A Unique RGO Aerogel/TiO2/MoS2 Composite Photocatalyst with a 3D Sandwich Network for the Removal of Organic Dyes by the Cooperative Action of Adsorption and Photocatalysis

Yujuan Zhang Xi'an University of Architecture and Technology Haojie Qi Xi'an University of Architecture and Technology Liang Zhang (  zl98zl@hotmail.com ) Xi'an University of Architecture and Technology https://orcid.org/0000-0002-4056-9803 Yao Wang Xi'an University of Architecture and Technology Lvling Zhong Xi'an University of Architecture and Technology Yage Zheng Xi'an University of Architecture and Technology Xin Wen Xi'an University of Architecture and Technology Xiaomin Zhang Xi'an University of Architecture and Technology Juanqin Xue Xi'an University of Architecture and Technology

The combination of adsorption and photocatalytic technologies is promising because the cooperation between these two processes can achieve a better removal e ciency of organic dyes compared to their individual processes (Dursun et al. 2020; El Mersly et al. 2021; Zhang et al. 2020). Strong interactions between dye molecules and functional groups on the surface of a catalyst ensued from adsorption can facilitate rapid and e cient reactivity between dye molecules and short-lived reactive materials, such as reactive oxygen species (ROS), produced on photocatalyst surface, resulting in photodegradation (Martins et al. 2020). Upon photocatalytic degradation, the products are desorbed from the catalyst surface, regenerating the adsorption sites to assist in further dye removal (Guo et  To circumvent this, modi ers can be added into the structure of a photocatalyst. Molybdenum disul de (MoS 2 ) has demonstrated excellent promise as a modi cation for photocatalysts

Preparation of the RGO aerogel/TiO 2 /MoS 2 composite
The RGO aerogel/TiO 2 /MoS 2 composite was synthesized via the sol-gel and physical vapor deposition (PVD) methods. First, graphene oxide (GO) was prepared by Hummers' method and treated by freezedrying. The resulting aerogel GO (0.4 g) was added to 20 mL of absolute ethanol, and the mixture was ultrasonically exfoliated at room temperature for 2 hours. Then, 1.5 mL of tetrabutyl titanate was dispersed into the mixture, and 3 mL of DI water was added. The precipitate was washed with DI water and treated via freeze-drying to obtain the RGO aerogel/TiO 2 .
MoS 2 (0.5 g) was added to a ceramic boat that was then wrapped with aluminum paper and tied with several holes. The ceramic boat and 1.0 g of the RGO aerogel/TiO 2 composite were placed in a reactor and heated at 570℃ for 3 hours at a heating rate of 5℃/min, after which they were cooled to room temperature and removed from the reactor to obtain the nal RGO aerogel/TiO 2 /MoS 2 composite material. The RGO aerogel/TiO 2 was prepared the same way but without adding MoS 2 .

Characterization methods
X-ray diffraction (XRD, Bruker D8 Advance X-ray diffractor, Karlsruhe, Germany) using Cu Kα radiation was performed on the RGO aerogel/TiO 2 /MoS 2 composite material to determine the crystal phase structure. Raman spectroscopy (LabRAM HR Evolution) with an excitation of 532 nm was performed to determine the phase composition of the composite material. A survey of the chemical bonds and functional groups within the material was conducted by fourier transform infrared spectroscopy (FT-IR, Nicolet iS50 FT-IR spectrometer). The morphology, size, and microstructural properties of the composite were investigated by scanning electron microscopy (SEM, Verios 460) and transmission electron microscopy (TEM, Tecnai G2 F20). X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi) with Al Kα radiation (hv = 1486.6 eV) was performed to determine to the elemental composition of the composite material. To assess the incorporation of MoS 2 into the material, the sample was characterized by energydispersive X-ray spectroscopy (EDS), as shown in Fig. S1. Ultraviolet-visible diffuse re ectance spectroscopy (UV-Vis DRS, Shimadzu UV2600 UV-Vis spectrophotometer) was applied for studying the light absorption properties of materials.. The recombination rate of the photogenerated electron-hole pairs was determined using photoluminescence (GangDong F-320 uorescence spectrophotometer). The Brunauer-Emmett-Teller (BET) surface areas, pore volumes, and pore size distributions of the materials were measured using an ASAP 2460 surface area and porosity analyzer (Micrometrics Instrument Corp.). Electron paramagnetic resonance (EPR) spectroscopy was conducted by an EMXmicro-6/1/P/L EPR spectrometer to identify the spin-trapped oxidative radical species. The spectrometer was equipped with a xenon lamp and a UV-cutoff lter (λ ≥ 400 nm). The center magnetic eld was set to 3500 G, the power was set to 6.35, and microwave frequency was 15 dB. The degradation intermediates of Rhodamine B (RhB) were analyzed by gas chromatography-mass spectrometry (GC-MS, Agilent 7890A/5975C).

Photodegradation of the organic dye solutions
We designed in-house a photocatalytic reaction device, a photo of which is shown in Figure S6. Up to 10 sets of samples were able to be placed at equal distances around the device. The light source of the device was set in the center of the device, the dye solution was placed in a quartz test tube, with the distance between the light source and the dye solution being 6 cm. An air pump was used for gas stirring.
circulating air was used for cooling. Took samples while stirring, so that the ratio of solution to catalyst remained unchanged, and the sample did not return to the main solution. A certain amount of the photocatalyst was dispersed into an organic dye standard solution. Upon reaching adsorption-desorption equilibrium between the dye and the photocatalyst, the suspensions were continuously stirred in the dark for 30 min. After the suspension was centrifuged, it was measured by UV-vis spectroscopy (722G UV-vis spectrophotometer, Shanghai Chuang Yi Science & Education Equipment Co., Ltd.) The mixed suspensions were irradiated using a high-pressure sodium lamp (HPSL, 278 W/m 2 intensity), in the intervals of illumination 3 ml suspension every 30 min was withdrawn and centrifuged to remove the photocatalyst particles for measurement. The removal e ciencies of the organic dyes were obtained using Eq. 1 below: where C 0 was the initial concentration, and C t was the concentration at time t.
The photodegradation kinetics of the dyes were tted with the Langmuir-Hinshelwood pseudo-rst-order kinetic equation (Eq. 2): where C 0 was the adsorption-desorption equilibrium concentration, C t was the concentration at time t, and The SEM image of the RGO aerogel indicated a 3D hierarchical network structure with an interlayer distance equal to dozens of nanometers ( Fig. 2A). Upon the incorporation of TiO 2 and MoS 2 into the RGO aerogel, the interlayer distance of the RGO aerogel sheets in the RGO aerogel/TiO 2 /MoS 2 increased ( Fig. 2B), which was conducive to the adsorption of more dye molecules. The thickness of the graphene was approximately 30 nm, and the 3D network structure demonstrated clear pores that would effectively enable the adsorption of the organic dyes. The TiO 2 nanoparticles (NPs), which had sizes of approximately 20 nm, were uniformly layered on the graphene sheet, so that more active sites were exposed, which was conducive to facilitating more e cient photocatalysis compared to when fewer active sites were exposed. In addition, the material was highly crystalline, which was consistent with the XRD results.
During the preparation of TiO 2 using the sol-gel method, the interactions between the TiO 2 precursors and the GO sheets were strong. However, the interactions between the individual GO sheets were weak, resulting in an increased interlayer distance (Wang et  The removal e ciency of the RGO aerogel/TiO 2 /MoS 2 composite under HPSL irradiation was evaluated based on the change in initial RhB concentration (C RhB ), as shown in Fig. 5A. In a dark environment, as the initial RhB concentration increased, the RhB removal e ciency by the RGO aerogel/TiO 2 /MoS 2 composite decreased. When the initial RhB concentration increased, more dye molecules were adsorbed onto the catalyst, but after the catalyst reached adsorption-desorption equilibrium, the concentration of dye molecules in the solution no longer changed. As the initial dye concentration increased, more dye molecules were excited by absorbed photons, which resulted in greater energy transfer during the photocatalytic degradation process, and adsorption sites were created to help remove the dye (Luo et al.

2019; Zhang et al. 2019
). However, the high concentration of dyes at the surface of the catalyst caused the transfer of photons to the photocatalyst to be blocked by the absorption by the adsorbed dye molecules, reducing the number of photons reaching the catalyst surface (Guo et al. 2014). The corresponding photodegradation kinetics data of RhB at different concentrations were tted to a pseudorst-order model (Fig. 5B). The apparent constants calculated from the model ( Table 1) indicated that the optimal rate constant (0.0169 min − 1 ) was achieved at an RhB concentration of 40 mg/L. At this concentration, the optimal removal e ciency of RhB was determined to be 95% after a duration of 150 min. This removal e ciency was achieved by the combination of adsorption and photocatalytic degradation.
The pH of the medium was also a factor to consider because it affected the adsorption capacity and photocatalytic performance of the composite (Chen and Bai 2013; Yahia Cherif et al. 2014). The removal of RhB from the corresponding RhB solutions at different pH's was performed by the RGO aerogel/TiO 2 /MoS 2 composite (Fig. 5C). The adsorption capacity of the composite in the dark increased between pH 1.0 and 7.0 but decreased when the pH exceeded 7.0. Therefore, the optimal adsorption capacity was achieved at pH 7.0. The decreased removal e ciency of the composite photocatalyst at lower pH was attributed to the protonation of surface-active carboxylic acid groups, which prevented the adsorption of RhB molecules at the adsorption sites. As the pH increased, the functional groups on the catalyst surface became increasingly deprotonated, which enabled the binding of the cationic RhB molecules via electrostatic interactions. However, when the pH exceeded 7.0, the reduction in the adsorption capacity of the composite was due to the competition between the high concentration of hydroxide ions in solution and the RhB molecules for absorption sites.
The photodegradation performance also demonstrated the same behavior as the adsorption capacity. The strong adsorption of the dyes onto the composite surface due to increased localized dye concentration enabled rapid and e cient collisions between the dye molecules and the reactive species . The corresponding photodegradation kinetics data of RhB at different pH's were also tted to a pseudo-rst-order model (Fig. 5D), and the calculated apparent rate constants are shown in the Table 1. From the comprehensive kinetics data, the optimal removal e ciency was achieved at a pH of 7, with a corresponding removal e ciency of 97 %.  which produced excited species that generated additional electron-hole pairs for improving photocatalysis. At the end of photocatalytic degradation of RhB (after 120 min), the reaction solution was analyzed by GC-MS. The electron donor and acceptor groups (diethylanilines and carboxylic acid, respectively) in RhB could be easily oxidized by free radicals generated during the photocatalytic process, which resulted in the formation of some colorless organic intermediates that decomposed into smaller organic oxidation products. Based on the GC-MS results in Table S1, it was indicated that the structure of RhB had been degraded.

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The removal e ciency of RhB from solution by the RGO aerogel/TiO 2 /MoS 2 composite was compared to the corresponding performance of the RGO non-aerogel/TiO 2 /MoS 2 (Fig. 7A). The RGO aerogel/TiO 2 /MoS 2 demonstrated a stronger adsorption capacity and higher photocatalytic degradation e ciency than the RGO non-aerogel-based composite. This was attributed to the uniformly distributed TiO 2 NPs throughout the surface of the RGO aerogel sheets in the RGO aerogel/TiO 2 /MoS 2 composite ( Fig. 7B and D, inset), which manifested larger interlayer pores and a higher speci c surface area compared to the non-aerogel samples, as observed in the SEM images ( Fig. 7B and D). The N 2 adsorption-desorption isotherms (Fig. 7C) of the aerogel and non-aerogel composites, both of which exhibited a type IV isotherm and hysteresis loops similar to the H3 type, further con rmed that the interlayer pores were formed in the stacked sheet-like materials (Li et al. 2013;Lin et al. 2019). The number of pores in the RGO aerogel/TiO 2 /MoS 2 was signi cantly higher than in the RGO nonaerogel/TiO 2 /MoS 2 composite, and the pore diameters were larger in the RGO aerogel-based composite compared to the non-aerogel-based composite (inset, Fig. 7C). The large number of mesopores and macropores facilitated the transfer of photons to the internal surface of the catalyst, thereby improving its photoelectric performance. In addition, the BET surface area of the aerogel-based composite was nearly 4.5 times larger than the non-aerogel composite ( Table 2). A high speci c surface area entails more surface active sites, which improves the carrier mobility rate, adsorption capacity, and photocatalytic performance of the photocatalyst (Li et al. 2013). In addition, a strong adsorption capacity causes more dye molecules to be concentrated at the catalyst surface, thereby increasing the probability of collisions between the dye molecules and reactive species at the surface active sites of the photocatalyst under visible light. Based on these considerations and the removal performances measured, the RGO aerogel/TiO 2 /MoS 2 demonstrated a superior removal performance .

Removal application of different organic dyes
The removal performance of additional cationic dyes (e.g., Rhodamine B (RhB), crystal violet (CV), and methylene blue (MB)) and anionic dyes (e.g., Alizarin red (AR), xylenol orange (XO), and methyl orange (MO)) by the RGO aerogel/TiO 2 /MoS 2 composite was studied (Fig. 8). The removal e ciencies of MB, CV, and RhB by the RGO aerogel/TiO 2 /MoS 2 composite were all around 97 %, which were attributed to the presence of negatively charged carboxylate groups on the surface of the RGO aerogel (Fig. 1C). In aqueous solution, the carboxylic acids ionized to generate a large number of negatively charged carboxylates, which enhanced the electrostatic interactions between the catalyst and the cationic dyes and repelled the anionic dyes. The high removal e ciencies were also facilitated by hydrogen bonding interactions formed between the functional groups of the catalyst and the dyes, which increased the adsorption capacity of the dyes onto the catalyst. Moreover, under strong adsorption, more dye molecules was attached to the surface of the catalyst, making fast and e cient contact between dye molecules and short-lived reactive species generated on the photocatalyst surface (Luo et al. 2019).
In addition, the degradation experiment of the RGO aerogel/TiO 2 /MoS 2 composite material in simulated real dye wastewater (Fig. S2), the degradation of anion dye MO under acidic conditions (Fig. S4) and the degradation experiment of the antibiotic tetracycline hydrochloride by the RGO aerogel/TiO 2 /MoS 2 composite material (Fig. S5) were also added. The results show that the composite has good degradation performance in simulating the real dye wastewater environment, under acidic conditions, the composite material has a good adsorption effect on the anion dye MO, but the photocatalytic effect is not obvious, and the composite material also has a certain degradation effect on tetracycline hydrochloride.
The corresponding photodegradation kinetics were fairly consistent with a pseudo-rst-order reaction ( Fig. 8B), and the apparent rate constants calculated for MR, XO, MO, MB, CV, and RhB are shown in Table 3. The apparent rate constants for the photodegradation of the cationic dyes were signi cantly higher than those of the anionic dyes. The various noncovalent interactions between the cationic dyes and the negatively charged carboxyl groups in the composite can improve the speci c surface area, dispersion, and stability of the material, as well as increase the layer spacing. These features provided the RGO aerogel/TiO 2 /MoS 2 composite with an excellent adsorption capacity for cationic organic dyes, and made the dye molecules on the surface of the catalyst concentration, in order to realize the dye removal. These results indicated that the RGO aerogel/TiO 2 /MoS 2 composite was very effective for the removal of cationic organic dyes from solution. The photocatalytic degradation performance of RhB by the composite catalyst for under HPSL irradiation was repeatedly tested to evaluate the reusability of the photocatalyst. The reusability tests were performed ve times under the same conditions. After each test, the catalyst was ltered and washed three times with ethanol and three times with deionized water. The catalyst was dried in a vacuum oven at 80°C for 12 hours until the next round of tests. The RGO aerogel/TiO 2 /MoS 2 was subjected to ve separate cycles of the photocatalytic experiments under HPSL radiation (Fig. 9). After the ve cycles were completed, the degradation e ciency of RhB by the photocatalyst decreased from 95-80%. The photodegradation e ciency of the composite catalyst under HPSL radiation decreased because, after each test, the number of active sites on the surface of the catalyst that were responsible for capturing photons decreased, which decreased the ability of the photocatalyst to degrade the organic dyes.  Fig. 10A (a)), while six characteristic signals corresponding to the O 2 •− -DMPO adducts were observed in the EPR spectra ( Fig. 10A  Therefore, the observed • OH was generated from the O 2 •− or holes produced in the VB of TiO 2 by the photochemical reaction. To further con rm the contribution of the reactive species to photodegradation, the photocatalytic degradation of RhB using different scavengers was performed by the RGO aerogel/TiO 2 /MoS 2 composite under HPSL irradiation (Fig. 10C). Benzoquinone (BQ) was found to inhibit the photocatalytic degradation e ciency of RhB by the RGO aerogel/TiO 2 /MoS 2 composite when it was added to scavenge the O 2 •− in the solution, which indicated that O 2 •− was the essential active species in these photocatalytic degradation reactions. When triethanolamine (TEA) and tert-butyl alcohol (TBA) were added to scavenge the h + and • OH, respectively, the photocatalytic degradation e ciency of RhB by the RGO aerogel/TiO 2 /MoS 2 composite was slightly lower than without any scavenger present, meaning h + had a slight in uence on the photocatalytic performance, and low amounts of • OH were produced Zhang et al. 2016). These consequences were consistent with the EPR analysis.
Considering the 3D sandwich network structure of the RGO aerogel/TiO 2 /MoS 2 composite (Fig. 11A), as well as the above results and analyses, a potential adsorption mechanism was proposed (Fig. 11B). The 3D sandwich network composite had a large speci c surface area (Table 2) and a signi cant number of negatively charged carboxylate groups on the surface of the RGO aerogel (Seen in IR, XPS), which enabled the formation of the electrostatic and hydrogen bonding interactions between the composite and the cationic dyes. These interactions provided the composite photocatalyst with a superior absorption capacity for cationic organic dyes (Fig. 8), causing the dye molecules to be removed from solution and concentrated on the surface of the catalyst (Fig. 11B). According to the above results, a potential mechanism for the electron-hole migration was developed (Fig. 11C).

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The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper.

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Data Availability Statement
All data generated or analysed during this study are included in this published article and its supplementary information les. The datasets used or analysed during the current study are available from the corresponding author on reasonable request.

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