Cerium-based porous coordination polymers with hierarchical superstructures: fabrication, formation mechanism and their thermal conversion to hierarchical CeO₂

Abstract

In this work, cerium-based porous coordination polymers (Ce-CPs) with hierarchical superstructures have been successfully synthesized on a large scale employing cerium nitrate as the metal salt and 2,5-pyridinedicarboxylic acid (2,5-H₂pdc) as the ligand by a mixed-solvothermal route. The as-obtained products were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetry and differential thermal analysis (TG-DTA). It was found that the hierarchical superstructures are composed of asymmetric branches with lengths ranging from 10 to 50 μm. The reaction parameters such as reaction temperature, total concentrations of the reactants, solvent composition, and the reaction time were investigated systematically. A possible formation mechanism for the hierarchical superstructures has been proposed to interpret the growth process. CeO₂ with similar hierarchical structures could be obtained after thermolysis of the Ce-CP precursors at 450 °C for 4 h. The UV-vis adsorption spectrum of the obtained CeO₂ shows that the band gap energy (E_g) is 2.64 eV, which is lower than that of bulk ceria. Moreover, the as-obtained CeO₂ also exhibited remarkable ability to remove rhodamine B (RhB).

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Introduction

Coordination polymers (CPs) as a class of captivating inorganic–organic hybrid materials have received a great deal of attention because of their unique and high degree of tailorability, which have great potential technological applications in heterogeneous catalysis,¹ gas storage,² medical imaging,³ biosensing and drug delivery.⁴ Up to now, with the on-going development of modern nanoscience and nanotechnology, a large number of CPs with a wide variety of morphologies and structures have been synthesized successfully. For example, Wang et al. prepared hollow iron-based ferrocenyl CP microspheres with microporous shells via a one-pot solvothermal method without any additional template and discussed the Ostwald ripening mechanism of the microspheres.⁵ Oh et al. also described a simple solvothermal approach for the synthesis of hexagonal-tube CPs.⁶ We have been engaged in the synthesis of CP micro/nanostructured materials during the past few years.⁷ All the previous studies suggested that suitable selection of the metal ions, building organic ligands and design of reaction routes would not only enrich their family but also bring us some unexpected results on morphology control or fascinating properties.⁸

The hierarchical self-assembly micro/nanostructured materials built from functional low dimensional nanostructures have attracted extensive research interest owing to their enhanced properties and potential applications over the past few decades.⁹ The chemical and physical properties of solid-state materials depend not only on their size, shape, and chemical composition, but their assemblies as well.¹⁰ Thus, control over size and shape of hierarchical micro/nanostructures is an important theme for understanding and exploiting their unique properties.^9c On the nanoscale, dendritic hyperbranched structures are formed by hierarchical self-assembly under non-equilibrium conditions.¹¹ Inspired by these results, many types of three dimensional (3D) micro/nanostructures with high complexity have been fabricated successfully, such as snowflakes,¹² nanocages,¹³ hyperbranches,¹⁴ multipods,¹⁵ dendrites,¹⁶ saws,¹⁷ flower-like microspheres,¹⁸ hollow nanospheres,¹⁹ airplane-like nanostructures,²⁰ and so on. However, these structures mainly focus on metal, metal oxides, and other semiconductors. Until now, there are only sporadic successful examples of the synthesis of uniform hierarchical CPs with rich morphologies.²¹ Therefore, it still remains a significant challenge to design and develop an effective and reliable route for the synthesis of CP hierarchical superstructures, regarding control over the complex structures.

Ceria is one of the most attractive rare-earth oxides, which has attracted extensive attention because of its candidate applications ranging from catalysis,²² fuel cells,²³ sensors,²⁴ to UV blocking,²⁵ and so forth. The morphologies and structures of CeO₂ have an important influence on its properties, the main emphasis on the CeO₂ has focused on the fabrication of various morphologies and structures.²⁶ So far, CeO₂ with various basic nanoscale morphologies and structures has been synthesized successfully through various methods, such as nanoparticles,²⁷ nanowires,²⁸ nanotubes,²⁹ nanorods,³⁰ and so on. Furthermore, it has been demonstrated that 3D CeO₂ hierarchical structures display enhanced activity towards the removal of As(V) and Cr(VI) in wastewater.³¹ In this respect, it is crucialto fabricate 3D hierarchical micro/nanostructured CeO₂ with various composites and morphologies based on different driving mechanisms, which endow them with some unique properties and novel functionalities. Yang's group fabricated 3D CeO₂ microflower structures via the calcination of a cerium oxalate precursor, and the as-prepared flowerlike CeO₂ displayed a high surface area (147.6 m² g⁻¹), which shows a higher catalytic activity for CO oxidation.³² However, to the best of our knowledge, there have been very few reports focusing on 3D CeO₂ hierarchical superstructures via the thermal treatment of the corresponding CP precursors.

In this work, the 2,5-H₂pdc building block is chosen as a multidentate ligand to generate organic–inorganic hybrid materials, which is based on the following reasons: (i) 2,5-H₂pdc as an N-heterocyclic analog of 1,4-benzenedicarboxylic (terephthalic) acid that combines the advantages of both organic multi-carboxylic acid and aromatic compounds.³³ (ii) The carboxylate groups of 2,5-H₂pdc can be completely or partially deprotonated to generate pdc²⁻ and Hpdc⁻, allowing various acidity-dependent coordination modes. (iii) The 2,5-H₂pdc ligand possesses rich coordination modes due to different –O and –N donors (carboxylate oxygen atoms and pyridine nitrogen) that may be suitable for connecting metal centers to generate metal–organic framework (MOF) structures. In addition, our previous studies have proven that the 2,5-H₂pdc is an excellent building block for the construction of lanthanide coordination polymers.^7c,f Herein, we present a mixed-solvothermal route to fabricate Ce-CP hierarchical superstructures from cerium(III) and a well-known 2,5-H₂pdc building block. Influential factors including reaction time, temperature, total concentrations, and solvent composition of the reactants for the formation of the Ce-CP superstructures have been studied systematically. A possible formation mechanism of the Ce-CP hierarchical superstructures was suggested based on the detailed experiments. After thermal decomposition of the Ce-CP, CeO₂ with similar hierarchical structures was successfully fabricated.

Experimental section

Materials and synthesis

All the reagents used in our experiment were of analytical grade and used without further purification. In a typical synthesis procedure of the hierarchical superstructures (sample 1), 0.2 mmol of Ce(NO₃)₃·6H₂O was dissolved in 10 mL of absolute ethanol, followed by the addition of 22 mL of dmf (N,N-dimethylformamide) containing 0.8 mmol of 2,5-pyridinedicarboxylic acid (2,5-H₂pdc). After vigorous stirring for 20 min under magnetic stirring, the resulting solution was transferred into a 40 mL Teflon-lined autoclave and heated at 160 °C for 12 h. After the autoclave was cooled to room temperature naturally, the fresh precipitates were collected by centrifugation, washed three times with absolute ethanol, and dried at 60 °C in a vacuum. The detailed experimental conditions for the as-synthesized products are listed in Table S1 (ESI†). Finally, mesoporous CeO₂ was obtained by calcination of the Ce-CP precursor under air at 450 °C for 4 h with a heating rate of 0.5 °C min⁻¹.

Characterization

The phase and composition of the resulting products were analyzed by XRD, in a two theta range from 5° to 90°, using a Rigaku/Max-3A diffractometer. The morphologies and superstructures of the as-prepared products were examined by using SEM (Hitachi, S-3400N). Transmission electron microscopy (TEM), high-resolution TEM (HRTEM), and selected area electron diffraction (SAED) images were obtained by using a JEM 2100 transmission electron microscope operated at an accelerating voltage of 200 kV. Elemental analysis (EA) data were obtained using an EA 300 instrument. FTIR spectra were recorded on a Nicolet 6700 spectrometer in the wavenumbers ranging from 4000 cm⁻¹ to 400 cm⁻¹. TG-DTA analysis was conducted with a TA-50 thermal analyzer instrument at a heating rate of 10 °C min⁻¹ from room temperature to 1000 °C under an air atmosphere. Nitrogen adsorption–desorption isotherms were collected at a liquid nitrogen temperature (77 K) using a BELSORP-mini II apparatus. The specific surface areas were calculated by the (Brunauer–Emmett–Teller) BET method, and the mesopore size distribution was calculated from the adsorption branch using the Barrett–Joyner–Halenda (BJH) theory. The Raman spectra were recorded using a LabRAM HR laser Raman spectrometer with a 633 nm excitation. XPS were measured using an EscaLab 250 X-ray photoelectron spectrometer with Al Kα radiation. An ultraviolet-visible (UV-vis) adsorption spectrum was recorded on a SOLID 3700 UV/VIS/NIR spectrophotometer. All the measurements were performed at room temperature.

Photocatalytic degradation experiments

Typical degradation experiments under a high-pressure mercury lamp (500 W) were performed at room temperature. Prior to irradiation, 20 mL of RhB aqueous solution (5 × 10⁻⁶ M) containing 0.05 g CeO₂ was magnetically stirred for 30 min in the dark continuously in order to achieve an adsorption/desorption equilibrium between the as-obtained CeO₂ and the RhB dye. Then the suspension was magnetically stirred under UV light irradiation. At regular intervals, aliquots of solution were withdrawn and centrifuged to separate solid particles for analysis of concentration. The characteristic absorbance of RhB at 554 nm was used to characterize the concentration of RhB. The photocatalytic reaction progress was monitored using a Lambda 35 UV-vis spectrometer.

Results and discussion

Characterization of the Ce-CP hierarchical superstructures

The morphology, structures and sizes of the as-synthesized Ce-CP hierarchical superstructures were examined using SEM, as shown in Fig. 1. Fig. 1(a) is the low magnification SEM image, revealing that the Ce-CP hierarchical superstructures were synthesized on a large scale and in high purity after a mixed-solvothermal reaction. The magnified image of Fig. 1(b) indicates that the Ce-CP superstructures exhibit a hierarchical structure. Each superstructure practically consisted of many branches attached to a mutual core. The length of the branches is in the range of 10–50 μm. By a close observation on the vertical direction of a single superstructure, two or more taper-shape-like rods with lengths of about 30 μm could be observed in Fig. 1(c) and (d). Perpendicular to the taper-shape-like rod direction, it can be clearly seen that four homogeneous dendritic structures with the lengths range from 10 to 25 μm were also observed, and a lot of sub-branches vertically distributed on the sides of the dendritic trunk (Fig. 1(e) and (f)). This asymmetric superstructure is not in the same plane but has a 3D superstructure, which is clearly different from the reported semiconductor superstructures.³⁴ Fig. S1(a)† shows the XRD pattern of Ce-CP hierarchical superstructures. It clearly displays that the peaks are strong and narrow, indicating the good crystallinity of the as-prepared products. However, it could not be finely indexed to a known phase compared with other references and all the standard XRD patterns in JCPDS cards. There are inherent difficulties in determining the exact crystalline information on Ce-CPs through HRTEM, but it is reasonable to believe that such Ce-CPs have been successfully prepared based on the above results.³⁵ Since CP micro/nanoparticles displayed unexplored crystalline structures, and the corresponding single crystals with a suitable size could hardly be obtained, researchers have usually devoted to apply multi-analysis methods to study the compositions and structures of the CP micro/nanoparticles.³⁶

Fig. 1 SEM images of the Ce-CP hierarchical superstructures (sample 1).

Therefore, Ce-CP hierarchical superstructures were carefully characterized by XPS, FTIR, EA, and TG-DTA analysis to investigate their chemical structures and compositions. The XPS spectrum of the as-synthesized Ce-CP powder clearly detects the obvious peaks of cerium, carbon, nitrogen and oxygen in the survey spectrum (Fig. S1(b)†). From Fig. S1(c),† four binding energy peaks in the 3d region at 881.18 eV, 884.38 eV, 900.18 eV, and 903.18 eV, in addition to the 4d region at 110.08 eV, indicate that the oxidation state of cerium in these samples is from Ce(III). The FTIR spectrum of the CP superstructure (Fig. S1(d)†) displays different characteristics compared with pure 2,5-H₂pdc, which further confirms the formation of Ce-CP hierarchical superstructures from the cerium ions and 2,5-H₂pdc. Firstly, the characteristic peaks at 1479 and 1417 cm⁻¹ are attributed to the stretching vibrations of the backbone of the benzene ring.³⁷ Secondly, comparing with the absorption band at 1731 and 1385 cm⁻¹ for the –COOH of the free 2,5-H₂pdc ligand, the bands around 1607 and 1353 cm⁻¹ could be assigned to the asymmetric ν_as(–COO⁻) and symmetric ν_s(–COO⁻) stretching vibrations of the ionized carboxylate groups, respectively.³⁸ Thirdly, the sharp band appearing at 506 cm⁻¹ is attributed to the Ce–O stretching vibration, which confirms that the Ce³⁺ ions are coordinated with the 2,5-H₂pdc to form a complex.³⁹ Elemental analysis (EA) was performed to study the chemical compositions of the Ce-CP, providing the elemental quantities as follows: C: 36.18, N: 8.31, H: 3.23. Combining the EA results and the building components determined by FTIR, the empirical formula of the Ce-CP could be proposed as Ce(pdc)(Hpdc)·2(dmf)·2(H₂O) (calcd C: 36.76, calcd N: 8.57, calcd H: 3.86%).⁴⁰ TG-DTA curves of Ce-CP taken under air (Fig. S1(e)†) were further conducted to support our conjecture. There are three weight loss steps in the thermal decomposition of this compound. The first stage occurs before 184 °C, and the weight loss is approximately 6.15%, corresponding to the weight of the physically absorbed and structured water molecules. The second weight loss in the range of 184–380 °C is 21.82%, which is related to the loss of dmf molecules.⁴¹ The last weight loss from 380 to 445 °C is ascribed to the decomposition of the organic ligand. N₂ adsorption–desorption isotherms were recorded after pretreatment at 150 °C for 4 h under dynamic vacuum. As shown in Fig. S1(f),† N₂ adsorption isotherm of Ce-CP hierarchical superstructures exhibits the behaviour for multilayer adsorption. The BET surface areas of Ce-CP obtained from the adsorption branch of the isotherm gave a relatively low BET surface area of 2.2 m² g⁻¹. The low N₂ adsorption is more likely attributed to the strong quadrupole interactions between N₂ molecules and the electrostatic field gradients near the surface of Ce-CP hierarchical superstructures. These considerable quadrupole interactions could block N₂ molecules from passing into the pore channels.⁴²

It is well known that the lanthanide elements are a group of 14 elements (except for Pm, which is a radioactive element), which can be divided into light (La → Eu) and heavy (Gd → Lu) rare earth groups. Under identical synthetic conditions, other lanthanide-based CPs were also prepared, just by replacing Ce³⁺ with other rare earth element ions.^7b It is clearly observed from SEM images that the morphologies of the as-synthesized products are different to some extent (Fig. S2 and S3†). From La to Gd, similar CP hierarchical superstructures with the length of branches in range of 30–120 μm were prepared, while other lanthanide-based CPs were close to a spherical shape with diameters of 5–20 μm and irregularly shaped microstructures with a random size distribution. Although the exact reason is not clear at present, it is reasonably believed that this difference seems to be closely related to the change in lanthanide ion radius.⁴³

Influences of different reaction parameters on the morphologies of Ce-CP hierarchical superstructures

In order to better understand the formation mechanism of the Ce-CP hierarchical superstructures, a series of controlled experiments (Table S1, ESI†) were conducted to systematically study the morphological evolution.

By fixing other reaction conditions, the effect of the reaction temperature on the morphology of the as-prepared products was investigated. At the reaction temperature of 80 °C (sample 2), the products were composed of uniform nanospheres with a size of about 500 nm in diameter (Fig. S4(a)†). Interestingly, with the increase in reaction temperature, the hierarchical structures began to form. At 100 °C (sample 3), a large amount of approximate hierarchical cubes with edge lengths of about 30 μm were obtained (Fig. S4(b)†). Furthermore, hierarchical superstructures were also prepared in high yield at 140 °C (sample 4). Compared with the typical hierarchical superstructures mentioned above (sample 1), these hierarchical superstructures possess a narrow size distribution (Fig. S4(c)†). However, when the reaction was carried out at 180 °C (sample 5), hierarchical superstructures had been destroyed (Fig. S4(d)†). From these results, we can conclude that the Ce-CP hierarchical superstructures can be selectively synthesized by controlling the reaction temperature rationally. The high temperature is helpful for the formation of hierarchical superstructures, and the typical Ce-CP hierarchical superstructures can be achieved at 160 °C.

To determine the effect of total concentrations of the reactants on the formation of the products while retaining other reaction conditions, the images of the products obtained from the solutions with a mixed-solvothermal treatment at 160 °C for 12 h are shown in Fig. S5(a–c).† It is obvious that the morphology changed greatly with increasing amounts of reactants. When the Ce³⁺ was 0.05 mmol (sample 6) and the molar ratio of Ce³⁺ to 2,5-H₂pdc was 1 : 4 with other reaction conditions unchanged, uniform spheres with diameters of about 500 nm were obtained (Fig. S5(a)†) on a large scale, which was much different from the typical sample. To investigate whether hierarchical superstructures can be obtained by increasing the reaction time, an experiment was carried out for 96 h (sample 7). However, no hierarchical superstructures were obtained. Instead, a large amount of octahedra with a size distribution of about 27 μm in diameter were observed and a small amount of nanoparticles with diameters of ca. 100–350 nm were also detected (inset in Fig. S5(a)†). While increasing the amount of Ce³⁺ to 0.1 mmol (sample 8), the as-synthesized product mainly takes on mixed morphologies of nanoparticles and complicated hierarchical superstructures (Fig. S5(b)†). Further increasing the amount of Ce³⁺ to 0.15 mmol (sample 9) resulted in the disappearance of the nanoparticles and the obtained hierarchical superstructures possess more complex structures (Fig. S5(c)†). Well-defined Ce-CP hierarchical superstructures were obtained at 0.2 mmol Ce³⁺ (sample 1). On the basis of the above results, we would like to point out that the concentration of reactants also plays a crucial role in determining the morphology of the products, and a high concentration of reactants favors the formation of pure and high yield Ce-CP hierarchical superstructures in a definite reaction time, and the typical Ce-CP hierarchical superstructures could be selectively obtained at a high concentration of reactants (sample 1).

It was found that the solvent composition also had a great effect on the morphology of the final product. Considering the poor solubility of 2,5-H₂pdc in ethanol, pure ethanol as the solvent could not be used in this experiment. When 12 mL of dmf and 20 mL of ethanol were used as the solvent media (sample 10), the product was mainly composed of typical hierarchical superstructures and some detached branches were also found in the product (Fig. S6(a)†). When the solvent was composed of 22 mL of dmf and 10 mL of ethanol (sample 1), well-defined Ce-CP hierarchical superstructures were obtained on a large scale (Fig. 1). No hierarchical superstructures were observable and only uniform spheres with diameters of about 1 μm were obtained (Fig. S6(b)†) when pure dmf was used as the solvent (sample 11). The results further confirm that the composition of the solvent is also an important factor to control morphological evolution of the as-prepared products.

In addition, time-dependent experiments were also conducted in this work. When the reaction time was 1 h (sample 12), a large quantity of uniform nanospheres with the diameters of about 500 nm were obtained (Fig. 2(a)). After 80 min (sample 13) of reaction, the nanospheres began to agglomerate and produced microstructured spheroids of about 20 μm in diameter (Fig. 2(b)). At the same time, some nanospheres adhering to the surface of spheroids could be found, which proved that these spheroid particles were composed of nanospheres. When the reaction time was prolonged to 1.5 h (sample 14), the product was mainly composed of poorly-formed hierarchical superstructures, representing the gradually extended 3D hierarchical superstructures (Fig. 2(c)). By extending the reaction time to 12 h (sample 1), hierarchical superstructures with the length of dendritic structures of 10–50 μm self-assembled by nanoparticles were obtained (Fig. 1). After further prolonging the reaction time to 24 h (sample 15), the obtained hierarchical superstructures became more complex and possessed a broad size distribution (Fig. 2(d)). These images clearly reveal that the Ce-CP hierarchical superstructures are composed of nanospheres and the size of the Ce-CP hierarchical superstructures increases by extending the reaction time.

Fig. 2 SEM images of the products obtained at 160 °C with different times (a) 1 h (sample 12), (b) 80 min (sample 13), (c) 1.5 h (sample 14), and (d) 24 h (sample 15), respectively.

Possible formation mechanism of the Ce-CP hierarchical superstructures

In order to explore the formation mechanism, we examined thoroughly the as-prepared products prepared at different reaction times ranging from 60 min to 12 h while maintaining other reaction conditions the same by XRD (Fig. S7†). It can be clearly seen that the peak intensity increases with increasing reaction time. After reaction for 80 min, the XRD patterns become much stronger and narrower, indicating that the amorphous aggregates have been transformed into crystalline solids, which are basically in agreement with the observation of SEM images.

The generation of branched or dendritic structures is always the result of kinetically and/or thermodynamically controlling the original nucleating stage and succeeding the crystal growth stage through changing the reaction parameters.⁴⁴ Although the exact mechanism of the assembled hierarchical structure is unclear, on the basis of the SEM observation, we consider that the formation of the porous Ce-CP hierarchical superstructures can be simply described as a self-assembly growth mechanism.^7b,18 A possible formation mechanism is illustrated in Scheme 1. At first, the Ce³⁺ reacts with 2,5-H₂pdc to produce primary nanospheres driven by the coordination force. Subsequently, the newly formed nanospheres grow and aggregate into microstructured spheroids driven by the minimization of interfacial energy. Furthermore, with continuous growth, the freshly formed spheroids are not stable, and they transform into multiple dendritic structures through a self-assembly process. Finally, the hierarchical superstructures were obtained via the further self-assembly process under mixed-solvothermal treatment.

Scheme 1 Schematic illustration of the formation of the Ce-CP hierarchical superstructures.

Crystal phase, morphology and properties of the as-obtained CeO₂

It has been demonstrated that micro/nanostructured CP could be used as the precursor for the preparation of the corresponding metal oxides since their organic components of the coordination polymers have been readily removed by thermal decomposition.^39,45Fig. 3 shows the XRD patterns of the Ce-CP hierarchical superstructures heat-treated at the temperature of 450–600 °C. The diffraction patterns exhibit the samples crystallized in the face-centered cubic pure phase (space group: Fm3m(225)) of CeO₂ (JCPDS card No. 34-0394). No signals of impurities are detected, implying that the Ce-CP has been decomposed into CeO₂ completely. In addition, it can be seen that the intensity of the diffraction peak increases and the half width of the diffraction peak decreases with the heat-treatment temperature, which indicates that the crystallite size of CeO₂ grain increases with the heat-treatment temperature. Furthermore, the broadening of the peaks indicates that the as-obtained CeO₂ hierarchical structures are composed of primary small crystal particles.³²

Fig. 3 XRD patterns of the Ce-CP hierarchical superstructures heat-treated at (a) 450 °C, (b) 500 °C, (c) 600 °C for 4 h with a heating rate of 0.5 °C min⁻¹, and the reference data of JCPDS card No. 34-0394 for pure CeO₂.

The morphology and architectures of the as-obtained CeO₂ were investigated by SEM and HTEM. As shown in Fig. 4(a), the SEM image reveals that most of the products inherit the morphology of their precursor. In comparison with the precursor, the shrinkage length and rough surface of CeO₂ (Fig. 4(b)) may be caused by the escape of H₂O and gas molecules from the surface during the thermal decomposition–oxidation to the CeO₂ process. The HRTEM images (Fig. 5a–c) clearly show that the CeO₂ consists of quantities of ceria nanocrystals which pack together in different directions to form the mesopores (Fig. 5a), and this is confirmed by N₂ adsorption–desorption isotherms (discussed later). The lattice fringes are clearly visible with a spacing of 0.31 nm and 0.27 nm, corresponding to the spacing of the (111) and (200) planes of cubic ceria. The SAED ring pattern (Fig. 5d) further indicates that the structure of the obtained product is a face-centered cubic CeO₂. The N₂ adsorption–desorption isotherms (Fig. 6) were used to characterize the porosity of the as-obtained CeO₂ after pretreatment at 150 °C for 4 h under vacuum conditions. It is found that the CeO₂ exhibits typical IV characteristics with an obvious hysteresis loop, suggesting that there are mesopores in CeO₂.⁴⁶ The specific surface area of the as-obtained CeO₂ calculated by the Brunauer–Emmett–Teller (BET) method is 35.9 m² g⁻¹, which is higher than that of commercial CeO₂ powder (a surface area of ca. 10.2 m² g⁻¹). The pore size distribution was calculated from the adsorption branch through the Barett–Joyner–Halenda (BJH) method and is shown in the inset of Fig. 6, which indicates that the pore distribution is in a wide range and the average pore diameter of the CeO₂ is about 4.89 nm.^45a Furthermore, no diffraction peaks could be observed in the small-angle XRD pattern of the as-prepared CeO₂ (Fig. S8†), suggesting that the mesopores are not periodically organized, which is in good accordance with the aforementioned HRTEM images. The Raman spectra of the as-obtained CeO₂ under ambient conditions are shown in Fig. S9.† The 464 cm⁻¹ band is attributed to the Raman-active symmetry vibrational mode (F_2g) of the CeO₂ fluorite-type structure, a weak band between 540 cm⁻¹ and 600 cm⁻¹ could prove the existence of oxygen vacancies.⁴⁷

Fig. 4 SEM images of the as-obtained CeO₂ after thermal decomposition of the Ce-CP hierarchical superstructures in air at 450 °C for 4 h.

Fig. 5 (a–c) HRTEM images and (d) selected-area electron diffraction (SAED) patterns of the as-obtained CeO₂.

Fig. 6 N₂ adsorption–desorption isotherms of the as-obtained CeO₂; the inset is the corresponding BJH pore size distribution curve.

To further characterize the product, XPS was conducted to further investigate the surface composition and chemical state of the as-obtained CeO₂. The wide-survey XPS spectrum of the as-obtained CeO₂ (Fig. 7(a)) reveals the predominant presence of Ce, O signals on the surface of the sample, which indicates the high purity of the as-obtained CeO₂ sample. The minor amount of the carbon element with a binding energy at 285.98 eV arises from the adventitious carbon. Fig. 7(b) displays the Ce 3d core level XPS spectrum of the as-obtained CeO₂, which consists of two series of V and U peaks corresponding to 3d_5/2 and 3d_3/2 states, respectively, consistent with the previous report of Ce⁴⁺ states. For 3d_5/2 of Ce(IV), the peaks of V and V′ can be attributed to a mixing of the 3d⁹4f²(O 2p⁴) and 3d⁹4f¹(O 2p⁵)Ce⁴⁺ states, and V′′ to the 3d⁹4f⁰(O 2p⁶)Ce⁴⁺ state. The origin of the U structures, due to the Ce 3d_3/2 level, can be carried out in the same way.⁴⁸ It is clear that the valley between V and V′ is assigned to the 3d⁹4f¹(O 2p⁶)Ce³⁺ final state, which indicates that a small quantity of Ce³⁺ is also present in the sample.²⁸ One can determine the relative amount of Ce³⁺ from the ratio of relative peak areas of Ce³⁺/total area.⁴⁹ However, it is difficult to obtain a definite value due to the complex backgrounds. Fig. 7(c) shows the O 1s core level XPS spectrum of the as-obtained CeO₂, which can be deconvolved into two peaks. The peak at 530.78 eV could be assigned to the absorbed oxygen, and another peak located at a lower BE (529.18 eV) originated from the lattice oxygen.⁵⁰

Fig. 7 (a) XPS wide-survey spectrum, (b) Ce 3d core level XPS spectrum and (c) O 1s core level XPS spectrum of the as-obtained CeO₂.

As is well known, CeO₂ can be used as a suitable ultraviolet-blocking material arising from the charge transfer transitions from O²⁻ in O 2p to Ce⁴⁺ in the Ce 4f state. The UV-vis absorption spectrum of the obtained CeO₂ sample is shown in Fig. 8. As we can see from the spectrum, the CeO₂ sample has a strong absorption in the UV region. The optical band gap E_g can be calculated based on the following equation: E_g = 1240/λ_AE, where λ_AE represents the edge wavelength of absorbance. The one set of absorption for our prepared CeO₂ is at 469 nm, corresponding to the band gap energy (E_g) of 2.64 eV, which is fairly lower than that of bulk ceria (E_g = 2.82 eV). In general, reducing the crystalline size would lead to the increase in the band gap width owing to the quantum size effect.⁵¹ However, our work is not in agreement with this viewpoint, and a similar result has also been observed in 3D hierarchical flowerlike CeO₂ microspheres reported by Lu and co-workers.⁵² There are two plausible theories for the expatiation of the final moderate red-shift of the CeO₂. The existence of the quantum confinement effect due to the nanoscale size of the primary particles forming the mesoporous CeO₂ with hierarchical structures resulted in a red-shift in the UV-vis spectrum.⁵³ On the other hand, ceria possesses intrinsic defects (Ce³⁺ and O vacancy concentration) in the fluorite structure, which generally introduces intermediate energy levels, resulting in a decrease in the band gap. Therefore, it can be speculated that a decrease in the band gap for the sample indicates that there has been more Ce³⁺ and O vacancy concentrations in mesoporous CeO₂.⁵²

Fig. 8 UV-vis spectrum of the as-obtained CeO₂.

To evaluate the photocatalytic activity of the as-obtained CeO₂, the photodegradation of RhB was carried out. Fig. 9(a) shows the visible spectral changes during the photocatalytic degradation of RhB in the presence of CeO₂ under UV light irradiation. The characteristic absorption of RhB at 554 nm was used to monitor the degradation process as a function of irradiation time. The change in the absorption of RhB aqueous solution represented the change in the concentration. It can be seen that the characteristic adsorption peak of RhB diminished quickly with the increase in irradiation time in the degradation process and almost disappeared after 5 h. The degradation rate (%) of RhB is defined as (C₀ − C)/C₀ × 100%, where C and C₀ represent the remnant and initial concentration of RhB, respectively. Fig. 9(b) displays the degradation rate of RhB on the two catalysts under UV light irradiation as well as the results of blank tests. After 5 h of illumination, compared to the blank experiment without a photocatalyst, the addition of CeO₂ catalysts obviously promoted the degradation of RhB. The photocatalytic degradation rate of RhB solution for the as-obtained CeO₂ reaches 99.6%, while the commercial CeO₂ powder is only 32.0%. These results suggest that the as-obtained CeO₂ exhibits good photocatalytic activity. The superior photocatalytic performances of CeO₂ can be attributed to two factors. One is that the as-obtained mesoporous CeO₂ possesses higher surface areas and facilitates the adsorption of the RhB molecules, which would greatly improve the catalytic efficiency.¹⁸ The other is assigned to the oxygen vacancies on the surface of mesoporous CeO₂, as revealed by XPS analysis mentioned above. These abundant surface oxygen vacancies can bear a negative charge, and thus they can conveniently combine with the cationic groups of RhB via strong electrostatic attraction to form V_o⁻⁺N (V_o, oxygen vacancy; ⁺N, the charged N in RhB) as well as hydrogen bonding with the nitrogen atoms of RhB.⁵⁴

Fig. 9 (a) Temporal UV-vis spectroscopic changes of the RhB aqueous solution in the presence of the as-obtained CeO₂ under exposure to UV light, (b) degradation rate of RhB in the dark under UV light: (blue line) the as-obtained CeO₂, (red line) commercial CeO₂, and (black line) without a photocatalyst.

Conclusions

In summary, we have successfully synthesized Ce-CP hierarchical superstructures by a simple one-pot mixed-solvothermal method without any additional template or surfactant. The results demonstrated that it is possible to control the size and morphology of the Ce-CP hierarchical superstructures by adjusting process parameters such as reaction temperature, reaction time, total concentrations, and solvent composition of the reactants. A possible growth mechanism called self-assembly has been proposed to explain the formation of the Ce-CP hierarchical superstructures, as shown by SEM and XRD investigations. Besides, CeO₂ with similar hierarchical structures was successfully synthesized by thermal conversion of the Ce-CP under air at 450 °C for 4 h. In addition, the as-obtained CeO₂ exhibits high activity towards the photocatalytic degradation for organic pollutants (RhB) under a high-pressure mercury lamp. Thus, the as-obtained mesoporous CeO₂ products may have potential application in eliminating organic pollutants from wastewater.

Acknowledgements

S. L. Zhong acknowledges the supporting projects from the National Natural Science Foundation of China (No. 21201089, 21261010, 61201104), Jiangxi Provincial Education Department (no. KJLD13021) and Jiangxi Provincial Department of Science and Technology (no. 20144BCB23039).

Notes and references

1. J. M. Amigo, T. Skov and R. Bro, Chem. Rev., 2010, 110, 4582–4605.
2. M. P. Suh, H. J. Park, T. K. Prasad and D.-W. Lim, Chem. Rev., 2012, 112, 782–835.
3. D. Liu, R. C. Huxford and W. Lin, Angew. Chem., Int. Ed., 2011, 50, 3696–3700.
4. F. Ke, Y.-P. Yuan, L.-G. Qiu, Y.-H. Shen, A.-J. Xie, J.-F. Zhu, X.-Y. Tian and L.-D. Zhang, J. Mater. Chem., 2011, 21, 3843–3848.
5. J. Huo, L. Wang, E. Irran, H. Yu, J. Gao, D. Fan, B. Li, J. Wang, W. Ding, A. M. Amin, C. Li and L. Ma, Angew. Chem., Int. Ed., 2010, 49, 9237–9241.
6. S. Jung, W. Cho, H. J. Lee and M. Oh, Angew. Chem., Int. Ed., 2009, 48, 1459–1462.
7. (a) S.-L. Zhong, R. Xu, L.-F. Zhang, W.-G. Qu, G.-Q. Gao, X.-L. Wu and A.-W. Xu, J. Mater. Chem., 2011, 21, 16574–16580; (b) S. Zhong, M. Wang, L. Wang, Y. Li, H. M. Noh and J. H. Jeong, CrystEngComm, 2014, 16, 231–236; (c) D. Zhao, L. Wang, Y. Li, L. Zhang, Y. Lv and S. Zhong, Inorg. Chem. Commun., 2012, 20, 97–100; (d) L. Luo, R. Xu, D. Zhao and S. Zhong, J. Rare Earths, 2010, 28, 106–110; (e) L. Wang, D. Zhao, S.-L. Zhong and A.-W. Xu, CrystEngComm, 2012, 14, 6875–6880; (f) S. Zhong, H. Jing, Y. Li, S. Yin, C. Zeng and L. Wang, Inorg. Chem., 2014, 53, 8278–8286.
8. (a) W. Lin, W. J. Rieter and K. M. L. Taylor, Angew. Chem., Int. Ed., 2009, 48, 650–658; (b) W. Cho, H. J. Lee and M. Oh, J. Am. Chem. Soc., 2008, 130, 16943–16946; (c) F.-X. Bu, M. Hu, L. Xu, Q. Meng, G.-Y. Mao, D.-M. Jiang and J.-S. Jiang, Chem. Commun., 2014, 50, 8543–8546; (d) S. Jung and M. Oh, Angew. Chem., Int. Ed., 2008, 47, 2049–2051.
9. (a) B. Lim and Y. Xia, Angew. Chem., Int. Ed., 2011, 50, 76–85; (b) V. Polshettiwar, M. N. Nadagouda and R. S. Varma, Chem. Commun., 2008, 6318–6320; (c) M. Cao, T. Liu, S. Gao, G. Sun, X. Wu, C. Hu and Z. L. Wang, Angew. Chem., Int. Ed., 2005, 44, 4197–4201.
10. J. Zhang, Q. Xu, Z. Feng, M. Li and C. Li, Angew. Chem., Int. Ed., 2008, 47, 1766–1769.
11. I. Lisiecki, P.-A. Albouy and M.-P. Pileni, Adv. Mater., 2003, 15, 712–716.
12. S. Bharathi, D. Nataraj, M. Seetha, D. Mangalaraj, N. Ponpandian, Y. Masuda, K. Senthil and K. Yong, CrystEngComm, 2010, 12, 373–382.
13. Z. Jiang, Z. Li, Z. Qin, H. Sun, X. Jiao and D. Chen, Nanoscale, 2013, 5, 11770–11775.
14. M. Chen, Y. N. Kim, C. Li and S. O. Cho, Cryst. Growth Des., 2008, 8, 629–634.
15. P. Chen, L. Gu and X. Cao, CrystEngComm, 2010, 12, 3950–3958.
16. Z. Yu, Z. Yao, N. Zhang, Z. Wang, C. Li, X. Han, X. Wu and Z. Jiang, J. Mater. Chem. A, 2013, 1, 4571–4576.
17. G. Shen, D. Chen and C. J. Lee, J. Phys. Chem. B, 2006, 110, 15689–15693.
18. L.-P. Zhu, N.-C. Bing, L.-L. Wang, H.-Y. Jin, G.-H. Liao and L.-J. Wang, Dalton Trans., 2012, 41, 2959–2965.
19. H.-J. Kim, K.-I. Choi, A. Pan, I.-D. Kim, H.-R. Kim, K.-M. Kim, C. W. Na, G. Cao and J.-H. Lee, J. Mater. Chem., 2011, 21, 6549–6555.
20. S. Z. Li, H. Zhang, J. B. Wu, X. Y. Ma and D. R. Yang, Cryst. Growth Des., 2006, 6, 351–353.
21. K. Liu, H. You, G. Jia, Y. Zheng, Y. Huang, Y. Song, M. Yang, L. Zhang and H. Zhang, Cryst. Growth Des., 2010, 10, 790–797.
22. S. Carrettin, P. Concepción, A. Corma, J. M. L. Nieto and V. F. Puntes, Angew. Chem., Int. Ed., 2004, 43, 2538–2540.
23. B. P. Pearman, N. Mohajeri, R. P. Brooker, M. P. Rodgers, D. K. Slattery, M. D. Hampton, D. A. Cullen and S. Seal, J. Power Sources, 2013, 225, 75–83.
24. X. Jiao, H. Song, H. Zhao, W. Bai, L. Zhang and Y. Lv, Anal. Methods, 2012, 4, 3261–3267.
25. N. Izu, T. Uchida, I. Matsubara, T. Itoh, W. Shin and M. Nishibori, Mater. Res. Bull., 2011, 46, 1168–1176.
26. (a) D. Zhang, X. Du, L. Shi and R. Gao, Dalton Trans., 2012, 41, 14455–14475; (b) D. Zhang, F. Niu, T. Yan, L. Shi, X. Du and J. Fang, Appl. Surf. Sci., 2011, 257, 10161–10167; (c) J. Zhang, S. Ohara, M. Umetsu, T. Naka, Y. Hatakeyama and T. Adschiri, Adv. Mater., 2007, 19, 203–206; (d) L. Wang, L.-F. Zhang, S.-L. Zhong and A.-W. Xu, Appl. Surf. Sci., 2012, 263, 769–776.
27. H. Mai, D. Zhang, L. Shi, T. Yan and H. Li, Appl. Surf. Sci., 2011, 257, 7551–7559.
28. Z. Sun, H. Zhang, G. An, G. Yang and Z. Liu, J. Mater. Chem., 2010, 20, 1947–1952.
29. S. Yang and L. Gao, J. Am. Chem. Soc., 2006, 128, 9330–9331.
30. Z. Ji, X. Wang, H. Zhang, S. Lin, H. Meng, B. Sun, S. George, T. Xia, A. E. Nel and J. I. Zink, ACS Nano, 2012, 6, 5366–5380.
31. L.-S. Zhong, J.-S. Hu, A.-M. Cao, Q. Liu, W.-G. Song and L.-J. Wan, Chem. Mater., 2007, 19, 1648–1655.
32. W. Liu, L. Feng, C. Zhang, H. Yang, J. Guo, X. Liu, X. Zhang and Y. Yang, J. Mater. Chem. A, 2013, 1, 6942–6948.
33. M. Eddaoudi, H. Li and O. M. Yaghi, J. Am. Chem. Soc., 2000, 122, 1391–1397.
34. A. Querejeta- Fernández, J. C. Hernández-Garrido, H. Yang, Y. Zhou, A. Varela, M. Parras, J. J. Calvino- Gámez, J. M. González-Calbet, P. F. Green and N. A. Kotov, ACS Nano, 2012, 6, 3800–3812.
35. Z. Shen, S. He, P. Yao, X. Lao, B. Yang, Y. Dai, X. Sun and T. Chen, RSC Adv., 2014, 4, 12844–12848.
36. R. Kaminker, R. Popovitz-Biro and M. E. van der Boom, Angew. Chem., Int. Ed., 2011, 50, 3224–3226.
37. K. Liu, H. Liu, L.-L. Yang, F.-Y. Zhao, Y. Li and W.-J. Ruan, RSC Adv., 2014, 4, 25160–25164.
38. H. Qiao, Y. Jia, Y. Zheng, N. Guo, Q. Zhao, W. Lv and H. You, CrystEngComm, 2012, 14, 5830–5835.
39. Y. Zheng, K. Liu, H. Qiao, Y. Zhang, Y. Song, M. Yang, Y. Huang, N. Guo, Y. Jia and H. You, CrystEngComm, 2011, 13, 1786–1788.
40. P. Silva, A. A. Valente, J. Rocha and F. A. A. Paz, Cryst. Growth Des., 2010, 10, 2025–2028.
41. V. I. Isaeva, E. V. Belyaeva, A. N. Fitch, V. V. Chernyshev, S. N. Klyamkin and L. M. Kustov, Cryst. Growth Des., 2013, 13, 5305–5315.
42. W. Steele, Chem. Rev., 1993, 93, 2355–2378.
43. C. Li, J. Yang, P. Yang, H. Lian and J. Lin, Chem. Mater., 2008, 20, 4317–4326.
44. L.-W. Qian, X. Wang and H.-G. Zheng, Cryst. Growth Des., 2012, 12, 271–280.
45. (a) Z. Shen, J. Liu, F. Hu, S. Liu, N. Cao, Y. Sui, Q. Zeng and Y. Shen, CrystEngComm, 2014, 16, 3387–3394; (b) Z. Sharifzadeh, S. Abedi and A. Morsali, J. Mater. Chem. A, 2014, 2, 4803–4810; (c) Y. Lu, H. Cao, S. Zhang and X. Zhang, J. Mater. Chem., 2011, 21, 8633–8639; (d) H.-Y. Shi, B. Deng, S.-L. Zhong, L. Wang and A.-W. Xu, J. Mater. Chem., 2011, 21, 12309–12315; (e) M. Wang, F. Wang, J. Ma and J. Xu, J. Mater. Chem. A, 2014, 2, 15480–15487.
46. G. Zhang, Z. Shen, M. Liu, C. Guo, P. Sun, Z. Yuan, B. Li, D. Ding and T. Chen, J. Phys. Chem. B, 2006, 110, 25782–25790.
47. M. Guo, J. Lu, Y. Wu, Y. Wang and M. Luo, Langmuir, 2011, 27, 3872–3877.
48. P. Ji, J. Zhang, F. Chen and M. Anpo, J. Phys. Chem. C, 2008, 112, 17809–17813.
49. H. Borchert, Y. V. Frolova, V. V. Kaichev, I. P. Prosvirin, G. M. Alikina, A. I. Lukashevich, V. I. Zaikovskii, E. M. Moroz, S. N. Trukhan, V. P. Ivanov, E. A. Paukshtis, V. I. Bukhtiyarov and V. A. Sadykov, J. Phys. Chem. B, 2005, 109, 5728–5738.
50. H.-I. Chen and H.-Y. Chang, Solid State Commun., 2005, 133, 593–598.
51. N. K. Renuka, J. Alloys Compd., 2012, 513, 230–235.
52. J. Li, G. Lu, H. Li, Y. Wang, Y. Guo and Y. Guo, J. Colloid Interface Sci., 2011, 360, 93–99.
53. S. Yuan, Q. Zhang, B. Xu, Z. Jin, Y. Zhang, Y. Yang, M. Zhang and T. Ohno, RSC Adv., 2014, 4, 62255–62261.
54. H. Xiao, Z. Ai and L. Zhang, J. Phys. Chem. C, 2009, 113, 16625–16630.

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Footnote

† Electronic supplementary information (ESI) available: Additional graphs. See DOI: 10.1039/c5qi00016e

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