High-Yield Synthesis of Janus Dendritic Mesoporous Silica

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High-Yield Synthesis of Janus Dendritic Mesoporous Silica @ Resorcinol - Formaldehyde Nanoparticles: A Competing Growth Mechanism Lili Qu, Huicheng Hu, Jiaqi Yu, Xiaoya Yu, Jian Liu, Yong Xu, and Qiao Zhang Langmuir, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 12, 2017

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High-Yield Synthesis of Janus Dendritic Mesoporous Silica @ Resorcinol - Formaldehyde Nanoparticles: A Competing Growth Mechanism Lili Qu,†,‡ Huicheng Hu,†,‡ Jiaqi Yu,†,‡ Xiaoya Yu,† Jian Liu,§,* Yong Xu,†,* and Qiao Zhang†,* †

Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for

Carbon-Based Functional Materials & Devices, Collaborative Innovation Center of Suzhou Nano Science and Technology, SWC for Synchrotron Radiation Research, Soochow University, Suzhou 215123, PR China §

Department of Chemical Engineering, Curtin University, Perth, WA 6845, Australia

KEYWORDS Janus nanostructures, Silica, Resorcinol formaldehyde resin, Concentric, Eccentric

ABSTRACT. Recently, Janus nanostructures that possess two or more different surface functions have attracted enormous attention because of their unique structures and promising applications in diverse fields. In this work, we present that Janus structured dendritic mesoporous silica @ resorcinol-formaldehyde (DMS@RF) nanoparticles can be prepared through a simple one-pot colloidal method. The Janus DMS@RF nanoparticle shows a bonsai-like morphology which is consisted of a dendritic mesoporous silica part and a spherical RF part. After a systematic study on the growth process, we proposed a competing growth mechanism that

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accounts for the formation of Janus nanostructures. It is believed that suitable polymerization rate of silica and RF resin is critical. Based on the competing growth mechanism, eccentric and concentric core-shell nanostructures have been successfully prepared by tuning the polymerization rates of silica and RF, respectively. Metal-contained ternary Janus nanoparticles that might be used for catalysis have also been prepared. This research may pave the way for the practical applications of delicate nanomaterials with desired structures and properties.

Introduction The ability to manipulate surface properties of colloidal nanoparticles inspired great improvement in nanoscience over the past decades.1-6 Among various shaped nanostructures, Janus or patchy nanoparticles which own two or more distinct chemical or physical surface properties have drawn much attention7-13 because of their diverse applications, including catalysis,14-16 drug delivery,17-19 imaging20-22 and self-assembly.23-25 Many methods have been developed to prepare Janus nanostructures, such as partial masking26-28 and selective growth.29-32 Although many efforts have been devoted to this field, many challenges still remain. For example, some reported examples require special reagents to modify the nanoparticle surface to realize a selective growth, which is difficult to be applied to other systems. Furthermore, most of the current methods need the complex and tedious preparation strategies, which usually suffered from time-consuming, high cost, and low yield.33 It is thus highly demanded to develop new strategies that can obtain Janus nanoparticles in a convenient, cost-effective and mass-produced way. Recently, resorcinol formaldehyde (RF) resin nanostructures have attracted much attention due to its widespread applications in imaging,34 catalysis35,36 and pollutant absorbent,37 and so forth.


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Since Liu and co-workers’ pioneering work on the colloidal synthesis of RF particles with controlled size and morphology,38 more and more sophisticated nanostructures such as core-shell nanostructure, yolk-shell nanostructures, hollow nanostructures and mesoporous nanostructures has been reported.39-44 Although so many works have been reported, most of them are focused on the isotropic nanostructures, while very a few of them have mentioned about anisotropic RFbased nanostructures, especially Janus nanostructures. The detailed formation mechanism of anisotropic needs to be explored. In this work, we present a simple one-pot colloidal synthesis method to prepare Janus dendritic mesoporous silica @ resorcinol-formaldehyde (DMS@RF) nanostructures. The Janus DMS@RF nanoparticle shows a bonsai-like morphology which is consisted of a dendritic mesoporous silica part and a spherical RF part. The morphology evolution process revealed a competition growth mechanism, in which silica and RF are polymerized simultaneously, leading to the separated growth process. Based on this growth mechanism, concentric core-shell and eccentric core-shell DMS@RF nanostructures have been obtained by tuning the growth kinetics. This method can also be extended to make metal-contained ternary system, in which platinum nanoparticles can be loaded into RF part only or dispersed uniformly through the whole nanoparticles. It is believed that this growth mechanism and preparation method will provide a new perspective in the design and synthesis of Janus nanoparticles.

Experimental section Materials Resorcinol (99.0 %), formaldehyde aqueous solution (37.0 wt. % aqueous solution stabilized by methanol), potassium tetrachloroplatinate (II) (K2PtCl4) and CTAT (hexadecyl-trimethylammonium p-toluenesulfonate) were all purchased from Sigma-


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Aldrich. Triethanolamine (TEA) was purchased from Aladdin. Tetraethoxysilane (TEOS) was purchased from TCI. 4-nitrophenol was purchased from SinoReagent (China). All materials were used without further purification. Characterizations The samples were characterized by a series of instruments including SEM, TEM and nitrogen sorption. Scanning electron microscopy (SEM) images were taken on a Zeiss Supra55 from Carl Zeiss, Germany. Transmission electron microscopy (TEM), HAADF-STEM and corresponding EDS elemental mappings data were all obtained with TECNAI G2 F20 from FEI, USA, operating at 200 KV. All samples are washed and centrifuged three times before dropping it on the TEM grid. The BrunauerEmmett-Teller (BET) data were collected from Micromeritics ASAP 2050 from Micromeritics, United States. Synthesis of Janus DMS@RF nanoparticles In a typical synthesis, a mixture of 0.96 g of CTAT, 0.16 g of TEA and 45 mL of deionized water was stirred in a three-neck bottle at 1150 rpm at 80 oC for 1 hour to form a clear surfactant solution. Then 2 mL of TEOS was quickly added into the solution. After 5 min, 5 mL aqueous solution containing 0.4 g resorcinol and 0.56 mL formaldehyde (37.0 wt. %) injected into the reaction mixture, which was stirred for another 3 hours at 80 oC under vigorous stirring. The synthesized Janus nanoparticles were washed for three times by ethanol and were collected after highrate centrifugation (9000 rpm). Synthesis of spherical mesoporous silica nanodendrites. It is according to a literature report.45 A mixture of 0.96 g of CTAT, 0.16 g of TEA and 45 mL of deionized water was stirred in a three-neck bottle at 1150 rpm at 80 oC for 1 hour to form a clear surfactant solution. Then 2 mL of TEOS was quickly added into the solution, and the mixture was


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stirred for another 3 hours at 80 oC under vigorous stirring. The final products were washed for three times by ethanol and were collected after high-rate centrifugation (9000 rpm). Synthesis of Janus DMS@RF nanoparticles with Pt loaded only in RF part A mixture of 0.96 g of CTAT, 0.16 g of TEA and 45 mL of deionized water was stirred in a threeneck bottle at 1150 rpm at 80 oC for 1 hour to form a clear surfactant solution. Then 2 mL of TEOS was quickly added into the solution. After 5 min, 2 mL of K2PtCl4 (40 mM) and 5 mL of RF mixture were injected into the bottle simultaneously. The mixture was stirred for another 3 hours at 80 oC under vigorous stirring. The final products were washed for three times by ethanol and were collected after high-rate centrifugation (9000 rpm). Synthesis of Janus DMS@RF nanoparticles with Pt dispersed through whole nanoparticles A mixture of 0.96 g of CTAT, 0.16 g of TEA and 45 mL of deionized water was stirred in a three-neck bottle at 1150 rpm at 80 oC for 1 hour to form a clear surfactant solution. Then 2 mL of TEOS was quickly added into the solution. After 5 min, 5 mL aqueous solution containing 0.4 g resorcinol and 0.56 mL formaldehyde (37.0 wt. %) injected into the reaction. 2 mL of K2PtCl4 (40 mM) was injected into the reaction after another 30 min. The mixture was stirred for another 2.5 hours at 80 oC under vigorous stirring. Catalytic reduction of 4-nitrophenol In a typical experiment, 0.10 mL of 0.01 M 4nitrophenol is mixed with 15 mL of H2O and 2 mL of 0.1 M NaBH4 under magnetic stirring. Certain amount of Pt-loaded Janus nanoparticles is injected to initiate the reduction process. 2 mL of reaction solution is taken out during the process to check the conversion by measuring the UV–vis spectra.


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Results and discussion The Janus DMS@RF nanoparticles are synthesized through a one-pot sol-gel process. In a typical experiment, 0.96 g hexadecyl-trimethylammonium p-toluenesulfonate (CTAT) and 0.16 g triethanolamine (TEA) are added into a three-neck bottle containing 45 mL H2O. The mixture is stirred at 80 oC for 1 hour to form a clear solution. 2 mL tetraethoxysilane (TEOS) is then rapidly injected into the solution. 5 minutes later, 5 mL solution containing 0.4 g resorcinol and 0.56 mL 37 wt. % formaldehyde is quickly injected into the mixture solution. The mixed solution is proceeded under 80 oC with vigorous magnetic stirring (1150 rpm) for another 3 hours. Monodisperse Janus DMS@RF nanoparticle can be obtained with a very high yield (almost 100%). As shown in Figure 1a and 1b, a clear Janus nanostructure can be observed. The size of the whole particle is about 93 nm (Figure S1). One side is a spherical nanoparticle, and the other side is a bonsai-like structure. The scanning electron microscopy (SEM) characterization also confirmed the Janus nanostructure (Figure 1c). To figure out the composition of each part, the Janus nanostructure has been characterized by the scanning transmission electron microscopy (STEM) and the energy-dispersive X-ray spectroscopy (EDS). As shown in Figure 1d, silicon can be found only in the bonsai-like part. Since the nanoparticles are dropped on a carbonsupported Cu grid, carbon can be found everywhere, but the bright area shows a clear spherical shape, which can be attributed to the RF part. The overlapped elemental mapping image shows a clear Janus nanostructure with one side is silicon and the other side is carbon.


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Figure 1 (a) Low magnification TEM image and (b) high magnification TEM image of the asprepared Janus DMS@RF nanoparticles. (c) Low magnification and high magnification SEM images of as-prepared Janus DMS@RF nanoparticles. The scale bar in the inset is 50 nm. (d) HAADF-STEM image and corresponding EDS elemental mappings of as-prepared Janus DMS@RF nanoparticles. The scale bars are 50 nm. The as-prepared Janus DMS@RF nanoparticles are porous nanostructures with a high surface area, as determined by the Brunauer–Emmett–Teller (BET) characterization. The total pore volume and surface area of as-prepared Janus DMS@RF nanoparticles are estimated to be ∼0.73cm3g-1 and ∼243 m2g-1, respectively (Figure 2a). The pore size distribution curve are calculated by the Barrett−Joyner−Halenda (BJH) method (Figure 2b). The pore size distribution curve shows a relatively sharp peak around 3.6 nm and a relatively broad peak in the large pore size region. Although RF resin are porous in nature, their pores are micro-pores with pore size


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smaller than 2 nm.47 As a result, the large surface area can be attributed to the porous nature of silica part. The sharp peak can be assigned to the porous structure of silica that formed through the templating effect of CTAT. Since the overall silica structure is an open bonsai-like structure, the whole structure also shows some large pore structure, which is consistent with the BJH pore size distribution.


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Figure 2 (a) N2 adsorption-desorption isotherms and (b) pore size distribution of Janus DMS@RF nanoparticles derived from desorption.

Figure 3 TEM images of a single Janus DMS@RF nanoparticle with different reaction time: (a) 3 min, (b) 7min, (c) 10 min, (d) 3h. All scale bars are 20 nm. It is believed that the TEA is a multi-functional reagent in the reaction system. First, TEA provides a basic condition for the polymerization of both silica and RF. In the presence of TEA, the pH of the solution is ∼8.25 at 80 oC. Without the addition of TEA, the pH value of the solution is ∼6.30 at 80 oC and no solid nanoparticles can be obtained. According to the classical Stöber method, forming silica nanoparticle by using TEOS as the silica precursor needs a basic environment which can be provided by TEA in this work.46 Meanwhile, Liu and co-workers have confirmed that uniform RF spheres can be obtained in a basic solution.37 Furthermore, the morphology of dendritic mesoporous silica can be determined by the mixture of TEA and


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CTAT.44 As shown in Figure S2, spherical mesoporous silica nanodendrites can be synthesized under the same reaction condition but without the addition of RF precursors. To figure out the growth mechanism, a systematic study has been conducted. First, the growth process has been carefully monitored by taking aliquots out from the reaction system at different growth stages. TEM image of the early stage (3 min after the addition of resorcinol and formaldehyde) showed that the nucleation of both silica and RF appeared almost at the same time (Figure 3a). Because of the different intrinsic properties between silica and RF, these two parts are totally separated from each other, as marked by the dashed lines (Figure 3a). With the reaction time prolonged, both silica part and RF part grew bigger. It is worth pointing out that the RF surface is always smooth, while the silica part shows a rough and porous surface (Figure 3b and 3c). Furthermore, the polymerization degree of RF became higher than the initial disordered state and the pore networks of silica became clear. The reaction process can be completed within three hours (Figure 3d). Since the growth kinetic is important, the injection time of RF precursors has been varied to investigate the growth mechanism. The polymerization rate of silica is lower than that of RF at the early stage, indicated by the fast colour change from colourless to brownish when TEOS and RF precursors were added simultaneously. And Janus DMS@RF nanoparticles can be obtained with a very low yield (∼30%, Figure S3). Therefore, in this system, TEOS was added 5 min earlier than the RF precursors to realize a close polymerization rate (Figure 1). In the absence of RF precursors, a rough and aggregated structured can be obtained (Figure S4a) after reacted for 8 min. A clear porous nanodendrite can be observed after reacted for 15 min (Figure S4b). Figure 4a shows the TEM image of the product obtained by injecting RF precursor 8 min later than the addition of TEOS. An eccentric silica-core and RF-shell has been prepared, in which the


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dendritic silica nanoparticle was encapsulated by the RF shell. When the interval time between the addition of TEOS and RF precursors was further increased to 15 min, only concentric coreshell nanostructure can be obtained (Figure 4b). It is worth pointing out that no nanoparticles can be obtained and only a big gel-like product can be prepared when RF precursors were added before the addition of TEOS, suggesting that the formation of silica nucleation is critical for the formation of Janus DMS@RF nanostructure.

Figure 4 TEM images of (a) eccentric core-shell and (b) concentric core-shell DMS@RF nanoparticles. The inset images showing the schematic illustration of the corresponding nanoparticles. Based on the above-mentioned results, a competing growth mechanism has been proposed, in which a close polymerization rate between silica and RF is critical for the formation of Janus nanostructure. When the reaction rates are very different, a core-shell nanostructure can be formed if the reaction condition is suitable. For example, we have successfully prepared coreshell structured metal@RF (metal can be Au, Ag, Pd and Pt) nanoparticles through a simple colloidal approach.47,48 Sevilla and co-workers synthesized core-shell structured silica@RF nanostructure with one-pot method by utilizing the big difference in reaction rates between the formation of silica and RF.49 In this work, the polymerization of silica and RF occurred at the same time in the basic environment in the early stage. With the assistance of TEA and CTAT,


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silica could form a dendritic structure, while RF preferred to form a spherical shape. Thanks to the positively charged surfactants, silica and RF tended to nucleate together heterogeneously to reduce the surface energy. By adjusting the reaction parameters, the polymerization rate of both silica and RF became very close. Thus, silica and RF grew simultaneously. According to the diffusion theory, the exhausting of the precursor (either TEOS or resorcinol-formaldehyde) happened quickly in each region, promoting the diffusion of the precursors to each part. As a result, silica and RF grew separately and formed the Janus nanostructure eventually. This competing growth mechanism has been confirmed by the fact that eccentric and concentric coreshell nanostructures could be obtained when the polymerization of silica and RF were separated (Figure 4).

Figure 5 TEM images of Janus DMS@RF nanoparticles with Pt (a) loaded only in the RF part and (b) dispersed through the whole nanoparticle. All scale bars are 50 nm. Because of the high surface area and large pore volume of Janus DMS@RF nanoparticles, it may be used as a catalyst carrier. Here, platinum (Pt) was demonstrated to extend the one-pot route to a ternary system. Two different strategies were designed to realize the Pt loading. In the first approach, K2PtCl4 solution and RF precursor were added into the reaction simultaneously. In this system, formaldehyde served as both the precursor of RF polymer and the reducing agent


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for K2PtCl4.46-47 Because of the high concentration of formaldehyde, K2PtCl4 was reduced immediately, indicated by the colour change from colourless to dark brownish. As shown in Figure 5a, Pt nanoparticles were loaded only in the RF part and formed a Pt@RF@DMS ternary Janus nanostructure. In the second approach, the same amount of K2PtCl4 was added into the reaction system 30 min later than the addition of RF precursor. Janus nanostructures could also be obtained in this way. Since most formaldehyde was reacted to form the RF nanoparticles, the reduction rate of K2PtCl4 was very low due to the low concentration of residual formaldehyde. As shown in Figure 5b, ultrasmall Pt nanoparticles (~ 1 nm in size) were uniformly dispersed in both the silica part and RF part. Pt-loaded Janus DMS@RF nanoparticles can be prepared in a large-scale through this approach. As shown in Figure S5, more than 1 gram of the product can be obtained in one batch, suggesting a bright future for nanocatalysis application. To demonstrate the potential applications of the as-prepared nanoparticles, the catalytic reduction of 4-nitrophenol has been demonstrated. As shown in Figure S6, in the presence of Pt-loaded Janus DMS@RF nanoparticles, 4-nitrophenol can be reduced effectively, suggesting the potential applications of such nanoparticles.

Conclusion In summary, we reported a simple one-pot colloidal synthesis method to prepare Janus DMS@RF nanoparticles with uniform size and high yield. The reaction system has been investigated systematically by tuning the reaction parameters. A competing growth mechanism has been proposed to explain the new strategy. Benefiting from the improved understanding, eccentric and concentric DMS@RF nanostructures have been successfully prepared. Owing to their high surface area and large pore volume, the Janus DMS@RF nanoparticles are expected as


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a great catalyst carrier. We have demonstrated that Pt nanoparticle can be loaded either only in the RF part or dispersed on the whole Janus nanoparticle by adjusting the addition time of the precursor. Ultrasmall Pt nanoparticles dispersed in the support can be obtained in a large scale, which might be useful for the industrial catalytic process. Further studies on using these unique nanostructures as nanoreactors for nanomotors are undergoing.

ASSOCIATED CONTENT Supporting Information. Additional size distribution histograms, photographs and TEM images are supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author [email protected] (J.L.); [email protected]; (Y.X.); [email protected]. (Q.Z.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21401135, 21673150) and the Natural Science Foundation of Jiangsu Province (BK20140304). This project


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is funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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Insert Table of Contents Graphic and Synopsis Here

Unique dendritic mesoporous silica and resorcinol formaldehyde polymers nanocomposites with nanostructures from Janus, eccentric and concentric core-shell have been prepared through a simple one-pot colloidal method.


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High-Yield Synthesis of Janus Dendritic Mesoporous Silica

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