Sulfur-Doped Mesoporous Carbon Nitride Decorated with Cu Particles

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Sulfur-Doped Mesoporous Carbon Nitride Decorated with Cu Particles for Efficient Photocatalytic Degradation under Visible-Light Irradiation Milad Jourshabani, Zahra Shariatinia, and Alireza Badiei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b05556 • Publication Date (Web): 21 Aug 2017 Downloaded from http://pubs.acs.org on August 21, 2017

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The Journal of Physical Chemistry

Sulfur-Doped Mesoporous Carbon Nitride Decorated with Cu Particles for Efficient Photocatalytic Degradation under Visible-Light Irradiation

Milad Jourshabani,† Zahra Shariatinia,†,* and Alireza Badiei‡,§

†

Department of Chemistry, Amirkabir University of Technology (Tehran Polytechnic),

P.O.Box:15875-4413, Tehran, Iran ‡

School of Chemistry, College of Science, University of Tehran, Tehran, Iran

§

Nanobiomedicine Center of Excellence, Nanoscience and Nanotechnology Research Center,

University of Tehran, Tehran, Iran

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ABSTRACT: Three simultaneous strategies were successfully combined, for the first time, in one material including sulfur doping, creating mesoporous structure, and deposition of Cu element as a low-cost non-noble metal to modify the internal and external surfaces of carbon nitride framework. For this purpose, sulfur-doped mesoporous graphitic carbon nitride (MCNS) materials were synthesized from thiourea as a low cost precursor and SiO2 gel solution as a template through a thermal condensation method. A series of copper deposited MCNS (Cu/MCNS) with Cu nominal loadings from 4 to 9 wt.% were fabricated by a facile precipitation-reduction method and the resulting samples were utilized in the photocatalytic degradation of methyl orange (MO) as a hazardous dye in aqueous solution under visiblelight

irradiation.

Phase

structures,

morphologies,

textural,

optical

as

well

as

photoelectrochemical properties of Cu/MCNS photocatalysts were well characterized. Results indicated that synergistic effects of the sulfur doping, mesostructure and especially the presence of Cu particles on the surface of MCNS extended the visible-light harvesting ability and facilitated the separation of photogenerated electron-hole pairs, and therefore the photocatalytic activity was dramaticaly enhanced. The optimal Cu nominal loading value was determined to be 7 wt.%, and the corresponding degradation rate of MO was about 100% during 90 min under the optimized operating variables conducted by the response surface methodology. In addition, the photocatalyst showed high reusability and stability without any significant decrease in its performace. Based on the trapping experiments results, the h+ and ⦁O2¯ radicals as the predominant active species were responsible for the oxidation of MO over the Cu/MCNS catalyst under visible-light irradiation.

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1. INTRODUCTION Various types of organic pollutants which create potential risk to human health are unintendedly released into the environment by many industries and need to be removed from wastewaters.1,2 Heterogeneous photocatalysis processes based on visible-light irradiation using semiconductors have received considerable attention, mainly owing to their promising applications for the photocatalytic degradation of several environmental pollutants in reactions mediated by solar irradiation.3 Although TiO2 is the most famous photocatalyst among other semiconductors, it can absorb only 3–5% of sunlight in the UV region which greatly limits its applications under the solar light.4 To utilize solar energy, well-organized photocatalysts should possess well architecture, efficient separation ability of electron-hole pairs, response to visible-light irradiation, and have suitable band gap. Recently, graphitic carbon nitride (g-C3N4) polymers which are analogues of graphite with high in-plane nitrogen number in heptazine units, was described as a new generation of photocatalysts.5,6 These materials have attracted considerable attention for the environmental remediation purposes because they are metal-free, eco-friendly, cost-effective, easily available as well as they have high thermal and chemical stability properties which are due to their π-conjugated frameworks.7,8 In addition, these materials have the energy band edges (ca. 2.7 eV) which can be activated by the visible-light irradiation with wavelengths up to 450 nm.9 However, the photocatalytic activity of the bulk g-C3N4 still suffers from the fast recombination of its photogenerated charge carriers and small surface area (420 nm.26 It is well known that photocatalysts with mesoporous structures and high surface areas not only can improve the visible-light harvesting capability via scattering light in their mesostructures, but also provide more surface sites for the diffusion of charge carriers and therefore their recombination is suppressed.27 Chen et al. reported the synthesis of ordered mesoporous graphitic carbon nitride (ompg-C3N4) using SBA-15 as a template and cyanamide as a precursor.10 They found that the ompg-C3N4 displayed much higher H2 evolution activity compared to bulk g-C3N4 under visible-light irradiation. In another modification of g-C3N4 materials, noble metals such as Au and Ag were conventionally dispersed on the surface of g-C3N4 to prolong the lifetime of electron-hole pairs and also to act as the electron sinks where oxygen was reduced to superoxide radicals. Cheng et al. synthesized Au nanoparticles loaded on the graphitic carbon nitride nanosheets (AuNP/gC3N4) and found that visible-light decomposition of methyl orange was greatly enhanced by AuNP/g-C3N4 compared to pure bulk g-C3N4.28 Yang et al. exhibited that the silver deposited g-C3N4 (Ag/g-C3N4) had better photocatalytic performance than pure g-C3N4 for the degradation of methyl orange and p-nitrophenol under visible-light irradiation.29 However, the above studies mainly focused on the photocatalytic behaviors of noble metals which are both expensive and rare. Consequently, it is important from an economic viewpoint, to seek and design photocatalysts to be low cost, stable, as well as strongly response to the visiblelight irradiation. For example, Yu et al. successfully deposited highly active and non-noble metal Cu(OH)2 cluster as a co-catalyst on TiO2 nanotube arrays to develop the photocatalytic activity of TiO2 for the H2-production.30 Zhou et al. prepared Cu(OH)2/g-C3N4 composite and found that hydrogen generation was greatly improved by the Cu(OH)2 clusters compared to 4 ACS Paragon Plus Environment

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the pure g-C3N4.31 In another study, Cu/TiO2 nanoparticles obtained by chemical reduction method showed higher photocatalytic efficiency, for the removal of phenyldiazenylpyridine2,6-diamine as a pollutant, compared with the pure TiO2 nanoparticles.32 On the basis of the related literature and the above reported results, it is inferred that the photocatalytic activity of g-C3N4 would be further improved by combining two or three strategies in one material. Hong et al. prepared in situ sulfur-doped mesoporous g-C3N4 (mpgCNS) using thiourea as a starting material and SiO2 nanoparticles as a template.33 They indicated that the combined two strategies in one material can significantly improve the photocatalytic activity of mpgCNS than the bulk g-C3N4 for the hydrogen evolution process. In another study, Chang et al. combined two strategies in one material through doping Pd on the surface of mesoporous graphitic carbon nitride (Pd/mpg-C3N4) and found that Pd/mpgC3N4 was more efficient than the bulk g-C3N4 for the photodegradation of bisphenol A.34 Considering the above concepts, herein we present the combined three strategies in one material. The mesoporous sulfur-doped graphitic carbon nitride (MCNS) materials were first successfully synthesized using thiourea as a low cost precursor and SiO2 gel solution as a template through a simple thermal condensation method. A series of the Cu-deposited MCNS (Cu/MCNS) catalysts were synthesized (for the first time to the best of our knowledge) using Cu as an inexpensive non-noble metal which was grown as clusters on the surface of MCNS by a facile precipitation-reduction approach. The resultant materials were subjected to several characterizations and their visible-light photocatalytic activities were evaluated by degradation of methyl orange (MO) as a model organic pollutant. The effects of operating variables on the photocatalytic activity were optimized using the response surface methodology (RSM) and the reusability of Cu/MCNS was investigated through recycling experiments. In addition, the active species generated during the photocatalytic process were revealed by the trapping experiments. 5 ACS Paragon Plus Environment


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2. EXPERIMENTAL SECTION 2.1. Materials and Methods. All chemicals were of analytical grade purity and used without further purification. Distilled water was used for the preparation of all solutions. Copper(II) nitrate trihydrate (Cu(NO3)2.3H2O), ammonium hydrogen bifluoride (NH4HF2), dicyandiamide and 12-nm SiO2 nanoparticles (Ludox HS40 Aldrich, diameter: 12 nm, 40 wt% suspension in water) were supplied by Sigma-Aldrich. Thiourea (CH4N2S), urea (CH4N2O), sodium hydroxide (NaOH), ethanol and methyl orange (MO) as a model pollutant in textile industry, were purchased from Merck Co. (Germany). TiO2 nanoparticles (P25, Degussa) were purchased from Degussa Company (Germany).

2.2. Synthesis of MCNS, CNS, CN, and bulk g-C3N4. Sulfur-doped mesoporous carbon nitride samples (MCNS) were synthesized as described previously with minor modifications.33 10.0 g thiourea was dissolved in deionized water (100 mL) and a desired amount of SiO2 gel solution was added to the solution so that the weight ratio of SiO2/thiourea = 0.3. After stirring for 30 min, the mixture was heated at 90 °C with stirring in an open system to evaporate water. Then, the dried solid was grounded to fine powder and calcined in air within a crucible covered by the lid at the temperature of 500 °C for 4 h. The SiO2 template was removed from the calcined solid by treatment in NH4HF2 (4 M) solution for 48 h with continuous stirring. The powder was then centrifuged and washed three times with distilled water and twice with ethanol until the pH of the supernatant solution reached around 7.0. Finally, the resultant sample was dried at 90 °C under vacuum overnight. To elucidate the effect of sulfur doping and mesoporosity strategies on the photocatalytic activity of the optimized final product, 10 g of urea (CN) and 10 g of thiourea without template (CNS) were separately synthesized under the same reaction conditions. Furthermore, the bulk 6 ACS Paragon Plus Environment

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g-C3N4 sample was prepared by condensation of dicyandiamide using the method described in literature.35

2.3. Synthesis of Cu/MCNS samples. The Cu/MCNS samples were synthesized via a simple precipitation-reduction method, according to Eqs. (1) and (2).36 For this purpose, 0.4 g of MCNS material was added to 25 mL deionized water and dispersed for 30 min in an ultrasonic bath. The required amounts of Cu(NO3)2.3H2O (0.261, 0.33, 0.47 and 0.62 mmol Cu) and NaOH (0.052, 0.066, 0.094 and 0.124 M) were added to the MCNS suspension. Then, required amounts of NaBH4 solution were added drop-wise to the reaction mixture for 4 h at room temperature under stirring. The obtained materials were washed with deionized water three times and the washed precipitates were dried at 80 °C for 12 h. The products are denoted as Cu(x)/MCNS, where x represents the nominal weight ratio of Cu to (MCNS + Cu) which are 4, 5, 7 and 9 wt.%. Figure 1 exhibits a schematic illustration of the deposition process of Cu metal on the surface of MCNS. The actual metal content of the samples were measured by the inductively coupled plasma atomic absorption spectrometry (ICP-AAS). Cu2+ + 2OH– Cu(OH)2

(1)

4Cu(OH)2 + BH4– 4Cu + B(OH)4– + 4H2O

(2)

2.4. Characterization. The X-ray diffraction (XRD) patterns were recorded on a Philips X’Pert X-ray diffractometer (XRD) using Cu Kߙ radiation (λ = 0.15406 nm) as the radiation source in the 2θ of 5–80°. The Fourier transformed infrared (FT-IR) spectra were recorded on an Equinox 55 Bruker spectrometer in the 400–4000 cm−1 wavelength range. The Brunauer-Emmett-Teller (BET) specific surface area and pore size distribution of the samples were determined by nitrogen adsorption-desorption at 77 K with a QuantaChrome NOVA 2000 instrument. The Diffuse reflection spectra (DRS) were obtained on an AvaSpec-2048 TEC spectrophotometer. The optical band gap value was calculated using Kubelka-Munk 7 ACS Paragon Plus Environment


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theory through the following equation (Eq. 3), where α is the optical absorption coefficient, k represents a constant which is dependent on the transition probability, h is the Plank’s constant, and ν is the frequency of the radiation. The n value is dependent to the type of optical transition of a semiconductor; the value of n for g-C3N4 materials with indirect transition is 2. The Eg values were calculated by plotting (αhν)1/2 versus hν, followed by extrapolation of the linear part of the spectra to the energy axis.

α(hν) = k(hν − E g )1/ n

(3)

The surface morphology and the elemental mapping analysis of samples were made using a scanning electron microscope (SEM, XMU, VEGA-II) equipped with an energy dispersive X-ray spectrometer (EDX) system for analyzing the chemical composition of the samples. The transmission electron microscopy (TEM) images were obtained by a Zeiss, EM 900 apparatus operated at an accelerating voltage of 120 kV. The photoluminescence (PL) spectra of the samples were acquired at room temperature excited by an incident light of 320 nm using a Perkin Elmer LS 55 spectrometer. The chemical composition of the sample was analyzed by an X-ray photoelectron spectrometry (XPS) instrument equipped with an Al-K X-ray source operated at 1486.6 eV.

2.5. Photoelectrochemical measurements. Electrochemical measurements were carried out with an Autolab potentiostat/galvanostat apparatus in a conventional three electrodes cell, using a Pt wire as the counter electrode and Ag/AgCl electrode (saturated KCl) as the reference electrode. The working electrode was prepared by spreading the slurry of 4 mg Cu/MCNS powder in 1 mL ethanol over 0.25 cm2 fluorine doped tin oxide (FTO) glass substrate. After air drying, the film was dried at 80 °C for 60 min in furnace to improve adhesion. To evaluate photoresponse behavior, visible-light (λ>400 nm) was illuminated onto the electrode with several on-off cycles of intermittent visible-light irradiation under its 8 ACS Paragon Plus Environment

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open-circuit voltage. The electrochemical impedance spectra (EIS) were also performed at open-circuit voltage. The frequency range was from 0.05 to 105 Hz, and the corresponding AC amplitude was 5mV. The equivalent circuits of measured spectra were fitted using the Zview program. The 0.5 M Na2SO4 solution (at pH=6.6) was used in all of the electrochemical measurements. Equation 4 was employed for converting the obtained potential (vs. Ag/AgCl) to RHE (NHE at pH=0).

E RHE = E AgCl + 0.059 pH + E° AgCl (E° AgCl = 0.197)

(4)

2.6. Visible-light Photocatalytic Activity. The photocatalytic activity of the prepared materials was investigated by monitoring the degradation of MO dye under visible-light irradiation which was provided by 300 W Halogen lamp with UV-stop feature. Photocatalytic degradation experiments were carried out in a self-made cylindrical Pyrex-glass reactor with 500 mL capacity (8 cm inner diameter, 10 cm height), in which the body of the reactor was cooled by circulating water to maintain the reaction temperature at room temperature. Artificial irradiation (λ > 400 nm) was positioned on top of the batch reactor and the focused intensity on the reactor using the Halogen lamp was ca. 21.9 mW cm2. Distance between the lamp and the reactor was maintained at 10 cm, in all of the measurements. In each run, a desired amount of photocatalyst (0.06–0.14 g) was dispersed in 100 mL of water and then, a desired concentration of MO (8–16 mgL−1) was transferred into the reactor and stirred for 60 min to reach the adsorption equilibrium in the dark before irradiation. The photocatalytic removal was initiated with turning on the light source. At given irradiation time intervals (30–90 min) according to experiments designed using Design Expert 7.1.3 Software, 3 mL of sample was taken out and centrifuged (Sigma 2-16P), and then, MO concentration was analyzed by a UV-Vis spectrophotometer (Perkin Elmer) at λ max=464 nm. For determination of the removal efficiency and kinetics model for the photocatalytic process of MO, Eqs. (5) 9 ACS Paragon Plus Environment


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and (6) were used, respectively, where C0 and C are the initial and final concentrations of MO after visible-light irradiation, respectively. The kapp is the pseudo first-order reaction rate constant (min−1) and t is the irradiation time (min). η=

ln

C0 − C × 100% C0

(5)

C0 = k app t C

(6)

2.7. Experimental Design. The response surface methodology (RSM) is an affordable and reliable method for optimizing certain processes. This technique reduces the number of designed experiments needed to investigate the effects of operating factors.37,38 The RSM coupled with central composite design (CCD) was applied to evaluate the influences of operating key factors namely catalyst dosage, initial dye concentration, and the irradiation time on the removal rate of MO. For this purpose, Design Expert Software (version 7.1.3) was applied for experimental design and analysis of the obtained results and the photocatalytic removal rate of MO (%) was selected as the response variable. The actual and coded levels of the independent factors are presented in Table 1. It should be noted that the pretests were conducted to achieve the domain values of operating factors.

3. RESULTS AND DISCUSSION 3.1. Phase Structures and Morphology. The XRD patterns were recorded to identify the phase structures of the prepared samples. Figure 2a shows the XRD patterns of the MCNS and Cu(x)/MCNS (x=4, 5, 7 and 9) catalysts. It can be seen that the graphitic stacking structure is identified for all of the synthesized materials. The strongest XRD peak at 27.4° is a characteristic (002) peak indicating the interlayer stacking of conjugated aromatic systems, with the corresponding d-spacing to be 0.326 nm, while the minor peak around 13.1° is 10 ACS Paragon Plus Environment

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associated to an in-plane structural packing motif (the hole-to-hole distance of the nitride pores), corresponding to a distance of d=0.687 nm.39 It is noteworthy that this d-spacing is below the size of one tris-s-triazine (ca. 0.713 nm) and may be caused by small tilts in the structure because of N-N repulsion.22 Figure 2a also exhibits that with increasing the Cu deposition amount, the intensities of the MCNS characteristic peaks slightly decrease, however there are no obvious diffraction peaks corresponding to copper metal when the loading level of Cu reaches to 7 wt.%. The reason for this phenomenon can be related to increasing the Cu content so that the Cu(9)/MCNS sample exhibits the lowest XRD peak intensity compared to that of the bare MCNS. It is suggested that the crystallinity and stacking degree of aromatic layers in MCNS is probably disrupted by the Cu species during the synthesis of Cu/MCNS samples. In a similar work, Ang et al. prepared TiO2-melon nanocomposites via mixing-calcination technique.40 It was believed that interactions between TiO2 and tri-s-triazine units of melon interrupted the melon layers stacking and also weakened the hydrogen bonding network between the melon strands.40 It seems that the copper species in our Cu/MCNS samples can uniformly be dispersed on the MCNS surface, which is consistent with similar reports.32 With further loading of Cu, the intensity of the diffraction peak at around 2θ = 13.1° decreases, but careful examination of the XRD patterns demonstrates that the two pronounced peaks of the parent MCNS sample do not have significant shifts after deposition of Cu, suggesting the Cu clusters do not incorporate into the MCNS framework, and are probably attached on the surface of MCNS layers. The FT-IR spectra of all prepared samples are displayed in Figure 2b, which can provide sufficient information about evolution of the g-C3N4 structure. For native MCNS, a series of bands located in the 1231–1630 cm-1 region are assigned to the stretching vibrations of the tri-s-triazine ring units, while the sharp peak at 804 cm−1 is considered as their out-ofplane bending vibrations, indicating there are the tri-s-triazine units in the local structure of 11 ACS Paragon Plus Environment


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the MCNS sample. The broad absorption band in the region of 3100–3400 cm-1 is related to the stretching modes of pending N-H bonds originating from uncondensed amino groups and O–H bands from absorbed H2O molecules. It is noteworthy that the FT-IR spectra of all mesoporous materials show an obvious signal at 2173 cm-1 assigned to the presence of C≡N bond. Cui et al. reported that the existence of C≡N bond in mesoporous carbon nitride confirms its less condensed texture with more pending nitrile groups, which can be helpful in retarding the recombination of electron-hole pairs.41 It can also be seen from Figure 2b that there are no changes in all of the characteristic peaks related to MCNS after the deposition of Cu particles, suggesting the structure of the synthesized catalysts do not collapse during a simple precipitation approach. The XPS technique was performed to further determine the chemical composition of Cu(7)/MCNS sample and the valence states of copper species presented therein (Figure 3). The XPS survey spectrum of the Cu(7)/MCNS (Figure 3a) reveals the coexistence of elements C, N, S, O, and Cu, with sharp photoelectron peaks located at binding energies of 288, (C 1s), 399 (N 1s), 532 eV (O 1s), and 932 eV (Cu 2p) as well as a very weak photoelectron peak at 163.9 eV (S 2p). As it is observed from Figure 3b, the high resolution C1s XPS spectrum of the sample could be fitted with three peaks located at binding energies of 285.07, 286.5, and 288.3 eV. The predominant peak at the binding energy of 288.3 eV is related to the sp2-bonded carbon of N=C-N in the graphitic carbon nitride, while the peak at 285.07 eV ascribed to the C-C coordination is due to carbon impurities originated from the environmental contamination of instrument itself.42,43 The characteristic peak centered at 286.5 eV, related to C-S bond, reveals that S atom is indeed covalently incorporated into the lattice of the sample.44 Figure 3c displays the high resolution N1s XPS spectrum of the sample, which can be separated into four peaks. The most intense peak at 398.6 eV is related to the sp2-bonded nitrogen atom of C=N-C. The peaks at 400.1 eV and 401.1 eV are 12 ACS Paragon Plus Environment

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attributed to tertiary nitrogen of N-(C)3 and the pending N-H groups, originating from uncondensed amino groups, respectively. A weak peak at 404.8 eV is assigned to the πexcitations.45 The high resolution S 2p XPS spectrum of sample obviously reveals two peaks centered at 163.9 and 168.5 eV (Figure 3d). The binding energies of S 2p in the S-N and S-C bonds are found at around 164 eV and 166 eV, respectively. In addition, the XPS peak in the region of 168–170 eV corresponds to the presence of sulfite species such as SO32– or SO42–.46 Therefore, the S 2p peak centered at 163.9 eV in our case can be assigned to the C-S chemical bonds formed in the g-C3N4 by substituting the lattice N atoms with the S atoms, and the peak located in 168.5 eV can also be attributed to SO32– or SO42– on the surface of Cu(7)/MCNS. The high resolution O1s XPS spectrum of the sample is depicted in Figure 3e in which the peak centered at 532.3 eV is due to the adsorbed CO2 and H2O on the sample, which agrees with the FT-IR spectra. The high resolution XPS spectrum of the Cu 2p region (Figure 3f) indicates the typical Cu 2p3/2 and Cu 2p1/2 peaks with the binding energies centered at 932.7 and 952.6 eV, respectively, indicating formation of the Cu metal on the surface of the parent MCNS.47 The morphologies of synthesized materials were further investigated by TEM images as presented in Figure 4. It can be seen that the bulk g-C3N4 displays a typical stacked lamellar morphology so that it can be inferred from the darker contrast of the TEM image (Figure 4a). In Figure 4b, the TEM image of MCNS sample obtained after treatment with the silica template indicates the presence of a well-developed mesoporous system with a diameter close to 12 nm, which is well corresponded to the original texture of the SiO2 template. The dark spots in Figures 4c and 4d directly represent that the Cu particles are attached on the surface of Cu(7)/MCNS photocatalyst. Furthermore, the elemental mapping image indicates that the synthesized sample consists of C and N elements along with substantial amounts of S element and copper particles are uniformly distributed on the surface of the MCNS sample (Figure 13 ACS Paragon Plus Environment


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4e). Atomic content of sulfur element is calculated to be 7.5 wt.% for the Cu(7)/MCNS sample (Figure S1, Supporting Information).

3.2. Textural properties. It is generally accepted that the photocatalysts with mesoporous structures and high surface areas not only can enhance the visible-light harvesting ability via scattering the light in their mesostructures, but also provide more surface sites for diffusion of reactant molecules and products through the porous networks and therefore enhance the catalytic activity.38 Figures 5a and 5b show the nitrogen adsorption/desorption isotherms and the corresponding pore-size distribution curves of CNS, MCNS, and Cu(7)/MCNS samples. It can be noticed from Figure 5a that the three samples have isotherms of type IV with hysteresis loops of type H3 according to the IUPAC classification, indicating the presence of mesopores (2–50 nm). Moreover, the hysteresis loop in CNS is due to aggregates of plate-like particles and does not exhibit any limiting adsorption at high P/P0, while those of MCNS and Cu(7)/MCNS samples are associated with capillary condensation taking place in mesopores, and the limiting adsorption is observed at high P/P0. This means that the two later samples have narrower pore size distributions and well-developed porous systems, as can be seen from Figure 5b. The pore size distribution curves achieved by the BJH method illustrate a narrow range of 2–17 nm with a peak pore diameter of about 10 nm for the MCNS and Cu(7)/MCNS samples which reflect the particle size of the silica template, further confirming the presence of mesopores. The data on the textural characteristics of the samples including BET surface area, pore volume, and average pore diameter are also given in Table 2. The loading of Cu metal on the MCNS decreases the surface area and increases the pore volume, but it has no significant effect on the pore diameter compared with the pure MCNS. These results suggest that the Cu nanoparticles are anchored on the surface of MCNS, which is in agreement with the XRD results. 14 ACS Paragon Plus Environment

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3.3. Optical Properties. The optical properties of all synthesized materials are studied by the DRS and the PL spectra, and the corresponding results are given in Figure 6. It can be seen from the Figure 6a that the bulk g-C3N4 reveals absorption in the range of UV to visible wavelength (about 450 nm), originating from the intrinsic band gap absorption of g-C3N4 because of charge transfer from its VB level to the CB level, which agrees with the value reported in literature.9 Compared with the bulk g-C3N4, the pure CNS sample shows an Urbach-like absorption tail in the visible-light area. In fact, the doped sulfur introduced as the impurity into the host backbone, i.e., carbon nitride (CN), leads to the formation of localized states between valance and conduction bands of CN which has been confirmed in other studies by density functional theory (DFT) calculations.26 These midgaps in turn facilitate separation of the photogenerated electron-hole pairs and consequently they can be helpful in increasing the lifetimes of electron-hole pairs. The light absorption of MCNS obviously upshifts as compared to the bulk g-C3N4 and CNS samples, which can be a result of its higher surface area, pore volume, and sulfur doping leading to scattering of light by its mesoporous wall and consequently enhanced light harvesting property. The reflectance spectra of bulk gC3N4, CNS, MCNS and Cu(7)/MCNS show absorption thresholds at 450, 458 and 624 nm, respectively. The Eg values for bulk g-C3N4, CNS, MCNS and Cu(7)/MCNS are 2.73, 2.68 and 1.99 eV, respectively. The results demonstrate that the reduced copper on the surface of the MCNS acts as an electron sink which diminishes its optical band gap energy and this phenomenon can enhance the photocatalytic efficiency of the MCNS under visible-light irradiation. However, the deposition of Cu particles on the surface of MCNS enhances the absorption significantly in the visible-light and near infrared regions (460–900 nm). This can also be deduced from the color change by Cu doping illustrated in the optical photograph (inset in Figure 6a), in which the color of MCNS sample changes from yellowish to dark 15 ACS Paragon Plus Environment


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olive after deposition of Cu particles. The broadened visible-light photoresponse may be attributed to the synergistic effects of the Cu particles deposited on the MCNS as an electron sink, which is helpful for retarding the recombination of photoinduced electrons and consequently increasing the photoactivity of the Cu/MCNS materials. The PL analysis is widely used to investigate the recombination of charge carriers in semiconductor materials and indeed it can provide valuable information about the transfer, separation, as well as the lifetime of photogenerated electrons and holes. Figure 6b shows the PL spectra of bulk g-C3N4, MCNS, and Cu/MCNS samples in the wavelength range of 400– 650 nm recorded at the excitation wavelength of 320 nm. The emission peak at about 460 nm can be attributed to the interband PL phenomenon, which results from the recombination of electrons and holes in the band gap of carbon nitride materials. In addition, the PL intensity of MCNS is much lower than that of the bulk g-C3N4 because of simultaneous modification including the sulfur doping and mesostructure in the carbon nitride texture. On the other hand, the defects in the framework of mesoporous samples act as centers to trap the photogenerated charge carriers, which can prevent recombination of electrons and holes and decreases the PL intensity. Compared with the parent MCNS, the emission intensities of Cu/MCNS samples obviously decrease, implying the recombination rates of electrons and holes in the mentioned materials are lower under the visible-light irradiation. As a result, the Cu(7)/MCNS displays the longest lifetime for the charge carriers among all catalysts. It is suggested that the excited electrons can migrate from the conduction band of MCNS to Cu particles which is very useful to decrease the direct recombination of electrons and holes.

3.4. Photoelectrochemical Properties. Impedance measurements were also performed to study the kinetics of charge transfer in the bulk g-C3N4, MCNS and Cu(7)/MCNS materials. The Nyquist plots show a semicircle for all samples which is related to the charger 16 ACS Paragon Plus Environment

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transfer resistances (Rct) of the double layer at the semiconductor/electrolyte interface. It is commonly recognized that lower charge transfer resistance can lead to the depressed recombination of charge carriers and in turn enhances the efficiency of the photocatalytic activity. The impedance data were fitted to an equivalent circuit which is presented in the inset of Figure 7a. Since the fitted Rct value of Cu(7)/MCNS sample is much smaller than those of CNS and bulk g-C3N4, it is expected that Cu(7)/MCNS will have the highest efficiency for the separation of the charge carriers. For the sake of further investigating the recombination of electrons and holes in our samples, transient photocurrent responses were carried out in the traditional three electrode system. As it is seen in Figure 7b, the transient photocurrent responses of bulk g-C3N4 and MCNS and Cu(7)/MCNS electrodes are done for several on-off cycles with chopped irradiation at its open-circuit voltage. Compared with the bulk g-C3N4, the photocurrent of the MCNS is negative even though the light turns off, indicating the sample is inherently conductive after sulfur doping and the holes dominate in the charge transport. In a similar study, Zhang et al. reported that the phosphorus doping led to creating the negative photocurrent and increasing the semiconductor conductivity.17 Interestingly, the photocurrent is positively rapidly climbed up to a high current level as the photoelectrode made of MCNS is irradiated, sugesting the electrons are produced in the conduction band of the system and are captured by a reducing agent and the holes are quenched in the presence of irradiation. Besides, it can be realized that the Cu(7)/MCNS electrode has a higher photocurrent intensity than both of the MCNS and the bulk g-C3N4 electrodes; this means that the Cu nanoparticles are very helpful in decreasing the electron-hole recombination and prolonging the life times of charge carriers. On the other hand, the photocurrent responses rapidly decrease in electrodes as soon as the irradiation of light turns off and the photocurrent quickly increases when the light turns on, confirming a good repeatability of the semiconductor performance. 17 ACS Paragon Plus Environment


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The photocurrent responses, impedance measurements, and the PL analyses approve that the Cu(7)/MCNS sample has the highest separation efficiency of the photoinduced charge carriers and the lowest recombination rate under visible-light in comparison with its competitors, which can be correlated to the three simultaneous modifications including the sulfur doping, creating mesoporous structure, and deposition of Cu clusters on the MCNS surface.

3.5. Photocatalytic Activity. The photocatalytic activities of the prepared samples were investigated by the degradation of MO (10 mg L−1, 100 mL) as a hazardous dye under visible-light irradiation (λ>400 nm). The MO is considered as a dye with stubborn azoic structure which has a weak adsorption on the photocatalysts surfaces.29 To evaluate the effects and importance of sulfur doping, mesoporous structure and the Cu loading on the photocatalytic efficiencies of the syntesized products, the CN, CNS, and bulk g-C3N4 samples were examined. Control experiments indicate that no significant photocatalytic degradation is detected in the absence of photocatalyst, suggesting photolysis of MO is very low in the presence of only visible-light irradiation for 90 min. As a comparison, the photodegradation activity of the commercial Degussa P25 TiO2 was also tested under the same conditions. Figure 8 presents the adsorption and photocatalytic activities of bulk g-C3N4, CN, CNS, MCNS, and Cu/MCNS photocatalysts. It is obvious that the adsorption capacity of MCNS is the highest among those of bulk g-C3N4, CN, and CNS. Also, it is inferred that the adsorption of dye molecules onto the MCNS originates from its high surface area, large pore volume and well-developed mesoporous system. What is important is that the adsorption ability is remarkably increased after loading the Cu particles on the MCNS sample regardless of Cu loading value, implying the Cu plays a key role in adsorption of MO molecules on the photocatalyst surface. Similar results were reported for Ag deposition on the carbon nitride 18 ACS Paragon Plus Environment

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materials.31 As it is seen in Figure 8a, the bulk g-C3N4 shows a very low photocatalytic activity because of the rapid recombination between CB electrons and VB holes. Despite the higher surface area of CN than that of the CNS, the photodegradation of CNS is higher and thus it can be deduced that the sulfur doping enhances the photocatalytic performance of carbon nitride materials. The degradation of MO is increased to 50.1% after 90 min when MCNS is used as the photocatalyst, in which the specific surface area is 6.7 times higher than that of the CNS sample. However, the surface area of Cu/MCNS samples are decreased after Cu loading, but their pore volumes are further increased as it is observed in Table 2. Figure 8b shows the variations of time-dependent UV-Vis absorption spectra of MO (at 464 nm) under visible-light irradiation by Cu(7)/MCNS as the photocatalyst. With increasing the loading of Cu particles to 7 wt.% (Cu(7)/MCNS), the photocatalytic activity reaches the highest values with 96.8% of MO removal within 90 min irradiation. Notably, further increasing the Cu content up to 9 wt.% leads to a decrease in the photocatalytic activity, which can results from the following effects, that is, the deposition of excessive Cu particles leads to shielding the MCNS surface active sites and subsequently decreasing the light irradiation crossing throughout the reaction solution. Another reason for the declined photocatalytic efficiency may be because of the increased size of Cu particles and change of their catalytic properties. In addition, it is known that the metal nanoparticles can behave as recombinant centers of charge carriers when their loading is further increased, leading to lower photocatalytic efficiency.34 No photodegradation can be detected when Cu particles are used as the catalyst alone. The reaction kinetics of MO photodegradation on the catalyst surface can be well fitted by the pseudo first-order equation (as shown in Figure 8c). The reaction rate constant of Cu(7)/MCNS is about 17.7 times higher than that of the bulk g-C3N4 under visible-light irradiation. Based on the above observed and discussed data, four main conclusions are suggested: (1) the bulk g-C3N4 without modification is inactive for the 19 ACS Paragon Plus Environment


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photodegradation of MO under visible-light irradiation, (2) the sulfur doping in the g-C3N4 lattice can improve the photodegradation of MO because of decreasing the recombination of electron-hole pairs through the formation of localized states between the valance and the conduction bands, (3) the presence of high surface area and mesostructure in the MCNS texture as well as the reflection or transmission of light by pore walls can enhance the photocatalytic activity, and (4) the photocatalytic activity of samples can further be increased after modification of MCNS with Cu particles.

3.6. Reusability. The reusability and stability are also crucial factors for the heterogeneous catalysts in practical applications. In this regard, the stability of the Cu(7)/MCNS catalyst was assessed in the photodegradation of MO for four consecutive runs, and the results are presented in Figure 8d. After each reaction, the photocatalyst was separated by centrifugation, washed several times with ethanol, and dried at 80 °C. As displayed in Figure 8d, it obvious that Cu(7)/MCNS is quite stable and no apparent decrease of the photocatalyst activity is detected after the 4th run. Furthermore, the XRD patterns of the photocatalyst do not reveal any differences before and after 4th run reaction, indicating its highly stability during the photocatalytic degradation of MO (Figure S2, Supporting Information).

3.7. Effect of operating variables on the degradation efficiency 3.7.1. Model results. In addition to develop a novel suitable catalyst, optimization of the operating variables plays a key role in achieving good catalytic activity. Literature review shows that factors such as the catalyst dosage, initial dye concentration and irradiation time have significant effects on the catalytic performance.48 In the next stage, the Cu(7)/MCNS catalyst with the highest photodegradation activity was selected, and the effects of 20 ACS Paragon Plus Environment

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operational variables on its performance were optimized using the RSM technique. The analysis of experimental data was done using the second-order polynomial model for the degradation rate of MO by Design Expert 7.1.3 Software, and corresponding results are presented in Table 3. The final empirical model based on the coded values for the degradation rate of MO is shown in Eq. (7). In this model, Y (degradation rate of MO) is expressed as a function of A (catalyst dosage), B (initial dye concentration) and C (irradiation time). The competence of the model was assessed using analysis of variance (ANOVA) and the Fisher test (F-test), with a 95% confidence level. The statistical terms related to the information of ANOVA is presented in Table S1, Supporting Information.

Degradation rate of MO = 95.91 + 7.71 Α − 8.99 Β + 9.35 C − 5.32 AB − 3.47 AC + 4.31 BC − 10.50 A 2 − 5.48 B2 − 4.99 C2

(7)

3.7.2. Optimization of Operating Variables. In this study, three-dimensional response surface and the two-dimensional contour plots were used to investigate the effects of the operating variables and their interactions on the degradation rate of MO. These curves show the simultaneous effects of two factors on a response variable, while other factors remain at the center point in the design space. Figure 9a demonstrates the effects of catalyst dosage and the initial dye concentration on the degradation rate of MO, while the irradiation time is kept at 60 min. From the results presented in Figure 9a, it can be seen that interactions between operating parameters of catalyst dosage and the initial dye concentration have a high impressive effect on the photocatalytic activity. Simultaneous increasing and decreasing the catalyst dosage and dye concentration around 0.12 g and 9 mgL–1, respectively, dramatically improves the photocatalytic performance, so that the degradation rate of MO is increased to about 100%. It may be a result of increasing more available catalytic active sites on the surface of Cu(7)/MCNS which act as harbors for dye molecules. As it can be seen from Figures 9b and 9c, the degradation rate of MO is decreased with a further increase in the 21 ACS Paragon Plus Environment


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catalyst dosage from 0.12 to 0.16 g, which may causes increased scattering and turbidity effects, leading to decrease in the visible-light penetration into the solution and on the catalyst surface. Besides, it is observed that the increase in the initial dye concentration from 12 to 16 mg L-1 decreases the photodegradation efficiency. It is suggested that increasing the dye concentration leads to a rise in the internal optical density which makes the solution become more impermeable to visible-light irradiation and consequently the photocatalytic efficiency decreases. In order to find out the effect of irradiation time on the photocatalytic activity of Cu(7)/MCNS, the reactions are conducted for different times from 10 to 120 min, and the results are presented in Table 3. The contour plots reveal that the highest degradation rate of MO is obtained with the maximum value of irradiation time (see Figure 9b). As a result, the findings also indicate that the interactions between the operating key factors have significant effects on the photocatalyst performance using carbon nitride materials. The degradation rate of MO is defined as a maximum in the design space using Design Expert Software. The optimal values of operating variables for the maximum degradation rate of MO (100%) are 0.07 g, 11 mgL-1, and 89 min for catalyst dosage, initial dye concentration, and irradiation time, respectively. To show the importance of non-metal doping and another modification on tuning the propertices of carbon nitride materials, Table 4 summarizes the photocatalytic performances of catalysts recently reported for pollutant degradation in aqueous solution. Of particular note is that the reaction conditions applied for every photocatalyst specially pollutant nature as well as light source differ with each other which greately influence the photocatalytic activity.

3.8. Possible Photocatalytic Mechanism. The highest photocatalytic activity is achieved over the Cu(7)/MCNS sample because of simultaneous modification of the g-C3N4 22 ACS Paragon Plus Environment

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including mesoporous manipulation, sulfur doping and surface deposition by the Cu nanoparticles. The textural modification enhances the visible-light harvesting ability of the photocatalyst and produces more photoexcited electron-hole pairs which transfer at the interface of MCNS and Cu nanoparticles. This phenomenon also inhibits the recombination of photoinduced charge carriers evidenced by the photoelectrochemical and the PL results. The seperated electron-hole pairs can be attacked into the oxygen and hydroxide molecules to create the active species which are responsible for the degradation of MO in aqueous solution (Eqs. 8–10). The active species generated in the photocatalytic system such as the photogenerated holes (h+), superoxide radicals (⦁O2¯), and hydroxyl radicals (⦁OH) play important roles during the process of photodegradation of pollutants. To further understand the mechanism of the degradation of MO over the Cu/MCNS photocatalyst under visiblelight irradiation, the active species are identified by adding 1 mmol isopropanol (IPA), 1 mmol benzoquinone (BQ) and 1 mmol ethylenediaminetetraacetic (EDTA) into the reaction solution as the ⦁OH, ⦁O2¯, and h+ scavengers, respectively. As can be seen in Figure 10a, the degradation of MO in the photocatalytic reaction is hardly inhibited after the IPA addition, denoting there are few •OH radicals as oxidative species in the photodegradation of MO by Cu/MCNS. However, the photodegradation efficiency of MO is significantly declined with the increase of the BQ and the EDTA amounts in the solution, implying the photogenerated h+ and ⦁O2¯ are the major oxidative species. Based on the above results from the trapping experiments, the h+ and ⦁O2¯ radicals as the predominant active species are responsible for the oxidization of MO over the Cu/MCNS catalyst under the visible-light irradiation, while the role of photogenerated ⦁OH could be insignificant. The Mott-Schottky plot corresponding to the electrochemical impedance spectra at a high frequency of 10 kHz was applied to determine the flat band potential of the MCNS sample, which indicates a typical n-type characteristic for this semiconductor. The flat band 23 ACS Paragon Plus Environment


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potential of MCNS is determined to be –1.1 eV which accompanies with the band gap energy obtained from the DRS spectra and is used for estimating the conduction band (CB) and valence band (VB) positions (Figure 10b). Therefore, the CB and VB of MCNS is calculated to be –1.1 and 1.31 V vs. Ag/AgCl, respectively. The absolute energy scheme, including the CB and VB positions of MCNS vs. NHE at pH=7 is presented in Scheme 1a. The reason why hydroxyl radicals have a very little role in the photadegradation efficiency of the Cu/MCNS is probably that the standard redox potential of ⦁OH /OH− is +1.99 (vs. NHE, at pH=7),49 which is higher than the valance band VB potential energy of the MCNS. Consequently, it is found that the photogenerated h+ cannot oxidize OH− to produce •OH radicals directly in native MCNS photocatalyst (Eq. 8), but the photogenerated h+ itself as active species attacks the organic pollutant molecules which is consistent with the trapping experiments. It should be noted that the standard redox potential of O2/⦁O2¯ is –0.13 V (vs. NHE, at pH=7),50 which is higher than the CB potential energy of the MCNS, and hence the photogenerated electrons could react with O2 to generate ⦁O2¯ radicals on the surface of photocatalyst to degrade the target pollutant (Eq. 9). The mechanism of the photocatalytic degradation of MO over the Cu/MCNS catalyst under visible-light irradiation is proposed in Scheme 1b. Under visiblelight irradiation, the VB electrons of MCNS are excited to its CB. The loaded Cu particles on the MCNS could act as electron traps not only to prolong the lifetimes of photogenerated electron-hole pairs but also efficiently to pass interfacial electrons to O2 in the solution to produce ⦁O2¯ radicals. In another pathway, the photogenerated h+ can directly capture electrons from the MO molecules adsorbed on photocatalyst surface and convert them into CO2, H2O and degradation products. OH– + hVB+ OH

(8)

O2 + eCB– O2–

(9)

O2 + 2H+ + 3eCB– OH + OH–

(10) 24


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4. CONCLUSIONS In summary, this work focused on the modification of carbon nitride materials via three simultanious strategies including sulfur doping, creating mesoporous structure, and deposition of Cu particles as an inexpensive non-noble metal on their surfaces which was done for the first time. In situ sulfur-doped mesoporous carbon nitride (MCNS) was successfully synthesized using thiourea as a low cost precursor and SiO2 gel solution as a template through a simple thermal condensation method. then, a series of the Cu deposited MCNS (Cu/MCNS) particles were obtained by a facile precipitation-reduction method and their photocatalytic activities were evaluated by the degradation of methyl orange (MO) as a model pollutant in aqueous solution under visible-light irradiation. The optimal Cu nominal loading value was determined to be 7 wt.%, and the corresponding degradation rate of MO was about 100% after 90 min under the optimized operating conditions, conducted by the response surface methodology (RSM), which was indeed the highest among those of bulk gC3N4, CN (un-doped g-C3N4), CNS (sulfur-doped g-C3N4 without template), and Degussa P25 TiO2. The characterization results indicated that synergistic effects of the sulfur doping, mesostructure and especially the presence of Cu nanoparticles on the surface of MCNS enhanced the visible-light harvesting ability and prolonged the lifetime of the photogenerated electron-hole pairs, which dramaticaly improved the photocatalytic activity. In adition, the phocatalyst could be recycled at least four times without any significant decrease in its efficiency. The photocatalytic mechanism was also discussed by capturing experiments in detail. As a result, the Cu nanoparticle can be apllied as an economical alternative for noble metals in preparing the genuine photocatalysts for the practical water treatment applications.



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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Analysis of variance (ANOVA) for removal of MO, the EDX analysis for the Cu(7)/CNS composite and the XRD patterns of Cu(7)/MCNS sample before and after the cycling photocatalytic experiments.

AUTHOR INFORMATION Corresponding Author *Tel.: +982164545810. E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The financial support of this work by the Research Office of Amirkabir University of Technology (Tehran Polytechnic) is gratefully acknowledged.


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Table 1. The Actual and Coded Levels of Independent Factors Independent factor

Unit

A: Catalyst dosage B: Initial dye concentration C: Irradiation time

(g) (mgL-1) (min)

-ߙ (-1.68) 0.03 5.27 9.55

-1 0.06 8 30


Level 0 0.1 12 60

+1 0.14 16 90

-ߙ (+1.68) 0.17 18.73 110.45

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Table 2. Physicochemical and Textural Properties of Synthesized Materials Sample x Cu (wt.%)a S.Ab (m2/g) P.D c (nm) P.Vd (cm3/g) MCNS 59.5 9.5 0.142 Cu(x)/MCNS 4 3.9 36.8 10.0 0.154 Cu(x)/MCNS 5 4.9 42.1 9.8 0.168 Cu(x)/MCNS 7 6.8 37.2 12.3 0.175 Cu(x)/MCNS 9 8.7 34.2 11.8 0.178 CNS 8.9 26.6 0.026 CN 25.1 32.2 0.090 a The actual chemical composition of the Cu was measured by AAS. b

Surface area (BET). cAverage pore diameter (BJH). dPore volume.



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Run 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Table 3. The Design Experiment Matrix Obtained Based on CCD Observed removal rate Predicted removal rate A (g) B (mg/L) C (min) (%) (%) 0.06 8.00 90.00 80.01 79.42 0.10 12.00 9.55 65.21 60.07 0.10 5.27 60.00 95.87 95.54 0.10 12.00 60.00 95.89 95.91 0.14 8.00 90.00 97.48 98.54 0.10 12.00 60.00 96.02 95.91 0.06 16.00 30.00 48.02 46.43 0.10 12.00 60.00 94.24 95.91 0.14 16.00 90.00 78.33 78.53 0.10 12.00 60.00 97.31 90.91 0.10 18.73 60.00 64.21 65.30 0.14 16.00 30.00 58.11 58.16 0.10 12.00 60.00 95.11 95.91 0.10 12.00 60.00 97.03 95.91 0.10 12.00 110.45 97.63 97.53 0.14 8.00 30.00 95.01 95.40 0.03 12.00 60.00 51.21 53.24 0.06 8.00 30.00 63.12 62.38 0.06 16.00 90.00 81.62 60.69 0.17 12.00 60.00 80.45 79.18


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Table 4. Comparison of Photodegradation Performances of Various Catalysts Used Two Simultaneous Modifications toward Pollutant Degradation in Aqueous Solution Catalyst

Modification strategies

Pollutant

Catalyst

(concentration)

dose (g)

C and Fe

Non-metal doping,

RhBa (10 mg/L)

0.025

codoped g-C3N4

co-catalyst Non-metal doping,

Fe and P co-doped g-C3N4

ZnTNPc/g-CNSb

RhB (10 mg/L)

0.05

Cu(7)MCNS-4

Activity

time (min)

(%)

180

~97

[43]

120

95.8

[49]

120

~92

[48]

90

~100

This study

high-pressure sodium lamp

MBc (15 mg/L)

500 W Xe lamp with an UV 0.02

cut-off filter (λ≥420 nm)

MO (11 mg/L)

0.07

300 W halogen lamp (λ>420)

co-catalyst a

Reference

(400-800 nm)

co-catalyst Non-metal doping, mesostructure,

300 W Xe lamp (400-700 nm)

Irradiation

250 W

co-catalyst Non-metal doping,

Light source

Rhodamin B. bZinc phthalocyanine (ZnTNPc) was combined with sulfur-doped graphitic carbon nitride (g-CNS). cMethylene blue.



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Figure 1. A schematic illustration of the deposition process of Cu metal on the surface of MCNS.


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Figure 2. The (a) XRD patterns and (b) the FT-IR spectra of the MCNS and Cu(x)/MCNS materials.



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Figure 3. (a) The XPS survey spectrum of the Cu(7)/MCNS. The high-resolution XPS spectrum expanded for the (b) C 1s, (c) N 1s, (d) S 2p, (e) O 1s and (f) Cu 2p regions. 40 ACS Paragon Plus Environment

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(e) Figure 4. The TEM images of (a) bulk g-C3N4, (b) MCNS as well as (c and d) the Cu(7)/MCNS at different magnifications. (e) The elemental mapping images of C, N, S, and Cu elements existing in the Cu(7)/MCNS sample. 41 ACS Paragon Plus Environment


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Figure 5. The (a) N2 adsorption−desorption isotherms and (b) pore size distributions of CNS, MCNS, and Cu(7)/MCNS.


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Figure 6. The (a) DRS spectra and (b) the photoluminescence (PL) spectra of the bulk g-C3N3, MCNS, and Cu/MCNS samples.



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Figure 7. (a) The EIS profiles and (b) transient photocurrent responses of bulk g-C3N4, MCNS and MCNS-4 samples.


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Figure 8. (a) The photocatalytic degradation of MO in aqueous solution over bulk g-C3N4, CN, CNS, P25 TiO2, MCNS, and Cu/MCNS photocatalysts. (b) Time-dependent UV-Vis absorption spectra for the catalytic degradation of MO over the Cu(7)/MCNS photocatalyst. (c) The plot of ln(C0/C) against reaction time for the catalytic degradation of MO using different photocatalysts, and (d) recyclability of the Cu(7)/MCNS photocatalyst in four experiments for the photocatalytic degradation of MO under visiblelight irradiation under the optimized conditions.



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Figure 9. Response surface plots showing the interaction effects of the operating variables on the photocatalytic removal of MO by Cu(7)/MCNS material. (a) Catalyst dosage and dye concentration, (b) irradiation time and catalyst dosage, (c) dye concentration and irradiation time.


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Figure 10. (a) Influence of various trapping agents on the MO catalytic degradation in the presence of Cu(7)/MCNS photocatalyst. (b) The Mott–Schottky plot for the MCNS sample.



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Scheme 1. (a) A schematic representation of the potential energy diagram for the parent MCNS electrode, and (b) mechanism of visible-light photocatalytic degradation of MO over the Cu(7)/MCNS photocatalyst.


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“TOC Graphic” 82x41mm (248 x 248 DPI)


Sulfur-Doped Mesoporous Carbon Nitride Decorated with Cu Particles

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