Article
pubs.acs.org/IC
Sol−Gel Synthesis of High-Purity Actinide Oxide ThO2 and Its Solid
Solutions with Technologically Important Tin and Zinc Ions
Vikash Kumar Tripathi and Rajamani Nagarajan*
Materials Chemistry Group, Department of Chemistry, University of Delhi, Delhi 110007, India
S Supporting Information
*
ABSTRACT: The applicability of epoxide-based sol−gel
synthesis for actinide oxide (thoria) starting from air-stable
salt, Th(NO3)4, has been examined. The homogeneous gel
formed from Th(NO3)4 when calcined at 400 °C yielded
nanostructured thoria, and with increasing tempeartures (600,
700, and 800 °C), the average crystallite size increased.
Successful Rietveld refinement of the powder X-ray diffraction
pattern of ThO2 in Fm3̅m space group was carried out with a =
5.6030(35) Å. The fingerprint vibrational mode of the fluorite
structure of ThO2 was noticed as a sharp band in the Raman
spectrum at 457 cm−1. In the SEM image, a near spherical
morphology of thoria was noticed. Samples showed blue
emission on exciting with λ = 380 nm in the photoluminescence spectrum indicative of the presence of defects. Following this approach, 50 mol % of Sn4+ could be substituted
for Th4+, retaining the fluorite structure as evidenced by the PXRD, Raman spectroscopy, electron microscopy, EDAX, and XPS
measurements. Randomization of the lattice was observed for the tin-substituted samples. A significant blue shift in the
absorption threshold along with a persistent blue emission in the photoluminesence spectra were evident for the tin-substituted
samples. The concentration of Zn2+ ion in thoria was limited to 15 mol % as revealed by PXRD and XPS measurements. The
Raman peak shifted to higher values for Zn2+-substituted samples. A change in the optical absorbance characteristics was
observed for the zinc-substituted thoria. A 50 mol % Sn4+-substituted thoria degraded aqueous Rhodamine 6G dye solutions in
the presence of UV−vis radiation following pseudo-first-order kinetics.
1. INTRODUCTION
Among many synthetic methods to generate solids of
technological importance, the sol−gel method has been
successfully demonstrated to produce high-purity products,
preferably at lower temperatures.7 If such a method can employ
an inorganic and air-stable salt, one can execute the synthesis
without much difficulty and with precision. Subsequently, pure
oxides can be obtained. The demonstration of ThO2:Y3+ as
oxygen sensor for the development of the Gen-IV nuclear
reactors illustrates the requirement of these materials in film
form.8 Gels can effectively be used to generate metal oxide
coatings by dip-coating method. The epoxide-mediated sol−gel
method has been successfully applied as a viable alternate either
to obtain metal oxides for which the metal alkoxide precursors
do not exist or to overcome the handling issues arising from
their air-sensitive and moisture-sensitive nature.9 Though many
questions such as the exact role of counteranion-promoting
gelation and metal-ion oxidation state persist and have not yet
been completely answered, the literature is devoid of the
applicability of epoxide gelation for the actinide ion, Th4+. Such
an exercise is important as nitrate salts of the chemically related
lanthanides (f-block elements) readily yield gels where as no
Research activity on the actinide oxides is being expanded as
they can offer possible solutions in the form of nuclear energy
to the ever-increasing energy demands facing human kind.1
Among the actinide oxides, thoria finds a unique and significant
place not only in nuclear industry but also in other applications
such as catalysts, electrodes, fuel cell electrolytes, and sensors.2
Owing to its low phonon energy, its role as a matrix to hold a
variety of spectroscopically rich rare-earth ions has been studied
in detail.3 They have been investigated as potential alternatives
to the existing luminescent materials in the field of plasmonics.3
The stability of thoria over a wide range of temperatures
without undergoing any phase transition has invited its use by
ceramic scientists.4 Thoria or thorium oxide (ThO2) crystallizes
in fluorite structure (space group Fm3̅m). Taking advantage of
the larger ionic radius of Th4+, substitution of rare-earth ions
has been carried out with ease as they prefer higher
coordination numbers with oxygen. 3 Thoria has been
synthesized by various wet-chemical methods including
chemical precipitation, combustion, solvothermal methods
using glycerol, and a complex sol−gel process.5 Starting from
acetylacetonate salts, nanocrystals of actinide oxides have also
been fabricated.6
© 2016 American Chemical Society
Received: August 30, 2016
Published: December 8, 2016
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DOI: 10.1021/acs.inorgchem.6b02086
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2. EXPERIMENTAL SECTION
gelling process takes place from their corresponding chloride
salts.9e Also, for some of the rare earths, oxychloride formation
was noticed on the calcination of gels.9e To address this gap,
the application of epoxide-mediated sol−gel synthesis of thoria
(ThO2) starting from Th(NO3)4 has been scrutinized.
Following this, the extent and effect of substitution of Th4+
with Sn4+ and Zn2+ ions in ThO2 has been examined as oxides
of these metals (SnO2 and ZnO) formed the core subject of
many scientific reports due to their wide application
capabilities.10
While SnO2 crystallizes in thermodynamically stable rutile
structure, high pressures have been found to be a necessary
condition to stabilize it in fluorite structure.11 Therefore,
determining the solubility limit of Sn4+ ion in ThO2 possessing
fluorite structure is of fundamental importance that can yield
useful information on the structure and their stability.
Interestingly, the ionic radius of Sn4+ in VIII-fold coordination
(0.81 Å) is very close to one of the members of rare-earth
family metal ions, viz., Tb4+ (0.87 Å).12 When formed as a solid
solution with fluorite-structured ceria, Sn4+ imparts higher
oxygen storage capacity (OSC) to it due to the availability of
two-electron exchange (Sn2+/Sn4+ redox couple).13 Such
compositions have found applicability as excellent catalysts
for CO oxidation.13 A similar situation prevails in the case of
ZnO, and when dissolved in ceria-possessing fluorite structure,
Zn-doped compositions find exhaustive catalytic applications.14
Both SnO2 and ZnO also share many similarities in terms of
defect chemistry and physics.10 Substitution of Sn4+ and Zn2+
for Th4+ in ThO2 can essentially lead to generation of novel
materials retaining the capabilities of defect structures of thoria,
tin oxide, and ZnO in the same structure. Such compositions
can potentially exhibit useful applications including high oxygen
storage materials and as catalysts. Interesting structural
chemistry also emerges when a nonstoichiometric fluorite
structure, MO2−X, is formed by substitution of the tetravalent
cations by cations of lower valence. The charge compensation
due to the substitution of the second cation is achieved by
oxygen vacancies leading to vacant lattice sites on the anion
sublattice and thus imparting higher anionic mobility. In these
nonstoichiometric oxides, both cation and anion sublattices can
exhibit either order or disorder so that a range of fluorite
derivative structures are possible.15,16 Bixbyite is one such
derivative structure usually considered to be a superstructure of
the simple fluorite lattice having one-fourth of the anion sites
vacant. The oxygen vacancies are in an ordered arrangement in
bixbyite. If the oxygen vacancies are disordered then the anion
lattice periodicity will be lost resulting in disordered (anion
deficient) fluorite structure. The presence of the complex
nature of defects in pure thoria also justifies the need of
investigating the substitution with Sn4+ and Zn2+ belonging to
the p and d block of the periodic table, respectively, and to
comprehend the defects caused by them.3e With this scientific
basis, the current study has been undertaken to seek answers
for some of the questions to the extent possible. The
synthesized samples have been extensively characterized by an
array of analytical techniques including high-resolution powder
X-ray diffraction (PXRD), Raman, UV−vis and photoluminescence spectroscopy, X-ray photoelectron spectroscopy,
and microscopy measurements. The samples have also been
evaluated for their catalytic role in the photodegradation of
Rhodamine-6G (Rh-6G) dye molecule, and the results are
discussed in this article.
2.1. Synthesis. Th(NO3)4·5H2O (Thomas Baker, 99%), SnCl2·
2H2O (Sigma-Aldrich, 99.9%), Zn (NO3)2·6H2O (Central Drug
House, 99%), absolute alcohol (Merck), and propylene oxide (P.O)
(Alfa aesar) were used for the experiments as purchased. A 0.570 g (1
mmol) amount of Th(NO3)4·5H2O was dissolved in 5 mL of absolute
ethanol by stirring for 15 min over a magnetic stirrer. To this 0.7 mL
(10 mmol) of propylene oxide was added slowly with stirring. It was
then subjected to sonication for nearly 15 min, after which a
transparent viscous gel formed. Similarly, other gels with tin chloride
and zinc nitrate salts were obtained using the following amounts of
reactants: 0.513 g (0.90 mmol) of Th(NO3)4·5H2O and 0.0225 g
(0.10 mmol) of SnCl2·2H2O, 0.399 g (0.70 mmol) of Th(NO3)4·
5H2O and 0.0675 g (0.30 mmol) of SnCl2·2H2O, 0.285 g (0.50 mmol)
of Th(NO3)4·5H2O, 0.1125 g (0.50 mmol) of SnCl2·2H2O, 0.130 g
(0.30 mmol) of Th(NO3)4·5H2O, 0.1575 g (0.70 mmol) of SnCl2·
2H2O, 0.513 g (0.90 mmol) of Th(NO3)4·5H2O, 0.0297 g (0.10
mmol) of Zn(NO3)2·6H2O, 0.4845 g (0.85 mmol) of Th(NO3)4·
5H2O, and 0.044 g (0.15 mmol) of Zn(NO3)2·6H2O in a way similar
to that described for Th(NO3)4·5H2O.
2.2. Characterization. Powder X-ray diffraction (PXRD) patterns
of the samples were recorded using a high-resolution PANanalytical
Empereyean diffractometer, equipped with a PIXcel3D detector
employing Cu Kα radiation (λ = 1.5418 Å) with a scan rate of
58.39 s/step and step size of 0.01313° over the range of 2θ = 20−70°
at 25 °C. The PXRD patterns were fitted using the Le Bail refinement
to obtain the cell dimensions. Riveted refinements of PXRD patterns
were carried out using the GSAS+EXPGUI program.17 The Raman
spectrum of the sample was collected in compact form using a
Renishaw spectrometer via a microscope system operating with an Ar+
laser (λ = 488 nm). Simultaneous thermogravimetric (TG) and
differential calorimetric analysis (DSC) of the xerogels was carried out
on a NETZSCH STA-449 F3 instrument in the temperature range of
30−900 °C at a scan rate of 10 °C/min under flowing nitrogen. FESEM micrograph and EDX analysis of the sample was obtained using a
Hitachi S-3700 M microscope. SAED patterns were recorded using a
FEI Technai G2 20 electron microscope operating at 200 kV. Diffuse
reflectance spectra of the samples were collected using a PerkinElmer
UV−vis spectrophotometer Lambda-35 attached with an integrating
sphere and using BaSO4 as the reference. The data were transformed
into absorbance using the Kubelka−Munk function. XPS spectra were
recorded using PHI 5000 versa prob II, FEI Inc., with Ar+ ion as well
as C60 sputter gun at a pressure better than 10−9 Torr. First, the sample
was cleaned by argon-ion bombardment in the sample compartment to
ensure that the surface was absolutely clean (without any
contamination), followed by its transfer to the analyzing chamber.
The core-level spectra were recorded using Al Kα radiation at a pass
energy of 50 eV, an electron take off angle of 90°, and a resolution of
0.1 eV. The core-level spectra were fitted after adjusting the baseline
relative to the signal background. The chemically distinct species were
resolved using a Gaussian distribution fitting procedure with the peak
positions, and areas were determined. The C 1s core-level spectra at
284.6 eV were taken as reference for the charge correction in the corelevel spectra, and the peak positions were calibrated with respect to it.
Photocatalytic degradation of aqueous dye solutions has been carried
out in an immersion-type, in-house-fabricated reactor under UV−vis
radiation employing a mercury vapor lamp with 125 W capacity
(Philips, India). A 50 mg amount of the catalyst was added to 50 mL
of the aqueous dye solution of Rh-6G with an initial concentration of
10 × 10−6 mol/L at room temperature. Prior to irradiation, the
suspension of the catalyst and dye solution was stirred in the dark for
30 min so as to reach the equilibrium adsorption. Periodically, 5−6 mL
of aliquots was taken out from the reaction mixture. The solutions
were centrifuged, and the concentration of the solutions was
determined by measuring the absorbance at λmax = 542 nm for Rh6G using a UV−vis spectrophotometer.
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TEM as well as from PXRD measurements.18 As the objective
of the present study was the application of the epoxide gel
method for the generation of monophasic thoria and not on the
possibility of fabricating thoria nanostructures, we centered the
rest of our investigation on the sample obtained after calcining
the xerogel at 800 °C. Successful Rietveld refinement of the
PXRD pattern in Fm3̅m space group with a = 5.6030 (35) Å
could be achieved for the thoria sample as shown by the
minimum difference between the observed and the simulated
profiles (Figure 2ii). The structural parameters and the refined
atom parameters are presented in Tables S1 and S2
(Supporting Information). The refined lattice parameter was
very close to the value reported for thoria obtained by the
oxalate decomposition method.5 In the SEM image, the near
spherical morphology of the crystallites was observed and they
were agglomerated (Figure 3a). A single vibration mode was
observed as a sharp band in the Raman spectrum at 457 cm−1,
confirming the fluorite structure (Figure 3b).14 The UV−vis
absorbance spectrum of the thoria sample is presented in
Figure 3c, from which an absorption threshold near about 430
nm is noticed. While the valence bands (VB) are constituted
mainly by the O 2p state and a few Th 6d and 5f states in the
electronics of thoria (bulk), Th 5f states together with Th 6d
and O 2p states build up the conduction band (CB).9 The
experimentally determined band gap of single-crystalline ThO2
is 5.75 eV. As thoria is a refractory and optically inert material,
researchers have concentrated on the sintering characteristics at
very high temperatures. The optical behavior of thoria obtained
at reasonably low temperatures (ca. 800 °C) has not been
investigated. It is relevant to point out at this juncture that
defect states arising from oxygen vacancy have been found near
about 3 eV above VB in thoria from theoretical calculations.3e It
is also quite interesting to note that another band gap value
such as 3.82 eV has been reported for thoria films by the spray
pyrolysis technique.19 It is noted that even the incorporation of
trace amounts of radioactive thorium in thoria alters drastically
the optical absorbance characteristics.20 In view of all these
facts, the observed deviation in the optical absorbance in our
samples from the ones reported in the literature might be
reasoned out to the creation of some intermediate energy levels
by defects (either trace amounts of carbon or by the oxygen
nonstochiometry). In the photoluminescence spectrum obtained using λex = 380 nm, emission in the blue region was
noticed (Figure 3d). On deconvolution, the presence of three
emissions at 415, 446, and 467 nm was evident. The
appearance of emission bands suggested the presence of
defects (possibly oxygen vacancies) in the system.
3. RESULTS AND DISCUSSION
An off-white-colored gel, from the reaction of Th(NO3)4 and
propylene oxide, was subjected to simultaneous thermogravimetric and differential calorimetric analysis. The results from
this experiment are presented in Figure 1. The gel showed a
Figure 1. Thermogravimetric and DSC traces of the gel obtained from
the reaction of Th(NO3)4 and propylene oxide.
gradual and continuous weight loss until 540 °C, after which no
appreciable weight loss was observed. This illustrated uniform
composition of the gel and calcination of it at around 500 °C
might be sufficient to produce crystalline thoria. The first step
of mass loss occurring between 50 and 156 °C in the TGA trace
corresponded to loss of occluded water molecules. The
observed weight loss between 156 and 350 °C corresponded
to the removal of organic moiety. These two steps were clearly
observed as exothermic events in DSC trace. The amorphous
nature of the xerogel was evident from its powder X-ray
diffraction pattern (Figure 2i(a)). The thermal evolution of
crystalline thoria was monitored by calcining the xerogel at 400,
600, 700, and 800 °C for a fixed duration of 2 h. From the
PXRD patterns presented in Figure 2i(b−e), diffraction peaks
pertaining to the cubic fluorite structure of thoria were
observable for all samples. However, the fwhm of the reflections
shifted from broad to sharp for the product calcined at 400,
600, 700, and 800 °C, suggesting an increase in the average
crystallite size of the samples. The crystallite size (estimated by
Scherrer analysis) of ThO2 obtained by calcining the xerogel at
400, 600, 700, and 800 °C was 4, 15, 22, and 29 nm,
respectively. Generally, the samples prepared from the epoxide
gel process show more or less the same crystallite size both in
Figure 2. (i) PXRD pattern of (a) xerogel and after calcining it at (b) 400, (c) 600, (d) 700, and (e) 800 °C for 2 h. (ii) Rietveld refinement of
PXRD pattern of ThO2 obtained at 800 °C.
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Figure 3. (a) FESEM image, (b) Raman spectrum, (c) UV−vis absorbance edge, and (d) photoluminescent emission spectrum of ThO2 sample
obtained with λex = 380 nm.
Our next target was to investigate the gel formation of
Th(NO3)4 along with tin and zinc salts along with subsequent
examination of their possible substitution for thorium. Toward
this, the gels with nominal molar compositions of 0.90:0.10,
0.70:0.30, 0.50:0.50, and 0.30:0.70 (in the case of Th:Sn) and
0.90:0.10, 0.85:0.15, and 0.80:0.20 (in the case of Th:Zn) were
prepared. First, the results from tin substitution in thoria will be
discussed followed by zinc substitution. Transparent gels
formed readily for all molar compositions investigated in the
case of the reaction between Th(NO3)4, SnCl2·2H2O, and
propylene oxide. This fact suggested that the rate of gelation
between these two salts was nearly the same. It is to be
recognized that these two metal salts contain cations belonging
to different groups of the Periodic Table with differing
oxidation state as well as with two different counteranions.
The thermal traces of these gels are presented in Figure S1
(Supporting Information), in which slow and steady weight loss
occurs hinting at the high homogeneity of the gels. The gel
obtained with a composition of 90:10 mol % of Th:Sn showed
slightly different behavior in its TG trace as compared to the gel
from thorium nitrate or with gels containing tin and thorium.
Nevertheless, the weight loss after 600 °C was negligible for all
these gels. Additionally, there was a systematic decrease in the
temperature ranges of exothermic events occurring for
increasing tin concentrations as compared to the gel from
pure thorium nitrate (Figure S2 Supporting Information).
Encouraged from these observations, the gels were calcined
under similar experimental conditions employed for obtaining
pure thoria, viz., calcination at 800 °C for 2 h followed by
cooling the furnace under natural conditions to room
temperature. The PXRD patterns of the products after
calcination are presented in Figure 4 and Figure S3 (Supporting
Information). From the PXRD patterns, certain conclusions
can be drawn. First, up to 50 mol % of tin can be substituted for
Th4+ in the fluorite structure, beyond which rutile-structured
SnO2 and fluorite-structured thoria separated (Figure S3
Supporting Information). Second, the reflections in the
PXRD patterns abruptly became very broad starting from 10
mol % tin substitution and continued until 50 mol %. There can
Figure 4. PXRD patterns of products obtained on calcining the gels
from (a) Th(NO3)4 and (b) 10, (c) 30, and (d) 50 mol % tinsubstituted samples.
be several reasons, such as instrumental artifacts (nonmonochromaticity of the source, imperfect focusing), crystallite
size, residual strain arising from defects (oxygen vacancies), and
dislocations, that lead to X-ray line broadening. Calcination of
gels containing tin and thorium at higher temperatures (900
°C) yielded more or less similar powder X-ray diffraction
patterns with no improvement in the broadening of reflections.
Such broadening in PXRD patterns has also been observed
when a second metal ion has been introduced in fluoritestructured ceria.21 Since all synthetic conditions for obtaining
tin-substituted samples have been kept the same as for pure
thoria, a comparison of the average crystallite size (from Schrrer
analysis) of thoria and tin-substituted samples was attempted.
The crystallite size decreased from 29 nm (for pure thoria) to a
range of 4−6 nm (for tin-substituted samples). The general
tendency of decrease in crystallite size with Sn substitution
suggests that tin hinders the crystallite growth at least at higher
molar concentrations. Such observations have been reported
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of tin-substituted thoria sample was sought from microscopy
techniques. The FESEM, EDX spectrum, and SAED pattern of
50 mol % tin-substituted thoria sample are presented in Figure
6. Uniform morphology was observed in the FESEM image of
the sample, and the EDX analysis on various locations yielded a
nearly equal percentage of thorium and tin in the sample.
Diffuse rings of bright spots were observed for this sample in
the SAED pattern, and they corresponded to the cubic unit cell
of the fluorite structure (Figure 6c and 6d). The presence of tin
and thorium was also evident in the X-ray photoelectron survey
spectrum of the 50 mol % tin-substituted sample (Figure 7).
The presence of a peak at a binding energy of 497 eV for Sn
3d3/2 indicated tin to be in the IV oxidation state.22 A clear blue
shift in the absorption threshold was noticed for the tinsubstituted thoria samples in their UV−vis spectra (Figure 8).
Generally, defects (such as oxygen vacancies) can be analyzed
by photoluminescence (PL) spectroscopy. PL emission and
excitation spectra of thoria are compared with the progressively
tin-substituted samples in Figure 9. The intensity of blue
emission peaks increased with increasing concentration of tin,
suggesting a possible increase in the defect concentration.
These might act as color-emitting centers.
After successfully establishing the extent of substitution of
tin, our next target was to determine the solubility of zinc in
thoria. Although SnO2 and ZnO share many similarities in
terms of defect chemistry and physics,4 zinc differs from tin in
its predominant existence in the +2 oxidation state (the most
stable oxidation state), and therefore, ascertaining its limit of
dissolution in the thoria lattice can in principle reveal the
complex oxygen vacancy arrangements existing in thoria.9
Additionally, it can append optoelectronic characteristics to the
refractory ceramic thoria. Homogeneous gels were formed from
the reactions of 0.90:0.10, 0.85:0.15, and 0.80:0.20 molar ratios
of Th(NO3)4 and Zn(NO3)2. On calcining the gels at 800 °C
for 2 h (a heating schedule employed for the parent thoria),
fluorite-structured products were obtained for 10 and 15 mol %
zinc-substituted samples (Figure 10i). The shift of reflections
toward higher two-theta values in their PXRD patterns
suggested the shrinking of the cubic unit cell and confirmed
incorporation of the smaller sized Zn2+ ion for the Th4+ ion in
the lattice (Figure S5, Table S3 Supporting Information). The
fingerprint reflections due to hexagonal wurtzite ZnO were
noticed in the PXRD pattern of the 20 mol % zinc-doped thoria
sample, suggesting the limit of solubility to be 15 mol % of zinc
under the present experimental conditions (Figure S6
Supporting Information). A similar conclusion was reached
from the Raman spectra of these compositions. The single
characteristic peak of the fluorite structure at 457 cm−1 (for
ThO2) shifted to 462 and 464 cm−1 for the 10 and 15 mol %
zinc-substituted samples, respectively (Figure 10ii). Marked
differences both in the PXRD patterns and in the Raman
spectra were observed for the zinc-substituted samples as
compared to the tin-substituted samples. First, there was no
drastic broadening of peaks in the PXRD patterns on
introduction of zinc in thoria. Instead, a slight shift of the
reflections toward the higher 2θ side was observed. Second, a
drastic reduction in the intensity of the Raman peak was not
noticed. Only a shift in its position to higher values was noticed.
It also ruled out the presence of other oxygen-deficient fluoritederived structures such as bixbyite or defect fluorite as a higher
number of bands were usually noticed.14 These differences
clearly highlighted the effects of incorporating elements from
different blocks of the Periodic Table in thoria. Also, the lower
earlier for transition-metal-ion or praseodymium-ion-doped
ceria compositions.21 From a close examination of the lattice
parameters derived by the Le Bail refinement of PXRD patterns
(Figure S4 Supporting Information), an initial dip in the cubic
lattice constant from 5.603 to 5.557 Å was observed for 10 mol
% tin-substituted sample indicating substitution of Th4+ (ionic
size 1.05 Å in VIII fold) with smaller sized Sn4+ (0.81 Å in VIII
fold).12 The extent of Sn4+ susbtitution in fluorite-structured
ThO2 was almost the same as that observed in CeO2; however,
peak broadening on the substitution of higher concentrations of
tin was not observed for CeO2−SnO2 solid solutions.13 Also,
with increasing tin concentrations up to 50 mol %, the cubic
lattice parameter was found to vary linearly in CeO2−SnO2
solid solutions.13 Such a trend was missing in the present case
(Table S3 Supporting Information). A slight increase in the
lattice parameter was noticed for 30 and 50 mol % tinsubstituted samples as compared to 10 mol % tin-substituted
sample. To understand this further and to ascertain the purity
of samples limited by PXRD measurements, they were
characterized by a more sensitive technique, viz., Raman
spectroscopy. In Figure 5, Raman spectra of these samples in
Figure 5. Raman spectra of (a) ThO2, (b) Th0.90Sn0.10O2, (c)
Th0.70Sn0.30O2, and (d) Th0.50Sn0.50O2.
the 200−650 cm−1 range are reproduced. The signature peak
for the fluorite structure observed at 457 cm−1 for thoria was
reduced in its intensity with increasing tin concentration,
suggesting the creation of randomness in the structure. The
nonobservance of any other peaks in the Raman spectrum due
to the rutile form of SnO2 negated phase segregation in the
samples and supported the substitution of tin for thorium. The
unique T2g mode observed for stoichiometric fluoritestructured MO2 is known to have a contribution from phonons
of all parts of the Bruilloiun zone due to the random
distribution of massive defects suppressing the translational
symmetry and leading to relaxation of the selection rule (K ≈
0).14 Under such circumstances, a broad spectrum, typical of
disordered phase, appears. It can get resolved only when a welldefined superstructure results by the ordering process. From
the increased bandwidth for the tin-substituted samples, it
appears that the long-range order may be present for the 10%
tin-substituted samples. With increased tin concentrations, this
seems to become short range. Therefore, the marginal
increment in the cubic lattice parameter for 30 and 50 mol %
tin-substituted samples (as compared to 10 mol % tinsubstituted sample) may be due to combined effects of shortrange order, crystallite size reduction, and the synthetic
conditions employed. Further evidence for the fluorite structure
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Figure 6. (a) FESEM image, (b) EDX spectrum, and (c) unindexed and (d) indexed SAED pattern of Th0.5Sn0.5O2.
Figure 7. Survey XPS spectrum of Th0.5Sn0.5O2 sample. (Inset) Corelevel spectrum of Sn 3d.
Figure 8. Optical absorbance of (a) Th0.90Sn0.10O2, (b) Th0.70Sn0.30O2,
and (c) Th0.50Sn0.50O2 samples. (Inset) Optical absorbance of ThO2.
concentration of zinc incorporated in thoria as compared to tin
may originate from its lower preference toward 8-fold
coordination with oxygen. The survey X-ray photoelectron
spectrum of 10 mol % Zn-substituted thoria sample along with
core-level analysis of Zn 2p are reproduced in Figure 11. This
reinforced the presence of both the metal ions in the sample
and the existence of zinc in the +2 oxidation state.22
The UV−vis spectrum and photoluminescent emission
spectrum of 10 mol % Zn2+-doped sample are reproduced in
Figure 12. The absorption edge moved to lower wavelength
(blue shift) with a distinct band at 366 nm for 10 mol % Zn2+substituted sample (Figure 12a). A similar blue shift has been
reported for the Zn2+-substituted ceria samples.10 Three
emissions centered at 413, 438, and 462 nm with enhanced
intensity (as compared to thoria) were observed for the zincsubstituted samples in the PL spectrum (Figure 12b). While the
blue emission can certainly be assigned to the defects (possibly
in the form of oxygen vacancies), the origin of other emissions
is not clear at present. Positron annihilation spectroscopy
experiments are being planned to unearth the nature of defects
present in these systems. However, all these observed changes
confirmed introduction of the second metal ion in thoria.
As substitution of Sn4+ and Zn2+ in thoria brought about
changes in the optical absorption characteristics, an application
involving the light was planned. A 50 mol % amount of Sn4+
and 10 mol % of Zn2+-substituted thoria samples were
evalauted for their role as catalyst for degradation of aqueous
Rh-6G dye solution under UV−vis irradiation. While
adsorption of dye molecules was predominant for Zn2+substituted thoria, degradation of nearly 80% of aqueous dye
concentration was effected by the tin-substituted thoria samples
within 60 min of irradiation (Figure 13 and Figure S7
Supporting Information). The inability of Zn2+-substituted
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Figure 12. (a) Optical absorbance of pure thoria and 10 mol % Zn2+substituted thoria. (b) Corresponding photoluminescent emissions of
these compounds.
Figure 9. Photoluminescence spectra at 380 (excitation) and 440 nm
(emission) for (a) ThO2, (b) Th0.90Sn0.10O2, (c) Th0.70Sn0.30O2, and
(d) Th0.50Sn0.50O2 samples.
Figure 10. (i) PXRD pattern of the product obtained on calcining the
gel of (a) Th(NO3)4 and (b) 90 mol %, 10 mol % and (c) 85 mol %,
15 mol % of nitrate salts of thorium and zinc, respectively. (ii)
Corresponding Raman spectra at room temperature.
Figure 13. Plot of C/C0 versus time for (a) photolysis and in the
presence of (b) thoria, (c) 10, (d) 30, and (e) 50 mol % tinsubstituted, and (f) 10 mol % zinc-substituted thoria samples as the
catalysts during the photodegradation of Rh-6G dye molecule.
Figure 11. Survey XPS spectrum of Th0.85Zn0.15O2 sample. (Inset)
Core-level spectrum of Zn 2p.
4. CONCLUSIONS
Taking advantage of a bottom-up approach, phase-pure thoria
in various crystallite sizes has been synthesized by epoxidemediated sol−gel synthesis from the nitrate salt of thorium.
The extent of substitution of Sn4+ ion in fluorite-structured
thoria has been determined to be 50 mol %. Modification of the
electronic strcture of thoria caused by Sn4+ substitution was
favorably utilized for the aqueous dye degradation process. The
incorporation of Zn2+ ion in thoria was restricted to 15 mol %
by this method. As sol−gel technology is an important thin film
technology to prepare many kinds of functional coatings, the
described method will be ideal to be developed further. The
results described in this study bear direct consequences to the
investigation of other related properties such as oxide ion
conductivity, oxygen sensing, and their use as electrolyte in
solid oxide fuel cells (SOFC).
■
thoria sample might be related to its lower concentration as
compared to the Sn4+-ion substitution in thoria in addition to
the intricate changes in the electronic structure. From the graph
it was obvious that the amount of dye degraded was
proportional to the amount of tin incorporated (15%, 30%,
and 80% dye degraded by 10, 30, and 50 mol % tin-substituted
thoria samples in about 60 min). Assuming the catalytic
degradation reaction to follow pseudo-first-order kinetics, the
estimated rate constants were 1.49 × 10−2, 2.40 × 10−2, 4.44 ×
10−2, and 7.72 × 10−2 min−1, respectively.
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02086.
TG and DSC traces of xerogels of varying compositions
of thorium nitrate and tin chloride, PXRD patterns of
calcined oxides from xerogels of SnCl2·2H2O and varying
compositions of thorium nitrate and tin chloride, Le Bail
fitting of PXRD patterns of tin- and zinc-substituted
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thoria samples, temporal changes in the absorbance
spectra of Rh-6G dye solutions in the presence of tinsubstituted thoria samples, crystallographic details of
Rietveld refinement of PXRD pattern of thoria (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected].
ORCID
Rajamani Nagarajan: 0000-0002-0983-7814
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
We thank DST (SB/S1/PC-08/2012), the University of Delhi,
and a DU-DST PURSE Grant for financial support to carry out
this research. V.K.T. thanks UGC, Government of India, for the
SRF fellowship. Useful discussions and use of DST-funded
facilities of Professor S. Uma of the Department of Chemistry,
University of Delhi, are gratefully acknowledged. We profusely
thank Prof. G. V. Prakash of the Department of Physics, Indian
Institute of Technology, Delhi, for help in Raman spectroscopy
measurements.
■
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