Graphitic Carbon

Research Article pubs.acs.org/journal/ascecg

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 College of Chemistry and Chemical Engineering, Southwest Petroleum University, 8 Xindu Avenue, Chengdu, Sichuan 610500, P. R. China Oil & Gas Field Applied Chemistry Key Laboratory of Sichuan Province, Southwest Petroleum University, Chengdu, Sichuan 610500, P. R. China S Supporting Information *

ABSTRACT: In this paper, polydopamine/graphitic carbon nitride (PDA/g-C3N4) has been synthesized by the dopamine (DA) polymerization modification of the surface of g-C3N4. For a study of the morphology and optical property of catalysts, the obtained PDA/gC3N4 composites were characterized by FTIR, XRD, SEM, TEM, BET, XPS, TGA, DRS (diffuse reflectance spectroscopy), photoluminescence, and photocurrent generation. Polydopamine (PDA) plays multiple roles as a light absorption substance, an electron transfer acceptor, and an adhesive interface in the design of PDA/g-C3N4 photosynthetic systems. The optical results demonstrate that PDA has an effect on the PDA/g-C3N4 composite light-harvesting capacity. With an increasing PDA ratio, the photocatalyst’s light-harvesting ability was gradually improved. In addition, the 10%PDA/g-C3N4 composite has been shown to be highly efficient for the degradation of the organic dyes methylene blue (MB), Rhodamine B (RhB), and phenol under visible-light irradiation. The degradation efficiency of MB is about 98% in 3 h, and the catalysts can have a degradation efficiency higher than 90% after four cycles. Polydopamine (PDA), as a surface-modified additive with abundant semiquinone and quinone functional ligands, was introduced for an improvement of the transfer ability of photoinduced electrons and accepts them from a 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 rapid industrial development, the amounts of various generated organic contaminants have had an influence on human habitats and are becoming serious threats to the longterm development of human society and environmental sustainability.1−3 As a result, novel discoveries and green sustainable methods in materials science and engineering have been pursued for a resolution of the crisis for environmental remediation.4 Visible-light-responsive semiconductor-based photocatalysis has attracted tremendous interest5 because it is considered an economic, renewable, clean, and safe technology, which only requires inexhaustible sunlight stimulation and suitable semiconductor materials.6 In addition, the photocatalytic process containing photon absorption, charge carrier transfer, and catalytic surface reactions can be enhanced and performed by the modification and change of the chemical and physical properties of the photocatalyst.4,7,8 Therefore, it is of great importance to seek advanced materials with a good visible-light response for environmental remediation. Recently, the design of visible-light-responsive semiconductor photocatalysts has been vastly pursued by researchers for the effective utilization of solar energy that consists of a large fraction of visible light.9,10 The traditional photocatalysts of © 2017 American Chemical Society

TiO2 with a large band gap (∼3.4 eV) are only active in the ultraviolet region and remain the bottleneck for the utilization of visible light.5,11,12 To date, the organic and metal-free polymeric two-dimensional graphitic carbon nitride (g-C3N4) photocatalyst, with a band gap of about 2.7 eV that can effectively utilize visible light, has attracted tremendous interesting in water splitting,13 contaminant degradation,14,15 CO2 conversion,16,17 and organic synthesis18 under visible-light irradiation. This is because g-C3N4 has excellent visible-light absorption, appealing electronic band structure, high physicochemical stability, and photocatalytic properties. Importantly, g-C3N4 can be simply prepared via the thermal polymerization of several low-cost nitrogen-rich precursors such as melamine,14 dicyandiamide,19 cyanamide,20 urea,17 thiourea,21 and ammonium thiocyanate.22 Nevertheless, in practical applications, the photocatalytic ability of pure g-C3N4 is still limited by several obstacles and shortcomings, because of the fast recombination of photogenerated electron−hole pairs, low electrical conductivity, and Received: April 26, 2017 Revised: July 2, 2017 Published: July 17, 2017 7840

DOI: 10.1021/acssuschemeng.7b01313 ACS Sustainable Chem. Eng. 2017, 5, 7840−7850

Research Article

ACS Sustainable Chemistry & Engineering

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

absorption which can effectively improve photocatalytic efficiency. Second, PDA behaved as an “electron acceptor” for the acceleration of the separation and transfer of the g-C3N4 photogenerated electrons and protons, avoiding electron−hole recombination. Third, PDA possesses an excellent adhesion ability which can efficiently absorb the dye pollutant on the surface of photocatalysts through an effective and direct reaction. Under visible light, the photocatalytic degradation of methyl blue (MB) was conducted for an evaluation of PDA/gC3N4 photocatalytic activity in the presence and absence of H2O2, and for an investigation into the influence of the added amount of PDA on g-C3N4 photocatalytic activity.

lack of absorption above 460 nm; these will weaken the photocatalytic activity of g-C3N4. As a response to these problems, there are several modified methods for pristine gC3N4 including semiconductor or surface-modification electron-acceptor coupling,3,23,24 metal−nonmetal doping,25,26 incorporation with carbonaceous materials,7,27,28 noble metal deposition,29 etc. The kind of surface-modification electronacceptor coupling associated with anchoring organic groups to g-C3N4 can significantly enhance light-harvesting ability and promote photoexcited electron−hole separation, and can then improve photocatalytic activity. For example, the typical surface modifier [Ru(bpy)3]3+ has been identified as an effective photosensitizer for the modification of semiconductors,30−32 resulting in good visible-light activation. Therefore, the search for stably immobilized surface modifiers for g-C3N4 photocatalysts is necessary for the effective enhancement of lightharvesting and the avoidance of electron−hole recombination, or acceleration of its separation, and then the efficient improvement of photocatalytic activity. Polydopamine (PDA), as a mimic-inspired black biopolymer of mussels and shellfish,33 has gathered a significantly broad range of interests in many different fields such as environment, energy, biomedicine, etc., because of its versatile adhesion ability, high-light-absorption ability, and outstanding biocompatibility and hydrophilicity.34 In addition, PDA as a versatile agent has displayed a good ability for surface functionalization, because it is without geometric hindrance and can readily form a thin film coating on different types of materials,31,35 such as metal−nonmetal oxides, various semiconductors, noble metals, and synthetic polymers.34 Importantly, PDA possesses a good UV- and visible-light-absorption ability and good photoconductivity under visible-light irradiation, implying an increase of photogenerated electrons and holes.30,33 In addition, PDA, as a catechol derivative, contains an amount of catechol groups, and under neutral and basic conditions, it has semiquinone or quinone groups;31,36 thus, it can accept electrons and protons from an electron donor. Therefore, PDA can effectively transfer and separate photoinduced electrons and protons as an electron gate for artificial photocatalysis systems, which will effectively avoid the electron−hole recombination of semiconductor photocatalysts and improve photocatalytic activity. Herein, we report the metal-free polymeric g-C3N4 using dopamine with a surface modification to form PDA/g-C3N4 composites, which were applied to degrade dye-contaminated water under visible irradiation. In the case of the PDA/g-C3N4 composite, the main role of PDA was the following three features: First, PDA was used to enhance visible-light

■

EXPERIMENTAL SECTION

Chemicals and Materials. Tris(hydroxymethyl) aminomethane (tris) and dopamine hydrochloride were obtained from Aladdin. CA (cellulose acetate) membrane, methylene blue, and melamine were obtained from Kelong Chemical Co., Ltd. (Chengdu, China). Deionized (DI) water from a Millipore Milli-Q system was used throughout. All used chemical reagents were analytical-grade unless otherwise marked. Characterizations. The surface morphology and microstructures of the as-prepared samples were observed through scanning electron microscopy (SEM; JSM-7500F, JEOL, Tokyo, Japan) and transmission electron microscopy (TEM; Tecnai G2 F20 with an 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) method. Before the measurements, the samples were outgassed under vacuum for 6 h at 150 °C. Fourier transform infrared (FTIR) spectra of the samples were obtained from an FTIR spectrometer (FTIR; WQF-520). The X-ray diffraction (XRD) analysis was conducted on an X’Pert Pro diffractometer (PANalytical) with a Cu Kα radiation source at a scan rate of 2° min−1 ranging from 5° to 70°. X-ray photoelectron spectroscopy (XPS; Thermo Fisher Scientific) was employed for analysis of the surface elemental composition. The as-prepared samples were analyzed by thermogravimetric analysis (TGA; STA449F3) from 30 to 800 °C with a heating rate of 5 °C min−1 under air atmosphere. The UV−vis diffuse reflectance spectra of the sample were measured on a PerkinElmer Lambda 850UV−vis−NIR spectrometer, using BaSO4 as reference. The photoluminescence (PL) measurements were carried out on a PerkinElmer LS55 device, and the samples were excited via a 330 nm laser illumination with a scanning speed of 600 nm min−1 at room temperature. The photoelectrochemical responses of the samples [deposited in FTO (fluorine-doped tin oxide) glass] were tested in a three-electrode experimental system by an electrochemical work-station (CS310). The concentrations of pollutants (MB, RhB, and phenol) were detected by UV−vis−NIR (UV-762; Shanghai Precision Scientific Instrument Co.). Preparation of the g-C3N4 Nanosheets. The g-C3N4 nanosheet was synthesized by the thermal treatment of melamine in a tube 7841


Research Article

ACS Sustainable Chemistry & Engineering furnace at 500 °C for 4 h (Figure 1, step 1). In detail, 10 g of melamine powder was transferred into a semiclosed quartz boat and was heated from room temperature to 500 °C with a heating rate of 5 °C min−1 under N2 atmosphere conditions. After calcination for 4 h, the sample was cooled naturally to room temperature. Upon collection of the creamy white sample and dispersion into the 500 mL of deionized water, the soya-like suspension solution was further dispersed and placed in an ultrasonic device (FS-600N, 400 W, 20 kHz) for 30 min. The sample was collected and dried at 60 °C for 24 h. Thus, a pure gC3N4 sample was obtained for further experiments. Polydopamine-Modified g-C3N4 (g-C3N4/PDA). PDA/g-C3N4 was prepared via a solution of dopamine hydrochloride added to the aqueous dispersion of g-C3N4 sheets at 60 °C for 24 h (Figure 1, step 2). In detail, 500 mg of g-C3N4 sheets was added to 100 mL of water, and the soya-like suspension was dispersed by sonication for 30 min. Subsequently, dopamine hydrochloride was added in different amounts, and the mixture was stirred at room temperature for 60 min (Table S1 in the Supporting Information). After 100 mL of a certain proportion of tris-HCl solution was added and the pH of the mixture solution was adjusted by the 1 M NaOH solution (pH = 8.5), the reaction mixture was magnetically stirred vigorously at 60 °C for 24 h. The solution was cooled down to room temperature and centrifuged under 4000 rpm for 10 min, and the collected solid samples were dispersed and thoroughly rinsed with deionized water 3 times for the removal of unreacted dopamine. The final sample was dried at 60 °C for 12 h, and a photograph of the collected material is shown in Figure S1. Photocatalytic Experiments. The photocatalytic efficiency for pure g-C3N4 and g-C3N4/PDA composites with different dopamine contents was evaluated by the degradation of methylene blue (MB) under visible-light irradiation, and the initial concentration of MB was 20 mg L−1. A 100 mg portion of photocatalyst was added into 100 mL of MB solution with ultrasound dispersion. Before light irradiation, the mixed suspensions were magnetically stirred in dark conditions for 30 min to ensure that MB molecules and the photocatalyst reached the adsorption equilibrium. Then a 500 Xe lamp with a cutoff filter (the reaction environmental temperature was kept at about 25 °C and illumination distance 15 cm) was used as the visible-light source. After certain time intervals, the reaction solution (∼3 mL) was collected and filtrated for the removal of the catalysts for analysis. The same experiment was conducted on the reaction system with 2 mL (30 wt %) of H2O2 and 50 mg samples of powder, MB (20 mg L−1), RhB (10 mg L−1), and phenol (20 mg L−1), in the solution. UV−vis spectroscopy was used to monitor the concentration of collected pollutant solutions, and maximum absorbance was recorded at the centered characteristic peak. The degradation efficiency of pollutants was calculated and converted by the following degradation equation: degradation efficiency ( ×100%) =

C0 − Ct A − At = 0 C0 A0

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

heterocycles.14 In addition, the bands at about 1325 and 1243 cm−1 are 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 samples.24 For the FTIR spectrum of PDA/gC3N4, with the different, additional content of PDA compared to pure g-C3N4, there are similar spectra and fewer differences, which reveal that the original structure of g-C3N4 was maintained in the PDA/g-C3N4 composites (Figure S2a). Additionally, the lower-intensity functional groups of PDA have not changed from g-C3N4, but the band intensity of PDA/gC3N4 has slightly weakened as the ratio of PDA increases. These results suggest that the surface of g-C3N4 was modified by PDA. X-ray diffraction (XRD) measurements were conducted for an investigation of the crystal structures of pure g-C3N4 and PDA/g-C3N4 with a different PDA content; the results are presented in Figure 3. The pure g-C3N4 was mainly shown by

(1)

where C0, Ct, A0, and At represent the temporary concentration at reaction times 0 and t, and absorbance of solution at reaction times at 0 and t, respectively.

■

DISCUSSION AND RESULTS Characterization. The FTIR spectra of the PDA, pure gC3N4, and 10%PDA/g-C3N4 samples were tested, and the result is shown in Figure 2. For the FTIR spectrum of PDA, the bands at 1508 and 1620 cm−1 are the stretching vibration of the 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 seen in all PDA/gC3N4 samples as well as a light red shift to a lower wavenumber (Figure S2b).27,37 The strong bands at the 1200−1650 cm−1 region are characteristic of aromatic CN heterocycles, and the absorption band at 1640 cm−1 is attributed to C−N stretching. The other three bands at 1549, 1458, and 1407 cm−1 are attributed to the typical stretching vibration modes of C−N

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

two diffraction peaks at 13.35° and 27.69°, which correspond to the (100) and (002) planes, 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 nm.6,39 In addition, the weak diffraction peaks at 13.35° of pure g-C3N4 represent the in-plane trigonal nitrogen linkage of tri-s-triazine units 7842


Research Article


Figure 4. SEM image of (a) pure 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 with white dotted circles that indicate pores.

corresponding to a distance of 0.662 nm.13,40 After a PDA modification, the crystal structure of g-C3N4 substantially contains the same diffraction peak at 27.69° with minor changes, and the peak’s intensity gradually weakens with the increase of PDA content, indicating that the g-C3N4 with a noncrystalline structure in the composites and the PDA surface were modified. However, the (100) peak of PDA/g-C3N4 shifts to a lower angle of 12.36°, and the diffraction peaks gradually reduced with the increase of PDA content, which is described by the adherence of PDA to the g-C3N4 surface and the weakening of the diffraction peak of g-C3N4, especially with an increasing amount of PDA. It is suggested that the PDA successful modifies the g-C3N4 and with a relatively interlayered structure for the convenient transfer of electrons. The microscopic morphologies and microstructures of pure g-C3N4 and 10%PDA/g-C3N4 samples were revealed using SEM and TEM, and the element distribution was displayed by elemental mapping. As shown in Figure 4a, the SEM images of pure g-C3N4 show a typically mixed aggregated morphology with sheet and some rodlike structures, with the short rods adhered to the sheet, and with some of them already mixed, indicating that the little rodlike structures were shaped under vacuum conditions with suitable power.41−43 In Figure 4b, compared with pure g-C3N4, the PDA/g-C3N4 samples had a rougher surface with many small sheets and short rods, which gave the PDA-modified g-C3N4 a good adhesive ability for the assembly of these hierarchical units. The element distribution

of PDA/g-C3N4 observed by the elemental mapping in Figure 4c showed that the elements of C, N, and O were symmetrically distributed on the PDA/g-C3N4 surface, and the uniform distribution of O on the PDA/g-C3N4 surface reveals that PDA was well-adhered to g-C3N4. In addition, energy-dispersive Xray spectroscopy (EDS) results are displayed in Figure S3. The morphology of pure g-C3N4 and PDA/g-C3N4 was further investigated using TEM in Figure 4d,e. Figure 4d shows that pure g-C3N4 has good sheet structures and some thin-rod carbon nitride sheets, and some of them have already fused on the fringes. In Figure 4e, TEM images of the PDA/g-C3N4 sample with different magnifications show that g-C3N4 has a typical sheetlike morphology structure, with some PDA coverage, and good interfacial contact between g-C3N4 and PDA. The diameter distance of the white diffraction ring corresponds well to the XRD results, and the HRTEM image of pure g-C3N4 and 10%PDA/g-C3N4 is exhibited in Figure 5. As Figure 5a shows, the pure g-C3N4 has a clear and uniform noncrystalline structure. However, the PDA-modified g-C3N4 (10%PDA/g-C3N4) displayed a blurry surface, and PDA can be observed on the edge of g-C3N4 (Figure 5b). Additionally, PDA/g-C3N4 also presents a heterogeneous noncrystalline structure. All of these observations indicate that the g-C3N4 was successfully modified by the PDA. For an investigation of the specific surface area and the pore size of pure g-C3N4 and PDA/g-C3N4, N2 physical adsorption tests were carried out, and the results are depicted in Figure S4. The surface-area data 7843


Research Article


PDA content, suggesting that the PDA coating on the surface of g-C3N4 makes the pore size relatively smaller. Surface stoichiometry information of the elemental and chemical environment of pure g-C3N4 and PDA/g-C3N4 samples was obtained by X-ray photoelectron spectroscopy (XPS) analysis, and these element compositions are presented in Table S2. From Figure 6a, the XPS survey spectra revealed that pure g-C3N4 and PDA/g-C3N4 contain the same C, N, and O elements. Compared with pure g-C3N4, PDA/g-C3N4 has a higher C and O percentage which is attributed to the PDA with an abundance of oxygen-containing functional groups and less N content. In Figure 6b, the XPS N 1s spectrum of g-C3N4 was fitted by a Gaussian curve into four component peaks, corresponding with the binding energies 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 PDA/g-C3N4 has the same binding energies (BEs) in each corresponding peak, but the peak intensity is relatively lower, which is attributed to the PDA adlayer and surface modification of g-C3N4. Additionally, the C 1s spectrum is shown in Figure 6c, and the pure g-C3N4 with mainly three 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, respectively.7,13 In addition, the NC(N)2 peak is the dominant peak which is due to the g-C3N4 with an aromatic nitrogen heterocycle structure. However, a new peak of CO

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

reveal that the PDA-modified g-C3N4 is not significantly different from the surface area of pure g-C3N4. In addition, the pore size of PDA/g-C3N4 slightly decreases with increasing

Figure 6. (a) XPS survey spectra and (b−d) N 1s, C 1s, and O 1s spectra for g-C3N4 and the PDA/g-C3N4 composite (10%PDA/g-C3N4 sample). 7844


Research Article


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

Figure 8. (a) Photoluminescence (PL) spectra of pure g-C3N4 and PDA/g-C3N4 samples under 330 nm laser illumination at room temperature. (b) Transient photocurrent response of pure g-C3N4 and 10%PDA/g-C3N4 samples (three-electrode systems in 0.5 mol L−1 Na2SO4 aqueous solution under visible-light irradiation).

the semiconductor was calculated by the Kubelka−Munk transformation, and the plots are depicted in Figure 7b. With the increasing PDA content, the estimated band gaps gradually reduce, and the band gaps of pure g-C3N4 and 20%PDA/gC3N4 are about 2.68 and 2.05 eV, respectively. The decrease of the band gap has further supported the observation of the PDA/g-C3N4 composite with a red shift at the absorption band edge when compared to pure g-C3N4. The Kubelka−Munk function is as follows: αhν = A(hν − Eg)n/2, where α, ν, Eg, and A represent the absorption coefficient, light frequency, bandgap energy, and a constant, respectively. The n value depends on the characteristics of the transition in a semiconductor, and the n value of g-C3N4 is 4 for the indirect transition.14,45,46 It is suggested that the PDA coating modification on the surface of semiconductor materials can facilitate light absorption and fast charge separation of proton-coupled electrons, and can then improve the catalytic performance of photocatalysts. In addition, the generation and recombination of charge carriers, and the transfer of electrons, are a vital factor in photocatalytic activity.47 Thus, the photoluminescence (PL) spectra were recorded for a study of the optical property of samples, and the experimental results are shown in Figure 8a. The PL spectrum of pure g-C3N4 displays a strong PL emission at 440 nm, due to the n−π* electronic transitions in g-C3N4,40 related to the highly radiative recombination of photoexcited electrons and holes. Obviously, the PDA-modified g-C3N4 has a lower emission intensity compared to pure g-C3N4, and with the increasing PDA amount, the emission intensity gradually

emerged in the PDA/g-C3N4 composite which arises from PDA on the resultant samples, and the CN peaks became stronger. This is because the PDA with an aromatic nucleus structure contains hydroxyl and amidogen. Additionally in Figure 6d, for the O 1s spectrum of PDA/g-C3N4, the peaks at 531.2 and 532.75 eV correspond to OCO and CO, respectively. In pure g-C3N4, this was accredited to the adsorption of H2O or CO2 , which is a general phenomenon shown in the literature.37,44 All of this indicates that the dopamine had been successfully introduced into g-C3N4. For an investigation into the effect of PDA content on the surface of PDA, the PDA/g-C3N4 samples with different PDA contents were tested by TGA (Figure S5). The optical absorbance property of pure g-C3N4 and the PDA/g-C3N4 samples with different PDA content was analyzed using UV−vis diffuse reflectance spectroscopy (DRS), and the result is shown in Figure 7. Figure 7a shows that pure g-C3N4 has an absorption edge in the visible-light region with a wavelength of about 460 nm, corresponding to a band gap of 2.68 eV. However, PDA modification has an important effect on the optical property of g-C3N4. With increasing PDA amounts in PDA/g-C3N4 composites, the absorption spectra exhibit a slight red shift and an enhanced absorption intensity of light in the visible-light region, suggesting that the PDA/gC3N4 composites have a good ability to harvest more visible light than pure g-C3N4. In addition, the 20%PDA/g-C3N4 sample exhibits an almost equally strong absorption intensity from the UV to the visible-light region. The band-gap energy of 7845


Research Article


Figure 9. (a) Photocatalytic degradation of MB for pure g-C3N4 and PDA/g-C3N4 samples under visible-light irradiation. (b) The UV−vis spectrum changes of MB with irradiation time for 10%PDA/g-C3N4 samples. (c) Kinetic linear simulation curves of MB photocatalytic degradation. (d) Photocatalytic degradation of MB for pure g-C3N4 and PDA/g-C3N4 samples with the addition of H2O2 (1 wt %).

transport ability of photogenerated charge carriers. It is acknowledged that the main reason for photocurrent generation is that the photoinduced electrons diffuse to the back contact, and then the photogenerated holes were accepted by the hole acceptors.48,49 This result was consistent with that of the PL measurement. Furthermore, the photogenerated charge separation process was also investigated by electrochemical impedance spectroscopy (EIS) methods (Figure S6). The semicircular Nyquist plots for pure g-C3N4 and the 10% PDA/g-C3N4 composite show that the arc radius of the 10% PDA/g-C3N4 composite is smaller than that of pure g-C3N4. This suggests that the 10%PDA/g-C3N4 composite has a relatively low charge carrier transfer resistance which can improve the separation and transfer efficiency of electron−hole pairs. Photocatalytic Evaluation. The photocatalytic activities of pure g-C3N4 and the PDA/g-C3N4 composite were tested by the degradation of organic dye (MB) under visible-light irradiation (λ > 420 nm). To reach the adsorption equilibrium, the mixed solution of MB and photocatalysts was magnetically stirred for 30 min under dark conditions. During the photodegradation process, the temporal Ct/C0 changes of MB were measured and are shown in Figure 9a; the different profiles show the effects of pure g-C3N4, PDA/g-C3N4 with different PDA contents, and the absence of a catalyst on the photocatalytic degradation efficiency curves of MB under identical experimental conditions. As shown, about 5% degradation of MB was observed without a photocatalyst

weakens which shows a lower photogenerated carrier recombination rate. That is, the g-C3N4 photogenerated electrons can be effectively transferred to PDA with less charge recombination, which forms more activated OH • for degradation of the MB. This result can be attributed to PDA with semiquinone and quinone functional ligands which act as an electron acceptor. Hence, both the enhanced light absorption and the lowered radiative electron−hole recombination will endow the PDA/g-C3N4 composite with high photocatalytic activity. The photoresponses of g-C3N4 and 10%PDA/g-C3N4 were tested by photocurrent experiments that applied an electric field, and the excited photoelectrons and holes moved in opposite directions, resulting in their improved separation rate and reduced possibility for recombination. The photocurrent− time (I−t) curves of samples are shown in Figure 8b with 60 s intermittent on−off cycles under visible-light irradiation. The photocurrent value almost equaled zero under dark conditions, and as with visible-light irradiation, the photocurrent sharply increased to a certain value. However, the photocurrent value sharply declined to zero when the illumination light was turned off, and then it quickly returned to a steady constant value with the illumination once again; the process was very renewable. The 10%PDA/g-C3N4 composite demonstrated photocurrent responses that were about 2−3 times higher than those of pure g-C3N4 under visible-light irradiation. This result indicated that the PDA surface-modified g-C3N4 composite could prolong the lifetime of photogenerated electron−hole pairs and improve its 7846


Research Article


MB degradation time of 10%PDA/g-C3N4 and 20%PDA/gC3N4 samples is about 120 min for the degradation efficiency to reach up to 99%. In addition, the reference sample without catalysts shows a significant degradation for MB of about 30%, which is because the H2O2 will generate the hydroxyl radical (OH•) which can directly degrade the MB. The improved photocatalytic results are due to the fact that the H2O2 can more quickly react with photoinduced holes (h+) to produce more hydroxyl radicals (OH•) and avoid electron−hole recombination. In addition, RhB and phenol were also chosen for an investigation into the photocatalytic activity of PDA/gC3N4 (Figure S7), which also shows that PDA/g-C3N4 has good photocatalytic activity compared to pure g-C3N4. These experimental results reveal that 10%PDA/g-C3N4 and 20% PDA/g-C3N4 show little difference in photocatalytic degradation, and thus the 10%PDA/g-C3N4 will be considered for further investigation in following work. The stability of the PDA/g-C3N4 catalyst was investigated via the recycle experiment for the degradation of MB with 2 mL of H2O2 under visible-light irradiation. For each recycle, the collected PDA/g-C3N4 was washed by deionized (DI) water 3 times and then dried at 60 °C for 12 h. As shown in Figure 10,

after 180 min of visible-light irradiation. As for pure g-C3N4, it displayed a lower degradation activity, and the photocatalytic efficiency for MB is about 28.56% after 180 min of photodegradation. Compared to pure g-C3N4, the PDAmodified g-C3N4 exhibited an increase in photocatalytic efficiency. With increasing PDA content, the photocatalytic activity of PDA/g-C3N4 photocatalysts was obviously enhanced, as the degradation efficiencies are about 34.54%, 45.20%, 61.06%, 96.57%, and 98.84%. The 10%PDA/g-C3N4 and 20% PDA/g-C3N4 samples exhibited a highly efficient photocatalytic performance, which was about 4 times higher than that of pure g-C3N4. In addition, upon comparison of the 5%PDA/g-C3N4 and 10%PDA/g-C3N4 samples, the 10%PDA/g-C3N4 composite has an obvious improvement in photocatalytic degradation efficiency. However, for the 10%PDA/g-C3N4 and 20%PDA/gC3N4 composites, there are no noticeable improvements in photocatalyst efficiency with increasing amounts of PDA. This phenomenon is attributed to the fact that PDA can accelerate the photoinduced electron rate through electron and proton redox-coupling methods, but superfluous PDA could not only hinder the absorption and utilization of stimulus light by gC3N4 but also harvest and shield the transferred photoelectron. All of these results reveal that PDA plays a vital role in the improvement of photocatalytic efficiency and separation of electron−hole pairs. The absorption spectrum change of MB solution for the 10% PDA/g-C3N4 photocatalysts is displayed in Figure 9b at different times. Figure 9b reveals the same strong maximum absorption peaks at 664 nm for each time. It is seen that the absorption curves gradually weaken as the exposure time increases, and the absorption peak at 664 nm nearly disappears after a certain time interval, indicating that the MB was degraded step-by-step by the PDA/g-C3N4 composite during the photocatalytic reaction processes under visible-light irradiation. The experimental data for MB degradation were fitted for a study of reaction kinetics by the first-order kinetics equation. Figure 9c shows that the fitted curves for MB photocatalytic degradation agree with first-order reaction dynamics. The firstorder equation is as follows:

⎛C ⎞ ln⎜ t ⎟ = −kt ⎝ C0 ⎠

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

the degradation efficiency of MB remains higher than 90% after a four-cycle decomposition process, revealing that the PDA/gC3N4 has excellent stability throughout the photocatalytic degradation process. The XRD and TGA of the 10%PDA/gC3N4 sample before and after the 4 photocatalytic reaction cycles are displayed in Figure S8. It was found that the diffraction peak of the recycled 10%PDA/g-C3N4 composite after 4 photocatalytic reaction cycles had no obvious discrepancy upon comparison to the fresh one (Figure S8a). However, an increase in diffraction peak intensity can be found through careful observation, indicating the loss of PDA from the g-C3N4 surface which is consistent with TGA results (Figure S8b). This is a possible reason for the decrease of photocatalytic efficiency of photocatalysts after each recycle. Photocatalytic Mechanism. There are a lot of possible reaction pathways for the intricate photocatalytic degradation process for the eventual photodegradation of the pollutants. However, the common reaction process can be summarized by two steps for the degradation of the organic contaminants. First, the organic pollutant was absorbed onto the surface of the photocatalyst. Second, the organic pollutant was degraded by a

(2)

where Ct and C0 are the dye concentrations in solution at each given time, and t = 0. In addition, the rate constant k (min−1) is the slope of the corresponding fitting curve, and t represents the reaction time. Figure 9c displays the effect of PDA content on the MB photodegradation rate for pure g-C3N4 and PDA/gC3N4. The irradiation time (t) of the fitted curves against −ln(Ct/C0) is a nearly straight line for the samples with varying PDA content. The reaction rate constant (k) values of pure gC3N4 and PDA/g-C3N4 with different PDA contents are about 0.0015, 0.0020, 0.0027, 0.0040, 0.0191, and 0.0234 min−1, separately. The reaction rate values of 10%PDA/g-C3N4 and 20%PDA/g-C3N4 are about 12.6 and 15.5 times higher, respectively, than that of pure g-C3N4 under the same experimental conditions. For an improvement in 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. Upon comparison of these results with those of the system without H2O2, although the amount of catalyst is only half, the 7847


Research Article

ACS Sustainable Chemistry & Engineering reactive species such as hydroxyl radicals (OH•), holes (h+), and superoxide radicals (O2•−). For an investigation of the main active species with an effect on PDA/g-C3N4 photocatalysts, different scavengers were introduced into the photocatalytic degradation reaction of MB, which perform the relative roles of the reactive species and allow evaluation of the photocatalytic mechanism of PDA/g-C3N4. These scavenger additives p-benzoquinone (BQ, 10 mM), disodium ethylenediaminetetraacetate (EDTA−2Na, 10 mM), and tertiary butanol (t-BuOH, 10 mM) were introduced into the photocatalytic reaction system for trapping the specific reactive species superoxide radicals (O2•−), holes (h+), and hydroxyl radicals (OH•), respectively. Figure 11 shows that the

via electrostatic interaction and hydrogen bonding. Moreover, the giant π conjugation between the MB molecule (benzene ring and the pyridine ring) and g-C3N4 (hexatomic ring of C− N) can form π−π stacking, and thus these organic dye molecules would be extracted from solution and then concentrated on the photocatalyst surface. The high photocatalytic activity can be attributed to the PDA’s good light-harvesting ability and fast transfer and separation ability of photogenerated electrons at the PDAmodified g-C3N4 interface. Under visible-light irradiation, the gC3N4 can induce a π−π* transition, and the excited-state electrons were transported from the VB (valence band) to the CB (conduction band), that is, from the VB formed by N 2p to the CB formed by C 2p orbitals of g-C3N4.14,37 Then, electrons (e−’s) accumulate in the CB, leaving holes (h+) in the VB: semiconductor (SC) + hν → h+(VB) + e−(CB).4 For the case of PDA/g-C3N4, the CB position value for g-C3N4 is about −1.42 eV versus NHE, which is beneficial for charge transfer from g-C3N4 to PDA (−0.08 eV versus NHE).31 In addition, the PDA on the g-C3N4 surface assembles a redox shuttle for the transfer of electrons and protons from donors to acceptors, via its abundance of catechol groups.31 Resembling a natural photosystem, quinone molecules functioning as a two-electron gate will directly improve the electron transfer efficiency from chlorophyll by a factor of 2.50 Then, the water molecule (H2O) or hydroxide ion (OH−) was captured by the valence band holes for production of the nonselective, extremely powerful, and oxidizing hydroxyl radical (OH•). The enriched electrons on the PDA would be trapped by the free molecular oxygen (O2) for a yield of superoxide radicals O2•−’s. Both the O2•− and OH• reactive radical can directly oxidize the molecular MB dye. Subsequently, introducing the H2O2 significantly enhances the degradation rate for photocatalytic action, as the Fentonlike excitation of H2O2 forms more hydroxyl radicals (OH•’s) compared to H2O molecules.51

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

degradation efficiency significantly decreases for the photocatalytic degradation of MB with the addition of t-BuOH and EDTA for 3 h of visible-light illumination, whereas BQ has a slight influence. This demonstrates that the major active species are h+ and OH• in the photocatalytic degradation process. For the PDA/g-C3N4 photocatalyst, the possible photocatalytic mechanism under visible-light irradiation is tentatively presented in Figure 12. The dopamine-modified g-C3N4 contains abundant functional groups such as amine groups, catechol groups, and aromatic rings,34 which can effectively absorb the cation dye MB. In addition, some of the active sites possessed by polydopamine can also absorb organic pollutants

■

CONCLUSIONS In summary, we have demonstrated a novel simple strategy to modify the g-C3N4 surface using polydopamine which not only improves light-harvesting but also acts as an electron acceptor for an enhancement of photocatalytic activity under visible-light irradiation, because PDA has multiple functional groups, possesses strong interfacial adhesion, and has an accelerated electron transfer ability. The influence of PDA/g-C 3N 4 composites with different PDA ratios on photocatalytic activity has been investigated in detail by the photocatalytic degradation of MB under visible-light irradiation. The results revealed that PDA/g-C3N4 has excellent photocatalytic activity for MB degradation. The degradation efficiencies of 10%PDA/ g-C3N4 and 20%PDA/g-C3N4 samples for MB are about 97% and 99% in 180 min, and the degradation rates 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 shows that PDA/g-C3N4 has good photocatalytic activity compared to pure g-C3N4. The improved photocatalytic activity of the PDA/g-C3N4 composite is attributed to PDA which can improve visible-light absorption and contain numerous catechol groups as electron acceptors for the transfer of photogenerated electrons, thus suppressing the recombination rate of electrons and holes. Therefore, surface-modified g-C3N4 composites provide new perspectives for organic degradation under visiblelight irradiation and have promising applications in water purity.

Figure 12. Schematic illustration of electron transfer and possible photocatalytic degradation mechanism scheme of PDA/g-C3N4 under visible-light irradiation. 7848


Research Article


■

(13) Gholipour, M. R.; Béland, F.; Do, T.-O. Post-Calcined Carbon Nitride Nanosheets as an Efficient Photocatalyst for Hydrogen Production under Visible Light Irradiation. ACS Sustainable Chem. Eng. 2017, 5 (1), 213−220. (14) Yang, X.; Qian, F.; Zou, G.; Li, M.; Lu, J.; Li, Y.; Bao, M. Facile fabrication of acidified g-C3N4/g-C3N4 hybrids with enhanced photocatalysis performance under visible light irradiation. Appl. Catal., B 2016, 193, 22−35. (15) Wang, X.; Hong, M.; Zhang, F.; Zhuang, Z.; Yu, Y. Recyclable Nanoscale Zero Valent Iron Doped g-C3N4/MoS2for Efficient Photocatalysis of RhB and Cr(VI) Driven by Visible Light. ACS Sustainable Chem. Eng. 2016, 4 (7), 4055−4063. (16) Ye, L.; Wu, D.; Chu, K. H.; Wang, B.; Xie, H.; Yip, H. Y.; Wong, P. K. Phosphorylation of g-C3N4 for enhanced photocatalytic CO2 reduction. Chem. Eng. J. 2016, 304, 376−383. (17) Qin, J.; Wang, S.; Ren, H.; Hou, Y.; Wang, X. Photocatalytic reduction of CO 2 by graphitic carbon nitride polymers derived from urea and barbituric acid. Appl. Catal., B 2015, 179, 1−8. (18) Lang, X.; Chen, X.; Zhao, J. Heterogeneous visible light photocatalysis for selective organic transformations. Chem. Soc. Rev. 2014, 43 (1), 473. (19) Liang, Q.; Li, Z.; Huang, Z. H.; Kang, F.; Yang, Q. H. Holey Graphitic Carbon Nitride Nanosheets with Carbon Vacancies for Highly Improved Photocatalytic Hydrogen Production. Adv. Funct. Mater. 2015, 25 (44), 6885−6892. (20) Zhu, J.; Wei, Y.; Chen, W.; Zhao, Z.; Thomas, A. Graphitic carbon nitride as a metal-free catalyst for NO decomposition. Chem. Commun. 2010, 46 (37), 6965−7. (21) Zhang, G.; Zhang, J.; Zhang, M.; Wang, X. Polycondensation of thiourea into carbon nitride semiconductors as visible light photocatalysts. J. Mater. Chem. 2012, 22 (16), 8083−8091. (22) Cui, Y.; Zhang, G.; Lin, Z.; Wang, X. Condensed and Lowdefected Graphitic Carbon Nitride with Enhanced Photocatalytic Hydrogen Evolution under Visible Light Irradiation. Appl. Catal., B 2016, 181, 413. (23) Lin, J.; Pan, Z.; Wang, X. Photochemical Reduction of CO2 by Graphitic Carbon Nitride Polymers. ACS Sustainable Chem. Eng. 2014, 2 (3), 353−358. (24) He, F.; Chen, G.; Yu, Y.; Zhou, Y.; Zheng, Y.; Hao, S. The synthesis of condensed C-PDA-g-C3N4 composites with superior photocatalytic performance. Chem. Commun. (Cambridge, U. K.) 2015, 51 (31), 6824−7. (25) Li, Z.; Kong, C.; Lu, G. Visible Photocatalytic Water Splitting and Photocatalytic Two-Electron Oxygen Formation over Cu- and FeDoped g-C3N4. J. Phys. Chem. C 2016, 120 (1), 56. (26) Hong, J.; Xia, X.; Wang, Y.; Xu, R. Mesoporous carbon nitride with in situ sulfur doping for enhanced photocatalytic hydrogen evolution from water under visible light. J. Mater. Chem. 2012, 22 (30), 15006−15012. (27) Xing, W.; Li, C.; Wang, Y.; Han, Z.; Hu, Y.; Chen, D.; Meng, Q.; Chen, G. A novel 2D/2D carbonized poly-(furfural alcohol)/g-C 3 N 4 nanocomposites with enhanced charge carrier separation for photocatalytic H 2 evolution. Carbon 2017, 115, 486−492. (28) Xing, W.; Li, C.; Chen, G.; Han, Z.; Zhou, Y.; Hu, Y.; Meng, Q. Incorporating a novel metal-free interlayer into g-C3N4 framework for efficiency enhanced photocatalytic H2 evolution activity. Appl. Catal., B 2017, 203, 65−71. (29) Fontelles-Carceller, O.; Muñoz-Batista, M. J.; Fernández-García, M.; Kubacka, A. Interface Effects in Sunlight-Driven Ag/g-C3N4 Composite Catalysts: Study of the Toluene Photodegradation Quantum Efficiency. ACS Appl. Mater. Interfaces 2016, 8 (4), 2617. (30) Mao, W. X.; Lin, X. J.; Zhang, W.; Chi, Z. X.; Lyu, R. W.; Cao, A. M.; Wan, L. J. Core-shell structured TiO2@polydopamine for highly active visible-light photocatalysis. Chem. Commun. 2016, 52 (44), 7122. (31) Kim, J. H.; Lee, M.; Park, C. B. Polydopamine as a biomimetic electron gate for artificial photosynthesis. Angew. Chem., Int. Ed. 2014, 53 (25), 6364−8.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01313. Matrix compositions, photographs of materials, FTIR spectra, EDS spectra, adsorption−desorption isotherms, elemental compositions, TGA curves, degradation curves, kinetics curves, Nyquist plots, and XRD patterns (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 86-15208397092. ORCID

Zongxue Yu: 0000-0002-5513-7421 Notes

The authors declare no competing financial interest.

■

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

■

REFERENCES

(1) Chen, D.; Zhu, H.; Yang, S.; Li, N.; Xu, Q.; Li, H.; He, J.; Lu, J. Micro-Nanocomposites in Environmental Management. Adv. Mater. 2016, 28 (47), 10443−10458. (2) Shannon, M. A.; Bohn, P. W.; Elimelech, M.; Georgiadis, J. G.; Mariñas, B. J.; Mayes, A. M. Science and technology for water purification in the coming decades. Nature 2008, 452 (452), 301−310. (3) Yang, J.; Chen, D.; Zhu, Y.; Zhang, Y.; Zhu, Y. 3D-3D Porous Bi 2 WO 6 /Graphene Hydrogel Composite with Excellent Synergistic Effect of Adsorption-Enrichment and Photocatalytic Degradation. Appl. Catal., B 2017, 205, 228. (4) Mamba, G.; Mishra, A. K. Graphitic carbon nitride (g-C 3 N 4) nanocomposites: A new and exciting generation of visible light driven photocatalysts for environmental pollution remediation. Appl. Catal., B 2016, 198, 347−377. (5) Zhao, Z.; Sun, Y.; Dong, F. Graphitic carbon nitride based nanocomposites: a review. Nanoscale 2015, 7 (1), 15−37. (6) Cao, S.; Low, J.; Yu, J.; Jaroniec, M. Polymeric photocatalysts based on graphitic carbon nitride. Adv. Mater. 2015, 27 (13), 2150− 76. (7) Tong, Z.; Yang, D.; Shi, J.; Nan, Y.; Sun, Y.; Jiang, Z. ThreeDimensional Porous Aerogel Constructed by g-C3N4 and Graphene Oxide Nanosheets with Excellent Visible-Light Photocatalytic Performance. ACS Appl. Mater. Interfaces 2015, 7 (46), 25693−701. (8) Cao, S.; Yu, J. Graphene-Based Photocatalysts for Hydrogen Generation. J. Phys. Chem. Lett. 2014, 5 (12), 2101−2107. (9) Ong, W. J.; Tan, L. L.; Ng, Y. H.; Yong, S. T.; Chai, S. P. Graphitic Carbon Nitride (g-C3N4)-Based Photocatalysts for Artificial Photosynthesis and Environmental Remediation: Are We a Step Closer To Achieving Sustainability? Chem. Rev. 2016, 116 (12), 7159− 329. (10) Cao, S.; Low, J.; Yu, J.; Jaroniec, M. Polymeric photocatalysts based on graphitic carbon nitride. Adv. Mater. 2015, 27 (13), 2150− 76. (11) Fujishima, A.; Rao, T. N.; Tryk, D. A. Titanium dioxide photocatalysis. J. Photochem. Photobiol., C 2000, 1 (1), 1−21. (12) Pol, R.; Guerrero, M.; García-Lecina, E.; Altube, A.; Rossinyol, E.; Garroni, S.; Baró, M. D.; Pons, J.; Sort, J.; Pellicer, E. Ni-, Pt- and (Ni/Pt)-doped TiO 2 nanophotocatalysts: A smart approach for sustainable degradation of Rhodamine B dye. Appl. Catal., B 2016, 181, 270−278. 7849


Research Article

ACS Sustainable Chemistry & Engineering (32) Wang, S.; Wang, X. Multifunctional Metal-Organic Frameworks for Photocatalysis. Small 2015, 11 (26), 3097−112. (33) Xie, A.; Zhang, K.; Wu, F.; Wang, N.; Wang, Y.; Wang, M. Polydopamine nanofilms as visible light-harvesting interfaces for palladium nanocrystal catalyzed coupling reactions. Catal. Sci. Technol. 2016, 6 (6), 1764−1771. (34) Liu, Y.; Ai, K.; Lu, L. Polydopamine and Its Derivative Materials: Synthesis and Promising Applications in Energy, Environmental, and Biomedical Fields. Chem. Rev. 2014, 114 (9), 5057−115. (35) Xu, L. Q.; Yang, W. J.; Neoh, K. G.; Kang, E. T.; Fu, G. D. Dopamine-Induced Reduction and Functionalization of Graphene Oxide Nanosheets. Macromolecules 2010, 43 (20), 8336−8339. (36) Yu, B.; Liu, J.; Liu, S.; Zhou, F. Pdop layer exhibiting zwitterionicity: a simple electrochemical interface for governing ion permeability. Chem. Commun. 2010, 46 (32), 5900−5902. (37) Ong, W.-J.; Tan, L.-L.; Chai, S.-P.; Yong, S.-T.; Mohamed, A. R. Surface charge modification via protonation of graphitic carbon nitride (gC 3 N 4) for electrostatic self-assembly construction of 2D/2D reduced graphene oxide (rGO)/gC 3 N 4 nanostructures toward enhanced photocatalytic reduction of carbon dioxide to methane. Nano Energy 2015, 13, 757−770. (38) Ma, T. Y.; Tang, Y.; Dai, S.; Qiao, S. Z. Proton-functionalized two-dimensional graphitic carbon nitride nanosheet: an excellent metal-/label-free biosensing platform. Small 2014, 10 (12), 2382−9. (39) Du, X.; Zou, G.; Wang, Z.; Wang, X. A scalable chemical route to soluble acidified graphitic carbon nitride: an ideal precursor for isolated ultrathin gC 3 N 4 nanosheets. Nanoscale 2015, 7 (19), 8701− 8706. (40) Li, Y.; Zhang, H.; Liu, P.; Wang, D.; Li, Y.; Zhao, H. Crosslinked g-C3 N4 /rGO nanocomposites with tunable band structure and enhanced visible light photocatalytic activity. Small 2013, 9 (19), 3336−44. (41) Li, H. J.; Qian, D. J.; Chen, M. Templateless Infrared Heating Process for Fabricating Carbon Nitride Nanorods with Efficient Photocatalytic H2 Evolution. ACS Appl. Mater. Interfaces 2015, 7 (45), 25162−70. (42) Dai, X.; Li, Z.; Ma, Y.; Liu, M.; Du, K.; Su, H.; Zhuo, H.; Yu, L.; Sun, H.; Zhang, X. Metallic Cobalt Encapsulated in Bamboo-Like and Nitrogen-Rich Carbonitride Nanotubes for Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2016, 8 (10), 6439−48. (43) Fu, J.; Zhu, B.; Jiang, C.; Cheng, B.; You, W.; Yu, J. Hierarchical Porous O-Doped g-C3 N4 with Enhanced Photocatalytic CO2 Reduction Activity. Small 2017, 13 (15), 1603938. (44) Teng, Z.; Lv, H.; Wang, C.; Xue, H.; Pang, H.; Wang, G. Bandgap engineering of ultrathin graphene-like carbon nitride nanosheets with controllable oxygenous functionalization. Carbon 2017, 113, 63−75. (45) Wang, J.; Tang, L.; Zeng, G.; Liu, Y.; Zhou, Y.; Deng, Y.; Wang, J.; Peng, B. Plasmonic Bi Metal Deposition and g-C3N4 Coating on Bi2WO6Microspheres for Efficient Visible-Light Photocatalysis. ACS Sustainable Chem. Eng. 2017, 5 (1), 1062−1072. (46) Chen, S.; Hu, Y.; Meng, S.; Fu, X. Study on the separation mechanisms of photogenerated electrons and holes for composite photocatalysts g-C 3 N 4 -WO 3. Appl. Catal., B 2014, 150−151 (9), 564−573. (47) Wang, S.; Lin, J.; Wang, X. Semiconductor-redox catalysis promoted by metal-organic frameworks for CO2 reduction. Phys. Chem. Chem. Phys. 2014, 16 (28), 14656. (48) Wang, S.; Wang, X. Photocatalytic CO 2 reduction by CdS promoted with a zeolitic imidazolate framework. Appl. Catal., B 2015, 162, 494−500. (49) Soedergren, S.; Hagfeldt, A.; Olsson, J.; Lindquist, S. E. Theoretical Models for the Action Spectrum and the Current-Voltage Characteristics of Microporous Semiconductor Films in Photoelectrochemical Cells. J. Phys. Chem. 1994, 98 (21), 5552−5556. (50) Blankenship, R. E. Electron Transfer Pathways and Components; Blackwell Science Ltd.: 2008; pp 124−156. (51) Li, X.; Pi, Y.; Wu, L.; Xia, Q.; Wu, J.; Li, Z.; Xiao, J. , Facilitation of the visible light-induced Fenton-like excitation of H 2 O 2 via

heterojunction of g-C 3 N 4 /NH 2 -Iron terephthalate metal-organic framework for MB degradation. Appl. Catal., B 2017, 202, 653−663.

7850


Graphitic Carbon

Recommend Documents