Two physical strategies to reinforce a nonmetallic photocatalyst, g-C₃N₄: vacuum heating and electron beam irradiation

Abstract

Herein, we demonstrated two physical strategies, namely, vacuum heating and electron beam irradiation, to reinforce a nonmetallic photocatalyst, g-C₃N₄. These two post-treatments also improved the visible light absorption properties of g-C₃N₄; however, electron beam irradiation was more destructive, and it caused a determined change in the chemical bonds and band structure of the compound. According to the post-processing parameters mentioned in this article, vacuum heating (38 ± 2 mTorr for 4 days at 200 °C) enhanced the photocatalytic efficiency of the original g-C₃N₄ by 2.5 times, whereas electron beam irradiation (760 kGy at 1.8 MeV and 8 mA s⁻¹) improved it by 4.5 times. Finally, the post-treated photocatalysts were stable during photocatalytic oxidation, which is important for practical applications.

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Introduction

Recently, a nonmetallic photocatalyst, graphitic carbon nitride (g-C₃N₄), composed only of C and N elements, drew world-wide attention due to its potential applications in solar energy conversion, photosynthesis, electrocatalysis, and bioimaging because of its appropriate bandgap, large surface area, and excellent chemical stability.^1–10 However, the efficiency of pure g-C₃N₄ was still far from satisfactory, owing to its high recombination rate of photoinduced electron–hole pairs.^11–16 To address this disadvantage, many strategies have been adopted,^17–19 such as metal loading (Ag,²⁰ Pd,²¹ Au²²), metal doping (Fe,²³ Zn,²⁴ Er,²⁵ Ni²⁶), non-metal doping (B,²⁷ O,²⁸ S²⁹), morphological control (nanosheets,³⁰ nanotubes,³¹ nanorods,³² quantum dots³³), compositing support (GO,³⁴ CNT,³⁵ SBA-15 ³⁶) and coupling with other semiconductors (S₈,³⁷ Ag₃PO₄,³⁸ SrTiO₃,³⁹ Bi₂WO₆,⁴⁰ ZnO,⁴¹ MoS₂,⁴² CeO₂,⁴³ etc.). Meanwhile, the majority of doping and composite attempts inevitably required the introduction of some other substances.

In our past work, we demonstrated a vacuum heating treatment with a low and easily achievable degree of vacuum to improve the photocatalytic properties of TiO₂.⁴⁴ Vacuum heating⁴⁵ as well as hydrogenation⁴⁶ could alter the surface structure of TiO₂ to enhance and extend the optical absorption region. On the other hand, radiation-induced effects in the TiO₂ crystal structure, especially electron beam irradiation, were reported.^47,48 The band gap of TiO₂ was decreased after electron beam irradiation without using any dopants.

Herein, we introduced, for the first time, vacuum heating and electron beam irradiation post-treatments for the strengthening of a nonmetallic photocatalyst, g-C₃N₄. The g-C₃N₄ photocatalysts were characterized after vacuum heating or electron beam irradiation by XRD, FT-IR, Raman, UV-Vis DRS, PL and XPS analysis. The photodegradation of RhB was employed to evaluate the photocatalytic activities under visible light irradiation, and the stability of the synthesized photocatalysts was also investigated through four successive experimental runs. Moreover, we compared the similarities and differences in the results resulting from these two post-treatments.

Results and discussion

Typically, g-C₃N₄ is synthesized by the thermal treatment of urea under ambient pressure directly in air.⁴⁹ In brief, 4.0 g of urea was transferred into a 25 mL crucible with a cover, and the crucible was heated in a muffle furnace at 80 °C (24 h) for drying and 550 °C (3 h) for the thermal reaction (ramp rate of 5 °C min⁻¹ from 80 to 550 °C), then air-cooled to the room temperature. Finally, the yellow product was washed with nitric acid (0.1 mol L⁻¹) and water several times, then dried at 60 °C overnight.

Vacuum heating

The resulting g-C₃N₄ was treated under a vacuum of 38 ± 2 mTorr for 2 (or 4) days at 200 °C. A degasser (FloVac, Quantachrome) was used to detect and control the vacuum, which was provided by a rotary vane vacuum pump (Duo 3, Pfeiffer Vacuum).

Electron beam irradiation

A mixture which contained 60 mg of obtained g-C₃N₄, 30 mg of water, 3 mL of isopropanol and 60 μL of ammonium hydroxide was sealed in a polythene bag (10 cm × 15 cm × 5 mm), and the bag was irradiated under a dynamiron (GJ-2-II, Shanghai Xianfeng Motor Factory Co., Ltd.) with an accelerating voltage of 1.8 MeV and an accelerating current of 8 mA s⁻¹ at room temperature. The irradiation dose was set at 380 (or 760) kGy.

We labelled these samples after the different post-treatments with CN-VA- (or CN-EB-) as a prefix, followed by the vacuum heating time (or the EB irradiation dose). For instance, CN-VA-4 indicates that the g-C₃N₄ was subjected to vacuum heating for 4 days, and CN-EB-380 denotes that the g-C₃N₄ underwent electron beam irradiation with a dose of 380 kGy. In addition, CN denotes the original g-C₃N₄ without any post-processing.

The typical graphitic carbon nitride was synthesized by the direct thermal polymerization of urea, as shown in Fig. 1. The TEM image presents a two-dimensional sheet structure with irregular wrinkles; furthermore, a number of pores are distributed over its surface according to the corresponding SEM image. The nitrogen adsorption–desorption isotherm and Barrett–Joyner–Halenda (BJH) pore size distribution curve (Fig. S1†) indicated that as-synthesized g-C₃N₄ exhibited a type IV isotherm with an H3 hysteresis loop. The Brunauer–Emmett–Teller (BET) specific surface area was calculated to be 150.4 m² g⁻¹, higher than a previous report,⁴⁹ which may lead to more efficient surface adsorption and faster photogenerated carrier separation, and thus to improvement of the photoactivity.

Fig. 1 TEM (a) and SEM (b) images of typical g-C₃N₄.

The original g-C₃N₄ appeared pale yellow (Fig. 2a). After vacuum heating or electron beam irradiation, the samples become darker in color; however, the color variation after the latter treatment was more obvious. Although the color changed, these two treatments did not produce significant changes in the morphology (Fig. S2†). Fig. 3 illustrates the XRD patterns of g-C₃N₄ with different post-treatments. The strongest peak at 27.7° is a characteristic interlayer stacking peak of aromatic systems, indexed as the (002) plane for graphitic materials.^27,50 The calculated interplanar distance of the aromatic units was d = 0.322 nm. The small-angle peak at 12.8° corresponded to a distance d = 0.692 nm which is indexed as the (100) plane and associated with an in-plane structural packing motif.^27,50 In addition, the other two weaker peaks at 17.6 and 21.7° are caused by the (011) and (110) plane reflections.⁵¹ It is worth noting that the (002) peak intensities of post-treated g-C₃N₄ decreased, indicating that vacuum heating as well as high-energy electron beam irradiation can partly damage the fine structure of g-C₃N₄ and reduce its crystallinity.⁴⁴

Fig. 2 Optical photographs of g-C₃N₄ with different post-treatments (a) and corresponding fluorescence phenomena under UV excitation (b).

Fig. 3 XRD patterns of CN, CN-VA-2, CN-VA-4, CN-EB-380 and CN-EB-760. The inset figure displays the intensity variations in the diffraction peaks.

In Fig. 4a, the FT-IR spectra revealed that the original g-C₃N₄ chemical structure remained mostly unchanged after vacuum heating or electron beam irradiation. The absorption bands at 1573 and 1637 cm⁻¹ were attributed to C=N stretching, while the four bands at 1240, 1317, 1403 and 1460 cm⁻¹ corresponded to aromatic C–N stretching.^52–54 The band at 812 cm⁻¹ was assigned to the breathing vibrational modes of s-triazine units.^52–54 The broad band at 3000 to 3700 cm⁻¹ belonged to the N–H vibration due to partial condensation and the adsorbed water molecules. In the Raman spectra presented in Fig. 4b, all samples exhibited three bands at 707, 769 and 975 cm⁻¹. The 769 cm⁻¹ band was attributed to the out-of-plane bending mode of the graphitic domains, and the remaining two bands arose from the breathing modes of the s-triazine rings.^55–57 It is noteworthy that both vacuum heating and electron beam irradiation increased the background fluorescence intensity of the corresponding samples so that the relative intensity of the Raman bands decreased, which is consistent with previous reports.^44,45 The greater the degree of processing, the more attenuated the band intensity.

Fig. 4 FT-IR (a) and Raman (b) spectra of CN, CN-VA-2, CN-VA-4, CN-EB-380 and CN-EB-760.

As can be seen in Fig. 5a, the optical properties of the above-mentioned g-C₃N₄ samples were measured by UV-Vis DR spectra. The absorption edge of post-treated g-C₃N₄ displayed a slight red shift in comparison with the original g-C₃N₄. The corresponding band gaps of CN, CN-VA-2, CN-VA-4, CN-EB-380 and CN-EB-760 were about 2.95, 2.94, 2.93, 2.97 and 2.97 eV, respectively (Fig. 5b). Vacuum heating resulted in a reduction in the band gap of the corresponding samples; however, electron beam irradiation brought about a change in the inflection point position of the absorption curve, leading to a slight increase of the calculated band gaps. In addition, the post-treated g-C₃N₄ exhibited more intense absorption in the visible range, which is in agreement with the color change from yellow to dark yellow. This prominent optical property might be due to the broadening of the band-tail, which resulted in more efficient light harvesting.

Fig. 5 UV-Vis DR (a), PL (c) spectra of g-C₃N₄ with different post-treatments and corresponding plots of Kubelka–Munk functions versus the energy of light (b); F(R) = (1 − R)² (2R)⁻¹ = K S⁻¹, where R, K and S are the reflectance, effective absorption and scattering coefficients, respectively.

Fig. 5c shows the PL spectra of the as-synthesized photocatalysts excited by 367 nm at room temperature. For these five samples, the main emission peak appeared at about 437 nm; this was assigned to the band–band PL phenomenon, with the energy of light approximately equal to the band-gap energy.^27,55 It is widely accepted that the enhancement of photocatalytic activity can be attributed to efficient photogenerated electron–hole pair separation.^28,58 Obviously, in comparison with the original g-C₃N₄, the intensity of the PL signal for CN-VA-2 and CN-VA-4 was somewhat lower, and that for CN-EB-380 and CN-EB-760 was significantly lower (also see Fig. 2b), which indicated that the recombination of electron–hole pairs might be effectively inhibited on the post-treated g-C₃N₄, especially after the electron beam irradiation treatment.

XPS was undertaken to accurately determine the chemical composition and specific electronic states of the different g-C₃N₄ samples. In the survey spectra of all five samples (Fig. 6a), the typical C 1s and N 1s peaks were observed, and a residual O 1s peak, probably due to calcination in air, was also present.^53,59 Fig. S3† illustrated two major C 1s peaks centered at 288.5 and 289.7 eV, corresponding to sp²-bonded carbon (C–N–C) and C–O respectively, and another trace amount of C–C bonding at 284.8 eV.^28,49 In the high-resolution XPS spectra of N 1s, three deconvolution peaks at 399.0, 400.3 and 401.5 eV should be ascribed to sp² C–N–C, sp³ N–[C]₃, and C–NH_x (amino functional groups), respectively.^28,49,59 The weak 404.5 eV peak was attributed to the π-excitations. The N 1s bonding ratios of g-C₃N₄ samples after different treatments are listed in Table 1. Vacuum heating did not notably change the proportions of the three N 1s bonds; however, electron beam irradiation might break the portion of the molecule with of C–N–C structure or convert it to an N–[C]₃ structure. That is to say, the partial tri-s-triazine structure of g-C₃N₄ could be rearranged or form an s-triazine structure under electron beam irradiation. The g-C₃N₄ samples based on the two structures were allotropes; however, the s-triazine in the theoretical calculations was in a metastable state.⁶⁰ These tiny structural distortions would lead to a size reduction of the nitrogen hole in the topological arrangement of the aromatic systems and cause the original flat surface to be curved and arched. The electronic energy state distributions of these raised positions were different from other locations, and readily form the separation point of the photogenerated electrons and holes, which would also cause a sharp decrease in the fluorescence effect as noted above (Fig. 5c).

Fig. 6 XPS survey (a) and VB (b) spectra of CN, CN-VA-2, CN-VA-4, CN-EB-380 and CN-EB-760.

Table 1 Ratios of bonds within the N 1s core-level peak in different samples and corresponding VBM energies

	C–N–C (%)	N–[C]3 (%)	C–NHx (%)	VBM (eV)
CN	71.1	22.6	6.3	1.77
CN-VA-2	70.8	22.8	6.4	1.79
CN-VA-4	71.2	22.6	6.2	1.81
CN-EB-380	67.3	26.2	6.5	1.70
CN-EB-760	68.0	25.8	6.2	1.62

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In addition, the valence band maxima (VBM) of the samples (Fig. 6b) also manifested the different results caused by the two post-treatments.^28,29,61 Vacuum heating increased the position of the VBM slightly; meanwhile, electron beam irradiation reduced it observably. Comparing the two post-treatments, vacuum heating is a relatively mild post-treatment, according to previous reports,^44,45 which would most likely produce an amorphous layer and would not lead to dramatic alteration in the chemical structure or band position. On the other hand, electron beam irradiation is a more destructive post-treatment, which could trigger a series of radiation chemical reactions simultaneously during the direct electron beam bombardment of the material. In this study, electron beam irradiation not only modified the optical properties of the samples but also caused decided changes in its chemical bonds and band structure.

The photocatalytic activities of different g-C₃N₄ samples were evaluated by the photodegradation of rhodamine B, a probe molecule, under visible light irradiation. All the photodegradation curves were consistent with pseudo-first-order kinetic reactions (Fig. 7a). Compared to the original g-C₃N₄, the post-treated g-C₃N₄ samples exhibited superior photocatalytic performance; notably, the photocatalytic property of CN-VA-4 was about 2.5 times higher than that of CN, and the efficiency of CN-EB-760 was about 4.5 times greater than that of CN. The greater the degree of treatment, the more remarkable the resulting upgrade. To test the stability and reusability of g-C₃N₄ after vacuum heating or electron beam irradiation, CN-VA-4 and CN-EB-760 were reused for the photocatalytic reaction four times or after a half-year of storage under the same conditions; these results are shown in Fig. 7b and c and S4.† The photocatalytic effects of the used samples after four photocatalytic degradation cycles and the samples after a half-year of storage were only slightly depressed, which indicated that these photocatalysts were stable during the photocatalytic oxidation of the pollutant molecules; this is important for many applications.

Fig. 7 Photocatalytic degradation of RhB in the presence of CN, CN-VA-2, CN-VA-4, CN-EB-380 and CN-EB-760 (a). Cycle and dark control degradation of RhB in the presence of CN-VA-4 (b) and CN-EB-760 (c).

In order to describe the mechanism of increased performance following the abovementioned physical post-treatments, scholars mainly analyze the generation of the amorphous layer and the variation of the element valency.^44–48 In this article, it is difficult to display the obvious lattice image of g-C₃N₄ via HRTEM; therefore, it is also difficult to directly observe the generation of the amorphous layer, which could be indirectly inferred by Raman, XRD and other characterizations. An amorphous or defect layer of g-C₃N₄ might lead to the broadening of the band-tail, amelioration of the energy band structure and red-shifting of the absorption spectrum. Furthermore, in accordance with the XPS results, we could speculate that the electron beam irradiation, different from vacuum heating, might bring about the transition and reconstruction of the partial chemical structure of the surface from a steady to a metastable state. These tiny structural distortions would cause the original flat surface topology arrangement to be curved and arched at the likely site of the active center of photocatalytic reaction, resulting in a considerable enhancement in the photocatalytic efficiency.

Conclusions

In summary, we successfully introduced, for the first time, vacuum heating and electron beam irradiation post-treatments to strengthen a nonmetallic photocatalyst, g-C₃N₄. These two post-treatments also improved the light absorption properties of g-C₃N₄, especially in the visible region. Vacuum heating was a relatively mild post-treatment, whereas electron beam irradiation was more destructive, and caused a determined change in the chemical bonds and band structure of g-C₃N₄. According to the post-processing parameters mentioned in this article, vacuum heating (38 ± 2 mTorr for 4 days at 200 °C) could enhance the photocatalytic efficiency of the original g-C₃N₄ by 2.5 times, whereas electron beam irradiation (760 kGy at 1.8 MeV and 8 mA s⁻¹) improved the efficiency by 4.5 times. Finally, the post-treated photocatalysts were stable during the photocatalytic oxidation, which is important for many applications.

Experimental section

Chemicals

Urea (NH₂ONH₂), ethanol (CH₃CH₂OH), nitric acid (HNO₃, ≥65%), isopropanol ((CH₃)₂CHOH) and ammonium hydroxide (NH₄OH, ≥25%) were of analytical grade and were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Deionized water was used throughout this study.

Photocatalysis

The photocatalytic performance was measured by analyzing the degradation of rhodamine B (C₂₈H₃₁ClN₂O₃, ≥95%, RhB, Sigma) under visible light irradiation after adding the photocatalysts (20 mg) to a RhB solution (50 mL, 20 mg L⁻¹) with continuous air bubbling. Prior to irradiation, the suspension was bubbled for 90 min in the dark to reach an adsorption equilibrium. A 350 W Xe lamp with a UV cut-off filter (420 nm) was used as the light source. The degradation of the dye was monitored by taking 3 mL aliquots at regular time intervals during the irradiation. After filtering through a 0.22 μm microporous membrane, the obtained supernatants were collected and the concentration of RhB was calculated by measuring the characteristic absorption peak of 550 nm recorded by a UV-Vis spectrophotometer (U-3010, Hitachi).

Characterization

The morphology of g-C₃N₄ was observed using a transmission electron microscope (TEM, JEM-200CX, JEOL) with an accelerating voltage of 120 kV and a field emission scanning electron microscope (FESEM, JSM-6700F, JEOL) with an accelerating voltage of 15 kV. Powder X-ray diffraction (XRD, D/max-2550, Rigaku) was employed to determine the crystalline phase of the samples, which deployed Cu Kα radiation (λ = 1.5418 Å) with 40 kV and 250 mA as the accelerating voltage and current, respectively. The Raman spectra were obtained at room temperature on a Raman microscope (inVia plus, Renishaw) and the excitation light was the 785 nm line of a laser diode. Nitrogen sorption isotherms were measured at 77 K using a surface area and pore size analyzer (Quadrasorb SI, Quantachrome). All of the samples were degassed under vacuum at 80 °C overnight prior to measurement. The Fourier transform infrared (FT-IR) spectra were performed on a FT-IR spectrometer (Avatar 370, Nicolet) in the mid infrared range (4000 to 400 cm⁻¹) for samples dispersed in KBr pellets. The diffuse reflectance (DR) spectra of the photocatalysts were collected at room temperature on a UV-Vis spectrophotometer (U-3010, Hitachi) with an integrating sphere accessory and BaSO₄ as the reference. The photoluminescence (PL) spectra of the materials were acquired on a fluorescence spectrophotometer (F-7000, Hitachi) at an excitation wavelength of 367 nm. The surface chemical bonding and valence band spectra of the samples were obtained with an X-ray photoelectron spectrometer (XPS, ESCALAB 250Xi, Thermo Scientific) equipped with an Al anode X-ray source with Kα radiation. The position of the peaks was calibrated to the C–C 1s peak at 284.8 eV.

Acknowledgements

This study was financially support by the National Natural Science Foundation of China (Grant No. 11275121, 21471096, 61174011 and 21371116).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22732a

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