Dense Silica Coatings on Micro- and Nanoparticles by Deposition of

28 Dense Silica Coatings on M i c r o -

and

Nanoparticles by Deposition Downloaded by STANFORD UNIV GREEN LIBR on March 20, 2013 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch028

of Monosilicic A c i d Horacio E. Bergna, Lawrence E. Firment, and Dennis G. Swartzfager D u P o n t Company, Wilmington, DE 19880-0228

The upper limit to the thickness of dense silica coatings on hydroxyl ated surfaces that can be obtained with conventional coating tech niques was found to be extended significantly by coating with monosilicic acid. An example with submicrometerα-aluminaparticles as a substrate showed dense, uniform coatings up to at least 800 Å thick. The silica coatings and coating mechanism were characterized by chemical analysis, electrokinetic potential, nitrogen surface area (Brunauer-Emmett-Teller), particle size, X-ray photoelectron spectroscopy, diffuse reflectance Fourier transform infrared spectroscopy, secondary ion mass spectroscopy, and transmission electron microscopy.

SILICA COATINGS ARE APPLIED TO PARTICULATE MATERIALS to modify surface characteristics that interfere with the exploitation of desired bulk properties, as with titania pigments that may photocatalyze the degradation of their vehicle, surfaces of selective zeolite catalysts that may promote undesired reactions, and fillers for plastics that may not disperse in their matrix. A n upper limit to the thickness of dense silica coatings on hydroxylated surfaces can be obtained with some of the conventional coating techniques (J). W e have found that this limit can be extended significantly by coating with monosilicic acid (MSA). The deposition of silica from water was discussed by Her (2). The mechanism of deposition of monomeric silica is different from that of the 0065-2393/94/0234-0561$08.00/0 © 1994 American Chemical Society

In The Colloid Chemistry of Silica; Bergna, H.; Advances in Chemistry; American Chemical Society: Washington, DC, 1994.

562

T H E C O L L O I D CHEMISTRY O F SILICA

Downloaded by STANFORD UNIV GREEN LIBR on March 20, 2013 | http://pubs.acs.org Publication Date: May 5, 1994 | doi: 10.1021/ba-1994-0234.ch028

deposition of colloidal particles. Monomeric silica is deposited from supersaturated solution i n two known ways: as a deposit of Si(OH)4 on a solid surface or as colloidal particles forming i n the supersaturated solution. Silanol groups S i O H condense with O H groups of M O H surfaces, where M is a metal that w i l l form a silicate at the p H and temperature involved. The reaction is represented as follows:

—Μ—ΟΗ

A — M — O H

OH

+

OH

-

\( OK

—¥~""°\

\ ) Η

O — Μ — Ο

/ °

H

SÎ^

+2H 0 2

(1)

OH

Further deposition of Si(OH)4 is on silica, thus a layer of silica is built up. The second known way of deposition of monomeric silica first involves polymerization in the solution. Her described this deposition as follows: " I f an insufficient area of a receptive solid surface is available to accept silica rapidly, and i f the concentration of Si(OH)4 is greater than 2 0 0 - 3 0 0 ppm (depending on p H ) , polymerization occurs with formation first of low polymers such as the cyclic tetramer; then these further condense to form small three-dimensional polymers which are colloidal particles" (2). Monomeric silica is also deposited in a third way by living organisms as biogenic amorphous silica through still-unknown mechanisms. The under lying principles of nature's mechanism of deposition are being investigated as a first step to biomimetic processing of ceramics (3). W i t h the knowledge that monomeric silica is deposited from supersa turated solution as a deposit of Si(OH)4 on a solid surface or as colloidal particles forming i n the supersaturated solution, a general picture of the deposition of M S A on a hydroxylated surface may be visualized as occurring in three steps. In the first step, even before supersaturation it is assumed that at least some M S A may react with the hydroxylated surface (Figure 1). After supersaturation a number of mechanisms involving homonucleation and heteronucleation of M S A take place (Figure 2). Part of the M S A may react directly with the hydroxylated surface, and another part may nucleate to form hydrated silica polymers. The hydrated silica polymers may take several different paths depending on the conditions of the system. In one case, the hydrous silica polymer forms porous deposits on the hydroxylated surface. In another, the hydrous silica polymerizes further, either gelling or forming discrete particles of colloidal silica. -Both the gel or the aquasol may attach to the hydroxylated surface, forming either " f l a k y " or porous deposits. Meanwhile, the M S A i n solution may deposit i n the interstices of the gel or i n the interstices of the flaky or porous deposits formed on the surface by either the gel or the aquasol.



28.

BERGNA E T AL.

Dense Coatings by Deposition of Monosilicic Acid

563

HYDROXYLATED SURFACE EX: ALUMINA

Figure 1. Silica deposition on hydroxylated surface;firststep: before supersatu ration. What path the M S A follows is determined by factors such as composi tion of the substrate surface, addition rate of the M S A i f it is being added to the system, slurry concentration i f the system involves a dispersion of particles as substrates, ionic strength of the electrolyte solution i n which the particles are dispersed, M S A concentration at any given time, and especially p H and temperature of the system. If all these factors are properly controlled, M S A can be deposited on the hydroxylated surface, forming dense silica layers. Once a layer that completely covers the original hydroxylated surface is formed, any further deposition is on silica, thus an increasingly thicker dense silica layer is built up on the substrate surface (Figure 3). In this chapter processes for coating α-alumina particles with M S A and for characterizing the coated particles to show the M S A is deposited as monomeric units to form a dense silica coating are described. The same procedures can be applied to coating titania and p-zeolite particles (4).

Experimental Details Materials. M S A was prepared by the following procedure (5, 6): a solution of sodium metasilicate was prepared by dissolving 30 g of pulverized, Fisher reagent grade Na Si03 · 9 H O i n 100 m L of 0.1 Ν N a O H . The silica content of this reagent, designated solution A , was 2.28%. A separate solution Β was prepared, consisting of 0.025 N H 2 S O 4 , and was cooled to 0 - 5 °C. Meanwhile, a quantity of sulfonic acid cation-exchange resin (Dowex H C R - W 2 - H ) was washed with distilled water until washings were colorless. A 1.5-g sample of the resulting washed resin was added to 100 m L of solution Β i n a beaker that was stirred and cooled in an ice bath at about 5 °C. At this point, 5 m L of solution A was added by intermittent jets of about 0.3 m L each, delivered by a 1-mL syringe and fine-tipped hypodermic needle. The p H of the resulting mixture was continuously maintained below 2.5 by delaying additions of solution A until the p H of the stirred, cooled mixture dropped below 2. After 5 m L of solution A had been added in this fashion, the p H of the mixture was about 2.15. The resulting clear solution of silicic acid was stored temporarily at 0 - 5 ° C i n an ice bath to prevent premature polymerization. The calculated concentra2

a



2

2

2

• Addition Rate • Surface Composition

• Temp.

• Ionic Strength

• MSA Cone.

• Slurry Cone.

-FLAKY"

GEL

χ sio · yH o

• pH

FAÇTQRg

\ INTERSTICES

1 ^ INTO

^

HOMONUCLEATION

POROUS

XT

COLLOIDAL

Figure 2. Silica deposition on hydroxylated surface; second step: after supersaturation.

ALUMINA SURFACE

POROUS DEPOSITS

2

χ sio »yH o

HOMONUCL.


2

7,

SI0

28.

BERGNA E T AL.


565

MSA


DENSE S i 0

2

LAYER

Τ

ALUMINA S U R F A C E

Figure 3. Silica deposition on hydroxylated surface; third step: after complete surface coverage. tion of this solution of silicic acid was 3 mg of S 1 O 2 per milliliter. The same procedure was used to prepare silicic acid solution with a concentration of 2 mg of S 1 O 2 per milliliter. The α-alumina was superground Alcoa A 1 6 alumina classified by sedimenta tion i n water and kept as a 10% aqueous slurry of p H 3.5-4.5. The B r u nauer-Emmett-Teller (BET) specific surface area of the alumina product was 10-13.5 m*/g. Equipment. A l l coating experiments were performed i n an automated reactor facility of a glass flask equipped with agitator, inlets for liquids, condenser, p H and temperature sensors, and controlling and recording instrumentation. The agitator consisted of a stir motor (Cole-Palmer model 4370-00) coupled to a glass stir rod with a Teflon blade positioned near the bottom of a glass flask. The flask was placed i n a heating mantle, and the M S A was kept between 2 and 3 °C by a chiller with a circulating pump. The aqueous alumina slurry was placed i n the flask. The temperature of the slurry and M S A feed, the p H , the feeding rates of M S A and acid (concentrated H C l ) or base (concentrated NH4OH), and the stirring rates were controlled, measured, and recorded automatically (Kaye III Digitstrip recorder). The information obtained was monitored on the color screen of a Digital V T 1 2 5 computer. Procedure. Temperature of the alumina slurry was adjusted to 60 °C, and p H was maintained at 8.5 with automatic additions of dilute acid or base as needed. At this temperature and p H the M S A solution was added at a rate of ca. 2 g of S 1 O 2 per 1000 m per hour to prevent formation of colloidal silica (2, 3). At the end of the M S A treatment, the slurry was allowed to cool to room temperature and centrifuged to separate the solid residue, made of coated alumina. The residue was washed by redispersing it in distilled water and centrifuging. The cake was dried i n vacuum at 110 °C for 12 h. 2

Characterization Techniques. The dry samples of silica-coated alumina were analyzed by inductively coupled plasma-atomic emission spectroscopy


566


(ICP-AES). Specific surface area was determined by nitrogen adsorption (BET). Particle-size determination was made by low-angle forward scattering of light from a laser beam (Leeds and Northrup's Microtrac particle sizer) and by monitoring sedimentation with a finely collimated beam of low-energy X-rays and a detector (Micromeritics Sedigraph 5100). The samples were characterized by X-ray diffraction (XRD); transmission and high-resolution electron microscopy ( T E M and H R E M ) ; microelectrophoresis; X ray photoelectron spectroscopy (XPS), also known as electron spectroscopy for chemical analysis (ESCA); secondary ion mass spectrometry (SIMS); and diffuse reflectance Fourier transform (DRIFT) infrared absorption spectroscopy. The transmission electron microscopy was done with a 100-kV accelerating potential (Hitachi 600). Powder samples were dispersed onto a carbon film on a C u grid for T E M examination. The surface analysis techniques used, X P S and SIMS, were described earlier (7). X-ray photoelectron spectroscopy was done with a D u Pont 650 instrument and M g K« radiation (10 k V and 30 mA). The samples were held in a cup for X P S analysis. Secondary ion mass spectrometry and depth profiling was done with a modified 3 M instrument that was equipped with an Extranuclear quadrupole mass spectrometer and used 2-kV Ne ions at a current density of 0.5 μΑ/cm . A low-energy electron flood gun was employed for charge compensation on these insulating samples. The secondary ions were detected at 90° from the primary ion direction. The powder was pressed into In foil for the SIMS work. Electrokinetic potential of the alumina samples redispersed i n 0.001 Ν ΚΝΟβ was measured over a p H range between 2 and 9.5 with a Pen Chem System 300 instrument. The coated alumina powder was ultrasonically dispersed at 0.001 wt% into 300 m L of 0.001 Ν KNO3 for ca. 15 min and immediately placed under N 2 atmosphere. A 50-mL portion was placed onto a titration stirrer under N 2 atmosphere that was fitted with a p H probe, a mechanical stirrer, and a port for addition of titrant. A portion of sample was pumped into the S3000 cell, which was fitted into a constant temperature bath set at 25 °C. This sample portion was used to rinse the cell of the previous sample. A second portion of sample was pumped into the cell. The p H was recorded and the zeta potential was measured. Two measurements were taken, one at the front stationary layer and the second at the back stationary layer. The histograms were then combined and averaged. The p H was adjusted with either 0.01 Ν K O H or 0.01 Ν HNO3, and measurements were repeated for each desired p H value. Once all the desired p H versus zeta potential data were obtained, the data were transferred to an I B M P C and plotted. Estimates of isoelectric points (IEPs) were made from the plots obtained. Infrared analysis of the coated and uncoated alumina particles was done by D R I F T with a 180° backscattering configuration and referenced to powdered (

2

-1

6

3

Results The original, uncoated α-alumina had a specific surface area of 10-13.5 m /g. Solid spherical particles of α-alumina 150 nm in diameter would have a surface area of 10 m /g. T E M s of the particles show that they are irregularly shaped and have a broad distribution of particle size between 0.1 and 0.8 μπι (Figure 4). 2

2



28. BERGNA ET AL.


567

Figure 4. Transmission electron micrograph of superground, dispersed, and peptized Alcoa A16 α-alumina. (Reprinted with permission from reference 7. Copyright 1989.) The silica content by chemical analysis, the specific surface area, and the particle-size analysis of the original alumina and the silica-coated products are included in Table I. For these alumina particles, theoretical monolayer coverage corresponds to 1 wt% silica. Within the error of the measurements, the specific surface area appears to remain fairly constant throughout the coating process. The constancy of surface area with silica level suggests that dense coatings are formed. Available particle-size measurements obtained by two different techniques suggest that the silica coating may cause some aggregation of the alumina particles at relatively low levels of silica and may become significant at higher coverages, starting at 6-9-wt% S i 0 . Figure 5 shows plots of electrokinetic potential versus p H for the alumina samples at various levels of silica coverage and the changing character of the interface of the particles in aqueous solution with increasing coverage with silica, from pure alumina (IEP ca. 9.0) to pure silica (IEP ca. 2.0). Most of the change i n electrokinetic potential has 2



568

Table t. MSA-Coated Submicrometer α-Alumina Particle-Size Analysis


Bulk Chemical Analysis (wt% S1O2)

Dense Silica Coatings on Micro- and Nanoparticles by Deposition of

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