Controllable Synthesis of Mesoporous Sulfur ... - ACS Publications

Article pubs.acs.org/Langmuir

Controllable Synthesis of Mesoporous Sulfur-Doped Carbon Nitride Materials for Enhanced Visible Light Photocatalytic Degradation 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, and §Nanobiomedicine Center of Excellence, Nanoscience and Nanotechnology Research Center, University of Tehran, Tehran 14174, Iran

‡

S Supporting Information *

ABSTRACT: Mesoporous sulfur-doped graphitic carbon nitride (MCNS) materials were successfully synthesized using thiourea as a low-cost precursor and SiO2 gel solution as a template through a simple thermal condensation method. The effects of three synthesis key factors, namely, the reaction temperature, the reaction time, and the weight ratio of SiO2/ thiourea, and also their interactions on the removal rate of methyl orange (MO) were investigated using response surface methodology, and the samples were subjected to several characterization techniques. Results showed that the optimized physicochemical properties could be achieved for the MCNS samples by controlling the synthesis key factors, and it was found that the reaction temperature and the reaction time had significant influences on the MO photocatalytic removal. Among bulk graphitic carbon nitride (g-C3N4), CN (undoped g-C3N4), CNS (sulfur-doped g-C3N4 without template), and TiO2 (Degussa P25) samples, the optimized MCNS-4 illustrated the highest photocatalytic activity toward the removal of MO under visible light irradiation. The enhanced performance originated from the synergistic effects of high surface area, mesoporous texture, sulfur doping, and high visible light absorption, which were helpful for the separation and transportation of the photogenerated electron−hole pairs. Furthermore, MCNS-4 revealed high reusability and stability without any significant decrease in its efficiency. Our findings not only confirm the importance of simultaneous sulfur doping and mesoporous structure to synthesize highly active photocatalysts but also might provide a new insight into textural engineering of carbon nitride materials only by the optimization of the synthesis key variables, considering their interactions without relying on extra metal oxides.

■

INTRODUCTION Heterogeneous photocatalysis processes, based on visible light irradiation, using semiconductors have received considerable attention, mainly owing to their promising applications in renewable energy and clean environment investigations.1 Also, the photocatalytic degradation of various environmental pollutants in reactions mediated by solar irradiation is of great interest.2 To utilize solar energy, well-organized photocatalysts should possess good architecture, efficiently separate electron−hole pairs, have suitable band gaps, and respond to visible light irradiation. The photogenerated electron−hole pairs can react directly or indirectly with a number of species in their surroundings.3 Recently, intensive studies have focused on graphitic carbon nitride (g-C3N4) as a new material because it is a nontoxic, metal-free, and cheap photocatalyst.4−7 It has been approved that g-C3N4 has a good visible light response at around 450 nm, which is related to its narrow band gap of 2.7 eV. In addition, these materials have high thermal and chemical stability because of their π-conjugated frameworks that connect two-dimensional (2D) layered structures of tri-s-triazine units. © 2017 American Chemical Society

Nevertheless, the two main drawbacks of bulk g-C3N4 for the photocatalytic activity include fast recombination of its electron−hole pairs and its small surface area ( 400 nm) was switched on and off, were measured at the open-circuit voltage. The electrochemical impedance spectra were also achieved at the opencircuit voltage. A sinusoidal AC perturbation of 5 mV was employed on the electrode over the frequency range of 0.05−105 Hz. The equivalent circuits of the measured spectra were fitted by using the ZView program. Eq 2 was used for converting the obtained potential (vs Ag/AgCl) to reversible hydrogen electrode (RHE) [normal hydrogen electrode (NHE) at pH = 0].

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

independent factor

A: reaction temperature B: reaction time C: weight ratio of SiO2/thiourea

ln

C0 − C × 100% C0

C0 = kappt C

level −α (−1.68)

−1

0

+1

−α (+1.68)

(°C)

416

450

500

550

584

(min)

139 0.13

180 0.2

240 0.3

300 0.4

340 0.47

experiments were done to distinguish the extreme values of the synthesis factors. For example, MCNS could not be formed efficiently when the weight ratio of SiO2/thiourea was around 0.7.

■

RESULTS AND DISCUSSION Model Results. A second-order polynomial model was obtained for the removal of MO using Design-Expert software; the experimental results are presented in Table 2. An empirical

Table 2. Design Experiment Matrix Based on the CCD Methoda name MCNS-2

(2) MCNS-4

Visible Light Photocatalytic Activity. The photocatalytic performances of the samples were investigated by the photocatalytic removal of MO. Photocatalytic removal processes were carried out in a home-made cylindrical Pyrex-glass reactor with 700 mL capacity (7 cm inner diameter, 17 cm height), in which the reaction system was cooled by circulating water to maintain the room temperature. Artificial irradiation (λ > 400 nm) was provided by a 300 W halogen lamp with UV-stop feature, positioned at the top of the batch reactor. In each run, 0.1 g of the catalyst was dispersed in 100 mL of water, and then, the desired concentration of MO (10 mg L−1) and photocatalyst were transferred into the reactor and stirred for 60 min to reach the adsorption equilibrium in the dark before irradiation. The photocatalytic removal was initiated by turning on the light source. The distance between the lamp and the reactor was maintained at 10 cm in all measurements. At given irradiation time intervals, the samples (3 mL) were taken out and centrifuged (Sigma 2-16P), and after that, the MO concentration was analyzed by a UV−vis spectrophotometer (PerkinElmer) at λmax = 464 nm. The removal efficiency and the kinetics model for the photocatalytic process of MO degradation were determined using eqs 3 and 4, respectively, where C0 and C are the initial and final concentrations of MO after visible light irradiation, respectively, kapp is the pseudo-first-order reaction rate constant (min−1), and t is the irradiation time (min).

η=

unit

MCNS-6

MCNS-9 MCNS-11 MCNS-12

MCNS-16

run

A (°C)

B (min)

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

550 500 450 500 550 500 450 500 500 450 500 584 500 500 550 416 500 500 550 450

300 240 300 240 180 240 300 240 139 180 340 240 240 240 180 240 240 240 300 180

C

observed removal rate (%)

predicted removal rate (%)

0.4 0.47 0.2 0.3 0.2 0.13 0.4 0.3 0.3 0.2 0.3 0.3 0.3 0.3 0.4 0.3 0.3 0.3 0.2 0.4

26.21 19.11 65.27 82.2 47.34 49.18 28.12 78.75 69.81 70.88 51.11 45.21 79.12 79.13 41.69 69.78 80.21 79.83 52.01 58.05

26.22 17.76 64.60 79.88 46.13 50.27 29.52 79.89 69.91 71.06 50.75 45.49 79.88 79.88 42.54 69.23 79.88 79.88 52.03 58.21

a

Reaction conditions: photocatalyst (0.1 g), concentration of MO (100 mL, 10 mg L−1).

relationship between the response variable and synthesis key factors, based on the coded values, is shown in eq 5. In this model, Y (removal of MO) is expressed as a function of A (reaction temperature), B (reaction time), and C (weight ratio of SiO2/thiourea). The details of the model competence were assessed using analysis of variance (ANOVA), and the related information is exhibited in Table S1.

(3) (4)

Experimental Design. RSM coupled with central composite design (CCD) was employed to investigate the effects of synthesis key factors, namely, the reaction temperature (°C), the reaction time (min), and the weight ratio of SiO2/thiourea, on the photocatalytic process. For this purpose, Design-Expert software (version 7.1.3) was used for experimental design and to analyze the obtained results; the photocatalytic removal of MO (%) was selected as the response

Y = +75.86 − 9.67A − 7.06B − 5.70C + 2.32AB − 5.56AC + 3.09BC − 14.86A2 − 6.59B2 − 5.54C 2 (5) 7064

DOI: 10.1021/acs.langmuir.7b01767 Langmuir 2017, 33, 7062−7078

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Figure 1. Response surface plots showing the interaction effects of the synthesis factors on the photocatalytic removal of MO by MCNS materials: (a) Reaction temperature and reaction time, (b) weight ratio of SiO2/thiourea and reaction temperature, and (c) weight ratio of SiO2/thiourea and reaction time.

Figure 2. (a) XRD patterns and (b) FT-IR spectra of the MCNS materials synthesized at different reaction temperatures: MCNS-16 (416 °C), MCNS-4 (500 °C), and MCNS-12 (584 °C), using various weight ratios of SiO2/thiourea: MCNS-6 (0.13) and MCNS-2 (0.47), and during different reaction times: MCNS-9 (139 min) and MCNS-11 (340 min).

Relationship between Synthesis Key Factors and the Photocatalytic Activity. Three-dimensional (3D) response surface and 2D contour plots were applied to investigate the effects of the synthesis variables and their interactions on the

catalytic performance of MO removal. These curves illustrate the simultaneous effects of two factors on the response variable while other factors remain at the center point in the design space.27,30 Figure 1a shows the effects of the reaction time and 7065


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The structural information of the prepared samples after increasing the temperature from 416 to 584 °C is also obtained by the FT-IR spectra, as shown in Figure 2b. At a low temperature of 416 °C, four absorption bands are located at 804, 1318, 1460, and 1630 cm−1 and strong N−H vibration signals exist at around 3100 and 3300 cm−1. The strong band at 804 cm−1 belongs to the out-of-plane bending vibration of tri-striazine rings, indicating that there are tri-s-triazine units in the structure of the MCNS samples. Other vibrational modes can be attributed to the incomplete formation of g-C3N4. A series of new peaks at 1231, 1417, and 1570 cm−1 are seen at 500 °C, which are characteristics of aromatic carbon nitride heterocycles and agree with the FT-IR analysis for the polymeric melon and g-C3N4. Upon increasing the temperature to 584 °C, no new peaks can be observed compared with samples prepared at 500 °C, but the intensity of the peaks at 1417 and 1570 cm−1 is enhanced, indicating the formation of a more condensed melon structure. Nevertheless, the broad absorption band in the range of 3000−3300 cm−1 originates from the stretching modes of N−H (uncondensed amino groups) and O−H bonds (absorbed H2O molecules). Next, the weight ratio of SiO2/thiourea severely affects the photocatalytic activity of the produced samples, as demonstrated by the experiments conducted with different ratios of SiO2/thiourea in the range of 0.13−0.47, as illustrated in Figure 2a (samples MCNS-6, MCNS-4, and MCNS-2). According to the XRD patterns, it can be concluded that by increasing the weight ratio of SiO2/thiourea from 0.13 to 0.3 (the reaction temperature and the reaction time are kept constant at 500 °C and 240 min, respectively), the (002) peak upshifts from 2θ ≈ 27.2° to 27.4° and the formation of the g-C3N4 structure can gradually be promoted (MCNS-4 sample), which is also in agreement with the FT-IR analysis (Figure 2b). Interestingly, in contrast to the two previous samples (i.e., ratios of 0.13 and 0.3), when the weight ratio of SiO2/thiourea reaches 0.47 (MCNS-2 sample), the intensity of the peak (002) is reduced, and a new single broad diffraction peak appears near 23.5°. A similar result was reported by Zimmerman et al. who found that a single broad peak at 2θ ≈ 25.8° was related to the hollow spherical carbon nitride nanostructure that was synthesized through the solution reaction of cyanuric chloride or fluoride with lithium nitride.34 In our case, the scanning electron microscopy (SEM) image was also used to study the morphology of this sample (Figure S1). As can be observed, the SEM micrograph includes not only the spherical morphology (which is due to the silica template) but also a layered shape because of the carbon nitride growth without template confinement, which is in agreement with the XRD result. Figure 2b also reveals some peaks in the 1200−1630 cm−1 range, as mentioned above, belonging to the C−N stretching modes of the carbon nitride heterocycles, whereas the peak at 804 cm−1 arises from the out-of-plane bending vibration of tri-s-triazine rings. The peak attributed to the surface sulfate species at 1093 cm−1 is also found in this sample. Possibly, the evolution trend of carbon nitride morphology toward the spherical structure is a result of using a higher weight ratio of SiO2/thiourea, leading to greater confinement effect by the template.34,35 Subsequently, we investigated the effect of reaction time on the structure of MCNS. Figure 2a also exhibits the XRD patterns of the samples prepared at various reaction times in the range of 139−340 min for the samples MCNS-9, MCNS-4, and MCNS-11 (the reaction temperature and the weight ratio

the reaction temperature on the removal rate of MO, when the weight ratio of SiO2/thiourea is kept constant at 0.3. Interestingly, simultaneously increasing the reaction temperature and the reaction time (around 500 °C and 240 min, respectively) dramatically improves the photocatalytic performance so that the removal efficiency of MO is increased to about 76%. Further increase in the mentioned parameters leads to a decrease in the removal rate of MO. Therefore, the MCNS sample tempered at around 500 °C and thermally treated for 240 min with a SiO2/thiourea weight ratio of 0.3 results in the highest photocatalytic activity among other samples. In our opinion, the nanoarchitecture properties and the photocatalytic capability of MCNS can be well-tailored by controlling the synthesis key factors. Besides, it is observed that the weight ratio of SiO2/thiourea has a crucial role in governing the photocatalytic activity of samples, as demonstrated by experiments designed with different ratios of SiO2/thiourea in the range of 0.2−0.4. As can be seen in Figure 1b,c, an increase in the weight ratio of SiO2/thiourea from 0.2 to 0.3 along with increasing temperature (around 500 °C) and time (around 240 min) increases the removal rate of MO up to 74%. More importantly, the findings indicate that the interactions between the synthesis key factors have significant effects on the photocatalytic performances of carbon nitride materials, which have not been considered previously in the literature until now. In addition, it is inferred that there is an optimal texture for the MCNS photocatalyst in terms of porosity and optical and photoelectrochemical properties to obtain the highest photocatalytic activity. For this reason, we focus on the characterization of the prepared samples under the conditions explained above at three levels (−1.68, 0, and +1.68, see Table 1) via several methods. Characterization of Photocatalysts. Structural Properties. Figure 2a shows the X-ray diffraction (XRD) patterns of the MCNS-16, MCNS-4, and MCNS-12 samples condensed under different temperatures, whereas the reaction time and the weight ratio of SiO2/thiourea are kept constant at 240 min and 0.3, respectively. A series of sharp peaks can be seen at the lowest reaction temperature of 416 °C (MCNS-16 sample), which presumably indicates that thiourea is not completely transformed into graphitic-like networks. Wang et al. reported that the material obtained by heating cyanamide at a temperature of 389 °C for 4 h could be attributed to the formation of polymeric melem (melon).31 In another study, the thermal condensation of melamine exhibited a variety of peaks, most of which could be assigned to melem.23 Nonetheless, when the temperature reaches 500 °C (MCNS-4 sample), only two distinct diffraction peaks are observed, which are located at 13.1° and 27.4°. The latter peak (≈27.4°) is assigned to the interplanar stacking unit (002) of the aromatic systems, with the corresponding d-spacing of 0.326 nm, whereas the peak at around 13.1° is associated with an in-plane structural packing motif (the hole-to-hole distance of the nitride pores), corresponding to a distance of d = 0.687 nm. This is a little below the size of one tri-s-triazine unit (approximately 0.713 nm) and may be caused by the N−N repulsion that leads to the creation of small tilts in the structure.32,33 With the further increase in temperature to 584 °C (MCNS-12 sample), the XRD peak at 13.1° shifts to 13.2°, implying that the in-plane spacing becomes smaller. This result indicates that a higher temperature leads to higher condensation and indeed more tilts in the samples. 7066


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Figure 3. (a) XPS survey spectrum of bulk g-C3N4 and MCNS-4 samples and (b) expanded high-resolution XPS spectrum for the C 1s, (c) N 1s, and (d) S 2p regions.

of SiO2/thiourea are kept constant at 500 °C and 0.3, respectively). It can be seen that the lowest reaction time of 139 min (MCNS-9 sample) is not adequate to obtain highly condensed MCNS. From the results, it is also found that the powders synthesized at 240 and 340 min have almost similar diffraction patterns, which point that poor condensation of MCNS can be overcome by prolonging the reaction time. A similar structural development can be deduced from the FT-IR spectra (Figure 2b). It seems that increasing the reaction time may lead to a decrease in the rate of producing self-supporting atmosphere that is derived from the covered crucible so that this can facilitate the polycondensation reaction of thiourea with carbon nitride. It is noteworthy that the FT-IR spectra of all synthesized materials, especially the MCNS-4 sample, display an obvious signal at 2173 cm−1 assigned to the existence of the CN band, which indicates that the mesoporous materials have a less condensed texture with more pending nitrile groups. These results are in agreement with a recent report on mesoporous carbon nitride compounds obtained by the pyrolysis of ammonium thiocyanate.36 The XPS technique was carried out for further elucidating the chemical composition of the bulk g-C3N4 and MCNS-4 samples and for investigating the local structural positions of elements. Figure 3a demonstrates the XPS survey spectra of both the samples, which consist of C, N, and O elements, with sharp photoelectron peaks appearing at binding energies of 288 (C 1s), 399 (N 1s), and 532 eV (O 1s) and a very weak

photoelectron peak at around 163 eV (S 2p) for the MCNS-4 sample. In addition, the binding energies of C 1s and N 1s in the XPS spectra do not show any obvious shifts (Figure 3a−c). On the basis of the XPS spectrum of the MCNS-4 sample, the calculated atomic ratio C/N (0.77) is greater than the theoretical value (0.75), which is due to the excess carbon that originates from the environmental contamination of the instrument itself. The high-resolution C 1s XPS spectra of the samples are presented in Figure 3b. Compared with the bulk gC3N4 sample, three peaks are found for the MCNS-4 sample at binding energies of 285.07, 286.5, and 288.3 eV. The main peak at the binding energy of 288.3 eV is assigned to the sp2-bonded carbon of NC−N in g-C3N4, whereas the peak at 285.07 eV ascribed to the C−C coordination is due to carbon impurities. The characteristic peak centered at 286.5 eV for MCNS-4 samples is related to the C−S bond, which reveals that the S atom is indeed covalently bound to the framework of the product.37 Figure 3c shows the expanded high-resolution N 1s XPS spectra of the samples, which can be deconvoluted into four peaks. The most intense peak at 398.6 eV is assigned to the sp2-bonded nitrogen of the CN−C group. The peaks at 400.1 and 401.1 eV are attributed to the tertiary nitrogen of N− (C)3 and the amino functional groups having hydrogen (C− N−H), corresponding to incomplete condensation, respectively. The weak peak at 404.8 eV can be assigned to π excitations.38 The expanded O 1s XPS spectrum of the samples is depicted in Figure S2, in which the peak centered at 532.3 eV 7067


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Figure 4. Final optimized structures of pure g-C3N4 and three CNS compounds in which the S atom has been replaced by one of the N atoms.

hysteresis loop area moves to the region with a lower relative pressure; the pore volume and the surface area enhance; and the sample indicates a sharp, well-developed, and narrow pore distribution at around 10 nm. This means that it has more absorptive capacity, which could be favorable for increasing the number of active sites and thereby facilitating the mass transfer. The BET surface areas, BJH pore sizes, and pore volumes of samples are summarized in Table 3. As can be seen, all samples (except MCNS-16) have average pore diameters close to 12 nm (9.3−12.9 nm), which reflect the particle size of the silica template. On the other hand, the hysteresis loop area again shifts to the region with a higher relative pressure; consequently, the surface area and the pore volume decrease with the excess sintering temperature up to 584 °C in the MCNS-12 sample. Similarly, it was found that a high degree of polycondensation of cyanamide at 550 °C produced carbon nitride materials with low surface areas (550 °C) the g-C3N4 sheets decomposed in the presence of oxygen to smaller particle sizes, which could lead to an increase in the surface area.41 In our samples, because of the decreased pore wall thickness in the presence of a template, possibly the pore walls of carbon nitride collapse at high temperature and consequently lead to a decrease in the surface area.36 TGA was also performed to determine the thermal stability of the MCNS-4 samples (Figure 5b). It can be observed that the weight of MCNS-4 decreases in the temperature range of 535− 680 °C, meaning that the decomposition of sample becomes complete in this temperature range. The effects of an SiO2/thiourea weight ratio on the textural characteristics of the samples are displayed in Table 3 and Figure 5a. On the one hand, the average pore diameter of the MCNS-6 sample is 20.7 nm, which is much higher than the size

is due to the adsorbed CO2 and H2O on the sample, which agrees with the FT-IR spectra. Figure 3d reveals the highresolution S 2p XPS spectrum, and the atomic content of the sulfur element is calculated to be 0.63 wt % for the MCNS-4 sample. The binding energies of S 2p in S−C and S−N bonds are found at around 163.9 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−. Therefore, the S 2p peak centered at 163.9 eV in our case can be assigned to the S−C chemical bonds formed in g-C3N4 by substituting the lattice N atoms with the S atoms.39 To estimate the preferred position of the S atom within the backbone of the CN framework, density functional theory (DFT) computations were carried out at B3LYP/6-31G(d) level of theory using Gaussian 98 software.40 For this purpose, structures of the pure g-C3N4 (1) and the three CNS compounds (2−4) in which the S atom has been replaced by one of the N atoms (Figure 4) are calculated. Comparing the binding energies (ΔEbinding) of compounds 1−4 reveals that the pure g-C3N4 (CN) has the most negative ΔEbinding value of −2473.59 kcal/mol, and among CNS materials, compound 3 shows the most negative ΔEbinding (−2368.58 kcal/mol), confirming that if the S atom is replaced by the N atom positioned at site 2, the CNS structure acquires the highest stability. The ΔEbinding values for compounds 2 and 4 are equal to −2339.81 and −2343.54 kcal/mol, respectively. Therefore, compound 3 is energetically the most favorable structure, indicating that the S atom is substituted by the N atom at site 2. Textural Properties. The nitrogen adsorption−desorption isotherms and the pore-size distributions (insets) of the MCNS-16, MCNS-4, MCNS-12 samples synthesized at different reaction temperatures are presented in Figure 5a. Clearly, the samples show the type IV isotherms, thereby indicating the existence of mesopores in these samples. As the reaction temperature increases from 416 to 500 °C, the 7068


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influence when the weight ratio of SiO2/thiourea is too low.42 As observed in the inset of Figure 5a, the pore size distribution curve of the MCN-6 sample is mostly distributed at about 5 nm and has a broad peak between 6 and 58 nm. Similarly, it was reported that the template-free thiourea can be decomposed into gas bubbles during the polycondensation process, leading to the formation of g-C3N4 with the disordered nanopores, which was mainly distributed at about 3.7 nm and had a broad peak between 5 and 70 nm.22,43 The MCNS-2 sample has the closest average pore diameter to the template size (12.9 nm), and this might be due to the severe confinement effect of the dense template when the weight ratio of SiO2/thiourea is too high (0.47), which agrees with the XRD result. To find out the effect of the reaction time on the textural characteristics in detail, the samples were heated for different durations of condensation (139−340 min). It can be seen from Figure 5a and Table 3 that MCNS-9, MCNS-4 and MCNS-11 have almost the same values of the specific surface areas, average pore diameters, and pore volumes. It is also indicated that the hysteresis loops shift to the region with lower P/P0 and that their hysteresis loop areas are enhanced when the reaction time increases from 139 to 340 min. In addition, the hysteresis loop in MCNS-11 is due to aggregates of platelike particles and does not exhibit any limiting adsorption at high p/p0, whereas those of MCNS-4 and MCNS-9 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 latter samples have narrower pore size distributions and well-developed pore systems, as can be seen from Figure 5a. EDX analysis technique was used to determine the composition of the prepared samples, as shown in Table 3. The sulfur content is in the range of 0.7−1.5 wt % for all samples, implying that sulfur is successfully introduced into the texture of g-C3N4 materials. Careful examination of Table 3 indicates that there is a relationship between the synthesis key factors and the sulfur doping level. In general, the sulfur doping level slightly increases as the weight ratio of SiO2/thiourea (for MCNS-6, MCNS-4, and MCNS-2) and the reaction time (for MCNS-9, MCNS-4, and MCNS-11) are increased in the identified range. On the contrary, as the reaction temperature increases from 416 to 584 °C, the sulfur doping level decreases from 1.5 to 0.7 wt %. Moreover, samples MCNS-4, MCNS-11, and MCNS-6 exhibit the highest C/N atomic ratios (0.61:0.65), demonstrating successful in situ synthesis of mpgCNS, which is consistent with a previously reported result.21 Morphology. The structural properties of the prepared samples were further investigated using TEM and SEM

Figure 5. (a) N2 adsorption−desorption isotherms and pore size distributions (insets) of MCNS samples at different reaction temperatures: MCNS-16 (416 °C), MCNS-4 (500 °C), and MCNS12 (584 °C), using various weight ratios of SiO2/thiourea: MCNS-12 (0.13), MCNS-4 (0.3), and MCNS-2 (0.47), and during different reaction times: MCNS-9 (139 min), MCNS-4 (240 min), and MCNS11 (340 min). (b) TGA diagram of the MCNS-4 sample.

of the silica template; on the other hand, its specific surface area is low compared with those of MCN-4 and MCN-2. These results may be due to the lack of template confinement

Table 3. Physicochemical and Textural Properties of the Synthesized MCNS Materials under Different Conditions

a

sample

S.Aa (m2/g)

P.Db (nm)

P.Vc(cm3/g)

C/N (at %)

S (wt %)

absorption edge (nm)

band gap (eV)

MCNS-16 MCNS-4 MCNS-12 MCNS-2 MCNS-6 MCNS-9 MCNS-11 CNS CN

25.6 59.5 32.9 56.8 35.3 57.0 50.6 8.9 25.1

10.0 9.5 9.3 12.9 20.7 10.0 9.6 26.6 32.2

0.047 0.142 0.098 0.183 0.183 0.128 0.122 0.026 0.09

0.56 0.65 0.57 0.56 0.65 0.59 0.61 0.65 0.69

1.5 1.2 0.7 1.3 0.8 0.8 1.3 1.0

428 486 478 520 460 456 464

2.76 2.41 2.50 2.23 2.65 2.56 2.59

Surface area (BET). bAverage pore diameter (BJH). cPore volume. 7069


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Figure 6. SEM images of (a) bulk g-C3N4 and (b) MCNS-4 samples. TEM images of (c) bulk g-C3N4 and (d) MCNS-4 catalysts. (e) Elemental mapping images of C, N, and S elements for the MCNS-4 sample.

size distribution in the mesoporous range among other samples. Three main hypotheses are suggested for the extended absorption range: (i) The red shift may result from the fact that the structural perfection of MCNS materials and consequently the electron delocalization extend with increasing the temperature to 500 °C, which is in agreement with the XRD and FT-IR results. (ii) The existence of high surface area and mesostructure in the MCNS texture as well as the reflection or transmission of light by the pore walls can enhance the visible-light-harvesting ability. (iii) Furthermore, sulfur doping in principle replacing either carbon or nitrogen atoms in the g-C3N4 lattice can enhance the light absorbance through the formation of localized states between the VB and CB,44−47 which is shown by comparing the absorbance spectra of bulk gC3N4 and CNS (Figure S3). Further increasing the temperature to 550 °C leads to a slight decrease in the absorption edge, and hence, the band gap of MCNS-12 increases to 2.50 eV. This blue shift of the absorption edge can presumably be originated from the strong quantum confinement effect, which may result in the thermal decomposition of MCNS into smaller particles, as has been reported elsewhere.36 The PL spectra of the MCNS samples synthesized under different reaction conditions and photoexcited at 320 nm are given in Figure 7b. The emission spectra of the MCNS-16 product synthesized at 416 °C exhibits at least two emission peaks located at 403 and 432 nm. On increasing the processing

analyses. Figure 6a demonstrates that the bulk g-C3N4 sample is composed of large particles with a layered shape, whereas Figure 6b shows that the particle size of the MCNS-4 sample clearly decreases after the removal of the silica frameworks. TEM images directly indicate the internal structures of the bulk g-C3N4 and MCNS-4 samples. The TEM image of the former displays a stacked lamellar structure that can be found out from the darker contrast of its image (Figure 6c). The TEM image of MCNS-4 reveals the presence of a well-developed mesoporous system with a pore diameter close to 12 nm, which excellently reflects the original texture of the SiO2 template (Figure 6d). These findings are in good agreement with the results obtained from the N2 physisorption measurements. The elemental mapping image indicates that the synthesized MCNS-4 sample consists of C and N along with substantial amounts of S and that the sulfur element is uniformly distributed in the framework of the sample (Figure 6e). Optical Properties. The relationship between the optical properties and the synthesis key factors of MCNS samples is provided by the DRS and the PL spectra. It can be found that on increasing the temperature from 416 to 500 °C, the absorption edge obviously shifts to higher wavelengths in the DRS, originating from a decrease in the band gap from 2.76 eV in MCNS-16 to 2.41 eV in MCNS-4 (Figure 7a). On the basis of our results in Table 3, the MCNS-4 sample has the highest surface area and the C/N ratio as well as the narrowest pore 7070


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temperature led to a stronger overlap of the π* antibonding state with the σ* antibonding state, which decreased the transition probability of σ* → LP.48 Moreover, the existence of mesostructure, high surface area (as indicated in Table 3), and surface terminal sites, that is, uncondensed nitride groups in the MCNS-4 framework, can also decrease the recombination rate of charge carriers, and more number of charge carriers can help the photocatalytic activity. It seems that MCNS-12 shows a slightly blue shift and a higher peak intensity compared with MCNS-4 synthesized by further increasing the reaction temperature, and this result agrees with the DRS and the nitrogen adsorption−desorption data. A red shift in the DRS spectra is observed for the MCNS samples synthesized on increasing the weight ratio of SiO2/ thiourea from 0.13 in MCNS-6 to 0.47 in MCNS-2, as presented in Figure 7a. Nevertheless, the photocatalytic activity of MCNS-2 is indeed the lowest among other samples for the removal of MO (19.11%), according to Table 2. In spite of similarity in the textural properties, MCNS-2 reveals higher fluorescence intensity than MCNS-4, indicating that the recombination rate of charge carriers in the MCNS-2 sample is higher (see Figure 7b). Notably, it is inferred that the textural properties are not the main factors in enhancing the photocatalytic activity of MCNS-2. On the basis of the results obtained from the XRD and SEM analyses for the MCNS-2 sample, it seems that the morphology acts as the light antenna, which is the most effective factor in the photocatalytic efficiency. That is, the layered shape with higher accessible contact areas can receive more light photons than the spherical shape; thus, a decrease in the light absorbance intensity is expected for the spherical morphology. Dai et al. found that the electron−hole separation in the stacked nanosheet/nanosheet heterojunction is higher because of its much larger area than that of spherical/spherical junction.49 In addition, agglomeration or deformation of the spherical structure can limit the photocatalytic reactions. To investigate the effect of the reaction time on the optical properties, the samples synthesized at different times from 139 min for MCNS-9 to 340 min for MCNS-11 are considered (Figure 6a). It is observed that the photocatalytic activities of MCNS-9, MCNS-4, and MCNS-11 are affected by the processed time so that the removal of MO is 69.8, 73.0, and 51.1%, respectively, by prolonging the time from 139 to 340 min. Figure 7a shows that the absorption edge of samples slightly increases with increase in the reaction time to 240 min and then decreases, which is well-matched with the results obtained for their textural and structural properties and the PL spectra (Figure 7b). It is noteworthy that the PL peak shape of MCNS-9 is similar to that of MCNS-16 explained above. Therefore, it may be inferred from the MCNS-9 and MCNS-16 spectra that controlling the time and temperature of condensation plays a very important role in the structural evolution of carbon nitride materials. To investigate whether the use of sulfur doping strategy has the desired synergistic effect on suppressing the recombination of photogenerated electron−hole pairs, the PL spectra of bulk g-C3N4 (undoped with S) and pure CNS (doped with S) samples are displayed in Figure S4. It is observed that the PL peak intensity is lower in the case of CNS, confirming that the S doping has led to decreasing the recombination of charge carriers. The natural bond orbital (NBO) calculations were performed to obtain the highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO),

Figure 7. (a) DRS and (b) PL spectra of the MCNS samples synthesized at different reaction temperatures: MCNS-16 (416 °C), MCNS-4 (500 °C), and MCNS-12 (584 °C), using different weight ratios of SiO2/thiourea: MCNS-12 (0.13), MCNS-4 (0.3), and MCNS-2 (0.47), and during different reaction times: MCNS-9 (139 min), MCNS-4 (240 min), and MCNS-11 (340 min).

temperature, the position of the emission peak of samples obviously illustrates a red shift from 432 nm in MCNS-16 to 445 nm in MCNS-4, and the intensities of the emission peaks located at 403 and 432 nm become weak. On the basis of the XRD and FT-IR results, it is obvious that MCNS-16 obtained at 416 °C has poor condensation compared with the sample synthesized at 500 °C so that MCNS-4 shows enhanced structural connections between the tri-s-triazine rings, which leads to the extension of electron delocalization in the aromatic units and a decrease in the band gap. It was believed that the emission peak located at around 403 nm was related to the transition between the lone pair (LP) VB formed via the electrons of the nitride groups and the σ* CB, which was correlated with the C−N bond (σ* → LP). The emission band located at 445 nm was also ascribed to the π* → LP transition. Increasing the delocalization of the electrons by increasing the 7071


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criterion of charge transfer through the double layer at the photocatalyst/electrolyte interface.50−52 As can be seen from Figure 9a, the EIS semicircle radius of the MCNS-4 sample is

band gap energies, and the partial density of states (PDOS) spectra. The PDOS spectra for the pure g-C3N4 and the CNS compound ( energetically the most favorable structure, i.e., S atom has been replaced by one of the N atoms at site 2, see Figure 4) are demonstrated in Figure 8. The PDOS spectra of

Figure 9. (a) EIS profiles and (b) transient photocurrent responses of bulk g-C3N4 and MCNS-4 samples.

dramatically smaller than that of the bulk g-C3N4 sample, indicating that the charge-transfer resistance of MCNS-4 in the double layer is lower than that of bulk g-C3N4, which can be correlated with its high surface area, large pore volume, welldeveloped pore distribution, and sulfur doping. As seen in Figure 9b, the transient photocurrent responses of bulk g-C3N4 and MCNS-4 electrodes are recorded for several on−off cycles of intermittent irradiation at its open-circuit voltage. The photocurrent of MCNS-4 is negative even though the light turns off, revealing that the semiconductor is inherently conductive and the holes dominate in the charge transport, which may be due to the delocalization of the lone pair of sulfur atom on the π-conjugated tri-s-triazine units. A similar result was observed when chemical doping with phosphorus was employed for modifying the electronic structure of a semiconductor.15 Interestingly, the photocurrent is positively enhanced as the photoelectrode is irradiated, suggesting that the electrons are dramatically generated in the CB of the MCNS-4 system and captured by a reducing agent and that the holes are extinguished in the presence of irradiation. In addition, it can be seen that the MCNS-4 electrode displays higher photocurrent intensity than the bulk g-C3N4 electrode; this means that the former has a lower electron−hole recombination and a prolonged lifetime for the charge carrier. Moreover, it is clear that photocurrent responses much rapidly

Figure 8. DOS spectra for the (a) pure g-C3N4 and (b) CNS compound in which the S atom has been replaced by one of the N atoms at site 2.

these structures exhibit the energy levels (including HOMO and LUMO levels) existing in a molecule. Comparison between the PDOS spectrum of pure g-C3N4 and that of the S-doped sample indicates that the introduction of the S atom creates midgap states between the VB and CB, as shown in Figure 8. These midgap states in turn facilitate the separation of the photogenerated electron−hole pairs and consequently can be helpful in retarding the lifetime of electron−hole pairs, which is confirmed by the PL results. Photoelectrochemical Properties. The basic process for the utilization of semiconductor photocatalysts is the creation, separation, and migration of photoinduced electron−hole pairs toward the photocatalyst surface to induce the surface reactions. It is generally accepted that the photoelectrochemical methods can give valuable information about the abovementioned processes. Electrochemical impedance spectroscopy (EIS) and transient photocurrent responses measurements of bulk g-C3N4 and MCNS-4 electrodes were conducted to investigate the simultaneous effects of sulfur doping and mesoporous structures on their electrochemical performances. In our samples, the EIS semicircle radius is considered as a 7072


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Figure 10. (a) Photocatalytic degradation of MO in aqueous solution over bulk g-C3N4, CN, CNS, P25 TiO2, and MCNS-4 photocatalysts. (b) Time-dependent UV−vVis absorption spectra for the catalytic degradation of MO over the MCNS-4 photocatalyst. (c) Plot of −ln(C/C0) against the reaction time for the catalytic degradation of MO using different photocatalysts. (d) Effects of SO32− and SO42− inorganic anions on the photocatalytic degradation rate of the MO dye.

210 min. However, the CN and CNS samples demonstrate higher photocatalytic performances compared to the bulk gC3N4 sample. Besides, the photocatalytic activity of CNS outperforms that of the CN sample, and according to their similar BET surface areas, it can be concluded that the enhanced performance of the CNS sample originates from sulfur doping. Further, when sulfur doping and the mesoporous structure are combined together to form MCNS-4, the degradation of MO increases to 82.7%, which is the highest among all compounds including CN, CNS, bulk g-C3N4, and P25 TiO2. Figure 10b shows the variations of time-dependent UV−vis absorption spectra (at λ = 464 nm) for the MO catalytic degradation under visible light irradiation by MCNS-4 as the photocatalyst. The reaction kinetics of the MO photodegradation on the catalyst surface can be well-fitted by the pseudo-first-order equation (Figure 10c). The reaction rate constant using MCNS-4 is about 17.7 times greater than that of bulk g-C3N4 under visible light irradiation. The enhanced photocatalytic ability of MCNS-4 as described above is consistent with the previous characterization results. It is found from the optical, textural, and structural analyses that the mentioned sample has a large surface area, which can provide more catalytic active sites for promoting the adsorption of MO dye molecules on its surface and a better mass transfer during photodegradation. In addition, the mesoporous structure leads to more light absorption via scattering of the light inside of the

decrease in the electrode as soon as the irradiation of light turns off and the photocurrent rapidly increases when the light turns on, exhibiting a good reversibility of the semiconductor. In total, the results of photoelectrochemical, optical, textural, and structural analyses and the RSM approach confirm that the MCNS-4 sample can be an appropriate candidate for removing the organic pollutants. Monitoring of the Photocatalytic Activity. The photocatalytic activity of the MCNS-4 sample is monitored in detail by the photodegradation of the MO dye (10 mg L−1, 100 mL) under visible light irradiation (λ > 400 nm). In the absence of the photocatalyst, the concentration of MO remains the same after irradiation for 210 min, indicating that MO is very stable, and its photolysis is insignificant after visible light irradiation. To further distinguish the effect and importance of sulfur doping and mesoporous structure on the photocatalytic activity of the MCNS-4 product, the photocatalytic activities of CN, CNS, and bulk g-C3N4 samples are recorded. The activity of the synthesized MCNS-4 is also compared with that of the commercial Degussa P25 TiO2 under the same conditions. Figure 10a illustrates that the MO concentration reaches the adsorption equilibrium after 60 min using the photocatalyst in the dark so that the adsorption ability of MO by P25 TiO2, bulk g-C3N4, CN, CNS, and MCNS-4 is equal to 16, 2, 10, 12, and 21%, respectively. The bulk g-C3N4 sample indicates lower photocatalytic activity, in which 8.8% of MO is degraded after 7073


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Langmuir carbon nitride body and consequently results in the high mobility of electron−hole and the photocatalytic activity. The presence of defects in the sample, according to the structural analysis, can also retard the recombination of electron−hole pairs and be helpful for the photocatalytic efficiency. In this study, the effect of unintentional SO42− or SO32− anions existing on the catalyst surface (which is confirmed by the XPS data) as inorganic oxidants on reducing the photocatalytic activity of MCNS-4 is investigated, and the results are presented in Figure 10d. A series of experiments were conducted to investigate the effect of SO42− as the inorganic oxidant (0.1 g/L Na2SO4) on the MO photocatalytic degradation using the optimized MCNS-4 sample. Moreover, to exclude the effect of the counter cation (i.e., Na+) on the photocatalytic activity, the influence of SO32− as another inorganic oxidant (0.1 g/L Na2SO3) was also assessed. The counter cation for the two added anions is Na+; therefore, only anions are involved in the final efficiency. We can observe from Figure 10d that both SO42− and SO32− anions have negative effects on the photocatalytic process and decrease the photocatalytic degradation rate of the MO dye. The reason why inorganic oxidants have negative effects on the photodegradation efficiency of MCNS-4 is probably due to the fact that SO32− and SO42− act as scavengers of •O2− and •OH radicals through the following reactions, which consequently terminate the radical formation.1 The negative effects of these inorganic anions are found to be in the order of SO32− > SO42−. SO4 2 − + •O2− → SO4•− + O2 2 −

(6)

SO4 2 − + •OH → SO4•− + OH−

(7)

SO32 − + •O2− → SO3•− + O2 2 −

(8)

SO4 2 − + •OH → SO3•− + OH−

(9)

Figure 11. Recyclability of the MCNS-4 photocatalyst in five experiments for the photocatalytic degradation of MO under visible light irradiation.

electronic structure of the photocatalyst. One of the reasons is that sulfur doping decreases the band gap of the semiconductor and results in the creation of more number of photoexcited charge carriers and consequently higher photocatalytic activity. In other words, sulfur doping along with the mesoporous structure obviously further suppresses the electron−hole pair recombination, as evidenced by the photoelectrochemical and PL results, which favorably increase the photocatalytic activity. It is generally accepted that the photogenerated holes (h+), superoxide radicals (•O2−), and hydroxyl radicals (•OH) are the three main active species generated in the advanced oxidation process, which can form by transferring electrons or holes to the surrounding molecules, as indicated in eqs 10−1112. The photocatalytic mechanism of the metal-free MCNS-4 is evaluated in detail via identifying the active species produced during the process of MO decomposition by the free radical and hole trapping experiments. In the present work, a series of trapping experiments were carried out by adding 5 mmol isopropanol (IPA), 5 mmol benzoquinone (BQ), and 5 mmol ethylenediaminetetraacetic acid (EDTA) into the reaction mixture as the •OH, •O2−, and h+ scavengers, respectively.55−58 As shown in Figure 12a, it can be found that the photodegradation rate of MO slightly decreases to ∼76% in the presence of IPA and EDTA, which is slightly lower compared with that of the reaction without using trapping agents. This result suggests that the •OH and h+ are not the crucial oxidative species for the photocatalytic removal of MO by MCNS-4. On the contrary, the photodegradation performance of MO by MCNS-4 is significantly declined by adding BQ into the reaction mixture, denoting that •O2− is the major oxidative species in the photodegradation of MO. On the basis of the above results, it can be seen that the superoxide radical can be a predominant active species that is responsible for the photocatalytic degradation of MO under visible light irradiation. The Mott−Schottky plot corresponding to the electrochemical impedance spectrum at a high frequency of 10 kHz is drawn to determine the flat band potentials of the MCNS-4 sample, which indicates a typical n-type characteristic (Figure

Reusability. Reusability is also a crucial parameter for a heterogeneous catalyst. In this regard, the reusability of the MCNS-4 catalyst was evaluated for the photocatalytic degradation of MO in five consecutive runs, and the results are shown in Figure 11. After each reaction, the photocatalyst was separated by centrifugation, washed several times with ethanol, and dried at 80 °C. As seen in Figure 11, MCNS-4 is quite stable after the fifth recycle, and no apparent deactivation of the photocatalyst is detected. Furthermore, no changes in the XRD and XPS results of MCNS-4 are observed before and after the fifth cycling reaction, indicating high stability of this catalyst (Figure S5). To show the importance of sulfur doping in modifying the carbon nitride materials, Table 4 summarizes the photocatalytic performances of sulfur-doped carbon nitride catalysts recently reported for pollutant degradation in aqueous solution.38,43,53,54 Of particular note is that the reaction conditions applied for every photocatalyst, especially pollutant in nature, and the light source differ from each other, which greatly affect the photocatalytic activity. Possible Photocatalytic Mechanism. The above experimental results, including textural, photoelectrochemical, and also photocatalytic properties, reflect that the photocatalytic activity of MCNS-4 having enhanced visible-light-harvesting ability is due to its high surface area and a well-developed mesoporous system. Indeed, the scattered light is collected by the carbon nitride framework through its mesopores as the antennas. In addition, chemical doping with sulfur modifies the 7074


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Table 4. Comparison of Photodegradation Performances of Various Pure S-Doped Carbon Nitride Catalysts Toward Pollutant Degradation in Aqueous Solution sulfur-doped carbon nitride CNS S-pg-C3N4 SCN g-CNS MCNS-4 a

pollutant concentration degradation of BPAa (50 mg L−1) degradation of RBb (10 mg L−1) degradation of RB (10 mg L−1) degradation of MBc (15 mg L−1) degradation of MO (10 mg L−1)

catalyst dose (g)

light source

irradiation time (min)

activity (%)

reference

0.06

150 W Philips

120

∼21

52

0.1

500 W xenon with 400 nm cutoff filter

120

∼77

44

0.05

150 W Philips with an UV cutoff filter (λ ≥ 420 nm) 500 W Xe lamp with an UV cutoff filter (λ ≥ 420 nm) 300 W halogen lamp (λ > 400)

120

∼40

53

120

∼60

38

240

∼82

this study

0.02 0.1

Bisphenol A. bRhodamine B. cMethylene blue.

Figure 12. (a) Influence of various trapping agents on the MO catalytic degradation in the presence of a MCNS-4 photocatalyst. (b) Schematic representation of the potential energy diagram for the MCNS-4 electrode.

S6). The flat band potential of MCNS is determined to be −1.1 eV, which accompanies the band gap energy obtained from the DRS and is used to estimate the CB and VB positions. Therefore, the CB and VB energies of MCNS-4 are calculated to be −1.1 and 1.31 V versus Ag/AgCl, respectively. The absolute energy scheme, including the CB and VB positions of MCNS versus NHE at pH = 7, is presented in Figure 12b. The reason why superoxide radical is found as the pivotal species in the photodegradation efficiency of MCNS-4 is probably due to the fact that the standard redox potential of O2/•O2− is −0.13 V (vs NHE, pH = 7),2 which is higher than the CB potential energy of sulfur-doped carbon nitride materials (−0.92 vs NHE), and therefore, eCB− can directly reduce O2 to generate •O2− radical on the surface of the photocatalyst to degrade the target pollutants (eq 11). Notably, the VB potential of a sulfurdoped carbon nitride (+1.49 V vs NHE, pH = 7)21 is lower than the standard redox potential of OH−/•OH (1.99 vs NHE, pH = 7);57 thus, it is deduced that the OH− anion cannot be oxidized to •OH by hVB+ (eq 10), but the •OH radicals can indirectly be produced in the reaction of oxygen reduction by eCB− (eq 12) to contribute in removing the organic pollutants. On the basis of the above discussion, it is expected that the •OH radicals play a minor role in the photodegradation of MO as confirmed by the trapping experiments. The basic mechanism of the photocatalytic MO degradation that takes place over the MCNS-4 photocatalyst is illustrated in Scheme 1. OH− + hVB+ → •OH

(10)

O2 + eCB− → •O2−

(11)

O2 + 2H+ + 3eCB− → •OH + OH−

(12)

Scheme 1. Mechanism of Visible Light Photocatalytic Degradation of MO over the MCNS-4 Photocatalyst

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SUMMARY AND CONCLUSIONS In summary, a series of MCNS materials were simply fabricated in situ by thermal condensation of thiourea in a SiO2 gel under air atmosphere, and their photocatalytic activities were investigated for the degradation of MO as a model pollutant in aqueous solution under visible light irradiation. The relationship between synthesis key factors and the photocatalytic activity was disclosed by the 3D response surface and the 2D contour plots. The photocatalysts obtained under different synthesis conditions were characterized by several techniques. Results demonstrated that the structural evolution 7075


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TiO2 nanotube arrays: A study of the structure−activity relationship. Appl. Catal., B 2016, 185, 119−132. (4) Zhao, Z.; Sun, Y.; Dong, F. Graphitic carbon nitride based nanocomposites: a review. Nanoscale 2015, 7, 15−37. (5) Thomas, A.; Fischer, A.; Goettmann, F.; Antonietti, M.; Müller, J.-O.; Schlögl, R.; Carlsson, J. M. Graphitic carbon nitride materials: variation of structure and morphology and their use as metal-free catalysts. J. Mater. Chem. 2008, 18, 4893−4908. (6) Li, S.; Dong, G.; Hailili, R.; Yang, L.; Li, Y.; Wang, F.; Zeng, Y.; Wang, C. Effective photocatalytic H2O2 production under visible light irradiation at g-C3N4 modulated by carbon vacancies. Appl. Catal., B 2016, 190, 26−35. (7) Hu, S.; Chen, X.; Li, Q.; Li, F.; Fan, Z.; Wang, H.; Wang, Y.; Zheng, B.; Wu, G. Fe3+ doping promoted N2 photofixation ability of honeycombed graphitic carbon nitride: The experimental and density functional theory simulation analysis. Appl. Catal., B 2017, 201, 58−69. (8) Chen, X.; Jun, Y.-S.; Takanabe, K.; Maeda, K.; Domen, K.; Fu, X.; Antonietti, M.; Wang, X. Ordered mesoporous SBA-15 type graphitic carbon nitride: A semiconductor host structure for photocatalytic hydrogen evolution with visible light. Chem. Mater. 2009, 21, 4093− 4095. (9) Wang, Y.; Zhang, J.; Wang, X.; Antonietti, M.; Li, H. Boron- and fluorine-containing mesoporous carbon nitride polymers: metal-free catalysts for cyclohexane oxidation. Angew. Chem., Int. Ed. 2010, 49, 3356−3359. (10) Ma, X.; Lv, Y.; Xu, J.; Liu, Y.; Zhang, R.; Zhu, Y. A strategy of enhancing the photoactivity of g-C3N4 via doping of nonmetal elements: a first-principles study. J. Phys. Chem. C 2012, 116, 23485− 23493. (11) Wang, Y.; Li, H.; Yao, J.; Wang, X.; Antonietti, M. Synthesis of boron doped polymeric carbon nitride solids and their use as metalfree catalysts for aliphatic C−H bond oxidation. Chem. Sci. 2011, 2, 446−450. (12) Wang, Y.; Di, Y.; Antonietti, M.; Li, H.; Chen, X.; Wang, X. Excellent visible-light photocatalysis of fluorinated polymeric carbon nitride solids. Chem. Mater. 2010, 22, 5119−5121. (13) Zhang, L.; Chen, X.; Guan, J.; Jiang, Y.; Hou, T.; Mu, X. Facile synthesis of phosphorus doped graphitic carbon nitride polymers with enhanced visible-light photocatalytic activity. Mater. Res. Bull. 2013, 48, 3485−3491. (14) Li, J.; Shen, B.; Hong, Z.; Lin, B.; Gao, B.; Chen, Y. A facile approach to synthesize novel oxygen-doped g-C3N4 with superior visible-light photoreactivity. Chem. Commun. 2012, 48, 12017−12019. (15) Zhang, Y.; Mori, T.; Ye, J.; Antonietti, M. Phosphorus-doped carbon nitride solid: enhanced electrical conductivity and photocurrent generation. J. Am. Chem. Soc. 2010, 132, 6294−6295. (16) Yan, S. C.; Li, Z. S.; Zou, Z. G. Photodegradation of rhodamine B and methyl orange over boron-doped g-C3N4 under visible light irradiation. Langmuir 2010, 26, 3894−3901. (17) Wang, X.; Blechert, S.; Antonietti, M. Polymeric graphitic carbon nitride for heterogeneous photocatalysis. ACS Catal. 2012, 2, 1596−1606. (18) Liu, G.; Niu, P.; Sun, C.; Smith, S. C.; Chen, Z.; Lu, G. Q.; Cheng, H.-M. Unique electronic structure induced high photoreactivity of sulfur-doped graphitic C3N4. J. Am. Chem. Soc. 2010, 132, 11642−11648. (19) Wang, K.; Li, Q.; Liu, B.; Cheng, B.; Ho, W.; Yu, J. Sulfur-doped g-C3N4 with enhanced photocatalytic CO2-reduction performance. Appl. Catal., B 2015, 176−177, 44−52. (20) Jourshabani, M.; Badiei, A.; Shariatinia, Z.; Lashgari, N.; Ziarani, G. M. Fe-Supported SBA-16 type cagelike mesoporous silica with enhanced catalytic activity for direct hydroxylation of benzene to phenol. Ind. Eng. Chem. Res. 2016, 55, 3900−3908. (21) 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, 15006−15012.

trend and consequently the textural and electronic properties of produced MCNS could be adjusted well by controlling the synthesis significant variables in mild conditions so that the sample synthesized at the reaction temperature of 500 °C for 240 min with the SiO2/thiourea weight ratio of 0.3 (MCNS-4) had the highest surface area, the most well-developed mesoporous system, enhanced visible-light-harvesting ability, and the narrowest band gap. More importantly, incorporation of sulfur into the carbon nitride framework inherently resulted in increasing charge transfer and electrical conductivity of the semiconductor. By monitoring the photocatalytic activity, it was found that the MCNS-4 powder indeed exhibited the highest MO degradation among bulk g-C3N4, CN (undoped g-C3N4), CNS (sulfur-doped g-C3N4 without template), and TiO2 (Degussa P25) samples, and this activity was elucidated via capturing experiments in detail. It was believed that the highest photoactivity of MCNS-4 was due to the prolonged lifetimes of photogenerated electron−hole pairs that originated from the co-contribution of their physical and chemical properties, which was confirmed by the photoelectrochemical and photoluminescence measurements.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.7b01767. SEM image of the MCNS-2 sample, expanded XPS spectrum of the O 1s region for the MCNS-4 sample, DRS and PL spectra of the bulk g-C3N4 and CNS samples, XRD patterns of the MCNS-4 sample before and after the cycling photocatalytic experiments, Mott− Schottky plot for the MCNS-4 sample, and ANOVA for the removal of MO (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zahra Shariatinia: 0000-0001-8533-6563 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support of this work by the Research Office of Amirkabir University of Technology (Tehran Polytechnic) is gratefully acknowledged. The authors are indebted to the High Performance Computing Research Center (HPCRC) at the Amirkabir University of Technology for providing all computer services (software and hardware).

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REFERENCES

(1) 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, 7159−7329. (2) Zhang, S.; Li, J.; Zeng, M.; Zhao, G.; Xu, J.; Hu, W.; Wang, X. In situ synthesis of water-soluble magnetic graphitic carbon nitride photocatalyst ands its synergistic catalytic performance. ACS Appl. Mater. Interfaces 2013, 5, 12735−12743. (3) Eskandarloo, H.; Hashempour, M.; Vicenzo, A.; Franz, S.; Badiei, A.; Behnajady, M. A; Bestetti, M. High-temperature stable anatase-type 7076


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