Materials Research Bulletin 47 (2012) 2726–2729
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Rapid synthesis of ordered hexagonal mesoporous silica and their incorporation with Ag nanoparticles by solution plasma Panuphong Pootawang a,*, Nagahiro Saito b, Osamu Takai b, Sang Yul Lee a a Center for Surface Technology and Applications, Department of Materials Engineering, Korea Aerospace University, 100 Hanggongdae-gil, Hwajeon-dong, Deogyang-gu, Goyang-city, Gyeonggi-do 412-791, Republic of Korea b EcoTopia Science and Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
A R T I C L E I N F O
A B S T R A C T
Article history: Available online 24 April 2012
Rapid synthesis of silica with ordered hexagonal mesopore arrangement was obtained using solution plasma process (SPP) by discharging the mixture of P123 triblock copolymer/TEOS in acid solution. SPP, moreover, was utilized for Ag nanoparticles (AgNPs) incorporation in silica framework as one-batch process using silver nitrate (AgNO3) solution as precursor. The turbid silicate gel was clearly observed after discharge for 1 min and the white precipitate formed at 3 min. The mesopore with hexagonal arrangement and AgNPs were observed in mesoporous silica. Two regions of X-ray diffraction patterns (2u < 28 and 2u = 35–908) corresponded to the mesoporous silica and Ag nanocrystal characteristics. Comparing with mesoporous silica prepared by a conventional sol–gel route, surface area and pore diameter of mesoporous silica prepared by solution plasma were observed to be larger. In addition, the increase in Ag loading resulted in the decrease in surface area with insigniﬁcant variation in the pore diameter of mesoporous silica. SPP could be successfully utilized not only to enhance gelation time but also to increase surface area and pore diameter of mesoporous silica. ß 2012 Elsevier Ltd. All rights reserved.
Keywords: A. Amorphous materials A. Metals A. Nanostructures
1. Introduction Mesoporous silica is deﬁned as the nanoporous material of polysilicate framework with diameter between 2 and 50 nm which can be easily synthesized via sol–gel method in acid or base condition containing the silica precursor and the surfactant micelles as the template. Mesoporous silica has been studied as a long history material which was ﬁrstly discovered and patented in 1971 by Chiola et al.  and it was further developed to notice as the popular nanoporous materials. Until now, there are many types of mesoporous silica which have been investigated for example MCM and SBA, SMS, MSU, and FDU with mesopore arrangements of hexagonal, sponge, wormlike, and cubic, respectively [2–4]. Mesoporous silica is promising material to be applicable in the supporting materials for catalyst, water puriﬁcation, chemical exchange system, and biosensor owing to its properties as high chemical and thermal resistant, inert, as well as non-toxic. Moreover, the modiﬁcations of mesoporous silica with nanometallic particles and the organic functions have been studied for catalyst, electrical and optical devices, imaging and sensing sensors, and biomedical applications [5–9].
* Corresponding author. Tel.: +82 10 5198 9987. E-mail address: [email protected]
(P. Pootawang). 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.04.040
The reduction of the process time in industrial scale is very crucial because of considerable cost saving. For mesoporous silica synthesis, there are many effective ways to reduce the manufacturing time with the satisﬁed properties. The changes in chemical and physical factors as pH and temperature are as primary solution to accomplish this propose. Since the isoelectric point of silica solution is at pH 2, the pH adjustment to obtain the optimum point for the fastest gelation and consolidation was investigated and reported to be at pH around 5–6, in which the suitable ionic interaction between silica species to form polysilicate network and become aggregation into solid framework . Over ambient temperature up to 50 8C and the additions of ionic species (e.g. SO42 , and CO32 ) in sol–gel process which could accelerate the gelation rate from silica sol to gel and enhance the rate and degree of condensation in silica were reported [11,12]. Solution plasma process (SPP) was successfully developed as one of high reaction rate processes in aqueous solution. This plasma is deﬁned by a discharge in aqueous solution and is stabilized by the exchanges of ions and electrons between gas and liquid phases. This plasma generates numerous numbers of high active species, e.g. hydrogen, oxygen, and hydroxy radicals to aqueous solution. The reactants of the desired reaction are added into aqueous solution and these substances are reacted to such generated species at the interface of gas and liquid phases. By using SPP, the nanoparticle formation via the reduction of Ag ions and hydrogen radicals without the addition of any reducing agent had
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demonstrated as a high effective process owing to rapid synthesis [13,14]. Moreover, SPP can also remove the organic template from the inorganic and organic compounds in mesoporous silica synthesis [15,16]. In this study, new approach of solution plasma technology for process improvement in mesoporous silica preparation with rapid consolidation was attempted. No previous report about the characteristic of SPP in this application has been made. Hence this work could be the ﬁrst report about the inﬂuence of physical plasma on the manufacturing time and characteristics of mesoporous silica. Moreover, the incorporation of AgNPs in silica framework was obtained by SPP. The physical and chemical properties of the obtained samples are further characterized by various scientiﬁc techniques including TEM, SEM, EDS, XRD, and nitrogen adsorption desorption measurement.
2. Materials and methods The hexagonal mesopore arrangement of silica and AgNPsincorporated mesoporous silica were synthesized using conventional sol–gel system and the gelation time was accelerated by SPP. 5 g of pluronic 123 triblock copolymer (EO20PO69EO20, Aldrich) was dissolved into 20.62 ml of 37% hydrochloric acid (HCl, Dae Jung) in solution plasma chamber. The prepared solution was adjusted by deionized water to obtain a 120 ml of 1.23 M HCl solution and further stirred for 1 h to form homogenous solution. Consequently, 10.6 g of tetraethyl oxysilane (TEOS, Dae Jung) was added and stirred until obtaining a clear solution. For conventional sol–gel route, the mixture solution was continuously stirred in room temperature for 24 h and then the white precipitate was formed. To accelerate the gelation time, the prepared mixture was
Fig. 1. Transmission electron microscope (TEM) images of the obtained mesoporous silica, (a) synthesized by conventional sol–gel method, (b) by SPP, (c) AgNPs-incorporated mesoporous silica using 1 mM AgNO3 concentrations, (d) 5 mM AgNO3, (e) 10 mM AgNO3, and scanning electron microscope (SEM) images of mesoporous silica, (f) synthesized by conventional sol–gel method and (g) by SPP.
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discharged by placing two rods of tungsten electrode and connecting to the unipolar pulse power supply. The experimental conditions were at 500 V, 20 kHz, 1 mm, and 3 min of applied voltage, frequency, distance between electrodes, and discharge time, respectively. For incorporation of AgNPs in mesoporous silica, the required silver nitrate (AgNO3, Sam Chun) solutions (1, 3, and 5 mM) were added into the clear solution before discharge. The precipitate was collected and washed several times with deionized water. Finally, the collected solid was dried at 80 8C for 24 h under ambient pressure. For characteristic analysis, transmission electron microscope (TEM) observation and energy-dispersive X-ray (EDX) measurement were performed using JEOL JEM-2010 microscope on the amorphous carbon ﬁlm covered on Cu grid with an acceleration voltage of 200 kV. X-ray diffraction patterns were collected using Rigaku Ultima with Cu Ka radiation (l = 0.154 nm) operating at 35 kV and 20 mA. Field emission scanning electron microscope (FE-SEM) observation was carried out with JEOL JSM-6700F using an acceleration voltage of 5 kV and magniﬁcation in the range of 1000–15,000 times. The structural parameters, surface area, pore diameter, and pore volume of all samples were calculated from nitrogen adsorption desorption isotherms which carried out on Micromeritics ASAP-2010. The Brunauer–Emmett–Teller method was applied for surface area calculation and the Barrett–Joyner– Halenda analysis was used to determine pore diameter and pore volume. All samples were degassed at 200 8C for 24 h prior to the measurement.
size of sample prepared by SPP might be remarked by the effect of SPP which could assist to expand pore size. For AgNPs-incorporated mesoporous silica samples using 1, 5, and 10 mM AgNO3 solutions, Fig. 1c–e gives the evidences of AgNPs incorporated in mesoporous silica matrix. The mean sizes of dispersed AgNPs in silica matrix were proportional to the increasing Ag loading and they were measured to be approximately 20, 22, and 25 nm using 1, 5, and 10 mM AgNO3 solutions, respectively. Besides, the percent content of Ag determined using wide scan mode of EDX were proportional to AgNO3 concentration (data not shown). SEM images in Fig. 1f and g give representative particle morphology of mesoporous silica prepared by different routes. The aggregated particle presented in the obtained mesoporous silica prepared by conventional sol–gel method showed the mean particle size and narrow size distribution (6 1.52 mm). Interestingly, the rope-like particles with diameter approximately 0.25 mm were formed by the synthesis route using SPP. This morphology could not be thermodynamically formed under normal conditions and the inﬂuence of SPP is noteworthy . Small angle X–ray diffraction patterns of mesoporous silica prepared by different synthesis routes were shown in Fig. 2a. These
3. Results and discussion In the last decade, SPP has been increasingly interested in the materials ﬁelds, especially for nanoparticle formation and organic decomposition with a short consumption time. SPP is extensively explored for many potential applications, owing to the great beneﬁts of its outcome. In this study, SPP was expected to enhance the gelation time for rapid preparation of mesoporous silica and to incorporate AgNPs in mesoporous silica framework since it provides high energetic species to accelerate the reaction rate and to create the reduction reaction in aqueous solution. The potential mechanism of mesoporous silica formation by the feasibility of SPP might be explained as follows. After mixing TEOS in the acid solution containing template micelles, the ethoxyl groups was chemically hydrolyzed to hydroxyl groups. This reaction requires enough time to complete the hydrolysis. In particular, use of acid or base condition is necessary for hydrolysis acceleration. After applying SPP, the hydrolysis time could be dramatically enhanced by the generated active species, especially hydroxyl radical [15,17,18]. Consequently, tetrahydroxyl silane species were condensed into the silicate gel and solidiﬁed to the polysilicate framework, respectively . The white precipitate of mesoporous silica could be easily formed in solution after discharge at 3 min, comparing with conventional sol–gel method which required at least 12 h for silica condensation. The mesopore features of mesoporous silica prepared by conventional method and solution plasma were studied by TEM as shown in Fig. 1a which shows hexagonal arrangement mesopore in mesoporous silica which was prepared using conventional sol–gel method for 24 h. The mean pore size was estimated to be about 3.9 nm. In the case of mesoporous silica prepared by SPP, hexagonal mesopore arrangement also represents with relative higher mesopore size (about 8 nm) as shown in Fig. 1b. Both synthesis routes were applicable for mesoporous silica synthesis by identical chemical reagent and condition. It notes that this chemical system could be stabilized the ionic surfactant micelles in form of hexagonal array, yielding SBA-15-like mesoporous silica with large pore size and high surface area [19,20]. Additionally, the result in higher pore
Fig. 2. X-ray diffraction patterns; (a) small angle X-ray patterns of mesoporous silica, (1) synthesized by conventional sol–gel method (2), by SPP, and (b) wide angle X-ray patterns of mesoporous silica synthesized by SPP (1) and AgNPsincorporated mesoporous silica, (2) using 1 mM AgNO3 and (3) 10 mM.
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large pore size could be noticeable [19,21]. By using these isotherms, the structural parameters, BET surface area, pore diameter, and pore volume were calculated and summarized in Table 1. BET surface area and pore diameter values of mesoporous silica prepared by SPP with and without AgNPs incorporation found to be higher than those values of conventional mesoporous silica. These results are consistent to the TEM results. The presence of AgNPs which obstructed the adsorption ability on the surface could be attributed to the decrease in surface area with insigniﬁcant variation in pore diameter and pore volume. This indicates that SPP route for AgNPs incorporation in mesoporous silica did not collapse the main structure of mesopores. 4. Conclusions
Fig. 3. Adsorption desorption isotherms using nitrogen adsorbed on mesoporous silica, (a) synthesized by conventional sol–gel method, (b) by SPP, and (c) AgNPsincorporated mesoporous silica using 5 mM AgNO3.
Table 1 Structural parameters, BET surface area, pore diameter, and pore volume of mesoporous silica samples. Sample
Conventional mesoporous silica Mesoporous silica (SPP) AgNPs in mesoporous silica (1 mM AgNO3) AgNPs in mesoporous silica (5 mM AgNO3) AgNPs in mesoporous silica (10 mM AgNO3)
Structural parameter BET surface area (m2 g 1)
Pore diameter (nm)
Pore volume (cm3 g 1)
515.15 780.16 696.36
4.47 8.32 8.15
0.43 0.47 0.48
diffraction patterns indicate the high degree of ordered mesoporous arrangement which show (1 0 0) main peak and the (1 1 0) peak. The (1 0 0) reﬂection peak in case of conventional mesoporous silica shifted to longer diffraction angle, meaning to lower distance between planes of hexagonal mesopore. Large unit cell of hexagonal mesoporous silica samples with high degree of ordered mesoporous structure was exhibited since the distance between planes was calculated to be over than 10 nm . In the wide angle mode of X-ray diffraction, Fig. 2b shows the diffraction patterns of the neat mesoporous silica compared with AgNPsincorporated mesoporous silica with various Ag precursors. The apparent diffraction peaks of mesoporous silica containing AgNPs in the range of 2u = 35–908 were indexed to plane reﬂections of (1 1 1), (2 0 0), (2 2 0), and (3 1 1), corresponding to Ag nanocrystal characteristic whereas there is no peak appeared in neat mesoporous silica sample . These results also demonstrated that the incorporation of AgNPs in mesoporous silica framework was successfully done by applied SPP. Nitrogen adsorption desorption isotherm were plotted to understand the relation of the relative pressure to the adsorbed amounts of nitrogen on sample surfaces at 196 8C as shown in Fig. 3. These patterns were classiﬁed to type IV adsorption desorption isotherms. The monolayer of adsorbed gas was formed at lower relative pressure and the ﬁlling of adsorbed gas in the tiny capillary was completed at the rapid transition of isotherm in unity of relative pressure. In these samples, the transition of relative pressure was in the range of 0.4–0.6 corresponding to capillary condensation characteristic of narrow mesopore. At this position, a
Comparing with conventional sol–gel method, SPP provides the new advanced beneﬁt to synthesize mesoporous silica with higher surface area and pore diameter as short time process. A rapid synthesis of mesoporous silica as well as AgNPs-incorporated mesoporous silica by SPP could be successfully made at 3 min of discharge time. Mesoporous silica with hexagonal mesopore arrangement and AgNPs incorporation in silica framework were conﬁrmed by TEM and the mean size of AgNPs incorporated in mesoporous silica increased against Ag precursor concentration. The formation of AgNPs was also conﬁrmed by the results of X-ray diffraction. The SEM observation showed that the rope-like silica particles were formed after SPP whereas the aggregated irregular particle was produced by conventional sol–gel method. BET surface area and pore diameter of mesoporous silica prepared by SPP were higher than that of conventional silica. With increasing Ag precursor, BET surface area tended to decrease with insigniﬁcant variation in pore diameter and pore volume. Acknowledgment This research was supported by National Research Foundation of Korea (NRF) grant funded by the Korea Government (MEST) (no. 2011-0117930). References  V. Chiola, J.E. Ritsko, C.D. Vanderpool, Application No. US 3556725D A ﬁled on 26Feb-1969, Publication No. US 3556725 A (Published on 19-Jan-1971).  A. Galarneau, F. Sartori, M. Cangiotti, T. Mineva, F.D. Renzo, M.F. Ottaviani, J. Phys. Chem. B 114 (2010) 2140–2152.  E. Prouzet, F. Cot, C. Boissiere, P.J. Kooyman, A. Larbot, J. Mater. Chem. 12 (2002) 1553–1556.  C. Yu, Y. Yu, D. Zhao, Chem. Commun. (2000) 575–576.  S.H. Joo, J.Y. Park, C.K. Tsung, Y. Yamada, P. Yang, G.A. Somorjai, Nat. Mater. 8 (2009) 126–131.  S. Gardelis, P. Manousiadis, A.G. Nassiopoulou, Nanoscale Res. Lett. 6 (2011) 2271–227-7.  H. Tian, J. Li, L. Zou, Z. Mu, Z. Hao, J. Chem. Technol. Biotechnol. 84 (2009) 490–496.  A. Auger, J. Samuel, O. Poncelet, O. Raccurt, Nanoscale Res. Lett. 6 (2011) 328-1– 328-12.  J. Andersson, J. Rosenholm, S. Areva, M. Linden, Chem. Mater. 16 (2004) 4160– 4167.  P.B. Sarawade, J.-K. Kim, A. Hilonga, H.T. Kim, Solid State Sci. 12 (2010) 911–918.  J.C. Ro, I.J. Chung, J. Non-Cryst. Solids 110 (1989) 26–32.  Y. Du, X. Lan, S. Liu, Y. Ji, Y. Zhang, W. Zhang, F.-S. Xiao, Microporous Mesoporous Mater. 112 (2008) 225–234.  J. Hieda, N. Saito, O. Takai, J. Vac. Sci. Technol. A 26 (2008) 854–856.  J. Hieda, N. Saito, O. Takai, Mater. Res. Soc. Symp. Proc. 1056 (2008), HH 03-39-44.  P. Pootawang, N. Saito, O. Takai, Jpn. J. Appl. Phys. 49 (2010) 126202-1–126202-7.  P. Pootawang, N. Saito, O. Takai, Thin Solid Films 519 (2011) 7030–7035.  S. Potocky, N. Saito, O. Takai, Thin Solid Films 518 (2009) 918–923.  C. Miron, M.A. Bratescu, N. Saito, O. Takai, Plasma Chem. Plasma Process. 30 (2010) 619–631.  D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science 279 (1998) 548–552.  Z. Jin, X. Wang, X. Cui, Colloids Surf. A: Physicochem. Eng. Aspects 316 (2008) 27– 36.  M. Kruk, M. Jaroniec, J.M. Kim, R. Ryoo, Langmuir 15 (1999) 5279–5284.  P. Pootawang, N. Saito, O. Takai, Mater. Lett. 65 (2011) 1037–1040.