Au-loaded porous graphitic C₃N₄/graphene layered composite as a ternary plasmonic photocatalyst and its visible-light photocatalytic performance

Abstract

A novel ternary plasmonic photocatalyst, Au-loaded porous graphitic C₃N₄/graphene layered composite (Au/pg-C₃N₄/GR), was fabricated by a facile sonication-photodeposition technique. In this hybrid structure, a polymeric semiconductor pg-C₃N₄ was immobilized on the surfaces of graphene sheets to form a layered composite with Au nanoparticles of sizes 10–15 nm uniformly deposited on it. The photocatalytic performance of the as-prepared Au/pg-C₃N₄/GR composite was evaluated by degradation of methylene blue (MB) and ciprofloxacin (CIP) as representative dye pollutant and antibiotic pollutant under visible light irradiation, respectively. The degradation rates of MB and CIP over the Au/pg-C₃N₄/GR photocatalyst were 4.34 and 3.05 times higher that of porous g-C₃N₄ (pg-C₃N₄), respectively, and even 7.42 and 6.09 times higher than that of pure g-C₃N₄, respectively. The results indicated that an improved photocatalytic efficiency was obtained when Au nanoparticles and graphene sheets co-incorporated in porous g-C₃N₄. The porous structure within the samples is advantageous to the adsorption capacity. The surface plasmon resonance (SPR) effect of Au and electron-acceptor role of graphene, which would improve the visible light harvesting ability, facilitate photogenerated charge carrier separation, as well as create more active reaction sites, and synergistically contribute to the enhancement of photocatalytic activity. Moreover, a possible photocatalytic mechanism was also tentatively proposed.

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Introduction

Over the past few decades, semiconductor-based photocatalysis has been intensively investigated as it is a promising and green technology to solve the problems of worldwide energy crisis and environmental pollution by utilization of sustainable solar energy.¹ In the domain of pollutant elimination, various types of semiconductors, such as metal oxides, sulfides, nitrides, solid solutions and so on have been exploited as effective photocatalysts for degradation of organic pollutants.^2–5 Nevertheless, the search for advanced photocatalysts with a relatively simple fabrication process and high activity is still underway. Among the new photocatalysts under investigation, graphitic carbon nitride (g-C₃N₄), a polymeric semiconductor, has attracted extensive attention recently due to its unique two-dimensional structure, excellent chemical stability and tunable electronic structure.^6,7 Although g-C₃N₄ has already shown great potential in photocatalysis field for water splitting, pollutant degradation and CO₂ reduction,⁸ the inevitable shortcomings of low visible light utilization efficiency, fast recombination of photogenerated electron–hole pairs and small specific surface area still limit its photocatalytic activity. Thus, it is of importance to explore suitable strategies to improve the quantum efficiency of g-C₃N₄. The combination of g-C₃N₄ with other materials, such as semiconductors (TiO₂, ZnO, CdS) with matched energy levels,^9–11 noble metals (Au, Ag, Pd),^12–14 and carbon materials (rGO, CNT, C₆₀)^15–17 to form heterostructured composites has been proved to be an effective way to enhance photocatalytic activity. Besides, the introduction of porous structure in g-C₃N₄ by soft or hard templates, which resulted in increased surface area and more active sites for adsorption and photocatalytic reaction, is also beneficial to the enhancement of photocatalytic efficiency.^18–20 Consequently, fabricating porous g-C₃N₄ based heterostructured photocatalysts is a promising way to further improve the photocatalytic activity of g-C₃N₄.

Graphene is a monolayer of carbon atoms sp²-hybridized into a two-dimensional (2D) honeycomb lattice with properties such as excellent mobility of charge carriers, large surface area, optical transparency and good chemical stability, which gives it potential applications in nanoelectronics, biosensing, polymer composites, capacitors, and catalysis.^21–24 Based on the good properties of g-C₃N₄ and graphene, the hybridization of the two layered materials into a composite appears to be an effective way to improve the photocatalytic performance of g-C₃N₄. Previous reports also demonstrated that graphene/g-C₃N₄ composites exhibited enhanced visible-light photocatalytic activity than pure g-C₃N₄ towards H₂ production via water splitting and CO₂ reduction due to the introduction of graphene sheets could act as electron-conducting channels to efficiently separate the photogenerated charge carriers.^25,26 Furthermore, plasmonic photocatalysis has recently come into focus as a very promising technology for high-performance photocatalysis. It involves dispersal of noble metal nanoparticles (mostly Au and Ag) on semiconductor photocatalysts and obtains drastic enhancement of photoreactivity under broad range of UV and visible light. Besides, it is reported that g-C₃N₄ and graphene can as act idea supports for noble metals,^27,28 and the surface plasmon resonance (SPR) effect of the deposited noble metals can increase visible-light harvesting ability as well as induce a strong localized electric field to facilitate photocatalytic reactions.¹ Cheng et al. successfully fabricated AuNPs/g-C₃N₄ nanohybrids, and the nanohybrids showed enhanced visible-light photocatalytic activity towards the degradation of methyl orange, which can be ascribed to AuNPs-facilitated separation of photogenerated electron–hole pairs, and the surface plasmon resonance excitation in AuNPs.¹² Yang et al. reported that the synthesized Ag/g-C₃N₄ plasmonic photocatalysts exhibited enhanced photocatalytic activity in the degradation of methyl orange (MO) and p-nitrophenol (PNP) compared to pure g-C₃N₄ under visible-light irradiation, and such improved photocatalytic performance can be explained in terms of the efficient visible-light utilization efficiency due to the SPR absorption of AgNPs as well as fast generation, separation and transportation of the photogenerated carriers.²⁹ In addition, hybridizing novel metal/g-C₃N₄ with other semiconductor or carbon material to form ternary visible-light-driven photocatalysts have also been developed, which can be applied for effective water splitting and degradation of organic pollutants.^30,31

In this work, inspired by the porous structure introduced in g-C₃N₄ to increase the specific surface area, the electron collector and transfer role of graphene to promote charge transfer and the SPR effect of Au nanoparticles to improve visible-light adsorption capacity, for the first time, we developed a heterostructured plasmonic photocatalyst composed of porous g-C₃N₄ (pg-C₃N₄), graphene (GR) and Au by a simple sonication-photodeposition method. First, the pg-C₃N₄/GR layered composite was fabricated via sonochemical approach, then Au nanoparticles were loaded on the above layered composite by photodeposition. The Experimental results demonstrated that the Au/pg-C₃N₄/GR composite enabled much higher photocatalytic activities than pure g-C₃N₄ for organic pollutants MB and CIP degradation and the composite photocatalyst was stable after cycling photocatalytic experiments. In addition, the photocatalytic mechanism of Au/pg-C₃N₄/GR plasmonic photocatalyst was proposed and discussed.

Experimental section

Materials

Graphene nanoplatelets (GR) were purchased from TCI, Japan (6–8 nm thick). Chloroauric acid (HAuCl₄·4H₂O), methylene blue (MB) and ciprofloxacin (CIP) were purchased from Aladdin Chemical Regent Co., Ltd. (Shanghai, China). Urea, melamine, isopropanol and ammonium hydroxide (NH₃·H₂O) were purchased from Sinopharm Chemical Reagent Co. Ltd. All the reagents in this experiment are analytically pure and used without further purification.

Sample preparation

Preparation of pg-C₃N₄. The pg-C₃N₄ power is prepared by directly polymerization of urea according to the literature.³² In a typical procedure, 25 g of the low-cost precursor urea was placed into a covered crucible, then thermally treated in a Muffle Furnace at 550 °C for 3 h. Gas bubbles generated from the calcination process were used as soft template to introduce porous structure in the sample.³³ The obtained light yellow-colored powder washed with nitric acid (0.1 M) and distilled water, finally dried at 80 °C overnight. Bulk g-C₃N₄ was prepared by melamine calcination in a tube furnace at 550 °C for 4 h with a heating rate of 2 °C min⁻¹ under air atmosphere, as the control sample for photocatalytic activity comparison.

Preparation of Au/pg-C₃N₄/GR ternary plasmonic photocatalyst. First, pg-C₃N₄/GR was fabricated through a sonochemical approach similar as previous reports.^34,35 In detail, 0.005 g of GR was initially well dispersed in 60 mL of deionized water with pH adjusted to 10 by NH₃·H₂O (25%).³⁶ Then, 0.1 g of pg-C₃N₄ powder was added into the above suspension and the mixture was ultrasonicated for 2 h to enable the connection between pg-C₃N₄ and GR via electrostatic attraction.²⁵ After that, photodeposition is applied to load Au nanoparticles on the layered pg-C₃N₄/GR composite. Typically, 105 μL of HAuCl₄ (10 mg mL⁻¹ Au) aqueous solutions was added into the above mixed suspension with ultrasonic treatment for 10 min, and 2.0 mL of isopropanol was added to act as a hole scavenger. Then the suspension was photoirradiated for 2 h by a mercury arc lamp (250 W, GY-250, λ = 365 nm) under magnetic stirring and with continuous nitrogen sparging. The Au(III) salt source was reduced by photogenerated electrons from the excited pg-C₃N₄ during the photodeposition procedure. Finally, the solid product was collected by centrifugation and washed with distilled water for several times, and dried at 80 °C overnight to obtain the Au/pg-C₃N₄/GR with the calculated amounts of hybridized GR and deposited Au to be 5% and 1% weight total to pg-C₃N₄, respectively. Au/GR sample was prepared using the same method without the presence of pg-C₃N₄ and pg-C₃N₄/GR sample was obtained without the sequential photodeposition process. Specially, the contents of GR and Au were fixed at 5 wt% and 1 wt% within the ternary composite because a lot of literatures have already carefully investigated the effect of the GR or noble metal content on photocatalytic activity when incorporating with g-C₃N₄ (ref. 25, 28, 37 and 38) and we selected the contents within the optimal range.

Characterization

X-ray diffraction (XRD) patterns were obtained by a SmartLab XRD spectrometer (Rigaku) with Cu Kα radiation in the range of 10–80° (2θ). Energy dispersive X-ray spectroscopy (EDS) was carried out on a Zeiss (Ultra Plus) field emission scanning electron microscopy to analyze the composition of samples. Fourier transform infrared (FT-IR) spectra were measured by using a BRUKER-ALPHA FT-IR spectrometer. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were obtained from a JEM-2100 high-resolution transmission electron microscope. The nitrogen adsorption and desorption isotherms were measured at 77 K on an ASAP 2020 (Micromertics USA). UV-vis diffuse reflectance spectra (DRS) of the samples were measured by using a UV-vis spectrophotometer (UV-3600, Shimadzu) with an integrating sphere attachment at a wavelength range of 200–800 nm. PL spectra were recorded on a Shimadzu RF-5301 fluorescence spectrophotometer at room temperature with an excitation wavelength at 385 nm.

Photocatalytic reaction

Water purification has became a highly essential issue since the wastewaters released into the aquatic environment normally contain organic pollutants such as dyes from textile industry and antibiotic residues from pharmaceuticals industry, which have potential harmfulness on human health and living ecosystems.^39,40 Herein, we choose the degradation of dye pollutant representative MB and antibiotic pollutant representative CIP under visible light irradiation to evaluated the photocatalytic performance of the as-prepared photocatalysts. The photocatalytic reaction was performed in a Pyrex reactor equipped with reflux water system to keep its temperature constant. In a typically photocatalytic test, prior to irradiation, the photocatalyst (100 mg) was dispersed in 100 mL of MB/CIP aqueous solution (10 mg L⁻¹) and magnetically stirred for 50 min in the dark to achieve absorption–desorption equilibrium. After that, the above suspension was irradiated by a light source of 500 W xenon lamp (Philips) with a 420 nm cutoff filter. At 30 min intervals, 3 mL of suspension was withdrawn and filtered through a 0.45 μm PTFE syringe filter to get clear liquid. The concentrations of MB/CIP in the liquid were analyzed by measuring the intensities of the characteristic adsorption peaks of 663 (λ_MB) and 272 nm (λ_CIP), respectively.

Results and discussion

Structure and morphology

Fig. 1 shows the XRD patterns of bare GR, pg-C₃N₄ and Au/pg-C₃N₄/GR composite. As can be seen, the bare GR shows two pronounced diffraction peaks of (002) and (100) planes at about 2θ = 26.3° and 54.5°.²⁷ Pure pg-C₃N₄ shows one characteristic diffraction peak of (100) plane at about 2θ = 12.7° related to in-planar structural packing motif, and another diffraction peak of (002) plane at 27.3° corresponding to the interlayer stacking of the aromatic system.⁴¹ In addition to the diffraction peaks of GR and pg-C₃N₄ included in the XRD pattern of Au/pg-C₃N₄/GR composite, two new diffraction peaks at 38.1° and 44.3° appeared in the curve, which can be indexed to the (111) and (200) planes of metallic Au (JCPDS no. 65-2870). Such observations indicated Au nanoparticles were successfully loaded on the layered composite after photodeposition process and the three components of Au, pg-C₃N₄, GR were finally hybridized. Besides, it should be noted that the typical interlayer-stacking peak (002) of pg-C₃N₄ in Au/pg-C₃N₄/GR composite had a little shift and increased to 27.6°, suggesting that the interlayer distance has a little change when coupling pg-C₃N₄ with GR via sonication and it is also indicative of intercalation of GR into pg-C₃N₄.⁴² The EDS pattern (Fig. 1(b)) further demonstrated that the Au/pg-C₃N₄/GR sample was only composed of C, N, and Au atoms, and no other impurity was observed.

Fig. 1 XRD patterns of (a) GR, pg-C₃N₄, Au/pg-C₃N₄/GR and (b) EDS spectrum of Au/pg-C₃N₄/GR composite.

Fig. 2 depicts the FT-IR spectra of the as-prepared photocatalysts. In the case of pure pg-C₃N₄, strong bands in the 1200−1700 cm⁻¹ correspond to the typical stretching vibration of C–N heterocycles, the peak at 813 cm⁻¹ presents the characteristic breathing mode of triazine units and the broad peak at 3000−3500 cm⁻¹ can be assigned to the stretching vibration of N–H and the stretching vibration of O–H of the physically adsorbed water.^43,44 As can be seen, Au/pg-C₃N₄, pg-C₃N₄/GR and Au/pg-C₃N₄/GR samples exhibited almost similar features as pure pg-C₃N₄. No characteristic peak at 1557 cm⁻¹ for the skeletal vibration of the graphene sheets⁴⁵ appeared in the curves of pg-C₃N₄/GR and Au/pg-C₃N₄/GR due to the low GR content. While small difference was still can be found with the introduce of GR in the composite, the characteristic peaks for C–N heterocycles and triazine units displayed a slight red shift to lower wavenumbers of 809, 1246, 1321, 1397, 1570 and 1639 cm⁻¹, instead of 813, 1252, 1327, 1403, 1574 and 1643 cm⁻¹ in pure pg-C₃N₄, respectively. The results implied that the bond strength of C–N was weakened, thus it is reasonable to conclude that there was an interaction between pg-C₃N₄ and graphene,²⁵ which is consistent with the XRD analysis.

Fig. 2 FT-IR spectra of the as-prepared pg-C₃N₄, Au/pg-C₃N₄, pg-C₃N₄/GR and Au/pg-C₃N₄/GR composite.

TEM and HRTEM were used to investigate the detailed morphology of and microstructure of Au/pg-C₃N₄/GR photocatalyst. As indicated in Fig. 3(a), pg-C₃N₄ possesses a special structure consisting of small flat sheets with wrinkles and irregular shape, and a typical porous morphology of the sample is also exhibited, which was highly similar to previous report.³³ The TEM image of GR (Fig. 3(b)) displays that it has a two-dimensional structure consisting of sheets with micrometer-long wrinkles. Fig. 3(c) shows a typical TEM image of Au/pg-C₃N₄/GR composite. It is observed that pg-C₃N₄ sheets were immobilized on the surfaces of GR sheets with most of the Au nanoparticles dispersed on the surfaces of pg-C₃N₄ and the other portion deposited on the GR surfaces. Such special sheet-on-sheet structure displayed a distinguished and close layered connection between pg-C₃N₄ and graphene. Furthermore, the interplanar distances of 0.234 nm measured out in the HRTEM image (Fig. 3(d)) can be indexed to the lattice spacing of the Au (111) plane.⁴⁶ All of the above observations suggested that the heterostructured Au/pg-C₃N₄/GR was indeed formed.

Fig. 3 Typical TEM images of (a) pg-C₃N₄, (b) GR, (c) Au/pg-C₃N₄/GR and HRTEM (d) image of Au/pg-C₃N₄/GR composite.

Nitrogen adsorption–desorption isotherms analysis is performed to investigate the porous nature and specific surface area of the samples. As shown in Fig. 4, nitrogen adsorption–desorption isotherm curves of the pg-C₃N₄ and Au/pg-C₃N₄/GR samples exhibit a type IV with a H3 hysteresis loop according to the IUPAC classification, which is a characteristic feature of the mesopores.¹⁸ Moreover, the adsorption branch of nitrogen isotherms shows a steady increase at P/P₀ approaching unity, suggesting the formation of large mesopores and small macropores.²⁶ The pore size distributions of both samples (inset of Fig. 4) are broad, which across the mesopore to macropore range and center at about 4 nm. Table 1 shows the parameters obtained from nitrogen desorption isotherms of the samples. As can be seen, the BET surface area, the single-point total pore volume and the mean pore size for Au/pg-C₃N₄/GR sample exhibit a small decrease than that of pure pg-C₃N₄, which is due to the introduction of Au nanoparticles might load in the porous structure.

Fig. 4 N₂ adsorption–desorption isotherms and the corresponding pore size distribution curves (inset) of the as-prepared pg-C₃N₄ and Au/pg-C₃N₄/GR composite.

Table 1 Parameters obtained from the nitrogen desorption isotherm experiments

Sample	Surface area m^2 g^−1	Pore volume cm^3 g^−1	Mean pore size nm
pg-C3N4	83.50	0.47	24.97
Au/pg-C3N4/GR	62.08	0.41	24.17

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UV-vis DRS and PL spectral analyses

The optical absorption properties of pg-C₃N₄, pg-C₃N₄/GR, Au/pg-C₃N₄ and Au/pg-C₃N₄/GR samples were characterized by UV-vis diffuse reflectance spectroscopy and the corresponding spectra are shown in Fig. 5. It is observed that pure pg-C₃N₄ shows a marginal adsorption edge at about 460 nm, corresponding to the reported optical band gap at around 2.7 eV, thus the photoconversion efficiency in the visible range is rather low. Moreover, the pg-C₃N₄/GR, Au/GR and Au/pg-C₃N₄/GR samples exhibit the similar characteristic absorption edge as pure pg-C₃N₄, indicating the basic framework of the host pg-C₃N₄ almost stayed unchanged after the hybridization.⁴⁷ Therefore, we deduce that carbon species from GR was not incorporated to the lattice of pg-C₃N₄ and the graphene sheets just act as a layered support for pg-C₃N₄. Whereas compared with the absorption spectrum of pg-C₃N₄, the pg-C₃N₄/GR sample displays a wide absorption ranging from 200 to 800 nm, indicating the introduction of GR has a positive effect on the optical property. Compared with pg-C₃N₄/GR, Au/pg-C₃N₄/GR composite shows more intensive absorption in the visible-light region, with a broad band peak located at around 550 nm, arising from the SPR effect of the Au nanoparticles.⁴⁸ On account of the improved optical adsorption property, it is anticipated that the Au/pg-C₃N₄/GR photocatalyst can achieve more efficient utilization of visible light and therefore show enhanced visible-light-driven photocatalytic activity.

Fig. 5 UV/vis diffuse reflectance spectra of pg-C₃N₄, pg-C₃N₄/GR, Au/pg-C₃N₄ and Au/pg-C₃N₄/GR samples.

PL spectral analysis was applied to investigate the transfer and recombination processes of photogenerated electron–hole pairs in the photocatalysts. PL emission intensity is related to the recombination rate of excited electron–hole pairs. It was generally believed that a lower PL emission intensity is an indication of a lower recombination of photogenerated electron–hole pairs.⁴⁹ Fig. 6 illustrates PL emission spectra of pg-C₃N₄, pg-C₃N₄/GR, Au/pg-C₃N₄ and Au/pg-C₃N₄/GR composite under an excitation wavelength of 385 nm. It can be seen that all samples exhibit an emission peak centered at about 460 nm, which could be ascribed to the band gap recombination of photoexcited electron–hole in pg-C₃N₄. The same emission peak also indicated that the band gap of composite samples is as same as that of pure g-C₃N₄ even if g-C₃N₄ was hybridized with Au or/and GR. As the band gap has no change, a lowering in the energy level of pg-C₃N₄ will not happen. To the best of our knowledge, suitable element doping in g-C₃N₄ would cause a lowering in the energy level, which further also can lower PL intensity. For example, B-doped g-C₃N₄, the PL emission peak would have a shift compared to that of g-C₃N₄. The PL band of B-doped g-C₃N₄ shifts toward longer wavelengths by about 10 nm, and this shift is associated with the 0.04 eV decrease in band gap for g-C₃N₄ by boron doping.⁵⁰ In our experiment, compared with pg-C₃N₄, the hybridization of Au or/and GR had not changed the position of PL emission peak, but lowed the relative intensity. In particular, the ternary Au/pg-C₃N₄/GR system reveals the lowest PL emission intensity among all samples. These results indicated that the heterojunction effect of Au and GR with pg-C₃N₄ accounted for the effective electron–hole pair separation and transfer, which is advantageous to the enhancement of photocatalytic efficiency.¹⁸

Fig. 6 PL emission spectra of pg-C₃N₄, pg-C₃N₄/GR, Au/pg-C₃N₄ and Au/pg-C₃N₄/GR samples.

Photocatalytic performance

Fig. 7 presents photocatalytic degradation of MB and CIP monitored according to the concentration change versus time in the presence of g-C₃N₄, pg-C₃N₄, pg-C₃N₄/GR, Au/pg-C₃N₄ and Au/pg-C₃N₄/GR photocatalysts under visible light irradiation, where C is the concentration of target pollutant remaining in the solution after irradiation time t, and C₀ is the initial concentration of target pollutant. As can be seen from Fig. 7(a) and (b), for the adsorption process in the dark, all of the pg-C₃N₄-based photocatalysts exhibited adsorption capacity of about 49% for MB and 22% for CIP, while g-C₃N₄ sample showed adsorption capacity of only 9.7% for MB and 5.1% for CIP. The porous structure within the samples probably contributed to such good adsorption performance of pg-C₃N₄-based photocatalysts. The blank experiments also demonstrated that the direct photolysis of MB and CIP almost can be ignored since the target pollutants were only slightly degraded without photocatalyst in the reaction. Fig. 7(a) clearly shows that the degradation rates of MB over pg-C₃N₄, pg-C₃N₄/GR, Au/pg-C₃N₄ and Au/pg-C₃N₄/GR photocatalysts were higher than that of pure g-C₃N₄. In the photocatalytic reaction, Au/pg-C₃N₄/GR displayed the best degradation efficiency, leading to almost 100% photodegradation of MB after irradiated for 180 min, whereas pure g-C₃N₄ can only photodegrade 47.4% MB after the same time. As for the degradation of CIP, it is observed from Fig. 7(b) that the degradation rate follows an order of Au/pg-C₃N₄/GR (96.7%) > Au/pg-C₃N₄ (88.3%) > pg-C₃N₄/GR (81.0%) > pg-C₃N₄ (72.6%) > g-C₃N₄ (44.5%) after 240 min visible light irradiation. The degradation efficiency order for CIP by the as-prepared photocatalysts is consistent with that for MB. In addition, photocatalytic degradation of organic pollutants generally follows pseudo-first-order kinetics if C₀ is within the millimolar concentration range, and the kinetic model can be expressed by equation ln(C₀/C) = kt, where k is the kinetic rate constant. The corresponding kinetic constants (k) were calculated and depicted in Fig. 8. Obviously, pg-C₃N₄ presents higher kinetic rate constant than g-C₃N₄, and the introduction of Au or GR with pg-C₃N₄ can further increase the kinetic rate constant to some extent. Moreover, Au/pg-C₃N₄/GR photocatalyst exhibits the highest rate constant among all of the samples for the degradation of both MB and CIP, which was 4.34 and 3.05 times higher that of pure pg-C₃N₄, respectively, and even 7.42 and 6.09 times higher than that of pure g-C₃N₄, respectively. The results suggested that the photocatalytic activity can be improved by introducing the porous structure within the sample and hybridizing Au or GR with pg-C₃N₄, particularly further enhanced over the Au and GR co-decorated pg-C₃N₄ photocatalyst. The probable reason for the enhanced photocatalytic activity will be discussed in the following proposed photocatalytic mechanism section in detail. Furthermore, the stability of Au/pg-C₃N₄/GR photocatalyst was investigated by recycling the photocatalyst for repeated degradation reactions, and the performed results are displayed in Fig. 9. It shows that the Au/pg-C₃N₄/GR sample exhibits a slight decline rather than obvious decrease of activity after four consecutive reaction cycles for both MB degradation (Fig. 9(a)) and CIP degradation (Fig. 9(b)), where the photocatalytic efficiency reduces only 5.2% and 9.4%, respectively, indicating the ternary plasmonic photocatalyst Au/pg-C₃N₄/GR has good photochemical stability.

Fig. 7 Photocatalytic activities of the as-prepared g-C₃N₄, pg-C₃N₄, pg-C₃N₄/GR, Au/pg-C₃N₄ and Au/pg-C₃N₄/GR for degradation of MB (a), and CIP (b) under visible light irradiation.

Fig. 8 The kinetic constants for the photocatalytic degradation processes of MB (a), and CIP (b) under visible light irradiation.

Fig. 9 Four photocatalytic degradation cycles of MB (a), and CIP (b) by Au/pg-C₃N₄/GR composite under visible light irradiation.

Proposed mechanism for enhanced photocatalytic activity

On the basis of the results described above, we proposed the following photocatalytic mechanism. Fig. 10 shows the schematic representation of the proposed mechanism for target pollutants degradation over Au/pg-C₃N₄/GR ternary plasmonic photocatalyst. When Au/pg-C₃N₄/GR is subjected to visible light irradiation, pg-C₃N₄ could be excited to produce photogenerated electrons (e⁻) and holes (h⁺) as well as the surface plasmon resonance (SPR)-excited Au nanoparticles generated hot electron–hole pairs, with the hot electrons transiently occupying empty states in the Au conduction band (CB) above the Fermi energy. The SPR effect of Au can not only adsorb visible light to improve the light utilization efficiency, but also cause intense local electromagnetic fields to speed up the formation rate of holes and electrons within the semiconductor.⁶ As the Fermi level of Au (0.6 eV) is higher than the CB value of pg-C₃N₄ (−1.13 eV) and the Fermi level of graphene (−0.08 eV),^25,51 the hot electrons generated by the SPR of Au could transfer toward the CB of pg-C₃N₄ and graphene sheets. While the hot holes left in Au nanoparticles would react with H₂O to produce active species ˙OH. The electrons in the CB of pg-C₃N₄ also had the tendency to flow into the graphene sheets since the CB of pg-C₃N₄ (−1.13 eV) is smaller than the work function of graphene (∼4.6 eV).⁵² As illustrated, the photogenerated electron–hole pairs separated effectively by the interface formed in the Au/pg-C₃N₄/GR. Furthermore, these captured electrons by graphene sheets which acted as extra reaction sites could react with oxygen to form active species ˙O₂⁻. Meanwhile, some of the photogenerated electrons in pg-C₃N₄ would migrate to the surface and reacted with dissolved oxygen molecules O₂ to generate active species ˙O₂⁻ owning to E_CB of pg-C₃N₄ (−1.12 eV) is more negative than the redox potential of O₂/˙O₂⁻ (−0.33 eV). With respect to the photogenerated holes left in the VB of pg-C₃N₄, they reacted with the target pollutants directly instead of oxidized H₂O to produce active species ˙OH to participate in the reaction, which can be explained that the E_VB value of pg-C₃N₄(+1.57 eV) is lower than the redox potential of ˙OH/H₂O (+2.68 eV).⁴⁹ These active species ˙O₂⁻ and ˙OH produced during the photocatalysis process would further react with the organic pollutants to achieve the degradation. In addition, in order to real the role of O₂, a control experiment for MB degradation by Au/pg-C₃N₄/GR photocatalyst was carried out under N₂ atmosphere. When N₂ continuous sparging through the reaction suspension to remove the O₂, the degradation rate of MB was remarkably decreased to 34.5% after the same irradiation time of 180 min, while the degradation rate of MB was almost 100% when the reaction was performed under air atmosphere. The result indicated that O₂ is an important factor for MB degradation over Au/pg-C₃N₄/GR photocatalyst. The dissolved O₂ was reduced by photogenerated electrons (e⁻) to form ˙O₂⁻ radicals, and ˙OH radicals also can produced via multistep reduction of O₂. Therefore, O₂ would affect the formation of active species ˙O₂⁻ and ˙OH in the reaction, which are significantly crucial to photocatalytic activity.⁵⁰ The major routes of photocatalytic degradation of MB/CIP under visible-light irradiation were proposed as follows:

pg-C₃N₄ + hν → pg-C₃N₄ (e⁻ + h⁺)

Au + hν → Au (e⁻ + h⁺)

Au (e⁻) + GR → Au + GR (e⁻)

pg-C₃N₄ (e⁻) + GR → pg-C₃N₄ + GR (e⁻)

pg-C₃N₄ (e⁻) + O₂ → pg-C₃N₄ + ˙O₂⁻

GR (e⁻) + O₂ → GR + ˙O₂⁻

˙O₂⁻ + H₂O → ˙OH + HO₂˙

Au (h⁺) + H₂O → ˙OH + H⁺

˙O₂⁻ + H⁺ → HO₂˙

2HO₂˙ → H₂O₂ + O₂

H₂O₂ + ˙O₂⁻ → ˙OH + OH⁻ + O₂

Au (h⁺) + OH⁻ → Au + ˙OH

˙O₂⁻, ˙OH, pg-C₃N₄ (h⁺) + MB/CIP → degradation products

Fig. 10 Proposed photocatalytic mechanism for degradation of MB/CIP by Au/pg-C₃N₄/GR composite under visible light irradiation.

By hybridizing both of Au nanoparticles and graphene sheets with pg-C₃N₄ to form the Au/pg-C₃N₄/GR heterostructure, mainly three advantages were achieved to enhance the photocatalytic activity: (i) taking efficient utilization of both UV and visible light; (ii) the photogenerated electron–hole pairs' recombination was effectively hindered; (iii) leading to more reaction sites with Au nanoparticles and graphene sheets. Therefore, such Au/pg-C₃N₄/GR composite is expected to be an excellent visible-light-driven plasmonic photocatalyst.

Conclusions

In summary, we have demonstrated a simple and efficient approach to prepare Au/pg-C₃N₄/GR composite by using sonication-photodeposition method. The ternary composite possesses particular structure, which is the porous semiconductor pg-C₃N₄ sheets immobilized on the graphene sheets to form layered composite with Au nanoparticles deposited on it. Such Au/pg-C₃N₄/GR hybrid structure provides efficient light harvesting, effective photogenerated electron separation and transportation as well as increase of the reaction sites compared to pure g-C₃N₄, which make it to be a good plasmonic photocatalyst for the degradation of organic pollutants MB and CIP, and the photocatalytic activity is apparently better than that of pure g-C₃N₄. In all, our work opens up new possibility in developing new effective and stable pg-C₃N₄-based photocatalytic materials for the applications of environmental remediation and even hydrogen production by utilizing solar energy.

Acknowledgements

The authors are grateful to the financial supports of National Natural Science Foundation of China (Grant No. 21376051, 21306023, 21106017), Natural Science Foundation of Jiangsu (Grant No. BK20131288), Fund Project for Transformation of Scientific and Technological Achievements of Jiangsu Province of China (Grant No. BA2014100) and the Fundamental Research Funds for the Central Universities (3207045301).

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Footnote

† J. J. Xue and S. S. Ma contributed equally to this work.

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