Solar-driven self-heating sponges for highly efficient

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Solar-driven self-heating sponges for highly efficient crude oil spill remediation† Chao Zhang,‡ Ming-Bang Wu,‡ Bai-Heng Wu, Jing Yang* and Zhi-Kang Xu

*

Received 13th March 2018 Accepted 17th April 2018 DOI: 10.1039/c8ta02336k rsc.li/materials-a

Herein, we report solar-driven self-heating hydrophobic/oleophilic sponges for the first time for fast adsorption of crude oil from water surfaces. The self-heating function benefits from the photothermal conversion effect of the polydopamine (PDA) coating, enabling the temperature of the sponges to quickly increase to in situ decrease the viscosity of crude oil. The adsorption capacity of the self-heating sponges can reach up to 1.29  0.37  106 g m 3, which is nearly 11 times higher than that of the sponges without the self-heating function (1.13  0.11  105 g m 3). Furthermore, the integration of the selfheating sponges with a peristaltic pump creates a self-heating vacuum cleaner to realize continuous clean-up or collection of crude oil from water surfaces.

Frequent oil spill accidents and industrial oily wastewater discharge have continuously caused a series of severe environmental and ecological damage to our home planet, and this problem will even grow much worse in the coming decades.1–3 To address this long-term issue and maintain ecological equilibrium, a number of human and nancial resources have been used to develop diverse treatment technologies to clean up or collect oil during oil spills.4–9 Among them, porous adsorption materials and advanced separation membranes or meshes have received signicant attention as potential candidates.10–19 However, the major issue that blocks the practical application of these separation materials is the high viscosity of crude oil (from 103 to 105 mPa s at room temperature).20–22 Viscous crude oil has poor mobility and thus low diffusion rates into and through the inner pores of these porous materials; this results in a low separation and treatment efficiency. Therefore, a great challenge lies in how to design the separation materials to

MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, China. E-mail: [email protected]; [email protected] † Electronic supplementary 10.1039/c8ta02336k

information

(ESI)

‡ The authors contribute equally to the work.

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decrease the viscosity and thus to increase the uidity of crude oil in situ for achieving highly efficient clean-up and collection. It has been widely accepted that the viscosity of crude oil usually decreases with the increasing temperature. Most recently, a Joule-heated graphene-wrapped sponge was proposed by Yu et al. to clean up the crude oil.23 They have found the Joule heat generated by electric power can signicantly reduce the viscosity of crude oil to further increase the oil diffusion coefficient. However, a remaining challenge is to nd a kind of on-the-spot available power supply for the emergency treatment of oil spills in offshore areas where electric power is usually inadequate to meet the demands. Therefore, novel separation materials and protocols are highly desired with selfheating features to decrease the viscosity and thus to increase the treatment efficiency of crude oil. The sun provides abundant energy for our planet. This energy can be converted into heat or electric power, and this concept has been applied in various ever-evolving elds such as in photocatalysis,24,25 solar cells26,27 and water treatment.28,29 In terms of solar heating, a variety of light-to-heat conversion materials, such as nanoparticles,30,31 polypyrrole,32 carbon,33 CNTs,34 MXene35 and graphene,34,36–38 have been integrated to fabricate steam-generation devices for clean water production. Inspired by this mechanism, we predict that this photothermal conversion effect can be used to develop self-heating materials for decreasing the viscosity of crude oils in situ without any extra power input. Herein, we propose novel hydrophobic/oleophilic sponges with self-heating properties via the photothermal conversion effect. The self-heating hydrophobic/oleophilic sponges are mainly composed of two crucial coatings: a polydopamine (PDA) layer with broadband sunlight absorbability to rapidly raise the local temperature and a polydimethylsiloxane (PDMS) layer to achieve hydrophobicity/oleophilicity to signicantly enhance the oil adsorption capacity. These sponges can be assembled with a peristaltic pump to form a self-heating vacuum cleaner for highly efficient and continuous clean-up or collection of crude oil from water surfaces (Fig. 1). This study not only offers new insights to clean up and collect crude

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Fig. 1 Schematic of the solar-driven vacuum cleaner assembled by a self-heating hydrophobic/oleophilic sponge and a peristaltic pump for the continuous clean-up or collection of high viscosity crude oil on water surfaces. The self-heating function is achieved via a photothermal conversion effect of a mussel-inspired PDA coating. The brown color represents the high-viscosity crude oil. The red color around the sponge shows that crude oil has relatively high temperature and low viscosity, ascribed to the solar-driven self-heating function.

oil in offshore areas, but also further broadens the practical applications of solar-driven photothermal materials. The self-heating hydrophobic/oleophilic sponges were synthesized by the sequential deposition of PDA and PDMS on melamine sponges (Fig. 2a and S1 in ESI†). At rst, the photothermal conversion coating was fabricated via a musselinspired PDA deposition. As is well-known, PDA can be stably coated on various substrates by an aqueous solution deposition process due to its superior interfacial adhesion.39–41 The asformed PDA coating has an analogous molecular structure to melanin,42,43 which exhibits a superior capability of absorbing broadband sunlight to generate sufficient heat.43–45 The PDA deposition has no inuence on the sponge morphology (Fig. 2b–e) although the sponges change from white to black (Fig. S2 in ESI†). The PDA coating can be further analyzed by the intensity increase of O1s and N1s peaks in the XPS spectrum (Fig. 2h). Subsequently, a hydrophobic/oleophilic coating was constructed via the condensation reaction between dihydroxylterminated polydimethylsiloxane (PDMS(OH)) and tetraethyl orthosilicate (TEOS) using dibutyltin dilaurate as a catalyst (Fig. S1 in ESI†).46 It is obvious that the PDA-coated sponges are uniformly and smoothly wrapped by the PDMS layer (Fig. 2f and g); this is further characterized by new peaks at 150 eV and 100 eV from Si2s and Si2p, respectively, in the corresponding XPS spectrum (Fig. 2h). Additionally, the water contact angle increases from 0 to 134 aer PDMS deposition (inset in Fig. 2f and S2 in the ESI†), whereas the oil contact angle is 0 because of the fast adsorption of oil droplets within the pore structures (Fig. S3 in ESI†). This means that the as-prepared sponges possess an excellent hydrophobicity/oleophilicity to facilitate oil capture. To recap, our two-step deposition strategy effectively endows the sponges with both hydrophobic/oleophilic and photo-thermal conversion properties. In view of the light-to-heat conversion, the light-absorption properties of the sponges are crucially important, and this has been quantitatively investigated by UV-vis absorption and

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Fig. 2 (a) Schematic of the fabrication process for the self-heating hydrophobic/oleophilic sponges. SEM images of (b and c) the nascent, (d and e) the PDA-coated and (f and g) the PDMS/PDA-coated sponges. The inset shows the corresponding water contact angle image. (h) XPS spectra of the nascent, the PDA-coated, and the PDMS/ PDA-coated sponges. (i) UV-vis absorption spectra and (j) diffusive reflectance spectra of different sponges: nacent, PDMS-coated, PDAcoated, and PDMS/PDA-coated sponges. In this part, the content of PDA in the PDA-coated and the PDMS/PDA-coated sponges is equally controlled, and the PDMS-coated and the PDMS/PDA-coated sponges have also similar amounts of PDMS.

diffusive reectance spectroscopy (Fig. 2i and j and S4 in the ESI†). The PDMS/PDA1-coated sponges have a signicant increase in the intensity of the broad absorption band from 200 nm to 800 nm as compared to the PDMS-coated and the nascent sponges. This difference is attributed to the specic p– p* transition in the PDA coating.47 The light absorbability constantly increases with the increasing number of PDA deposition cycle (Fig. S4 in ESI†). Note that the light adsorption has a slight reduction for the PDMS/PDA-coated sponges as compared to the case of the PDA-coated sponges. It demonstrates that the introduction of the PDMS layer has almost no inuence on the light adsorption property of the PDA coating; this can also be visually observed by the unchanged black color of the PDMS/PDA-coated sponges (Fig. S2 in the ESI†). Correspondingly, all the PDMS/PDA-coated sponges exhibit much lower reectivity than the PDMS-coated sponges (Fig. 2j); this shows that the PDA coating possesses excellent light absorbability from another perspective. To evaluate the photothermal conversion efficiency of the self-heating sponges, an infrared thermometer was employed to in situ detect the temperature change under the simulated sunlight irradiation (power density: 1.5 kW m 2). The surface

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equilibrium temperature of the PDMS-coated sponge has a slight increase from 33  C to 39  C aer 80 s of irradiation (Fig. 3a and b). It is very interesting that both the heating rate and the nal equilibrium temperature are much higher for the PDMS/PDA-coated sponges than those for the PDMS-coated sponges. The equilibrium temperature can even reach up to 79  C for the PDMS/PDA5-coated sponges; this is in good agreement with the above mentioned light absorbability results. The light-to-heat conversion ability is also enhanced with the increasing number of PDA deposition cycles. Furthermore, the temperature will rapidly decrease to near room temperature when the simulated sunlight is off. As the light is turned on again, the temperature can be recovered to 79  C. This alternate temperature up/down can be maintained under repeated light on/off (Fig. S5 in the ESI†) cycles; this demonstrates that the PDA coating is very stable during multiple photothermal conversion processes. Similar experiments were further executed under real sunlight irradiation (Fig. 3c and d). As expected, the surface temperature of the PDMS/PDA5-coated sponges can increase up to 63  C aer 60 s of sunlight irradiation, which is nearly 40  C higher than room temperature. Thereby, this solar-driven self-heating function provides a new route to facilely trigger the sponges to generate heat via a noncontact remote control mode, which is expected to decrease the viscosity of crude oil. The viscosity is 2.4  104 mPa s at room temperature for the crude oil used in this study (Fig. 4a). It has a distinct reduction

with the increasing oil temperature. The viscosity will decrease to below 100 mPa s when the temperature is above 50  C. Then, it maintains a viscosity of 15 mPa s, which is in the same order of magnitude of light oil (1–100 mPa s at room temperature), with a further increase in temperature to above 70  C. In addition, crude oil is in the solid-state at room temperature, but instantly turns into the liquid-state with high mobility at 75  C (Fig. 4b). As a consequence, increasing the temperature is an effective way to decrease the viscosity of crude oil and enhance the uidity. This protocol was also employed in the longdistance pipage transportation of crude oil.48 In our cases, the self-heating sponges are able to quickly achieve a high temperature of 70  C and 60  C under simulated and real sunlight irradiation, respectively, which is enough to ensure a fast viscosity reduction of the crude oil. Thus, the adsorption behavior of oil droplets by the sponges was used to qualitatively evaluate the uidity under simulated sunlight irradiation (Fig. 4c and d). For the self-heating sponges, their surface temperature can rapidly go up to above 50  C for thermally inducing the viscosity reduction of crude oil. As a result, the crude oil possesses high mobility to be quickly adsorbed into the hydrophobic/oleophilic sponges. The corresponding adsorption time is only 26.3  1.7 s and 19.7  0.5 s for the PDMS/PDA1-coated and the PDMS/PDA3-coated sponges,

Fig. 3 Heat localization behavior of self-heating sponges. (a) Thermal images and (b) time-dependent temperature evolution curves of different sponges under the simulated sunlight irradiation (power density: 1.5 kW m 2) after 80 s. (c) Thermal images and (d) timedependent temperature evolution curves of different sponges under sunlight irradiation. The orange and blue regions in (b) and (d) represent temperature-rise and temperature-fall period, respectively. PDMS/PDA1, PDMS/PDA3 and PDMS/PDA5 represent that the deposition cycle of PDA is 1, 3 and 5, respectively. The deposition time of every cycle is 40 min. The sponge size is 2  2  1 cm. The environment humidity is 60%. All of the experiments were executed at room temperature.

Fig. 4 (a) Viscosity change of crude oil as a function of oil temperature. The shear rate is 10 s 1. (b) Digital images of the crude oil at 25  C and 75  C. (c) Adsorption time of a crude oil drop on the surface of different sponges under simulated sunlight irradiation (sunlight power density: 1.5 kW m 2): PDMS-coated, PDMS/PDA1-coated and PDMS/ PDA3-coated sponges. PDMS/PDA1 and PDMS/PDA3 represent that the deposition cycle of PDA is 1 and 3, respectively. The deposition time of every cycle is 40 min. The sponge size is 2  2  1 cm. The environment humidity is 60%. All of the experiments were executed at room temperature. (d) Schematic of the adsorption behavior of high viscosity crude oils on the PDMS-coated and the PDMS/PDA-coated sponges under simulated sunlight irradiation.

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respectively. By contrast, 387.0  60.0 s is required to achieve full adsorption for the PDMS-coated sponges, which is 15 times that of the self-heating sponges (Fig. 4c). This is because the crude oil temperature only increases about 4  C aer 60 s sunlight irradiation (Fig. S6 in ESI†). Therefore, as illustrated in Fig. 4d, the difference in the viscosity of the crude oil droplets caused by the solar-driven self-heating effect gives rise to different adsorption rates. In addition, we used a homemade device to quantitatively compare the extrusion amount of crude oil from the sponges with full adsorption under sunlight irradiation (Fig. S7 in ESI†). It can be seen that the crude oil can be immediately squeezed from the PDMS/PDA-coated sponges when the sponges are compressed by 20% strain. With the increasing compressive time, a growing amount of crude oil is extruded; this indicates that the adsorbed crude oils can be recovered with high efficiency. In contrast, no crude oil outows from the PDMS-coated sponges upon the same compressive force. Therefore, the fast diffusion behavior of crude oil within these solar-driven self-heating sponges predicts a bright prospect in the application of highly efficient oil clean-up from water surfaces. Moreover, two strategies were employed to assess the cleanup or collection capability of the self-heating sponges for high viscosity crude oil: static adsorption and dynamic drawing with the assistance of a peristaltic pump. The adsorption capacity of the sponges are compared (Fig. 5a and S8 in ESI†). Obviously, the self-heating sponges possess much higher adsorption capacity than the PDMS-coated sponges. For instance, the PDMS/PDA3-coated sponges can adsorb 6.70  0.67  105 g m 3 crude oil, which is 7 times higher than that of the PDMS-coated sponges (only 0.91  0.07  105 g m 3) under simulated sunlight irradiation for 3 min. Similarly, the adsorption capacity under sunlight irradiation constantly increases with the increasing PDA deposition cycle on the sponges, and its value is 2.9 and 11.4 times higher for the PDMS/PDA1-coated and the PDMS/PDA3-coated sponges than that of the PDMS-coated sponges, respectively (Fig. 5b). These results are ascribed to the fact that the PDA coating bestows the hydrophobic/oleophilic sponges with a robust self-heating function via the photothermal conversion effect to spontaneously reduce the viscosity of crude oil and further enhance their uidity. Apart from the crude oil, the PDMS/PDA-coated sponges can also efficiently clean up non-polar solvents, such as n-hexane and 1,2-dichloroethane, from the surface or the bottom of water (Fig. S9 in ESI†). Moreover, Fig. 5c shows that the adsorption capacity has barely any loss and always maintains a stable value of around 7.30  105 g m 3 even aer 9 cycles; this indicates an outstanding reusability of these sponges in practical applications due to their good exibility, elastic property (Fig. S10 in ESI†), robust hydrophobicity/ oleophilicity (Fig. S11 in ESI†), and durable self-heating function. On the other hand, it should be kept in mind that the sponge shape will inuence the adsorption performance via the self-heating function because the bulky materials and porous structures can greatly impact the thermal conductivity pathway. Increasing the thermal conductivity of the PDA-based coatings is a reasonable way to overcome this challenge.

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(a) Crude oil adsorption capacity of different sponges under simulated sunlight irradiation at different points in time. (b) Crude oil adsorption capacity of different sponges after 5 min of sunlight illumination: PDMS-coated, PDMS/PDA1-coated and PDMS/PDA3coated sponges. (c) Reusability of the crude oil adsorption ability of the PDMS/PDA-coated sponges under simulated sunlight irradiation. Every adsorption experiment ensures that the sponges reach their maximum adsorption capacity. Continuously collecting the viscous crude oil from the water surface by in situ pumping through (d) PDMScoated and (e) PDMS/PDA-coated sponges under simulated sunlight irradiation. PDMS/PDA1 and PDMS/PDA3 represent that the deposition cycle of PDA is 1 and 3, respectively. The deposition time of every cycle is 40 min. The sponge size is 2  2  1 cm, and the initial contact area between the sponge and the crude oil is 2  2 cm. The environment humidity is 60%. All of the experiments were executed at room temperature. Sunlight power density is about 1.5 kW m 2.

Fig. 5

A dynamic oil drawing process from water surfaces was further performed to imitate real oil spills in a marine environment. In general, the hydrophobic/oleophilic porous materials can be elegantly integrated into a pumping system to continuously remove oil spills,10,11 but they are only suitable for various light oil. Thus, we designed a homemade device to equip the self-heating sponges with a peristaltic pump by assembling it into a self-heating vacuum cleaner to realize continuous and dynamic clean-up of crude oil from water surfaces. Under the simulated sunlight irradiation, the selfheating sponges can draw the crude oil to the right beaker with the assistance of the peristaltic pump aer only 6 min (Fig. 5e and Video S1 in ESI†). With prolonged operation time, more and more crude oil is successfully collected from the water surface. As long as the self-heating vacuum cleaner keeps running, the clean-up and collection of the crude oil will continue because these sponges have a robust self-heating function and excellent reusability for the adsorption of crude oil. As a contrast, the PDMS-coated sponge shows poor ability to draw oil. It requires 21 min for the start-up of the oil collection (Fig. 5d and Video S2 in ESI†), which is 3.5 times longer than that of the self-heating sponges.

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In summary, we used a mussel-inspired strategy to render the porous sponges with a self-heating function via a robust photothermal conversion effect. Our results demonstrate that the PDA coatings can effectively heat the hydrophobic/ oleophilic sponges to a suitable temperature of 79  C and 63  C under simulated sunlight and sunlight irradiation, respectively. This in situ solar-driven self-heating mechanism is able to effectively decrease the viscosity of crude oil from 104 mPa s to 10 mPa s, signicantly facilitating the uidity and diffusion of crude oil within the porous sorbents. Moreover, the self-heating sponges are equipped with a peristaltic pump to be integrated as a self-heating vacuum cleaner to realize highly efficient continuous clean-up and collection of crude oil on water surfaces; this broadens the application scope of these oilspill sorbents from light oil to heavy oil. This study immensely inspires us to design versatile self-heating porous materials with high thermal conductivity to achieve highly efficient remediation of crude oil spills on offshore seawater surfaces.

Conflicts of interest There are no conicts to declare.

Acknowledgements Financial support is acknowledged to the Zhejiang Provincial Natural Science Foundation of China (Grant no. LZ15E030001), and the National Natural Science Foundation of China (Grant no. 21534009).

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