Multifunctional Edge-Activated Carbon Nitride Nanosheet-Wrapped Polydimethylsiloxane Sponge Skeleton for Selective Oil Absorption and Photocatalysis.

Developing green 3D porous materials integrating multitasking environmental remediation with high efficiency and reusability is considered to be a promising sustainable approach and is urgently required. Herein, we have successfully prepared a facile, ecofriendly, and robust multifunctional composite sponge of carbon nitride (CN) nanosheets wrapping an elastomer polydimethylsiloxane (PDMS) skeleton without harsh treatments. The composite sponge (CN@PDMS) exhibits excellent hydrophobic and superoleophilic properties with a water contact angle of 133.2°. This sponge also shows high selective absorption of organic solvents and oils with high recyclability after 10 absorption cycles. Furthermore, the CN@PDMS sponge has a high ability for demulsification of the oil-in-water emulsion as well. The as-prepared sponge displays high thermal stability, retaining 82.16% of its original weight up to 550 °C, and extraordinary prolonged stability in harsh corrosive solutions over 35 h compared with the pristine PDMS sponge. Additionally, the CN@PDMS sponge exhibits a high ability for adsorption and photodegradation of rhodamine B under visible light irradiation with self-cleaning and high reusability over 5 runs. Such a sustainable strategy would provide new ways for broad environmental applications.

Introduction

With the rapid pace of industrialization and maritime transport growth, water pollution has become one of the major environmental problems around the world.[1−3] This has occurred through leaking of pollutants into the water either by accident or deliberately including petroleum products, dyes, pesticides, pharmaceuticals, and so on.[4] Among these organic pollutants, oily wastewater is highly prevalent and contains harmful chemicals, which causes deterioration side effect on aquatic lives and human health itself.[5] On the other hand, the dissolved organic pollutants such as the synthetic dyes are nonbiodegradable, persistent in the environment, and recalcitrant to degradation and have usually mutagenic effects on the aquatic lives.[6,7] Recently, numerous efforts have been devoted to develop advanced techniques for removal of oily wastewater including filtration, precipitation, absorption, bioremediation, combustion, and so forth.[8,9] However, in addition to the high costs and complicated processes, these techniques suffer from the degradation ability for dissolved organic pollutants in the water matrix. From a practical standpoint of water treatment, the development of a multifunctional material dealing with the aforementioned pollutants is highly desired.

The recent decade has witnessed an increasing amount of research interest with regard to fabrication of 3D porous polymers because of their high porosity, open-celled morphology, flexibility, and ease to modify.[10,11] Of these, polydimethylsiloxane (PDMS) sponge, a highly transparent and hydrophobic silicone polymer, has been widely used in various fields owing to its extraordinary properties, such as biocompatibility, low cost, thermal stability, low density, high porosity, and good elasticity.[12−15] Since the first successful fabrication of the PDMS sponge via a sacrificial sugar templating method,[16] the PDMS sponge has been immensely developed for diverse environmental and energy applications, including oil/water separation, catalysis, sensor, and energy harvesting and conversion.[17,18] For enhancing oil/water separation, Tran et al. developed a robust and eco-friendly graphene/PDMS composite sponge by embedding graphene sheets in the PDMS sponge structure.[19] The composite sponge showed a highly efficient absorption performance for different oils and organic solvents in different oil/water mixtures. Gupta and Kulkarni synthesized gold nanoparticle–PDMS nanocomposite foam.[20] The prepared foam exhibited the ability for absorption of diesel and odorous sulfur-containing contaminants from water with high thermal and chemical stability. As a photocatalyst, TiO2–PDMS composite sponge was developed via a simple injection process of commercial TiO2 anatase onto the PDMS sponge. The composite sponge showed high adsorption and photocatalytic performance for rhodamine B (RhB) under solar irradiation.[21] However, the applicability of previous reports would be limited because of the obstacles accompanied with the intricate synthesis process, utilization of harsh solvents, high cost, or low efficiency. Thus, green fabrication of nanoparticle–PDMS composite sponge with multifunctional applications and robust properties is highly anticipated.

Recently, carbon nitride (CN), a 2D metal-free polymeric and sustainable semiconductor, has attracted intensive attention in the field of photocatalysis. It possesses various fascinating features, such as low cost, high chemical and thermal stability, unique electronic properties, and highly efficient visible light response with an appropriate band gap.[22−25] These merits endow CN with widespread applications in water splitting, CO2 conversion, photocatalytic degradation of organic pollutants, and disinfection.[26−28] However, the photocatalytic efficiency of bulk CN is still limited because of the low specific surface area and the high recombination rate of the photogenerated charge carriers, which hinder its large-scale applications.[29,30] To address these limitations, various strategies have been developed, including doping with metallic or nonmetallic elements, coupling with inorganic semiconductors, and nanostructure engineering.[31,32] Among these approaches, ultrathin fabrication of 2D nanosheets has drawn much attention owing to their high surface area, abundant active sites, excellent charge transfer, and significant inhibition of the recombination rate of photogenerated charge carriers. Thus, the photocatalytic activity of these materials is significantly enhanced under visible light irradiation.[33,34] However, CN 2D nanosheets still suffer from poor chemical and structural stability during the photocatalytic process in the field application. To overcome these weaknesses, the photocatalysts could be immobilized onto the 3D insoluble porous materials which provide more active sites and large contact surface area with the pollutants, prevent stacking or aggregation of nanosheets, and make them ease to handle and reusable for prolonged times.

For the aforementioned merits, the PDMS sponge offers an ideal 3D porous candidate for immobilization of CN nanosheets and fabrication of the heterogeneous composite system for multifunctional applications. In this work, we present the fabrication of multifunctional CN nanosheets anchored on a PDMS sponge skeleton via a facile and eco-friendly approach. The morphologies, properties, and chemical structures of the as-prepared sponges were characterized, and the thermal and chemical stability of the as-fabricated sponges were also investigated. The synthesized CN@PDMS sponge was tested for oil/water separation and demulsification of an oil-in-water emulsion. Afterward, the photocatalytic performance and regeneration of the composite sponge were evaluated with the RhB degradation under visible light irradiation. The findings in this study elucidate that this novel composite sponge has the potential for broad wastewater treatment applications.

Results and Discussion

Fabrication and Characterization of As-Prepared Sponges

The schematic fabrication of the pristine and composite sponges is illuminated in Figure . The pristine sponge was fabricated using the sugar templating method, whereas ultrathin CN nanosheets with enriched active sites were synthesized by a green and facile approach in the wet atmosphere. This strategy depends on the coupling of melem segment polymerization at high temperature and delamination of condensed layers with water molecules entirely in one-pot without any other additives.[35] The as-synthesized CN appears as a foam-like white powder with high specific surface area (96 m2 g–1). The composite sponge was fabricated via a simple injection process and thermal annealing at 200 °C to remove the adsorbed water and enhance the attachment between the nanosheets and the PDMS surface. After annealing, the composite sponge appeared off-white, retaining its elastomer properties.

Figure 1

Schematic illustration of the green synthetic process of CN nanosheets and CN@PDMS sponge.

The morphological structure of the as-prepared sponges was characterized by scanning electron microscopy (SEM). As shown in Figure a, the SEM image of the PDMS sponge demonstrates a 3D interconnected porous framework with an average pore size of 150 μm. The high magnification view of Figure b,c reveals that the pristine PDMS sponge has a very smooth surface. For the CN@PDMS sponge, the SEM images clarify the stability of wrapped CN nanosheets on the sponge surface after thermal annealing and rinsing with ethanol and show the uniform coating of nanosheets on the sponge skeleton with roughness structure (Figure e–g). Furthermore, the composite sponge after the thermal annealing exhibits excellent compliance and springiness after manual compression and it can perfectly retrieve its original shape without breaking apart (Figure S1).

Figure 2

(a) SEM images of the pristine PDMS sponge with magnification (b,c). (e) SEM images of the CN@PDMS sponge with magnification (f,g). (d,h) EDS of the pristine and the composite sponges, respectively; inside tables are the related elemental analysis for the as-synthesized sponges.

To confirm the elemental composition of the pristine and coated PDMS sponges, energy dispersive X-ray spectroscopy (EDS) and elemental analysis were conducted. As described in Figure d, The EDS analysis shows that the pristine sponge comprises Si, C, O, and traces of nitrogen (nitrogen traces may be residues from the sugar template), which are basic elements in the PDMS polymer. On the other hand, the amount of C, N, and O increases in the CN@PDMS sponge, confirming the introduction of CN nanosheets on the surface of PDMS sponge (Figure h). The elemental mapping images manifest the distribution of even elements on the surface of the pristine and coated sponges, as illustrated in Figure , further clarifying the good distribution of CN nanosheets on the sponge surface.

Figure 3

Elemental mapping images of (a) PDMS sponge and (b) CN@PDMS sponge for C, N, O, and Si elements.

To investigate the active groups in the as-fabricated materials, Fourier transform infrared (FTIR) spectral analysis was carried out, and the spectra of melem and CN nanosheets are shown in Figure S2. For melem, there are 3 distinguished peaks at 1612, 1467, and 803 cm–1, which ascribe to the characteristic absorption of melem.[36−38] Furthermore, the broad band centered at 3129 cm–1 is related to uncondensed terminal amino groups. The characteristic peaks of CN nanosheets confirm the formation of CN with enriched active sites. The peak at 810 cm–1 refers to bending vibration of triazine rings. The intensive band between 1200 and 1638 cm–1 regions is attributed to typical stretching vibration modes of either bridging (−C–NH–C−) or trigonal N–(C3) in the s-triazine heterocyclic ring (C6N7) units. Furthermore, the broad band between 2900 and 3600 cm–1 indicates the enriched active edges of −NH and −OH stretching. For the PDMS sponge, the broad band centered at 1007 cm–1 is assigned to −Si–O–Si– stretching vibration (Figure a), whereas the peak at 2962 and 1257 cm–1 and band centered at 786 cm–1 are referred to symmetric bending of the methyl groups (−CH3) in Si–C bonds.[39,40] For the CN@PDMS sponge, a new broad band appears between 1287 and 1705 cm–1 (the black dashed rectangle), referring to the successful introduction of tri-s-triazine heterocyclic ring units on the surface of the sponge (Figure a). Furthermore, a new observed band between 3014 and 3376 cm–1 (the red dashed rectangle) is attributed to terminal active groups on the CN polymer, asserting the activation of the composite sponge surface by the active sites. However, this band is rather weak probably because of the elimination of most hydroxyl groups after annealing at 200 °C or attaching with the oxygen molecules in the PDMS polymer via hydrogen bonds.

Figure 4

(a) FTIR spectra and (b) thermogravimetric analysis (TGA) of the pristine and composite PDMS sponges.

TGA was conducted to examine the stability and regeneration of the as-synthesized sponges. As illustrated in Figure b, the PDMS sponge is stable until 220 °C and then shows a gradual decomposition rate up to 420 °C. This weight loss may be due to the depolymerization of PDMS chains to volatile oligomers.[41] In contrast, the CN@PDMS sponge shows high thermal stability up to 420 °C without an observed decline. Furthermore, 43.89% of the PDMS sponge weight is lost (black and red arrows) at 550 °C, whereas merely 17.84% of the CN@PDMS sponge weight is decomposed (black arrow). The CN@PDMS sponge exhibits a much broader thermal stability compared with the gold nanoparticle–PDMS nanocomposite foam (Au–PDMS) as reported before.[20] The thermal stability of Au–PDMS foam sharply decomposed at 506 °C with a total weight loss of 71%. The total weight loss for the CN@PDMS sponge (57.87%) is lower than that of the pristine sponge (62.23%) at 800 °C, confirming the ability of CN nanosheets to improve the thermal stability of PDMS sponge at high temperatures because of the inherent thermal stability of the heptazine-based CN polymer.[42]

Oil Absorption and Oil-in-Water Emulsion Separation

To investigate the absorption capacities of synthesized sponges, various kinds of organic solvents and oils were employed and the as-prepared sponge was immersed in the solution for a minute. During this period, both of the pristine and composite sponges were quickly wetted and swelled by the oil and no dripping of the absorbed oil was observed after the handling process. Then, the absorption capacity was calculated using the weight percentage (wt %) of the absorbed oil weight to the own weight of the sponge. As shown in Figure a, the absorption capacities of the pristine and composite sponges are almost close and ranged between 170 and 864 wt %. This range is in accordance with the as-fabricated PDMS/graphene composite sponge with the range from 220 to 800 wt %.[19] The insignificant change in the absorption capacity between the pristine and composite sponges may be attributed to the fact that the CN nanosheets’ thin layer coated on the PDMS surface does not affect the porosity of the composite sponge. As can be seen in the inset of Figure a, a water droplet can stand freely on the surface of the CN@PDMS sponge with a high contact angle (133.2°), whereas the oil droplet can absolutely wet its surface, affirming the excellent hydrophobic and superoleophilic properties of the composite sponge. On the other hand, the pristine PDMS sponge exhibits lower wettability with a contact angle of 116.3°.

Figure 5

(a) Absorption capacity of the as-synthesized sponges; insets show water and n-hexane droplets colored with methylene blue and Sudan red, respectively, and the contact angle of a water drop (133.2°) on the surface of the CN@PDMS sponge. (b) 10 cycles of testing the n-hexane and chloroform absorption capacities using the composite sponge.

The recyclability of the CN@PDMS sponge is crucial to its widespread application. As lighter and heavier solvents than water, n-hexane and chloroform were selected to test the reusability, respectively. As shown in Figure b, the CN@PDMS sponge exhibits excellent reusability performance over 10 times with no observed change in the absorption capacity. Furthermore, the absorbent sponge could be recycled again via simple squeezing, washing with ethanol, and finally drying in the oven because of its intrinsic elastic nature. In spite of the swellable property of the CN@PDMS sponge during the absorption and recyclability tests, the morphology of the composite sponge after 10 absorption runs of n-hexane and chloroform shows high stability of CN nanosheets on the PDMS skeleton (Figure S3). The high fixability of the CN nanosheets on the PDMS surface may be attributed to their intrinsic hydrophobicity which makes them attach well with the terminal hydrophobic parts of the PDMS chain. Moreover, the enriched CN edges with active groups such as amino and hydroxyl groups may play another role for enhancing the fixability via the adsorption with oxygen molecules in the (−O–Si–O−) repeated units through hydrogen bonds.

It is of great importance to test the prepared sponges under a harsh condition taking into account the complex practical environments. Prolonged exposure of the pristine and composite sponges for corrosive aqueous liquids (2 M HCl, 2 M NaOH and saturated NaCl) has been investigated, as exhibited in Figure . After immersing the as-synthesized sponges in the strongly acidic and alkaline solutions, the wettability of the PDMS sponge is decreased to approximately 95° after 35 h. To understand the wettability changing of the PDMS sponge after acidic and basic treatment, the FTIR spectral analysis was conducted (Figure S4). At an absorption peak of 3440 cm–1 (Figure S4b), the peak intensity of NaOH treatment is stronger than that of the pristine sponge, which confirms the effect of alkaline corrosion on the etching of the PDMS surface with introducing more hydroxyl groups. In addition to the alkaline etching, HCl also has a distortion effect on the PDMS surface after 35 h. This is affirmed by the decline of symmetric bending peaks of the methyl groups in Si–C at 1257 and 2962 cm–1 (Figure S4c,d). The corrosive effects of the acidic and alkaline treatment were further established via SEM images (Figure S5). In contrast to its smooth surface, the PDMS surface morphologies change significantly after the treatment, retaining its skeleton unchanged. The SEM images confirm the etching effect on the surface of PDMS with sinuous and roughness morphology and dense pitting. These results have also been observed by Zhang et al. after treating the PDMS sponge with 5 M NaOH for 6 h at 80 °C.[43] On the other hand, the CN@PDMS sponge shows outstanding stability over 35 h after acidic and alkaline treatment, maintaining the contact angle for about 120°. This stresses that CN does not only enhance the wettability and thermal stability of the PDMS sponge but also improve the chemical stability for prolonged times. The enhanced chemical stability is owing to the strong bonding between conjugated tri-s-triazine polymers, which far protects the surface of PDMS from wettability changing. The stability of CN nanosheets on the surface of the PDMS polymer during the prolonged harsh treatment is confirmed by SEM images as illustrated in Figure S6. Additionally, the FTIR spectra further emphasize that the PDMS sponge still retains its activation surface via anchoring of CN nanosheets on its surface (Figure S7). In a neutral medium, the PDMS and CN@PDMS sponges show excellent stability and the contact angles remain unaltered with no obvious decline in the contact angle after 24 h (Figure c).

Figure 6

Water contact angles after immersing the pristine and composite sponges at various times in the harsh conditions: (a) 2 M HCl, (b) 2 M NaOH, and (c) saturated NaCl.

To test the potential of the CN@PDMS sponge for selective absorption of oils from water, a piece of the composite sponge was immersed in the n-hexane/water mixture and the solvent was quickly absorbed by the sponge and completely soaked up in 35 s as illustrated in Figure a and Video S1. Also, the chloroform drops sunk at the bottom of the water were immediately absorbed by the composite sponge merely in 10 s as shown in Figure b and Video S2. Furthermore, the composite sponge exhibits high hydrophobic and superoleophilic properties surrounding by trapped water bubbles (mirror-like surface) and no water drops were absorbed (Figure b and Video S2).

Figure 7

Snapshots of the composite sponge (CN@PDMS) during absorption of (a) n-hexane and (b) chloroform from water.

In addition to the oil/water immiscible mixtures, oil-in-water emulsions resulted from diffused surfactant stabilizers are widely available in oily wastewater which requires high cost and complex treatments.[44] Here, we simulated the oil-in-water emulsion using toluene as an oil and Tween 20 as an emulsifier in water. The emulsion solution was still stable over 1 week, as seen in Figure S8, and appears white milky color. After immersing the composite sponge for 1 h, all the oil absorbed and the solution became transparent. The oil-in-water emulsion separation was confirmed by the optical microscopy images. As illustrated in Figure , the emulsion solution comprises numerous toluene droplets and no droplet is observed after adding the sponge. The ability of the composite sponge for demulsification may be attributed to the excellent hydrophobic properties of the composite sponge and further enhancement of the adsorption of toluene molecules by active groups on the CN nanosheets. These findings offer clear evidence that the CN@PDMS sponge provides a promising and clean approach for highly efficient oily wastewater treatment.

Figure 8

Optical microscopy photographs of toluene drops in the emulsion before (left) and after (right) separation.

Photocatalytic Performance

To investigate the adsorption and photocatalytic activity of the CN@PDMS sponge toward azo dyes, RhB is used as a model. For the pristine PDMS sponge, it is known that the PDMS polymer has a high ability to adsorb small organic molecules such as RhB through its nonpolar hydrophobic end.[21] Here, the PDMS sponge shows good ability for adsorption of RhB with an adsorption of 19.17% after 1 h and slightly increases after another 1 h. For the CN@PDMS sponge, it shows a higher ability to adsorb RhB than the pristine sponge in the absence of light irradiation as illustrated in Figures a and S9, with an adsorption of 49.39% of RhB. The higher adsorption of RhB on the composite sponge over the pristine sponge may be attributed to the activation of the PDMS surface via enriched active groups of CN nanosheets. This finding is also reported by Zhang et al. where they observed that the addition of CN with graphene oxide (GO)-wrapped melamine sponge exhibited the high adsorption capacity for RhB compared to the GO-wrapped sponge and the pristine sponge.[45]

Figure 9

(a) Degradation efficiency of RhB under visible light irradiation with the pristine and composite sponges. (b) Recyclability of CN@PDMS sponge for the degradation of RhB under visible light irradiation.

After adsorption of RhB by the sponges in the dark, the solution is exposed to visible light irradiation for 1 h. The RhB solution shows negligible photodegradation under irradiation. Also, the pristine sponge does not show photodegradation activity, and the slight decline after irradiation may be due to further adsorption of RhB on the surface of the sponge. For the composite sponge, it exhibits a high photocatalytic performance under visible light and 98% of RhB degrades after 45 min and completely degrades after 1 h (Figure a). The high photocatalytic activity of the composite sponge is achieved because of the unique structure of the as-synthesized CN in the wet atmosphere with a high surface area and enriched active sites. The CN@PDMS sponge shows higher RhB photodegradation efficiency under visible light compared to the TiO2–PDMS composite sponge where 80% of RhB was removed over TiO2–PDMS after overnight adsorption in the dark and then exposure to solar irradiation for 1 h.[21]

The reusability of the CN@PDMS sponge was also evaluated, and five consecutive photocatalytic runs were measured. The composite sponge displays extraordinary stability over 300 min adsorption in the dark and 300 min visible light irradiation (Figure b). After five runs, the sponge retains its high performance for RhB degradation with 96.5% efficiency, suggesting excellent potentials for the field application. The stability of CN nanosheets on the PDMS surface is asserted by the SEM image and FTIR analysis, as shown in Figures S10 and S11, respectively.

This excellent regeneration is attributed to the self-cleaning property of the CN@PDMS sponge. After immersing the composite sponge in the RhB solution for 1 h, it is exposed to visible light irradiation for another 1 h without any solvent (Figure ). After that, the composite sponge turns visibly to its original appearance. By virtue of the enhanced photocatalytic performance of CN nanosheets, these nanosheets on the sponge surface endow it with the ability to degrade residues of the RhB molecules. Such a self-cleaning feature under visible light promotes further adsorption of RhB and bears it the recyclability several times with the ease of regeneration.

Figure 10

Self-cleaning feature of CN@PDMS sponge after adsorption of RhB and exposing for 1 h under visible light irradiation.

The enhanced composite sponge surface can be explained with UV–visible diffuse reflectance spectra, as shown in Figure S12. Obviously, the PDMS sponge exhibits only UV absorption performance with an absorption edge of 322 nm, and the band gap accordingly is calculated and shows a wide band gap (3.84 eV). After the introduction of CN, the CN@PDMS sponge shows an enhanced visible light absorption over the range of 450–700 nm with an absorption edge of 432 nm and the band gap subsequently decreases to 2.9 eV. Furthermore, the XPS valence band (VB-XPS) spectrum of CN@PDMS was recorded to deduce the valence band (VB) value to be 2.43 eV. Thus, the conduction band (CB) is calculated to be −0.47 eV which is more negative than that of the reduction potential of O2/•O2– (−0.28 eV).[45]

To elucidate the dominant reactive species generated in the photocatalytic process of RhB degradation, the trapping experiments were carried out with the addition of p-benzoquinone (BQ, 1 mM), isopropanol (IPA, 1 mM), and triethanolamine (TEOA, 1 mM), separately, as scavengers for superoxide (•O2–), hydroxide (•OH), and holes (h+) radicals, respectively. As illustrated in Figure , the addition of TEOA and BQ exhibits the higher inhibition of RhB photodegradation with a suppression rate of 36.76 and 34.8%, respectively, whereas the addition of IPA exhibits the lower suppression with a decline rate of 4.77%. These findings confirm that the holes and •O2– radicals are the major reactive species in the photocatalytic process.

Figure 11

Photocatalytic activity of the CN@PDMS sponge for the degradation of RhB in the presence of different scavengers.

On the basis of the abovementioned results, the possible mechanism of RhB degradation is well-aligned with the adsorption of RhB on the surface of the composite sponge. After absorption of photons from the light source by the photocatalyst, the electron is excited from the valence band to the CB and forms electron–hole pairs. The excited electron has the ability to reduce the dissolved oxygen to produce superoxide radicals which in turn degrade and mineralize the RhB molecules. At the same time, the produced active holes as oxidizing agents also participate in the photodegradation of RhB by means of the oxidation process. Importantly, owing to its high transparency, the PDMS polymer plays a key role in simulating the photocatalytic process.[18,46] The transparent PDMS sponge works as a light-absorbing and -intensifying material and lets the incident light smoothly permeate through it, whereby the photogenerated charge carriers are inducibly produced and the photocatalytic activity is subsequently enhanced. The overall reaction of the RhB photodegradation can be summarized as in the following equations

It is worth mentioning that there are two pathways for RhB photodegradation over the photocatalyst: N-deethylation and decomposition of the conjugated xanthene ring in RhB.[47,48] According to the UV–visible spectra of RhB degradation over the composite sponge (Figure S9a), there is no a hypsochromic shift during 30 min, indicating the high efficiency of the composite sponge for destructing the xanthene ring and RhB mineralization. Afterward, a slight blue hypsochromic shift is observed after 30 min, suggesting the formation of N-deethylation species.[49] Furthermore, in addition to the high adsorption feature of the PDMS elastomer for RhB, the nucleophilic active sites on the CN edges may play an important role in further enhancing the degradation of RhB molecules. This enhanced degradation may occur via the electrostatic attraction between the lone pairs of these groups and the cationic part of RhB and through hydrogen bonds between carboxyl groups in RhB and the active groups.

Conclusions

We demonstrated a green fabrication of the CN nanosheet-doped PDMS sponge skeleton via simple injection and annealing without tedious or harsh conditions. The CN@PDMS sponge illustrated extraordinary features including excellent thermal and chemical stability over harsh corrosive conditions owing to the inherent fascinating durability of CN nanosheets. The composite sponge showed fast and aloft selective absorption of various oils from water, highly efficient oil-in-water emulsion separation, and high reusability. Furthermore, the as-fabricated sponge showed high adsorption and photocatalytic performance toward RhB removal. Additionally, the prepared sponge showed high stability and recyclability of RhB photodegradation with a self-cleaning feature as well. These findings provide clear evidence that the CN@PDMS sponge with such multifunctional and robust properties can be scaled up and employed as a multitasking platform for broad environmental applications.

Experimental Section

Materials

PDMS resin and curing agent (Sylgard 184) were purchased from Dow Corning Corporation, Midland, United States. Melamine, RhB, and TEOA were purchased from Aladdin, China. Vegetable oil was purchased from a local market. IPA and other oils used in the separation experiments were purchased from Sinopharms, China. BQ was obtained from Shanghai Macklin Biochemical Company, China. All chemicals were used as received without further purification.

Fabrication of CN@PDMS Sponge

The synthesis of the ultrathin CN nanosheets was fabricated as reported in our previous report.[35] Simply, melamine was condensed to melem, and then, the fine melem powder was moved to the tube furnace and heated to 550 °C for 4 h under the wet atmosphere. After cooling to room temperature, the ultrathin foam-like CN nanosheets were obtained. The preparation of PDMS sponge was achieved via the conventional sugar templating method as in the previous reports.[15,16] Briefly, 5 g of PDMS was mixed well with 0.5 g of the curing agent (10:1 mass ratio) in a Petri dish, and then, the air trapped was degassed under air vacuum until all bubbles were removed. Sugar cubes were immersed in the polymer and then placed in a vacuum chamber to further promote the diffusion of the polymer through sugar particles. After curing at 100 °C for 2 h, the cubes were sonicated in a water bath to remove the sugar particles and obtain the PDMS sponge. The CN@PDMS sponge was fabricated by dispersing 50 mg of CN powder in 40 mL of IPA with ultrasonication for 30 min. The well-dispersed solution was injected into the PDMS sponge using a syringe and then dried in the oven at 60 °C for 1 h. Then, this process was repeated three times to ensure the good dispersion of the nanosheets on the PDMS skeleton. Finally, the composite sponge was annealed at 200 °C for 2 h to enhance the hydrophobicity and adsorption of CN nanosheets on the surface of PDMS. The composite sponge was rinsed thoroughly with ethanol to remove the unanchored CN nanosheets and air-dried overnight.

Characterization

The morphology of the as-prepared sponges was imaged using a scanning electron microscope (Phenom Pro). Water contact angles were measured on a contact angle meter (OCA, DataPhysics) at room temperature. FTIR spectra were recorded on a Nicolet 6700 FTIR spectrometer (Thermo Fisher Scientific). TGA was conducted under a nitrogen atmosphere from 25 to 800 °C (TA TGA Q5000), and elemental analysis was performed using an elemental analyzer (Elementar Vario EL Cube). Energy dispersive spectra of the as-synthesized sponges were recorded using an energy dispersive spectrometer (Bruker QUANTAX).

Oil Absorption Capacity

The pristine and composite PDMS sponges were cut to small pieces and used for testing oil absorption capacity. The as-synthesized sponges were immersed in various oils and organic solvents for about 1 min and quickly weighed to avoid evaporation of the oil. The absorption capacity is calculated from the following equationwhere C0 is the initial weight of the sponge and C is the weight of the sponge after absorption.

Oil-in-Water Emulsion Separation

The oil-in-water emulsion was formed by mixing water, toluene, and the emulsifier Tween 20 in the mass ratio 99:1:0.05, respectively, and ultrasonicated for 15 min. After immersing the sponge in the emulsion for 1 h, the emulsified oil drop removal was detected under an optical microscope (Shanghai Cewei Photoelectric Technology).

Photocatalytic Test

The photodegradation of RhB as a dye pollutant was evaluated under visible light irradiation. In detail, one piece of the as-synthesized sponge (2 × 2 × 1 cm3) was cut to pieces and immersed in 10 mL of RhB (10 mg L–1) in a quartz tube. Prior to the irradiation, the sponge was allowed to come into contact with RhB solution in the dark for a certain time to achieve adsorption–desorption equilibrium. After that, the solution was irradiated using a 300 W xenon lamp (PLS-SXE300C, Perfect Light Limited, Beijing) with a 420 nm cutoff filter-provided visible light irradiation, and the distance between the light source and the sample was 12 cm. Aliquots of RhB solution were collected every 15 min. The concentration changes of RhB were measured using a UV–visible 6000 spectroscope at a wavelength of 554 nm.