Anal. Chem. 2008, 80, 651-656
Electrodeposited Silicate Films: Importance of Supporting Electrolyte Maryanne M. Collinson,*,† Daniel A. Higgins,‡ Roshna Kommidi,† and Debbie Campbell-Rance†
Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23229, and Department of Chemistry, Kansas State University, Manhattan, Kansas 66506
Silica and hybrid organic-inorganic films, ca. 100-200 nm thick, can be grown on glassy carbon electrodes through reactions initiated by electrogenerated hydroxide or hydronium ions in water under reductive and oxidative conditions, respectively. A variety of different alkoxysilanes (tetramethoxysilane and organoalkoxysilanes) and supporting electrolytes were used to evaluate whether film formation takes place on glassy carbon electrodes. The results of the study indicate that the acid-base properties of the supporting electrolyte are an important factor in determining whether film formation will take place. For cathodic electrodeposition, thin films can be formed using supporting electrolytes that are close to neutral, such as KCl, KNO3, and NaClO4. For anodic electrodeposition, thin films can be formed using supporting electrolytes that are acidic, such as, KH2PO4, HNO3, H2SO4, etc. The acidity/basicity effects of the electrolytes arise in part from the strong dependence of the hydrolysis and condensation rates of the silicon alkoxide precursors on pH. Sol-gel thin films are typically made by either spin coating or dip coating with a sol prepared by hydrolyzing/condensing an alkoxysilane in a water/alcohol solution on a suitable substrate such as a glass slide, an electrode, or a silicon wafer.1,2 An alternative method, which has been the focus of several recent studies, is a relatively old electrochemical method called electrodeposition.3-5 In this technique, a sufficiently positive or negative potential is applied to a conductor immersed in a silica sol (for example) to electrochemically generate either hydronium ions or hydroxide ions at the electrode surface via oxidation of water or by reduction of oxygen or water, respectively.6,7 Hydronium ions or hydroxide ions serve as a catalyst and cause the sol to gel at the electrode surface (but not in the bulk of the solution), thus forming a thin film.6 * Corresponding author. E-mail:
[email protected]. † Virginia Commonwealth University. ‡ Kansas State University. (1) Brinker, C. J.; Frye, G. C.; Hurd, A. J.; Ashley, C. S. Thin Solid Films 1991, 201, 97-108. (2) Brinker, C. J.; Hurd, A. J.; Schunk, P. R.; Frye, G. C.; Ashley, C. S. J. NonCryst. Solids 1992, 147-148, 424-436. (3) Boccaccini, A. R.; Zhitomirsky, I. Curr. Opin. Solid State Mater. Sci. 2002, 6, 251-260. (4) Zhitomirsky, I. Adv. Colloid Interface Sci. 2002, 97, 279-317. (5) Therese, G. H. A.; Kamath, P. V. Chem. Mater. 2000, 12, 1195-1204. (6) Collinson, M. M. Acc. Chem. Res. 2007, 40, 777-783. (7) Walcarius, A.; Mandler, D.; Cox, J.; Collinson, M. M.; Lev, O. J. Mater. Chem. 2005, 15, 3663-3689. 10.1021/ac7017124 CCC: $40.75 Published on Web 01/04/2008
© 2008 American Chemical Society
Electrodeposited films, in contrast to spin-coated or dip-coated thin films, have many promising attributes and are worth investigating in greater detail.6,7 Silica films prepared via electrogeneration of base can be more porous than spin-coated films because they are prepared via a base-catalyzed mechanism (which gives rise to more particulate sols) and because gelation and evaporation can be separated in time.6,8 This method of film formation could technically be used to pattern surfaces, because film formation will “ideally” take place only on conducting surface regions. Indeed, there are many examples in the literature that use these materials in chemical sensing,9-12 corrosion protection,13 and in ion-exchange applications.14 Electrodeposition, although conceptually easy to understand, is complex, especially when used to deposit thin silica films on conducting substrates. There are many conflicting reports in the literature, undoubtedly because the processes are complicated. Two very important factors to be considered are the (1) rates of hydrolysis and condensation of the alkoxysilanes under acidic and basic conditions and (2) the type of electrolyte used in the electrochemical experiment. The electrolyte in an electrochemical experiment is usually chosen to be an “inert” species that does not directly participate in the electron-transfer reaction. However, the electrolyte’s chemical properties (e.g., acidic/basic, electron transfer, complexation) can enable direct reactions at the electrode surface or reactions with species electrogenerated at an electrode surface, all of which can be dependent on the applied potential. In this paper, we examined the importance of electrolyte on film formation on glassy carbon surfaces from sols containing a variety of alkoxysilanes and/or organoalkoxysilanes via anodic or cathodic electrogeneration of H3O+ and OH-, respectively. The results show that film formation may or may not take place in a given sol depending on the supporting electrolyte and/or alkoxysilane used. EXPERIMENTAL SECTION Reagents and Materials. Tetramethoxysilane (TMOS, 99%, Acros), tetraethoxysilane (TEOS, 98%, Acros), ethyltrimethoxysi(8) Deepa, P. N.; Kanungo, M.; Claycomb, G.; Sherwood, P. M. A.; Collinson, M. M. Anal. Chem. 2003, 75, 5399-5405. (9) Walcarius, A.; Sayen, S. Electrochem. Commun. 2003, 5, 341-348. (10) Carrington, N. A.; Yong, L.; Xue, Z. L. Anal. Chim. Acta 2006, 572, 17-24. (11) Zhang, Z. H.; Nie, L. H.; Yao, S. Z. Talanta 2006, 69, 435-442. (12) Walcarius, A.; Sibottier, E. Electroanalysis 2005, 17, 1716-1726. (13) Sheffer, M.; Groysman, A.; Mandler, D. Corros. Sci. 2003, 45, 2893-2904. (14) Collinson, M. M.; Moore, N.; Deepa, P. N.; Kanungo, M. Langmuir 2003, 19, 7669-7672.
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lane (ETMOS, 97%, Aldrich), methyltrimethoxysilane (MTMOS, 98%, Aldrich), isobutyltrimethoxysilane (iBTMOS, 97%, Aldrich), phenyltrimethoxysilane (PMTOS, 97%), and 3-(triethoxysilyl)propionitrile (CNTEOS, 97%, Aldrich) were used as received. The electrolytes (i.e., potassium chloride, potassium nitrate, ...) used in this study were typically from Fisher Scientific. The working electrode was a 5 mm diameter glassy carbon electrode fabricated by sealing a glassy carbon rod in a glass tube with epoxy and polishing to a mirror finish with 0.05 µm alumina on a napless polishing cloth (Buehler). The working electrode was subsequently cleaned by sonication in water for at least 10 min prior to use. For the electrodeposition experiments, the auxiliary electrode was a 2 mm diameter graphite rod (Ted Pella) and the reference electrode (quasi-reversible) was a AgCl-coated Ag wire electrode (1 mm diameter). Instrumentation. Water was purified to type I by a Millipore water purification system. A one-chamber, three-electrode cell was used with a Bioanalytical Systems (BAS) CV50 or CV100 potentiostat to carry out the electrochemical work. Film thickness was measured with a Tencor Alpha Step 500 surface profilometer on films prepared on glassy carbon chips. Procedures. The electrochemical deposition of silica on conducting substrates was carried out using amperometry. The one-chamber electrochemical cell typically contained 4 mL of 0.2 M electrolyte solution, 3 mL of deionized water, 1 mL of absolute ethanol, and 0.2 mL of the alkoxysilane. The solution was stirred for 30 min. Potentials in the range of -700 to -1500 mV or 17002300 mV were applied to the working electrode with respect to a silver chloride coated silver wire in an unstirred sol for a period of 30 min. The auxiliary electrode was typically placed a few centimeters away from the working electrode to minimize cross contamination from byproducts formed at it during the 30 min deposition period. After electrodeposition, the working electrode was immediately taken out of the cell, rinsed with deionized water, and dried overnight at a relative humidity of ∼35%. The pH of each sol was measured before electrodeposition with short-range pH paper. RESULTS Cathodic Electrodeposition. Application of sufficiently negative potentials to an electrode surface in a nondeoxygenated, watercontaining solution results in an increase in the pH at the electrode surface.3-7 It is well-known that hydroxide ions catalyze the hydrolysis and condensation of alkoxysilanes and gelation proceeds fairly rapidly at pHs significantly greater than the isoelectric point of silica.15 Thus, the electrogeneration of hydroxide ions results in rapid film formation on the electrode surface, while the bulk sol remains a sol.6 The following are a list of reactions that are believed to be the most important under the conditions utilized in this work; however, they are by no means exclusive:
2H2O + 2e- f 2OH- + H2 -
O2 + 2H2O + 4e f 4OH
-
2H+ + 2e- f H2
(1) (2) (3)
Equations 4-7 may also be possible if nitrate or perchlorate are used as supporting electrolytes.4,5,16 652
Analytical Chemistry, Vol. 80, No. 3, February 1, 2008
NO3- + 7H2O + 8e- f NH4+ + 10OH-
(4)
NO3- + H2O + 2e- f NO2- + 2OH-
(5)
ClO4- + H2O + 2e- f ClO3- + 2OH-
(6)
ClO4- + 4H2O + 8e- f Cl- + 8OH-
(7)
In previous work, our group and others have shown that thin silica films that range in thickness from nanometers to micrometers can be easily formed on glassy carbon, gold, ITO (indium tin oxide), etc. at potentials more negative than ∼ -900 mV from sols containing TMOS or various organoalkoxysilanes.8-12,14,16,17 The supporting electrolytes used were typically potassium chloride or potassium nitrate, and in many cases, the sol was prehydrolyzed at a pH of 3 with addition of a small amount of acid. Often the sol was stirred during deposition as a means to improve film uniformity. To keep the experiment as “simple” as possible, in this study the sols were not prehydrolyzed or stirred. By not adding acid to the sol, one less source of ions is included, and furthermore, hydrogen generation (bubble formation) at the working electrode is greatly reduced, especially at more cathodic potentials. By not stirring the solution, interference from products generated at the counter electrode will be less significant since a one-chamber electrochemical cell is used. Figure 1 shows a cyclic voltammogram (CV) acquired at a 5 mm diameter glassy carbon electrode in a sol containing potassium nitrate as the supporting electrolyte. The peak at E ∼ -0.9 V is attributed to oxygen reduction (eq 2) where the increasing current noted near E ∼ -1.2 V is due to the reduction of solvent and oxygen. As noted in prior work, application of potentials more negative of -0.9 V versus a silver chloride coated silver wire results in film formation.8 The inset in Figure 1 shows a photograph of a thin “brown” film formed on glassy carbon at -1.3 V. No visible bubble formation was evident during deposition. The film color, which arises from an optical interference effect, depends on film thickness and its refractive index.18,19 As evaluated by atomic force microscopy (AFM) and/or scanning electron microscopy (SEM), the surfaces of these films are macroscopically rough,8 and the degree of roughness has been shown to be dependent on deposition potential and time.17 It is important to note that in this study a quasi-reversible reference electrode was used. Ideally, it would be best to use a reference electrode with a constant potential (for example, a silver-silver chloride reference in 1 M KCl), but the porous frit in this traditional reference electrode becomes clogged with silane, giving rise to large and irreproducible junction potentials. Because the reference electrode used is quasi-reversible, its potential will vary day to day and from experiment to experiment, especially in (15) Brinker, J.; Scherer, G. Sol-Gel Science; Academic Press: New York, 1990. (16) Shacham, R.; Avnir, D.; Mandler, D. Adv. Mater. 1999, 11, 384-388. (17) Sibottier, E.; Sayen, S.; Gaboriaud, F.; Walcarius, A. Langmuir 2006, 22, 8366-8373. (18) Manifacier, J. C.; Gasiot, J.; Fillard, J. P. J. Phys. E: Sci. Instrum. 1976, 9, 1002-1004. (19) Schauer, C. L.; Chen, M. S.; Chatterley, M.; Eisemann, K.; Welsh, E. R.; Price, R. R.; Schoen, P. E.; Ligler, F. S. Thin Solid Films 2003, 434, 250257.
Figure 1. Cyclic voltammogram at a glassy carbon electrode in a sol containing TMOS and KNO3. Scan rate: 100 mV/s. Inset: digital picture of a film formed via application of -1.3 V for 30 min. Table 1. Film Formation at Eapplied ) -1.3 V for 30 Minutes (Electrolyte Concentration in Sol ) 0.1 M) -1.3 V
TMOSa
MTMOSa
ETMOSa
iBTMOSa
pH of sol
KNO3 KCl NH4Cl NaBr NaAc NaClO4 KH2PO4
brown brown brown brown brown -
very thin film -
-
-
4 4 4
-
-
-
8
-
-
-
4
a
“-” indicates no film can be visibly seen on the electrode surface.
the different electrolyte solutions. The redox potential of ferrocene methanol (1 mM) in 0.1 M KCl was ca. 150 mV more positive than it was when the electrolyte solution was 0.10 M KNO3 using this quasi-reversible electrode. In contrast, it differed by less than 5 mV using a traditional reference electrode. Day-to-day variations typically are on the order of a few millivolts. Changes of this magnitude will not significantly alter whether film formation will or will not take place but can and most likely do affect film thickness and its day-to-day variability, especially when different electrolyte solutions are used. Table 1 lists the supporting electrolytes used and whether film formation was visibly observed. A number of other electrolytes were also used (i.e., Na2CO3, K2HPO4), but they either did not fully dissolve in the sol or the sol became cloudy and started to gel during deposition. The pH of the TMOS sol prior to deposition is also listed for some electrolyte/sols. The pH of a sol prepared from TMOS without supporting electrolyte is ca. 4. Nitric acid and hydrofluoric acid were also used, but significant hydrogen evolution was noted during deposition. Since significant hydrogen evolution can interfere with film formation, strong acids were not used when negative potentials were applied to the working electrode. To evaluate whether or not the magnitude of the applied potential, the length of time it was applied, or slight changes in the value of the reference electrode potential makes a difference, potentials of -1.3 to -1.5 V were applied for up to 60 min to a glassy carbon electrode in a TMOS sol containing KH2PO4. No film formation was evident. For the sol/electrolyte systems that give rise to a film (i.e., KNO3, KCl, ...), application of more negative potentials results in thicker films. The lack of a potential
Figure 2. Cyclic voltammogram at a glassy carbon electrode in a sol containing iBTMOS with KH2PO4 as the supporting electrolyte. Scan rate: 100 mV/s. Top panel: images of films formed on glassy carbon at Eapplied of 1.8, 2.0, and 2.2 V for 30 min, respectively.
dependence suggests that the lack of film formation is not due to slight differences in the overpotential associated with oxygen/ water reduction or to variations in the reference electrode potential but due to the electrolyte. A mixed electrolyte system was also evaluated. A TMOS sol containing 0.05 M KNO3 and 0.05 M KH2PO4 was prepared. No film formation was evident upon application of -1.3 V for 30 min. The presence of KH2PO4 in the sol appears to be enough to prevent film formation. Anodic Electrodeposition. Most of the literature on electrodeposition of alkoxysilanes involves the electrogeneration of base, primarily because gelation proceeds rapidly under basic conditions. In prior work, we have noted film formation via the electrogeneration of hydronium ions in a sol containing TMOS did not take place, presumably because of the slow hydrolysis/ condensation rates of TMOS under acidic conditions.8 However, in this work, we show that it is possible to form beautiful thin films on glassy carbon from sols containing TMOS with certain supporting electrolytes. Because organoalkoxysilanes hydrolyze fast under acidic conditions,15 electrogeneration of H3O+ also provides an important means to form thin hybrid films from a variety of silanes without prehydrolysis. Figure 2 shows a CV acquired at a 5 mm diameter glassy carbon electrode in a sol containing iBTMOS with KH2PO4 as the supporting electrolyte. The increasing current noted near E ∼ + 1.8 V is due to the oxidation of solvent (eq 8) to generate hydronium ions, thus decreasing the pH at the electrode interface. It is also possible to add ascorbic acid to the sol, which, when oxidized, produces hydronium ions as well.
2H2O f O2 + 4H+ + 4e-
(8)
Digital images of three different films formed on three different electrodes at three different potentials (1.8, 2.0, 2.2 V) are shown in Figure 2. Differences in film color represent differences in film thickness and/or refractive index.18,19 Table 2 lists the supporting electrolytes used and whether film formation was evident. Analytical Chemistry, Vol. 80, No. 3, February 1, 2008
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Table 2. Film Formation at Eapplied ) +1.8 V for 30 Minutes (Electrolyte Concentration in Sol ) 0.1 M)a +1.8 V
TMOSb
MTMOSb
ETMOSb
iBTMOSb
KNO3 KCl NH4Cl NaAc NaClO4 KH2PO4 HNO3 HCl HClO4 H3PO4 H2SO4
brown blue
thin film brown blue brown/green violet blue gold
brown green
brown/blue orange green
violet blue gold
violet blue gold
a
violet blue gold
CNTEOS
PTMOS
pH of sol
blue
blue
4 4 4 8
gold blue
blue blue
4 1 1 1 1.3-1.6 1
For the acids, the concentration was 0.05 M in the sol. b “-” indicates no film can be visibly seen on the electrode surface.
electrolytes, the addition of AA to the sol still does not result in film formation from a sol containing MTMOS.
Figure 3. Film thickness vs applied potential. Sols were prepared according to the Experimental Section using KH2PO4 as the supporting electrolyte and either MTMOS, ETMOS, iBTMOS, or TMOS as the silicon precursor. Deposition time: 30 min.
To evaluate whether or not the magnitude of the applied potential, the length of time it was applied, or slight changes in the value of the reference electrode potential makes a difference, potentials of +1.8 to +2.2 V were applied to a glassy carbon electrode in an MTMOS sol containing KCl for up to 60 min. No film formation was evident, again suggesting that lack of film formation is not due to slight differences in the overpotential associated with water oxidation in the different electrolyte solutions or small variations in the reference electrode potential. Also, if a mixed electrolyte system is used (TMOS sol containing 0.05 M KCl and 0.05 M KH2PO4) no film formation was evident upon application of +1.8 V for 30 min. The presence of KCl in the sol is apparently enough to prevent film formation. Also, a mixture of HCl/HNO3 as the electrolyte also does not give rise to film formation. Figure 3 shows a plot of film thickness versus deposition potential for the different silanes using KH2PO4 as the electrolyte. As with cathodic-based electrodeposition, film thickness depends on the applied potential: the more positive the potential, the thicker the film. Addition of a small amount of ascorbic acid (AA) to the sol also increases film thickness. Much thicker films can be obtained for MTMOS/KNO3 at lower potentials when 10-20 mg of AA is added to the sol. AA can be oxidized at ca. 0.6 V, giving rise to films at a much lower potential than in the case of solvent oxidation. However, if KCl, NH4Cl, or NaAc are used as 654 Analytical Chemistry, Vol. 80, No. 3, February 1, 2008
DISCUSSION It is well-known in the sol-gel literature that the rates of hydrolysis and condensation of alkoxysilanes are highly dependent on the strength of the catalyst and its concentration.15 The pH dependence of hydrolysis/condensation of the silanes is fairly complicated. It has been shown in the literature that a parabolalike curve is obtained when the gelation (or condensation) rate is plotted versus pH. The minimum rate is observed near the isoelectric point of the silane (close to 2).15 Thus, when the sol becomes really acidic, (pH , 2) or more basic (pH . 2), gelation proceeds fast (until pH > ∼8). Film formation from silica-based sols will also be highly dependent on the local concentration of catalyst at the electrode interface, which will depend on the rate at which it is produced. Indeed, Sibottier et al. have recently noted that the amount of OH- produced is important in thin-to-thick film formation.17 Any chemical reactions taking place at the interface that result in the depletion of the catalyst will inevitably affect film formation. In some respects, the pH of the bulk sol is irrelevant since the chemistry occurring at the electrode/solution interface is most important. Another factor that could influence film formation is the rate of hydrolysis/condensation of the individual alkoxysilanes under basic conditions (cathodic electrodeposition) and acidic conditions (anodic electrodeposition). Since the alkoxysilanes were not prehydrolyzed, hydrolysis is a concern. It can be postulated that without sufficient hydrolyzed precursors, film formation can be limited. Under basic conditions, “simple” (i.e., nonsterically hindered) organoalkoxysilanes hydrolyze slower than TMOS, whereas the reverse is true under acidic conditions.15 Therefore, it is expected that it would be technically easier to form an organic-inorganic hybrid film from sols containing organoalkoxysilanes under acidic conditions (anodic electrodeposition) compared to TMOS. In this study, sols prepared with some of the more nonpolar organoalkoxysilanes, particularly CNTEOS, PTMOS, and iBTMOS, were not fully miscible. In these cases, a few droplets of silane were noted in the glass vial after 30 min of vigorous stirring. Sols prepared from TMOS, MTMOS, or ETMOS appear miscible. Cathodic Electrodeposition. Application of sufficiently negative potentials at the electrode surface results in the production
of hydroxide ions. Upon examination of Table 1, it can be observed that film formation using organoalkoxysilanes as precursors is limited under these conditions. Since sols containing these precursors were not prehydrolyzed and because hydrolysis of organoalkoxysilanes under basic conditions is slow, it is possible the lack of film formation is caused by their slow rate of hydrolysis under basic conditions (possibly in addition to electrolyte effects, as described below). For sols containing TMOS, it can be observed that film formation is highly dependent on the supporting electrolyte used. It is unlikely that the results are due to a catalytic effect of the counterions as it has been shown that for the case of acid-catalyzed hydrolysis, no catalytic effect of the counterions was observed when the pH of the sol was directly compared.15 What appear to be most important are the acidic/basic properties of the electrolyte. Film formation is evident for the “neutral” electrolytes (KNO3, KCl, NaClO4) but not using electrolytes that are weakly acidic (KH2PO4). The presence of anions (i.e., H2PO4-) that can act as weak acids and react with electrogenerated base will decrease the rate at which the OH- is generated and the magnitude of the pH change at the interface and will interfere with film formation. Of all the salts examined, potassium nitrate and potassium chloride would be least acidic and least likely to react with the electrogenerated base. These are the two electrolytes that almost always give rise to films and are the most widely used in the literature. Both nitrate and chloride ions are conjugate bases of very strong acids and are thus very weak in water. The pKa’s of HCl and HNO3 in pure water or pure ethanol are -3.7 and -1.8 and 2.1 and 4.0, respectively.20 Under the solution conditions (water/ethanol/ silane) used in this work, these anions may be slightly basic and thus not likely to react with the electrogenerated base. For potassium nitrate or sodium perchlorate, it is also possible that reactions 4-7 also take place resulting in a greater rate of OHproduction, thus aiding film formation. However, if KNO3 is mixed with KH2PO4, no film formation is evident suggesting that the presence of an acidic salt at a high enough concentration is enough to prevent film formation on glassy carbon. One major exception is NH4Cl, which has a pKa ) 9.24 (in water), and thus is a very weak acid. The ammonium ion will be an even weaker acid in the water/ethanol/silane mixture used in this work. Because it is so weak, it may have little affinity for the electrogenerated hydroxide ion and film formation can still take place. Another exception is NaAc. Since it is a basic salt, film formation would be expected. However, the pH of the initial sol is relatively high (∼8) because of the basicity of the salt. The condensation rates of silanes start to become significantly slower as the pH increases beyond 8.15 It is thus possible that the electrogeneration of base in a sol that initially has a basic pH will increase the pH enough to put it in the regime where condensation is slow; hence, no film formation is observed. The sol containing sodium acetate was not as stable as the other sols and becomes cloudy with time, further confirming the pH is high enough to start the sol to gel process. Anodic Electrodeposition. Application of sufficiently positive potentials to the electrode surface results in the production of hydronium ions. As shown in Table 2, film formation was observed (20) Charlot, G.; Tremillon, B. In Chemical Reactions in Solvents and Melts; Pergamon Press: Oxford, U.K., 1969; pp 272-313 (translated to English by P. T. T. Harvey).
for all the alkoxysilanes used in this work. Films can easily be formed from TMOS- (or TEOS)-containing sols using a strong acid (HClO4, H3PO4, H2SO4, HNO3) or KH2PO4 or NaClO4 as the electrolyte. Likewise, these electrolytes can also be used to electrodeposit hybrid films on glassy carbon from a variety of organoalkoxysilanes. Films can even be formed from sols containing iBTMOS, PTMOS, CNTEOS, and TEOS, with KH2PO4, which are not completely miscible, in contrast to that observed during cathodic electrodeposition. As described in previous work, it takes very little silane to actually form a film.8 It is believed that the electrogeneration of H3O+ at the electrode interface hydrolyzes and then condenses the small amount of silane in solution at the surface resulting in film formation. As with cathodic electrodeposition, what appear to be most important are the acidic/basic properties of the electrolyte. Film formation is evident for the “acidic” electrolytes (KH2PO4, HNO3, H2SO4, ...) but not using electrolytes that are likely close to neutral or slightly basic (see below) under these conditions (KCl, NaAc, ...). Electrolytes that are slightly basic could react with the electrogenerated acid, affecting the rate at which it is produced and, hence, film formation. ClO4- is an anion derived from the very strong acid, HClO4, and should not have any base strength in water. Cl- and NO3- are also anions derived from strong acids and thus should also have little, if any, base strength in water. However, films can be electrodeposited using ClO4- or NO3- as an electrolyte, whereas films are not obtained using Cl-. In water, perchloric acid is believed to be the strongest acid, followed by HBr and the other acids. pKa values, however, are hard to compare because there are significant discrepancies noted in the literature. HCl, for example, has been reported to have a pKa ranging from -3 to -7, HBr from -4 to -9, and HNO3 from -1 to -2.20-24 The addition of ethanol and silane to the sol will also increase the pKa,20,24 thus making it difficult to compare actual numbers. For HNO3, the pKa in ethanol is estimated to be ca. 4 compared to -1.8 in water at an ionic strength of 0.20 Although actual pKa values are not available, it can be said that under these conditions, the anions will be more basic since the pKa of their acid counterpart will be larger in the water/ethanol/silane sol compared to pure water. What is believed at this point in time is that the basicity of the electrolyte is an important factor in anodic electrodeposition of alkoxysilanes. If the electrolyte ions have basic properties and can react with the electrogenerated acid, film formation will not take place. In the case of NH4Cl, the chloride ion could be reacting with the electrogenerated acid as observed when KCl is also used as an electrolyte. The differences observed between the strong acids used likely results from subtle differences in the basicity of their anions. From this work, it appears that ClO4- has the least tendency to react with electrogenerated H3O+, whereas Cl- has the most. It is interesting to note that films cannot be formed using HCl, KCl, or NH4Cl as the electrolyte, whereas films can be formed using HClO4/NaClO4 or HNO3/KNO3. Previous studies have shown that there does not appear to be a significant catalytic effect of the counterions (except for NaF, where F- acts as a catalyst in (21) Pliego, J. R.; Riveros, J. M. J. Phys. Chem. A 2002, 106, 7434-7439. (22) Petkovic, D. M. J. Chem. Soc., Dalton Trans. 1982, 2425-2427. (23) Schmid, R.; Miah, A. M. J. Chem. Educ. 2001, 78, 116-117. (24) Kissel, T. R. In Physical Methods of Chemistry, 2nd ed.; Rossiter, B. W., Hamilton, J. F., Eds.; John Wiley and Sons: New York, 1986; Vol. II, p 129.
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a manner similar to hydroxide ions).15 A decrease in gel time was noted using acetic acid, but this was attributed to the decrease in basicity of the acetate anion in alcohol and not to a catalytic effect.15 No film formation is evident if the pH is buffered to 3.8 using acetate with any of the salts in Table 1 or Table 2 via application of either -1.3 or 1.8 V to a TMOS or MTMOS sol, indicating the importance of the basic acetate ion to anodic-based electrodeposition and the acidic pH to cathodic electrodeposition. Equally interesting, the pH of the sols containing the strong acids are almost the same, yet film formation is evident with one strong acid (HNO3) but not with another (i.e., HCl). For the electrolyte ions that do not result in film formation application of potentials several hundred millivolts more positive or negative and/or for longer times (60 min) does not seem to help. Also, if KCl is mixed with KH2PO4 or if HCl is mixed with HNO3, no film formation is evident indicating that the presence of a “slightly neutral to basic” salt (Cl-) in a relatively high amount is enough to prevent film formation on glassy carbon. Another possibility to consider is that the chloride ion can be oxidized to form chlorine, which could affect film formation. In this study, no bubble formation indicative of the evolution of chlorine was observed during anodic electrodeposition. To further evaluate whether chlorine was formed in the sol, orthotolidine, a common chemical used to detect chlorine in pool water at the ppm levels, was added to the different sols (TMOS with KCl or MTMOS with HCl) after electrodeposition (Eapplied ) 1.8 or 2.0 V for 30 min). In the presence of chlorine, orthotolidine turns yellow. The sols, however, remained colorless, indicating that chlorine (if present) was too low to detect using this simple color test. In contrast, when NaBr was used as an electrolyte, it was quite obvious that bromide was being oxidized to bromine. Not only did the sol turn a faint yellow during electrodeposition, it reacted positively upon addition of orthotolidine, producing a dark yellow solution. Because the formation of Br2 could impact film formation,
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Analytical Chemistry, Vol. 80, No. 3, February 1, 2008
data collected from NaBr was not included in Table 2. Bromide and, especially, iodide are easier to oxidize than chloride at a carbon electrode. CONCLUSION Electrodeposition can be used to form thin silica and hybrid films from a variety of different alkoxysilanes via electrogeneration of hydroxide ions or hydronium ions. Although the rates at which these precursors hydrolyze and condense is important, the most important factor in determining whether film formation will take place is the supporting electrolyte. The acidity and basicity of the electrolyte ions appears to be the deciding factor, likely due in part to the strong dependence of the hydrolysis and condensation rates of alkoxysilanes on pH. Cathodic electrodeposition requires supporting electrolytes that have no acidic properties and do not react with the electrogenerated base. Anodic electrodeposition requires supporting electrolytes that have no basic properties and hence little affinity for the electrogenerated acid. Indeed, films can easily be formed anodically using strong acids as the electrolyte. Results obtained in this study are likely also applicable to the formation of films from non-silicon precursors such as titanium or zirconium alkoxides. ACKNOWLEDGMENT We acknowledge support of this work by the National Science Foundation through CHE-0618220 and more recently through CHE-0647849 and CHE-0648716. Professor Julio Alvarez and Professor Fred Hawkridge are also gratefully acknowledged for helpful suggestions regarding this manuscript.
Received for review August 13, 2007. Accepted October 29, 2007. AC7017124
