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Development of SiO2@TiO2 core-shell nanospheres for catalytic applications
Accepted Manuscript Full Length Article Development of SiO2@TiO2 core-shell nanospheres for catalytic applications I. Kitsou, P. Panagopoulos, Th. Maggos, M. Arkas, A. Tsetsekou PII: DOI: Reference:
S0169-4332(18)30351-9 https://doi.org/10.1016/j.apsusc.2018.02.008 APSUSC 38466
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
4 January 2018 26 January 2018 1 February 2018
Please cite this article as: I. Kitsou, P. Panagopoulos, Th. Maggos, M. Arkas, A. Tsetsekou, Development of SiO2@TiO2 core-shell nanospheres for catalytic applications, Applied Surface Science (2018), doi: https://doi.org/ 10.1016/j.apsusc.2018.02.008
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Development of SiO2@TiO2 core-shell nanospheres for catalytic applications I. Kitsou1, P. Panagopoulos2, Th. Maggos2, M. Arkas3, A. Tsetsekou1* 1
School of Mining and Metallurgical Engineering, National Technical University of Athens, 157 80 Zografos, Athens, Greece
2
Institute of Nuclear & Radiological Sciences & Technology, Energy & Safety, NCSR “Demokritos”, 15310 Agia Paraskevi, Attiki, Greece 3
Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, 15310 Aghia Paraskevi, Attiki, Greece
*e-mail: [email protected], Tel.: +30 210 7722213, Fax.: +30 210 772119
Abstract Silica-titania core-shell nanospheres, CSNp, were prepared via a simple and environmentally friendly two step route. First, silica cores were prepared through the hydrolysis-condensation reaction of silicic acid in the presence of hyperbranched poly(ethylene)imine (HBPEI) followed by repeating washing, centrifugation and, finally, calcination steps. To create the core-shell structure, various amounts of titanium isopropoxide were added to the cores and after that a HBPEI-water solution was added to hydrolyze the titanium precursor. Washing with ethanol and heat treatment followed. The optimization of processing parameters led to well-developed core-shell structures bearing a homogeneous nanocrystalline anatase coating over each silica core. The photocatalytic activity for NO was examined in a continuous flux photocatalytic reactor under real environmental conditions. The results revealed a very potent photocatalyst as the degradation percentage reached 84.27 % for the core-shell material compared to the 82 % of pure titania with the photodecomposition rates measured at 0.62 and 0.55 μg·m-2·s-1, respectively. In addition, catalytic activities of the CSNp and pure titania were investigated by monitoring the reduction of 4-nitrophenol to 4-aminophenol by an excess of NaBH4. Both materials exhibited excellent catalytic activity (100 %), making the core-shell material a promising alternative catalyst to pure titania for various applications.
Keywords: Core-shell; Titanium dioxide; Silicon dioxide; Nitrogen oxides; 4nitrophenol
1. Introduction Among the various photocatalysts, titania has high potential to reduce the environmental pollution through decomposing many inorganic and organic pollutants, hence it is the most used one [1-4]. Its photocatalytic properties stem from the semiconductor bandgap effects occurring when it is irradiated with ultraviolet light [5-6]. Consequently, holes and electrons are photogenerated and then diffuse to the surface, where oxidation and reduction phenomena take place. It is well known that the photocatalysis procedure depends on the physical properties of the material such as morphology, surface area, particle size and crystalline phase of titania, with anatase being the most promising active phase of TiO2 [5,7]. However, titania, in powder form, is thermally unstable and as a solution, it dissolves and loses its surface area, making the recovery of the catalyst almost impossible [8-9]. In addition, it has been reported that titania particles, in the scale of only few nanometers, can have adverse effects on health [10-11]. To overcome these problems, many researchers, focus on the production of coreshell nanoparticles with titania as shell. Core-shell nanoparticles have many advantages, as they can combine the properties of both the core and the shell or they can give more effective properties than those of the pure materials used as core and shell [12-13]. Changing the size of the core and the thickness of the shell leads to the production of modified materials which can be used in different ways [12,14]. In addition, they can be also employed to reduce the cost of an expensive material as only a small amount of the expensive material can be used to cover a cheap core [12-13, 15-16]. Among the various cores that have been used, such as Au [17], ZnO [18], CeO2 [19], Fe3O4 [20], silica has found to be the most effective, due to its special characteristics: well-known surface chemistry and adsorption capacity, thermal and mechanical stability as well as low cost and optical transparency in the wavelength region where titanium dioxide absorbs [1]. Furthermore, the low toxicity of silica is beneficial in many applications [21-22]. Last but not least, silica can be easily removed in alkaline conditions in order to produce hollow spheres of the shell material with enhanced properties [1]. Many researchers have focused on the synthesis of core shell nanomaterials with silica as a core and titania as a shell for many applications [9, 23-26]. In order to overcome several problems when pure titania is used as a photocatalyst, such as high degree of aggregation, phase transformation etc, Kamaruddin and Stephan [1] produced silica-titania core shell nanoparticles and they investigated the photocatalytic activity of this material as an alternative to pure titania over the organic pollutant methylene blue. More than a few
methods are known to prepare SiO2@TiO2 core shell particles. Kalele et al. [27] synthesized SiO2@TiO2 core shell particles by hydrolysis and condensation of titanium butoxide on the silica cores. Hu et al. [28] produced core–shell particles by a flame aerosol process. The photocatalytic activity of SiO2@TiO2 core shell nanoparticles over the inorganic air pollutant NOx and the organic water pollutant 4-nitrophenol has not been investigated in depth yet. As far as NOx photocatalysis is concerned, only a few studies have been carried out, the majority of which are referred to mixed oxides of SiO2 and TiO2 [29-32]. Signoretto et al. [30] produced titania nanoparticles supported on mesoporous micrometer scale silica via an incipient wetness impregnation method and they studied their photocatalytic activity. The production of mesoporous silica [33] took place through a time-consuming procedure at elevated temperature. Subsequently, the titanium was added by the incipient wetness impregnation method using an alcoholic solution. The study of photocatalytic activity showed that there is a critical limit of addition of titanium beyond which the photocatalytic activity decreases. The material with 16 % wt titanium had the maximum photocatalytic yield, which was recorded at 60 %. In the majority of the research works that have been carried out for the production of core-shell nanoparticles, either toxic solvents have been used or increased temperature has been required. In the present study, aiming at simplifying the procedures and avoiding the use of toxic solvents, the entire process took place at ambient temperature and in aqueous solutions. By optimizing the required amount of titanium precursor, core-shell particles bearing a homogeneous anatase shell were developed which showed excellent photocatalytic efficiency for the air pollutant NOx degradation and the organic pollutant 4nitrophenol (4NP) reduction.
2. Materials and methods
2.1 Synthesis The silica nanospheres were prepared through a biomimetic precipitation process [34]. Briefly, a solution of 20 mM hyperbranched poly(ethylene)imine (HBPEI, Mn=25,000, Sigma-Aldrich) - phosphate buffer (p.b, 100 mM) was prepared. To the above solution, an appropriate amount of silicic acid (1 M) was added dropwise (derived from the hydrolysis of tetraethyl orthosilicate –TEOS 98%, Sigma-Aldrich- in 1 mM hydrochloric acid solution). The precipitation of the nanospheres was carried out instantaneously, however, the suspension
was allowed to stir for 5 min. The suspension was centrifuged and the resulting precipitate was washed with deionized water several times. Finally, the resulting material was freeze dried and calcined in air at 800oC for 3h. For the synthesis of the CSNp SiO2@xTiO2, various amounts of Titanium (IV) Isopropoxide (TTIP 97%, Sigma-Aldrich) were added to the calcined cores which remained under sonication so that adsorption of the precursor on core’s surface takes place. Then, an appropriate amount of 10% w/v HBPEI-H2O solution was added to hydrolyze the titanium precursor. HBPEI was added to act as dispersant contributing in developing a homogeneous shell of very fine nanostructure. The suspension was left for 24 h under magnetic stirring and afterwards it was centrifuged and washed with ethanol to remove the excess of titanium precursor. Finally, the precipitated CSNp were dried at room temperature and calcined at 600 oC for 4 h under air flow. The w/w ratios of TTIP:SiO2 used for the synthesis of the CSNp as well as their code names are presented in Table 1. Pure titanium dioxide was also produced following the same procedure with the aim to compare its catalytic efficiency to that of CSNp material and evaluate the potential of CSNp to be used as an alternative material to pure titania.
2.2 Materials Characterization Transmission electron microscopy (TEM) images were taken using a high-resolution Transmission Electron Microscope (HRTEM, JEOL 2100 HR). The powders were characterized by FTIR spectroscopy employing a Nicolet Magna-IR Spectrometer 550. Further, powder Xray diffraction (XRD) analysis was performed on a diffractometer (Bruker D8 Focus) with nickel-filtered CuKa radiation (k= 1.5406 Å), at 40 kV and 40 mA, over the angular range of 10-80°. Specific surface area and pore size distribution were obtained from N 2 adsorption– desorption isotherms at 77 K (MICROMERITICS ASAP 2000 Analyser). The surface area was calculated by the BET equation.
2.3 Catalytic tests Based on the results of physicochemical characterization, the material with the optimum shell structure was selected for studying its photocatalytic activity against the inorganic NO gaseous and the catalytic reductive degradation of the organic aqueous pollutant 4-nitrophenol (4NP).
2.3.1 NOx Photocatalysis The samples in powder form were coated on a specific sampler having a final area of 59.28 cm2. The sampler was then placed in the optical quartz window of a continuous flux photoreactor parallel to the surface of the irradiation lamp. The window, from where the gaseous pollutant flows, is transparent to the ultraviolet radiation and has a distance of 5 cm from the sampler. The photoreactor is constructed from materials that do not adsorb the gas for minimizing any losses. Initially the pollutant enters the system in darkness until balance is reached. The measurements were performed at ambient temperature, whereas the total gas (a mixture of air, consisting of 20.8 % O2 and 79.2 % N2, and NO the concentration of which was set at ≈500 ppb) flow was ≈1.65 dm3/min, part of which passes through a scrubber in order to achieve a relative humidity of approximately 40 %. Both the concentration levels of the pollutant in the cuvette as well as the temperature and humidity values were selected in order to carry out the experiment under conditions simulating the real environment. A fan was installed inside the chamber to ensure gas dispersion during the experiment [33-34]. Thereafter the samples were irradiated with ultraviolet radiation (Omicron FSLED lamp) with an intensity equal to 10.0 W/m² ± 5 %. The initial NO concentration was around 450 ppb while the irradiation time was 30 min. The photocatalytic yield of the materials is determined by the NO reduction rate according to the formula (1) [35-36]
(1) While the rate of conversion of NO to NO2 is given by the formula (2):
(2) Where
and
: the concentration of NO and NO2 respectively, at reactor inlet and
: the concentration of NO and NO2 respectively, at
reactor outlet under stable conditions with irradiation (lamp on)
The rate of the photocatalytic NO reduction as well as the rate of photocatalytic production of NO2 are given by the following formulas (3) and (4):
(3)
(4) Where F: the gas flow (m3 h-1) S: The area of the test surface (m2) The rate of photocatalytic yield of the material is calculated by the formula 5:
(5) All rates are expressed in μg·m-2·h-1 As it was previously mentioned, the losses in the system are minimal to nonexistent, and as a consequence the fractions
and
are zero.
2.3.2 4NP reduction catalysis In order to study the catalytic activity over 4-nitrophenol (Fluka), 1.6 g/l NaBH4 (Sodium borohydrate 99%, Sigma-Aldrich) and 0.2 g/l of catalyst were mixed with 50 ml of 4NP aqueous solution (4 ppm). The reaction was carried out at room temperature with continuous stirring. Parts of the mixture were filtered through a 0.2 mm membrane filter for the measurement with UV-vis absorption spectroscopy (UV-Vis Spectrometer, Cary 100) in the range of 200–800 nm, at various time intervals. To study the reusability of catalysts the used sample was centrifuged and separated from the solution. The recycled sample was washed with water several times for the next cycle. The same procedure, as previously described, was performed for 5 cycles.
3. Results and discussion 3.1 Core-shell nanostructures synthesis TEM studies were carried out to evaluate the morphology of titania nanopowder after calcination at 600 °C (Fig. 1a and b). The material consists of grains of irregular shape with a mean grain size at around 15-30 nm. SAED analysis (Fig. 1 c), in combination with the results of XRD analysis shown below, reveals the crystalline structure of the material, whereas the EDS analysis (Fig. 1d) confirms that the material is pure titanium dioxide as only
the Ti and O peaks are present. Some minor peaks, i.e. C, Ni and Cu come from the sampler used for the analysis. Comparing the images of the uncoated silica nanospheres (Fig. 2) to those of Sc@xTiP (Fig. 3-5) and in conjunction with the EDS analysis, it can be derived that a titania coating was successfully developed upon silica cores. As it is seen, however, the homogeneity and integrity of this is greatly dependent on the ratio of titania to silica employed. When a ΤΤΙΡ:SiO2 1:1 w/w ratio, was used, the final morphology is inhomogeneous as the coating is incomplete and quite thin, whereas at the same time several titania particles coexist that do not cover the silica cores. By increasing the amount of the precursor to ΤΤΙΡ:SiO2 2:1 w/w, the coating is very satisfactory with visibly increased thickness and no separate TiO2 particles in the test sample. However, further increase (3:1), leads to the formation of defects in the coating which presents an inhomogeneous surface bearing bumps. In this case, creation again of free or coreless TiO2 nanoparticles and development of necks between the particles and hence an increase in the phenomenon of aggregation are observed. It is also noteworthy that the titanium dioxide particles in coatings have an average size of ~ 7 nm, almost the half of that of pure titania. In addition, from the image of the SAED analysis (Fig. 4 c) and in combination with the results of X-ray diffraction, it is concluded that titanium dioxide has been crystallized in the structure of anatase. The interplanar distances are 0.374 nm, 0.259 nm, 0.219 nm, 0.187 nm and 0.164 nm (±0.02 nm) which correspond to (101), (103), (004), (200) and (105) crystal planes of anatase (JCPDS 21-1272), respectively [7]. Fig. 6a and b present the XRD patterns for pure titania and CSNp prepared employing different amounts of titania precursor after their thermal treatment at 600oC. As it can be seen in Fig. 6a, in pure titania sample, anatase and rutile phases coexist as the characteristic peaks corresponding both to the lattice of anatase (101) (103) (004) (112) (200) (105) (211) (213) (204) (116) (JCPDS No. 21-1272) [37-39] and that of rutile (110) (200) (111) (210) (211) (220) (002) (310) (221) (301) (JCPDS No. 21-1276) [40] can be detected. In contrast, in the CSNp patterns (Fig. 6b), only the main peaks ((101), (004), 200), (105), (211), (204) (116) (220) (215)) of the anatase phase can be detected (JCPDS No. 21-1272) [35-37], which increase in intensity as the amount of titanium precursor during synthesis increases. Additionally, in this case, the characteristic broad peak at approximately 22 degrees corresponding to the amorphous structure of silica [41] is also present due to the presence of the silica cores. It should be emphasized, at this point, the difference in the crystal structure between the pure titania sample being a mixture of anatase and rutile and the
shell material in the core-shell samples being pure anatase. This difference could be attributed to the reduced particle size of titania and the good dispersion of it onto the silica cores. Many studies have shown that the thermal stability of anatase is much higher when the crystallite’s size is smaller than 14 nm and in the case of this work is about 10 nm [4245]. The FTIR spectra of uncoated SiO2, pure TiO2 and CSNp are compared in Fig. 7. With the increase of titanium precursor, there are very small changes as shown in the figure. The formation of the silica network by concentration of silanols to Si-O-Si is indicated by the appearance of three characteristic peaks at 1068, 798 and 445 cm -1 [34, 46-47] corresponding to extensive vibrations and Si-O-Si bending bonds. However, a slight shift of peak at 1068 cm-1 to 1080 cm-1 is observed in the curves of coated particles, most likely due to the successful creation of the coating. Further, in the spectra of coated particles the peak reported in the literature around 970 cm-1 corresponding to Ti-O-Si bonds [48-49] does not appear, but instead two new peaks at 421 and 402 cm-1 have been introduced, as shown in the enlargement in Figure 6 b, which are probably due to the formation of Ti-O-Ti [50] and Si-O-Ti bonds. N2 porosimetry measurements were carried out only to the material with the optimum shell structure, Sc1TiP, and the pure titania sample. The results (Fig. 8) revealed that the CSNp Sc1TiP is essentially non porous with a higher BET surface area (42.9 m2/g) in comparison to pure titania (15.8 m2/g). This sharp increase in the specific surface area of the final core-shell structure Sc1TiP compared to pure titanium dioxide could be attributed to the much lower particle size of titania in the core-shell material as discussed in the TEM analysis, leading to the conclusion that the preparation of such a core-shell structure presents additionally the advantage of increasing the specific surface area of the final material.
3.2 Photocatalytic activity of Sc1TiP and TiO2 for NOx Catalysis Both materials, Sc1TiP and pure TiO2, showed excellent photocatalytic activity leading to high NO degradation rates. More specifically the core-shell nanomaterial Sc1TiP showed slightly better photocatalytic activity than that of pure titania, i.e. 89.7 % degradation degree vs. 88.5 %, whereas it also led to the production of smaller amount of the undesired byproduct NO2. This degradation percentage of Sc1TiP is quite high and much higher compared to the results from similar materials found in literature. For example Signoretto et al. [30] report degradation only 60 %. Moreover, it showed a higher rate of
degradation than that of pure titania (Table 2). Considering that the amount of titania in the core-shell material is much lower than that used to determine the photocatalytic activity of pure titania as well as the fact that the CSNp showed better photocatalytic activity, it can be easily deduced that the Sc1TiP material is an excellent photocatalyst. Therefore it can be used as alternative to pure titanium dioxide being not only a more economical material but also offering a solution to the various problems arising when pure nanotitania is used, such as intense degree of aggregation, phase transformations during operation and even various health problems listed in literature [10-11]. Fig. 9b presents the change of NO and NO2 concentrations versus time during the experimental procedure. In the first 20 min, the system remained in equilibrium and no significant change in the concentrations was observed. At the onset of irradiation there was a sharp decrease in NO concentration in both cases, which remained stable for the next 10 minutes where the irradiation stopped and the system returned to its original levels. At the same time, as the NO concentration was being reduced, there was a slight increase of NO2 production up to 5.5 %, as shown in Table 2, which is quite low to say that photocatalysis is not profitable. Therefore, it can be concluded that the process of photocatalysis takes place rapidly and at an extremely satisfactory level for both materials.
3.3 Catalytic activity of Sc1TiP and TiO2 for 4NP reduction The reduction of 4NP was carried out by NaBH4 in the presence of the catalytic material. The 4NP organic pollutant exhibits a characteristic absorption peak at 317 nm. After the addition of the reducing agent, this peak shifts to 400 nm (Fig. 10) indicating the reduction of 4NP to 4-nitrophenolate ion [51], whereas, a color change of the solution from light yellow to deep yellow occurs. The reduction of the 4-nitrophenol to 4-aminophenol is determined by the decrease of the intensity of the absorption peak at 400 nm and the appearance of the new peak at ~300 nm corresponding to aminophenol [52-53] (Fig. 11 a and d) and the discoloration of the solution (Fig. 11 c and f). It is worth mentioning that in the absence of the catalyst, the intensity of the peak of 4-nitrophenolate ion remained unchanged even after 24 hours indicating that the reduction of the pollutant did not occur even in a small percentage. In all cases the concentration of the reducing agent was in excess compared to the concentration of the pollutant. With the addition of the catalyst, adsorption of the two substances is carried out on the surface of the catalyst and catalytic reduction is initiated by transferring electrons from the BH4- donor to the 4NP acceptor. The excess of the reducing
agent increases the pH of the suspension resulting in slowing down the degradation of the borohydride ions. The free hydrogen from the borohydride ions purged out the air, preventing the oxidation of the 4-aminophenol product [54]. As it is clear from the figures, both materials perform 100 % conversion of the 4NP, with the pure oxide carrying out the degradation in half the time. However, the amount of titanium in CSNp Sc1TiP is much lower than the one tested for the pure oxide. So we conclude that with less quantity and thus a more economical material, the desired result can be achieved in a little higher time. An important advantage of the CSNp Sc1TiP is the fact that the final particle’s size is bigger, which facilitates their separation and collection by a heterogeneous system, a problem that is often encountered when pure nanotitania is used [7-8]. As it was mentioned above, the reducing agent was at a much higher concentration than the 4NP. This allows us to assume that the rate of reduction is independent of the concentration of the NaBH4, Thus, a pseudo-first-order rate kinetics with regard to the 4nitrophenolate concentration could be used to evaluate the apparent rate constant [55-56]. The rate constants k of the reactions were calculated from the slope of the straight lines in Fig. 12. As it was expected, the pure oxide exhibits a higher rate of degradation than Sc1TiP. It is also noteworthy, the fact that in the case of CSNp, at the beginning of the reaction there is an induction time (t0) (approximately up to 30 minutes), which is attributed to the rearrangement of the surface atoms of the nanoparticle in order to be activated before the progress of the reaction starts [57]. This is not observed in the case of pure oxide as the reaction is rapid.
Reusability of ScTiP and TiO2 The recyclability of a catalyst is an important factor, hence the reusability of Sc1TiP and TiO2 was investigated. Fig. 13 shows the results of the reusability investigation for 5 cycles. It was observed that the ability of both catalysts remained almost unchanged for all cycles. Thus, it can be assumed that the two catalysts were not deactivated or poisoned during the whole process.
4. Conclusions In this study, core-shell nanospheres, consisting of silica as a core and titanium oxide as a shell, have been successfully prepared through a water-based environmentally friendly route based on the use of hyperbranched poly(ethylene)imine and the proper optimization
of processing parameters. Pure titanium oxide was also prepared by the same procedure (but without adding silica cores in the synthesis solution) in order to compare the two materials regarding their catalytic and photocatalytic activity. The analysis revealed the successful creation of CSNp, with the optimum material being the one produced by employing a 2:1 ΤΤΙΡ: SiO2 weight ratio. The shell consists of very fine pure anatase particles (~ 7 nm) in contrast to the pure titanium dioxide material which consists of greater particles (ranging between 15 to 30 nm) of both anatase and rutile phases. As a result, the new, optimized, core-shell structure exhibits a significantly increased specific surface area (42.9 m2 /g) compared to pure titanium dioxide (15.8 m2/g). The results from the photocatalytic and catalytic investigations showed that the two materials are excellent catalysts in both cases. In the first one, the optimum CSNp material (Sc1TiP) showed better performance than that of pure titania, i.e. 89.71 % vs. 88.54 % degradation percentage and a higher degradation rate (0.62 vs 0.55 μg·m-2·s-1). In the case of conversion of 4NP to 4AP, the two materials exhibited excellent catalytic activity, too. Specifically, Sc1TiP performed 100 % conversion within 240 min, while the pure oxide within 120 min (for 4 ppm 4NP concentration, 0.2 g/l catalyst and a 1.6 g/l reducing agent NaBH4). It should be considered, that the amount of titania in the CSNp was much lower, as the surface consists of just a thin nanoscaled coating. Consequently, CSNp has the potential to be used as an alternative material to pure nanotitania, as a cheaper material but also as a means for avoiding various problems arising when using TiO2, such as intense degree of aggregation, phase transformations and even various health problems listed in literature when TiO2 is in nanoscale.
Acknowledgement This work was partially funded by the Research Committee of the National Technical University of Athens as a grant to Mrs Ioanna Kitsou to accomplish her PhD studies.
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Figure and Table captions Figure 1: Bright field images of TiO2 (a,b), SAED pattern (c) and EDS spectra (d) Figure 2: Bright field image of uncoated SiO2 Figure 3: Bright field images of Sc0.5TiP (a,b) and EDS spectra (c) Figure 4: Bright field images of Sc1TiP (a,b), SAED pattern (c) and EDS spectra (d) Figure 5: Bright field images of Sc1.5TiP (a,b) and EDS spectra (c) Figure 6: XRD patterns of pure TiO2 (a) and of CSNp for 0.5, 1 and 1.5 ml titanium precursor (b), after calcination at 600oC. Figure 7: FTIR spectra of uncoated SiO2, pure TiO2 and CSNp (a) and spectra details (enlargement) in the area 700-400 cm-1 (b). Figure 8: N2 Adsorption–desorption isotherms of CSNp Sc1TiP and pure TiO2 material. Figure 9: Photocatalytic yields (% η) for the two catalysts (a), concentration profile of NOx vs. time for the photo-oxidation of NO in the presence of Sc1TiP and TiO2 (b). Figure 10: UV–Vis spectra of 4NP before and after adding NaBH4. Figure 11: Time-dependent UV-Vis spectra for the catalytic reduction of 4NP by NaBH4 in the presence of Sc1TiP (a) and TiO2 (d), plots of 4NP concentration vs time for the reduction of 4NP by NaBH4 in the presence of Sc1TiP (b) and TiO2 (e), discoloration of the solution after the catalytic reduction in the presence of Sc1TiP (c) and TiO2 (f). Figure 12: Comparative plots of ln(A/A0) for Sc1TiP and TiO2 toward the reduction of 4NP. Figure 13: The reusability of Sc1TiP and TiO2 as catalysts for the reduction of 4NP with NaBH4.
Table 1: w/w ratios of TTIP:SiO2 and code names of the as-prepared CSNp SiO2@xTiO2. Table 2: Photocatalytic parameters.
Table 1: w/w ratios of TTIP:SiO2 and code names of the as-prepared CSNp SiO2@xTiO2. Code names
TTIP (gr)
SiO2 (gr)
0.49 0.98 1.47
0.5 0.5 0.5
Sc0.5TiP Sc1TiP Sc1.5TiP
ΤΤΙΡ: SiO2weight ratio 1:1 2:1 3:1
Table 2: Photocatalytic parameters %ηNO
%ηNOx
%ηNO2
rNO
rNO2 -2
TiO2 88.54 81.99 Sc1TiP 89.71 84.28
5.95 5.43
rNOx -1
(μg·m ·s ) 0.3665 0.0046 0.5561 0.4134 0.0041 0.6283
Highlights
Fabrication of core-shell silica-titania nanostructures. Significant NO photocatalysis with a photooxidation rate up to 0.62 μg·m−2·s−1. 100% reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP)
S0169-4332(18)30351-9 https://doi.org/10.1016/j.apsusc.2018.02.008 APSUSC 38466
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
4 January 2018 26 January 2018 1 February 2018
Please cite this article as: I. Kitsou, P. Panagopoulos, Th. Maggos, M. Arkas, A. Tsetsekou, Development of SiO2@TiO2 core-shell nanospheres for catalytic applications, Applied Surface Science (2018), doi: https://doi.org/ 10.1016/j.apsusc.2018.02.008
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Development of SiO2@TiO2 core-shell nanospheres for catalytic applications I. Kitsou1, P. Panagopoulos2, Th. Maggos2, M. Arkas3, A. Tsetsekou1* 1
School of Mining and Metallurgical Engineering, National Technical University of Athens, 157 80 Zografos, Athens, Greece
2
Institute of Nuclear & Radiological Sciences & Technology, Energy & Safety, NCSR “Demokritos”, 15310 Agia Paraskevi, Attiki, Greece 3
Institute of Nanoscience and Nanotechnology, NCSR “Demokritos”, 15310 Aghia Paraskevi, Attiki, Greece
*e-mail: [email protected], Tel.: +30 210 7722213, Fax.: +30 210 772119
Abstract Silica-titania core-shell nanospheres, CSNp, were prepared via a simple and environmentally friendly two step route. First, silica cores were prepared through the hydrolysis-condensation reaction of silicic acid in the presence of hyperbranched poly(ethylene)imine (HBPEI) followed by repeating washing, centrifugation and, finally, calcination steps. To create the core-shell structure, various amounts of titanium isopropoxide were added to the cores and after that a HBPEI-water solution was added to hydrolyze the titanium precursor. Washing with ethanol and heat treatment followed. The optimization of processing parameters led to well-developed core-shell structures bearing a homogeneous nanocrystalline anatase coating over each silica core. The photocatalytic activity for NO was examined in a continuous flux photocatalytic reactor under real environmental conditions. The results revealed a very potent photocatalyst as the degradation percentage reached 84.27 % for the core-shell material compared to the 82 % of pure titania with the photodecomposition rates measured at 0.62 and 0.55 μg·m-2·s-1, respectively. In addition, catalytic activities of the CSNp and pure titania were investigated by monitoring the reduction of 4-nitrophenol to 4-aminophenol by an excess of NaBH4. Both materials exhibited excellent catalytic activity (100 %), making the core-shell material a promising alternative catalyst to pure titania for various applications.
Keywords: Core-shell; Titanium dioxide; Silicon dioxide; Nitrogen oxides; 4nitrophenol
1. Introduction Among the various photocatalysts, titania has high potential to reduce the environmental pollution through decomposing many inorganic and organic pollutants, hence it is the most used one [1-4]. Its photocatalytic properties stem from the semiconductor bandgap effects occurring when it is irradiated with ultraviolet light [5-6]. Consequently, holes and electrons are photogenerated and then diffuse to the surface, where oxidation and reduction phenomena take place. It is well known that the photocatalysis procedure depends on the physical properties of the material such as morphology, surface area, particle size and crystalline phase of titania, with anatase being the most promising active phase of TiO2 [5,7]. However, titania, in powder form, is thermally unstable and as a solution, it dissolves and loses its surface area, making the recovery of the catalyst almost impossible [8-9]. In addition, it has been reported that titania particles, in the scale of only few nanometers, can have adverse effects on health [10-11]. To overcome these problems, many researchers, focus on the production of coreshell nanoparticles with titania as shell. Core-shell nanoparticles have many advantages, as they can combine the properties of both the core and the shell or they can give more effective properties than those of the pure materials used as core and shell [12-13]. Changing the size of the core and the thickness of the shell leads to the production of modified materials which can be used in different ways [12,14]. In addition, they can be also employed to reduce the cost of an expensive material as only a small amount of the expensive material can be used to cover a cheap core [12-13, 15-16]. Among the various cores that have been used, such as Au [17], ZnO [18], CeO2 [19], Fe3O4 [20], silica has found to be the most effective, due to its special characteristics: well-known surface chemistry and adsorption capacity, thermal and mechanical stability as well as low cost and optical transparency in the wavelength region where titanium dioxide absorbs [1]. Furthermore, the low toxicity of silica is beneficial in many applications [21-22]. Last but not least, silica can be easily removed in alkaline conditions in order to produce hollow spheres of the shell material with enhanced properties [1]. Many researchers have focused on the synthesis of core shell nanomaterials with silica as a core and titania as a shell for many applications [9, 23-26]. In order to overcome several problems when pure titania is used as a photocatalyst, such as high degree of aggregation, phase transformation etc, Kamaruddin and Stephan [1] produced silica-titania core shell nanoparticles and they investigated the photocatalytic activity of this material as an alternative to pure titania over the organic pollutant methylene blue. More than a few
methods are known to prepare SiO2@TiO2 core shell particles. Kalele et al. [27] synthesized SiO2@TiO2 core shell particles by hydrolysis and condensation of titanium butoxide on the silica cores. Hu et al. [28] produced core–shell particles by a flame aerosol process. The photocatalytic activity of SiO2@TiO2 core shell nanoparticles over the inorganic air pollutant NOx and the organic water pollutant 4-nitrophenol has not been investigated in depth yet. As far as NOx photocatalysis is concerned, only a few studies have been carried out, the majority of which are referred to mixed oxides of SiO2 and TiO2 [29-32]. Signoretto et al. [30] produced titania nanoparticles supported on mesoporous micrometer scale silica via an incipient wetness impregnation method and they studied their photocatalytic activity. The production of mesoporous silica [33] took place through a time-consuming procedure at elevated temperature. Subsequently, the titanium was added by the incipient wetness impregnation method using an alcoholic solution. The study of photocatalytic activity showed that there is a critical limit of addition of titanium beyond which the photocatalytic activity decreases. The material with 16 % wt titanium had the maximum photocatalytic yield, which was recorded at 60 %. In the majority of the research works that have been carried out for the production of core-shell nanoparticles, either toxic solvents have been used or increased temperature has been required. In the present study, aiming at simplifying the procedures and avoiding the use of toxic solvents, the entire process took place at ambient temperature and in aqueous solutions. By optimizing the required amount of titanium precursor, core-shell particles bearing a homogeneous anatase shell were developed which showed excellent photocatalytic efficiency for the air pollutant NOx degradation and the organic pollutant 4nitrophenol (4NP) reduction.
2. Materials and methods
2.1 Synthesis The silica nanospheres were prepared through a biomimetic precipitation process [34]. Briefly, a solution of 20 mM hyperbranched poly(ethylene)imine (HBPEI, Mn=25,000, Sigma-Aldrich) - phosphate buffer (p.b, 100 mM) was prepared. To the above solution, an appropriate amount of silicic acid (1 M) was added dropwise (derived from the hydrolysis of tetraethyl orthosilicate –TEOS 98%, Sigma-Aldrich- in 1 mM hydrochloric acid solution). The precipitation of the nanospheres was carried out instantaneously, however, the suspension
was allowed to stir for 5 min. The suspension was centrifuged and the resulting precipitate was washed with deionized water several times. Finally, the resulting material was freeze dried and calcined in air at 800oC for 3h. For the synthesis of the CSNp SiO2@xTiO2, various amounts of Titanium (IV) Isopropoxide (TTIP 97%, Sigma-Aldrich) were added to the calcined cores which remained under sonication so that adsorption of the precursor on core’s surface takes place. Then, an appropriate amount of 10% w/v HBPEI-H2O solution was added to hydrolyze the titanium precursor. HBPEI was added to act as dispersant contributing in developing a homogeneous shell of very fine nanostructure. The suspension was left for 24 h under magnetic stirring and afterwards it was centrifuged and washed with ethanol to remove the excess of titanium precursor. Finally, the precipitated CSNp were dried at room temperature and calcined at 600 oC for 4 h under air flow. The w/w ratios of TTIP:SiO2 used for the synthesis of the CSNp as well as their code names are presented in Table 1. Pure titanium dioxide was also produced following the same procedure with the aim to compare its catalytic efficiency to that of CSNp material and evaluate the potential of CSNp to be used as an alternative material to pure titania.
2.2 Materials Characterization Transmission electron microscopy (TEM) images were taken using a high-resolution Transmission Electron Microscope (HRTEM, JEOL 2100 HR). The powders were characterized by FTIR spectroscopy employing a Nicolet Magna-IR Spectrometer 550. Further, powder Xray diffraction (XRD) analysis was performed on a diffractometer (Bruker D8 Focus) with nickel-filtered CuKa radiation (k= 1.5406 Å), at 40 kV and 40 mA, over the angular range of 10-80°. Specific surface area and pore size distribution were obtained from N 2 adsorption– desorption isotherms at 77 K (MICROMERITICS ASAP 2000 Analyser). The surface area was calculated by the BET equation.
2.3 Catalytic tests Based on the results of physicochemical characterization, the material with the optimum shell structure was selected for studying its photocatalytic activity against the inorganic NO gaseous and the catalytic reductive degradation of the organic aqueous pollutant 4-nitrophenol (4NP).
2.3.1 NOx Photocatalysis The samples in powder form were coated on a specific sampler having a final area of 59.28 cm2. The sampler was then placed in the optical quartz window of a continuous flux photoreactor parallel to the surface of the irradiation lamp. The window, from where the gaseous pollutant flows, is transparent to the ultraviolet radiation and has a distance of 5 cm from the sampler. The photoreactor is constructed from materials that do not adsorb the gas for minimizing any losses. Initially the pollutant enters the system in darkness until balance is reached. The measurements were performed at ambient temperature, whereas the total gas (a mixture of air, consisting of 20.8 % O2 and 79.2 % N2, and NO the concentration of which was set at ≈500 ppb) flow was ≈1.65 dm3/min, part of which passes through a scrubber in order to achieve a relative humidity of approximately 40 %. Both the concentration levels of the pollutant in the cuvette as well as the temperature and humidity values were selected in order to carry out the experiment under conditions simulating the real environment. A fan was installed inside the chamber to ensure gas dispersion during the experiment [33-34]. Thereafter the samples were irradiated with ultraviolet radiation (Omicron FSLED lamp) with an intensity equal to 10.0 W/m² ± 5 %. The initial NO concentration was around 450 ppb while the irradiation time was 30 min. The photocatalytic yield of the materials is determined by the NO reduction rate according to the formula (1) [35-36]
(1) While the rate of conversion of NO to NO2 is given by the formula (2):
(2) Where
and
: the concentration of NO and NO2 respectively, at reactor inlet and
: the concentration of NO and NO2 respectively, at
reactor outlet under stable conditions with irradiation (lamp on)
The rate of the photocatalytic NO reduction as well as the rate of photocatalytic production of NO2 are given by the following formulas (3) and (4):
(3)
(4) Where F: the gas flow (m3 h-1) S: The area of the test surface (m2) The rate of photocatalytic yield of the material is calculated by the formula 5:
(5) All rates are expressed in μg·m-2·h-1 As it was previously mentioned, the losses in the system are minimal to nonexistent, and as a consequence the fractions
and
are zero.
2.3.2 4NP reduction catalysis In order to study the catalytic activity over 4-nitrophenol (Fluka), 1.6 g/l NaBH4 (Sodium borohydrate 99%, Sigma-Aldrich) and 0.2 g/l of catalyst were mixed with 50 ml of 4NP aqueous solution (4 ppm). The reaction was carried out at room temperature with continuous stirring. Parts of the mixture were filtered through a 0.2 mm membrane filter for the measurement with UV-vis absorption spectroscopy (UV-Vis Spectrometer, Cary 100) in the range of 200–800 nm, at various time intervals. To study the reusability of catalysts the used sample was centrifuged and separated from the solution. The recycled sample was washed with water several times for the next cycle. The same procedure, as previously described, was performed for 5 cycles.
3. Results and discussion 3.1 Core-shell nanostructures synthesis TEM studies were carried out to evaluate the morphology of titania nanopowder after calcination at 600 °C (Fig. 1a and b). The material consists of grains of irregular shape with a mean grain size at around 15-30 nm. SAED analysis (Fig. 1 c), in combination with the results of XRD analysis shown below, reveals the crystalline structure of the material, whereas the EDS analysis (Fig. 1d) confirms that the material is pure titanium dioxide as only
the Ti and O peaks are present. Some minor peaks, i.e. C, Ni and Cu come from the sampler used for the analysis. Comparing the images of the uncoated silica nanospheres (Fig. 2) to those of Sc@xTiP (Fig. 3-5) and in conjunction with the EDS analysis, it can be derived that a titania coating was successfully developed upon silica cores. As it is seen, however, the homogeneity and integrity of this is greatly dependent on the ratio of titania to silica employed. When a ΤΤΙΡ:SiO2 1:1 w/w ratio, was used, the final morphology is inhomogeneous as the coating is incomplete and quite thin, whereas at the same time several titania particles coexist that do not cover the silica cores. By increasing the amount of the precursor to ΤΤΙΡ:SiO2 2:1 w/w, the coating is very satisfactory with visibly increased thickness and no separate TiO2 particles in the test sample. However, further increase (3:1), leads to the formation of defects in the coating which presents an inhomogeneous surface bearing bumps. In this case, creation again of free or coreless TiO2 nanoparticles and development of necks between the particles and hence an increase in the phenomenon of aggregation are observed. It is also noteworthy that the titanium dioxide particles in coatings have an average size of ~ 7 nm, almost the half of that of pure titania. In addition, from the image of the SAED analysis (Fig. 4 c) and in combination with the results of X-ray diffraction, it is concluded that titanium dioxide has been crystallized in the structure of anatase. The interplanar distances are 0.374 nm, 0.259 nm, 0.219 nm, 0.187 nm and 0.164 nm (±0.02 nm) which correspond to (101), (103), (004), (200) and (105) crystal planes of anatase (JCPDS 21-1272), respectively [7]. Fig. 6a and b present the XRD patterns for pure titania and CSNp prepared employing different amounts of titania precursor after their thermal treatment at 600oC. As it can be seen in Fig. 6a, in pure titania sample, anatase and rutile phases coexist as the characteristic peaks corresponding both to the lattice of anatase (101) (103) (004) (112) (200) (105) (211) (213) (204) (116) (JCPDS No. 21-1272) [37-39] and that of rutile (110) (200) (111) (210) (211) (220) (002) (310) (221) (301) (JCPDS No. 21-1276) [40] can be detected. In contrast, in the CSNp patterns (Fig. 6b), only the main peaks ((101), (004), 200), (105), (211), (204) (116) (220) (215)) of the anatase phase can be detected (JCPDS No. 21-1272) [35-37], which increase in intensity as the amount of titanium precursor during synthesis increases. Additionally, in this case, the characteristic broad peak at approximately 22 degrees corresponding to the amorphous structure of silica [41] is also present due to the presence of the silica cores. It should be emphasized, at this point, the difference in the crystal structure between the pure titania sample being a mixture of anatase and rutile and the
shell material in the core-shell samples being pure anatase. This difference could be attributed to the reduced particle size of titania and the good dispersion of it onto the silica cores. Many studies have shown that the thermal stability of anatase is much higher when the crystallite’s size is smaller than 14 nm and in the case of this work is about 10 nm [4245]. The FTIR spectra of uncoated SiO2, pure TiO2 and CSNp are compared in Fig. 7. With the increase of titanium precursor, there are very small changes as shown in the figure. The formation of the silica network by concentration of silanols to Si-O-Si is indicated by the appearance of three characteristic peaks at 1068, 798 and 445 cm -1 [34, 46-47] corresponding to extensive vibrations and Si-O-Si bending bonds. However, a slight shift of peak at 1068 cm-1 to 1080 cm-1 is observed in the curves of coated particles, most likely due to the successful creation of the coating. Further, in the spectra of coated particles the peak reported in the literature around 970 cm-1 corresponding to Ti-O-Si bonds [48-49] does not appear, but instead two new peaks at 421 and 402 cm-1 have been introduced, as shown in the enlargement in Figure 6 b, which are probably due to the formation of Ti-O-Ti [50] and Si-O-Ti bonds. N2 porosimetry measurements were carried out only to the material with the optimum shell structure, Sc1TiP, and the pure titania sample. The results (Fig. 8) revealed that the CSNp Sc1TiP is essentially non porous with a higher BET surface area (42.9 m2/g) in comparison to pure titania (15.8 m2/g). This sharp increase in the specific surface area of the final core-shell structure Sc1TiP compared to pure titanium dioxide could be attributed to the much lower particle size of titania in the core-shell material as discussed in the TEM analysis, leading to the conclusion that the preparation of such a core-shell structure presents additionally the advantage of increasing the specific surface area of the final material.
3.2 Photocatalytic activity of Sc1TiP and TiO2 for NOx Catalysis Both materials, Sc1TiP and pure TiO2, showed excellent photocatalytic activity leading to high NO degradation rates. More specifically the core-shell nanomaterial Sc1TiP showed slightly better photocatalytic activity than that of pure titania, i.e. 89.7 % degradation degree vs. 88.5 %, whereas it also led to the production of smaller amount of the undesired byproduct NO2. This degradation percentage of Sc1TiP is quite high and much higher compared to the results from similar materials found in literature. For example Signoretto et al. [30] report degradation only 60 %. Moreover, it showed a higher rate of
degradation than that of pure titania (Table 2). Considering that the amount of titania in the core-shell material is much lower than that used to determine the photocatalytic activity of pure titania as well as the fact that the CSNp showed better photocatalytic activity, it can be easily deduced that the Sc1TiP material is an excellent photocatalyst. Therefore it can be used as alternative to pure titanium dioxide being not only a more economical material but also offering a solution to the various problems arising when pure nanotitania is used, such as intense degree of aggregation, phase transformations during operation and even various health problems listed in literature [10-11]. Fig. 9b presents the change of NO and NO2 concentrations versus time during the experimental procedure. In the first 20 min, the system remained in equilibrium and no significant change in the concentrations was observed. At the onset of irradiation there was a sharp decrease in NO concentration in both cases, which remained stable for the next 10 minutes where the irradiation stopped and the system returned to its original levels. At the same time, as the NO concentration was being reduced, there was a slight increase of NO2 production up to 5.5 %, as shown in Table 2, which is quite low to say that photocatalysis is not profitable. Therefore, it can be concluded that the process of photocatalysis takes place rapidly and at an extremely satisfactory level for both materials.
3.3 Catalytic activity of Sc1TiP and TiO2 for 4NP reduction The reduction of 4NP was carried out by NaBH4 in the presence of the catalytic material. The 4NP organic pollutant exhibits a characteristic absorption peak at 317 nm. After the addition of the reducing agent, this peak shifts to 400 nm (Fig. 10) indicating the reduction of 4NP to 4-nitrophenolate ion [51], whereas, a color change of the solution from light yellow to deep yellow occurs. The reduction of the 4-nitrophenol to 4-aminophenol is determined by the decrease of the intensity of the absorption peak at 400 nm and the appearance of the new peak at ~300 nm corresponding to aminophenol [52-53] (Fig. 11 a and d) and the discoloration of the solution (Fig. 11 c and f). It is worth mentioning that in the absence of the catalyst, the intensity of the peak of 4-nitrophenolate ion remained unchanged even after 24 hours indicating that the reduction of the pollutant did not occur even in a small percentage. In all cases the concentration of the reducing agent was in excess compared to the concentration of the pollutant. With the addition of the catalyst, adsorption of the two substances is carried out on the surface of the catalyst and catalytic reduction is initiated by transferring electrons from the BH4- donor to the 4NP acceptor. The excess of the reducing
agent increases the pH of the suspension resulting in slowing down the degradation of the borohydride ions. The free hydrogen from the borohydride ions purged out the air, preventing the oxidation of the 4-aminophenol product [54]. As it is clear from the figures, both materials perform 100 % conversion of the 4NP, with the pure oxide carrying out the degradation in half the time. However, the amount of titanium in CSNp Sc1TiP is much lower than the one tested for the pure oxide. So we conclude that with less quantity and thus a more economical material, the desired result can be achieved in a little higher time. An important advantage of the CSNp Sc1TiP is the fact that the final particle’s size is bigger, which facilitates their separation and collection by a heterogeneous system, a problem that is often encountered when pure nanotitania is used [7-8]. As it was mentioned above, the reducing agent was at a much higher concentration than the 4NP. This allows us to assume that the rate of reduction is independent of the concentration of the NaBH4, Thus, a pseudo-first-order rate kinetics with regard to the 4nitrophenolate concentration could be used to evaluate the apparent rate constant [55-56]. The rate constants k of the reactions were calculated from the slope of the straight lines in Fig. 12. As it was expected, the pure oxide exhibits a higher rate of degradation than Sc1TiP. It is also noteworthy, the fact that in the case of CSNp, at the beginning of the reaction there is an induction time (t0) (approximately up to 30 minutes), which is attributed to the rearrangement of the surface atoms of the nanoparticle in order to be activated before the progress of the reaction starts [57]. This is not observed in the case of pure oxide as the reaction is rapid.
Reusability of ScTiP and TiO2 The recyclability of a catalyst is an important factor, hence the reusability of Sc1TiP and TiO2 was investigated. Fig. 13 shows the results of the reusability investigation for 5 cycles. It was observed that the ability of both catalysts remained almost unchanged for all cycles. Thus, it can be assumed that the two catalysts were not deactivated or poisoned during the whole process.
4. Conclusions In this study, core-shell nanospheres, consisting of silica as a core and titanium oxide as a shell, have been successfully prepared through a water-based environmentally friendly route based on the use of hyperbranched poly(ethylene)imine and the proper optimization
of processing parameters. Pure titanium oxide was also prepared by the same procedure (but without adding silica cores in the synthesis solution) in order to compare the two materials regarding their catalytic and photocatalytic activity. The analysis revealed the successful creation of CSNp, with the optimum material being the one produced by employing a 2:1 ΤΤΙΡ: SiO2 weight ratio. The shell consists of very fine pure anatase particles (~ 7 nm) in contrast to the pure titanium dioxide material which consists of greater particles (ranging between 15 to 30 nm) of both anatase and rutile phases. As a result, the new, optimized, core-shell structure exhibits a significantly increased specific surface area (42.9 m2 /g) compared to pure titanium dioxide (15.8 m2/g). The results from the photocatalytic and catalytic investigations showed that the two materials are excellent catalysts in both cases. In the first one, the optimum CSNp material (Sc1TiP) showed better performance than that of pure titania, i.e. 89.71 % vs. 88.54 % degradation percentage and a higher degradation rate (0.62 vs 0.55 μg·m-2·s-1). In the case of conversion of 4NP to 4AP, the two materials exhibited excellent catalytic activity, too. Specifically, Sc1TiP performed 100 % conversion within 240 min, while the pure oxide within 120 min (for 4 ppm 4NP concentration, 0.2 g/l catalyst and a 1.6 g/l reducing agent NaBH4). It should be considered, that the amount of titania in the CSNp was much lower, as the surface consists of just a thin nanoscaled coating. Consequently, CSNp has the potential to be used as an alternative material to pure nanotitania, as a cheaper material but also as a means for avoiding various problems arising when using TiO2, such as intense degree of aggregation, phase transformations and even various health problems listed in literature when TiO2 is in nanoscale.
Acknowledgement This work was partially funded by the Research Committee of the National Technical University of Athens as a grant to Mrs Ioanna Kitsou to accomplish her PhD studies.
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Figure and Table captions Figure 1: Bright field images of TiO2 (a,b), SAED pattern (c) and EDS spectra (d) Figure 2: Bright field image of uncoated SiO2 Figure 3: Bright field images of Sc0.5TiP (a,b) and EDS spectra (c) Figure 4: Bright field images of Sc1TiP (a,b), SAED pattern (c) and EDS spectra (d) Figure 5: Bright field images of Sc1.5TiP (a,b) and EDS spectra (c) Figure 6: XRD patterns of pure TiO2 (a) and of CSNp for 0.5, 1 and 1.5 ml titanium precursor (b), after calcination at 600oC. Figure 7: FTIR spectra of uncoated SiO2, pure TiO2 and CSNp (a) and spectra details (enlargement) in the area 700-400 cm-1 (b). Figure 8: N2 Adsorption–desorption isotherms of CSNp Sc1TiP and pure TiO2 material. Figure 9: Photocatalytic yields (% η) for the two catalysts (a), concentration profile of NOx vs. time for the photo-oxidation of NO in the presence of Sc1TiP and TiO2 (b). Figure 10: UV–Vis spectra of 4NP before and after adding NaBH4. Figure 11: Time-dependent UV-Vis spectra for the catalytic reduction of 4NP by NaBH4 in the presence of Sc1TiP (a) and TiO2 (d), plots of 4NP concentration vs time for the reduction of 4NP by NaBH4 in the presence of Sc1TiP (b) and TiO2 (e), discoloration of the solution after the catalytic reduction in the presence of Sc1TiP (c) and TiO2 (f). Figure 12: Comparative plots of ln(A/A0) for Sc1TiP and TiO2 toward the reduction of 4NP. Figure 13: The reusability of Sc1TiP and TiO2 as catalysts for the reduction of 4NP with NaBH4.
Table 1: w/w ratios of TTIP:SiO2 and code names of the as-prepared CSNp SiO2@xTiO2. Table 2: Photocatalytic parameters.
Table 1: w/w ratios of TTIP:SiO2 and code names of the as-prepared CSNp SiO2@xTiO2. Code names
TTIP (gr)
SiO2 (gr)
0.49 0.98 1.47
0.5 0.5 0.5
Sc0.5TiP Sc1TiP Sc1.5TiP
ΤΤΙΡ: SiO2weight ratio 1:1 2:1 3:1
Table 2: Photocatalytic parameters %ηNO
%ηNOx
%ηNO2
rNO
rNO2 -2
TiO2 88.54 81.99 Sc1TiP 89.71 84.28
5.95 5.43
rNOx -1
(μg·m ·s ) 0.3665 0.0046 0.5561 0.4134 0.0041 0.6283
Highlights
Fabrication of core-shell silica-titania nanostructures. Significant NO photocatalysis with a photooxidation rate up to 0.62 μg·m−2·s−1. 100% reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP)