Reduced graphene oxide/gold nanoparticle aerogel for catalytic reduction of 4-nitrophenol

Abstract

Currently, it is of great significance and a challenge to develop facile synthetic routes to obtain a novel plasmonic heterogeneous catalyst with high activity and long lifetime for the reduction of a refractory organic compound like 4-nitrophenol (4-NP). To this end, a three-dimensional (3D) porous framework named reduced graphene oxide/gold nanoparticle aerogel (rGO/Au NPA) was constructed by individual GO sheets and HAuCl₄ under the reduction of trisodium citrate dihydrate (Na₃Cit) via a one-step hydrothermal method. The abundant Au NPs having a diameter of 7–160 nm can be easily in situ incorporated into graphene sheets to form a 3D hierarchical monolith by the reduction of Na₃Cit, which was well-disclosed by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Such 3D rGO/Au NPA with interconnected porous structure displays a good thermal stability and large Brunauer–Emmett–Teller specific surface area of 37.8325 m² g⁻¹. More importantly, the fabricated rGO/Au NPA can act as a heterogeneous catalyst, exhibiting an outstanding catalytic activity and good reusability towards the reduction of 4-NP due to the synergetic effect between Au NPs and graphene sheets. Additionally, the mechanism of enhanced catalytic efficiency for the 3D rGO/Au NPA catalyst has also been proposed.

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1. Introduction

Nowadays, a variety of organic pollutants from industrial wastewater have brought serious hazards to human beings due to their carcinogenic, bactericidal, and mutagenic properties.^1–3 Aromatic nitro-compounds are toxic organic compounds and a potential threat affecting millions of people worldwide.⁴ Therefore, efficient removal/utilization of organic pollutants has become an important issue for environmental protection.⁵ Various methods including solvent extraction, biodegradation, adsorption, chemical oxidation and photocatalytic degradation have been applied to remove organic pollutants from wastewater.^6–9 Among them, catalytic reduction technique is of particular interest, as it is green, low-cost and economic-efficient for industrial manufacture.^10,11 Selecting an appropriate catalyst plays an important role in the industrial catalysis. Based on these considerations, noble metallic nanoparticle catalysts such as silver (Ag),¹² gold (Au),¹³ platinum (Pt)¹⁴ and palladium (Pd),¹⁵ have been used in various catalytic reduction reactions owing to their efficient mediation of the electron transfer process. However, noble metal nanocatalysts generally suffer from poor stability and show a tendency to aggregate in harsh environments due to their small-size effect, resulting in a serious low catalytic activity. Thus, it is highly desirable to develop a new strategy to prepare more durable, efficient and eco-friendly catalysts to overcome all these inadequacies. One of the ideal strategies is, to immobilize them onto a certain support such as polymeric materials, silica, metal oxides or carbon nanotubes, which effectively prevents the aggregation and further maintains their activity and lifetime.^16–20

Recently, graphene-based composites with plasmonic nanoparticles as an alternative catalyst offered a unique advantage for the removal of organic pollutants.^21–24 These graphene-based carbocatalysts possess a large specific surface area, superior charge mobility, π–π conjugated structure and sp²sp³ hybrid carbon network with abundant oxygen-containing groups, making them ideal candidates as environmental scavengers. So far, several research groups have been devoted to the design and utilization of plasmonic nanoparticles coated on graphene sheets. For instance, Fang et al. prepared a 2D graphene/CuNi nanocomposite with an outstanding recyclability and activity as high as 99% toward catalyzing the reduction of various aromatic nitro-compounds.⁴ Xu et al. have obtained a heterogeneous Pd/PDA-rGO (PDA = 1,4-phenylenediamine) nanocatalyst for catalyzing decomposition of formic acid with the turnover frequency of 3810 h⁻¹ at 323 K.¹⁵ Similarly, Jana et al. employed rGO/Ag composites as photocatalyst for ruining colorless organic pollutants such as phenol, bisphenol A and atrazine under UV and visible light.⁵ In another research, Zhang et al. developed Au-GO and Au-rGO nanocomposites with 2-mercaptopyridine premodified onto GO and rGO sheets for catalytic reduction of o-nitroaniline to 1,2-benzenediamine in the presence of NaBH₄.²⁵ However, most of the graphene-based carbocatalysts involve multiple complex procedures to separate from the organic pollutants and possess low loading amount of metallic nanoparticles, which prevent them from practical applications. On the other hand, lower yield, complex preparation processes and relatively lower specific surface area also limit their practical applications. Hence, exploring new graphene-based materials with improved stability and recyclability for catalytic reactions is still highly desired.

To address the challenge, herein we present a facile one-step hydrothermal approach to fabricate a 3D rGO/Au NPA with a mixture of HAuCl₄, GO sheets and Na₃Cit. The additive Na₃Cit acts as a reducing agent, which is highly beneficial to the scale-up and mass-production of Au NPs loading on graphene aerogel. The as-obtained 3D rGO/Au NPA exhibited interconnected porous structure with a high specific surface area (37.8325 m² g⁻¹) and good thermal stability. More importantly, the resulted 3D rGO/Au NPA demonstrated an excellent catalytic activity for the reduction of 4-NP to 4-AP (p-aminophenol) in the presence of NaBH₄ as compared with pure Au NPs or independent rGO aerogel. Moreover, conversion rate for the reduction of 4-NP was still maintained above 87% after a recycle lifetime for 5 times. Meanwhile, the corresponding synergistic catalytic mechanism has also been proposed.

2. Experimental methods

2.1. Chemicals and materials

All chemicals and solvents were of analytical grade and used as received unless specified otherwise. Graphite powder (325 mesh, 99.8%), 4-nitrophenol (4-NP), and sodium borohydride (NaBH₄) were supplied by Alfa Aesar. Hydrogen peroxide 30% (H₂O₂), potassium permanganate (KMnO₄), sulfuric acid (H₂SO₄), sodium nitrate (NaNO₃), chloroauric acid (HAuCl₄·4H₂O), trisodium citrate dihydrate (Na₃Cit), and hydrochloric acid (HCl) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). In all cases, Millipore water (18.2 MΩ) was used to prepare solutions.

2.2. Preparation of GO

GO was synthesized using high-purity graphite powder according to the previously reported Hummers' method.²⁶ In brief, 0.6 g of graphite powder was mixed with 1 g of NaNO₃ and 35 mL of cold concentrated H₂SO₄ in a 250 mL round-bottom flask under stirring in an ice bath for 1.5 h. Afterwards, 3 g of KMnO₄ was gradually added into the mixture, maintaining the temperature below 20 °C for 4 h. Then the suspension was transferred to a 35 °C water bath and magnetically stirred for 30 min. Subsequently, 150 mL of water was slowly poured into the mixture and the temperature was raised to 98 °C and kept stirring for 25 min. Thereafter, the suspension was diluted with another 200 mL of water and then 10 mL of 30% H₂O₂ aqueous solution was added to obtain an earthen yellow suspension. Finally, the mixture was subjected to repeated centrifugation and suspension in 5% HCl solution to remove metal ions. The as-obtained brown-colored slurry was washed with a copious amount of ultrapure water and freeze-dried overnight for further use.

2.3. Synthesis of 3D rGO/Au NPA

A homogeneous GO colloidal dispersion (2.5 mg mL⁻¹) was prepared by performing ultrasonication of solid GO (125 mg) in double-distilled water (50 mL) for 2 h. Typically, 1 mL of HAuCl₄ solution (24.3 mM) was added into 10 mL of GO suspension (2.5 mg mL⁻¹) and stirred for 1 h at room temperature. Then, 97.1 mg of Na₃Cit (0.33 mmol) was added into the above mixture, followed by a 15 min stirring procedure. The resultant solution was sealed in a 50 mL Teflon-lined autoclave, kept at 180 °C for 16 h and allowed to reach the ambient temperature to form a hydrogel. The as-obtained hydrogel was rinsed thoroughly with water for several times, and then was freeze-dried to yield a 3D rGO/Au NPA. Finally, the as-made 3D specimen was calcined in a tubular furnace at 800 °C for 3 h under Ar to remove the impurities. A schematic representation of the 3D rGO/Au NPA fabrication has been illustrated in Fig. 2a. For comparison, rGO aerogel was also fabricated in a similar method without the addition of HAuCl₄ solution. The pure Au NPs were prepared according to the Na₃Cit reduction method by boiling the same dosage of HAuCl₄ aqueous solution.²⁷

2.4. Catalytic reduction of 4-NP

The reduction of 4-NP to 4-AP in the presence of NaBH₄ was chosen as a model reaction to evaluate the catalytic activity of a prepared 3D rGO/Au NPA catalyst. Briefly, 50 mg of the as-prepared catalyst was placed in the mixture containing 250 μL of 4-NP (10 mM), 19.5 mL of ultrapure water, and 250 μL of NaBH₄ (5 M) with continuous stirring (100 rpm) in a 50 mL beaker under ambient conditions. At a specific time interval (3 min), a small portion of the solution (500 μL) was injected into a quartz cuvette for immediate analysis with an UV-vis spectrophotometer. To study the reusability of our 3D rGO/Au NPA, the catalysts were quickly collected in a funnel with filter paper, washed three times with deionized water and reused in the next cycle.

2.5. Characterization

X-ray diffraction analyses (XRD) were carried out using a Bruker D8 Discover X-ray diffractometer with Cu-Kα radiation (λ = 0.15406 nm) over 2θ range of 5–90° at room temperature. The morphological characterizations were investigated by field-emission scanning electron microscopy (FESEM, UltraPlus Zeiss) equipped with an Oxford-INCA energy dispersive X-ray spectroscopy (EDS) system. Transmission electron microscopy (TEM) images were taken with a JEM-2100 Electron Microscope operating at 200 kV. Fourier transform infrared spectrum (FTIR) was conducted on Thermo Scientific Nicolet iS50 FTIR spectrometer. The Raman spectra were collected on a Renishaw Invia Reflex system with a laser wavelength of 514.5 nm. Thermogravimetric-differential scanning calorimetry (TG-DSC) measurements were performed on a Netzsch STA 449F3 Jupiter differential thermal analyzer at a heating rate of 10 °C min⁻¹ within 25–700 °C in flowing air. Nitrogen adsorption–desorption isotherms were obtained at 77 K by using Micrometrics ASAP 2020 equipment. The pore diameter, pore volume and pore diameter distribution were calculated by applying the Barrett–Joyner–Halenda (BJH) approach to the desorption branch of the isotherms. Ultraviolet-visible (UV-vis) spectra were collected with a Shimadzu UV-1800 spectrophotometer.

3. Results and discussion

The crystal phase compositions of the as-synthesized GO, rGO aerogel and 3D rGO/Au NPA were confirmed by XRD patterns as given in Fig. 1. It is clear that a characteristic sharp peak centered at 2θ = 7.2° could be indexed to the (0 0 2) plane of GO sheets (black curve). For rGO aerogel (blue curve), it exhibited two broad peaks at around 24.3° and 42.7°, which corresponded to the (0 0 2) and (1 0 0) crystal faces, indicating the successful conversion of GO to graphene after the hydrothermal reaction at 180 °C.²⁸ Compared with rGO aerogel sample, the XRD pattern of the 3D rGO/Au NPA (red curve) showed all of the major identified diffraction peaks at 38.2°, 44.4°, 64.7°, 77.7° and 81.7°, which can be assigned to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of gold structure (JCPDS file card no. 04-0784), respectively.²⁹ A broad and weak peak which was noted at about 24° in the XRD pattern of 3D rGO/Au NPA demonstrated the characteristic reflection from graphene, attributing to the removal of the most oxygen-containing groups from GO, similar to the reported 3D graphene/MnO₂ xerogel.³⁰

Fig. 1 XRD patterns of the as-prepared 3D rGO/Au NPA, rGO aerogel and GO.

Our strategy for the fabrication of 3D rGO/Au NPA has been illustrated in Fig. 2a. Initially, a 3D rGO/Au NP hydrogel was one-step hydrothermally prepared from the homogeneous dispersion containing GO, HAuCl₄ and Na₃Cit. A freeze-drying and post-annealing treatment was performed to obtain an unchanged and pure 3D xerogel. The as-prepared monolith was in a well-defined 3D cylinder shape with a diameter and height of about 10 and 8 mm, respectively. The 3D xerogel possessed a typical interconnected porous structure with the pore size in the range of several micrometers to hundreds of micrometers within the interwoven graphene layers, as revealed in FESEM images (Fig. 2b and c). In an enlarged view (Fig. 2d), an abundance of Au NPs were randomly distributed on the thin layers of stacked graphene sheets. On comparison, the rGO aerogel also showed a hierarchical pore structure consisting of mutual cross-linked corrugated graphene sheets but no other species could be seen on the surface of the graphene (Fig. S1a and b in the ESI†). Further, the magnified FESEM image (Fig. 2e) showed that the large-scale Au NPs with a diameter of 7–160 nm were uniformly decorated on both sides of the graphene sheets. Energy dispersive spectroscopy (EDS) analyses from the selected-area FESEM image (Fig. 2e) confirmed the components of the 3D rGO/Au NPA sample. As can be seen from Fig. 2g, there exist some signals of carbon, oxygen and aurum elements with a weight ratio of C/O/Au = 62.74 : 4.20 : 33.06. A high-resolution FESEM image (Fig. 2f) showed a cross-section of the rGO/Au NP hybrid sheet with an average thickness of 56 nm. The unique interconnected porous nanostructure with some ripples and corrugations on the sheets of rGO/Au NPA sample was further confirmed with TEM (Fig. 3a). A large number of Au NPs were well immobilized on the rGO surface, which was in accordance with the previous FESEM results (Fig. 2e). The size distribution shown in Fig. 3b demonstrated that the diameters of Au NPs were also in a range of 7–160 nm, and the average diameter of the Au NPs was around 31.6 nm. In addition, a high resolution TEM image of a single gold nanoparticle (Fig. 3c) showed uniform lattice fringes at a spacing of about 0.206 nm, matching well with the (2 0 0) plane of crystalline gold.³¹ The aforementioned analyses indicate that a 3D rGO/Au NPA material with porous microstructure was successfully fabricated.

Fig. 2 (a) Schematic flowchart for preparation of 3D rGO/Au NPA via hydrothermal, freeze drying and annealing treatment. (b–d) FESEM images of 3D rGO/Au NPA at lower magnification. (e) High-magnification FESEM image of 3D rGO/Au NPA. (f) High-magnification image of the cross section of a single rGO/Au nanosheet composite. (g) The EDS spectrum of rGO/Au NPA corresponding to red rectangle in the image shown in (e).

Fig. 3 (a) Representative TEM image of rGO/Au NPA sample. (b) Size distribution of Au NPs immobilized on rGO aerogel. (c) High-resolution TEM image of one single Au NP anchored on graphene from the selected area in image (a).

Fourier transform infrared spectroscopy (FTIR) analysis was then used to investigate the changes occurred in the chemical bonds during the reaction and the molecular structures were ascertained. As depicted in Fig. 4a, a strong and overlapping band at 3420 cm⁻¹ is attributed to the O–H stretching vibration of adsorbed water molecules and structural OH groups. The characteristic peaks for pristine GO spectra (black curve) exhibited stretching vibrations of C–H (2915 and 2849 cm⁻¹), C=O (1730 cm⁻¹), C=C (1580 cm⁻¹), C–O (1165 cm⁻¹) and C–O–C (1058 cm⁻¹), respectively.³² The peak at 1405 cm⁻¹ should be assigned to the bending vibration of O–H in the carboxylic group. It was noticed that the band centered at 1730 cm⁻¹ (C=O) was disappeared and other O-containing stretches were also decreased dramatically in rGO (blue curve) and 3D rGO/Au NPA (red curve). This observation revealed that most oxygen moieties in the GO were almost removed during the reaction. Furthermore, Raman spectrum was employed to understand the electronic interaction between gold nanostructures and graphene sheets. The different Raman spectra of GO (black curve), rGO aerogel (blue curve) and 3D rGO/Au NPA (red curve) are shown in Fig. 4b. As for pristine GO, there were two characteristic peaks at 1364 cm⁻¹ and 1599 cm⁻¹, which could be assigned to the disordered D-band (k-point phonons of A_1g symmetry) and graphitic G-band (E_2g phonon of the sp² carbon atoms) of carbon-based materials, respectively.³³ In the case of rGO aerogel and 3D rGO/Au NPA, the D and G bands were shifted to 1356 and 1603 cm⁻¹ after hydrothermal and annealing treatment. The relative intensity of D-band and G-band (I_D/I_G) for graphene-based materials generally reveals the electronic conjugation state.³⁴ The intensity ratios of I_D/I_G for GO and rGO aerogel were 0.919 and 1.066, respectively; while it significantly increased to 1.089 for 3D rGO/Au NPA, which indicated that there had been an increase in the number of sp² domains during the reaction and confirmed the presence of strong electronic interaction between Au NPs and graphene sheets. This unexpected phenomenon agrees well with the Raman spectrum of graphene-Au nanocomposites reported by other groups.³⁵ Additionally, the thermogravimetric-differential scanning calorimetry (TG-DSC) analysis in a flowing air was also carried out to investigate the thermal stability and composition of GO (black curve), rGO aerogel (blue curve) and 3D rGO/Au NPA (red curve). As could be seen from Fig. 4c, the pure GO showed a three-stage degradation process from room temperature to 700 °C. There was a mass loss of 16.98% in the first decomposition step occurring at the temperature below 140 °C, which could be attributed to the evaporation of physically adsorbed water. From the second weight loss of 76.15%, a sharper decomposition rate was observed over the temperature range between 140 °C and 180 °C, which could be assigned to the degradation of labile oxygen-containing functional groups. The third weight loss of 6.87% in the temperature range of 320–550 °C was associated with the decomposition of more stable oxygen bearing functional groups and carbon oxidation from GO sheets.³⁶ As for rGO aerogel, the TG curve exhibited two weightlessness ladders. The first weight loss (∼28.71%) appeared at 25–240 °C was due to the removal of adsorbed water and labile oxygen functional groups, while the major weight loss (∼60.65%) in the second decomposition step at 240–700 °C was related to the thermal decomposition of carbon networks and residual oxygen functionalities.⁷ On comparison, it can be found that the weight loss of 3D rGO/Au NPA sample was greatly restricted, probably due to strong interaction between Au NPs and rGO sheets. The initial weight loss (∼17.53%) of 3D rGO/Au NPA up to 280 °C belonged to the elimination of trapped/adsorbed water molecules and unstable oxygenated functional groups. The majority of the mass loss (∼46.88%) from 280 °C to 700 °C corresponded to the gradual pyrolysis of carbon skeleton and those remaining oxygen functional groups. The reason for the differences in the TG analysis of GO, rGO aerogel and 3D rGO/Au NPA can be as follows: there were a large number of oxygen-containing functional groups such as carbonyl, hydroxyl, epoxy and carboxyl on the surface of GO sheets.³⁷ When the GO was reduced by Na₃Cit during the hydrothermal reaction, some labile oxygen-containing functional groups like carbonyl were thoroughly removed from the surface of GO sheets, which could be verified by previous FTIR analysis. The decoration of GO sheets with Au NPs could impose great restriction on mobilization of GO sheets, thus resulting in strong interaction between Au NPs and graphene sheets during the heating process.^38,39 As a result, the reduction and decoration alter the number of functional groups within GO sheets and exert an effect on the thermal behavior of GO. For clarity, the corresponding DSC curves of the samples were also obtained. As presented in Fig. 4d, two exothermic peaks at 158.4 °C and 491 °C of GO were observed. For rGO aerogel, there was a broad exothermic peak at 372.4 °C and a weak exothermic one at 474.8 °C. While for the 3D rGO/Au NPA sample, a strong and sharp exothermic peak at 392.4 °C combined with an adjacent small exothermic one at around 488 °C could be observed. Therefore, it is apparent that the 3D rGO/Au NPA has a much higher thermal stability than rGO aerogel, which is an important property for subsequent applications.

Fig. 4 (a) FTIR spectra of GO, rGO aerogel and 3D rGO/Au NPA. (b) Comparison of the Raman spectra for the GO, rGO aerogel and rGO/Au NPA samples at laser excitation wavelength of 514.5 nm. (c, d) Thermogravimetric (TG) and differential scanning calorimetry (DSC) analyses of GO, rGO aerogel and rGO/Au NPA samples from 25 to 700 °C with heating rate of 10 °C min⁻¹ under air condition.

Furthermore, the Brunauer–Emmett–Teller (BET) surface area and porous characteristics of rGO aerogel and 3D rGO/Au NPA were characterized by N₂ adsorption and desorption experiments at 77 K. The nitrogen adsorption/desorption isotherms and pore size distributions of the two samples were presented in Fig. 5. It could be seen that the 3D rGO/Au NPA demonstrated a much higher surface area (37.8325 m² g⁻¹) than rGO aerogel (17.5871 m² g⁻¹) due to the introduction of Au NPs. The intercalation of Au NPs significantly reduced the π–π stacking between rGO sheets and increased the specific surface area of 3D rGO/Au NPA to a certain extent. Besides, the low specific surface area for rGO aerogel could be ascribed to the collapse of the carbon skeleton structure and π–π stacking of rGO sheets. In addition, the N₂ adsorption/desorption isotherms of 3D rGO/Au NPA (Fig. 5c) exhibited a typical type-IV behavior with a distinct H3 hysteresis loop according to the IUPAC classification, which indicated the mesoporous characteristic within the obtained composites. Meanwhile, the BJH mean pore diameter for 3D rGO/Au NPA sample (36 nm) was larger than rGO aerogel (19.6 nm), which suggested that the insertion of Au NPs between graphene sheets prevented the graphene from restacking effectively.⁴⁰ And the total pore volume for 3D rGO/Au NPA sample had been calculated to be 0.1323 cm³ g⁻¹ (Fig. 5d), which was also slightly higher than rGO aerogel (0.1204 cm³ g⁻¹) in Fig. 5b. All of these above mentioned results suggest that large specific surface area and remarkable porous structure of 3D rGO/Au NPA will be highly favorable for subsequent catalytic applications.

Fig. 5 (a, c) Typical N₂ gas adsorption–desorption isotherms of rGO aerogel and rGO/Au NPA. (b, d) Barret–Joyner–Halenda desorption pore diameter distribution profiles of rGO aerogel and rGO/Au NPA.

The catalytic performance was then investigated by a typical liquid-phase reduction of 4-NP to 4-AP by NaBH₄ in the presence of metal-based catalysts under ambient temperature. 4-NP is a well-known carcinogenic compound with high toxicity, whereas 4-AP is a potential chemical intermediate that may be used in corrosion inhibitor, anticorrosion lubricant, analgesic and antipyretic drugs, photographic developer, etc.⁴¹ The reduction product 4-AP in aqueous solution can be easily obtained through the process of filtration and solvent extraction.^42–44 Thus the catalytic reduction is essential and beneficial for industrial applications. The reduction of 4-NP can be easily monitored by UV-vis spectroscopy through the disappearance of the absorbance peak (400 nm) of 4-nitrophenolate anion and appearance of two small features at 234 and 298 nm of the 4-AP product.¹⁰ Normally, the characteristic absorption bands for pure 4-NP aqueous solution were centered at 226 and 318 nm (Fig. 6a, red curve). Upon addition of NaBH₄, the maximum absorption band around 318 nm immediately red-shifted to 400 nm which suggested the formation of 4-nitrophenolate ions within the deep yellow aqueous solution. And there were no obvious changes in the absence of any metal catalyst in an extended 60 minutes (Fig. 6a). However, a significant decrease with elapsed time at 400 nm for the characteristic absorbance of 4-nitrophenolate ions could be seen after the introduction of 3D rGO/Au NPA catalyst; meanwhile, two new peaks at 298 and 234 nm appeared due to the successful conversion of 4-NP to 4-AP (Fig. 6b). Moreover, full reduction of 4-NP by NaBH₄ in the presence of 3D rGO/Au NPA catalyst was quickly completed within 18 min with the observation of a fading process of the deep-yellow color of aqueous solution. Only a slow decreased absorption intensity at 400 nm occurred probably due to the absorption of 4-NP by the 3D rGO/Au NPA catalyst in the absence of NaBH₄ (Fig. S2a in the ESI†). Similar slow absorption behavior could also be observed for rGO aerogel in the presence of NaBH₄ (Fig. S2b in the ESI†). For pure Au NPs (Fig. S2c in the ESI†), a longer reaction time of 39 min was required to achieve full reduction of 4-NP in the presence of NaBH₄. Since the concentration of NaBH₄ greatly exceeded that of 4-NP in the reaction (C_NaBH4/C_4-NP = 500), the pseudo-first-order kinetic model was employed to evaluate the rate constant.⁴⁵ The apparent rate constant k was defined according to the following kinetic equation: ln(C_t/C₀) = ln(A_t/A₀) = −kt, where C₀ and A₀ were the initial concentration and absorbance of 4-NP, while C_t and A_t were the concentration and absorbance values of 4-NP at a specific time t, respectively. And k was the apparent rate constant taken from the slope of linear section of the resultant dataset. Here, the characteristic peak at 400 nm for 4-nitrophenolate ions was monitored as a function of time to C_t/C₀ after the addition of catalysts. The plots of C_t/C₀ vs. time (t) for the reduction of 4-NP in the presence of different catalysts were shown in Fig. 6c. It could be clearly seen that the reduction reaction did not take place in the absence of catalyst or NaBH₄. There was also no catalytic activity except some adsorption of 4-NP, even when the rGO aerogel was used in the presence of NaBH₄. Significantly, the 3D rGO/Au NPA induced catalytic reduction of 4-NP was more effective as compared to reduction by pure Au NPs or rGO aerogel; and the corresponding apparent rate constant k was estimated to be 0.18952 min⁻¹ (Fig. 6d), which compared favorably to the reported AuNS/SnO₂ composites.¹³ Additionally, reusability should also be undoubtedly taken into account for the practical applications. To explore the recycling performances of the 3D rGO/Au NPA, we carried out five successive catalytic tests under the same conditions. As displayed in Fig. 7, the conversion efficiency of 4-NP remained as high as 87.1% even after 5 cycles, which suggested the extraordinary recyclability and stability of the obtained 3D rGO/Au NPA material. These results mentioned above demonstrated that the 3D rGO/Au NPA can exhibit an excellent catalytic activity and good recycle stability toward reduction of 4-NP reaction.

Fig. 6 (a) UV-vis absorption spectrum of 4-nitrophenol (4-NP) solution (red curve) and time dependent absorption spectra for the catalytic reduction of 4-NP by NaBH₄. (b) The reduction profiles of 4-NP by NaBH₄ in the presence of rGO/Au NPA catalyst at various time intervals at room temperature. (c) Plot of C_t/C₀ versus time for the reduction of 4-NP to 4-AP with different catalysts. (d) Rate constant of the catalytic reductive reaction of 4-NP by NaBH₄ for the rGO/Au NPA catalyst.

Fig. 7 (a) Activity of 3D rGO/Au NPA in multiple cycles of catalytic reduction of 4-NP at room temperature. (b) Conversion efficiency of 4-NP in five repeated reaction cycles in agreement with part (a).

Additionally, a tentative reaction mechanism of catalytic reduction of 4-NP to 4-AP had also been proposed on the basis of the aforementioned results (Fig. 8). To sum up, three major points should be responsible for the superior catalytic performance of 3D rGO/Au NPA catalyst in our work. First of all, the rGO sheets with a large specific surface area dramatically increased the chances for adsorption of π-rich 4-NP molecules on account of π–π stacking interactions.⁴⁶ Such adsorption increased the local concentration of 4-NP and thus helped Au NPs to capture and widespread the electrons. Secondly, abundant electrons injected by the donor BH₄⁻ were transferred to the surface of Au NPs and then to rGO sheets. The conductive rGO surface adjacent to Au NPs continued to quickly transport those electrons to acceptor 4-NP via extended π-conjugation structure, leading to the formation of 4-AP.^4,47,48 Third, the generated 4-AP molecules desorbed from the rGO sheets and dissolved in the solution.⁴⁹ In addition, high porosity and abundant reactive sites in the 3D rGO/Au NPA were beneficial for the electron transfer to remove the kinetic barrier. Therefore, it was speculated that a synergistic effect between Au NPs and rGO sheets significantly contributed to the enhanced catalytic activity of 3D rGO/Au NPA material.

Fig. 8 Illustration of the proposed mechanism for the reduction of 4-NP to 4-AP by 3D rGO/Au NPA in the presence of NaBH₄.

4. Conclusion

In conclusion, a macroscopic 3D rGO/Au NPA material has been successfully produced via a green and facile process through the combination of hydrothermal reduction, freeze-drying and annealing treatment. The resulting 3D rGO/Au NPA was constructed with cell walls of graphene sheets which were anchored by abundant Au NPs with a diameter of 7–160 nm. Such monolith possessed a hierarchical porous architecture with a large specific surface area of 37.8325 m² g⁻¹ and good thermal stability. Furthermore, the 3D rGO/Au NPA also exhibited a highly enhanced catalytic activity with good cycling stability for the reduction of 4-NP to 4-AP, giving a potential application in the fields of catalysis and environmental treatment.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (973 Program 2013CB932902), the National Natural Science Foundation of China (NSFC) (No. 61071047, 31371003), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20130092110030), the Postdoctoral Science Foundation of Jiangsu Province (1501017C), the Fundamental Research Funds for the Central Universities (2242016R20026) and the Analysis Foundation of Southeast University (1160739030).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra09386h

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