Graphitic carbon nitride (g-C3N4), a non-metal semiconductor consisting of only carbon and nitrogen, has attracted interest in the photocatalysis field [1, 2, 3, 4, 5, 6]. Its band gap is 2.7 eV, which is much smaller than that of TiO2 (3.2 eV) [7]. So, compared to TiO2, g-C3N4 can absorb visible light, and its electrons can be excited from the valence band (VB) to the conduction band (CB) under visible light irradiation. Apart from its excellent sunlight harvesting ability, g-C3N4 is very stable under irradiation even in acid and base solutions, which makes it a good photocatalyst in various hazardous environments [3, 4, 5, 6].
However, the high recombination rate of the photo- generated e--h+ pairs limits its practical applications. Many modification methods have been tried to improve its photocatalytic activity, such as physical coupling [8, 9], chemical doping [10, 11], microstructrual shaping [12, 13, 14, 15] and surface modification [16]. Among these methods, physical coupling is very effective and easy to perform. Due to the different potential levels of the CB and VB in the two different coupling materials, the photo-generated e− and h+ of g-C3N4 can be separated efficiently at the heterojunction, giving rise to enhanced photocatalytic activity. The materials chosen for the counterparts can be divided into four classes: metal sulfide or oxide (e.g., CdS [17] and TiO2 [18]), noble metals (e.g., Au [19]), polymers (e.g., P3HT [20] and PANI [9]), and graphene-based materials (e.g., reduced graphene oxide (rGO) [8, 21, 22, 23, 24, 25], GO [26] and carbon nanotubes [27]).
Graphene or rGO is a very good candidate for the coupling because g-C3N4 and graphene are both 2D materials, and the atomic sheets of graphene can facilitate electron transfer. g-C3N4/rGO has been synthesized from the molecular-level mixture of urea/GO [8], cyanamide/GO [21, 22], dicyandiamide/GO [23], and melamine/GO [24, 25]. The heterojunction between g-C3N4 and rGO is formed by N bridging [8, 23], O bridging [21], or van der Waals π-π stacking [25]. The introduction of rGO has led to increased photocatalytic activity [21, 22, 23, 24, 25]. However, all these coupling processes use a non-oxygen environment to make the hybrid, such as N2 [22, 25], Ar [8, 21, 23], H2 [24] or hydrazine hydrate environment [25]. This is because GO can be reduced to rGO at high temperature in an inert or reductive atmosphere [28], which generally needs a complicated fabrication process.
GO itself is effective in modifying the photocatalysis of g-C3N4 when overlaid on the surface of g-C3N4 as a GO/g-C3N4 hybrid [26], even though the mechanism is not very clear. So, it may not be necessary to use a non-oxygen environment to fabricate the hybrid. That is, a g-C3N4/rGO hybrid can be prepared with the existence of oxygen, e.g., in air. On the one hand, g-C3N4 has been polymerized in air and it showed increased photocatalytic activity due to the increased specific surface area [29]. On the other hand, GO can be reduced in air in some mixtures, as reported in our previous experiments [30]. During the heat treatment in air, GO is oxidized and splits into smaller unit, some down to the quantum dot level. The high temperature makes the major contributions to the removal of the epoxy, carbonyl and carboxyl groups, leading to a partially or fully reduced GO, i.e., rGO. Local reductive gases, such as the released NH3, also contribute to the reduction [30].
In this work, we carried out a simple method to fabricate a g-C3N4/rGO hybrid by directly heating a mixture of melamine/GO in air. The chemical microstructure of the hybrid, photocatalytic performance for the degradation of rhodamine B (RhB), pH-response behavior, and the photocatalytic mechanism were studied. The hybrid showed much increased photocatalytic performance in acidic conditions under visible light irradiation.
GO dispersion was performed according to the modified Hummers method [27, 29]. One gram of graphite and 6 g of KMnO4 were put into a solution composed of concentrated H2SO4 (120 mL) and H3PO4 (13.3 mL). After 18 h under vigorous stirring at 50 °C, the mixture was cooled to room temperature. This was followed by adding some H2O2 to react with the residual KMnO4. After filtration and washing, most of the acid and ions were removed. The GO was re-dispersed in water and dialyzed by dialysis bags of 8000-14000 Da to remove acid and ions completely.
Four grams of melamine and an amount of GO dispersion were mixed and put into deionised water. After vigorous stirring and oven drying, a grey melamine/GO mixture was obtained. The mixture was put into a crucible and heated to 550 °Cfor 1 h in air. The g-C3N4/rGO hybrid was obtained after grinding and denoted as g-C3N4/rGO-x, where x means the mass ratio of melamine/GO. Pristine g-C3N4 was prepared in a similar procedure using only melamine. rGO was prepared by heating GO under the same heating conditions.
Fourier transform infrared spectroscopy (FT-IR) spectra were recorded on a Bruker TENSOR-27 Fourier transform infrared spectrophotometer. The crystalline structure was investigated by X-ray diffraction (XRD) on a D8 ADVANCE instrument using Cu Kα radiation. The scanning electron micrographs (SEM) were obtained on a JSM-6700F microscope. Transmission electron microscopy (TEM) images were obtained by a JEM-2100F electron microscope at an acceleration voltage of 200 kV with a CCD camera. X-ray photoelectron spectroscopy (XPS) was performed using a Kα 1063 instrument with focused monochromatized Al Kα radiation. UV-Vis diffuse reflectance spectra (DRS) were obtained on a HITACHI U4100 spectrophotometer using BaSO4 as the reference. The BET specific surface area (ABET) was determined by N2 adsorption isotherm measurement at -196 °C on a Micromeritics ASAP 2010 system. Photoluminescence (PL) spectra in reflectance mode were recorded using an FL-3 transient fluorescence spectrometer. The dried sample was pressed into a solid state powder film with a relatively uniform thickness more than 1.0 mm.
The photocatalytic activity of the as-synthesized samples was evaluated by the degradation of RhB under a 350 W Xe Lamp (XD350W-I, Changzhou Siyu Environmental Sci-Tech Co., Ltd.) with a 400 nm cut off filter. A photocatalyst sample (100 mg) was added into a 100 mL aqueous solution of RhB (10 mg/L). Prior to irradiation, the suspension was treated using ultrasound in the dark for 30 min to achieve adsorption- desorption equilibrium. A 350 W Xe Lamp was used as the visible light source to perform the photocatalytic experiment. During the degradation of RhB, 5 mL samples were taken out and centrifuged to remove the solid particles for analysis. The concentration of aqueous RhB was determined using a UV-vis spectrophotometer (SHIMADZU UVmin-1240) at 552.0 nm by measuring its absorbance. The RhB concentration was calculated by the Lambert-Beer equation. The photodegradation of RhB under visible light irradiation in the absence of the photocatalyst was also evaluated.
The chemical and crystalline structures of as-synthesized rGO, g-C3N4/rGO and g-C3N4 were studied first. The FT-IR spectra and XRD patterns are shown in Fig. 1. In the FT-IR spectrum of rGO shown in Fig. 1(a), there are no obvious absorption bands that can be assigned to oxygen-containing groups, which is a typical characteristic feature of graphene. This indicated that GO was mostly reduced to rGO at the high temperature even in air. In the FT-IR spectra of g-C3N4/rGO and g-C3N4, there were three obvious bands at 810, 1200−1650 and 3150 cm-1. The first two bands were assigned to the breathing mode and stretching modes of CN heterocyclic groups, while the last band belonged to the residual N-H groups [15].
XRD patterns are presented in Fig. 1(b). rGO only showed a graphitic peak at 2θ = 26.2° with an interlayer spacing of 0.34 nm, indicating the existence of graphene atomic layers [21, 22, 23, 24, 25]. For g-C3N4/rGO and g-C3N4, there was one weak peak at 2θ = 13.1°, which was due to the (100) crystal face and showed the interplanar distance between the nitride pores [3]. The strong peak at 2θ = 27.4° is the characteristic feature of the (002) crystal face and showed the interlayer stacking of the conjugated aromatic systems [1].
Both the FT-IR spectra and the XRD patterns were almost identical for g-C3N4/rGO and g-C3N4, revealing that GO had little effect on the thermal polymerization of melamine and the network of g-C3N4 was retained.
The microstructures of the as-synthesized rGO, g-C3N4/rGO and g-C3N4 were investigated using SEM and TEM, and shown in Fig. 2. At the high temperature, GO was reduced and curved flexible rGO was formed, as shown in Fig. 2(a) and (d). The diffraction ring indicated the graphene atomic sheets [27]. Pristine g-C3N4 consists of irregular thick graphitic sheets as shown in Fig. 2(c) and (f). In the hybrid product of g-C3N4/rGO shown in Fig. 2(b) and (e), rGO was observed sandwiched in the matrix of g-C3N4 even though this was hard to recognize due to the very limited ratio of rGO. The overall microstructure of g-C3N4/ rGO was very similar to that of pristine g-C3N4.
XPS was utilized to investigate the chemical composition and chemical bonding states of g-C3N4/rGO and g-C3N4, and shown in Fig. 3 and Table 1.
In Fig. 3(b), most of the oxygen molecules were from adsorbed oxygen on the surface of the sample. In the C 1s spectra, both samples contained two common carbon species at 288.3 and 284.6 eV, corresponding to the C-N=C coordination and graphitic carbon [9]. The carbon contents were 42.41% in g-C3N4 and 43.69% in g-C3N4/rGO-800. The latter was slighter higher due to the trace amount of rGO introduced. Compared to g-C3N4 (0.02), the ratio of graphitic carbon in all the carbon species in g-C3N4/rGO-800 was higher (0.04). The N 1s of g-C3N4/rGO and g-C3N4 can be deconvoluted into three peaks at 398.7, 399.7 and 401.0 eV. The main peak at 398.7 eV originated from the sp2-bonded N in the triazine rings in g-C3N4. The weak peaks at 399.7 and 401.0 eV indicated the presence of the tertiary nitrogen N-(C)3 group and amino N-H [17]. As shown in Table 1, the ratio fraction of the tertiary nitrogen N-(C)3 group was lower in g-C3N4/rGO-800. This may be due to less N-(C)3 linkers in the hybrid material from the lowering of the dimensions of g-C3N4. In general, it can be deduced that there were no new groups in quantity in the hybrid. So π-π interaction was the main interaction between g-C3N4 and rGO.
The optical absorption features are a reflection of the photocatalytic activity. This was studied using the DRS technique and shown in Fig. 4(f). The visible light absorption coefficient of g-C3N4/rGO showed an upshift with the increase of GO loading. The optical images shown in Fig. 4(a)-(e) had a good appearance as an indication for this result. This is because rGO has a high visible light absorption coefficient. But the wavelength threshold remained relatively stable and the band gap of g-C3N4/rGO was almost identical to that of pristine g-C3N4 (Fig. 4(g) and Table 2). This phenomenon also demonstrated the non-chemical bond contact between g-C3N4 and rGO, in agreement with the XPS analysis.
The photocatalytic activity of g-C3N4/rGO and g-C3N4 was evaluated for the photodegradation of RhB under visible light irradiation. As shown in Fig. 5(a), RhB is an anti- photocorrosion material and has no detectable degradation without a photocatalyst under irradiation. The initial lower concentration of RhB was due to the adsorption of RhB on the surface of the photocatalyst. Both g-C3N4/rGO and g-C3N4 can initiate the degradation of RhB under visible light irradiation.
It is known that the photodegradation of RhB follows first order reaction kinetics. Its rate constant (k) can be calculated by Eq. 1 [17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27]:
where c is the concentration of RhB at time t, and c0 is the initial concentration of RhB [18]. As shown in Fig. 5(b), the photocatalytic efficiency of g-C3N4/rGO-5000 (0.0199 min-1), g-C3N4/ rGO-800 (0.0411 min-1) and g-C3N4/rGO-600 (0.0241 min-1) for the degradation of RhB were 1.2, 2.6 and 1.5 times that of pristine g-C3N4 (0.0160 min−1). Only g-C3N4/rGO-200 showed a decreased photocatalytic activity. Among the photocatalysts, g-C3N4/rGO-800 had the highest photocatalytic activity.
The influence of the pH value on the photocatalytic activity of g-C3N4/rGO for the degradation of RhB was also studied using g-C3N4/rGO-800 as the catalyst. This is shown in Fig. 5(c) and (d). The pH value was adjusted using HCl and NaOH solution. In the absence of g-C3N4/rGO-800, RhB has no detectable degradation in both acidic and alkaline conditions, which implies that RhB is resistant to acid and alkaline corrosion. The photodegrading rate of RhB has no significant change after 100 mg of NaCl was added into the photocatalytic system, which implies that free Na+ and Cl- have no significant influence. As shown in Fig. 5(c) and (d), the photocatalytic activity of g-C3N4/rGO is pH sensitive, with a much increased photodegrading rate at low pH values. The first order rate constant (0.1389 min-1) was 8.6 times that with pristine g-C3N4 at pH = 1.98. With the increase of pH value, the photodegrading rate of RhB decreased sharply. When the pH value was above 7, the degradation rate decreased to almost zero.
The photocatalytic process of g-C3N4 for the degradation of RhB can be divided into three steps, i.e., capture of photon, separation of e--h+ pair and generation of reactive species. The three steps are affected by the band gap, recombination rate of e--h+ pairs and specific surface area. As shown in Fig. 4 and Table 2, the band gap of g-C3N4/rGO was almost identical to that of pristine g-C3N4. Their optical absorption spectra are also similar. Compared to that of pristine g-C3N4 (47.87 m2/g), the specific surface area (ABET) of g-C3N4/rGO (12.16 m2/g) decreased when rGO was added. We think that it is the mixing, stirring and oven drying process that led to a highly dense state in the melamine/GO mixture. This was difficult to make porous unlike the original melamine. This indicated that the surface area made little contribution to the enhanced photocatalytic activity. The recombination rate of e--h+ pairs would be the key in the photocatalytic activity of g-C3N4/rGO for the degradation of RhB.
Photoluminescence (PL) is caused by the recombination of photo-generated e- and h+. So the PL spectrum can be used to evaluate the recombination rate of e- and h+. This is shown in Fig. 6(a). It was found that the fluorescence of g-C3N4/rGO was weaker than that of g-C3N4. A higher mass ratio of the GO loading gave a weaker fluorescence. So, it can be inferred that rGO transfers e- from g-C3N4 and facilitates the separation of e- and h+ [17, 26]. The separation of e- and h+ led to increased photocatalytic activity. However, the loading of rGO has an optimal content because excess rGO would have a light screening effect on g-C3N4 to decrease the photocatalytic activity.
Under visible light irradiation, an e- of g-C3N4 is excited from the VB to the CB, which generates a pair of free e- and h+. e- can react with oxygen to produce superoxide radicals (·O2-), hydrogen peroxide (H2O2) and hydroxyl radicals (·OH) by the following reactions [31]:
The effects of h+, ·O2-, H2O2 and ·OH on the photodegradation of RhB are different. In order to evaluate their effects, the photodegradation of RhB was measured after adding different scavengers. This is shown in Fig. 6(b). The degradation of RhB increased after adding H2O2 (100 mmol/L) in g-C3N4, indicating that ·OH accelerated the degradation of RhB. H2O2 by itself has no effect on the photodegradation. The degradation rate of RhB decreased slightly after adding isopropanol (IPA (100 mmol/L), a ·OH scavenger [32]). So, in terms of thermodynamics and kinetics, ·OH is not an effective contributor. The degradation of RhB slowed down significantly on adding triethanolamine (TEOA (100 mmol/L), a h+ scavenger [31]), indicating that h+ is the major contributor, and ·O2- and ·OH are comparatively minor contribution species. Even though adding either IPA or TEOA can retard the catalytic process, the amount of the scavenger was quite different. When the amount of IPA was 100 mmol/L, while that of TEOA was 10 mmol/L, 10 mmol/L TEOA has a larger effect in retarding the catalytic process than 100 mmol/L IPA, so we deduce that holes contribute more than superoxide radicals.
Furthermore, it was obvious that O2 was indispensable for the photocatalysis of g-C3N4/rGO-800 for the degradation of RhB because O2 can consume e- and prevent the accumulation of e-. So under acidic conditions, H+ can react with e-and promote the separation of e- and h+ indirectly, speeding up the photodegradation of RhB. Under alkaline conditions, OH- consumes the majority of h+, resulting in a significant decrease of the photodegrading rate of RhB. After a series of reactions, H+ and OH- can be recovered and the pH value remains relatively stable.
In theory, even OH- anions can completely consume holes, and photogenerated electrons are free to react with adsorbed water to form superoxide radicals that still have roles significantly for the degradation of RhB. But we know that holes can react with RhB directly in one step, while photogenerated electrons need two or more steps to react with RhB via superoxide radicals. OH- anions can retard the formation of superoxide radicals by some thermodynamic reasons.
The photocatalytic mechanism of g-C3N4/rGO for the degradation of RhB is summarized in Fig. 7. Under visible light, an e- of g-C3N4 is excited from the VB to the CB and the e--h+ pair is generated. h+ makes the major contribution to the degradation of RhB. e- transfers efficiently through the atomic sheets of rGO, so rGO can accelerate the separation of e--h+ pairs. In addition, e- can further react with O2 to produce ·O2- and ·OH, which also make contributions to the degradation of RhB.
A hybrid catalyst of g-C3N4/rGO formed by π-π interaction was made successfully from melamine and GO in air. The hybrid catalyst of g-C3N4/rGO synthesized with a mass ratio of 800/1 showed the best photocatalytic activity (0.0411 min-1) for the degradation of RhB, which was 2.6 times that of pristine g-C3N4 (0.0160 min-1). The catalyst also showed a pH-sensitive phenomenon with a much increased activity at low pH values (0.1389 min-1 at pH = 1.98). rGO, even in a very lower ratio in the hybrid, plays an important role in promoting the separation of e- and h+. h+ is the major contribution species while ·O2- and ·OH are minor contribution species in the photodegradation of RhB in the g-C3N4/rGO system.
2015, Vol. 36 Issue (7): 1009-1016
PDF (1341 KB)
2015, Vol. 36 Issue (7): 1009-1016
PDF (1341 KB)