Synthesis of Polystyrene@TiO 2 Core–Shell Particles and Their Photocatalytic Activity for the Decomposition of Methylene Blue

: In this study, we investigated the preparation conditions of polystyrene (PS)@TiO 2 core– shell particles and their photocatalytic activity during the decomposition of methylene blue (MB). TiO 2 shells were formed on the surfaces of PS particles using the sol–gel method. Homogeneous PS@TiO 2 core–shell particles were obtained using an aqueous NH 3 solution as the promoter of the sol–gel reaction and stirred at room temperature. This investigation revealed that the temperature and amount of the sol–gel reaction promoter inﬂuenced the morphology of the PS@TiO 2 core–shell particles. The TiO 2 shell thickness of the PS@TiO 2 core–shell particles was approximately 5 nm, as observed using transmission electron microscopy. Additionally, Ti elements were detected on the surfaces of the PS@TiO 2 core–shell particles using energy-dispersive X-ray spectroscopy analysis. The PS@TiO 2 core–shell particles were used in MB decomposition to evaluate their photocatalytic activities. For comparison, we utilized commercial P25 and TiO 2 particles prepared using the sol–gel method. The results showed that the PS@TiO 2 core–shell particles exhibited higher activity than that of the compared samples.


Introduction
As environmental pollution continues to worsen, technologies and materials that mitigate this issue have become widely researched topics [1,2].Organic dyes primarily released from industrial plants are among such pollutants that form part of effluents in various water bodies [3,4].Dyes such as methylene blue (MB) are toxic and nonbiodegradable, and they remain in the water for long periods of time [5].Therefore, developing methods for removing dyes from water bodies is crucial.
Titanium dioxide (TiO 2 ) is well known for being an excellent photocatalyst.This oxide has attracted attention for use in pollutant removal because it offers high conversion efficiency, good chemical stability, and low cost [14][15][16].The sol-gel reaction has been reported as a synthetic method for generating TiO 2 particles [17,18].In general, TiO 2 particles with a small particle size can efficiently act as a photocatalyst; however, these tend to agglomerate due to their high energy and reduce the number of available active sites [19].To solve this problem, TiO 2 particles have been immobilized on polymer substrates such as polymers, including polyethlene [20,21] and poly (dimethylsiloxane) beads, [22] and so on [23].Moreover, it is difficult to obtain TiO 2 particles with uniform size due to the fast sol-gel reaction rates exhibited by Ti precursors [24,25].Therefore, the challenge is to immobilize TiO 2 particles uniformly on the surfaces of substrates.Additionally, many complex processes are required in order to obtain substrate-supported TiO 2 particles due their long synthesis time [26][27][28].We propose a facile method for the immobilization of TiO 2 particles on the surfaces of polystyrene (PS) particles.
Monodisperse PS particles with a positive charge were prepared using emulsifier-free emulsion polymerization using 2 2-azodiisobutyramidine dihydrochloride as the initiator and poly (vinylpyrrolidone) as the stabilizer [29][30][31].Previous studies found that SiO 2 -Al 2 O 3 nanoparticles could be collected on the surfaces of PS particles.In this method, the SiO 2 -Al 2 O 3 nanoparticles were prepared using the sol-gel method, and the nanoparticles with negative charges were attracted to the surfaces of the PS particles because of their positive charges, thus forming SiO 2 -Al 2 O 3 shells [32].With this knowledge in mind, we attempted to collect TiO 2 particles on the surfaces of PS particles using this simple method.
Herein, we report PS@TiO 2 core-shell particles synthesized using the sol-gel method.In this method, TiO 2 shells were formed on the surfaces of PS particles.Homogeneous spherical particles were obtained by adjusting the reaction temperature and amount of the promoter aqueous NH 3 solution.A schematic illustration of the synthesis of the PS@TiO 2 core-shell particles is shown in Figure 1.Experimental observations confirmed that the diameters of PS@TiO 2 core-shell particles were approximately 220 nm.Furthermore, TiO 2 was confirmed to be present on the surfaces of the PS@TiO 2 core-shell particles.The photocatalytic activity of the PS@TiO 2 core-shell particles was evaluated using MB decomposition.The PS@TiO 2 core-shell particles showed better performance than that of the commercial P25.particles with a small particle size can efficiently act as a photocatalyst; however, these tend to agglomerate due to their high energy and reduce the number of available active sites [19].To solve this problem, TiO2 particles have been immobilized on polymer substrates such as polymers, including polyethlene [20,21] and poly (dimethylsiloxane) beads, [22] and so on [23].Moreover, it is difficult to obtain TiO2 particles with uniform size due to the fast sol-gel reaction rates exhibited by Ti precursors [24,25].Therefore, the challenge is to immobilize TiO2 particles uniformly on the surfaces of substrates.Additionally, many complex processes are required in order to obtain substrate-supported TiO2 particles due their long synthesis time [26][27][28].We propose a facile method for the immobilization of TiO2 particles on the surfaces of polystyrene (PS) particles.
Monodisperse PS particles with a positive charge were prepared using emulsifierfree emulsion polymerization using 2′2-azodiisobutyramidine dihydrochloride as the initiator and poly (vinylpyrrolidone) as the stabilizer [29][30][31].Previous studies found that SiO2-Al2O3 nanoparticles could be collected on the surfaces of PS particles.In this method, the SiO2-Al2O3 nanoparticles were prepared using the sol-gel method, and the nanoparticles with negative charges were attracted to the surfaces of the PS particles because of their positive charges, thus forming SiO2-Al2O3 shells [32].With this knowledge in mind, we attempted to collect TiO2 particles on the surfaces of PS particles using this simple method.
Herein, we report PS@TiO2 core-shell particles synthesized using the sol-gel method.In this method, TiO2 shells were formed on the surfaces of PS particles.Homogeneous spherical particles were obtained by adjusting the reaction temperature and amount of the promoter aqueous NH3 solution.A schematic illustration of the synthesis of the PS@TiO2 core-shell particles is shown in Figure 1.Experimental observations confirmed that the diameters of PS@TiO2 core-shell particles were approximately 220 nm.Furthermore, TiO2 was confirmed to be present on the surfaces of the PS@TiO2 core-shell particles.The photocatalytic activity of the PS@TiO2 core-shell particles was evaluated using MB decomposition.The PS@TiO2 core-shell particles showed better performance than that of the commercial P25.

Results and Discussion
First, we investigated the influence of preparation conditions such as the preparation temperature and amount of aqueous NH3 solution on the morphology of PS@TiO2 coreshell particles.Figure 2a-c show scanning electron microscopy (SEM) images of PS@TiO2 core-shell particles prepared at various preparation temperatures and with varying amounts of aqueous NH3 solution.Uniform spherical particles were prepared using 300 μL of aqueous NH3 solution at room temperature (Figure 2a), whereas spherical particles and particles with irregular shapes were prepared using 300 μL of aqueous NH3 solution at 323 K (Figure 2b).Meanwhile, the rough layers, including spherical particles on the surface, were observed in SEM images of Figure 2c when prepared using 1000 μL of aqueous NH3 solution at room temperature.Using this process, the TiO2 nanoparticles prepared via the sol-gel reaction were collected on the surfaces of PS particles.Reportedly, the sol-gel reaction rate enhances higher temperatures and increases the amounts of promoters such as NH3 [33].Consistent with the above reports, a faster sol-gel reaction rate

Results and Discussion
First, we investigated the influence of preparation conditions such as the preparation temperature and amount of aqueous NH 3 solution on the morphology of PS@TiO 2 core-shell particles.Figure 2a-c show scanning electron microscopy (SEM) images of PS@TiO 2 core-shell particles prepared at various preparation temperatures and with varying amounts of aqueous NH 3 solution.Uniform spherical particles were prepared using 300 µL of aqueous NH 3 solution at room temperature (Figure 2a), whereas spherical particles and particles with irregular shapes were prepared using 300 µL of aqueous NH 3 solution at 323 K (Figure 2b).Meanwhile, the rough layers, including spherical particles on the surface, were observed in SEM images of Figure 2c when prepared using 1000 µL of aqueous NH 3 solution at room temperature.Using this process, the TiO 2 nanoparticles prepared via the sol-gel reaction were collected on the surfaces of PS particles.Reportedly, the sol-gel reaction rate enhances higher temperatures and increases the amounts of promoters such as NH 3 [33].Consistent with the above reports, a faster sol-gel reaction rate promoted TiO 2 particle growth, leading to the formation of particles with irregular shapes and rough layers, as shown in Figure 2b,c.Moreover, we confirmed the presence of TiO 2 on the surfaces of uniform PS@TiO 2 core-shell particles prepared using 300 µL of aqueous NH 3 solution at room temperature through energy-dispersive X-ray spectroscopy (EDX) measurements.Figure 3 shows elemental mapping images of O, Ti, and C for the prepared PS@TiO 2 core-shell particles.O and Ti were observed in the overall images, and the ratio of O:Ti was approximately 1:1.Additionally, the areas of C elemental mapping overlapped the areas of Ti and O elemental mapping.This result suggests that very thin TiO 2 shells can be formed on the surfaces of PS particles.Meanwhile, we measured the specific surface area of the PS@TiO 2 core-shell particles using the Brunauer-Emmett-Teller (BET) method.The results showed that the specific surface area of the PS@TiO 2 core-shell particles was 125 m 2 g −1 .
Inorganics 2023, 11, x FOR PEER REVIEW 3 of 12 promoted TiO2 particle growth, leading to the formation of particles with irregular shapes and rough layers, as shown in Figure 2b,c.Moreover, we confirmed the presence of TiO2 on the surfaces of uniform PS@TiO2 core-shell particles prepared using 300 μL of aqueous NH3 solution at room temperature through energy-dispersive X-ray spectroscopy (EDX) measurements.Figure 3 shows elemental mapping images of O, Ti, and C for the prepared PS@TiO2 core-shell particles.O and Ti were observed in the overall images, and the ratio of O:Ti was approximately 1:1.Additionally, the areas of C elemental mapping overlapped the areas of Ti and O elemental mapping.This result suggests that very thin TiO2 shells can be formed on the surfaces of PS particles.Meanwhile, we measured the specific surface area of the PS@TiO2 core-shell particles using the Brunauer-Emmett-Teller (BET) method.The results showed that the specific surface area of the PS@TiO2 core-shell particles was 125 m 2 g -1 .Next, the morphology of PS@TiO2 core-shell particles were observed in detail using transmission electron microscopy (TEM).For comparison, we used TiO2 particles promoted TiO2 particle growth, leading to the formation of particles with irregular shapes and rough layers, as shown in Figure 2b,c.Moreover, we confirmed the presence of TiO2 on the surfaces of uniform PS@TiO2 core-shell particles prepared using 300 μL of aqueous NH3 solution at room temperature through energy-dispersive X-ray spectroscopy (EDX) measurements.Figure 3 shows elemental mapping images of O, Ti, and C for the prepared PS@TiO2 core-shell particles.O and Ti were observed in the overall images, and the ratio of O:Ti was approximately 1:1.Additionally, the areas of C elemental mapping overlapped the areas of Ti and O elemental mapping.This result suggests that very thin TiO2 shells can be formed on the surfaces of PS particles.Meanwhile, we measured the specific surface area of the PS@TiO2 core-shell particles using the Brunauer-Emmett-Teller (BET) method.The results showed that the specific surface area of the PS@TiO2 core-shell particles was 125 m 2 g -1 .Next, the morphology of PS@TiO2 core-shell particles were observed in detail using transmission electron microscopy (TEM).For comparison, we used TiO2 particles Next, the morphology of PS@TiO 2 core-shell particles were observed in detail using transmission electron microscopy (TEM).For comparison, we used TiO 2 particles prepared using sol-gel reactions and commercial P25. Figure 4a shows a TEM image of PS particles.In this image, the diameters of the PS particles are approximately 280 nm. Figure 4b,c show TEM images of the PS@TiO 2 core-shell particles.The obtained images show that the diameters of the spherical particles were approximately 290 nm.The presence of the TiO 2 shell was confirmed by the different contrasts between core and shell.As shown in Figure 4a-c, the shell thickness of the PS@TiO 2 core-shell particles could be ~5 nm.This result is also consistent with the results of the SEM/energy-dispersive X-ray spectroscopy (EDX) element mapping images, which show the detection of C (Figure 3).Meanwhile, the presence of prepared TiO 2 particles and P25 was also confirmed using the TEM images.In these results, the prepared TiO 2 is presented in random shapes (Figure 4d), whereas crystal particles with diameters of 20-50 nm are observed in the TEM image of P25 (Figure 4e).
prepared using sol-gel reactions and commercial P25. Figure 4a shows a TEM ima PS particles.In this image, the diameters of the PS particles are approximately 280 Figure 4b,c show TEM images of the PS@TiO2 core-shell particles.The obtained im show that the diameters of the spherical particles were approximately 290 nm.The ence of the TiO2 shell was confirmed by the different contrasts between core and she shown in Figure 4a-c, the shell thickness of the PS@TiO2 core-shell particles could b nm.This result is also consistent with the results of the SEM/energy-dispersive X-ray troscopy (EDX) element mapping images, which show the detection of C (Figure 3).M while, the presence of prepared TiO2 particles and P25 was also confirmed using the images.In these results, the prepared TiO2 is presented in random shapes (Figure whereas crystal particles with diameters of 20-50 nm are observed in the TEM ima P25 (Figure 4e).The crystallinity of the samples was measured using powder X-ray diffraction (X Figure 5 shows the XRD patterns of the PS@TiO2 core-shell particles, the prepared and the commercial P25.The PS@TiO2 core-shell particles showed a broad diffra peak, whereas the prepared TiO2 showed no peak, indicating that both samples were posed of an amorphous phase.Meanwhile, the commercial P25 showed sharp peak dicating that it was composed of rutile and anatase phases [34].The crystallinity of the samples was measured using powder X-ray diffraction (XRD).Figure 5 shows the XRD patterns of the PS@TiO 2 core-shell particles, the prepared TiO 2 , and the commercial P25.The PS@TiO 2 core-shell particles showed a broad diffraction peak, whereas the prepared TiO 2 showed no peak, indicating that both samples were composed of an amorphous phase.Meanwhile, the commercial P25 showed sharp peaks, indicating that it was composed of rutile and anatase phases [34].Finally, the photocatalytic activities of the PS@TiO2 core-she TiO2, and the commercial P25 were evaluated through MB decom of MB was observed at 664 nm.The UV visible (UV-Vis) absorptio core-shell particles, the prepared TiO2, the commercial P25, an tored at different times are shown in Figure 6a-d.As seen in thes the absorption peak gradually decreased with the reaction tim PS@TiO2 core-shell particles, the prepared TiO2, and the comm intensity of the absorption peak was not changed in the presence 6e shows the reaction time dependence of MB decomposition samples with UV irradiation.The relative intensity of the absorb to calculate the value of C/C0, where C0 and C are the MB conce MB solution at the beginning and reaction time, respectively PS@TiO2 core-shell particles and the prepared TiO2, 84% and 38% 3 min.Meanwhile, 84% and 60% of MB degraded within 24 min, ples did not promote MB decomposition after 4 min.In the cas the intensity of the main peak gradually decreased with the incre 58% of MB degraded after 24 min.On the other hand, the PS par These results indicate that the PS@TiO2 core-shell particles demo rate than those of the prepared TiO2 and the commercial P25.Finally, the photocatalytic activities of the PS@TiO 2 core-shell particles, the prepared TiO 2 , and the commercial P25 were evaluated through MB decomposition.The main peak of MB was observed at 664 nm.The UV visible (UV-Vis) absorption spectra of the PS@TiO 2 core-shell particles, the prepared TiO 2 , the commercial P25, and the PS particles monitored at different times are shown in Figure 6a-d.As seen in these results, the intensity of the absorption peak gradually decreased with the reaction time in the presence of the PS@TiO 2 core-shell particles, the prepared TiO 2 , and the commercial P25.However, the intensity of the absorption peak was not changed in the presence of the PS particles.Figure 6e shows the reaction time dependence of MB decomposition in the presence of these samples with UV irradiation.The relative intensity of the absorbance at 664 nm was used to calculate the value of C/C 0 , where C 0 and C are the MB concentrations of the aqueous MB solution at the beginning and reaction time, respectively.In the presence of the PS@TiO 2 core-shell particles and the prepared TiO 2 , 84% and 38% of MB degraded within 3 min.Meanwhile, 84% and 60% of MB degraded within 24 min, indicating that both samples did not promote MB decomposition after 4 min.In the case of the commercial P25, the intensity of the main peak gradually decreased with the increase in reaction time, and 58% of MB degraded after 24 min.On the other hand, the PS particles showed no activity.These results indicate that the PS@TiO 2 core-shell particles demonstrated a faster reaction rate than those of the prepared TiO 2 and the commercial P25.
The PS@TiO 2 core-shell particles were used in MB decomposition with and without UV irradiation to evaluate their photocatalytic activity.Figure 7a shows the UV-Vis absorption spectra of the reaction solutions with the PS@TiO 2 core-shell particles without UV irradiation.The intensity of the absorption peak at 664 nm was unchanged without UV irradiation.Figure 7b shows the reaction time dependence in the presence of the PS@TiO 2 core-shell particles with and without UV irradiation.With UV irradiation, 87% of MB degraded after 24 min, as described in the above results, whereas without UV irradiation, MB decomposition was not promoted.The PS@TiO2 core-shell particles were used in MB decomposition with and without UV irradiation to evaluate their photocatalytic activity.Figure 7a shows the UV-Vis absorption spectra of the reaction solutions with the PS@TiO2 core-shell particles without UV irradiation.The intensity of the absorption peak at 664 nm was unchanged without UV irradiation.Figure 7b shows the reaction time dependence in the presence of the PS@TiO2 core-shell particles with and without UV irradiation.With UV irradiation, 87% of MB degraded after 24 min, as described in the above results, whereas without UV irradiation, MB decomposition was not promoted.The PS@TiO2 core-shell particles were used in MB decomposition with and withou UV irradiation to evaluate their photocatalytic activity.Figure 7a shows the UV-Vis ab sorption spectra of the reaction solutions with the PS@TiO2 core-shell particles withou UV irradiation.The intensity of the absorption peak at 664 nm was unchanged withou UV irradiation.Figure 7b shows the reaction time dependence in the presence of th PS@TiO2 core-shell particles with and without UV irradiation.With UV irradiation, 87% of MB degraded after 24 min, as described in the above results, whereas without UV irra diation, MB decomposition was not promoted.Herein, we discussed the differences in the photocatalytic activities of the PS@TiO 2 core-shell particles, the prepared TiO 2 , and the commercial P25.First, we noted that PS@TiO 2 core-shell particles exhibited the highest MB decomposition activity.In this activity evaluation, the masses of the samples were equal, whereas the masses of TiO 2 were lower than the practical masses of the samples because PS@TiO 2 core-shell particles contained PS particles as the core.Nevertheless, the PS@TiO 2 core-shell particles exhibited the highest MB decomposition activity among all samples.This may have been because TiO 2 shells with a shell thickness of about 5 nm were uniformly coated on surface of PS particles leading to efficient activity promotion.Therefore, the PS@TiO 2 core-shell particles, the prepared TiO 2 , and the PS particles were investigated via photoluminescence (PL) measurements.Figure 8 shows the PL spectra of the PS@TiO 2 core-shell particles, the prepared TiO 2 , and the PS particles.From this result, a broad peak was observed at around 390 nm for the PS@TiO 2 core-shell particles and the prepared TiO 2 , while no peak was observed for the PS particles.In addition, the PS@TiO 2 core-shell particles exhibited a higher peak intensity than the prepared TiO 2 .This peak was attributed to band-to-band transition [35].Previous studies have reported that a peak was not observed for amorphous TiO 2 because of the lack of fluorescence [36], whereas for the TiO 2 with anatase crystalline phases, a peak was observed [35].It may be that the TiO 2 shells on the surface of the PS@TiO 2 core-shell particles formed very tiny crystal particles with poor crystalline phases.The broad peaks observed at 20 • in the XRD results may imply the existence of poor crystalline phases.In other reports, the peak intensity of the amorphous TiO 2 coating on the surface of ZnO particles increased compared with that of ZnO particles, indicating that the fine control of TiO 2 shell thickness on the surface of ZnO nanoparticles was very important for the enhancement of PL intensity [37].Considering this, the crystallinity and thickness of the TiO 2 shell of PS@TiO 2 core-shell particles may play an important role in MB decomposition.
measurements.Figure 8 shows the PL spectra of the PS@TiO2 pared TiO2, and the PS particles.From this result, a broad pe 390 nm for the PS@TiO2 core-shell particles and the prepare observed for the PS particles.In addition, the PS@TiO2 core higher peak intensity than the prepared TiO2.This peak was transition [35].Previous studies have reported that a peak w phous TiO2 because of the lack of fluorescence [36], wherea crystalline phases, a peak was observed [35].It may be that th of the PS@TiO2 core-shell particles formed very tiny crystal pa phases.The broad peaks observed at 20° in the XRD results poor crystalline phases.In other reports, the peak intensity of t on the surface of ZnO particles increased compared with that that the fine control of TiO2 shell thickness on the surface of Z important for the enhancement of PL intensity [37].Consideri thickness of the TiO2 shell of PS@TiO2 core-shell particles ma MB decomposition.Next, a comparison of the MB decomposition obtained other previously reported studies [38][39][40][41][42] is listed in Table 1.core-shell particles show higher activity than most TiO2-poly lysts.However, the MB degradation rate per dosage of the P (0.379% min −1 mg −1 ) was lower than that of TiO2/graphene min −1 mg −1 ).This result might be ascribed to the different cry shells of the PS@TiO2 core-shell particles were probably amo Figure 5a.On the other hand, the TiO2 on the graphene porous phase [42].Indeed, the band gap of the amorphous TiO2 is h and anatase TiO2 [43].Moreover, there may also be a differenc Next, a comparison of the MB decomposition obtained in this study with those of other previously reported studies [38][39][40][41][42] is listed in Table 1.It was found that PS@TiO 2 coreshell particles show higher activity than most TiO 2 -polymers and their related catalysts.However, the MB degradation rate per dosage of the PS@TiO 2 core-shell particles (0.379% min −1 mg −1 ) was lower than that of TiO 2 /graphene porous composites (0.64% min −1 mg −1 ).This result might be ascribed to the different crystallinity of TiO 2 .The TiO 2 shells of the PS@TiO 2 core-shell particles were probably amorphous phases, as shown in Figure 5a.On the other hand, the TiO 2 on the graphene porous composites was an anatase phase [42].Indeed, the band gap of the amorphous TiO 2 is higher than that of the rutile and anatase TiO 2 [43].Moreover, there may also be a difference in the mechanism of photocatalysis.In addition, we discussed the differences in reaction rates between the PS@TiO 2 coreshell particles and the commercial P25.The reaction rates of MB decomposition in the presence of the PS@TiO 2 core-shell particles and the commercial P25 from 0 to 4 min were 21.3% min −1 and 3.7% min −1 , respectively.Meanwhile, the reaction rates of MB decomposition from 4 to 24 min were 0.13% min −1 and 2.1% min −1 , respectively.The significantly faster reaction rate of the PS@TiO 2 core-shell particles might be related to the zeta potential of the samples.TiO 2 samples such as P25 reportedly have a negatively charged surface at a pH value of greater than 7 [44].Meanwhile, the PS@TiO 2 core-shell particles have a negatively charged surface at a pH value of more than 4-5 [45,46].Because the pH of the reaction solution was 6 in this study, the surface of the PS@TiO 2 core-shell particles was negatively charged, whereas the surface of the commercial P25 was positively charged.MB which have cationic ions were electrochemically attracted to the surface of the PS@TiO 2 core-shell particles, leading to an increase in the initial reaction rate.Meanwhile, the MB was electrochemically repelled from the surface of the commercial P25.The photocatalytic activity decreased after 4 min, probably because the rate of MB adsorption on the surface was faster than the rate of decomposition induced by the photocatalyst, and the MB covered the TiO 2 layer as it prevented the blocking of UV irradiation.In order to solve this problem, we suggest that TiO 2 hollow spheres are prepared via calcination of the PS@TiO 2 core-shell particles.The hollow spheres have high specific surface areas because of the void spaces, leading to an increase in contact frequency with the MB.We found that the PS particles can be decomposed via calcination at 723-873 K [47].The crystallinity of the TiO 2 simultaneously grows during calcination.Based on this knowledge, we expect that TiO 2 hollow spheres with crystalline shells can efficiently act as photocatalysts.

Materials and Methods
3.1.Synthesis of PS@TiO 2 Core-Shell Particles and TiO 2 particles PS particles were prepared with reference to a previous study [29].The collected PS was washed with ethanol (10 mL, Kanto Chemical Co., Tokyo, Japan, >99.5%) via centrifugation at 3800 rpm for 10 min, twice.The final PS contents were dispersed in ethanol (15 mL).This PS suspension (10 g), titanium-n-butoxide (1000 µL, Kanto Chemical Co., >98.0%), and ethanol (40 mL) were stirred at room temperature for 24 h.The obtained suspension was centrifuged at 3800 rpm for 10 min and washed with ethanol to obtain the precursor of the PS@TiO 2 core-shell particles.The precursor was spread in ethanol (20 mL).After that, ethanol and an aqueous NH 3 solution (300 or 1000 µL, Kanto Chemical Co., 28.0-30.0%NH 3 ) were added to the precursor suspension, which was then stirred at room temperature or 323 K for 17 h.Then, the suspension was centrifuged at 3800 rpm for 10 min, and the collected white contents were dried in a desiccator overnight to obtain the PS@TiO 2 core-shell particles.For comparison, we utilized commercial P25 (Kanto Chemical Co.) and TiO 2 particles prepared using the sol-gel method.The TiO 2 particles were prepared as follows: The titanium-n-butoxide (1000 µL), aqueous NH 3 solution (300 µL), and ethanol were stirred at room temperature for 1 h.After that, the white suspension was separated via filtration.The collected white powders were dried in a desiccator overnight to obtain the TiO 2 particles.

Characterization
The morphologies of the samples were observed via TEM (JEM 2010F, JEOL), operating at an accelerating voltage of 200 kV, and SEM (SU3500, JEOL), operating at an accelerating voltage of 15 kV.The chemical compositions and mapping images of the samples were determined using EDX spectroscopy.The specific surface areas of the samples were measured using N 2 sorption at 77 K using the BET method (Belsorp-mini, MicrotracBEL).The crystallites of the samples were determined using XRD (MultiFlex X-ray diffractometer, Rigaku) at 30 kV and 15 mA of CuKα radiation.PL measurement was performed to evaluate the samples using a fluorospectrophotometer with a Xe lamp light source (HITACHI, F-4500).The measurement excitation wavelength was set to 365 nm.All measurements of the sample were carried out at room temperature.

Photocatalytic Activity in MB Decomposition
The photocatalytic activities of the PS@TiO 2 core-shell particles, the prepared TiO 2 , the commercial P25, and the PS particles were evaluated using MB decomposition.Samples (10 mg) were added to 7 ppm aqueous MB solutions (100 mL).Thereafter, this suspension was stirred at 30 min in a black box (40 cm × 40 cm × 40 cm).Then, the photocatalytic reaction was started after irradiation with a UV lamp (365 nm) with the irradiation power (150 mW/cm 2 ) in the black box.A total of 3 mL of the mixture was withdrawn, and the pure aqueous MB solution was separated from the suspension by centrifugation at 3800 rpm for 5 min.The adsorption spectra of these solutions were recorded on spectrophotometer (UV-1800, Shimazu).

Conclusions
We synthesized PS@TiO 2 core-shell particles and demonstrated their photocatalytic activity in MB decomposition.In this method, TiO 2 nanoparticles prepared using the sol-gel method were collected on the surfaces of PS particles.Homogeneous PS@TiO 2 coreshell particles were obtained using 300 µL of an aqueous NH 3 solution and stirred at room temperature.TEM images showed that the TiO 2 shell thickness on the surfaces of the PS particles was approximately 5 nm.Additionally, Ti elements were confirmed to be present on the surfaces of the PS@TiO 2 core-shell particles using EDX analysis.The photocatalytic activities of the PS@TiO 2 core-shell particles were evaluated using MB decomposition.For comparison, we utilized TiO 2 , prepared using the sol-gel method, and commercial P25.The MB decompositions of the PS@TiO 2 core-shell particles, the prepared TiO 2 , and the commercial P25 after 24 min were 87%, 60%, and 58%, respectively.The crystallinity and shell thickness of TiO 2 on the surface of PS particles might play important role in MB decomposition.On the other hand, the reaction rates of the PS@TiO 2 core-shell particles between 0 and 4 min (21.3%min −1 ) were faster than those of the commercial P25 (3.7% min −1 ).Meanwhile, the rate of MB decomposition from 4 to 24 min was likely promoted in the presence of the PS@TiO 2 core-shell particles, probably because the surface of the PS@TiO 2 core-shell particles may have adsorbed the MB and its related decomposition products, leading to a decrease in photocatalytic activity.As one of the approaches to solve this problem, PS@TiO 2 core-shell particles can be synthesized via calcination to obtain TiO 2 hollow spheres.

Figure 1 .
Figure 1.Schematic illustration of the formation process of PS@TiO2 core-shell particles.

Figure 1 .
Figure 1.Schematic illustration of the formation process of PS@TiO 2 core-shell particles.

Figure 2 .
Figure 2. SEM images of PS@TiO 2 core-shell particles prepared at (a) room temperature, (b) 323 K using 300 µL of aqueous NH 3 solution, and (c) room temperature using 1000 µL of aqueous NH 3 solution.

Figure 6 .
Figure 6.UV-Vis absorption spectra of the reaction solutions with (a) PS@TiO2 core-shell particles, (b) prepared TiO2, (c) commercial P25, and (d) PS particles.(e) Relationship between C/C0 and the reaction time for MB decomposition.

Figure 7 .
Figure 7. UV-Vis absorption spectra of the reaction solutions with (a) PS@TiO2 core-shell particles without UV irradiation.(b) Relationship between C/C0 and the reaction time for MB decomposition in the presence of the PS@TiO2 core-shell particles with and without UV irradiation.

Figure 6 .
Figure 6.UV-Vis absorption spectra of the reaction solutions with (a) PS@TiO 2 core-shell particles, (b) prepared TiO 2 , (c) commercial P25, and (d) PS particles.(e) Relationship between C/C 0 and the reaction time for MB decomposition.

Figure 6 .
Figure 6.UV-Vis absorption spectra of the reaction solutions with (a) PS@TiO2 core-shell particles (b) prepared TiO2, (c) commercial P25, and (d) PS particles.(e) Relationship between C/C0 and th reaction time for MB decomposition.

Figure 7 .
Figure 7. UV-Vis absorption spectra of the reaction solutions with (a) PS@TiO2 core-shell particle without UV irradiation.(b) Relationship between C/C0 and the reaction time for MB decomposition in the presence of the PS@TiO2 core-shell particles with and without UV irradiation.

Figure 7 .
Figure 7. UV-Vis absorption spectra of the reaction solutions with (a) PS@TiO 2 core-shell particles without UV irradiation.(b) Relationship between C/C 0 and the reaction time for MB decomposition in the presence of the PS@TiO 2 core-shell particles with and without UV irradiation.

Table 1 .
Comparison of the MB decomposition of TiO 2 -polymer photocatalysts in previous reports.