4 surface using the polydopamine which not only improve light-harvesting but act as an electron acceptor to enhance the photocatalytic activity under visible light irradiation, owing to the PDA have a lots of multiple functional groups, possess strong interfacial adhesion, and own the accelerated electron transfer ability. The influence of PDA/g-C3N4 composites with different PDA ratio to the photocatalytic activity has been investigated detailedly by photocatalytic degradation of MB under visible light irradiation. The results revealed that the PDA/g-C3N4 have an excellently photocatalytic activity for MB degradation. Among degradation efficiency of 10%PDA/g-C3N4 and 20% PDA/g-C3N4 samples for MB is about 97% and 99% in 180min, and the degradation rate are 12.6 and 15.5 times higher than that of pure g-C3N4, respectively. At the same time, the photocatalytic degradation of RhB and phenol also shown that the PAD/g-C3N4 have a well photocatalytic activity comparing with pure g-C3N4. The importantly improved photocatalytic activity of PDA/g-C3N4 composite is attributed to the PDA which can improve the visible light absorption and contain numerous catechol groups as an electron acceptor to transfer the photogenerated electrons, thus, suppressing the recombination rate of electrons and holes. Therefore, surface modified g-C3N4 composites provide new perspectives for organic degradation under visible light irradiation which has a promising for application in water purity.

ACKNOWLEDGEMENT This work was supported by Open Fund(TLN201617) of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation（Southwest Petroleum University）

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Abstract Graphic:

Synopsis: The PDA modified g-C3N4 photocatalysts can efficiency enhance the visible-light absorption and accelerate the separation of hole-electron of g-C3N4, which exhibit well ability in removing dye pollutants.



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105x56mm (300 x 300 DPI)


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Graphitic Carbon

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