Ultrasonically Engineered Ceria-Titania Nanostructure Mediated Photocatalytic and Sonocatalytic Degradation of Organic Dye

    

    

    Figure 1: Ultrasound equipment set up.
    
    

Make: Ace, USA.

Operating frequency: 22 kHz,

Rated output power: 750 W,

Diameter of stainless steel tip of horn: 1.3 * 10^-2 m,

Surface area of ultrasound irradiating face: 1.32 * 10^-4 m²,

Expected ultrasound intensity 3.4*10⁵ W/m²

Synthesis of CeO₂ via ultrasound assisted Precipitation technique

Cerium nitrate hexahydrate, 0.9g (0.1g mol) was dissolved in 30ml distilled water and 0.24g, (0.3g mol) sodium hydroxide was dissolved in 30 ml distilled water separately and was kept ready. The sodium hydroxide solution was then added drop wise to the cerium nitrate hexahydrate solution under sonication using an Ultrasonic Horn (ACE 22 kHz) at 40% amplitude for 2min with a 5s pulse and 5s relaxation cycle at time t = 0h. Precaution was taken to see to it that the addition was done in conjunction with the sonic pulse created by the transducer (Horn). After addition, the solution was again exposed to acoustic cavitation (by using ultrasonic horn) for further 18min, by keeping all sonication parameters constant (same as that was used during mixing), to carry out the complete reaction of cerium nitrate hexahydrate with sodium hydroxide. The temperature of the reaction mixture was maintained at 35 ± 2°C by circulating water in jacketed reactor which was used for synthesis work. Further, the solution was centrifuged at 8000rpm for 10min to separate the product and the settled product was washed thrice using distilled water to remove the byproducts. After complete washing, the product was dried at 100°C for 3h to complete the conversion of Ce(OH)3 to CeO₂. After complete conversion, CeO₂ powder obtained was (light yellowish in nature), cooled, ground by mortar, and weighted to check for yield of the process.

Synthesis of TiO₂ via ultrasound assisted Sol-Gel technique

5 mL of Titanium Tetraisopropoxide (TTIP) precursor was added drop wise to 30mL of 2- propanol. The resulting clear solution was added drop wise to 5mL of glacial acetic acid solution. The mixture of TTIP precursor, 2-propanol and glacial acetic acid solution was subjected to sonication using an Ultrasonic Horn (ACE 22 kHz) at 40% amplitude actually delivering 29.2W of power for 10 min with a 5s pulse and 5 s relaxation cycle. Thus, the sol obtained after keeping it for 24h was again ultrasonically irradiated by keeping same parameters. It was then converted into a gel by adding it to 30mL of distilled water. The obtained gel was then dehydrated in an air circulating oven at 110°C for 3h. The white powder obtained was cooled, ground and weighed to check for yield of the process.

Synthesis of CeO₂/TiO₂ composite nanoparticle via ultrasound assisted technique

The synthesis of CeO₂/TiO₂ composite nanoparticles via ultrasound assisted method, Proceeded as follows. In a typical procedure, 1.5g (0.1g mol) of Ce( NO₃)₃. 6H₂O was dissolved in 50ml of deionized water and 0.6g, (0.3g mol) sodium hydroxide was also dissolved in 50 ml of deionized water and kept ready. The sodium hydroxide solution was Then added drop wise to the cerium nitrate hexahydrate solution under sonication using an Ultrasonic Horn (ACE 22 kHz) at 40% amplitude for 2min with a 5s pulse and 5 s relaxation cycle at time t = 0h. Care was taken to see to it that the addition was done in conjunction with the sonic pulse afforded by the transducer (Horn). After addition, the solution was again exposed to acoustic cavitation (by using ultrasonic horn) for further 18 min, by keeping all sonication parameters constant (same as that was used during mixing), to carry out the complete reaction of cerium nitrate hexahydrate with sodium hydroxide. The temperature of the reaction mixture was maintained at 35 ± 2°C by circulating water in jacketed reactor which was used for synthesis work. A purple colored colloidal dispersion was thus formed. 5mL of Titanium Tetraisopropoxide (TTIP) precursor was added dropwise to 30mL of 2- propanol. The resulting clear solution was added dropwise to 5mL of glacial acetic acid solution. The mixture of TTIP precursor, 2-propanol and glacial acetic acid solution was subjected to sonication using an Ultrasonic Horn (ACE 22 kHz) at 40% amplitude actually delivering 29.2W of power for 10 min with a 5 s pulse and 5 s relaxation cycle to form a sol which was allowed to rest for 24 hours. Further, this sol was then agitated using a magnetic stirrer added drop wise to the colloidal dispersion prepared before. This mixture was further sonicated for 10min with a 5 s pulse and 5 s relaxation cycles. The resulting mixture was further centrifuged at 8000 rpm for 10min to separate the product. The separated product was dried at 110°C for 3h. After complete conversion obtained powder was cooled, ground by mortar, and weighted to check for yield of the process.

Characterization

The phase structures of CeO₂, TiO₂, and CeO₂/TiO₂ composite nanoparticles were studied on a Rigaku Mini-Flex X-Ray Diffractometer instrument using CuKa radiation source (λ=0.154 nm). XRD patterns were recorded at angles between 20° and 80°, with a scan rate of 2°/min. Particle sizes were determined using the Debye– Scherrer equation. The thermal behavior of the composite CeO₂/TiO₂ composite nanoparticles was investigated by using thermo gravimetric analysis. The method used a Q50 TGA with standard furnace. The TGA investigation was carried out at a temperature ramp of 20°C/min, from ambient temperature (25°C) till 10007°C. Sample preparation for Scanning Electron Microscopy (SEM) includes the deposition of platinum on powder of the composite particle. The morphology and diameter of individual CeO₂ and TiO₂ composite nanoparticles were characterized by a JEOL, JSM-6380 LA 15KV Scanning electron microscope. Scanning Electron Microscopy (SEM) was carried out to provide evidence to the morphology of the individual CeO₂ and TiO₂ as well as the CeO₂/TiO₂ composite nanoparticles synthesized via US techniques. The morphology, size and structure of the particles have been investigated in detail using SEM. The morphology of CeO₂/TiO₂ composite nanoparticles and diameter of core and thickness of shell were characterized by Philips Model CM200 Transmission Electron Microscope having 177 operating voltage from 20kv to 200kv with the resolution of 2.4 A°. The Photocatalytic and sonocatalytic degradation of RhB, using the synthesized particles as catalysts, was investigated using a Perkin–Elmer UV–visible Spectrophotometer. Demineralized water was used as a blank reference.

Photocatalytic activity

Degradation experiment: The photocatalytic activity of the individual CeO₂ and TiO₂ as well as CeO₂/TiO₂ nanoparticles as catalyst material was determined by degradation of aqueous solutions of Rhodamine B (RhB) dye, by using a photoreactor. The photoreactor is a closed box with a UV lamp Spectroline XX-15 N that emits radiation at 365 nm with intensity of 200 W/cm². The solution of RhB was made at 25ppm concentration each, in order to compare the activity of the composite CeO₂/TiO₂ composite nanoparticle catalyst with the photocatalytic activity of individual CeO₂ and TiO₂ nanoparticle catalysts. Solutions with 25 mg of CeO₂, TiO₂ and CeO₂/ TiO₂ photocatalysts suspended in 100 ml dye solution were prepared to make aqueous solution. The experiments performed with different addition amounts showed that the photocatalytic degradation ratios of Rhodamine B gradually increase with the increase of nano-sized catalyst powder up to 25 and then the degradation of Rhodamine B reached a constant value for any additional amount of catalyst. A

constant concentration of photocatalyst 0.05% was hence maintained during the degradation experiment. The suspension was subjected to irradiation under UV–vis light for a span of 90 minutes. Prior to photoreaction, the suspension was magnetically stirred in dark condition for 30 min to establish absorption/desorption equilibrium condition. Also during the photocatalytic reaction, the aqueous suspension was magnetically stirred. The solutions were exposed to ambient temperature of 35°C and pH of 7 was maintained during the process. Aliquots of mixture were taken out with the help of a syringe at periodic intervals during the irradiation, using a micropipette. These samples were subjected to an ultracentrifuge in order to separate any suspended solids and then analysed in the double beam UV–vis spectrophotometer. The wavelength of maximum absorbance (λmax) of dye was found to be 554 nm for RhB dye. Reproducibility of the obtained experimental data is very important in investigation related to the effects of the operating parameters. In the current work, all the experiments were carried out three times to estimate the reproducibility of the obtained data. The graphs were plotted using mean values obtained from the data.

Sonocatalytic activity

Degradation experiment: The sonocatalytic activity of the individual CeO2 and TiO2as well as CeO2/TiO2 composite nanoparticles as catalyst material was determined by degradation of aqueous solutions of Rhodamine B (RhB) dye, by using an ultrasound bath. Sonocatalytic degradation process of Rhodamine B was carried out in a glass reactor filled with 100 ml of aqueous dye solution at a concentration of 25 mg/L. The experiments performed with different addition amounts showed that the sonocataytic degradation ratios of Rhodamine B tempestuously increase with the increase of nanosized catalyst powder up to 0.05% and then the degradation ratio of Rhodamine B reached a constant value for any additional amount of catalyst. A constant concentration of sonocatalyst 0.05% was hence maintained during the degradation experiment. The suspension was magnetically stirred in dark condition for 30 min to establish absorption/desorption equilibrium condition. The solutions were exposed to ambient temperature of 35°C and pH of 7 was maintained during the process. Ultrasonic irradiation was achieved by means of an ultrasonic bath (Dakshin Ultrasonic Model 9L 300 H/DF), which was operated at a frequency of 20 kHz and an effective power output of 80 W through manual adjusting. Under these mild ultrasonic conditions, degradation of organic substances due to ultrasound is generally reported to be quite low [35,36].

Results and Discussion

X ray diffraction (XRD)

The phase identification and structural changes were investigated with the help of X ray diffraction (XRD) technique. Figure 2 shows the typical XRD patterns recorded over 20°-80° for the all synthesized sample which gives a good insight into the crystallinity of the products. Figure 2 shows the XRD patterns for the individual as well as CeO₂/TiO₂ nanoparticles. From the XRD pattern of CeO₂/ TiO₂ in Figure 2, it is seen that there are two sets of diffraction peaks for the sample, which indicates that the as-synthesized samples are composite materials. The XRD peaks at 2θ 28.541° (111) , 33.071°(200) , 47.471°(220) , 56.331°(311) , 59.071°(222) and 69.401°(400) are attributable to CeO₂ as shown in Figure 1. According to the Debye Scherrer formula, the strongest peak (111) at 2θ = 28.575°, the peak (200) at 2θ = 33.139°, the peak (220) at 2θ = 47.572° and the peak (311) at 2θ = 56.402° were used to calculate the average crystal size of the CeO₂ nanoparticles, which was determined to be around 12 nm. These diffraction peaks are identified as that of pure CeO₂ consistent with JCPDS Card No.: 34-0394. The XRD peaks at 2θ 25.2° (101), 38.5° (004), 44.1° (211), 53.8° (204), 55.0° (310) are attributed to anatase TiO₂ as seen in Figure 2. These diffraction peaks are identified to be of pure anatase form of TiO₂ as per JCPDS Card No. 21-1272. The peaks at 2θ 27.4° (110), 36.0° (101), 41.2° (111), 54.3° (211) are attributed to rutile TiO₂ as seen in Figure 2. These diffraction peaks are identified to be of pure anatase form of TiO2 as per JCPDS Card No. 21-1276. These observations indicate the presence of multiphase (anatase and rutile) form of TiO₂.According to the Debye-Scherrer formula, the strongest peak (101) at 2θ = 25.2°, the peak (004) at 2θ = 38.5°, the peak (211) at 2θ = 44.1°, the peak (211) at 2θ = 54.3°, the peak (310) at 2θ = 55.0° were used to calculate the average crystal size of the TiO₂ nanoparticles, which was determined to be around 36 nm. Peak positions for both CeO₂ and TiO₂ in the XRD patterns have no obvious shift comparing with their standard patterns. Therefore, the oxides of TiO₂ and CeO₂ should form a composite. No peaks of any other phase were observed, which indicated that the product is pure and well crystallized. The following table (Table 1) summarizes the crystal sizes and % crystallinity observed in individual as well as composite core/shell particles by X - Ray diffraction.

Table 1: Average Crystal size and % crystallinity of US synthesized CeO₂, TiO₂, CeO₂/TiO₂ nanoparticles.

  

    
Sr No
    
Sample Name
    
% Crystallinity
    
Crystal size
    
560
  
  

    
561
  
  

    
1
    
US CeO2
    
13.69
    
55.09
    
562
  
  

    
2
    
US TiO2
    
40.35
    
57.37
    
563
  
  

    
3
    
US CeO2 / TiO2
    
28.12
    
30.61
    
564
  

Table 1: Average Crystal size and % crystallinity of US synthesized CeO2, TiO2,
CeO2/TiO2 nanoparticles.

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Figure 2: XRD plot of US synthesized CeO₂, TiO₂, CeO₂/TiO₂ composite nanoparticles.

    

    

    Figure 2: XRD plot of US synthesized CeO2, TiO2, CeO2/TiO2 composite
nanoparticles.
    
    

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Thermogravimetric analysis

The thermal behaviour of the CeO₂/TiO₂ composite nanoparticles synthesized by ultrasound- assisted (US) technique was investigated by using thermo gravimetric analysis. Figure 3 shows the thermo gravimetric (TGA) analysis, analyzing the weight loss of the sample with respect to a temperature gradient. A continuous weight loss was observed for the sample as the temperature was increased from 35°C to 600°C. At 50°C, it was observed that the weight loss for CeO₂/TiO₂ composite nanoparticle synthesized by US technique was 0.14% and for that of CeO₂ and TiO₂ particles was 0.19% and 0%, respectively. As the samples were heated further, a notable weight loss was observed at 145°C for CeO₂/TiO₂ composite nanoparticles, which was 0.85% and for CeO₂ and TiO₂ nanoparticles it was 1.79% and 0% respectively. This loss of weight can be attributed to the disassociation of free and physically adsorbed water molecules on the nanoparticles. On heating the samples further, another notable weight loss was found at 330°C for CeO₂/TiO₂ composite nanoparticles, which was 1.42% % and for CeO₂ and TiO₂ nanoparticles it was 7.68% and 0% respectively. The reason for this reduction in weight may be the loss of chemisorbed water molecules and to dehydroxylationie loss of OH- ions from the structure. At 600°C, the final weight loss of US synthesized CeO₂/ TiO₂, CeO₂ and TiO₂ nanoparticels was observed to be 1.71%, 11.75% and 0.03% respectively. Throughout the temperature gradient, the degradation characteristics of the CeO₂/TiO₂ composite nanoparticles lie between the individual CeOM₂ and TiO₂ nanoparticles which suggest the formation of a composite between the two individual compounds. Ultrasound assisted technique causes a change in the crystallography of the nanoparticles. This change is caused by the formation of hotspots during acoustic cavitation and micro mixing leads to an increase in the Brownian motion of molecules. This does not allow the formation of a regular crystal structure during the core/shell particle synthesis, which leads to a reduced thermal stability of the particles synthesized using ultrasound assisted (US) technique, which may be the reason for the reduction in weight. Another reason for the weight loss exhibited by the particles synthesized using the US technique may be that sonication causes a reduction in the average particle size, which increases the efficiency of thermal conduction. Hence, at a lower temperature US particles will feel a higher effect of thermal conductive currents, which may cause a phase transformation, thus leading to the significant weight loss. This phenomenon may be investigated further, to conclusively say so.

Figure 3: TGA curves of US synthesizedCeO₂, TiO₂ and CeO₂/TiO₂ nanoparticles.

    

    

    Figure 3: TGA curves of US synthesizedCeO2, TiO2 and CeO2/TiO2
nanoparticles.
    
    

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Scanning Electron Microscopy (SEM): Figure 4 shows the SEM micrographs of the individual CeO₂, TiO₂ and CeO₂/TiO₂ nanoparticles respectively. The micrographs of the synthesized particle support the observations from the XRD patterns. SEM results were compared at the same magnification of 500X and 1000X. CeO₂ micrograph depicts the presence of agglomerated particles stacked on each other to give a rough surface morphology which you can see it from Figure 4 (a-1, & a-2). TiO₂ micrographs however indicate a smooth surface morphology from Figure 4 (b-1 & b-2). The smooth flat plate like surface of the CeO₂/TiO₂ nanoparticles as seen in the SEM image Figure 4 (c-1 & c-2) indicate that CeO₂ must have been coated with TiO₂ to impart its surface morphology. The SEM images also depict that the composite particles have smooth and sharp cuts which conforms the formation of CeO₂/TiO₂ in support to the XRD and TEM images.

Figure 4: SEM micrographs at 500X and 1000X magnifications respectively of a 1) CeO₂ 500X a 2) CeO₂ 1000X, b 1) TiO₂ 500X b 2) TiO₂ 1000X, c 1) CeO₂/TiO₂ 500X c 2) CeO₂/TiO₂ 1000X nanoparticles.

    

    

    Figure 4: SEM micrographs at 500X and 1000X magnifications respectively of a 1) CeO2 500X a 2) CeO2 1000X, b 1) TiO2 500X b 2) TiO2 1000X, c 1) CeO2/TiO2
500X c 2) CeO2/TiO2 1000X nanoparticles.
    
    

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Transmission electron microscopy (TEM)

Figure 5 shows the TEM image of the composite CeO₂/TiO₂ particle generated via US synthesis. The images clearly confirm the formation of CeO₂/TiO₂ composite particles, as proposed. The average size of particles as seen in the TEM graphs is in good agreement with the calculated crystallite size from XRD analysis. The dark grey or black colored portions represent the CeO₂ core, while the light grey color surrounding represents the TiO₂ shell. The particles were formed with a fairly narrow size distribution and uniform shape could be observed. The average particle size was 10 nm with the average diameter of the CeO₂ core being 8 nm and the average width of the TiO₂ coating being 2 nm. It can be clearly seen from the TEM image that the individual particles have aggregated to form secondary particles of larger size, as seen even in the SEM image (Figure 4). The aggregation can be associated to the high surface energy of the nano particles due to presence of surface charge. Then select area electron diffraction (SAED) patterns correspond to the CeO₂/TiO₂ nanoparticles (Figure 5). The amorphous nature of TiO₂ could be observed based on the particles and their corresponding SAED patterns.

Figure 5: TEM image and SAED pattern of CeO₂/TiO₂ nanoparticles.

    

    

    Figure 5: TEM image and SAED pattern of CeO2/TiO2 nanoparticles.
    
    

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Photocatalytic activity

The photocatalytic activity of the prepared samples was tested for the degradation of RhB constant optimum loading of photocatalyst. The effect of change in photocatalyst on the extent of degradation was investigated and has been discussed in the following sections.

Effect of change in photocatalyst: Photocatalytic activity of CeO₂, TiO₂ and CeO₂/TiO₂ core/shell photocatalysts were evaluated through the degradation of a basic dye i.e. Rhodamine B in an aqueous solution under UV radiation, as mentioned in the degradation experiment. The removal of Rhodamine B in the presence of all three type of catalyst powders was confirmed by a drop in the absorbance peak within visible wavelength as shown in Figure 6.

Figure 6: UV-vis spectra of RhB during photocatalytic degradation.

    

    

    Figure 6: UV-vis spectra of RhB during photocatalytic degradation.
    
    

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Abs vsλ for RhB proving λmax = 554 nm

It has been reported by Minero et al. [34], that when the alteration of the shape of the spectrum is not significant, it indicates that there is no colored degradation intermediates produced. Thus, the spectral interference during our reaction was negligible and the change in absorbance value can be due to dye degradation in presence of photocatalyst, after 90mins. The concentration of Rhodamine B was detected at its maximum absorbance of 554 nm as found from the absorption peak in Figure 6. The degradation efficiency of Rhodamine B is defined as follows:

Degradation efficiency (%) = {C₀ – C_t)/C₀ × 100%

where C₀ is the initial concentration of Rhodamine B and Ct is the concentration of Rhodamine B at reaction time t (min). Figure 7 shows the comparison of degradation efficiency rhodamine B in the presence of the synthesized nano-sized powders, under UV radiation at different moments. It can be seen that the concentration of rhodamine B solutions gradually decrease along with irradiation time, which indicates that rhodamine B in aqueous solutions is decomposed piece by piece under UV light. The detailed degradation processes are shown in Figure 8. It could be seen that the degradation ratio of Rhodamine B in the presence of nano-sized CeO₂ powder attains 55.11% within 90 minutes of exposure to UV light, while it was 80.51% for rhodamine B in presence of TiO₂ and 69.35% in presence of composite CeO₂/TiO₂ nano-sized photocatalyst, respectively, at the same time. Correspondingly, the degradation ratio of rhodamine B in the absence of any catalyst under only UV irradiation was only 7.05% at the same moment. These results indicate that the degradation effects of rhodamine B in the presence of nano-sized catalyst powders combining with UV light are more obvious than corresponding to those using only UV light. The increase in the degradation rate in the presence of catalyst can be attributed to the semiconducting nature of the catalyst particles. Irradiation of metal oxide particles with photons of energy equal to, or greater than their band gap energy, results in the promotion of an electron from the Valence Band (VB) to the Conduction Band (CB) of the particle. The outcome of this process is a region of positive charged hole, in the valance band. The charge can migrate to the particle surface, where the holes can react with surfacebound hydroxyl groups (OH^-) and adsorbed water molecules to form hydroxyl radicals (OH•) ie carry out water dissociation reaction. The high intensity of UV light generated in the reactor leads to generation of large amount of free radicals in the aqueous system, hence causing an increased degradation. In addition, to infer the reaction kinetics of photocatalytic degradation of rhodamine B, the data of -ln (C_t/C₀) for first-order reaction as a function of irradiation time (t) were calculated. In Figure 9, the results indicate that all calculated values of -ln (C_t/ C0) are approximately linear with the irradiation time all through for photocatalytic degradation of Rhodamine B for all cases; ie they conform to pseudo first-order kinetics reactions. The rate constants of photo degradation processes of rhodamine B in presence of CeO₂, TiO₂ and CeO₂/TiO₂ composite particles are much larger than that of photo degradation of RhB solution without catalysts. This supports our intention for synthesis of photocatalytic materials for degradation of RhB dye. The Photocatalytic activities of these nanoparticles show significant differences. That is, the order of Photocatalytic activities is TiO₂ > CeO₂/TiO₂ > CeO₂. The corresponding degradation ratios calculated are 80.51%, 69.35% and 55.11%, respectively, and that the degradation ratio under UV radiation alone is only 7.05%. The result confirms the intention of coating CeO₂ particles with more reactive TiO₂, to increase its photocatalytic activity.

Figure 7: Degradation of RhB with respect to time in presence of photocatalyst.

    

    

    Figure 7: Degradation of RhB with respect to time in presence of photocatalyst.
    
    

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Figure 8: Effect of photocatalyst on degradation of RhB.

    

    

    Figure 8: Effect of photocatalyst on degradation of RhB.
    
    

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Figure 9: Influence of photocatalyst on rate of degradation of RhB.

    

    

    Figure 9: Influence of photocatalyst on rate of degradation of RhB.
    
    

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Sonocatalytic activity

Aliquots of mixture were taken out at periodic intervals during the irradiation, using a micropipette. These samples were subjected to an ultracentrifuge in order to separate any suspended solids and then analyzed in the double beam UV–v is spectrophotometer. The wavelength of maximum absorbance (λmax) of dye was found to be 554 nm for RhB dye. Reproducibility of the obtained experimental data is very important in investigation related to the effects of the operating parameters. In the current work, all the experiments were carried out three times to estimate the reproducibility of the obtained data. The graphs were plotted using mean values obtained from the data.

Effect of change in sonocatalyst: Sonocatalytic activity of CeO₂, TiO₂ and CeO₂/TiO₂ core/shell sonocatalysts was evaluated through the degradation of a basic dye i.e. Rhodamine B in an aqueous solution under ultrasonic irradiation as mentioned in the degradation experiment. The removal of Rhodamine B in the presence of all three type of catalyst powders was confirmed by a drop in the absorbance peak within visible wavelength as shown in Fig. 10. It has been reported by Minero et al. [34], that when the alteration of the shape of the spectrum is not significant, it indicates that there is no colored degradation intermediates produced. Thus, the spectral interference during our reaction was negligible and the change in absorbance value can be due to dye degradation in presence of sonocatalyst, after 90 mins. The concentration of Rhodamine B was detected at its maximum absorbance of 554 nm as found from the absorption peak in Figure 10. The degradation efficiency of Rhodamine B is defined as follows:

Figure 10: UV-vis spectra of RhB during sonocatalytic degradation.

    

    

    Figure 10: UV-vis spectra of RhB during sonocatalytic degradation.
    
    

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Degradation efficiency (%) = {C₀ – C_t)/C₀ × 100%

Where, C₀ is the initial concentration of Rhodamine B and C_t is the concentration of Rhodamine B at reaction time t (min). Figure 11 shows the comparison of degradation efficiency Rhodamine B in the presence of the synthesized nano-sized powders, under ultrasonic irradiation at different moments. It can be seen that the absorption peaks of Rhodamine B solutions gradually decrease along with irradiation time, which indicates that Rhodamine B in aqueous solutions is decomposed piece by piece under ultrasonic irradiation. The detailed degradation processes are shown in Figure 12. It could be seen that the degradation ratio of Rhodamine B in the presence of nano-sized CeO₂ powder attains 36.82% within 90 minutes of ultrasonic irradiation, while it was 58.96% for Rhodamine B in presence of TiO₂ and 46.41% in presence of composite CeO₂/TiO₂ nano-sized sonocatalyst, respectively, at the same time. Correspondingly, the degradation ratio of Rhodamine B in the absence of any catalyst under only ultrasound irradiation was only 13.9% at the same moment. These results indicate that the degradation effects of Rhodamine B in the presence of nanosized catalyst powders combining with ultrasonic irradiation are more obvious than corresponding to those using one fold ultrasonic irradiation. The increase in the degradation rate in the presence of catalyst can be attributed to the presence of solid particles in a liquid which increased the nucleation sites for cavity formation. Once undergoing electron shifts from the valence band to the conduction band, the metal oxide particles itself could also act as a catalyst to promote water dissociation reactions. The combined effects led to the generation of more free radicals in the aqueous system, hence causing an increased degradation. In addition, to infer the reaction kinetics of sonocatalytic degradation of rhodamine B, the data of -ln (C_t/C₀) for first-order reaction as a function of irradiation time (t) were calculated. In Figure 13, the results indicate that all calculated values of -ln (C_t/C₀) are approximately linear with the irradiation time all through for sonocatalytic degradation of Rhodamine B for all cases ie they conform to pseudo first-order kinetics reactions. Nevertheless, the rate constants of degradation processes for ultrasonic irradiation of rhodamine B in presence of CeO₂, TiO₂ and CeO₂/TiO₂ composite particles are much larger than that of RhB solution without catalysts. This supports our intention for synthesis of sonocatalytic materials for degradation of RhB dye. The sonocatalytic activities of these nanoparticles show significant differences. That is, the order of sonocatalytic activities is TiO₂ > CeO₂/TiO₂ > CeO₂. The corresponding degradation ratios calculated are 53.96%, 46.41% and 36.82% respectively. And that the degradation ratio under ultrasonic irradiation alone is only 13.9%. The results demonstrate that CeO₂/TiO₂ composite takes co-operative effects from individual components to demonstrate sonocatalytic activity at the used intensity and frequency of ultrasound.

Figure 11: Degradation of RhB with respect to time in presence of sonocatalyst.

    

    

    Figure 11: Degradation of RhB with respect to time in presence of
sonocatalyst.
    
    

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Figure 12: Effect of sonocatalyst on degradation of RhB.

    

    

    Figure 12: Effect of sonocatalyst on degradation of RhB.
    
    

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Figure 13: Influence of sonocatalyst on rate of degradation of RhB.

    

    

    Figure 13: Influence of sonocatalyst on rate of degradation of RhB.
    
    

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Conclusion

Nano-sized CeO₂, TiO₂ and CeO₂/TiO₂ particles were successfully prepared via eco-friendly ultrasound-assisted technique. The evidence of formation of CeO₂/TiO₂ core/shell is given by the TEM images. The as prepared particles were further tested for its catalytic activity by using Rhodamine B dye. Degradation characteristics confirmed the inferior catalytic activity of CeO₂ nanoparticles in comparison to TiO₂ nanoparticles and thus support our intention of coating CeO₂ particles with a TiO₂ shell to increase the activity of CeO₂ as a catalyst. Superior photocatalytic degradation over sonocatalytic degradation was also confirmed by higher % degradation of Rhodamine B and rate of reaction.

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Citation: Shah NH, Bhangaonkar KR, Pinjari DV and Mhaske ST. Ultrasonically Engineered Ceria-Titania Nanostructure Mediated Photocatalytic and Sonocatalytic Degradation of Organic Dye. Austin Chem Eng. 2016; 3(3): 1032.

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