Recyclable Ag/TiO2@PDMS Coated Cotton Fabric with Visible-Light Photocatalytic for Ecient Water Purication

Multifunctional materials for water purication have attracted great attention due to the increased water pollution problems. However, fabricating the low-cost and recyclable separation material is still a challenge. Herein, we developed an Ag/TiO 2 @PDMS coated cotton fabric with self-clean ability, high ux, superior visible-light photocatalysts ability, and recyclability via the “powder + glue” strategy. The composites exhibit superhydrophobic (water contact angle 157°), and high separation eciency. The separation eciency of 20 times of repeated use remains 16322 Lm −2 h −1 and the degradation rate of methylene blue (MB) remains almost no change. The high oil purication, catalytic property, excellent stability in harsh condition and recyclability enables the material as a satisfactory candidate for water purication.


Introduction
Water pollution, including frequent crude oil leakages and the discharge of the organic solvent, has become a serious threat to marine species, human beings, and the ecological environment. (Fan et al. 2021) E cient, selective, economical, and eco-friendly materials for water puri cation are highly required. is only achieved in the ultraviolet region, which greatly limits its application potential.
Herein, we deposited an eco-friendly and low-cost method to fabricate recyclable Ag-TiO 2 @PDMS coated cotton fabric for oil/water separation and organic contaminates degradation. By combining the advantages of silver nanoparticles (Ag), we fabricated a heterojunction structured photocatalyst, which changed the light absorption range of TiO 2 and improved photocatalytic performance. After, polydimethylsiloxane (PDMS) was introduced as a "glue" attached the Ag-TiO 2 particles to form a micro−nano structure on the surface of cotton fabric, endowing with the superhydrophobic properties and high photocatalytic activity. The superhydrophobic/superlipophilic fabric can effectively separate the oil/water mixture and emulsion with high ux and high purity. Importantly, large-scale water-soluble organic contaminants are transferred into the photocatalytic activity sites due to the high photocatalytic effect. The separation e ciency of oil/water emulsion is as high as 16482 Lm −2 h −1 , and the purity of separated oil is 99.98%, the tensile strength is 74.785 MPa. Moreover, the tightly attached Ag-TiO 2 @PDMS on the exible cotton fabric makes it easy to recycle and reuse after sunlight irradiation.
Hence, the environmentally friendly, easy accessibility composite with high oil−water separation e ciency and visible-light photocatalytic activity, provides a novel approach to deal with the water pollution problems.
Preparation of silver decorated TiO 2 (Ag/TiO 2 ) particles TiO 2 (0.2 g) and silver nitrate (5%, 10%, 15%, 20% and 25% of TiO 2 mas ration) were dispersed in distilled water (100 mL) and ultrasonic for 5 min. Then the pH of the solution was adjusted to 10 with ammonia and magnetic stirring for 10 min to get liquid A. The same molar amount of sodium borohydride as silver nitrate was weighed and dissolved in distilled water (10 mL) to obtain liquid B. Then, liquid A and liquid B were mixed and dispersed by ultrasonic for 30 min. The precipitate was collected and the precipitate was cleaned and then dried overnight in a drying furnace at 50℃ to get Ag-TiO 2 powder. Then, the asprepared Ag-TiO 2 was dispersed in 50 mL tetrahydrofuran, followed by adding 10uL, 20uL, 30uL, and 40uL cetyltrimethoxy silane with ultrasonic for 15 min, and then the fully stirred solution was ltered and the powder was dried in the oven at 50℃ for 6 h. The prepared samples were named 5% Ag-TiO 2 , 10% Loading [MathJax]/jax/output/CommonHTML/jax.js Ag-TiO 2 , 15% Ag-TiO 2 , 20% Ag-TiO 2, and 25% Ag-TiO 2 according to the mass ratio of silver nitrate in titanium dioxide.

Preparation of superhydrophobic cotton-based material
The modi cation process of TiO 2 particles and the fabrication schedule of Ag-TiO 2 @PDMS coated fabric as illustrated in Scheme 1. The polydimethylsiloxane (PDMS) and curing agent were mixed with the mass ratio of 10:1 and dispersed into 50 mL cyclohexane and stirred for 30 min. Then, a certain amount of Ag/TiO 2 particles was added to cyclohexane containing PDMS and ultrasonic dispersion for 30 min. The cotton fabric was cut into 6×6 cm 2 size and put into Ag-TiO 2 /PDMS solution with magnetic stirring for 2 h at 500 rpm. Finally, the soaked fabric was dried at 135℃ for 3 h to obtain the Ag-TiO 2 @PDMS coated cotton fabric.
Thermogravimeter (Diamond TG/DTA, PerkinElmer) was used to test the thermal stability by heating from 30℃ to 800℃ at a heating rate of 10 ℃/min in an N 2 atmosphere. The water contact angles (WCAs) were carried out with a contact angle meter (OCA40 Micro, Data Physics, Germany). The droplet sizes of the feed and ltrate were tested by a dynamic laser scattering (DLS) analyzer measured (Malvern Zeta ZS). The ultraviolet-visible spectroscope (UV-vis, UV-2550) was used to assessing the MB concentrations in the feed and ltrate.

Water-in-oil immiscible emulsion separation
The Span 80, deionized (DI) water and oil were mixed at the weight ratio of 1:10:100 and vigorously stirred for 6 h. During the water-in-oil emulsion separation experiments, the as-prepared superhydrophobic fabric was xed inherently between two tubes. The water-in-oil emulsions were poured into the container and permeated through the superhydrophobic coated cotton fabric. After separation, pure oil was owed down and collected in the bottom vessel. The ux was calculated by calculating the volume of the collected oil within unit time by the following equation: where V is the volume of collected emulsion permeated through the coated cotton fabric, A is the valid test area of the fabric, and t represents the valid time.

Photocatalytic Performance measurements
To evaluate the photocatalytic performance of Ag-TiO 2 @PDMS coated cotton fabric, MB was used as the model pollutants probes. Photocatalysts (40 mg) were added into MB dye solution (50 mg/L, 100 mL).
The solution was placed in the dark for 30 min to achieve adsorption equilibrium. Then, the photodegradation test was employed using a 300 W Xe lamp (BL-CHI-Xe-300) without any lter (320−2500 nm). A series of reaction solutions were collected at 10min intervals. The percentage of degradation is expressed as C/C 0 . Here, C 0 is the original concentration of the dye solution, and C is the dye concentration obtained each time.

Self-cleaning properties and reuse ability
The modi ed cotton fabric's self-cleaning property was tested by removing the dust and MB particles sprinkled on the cotton fabric using water droplets. For recycle and reuse ability measurement, the coated cotton fabric after oil/water separation was placed in the sunlight for a photocatalytic reaction for 1h and then again for ltration separation, repeated 20 times to verify the recycling performance of the separation material. Each analyzed sample was lled back for the next period of irradiation.

Results And Discussion
Hydrophobic Ag-TiO 2 particles with high visible light photocatalytic ability The morphology and structure of the original TiO 2 particles (Fig. S1) and the hydrophobic 15% Ag-TiO 2 were observed by SEM and TEM imaging. TEM analysis shows that the silver particles in hydrophobic 15%Ag-TiO 2 are uniformly attached upon the TiO 2 particles and both Ag and TiO 2 nanoparticles presented nearly spherical shapes. In addition, the diameter of TiO 2 particles is about 100 nm while the diameter of silver particles is about 5 nm to 40 nm and mainly concentrates on 20 nm (Fig. 1a). corresponding. In general, the sharp diffraction peak of silver nanoparticles can be found in the XRD diffraction pattern, which indicates that the surface of modi ed titanium dioxide is deposited with good Loading [MathJax]/jax/output/CommonHTML/jax.js crystalline silver nanoparticles. The FTIR spectrum in Fig. S2 also demonstrated the Ag presence. The 15% Ag-TiO 2 presents hydrophobic property (CA=157°) and the water droplets can slide freely on the lter cake, while the TiO 2 particles easy to absorb water (Fig. S3).
UV-vis diffuse re ectance spectra (UV-vis DRS) were used to assessing the light absorption property and electronic band structure of pure TiO 2 and Ag-doped TiO 2 particles. As shown in Fig. 1e As shown in Fig. 1g, the as-prepared Ag-TiO 2 particles exhibit excellent photocatalytic activities for MB degradation under visible light irradiation. The photocatalytic performance increase to 80% as the Ag amount increased from 5 and 10 %, and the 15% Ag-TiO 2 particles show the best photocatalytic activity for visible light photocatalytic tetracycline degradation, which is far superior to commercial TiO 2 .
However, the photodegradation activity decreased while further increasing the Ag doping, the reason is that small amounts of Ag-doping increase the speci c surface area and decreasing the nanoparticle size, providing a higher number of reactive sites for photocatalytic processes. But a higher Ag content strongly re ected the incident UV beam, decreasing the generation of electron-hole pairs, leading to a declination of the photocatalytic degradation ability.
The simultaneous photocatalysts experiments of TiO 2 and Ag-TiO 2 particles were carried out on the 50 mL of mixture solution of MB under visible light. As shown in Fig. 1h, there is almost no MB degradation of irradiation for TiO 2 under visible light. After modi cation, the 15% Ag-TiO 2 particles displayed enhanced dye degradation activity relative to TiO 2 both under sunlight (Fig. 1i) The morphologies of Ag-TiO 2 @PDMS coated cotton fabric obtained under different weight ratios of PDMS to Ag-TiO 2 are shown in Fig. S5, S6. As shown in Fig. 2a, a rough hierarchical surface was formed via attached the Ag-TiO 2 particles upon cotton fabric through PDMS polymer. The EDX elemental mappings also demonstrated the C, O, Ag, Ti, and Si elements are uniformly distributed in the prepared Ag-TiO 2 @PDMS coated fabric, indicating the successful fabrication of superhydrophobic Ag-TiO 2 @PDMS coated fabric (Fig. 2b). The FTIR spectrum of cotton fabric before and after coating modi cation was shown in Fig. 2c. The wettability is a critical property of wastewater treatment, which is determined by the surface structure and chemical composition. Fig. 3a demonstrated the superhydrophobic mechanism of coated fabric, the PDMS functioned as a "glue" to embedded the Ag-TiO 2 particles on the cotton fabric surface to form superhydrophobic lter materials. The surface roughness of the Ag-TiO 2 @PDMS coated fabric was also measured by AFM. The micro-sized roughness is evident on the coated cotton fabric compared with the pristine cotton fabric (Fig. S9). As shown in Fig. 3b, the increase of the mass ratio of Ag-TiO 2 to PDMS leading to a higher water contact angle.
To illustrate the superhydrophobic durability and stability of the Ag-TiO 2 @PDMS composite, we investigated the WCAs under harsh conditions, including chemical oxidation, strong light, and physical rubbing. As presented in Fig. 3c,  high-temperature treatment and air storage, even after 200 min of air exposure (Fig. 3e) and 150 ℃ of high-temperature heating (Fig. 3f). As illustrated in Fig. 3g. the Ag-TiO 2 @PDMS cotton fabric shown improved thermal stability compared with the pristine cotton fabric. The thermal stability of the Ag-TiO 2 @PDMS composite was also con rmed by TG testing. The weight decreased 15.05% for cotton fabric and 24.09% for coated cotton fabric after 412 ℃, and about doubled weight remained for coated fabric compared with pristine cotton fabric at 800 ℃ (Fig. 3g). The DTG curves (Fig. S10) show that the peak degradation temperature of the cellulose is 392.16 ℃ in the natural balsa, and shifts to 419.87 ℃ after coating.
The pore structure and pore size are critical paraments that in uence the separation e ciency of the water puri cation materials. We investigated the pore structure of the cotton fabric and Ag-TiO 2 @PDMS coated cotton fabric by using nitrogen adsorption/desorption measurements. As illustrated in Fig.s 3h, the nitrogen physisorption isotherm features type-IV behavior, suggesting the coated cotton fabric maintained a hierarchical porous structure. (Zhou et al. 2021) The pore sizes are distributed continuously, and the adsorption average pore diameter is 5.1 nm for the cotton fabric and 6.1 nm for coated cotton fabric. (Lei et al. 2021) This interconnected hierarchical porous architecture would provide more catalytic sites and improved photocatalytic performance of Ag-TiO 2 @PDMS coated cotton fabrics. In addition, the tensile measurement was conducted to study the stretching deformation of Ag-TiO 2 @PDMS coated fabric. It is found that the tensile force reaches 74.785 MPa after modi cation (Fig. 3i).
Separation e ciency of oil/water mixtures and water in oil emulsion Loading [MathJax]/jax/output/CommonHTML/jax.js The super-hydrophobic/super-oleophilic Ag-TiO 2 @PDMS coated cotton fabric has low adhesion to water droplets but selectively allows oil pollution to pass through, leading to a high separation e ciency for oilwater mixtures. The was evaluated. A series of oil/water mixtures were applied to evaluate the potential oil/water separation performance of the composite for the complex e uents system.
The separation uxes of Ag-TiO 2 @PDMS coated cotton fabric for various oil/water mixture (Fig. 4a) and water-in-oil emulsions (Fig. 4b) were measured. The results revealed the modi ed cotton fabric present a higher separation ux. For example, the ux of hexane/water mixture and hexane in water emulsion were 14592 ± 32.1 Lm −2 h −1 and 11397.2 ± 66.5 Lm −2 h −1 , respectively. Moreover, the oil purity of all coated cotton fabrics was greater than 99.9% (Jiang et al. 2019). As shown in Fig. 4c, a mixture of dichloromethane and pure water was poured for separation. The dichloromethane quickly passed through the fabric and fall into the container driven by gravity, while the blue water was retained upon the coated cotton fabric. The separation process of the oil/water mixture and the optical microscopy images of feed emulsion and the associated ltrate was shown in Fig. 4d. where ΔP is the Laplace pressure, θ liquid is the CA of the liquid, and D pore stands for the pore diameter.
Photocatalytic ability and recycling performance Organic dyes are ubiquitous in wastewater, not only posing a challenge in wastewater treatment but causing membrane contamination. We glue the photophobic Ag-TiO 2 particles to cotton fabric by lowenergy PDMS polymer to construct a superhydrophobic surface. Simultaneously, the particles endow the composites with excellent visible light catalytic degradation performance. UV-vis diffuse re ectance Loading [MathJax]/jax/output/CommonHTML/jax.js spectroscopy (DRS) was used to investigate the optical property of the Ag-TiO 2 @PDMS coated cotton fabric and MB was used as a target organic pollutant to conduct photocatalytic degradation.
As shown in Fig. 5a, the traditional cotton fabric has almost no photocatalytic degradation ability while the modi ed cotton fabric completely degrades MB in about 50 min. Meanwhile, it can be seen that the photocatalytic performance of the modi ed cotton fabric is better than that of the fabric without Ag modi cation (70%). The results illustrate that the combination between Ag nanoparticles and TiO 2 can enhance the photocatalytic property under simulated sunlight. The main absorption peak at 464 nm gradually weakened with the increase of irradiation time, indicating the decomposition of MB (Fig. 5b), which is also con rmed by the statistics of the absorption spectrum in Fig. 5c. The enhanced photocatalytic degradation abilities can be explained by the following reasons: 1) the Ag doping upon TiO 2 lower the band-gap energy, rendering the electrons transfer from the valence band to the particles conductive band more easily; 2) the Ag-TiO 2 particles longer the wavelength, extended adsorption peak from UV-light wavelength to visible light; 3) the porous cotton fabric with Ag-TiO 2 nanoparticles homogenously immobilized, providing more catalytic sites (Zhou et al. 2021).
The pollutants attached to the surface always lower the photocatalytic activity and separation e ciency of the separation material. Thus, self-clean property and reuse ability are critical for water puri cation materials. Fig. 5d shows The photocatalytic degradation mechanism of the Ag-TiO 2 @PDMS coated cotton fabric for oil and MB is displayed in Fig. 5e The recyclability of Ag-TiO 2 @PDMS coated cotton fabric is a pivotal criterion for the engineering application in the water puri cation area. The SEM images also illustrated the stability of morphology of Ag-TiO 2 @PDMS coated cotton fabric, maintaining the properties of Ag-TiO 2 @PDMS coated cotton fabric (Fig. 5f). The reusability of Ag-TiO 2 @PDMS coated cotton fabric was evaluated by simultaneous removal of MB in ve cycles. The Ag-TiO 2 @PDMS coated cotton fabric still exhibits e cient photocatalytic ability after ve consecutive cycles (Fig. 5g), thereby indicating promising recyclability. In summary, the Ag-TiO 2 @PDMS coated cotton fabric photocatalyst possesses acceptable reusability and stability. As shown in Fig. 5h, ten cycles of the water-in-oil emulsion separation were performed. After each cycle of separation, the lter fabric was subjected to photocatalysis degradation. Though the separation ux decreased slightly, the separation e ciency after separation was maintained above 99 %, demonstrating the as-prepared cotton fabric could be used as a low-cost effective water puri cation material with recycling and reuse ability (Lei et al. 2021;Li et al. 2020).

Conclusions
In summary, we prepared an effective material for water puri cation via constructing the micro/nano hierarchy surface by immobilization of Ag-TiO 2 and PDMS on cotton fabric. The coated cotton fabric exhibits superhydrophobic properties (CA=157°), high ux (16482 Lm −2 h −1 ), high oil purity up to 99.98% and exhibits excellent durability in harsh conditions. In addition, the material demonstrated satisfactory photocatalytic activity and the combination of Ag and TiO 2 , reducing the bandgap and enhances the photocatalytic e ciency under visible-light irradiation. Thus, the material could degrade the organic dyes in the wastewater and could be recycled and reused after photocatalysis without performance reduction after 20 cycles. The one-pot process, low cost, high e ciencies, outstanding stability, and can be reused to avoid the environmental pollution and resource waste during the water puri cation process of the coated cotton fabric, providing a new sight for the advanced materials for large-scale application on water puri cation area.
Declarations Table   Table 1 Comparison of separation e ciency and reuse ability.