polyacrylamide gel method: synthesis and property of beo nanopowders

ORIGINAL PAPER

Polyacrylamide gel method: synthesis and propertyof BeO nanopowders

Xiaofeng Wang • Richu Wang • Chaoqun Peng •

Tingting Li • Bing Liu

Received: 6 August 2010 / Accepted: 7 September 2010 / Published online: 24 September 2010

� Springer Science+Business Media, LLC 2010

Abstract Effects of monomer (AM) concentration,

monomer/crosslinker (AM/MBAM) ratio and salt concen-

tration on the thermal behavior of precursor gel and the

properties of BeO nanopowder synthesized by polyacryl-

amide gel method were investigated. The decomposition

process of precursor gel was also studied. The decompo-

sition process of precursor gel is that, first, the extraction of

free and crystallized water, and then the thermal degrada-

tion of polymeric network under temperature higher than

600 �C, final, the decomposition of nanoscale beryllium

sulfate to BeO nanopowder. As the monomer concentration

increases, the calcination temperature of precursor gel

decreases due to more compact network structure of gel

and thus smaller size of salt in nanocaves in gel. The

average particle size of nanopowder reduces correspond-

ingly. The AM/MBAM ratio also has significant effect on

the thermal behavior of precursor gel and the average

particle size of product. When the ratio of AM to MBAM is

6, the calcination temperature of precursor gel is the low-

est, the average particle size of powders is the smallest,

because the network structures of gel is the tightest and

thus the sizes of salts in precursor gels are the smallest. As

the AM/MBAM ratio deviates from this value, the network

structures of gel becomes looser and thus the size of salt in

precursor gel becomes larger, so the calcination tempera-

ture increases and the average particle size of powders

becomes larger certainly. For the same reason, both the

calcination temperature and the average particle size of

powders increases with increasing the salt concentration.

The synthesis conditions have no effect on the particle size

distribution of the final product due to the natural random

distribution of porosity in gel.

Keywords Polyacrylamide gel method � Synthesis �Nanopowder � BeO � Property

1 Introduction

Most of the research in the broad field of Nanoscience is

dedicated to the development of synthesis routes to nano-

particles, which is the base of the application of nano-

technology [1]. The aim of these efforts is to get

nanoparticles with a wide range of homogeneous compo-

sitions, monodisperse crystallite sizes, unprecedented

crystallite shapes, and complex assembly properties.

Obviously, conventional solid state chemistry paths,

involving the reaction of a mixture of powders, are inef-

ficient due to uncontrolled crystal growth during the step of

high temperature calcinations [1]. Composition inhomo-

geneities, grain size non-uniformity, uncontrolled shape are

other disadvantages of classical methods.

Wet chemical routes provide sophisticated alternatives

because the mixing of precursors occurs at a molecular

level, with constituent metal cations being dissolved in

solution. The state-of-the-art methods are precipitation [2]

and co-precipitation [3], hydrothermal method [4–6],

sol–gel [7, 8], and so on. One of the most attractive syn-

thesis methodologies is the sol–gel method [9]. In this

process, the solid oxidic network, in which metal ions are

uniformly distributed, is formed with the hydrolysis and

condensation of molecular precursors in solution. The

aqueous route uses inorganic salts dissolved in water

whereas alkoxides dissolved in organic solvents are used in

the metal-organic route. However, these precursors are

X. Wang � R. Wang (&) � C. Peng � T. Li � B. Liu

School of Materials Science and Engineering,

Central South University, 410083 Changsha, China

e-mail: [email protected]

123

J Sol-Gel Sci Technol (2011) 57:115–127

DOI 10.1007/s10971-010-2331-5

much more expensive and difficult to handle than aqueous

solutions. Furthermore, it is difficult to control the reaction

mechanisms for the synthesis of binary or ternary oxides

due to the different chemical behavior of each cation [10].

Thus, the sol–gel method should be modified and the

synthesis strategy of network might be changed. In fact, a

rational route to form network has been proposed with the

aid of organic polymers or chelates [11].

Many years ago, a modified sol–gel process was

developed by Pechini for metals that are not suitable for

traditional sol–gel type reactions due to their unfavorable

hydrolysis equilibria [11]. Although the original method

was developed specifically for the preparation of thin

electronic ceramics such as titanates and niobates for

capacitors, it was later adapted to the synthesis of pow-

dered products. In this process, the network is formed via

polymeric precursors made from hydroxycarboxylic acid

(citric acid) and polyhydroxyl alcohol (ethylene glycol).

Citric acid is used to chelate with various cationic pre-

cursors by forming a polybasic acid. In the presence of

ethylene glycol, these chelates react with it to form organic

esters and water by-products. When the mixture is heated,

polyesterification occurs and leads to a homogeneous sol,

in which metal ions are uniformly distributed throughout

the organic matrix. When the sol is further heated to

remove the excessive solvents, an intermediate resin with

polymeric network structure will be formed. Burning this

resin results in ceramic powders with better chemical

homogeneity and smaller particle size. The morphology of

its resin intermediate influences the final oxide powder and

can modify their properties within a given composition.

Later, numerous variations of the Pechini method have

been proposed, most involving alternative chelating agents.

Ethylenediaminetetraacetic acid (EDTA) [12, 13], oxalic

acid [14], starch type [15] and poly(vinyl alcohol) (PVA)

[14, 16] are occasionally substituted for citric acid. How-

ever, citric acid is still the most widely used chelating agent

for the carboxylate gel process and forms relatively stable

complexes with a variety of metal cations in its ionized

form in the pH range from about 3 to 10. Although the

Pechini method has been previously studied for the prep-

aration of fine powders, such as Y2O3 [17], and yttria-

stabilized zirconia [18], Bi2Ru2O7 [19], Y3Al5O12(YAG)

[20], and numerous perovskite-structured materials [21–24],

it turns out that the specific synthesis route cannot be

efficient. The reason lies in the fact that it has a time

consuming step of concentration and dehydration in a

revolving evaporator under reduced pressure and complete

drying in a vacuum oven below 100 �C.

In 1989, an improved sol–gel method was developed

again by Odier to produce YBa2Cu3O7-x [10]. It takes

advantage of the auxiliary three-dimensional (3D) tan-

gled polyacrylamide gel network formed in solution

polymerization. Hence, cations that are entrapped into a

solution in nanocavities form inside the gel. A steric

entrapment of stoichiometric cation solution occurs, that is,

an homogeneous microsolution with cations in the desired

stoichiometry is formed. In contrast to the progressive

transformation from viscous to resin in the Pechini method,

this method is a time-saving method because the artificial

gel formation at low temperature is rapid [9, 25, 26]. It is

also a simple and cheap method used for the synthesis of

fine powders in that the raw materials for polyacrylamide

gel are inexpensive and the formation of gel is generally

easy [27]. Nowadays, more and more researchers are

interested in this method and it has been used to prepare

different oxide ultrafine powders or nanoparticles, metallic

and oxide compounds especially, such as 2SiO2–3Al2O3

(mullite) [10], a-Al2O3 [25, 27], Zr2O3 [9], BiO2 [28],

nanocrystalline YVO4:Eu [29], SnO2:Eu [30] and ultrafine

powders for solid oxide fuel cells (SOFCs) such as

Zr0.84Y0.16O1.92 [26], Ce0.8Gd0.2O1.9 [26] and La0.85Sr0.15-

Ga0.85Mg0.15O2.85 [31]. Although this method has been

used more and more extensively, synthesis conditions on

the thermal decomposition of precursors and the size of

nanopowders have not been systematically studied.

The main aim of the present work are: (a) to understand

the decomposition process of precursor gel; (b) to investi-

gate the influence of synthesizing parameters (AM con-

centration, AM/MBAM ratio and salt concentration) of

precursor gel on thermal behavior; (c) to discuss the rela-

tionships between these synthesizing parameters and the

size, particle distribution and morphology of nanopowders.

The effects of monomer (AM) concentration, the ratio (AM/

MBAM) of monomer to crosslinker (MBAM) and salt

concentration were tested. In order to achieve these aims,

Thermogravimetric analysis (TGA-DSC/TGA), X-ray dif-

fraction (XRD), Brunauer-Emmett-Teller (BET) isotherm

technique with nitrogen adsorption and Transmission

electron microscopy (TEM) techniques were employed.

2 Experimental

2.1 Raw materials

The characteristics of the materials used are summarized in

Table 1. All of the reagents are of AR grade, except dis-

tilled water produced in our laboratory is of chemically

pure (CP) grade.

2.2 Synthesis procedure

The typical experimental procedure of polyacrylamide gel

route for preparing nanopowders is as follows. Firstly,

transparent BeSO4 solutions with different concentrations

116 J Sol-Gel Sci Technol (2011) 57:115–127

123

were prepared by dissolving the beryllium sulfate in the

distilled water with moment stirring. After that, some

amount of organic agents AM and MBAM monomers

with different molar ratios were added to the pre-mixed

solutions with constant stirring till clear solution was

observed. Then, freshly made 10% (mass fraction)

ammonium persulfate (APS) as initiator was added, and

the temperature of the solution was increased slowly to

60 �C with water bath. The free-radical crosslinking

copolymerization of AM and MBAM was initiated by the

initiator APS. The mixture was turned gradually to

transparent hydrogel. In order to make sure that organic

monomers reacted absolutely, the condition was held for

1 h. The gel was dried at 80 �C for 48 h in vacuum drier.

The dried gel thus formed was homogenized in a ceramic

mortar and calcined with a laboratory furnace in an

oxygen atmosphere to obtain nanopowders of a pure BeO

phase. The thermal treatment was applied at a heating rate

of 5 �C/min up to the desired holding temperature during

2 h, followed by the same cooling rate.

Obviously, the original materials of the polyacrylamide

gel method should be soluble in water. In order to study the

effect of synthesis conditions (AM/MBAM ratio, monomer

concentration and salt concentration) on preparation process

and properties of powders, the solubility of AM, MBAM

and sulfate salt should be taken into account. According to

the solubility of raw materials (Table 1), the ranges of

monomer AM concentration, mass ratio of AM/MBAM and

salt molar concentration are \67%(mass fraction), \107.5

and\2.2 mol/L, respectively. In our experiments, however,

MBAM more than 10% (mass fraction) in mixture solution

is insoluble. Hence, three sets of experiments under differ-

ent synthesis conditions of precursor gel samples were

designed, as shown in Table 2. In each set, one of the

parameters (i.e. monomer AM concentration, mass ratio of

AM/MBAM and salt molar concentration) varied while the

other parameters were kept constant. Experiments based on

the above sets were designed as follows: Conditions of set 1

in Table 2 are designed to study the effect of the ratio of

AM/MBAM. Samples prepared under conditions of set 3 in

Table 2 are used to study the influence of monomer AM

concentration. Conditions of set 2 in Table 2 are for

studying the effect of salt concentration.

2.3 Characterization

To define the precise calcinations temperature, the beryllium

sulfate salt and precursor gels (dried gels) were studied by

thermogravimetric analysis (TGA-DSC/TGA, STA 449C,

Netzsch) at a heating rate of 5 �C/min from room tempera-

ture to 1,000 �C, and an air flow of 30 cm3/min. X-ray dif-

fraction (XRD) profiles were obtained from synthesized

powders for phase identification and determination of crys-

tallite size using Cu–Ka radiation (D/Max2550VB?, Riga-

ku) source in the range of 2h = 20�–90�. The mean diameter

of crystallites was determined from X-ray diffraction line

broadening using the Scherrer formula. The particle size,

size distribution and morphology of the powders prepared

Table 1 Characteristics of raw materials

Raw material Function Molecular formula Solubility in 100 mL water/g [32]

Beryllium sulfate tetrahydratea Initial salt BeSO4�4H2O 39.1 (20 �C)

Acrylamide (AM)b Monofunctional monomer C2H3CONH2 215 (30 �C)

N,N0-Methylene bis acrylamide (MBAM)b Difunctional monomer(crosslinker) (C2H3CONH2)2CH2 2 (45 ± 5 �C)

Ammonium persulfate (APS)b Initiator (NH4)2S2O8 –

Water Solvent H2O –

a Supplier is Shuikoushan Nonferrous Metals Co., Ltd., Chinab Supplier is Shanghai Chemical Reagent Co., Ltd., China

Table 2 Experimental design for studying the effect of synthesis

conditions on the thermal decomposition of precursors gel and the

size of nanopowders

Set

number

Sample

number

Ratio

of AM/

MBAM

Monomer

concentration/%

Salt

concentration/

(mol L-1)

1 1 3:1 3 1.5

2 4:1 3 1.5

3 5:1 3 1.5

4 6:1 3 1.5

5 7:1 3 1.5

6 10:1 3 1.5

7 20:1 3 1.5

2 8 20:1 5 0.25

9 20:1 5 0.5

10 20:1 5 1

11 20:1 5 1.5

3 12 20:1 5 1.5

13 20:1 10 1.5

14 20:1 15 1.5

15 20:1 20 1.5

J Sol-Gel Sci Technol (2011) 57:115–127 117

123

were examined by transmission electron microscopy (TEM,

CM-300, Philips) or field-emission scanning electron

microscopy (SEM, Sirion-2000, Philips). Specimens for

transmission electron microscopy were prepared by sus-

pending the fine powder in alcoholic on carbon grids.

Specific surface areas of synthesized powders were mea-

sured by the Brunauer-Emmett-Teller (BET) isotherm

technique with nitrogen adsorption using a Micromeritics

ASAP 2000 surface area analyzer.

3 Results

3.1 Thermal decomposition of beryllium sulfate

For reference, thermal analysis (TG-DSC) of beryllium

sulfate was carried out. The results given in Fig. 1 dem-

onstrate two-stage dehydration between 100 and 300 �C

and confirm the formulas as follows.

BeSO4 � 4H2O! BeSO4 � 2H2Oþ 2H2O " ð1ÞBeSO4 � 2H2O! BeSO4 þ 2H2O " ð2Þ

This is followed by breakdown of the anhydrous sulfate

above 670 �C to BeO, corresponding to the following

formula

BeSO4 ! BeOþ SO3 " ð3ÞAs can be seen, complete conversion from BeSO4 to

BeO occurs at temperatures higher than 867 �C, which is

consistent with the research of Dollimore and Konieczay

[33]. Thus, Beryllium sulfate has a simple thermal

decomposition process during calcination. This confirms

that it is an excellent model material for investigation of

the calcination process of precursor gel in the polyacryl-

amide gel method.

3.2 Effect of monomer concentration

In order to investigate the effect of monomer concentration

on the synthesis process of polyacrylamide gel method, 4

kinds of precursor gel (dried gel) made from different

monomer concentration (5, 10, 15, 20%, mass fraction) were

prepared. Figure 2 illustrates the thermogravimetric analy-

sis (TGA) results of precursor gels with different monomer

concentrations, examined also under the same conditions

preciously used for beryllium sulfate. The mass loss of

precursors is found with a rise of temperature, which is

similar to the observations of sulfate (as shown in Fig. 1).

However, differences still exist. It is seen that as the

monomer concentration goes down, steps on the TGA

curves corresponding to the decomposition of sulfate con-

tained in precursors (the same kind as Fig. 1) are more and

more obvious. This means that the monomer concentration

play an important role in the decomposition step of pre-

cursor gels. Besides, the temperature of complete decom-

position of precursor gel (ending up temperature) increases

with the increasing of monomer concentration. For example,

the temperature of complete decomposition of the precursor

with 20% monomer is 675 �C, while that of the one with

20% monomer is 820 �C. Note that there is more ‘‘frilling’’

on TGA curves than that on other curves (the reason of this

strange observation will be discussed in later section).

All of the precursors made from different monomer

concentrations were calcinated at 700 �C for 2 h. Figure 3

displays the X-ray diffraction patterns (XRD) of the

obtained powders. It can be seen that powders from pre-

cursor gels with higher monomer concentrations (10%, 15,

and 20%, mass fraction) are composed of pure a-BeO, but

the one from precursor with lower monomer concentration

(5%, mass fraction) is composed of impure a-BeO, which

covers anhydrous sulfate. This suggests that the precur-

sor with 5% (mass fraction) monomer concentration is

incompletely decomposed due to its higher temperature of

complete decomposition (820 �C, in Fig. 2). The results

of X-ray diffraction support those of thermogravimetric

analysis.

The average particle size of powders (given in Table 3)

was determined from X-ray diffraction patterns by the

same method as before. The particle size increases with

decreasing monomer concentration, till that powders con-

taining BeSO4, which was synthesized with the precursor

made from less monomer concentration = 5% (mass

fraction). When the monomer concentration for precursor

was 20% (mass fraction), the size of powders is the

smallest, 11 ± 1 nm. The results of surface area mea-

surements are also shown in Table 3, which are fitted well

with those of X-ray diffraction patterns.

Figure 4 shows the TEM micrographs of BeO powders

synthesized by polyacrylamide gel method with differentFig. 1 Thermal analysis of beryllium sulfate. (5 �C/min in air)


123

monomer concentrations. The average size of powders is

obviously reduced with increasing the monomer concen-

tration, which is also consistent with the other results.

Besides, agglomeration of powder with nearly spherical

morphology is found, of which the degree is increased due

to the smaller size.

3.3 Effect of AM/MBAM ratio

Figure 5 presents the thermal analysis (TG) results of

precursor gel (dried gel) elaborated with various AM/

MBAM ratios for BeO, examined under the same condi-

tions preciously used for beryllium sulfate. Obviously, the

decomposition of precursor gel is very different from that

of beryllium sulfate (compared with Fig. 1). The decom-

position of precursor gel starts and ends up at lower tem-

perature. For example, the decomposition of the precursor

gel made from the AM/MBAM = 3 starts at around 100�C

and ended up at 500�C while the one without AM and

MBAM starts at 200�C and ends up at 450�C.

The TGA data in Fig. 5 also indicate that decomposition

of the precursor gel is strongly affected by the AM/MBAM

ratio. The appearance of ‘‘frilling’’ on the curve of AM/

MBAM = 3 shown in Fig. 5 is similar to the phenomenon

occurring in pure salt sample as seen in Fig. 1, which means

that the decomposition progresses in several steps as men-

tioned above. However, an interesting observation from the

curves is found when AM/MBAM ratio increases. With

increasing the ratio, the ‘‘frilling’’ of curves disappears

gradually as AM/MBAM \ 6, but it appears again gradu-

ally as AM/MBAM [ 6. This means AM/MBAM = 6 is a

critical value, suggesting that the progressing step of

decomposition of precursor gel should be changed with the

increasing of AM/MBAM ratio. As AM/MBAM \ 6, the

decomposition of precursor gel turns from several steps to

one step; but this turn over as AM/MBAM [ 6.

Besides, the decomposition of precursor gel ends up at

lower temperature and then at higher temperature when

AM/MBAM ratio increases, that is, the temperature of

complete decomposition of precursor decreases primarily

and then increases again (in Fig. 5). The AM/MBAM ratio

of 6 is also a critical value. For example, the decomposi-

tions of the precursor gels made from the AM/MBAM = 3

and AM/MBAM = 20 ended up at 800 and 830 �C,

respectively, while the one with AM/MBAM = 6 ends up

at 690 �C. Especially, the difference is more obvious

when AM/MBAM ratio increases, that is, temperature of

Fig. 2 TG analysis of precursor as a function of monomer concen-

tration. (5 �C/min in air)

Fig. 3 X-ray diffraction patterns of powders synthesized by poly-

acrylamide gel method with different monomer concentrations.

(700 �C, 2 h)

Table 3 Specific surface area, average particle size of powders as a

function of monomer concentration calcined at 700 �C for 2 h

determined by XRD and BET, respectively

Monomer concentration,

% (mass fraction)

Specific surface

area/(m2 g-1)

Average particle size

from XRD patterns/nm

5* – –

10 121.81 14 ± 1

15 143.11 12 ± 2

20 162.01 11 ± 1

* Powders containing BeSO4, as shown in Fig. 3


123

complete decomposition of precursor gel (ending temper-

ature) decreases.

XRD was performed to investigate the influence of

AM/MBAM ratio on properties of powders. Figure 6

shows the X-ray diffraction patterns of the powders,

made from precursors with different AM/MBAM ratios

and calcinated at 800 �C for 2 h. In all cases, the peak

pattern corresponds the characteristic a-BeO pattern

(JPCDS 35-0818), confirming the formation of powders

with a pure phase. The significant peak broadening in the

XRD patterns corroborates that the material consists of

fine crystallites. Most significantly, the full-width for the

half-maximum (FWHM) intensity peak of the powder,

made from precursors with different AM/MBAM ratios,

is strongly affected. When the ratio of AM to MBAM is

6:1, the FWHM is the broadest. Given the ratio deviates

from the value 6, the FWHM reduces. This indicates that

the average particle size of powders is also affected

by the AM/MBAM ratio. In order to confirm this influence,

the average particle size of samples was calculated with the

data obtained from the XRD patterns and Scherrer formula.

The BET surface area of powders was also investigated.

All the results are listed in Table 4. Both kinds of results are

in accordance with each other.

The average particle size of powders, prepared by cal-

cining the precursor gel with the AM/MBAM ratio of 6, is

found to be 17 ± 2 nm from X-ray line broadening mea-

surements, which is the smallest. When the ratio deviates

from 6, the average particle size of powders increases.

Transmission electron microscopy images, as shown in

Fig. 7, also support this result. It is obvious that as the ratio

of AM/MBAM goes up, the size of the particles decreases

from 22–26 nm for AM/MBAM = 3 to 15–19 nm for AM/

MBAM = 6, and then increases to 18–20 nm for AM/

MBAM = 10, which is in basic agreement with the results

of XRD and specific surface area measurement. These

results also suggest that the particles have nearly spherical

morphology.

3.4 Effect of salt concentration

Figure 8 shows the thermogravimetric analysis (TGA)

results of precursors with different salt concentrations, which

suggests that the salt concentration also has a strong effect on

the thermal decomposition of synthesis process. The turning

points on TGA curves are still not very distinct, which

indicates that the decomposition of precursor gels does not

progress distinctly in several steps compared with that of

beryllium sulfate (Fig. 1). The observation confirms again

that the pyrolysis of the organic gel has a crucial effect on that

of salt in precursors during the decomposition process.

Similarly, the temperature of complete decomposition of

precursor gels with different salt concentrations is also

affected, which increases with increasing the salt concen-

tration. For example, the temperature of complete decom-

position of precursors with 0.25 mol/L salt is 670 �C, while

the one with 1.5 mol/L salt is 780 �C.

Figure 9 shows the X-ray diffraction patterns of the

powders obtained by calcinating precursor gels with dif-

ferent salt concentrations at 700 �C for 2 h. The powders

made from precursors with lower salt concentrations (0.25

and 0.5 mol/L) are composed of pure a-BeO, while those

made from precursors with higher salt concentrations (1

and 1.5 mol/L) contain BeSO4. This suggests that complete

decomposition of precursor gels under the same condition

is more and more difficult with the increase of salt con-

centration, which also supports the results of TGA.

All the precursor gels with different salt concentrations

were calcined at 700 �C for 2 h in air, and white powders

were obtained. The same methods as before were applied

to achieve the specific surface areas and average particle

sizes of the powders, which is reported in Table 5. The

Fig. 4 Transmission electron microscopy (TEM) images of BeO powders synthesized by polyacrylamide gel method with monomer

concentrations of a 10%, b 15% and c 20%. The precursor gel calcinated at 700 �C for 2 h in air


123

results, corroborating each other, show that the specific

surface area (or average particle size) of powders reduces

(or increases) with the increase of salt concentration.

Transmission electron microscopy (TEM) images of BeO

powders, as shown in Fig. 10, confirm that the property of

synthesized powders is significantly affected by the salt

concentration. It is interesting to note that very small pri-

mary particles with a size of around 8 nm are got in our

experiments. Their specific surface area is very high, about

151 m2/g.

4 Discussion

4.1 Formation of network structures

Polyacrylamide (PAAm) gels with excellent network

structures have been extensively used in electrophoresis for

protein separations [34–36] or as membranes for protein

isolations or blood purification [36]. Besides, network

structures of PAAm gels are easily controlled by adjusting

the conditions under which the gels are formed. Similarly,

polyacrylamide gel method takes advantage of the network

structures of gels and, more significantly, the polymeriza-

tion reaction forming gel. During this process, the free-

radical crosslinking copolymerization of acrylamide (AM)

and N,N0-Methylenebis(acrylamide) (MBAM) monomers

happens in solutions containing cations and anions. The

result of polymerization is that the gels with network

structures are obtained and micro-solutions with ions in

nanocaves of gels are formed.

Figure 11 shows the structure evolution of the reaction

solutions. Initially, inorganic ions and organic molecules

(Be2?, SO42-, AM and MBAM) move freely in solvent

(distilled water) (Fig. 11a). Then, ammonium persulfate

(APS) is added, which solves in solution (Fig. 11b). And

then it is activated by heating to generate free-radicals.

These are persulphates with unpaired electron which is

transferred to the pendant vinyls of acrylamide unit form-

ing monomer radical, so that it in turn becomes reactive.

Another monomer, MBAM, can therefore be attached and

activated in the same way. So, the chain reaction of poly-

merization is initiated and called chain initiation reaction

Fig. 5 TG analysis of precursor gel as a function of ratio of AM/

MBAM. (5 �C /min in air)

Fig. 6 X-ray diffraction patterns (XRD) of BeO powders synthesized

by polyacrylamide gel method with the different ratios of AM/

MBAM. (800 �C, 2 h)


123

(Fig. 11c). Next, the initiated chains of monomers (AM

and MBAM) attach with the rest monomers, so that these

chains grow and become active with the active center being

continually shifted to the free end of the chain. Therefore,

the polymer continues growing indefinitedly (or until the

supply of monomers is exhausted). This reaction stage is

called chain propagation reaction (Fig. 11d). The forma-

tion of chain propagation may be straight, cross or bran-

ched. However, the microgels with networks are formed

prior to the onset of macrogelation [37]. Hence, a part of

cations and anions are trapped in gels (Fig. 11e). As the

reaction proceeds, microgels are connected to a macrogel

through their peripheral pendant vinyls and radical ends,

whereas those in their interior remain intact. Finally, all

ions are entrapped into gels and separated by network

structures of gels (Fig. 11f).

In general case, the same method, based on the formation

of polymer network structures through the copolymerization

of acrylamide (AM) and N,N0-Methylenebis(acrylamide)

(MBAM) monomers, has already been used. The formation

of PAAm gels is simple, efficient and cheap. It was used for

preparing polymer-based metal–polymer nanocomposite,

PAAm-M (M = Au, Pd), by Zhou et al. [38], for example. It

is also used for gelling aqueous suspensions of powders to

fabricate ceramic green, which is named gelcasting [39, 40],

an advanced colloidal processing of ceramics combining

advantages such as the microstructure homogeneity and the

ability to form complex-shaped articles.

4.2 Decomposition of precursor gels

Drying gels (precursor gels) mainly consist of the polymer

PAAm xerogels and salts. As the calcination proceeds,

xerogels and salts are decomposed at the same time. The

decomposition of the PAAm gel has been extensively

investigated [9, 41]. The results are summarized as follows.

The gel decomposes at temperatures between 160 and

300 �C in several steps, which is completed at about

600 �C. In the literature [41], two strong exothermic peaks

Table 4 Specific surface area, average particle size of powders as a

function of AM/MBAM ratio calcined at 800 �C for 2 h determined

by XRD and BET, respectively

AM/MBAM

ratio

Specific surface

area/(m2 g-1)



3:1 74.04 24 ± 2

4:1 70.73 25 ± 1

5:1 95.01 19 ± 2

6:1 106.86 17 ± 2

7:1 100.62 18 ± 2

10:1 93.98 19 ± 1

Fig. 7 Transmission electron microscopy (TEM) images of BeO powders synthesized by polyacrylamide gel method with different ratios

of AM/MBAM; a 3:1, b 4:1, c 5:1, d 6:1, e 7:1, f 10:1


123

appeared at about 330 �C (step I) and 500 �C (step II),

representing the exothermic reaction of the oxidative

decomposition of organic. The first one is attributed to the

decomposition of side-chain of PAM. The other one, less

intense than the first exothermic peak, is assigned to the

decomposition of PAM’s backbone. This indicates that

xerogel in precursor gels degraded completely below

600 �C, and so the decomposition of precursor gel above

that temperature is attributed to that of the sulfate. That is,

as the temperature grows, volatile such as water and

polymer is firstly removed from precursor gels and then

anhydrous salt in the form of powder is obtained. However,

the powder is in nanoscale, which is confirmed by scanning

electronic microscopy. Figure 12 shows the SEM image of

powder consisting of anhydrous sulfate and beryllia

obtained by calcining the precursor gel (sample 11) at

700 �C for 2 h.

Nanosized salt with higher specific surface energy is

easier to decompose, because its activation energy of

decomposition (from sulfate to oxide) is lower. Therefore,

the calcination temperature of salt completely turning to

oxide is lower than that with traditional methods. In fact,

the phenomenon agrees with the reports of Pan et al. [28]

and Wang et al. [25]. Pan et al. [28] used the polyacryl-

amide gel method to prepare bismuth oxide nanoparticles

and they found that the precise calcination temperature is

much lower than that with the traditional precipitation

method. In the study of Wang and co-workers [25], calci-

nation temperature to obtain a-Al2O3 in the polyacrylamide

gel is 100 �C less than traditional methods in that c-Al2O3

more easily transform to a-Al2O3 for smaller grains size.

Note that the lower calcination temperature is attributed to

the smaller size of precursory salt or phase. Hence, the

calcination temperature should change with the size of

precursory salt or phase. As mentioned in Sect. 4.1, the size

of precursory salt in precursor gel is decided by the net-

work structure of PAAm gel. Nevertheless, it is strongly

affected by polymerization conditions such as monomer

(AM) concentration and AM/MBAM ratio.

Several studies showed that the network structure of gel

strongly depends on the initial monomer concentration [37,

42–44]. As the amount of solvent (water) present in poly-

merization increases, the network structure becomes

increasingly loose. No continuous network is even formed

Fig. 8 TG analysis of precursor gel as a function of salt concentra-

tion. (5 �C/min in air)

Fig. 9 X-ray diffraction patterns (XRD) of BeO powders synthesized

by polyacrylamide gel method with different sulfate salt concentra-

tions. (700 �C, 2 h)

Table 5 Specific surface areas, average particle size of powders as a

function of salt concentration calcined at 700 �C determined by XRD

and BET, respectively

Salt concentration/

mol L-1Specific surface

area/(m2 g-1)



0.25 151.29 8 ± 2

0.5 136.75 14 ± 2

1.0* – –

1.5* – –

* Powders containing BeSO4, as shown in Fig. 9


123

above a critical amount of solvent. Therefore, the amount

and size of salt contained in network structure of precursor

gel decrease with increasing the monomer concentration.

The temperatures of complete decomposition of the pre-

cursors correspondingly reduce, which is confirmed in

Fig. 2.

The ratio of AM/MBAM also has a significant effect on

network structure of gel. At high AM/MBAM ratio, science

the local pendant vinyl group concentration from MBAM

surrounding the growing macroradical is relatively low, only

a few multiple crosslinks are expected to occur after each

crosslinking reaction and the network structure of gel is

loose. At lower AM/MBAM ratio, the crosslinker (MBAM)

contents is higher, so the network structure of gel with more

multiple crosslinks is tighter due to the higher pendant vinyl

group concentration. Thus, the lower the AM/MBAM ratio,

the higher the crosslinker (MBAM) content, the larger the

number of multiple crosslinks will be and the compacter the

network structure of gel. Hence, there is theoretically a

corollary to it: decreasing AM/MBAM ratio (increasing

crosslinker content) increases the compactness of network

structure, and the complete calcination temperature of pre-

cursor gel in polyacrylamide gel method decreases. How-

ever, the critical complete calcination temperature of

precursor gel in Fig. 5 shows a minimum at AM/MBAM

ratio of 6, which suggests that the network structure of gel

becomes loose again when the ratio of AM/MBAM is

larger than 6. The reason is the spatial inhomogeneity in

poly(acrylamide) gels, of which the degree increases with

increasing crosslinker content (decreasing AM/MBAM

ratio) [45]. Decreasing AM/MBAM ratio (increasing cross-

linker concentration) increases both the effective cross-link

density and the degree of inhomogeneity in PAAm gels. The

network structure of gels becomes tighter due to the first

effect, while the latter effect enlarges the apparent network

structure of gels. The interplay of these two opposite effects

Fig. 10 Transmission electron

microscopy (TEM) images of

BeO powders synthesized by

polyacrylamide gel method

with different sulfate salt

concentrations; a 0.25 mol/L,

b 0.5 mol/L. (700 �C, 2 h)

.

.

. .

.

.

.

.

.

.

.

.

. .

. .

.

Be2+

SO42-

AM

MBAM

APS

. Monomer radical

Macro radical (propagation

chain with radical)

.

Microgel

Fig. 11 Model of structures

evolution of the reaction

solutions with polymerization of

AM-MBAM system


123

determines the network structure of gels. As AM/MBAM

ratio is less than 6, the effect of crosslinker on spatial inho-

mogeneity becomes to be in the ascendancy, and the network

structure of gels becomes loose again. Thus, as AM/MBAM

ratio is 6, the network structure of gels is the tightest and

the complete calcination temperature of precursor gel is the

lowest. When the AM/MBAM ratio deviates from 6:1, the

network structure of gels becomes loose and the complete

calcination temperature of precursor gel increases.

Besides, Fig. 8 indicates that salt concentration also

plays a strong role in the decomposition of precursor gel.

As pointed out in the Introduction, ions are entrapped into

nanocavities inside the gel and nanoscale solutions form by

polyacrylamide gel method. The cubages of nanocavities

are ascertained under the fixed synthesis conditions (AM

concentration and AM/MBAM ratio). Therefore, the

amounts of salt in nanocavities are determined by the salt

concentration. As expected, at high salt concentration, the

size of salt in precursor gel is larger and its complete cal-

cination temperature is higher, which is confirmed in our

experiment (Fig. 8) [46].

In addition, network structure of gel is also affected by

other factors such as the pH value of solution and copo-

lymerization temperature, and further investigation is

needed.

4.3 Properties of powder

According to crystallography, the size, monograph and

other properties of powder are mainly decided by its pre-

cursory salt, which is strongly affected by synthesization

conditions (AM concentration, AM/MBAM ratio and

solution concentration) of polyacrylamide gel method.

Therefore, properties of powder are influenced in turn. As

the monomer concentration increases, network structures

of gels become compact, the sizes of salt are smaller, and

thus the average size of powders is obviously reduced. The

AM/MBAM ratios also affect the network structures of

gels and thus the sizes of salts in precursor gels. When the

ratio of AM to MBAM is 6:1, the average particle size of

powders is the smallest due to the tightest network struc-

tures of gels and thus the smallest sizes of salts in precursor

gels are obtained. As AM/MBAM ratio deviates from this

value, the average particle size of powders becomes larger

in the same reason. Besides, the amount of salt in nanoc-

aves of gels is decided by the initial salt concentration,

so the average particle size of powders decreases with

increasing the salt concentration.

Besides, the particle size distribution (PSD) of nano-

particles is another key property. As shown in TEM images

(Figs. 4, 7, and 10), no significant regularity is observed in

particle size distribution of powder synthesized under dif-

ferent conditions. The reason should be attributed to PAAm

gel always with spatial inhomogeneity in that gelation

during copolymerization occurs non-randomly. Therefore,

the size of salt in precursor gel is in a wide range, which

certainly decides the PSD of nanopowder with non-regu-

larity. That is, the PSD of nanoparticles is determined by

inhomogeneous crosslink density distribution in gel, which

is almost immune to synthesis conditions. Tahmasebpour

et al. [27] attributed the irregular PSD to the interactions

between crystallization and polymeric network degradation

during calcination process, which is not supported in this

study. Furthermore, the observation that a few large par-

ticles appeared in TEM images (especially in Figs. 4, 7) is

reasonable, because some large singular holes are gener-

ated as gel forms. Thus, in order to achieve nanopowder

with narrow size distribution, spatial gel inhomogeneity is

undesirable and network structures of PAAm gels should

be tailored.

Although the inhomogeneity control in gels is quite

sophisticated, there are still a lot of methods. Decreasing

the crosslinker concentration used in the gel preparation

also reduces the degree of spatial inhomogeneity due to the

increasing distance between the pendant vinyl groups

during the gelation process. On the other hand, the work of

Kwok et al. [47] suggests that the polymerization envi-

ronments (solvent) affects the network structure of PAAm

gels, which may be also taken advantage of. However,

whether these methods will perform well or not in poly-

acrylamide gel method to prepare nanoparticles with nar-

row PSD, further researches are needed. In general case,

PAAm gels with more homogenous network structures is

also beneficial to prepare composites with more excellent

microstructures and thus properties. Meanwhile, the pro-

cess of gelcasting technique in a closely related PAAm gels

may be greatly improved.

In fact, these methods based on making use of poly-

merization process and its resulted network structures of

PAAm gels, including polyacrylamide gel method, can be

Fig. 12 The morphology of powder obtained by calcining the

precursor gel (sample 11) at 700 �C for 2 h. The powder is anhydrous

sulfate with little beryllia as shown in Fig. 9


123

seen as one kind of template methods. In this method,

substances with specific structure used as templates are

used to control the constituent, structure, morphology, size,

configuration, and arrangement of the target materials.

Considering the similarity of methods, some template in

these methods may be borrowed to prepare nanoparticles.

For example, the gel system of 2-hydroxyethyl methacry-

late (HEMA) and N,N0-Methylenebis(acrylamide) (MBAM)

[48] which has been used in gelcasting may be also suitable

to synthesize nanopowders. Moreover, some organic gels

may be used as template to prepare nanoparticles, which

play the same role as polyacrylamide gel in this method.

For example, nanopowders might be prepared with

N-Isopropylacrylamide-based hydrogels and PEO/PPO-

based hydrogels [49], which are have been extensively

used in applications such as drug and gene delivery. These

methods show great advantages in synthesis of nanostruc-

ture materials and preparation of inorganic products with

the addition of polymer.

5 Conclusions

The decomposition procedure of precursor and the effects

of various experimental conditions on synthesis and prop-

erties of BeO nanopowders through polyacrylamide gel

method have been investigated. The following conclusions

can be offered through this study.

1. As calcination processes, precursor composed of

polyacrylamide gel and sulfate decomposes. The free

and crystallized water in gel lose and the pyrolysis of

the polymeric network of gel happens under temper-

ature higher than 600 �C. The nanoscale beryllium

sulfate formed in gel decomposes to be BeO nano-

powders with the inhibitor effect of polymer-network.

2. As the monomer concentration increases, the calcina-

tion temperature of precursor gel determined by

thermogravimetric analyses decreases due to more

compact network structure of gel and thus smaller size

of salt in nanocaves in gel. Correspondingly, the

average particle size of nanopowder also reduces, as

illustrated by transmission electron microscopy

images.

3. The ratio of AM to MBAM also plays an important

role. With the increasing of AM/MBAM ratio, the

calcination temperature decreases firstly and then

increases again. When the AM/MBAM ratio is 6, the

calcination temperature of precursor gel is the lowest,

the average particle size of powders is the smallest.

The reason lies in that the network structures of gel is

the tightest and thus the sizes of salts in precursor gels

is the smallest. As AM/MBAM ratio deviates from this

value, the network structures of gel becomes loose and

thus the size of salt in precursor gel becomes larger, so

the calcination temperature increases and the average

particle size of powders becomes larger certainly.

4. The amount of salt in nanocaves of gels with fixed

structure is decided by the initial salt concentration, so

both the calcination temperature and the average

particle size of powders increase with increasing the

salt concentration.

5. The synthesis conditions have no effect on the particle

size distribution of the final product due to the natural

random distribution of porosity in gel.

References

1. Djerdj I, Arcon D, Jaglicic Z, Niederberger M (2008) Nonaque-

ous synthesis of metal oxide nanoparticles: Short review and

doped titanium dioxide as case study for the reparation of tran-

sition metal-doped oxide nanoparticles. J Solid State Chem

181:1571–1581

2. Ibarguen CA, Mosquera A, Parra R, Castro MS, Rodrıguez-Paez

JE (2007) Synthesis of SnO2 nanoparticles through the controlled

precipitation route. Mater Chem Phys 101:433–440

3. Jiang W, Hua X, Han Q, Yang X, Lu L, Wang X (2009) Prep-

aration of lamellar magnesium hydroxide nanoparticles via pre-

cipitation method. Powder Technol 191:227–230

4. Pu Y, Tao X, Zhai J, Chen J (2010) Hydrothermal synthesis and

magnetic properties of Co0.2Cu0.03Fe2.77O4 nanoparticles. Mater

Res Bull 45:616–620

5. Castro AL, Nunes MR, Carvalho AP, Costa FM, Florencio MH

(2008) Synthesis of anatase TiO2 nanoparticles with high tem-

perature stability and photocatalytic activity. Solid State Sci

10:602–606

6. Somiya A, Hishinuma K, Akiba T (1995) A new materials pro-

cessing-hydrothermal processing. Bull Mater Sci 18:811–818

7. Livage J, Henry M, Sanchez C (1998) Sol–gel chemistry of

transition metal oxides. Progress in Solid State Chem 18:259–342

8. Lee B, Han J, Saito F (2005) Microstructure of sol–gel synthe-

sized Al2O3–ZrO2(Y2O3) nano-composites studied by transmis-

sion electron microscopy. Mater Lett 59:355–360

9. Tahmasebpour M, Babaluo AA, Aghjeh MKR (2008) Synthesis

of zirconia nanopowders from various zirconium salts via poly-

acrylamide gel method. J Eur Ceram Soc 28:773–778

10. Douy A, Odier P (1989) The polyacrylamide gel: a novel route to

ceramic and glassy oxide powders. Mater Res Bull 24:1119–1126

11. Pechini MP (1967) Method of preparing lead and alkaline earth

titanates and niobates and coating method using the same for a

capacitor. US Patent 3,330,697

12. Xu Y, He Y, Wang L (2000) Synthesis of Ba2Ti9O20 via ethy-

lenediaminetetraacetic acid precursor. J Mater Res 16:1195–1199

13. Hodgson SNB, Shen X, Sale FR (2000) Preparation of alkaline

earth carbonates and oxides by the EDTA-gel process. J Mater

Sci 35:5275–5282

14. Chiang C, Shei CY, Wu SF, Huang YT (1991) Preparation of

high-purity TI-based ‘‘1223’’ superconductor phase by modified

Pechini process in water solution. Appl Phys Lett 58:2435–2437

15. Pramanik P (1999) A novel chemical route for the preparation of

nanosized oxides, phosphates, vanadates, molybdates and tung-

states using polymer precursors. Bull Mater Sci 22:335–339


123

16. Zhang L, Xue D (2002) Preparation and magnetic properties of

pure CoO nanoparticles. J Mater Sci Lett 21:1931–1933

17. Roy S, Sigmund W, Aldinger F (1999) Nanostructured yttria

powders via gel combustion. J Mater Res 14:1524–1531

18. Laberty-Robert Ch, Ansart F, Deloget C, Gaudon M, Rousset A

(2001) Powder synthesis of nanocrystalline ZrO2–8%Y2O3 via a

polymerization route. Mater Res Bull 36:2083–2101

19. Kahoul A, Nkeng P, Hammouche A, Naamoune F, Poillerat G

(2001) A sol–gel route for the synthesis of Bi2Ru2O7 pyrochlore

oxide for oxygen reaction in alkaline medium. J Solid State Chem

161:379–384

20. Zhang J, Ning J, Liu X, Pan Y, Huang L (2003) Synthesis of

ultrafine YAG:Tb phosphor by nitrate–citrate sol–gel combustion

process. Mater Res Bull 38:1249–1256

21. Chahara K, Ohno T, Kasai M, Kozono Y (1993) Magnetoresis-

tance in magnetic manganese oxide with intrinsic antiferromag-

netic spin structure. Appl Phy Lett 63:1990–1992

22. Popa M, Kakihana M (2002) Synthesis of lanthanum cobaltite

(LaCoO3) by the polymerizable complex route. Solid State Ionics

151:251–257

23. Fuentes AF, Hernandez-Ibarra O, Mendoza-Suarez G, Esca-

lante-Garcıa JI, Boulahya K, Amador U (2003) Structural

analysis of several W(VI) and Mo(VI) complex perovskites

prepared by the polymeric precursors method. J Solid State

Chem 173:319–327

24. Chang YS, Chang YH, Chen IG, Chen GJ, Chai YL (2002)

Synthesis and characterization of zinc titanate nano-crystal

powders by sol–gel technique. J Cryst Growth 243:319–326

25. Wang H, Gao L, li W, li Q (1999) Preparation of nanoscale a-

Al2O3 by the polyacrylamide gel method. NanoStructured Mater

11:1263–1267

26. Tarancon A, Dezanneau G, Arbiol J, Peiro F, Morante JR (2003)

Synthesis of nanocrystalline materials for SOFC applications by

acrylamide polymerization. J Power Sources 118:256–264

27. Tahmasebpour M, Babaluo AA, Shafiei S, Pipelzadeh E (2009)

Studies on the synthesis of a-Al2O3 nanopowders by the poly-

acrylamide gel method. Powder Technol 191:91–97

28. Pan C, Li X, Wang F, Wang L (2008) Synthesis of bismuth oxide

nanoparticles by the polyacrylamide gel route. Ceram Inter

34:439–441

29. Zhang H, Fu X, Niu S, Sun G, Xin Q (2004) Low temperature

synthesis of nanocrystalline YVO4:Eu via polyacrylamide gel

method. J Solid State Chem 177:2649–2654

30. Fu X, Zhang H, Niu S, Xin Q (2005) Synthesis and luminescent

properties of SnO2:Eu nanopowder via polyacrylamide gel

method. J Solid State Chem 178:603–607

31. Liu N, Yuan Y, Majewski P, Aldinger F (2006) Synthesis of

La0.85Sr0.15Ga0.85Mg0.15O2.85 materials for SOFC applications by

acrylamide polymerization. Mater Res Bull 41:461–468

32. Dean JA (eds) Shang F et al. (Translation) (1999) Lange’s

handbook of chemistry, 13th edition. Science press, Beijing

33. Dollimore D, Konieczay JL (1998) The thermal decomposition of

beryllium oxalate and related materials. Thermochim Acta

318:155–163

34. Patras G, Qiao GG, Solomon DH (2000) Characterization of the

pore structure of aqueous three-dimensional polyacrylamide gels

with a novel cross-linker. Electrophoresis 21:3843–3850

35. Caulfield MJ, Purss HK, Solomon DH (2001) Novel cross-linked

polyacrylamide matrices: An investigation using gradient gel

electrophoresis. Electrophoresis 22:4297–4302

36. Horvath ZS, Corthals GL, Wrigley CW, Margolis J (1994)

Multifunctional apparatus for electrokinetic processing of pro-

teins. Electrophoresis 15:968–971

37. Naghash HJ, Okay O (1996) Formation and structure of poly-

acrlamide gels. J Appl Polymer Sci 60:971–979

38. Zhou Y, Hao LY, Zhu YR, Hu Y, Chen ZY (2001) A novel

ultraviolet irradiation technique for fabrication of polyacryl-

amide–metal (M = Au, Pd) nanocomposites at room tempera-

ture. J Nanoparticle Res 3:379–383

39. Young AC, Omatete OO, Janney MA, Menchhofer PA (1991)

Gelcasting of alumina. J Am Ceram Soc 74:612–618

40. Shanti NO, Hovis DB, Seitz ME, Montgomery JK, Baskin DM,

Faber KT (2009) Ceramic laminates by gelcasting. Inter J Appl

Ceram Technol 6:593–606

41. Wu S, Liu Y, He L, Wang F (2004) Preparation of b-spodumene-

based glass–ceramic powders by polyacrylamide gel process.

Mater Lett 58:2772–2775

42. Baker JP, Hong LH, Blanch HW, Prausnitz JM (1994) Effect of

Initial Total Monomer Concentration on the Swelling Behavior of

Cationic Acrylamide-Based Hydrogels. Macromolecules 27:

1446–1454

43. Kizilay MY, Okay O (2003) Effect of initial monomer concen-

tration on spatial inhomogeneity in poly(acrylamide) gels. Mac-

romolecules 36:6856–6862

44. Gelfi C, Righetti PG (1981) Polymerization kinetics of poly-

acrylamide gels I. Effect of different cross-linkers. Electropho-

resis 2:213–219

45. Yazici I, Okay O (2005) Spatial inhomogeneity in poly(acrylic

acid) hydrogels. Polymer 46:2595–2602

46. Oppermann W, Lindemann B, Vogerl B (2000) Network inho-

mogeneities in polymer gels. ACS Symp Ser 833:37–50

47. Kwok AY, Qiao GG, Solomon DH (2003) Synthetic hydrogels. 1.

Effects of solvent on poly(acrylamide) networks. Polymer

44:6195–6203

48. Cai K, Huang Y, Yang J (2005) Alumina gelcasting by using

HEMA system. J Eur Ceram Soc 25:1089–1093

49. Klouda L, Mikos AG (2008) Thermoresponsive hydrogels in

biomedical applications. Eur J Pharm Biopharm 68:34–45


123

polyacrylamide gel method: synthesis and property of beo nanopowders

Documents