Preparation and Properties of CaCO3-Supported Nano-TiO2 Composite with Improved Photocatalytic Performance

In order to improve the photocatalytic degradation efficiency of nano-TiO2, reduce its usage and realize recycling and reuse, CaCO3–TiO2 composite photocatalyst was prepared with calcium carbonate (CaCO3) and TiO2 in a grinding machine through the integration of grinding depolymerization, dispersion and particle composition. The photocatalytic degradation performance, recycling performance, structure and morphology of CaCO3–TiO2 were studied. The interaction mechanism between CaCO3 and TiO2 and the improvement mechanism for the photocatalytic performance of TiO2 were also discussed. The results show that under the UV light irradiation for 20 and 40 min, the degradation efficiency of methyl orange by the composite photocatalyst with 40% TiO2 (mass fraction) was 90% and 100%, respectively. This was similar to that of pure TiO2, and the performance of the composite photocatalyst was almost unchanged after five cycles. CaCO3–TiO2 is formed by the uniform loading of nano-TiO2 particles on the CaCO3 surface, and the nano-TiO2 particles are well dispersed. Due to the facts that the dispersion of nano-TiO2 is improved in the presence of CaCO3 and the charge transport capability is improved through the interfacial chemical bonds between CaCO3 and TiO2, the formation of this complex is an intrinsic mechanism to improve the photocatalytic efficiency of nano-TiO2 and reduce its usage in application processes.


Introduction
In recent decades, with the rapid development of industries such as chemical industry, mining industry and agricultural breeding industry, environmental pollution problems, including water pollution, have become increasingly serious. Photocatalytic techniques are considered to be one of the most ideal ways to combat environmental pollution problems such as water pollution [1,2]. Therefore, it is of great significance to optimize the use conditions of photocatalysts, improving the function of photocatalyst technology and reducing the use amount of photocatalyst required to reduce the overall cost. Nano-TiO 2 has become the most widely studied and applied semiconductor photocatalytic materials owing to its non-toxic, non-polluting and stable properties [3,4]. Under the excitation of ultraviolet light, electron-hole pairs are produced by electron transition in nano-TiO 2 , following which the separated electrons and holes migrate to the surface of TiO 2 to produce active groups or directly react with pollutants. Thus, the degradation of pollutants such as dyes, heavy metal ions, phenols, dichlorophenol and antibiotics can be achieved through the further redox action [5][6][7][8]. However,

Raw Materials and Reagents
The CaCO 3 raw material was calcite grinding powder produced in Jilin province, China, with a purity of 100% and whiteness of 95%. The D 97 (particles with size ranging from 0 to D 97 account for 97% of the weight ratio) of the used CaCO 3 was 25 µm, 15 µm and 10 µm. The raw material of nano-TiO 2 used in this study was the commercial degussa P 25 product, which is composed of the mixed phase of anatase and rutile.

Preparation of CaCO 3 -TiO 2 Composite Photocatalyst
According to the composite ratio, a certain amount of nano-TiO 2 powder and CaCO 3 powder with specific particle sizes were put into an oven and dried at 80 • C for 2 h. Then, the nano-TiO 2 powder and CaCO 3 powder were well mixed, and the mixed powder was added to an RK/XPM-Φ120 grinding machine. After grinding for 30 min (the diameter of grinding bowl was 120 mm, the speed was 9 r/min and the speed of the grinding rod was 220 r/min), the CaCO 3 -nano-TiO 2 composite photocatalyst was obtained.

Photocatalytic Properties Test
The photocatalytic performance of CaCO 3 -TiO 2 composite photocatalyst was tested with methyl orange as the target degradation product. A 300 W mercury lamp with a dominant wavelength of 254 nm was used as the light source. First, 50 mg of CaCO 3 -TiO 2 composite photocatalyst was added to 50 mL of as-prepared methyl orange solution (the concentration was marked as C 0 , 10 mg/L) to obtain a suspension. In order to reduce the measurement error caused by sample adsorption, dark reaction was carried out for 1 h. After turning on the light source, the residual concentration of methyl orange was tested at specified intervals and marked as C. The photocatalytic degradation performance of the samples was characterized and evaluated by the change of C/C 0 . Linear regression was performed on the relationship between -Ln (C/C 0 ) and t (lighting time), and the linear slope was used to characterize the photocatalytic degradation rate.
The measurement of the concentration of methyl orange solution after photodegradation was as follows: the solution was centrifugated, the supernatant was removed and its absorbance was measured by a UV-VIS spectrophotometer (Cary 5000, Varian company, Palo Alto, CA, USA). The concentration of methyl orange in the solution was obtained through the relationship between the absorbance and concentration.
After the photocatalytic degradation of methyl orange, the suspension was centrifuged to obtain the sediments (photocatalyst). After being dried at a low temperature, the composite photocatalyst was tested for the photocatalytic degradation performance again, which was regarded as one cycle. The above process was repeated multiple times to obtain the cycle degradation performance of the samples.

Characterization
An X-ray powder diffractometer (D/MAX2000, Rigaku Corporation, Tokyo, Japan) was used to analyze the phase composition of the CaCO 3 -TiO 2 composite photocatalyst. A scanning electron microscope (SEM, S-3500N, HITACHI, Tokyo, Japan) was used to observe the sample morphology. The UV-VIS diffuse reflection absorption spectrum (Cary 5000, USA Varian, Palo Alto City, CA, USA) was used to characterize the light absorption properties of the samples. Fourier transform infrared spectroscopy (Spectrum 100, PerkinElmer Instruments (Shanghai) Co., Ltd., Shanghai, China) was used to test the binding properties of CaCO 3 and TiO 2 . In order to investigate the effect of the particle size of CaCO 3 on the properties of the prepared CaCO 3 -TiO 2 composite photocatalyst, CaCO 3 -TiO 2 composite photocatalysts were prepared with 25 µm, 15 µm and 10 µm CaCO 3 as the carrier (the mass ratio of nano-TiO 2 was 40%). Figure 1 shows the photocatalytic degradation of methyl orange by CaCO 3 -TiO 2 and pure TiO 2 , and a blank experiment was also conducted. It can be seen that when the pure methyl orange solution was illuminated by the ultraviolet light only, the C/C 0 remained almost unchanged with the increase of illumination time, indicating that there was no degradation effect on the methyl orange. By contrast, treatment with the CaCO 3 -TiO 2 composite photocatalyst (the size of CaCO 3 was 15 µm) and pure TiO 2 both resulted in the effective degradation of methyl orange. After 30 min of irradiation, the C/C 0 was reduced to less than 0.05, indicating that the degradation efficiency was higher than 95%; after 40 min of irradiation, the degradation efficiency reached 100%. Obviously, CaCO 3 -TiO 2 has excellent photocatalytic performance, which is similar to that of nano-TiO 2 . Since the mass ratio of TiO 2 to CaCO 3 -TiO 2 was only 40%, the above results indicate that CaCO 3 has an improving effect on the performance of TiO 2 . Figure 1 also shows that among the three CaCO 3 -TiO 2 samples, the composite with 15 µm CaCO 3 as the carrier exhibited the highest degradation efficiency. After 20 min of irradiation, the degradation efficiency of methyl orange was 90%, and the samples with 25 µm and 10 µm CaCO 3 as the carrier only reached about 82%. When the irradiation time increased to 30 min, the methyl orange degradation efficiencies of the three samples were 98%, 86% and 89%, respectively. The above results indicate that the particle size and specific surface area of CaCO 3 are important, and that CaCO 3 with a particle size of 15 µm properly matched with the nanoTiO 2 particles to form an optimal composite relationship. Therefore, CaCO 3 with a particle size of 15 µm was chosen as the carrier to be compounded with nano-TiO 2 .

Results and Discussion
Materials 2019, 12, x FOR PEER REVIEW 4 of 12 treatment with the CaCO3-TiO2 composite photocatalyst (the size of CaCO3 was 15 μm) and pure TiO2 both resulted in the effective degradation of methyl orange. After 30 min of irradiation, the C/C0 was reduced to less than 0.05, indicating that the degradation efficiency was higher than 95%; after 40 min of irradiation, the degradation efficiency reached 100%. Obviously, CaCO3-TiO2 has excellent photocatalytic performance, which is similar to that of nano-TiO2. Since the mass ratio of TiO2 to CaCO3-TiO2 was only 40%, the above results indicate that CaCO3 has an improving effect on the performance of TiO2. Figure 1 also shows that among the three CaCO3-TiO2 samples, the composite with 15 μm CaCO3 as the carrier exhibited the highest degradation efficiency. After 20 min of irradiation, the degradation efficiency of methyl orange was 90%, and the samples with 25 μm and 10 μm CaCO3 as the carrier only reached about 82%. When the irradiation time increased to 30 min, the methyl orange degradation efficiencies of the three samples were 98%, 86% and 89%, respectively. The above results indicate that the particle size and specific surface area of CaCO3 are important, and that CaCO3 with a particle size of 15 μm properly matched with the nanoTiO2 particles to form an optimal composite relationship. Therefore, CaCO3 with a particle size of 15 μm was chosen as the carrier to be compounded with nano-TiO2.

Effect of Loading Amount of Nano-TiO2
As the photocatalytic active component in CaCO3-TiO2, the loading amount of nano-TiO2 should have a significant effect on the performance of the composite photocatalyst. Therefore, using 15 μm CaCO3 as the carrier, CaCO3-TiO2 composite photocatalysts with different mass ratios of nano-TiO2 were prepared, and the degradation performance of methyl orange under ultraviolet light was tested. The results are shown in Figure 2. As the mass ratio of nano-TiO2 increased from 10% to 40%, the degradation efficiency of CaCO3-TiO2 increased gradually. When the composite photocatalyst with a TiO2 mass ratio of 40% was illuminated for 20 min, the C/C0 was lower than 0.1, indicating that the degradation efficiency reached higher than 90%. And when the irradiation time increased to 40 min, the degradation efficiency reached 100%, which is equivalent to pure TiO2. When the mass ratio of nano-TiO2 increased to 50%, although the degradation efficiency was improved after 10 min of illumination, the degradation efficiency remained almost unchanged when the irradiation time was extended, with a maximum of only 90%. Obviously, the mass ratio of nano-TiO2 in CaCO3-TiO2 should be set as 40%.
Since CaCO3-TiO2 with only a small mass ratio of nano-TiO2 reaches the photocatalytic degradation effect similar to that of pure nano-TiO2, the use amount of TiO2 can be significantly

Effect of Loading Amount of Nano-TiO 2
As the photocatalytic active component in CaCO 3 -TiO 2 , the loading amount of nano-TiO 2 should have a significant effect on the performance of the composite photocatalyst. Therefore, using 15 µm CaCO 3 as the carrier, CaCO 3 -TiO 2 composite photocatalysts with different mass ratios of nano-TiO 2 were prepared, and the degradation performance of methyl orange under ultraviolet light was tested. The results are shown in Figure 2. As the mass ratio of nano-TiO 2 increased from 10% to 40%, the degradation efficiency of CaCO 3 -TiO 2 increased gradually. When the composite photocatalyst with a TiO 2 mass ratio of 40% was illuminated for 20 min, the C/C 0 was lower than 0.1, indicating that the degradation efficiency reached higher than 90%. And when the irradiation time increased to 40 min, the degradation efficiency reached 100%, which is equivalent to pure TiO 2 . When the mass ratio of nano-TiO 2 increased to 50%, although the degradation efficiency was improved after 10 min of illumination, the degradation efficiency remained almost unchanged when the irradiation time was extended, with a maximum of only 90%. Obviously, the mass ratio of nano-TiO 2 in CaCO 3 -TiO 2 should be set as 40%.
Since CaCO 3 -TiO 2 with only a small mass ratio of nano-TiO 2 reaches the photocatalytic degradation effect similar to that of pure nano-TiO 2 , the use amount of TiO 2 can be significantly reduced and thus cost can be saved in practical applications. This is a result of the significant improvement in the photocatalytic efficiency of TiO 2 by its combination with CaCO 3 . It is speculated that this improvement results from the improvement of the dispersibility of nano-TiO 2 and the formation of a combination interface between TiO 2 and CaCO 3 . reduced and thus cost can be saved in practical applications. This is a result of the significant improvement in the photocatalytic efficiency of TiO2 by its combination with CaCO3. It is speculated that this improvement results from the improvement of the dispersibility of nano-TiO2 and the formation of a combination interface between TiO2 and CaCO3.

Photocatalytic Degradation of Different Concentrations of Methyl Orange Solution by CaCO3-TiO2
The investigation of the degradation performance of CaCO3-TiO2 in different concentrations of methyl orange solution can not only further verify the photocatalytic degradation ability of CaCO3-TiO2, but it can also determine its reasonable scope of application. Figure 3 shows the photocatalytic degradation effect of CaCO3-TiO2 for different concentrations of methyl orange solution. It can be seen that the degradation efficiency of the CaCO3-TiO2 composite photocatalyst on each concentration of methyl orange solution increased with the increase of the illumination time, and finally reached a higher degradation effect. Comparing the degradation effect of different concentrations, it was found that the degradation efficiency of methyl orange solution with concentrations of 10 and 20 ppm by CaCO3-TiO2 was higher than that of concentrations of 50 and 80 ppm. Among them, the methyl orange solution with concentrations of 10 and 20 ppm could be completely degraded (C/C0 = 0) by CaCO3-TiO2 at illumination times of 60 and 80 min, respectively, and the degradation rate was faster. In contrast, the degradation rates of 50 and 80 ppm methyl orange solution by CaCO3-TiO2 were reduced, and the degradation efficiencies were 90% and 80%, respectively (C/C0 was 0.1 and 0.2, respectively) at illumination times of 60 and 90 min. These results show that the CaCO3-TiO2 composite photocatalyst can effectively degrade methyl orange solution with concentrations of 10-80 ppm. The reason why the degradation rate decreases with the increasing concentration of methyl orange solution is that high concentrations of pollutants require longer reaction times. The investigation of the degradation performance of CaCO 3 -TiO 2 in different concentrations of methyl orange solution can not only further verify the photocatalytic degradation ability of CaCO 3 -TiO 2 , but it can also determine its reasonable scope of application. Figure 3 shows the photocatalytic degradation effect of CaCO 3 -TiO 2 for different concentrations of methyl orange solution. It can be seen that the degradation efficiency of the CaCO 3 -TiO 2 composite photocatalyst on each concentration of methyl orange solution increased with the increase of the illumination time, and finally reached a higher degradation effect. Comparing the degradation effect of different concentrations, it was found that the degradation efficiency of methyl orange solution with concentrations of 10 and 20 ppm by CaCO 3 -TiO 2 was higher than that of concentrations of 50 and 80 ppm. Among them, the methyl orange solution with concentrations of 10 and 20 ppm could be completely degraded (C/C 0 = 0) by CaCO 3 -TiO 2 at illumination times of 60 and 80 min, respectively, and the degradation rate was faster. In contrast, the degradation rates of 50 and 80 ppm methyl orange solution by CaCO 3 -TiO 2 were reduced, and the degradation efficiencies were 90% and 80%, respectively (C/C 0 was 0.1 and 0.2, respectively) at illumination times of 60 and 90 min. These results show that the CaCO 3 -TiO 2 composite photocatalyst can effectively degrade methyl orange solution with concentrations of 10-80 ppm. The reason why the degradation rate decreases with the increasing concentration of methyl orange solution is that high concentrations of pollutants require longer reaction times.

Recycling Performance of CaCO3-TiO2 Composite Photocatalyst
The recycling performance of the catalyst is an important factor reflecting its stability and practical application performance. Therefore, we investigated the recycling performance of the optimum CaCO3-TiO2 composite photocatalyst (the particle size of CaCO3 was 15 μm, the mass ratio of TiO2 was 40%). The results are shown in Figure 4. The results show that the photocatalytic degradation performance of CaCO3-TiO2 after five cycles is similar to that of the first use, indicating that recycling does not reduce its performance. This indicates that the CaCO3-TiO2 composite photocatalyst prepared in this study has excellent recycling performance and can be recycled and reused several times, thereby reducing the cost. Obviously, this is due to the loading of TiO2 on the surface of CaCO3, which results in the nano-TiO2 being firmly fixed on the surface of CaCO3. Therefore, the loss of TiO2 is prevented, and CaCO3-TiO2 can easily be separated from water due to its large particle size. However, single nano-TiO2 particles have a small particle size, so they are difficult to precipitate and easily lost when separated from their carrier. Therefore, it is believed that the CaCO3-TiO2 composite photocatalyst has practical application prospects for sewage treatment.

Recycling Performance of CaCO 3 -TiO 2 Composite Photocatalyst
The recycling performance of the catalyst is an important factor reflecting its stability and practical application performance. Therefore, we investigated the recycling performance of the optimum CaCO 3 -TiO 2 composite photocatalyst (the particle size of CaCO 3 was 15 µm, the mass ratio of TiO 2 was 40%). The results are shown in Figure 4. The results show that the photocatalytic degradation performance of CaCO 3 -TiO 2 after five cycles is similar to that of the first use, indicating that recycling does not reduce its performance. This indicates that the CaCO 3 -TiO 2 composite photocatalyst prepared in this study has excellent recycling performance and can be recycled and reused several times, thereby reducing the cost. Obviously, this is due to the loading of TiO 2 on the surface of CaCO 3 , which results in the nano-TiO 2 being firmly fixed on the surface of CaCO 3 . Therefore, the loss of TiO 2 is prevented, and CaCO 3 -TiO 2 can easily be separated from water due to its large particle size. However, single nano-TiO 2 particles have a small particle size, so they are difficult to precipitate and easily lost when separated from their carrier. Therefore, it is believed that the CaCO 3 -TiO 2 composite photocatalyst has practical application prospects for sewage treatment.

Recycling Performance of CaCO3-TiO2 Composite Photocatalyst
The recycling performance of the catalyst is an important factor reflecting its stability and practical application performance. Therefore, we investigated the recycling performance of the optimum CaCO3-TiO2 composite photocatalyst (the particle size of CaCO3 was 15 μm, the mass ratio of TiO2 was 40%). The results are shown in Figure 4. The results show that the photocatalytic degradation performance of CaCO3-TiO2 after five cycles is similar to that of the first use, indicating that recycling does not reduce its performance. This indicates that the CaCO3-TiO2 composite photocatalyst prepared in this study has excellent recycling performance and can be recycled and reused several times, thereby reducing the cost. Obviously, this is due to the loading of TiO2 on the surface of CaCO3, which results in the nano-TiO2 being firmly fixed on the surface of CaCO3. Therefore, the loss of TiO2 is prevented, and CaCO3-TiO2 can easily be separated from water due to its large particle size. However, single nano-TiO2 particles have a small particle size, so they are difficult to precipitate and easily lost when separated from their carrier. Therefore, it is believed that the CaCO3-TiO2 composite photocatalyst has practical application prospects for sewage treatment.   Figure 5 shows the UV-VIS diffuse reflectance absorption spectra (DRS) of the CaCO 3 -TiO 2 composite photocatalyst as well as the raw materials of CaCO 3 and TiO 2 . It can be seen that CaCO 3 -TiO 2 and TiO 2 exhibit similar light absorption characteristics. They show no light absorption in the visible light region (wavelengths of 400-800 nm) and show strong absorption in ultraviolet light (wavelengths of 200-400 nm), which is consistent with the bandgap characteristics of the TiO 2 semiconductor (3.2 eV of bandgap width). By contrast, CaCO 3 shows almost no absorption of both visible light and ultraviolet light. The results show that CaCO 3 -TiO 2 has similar properties to TiO 2 , consistent with the observed strong photocatalytic degradation performance of CaCO 3 -TiO 2 and TiO 2 . Meanwhile, it is inferred that a composite structure of CaCO 3 with surface-loaded nano-TiO 2 is formed in the creation of CaCO 3 -TiO 2 .

Optical Performance Test
Materials 2019, 12, x FOR PEER REVIEW 7 of 12 Figure 5 shows the UV-VIS diffuse reflectance absorption spectra (DRS) of the CaCO3-TiO2 composite photocatalyst as well as the raw materials of CaCO3 and TiO2. It can be seen that CaCO3-TiO2 and TiO2 exhibit similar light absorption characteristics. They show no light absorption in the visible light region (wavelengths of 400-800 nm) and show strong absorption in ultraviolet light (wavelengths of 200-400 nm), which is consistent with the bandgap characteristics of the TiO2 semiconductor (3.2 eV of bandgap width). By contrast, CaCO3 shows almost no absorption of both visible light and ultraviolet light. The results show that CaCO3-TiO2 has similar properties to TiO2, consistent with the observed strong photocatalytic degradation performance of CaCO3-TiO2 and TiO2. Meanwhile, it is inferred that a composite structure of CaCO3 with surface-loaded nano-TiO2 is formed in the creation of CaCO3-TiO2.  Figure 6 shows SEM images of CaCO3, nano-TiO2 raw materials, CaCO3-TiO2 composite photocatalyst with different mass ratios of TiO2 and the physical mixtures of CaCO3 and TiO2 (40% TiO2 mass ratio). Figure 6a shows that the CaCO3 particles have a bulk shape with a particle size of about 1-3 μm, as well as smooth surfaces without coatings. Figure 6b,c shows that the nano-TiO2 particles are fine, but their agglomeration is severe, with an aggregate size of up to 2 μm. Figure 6dg show that when CaCO3 is combined with nano-TiO2, the surface of CaCO3 particles is uniformly covered by fine particles, and the surface becomes rough. Figure 6i,j are the mapping results of Ca and Ti elements corresponding to Figure 6f; it can be seen that Ca distributes in the particle contour range of CaCO3, reflecting the characteristic of CaCO3. The distribution of Ti in the scanning area is uniform, which corresponds to the location of the particles. This proves that the TiO2 is distributed on the surface of CaCO3 uniformly. Obviously, the CaCO3-TiO2 composite photocatalyst is composed of composite particles characterized by nano-TiO2 loaded on the CaCO3 surface. Therefore, it is inferred that CaCO3-TiO2 should exhibit the properties of nano-TiO2 (photocatalytic properties). In addition, it was found that the dispersion of TiO2 loaded on the surface of CaCO3 was significantly higher than that of TiO2 alone. The particle unit size is generally reduced to less than 0.2 μm, which allows the active sites of TiO2 to be more exposed, thereby increasing its photocatalytic performance. This is undoubtedly the intrinsic mechanism enabling CaCO3-TiO2 to exhibit a photocatalytic degradation performance comparable to that of pure TiO2 when the mass ratio of TiO2 is only 40%. Figure 6d-f shows that as the mass ratio of TiO2 increased from 20% to 50%, the surface of the CaCO3 was coated by nano-TiO2 particles more and more uniformly and completely. Especially when the mass ratio of TiO2 was 40%, the surface of CaCO3 was almost completely covered by TiO2 particles. However, in the physical mixture of CaCO3 and TiO2 with the same composite proportion  Figure 6 shows SEM images of CaCO 3 , nano-TiO 2 raw materials, CaCO 3 -TiO 2 composite photocatalyst with different mass ratios of TiO 2 and the physical mixtures of CaCO 3 and TiO 2 (40% TiO 2 mass ratio). Figure 6a shows that the CaCO 3 particles have a bulk shape with a particle size of about 1-3 µm, as well as smooth surfaces without coatings. Figure 6b,c shows that the nano-TiO 2 particles are fine, but their agglomeration is severe, with an aggregate size of up to 2 µm. Figure 6d-g show that when CaCO 3 is combined with nano-TiO 2 , the surface of CaCO 3 particles is uniformly covered by fine particles, and the surface becomes rough. Figure 6i,j are the mapping results of Ca and Ti elements corresponding to Figure 6f; it can be seen that Ca distributes in the particle contour range of CaCO 3 , reflecting the characteristic of CaCO 3 . The distribution of Ti in the scanning area is uniform, which corresponds to the location of the particles. This proves that the TiO 2 is distributed on the surface of CaCO 3 uniformly. Obviously, the CaCO 3 -TiO 2 composite photocatalyst is composed of composite particles characterized by nano-TiO 2 loaded on the CaCO 3 surface. Therefore, it is inferred that CaCO 3 -TiO 2 should exhibit the properties of nano-TiO 2 (photocatalytic properties). In addition, it was found that the dispersion of TiO 2 loaded on the surface of CaCO 3 was significantly higher than that of TiO 2 alone. The particle unit size is generally reduced to less than 0.2 µm, which allows the active sites of TiO 2 to be more exposed, thereby increasing its photocatalytic performance. This is undoubtedly the intrinsic mechanism enabling CaCO 3 -TiO 2 to exhibit a photocatalytic degradation performance comparable to that of pure TiO 2 when the mass ratio of TiO 2 is only 40%.

SEM Analysis
Figure 6d-f shows that as the mass ratio of TiO 2 increased from 20% to 50%, the surface of the CaCO 3 was coated by nano-TiO 2 particles more and more uniformly and completely. Especially when the mass ratio of TiO 2 was 40%, the surface of CaCO 3 was almost completely covered by TiO 2 particles. However, in the physical mixture of CaCO 3 and TiO 2 with the same composite proportion (Figure 6h), the loading of nano-TiO 2 was very poor. Only a small part of the surface of CaCO 3 particles contained loadings, and most of them were bare. The above results indicate that the co-grinding of CaCO 3 and TiO 2 greatly promotes the loading function, thereby improving the performance of CaCO 3 -TiO 2 .
Materials 2019, 12, x FOR PEER REVIEW 8 of 12 (Figure 6h), the loading of nano-TiO2 was very poor. Only a small part of the surface of CaCO3 particles contained loadings, and most of them were bare. The above results indicate that the cogrinding of CaCO3 and TiO2 greatly promotes the loading function, thereby improving the performance of CaCO3-TiO2.  Figure 7 displays the XRD spectra of CaCO3, nano-TiO2 raw material, CaCO3-TiO2 composite photocatalyst and the physical mixture of CaCO3 and TiO2. The results reveal that only the characteristic diffraction peaks of calcite appeared in the XRD spectrum of CaCO3, indicating its high purity. The characteristic diffraction peaks of rutile and anatase appeared in the spectrum of nano-TiO2, indicating that it consisted of rutile and anatase mixed phase, consistent with the phase composition of P25. In the spectrum of the CaCO3-TiO2 composite photocatalyst, only calcite, rutile and anatase crystal phases were observed, indicating that the compounding of CaCO3 and TiO2 does not produce a new phase. It was concluded that the combination of CaCO3 and TiO2 occurs in their interface region, and the binding properties should be chemical or physical action within the interface area. The phase composition of the physical mixture of CaCO3 and TiO2 was found to be the same as that of CaCO3-TiO2 composite photocatalyst, but the intensity of the characteristic peaks of CaCO3 in CaCO3-TiO2 composite photocatalyst was weaker than that in the physical mixture. This may be due to the reduction of CaCO3 exposure in CaCO3-TiO2 due to the coating of the surface with titanium dioxide, while most of the surface of CaCO3 in the physical mixture was still bare [23]. The above results also indicate the successful compounding of CaCO3 and TiO2 in CaCO3-TiO2.  Figure 7 displays the XRD spectra of CaCO 3 , nano-TiO 2 raw material, CaCO 3 -TiO 2 composite photocatalyst and the physical mixture of CaCO 3 and TiO 2 . The results reveal that only the characteristic diffraction peaks of calcite appeared in the XRD spectrum of CaCO 3 , indicating its high purity. The characteristic diffraction peaks of rutile and anatase appeared in the spectrum of nano-TiO 2 , indicating that it consisted of rutile and anatase mixed phase, consistent with the phase composition of P 25 . In the spectrum of the CaCO 3 -TiO 2 composite photocatalyst, only calcite, rutile and anatase crystal phases were observed, indicating that the compounding of CaCO 3 and TiO 2 does not produce a new phase. It was concluded that the combination of CaCO 3 and TiO 2 occurs in their interface region, and the binding properties should be chemical or physical action within the interface area. The phase composition of the physical mixture of CaCO 3 and TiO 2 was found to be the same as that of CaCO 3 -TiO 2 composite photocatalyst, but the intensity of the characteristic peaks of CaCO 3 in CaCO 3 -TiO 2 composite photocatalyst was weaker than that in the physical mixture. This may be due to the reduction of CaCO 3 exposure in CaCO 3 -TiO 2 due to the coating of the surface with titanium dioxide, while most of the surface of CaCO 3 in the physical mixture was still bare [23]. The above results also indicate the successful compounding of CaCO 3 and TiO 2 in CaCO 3 -TiO 2 .

Infrared Spectrum Analysis
The infrared spectra of CaCO3, nano-TiO2 raw material and CaCO3-TiO2 composite photocatalyst were measured, as shown in Figure 8. In the infrared spectrum of nano-TiO2, the wide absorption band in the range of 3250-3700 cm −1 and the absorption peak at 1636 cm −1 respectively represent the stretching vibration and bending vibration of the O-H bond in the water molecules and hydroxyl groups on the surface of TiO2. The absorption band in the range of about 3300-3500 cm −1 in the CaCO3 spectrum represents the characteristics of adsorption water and hydroxyl groups on its surface. In the infrared spectrum of the CaCO3-TiO2 composite photocatalyst, the absorption peak at 1636 cm −1 of TiO2 disappeared, and the absorption peak of CaCO3 ranging from 3300 to 3500 cm −1 was weakened. It has been suggested that CaCO3 and TiO2 form a firm chemical combination through their surface hydroxyl groups [24]. Based on the above study, the mechanism of CaCO3 as a carrier to improve the photocatalytic performance and utilization efficiency of nano-TiO2 can be summarized as follows: (1) The dispersibility of TiO2 is improved by uniformly loading TiO2 on the surface of CaCO3, thereby causing an increase in active sites and light absorption areas. (2) A chemical bond is formed between TiO2 and CaCO3 at their interface, which provides a new transport channel for the photo electrons [25,26], and further improves the separation efficiency of electrons and holes. (3) In CaCO3-TiO2, nano-TiO2 and CaCO3 are firmly combined due to their surface chemical bonding, which prevents the loss of TiO2 when degrading pollutants in water [9,27]. Therefore, this composite photocatalyst can be easily

Infrared Spectrum Analysis
The infrared spectra of CaCO 3 , nano-TiO 2 raw material and CaCO 3 -TiO 2 composite photocatalyst were measured, as shown in Figure 8. In the infrared spectrum of nano-TiO 2 , the wide absorption band in the range of 3250-3700 cm −1 and the absorption peak at 1636 cm −1 respectively represent the stretching vibration and bending vibration of the O-H bond in the water molecules and hydroxyl groups on the surface of TiO 2 . The absorption band in the range of about 3300-3500 cm −1 in the CaCO 3 spectrum represents the characteristics of adsorption water and hydroxyl groups on its surface. In the infrared spectrum of the CaCO 3 -TiO 2 composite photocatalyst, the absorption peak at 1636 cm −1 of TiO 2 disappeared, and the absorption peak of CaCO 3 ranging from 3300 to 3500 cm −1 was weakened. It has been suggested that CaCO 3 and TiO 2 form a firm chemical combination through their surface hydroxyl groups [24].

Infrared Spectrum Analysis
The infrared spectra of CaCO3, nano-TiO2 raw material and CaCO3-TiO2 composite photocatalyst were measured, as shown in Figure 8. In the infrared spectrum of nano-TiO2, the wide absorption band in the range of 3250-3700 cm −1 and the absorption peak at 1636 cm −1 respectively represent the stretching vibration and bending vibration of the O-H bond in the water molecules and hydroxyl groups on the surface of TiO2. The absorption band in the range of about 3300-3500 cm −1 in the CaCO3 spectrum represents the characteristics of adsorption water and hydroxyl groups on its surface. In the infrared spectrum of the CaCO3-TiO2 composite photocatalyst, the absorption peak at 1636 cm −1 of TiO2 disappeared, and the absorption peak of CaCO3 ranging from 3300 to 3500 cm −1 was weakened. It has been suggested that CaCO3 and TiO2 form a firm chemical combination through their surface hydroxyl groups [24]. Based on the above study, the mechanism of CaCO3 as a carrier to improve the photocatalytic performance and utilization efficiency of nano-TiO2 can be summarized as follows: (1) The dispersibility of TiO2 is improved by uniformly loading TiO2 on the surface of CaCO3, thereby causing an increase in active sites and light absorption areas. (2) A chemical bond is formed between TiO2 and CaCO3 at their interface, which provides a new transport channel for the photo electrons [25,26], and further improves the separation efficiency of electrons and holes. (3) In CaCO3-TiO2, nano-TiO2 and CaCO3 are firmly combined due to their surface chemical bonding, which prevents the loss of TiO2 when degrading pollutants in water [9,27]. Therefore, this composite photocatalyst can be easily Based on the above study, the mechanism of CaCO 3 as a carrier to improve the photocatalytic performance and utilization efficiency of nano-TiO 2 can be summarized as follows: (1) The dispersibility of TiO 2 is improved by uniformly loading TiO 2 on the surface of CaCO 3 , thereby causing an increase in active sites and light absorption areas. (2) A chemical bond is formed between TiO 2 and CaCO 3 at their interface, which provides a new transport channel for the photo electrons [25,26], and further improves the separation efficiency of electrons and holes. (3) In CaCO 3 -TiO 2 , nano-TiO 2 and CaCO 3 are firmly combined due to their surface chemical bonding, which prevents the loss of TiO 2 when degrading pollutants in water [9,27]. Therefore, this composite photocatalyst can be easily recovered and recycled from water by sedimentation, thereby reducing the use amount of TiO 2 as well as the cost. Figure 9 is a schematic diagram reflecting the above principles. recovered and recycled from water by sedimentation, thereby reducing the use amount of TiO2 as well as the cost. Figure 9 is a schematic diagram reflecting the above principles. Figure 9. Improvement mechanism of photocatalytic performance.

Conclusions
(1) The CaCO3-TiO2 composite photocatalyst was prepared by the co-grinding of CaCO3 and TiO2.
The optimal CaCO3-TiO2 composite photocatalyst, in which the particles size of CaCO3 is 15 m and the mass ratio of TiO2 is 40%, shows excellent photocatalytic degradation performance towards methyl orange and good recovery performance. The degradation efficiency of optimal CaCO3-TiO2 composite photocatalyst was found to be 90% and 100% after 20 and 40 min of ultraviolet light illumination, respectively. The degradation effect is comparable to pure TiO2. Moreover, its degradation effect on methyl orange is not significantly reduced after five cycles. (2) CaCO3-TiO2 composite photocatalyst is characterized by CaCO3 loaded by nano-TiO2 uniformly and completely. The dispersibility of the loaded TiO2 is significantly enhanced compared to pure TiO2, and a strong chemical bond is formed between CaCO3 and TiO2 particle interfaces. These are important mechanisms for improving the photocatalytic efficiency of nano-TiO2 and reducing the its amount in CaCO3-TiO2.