Preparation of AuNPs/GQDs/SiO2 Composite and Its Catalytic Performance in Oxidation of Veratryl Alcohol

Composites of gold nanoparticles and graphene quantum dots (AuNPs/GQDs) exhibit excellent dispersibility in aqueous solutions. Thus, it is difficult to separate them fromwet reaction systems when they are used as catalysts. To resolve this issue, in this study, an AuNPs/GQDs composite was immobilized on silicon dioxide through the hydrothermal method, which involved the formation of an amide bond between the surface GQDs of the AuNPs/GQDs composite and the amino group of the silane. The as-synthesized AuNPs/GQDs/SiO2 composite was found to be suitable for use as a heterogeneous catalyst for the oxidation of veratryl alcohol in water and exhibited catalytic activity comparable to that of bare AuNPs/GQDs as well as better recyclability.


Introduction
Graphene oxide (GO) and graphene quantum dots (GQDs) have attracted increasing attention in recent years, and early reports of gold nanoparticles (AuNPs)/GQDs composites appeared in 2013 and 2014.Generally, synthesis of these composites was carried out using several steps.In most cases, AuNPs and GQDs were modified separately before they were compounded [1][2][3][4].However, other reports detailed the onestep generation of gold nanoparticles functionalized with graphene quantum dots (AuNPs/GQDs) [5,6].For example, GQDs with a two-dimensional lateral size of less than 100 nm, chloroauric acid (HAuCl 4 ), and sodium citrate were mixed, stirred, and heated to boiling in an aqueous solution to generate AuNPs/GQDs with uniform size [6].Because the GQDs were obtained from the GO photo-Fenton reaction [7], they retained good chemical stability and the electronconjugated state of GO.Further, they had more edge carboxyl groups, thus improving the stability and water dispersibility of AuNPs/GQDs.Studies of AuNPs/GQDs composites mainly focus on the field of gene detection [8,9], biosensors [10,11], and electrochemistry [12].However, the very good water dispersibility of AuNPs/GQDs composites may result in great losses during circular catalytic reactions and this issue has not yet been addressed.Silica (SiO 2 ) is used widely as the supporting matrix for catalysts owing to its unique physical/chemical properties, such as good thermal stability, large specific surface area, and good mechanical strength.Among the various types of silica available, mesoporous silica (such as MCM-41 and SBA-15), which has a high specific surface area and an ordered threedimensional channel-like structure, has attracted even more attention in recent years [13][14][15][16][17][18][19].For example, nanoparticles of certain noble metals can be supported on mesoporous silica by modifying the silanol bonds on their surfaces [20][21][22].Accordingly, it can be assumed that AuNPs/GQDs [6] can be supported on silica and that doing so will improve their recyclability during the catalytic reaction.Thus, in this work, an AuNPs/GQDs/SiO 2 composite was designed, synthesized, and characterized for use as a catalyst for the oxidation of veratryl alcohol in aqueous solution.

Preparation of AuNPs/GQDs Aqueous Suspension.
First, the AuNPs/GQDs composite was prepared according to the method reported in our previous study [6].In brief, 10 mg of the GQDs (10 mg/mL GQD solution, 1 mL), 20 mg HAuCl 4 (2% w/w aqueous solution of chloroauric acid, 1 g), and 5 mL of deionized water were added to a 50 mL flask, which was placed in a 100 ∘ C oil bath, and stirred.Then, 10 mL of a sodium citrate solution (58.8 mg of sodium citrate and 10 mL of deionized water) was added to the flask immediately.The flask was kept at 100 ∘ C to prepare the GQDs-functionalized gold nanoparticles (i.e., AuNPs/GQDs composite).The flask was removed from the oil bath after 30 min, and the solution in the flask was transferred to a 25 mL volumetric flask.The volume was adjusted to 25 mL by adding deionized water.The thus-obtained dispersion of the AuNPs/GQDs was then left to rest.

Synthesis of AuNPs
/GQDs/SiO 2 .First, 1.26 g of TEOS, 8.4 g of APTES, and 20 mL of ethanol were stirred at 40 ∘ C for 1 h.Next, a Pluronic F-127 solution (0.38 g of F127, 2 mL of HCl, and 20 mL of ethanol) and 24 mL of the AuNPs/GQDs dispersion were added to this mixture.The reaction was allowed to occur for 1 h at 80 ∘ C.Then, the temperature was increased to 100 ∘ C, and the mixture was refluxed for 24 h.Finally, the hydrothermal treatment was performed on the mixture at 150 ∘ C for 24 h.This yielded a black solid product (AuNPs/GQDs/SiO 2 ), which was washed thrice with deionized water and dried in a vacuum drier at 60 ∘ C for 12 h.

Oxidation of Veratryl Alcohol in Aqueous Solution.
The catalytic performance of the obtained composite was evaluated based on the oxidation of veratryl alcohol in an aqueous solution.The standard reaction conditions were as follows: 2 mol of veratryl alcohol, 20 mol of hydrogen peroxide, 1 mL of deionized water, and 2 mg of the synthesized composite were added to a 5 mL centrifuge tube.The centrifuge tube was then shaken at a temperature of 50 ∘ C for different amounts of time.After the reaction, 2 mL deionized water was added to stop the reaction and then the composite catalyst was removed by centrifugation at 10,000 rpm for 15 min.The solutions were diluted to 10 mL for UV-vis measurement.The conversion ratio of the reaction was calculated from the UVvis test and averaged over three independent experiments.

Result and Discussion
3.1.Morphology of AuNPs/GQDs/SiO 2 Composite.First, the particle size and morphology of the AuNPs/GQDs composite were determined using AFM imaging (Figure 1(a)).The size of the composite particles as estimated from the AFM images was mainly in 5-10 nm; this was confirmed by TEM imaging as well (Figures 1(b) and 1(d)).HRTEM image (Figure 1(c)) showed the lattice parameters of 0.124 nm and 0.243 nm which were corresponding to Au (311) lattice plane [24] and GQDs (111) lattice plane [25], respectively.These TEM images revealed the structure of AuNPs/GQDs in detail; the GQDs are attached on the surfaces of the AuNPs same as our previous study [6], though the sickness of GQDs layer (the edge of particles in Figure 1(b)) is much thinner due to the lower feed ratio than before; it is likely that the attached GQDs prevented AuNPs from further aggregation.The as-prepared AuNPs/GQDs composite was used as the raw material to synthesize the AuNPs/GQDs/SiO 2 composite.In principle, the GQDs used in this study, which were synthesized by the photo-Fenton reaction, would have contained unbound carboxylic groups, which can be activated by the hydrochloric acid in the reaction solution.Therefore, the activated GQDs on the surfaces of the AuNPs/GQDs particles could react with the amino groups of APTES during the reflux and hydrothermal processes.This was probably the main step in the bonding of the AuNPs/GQDs particles on the SiO 2 support.
To confirm whether this was indeed the case, FT-IR and XPS measurements were performed, in order to further analyze the AuNPs/GQDs/SiO 2 composite.As shown in Figure 2(a), peaks related to the stretching vibrations of the N-H bond (amino, 3400 cm −1 and 3500 cm −1 ), C-N bond (1420 to 1400 cm −1 ), and Si-O bond (798 cm −1 ) can be observed clearly in the FT-IR spectrum of APTES.Further, peaks ascribable to the stretching vibrations of the N-H bond (amido, 3350 cm −1 and 3180 cm −1 ) and C=O bond (1630 cm −1 ) can be seen in the spectrum of the AuNPs/ GQDs/SiO 2 composite (Figure 2(b)), confirming the formation of the amide bond.In addition, the fact that the intensity of the peak related to the Si-O bond (798 cm −1 ) decreased and the peak related to the Si-O-Si bond (1083 cm −1 ) remained unchanged confirmed the formation of the SiO 2 support.The concentrations of the constituent elements of the AuNPs/GQDs/SiO 2 composite, determined from the XPS measurements, are shown in Table 1.Further, the C 1s, N 1s, O 1s, Si 2p, and Au 4f spectra are shown in Figure 3.The C 1s spectra indicated that the binding energy for the C-C and C=C bonds was 284.7 eV, while those for the C-N, C-Si, C=O, and C-O bonds were 285.9 eV, 284.0 eV, 288.3 eV, and 287.6 eV, respectively.These results further confirmed the formation of an amide bond between the GQDs and SiO 2 .
Next, the morphology and size of the AuNPs/GQDs/SiO 2 composite were analyzed through FE-SEM, TEM, and XRD measurements.The FE-SEM image in Figure 4(a) shows clearly that the AuNPs/GQDs/SiO 2 composite has a threedimensional continuous structure.By measuring the surface area and porosity of the AuNPs/GQDs/SiO 2 composite, its BET surface was determined and found to be approximately 64.7 m 2 g −1 .Further, the pores of the composite were 5-50 nm in size (as shown in Fig. S1b in Supplementary Material available online at https://doi.org/10.1155/2017/4130569).Figures 4(b) and 4(c) are TEM and HRTEM images of the AuNPs/GQDs/SiO 2 composite, respectively, and further confirm that the AuNPs/GQDs particles are embedded in the SiO 2 support.Finally, the XRD pattern of the AuNPs/GQDs/SiO 2 composite exhibits peaks of 2 at 38.1 ∘ , 44.4 ∘ , 64.6 ∘ , and 77.6 ∘ ; these correspond to the (111), ( 200), (220), and (311) planes, respectively, of gold crystallites and confirm the existence of AuNPs [26].

Catalytic Performance of As-Prepared AuNPs/GQDs/SiO 2
Composite.It is well known that Au nanoparticles supported on various matrices can be employed to catalyze alcohol oxidation [27].In this work, oxidation of veratryl alcohol in an aqueous solution using H 2 O 2 as an oxidizing agent was employed as a model system.The catalytic performance of the synthesized AuNPs/GQDs/SiO 2 composite with respect to the oxidation of veratryl alcohol was evaluated through UV-vis spectroscopy and HPLC measurements.The characteristic UV-vis absorption peaks of veratryl alcohol, veratraldehyde, and veratric acid appear at 277 nm, 308 nm, and 252 nm, respectively (Fig. S2a).As shown in Figure 5(a), with an increase in the reaction time, the veratraldehyde yield increased gradually.The spectra acquired after reaction times of 6 h, 9 h, and 12 h indicated that the conversion of the alcohol to the aldehyde was almost complete within 9 h and that the conversion of the aldehyde to the acid started subsequently.For durations of 27 h and 42 h, the generated veratraldehyde of oxidation reaction catalyzed by AuNPs/GQDs/SiO 2 composite was then partly converted into veratric acid; this trend is well consistent with our previous work [6].It was generally accepted that the residual oxygen-containing functional groups on the surface of supported Au catalysts play a key role in their catalytic performances, especially for the oxidization reactions [28].Besides, it was also reported that the GQDs with plenty of the periphery carboxylic groups show high peroxidase-like activity under acidic environment [29].Thus, the mechanism of the oxidation reaction catalyzed by AuNPs/GQDs/SiO 2 composite is summarized as follows.During the reaction, veratryl alcohol was absorbed onto the AuNPs/GQDs through p-p stacking firstly; then, the superoxide anion O 2 −• and singlet oxygen 1 O 2 generated by both AuNPs and GQDs with H 2 O 2 oxidize the reactant to veratraldehyde and veratric acid.The selectivity and conversion efficiency of AuNPs/GQDs/SiO 2 composite are mainly contributed by AuNPs, GQDs and their interaction.
To evaluate the recyclability of the AuNPs/GQDs/SiO 2 composite catalyst, after the 2 h reaction, the composite was separated from the reaction system by centrifugation at 10,000 rpm for 15 min, and the reaction products were analyzed.This process was repeated four times.As shown in Figure 5(b), the yield of veratraldehyde remained constant (over 80%) from the second cycle onwards; these results are listed in Table 2. Thus, it was confirmed that, when used as a catalyst, the as-prepared AuNPs/GQDs/SiO 2 composite exhibited greater reusability as compared to the AuNPs/GQDs composite, which cannot be separated from the reaction system by centrifugation.Besides, TEM images suggest that the morphology of the AuNPs/GQDs/SiO 2 composite underwent almost no change, as shown in Figure 6.

Conclusion
AuNPs/GQDs were immobilized on silicon dioxide through the hydrothermal method, which involved the formation of   an amide bond between the surface GQDs and the silane.The as-obtained AuNPs/GQDs/SiO 2 composite exhibited a surface area of 64.7 m 2 g and had a good dispersion of AuNPs/GQDs.Using AuNPs/GQDs/SiO 2 composite as a heterogeneous catalyst and hydrogen peroxide as the oxidant, veratryl alcohol could be oxidized to veratraldehyde and veratric acid in a step-wise manner.It was also confirmed that the recyclability of the AuNPs/GQDs composite improved after immobilization on SiO 2 .Work is in progress to obtain insight into the details regarding the morphology and catalytic performance of AuNPs/GQDs/SiO 2 composites fabricated under a wider range of conditions.

Figure 5 :Figure 6 :
Figure 5: UV-vis spectra of aqueous solutions containing veratryl alcohol after being subjected to the catalytic oxidation reaction for various durations.Reaction conditions: 2 mol of veratryl alcohol, 20 mol of hydrogen peroxide, 1 mL of deionized water, 2 mg of composite, and temperature of 50 ∘ C.

Table 2 :
Catalytic ability of AuNPs/GQDs/SiO 2 with respect to oxidation of veratryl alcohol.Reaction conditions: 2 mol of veratryl alcohol, 20 mol of hydrogen peroxide, 1 mL of deionized water, 2 mg of composite, and temperature of 50 ∘ C. The standard curve whose -coordinate is the conversion ratio and -coordinate is the UV absorbance is shown in Fig.S2b.b Conversion ratios are the average values for three reaction runs.c The peak area of the HPLC curve for a 2 mmol/L veratraldehyde solution (corresponding to 100% conversion) is 8584 mAU × S. a