Water repellent Bi2MoO6 cotton fabric for implementing water-source purication

As a low-budget, facile-availability and eco-friendly substrate material, cotton fabric after hydrophobic modication has been applied to sewage purication. While, a single superhydrophobic function is unable to decompose toxic water-soluble organic dye. In this paper, pathways are propounded for the simultaneous achievement of the removal of insoluble spilling oil and organic dye contaminants. Particularly, the hydrophobic ZnSnO 3 after stearic acid modication and Bi 2 MoO 6 catalysts are introduced into the cotton fabric substrate through solution dip-coating. The durability of the prepared fabric suffers from the acid-base corrosion, thermal treatment and mechanical wear, meanwhile, it still exhibits remarkable water repellent (WCA > 150°) property. Furthermore, the remarkable photocatalytic activity makes it possible for reusable degradation and the primary active species, namely the holes, to be veried by the radicals-capturing experiment.


Introduction
In terms of polarity difference, the major contaminant categories of aquatic ecosystems are generally classi ed into water-soluble and spilling oil Yue et al. 2020;Ma et al. 2020;. For the treatment of aquatic pollution, the ecosystem lacks adequate self-cleaning function.
Therefore, adopting reasonable manual strategies to achieve oil-water separation from sewage is quite signi cant for implementing water puri cation (Zhou et al. 2013;Shang et al. 2020;Wang et al. 2019;. Superhydrophobic function provides an opportunity for selective oil-water separation, which is bene cial to the whole water circulation system (Kang et al. 2014;Qin et al. 2019). The construction methods of superhydrophobic surface were generalized by the following types Boreyko et al. 2013;Han et al. 2018): the surface of superhydrophobic materials was accustomed to building rough construction via in situ modi cation sundry substrates, such as glass (Isakov et al. 2020;Long et al. 2017), copper wire mesh (Torresin et al. 2013;, magnesium alloy Wu et al. 2017) and fabric (Sun et al. 2020;Yang et al. 2019), etc. Furthermore, through grafting superhydrophobic particles on rough substrates, a superhydrophobic surface could be obtained, which was easier to implement. Among various arti cial hydrophobic substrates, water repellent fabric is regarded promising due to its low-budget and environmental non-toxicity (Fang et al. 2020;Bai et al. 2020;Xue et al. 2015). Therefore, synthesizing superhydrophobic fabric with the preeminent waterproof performance and durability is regarded as an effective strategy to achieve the selective removal oil spilling from the sewage.
While, water-soluble contaminants are unable to be treated by the single superhydrophobic function. How to realize the simultaneous removal of the water-soluble dye and insolubility oil spilling becomes a crucial strategy to mitigate the damage to ecological constellation. ) prepared a superhydrophobic P25 nanoparticle and polydimethylsiloxane on alkaline-etched copper mesh (P25@PDMS@COM), achieving the combination of water-soluble contaminant degradation under ultraviolet irradiation and the removal of oil phase. While, the traditional titanium dioxide (P25) catalyst can only respond to ultraviolet light. To make the utmost of the visible light in solar energy, research on appropriate photocatalysts possessing wide-range solar light response has raised widespread attention.
Among the numerous semiconductor photocatalysts, bismuth molybdate (Bi 2 MoO 6 ) has presented vast potential in multiple applications, relying on its features including controllable morphology, relatively narrow band-gap, eco-friendliness and remarkable thermal stability Zhang et al. 2020;Kash -Sadabad et al. 2016). As a typical layered Aurivillius metal-oxide, it takes on sandwich structure and is mainly constructed by (MoO 4 ) 2− perovskite slabs as well as (Bi 2 O 2 ) 2+ slices (Salari et al. 2020;Mei et al. 2019;Wang et al. 2016). But the most regret is that the single photocatalytic function is unable to resolve the oil-phase pollution, achieving an incorporation function of water repellent and the photocatalytic function ought to be a feasible strategy.
Herein, superhydrophobic SA-ZnSnO 3 is prepared via stearic acid (SA) modi ed on the surface of zinc stannate (ZnSnO 3 ) particles, reducing the damage of traditional superhydrophobic solutions (mercaptan ) and uorine-containing (Ruan et al. 2014), etc) to the ecological environment. Then, the superhydrophobic SA-ZnSnO 3 and Bi 2 MoO 6 loaded fabric (denoted as SH BM fabric) are fabricated via physical dip-coating, achieving the removal of the insoluble spilling oil and organic dye contaminants, meanwhile resolving the recovery troubles of the catalysts. The remarkable properties of oil-water separation and decomposing organic substances are simultaneously applied into practical, indicating that SH BM fabric has a vast development prospect. The prepared product has erce response for the visible-light, and the degradation e ciency remains at 94 % after being used repetitively. The relevant energy band con guration of Bi 2 MoO 6 catalysts is herein investigated in detail, and the holes serve as the main active species via the trapping experiment. To synthesize the target product, the purpose of additive metal-organic framework ZIF-8 was utilized to provide Zn source. The detailed fabrication procedures were shown: rstly, 0.9 g Zn(NO 3 ) 2 •6H 2 O was dispersed into 30 mL methanol until transparent solution was formed (solution A). 1.6 g 2methylimidazole was dissolved in 20 mL methanol (solution B), and was then gradually poured into the above solution A under stirring for 1 h. After aging for an overnight, the white precipitation was further puri ed, and dried at 80 ℃ for 12 h.

Experimental
Subsequently, in order to obtain ZnSnO 3 particles, 0.6 mmol ZIF-8 was scattered into 5 mL ethanol driven by the ultrasound (the ultrasonic time was approximately 1 h). Besides, the Na 2 SnO 3 •3H 2 O with an identical molar ratio was dissolved in 30 mL deionized water, and then ZIF-8 dispersion was added drop by drop. After reaction for 30 min, the oyster white solution was puri ed by deionized water and ethanol, after which it was dried completely in an oven. Subsequently, the ZnSn(OH) 6 precursor was conducted at 250 ℃ for 4 h (the heating rate was about 1.5 ℃/ min), aimed at dehydration.
Fabrication of SA-ZnSnO 3 0.2 g stearic acid was dissolved in 50 mL ethanol to form solution, and 80 mg ZnSnO 3 was further introduced into the above mixture accompanied with ultrasonic dispersion for 10 min. After being stirred for 24 h, the reaction substance was obtained after several times of centrifugal puri cation by ethanol and dried at 80℃ for 6 h.
Fabrication of superhydrophobic Bi 2 MoO 6 fabric 100 mg PDMS, 0.04 g curing agent, and 20 mL ethyl acetate were agitated to form a homogeneous solution. 60 mg Bi 2 MoO 6 catalyst and 60 mg SA-ZnSnO 3 were dispersed into the above solution completely, and then, the pristine fabric (the size was about 4 cm × 4 cm) was immersed in the mixed solution to perform a physical sedimentation, after which it was dried at 120 ℃ for 1 h. The repeated sedimentation process was conducted for 5 times, aiming to aggrandize the load content of Bi 2 MoO 6 catalyst. To achieve an absolutely cure, the nal fabric was dried at 120 ℃ for 2 h.

Characterization
The qualitative and quantitative analysis of the crystal phase are researched by Bruker D8w X-ray diffractometer. The surface micro-morphology and phase component structure are analyzed by scanning electron microscope (Zeiss Company, FESEM) and transmission electron microscope (FEI Tecnai F30, TEM). The Fourier infrared spectrum analysis (Nicolet iS50, FTIR) is utilized to investigate molecular structure and chemical bonds. The thermogravimetric curves of ZnSnO 3 and SA-ZnSnO 3 are analyzed by the thermogravimetric analyzer (STA 449C, TG). The X-ray photoelectron spectroscopy (ESC PHI500, XPS) is adopted to research the valence state discrimination of relevant chemical elements. The energy dispersive spectrum analysis (EDS) is used to characterize the distribution and the content of surface elements. The photocatalytic activity of degrading contaminants is evaluated via UV-visible Page 5/20 spectrophotometer (752B). The photocurrent test, Mott-Schottky (MS) and Nyquist impedance spectrum (EIS) of Bi 2 MoO 6 catalyst are detected by a three-electrode electrochemical workstation. The separation and recombination situation of photo-induced carrier is measured by the photoluminescence spectroscopy (FLS 920, PL). The optical property of as-synthesized Bi 2 MoO 6 is determined on diffuse re ectance spectrum (UV-3600 spectrophotometer, DRS). Brunauer-Emmett-Teller surface analysis (Belsorp-max, BET) of Bi 2 MoO 6 are utilized to study speci c surface and porosity. The superhydrophobic property of SH BM fabric is measured by contact angle meter (KrüssDSA100).

Results And Discussion
The crystallographic structure of as-synthesized products is represented by XRD. Fig. 1a shows the XRD patterns of orthorhombic Bi 2 MoO 6 , the 2θ value of apparent characteristic diffraction peaks at 28.25°, 32.61°, 47.15° and 55.56°, which are indexed to crystal planes of (131), (200), (260) and (331) (200), (220) and (420) planes and well matched with the cubic phase of ZnSn(OH) 6 (JCPDS card No.73-2384). There's no redundant impurity peak, and the result demonstrates that the purity of the synthesized ZnSn(OH) 6 precursor is high. Observably, the XRD patterns of dehydration ZnSnO 3 take on no apparent intense peak because the structure of the ZnSnO 3 sample is amorphous. During the process of annealing treatment, dehydration of the ZnSn(OH) 6 precursor occurs. The initial hydrogen-oxygen bonds (-H-O-) cause breakage, forming plentiful dangling bonds on the external surface and disturbing the inner lattice, and then the amorphous structure of ZnSnO 3 nally comes into being.
After superhydrophobic SA modi cation, the XRD results of SA-ZnSnO 3 indicate that an amorphous structure is retained. To investigate the situation of surface-grafted SA, the FTIR analysis of ZnSnO 3 and SA-ZnSnO 3 are performed in Fig. 1c. The FTIR characteristic absorption peak of SA-ZnSnO 3 appears approximately at 1340-1465 cm -1 and 2850-3000 cm -1 , which respectively contributes to the bending and stretching vibration of -C-H-bonds. Besides, to further verify the successful graft of SA, the thermal treatment of nitrogen protected atmosphere at 800 ℃ is conducted in Fig. 1d. There is an obvious thermal weight loss of about 8.6 % at 800 ℃, caused by the disappearance of SA. Therefore, combined with the above measurement analysis, the results show the successfully grafted SA on the surface of ZnSnO 3 products.
The SEM technique is utilized to re ect surface morphology of the as-prepared products. It can be seen from the low magni cation image (Fig. 2a) that ZnSnO 3 shows uniform cubic morphology. Fig. 2b presents the high magni cation morphological image of SA-ZnSnO 3 , which exhibits that plentiful irregular nanoparticles exist on the surface of cubes. Especially, each cube surface contains a porous structure, taking on the irregular hollow crystal-structure boxes. The elemental mapping image of SA-ZnSnO 3 is carried out via an EDS analysis. As is presented in Fig. 2c, the element distribution of Zn, Sn and O are considerably uniform. Besides, the TEM analysis of SA-ZnSnO 3 is further investigated to determine its internal structure. In Fig. 2d, the dark border and whitish heartland is in stark contrast, whose conclusion precisely demonstrates that SA-ZnSnO 3 displays a hollow box structure. In order to analyze the electronic structure and chemical state of the SA-ZnSnO 3 nanoparticles, the relevant XPS analysis is employed. Fig. 2e displays the Sn 3d spectrum, the peak position occurs at 486.7 eV and 495.2 eV, which are assigned to the Sn 3d 5/2 and 3d 3/2 . The peak of binding energy of Sn 3d spinning orbits is consistent with the quintessential tetravalent Sn ironic (Sn 4+ ) in ZnSnO 3.
The surface morphology of Bi 2 MoO 6 photocatalyst is conducted by scanning electron microscope. Fig.   3a and b intuitively represent the low and high magni cation morphological images, numerous smaller nanoparticles aggregate with each other to constitute the Bi 2 MoO 6 particles. Fig. 3c depicts the XPS measurement of Bi 4f orbitals from the Bi 2 MoO 6 products. The peaks sited at 158.9 eV and 164.2 eV in the Bi 4f spectrum are assigned to 4f 7/2 and 4f 5/2 . To dig deeper into the microstructure of the Bi 2 MoO 6 catalyst, the TEM analysis is performed. Fig. 3d further veri es that the morphology of Bi 2 MoO 6 is constructed by the accumulation of abundant smaller particles. Besides, the Fig. 3e shows the HRTEM image of Bi 2 MoO 6 as well as the lattice fringes spacing of 3.16 nm and 2.74 nm, assigned to the crystal planes of (131) and (200). Selective electron diffraction analysis is used to calibrate the diffraction crystal planes of (131) and (200) in Fig. 3f. Besides, the diffraction ring spectrum indicates the polycrystalline structure of Bi 2 MoO 6 .
The surface micro-morphology of pristine fabric and superhydrophobic Bi 2 MoO 6 fabric is investigated through FESEM. As displayed in Fig. 4a, the pristine fabric is constructed by the multitudinous transverse and longitudinal cross-linking of bers. Noticeably, the high magni cation SEM image of pristine fabric shows that the skeleton surface is fairly smooth. While, after loading the SA-ZnSnO 3 and Bi 2 MoO 6 particles, rough nano-structures are formed on the ber surface (Fig. 4b). Besides, the synergistic effect between the rough nano-structures on the ber surface and superhydrophobic SA-ZnSnO 3 is contributed to the achievement of the superhydrophobic function. The EDS mapping is utilized to characterize the surface chemical compositions (Fig. 4c) and element distribution (Fig. 4d) of the superhydrophobic Bi 2 MoO 6 fabric. The content of elements including C, O, Zn, Sn, Bi and Mo of SH BM fabric is 43.9 %, 45.9 %, 4.7 %, 4.8 %, 0.3 % and 0.4 %, respectively. It can be observed that the elements of zinc (Zn) and molybdenum (Mo) are uniformly distributed, and this phenomenon demonstrates that the internal skeleton of the fabric is covered by SA-ZnSnO 3 and Bi 2 MoO 6 particles.
To investigate the photocatalytic activity, the photodegradation experiment of SH BM fabric (4 cm 4 cm) is conducted under visible-light irradiation. Fig. 5a shows the recycle degradation RhB solution (50 mL, 10 mg/L) of SH BM fabric, and a remarkable degradation e ciency is still maintained after going through four cycles of degradation tests (the degradation e ciency is higher than 94 %). Subsequently, to research the degradation circumstance of the SH BM fabric under the natural sunlight, the identical scale superhydrophobic fabric is utilized to carry on the outdoor sunlight photocatalytic tests (Fig. 5b). The preeminent catalytic property is contributed to the organic dye contaminants resolved into nontoxic single molecules. The relevant photocatalytic reaction mechanism is explored through the active species capture experiments of Bi 2 MoO 6 powder (Fig. 5c). Edetate disodium (0.5 mmol/L, EDTA-2Na), isopropyl alcohol (1 mmol/L, IPA) and p-Benzoquinone (1 mmol/L, p-BQ) respectively serve as the quenchers of vacancy (h + ), hydroxyl free radical (•OH) and superoxide anion (•O 2 − ) (Yue et al. 2021;Tang et al. 2020;Deng et al. 2020;Li et al. 2019). The RhB solution (50 mL, 10 mg/L) is regarded as the principal degradation target, and the above scavengers are appended in the reaction system to investigate the principles of photocatalytic response. Correspondingly, Fig. 5d is the column chart of degradation e ciency with various scavengers added in reaction system. Compared with non-scavengers (99.7 %), the degradation e ciency of the additive IPA (99.6 %) and p-BQ (93.1 %) has barely changed, while it signi cantly decreases after adding EDTA-2Na (19.9 %) into the mixed solution. This result intuitively re ects that h + is the major active species that plays a crucial character in the entire photocatalytic reaction process. Fig. 6a represents the adsorption-desorption isotherms of Bi 2 MoO 6 , and the typical IV-type curves present hysteresis loops at the medium relative pressures (P/P 0 ) area, demonstrating the existence of abundant mesopores in Bi 2 MoO 6 . Characterized by the BET analysis, the speci c surface area of Bi 2 MoO 6 is 65.801 m 2 g -1 . Fig. 6b shows that the primary pore size distributions of Bi 2 MoO 6 is approximately at 13.7 nm, further verifying the structure of the mesopores materials. The sequence photographs of removal oil phase (methylbenzene, dyed by oil red) are displayed in Fig. 6c. As is presented in the diagram, a facile oil skimmer is fabricated and applied to selective oil-water separation, exhibiting the phenomenon that the oil oating on the water surface is entirely picked up by the oil skimmer. The behavior of water phase (dyed by RhB) on the SH BM fabric surface is researched in Fig. 6d. The spherical water is standing on the as-synthesized sample, and the static contact angle indicates that the superhydrophobic performance of the SH BM fabric is pretty remarkable, as is shown in the illustration. Subsequently, to research the resistance stability of the SH BM fabric to acid and alkali corrosion, the samples are soaked in solutions of diverse pH values, namely 3, 5, 7, 9, 11 and 13. As is depicted in Fig. 6e, the water contact angles (WCA) remain surpassing 150°, therefore, the hydrophobic stability of the SH BM fabric is advantageous in the rigorous atmosphere.
In order to evaluate the superhydrophobic stability of SH BM fabric, it has been subjected to various harsh treatments. During the treatment process at a temperature of 100 ℃, the fabric samples are taken out with an interval of about 30 min. After dried at 120 ℃ for 2 h, the water contact angles are detected to characterize the in ltration degree. As is displayed in Fig. 7a, it can be discovered that the water contact angles of SH BM fabric remain larger than a baseline degree of 150° after undergoing a series of high-temperature treatments.
In addition, the mechanical stability is explored by frictional wear experiment (Fig. 7b). The type of friction pair is a 6 bearing steel ball and the supplementary loading is 1 N, with 200, 300 ,400, 500 and 600 friction cycles conducted. The images of water on the SH BM fabric appear spherical, hence the results obviously indicate that the sample is of a remarkably durable superhydrophobic stability. To investigate the relevant relation between the polarization signal excitation of electricity and potential, a series of electrochemical measurements are performed in the article. Fig. 7c shows the electrochemical impedance spectroscopy of Bi 2 MoO 6 powder, in which Z and Z represent real and imaginary component of impedance. The photoluminescence spectra is utilized to investigate the photocatalytic activity and separation e ciency of photogenic carriers. As is displayed in the illustration, under the excitation wavelength of 298 nm, the electrons among the Bi 2 MoO 6 catalyst in the upper state fall back to the ground state, generating the maximum emission wavelength of 468 nm. Furthermore, the photocurrent response is utilized to research the recombination mechanism of relevant electron-hole pairs (Fig. 7d). It is worth noting that the photocurrent value of Bi 2 MoO 6 raises to a stable region when the light source is opened instantly, attributing to cathodic photocurrent signals generated by photo-induced electron transitions. Besides, the photocurrent signals show a photo-dependent performance, whose concrete expression is annihilated accompanied with the light source shutoff.
To determine the mechanism of photocatalytic activity, the relevant electronic band-gap structure of Bi 2 MoO 6 is investigated. As is shown in Fig. 8a, the positive slope of the Mott-Schottky curves demonstrates that Bi 2 MoO 6 is a typical n-type semiconductor, and the intercept of the abscissa axis represents that the value of at band potential is about -0.925 eV. Correspondingly, the at band potential is estimated to be -0.68 eV (vs. NHE). In Fig. 8b, the XPS valance band spectra are utilized to obtain the valance band information. From the valance band diagram, it can be summarized that the distance between the valance band and the at band potential is determined to be 1.76 eV, therefore, the position of valance band (VB) is calculated to be 1.08 eV. In addition, to research the light-absorption property and the bandgap energy of the Bi 2 MoO 6 catalyst, the UV-Vis diffuse re ectance analysis is performed. As is shown in Fig. 8c, it is noteworthy that the visible-light absorption edge of Bi 2 MoO 6 is approximately 540 nm. Besides, according to the illustration of the (αhν) 2 versus hν, the band gap energy (Eg) is calculated to be about 2.72 eV. Therefore, combined with the above calculation, the position of the conduction band (CB) is evaluated to be -1.64 eV.
Based on the above calculation and the analysis of energy band structure, the rational photocatalytic enhanced mechanism diagram of Bi 2 MoO 6 is given in Fig. 8d. In this paper, RhB solution is regarded as the primary degradation target contaminant, and the oxidation-reduction ability of Bi 2 MoO 6 semiconductor catalyst is utilized to decompose into eco-friendly components (H 2 O, CO 2 , etc). Driven by the irradiation of visible-light, the electrons (e -) of VB acquire energy to struggle free of forbidden band barriers and nally form an equal number of holes. The numerous aggregation h + of VB can oxidize aromatic structure dyes, therefore, the existence of h + is extremely crucial in the entire RhB degradation process.

Conclusion
Herein, the stearic acid modi ed ZnSnO 3 superhydrophobic nanoparticles and Bi 2 MoO 6 catalysts are deposited on the surface layer of the cotton fabric via a facile dip-coating solution. The as-prepared SH BM fabric exhibits a preeminent superhydrophobic and photodegradation performance, which are simultaneously utilized to selective oil-water separation and the degradation of organic contaminants from sewage. No matter in what a harsh environment (thermal, acid and alkali corrosion as well as mechanics abrasion), the SH BM fabric is always of an excellent water non-wetting behavior, and its durability makes the multiple reusability realized. Furthermore, the photocatalytic stability of SH BM fabric is quite remarkable, whose degradation e ciency is still approximately 94 % after suffering cyclic application.
Declarations Figure 1 The XRD patterns: (a) Bi2MoO6, (b) ZnSn(OH)6 precursor, ZnSnO3 and SA-ZnSnO3; (c) the FTIR spectrum analysis; (d) the TG curves of ZnSnO3 and SA-ZnSnO3 at the nitrogen protected atmosphere.       The superhydrophobic stability of the SH BM fabric: (a) thermal treatment at 100 ℃, (b) frictional wear treatment; (c) the EIS analysis and PL spectrum of Bi2MoO6; (d) the photocurrent response of Bi2MoO6 powder.