Effect of the temperatures on structural and optical properties of tin oxide (SnOx) powder

Article abstract of DOI:10.1016/j.physb.2009.08.082

A series of composite SnO_x materials were prepared by SnO powder heating at different temperatures in air for 2 hr. Depending on treatment conditions it was found that SnO, with its tetragonal structure, could transform into the Sn phase or tetragonal SnO and tetragonal SnO₂. The influence of heating at different temperatures on the properties of SnO_x has been studied. The influences of annealing temperatures in air on the structural and optical properties of SnO_x have been investigated. Correlations between the structural and optical properties of the products were found.

Full text of DOI:10.1016/j.physb.2009.08.082

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Physica B 405 (2010) 313–317
Contents lists available at ScienceDirect
Physica B
journal homepage: www.elsevier.com/locate/physb
Effect of the temperatures on structural and optical properties
of tin oxide (SnO ) powder
x
a,b,Ã
b
b
b
N.M.A. Hadia
, S.V. Ryabtsev , P.V. Seredin , E.P. Domashevskaya
a
Department of physics, Faculty of Science, Sohag University, 82524-Sohag, Egypt
Voronezh State University, Universitetskaya pl. 1, 394006, Voronezh, Russia
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of composite SnO materials were prepared by SnO powder heating at different temperatures in
x
Received 8 November 2008
Received in revised form
air for 2 hr. Depending on treatment conditions it was found that SnO, with its tetragonal structure,
could transform into the Sn phase or tetragonal SnO and tetragonal SnO
different temperatures on the properties of SnO has been studied. The influences of annealing
temperatures in air on the structural and optical properties of SnO have been investigated. Correlations
between the structural and optical properties of the products were found.
2
. The influence of heating at
18 August 2009
x
Accepted 18 August 2009
x
PACS:
&
2009 Elsevier B.V. All rights reserved.
61.10.Nz
74.25.Gz
7
7
8.30.-j
8.40.Fy
Keywords:
Sno powder
SnO
2
Temperatures
Structural and optical
1
. Introduction
more desirable than high transmittance [5], additionally it is
useful to have some water content in the resultant film [4], which
plays key role in inducing electrochromic (EC) effect.
It is noticed from the literature survey that the variety of
methods of preparation will lead to the layers having different
optical and electrical properties, which evokes critical influence of
oxygen vacancies, serving as donor in tin oxide films [5]. In
principle physical methods such as sputtering [3], and thermal
evaporation [6], lead to weakly non-stoichiometric tin oxide with
co-existence of other insulating phases like SnO, resulting into
Tin oxide is of great technological interest as transparent
conducting electrodes, IR reflecting heat mirrors, and SnO/Si solar
2
cell devices with high conversion efficiency [1]. Moreover, SnO -
based sensor devices have been used as a prototype for detecting
reducing and inflammable gases [2]. Tin oxide thin films have
been successfully demonstrated as transparent conductors (TC),
optical windows for the solar spectrum, stability resistors, touch-
sensitive switches, digital displays, light emitting diodes (LEDs),
electrochromic displays (ECDs), etc. [3], mainly due to their
outstanding properties.
relatively high resistive films. The range of resistivity of as
À3
deposited SnO
2.5 Â10
x
films typically varies from 6.6 Â10
O cm to
À3
The consensus of the researchers is that for TC, high
O cm [3]. On the other hand chemical methods
transmittance (T%) and relatively low electrical resistivity (
desirable while for applications such as display devices and LEDs,
low electrical resistivity and moderate transmittance are desirable
r
) is
especially spray pyrolysis technique, lead to strongly non-
stoichiometric tin oxide films without co-existence of insulating
phases, resulting into comparatively low resistive films [7]. The
[
4]. These applications rely on its itinerant electrons that stem
electrical resistivity of as deposited SnO
x
films typically varies
cm, which is several times
less than the films deposited by physical methods. Therefore, it
can be concluded that the SnO films deposited by spray pyrolysis
technique are more susceptible to oxygen deficiencies [8].
We are interested in SnO films in connection with the
À3
À3
from the ionization of the dopants and enter the conduction band.
For ECDs, which hinges on the ability of the material to sustain
mixed conduction of ions and electrons, low electrical resistivity is
from 1.45 Â10
O
cm to 0.45 Â10
O
x
x
Ã
electrochromism. Electrochromic tin oxide films were described
recently by Orel et al. [9] and Olivi et al. [10] who prepared their
samples by dip-coating and Isidorosson et al. [11] who prepared
Corresponding author at: Voronezh State University, Universitetskaya pl. 1,
3
0
94006, Voronezh, Russia. Tel/fax: +7 0732 208 363.
E-mail address: Nomery_abass@yahoo.com (N.M.A. Hadia).
921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.physb.2009.08.082

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their samples by sputtering. They emphasize the importance of
Ultraviolet photoelectron spectroscopy experiments for surface
electronic structure of tin oxide were performed only on single
various properties that SnO
nounced electrochromism.
x
should exhibit for attaining pro-
2 2
crystal SnO (0 01) and SnO (110) surfaces using synchrotron
In this investigation, we have employed spray pyrolysis
technique for SnO thin film deposition and discussed their
light and He I UV sources [20]. Cox et al. [21] found unusual band
+
x
2
gap emission at the clean SnO crystal surface after Ar bombard-
structural, electrical and optical properties. The deposition has
been carried out from aqueous stannic chloride solution, with a
postulation that the resultant films may have some water content
ment and Themlin et al. [35] also reported that the tin-derived
character of the band gap defect states was found in the perturbed
2
SnO (110) surface by the ion bombardment.
[
11], which would be in turn beneficial for better electrochromic
effect. Several experiments on electrochromism in SnO thin films
are underway and results will be disseminated elsewhere.
SnO is an n-type semiconductor with an optical band gap of
about 3.6 eV. SnO
rutile SnO , each atom is surrounded by a distorted octahedron of
O atoms with all Sn distances equal to 2.05 A. In comparison with
SnO , the structure and physical properties of SnO have not been
extensively investigated. The structure of SnO is layered, similar to
In this present work, we thoroughly examine the oxidation
processes of SnO powder heated at different temperatures in air
x
for 2 hr. The variations of the structural properties of SnO
x
that
2
have happened through SnO to SnO transformation are investi-
2
1
4
4
h
2
crystallizes in the rutile structure D [12]. In
gated. The effects of annealing temperatures on the optical and
structural properties of the products are discussed.
2
˚
2
2. Experimental details
1
4
that of PbO, with tetragonal structure D [12]. In this structure
the Sn is situated at the apex of a square pyramid with Sn-O
4
h
2
+
99.99% pure SnO powders were placed in an alumina boat
˚
positioned at the centre of the quartz tube. The temperature of the
furnace was rapidly ramped up to 250, 450, 650 and 850 1C and
kept for 2 hr. During the process, a constant flow of air was
distances equal to 2.224 A and O-Sn-O angles 11.731. Also, SnO
exists in an orthorhombic phase depending on the preparation
procedures. The optical band gap is in the range 2.5–3 eV.
The oxidation of tin and in particular the mechanism of the
adjusted. The crystal structure of the SnO
X-ray diffraction (XRD) using a DRON 4 utilizing Cu K
UV/visible absorption measurements were carried out on the SnO
x
were characterized by
oxidation process from SnO to SnO
2
have been extensively studied
a
radiation.
using various kinds of preparation techniques [13]. In general,
when tin oxide films were deposited on a high temperature
substrate by several deposition techniques, since it dissociates in
the gaseous SnO and forms oxygen-deficient SnO
post-annealing in an O environment should be given. Geurts
x
using a spectrophotometer (UV-210A, Shimadzu) in the wave-
length range between 190 and 900 nm. The IR studies were
carried out at room temperature using FIR -spectrometer Vertex
70 (Bruker).
x
(xo2) films,
2
et al. [14] and Reddy et al. [13] found that the annealed films reach
the final oxidation state either through simple oxidation of SnO
3. Results and discussions
(
(
direct transition) or through intermediate oxidation states
indirect transition), namely Sn or Sn , depending on the
2
O
3
3 4
O
3
.1. X-ray diffraction studies
deposition parameters.
According to most previous studies [13], the oxidation of SnO
and SnO was carried out without knowledge of the initial oxygen
content in as-deposited films and it was reported that the perfect
SnO formation could only be attained after above 600 1C
annealing in O exposure. Therefore the influences of initial
composition, which intimately depend on deposition parameters,
on the oxidation process and crystallization from SnO and SnO
The crystal structure and phase of the powder samples were
2
determined from the XRD patterns. Fig. 1 shows X-ray diffraction
(XRD) patterns of the SnO powder heating at different
temperatures (250, 450, 650 and 850 1C) for 2 hr in air and SnO
2
powder. All the peaks in Fig. 1(a and b) can be readily indexed as
2
2
tetragonal SnO (JCPDS, no. 06-0395) with cell parameters
2
˚
˚
were not yet systematically clarified as the annealing temperature
increased.
a=3.80 A and c=4.84 A. Comparing the XRD spectrum of the
standard tetragonal SnO powder (Fig. 1a), the XRD spectrum of
SnO powder heated at 450 1C (Fig. 1c) shows a very strong feature
of texture structure. Fig. 1c is indexed as (2 0 0), (110), (10 2),
(211), (3 01), (4 0 0) for Sn, (101), (110), (0 0 2), (2 0 0), (112),
(211), (10 3), (2 2 0), (0 0 4) for SnO and (110), (101), (2 0 0),
Moreover, many extensive investigations of the oxidation state
of tin oxide have been carried out using Auger electron spectro-
scopy (AES), core-level and valence-band (VB) X-ray photoelec-
tron spectroscopy (XPS), ion scattering spectroscopy (ISS), and
ultraviolet photoelectron spectroscopy (UPS) [15], but quantita-
tive analysis is complicated by the difficulty of preparing standard
samples with an accurately known composition, and the mod-
ification of surface composition by an incident electron or ion
beam.
Detailed analyses of the VB region led to the distinction
2
between SnO and SnO by UPS and VB XPS, where the presence
of the prominent leading peak of a Sn 5s-derived origin for SnO
and that of an O 2p-derived structure at the lower binding energy
side of the VB in SnO
the SnO and SnO phases [16]. Recently Themlin et al. [17]
and Sanjinrs et al. [18] reported that a sizable chemical shift of
2
uses the characteristic peak to distinguish
2
2
+
4+
0
.7 eV between Sn (SnO) and Sn (SnO
2
) by XPS. In a previous
2
+
report [19], we also found that the chemical shift between Sn
4
+
and Sn occurred as much as 1.070.02 eV in XPS analyses and 2–
71 eV in AES spectra. However, the chemical shift measured
from AES spectra was not reliable because main doublet Auger
4
transitions (M
resolved.
5 4 5 4 4 5
N , N4,5,M N , N4,5) in tin oxide were not well
Fig. 1. XRD spectra of source material SnO powders annealing at different
2
temperatures 250, 450, 650 and 850 1C for 2 h in air and SnO powder.

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315
2
2
Fig. 2. (a) Spectral variation of reflectance (R) for all the samples SnO annealed at T=450 1C, 550 1C, 650 1C, 850 1C for 2 h in air and SnO powder). (b) The variation of (ahn)
2
versus hn for all the samples, SnO annealed at T=450 1C, 550 1C, 650 1C, 850 1C for 2 h in air and SnO powder).
(211), (2 2 0), (0 0 2), (310), (112), (3 01), (2 0 2), (3 21) for
2
SnO intimately depends on the initial content of oxygen and the
SnO . Therefore, Sn, SnO and SnO are simultaneously present
2
2
annealing temperature.
at the annealing temperature of 450 1C, and the intergrowth
mechanisms may occur at this thermal oxidizing temperature
[
22]. Increasing the annealing temperature further to 650 1C, the
3.2. Optical properties of SnO
x
Sn and SnO diffraction peaks disappeared and a complete
transformation of SnO to a tetragonal rutile structure SnO2
powders is observed. The thermal oxidation process of SnO to
It is well known that SnO
band gap energy (E ) in the range of 3.4–4.6 eV [25]. This scatter in
band gap energy (E ) values of SnO may be due to wide extent of
non-stoichiometry of the deposited layers. The dependency of the
band gap energy on the carrier concentration has been explicitly
given in the literature [25]. It has been apprehended that band gap
energy increases linearly with the increase in carrier concentra-
tion to the power 2/3.
2
is a degenerate semiconductor with
g
SnO
of oxygen environment, the oxidation begins at the surface of the
Sn particles, and SnO and SnO phases are nucleated and formed
as dispersed clusters on the surface of the Sn particles. The
SnO clusters keep on growing into nanoparticles when the
temperature is high enough to allow oxygen to diffuse into
the SnO particles. Fig. 1(d, and f) represent only the
characteristic SnO peaks corresponding to (110), (101), (2 0 0),
111), (210), (211), (2 2 0), (0 0 2), (310), (112), (3 01), (2 0 2) and
3 21) planes and the SnO powder is believed to be perfectly
. As temperature increases from 650 to 850 1C, no
2
may be described as follows [23]: due to the existence
g
2
2
2
e
Fig. 2(a and b) shows the reflectance and the variation of
2
2
(a
hn)
versus
h
n
for SnO powder annealed at different
powder . The
nature of the plots indicates the existence of direct optical
transitions. The band gap (E ) is determined by extrapolating
the straight-line portion of the plot to the energy axis. The
intercept on energy axis gives the value of band gap energy E for
(
(
temperatures (450, 550, 650 and 850 1C ) and SnO
2
oxidized to SnO
2
g
changes in positions and intensities of the XRD peaks are found.
From this observation it is proved that the annealing
temperature of 450 1C is not high enough to form the highest
g
all the samples and the values lie in the range of 3.58–3.73 eV. It is
noticed that band gap energy value is minimum (3.58 eV) for SnO
samples annealed at 450 1C. The samples annealed at 550 and
oxidation state of SnO
of polycrystalline SnO could completely reach pure polycrystalline
SnO after post-annealing at temperatures higher than 600 1C in
an O atmosphere. In consequence, the oxidation from SnO to
2
. It was reported in [24] that the annealed
2
650 1C, have E
g
values amongst all other samples, owing to lower
2
carrier concentration. Increasing the annealing temperature to

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above discussion is clear enough to understand s-p direct optical
transition in SnO
2
.
3.3. IR studies of SnO
x
The IR transmittance spectra of the SnO powder heating at
different temperatures (250, 450, 650 and 850 1C) for 2 hr in air
–1
2
and SnO powder in the low frequency range 550–800 cm are
shown in Fig. 3. Several bands due to fundamentals, overtones and
combinations of OH, Sn–O and Sn–O–Sn entities appear in the
À1
À1
4000–800 cm
range. Below 800 cm , the cut-off arising from
lattice vibrations is occurred.
The results and the proposed attributions are presented in
Table 1 and compared with the data published in the literatures
[
(
27–31]. The discrepancies concerning are due to several factors:
i) the nature of the sample (monocrystal, powder, colloidal
suspension) and the proportion of low-coordination sites [32]; (ii)
the stoichiometry of the oxide, i.e. the presence of intrinsic
defects; (iii) the presence of impurities, i.e. extrinsic defects; (iv)
the size and shape of the particles [33]; (v) the hydroxyl groups
concentration.
Fig. 3. (a) IR spectra of source material SnO powders annealing at different
temperatures 250, 450, 650 and 850 1C for 2 h in air and SnO powder. (b) IR
spectra of source material SnO powders annealing at different temperatures 250,
50, 650 and 850 1C for 2 h in air and SnO powder.
2
4
2
4. Conclusions
Table 1
IR band positions and assignments for SnO powder annealing in air at different
The oxidation of SnO to SnO
2
is strongly depends on the
temperatures (250, 450, 650 and 850 1C) for 2 h and SnO
2
powder.
preparation method, the initial oxygen content, and the annealing
temperature. The oxidation is started with internal disproportio-
nation and evolved through direct/indirect transformation. During
the indirect phase transformation, intermediate phases were
involved and the distorted o-SnO phase was also observed. On
the other hand, in the case of direct transformation, when SnO
–
1
n
(cm
)
Reference
Fundamental
a
Vibrations
*
*
5
40 [30]; 555.7 ;
[30,31],this work
v(Sn–O, T)
*
*
5
58.1 ; 559.5 ; 561
*
*
[
31]; 562.2 ; 564 ;
*
*
comprised a tin matrix similar to the SnO
transformed into SnO with preferred orientation along the axis
similar in the atomic distance of the tin matrix.
Optical properties of SnO were investigated in UV, VIS, and IR
ranges. It was found that the optical bandgap lies between 3.58
and 3.72 eV. The IR transmission of SnO , n-type semiconductor, is
2
plane it can be easily
5
65.3 ; 569.7 ;
*
*
5
74.9 ; 579.7 ;
2
*
5
5
5
6
80.1 ;
*
*
86.8 ,596.2 ,
*
*
x
99.6 ; 601.1 ;
*
*
04.1 ; 605.5
*
*
6
10 [29]; 617.6 ;
[29], this work
v(Sn–O)
x
*
*
6
6
6
19.2 ; 631.1 ;
sharply decreased by electron absorption and the quality of
vibrational information concerning form the surface species
depends strongly on the nature of the treatment.
*
*
32.2 ; 636.7 ;
*
47.1
6
50 [28];
[28]
v(Sn–O)
*
(665,667) [30];
[30,31], this work
v(Sn–O–Sn)
*
6
68.6 ; 680 [31];
*
*
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v (Sn–O)
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*
*
7
42.2 ; 757.1 ;
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