English

• 0.   Introduction

  In the past decades, volatile organic compounds (VOCs) emitted from various industrial processes, fuel combustion, building materials and furniture are major gaseous pollutants that adversely affect human health^[1]. For instance, toluene is a typical VOCs that can cause skin inflammation, respiratory diseases, acute and chronic poisoning^[2]. Up to now, different methods have been developed to control VOCs in the ambient air, including absorption, condensation, membrane separation, biological degradation and photocatalytic oxidation (PCO) ^[3-8]. Among them, PCO technology is a new advanced technology for VOCs degradation with low cost, mild operation conditions and no secondary pollution. In particular, graphitic carbon nitride (g-C₃N₄) is a typical metal-free polymer semiconductor material with a band gap suitable for absorbing visible light radiation, unique 2D structure, good chemical stability and adjustable electronic structure^[9-10]. However, its photocatalytic activity is still limited by the inevitable disadvantages such as low efficiency of visible light utilization, fast recombination speed of photoelectron-hole pair and insufficient specific surface area. Therefore, some approaches have been employed to improve the photocatalytic activity of g-C₃N₄, including nano/mesoporous structures design, elements doping, forming heterojunction structure, and so forth^[11-13]. Particularly, the modification of semiconductors with precious metal particles, such as platinum nanoparticles (Pt NPs), can significantly improve the photocatalytic activity of semiconductors due to the surface plasmon resonance (SPR) effect^[14]. However, the rare and expensive precious metal platinum greatly hindered the expansion of industrial scale. Consequently, it is necessary to reduce the amount of Pt without degradation of photocatalytic performance. It is reported that the size effect of metal particles has a great influence on the catalytic performance^[15-16]. The catalytic efficiency of subnanoscale metal clusters is always better than that of nano-scale metal clusters. Ideally, reducing the size of platinum catalysts to atomic clusters or even single atoms is expected to maximize the utilization efficiency of atoms, which has become the most active new frontier in various catalytic reactions^[17-18]. For example, Xiong et al. demonstrated that the synergistic effect between monatomic Pt and C₃N₄ can expand the light absorption and enhance the photocatalytic performance^[19]. Hu et al. synthesized a single-atom dispersed g-C₃N₄-Pt nanohybrids which showed an enhanced catalytic activity and high stability for methanol oxidation^[20]. However, as far as we know, no studies have investigated the photocatalytic degradation of toluene by g-C₃N₄ supported Pt photocatalyst.

  We synthesized a holey ultrathin g-C₃N₄ nanosheets (CNHS) supported Pt photocatalyst (Pt-CNHS) by a two steps method: thermal oxidation etching and photochemical reduction. The internal pores in the g-C₃N₄ layer can provide more active sites for photocatalytic reaction^[21]. In addition, the crosslayer diffusion path generated by the holes can improve the mass transfer efficiency and electron distribution efficiency of CNHS, which is beneficial to the improvement of photocatalytic performance. The photocatalytic properties of the samples were studied by photodegradation of toluene in gas phase. As expected, the photocatalytic activity of Pt-CNHS was higher than that of pure g-C₃N₄ and CNHS under visible light. In addition, the path for the degradation of toluene by Pt-CNHS photocatalyst was also proposed and discussed.

  1.   Experimental

  1.1   Preparation of the samples

  CNHS were prepared via a thermal oxidation etching method according to the literature^[12]. In a typical process, 10 g melamine powder was uniformly spread into an alumina crucible and heated at 550 ℃ for 10 h with a heating rate of 2 ℃·min^-1 under air atmosphere. g-C₃N₄ (CNB) was prepared by melamine calcination for 4 h under the same condition.

  The synthesis of Pt-CNHS was slightly modified on the basis of previous reports^[22-24]. Typically, CNHS (50 mg) was dispersed in 20 mL water under ultrasound and 2.3 mL isopropanol was added as hole scavenger. Then, 0.5 mL of K₂PdCl₆ solution (5 mmol·L^-1) was added into the CNHS dispersion under stirring. The suspension was rapidly frozen by liquid nitrogen and followed by irradiating under a 500 W Xe light with the light filter of 420 nm for 10 min. The obtained precipitates were collected by centrifugation and washed with water and ethanol. Finally, the precipitates were dried in an oven at 60 ℃ for 12 h.

  1.2   Characterization of the photocatalysts

  X -ray diffraction (XRD) data were obtained on an X-ray diffractometer (Smart Lab, Rigaku) operated at 40 kV and 30 mA with Cu Kα X-ray radiation source (λ =0.154 nm) and 2θ range of 10° ~60°. Field emission scanning electron microscopy (FESEM) and energy disperse spectroscopy (EDS) spectra were obtained on a SUPRA55 FESEM at the acceleration voltage of 5 kV. High resolution images were taken by transmission electron microscopy (TEM, JEM-2100) at 160 kV. Fourier transform infrared (FT -IR) spectroscopy were performed on a BRUKER-ALPHA FT-IR spectrometer. The X-ray photoelectron spectroscopy (XPS) were carried out on Thermo Scientific Escalab 250Xi equipped with an Al Kα monochromatic X-ray source (hν = 1 486.7 eV) with a line width of 0.20 eV in an analysis chamber at a bass pressure of less than 4.3×10^-8 Pa. UV-Vis diffuse reflectance spectra (UV-Vis DRS) of the samples were measured by using a UV-Vis spectrophotometer (UV-3600, Shimadzu) with an integrating sphere attachment. Shimadzu RF-5301 fluorescence spectrophotometer was used to obtain photoluminescence (PL) with an excitation wavelength of 325 nm. The content of Pt elements in the as-prepared sample was analyzed by an inductively coupled plasmaatomic emission spectrometer (ICP-AES) on Perkin Elmer Dptima 2100DV. The N₂ adsorption-desorption of the samples was tested with the Micromeritics ASAP2020 nitrogen adsorption apparatus, and Brunauer-Emmett-Teller (BET) specific surface areas of the samples were calculated. The electrochemical properties of the samples were investigated on an electrochemical workstation (CHI660B, Chen Hua Instruments, Shanghai, China).

  1.3   Photocatalytic activity

  The photocat alytic degradation of gaseous toluene was carried out in a high-pressure cylindrical quartz glass reactor with an effective volume of 0.8 L with reflux water. The UV and visible light were provided by a 250 W high-pressure mercury lamp (GY-250) and a 500 W xenon lamp (GX500) with a UV-cutoff filter (λ ≥420 nm), respectively. In a typical experiment, the catalyst (0.2 g) was dispersed in 5 mL ethanol and then ultrasonically treated for 30 min and uniformly coated on polymethyl methacrylate (PMMA, 2 cm×15 cm) substrate. The catalyst was dried and placed at the bottom of the reactor. The gaseous toluene was then mixed with the synthetic air (Volume fraction: 79.5% for nitrogen, 20.5% for oxygen) at room temperature into the reactor until the concentration of the gaseous toluene stabilized at 370 mg·L^-1. After 1 h of adsorption equilibrium in the dark, the photoreaction started. With the proceeding of reaction, 100 µL of gas samples were taken from the reactor every once in a while, and the concentration of gaseous toluene was analyzed by gas chromatogram (GC1100, Persee, Beijing, China) equipped with a flame ionization detector. For comparison, the reactions were carried out under the same conditions in the presence of CNB or CNHS or Pt-CNHS or in the absence of catalyst. The degradation rate was calculated as c/c₀, where c is the gaseous toluene concentration at time t and c₀ is the initial concentration at the beginning of photoreaction after adsorption equilibrium.

  2.   Results and discussion

  The XRD patterns of as-prepared CNB, CNHS and Pt-CNHS are shown in Fig. 1a. It was observed that CNB showed one diffraction peak of (100) plane at 2θ= 12.9° with respect to the characteristic interlayer structural packing, and another diffraction peak of (002) plane at 27.4° corresponding to the interplanar stacking peaks of the aromatic systems^[25]. The decreased intensity of peak at 12.9° is mainly due to the fact that the oxidation etching parts of tri-s-triazine (melem) units during the long-time calcination may decrease the ordering degree of in-plane structural units. The decreased intensity of peak at 27.4° verified that the layered CNB has been successfully exfoliated into nanosheets^[26]. The peak of CNB shifted slightly to the right, indicating that the channel distance between the nanosheets was reduced^[27]. Meanwhile, the introduction of Pt may slightly reduce the interlayer spacing of the nanosheets, thus leading to an increase in the peak strength of Pt-CNHS at 27.7°. Compared with bare CNHS, the diffraction pattern of Pt-CNHS has no obvious difference, which indicates that the introduction of Pt has no obvious effect on the crystal structure of CNHS. Nevertheless, the diffraction peaks of Pt element did not be detected in the pattern of Pt-CNHS, which may be due to its low loading content and small size. Fig. 1b depicts the FT -IR spectra of CNB, CNHS and Pt-CNHS. As for bare CNB, the peak at 813 cm^-1 presents the characteristic breathing mode of triazine units, the strong band of 1 200~1 700 cm^-1 corresponds to the typical stretching vibration of C—N heterocycles, and the broad peak around 3 000~3 500 cm^-1 can be assigned to the stretching vibration of N—H^[28]. It is clear from Fig. 1b that the structures of CNHS and PtCNHS have not changed. These results confirm that Pt loading has no effect on the CNHS structure.

  Figure 1

  

  Figure 1.  (a) XRD patterns and (b) FT-IR spectra of CNB, CNHS and Pt-CNHS

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  The morphology and detailed structure of the prepared samples were investigated by FESEM and TEM as shown in Fig. 2. In Fig. 2a, the aggregated edge of CNB displays 2D lamellar structures. Therefore, CNBs can be stripped into nanosheets by further heat treatment. The TEM image shown in Fig. 2b clearly shows that CNHS has large pores and the corresponding FESEM image inset of Fig. 2b demonstrate that CNHS has a large number of in-plane holes, the surface is no longer smooth, and the surface becomes rougher due to oxidation corrosion. After Pt was loaded on CNHS, its structure did not change significantly (Fig. 2c). Specifically, no obvious Pt particle or cluster was observed, thus suggesting highly uniform Pt loading on the CNHS. The elemental mapping of Pt-CNHS (Fig. 2d) shows that the Pt element was homogeneously dispersed in the whole region, which is highly consistent with the above TEM observations. And the corresponding EDS spectrum of Pt-CNHS is shown in Fig. 2e, indicating that Pt is definitely present on the photocatalyst.

  Figure 2

  

  Figure 2.  TEM images of (a) CNB, (b) CNHS (Inset: FESEM image) and (c) Pt-CNHS; (d) Element mappings of Pt-CNHS; (e) EDS spectrum of Pt-CNHS

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  The surface chemical composition of the composite material was analyzed by XPS. As shown in Fig. 3a, the C1s peaks were at 285.5 and 284.8 eV, assigned to C—(N)₃ in CNHS. The characteristic peak at 281.5 eV is attributed to the C—C bond in the materials^[29]. Fig. 3b shows the N1s XPS spectrum of Pt-CNHS. The main peak at 395.3 eV can be attributed to C—N=C (sp² hybridized nitrogen), which consists of the triazine ring of CNHS. The peaks at 396.1 and 397.8 eV correspond to the N—(C)₃ and C—N—H groups, respectively. And the peak at 401.1 eV is ascribed to amino functional group (C—N—H) of CNHS^[30]. The Pt4f XPS spectrum in Fig. 3c can be fitted into two peaks for Pt4f_7/2 at 69.6 eV and Pt4f_5/2 at 72.8 eV^[31]. According to the ICP result, the mass fraction of Pt element in Pt-CNHS was approximately 0.83%.

  Figure 3

  

  Figure 3.  XPS high-resolution spectra of Pt-CNHS: (a) C1s, (b) N1s and (c) Pt4f

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  As depicted in Fig. 4, CNHS exhibited a typical Ⅳ isotherm with a high adsorption capacity in a p/p₀ range of 0.5~1, suggesting the presence of abundant meso- and macropores. The calculated BET surface area of CNHS and Pt-CNHS composites were 203 and 139 m²·g^-1, respectively, which were much higher than that of CNB (26 m²·g^-1). The pore size distribution peak of CNB in Fig. 4b was not obvious, while those of CNS and Pt-CNHS at 2.7 nm increased slightly. Notably, the pore size distributions of both samples are broad, which across the mesopore to macropore range and center at about 2.7 nm.

  Figure 4

  

  Figure 4.  (a) N₂ adsorption-desorption isotherms and (b) corresponding pore size distribution curves of as-prepared CNB, CNHS and Pt-CNHS

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  The optical properties of CNB, CNHS and Pt-CNHS were investigated by UV-Vis DRS. The results are shown in Fig. 5a. UV-Vis DRS spectra showed that, compared with CNB, the intrinsic absorption edge of CNHS had a slight blue shift. One reason may be the well-known quantum confinement effect^[21]. Another reason for the larger band gap is that the presence of holes in the plane will reduce the conjugated system of g-C₃N₄. Compared with the absorption spectra of CNB and CNHS, Pt-CNHS had a wider absorption range in a range of 200~800 nm, indicating that the introduction of Pt has a positive effect on the optical properties. The photoluminescence spectroscopy was used to study the recombination rate of photoinduced electron-hole pairs. It is generally believed that lower emission intensity of PL indicates lower recombination of photogenerated electron-hole pairs^[32]. As shown in Fig. 5b, Pt-CNHS showed the lowest emission peak intensity relative to CNB and CNHS, indicating that Pt-CNHS has the lowest photoexcited electron and hole recombination rate. The results show that the introduction of Pt can effectively inhibit the recombination rate of photocarriers, thereby generating more active groups and improving photocatalytic performance.

  Figure 5

  

  Figure 5.  (a) UV-Vis DRS spectra and (b) PL emission spectra of as-prepared samples

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  In order to study the separation efficiency of photogenerated carriers, photochemical measurements were carried out. Fig. 6a displays the transient photocurrent responses of CNB, CNHS and Pt-CNHS in several light on-off cycles. The intensity of photocurrent for CNB was weak, indicating the quantity and migration speed of charge carriers is low. Compared with CNB, CNHS exhibited a higher transient photocurrent intensity, which may be due to the presence of a large number of in-plane holes in CNB, which facilitates mass transfer and improves the mobility of photogenerated charges. Obviously, Pt-CNHS exhibited a much higher photocurrent density than CNHS and CNB, which indicates that the introduction of Pt can further reduce the electron and hole recombination rate. Electrochemical impedance spectroscopy (EIS) was also performed for the samples, and the results are shown in Fig. 6b. Obviously, Pt-CNHS had the smallest arc curvature radius, indicating that its electron-hole pair separation and electron transfer efficiency were the highest, which is consistent with the photocurrent response results.

  Figure 6

  

  Figure 6.  (a) Photocurrent response curves and (b) Nyquist plots of CNB, CNHS and Pt-CNHS

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  The photocatalytic activities of the as-prepared samples were evaluated by the photodegradation of gaseous toluene under UV and visible-light irradiation. As shown in Fig. 7a, the blank experiment indicated that the degradation rate of gaseous toluene was 19% by direct UV photolysis in the absence of photocatalyst. For comparison, the activities of CNB, CNHS and Pt-CNHS were also tested under the same conditions. CNB and CNHS displayed a certain photocatalytic efficiency of 29% and 52% after UV light irradiation for 50 min, respectively. As expected, Pt-CNHS exhibited higher photocatalytic activity than CNHS, and provided degradation rate of gaseous toluene being 84% under UV light irradiation. As shown in Fig. 7b, the photocatalytic degradation rates of gaseous toluene over asprepared catalysts followed pseudo-first-order kinetics and the kinetic model can be expressed by equation ln(c₀/c)=kt, where k is the kinetic rate constant. It can be found that the k of Pt-CNHS (0.036 7 min^-1) was about 2.5 times that of CNHS (0.014 7 min^-1) and about 5.4 times that of CNB (0.006 8 min^-1). To broaden its application in the whole range of sunlight, the photocatalytic performance of the catalysts for gaseous toluene photodegradation was also conducted under visiblelight irradiation, as shown in Fig. 7c. It can be found that gaseous toluene was rarely degraded without photocatalysts in the control test, indicating that the self-photolysis of gaseous toluene could be ignored. Obviously, the photocatalytic activity of Pt-CNHS was much higher than those of CNHS and CNB, indicating that the introduction of Pt has a significant effect on their photocatalytic performance. As shown in Fig. 7d, the apparent rate constant of gaseous toluene photodegradation can be calculated to be 0.38 h^-1 for Pt-CNHS, which was 7.6 and 3.1 times higher than those of CNB (0.05 h^-1) and CNHS (0.124 h^-1), respectively. Furthermore, the stability of Pt-CNHS was investigated by recycling the photocatalyst for repeated visible light driven photodegradation reactions. The results are displayed in Fig. 8a. Pt-CNHS for photocatalytic decomposition of gaseous toluene showed a slight decline rather than a significant loss of activity after five cycles, where the photocatalytic efficiency reduced only 0.052%, suggesting that the photocatalyst was stable. In addition, the TEM image of Pt-CNHS after five cycles is shown in Fig. 8b. Obviously, the morphology of Pt-CNHS hardly changed during the cycle, which indicated that prepared Pt-CNHS did not undergo photodissolution.

  Figure 7

  

  Figure 7.  Photocatalytic activity and kinetics of as-prepared photocatalyst to degrade gaseous toluene under (a, b) UV light and (c, d) visible light

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  Figure 8

  

  Figure 8.  (a) Cycle stability of Pt-CNHS under visible light irradiation, and (b) TEM image of Pt-CNHS after visible light photocatalytic degradation of gaseous toluene

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  Fig. 9a shows the GC -MS chromatogram of organic by-products produced in the process of photocatalytic degradation of gaseous toluene by Pt-CNHS. As can be seen from Fig. 9a, five by-products were identified, including benzaldehyde, benzoic acid, phenol, formic acid and acetic acid. Fig. 9b shows the possible pathways of toluene decomposition by Pt-CNHS under light, which based on the suggestion that toluene could be destructed mainly by electron impact and active species oxidation. Generally, the destruction pathway of toluene is closely related to the bond energy of chemical groups. The dissociation energy of C—H bonds in methyl is 3.7 eV, which is smaller than that in aromatic rings (4.3 eV), C—C bond energy between methyl and aromatic rings (4.4 eV), C—C bond energy (5.0~5.3 eV) and C=C bond energy (5.5 eV) on aromatic rings^[33]. The main pathway of toluene oxidation is to extract H from methyl group by high energy electron. Hydrogen is extracted from methyl to form benzyl radical, which reacts with O or ·OH to form benzaldehyde^[34]. Benzaldehyde may be further oxidized to benzoic acid. These aromatic intermediates are further attacked by high-energy electrons, causing the aromatic rings to break. The C—C between methyl and toluene rings can be interrupted to form phenyl groups, which can combine with OH to form phenol^[35]. The compounds generated after the ring opening are substances with small molecular mass, such as formic acid and acetic acid. The reaction proceeds by a series of oxidation step by ·OH/O attack, eventually producing harmless CO₂ and H₂O.

  Figure 9

  

  Figure 9.  (a) GC-MS chromatogram of the organic by-products in degradation of toluene by Pt-CNHS and (b) pathway of degradation of toluene by Pt-CNHS

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  3.   Conclusions

  To summarize, Pt-CNHS photocatalyst was synthesized via thermal oxidation etching and in-situ photocatalytic reduction method. The as-prepared Pt-CNHS photocatalyst exhibited significantly enhanced photocatalytic activities toward gaseous toluene degradation and the degradation rate was nearly 7.6 and 3.1 times higher than those of CNB and CNHS under visible light, respectively. Various in-plane pores on CNHS layer can provide more active sites for the photocatalytic reaction, and the introduction of Pt expands the absorption range, and the combination with CNHS can effectively separate photogenerated carriers and improve photocatalytic activity. In addition, Pt-CNHS photocatalyst showed good stability in five consecutive runs. The research results could provide an effective approach for design of high-efficiency photocatalyst materials under lower cost conditions.

  
  Acknowledgements: This work was supported by the National Natural Science Foundation of China (Grants No. 21808019, 41772240), the Natural Science Foundation of Jiangsu Province (Grants No. BK20181048, BK20180958) and the Science and Technology Bureau of Changzhou (Grant No. CJ20190074).
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• 

  Figure 1  (a) XRD patterns and (b) FT-IR spectra of CNB, CNHS and Pt-CNHS

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  Figure 2  TEM images of (a) CNB, (b) CNHS (Inset: FESEM image) and (c) Pt-CNHS; (d) Element mappings of Pt-CNHS; (e) EDS spectrum of Pt-CNHS

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  Figure 3  XPS high-resolution spectra of Pt-CNHS: (a) C1s, (b) N1s and (c) Pt4f

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  Figure 4  (a) N₂ adsorption-desorption isotherms and (b) corresponding pore size distribution curves of as-prepared CNB, CNHS and Pt-CNHS

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  Figure 5  (a) UV-Vis DRS spectra and (b) PL emission spectra of as-prepared samples

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  Figure 6  (a) Photocurrent response curves and (b) Nyquist plots of CNB, CNHS and Pt-CNHS

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  Figure 7  Photocatalytic activity and kinetics of as-prepared photocatalyst to degrade gaseous toluene under (a, b) UV light and (c, d) visible light

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  Figure 8  (a) Cycle stability of Pt-CNHS under visible light irradiation, and (b) TEM image of Pt-CNHS after visible light photocatalytic degradation of gaseous toluene

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  Figure 9  (a) GC-MS chromatogram of the organic by-products in degradation of toluene by Pt-CNHS and (b) pathway of degradation of toluene by Pt-CNHS

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