Rapid and Scalable Fabrication of TiO2@g-C3N4 Heterojunction for Highly Efficient Photocatalytic NO Removal under Visible Light

TiO2@g-C3N4 heterojunction composites were successfully synthesized via one-step pyrolysis using commercial TiO2 and urea. The photocatalytic performance of a composite was investigated by measuring the degradation of nitric oxide (NO) under visible light. The light absorption and bandgap of the selected materials were determined using diffuse reflectance spectroscopy (DRS), which confirmed the strong photocatalytic activity of the materials under visible light. The NO photodegradation efficiency of the composite was almost 90% under visible irradiation. The generation of by-products and nitrogen dioxide (NO2) was calculated to determine the conversion pathway of NO during the photocatalytic reaction. Furthermore, the results for recycling indicated that the TiO2@g-C3N4 composite exhibited excellent stability. Also, the morphology and the chemical compositions of the materials were observed using high-resolution transmission electron microscopy (HR-TEM) images and X-ray photoelectron spectroscopy (XPS). Raman spectroscopy was used to detect the structural vibrations in the materials. Through electron spin resonance (ESR) and trapping tests, the photocatalytic mechanism of the TiO2@g-C3N4 composite was also elucidated, in which superoxide radical anions were found to play a significant role in NO removal.


INTRODUCTION
Climate change has been one of the most critical problems globally and has been significantly affecting air quality. Thus, it is highly desirable to address this overwhelming problem by amending major air pollutants (nitrogen oxides (NO x ), sulfur dioxide, and volatile organic compounds). In 2012, diesel engines were identified as cancer-causing by the World Health Organization (Espina et al., 2015). NO x is primary air pollutants, causing severe impacts on human health. In particular, it causes lung problems, headaches, eye damage, loss of taste, and corroded teeth. In addition, NO x contributes to the formation of photochemical smog when exposed to sunlight (Cheng et al., 2018). People with lung diseases such as asthma may experience damage to lung tissue and reduced lung function when working outdoors when a high concentration of NO x is present. Recent advancements in science and technology have inspired several practical approaches to addressing air pollution (Liu et al., 2013;Chen et al., 2021;Wu and Ku, 2021;Zhang et al., 2021).
Several strategies have been proposed to suppress air pollution, including physical, chemical, and biological treatments through recovery or degradation. In particular, methods such as adsorption, thermal oxidation, microorganisms, selective catalytic reduction, selective noncatalytic reduction, and activated carbon injections have been used. Given the high costs, poor efficiency of secondary products, and complicated operating processes , the use of these technologies has been limited. (Jiang et al., 2012). Nevertheless, plant operators should consider implementing the most advanced technology available to achieve high NOx removal while holding operational and investment costs to a minimum.
In recent years, photocatalysis research has gained significant attention by taking advantage of the abundance of solar light for the purpose of addressing environmental pollution issues. In particular, titanium dioxide (TiO 2 ) has been identified as one of the most efficient photocatalysts because of its high photoactivity, low cost, non-toxicity, and highly tuneable bandgap (Huang et al., 2017;Huy et al., 2019a;Nasrabadi et al., 2021). However, the practical use of TiO 2 in photocatalytic NO removal has been limited because of its large bandgap and rapid charge recombination (Van Viet et al., 2018). Therefore, several strategies have been implemented further to improve the performance of TiO 2 , such as doping, modifying its surface and bandgap, forming heterojunctions (Tran et al., 2018;Van Viet et al., 2019). Notably, coupling with metal-free graphitic carbon nitride (g-C 3 N 4 ) presents great promise in generating highly efficient photocatalysts. This effective combination originates from the unique advantages of g-C 3 N 4 , showing an excellent response under visible light, non-toxicity, low cost, and ease of fabrication (Yu et al., 2016;Wang et al., 2019;Baudys et al., 2020;Van Viet et al., 2021;Wang et al., 2021). Specifically, TiO 2 @g-C 3 N 4 heterojunction materials (TiO 2 @g-C 3 N 4 ) have led to great improvement in photocatalysis-induced pollutant degradation Pareek and Quamara, 2018;Wang et al., 2018;Chen, 2021;Gahlot et al., 2021;Xiong et al., 2021). The underlying physics for such a successful combination comes from the proper bandgap alignment between TiO 2 and g-C 3 N 4 . However, the use of these TiO 2 @g-C 3 N 4 heterojunctions has rarely been studied in an NO pollution context. In addition, the typical approaches for generating these structures are generally complicated, laborious, have poor stability and reproducibility, and are time-consuming, which hinders their potential for scalable manufacturing.
In this study, we report on a scalable fabrication of TiO 2 @g-C 3 N 4 heterojunction materials for the purpose of addressing NO pollution. By forming a functional interface between TiO 2 and g-C 3 N 4 , TiO 2 @g-C 3 N 4 heterojunctions exhibit excellent photocatalytic NO removal under visible light. The role of the main radical species, involved in the photocatalytic activity is also thoroughly investigate in the present study. Such a TiO 2 @g-C 3 N 4 heterojunction, prepared via scalable fabrication without the generation of any harmful by-products is shown to be highly efficient in terms of removing NO pollution, and thus to have great promise for practical and industrial implementation.

Materials and Chemicals
All chemicals were of analytic grade and used without being further treated. Urea 99% (NH 2 CONH 2 ) was purchased from Alfa Aesar. TiO 2 (mass fraction 0.990) was provided by Xilong Chemical Co., Ltd., Beijing, China. Nitric oxide (NO) was purchased from Ming Yang special gas company. Deionized water (DI) was obtained from Thermal Scientific.

Synthesis of Materials
A schematic illustration for the synthesis of g-C 3 N 4 and TiO 2 @g-C 3 N 4 is shown in Fig. 1. For the synthesis of g-C 3 N 4 , 30 g of Urea were ground and placed in a 100 mL crucible. This crucible was then annealed at 550°C for 120 min. The sample was then cooled down to room temperature naturally.
For the synthesis of the TiO 2 @g-C 3 N 4 , 30 g of urea and a desirable amount of the TiO 2 (0.8 g) were ground and placed in a crucible. This crucible was then annealed at 550°C for 2 h. The sample was then cooled down to room temperature naturally. The ratio of the TiO 2 and g-C 3 N 4 was calculated based on the desirable ratio.

Photocatalytic Activity Test
The photocatalytic performance of the materials was evaluated by measuring the degradation of gaseous NO under visible light. 0.2 g of the samples were placed in a petri dish, after which it was dispersed in 10 mL DI for 5 min under an ultrasonic cleaner, and then dried at 60°C for 30 min to remove the water via evaporation. The reactor was a cylinder reactor (Fig. 2). The reaction chamber which had a volume of 4.5 L (30 cm × 10 cm × 15 cm), was made of stainless steel, and quartz was used to seal the top of the reactor to allow light to pass through. A xenon lamp (300 W) was placed vertically 40 cm above the reaction chamber. Firstly, the initial 100 ppm concentration of NO was diluted with zero-air (Zero-air generator, Model 1001, Sabio Instruments). The input flow rate and the concentration of NO and zero-air were controlled by a mass control flow rate to reach an NO concentration of 500 ppb. Finally, the lamp was turned on after the catalyst reached adsorption/desorption equilibrium. The sampling rate of the experiment was 0.5 L min -1 . In this study, the photocatalytic NO removal experiment followed the U.S. EPA requirements for RFNA-1289-074 and the international standard-ISO 22197-1.
The factors considered for the photocatalytic degradation of NO included the hydroxyl radicals, electrons, and hole pairs. To explain the photocatalytic mechanism, a trapping experiment was performed during the photocatalytic reaction in the presence of the materials to detect active species. The capture experiments were carried out by mixing 0.2 g of the photocatalyst with trapping agents, including potassium iodide (KI), potassium dichromate (K2Cr2O7), and isopropyl alcohol (IPA), which served as a radical scavenger for the hole (h + ), one electron (e -), and the The NO degradation, and NO 2 conversion, were calculated by Eqs. (1) and (2), respectively (Ohtani, 2011;Guerrand et al., 2015);.
where C NO (ppb) is the concentration of NO. C NO 2 is the concentration of NO 2 , i is the initial concentration, and f is the final concentration.

Reusability of Photocatalysts
The ability to reuse the materials used for removing NO was evaluated for four cycles. After the first test, the sample was washed carefully with DI and dried in an oven at 80°C for 20 min before the rest of the photocatalytic test was performed. The degradation efficiency was almost the same as that in the previous test.

Characterizations
The X-ray diffraction (XRD) patterns were studied to determine the crystal structures of the materials in the range of 10° to 80°. Fourier-transform infrared spectroscopy (FTIR) was used to investigate the structural vibrations of the materials in the range of 4000 cm -1 to 400 cm -1 . The morphology of the materials was observed using high-resolution transmission electron microscopy (HR-TEM) images. X-ray photoelectron spectroscopy (XPS) was used to determine the chemical compositions of the materials. Raman spectroscopy was used to detect the structural vibrations of the materials in the range of 100 cm −1 to 500 cm −1 . Diffuse reflectance spectroscopy (DRS) was used to observe the bandgap of the materials. The electron spin resonance (ESR) was implemented to capture the presence of oxygen free radicals (•O 2 ) and (•OH) in the photocatalytic processes.

Structural and Compositional Characterizations
XRD patterns of the materials are shown in Fig. 3(a). In the XRD pattern of the pure g-C 3 N 4 , significant diffraction peaks appeared at positions of 27.3° and were indexed to the (002) plane of g-C 3 N 4 , indicating a typical graphitic structure (JCPDS 87-1526). However, no other impurity peaks were detected in the XRD pattern, indicating that the synthesized g-C 3 N 4 exhibited high purity crystalline characteristics. The flat cell, repeating cell structure, and sandwich stack reflection of g-C 3 N 4 have been suggested to be responsible for the sheet structure of g-C 3 N 4 (Yang et al., 2013;Zhang et al., 2013).
Compared with the diffraction peaks of pristine g-C 3 N 4 and TiO 2 , the diffraction peak corresponding to the g-C 3 N 4 (0 0 2) crystal plane in the composite is relatively broader, and the intensity was weaker. The peaks of g-C 3 N 4 and anatase TiO 2 could be observed in the TiO 2 @g-C 3 N 4 sample, thus indicating that the TiO 2 @g-C 3 N 4 composite was successfully synthesized. Fig. 3(b) shows the FT-IR spectra of the TiO 2 @g-C 3 N 4 , pure g-C 3 N 4 , and pure TiO 2 composite. The absorption band of the pure g-C 3 N 4 in the range of 1220-1650 cm -1 corresponds to the typical stretching vibration of C-N heterocycles . The absorption band around 400-800 cm -1 was attributed to the Ti-O bonds (Fu et al., 2013). The peaks at 809 cm -1 and 1220-1650 cm -1 were attributed to the characteristic stretching modes and suggest the existence of g-C 3 N 4 (Fu et al., 2014). In the region of 2900-3500 cm -1 , the O-H vibration broadband was observed. The spectrum showed the main features of the g-C 3 N 4 and TiO 2 in the TiO 2 @g-C 3 N 4 samples, suggesting that a mixture of g-C 3 N 4 and TiO 2 was formed.

Morphology Characterizations
The morphology of the TiO 2 @g-C 3 N 4 , TiO 2 , g-C 3 N 4 observed at the HR-TEM scale was 50 nm, as shown in Fig. 4. The structure of the TiO 2 @g-C 3 N 4 was observed as presented in Fig. 4(a). The d-spacing was 0.48 nm, which corresponded to the plane (002) of the TiO 2 (Štengl and Grygar, 2011). The TEM results further demonstrated that the TiO 2 @g-C 3 N 4 composites were successfully synthesized via one-step pyrolysis. In addition, the particle length of the TiO 2 @g-C 3 N 4 is shown in Fig. 5. The average particle length of TiO 2 in the TiO 2 @g-C 3 N 4 was 14.4 nm. Furthermore, the effect of the synthesis method on the specific surface area of the materials and the effect of the specific surface area on the photocatalytic activity were confirmed as indicated in previous studies, as shown in Table 1. Previous studies indicated that the photocatalytic activity of these materials is better when the specific surface area is higher. Fig. 6(a) shows the Raman spectra from 100 cm -1 to 450 cm -1 of the materials. The characteristic peaks of TiO 2 can be observed at 139.2 cm -1 and 392.6 cm -1 in TiO 2 @g-C 3 N 4 and bare TiO 2 . The  Fig. 4. TEM and HR-TEM of (a, b) TiO 2 @g-C 3 N 4 , (c, d) TiO 2 , (e, f) g-C 3 N 4 . The SEAD of TiO 2 @g-C 3 N 4 (insert b).  Table 1. The specific surface area of g-C 3 N 4 , TiO 2 , and TiO 2 @g-C 3 N 4 heterojunction.

No Pollutant
BET surface area (m 2 g -1 ) References g-C 3 N 4 TiO 2 TiO 2 @g-C 3 N 4 1 Methylene Blue 6.  peak at 139.2 and 392.6 corresponded to the Ti-Ti vibrational modes, and the Ti-O vibrational modes, respectively. On other hand, no peak shifts of g-C 3 N 4 were observed within the Raman shift from 100 cm -1 to 450 cm -1 (Lin and Zhao, 2019). The surface chemistry of the materials is shown in Figs. 6(b-f). In the XPS survey of the materials shown in Fig. 6(b), the peaks of C 1s, N 1s, O 1s, and Ti 2p can be observed at 288.2 eV, 398.8 eV, 531.7, and 460.5 eV, respectively. Furthermore, the high-resolution XPS (HR-XPS) of C 1s, N 1s, O 1s, and Ti 2p was investigated to determine the effect of the synthesis method on the surface chemistry of the materials (Tang et al., 2021;Toghan et al., 2021). In Fig. 6(c), the peak at 288.2 eV was attributed to the N-C=N 2 groups. Interestingly, the strong peak at 284.7 eV in the TiO 2 @g-C 3 N 4 was attributed to the C-C groups (Štengl and Grygar, 2011). In contrast, the N 1s, O 1s, and Ti 2p spectra were similar for the three materials. Fig. 7(a) depicts the optical properties of the synthesized TiO 2 @g-C 3 N 4 sample as characterized using DRS. These spectra are noticeably different between TiO 2 @g-C 3 N 4 and the two bare materials. As shown in the results, the TiO 2 and g-C 3 N 4 both exhibited an absorption band at ~380 nm and ~475 nm, respectively, which is consistent with previous reports (Boonprakob et al., 2014;Huang et al., 2015;Xiao et al., 2015). Therefore, the TiO 2 @g-C 3 N 4 had coherent TiO 2 and g-C 3 N 4 optical properties, and the absorption edge of the composite showed a remarkable redshift to a larger wavelength at ~600 nm. These results suggested that the as-prepared composite can improve utilization in the visible-light region compared with the single material. As shown in Fig. 7(b), absorption occurred at wavelengths shorter than 410 nm and 460 nm, which agreed well with the bandgap of the TiO 2 (3.31 eV) and g-C 3 N 4 (2.92 eV), respectively (Huang et al., 2013;Huang et al., 2015). The energy bandgap of the synthesized composite was estimated using the Kubelka-Munk function, where the bandgap of the TiO 2 @g-C 3 N 4 composite was 3.03 eV, leading to the ability to absorb light in the visible region as well as reducing the recombination of electrons and hole pairs.

Photocatalytic Performance
The efficiency of photocatalytic NO removal of the materials is shown in Fig. 8(a). The NO photodegradation efficiency of the single TiO 2 , g-C 3 N 4 , and TiO 2 @g-C 3 N 4 was 76.4 %, 71.6%, and 90.2%, respectively. The enhancement of the removal efficiency indicated that the synthesized composite can utilize visible light, parallel with the improvement in the electron and hole recombination rates.
Specifically, the products of the NO removal process were also considered, for which the results are shown in Fig. 8(b). The amount of NO2 that converted from NO after the degradation process was significantly decreased from 27.5%, 6.8%, and 5.3% for g-C 3 N 4 , TiO 2 , and TiO 2 @g-C 3 N 4 , respectively. In addition, the results indicated that the other nitrogen products produced when using the synthesized composite were higher than those produced using a single catalyst. The other nitrogen products were various types of nitrogen gas (nitrate radical (NO 3 ), nitrogen trioxide,  The reusability of the materials was determined using a recycling test and the XRD pattern of the samples before and after the 4 th recycling process. As shown in Fig. 9(a), the photocatalytic efficiency of TiO 2 @g-C 3 N 4 decreased slowly and reached 86.4% at the 4 th recycling cycle. In addition, the XRD pattern ( Fig. 9(b)) indicated that the crystal structure of the TiO 2 @g-C 3 N 4 was not affected during the photocatalytic reaction. These results indicated that TiO 2 @g-C 3 N 4 is a high reusable material. Table 2 shows a comparison of this study and previous studies related to TiO 2 @g-C 3 N 4 .

Photocatalytic NO Degradation Mechanism
The ESR signals of the •OH and •O 2 radicals were studied to validate the generation of the radical from the materials. The ESR signals of the •OH and •O 2 radicals under dark conditions and visible conditions for 300s are shown in Fig. 10(a), (b). Under the visible condition, the generation of •OH was higher than •O 2 . In contrast, there were no •OH and •O 2 signals under dark conditions. A trapping experiment was conducted to consider the critical factors leading to photodegradation of NO by adding the K 2 Cr 2 O 7 , KI, and IPA scavengers. Herein, the K 2 Cr 2 O 7 , KI, and IPA scavengers were mixed with the materials to trap e -, h + , and •OH, respectively. The results of the trapping experiments are shown in Fig. 10(c). The trapping results demonstrated that the contribution of eto the photocatalytic activity of the three materials was higher than that of h + and •OH. These results were consistent with the ESR data signal.   The photocatalytic mechanism of TiO 2 @g-C 3 N 4 for NO degradation under visible light is presented in Fig. 11. Under visible light irradiation, the generated emoved from the valance band to the conduction band of TiO 2 created excess h + in the TiO 2 valence band. Then, the etransferred from the TiO 2 conduction band to the valence band of g-C 3 N 4 . Finally, the emoved to the g-C 3 N 4 conduction band. The ein the g-C 3 N 4 conduction band reacted with O 2 to form the Fig. 11. The proposed photocatalytic mechanism of the TiO 2 @g-C 3 N 4 heterojunction base on the Z-scheme interface for NO degradation under visible light.
•O 2 -. The generated O 2oxidized NO to form NO 3and prevented the formation of NO 2 (Huy et al., 2019a). On the other hand, the h + in the valence band of TiO 2 reacted with H 2 O to form •OH. The •OH degraded NO to NO 2 and NO 3 - . From the ESR and trapping results, the materials generated •OH more than •O 2 radicals.

CONCLUSIONS
A one-step pyrolysis method was employed in the present work to synthesize the TiO 2 @g-C 3 N 4 heterojunction. Outstanding photocatalytic activity and stability was demonstrated for photocatalytic NO removal and low NO 2 generation (5.3%) under visible light. The photocatalytic efficiency of TiO 2 @g-C 3 N 4 reached 86.1% after the first 10 min under visible light irradiation, after which it reached the highest efficiency (90.1%) 30 min later. Meanwhile, the efficiency of pure TiO 2 (76.34%) and pure g-C 3 N 4 (71.6%) was lower than that of TiO 2 @g-C 3 N 4 . This result demonstrated that the heterojunction structure greatly affects the photocatalytic performance of these materials. Herein, the heterojunction structure reduced the recombination of electron-hole pairs and extended the light absorption of the composite under visible light. The reduction in the recombination of electron-hole pairs decreased photocatalytic inactivation, which led to an increase in the photocatalytic activities of TiO2@g-C3N4. Furthermore, after four reusability tests, the TiO 2 @g-C 3 N 4 showed good stability with only a 3.9% decrease in reusability. The influence of the heterojunction structure on the optical properties of the materials was determined using DRS. Finally, the photocatalytic mechanism of the materials was confirmed and discussed through trapping experiments, where ESR, h + , and •OH were the main factors contributing to the photocatalytic activity of TiO 2 @g-C 3 N 4 during photodegradation of NO under visible light. The material worked well under visible light and overcame the biggest drawback of photocatalyst technology by reducing the energy consumption required for the photocatalyst reaction. From the above results, the TiO 2 @g-C 3 N 4 material can be considered to be a material with high applicability, a simple manufacturing method, and low cost.