Merck’s 99.99% pure tin (IV) chloride pentahydrate (SnCl₄.5H₂O), hydrazine hydrate (N₂H₄.H₂O), commercial titanium dioxide powder (TiO₂, Merck, 99.99%), and sodium hydroxide (NaOH, Merck, 99%) were used in the experiment. In this study, the MMT used was sourced from the Binh Dinh province, Vietnam.

The synthesis of SnO₂/TNTs heterojunction and the preparation of TNTs and SnO₂ NPs was carried out using a one-step hydrothermal method, following the procedures outlined in our previous publications [16]. The composite material was prepared using the ball milling method, combining MMT with SnO₂/TNTs. A specified amount of MMT x (g) and 2 g of SnO₂/TNTs were used to ensure a consistent ratio for composite formation. During ball milling, the mechanical forces led to the intimate mixing and interfacial contact between the two components. The repeated collisions and grinding actions resulted in the breakdown of particle agglomerates and the formation of a homogeneous mixture at the nanoscale, ensuring thorough dispersion of MMT within the SnO₂/TNTs matrix. Composite samples with different compositions (5%, 10%, and 15% MMT content relative to SnO₂/TNTs) were obtained by adjusting the amount of MMT added to the initial mixture.

The characterization of the synthesized material involved the use of various techniques. X-ray diffraction (XRD) was conducted using a D8 ADVANCED instrument from Bruker AXS with Cu radiation to determine the material’s crystal structure. The Fourier Transform Infrared Spectroscopy (FT-IR) was performed using a Jasco FT-IR V-4700 spectrometer to analyze the vibrational modes of chemical bonds. Diffuse Reflectance Spectroscopy (DRS) was used to study the absorption wavelengths of the material within the UV-Vis range, employing a Jasco V-770 UV-VIS spectrophotometer. Electron Spin Resonance (ESR) was also employed to examine other material properties.

The photocatalytic capability of the MMT/SnO₂/TNTs material in treating NO gas was evaluated using an experimental setup as Scheme 1. The initial NO gas with 100 ppm of concentration was diluted by atmospheric air via a zero-air generator to achieve an inlet NO gas concentration of 500 ppb. The humidity within the reaction chamber was controlled using an airflow control system and a humidification system. The gas mixture was exposed to photoexcitation using a solar spectrum lamp and filter system, allowing the NO gas to react with the catalyst. The concentration of initial NO gas, the concentration change of NO and the formation of NO₂ gas were identified by a NO_x spectrometer system (Sabio, 6040).

For the preparation of the synthesized material, 0.2 g of the material was evenly spread on a 12 cm diameter petri dish using DI water as the medium. After undergoing a ten-minute ultrasonic bath, the material was dried at 60°C. The NO removal efficacy of the prepared sample was evaluated under visible light within a reaction chamber as in previous study [16]. Besides, the products of the photocatalytic reaction between semiconductors with NO gas were created via the green products of the NO photocatalytic, including nitrate (NO₃⁻), nitrite (HNO₂), and nitric acid (HNO₃), which are easily removable from the photocatalyst surface through washing with water. In contrast, nitrite (NO₂) is regarded as an undesirable byproduct due to its toxicity. Therefore, the NO removal efficiency (η, %), NO₂ conversion performance (η, %), and the conversion efficiency of NO into green products (ϕ, %) were calculated using Eq. (1–3), respectively.

To compare the efficiency of using light for photocatalytic reactions, the quantum efficiency is calculated using Eq. (4)

In Eq. (4), N_A (mol⁻¹) represents Avogadro’s constant, V_t (L min⁻¹) is the flow rate of NO, and M (g mol⁻¹) is the molar mass of NO. In the experiment, the photon flux was measured to be 2.72×1019 cm⁻² min⁻¹, and the irradiated area was 103.4 cm² for a 12 cm diameter petri dish.

Furthermore, trapping investigations were performed to comprehend the photocatalytic mechanism and the impact of crucial factors, including the participation of holes, electrons, and redox reactions, on the elimination of NO through photocatalyst processes. These experiments involved using 0.2 g of the synthesized sample with a trapping agent mass equal to 1% of the catalyst mass. Potassium iodide (KI), potassium dichromate (K₂Cr₂O₇), and terephthalic acid were employed as trapping agents to turn off photogenerated electrons (e⁻), holes (h⁺), and hydroxyl radicals (•OH), respectively. Additionally, recycling experiments were carried out, where the photocatalyst dispersed on the petri dish was removed from the reaction chamber after each cycle. The photocatalytic degradation experiment was then repeated under the same conditions.

X-ray diffraction (XRD) analysis was conducted to examine the crystalline phase states and identify the characteristic bonds present in the SnO₂/TiO₂ and MMT/SnO₂/TiO₂ materials under investigation. The XRD analysis of the MMT/SnO₂/TNTs composite material in Fig. 1a revealed the presence of three main phases: SiO₂, SnO₂, and TiO₂. The TiO₂ phase in the MMT/SnO₂/TNTs sample exhibited peaks at 2θ values of 25.20°, 37.83°, 40.00°, 53.96°, 62.01°, and 71.03°, corresponding to the (101), (004), (200), (105), (211), and (220) crystallographic planes of TiO₂. Additionally, the SnO₂ phase displayed peaks at 2θ values of 21.48°, 25.01°, 30.60°, 36.18°, 41.76°, and 47.35°, corresponding to the (110), (101), (200), (211), (220), and (311) planes. The MMT clay component was characterized by SiO₂ peaks at 2θ values of 22.02°, 26.64°, 31.48°, 36.67°, 42.25°, and 46.8°, corresponding to the (100), (011), (110), (102), (112), and (200) planes [17].

The FTIR spectrum was conducted to facilitate the observation of bond vibrations and the formation of SnO₂/TNTs and MMT/SnO₂/TNTs composite materials. The FTIR spectrum allows for the analysis of specific chemical bonds and the presence of functional groups in the region spanning wavenumbers from 4000 to 400 cm⁻¹. The FTIR analysis in Fig. 1b confirmed the presence of specific bands in the MMT sample, such as the O-H stretching vibration of water at 3440 cm⁻¹, the Si-O stretching vibration at 1630 cm⁻¹, and the Si-O-Si bending vibration at 785 cm⁻¹, indicating the presence of silica. In the FTIR spectra of the 5% MMT, 10% MMT, and 15% MMT samples, similar bands to those observed in the SiO₂ sample were present, confirming the presence of silica. Additionally, evidence of SnO₂ and TiO₂ in the composite was provided by the Sn-O stretching vibration at 1050 cm⁻¹ and the Ti-O stretching vibration at 880 cm⁻¹. The 5% MMT sample exhibited a slightly weaker intensity of the SnO₂ and TiO₂ bands compared to the 10% MMT sample, indicating a lower concentration of SnO₂ and TiO₂.

X-ray photoelectron spectroscopy (XPS) was utilized to gain valuable insights into the chemical within the 10%MMT/SnO₂/TNTs. Fig. 2a shows the XPS of MMT/SnO₂/TNTs, including binding energy peaks of Si, Sn, Ti and O. The carbon is associated with unexpectedly adsorbed hydrocarbons. The high-resolution XPS spectrum of Sn 3d (Fig. 2b) displays two binding energy (BE) peaks at 494.1 and 484 eV, Sn is present in Sn (IV) state. Fig. 2c shows the high resolution XPS for Ti with two main peaks. In detail, the BEs of two peaks located at 456.55 eV for 2p_3/2 and a weak peak at 462.3 eV for 2p_1/2, corresponding to the Ti⁴⁺ oxidation state of TiO₂. The deconvoluted HRXPS spectra for O 1s indicate two contributions for oxygen species as shown in Fig. 2d. One of which corresponds to Sn-O-Sn and the other Ti-O-Ti. These XPS evidence demonstrated the formation of SnO₂/TNTs.

Fig. 3 displays the MMT clay containing Si⁴⁺, Na⁺, and K⁺ cations, which enhances the mechanical properties of the composite material [18], including tensile strength, compressive strength, hardness, and flexibility. Tin (Sn) is distributed evenly throughout the sample, while titanium (Ti) is concentrated in specific regions. Silicon (Si) is relatively evenly distributed, and sodium (Na) and potassium (K) are uniformly distributed.

The composition and content of the elements in the sample are as follows: Tin (Sn) constitutes 48.7%, titanium (Ti) constitutes 13.4%, silicon (Si) constitutes 13.4%, sodium (Na) constitutes 5.3%, potassium (K) constitutes 0.5%, and oxygen (O) constitutes 27.1%. The presence of these elements in MMT can enhance its electrical conductivity, corrosion resistance, mechanical strength, and catalytic efficiency [19]. In addition, the BET surface area of SnO₂/TNTs and 10% MMT/SnO₂/TNTs was determined.

In addition, the BET surface area of SnO₂/TNTs and 10% MMT/SnO₂/TNTs was determined. In detail, Fig. 4 and Table S1 compared the parameters for the two samples, MMT and 10% MMT/SnO₂/TNTs. In detail, the presence of SnO₂/TNTs significantly affected the surface area and pore size of the material. SnO₂/TNTs not only penetrate the pores of MMT but also distribute on its surface, leading to a reduction in the sample’s surface area.

This is evident from the BET surface area of the 10% MMT/SnO₂/TNTs sample, which is only 4.5948 m²/g, considerably lower than the original MMT sample’s 77.7942 m²/g. Additionally, the presence of SnO₂/TNTs within and on the pore surfaces also reduces the average pore size from 18.3027 nm (adsorption) and 13.5354 nm (desorption) in MMT to 14.6424 nm and 8.3113 nm in the 10% MMT/SnO₂/TNTs sample. Furthermore, the pore volume of the 10% MMT/SnO₂/TNTs sample is also reduced compared to MMT, indicating that SnO₂/TNTs have occupied part of the original pore volume of MMT.

The optical properties of semiconductor materials are crucial for their applications in various fields as they govern the interactions between light and matter. To gain a deeper understanding of these properties, the synthesized materials were analyzed using the diffuse reflectance spectroscopy (DRS) technique, as shown in Fig. 5a. The DRS analysis provided significant findings, revealing that the SnO₂/TNTs material exhibited an absorption wavelength at 395 nm [17]. Interestingly, the incorporation of MMT into the SnO₂/TNTs composite greatly enhanced its light absorption capacity. Among the samples synthesized with different radiation doses of MMT, the sample with a 10% MMT dose displayed the highest light absorption capability, particularly at a wavelength of 410 nm. This observation suggests that the addition of MMT positively influences the material’s ability to absorb light. Furthermore, an intriguing relationship was observed between the radiation dose of MMT and the mass percentage of the MMT integrated into the SnO₂/TNTs material. As the radiation dose of MMT increased, the mass percentage of the MMT incorporated into the composite structure also increased. This increase in MMT content corresponded to a significant reduction in the material’s absorption wavelength. Increasing the dosage of the MMT resulted in a greater concentration of the MMT within the composite, which caused a decrease in the absorption wavelength.

Additionally, it was noted that the bandgap energy of the material decreased as the mass percentage of MMT increased in Fig. 5b. Specifically, the bandgap energy decreased from 3.2 eV for the SnO₂/TNTs material to 3.12 eV for the sample with a 10% MMT dose [17]. This indicates that the incorporation of MMT not only improves light absorption but also affects the fundamental electronic properties of the material, resulting in a narrower bandgap. The application of DRS provided valuable insights into the optical properties of the synthesized materials. The inclusion of MMT in the SnO₂/TNTs composite significantly enhanced its light absorption capacity, with the sample containing a 10% MMT dose exhibiting the highest absorption capability at a specific wavelength. Moreover, increasing the radiation dose of MMT led to a higher MMT content within the composite, resulting in a noteworthy decrease in the material’s absorption wavelength. These findings contribute to a comprehensive understanding of the optical properties of the synthesized materials and hold potential for various applications.

Fig. 6a illustrates the NO gas removal efficiency of the SnO₂/TNTs and MMT/SnO₂/TNTs materials. It is evident that the photocatalytic efficiency of the material significantly increases during the initial 5 minutes of irradiation [20]. Subsequently, the efficiency stabilizes for the remaining duration of the experiment. Notably, the incorporation of MMT into the composite material significantly enhances its effectiveness. Moreover, the MMT sample shows negligible impact on NO gas decomposition, as depicted in Fig. 6a. Fig. 6b shows the SnO₂/TNTs sample achieves a green product transformation efficiency of 34.2%. In contrast, the 10% MMT sample achieves a green product transformation efficiency of 54.3%, with a low NO₂ conversion efficiency of 0.52%. Additionally, the 5% and 15% MMT samples also exhibit green product conversion efficiencies of 42.74% and 45.47%, respectively. The MMT acts as a promoter for the photocatalytic effect, acting as a matrix that carries the nano-sized particles of SnO₂/TNTs, thanks to its components such as Si⁴⁺, Na⁺, and K⁺, which enhance the photocatalytic process. Furthermore, it is evident in Fig. 6c that both SnO₂/TNTs and MMT-MMT-containing samples demonstrate higher quantum efficiencies compared to the base SnO₂/TNTs sample, confirming the enhanced photocatalytic effectiveness of SnO₂/TNTs with MMT. Notably, the quantum efficiency peaks at 10.35 (10⁻⁴%) for the 10% MMT sample, indicating that this sample exhibits the highest photocatalytic effectiveness for removing NO gas from the atmosphere.

To insight into the photocatalytic mechanism and understand the impact of key factors such as electron-hole pairs and redox reactions on the efficiency of the photocatalytic reaction, a trap experiment was conducted. The experiment aimed to capture and analyze the main factors, namely h⁺, e⁻, and •OH, and their respective influence on the photocatalytic process. To trap these factors, KI, K₂Cr₂O₇, and terephthalic acid (TPA) were utilized.

Fig. 6d illustrates the photocatalytic removal efficiency of NO in the presence of KI, K₂Cr₂O₇, and terephthalic acid. The results demonstrate that electrons (e⁻) and holes (h⁺) play a crucial role in driving the photocatalytic reaction, whereas •OH radicals have a lesser contribution. This is evident from the observed decrease in NO removal efficiency when KI, K₂Cr₂O₇, and terephthalic acid are introduced to the reaction. Specifically, the NO removal efficiency decreases by 17.71% with the addition of KI, 5.51% with K₂Cr₂O₇, and 44.76% with terephthalic acid. These findings highlight the significance of electrons and holes in the photocatalytic process, underscoring their primary involvement in the desired reaction. The limited contribution of •OH radicals suggests that their presence has a relatively minor impact on the overall efficiency of NO removal in this particular system. Further investigations could delve into these factors’ underlying mechanisms and interactions of these factors to gain a deeper understanding of the photocatalytic reaction and optimize its performance.

To evaluate the stability and practical applicability of the 10% MMT material, photocatalytic ability was evaluated via recycling tests under visible light conditions. Fig. 7a shows the NO gas photocatalytic removal efficiency of the material under visible light irradiation. It is noteworthy that the material exhibited a remarkable consistency in its performance, as there was no significant decrease in its efficiency after undergoing 5 consecutive cycles. The initial efficiency was measured at 54.82%, and even after the 5th cycle, it remained high at 53.60%.

These findings indicate the promising stability and durability of the 10% MMT material as a photocatalyst for NO gas treatment under visible light. The negligible decrease in its efficiency over multiple cycles suggests its potential for practical application in real-world scenarios where continuous, long-term performance is required. Further research and testing could be conducted to validate and optimize the performance of the 10% MMT material, exploring its suitability for large-scale applications and investigating any underlying mechanisms contributing to its sustained photocatalytic efficiency. Furthermore, the FTIR analysis conducted after undergoing five cycles of reuse reveals a striking resemblance between the spectra, affirming the structural consistency of the photocatalyst despite prolonged exposure to wide-spectrum light in Fig. 7b. As a result, these outcomes provide encouraging evidence regarding the material’s durability when employed in practical settings.

To provide additional evidence of the participation of MMT and SnO₂/TNTs nanocomposite in the generation of reactive oxygen species (ROS) during photocatalytic reactions, an EPR technique was employed to monitor the signal of these species under varying conditions. The results obtained are presented in Fig. S1a, b showcasing the outcomes obtained from the EPR measurements. In the dark conditions, the results of the investigation revealed no detectable signals of reactive species such as •OH and •O₂⁻ radicals. This indicates that, without illumination, the generation of these radicals is negligible. However, when the MMT and SnO₂/TNTs nanocomposite was exposed to light in the visible range, distinct and strong signals from reactive species were detected. This observation unequivocally demonstrates that the heterojunction formed by the nanocomposite is effectively activated by the incident light, leading to the formation of reactive species. The capability of the 10% MMT/SnO₂/TNTs nanocomposite to facilitate both oxidation and reduction reactions is noteworthy. This property enables spatial distributions of oxidation, indicating that the composite material possesses the ability to promote simultaneous electron transfer processes, involving oxidation and reduction reactions.

Based on the trapping results and ESR results, the photocatalytic mechanism of MMT/SnO₂/TNTs for NO photocatalytic removal is indicated in Fig. 8. The MMT/SnO₂/TNTs heterojunction allows both reduction and oxidation reactions, which leads to the distribution of reduction sites (SnO₂ part) and oxidation sites (TNTs) via charge transfer interface. When SnO₂ and TNTs combined, the photogenerated electrons from high energy level will transfer to low energy level and establishing the internal electric field formed at the contact surface of the two semiconductors as a result [21]. At the same time, under the internal electric field, the holes in TNTs and electrons in SnO₂ transfer and recombine. Consequently, the residual electrons in the valence band (VB) of SnO₂ and the conduction band (CB) of TNTs are utilized in the photocatalytic process, exhibiting significant redox potential values. As a result, it is able for the efficient harnessing of the generated electrons (e⁻) and holes (h⁺) to enhance redox capability and effectively remove NO_x gases as Eq. (5–11) as follows.