Photocatalytic degradation of dye wastewater by stepwise assembling PVA aerogel/TiO₂/MoS₂/Au composites in visible light

In the past few decades, the massive consumption of chemicals in industrial production has led to the widespread presence of hazardous substances in the water environment, which has affected the entire ecosystem (Heidarpour et al. 2020). Dye is a common environmental pollutant, widely used in textile, paper, plastic, pharmaceutical and cosmetic industries (Yan et al. 2015). Traditional wastewater treatment methods, such as flocculation, biodegradable, physical adsorption and chemical oxidation process are used to treat dye-polluted wastewater (Liang et al. 2014), but these methods have certain shortcomings, such as the biodegradation method time is long, with a slow effect; the cost of physical adsorption is high and the adsorbent is difficult to reuse; the removal effect of chemical oxidation is low. Therefore, the development of effective advanced treatment methods to treat dye-containing wastewater is very important for the entire ecosystem. In recent years, photocatalytic degradation of organic pollutants has been considered as a sustainable technology for wastewater treatment (Li et al. 2017).

Titanium dioxide (TiO₂) is the most widely used photocatalytic material at present. It has attracted the interest of researchers because of its biocompatibility, chemical stability, and no secondary environmental pollutants. However, its large-scale application is inhibited because of some shortcomings, such as high recombination rate of photogenerated e⁻-h⁺ pairs, low utilization rate of solar energy, poor utilization prospect and so on. Researchers have made great efforts in the exploration of new photocatalytic materials and material modification, especially in the construction of heterojunctions (Chen et al. 2020). An et al. (2014) found that the light absorption capacity of TiO₂/Bi₂O₃ nanocomposite was improved compared with that of Bi₂O₃ with a narrow band gap before coupling, and a part redshift occurred, which is known to be beneficial to the degradation of organic dyes. Mingmongkol et al. (2021) synthesized 0.1% CuO/TiO₂ using the sol-gel method, which displays the best removal effect on Methylene blue(MB). This is because the heterojunction of CuO/TiO₂ has the rapid recombination of photogenerated e⁻-h⁺ pairs. These heterojunction catalysts have good utilization efficiency under visible light, and good catalytic effects on organic pollutants, but the catalysts have some problems such as large loss, difficult reuse and difficult practical application.

Providing appropriate carriers may be an effective way to solve the above problems (Nurdin et al. 2016; Mangindaan et al. 2020). Polyvinyl alcohol (PVA) aerogel with its three-dimensional structure and larger specific surface area has become an ideal carrier for nanomaterials because of its good mechanical properties, biocompatibility and non-toxic performance, which helps to expose more active sites and provide adhesion for semiconductor catalyst. In our previous work, a PVA/RGO/TiO₂/Au granular structure was prepared, and PVA showed good carrier performance. If the catalytic performance decline is not obvious, the semiconductor catalyst can be well recovered, which improved the utilization efficiency of the composite materials. However, RGO in this material is a zero-band gap material and only forms conduction to electrons. RGO/TiO₂ cannot construct effective heterojunctions, and are poor for the absorption of visible light excitation electrons.

Molybdenum disulfide (MoS₂) has a good application prospect in photocatalytic modification due to its 2D materials, unique physical and chemical properties (Gao et al. 2019), graphene-like nanostructures, relatively high activity, and unique semiconducting properties. When TiO₂ (Eg = 3.2 eV) is modified by MoS₂ (Eg = 1.8–1.9 eV) (Cao et al. 2021), heterojunctions can be formed and the band gap will decrease. Because the position of the conduction band (CB) and valence band (VB) of MoS₂ is higher than that of TiO₂, under visible light irradiation, the photoelectrons in MoS₂ CB transfer to the CB of TiO₂, and the holes in TiO₂ VB are injected into the VB of MoS₂. The heterojunction of TiO₂ and MoS₂ facilitates acceleration of the separation of photogenerated e⁻-h⁺ and the extension of electrons lifetime (Lin et al. 2019; Cao et al. 2021). However, TiO₂/MoS₂ nanocatalysts face great challenges in a wide range of practical applications. On the one hand, nanoparticles are easy to agglomerate irreversibly during the process of wastewater treatment, resulting in the decline of both active sites and photocatalytic performance. On the other hand, part of the photocatalyst is lost in the recycling process, it is very necessary to solve this problem. Moreover, the recovery and purification of the reaction mixture are also difficult. Based on our previous work, we proposed to use the PVA aerogel as a carrier to load a TiO₂/MoS₂ nanocatalysts formation composite for the degradation of organic pollutants. The PVA aerogel with the large surface area in the new system is conducive to the dispersion and load of the semiconductor catalyst, and is more conducive to the adsorption of the dyes, which helps to improve the catalytic performance of the materials. Doping precious metal gold nanoparticles have been confirmed to improve the photocatalytic performance. Gold nanoparticles with good electrical conductivity can provide convenience for the migration of photoexcited electrons. In addition, they can absorb photons to excite electrons, which is caused by the surface plasmon resonance effect in its region (Nurdin et al. 2016).

In this work, we prepared successfully PVA aerogel/TiO₂/MoS₂/Au composites with a three-dimensional network structure using hydrothermal method. The material has better adsorption of pollutants and recovery, which can apply to the photodegradation of organic dyes. The evaluation of the degradation process shows that the composite not only improves the visible light response range and photodegradation efficiency, but also reduces the agglomeration effect and improves reuse performance. The study shows that the composite has a wide application prospect in sewage treatment processes.

Molybdenum disulphide (MoS₂) was purchased from the Tianjin Fushen Chemical Reagent Factory, China. Tetrabutyl titanate (4.36 × 10⁻³mol/l) was recieved from Shanghai Sampu Chemical Co., Ltd. Glutaraldehyde (GA), Rhodamine B (RhB), Sodium hydroxide (NaOH) and hydrogen peroxide (30%) were purchased from the Tianjin Kemiou Chemical Reagent Co., Ltd. Sodium nitrate, sulfuric acid (98%) and hydrochloric acid were purchased from Xi ‘an Chemical Reagent Factory, China. Anhydrous ethanol (99.5%) was purchased from the Tianjin Tianli Chemical Reagent Co., Ltd. All of these reagents were received from the manufacturer. Deionized (DI) water was used throughout the experiment.

Polyvinyl alcohol-124 (PVA-124) (2.4 g) was transferred to DI solution and heated at 90 °C. When PVA was completely dissolved, HCl was slowly added to adjust the pH to 4, and following slowly adding 10 ml GA with continuous stirring. After 10 min, the system temperature dropped to 35 °C, remaining for 12 h, and slowly cooled to room temperature. The mixture was filtered, washed with ethanol and washed with deionized water for three times, and then non-aerogel and aerogel PVA were prepared by drying and freeze-drying, respectively. PVA aerogel (1.0 g) was dissolved in 20 ml deionized water with continuous stirring. Then, 0.5 g of sodium molybdate and 0.53 g of thioacetamide were dispersed into the mixture, and HCl was slowly added to adjust pH to 1. Afterwards, the mixture was heated in a high-pressure reactor at 180 °C for 12 h. The PVA aerogel/MoS₂ composite material was prepared after the mixture was filtered and washed. The compound was added to 20 ml anhydrous ethanol, and 1.0 ml tetrabutyl titanate was dispersed into the mixture, then 1.0 ml DI water was dropped into the mixture under continuous agitation. The mixture was then heated at 180 °C in a high-pressure reactor. After 24 h, the system was slowly cooled down to room temperature. PVA aerogel/TiO₂/MoS₂ composites were prepared.

The as-synthesized PVA aerogel /TiO₂/MoS₂ composite material of 0.1 g and chloroauric acid of 5.8 ml was added into a 100 ml beaker containing 50 ml deionized water. After stirring for 2 h, 5.8 ml sodium borohydride (1.09 × 10⁻² mol/l) was added. After stirring continuously for 24 h, the solution was filtered, washed with deionized water, and freeze-dried. The final PVA aerogel/TiO₂/MoS₂/Au composite was obtained.

The crystal phase structure was obtained by Cu-Kα illuminated X-ray diffraction (XRD, Bruker D8, Germany). Surface morphology and nanoparticle size were observed under scanning electron microscopy (SEM, FEI verous 460, US) and transmission electron microscopy (TEM, FEI Tecnai G2 F20, US). The chemical composition and state were determined by X-ray photoelectron spectroscopy (XPS, ThermoFisher ESCALAB 250Xi, USK) under Al-Kα radiation (hv = 1486.6 eV). UV-visible diffuse reflectance spectroscopy (DRS, UV 2600, Shimadzu, Japan) was used to determine the optical properties of the samples. Photoluminescence (PL) spectra obtained at 397 nm excitation using a Guangdong F-320 spectrophotometer equipped with a 200 W Xenon lamp were used to determine the recombination rate of photogenerated electron-hole pairs. The Brunauer-Emmett-Teller (BET) surface area, pore volume and pore diameter distribution of the sample were obtained on the automatic specific surface area and porosity analyzer (Micromeritics, ASAP 2460, US). After pretreatment at 130 °C for 6 h, absorption and desorption of nitrogen (N₂) were successively carried out. The BET surface area was calculated by multi-point BET method using nitrogen adsorption data. The total pore volume was calculated by single point method using the adsorption data when the relative pressure (P/P₀) was 0.99. The pore size distribution and mean pore size were calculated from the nitrogen desorption data using the Barrett-Joyner-Halenda (BJH) method.

XRD patterns of PVA aerogel, PVA aerogel/MoS₂, PVA aerogel/TiO₂/MoS₂ and PVA aerogel/TiO₂/MoS₂/Au are shown in Figure 1(a). The peak at the 2θ value of 19.14° corresponds to the (101) plane of the PVA aerogel. The main peaks with 2θ values of 14.37°, 33.50°, 39.53° and 58.33° correspond to the (002), (101), (103) and (110) planes (Lin et al. 2019) of MoS₂, respectively, which are a good match with the JCPDF powder diffraction pattern 37-1492. The main peaks with 2θ values of 25.14°, 37.77°, 48.01°, 53.77°, 54.95° and 74.88° correspond to (101), (103), (004), (200), (211), (204) and the face of anatase TiO₂ (215) (Gao et al. 2019), matching well with the JCPDS powder diffraction pattern 21-1272. The (111), (200), (220) and (311) peaks are attributed to the diffraction peaks of Au, confirming the coexistence of metal Au. With the addition of MoS₂ and TiO₂, the crystallization performance of the PVA was further reduced, so the peak intensity of the symbolic PVA was gradually weakened. The peak intensity of MoS₂ is clearly visible in the PVA/MoS₂ material, but with the deposition of TiO₂ and gold, the MoS₂ peaks also weakened and partially disappeared. The reason is that the peak intensity of MoS₂ is not high in the PVA/MoS₂, and the relative content is further reduced in the subsequent materials, and the peaks of MoS₂ are further masked due to the higher peak intensity of TiO₂ and gold and the overlap with the peaks of MoS₂. (Tsou et al. 2015; Yadav et al. 2020) The chemical composition and surface state of PVA aerogel/MoS₂/TiO₂/Au composites were analyzed by XPS. XPS signals of C 1s, Ti 2p, O 1s, Mo 3d, S 2p and Au 4f peaks are observed in Figure 1(b). The C 1s spectrum shows two binding energy peaks at 286.37 eV and 284.8 eV, which are attributed to the C-O and C-C/C = C bonds respectively (Guo et al. 2014). The Ti 2p (Chen et al. 2020) spectrum corresponds to the spin orbits of Ti 2p_3/2 458.17 eV and Ti 2p_1/2 463.9 eV in PVA aerogel/MoS₂/TiO₂/Au, respectively. As shown in Figure 4, the XPS spectrum of high-resolution O1s was split into two peaks, located at 529.2 eV and 530.7 eV, corresponding to Ti-O-Ti and Ti-O-Mo bonds. Both Figure 4(f) and 4(g) show the Mo 3d_5/2, Mo 3d_3/2, S 2p_3/2 and S 2p_1/2 peaks are assigned to the binding energies at around 229.75 eV, 232.9 eV, 162.35 eV and 163.9 eV, respectively (Gao et al. 2019). These are typical values for Mo⁴⁺ and S²⁻ in MoS₂. Au 4f spectra (Shi et al. 2017; Zhang et al. 2019) in Figure 1(h) are observed at the binding energies of 83.44 eV (Au 4f_7/2) and 87.09 eV (Au 4f_5/2). XRD and XPS analysis shows that the prefabricated composite material was successfully prepared.

The morphology and microstructure of non-aerogel PVA, PVA aerogel and PVA aerogel/TiO₂/MoS₂/Au were studied by FE-SEM and TEM, as shown in Figure 2.

The panoramic view of the non-aerogel PVA in Figure 2(a) shows a uniform, layered network structure with many holes. Compared with non-aerogel samples, PVA aerogel has larger interlayer pores and higher specific surface area, as observed in the SEM images in Figure 2(a) and 2(b). This result is similar to that of the N₂ absorption–desorption analysis test. The spatial structure of the PVA aerogel carrier was not been destroyed during the preparation process of the PVA aerogel/TiO₂/MoS₂/Au composite, which is confirmed in Figure 2(c). Further zooming into the microscopic area in Figure 2(d), the clear surface morphology of the PVA aerogel/TiO₂/MoS₂/Au composite presents a uniform roughness, indicating an uniform and orderly load of nanomaterials, which is beneficial to photocatalytic degradation.

The TEM image of PVA aerogel/TiO₂/MoS₂/Au in Figure 3(a) shows the porous structure of the composite, which is consistent with the SEM results discussed above. The high-resolution TEM (HRTEM) image in Figure 3(b) clearly reveals the lattices of Au and TiO₂, but the MoS₂ nanoparticles cannot been clearly distinguished. The probable cause is that MoS₂ is blocked by Au and TiO₂, but the presence of MoS₂ can be still demonstrated by energy dispersive spectroscopy (EDS) mapping. In Figure 3(c) the lattice spacing is about 0.23 nm, which is assigned to the (111) face of Au nanocrystals; the lattice spacing at about 0.35 nm corresponds to the (101) face of TiO₂ (Shi et al. 2017). These results are consistent with XRD analysis. The EDS-mapping results of the PVA aerogel/TiO₂/MoS₂ composite material confirmed that the MoS₂ had been attached to the surface of the PVA aerogel, and that TiO₂ had grown uniformly on the PVA aerogel/MoS₂, which also confirmed that gold nanoparticles are distributed on PVA aerogel/TiO₂/MoS₂. This shows that the prefabricated composite material had been successfully prepared.

N₂ absorption–desorption type IV isotherms of non-aerogel PVA, PVA aerogel, PVA aerogel/TiO₂/MoS₂ and PVA aerogel/TiO₂/MoS₂/Au are shown in Figure 4, and pore size distribution can be seen in the inset. It presents an obvious hysteresis loop close to the H3 type in the relative pressure range of 0.45–1.0, which is a typical feature of mesopores (2–50 nm) and macropores (>50 nm) (Sun et al. 2017). Also, the inset shows that the pore size is several to several hundred nanometers. PVA aerogel has larger interlayer pores and higher specific surface area than non-aerogel samples, and average pore size in the PVA aerogel is significantly greater than that in PVA/TiO₂/MoS₂. The BET surface area, pore volume and pore diameter values of the as-prepared samples are listed in Table 1. The BET special surface area and pore volume of the PVA aerogel/TiO₂/MoS₂/Au are 26.995 m²/g and 0.15218 cm³/g are a slight decrease than those of PVA aerogel (28.879 m²/g and 0.19241 cm³/g). It can be attributed to the adhesion of TiO₂/MoS₂/Au nanoparticles on the surface of PVA aerogel. The high specific surface area can provide more surface-active sites, and the mesopores and macropores can promote the injection of dyes and photons. These factors would be helpful to the improvement of the photocatalytic performance of nano composite material (Reza et al. 2015).

The light absorption properties of PVA aerogel/MoS₂, PVA aerogel/TiO₂/MoS₂, and PVA aerogel/TiO₂/MoS₂/Au were revealed by UV-vis diffuse reflectance spectroscopy (UV-vis DRS) in Figure 5(a). MoS₂ with a unique layered structure presents a larger specific surface area and a higher theoretical capacity, so the PVA aerogel/MoS₂ shows a light absorption rate (Ali & Sandhya 2016). In addition, the presence of Au will have the plasma resonance effect on the surface of the material, which increases the absorption of 400–500 nm to visible light (Shi et al. 2017). The inset shows the Kubelka–Munk function plot of the spectrum. The band gap of PVA aerogel/TiO₂/MoS₂/Au is estimated to be about 2.8 eV.

The recombination rate of photogenerated e⁻-h⁺ pairs is studied. The photoluminescence (PL) spectra of PVA aerogel /MoS₂, PVA aerogel/TiO₂/MoS₂ and PVA aerogel/TiO₂/MoS₂/Au are shown in Figure 5(b). The PL intensity of PVA aerogel TiO₂/MoS₂ is lower than that of PVA aerogel/MoS₂, indicating a lower recombination rate of e⁻-h⁺ pairs (Guo et al. 2014). This also proves that the heterojunction formed between MoS₂ and TiO₂ improves the separation efficiency of photogenerated carriers, and the contribution of MoS₂ may be greater to the increase of the concentration of photoinduced electrons per unit volume. In addition, the possibility of electrons being captured by oxygen increases (Zhang et al. 2015b). The lowest PL intensity of PVA aerogel/TiO₂/MoS₂/Au indicates a low recombination rate of e⁻-h⁺ pairs, which indicates that the composition of Au nanoparticles further improves the separation efficiency of photogenerated carriers. This can be attributed to the excellent electrical conductivity of gold nanoparticles to the transfer of photoexcited electrons, on the other words, it means that they act as electron receptors (Shi et al. 2017).

By comparing the degradation performance of PVA aerogel, PVA aerogel/MoS₂, PVA aerogel/TiO₂/MoS₂ and PVA aerogel/TiO₂/MoS₂/Au to RhB under HPSL irradiation, the degradation performance of PVA aerogel/TiO₂/MoS₂/Au was evaluated. The experimental results are shown in Figure 6. Compared with non-catalyst, pure PVA aerogels show stronger degradation performance because PVA aerogels can absorb a certain amount of dye molecules. The PVA aerogel/MoS₂ composite shows a greater photocatalytic activity. This is due to the two-dimensional layered structure, narrow band gap and the ability to absorb photons of MoS₂, resulting in the promotion of the degradation efficiency of dyes (Li et al. 2018). Furthermore, the photocatalytic performance of PVA aerogel/TiO₂/MoS₂ composite is obviously better than that of PVA aerogel/MoS₂ because the formation of heterojunction between MoS₂ and TiO₂ accelerates the separation of photogenerated electrons and holes and prolongs the electron lifetime (Zhang et al. 2015a). As expected, PVA aerogel/TiO₂/MoS₂/Au composite shows the best photocatalytic activity. The photoelectrons could be captured by Au nanoparticles with the good electrical conductivity, thus further improving the separation rate of photogenerated electrons and holes. In addition, it can absorb photons to excite electrons due to the surface plasmon resonance effect in its region (Li et al. 2017). This result is also consistent with the corresponding degradation kinetics results. It indicates that PVA aerogel/TiO₂/MoS₂/Au composite material has excellent removal ability for dyes. Compared with other related catalysts, our catalyst has a better degradation rate for RhB as shown in Table 2. PVA aerogel as a carrier, the catalyst is easier to recycle than the powder catalysts, which greatly reduce the loss of catalyst. As the carrier of PVA. In addition, as compared with the same weight of the other catalyst the same weight of PVA aerogel/TiO₂/MoS₂/Au composites, the proportion of semiconductor catalyst in PVA aerogel/TiO₂/MoS₂/Au composites is smaller, but the degradation effect was not significantly reduced.

The degradation effect of RhB is different with the amount of catalyst. The degradation effect of RhB under the condition of different contents of PVA aerogel/TiO₂/MoS₂/Au catalyst is shown in Figure S2A. With the increase of catalyst dosage from 0.025 g to 0.05 g, the removal effect of RhB in the solution also increases gradually. Once the amount of catalyst exceeds 0.05 g, the removal efficiency of RhB in the solution decreases. The typical first-order photodegradation characteristics can be seen at different amounts of catalyst, shown in Figure S2B. The increase in RhB removal efficiency is due to the increase in the amount of composite material, which provides a large special surface area and additional active sites for the generation of oxidizing substances. When excess catalyst is present, the effective surface area would reduce and the incident light would be blocked due to the agglomeration of nanoparticles, further reducing the light transmittance of the solution (Davididou et al. 2018). This result is also consistent with the corresponding degradation kinetics results. This indicates that 0.05 g PVA aerogel//TiO₂/MoS₂/Au composite material has the best dosage for RhB removal.

Under the condition of both fixed HPSL and content of PVA aerogel/TiO₂/MoS₂/Au composite, the initial RhB with different concentrations (C_RhB) was used to evaluate its impact on photocatalytic performance. Figure S2C shows that as the initial concentration of RhB increases from 10 mg/l to 50 mg/l, the removal efficiency of RhB decreases. The typical first-order photodegradation characteristics can be seen at different initial RhB concentrations, shown in Fig. S 2D. Both the possibility of forming reactive species on the surface of catalyst and the reactive species accepted by RhB determine the degradation rate. As the initial dye concentration increases, more dye molecules can be used for excitation and energy transfer (Chen & Bai 2013). When the dye concentration is too high, however, the adsorption capacity of the catalyst reaches equilibrium, resulting in a the relatively large residual amount of dye molecules. Moreover, the existence of excessive organic substrate would also block photons reaching at the catalyst surface (Sun et al. 2017).

The pH value is an important factor in the photocatalytic reaction because it is closely related to the surface charge of the catalyst, the adsorption of organic matter on the catalyst surface (Sun et al. 2017) and the number of charged free radicals generated during the photocatalysis process. The degradation rate of RhB solution with different pH values was tested under HPSL irradiation. The experimental results in Figure 7(a) show that the pH value affects significantly the adsorption capacity and degradation performance (Abdullah & Kuo 2015). When the pH value increases from 2 to 10, the adsorption capacity percentage of RhB on the surface of catalyst firstly increases and then decreases. The best adsorption capacity can be obtained at pH of 8. Due to the protonation of the active functional groups on the catalyst surface, a part of the composite material is positively charged at a lower pH. This positive surface charge results in competition with the cation RhB for adsorption sites. As the pH value increases, the surface of the catalyst may be negatively charged, thereby enhancing the electrostatic attraction between catalyst and dye. When the pH is higher than 8, the reduction of adsorption efficiency may be attributed to the increase in the strength of hydroxyl ions in the solution, which leads to more competition for adsorption sites (Koutahzadeh et al. 2016). The degradability basically shows a similar behavior with the adsorption capacity. In addition, the corresponding photodegradation kinetics of RhB roughly follows the pseudo-first-order reaction in Figure 7(b). PVA aerogel/TiO₂/MoS₂/Au shows excellent photocatalytic activity at pH of 8.

The reusability of the photocatalyst was tested for evaluating the photocatalytic performance of the composite catalyst to degrade RhB under HPSL irradiation. Under the same test conditions, the reusability test was carried out five times. After degradation, the catalyst was filtered and washed three times with ethanol and deionized water after each test. The catalyst was subsequently dried in a vacuum oven at 80 °C for 12 h and then used for the next run. Under HPSL irradiation, five cycles were performed with PVA aerogel/TiO₂/MoS₂/Au for photocatalysis experiments in Figure 7(c).

After five cycles, the degradation ability of PVA aerogel/TiO₂/MoS₂/Au composite material to RhB decreases slightly. According to the reusability test, the composite catalyst can be used for long-term photodegradation under HPSL irradiation. The reason for the decrease is that some catalyst surfaces were covered by a thin layer of intermediate dye after the first operation, so the adsorption capacity of the composite material reduces in subsequent operations. After each run, the covered catalyst surface loses some photon-capturing active centers, thereby reducing the reactive intermediates generated by the reaction with dye molecules under HPSL irradiation (Ali & Sandhya 2016). As the number of runs increases, the degradation capacity decreases.

During the photodegradation process, the short-lived reactive species generated on the active site of the photocatalyst would contact the dye molecules and degrade. Reactive substances include superoxide radicals (•O₂⁻), holes (h⁺) and hydroxyl radicals (•OH). In order to further confirm the contribution of reactive substances in the presence of PVA aerogel/TiO₂/MoS₂/Au under HPSL irradiation, RhB was photocatalytically degraded in solution with different scavengers in Figure 8(a). It has been observed that the photocatalytic degradation efficiency is significantly inhibited when nitrogen (N₂) used to remove O₂ and ammonium oxalate (AO) used to remove h⁺ are added, which means that •O₂⁻ and h⁺ are the main reactive species. In the case of adding tert-butanol (TBA) used to remove •OH, the degradation efficiency of photocatalysis is lower than that without adding any scavenger, which indicates that a small amount of •OH produced has a slight impact on the photocatalytic performance (Vergili et al. 2012;Cao et al. 2021). As is well known, the VB energy level of MoS₂ (about 1.39 eV to ordinary hydrogen electrode (NHE)) cannot directly oxidize water or OH⁻ to generate •OH (OH⁻/•OH is about 1.99 eV, H₂O/•OH is about 2.33 eV) (Zhang et al. 2015a). The observed •OH is produced by •O²⁻ or holes generated on the surface of gold nanoparticles through photochemical reactions.

Based on the above results and analysis, Figure 8(b) shows graphically the possible mechanism of electron-hole migration. MoS₂ and Au (Shi et al. 2017) in PVA aerogel/TiO₂/MoS₂/Au absorb visible light to excite many electrons, and then the electrons transfer from the CB between the excited MoS₂ and Au to TiO₂; The electrons in MoS₂ CB can also be transferred to the surface of Au, which can realize the absorption of visible light and electron-hole separation. The transferred electrons are captured by O₂ on the surface of the catalyst to generate superoxide anion radicals (•O²⁻), and the h⁺ reacts with H₂O to generate •OH radicals. These powerful oxidizing radicals •O²⁻ and •OH are accepted by RhB adsorbed in water. The degradation of RhB was realized on the surface of catalyst in the reaction system.

This article addresses the problems of low degradation efficiency and difficulty in recovery during the photocatalytic treatment of organic dye wastewater using titanium dioxide semiconductors. PVA aerogel/TiO₂/MoS₂/Au composite material has been successfully synthesized by physical vapor deposition and hydrothermal methods. SEM and TEM results show that the composite material has a uniform and well defined porous network structure, which also confirms the large specific surface area and large pore size of the PVA aerogel. The results are consistent with the nitrogen adsorption analysis test. UV-vis DRS and PL indicate that the composite shows a high absorption rate in the visible range, and the recombination of photogenerated electron-hole pairs could be effectively inhibited. In brief, PVA aerogel/TiO₂/MoS₂/Au composite material has excellent photocatalytic performance and good reusability for the degradation of RhB dye. This research illustrates that the composite material has great application potential in degradation of dye wastewater.

The authors acknowledge partial support from the National Natural Science Foundation of China (51874223), the Natural Science Major Research Plan in Shaanxi Province, China (2017ZDJC-25), National College Students’ Innovative Enterpre neurial Training Plan (202110703020), Shaanxi Key Laboratory of Environmental Engineering of Xi'an University of Architecture and Technology, and Key Laboratory of Northwest Water Resource, Environment and Ecology, MOE.

Liang Zhang, Lvling Zhong, and Xiaomin Zhang conceptualized the study. Liang Zhang, Juanqin Xue acquired the funding. Liang Zhang and Yao Wang supervised the work. Haojie Qi, Yage Zheng and Yujuan Zhang performed the experiments and wrote the draft manuscript. Liang Zhang and Yage Zheng reviewed and revised the manuscript.

The authors acknowledge partial support from the National Natural Science Foundation of China (51874223), Natural Science Major Research Plan in Shaanxi Province, China (2017ZDJC-25), Shaanxi Key Laboratory of Environmental Engineering of Xi'an University of Architecture and Technology, and Key Laboratory of Northwest Water Resource, Environment and Ecology, MOE.

I would like to declare that, on behalf of my co-authors, the work described in this manuscript is original research that has not been published previously and is not under consideration for publication elsewhere, in whole or in part.

The authors declare no conflicts of interest in the submission of this manuscript, and this manuscript has been approved by all authors for publication. All authors consent to participate and consent to be published.

The authors declare no known competing financial interests or personal relationships that could have influenced the work reported in this paper.

Informed consent was obtained from all individual participants included in the study.

The participants have consented to the submission of the case report to the journal.

This article does not contain any studies with human participants or animals performed by any of the authors.

All relevant data are included in the paper or its Supplementary Information. All data generated or analyzed during this study are included in this published article and its supplementary information files. The datasets used or analyzed during the current study are available from the corresponding author upon reasonable request.