**1. Introduction**

In recent years, substances that cause water pollution are mainly refractory pollutants in industrial and agricultural wastewater and domestic sewage, such as volatile halogenated hydrocarbons, phenols, nitrobenzene and polycyclic aromatic hydrocarbons. Much effort has been devoted to the photocatalysis field in water treatment to efficiently remove nonbiodegradable compounds and avoid secondary pollution. The photocatalytic reaction transfers an electron from a valance band electron to an empty conduction band by absorbing photon energy equal to or more than the semiconductor band gap. The resulting electron–hole pairs contribute to the degradation reaction of the pollutant. Phenolic compounds are highly toxic and carcinogenic; they are present in wastewater, mainly in effluents from the production of pharmaceuticals, plastics, pesticides, oil and petrochemicals. Phenol was used as a model form of pollution since its molecular structure contains a benzene ring, making it quite stable and difficult to be biodegraded.

Common semiconductor photocatalysts include TiO2, WO3, ZnO and NiO. In order to improve the photocatalytic degradation activity, many technologies were proposed, such as metal or nonmetal doping, the development of microspheres and coupling semiconductors together. A pure TiO<sup>2</sup> photocatalyst often has some shortcomings, such as easy recombination of photogenerated electrons and holes, low catalytic efficiency and only responds to UV light, which limits its practical application. To solve these problems, current research focuses on metal/nonmetallic element doping, precious metal deposition and the construction of composite photocatalysts. TiO<sup>2</sup> of ultrafine nanoparticle size is considered to be

**Citation:** Dou, Y.; Chang, Y.; Duan, X.; Fan, L.; Yang, B.; Lv, J. The Preparation of N-Doped Titanium Dioxide Films and Their Degradation of Organic Pollutants. *Int. J. Environ. Res. Public Health* **2022**, *19*, 15721. https://doi.org/10.3390/ ijerph192315721

Academic Editors: Xun Wang, Zhiyuan Wang and Xin Zhao

Received: 20 October 2022 Accepted: 23 November 2022 Published: 25 November 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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the most useful photocatalyst for its excellent properties, such as high-light-conversion efficiency, chemical stability, nontoxic nature and low cost [1–13].

The band gap energy of TiO<sup>2</sup> is 3.2 eV (for anatase) and 3.0 eV (for rutile), and the maximum absorption wavelength of TiO<sup>2</sup> is 387.5 nm (anatase); that is to say, TiO<sup>2</sup> can only assimilate UV light rather than generate electrons (e−) and holes (h<sup>+</sup> ), which can subsequently induce redox reactions for the degradation of nonbiodegradable organics in water [14]. Under the irradiation of UV light, electrons are promoted from the valence band to the conduction band of the semiconductor, creating electron–hole pairs, which can cause highly oxidizing hydroxyl and highly reduced superoxide radicals [15]. While the energy of UV light only takes 4–5% of the solar energy, how to enlarge the maximum absorption wavelength of TiO<sup>2</sup> for visible light and cause TiO<sup>2</sup> to absorb more visible light has become a research hotspot in recent years [16]. Ion doping, semiconductor complexes and surface photosensitized methods were employed to cause TiO<sup>2</sup> to be responsive to visible light. Nonmetallic ion doping is one of the widely studied ways of inducing new electronic bands and optical transitions, which involves the inclusion or substitution of a foreign atom, such as nitrogen, sulfur, fluorine or sulfur, that replaces the oxygen atom in the TiO<sup>2</sup> crystal lattice [17–19]. Doping TiO<sup>2</sup> with nitrogen can create a redshift in the absorption wavelength from UV to the visible range because of the formation of new states inside the TiO<sup>2</sup> bandgap. This shift could enable photocatalytic reactions to produce a high degradation rate under sunlight illumination [20–23].

Different nano-TiO<sup>2</sup> photocatalytic systems in suspension were studied, such as nanoparticles, nanobelts and nanotubes, where TiO<sup>2</sup> has a larger specific surface area and high absorption of light, and all show increased photocatalytic activity when excited under visible light toward the degradation of different chemical species [24–26]. However, difficulties in separation and recycling lead to the smaller possibility of industrial application. Furthermore, the dosage of TiO<sup>2</sup> is difficult to control, where too little will lead to low photocatalytic efficiency and too much will cause light scattering that influences the absorption of light. Thus, much effort was expended to immobilize photocatalysts in the form of thin films on a stable support to avoid the problems associated with disposing photocatalyst suspensions [27]. Different contents of doped nitrogen have different impacts on the photocatalytic efficiency, for disparities in the replaced oxygen atom can lead to variations in the photocatalyst activity. Frequently used nitrogen sources include urea, triethylamine, ammonia and ethylmethylamine [25].

N-doped TiO<sup>2</sup> thin films can effectively solve the previous problems by enlarging the maximum absorption wavelength and realizing immobilization together. Nitrogen is doped into TiO<sup>2</sup> using the sol–gel method, which is easy to operate and the reaction condition is mild. The formation of catalyst needs to go through the procedure of dipping, pulling out, drying, annealing, and chilling [28].

The purpose of this research was to prepare nitrogen-doped TiO<sup>2</sup> thin films on sheet substrates of different nitrogenous amounts using the sol–gel method. The advantages of this method are that TiO<sup>2</sup> can be stimulated with visible light, it solves the traditional issue of photocatalytic technology being difficult to control, problems such as high cost and effective components being easily lost are solved, and it greatly improves the possibility of a large-scale practical application in wastewater treatment engineering. We compared the degradation properties of these samples, investigated degradation results and their characterization consequences, analyzed the influence of the amount of nitrogen, and addressed the relationship between the concentration of zymolyte and the reaction time.

#### **2. Materials and Methods**

#### *2.1. Materials and Reagent*

Tetrabutyltitanate (CP, 98%), acetylacetone (AR, 98.0%), polyethylene glycol, acetone (AR, 99.5%) and hydrofluoric acid were provided by GuangFu Fine Chemical Research Institution in TianJin. Anhydrous ethanol (AR, 99.7%) and carbamide (AR, 99.5%) were made by ShuangShuang Chemical Co., Ltd., in YanTai, China. Deionized water was created in our lab. The glass substrates used were common commercial glass sheets cut to the needed size.
