Green Synthesis of Au-NPs on g-C3N4 Hybrid Nanomaterials Based on Supramolecular Pillar[6]arene and Its Applications for Catalysis.

Gold nanoparticles (Au NPs) are installed in situ on the surfaces of graphitic carbon nitride (g-C3N4) based on supramolecular hydroxylatopillar[6]arene (P6). The Au NPs can be obtained via the redox reaction between HAuCl4 and P6 without any NH2-NH2, NaBH4, and other reductants, where AuCl4 - is reduced to Au0 by the -OH groups in the presence of OH-, and the -OH groups are oxidized into -COOH. First, P6 is loaded onto the surface of g-C3N4 via π-π interaction between P6 and g-C3N4, which offers a stabilized and reduced site for in situ anchoring of Au NPs. The hybrid nanomaterial Au-NPs@P6@g-C3N4 exhibits higher catalytic capability than the Pd/C catalyst in 4-nitrophenol (4-NP) reduction and methylene blue degradation, which opens a new avenue for designing more efficient hybrid nanomaterials for application in catalysis, sensing, and other fields.

Introduction

Gold nanoparticles (Au NPs) have attracted extensive attention and application in many fields of nanoscience and nanotechnology due to their excellent properties. The physicochemical features of Au NPs have attracted forceful repercussions in biomedicine, nanoelectronics, sensors, and catalysis.[1−5] They play a crucial role in novel hybrid nanomaterials in light of their diverse size, easy synthesis, facile functionalization, and high stability. In parallel, macrocyclic hosts of cyclodextrin, calixarene, cucurbiturils, and others with size-tunable cavity structures and unique properties have been well-applied in separation, sensing, drug delivery, and self-assembly.[6−8] In recent years, the study of hybrid nanomaterials between metal nanoparticles and macrocyclic molecule has been reported.[9−11] Thus, the integration of metal nanoparticles and host compounds not only remarkably combines and enhances the advantages and characteristics of metal nanoparticles and macrocyclic hosts, including optical, catalytic, and host–guest recognition performances, but also paves the way for bringing new functions and applications.[12,13]

Although the studies on hybrid nanomaterials consisting of Au NPs and macrocyclic hosts have been extensive, there are some potential issues that are still worth focusing on. A crowd of harsh reagents or conditions including N2H4, NaBH4, thiols, and high temperature are commonly used to prepare the hybrid nanomaterials.[14−18] These conditions go against the concept of sustainable and friendly development, and the combination of different macrocyclic hosts with Au NPs may bring unwanted performances in the desired applications.[19,20] Therefore, it is significant and urgent to develop a green and facile method to prepare multifunctional hybrid nanomaterials between macrocyclic hosts and metal nanoparticles.

As a risen star, macrocyclic pillararenes were first reported by Ogoshi and co-workers in 2008 and have attracted wide attention based on their facile synthesis, accessible modification, preeminent host–guest chemistry, and excellent size regulating effect for metal nanoparticles.[21−26] The performances of drug delivery, synthesis, supramolecular self-assembly, and host–guest chemistry based on the pillararene have been broadly studied, but the applications of pillararenes combining with metal nanoparticles and two-dimensional layered material are rarely explored.[27−29] Yang’s group reported the synthesis of Au NPs via the reduction of HAuCl4 by NaBH4 based on carboxylatopillar[5]arene.[14] Yao et al. reported the synthesis of Au NPs based on imidazolium-functionalized pillar[5]arene.[15] However, these approaches for obtaining Au NPs have a common trait of using a harsh agent, NaBH4, which is unfriendly to the environment. Recently, Xia and co-workers reported that Au NPs can be obtained by heating the mixed solution of HAuCl4 and cyclodextrin.[18] Zhao et al. reported that hyamine hydroxylatopillar[5]arene could reduce HAuCl4 under the alkaline condition but could not integrate two-dimensional materials, such as graphitic carbon nitride (g-C3N4), which constrains their applications in sensing and other fields.[30] Therefore, exploring multicomponent hybrid nanomaterials based on pillararenes, Au NPs, and g-C3N4 is crucial in sensing, catalysis, electronics, biology, and so forth.[31−33]

Graphitic carbon nitride with a graphene-like structure, which shows many excellent features including easy synthesis, low toxicity, and high surface area, is widely used in sensing, catalysis, and other nanoscience and nanotechnology applications.[34−38] However, to the best of our knowledge, there is no literature report on the catalytic application based on pillar[6]arene, g-C3N4, and Au NPs. In this work, the Au NPs are prepared in situ based on the surfaces of hydroxylatopillar[6]arene (P6)-functionalized g-C3N4 (P6@g-C3N4). P6@g-C3N4 is first obtained via π–π and H-bond interactions between P6 and g-C3N4, which offers stabilized and reduced sites for in situ anchoring of Au NPs. Au is reduced and stabilized by P6 under the NaOH alkaline condition (Scheme ). The catalytic capability of Au-NPs@P6@g-C3N4 hybrid nanomaterials is studied via catalytic reduction of 4-nitrophenol (4-NP) and degradation of methylene blue (MB), which display high catalytic activity compared to control materials of commercial Pd/C. The high catalytic performance of Au-NPs@P6@g-C3N4 is ascribed to the excellent size regulation effects of P6.

Scheme 1

Schematic Representation for the Preparation Procedure of Au-NPs@P6@g-C3N4 and Its Application in Catalytic 4-NP Reduction and MB Degradation

Experimental Section

Chemicals and Materials

HAuCl4, NaOH, Pd/C, 4-NP, and MB were purchased from Shanghai Titan Scientific Co. Ltd (Shanghai, China). Other chemicals are of analytical grade and used without further purification. Deionized water (DW, 18 MΩ cm) was used for all experiments. g-C3N4 was prepared by pyrolyzing thiourea at 550 °C for 2 h.[39] P6 was synthesized following the reported method, and the synthetic route and NMR data (Figures S1 and S2) of P6 are shown in the Supporting Information.[40,41]

Preparation of P6@g-C3N4

g-C3N4 (10.0 mg) was added to the solution of P6 (20.0 mL, 1 mg/mL). The mixtures were sonicated for 5 h at room temperature. The above solution was then centrifuged at 18,000 rpm to remove unbounded P6 molecules.

Preparation of Au-NPs@P6@g-C3N4

The as-synthesized P6@g-C3N4 (10 mg) was dispersed in DW (20 mL) via sonication, and HAuCl4 (0.2 mL, 10 mM) was added to the solution and stirred for 5 min at room temperature. Afterward, the NaOH solution (0.1 mL, 1M) was added to the mixture and stirred for 30 min at 25 °C. The Au-NPs@P6@g-C3N4 hybrid nanomaterials were obtained by centrifuging and washing with DW three times.

Catalytic Reduction of 4-NP and Degradation of MB

The experiment of 4-NP reduction was investigated as follows. First of all, NaBH4 (1.5 mL, 20 mg/mL) was added to the 4-NP solution (1.5 mL, 0.4 mM) in a standard quartz cell with a 4 mL volume and a 1 cm path length. Then, Au-NPs@P6@g-C3N4 (10 μg) was added to the above mixtures. The catalytic process was characterized via UV–vis adsorption in the 250–500 nm wavelength range. The comparative experiment of the Pd/C catalyst was further studied following a similar procedure, and the only difference was that Au-NPs@P6@g-C3N4 was replaced by Pd/C.

Degradation of MB was also studied by Au-NPs@P6@g-C3N4 in the presence of NaBH4. NaBH4 (1.5 mL, 20 mg/mL) was added to the MB solution (1.5 mL, 100 μM) at room temperature. Then, the Au-NPs@P6@g-C3N4 (10.0 μg) catalyst was added, and the UV–vis spectrum was obtained at the 400–800 nm wavelength range. The comparative experiment using the Pd/C catalyst was carried out following a similar process, and the only difference was that Au-NPs@P6@g-C3N4 was replaced by Pd/C.

Results and Discussion

Characterization of P6@g-C3N4 and Au-NPs@P6@g-C3N4

The Fourier transform infrared (FTIR) spectra of g-C3N4, P6, and P6@gC3N4 are shown in Figure a. As we can observe from the spectrum of g-C3N4, several peaks appear in the range 1640–1200 cm–1 attributed to the typical CN heterocycle stretching modes. The characteristic stretching of triazine units on g-C3N4 occurs at 805 cm–1. The above results are consistent with the published literature,[42] suggesting the successful preparation of the two-dimensional carbon materials g-C3N4. In the FTIR spectrum of P6, a number of peaks are seen at 3395, 1642, 1489, 1409, 1207, and 1072 cm–1. The peak at 3395 cm–1 is ascribed to bending vibrations of O–H groups, and the peaks at 1642, 1489, 1409, and 1207 cm–1 are attributed to the characterized peaks of the benzene ring in P6, and the peak at 1072 cm–1 belongs to C–O–C. By comparing the FTIR spectra of g-C3N4 and P6, it is observed that P6@g-C3N4 exhibits common peaks of P6 and g-C3N4, which are also observed in g-C3N4 and P6. Furthermore, the intensity of −OH groups is slightly decreased and shifted to a low wavelength, and this is caused by the H-bond interactions between −OH and the N atom. Therefore, the FTIR results indicate strong evidence that the P6 molecules are adsorbed onto the surfaces of g-C3N4 via H-bonding and π–π interaction.[43,44] Thermostability of P6@g-C3N4 and the loaded content of P6 are investigated using thermogravimetry analysis (TGA), and the result is illustrated in Figure b. A considerable mass loss of P6@g-C3N4 complexes is viewed at ∼600 °C, which is mainly caused by the decomposition of macrocyclic P6. However, a feeble mass loss is observed in pure g-C3N4 at the same temperature. This suggests that the two-dimensional carbon material g-C3N4 shows excellent structural stability at high temperature. Consequently, with these results in mind, the content of P6 is calculated to be 43 wt % when the mass loss of g-C3N4 is deducted. The TGA data can further demonstrate that P6@g-C3N4 is successfully prepared.

Figure 1

(a) FTIR spectra of g-C3N4, P6@g-C3N4, and P6 and (b) TGA curves of g-C3N4 and P6@g-C3N4.

Characterization of Au-NPs@P6@g-C3N4

The microstructure of synthetic hybrid nanomaterials of Au-NPs@P6@g-C3N4 is characterized using transmission electron microscopy (TEM). The typical two-dimensional nanosheet of g-C3N4 is obviously seen in the TEM image (Figure a), and the uniform Au NPs uniformly disperse on the surfaces of g-C3N4 instead of being in a random distribution. The high-resolution TEM (HRTEM) image of Au-NPs is exhibited in Figure b, which displays distinct lattice fringes. Additionally, the crystal lattice spacing and diameter of prepared Au NPs are 9–10 and 0.255 nm, respectively, suggesting preeminent crystallinity and high catalytic capability. The uniform and small-size Au NPs can be obtained due to the outstanding size regulating effect of P6 in the formation of Au NPs. At the same time, the energy-dispersive X-ray spectroscopy (EDS) mapping can reveal that the Au NPs are homogeneously anchored onto the surfaces of g-C3N4 (Figure c–h).

Figure 2

(a) TEM image of Au-NPs@P6@g-C3N4; (b) HRTEM image of Au-NPs@P6@g-C3N4 and HRTEM mapping image of Au-NPs@P6@g-C3N4; (c) mixed element map; and (d) dark-field images and mappings of (e) C, (f) N, (g) O, and (h) Au.

The crystal structures of Au-NPs@P6@g-C3N4 and g-C3N4 are also studied using X-ray diffraction (XRD), and the results are illustrated in Figure a. g-C3N4 shows two characterized peaks at 13.0 and 27.5°, which are attributed to the (100) crystal plane of interplanar stacking of CN aromatic units and the (002) crystal plane of in-plane packing for the tri-s-triazine units, respectively. The Au-NPs@P6@g-C3N4 catalyst shows four peaks for Au NPs [(111), (200), (220), and (311)].[15] Also, the crystal lattice spacing for (111) planes of Au NPs is 0.255 nm, and this result corresponds to the TEM result. Au-NPs@P6@g-C3N4 and g-C3N4 are further assessed using X-ray photoelectron spectroscopy (XPS). The XPS spectrum of g-C3N4 shows two peaks of N 1s and C 1s (Figure b). Further, two peaks of O 1s and Au 4f are observed (Figure b), which were attributed to P6 and HAuCl4, respectively. The high-resolution XPS spectrum of Au 4f is shown in Figure c, with two peaks at 87.6 and 83.9 eV. These peaks are attributed to Au 4f5/2 and Au 4f7/2, respectively. Moreover, the O and Au elements can be characterized from the EDS spectrum (Figure d), which reveals a Au content of 39.04 (wt.) %. As a consequence, these results show overwhelming evidence and declare the successful preparation of Au-NPs@P6@g-C3N4.

Figure 3

(a) XRD patterns of g-C3N4 and Au-NPs@P6@g-C3N4; (b) XPS survey scan spectra of P6, g-C3N4, and Au-NPs@P6@g-C3N4; (c) high-resolution XPS spectrum of Au 4f; and (d) EDS spectrum of the Au-NPs@P6@g-C3N4 nanocomposite.

For clarification of the mechanism of formation of Au NPs via the reaction between P6 and HAuCl4, the high-resolution XPS spectra of C 1s, O 1s, and N 1s are evaluated. First, the XPS spectrum of C 1s for Au-NPs@P6@g-C3N4 (Figure a) displays five peaks at 284.8, 286.5, 287.8, 288.1, and 288.9 eV, which are ascribed to C–C, C–O, C=O, C–N3+, and O–C=O, respectively. Three peaks at 531.3, 532.4, and 533.4 eV are seen in the XPS spectrum of O 1s for Au-NPs@P6@g-C3N4 (Figure b). These peaks are attributed to O=C–O, C–O–C, and O=C–O, respectively. Nevertheless, the high-resolution XPS spectra of C 1s and O 1s of P6 (Figure d,e) only present C–O, C–C, C–N3+, and C–O–C. Therefore, from the high-resolution XPS results of C 1s and O 1s for Au-NPs@P6@g-C3N4 and P6, it is observed that the carboxyl groups are present in Au-NPs@P6@g-C3N4 nanomaterials, which are produced by the oxidized hydroxyl groups in P6.[30] The high-resolution XPS spectra of N 1s for Au-NPs@P6@g-C3N4 (Figure c) and P6 (Figure f) can further suggest that the Au-NPs@P6@g-C3N4 catalyst is successfully prepared, in which C=N–C is ascribed to g-C3N4 and C–N3+ is attributed to P6.

Figure 4

High-resolution XPS spectra of C 1s (a), O 1s (b), and N 1s (c) for Au-NPs@P6@g-C3N4 and high-resolution XPS spectra of C 1s (d), O 1s (e), and N 1s (f) for P6.

Catalytic Reduction of 4-NP and Degradation of MB

With Au-NPs@P6@g-C3N4 in hand, therefore, the catalytic activity of the synthesized catalyst is evaluated for reduction of 4-NP and degradation of MB, and they are frequently utilized as the reactive model for assessing the catalytic capability of nanomaterials.[30,45−48] For the 4-NP reduction, a distinct peak can be found at ∼400 nm after the addition of the NaBH4 solution due to the 4-NP ion formation (Figure a). The adsorption peak at ∼400 nm is gradually decreased as the reaction proceeds due to the addition of 7.0 μg of the Au-NPs@P6@g-C3N4 catalyst. At the same time, a fresh peak is observed at ∼300 nm (Figure a), revealing that 4-NP is reduced to 4-nitroaniline (4-AP) by NaBH4 under the catalytic action of the Au-NPs@P6@g-C3N4 catalyst according to the published literature.[15,45] Since there is more NaBH4 than 4-NP, this reaction is considered a pseudo-first-order reaction only for 4-NP. For the control study, the catalytic activities of Au-NPs@P6@g-C3N4 and commercial Pd/C are used to assess the catalytic performance of the prepared nanomaterial. Figure b shows the catalytic activity of Pd/C for the reduction of 4-NP under the same conditions with Au-NPs@P6@g-C3N4. The UV–vis spectrum is proportional to the concentration of 4-NP in this work. Therefore, the value of ln (A/A0) reflects that of ln (C/C0), with C and C0 representing the concentrations of 4-NP at times t and 0, respectively. In consequence, the reactive rate constant k could be calculated from the rate equation ln (C/C0) = kt.[49]Figure c exhibits the time-dependent UV–vis spectrum of 4-NP reduction. The reactive rate k of 4-NP reduction catalyzed by the Au-NPs@P6@g-C3N4 catalyst is obtained from the slope of the fitted line and is calculated to be 0.291 min–1. Also, the reactive rate k of 4-NP reduction catalyzed by the Pd/C catalyst is calculated to be 0.057 min–1 following a similar procedure to that of Au-NPs@P6@g-C3N4 (Figure c). The k value for catalytic reduction of 4-NP by Au-NPs@P6@g-C3N4 is 5.1 times higher than that by commercial Pd/C. Compared with the similar reported literature results, the Au-NPs@P6@g-C3N4 catalyst exhibits higher catalytic activities, as displayed in Table S1. The predominant catalytic performance of Au-NPs@P6@g-C3N4 is attributed to the uniform and small size of Au NPs on g-C3N4. This is based on the outstanding size regulating effects of supramolecular P6 via the coordination among Au NPs and carboxyl groups of the oxidation product of P6 during the formation of Au NPs.[50−52]

Figure 5

Time-dependent UV–vis absorption spectra for catalytic reduction of 4-NP in the presence of Au-NPs@P6@g-C3N4 (a) and Pd/C catalysts (b); plots of ln[C/C0] as a function of the reaction time for reduction of 4-NP by Au-NPs@P6@g-C3N4 and Pd/C (c); time-dependent UV–vis adsorption spectra for catalytic degradation of MB in the presence of Au-NPs@P6@g-C3N4 (d) and Pd/C catalysts (e); and plots of ln[C/C0] as a function of the reaction time for degradation of MB by Au-NPs@P6@g-C3N4 and Pd/C (f).

The catalytic performance of Au-NPs@P6@g-C3N4 is further studied by catalyzing the degradation of MB. Moreover, the catalytic degradation procedure of MB by Au-NPs@P6@g-C3N4 is monitored accurately via the time-dependent adsorption spectrum at various regular times. As displayed in Figure d, two characterized peaks are observed at ∼613 and ∼664 nm, which are ascribed to the dimer and monomer of MB, respectively. Furthermore, these peaks decrease gradually until they are completely disappeared within 330 s after the addition of Au-NPs@P6@g-C3N4, suggesting the ultrahigh catalytic activity of Au-NPs@P6@g-C3N4. For comparison, the catalytic performance of Pd/C with the same conditions is also evaluated for MB degradation (Figure e). The k values of catalytic degradation of MB by Au-NPs@P6@g-C3N4 and Pd/C are calculated from the fitted line to be 0.849 and 0.041 min–1 (Figure f), respectively. The k value of MB degradation by Au-NPs@P6@g-C3N4 is 20.7 times higher than that by Pd/C, suggesting the excellent catalytic capability of the Au-NPs@P6@g-C3N4 catalyst.

The time when 4-NP is changed into 4-AP by the Au-NPs@P6@g-C3N4 catalyst is studied by the amount of the catalyst. Figure S3 shows that the reactive time is gradually decreased under the enhancement of the Au-NPs@P6@g-C3N4 catalyst. Also, the time is almost unchanged with the addition of excess catalyst, illustrating that the optimal number of the prepared catalyst is 8 μg for catalytic reduction of 4-NP and degradation of MB. The reusability of Au-NPs@P6@g-C3N4 nanomaterials is also investigated, and the results demonstrate similar catalytic activity without obvious recession in conversion after six cycles (Figure S4).

With the above results in mind, the proposed reaction process using Au-NPs@P6@g-C3N4 as the catalyst is shown in Figure .[53] Besides, macrocyclic P6 has excellent host–guest recognition ability. As a result, the 4-NP and MB molecules might be interacting with P6 via host–guest interactions, which may also accelerate the catalytic reaction procedure.[54] Simultaneously, 4-NP and MB also might be anchored via π–π interaction between the two-dimensional material g-C3N4 and the benzene ring structure in 4-NP and MB molecules. According to these data, the resulting ultrahigh catalytic activity of Au-NPs@P6@g-C3N4 should be ascribed to the two-dimensional structure of g-C3N4, positively charged Au NP surface, and the merits of the ternary hybrid material composed of Au, g-C3N4, and P6.

Figure 6

Proposed mechanism for the Au-NPs@P6@g-C3N4 catalytic reduction of 4-NP.

Conclusions

A green, novel, and facile synthesis of ternary hybrid nanomaterial Au-NPs@P6@g-C3N4 using chloroauric acid, graphitic carbon nitride, and supramolecular hydroxylatopillar[6]arene is developed. The Au NPs are obtained in situ via a redox reaction between P6 and HAuCl4 with a handful of OH– at room temperature. For the formation process of Au NPs, the Au3+ in HAuCl4 is reduced to Au0 by hydroxyl groups in P6, and the hydroxyl is oxidized to carboxyl, which can coordinate with Au via O–Au conjunction and restrain limitless agglomeration of the Au NPs. The Au-NPs@P6@g-C3N4 hybrid nanomaterial has shown high catalytic performance toward catalytic 4-NP reduction and MB degradation than the commercial Pd/C catalyst. Investigating the strategy of utilizing pillar[6]arene, two dimensional g-C3N4, and Au NPs for application in catalytic reduction of 4-NP and degradation of MB might have potential for green and highly efficient synthesis of hybrid nanomaterials for application in catalysis, nanoelectronics, and sensing fields.