Sol–Gel Synthesis of High-Purity Actinide Oxide ThO2 and Its Solid

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 Fm3m ̅ 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 Fm3m ̅ ). 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 © XXXX American Chemical Society

Received: August 30, 2016

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DOI: 10.1021/acs.inorgchem.6b02086 Inorg. Chem. XXXX, XXX, XXX−XXX

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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. B

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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. 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 D

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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 E

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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 F

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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).

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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 G

DOI: 10.1021/acs.inorgchem.6b02086 Inorg. Chem. XXXX, XXX, XXX−XXX

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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.

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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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DOI: 10.1021/acs.inorgchem.6b02086 Inorg. Chem. XXXX, XXX, XXX−XXX