J Mater Sci (2015) 50:7159–7181 DOI 10.1007/s10853-015-9281-9
REVIEW
Silylation from supercritical carbon dioxide: a powerful technique for modification of surfaces Deniz Sanli1 • Can Erkey1,2
Received: 24 February 2015 / Accepted: 20 July 2015 / Published online: 18 August 2015 Ó Springer Science+Business Media New York 2015
Abstract Silylation is one of the most frequently employed surface-functionalization techniques. Silylation of surfaces from supercritical CO2 (scCO2) solutions, which is carried out by exposing the surface to a solution of a silane-based modifying agent dissolved in scCO2, has been attracting increased attention due to its numerous advantages over the conventional silylation techniques which utilize liquid solutions or vapor phase. Besides being a green and environmentally friendly route, silylation using scCO2 provides solvent-free materials after processing, enhanced diffusion and mass-transfer rates, faster reactions, homogenous and uniform surfaces, and control over the properties of the surface. Such advantages have led to many interesting studies on the development of novel scCO2-based silylation technologies in various fields ranging from porous materials to microelectronic processing, and from thin films to nanocomposites. In this article, we give an overview of the fundamental aspects of silylation from scCO2 and summarize the studies in the literature in various fields.
& Can Erkey
[email protected] 1
Department of Chemical and Biological Engineering, Koc¸ University, Rumeli Feneri Yolu, 34450 Sariyer, Istanbul, Turkey
2
Koc¸ University Tu¨pras¸ Energy Center (KUTEM), Koc¸ University, 34450 Sariyer, Istanbul, Turkey
Introduction The presence of a critical point was first recognized in 1869 by Thomas Andrews who at the time was probably not aware that his discovery would pave the way to the development of many industrial processes which use supercritical fluids (SCFs) [1]. A SCF can simply be defined as a fluid that is heated up and compressed to above its critical temperature and pressure. SCFs have liquid-like viscosities and gas-like densities; hence, they can be considered as hybrid solvents. One important characteristic of SCFs is that their properties can be tuned with small variations in temperature and/or pressure without crossing any phase boundaries. Thus far, SCFs have been successfully utilized in many large-scale industrial processes [2]. The majority of the established SCF plants are utilized for food-related applications including coffee and tea decaffeination and color, flavor, aroma, spices, nicotine, and hops extraction [2–4]. There are currently around 300 supercritical extraction plants worldwide. Besides extraction, SCFs have also been employed in large-scale material-processing applications including wood impregnation and catalyst regeneration [2]. Moreover, an important large-scale SCF drying process was developed and implemented by Aspen Aerogels to produce aerogel blankets on a commercial scale. Another area that may benefit from the favorable properties of SCFs is surface modification, which is used in many fields to incorporate new physical, chemical, and/or biological characteristics to surfaces [5, 6]. Different methods can be employed for the modification of surfaces to alter a wide range of functional properties such as roughness, hydrophilicity, surface charge, surface energy, electronic, magnetic, mechanical properties, corrosion resistance, biological functionality, and reactivity. Among
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others, silylation, i.e., the introduction of a functional group to a surface by using silane-based agents, is one of the most frequently utilized surface modification techniques. Silylation of the surfaces can be performed using supercritical CO2 (scCO2), which provides solvent-free materials after processing, enhanced diffusion and mass-transfer rates, faster reactions, homogenous and uniform surfaces, and control over the properties of the surfaces. Such advantages have led to many interesting studies on the development of novel scCO2-based silylation technologies in various fields ranging from porous materials to microelectronic processing, and from thin films to nanocomposites. In this article, we give an overview of the fundamental aspects of silylation from scCO2 and summarize the studies in the literature in various fields.
Conventional silylation and silylation chemistry There are basically two conventional routes that are followed for silylating a surface. First route is to expose the surface to the vapor of a silane agent at high temperatures [7–9]. One requirement for this technique is that the silylation agent should have a high vapor pressure at the working temperatures. It was reported that silanes that have vapor pressure [5 torr at 100 °C have achieved commercial utilization [8, 10]. In vapor-phase treatment, the temperature of the substrate should be held between 50 and 120 °C to promote the reaction with the silane agent. Alternative to high temperature, vacuum can also be applied to enable the vaporization of the silane agent. Cyclic azasilanes were reported to deposit quickly within 5 min, while aminosilanes can also provide fast deposition (within 30 min) without a catalyst. Other silanes require prolonged reaction times (4–24 h) [10]. Hence, the vaporphase treatment can only be employed with a limited number of silylation agents. The second route is the modification from the liquid phase, which is performed by contacting the surface with a liquid solution of the silylation agent [9, 11–13]. This route is usually employed at low temperatures, which can be advantageous in terms of energy savings. However, at these low temperatures, there is usually no direct reaction between the silylation agent and the surface groups [14, 15]. Hence, a subsequent curing step is usually employed to facilitate the reaction of the silane and the substrate surface. During such so-called indirect routes, the silylation agents can undergo self-polymerization before reaching and reacting with the surface groups, which reduces the effectiveness of the technique, and limits the control over the silane concentration on the surface giving rise to nonuniform surfaces. The competition between the surface reaction and self-polymerization is dictated by various
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factors including type of solvent and organosilane, working temperature, and the amount of water adsorbed on the surface. Different methodologies can be followed for the deposition of silanes from liquid phase. The most commonly used technique is the deposition from aqueous alcohol solutions. Silylation agent is hydrolyzed in an alcohol–water mixture, and the substrate is dipped into that solution. After the exposure, the substrate is rinsed with alcohol and then cured at high temperatures. A similar method is the deposition from aqueous solution; however, this method requires silane agents to be soluble in water. Due to poor solubility parameters, long chain alkyl silanes and aromatic silanes are not suitable for this technique [10]. Bulk deposition onto powders can also be performed by means of spray-on method, where silanes are deposited from alcohol solutions. However, this method requires that the powders have sufficient moisture content to hydrolyze the silane reagents. Anhydrous deposition of chlorosilanes, methoxysilanes, aminosilanes, and cyclic azasilanes can be employed for the deposition onto small particles or nanofeatured substrates. Solutions of silanes in toluene, tetrahydrofuran, benzene, and carbon tetrachloride are prepared, and the substrates are treated for 12–24 h. Subsequently, washing with the solvent is performed. However, this method can be cumbersome and require rigorous control [8, 10]. Regardless of the technique employed, the underlying mechanism for silylating a surface is the same. Silanes, especially organosilanes of the form RnSiX4-n are generally employed as reagents, where R is a non-hydrolyzable organic group that possesses the functionality, and X is either a halogen, alkoxy, acyloxy, or amino group [10, 16, 17]. When X is a halogen or an amino group, the silane reagent can readily react with the surface hydroxyl groups of the substrates. On the other hand, a priori hydrolysis of the alkoxy and acyloxy groups is required. Water, either adsorbed on the surface or present at trace amounts in the silane solution, can be used to convert the alkoxy or acyloxy organosilane to organosilanol (RnSi(OH)4-n) which then reacts with the appropriate surface groups. Alkoxy
Fig. 1 Silylation of inorganic materials with alkoxy or acyloxy silanes
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groups can also be directly attached to the surface at high temperatures without necessitating hydrolysis. Figure 1 depicts the general mechanism of silane coupling on inorganic materials, whereas Fig. 2 displays the anhydrous deposition of alkoxy silanes. It should be noted that silylation agents having hydroxyls (–OH) as X groups are also commercially available and utilization of such agents also eliminates the need for hydrolysis reactions since the hydroxyl groups can readily react with the surface groups. In addition, cyclic azasilanes can also readily react with the surfaces containing hydroxyl groups by ring-opening reaction. The deposition of cyclic azasilanes on substrates is particularly interesting since the reaction can be performed rapidly at low temperatures without requiring water and with no byproducts. As a result of these reactions, covalent attachment of the organosilane to the surface is achieved where the organic group, R, extends from the surface and imparts the desired functionality [7, 10, 16]. The reactive groups of the silanes are of prominent importance in silylation of the surfaces. Prior to surface modification, the surface chemistry should be specifically evaluated, and the appropriate modifying agent should be chosen, accordingly. The reactivity of silylating agents varies depending on the reactive groups they possess. Hydroxyl groups are the most reactive moieties followed by phenol, carboxyl, amino, and mercapto groups as shown in Fig. 3 [18]. On the other hand, the type of the R group determines the final physical and chemical properties of the surface, and thus, during the selection of the silylation
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Fig. 3 Reactivity of silylating agents containing different functional groups [18]
agent, the desired material properties should also be considered carefully. The most widely utilized organosilanes have alkyl groups as R group, and they are used to make the substrates water repellent by altering the surface energy or wetting characteristics [19]. Methyltrimethoxysilane, methyltriethoxysilane, ethyltriethoxysilane, methyltrichlorosilane (MTCS), and ethyltrichlorosilane are the most common examples of such silylation agents. When the terminal group is bromine, SCN, N3, and S terminal groups can be obtained by reactions with KSCN, NaN3, and Na2S, respectively. Being highly reactive, surfaces silylated with amine functional groups can readily react with numerous chemical groups (e.g., Cl–, ClCO–, OCN–, OH–, H2N–, HOOC–, HOSO2–) which leads to a wide variety of surface characteristics. Surfaces silylated with amine functional groups are also utilized in the immobilization of enzymes and adsorption of proteins [20]. Silylation agents having cyanide functional groups can additionally be utilized in functionalization of the surfaces. These cyanide groups can be converted into amine groups which can then be used in the attachment of b-cyclodextrins [21].
The role of surface groups
Fig. 2 Anhydrous deposition of alkoxy silanes [10]
Regarding the effectiveness of the silylation process, the surface chemistry of the substrates to be modified plays a key role. All silylation agents can form stable siloxane bonds with the oxides of silicon, aluminum, zirconium, tin, titanium, and nickel as a result of the condensation reactions, whereas less stable bonds are formed with oxides of boron, iron and carbon [10]. The effectiveness of silylation agents on different substrates are summarized in Fig. 4, which represents the strength of the covalent bonds that silylation agents form with surface groups of different materials [10]. When metal oxides are used as the substrate, almost all of the silylation reactions utilize surface hydroxyl groups. Metal oxide substrates can possess different types of surface hydroxyl groups, and the efficiency of the silylation reactions depend on the type and concentration of these hydroxyl groups [22, 23]. Among other substrates, the structural properties and chemistry of silica surfaces have been frequently studied, and three different types of hydroxyl groups termed isolated, vicinal, and geminal were revealed to exist on silica surface, which are displayed in Fig. 5 [23–25]. Although both isolated and
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chlorotrimethylsilane (CTMS) reacts only with the isolated hydroxyl groups on the surface [27]. Tripp and Hair also studied the reactions of TCMS, CTMS, and DCDMS with the silica surface by employing transmission infrared spectroscopy and revealed valuable information about the reaction mechanisms and the role of surface –OH groups of silica [7]. These studies remark the significance of the role of different types of surface groups. Besides, surface hydroxyl groups also play a determinant role in the acidity and basicity of the surfaces. Surface hydroxyl groups may act as Brønsted acid sites having the ability to protonate the adsorbed basic species by dissociation [28].
Silylation from supercritical CO2
Fig. 4 Effectiveness of silylation agents on inorganic materials [10]
Fig. 5 Types of silanol groups on amorphous silica. Reproduced from Ref. [24] with permission from Elsevier Science B.V.
geminal groups are considered as reaction sites, the reactivity of isolated silanol groups is greater than the geminal silanols. On the other hand, the vicinal silanol groups are considered as the least reactive sites since two hydroxyl groups are bound to each other with H-bonds [26]. In the work of Amistead and Hockey, trichloromethylsilane (TCMS) was reported to react with both hydrogen bonded and isolated hydroxyl groups. In the same study, it was also revealed that dichlorodimethylsilane (DCDMS) and
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Silane-based compounds are known to have substantial solubility in scCO2, which enables silane–scCO2 mixtures to be easily employed in surface modification. In this technique, the surface is exposed to a solution of silane agent dissolved in scCO2. ScCO2-based processes possess numerous advantages over the conventional routes that are used to silylate surfaces. Besides being nonflammable, nontoxic, and naturally abundant, scCO2 exhibits various desired physical and chemical properties. ScCO2 is relatively chemically inert, has no surface tension, excellent wetting characteristics, and highly tunable solvent behavior, facilitating easy separation of the products. Moreover, scCO2 has low viscosity and high diffusivity, which are favorable since such properties reduce the diffusion and mass-transfer limitations, especially when porous materials are of interest. Among the main advantages of silylation from scCO2 over conventional techniques are the high diffusion and fast penetration rates provided by the low viscosity and high diffusion coefficients of scCO2, which result in enhanced mass-transfer rates and enable faster reactions. Another important feature is that, CO2 has relatively mild critical temperature and pressure (Tc = 304.1 K, Pc = 73.8 bar), which can be accessed easily on an industrial scale. In addition, the final material properties can be fine tuned by simply adjusting the processing parameters such as pressure and/or temperature when silylation from scCO2 is employed. In Fig. 6, the P–V diagram of pure CO2 is displayed. The density of scCO2 close to critical point is highly sensitive to pressure, and therefore small changes in pressure can lead to large changes in density and thus the solvation power of scCO2. Another prominent advantage is that one can achieve the uniformity of the introduced characteristics throughout the entire surface when scCO2-based processes are employed. In the case of graded surfaces, the aforementioned uniformity is only desired for a definite region, which can be accomplished merely by a controlled process. Since
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Fig. 6 P–V diagram for pure CO2 where single- and two-phase regions are emphasized along with the supercritical region. The dashed lines represent the isotherms. Reproduced from Ref. [29] with permission from Elsevier Science B.V.
achieving homogenous surface characteristics is the biggest challenge for conventional techniques, scCO2-based surface silylation becomes an intriguing alternative affording easy control over the process. Due to its large quadrupole moment, CO2 may establish strong molecular interactions with different surfaces depending on the surface chemistry, which in turn may play a dominant role in silylation processes. The molecular interactions of CO2 with various surfaces such as silica– alumina and c-alumina [30], a-alumina [31], zirconia, and magnesium oxide surfaces [32–34] were investigated in several studies thus far. Although weakly, CO2 is known to adsorb on silica surfaces due to dispersion and electrostatic forces and form Si–OHd?d-O=C=Od- moieties with the surface hydroxyl group [35, 36]. In addition, although the solubility of water in scCO2 is very low, it was demonstrated that scCO2 extracts residual water that is adsorbed on the silica surface, exposing the isolated silanol (Si–OH) groups. In 2006, Gu and Tripp studied the reaction of hexamethyldisilizane (HMDZ) and TMCS with TiO2 and Al2O3 in scCO2 [37]. They revealed that the interactions of scCO2 with TiO2 and Al2O3 result in removal of water layer and the formation of carbonate, bicarbonate, and carboxylate species on TiO2 surface, and carbonate species on Al2O3 surface. Formation of such carbonate species poisoned the surface and impeded the reaction of silylation agents with the surface groups. However, the effects of such CO2–surface interactions remain unclear, and it is highly probable that they play an essential role by means of altering the reactivity of the surface groups and their presentation to the silylation agents during the surface modification processes [38].
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In silylation of surfaces from scCO2, CO2 acts as the solvent and reaction medium that dissolves silylation agent and makes the silylation agent establish contact with the surface to be modified. As a solvent and reaction medium, CO2 should be inert against the silylation agent and the surface to be modified, i.e., it should not react neither with the silylation agent nor with the surface groups. Being nonpolar and nonreactive, CO2 does not react with most of the silanes and the chemical groups on the surface of materials, allowing for retention of the reactivity of the silanes and surface groups throughout the silylation process. However, in case of amino silanes, CO2 reacts with the amine group to form carbamates, which results in loss of reactivity of the reagent. Therefore, the inertness of CO2 against the silylation reagent and the surface groups of the substrate should be considered carefully in order to carry out an effective silylation process. Water that may be produced as a result of silylation reactions can either be adsorbed on the substrate surface or extracted with CO2 if it is present in trace amounts. Silylation of surfaces of porous materials can be performed by following a few basic steps. First of all, the silane molecules are dissolved in scCO2, the binary mixture of silane–CO2 then diffuses through the material and reaches to the reaction site on the surface, and finally the silane reacts with surface groups of the material. These steps can be generalized as the dissolution, external mass transfer, internal diffusion, adsorption, and surface reaction and are represented schematically in Fig. 7. In the case of nonporous materials, the diffusion step is eliminated since there are no pores for the molecules to diffuse through, and as a result, the mass-transfer step reduces to only external mass transfer, which is the transfer from bulk fluid to the surface. Although the mechanism of silylation is not fully understood, it is well known that the mass-transfer and reaction rates govern the overall surface modification process. The overall rate of the silylation process is determined by the slowest step, namely, rate-limiting step, which can be either diffusion or surface reaction. The time needed to achieve a desired level of silylation thereof can be identified considering the rate-limiting step. There are several factors that affect the mass-transfer and reaction rates to some extent, the most important ones being the solubility of the silane in scCO2, molecular weight and the functional groups of the silane, concentration, temperature, and pressure. Considering all those factors, one can conclude that the overall efficiency of silylation of surfaces from scCO2 depends on several parameters such as types of surface and silylation agents; concentration and type of surface groups; reactivity of silylation agents against specific surfaces; diffusion and mass-transfer coefficients; viscosity, temperature, pressure, concentration, and molecular weight of
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Fig. 7 Fundamental steps of silylation of porous materials with scCO2
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the silylation reagent. Moreover, some of these parameters also depend on each other. Therefore, prior to the modification of a specific surface, all of these parameters should be carefully considered in order to achieve the desired properties of the surface in an effective manner. Thus far, there have been numerous studies in various fields that involve silylation of different substrates from scCO2. Table 1 lists the substrates and silylation agents that were employed in such studies in the literature, so far.
Thermodynamics of silane–supercritical CO2 binary mixtures In silylating the surfaces from scCO2, one of the most important parameters is the concentration of the silane in scCO2. From the kinetic point of view, a rate of a reaction depends on the concentration of the reactants at the surface or the concentration of the reactants in the fluid phase or both. In a simplistic manner, the effect of concentration on reaction rate is determined by the order of reaction on reactants. The concentration of the silane reagent in the fluid phase at a specific temperature and pressure is therefore a crucial parameter that determines the amount of silane groups introduced to the surfaces. With the increasing concentration, the probability of the silane molecules coming across the surface groups to react increases. The solubility of silane reagent in scCO2 is essential, since such information allows for the determination of the maximum concentration of the silane reagent in scCO2 during the surface modification process. The solubility is one of the key aspects for controlling the surface modification process, which would allow attaining the desired surface characteristics. The solubility of silanes in scCO2 can be acquired from the phase behavior measurements of silane–CO2 binary mixtures which provide the information about the vapor liquid equilibrium (VLE) or single-phase and two-phase regions of the mixtures. The amount of a certain silane that can be dissolved in scCO2 at a specific pressure and temperature can be determined from such phase behavior data, and the desired silane concentrations can be obtained by changing the temperature and pressure based on the solubility information. The phase behavior data of silanes in scCO2 enables the tuning of pressure, temperature, and concentration conditions in such a way that the desired degree of silylation by means of grafting density, number of silane molecules, and thickness of the silane layer on the surface can be achieved. The phase behavior studies of binary mixtures are generally supported through modeling of the experimental data by means of appropriate equation of states. For binary mixtures of CO2 and compounds with a small molecular
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weight, the most widely utilized equation of state is the Peng–Robinson equation of state which provides accurate predictions of the thermodynamic data of such binary mixtures [81]. Sanchez–Lacombe equation of state is another commonly used equation of state which is derived based on the Flory’s lattice fluid model and is appropriate for describing CO2–polymer systems [82]. Such equations of state have at least one binary interaction parameter, which defines the molecular interactions between the components of the binary mixtures. The binary interaction parameters are regressed from the experimental phase behavior data such that they can predict the whole phase region for a specific mixture with high enough accuracy. The knowledge of the binary interaction parameter of a specific binary system allows for estimation of different regions of the global phase diagram without the need for any experimental data. HMDZ has been one of the most commonly utilized silylation reagents thus far, mostly for rendering the surfaces hydrophobic. HMDZ has two tri(methyl)silyl (–Si(CH3)3) groups bound to a central amine moiety. The frequent usage of HMDZ is due to its high reactivity against oxide surfaces and surfaces that have hydroxyl groups. The oxygen atom of the surface hydroxyl group interacts with the silicon of HMDZ that is bound to the amine moiety, and the substitution reaction occurs by replacing the H of the surface –OH group with –Si(CH3)3 group of HMDZ. As a result, hydrophilic surfaces can be modified to be hydrophobic, and even super-hydrophobic surfaces. Of course, the degree of hydrophobicity depends on the efficiency of the surface reaction, which in turn is related to the processing conditions, namely, temperature, pressure, and HMDZ concentration. The optimal processing conditions to attain a certain degree of hydrophobicity can be determined based on the thermodynamic information about the HMDZ–CO2 binary mixture. The phase behavior of HMDZ–CO2 binary mixture was first studied by Kartal and Erkey in 2010 for a wide range of composition and at four temperatures [73]. The two-phase and single-phase regions for the HMDZ–CO2 binary mixture were investigated with the bubble point measurements up to 11 MPa. It was found that HMDZ and CO2 formed miscible mixtures at low temperatures and low pressures. It was demonstrated that the bubble point pressures increased with the increasing temperature and CO2 mol fractions. The experimental data were modeled using Peng–Robinson Stryjek–Vera equation of state, and the binary interaction parameters were regressed. Figure 8 displays the experimental bubble point data together with the model results, which, indeed, demonstrates the phase envelopes for HMDZ–CO2 binary mixtures. With such P–T–x diagram, one can, for instance, determine the pressure to be attained at a specific
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Table 1 Summary of the literature studies that involve silylation from scCO2 Substrate Oxidized Si
Silylation agent
References
Aminopropyltrimethoxysilane (APTMS)
[39]
Mercaptopropyltrimethoxysilane (MPTMS) Aminobutyltrimethoxysilane (ABTMS) Aminoethylaminopropyltrimethoxysilane (AEAPTMS) Si wafer Polystyrene (PS) poly(methylmetacrylate) (PMMA) Fumed silica
(Tridecafluoro-1,1,2,2tetrahydrooctyl)dimethylchlorosilane (FDCS)
[40]
Hexamethyldisilizane (HMDZ)
[16]
Octadecyltrichlorosilane (OTCS) Anhydride-derivatized SiO2
p-Aminophenyltrimethoxysilane (APhS)
Tetrahydropyranyl methacrylate (THPMA)-fluorinated methacrylate (F7MA) block copolymer (P(THPMA-block-F7MA))
Hexamethyldisilizane (HMDZ) Tetramethyldisilazane (TMDS)
[42]
Si wafer
Heptadecafluoro-1,1,2,2-tetrahydrodecyl trichlorosilane
[43]
[41]
3-Cyanopropyltrichlorosilane (CPS)
n-Octadecyltrichlorosilane (n-OTCS) Fumed silica MCM-48
Octadecylsilane (ODS)
[44]
Si wafer
octadecyltrichlorosilane (OTCS)
[45]
Poly(tetrahydropyranyl methacrylate-co-1H,1H perfluorooctyl methacrylate) (THPMA-F7MA)
Hexamethyldisilizane (HMDZ)
[46]
Si wafer
N-(3-Triethoxysilylpropyl)-4-azidotetra-fluorobenzoate (1, PFPA-silane)
[47]
Si wafer Nanoporous silica gel
(Dimethylamino)octadecyldimethylsilane (DODDS) (Dimethylamino)trimethylsilane (DTMS)
[48]
Fused silica slides Microstructured optical fiber capillaries Tetramethyldisilizane (TMDS)
(Dimethylamino)-n-octyldimethylsilane (DODS) Me3SiCl C8H17SiMe2Cl C10H21SiMe2Cl Me2SiCl2 C8H17SiMeCl2 MeSiCl3 C8H17SiCl3 C8F17(CH2)2SiMe2Cl C6F13(CH2)2SiCl3 BrCH2SiMe2Cl Cl(CH2)3SiMe2Cl Poly(vinyl acetate) (PVAc)
Tetraethoxysilane (TEOS)
[49]
Tetramethoxysilane (TMOS) Nanometric powders of hydroxyapetite (HA) and titanium dioxide (TiO2)
Vinyltrimethoxysilane (VTMO) c-Methacryloxypropyltrimethoxysilane (MPTMS)
[50]
SiO2 microparticles TiO2 nanoparticles
Octadecyltrimethoxysilane (OTMS)
[51]
Hydroxyapatite (HA)
c-methacryloxypropyltrimethoxysilane (MPTMS)
[52]
[53–55]
Titanium dioxide (TiO2) Hectorite (HE) TiO2 nanoparticles
Octyltriethoxysilane (OTES)
Nanometric
Methyltrimethoxysilane
c-Fe2O3 powder
Isobutyltriethoxysilane Octyldimethylmethoxysilane
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Table 1 continued Substrate
Silylation agent
References
SiO2 nanoparticles
c-Methacryloxypropyltrimethoxysilane (MPTMS)
[56]
TiO2
Hexamethyldisilizane (HMDZ)
[37]
Octadecyltrimethoxysilane
Aluminum oxide (Al2O3)
Trimethylchlorosilane (TMCS)
SiO2
Octyltriethoxysilane (OTES)
[57]
Porous methylsilsesquioxane (pMSQ)
Hexamethyldisilizane (HMDZ)
[58–62]
Tetramethyldisilazane (TMDS) Trimethylchlorosilane (TMCS) Dimethyldichlorosilane (DMDCS) Methyltrichlorosilane (MTCS) Trimethylbromosilane (TMBS) Trimethyliodosilane (TMIS) Plasma-damaged
Hexamethyldisilizane (HMDZ)
p-SiOCH low-k dielectric films
Trimethylchlorosilane (TMCS)
[63]
Dimethyldichlorosilane (DMDCS) Plasma-damaged
Dimethyldichlorosilane (DMDCS)
Methylsilsesquioxane films (MSQ)
Diethyldichlorosilane (DEDCS)
[64]
Dibutyldichlorosilane (DBDCS) Mesoporous SiO2 membrane
Octadecyldimethylchlorosilane (ODMCS)
[65]
MCM-41
Mercaptopropyltrimethoxysilane (MPTMS)
[66]
Silica aerogels
Trimethylethoxysilane (TMES)
[67]
Octyltrimethoxysilane (OTMS) Chlorotrimethylsilane (CTMS) Silica gels and monomeric type octadecyl-silylated (ODS)silica gels
Hexamethyldisilizane (HMDZ)
[68]
Perfluorosulfonic acid (PFSA) membrane
(3-Mercaptopropyl)methyldimethoxysilane (MPMDMS)
[69, 70]
Various macroporous, mesoporous and microporous silica substrates
Octyltriethoxysilane (OTES)
[71]
Zeolite, silica gel, mesoporous silica
Tris(-methoxy)mercaptopropylsilane (TMMPS)
[72]
Silica aerogels
Hexamethyldisilizane (HMDZ)
[73]
Mesoporous silica SBA-15
(N,N-dimethylaminopropyl)trimethoxysilane (DMAPTS)
[74]
Silica aerogels
Hydroxy-terminated poly(dimethylsiloxane) (PDMS(OH))
[75]
TiO2
3-(Trimethoxysilyl)propylmetacrylate (MPTMS), 3-chloropropyltriethoxysilane (CPTES)
[76]
4-Nitrophenyl-(trimethoxysilyl)propyl-methanimine (NPTMS) 4-(((3-Trimethoxysilyl)propyl)imino)methyl-benzaldehyde (FPTMS) Silica gels, mesoporous silica MCM-41, microporous faujasite of type Y
3-(Methylamino)propyltrimethoxysilane (MAP) n-(2-amino ethyl)3-aminopropyltrimethoxysilane (AEAP)
[77]
Si/SiO2
Tetraethoxysilane (TEOS)
[78]
Alkyl poly(ethylene oxide) surfactants
Tetraethoxysilane (TEOS)
[79]
Poly(ethylene oxide) (PEO)–poly(propylene oxide)– poly(ethylene oxide) tri-block copolymers
Methyltriethoxysilane (MTES)
Cement paste
Octyltriethoxysilane (OTES)
temperature and HMDZ concentration in order to reach a single-phase state during the surface modification. Chlorosilanes is another class of compounds that have been widely employed as a silylation agent in the surface
[80]
modification studies. Among other halosilanes, chlorosilanes were shown to provide better control over the surface reaction. Having the chemical form [ClnSiR4-n], with R being an alkyl group, chlorosilanes include a central silicon
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Fig. 9 Mixtures’ critical pressures at different mole fractions of chlorosilane in the binary mixtures. (Filled triangle) trimethylchlorosilane (TMCS)–CO2; (filled circle) diethyldichlorosilane (DEDCS)–CO2; (filled diamond) methyltrichlorosilane (MTCS)– CO2; (asterisk) dimethyldichlorosilane (DMDCS)–CO2 binary mixtures. Reproduced from Ref. [83] with permission from Elsevier Science B.V.
Fig. 8 Bubble point pressure data for HMDZ–CO2 binary mixture for various CO2 mole fractions and at four temperatures. Reprinted from Ref. [73] with permission from Elsevier Science B.V.
atom and one to four chlorine atoms that are bound to silicon. The functionality of the chlorosilanes is determined by the number of chlorine atoms (n). The phase behavior of chlorosilanes with CO2 was studied by Vyhmeister et al. [83]. The phase envelopes of four chlorosilanes with different functionalities were determined at various temperatures up to 0.14 chlorosilane mole fractions and 12 MPa pressures. Two types of phase separation—bubble point and dew point—were observed for the mixtures. Mixtures’ critical pressures and temperatures were observed to increase with the increasing chlorosilane mole fraction. The experimental data were modeled using Peng–Robinson equation of state, and the binary interaction parameters for the binary mixtures were regressed. It can be concluded that, at a constant mole fraction and temperature, chlorosilanes employed in this study have phase separation pressures similar to HMDZ. Figure 9 shows a P–x diagram of chlorosilane–CO2 binary mixtures for various chlorosilane mole fractions. The thermodynamic data presented in the study were employed for the determination of the chlorosilane concentration in scCO2, which was necessary for understanding the kinetics of the chemical reactions of chlorosilanes with the surfaces in thin film-repair applications. In addition to HMDZ and chlorosilanes, alkoxysilanes are also employed for surface modification. Alkoxysilanes have the chemical form [RnSi(OR0 )4-n], where R is an organic group with or without any reactivity and OR’ is the reactive alkoxy group. In 2009, Garcia-Gonzales et al.
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silylated the surface of TiO2 nanoparticles by using octyltriethoxysilane (OTES) dissolved in scCO2 [53]. In this study, the P–x–y diagram of OTES–CO2 binary mixture was determined at 318 and 348 K and pressure range of 8–18 MPa. The experimental data were correlated with Chrastil equation [84] which relates the solubility to the density of CO2 and temperature, and the solubilities were determined at the studied conditions. The solubilities increased with the increasing pressure. A crossover phenomenon was also observed at 16 MPa and 0.014 silane mole fractions. At less than the crossover pressure, the solubilities increased with the decreasing temperature, while at higher than the crossover pressure, the solubilities increased with the increasing temperature. The solubility of OTES in scCO2 was lower compared to HMDZ and chlorosilanes. The measured solubility data were then utilized to determine the processing conditions for the silylation of TiO2 and c-Fe2O3 nanoparticles. In a recent study, Sanchez-Vicente et al. investigated the solubility of (N,Ndimethylaminopropyl)trimethoxysilane (DMAPTS) in CO2 by measuring the bubble and dew points up to pressures of 13 MPa and DMAPTS mole fractions of 0.07. With the increasing CO2 mole fraction, the dew point pressures decreased at constant temperature, whereas the bubble point pressures increased. In addition, both bubble and dew point pressures were found to increase with the increasing temperature. The solubilities of DMAPTS in scCO2 were observed to be very similar to that of HMDZ. The solubility data were similarly employed in the determination of the experimental conditions for the silylation of mesoporous silica SBA-15 from scCO2 [74]. In another recent study, the phase behavior studies were performed with 3-(methylamino)propyltrimethoxysilane (MAP) and n-(2amino ethyl)-3-aminopropyltrimethoxysilane (AEAP) in
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scCO2, and the solubilities of the aminosilanes in scCO2 were determined. The phase behavior information was then utilized in the modification of MCM-41 and microporous faujasite of type Y surfaces with the aminosilanes [77]. In another recent study, phase behavior of hexamethyldisiloxane (HMDS)–CO2 was studied [85]. The bubble point pressures of the binary mixtures were measured at five different temperatures, and the binary interaction parameters were regressed with Peng–Robinson Stryjek–Vera equation of state. In addition, the P–T projection of the critical locus of HMDS–CO2 binary mixture was computed with PRSVEoS using GPEC software [86]. As shown in Fig. 10, HMDS–CO2 binary mixture was found to exhibit Type II phase behavior according to the general classification of fluid-phase equilibria established by Van Konynenburg and Scott [87] where, in addition to the continuous l = g curve, one can also observe a l2 = l1 critical curve and a l2l1 g three-phase curve at low temperature values, intersecting at upper critical end point (UCEP) ((l2 = l1) ? g). The l2 = l1 critical curve runs steeply to high pressure values and represents upper critical solution temperatures. There is no experimental evidence about the existence of a l2 = l1 critical curve and a l2l1g three-phase curve since there was no previous study of this binary system. Thus, the critical locus displayed in Fig. 10 should further be investigated experimentally to test the reliability of the model predictions. In P–T–x space, a two-phase (l2 ? l1) region is observed at temperatures lower than the l2 = l1 critical curve and pressures higher than the l2l1g three-phase curve. From the above discussions, it is apparent that the phase behavior of different silanes in scCO2 has been extensively studied, thus far. The thermodynamic data obtained from such studies have been utilized as key information in the
Fig. 10 P–T projection of HMDS–CO2 binary mixture for kij = 0.0947. Reproduced from Ref. [85] with permission from Elsevier Science B.V.
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surface silylation of various materials to determine the experimental conditions such as temperature, pressure, and silane concentration in scCO2. It can be deduced that silanebased compounds have, in general, substantial solubility in scCO2 and high silane concentrations can be achieved at moderate pressures. The phase separation pressures generally increases with the increasing silane concentration in the mixture and the increasing temperature. However, crossover behavior can be observed for some silanes such as OTES, where the relationship between the silane concentration, temperature, and pressure can be reversed. Besides various silane compounds, silicon polymers are also attracting attention in surface modification studies. Being one of the cheapest polymers, poly(dimethylsiloxane) (PDMS) is very well known to be miscible with scCO2, especially for low polymer molecular weights, and thus it is one of the very few polymer candidates to be used as a silylating agent. Its siloxane backbone is the most flexible polymer backbone known, and it has considerably high solubility in scCO2 at moderate-to-high pressures depending on the molecular weight. Thus far, the phase behaviors of PDMS–CO2 binary mixtures were studied for different polymer molecular weights, and the phase envelopes at different temperatures, pressures, and concentration ranges were revealed [88–91]. The results demonstrated that at constant temperature, the increasing CO2 fraction increases the phase separation pressure, namely, the demixing pressure, and at a fixed composition, the demixing pressure increases with the increasing temperature. Normally, PDMS is highly stable having methyl groups attached to silicon of the siloxane backbone. However, in order to be used for surface modification, the polymer needs to have reactive groups that would react with the functional groups on the surface. Various forms of PDMS with different functional groups such as bis(3-aminopropyl) terminated, vinyl terminated, and bis(hydroxyalkyl) terminated are now commercialized. Among those different types, hydroxyl-terminated PDMS (PDMS(OH)) is the one potential functionalized form of PDMS due to the affinity of terminal hydroxyl groups toward the oxide surfaces, and in a recent study, it was demonstrated that this type reacts with the surface hydroxyl groups of silica aerogels [75]. Phase behavior measurements of PDMS(OH)–CO2 binary mixtures were conducted at three different temperatures and up to 30 MPa pressures by utilizing polymers with different molecular weights. Three different phase characteristics were detected for the polymer mixtures; bubble points were observed at low CO2 weight fractions, cloud points were detected at high CO2 weight fractions, and color changes of the binary mixtures were observed at around 0.75 CO2 weight fraction, which was attributed to the mixtures’ critical points. The demixing pressures increased with the increasing temperature and CO2 concentration. The bubble point pressure data
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were modeled using Sanchez–Lacombe equation of state, and the binary interaction parameters were determined [92]. Figure 11 shows the demixing pressures of this binary mixture for various CO2 weight fractions where the singlephase and two-phase regions were designated. Furthermore, the effects of polymer molecular weight and type of terminal groups on the demixing pressures were also investigated. Figure 12 displays the demixing pressures of PDMS(OH)–CO2 and PDMS(CH3)–CO2 binary mixtures at a fixed composition and temperature. It is evident that the demixing pressures increase with the increasing polymer molecular weight. More importantly, the demixing pressures fall on a smooth line with the increasing molecular weight regardless of the terminal group, which suggests that the terminal groups of the polymer do not have a significant effect on the demixing pressures [91, 92]. The phase behaviors of solutions of different silanes dissolved in scCO2 have been frequently studied thus far, and P–T–x data of these solutions were successfully measured. The information gained from such thermodynamic data plays a vital role in the determination of the experimental conditions during the silylation process.
Microelectronic processing In recent years, utilization of scCO2 has been attracting increased attention in the semiconductor and microelectronic processing for the reduction of the amount of toxic solvents [93]. So far, scCO2 was utilized for wafer cleaning, resist stripping, drying, resist development [42, 46] and grafting of metals. In addition to these applications, scCO2 was also
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Fig. 12 The effect of polymer molecular weight on the demixing pressures of PDMS(OH)–CO2 and PDMS(CH3)–CO2 binary mixtures at 323.2 K for 5 wt% of polymer composition. Reproduced from Ref. [92] with permission from Elsevier Science B.V.
employed in the grafting of self-assembled monolayers (SAMs). SAMs are spontaneously formed molecular assemblies that occur upon chemisorption on a surface. SAMs are built up from molecules that have a head group where the chemical attachment on a surface takes place, a functional group that protrudes from the surface and a tail group that links the head and functional groups. When silylation agents are utilized as the precursor molecules, a silyl monolayer is formed on the surfaces. SAMs have prominent importance in nanodevices and microelectronic components due to their nanometer scale layer thicknesses and well-defined surface structures [43]. Therefore, scCO2 is considered as a promising
Fig. 11 P–wCO2 diagrams of PDMS(OH)–CO2 binary mixtures at three temperatures for a MnPDMS(OH) = 2750 (g/mol); and b MnPDMS(OH) = 18,000 (g/mol). Reproduced from Ref. [92] with permission from Elsevier Science B.V.
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medium for the formation of SAMs since the deposition from scCO2 results in homogenous surface coverage with fine control over the layer thicknesses. Moreover, scCO2 was also demonstrated to be a perfect medium for the silylation of native silicon oxide surfaces [94]. The usage of scCO2 as the carrier solvent constitutes a green alternative to processes that are carried out from vapor or solution phase [93]. There have been various surface modification studies related to microelectronic processing involving a silane-based compound dissolved in liquid or scCO2. Silylation of metal oxide surfaces either from vapor or solution phases has been conventionally employed in microelectronic processing for forming monolayers for lithography, micropatterning, and sensors [95–98]. In the pioneering study of Tripp and coworkers in 1998, the surfaces of silica were chemically modified with HMDZ and octadecyltrichlorosilane (OTS) from scCO2 and silylation from scCO2 emerged as a novel and efficient technique [16, 38]. The study of Tripp and coworkers triggered novel ideas in microelectronic processing since silicon wafers and their derivatives are the key components in this field. Following that study, Cao et al. prepared various alkylsilyl monolayers on polished silicon wafers and nanoporous silica by covalent attachment of several silanebased compounds including monofunctional dimethylaminosilanes, monofunctional chlorosilanes, dichlorosilanes, and trichlorosilanes using scCO2 as the solvent and compared the results to those obtained by using organic solvents [48]. The kinetics of the reactions of (dimethylamino)trimethylsilane (DTMS) and (dimethylamino)-noctyldimethylsilane (DODS) with silicon wafer were investigated by measuring water contact angles as a function of time. It was demonstrated that the initial stages of the silylation was extremely fast, and rapid hydrophobization of the surface was achieved in the first 3 min of the reaction resulting in 83° and 92° contact angles for silylation with DTMS and DODS, respectively. It was also shown that the silylation with DTMS in scCO2 is faster than the one in (80° contact angles after 6 min of silane exposure). In addition, the contact angles that were obtained by silylation with monochlorosilanes in scCO2 were slightly lower than the ones obtained in toluene, whereas dialkyldichlorosilanes and alkyltrichlorosilanes exhibited similar contact angles for the samples prepared in scCO2 and toluene. In 2006, Puniredd and Srinivasan obtained ultrathin films of oligoimide on anhydride-derivatized silicon surfaces which were generated by the grafting of 3-cyanopropyltrichlorosilane and following treatment with trifluoro aceticanhydride in scCO2 [41]. In 2009, Re´biscoul et al. deposited mercaptopropyltrimethoxysilane (MPTMS) and APTMS on SiO2 substrates from scCO2 to replace the metallic barriers that are used in semiconductor devices with SAMs [39]. Polycondensed layers were obtained from the grafting of APTMS, whereas the grafting of MPTMS resulted in SAMs [39]. In an
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interesting study published in 2005, Chow et al. demonstrated an alternative strategy toward monolayer preparation by removing multilayer defects via CO2 treatment [78]. They utilized the so-called CO2 snow-cleaning technique in which dry ice and liquid CO2 were employed together to remove the loosely bound and hydrolyzed silane islands resulting in the conversion of imperfect siloxane films to nearly perfect ones. In 2003, Pham et al. described a method for the development of positive-tone photoresist by in situ chemical modification of tetrahydropyranly metacrylate (THPMA)fluorinated metacrylate (F7MA) through silylation [46]. Pattern imaging at 248 nm generated acids in the exposed regions that cleave tetrahydropyranly (THP) groups, which resulted in methacrylic acid with significantly lower solubility in scCO2. Treatment with scCO2 resulted in negativetone photoresist with methacrylic acid regions forming the patterns since they were not extracted due to the low solubility in scCO2. During subsequent silylation, HMDZ diffused into the film and reacted with free carboxylic acid groups, forming O–Si(CH3)3. Patterned regions thus were soluble in scCO2 due to the addition of the organosilicon protecting groups. Subsequent UV exposure activated unreacted photoacid generators throughout the sample. Cleavage of nonpolar THP protecting groups occurred everywhere except in the originally patterned regions. Finally, development in scCO2 removed the patterned regions, making a positive-tone resist [46]. Figure 13 summarizes the processing steps for the development of positive-tone photoresist in scCO2. Figure 14 shows the SEM images of negative- and positive-tone photoresists developed in scCO2. Kim et al. formed thin polymer films on silicon wafer by generating SAMs of N-(3-triethoxysilylpropyl)-4-azidotetrafluorobenzoate (PFPA-silane) using scCO2 [47]. The silylated surfaces were spin coated with (3-aminopropyl)triethoxysilane (PEOX) and polystyrene (PS) after which the substrates were subjected to UV irradiation. Upon UV treatment, PS and PEOX were immobilized on the surface forming uniform thin film layers [47]. It was stated that the deposition time in scCO2 was shorter than that in toluene. In addition, optimal amount of PFPA-silane used for the deposition was reduced compared to toluene. Therefore, scCO2 was concluded to be an effective solvent for the formation of SAMs compared to the conventional solvents such as toluene. The method was a combination of immobilization chemistry and photolithography, which can be used to generate patterned polymer films and arrays with unique surface topographies that can be utilized in various application fields such as biosensors and molecular electronics. Figure 15 shows the processing steps employed in this study as well as the AFM image of the patterned array. In another study, SAMs were developed on SiO2 wafers by depositing semi-fluorinated and hydrocarbon trichlorosilane
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Fig. 13 Schematics of the processes for positive- and negative-tone scCO2 development. Adapted from Ref. [42] with permission from Society of Chemical Industry
precursors from liquid CO2 [43]. The results were compared with the grafting from vapor phase and from organic solvent, and the deposition rates were revealed to be several orders of magnitude greater with liquid CO2 than the other methods [43]. In addition, the analysis results showed a rapid initial adsorption, and as a result, high coverage of the substrates by SAM were achieved within first few minutes. The change in SAM thicknesses with deposition time was depicted in Fig. 16. In an interesting study of Danıs¸ man et al., an approach for fabricating SAMs within microstructured optical fibers using near critical or scCO2 as the reaction solvent was reported, and the results of contact angle measurements within capillaries were presented [45]. The formation of SAMs was achieved by means of OTS dissolved in CO2. It was shown that the films in the capillaries exhibit alkyl chains with a high degree of conformational ordering and dense packing. Furthermore, water contact angle measurements were performed at an air/liquid interface of a column of water (or a bubble of air in between two columns of water) inside the silica capillaries. A slight pressure was applied to force the water to move, and the meniscus position was monitored with an optical microscope. The wetting measurements were stated to be very useful for determining the film coverage and homogeneity
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in the capillaries [45]. Figure 17 displays images of air bubble inside an OTS-coated capillary. In 2009, Sha and Ober published a mini-review which described the lithographic patterning of fluorine- and siloxane-containing polymers using scCO2 as the developer solvent [42]. Silylation has been attracting significant interest in microelectronic processing and has been employed in wafer cleaning, resist stripping, drying, resist development, grafting of metals, formation of SAMs, and monolayers for lithography, micropatterning, and sensors, thus far. Many silane reagents dissolved in scCO2 have been successfully utilized for modification of surfaces, which may lead to the reduction of toxic organic solvents in microelectronics processing if utilized on an industrial scale.
Porous materials: gels, aerogels, membranes, and polymer films Modification of the surface chemistry of porous materials has always been challenging when conventional vapor or liquid-phase treatments are employed. The diffusion and homogenous distribution of the molecules on the surfaces is problematic due to the fine pore sizes which forms the major limitations of the conventional liquid-phase
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Fig. 14 SEM images of THPMA–F7MA random copolymer resist patterned by 248-nm exposure: a scCO2 processed negative-tone images; b positive-tone images processed after silylation with
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HMDZ; c positive-tone images processed after silylation with tetramethyldisilizane (TMDS). Reprinted with permission from [46]. Copyright (2003) American Chemical Society
Fig. 15 Formation of SAM on silicon wafer and immobilization of PS thin film (left); AFM image of patterned PEOX/PS array (right) [47]
processes. In addition, for some porous materials such as aerogels, liquid-phase surface modification damages the porous structure due to the high capillary pressures, which originate from the vapor–liquid-phase boundary in nanosized pores. Such phase boundaries can also occur in the
vapor-phase treatments since condensation of the silylation reagents can occur in the pores due to very fine sizes when the treatment is carried out at the saturation pressure of the silane reagent. Grafting of the modifying agents from scCO2 is considered as an alternative route, which
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Fig. 16 Ellipsometric thicknesses of semi-fluorinated (F8H2-SAM) and hydrocarbon (H18-SAM) SAMs as a function of the deposition time. Reprinted with permission from [43]. Copyright (2002) American Chemical Society
eliminates such drawbacks of the conventional techniques. More importantly, utilization of scCO2-based surface modification eliminates the possible damages in the porous structure; therefore, the original porous structure can be retained after the surface modification. In 1996, Yarita, Nomura, and Horimoto carried out in situ silylation of silica gel column packing using HMDS dissolved in scCO2, and the performance of the packing was evaluated by pyridine/phenol test in SCF chromatography as well as high-performance liquid chromatography (HPLC) [68]. The surface coverage was calculated as 3.91 lmol/m2 for silylation from toluene, while it was found as 4.13 lmol/
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m2 for silylation from scCO2. Based on these results, scCO2 was demonstrated to be a more effective reaction medium than organic solvents. In the study of Shin et al., commercial zeolite beta was functionalized by tris(methoxy)mercaptopropylsilane (TMMPS) from scCO2 [72]. Grafting from scCO2 was shown to be an effective approach for functionalization of microporous materials since it provides enhanced diffusivity in micropore channels and accelerated reaction kinetics, and the accessibility of the internal micropores is not hindered. In another study, scCO2 was used as the reaction media for the grafting of SAMs of MPTMS on mesoporous silica MCM-41, and the results were compared with the conventional liquid-phase silylation from toluene [66]. In a recent study, silylation of mesoporous silica SBA-15 with DMAPTS was carried out from scCO2 [74]. The results were compared with the conventional silylation from toluene, which indicated that silylation from scCO2 preserves the mesoporous structure of silica and provides faster reaction rates resulting in larger silane loadings. In a recent study of Domingo’s group, solid sorbents for use in CO2 capture were developed by grafting aminosilanes onto porous silica from scCO2 [77]. In another recent study, macro-(perlite), meso-(silica gels and agglomerated nanoparticles),and microporous (zeolite) silica-based substrates were functionalized by the impregnation of OTES from scCO2 to obtain high capacity oil adsorbents [71]. Higher grafting densities were obtained for macro- and microporous matrices compared to the mesoporous ones. The results also revealed that silylation from scCO2 allowed for the preservation of the porous character of the bare supports. In 2002, Jia and McCarthy carried out a study on the surface modification of the interfaces between the silicon wafer and PS or poly(methylmetacrylate) (PMMA) from scCO2 [40]. The
Fig. 17 Images of air bubble moving (to the right) inside an OTS-coated 50-lm capillary, after UV/ozone treatment. Reprinted with permission from [45]. Copyright (2008) American Chemical Society
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silanol groups that were buried at the interface of silicon wafer and polymer were utilized for silylation with (tridecafluoro1,1,2,2-tetrahydrooctyl)dimethylchlorosilane (FDCS) in liquid or scCO2. CO2 was employed as swelling agent as well as solvent for the silylation agent [40]. Higher pressures and temperatures resulted in higher amount of FDCS bound at the SiO2/PS interface since the polymer chains are more mobile at high temperatures and pressures providing faster diffusion and exposing more silanols. Lower silylation was obtained at SiO2/ PMMA interface due to the strong hydrogen-bonding interactions between PMMA and silanols, which make silanols unavailable for silylation agent and limit the chain mobility of polymer reducing the diffusion rate. It was concluded that scCO2 allows for the chemical modification of the interfaces between the silicon wafer and PS or PMMA by making the buried interfaces accessible to the modifying reagent, and the method can be effectively employed to rehabilitate polymer coatings and reinforce polymer composites. In a similar study, McCool and Tripp demonstrated that ‘‘inaccessible’’ hydroxyl groups of mesoporous silica MCM-48 are in fact accessible and participate in reactions with octadecyldimethylchlorosilane (ODMCS) when scCO2 is used as the delivery medium [44]. Reactive deposition with scCO2 possesses significant advantages for surface modification in the case of aerogels. Aerogels are highly porous nanostructured materials with unique properties that make them potential candidates for several application areas varying from catalysis to thermal insulation and CO2 capture and storage [86]. Aerogels are obtained by the sol–gel process usually with a supercritical drying step where the pore liquid is extracted most commonly with scCO2. Among various types, silica aerogels are the most widely studied aerogels. Silica aerogels are transparent materials that have high surface areas, low densities, and very low thermal conductivities. They are highly hydrophilic materials by nature, due to their high surface areas and pending –OH groups covering the surface. In addition, they can be processed as monoliths in various sizes and shapes. One obstacle that limits their widespread use is their fragile and brittle nature. Moreover, their hydrophilic nature is considered as an additional drawback since the adsorption of water vapor on the surface destroys the porous structure and shortens the lifetime. There have been several studies aiming to improve the poor mechanical properties as well as to adjust the surface chemistry of silica aerogels to render them hydrophobic. In 2010, Kartal and Erkey demonstrated that hydrophobic silica aerogels can be obtained by the reactive deposition of HMDZ from scCO2 [73]. The modification of surface chemistry was achieved by the replacement of surface –OH groups of silica aerogel with trimethylsilyl (Si(CH3)3) groups of HMDZ which resulted in 130° contact angels. The changes in the surface chemistry can be observed from
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Fig. 18, which shows the FTIR spectra of silica aerogel before and after HMDZ treatment. It is evident from the figure that before HMDZ treatment, there is a broad band of Si–OH peak at 3000–3700 cm-1 wavenumbers and upon treatment, this broad band vanished since the Si–OH groups of silica aerogel were consumed after the reaction with HMDZ. A similar surface modification study was carried out by the grafting of trimethylethoxysilane (TMES), OTES, and CTMS from scCO2 [67]. It was revealed by those authors that by varying the silylation agent and grafting conditions such as pressure, it is possible to control the degree of functionalization, which allows for gradual variation of hydrophobicity. Recently, we demonstrated that the surfaces of silica aerogels can also be functionalized by hydroxyl-terminated PDMS (OH) which is a silane-based CO2-philic polymer [75]. According to the phase behavior measurements of PDMS(OH)–CO2 binary mixtures, the experimental conditions were determined, and PDMS(OH) was deposited from scCO2 on to the silica aerogels. Polymer molecules were revealed to react with surface –OH groups of silica aerogels. The thickness of the polymer layer was calculated for various pore sizes, and the polymer molecules were found to form a coating layer of *1 nm on silica aerogel surface. The polymer uptakes of silica aerogels were observed to increase with the increasing polymer concentration, the deposition time, and the deposition temperature. The resulting nanocomposite material was additionally revealed to be hydrophobic with varying contact angles depending on the polymer amount, which is given in Fig. 19. Silylation from scCO2 is also considered as an effective way for modifying surfaces of different types of membranes that are used for separation processes. Higgins et al.
Fig. 18 FTIR spectra of (a) silica aerogel after HMDZ treatment; (b) native silica aerogel. Reproduced from Ref. [73] with permission from Elsevier Science B.V
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Fig. 19 Water droplets on the deposited aerogels with 36.6 wt% (left) and 75.4 wt% (right) of PDMS(OH) uptake. Reprinted with permission from [75]. Copyright (2013) American Chemical Society
reported the preparation of organically modified mesoporous silica membranes using reactive deposition with scCO2 [65]. The mesoporous silica membranes which were prepared by surfactant-templated synthesis on a-alumina supports were modified by contacting them with ODMCS in scCO2. The permeations of light gases such as He, Ar, N2, and CH4 were significantly reduced after the silylation of the membrane, which originated from the reduction of the pore size from 5 to 1 nm. In addition, no change in separation of methane and propane over N2 was observed between the unmodified and silylated membrane. In another study on modification of membranes, Su et al. developed polysiloxane-modified perfluorosufonic acid (PFSA) membranes in scCO2 for utilization in direct methanol fuel-cell applications [69, 70]. (3-mercaptopropyl) methyldimethoxysilane (MPMDMS) was impregnated into PFSA membranes from scCO2. The results showed that the modified membranes have higher proton conductivity and lower methanol permeability, which resulted in 75.6 % higher selectivity compared to the unmodified PFSA membranes. Silylation from scCO2 was also employed for the modification of porous polymer films. In 2004, Xie and Muscat performed silylation of porous methylsilsesquioxane (MSQ) thin films with HMDZ, tetramethyldisilizane (TMDS), and trimethylchlorosilane (TMCS) dissolved in scCO2 [61]. Those authors reported the reactions of disilizanes and chlorosilane with both lone and H-bonded silanol (Si–OH) groups on the MSQ surface. As a result of the modification process, the damage in MSQ films, which occurred due to plasma ashing, was repaired, and the hydrophobicity was recovered. The following year, same group reported the repair and capping of porous MSQ thin films using different trimethylhalosilanes (TMCS), trimethylbromosilane
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(TMBS), and trimethyliodosilane (TMIS)) dissolved in scCO2 [58]. According to the contact angle measurements, the reactivity of the silylation agents were shown to increase in the order: TMCS \ TMBS \ TMIS. Another study by the same group investigated the repair and capping of porous MSQ thin films using different chlorosilanes (TMCS), dimethyldichlorosilanes (DMDCS), and MTCS in scCO2 [62]. The results indicated that intermolecular linking occurred when more reactive bi- and tri- functional chlorosilanes were employed. In a recent study of the same group, the silylations of low-k films with HMDZ from scCO2 were investigated with in situ FTIR analysis [60]. In a similar study, low-k films were silylated with HMDZ from scCO2 [59]. In another study, the surface of plasma-damaged porous MSQ films was modified using DMDCS, diethyldichlorosilane (DEDCS), and dibutyldichlorosilane (DBDCS) dissolved in scCO2 [64]. The reactions between chlorosilanes and substrate hydroxyls were shown to strongly depend on the length of the alkyl group of the chlorosilane, regardless of the concentration. The increasing alkyl chain lengths caused steric effects and shielded the adjacent silanols making them unavailable for further reaction. In addition, silanes were in excess amount in the studied concentration ranges. Moreover, silanes with small alkyl chains were shown to penetrate and react more effectively through MSQ films, while silanes with bulky alkyl chains primarily deposited on the external surface. In another study, Jung et al. investigated the repair of plasmadamaged thin films with low dielectric constant by silylation from scCO2 using trimethychlorosilane (TMCS), hexamethyldisilazane (HMDZ), and dimethyldichlorosilane (TMDCS) as the silylation agents. It was found that the hydrophobicity of the thin films that was lost during the plasma etching [63] was recovered. In an interesting study of Pai and Watkins, the synthesis of mesoporous organosilicate
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Fig. 20 TEM image and electron diffraction pattern of a calcined mesostructured organosilicate film. Reproduced from Ref. [79] with permission from Wiley-VCH Verlag GmbH & Co.
films in scCO2 was reported [79]. The films were synthesized by first spin coating of the organic templates on a silicon wafer, followed by the exposure to inorganic oxide precursor solution in humidified scCO2. After the film syntheses, the organic template was removed with calcination, which yielded porous organosilicate films. Figure 20 displays the TEM image of a calcined mesostructured organosilicate film. In another interesting study, Garcia-Gonzales et al. silanized mortar using OTES in scCO2 [80]. Silane coating of mortar was shown to have remarkable effectiveness for rendering mortar hydrophobic which reduced the water absorption. Silylation of surface of porous materials from scCO2 have a vital importance since it eliminates the drawbacks of the conventional liquid and vapor-phase techniques by retaining the pore structure intact, providing homogenous distribution of the silane layer on the surface and enables fast diffusion through the pores. Thus far, silylation from scCO2 have been successfully utilized to modify the surfaces of porous membranes, column packing, mesoporous silica, aerogels, and MSQ thin films.
Nanoparticles and nanocomposites Surface modification of nanoparticles has been gaining increasing attention since the ability to control the physical and chemical properties of the surface is crucial for many applications. The coating of the surfaces of micrometric and nanometric powders of hydroxyapatite (HA), hectorite (HE), and TiO2 from scCO2 was performed using c-methacryloxypropyltrimethoxysilane (MPTMS) as the silylation agent [50, 52]. The modification resulted in the formation of thermally stable self-assembled silane
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monolayers by the covalent attachment of MPTMS on the surface. MPTMS was similarly used as the silylation agent for the coating of SiO2 nanoparticles in both scCO2 and scCO2–ethanol mixtures [56]. Gu and Tripp used HMDZ and TMCS for silylating TiO2 and Al2O3 surfaces from scCO2 [37]. The results indicated that the presence of ethanol together with scCO2 plays an important role for deagglomeration of the coated nanoparticles. In 2007, Charpentier et al. synthesized silica–poly(vinylacetate) (SiO2– PVAc) nanocomposites with a one-step synthesis procedure in scCO2 [49]. The free radical polymerization, hydrolysis, condensation reactions, and linkage to the polymer matrix that took place in scCO2 during the synthesis were monitored by in situ ATR-FTIR spectroscopy. Nanoparticles of 10–50 nm were observed to form during the synthesis [49]. In 2009, three studies were published which investigated the functionalization of the surfaces of c-Fe2O3 and TiO2 with various organosilanes dissolved in scCO2 as well as the kinetics of the silylation reactions [53–55]. It was demonstrated that the use of scCO2 provided an effective approach to functionalize individual inorganic nanoparticles due to the enhanced diffusivity of the silylation agents in the aggregates’ inter-particle voids. Figure 21 shows a schematic representation of the surface structure of the silanized particle and respective TEM image. In a similar study, the silylation of TiO2 nanoparticles was performed with octadecyltrimethoxysilane (ODTMS) in scCO2 [51]. The prepared TiO2-polysiloxane particles were analyzed with vibrational spectroscopy, and the covalent attachment of ultra-thin film on TiO2 nanoparticles was reported. In a recent study, Lopez-Periago et al. prepared TiO2 nanoparticles functionalized with different silanes [76]. The functionalizations were carried out by dissolving four different silanes containing nitrogen moieties in scCO2. 3-(trimethoxysilyl)propylmetacrylate (MPTMS), 3-chloropropyltriethoxysilane (CPTES), 4-nitrophenyl-(trimethoxysilyl)propyl-methanimine (NPTMS), and 4-(((3-trimethoxysilyl)propyl)imino)methyl-benzaldehyde (FPTMS) were successfully deposited onto TiO2 nanoparticles. The presence of the silanes on TiO2 was confirmed by FTIR analysis, and the grafting densities were calculated according to the TGA analysis results. In another recent study, Purcar et al. modified the surface of silica particles with OTES from scCO2 and obtained hydrophobic surfaces with different contact angles [57]. Silylation of surfaces of nanoparticles and nanocomposites from scCO2 have been used as a successful method since the process provides uniform distribution of the surface layer and fine tuning of the grafting density by adjusting the deposition conditions such as temperature, pressure, and silane concentration. Thus far, surfaces of
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Fig. 21 Schematic representation of the a surface structure of a silanized particle and b TEM image of a silanized TiO2 particle. Reprinted with permission from [54]. Copyright (2009) American Chemical Society
TiO2, Al2O3, and Fe2O3 were successfully silylated from scCO2.
Conclusions and future perspectives Supercritical carbon dioxide (scCO2) is attracting increasing interest as a medium for processing of materials. It has been utilized in various application areas as a nonflammable and nontoxic green solvent due to its easily accessible critical conditions. Moreover, the diversity of application fields that utilize scCO2 is expanding day-by-day paving the novel green routes for processing of materials. Silylation of the surfaces is by far the most frequently utilized surface-functionalization technique, which allows for the processing of various types of materials. Considering the extensive silane family of chemicals with numerous chemistries and substantial solubilities in scCO2, combining silylation and scCO2 promises intriguing materials with different properties, which can be used in a wide range of application fields. Thus, silylation of the surfaces with scCO2 is of prominent interest and becoming more important as the advantages of scCO2 over the conventional silylation processes is uncovered. In addition to the apparent advantages, scCO2 route provides control over the process, unlike conventional liquid-phase or gas-phase silylation techniques, which is one of the key issues in surface modification. The physical and chemical properties of materials can be fine-tuned in processes with scCO2 by simply adjusting the temperature and/or pressure. Such a control gave rise to employment of silylation of surfaces from scCO2 in diverse areas ranging from porous materials to microelectronic processing, as well as from thin films to nanocomposites.
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Silylation from scCO2 is a relatively new field, and the studies that were carried out thus far are quite few. Considering the wide class of silane reagents, there are numerous studies that can be performed involving different types of silane reagents, their phase behavior in scCO2, and modification of specific materials with them. In addition, there are no literature data on the diffusion and mass-transfer coefficients of silanes in scCO2. Measurements can be performed, and several correlations which allow for the predictions of the diffusion and mass-transfer coefficients of systems in SCF based on the system properties such as density, viscosity, molecular weight, and temperature can be developed. Moreover, there is also a substantial need for fundamental studies in the field. The kinetics of surface reactions during the silylation from scCO2 should additionally be studied in order to understand the fundamental aspects of the overall process. The reaction mechanisms can be revealed by implementing in situ techniques such as IR. The silane reagents that are commercially available were developed for aqueous- and vapor-phase techniques, and some of them have limited solubility in scCO2. Novel silane agents that have enhanced intermolecular interactions with CO2 can be synthesized to increase the solubility in scCO2. In addition, different co-solvents can be utilized in silylation, and the effects of using such chemical agents on surface reactivity and the overall silylation process can be studied. Regarding these considerations, it is reasonable to believe that in the near future, the diversity of the materials and applications utilizing silylation from scCO2 will rapidly rise. Acknowledgements This work was funded by the European Union Seventh Framework Program (FP7/2007 – 2013) under Grant Agreement No. NMP4-SL-201[20]0-260086.
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