Journal of Materials Science & Technology  2020 , 40 (0): 176-184 https://doi.org/10.1016/j.jmst.2019.08.031

Uniform assembly of gold nanoparticles on S-doped g-C3N4 nanocomposite for effective conversion of 4-nitrophenol by catalytic reduction

Vellaichamy Balakumar, Hyungjoo Kim, Ji Won Ryu, Ramalingam Manivannan, Young-A Son*

Department of Advanced Organic Materials Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon, 305-764, South Korea

Corresponding authors:   *Corresponding author.E-mail address: yason@cnu.ac.kr (Y.-A. Son).*Corresponding author.E-mail address: yason@cnu.ac.kr (Y.-A. Son).

Received: 2019-07-14

Accepted:  2019-09-23

Online:  2020-03-01

Copyright:  2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

More

Abstract

In this work, a simple synthesis of sulfur doped graphitic carbon nitride (S-g-C3N4) act as a support cum stabilizers for gold nanoparticles (Au) and its was characterized by UV-vis and XRD to measure the absorbance and crystallinity, respectively. The functional group and morphology of the samples were identified using FT-IR and TEM. Finally, the Au@S-g-C3N4 nanocatalyst exhibits good catalytic performance and stability in the reduction of hazardous 4-nitrophenol (NP) compared to S-g-C3N4 using NaBH4. Moreover, the Au@S-g-C3N4 nanocomposite holds a good catalytic efficiency (near 100%) achieved by within 5 min. The highest catalytic reduction of NP is due to the synergistic effect of Au nanoparticles decorated on S-g-C3N4. The fast electron transfer reduction mechanism was elucidated and discussed. Excellent reusability and stability of the developed nanocomposites were also observed in consecutive reduction experiments. The filtering and catalyzing device was used for the direct conversion of NP polluted water. This method can open a new avenue for the metal nanoparticles based carbon materials heterogeneous catalyst and its reduction of toxic contaminants.

Keywords: Gold nanoparticles ; S-doped g-C3N4 ; Catalytic reduction ; 4-nitrophenol

0

PDF (4234KB) Metadata Metrics Related articles

Cite this article Export EndNote Ris Bibtex

Vellaichamy Balakumar, Hyungjoo Kim, Ji Won Ryu, Ramalingam Manivannan, Young-A Son. Uniform assembly of gold nanoparticles on S-doped g-C3N4 nanocomposite for effective conversion of 4-nitrophenol by catalytic reduction[J]. Journal of Materials Science & Technology, 2020, 40(0): 176-184 https://doi.org/10.1016/j.jmst.2019.08.031

1. Introduction

Catalysis is a powerful approach to synthesize various fine chemicals. Generally, metal nanoparticles have been acts to be a good catalyst for various reactions, including alcohol oxidation, carbon-carbon bond formation, selective hydrogenation, etc [[1], [2], [3]]. Among them, gold nanoparticles (Au) are highly stable and efficient in a wide variety of catalytic reactions, sensors and drug deliveries [[3], [4], [5]]. Since the pure Au has some disadvantages including a strong tendency to form large aggregate, reduce the specific surface area, hinder the surface-active site and decrease the catalytic activity [6]. Therefore, it is critical to prevent the aggregation of Au supported on solid matrices that are both chemically and physically robust. Over the past several decades, gold nanoparticles are supported by various metal oxides like TiO2 [7], CeO2 [8], Fe2O3 [9] and carbon based materials (activated carbon [10], graphene [11], carbon nanotubes [12], polymers [13] and graphitic carbon nitride [14]). Among various carbon based supports, graphitic carbon nitride is considered as an excellent support for various metal nanoparticles, due to its good dispersion [15,16]. The bulk g-C3N4 is limited for good support of catalytic metal nanoparticles due to a smaller specific surface area. To solve this problem, enormous attempts have been made to enhance the performance of g-C3N4 doping with heteroatoms like B, P and S [[17], [18], [19]]. Among them S-g-C3N4 is considerably improved the electron transfer and catalytic performances [[19], [20], [21]]. However, to the best of our knowledge, it was the first investigation of the assembly of Au with S-g-C3N4 which can be utilized as robust catalyst for the reduction of NP.

Nitro aromatic compounds, 4-NP are well known to be toxic, carcinogenic and mutagenic to human and aquatic environments. In aquatic system, the trace level of NP and its derivatives are carcinogenic because of non-biodegradable and high toxicity [22]. Therefore, NP has become anthropogenic pollutant and posed a threat to the environment and human health. Up to now various technologies (adsorption [23], photocatalytic degradation [24], ozonation [25] and catalytic reduction [3]) have been explored to removal the toxic nitroaromatics. Among them, the catalytic reduction process is very simple and efficient method to convert toxic NP to benign AP. The AP is one of the important intermediates for the synthesis of pharmaceuticals and analgesic and antipyretic drugs preparation.

In this work, we demonstrate a simple assembly of Au on the surface of S-g-C3N4 and its catalytic reduction performance towards NP. The Au assembled on S-g-C3N4 exhibits an excellent NP reduction compared to single S-g-C3N4. The reduction percentage and kinetic rate constants were estimated and found to be 99% and 0.5 × 10-3 respectively. The reusability and stability were also examined for four cycles revealing good results. Still, there is no report on the detailed kinetic analysis of the reduction of 4-nitrophenol using Au assembled on S-g-C3N4 catalysts.

2. Experimental

2.1. Materials

Thiourea (CH4N2S), chloroauric acid (HAuCl4), sodium borohydride (NaBH4) and 4-nitrophenol were acquired from Sigma-Aldrich. All the other chemicals were purchased from commercial sources and used as received.

2.2. Synthesis of S-g-C3N4

According to previous literature [19], 5 g of thiourea was placed in a sealed crucible and calcined at 550 °C for 4 h in a muffle furnace at N2-atmosphere. After calcination, the obtained bright yellow solid product is labeled as S-g-C3N4. These results are confirmed by FT-IR and XRD.

2.3. Synthesis of Au@S-g-C3N4

The 0.002 g of S-g-C3N4 was dissolved in 5 mL of distilled water and ultra-sonication for 15 min. After that 5 mL of 1 mM HAuCl4 was added into the above solutions. Then, drop by drop 5 mL of 0.05 M NaBH4 was added and stirred continuously for 30 min. Finally, the reddish yellow powder of Au@S-g-C3N4 nanocomposite was filtered, characterized and used to application of reduction of NP (Scheme 1).

Scheme 1.   Schematic illustration of the synthetic procedure of Au@S-g-C3N4 nanocomposite.

2.4. Instrumentations

X-ray diffraction (XRD) measurements were performed on a Cu radiation (λ = 1.54056 Å) using (PHILIPS, X'Pert-MPD System, Max P/N: 3 kW/40 kV, 45 Ma, Netherlands). Infrared spectra experiments were collected on ALPHA-P spectrometer. The surface composition of the nanocomposite was confirmed by XPS using K-Alpha + XPS Spectrometer with Al (1350 eV) radiation source. The UV-vis spectra was used to record the absorption value of samples (Shimadzu Spec-2600 instrument). The High-resolution surface structure and selected area electron diffraction (SAED) were obtained with a transmission electron microscope (TEM, FEI TECNAI T20 G2) operating at 100 kV. The as-prepared nanocomposite energy dispersive spectrum (EDS) and mapping analysis were identified by scanning electron microscope (SEM, LYRA3, XMU).

2.5. Procedure for NP reduction

According to the literature, 10 mL (1.0 mM) aqueous solution of NP was mixed with 0.5 mg of the Au assembled S-g-C3N4 nanocomposite catalysts by 30 min sonication. Afterwards, 1 mL (1 mM) of freshly prepared NaBH4 aqueous solution was added and monitored by UV-vis spectrophotometer. The NP absorbance peak 317 nm was red shifted to 400 nm (4-nitrophenolate ions). Every one min, the solution was monitored by time-dependent UV-vis spectrophotometer.

3. Results and discussion

3.1. Catalyst characterizations

The crystallinity and phase structure of the samples were analyzed by XRD as shown in Fig. 1. The two major peaks were observed at 13.02° and 27.32° (Fig. 1(a)) which are corresponding to (100) and (002) planes of g-C3N4 and conjugated aromatic systems with interlayer stacking, respectively [19]. Notably, the 13.1° characteristic diffraction peak intensity was decreased compared to reported g-C3N4 [18], which indicates the doping sulfur. Interestingly, after assembly of Au on the surface of S-g-C3N4 (Fig. 1(b)), four new diffraction peaks (37.74°, 43.83°, 45.83°, 63.66° and 76.42°) were observed and they were corresponding to the (111), (200), (220), (311) and (222) face centred cubic crystalline planes of Au [13]. These results indicate that the Au have been successfully assembled on the surface of S-g-C3N4.

Fig. 1.   XRD patterns of S-g-C3N4 (black line) and Au@S-g-C3N4 nanocomposite (red line).

The FT-IR spectra of S-g-C3N4 and Au@S-g-C3N4 nanocomposite are presented in Fig. 2. The broad peak at 3100-3300 cm-1 is ascribed to the N—H bonds of stretching vibration. The distinct peaks observed at 1250, 1336, 1411, 1564 and 1655 cm-1 are corresponding to S-g-C3N4 (Fig. 2 (black line)), C—N heterocycle of stretching vibration modes [17,18]. The peaks at 811 and 889 cm-1 are ascribed to breathing mode of tri-s-triazine unit of CN heterocycles and the cross-linked deformation mode of N—H bonds, respectively. The peak observed at 707 cm-1 was attributed to the C—S stretching vibration of sulfur doping [19]. The FT-IR spectrum of Au@S-g-C3N4 nanocomposite (Fig. 2 (red line)) showed similar peaks, but the characteristic peak intensity was decreased compared to S-g-C3N4, indicating the successful assembly of Au on the surface.

Fig. 2.   FT-IR spectra of S-g-C3N4 (black line) and Au@S-g-C3N4 nanocomposite (red line).

To investigate the chemical composition of the as-prepared Au@S-g-C3N4 nanocomposite, XPS was analyzed. Survey spectrum (Fig. 3(A) of the Au@S-g-C3N4 nanocomposite revealed four elements like C 1s, N 1s, S 2p and Au 4f. Fig. 3(B) shows the C 1s peaks at 285.34 eV and 288.8 eV is attributed to the C=C and C-(N)3 bonds, respectively [19]. The N 1s spectrum could be fitted by three peaks at 399.34, 399.51, and 400.96 eV (Fig. 3(C)), which corresponded to C—N—C, N-(C3) and N—H, respectively [20]. The peaks at 165.31 eV and 163.90 eV (Fig. 3(D)) are corresponding to C—S bonds, which were formed by substitution of sulfur with lattice nitrogen atoms [21]. The Au 4f spectra (Fig. 3(E)) of Au@S-g-C3N4 catalysts are measured. The two peaks at 84.14 eV and 87.77 eV is ascribed to Au 4f7/2 and Au 4f5/2, which is in good agreement with reported Au [26].

Fig. 3.   XPS spectra of Au@S-g-C3N4 nanocomposite: (A) the survey spectra; (B) C 1s; (C) N 1s; (D) S 2p; (E) Au 4f.

To investigate the optical properties of the S-g-C3N4 and Au@S-g-C3N4 nanocomposite using UV-vis spectroscopy. As seen in Fig. 4, the absorption band observed at 385 nm (Fig. 4 (black line)) is corresponding to the S-g-C3N4 [19]. After assembling of Au on the surface of S-g-C3N4 (Fig. 4 (red line)), new absorption band at 539 nm was appeared due to the SPR of Au nanoparticles. These results coincided with the earlier reported gold nanoparticles [[14], [15], [16]]. Further, HRTEM used to identify the morphology and size of the S-g-C3N4 and Au@S-g-C3N4 nanocomposite. As shown in Fig. 5(A), the TEM image of S-g-C3N4 showed the stacked layers structure. The surface morphology of the Au@S-g-C3N4 nanocomposite (Fig. 5(B) and (C)) has been changed and clearly showing the uniform sized (12 nm) Au assembled on to the surface of S-g-C3N4. the crystalline nature of the Au nanoparticles on the surface of S-g-C3N4 is further confirmed by SAED as shown in Fig. 5(D). The EDS spectrum (Fig. 6) confirms the formation of Au@S-g-C3N4 nanocomposite and the quantitative elemental composition is shown in the inset. Further, the EDS mappings can verify the elements distribution of Au and S-g-C3N4 components in the nanocomposite as shown in Fig. 7(A)-(E).

Fig. 4.   UV-vis spectra of S-g-C3N4 (black line) and Au@S-g-C3N4 nanocomposite (red line).

Fig. 5.   TEM images of S-g-C3N4 (A), Au@S-g-C3N4 nanocomposite (B, C) and SAED pattern of Au@S-g-C3N4 nanocomposite (D).

Fig. 6.   EDS spectrum of Au@S-g-C3N4 nanocomposite and the quantitative elemental composition of the nanocomposite (the inset).

Fig. 7.   (A) SEM mapping analysis of Au@S-g-C3N4 nanocomposite and corresponding elements (B) C, (C) N, (D) S and (E) Au.

3.2. Effecting parameters for NP reduction

Herein, we have confirmed the effect of parameters (initial NP concentration and catalyst dosage) on NP reduction. Table 1 shows the different NP concentration (0.5-1.5 mM) on the reduction efficiency over Au@S-g-C3N4 nanocomposite. As much as NP concentration increased from 0.5 to 1.5 mM, the reduction rate was decreased. However, all the concentration of NP was reduced above 90% after 5 min in reaction time. Thus, the optimum concentration NP (1.0 mM) was selected and used in all experiments. These similar results were reported by earlier [27]. Further, we investigated the effect of different weigh percentage (0.1-1.0 mg) of catalyst dosage on reduction of NP as shown in Table 2. Generally, the enhancement of catalyst dosage increases the catalytic reduction of NP. The reduction rate increased highly up to 0.5 mg. However, the reduction rate increased slightly when catalyst weight increased over 1.0 mg. With above results, to reduce cost in real filed application the highest rate and minimum dosage of catalyst were used in the whole experiment.

Table 1   Reduction efficiency and their rate constant at various condition in 5 min with different NP concentarion.

NP concentration (mM)Reduction (%)Rate constant (min-1)Correlation co-efficient, R2
0.599.20.7960.9938
1.098.60.7510.9977
1.590.10.4930.9876

New window

Table 2   Reduction efficiency and their rate constant at various condition in 5 min with different catalyst weight.

Catalyst weight (mg)Reduction (%)Rate constant (min-1)Correlation co-efficient, R2
0.168.20.0470.9862
0.598.60.7510.9977
1.099.00.7720.9984

New window

3.3. Catalytic activity

The catalytic reduction of NP was used as a model probe to check the catalytic performance of the Au@S-g-C3N4 nanocomposite. The time dependent UV-vis spectroscopy was used to monitored the reduction progress. The band was observed at 317 nm is corresponding to the NP. This band at 317 nm was red shifted, which was observed at 400 nm after the addition of freshly prepared NaBH4 solution as shown in Fig. 8. These results certainly confirmed to the formation of 4-nitrophenolate ions with more pronounced π-conjugated donor-acceptor property [[14], [15], [16]]. The color of the solution also changed from light to intense yellow as shown in Fig. 8. Without adding any catalyst, the absorbance intensity and the intense yellow color of NP (400 nm) was observed without any changes. Interestingly, the absorbance intensity and the intense yellow color of NP decreased and disappeared after the addition of Au@S-g-C3N4 nanocomposite as a function of time Fig. 9(A) and (B). In addition, as the time increased, the new peak was also observed at around 300 nm (Fig. 9(B)) and gradually increased the peak intensity, which is due to the formation of reduction products of AP [22]. With the addition of S-g-C3N4, the reduction reaction initiated by the catalytic conversion of the 4-nitrophenolate solution but complete conversion does not occur as shown in Fig. 10(A) and (B). The reduction efficiency (Fig. 10(A)) and rate constant was calculated by the linear relationship between ln(Ct/C0) and reaction time t as shown in Fig. 10B. The rate constant was found to be 0.0896 min-1 and 0.751 min-1 for S-g-C3N4 and Au@S-g-C3N4 nanocomposite respectively. Thus, the present catalyst has comparable catalytic activity to other earlier reported catalysts [[28], [29], [30]] and also pure S-g-C3N4. The reason of high catalytic activity was due to the uniform Au nanoparticles assembled on to the surface S-g-C3N4 creates a synergistic effect and the accessibility of fast electron transfer. It is concluded that the synthesized Au@S-g-C3N4 nanocomposite can be used as an effective catalyst for the reduction reaction of NP. Finally, to check the reusability of the Au@S-g-C3N4 nanocomposite catalyst was decreased by around 5% (fifth cycle) compared to the first reduction of 4-nitrophenol as shown in Fig. 11(A). This may be attributed to the loss of catalysts weight when the filtration step of each catalytic cycle. As shown in the XRD patterns of Au@S-g-C3N4 nanocomposite before and after (Fig. 11(B)) the catalytic reaction no significant change was observed. These results indicate that the Au@S-g-C3N4 nanocatalyst had good catalytic reusability and stability.

Fig. 8.   UV-vis spectra of NP (curve a) and after addition of NABH4 in NP (curve b).

Fig. 9.   Photographic image of 4-nitrophenolate ions with Au@S-g-C3N4 nanocomposite at different time intervals (a) and UV-vis spectra of 4-nitrophenolate ions with Au@S-g-C3N4 nanocomposite at different time intervals (b).

Fig. 10.   The plot of reduction vs. reaction time (A) and ln(C/C0) against the reaction time for reduction kinetics of NP (B).

Fig. 11.   Reusability of NP catalytic reduction by Au@S-g-C3N4 nanocomposite (A) and XRD patterns of Au@S-g-C3N4 nanocomposite before and after being used for 5 cycles (B).

3.4. Catalytic mechanism

According to the Langmuir-Hinshelwood mechanism, the electron donor properties of 4-nitrophenolate and acceptor properties of BH4- were adsorbed on the surface of Au@S-g-C3N4 nanocomposite catalyst prior to reaction as shown in Scheme 2. The synergistic catalyst of Au@S-g-C3N4 can lead to the improvement of electron transfer from the adsorbed BH4- to 4-nitrophenolate and the 4-nitrophenolate was converted to benign form of AP. Finally, the conversion product of AP was desorbed on to the surface of Au@S-g-C3N4 nanocomposite. These type of catalytic mechanism were reported earlier [31,32].

Scheme 2.   Mechanism for the fast electron transfers reduction mechanism for the reduction of NP in the presence of Au@S-g-C3N4 nanocomposite.

3.5. Purification of NP-contaminated water

According to the literature [30,33,34], the filtering and catalyzing set up (Fig. 12(A)) was used to purify the NP containing water pollutants. 50 mL of NP (0.12 mM) containing water and NaBH4 (12 mM) in a beaker was stirred at 800 rpm, and then 5 mL of Au@S-g-C3N4 nanocomposite suspension (0.5 mg/mL) was added into the above solution (Fig. 12(B)). Afterwards, the mixture was quickly filtered through vacuum pump using a decompress filter. The filtrated and purified water showed pale yellow within 3 min (Fig. 12(C)) and finally (5 min) the pale yellow was changed to colorless. The absorbance peak at 400 nm was absent (Fig. 12(D)). Based on the overall experimental results indicates that the Au@S-g-C3N4 can be simply used for an efficient reduction of NP.

Fig. 12.   Digital photographs of the device of NP containing water purification (A) and UV-vis spectra of 0 min (B), 3 min (C) and 5 min (D). The insets correspond to their photographs.

4. Conclusion

A simple synthesis of sulfur doped graphitic carbon nitride act as a support cum stabilizers for gold nanoparticles towards the reduction of hazardous NP to benign AP. The Au@S-g-C3N4 nanocomposite showed superior catalytic reduction performance as compared to pure S-g-C3N4 and other reported results. The Au@S-g-C3N4 nanocomposite was found to be an efficient and recycled catalysts for the catalytic reduction of NP in the presence of NaBH4. The catalytic reduction percentage was observed at nearly 100% within 5 min and the kinetic rate constant was found to be 0.751 min-1. The highest catalytic reduction of NP is due to the synergistic effect of surface plasmon resonance of Au nanoparticles assembled on to the surface of S-g-C3N4. We hope that our work can be used for preparation of sustainable catalysts, various metal nanoparticles assembled with S-g-C3N4 for advanced reduction process in organic and pharmaceuticals.

Acknowledgements

This work was supported financially by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. NRF-2017R1E1A1A01074266) and the Industrial Fundamental Technology Development Program (No. 10076350) funded by the Ministry of Trade, Industry and Energy (MOTIE) of Korea.


/