Synthesis and characterization of ceria-coated silica nanospheres: their application in heterogeneous catalysis of organic pollutants
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Τhe present work investigates the parameters for the successful coating of silicon oxide nanoparticles surface with a homogeneous cerium oxide shell in an effort to develop core–shell nanostructures. For this, spherical silica nanoparticles (~ 300 nm) were developed by a biomimetic approach and their coating with ceria was performed through a precipitation method. Several processing conditions, such as the precipitation pH, the cerium precursor concentration and the treatment of silica cores (i.e., their use either as-received after their biomimetic formation, or after their calcination followed by surface modification), were examined and optimized. The as-obtained powder material was characterized by scanning electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction analysis, N2 adsorption and UV–Vis reflectance spectroscopy. The analysis revealed the formation of spherical core–shell nanostructures bearing a uniform shell layer of crystalline cerium oxide around each silica core which after calcination at 600 °C was comprised of cubic CeO2 nanocrystals with sizes ranging between 2 and 6 nm. The material with the optimum core–shell structure and pure cerium dioxide were studied in terms of their catalytic reduction activity over 4-nitrophenol to 4-aminophenol.
KeywordsCore–shell Cerium oxide Silicon oxide Environmentally friendly procedure Catalytic reduction
Recent technological breakthroughs and the desire for new features have created a huge demand for new materials. Many of the usual materials, such as metals, ceramics or plastics, cannot fulfill all of the technology requirements for many novel and demanding applications. In recent years, the research for the preparation of core–shell nanoparticles with a well-defined structure has attracted considerable interest due to their excellent physical and chemical properties, such as optical, electrical, thermal, mechanical, magnetic and catalytic ones, compared to pure materials. [1, 2].
Nanomaterials consisting of a core and a shell are commonly referred as core–shell nanomaterials/nanoparticles (CSNs or CSNp) [3, 4]. Core–shell nanomaterials are extremely functional materials with modified properties . The majority of the CSNs are manufactured in order to combine two materials—and therefore two or more properties in one structure. Thereby, the new structure is characterized by the properties of both core and shell, giving a variety of new potentials and countless combinations . Their production can be realized either in a one-step process where the core particles are synthesized in situ, this being followed by the coating of the shell material, or in a two-step process. In the latter, the core is firstly synthesized and then the synthesis of the shell follows [5, 7] employing several synthetic procedures such as sol–gel , microemulsion , polymerization  or deposition–precipitation . By this way, various types of materials can be combined such as dielectric materials, metals and semiconductors, where the core consists of one of the materials and the shell of another or the same material [5, 12].
There are manifold purposes of coating the core particle, for instance, surface modification, the ability to increase the functionality, stability and dispersibility, the controlled release of the core, the reduction in consumption of precious materials and so forth [6, 13]. Apart from the improved material properties, core/shell materials are also significant from a financial viewpoint. For example, a precious material can be coated over an inexpensive material in order to reduce its consumption as opposed to making the same size structure composed of the pure precious material [4, 5, 14, 15].
Cerium dioxide or ceria (CeO2) is considered as a very useful material as it can be used in various applications such as polishing material  or solid oxide fuel cell electrolyte . Moreover, it is not toxic, a fact that makes it useful in optoelectronics and photocatalysis . Owing to the high oxygen storage capacity and the ability to stabilize the dispersed metal cation species, ceria has attracted researchers’ attention finding many applications in environmental catalysis . Environmental pollution is a major issue and has attracted the interest of researchers, primarily because of adverse effects on human health and all living organisms. Industry causes many environmental problems, releasing a wide range of toxic substances into the environment . Heterogeneous catalysis, as a means of anti-pollution, has also attracted much interest and has become an emergent technology approach for a sustainable environment, such as the removal of organic pollutants from aqueous effluents .
It is well known that size, shape, surface state and crystallinity are crucial factors for the catalytic performance of nanomaterials. Due to their tendency for aggregation during the catalytic process, their catalytic activity and stability are declined .
Therefore, different approaches have been investigated to control these properties. Conventional production methods such as precipitation and sol–gel usually result in large particles (about 50 nm), whereas the thermal degradation of cerium salts leads to particles of high porosity and specific surface area, but the control of particle size and morphology is very limited . Many researchers wishing to overcome these problems have developed various methods of preparing silica–ceria core–shell particles as silica can offer many advantages such as good dispersion, narrow size distribution and controlled particle size [24, 25, 26, 27, 28, 29]. However, in previous studies , the final particles showed intense agglomeration due to the formation of hydrogen bonds because of the presence of water during the precipitation process. In addition, the resulting suspension contained both coated silica particles and pure ceria particles. Thus, the development of core–shell structures which leads to good dispersion, narrow size distribution and controlled particle size will favor the catalytic process.
In the present study, the main objective is to develop a core–shell material comprising a uniform nano-CeO2 shell onto silica cores produced by an environmentally friendly method, as well as to reduce the cost of the final material as cerium oxide is an expensive material. The experimental procedure took place at ambient temperature, while the neutralizing agent for the precipitation of cerium oxide on the silica surface was a polymer–water solution. Finally, the optimum core–shell structure and the pure CeO2 were studied for their catalytic properties by their ability to catalyze the reduction of 4-nitrophenol to 4-aminophenol in the presence of sodium borohydride. The results showed that the development of core–shell structure affects the catalytic performance of CeO2.
2 Materials and methods
Tetraethyl orthosilicate (TEOS, 98%) was used as a precursor for the silica nanoparticles, and hyperbranched poly(ethylene)imine was used to modify the surface of the silica with the amine groups. Cerium (III) nitrate hexahydrate (99%) was used as a precursor for the ceria shell layer. 4-Nitrophenol (Fluka) was used as the organic pollutant, and sodium borohydrate (NaBH4, 99%) was used as the reducing agent. All materials, except 4-nitrophenol, were purchased from Sigma-Aldrich and used without further purification.
2.2 Synthesis of the core–shell nanoparticles
Firstly, silica cores were prepared through the hydrolysis–condensation reaction of silicic acid in the presence of hyperbranched poly(ethylene)imine . To create the core–shell structure, the silica cores were calcined at 800 °C for 3 h under airflow and then they were redispersed in water.
Codes and w/w ratios of the precursors for the final synthesized materials
w/w Ce(NO3)3·6H2O/SiO2/ΗΒΡΕΙ ratio
2.3 Characterization techniques
The morphology of the core–shell structures as well as of pure SiO2 and CeO2 was examined via transmission electron microscopy (TEM) with a high-resolution transmission electron microscope (HRTEM, JEOL 2100 HR) which is equipped with a JED-2300T energy-dispersive X-ray spectrometer and a digital scanning image observation device (STEM, JEOL EM 24511 SIOD). Scanning electron microscopy (SEM) images were recorded with JEOL 6380LV. X-ray diffraction (XRD) patterns were obtained on a diffractometer (Bruker D8 Focus) with nickel-filtered CuKa radiation (k = 1.5406 Å, 40 kV, 40 mA), in the range of 10–80°. FTIR spectroscopy was also conducted using a Nicolet Magna-IR spectrometer 550. N2 adsorption/desorption measurements were taken using a Micromeritics ASAP 2000 apparatus, and the surface area was calculated by the BET equation. A UV–Vis spectrophotometer (Hitachi U-3010) was used in order to obtain diffuse reflectance spectra.
2.3.1 4NP reduction catalysis
The catalytic properties of the material with the optimum core–shell structure and of pure CeO2 were examined through the reduction of 4-nitrophenol (Fluka) to 4-aminophenol in the presence of NaBH4 (sodium borohydrate 99%, Sigma-Aldrich) as the reductant under room temperature. In aqueous solutions of 4NP in a concentration of 4 ppm, 1.6 g/l NaBH4 and 0.2 g/l of catalyst were added. The progress of the reaction was monitored using UV–Vis spectroscopy (UV–Vis Spectrometer, Cary 100). The samples were filtered and measured in the range of 200–800 nm at different times.
3 Results and discussion
3.1 Silica–ceria core–shell nanoparticles
The preparation of core–shell nanoparticles is a very difficult process as various difficulties are encountered, namely the agglomeration of the cores, the tendency to create separate particles from the coating material instead of coating the cores, the incomplete coating and the difficult control of the reaction degree .
Firstly, an attempt was made for biomimetic synthesis, where the cerium precursor was added to the silica nanospheres without being calcined and chemically surface-modified. However, inhomogeneities in the final coating as well as the creation of independent ceria nanoparticles were observed (Fig. S1). A pH study was also conducted, using phosphate buffer at various pH values but without leading to the desired result (Fig. S2). For the above reasons, the nanospheres were calcined in order to remove any free amino groups from the surface and then they were subjected to surface chemical modification using HBPEI. In order to simplify the process as well as to minimize the use of harmful substances, the process was carried out in an aqueous medium and at ambient temperature. In similar studies [26, 27, 32, 33], part or the whole reaction was carried out at higher temperature (70–100 °C) as well as with the use of toxic agents such as nitric acid or organic solvents.
3.2 Morphology of silica cores, cerium oxide and silica–ceria nanoparticles
Comparing the pure ceria material to that formed on silica cores, it should be pointed out that the ceria of the shell material presents a significantly reduced particle size. In particular, after calcination at 600 °C, the size of cerium oxide in the core–shell material varies between 2 and 6 nm, whereas that of the pure cerium oxide was 10–20 nm. The significantly reduced size was achieved possibly due to two key reasons. Firstly, the presence of the SiO2/HBPEI cores facilitates the nucleation rate. Secondly, the good dispersion of the CeO2 particles on the silica cores leads to the reduction in the grain growth during the final calcination process.
% CeO2 content of each silica–ceria core shell structure
2.2 ± 0.5
3.3 ± 0.5
5.0 ± 1.0
3.3 XRD measurements
3.4 FTIR analysis
3.5 N2 adsorption analysis
3.6 UV–Vis measurements
The absorption spectrum of cerium dioxide, as well as the value of the energy band gap Ebg = 2.53 eV, coincides with the literature data on doped CeO2  as the usual Ebg value for ceria is 3.1–3.3 eV [50, 51, 52]. Therefore, it can be concluded that the experimental process described above leads to the preparation of CeO2 with a sufficiently reduced energy band gap. As it is evident and consistent with the leterature [53, 54], the reflection spectrum for pure SiO2, given in Fig. 8a, exhibits negligible absorption only in the ultraviolet region. Instead, the pure CeO2 as well as the core–shell material ScP33%CeP absorbs in both the visible and ultraviolent regions. The measurement of the energy gap of SiO2 material due to the very low absorption was not reliable, but in the literature it is given at around 9 eV . Comparing the spectra and the energy gap values of pure CeO2 and the core–shell material ScP33%CeP, a change in absorbance after the creation of the new core–shell structure is shown. In particular, the absorption spectrum of the core–shell structure is shifted to smaller wavelengths, while it exhibits a higher reflectance (Fig. 8a). The core–shell material has a higher energy gap value of 3.01 eV than that of pure CeO2, which is could be attributed to the smaller particle size of CeO2  in the core–shell structure, as it was observed by TEM analysis.
4 Catalytic activity of ScP33%CeP and CeO2 for 4NP reduction
The concentration of NaBH4 exceeded the concentration of 4-NP. The high concentration of the reducing agent results in high pH values and as a consequence in deceleration of the degradation of the borohydride ions. In addition, the oxidation of the 4-aminophenol product is prevented due to the presence of free hydrogen from the borohydride ions which purged out the air. After the addition of the catalyst, NaBH4 and 4-NP are absorbed onto the catalysts surface and the reduction process is started by transferring electrons from the BH4− donor to the 4NP acceptor. At this point, it must be pointed out that in the absence of catalyst the reaction was not taken place even after 24 h [39, 57, 58, 59]. Consequently, the presence of the catalytic nanoreactor is crucial.
The catalytic reduction of 4NP to 4-AP in excess of NaBH4 is commonly described by a first-order rate kinetic equation with respect to nitrophenolate ion concentration. Hence, the apparent rate constant (k) for each material was estimated from the slope of the straight lines in Fig. 10d [57, 58, 59]. The degradation rate for the core–shell structure was estimated at 0.003 min−1, whereas that for the pure cerium oxide was much lower (0.0002 min−1) in accordance with the conversion rates discussed above. Furthermore, in both cases, an induction time (t0) for the first 15 min is observed, indicating the surface atoms’ of the nanoparticle reorganization in order to be activated before the beginning of the reaction .
% Conversion and the respective rate constants of the catalytic systems
Catalyst concentration (g/l)
k × 10−2 (min−1)
k × 10−2 (min−1)
From the above results, it is evident that the development of a new SiO2@CeO2 core–shell structure promotes the catalytic activity of ceria. Using only a small amount of CeO2 as a shell on silica cores, the catalytic activity of the final material is five times higher than that of pure CeO2.
Uniform SiO2@CeO2 core–shell nanoparticles were prepared through an environmentally friendly chemical precipitation method. The whole process was carried out with the objective to establish an economical and environmentally friendly process, so the temperature used was that of the environment, while only distilled water was employed as solvent. Important parameters proved the calcination and surface modification of silica cores by HBPEI, the w/w Ce(NO3)3·6H2O/SiO2/ΗΒΡΕΙ ratio employed (with the optimum one being at 0.33:1) and the CeO2 precipitation pH which was adjusted again by HBPEI. By this way, homogeneous, spherical SiO2@CeO2 core–shell nanoparticles of a mean size at 300 nm comprising a uniform ceria layer were developed. The CeO2 shell on top of SiO2 cores had a very fine nanostructure consisted of nanocrystals of the cubic CeO2 with sizes ranging between 2 and 6 nm. Measurements on pure ceria particles (not on top of silica cores) developed using the same procedure revealed significantly higher particle size, about 10–20 nm, compared to that of ceria particles in the core–shell material. The N2 porosimetry study showed that the material with the optimum coating had a reduced specific surface area compared to pure cerium dioxide, i.e., 27.5 m2/g versus 48.7 m2/g due to the much smaller amount of ceria particles impacted on much larger silica cores. The catalytic activity of the core–shell structure over 4-nitrophenol was found at 47.8% conversion, while that of the pure CeO2 was 8.5%, indicating the high efficacy of the core–shell structure and the good dispersion of CeO2 particles on the silica cores.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- 14.Kalele S, Gosavi SW, Urban J, Kulkarni SK (2006) Nanoshell particles: synthesis, properties and applications. Curr Sci 91:1038–1052Google Scholar
- 34.Hamlaoui Y, Pedraza F, Remazeilles C, Cohendoz S, Rébéré C, Tifouti L, Creus J (2009) Cathodic electrodeposition of cerium-based oxides on carbon steel from concentrated cerium nitrate solutions. Part I. Electrochemical and analytical characterization. Mater Chem Phys 113:650–657CrossRefGoogle Scholar
- 40.Motahari S, Abolghasemi A (2015) Silica aerogel–glass fiber composites as fire shield for steel frame structures. J Mater Civ Eng 27:04015008/1-7Google Scholar
- 45.Gregg SJ (1982) Adsorption surface area and porosity, 2nd edn. Academic Press Inc., LondonGoogle Scholar
- 46.Kubelka P, Munk F (1931) An article on optics of paint layers. Z Tech Phys 12:593–608Google Scholar
- 47.Morales AE, Mora ES, Pal U (2007) Use of diffuse reflectance spectroscopy for optical characterization of un-supported nanostructures. Rev Mexikana Fis 53:18–22Google Scholar
- 51.Babitha KK, Sreedevi A, Priyanka KP, Sabu B, Varghese T (2015) Structural characterization and optical studies of CeO2 nanoparticles synthesized by chemical precipitation. Indian J Pure Appl Phys 53:596–603Google Scholar