Highly efficient visible-light photocatalysts: reduced graphene oxide and C₃N₄ nanosheets loaded with Ag nanoparticles

Abstract

In the current study, the degradation of methyl orange under visible-light irradiation was performed to gain an insight into the performance of the novel Ag/C₃N₄/reduced graphene oxide photocatalyst (Ag/C₃N₄/RGO). Using N,N-dimethylformamide (DMF) as an effective reducing agent as well as solvent, Ag nanoparticles were well anchored on C₃N₄/RGO nanosheets, which were prepared by a facile hydrothermal reaction. Inexpensive, stable C₃N₄ was coupled with well-conductive RGO and the plasmon resonance effect of Ag exhibited higher activities compared with pure C₃N₄, C₃N₄/RGO, Ag/RGO, and Ag/C₃N₄, respectively. The presence of Ag and RGO was confirmed by FTIR, XRD, and TEM, and their influence on the activity of C₃N₄ was demonstrated with the photocatalytic results, detection of reactive species and mechanism analysis. The photoluminescence spectra (PL) and the transient photocurrent response were also tested to determine the enhanced separation of photogenerated charge carriers. The facile synthesis of Ag/C₃N₄/RGO, together with their superior catalytic performance, successfully provide a promising route for the rational design of comparatively inexpensive and highly active C₃N₄-based ternary nanostructured composites.

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1. Introduction

Semiconductor photocatalysis has attracted intense attention as a potential solution to the worldwide energy shortage and in environmental purification, especially in the photodegradation of organic pollutants.^1–5 Over the past years, various semiconductor photocatalysts, such as TiO₂, SnO₂, ZnO, WO₃, Fe₂O₃, Bi₂WO₆, and In₂S₃, have been studied and employed in environmental remediation.⁶ However, the application of these pure semiconductor catalysts is limited due to various factors, including the high recombination rate of photogenerated electron–hole pairs, low absorption coefficients, or mismatch with the solar spectrum. Constructing efficient sunlight or visible-light-driven photocatalysts with an appropriate bandgap, strong oxidative/reductive ability, and high stability in water solution systems is thus a promising but challenging task.

Recently, a polymeric semiconductor material graphitic carbon nitride (C₃N₄), only made up of carbon and nitrogen, was used as a metal-free photocatalyst for solar energy conversion, hydrogen production, and environment purification, due to its cheap, abundant, medium bandgap (2.7 eV) and stable merits.^7–14 Nevertheless, pure g-C₃N₄ suffers from a high recombination rate of photogenerated electron–hole pairs, low specific surface area, and low visible-light utilization efficiency, which limit its photocatalytic performance. To solve these problems, many methods, such as chemical doping and modification, have been proposed. Silver nanoparticles¹⁵ (AgNPs) are often used in combination with a semiconductor as an efficient photocatalyst, because of their specific optical, electrical, magnetic, catalytic, and antibacterial features, and particularly due to their cheap price compared with platinum or gold. Some Ag-based composite have been demonstrated to be efficient photocatalysts under visible light by our group.^16,17 We have recently designed and prepared Ag/C₃N₄ composites with different AgNPs contents by a facile one-pot hydrothermal method and demonstrated their significantly enhanced photocatalytic efficiency, which was attributed to the more efficient separation of photoinduced electrons from holes and from more reactive species.¹⁸ In addition, graphene is known to be an exciting material in material science¹⁹ and has recently attracted much attention in photocatalysis,^20–25 due to its optical transmittance, large surface area, and high electrical conductivity.²⁶ It has been reported that graphene is also an excellent electron-donating modifier for g-C₃N₄, due to their similar layered structure and suitable electronic, mechanical, and chemical properties.²⁷ The binary photocatalyst graphene/g-C₃N₄, synthesized from the combination of graphene and g-C₃N₄, has also been used for hydrogen production, and showed that the H₂-production rate of the composite exceeded that of pure g-C₃N₄ by more than 3.07 times.²⁸ The combination of graphene sheets and AgNPs to C₃N₄ may further increase the photocatalytic activity, but this has never been studied.

Herein, we first report a ternary photocatalyst by anchoring AgNPs onto C₃N₄ and reduced graphene oxide (Ag/C₃N₄/RGO) sheets, as depicted in Fig. 1, for the degradation of methyl orange (MO) under visible light. We found that the prepared Ag/C₃N₄/RGO composite exhibited a significantly more rapid photodegradation activity compared with pure C₃N₄ and the binary photocatalysts Ag/C₃N₄, Ag/RGO, and C₃N₄/RGO. We also discuss possible photodegradation mechanisms of the composites in detail based on the experimental results, the PL spectra, their detection of reactive species, and the transient photocurrent response.

Fig. 1 Schematic of the formation process of Ag/C₃N₄/RGO composite.

2. Experimental

2.1. Preparation of graphene oxide (GO)

GO was synthesized using the modified Hummers method from natural graphite powder (natural, −325 mesh, Alfa Aesar).²⁹ Concentrated H₂SO₄ (69 mL) was added to a mixture of graphite powder (3.0 g) and NaNO₃ (1.5 g) in a 100 mL three-neck boiling flask, and the mixture was cooled to 0 °C using an ice bath. KMnO₄ (9 g) was very slowly added to the mixture under continuous stirring, and the temperature was kept below 25 °C. The mixture was then warmed to 35 °C and stirred for 7 h. Additional KMnO₄ (9 g) was added and the mixture was continuously stirred for 12 h at 35 °C. After cooling down to room temperature, the mixture was then diluted with 400 mL of deionized water, followed by the addition of 30% H₂O₂ (3 mL), causing the color of the mixture to change to bright yellow. Afterwards, the mixture was centrifuged and washed successively with 1 M HCl solution (at least three centrifugation cycles, 8000 rpm for 5 min, TG16-WS, Hunan Xiangyi Laboratory Instrument Development Co., Ltd. China) and deionized water (at least five centrifugation cycles, 10 000 rpm for 10 min) until the filtrate became neutral. The product was separated by centrifugation, washed with ethanol, and dried under vacuum at 60 °C overnight. The resulting product was dispersed in deionized water, followed by ultrasonic treatment for 2 h. The final product was separated by centrifugation, washed with ethanol, and dried under vacuum at 60 °C.

2.2. Preparation of C₃N₄/RGO

C₃N₄ was prepared according to our previous report.¹⁸ In a typical case, 0.21 g of as-prepared C₃N₄ was dispersed into 20 mL of deionized water by stirring for 10 min, followed by ultrasonication for 30 min. Afterwards, 0.09 g of GO was dispersed into 20 mL of deionized water, and sonicated for 1 h. Then, it was added to the above solution dropwise. The mixture was stirred for 30 min, and then transferred into a 50 mL Teflon-lined stainless steel autoclave. The temperature was increased to 140 °C and maintained for 6 h.³⁰ Afterwards, the mixture was cooled down to room temperature and washed with deionized water and ethanol several times, then dried under vacuum at 60 °C.

2.3. Preparation of Ag/C₃N₄/RGO, Ag/C₃N₄ and Ag/RGO

The typical procedures for the fabrication of the Ag/C₃N₄/RGO ternary hybrid were as follows: first, 0.17 g of dried C₃N₄/RGO powder was sonicated in 30 mL N,N-dimethylformamide (DMF) for 1 h in a beaker. In a separate beaker, 0.047 g of AgNO₃ and 0.05 g of PVP were stirred in 10 mL DMF for 30 min. Then, the two mixtures were poured into a screw-capped test tube and heated over a silicon oil bath for 20 h. A schematic of the preparation of Ag/C₃N₄/RGO nanocomposite is shown in Fig. 1. After being centrifuged repeatedly at 4500 rpm with deionized water to remove the adsorbed Ag⁺ ions and the unbound Ag NPs, the final product was obtained after washing with ethanol and drying at 50 °C under vacuum for 12 h. The final product was then stored for further characterization and photocatalytic study. Ag/RGO and Ag/C₃N₄ photocatalysts were prepared using the same method without the presence of C₃N₄ and GO, respectively.

2.4. Characterization

The crystal structure and phase purity of the Ag/C₃N₄ were analyzed by X-ray diffraction (XRD) using a D8 Advance X-ray diffraction (Bruker axs company, Germany) equipped with Cu-Kα radiation (λ = 1.5406 Å), employing a scanning rate of 7° min⁻¹ in the 2θ range from 5° to 80°. Fourier transform infrared spectroscopy (FT-IR) analysis was carried out on Nicolet Model Nexus 470 IR equipment to determine the specific functional groups present on the surface. The morphology of the as-prepared catalysts were also examined by transmission electron microscopy (TEM), which was recorded on a JEOL-JEM-2010 (JEOL, Japan) operating at 200 kV. UV-vis diffuse reflectance spectra (UV-vis/DRS) were recorded on a Shimadzu UV2450 spectrophotometer. The PL of the samples was obtained using a Varian Cary Eclipse spectrometer.

2.5. Photocatalytic experiments

The photocatalytic activity of the as-prepared Ag/C₃N₄/RGO composite was evaluated by the degradation of MO. All the experiments were performed in a self-made quartz photoreactor fitted with a circulation water system to maintain a constant temperature and under visible-light irradiation of a 300 W xenon lamp (PLS-SXE300, Trusttech Co. Ltd., Beijing) with a 400 nm cutoff filter to remove the UV irradiation. In all the photocatalytic degradation experiments, the catalyst (50 mg) was suspended in 100 mL of 10 mg L⁻¹ MO aqueous solution. Before the illumination, the suspension was magnetically stirred for 30 min in the dark to reach the adsorption–desorption equilibrium between the dye and the catalyst. At given irradiation time intervals, 3 mL of the suspension was collected and subsequently centrifuged to remove the catalyst particles. The concentration was analyzed by measuring the maximum absorbance at 464 nm for MO using a Shimadzu UV-2450 spectrophotometer. The degradation efficiency of methyl orange was calculated by the following equation:where C₀ and A₀ are the initial concentration and absorbance of methyl orange solution at 464 nm, corresponding to maximum absorption wavelength; C_t and A are the concentration and absorbance of methyl orange solution at 464 nm after UV light irradiation at any time.

2.6. Photoelectrochemical measurements

The photocurrents were measured using an electrochemical analyzer (CHI660B, ChenHua Instruments, Shanghai, China) to investigate the transition of the photogenerated electrons in C₃N₄, C₃N₄/RGO, Ag/C₃N₄, and Ag/C₃N₄/RGO, respectively. A standard three-electrode cell was used equipped with a platinum wire as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and an indium-tin oxide glass (ITO) as the working electrode, respectively. A 300 W Xe arc lamp with λ ≥ 400 nm was utilized as the light source. 0.1 M Na₂SO₄ aqueous solution was used as the electrolyte solution. For the working electrodes, 20 μL 5.0 mg mL⁻¹ C₃N₄, C₃N₄/RGO, Ag/C₃N₄, and Ag/C₃N₄/RGO solution (dispersed in ethanol) were dropped onto the pretreated ITO with a 1 cm × 0.5 cm area and dried to form the modified ITO.

3. Results and discussion

3.1. Characterization of photocatalysts

As revealed by the XRD pattern (Fig. 2a), two distinct peaks at 13.0° and 27.4° (corresponding to 0.681 nm and 0.326 nm, respectively), in the pure C₃N₄ sample can be indexed as the (100) and (002) diffractions for the graphitic materials, respectively, corresponding to the in-planar structural packing motif and the interlayer stacking of the aromatic system.⁷ In the XRD pattern of GO, the intense and sharp diffraction peak at 2θ = 9.8° is attributed to the (001) lattice planes. The C₃N₄/RGO exhibits a similar XRD pattern as pure C₃N₄, suggesting that the incorporation of GO sheets has little influence on the crystallite size and phase structure of C₃N₄. The four diffraction peaks of (111) at 38.2°, (200) at 44.6°, (220) at 64.8°, and (311) at 77.9° in the patterns of Ag/C₃N₄ and Ag/C₃N₄/RGO match well with the face-centered cubic structure Ag, indicating the formation of metallic Ag in these nanocomposites. We further analyzed the formation of GO, C₃N₄, C₃N₄/RGO, Ag/C₃N₄, and Ag/C₃N₄/RGO composites by using FTIR. The broad absorption at 3000–3700 cm⁻¹ and 1625 cm⁻¹ of GO correspond to the stretching mode of hydroxyl, and the stretching vibration at 1735 cm⁻¹ and 1061 cm⁻¹ are assigned to the C=O and C–O–C groups, respectively (Fig. 1b). The weak bending vibration and stretching vibration at 1424 cm⁻¹ and 1218 cm⁻¹ are assigned to the O–H deformations of the C–OH groups. However, the FTIR spectra of C₃N₄/RGO and Ag/C₃N₄/RGO composites show no characteristic peaks of GO, indicating the reduction of GO. The result is consistent with the result of the XRD pattern. The FTIR spectra of all the doped C₃N₄ samples show the typical stretching modes of CN heterocycles in the 1200–1650 cm⁻¹ range, and the breathing mode of the tri-s-triazine units at 808 cm⁻¹ indicating that modification with GO or/and AgNPs does not influence the structure of the composites. Therefore, the FTIR analysis, as well as the results of the XRD, supports the formation of the Ag/C₃N₄/RGO composite.

Fig. 2 XRD patterns (a) and FTIR spectra (b) of the as-prepared GO, C₃N₄, C₃N₄/RGO, Ag/C₃N₄, and Ag/C₃N₄/RGO composites.

The morphology and microstructure of the samples are examined by TEM and high-resolution TEM (HR-TEM). The TEM image of GO reveals that the surface morphology of GO is smooth and has a wrinkle-like structure (Fig. 3a), and the pure C₃N₄ structure is made up of a few nanometer-sized sheets and pores (Fig. 3b). As seen from the TEM image of Ag/C₃N₄, spherical-shaped Ag nanoparticles are observed uniformly on the surface of C₃N₄ (Fig. 3c), since adding PVP during the synthesis affects the molecular motion of the reduced silver and subsequently limits the aggregation of colloids.¹⁸ Though they are tightly connected with each other, we can also observe the intrinsic structure of C₃N₄ and reduced GO (Fig. 3d). From the TEM image of Ag/C₃N₄/RGO (Fig. 3e), it can be seen that the AgNPs and C₃N₄ sheets completely cover the RGO sheets to form the Ag/C₃N₄/RGO composites. The average diameter of AgNPs is about 10 nm. The existence of AgNPs is also confirmed by the HRTEM image in Fig. 3f, the lattice distance of 0.236 nm corresponding to the d-spacing of the (111) crystallographic planes of metallic Ag.

Fig. 3 TEM of the as-prepared GO (a), C₃N₄ (b), Ag/C₃N₄ (c), C₃N₄/RGO (d), Ag/C₃N₄/RGO (e), and HRTEM of Ag (f).

3.2. Photocatalytic activity and mechanisms

Fig. 4a shows the photocatalytic activity of all the samples under visible light. It clearly shows that the degradation efficiency of MO over Ag/C₃N₄/RGO was higher than that of other photocatalysts. MO degradation follows a pseudo-first-order kinetics (Fig. 4b), and the kinetic constant with C₃N₄, Ag/RGO, C₃N₄/RGO, Ag/C₃N₄ and Ag/C₃N₄/RGO are 0.035, 0.060, 0.085, 0.117, and 0.432 h⁻¹, respectively. The obtained degradation rate on Ag/C₃N₄/RGO was 12.3, 7.2, 5.1, and 3.7 times larger than that on C₃N₄, Ag/RGO, C₃N₄/RGO and Ag/C₃N₄, respectively.

Fig. 4 (a) Degradation rate of methyl orange under visible-light irradiation in the presence of catalysts: pure C₃N₄; Ag/RGO; C₃N₄/RGO; Ag/C₃N₄ and Ag/C₃N₄/RGO, and (b) MO degradation curves of ln(C/C₀) versus time for different catalysts.

Since the absorption property of a catalyst is very important, UV-vis diffuse reflectance spectra (DRS) was used to test the optical absorption properties of the photocatalysts (Fig. 5). Coupling with RGO, the absorption intensity in the visible-light region of C₃N₄ was clearly improved, albeit, modified with AgNPs, leading to the absorption edge being shifted towards a higher wavelength, indicating a decreased bandgap.¹⁷ Both Ag/C₃N₄ and Ag/C₃N₄/RGO show intensive absorption in the visible region, which is consistent with the surface plasmon resonance (SPR) effect of Ag, further confirming the formation of AgNPs. The bandgaps of C₃N₄, C₃N₄/RGO, Ag/C₃N₄, and Ag/C₃N₄/RGO are estimated, from their UV-vis spectra, to be 2.61, 2.37, 2.02, and 1.78 eV, respectively, showing an intrinsic semiconductor-like absorption in the blue region of the visible spectrum.

Fig. 5 UV-vis diffuse reflectance spectra (DRS) of bulk C₃N₄, C₃N₄/RGO, Ag/C₃N₄, and Ag/C₃N₄/RGO composites.

The improved photocatalytic performance of Ag/C₃N₄/RGO can be further explained by the PL spectra in Fig. 6. The pure C₃N₄ sample was found to have a higher PL intensity than the composites. This indicates that C₃N₄ had the highest optical recombination rate, which could deteriorate the photodegradation efficiency. However, PL quenching was observed in the composites, and was most significant in Ag/C₃N₄/RGO, which indicates that this composite had a lower rate of recombination of photoelectrons compared with pure C₃N₄ and the other composites, thus greatly improving the photocatalytic activity. To provide additional evidence for the separation of the photoelectrons and holes, the transient photocurrent responses of the C₃N₄, C₃N₄/RGO, Ag/C₃N₄, and Ag/C₃N₄/RGO samples were recorded over several cycles of on-off visible-light irradiation, as shown in Fig. 7. The on–off cycles of the photocurrent are highly reproducible, and all the samples show an apparently boosted photocurrent response under visible-light illumination, which is attributed to the separation of the photoelectrons and holes within the photo-electrode. The photoelectron may transport to the cathode, via C₃N₄ or RGO sheets, and photoinduced holes are trapped or captured by a reduced species in the electrolyte. Compared to the C₃N₄, C₃N₄/RGO, and Ag/C₃N₄ samples, the Ag/C₃N₄/RGO sample shows the highest transient photocurrent. This is presumably due to the following two reasons: (1) in the presence of AgNPs, the interface of Ag/C₃N₄ can form the Schottky junction, which results in the separation of the photoelectrons and holes; (2) RGO sheets, due to their two-dimensional π-conjugation structure, can act as an electron collector and transporter in the Ag/C₃N₄/RGO sample, which effectively suppresses the charge recombination, leaving more charge carriers to form reactive species.

Fig. 6 Room-temperature PL emission spectra of different photocatalysts excited by wavelength at 393 nm.

Fig. 7 The transient photocurrent response of C₃N₄, C₃N₄/RGO, Ag/C₃N₄, and Ag/C₃N₄/RGO composites.

To further explore the mechanism, reactive oxidative species trapping experiments were performed to investigate the main reactive oxidative species involved in the photocatalytic process by using three different scavengers: disodium ethylenediaminetetraacetate (EDTA, holes scavenger), tert-butanol (t-BuOH, OH˙ radicals scavenger), and p-benzoquinone (BQ, O₂˙⁻ radicals scavenger) (Fig. 8). The photodegradation of MO was significantly suppressed by the introduction of 1 mmol BQ, indicating that the O₂˙⁻ radical species are the main reactive species involved in the photocatalysis.³¹ In contrast, the photodegradation of MO undergoes no conspicuous changes by the introduction of 1 mmol EDTA anions, implying that the holes make no contribution to the degradation process.³² When 1 mmol of t-BuOH was added, the degradation rate of MO was reduced from 93.4% to 76%, indicating that OH˙ radical species are minor reactive species involved in the photocatalytic oxidation process.³³

Fig. 8 Degradation ratio of Ag/C₃N₄/RGO composites with different scavengers.

On the basis of the above experimental results and discussion, the improved photocatalytic performance arises from the enhanced light harvesting and more efficient separation of photogenerated electron–hole pairs due to the Ag/C₃N₄/RGO composite. The possible mechanism for the photocatalytic degradation of MO by Ag/C₃N₄/RGO is thus proposed and schematically illustrated in Fig. 9.

Fig. 9 Schematic of methyl orange degradation over Ag/C₃N₄/RGO composite photocatalyst under visible light.

The relevant reactions are listed in eqn (2)–(8). Under visible-light irradiation, owing to the visible-light wavelength bandgap energy of pure C₃N₄, it may absorb visible light and generate electron–hole pairs when it is irradiated under the light with photon energy equal to or greater than approximately 2.61 eV (eqn (2)). For the present Ag/C₃N₄/RGO composite, the process becomes quicker, because it has a smaller bandgap than pure C₃N₄. The photogenerated electrons from C₃N₄ can then transfer easily to the RGO sheet due to their suitable energy levels (eqn (3)),³⁴ leaving positively charged holes behind. The electrons were captured by the RGO sheet and utilized for the reduction of oxygen, thus forming O₂˙⁻ radical species (eqn (4)). Meanwhile, under light irradiation, the plasmonic Ag nanocrystals also generate electron–hole pairs through surface plasmon resonance (SPR), in which the active photo-induced electrons can reduce O₂ molecules to form O₂˙⁻ radicals,³⁵ leaving behind holes (Ag⁺) that can be filled by electrons from C₃N₄/RGO (eqn (5)–(7)). The O₂˙⁻ radicals together with the photo-generated holes in the VB of C₃N₄ can oxidize organic dye directly (eqn (8)).

C₃N₄ + hv → C₃N₄(e⁻ + h⁺) (2)

C₃N₄(e⁻ + h⁺) + RGO → C₃N₄(h⁺) + RGO(e⁻) (3)

RGO(e⁻) + O₂ → RGO + O₂˙⁻ (4)

Ag + hv → Ag* (5)

Ag* + O₂ → O₂˙⁻ + Ag⁺ (6)

Ag⁺ + RGO(e⁻) → Ag + RGO (7)

O₂˙⁻/C₃N₄(h⁺) + MO → CO₂ + H₂O (8)

Therefore, the RGO sheet and Ag nanocrystals play important roles in the efficient separation of photo-generated electron–hole pairs and the generation of reactive oxidation species of O₂˙⁻, and thereby enhance the photodegradation activity of ternary composites. In addition, the relative narrow bandgap, as well as the increased absorbance in the visible-light region, also contributes to the improved photocatalytic activity of the Ag/C₃N₄/RGO composite.

4. Conclusion

We synthesized a ternary photocatalyst Ag/C₃N₄/RGO by anchoring Ag nanoparticles onto C₃N₄ and RGO sheets, which exhibited superior photocatalytic activity over pure C₃N₄ and binary C₃N₄/RGO, Ag/RGO, and Ag/C₃N₄ photocatalysts under visible light. The enhanced photocatalytic activity could be attributed to the enhanced light harvesting and more efficient separation of photogenerated electron–hole pairs due to the addition of AgNPs and the RGO sheets. The Ag/C₃N₄/RGO photocatalyst may have potential applications in the fabrication of industrial photocatalytic devices and in environmental conservation.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no. 21003065). This work was partly financially supported by the Scientific Innovation Research of College Graduate in Jangsu Province (SJLX_0480) and Jiangsu Province Research Joint Innovation Fund-Prospective Joint Research Project (BY2014123-08). We also thank support of China Council Scholarship.

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