Facile preparation of a 3D rGO/g-C₃N₄ nanocomposite loaded with Ag NPs for photocatalytic degradation

Abstract

Graphene-based and g-C₃N₄-based nanocomposites can effectively remove organic pollutants from water. However, the reasonable design and scale preparation of hybrid nanomaterials of reduced graphene oxide (rGO), g-C₃N₄ and silver nanoparticles (Ag NPs) with improved performance for practical application need to be further explored. Herein, a 3D rGO/g-C₃N₄ nanocomposite loaded with Ag NPs was successfully fabricated through a facile three-step synthetic route. The microstructure and morphology of GO, g-C₃N₄, rGO/g-C₃N₄ and rGO/g-C₃N₄ nanocomposite loaded with Ag NPs were characterized and analyzed. The experimental results show that the as-prepared nanocomposite loaded with Ag NPs has excellent activity to remove methylene blue (MB) from water under visible light irradiation, and its maximum removal capacity is high as 49.60 mg g⁻¹ within 60 min. Based on its possible catalytic process and kinetic analysis, the adsorption and catalytic performance of this nanocomposite may be attributed to a synergistic effect of rGO, g-C₃N₄ and Ag NPs. In addition, it can provide a useful reference for the rational design and scale preparation of rGO/g-C₃N₄ nanocomposite for practical applications.

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1. Introduction

At present, organic pollutants in water can cause a serious threat to human health and the safety of the ecological environment.^1,2 Moreover, organic dyes and aromatic compounds are generally recognized as the main organic pollutants in water.^3,4 To solve this problem, numerous approaches have been employed to remove organic pollutants from water.^1–5 Among them, adsorption and photocatalytic degradation are generally considered as efficient methods, especially graphene-based and g-C₃N₄-based nanocomposites have attracted extensive research attention.^3–7

On the one hand, graphene as a 2D nanocarbon material, has lots of novel functional applications due to its high strength, high conductivity and large specific surface area and so on.^8–10 Graphene-based nanomaterials are generally used as excellent adsorbents and catalysts owing to their excellent physical and chemical properties.^11–14 However, graphene has no functional groups (such as carboxyl group, hydroxyl group and epoxy group) on its surface, and van der Waals forces lead to easy agglomerate between different layers,^15,16 thus it is very difficult to interact with other molecules or ions, which hinders its practical functional application.

On the other hand, graphene oxide (GO), as a derivative of graphene, has some advantages such as rich pore structure and some oxygen-containing groups on its surface.^17–19 Importantly, these oxygen-containing groups can provide numerous reactive sites to enhance interactions with other molecules or ions.^20–22 Moreover, the negatively charged GO with excellent hydrophilicity can strongly adsorb heavy metal cations and cationic dyes in wastewater through electrostatic actions and π–π interactions.^23–25

At the same time, to improve the adsorption and catalytic performance of GO, it is usually hybridized to prepare GO-based nanocomposites, especially nanomaterials hybridized with Ag NPs have attracted intense attention because of their excellent performance.^26–28 For example, Rath and co-workers²⁸ designed GO/Ag nanocomposite to adsorb crystal violet dye at pH of 8, and the maximum adsorption capacity of crystal violet was as high as 48.78 mg g⁻¹. Moreover, after many adsorption and desorption cycles, its recyclability and stability were also satisfactory. Naeem and co-workers⁴ prepared GO/Ag nanocomposite not only as adsorbent to adsorb multiple pollutants, but also as catalyst to catalyze the degradation of dyes. Mohamed and co-workers²⁹ designed rGO impregnated with Ag NPs to catalyze the degradation of MB, and this nanocomposite of rGO with Ag NPs was efficiently used as adsorbent and catalyst to remove organic dyes because of its excellent electrical conductivity and highly catalytic active feature of Ag NPs.

Recently, graphitic carbon nitride (g-C₃N₄), as a novel 2D graphene analogue, has emerged as a promising photocatalyst since 2009 (ref. 30) due to its excellent physical and chemical properties such as tunable band structure, non-toxicity, facile synthesis and chemical stability.^31–33 However, the pristine g-C₃N₄ catalyst still has some inherent disadvantages such as its poor charge separation, limited light absorption and low surface area, thus leading to its moderate photocatalytic activity.^34–36

To improve the photocatalytic performance of g-C₃N₄-based photocatalysts, some further modification strategies (such as vacancy engineering, nanostructure design, heteroatom doping and forming heterostructure) have been attempted.³⁷ Particularly, g-C₃N₄ with 3D nanostructure can increase the specific surface area and improve the anchoring stability of cocatalysts. Moreover, graphene with 3D nanostructure can promote light absorption and enhance charge migration.³⁸ because graphene acts as a light-transmitting material and a conductor.³⁹ At the same time, the loaded cocatalysts can be used as the electron acceptors to remarkably promote the electron–hole separation.³⁷

More recently, many efforts have been focused on the internal electrons of g-C₃N₄ at the atomic scale through monatomic engineering.^40–43 The metal atoms can be uniformly anchored on the network by forming coordination bonds with lone pair electrons of the nitrogen contained in g-C₃N₄. Specifically, Ag NPs dispersed in the g-C₃N₄ nanosheets can significantly accelerate the charge separation to improve the photocatalytic activity.^44,45 For example, Jiang and co-workers⁴⁰ designed the g-C₃N₄ catalyst anchored with Ag NPs, which exhibited excellent catalytic activity and durability for efficient photocatalytic hydrogen evolution, because the Ag NPs not only extend the visible light absorption region but also facilitate electron transfer.^41–43

Although numerous results of g-C₃N₄-based photocatalysts have been achieved, however, to overcome their serious drawbacks such as small specific surface area, fast charge recombination and limited visible-light absorption,^46,47 the reasonable design of g-C₃N₄-based photocatalysts with rGO and Ag NPs is still a research hotspot. In particular, Ag doping can not only enhance the light absorption but also accelerate the photogenerated carrier transfer process, and its related mechanism needs to be given intensified attention. Therefore, the reasonable design and scale preparation of hybrid nanomaterials of rGO, g-C₃N₄ and Ag NPs with improved performance for practical applications need to be further explored.

In this paper, we successfully fabricated a 3D rGO/g-C₃N₄ nanocomposite loaded with Ag NPs (rGO/g-C₃N₄/Ag NPs) through a facile three-step synthetic route. At the same time, the adsorption and catalytic properties of this nanocomposite were analyzed. Moreover, its possible catalytic process and kinetic analysis were discussed.

2. Experimental section

2.1 Materials and reagents

All the chemicals were of analytical grade and purchased from National Pharmaceutical Reagent Company, including graphite powders (99.5 wt% purity, 200 mesh), concentrated sulfuric acid (H₂SO₄, 98 wt%), sodium nitrate (NaNO₃), potassium permanganate (KMnO₄), hydrogen peroxide (H₂O₂, 30 wt%), sodium chloride (NaCl), silver nitrate (AgNO₃), ammonia (NH₃·H₂O), methylene blue (MB), melamine and glucose. All the chemicals were used without further purification. Deionized water (a resistance of 18 MΩ) made from a Milli-Q solvent system in our own lab was used through all the experiments.

2.2 Methods

2.2.1 Synthesis of GO sheets. GO was prepared from natural graphite powders based on the oxidation intercalation of the improved Hummers' method^18,19,48 and subsequent exfoliation under ultrasound.^49–51 As a typical process, firstly, 10.0 g of graphite powders, 5.0 g of NaNO₃ and 240 mL of H₂SO₄ (98 wt%) were mixed in a beaker under an ice-water bath with electromagnetic stirring for 30 min. Secondly, 30.0 g of KMnO₄ was slowly added to the mixture under an ice-water bath, and this addition was completed within 30 min under electromagnetic stirring. Thirdly, the obtained mixture was stirred for 3 h at room temperature, and 300 mL of deionized water was slowly added into the above mixture, keeping stirring for 1 h. Fourthly, 35 mL of H₂O₂ (30 wt%) was dropped into the above mixture to eliminate unreacted KMnO₄ under stirring, and then graphite oxide was obtained after the precipitate was washed with deionized water several times. Fifthly, an aqueous suspension of GO sheets was prepared by ultrasonic stripping of graphite oxide in deionized water for 2 h. Finally, a sample of GO powders was obtained after GO suspension was dried in a vacuum drying oven at 60 °C for 24 h.

2.2.2 Preparation of g-C₃N₄ sheets. The bulk g-C₃N₄ powders were synthesized by calcinating a mixture of melamine and NaCl based on a thermal polymerization reaction.⁵² Typically, firstly, 10.0 g of melamine and 8.0 g of NaCl were mixed and evenly ground, and then placed in a porcelain crucible with a cover. Subsequently, the mixture in the crucible was placed in the muffle furnace and heated to 550 °C with a heating rate of 5 °C min⁻¹, keeping at this temperature for 5 h. Thirdly, the sample in the crucible was naturally cooled to room temperature, after several washing with deionized water to remove NaCl, the product of bulk g-C₃N₄ powders was dried and ground into fine powders for characterization and use. Finally, the g-C₃N₄ sheets were prepared by sonicating fine g-C₃N₄ powders in deionized water for 2 h. As control experiments, the samples of 500 °C and 600 °C were prepared by only changing the thermal polymerization reaction temperature. These preparation processes were easily operated under ambient conditions, and no inert gas protection was required.

2.2.3 Preparation of rGO/g-C₃N₄ nanocomposite. The GO/g-C₃N₄ nanocomposite was fabricated via a simple ultrasound-assisted method. Typically, 0.1 g of GO powders was dispersed in 100 mL of deionized water and sonicated for 2 h to produce a GO suspension (1 mg mL⁻¹). Similarly, 0.1 g of fine g-C₃N₄ powders was dispersed in 100 mL of deionized water to obtain g-C₃N₄ suspension (1 mg mL⁻¹) through an ultrasonic process. Subsequently, the obtained g-C₃N₄ suspension was slowly added into the above GO suspension under ultrasound for 30 min to fabricate GO/g-C₃N₄ nanocomposite. Thirdly, the obtained GO/g-C₃N₄ nanocomposite was placed in a high-pressure reactor and heated to 200 °C, keeping for 5 h. After cooling to room temperature, the sample was taken out from the high-pressure reactor. Finally, a sample of rGO/g-C₃N₄ nanocomposite was obtained after being filtered and dried, which can be used for subsequent use.

On the basis of the as-prepared GO and g-C₃N₄ suspension, GO nanosheets with excellent hydrophilicity can be evenly dispersed in an aqueous solution, and they are beneficial to interact with g-C₃N₄ nanosheets to form GO/g-C₃N₄ nanocomposite.^53,54 Moreover, during the hydrothermal reaction, GO and g-C₃N₄ nanosheets can be self-assembled to rGO/g-C₃N₄ nanocomposite with 3D network structure, similar to the self-assembly of GO nanosheets.⁵⁵

2.2.4 Preparation of rGO/g-C₃N₄/Ag NPs. An appropriate amount of rGO/g-C₃N₄ nanocomposite was dispersed in deionized water under ultrasound, and a solution of silver ammonium complex ion was slowly added to the dispersion of rGO/g-C₃N₄ nanocomposite under stirring. Subsequently, an aqueous solution of glucose (5 wt%) was added to the above mixture under 70 °C water bath with ultrasound. After 30 min, the rGO/g-C₃N₄ nanocomposite was in situ loaded with Ag NPs. After several washing with deionized water, the sample was filtered and dried in a vacuum drying oven at 60 °C for 24 h for characterization and use.

2.3 Characterizations

Their microstructure and morphology of GO, g-C₃N₄, rGO/g-C₃N₄ and rGO/g-C₃N₄/Ag NPs were detected by the scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEM-2100). Fourier transform infrared spectroscopy (FTIR, Nicolet5700), Raman spectroscopy (M00-141) and X-ray diffractometer (XRD, Bruker D8 diffractometer) were used to analyze structural compositions of the relevant samples.

3. Results and discussion

3.1 Structure analysis

Three steps were rationally designed to fabricate rGO/g-C₃N₄/Ag NPs, and a schematic procedure was shown in Fig. 1. After the ultrasonic stripping of g-C₃N₄ powders in deionized water, an aqueous suspension of g-C₃N₄ nanosheets can be prepared, and its color is milky yellow (Fig. 1a). Subsequently, an aqueous suspension of g-C₃N₄ nanosheets was gradually added to an aqueous suspension of GO (Fig. 1b) under ultrasonic treatment. Then, a nanocomposite of GO/g-C₃N₄ was easily obtained, which has a milky grey colour, as shown in Fig. 1. After a hydrothermal reaction in a high pressure reactor at 200 °C for 5 h, the above GO/g-C₃N₄ nanocomposite was transformed into rGO/g-C₃N₄ nanocomposite, as shown in Fig. 1d. Finally, the rGO/g-C₃N₄ nanocomposite was in situ loaded Ag NPs to fabricate 3D nanocomposite hybridized with Ag NPs, as shown in Fig. 1e and f.

Fig. 1 A peparation schematic illustration of rGO/g-C₃N₄/Ag NPs. Optical photos of aqueous suspensions of g-C₃N₄ (a), GO (b), GO/g-C₃N₄ (c), rGO/g-C₃N₄ (d), and rGO/g-C₃N₄/Ag NPs (e). SEM image of rGO/g-C₃N₄/Ag NPs (f).

The rGO/g-C₃N₄/Ag NPs mainly consists of three active ingredients (rGO, g-C₃N₄ and Ag NPs), which are effectively combined together. Each of them has a very important role in the nanocomposite for functional applications, and thus each ingredient needs to be considered in the preparation process in detail.

Based on the improved Hummers' method^18,19,48 and subsequent ultrasonic stripping,^49–51 the graphite powders were converted into GO sheets, accompanied with a slight color changing from dark black to grey black, as shown in Fig. 2a and b. The obtained GO sheets are single layer or several layers with loose, stacked and cross-linked structure, as displayed in Fig. 2c and d.

Fig. 2 Optical photos of graphite powders (a) and GO powders (b). SEM image of GO nanosheets (c) and TEM image of GO nanosheets (d).

The microstructure of g-C₃N₄ powders was shown in Fig. 3. The melamine powders were transformed into g-C₃N₄ powders after a thermal polymerization reaction of 550 °C for 5 h in a covered porcelain crucible. Moreover, the colors of the samples change from cream white color of melamine powders to light yellow color of g-C₃N₄ powders, as shown in Fig. 3a and b. Seen from SEM images in Fig. 3c, the g-C₃N₄ powders consist of numerous nanosheets with mixed lamellar and stacked structure. In particular, the micromorphology and size of the g-C₃N₄ nanosheets are more uniform after ultrasonication, as shown in Fig. 3d.

Fig. 3 Optical photos of melamine powders (a) and g-C₃N₄ powders (b). SEM images of g-C₃N₄ powders (c) and g-C₃N₄ sheets (d).

During the preparation of rGO/g-C₃N₄/Ag NPs, an aqueous solution of glucose (5 wt%) was slowly added into a dispersion of rGO/g-C₃N₄ and silver ammonia solution, and then [Ag(NH₃)₂]⁺ was reduced by the green reducing agent of glucose under 70 °C water bath.

When the above hybrid mixture was remained static for 20 min, such that a bright silver mirror was attached onto the inside of the beaker, as shown in Fig. 4a. Interestingly, the ultrasonic treatment was instead of resting state, [Ag(NH₃)₂]⁺ was reduced not to form bright and uniform silver mirror, but to load into the rGO/g-C₃N₄ nanocomposite as Ag NPs, as shown in Fig. 4b.

Fig. 4 Optical photos of silver mirror attached to the inside of a beaker (a) and rGO/g-C₃N₄/Ag NPs (b). SEM images of Ag NPs with different magnifications (c and d). The inset of (d) is TEM image of Ag NPs.

As a contrast, without the rGO/g-C₃N₄ nanocomposite as a load carrier, the Ag NPs were prepared under 70 °C water bath and ultrasonic treatment, and the microstructure of Ag NPs was analyzed by SEM and TEM. The obtained results show that the particle sizes of Ag NPs are 50–100 nm, as shown in Fig. 4c and d. In the formation process of Ag NPs without a carrier, the sizes of Ag NPs are not uniform, which may be attributed to some influencing factors such as agglomeration, surface coatings and overlapping.

Compared with g-C₃N₄ sheets in Fig. 3d, the rGO/g-C₃N₄ nanocomposite presents a 3D porous structure, which can effectively increase specific surface area and adsorption capacity, as shown in Fig. 5a and b. At the same time, the rGO/g-C₃N₄ nanocomposite was successfully prepared in a high-pressure reactor at 200 °C for 5 h, thus some oxygen-containing groups were removed from the GO surface. Moreover, the electrical conductivity of rGO sheets will be greatly improved, as well as rGO and g-C₃N₄ sheets are tightly bonded and their π–π interactions has been strengthened, which are beneficial to promote the electron–hole separation.

Fig. 5 SEM images of rGO/g-C₃N₄ nanocomposite with different magnifications (a and b), and rGO/g-C₃N₄/Ag NPs with different magnifications (c and d).

As shown in Fig. 5c and d, the rGO/g-C₃N₄/Ag NPs has a 3D porous structure, while its microstructure can be further optimized, and numerous of Ag NPs are in loaded onto the surface or into inside of the nanocomposite.

Based on the above prepared GO, g-C₃N₄ and rGO/g-C₃N₄, the rGO/g-C₃N₄ loaded with Ag NPs was rationally constructed. Compared to g-C₃N₄ sheets in Fig. 3d, the rGO/g-C₃N₄ nanocomposite presents a 3D porous structure, which effectively increases its specific surface area. Moreover, the hydrothermal reaction caused GO/g-C₃N₄ to rGO/g-C₃N₄, and some oxygen-containing groups on GO surface were removed. Compared to rGO/g-C₃N₄, the rGO/g-C₃N₄/Ag NPs also presents a 3D porous structure, and its morphology can be further optimized due to in situ loading of Ag NPs, as shown in Fig. 5c and d.

To analyze the composition of rGO/g-C₃N₄/Ag NPs, its elemental mapping for elements (including C, O, N and Ag) was performed, as shown in Fig. 6. The elemental mappings of a selected region clearly demonstrate the corresponding distributions and atomic percents (C, O, N and Ag atoms).

Fig. 6 (a) SEM image of rGO/g-C₃N₄/Ag NPs in a selected region, and (b–e) Elemental mappings of C, O, N and Ag.

As the same time, FTIR spectra are employed to analyze the functional groups of GO, g-C₃N₄ and rGO/g-C₃N₄/Ag NPs. It can be seen from Fig. 7a that the characteristic peaks located at 3000–3500 cm⁻¹ should be caused by the stretching vibration of –OH and –NH₂ groups.^56,57 While the peaks of 1200–1700 cm⁻¹ are due to the stretching vibrations of CN heterocycles, and the peak of 806 cm⁻¹ is related to the breathing and stretching mode of CN-heterocyclic triazine. As mentioned above, the characteristic peaks of FTIR spectra are completely consistent with the reported literature.^56,57

Fig. 7 FTIR spectra of g-C₃N₄ sheets prepared at different temperatures (a), and FTIR spectra of GO, g-C₃N₄ and rGO/g-C₃N₄/Ag NPs (b).

The reaction temperature is considered as a crucial influencing factor on the crystallinity of g-C₃N₄ sheets, which has played an important role on its photocatalytic activity.⁵⁸ Thus, different reaction temperatures (such as 500 °C, 550 °C and 600 °C) were considered based on the thermal polymerization reaction.^52,58 Compared with g-C₃N₄ sheets prepared at 550 °C and 600 °C, the characteristic FTIR peaks of g-C₃N₄ sheets prepared at 500 °C are not obvious. Although the peak positions of g-C₃N₄ sheets prepared at 550 °C and 600 °C are almost no difference, while there are obvious differences in their colors, that is, the color of g-C₃N₄ sheets at 550 °C is light-yellow and the color of g-C₃N₄ sheets at 600 °C is deep-yellow.

Seen from the characteristic FTIR peaks in Fig. 7b, the rGO/g-C₃N₄/Ag NPs contains some oxygen-containing functional groups. The absorption peak around 1600 cm⁻¹ is the basic skeleton of C=C bond, and the absorption peak near 1750 cm⁻¹ is the C=O stretching vibration absorption peak, as well as the absorption peak near 3400 cm⁻¹ is the O–H stretching vibration absorption peak.^50,51 Therefore, the rGO/g-C₃N₄/Ag NPs has some functional group structures, which will help it absorb pollutants from water for degradation.

Raman spectra in Fig. 8a show that the characteristic peak near 800 cm⁻¹ corresponds to the bending vibration of the triazine ring, and the characteristic peak near 1600 cm⁻¹ is caused by the stretching vibration of the CN heterocyclic compound.⁵⁹ The Raman spectral characteristic peaks of g-C₃N₄ prepared at 500 °C and 550 °C are more obvious compared with that of g-C₃N₄ prepared at 600 °C. Based on above relevant analysis of FTIR and Raman spectra of g-C₃N₄ samples prepared 500 °C, 550 °C and 600 °C, the g-C₃N₄ powders in our experiment were deliberately prepared at 550 °C.^52,58

Fig. 8 Raman spectra of g-C₃N₄ sheets prepared at different temperatures (a), and GO/g-C₃N₄, rGO/g-C₃N₄ and rGO/g-C₃N₄/Ag NPs (b).

It can be observed from Fig. 8b that the characteristic peaks near 1350 cm⁻¹ and 1600 cm⁻¹ correspond to the D and G bands of the graphite structure, respectively. Moreover, the ratio of D peak intensity to G peak intensity (I_D/I_G) is generally used to represent the degree of graphitization of graphite,⁵⁰ after the hydrothermal reaction of rGO/g-C₃N₄ and in situ loading of Ag NPs, the value of I_D/I_G increases from GO/g-C₃N₄ and rGO/g-C₃N₄ to rGO/g-C₃N₄/Ag NPs, indicating that some oxygen-containing groups in rGO/g-C₃N₄ nanocomposite can be removed by heating reduction⁵⁰ and the Ag NPs have Raman enhancement effects.⁶⁰

Seen from XRD analysis in Fig. 9a, there are a weak peak (13.2°) and a prominent peak (27.5°), which corresponds to the (100) and (002) crystal faces of g-C₃N₄ sheet,^52,56 respectively. Moreover, the XRD patterns of g-C₃N₄ sheets prepared at different temperatures (inset of Fig. 9a) show that the g-C₃N₄ sheets prepared at 550 °C have more obvious characteristic peaks compared with that of g-C₃N₄ sheets prepared at 500 °C and 600 °C.

Fig. 9 XRD patterns of (a) g-C₃N₄ sheets prepared at 550 °C, (b) rGO/g-C₃N₄, (c) rGO/g-C₃N₄/Ag NPs and (d) g-C₃N₄, GO and rGO/g-C₃N₄/Ag NPs. The inset of (a) is XRD patterns of g-C₃N₄ sheets prepared at different temperatures.

For rGO sheets, the diffraction peak at 10.5° is a characteristic diffraction peak (Fig. 9b) of the crystal face of (001).^50,51 Especially, as displayed in Fig. 9c, the diffraction peaks of rGO/g-C₃N₄ nanocomposite with Ag NPs located at 38.2° (111), 44.5° (200), 64.5° (220) and 77.5° (311) are the characteristic diffraction peaks of Ag NPs (JCPDS No. 04-0783),^40,41 respectively, indicating that Ag NPs were successfully introduced into the rGO/g-C₃N₄ nanocomposite. The diffraction peaks of Ag NPs are strong and there are no other miscellaneous peaks, which confirms the high purity of Ag NPs.^40,41

Very significantly, there is a noticeable change after in situ loading of Ag NPs, especially their intensity of the XRD patterns has changed in rGO/g-C₃N₄/Ag NPs. That is, a prominent peak at (002) and a weak peak at (100) crystal faces of g-C₃N₄ sheets are shown in Fig. 9a, as well as a prominent peak at (002) crystal face of g-C₃N₄ sheets and a weak peak at (001) crystal face of rGO sheets are shown in Fig. 9b. However, the diffraction peaks of Ag NPs are very strong, such that the diffraction peaks of g-C₃N₄ sheets and rGO are too weak to be seen in rGO/g-C₃N₄/Ag NPs, as shown in Fig. 9c. Moreover, the crystallite size of Ag NPs can play an important role on photocatalytic efficiency. Generally, smaller crystallites imply higher surface-to-volume ratio with enhancing photocatalytic activity.⁶¹

Thermogravimetric analysis was performed to calculate the water adsorption capacity of 3D porous structure.⁶² As the adsorbed water evaporates at room temperature, the weight loss of this nanocomposite is about 56.7 wt%, and then the labile oxygen-containing functional groups are broken down to produce water at around 200 °C, the weight loss of hydrogel is about 20.7 wt%.⁶³

3.2. Adsorption and catalytic properties

MB is generally employed as the model compound to detect the adsorption and catalytic properties of an adsorbent. The obtained rGO/g-C₃N₄/Ag NPs was used as an adsorbent and catalyst to remove MB from water. As control experiments, the pristine g-C₃N₄ sheets and rGO/g-C₃N₄ nanocomposite were also employed as adsorbents and catalysts to remove MB from water under the same experimental conditions, respectively.

Firstly, the test experiments were carried out under alkaline medium, that is, the pH value of an aqueous solution containing MB was about 8.²⁸ Typically, 100 mg of adsorbent (such as g-C₃N₄ sheets, rGO/g-C₃N₄ and rGO/g-C₃N₄/Ag NPs) was immersed into 100 mL of MB solution (50 mg L⁻¹) in the dark, followed by stirring at 200 rpm at room temperature. At time intervals of 10 min, the adsorbent was separated by filtration and the concentration of residual MB in the filtrate was calculated by Beer's law based on the absorption peak at 662 nm with a UV-visible spectrophotometer (Shimadzu UV-2450).^51,63–65

Secondly, under simulated solar irradiation, a light resource was introduced using a 300 W Xe lamp. Typically, 100 mg of adsorbent was immersed into 100 mL of MB solution (50 mg L⁻¹) under simulated solar irradiation, followed by stirring at 200 rpm at room temperature. At time intervals of 10 min, the adsorbent was separated by filtration and the concentration of residual MB in the filtrate was calculated by Beer's law based on the absorption peak at 662 nm with a UV-visible spectrophotometer (Shimadzu UV-2450).^51,63–65

Finally, the ratio of temporal concentration to initial concentration (C/C₀) was analyzed with the change of adsorption time. The elimination efficiency was assessed by using C/C₀, which is equal to A/A₀. Where A₀ and C₀ were the initial absorbance and concentration of MB, respectively. The A and C were the corresponding absorbance and concentration of MB to be tested at different adsorption times, respectively.

As can be seen from Fig. 10a, when the pristine g-C₃N₄ sheets acts as an adsorbent to adsorb MB from water in the dark, it takes about 60 min to reach adsorption equilibrium and the value of C/C₀ decreases to 21.80%. Moreover, the rGO/g-C₃N₄ nanocomposite acts as an adsorbent, the adsorption reaches equilibrium at 60 min and the value of C/C₀ decreases to 15.80%. While the rGO/g-C₃N₄/Ag NPs is used as the adsorbent, the adsorption equilibrium time is also 60 min and the value of C/C₀ decreases to 12.80%.

Fig. 10 The ratio of temporal concentration to initial concentration (C/C₀) with the change of adsorption time (a) in the dark and (b) under visible light. MB removal capacity with the change of adsorption time (c) in the dark and (d) under visible light.

As a control experiment, the visible light was introduced on the basis of the above same experimental conditions, the pristine g-C₃N₄ sheets, rGO/g-C₃N₄ and rGO/g-C₃N₄/Ag NPs were employed as adsorbents and catalysts, respectively. At equilibrium, their values of C/C₀ decrease to 18.80%, 10.80% and 0.80%, respectively, as shown in Fig. 10b.

From the above results, it was reasoned that the pristine g-C₃N₄ sheets, rGO/g-C₃N₄ and rGO/g-C₃N₄/Ag NPs were only employed as adsorbents in the dark, respectively, and their removal capacities (Fig. 10c) were calculated as 39.10 mg g⁻¹, 42.10 mg g⁻¹, and 43.60 mg g⁻¹, respectively. However, as the visible light is introduced, they are not only adsorbents but also catalysts, such that there is a significant decline in their values of C/C₀ (from 21.80%, 15.80% and 12.80% to 18.80%, 10.80% and 0.80%), as well as their removal capacities (Fig. 10d) are up to 40.60 mg g⁻¹, 44.70 mg g⁻¹, and 49.60 mg g⁻¹, respectively. Compared with the pristine g-C₃N₄ sheets and rGO/g-C₃N₄, the rGO/g-C₃N₄/Ag NPs has best performance.

Among them, a 3D porous structure of rGO/g-C₃N₄ nanosheets with rich functional groups provides an effective carrier to hybridize with Ag NPs and adsorb MB, as well as enhance light absorption.³⁹ The rGO nanosheets act as the charge transmission bridge between g-C₃N₄ nanosheets and Ag NPs, which not only facilitates the separation of electrons and holes but also improves the stability of the photocatalyst of Ag NPs.^37,39 The Ag NPs can display a plasmon effect of the localized surface plasmon resonance, such that the Ag NPs not only act as photocatalyst, but also enhance light absorption and promote charge separation.^40–45,60

Based on the above analysis, the rGO/g-C₃N₄/Ag NPs was employed as not only adsorbent but also catalyst under visible light irradiation, and their values of C/C₀ decrease from 12.80% to 0.80%, with MB removal capacity increase from 43.60 mg g⁻¹ to 49.60 mg g⁻¹. Under visible light irradiation, the adsorption and catalytic performance of rGO/g-C₃N₄/Ag NPs is better than that reported in many literature, which is compared with the related reports,^{51,53,63,64,66,67} as shown in Table 1.

Table 1 Adsorbents or catalysts for removal of MB

Adsorbents or catalyst	Pollutants	Removal capacity (mg g^−1)	Ref.
N-RGO	MB	94.4	63
rGO/g-C3N4/Ag	MB	49.60	This work
g-C3N4/GO	MB	30.0	53
Biomass activated carbon	MB	24.0	64
GO	MB	23.8	51
GO/polyaniline	MB	6.70	66
TiO2/GO composites	MB	5.01	67

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The pH of the system is a very important factor, which influences the photocatalytic degradation activities of the catalysts.^28,68 Fig. 11a demonstrates the performance of this catalyst of rGO/g-C₃N₄/Ag NPs at different pH values towards the degradation of MB. There is a basic tendency to increase catalytic activity as pH increases, reaching a maximum value at pH value of 11.⁶⁹

Fig. 11 Photocatalytic degradation of MB under visible light irradiation. (a) Effect of solution pH and (b) stability experiments for MB degradation.

In our experiment, MB is selected as a model compound that is a cationic dye. Although the pH of the system is a very important factor, at the same time, positively charged ions with strong electrostatic interactions will interfere with the efficiency of the method. Moreover, if some ions get through π–π interactions to compete for active sites on surface of catalyst, they can also interfere with the efficiency of this method.

The stability and reusability of a catalyst is very important for assessing the practical applicability of the catalyst. After a run, the filtered photocatalyst was washed for three times with ethanol and deionized water, respectively, and dried at 80 °C for 5 h. After 4 cycles, the efficiency of this photocatalyst is still more than 90%, as shown in Fig. 11b.

3.3. Analysis of catalytic process and mechanism

In our experiments, a possible mechanism was proposed to explain photocatalytic degradation of MB under visible light irradiation. The evaluations of various reactive species were done to confirm their contribution towards MB degradation. Several chemical agents were selected as quenchers,^62,68,70 such as ethylenediaminetetraacetic acid disodium (EDTA-2Na) for holes (h⁺), methanol for hydroxyl radicals (·OH) and benzoquinone for superoxide (·O₂⁻), respectively. The production of ·O₂⁻ radicals was confirmed as the most dominant radicals that can lead to enhanced photocatalytic degradation of MB on the catalyst surface. Moreover, the in situ loaded Ag NPs synergistically increase the generation of reactive oxygen species proposed to be ·O₂⁻ through redox reactions.

There are three kinds of heterojunctions existing in rGO/g-C₃N₄/Ag NPs, that is, the heterojunctions exist between rGO/g-C₃N₄, g-C₃N₄/Ag and rGO/Ag, and they will play relevant roles in the photocatalytic process. As a typical n-type semiconductor, the g-C₃N₄ nanosheet can be excited to generate photogenerated electrons and holes under visible light irradiation. In this 3D rGO/g-C₃N₄/Ag NPs, rGO is not only used to enhance light absorption, but also used as an excellent conductor of electrons to promote the separation of photogenerated electron–hole pairs.⁵³ At the same time, Ag NPs has a strong localized surface plasmon resonance effect under visible light irradiation, which not only enhances light absorption, but also increases the generation rate of photogenerated electron–hole pairs on the surface of g-C₃N₄.^{40,41,44,45,54} In the 3D network structure, rGO, g-C₃N₄ and Ag NPs play an effective synergistic role to overcome the drawbacks of g-C₃N₄-based photocatalyst such as small specific surface area, fast charge recombination and limited visible-light absorption. Thus the photogenerated electrons generated by g-C₃N₄ nanosheet under the excitation of visible light can effectually react with O₂ to form the active species ·O₂⁻ to catalyze the decomposition of MB.

Based on the above photocatalytic analysis and the mechanism proposed, the kinetic of adsorption and degradation of MB belongs to pseudo-first order kinetics.^34,54,68 Under visible light irradiation, the reaction rate constants of g-C₃N₄ sheets and rGO/g-C₃N₄ were calculated as 0.02622/min and 0.03608/min, respectively. While the reaction rate constant of rGO/g-C₃N₄/Ag NPs was calculated as 0.07795/min, which is higher than that g-C₃N₄ sheets and rGO/g-C₃N₄.

4. Conclusion

In conclusion, we developed a facile step-by-step approach (including ultrasonic mixing, hydrothermal reaction and in situ loading) to fabricate a 3D rGO/g-C₃N₄ nanocomposite loaded with Ag NPs. Due to the synergistic effect of rGO, g-C₃N₄ and Ag NPs, this obtained nanocomposite with excellent performance can be used as adsorbent and catalyst to efficiently remove MB (50 mg L⁻¹) from water under visible light, and its MB removal capacity is high as 49.60 mg g⁻¹ within 60 min. This work provides a useful reference for the rational design and scale preparation of rGO/g-C₃N₄ nanocomposites for practical applications.

Data availability

The authors declare that the raw data supporting this article are available from the corresponding author upon reasonable request.

Author contributions

Kesheng Cao, and Xueyu Ge: conceptualization, methodology, and writing original draft; Shuang Li, Suya Cui, Guijin Guo, and Liuqing Yang: investigation, validation, and data analysis; Xingwu Li, Yabo Wang, and Suzhen Bai: data analysis, review, and editing; Zhengshan Tian, Qian Wei, and Wei Li: writing, review, editing, and supervision.

Conflicts of interest

The authors declare no confict of interest.

Acknowledgements

This work was supported by the Henan Province science and technology research project (242102230037 and 252102231024) and Henan Province Science and Technology Research and Development Plan Joint Fund (Industry Category) Key Project“Key Technologies and Applications of High-Performance Para-Aramid Fiber for Insulation Tie Rod”.

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