**1. Introduction**

Photocatalysis is a technology that can use semiconductor materials to remove pollutants from the environment under different lighting conditions [1–3]. Most materials require ultraviolet conditions to produce redox effects. The visible part of sunlight irradiate that reaches the water surface only occupies 45%, while the ultraviolet part occupies less than 4% [4]. At the same time, water absorbs and reflects sunlight in different wavelengths, which seriously limits the removal efficiency of photocatalytic materials in a water environment [5–7].

g-C3N<sup>4</sup> is a good semiconductor material with application potential that is metal free and has visible light responsiveness [8–10]. However, researchers found that g-C3N<sup>4</sup> still has many disadvantages such as low utilization of visible light, poor photoelectric conversion efficiency, and low specific surface area [11]. Since Serpone et al. [12] first reported that a solid–solid heterojunction interface with good contact can be constructed from different coupled semiconductor materials to promote electron transfer between particles, more and more researchers have paid attention to different semiconductor materials. Among the many photocatalytic semiconductor materials, TiO<sup>2</sup> is widely used in sewage treatment [13], photocatalytic synthesis [14], and self-cleaning [15]. Due to its easy availability, low cost, stable chemical properties, corrosion resistance, non-toxicity, and strong oxidizing properties [16], its air purification [17] and antibacterial properties [18] have been extensively

**Citation:** Guo, X.; Rao, L.; Shi, Z. Preparation of High-Porosity B-TiO2/C3N<sup>4</sup> Composite Materials: Adsorption–Degradation Capacity and Photo-Regeneration Properties. *Int. J. Environ. Res. Public Health* **2022**, *19*, 8683. https://doi.org/10.3390/ ijerph19148683

Academic Editors: Paul B. Tchounwou, Xin Zhao, Xun Wang and Zhiyuan Wang

Received: 24 June 2022 Accepted: 15 July 2022 Published: 17 July 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**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/).

studied. Therefore, using the advantages of TiO<sup>2</sup> material properties to composite it with g-C3N<sup>4</sup> to become a more competitive material has attracted the attention of more and more researchers [19–21].

In the treatment of water environment pollution, the adsorption performance and visible light response ability of photocatalytic materials are two important factors that determine whether the photocatalytic technology can be effectively promoted [22,23]. The high-efficiency adsorption and enrichment ability of the material reduce the concentration of pollutants in water and provide a high-concentration contact environment that is conducive to the photocatalytic reaction for the material [24,25]. The melting characteristics of B2O3-modified g-C3N<sup>4</sup> are effective for improving its specific surface area and adsorption performance, but its response to visible light is limited, which affects the visible light photocatalytic activity [26].

Effectively improving the adsorption performance and visible light catalytic activity of materials at the same time is a key issue for researchers. Functional coupling through different materials is an idea to solve this problem. Here, TiO<sup>2</sup> and g-C3N<sup>4</sup> were formed into a composite heterostructure to improve the photocatalytic oxidation ability of the material. Then, the melting characteristics of B2O<sup>3</sup> during the heating process were used as a "reaction environment regulator", and TiO2/C3N<sup>4</sup> was co-calcined to prepare a composite material. The results of the experiment showed that B element doped the composite-modified materials, and, finally, the adsorption behavior and visible light catalytic degradation ability of B-TiO2/C3N<sup>4</sup> were analyzed by using MB and RhB organic pollutants. This method effectively increased the specific surface area of the material, increased the conjugated system, improved the adsorption capacity, and effectively improved the visible light catalytic effect of the material. This provides a technical idea for the promotion and application of a low-cost preparation of an efficient adsorption–degradation photocatalytic composite material.
