Next Article in Journal
Research Progress of Macrocell Corrosion of Steel Rebar in Concrete
Next Article in Special Issue
Migration of TiO2 from PET/TiO2 Composite Films Used for Polymer-Laminated Steel Cans in Acidic Solution
Previous Article in Journal
Effectiveness of Lubricants and Fly Ash Additive on Surface Damage Resistance under ASTM Standard Operating Conditions
Previous Article in Special Issue
Fabrication of Modified Polyurethane Sponge with Excellent Flame Retardant and the Modification Mechanism
 
 
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Effect of Calcination Temperature on the Structure, Crystallinity, and Photocatalytic Activity of Core-Shell SiO2@TiO2 and Mesoporous Hollow TiO2 Composites

1
School of Environmental and Municipal Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
2
Key Laboratory of Yellow River Water Environment in Gansu Province, Lanzhou Jiaotong University, Lanzhou 730070, China
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(5), 852; https://doi.org/10.3390/coatings13050852
Submission received: 5 April 2023 / Revised: 25 April 2023 / Accepted: 27 April 2023 / Published: 30 April 2023

Abstract

:
TiO2 and core–shell SiO2@TiO2 nanoparticles were synthesized by sol-gel process at different calcination temperatures. Mesoporous hollow TiO2 composites were prepared by etching SiO2 from SiO2@TiO2 nanoparticles with alkali solution. X-ray diffraction (XRD), Scanning electron microscope (SEM),Transmission electron microscope (TEM), and N2 adsorption–desorption isotherms, and Roman and Diffuse reflectance spectroscopy (DRS) were employed to characterize the synthesized materials. The effects of different calcination temperatures on the morphology, crystallinity, phase composition, and photocatalytic activity of the prepared materials were investigated in detail. It was found that the calcination temperature altered the phase structure, crystallinity, morphology, specific surface area, and porous structure. Additionally, it was verified that SiO2 could inhibit the transfer of TiO2 from anatase phase to rutile phase under high temperature calcination (850 °C). The hollow TiO2 calcined at 850 °C showed the highest photocatalytic efficiency of 97.5% for phenol degradation under UV irradiation.

1. Introduction

Titanium dioxide (TiO2), as one of the most promising photocatalyst materials, has many advantageous properties, such as low toxicity, high photocatalytic activity, low cost, and high chemical stability [1,2,3]. TiO2 can generate electron-hole pairs and react with H2O/O2 to generate •O2/•OH radicals for degradation of organic pollutants in water [4]. However, pure TiO2 has large band gap(Eg 3.0–3.2 eV), easy agglomeration, phase transformation, and high electron-hole pair recombination rate, which reduces its photocatalytic activity [5]. It has been reported that nanoscale TiO2 exhibits enhanced photocatalytic activity under the morphological control of nanorods, nanotubes, core–shell nanospheres, and hollow spheres [6]. In general, these nanostructured TiO2 have much higher specific surface areas than bulk TiO2 with improved light utilization, which can shorten the charge carrier diffusion path length of the electron-hole pair and promote the increase in electrochemical reaction at the aqueous interface [7].
Among these controlled morphologies, core–shell nanospheres and hollow spheres have attracted wide attention due to their improved physical and chemical properties, such as higher surface area, low density, controlled morphology, surface permeability, and improved light-trapping effect [8]. It has been reported that SiO2 can be used as solid core and TiO2 as shell to synthesis core–shell SiO2@TiO2 nanospheres for photocatalytic degradation of dyes and other organic pollutants [9]. Similarly, hollow TiO2 spheres can be synthesized by “hard templating” method, in which a SiO2 core is etched with an alkaline solution [10].
During the process of synthesizing the core–shell SiO2@TiO2 nanospheres and hollow TiO2 spheres, calcination process is usually used to form photocatalyst to promote phase transformation, thermal decomposition and crystallinity [11]. Therefore, applying calcination to the formation of doped TiO2 can also improve its photocatalytic activity, morphology, surface area, crystallinity, and the photoabsorption of the photocatalyst.
For TiO2 photocatalyst, phase composition, surface area, and crystallinity are three crucial factors that affect the photocatalytic activity [4]. It is well known that TiO2 has three crystal forms of phase: brookite, anatase, and rutile. Among them, brookite is unstable. Anatase has higher reduction potential and lower recombination rate of electron-hole pairs, and can also produce more oxygen vacancies to capture electrons; thus, being identified as the most active phase from a photocatalytic point. Rutile has a lower band-gap energy (3.0 eV) than anatase with more stable crystalline structure, resulting in easy recombination of electrons-holes and almost no photocatalytic activity [8]. At the optimum calcination temperature [12], a well crystallized TiO2 anatase phase material has large surface area and small grain size, which is the preferred form of TiO2 based photocatalyst. However, the perfectly crystallized hollow TiO2 often conflicts with the realization of large surface area, because the increased calcination temperature used for TiO2 crystallization may lead to further TiO2 growth or agglomeration, ultimately reducing the specific surface area [13]. Generally, in the process of preparing hollow TiO2 by alkaline etching method, the phase structure of hollow TiO2 is unstable and easy to be destroyed. Additionally, the hollow TiO2 cannot form a uniform and complete hollow structure if the calcination temperature is too low; while if the calcination temperature is too high, the core–shell SiO2@TiO2 is easy to be destroyed and the hollow TiO2 will form rutile phase, which is not conducive to improve the photocatalytic activity [14]. Therefore, how to control the appropriate calcination temperature to form hollow TiO2 with complete morphology and stable phase composition is very important for the preparation of hollow TiO2. At present, few articles focus on systematic research and comparison of the influence of different calcination temperatures on the phase change structure and morphology characteristic of SiO2@TiO2 nanospheres and mesoporous hollow TiO2 nanoparticles [4,7,14].
In this paper, the TiO2, core–shell SiO2@TiO2 and mesoporous hollow TiO2 were prepared under different calcination temperatures. The effects of calcination temperature on microstructure, crystallinity, phase composition, and photocatalytic activity of TiO2, core–shell SiO2@TiO2 and mesoporous hollow TiO2 were investigated. Moreover, the correlation between calcination temperatures and microstructure, crystallinity and photocatalytic performance was discussed comparatively.

2. Experimental Procedure

2.1. Experiment Materials

Tetraethyl orthosilicate (TEOS), tetrabutyl titanate (TBT), ethanol, phenol, methylene blue (MB), and ammonia were purchased from Sinopharm chemical reagent company (China, Shanghai). None of the agents were purified.

2.2. Synthesis of SiO2 Core Spheres

The SiO2 core spheres were synthesized using the Stöber method according to the reported paper [15]. First, 10 mL TEOS was added to 90 mL ethanol. After continuous stirring, solution A was formed. At the same time, 20 mL water and 10 mL aqueous ammonia were added to 70 mL ethanol. After vigorous stirring, solution B was formed. Then, solution A and solution B were mixed under continuous stirring for 3 h at 40 °C. Finally, the products were centrifuged at 4000 rpm and washed twice with methanol and once with water. The final obtained products were dried at 70 °C for at least 20 h.

2.3. Synthesis of Core-Shell SiO2@TiO2 Nanospheres

The core–shell SiO2@TiO2 nanospheres were prepared by sol-gel process according to the literature [16]. First, 0.3 g SiO2 was sonicated in 100 mL ethanol for 30 min to obtain solution A. 4 mL TBT was added into 100 mL ethanol to obtain solution B. Then, solution B and 1.5 mL aqueous ammonia were added to solution A under vigorous stirring at 60 °C for 3 h. The resulting precipitates were centrifuged at 8000 rpm, washed twice with ethanol and once with water. Finally, these obtained products were dried at 70 °C for at least 20 h. Additionally, through the same procedures, unsupported TiO2 was also synthesized using 4 mL TBT by the same above procedures without SiO2 in the mixture.

2.4. Synthesis of TiO2 and SiO2@TiO2 Nanoparticles at Different Calcination Temperatures

The prepared TiO2 was calcined at 450 °C, 650 °C, 850 °C, and 1050 °C for 2 h, and the heating rate was 10 °C/min. The synthesized SiO2@TiO2 core–shell nanospheres were calcined for 2 h at 450 °C, 550 °C, 650 °C, 750 °C, 850 °C, 950 °C, and 1050 °C with the calcination heating rate of 10 °C/min, respectively. All the samples were synthesized in air atmosphere during calcination.

2.5. Synthesis of Mesoporous Hollow TiO2 Nanoparticles

An amount of 0.5 g core–shell SiO2@TiO2 nanoparticles calcined at different temperatures were dispersed in 60 mL water under ultrasonication for 30 min. Subsequently, 3 mL 2.5 M NaOH solution was added into the solution to etch the SiO2 core form SiO2@TiO2 nanoparticles under vigorous stirring at 30 °C for 6 h. Then, the etched hollow TiO2 nanoparticles were isolated by centrifugation at 8000 rpm, washed twice with ethanol and once with water. Finally, the obtained hollow TiO2 nanoparticles were dried at 70 °C for at least 20 h.

2.6. Characterization

The morphology of samples was characterized by scanning electron microscope (SEM, JSM-6701F, Japan), the sample was dispersed in ethanol solution and dropped onto a copper sample table. After spraying gold three times to increase conductivity, it was tested using SEM. Transmission electron microscopy (TEM, TECNAI G2,USA) was also used to test the morphology of samples with copper mesh as support film. The phases of the samples were performed on an X-ray diffraction (XRD), which was carried out by a RINT 2000 equipment with Cu Kα radiation (40 Kv/30 mA). The pore structure and Brunauer–Emmett–Teller (BET) specific surface area of the composite particles were determined from nitrogen gas absorption-desorption using an ASAP 2020 instrument. Raman spectroscopy was characterized by a Raman spectrometer (Renishaw 2000L, Britain, UK). UV-Vis diffuse reflectance spectra (UV-vis DRS) was tested by a UV-Vis spectrophotometer (Lambda 950) with a wavelength range of 200–800 nm and a reflectance standard of BaSO4. The photodegradation of phenol was detected by UV spectrophotometer(UV-3100).

2.7. Measurement of Photocatalytic Performance

The photocatalytic performance of the samples was evaluated by measuring the photodegradation of phenol in a reactor under UV light irradiation (500 W Hg lamp) for 180 min. In general, 75 mg of prepared photocatalysts was dispersed in 300 mL phenol aqueous solution with an initial concentration of 20 mg/L and stirred in the dark for 30 min to reach the adsorption–desorption equilibrium, then carried out under UV light. An amount of 4 mL solution was collected from the reactor at different irradiation intervals and the suspended particles were separated by centrifugation. The concentration of phenol was determined at 510 nm by colorimetric method of 4-amino antipyrine.

3. Results and Discussion

3.1. Structure and Characterization

3.1.1. SEM and TEM

The morphology and crystallinity of TiO2, SiO2@TiO2, and hollow TiO2 were characterized and compared by TEM and SEM, as shown in Figure 1, Figure 2, Figure 3 and Figure 4. Figure 1 showed the morphology and crystallinity of SiO2@TiO2 prepared at different calcination temperatures. The overall morphology of SiO2@TiO2 nanoparticles was spherical and the TiO2 was loaded on the surface of SiO2, thus forming a core–shell structure with a thickness of about 40–70 nm. When the calcination temperatures increased from 450 °C to 1050 °C, the shell surface TiO2 particle size of SiO2@TiO2 gradually increased and the core–shell structure became more obvious as shown in Figure 1a–d. In Figure 2a–d, the crystal structures of SiO2@TiO2 were further characterized by HRTEM. The images showed that the particle size of TiO2 gradually increased and the thickness of TiO2 shell structure decreased with the increase in calcination temperatures. The TiO2 shell sizes of SiO2@TiO2 were about 7 nm, 10 nm, 15 nm, and 30–50 nm when the calcination temperatures were 450 °C, 650 °C, 850 °C, and 1050 °C, respectively. Compared with the size of individual TiO2 particles prepared at different calcination temperatures, the particle size of TiO2 from SiO2@TiO2 was smaller than that of pure TiO2, which was consistent with the XRD characterization results of SiO2 inhibiting the nucleation and growth of TiO2 [17].
Figure 3 showed SEM images of hollow TiO2 prepared at different calcination temperatures. As shown in Figure 3, hollow TiO2 prepared at 450 °C and 650 °C had more obvious hollow structure than the hollow TiO2 prepared at 850 °C and 1050 °C. At the same time, with the increase in calcination temperature, TiO2 on the surface of SiO2 is easy to fall off and form agglomerated particles in the process of calcination and alkali corrosion. This was because with the increase in calcination temperature, the structure of SiO2@TiO2 core–shell photocatalyst became more stable, the binding force of Si-O-Ti bond was enhanced, and it was more difficult to remove SiO2 with alkaline solution. With the increase in the calcination temperature, the TiO2 particle size on the surface of SiO2 became larger during the calcination process, which was easy to fall off and to form agglomerated particles. At the same time, the increase in calcination temperatures would enhance the binding force of Si-O-Ti bond and make the SiO2@TiO2 core–shell structure more stable; thus, it was more difficult to etch SiO2 with alkaline and the hollow structure morphology was not obvious [18].
Figure 4 showed the TEM of hollow TiO2 prepared at different calcination temperatures. Compared with the SiO2 and SiO2@TiO2, the morphology of hollow TiO2 prepared at different calcination temperatures had hollow structure. However, there were two differences in the morphology characteristics of hollow TiO2 prepared at different calcination temperatures: First, with the increase in calcination temperature, the particle size of SiO2@TiO2 increased, and the crystal structure became more stable, and the TiO2 network structure containing silicate became firmer [19]. Therefore, SiO2 was more difficult to remove, and the hollow structure was less obvious, which was consistent with the previous SEM results. TEM results also showed that the hollow structure of TiO2 prepared at 450 °C was very obvious, while the hollow structures of TiO2 prepared at 650 °C, 850 °C and 1050 °C were not very obvious, and only part of SiO2 was removed. Second, as the calcination temperature increased, the thickness of TiO2 shell structure of SiO2@TiO2 decreased. The thickness of the hollow TiO2 shell at 450 °C, 650 °C, 850 °C, and 1050 °C calcination temperature was about 70 nm, 50 nm, 40 nm, and 20 nm, respectively (Figure 4c–f), which showed that the calcination temperature directly affected the morphology and structural stability of hollow TiO2 [20].

3.1.2. XRD

Figure 5 showed the XRD patterns of prepared TiO2, SiO2@TiO2 and hollow TiO2 nanoparticles, respectively. According to the XRD pattern in Figure 5a, different calcination temperatures had an obvious effect on the crystal structure of TiO2. After calcination at 450 °C, TiO2 crystal structure was anatase type (JCPDS No. 21-1272) with characteristic diffraction peak 2θ = 25.3°, 38.0°, and 48.2°. However, the characteristic diffraction peak intensity of anatase was weak and the peak shape was not sharp, indicating that TiO2 had low crystallinity and small particle size. Compared with TiO2 calcined at 450 °C, the crystal structure of TiO2 calcined at 650 °C was more obvious, and the characteristic diffraction peaks of anatase were 2θ = 25.3°, 38.0°, 48.2°, 54.0°, and 62.8°, corresponding to (101), (004), (200), (105), and (204) planes of anatase TiO2, respectively [21]. At the same time, the characteristic diffraction peak intensity was significantly enhanced, and the peak shape was sharp, indicating that the crystal structure of TiO2 anatase was more obviously stable and the crystallinity was higher after being calcined at 650 °C. When the calcination temperature increased to 850 °C, the crystal structure of TiO2 shifted from anatase phase to rutile phase, and obvious characteristic diffraction peaks at 27.5°, 36.1°, 54.3°, 56.8°, and 69.1° were shown, corresponding to (110), (101), (211), (220), and (301) planes of rutile TiO2 (JCPDS No. 21-1276), respectively [9]. When the calcination temperature increased to 1050 °C, the crystal structure of TiO2 rutile became more obvious, and the intensity of characteristic diffraction peak also increased, indicating that the particle size of TiO2 rutile phase became larger. According to the characteristic diffraction peak properties of XRD after calcination at different temperatures, the particle sizes of TiO2 at different calcined temperatures (450 °C, 650 °C, 850 °C, and 1050 °C) were 14.4 nm, 28.0 nm, 57.1 nm, and 95.5 nm, respectively, calculated by Scherrer equation (Rm = kλ1/2cosθ). The results showed that with the increase in calcined temperature, the particle size of TiO2 increases gradually [7].
Figure 5b showed the XRD results of SiO2@TiO2 nanoparticles prepared at different calcination temperatures. The XRD results of SiO2@TiO2 nanoparticles calcinated at 450 °C showed amorphous structure without obvious characteristic diffraction peak. The XRD results presented the anatase crystal diffraction peak with low intensity when the calcination temperature gradually increased from 550 °C to 750 °C. As the calcination temperature rose to 850 °C, the obvious anatase diffraction peak of SiO2@TiO2 nanoparticles appeared, while pure TiO2 belonged to obvious anatase with the calcination temperatures of 450 °C and 650 °C, which indicated that the nucleation and growth of TiO2 crystal were significantly inhibited due to the doping of SiO2, and the structural transfer of anatase phase to rutile phase was also inhibited during the synthesis process. The reported research also indicated that the doping of SiO2 would inhibit the growth and phase transfer of TiO2 at higher calcination temperature [22]. When the calcination temperature rose to 950 °C and 1050 °C, the phase structure of SiO2@TiO2 nanoparticles was composed of anatase and rutile homogenous structure. The crystal structure calcinated at 950 °C transferred with some rutile phase characteristic diffraction peak at 27.5° and 36.2°, but the main diffraction characteristic peak was 2θ = 25.3°, which belonged to anatase. The SiO2@TiO2 nanoparticles calcinated at 1050 °C had obvious rutile phase characteristic diffraction peak at 27.5°, 36.2°, 41.4°, 54.4°, and 56.8°, and the crystal structure of anatase was significantly weakened.
Compared with the pure TiO2 crystal structure of rutile at the calcination temperature of 1050 °C, SiO2@TiO2 nanoparticles still had part of anatase crystal structure at 1050 °C, which further showed that SiO2 could inhibit the transfer of TiO2 crystal structure from anatase to rutile phase. The main reason SiO2 could inhibit the phase transfer of TiO2 was that Ti-O-Si bond was formed during the synthesis of SiO2@TiO2 nanoparticles. As the calcination temperature increases, the Ti-O-Si bond became stronger, and the transfer of TiO2 from anatase to rutile became more difficult, thus inhibiting the growth of TiO2 particle size and crystal phase transfer [23,24].
Figure 5c showed the XRD of hollow TiO2 nanoparticles prepared by SiO2@TiO2 nanoparticles after etching with alkali. The crystal structure was basically consistent with that of SiO2@TiO2 nanoparticles at different calcination temperatures. The hollow TiO2 nanoparticles prepared at 450 °C belonged to amorphous structure. The XRD diffraction peaks of hollow TiO2 nanoparticles prepared after calcination temperature from 550 °C to 850 °C belonged to anatase crystal type. With the increase in calcination temperature, the intensity of its anatase characteristic diffraction peak increased, the peak shape became sharp, the crystallinity and the nanoparticle size also increased. The hollow TiO2 nanoparticles prepared after calcination at 950 °C and 1050 °C belonged to the mixed crystal structure of anatase and rutile, which indicated that the process of alkali corrosion did not destroy the crystalline structure of TiO2. At the same time, the characteristic diffraction peak of hollow TiO2 crystalline structure was more obvious and the crystallinity was higher compared with SiO2@TiO2 nanoparticles [25].
Figure 6 showed the XRD comparison diagram of TiO2, SiO2@TiO2, and hollow TiO2 prepared at different calcination temperatures. As can be seen from Figure 6a, when the calcination temperature was 450 °C, the crystalline structure of TiO2 was anatase, while SiO2@TiO2 and hollow TiO2 belonged to amorphous structure. The crystal structure of TiO2, SiO2@TiO2, and hollow TiO2 presented anatase crystal structure, while the characteristic diffraction peak intensity of TiO2 anatase was sharper than SiO2@TiO2 and hollow TiO2 when the calcination temperature was 650 °C (Figure 6b). When the calcination temperature increased to 850 °C (Figure 6c), the crystalline structure of TiO2 belonged to rutile phase, but SiO2@TiO2 and hollow TiO2 crystal structure was still anatase. When the calcination temperature increased to 1050 °C (Figure 6d), the crystalline structure of TiO2 was rutile phase, but the crystal structure of SiO2@TiO2 and hollow TiO2 belonged to the mixed crystal structure of rutile and anatase, which further indicated that SiO2 inhibited the transfer of TiO2 crystal structure from anatase phase to rutile phase. In addition, the XRD comparison diagram showed that the characteristic diffraction peak intensity of hollow TiO2 crystal structure was higher than that of SiO2@TiO2 for high purity and crystallinity of TiO2 [26].

3.1.3. BET

Figure 7 and Figure 8 showed the N2 adsorption–desorption isotherms of TiO2, SiO2@TiO2, and hollow TiO2. The Brunauer–Emmett–Teller (BET) method was used to calculate the surface area and pore size parameters of the synthesized samples, which were shown in Table 1. Figure 7a indicated that the N2 adsorption–desorption curves calcined at 450 °C and 650 °C belonged to type IV adsorption curves with H2 type hysteresis loops and were mesoporous materials. Additionally, the N2 adsorption–desorption curves of TiO2 calcined at 850 °C and 1050 °C belonged to type II isotherm, which belonged to non-porous materials. As shown in Figure 7b, N2 adsorption–desorption curves of SiO2@TiO2 calcined at different temperatures were quite different. The N2 adsorption–desorption curves of SiO2@TiO2 calcined at 450 °C, 650 °C, and 850 °C could be categorized as type IV with H2 type hysteresis loops, belonging to mesoporous materials. The N2 adsorption–desorption curve of SiO2@TiO2 calcined at 1050 °C belonged to type II isotherm and was non-porous material, which was consistent with the results of TiO2 calcined at 1050 °C. As shown in Figure 7c, N2 adsorption–desorption curves of hollow TiO2 calcined at different temperatures belonged to type IV adsorption curves with H2 type hysteresis loops (calcined at 450 °C, 650 °C, and 850 °C) and H3 type hysteresis loop (calcined at 1050 °C). Additionally, the N2 adsorption–desorption curves of hollow TiO2 calcined at 650 °C and 850 °C showed more obvious H2 type hysteresis loops and more obvious mesoporous material properties.
Figure 8 showed the comparison diagram of N2 adsorption–desorption curves of TiO2, SiO2@TiO2, and hollow TiO2 calcined at different temperatures. The results indicated that SiO2@TiO2 had more obvious mesoporous material properties with H2 type hysteresis loop when calcined at 450 °C. However, as the calcination temperatures increased to 650 °C, 850 °C, and 1050 °C, the hollow TiO2 showed more obvious hysteresis ring structure, which indicated that hollow TiO2 had more obvious mesoporous material properties. Compared with TiO2 and SiO2@TiO2, hollow TiO2 had higher specific surface area, smaller density and better dispersion [27].
Table 1 showed the specific surface area, pore size, and pore volume of the prepared photocatalysts. The results showed that the specific surface area of TiO2, SiO2@TiO2, and hollow TiO2 decreased with the increase in calcination temperature, which was related to the increase in particle size and the easy destruction of core–shell structure at higher calcination temperature. The results also showed that SiO2@TiO2 presented higher specific surface area than TiO2 and hollow TiO2 after calcinated at 450 °C and 650 °C, this was because the hollow TiO2 calcinated at 450 °C and 650 °C could be destroyed by alkaline solution and the etched TiO2 was easy to agglomerate; thus, the core–shell structure was destroyed and the specific surface area decreased. However, the specific surface area of hollow TiO2 calcinated at 850 °C and 1050 °C was higher than SiO2@TiO2 calcinated at 850 °C and 1050 °C because the morphology and crystal structure were more stable, the TiO2 shells were not easy to be destroyed at 850 °C and 1050 °C, and the hollow structure was conducive to increasing the specific surface area [28]. The determination results of pore size and pore volume of TiO2, SiO2@TiO2, and hollow TiO2 were consistent with specific surface area. The results showed that the smaller the pore size, the larger the specific surface area, and the larger the corresponding pore volume. In summary, the calcination temperature greatly affected the specific surface area and mesoporous properties of TiO2, SiO2@TiO2, and hollow TiO2, and hollow TiO2 showed more obvious mesoporous material properties.

3.1.4. Raman Spectra

Raman spectroscopy was also used as an additional characterization method to study the phase transformation of TiO2 at different calcination temperatures for the anatase phase and the rutile phase have different Raman active modes. Figure 9 showed the Raman spectra of hollow TiO2 prepared at different calcination temperatures. As shown in Figure 9, the hollow TiO2 calcined at 450 °C was amorphous due to its unstable crystal structure and did not have Raman characteristic absorption peak. The hollow TiO2 calcined at 650 °C and 850 °C had Raman characteristic absorption peaks at 385 cm−1, 505 cm−1 and 626 cm−1, which belonged to anatase phase Raman characteristic absorption peaks [29]. Additionally, the Raman characteristic absorption peak intensity of anatase calcined at 850 °C was significantly higher than that of calcined at 650 °C, this was because the crystallinity of hollow TiO2 calcined at 850 °C was higher and the crystal structure was more stable, which was consistent with the XRD results of hollow TiO2 calcined at 850 °C (Figure 1c). The XRD results of hollow TiO2 calcined at 1050 °C showed that it was a mixed crystal phases of rutile and anatase. The Raman spectrum results also indicated that in addition to the Raman characteristic absorption peaks of anatase phase at 385 cm−1, 505 cm−1 and 626 cm−1, rutile phase also had Raman characteristic absorption peaks at 435 cm−1 and 601 cm−1 [30,31]. The Raman spectra of hollow TiO2 calcined at different temperatures were consistent with the previous XRD characterization results.

3.1.5. Diffuse Reflectance Spectra

Figure 10 showed the UV-vis absorption spectra and the band gap energy of TiO2, SiO2@TiO2, and hollow TiO2 samples calcinated at 850 °C. The band gap energy was estimated by extrapolating the linear region of the plot of (αhv)2 versus photon energy (hv). The optical adsorption spectra of the nanocomposites were shown in Figure 10a, the TiO2 had an absorption band in the region of 380–430 nm because of the crystal structure in rutile phase as shown in XRD results (Figure 1a). The hollow TiO2 showed absorption band around 320–420 nm for the anatase crystal structure as shown in Figure 1c. However, due to the synergistic effect of Ti-Si, the absorption band of SiO2@TiO2 nanoparticles had a blue shift to higher wavelength region [32]. The calculated band gap energies of TiO2, SiO2@TiO2 and hollow TiO2 were 2.88, 3.20, and 3.30, respectively (Figure 10b). The lower band gap energy of TiO2 nanoparticles (2.88 eV) was aroused from the rutile phase, the higher band gap width of hollow TiO2 (3.30 eV) could cause a lowering in the energy of valence band and an increase in the conduction band edge, which can promote the separation of electrons and holes and inhibit their recombination, resulting in higher photocatalytic activity [33].

3.1.6. Photoluminescence Spectra

Figure 11 showed the photoluminescence spectra of TiO2, SiO2@TiO2, and hollow TiO2 samples calcinated at 850 °C. The photoluminescence spectra of semiconductor can provide important information about interfacial electron transfer and the charge carrier recombination process [32]. After excitation at a wavelength of about 400 nm, a broad signal (360–440 nm) was obtained. The results presented that the hollow TiO2 had minimum photoluminescence emission compared with TiO2 and SiO2@TiO2, which indicated that the thin shell layer of hollow structure reduced the transmission distance of charge carriers and suppressed charge recombination.

3.2. Photocatalytic Degradation of Phenol

The photocatalytic activities of TiO2, SiO2@TiO2, and hollow TiO2 prepared at different calcination temperatures for phenol degradation under UV light irradiation were studied in Figure 12. As shown in Figure 12a, TiO2 calcined at different temperatures did not absorb phenol in the dark reaction stage. At the 180 min light reaction stage, the degradation efficiencies of TiO2 calcined at 450 °C and 650 °C were significantly higher than that of TiO2 calcined at 850 °C and 1050 °C, which indicated that the crystal structure of TiO2 was closely related to photocatalytic activity. The XRD results (Figure 1a) showed that TiO2 nanoparticles calcined at 450 °C and 650 °C were mainly anatase phase, while TiO2 nanoparticles calcined at 850 °C and 1050 °C were mainly rutile phase. It has been reported that anatase TiO2 had better photocatalytic performance than rutile TiO2 [33]. The degradation efficiency of TiO2 calcinated at 650 °C was the highest, 95.2%, which was related to the high crystallinity of anatase TiO2. The photocatalytic activity of SiO2@TiO2 calcined at different temperatures was shown in Figure 12b. The preparedSiO2@TiO2 also had no adsorption on phenol at the dark stage, and the photocatalytic efficiency in the light reaction followed the order of 850 °C > 650 °C > 1050 °C > 450 °C. Compared with phenol degradation by TiO2 calcined at different temperatures, SiO2@TiO2 calcined at different temperatures showed quite different photocatalytic performance due to different phase structure and specific surface area. The crystal structures of SiO2@TiO2 after calcination at 450 °C and 650 °C were anatase phase, but the crystallinity of SiO2@TiO2 was lower than that of TiO2 calcined at 450 °C and 650 °C, as shown in Figure 1b. Therefore, the photocatalytic efficiency of SiO2@TiO2 calcined at 450 °C and 650 °C was lower than that of TiO2 calcined at 450 °C and 650 °C. Additionally, SiO2@TiO2 calcined at 1050 °C also had higher photocatalytic efficiency than TiO2 calcined at 1050 °C for the anatase and rutile mixed crystal structure. However, the SiO2@TiO2 calcined at 850 °C had the best photocatalytic performance of 90.9%, which was due to its high anatase content and high crystallinity. It has been reported that high crystallinity is essential to improve the generation and migration of electron/hole pairs on the surface of TiO2 [34,35,36,37].
Figure 12c presented that hollow TiO2 calcined at different temperatures had better photocatalytic activity than SiO2@TiO2 calcined at different temperatures. The photocatalytic activity of hollow TiO2 calcined at 850 °C was the highest photocatalytic efficiency of 97.5%, which was related to the results of SiO2@TiO2 calcined at 850 °C. As shown in Table 1, hollow TiO2 nanoparticles calcined at 850 °C had larger specific surface area than SiO2@TiO2, which increased light absorption, diffraction and reflection. At the same time, the thin shell layer reduced the transmission distance of charge carriers and inhibited charge recombination [38]. In addition, the hollow structure increased the dispersion of photocatalysis in the liquid reaction system, supplied more active sites, and improved the photocatalytic efficiency. Figure 12d compared and showed the photodegradation of TiO2, SiO2@TiO2, and hollow TiO2 prepared at different calcination temperatures for phenol degradation under UV light. In summary, calcination temperature had a great influence on photocatalytic performance. The photocatalytic activities of the three main different nanoparticles abided by the following order: hollow TiO2 calcined at 850 °C > TiO2 calcined at 650 °C > SiO2@TiO2 calcined at 850 °C > hollow TiO2 calcined at 650 °C > SiO2@TiO2 calcined at 650 °C > hollow TiO2 calcined at 1050 °C. The photocatalytic activity of TiO2 with different structures mainly depended on the crystal structure and specific surface area. TiO2 with different structures and morphology should control its crystal structure and physical properties to obtain higher photocatalytic efficiency [39,40]. Compared with reported paper about the TiO2-SiO2 and hollow TiO2 composites for photodegradation of phenol as shown in Table 2 [23,36,37,40,41,42], the hollow TiO2 calcined at 850 °C showed higher photocatalytic efficiency with less concentration of photocatalysis (0.25 g/L).

4. Conclusions

The effects of different calcination temperatures on the morphology, crystal structure, specific surface and photocatalytic efficiency of TiO2, SiO2@TiO2, and hollow TiO2 were deeply and systematically studied. The results verified that SiO2 could inhibit the transfer of TiO2 from anatase phase to rutile phase under high temperature calcination (850 °C). The photocatalytic activity results showed that the photocatalytic efficiency of hollow TiO2 at different temperatures was higher than SiO2@TiO2. The hollow TiO2 calcined at 850 °C presented the highest photocatalytic efficiency for phenol degradation for the higher crystallinity of anatase structure and specific surface area than TiO2. Therefore, the experimental results showed that the calcination temperature had a great influence on the morphology and photocatalytic activity of TiO2, SiO2@TiO2, and hollow TiO2. The influence of calcination temperature should be fully considered in the preparation and application of TiO2 photocatalyst.

Author Contributions

Methodology, N.F., H.C., R.C. and S.D.; writing—review and editing, X.R. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Yong Scholars Science Foundation of Lanzhou Jiaotong University (2022044).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Jaleh, B.; Nasrollahzadeh, M.; Mohazzab, B.F.; Eslamipanah, M.; Sajjadi, M.; Ghafu, H. State-of-the-art technology: Recent investigations on laser-mediated synthesis of nanocomposites for environmental remediation. Ceram. Int. 2021, 47, 10389–10425. [Google Scholar] [CrossRef]
  2. Tang, J.J.; Chen, Y.Q.; Dong, Z.J. Effect of crystalline structure on terbuthylazine degradation by H2O2-assisted TiO2 photocatalysis under visible irradiation. J. Environ. Sci. 2019, 79, 153–160. [Google Scholar] [CrossRef] [PubMed]
  3. Nakata, K.; Fujishima, A. TiO2 photocatalysis: Design and applications. J. Photochem. Photobiol. C Photochem. Rev. 2012, 13, 169–189. [Google Scholar] [CrossRef]
  4. Chen, J.; Qiu, F.; Xu, W.; Cao, S.; Zhu, H. Recent progress in enhancing photocatalytic efficiency of TiO2-based materials. Appl. Catal. Gen. 2015, 495, 131–140. [Google Scholar] [CrossRef]
  5. Li, W.; Yang, J.P.; Wu, Z.X.; Wang, J.X.; Li, B.; Feng, S.S.; Deng, Y.H.; Zhang, F.; Zhao, D.Y. A versatile kinetics-controlled coating method to construct uniform porous TiO2 shells for multifunctional core shell structures. J. Am. Chem. Soc. 2012, 134, 11864–11867. [Google Scholar] [CrossRef]
  6. Yang, H.G.; Sun, H.S.; Qiao, S.Z.; Zou, J.; Liu, G.; Smith, S.C.; Cheng, H.M.; Lu, G.Q. Anatase TiO2 single crystals with a large percentage of reactive facets. Nature 2008, 453, 638–641. [Google Scholar] [CrossRef]
  7. Tian, G.H.; Fu, H.G.; Jing, L.Q.; Tian, C.J. Synthesis and photocatalytic activity of stable nanocrystalline TiO2 with high crystallinity and large surface area. J. Hazard Mater. 2009, 161, 1122–1130. [Google Scholar] [CrossRef]
  8. Anitha, B.; Khadar, M.A. Anatase-rutile phase transformation and photocatalysis in peroxide gel route prepared TiO2 nanocrystals: Role of defect states. Solid State Sci. 2020, 108, 106392. [Google Scholar] [CrossRef]
  9. Zhang, W.; He, H.L.; Tian, Y.; Li, H.Z.; Lan, K.; Zu, L.H.; Xia, Y.; Duan, L.L.; Li, W.; Zhao, D.Y. Defect-engineering of mesoporous TiO2 microspheres with phase junctions for efficient visible-light driven fuel production. Nano Energy 2019, 66, 104113. [Google Scholar] [CrossRef]
  10. Li, L.; Chen, X.H.; Xiong, X.; Wu, X.P.; Xie, Z.N.; Liu, Z.H. Synthesis of hollow TiO2@SiO2 spheres via a recycling template method for solar heat protection coating. Ceram. Int. 2021, 47, 2678–2685. [Google Scholar] [CrossRef]
  11. Nemiwal, M.; Kumar, D. TiO2 and SiO2 encapsulated metal nanoparticles: Synthetic strategies, properties, and photocatalytic applications. Inorg. Chem. Commun. 2021, 128, 108602. [Google Scholar] [CrossRef]
  12. Wang, S.; Zhao, L.; Bai, L.N.; Yan, J.M.; Jiang, Q.; Lian, J.S. Enhancing photocatalytic activity of disorder-engineered C/TiO2 and TiO2 Nanoparticles. J. Mater. Chem. A 2014, 2, 7439–7445. [Google Scholar] [CrossRef]
  13. Dhaifallah, D.; Xia, Y.Y.; Zhao, D.Y. Mesoporous TiO2 microspheres with precisely controlled crystallites and architectures. Inside Chem. 2018, 4, 2436–2450. [Google Scholar] [CrossRef]
  14. Joo, J.B.; Zhang, Q.; Lee, I.; Dahl, M.; Zaera, F.; Yin, Y.D. Mesoporous anatase titania hollow nanostructures though silica-protected calcinations. Adv. Funct. Mater. 2012, 22, 166–174. [Google Scholar] [CrossRef]
  15. Fu, N.; Ren, X. Synthesis of double-shell hollow TiO2@ZIF-8 nanoparticles with enhanced photocatalytic activities. Front. Chem. 2020, 8, 578847. [Google Scholar] [CrossRef]
  16. Meng, H.L.; Cui, C.; Shen, H.L.; Liang, D.Y.; Xue, Y.Z.; Li, P.G.; Tang, W.H. Synthesis and photocatalytic activity of TiO2@CdS and CdS@TiO2 double-shelled hollow spheres. J. Alloys Compd. 2012, 527, 30–35. [Google Scholar] [CrossRef]
  17. Guo, N.; Liang, Y.; Lan, S.; Liu, L.; Ji, G.; Gan, S.; Zou, H.; Xu, X. Uniform TiO2-SiO2 hollow nanospheres: Synthesis, characterization and enhanced adsorption-photodegradation of azo dyes and phenol. Appl. Surf. Sci. 2014, 305, 562–574. [Google Scholar] [CrossRef]
  18. Joo, J.B.; Zhao, Q.; Dahl, M.; Lee, I.; Goebl, J.; Zaera, F.; Yin, Y.D. Control of the nanoscale crystallinity in mesoporous TiO2 shells for enhanced photocatalytic activity. Energy Environ. Sci. 2012, 5, 6321–6327. [Google Scholar] [CrossRef]
  19. Liu, J.X.; Shi, F.; Bai, L.N.; Feng, X.; Wang, X.K.; Bao, L. Synthesis of TiO2-SiO2 aerogel via ambient pressure drying: Effects of sol pre-modification on the microstructure and pore characteristics. J. Sol. Gel Sci. Technol. 2013, 69, 93–101. [Google Scholar] [CrossRef]
  20. Neshchimenko, V.V.; Li, C.D.; Mikhailov, M.M. Radiation stability of TiO2 hollow particles pigments and coatings synthesis by hydrothermal methods from TTIP. Dyes Pigments 2017, 145, 354–358. [Google Scholar] [CrossRef]
  21. Ullah, S.; Ferreira-Neto, E.P.; Pasa, A.A.; Alcântara, C.C.J.; Acuña, J.J.S.; Bilmes, S.A.; Ricci, M.L.; Landers, R.; Fermino, T.Z.; Rodrigues-Filho, U.P. Enhanced photocatalytic properties of core@shell SiO2@TiO2 nanoparticles. Appl. Catal. B Environ. 2015, 179, 333–343. [Google Scholar] [CrossRef]
  22. Dahl, M.; Dang, S.; Joo, J.B.; Zhao, Q.; Yin, Y.D. Control of the crystallinity in TiO2 microspheres through silica impregnation. CrystEngComm 2012, 14, 7680–7685. [Google Scholar] [CrossRef]
  23. Wang, Y.B.; Xing, Z.P.; Li, Z.Z.; Wu, X.Y.; Wang, G.F.; Zhou, W. Facile synthesis of high-thermostably ordered mesoporous TiO2/SiO2 nanocomposites: An effective bifunctional candidate for removing arsenic contaminations. J. Colloid Interface Sci. 2017, 485, 32–38. [Google Scholar] [CrossRef] [PubMed]
  24. Chang, W.; Yan, L.; Bin, L.; Sun, R. Photocatalyic activity of double pore structure TiO2/SiO2 monoliths. Ceram. Int. 2017, 43, 5881–5886. [Google Scholar] [CrossRef]
  25. Liu, H.Y.; Joo, J.B.; Dahl, M.; Fu, L.S.; Zeng, Z.Z.; Yin, Y.D. Crystallinity control of TiO2 hollow shells through resin-protected calcination for enhanced photocatalytic activity. Energy Environ. Sci. 2014, 8, 286–296. [Google Scholar] [CrossRef]
  26. Wang, X.; Bai, L.; Liu, H.; Yu, X.F.; Yin, Y.D.; Gao, C.B. A unique disintegration-reassembly route to mesoporous titania nanocrystalline hollow spheres with enhanced photocatalytic activity. Adv. Funct. Mater. 2018, 28, 1704208. [Google Scholar] [CrossRef]
  27. Kitsou, I.; Panagopoulos, P.; Maggos, T.; Arkas, M.; Tsetsekou, A. Development of SiO2@TiO2 core-shell nanospheres for catalytic applications. Appl. Surf. Sci. 2018, 441, 223–231. [Google Scholar] [CrossRef]
  28. Bao, Y.; Kang, Q.L.; Ma, J.Z. Structural regulation of hollow spherical TiO2 by varying titanium source amount and their thermal insulation property. Colloids Surf. A 2018, 537, 69–75. [Google Scholar] [CrossRef]
  29. Thakur, A.; Hamamoto, T.; Ikeda, T.; Chammingkwan, P.; Wada, T.; Taniike, T. Microwave-assisted polycondensation for screening of organically-modified TiO2/SiO2 catalysts. Appl. Catal. A-Gen. 2020, 595, 117508. [Google Scholar] [CrossRef]
  30. Nguyen, D.T.; Kim, K.S. Self-development of hollow TiO2 nanoparticles by chemical conversion coupled with Ostwald ripening. Chem. Eng. J. 2016, 286, 266–271. [Google Scholar] [CrossRef]
  31. Yu, K.F.; Ling, M.Q.; Liang, J.C.; Liang, C. Formation of TiO2 hollow spheres through nanoscale Kirkendall effect and their lithium storage and photocatalytic properties. Chem. Phys. 2019, 517, 222–227. [Google Scholar] [CrossRef]
  32. Chowdhury, I.H.; Roy, M.; Kundu, S.; Naskar, M.K. TiO2 hollow microspheres impregnated with biogenic gold nanoparticles for the efficient visible light-induced photodegradation of phenol. J. Phys. Chem. Solids 2019, 129, 329–339. [Google Scholar] [CrossRef]
  33. Fu, X.X.; Fan, C.Y.; Shi, L.; Yu, S.Q.; Wang, Z.Y. Hollow TiO2 microspheres: Template-free synthesis, remarkable structure stability, and improved photoelectric performance. New J. Chem. 2016, 10, 1039. [Google Scholar] [CrossRef]
  34. Shen, H.L.; Hu, H.H.; Liang, D.Y.; Meng, H.L.; Li, P.G.; Tang, W.H.; Cui, C. Effect of calcination temperature on the microstructure, crystallinity and photocatalytic activity of TiO2 hollow spheres. J. Alloys Compd. 2012, 542, 32–36. [Google Scholar] [CrossRef]
  35. Zhang, H.; Sun, S.; Ding, H.; Deng, T.; Wang, J. Effect of calcination temperature on the structure and properties of SiO2 microspheres/nano-TiO2 composites. Mat. Sci. Semicon. Proc. 2020, 115, 105099. [Google Scholar] [CrossRef]
  36. Babu, B.; Shim, J.; Yoo, K. Effects of annealing on bandgap and surface plasmon resonance enhancement in Au/SnO2 quantum dots. Ceram. Int. 2020, 46, 17–22. [Google Scholar] [CrossRef]
  37. Babu, B.; Talluri, B.; Gurugubelli, T.R.; Kim, J.; Yoo, K. Effect of annealing environment on the photoelectrochemical water oxidation and electrochemical supercapacitor performance of SnO2 quantum dots. Chemosphere 2022, 286, 131577. [Google Scholar] [CrossRef]
  38. Li, X.; He, J. Synthesis of raspberry-like SiO2-TiO2 nanoparticles toward antireflective and self-cleaning coatings. ACS Appl. Mater. Interfaces 2013, 5, 5282–5290. [Google Scholar] [CrossRef]
  39. Sun, S.; Deng, T.; Ding, H.; Chen, Y.; Chen, W. Preparation of nano-TiO2-coated SiO2 microsphere composite material and evaluation of its self-cleaning property. Nanomaterials 2017, 7, 367. [Google Scholar] [CrossRef]
  40. Alanazi, H.; Hu, J.; Kim, Y.R. Effect of slag, silica fume, and metakaolin on properties and performance of alkali-activated fly ash cured at ambient temperature. Construct. Build. Mater. 2019, 197, 747–756. [Google Scholar] [CrossRef]
  41. Wang, J.; Sun, S.; Ding, H.; Chen, W.; Liang, Y. Preparation of a composite photocatalyst with enhanced photocatalytic activity: Smaller TiO2 carried on SiO2 microsphere. Appl. Surf. Sci. 2019, 493, 146–156. [Google Scholar] [CrossRef]
  42. Fuentes, K.M.; Betancourt, P.; Marrero, S.; García, S. Photocatalytic degradation of phenol using doped titania supported on photonic SiO2 spheres. React. Kinet. Mech. Cat. 2017, 120, 403–415. [Google Scholar] [CrossRef]
Figure 1. TEM images of SiO2@TiO2 prepared at different calcination temperatures (a) 450 °C, (b) 650 °C, (c) 850 °C, (d) 1050 °C.
Figure 1. TEM images of SiO2@TiO2 prepared at different calcination temperatures (a) 450 °C, (b) 650 °C, (c) 850 °C, (d) 1050 °C.
Coatings 13 00852 g001
Figure 2. HRTEM images of SiO2@TiO2 prepared at different calcination temperatures (a) 450 °C, (b) 650 °C, (c) 850 °C, (d) 1050 °C.
Figure 2. HRTEM images of SiO2@TiO2 prepared at different calcination temperatures (a) 450 °C, (b) 650 °C, (c) 850 °C, (d) 1050 °C.
Coatings 13 00852 g002
Figure 3. SEM images of hollow TiO2 prepared at different calcination temperatures (a) 450 °C, (b) 650 °C, (c) 850 °C, (d) 1050 °C.
Figure 3. SEM images of hollow TiO2 prepared at different calcination temperatures (a) 450 °C, (b) 650 °C, (c) 850 °C, (d) 1050 °C.
Coatings 13 00852 g003
Figure 4. TEM images of hollow TiO2prepared at different calcination temperatures (a) SiO2, (b) SiO2@TiO2 (450 °C), (c) 450 °C, (d) 650 °C, (e) 850 °C, (f) 1050 °C.
Figure 4. TEM images of hollow TiO2prepared at different calcination temperatures (a) SiO2, (b) SiO2@TiO2 (450 °C), (c) 450 °C, (d) 650 °C, (e) 850 °C, (f) 1050 °C.
Coatings 13 00852 g004
Figure 5. XRD spectra of (a) TiO2, (b) SiO2@TiO2, and (c) hollow TiO2 prepared at different calcination temperatures.
Figure 5. XRD spectra of (a) TiO2, (b) SiO2@TiO2, and (c) hollow TiO2 prepared at different calcination temperatures.
Coatings 13 00852 g005aCoatings 13 00852 g005b
Figure 6. XRD spectra of prepared nanoparticles calcined at different temperatures (a) 450 °C, (b) 650 °C, (c) 850 °C, (d) 1050 °C.
Figure 6. XRD spectra of prepared nanoparticles calcined at different temperatures (a) 450 °C, (b) 650 °C, (c) 850 °C, (d) 1050 °C.
Coatings 13 00852 g006
Figure 7. N2 absorption-desorption isotherms of (a) TiO2, (b) SiO2@TiO2, and (c) hollow TiO2 prepared at different calcination temperatures.
Figure 7. N2 absorption-desorption isotherms of (a) TiO2, (b) SiO2@TiO2, and (c) hollow TiO2 prepared at different calcination temperatures.
Coatings 13 00852 g007
Figure 8. N2 absorption-desorption isotherms of prepared nanoparticles calcined at different temperatures (a) 450 °C, (b) 650 °C, (c) 850 °C, (d) 1050 °C.
Figure 8. N2 absorption-desorption isotherms of prepared nanoparticles calcined at different temperatures (a) 450 °C, (b) 650 °C, (c) 850 °C, (d) 1050 °C.
Coatings 13 00852 g008
Figure 9. Raman spectra of hollow TiO2 at different calcination temperatures (a) 450 °C, (b) 650 °C, (c) 850 °C, (d) 1050 °C.
Figure 9. Raman spectra of hollow TiO2 at different calcination temperatures (a) 450 °C, (b) 650 °C, (c) 850 °C, (d) 1050 °C.
Coatings 13 00852 g009
Figure 10. (a) Diffuse reflectance spectra and (b) band gap energy calculation of TiO2, SiO2@TiO2, and hollow TiO2 prepared at 850 °C.
Figure 10. (a) Diffuse reflectance spectra and (b) band gap energy calculation of TiO2, SiO2@TiO2, and hollow TiO2 prepared at 850 °C.
Coatings 13 00852 g010
Figure 11. Photoluminescence spectra obtained for TiO2, SiO2@TiO2, and hollow TiO2 prepared at 850 °C.
Figure 11. Photoluminescence spectra obtained for TiO2, SiO2@TiO2, and hollow TiO2 prepared at 850 °C.
Coatings 13 00852 g011
Figure 12. Photocatalytic activity of (a) TiO2, (b) SiO2@TiO2, (c) hollow TiO2, and (d) photocatalytic degradation efficiency diagram prepared at different calcination temperatures.
Figure 12. Photocatalytic activity of (a) TiO2, (b) SiO2@TiO2, (c) hollow TiO2, and (d) photocatalytic degradation efficiency diagram prepared at different calcination temperatures.
Coatings 13 00852 g012
Table 1. BET specific surface area and fitting parameters of TiO2, SiO2@TiO2, and hollow TiO2 prepared at different calcination temperatures.
Table 1. BET specific surface area and fitting parameters of TiO2, SiO2@TiO2, and hollow TiO2 prepared at different calcination temperatures.
Calcination Temperatures (°C)BET Surface Area (m2g−1)Pore Size
(nm)
Pore Volume
(cm3g−1)
TiO245087.01.260.309
65014.43.850.164
8502.626.500.036
10500.8681.870.002
SiO2@TiO24502803.190.256
6501054.120.120
85014.525.50.059
10505.5562.30.049
hollow TiO24502134.840.157
65073.11.480.172
85029.21.740.113
105015.44.680.127
Table 2. A comparison of TiO2-SiO2 and hollow TiO2 composites for photodegradation of phenol.
Table 2. A comparison of TiO2-SiO2 and hollow TiO2 composites for photodegradation of phenol.
PhotocatalystSynthesis MethodLight SourceInitial Concentration
of Phenol (mg/L)
Concentration
of Photocatalysis (g/L)
Reaction Time (min)Efficiency
(%)
Ref.
TiO2-SiO2 phtocatalystSol-gelUV light (150 W)501.012048.0[36]
Titania-silica compositesNon-aqueous approachUV light (8 W)1003.018067.0[37]
TiO2/SiO2 nanoparticlesHydrothermal methodUV light (30 W)101.03596.4[40]
Hollow TiO2-MoS2Hydrothermal methodVisble light (300 W Xe lamp)101.6715078.0[41]
Hollow TiO2 nanocompositeSol-gel processUV light (6 W)23.50.506021.4[42]
TiO2-SiO2 hollow nanospheresSol-gel methodUV light (175 W)802.514090.0[23]
Hollow TiO2 nanocompositeSol-gel processUV light (500 W)200.2518097.5This paper
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Fu, N.; Chen, H.; Chen, R.; Ding, S.; Ren, X. Effect of Calcination Temperature on the Structure, Crystallinity, and Photocatalytic Activity of Core-Shell SiO2@TiO2 and Mesoporous Hollow TiO2 Composites. Coatings 2023, 13, 852. https://doi.org/10.3390/coatings13050852

AMA Style

Fu N, Chen H, Chen R, Ding S, Ren X. Effect of Calcination Temperature on the Structure, Crystallinity, and Photocatalytic Activity of Core-Shell SiO2@TiO2 and Mesoporous Hollow TiO2 Composites. Coatings. 2023; 13(5):852. https://doi.org/10.3390/coatings13050852

Chicago/Turabian Style

Fu, Ning, Hongjin Chen, Renhua Chen, Suying Ding, and Xuechang Ren. 2023. "Effect of Calcination Temperature on the Structure, Crystallinity, and Photocatalytic Activity of Core-Shell SiO2@TiO2 and Mesoporous Hollow TiO2 Composites" Coatings 13, no. 5: 852. https://doi.org/10.3390/coatings13050852

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop