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UNIVERSITY OF ZARAGOZA Chemical Engineering and Environmental Technology Department MICROPATTERNED SUBSTRATE MODIFIED WITH HYDROPHIC COATING FOR DEWATERING APPLICATIONS Master´s Degree in Nanostructured Materials for Nanotechnology Applications Author: Leidys Marleyn Rodríguez Castro Tutor: María Pilar Pina Iritia Tutor: Miguel Ángel Urbiztondo Castro Enero 2013 2 3 My grateful and sincere thanks to Professors María Pilar Pina and Miguel A. Urbiztondo, for listening me at all moments, discussing with me and supervising all the details of this work. I would like to thank scholarship program of the Fundación Carolina and University of Zaragoza for giving me the opportunity to develop my research in nanotechnology. … To God and all my family 4 OUTLINE 1. INTRODUCTION 11 1.1. General description of zeolites. 11 1.2. Properties of zeolite and applicatons of micro-scale 13 1.3. General about μ-preconcentrators 14 1.4. Zeolites crystallization 16 2. AIM OF THIS WORK 18 3. EXPERIMENTAL 19 3.1. Preparation of zeolite LTA seeds 19 3.2. Preparation and fabrication of zeolite composite substrate 20 3.2.1. Preparation of flat and 3D substrates. 20 3.2.2. Seeding process zeolite A 23 3.2.3. Hydrothermal Synthesis of layer A-type zeolite. 28 3.3. Adsorption/desorption analysis 29 3.4 Sample characterization. 30 4. RESULTS AND DISCUSSION 32 4.1. Synthesis of LTA-zeolite seeds 32 4.1.1 Morphology and average seed size. 32 4.1.2. The chemical composition 33 4.2. Synthesis of zeolite composite substrate 34 4.2.1. Synthesis Boehmite (γ-AlOOH) 34 4.2.2. Optimization of seeding process. 36 4.2.3. Hydrothermal Synthesis of layer A-type zeolite. 39 4.2.3. Elemental analysis of zeolite crystals (discrete and deposited form) 42 4.3. Adsorption /desorption analysis. 45 FUTURE WORK AND A PARTICULAR APPLICATION: SYNTHESIS OF 5 ZEOLITE Y LAYERS ONTO SILICON MICROSTRUCTURES. CONCLUSIONS REFERENCES APPENDEX I. EQUIPMENT USED FOR THE SYNTHESIS OF ZEOLITES II. CHARACTERIZATION TECHNIQUES 6 LIST OF FIGURES FIGURE Figure 1. Comparison of pore sizes of different framework structures 12 Figure 2. Structure and dimensional pore system for LTA zeolite. Framework viewed along. 13 Figure 3. Hydrothermal zeolite synthesis. 17 Figure 4. Two possibilities for synthesis of zeolite polycrystalline layers. Mode exsitu or in-situ 17 Figure 5. Process flow for the fabrication of microchannels onto silicon support. 21 Figure 6. Scanning electron microscopic images of the microchannels on silicon wafer: (a),(d) Top view two different patterns, (e) prospective view, (b) crosssection and (c),(d) The microchannel patterns. 22 Figure 7. Photolithography of microchannels mask. 23 Figure 8. a) Microchannels pattern on SU-8 resin, b) microdevices for seeding process of microchannels fabricated in SU-8 resin. 23 Figure 9. Scheme of the hydrothermal synthesis of a zeolite layer using poly(diallyldimethylammonium chloride) (PDDA) or Boehmite as interlayer. 24 Figure 10. Spin-coating process. Dispensing of solution (A). Distribution (B). Uniform thinning (the solvent evaporation). 25 Figure 11. (A) Boehmite structure and unit cell. (B) Boehmite model of nano-sized boehmite crystallites. 26 Figure 12. Stages of the dip coating process: dipping of the substrate into the coating solution, wet layer formation by withdrawing the substrate and gelation of the layer by solvent evaporation. 27 Figure 13. Fabrication of microchannel pattern with zeolite layer. 28 Figure 14. Schematic of experimental flow loop used in microchannel. 28 Figure 15. Schematic diagram of the experimental setup adsorption/desorption analysis. 30 7 Figure 16. SEM micrographs of Na-A zeolite synthesized from 13.4(TMA)2O:0.3Na2O:1.8Al2O3: 11.25SiO2:700H2O and 100°C for 15 h a) Particle size b) Morphology of zeolite seed. 32 Figure17.DLS of zeolite A nanocrystals obtained from 13.4(TMA)2O:0.3Na2O:1.8Al2O3: 11.25SiO2:700H2O.and 100°C for 15 h a) Mean iam 1 nm b ean iam 17.1 nm 33 Figure 18. a) TEM image of boehmite nanoparticles synthesized from the (Al(OCH(CH3)2)3 precursor in acid at 90 °C. b) SEM micrograph of seeded support with boehmite layer: (solution 9 wt% and pH 4) 35 Figure 19. IR spectra of Boehmite measured in the range 4000 - 600cm-1 and a band resolution of 35 Figure 20. XDR patterns of the calcinated sample (black) and JCPDS card No. 10- 0425 (red 36 Figure 21. Zeolite seed coating using a) PDDA b) Boehmite, as an interlayer 37 Figure 22. Multi-stage coating method: Boehmite solution (9 wt%) was spinned a) Once b) 3 times c) 5 times d) 8 times. Zeolite seed solution (1 wt%) was spinned 5 times onto silicon dioxide wafers 37 Figure 23. Scanning electron micrographs of seeding process microchannels on silicon substrates: (a) Top view (b) cross-section. 38 Figure 24. Scanning electron micrographs of seeding process microchannels on SU-8 support to mag x 1500, 6000 and 13000. 39 Figure 25. Micrographs (Top view and cross-section) of pieces wafer to different hydrothermal synthesis time. a h 3 nm b h nm c h nm d 1 h 1. μm e 1 h 1. 3μ m f h .1 μm . 40 Figure 26. a) Top view and b) Cross-section image of A-type zeolite layer intergrown onto a 3 inch silicon dioxide wafer. 41 Figure 27. SEM image of the zeolite A layer on microchannel pattern obtained by hydrothermal synthesis at 90°C by 7.5 hours 42 Figure 28. Discrete zeolite crystals obtained by hydrothermal synthesis without support to 90°C by 7.5 hours. 43 Figure 29. EDX analysis points on cross-section of zeolite layer intergrown onto 44 8 silicon support synthesized to 90°C by 20 hours. 9 LIST OF TABLES TABLE Table 1: Chemical composition of the as-synthesized nano zeolite seed of LTA by Energy-Dispersive X-ray Spectroscopy (EDXS) 33 Table 2: Comparison between chemical composition of the as-synthesized zeolite crystal of LTA and zeolite film deposited on silicon substrate by Energy-Dispersive X-ray Spectroscopy (EDXS) 43 Table 3: Comparison Si/Al ratios measured by EDX, to different hei hts of -t pe eolite film μm deposited onto silicon dioxide substrate. 44 16 the fabrication of micro-channels for micro fluidic and bioMEMS devices. Grzelka et al. used a preconcentrator with SU-8 technology for analysis of different kinds of cells/microparticles. The adsorption capacity of the preconcentrator is a critical issue in the design and operation of the preconcentrator and it governs the minimum size of the microdevice. The selection of the adsorption materials is mainly based on experimental characterization of the adsorbents and their behavior with the target gases molecules [17]. zeolite can be an interesting candidate for preconcentration applications because have a good selectivity due to their homogeneous pore size, moreover exhibit a high thermal stability, high resistance to corrosive gases and strong adsorption affinity allowing the capture of gases at very low concentration. Indeed, zeolites are capable of separating molecules by their size, shape and polarity. Moreover, zeolite coatings and films have been successfully grown on flat and micropatterned substrates using various synthesis approaches [3-5,12]. This enabled to Lahlou et al, the incorporation of zeolites (i.e., FAU) as adsorbent potential in a 3D micropreconcentrator for analysis of orthonitrotoluene. The ability to grow a stable of zeolite film in a desired area is a critical issue to their use in micropreconcentration. A zeolite coating/film with high quality can be fabricated by using zeolite nanocrystals as building blocks [29]. 1.4. Zeolites crystallization Zeolite synthesis involves the hydrothermal crystallization of active hydrated aluminosilicate gels or sols in a basic environment. The gel is defined as an aqueous solution containing a silica source, an aluminum source and an alkali source. Hydrothermal synthesis usually refers to reactions occurring under conditions of high temperature–high pressure (>100 ºC, >1 bar) in a closed system (Figure 3) [7]. The duration required for crystallization varies from a few hours to several days. The general factors influencing hydrothermal zeolite synthesis are well known (reactant sources, Si/Al ratio, alkalinity, water content, inorganic cations, organic templates, solvents, temperature, aging, stirring, and seeding) [29]. 17 Figure 3. Hydrothermal zeolite synthesis. The precursor materials are converted into the crystalline product whose microporosity is defined by the crystal structure. During the hydrothermal reaction in the presence of a ‘‘mineralisin ’’ a ent most commonl an alkali metal h droxide), the crystalline zeolite product (e.g. zeolite A) containing Si-O-Al linkages is created. Basically, the hydrothermal synthesis procedure can be ex situ and in situ methods (figure 4), that is with and without a previous seeding step. The in situ hydrothermal synthesis is method in which the support is immersed into the synthesis solution and the layer/film is formed by direct crystallization. Coating the zeolite seed on the support surface before hydrothermal synthesis, which is also called a secondary growth method (ex situ), is an effective approach to develop a high-quality zeolite coatings. Figure 4. Two possibilities for synthesis of zeolite polycrystalline layers. Mode ex-situ or in-situ 18 2. AIM OF THIS WORK This work is part of the research line focused in developing microsystems with layers/coatings zeolitic as a hallmark. Therefore, A-type zeolite has been investigated for its potential use as adsorbent material for to create micro-traps/micro-preconcentrators of water in gas streams. In order to accomplish this objective, next steps have been followed:  The first part of this work was aimed at the optimization of method for deposition and intergrowth of coating/films A-type zeolite defect-free onto silicon dioxide wafer (flat substrate), since previous work by our group demonstrated that the main problem associated with zeolite coating synthesis is crack formation,  In the second part, was devoted to seeding the microchannels fabricated on silicon oxide and photoresin SU-8 support with A-type zeolites enabling the deposition and growth of defect-free eolite film b h drothermal s nthesis in the case of silicon substrate.  In the end, we have evaluated the possibility of preconcentration on microchannels for water/nitrogen in gas stream, through analysis adsorption/desorption of seed A- type zeolite, nanocrystal zeolite A and coatings onto silicon plane substrates. 19 3. EXPERIMENTAL The zeolite layers characterized in this work were prepared according to the method described by previous work [30]. Basically the deposition of zeolite LTA layer onto the substrate involves two main steps, namely seeding and hydrothermal growth. In the first step, seeding process, the surface of silicon oxide support is modified in order to promote the adhesion of colloidal zeolite seeds, oppositely charged. The surface of the substrate was modified by reacting with two different compounds: an cationic polymer, poly(diallyldimethylammonium chloride) (PDDA) and Boehmite, an aluminium oxide hydroxide (γ-AlOOH) mineral. A zeolite nanocrystals colloidal solution with a concentration of the solid nanocrystals of 2 wt% was spread and spinned onto modified support. The seeded support is then hydrothermally treated in the same synthesis solution to allow the growth of zeolite seeds into a dense film. The directed growth of zeolite A within the confined space of microfabricated channels on silicon dioxide substrates has been developed in the second part of this work. The surface of the substrate was modified by reacting with Boehmite (shows better results than the PDDA). Following, the seeding processes and hydrothermal synthesis were carried out. Finally, the incorporation of zeolite seeds on micro-channel of SU-8 resin was realized by seeding continuous flow process, with a volumetric flow 0,1 mL/min. The SU-8 support was modified with PDDA covalent linker. The microchannels in SU-8 resin cannot undergo hydrothermal synthesis due to the instability of the material at the temperature for zeolite synthesis. 3.1. Preparation of zeolite LTA seeds The A nano-zeolite seeds were prepared by hydrothermal synthesis according to the procedure reported in a previous work by the authors [3,12,30]. In a typical preparation of 100 g of zeolite seed, 12.78 g of LUDOX (AS-30 Aldrich) were diluted in 11.36 g of DDI water in a polypropylene vessel and stirred at room temperature until completely dissolved 20 (solution 1). In a separate vessel, 4.26 g of aluminum isopropoxide (C9H21O3Al3 98 wt % Aldrich), 28.4 g of Tetramethylammonium hydroxide pentahydrate (TMAOH.5H2O 97 wt% Sigma Aldrich), 39.76 g of DDI water and 3.4 g of 1M sodium hydroxide (NaOH97 wt% Sigma Aldrich) solution were combined and stirred during 1 h until completely dissolved (solution 2). Finally, solution 2 was added to solution 1 under stirring during 48 h, at room temperature, until homogeneous dissolution, then transferred to a polypropylene bottle and heated to 100°C in rotary evaporator for 15 h under static conditions. The final mixture having a molar composition of 13.4(TMA)2O:0.3Na2O:1.8Al2O3: 11.25SiO2:700H2O. The zeolite product was then washed with deionized water and dispersed in DDI H2O, and then it was centrifuged at 10.000 r.p.m. for 30 min and redispersed in DDI H2O again under ultrasonic bath. The washing procedure was repeated several times obtaining a final dispersion of nanocrystals seeds at pH 10. 3.2. Preparation and fabrication of zeolite composite substrate 3.2.1. Preparation of flat and 3D substrates. The flat substrates used for the preparation of the zeolite-silicon composites were commercial 3 inch silicon wafers with a <100> crystal orientation. These silicon wafers were thermally oxidized at 1000°C in order to obtain silicon oxidized wafers with a SiO2 thickness of 700 nm. A tubular oven (Carbolite) has been used for growing thermal SiO2 layers onto silicon wafers . We have used two different processes: dry oxidation, using O2 and N2 , and wet oxidation, bubbling O2 and N2 through DDI water. The usual procedure is to grow a mixed oxide sandwich: dry oxidation + wet oxidation + dry oxidation. The water required for the wet oxidation is achieved by saturating the input stream of oxygen through a bubbler to 100 °C. The process variables are oxidant gas composition and the oven temperature. Before seeding process, silicon oxide substrates were sonicated in acetone for 10 min, a mixture of acetone (50%) and ethanol (50%) for others 10 min, and rinsed with a copious amount of deionized water. After that, silicon oxide wafers were cleaned in freshly prepared Piranha solution (a mixture of 30% H2O2 (35% wt% Sigma Aldrich) and 70% of 21 H2SO4 (96 wt% Panreac)) for at least 0.5 h, rinsed exhaustively with deionized water, and dried in a stream of nitrogen gas. On the other hand, the 3D-substrates (microchannels) were fabricated onto silicon wafers using the following procedure. A thick layer of polymer resist (AZ 6624) was spin-coated onto the silicon oxide wafer, and micropatterned using standard photolithography techniques. The schematic drawing in figure 5 illustrates the procedure for the fabrication of microchannel onto silicon wafer. After exposure the wafers to ultra violet light for final pattern transfer, the support was attacked with strong alkaline substances (pH > 12) such as aqueous KOH or TMAH- solutions (Anisotropic Silicon Etching) Si + 4OH- Si(OH)4 + 4e- Since the bonding energy of Si atoms is different for each crystal plane, and KOH/TMAH Si etching is not diffusionbut etch rate limited, Si etching is highly anisotropic: the (100)- orientated wafers form square-based pyramids with (111) surfaces. Finally, silicon wafers were again thermally oxidized and the microchannels were seeded with zeolite A. Figure 5. Process flow for the fabrication of microchannels onto silicon support. The patterns, shown in figure 6, consists of 60 (Fig. 6a) and 47 (Fig. 6d) microchannels with a len th of 3 and μm respectivel and line spacin of μm approximatel into the plate with area of 1 cm2. The channels measurin around μm wide and 3 μm deep. The pattern was coated with Boehmite onto which was adsorbed a layer of nanometer sized zeolite seeds (i.e., 90.8 nm). A layer of a-type zeolite film was grown onto the seeded pattern following hydrothermal synthesis at 90°C for 7,5 h. A Teflon-lined stainless steel autoclave with a reactor volume of 35 cm3 was used in the zeolite crystallization and growth. 22 Figure 6. Scanning electron microscopic images of the microchannels on silicon wafer: (a),(d) Top view two different patterns, (e) prospective view, (b) cross-section and (c),(d) The microchannel patterns. Refer to figure 7 below for step-by-step pattern transfer and etching of the channels mask the cross-sectional of the SU-8 substrate. The etch process began with deposition of a layer of phothoresin SU-8 onto kapton support. To apply the pattern of the channels to the SU-8 layers, the contact photolithography of the channels mask was performed using an SÜSS MICROTEC MA6 model Series Mask Aligner, the channels mask was aligned over the front-side of the coated SU-8 substrate. The support were then exposed to ultra violet light for final pattern transfer, after deposition of a second SU-8 layer. Figure 8 illustrates of images of the microchannels fabricated in SU-8 resin. and microdevices used for the seeding process. The channels measuring around 100 μm wide and 100 μm deep. 23 Figure 7. Photolithography of microchannels mask. A) Clean Kapton support, B) spin photoresist of first layer and cured. C) spin photoresist osf second layer, D) expose Channels mask, E) develop, F) seeding process G) microdevice closed Figure 8. a) Microchannels pattern on SU-8 resin, b) microdevices for seeding process of microchannels fabricated in SU-8 resin 3.2.2. Seeding process zeolite A Surface seeding accelerates the zeolite crystallization process on the support surface, and also enhances the formation of homogeneous zeolite A layer. But the main problem associated with coating synthesis is crack formation. In previous works in this research group have used PDDA as covalent linker between zeolite seed and silicon support however, forming cracks in the coatings zeolitic cannot be avoided. Formation of crack was reduced by applying intermediate layer, between the support surface and seed layer 24 The linkers PDDA and Boehmite adsorbed on the support surface are used to capture LTA zeolite particles during hydrothermal synthesis. It is expected that because of the electrostatic interaction, the negatively charged LTA zeolite particles can homogenously and easily migrate to the positively charged support surface, facilitating the formation of a uniform and dense zeolite LTA layer. The preparation scheme is shown in Figure 9. Figure 9. Scheme of the hydrothermal synthesis of a zeolite layer using poly(diallyldimethylammonium chloride P or Boehmite as interla er. In the first step the support was modified b P and Boehmite, following seeding steps and finally, hydrothermal synthesis. Cationic polymer, poly(diallyldimethylammonium chloride) ((C8H16ClN)n 20 wt% in water Aldrich), was deposited on the substrates prepared as above. The solution of PDDA was prepared solving 1.47 g of NaCl (99 wt% Panreac) and 0.25 g of PDDA in 50 ml of DDI water, the solution was stirred till completely dissolve the poly(diallyldimethylamonium) chloride and after the solution was spinned onto the silicon substrate. Finally, the silicon substrate was seeded with zeolite nanocrystals before hydrothermal synthesis. The colloidal solution seeds of zeolite spread and spinned onto the substrate. The angular speed used was initially 170 r.p.m by 1.5 minutes, to spread all the deposited material onto the substrate and then was spinned at 4000 r.p.m by 2 minutes, in order to finish the covering and evaporate the solvent. These parameters reproduce the optimal conditions reported by 25 previous work [30]. The use of PDDA allows reversing the charge of the SiO2 surface in order to obtain a positively charged substrate and produce attractive electrostatic interactions between the surface of the substrate and the zeolite nanocrystals, as shown in figure 9. PDDA is always cationic, regardless of pH. The modified support were coated with LTA-type zeolite crystals as nucleation seed. To get a uniform layer on the support surface, the nucleation seeds should be very small and uniform in size. The particle size of seed crystals was 90.8 nm (average particle size measured from SEM and DLS). In order to get a uniform seed layer on the support surface, the seeds should be dispersed homogeneously on the support surface. In this seeds coating process, the support substrate was spinned with a 2 wt% LTA-type zeolite suspension in deionized water 5 times for a duration of 1 minute at 100 r.p.m for to thin the fluid and then 0.5 minute at 3000 r.p.m for to eliminate excess solvents from the resulting film. After the spinning procedure, the seeded supports were dried at 150°C for 24 h. The zeolite A layer were synthesized hydrothermally on silicon dioxide modified, seeded support wafer. A typical spin process consists of a dispense step in which the material is deposited onto the substrate surface, a high speed spin step to thin the fluid, and a drying step to eliminate excess solvents from the resulting film. Figure 10 shows an schematic description of the followed spin coating process. Figure 10. Spin-coating process. Dispensing of solution (A). Distribution (B). Uniform thinning (the solvent evaporation). Boehmite nanoparticles were prepared by a peptization method according to a procedure developed by Yoldas [31] Twenty five grams of aluminum tri-sec-butoxide Al[OCH(CH3)C2H5]3, 97 wt% Aldrich) precursor was hydrolyzed in 100 mL of distilled water at 80°C for 1 h, under vigorous stirring to form a white precipitate. Then 7 mL of 32 4. RESULTS AND DISCUSSION 4.1. Synthesis of LTA-zeolite seeds 4.1.1 Morphology and average seed size. Synthesis of zeolite A nanocrystals were prepared as described above. The morphology of the nanocrystals obtained and the particle size deposited onto silicon dioxide wafer were characterized via Scanning Electron Microscopy. Dynamic Light Scattering was used in order to obtain the particle size distribution in suspension. Results obtained from the SEM image fit with the results obtained from DLS. Two different populations are observed with DLS technique for the synthesis 48 hours of aging time and 15 hours of synthesis time, one of around 108,4 nm and the other of 17,1 nm. The average cr stal si e reported b LS was 90,8 nm and the polydispersity index was 0,16. As the value of the pol dispersit index is si nificantl lar er than ,2 and the calculated hydrodynamic radius is no longer reliable. Figure 16 and 17 shows SEM images and DLS results, respectively, for the nanocrystals obtained. Figure 16 (b) shows the SEM image of zeolite seeds deposited onto silicon dioxide support modified with boehmite solution (9 wt%). The seeds have a typical cubic form from the LTA zeolite. Figure 17 show DLS results for the nanocrystals obtained. Figure 16. SEM micrographs of Na-A zeolite synthesized from 13.4(TMA)2O:0.3Na2O:1.8Al2O3: 11.25SiO2:700H2O and 100°C for 15 h a) Particle size b) Morphology of zeolite seed. 33 Figure 17: DLS of zeolite A nanocrystals obtained from 13.4(TMA)2O:0.3Na2O:1.8Al2O3: 11.25SiO2:700H2O.and 100°C for 15 h a ean iam 108,4 nm b) Mean Diam 17.1 nm 4.1.2. The chemical composition Chemical composition of the as-synthesized nano zeolite seed can be found in Table 1. EDX-SEM results show that the synthesized product only contained Silicon, Aluminum, Sodium and Oxygen atoms. The Si/Al ratio of the as-synthesized zeolite was 1.57, which is in the reported range of zeolite A compositions. Using of template in the synthesis of zeolites tends to change the Si/Al ratio of the final products. For example, zeolite A, synthesized by using template (tetramethylammonium), has a Si/Al ratio about 2. In contrast, template-free synthesis leads to Si/Al =1 [32]. As results of increasing of Si/Al ration, thermal and chemical stability of the product will be improved remarkably. Table 1: Chemical composition of the as-synthesized nano zeolite seed of LTA by Energy- Dispersive X-ray Spectroscopy (EDXS) Element Weight % Atomic% Oxygen 49,45 61,99 Sodium 9,03 7,88 Aluminum 16,15 12 Silicon 25,38 18,13 Si/Al 1,57 -- Total 100 100 34 4.2. Synthesis of zeolite composite substrate 4.2.1. Synthesis Boehmite (γ-AlOOH) Boehmite nanoparticles were prepared by a peptization method in an aqueous nitric acid solution. Peptization is a process in which coagulated precipitates dissolve and recrystallize to form nanoparticles with an acid or base. Boehmite nanoparticles were characterized using powder X-ray diffraction and in-situ IR spectroscopy. The morphology of the nanoparticles obtained and deposited onto silicon dioxide wafer were characterized via Transmission Electron Microscopy and Scanning Electron Microscopy, respectively. Dynamic Light Scattering was used in order to obtain the particle size distribution in suspension. In the formation of boehmite, aluminum tri-sec-butoxide Al[OCH(CH3)C2H5]3 is easily hydrolyzed with excess water because the water molecule is more nucleophilic than the alkoxyl group. Consequently, water molecules are formed by the reaction from the water molecules and the aluminum atoms in precursor. In our experiment, the hydrolysis reaction in acid was markedly expedited by the reaction temperature (90 °C) employed, giving rise to the formation of whitish boehmite nanoparticles. The formation of boehmite from the hydrolysis of aluminum tri-sec-butoxide in acid may be sumarized as follows [33] Al[OCH(CH3)C2H5]3(l) + (n+2)H2O(l) + HNO3 AlO(OH).H2O (l) + 3C4H9OH(g) Results obtained from DLS indicated a polydispersity sample with an average size of 80 nm and polidispersity of 0,22. A TEM observation indicates that the boehmite nanoparticles were about spherical in shape (Fig. 18a) and tended to form aggregated clusters with an apparent ‘ ‘particl e’’ si e in a ran e similar to that determined b the li ht-scattering method. The ‘‘real’’ particle si e of boehmite ma be in the 3 nm ran e. In contrast fi ure 18(b) is an SEM micrograph of the boehmite solution prepared at pH 4. Particle 35 aggregation is apparent, suggesting a formation of continuous particulate network in the suspension structure. Figure 18. a) TEM image of boehmite nanoparticles synthesized from the (Al(OCH(CH3)2)3 precursor in acid at 90 °C. b) SEM micrograph of seeded support with boehmite layer: (solution 9 wt% and pH 4) Figure 19 show the infrared absorption spectra of the boehmite with 9 wt% of concentration before calcination at 600 °C. Typical bands for crystallized boehmite [34] can be observed: 3000 - 3600 cm-1 , O-H stretching vibrations; 1150 cm-1, O-H bending vibrations; 1637 cm-1, 1355 cm-1 , and 1078 cm-1 which can be assigned to Al=O, Al–O–H, and Al–O–Al bonds, respectively. The vibrations appear in the 3000-3600 cm-1 range as strong peaks are characteristic for -OH fragments of physisorbed water. Figure 19. IR spectra of Boehmite measured in the range 4000 - 600cm-1 and a band resolution of 5 cm-1. 36 The Boehmite synthesized was calcined (600°C during 6h) in order to get its dehydrated alumina form. The XRD pattern of the calcined sample (figure 20) indicates that a transformation form boehmite phase to γ-alumina after calcination occurred. All the peaks can be indexed to a cubic unit cell of γ-alumina (a = 7.900, b= 7.900, c= 7.900, JCPDS PDF No. 10-0425) Figure 20. XDR patterns of the calcinated sample (black) and JCPDS card No. 10-0425 (red) 4.2.2. Optimization of seeding process. The clean and modified silicon substrate was seeded using the procedure described in Section 3.2.2 in order to obtain a complete and uniform seed coverage. Different from the previous study, in the present work, the Boehmite was adsorbed on the support surface and was used to capture LTA zeolite particles during seeding process onto and subsequent ex situ hydrothermal synthesis. The main cause of crack of membrane layer was lack of good adherence between zeolite layer and substrate layer. As expecting the boehmite intermediate layer enhance the adherence between zeolite seed layer and substrate by hydrogen bonding. In the figure 21 is possible compare the zeolite seed (2 wt%) coating using PDDA and Boehmite as intermediate layer. The type of zeolite loading depended on 37 the coating of surface. Deposition parameters of PDDA layers reproduce the optimal conditions reported by previous work [30]. Figure 21. Zeolite seed coating using a) PDDA b) Boehmite, as an interlayer The quality of the zeolite seed coating can be improved by employing the multi-stage coating methods such as is illustrated in the figure 22. The boehmite solution was spinned repeatedly on the silicon wafer in order to enhance zeolite seed coating. In all cases the zeolite seed solution (2 wt%) was spinned five times onto boehmite layer. Optimization of seeding process was carried out through the multi-stage coating methods and use of a boehmite interlayer. Figure 22. Multi-stage coating method: Boehmite solution (9 wt%) was spinned a) Once b) 3 times c) 5 times d) 8 times. Zeolite seed solution (1 wt%) was spinned 5 times onto silicon dioxide wafers. 38 Before the hydrothermal synthesis, the quality of zeolite seed layer on the microchannels structure were examined by scanning electron microscope. This is essential to check for imperfection in the zeolite layer which can result in formation of crack. Fabricated microchannel structures is shown in figure 23. The Zeolite A layer was uniform, compact and have a thickness of about 300 nm. It is important to note that prior to the sectioning of the sample for analysis with a scanning electron microscope, the zeolite film uniformly covers the entire substrate. Fig. 23 (a and b), show a top view and cross-section of microchannels fabricated onto the silicon substrate. In both samples a 300 nm thick was observed. Initially the microchannels on silicon dioxide substrate were dipped into boehmite sol. In the seeding process of microchannels, the size of the zeolite seed must be uniform and smaller than the channel width in order to prevent blockage and to ensure a uniform deposition. The zeolite loading can be controlled by changing the concentration of zeolite in the slurry and through repeated coating of the microchannel. The seed adhesion was strong and the zeolites remained onto substrate surface. Microchannels were dipped into boehmite solution (9 wt%) with a dipping speed of 0.1 mL/s and repeated 5 times. Following the substrate was dipped in a 1 wt% of A-type zeolite suspension in deionized water 5 times with same dipping speed. 39 Figure 23. Scanning electron micrographs of seeding process microchannels on silicon substrates: (a) Top view (b) cross-section. A cross section of the microchannel fabricated in SU-8 resin (Figure 24) shows the A-zeolite seed layer (1 wt %) deposited on the microchannel wall. The zeolite was uniformly deposited along the channel walls. The zeolite seed film displayed good adhesion to the channel but SU-8 resin was not able to withstand temperature treatments above 80°C without deforming of material substrate. The SU-8 support was modified with PDDA covalent linker at flow rate of 0.1 mL/min by 1 hour, changing the direction of flow for periods of 10 minutes. Then zeolite A was seeded with the same parameters of time and volumetric flow but at concentration of 1 wt%. . Figure 24. Scanning electron micrographs of seeding process microchannels on SU-8 support to mag x 1500, 6000 and 13000 4.2.3. Hydrothermal Synthesis of layer A-type zeolite. The next step in the fabrication of zeolite-silicon composite substrate is hydrothermal synthesis. The seeding procedure involved the functionalization of the surface flat and 3D substrate with boehmite solution (9 wt%) utilizing different seeding 40 techniques (spin and Dipcoating) according type substrate. The composition of sol-gel precursor and parameter operating for hydrothermal synthesis are shown in the section 3.2.3. In order to obtain a high quality of zeolite intergrown onto substrates, synthesis time was evaluated. Six pieces of silicon dioxide wafer were modified with boehmite solution and seeded with zeolite nanocrystals. The seeding process steps was repeated such as written in section 3.2.2. A uniform and homogeneus zeolite seed coating, was obtained. A layer of zeolite was grown onto the seeded surfaces by placing the pieces of wafer horizontally in synthesis mixture contained in a teflon and with the molar composition is 2.8Na2O:2.7SiO2:1Al2O3:174-347H2O:0.05(TMA)2. The zeolite-silicon composite after the synthesis was rinsed in deionized water and dried in a plate heating at 150°C for 24 h. Scanning electron microscopy was used to examine the quality of the synthesis for crackfree zeolite layer. The figure 25 illustrates to result. The micrographs reveal that optimum time for hydrothermal synthesis was 12 hours (Fig 25d). The zeolite layers synthesized below this value (Fig 25a-c) have few thickness and above 12 hours the zeolite layer reveal cracks (Fig 25e,f). 41 Figure 25. Micrographs (Top view and cross-section) of pieces wafer to different hydrothermal synthesis time. a h 3 nm b h nm c h nm d 1 h 1. μm e 1 h 1. 3μm f h .1 μm According to written above. The optimum conditions to obtain a layer of intergrown zeolite of high quality on flat substrates are summarized as follow. The silicon wafer with silicon dioxide layer was modified with a boehmite solution (9 wt%) utilizing spin-coating technique. The solution was spinned onto subtrate 8 times at pH 4. Then, support was dired to 200°C for 2 h. Following, seed zeolite A was spinned onto boehmite layer 5 times and finally dried to 150°C for 24 h. The angular speed used was initially 100 r.p.m by 1 minute, and then was spinned at 3000 r.p.m by 0.5 minute. Hydrothermal synthesis was carried out at 90°C by 12 hours. Figure 26 shows scale-up procedure for a 3 inch silicon wafers. The zeolite layer obtained have a thickness 1 μm and cracks-free. Figure 26. a) Top view and b) Cross-section image of A-type zeolite layer intergrown onto a 3 inch silicon dioxide wafer. The figure 27 (a) and (b) show SEM image of top view and cross-section , respectively, of a high quality zeolite coatings intergrown on microchannel fabricated in silicon wafer. In this case, the dip-coating tecnique was preferred as seeding method. 3D substrate was dipped into boehmite solution (9 wt%) and A-type zeolite seed (1wt%) 5 times. The thickness of the films was similar as that for sample synthesized by 12 hours onto flat 48 [19] R. H. Brown., Collection and Analysis of Trace Organic Vapour Pollutants in Ambient Atmospheres: The Performace of a Tenax-Gc Adsorbent Tube. Journal of Chromatography A 178 (1979) 79-90. [20] E. D. Pellizzar., Collection and Analysis of Trace Organic Vapor Pollutants in Ambient Atmospheres. The Performance of a Tenax GC Cartridge Sampler for Hazardous Vapors. Analytical Letters 9 (1976) 45 - 63. 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