Ternary Ag nanoparticles/natural-magnetic SiO2-nanowires/reduced graphene oxide nanocomposites with highly visible photocatalytic activity for 4-nitrophenol reduction
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Agglomerate and reuse limit the promising application of silver nanoparticles (AgNPs) as catalyst. To eliminate those disadvantages, herein, Fe-containing silica nanowires (SiO2NWs) and reduced graphene oxide (RGO) are used as suitable substrates to prepare AgNPs/SiO2NWs/RGO nanocomposite via self-assembly approach. The nanocomposite mostly assembled with each other via intermolecular hydrogen bond and electrostatic adsorption to form a three-dimensional network structure. The AgNPs/SiO2NWs/RGO nanocomposite exhibit excellent photocatalytic activity for 4-nitrophenol reduction by NaBH4, originating from that the nearly mono-dispersed AgNPs are adhered on the surface of the SiO2NWs and RGO, allowing the effective contact of reactants with catalyst and facilitating the electron transfer between them in the reaction. The obtained nanocomposites exhibit the superior stability and can be easily recovered with their fully catalytic activities due to the hydrophobic and magnetic properties of the nanocomposites. It shows the great prospect for the 4-NP reduction in practice and is promising for wide applications in visible light catalytic reaction.
KeywordsReduced graphene oxide SiO2 nanowires Silver nanoparticles 4-Nitrophnol reduction Photo-catalytic activity
Due to the outstanding catalytic properties of silver nanoparticles (AgNPs), it was considered as one of the most promising functional materials in the field of electronics, chemicals, biologics and catalyst for a long time [1, 2, 3]. However, agglomerate and reuse were the main drawbacks for limiting its application. To solve the disadvantage of AgNPs, traditional strategies of dispersed AgNPs on a suitable substrate were used to form hybrid catalysts by chemical synthesis methods (such as polymers, metal oxides, silica nanotubes, carbon nanofibers, etc.) [4, 5, 6, 7, 8]. Silica nanomaterial was one of the suitable substrates because of material availability and environmental friendly [3, 7, 9, 10, 11, 12]. In recent years, many silicon oxide nanostructures have been studied to assemble AgNPs via different methods include chemical plating [13, 14, 15], ultrasonication , in situ assembly and in situ reduction [12, 17, 18], electro static interaction  etc. Conventional methods, using silane and other organic reagent to prepare nano silicon dioxide, had a harmful effect on the environment in many previous studies, and also lack of sustainability. Therefore, it is necessary to develop a new synthesis method of nanometer silicon dioxide.
Silica nanowires prepared from Chrysotile (Mg6[Si4O10](OH)6) was an excellent natural catalyst support candidate because of its outstanding physicochemical properties [12, 20, 21] and simple synthesis method . As material sources are abundant and the reuse of Chrysotile asbestos tailings, the natural Chrysotile-based silica nanowires were comparatively cheap and became the better choice of catalyst support [12, 22]. In addition, the presence of associated mineral of Chrysotile made the prepared silica nanowires containing iron, which introduced the new property: magnetic property. Although various Ag/SiO2 composites had effectively prevented the agglomerates of Ag NPs, the problem of the catalyst reuse still hadn’t been solved very well due to the size of nanometer materials. It seems that the use of graphene could effectively solve this problem. As nanoscale silicon dioxide could be coated by graphene to form a hydrophobic composite , and graphene was another suitable holder which was studied due to its large surface area and unique optical, electronic, mechanical, catalytic properties in recent years [23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39]. To improve the catalytic property, these two suitable holders also were used together to combined with AgNPs [40, 41]. At the same time, it had great help for recyclable property.
Silicon dioxide nanowires (SiO2NWs) , 3-aminopropyltriethoxysilane (γ-APS, 98%),acetic acid (CH3COOH, 98%), silver nitrate (AgNO3, 99.8%), sodium citrate (C6H5Na3O7·2H2O, 99%), and ethanol (EtOH, 99.7%) were supplied by Kelong Chemical Factory (Chengdu, China). Sodium borohydride (NaBH4, 97%) and 4-nitrophenol (C6H5NO3, 99.7%) were purchased from Aladin Ltd. (Shanghai, China). All chemicals were used without further purification. GO nanosheets were obtained from flake graphite (< 30 μm, Qingdao, China) by using the modified Hummers method . The water used was purified through a Youpu system.
2.2 Preparation of AgNPs/SiO2NWs/RGO nanocomposites
SiO2NWs prepared from Chrysotile were aminated firstly by electron-rich 3-aminopropyltriethoxysilane. Procedure for amination was according to the method in Ref. . 0.1 mg mL−1 of GO solution was prepared. GO nanosheets were obtained from flake graphite. The AgNPs were restored by sodium borohydride while electron-deficient sodium citrate acted as stabilizer. Typically, 25 mL of AgNO3 (2 mmol L−1) and 25 mL of sodium citrate (4 mmol L−1) solution were mixed and stirred at 333 K for about 20 min. After addition of 0.6 mL of NaBH4 (10 mmol L−1), the mixed solution changed from colorless to yellow. Then, 0.01 g of modified SiO2NWs was dissolved into 60 mL water. After ultrasonication at 323 K for 1 h, the suspensions were mixed with different volumes of AgNPs and 18 mL 0.1 mg mL−1 of GO (the maximum amount of GO combined with SiO2NWs which was found by the experiment). After that, it was stirred for 2 h. Subsequently, the mixture was centrifuged at 4000 r min−1 for 5 min, washed with water for 5 times. The precipitates were re-dispersed in 100 mL of water and reduced by excess NaBH4. Finally, the composites were dried at 333 K. The added amount of AgNPs by different volumes (1, 2, 3, 5, 8, 10 mL) were 0.89, 1.77, 2.64, 4.32, 6.74, 8.28 wt%, respectively. The number of added AgNPs volumes was used to name the different AgNPs/SiO2NWs/RGO-X Nanocomposites as the X.
The crystalline phases of composites were examined by X-ray diffraction (XRD, Panalytical X’Pert Pro) using Cu Kα radiation (λ = 0.03343).The composites morphologies were analyzed by scanning electron microscope (SEM, Zeiss Libra, Germany). AgNP size was tested and the microstructure of composite was analyzed by transmission electron microscope (TEM: 200FE, Zeiss Libra, Germany). Identification of the different chemical states of elements was carried out by X-ray photoelectron spectroscopy (XPS, SSX-100). Magnetic hysteresis loops was measured by vibrating sample magnetometer (VSM: BKT-4500Z, China). The nitrogen adsorption–desorption isotherm was measured at 77 Kusing Micromeritics ASAP 2020 adsorption apparatus. The Brunauer–Emmett–Teller (BET) surface area of the sample was evaluated using the nitrogen adsorption isotherms.
The photocatalytic activity of the AgNPs/SiO2NWs/RGO nanocomposites were evaluated for 4-nitrophenol reduction by using NaBH4 in the photo reaction apparatus (BL-GHX-V, Bilang Biological Science and Technology Co., Ltd., Xi’an) using a 300 W Xe lamp with an ultraviolet cutoff filter (providing visible light ≥ 400 nm) as the light source to trigger the photocatalytic reaction.
A 10 mL portion of 4-nitrophenol solution (4-NP, 100 mg L−1) and 10 mL of sodium borohydride (NaBH4, 2.7 g L−1) were dropped into quartz test tubes. Next, 10 mg AgNPs/SiO2NWs/RGO nanocomposite was dropped into the mixture solution, and the reaction was maintained at an appropriate time. The reaction was measured by using an UV–vis spectrophotometer (UV2600A UV–vis spectrophotometer). The composite was recovered by vacuum suction filtration quickly after the photocatalytic reaction.
3 Results and discussion
3.1 Characterization of AgNPs/SiO2NWs/RGO nanocomposites
In order to study the morphology of RGO and AgNPs on SiO2NWs surface, the microstructure transformations of SiO2NWs and the AgNPs/SiO2NWs/RGO nanocomposites were analyzed by SEM. As shown in Figure S2(a, b), the SEM images of the SiO2NWs and AgNPs/SiO2NWs/RGO nanocomposites indicate that RGO nanosheets and AgNPs on SiO2 NWs surfaces are well-assembled and the integrated material possesses a three-dimensional network structure consisting of mutual cross-linked RGO nanosheets and SiO2NWs adhered AgNPs. And there is no obvious preferred orientation between RGO sheets and SiO2NWs, which is in agreement with the existence of strong intermolecular hydrogen bonds. The diameter of SiO2NWs is almost 50 nm. The three-dimensional network structure indicates that the amino groups modified silica surface is helpful for bonding with graphene oxide and well-distribution of silver nanoparticles. The aminated SiO2 NWs are negatively charged. Intermolecular hydrogen bonds between amino groups and functional groups (−OH and −COOH groups carboxyl) of GO also exist. The results of FT-IR spectra also proved the presence of hydrogen bonds (see the supporting Information, Figure S3). The electron-deficient AgNPs adhered on the surface of RGO nanosheets and electron-rich amino groups functionalized SiO2NWs are interacted through electrostatic attraction.
3.2 Catalytic reduction of 4-nitrophenol
The reduction of 4-nitrophenol (4-NP) is one of the model reactions for appraising the catalytic activity of noble metal nanoparticle [6, 26]. So the photo-catalytic reduction of nitroaromatic compounds is chosen as a test reaction to investigate the photo-catalytic activity of as-prepared AgNPs/SiO2NWs/RGO nanocomposite. In fact, the absorption peak of 4-NP solutions is at 317 nm under non-alkaline conditions. The peak is red-shifted to 400 nm because of the formation of 4-nitrophenolate ion after being treated by NaBH4 (see the supporting Information, Figure S5). The color of the 4-NP solutions changes from light-yellow to yellow-grown at the same time.
The repeatability test was used to investigate the stability of the photochemical catalytic properties of AgNPs/SiO2NWs/RGO nanocomposites, and the results show that the photocatalytic activity of AgNPs/SiO2NWs/RGO-10 is outstanding among all kinds of AgNPs/SiO2NWs/RGO nanocomposites prepared in the current reaction system (Fig. 6c). The high activity after undergoing four catalysis cycles suggesting the composite’s good recyclability. After recycling, the structure and morphology of the AgNPs/SiO2NWs/RGO catalyst is stable, and the three-dimensional network structure is remain exist (see the supporting Information, Figure S8). The catalytic reduction is accompanied by the rapid color change (see the supporting Information, Figure S4). As the hydrophilic surface of the SiO2 nanowires became hydrophobic after wrapped with RGO , the hydrophobic AgNPs/SiO2NWs/RGO nanocomposites make it easily to be recycled, forming film via filtration process. The kinetics of decomposition can be understood according to physical chemistry principles. The results shown in Fig. 6 imply that the previous catalytic reduction reactions are consistent with the Langmuir–Hinshelwood apparent first order kinetics model because of superfluous NaBH4 used to protect the 4-AP from aerial oxidation compared with 4-NP and catalyst .
Figure 6d shows the linear relationship of lnC/C0 versus t, and which indicates that the reaction of 4-NP in the presence of AgNPs/SiO2NWs/RGO nanocomposites followed pseudo-first-order kinetics. It can be observed that AgNPs/SiO2NWs/RGO exhibits high catalytic activity. While, the samples of AgNPs/SiO2NWs/RGO-1, 2, 3, 5, 8, 10 result in the reaction rate constants of 1.628, 1.717, 1.820, 1.942, 3.254, and 3.711 × 10−3 s−1, which are some higher than Ag-SiO2NWs and AgNPs (34.8 mg L−1 4-NP, 2.52 × 10−3 s−1, 2.38 × 10−3 s−1) . This indicates that the catalytic efficiency is significantly enhanced with the increasing silver nanoparticles on the SiO2NWs and RGO. In addition, our results also imply that AgNPs/SiO2NWs/RGO nanocomposites would greatly promote the industrial potential application of pristine SiO2NWs, AgNPs and RGO-SiO2NWs. Nevertheless, the reaction rate constants are lower than Ag-RGO (10 mg L−1 4-NP, 6.49 × 10−3 s−1) . Although the constant of Ag-RGO is much higher, there is no actual comparability because of the lacked Ag concentration in the paper.
All above analyze show that the insulator SiO2NWs provide the framework and form the stable three-dimensional network structure with RGO. The adhered AgNPs have the photocatalytic activity, the graphene facilities make the charge separation of the photocatalyst, and the maximum load of GO combined with SiO2NWs is found to be 18 wt%. Moderate graphene and Ag NPs load lead to the increased photocatalytic activity because of the increase of the available surface area for 4-NP adsorption.
3.3 Catalytic mechanism
In summary, we reported a novel and scalable preparation procedures of AgNPs/SiO2NWs/RGO nanocomposites with three-dimensional network structure. It was synthesized by using SiO2NWs prepared from Chrysotile and homemade GO as the suitable holder to combine with AgNPs under the strong hydrogen-bonding and electrostatic adsorption between SiO2NWs, GO nanosheets and AgNPs. The SiO2NWs provide the framework and form the stable three-dimensional network structure with RGO. The photocatalytic activity of the AgNPs/SiO2NWs/RGO was evaluated for 4-nitrophenol reduction by using NaBH4. The composites exhibited high catalytic activity because the nearly mono-dispersed AgNPs were adhered on the surface of SiO2NWs and RGO, allowing effective active contact and electron transfer between the reactants and catalysis of the reaction. In particular, the as-prepared AgNPs/SiO2NWs/RGO nanocomposites with 10 mL AgNPs (AgNPs/SiO2NWs/RGO-10) exhibited excellent catalytic activity. Significantly, these AgNPs/SiO2NWs/RGO nanocomposites exhibit the superior stability and can be easily reused with a little decline of the catalytic activity due to SiO2 nanowires natural mineral frameworks with large amounts of active sites and the hydrophobic surface and soft magnetic property of AgNPs/SiO2NWs/RGO materials. These nanocomposites show the great prospect for the 4-NP reduction in practice and are promising for wide applications in visible light catalytic reaction.
This study was funded by the Open Foundation of Joint Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology and Research Center of Laser Fusion, CAEP (14tdjk02); the Open Foundation of Nuclear waste and environmental safety National defense key discipline laboratory (15kffk07); Basic Scientific Research Key Project (JCKY2016208B012).
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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