101

Preparation of Glass-like Materials by Sol-Gel Method Using

Alkoxide Precursor

Dr. Adam Hassan Elhaj Yousif, Department of chemistry, Faculty of

Education, University of Alfashir, Al Fashir , Sudan, E-mail :

Adamhassan43@gmail.com

جل باستخدام -تحضير المواد التي تشبه الزجاج بطريقة الصول

مادة أولية الكوكسيدية

: املستخلص

وتتلخص . يثوكسي السيالن لتكوين مادة اجللتشمل هذه الدراسة حلمأة رباعي إ

يثوكسي سيالن من تفاعل رباعي كلورو السيالن ذا الدراسة يف تصنيع مادة رباعي إه

ثو . يثوكسوي السويالن يف حالوة ة يوة مون النوات يفصول ربواعي إ و. مع الكحوو اييثيلوي

ين ى ويضاف إليه قليل من املاء يف وجوود عامول مسواعد ضوي او قاعود حتوى ينت وي

م 000ويوو ا اجلوول ليثووحير ثوو دووة عنوود درجووة حووةارة . التفاعوول بتكوووين مووادة اجلوول

ضوةة باسوتخدام ا و ة ومن ث حتلل النووات ام . للحصو على املادة اليت تشبه الزجاج

حيووأ اكودت ةتوواه هوذه التحاليوول تكوون مووادة . حتو اممووةاء وحيوود ا وو ة السوينية

. اجلل واملادة الصلبة اليت تشبه الزجاج على درجة عالية من الن اء واجلودة

Abstract:

This study involves simple treatment of tetraethoxysilane with

water which results into formation of gel. This work shows the

conversion of tetrachlorosilane to tetraethoxysilane. The hydrolysis

and poly-condensation reactions of this product, the backbone of this

study which result in formation of the gel. The aging process of

formed gel followed by drying and firing at 6000 C gave a solid glass-

like material. The obtained products were analyzed by IR and XRD.

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The results of these analyses confirmed the formation of gel and high

purity glass-like material.

Keywords : sol-gel , tetraethoxysilane, Glass-like material, Gelation

time.

1- Introduction:

Interest in sol-gel processing of inorganic ceramics and glass

materials began as early as the mid 80s with (Paul 1990) Ebelman and

Graham’s studies on silica gel. These early investigators concluded

that the hydrolysis of tetraethoxysilane (TEOS), Si(OC2H5)4 , under

acidic condition yielded SiO2 in the form of “ Glass-like material”,

essentially, this study centers round glasses or – materials derived

from the sol gel process. The use of this process in the preparation of

silicate gel, glasses, and ceramics has been an active area of research

for the past three decades (Bryans et. al 2000)

Sol- gel process used liquid alkoxide precursor such as

tetraethoxysilane to react with water in the presence of acid or base

catalyst at room temperature to produce gel. (Chrusciei & Slusarski

2003)

Si(OC2H5)4 + 2H2O SiO2 + 4C4H5OH 1

The gel product is then aged and dried to the formation of bulk

glasses and ceramics, ultra porous materials (aerogels and Xerogles),

and thin films. The stages of sol – gel process using tetraethoxysilane

precursor are presented in the following scheme 1 (khimich 2004):

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Scheme1: the stages of sol-gel process using TEOS precursor

The synthetic routes to glass formation by sol gel processes can be

divided into two distinct areas, namely the alkoxide route and the

aqueous route (Hench & West 1990). The latter route rely on

generating a stable dispersion of colloidal sized particles in a solvent

(in this case water) and destabilizing this sol in a controlled manner to

give a solid gel from which the solvent can be removed and then

sintering to give a dense amorphous mass.

The schematic representation of aqueous route to silica glass is shown

below (Bradley 1960)

Sodium silicate solution Na2 SiO3

Silicic acid Si(HO)4 - NaO - resin

(HO)3Si – O – Si(OH)3 condensation

H2O

Dense silicate particle with hydroxylated surface for the condensation and agglomeration

Silica gel

Scheme 2 : preparation of silica glass by aqueous route.

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The alkoxide route has attracted more attention in recent years.

This route is also divided into two types. These types are aqueous and

nonaqueous (Fig. 3) (Kichelbick 2008):

Sol – gel process

aqueous: step 1 step 2

Hydrolysis condensation between

M(OR)4 H

2O / (cat)

(RO)3M-OH active species

(RO)3M-O-

M(OR)3+ROH

Precursor active species R= organic rest or H

Metal – Alkoxide

Nonaqueous:

M(OR)4 (RO)

3M – O – R

condensation

(RO)3M-O-

M(OR)3+ROR

Precursor (RO)3

M – O – R

Metal – Alkoxide

Fig. 3 Similarities and differences of sol – gel process using metal

Alkoxide precursor:

The following equations describe the fundamental reactions

which allow the conversion of monomeric organ metallic precursor

into gels and ultimately glasses or Ceramics (Lukowiak & Strek

2009):

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Hydrolysis

M(OR)X + H2O M(OR)n-1 OH + ROH

H

2O

M(OR)n-2 (OH)2 + ROH

Condensation

M-OH + HO-M M-O-M + H2O

M-OH + RO-M M-O-M + ROH

M-OR + OR-M M-O-M + ROR

[M is a metal chosen from (Al, In, Si, Ti, Zr, Sn, Pb, Ta, Cr, Fe, Ni,

Co) and others, R represents alkyl group].

In general the sol-gel process is a low temperature glass

formation method that consists of hydrolysis and condensation

reactions (see scheme 4) of metal alkoxide, such as tetramethoxysilane

(TMOS) or Tetraethoxysilane (TEOS) (Bryans et. al 2000)

≡SiOR + H2O

≡ SiOH + ROH (1)

2≡SiOH

≡Si – O – Si≡ + H2O (2)

≡SiOH + ROSi≡

≡Si – O – Si≡ + ROH (3)

Scheme4: Representation of the sol –gel process.

In the hydrolysis reaction, (Eq.1), the addition of water replaces

alkoxide group, (OR), with hydroxyl Group, (OH). Subsequent

condensation reactions (Eq.2 and 3) involving the silanol groups (Si-

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OH) produce siloxane bonds (Si – O – Si) Plus the byproducts water

or alcohol (keefer 1990)

The gelation point of any system, including sol-gel silica, is

easy to observe qualitatively and easy to define in abstract terms but

extremely difficult to measure analytically. As the sol particles grow

and collide, condensation occurs and macro particles form. The sol

becomes agel when it can support a stress elastically. This is typically

defined as the gelation point or gelation t, (t gel). Gelation time (t gel)

of hydrolyzed tetraethoxysilane depends on some experimental

parameters. These parameters include catalyst nature, PH value of

solution, (Klein et. al 2010), the ratio of water to alkoxysilane, Alkoxy

group (OR), (Rakhimov et. al 2009) Temperature and solvent

(chrusciei & Slusarski 2003). When a gel maintained in its pore liquid,

its structure and properties continue to change long after the gel point.

This process called aging. The gel is dried by one of a number of

methods.

The method used here had been indicated by the characteristics

and composition of the material. Drying techniques which have been

employed include filtration, oven drying, vacuum drying, spray

drying, microwave drying, liquid drying and freeze drying (Aravind

et. al 2010). Drying time has not been qualified. Firing of the hydrated

species is converted to oxides and clear glass-like materials are

obtained. Thus the glass can be made by sol-gel process (Voronkov et.

al 2008). This process of disordered ceramics from liquid chemical

solution, (referred to as sol - gel processing), followed by

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densification, convert into glass (Chiang et.al 1997). Despite sol-gel is

widely applied, the chemistry utilizing metal alkoxide precursor is

hardly understood (kesslar et. al 2006). Therefore, the aim of the

present work is to prepare glass-like materials using tetraethoxysilane

as alkoxide precursor by sol-gel method and study the reactions

parameters involved in this process.

2. Experimental

2.1 Chemicals

All chemical used were of analytical grade type.

2.2 Preparation of tetraethoxysilane by Halosilane Route.

50 ml – 150 ml of absolute ethanol were placed in the reaction

flask , 6-18 cm3 of tetrachlorosilane were then added drop wise from a

separatery funnel. The mixture was then brought to the programmed

temperature range of 60 – 70 Co. the reaction was allowed to proceed

under reflection was stopped by rapid cooling. A colorless liquid was

obtained. The resulting product was analyzed by IR. For results see

Figure 0 & Table1.

2.3Preparation of Gels by Acid or Base catalyzed hydrolysis of

tetraethoxysilane and condensation of their hydrolyzed species:

2.3.1 Preparation of Gel-1 by Acid catalyzed hydrolysis of

tetraethoxysilane and condensation of their hydrolyszed species

tetraethoxysilane (6 ml, 5.556 g/cm3) was placed in a test tube, also

hydrochloric acid ( 1 ml, ph 1) was added. 1 ml of absolute ethanol

used as solvent dropwise and 1 ml of double distillated water. The

mixture began to form gel. After 144 hrs there was a finger of gel in

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the test tube at room temperature, after that test tube was wormed to

take out figure of the gel and weight. The gel was analyzed by IR

spectroscopy. For results see Table (2), Figure 1 & Table 3.

2.3.2 Preparation of gel-2 by base catalyzed hydrolysis reaction of

TEOS and condensation of its hydrolyzed species.

6 ml of tetraethoxysilane was placed in a test tube and ammonium

hydroxide ( 1 ml , PH = 13) was also added 1 ml of absolute ethanol

used as solvent & drop wise 1 ml of double distilled water. The

mixture began to form a gel. After 261 hrs there was a finger of gel in

the test tube. After that test tube was wormed to take out finger of the

gel and weigh it. The gel was analyzed by IR for results see Table 4,

Figure 1.

2.4 Preparation of Glass-like materials from gels prepared by

Acid or base catalyzed hydrolysis reaction of TEOS and

condensation of their hydrolyzed species.

Glass–like materials have been prepared by drying and firing of

the prepared gels according to programmed temperature as follows:

1- Heat at 650 C for 3 – 4 hours

2- Heat at 950 C

for 2 – 3 hours

3- Heat at 1200 C for 3 hours

After that the dried gel heat at 180 Co for 2 hours. Then the obtained

gel placed in porcelain curricible and fired at 600o C.

2.4.1 Preparation of Glass-like material – 1 by Drying & firing of

gel-1 prepared from TEOS precursor.

Glass-like material -1 has been prepared by firing the gel-1

prepared in experiment (2.3.1) using programmed temperature to give

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dried gel. Dried gel was heated at 180 Co for 2 hours. Then the

resulting gel was placed in porcelain curricible and fired at 600 Co for

2 hours. The final product of glass-like material was analyzed by IR

and XRD. For results see Fig. 1 & Fig. (11).

2.4.2 Preparation of glass-like material – 2 by Drying & firing of

gel-2 prepared from TEOS precursor.

Glass-like material -2 has been prepared by firing of the gel-2

prepared in experiment (2.3.2) using programmed temperature to give

dried gel. Dried gel was heated at 180 Co for 2 hours. Then the

obtained gel was placed in porcelain curricible and fired at 600 Co for

2 hours. The ultimate product of glass-like material was analyzed by

IR and XDR. For results see Figure 10 and Figure 11.

2.5 Instrumentation

The prepared compounds such as TEOS, gels, & Glass-like

materials were analyzed by IR spectroscopy. Infrared spectra were

recorded on a 800 – pc FT IR schimadzu spectrophotometer using K

Br pellets in the mid infrared region 4000 – 400 Cm-1

. Also infrared

spectra were recorded on a 4100 – FT IR Jacso spectrophotometer

using K Br pellets in region 4000 – 400 Cm -1

.

The obtained glass-like materials were also characterized by X-ray

diffraction (XDR) analysis using AD8 advanced X-ray from Bruker

Analytical X-ray system model.

3. Results & Discussion:

Two glass-like materials were prepared by three stages. The first

stage is the preparation of tetraethoxysilane (TEOS) by halosilane

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route. The second stage includes the synthesis of gels by hydrolysis of

tetraethoxysilane (TESO) in the presence of acid or base catalyst and

condensation of their hydrolyzed species. The third stage involves

drying of the prepared gels followed by firing at 600 Co to convert to

glass-like material. These stages will be illustrated as following.

3.1 Synthesis of TEOS by the reaction tetrachlorosilane and

absolute ethanol:

Tetraethoxysilane was prepared by the reaction of

tetrachlorosilane and absolute ethanol (2-2) according to the following

equation:

SiCl4 + 4EtOH Si(OEt)4 + 4HCl

The obtained colourless liquid analyzed by FT IR analysis

showed the presence of the bands 88 1 cm-1

, 1090 cm-1

, 1050 cm-1

&

804 cm-1

. These results indicate the presence of TEOS, which is in

good agreement with the reported values (Zaman & Bhuiyan 2009)

for tetraethoxysilane.

111

Fig. 6 In

frared sp

ectra of th

e pro

du

ct ob

tained

form

the reactio

n o

f tetrach

loro

silane an

d ab

solu

te ethan

ol

111

. Table:(1) Assignments of IR absorption peaks for TEOS prepared by

the reaction of tetrachlorosilane and absolute ethanol.

Frequency cm-1

Assignment

3345 B O-H stretch

2974 M C-H asymmetric stretch

2256 W

1925 W

C-H asymmetric stretch

1381 S

1332 W

C-H bend

1275 W CH3 rocking

1090 M

1050 S

C-O stretch

881 S C – C stretch

804 W SiO4 asymmetric stretch

B= broad, S = strong, W = weak, M = moderate

3.2 Characterization of gels prepared by hydrolysis of

tetraethoxysilane

Gels were prepared by hydrolysis of tetraethoxysilane using acid

or base catalyst. These products were characterized by FT IR

spectroscopy and their results will be discussed in the following

sections (3.2.1, 3.2.2 & 3.2.3)

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3.2.1 Characterization of gel-1 prepared by acid catalyzed

hydrolysis of tetraethoxysilane.

Gel-1 has been prepared by hydrochloric acid catalyzed

hydrolysis of TEOS (2.3.1). The results of this experiment were

summarized in Table 2.

The final product was analyzed by FT IR spectroscopy. FT IR

analysis of obtained product was given in Fig.1 & Table 3.

Table (2): Summary of gel-1 prepared by acid catalyzed

hydrolysis reaction of TEOS and its hydrolyzed species of

condensation.

TEOS 6 ml

HCl 1 ml, PH = 1

H2O 1 ml

C2H5OH 1 ml

Gelation time / hrs 144

Product weight / g 1.937

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Fig. 7 In

frared sp

ectra of th

e gel-1 p

repared

by acid

catalyzed h

ydro

lysis reaction

of TEO

S and

con

den

sation

of its

hyd

rolyzed

species.

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Table 3: Reported FT IR characteristic bands for gel-1 prepared

by acid catalyzed hydrolysis of TEOS and condensation of its

hydrolyzed species.

Assignments Frequency / Cm-1

References

Obtained Literature

O-H

stretching

3433 3450 – 3400 (Al-owein & El-Rasy

2009; Nadargi et, al

2009)

O-H

deformation

1634 1653 – 1634 (Socrtaes 2001)

Si-O

stretching

1082 1200 – 100 (Estella et al 2007;

Dobryanska et. al

2006; Smith 1974)

Si-O –Si

stretching

795 800 (Al-owein & El-Rasy

2009; Nadargi et, al

2009)

Si-O –Si

bending

467 467 (Al-owein & El-Rasy

2009; Nadargi et, al

2009)

Fig. 10 & Table 3 show the results of FT IR analysis for the

prepared gel-1. The FT IR absorption band near 3450 cm-1 is

attributed to stretching modes of residual Si-OH and some adsorbed

water. This band goes often accompanied by another at 1634 cm-1

,

which is attributed to – OH vibration of molecular water that is

physically adsorbed in the net work. Because the low intensity of 1634

Cm-1

band, the 3433 Cm-1

band would reflect the contribution of

silanol groups. The smaller the amount of residual Si-OH, the larger

degree of condensation. The most intense peak was observed at 1082

cm-1

due to the asymmetric stretching vibration of (Si-O) bond.

Oxygen atoms play the role of bridges between each two silicon sites

110

in these silicon- oxygen covalent bonds. On the other hand, the

symmetric stretching vibration of (Si –O– Si) appear at 550 cm-1 and

its bending mode appear at 467cm1. The low energy band at a round 5

Cm-1

is assigned to (Si – O – Si) stretching of the (SiO2 ) network

defects.

3.2.2 Characterization of gel – 2 prepared by base catalyzed

hydrolysis reaction of TEOS and condensation of its hydrolyzed

species:

Gel-2 has been prepared by hydrolysis reaction between TEOS

and water using ammonium hydroxide as catalyst and absolute ethanol

as solvent (2.3.2) the results of this experiment were presented in

Table 4. The obtained gel-2 product was analyzed by FT IR

spectroscopy. FT IR analysis of gel-2 product was shown in Fig. 11

below.

Table 4: Summary of gel-2 prepared by base catalyzed hydrolysis

reaction of TEOS and condensation of its hydrolyzed species:

TEOS 6 ml

NH4OH 1 ml, PH = 13

H2O 1 ml

C2H5OH 1 ml

Gelation time / hrs 216

Product weight / g 0.941

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Fig. 8 In

frared sp

ectra of gel-2

prep

ared b

y base catalyzed

hyd

rolysis reactio

n o

f TEOS &

con

den

sation

of its h

ydro

lyzed

species.

111

The prepared gel-2 was characterized by FT IR spectroscopy. Fig.

11 illustrates the results of FT IR analysis for gel-2. These results

were found to be similar to the results of FT IR analysis of gel –

1prepared by acid catalyzed hydrolysis of TEOS and water. The

results are in good agreement compared with the literature values (Al-

Owein & El-Rasy 2009) indicating the formation of the gel-2.

3.2.3 Gelation time

In the current work the effect of the PH value (type of catalyst) on

gelation time for hydrolysis reaction of TEOS has been carried out in

a similar experimental condition. Gelation time for base catalyzed

hydrolysis reaction of TEOS is longer than the gelation time of acid

catalyzed hydrolysis of TEOS see Table 2 & Table 4. So that the

acidic catalysis results in much quicker hydrolysis than the basic one

at the same concentration of a protolytic catalyst, indicating that

tetraethoxysilane is stronger Lewis base than Lewis acid.

3.3Characterization of Glass-like materials synthesized from the

prepared Gels.

Two glass-like materials such as glass-like material-1 and glass-

like material-2 were analyzed according to sections (2.4.1 & 2.4.2)

respectively. The ultimate glass-like materials were characterized by

FT IR & XDR for results see Fig.1, Fig. 10, Fig. 11 & Fig. 11.

111

Fig 9 in

frared sp

ectra of glass-like

material-1

prep

ared fo

rm G

el-1

.

110

Fig 10

infrared

spectra o

f glass-like m

aterial-2 p

repared

from

Ge

l-2.

111

Fig.1 & fig. 10 show infrared spectra of glass-like material-1&

glass-like material-2 respectively. The FT IR absorption band at

1107cm-1

due to asymmetric stretching vibration of the Si-O bond

constitutes asketel SiO2 network. This band goes together with two

bands a round 800 Cm-1

and 471 Cm-1

which are assigned,

respectively, to bond bending and bound rocking vibration of Si-O

bonds in the three dimensional net work. The absent of the absorption

bands in the FT IR region from 4000 – 1200 cm-1

indicate that the

obtained products have not any organic groups in their structures.

Because all carbon and hydrogen atoms burned off at 600 C during the

firing leaving glass-like materials as high purity products.

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Fig 11

x-ray po

wd

er diffractio

n o

f Glass-like

material-1

prep

ared fro

m G

el – 1

111

Fig 12

x-ray po

wd

er diffractio

n o

f Glass-like

material-2

prep

ared fro

m G

el – 2

111

X-ray diffraction analysis of the obtained glass-like materials was

carried out and the results were presented in Fig.11 & Fig.11. It was

found that the prepared glass-like materials were amorphous solid,

(Ameen et. al 2008), because no crystalline phase was detected by

conventional powder XDR.

4- Conclusion:

The glass-like materials were prepared from the densed gels

which are the hydrolytic products of metal alkoxides or semi metal

alkoxides such as tetraethoxysilane. Kinetic studies of gelation for

hydrolysis reaction of TEOS in the acid or base catalyst were

investigated in the present work using similar experimental

conditions. It was found that gelation or rate of hydrolysis reaction of

TEOS using acid catalyst is higher than using base catalyst because,

gelation depends on PH value. Sol – gel process has been described to

be the technological back up for the production of these glasses with

better purity and homogeneity than high temperature conventional

method or process.

111

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Acknowledgement

The editorial board would like to thank the reviewers, who have

reviewed the manuscripts submitted to the journal. Their efforts

have been indispensible for the publication of this issue. They

are:

1. Prof Mukhtar Ahmed Mustafa Uof K

2. Prof Kamal Fadl Elseid Elkhalifa U of K.

3. Prof.Sanaa Osman YagoubEl Neelain University

4. Prof. Mohamed Ali Hussain. Africa International University

5. Prof.Yousif Rizgalla Sulieman Bahri University

6. Prof.Omer Yousif Alhussain U of K

7. Prof. Aisha Zohair Almagboul Medicinal and Aromatic Plants

Research Institute

8. Prof. Osman Taha Elzaki Institute of Technological Research

9. Dr. Dr. Zaynab Abdallah Yosuf Bahri University

10. Dr. Ahmed Mohamed Adam Eldoma Sudan University of

Technology

11. Dr.Eltom Elsadig Ali Desertification and Desert cultivation

Institute.U of K

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12. Dr.Eltayeb Elhag Ali Ahmed UNESCO Chair of Desertification

U of K

13. Dr.Elmahdi Ahmed Haroun Bahri University