The gas sensitivity experiments have demonstrated that the as-synthesized SnO2especially, flower-like morphology, exhibit high sensitivity to CO, which may offer potential applications in
Trang 1Materials Chemistry and Physics xxx (2008) xxx–xxx
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Materials Chemistry and Physics
j o u r n a l h o m e p a g e :w w w e l s e v i e r c o m / l o c a t e / m a t c h e m p h y s
Effects of flower-like, sheet-like and granular SnO 2 nanostructures
prepared by solid-state reactions on CO sensing
Azam Anaraki Firooza, Ali Reza Mahjouba,∗, Abbas Ali Khodadadib
aDepartment of Chemistry, Tarbiat Modares University, 14115-175 Tehran, Iran
bSchool of Chemical Engineering, University of Tehran, 11155-4563 Tehran, Iran
a r t i c l e i n f o
Article history:
Received 19 August 2008
Received in revised form 29 October 2008
Accepted 17 November 2008
Keywords:
SEM
XRD
Semiconductor
Nanostructures
a b s t r a c t
Nanostructured SnO2with different morphologies of flower-like, sheet-like and granular have been suc-cessfully prepared via a solid-state reaction in the presence of NaBr, NaCl, and NaF, respectively The added salts not only prevent a drastic increase in the size of the tin species but also provide suitable conditions for the oriented growth of primary nanoparticles The formation mechanisms of these materials by solid-state reaction at ambient temperature are proposed The gas sensitivity experiments have demonstrated that the as-synthesized SnO2especially, flower-like morphology, exhibit high sensitivity to CO, which may offer potential applications in gas sensors
© 2008 Elsevier B.V All rights reserved
1 Introduction
Hierarchical self-assemblies of nano-/micro-crystallites with
specific morphology are of great interest in areas of chemistry and
materials science because of their unique and exciting properties
[1] Some reports indicate that the catalytic selectivity and
sensitiv-ity of these structures are significantly improved, compared with
other shapes[2,3] SnO2 is an n-type semiconductor with a long
band gap[4], and is well known for its applications in gas sensors
[5,6], dye-base solar cells[7], optoelectronic devices[8], electrode
materials[9]and catalysts[10] Thus, designing and preparing SnO2
materials with novel morphology are of significant importance in
meeting the scientific and technological applications There are
many methods to synthesize of different morphology of this
mate-rial Ohgi et al reported the evolution of nanoscale SnO2flakes into
hierarchically structures by subsequent hydrothermal treatment
[11] Xie and co-workers prepared 2D hierarchical SnO2
flower-like nanostructures without post-treatment of calcinations, taking
advantage of slow oxidation of tin foil by the solution of KBrO3and
NaOH[12] Mu and co-workers synthesized the flowerlike SnO2
quasi-square submicrotubes by reaction between SnCl2and oxalic
acid in ethanol solution, followed by calcination in air[13] All these
methods to dioxide nanoparticles are in general complicated and
expensive There are many advantages in the solid-state reaction
approach such as: (a) simple, cheaper and convenient; (b) involve
∗ Corresponding author Tel.: +98 21 82883442; fax: +98 21 88007930.
E-mail address:mahjouba@modares.ac.ir (A.R Mahjoub).
less solvent and reduce contamination; (c) give high yields of prod-ucts[14]
In this paper, we synthesize SnO2nanostructures with differ-ent morphologies by solid-state reaction method Our studies show that this method is not only a simple process but also gives as uniform and monodisperse products as those by other lucrative methods We have also investigated the effect of halogen salts on the morphology and explained in light of the proposed mechanisms
We found that the as-prepared SnO2 materials with flower-like morphology exhibit higher sensitivity to CO gas and thus are expected to be useful in industrial applications such as gas sensors
2 Experimental
2.1 Preparation
A mixture of SnCl 2 ·2H 2 O (0.01 mol, 3.51 g) and NaOH (0.038 mol, 2.13 g) pow-ders was ground for 30 min Then, each of NaBr, NaCl, and NaF halogen salt with a weight ratio of 2:1 was added to the system and ground for another 30 min at room temperature The reaction began immediately during the mixing process (accom-panying an emission of water vapor from the system) The products dried in air to yield black SnO powder The powder was calcined at 400 and 600 ◦ C for 2 h in air and washed with distilled water for removing the halogen salt and dried in 80 ◦ C.
2.2 Characterization
The morphology of tin oxide powders was determined by using scanning electron microscopy (SEM) of a Holland Philips XL30 microscope X-ray powder diffraction (XRD) patterns of the powders were recorded in ambient air with using
a Holland Philips Xpert X-ray powder diffraction (Cu K␣, = 1.5406 Å), at scanning speed of 2◦min−1from 20◦to 80◦(2) Specific surface area of SnO 2 nanoparticles were determined by nitrogen adsorption, after degassing at 300◦C for 2 h, using a surface area analyzer (CHEMBET 3000) and BET method.
0254-0584/$ – see front matter © 2008 Elsevier B.V All rights reserved.
doi: 10.1016/j.matchemphys.2008.11.028
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Fig 1 XRD patterns of flower-like (NaBr) calcined at: (a) 400◦C (NaBr400); (b)
600 ◦ C (NaBr600) for 2 h.
2.3 Gas sensing measurement
A paste of tin oxide powder was applied on an alumina tube with gold electrodes
already deposited on it The sample was dried and calcined at 400 ◦ C Thus obtained
sensor was placed in a glass holder immersed in a molten salt bath, temperature
of which was accurately controlled by a PID temperature controller The sensor was
connected to an electrical circuit using platinum electrodes The DC electrical
mea-surement was made by using an applied voltage of 4.0 V onto a known resistance in
series with the sensor The DC voltage across the sensor was read out using an A/D
converter and data was transferred to the computer for further processing The
sen-sor response was measured at different temperatures in the presence of 1000 ppm
CO in air.
3 Result and discussion
3.1 Tin oxide morphology
The structure of tin oxide powders was determined by XRD,
as shown in Figs 1–3 Major SnO2 cassiterite structure (JCPDS
no 41-1445) is observed in all XRD patterns The marked peaks
Fig 2a and 3aare attributed to SnO phase (JCPDS no 6-395), which is
stable up to 400◦C The ratio of (1 0 1) SnO peak to (1 1 0) SnO2peak
intensities, as a semiquantitative measure of SnO/SnO2ratio, are
included inTable 1 After calcination at 600◦C, the (1 0 1) diffraction
peak intensity of SnO is reduced, as shown inFig 2b and 3b All XRD
patterns show that, when the calcination temperature increases,
the intensity of the diffraction peaks increases, indicating a high
degree of crystallinity and grain sizes of the nanoparticles The
crys-tal grain sizes were calculated from the FWHM in XRD pattern using
the Debye–Scherrer’s equation and listed inTable 1
Scanning electron microscopy was employed to study the
mor-phologies of the tin oxide samples.Fig 4a–c shows that flower-like,
sheet-like, and granular morphologies of tin oxide are formed,
when NaBr, NaCl, or NaF halogen salts are used in the salt-assisted
solid-state synthesis, respectively The flower-like morphology
reveals that the structure is built up many sheet-like nanoparticles
Fig 2 XRD patterns of sheet-like (NaCl) calcined at: (a) 400◦ C (NaCl400); (b) 600 ◦ C (NaCl600) for 2 h.
Fig 3 XRD patterns of nanoparticle (NaF) calcined at: (a) 400◦ C (NaF400); (b) 600 ◦ C (NaF600) for 2 h.
3.2 Growth mechanism of SnO 2 nanostructures
Li et al have proposed that in the first step of the solid-state reaction, SnO fine particles are formed[15] The reaction is often self-initiated and self-sustained with H2O vapor releasing after grinding of the mixture of the precursors After the calcination at
400 or 600◦C, the SnO is mostly converted to SnO2nanoparticles
It is well known that the structure of products by a solid-state reac-tion depends on the rate of nucleareac-tion and growth It is also thought that adding inorganic salts causes to reduce the overall reaction rate and broaden the distribution of product NaBr, NaCl, and NaF
as salt-assisted additives are expected to cause cage-like shells surrounding the SnO particles, preventing their growth Adjacent nanoparticles rotate to find the low-energy configuration repre-sented by a coherent particle–particle interface[16] As a result, the added salts help to form of suitable morphology with high yields
Table 1
The sizes, sensitivity, semiquantitative measure of SnO/SnO 2 ratio and physical properties of the as prepared materials.
Samples Color Morphology Crystallite size (nm) Sensitivity Surface area (m 2 g−1) Grain size (nm) SnO/SnO 2 (ratio of peaks intensity)
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Fig 4 The SEM images of (a) NaBr (flower-like), (b) NaCl (sheet-like), and (c) NaF (granular).
3.3 CO sensing
The CO sensitivity is defined as S = Rair/RCOwhere Rairand RCO
are resistances of the tin oxide in air and in CO, respectively[17]
Figs 5–7show the changes in sensitivity of the tin oxide
materi-als calcined at 400 and 600◦C, when their thick-film sensors were
exposed to CO at various temperatures Maximum sensitivities
occur at about 275–300◦C for all sensor materials at the two
calci-Fig 5 Temperature-dependent sensitivity of SnO2 flower-like (NaBr) (a) NaBr 400;
(b) NaBr 600.
nation temperatures The highest maximum sensitivity is observed for the flower-like morphology calcined at 400◦C Calcination at the higher temperature of 600◦C leads to an increase in the grain sizes, as indicated by XRD and BET results (Table 1) Lower sensitiv-ities are observed for the sheet-like and granular morphologies As the calcination temperature increases from 400 to 600◦C, the max-imum sensitivities of sheet-like and granular tin oxides increase, while their grain sizes increase (seeTable 1andFigs 6 and 7) The
Fig 6 Temperature-dependent sensitivity of SnO2 sheet-like (NaCl) (a) NaCl 400; (b) NaCl 600.
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Fig 7 Temperature-dependent sensitivity of SnO2 granular (NaF) (a) NaF 400; (b)
NaF 600.
XRD patterns of the same samples show the presence of stannic
suboxide (i.e SnO), which decreases as the calcination temperature
increases
The sensitivity to a target gas strongly depends on the ease of
diffusion of gas molecules inside the sensor[18–20] Thus, the
struc-ture and morphology of materials can be correlated with the sensor
performance Flower-like SnO2consists of numerous nanoparticles
joined together into flower-like structure, resulting in much more
active exposed sites for gas chemisorptions Thus, the realization of
a high sensitivity of flower-like morphology may be explained in
terms of rapid gas diffusion onto the entire sensing surface due to
the specific morphology of this material
Usually the gas sensitivity of metal oxide semiconductors
decreases, as their sizes increase, due to lower surface areas and
defect density[21] It sounds that, the presence of tin with lower
oxidation state in tin oxide sensors diminishes their sensitivity to
reducing gases such as CO[22,23] On the other hand, the presence
of SnO causes a decrease in oxygen vacancies in the samples
Therefore, not only the morphology but also the presence of
SnO in sheet-like and granular tin oxides, contributes to their lower
sensitivity to CO
4 Conclusion
We report the preparation of flower-like, sheet-like and
granu-lar morphologies of SnO2by solid-state reactions in the presence of
NaBr, NaCl, and NaF salts, respectively The salts added are expected
to cause cage-like shells surrounding the SnO particles, prevent-ing their growth of nanoparticles Adjacent nanoparticles rotate
to find the low-energy configuration represented by a coherent particle–particle interface, resulting in particular morphologies The gas sensitivity experiments showed that the SnO2 flower-like offered higher sensitivity than SnO2 sheet-like and granular The high sensitivity may be explained in terms of rapid gas dif-fusion onto the sensing surface In addition, the XRD results reveal the presence of a stannic suboxide, which explains in part the lower sensitivity to CO shown by the sheet-like and granular nanostruc-tures
Acknowledgments
Supports for this investigation by Tarbiat Modares University and University of Tehran are gratefully acknowledged
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