Unusual electronic state of tin dopant atoms in surface layers of NiTiO3

Article abstract of DOI:10.1134/S0036023611040048

The ¹¹⁹Sn M?ssbauer spectra of polycrystalline NiTiO ₃ samples impregnated with a solution containing 0.3 at % Sn ⁴⁺ are evidence that annealing in H₂ converts tin into the state with the electron density |Ψ(0)|² on ¹¹⁹Sn nuclei corresponding to Sn³⁺ ions. The stabilization of tin atoms in such an untypical formal oxidation state occurs at a depth of no more than 2-3 nm from the surface of titanate crystallites. It was revealed that the Sn³⁺ ions are not subjected to spin polarization even at temperatures considerably lower than the Néel temperature of NiTiO ₃, which can be explained by their location in the Ni²⁺ positions. The formation of Sn3+ prevents the further reduction of tin to the divalent state and, hence, precludes localization of ¹¹⁹Sn probe cations in positions at the interface.

Full text of DOI:10.1134/S0036023611040048

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2011, Vol. 56, No. 4, pp. 606–611. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © R.A. Astashkin, M.V. Korolenko, M.I. Afanasov, P.B. Fabrichnyi, A. Wattiaux, C. Labrugère, C. Delmas, 2011, published in Zhurnal Neorganicheskoi
Khimii, 2011, Vol. 56, No. 4, pp. 650–656.
PHYSICAL METHODS
OF INVESTIGATION
Unusual Electronic State of Tin Dopant Atoms
in Surface Layers of NiTiO
3
a
a
a
a
R. A. Astashkin , M. V. Korolenko , M. I. Afanasov , P. B. Fabrichnyi ,
b
b
b
A. Wattiaux , C. Labrugère , and C. Delmas
a
Moscow State University, Leninskie gory, Moscow, 119992 Russia
b
Institut de la Chimie de la Matière Condensée de Bordeaux, Université Bordeaux 1,
CNRSꢀUPR 4048, Avenue Dr. A. Schweitzer 33608 Pessac, Cedex, France
Received May 5, 2010
Abstract—The 119Sn Mössbauer spectra of polycrystalline NiTiO3 samples impregnated with a solution conꢀ
4+
taining 0.3 at % Sn are evidence that annealing in H2 converts tin into the state with the electron density
2
119
3+
|Ψ
(0)
|
on Sn nuclei corresponding to “Sn ” ions. The stabilization of tin atoms in such an untypical forꢀ
mal oxidation state occurs at a depth of no more than 2–3 nm from the surface of titanate crystallites. It was
3+
revealed that the Sn ions are not subjected to spin polarization even at temperatures considerably lower than
2+
the Néel temperature of NiTiO3, which can be explained by their location in the Ni positions. The formaꢀ
3+
tion of Sn prevents the further reduction of tin to the divalent state and, hence, precludes localization of
119
Sn probe cations in positions at the interface.
DOI: 10.1134/S0036023611040048
Recent 119Sn Mössbauer study [1] has shown that surface of MgO crystallites, for which an increase in
the synthesis of MgTiO3 through annealing coprecipiꢀ annealing temperature did not affect the kinetics of
tated titanium and magnesium hydroxides with Sn4
+
Sn oxidation.
ii) In addition to passivation of Sn2+, annealing in
Н2 at 900 led to the appearance of a new spectral
component with the isomer shift 1. 6 mm/s. The
2+
additions in H2 results in stabilization of tin ions in the
oxidation state +2 on the crystallite surface. Thus, this
double oxide with an ilmenite structure turned out to
be suitable for studying chemical reactions of Sn
probe atoms at the solid–gas interface, in addition to
simple oxides previously used for this purpose [2]. As
3dꢀmetal titanates, an
advantage of MgTiO3 is the extremely high stability of
Mg cations under both oxidative and reductive conꢀ
(
°С
δ
295 K
119
δ
value, intermediate between the typical values for
tetraꢀ and divalent tin oxides ( ~ 0 and ~ 3 mm/s,
respectively), can be assigned neither to ꢀSn (
.56 mm/s) nor to a tin compound capable of forming
as clusters on annealing MgTiO3 in Н2
δ
β
δ
=
compared with isostructural
2
.
2+
To explain these features, it was suggested in [1]
that Тi ions are partially reduced, which leads to the
appearance of delocalized electron density in their t2g
orbitals. As a result of interaction of electrons transꢀ
ditions, which makes it possible to vary in a wide range
4+
the type of reaction involving 1 Sn. The stability of
19
MgTiO3 is especially important in annealing in Н2
.
4+
This annealing is necessary to convert Sn cation into
the divalent state suitable for completing the solid–gas
interface. This specific feature is caused by the fact
that Sn has a sterically active lone electron pair,
which allows these cations to occupy positions with a
low coordination number energetically unfavorable
for major (structureꢀforming) cations [2, 3].
Study of 119Sn Mössbauer spectra recorded in situ
in an Н2 atmosphere revealed two interesting effects,
which were not observed for simple oxides:
ferred from hydrogen with adsorbed О2 molecules, the
2+
latter lose the ability to oxidize neighboring Sn catꢀ
ions at room temperature. The subsequent appearance
of the spectral component with an intermediate isoꢀ
mer shift value (and, hence, intermediate electron
2+
2
119
density |Ψ(0) on Sn nuclei) corresponding to tin
|
atoms in the oxidation state +3 was explained by an
increase in the number of electrons transferred to the
titanate with an increase in annealing temperature and
by partial localization of their charge density on Тi
ions in the vicinity of Sn ions.
Additional information on the Sn3+ dopant cenꢀ
4+
4+
i) The Sn2+ that appeared on the surface of MgTiO3
crystallites in the course of annealing in Н2 at 600
(
°
С
2+
immediately converted into the tetravalent state on ters, as well as on the local environment of Sn ions
contact with air. However, after further annealing in on the crystallite surface, could be provided by studyꢀ
2+
Н2 at 900
on longꢀterm exposure to air. Such passivation of Sn
ions was in sharp contrast with their behavior on the magnetic Мg2 cations. This is caused by the fact that
°
С, the Sn ions ceased to be oxidized even ing isostructural titanates containing magnetically
2+
active cations of one of the 3d metals rather than diaꢀ
+
606

UNUSUAL ELECTRONIC STATE
607
spin polarization of the electronic shell of tin leads to
the appearance of magnetic hyperfine splitting in the
[Sn]
Table 1. Ratios of atomic concentrations
in samꢀ
[
Ni
]
+
[ ]
Ti
119
Sn spectrum; parameters of this splitting are specifꢀ
ples I and II as determined by XPS
ically sensitive to the cationic environment of the
probe atom. In the present paper, we report the results
of studying the Mössbauer spectra of NiTiO3 doped
with 1 Sn. For this titanate, an antiferromagnet with
the Néel temperature ТN = 23 K [4], Mössbauer
[
Sn
]
[
Sample (annealing conditions)
19
[
Ni
]
+
Ti
]
parameters of 1 Sn in the titanium substitution posiꢀ
tions in the crystallite bulk are known [5]. This fact
facilitates the interpretation of the 1 Sn Mössbauer
spectra of samples with a more complicated distribuꢀ
tion of probe cations with respect to the interface. In
addition, nickel titanate is active in many catalytic
reactions, in particular, in the CO oxidation by oxygen
19
4+
I (N , 450
°
C, 2 h + H , 350
°
C, 2 h)
C, 2 h)
0.18
0.07
2
2
19
II (N , 900
°
C, 2 h + H , 350
°
2
2
Note: Xꢀray photoelectron spectra revealed that the particle surface in
both samples was contaminated with products of hydrolysis of
sodium silicate. Their formation is caused by the interaction of a
concentrated alkali solution with the walls of glassware. These
contaminations are not detected by Xꢀray powder diffraction and
they cannot be removed by additional washing of the samples.
Therefore, in the synthesis of samples intended for catalytic tests,
hydroxide coprecipitation should be carried out by less alkaline
solutions stored in Teflon bottles.
[
6]. This allows using this reaction in future for eluciꢀ
dating the modifying action of different tin forms
identified by Mössbauer spectroscopy.
EXPERIMENTAL
Nickel titanate samples free of tin were obtained by
2+
4+
coprecipitation of equimolar amounts of Ni and Ti
ions from aqueous hydrochloric acid solutions. A hot
M NaOH solution was used for precipitation, which
The Mössbauer spectra were recorded on a conꢀ
stantꢀacceleration spectrometer. The source was
2
119m
Сa SnO at 295 K. Isomer shifts were referenced to
3
ensured quantitative deposition of both cations [5].
The resulting hydroxide precursor was dried in air and
119
the Сa SnO3 absorber at 295 K. The spectra were
processed using routine software.
annealed in a nitrogen flow at 1000 С for 2 h. Xꢀray
°
Xꢀray photoelectron spectra were recorded on an
ESCALAB VG 220iꢀXL spectrometer. Samples were
pressed into indium foil. Atomic concentrations were
calculated from the integrated intensities of the
powder diffraction analysis of the samples thus syntheꢀ
sized showed that they were singleꢀphase and repreꢀ
sented rhombohedral NiTiO3 (space group R3 [7]).
Synthesis of samples of NiTiO doped with Sn4+
3
ions (0.3 at %, 92% enrichment in 1 Sn) located in the
bulk of the crystallites (reference) has also involved the
preparation of the hydroxide precursor. However, it
turned out impossible to simultaneously introduce
19
Sn3d5/2
Ni2p3/2
(
Eb = 486.6 eV), Ti2p3/2
(Еb = 458.8 eV), and
(Еb = 854.6 eV) peaks with inclusion of effecꢀ
tive photoionization crossꢀsections and electron
absorption in a matrix [8]. The excitation source was
4+
nonmonochromated Mg
K
line (
hν = 1253.6 eV).
Sn cations into the precursor because of the formaꢀ
α
Highꢀresolution spectra were processed with the
AVANTAGE program package supplied by the Therꢀ
moFischer Scientific Company.
tion of soluble sodium stannate at high pH values.
Therefore, an acidified 119SnCl4 solution was poured
dropwise onto the preliminarily coprecipitated nickel
and titanium hydroxides washed with distilled water
and dried in air at 100 С. Before impregnation, the
°
RESULTS AND DISCUSSION
precursor was moistened with a 10% ammonia soluꢀ
tion for neutralization of the impregnation solution to
Our first experiments showed that the procedure
analogous to that used for localization of tin on the
surface of MgTiO3 crystallites turned out to be ineffiꢀ
cient in the case of NiTiO3. In particular, we demonꢀ
form SnО2 nН О. The resulting solid was heat treated
⋅
2
under the same conditions as those used in synthesis of
NiTiO3 free of tin additions.
4+
The NiTiO3 samples with tin additives in the surface
layers were obtained by the following procedure. A
required amount of tin(IV) chloride, corresponding as in
the above case to the overall tin concentration of 0.3 at %,
was poured dropwise onto a powder of crystalline
NiTiO3 preliminarily moistened with an ammonia
solution, The resulting solid was dried in air, and
strated that annealing a sample containing Sn ions in
the bulk of NiTiO3 crystallites in Н2 for 2 h at 350 did
°С
119
not lead to a change in Sn Mössbauer spectra. The
spectrum at 4.2 K was as previously dominated by the
system of magnetic hyperfine splitting lines due to
119
4+
4+
Sn ions in the Ti substitution position [5]. This is
4+
evidence that the diffusion of Sn ions in the titanate
lattice at 350 is too slow for a noticeable enrichment
of the particle surface with probe cations. According to
Xꢀray powder diffraction analysis showed that the Xꢀray diffraction data, increasing temperature of
presence of tin did not affect the Xꢀray powder diffracꢀ annealing in Н2 to 600 resulted in partial decompoꢀ
tion patterns of the samples. sition of NiTiO3 yielding nickel metal and titanium
annealed first in an N2 flow for 2 h at 450 or 900
°С and
°C
then in an Н2 flow for 2 h at 350
°
С.
°С
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 4 2011

608
ASTASHKIN et al.
Table 2. 119Sn Mössbauer parameters of two NiTiO samples obtained under different conditions of introduction of tin dopant
3
atoms (0.3 at %)
Δ
or 2
ε
,
Γ
,
Sample (annealing conditions)
Trec, K
δ
,
±
0.05 mm/s
H
,
kOe
A, ±9%
±
0.07 mm/s
±
0.1 mm/s
I (N , 450
°
C, 2 h + H , 350
°
C, 2 h)
4.2
(1) 0.13
2) 1.7
(1) 0.03
2) 1.63
(1) 0.25
0.4*
–
–
1.2*
1.5
20
80
31
69
2
2
(
–
–
–
295
0.4
–
1.2*
1.5
(
II (N , 900
°
C, 2 h + H , 350
°
C, 2 h)
4.2
+0.23
2.03
0.46
0.53
1.8
50
±
–
–
–
–
2
1.0
1.0
1.0
1.0
1.1
32
30
38
83
17
2
2
(
2) 3.02
3) 0.11
(
295
(1) 0.06
2) 2.84
(
*
Values fixed upon fitting.
dioxide. It is noteworthy that the Mössbauer spectrum ple II, which is consistent with the lower temperature
of this sample at 295 K showed as previously only the of the preliminary annealing of I in an N2 atmosphere.
peak with the isomer shift = 0.08 mm/s, which was However, XPS did not reveal differences in the elecꢀ
δ
evidence of the persistence of tin atoms in the oxidaꢀ tronic state of tin in two samples under consideration.
tion state +4.
The 119Sn Mössbauer spectra in Figs. 1 and 2 show
119
For this reason, we tried another method to stabiꢀ that the hyperfine coupling parameters of Sn in I
2+
lize Sn ions in surface positions, which was based on and II are not equivalent (Table 2).
impregnation of polycrystalline NiTiO3 with a tin(IV)
In the spectrum of I at 4.2 K (Fig. 1a), the strong
chloride solution. The aim was to obtain samples conꢀ
4+
peak with the isomer shift
δ
= 1.7 mm/s close to that
1
taining Sn ions at a rather small distance from the
observed for 1 Sn in MgTiO3 is worth noting. However,
19
surface (owing to the use of relatively mild annealing
conditions in N2) so that these ions could return to it in the case of NiTiO
, the appearance of this peak
3
at moderate annealing temperature in Н2 (at which the could be a priori caused by the reduction in Н2 of some
titanate is still not decomposed). Two samples were surface nickel and tin cations to the metals yielding the
prepared at different temperatures of annealing in N2 alloy (which cannot be detected by Xꢀray powder difꢀ
fraction). Moreover, 1 Sn atoms in nickel have a close
19
(
Tanneal = 450 and 900 С for sample I and II, respecꢀ
°
4+
tively, = 2 h). Thus, Sn ions in II could migrate to a
larger depth from the surface than in I. Further
annealing of I and II in Н2 were carried out under the
same conditions (Tanneal = 350
τ
isomer shift value (at 295 K,
corresponds, with taking into account the secondꢀ
order Doppler shift, to ~ 1.6 mm/s at 4.2 K). Thereꢀ
fore, to unambiguously interpret the spectrum of I, it
is important that, below the Curie point ТС = 631 K of
the nickel metal, the tin atoms should be subjected to
spin polarization (according to [10], for the Ni–0.5%
δ
= 1.5 mm/s [9], which
δ
°
С; = 2 h).
τ
XPS analysis (Table 1) indicates that in both samꢀ
ples, the atomic concentration of tin in surface layers
about 2–3 nm thick considerably exceeds the average
value corresponding to the amounts of tin, nickel, and
Sn alloy at
Т = 78 K, Н = 19 kOe), which is not the
case. In addition, the very low temperature of recordꢀ
ing the spectrum shown in Fig. 1a does not allow one
to explain the lack of the magnetic splitting structure
⎛
[
Sn
]
⎞
titanium
= 0.003 , used in the synthesis.
⎜
⎝
⎟
⎠
[
Ni
]
+
[
Ti
]
It is worth noting that the enrichment of the surface by superparamagnetic relaxation of Ni moments. All
layer in I with tin turned out to be higher than in samꢀ these facts mean that the appearance of the compoꢀ
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 4 2011

UNUSUAL ELECTRONIC STATE
а)
609
(
100
98
96
94
–10
–5
0
5
10
(b)
100
99
98
97
–10
–5
0
5
10
Velocity, mm/s
Fig. 1. 119Sn Mössbauer spectra of sample I (annealing conditions: 2 h at 450
95 K.
°С
in N2 + 2 h at 350
°С
in Н2). Тrec is (a) 4.2 and (b)
2
nent with
δ
= 1.7 mm/s in the spectrum of sample I is considerably larger width at half maximum of the comꢀ
1
1
ponents of the “averaged doublet” ( 2
compared with the values for the spectrum of a thin
reference sample of СаSnO3 Гref = 0.88 mm/s).
Г = 1.2 mm/s) as
not related to the formation of the alloy; rather, this is
evidence of the stabilization of tin atoms in the untypꢀ
Г
(
3+
ical Sn state, as was observed in MgTiO3
.
With allowance for the high tin content in the nearꢀ
surface layers of crystallites (Table 1), the distribution
of electric field gradients corresponding to the distriꢀ
The spectrum in Fig. 1a also shows the resonance
absorption at = 0.13 mm/s caused by tin atoms
δ
2
retaining the oxidation state +4. Analysis of this specꢀ bution of
Δ
values can be caused by fluctuations of
2
tral component shows that it can be assigned to Sn4
+
4+
interatomic distances for some Sn ions with a differꢀ
ions involved in quadrupole interactions of different ent number of dopant cations in their local environꢀ
strength corresponding to the mean quadrupole splitꢀ ment.
ting
Δ
= 0.4 mm/s. This is also supported by the
2
Measurements at 295 K showed (Fig. 1b) that the
2+
disappearance of magnetic order of Ni moments
upon the transition of NiTiO3 into the paramagnetic
1
This conclusion was confirmed in studying a NiO sample conꢀ
4+
taining 0.3 at % Sn in the bulk of crystallites [11] after annealꢀ
3+
state was not accompanied by narrowing of the Sn
ing in H . Indeed, according to Xꢀray powder diffraction eviꢀ
2
peak. This means that its broadening at 4.2 K as comꢀ
dence, this treatment at 600°C led to almost complete reduction
of nickel oxide to the metal. Nevertheless, the 1 Sn Mössbauer pared with the spectrum of the reference absorber was
19
spectra pointed to the persistence of tin atoms in the oxidation
of nonmagnetic origin and, hence, was caused by weak
state +4 ( ~ 0 mm/s), most likely, as NiSnO clusters [12].
δ
3
quadrupole interactions. Noteworthy is also a rather
Thus, this experiment demonstrated that the tin–nickel alloy
did not form even under more favorable conditions than the
annealing conditions for NiTiO3.
3+
weak decrease in the spectral contribution of Sn with
an increase in temperature, which is comparable with
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 4 2011

6
10
ASTASHKIN et al.
а)
(
100
9
8
6
9
–10
–5
0
5
10
(
b)
100
99
98
97
–10
–5
0
5
10
Velocity, mm/s
Fig. 2. 119Sn Mössbauer spectra of sample II (annealing conditions: 2 h at 900
b) 295 K.
°С in N2 + 2 h at 350°С in Н2). Тrec is (a) 4.2 and
(
that observed for Sn4+ ions in Тi4+ substitution posiꢀ
To determine the conditions of formation of Sn
3+
119
tions [5]. This result, indicative of the similarity of the dopant centers, it is expedient to consider the Sn
3+
4+
coordination polyhedra of Sn and Sn , is another Mössbauer parameters for sample II annealed in
N at
2
3+
a higher temperature (Table 2). The main specific feaꢀ
ture of its spectrum at 4.2 K (Fig. 2a) is that it lacks the
component corresponding to Sn ions. Instead of this
component, the spectrum shows the sextet of magꢀ
netic hyperfine splitting corresponding to Sn ions in
Тi substitution positions in the bulk of crystallites
= 52.5 kOe) [5], which was absent in the spectrum of
I. In addition, the spectrum shows a quadrupole doublet
with hyperfine splitting parameters ( = 3.02 mm/s,
= 2.03 mm/s) typical of Sn ions located on posiꢀ
tions on the interface [1, 13]. Finally, the spectrum
also shows a peak due to Sn ions with nonpolarized
argument in favor of the existence of Sn ions in the
NiTiO3 lattice, in spite of the value untypical of the
δ
1
3+
4+
oxide matrix. Taking into account that the Тi posiꢀ
tions have a magnetically active cationic environment
4+
[
5], the lack of magnetic hyperfine splitting in the
4
+
3+
spectrum at 4.2 K allows us to assume that the Sn
ions are located in Ni positions. In this case, the
completely occupied t2g orbitals of Ni are not able to
induce spin polarization of the electronic shell of a
neighboring coplanar tin cation, whereas 90ꢀdegree
superexchange interactions involving the halfꢀfilled eg
(
Н
2+
2+
δ
2+
Δ
4+
orbitals are negligible. The idea that sample I contains spins (
= 0.12 mm/s, Н = 0 kOe).
δ
3
+
4+
Sn ions (and corresponding Sn parent cations) in
2+
An increase in the recording temperature to 295 K
led to the following changes (Fig. 2b):
the Ni layer allows us to elucidate the reason for the
increased electron density on Ni cations in the vicinꢀ
2
+
ity of tin cations. Indeed, this effect can be explained
(i) the disappearance of the magnetically split comꢀ
by partial local compensation of the excess positive ponent caused by the NiTiO3 transition into the paraꢀ
4+
charge of Sn
.
magnetic state, which is quite expected;
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 4 2011

UNUSUAL ELECTRONIC STATE
611
(
ii) the sharper decrease in the spectral contribuꢀ the Ti4+ substitution positions. Comparison of two
2+
4+
tion for Sn than for Sn , which is consistent with the samples obtained under different conditions for
sharper temperature dependence of the probability of migration of Sn ions into the nearꢀsurface layers
recoilless transitions
Thus, comparison of information provided by the
Sn Mössbauer spectra of samples I and II allows us
to draw the following conclusions.
4+
3
+
f(T)
for divalent tin.
showed that the formation of Sn prevents the transiꢀ
tion of tin into the divalent state and, hence, precludes
119
119
the location of Sn probe cations in positions at the
interface.
(
i) The fact that the 119Sn3+ component is absent in
the spectrum of II, although the layers of I and II
examined by XPS contained comparable tin amounts,
means that the formation of Sn occurs at a very small
distance from the interface. This result also points to a
sharp difference in the tin concentration profiles in the
nearꢀsurface layers of samples I and II.
ACKNOWLEDGMENTS
3+
This work was supported by the Russian Foundaꢀ
tion for Basic Research, project no. 10ꢀ03ꢀ00002.
REFERENCES
3
+
(
ii) The formation of Sn prevents the further
1
2
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(
iii) The Sn2+ ions formed in sample II are not subꢀ
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+
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19
3
face 1 Sn cations at
19
2+
Т
Ӷ
TN was also represented by
nonmagnetic” (quadrupole) doublet [13].
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“
5
(
2+
showed that Sn ions almost does not interact with
oxygen at room temperature, exactly as it was observed
for MgTiO3 [1] and MnTiO3 [13]. This allows us to sugꢀ
gest that annealing these compounds in hydrogen
leads to passivation of О2 molecules adsorbed on them
by a common mechanism.
Analysis of the Mössbauer spectra of 119Sn in
NiTiO3 revealed the stabilization of tin dopant atoms
in the oxidation state +3. XPS showed that Sn ions
are formed at a distance of no more than 2–3 nm from
the surface of particles. The spectra of 1 Sn in antiferꢀ
8
8, 1396 (1976).
1
1
9,
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3+
1
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19
1
3. M. V. Korolenko, M. Danot, P. B. Fabrichnyi, et al.,
Zh. Neorg. Khim. 50 (9), 1423 (2005) [Russ. J. Inorg.
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romagnetic NiTiO revealed the absence of spin polarꢀ
3
3+
ization of Sn ions, which excludes their location in
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 56 No. 4 2011