Facile preparation of CuS microspheres based superhydrophobic paper with self-cleaning, high chemical stability, and photothermal effect enhanced separation of oil/water mixtures and emulsions

Frequent oil spill accidents and the discharge of oily wastewaters cause signi�cant threats to the marine ecological systems and human health. Herein, a CuS microspheres based superhydrophobic paper (CuS SP) has been prepared with the photothermal property for e�cient oil/water separation and emulsions. To increase the durability, polydopamine is introduced to cellulosic �bers of the �lter paper via self-assembly of dopamine. The CuS SP exhibits a water contact angle of 150.5° and an oil contact angle of ~ 0°, displaying excellent superhydrophobicity and superoleophilicity. Also, the CuS SP possesses excellent chemical resistance, thermal stability, mechanical durability and self-cleaning property. More importantly, the surface temperature of the CuS SP can reach about 48 ℃ after 100 s under one sunlight irradiation (1.0 kW·m -2 ). The separation �ux of CCl 4 can be increased about 14.3% under one sunlight irradiation by using the CuS SP when compared with that without sunlight irradiation. Although the photothermal effect has no obvious in�uence on the separation e�ciency, the CuS SP still shows a high separation e�ciency (> 99%) for CCl 4 under 15 cyclic separation tests with/without sunlight irradiation. Besides, in situ experimental observations for the separation of oil-in-water emulsions have been investigated with the CuS SP by using an optical microscopy, and the possible mechanisms for the separation of oil-in-water emulsions are discussed. Thus, the as-prepared CuS SP shows great potentials in the practical applications of self-cleaning, and the separation of oil/water mixtures and emulsions.


Introduction
Oil spill accidents and the discharge of oily wastewaters cause serious threats to ecological systems and human health (He et al., 2022c).To solve these problems, traditional methods have been developed to deal with oily wastewaters, including in situ burning, bioremediation, oil skimming, air otation, and chemical dispersion (He et al., 2022a;Ma et al., 2023;Shi et al., 2021).These methods, however, suffer from complex operations, high energy consumption, low separation e ciency, and secondary pollutions (He et al., 2022d;Satapathy et al., 2017).Thus, it is urgent to nd a feasible, low-cost and e cient approach to the treatment of oily wastewaters.Superhydrophobic/superoleophilic materials have been prepared on various substrates for the separation of oily wastewaters, including papers (Zhang et al., 2020a), fabrics (Xu et al., 2020), meshes (Wang et al., 2021), sponges (He et al., 2022b) and membranes (Li et al., 2021a).Among them, superhydrophobic paper (SP) shows great advantages in the application of oil/water separation because of its recyclability in nature.Generally, SP can be obtained by decorating nanoparticles (NPs) and chemically modifying with low surface energy materials (Ge et al., 2015;He et al., 2022a;Xi et al., 2021;Zhang et al., 2017).He et al. successfully achieved a durable TiO 2 -based SP by rmly anchoring TiO 2 NPs on cellulosic micro bers, and this SP maintained high separation e ciency of 94.3% for oil/water mixtures even after 15 cyclic tests (He et al., 2022c).Li et al. prepared a durable and sustainable SP by the layer-by-layer assembly of ZnO NPs and carrageenan and the modi cation of polydimethylsiloxane, and this SP showed excellent separation performances for oil/water mixtures and water-in-oil emulsions (Li et al., 2021b).Khan et al. synthesized a SP by introducing a hydrophobic silica NPs via a dip coating technique, which was proved to be an e cient medium for the separation of oil/water mixtures (Khan et al., 2017).
In addition, the durability is a very important property for the real application of SPs.As inspired by additive mussel proteins, polydopamine (PDA) has been introduced to enhance the adhesion between NPs (or microparticles) and cellulosic micro bers, and thus improve the durability and recyclability of SPs in the oil/water separation (He et al., 2022b;Jiang et al., 2011).For example, Li et al. obtained superhydrophobic cellulose-based paper through PDA-induced in situ growth of Ag NPs and chemical treatment with n-dodecyl mercaptan, and achieved separation e ciency above 96% for oil/water mixtures and water-in-oil emulsions by using this kind of SP (Li et al., 2022b).Ruan et al. realized a superhydrophobic tissue paper by the self-assembly of PDA and coating the mixture of SiO2 NPs and PDMS, and this SP exhibited a separation e ciency of oil/water mixtures larger than 99% (Ruan et al., 2020).In short, cellulosic paper based superhydrophobic materials are suitable for the e cient separation of oil/water mixtures and emulsions.
Photothermal effect has been introduced to superhydrophobic materials with enhanced performances for the oil/water separation (Hussain et al., 2021;Wu et al., 2022).Usually, photothermal heating absorbers (i.e., CuS (Bu et al., 2014) In this work, a CuS microspheres based superhydrophobic paper (CuS SP)has been successfully obtained with the excellent photothermal property, superhydrophobicity and superoleophilicity.Also, this CuS SP possesses excellent chemical resistance, thermal stability and mechanical durability.Furthermore, the CuS SP can absorb photothermal heating and reach a surface temperature of 48 ℃ after 100 s under one sunlight irradiation.It can be found that the photothermal effect only promotes the separation ux of oil-in-water emulsions, while it doesn't in uence the separation e ciency (> 99%) for CCl 4 under 15 cyclic separation tests.Based on in situ experimental observations, possible mechanisms for the separation of oil-in-water emulsions are discussed.Therefore, the rational design of the CuS SP provides new insights to the preparation of superhydrophobic materials and thus can be used in the practical applications for the separation of oil/water mixtures and emulsions.

Preparation of CuS microspheres
CuS microparticles were prepared based on a hydrothermal synthesis method (Fang et al., 2018;Huang et al., 2021;Lu et al., 2015).Brie y, 0.8 g copper sulfate, 0.5 g sodium thiosulfate, and 0.5 g PVP were dispersed in 60 mL of deionized water under continuous stirring.Then, the solution was completely dissolved and transferred to a 100 mL Te on-lined autoclave which was sealed and kept in an oven at 160°C for 12 h.Afterwards, the autoclave was naturally cooled to room temperature, and the black-brown precipitate was obtained by centrifuging and washing for more than three times.Finally, CuS powders were dried in an oven at 90 ℃ for 4 h.

Preparation of CuS microspheres based superhydrophobic paper
The preparation of CuS microspheres based superhydrophobic paper (labeled as CuS SP) was achieved by using a previous method (He et al., 2022c).First, 0.1 g dopamine hydrochloride was fully dissolved in 100 mL Tris-HCl buffer solution with the pH of 8.5.Then, the deposition of polydopamine (PDA) was realized by immersing a pristine lter paper in the above solution for 24 h, and the obtained lter paper was rinsed with deionized water and dried in an oven at 40°C for 2 h.After that, different amounts of CuS microspheres (i.e., 0.5, 1, 1.5, 2, 2.5, 3, 4, and 5 g) were dispersed in 100 mL of Tris-HCl buffer solution (pH = 8.5), which were ultrasonically dispersed for 1 h and magnetically stirred at room temperature for 3 h.Furthermore, a PDA modi ed lter paper was immersed into the suspension of CuS microspheres with different concentrations for 2 h, followed by rinsing with deionized water and drying at 40°C for 2 h.Finally, the CuS microspheres based superhydrophobic paper was obtained by chemically modifying with an n-hexane solution of FDTS (0.5 wt.%) for 10 min and drying in an oven at 60°C for 1 h.

Characterizations
The surface morphologies of pristine lter paper, CuS microspheres and CuS SP were characterized by using a scanning electron microscopy (SEM, Apreo).The chemical composition of CuS SP was investigated by using an energy-dispersive spectrometry (EDS) equipped with an SEM and a Fourier Transform Infrared Spectroscopy (FTIR, Nicolet iS 10, Thermo Fisher).The crystal structures of pristine lter paper and CuS SP were determined by using an X-ray diffractometer (XRD, SmartLab, Rigaku).Water contact angle (WCA) was evaluated and repeated with a deionized water droplet of ~ 15 µL at room temperature by using a contact angle goniometer (JY-82A).The size distribution of water-in-oil (or oil-inwater) emulsions was characterized by using dynamic light scattering (Nano BT-90) (He et al., 2022d).

Stability measurements
The chemical stability of CuS SPs was tested by immersing CuS SPs in the saline solution and organic solvents (i.e., 3.5 wt.% NaCl, n-hexane, ethanol, and DMF) for 1 h and solutions with different pH values (i.e., pH = 1 and pH = 13) for 3 h, respectively.To characterize the thermal stability, CuS SPs were immersed in boiling water for different time (i.e., 0 ~ 120 s).Furthermore, the mechanical durability of CuS SPs was evaluated by the changes of WCAs after cyclic sandpaper abrasion tests.Brie y, a CuS SP was pressed against a sandpaper (800 mesh) with a 100 g stainless steel and pulled with a pair of tweezers for 10 cm at a speed of 0.1 cm/s (He et al., 2022d).

Separation of oil/water mixtures and emulsions
The oil/water separation performances of CuS SPs were investigated by using a gravity-assisted oil/water separation method.In general, the CuS SP was xed in a self-made experimental device, and then a mixture of tetrachloromethane (dyed with Sudan I) and deionized water (dyed with Brilliant Green) was poured into the device.During an oil/water separation process, tetrachloromethane can easily permeate through the CuS SP into the beaker below, while water remains above the CuS SP.The separation e ciency of oil/water mixtures (or emulsions) is calculated as follows: where m 0 (g) and m (g) are the weights of water before and after an oil/water separation process, and η (%) is the separation e ciency of CuS SPs for obtaining oil from oil/water mixtures (or emulsions) (He et al., 2022c).In addition, water-in-oil emulsions were prepared by ultrasonically dispersing deionized water (1 wt.%) and Span-80 (0.1 wt.%) to tetrachloromethane for 1 h, and oil-in-water emulsions were prepared by ultrasonically dispersing tetrachloromethane (1 wt.%) to deionized water for 1 h.

Photothermal enhanced separation of emulsions
The sunlight irradiation was simulated by using a sunlight simulator (PerfectLight, PLS-SXE300, China) with a standard solar spectrum AM 1.5 G as a reference.The sunlight illumination intensity was characterized by a Power Meter (PerfectLight, PL-MW2000, China).Infrared images were recorded with an infrared camera (FLIR One Pro, America).The oil/water separation ux is calculated as follows: where V (L) is the permeated volume of oils, S (m 2 ) is the effective contact area of CuS SP, and t (h) is the oil/water separation time (Li et al., 2021a).

Preparation of CuS microspheres based superhydrophobic paper
CuS microspheres have been prepared by a modi ed hydrothermal synthesis method (He et al., 2022c).The formation of CuS microspheres is achieved by the self-assembly of CuS nanorods as illustrated in

WCAs of CuS microspheres-based papers
To obtain CuS SP, the concentration of CuS microspheres in the dispersion is important for the preparation of CuS based papers, which also in uences the surface roughness as well as the corresponding WCAs.To optimize the concentration of CuS microspheres for the preparation of CuS SP, a series of CuS dispersions with different concentrations (i.e., 5, 10, 15, 20, 30, 40, and 50 mg/mL) were used.It can be seen in Fig. 2 that the WCA of CuS microspheres-based paper reaches a maximum contact angle of 150.5° (the inserted gure in Fig. 2) when the concentration of CuS microspheres is selected as 15 mg/mL.When the concentration of CuS microspheres is less than 15 mg/mL (i.e., 5 and 10 mg/mL), the WCAs of CuS microspheres-based paper are 147.5°and 149.7°.The reason why the WCAs cannot reach 150° is possible that the concentration of CuS microspheres is not enough to form hierarchical surface roughness to achieve surface superhydrophobicity.When the concentration of CuS microspheres is between 15 mg/mL and 50 mg/mL, the WCAs are still larger than 142.3°.As the concentration of CuS microspheres increases, there are not enough anchoring sites for extra CuS microspheres, and thus the aggregation of extra CuS microspheres appears.Actually, the aggregation of CuS microspheres not only in uences the formation of surface roughness for surface superhydrophobicity but also weaken the binding between CuS microspheres and the lter paper.Therefore, the concentration of CuS microspheres is chosen as 15 mg/mL in this study during the preparation of CuS SP.

Surface Morphologies and Chemistry
The SEM images of the pristine lter paper and CuS SP are shown in Fig. 3.The surfaces of cellulosic bers in the pristine lter paper are smooth, and some tiny cellulosic bers exist among cellulosic micro bers (Fig. 3a).The CuS microspheres are self-assembled by CuS nanorods (Fig. 1 and Fig. 3b), and the diameter of CuS microspheres as prepared by the hydrothermal method is about 1-2 µm.It can be seen in Fig. 3c-e that CuS microspheres are uniformly distributed among cellulosic micro bers, and no obvious aggregation of CuS microspheres is found.Also, some sub-microscale (or nanoscale) CuS based particles exist among CuS microspheres (Fig. 3f), which is bene cial to achieve surface superhydrophobicity.Although CuS microspheres consist of CuS nanorods, the maximum WCA of CuS SP only reaches 150.5°.It indicates that surface superhydrophobicity of CuS SP cannot be enhanced by simply adjusting the concentrations of CuS microspheres, which is in agreement with the results in Fig. 2.
To investigate chemical composition, the EDS analysis for the CuS SP is carried out.It can be seen in Fig. 4a that the CuS SP contains the elements of C, N, O, F, Si, Cu, and S. The peak intensity of the elements of Cu and S is very strong, showing that CuS microspheres are fully covered on the cellulosic bers.The peaks for the elements of C, O and N are attributed to cellulosic bers and PDA, and the peaks for the elements of F and Si is due to the chemical modi cation of FDTS, indicating that CuS SP has been successfully prepared.
The FTIR spectra of the pristine lter paper and CuS SP were characterized to analyze the chemical compositions (Fig. 4b).The absorption bands at 3333 cm − 1 , 1157 cm − 1 , 2882 cm − 1 , and 897 cm − 1 belong to the O-H stretching vibration, the C-O-C peak, and the C-H stretching and bending vibration, respectively (Zhang et al., 2020a).The peaks at 1418 cm − 1 and 1314 cm − 1 belong to the vibrations of C-H and C-CH (Ibrahim et al., 2011).All these absorption bands belong to the absorption bands of glucose monomers in paper bers, whereas the absorption bands at 1028 cm − 1 and 1054 cm − 1 are attributed to the C-O stretching in the cellulose backbone (Wu et al., 2018).Furthermore, the CuS SP exhibits different characteristic peaks.For example, the absorption bands at 1506 cm − 1 , 1614 cm − 1 and 3420 cm − 1 belongs to the shear vibration of N-H, the C = C stretching, and the stretching vibration of phenol O-H and N-H, respectively (Fu et al., 2015;Jiang et al., 2011).These peaks of the CuS SP are attributed to PDA, indicating the successful self-polymerization of dopamine on the lter paper during the immersion process.The absorption band at 610 cm − 1 is attributed to the vibration of CuS (Lv et al., 2022), demonstrating the successful deposition of CuS microspheres among the cellulosic bers of the CuS SP.
The peak at 1236 cm − 1 is attributed to the stretching vibration of C-F in terms of either -CF 2 or -CF 3 , exhibiting the successful modi cation of FDTS for the CuS SP (Teng et al., 2020a).Thus, the FTIR analysis proves that the CuS SP has been successfully obtained through the self-assembly of PDA, the decoration of CuS microspheres and chemical modi cation of FDTS when compared with the pristine lter paper.
Besides, the XRD patterns of the pristine paper, CuS microparticles and CuS SP are displayed in Fig. 4b.It can be seen that the diffraction peaks of the CuS microspheres and CuS SP locate at 29.3°, 31.8°,32.8°, and 48°, which is corresponding to the crystal planes of (102), ( 103), (006), and (110) for CuS (PDF card # 06-464), respectively (Zhang et al., 2020b).In contrast, the pristine lter paper does not have these diffraction peaks as mentioned above, indicating that CuS microspheres have been successfully decorated on the cellulosic bers of the lter paper.

Chemical, thermal and mechanical stability
The chemical resistance of CuS SP is analyzed by immersing it in 3.5 wt% NaCl solution, DMF, ethanol, and CCl 4 for different time (i.e., 0 ~ 60 min), respectively.The water contact angles (WCAs) of CuS SP after being immersing in above liquids are shown in Fig. 5a.It can be found that WCAs of CuS SP show a slight decrease after immersion for even 60 min, and WCAs are still higher than 145°, showing highly hydrophobicity and excellent chemical resistance of CuS SP.
To investigate the acid and alkali corrosion resistance, the CuS SP is tested by being immersing in HCl solution (pH = 1) and NaOH solution (pH = 13) for different time (i.e., 0 ~ 180 min).It can be seen in Fig. 5b that WCAs of CuS SP are still greater than 145° after being immersing for 3 h, and CuS SP shows excellent corrosion resistance for acid and alkali solutions.Generally, the WCAs of CuS SP in acid solution are slightly higher than that in alkali solution, and thus CuS SP shows better chemical resistance in alkali solution.The reason is that the hydrogen bonds in the PDA layers are easy to be broken in alkaline solution (He et al., 2022c).Consequently, the adhesion between CuS microspheres and cellulosic bers is reduced, and then the microstructures of CuS SP are gradually destroyed as the immersion time increases.
The thermal stability is an important parameter for superhydrophobic materials.As shown in Fig. 5c, the CuS SP is being immersed in boiling water for different time (i.e., 0 ~ 120 s).As the immersing time increases, the WCAs of CuS SP show a gradual decrease, and maintain above 145° even after being immersing in boiling water for 120 s, indicating good thermal stability of CuS SP.In addition, the abrasion resistance of the CuS SP is also analyzed by pulling it against a sandpaper (800 mesh) with a 100 g stainless steel for 30 times (Fig. 5d).It can be seen that the WCAs of CuS SP vary between 145° and 150°d uring 15 cyclic abrasion tests, and they gradually decrease and maintain above 140° after 30 cyclic abrasion tests.The reduction of WCAs is possibly due to the slight destruction of cellulosic bers during cyclic abrasion tests (He et al., 2022c).In short, the as-prepared CuS SP exhibits excellent stabilities and maintains highly hydrophobicity after chemical resistance tests, boiling water tests and mechanical durability tests, showing great potentials in the application of oil/water separation.

Wetting and self-cleaning properties
A series of wettability experiments are carried out to investigate wettability and self-cleaning properties of the CuS SP (Fig. 6).As shown in Fig. 6a-b, droplets of deionized water (dyed with Brilliant Green) and nhexane (dyed with Sudan I) are placed on the pristine lter paper and CuS SP.Both water and oil droplets can easily penetrate into the pristine lter paper, whereas water droplets exist on the CuS SP and only oil droplets penetrate into the CuS SP.It indicates that the CuS SP shows surface superhydrophobicity and superoleophilicity.To further investigate wettability, various liquid droplets (i.e., juice, tea, coffee, water and milk) are dropped on the CuS SP, and the CuS SP shows excellent repellency for all these droplets (Fig. 6c).In Fig. 6d, a water jet can rebound on the CuS SP without leaving any trace, indicating that the CuS SP has a good impact resistance and excellent superhydrophobic stability.In addition, the selfcleaning properties of the CuS SP are characterized by pouring various liquids (i.e., tea, juice, coffee and milk) (Fig. 6e-h), and these liquids can easily move away from the CuS SP.When the CuS SP is tilted at 20°, blue chalk powders can be carried away by water droplets (Fig. 6i1-i4), showing that the CuS SP has a good ability to remove solid pollutants with the assistance of water droplets.

Oil/water separation
The oil/water separation ability is an important property for superhydrophobic materials.As shown in Fig. 7a-b, n-hexane and CCl 4 (both dyed with Sudan I) are dropped into the deionized water (dyed with Brilliant Green).Due to the difference of densities, n-hexane oats on the surface of water and CCl 4 sinks to the bottom of water.When the CuS SP is gently placed on oil droplets oating on water, and n-hexane droplets can be easily adsorbed (Fig. 7a).Similarly, CCl 4 droplets at the bottom of water can be adsorbed by the CuS SP because of its superhydrophobicity and superoleophilicity (Fig. 7b).Furthermore, Fig. 7c shows the continuous oil/water separation ability of the CuS SP for the mixture of CCl 4 and water.It can be seen that CCl 4 can pass through the CuS SP with the force of gravity, showing great potentials for the practical application of oil/water separation.

Photothermal effect enhanced oil/water separation
As a semiconductor photosensitizer, CuS can absorb light and generate heat through d-d* energy band jump and plasma resonance (Bu et al., 2014;Mathew et al., 2021).Thus, CuS based superhydrophobic materials (i.e., CuS SP) have a good photothermal effect under solar irradiation.In Fig. 8a, UV absorption tests are carried out on the pristine lter paper and the CuS SP.Due to the presence of CuS microspheres, the CuS SP shows a good absorption under the wavelengths of 200 ~ 800 nm when compared with the pristine lter paper.To verify the photothermal performance, the CuS SP and the pristine lter paper are exposed under arti cial sunlight with a xenon lamp, and the corresponding temperatures are varied as time goes (Fig. 8b).It can be seen that the surface temperature of the CuS SP obviously increases in the rst 30 s under one sunlight irradiation (1.0 kW•m − 2 ) as time goes, and it gradually stabilizes and maintains at about 48°C after 100 s.Compared with the CuS SP, the surface temperature of the pristine lter paper only slightly increases under one sunlight irradiation, indicating that the CuS SP has a good photothermal heating conversion performance.Also, the optical and infrared photographs of the CuS SP under light illumination for the separation of the mixture of CCl 4 and water are illustrated in Fig. 8c-d, showing indicating the excellent photothermal effect of the CuS SP during the separation process.
The separation e ciencies of the CuS SP for the mixture of CCl 4 and water with/without one sunlight irradiation are investigated as shown in Fig. 8e.The CuS SP exhibits a high separation e ciency (> 99%) for CCl 4 under 15 cyclic separation tests with/without one sunlight irradiation, and the photothermal effect almost has no in uence on the separation e ciency.Furthermore, the separation ux of the CuS SP is also investigated with/without light (Fig. 8f).The separation ux of the CuS SP for CCl 4 without light is stabilized at about 1400 L•m − 2 •h − 1 during ve cycles, while the separation ux has an obvious increase in the light and maintains above 1600 L•m − 2 •h − 1 .The reason is that the increase of the surface temperature of the CuS SP reduces the viscosity of CCl 4 and thus enhances the separation ux.

Emulsion separation and possible mechanisms
The separation properties of water-in-oil (or oil-in-water) emulsions are very important for superhydrophobic materials.In Fig. 9a, the separation of water-in-oil emulsion (CCl 4 ) by using the CuS SP is investigated.It can be found that the original sizes of water-in-oil emulsions range from 0.15 µm to 0.25 µm as shown in the inserted gures in Fig. 9a.After the separation by using the CuS SP, no obvious droplets can be observed under an optical microscope (Fig. 9b), indicating that the CuS SP has an excellent performance for the separation of water-in-oil emulsions.Furthermore, the separation of oil-inwater emulsion (CCl 4 ) is also investigated by using the CuS SP under microscopic observation.It can be seen that the sizes of oil-in-water emulsions range from 0.35 µm to 0.57 µm (Fig. 9c).
To study the possible mechanisms for the separation of oil-in-water emulsions, in situ experimental observations are utilized with the assistance of an optical microscopy (Fig. 9d1-d8, Fig. 9e1-e8 and Movie S1).It can be seen that microscale oil droplets gradually grow bigger on cellulosic bers because of the superoleophilicity of the CuS SP, and demulsi cation and coalescence of oil-in-water emulsions (Fig. 9d1-d2).Interestingly, oil liquid bridge can form and spread among small pores of cellulosic bers, and then microscale oil droplets form as the coalescence of oil-in-water emulsions continuously occurs near the oil liquid bridge (Fig. 9d3-d4).As for microscale oil droplets, the coalescence between two adjacent microscale oil droplets can also happen as long as the distance of two oil droplets is close enough (Fig. 9d5-d6).During the coalescence process, the extra energy releases and thus results in the moving of coalesced oil droplets (Fig. 9d7-d8 and Fig. 10a-b).Once microscale oil droplets form, they can be easily separated by using the CuS SP.
To further demonstrate the possible mechanisms, the coalescence behaviors of oil-in-water emulsions are in situ observed focusing on one certain cellulosic ber.It can be seen that an oil liquid bridge gradually forms during the demulsi cation processes, starts to coalesce with adjacent oil droplets, and then moves away after coalescence (Fig. 9e1-e5, Movie S2, and Fig. 10b).Of course, the formation of an oil liquid bridge only occurs around proper pores of cellulosic bers where the capillary force among the pores and surface superhydrophobicity of cellulosic bers reach a balance.Surprisingly, a very tiny cellulosic ber near the oil liquid bridge is destroyed by the released energy during the coalescence process.It indicates that the micro/nanostructures of cellulosic bers can be gradually destroyed during cyclic separation tests.As shown in Fig. 9e1-e8 and Movie S2, oil-in-water emulsions are found to gradually aggregate and coalesce around the cellulosic ber (Fig. 10a).When oil droplets around the cellulosic ber grow bigger enough, microscale oil droplets can be easily observed by an optical microscopy (Fig. 9e6-e8), which are then easily separated by using the CuS SP.Besides, the dynamic speeds of oil-in-water emulsions are found to be increased around cellulosic bers under one sunlight irradiation, and thus the photothermal effect enhances the separation ux of emulsions.

Conclusions
In summary, a photothermal CuS SP has been successfully fabricated via the self-assembly of dopamine between CuS microspheres and cellulosic bers of the lter paper.The as-prepared CuS SP possesses excellent self-cleaning property, superhyrophobicity and superoleophilicity, and also shows excellent chemical resistance, thermal stability and mechanical durability.Furthermore, the CuS SP exhibits good photothermal effect under one sunlight irradiation and reaches about 48 ℃ after 100 s under one sunlight irradiation (1.0 kW•m − 2 ).The results demonstrate that the separation ux of CCl 4 by using the CuS SP can be increased about 14.3% under one sunlight irradiation when compared with that without sunlight irradiation.However, the photothermal effect has no obvious in uence on the separation e ciency of emulsions.Based on in situ experimental observations, the possible mechanisms for the separation of oil-in-water emulsions are discussed.Thus, the obtained CuS SP can be utilized in the practical elds of self-cleaning, anti-fouling, and the separation of oil/water mixtures and emulsions.

Fig. 1a .
Fig. 1a.To realize CuS microspheres based superhydrophobic paper (labeled as CuS SP), polydopamine (PDA) is introduced via the self-polymerization of dopamine to enhance the adhesion between CuS microspheres and pristine lter paper in the alkaline condition (pH = 8.5) (He et al., 2022c).During the polymerization process, dopamine is oxidized to form dopamine-quinone in an alkaline solution and subsequently undergoes intermolecular interactions to attach to the lter paper, providing anchoring sites for CuS microspheres (Fig. 1b) (Cheng et al., 2020; Cui et al., 2019).Once CuS microspheres are decorated on the lter paper, PDA continues to form around CuS microspheres and thus rmly binds CuS to the pristine lter paper.Finally, the CuS SP has been successfully obtained after the chemical modi cation of FDTS solution.

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Figure 9 Optical
Figure 9