3.2. XRD Analysis
Figure 2 shows the X-ray diffraction (XRD) spectra of CdS/F–TiO
2 composites with different proportions. It is known that the diffraction angles of the characteristic peaks on the surface of anatase titanium dioxide (101) and rutile titanium dioxide (110) are 2θ = 25.3° and 2θ = 27.5°, respectively. It can be seen from the four curves that only the characteristic peak of anatase type titanium dioxide were observed in the samples and there were no diffraction peaks of rutile type and plate titanium type titanium dioxide and no diffraction peaks of fluorine-related compounds were detected. When the contents of CdS were 0.5% and 2%, the characteristic peak of CdS appeared in CdS/F–TiO
2 composite but the intensity is poor. However, there are no diffraction peaks related to CdS in the
Figure 2c curves. For the sample 1% CdS/F–TiO
2, CdS and F–TiO
2 may be well compounded and the whole concentration of CdS is below the detection limit of XRD. However, it is deserved to be mentioned that the results cannot exclude the presence of small nanocrystalline rutile TiO
2 or CdS particles, with the sizes below detection limit. In addition, after composite, some lattice defects may be caused, which resulted in the diffraction peaks broadening or even not be detected. However, 0.5% CdS/F–TiO
2 showed a weak CdS diffraction peak, which may be due to the fact that CdS and F–TiO
2 samples may be not compounded evenly, leading to a high concentration of CdS in some areas and the appearance of weak X-ray diffraction peak. In the whole, the crystallinity of the four samples is high and the diffraction peaks of anatase are obvious. With the increase of CdS content, the diffraction peak strength of anatase in CdS/F–TiO
2 composites decreases significantly. It may be related that CdS is attached to the surface of TiO
2, which causes its overall crystallinity to become worse. It is also possible that CdS enters the TiO
2 lattice, resulting in lattice defects [
21]. As a result, the intensity of the diffraction peak intensity is weakened. According to the basic theory of X-ray diffraction, the intensity and sharpness of the diffraction peak of the sample are not only related to the crystallinity of TiO
2 but also to the grain size. The grain size of the sample can be calculated by the Scherrer Equation (2) and the results are shown in
Table 1 [
22]. According to Scherrer formula and the data in
Table 1, the grain size of CdS/F–TiO
2 composites decreases with the increase of CdS content. It is indicated that the modification of CdS has an inhibitory effect on the crystallinity and grain growth of fluorinated titanium dioxide, which can improve the photocatalytic performance of TiO
2 to a certain extent.
Figure 3 shows the crystallinity spectrum of TiO
2 and 1% CdS/F–TiO
2. Phase analysis employs the diffractograms of the pure crystalline and pure amorphous phases to decompose the X-ray diffraction pattern of the mixture sample. By setting the Bragg-Brentano geometry and polarization value as 0.5. The results showed that the crystallinity of pure TiO
2 was 100% and that of 1% CdS/F–TiO
2 was 62.75%, which demonstrates that CdS has an inhibitory effect on the crystallinity of fluorinated titanium dioxide.
3.3. UV-Vis-Abs Analysis
Figure 4 shows the ultraviolet-visible absorption spectra (UV-Vis-Abs) of CdS/F–TiO
2 particles with different proportions. It can be seen from
Figure 4 that the absorption intensity of the fluorinated titanium dioxide modified by CdS in the optical region of 470–540 nm increased to a large extent, while the absorption intensity in the visible light region with a wavelength more than 540 nm is decreased. When the content of CdS is 1%, the absorption intensity of the sample in the light region less than 470 nm also increases. The light absorption ability of photocatalyst is closely related to the photocatalytic activity of the catalyst. Under the same conditions, the stronger the photocatalyst’s ability to absorb light, the higher its photocatalytic activity. Two threshold wavelengths appeared after the combination of cadmium sulfide and titanium dioxide. The threshold wavelength at about 400 nm should be titanium dioxide and the threshold wavelength at about 550 nm should be cadmium sulfide. It can be seen from
Figure 5 that the band gap energy of F–TiO
2 particles is about 3.15 eV and the band gap energy of 0.5% and 2% CdS/F–TiO
2 composites is 3.10 eV, while that of 1% CdS/F–TiO
2 is 2.96 eV. The threshold wavelength (λ
g) of 1% CdS/F–TiO
2 is the largest about 550 nm, that is, the band gap energy is the smallest. Generally speaking, the band gap energy of anatase titanium dioxide is about 3.2 eV. It indicates that the band gap energy of F–TiO
2 in this work decreases to a certain extent compared with that of general anatase-type titanium dioxide and the band gap energy of F–TiO
2 can also be significantly reduced by CdS modification. The results showed that the absorption band edges of F–TiO
2 were redshifted obviously after CdS modification.
The absorption band edge of fluoride titanium dioxide was redshifted after modification with CdS. The reason is that after CdS is combined with TiO
2, when a certain amount of light is irradiated, the electrons on CdS are transitioned. Because the conduction band of CdS is higher than that of TiO
2, that is, photogenerated electrons can be transferred more to TiO
2. On the conduction band and photogenerated holes remain in the valence band of CdS, which effectively reduces the electron-hole pair recombination [
23]. In addition, the band gap width decreases and the band gap structure changes after the combination of F–TiO
2 and CdS. The above two broaden the response range of CdS/F–TiO
2 to visible region [
24]. The reason for the enhancement of light absorption intensity is that CdS has a strong light absorption in the light region with a wavelength less than 500 nm. TiO
2 combined with CdS has some advantages of cadmium sulfide, so the absorption of light in the region of 380–500 nm is obviously enhanced, which is consistent with Hu et al. [
25].
3.5. XPS Analysis
The effect of CdS modified fluorinated titania catalyst on surface element composition and valence state was investigated by X-ray photoelectron spectroscopy XPS characterization. The results are shown in
Figure 7. Among them, a is the XPS full spectrum of the sample, b, c, d, e and f are the spectra of Ti, O, F, S and Cd, respectively. It can be seen from
Figure 7 that F–TiO
2 and CdS/F–TiO
2 are mainly composed of Ti, O and C, among which C element was introduced into the sample during the test. Both CdS/F–TiO
2 and F–TiO
2 contain a small amount of F, while CdS/F–TiO
2 contains a certain amount of Cd and S.
Figure 7b is the XPS spectrum of Ti element, in which the binding energies of the spin orbitals Ti 2
p3/2 and Ti 2
p1/2 of Ti in fluorinated titanium dioxide are 458.63 eV and 464.43 eV, respectively. The peaks at 458.63 eV belong to Ti
3+ while those at 464.43 eV belong to Ti
4+ [
28]. Moreover, the binding energy of Ti elements in the sample is larger than that of standard Ti in TiO
2, which indicates that Ti mainly exists in the form of Ti
4+ [
29]. Compared with F–TiO
2, the spin orbital binding energies of Ti elements in CdS/F–TiO
2 samples decreased by 0.19 eV and 0.19 eV, respectively, indicating that the modification of CdS has a great change in the chemical bond environment around Ti
4+. It may be due to that some S atoms replace the O atoms in the crystal lattice to form Ti–S bonds, some Cd atoms enter into the lattice of titanium fluoride to form Ti–O–Cd bonds and the composite CdS on the surface of F–TiO
2 also has some influence on the electrons state around the Ti atom. The two combined action makes the electron cloud density of Ti atom increase, which results in the peak of the latter Ti element moves towards the direction of low energy after compound [
30]. XPS spectra of O element are shown in
Figure 7c. The binding energy of CdS/F–TiO
2 in O 1
s orbital is 0.22 eV smaller than that of F–TiO
2. The first reason is that S atom enters F–TiO
2 lattice to form O–Ti–S bond and the second reason is that Cd atom replaces titanium atom in the lattice to form Ti–O–Cd bond. That is, the density of electron cloud around O atom is affected, the lattice constant of F–TiO
2 is changed, resulting in an increase in the distortion energy and the density of the ambient electron cloud around the O atoms after compound. The broader peak at 531.87 eV can be attributed to hydroxyl oxygen (·OH) [
31]. It can be seen that the peak of CdS/F–TiO
2 is slightly higher than that of F–TiO
2, that is, the modification of CdS may be beneficial to the formation of ·OH on the surface of F–TiO
2. The formation of ·OH with a high catalytic activity by hole oxidation is beneficial to enhance photocatalytic activity [
28]. The reason for modification of CdS may be conducive to the generation of ·OH on the surface of F–TiO
2 may be given as the following. The essence of modification of TiO
2 by CdS is that CdS and TiO
2 semiconductors with different band gaps are coupled at a certain proportion. When exposed to metal halide light, the electrons generated by CdS excitation are more likely to transition to the conduction band of TiO
2. The holes remain on the valence band of CdS, enhancing the charge separation effect. Thus, the generated number of ·OH may be increased. In
Figure 7d, it is easy to see that both samples have peaks of F and each has two characteristic peaks and the intensity of CdS/F–TiO
2 peak is not as high as that of F–TiO
2 peak.
The peak at 684 eV is caused by the fluoride ion that replaced the hydroxyl group on the surface of TiO
2 [
31]. The binding energies of the two peaks of F 1
s move toward the lower energy level. The former is caused by CdS entering into the lattice of TiO
2, which is consistent with the previous analysis. The latter is that the CdS composite is mainly distributed on the surface of F–TiO
2, which has a great impact. From the e and f of
Figure 7, it can be seen that the peaks of S and Cd are obvious, indicating the existence of the two elements in CdS/F–TiO
2. The binding energy of S in S 2
p2/3 is 160.96 eV, that is to say, S mainly exists in the form of S
2− in the sample. The binding energy of Cd in Cd 3
d5/2 and Cd 3
d3/2 is 404.63 and 411.41 eV, indicating that Cd is mainly in the form of Cd
2+ [
32].
In addition, the quantitative analysis results of XPS showed that the element composition of F–TiO2 was Ti, O, C and F and the contents of each element were 26.76, 53.45, 17.20 and 2.59 at.%, respectively. The elements of CdS/F–TiO2 are Ti, O, C, F, Cd and S and the contents of each element are 28.11, 55.34, 12.79, 1.34, 1.21, 1.21 at.%, respectively. It can be seen that the ratio of O/Ti content in F–TiO2 sample is slightly less than the stoichiometric ratio of standard TiO2 2:1. This is because part of F− enters into the lattice of TiO2 and occupies the position of O, resulting in the decrease of O content. The ratio of O/Ti in CdS/F–TiO2 sample is 1.969, which is slightly less than that of O/Ti in F–TiO2 sample 1.997. The first reason is that the decrease of F– content on the surface of TiO2 results in the decrease of ·OH. Secondly, it is possible that part of S enters into the lattice of TiO2 and replaces the position of O. The content of F in CdS/F–TiO2 decreased. First, CdS and F–TiO2 are mainly attached to the surface of F–TiO2 after compounding and the surface is covered by F−. The elements on the surface of the main sample measured by XPS have a certain depth, so that the measured F content decreases. Secondly, the absorbed fraction of F− falls off during centrifugal washing after compounding.
3.7. Photocatalytic Performance
Under the same reaction conditions, photocatalytic degradation of methyl orange solution with initial mass concentration of 10, 20 and 40 mg/L was carried out respectively to explore the influence of initial concentration of methyl orange on photocatalytic degradation and the results were shown in
Figure 9. As the initial mass concentration of methyl orange increased from 10 to 20 mg/L, the photocatalytic degradation rate decreased significantly but it did not affect the final degradation rate. The degradation was basically complete at 20 min. When the initial mass concentration of methyl orange was 40 mg/L, the degradation rate decreased sharply and the degradation rate was only 57% at 20 min. On the one hand, the increase of methyl orange concentration exceeds the maximum value that ·OH can oxidize. On the other hand, the concentration is too high and the light scattering is strong, which makes it difficult for light to enter into the solution, so that the titanium dioxide surface cannot form enough ·OH [
34]. Taking into consideration of the degradation rate and treatment cost of methyl orange, the concentration of methyl orange should be controlled at 20 mg/L.
The effect of H
2O
2 dosage on the degradation of methyl orange by 1% CdS/F–TiO
2 is shown in
Figure 10. Under the irradiation of the metal halide lamp, the photocatalytic activity of the system with H
2O
2 was obviously better than that without H
2O
2. When the concentration of H
2O
2 was 3%, the reduction rate of methyl orange (20 mg/L) by 1% CdS/F–TiO
2 reached 93.69% after 8 min. It indicates that the addition of a certain amount of H
2O
2 is beneficial to the degradation of methyl orange by CdS/F–TiO
2. This is because H
2O
2 can generate hydroxyl radicals (·OH). When H
2O
2 is added, the light irradiated by the metal halide lamp and H
2O
2 will have a synergistic effect, making the excited state H
2O
2 split into ·OH; Moreover, H
2O
2 can be used as an electron acceptor, generating ·OH with photogenerated electrons, avoiding recombination of photogenerated electrons and holes and improving the quantum efficiency of light [
35,
36]. And ·OH is the most powerful oxidant in water, which can rapidly degrade methyl orange into CO
2 and H
2O. Therefore, a certain amount of H
2O
2 is beneficial to photocatalytic reaction.
Figure 11 shows the time-dependent curve of degradation rate of methyl orange by 1% CdS/F–TiO
2 at different pH. It can be seen from
Figure 11 that the photocatalytic performance in acidic and alkaline environments is superior to that under neutral conditions and the acidic conditions are better than those under alkaline conditions. The reasons for the above phenomena are—First, methyl orange has a quinoid structure in acidic conditions and an azo structure in alkaline conditions. When methyl orange is degraded under light, the quinone structure is more susceptible to be degraded than the azo structure under alkaline conditions [
37]. Second, it is determined by the photocatalytic mechanism of titanium dioxide. Hydroxyl radical is one of the main active substance in photocatalytic reaction, which plays a decisive role in photocatalytic oxidation. OH
−, H
2O adsorbed on the surface of the catalyst and in hydrated suspension can produce this substance. When water is adsorbed on the surface of titanium dioxide, the reaction mechanism is expressed as follows—TiO
2 + hv → e
− + h
+, h
+ + H
2O→·OH + H
+. In the strong alkali environment, there is a large amount of OH
-, which is beneficial to the formation of ·OH. When the titanium dioxide adsorbs O
2 on the surface, the mechanism can be expressed as follows—O
2 + e
− →·O
2−, ·O
2− + H
+ + e
− → HO
2−, ·OH + H
+ dye →···→ CO
2 + H
2O. There is a large amount of H
+ in a strong acid environment and it can be seen from the expression of reaction mechanism that it is beneficial to the formation of ·OH under a large amount of H
+. The large number of hydroxyl radicals formed in both acid and alkali environment and pH plays an important role in the whole catalytic oxidation process.
In addition, the pH has a great influence on the surface electrostatic charge of CdS/F–TiO
2 photocatalyst. When the pH is lower than the isoelectric point of CdS/F–TiO
2, the catalyst surface is positively charged and Na
+ and negatively charged chromogenic groups are ionized by methyl orange. The chromogenic groups are captured by electrostatic action, that is, the removal rate is high. When the pH is higher than the isoelectric point of CdS/F–TiO
2, the catalyst surface is negatively charged and the chromophore is negatively charged, which results in a lower degradation rate due to electrostatic repulsion. In summary, the acidic environment has the best photocatalytic effect, followed by the alkaline environment and finally the neutral environment [
38].
Figure 12 shows the decolorization rate of methyl orange solution with a concentration of 20 mg/L after illumination for 16 min by 1% CdS/F–TiO
2. It can be seen from
Figure 12 that the decolorization rate of methyl orange solution increase with the increasing of catalyst dosage. When it reaches 2 g/L, the decolorization rate is quite high. The decolorization rate of methyl orange solution increases slightly when it is higher than 2 g/L. This is because the photocatalysis reaction is carried out on the catalyst surface. When the concentration of methyl orange solution is fixed, the active sites provided for the reaction increase with the increased catalyst amount and the active groups produced by the illumination also increase. Thus, the photocatalysis reaction speed increases. With the further increased of the catalyst amount, enough active sites are provided for the reaction and the reaction reaches saturation, which results in the gentle change in decolorization rate of methyl orange solution. In this reaction system, the optimal catalyst dosage is 2 g/L when the concentration of methyl orange solution is 20 mg/L [
39].
To confirm the degradation, the chemical oxygen demand value (COD) of the initial and degraded solution were tested. 30 mL of the degraded methyl orange solution (20 mg/L) was added into a quartz tube, followed by adding 0.02 g 1% CdS/F–TiO
2. The degradation rate is 90.01% after irradiation with a 300 W metal halide lamp for 20 min (with a wavelength range of 280–780 nm). The chemical oxygen demand (COD) of methyl orange solution (20 mg/L) before and after degradation is determined by the Inspection and Testing Co., Ltd., of Neijiang Normal University. The results showed that the COD of the initial solution was 31 mg/L and that of the degraded solution was 23 mg/L. According to the value of COD, the COD removal rate was calculated to be 25.81%. However, according to the absorbance change, the degradation rate was 90.01%, shown in
Figure 13. The reason for the great difference between the two test results are that when 1% CdS/F–TiO
2 is used to degrade methyl orange solution, most of the chromogenic groups of methyl orange have been destroyed and methyl orange has been changed into the colorless substances, however only a part of them has been degraded into CO
2.
Stability is an important factor to evaluate the catalyst in practical application. In order to evaluate the reuse of synthesized photocatalytic material, the 1% CdS/F–TiO
2 catalyst was recovered after photocatalysis reaction and it was reused to degrade methyl orange with metal halide. As shown in
Figure 14, methyl orange was degraded by 1% CdS/F–TiO
2 for the first time with a degradation rate of 93.36%, followed by continuous experiments with a degradation rate of 90.37%, 89.05%, 87.94% and 86.54%, respectively. The photocatalytic activity did not show obvious decrease in 5 consecutive experiments, which illustrates that 1% CdS/F–TiO
2 has a good recycling performance.
During the photocatalytic performance test, the dark reaction of each sample was processed for 30 min to obtain the adsorption data, as shown in
Table 3. According to
Table 3, after dark reaction treatment, the absorbance of the solution hardly changed significantly, that is, the photocatalyst had no obvious adsorption effect on methyl orange. After the photoreaction, the decolorization rate of the sample was calculated by Equation (1) and the results are shown in
Figure 15. It can be seen from
Figure 14 that under the same conditions, the degradation rates of methyl orange by CdS/F–TiO
2 with different mass ratios are 1% > 2% > 0% > 0.5% > blank after illumination for 20 min. The absorbance of methyl orange solution without photocatalyst basically remained unchanged, indicating that the degradation of methyl orange was mainly achieved by photocatalysis of CdS/F–TiO
2 samples. The decolorization rate of 0.5% CdS/F–TiO
2 is the lowest (64.45%), while that of 1% CdS/F–TiO
2 is the highest (93.36%). It can be seen that the photocatalytic activity of F–TiO
2 is inhibited when the amount of CdS is too small. With the increase of CdS content, the photocatalytic activity of the sample is promoted. This indicates that CdS has a significant effect on the photocatalytic performance of F–TiO
2 after compound modification and only by adding an appropriate amount of CdS can the photocatalytic efficiency be improved effectively.
In this work, CdS can effectively improve the photocatalytic performance after F–TiO
2 modification. The reasons are as follows—First, the specific surface area of the sample increases and the grain size decreases after compound; Second, the modification of CdS causes a redshift of the threshold wavelength (λ
g) of F–TiO
2 absorption spectrum and the absorption intensity in the ultraviolet light region is obviously increased. The composite of the two materials shows a new threshold wavelength corresponded to CdS, which can effectively improve its photocatalytic activity. Third, compared with F–TiO
2, CdS/F–TiO
2 has a lower probability of photogenerated electrons and hole recombination under illumination. This is due to the large difference in the band structure of CdS and TiO
2. The band gap of CdS is narrower than that of TiO
2, so photogenerated electrons are transferred from the conduction band of CdS to the conduction band of TiO
2 and the holes on the valence band of TiO
2 are transferred to the lower valence band of CdS, thereby further promoting photocatalytic activity [
40] (as shown in
Figure 16). The essence of modification of TiO
2 by CdS is that CdS and TiO
2 semiconductors with different band gaps are coupled at a certain proportion. When exposed to metal halide light, the electrons generated by CdS excitation are more likely to transition to the conduction band of TiO
2. The holes remain on the valence band of CdS, enhancing the charge separation effect. When the incorporation amount is small, the modification of TiO
2 is not complete and when the amount of CdS is intensive, the surface, chain or island configuration centered on the incorporation may be formed, which weakens the coupling effect. Therefore, only by adding an appropriate amount of CdS can the absorption of light have a better effect [
41].
The first-order reaction kinetic has been studied. The kinetic curve of photocatalytic degradation was obtained by plotting the reaction time t with ln
Ct/
C0 (
C0: the initial concentration of methyl orange solution,
Ct: the concentration of methyl orange solution at time t in the degradation process), shown in
Figure 17. The six degradation kinetic curves all showed the approximate linear relationship. The reaction rate constant can be calculated according to the formula ln
Ct/
C0 =
Kt and the maximum reaction rate constant
K is 0.122 min
−1 (1% CdS/F–TiO
2). The minimum constant
K is 0.039 min
−1 (TiO
2). The results show that the photocatalytic degradation of methyl orange by TiO
2 and 1% CdS/F–TiO
2 follows the Langmuir-Hinshelwood first-order kinetic model.