# Low Temperature Synthesis ofLi2SiO3: Effect on Its Morphological and Textural Properties

Authors: Georgina Mondragón-Gutiérrez; Daniel Cruz; Heriberto Pfeiffer; Silvia Bulbulian
Journal: Research Letters in Materials Science
 (2008)

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## Abstract

Synthesis, at low temperature, ofLi2SiO3 was investigated using different Li : Si molar ratios and urea, which was used as template. This new synthesis was performed in order to look for different textural and morphological properties than those obtained usually by conventional methods in this kind of ceramics. XRD and SEM analyses showed that Li2SiO3 was obtained pure and with ceramic particle morphology of hollow spheres of 2–6 μm. TEM analysis showed that those spheres were composed by needle-like particles crosslinked among them. This morphology provided a high surface area, probed by N2 adsorption. Therefore, this method of synthesis may be used to obtain other similar ceramics and test them in different applications.


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## Body

## 1. Introduction

Lithium ceramics are of research interest because of their technological applications. Thus, for
example, there has been research in recent years on their application as
electronic devices, as CO2 captors and as breeder materials for
nuclear fusion reactors, in addition to other more well-known applications such
as in batteries and in low thermal expansion glass-ceramics used in ceramic
hobs [1–10]. Among these ceramics, lithium silicates (Li2SiO3 and Li4SiO4) seem to present very good properties as
materials in nuclear research as tritium (3T) breeding materials and as
materials to absorb carbon dioxide [11–18]. Nevertheless, the main problem in
this application is related to the lithium diffusion and
release from the ceramic. Therefore, the aim of this work was to study a new
synthesis method at low temperature for one of these lithium silicates, lithium
metasilicate Li2SiO3, in order to look for different
textural and morphological properties than those obtained usually by
conventional methods.


## 2. Experimental Section

Lithium metasilicate (Li2SiO3) was
synthesized by solid state reaction for comparison reasons. In this case, a
stoichiometric mixture of lithium carbonate (Li2CO3, Sigma, Saint Louis, Mo, USA) and silicic acid (H2SiO3, Mallinckrodt, Hazelwood, Mo, USA) were mixed and
then heat treated at 900°C for 4 hours. By contrast, a different synthesis was
performed at low temperature, using lithium hydroxide (LiOH, Merck, Whitehouse Station, NJ, USA) and silicic acid (H2SiO3) with different Li : Si molar
ratios (2 : 1, 3 : 1, 4 : 1, and 6 : 1), and urea as template (1, 2, and 4 M, Merck, Whitehouse Station, NJ, USA). First, LiOH was
dissolved on the aqueous solution and then H2SiO3 was added. The suspension
was stirred during 1 hour, and later, dried at 90°C to obtain a white powder. The produced
powders were washed and centrifuged three times with deionized water. The
composition, surface area, total porous volume, crystal size, and
particle size of the prepared compounds were obtained by X-ray diffraction
(XRD), scanning electron microscopy (SEM), transmission electron microscopy
(TEM), and N2 adsorption (BET). A diffractometer Siemens D-5000,
coupled to a copper-anode X-ray tube, was used to obtain the experimental X-ray
diffraction patterns. Additionally, the crystal size of Li2SiO3 was calculated by the Debye-Scherrer
equation [19]. For the SEM analysis, the samples were covered with gold to
overcome the lack of electrical conductivity. Then, samples were analyzed in a
Cambridge/Leica Stereoscan 4400 to find out morphology and particle size. The
transmission electron analysis was performed using a Jeol 2010HT microscope.
Samples were prepared using standard gravimetric procedures. Finally, the
surface area was determined by the N2 adsorption technique using the BET method, in a surface area analyzer,
Micromeritics Gemini 2375 V5.01.


## 3. Results and Discussion

A set of Li2SiO3 samples was prepared using different urea
solutions. Figure 1 shows
the XRD patterns of the samples varying the Li : Si molar ratio in 1 M urea solution. In all the cases, Li2SiO3,
with different yields and crystal sizes, was obtained. At low Li : Si molar ratios (2 : 1 and
3 : 1) two other phases were detected, Li2Si2O5 and an amorphous phase. The presence of the amorphous phase and a lithium poor phase
(Li2Si2O5) indicated an incomplete reaction.
Therefore, higher Li : Si molar ratios were used (4 : 1 and 6 : 1). In these cases,
Li2SiO3 was obtained with a yield of 100% according to
the XRD patterns. In all these samples, the wideness of the XRD peaks strongly
suggested the presence of very small crystals. The crystal size was calculated by
the Scherrer formula [19]. The data showed the presence of tiny crystals, where
the smaller Li2SiO3 crystals were
obtained using a Li : Si molar ratio of 4 : 1, 78 Å. In order to determine the effect of
urea concentration in the Li2SiO3 synthesis, Li : Si = 4 : 1
samples were prepared utilizing different urea concentrations (2 and 4 M).
Nevertheless, none of these experiments showed any change by XRD (data not
shown).Figure 1
XRD
patterns of the Li : Si samples prepared in urea 1 M: (A) Li : Si = 2 : 1; (B) Li : Si
= 3 : 1; (C) Li : Si = 4 : 1; (D) Li : Si = 6 : 1. The
labels correspond to  Li2SiO3(▪) and Li2Si2O5(▵). The
composition of each sample is indicated as well.According to the XRD results, the sample analyzed by SEM and
TEM was the one obtained using a Li : Si = 4 : 1 molar ratio, which produced Li2SiO3 with a yield of 100% and has a crystal size of 78 Å. The particles presented a
very singular morphology (Figure 2). They seemed to be hollow spheres with
diameters between 2 μm  and
6 μm, which were aggregated in bigger
agglomerates of about 77 μm. Additionally, the surface of these
particles seemed to be corrugated and highly porous. In fact, this kind of
morphology has been already reported for metal oxides synthesized by using soluble organic templates [20–22]. In
this case, urea must produce core particles where metal ions are deposited.
Later, during the heat treatment, urea decomposes producing gases (CO2 and NH3), which form the hollow spheres. Additionally, the production
and release of the gases, from the core of the particles, inhibited the
densification of the surfaces and produced porous materials.Figure 2
SEM
image of the Li  :Si = 4 : 1 sample prepared in a 1 M urea solution.TEM results supported this
hypothesis (Figure3). The image corresponds, approximately, to the diameter of
one hollow sphere. As it can be seen, this sphere is composed by much smaller particles, which have a needle-like structure.
These needles
are crosslinked among them, producing a very porous structure. The average size
of the needles was calculated to be 200×25 nm width. The width calculated
indicates that these needle-like particles are polycrystalline, since the
crystal size determined by XRD is smaller. The needle-like structure could be produced due to the union
of several Li2SiO3 chains. Li2SiO3 is a silicate which has a linear (SiO3)2- structure [23].
Hence, if several of these chains are linked, the needle-like structure should
be formed. In fact, this kind of structure had been already reported several
years ago by Hesse [24].Figure 3
TEM
image of the Li : Si = 4 : 1 sample prepared in a 1 M urea solution.Finally, the surface area and porous volume were determined for all the samples. As
expected, these samples presented very high surface areas. Figure4 presents the surface area
and porous volume of samples prepared in the different urea solutions. Although
the compounds,
prepared in different urea concentrations, were similar according to XRD
results, the textural properties increased as a function of the urea
concentration. The samples prepared with 1 M and 2 M urea solutions had similar
surface areas (20 m2/g)
but the one prepared with a 4 M urea solution increased its surface area up to
75 m2/g. The
explanation to this result can be simply associated to the gas formation during
the urea decomposition. As more urea is present in the sample, more gases were
produced and therefore the porosity was increased significantly. A similar
trend was observed for the porous volume, which increased from 0.07 cm3/g
to 0.27 cm3/g. Additionally, when urea concentration increases, it
tends to produce dendritic crystals, which may offer a higher surface area to
the Li2SiO3 deposition and formation. If that were the
case, it could reinforce the explanation of why the surface area stays
relatively constant at low urea concentrations (1 and 2 M) and it increases
dramatically with a 4 M urea solution. These results are in very good agreement
with the previous results, which proved the presence of particles with a
needle-like structure highly porous.Figure 4
Surface area and
porous volume of the Li : Si = 4 : 1 samples prepared with different urea
concentrations.


## 4. Conclusions

Lithium metasilicate (Li2SiO3) was obtained at low
temperature using different Li : Si molar ratios in
different urea solutions. The low temperature method prepared in the presence of urea
solutions produced hollow sphere materials, which were
composed by needle-like particles. The needles were crosslinked among them,
producing a very porous structure due to the union of several Li2SiO3 chains. Consequently, the surface area obtained for these samples was very
large.Obtaining Li2SiO3 with small particles and high surface
areas should facilitate the diffusion and release of lithium, which is the
limiting step on several applications
of this kind of materials. Therefore, this method of synthesis may be used to
obtain other similar ceramics and test them in the application to see the
results as a function of the particle size and surface area, in comparison to
the same materials obtained by other methods.



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*Source: 908654-2008-03-30.xml*