Solventless Preparation of Thoria and Its Inclusion into SiO2 and TiO2: A Luminescence and Photocatalysis Study.

Thoria was prepared using a solid-state method from the macromolecular precursor Chitosan·Th(NO3)4 (chitosan) and PS-co-4-PVP·Th(NO3)4 (PVP). The morphology and the average size of ThO2 depend of the chitosan and PS-co-4-PVP polymer forming the precursor. Their photoluminescent properties were investigated, finding a dependence of their intensity emission maxima, with the nature of the precursor polymer. The photocatalytic activity of ThO2 toward the degradation of methylene blue was measured for the first time, finding a degradation of about 66% in 300 min. The inclusion of ThO2 into SiO2 and TiO2 was achieved by the solid-state pyrolysis of the macromolecular composites Chitosan·Th(NO3)4//MO2 and PS-co-4-PVP·Th(NO3)4//MO2, MO2 = SiO2 or TiO2. The ThO2 exhibits a homogeneous dispersion inside the silica, showing sizes of about 40 and 50 nm for the chitosan and PVP polymer precursors, respectively. The luminescent properties of the ThO2/SiO2 and ThO2/TiO2 composites were also studied, finding a decrease in intensity when introducing the SiO2 or TiO2 matrices. The photocatalytic behavior to methylene blue degradation of ThO2 and their composites ThO2/SiO2 and ThO2/TiO2 was investigated for the first time, with them in the following order: ThO2 > ThO2/TiO2 > ThO2/SiO2.

Introduction

Among actinides oxides, thoria is an important and promising material used in ceramic catalyst sensor solid electrolytes, catalysis and optical materials, and in the traditional nuclear industry.[1−4] In spite of this, few papers related to the preparation and properties of nanostructured ThO2 have been reported. Among other methods, thermolysis, precipitation, sol–gel, or hydrothermal synthesis under supercritical conditions were proposed. Dash et al.[5] prepared ThO2 by thermal decomposition of Th(CO3)2, while Tabakova[6] prepared thoria starting from Th(NO3)4 and then precipitating the thorium hydroxide with K2(CO3)2 followed by thermal treatment. A similar route was followed by Reibold et al.,[7] preparing thoria by hydrolysis of Th(NO3)4 in the presence of ammonium hydroxide and propylene oxide. In another approximation, Moeini et al.[8] prepared ThO2 by a hydrothermal process employing supercritical water. Most recently, Hudry et al.[3] used the oleyamine method to prepare ThO2 of controlled morphology, while Shi and co-workers also prepared ThO2 using a hydrothermal method starting from Th(NO3)4 pentahydrate.[9] Recently, Pinkas[10] reports the preparation of thorium dioxide with nanofibrous morphology by the electrospinning method. Also, Romanchuk[11] reported that the facile chemical precipitation method and subsequent thermal treatment were shown to be suitable for preparation of crystalline ThO2 nanoparticles. Verma[12] has newly reported that ThO2 nanoflowers can be successfully synthesized using thorium nitrate pentahydrate as the metal source along with two different capping agents.

Almost all of these preparation methods involve solution procedures. However, several of the abovementioned practical applications require the incorporation of ThO2 into solid-state devices. Generally, the incorporation of metal-oxide nanoparticles into solid devices is problematic when those have been produced via a solution phase method because the solid-state isolation could cause nanoparticle agglomeration.[13−16] In this regard, the synthesis of nanoparticles directly from a solid-state approach might represent a more reliable method to achieve the incorporation of metal oxides into practical applications.

In addition, various practical applications—for instance, catalysis—involving solid-state devices are formed by nanoparticles and/or nanostructures inside a solid matrix, such as SiO2, TiO2, Al2O3, glasses, and so on.[17,18] For this reason, we will show results about the incorporation of thoria inside SiO2 and TiO2 matrices. This solid-state route synthesis method to prepare nanostructured metal and metal oxides materials from thermal treatment of the Chitosan·(MLn) and PS-co-4-PVP·(MLn) precursors under an air atmosphere was developed recently.[19−21]

In this paper, we have applied this methodology to prepare nanostructured ThO2 from the chitosan and PVP precursors. We also present an easy alternative procedure to prepare nanostructured thoria and their inclusion inside SiO2 and TiO2, as seen in Scheme . Their photoluminescent properties and the photocatalytic activity toward the degradation of methylene blue were also investigated.

Scheme 1

Schematic Representation of the Preparation of the Composites ThO2/SiO2 and ThO2/TiO2

This novel and original work includes the first systematic study of the effect of different matrices—SiO2 and TiO2—in a metal oxide such as thoria, determining their optical and photocatalytic properties. There are very limited studies regarding ThO2 photocatalytic activity and more especially regarding its combined effect with different matrices.

The inclusion of thoria will be performed by a novel solid-state thermolysis of different chitosan and PVP precursors, ensuring a regular distribution of thoria with the different Ti and Si oxides.

Methodology Description

Reagents

Th(NO3)4, TEOS (tetraethylortosilicate), chitosan, and poly(styrene-co-4-vinilpyridine) PS-co-4-PVP were purchased from Sigma-Aldrich and were used as-received.

Preparation: Synthesis of the Precursors

Chitosan·Th(NO3)4 and PS-co-4-PVP·Th(NO3)4

The typical procedure is described as follows: in a Schlenk flask, an appropriate amount of Th(NO3)4 and chitosan or PS-co-4-PVP were added into CH2Cl2 at different [polymer/Th(NO3)4] molar ratios (1:1; 1:5). The heterogeneous mixture was stirred at room temperature for a given reaction time (reaction time and additional details for each Th(NO3)4 reaction are explained in Table S1 of the Supporting Information). After removing the supernatant solution by decantation, the remaining solid was dried under reduced pressure to give a white solid.

Chitosan·Th(NO3)4//SiO2 and PS-co-4-PVP·Th(NO3)4//SiO2

SiO2 was prepared according to the following literature procedure.[21] Briefly, tetraethoxysilane (TEOS), ethanol, and acetic acid in a molar ratio of 1:4:4 were mixed with water (nanopure), and the mixture was stirred for 3 days. The obtained gel was dried at 100 °C under reduced pressure in a vacuum furnace.

Pyrolysis of the Precursors

Pyrolysis experiments were performed using 0.05–0.15 g of the metallic Chitosan·Th(NO3)4 and PS-co-4-PVP·Th(NO3)4 precursors, as well as their composites with SiO2. The samples were placed in alumina boats and heated in a furnace (Daihan oven model Wise Therm FHP-12) under an airflow up to 200 °C and then to 800 °C, followed by annealing for 2–4 h. The heating rate was fixed at 10 °C min–1 for all experiments.

Characterization of the Pyrolytic Products

The solid pyrolytic samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), Fourier transform infrared (FT-IR) spectroscopy, and thermogravimetric and differential scanning calorimetric analysis. SEM images were acquired with a Philips EM 300 scanning electron microscope. Energy-dispersive X-ray analysis was performed on a NORAN Instrument microprobe attached to a JEOL 5410 scanning electron microscope. TEM data were acquired using a JEOL SX100 and a JEOL 2011 transmission electron microscope. HRTEM observations were performed using a JEOL 2000FX microscope at 200 kV. TEM samples were prepared by dispersing the pyrolyzed material onto copper grids in ethanol media and then dried at room temperature. XRD was conducted at room temperature on a Siemens D-5000 diffractometer with θ–2θ geometry. XRD data were collected using Cu Kα radiation (40 kV, 30 mA). FTIR measurements were performed on a PerkinElmer FT-IR spectrophotometer model Spectrum BXII.

Photocatalytic Measurements

For the evaluation of the photocatalytic activity of the composites, 30 mL of a buffer solution at pH 7 of 1 × 10–5 M methylene blue (AM) was used. To reduce the adsorption of the dye on the surface, a buffer solution of pH = 7 was used to reach a zero isoelectric point. The same amount of mass (5.6 mg) was added for the composites dissolved in 300 μL of ethanol.

Irradiation was performed with a 300 W xenon lamp model 6258, which presented an incident light intensity of 1250 mW/m2, measured with a pyranometer. The distance between the lamp and the suspension was 16 cm. The AM concentration for each sample was determined by UV–vis spectrophotometry. Specifically, the intensity of the maximum absorbance corresponding to the lowest energy peak of the characteristic spectrum of methylene blue at 664 nm was measured. The photocatalytic efficiency of the products is expressed as the variation of the percentage of degradation with respect to the irradiation time.

Results and Discussion

ThO2 was prepared by thermal treatment of the chitosan and PVP precursors. The XRD pattern shows the pure phase of ThO2 arising from these two precursors (see Supporting Information S2), in concordance with the other XRD pattern reported from solution methods.[1,4,5,8,9] SEM images evidence a morphology composed of porous grains of ThO2 particles arising from both macromolecular complexes (see Figure ). Similar morphology was observed for ThO2 obtained by Moeini et al.,[8] in contrast with more densified grains observed from other solution methods.[4,5] The SEM–EDS analysis for ThO2 from both precursors shows the expected presence of Th and O (see Supporting Information S3).

Figure 1

SEM image of ThO2 from the Chitosan·Th(NO3)4 precursor (a) and from PS-co-4-PVP·Th(NO3)4 (b).

TEM images show nanoparticles of average sizes of 50 and 40 nm for ThO2 from chitosan (Figure a) and PVP (Figure b) precursors, respectively. Similar particle sizes were observed for ThO2 obtained by the hydrothermal method.[8]

Figure 2

TEM image of ThO2 from the Chitosan·Th(NO3)4 precursor (a) and from PS-co-4-PVP·Th(NO3)4 (b).

In Figure , HR-TEM images are shown for ThO2 obtained from the chitosan precursor. Nanoparticles of ThO2 with sizes of about 10 nm (Figure a,b) are observed, which are typically concatenated. Several interplanar distances were indexed, such as 0.32 and 0.28 nm, which correspond to the (111) and (200) interplanar spacings, respectively. Similar results were obtained for the ThO2 obtained from PVP, as observed in Figure a, with a mean size of about 45 nm. An interplanar distance of 0.31 nm corresponding to the (111) interplanar spacing of the ThO2 was measured (Figure d).

Figure 3

HRTEM image of ThO2 from Chitosan·Th(NO3)4 precursors. 50 (a), 10 (b), and 5 μm (c,d).

Figure 4

HRTEM image of ThO2 from PS-co-4-PVP·Th(NO3)4. 50 (a), 10 (b), 20 (c), and 10 μm (d).

In terms of their photoluminescence properties, a very few studies have been reported for ThO2.[22−26] Our photoluminescence study for the obtained ThO2 particles is shown in Figure . The excitation spectra (not shown), which reveal one broad band at 366 nm, produce an emission wavelength at 697 nm. The position of the band at 366 nm is attributed to absorption of charge transfer ThIV/ThIII → ThIII/ThIV under nonstoichiometric conditions (ThO2–).[24] The two main emission peaks, around 400 and 420 nm, have been assigned to the Th4+ typical center,[22,24] while those at 680 and 710 nm could be ascribed to the formation of oxygen vacancies on the ThO2 surface, which could be occupied by either oxygen or other impurities.[22,26] Nevertheless, the exact origin of these emissions is not fully clear.[22−26] Additionally, an effect on the emission intensity depending on the nature of the polymer and the metal/polymer ratio of the precursor was also observed (see Figure ). The ThO2 prepared from the PVP precursor using a 1:1 metal/polymer ratio exhibits the most intense emission of the maxima peaks at 400 and 420 nm, while that for the PVP precursor using the 1:5 metal/polymer ratio exhibits the most intense emission in the emission at 680 and 710 nm.

Figure 5

Luminescence spectra of the ThO2 from the precursors Chitosan·Th(NO3)4 and PS-co-4-PVP·Th(NO3)4 in 1:1 and 1:5 molar ratios.

Different composites of ThO2 with SiO2 were also prepared by pyrolysis at 800 °C under air of the Chitosan·Th(NO3)4//SiO2 and PS-co-4-PVP·Th(NO3)4//SiO2 precursors. The diffraction pattern of the different obtained products is shown in Supporting Information S4. ThO2 is weakly observed by XRD due to the dilution of the precursors containing the thorium salts with respect to SiO2 (1:100), as also previously reported for other metals[27,28] or metal oxides[29] with respect to the amorphous silica. The presence of ThO2 inside silica was corroborated by SEM–EDS mapping by element analysis. This technique gives also information about the distribution of the thoria into SiO2. From Figure , ThO2 agglomerated nanoparticles of about 250 nm were observed to be homogeneously dispersed into bigger silica particles. A similar distribution of the ThO2 nanoparticles, but with bigger sizes of about 950 nm, was observed for ThO2 inside SiO2 obtained from the chitosan precursor (Figure ). The representative SEM image of the ThO2/SiO2 composites arising from both macromolecular precursors is displayed in Supporting Information S5. For the ThO2/chitosan sample, typical nanostructure morphology of that obtained from solid-state pyrolysis can be seen.[13] In some areas, a dense 3D morphology with several shapes joined between them with the presence of pores is exhibited, as shown in Figure S5a,b. On the other hand, sphered shapes were also observed, Figure S5c,d. For ThO2 from the PS-co-4-PVP·Th(NO3)4//SiO2 precursor, big agglomerates composed mainly of spheres of different sizes were observed, see Figure S5a.

Figure 6

SEM–EDS mapping by elements of ThO2 inside SiO2 from the Chitosan·Th(NO3)4//SiO2 precursor.

Figure 7

SEM–EDS mapping by elements of ThO2 inside SiO2 from the precursor PS-co-4-PVP·Th(NO3)4//SiO2.

XRD patterns of the ThO2//TiO2 composites from the chitosan and PVP precursors are shown in Figure S6 of the Supporting Information Again, the TiO2 matrix presents more intensity than ThO2, for both cases. Nevertheless, the presence of ThO2 was clearly identified from the SEM–EDS mapping image (see S7, Supporting Information). Several SEM images of the ThO2//TiO2 composite obtained from the chitosan precursor are also shown in Figure , where dense grains can be observed. Similar morphology was obtained for ThO2//TiO2 composites arising from PVP precursors (see Figure ).

Figure 8

SEM image of the ThO2//TiO2 composites arising from the Chitosan·Th(NO3)4//TiO2 precursors: 20 (a), 5 (b), and 2 μm (c,d).

Figure 9

ThO2//TiO2 composites arising from the PS-co-4-PVP·Th(NO3)4//TiO2 precursors: 20 (a), 10 (b), and 2 μm (c,d).

Finally, SEM–EDS mapping (see Figure S7) exhibits a homogeneous distribution of ThO2 inside the TiO2 matrix for ThO2//TiO2 composites produced from both precursors.

Photoluminescence of ThO2/SiO2 and ThO2/TiO2 Composites

In order to investigate the effect of the SiO2 and TiO2 matrices on the intensity of ThO2 particles, the luminescence spectra of ThO2/SiO2 and ThO2/TiO2 composites were recorded and compared with those of pure ThO2. The results are plotted in Figure .

Figure 10

Luminescence spectra of the ThO2 from the precursors ThO2, Chitosan·Th(NO3)4//SiO2 (a), and PS-co-4-PVP·Th(NO3)4//TiO2 (b) λexc. = 366 nm.

For the chitosan precursor (Figure a), it can be observed that the matrix inclusion produces a decrease in the emission intensity, showing the order ThO2 > ThO2/SiO2 > ThO2/TiO2 for both main emissions. In the case of the samples produced from the PVP precursor, Figure b, a similar trend was observed for the emission maxima at 680 and 710 nm but an inversion occurs for the emission at 400 and 420 nm, with the order ThO2 > ThO2/TiO2 > ThO2/SiO2. These variations of the maxima intensity at 680 and 710 nm could be related with the presence of surface oxygen or impurities generated in the SiO2 and TiO2, leading to different morphologies and surface defects induced by these matrices. The detailed mechanism of how this effect occurs is unknown. In this sense, it was reported that an increase in the emission intensity is normally associated with an increase in the surface defects on the ThO2 which are occupied by oxygen.[26] As a consequence, according to Figure a, the oxygen absorbed on the surface of thoria decreased on the inclusion on SiO2 and TiO2. A similar explanation for the observed intensity variation could hold for the ThO2/PVP sample.

Photocatalytic Activity

Although there are scarce reports about the catalytic activity of thoria, no studies about the photocatalytic activity of ThO2 neither ThO2/SiO2 nor ThO2/TiO2 composites toward contaminant dyes have been reported. We have performed studies about the use of thoria acting as a photocatalyst for the degradation of methylene blue under solar radiation. As is shown in Figure , thoria exhibited an activity of 66 and 67% in 300 min for ThO2 prepared form the chitosan and PVP precursors, respectively. The absorption decrease in the absorbance at 655 nm of methylene blue versus wavelength at different time intervals for the different studied samples can be observed in Supporting Information S8. The kinetic degradation of methylene blue with ThO2/chitosan followed a zero order, while that for ThO2/PVP followed a first order. Similar kinetic behavior was observed for the ThO2/SiO2 and ThO2/TiO2 composites, as summarized in Supporting Information S9. On the other hand, the photocatalytic efficiency of the ThO2/SiO2 composite decreased to 25 and 28% for the chitosan and PVP precursors, respectively. As for the case of the ThO2/TiO2 composite, the photocatalytic efficiency decreased to 39.5 and 27% for the chitosan and PVP precursors, respectively. A summary of the kinetic data is shown in Table .

Figure 11

Normalized concentration change in MB without the catalyst and in the presence of ThO2 from the PS-co-4-PVP·Th(NO3)4 (a) and Chitosan·Th(NO3)4 (b) precursors.

Table 1

Kinetic Data for the Photodegradation Process of MB with ThO2 and with the Composites ThO2/SiO2 and ThO2/TiO2

photocatalyst	apparent photodegradation	discoloration rate (%)	r2 linear fit (%)
ThO2-Chitosan	3.7 × 10–3	67	0.992
ThO2-PS-4-PVP	2.2 × 10–3	66	0.967
ThO2/SiO2-Chitosan	7.7 × 10–4	24	0.979
ThO2/SiO2-PS-4-PVP	8.5 × 10–4	25	0.923
ThO2/TiO2-CHITOSAN	1.4 × 10–3	39	0.815
ThO2/TiO2-PS-4-PVP	8.7 × 10–4	27	0.941

Table shows the obtained values for photodegradation, the speed constant, and the correlation coefficient. In all cases, it is observed that the correlation coefficient (r2) is close to unity, which indicates that the photocatalytic degradation process, mediated by the synthesized thorium oxides, adjusts to the zero-order and first-order kinetics. The estimated rate constant for the degradation of methylene blue in the presence of thorium prepared without the SiO2 and TiO2 matrices is greater than that of the pristine compounds, suggesting that the structural modification and synergy of the inorganic component play a fundamental role. This is related with the increase in the photocatalytic efficiency of the semiconductor, due to the greater number of active sites available by the (ThO2) n-PVP and (ThO2) n-chitosan precursors. The increase in photocatalytic activity of only ThO2 with respect to the composites ThO2/SiO2 and ThO2/TiO2 may be due to the greater porosity of thoria alone with respect to the porosity of thoria included in SiO2 and TiO2 matrices. This can be observed from the SEM images of only ThO2, Figure , with respect to the SEM images of i ThO2 included in silica and titania, see Figures and S5 of the Supporting Information. Clearly, the SEM image of ThO2 is porous, in contrast with the SEM images of ThO2 including inside SiO2 and TiO2 matrices where a dense material is observed.

Band Gap Study

The band gap of metal oxides is an interesting property, which is involved in electronic applications.[30,31] Materials based on thoria are viewed as wide band gap semiconductors. For the case of thoria, there is little information in the literature regarding its band gap. For ThO2 nanoparticles, Aller et al.[23] reported values in the range 6.22–5.69 eV, while Buono-Core et al.[24] reported values from 4.5 to 4.61 eV for ThO2 thin films. The study of Aller et al. also reported a wide range of values from 3.84 to 6.9 eV, although some of these values are based on theoretical calculations. Using the solid-state UV–visible absorption and with the Tauc plot for ThO2, we estimated a value of 5.66 and 5.76 eV for chitosan and PVP precursors, respectively (see Supporting Information S11). These values are in concordance with those previously reported in the literature.[29] For the ThO2/SiO2 composite, similar values of 5.50 and 5.60 eV were estimated. In this case, there are no literature data for these composites. On the other hand, the band gap values for thoria included in TiO2 matrices exhibited values of 3.14 and 3.15 eV for the chitosan and PVP precursors, respectively. These values are significantly lower than those of ThO2 and ThO2/SiO2. As pointed by Buono-Core et al.,[24] the optical band gap energy is very sensitive to the preparation method and the experimental parameters applied in the synthesis.[24] In fact, Mahmoud also reported a value of 3.82 eV for ThO2 prepared by a spray pyrolysis technique.[25]

Formation Mechanism Probable

A proposed formation mechanism is discussed here based on previous studies of solid-state nanostructures using the same synthetic approach.[19,32] According to these studies, the first step on heating the samples involves the formation of a 3D network to produce a thermally stable matrix. This step is crucial as it offsets the sublimation (see Figure ). The first heating step could involve a cross linking of the chitosan or PSP-4-PVP polymer, giving a 3D matrix containing the Th(NO3)4 compound linked to the polymeric chain. The following steps could involve the organic carbonization, producing holes where the nanoparticles begin to nucleate. As confirmed in earlier studies,[32] the ThO2 oxide could grow over the layered graphitic carbon host which is lost during the final annealing temperature, that is, 800 °C.

Figure 12

Schematic representation of the proposed mechanism of the formation of the metal oxide nanoparticles. MX represents the general formula of the metallic salt coordinated to the chitosan and PSP-4-PVP polymer, Th(NO3)4 and }}}}}}} represent the chitosan and PSP-4-PVP polymer, respectively. The temperatures are referential general values.

Conclusions

ThO2 nanoparticles were satisfactorily prepared by solid-state pyrolysis of the Chitosan·Th(NO3)4 and PS-co-4-PVP·Th(NO3)4 precursors. The particle sizes were in the range of 40–50 nm depending on the polymer solid-state template. The luminescence of ThO2 arising from both polymers exhibits a dependence with the nature of the precursor and with the metal/polymer ratio being the most intense emission for the ThO2 arising from PS-co-4-PVP·Th(NO3)4 in the molar ratio 1:1. The photocatalytic efficiency of ThO2 toward the degradation of methylene blue was around 66% in 300 min for the thoria obtained from both precursors. The inclusion of thoria into the SiO2 and TiO2 matrices was achieved by solid-state thermolysis of the solid Chitosan·Th(NO3)4//MO2 and PS-co-4-PVP·Th(NO3)4//MO2 precursors, where MO2 = SiO2 and TiO2 to give the ThO2/SiO2 and ThO2/TiO2 composites. SEM–EDS mapping analysis showed a regular dispersion of the thoria into the SiO2 and TiO2 matrices. The particle size of ThO2 increases by about 19 times for the nanoparticles obtained from chitosan as they are included in SiO2, while for the ThO2 obtained from PS-co-4-PVP, an increase of 6 times is observed when the ThO2/SiO2 composite is formed. The effect of the polymer precursors on the particle size is little for the free matrices of ThO2, while that when included inside silica is in the particle size order chitosan > PS-co-4-PVP by nearly four times.

The intensity of the emission at 420 nm followed the order ThO2 > ThO2/SiO2 > ThO2/TiO2 which was explained by the surface’s defects on the ThO2 associated with their inclusion into SiO2 and TiO2 matrices. The photocatalytic activity toward methylene blue degradation follows the order of ThO2 > ThO2/TiO2 > ThO2/SiO2 and was attributed to the encapsulation of the ThO2 into the different matrices. Similar results on the effect of the SiO2, TiO2, and Al2O3 on the nanostructured NiO have recently appeared.[33]