*3.2. SEM Analysis*

sizes ranging from 30 nm to 80 nm.

*3.2. SEM Analysis* Figure 2 shows the SEM photographs of a clean glass substrate and N‐doped TiO2 films. The glass substrate after being washed with acetone and eroded with hydrofluoric acid was quite clean and had an even roughness with a great transmission of light. A rough surface can increase the capacity of the load [27]. The microstructures of the six samples had no obvious differences, which indicated that the quantity of carbamide had little impact on the surface of the coatings. Figure 2d, e demonstrate that the films made using the sol–gel method were porous and the granules dispersed uniformly with particle Figure 2 shows the SEM photographs of a clean glass substrate and N-doped TiO<sup>2</sup> films. The glass substrate after being washed with acetone and eroded with hydrofluoric acid was quite clean and had an even roughness with a great transmission of light. A rough surface can increase the capacity of the load [27]. The microstructures of the six samples had no obvious differences, which indicated that the quantity of carbamide had little impact on the surface of the coatings. Figure 2d, e demonstrate that the films made using the sol–gel method were porous and the granules dispersed uniformly with particle sizes ranging from 30 nm to 80 nm.

**Figure 2.** SEM photographs of a clean glass substrate (**a**,**b**) (TN0) and SEM photographs of sample TN9 (**c**–**e**). **Figure 2.** SEM photographs of a clean glass substrate (**a**,**b**) (TN0) and SEM photographs of sample TN9 (**c**–**e**).

#### *3.3. UV‐Vis DRS Analysis 3.3. UV-Vis DRS Analysis*

UV‐vis DRS results are shown in Figure 3. The maximum absorption wavelength of TN0 was about 380 nm, indicating the main components of samples were naked titanium dioxide with little impurities. Different amounts of N‐doping replaced oxygen atoms in the TiO2 crystal lattice with nitrogen atoms to different degrees, which was also the reason for different degrees of redshift when compared with the maximum absorption wave‐ length of TN0. It is worth noting that the nitrogen doping amount did not follow a "the more, the better" pattern within a certain range since the maximal redshift occurred for TN5, not TN9. Different amounts of N‐doping can reduce the bandwidth of TiO2, enhance the transfer of electrons from the valence band to the conduction band and improve the photocatalytic rate. However, when the amount of N‐doping is too much, the nitrogen atom will become the center of electron recombination and accelerate the recombination rate of electrons and holes, thus affecting the photocatalytic rate [31,32]. UV-vis DRS results are shown in Figure 3. The maximum absorption wavelength of TN0 was about 380 nm, indicating the main components of samples were naked titanium dioxide with little impurities. Different amounts of N-doping replaced oxygen atoms in the TiO<sup>2</sup> crystal lattice with nitrogen atoms to different degrees, which was also the reason for different degrees of redshift when compared with the maximum absorption wavelength of TN0. It is worth noting that the nitrogen doping amount did not follow a "the more, the better" pattern within a certain range since the maximal redshift occurred for TN5, not TN9. Different amounts of N-doping can reduce the bandwidth of TiO2, enhance the transfer of electrons from the valence band to the conduction band and improve the photocatalytic rate. However, when the amount of N-doping is too much, the nitrogen atom will become the center of electron recombination and accelerate the recombination rate of electrons and holes, thus affecting the photocatalytic rate [31,32].

**Figure 3.** UV-vis DRS of TN0, TN1, TN3, TN5, TN7 and TN9.

#### **Figure 3.** UV‐vis DRS of TN0, TN1, TN3, TN5, TN7 and TN9. *3.4. Photocatalytic Activity under UV and Visible Light Irradiation*

#### *3.4. Photocatalytic Activity under UV and Visible Light Irradiation* 3.4.1. Photocatalytic Activity under UV Light Irradiation

3.4.1. Photocatalytic Activity under UV Light Irradiation With regard to absorption, the dark test showed that the variation of phenol concen‐ tration caused by absorption was lower than 3%, which meant that the absorption had With regard to absorption, the dark test showed that the variation of phenol concentration caused by absorption was lower than 3%, which meant that the absorption had little influence on the degradation ratio of the photocatalytic procedure.

little influence on the degradation ratio of the photocatalytic procedure. The variation of each sample over time is shown in Figure 4. The phenol concentra‐ tion at 0 min was measured just after absorption. It can be observed that the rank of the six samples regarding photocatalytic activity was TN5 > TN3 > TN7 > TN1 > TN9 > TN0, i.e., TN5 had the highest degradation over the others during the same period. The degra‐ dation ratios were 93.77%, 91.32%, 90.82%, 88.89%, 85.73% and 80.10%, respectively. The maximum absorption wavelength of the five N‐doped samples were all redshifted to vis‐ ible light to different degrees, while the photocatalytic activities of the five samples were also enhanced, which should have been caused by the doping nitrogen leading to the en‐ ergy structure of titanium dioxide changed; that is to say, the optical energy band gap of TiO2 diminished. A reduction in the optical energy band gap will enhance the transfer of electrons from the valence band to the conduction band under visible light, which may have been the reason for the better relative performance of the N‐doped TiO2 [33]. The The variation of each sample over time is shown in Figure 4. The phenol concentration at 0 min was measured just after absorption. It can be observed that the rank of the six samples regarding photocatalytic activity was TN5 > TN3 > TN7 > TN1 > TN9 > TN0, i.e., TN5 had the highest degradation over the others during the same period. The degradation ratios were 93.77%, 91.32%, 90.82%, 88.89%, 85.73% and 80.10%, respectively. The maximum absorption wavelength of the five N-doped samples were all redshifted to visible light to different degrees, while the photocatalytic activities of the five samples were also enhanced, which should have been caused by the doping nitrogen leading to the energy structure of titanium dioxide changed; that is to say, the optical energy band gap of TiO<sup>2</sup> diminished. A reduction in the optical energy band gap will enhance the transfer of electrons from the valence band to the conduction band under visible light, which may have been the reason for the better relative performance of the N-doped TiO<sup>2</sup> [33]. The kinetics of phenol removal followed the Langmuir–Hinshelwood kinetic equation [34]:

$$\mathbf{R} = d\_{\mathbf{c}} / d\_{l} = \mathbf{k} \mathbf{K}\_{\mathbf{c}} / (1 + \mathbf{K}\_{\mathbf{c}}) \tag{1}$$

ܴൌ݀⁄݀௧ ൌ ݇ܭ/ሺ1 ܭሻ (1) where ܴ represents the reaction rate, ܿ is the concentration of the substrate at the time, ݇ is the reaction rate constant and ܭ is the adsorption constant. When the concentration where *R* represents the reaction rate, *c* is the concentration of the substrate at the time, *k* is the reaction rate constant and *K<sup>C</sup>* is the adsorption constant. When the concentration was very low, *K<sup>c</sup>* << 1; therefore, the relevant equation turned out to be

$$
\ln(c\_0/c) = k\_l + b \tag{2}
$$

where ܿ is the initial concentration, ݐ is the reaction time and ݇௧ is the apparent reac‐ tion rate constant. Figure 5 shows the kinetics of the six samples. The apparent reaction rate constants ranged from 0.00614 to 0.01048. where *c*<sup>0</sup> is the initial concentration, *t* is the reaction time and *k<sup>t</sup>* is the apparent reaction rate constant. Figure 5 shows the kinetics of the six samples. The apparent reaction rate constants ranged from 0.00614 to 0.01048.

*Int. J. Environ. Res. Public Health* **2022**, *19*, 15721 7 of 10

**Figure 4.** Phenol concentration trend under UV light. **Figure 4.** Phenol concentration trend under UV light.

**Figure 4.** Phenol concentration trend under UV light.

**Figure 5.** Reaction kinetics trend under UV light. **Figure 5.** Reaction kinetics trend under UV light.

3.4.2. Photocatalytic Activity under Visible Light Irradiation

**Figure 5.** Reaction kinetics trend under UV light. 3.4.2. Photocatalytic Activity under Visible Light Irradiation 3.4.2. Photocatalytic Activity under Visible Light Irradiation The absorption under visible light in the dark period made little difference compared with UV light, while the variation in phenol concentration during illumination time made The absorption under visible light in the dark period made little difference compared with UV light, while the variation in phenol concentration during illumination time made a significant difference.

The absorption under visible light in the dark period made little difference compared with UV light, while the variation in phenol concentration during illumination time made a significant difference. It can be observed from Figure 6 that the degradation rates of the N‐doped samples were obviously better than that of the naked TiO2 (TN0). TN5 still possessed the fastest degradation rate, followed by TN3, TN7, TN1 and TN9; that is to say, the best degradation a significant difference. It can be observed from Figure 6 that the degradation rates of the N‐doped samples were obviously better than that of the naked TiO2 (TN0). TN5 still possessed the fastest degradation rate, followed by TN3, TN7, TN1 and TN9; that is to say, the best degradation efficiency appeared at the point where the molar ratio of N/Ti was 0.5, rather than the more nitrogen doping, the higher the degradation rate. The degradation results were also It can be observed from Figure 6 that the degradation rates of the N-doped samples were obviously better than that of the naked TiO<sup>2</sup> (TN0). TN5 still possessed the fastest degradation rate, followed by TN3, TN7, TN1 and TN9; that is to say, the best degradation efficiency appeared at the point where the molar ratio of N/Ti was 0.5, rather than the more nitrogen doping, the higher the degradation rate. The degradation results were also coincident with the UV-vis DRS results, where TN5 had the greatest redshift.

efficiency appeared at the point where the molar ratio of N/Ti was 0.5, rather than the more nitrogen doping, the higher the degradation rate. The degradation results were also

coincident with the UV‐vis DRS results, where TN5 had the greatest redshift.

coincident with the UV‐vis DRS results, where TN5 had the greatest redshift.

**Figure 6.** Phenol concentration trend under visible light. The kinetics of phenol removal under visible light also followed the Langmuir–Hin‐

*Int. J. Environ. Res. Public Health* **2022**, *19*, 15721 8 of 10

**Figure 6.** Phenol concentration trend under visible light. The kinetics of phenol removal under visible light also followed the Langmuir–Hin‐ shelwood kinetic equation (the kinetic Equation (1) in 3.4.1), as Figure 7 shows. The ap‐ The kinetics of phenol removal under visible light also followed the Langmuir– Hinshelwood kinetic equation (the kinetic Equation (1) in 3.4.1), as Figure 7 shows. The apparent reaction rate constants ranged from 0.00156 to 0.04893. shelwood kinetic equation (the kinetic Equation (1) in 3.4.1), as Figure 7 shows. The ap‐ parent reaction rate constants ranged from 0.00156 to 0.04893.

**Figure 7.** Reaction kinetics trend under visible light.

#### **Figure 7.** Reaction kinetics trend under visible light. **4. Conclusions**

**Figure 7.** Reaction kinetics trend under visible light. **4. Conclusions** N‐doped TiO2 thin films were successfully immobilized on commercial glass sub‐ strates via the sol–gel method starting from tetrabutyltitanate dissolving in anhydrous ethanol as a precursor. The formulation of sol and an annealing temperature of 490 °C were optimal, as seen by the highly uniform lattice structure that was mainly constituted **4. Conclusions** N‐doped TiO2 thin films were successfully immobilized on commercial glass sub‐ strates via the sol–gel method starting from tetrabutyltitanate dissolving in anhydrous ethanol as a precursor. The formulation of sol and an annealing temperature of 490 °C were optimal, as seen by the highly uniform lattice structure that was mainly constituted of anatase and the TiO2 granules being evenly distributed with ultrafine nano‐particle N-doped TiO<sup>2</sup> thin films were successfully immobilized on commercial glass substrates via the sol–gel method starting from tetrabutyltitanate dissolving in anhydrous ethanol as a precursor. The formulation of sol and an annealing temperature of 490 ◦C were optimal, as seen by the highly uniform lattice structure that was mainly constituted of anatase and the TiO<sup>2</sup> granules being evenly distributed with ultrafine nano-particle sizes ranging from 30 to 80 nm. The surface morphology of the coating was basically unaffected by different nitrogen contents. Under the same experimental conditions, the degradation efficiency of phenol in the experimental group under visible light irradiation reached about 90% of that

of anatase and the TiO2 granules being evenly distributed with ultrafine nano‐particle

under UV light, indicating that N-doping caused the optical energy band gap of TiO<sup>2</sup> to diminish; therefore, the maximum absorption wavelength had obvious redshifts, leading to the doped films having more efficient photocatalytic activity, both under UV light and visible light. The optimal photocatalytic efficiency was realized at an N-doping ratio of 0.5, rather than the more dopant, the better the photocatalytic efficiency since an excess Ndoping ratio leads to an increased recombination ratio of electrons and holes, which reduces the photon utilization factor. The pollution absorption ability of the TiO<sup>2</sup> and glass sheet was quite feeble; thus, the kinetics of degradation followed the Langmuir–Hinshelwood kinetic equation, which describes first-order reaction kinetics.

**Author Contributions:** Y.D. and L.F. contributed substantially to the design of the study, the analyses, the writing of the manuscript and the original draft preparation. X.D. and J.L. contributed to the project administration, the funding acquisition and verifying the experimental data. Y.C. and B.Y. took responsibility for performing the experiments and validation and made the figures, replied to comments and corrected the draft. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Project of Guizhou Education, Science and Technology Project of Guizhou Province ([2016]1163), the Project of Guizhou Education Department([2016]047), the Training Program for Young Backbone Teachers in Colleges and Universities of Henan Province, (no. 2019GGJS140), the Zunyi Science and Technology Planning Project (no. ZSKH(2021)199), the 2021 Rural Revitalization Project of Zunyi Normal University (Special Scientific Research Fund of Guizhou Provincial Department of Education) (QJHKY [2021]017-9), the Zunyi Normal University Serving Local Industrial Revolution project (N0.ZSCXY [2021]04) and the "2021 academic new seedling cultivation and innovation exploration project" of Zunyi Normal University (no. ZSXM [2021]1-10).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

