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Adsorption of octadecyltrichlorosilane on mesoporous SBA-15

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Adsorption of octadecyltrichlorosilane on mesoporous SBA-15
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  Adsorption of octadecyltrichlorosilane onmesoporous SBA-15 S.A. Mirji a, *, S.B. Halligudi b , Dhanashri P. Sawant b ,Nalini E. Jacob b , K.R. Patil c , A.B. Gaikwad c , S.D. Pradhan c a Physical Chemistry Division, National Chemical Laboratory, Pune 411008, India b  Inorganic & Catalysis Division, National Chemical Laboratory, Pune 411008, India c Center for Material Characterization, National Chemical Laboratory, Pune 411008, India Received 1 March 2005; received in revised form 6 June 2005; accepted 6 June 2005Available online 11 July 2005 Abstract Adsorption of octadecyltrichlorosilane (OTS) on mesoporous SBA-15 has been studied by using Brunauer–Emmett–Teller(BET) surface area analysis,scanning electron microscopy (SEM), energydispersive X-ray analysis (EDAX), Fouriertransforminfrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and thermo-gravimetric analysis (TGA) techniques.BET surface area analysis shows decrease of surface area from 930 to 416 m 2  /g after OTS adsorption. SEM pictures show closeattachment of SBA-15 particles. EDAX measurements show increase of carbon weight percentage and decrease of oxygen andsilicon weight percentage. XPS results closely support EDAX analysis. FTIR spectra shows presence of methyl (–CH 3 ) andmethylene (–CH 2 ) bands and oriented OTS monolayer on SBA-15. Thermo-gravimetric analysis shows that the OTS adsorbedon SBA-15 are stable up to a temperature of 230  8 C and that the OTS monolayers decompose between 230 and 400  8 C. # 2005 Elsevier B.V. All rights reserved. PACS:  68.18.  g; 68.43.  h; 68.47.Pe; 81.16.Dn Keywords:  SBA-15; OTS; Self-assembly; Adsorption; Thermal stability 1. Introduction Self-assembled organic monolayers, which formspontaneously on solid surfaces, have many applica-tions such as wafer level anti-stiction coating formicro-electromechanical systems (MEMS), passivat-ing layers, chemical sensing, ultra-fine scale litho-graphy, protection of metals against corrosion, in thepreparationofnewcomposite and functional materials[1–3]. Different types of organic compounds are usedfor self-assembled monolayer formation on differentsurfaces. These include alkanethol monolayers ongold, silver and copper [1], organosilicon on SiO 2 ,Al 2 O 3 , TiO 2 , Si 3 N 4 , mica and glass [1]. Recently, www.elsevier.com/locate/apsuscApplied Surface Science 252 (2006) 4097–4103* Corresponding author. Tel.: +91 20 25893400x2476;fax: +91 20 25893044. E-mail address:  mirji@dalton.ncl.res.in (S.A. Mirji).0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved.doi:10.1016/j.apsusc.2005.06.009  n -octadecyltrichlorosilane (OTS) self-assembledmonolayers have been extensively studied becauseof its simplicity in the growth technique andpossibility of its wide use in MEMS as anti-stictioncoating [4]. OTS monolayer growths on variety of substrates [4–6] and on silica nanobeads [7] have been investigated.Adsorption of OTS on SiO 2  takes place throughhydrolysis of the Si–Cl bonds to form Si–OH groups.All Si–Cl bonds become hydrolyzed as no Cl isdetected after monolayer formation [8]. The OHgroups experience condensation reaction to form Si–O–Si cross-linking bonds between adjacent headgroups. The OH groups in OTS also interact with OHgroups on silica forming Si–O–Si bonds to thesubstrate. Formation of OTS monolayer also dependson the presence of thin water layer on silica surface.Presence of too much water causes polymerization of OTS molecules and hence leads to poor quality of monolayers[9].Sincesilicaishydrophilic,athinlayerof water exists on its surface, which helps physisorp-tion of OTS molecule. At this stage, OTS moleculesare mobile. They move freely and pack themselvesclosely before condensation reaction takes place, atwhich point they bond to the silica surface [10].ThoughOTSmonolayergrowthonmanysubstrates[4–6] and silica nanobeads [7] have been studied extensively, to our knowledge, there is no study of OTS monolayer on recently discovered mesoporous[11] materials. Mesoporous materials have very largesurface area of the order of 1000 m 2  /g, ordered porestructure and extremely narrow pore size distribution[2]. OTS monolayer growth on these mesoporousmaterials can be interesting because of large surfacearea and hence large adsorption sites, which may helptogrowalargequantityofmonolayers.Manydifferentmonolayers such as 3-mercaptopropyl groups and 3-aminopropyl groups have been grown on mesoporoussilica such as SBA-15 and studied for their selectiveadsorption property of heavy metal ions [2,12].In this paper, we study OTS adsorption and itsthermal decomposition on SBA-15 mesoporous silica.Brunauer–Emmett–Teller (BET) surface area analy-sis, energy dispersive X-ray analysis (EDAX),scanning electron microscopy (SEM), Fourier trans-form infrared (FTIR) spectroscopy and X-ray photo-electron spectroscopy (XPS) techniques have beenused for confirmation of OTS adsorption on SBA-15.Thermo-gravimetric analysis (TGA) in air is used tostudy the thermal degradation of OTS on SBA-15. 2. Experimental 2.1. Preparation of mesoporous SBA-15 MesoporousSBA-15wassynthesizedbyastandardmethod [13,14]. Amphiphilic triblock copolymer,P123 of 4 g was dispersed in 30 g of water andstirred for 4 h and then 120 g of 2 M HCl solution wasadded and stirred for 2 h. Then, 8.54 g of tetra-ethylorthosilicate (TEOS) was added to the homo-geneous solution under stirring. The resulting gel wasaged at 40  8 C for 24 h and finally heated to 100  8 C for48 h. After synthesis, the solid was filtered, washedwith distilled water and dried in an air oven at 100  8 Cfor5 hfollowed by calcination inflowing airat 540  8 Cto decompose the triblock copolymer. 2.2. Adsorption study The chemicals used for adsorption study are:  n -octadecyltrichlorosilane, CH 3 (CH 2 ) 17 SiCl 3  (95%)obtained from Aldrich and toluene (99.5%) obtainedfrom Qualigens. One micromolar OTS solution intoluene was prepared for OTS adsorption study in anitrogen filled glove box. The amount of toluene takenis 10 ml. SBA-15 powder of 0.2 g quantity wasimmersed in OTS solution for 24 h with intermediatestirring. After 24 h the SBA-15 powder with OTSadsorbedwasrecoveredafterevaporationoftolueneina rotary vacuum evaporator at 80  8 C. 2.3. Characterization2.3.1. Surface area analysis BET surface area, pore volume and pore diameterof SBA-15 mesoporous material were measuredbefore and after OTS monolayer growth by usingQuantachrome Autosorb automated gas sorptionsystem model number NOVA 1200. 2.3.2. Scanning electron microscopy and energydispersive X-ray analysis The surface morphology of SBA-15 particles wascarried out before and after OTS monolayer growth by S.A. Mirji et al./Applied Surface Science 252 (2006) 4097–4103 4098  using Leica Stereoscan model 440 SEM. Theelemental composition of the Si(1 0 0) and SBA-15powder were also carried out before and after OTSmonolayer growth by EDAX analysis coupled withLeica Stereoscan model 440 SEM. 2.3.3. Fourier transform infrared spectroscopy The FTIR of SBA-15 particles was carried outbefore and after OTS monolayer growth on Perkin-Elmer 1615 FTIR spectrometer. 2.3.4. X-ray photoelectron spectroscopy XPS of SBA-15 mesoporous silica was donebefore and after OTS monolayer growth using VGMicro Tech ESCA 3000 instrument at a pressurebelow 10  9 Torr. The sample was mounted onsample stubs. The wide scan, C 1s, O 1s and Si2p core level spectra were recorded with amonochromatic Al K  a  radiation (photon ener-gy = 1486.6 eV) at pass energy of 50 eVand electrontakeoff angle of 60 8 . The core-level binding energieswere aligned taking ‘adventitious’ of carbon bindingenergy as 284.6 eV. All peaks were fitted withGauss–Lorentz peaks using XPSPEAK41 [15] soft-ware to obtain peak information. A Shirley’s baseline was used in the fitting process. 2.3.5. Thermal behavior by thermo-gravimetricanalysis TGA measurements of SBA-15 before and afterOTS monolayer growth were performed on Seikoinstruments thermal analyzermodelno.:TCA/DIA-32in the temperature range from ambient to 800  8 C. Theramp rate used was 10  8 C/min. 3. Results and discussion 3.1. General observations When the SBA-15 particleswith OTSmonolayerintoluene are kept under illumination (under 200 Welectric bulb), the functionalized SBA-15 particles arefound to assembly to form a spherical shape of 2–5 mm diameter. However, the bonding was not thatstrong and when the light was switched off, theparticles dispersed. Upon functionalization of SBA-15mesoporous silica with OTS monolayer, the milkywhitecolorofSBA-15changedtooffwhitecolor.Thiscould be due to change in the refractive index 3.2. BET surface area analysis BET surface area analysis before and after OTSadsorption on SBA-15 shows decrease of surface areafrom 930 to 416 m 2  /g, decrease of total volume from1.25 to 0.9 cm 3  /g and decrease of pore size/diameterfrom9.2to8.65 nm.Thisresultclearly revealsthatthesurface modification indeed occurred inside theprimary mesopores of the SBA-15. Deposition of mercaptopropyltrimethoxysilane self-assembledmonolayer on mesoporous silica from supercriticalfluids also shows a similar decrease of surface area(approximately half) [16]. 3.3. Scanning electron microscopy The SEM pictures of mesoporous SBA-15 particlesbefore and after surface modification by OTSmonolayer are shown in Fig. 1a and b, respectively.The SBA-15 particles are something like sphericallyshaped with size approximately in the range of 1–2 m m. The particles are clearly visible and areseparate. Agglomeration of these particles is notobserved. After modification of SBA-15 by OTSmonolayers,theparticlesarefoundthemselvescloselyattached with each other. The SEM picture in Fig. 1bclearly shows a large number of SBA-15 particlesattaching closely with one another. These phenomenaobserved both in microscopic and macroscopic scalecould be due to spine (alkyl chain) like OTSmonolayer growth on SBA-15. These spines of oneSBA-15particlemaybe caughton the spineofanotherSBA-15. This way the bonding of large number of SBA-15 particles may take place. On the other hand, itcould be due to van der Walls and electrostaticinteraction of alkyl chains between the OTS mono-layers of adjacent SBA-15 particles. 3.4. Energy dispersive X-ray analysis The presence of silicon, oxygen and carbon inSBA-15 before and after OTS monolayer growth wasexamined with EDAX measurements. The EDAXspectrum was used to obtain qualitative informationabout OTS adsorption. The EDAX spectrum shows S.A. Mirji et al./Applied Surface Science 252 (2006) 4097–4103  4099  53.1 wt% oxygen and 46.9 wt% silicon, which closelymatches with the calculated weight percent age of 53.3 wt% oxygen and 46.7 wt% of silicon in SiO 2 .Carbon peak was not detected in SBA-15. After OTSadsorption for 24 h on SBA-15 the EDAX spectrumshows 7 wt% carbon, 51.5 wt% oxygen and 41.5 wt%silicon. OTS molecule predominantly consists of carbon; therefore, presence of carbon peak indicatesOTS adsorption. 3.5. Fourier transform infrared spectroscopy The FTIR spectra in the C–H stretching region of SBA-15 after OTS adsorption are shown in Fig. 2.Methylene (–CH 2 ) and methyl (–CH 3 ) asymmetricand symmetric C–H bands are observed. Theseobservations clearly suggest OTS adsorption. Highlyordered, densely packed OTS monolayer with close toperpendicular chain axis orientation has a methylene(–CH 2 ) asymmetric and symmetric stretching at 2920and 2850 cm  1 , respectively, and methyl (–CH 3 )asymmetric and symmetric stretching at 2968 and2879 cm  1 ,respectively[17,18].CH 2 asymmetricandsymmetric stretching observed for OTS on SBA-15are 2923 and 2855 cm  1 , respectively. CH 3  asym-metric and symmetric stretching observed are at 2956and 2879 cm  1 ,respectively.The resultsare closely inagreement with reported data [17,18]. The intensityratio of   n a (–CH 3 ) to  n a (–CH 2 ) vibrations indicate theorientation of alkyl chain relative to the surface [19]. S.A. Mirji et al./Applied Surface Science 252 (2006) 4097–4103 4100Fig. 1. SEM monograph of mesoporous SBA-15: (a) before OTSmonolayer growth and (b) after OTS monolayer growth.Fig. 2. FTIR spectra in the C–H stretching region of SBA-15 withOTS adsorption showing –CH 2  and –CH 3  stretching bands.Fig. 3. X-ray photoelectron spectra showing C 1s binding energyrange:(a) SBA-15beforeOTSadsorption and(b)SBA-15after OTSadsorption.  This ratio is   1, closely in agreement with reporteddata [19]. Therefore, the OTS adsorption on SBA-15forms monolayers with 72–80 8  oriented alkyl chainsrelative to the surface [19]. 3.6. X-ray photoelectron spectroscopy Figs. 3–5 show X-ray photoelectron spectra in theC 1s, O 1s and Si 2p binding energy range,respectively. The spectra are fitted with Gauss–Lorentzpeaks using XPSPEAK software to obtain peak information. A Shirley’s base line was used in thefitting process. For C 1s binding energy spectra, threepeaks are required to get a good fit to experimentaldata. The peak binding energies obtained areapproximately 284.6, 286.4 and 288.5 eV. Peak 1 isimportant to us because it belong to OTS adsorption[20]. Relatively small peaks, 2 and 3, may be due toimpurities in the system or may be due to ‘‘ghost’’peaks. After OTS adsorption the integrated area of peak 1 increases by about 1.9 times as compared tobare SBA-15 ‘adventitious’ carbon peak. This largeincrease could be due to large adsorption of OTS duetoverylarge surface area of SBA-15. It can also be dueto combined adsorption of OTS and toluene solvent.The toluene adsorption also gives C 1s binding energyspectra close to 284.7–285.1 eV [21]. After OTSadsorption O 1s peak integrated area is found todecrease as shown in Fig. 4. The decrease of O 1sintegrated area is attributed to screening of electronsfrom the oxide layer below the OTS layer. Moredecrease of integrated area of O 1s, therefore,corresponds to better quality (densely packed) OTSmonolayer [20]. The Si 2p peak integrated area alsodecreases after OTS adsorption. The decrease is due toscreening of Si 2p electrons from the SiO 2  due to thepresence of OTS monolayer. More screening of electrons, therefore, implies more decrease of inte-grated area of Si 2p [20]. Full width at half middle(FWHM) of C 1s, O 1s and Si 2p peaks slightlyincrease after OTS adsorption. This could be due toaggregation of SBA-15 particles (Fig. 1) after OTSadsorption. 3.7. Thermo-gravimetric analysis Thermo-gravimetric analysis of SBA-15 powderbefore and after OTS adsorption is shown in Fig. 6.The thermal analysis was carried out in thetemperature range from 30 to 800  8 C, in air at aheating rate of 10  8 C/min. The TGA pattern of SBA-15 shows a total weight loss of 15% over thetemperature range from ambient to 600  8 C. Theunderstanding of this weight loss requires knowledgeof thermally adsorbed species on the surface of SBA-15. The surface of SBA-15 like any SiO 2  surfaceconsists of a small portion of free silanols; a largeamount of hydrogen bonded silanols and adsorbed S.A. Mirji et al./Applied Surface Science 252 (2006) 4097–4103  4101Fig. 4. X-ray photoelectron spectra showing O 1s binding energyrange: (curve 1) SBA-15 before OTS adsorption and (curve 2) SBA-15 after OTS adsorption.Fig. 5. X-ray photoelectron spectra showing Si 2p binding energyrange: (curve 1) SBA-15 before OTS adsorption and (curve 2) SBA-15 after OTS adsorption.
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