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Controlled Synthesis of Noble Metal@Mesoporous Carbon Colloid as Highly Active Nanocatalysts Tao Wang, Ying Jing, Yan Sun, Yali Ma, Kaiqian Li, Yunling Liu, Ling Zhang, Qisheng Huo, and Zhen-An Qiao ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00049 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Applied Nano Materials

Controlled Synthesis of Noble Metal@Mesoporous Carbon Colloid as Highly Active Nanocatalysts †

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Tao Wang, Ying Jing, Yan Sun, Yali Ma, Kaiqian Li, Yunling Liu, Ling Zhang, ,* Qisheng Huo,† and Zhen-An Qiao†,* †

State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of Chemistry,

Jilin University, Changchun 130012, China ‡

College of Chemistry, Jilin University, Changchun 130012, China

KEYWORDS colloid, mesoporous materials, core-shell structure, noble metal nanoparticles, carbon materials

ABSTRACT

The key concept to simplify the synthesis steps of core-shell nanocomposites is to synthesize the core and shell components together, without tedious separation and purification steps. In addition, the sintering problems of carbon spheres during high-temperature carbonization process need to be addressed aiming to obtain monodisperse carbon nanospheres. Through extension of our reported silica-assisted strategy, a facile one-pot strategy is reported here to synthesize noble metal@mesoporous carbon (NM@MC) core-shell colloids, in which the formation and encapsulation process of noble metal nanoparticles happened simultaneously through a series of

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synergetic reactions. The resulting NM@MC colloidal nanospheres show uniform and highly dispersed morphology, tunable size, high surface area, and clear core-shell structure with abundant mesopores. In particular, Au@MC colloid shows significantly high catalytic activity in reduction of 4-nitrophenol to 4-aminopheol thanks to the excellent catalytic performance of Au inner core and the fast mass transfer of mesoporous carbon shell.

INTRODUCTION Core-shell nanostructure as a type of vibrant nanoarchitecture consisting of a guest inner core nanoparticle and a different material shell, provides a great opportunity to control the interaction among the different components, which is useful in many fields, such as catalysis, sustainable energy, and biomedical applications.1-5 Most current research efforts in this field are directed to the development of novel synthetic strategies,6-10 such as bottom-up or soft-templating, top-down or selective etching approaches, the Ostwald ripening or galvanic replacement process, ship in a bottle strategies, and Kirkendall effect–based methods for core-shell materials with various types including organic@organic, inorganic@inorganic, organic@inorganic, and inorganic@organic core-shell nanostructures.11, 12 In particular, noble metal@ mesoporous carbon (NM@MC) nanocomposites, with a noble metal nanoparticle as a core and a mesoporous carbon as a shell, combine the excellent performance of noble metal nanoparticles13-18 with the high stability and fast mass transfer rate of mesoporous carbon shell,19-24 which are attractive for applications as nanoreactors, surface-enhanced Raman scattering (SERS) technologies, and antibacterial agents.25-29 However, the synthesis of NM@MC core-shell nanocomposite generally contains several tedious steps: growth, separation and purification of noble metal nanoparticles;


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encapsulation of noble metal in carbon precursor; pyrolysis of carbon precursor to obtain porous carbon shell.30-34 Even so, the obtained shell is mostly microporous carbon, which intensely limits the mass transfer efficiency of guest molecule.30, 35 Therefore, developing a concise synthetic route to obtain NM@MC core-shell nanocomposites is still challenging but worth expecting. Mainwhile, we have reported the silica-assisted strategy to synthesis diverse mesoporous carbon colloidal spheres by using phenolic resols, silicate oligomers as precursors, and the cationic surfactant hexadecyl trimethylammoniumchloride (CTAC) as a template.24 The surfactant, CTAC has been reported to be utilized as both a reductant and stabilizer for the synthesis of noble metal nanoparticles.36 Choma et al. reported that microporous carbon/Ag nanocomposites can be obtained by addition of F127 and silver nitrate during the synthesis of resorcinol–formaldehyde resin.37 The polymerizable monomer of phenolic resols, formaldehyde can serve as a reducing agent for the synthesis of Ag nanoparticles.38 Choma et al. also prospected a possible extended Stöber method to obtain Ag nanoparticles embedded rattle-type micro–mesoporous carbon spheres.37 Combining these work, the “silica-assisted strategy” is hopefully applied to the succinct synthesis of monodisperse NM@MC colloids. Herein, we report an extended silica-assisted strategy to synthesize monodisperse NM@MC core-shell nanocomposites, which simplifies the general production processes of core-shell nanocomposites as reported previously. Three typical noble metal Au, Ag and Pt nanoparticles are encapsulated in MC nanoshell with controllable shell thickness respectively to obtain corresponding core-shell nanocomposites, which are respectively denoted as Au@MC, Ag@MC, and Pt@MC. The uniqueness of our synthesis approach is that the reduction process of noble metal ions to noble metal nanoparticles and the encapsulation process of noble metal nanoparticles by resorcinol–formaldehyde resin (RF) and silica composites are happened


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simultaneously in a one-pot synthesis system (Figure 1a). Different from the reported works, the main reductant for the formation of noble metal nanoparticles is resorcinol, not CTAC or formaldehyde. CTAC, which is used as the assistant reductant and stabilizer for the noble metal nanoparticles,38 also plays an important role as templates to originate mesopores in carbon shell. MC shell was synthesized based on our previously reported “silica-assisted” strategy,24 in which tetraethylorthosilicate (TEOS) as an inorganic precursor not only assembles with the surfactants but also condenses and cross-links with phenolic resoles to form the framework of mesoporous carbon. Phenolic resin as the carbon precursor also prevents the sintering of core particles during the carbonization process at high temperature. The excellent catalytical property of noble metal cores and the highly efficient mass transfer ability of MC shell enable Au@MC collodial nanocomposites with significantly high catalytic activity during the catalytic reduction of 4nitrophenol (4-NPh) to 4-aminopheol (4-APh) by NaBH4.

Figure 1. (a) Schematic illustration of the synthetic process of NM@MC. (b) Hydrodynamic diameters distribution of Au@RF-SiO2 nanospheres measured by DLS. (c) Relationship between the main particle size and reaction time for Au@RF-SiO2 and RF. (d) SEM image of Au@RFSiO2. (e)TEM image of Au@RF-SiO2.


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EXPERIMENTAL SECTION Chemicals. Cetyltrimethylammonium chloride (CTAC), ethanol, Ammonia (27 wt% aqueous solution), hydrofluoric acid solution (40 wt% aqueous solution) and formaldehyde solution (40 wt% aqueous solution) were obtained from Beijing Chemical Works. Resorcinol, 4-nitrophenol (4-NPh), sodium borohydride (NaBH4), chlorine acid (HAuCl4), potassium hexachloroplatinate (K2PtCl6), and silver nitrate (AgNO3) were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China). Synthesis of NM@RF-SiO2. For a typical synthesis of Au@RF-SiO2 with the average size of 500 nm, 0.1 mL of ammonia was mixed with 7 mL of EtOH, 1.04 g of CTAC solution (25 wt% aqueous solution), and 19 mL of deionized water, then stirred for 30 mins. Subsequently, 0.2 g of resorcinol was added to the solution under stirring for 0.5 h. Then, 1 mL of HAuCl4 (1 wt% aqueous solution) was dropped to the solution. Then 0.36 mL of TEOS and 0.28 mL of formaldehyde solution were sequentially dropped into the reaction solution and continually stirred for 24 h at 30 oC. The product was collected by centrifugation, and washed with deionized water and EtOH for twice. Au@RF-SiO2 with other sizes were synthesized by adjusting the amount of EtOH. Pt@RFSiO2 and Ag@RF-SiO2 were synthesized similar to Au@RF-SiO2, by changing the HAuCl4 to the same amount of K2PtCl6 and AgNO3, respectively. Synthesis of NM@MC. The solid product Au@ RF-SiO2 was air-dried overnight, and heated in a tubular furnace at 200, 350, 500, 600 oC for 2 h with a heating rate of 1 oC min-1 and 5 oC min-1 to 800 oC for 5 h under N2 flow. The pyrolyzed product was treated with a 20% hydrofluoric acid


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solution for 6 h to remove silica, washed with deionized water more than 3 times, and air-dried overnight to obtain Au@MC. Ag@MC and Pt@MC were obtained the same as Au@MC. Catalysis. Typically, 5 mg of catalyst was dispersed in 1 mL of H2O to form an ink. Then, 100 µL of catalyst ink was added into the mixture of reaction solution containing 3 µmol of 4-NPh, 22.5 µmol of NaBH4, and 3 mL of H2O. The absorbance change was monitored by a Shimadzu UV-2450 spectrometer in the spectral range of 280 nm to 550 nm. Characterization. Transmission electron microscope (TEM) analysis was performed on a FEI Tecnai G2 F20 s-twin D573 field emission transmission electron microscope operated at 200 kV. Scanning electron microscopy (SEM) analysis was performed on a JEOL JSM-6700F field emission scanning electron microscope operated at 5 kV. Powder X-ray diffraction (XRD) patterns were collected by a Rigaku 2550 diffractometer with Cu Kα radiation (λ = 1.5418 Å). The sizes of Au@RF nanospheres were measured by a Nano ZS90 laser particle analyzer (Malvern Instruments, UK). UV-vis spectra were performed with a Shimadzu UV-2450 spectrometer. N2 adsorption-desorption isotherms were obtained at 77 K on a Micromeritics 2420 instrument. Samples were degassed by vacuum degassing at 150 oC for 12 h before analysis. RESULTS AND DISCUSSION Synthesis and characterization of NM@MC. In a typical synthesis of Au@MC, chloroauric acid solution was dropped into the mixture solution containing CTAC, resorcinol, ethanol and ammonia. The reaction solution quickly changed from colorless to dark, indicating the formation


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of Au nanoparticles (Figure S1a). The average diameter of Au nanoparticles can be adjusted from 63 to 122 nm by the concentration of HAuCl4 (Figure S2), according to the results of dynamic light scattering (DLS). The RF-SiO2 shell began to grow as soon as TEOS and formaldehyde were added in the reactant system (Figure 1b). Especially, the particle size increased fast from 86 to 470 nm in the first 30 minutes (Figure 1c), after that, the particles performed a slow growth to reach their final size of 497 nm. The product (denoted as Au@RFSiO2) was collected by centrifugation. The morphology and structure of Au@RF-SiO2 were characterized by SEM and TEM. Au@RF-SiO2 exhibited monodispersed spheres in SEM image (Figure 1d) and clear core-shell structure in TEM image (Figure 1e). The Au core of Au@RFSiO2 is about 80 nm and the thickness of the shell is about 220 nm, which is in agreement with the results of DLS. Au@RF-SiO2 was pyrolyzed under nitrogen atmosphere to achieve carbonization of RF, and then treated in hydrofluoric acid (HF) solution to remove silica in the framework to obtain the final production of Au@MC core-shell nanocomposites.

Figure 2. (a, b, c) SEM images of Au@MC (a), Ag@MC (b) and Pt@MC (c); (d, e, f) TEM images of Au@MC (d), Ag@MC (e) and Pt@MC (f).


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Figure 3. (a) Photographs of Au@MC dispersed in hexane and water. (b) DLS results of Au@MC in water; inset: photograph of Au@MC dispersed in water under a red laser. The resulting Au@MC nanocomposites maintain monodispersed spherical morphology (Figure 2a) and core-shell structure (Figure 2d) of Au@RF-SiO2. In general, phenolic resin derived porous carbon spheres are hydrophobic ones after high temperature treatment.39 Au@MC nanocomposites here are hydrophilic that could be dispersed in water but not in hexane (Figure 3a). The possible reason is that SiO2 is used to cross-link with phenolic resoles to form the frameworks of MC and the removal of SiO2 leads to the residual hydroxyl groups in MC frameworks. The stable Au@MC colloid in water (Figure 3b) exhibits a particle dispersion index (PDI) of 0.023, indicating an excellent monodisperse system, which favors the catalytic reaction in water. Fourier transform infrared (FTIR) spectroscopy was used to confirm the existence of hydroxyl groups in Au@MC (Figure S6), which is critical to the hydrophilic. Similar to Au@mesoporous silica nanoparticles (Au@MSN)， Au@MC also exhibited strong hydroxyl absorption peak at 3400 cm-1 in FTIR spectra, indicating abundant hydroxyl groups in Au@MC. The Au cores did not agglomerate during the pyrolysis process, benefiting from the protection of the outer shell. The dendritic mesopores about 3 nm in the shell can be directly observed in the TEM image of Au@MC (Figure 2d). The average diameter of Au@MC nanospheres can be tailored from 280 to 800 nm by varying the ethanol/water volume ratio from 0.35 to 0.45 in synthesis solutions (Figure S4). In consideration of the negligible change of Au cores, the growth


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of particle size is mainly attributed to the increased shell thickness. Thus, the shell thickness can be adjusted from about 100 to 350 nm. With the increase of shell thickness, the worm-like mesopores in out shell became larger, straighter and more orderly. Powder X-ray diffraction (PXRD) was used to confirm the crystalline phase of metal nanoparticle core in NM@MC (Figure S5). PXRD pattern of Au@MC (Figure S5a) shows a series of peaks at 2θ = 38.2°, 44.4°, and 64.6°, assigning to (111), (200), and (220) reflection of Au cubic (fcc) lattice. The broad low intensity peak at 2θ = 23.4° corresponds to the (002) planes of graphite, indicating the amorphous carbon structures in Au@MC. Moreover, our synthetic strategy can be extended to one-pot encapsulate other nanoparticles within MC shell, such as Ag and Pt to obtain Ag@MC and Pt@MC (Figure 2e and 2f), respectively.

Figure 4. (a) N2 adsorption−desorption isotherms of Au@MC, Pt@MC and Ag@MC at 77 K. (b) PSD analysis for Au@MC, Pt@MC and Ag@MC according to the BJH model. PSD curves are offset by 1 cm3 g−1 Å−1 along the vertical axis for clarity. The pore characteristics of NM@MC were further examined by the nitrogen adsorption experiment at 77 K (Figure 4a). All the nitrogen adsorption isotherms for NM@MC core-shell nanocomposites exhibit typical characteristics of type IV according to the classification of the International Union of Pure and Applied Chemistry (IUPAC), suggesting the mesoporous


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frameworks of the carbon shell. The adsorption isotherms show obvious increase at a low relative pressure (P/P0 < 0.1), suggesting the existence of numerous micropores in the carbon shell. Adsorption hysteresis originated from mesopores can be found in adsorption isotherms at the relative pressure between 0.4 and 0.7. The increases of N2 adsorption at a higher pressure (P/P0 > 0.9) reflect the interparticle texture between the nanocomposites. As shown in Table 1, the Brunauer-Emmett−Teller surface areas (SBET) are calculated to be 915 m2/g for Au@MC, 869 m2/g for Pt@MC, and 625 m2/g for Ag@MC, based on the N2 adsorption isotherms. The pore size distribution (PSD) curves (Figure 4b) caculated by using the Barrett–Joyner–Halenda (BJH) method exhibit a pore size distribution with a sharp maximum at 3.1 nm for Au@MC, 2.1 nm for Pt@MC, and 3.9 nm for Ag@MC. The results based on the N2 adsorption isotherms are in agreement with the structures observed in the TEM images (Figure 2b, d and f). All the above results demonstrate the successful synthesis of NM@MC nanoparticles. Table 1. Structural properties of Au@MC, Pt@MC and Ag@MC. Materials SBETa (m2/g) Smicrob (m2/g) Smesoc (m2/g) Vtotald (cm3/g) Pore width (nm) Au@MC

915

209

706

0.76

3.1

Pt@MC

869

218

651

0.61

2.9

Ag@MC

625

267

358

0.54

3.9

a

SBET is the BET specific surface area obtained from nitrogen adsorption isotherms at 77 K in the P/P0 range from 0.05 to 0.3; b

Smicro is the cumulative surface area calculated in the range of pore widths below 2 nm by using t-plot analysis; c

Smeso is the cumulative surface area calculated in the range of pore widths above 2 nm by using t-plot analysis; d

Vtotal is the single point pore volume calculated from adsorption isotherm at P/P0 = 0.985.


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Discussion of the synthesis mechanism. The one-pot strategy for NM@MC nanoparticle synthesis contains a series of synergetic reactions. At the beginning of the reaction, the raw material resorcinol serves as the main reductant and coordination stabilizer for the growth of Au cores because the Au nanoparticles will be flocculent precipitate in absence of resorcinol (Figure S1b). The formation of Au nanoparticles completes before the addition of formaldehyde (Figure S1a). Thus, formaldehyde does not serve as a reductant here. Without CTAC or TEOS, the carbon shell will be microporous one, suggesting the key roles of CTAC and TEOS for the formation of mesoporous structure (Figure S1c). After the addition of TEOS and formaldehyde, the generated RF-SiO2-CTAC mesostructures grow around Au cores to form RF-SiO2 shell. The growth of Au@RF-SiO2 is faster than pure RF-SiO2 (Figure 1c) because the noble metal nanoparticles accelerate the growth of RF-SiO2 shell by decreasing the nucleation process. Then, during the calcination process, the uniform RF-SiO2 shell protects the noble metal cores and carbon spheres from agglomeration at high temperature up to 800 oC. Moreover, the removal of SiO2 in the framworks lead to abundant oxygen species, and hydrophilic nature of NM@MC. The obtained NM@MC collodial nanocomposites show uniform and highly dispersed morphology, and clear core-shell structure with abundant mesopores on the shell. Catalytic activity of NM@MC The catalytic activity of NM@MC has been tested through the reduction of 4-NPh to 4-APh by NaBH4, which will not occur without catalysts. The reaction was real-time monitored by UV–vis spectroscopy because of the distinct absorption peaks of 4-APh at 300 nm and 4-NPh at 400 nm in alkaline reaction media. The catalyst particles with the same weight was used in each reaction. An obvious decrease of the absorbance at 400 nm could be observed as soon as the nanocatalysts were added into the reaction mixture (Figure 5a). Compared with MC, Pt@MC, and Ag@MC,


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Au@MC exhibited the best catalytic performance. The kinetic analysis of the reduction reaction can performed with the temporal decay of the distinct absorption peaks. The ratio of Ct to C0, was calculated from the relative intensity of the distinct absorption peaks, where Ct is 4-NPh concentration at time t and C0 is the initial concentration. The linear relations of ln(Ct/C0) versus time indicate first-order kinetics of the reactions except the first plot, because the thick porous shell showed significantly fast adsorption at the initial stage of reaction. The fast adsorption of 4NPh by MC shell can also be identified by the obvious decrease of Ct/C0 for MC (Figure 5b). The fast adsorption of reactant may lead to the accumulation of 4-NPh in the shell, and thus accelerates the catalytic rate of the reaction. The pseudo-first-order kinetics are in line with expectations because the amount of NaBH4 is in large excess relative to the amount of 4-NPh. The rate constant (1.1 min−1) of Au@MC core-shell nanocomposite is the largest among Au@MC, Ag@MC and Pt@MC (Fig 5(c)), indicating the highest activity of Au in the catalytic reduction of 4-NPh to 4-APh. Moreover, the rate constant of Au@MC (1.1 min−1) is much higher than that of Au@microporous carbon (Au@MicroC, Fig 5(d)) and comparable to that of reported ultra-small Au nanoparticles (1.2 min−1),15 indicating the better mass transfer of mesoporous channels in the carbon shell. Compared to MicroC, MC exhibited faster adsorption speed and larger adsorption of 4-NPh, which ensures the excellent catalytic activity of Au@MC. Thus, the catalytic performance of core-shell nanocomposite depends on both the inner core and the mass transfer channel in the out shell.


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Figure 5. (a) Time-dependent UV–vis spectra of the reaction mixture of Au@MC catalytic reduction of 4-NPh. (b) Absorbance of the reaction mixture at 400 nm during the catalytic reaction in presence of NM@MC or MC. (c) Kinetic analysis of the catalytic reduction of 4-NPh based on plot of ln(Ct/C0) versus time for 4-NPh. (d) Absorbance of the reaction mixture at 400 nm during the catalytic reaction in presence of Au@MC, MC, Au@MicroC or MicroC. CONCLUSION In summary, we designed an extended silica-assisted strategy to synthesize monodisperse NM@MC core-shell colloidal nanocomposites with high surface area, abundant mesopores and tunable particle size. The particle size of Au@RF-SiO2 was real-time monitored by DLS to shed light on the growth process of core-shell nanocomposites. Au@MC colloid exhibits the excellent catalytic performance in the reduction of 4-NPh to 4-APh, benefiting from the active Au core and the mesoporous carbon shell. The present work may arouse the study of more simple and effective designed synthesis of core-shell nanocomposites.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. SEM images of Au nanoparticles, Au@MC and Ag@MC nanoparticles with different sizes. XRD patterns for NM@MC. AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Young Thousand Talented Program and the National Natural Science Foundation of China (No. 21671073, 21621001, 21604030 and 21671074), and the “111” Project of the Ministry of Education of China (B17020). REFERENCES (1)

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