Novel TiO2/GO/M-MMT nano-heterostructured composites exhibiting high photocatalytic activity

This study proposed a technique to enhance the photocatalytic properties of TiO2 using graphene oxide (GO) and modified Montmorillonite (M-MMT). TiO2/GO/M-MMT nano-heterostructured composites were prepared via hydrothermal and co-precipitation. The photocatalytic performance was evaluated by investigating the photodegradation rate and absorption behavior of methyl orange (MO) under visible light irradiation. The results showed that TiO2/GO/M-MMT heterojunction exhibited excellent photocatalytic degradation performance, as the degradation rate of MO was observed to be 99.3% within 150 min. The density of adsorbed MO decreased by 62.1% after 210 min of dark adsorption using the TiO2/GO/M-MMT composite, which was significantly higher than that achieved using M-MMT, GO/M-MMT, and TiO2/M-MMT. The nano-heterostructure increased the effective interface between TiO2, GO, and MMT, which increased the charge transfer ability and prolonged the electron-hole separation time. Therefore, the results of this study can be used to design novel photocatalysts to eradicate environmental pollutants.


Introduction
Environmental pollution engendered from rapid advancements in the modern chemical industry has attracted the attention of researchers worldwide. Secondary chemical contamination can be caused during the degradation of pollutants using chemical methods (Laysandra et al., 2017). Photocatalytic degradation has been used to eradicate organic pollutants from wastewater owing to its several advantages such as operation simplicity, low cost and pollution-free nature (Lee et al., 2018;El-Kousy et al., 2020;Dao et al., 2021;Liu et al., 2022).
Titanium dioxide (TiO 2 ) is one of the most efficient photocatalysts used to produce hydrogen owing to its low cost, high stability, corrosion resistance, and environmental friendliness (Tao et al., 2020;Wang et al., 2021). TiO 2 photocatalysts have been used for industrial wastewater treatment using solar energy by converting the wastewater into nontoxic chemical products without producing any other pollutants. However, because of the wide band gap, TiO 2 must be irradiated with ultraviolet light, which constitutes approximately only 4% of visible light and provides low quantum yield (Wang et al., OPEN ACCESS EDITED BY Meng Zheng, Qingdao Haiwan Science and Technology Industry Research Institute Co., Ltd., China 2019). TiO 2 activity can be enhanced by coupling it with other semiconductors or doping with higher work-function metals (Yadav and Ahmad, 2015;Umer et al., 2019;Yang et al., 2020;Xiang et al., 2021;Li et al., 2022a;Dong et al., 2022;Jing et al., 2022;Xu and Li, 2022). Montmorillonite (MMT) is the most widely used material in clay semiconductor nanocomposites owing to its layered structure, high cation exchange capacity, excellent charge trapping ability, and considerable adsorption potential for semiconductor particles (Wang et al., 2011;Paiva et al., 2014;Sharma et al., 2018). Clay-TiO 2 /MMT heterostructured composites can provide additional number of sites for trapping photo-generated electrons, which ultimately enhances the photocatalytic activity (Liao et al., 2022;Wang et al., 2022). Additionally, graphene oxide (GO) can enhance the catalytic effect of semiconductors (Joshi et al., 2020) and can be used to provide electric carriers with more active attachment-sites for photocatalysis. This enables a faster transfer of photoelectrons, avoids accumulation because of its high electrical conductivity and large specific surface area, thereby reducing the possibility of electron-hole recombination. Liu et al. (2021) prepared N-TiO 2 / GO photocatalyst via sol-gel and hydrothermal methods and analyzed its adsorption performance for RhB.
Photocatalysis is a new, efficient and potential discovery. It uses renewable energy to decompose organic pollutants by sunlight. At present, the known photocatalytic materials are semiconductor materials and polymer materials. In the past decade, polymer photocatalysts have been developed rapidly, and many polymer photocatalysts with catalytic activity have been found (Kumar et al., 2021;Nadali et al., 2021;Yin et al., 2022). However, polymer photocatalysts have limited photochemical stability, lack of understanding of the reaction mechanism, balance between charge carrier lifetime and catalytic time, and the use of unsustainable sacrificial reagents (Tahir, 2017;Dao et al., 2021). However, the photocatalytic performance of TiO 2 /GO/MMT nanoheterostructured composite has not been reported thus far. In this study, a TiO 2 /GO/MMT nanocomposite was synthesized via hydrothermal and co-precipitation methods and the photodegradation of methyl orange (MO) using the nanocomposite was studied.

Experimental process
Modified MMT (M-MMT; 1 g) and cetyltrimethylammonium bromide (CTAB; 1 g) (~1% of MMT mass) were added to distilled water (200 mL) under ultrasonic conditions for 60 min. A certain amount of GO [prepared via the modified Hummer method (Pu et al., 2019)] was added to the M-MMT solution and stirred for 12 h using a magnetic stirrer. The solution was kept idle at room temperature for 24 h. Subsequently, the precipitates were washed, dried, and grinded to obtain the GO/M-MMT composite.
CTAB and butyl titanate were weighed at a ratio of 1:2 (n (CTAB) / n (Ti) = 0.5). The CTAB was then dispersed in 30 mL of distilled water and butyl titanate was placed in a beaker. A certain amount of the GO/M-MMT composite was added to the CTAB solution. After stirring for 30 min, butyl titanate was dropped into the solution at a rate of 0.5 drop/s using a dropper. After titration, the as-prepared solution was poured into a reactor and hydrothermally treated at 180°C for 10 h. A white powder was obtained after filtration and drying. The composite, TiO 2 /GO/M-MMT, was obtained by calcining the powder at 300°C for 50 min in a muffle furnace.
The irradiation light power is 1380 W/m 2 . The wavelength range of visible light is 400-760 nm. Visible light is used to study the photocatalytic performance in the experiment. The rate of photodegradation was tested using a 722S visible spectrophotometer. The photocatalytic performance of the composite was evaluated directly by measuring the change in the rate of degradation MO. The initial absorbance of MO (denoted by A 0 ) without the catalyst before illumination was determined by adjusting the wavelength. First, the suspension was stirred in dark for 60 min to achieve an adsorption-desorption equilibrium, and the solution was sampled every 20 min during the experiment. Next, the samples were centrifuged for 5 min at a speed of 4000 rpm using a high-speed centrifuge, following which the supernatant was collected to measure the absorbance (denoted by A). The following equation (Eq. 1) was used to calculate the degradation rate (η).
The composite (0.05 g) was placed in a MO solution (100 mL, 10 mg/L). The solution was irradiated with ultraviolet light and stirred in a dark box for 40 min. After centrifugation, the absorbance of the MO solution was measured at a wavelength of 460 nm. The amount of MO adsorbed by the composite was calculated using the following equation (Eq. 2).
where q t , V, and W represent the amount of MO adsorbed (mg/g), initial volume of the MO solution (L), and the quantity of the composite (g), respectively. C 0 and C t denote the initial density (mg/ L 3 ) and the concentration of the MO solution (mg/L) at time t, respectively. The activity of TiO2/GO/M-MMT photocatalyst will be evaluated by the change of methyl orange concentration and its adsorption capacity. At the same time, the pseudo-first-order kinetic equation will be given to further study the kinetics of photocatalytic degradation, and the mechanism of photocatalytic degradation will be discussed. Figure 1A shows the image of M-MMT, which displays a layered structure. Figure 1B shows the morphology of the GO/M-MMT composite. Several layers of the GO sheets are rough and wrinkled, which might provide more active sites for other components (TiO 2 ). As can be observed from Figure 1C The TEM images of TiO 2 /GO/M-MMT nanocomposites in Figure 2A, GO is circled by the orange ring, M-MMT is circled by the white line. The growth on M-MMT is TiO 2 , Which marked with a red circle. In the figure, it can be seen that the large layer M-MMT and TiO 2 particles was tightly wraped with the film GO, and the TiO 2 particles with a diameter of 15-20 nm grow uniformly on the M-MMT, which is consistent with the SEM in Figure 1D. The   Figure 3A. The XRD patterns obtained for M-MMT display a weak diffraction peak near 7°, which corresponds to the (100) characteristic diffraction peak of M-MMT. In addition to the characteristic M-MMT peaks, the GO/ M-MMT spectrum displays an additional strong diffraction peak at 10°, which corresponds to the (002) characteristic peak of GO. A weak diffraction peak is observed in the TiO 2 /M-MMT spectrum at 25.5°, corresponding to the (101) peak of TiO 2 . Diffraction peaks corresponding to GO and TiO 2 are observed in the spectrum of TiO 2 /GO/M-MMT. The diffraction-peak intensity corresponding to TiO 2 is higher than that corresponding to TiO 2 /M-MMT, In TiO 2 / M-MMT, the ratio of M-MMT is much higher than that of TiO 2 , the grains are closely arranged in the same direction and the crystallinity is better in the diagram, which makes the intensity of the M-MMT diffraction peak is higher than that of others. The higher diffraction peak shows that the TiO 2 diffraction peak is very small. In TiO 2 /GO/ M-MMT composites, the addition of GO makes it enter into the layered M-MMT layers, which changes the distance between layers, so that the diffraction peak of M-MMT becomes shorter and wider, and the intensity of TiO 2 diffraction peak increases after data normalization; however, the peak at 25.5°shifts to a lower angle. Because the M-MMT load is a lamellar structure, the layer spacing becomes wider because the addition of TiO 2 and GO enter the interlayer. According to the Bragg equation 2dsinθ = nλ, the value of d increases and the value of θ decreases. The diffraction peak of M-MMT is relatively weak, indicating that TiO 2 is attached to the surface of M-MMT. In addition, the diffraction peak of the composite is broadened. This could be because TiO 2 and GO entered the M-MMT interlayer via intercalation.  measured as 12 m 2 /g, 42 m 2 /g, and 117 m 2 /g, respectively. The specific surface area of the TiO 2 /GO/ M-MMT composite is approximately ten and three times higher than that of TiO 2 /M-MMT and GO/M-MMT, respectively. The large specific area opened more number of ion-transportation channels, resulting in a faster electron transport rate and more number of adsorption active sites, thereby improving the photocatalytic performance. Supplementary Figure S2 shows the pore-size distributions of the GO/M-MMT, TiO 2 /M-MMT, and TiO 2 /GO/M-MMT nanocomposites, which have average pore sizes of 6.7, 9.2, and 13.4 nm, respectively. The average pore size of the TiO 2 /GO/M-MMT nanocomposite is the highest, indicating that the ion transport channels become wider, resulting in higher electron transport rates and photocatalytic degradation rates (Kočí et al., 2014;Mottola et al., 2022).

Results and discussion
The characteristic peaks of Al, Si, C, Ti, and O are observed in the XPS full spectrum of the TiO 2 /GO/M-MMT nanocomposite ( Figure 3C). The peaks of 458.6 and 464.4 eV observed in the Ti 2p high-resolution spectrum ( Figure 3D) can be attributed to Ti 2p 3/2 and Ti 2p 1/2 , respectively, indicating that Ti bonded to oxygen and exhibited a +4 valence in the composites. The O 1s peak is separated and fitted as shown in Figure 3E. The peaks at 531.5, 532.0, and 532.5 eV correspond to the O-O, Ti-O, and Si/Al-O bonds, respectively. The characteristic peak at 74.6 eV in the Al 2p XPS profile ( Figure 3F) represents the Al-O bond in M-MMT. Figure 4A shows EIS Nynquist plots of different composites. Because M-MMT is a natural mineral material, the conductivity is weak and the slope is low. With the addition of GO and TiO 2 , the conductivity is improved, and the slope is increased. In TiO 2 /GO/M-MMT nanocomposites, due to the synergistic effect of GO and TiO 2 , the separation of electron-hole pairs is accelerated and the lifetime of photoinduced carriers is prolonged, thus enhancing the photocatalytic activity. The conductivity is enhanced, and the slope value is gradually close to 1, showing strong conductivity. Figure 4B shows the currentpotential curves of different composites. The area enclosed by the CV curve is the activity of the material, and the more active the photocatalytic performance is. The CV curve of M-MMT in the diagram is almost a straight line, and the macroscopic performance is poor catalytic performance. In GO/M-MMT and TiO 2 /M-MMT, with the addition of GO and TiO 2 to M-MMT, the activity gradually increased and the reduction peak appeared. In TiO 2 /GO/M-MMT, the area is the largest, the position of the reduction peak is the lowest, and the activity is the best, indicating that its photocatalytic performance is the best. Figure 5A shows the rate of degradation of MO using the photocatalysts prepared in the study. The degradation test conducted for 240 min reveals that the rate of degradation of MO using M-MMT is only 15%. As the amounts of GO and TiO 2 increase, the rate of degradation of MO increases to 41% and 79%, respectively. With the TiO 2 /GO/M-MMT nanocomposite, the rate of degradation of MO is 99%, which is twice as high as that achieved with GO/M-MMT. These results indicate that the TiO 2 component plays an important role in the MO degradation process. Figure 5B shows  (Lin et al., 2020;Xiang et al., 2021).
The kinetics of the photocatalytic degradation was studied further to investigate the mechanism of the photocatalytic degradation. A pseudo-first-order kinetic equation is given in the following equation (Eq. 3) (Sharma et al., 2018).
where k is the quasi-first-order reaction rate constant (min-1) and t is the irradiation time (min). Figure 5C shows the change in the concentration of MO after adsorption using the different composites. For all composites, the concentration of MO decreases after adsorption. The change in the Accordingly, the TiO 2 /GO/M-MMT composite had a high photocatalytic performance with excellent stability and can be recommended for removing the antibiotic compounds. The concentration of MO with different adsorption times is fitted for different photocatalysts, as shown in Figure 5D. The k value estimated for M-MMT is observed to range from minimum to zero, indicating a weak response to visible light. The k value for the TiO 2 /M-MMT nanocomposite is higher than that of the GO/ M-MMT nanocomposite, indicating that the response time of TiO 2 /M-MMT to visible light is longer than that of GO/ M-MMT. The degradation rate can be significantly increased using a combination of the three components (the k value increases). GO can reduce the space charge region of TiO 2 and induce its electric field to separate photogenerated carriers effectively, which increases the photocatalytic activity. However, when the space charge region becomes too narrow, the dopant concentration increases and the recombination of photogenerated carriers becomes faster (Lincho et al., 2021;Olea et al., 2021;Li et al., 2022b). According to experimental results explained above, the mechanism of the photocatalytic degradation of MO using TiO 2 /GO/M-MMT is proposed, as shown in Figure 6. When the MO dyes are irradiated with visible light, the MO dyes become photosensitized and initiate the photocatalytic process. Visible light is absorbed by the photosensitized MO dye molecules, which excites the MO-dye electrons to a higher energy level. Furthermore, the light-triggered electrons are transported from the high-energy state to the conduction band of TiO 2 , and MO is degraded by the active TiO 2 . Moreover, some electrons are transferred from TiO 2 to GO. Because the two-dimensional π-conjugated structure in GO is the electron acceptor, this special structure can effectively suppress the recombination between charges and carriers. The recombination between the light-triggered electrons and oxygen produces superoxide radicals (•O 2-) (Tahir, 2017). MO dye molecules are oxidized by these superoxide radicals to produce CO 2 ,  Frontiers in Chemistry frontiersin.org H 2 O, and intermediates. In addition, the layered structures of M-MMT enhance the adsorption capacity of the composites, which improve the photocatalytic performance (Chen et al., 2018;Yang et al., 2019;Foroutan et al., 2020;Mallik et al., 2021).

Conclusion
To promote the uniform dispersion of nano-TiO 2 and improve the adsorption capacity of the photocatalyst, TiO 2 /GO/M-MMT nanocomposites were prepared. The main conclusions are as follows: 1) The interlayer space in the nanocomposites increased with the addition of GO, providing more space for TiO 2 to enter the interlayer. The uniform nano-sized TiO 2 particles were distributed in the interlayer and on the surface of the TiO 2 /GO/ M-MMT nanocomposite, which formed ideal nanostructures. 2) The photocatalytic degradation rate of MO by the TiO 2 /GO/ M-MMT nanocomposite was up to 99.3%. The nanocomposite exhibited a better adsorption performance, which conformed to the pseudo-first-order kinetic equations.

Data availability statement
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.