*2.2. Preparation of N-Doped TiO<sup>2</sup> Films*

The dip-coating method was utilized to immobilize the films. Precursor solutions for N-doped TiO<sup>2</sup> coatings were prepared with tetrabutyltitanate, anhydrous ethanol, acetylacetone, carbamide, deionized water and polyethylene glycol (1 wt%). Tetrabutyltitanate (20 mL), anhydrous ethanol (80 mL) and acetylacetone (3 mL) were mixed using magnetic stirring for 20 min, followed by a certain quality of carbamide added into the solution as the donor of nitrogen (the molar ratios of N/Ti used were 0.1, 0.3, 0.5, 0.7 and 0.9). Deionized water (15 mL) was dropwise added at the speed of one drop per second and left to stir for one hour; then, the procedure was finished after the addition of polyethylene glycol (1 mL) to avoid fracturing of the films [28]. Undoped TiO<sup>2</sup> sol with the same reagents and procedures was also prepared to compare the photocatalytic activity with N-doped coatings. The sol was aged for 24 h at 35 ◦C and then used to make the coatings.

Commercial glass substrates (30 mm × 30 mm × 2 mm) cleaned with acetone (10 wt%) and eroded with hydrofluoric acid (10 wt%) for 3 h were used as the support of the N-doped TiO<sup>2</sup> films. The substrates were maintained in the aged sol for 10 min and then pulled out at the rate of 2 cm/s using a dip coater (CZ-4200, Qingdao Zhongrui Intelligent Instrument Co., LTD, Qingdao, China). After the films dried out, the previous process was repeated another three times; the product was then calcined for 3 h at 490 ◦C. The films were finally even and tight on the glass sheet. The coatings obtained were named TN0, TN1, TN3, TN5, TN7 and TN9 according to the amount of doped nitrogen.

### *2.3. Characterization*

After the deposition of the TiO<sup>2</sup> films onto glass substrates, all of the remaining sol was dried at 80 ◦C for 20 h in order to obtain dried gels, which were then calcined at 490 ◦C for 3 h to prepare the powders with the same crystal phase as the coatings. These powders were synthesized to analyze the phase compositions of titania by means of X-ray diffraction (XRD) using a D/Max-2200 Powder X-ray Diffractometer (XRD, D/Max-2200, Nippon Science Corporation, Tokyo, Japan) with Cu-Kα radiation at 40 kV and 20 mA.

Scanning electron microscopy (SEM, SU8220, Hitachi hi-tech, Shanghai, China) was utilized in an air atmosphere to examine the morphological structure and grain size of the films coated on the glass. The vapor has its considerable impact on the dried sample before observation [29].

The UV-vis DRS measurements were recorded at room temperature for the dry-pressed disk samples using a UV-3600 UV-vis spectrophotometer (UV-3600 UV-vis, Shimadazu, Tokyo, Japan) equipped with an integrating sphere assembly within the range of 300–900 nm.

#### *2.4. Measurement of Photocatalytic Activity*

The experiments were carried out with the initial concentration of phenol equal to 10 mg/L at an ambient temperature (approximately 25 ◦C) and pressure. The photoreactor used an acrylic cuboid static opaque chamber (700 mm × 450 mm × 250 mm) equipped with a thermocouple to monitor the temperature during irradiation. The UV source was supplied by 6 ultraviolet tubes (20 W), which had a dominant wavelength of 254 nm. As for the visible light source, 6 ordinary fluorescent lamps (20 W) were employed to produce the longer wavelength light.

Each beaker contained 8 pieces of glass sheet supporting N-doped TiO<sup>2</sup> coatings of the same amount of doped nitrogen. The illuminant was about 15 cm from the bottom of the solution. The system was left in the dark for 30 min until reaching phenol adsorption equilibrium, and then a photocatalytic reaction was carried out under UV light or visible light. The samples were taken from the reactor for analysis every 30 min, where the samples were placed in a 2 cm quartz dish and the remaining concentrations were analyzed using 4-AAP

4 h.

extraction spectrophotometry and UV-vis spectrophotometry (Photo Lab 6600 UV–Vis, WTW, Munich, Germany) at 510 nm. The photocatalysis reaction lasted for 4 h. **3. Results and Discussion**

forthe visible light source, 6 ordinary fluorescent lamps (20 W) were employed to produce

6600 UV–Vis, WTW, Munich, Germany) at 510 nm. The photocatalysis reaction lasted for

Each beaker contained 8 pieces of glass sheet supporting N‐doped TiO2 coatings of the same amount of doped nitrogen. The illuminant was about 15 cm from the bottom of the solution. The system was left in the dark for 30 min until reaching phenol adsorption equilibrium, and then a photocatalytic reaction was carried out under UV light or visible light. The samples were taken from the reactor for analysis every 30 min, where the sam‐

#### **3. Results and Discussion** *3.1. XRD Analysis*

*Int. J. Environ. Res. Public Health* **2022**, *19*, 15721 4 of 10

#### *3.1. XRD Analysis* Figure 1 shows the XRD patterns of the six powdery samples. For all samples, it can

the longer wavelength light.

Figure 1 shows the XRD patterns of the six powdery samples. For all samples, it can be observed that where the 2θ was 25.4◦ (101), the diffraction peak was especially distinct, and at 2θ = 30.7◦ (121), the relative intensity is quite small, which means that the anatase phase was dominant and the rutile phase was hardly existing. In addition, other characteristic peaks (2θ = 25.34◦ , 48.11◦ and 44.49◦ ) were all accordant with JPDS-21-1272 [30] (anatase standard card). It can be confirmed that N-doped TiO<sup>2</sup> mainly existed in the form of anatase. With the increase in the N doping amount, the sample pattern almost did not change, indicating that N doping had little effect on the crystal structure. The difference being all samples on the 101 crystal plane may have been caused by the grinding of the samples with different N contents. be observed that where the 2θ was 25.4° (101), the diffraction peak was especially distinct, and at 2θ = 30.7° (121), the relative intensity is quite small, which means that the anatase phase was dominant and the rutile phase was hardly existing. In addition, other charac‐ teristic peaks (2θ = 25.34°, 48.11° and 44.49°) were all accordant with JPDS‐21‐1272 [30] (anatase standard card). It can be confirmed that N‐doped TiO2 mainly existed in the form of anatase. With the increase in the N doping amount, the sample pattern almost did not change, indicating that N doping had little effect on the crystal structure. The difference being all samples on the 101 crystal plane may have been caused by the grinding of the samples with different N contents.

**Figure 1.** XRD patterns of TN0, TN1. TN3, TN5, TN7 and TN9. **Figure 1.** XRD patterns of TN0, TN1. TN3, TN5, TN7 and TN9.
