Oxidation behavior of amorphous metallic Ni3(SbTe 3)2 compound

Article abstract of DOI:10.1016/j.materresbull.2005.09.003

Amorphous Ni₃(SbTe₃)₂ compound was prepared from a metathesis between Zintl phase K₃SbTe₃ and NiBr₂ in solution and its oxidation behavior was investigated in the temperature range of 200-700°C in air. To characterize the sample, thermogravimetry (TG), X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive analysis by X-ray (EDAX) analyses were performed and electrical conductivity was measured as a function of temperature in the range of 25-800°C in air. The specimen showed a metallic conducting-like behavior below 585°C while a semiconducting-like behavior above 585°C. At a first oxidation step of Ni₃(SbTe ₃)₂ below 500°C, TeO₂ phase is formed. Above 500°C, NiO phase is formed, then some NiO reacts with TeO₂ to form NiTeO₃ and NiSb₂O₆ is simultaneously formed. Above 700°C, NiTeO₃ is further reacted with TeO ₂ to form NiTe₂O₅. Both NiTeO₃ and NiTe₂O₅ are decomposed above 774°C.

Full text of DOI:10.1016/j.materresbull.2005.09.003

Materials Research Bulletin 41 (2006) 628–634
www.elsevier.com/locate/matresbu
Oxidation behavior of amorphous metallic
Ni (SbTe ) compound
3
3 2
a
Jong-Ho Jun , Jin-Seung Jung , Seung-Lim Oh , Yong-Rok Kim ,
b,*
c
c
c
Sung-Han Lee , Charles J. O’Connor
d
a
Department of Applied Chemistry, Konkuk University, Choongju 380-701, South Korea
b
Department of Chemistry, Kangnung National University, Kangnung 210-702, South Korea
c
Department of Chemistry, Yonsei University, Seoul 120-749, South Korea
d
Advanced Materials Research Institute, University of New Orleans, New Orleans, LA 70148, USA
Received 15 June 2005; received in revised form 25 August 2005; accepted 2 September 2005
Available online 27 September 2005
Abstract
Amorphous Ni (SbTe ) compound was prepared from a metathesis between Zintl phase K SbTe and NiBr in solution and its
3
3 2
3
3
2
oxidation behavior was investigated in the temperature range of 200–700 8C in air. To characterize the sample, thermogravimetry
TG), X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive analysis by X-ray (EDAX)
(
analyses were performed and electrical conductivity was measured as a function of temperature in the range of 25–800 8C in air. The
specimen showed a metallic conducting-like behavior below 585 8C while a semiconducting-like behavior above 585 8C. At a first
oxidation step of Ni
3
(SbTe
3
)
2
below 500 8C, TeO phase is formed. Above 500 8C, NiO phase is formed, then some NiO reacts with
2
TeO to form NiTeO and NiSb O is simultaneously formed. Above 700 8C, NiTeO is further reacted with TeO to form NiTe O .
2
3
2
6
3
2
2 5
Both NiTeO
#
3
2 5
and NiTe O are decomposed above 774 8C.
2005 Elsevier Ltd. All rights reserved.
Keywords: A. Alloys; A. Amorphous materials; C. Thermogravimetric analysis (TGA); C. X-ray diffraction; D. Catalytic properties
1
. Introduction
Metal chalcogenides are known to be suitable materials for optical data storage. We have reported the synthesis and
characterization of a series of M (SbTe ) during the past 20 years. These ternary metal chalcogen compounds have
3
3 2
been found to show a photomagnetic property at low temperatures [1–6]. M–Sb–Te compounds may possess potential
applications as heterogeneous catalysts because it has been known that metal–metalloid alloys are active catalysts for
the hydrogenation of carbon oxides [7–10]. In the previous paper [11], we reported amorphous Ni (SbTe ) compound
3
3 2
to show catalytic activity for the partial oxidation of methane when it was treated in gaseous oxygen prior to the
reaction. Its catalytic activity is considered to be due to active oxide phases formed by the O -treatment, and thus the
2
investigation of its oxidation at high temperatures may be helpful to understand its active sites for the methane
activation. In this work, the oxidation behavior of amorphous Ni (SbTe ) was studied in the temperature range of
3
3 2
2
00–700 8C. To characterize the samples treated at various temperatures in gaseous oxygen, thermogravimetry (TG),
*
Corresponding author. Tel.: +82 33 640 2305; fax: +82 33 647 1183.
E-mail address: jjscm@kangnung.ac.kr (J.-S. Jung).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.09.003

J.-H. Jun et al. / Materials Research Bulletin 41 (2006) 628–634
629
X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive analysis by X-ray
EDAX) analyses were performed and the electrical conductivity was measured in the temperature range of 25–800 8C
in air.
(
2
. Experimental
Ni (SbTe ) was prepared by the reaction of aqueous solutions of K SbTe and NiBr . The ternary Zintl phase
2
3
3 2
3
3
K SbTe prepared from a direct combination of the elements is soluble in polar solvents, such as ethyl alcohol and
3
3
+
3ꢀ
water by the reaction: K SbTe (s) ! 3 K + SbTe , and the Zintl solution allows subsequent metathesis reaction
3
3
3
3
ꢀ
anion to the nickel cation
with nickel salt to form amorphous material due to the transfer of electrons from the SbTe
3
2
+
3ꢀ
according to the reaction: 3Ni + 2SbTe3 ! Ni (SbTe ) . A stoichiometric quantity of K SbTe aqueous
3
3 2(s)
3
3
solution was added slowly while stirring the NiBr solution. A fine black precipitate was formed immediately, which
2
was then separated by solution filtration, washed with deionized water and acetone, and dried under vacuum. All
manipulations were carried out in an argon-filled glove-box because of the air sensitivity of the compounds. The
experimental empirical formula of the resultant product determined by atomic absorption spectroscopy (AAS) was
Ni1.61Sb1.00Te3.59
.
XRD analysis was performed for the samples treated at various temperatures in a flow of dry air for 1 h. TG analysis
was carried out for the samples at a heating rate of 20 8C/min in the temperature range of 25–900 8C in flows of both air
and argon gas. SEM analysis was performed for the samples to investigate the morphological change with the heating
temperature. EDAX analysis was performed for the samples to investigate the chemical composition. Electrical
conductivity was measured as a function of temperature in the range of 25–800 8C in air by means of the four-probe
method. To measure the electrical conductivity, the pellet with a diameter of 12 mm and a thickness of about 3 mm was
connected to the Pt probes using nickel paste and then inserted into the quartz sample container which was placed in an
electrical furnace to control the temperature.
3
. Results and discussion
Fig. 1 presents the TG curves of Ni (SbTe ) measured at a heating rate of 20 8C/min in flows of both Ar and air
3
3 2
3
(
20 cm /min). In Fig. 1, small weight loss is observed in the temperature range of 25–488 8C in a flow of Ar and of 25–
85 8C in a flow of air, which is due to the vaporization of residual solvent molecules. In a flow of air, a weight gain is
observed in the temperature range of 285–508 8C and then a small weight loss is observed in the temperature range of
08–573 8C. Beyond 573 8C, a weight gain occurs again up to 774 8C and a large weight loss begins at 774 8C. The
2
5
uptake in weight is larger in dry air than in argon gas, which means that a lot of gaseous oxygen is chemisorbed on the
surface, leading to an oxidation of sample. To provide more information on the oxidation of Ni (SbTe ) , TG analysis
3
3 2
of Sb and Te elements was performed. Fig. 2 presents the TG curves of Sb and Te elements measured in flows of both
Ar and air in the temperature range of 25–800 8C. In a flow of Ar, a weight loss of Te begins at 450 8C, which
corresponds to the melting point of Te at 1 atm. The vapor pressure of Te element is known to be 1 and 10 Torr at 520
and 633 8C, respectively. When Te is vaporized above 450 8C, the weight will be decreased as shown in Fig. 2.
Accordingly, a weight loss above 488 8C in Fig. 1(A) is believed to be due to the vaporization of Te. In Fig. 1(B), the
weight of Ni (SbTe ) increases in the temperature range of 285–774 8C in a flow of air. Fig. 3 presents the TG curves
3
3 2
of the samples treated at 200, 300, 400, 500 and 600 8C for 1 h in a flow of air, in which the samples treated at 200 and
00 8C show a small weight gain at 285 8C, indicating the samples to be oxidized. The TG data of the samples treated
3
below 500 8C show a weight loss at a temperature below 560 8C, but the TG data of the sample treated at 600 8C show
a weight loss at about 780 8C, implying that rather stable metal oxides are produced on the sample when Ni (SbTe ) is
3
3 2
treated in air above 600 8C. The cause of the weight decrease above 780 8C is mainly the evaporation of tellurium.
Fig. 4 presents the X-ray diffraction patterns of the samples. The samples treated at 200 and 400 8C in air show the
presence of TeO as a XRD detectable phase. The sample treated at 500 8C in air shows no crystalline peak as shown in
2
Fig. 4(C), indicating the sample to be amorphous phase. Because TeO is easily combined with other metal oxides to
form glasses [12], the sample treated at 500 8C in air is considered to be metal tellurium oxide glasses. The sample
treated at 600 8C in air shows the presence of NiO, NiTeO , NiSb O and Ni Te phases as presented in Fig. 4(D).
2
3
2
6
3
2
Fig. 5(A) presents the SEM photograph of amorphous Ni (SbTe ) . Fig. 5(B) presents the Ni (SbTe ) treated at
3
3 2
3
3 2
4
00 8C in air, showing a rod-shaped crystallite. From the EDAX result, the rod-shaped crystallite was found to be TeO2

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J.-H. Jun et al. / Materials Research Bulletin 41 (2006) 628–634
3 3 2
Fig. 1. TG curves of amorphous Ni (SbTe ) in flows of (A) Ar and (B) air.
phase, which was identified by XRD as shown in Fig. 4(B). Fig. 5(D) presents the SEM photograph of the sample
treated at 600 8C, showing a microcrystallite, which was found to be NiTeO by EDAX analysis.
3
The electrical conductivity of amorphous Ni (SbTe ) was measured with increasing the temperature at a heating
3
3 2
rate of 5 8C/min in the range of 25–800 8C in air. Fig. 6 shows the plot of log conductivity versus temperature (8C), in
Fig. 2. TG curves of Sb and Te in flows of Ar and air.

J.-H. Jun et al. / Materials Research Bulletin 41 (2006) 628–634
631
3 3 2
Fig. 3. TG curves of Ni (SbTe ) treated at various temperatures in air for 1 h.
3 3 2
Fig. 4. X-ray diffraction patterns of Ni (SbTe ) samples treated at (A) 200 8C, (B) 400 8C, (C) 500 8C and (D) 600 8C in air for 1 h.

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J.-H. Jun et al. / Materials Research Bulletin 41 (2006) 628–634
3 3 2 3 3 2
Fig. 5. SEM photographs of (A) Ni (SbTe ) and Ni (SbTe ) samples treated at (B) 400 8C, (C) 500 8C and (D) 600 8C in air for 1 h.
which the electrical conductivity decreases with increasing temperature up to 585 8C, while above 585 8C the
electrical conductivity increases with increasing temperature. The result indicates the electrical conductivity of the
specimen to be changed from a metallic conducting-like behavior to a semiconducting-like behavior at 585 8C.
According to the TG data in Fig. 1(B), a small weight gain is observed in the temperature range of 285–400 8C,
indicating the oxidation of the specimen to begin at 285 8C. The weight is steeply increased in the temperature range of
4
00–508 8C. Considered the electrical conductivity data in conjunction with the TG result, in the temperature range of
00–585 8C the specimen seems to be decomposed and simultaneously oxidized to form a mixture, which is
amorphous as shown in the XRD result of Fig. 4(C).
4
In general, metal tellurides are known to be readily oxygenated in the presence of gaseous oxygen and the present
result is similar to those for other metal tellurides [10,13,14]. According to the study on the oxidation of nickel
tellurium compounds [15], NiO is produced in the first stage of oxidation, various nickel tellurium oxides are slowly
formed above 450 8C, and the metal oxides decompose above 900 8C, leaving NiO as the final solid product. However,
the present samples treated at 200, 300 and 400 8C revealed the existence of TeO phase and NiO phase was not
2
observed as shown in Fig. 4. Namely, TeO phase is formed in the first stage of oxidation of Ni (SbTe ) according to
the reaction: Ni Sb Te + xO ! xTeO + Ni Sb Te . The sample treated at 600 8C did not show the presence of
2
3
3 2
3
2
6
2
2
3
2
6ꢀx
TeO phase, but showed the presence of NiO phase as in Fig. 4(C).
2

J.-H. Jun et al. / Materials Research Bulletin 41 (2006) 628–634
633
3 3 2
Fig. 6. Electrical conductivity of Ni (SbTe ) as a function of temperature in the temperature range of 25–800 8C at atmospheric pressure.
The TG curve in Fig. 1(B) indicates that the sample is largely oxidized in the temperature range of 573–774 8C. The
XRD result of the sample treated at 600 8C exhibits the presence of NiO, NiTeO and NiSb O phases as shown in
3
2 6
Fig. 4(D). The NiTeO can be formed by the reaction of NiO and TeO : NiO + TeO ! NiTeO . A small weight loss
3
2
2
3
(
about 0.2%) in the temperature range of 508–573 8C seems to be due to the decomposition of some TeO uncombined
2
with nickel oxide. The presence of NiSb O suggests that some NiO reacts with Sb O to form NiSb O above 600 8C:
2
6
2
5
2 6
NiO + Sb O ! NiSb O . On the other hand, the XRD pattern of the sample treated at 700 8C showed the existence of
2
5
2 6
NiTe O as well as NiO, NiTeO and NiSb O phases, suggesting that NiTeO can be further reacted with TeO to
2
2
5
3
2
6
3
form NiTe O : NiTeO + TeO ! NiTe O . According to the TG data in Fig. 1(B), a large weight loss begins at
2
5
3
2
2 5
7
74 8C, which is believed to be due to the decomposition of NiTeO and NiTe O . Namely, the nickel tellurium oxides
3
2 5
are considered to be decomposed into TeO and NiO above 774 8C. The TeO can be subsequently decomposed,
2
2
resulted in a rapid weight loss as shown in Fig. 1(B).
As shown in Fig. 6, a semiconducting-like behavior above 585 8C may originate from metal oxide networks, such
as NiO, NiTeO , NiSb O and NiTe O . The activation energy calculated from the slope of ln conductivity versus
6
3
2
2 5
1
000/T plot in the temperature range of 700–800 8C was about 0.9 eV and this value is in the range of 0.78–1.07 eV
measured by other investigators for NiO [16], which enables us to consider that the semiconducting behavior above
85 8C arises from NiO phase. The XRD signal intensity of NiO for the sample treated at 700 8C was stronger than that
5
for the sample treated at 600 8C, while the XRD signal intensity of NiTeO for the sample treated at 700 8C was less
3
than that for the sample treated at 600 8C. The result suggests that some NiTeO is decomposed at higher temperatures,
3
leaving NiO on the surface. When NiO particles are either making contact among themselves or are separated by very
small gaps, the charge carriers can migrate from one aggregate to a neighboring one through hopping.
Consequently, in the oxidation of Ni (SbTe ) , TeO phase is formed as a first oxidation step below 500 8C, NiO is
3
3 2
2
formed above 500 8C, then some NiO reacts with TeO to form NiTeO , and NiSb O is simultaneously formed with
2
3
2 6
the formation of NiTeO . Above 700 8C, NiTeO further reacts with TeO to form NiTe O . Both NiTeO and NiTe O
3
3
2
2
5
3
2 5
are decomposed above 774 8C, leaving NiO on the sample as the final product.
Acknowledgement
This paper was supported by Konkuk University in 2003.
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