Langmuir 2004, 20, 273-275
273
KHCO3 Mineralization Self-Assembled on Aminopropyl Organosilica Joo-Woon Lee, Allison B. Carroll, Amy E. Patenaude, Sungwook Kim, and J. M. White* Center for Materials Chemistry, Department of Chemistry and Biochemistry, University of Texas, Austin, Texas 78712 Received July 23, 2003. In Final Form: September 10, 2003
Controlling biomineralization can, in principle, provide routes to different polymorphs of crystalline materials.1 Although the precise mechanisms of biomineralization are still poorly understood, it is noteworthy that they are very specific in directing the shape, size, and crystallographic organization of specific inorganic species.2 Therefore, understanding the chemical routes may enable control over crystal orientation, polymorphs, or morphology. Previously, we reported self-organized nanoscale fibrous mineralization at the interface between air and poly(γaminopropyl triethoxysilane) (PAPS).3 The phenomenon was related to atmospheric CO2 and H2O and to Na+ ions leached out from soda-lime glass substrates.3a The growth of fibrous networks was dependent on the Na+ concentration segregated at the air-exposed surface of PAPS matrixes. Recently, on the basis of spectroscopic and microscopic evidence, we have reported for the first time the growth of fluid-filled fibrous KHCO3 microtubes from the surface of PAPS matrixes solvent-cast on boron-doped SiO2/Si(100) substrates.4 Unexpectedly, the fluid was aqueous potassium formate (HCO2K). While evidence regarding the chemical composition is strong, mechanistic details have remained debatable. Here, we present evidence regarding mechanisms leading to the mineralization of KHCO3 matrix-mediated at the exposed interface with air of PAPS and the formation of aqueous potassium formate (HCO2K) at the buried interface with the polymer of boron-doped SiO2/Si(100) substrates. A typical precursor solution (pH ∼ 11.6) was prepared by adding 0.125 g of KOH to 30 g of an aqueous 5.0 wt % γ-aminopropyl triethoxysilane (APS) solution (pH ) 10.5).4 Transparent PAPS matrixes (ca. 40-µm thick), denoted as K+/PAPS, were prepared by casting ∼0.4 g of the basic silica sol on 1 × 2 cm2 SiO2/Si(100) wafers. The effects on KHCO3 mineralization of degree of polycondensation, humidity, pH, catalyst, illumination, and concentration of CO2 were investigated. To increase the degree of PAPS polycondensation, the KOH-catalyzed silica sol was allowed to stand for up to 7 days at ambient conditions before preparing the thin film. Because monomeric γ-aminopropyl silanetriol5a hydrolyzed from APS is sta* To whom correspondence should be addressed. Telephone: 512-471-3704. Fax: 512-471-9495. E-mail: jmwhite@ mail.utexas.edu. (1) Mann, S. Biomineralization: Principles and Concepts in Bioinorganic Materials Chemistry; Oxford University Press: Oxford, 2001. (2) (a) Han, Y.-J.; Aizenberg, J. J. Am. Chem. Soc. 2003, 125, 4032. (b) Volkmer, D.; Tugulu, S.; Fricke, M.; Nielsen, T. Angew. Chem., Int. Ed. 2003, 42, 58. (3) (a) Cabibil, H. L.; Pham, V.; Lozano, J.; Celio, H.; Winter, R. M.; White, J. M. Langmuir 2000, 16, 10471. (b) Cabibil, H.; Celio, H.; Lozano, J.; White, J. M.; Winter, R. Langmuir 2001, 17, 2160. (4) Celio, H.; Lozano, J.; Cabibil, H.; Ballast, L.; White, J. M. J. Am. Chem. Soc. 2003, 125, 3302. (5) (a) Gelest, Inc., Catalog Number SIA0608.0. (b) Hench, L. L.; West, J. K. Chem. Rev. 1990, 90, 33.
Figure 1. Bright-field optical micrographs (A and B) and scanning electron micrographs (C and D) demonstrating fibrous KHCO3 microtubes mineralized at the surface of K+/PAPS matrixes: parts A and C show tubes emerging from nucleation sites, part B is an aqueous HCO2K-filled microtube, and part D is a split microtube.
bilized as a result of intramolecular hydrogen bonding between amine groups and a silanol group, polycondensation occurs slowly. Because sol-gel synthesis is a process, not a product, the three-dimensional networks remain dynamic during gel aging; condensation reactions continue to occur as long as the remaining silanol and ethoxy groups move close enough to react.5b The rate of polycondensation can be increased by initially adding ethanol, the byproduct of hydrolysis, as a solvent. Qualitatively, as the degree of polycondensation increased, the areal number density of the KHCO3 fibers increased. Forming fibers does not depend on bathing in CO2; the amount in ambient air is sufficient. Control of humidity is critical. Because CO2(g) is highly soluble in H2O (0.5 mg/L at ambient conditions and 25 °C), the extent of [CO2]aq dissolved from air can be affected by the extent to which polymeric matrixes are hydrated, a factor controlled by relative humidity. Fibrous microtubes grow when the atmospheric relative humidity lies in the range 50-60%. Thus, the main effect of exposing K+/PAPS to a stream of cool CO2, used in our prior report,4 was the generation of a moisture layer on the matrixes. Fibrous KHCO3 microtubes are always high-aspect-ratio structures but are otherwise diverse. Figure 1 shows typical bright-field optical micrographs (A and B) and scanning electron micrographs (C and D). Structures such as A and C dominate, B is filled with aqueous HCO2K, and D exhibits a Y-shaped structure. Adequate carbon and oxygen for the bicarbonate species is derived from the dissolution of CO2(aq) from air (CO2: ∼0.03% v/v at 15 °C), and the concentration in PAPS is influenced by the concentration of KOH doped in the matrixes under humidity control (50-60%) at ambient temperature (24 ( 0.5 °C). In complementary experiments, the growth rate and areal number density of microtubes increased when [KOH] was doubled, but very few microtubes were found when KOH was replaced by an equal concentration of KHCO3 or K2CO3. These observations indicate the importance of pH on the absorption of CO2 from ambient air into hydrated K+/PAPS matrixes. In accounting for the influence of pH on KHCO3 mineralization, we postulate that the localized chemistry at the exposed interface between K+/PAPS and air is analogous to a bulk aqueous
10.1021/la035341f CCC: $27.50 © 2004 American Chemical Society Published on Web 11/19/2003
274
Langmuir, Vol. 20, No. 1, 2004
Notes Scheme 1. KHCO3 Mineralization Process at the Air-K+/PAPS Interface
Figure 2. Characterization of a crystal polymorph using XRD: (A) diffraction pattern of fibrous KHCO3 microtube clusters mineralized at the air-PAPS interface and (B) reference XRD pattern of KHCO3 (PDF number: 85-2382).8
system where dissolved CO2(aq) reacts with OH- to form HCO3- and further to CO32- depending on the pH.6 The pH of the K+/PAPS silica sol solution is 11.6 after mixing KOH into aqueous APS solution (pH ) 10.5); the pH of the polymeric matrixes hydrated under 50-60% relative humidity likely exceeds 11.6. At pH 11.6, the dissolved CO2 in an aqueous solution is predominately carbonate, CO32- (∼97%).7 Hence, it can be concluded that CO32- is the dominant inorganic carbon species derived from the dissolved CO2(aq) deep within the hydrated K+/PAPS system. Local pH changes, however, may occur at the exposed air-K+/PAPS interface as a result of the acidbase reaction between CO2(aq) and the primary amine functionality of the polymeric matrixes. This would induce local reduction of the pH resulting from the formation of carbamate, -NHCO2-. There is X-ray photoelectron spectroscopic (XPS) evidence that the functional amine group of PAPS reacts with CO2(aq) to form the corresponding sodium carbamate (-NHCO2Na) in the presence of Na+; deconvolution of the carbon 1s (286.0 eV) and nitrogen 1s (400.3 eV) lines provides the positions and fractions of the carbamate carbon (-NHCO2-) and nitrogen (-NHCO2-) at 289.5 and 401.3 eV, respectively.3a X-ray powder diffraction (XRD), Figure 2A, confirms the presence of crystalline KHCO3 in the fibers by comparison with a standard spectrum, Figure 2B. The sample for XRD was prepared by removing clusters of fibers (inset is an optical microscopic image) from the SiO2/Si(100) substrate and packing them on a glass substrate. On the basis of the results from XPS3a and XRD previously described, a mineralization mechanism, Scheme 1, is proposed. This involves potassium carbamate (-NHCO2K; 2) derived from the reaction between CO2(aq) and primary amine species 1 of PAPS. The carbamate 2 forms a complex hydrogen-bonded intermediate 3 with KHCO3 previously formed at the interface of KOHcatalyzed PAPS matrixes with air. The intermediate 3 subsequently undergoes rearrangement to mineralize the crystalline dimeric anion of KHCO3 5 and to reproduce carbamate 2 through the isocyanate derivative 4, thereby establishing a catalytic KHCO3 mineralization pathway. Although 3 was not directly observed, it is consistent with the mineralization to form the observed, Figure 2, (6) Keene, F. R. In Electrochemical and Electrocatalytic Reactions of Carbon Dioxide; Sullivan, B. P., Krist, K., Guard, H. E., Eds.; Elsevier: Amsterdam, 1993. (7) Wetzel, R. W. Limnology: Lake and River Ecosystems, 3rd ed.; Academic Press: San Diego, 2001.
crystalline structure that contains dimeric anion moieties 5.9 To explain the formation of aqueous HCO2K followed by filling into fibrous KHCO3 microtubes, a photoelectrocatalytic reduction of carbonate (CO32-) ions is proposed at the interface between boron-doped Si and alkaline K+/ PAPS polymeric matrixes. The stepwise photoelectrochemical reduction of CO2(aq) at p-type semiconductors, like the boron-doped SiO2/Si(100) wafer used here, has been reported.10 Hence, the two-electron reduction of CO32to formate (HCO2-) can be considered:
CO32-(aq) + 2H2O + 2e- f HCO2-(aq) + 3OH-
(1)
The calculated standard ∆Gf° and corresponding E° (vs normal hydrogen electrode) are 195.9 kJ/mol and -1.015 V, respectively.11 While these data indicate that the reaction in eq 1 is not feasible, it becomes so under room light and can be accelerated using a universal fluorescence lamp (47.9 nW/cm2 in the range of 400 < λ < 1100 nm) placed 135-cm away from the sample. The p-type Si wafer (Eg ) 1.12 eV) acts as the photocathode. Once formed, aqueous potassium formate (HCO2-K+) produced at the buried interface between K+/PAPS and the SiO2/Si(100) substrate fills the fibrous KHCO3 microtubes by virtue of capillary action (Figure 3). Supporting the photon-driven aspects of this model, fibrous KHCO3 microtubes were formed but no fluid filled them when the boron-doped SiO2/ Si(100) wafer was replaced by soda-lime glass under the same illumination or when exposure to light was eliminated. In conclusion, a novel catalytic mineralization process is proposed for the growth of tubular KHCO3 microfibers filled with HCO2-K+. It involves (1) a catalytic cycle via (8) Powder Diffraction File, Release 2001, International Centre for Diffraction Data, Newton Square, PA. (9) Kashida, S.; Yamamoto, K. J. Solid State Chem. 1990, 86, 180. (10) (a) Halmann, M. Nature 1978, 275, 115. (b) Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K. Nature 1979, 277, 637. (11) Latimer, W. M. The Oxidation States of the Elements and their Potentials in Aqueous Solutions, 2nd ed.; Prentice Hall: New York, 1952.
Notes
Figure 3. Schematic of the K+/PAPS system illustrating the growth of aqueous HCO2K-filled fibrous KHCO3 microtubes at the interface between air and the polymeric matrix, the latter providing microchannels for HCO2- ions to be collected into the microtubes by capillary action.
the reproduction of potassium carbamate (-NHCO2K; 2) for growing the KHCO3 microtubes at the air-K+/PAPS
Langmuir, Vol. 20, No. 1, 2004 275
interface, (2) the photocatalytic reduction of CO32- to HCO2- at the buried interface between K+/PAPS and p-type SiO2/Si(100), and (3) the capillary action of the microtubes to acquire aqueous HCO2-. The carbon source for bicarbonate and formate is CO2 present in ambient air. This mechanism may give rise to novel biomimetic biomineralization strategies for producing hierarchically structured inorganic biomaterials. The Na+/PAPS system is under investigation. Acknowledgment. The authors acknowledge support from the National Science Foundation (CHE 0070122), the Robert A. Welch Foundation, and, for support of the electron microscopy, the Texas Materials Institute. LA035341F
