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Surface & Coatings Technology 190 (2005) 110–114

Dense silica-based coatings prepared from colloidal silica

J.A. Calderon-Guillena,b,*, L.M. Aviles-Arellanoa,c, J.F. Perez-Roblesa,J. Gonzalez-Hernandeza, E. Ramos-Ramırezc

aCINVESTAV-IPN, Unidad Queretaro, Libramiento Norponiente No. 2000, Fracc. Real de Juriquilla, 76010 Queretaro, Qro., MexicobUniversidad Autonoma de Sinaloa, Angel Flores y Riva Palacio s/n, Col, centro, C.P. 81010, Culiacan, Sinaloa, Mexico

cCentro de Investigacion en Quımica Inorganica, Universidad de Guanajuato. Noria Alta s/n, Col Noria Alta, C.P. 36050 Guanajuato, Gto., Mexico

Received 15 July 2003; accepted in revised form 8 April 2004

Available online 2 June 2004

Abstract

Two sets of silica-based coatings have been prepared using two different wet methods. Set Awas obtained using the conventional sol–gel

method from thetraethyl-orthosilicate (TEOS) as the main precursor. Set B was produced from commercial colloidal silica added with two

structural modifiers, KOH and sodium metasilicate (Na2SiO3S5H2O). The structure of both sets of coatings was characterized using infrared

absorption, X-ray diffraction and nitrogen adsorption isotherms measurements. The results show a much lower surface area and a slower

oxygen diffusion rate for coatings of set B, indicating that these coatings have a denser structure and therefore are more effective as oxygen

barriers.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Sol–gel; Colloidal silica; Diffusion of oxygen; Dense silica

1. Introduction way of the removed materials. Many applications of silica

In recent years, several publications have described low-

temperature preparation and processing of ceramic coatings

on various types of substrates. Due to their versatility and

low cost, the synthesis of inorganic oxides by wet chemistry

methods, such as the sol–gel technique, has been largely

used for the preparation of these oxides. In particular, silica-

based coatings with various structural characteristics have

been prepared for multiple applications [1–4]. In the tradi-

tional sol–gel method, the synthesis of inorganic oxides

starts from molecular precursors (metal alkoxides); the

oxide network is obtained via hydrolysis and condensation

reactions, which occur in the solution [1]. However, in

general, the sol–gel-derived amorphous coatings usually

have a low density and a large surface area due to their

opened porous structure [5,6]. The pore texture arises from

the preparation method, which involves the elimination of

volatile materials during condensation. These cavities are

produced as a result of both solid rearrangement and exit

0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2004.04.068

* Corresponding author. CINVESTAV-IPN, Unidad Queretaro, Libra-

miento Norponiente No. 2000, Fracc. Real de Juriquilla, 76010 Queretaro,

Qro., Mexico. Tel.: +52-442-4414925; fax: +52-442-4414939.

E-mail address: cajoel_99@yahoo.com (J.A. Calderon-Guillen).

films, such as oxygen barriers, anticorrosive barriers, optical

and electronic coatings require dense films. A curing pro-

cess may achieve densification of these coatings; neverthe-

less, in general, relatively high temperatures are required.

There are some examples of densification of silica coatings

using low temperatures [7–10]. For instance, partial densi-

fication in not intentionally heated sol–gel-derived coatings

have been achieved by UV irradiation [7,8], ion implanta-

tion [9] and laser radiation [10].

Few reports have been published concerning the use of

another method employed to produce silica-based coatings

with different characteristics. The method is based on the

use of aqueous colloidal silica suspensions combined with

inorganic binding and/or structural modifier agents [11–14].

The use of appropriate binders may produce coatings with

dense structures. The term colloidal silica refers to a stable

dispersion or sols of discrete nanometric particles of amor-

phous silica, commonly suspended in water with a size of

about 5 nm in diameter. During the evaporation of water, in

which the colloidal silica particles and binders are sus-

pended, some polymerization/condensation reactions take

place, and a solid dried coating is obtained after the drying

process is completed. This process differs from the conven-

tional sol–gel method because a great variety of compounds

J.A. Calderon-Guillen et al. / Surface & Coatings Technology 190 (2005) 110–114 111

that are compatible with the aqueous system can be added

and no hydrolysis reactions take place. The coatings are

formed via condensation and/or polymerization reactions.

The study of the mechanical properties of thin coatings has

gained considerable interest in recent years.

The aim of this work is to compare the properties of

coatings prepared using the conventional sol–gel method

with those of coatings prepared using colloidal silica. The

first set was prepared from precursor solutions known to

produce dense coatings. The preparation of coatings using

the colloidal silica includes the addition of two specific

inorganic structure modifier agents, which produce dense

films at low curing temperatures with a closed porous

structure and a low surface area.

Fig. 1. IR absorption for samples of set A and B after annealing at 100 jC.

2. Experimental procedure

For this study, two sets of samples were prepared: set A,

consisted of samples prepared by the traditional sol–gel

process; and set B, formed by samples prepared using

colloidal silica. Both sets of samples were prepared in the

form of powders and of coatings on silicon wafers and on

polished copper substrates. The samples were heated in air,

the powders at 100 jC and the coatings in the range of 100

to 500 jC.Samples of set A were prepared from precursor solutions

formed by mixtures of thetraethyl-orthosilicate (TEOS),

water and ethanol. The molar ratios of water-to-TEOS and

ethanol-to-TEOS were 11.7 and 4, respectively. These

preparation conditions are known to produce coatings with

dense structures; more details about the preparation of these

samples can be found elsewhere [15]. To catalyze the

reactions, a small amount (0.03 mol) of HNO3 was added

to the solutions. The substrates were deposited on micro-

scope slide glasses using a conventional dipping apparatus

when the viscosity of the precursor suspensions was about 3

cP. The coating were immersed into the precursor suspen-

sions, maintained there for about 30 s and then removed at a

constant speed of about 5 mm/s. At this removal speed, the

coating thickness was about 500 nm. Samples in the form of

powders were obtained from the same solutions after they

were dried at 100 jC for about 5 h.

Samples of set B were prepared from the condensation of

aqueous suspensions consisting of a mixture of colloidal

silica added with two structural modifiers, KOH and sodium

metasilicate (Na2SiO3S5H2O). The aqueous colloidal silica

was obtained from the company OPTA (Mexico, D.F), with

40 wt.% of silica particles of about 20 nm in diameter. The

structural modifiers were commercial grade. The aqueous

suspension was prepared in the following way: 50 ml of

colloidal silica was added to 70 ml of distilled water, the

suspension was homogenized using magnetic stirring for 5

min, then 2.4 g of KOH was added to this suspension under

magnetic stirring for another 10 min. In a separate vessel,

12.5 g of sodium metasilicate was dissolved in 60 ml of

distilled water. This solution was mixed using magnetic

stirring for 20 min, with the suspension containing the

colloidal silica to form the starting materials. More details

about the procedure used in the preparation of these samples

can be found elsewhere [16]. The coatings were obtained

using the same apparatus and the same removal speed used

for samples in set A. Under these conditions, the coating

thickness was about 500 nm. Samples in the form of

powders were obtained by drying the precursor suspension

in air at a temperature of 100 jC for about 5 h.

The X-ray diffraction measurements were carried out

using a 2100-Rigaku diffractometer equipped with the Cu

Ka radiation. The IR measurements were performed in a

Perkin Elmer 1600 with software Spectrum 80. The atomic

composition of coatings of set B deposited on copper

substrates was determined using electron dispersive spec-

troscopy (EDS) from a Phillips environmental scanning

electron microcopy model XL30ESEM. The determined

percentage atomic composition was O (45%), Si (38%),

Na (12%) and K (5%).

3. Experimental results

Fig. 1 shows typical IR spectra for samples deposited on

Si substrates heated in air at 100 jC for 1 h. Spectra A and B

correspond to samples of set A and B, respectively. Similar

spectra have been reported in previous IR investigations for

silica-based materials [17–19]. In Fig. 1, the most promi-

nent absorption bands have been denoted with the letters r at

450 cm� 1, b at 800 cm� 1, s in the range of 1000–1300

cm� 1 and OH in the range of 3000–4000 cm� 1. The r, b

and s bands have been assigned to bond rocking, bond

bending and bond stretching vibrations of the Si–O bonds

in the three-dimensional SiO2 network, respectively [17,20],

and the broad OH band with stretching vibrations of the O–

H bond in hydroxyl groups of varying strengths. The

hydroxyl groups are generally attached to the internal walls

of the oxide [21]. The band at about 1630 cm� 1, marked

with an asterisk in spectrum A, is associated with vibrations

Fig. 3. Nitrogen adsorption isotherms for powder corresponding to samples

of sets A and B.

J.A. Calderon-Guillen et al. / Surface & Coatings Technology 190 (2005) 110–114112

of molecular water adsorbed on internal walls of voids in the

oxide matrix [22]. Notice that in sample of set B, the

hydroxyl band is weaker and the adsorbed H2O signal does

not appear.

Fig. 2 shows details of the s band, in the range of 800 to

1400 cm� 1, for the same samples shown in Fig. 1; that is,

spectra A and B correspond to samples of set A and B,

respectively. Both spectra have been decomposed into four

Gaussian bands, which provide the best fit to the experi-

mental data. For spectrum A, the bands 1, 2, 3 and 4 are

located at about 928, 1079, 1210 and 1280 cm� 1, and for

spectrum B, at 965, 1068, 1200 and 1300 cm� 1, respec-

tively. In both spectra, band 2 is the most intense and has

been assigned to transverse optical (TO) asymmetric stretch-

ing vibrations of Si–O–Si units [17,20]. This band has been

reported in the range of 1060 to 1085 cm� 1, depending on

the structural ordering and purity of the SiO2 network

[7,17,20,23,24]. For bands 3 and 4, different assignments

have been documented. Some authors relate them with

longitudinal optical (LO) modes of the same structural units

[25], while others, with breathing vibrations of rings formed

by five and six Si–O–Si units [24]. Band 1 in spectrum A is

associated with silanol (Si–OH) vibrations, which has been

reported in the range of 920–940 cm� 1 depending on the

specific configuration of the local environment [15,19,

20,26]. Band 1 in spectrum B is assigned to TO asymmetric

stretching vibrations of Si–O–Na units [26]; this agrees

with other reports in silica glasses containing sodium, where

a similar band was observed at about 970 cm� 1 [27]. Notice

that this band is near the Si–OH absorption range, and

therefore it could be erroneously assigned to vibrations of

this type; however, if this were the case, one would expect a

much larger intensity of the hydroxyl and adsorbed water

bands in spectrum B of Fig. 1.

Fig. 3 shows the nitrogen adsorption isotherms for

powders corresponding to the samples of set A and B.

Nitrogen adsorption at boiling temperature (� 196 jC)represents the most widely used technique to determine

surface area and to characterize porous structure. Brunauer,

Fig. 2. IR absorption in the range of 800–1400 cm� 1 for the samples in

Fig. 1.

Emmet and Teller (BET) developed a model to determine

the surface area using the adsorption isotherm plots [28].

The starting point is the determination of the adsorption

isotherm, that is, the nitrogen adsorbed volume against its

relative pressure P/Po, where Po is the saturated vapor

pressure of nitrogen at � 196 jC. Following the BET

method, the surface area for the two samples in Fig. 3

was determined, being of 216 and 0.55 m2/g for samples of

set A and B, respectively. For sol–gel-derived samples, a

surface area in the range of 18 [28] to about 550 m2/g [5]

have been previously reported.

Figs. 4 and 5 show the X-ray diffraction data in the range

of 30j to 60j for coatings, on copper substrates, of sets A

and B, respectively. Both samples were annealed in air at

the indicated temperatures for 1 h. A weak and broad X-ray

signal, from the thin amorphous coating, was observed at

about 22j (not shown in Figures). In the patterns corres-

ponding to the samples annealed at 200 jC, only the (111)

at about 43j and the (200) at about 51j diffraction lines

from the copper substrate are observed. After the annealing

Fig. 4. X-ray diffraction patterns for samples of set A on copper substrates,

heat treated at the indicated temperatures.

Fig. 5. X-ray diffraction patterns for samples of set B on copper substrates,

heat treated at the indicated temperatures.

J.A. Calderon-Guillen et al. / Surface & Coatings Technology 190 (2005) 110–114 113

at 300 jC, besides the copper lines, the pattern

corresponding to the coating of set A (Fig. 3) shows the

(111) line at 36j from the Cu2O cubic phase. The latter

phase does not appear in samples of set B (Fig. 5) annealed

at the same temperature. With the annealing at 400 jC, theintensity of the diffraction lines corresponding to the Cu2O

phase increases, and this phase starts to appear in samples

of set B. After the annealing at 500 jC, the CuO phase

appears in the patterns corresponding to both sets of

samples. The formation of the copper oxides is due to the

reaction of atmospheric oxygen, diffusing through the silica

coating, with the copper substrate at the silica/substrate

interface.

4. Discussion of results

The larger surface area found in samples of set A, or in

other words the smaller volume fraction of voids in samples

of set B, is supported by the IR results. In the latter, the

absence of silanol and hydroxyl absorption bands in the

spectra of samples of set B also indicates that these samples

have less amount of pores than those corresponding to

samples of set A.

According to the X-ray data, oxygen diffuses through the

silica coating, reaches the copper substrate and forms a

copper oxide layer at the coating/substrate interface. The

diffusion of oxygen has been previously used to characterize

the structure of several vitreous silica coatings [29–31].

From these works, values for the diffusion coefficient (D),

as low as 10� 19 cm2/s in silica glass at T= 1000 jC [30] and

as high as 10� 3 at room temperature in largely porous silica

gel glasses [31], have been reported. In porous materials, the

value of D depends on the porous structure of the material.

Assuming that the diffusion process is thermally activated,

that is, D =Doexp(�Q/kT), an average activation energy Q

of about 1 eV has been found for the diffusion process of

oxygen in silica-based materials [29–31]. In a previous

report, a model has been developed to calculate D in sol–

gel-derived silica samples deposited on copper substrates

[29] and heated in air at various temperatures. The authors

have proposed an expression which relates Q, Do and to,

where to is the minimum time required at a given temper-

ature to observe the X-ray signal from the Cu2O formed at

the coating/copper substrate interface. The expression de-

scribing the model is, 1/to=(CDo/Nd)exp(�Q/kT) [29],

C = 5.4� 1018 cm� 3 is the amount of oxygen atoms in the

atmosphere, N = 1.2� 1016 cm� 2 is the total amount of

oxygen atoms in the copper oxide layer formed at the

coating/copper interface at the time to, and d = 500 nm is

the coating thickness. From the X-ray data, we estimate the

temperature necessary to observe the Cu2O signal for to = 1

h as 500 K for samples of set A and as 700 K for samples of

set B. Taking Q = 1 eV from the expression above, we

estimate that Do has a value of about 4.5� 10� 2 and

5� 10� 4 cm2/s for samples corresponding to set A and B,

respectively. The estimated D values, at room temperature,

are of 1.2� 10� 16 and 1.3� 10� 18 cm2/s for samples of set

A and B, respectively. The latter means that in samples of

set B, the copper oxide layer at the coating/copper interface

will start forming after about 10 years of exposure to

atmospheric conditions (25 jC).At the present time, we do not have enough elements that

allow us to fully understand the lower oxygen diffusion rate

in the colloidal silica-derived coatings. However, in previ-

ous works [29–31], it has been reported that in silica-based

coating, the diffusion coefficient may be used as a qualita-

tive measured of their porous structure. According to these

results, denser materials have lower diffusion rates, there-

fore one could conclude that the structure of the coatings of

set B is denser or with fewer voids than that of the coatings

of set A. A possible explanation to this observation are the

following. It is known that the introduction of sodium or

any other alkali metal (M) into silica glasses produces

significant changes in glass structure. In pure SiO2 glasses,

all the O2� ions are bound to two Si4 + ions. Thus, the O2�

ions form bridges between neighboring Si4 + ions. The

incorporation of an alkali ion M+ splits open the oxygen

bridging bonds, and each M+ ion gets attached to a

neighboring Si4 + ion, forming u Si–O–M structures

[32,33]. The addition of a few percentage of randomly

distributed alkali metals into the glass structure will break

up the SiO2 structure into chains cross-linked among each

other, forming an entangled type structure. This structure is

probably denser than that of pure SiO2 coatings, where the

small silica particles aggregate to form a granular structure

leaving intraparticle cavities that facilitate the diffusion of

oxygen.

Knowing that the diffusion of oxygen is faster at cracks

and at grain boundaries, we analyzed the surface of both

types of coatings (using Scanning Electron Microscopy)

before and after heat treated at various temperatures. No

J.A. Calderon-Guillen et al. / Surface & Coatings Technology 190 (2005) 110–114114

cracks and no granular structure were detected in the film,

which indicates that the diffusion of oxygen is uniform

through the whole front surface of the coatings.

5. Conclusions

In summary, silica-based coatings were prepared at low

temperature from the condensation of a suspension of

commercial colloidal silica added with two specific struc-

tural modifiers. According to the characterization, these

coatings have a dense structure which acts as an effective

oxygen barrier for temperatures lower that about 300 jC.According to the results, these coatings have a closer

structure than those prepared using the traditional sol–gel

method.

Acknowledgements

We acknowledge the assistance of M.A. Hernandez-

Landaverde and I.E.J. Eleazar Urbina-Alvarez in the X-ray

and ESEM measurements. This work was partially sup-

ported by CONACyT of Mexico.

References

[1] C.J. Brinker, G.W. Sherer, Sol–Gel Science: The Physics and Chem-

istry of the Sol–Gel Process, Academic Press, San Diego, 1990.

[2] J. Gonzalez-Hernandez, F. Perez-Robles, A. Manzano-Ramırez, R.

Ramırez-Bon, E. Prokhorov, Y.V. Vorobiev, F.J. Garcıa-Rodrıguez,

Appl. Phys. Lett. 75 (1999) 3192.

[3] T.P. Ma, K. Miyauchi, Appl. Phys. Lett. 34 (1979) 88.

[4] R.A. Weimer, P.M. Lenahan, T.A. Marchione, Appl. Phys. Lett. 51

(1987) 1179.

[5] S.L. Hietala, D.M. Smith, J.L. Golden, C.J. Brinker, Comm. Am.

Ceram. Soc. 72 (1989) 2354.

[6] P.M. Glaser, C.G. Pantano, J. Non-Cryst. Solids 63 (1984) 209.

[7] H. Imai, M. Yasumori, H. Hirashima, K. Awazu, H. Onuki, J. Appl.

Phys. 79 (1996) 8304.

[8] S. Maekawa, T. Ohishi, J. Non-Cryst. Solids 169 (1994) 207.

[9] T.E. Levine, J.L. Keedie, P. Revesz, J.W. Mayer, E.P. Giannelis,

J. Am. Ceram. Soc. 76 (19993) 1369.

[10] C. Fiori, R.A.B. Devine, Phys. Rev., B 33 (1986) 2972.

[11] T. Mori, J. Non-Cryst. Solids 100 (1988) 523.

[12] R.G. Horn, J. Am. Ceram. Soc. 73 (1988) 1117.

[13] J.M. Fitzgibbons, E. James-French, United State Patent No.

5,626,923, May 6 (1997).

[14] C. Williams Perkins, W. Lewis, United State Patent, No.

5,221,560, June (1993) 22.

[15] J.F. Perez-Robles, L.A. Garcıa-Cerda, F.J. Espinoza-Beltran, M.

Yanez-Limon, J. Gonzalez-Hernandez, J.R. Parga-Torres, F. Ruiz, J.

Mendez-Nonell, Phys. Status Solidi 172 (1999) 49.

[16] J.L. Almaral-Sanchez, J. Alvarez-Quintana, C. Araujo-Andrade,

J.A. Calderon-Guillen, H. Carrillo-Esquivel, E.A. Elizalde-Pena,

N. Flores-Ramırez, F.A. Garcia-Pastor, O. Gomez-Guzman, L.

Licea-Jimenez, D. Meneses-Rodrıguez, A.E. Pena-Hernandez,

S.A. Perez-Garcia, J.C. Rubio-Avalos, A. Salazar-Flores, M. Tala-

vera-Ortega, G. Vazquez-Garcıa, L.D. Vazquez-Santoyo, J. Gon-

zalez-Hernandez, Thin Solid Films 423 (2003) 196.

[17] F.L. Galeener, Phys. Rev., B 15 (1979) 4292.

[18] R.J. Bell, P. Dean, Nature 121 (1966) 1353.

[19] G. Lucovsky, J.T. Fitch, E. Kobeda, E.A. Irene, in: R. Helms, B.E.

Deal (Eds.), The Phys. and Chem. of SiO2 and the Si–SiO2 Interfa-

ces, Plenum Press, 1998, p. 139.

[20] P. Shen, M.F. Thorpe, Phys. Rev., B 15 (1979) 4030.

[21] D.L. Wood, E.M. Rabinovich, J. Non-Cryst. Solids 107 (1989) 199.

[22] G. Cordoba, R. Arroyo, J.L.G. Fierro, M. Viniegra, J. Solid State

Chem. 123 (1996) 93.

[23] I.W. Boyd, J.I.B. Wilson, Appl. Phys. Lett. 50 (1987) 320.

[24] J.R. Martınez, F. Ruiz, Y.V. Vorobiev, F. Perez-Robles, J. Gonzalez-

Hernandez, J. Chem. Phys. 109 (1998) 7511.

[25] C.M. Parler, J.A. Ritter, M.D. Amiridis, J. Non-Cryst. Solids 279

(2001) 119.

[26] A. Duran, C. Serna, V. Fornes, J.M. Fernandez-Navarro, J. Non-Cryst.

Solids 63 (1984) 45.

[27] H. Isobe, I. Tokunga, N. Nagai, K. Kaneko, J. Mater. Res. 11 (1996)

2908.

[28] S. Brunauer, P.H. Emmet, J. Am. Chem. Soc. 60 (1938) 309.

[29] M. Jaroniec, M. Kruk, J.P. Oliver, Langmuir 15 (1999) 5410.

[30] F.J. Garcıa-Rodrıguez, F. Perez-Robles, A. Manzano-Ramırez, Y.V.

Vorobiev, J. Gonzalez-Hernandez, Solid State Commun. 111 (1999)

717.

[31] J.C. Mikkelsen Jr., Appl. Phys. Lett. 45 (1984) 1187.

[32] S. Satoh, I. Matsuyama, K. Susa, J. Non-Cryst. Solids 190 (1995) 206.

[33] Glass, Nature, Structure and Properties, Horst Scholze, Springer-Ver-

lag, New York, 1991.