Facile Two-Step Synthesis of Porous Carbon Nitride with Enhanced

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Facile two-step synthesis of porous carbon nitride with enhanced photocatalytic activity using a soft template Qingyun Yan, Chaocheng Zhao, Liang Zhang, Yalu Hou, Shuaijun Wang, Pei Dong, Feifei Lin, and Yongqiang Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04873 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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ACS Sustainable Chemistry & Engineering

Facile two-step synthesis of porous carbon nitride with enhanced photocatalytic activity using a soft template Qingyun Yan1, Chaocheng Zhao1, *, Liang Zhang1, Yalu Hou1, Shuaijun Wang1, Pei Dong1, Feifei Lin1, Yongqiang Wang1, *

1.State Key Laboratory of Petroleum Pollution Control, China University of Petroleum (East China), Qingdao, 266580, PR China

*Corresponding Author Email:

[email protected] (C. Zhao) [email protected] (Y. Wang)

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Abstract: In this study, we successfully synthesized a thin-slice layer of graphitic carbon nitride (g-C3N4) with abundant irregular holes by a facile two-step way using Pluronic P123 as a template (CN-P123-x, where x represents the mass ratio of melamine to Pluronic P123). The characterization data suggest that the introduction and removal of Pluronic P123 altered the chemical material structure of the carbon nitride. The CNP123-x presented lamellar structure with irregular holes, whereas H-g-C3N4 (g-C3N4 prepared using a mild hydrothermal and calcination method without Pluronic P123 or HCl) has a dense blocky structure. Additionay, the prepared CN-P123-x exhibited an effective Rhodamine B (RhB) degradation rate of 98.7% within 40 min of illumination. The optimal photocatalytic activity of CN-P123-6 for degrading RhB was 13.9 times greater than that of H-g-C3N4 in terms of the kinetic constant. Futhermore, the H2 evolution rate of CN-P123-6 can reach 1074.9 μmol∙g−1∙h−1, whereas that of H-g-C3N4 is only 3.1 μmol∙g−1∙h−1. It is worth noting that the adoption of HCl (H-g-C3N4-HCl) and Pluronic P123 (CN-P123-6 without HCl) alone has no insignificant effect on photocatalytic performance. The intensive activities are on account of the irregular pores in the thin slice, which increase the specific surface area of the sample and provide additional active sites for reaction. This work provides an excellent basis for improving the performance of the photocatalytic degradation and hydrogen production of carbon nitride. Keywords: H-g-C3N4; CN-P123-x; Rhodamine B (RhB); photocatalytic degradation; hydrogen production


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Introduction Semiconductor photocatalysts can covert solar energy into clean fuels, which is important for the sustainable development of human society. This has become a popular strategy worldwide.1–7 To date, many active photocatalysts have been prepared and reported, including (oxy) nitrides, (oxy) sulfides, and silicides. However, reported semiconductor photocatalysts have the most significant restrictions on their solar light energy conversion efficiency based on their limited UV-light responsiveness and wide bandgap.8–14 Therefore, it is imperative to develop highly stable, easily accessible, and responsive materials in the visible range to with appropriate bandgaps to trigger efficient degradation and hydrogen generation in the visible region. Lately, graphitic carbon nitride (g-C3N4) has been researched as a promising polymer photocatalyst due to its unique properties, including a two-dimensional metalfree structure, excellent thermal and chemical stability, low cost, and relatively applicable band energy (~2.7 eV).15–21 This material has already seen promising applications in several relevant fields, such as the photodegradation of organic pollutants,22–23 photolysis of water,24–26 and reduction of CO2.27–29 Currently, the conventional g-C3N4 material is generally prepared via self-condensation of melamine at a high temperature of 550 °C.30–31 Although this method is simple and easy to implement, the obtained g-C3N4 tends to have a small surface area (100 nm). The large pores may be caused by the agglomeration of Pluronic P123 during its introduction. The BET results for CN-P123-6 without HCl and H-g-C3N4 show no significant differences, suggesting that the presence of HCl is beneficial for the introduction and removal of



Pluronic P123, which is pivotal for the construction of porous frameworks. In comparison with H-g-C3N4, H-g-C3N4-HCl with HCl alone exhibited increased specific surface area, but the results are entirely different from those of CN-P123-x. This may be caused by the etching effect of HCl on the structure, which would explain why the combination of Pluronic P123 and HCl can affect the specific structure of a sample significantly. The data which listed in Table 1 reveals that CN-P123-x has a significantly higher specific surface area compared to H-g-C3N4, as well as a larger pore volume. With an increase in the ratio of melamine to Pluronic P123, the specific surface area first increased, then decreased, whereas the pore size and pore showed an opposite trend. The CN-P123-6 sample has the largest specific surface area. Additionally, CNP123-x has the same C/N ratio as H-g-C3N4, CN-P123-6 without HCl, and H-g-C3N4HCl, indicating that Pluronic P123 only played a structurally orientation role and did not serve as a C source during sample preparation.


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Fig. 5.

N2 absorption-desorption isotherms (a), and the pore size distribution curves (b) of the CN-P123-x, H-g-C3N4, CN-P123-6 without HCl and H-g-C3N4-HCl samples.

Table 1

Texture parameters of the CN-P123-x, H-g-C3N4, CN-P123-6 without HCl and H-gC3N4-HCl samples.

Samples

SBET /(m2/g)

Dpore /(nm)

Vpore /(cm3/g)

C/N

CN-P123-4

63.69

25.04

0.40

0.66

CN-P123-5

69.22

23.52

0.40

0.66

CN-P123-6

73.29

15.01

0.27

0.66

CN-P123-7

63.76

17.65

0.29

0.66

H-g-C3N4

9.75

27.06

0.061

0.66

CN-P123-6 without HCl

11.63

23.37

0.066

0.66

H-g-C3N4-HCl

40.21

24.43

0.35

0.66

To analyze the chemical compositions and states of the CN-P123-6 and H-g-C3N4 samples, we performed high-resolution XPS characterization. From Fig. 6a, one can conclude that the CN-P123-6 and H-g-C3N4 samples consist of C, O, and N elements in the survey spectra. The C 1s spectra (Fig. 6b) can be deconvoluted into three peaks. The first is a peak at 288.1 eV, which is vest in sp2 N-C=N. A second peak at 286.1 eV can be identified as sp2-coordinated carbon in the C-N-C groups. Finally, according to



a previous study,38 the 284.7 eV peak is contributed from sp2 graphitic carbon (C-C bonding). In Fig. 6c, the N 1s XPS spectra of the CN-P123-6 and H-g-C3N4 samples were resolved into four peaks at 404.2 eV, 400.7 eV, 399.6 eV, and 398.6 eV, which are consistent with sp2-bonded N in the aromatic heterocycles, surface uncondensed amino functions (C-N-H), N-(C)3 groups, and sp2 C-N=C bonds,39 respectively. The high-resolution O 1s peak (Fig. 6d) exhibits two prominent peaks at approximately 532.1 eV and 533.4 eV, which may be due to C-OH bonding and the O2 adsorbed on the surface.40 The above results indicate that the introduction and removal of Pluronic P123 did alter the chemical compositions and states of the samples.


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Fig. 6.

The XPS of the CN-P123-6 and H-g-C3N4 samples for the survey spectrum (a), C 1s (b),



N 1s (c), and O 1s (d).

Figure 7a presents the UV–Vis diffuse reflection spectra of the CN-P123-x, H-gC3N4, CN-P123-6 without HCl, and H-g-C3N4-HCl samples. Compared to H-g-C3N4, the absorption edge shows a slight hypsochromic shift for CN-P123-x, which may be caused by quantum confinement effects. Additionally, the H-g-C3N4-HCl sample shows an apparent blue shift. CN-P123-6 without HCl and H-g-C3N4 show very similar absorption edges. We plotted (αhν)1/2 against photon energy (hν) to obtain the corresponding bandgaps, as shown in Fig. 7b). The bandgaps of CN-P123-6 and H-gC3N4 were estimated as 2.75 and 2.70 eV. Furthermore, the Mott-Schottky curves presented in Fig. 6c identified the conduction band minimums. According to the intersection of the curve extension line and abscissa, the flatband potentials of CNP123-6 and H-g-C3N4 are approximately −0.86 and −0.75 V versus the SCE (−0.62 and −0.51 V versus NHE). Based on the bandgap data (Fig. 7b), the approximated positions of the valence band maximums are 2.13 and 2.19 V versus the NHE for CN-P123-6 and H-g-C3N4, respectively. The schematic bandgap structures of the CN-P123-x and H-gC3N4 samples are presented in Fig. 7d.


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Fig. 7.

UV–vis diffuse reflection spectra of CN-P123-x, H-g-C3N4, CN-P123-6 without HCl and

H-g-C3N4-HCl samples (a); the bandgap determination plots (b); Mott–Schottky plots (c); schematic bandgap structures (d) of CN-P123-6 and H-g-C3N4 samples.

To analyze the separation of photogenerated charge carriers, PL spectroscopy measurements were performed. As shown in Fig. 8, the PL peaks of all CN-P123-x samples shifted slightly toward longer wavelengths as the ratio of melamine to Pluronic P123 increased with an excitation wavelength of 320 nm. The CN-P123-6 sample presented the lowest peak strength, indicating that it provides high-efficiency termination of the photogexcited electron-hole pairs. Additionally, the PL peaks of CNP123-6 without HCl and H-g-C3N4-HCl samples show a slight shift, but no noticeable change in intensity, implying that the introduction of either Pluronic P123 or HCl alone did not cause any significant changes in photogenerated charge carrier transfer efficiency.


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Fig. 8.

The photoluminescence spectra of CN-P123-x, H-g-C3N4, CN-P123-6 without HCl and H-g-C3N4-HCl samples.

In order to elucidate the separation and transfer efficiency of photogenerated electron-hole pairs in the photocatalysts, the CN-P123-x, H-g-C3N4, CN-P123-6 without HCl, and H-g-C3N4-HCl samples were analyzed through photoelectrochemical (electrochemical impedance spectroscopy (EIS) and photocurrent) experiments. The magnitude of the Nyquist plot radius from the EIS experiment reflects the magnitude of the charge transfer rate. From Fig. 9a, one can see that compared to H-g-C3N4, the CN-P123-x samples have smaller radii, particularly CN-P123-6, implying that CNP123-6 has efficient charge separation capability and fast interface charge transfer capability. CN-P123-6 without HCl has a similar Nyquist radius to H-g-C3N4, whereas H-g-C3N4-HCl has a smaller radius than H-g-C3N4, which may be the result of protonation by the HCl, but its radius is far smaller than that of CN-P123-x, suggesting that the samples achieve optimal separation and transfer of photoexcited charge carriers when Pluronic P123 is introduced and removed using HCl. Additionally, from Fig. 9b,



one can see that the CN-P123-x samples have larger photocurrent densities than H-gC3N4, with CN-P123-6 exhibiting the largest value at nearly three times the value of Hg-C3N4. Consistent with the previous results, CN-P123-6 without HCl and H-g-C3N4HCl did not exhibit high photocurrent strengths.

Fig. 9.

The electrochemical impedance spectra (a) and periodic on/off photocurrent response

under visible light irradiation (b) of CN-P123-x, H-g-C3N4, CN-P123-6 without HCl and H-gC3N4-HCl samples.


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The photocatalytic activity of the CN-P123-x, H-g-C3N4, CN-P123-6 without HCl, and H-g-C3N4-HCl samples was measured based on the photodegradation of RhB. As shown in Fig. 10a, after 40 min of photocatalytic reaction, H-g-C3N4 and CN-P123-6 without HCl showed relatively poor photocatalytic performance. In contrast, the photocatalytic activity of the CN-P123-x samples were remarkable, particularly CNP123-6, which removed 98.7% of the RhB within 40 min. This excellent performance can be attributed to a loose and porous structure, which provides more active sites for reaction. It is worth noting that H-g-C3N4-HCl exhibits high adsorption capacity based on protonation, but its capacity for the photocatalytic degradation of RhB is still inferior to that of CN-P123-x within 40 min of illumination. The above experimental results agree well with first-order kinetic calculations. We can derive the rate constant (k) based on the fitting equation ln(C0/C) = 𝑘𝑡,

(1)

where C0 and C are the initial concentration and the instant concentration at reaction time t under illumination of the system, respectively, k is the first-order reaction kinetic rate constant (min−1), and t is the visible light illumination time (min) (Fig. 10b). Fig. 10c displays the rate constants for each sample, indicating that the k values of CN-P123-x are significantly higher than those of H-g-C3N4, CN-P123-6 without HCl, and H-g-C3N4-HCl. Among the three latter samples, H-g-C3N4-HCl has the greatest k value based on protonation, but its value is still far less than that of CNP123-x. The first-order rate constant is 13.9 times higher for CN-P123-6 compared to H-g-C3N4.



After five subsequent experimental cycles, no apparent decreases were observed in the data presented in Fig. 10d, indiciating that the CN-P123-6 sample has excellent chemical stability and recyclability. These results demonstrate that compared to H-gC3N4, CN-P123-x has greater photocatalytic degradation activity, which may be due to the porous structure obtained through the introduction and removal of Pluronic P123 using a mild hydrothermal method. This structure provides additional active sites for reaction.


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Fig. 10.

The RhB degradation under visible light irradiation (a); the corresponding first-order-

kinetic plots (b); the corresponding apparent rate constants k of CN-P123-x, H-g-C3N4, CN-P1236 without HCl and H-g-C3N4-HCl samples. The stability test of CN-P123-6 driven by visible light (d).

The photodegradation mechanism of the system was determined through the addition of a series of radical scavengers, namely IPA (•OH scavenger),41 TEOA (h+ scavenger),6, 42 and BQ (•O2− scavenger). Fig. 11 reveals that the degradation of RhB decreased insignificantly when IPA was injected into the system, but was noticeably inhibited following the dispersion of TEOA and BQ, indicating that the radicals that play major roles in the RhB degradation process are both holes (h+) and •O2−, rather than •OH radicals.


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Fig. 11.

The RhB photodegradation of the CN-P123-6 with the addition of different scavengers.

We further evaluated the photocatalytic activities of the CN-P123-6, H-g-C3N4, CN-P123-6 without HCl, and H-g-C3N4-HCl samples based on the level of H2 generation under illumination. From Fig. 12, one can see that CN-P123-6 showed remarkably enhanced photocatalytic activities in comparison with H-g-C3N4, CNP123-6 without HCl, and H-g-C3N4-HCl. The H2 evolution rate of CN-P123-6 was determined to be 1074.9 μmol∙g−1∙h−1, which is dramatically higher than that of H-gC3N4 (3.1 μmol∙g−1∙h−1), CN-P123-6 without HCl (2.6 μmol∙g−1∙h−1), or H-g-C3N4-HCl (4.3 μmol∙g−1∙h−1). These results reveal that CN-P123-6 not only has high photocatalytic degradation activity, but also has excellent photocatalytic hydrogen production performance.



Fig. 12.

photocatalytic hydrogen evolution of CN-P123-6 and H-g-C3N4 under visible light irradiation.

To determine the stability of CN-P123-6, XRD, FT-IR, and XPS analyses of both fresh and used photocatalysts were performed. The XRD patterns (Fig. 13a) and FT-IR spectra (Fig. 13b) imply that there were no noticeable changes in the crystal or chemical structures of the CN-P123-6 following the photocatalytic reactions. The XPS spectral data also corroborates this result (Figs. 13c and 13d). The C 1s and N 1s spectra of CNP123-6 before and after photocatalytic reaction show no differences, indicating that the chemical states of the C and N elements of CN-P123-6 remained unchanged, demonstrating the excellent stability of the CN-P123-6 photocatalyst.


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Fig. 13.

XRD (a), FT–IR (b), XPS C 1s (c) and N 1s (d) patterns for used sample and fresh sample.

Conclusions We successfully synthesized porous carbon nitride using a facile two-step synthesis method. Based on the original state of melamine, Pluronic P123 was introduced and removed using a mild hydrothermal method and calcination method, and thin-slice layers labelled as CN-P123-x with abundant irregular holes were obtained. The optimal sample, namely CN-P123-6, exhibited excellent photocatalytic degradation activity and photocatalytic hydrogen production performance, performing 13.9 times better than H-g-C3N4 in terms of the kinetic constant and achieving an H2 production rate of 1074.9 μmol∙g−1∙h−1. Additionally, the introduction of Pluronic P123 or HCl alone did not cause remarkable changes in the structural compositions and properties of the samples. A thin-slice structure with irregular pores can effectively increase the specific surface area of as-prepared samples and provide additional reactive sites for photocatalytic activity. In summary, this work provides a basis for improving


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photocatalysts by controlling their morphology and provides useful information regarding photocatalytic degradation and H2 evolution driven by visible light. Acknowledgments The authors thank for the National Science and Technology Major Project (NO. 2016ZX05040003) and the Fundamental Research Funds for the Central Universities (NO. 17CX06027, NO. 18CX06068A). Notes There is no conflict to declare.



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Graphical Abstract The graphitic carbon nitride (g-C3N4) with porous thin-slice layer was synthesized by a facile two-step way using Pluronic P123 as a template.

Illustration of the preparation process of CN-P123-x and H-g-C3N4 samples.


Facile Two-Step Synthesis of Porous Carbon Nitride with Enhanced

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