Journal of Chemistry
Volume 2019 (2019), Article ID 8587949, 10 pages
https://doi.org/10.1155/2019/8587949

Research Article

Preparation of Highly Stable and Effective N-Doped TiO₂@SiO₂ Aerogel Catalyst for Degradation of Organic Pollutants by Visible Light Catalysis

Shiyun Tang,1,2 Jingyu Ran,1,2 Junjiang Guo,1,2 and Anjiang Tang1,2

¹College of Chemical Engineering, Guizhou Institute of Technology, Guiyang 550003, China
²Engineering Technology Research Center of Fluorine Silicon Material, Guiyang 550003, China

Correspondence should be addressed to Shiyun Tang; 524506512@qq.com

Received 20 July 2019; Revised 7 October 2019; Accepted 15 October 2019; Published 3 November 2019

Guest Editor: Fada Feng

Copyright © 2019 Shiyun Tang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

To obtain high stable and effective TiO₂ photocatalyst, nano-N-doped TiO₂@SiO₂ (TiO2−xNx@SiO₂, 0 ≤ x ≤ 2) composite aerogels were synthesized by the sol-gel method combined with supercritical drying and direct oxidation process. The adsorption/photocatalytic degradation efficiency of TiO2−xNx@SiO₂ aerogels was evaluated by the degradation of RhB in aqueous solution under visible light irradiation. The physiochemical properties of the aerogels were examined by XRD, FT-IR, TEM, SEM, TG/DTA, and BET methods. It was found that the specific surface areas of all TiO2−xNx@SiO₂ samples exceeded 700 m²/g and exhibited a honeycomb porous structure with fine particulate morphology. Photocatalytic activity tests show that the 500-TiO₂@SiO₂ composite aerogel exhibits the best adsorption/photocatalytic degradation rate for RhB, which obtained about 80% of the degradation rate in 30 min under visible light and over 95% after 120 min. On the one hand, the SiO₂ aerogels can significantly inhibit the phase transition of TiO₂, and the nano TiO₂ can be highly dispersed in the SiO₂ aerogels; On the other hand, if the oxidation temperature is selected properly, the N-doped TiO2−xNx@SiO₂ aerogel can be obtained by simple TiN@SiO₂ aerogel oxidation.

1. Introduction

Titanium oxide (TiO₂) is an excellent semiconductor photocatalyst because of its high photocatalytic activity, good chemical stability, safety, nontoxicity, and low cost without secondary pollution [1–3]. In the past decades, TiO₂ as photocatalytic materials has become the important subject of attention by many researchers at home and abroad, in the water/air treatment [4], solar cell electrodes [5], water decomposition for hydrogen production [6], and other fields. It has three polymorphs in nature: anatase, rutile, and brookite. The main photocatalytic activities are anatase and rutile TiO₂. Rarely researchers focus on brookite TiO₂ due to its little photocatalytic activity and low thermal stability [7]. However, owing to the rather high intrinsic bandgap of pure TiO₂ (about 3.0 eV), only 3%–5% of the incoming solar energy on the earth’s surface can be utilized. Many problems still restrict the practical application of TiO₂ as a photocatalyst with excellent performance, such as low quantum efficiency, narrow light absorption wavelength, low solar energy utilization, and poor adsorption and recycling in the suspended phase photocatalytic system of water treatment.

To improve the photocatalytic efficiency of TiO₂, an extensive interest is invested in the use of metal or nonmetal as dopant during the preparation process. This principle is based on the “band filling mechanism;” the bandgap energy can be increased by doping metallic ions or nonmetallic element because the photoexcited electrons can be confined in the conduction band; thus, their lifetime can be prolonged [8]. Although metal and nonmetallic ion modification methods can improve the performance of TiO₂ photocatalyst to some extent, TiO₂ doped with metal ions is often thermounstable and the carrier recombination rate increases, thus sacrificing its photocatalytic ability in the ultraviolet (UV) region. On the contrary, TiO₂ doped with nonmetallic can not only enhance the visible light response ability but also maintain the photocatalytic activity in the UV region. Therefore, the nonmetallic ions doping in TiO₂ photocatalyst has shown great industrial application prospects. At present, the research studies on doping of nonmetallic ions are mainly focused on C [9], N [10], S [11], and F [12]. Among them, N is easy to be introduced into the structure of TiO₂ because of the comparable atomic radius to O atom, low ionization energy, and high stability. N-doped TiO₂ composite oxides always use the concentrated NH₃·H₂O or NH₄F as nitrogen and fluorine source [13]. The photocatalytic activity of TiO₂ can be significantly improved with the addition of N [14, 15]. Therefore, N-doped TiO₂ has become the most promising doping method for nonmetallic doping.

Moreover, to obtain the high-performance photocatalyst, nanosized TiO₂ is often used due to the higher photocatalytic activity than ordinary TiO₂. Nevertheless, TiO₂ nanoparticles tend to aggregate, which reduce its efficiency. To solve this problem, this work employs the SiO₂ aerogel as a support for TiO₂ nanoparticles, which make TiO₂ nanoparticles highly dispersed in SiO₂ aerogels, thus preventing its aggregation. SiO₂ aerogel is a solid material composed of nanoparticles or polymer molecules, which are crosslinked to form nanoporous skeleton structure. The unique nanoscale skeleton and pore distribution feature endow its excellent performance, such as ultralow density (as low as 0.003 g/cm³), high porosity (above 90%), huge specific surface area (500–1200 m²/g), strong transmittance (about 93%), low refractive index, and small dielectric constant (<1.01) [16–18]. Owing to these characteristics, SiO₂ aerogel has a broad application prospect in the field of thermal insulation materials, sound insulation materials, filter materials, catalysts, adsorbents, sensors, fuel cells, etc. [19–21].

It is important to combine the excellent properties of aerogels with nanophotocatalysts to take full advantage of the high adsorption efficiency and strong transmittance of SiO₂ aerogel and photocatalytic activity of nanosized TiO₂ photocatalyst simultaneously. The nano-TiN@SiO₂ composite aerogel materials were prepared by the sol-gel method combined with supercritical drying process. Then, the nano-N-doped TiO₂@SiO₂ (TiO2−xNx@SiO₂, 0 ≤ x ≤ 2) composite aerogels were obtained by the direct oxidation method. Finally, the adsorption/photocatalytic degradation efficiency for Rhodamine B (RhB) in aqueous solution of the TiO2−xNx@SiO₂ composite aerogels have been analyzed and discussed. This preparation process of aerogels can be either supercritical drying or ambient pressure drying [22, 23], and it can be easily scaled up for industrial production of visible light-driven N-doped TiO₂ photocatalyst for pollutants removal because of its convenience and energy-saving properties.

2. Experimental Details

2.1. Materials

Nano-TiN powder (≥99.9%) with a particle size of 20 nm and density of 5.24 g/cm³ was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Tetraethoxysilane (TEOS), absolute ethanol (C₂H₅OH), hydrochloric acid (HCl), ammonia (NH₃·H₂O), and RhB were obtained from ScienceLab Instrument Inc., Guizhou. All the reagents were of analytical grade purity and used without further purification. Deionized water was used throughout the experiment.

2.2. Preparation of TiO2−xNx@SiO₂ Composite Aerogel

Nano-TiN@SiO₂ composite aerogel was synthesized through the sol-gel method with the high-temperature supercritical drying. The procedure is as follows: Firstly, a certain amount of C₂H₅OH and TEOS were put into water (The volume ratio of C₂H₅OH : TEOS : H₂O is 1 : 3 : 6) and dispersed by ultrasonic oscillation for 30 min at room temperature. Then, adding 1 mol/L HCl as catalyst, adjusting pH value to 1.5–2.5 under magnetic stirring for 2 hours, followed by addition of 0.2 mol/L NH₃·H₂O slowly until the pH value is 7-8. At the same time, appropriate amount of nano-TiN powder (the theoretical wt.% of TiN is 10%) was added rapidly under intense stirring. The solution was continuously stirred for 2.5 h and subsequently aged for 2 h to obtain TiN@SiO₂ gel with appropriate viscosity. Prior to supercritical drying, the gel was soaked in ethanol for 4-5 times, and the aging solution was replaced every 24 h. Finally, the gel was transferred to an autoclave with admitting nitrogen gas into the chamber at about 2 MPa. Heating was carried out at a rate of 1°C/min until 260°C. This was chosen as the supercritical temperature for the ethanol solvents and the necessity to avoid any thermal decomposition of the solvent. The pressure inside the autoclave gradually increases to about 8 MPa when the supercritical temperature was achieved. The final pressure and temperature were maintained for at least 15 min. After that, the solvent was rapidly removed by opening the outlet valve to vent, and the autoclave was then allowed to cool to room temperature and disassembled. A black-colored aerogel powder of TiN@SiO₂ was obtained at this stage. To gain the TiO2−xNx@SiO₂ composite aerogel, the TiN@SiO₂ was ground into a finer powder and calcinated at the fixed temperature for 2 h in a furnace under air atmosphere. The samples were coded as calcinated temperature- TiO₂@SiO₂, e.g., the TiN@SiO₂ aerogels calcined at 300°C were recorded as 300-TiO₂@SiO₂. Color changed from black to light yellow N-doped TiO₂@SiO₂ (TiO2−xNx@SiO₂, 0 ≤ x ≤ 2) composite aerogels after different temperature calcined.

2.3. Photocatalytic Activity Evaluation

The photocatalytic activity of TiO2−xNx@SiO₂ composite aerogel was evaluated by the degradation of RhB in aqueous solution under simulated solar irradiation with a 500 W Xenon lamp (Beijing Perfectlight Technology Co. LTD., PLS-SXE300/300 UV). The lamp was positioned inside a cylindrical vessel and surrounded by a circulating water jacket to cool it. An amount of 10 mg photocatalyst was totally suspended in a 100 ml aqueous solution of 10 mg·L⁻¹ RhB. The solution was treated by ultrasonic oscillation for 15 min at room temperature. Then, the solution was shined with visible light by removing ultraviolet light using a filter. The distance between light source and the bottom of the solution was about 15 cm. At each time interval, 4 ml of the suspension was extracted for analysis during each photocatalytic process. Finally, the concentration of RhB was monitored by colorimetry with a UV-Vis spectrophotometer (Lambda35, PerkinElmer) at its maximum absorption wavelength. The adsorption/photocatalytic degradation efficiency of RhB was defined according to the following equation:

where η is the adsorption/photocatalytic degradation rate, C₀ was the initial concentration of RhB, and Ct was the corresponding concentration of RhB at certain reaction time.

2.4. Characterization

The specific surface area and pore size distribution of aerogels were determined by the Brunauer–Emmitt–Teller N₂ adsorption-desorption method (BET, HYA2010-B1). Pore size distribution and specific desorption pore volumes were obtained using the Barrett-Joyner-Halenda (BJH) method using the desorption branches. The surface morphology and porous structure of aerogel photocatalysts were observed by scanning electron microscopy (SEM, Nova Nano 450, FEI, USA) and transmission electron microscopy (TEM, Tecnai G² F20 S-TWIN, FEI, USA). The phase compositions of the samples were determined by X-ray diffraction analysis (XRD, Ultima IV, Rigaku Corporation, Japan), which was carried out in a DX-2500 rotating anode X-ray diffractometer using Cu Kα (λ = 0.15406 nm) radiation within the 2θ range of 10–80°. Thermogravimetric and differential thermal analysis (TG/DTA, Beijing Everlasting Scientific Instrument Factory, ZH‐1450) were performed in air atmosphere at a heating rate of 10°C/min from the room temperature to 750°C. Fourier transform infrared (FT-IR) spectra were recorded on a Spectrum VERTEX 80 spectrometer in the range of 400–4000 cm⁻¹.

3. Results and Discussion

3.1. Morphology and Structures of Composite Aerogels

The N₂-BET is used to characterize the porosity and surface area of the as-prepared TiN@SiO₂ and TiO2−xNx@SiO₂ composite aerogels. A typical nitrogen adsorption-desorption isotherm and pore size distribution of TiN@SiO₂ composite aerogel is presented in Figure 1. According to IUPAC classification, the adsorption isotherm of TiN@SiO₂ aerogel corresponds to type-IV, which is considered to be due to the presence of mesopores in aerogel. The pore shape can be identified by the hysteresis loop, the TiN@SiO₂ aerogel shows the type-H2 hysteresis loop in relative pressure (P/P₀) ranges from 0.70 to 0.98, which suggests that the pore structures are complex and tend to be made up of interconnected networks with different sizes and shapes. Furthermore, the illustration of Figure 1 shows the pore size distributions measured by the BJH model. It is evident that there is a bimodal distribution, which was concentrated at 9.90 nm and 13.21 nm, respectively.

Figure 1: Adsorption-desorption isotherm and pore size distribution of TiN@SiO₂ composite aerogel.

The specific surface areas of as-prepared TiN@SiO₂, 300-TiO₂@SiO₂, 500-TiO₂@SiO₂, and 700- TiO₂@SiO₂ composite aerogels are listed in Table 1. All samples were porous and developed a huge specific area (exceed 700 m²/g). Specifically, the specific surface of the as-prepared TiN@SiO₂ aerogel sample is 711 m²/g and slightly increases reaching about 743 m²/g after thermal oxidation at 300°C for 2 h, then further increased to 770 m²/g after calcined at 500°C for 2 h. However, the specific surface of 700-TiO₂@SiO₂ aerogel decreases to 703 m²/g as expected. It can be seen that the composite aerogels manage to preserve a substantial specific surface even with calcination at 700°C. The average diameters of the pores obtained by BET analysis are 16.32 nm for as-prepared TiN@SiO₂, 12.97 nm for 300-TiO₂@SiO₂, 11.29 nm for 500-TiO₂@SiO₂, and 12.66 nm for 700-TiO₂@SiO₂, and the corresponding mesopore volumes are 4.19 cm³/g, 3.18 cm³/g, 2.85 cm³/g, and 2.93 cm³/g, respectively. It is obvious that the pore size of aerogels is relatively stable after calcination, which means that the aerogels have the high thermal stability.

Table 1: Specific surface area (SBET), average diameter (d), and mesopore volume (Vp) of the aerogel samples.

Sample
SBET (m²/g)
d (nm)
Vp (cm³/g)

As-prepared TiN@SiO₂
711
16.32
4.19

300-TiO₂@SiO₂
743
12.97
3.18

500-TiO₂@SiO₂
770
11.29
2.85

700-TiO₂@SiO₂
703
12.66
2.93

SEM and TEM images are presented in Figure 2 for a few structural characteristic TiN@SiO₂ composite aerogel before and after thermal treatment. The SEM image of the as-prepared TiN@SiO₂ sample is shown in Figure 2(a). It can be seen that it exhibits a honeycomb porous structure with fine particulate morphology. The structure is quite uniform and amorphous flocculent on the whole. However, the accurate determination of crystallite size cannot be made by SEM. This was subsequently observed by using the TEM image (Figure 2(b)), which revealed a porous nanostructure built up of small nanoparticles of size about 10 nm. After thermal oxidation at 300°C for 2 h, the bonding between particles is relatively close by SEM (Figure 2(c)), and the particles grow and sinter slightly at high temperature from TEM with higher magnification (Figure 2(d)). With the increase of oxidation temperature to 500°C, the microstructures of the samples hardly changed compared to 300-TiO₂@SiO₂ (see Figures 2(e) and 2(f)). However, when the oxidation temperature is further increased to 700°C, it can be observed that serious sintering phenomena with the number of large particles increase obviously (see Figures 2(g) and 2(h)). This trend is consistent with the previous BET results. When calcined at less than 500°C, the specific surface area of the material increases gradually due to the volatilization and pore expansion of residual ethanol in the aerogel. However, when the calcination temperature is up to 700°C, the specific surface area of the material begins to decrease due to the strong sintering effect.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Figure 2: SEM and TEM images of TiN@SiO₂ composite aerogels before and after thermal treatment: SEM (a) and TEM (b) of TiN@SiO₂; SEM (c) and TEM (d) of 300-TiO₂@SiO₂; SEM (e) and TEM (f) of 500-TiO₂@SiO₂; SEM (g) and TEM (h) of 700-TiO₂@SiO₂.

3.2. Photocatalytic Activity of TiO2−xNx@SiO₂ Aerogels

As is well known, RhB as an organic dye existed in water can cause serious water pollution problems [24]. In this work, the RhB was chosen as model molecules to investigate the adsorption/photocatalytic activity of TiO2−xNx@SiO₂ composite aerogels. The adsorption/photocatalytic degradation efficiency is a combination effect of adsorption and photodegradation. Figure 3 shows the adsorption/photocatalytic degradation efficiency (η) curves of TiO2−xNx@SiO₂ composite aerogels for RhB under visible light irradiation. It can be found that the η values for RhB increases with the extension of reaction time, and the 500-TiO₂@SiO₂ composite aerogel exhibits the best adsorption/photocatalytic degradation rate for RhB, which obtained about 80% of the degradation rate in 30 min under visible light and over 95% after 120 min. For the as-prepared TiN@SiO₂, the adsorption/photocatalytic degradation rate quickly reached about 60% in the first 30 minutes due to the strong adsorption capacity of aerogel, and it hardly increases with the increase of reaction time. According to previous BET analysis results, the mesoporous structure with higher pore volume of as-prepared TiN@SiO₂ composite aerogel is favorable for physical adsorption of RhB. Similar to as-prepared TiN@SiO₂, the adsorption/photocatalytic degradation rate of the 300-TiO₂@SiO₂ also quickly reached about 62% in the first 30 minutes, but it increases slowly with the increase of reaction time. This indicates that the as-prepared TiN@SiO₂ composite aerogel has almost no visible-light photocatalytic activity, while the 300-TiO₂@SiO₂ has very weak visible photocatalytic activity. Furthermore, as the oxidation temperature increases, the adsorptivity capacity of 500-TiO₂@SiO₂ and 700-TiO₂@SiO₂ aerogels is significantly increased after heat treatment because of the improved hydrophilicity of TiN@SiO₂ composite aerogel in the heat treatment process.

Figure 3: Adsorption/photocatalytic degradation rate curves of TiO2−xNx@SiO₂ composite aerogels for RhB under visible light irradiation.

3.3. Possible Photocatalytic Reaction Mechanism of TiO2−xNx@SiO₂ Aerogels

To obtain the possible photocatalytic reaction mechanism of TiO2−xNx@SiO₂ aerogels, the phase composition, thermal stability, surface group, and absorbance properties as the increase of oxidation temperature were characterized by XRD, TG-DSC, FT-IR, and UV-Vis spectroscopy, respectively.

Figure 4 shows the XRD patterns of TiN@SiO₂ composite aerogels and TiN powers with different oxidation temperature. From Figure 4(a), it can be seen that all the samples have a dispersion characteristic peak at 15°–30°, which indicates that these samples are mainly amorphous with a relatively low crystallinity. This is consistent with the results of SEM and TEM. For the as-prepared TiN@SiO₂ composite aerogel, the reflections at 2θ angles of 36.8°, 42.7°, and 62.0° belong to (111), (200), and (220) crystal face of NaCl-type structure of TiN (JCPDS card number 38-1420). As the oxidation temperature increases to 300°C, there is no obvious change between the as-prepared TiN@SiO₂ and 300-TiO₂@SiO₂ composite aerogel, and only the characteristic diffraction peaks corresponding to the (111), (200), and (220) crystal faces of TiN are presented. According to previous work [25], this can be explained that the oxidation of TiN at 300°C is very weak, and its significant initial oxidation temperature is about 350°C. When the oxidation temperature is above 500°C, the characteristic diffraction peaks of TiN are completely disappeared, and two very weak and dispersion diffraction peaks at 25.4° and 54.4° which belong to the (101) and (211) crystal faces of anatase-TiO₂ appear. Comparing 500-TiO₂@SiO₂ with 700-TiO₂@SiO₂ samples, it can be found that the intensity of (101) crystal plane of 700-TiO₂@SiO₂ is slightly strong than that of 500-TiO₂@SiO₂. This indicates that the TiO2−xNx@SiO₂ aerogels have an excellent thermal stability.

(a)

(b)

Figure 4: XRD patterns of TiN@SiO₂ composite aerogels (a) and TiN powers with different oxidation temperatures (b).

Figure 4(b) gives the XRD patterns of pure TiN powers after oxidation at 500°C and 700°C. It can be seen that the pure TiN powers convert to anatase and rutile TiO₂ after oxidation at above 500°C. The TiO₂ by calcined TiN powers has a series of obvious diffraction peaks with relatively narrow and acuity. Halo peaks or other signs of amorphous phases are not observed, which implies that the crystal phase structures of TiO₂ are relatively complete with a high crystallinity. It is noteworthy that the (101) and (200) reflections at 2θ angles of 25.4° and 48.2° of anatase-TiO₂ are more pronounced than (110) and (211) peaks at 27.5° and 54.4° of rutile TiO₂ for oxidation at 500°C, respectively. But the case of the 700°C oxidation is just the opposite. This is due to the fact that other crystalline TiO₂ is easily transformed into more stable rutile structure at high temperatures [26]. The results are very different from TiO2−xNx@SiO₂ aerogels, because there are only weak and dispersed diffraction peaks of anatase TiO₂ observed in the latter. Considering the TiN wt.% of TiN@SiO₂ composite aerogels as 10%, the theoretical TiO₂ wt.% of TiO2−xNx@SiO₂ composite aerogels obtained by oxidation is 12.5%. The value is much higher than the detection limit of XRD. It means that the SiO₂ aerogels can significantly inhibit the phase transition of TiO₂ and the nano-TiO₂ can be highly dispersed in the SiO₂ aerogels.

From the XRD results, it seems that TiN in the TiN@SiO₂ composite aerogels can be completely oxidized to TiO₂ above 500°C. However, it is well known that pure TiO₂ has good photocatalytic activity only under ultraviolet irradiation, which is inconsistent with our experimental results. To further illustrate the visible light photocatalytic activity and thermal stability of TiO2−xNx@SiO₂ aerogels, TG-DTA experiments were carried out with as-prepared SiO₂ aerogel and as-prepared TiN@SiO₂ composite aerogel, as shown in Figure 5. From Figure 5(a), pure SiO₂ aerogel has a small weight loss (about 5%) from room temperature to 215°C which is attributed to desorption of physical adsorption water. Then, a notable weight loss (about 15%) is observed from 215°C to 310°C, which is attributed to the residual organic solvents in the aerogel. In the DTA curve, the decomposition of organic residues is reciprocated by the appearance of a remarkable endothermic peak with the maximum peak temperature (Tmax) of 245°C. Further weight loss (about 7.5%) is observed at a temperature range of 310–750°C. This might be ascribed to the organic macromolecular formed by condensation of organic solvents during supercritical drying and further condensation of free −OH groups on the SiO₂ network to form Si–O–Si bridges [27]. The TG curve of TiN@SiO₂ composite aerogel is similar to that of as-prepared SiO₂ on the whole (see Figure 5(b)). About 5% of weight loss belongs to the physical adsorption water at a temperature range of room temperature −250°C. A sharp weight loss (about 5%) of the residual organic solvents at a temperature range of 250–290°C is also observed, which shows a remarkable endothermic peak with Tmax of 258°C in the corresponding DTA curve. However, there is obvious difference that the TiN@SiO₂ composite aerogel has a broad endothermic peak at a temperature range of 290–570°C with Tmax of 512°C in the DTA curve, which should be attributed to the gradual oxidation of TiN in the SiO₂ aerogel. It indicates that the TiN@SiO₂ needs to be completely oxidized above 570°C to form the TiO₂@SiO₂. According to our previous works about the oxidation of TiN coatings [25], the oxidation of TiN coating can be divided into three stages: mild oxidation below 500°C, moderate oxidation between 550 and 600°C, and severe oxidation between 650 and 750°C. Specifically, initial oxidation of TiN with a partial color change occurs at 350°C and remarkable oxidation of TiN occurs between 400 and 450°C. Therefore, if the oxidation temperature is selected properly, the N-doped TiO2−xNx@SiO₂ aerogel can be obtained by simple TiN@SiO₂ aerogel oxidation.

(a)

(b)

Figure 5: TG-DTA curves of SiO₂ aerogel (a) and TiN@SiO₂ composite aerogels (b).

Figures 6(a) and 6(b) present the FT-IR and UV-Vis spectra of TiN@SiO₂ composite aerogels with different oxidation temperature, respectively. From Figure 6(a), the bands at about 1640 cm⁻¹ and 3450 cm⁻¹ can be assigned to the bending and stretching vibrations of the O-H groups, respectively. This is mainly due to the hydrophilicity of aerogels prepared by supercritical drying, which makes it easy to absorb water and form surface hydroxyl groups. According to the results of Huang et al. [28], the hydroxyl groups and hydrogen bonds on the surface of aerogels could enhance its adsorption capacity for RhB; thus, it will facilitate the subsequent photocatalytic degradation. The peaks assignable to symmetrical and asymmetrical stretching of Si-O-Si vibrations from the silica framework can be seen at 790–815 cm⁻¹ and 1080–1100 cm⁻¹, respectively. It should be noted that there is the hetero-linkage Ti-O-Si bond at 940–960 cm⁻¹ which indicates the incorporation of TiO₂ into SiO₂ to form binary TiO₂-SiO₂ systems. However, the expected Ti-N absorption bands (950–1000 cm⁻¹, 1100 cm⁻¹ and 1300 cm⁻¹) [29] could not be observed because they were obscured by the strong absorptions bands in the region 950–1300 cm⁻¹ from the silica framework. From Figure 6(b), the raw material SiO₂ aerogel exhibits almost no absorption from 300 to 800 nm. However, unlike pure TiO₂, which has no visible light absorption, the absorption of the TiN@SiO₂ composite aerogels becomes more obvious visible light absorption (>400 nm). And the increasing order of visible light absorption intensity with different oxidation temperatures is 300-TiO₂@SiO₂, 700-TiO₂@SiO₂, 500-TiO₂@SiO₂.

(a)

(b)

Figure 6: FT-IR (a) and UV-Vis (b) spectra of TiN@SiO₂ composite aerogels with different oxidation temperature.

Based on the above discussion and experimental results, it can be inferred that the possible reasons of highest efficiency of 500-TiO₂@SiO₂ composite aerogel are as follows: Firstly, it has the highest specific surface area of all samples. Secondly, N-doping can be formed by incomplete oxidation for the 500-TiO₂@SiO₂ composite aerogel, and the content of N-doped is moderate for photocatalysis. Moreover, the 500-TiO₂@SiO₂ composite aerogel exhibits the best visible light absorption after oxidation. It can also draw a schematic picture of the mechanism connected to the interaction of TiO2−xNx@SiO₂ aerogels under visible light, as shown in Figure 7. TiN@SiO₂ composite aerogel is oxidized to TiO2−xNx@SiO₂ aerogel at a certain temperature under air atmosphere; the reaction is shown in equation (1). Theoretically, if the oxidation temperature and time are chosen properly, the N-doped TiO₂@SiO₂ aerogels can be obtained due to the incomplete oxidation. Irradiation with visible light promotes the formation of the active oxygen species such as superoxide radicals and hydroxyl radicals [30, 31]. Numerous studies have shown that hydroxyl radicals are highly oxidative species which are considered to be the main species responsible for the photodegradation of the organic contaminants either adsorbed on the surface of the photocatalyst or in the bulk solution [32–34]. Therefore, the photocatalytic reaction can completely oxidize the RhB to form CO₂ and H₂O, as shown in equation (2). In fact, two effects must be considered with the final degradation efficiency when porous materials used as catalyst, the adsorption and degradation of the degradants. TiO₂ powers have a weak adsorption capacity, but SiO₂ aerogels can provide more adsorption centers; thus TiO₂@SiO₂ aerogels combine the advantage of high surface area and porous SiO₂ aerogels and semiconductor properties of TiO₂ to significantly yield novel materials appropriate for heterogeneous photocatalysis. When the N atoms get into the skeleton of TiO₂@SiO₂ aerogels to form the N-doped TiO₂@SiO₂ aerogels, a blue shift will appear with N-doped because not only is the top of the TiO₂ valence band lowered but the inserted N2p levels are also lower in energy than the valence band of pure TiO₂; thus, the N-doped TiO₂@SiO₂ aerogels have a higher visible light active properties [33]:

Figure 7: Sketch of the proposed mechanism for the processes induced by visible light irradiation of the TiO2−xNx@SiO₂ aerogels.

4. Conclusions

To improve the photocatalytic efficiency of TiO₂, this work combines the excellent properties of aerogels and nanophotocatalysts to take full advantage of the huge specific surface area, high adsorption efficiency, and strong transmittance of SiO₂ aerogel and photocatalytic activity of nanosized N-doped TiO₂ simultaneously. Sol-gel method, supercritical drying, and direct oxidation process were adopted to prepare the nano-N-doped TiO₂@SiO₂ (TiO2−xNx@SiO₂, 0 ≤ x ≤ 2) composite aerogels. The specific surface areas of all TiO2−xNx@SiO₂ samples exceeded 700 m²/g and exhibited a honeycomb porous structure with fine particulate morphology. The 500-TiO₂@SiO₂ composite aerogel exhibits the best adsorption/photocatalytic degradation rate for RhB, which obtained about 80% of the degradation rate in 30 min under visible light and over 95% after 120 min. Furthermore, as the increase of oxidation temperature, the adsorptivity capacity of 500-TiO₂@SiO₂ and 700-TiO₂@SiO₂ aerogels is significantly increased after heat treatment because of the improved hydrophilicity of TiN@SiO₂ composite aerogel after air treatment. The SiO₂ aerogels can significantly inhibit the phase transition of TiO₂, and the nano-TiO₂ can be highly dispersed in the SiO₂ aerogels. If the oxidation temperature is selected properly, the N-doped TiO2−xNx@SiO₂ aerogel can be obtained by simple oxidation of TiN@SiO₂ aerogel.

Data Availability

All data included in this study are available upon request by contact with the corresponding author.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

This research was supported by the National Natural Science Foundation of China (No. 2176605), S&T Plan Project Approving in Guizhou (Nos. Qiankehe LH Zi[2016] 7104, Qiankehe SY Zi[2014] 3058, and Qiankehe Jichu[2019]1135), Natural Science Foundation of Guizhou Provincial Department of Education (Qiankehe KY Zi[2016]014), Science and Technology Services for Rural Industrial Revolution and Poverty Alleviation and Undergraduate Training Programs for Innovation and Entrepreneurship.

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