Atomic Layer Deposition of a High-Density Aminopropylsiloxane

P.O. Box 6100, FIN-02015 Espoo, Finland, Department of Chemistry, University of Joensuu,. P.O. Box 111, FIN-80101 Joensuu, Finland, and Institut de ...
0 downloads 0 Views 189KB Size
Download PDF
Langmuir 2003, 19, 10601-10609

10601

Atomic Layer Deposition of a High-Density Aminopropylsiloxane Network on Silica through Sequential Reactions of γ-Aminopropyltrialkoxysilanes and Water Satu Ek,*,† Eero I. Iiskola,† Lauri Niinisto¨,† Jari Vaittinen,‡ Tuula T. Pakkanen,‡ Jetta Kera¨nen,†,§ and Aline Auroux§ Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, P.O. Box 6100, FIN-02015 Espoo, Finland, Department of Chemistry, University of Joensuu, P.O. Box 111, FIN-80101 Joensuu, Finland, and Institut de Recherches sur la Catalyse - UPR (CNRS) 5401, 2 Avenue A. Einstein, F-69626 Villeurbanne Cedex, France Received August 12, 2003. In Final Form: October 3, 2003 A novel gas-phase procedure for the control of amino group density on porous silica through consecutive reactions of aminopropylalkoxysilanes and water vapor was developed. First heat-treated silica was saturated with trifunctional γ-aminopropyltrimethoxysilane (APTMS) or γ-aminopropyltriethoxysilane (APTS) in an atomic layer deposition reactor. During this step, precursor molecules were bound onto the surface both mono- and bidentately forming siloxane bridges with the silanol groups of silica. Then surface densities of 1.8 APTMS or 2.0 APTS molecules/nm2 were achieved. Next the aminosilylated surface was treated with water vapor in order to hydroxylate the free alkoxy groups of chemisorbed aminosilane molecules. At the same time, the silanol groups on the silica surface, which had remained unreacted during the first step, were revealed below the hydrolyzed alkoxy groups. These silanol groups of silica and hydrolyzed alkoxy groups were able to react further with the next feed of aminosilane molecules. The above-mentioned aminosilane/water vapor cycles, that is, two consecutive steps, could be repeated several times, and the amino group content on silica could be controlled through the number of aminosilane/water cycles. After four cycles, the surface was observed to be saturated and maximum amino group density was achieved. Then, by performing four or five cycles, surface densities of up to 3.0 APTS or APTMS molecules/nm2 were obtained. With this procedure, a high-density aminopropylsiloxane network is grown through horizontal polymerization of aminosilane molecules on the surface. With bifunctional γ-aminopropyldiethoxymethylsilane (APDMS), the repetition of aminosilane/water cycles did not increase the amino group content because of a lack of free and reactive ethoxy groups on the aminosilylated silica surface due to the bidentate bonding of APDMS molecules on silica.

1. Introduction Amino-functionalized silicon dioxide surfaces are mostly used for the immobilization of inorganic ions and molecules or organic and biochemical molecules onto the surface.1,2 The main applications of silane coupling agents are in the fields of analytical chemistry,3 biochemistry,4,5 catalyst technology,6-9 and electronics.1,10 One of the main commercial applications for aminosilylated porous silica is its * Corresponding author. Tel: 358-9-451 2602. Fax: 358-9-462 373. E-mail: [email protected]. † Helsinki University of Technology. ‡ University of Joensuu. § Institut de Recherches sur la Catalyse - CNRS. (1) Vansant, E. F.; Van Der Voort, P.; Vrancken, K. C. Characterization and Chemical Modification of the Silica Surface; Elsevier: Amsterdam, 1995; Vol. 93, Chapters 8 and 9. (2) Plueddemann, E. P. In Silane Coupling Agents; Plenum Press: New York, 1991; pp 55-63. (3) O’Gara, J. E.; Walsh, D. P.; Phoebe, C. H., Jr.; Alden, B. A.; Bouvie, S. P.; Iraneta, P. C.; Capparella, M.; Walter, T. H. LC-GC 2001, 19, 632. (4) Schena, M. Microarray Biochip Technology; Eaton: Natick, MA, 2000. (5) Janowski, F.; Fischer, G.; Urbaniak, W.; Foltynowicz, Z.; Marcimiec, B. J. Chem. Technol. Biotechnol. 1991, 51, 263. (6) Juvaste, H.; Iiskola, E. I.; Pakkanen, T. T. J. Mol. Catal. A: Chem. 1999, 150, 1. (7) Juvaste, H.; Iiskola, E. I.; Pakkanen, T. T. J. Organomet. Chem. 1999, 587, 38. (8) Jaroniec, C. P.; Kruk, M.; Jaroniec, M. J. Phys. Chem. B 1998, 102, 5503. (9) Yang, Y.; Zhou, R.; Zhao, S.; Li, Q.; Zheng, X. J. Mol. Catal. A: Chem. 2003, 192, 303.

use as a stationary phase in various types of chromatography.1,3 In the various applications, high surface coverage of amino groups is aimed at in order to achieve a large number of reactive sites on the surface and to maximize the performance of the silylated surface. In addition, a simple and reproducible preparative route for silylated surfaces is preferred especially in productions on a larger scale. The preparative method of silylated surfaces has a remarkable effect on the coating morphology, that is, layer thickness, surface density, orientation of the surface molecules, and the type of interactions between the surface groups and the precursor molecules.1 The presence of water has a great influence on the mechanism of overlayer formation and further on the structure of overlayers.11 In aqueous conditions, multilayers are formed with bi- and trifunctional alkoxysilanes because of hydrogen-bonding and hydrolysis of precursor molecules before deposition onto the surface.1 This usually happens also in the depositions performed in organic solvents regardless of careful dehydration treatments. For example, according to the studies of Moon et al.12,13 multilayers were obtained (10) Burtman, V.; Zelichenok, A.; Yakimov, A.; Yitzchaik, S. In Semiconducting Polymers: Applications, Properties, and Synthesis; Hsieh, B. R., Wei, Y., Eds.; ACS Symposium Series No. 735; American Chemical Society: Washington, DC, 1999; Chapter 25. (11) Krasnoslobodtsev, A. V.; Smirnov, S. N. Langmuir 2002, 18, 3181. (12) Moon, J. H.; Shin, J. W.; Kim, S. Y.; Park, J. W. Langmuir 1996, 12, 4621.

10.1021/la035472g CCC: $25.00 © 2003 American Chemical Society Published on Web 11/06/2003

10602

Langmuir, Vol. 19, No. 25, 2003

when tri- and bifunctional aminosilanes were deposited onto planar silicon wafers in a dry organic solvent but a molecular surface-saturated overlayer only was obtained with a monofunctional precursor. Elimination of disturbing water molecules and deposition of surface-saturated molecular layers of alkoxysilanes can be most easily performed in the gas phase. When the deposition of 3-glycidoxypropyltrimethoxysilane onto silica was compared under various conditions, viz., in aqueous conditions, in dry organic solvent, or in the gas phase, the following surface densities were obtained: 3.5, 1.7, and 1.9 molecules/nm2, respectively.14 The most dense molecular surface-saturated overlayer was achieved through the gasphase method although an almost equal content of amino groups on silica was obtained by deposition in well-dried organic solvent. In addition, in the same study the product deposited via the gas phase for chromatographic applications was observed to possess a significantly improved performance and to be well-defined independent of the quality of the reagents.14 Also, alkylsilane overlayers deposited in the gas phase are better ordered than layers deposited in toluene solutions.15 Silylation of silicon dioxide or other surfaces, in general, is most often performed by liquid-phase methods, organic solvent techniques being industrially the most important.1 Only a few studies have been reported on the gas-phase modification with aminopropylalkoxysilanes on planar16,17 or porous6,7,14,18-21 surfaces. Nevertheless, gas-phase techniques offer major advantages both from a scientific and industrial point of view. The preparation and study of modified surfaces and modification mechanisms are facilitated by the absence of solvents because the use of gas-phase deposition eliminates many of the tedious operations of the alternative methods, such as controlled hydrolysis of alkoxysilanes, solvent removal and recovery, and washing procedures.22 Amino-functionalized overlayers can be deposited onto silicon dioxide surfaces in the gas phase, thus eliminating hydrogen-bonding, hydrolysis, and condensation of aminopropylalkoxysilane molecules before the deposition on the surface. Then surface-saturated molecular layers with the surface densities of 2.0 APTS (γ-aminopropyltriethoxysilane) groups/nm2 and 1.8 APTMS (γ-aminopropyltrimethoxysilane) groups/nm2 can be achieved on porous silica surfaces.20 However, controlled hydrolysis of aminosilylated silica, that is, a treatment of the remaining unreacted alkoxy groups with water, followed by a silanization step can result in the controlled formation of a high-density amino-terminated siloxane network. When consecutive aminopropyltrialkoxysilane and water cycles have been performed, an increased content of amino groups on porous silica has been observed.11 When APTS dissolved in organic solvent was deposited onto silica and the modified surface was treated with water, a surface concentration of up to 2.7 amino groups/nm2 was achieved. The same kind of procedure has been performed also in (13) Moon, J. H.; Kim, J. H.; Kim, K.-J.; Kang, T.-H.; Kim, B.; Kim, C.-H.; Hahn, J. H.; Park, J. W. Langmuir 1997, 13, 4305. (14) Wikstro¨m, P.; Mandenius, C. F.; Larsson, P.-O. J. Chromatogr. 1988, 455, 105. (15) Duchet, J.; Chabert, B.; Chapel, J. P.; Gerard, J. F.; Chovelon, J. M.; Jaffrezic-Renault, N. Langmuir 1997, 13, 2271. (16) Kurth, D. G.; Bein, T. Langmuir 1993, 9, 2965. (17) Haller, I. J. Am. Chem. Soc. 1978, 100 (26), 8050. (18) Basiuk, V. A.; Chuiko, A. A. J. Chromatogr. 1990, 521, 29. (19) White, L. D.; Tripp, C. P. J. Colloid Interface Sci. 2000, 232, 400. (20) Ek, S.; Iiskola, E. I.; Niinisto¨, L. Langmuir 2003, 19, 3461. (21) Ek, S.; Iiskola, E. I.; Niinisto¨, L.; Pakkanen, T. T.; Root, A. Chem. Commun. 2003, 2032. (22) Scott, R. P. W. In Silica Gel and Bonded Phases; John Wiley & Sons Ltd.: Chichester, 1993; pp 139-174.

Ek et al.

the gas phase for epoxysilane with triethylamine (TEA) as a catalyst when a surface density of 2.2 groups/nm2 was achieved, whereas with a procedure without water treatment and the additional silane step, a somewhat lower surface density, that is, 1.9 silane groups/nm2, was obtained.14 The aim of this study was to develop a procedure for the preparation of a high-density aminopropylsiloxane network on porous silica. This was achieved through several consecutive reactions of vaporized aminopropylalkoxysilanes and water on the silica surface. Atomic layer deposition (ALD), also known as atomic layer epitaxy (ALE), was used as the technique for the deposition of surface-saturated molecular layers on silica.23,24 By means of ALD, vaporized precursors can be directly deposited in one step onto a silica bed at elevated temperatures.25 The deposition process is based on alternating chemisorption of the precursors, surface reaction, and desorption of the gaseous side-products.26,27 Physisorption of precursors can be avoided by purging physisorbed molecules from the surface with inert gas and using elevated temperatures. Due to the surface-limiting growth mechanism, conformal surface-saturated overlayers can be reproducibly deposited onto the solid surface. The gas-solid reactions of precursor molecules on porous supports can be studied by performing only one deposition step, viz., deposition of one molecular surface-saturated overlayer on the support. However, also multilayered structures, that is, titania,28,29 alumina,30 aluminum nitride,31 and cobalt silicate32 overlayers, can be deposited onto porous supports by ALD. When metal alkoxides, for example, titanium isopropoxide, have been used for the layer-by-layer growth of metal oxides on porous substrates, a lateral growth of oxides has been obtained.30,31 To the best of our knowledge, this was the first time when from one to five aminoalkoxysilane/water cycles were performed on a heat-treated silica surface in the gas phase in order to achieve a high surface coverage of amino groups on the surface. The gas-solid reactions of aminopropylalkoxysilanes with the porous silica surface have been previously studied by us,20,21 but in the present study the hydrolysis for alkoxy groups of aminosilylated silica was also studied in the gas phase. Diffuse reflectance Fourier transform spectroscopy (DRIFTS), solid-state 13C NMR, and elemental analyses were used for the characterization of aminosilylated and hydrolyzed silica samples. 2. Experimental Section 2.1. Chemical Reagents. APTMS (1), from Aldrich, U.S.A., and ABCR, Germany, APTS (2), from Merck, Germany, and γ-aminopropyldiethoxymethylsilane, APDMS (3), from ABCR, Germany, were used without further purification (Chart 1). Purity of the reagents, as reported by the manufacturers, was >9799%. For the deuteration experiments, deuterium oxide (99.8%) from Acros Organics, U.S.A., was used. (23) Suntola, T. Mater. Sci. Rep. 1989, 4, 261. (24) Suntola, T. In Handbook of Crystal Growth; Hurle, D. T. J., Ed.; Elsevier: Amsterdam, 1994; Vol. 3B, pp 601-603, 605-663. (25) Lakomaa, E.-L. Appl. Surf. Sci. 1994, 75, 185. (26) Niinisto¨, L. Proc. Int. Semicond. Conf. 2000, 1, 33. (27) Ritala, M.; Leskela¨, M. In Handbook of Thin Film Materials; Nalwa, H. S., Ed.; Academic Press: San Diego, 2002; Vol. 1, pp 103159. (28) Lindblad, M.; Haukka, S.; Kyto¨kivi, A.; Lakomaa, E.-L.; Rautiainen, A.; Suntola, T. Appl. Surf. Sci. 1997, 121/122, 286. (29) Kera¨nen, J.; Iiskola, E.; Guimon, C.; Auroux, A.; Niinisto¨, L. Stud. Surf. Sci. Catal. 2002, 143, 777. (30) Lakomaa, E.-L.; Root, A.; Suntola, T. Appl. Surf. Sci. 1996, 107, 107. (31) Puurunen, R. L.; Root, A.; Sarv, P.; Viitanen, M. M.; Brongersma, H. H.; Lindblad, M.; Krause, A. O. I. Chem. Mater. 2002, 14, 720. (32) Rautiainen, A.; Lindblad, M.; Backman, L. B.; Puurunen, R. L. Phys. Chem. Chem. Phys. 2002, 4, 2466.

Deposition of a High-Density Network on Silica Chart 1

Langmuir, Vol. 19, No. 25, 2003 10603 condition for cross-polarization and the magic angle were set using adamantane and glycine, respectively. The 13C CP/MAS NMR spectra were obtained at a spinning speed of 4.5 kHz using a 50 kHz spectral window with 6.1 Hz data points. The single contact pulse sequence used a 3 ms contact time. Scans of 14 000 were acquired with a 4 s recycle delay. The chemical shifts are reported relative to TMS with use of an external sample of adamantane as a reference.

3. Results and Discussion

2.2. Silica Support. The support was EP 10x silica gel from Crosfield Ltd., U.K. The average particle size, as reported by the manufacturer, was 100 µm. Silica was heat-treated at 450 or 600 °C for 16 h in a muffle furnace in air. 2.3. Characterization of Pure Silica and Aminosilylated Silica Samples by Nitrogen Adsorption. The BrunauerEmmett-Teller (BET) measurements were performed by nitrogen adsorption at 77 K. Prior to the measurements, silica and aminosilylated samples were degassed under vacuum for 3.5 h at 623 and 473 K, respectively. The BET surface area for pure silica was 300 m2/g, the average pore volume was 1.2 cm3/g, and the average pore diameter was 20 nm. For modified silica, the specific surface area and pore volume were found to be diminished, which will be further discussed in section 3.5.3. 2.4. Gas-Phase Deposition of Aminopropylalkoxysilanes on Silica. Precursors were deposited onto a porous silica support by a gas-phase technique, atomic layer deposition. Prior to the depositions, the support (10 g) was pretreated in an ALD reactor at 150-180 °C in order to remove physisorbed water molecules from the silica surface. The pressure in the ALD reactor (F-120, ASM Microchemistry Ltd., Espoo, Finland) was 20-50 mbar. The excess of precursors 1-3 needed for the reaction with silanol groups was vaporized at 90-130 °C (first step). The reaction temperature and reaction time were high enough to avoid condensation of the precursor molecules onto the pore walls. At a reaction temperature of 150 °C, the gas-solid reactions were surface-limiting, indicating typical ALD growth. Next the modified silica was treated with water vapor at a reaction temperature of 150 °C (second step) when free alkoxy groups were hydroxylated. These two steps (referred to here as one cycle) were repeated one to five times. For the removal of unreacted reactants, each reaction step was followed by a nitrogen purge at the reaction temperature. Regarding the precursor 1, the first hydrolysis step was performed with deuterium oxide at 150 °C in order to show the existence of unreacted silanol groups on silica. In addition, the hydroxylation of alkoxy groups on silica, modified with precursor 2, was studied in the ALD reactor by treating the aminosilylated sample with water vapor at 100300 °C. First silica was treated with 2 as described above, and next the aminosilylated silica was treated with water in the reactor at 100, 150, 200, 250, and 300 °C. 2.5. DRIFTS Measurements. The DRIFT spectra of the heattreated support and aminosilylated silica samples were measured in a Nicolet Magna IR 750 spectrometer equipped with a SpectraTech diffuse reflectance accessory. The spectrum obtained from a steel mirror was used as the background. The samples were prepared in air as quickly as possible to avoid additional hydrolysis of alkoxy groups. The measurements were made in a microsize sampling cup. The spectra were collected in the region of 4000-400 cm-1 at a 2 cm-1 resolution over 64 scans. 2.6. Carbon and Nitrogen Analysis. The carbon and nitrogen contents of aminosilylated silica samples were determined in the LECO CHN-600 elemental analyzer. Samples of 100-150 mg were weighed in tin capsules which were burned in oxygen at 950 °C for 150 s, whereupon organic matter was decomposed. The evolved combustion gases CO2 (and H2O) were monitored by infrared detectors. NOx gases were reduced to N2 which was quantitatively measured by thermal conductivity. 2.7. 13C CP/MAS NMR Analysis. Solid-state 13C crosspolarization/magic-angle spinning (CP/MAS) NMR spectra were recorded on a Bruker AMX 400 standard bore, high-resolution NMR spectrometer operating at 100.6 MHz. The powder samples were placed in zirconia rotors. The Hartmann-Hahn matching

3.1. Silica Surface. EP10x silica was used as the silica support because of its large average pore diameter, viz., 20 nm, which enabled migration of precursor molecules into the pores. Silica was heat-treated at a relatively low pretreatment temperature, that is, 450 °C, to maintain a large coverage of reactive surface groups, viz., silanol groups, on silica. When the silica was heat-treated at 450 °C, there were 2.1 vicinal, that is, H-bonded, OH groups per nm2 and 2.0 free, that is, isolated, OH groups per nm2 on the silica surface according to the 1H MAS NMR measurements.33 For the deuteration experiments, silica was heat-treated at 600 °C leaving on the surface 1.6 isolated OH groups/nm2 but only 0.5 H-bonded OH groups/ nm2. Both single and geminal silanols are included in the number of isolated silanols. 3.2. Deposition of Aminopropylalkoxysilanes on Silica. In our previous studies, the reaction of vaporized aminopropylalkoxysilanes with the porous silica surface was observed to be surface-limiting at a reaction temperature of 150 °C and at a pressure of 20-50 mbar.20,21 This was concluded on the basis of the very similar results of elemental analyses and identical DRIFT spectra of samples taken from the surface and bottom of the silylated silica bed.20 Aminosilane molecules were observed to interact with the silica surface through a site adsorption mechanism because after the reaction the band for isolated silanols approximately at 3750 cm-1 disappeared. After the silylation reaction, a strong and broad band next to that of free silanols at about 3740-3600 cm-1, with a peak minimum at 3674 cm-1, can be seen in the DRIFT spectrum (Figure 1). This band is likely to be due to both internal silanols and unreacted silanols, discussed in section 3.3, perturbed by hydrogen bonds with oxygen atoms of the alkoxy groups of aminosilanes.34 The most important IR vibrations for N-H and C-H bonds for aminosilylated silica are shown in Table 1. The effect of hydrogen-bonding interactions can also be observed as a perturbation of free N-H bands35 at 3376-3365 and 3301 cm-1 caused by hydrogen bonds between amino and unreacted silanol groups.6 The strongest stretching vibrations of C-H bonds are observed in the region 2979-2864 cm-1, and weaker bands approximately at 2900-2700 cm-1. Trifunctional aminopropylalkoxysilanes can be bound onto the silica surface through one, two, or three alkoxy groups. Aminopropylalkoxysilane precursors have previously been observed to react with isolated silanol groups of silica forming siloxane bonds.20 Both mono- and bidentate bonding modes of APTS (2) were observed on silica pretreated at 450 °C in recent gas-phase studies.6,21 Thus, there remain unreacted alkoxy groups on the surface which can be hydroxylated in the presence of water at an adequately high temperature resulting in free hydroxyl groups on the silylated surface. In addition to these OH groups, there are also silanol groups on the silica surface (33) Haukka, S.; Lakomaa, E.-L.; Root, A. J. Phys. Chem. 1993, 97, 5085. (34) Iiskola, E. I.; Timonen, S.; Pakkanen, T. T.; Ha¨rkki, O.; Lehmus, P.; Seppa¨la¨, J. V. Macromolecules 1997, 30, 2853. (35) Hertl, W. J. Phys. Chem. 1973, 77, 1473.

10604

Langmuir, Vol. 19, No. 25, 2003

Ek et al.

Figure 1. DRIFT spectra of APTS (2)-modified silica (heat-treated at 450 °C) with water treatments at 150 °C: (a) first APTS treatment, (b) first water treatment, (c) second APTS treatment, (d) second water treatment, (e) third APTS treatment, and (f) third water treatment. Table 1. The Most Intense Vibrational Frequencies and Their Assignments in the DRIFT Spectra of 1 and 2 on Silica Heat-Treated at 450 °C wavenumber/cm-1 assignmentsa

APTMS(1)

APTS(2)

ν (O-H)b νas (N-H) νas (N-H) νas (CH3) νas (CH2) νs (CH3) νs (CH2) δ (NH2)

3673 3376 3301 2979 2932 2889 2870 1592

3674 3375 3302 2978 2933 2904 2864 1594

a ν ) stretching, δ ) bending. b Broad band (peak minimum presented here) for silanols hydrogen-bonded to alkoxy groups of aminosilanes.

which have not been reacted with the precursor molecules. The presence of these unreacted hydroxyl groups is discussed in section 3.3. 3.3. Existence of Unreacted OH Groups on the Silica Surface. Due to steric hindrance or the presence

of strong hydrogen bonding, there are unreacted silanol groups under the adsorbed aminosilane groups on silica. These silanols can be observed by IR spectroscopy after treatment with deuterated water.20,36 In this way, all surface hydrogen atoms are exchanged with deuterium and at the same time alkoxy groups are hydroxylated and deuterated resulting in the formation of OD groups on the surface. The formation of deuterioxyl groups can be observed in the DRIFT spectrum (Figure 2) as the appearance of a small peak at 2760 cm-1.37 At the same time, a sharp peak corresponding to free OH groups at 3744 cm-1 appears in the DRIFT spectrum which can be assigned to undeuterated OH groups of silica under the alkoxy groups. These unreacted OH groups on silica pretreated at 600 °C can now be seen by IR because largersized alkoxy groups have reacted leaving smaller hydroxyl groups on the surface. At lower calcination temperatures (36) Iiskola, E. Second International Conference on Silica (SILICA 2001), extended abstract on CD-ROM. (37) Davydov, V.; Kiselev, A. V.; Zhuravlev, L. T. Trans. Faraday Soc. 1964, 60, 2254.

Deposition of a High-Density Network on Silica

Langmuir, Vol. 19, No. 25, 2003 10605

Figure 2. DRIFT spectra of APTMS (1) on silica (heat-treated at 600 °C) and after treatment with deuterated water (at 200 °C).

of silica, for example, at 450 °C, the appearance of this peak is not observed by IR because of strong hydrogenbonding interactions of silanol groups on silica. 3.4. Hydrolysis of Aminosilylated Silica Samples. Alkoxy groups of aminopropylalkoxysilanes are quite easily hydroxylated in the presence of water due to the self-catalytic nature of the amino group. Hydrolysis may begin at room temperature due to the humidity in air especially during longer periods of time. According to DRIFTS, an indication of the hydrolysis of methoxy groups of 1 on silica was not found until the sample was stored in air for over 24 h. Modified samples were assumed to remain unchanged during storage in a desiccator according to the measured DRIFT spectra. The hydroxylation reactions of silica samples modified with γ-aminopropyltriethoxysilane (2) were studied in the ALD reactor at elevated temperatures: 100, 150, 200, 250, and 300 °C. Hydroxylation of alkoxy groups can be seen as a decrease in the intensity of IR vibrations for methyl groups (-OCH2CH3) in the DRIFT spectra (Figures 1 and 3). This effect is most easily seen in the DRIFT spectra of the most intense methyl band at 2979-2978 cm-1. In addition, the appearance of a new band for free OH groups, that is, approximately at 3750 cm-1, can be

observed by DRIFTS at high pretreatment temperatures of silica, that is, at 600-800 °C. Nevertheless, this peak is not seen in this case at a low pretreatment temperature of silica, viz., 450 °C, because of strong hydrogen bonds between the hydroxylated alkoxy groups and silanol groups on silica. The band at 3750 cm-1 was observed to be absent also with experiments on deuterated water at low pretreatment temperatures of silica. An obvious decrease in the intensity of the above-mentioned methyl band of the ethoxy groups in the DRIFT spectra is clearly seen when the reaction temperature is raised from 100 to 150 °C. A distinct decrease in the intensity of signals for ethoxy groups at 58 and 15 ppm is observed also in the 13 C CP/MAS NMR spectra of 2 on silica after treatment with water at 150 °C compared to the sample before hydrolysis (Figure 4 and Table 2). Upon further heating to 200 °C, only a small number of ethoxy groups have further reacted according to DRIFTS but no further hydrolysis is observed at 250-300 °C. Also, according to the solid-state 13C NMR only very weak bands for ethoxy groups are seen after hydrolysis at 200 °C, but at 250 °C these bands have totally vanished (Figure 4). Heat treatments as high as at 300 °C should not be used for aminosilylated silica samples because distinct chemical

10606

Langmuir, Vol. 19, No. 25, 2003

Ek et al.

Figure 3. DRIFT spectra of (a) APTS (2) on silica (heat-treated at 450 °C) and treatment of modified samples with water at reaction temperatures of (b) 100 °C, (c) 150 °C, (d) 200 °C, (e) 250 °C, and (f) 300 °C.

changes of aminoalkoxysilanes on silica, for example, decomposition, are likely to occur.20,21 According to Linde and Gleason, APTS species on silica are observed to decompose upon heat treatment in air at temperatures higher than 200 °C, resulting in the oxidation of methylene groups in the propyl chain and leading during continued air oxidation to the formation of silicon dioxide.38 In the present study, the chosen temperature for the hydroxylation of alkoxy groups was kept as low as possible to avoid condensation reactions between adjacent silanol groups, that is, cross-polymerization reactions. Thus, the temperature for the hydrolysis step was chosen to be 150 °C. According to the elemental analyses on aminosilylated silica samples after hydrolysis, the C/N ratio was observed to be near 3.0 (Table 3) indicating quite complete hydroxylation of alkoxy groups for trifunctional precursors because only the propyl chain remains on the samples after hydrolysis. However, as discussed above, the temperature of 200 °C under reduced pressure would be adequate to achieve complete hydrolysis of aminoalkox(38) Linde, H. G.; Gleason, R. T. J. Polym. Sci. 1984, 22, 3043.

ysilanes. The most convenient way to achieve complete hydrolysis of alkoxy groups in the gas phase is to raise the reaction temperature. The reaction time for hydrolysis can also be extended, which however leads to longer running times. 3.5. Repetition of Aminopropylalkoxysilane/Water Cycles. A simplified description of the sequential deposition of aminosilane and water vapor on the silica surface is shown below in Scheme 1. The surface was simplified by exhibiting a planar surface with isolated silanols, while the H-bonded silanols are excluded from the surface for simplicity. The DRIFT spectra of the first, second, and third aminosilane/water cycles on silica are shown in Figure 1. Because of the similarity of the DRIFT spectra of samples after the fourth and fifth cycles to the spectrum after the third cycle, those spectra are not shown here. 3.5.1. Aminosilane/Water Cycles with APTMS (1) or APTS (2) as a Precursor. When consecutive γ-aminopropylalkoxysilane/water cycles were repeated, the nitrogen contents in the APTMS (1)- and APTS (2)modified samples distinctly increased (Table 3). The numbers of nitrogen and carbon atoms on silica modified

Deposition of a High-Density Network on Silica

Langmuir, Vol. 19, No. 25, 2003 10607 Scheme 1

Figure 4. 13C CP/MAS NMR spectra of (a) APTS (2)-modified silica (heat-treated at 450 °C) and treatment with water at (b) 150 °C, (c) 200 °C, and (d) 250 °C. Table 2.

cycles, surface densities up to 3.0 APTMS or APTS/nm2 on silica pretreated at 450 °C were achieved. When aminosilane/water cycles were performed on silica pretreated at 600 °C, a somewhat lower amino group content was achieved, viz., 2.3 amino groups/nm2 after three cycles, due to the smaller number of silanol groups on silica. The sequential increase and decrease in the number of carbon atoms per nm2 of silica during APTS/water cycles are clearly seen in Figure 5 and in Table 3. With this procedure, the amino group content on silica can be controlled through the number of aminosilane/water cycles in a reproducible way. The 13C CP/MAS NMR spectra of the modified silica samples after aminosilane (2) and water steps are shown in Figure 6. In the 13CP/MAS NMR spectrum of 2-modified silica, five signals can be seen: two signals of unreacted ethoxy groups at 58 ppm (-OCH2-) and 15-16 ppm (-CH3) and three signals due to the propyl chain (H2NCH2(3)CH2(2)CH2(1)Si) at 44 ppm (C3), 27 ppm (C2), and 8-9 ppm (C1) (Table 2).1,6 In our spectra, the decrease

13C

CP/MAS NMR Chemical Shifts (δ) for APTS (2)- and APDMS (3)-Modified Silica chemical shifts (δ)

NH2-CH2-CH2-CH2-Si

Si-OCH2CH3

precursor 2 3

Si-CH3 44 44

27 26

9 13

58 (58)

16 (16)

-4

with 2 are also depicted in Figure 5. A somewhat larger number of nitrogen atoms was obtained with precursor 2 than with 1 after the first cycle. However, after several aminosilane/water cycles equal nitrogen and carbon densities were achieved with both precursors. The largest increase was observed during the first cycle, but the nitrogen content still increased during the next three or four cycles. The fifth aminosilane/water cycle did no longer distinctly increase the nitrogen content. After four or five

Table 3. Surface Densities of Nitrogen and Carbon Atoms on Silica (Pretreated at 450 °C) in Depositions Performed with Sequential Reactions of Vaporized Aminosilanes and Water (at Reaction Temperatures of 150 °C) APTMS (1)

APTS (2)

APDMS (3)

steps

N atoms/nm2

C atoms/nm2

C/N ratio

N atoms/nm2

C atoms/nm2

C/N ratio

first aminosilane first water second aminosilane second water third aminosilane third water fourth aminosilane fourth water fifth aminosilane fifth water

1.7 1.7 2.3 2.3 2.7 2.7 2.9 2.9 3.0 3.0

7.6 5.5 7.5 6.9 8.5 8.1 8.8 8.9 9.2 9.0

4.6 3.3 3.3 3.1 3.2 3.0 3.1 3.1 3.1 3.0

1.9 1.9 2.3 2.4 2.7 2.7 2.9 2.9 3.0 3.0

7.1 5.2 8.4 6.8 8.8 7.8 8.7 8.5 8.9 8.7

3.9 2.8 3.7 2.9 3.2 2.9 3.0 2.9 3.0 2.9

N atoms/nm2

C atoms/nm2

C/N ratio

2.1 2.0 2.1 2.1 2.1 2.0

7.1 6.0 7.6 7.4 8.4 7.9

3.5 3.1 3.7 3.6 4.0 4.0

10608

Langmuir, Vol. 19, No. 25, 2003

Ek et al.

Figure 7. 13C CP/MAS NMR spectra of (a) APDMS (3)-modified silica (heat-treated at 450 °C) and (b) after treatment with water (i.e., first cycle). Chart 2

Figure 5. Effect of APTS/water cycles on the surface density of nitrogen and carbon atoms on silica pretreated at 450 °C.

Figure 6. 13C CP/MAS NMR spectra of APTS (2)-modified silica (heat-treated at 450 °C) with water treatments: (a) first APTS treatment, (b) first water treatment, (c) second APTS treatment, (d) second water treatment, (e) third APTS treatment, (f) third water treatment, (g) fourth APTS treatment, (h) fourth water treatment, (i) fifth APTS treatment, and (j) fifth water treatment.

and increase in the intensity of bands at 58 and 15-16 ppm for carbon atoms of the ethoxy groups are seen during the hydrolysis and aminosilane steps, respectively. Hydroxylation of the ethoxy groups for the aminosilylated

silica samples during water treatment was clearly observed also in the bands for methyl groups in the DRIFT spectra (Figure 1) and in the number of carbon atoms (Table 3 and Figure 5), as discussed above. After the fourth deposition step of APTS on silica, only very weak peaks of carbon atoms for ethoxy groups appear in the solidstate NMR spectrum. Thus, after four APTS/water cycles the surface seems to be saturated and maximum amino group density achieved. After all the reactive sites, that is, OH groups, are consumed, the surface is saturated and the maximum amino group density is achieved. This seems to happen by performing four APTS/water cycles at deposition temperatures of 150 °C (Figures 5 and 6). In the end, the repetition of aminosilane/water cycles results in the formation of a high-density aminopropylsiloxane network on silica which is assumed to grow through horizontal polymerization of silane molecules because of the saturation achieved. An ideal scheme of a dense aminoterminated siloxane network is shown in Chart 2. 3.5.2. Aminosilane/Water Cycles with APDMS (3) as a Precursor. Aminosilane/water cycles were also performed with the bifunctional precursor APDMS (3), but no remarkable increase in the nitrogen content on silica was observed after three APDMS/water cycles as the nitrogen content remained constant at 2.0-2.1 N/nm2. This is due to the bidentate attachment of 3 on silica,20,39 in which case there are no unreacted ethoxy groups on the surface and thus hydrolysis cannot occur. As a consequence, no decrease in the intensity of methyl groups of ethoxy groups during hydrolysis could be observed in the DRIFT spectra of 3 on silica. The bidentate attachment of 3 on silica can also be seen in the 13C CP/MAS NMR spectrum (Figure 7a). Only four signals are visible in the 13C CP/MAS NMR spectrum of precursor 3 on silica: three signals due to the propyl chain (H2NCH2(3)CH2(2)CH2(1)Si) at 44 ppm (C3), 26-27 ppm (C2), and 13 ppm (C1) and the signal of methyl groups bound to silicon at -4 ppm (Table 2). Only a very weak signal for free ethoxy groups at 58 ppm (-OCH2-) and 16 ppm (-CH3) can be seen in the spectrum indicating the reaction of both ethoxy (39) Ek, S.; Iiskola, E. I.; Niinisto¨, L.; Vaittinen, J.; Pakkanen, T. T.; Root, A. Langmuir, submitted.

Deposition of a High-Density Network on Silica

Langmuir, Vol. 19, No. 25, 2003 10609

Table 4. BET Parameters of Pure Silica and Silica after 1, 3, and 5 Aminosilane/Water Cycles surface area/m2/g

pore volume/cm3/g

average pore diameter/nm

cycles

2

3

2

3

2

3

pure silica first aminosilane/water third aminosilane/water fifth aminosilane/water

300 273 244 234

300 251 241

1.2 1.1 1.0 0.9

1.2 1.1 1.1

20 20 19.5 19

20 20 20

groups with the surface silanols, that is, bidentate bonding of 3 on silica. No hydroxylation for this small number of ethoxy groups seems to have occurred because this weak band still remains unchanged after hydrolysis (Figure 7b). The above-mentioned assignments for chemical shifts of 3 on silica were based on the solid-state 13C NMR studies for the monofunctional precursor, γ-aminopropyldimethylethoxysilane (APDMES),1,7,40 because no reports on the 13 C NMR studies of silica modified with 3 were found in the literature. 3.5.3. Effect of APTS or APDMS/Water Cycles on the Specific Surface Area and Pore Volume of Silica Samples. The BET surface area and pore volume were observed to decrease 8-9% after one APTS (2)/water cycle, respectively (Table 4). The most remarkable diminishment of the specific surface area is just observed after the first aminosilane/water cycle and only to a smaller extent after the third and fifth cycles. With the bifunctional precursor APDMS (3) on silica, the BET surface area was also reduced by 16% after the first aminosilane/water cycle but after three cycles the change was only 4% compared to the surface area after the first cycle, whereas with trifunctional 2 the decrease in surface area was as much as 11% after three cycles. This small decrease in the specific surface area for precursor 3 after the third cycle supports the conclusion that a high-density aminopropylsiloxane network cannot be deposited by performing several APDMS/water cycles. A more notable diminishment of BET parameters has been observed in the aminosilane samples prepared in the liquid phase on silica41,42 and porous silica glass.43 When 3-aminopropyldimethylmethoxysilane (APDMMS)41 or APTS42 was deposited onto wide pore size silica in the liquid phase, resulting in a surface density of 1.7-2.0 N atoms/nm2, the BET surface area and pore volume, which were initially 344-354 m2/g and 0.91-1.14 cm3/g, respectively, were observed to decrease 14-25%.41 In our study, the overall decrease of specific surface area and pore volume after five APTS (2)/water cycles was about (40) Kallury, K. M. R.; Macdonald, P. M.; Thompson, M. Langmuir 1994, 10, 492. (41) Gun’ko, V. M.; Sheeran, D. J.; Augustine, S. M.; Blitz, J. P. J. Colloid Interface Sci. 2002, 249, 123. (42) Murphy, E. F.; Ferri, D.; Baiker, A.; Van Doorslaer, S.; Schweiger, A. Inorg. Chem. 2003, 42, 2559. (43) Takei, T.; Yamazaki, A.; Watanabe, T.; Chikazawa, M. J. Colloid Interface Sci. 1997, 188, 409.

equal compared to results of the study performed in the liquid phase by Gun’ko et al.41 The average pore diameter was observed to increase 5-6% in both liquid-phase studies, but in the present gas-phase study after five cycles the pore diameter decreased 5%. However, no change in the average pore diameter of APTS (2) (and also of APDMS (3)) was observed after the first cycle. With precursor 2, a minor decrease in average pore diameter was seen after the following four cycles. 4. Conclusion According to our previous studies,20,44 the surface density of amino groups on silica can be controlled through the appropriate selection of an aminopropylalkoxysilane precursor or heat-treatment temperature of silica. However, successful control of the surface density of amino groups on silica in a larger scale can be performed with the presently developed gas-phase procedure involving sequential reactions of trifunctional aminopropylalkoxysilanes and water. By increasing the number of aminopropyltrialkoxysilane/water cycles from one to four (or five), the amino group content could be increased from 1.7 or 1.9 to 3.0 amino groups/nm2 of silica. In addition, exceptionally high surface coverage of amino groups on silica was achieved with trifunctional APTS (2) and APTMS (1) because the repetition of four or five aminosilane/water cycles resulted in surface densities of up to 3.0 amino groups/nm2 on silica. This high surface density of amino groups on porous silica or planar silicon has not been reported earlier in the literature for gas-phase studies. Furthermore, the above-described procedure enables the preparation of a well-ordered aminopropylsiloxane network on a porous silica surface in a solventfree process. These results can further be applied for the depositions on planar substrates. On the other hand, the amino group content of bifunctional APDMS (3) on silica could not be notably increased with this procedure because of the lack of free alkoxy groups due to the bidentate bonding mode of the precursor molecules on silica. The hydroxylation reactions of γ-aminopropylalkoxysilanes deposited onto silica were also studied in the gas phase. Most of the free alkoxy groups of aminosilanes on silica were observed to be hydroxylated at 150 °C, but further hydrolysis was still observed upon heating to 200 °C according to DRIFTS and 13C CP/MAS NMR. Acknowledgment. Financial support for S.E. and E.I. from the Fortum Foundation and for S.E. from the Jenny and Antti Wihuri Foundation is gratefully acknowledged. LA035472G (44) Ek, S.; Iiskola, E. I.; Niinisto¨, L. In Chemical Vapor Deposition XVI/EUROCVD 14, Proceedings of the International Symposium; Allendorf, M. D., Maury, F., Teyssandier, F., Eds.; The Electrochemical Society: Pennington, NJ, 2003; Vol. 1, pp 22-29.