Nanonization of g-C₃N₄ with the assistance of activated carbon for improved visible light photocatalysis

Abstract

A visible light-driven g-C₃N₄/activated carbon composite photocatalyst (g-C₃N₄/AC) was prepared by a polymerization reaction of melamine and activated carbon. The photocatalytic activity of g-C₃N₄/AC was investigated by the degradation of phenol under visible light and sunlight irradiation. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence (PL) emission spectroscopy, Fourier transform infrared spectroscopy (FTIR), and N₂ adsorption–desorption isotherms were used for catalyst characterization. The results showed that g-C₃N₄/AC with 8 wt% AC had the best photocatalytic activity under visible light and sunlight irradiation. The mesoporous AC composite enables g-C₃N₄ to have a uniform particle distribution, less particle aggregation, and an increased specific surface area. A mechanism for the g-C₃N₄/AC photocatalyst was proposed, and the good efficiency for photodegradation was attributed to the increased surface area of the AC composite and decreased aggregation of g-C₃N₄, which facilitated the separation of photo-excited electron–hole pairs, and the strong adsorption capability of activated carbon, which accelerated the pollutant transfer rate and accumulation. Therefore, g-C₃N₄ can easily execute photocatalytic degradation in a pollutant-rich environment.

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1. Introduction

Graphitic carbon nitride (g-C₃N₄) is a novel, metal-free, and visible light-responsive semiconductor with an optical band gap of E_g = 2.7 eV.¹ Wang et al. first reported in 2009 that g-C₃N₄ has photocatalytic activity for the production of hydrogen or oxygen during water splitting under visible light irradiation.^2,3 Yan et al. reported that metal-free g-C₃N₄ has a good performance in the photodegradation of organic pollutants,⁴ which indicates that the metal-free g-C₃N₄ also has promising potential in the photocatalysis field. The application of g-C₃N₄ in photocatalysis has gained considerable scientific attention because of its outstanding optical, electronic, and catalytic properties.^1,2,5 Compared with other allotropes of carbon nitride, g-C₃N₄ shows the most stable structure because of the formation of strong covalent bonds between the carbon and nitrogen atoms.^6,7 Meanwhile, it also can be synthesized from available precursors, such as melamine,⁸ dicyandiamide,⁹ cyanamide,¹⁰ urea and thiourea,¹¹ etc.

However, g-C₃N₄ suffers from the disadvantages of a low specific surface area and low quantum efficiency, both of which have limited its photocatalytic performance.¹² In order to overcome these drawbacks, many attempts have been made to improve the photocatalytic capability of g-C₃N₄. The first approach is to use metal or non-metal element doping for improving the photoactivity of g-C₃N₄ (such as Fe-doped, Cu-doped, P-doped, N-doped, or S-doped g-C₃N₄),^13–17 which is based on energy band engineering. The second way is to form composite catalysts with a second photocatalyst, such as TiO₂/g-C₃N₄,¹⁸ graphene/g-C₃N₄,¹⁹ SmVO₄/g-C₃N₄, TaON/g-C₃N₄,^20,21 N–TiO₂/g-C₃N₄,²² g-C₃N₄/Cu₂O,^23,24 and SnS₂/g-C₃N₄,²⁵ etc. These composite photocatalysts are mainly developed based upon the coupling effect. The third one is to go through copolymerization with the nitrogen precursor²⁶ and protonation.²⁷ In addition to the three aforementioned methods, increasing the surface area of g-C₃N₄ has also been effective, as a photocatalytic reaction can occur on the surface nearby the photocatalytically active center. So the photocatalyst coating on the inert support surface still has the photocatalytic ability to degrade pollutants.^28–32 Cui et al.³³ used nano-SiO₂ as a hard template to prepare mesoporous g-C₃N₄ with a large surface area and a considerably higher photoactivity than that of bulk g-C₃N₄ in the degradation of 4-chlorophenol. Similar studies have also been reported by Lee and Bai.^34,35 However, their method is complicated, requiring the use of expensive raw materials and the unfavourable HF to remove SiO₂. Therefore, a cheap and simple approach to increase the surface area of g-C₃N₄ is preferred. Lin et al.¹² prepared core–shell nanospheres of SiO₂/g-C₃N₄ using a heating method to anneal the mixture of silicon dioxide nanospheres and molten cyanamide in nitrogen atmosphere. The photocatalytic activities of the SiO₂/g-C₃N₄ composites showed the highest activity with a RhB conversion of 94.3% after 150 min visible light irradiation, which is 3.5 times higher than that of the pure g-C₃N₄. Wang et al.³⁶ and Xiao³⁷ also prepared a SiO₂/g-C₃N₄ composite photocatalyst by a simple method by heating a mixture of SiO₂ and melamine. Actually, the SiO₂-modified g-C₃N₄ photocatalyst showed that g-C₃N₄ with SiO₂ as a support achieves a higher surface area, promotes the adsorption of pollutants, and enhances the photocatalytic activities under visible light irradiation. Activated carbon has a well-developed pore structure with a very large surface area and strong adsorption capacity and is widely used as an adsorbent and catalyst support. In addition, activated carbon surface-rich carboxyl groups, phenolic hydroxyl groups, carbonyl groups, lactone groups, and amide groups, etc. can chemically react with polymers to form hybrid composites. Furthermore, activated carbon is biologically renewable and inexpensive. However, there are few reports using activated carbon for synthesizing a g-C₃N₄/AC composite photocatalyst.

In this work, we attempt to use activated carbon as a adsorption center to prepare a g-C₃N₄/AC composite photocatalyst. With the strong adsorption ability of activated carbon to enrich the pollutants around g-C₃N₄, the catalyst is expected to accelerate photodegradation reactions.

2. Methods

2.1 Catalyst preparation

5 g melamine was mixed with suitable activated carbon and they were fully ground together. The ground powder was loaded in a tube furnace under flowing argon. The mixed powder was firstly heated at 500 °C for 1 h with a heating rate of 5 °C min⁻¹, then with a ramp rate of 0.2 °C min⁻¹ to 520 °C for 2 h. After cooling to room temperature the g-C₃N₄/x AC composite photocatalyst was obtained. In this preparation, the amount of added 5, 8, or 12 wt% AC in the g-C₃N₄/x AC composite was based upon the g-C₃N₄ production yield of 38.5% from the added melamine. All the AC before composite processing was washed with boiled deionized HCl solution and distilled water until it was at pH 7, then dried under vacuum at 85 °C for 24 h, so there was no metal impurity in the AC. For comparison purposes, pure g-C₃N₄ catalyst was prepared under the same conditions without the activated carbon.

2.2 Catalyst characterization

The crystal structure of the samples was characterized using X-ray diffraction analysis (Rigaku, Japan) using Cu Kα radiation. The surface composition and chemical states of the catalyst were investigated using an XPS Physical Electronics PHI5700 photoelectron spectrometer under Al Kα X-ray (hv = 1486.6 eV) radiation and calibrated with carbon C 1s (E_a = 284.62 eV). Inter-phase development was studied on a Nicolet-380 Fourier transform infrared spectrometer with samples embedded in a KBr pellet. The particle size and morphology of the fired powders were examined using JSM-7610F field-emission scanning electron microscopy (FE-SEM) and Tecanai 10 transmission electron microscopy (TEM). The energy threshold structure and light absorption of the samples were analyzed using a TU-1901 UV-vis spectrophotometer equipped with an integrating sphere using BaSO₄ as a reference. Photoluminescence (PL) emission spectra were measured on a JASCD FB-8500 fluorescence spectrophotometer with a 315 nm excitation wavelength at room temperature. N₂ adsorption–desorption experiments were performed on an ASAP 2020 porosity and specific surface area analyzer with the sample degassed at 200 °C for 2 h before the test. The specific surface area was calculated according to the BET equation.

2.3 Photocatalytic measurements

Photocatalytic reactions were performed in a home-made and jacketed quartz reactor (250 mL).³⁸ A 350 W Xe lamp with a filter to cut the light wavelength to shorter than 400 nm was built in the quartz tube for the visible light source. The reaction temperature was maintained at 25 °C by flushing with cooling water over the outer jacket of the reactor. The reactor was wrapped by aluminum foil in order to avoid additional interference. The photocatalytic degradation was tested in a 250 mL phenol solution of 50 mg L⁻¹ with the photocatalyst concentration kept at 1.0 g L⁻¹. The reactant mixture was under magnetic stirring in the dark for 30 min to obtain an adsorption–desorption equilibrium between phenol and the catalyst before illumination. The air supply to the reactor was maintained at a flow rate of 60 mL min⁻¹ in order to mix phenol with dissolved oxygen. A 5 mL sample was taken every 20 min from the reactor, followed by centrifugation. Absorbance of the supernatant was measured with a TU-1901 UV-vis spectrophotometer at 270 nm. The concentration of phenol was calculated based on the Lambert–Beer law.

The photocatalytic activity under sunlight was also tested. 0.1 g photocatalyst was added into 100 mL phenol solution of 50 mg L⁻¹ in a Petri dish with a diameter of 15 cm. The reactant mixture was stirred for 30 min to obtain an adsorption–desorption equilibrium between phenol and the catalyst. Then the Petri dish was sealed with plastic wrap and exposed directly to sunlight.

To evaluate the reusability of the pure g-C₃N₄ and g-C₃N₄/8 AC photocatalyst, 0.25 g photocatalyst was added into 250 mL phenol solution of 50 mg L⁻¹. The reactant mixture was magnetically stirred in the dark for 30 min, and then reacted under visible light irradiation for 160 min. After gravity settling, the supernatant solution was discarded by decantation, and the remaining catalyst was reused for the next photocatalytic reaction.

3. Results and discussion

3.1 XRD analysis

Fig. 1 shows the XRD patterns of pure g-C₃N₄ and g-C₃N₄/AC. It was observed that both pure g-C₃N₄ and g-C₃N₄/AC have characteristic peaks at 13.2° and 27.5°. The sharp and strong diffraction peak at 2θ = 27.5° with the d-spacing of 3.22 Å corresponded well to the (002) lattice plane of g-C₃N₄.^39,40 The peak at 2θ = 13.2° can be ascribed to the (100) plane of g-C₃N₄. The XRD pattern suggests that the activated carbon does not affect the g-C₃N₄ formation. The intensity of the diffraction peaks of g-C₃N₄ slightly decreased at the higher activated carbon content, but activated carbon did not affect the characteristic peak position of g-C₃N₄.

Fig. 1 XRD patterns of pure g-C₃N₄ and the g-C₃N₄/AC.

3.2 XPS analysis

XPS was carried out to analyze the surface chemical composition of pure g-C₃N₄ and g-C₃N₄/AC. The C 1s high resolution XPS spectra of pure g-C₃N₄ and g-C₃N₄/AC (Fig. 2a) show three peaks at 285.0, 286.2, and 288.5 eV. The peak at 285.0 eV was assigned to the C–C coordination including the adventitious hydrocarbon from the XPS instrument itself and sp²-hybridized carbon atoms present in g-C₃N₄, whereas the peaks at 286.2 and 288.5 eV were ascribed to C–NH₂ species and N=C–N groups, respectively, of the triazine rings on g-C₃N₄.^4,19 The N 1s spectra of pure g-C₃N₄ and g-C₃N₄/AC (Fig. 2b) could be deconvoluted into three peaks at 398.0, 399.5, and 400.9 eV, corresponding to the nitrogen atoms in the aromatic rings (C=N–C), tertiary nitrogen (N–(C)₃), and C–N–H, respectively.^41,42 The XPS results clearly indicate that the activated carbon composite is only a simple physical mixture and did not have chemical bonding between g-C₃N₄ and AC.

Fig. 2 XPS spectra of pure g-C₃N₄ and g-C₃N₄/AC.

3.3 FTIR analysis

Inter-phase development was characterized by FTIR. Fig. 3 shows the FTIR spectra of pure g-C₃N₄ and g-C₃N₄/AC. For the FTIR spectrum of pure g-C₃N₄, the small and sharp peak located at 809 cm⁻¹ originated from the characteristic breathing vibration of the triazine ring,⁴ whereas that at 889 cm⁻¹ was associated to a deformation mode of cross-linked heptazine deformation.^43–45 The peak at 1638 cm⁻¹ was attributable to the C=N stretching vibration mode, while those at 1242, 1325, 1408, 1465, and 1568 cm⁻¹ could be ascribed to aromatic C–N stretching vibration modes.^19,46 The broad peak centered at 3178 cm⁻¹ could be assigned to the stretching mode of the N–H bond.⁴⁷ It can be clearly seen that the main characteristic peaks of g-C₃N₄ all appear in the spectrum of g-C₃N₄/AC, confirming that the composite was composed of g-C₃N₄. We also observed that the intensity of all peaks decreased with the increase in the activated carbon content.

Fig. 3 FTIR spectra of pure g-C₃N₄ and g-C₃N₄/AC.

3.4 FE-SEM and TEM analysis

Fig. 4 shows the FE-SEM images of pure g-C₃N₄ and g-C₃N₄/AC. It can be seen from Fig. 4a that pure g-C₃N₄ particles have a considerable degree of agglomeration and a larger particle size with an average diameter of 1–3 μm. In contrast, it can be seen from Fig. 4b and c that g-C₃N₄ on the g-C₃N₄/AC sample not only demonstrates superior particle dispersion but also presents a smaller particle size. It is interesting to note that g-C₃N₄ particles uniformly distribute on the surface of AC. The TEM image in Fig. 4d confirms further that g-C₃N₄ is formed on the surface of AC. It can be viewed that activated carbon could avoid the g-C₃N₄ aggregation, reduce its particle aggregate size, and narrow its particle size distribution. The advantages of g-C₃N₄ on AC with a smaller and uniform particle size could improve its adsorption performance and enable rapid transfer of light-excited carriers to the particle surface for effectively lowering the carrier recombination rate and accelerating the photocatalytic reactions.^48–50

Fig. 4 FE-SEM images of (a) pure g-C₃N₄ and (b and c) g-C₃N₄/AC. TEM image of g-C₃N₄/AC (d).

3.5 Surface area analysis

To obtain the specific surface area of g-C₃N₄, N₂ adsorption–desorption isotherms were measured via a multi-point BET method. The BET surface area of all samples is summarized in Table 1, where the specific surface area of g-C₃N₄/AC is much higher than that of pure g-C₃N₄ prepared by a direct pyrolysis method. The increase in the specific surface area is attributed to the activated carbon to inhibit the g-C₃N₄ agglomeration and to improve the g-C₃N₄ specific surface area. From the computational results of ΔA = A_g-C3N4/AC − A_AC × (x/100) − A_g-C3N4 × [1 − (x/100)], where x is the AC content in weight percent, it is obvious that the addition of AC increases the specific surface area of g-C₃N₄ to 22 m² g⁻¹, as compared to 11.5 m² g⁻¹ for pure g-C₃N₄. An important step in the photocatalytic process is the adsorption of reacting substances onto the surface of the catalyst for heterogeneous reactions. If the lattice defects and other factors are the same, the higher specific surface area of g-C₃N₄/AC is expected to give a higher adsorption capacity to enhance the heterogeneous reaction.⁵¹

Table 1 Characteristics of pure g-C₃N₄ and g-C₃N₄/AC

Sample	A (m^2 g^−1)	ΔA (m^2 g^−1)	Phenol removal^a (%)	k (s^−1)	R
a At 160 min, k: reaction constant, ΔA = Ag-C3N4/AC − AAC × (x/100) − Ag-C3N4 × [1 − (x/100)], where x is the AC content in weight percent, Ag-C3N4/AC: surface area of g-C3N4/AC, AAC: surface area of activated carbon, Ag-C3N4: surface area of g-C3N4.
AC	1246.1	—	—	—	—
g-C3N4	11.5	—	37.3	0.077	0.983
g-C3N4/5% AC	95.8	22.6	85.9	0.315	0.999
g-C3N4/8% AC	136.2	25.9	100	0.630	0.976
g-C3N4/12% AC	186.3	26.7	95.0	0.482	0.993

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3.6 DRS analysis

The effect of the activated carbon content on the band energy of g-C₃N₄ was investigated by DRS (Fig. 5). It can be observed that the light reflection of the g-C₃N₄/AC catalysts in the range 460–700 nm decreased when the AC content increased, which may be due to the visible light absorption characteristic of activated carbon. However, the absorption edges of the g-C₃N₄/AC catalysts remained unchanged and no significant changes in band energy were observed. According to the first derivative calculation of the DRS spectra, the threshold values for optical absorption of pure g-C₃N₄ and g-C₃N₄/AC were both at 460 nm. According to the formula of the band gap E_g = 1240/λ, the E_g value was 2.7 eV for both pure g-C₃N₄ and g-C₃N₄/AC.

Fig. 5 Diffuse reflectance spectroscopy of pure g-C₃N₄ and g-C₃N₄/AC.

3.7 PL analysis

Fig. 6 shows the PL spectra of pure g-C₃N₄ and g-C₃N₄/AC. It was observed that g-C₃N₄/AC had a lower PL intensity in comparison with that of pure g-C₃N₄. It is well known that the PL intensity is positively correlated with the recombination probability of the photogenerated electron–hole pairs, thus related to the photocatalytic performance.^52,53 Therefore, as previously reported,^36,54 the weak intensity indicates that the addition of AC has a positive effect on restraining the recombination of the photogenerated electron–hole pairs by trapping photo-excited electrons, which is similar to the action of Al₂O₃ in TiO₂/Al₂O₃ photocatalysts,⁵⁵ and SiO₂ in SiO₂/g-C₃N₄ photocatalysts.³⁶ AC aids the formation of g-C₃N₄ with a smaller and uniform particle size for enabling rapid transfer of light-excited carriers to the particle surface and to avoid the carrier recombination.^48–50

Fig. 6 PL spectra of pure g-C₃N₄ and g-C₃N₄/AC.

3.8 Photocatalytic activity

Fig. 7a shows the phenol adsorption curves in the dark over different photocatalysts. The g-C₃N₄/AC composite photocatalyst has a stronger phenol adsorption ability than that of pure g-C₃N₄, which could be attributed to its larger surface area. The adsorption of phenol in the dark increased with the increase in AC content, which is consistent with the BET result. The other reason is attributed to AC having fine g-C₃N₄ particles with a high specific surface area for adsorption. The photocatalytic activities of pure g-C₃N₄ and g-C₃N₄/AC were evaluated by the photodegradation of phenol aqueous solution at room temperature under visible light irradiation. The results are shown in Fig. 7b. It can be observed that the photocatalytic degradation of phenol on g-C₃N₄/AC was more efficient than that on the pure g-C₃N₄ prepared in this work. g-C₃N₄/8% AC completely removed phenol after reaction for 160 min. According to the kinetic analysis, the photocatalytic phenol degradation fitted pseudo-first-order kinetics well. The first-order reaction rate constant (k) is shown in Table 1 and follows the sequence: g-C₃N₄/8 AC (k = 0.630 s⁻¹) > g-C₃N₄/12 AC (k = 0.482 s⁻¹) > g-C₃N₄/5 AC (k = 0.315 s⁻¹) > pure g-C₃N₄ (k = 0.077 s⁻¹). The optimum AC content in g-C₃N₄/AC was found to be 8 wt%. The AC alone did not show catalytic activity for the degradation of phenol. In conjunction with the adsorption experiments shown in Fig. 7a, it could be found that the stronger adsorption capacity for g-C₃N₄/AC does not guarantee a higher photocatalytic activity. Although the adsorption is the highest for g-C₃N₄/12 AC, the black AC with a high content blocks the light absorption of g-C₃N₄ and reduces the photocatalytic performance. The experimental results obtained for the photocatalysts under sunlight illumination are shown in Fig. 7c. It manifests that g-C₃N₄/8 AC has a higher phenol photocatalytic activity than that of pure g-C₃N₄ under sunlight. Fig. 7d shows the recycling test of pure g-C₃N₄ and g-C₃N₄/AC for the photocatalytic degradation of phenol under visible light irradiation. It was seen that the activity of pure g-C₃N₄ greatly reduced and significant deactivation occurred. After six runs for the reusability testing, only 3.2% of phenol could be removed within 160 min. As g-C₃N₄/8 AC has shown a higher photocatalytic performance, it can still degrade 81.5% phenol after six runs. Due to the addition of activated carbon, the stability and reusability of g-C₃N₄/AC have been significantly improved compared to those of pure g-C₃N₄.

Fig. 7 (a) Phenol adsorption curves of g-C₃N₄ and g-C₃N₄/AC, photocatalytic activity of g-C₃N₄ and g-C₃N₄/AC (b) under visible light and (c) under sunlight, and (d) reusability of g-C₃N₄ and g-C₃N₄/AC for phenol degradation under visible light.

3.9 Mechanism for AC-enhanced photoactivity of g-C₃N₄

The surface area of the catalyst is an important factor that affects the photocatalytic activity, because pollutants have to be adsorbed on the catalyst surface before the photocatalytic reaction occurs. A large surface area of the photocatalyst is usually required for satisfactory adsorption of the reactants. AC in the g-C₃N₄/AC catalysts can act as an adsorbing center for pollutants before transferring to the decomposition center of g-C₃N₄ to be illuminated on the AC surface. However, strong adsorption of pollutant molecules may make it difficult to move and inhibit subsequent photocatalytic reactions. Therefore, the photoactivity of the composite catalyst is enhanced with the addition of the AC composite to significantly improve its surface area and reduce the particle size. g-C₃N₄ with the smaller and uniform particle size on the AC surface enables rapid transfer of light-excited carriers to the particle surface for effectively reducing the carrier recombination rate, which might also contribute to the enhanced photoactivity. A mechanism for the enhanced photoactivity of g-C₃N₄/AC is shown in Fig. 8.

Fig. 8 Schematic mechanism for AC-enhanced photoactivity of g-C₃N₄.

4. Conclusions

A g-C₃N₄/AC composite photocatalyst was successfully synthesized by pyrolyzing melamine-mixed AC powder at 520 °C. Results of the photocatalytic tests showed that g-C₃N₄/AC had a much higher photocatalytic activity than pure g-C₃N₄ did. The reaction center for the photodegradation of pollutants in the g-C₃N₄/AC system is the g-C₃N₄ photocatalyst. However, the improved activity of this AC composite is mainly attributed to the large surface area and smaller particle aggregation size of g-C₃N₄ on the AC surface, which enhance the adsorption of pollutants on the AC, increase the mass transfer rate of pollutants through the solution environment, and allow more pollutant accumulation on the AC surface for the nearby g-C₃N₄ catalyst to react. Therefore, the heterogeneous catalytic reaction of phenol photodegradation by g-C₃N₄ is enhanced by the AC composite with a high adsorption capability.

Acknowledgements

This work was supported by Ministry of Science and Technology of the Republic of China under the Grant No. MOST 104-2221-E-011-169-MY3 and by National Natural Science Foundation of China under the Grant No. 31000269.

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