INVESTIGATIONS OF STRUCTURAL AND SPECTROSCOPIC CHARACTERIZATION OF LANTHANUM SILICATE...

ISSN 2394–7799 IJAETCS (2014) Vol.1, No.1, 28-35

Research Article

International Journal of Advancement in Engineering, Technology and

Computer Sciences

INVESTIGATIONS OF STRUCTURAL AND

SPECTROSCOPIC CHARACTERIZATION OF

LANTHANUM SILICATE NANOCRYSTALLINE

A. M. El NahrawyI*

, H. H. AfifyI, A. I. Ali

II,III

I*National Research Center (NRC) Department of Solid State Physics, Doki, Giza, Egypt ([email protected])

IIEnergy Harvest-Storage Research Center and Department of Physics, University of Ulsan, Ulsan 680-749 South Korea.

III Basic Science Department, Faculty of Industrial Education, Helwan University, Saray El-Quba, 11281 Cairo, Egypt.

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ABSTRACT: Oxyapatite nanostructure pure silica and lanthanum silicate (10 La2O3-90 SiO2)

nanoparticles were synthesized using sol–gel technique. La2O3 ions were introduced in sol–gel derived

silica matrix by direct mixing dissolved lanthanum nitrate into a silica sol. The effects of H2O/TEOS

ratio on the structural and spectroscopic properties of (La2O3-SiO2) and the effect of La2O3 ions on

the silica gel were studied. The gel and oxide powders were characterized by X-ray diffraction (XRD),

Fourier transformed infrared (FTIR) spectroscopy, transmission electron microscopy (TEM),

scanning electron microscopy (SEM/EDS) and optical properties. The observed XRD patterns for the

prepared samples calcined at different temperature confirmed the formation of nano-crystalline

lanthanum silicate phases. The refractive indices and UV-Vis absorption for pure and doped silicate

systems have been achieved.

KEY WORDS: lanthanum silicate, nano-particles, sol-gel process, XRD and spectroscopic

properties.

----------------------------------------------------------------------------------------------------------------------------------------------------------- 1. INTRODUCTION: Recently, the development of novel low-cost engineering materials with promising

properties and stability are of vital importance. Oxyapitatite materials based silicate have a great interest due

their uses in different industrial application especially as electrolytes for intermediate temperature solid

oxide fuel cells (SOFC) because of their high ionic conductivity at low temperatures as well as the low cost

of raw materials [(1)-(4)]. For modified silica-based nano-crystalline materials obtained using sol–gel

technique, the chemical composition and the stabilization temperature are the main required synthesis

conditions that will define the final network product. Oxyapitate silicate materials is the most common type

and has the formula (M10 (AO4)6O2-δ) where M are the rare-earth and alkaline earth metal cations and A is a

p-block element such as P,As, Si, Ge,V,Zn [(5)-(7)]. While the apatite structure of lanthanum silicates

consists of isolated SiO4 tetrahedra and M-site cations, occupying cavities between SiO4 units creating a

new silicate systems ; and additional oxygen sites (O4) surrounded by La cations.

Over the past decades, it has been well known that the sol–gel technique has become a simple,

attractive and a potential alternative to the traditional methods. The sol–gel process, in which metal alkoxide

precursors could readily undergo, catalyzed hydrolysis and poly-condensation reactions in the formation of

a sol with controllable size and narrow particle size distribution. Sol gel technology used as alternative

technology to prepare a wide variety of applications such as powders, monoliths, thin films, nanotube and

nanofibers [(8)-(10)]. Therefore, in order to reduce the synthesis temperature, sol gel technique was used as

synthetic procedure for pure and doped lanthanum silicate oxyapatites instead of the conventional solid-state

reaction method

The aim of the preset work is to explore and decrease the synthesis temperature of lanthanum silicate

(10 La2O3-90 SiO2) nanoparticles in comparing with pure silica and study the effect of lanthanum on pure

silica gel. In this paper, the structural evolution of pure silica and lanthanum silicate (La2O3-SiO2) with

El Nahrawy, A. M. et al./ International Journal of Advancement in Engineering, Technology and Computer Sciences, Vol.1, No.1 29

annealing temperature was investigated by X-ray diffraction (XRD), Fourier transform infrared

spectroscopy (FTIR), transmission electron microscopy, scanning electron microscopy (SEM/EDX) The

relationship between preparation conditions, structures and optical properties of the prepared samples were

also discussed.

2. EXPERIMENTAL:

2.1. MATERIALS: Tetraethylorthosilicate (TEOS, Aldrich Chemicals, 98%), ethanol and lanthanum

nitrate (La (NO3)

3·6H

2O (Merck)) as source of La2O3 ions, nitric acid. Water for preparing the samples was

purified by ion exchange followed by distillation. These chemicals were in the analytical grade and used as

provided.

2.2. SAMPLE PREPARATION: Pure silica and lanthanum silicate (10 La2O3-90 SiO2) nanoparticles

were prepared using sol gel process. Tetraethylorthosilicate was dissolved in excess of absolute ethanol and

distilled water in the presence of HNO3 (N.A) under vigorous stirring (950 rpm) for 2h. About lanthanum

silicate system, tetraethyorthosilicate mixed with lanthanum hexahydrate nitrate dissolved in a mixture of

ethanol and nitric acid and distilled water under vigorous stirring for 2h at 750C, and the solution gradually

became clear sol. The experimental procedures flow chart as in Fig. I. The sol transferred to a clear gel after

one night. The wet gel was dried at 100 ºC for 24 h. Then the gel was heated at different temperatures from

200 to 8000C for 4 h to remove the water, organic components and decompose the nitrates. The influences

of different H2O/TEOS ratio on the physicochemical properties are studied as summarized in Table 1.

sample N.A/TEOS volumetric

ratio

EtOH/TEOS

molar ratio

H2O/TEOS

molar ratio= r

La2O3-SiO2 %

Gel 1 2 4 4 0-100

Gel 2 2 4 6 10-90

Gel 3 2 4 8 10-90

Gel 4 2 4 11 10-90

Table 1: Composition and H2O/TEOS ratio for lanthanum silicate.

2.3. X-RAY DIFFRACTION: The phase purity and the crystal structure of the powders were checked

using X-ray diffraction (XRD, D8–ADVANCE).The measurements were done using Cu-Kα radiation (λ

=1,5418Å). The average crystallite sizes for the samples were calculated using the Debye-Sherrer equation:

D= , Where is the X-ray wavelength (1.54 Å), is the Bragg diffraction angle, and B is full width at

half maximum [(11)].

2.4. TRANSMISSION AND SCANNING ELECTRON MICROSCOPY (SEM/EDS): The particle

distribution and the surface morphology with element ration were examined by transmission and scanning

electron microscopy ((JEOL model JSM 840) with energy dispersive analyser (SEM/EDS).

2.5. FOURIER TRANSFORMS INFRARED SPECTROSCOPY (FTIR): FTIR spectra were recorded

using a Perkin Elmer FTIR spectrometer in KBr medium in the wavenumber range 470–4000 cm-1

.

2.6. OPTICAL PROPERTIES: The refractive indices of these materials calculated using nearly normal

transmittance and reflectance spectra were done by Jasco V-570 spectrophotomete and UV–vis spectra were

recorded by means of spectrophotometer (Mini-D2T, Ocean Optics Inc.).


Stirring for 2hTEOS + ETOH +H2O+nitic acid

Silica (SiO2)(solution)

TEOS+ETOH+H2O+La(NO3) 3.6H2O+ nitric acid

La2O3-SiO2 solution

Oxyapatite lanthanum silicate

Draying 1 night at 750C

Calcinations at different temperatures

Stirring for 2h

transparent gel

Fig.I. Flow chart of sol gel process for the preparation of pure silica and lanthanum silicates nanoparticles.

3. RESULTS AND DISCUSSION:

3.1. SOL–GEL PROCESS: One of the most advantages of sol–gel technique comparing with other

chemical techniques is that they can be synthesized different organic / inorganic materials in normal

ambient conditions; they do not require high temperatures, high pressures, vacuum media, gas or water flow

or critical chemical conditions for production of organic and inorganic materials. The sol–gel mechanisms

involve hydrolysis and poly-condensation reactions of molecular precursors in water and co-solvent media

[(12), (13), (14)]. It is well known that the sol–gel preparation condition has a significant effect on the

response of the sol–gel-derived nanoparticles. The formation of sol gel pure silica (SiO2) and lanthanum

silicate (La2O3-SiO2) occurred by direct decomposition of tetraethoxysilane in the presence of dispersed

La2O3 ions under vigorous stirring at room temperature for 2 h. the ratio of water/alkoxide in the sol–gel

solution for the acid-catalyzed hydrolysis strongly influences the nature of the sol gel end product.

The mechanism involves hydrolysis of a pure and doped alkoxysilane and condensation of alcohol and

water to yield the silica gel and lanthanum silicate gel as in the following [(8), (12), (15), (16)]:

≡Si-OC2H5 + H2O ≡Si-OH C2H5OH hydrolysis

≡Si OH HO Si ≡Si O Si≡ H2O water condensation

≡Si OC2H5+ HO Si ≡ ≡Si O Si≡ + C2H5OH alcohol condensation

≡Si OH+ La (OH)n+ Si OH≡→≡Si O La Si≡+ n H2O

≡Si OC2H5 + La (OH)n+ Si OC2H5 ≡→≡Si O La Si≡ + n C2H5OH

When producing a multi-component silicate as lanthanum silicate, tetraethylorthosilicate as a source of SiO2

(which only takes minutes to hydrolyze is the chosen precursor in order to have better control on the rate of

hydrolysis) and lanthanum nitrate as a source of La2O3 were used in the preparation of apatite silicate.

Condensation reaction was catalyzed by using nitric acid result in sol particles grow and collide poly-

condensation occurs and macro-particles form. The hydrolysis is not completed before gelation occurs and

the sol becomes a gel when it can support a stress elastically [(17)-(20)]. The gelation process changed

slowly during the aging one night at 750C for lanthanum silicate system with different molar H2O ratio,

during the careful removal of the solvent from the liquid phase to yields the dense xerogel but pure silica

was completely changed to gel. The removal of excess liquids during the gel shrinkage can be represented

as in the previous equations for both pure and doped silicate systems. By increasing the calcination

temperature from 100oC up to 800

oC for 4 h, the mass loss increased and the contours changes from the

amorphous phase to the crystalline material resulted due to the dehydroxylation process in pure and doped

silicate systems includes the breaking of the Si-OH bond and La-OH that leads to the densification and the

formation of new bonds.


3.2. CRYSTALLIZATION BEHAVIOR OF THE GEL- POWDERS BY XRD: To study the

crystallization behavior of the phases appeared at different temperatures for pure silica and lanthanum

silicate (10 La2O3- 90 SiO2), XRD patterns of the gel –nanocrystalline powders are shown in Figs. II(a, b)

and 3, calcined at different temperatures 1000C, 400

oC and 800

oC for 4h.

All the samples remained amorphous materials at 100oC. By increasing the calcinations temperature

up to 400oC the crystallization of lanthanum silicate developed with low intensity peaks observed at 28.29,

30.06, 33, 47and 55.560 due to the formation of short range chains of lanthanum silicate composed in silica

system but pure silica was still in the amorphous form as shown in Fig.II(a, b). Fig. III shows XRD patterns

for lanthanum silicate samples (gel 2, gel 3 and gel 4) were found to exhibit similar behavior with increasing

the calcinations temperature from 400 up to 800oC, except the sample gel 4 that prepared with higher H2O

ratio the diffraction peaks become sharper and intensify indicating a higher degree of crystallinity. This

figure indicates that the nano-crystalline powders calcined at 800oC consist of two phases tetragonal

lanthanum silicate and hexagonal lanthanum silicate according to (JCPDS -72-2456) and (JCPDS-49-0443).

The average crystallite sizes for lanthanum silicate samples between 16 and 27 nm. The inset in Fig.IIIfor

pure silica reveals that up to 800oC, the crystallization did not occur. Which, indicate that the addition of

lanthanum ions into the silicate system rearrangement and reduce the crystallization temperature of silicate.

10 20 30 40 50 60 70 80

(a)

gel 2

gel 3

gel 4

In

tens

ity (a

.u.)

20

gel 1

10 20 30 40 50 60 70 80

(b)

gel 3

gel 4

Inte

nsit

y (

a.u

.)

gel 2

Fig. II XRD patterns of pure silica (gel 1) and lanthanum silicate with different H2O ratio gel 2 (r = 6), gel 3

(r = 8) and gel 4(r = 11) calcined at two different temperatures 100oC and 400

oC for 4 h.

10 20 30 40 50 60 70 80

10 20 30 40 50 60 70 80

SiO2

Inte

nsit

y (

a.u

.)

20

gel 3

gel 4

gel 2

Fig. III XRD patterns of nano-particles lanthanum silicate obtained after decomposition and calcination at

800oC for 4h of gel 2, gel 3 and gel 4 and the inset for pure silica calcined at 800

0C for 4h , o La9.33 Si6O26

phase and * La2 Si2O7 phase.


3.3. TEM ANALYSIS: To confirm the structure, TEM analysis has been performed for two samples gel 1

and gel 4 calcined at 800oC for 4 h. The micrograph show approximately spherical shape of pure silica and

lanthanum silicate nanoparticles synthesized, as shown in Fig. IV (a, b). In addition, there are

agglomerations as can be seen in Fig. IV (a, b). The diameter of the nanoparticles mainly ranged from 10 to

25 nm, which is in good agreement with that estimated from the XRD pattern.

Gel 4Gel 1

Fig. IV(a, b) TEM pictures for gel 1 and gel 4 calcined at 800oC for 4 h.

3.4. SEM-EDX ANALYSIS: Fig. (V, VI) show the morphology and the elements ration in pure and dope

were had silicate system. The morphological images of pure silica (gel 1) and lanthanum silicate (gel 2, gel

3 and gel 4) nanoparticles after calcinations at 8000C for 4h were observed by using scanning electron

microscopy (SEM) as shown in Fig. VI. It is observed that pure and doped silicate particles and are nano-

sized with nearly spherical shapes. While, the aggregation (interconnected mass of fine, nearly spherical

grains) of silica nanoparticles is very serious and appeared in all images, which is a typical problem for

nano-sized particles. It is found that the morphology of the obtained lanthanum silicate systems is different

from that of pure silica especially gel 4 has very fine particles. The average grain sizes measured from the

SEM micrographs are in the range 75 nm to 29 nm for gel 4, and all grains exhibit nearly spherical shape. It

is clear that the particle sizes evaluated from the SEM micrographs are larger than those evaluated from the

XRD patterns. Fig. VI show the energy dispersive spectra of the precursors used in synthesis of pure silica

(gel 1) and lanthanum silicate (gel 2, gel 4) calcined at 800oC for 4h, indicating the existence of O and Si in

pure silica also the presence of O, Si and La element in the lanthanum silicate nanoparticles.

gel 1

gel 3

gel 2

gel 4

Fig. V: SEM micrographs of pure silica (gel 1) and lanthanum silicate (gel 2, gel 3& gel4), calcined at

800oC for 4h.


Eleme

nt

Weight

%

Atomic

%

O K 61.13 73.41

Si K 38.87 26.59

Totals 100.00

Eleme

nt

Weight

%

Atomic

%

O K 48.59 81.35

Si K 11.48 10.95

La L 39.93 7.70

Totals 100.00

Elemen

t

Weight

%

Atomic

%

O K 57.98 83.08

Si K 15.33 12.51

La L 26.69 4.40

Totals 100.00

Gel 1

Gel 3

Gel 4

Fig. VI: EDX spectra of gel 1, gel 3and gel 4 calcined at 800oC for 4h.

3.5. FTIR SPECTROSCOPY: To investigate the structural changes with different H2O content occurred

towards the formation of pure silica and lanthanum silicate through different phase transition from gel to the

nano-crystalline powders calcined at 100oC, 400

oC. FTIR spectra were performed in the wavenumber range

470–4000 cm-1

for the prepared samples. The broad band at 3394 cm-1

are assigned to the symmetric

stretching mode of interstitial H2O molecules in both pure and doped silicate and become more intense in

gel 4 [(21)], which disappeared with increasing the temperature degree up to 400oC, as in Fig.VI(a, b).

The band at 1635 cm−1

is due the deformation vibration of the H–O–H belonging to the water

absorption in the prepared gels and gradually has a higher intense as in gel 4but its intensity decrease with

increasing the calcinations temperature [(22)]. The important decrease of the absorption bands from 3394 to

1342 cm-1

after the calcination at 4000C can be explained by the polycondensation and densification of

lanthanum silicate system that lead to the formation of new Si–O–Si, La–O–Si links and eliminate -OH

groups [(23), (24)]. The bands at 1342- 1466 cm-1

region may be correspond to La–O asymmetric stretching

vibration in silicate system bonded with nitrate or the residual organic coming from (La(NO3)3) as La

source and the used solvents [(25), (26)].The absorption peaks corresponding to metal-attached hydroxyl

groups becomes weaker and the peaks of Si–O–Si and La-O-Si bonds become stronger, which indicates the

gradual formation of pure and doped silicate network by the condensation–polymerization reaction. The

absorption bands arising from the formation of lanthanum silicate network, for wavenumbers from 1000 to

1466 cm−1

, with a peak centered at 1095 cm−1

attributed to asymmetric stretching Si-O-Si [(26)-(28)]. The

weak peaks near 925 cm-1

is and 987 cm-1

due to Si-O and La-O stretching vibrations in pure silica and

lanthanum silicate [(26)]. The weak bands at 648 cm−1

, 725 cm−1

may be due to the stretching mode of

broken Si–O_ bridges and formation of Si-O-La [29-31]. The bands at 725 and 944 cm-1

are respectively

associated with the formation of a ring structure of SiO4 tetrahedral and with Si–O–M bond formation,

where M (Al, P, Li, La,….etc), which is significant in crystalline phases [(25)-(31)]. At around 509 cm−1,

the absorption band found in all the spectra was the characteristic band of Si–O–Si bending vibration in

SiO4 tetrahedra [(17), (29)]. In general, the FTIR spectra were consistent with the XRD patterns (Fig. III),

which showed sharp and narrow peaks corresponding to the formation of lanthanum silicate nano-crystalline

powder. Since the addition of La2O3 ions to the silica system enhancement the physicochemical properties

of silica gel and increased the transformation rate to lanthanum silicate system.


500 1000 1500 2000 2500 3000 3500 4000

(a)

64

8

3394

16

35

14

65

13

73

13

42

10

95

98

7

92

5

Ab

so

rb

an

ce(a

.u.)

Wavenumber (cm-1)

gel 2

gel 3

gel 4

gel 1

72

5

50

9

gel 4 at 4000C

(b)

Fig. VII(a, b) The FTIR absorbance spectra in wide spectral region 470–4000 cm

-1 of (a) pure silica (gel1)

and lanthanum silicate with different H2O ratio (gel 2, gel 3 & gel 4) and (b) gel 4 calcined at temperature

400oC, for 4h.

3.6. OPTICAL PROPERTIES: Fig. VIII (a, b) shows the UV–vis spectra (300–1100 nm range) and the

refractive indices of pure SiO2 (gel 1) and 10 La2O3-90 SiO2 (gel 2, gel 3& gel 4) samples calcined at 600oC

as representative curves. It is noticeable that the gel 1 is a highly transparent glass with no absorption in the

visible range as in Fig. VIII(a). After introduction of La2O3 ions into the silicate system and hydrolysis of

lanthanum silicate with different H2O ratios (gel 2, gel 3& gel 4), common features can be observed in the

spectra of all doped samples, that is, an absorption beginning at approximately 335 nm. The sample gel 4

showed triply split peaks at 208, 231 and 303 nm, respectively. These peaks may be assigned to La2O3 in

small oligomeric LaxOy clusters through the silicate system. Fig. VIII(a) also indicates the presence of broad

band at 300 nm and the 231 nm for the gel 4, which are missing in gels (2, 3) for lanthanum silicate system.

The refractive indices of pure and doped silicate systems were calculated from the normal transmission and

reflection using Fresnel equation.

n = [1+R/ 1-R] + [((4R/ (1-R) 2) - K

2]

½

The refractive index values were found to be strongly affected by the composition and the calcinations

temperature. Fig. VIII(b) shows the change in the refractive index (n) values with the calcinations

temperature from 200 up to 600oC for 4 h for gel 1 and gel 4. It is found that by increasing the calcinations

temperature the refractive index for lanthanum silicate (gel 4) increases. This indicated that the

incorporation of La2O3 ions through the silicate network alters fundamentally the pure silicate properties.

The n values of pure SiO2 were found to be in the range from 1.45 up to 1.47 and for 10 La2O3-90 SiO2 (gel

4) ranging from 1.67 up to 1.77. This increase is associated with the densification of the materials.


200 300 400 500 600

1.45

1.50

1.55

1.60

1.65

1.70

1.75

1.80 (a)

gel 4

gel 1Re

fra

cti

ve

in

de

x (

n)

Temperature ( 0C)

(b)

200 300 400 500 600 700 800 900

(a)

gel 4

gel 2

gel 3

Ab

so

rba

nc

e

Wavelength (nm)

gel 1

Fig. VIII(a, b) UV–vis spectra of gel 1, gel 2gel 3and gel 4 calcined at 6000C for 4h and (b) the refractive

index as a function in temperature for two samples gel1 and gel 4.

4. CONCLUSIONS: SiO2 and 10 La2O3- 90 SiO2 nano-crystalline have been synthesized successfully

using sol–gel technique. The crystallinity of 10 La2O3- 90 SiO2 powders increases with increasing

calcinations temperature and H2O content and the grain size of nanoparticles mainly ranged from 10 to 27

nm. XRD and FTIR studies confirmed that a better homogeneous reaction took place with the constituents

La2O3 and SiO2 in lanthanum silicate powders. The optical properties changed gradually with increasing the

calcinations temperature. SEM indicated the spherical nanoparticles morphology for the prepared samples.

The reported results are encourage to confirm the possibility to incorporation of different molecules through

silicate matrix as nanocomposite could be identified as potential materials towards the development of

silicate system for future applications.

ACKNOWLEDGEMENTS: Dr. A. M. El Nahrawy is expressing their grateful thanks to Dr. A. I. Ali,

from Energy Harvest-Storage Research Center and Department of Physics, University of Ulsan, Ulsan,

Korea and prof. G. Turky, from Microwave physics and Dielectrics Department, National research centre

(NRC),Cairo, Egypt for your great help to me.

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