An aerogel adsorbent with bio-inspired interfacial adhesion between graphene and MoS2 sheets for water treatment

Journal Pre-proofs Full Length Article An aerogel adsorbent with bio-inspired interfacial adhesion between graphene and MoS2 sheets for water treatment Wuqing Zhu, Yinlei Lin, Wanwen Kang, Haiyan Quan, Yuyuan Zhang, Menglei Chang, Kun Wang, Min Zhang, Weibin Zhang, Zhiqiang Li, Hongyang Wei, Ting Fan, Dongchu Chen, Huawen Hu PII: DOI: Reference:

S0169-4332(20)30473-6 https://doi.org/10.1016/j.apsusc.2020.145717 APSUSC 145717

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

18 January 2020 30 January 2020 9 February 2020

Please cite this article as: W. Zhu, Y. Lin, W. Kang, H. Quan, Y. Zhang, M. Chang, K. Wang, M. Zhang, W. Zhang, Z. Li, H. Wei, T. Fan, D. Chen, H. Hu, An aerogel adsorbent with bio-inspired interfacial adhesion between graphene and MoS2 sheets for water treatment, Applied Surface Science (2020), doi: https://doi.org/ 10.1016/j.apsusc.2020.145717

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An aerogel adsorbent with bio-inspired interfacial adhesion between graphene and MoS2 sheets for water treatment

Wuqing Zhua, Yinlei Lina, Wanwen Kanga, Haiyan Quana, Yuyuan Zhanga, Menglei Changa, Kun Wanga, Min Zhanga, Weibin Zhanga Zhiqiang Lia, Hongyang Weia Ting Fana, Dongchu Chena,*, Huawen Hua,b,c,*

a

School of Materials Science and Energy Engineering, Foshan University, Foshan, Guangdong

528000, China. b

Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development,

Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China c

Guangdong Provincial Key Laboratory of Industrial Surfactant, Guangdong Research Institute of

Petrochemical and Fine Chemical Engineering, Guangzhou 510640, China

Corresponding Emails: [email protected]; [email protected]

1

Abstract

The increasing environmental pollution calls for the development of stable functional materials for efficiently binding contaminants, while they can be synthesized and recycled easily. Herein, a three-dimensional (3D) aerogel is constructed by bio-inspired adhesion of graphene and MoS2 sheets with polydopamine (PDA) through a one-step hydrothermal route for the adsorption of the water-soluble organic contaminants. The impact of the bio-inspired interfacial adhesion on the resulting composite aerogel is thoroughly investigated, e.g., the interfacial PDA layer renders the composite aerogel considerably more porous, together with the much higher specific surface area and pore volume, as well as strikingly smaller average pore size and superior stability upon exposure to air, relative to the counterpart without the bio-inspired adhesion. The 3.2 wt.% PDA composition is adequate to yield a composite structure with small MoS2 nanocrystallites uniformly dispersed over the modified graphene surface without aggregation. The adsorption of methylene green onto such a 3D composite architecture is spontaneous and endothermic and obeys the Langmuir isotherm model and pseudo-second-order kinetics, with the maximum adsorption capacities over 200 mg/g at all the operating temperatures and the satisfactory recycling properties without significant degradation after 5 cycles. Keywords: graphene; MoS2; aerogel; composite; interfacial; adsorption

2

1. Introduction

The fast development of various industries brings increasing global contamination problems, which are posing a severe threat to the ecological system and public healthcare [1, 2]. Functional materials, capable of removing contaminants, are in urgent demand. Many remediation methods, based on various functional materials, have been developed to deal with the water contaminants [3-5], such as electrochemical treatment [6], photocatalysis [7], biological degradation [8], flocculation [9], advanced oxidation [10], nanofiltration [11], and adsorption [12]. Among them, adsorption is considered to be the most feasible one due to its ease of operation, low cost, high efficiency, and no secondary pollution [4]. Although a variety of adsorbents have been reported to handle different kinds of water contaminants, most of them are in the powder form, making them extremely difficult to recycle since high-speed centrifugation and vacuum filtration are generally needed for the recycling. Therefore, the adsorbents bearing a three-dimensional (3D) macroscopic structure are highly desirable, especially hydrogels and aerogels [13-16]. Aerogels have been captured much attention over recent decades owing to their self-standing structure, an extensive network of channels, large specific surface area, and multidimensional mass transport pathways [17]. Diverse materials have been exploited for the construction of 3D aerogels. Typically, the world's lightest carbon aerogel was prepared using carbon nanotube as the ribs and graphene sheets as the cell walls, which exhibited remarkable adsorption capacity for oil contaminants [16]. Besides, hydroxypropyl methylcellulose [18], ZrO2 and TiO2 [19], TiO2-SiO2 combined with transition metals [20], and a cellulose nanofibril loaded with graphene oxide (GO)-Fe (III) [21] were employed to synthesize aerogels for water treatment. Despite the progress in the fabrication of aerogel adsorbents, carbon-based aerogels have rarely been reported for the satisfactory adsorption of water-soluble 3

contaminants, and there remains a pressing need for the development of novel carbon-based aerogel adsorbents with high efficiency and convenient recycling properties for processing water-soluble contaminants. Graphene-based aerogels are usually explored to tackle oily contaminants due to the hydrophobicity of the graphene surface [16, 22-26]. The modification of the graphene structure is thus essential to construct a graphene-based aerogel for processing water-soluble contaminants. The most frequently adopted strategy is to prepare GO incorporating numerous oxygen functionalities. However, the active oxidation conditions result in GO with the sp2-conjugated structure damaged to a substantial extent, considerably sacrificing the fascinating 2D in-plane structure of graphene, e.g., the damaged conjugation structure weakens the π-π stacking interactions with aromatic contaminants. As a result, GO is generally used as a precursor to synthesize diverse functional materials. Before environmental applications, GO is usually converted to reduced graphene oxide (RGO) with restored in-plane conjugation structure. Nature-inspired simultaneous reduction and surface modification of GO with dopamine (DA) is an effective and environmentally benign way to produce a surface-modified RGO. The polydopamine (PDA) layer generated on the RGO surface is nano-thin [27], revealing that the properties of RGO would not be blocked [14, 27-32]. To the best of our knowledge, most of the materials involving the PDA-modified RGO (named as PG hereafter) are in the powder form [28-32], and it is, therefore, highly desirable to explore a PG-based 3D architecture to overcome the weakness from which the powdery materials are suffered. Considering that molybdenum disulfide (MoS2) also has the 2D planar structure (similar to graphene) and unique physicochemical properties [33], it was employed to construct a 3D ternary composite having the maximum interfacial contact area between graphene and MoS2 sheets as a 4

result of the 2D-2D stacking. The interactions between MoS2 and RGO can also avoid the common aggregation problem encountered by graphene to some extent. Although composites of GO (or RGO) and MoS2 have been explored for many applications [34-45], the interfacial adhesion between them remains underexplored. For example, MoS2-graphene hybrids were fabricated by hydrothermal and calcination treatments and employed for capacitive deionization [34], a counter electrode of dye-sensitized solar cells [46], water splitting and lithium storage [35], and microwave absorption [36]. All of these studies were devoted to the construction of Van der Waals's stacks of MoS2 and graphene sheets, and the interfacial interactions between MoS2 and graphene were thus primarily dictated by the Van der Waals force with limited strength [34, 35, 38, 46]. A lack of strong interfacial interactions between RGO and MoS2 would lead to the instability of the binary composite of MoS2 and RGO as caused by the aggregation of MoS2 and RGO sheets. The novelty of this work lies in the bio-inspired fabrication of a novel composite aerogel (consisting of MoS2 and RGO sheets with the strengthened interfacial adhesion given by PDA) for the adsorption of water-soluble organic contaminants, with ease of handling and recycling. The bio-inspired preparation process can be realized easily through a one-step hydrothermal reaction, without the energy-consuming calcination treatment, thus having advantages over the complex multi-step preparation process [45]. The role that PDA played in yielding a stable ternary composite aerogel was disclosed through an in-depth comparison between the graphene/MoS2 composites with and without the interfacial PDA layer. The microstructures of the prepared binary MoS2/RGO composite aerogel (termed as MG hereafter) and ternary MoS2/PDA/RGO composite aerogel (termed as PMG) were compared by various characterization techniques and systematic adsorption experiment. A toxic and carcinogenic nitro-aromatic compound (i.e., methylene green) [47] was 5

selected as a typical water-soluble organic contaminant to investigate the adsorption kinetics, isotherms, and thermodynamics. As a synthetic dye, methylene green is widely used in various industries including textiles, paper, carpets, ceramics, leather, printing, pigments, and cosmetics, etc. [48, 49], which could pose a long-term threat to the living organisms relying on water in that its recalcitrant structure makes it resistant to the degradation by nature [50]. This study opens up a new avenue for the construction of various composite materials with adequate interfacial interactions between different kinds of functional compositions for a broad spectrum of applications even beyond environmental remediation.

2. Experimental

2.1. Materials

Dopamine hydrochloride (98%) was supplied by Sigma-Aldrich. Extra-pure graphite fine powder was obtained from Tianheng Technology Co., Ltd., Hong Kong. Anhydrous sodium molybdate (AR grade) was obtained from Shanghai Aladdin Bio-Chem Technology Co., Ltd. Tris(hydroxymethyl) aminomethane (AR grade) and L-cysteine (AR grade) were purchased from Beijing Innochem Science & Technology Co., Ltd. Methylene green (AR grade) was supplied by Shanghai Adamas Reagent Co., Ltd. All of the other chemicals were analytical reagent and used without further purification unless otherwise stated.

6

2.2. One-step preparation of PG, MG, and PMG

2.2.1. Synthesis of PG

The GO was synthesized by a modified Hummers’ method as reported elsewhere [51, 52]. The synthesized GO (100 mg) was mixed into deionized water (50 mL) by sonication treatment for 1 h. The monomer DA (50 mg) and a tris(hydroxymethyl) aminomethane solution (20 mL, 50 mM) were subsequently added, followed by a hydrothermal reaction at 200 oC for 12 h. The reaction solution was cooled down, vacuum-filtered, and then thoroughly washed with deionized water. The collected filter cake was finally freeze-dried and kept in a desiccator before use.

2.2.2. Preparation of MG

The prepared GO powder (100 mg) was homogenized into deionized water (50 mL) by sonication treatment for 1 h, and then anhydrous sodium molybdate (300 mg) and L-cysteine (1.2 g) were mixed by magnetic stirring. Afterward, a tris(hydroxymethyl) aminomethane solution (20 mL, 50 mM) was added. The mixture was then hydrothermally treated at 200 oC for 12 h. After cooling down, it was vacuum-filtered, thoroughly washed with deionized water, and finally freeze-dried. The obtained MG sample was kept in a desiccator before use.

2.2.3. Preparation of PMG

The GO powder (100 mg) was mixed into deionized water (50 mL) by sonication treatment for 1 h, and then anhydrous sodium molybdate (300 mg), L-cysteine (1.2 g) and DA (50 mg) were stepwise introduced. After the mixture had been homogenized by magnetic stirring, a tris(hydroxymethyl) aminomethane solution (20 mL, 50 mM) was added to adjust the pH to around 7

8.5, and then the mixture was subject to a hydrothermal reaction at 200 oC for 12 h. The reaction solution was cooled down, suction-filtered, thoroughly washed, and finally freeze-dried.

2.3. Characterizations

The surface morphologies of the prepared samples were characterized using an S-4800 field-emission scanning electron microscope (SEM, Hitachi, Japan), equipped with an energy-dispersive X-ray (EDX) detector. A JEM-2100F field-emission transmission electron microscope (TEM, Japan), equipped with selected area electron diffraction (SAED), was used to further clarify the structural features of the prepared GO, MG, and PMG specimens. For the preparation of the TEM specimen, an ethanol dispersion of the sample was withdrawn and deposited onto a copper grid, followed by air-drying. X-ray diffraction (XRD) patterns were captured using a Bruker D8 Advance X-ray diffractometer (Bruker AXS, Karlsruhe, Germany). The functional groups of the GO, MG, and PMG samples were analyzed using a Fourier transform infrared (FTIR) spectrometer (Thermo Fisher, USA). The chemical states of the samples were analyzed using an XSAM800-XPS X-ray photoelectron spectrometer (XPS) with a Ma-Kα X-ray source. The N2 adsorption-desorption isotherms and the pore size distribution plots were obtained using an auto fast specific

surface

area

analyzer

(Chemstrat

ASIQMO002-2,

USA)

based

on

the

Barrett-Joyner-Halenda (BJH) approach. For the adsorption experiment, the monitoring of the dye concentration was carried out using an Ultraviolet-Visible (UV/Vis) spectrophotometer (UU-1800, Rigaku, Japan). Raman spectra were recorded using a Witec alpha 300 conformal Raman microscope under the 532 nm laser excitation. Atomic force microscopy (AFM) image was captured using a 8

Bruker Dimension Icon System (Bruker Biosciences, Billerica, Massachusetts).

2.4. Adsorption experiment

2.4.1. Comparison of the adsorption efficiency between the prepared MG and PMG samples

Considering that the present research focuses on the fabrication of a 3D self-standing aerogel as the adsorbent to remove organic contaminants, the adsorption performance of the PG sample in the powder form was not investigated since the PG sample could be homogenously and stably dispersed in water media, making it extremely challenging to manipulate. We only placed the attention to the MG and PMG samples, and both of them could be easily handled for the adsorption test. The detailed procedures were presented as follows for the comparison of the adsorption efficiency between the MG and PMG samples. A given mass (15 mg) of the MG or PMG sample was mixed into an aqueous solution of methylene green (50 mL, 100 mg/L), which was then subject to the oscillation at 323 K. At given intervals, the dye solution was withdrawn for the UV/Vis spectroscopic measurements, and the typical absorbance intensity at 617 nm was recorded. The Lambert-Beer law facilitated the conversion of the UV/Vis absorbance to the concentration of the dye molecules that remained in the solution, and consequently, the adsorption amount could be calculated by Eq. 1: 𝑞𝑡 =

𝐶0 ― 𝐶𝑡 𝑚

(1)

×𝑉

where C0 is the initial dye concentration, Ct represents the dye concentration at adsorption time t, m is the mass of the prepared adsorbent, V is the volume of the solution used for the adsorption test, and qt is the amount of the dye adsorbed by the sample at adsorption time t. 9

2.4.2. Test on the isothermal adsorption

The isothermal adsorption was investigated at different temperatures including 303, 313 and 323 K. Typically, 7 portions of the PMG samples (each of 3 mg) were added into a series of methylene green solutions with a range of concentrations including 30, 40, 50, 60, 80, 100 and 120 mg/L. The mixtures were subsequently placed into a thermostatic oscillation incubator for the adsorption processing for 24 h at 303, 313 or 323 K. The equilibrium adsorption amount (qe) was calculated based on Eq. 2. 𝑞𝑒 =

𝐶0 ― 𝐶𝑒 𝑚

(2)

×𝑉

where C0 is the initial dye concentration, Ce represents the dye concentration under equilibrium, m is the mass of the prepared adsorbent, V is the volume of the solution adopted for the adsorption test, and qe is the equilibrium adsorption capacity. 2.4.4. Adsorption kinetics analysis

A given amount of the PMG adsorbent (15 mg) was mixed into a methylene green solution (50 mL, 100 mg/L), and the mixture was subject to the thermostatic oscillation treatment at 323 K. At given intervals, the solution was withdrawn for the UV/Vis spectroscopic measurement, and the results of the adsorption kinetics were plotted and analyzed by the fitting.

2.4.5. Evaluation of the recycling properties of PMG

The used PMG adsorbent was collected after the adsorption reaction, which was then sufficiently washed with deionized water several times. Afterward, it was successively washed with an HCl solution (0.1 M) and a NaOH solution (0.1 M) each for 30 min, followed by thoroughly 10

washing with deionized water until the pH of the washing fluid was kept unchanged. The recycled and cleaned PMG adsorbent was reused in a new cycle, and a total of five runs were considered. The removal percentage was evaluated by Eq. 3. 𝑅=

𝐶0 ― 𝐶𝑒 𝐶0

(3)

× 100%

where R represents the removal percentage, and C0 and Ce are the initial and equilibrium dye concentrations, respectively.

3. Results and Discussion

3.1. Comparison of the microstructure, morphologies, and properties between MG and PMG

The main preparation scheme of the present study is depicted in Figure 1. The synthesis is mainly based on a facile one-step hydrothermal reaction without energy-consuming calcination treatment. There is a distinct difference in the structure and properties between the binary composite (without the interfacial adhesion provided by PDA) and the ternary composite of MoS2 and RGO bridged by PDA.

11

Figure 1. Schematic illustration of the preparation of the MG and PMG samples based on the one-step hydrothermal reaction and ice-templated lyophilization.

The microscopic morphologies of the composite aerogels (i.e., MG and PMG) are observed by SEM (Figure 2). Porous structure can be seen for both MG (Figure 2(a-c)) and PMG (Figure 2(d-f)). Through comparison of the microstructure of MG and PMG, it can be noted that the average pore size of PMG is smaller than that of MG, especially when one compares the SEM images of MG and PMG as shown in Figure 2(c) and Figure 2(f), respectively. Considering that the micropores are generated by the stacking of graphene or MoS2 sheets, the smaller pore size is an indication of a more uniform dispersion of the graphene or MoS2 sheets. This result also reveals that a higher extent of aggregation of graphene and MoS2 sheets most likely exists in MG as compared to that in PMG. 12

Therefore, the introduction of PDA into the fabrication process of graphene/MoS2 can help to stabilize the dispersion of graphene and hence facilitate the formation of the PMG aerogel with a well-constructed microstructure. The photoimage of the as-prepared PMG is also shown in the inset of Figure 2(d). The delicate flower can even support the whole body of the PMG sample even though the density of MoS2 is approximately 4-5 g/cm3 [34], which is much higher than that of carbon materials. The lightweight of the PMG aerogel most likely arises from its high porosity and highly exfoliated state of the graphene and MoS2 sheets.

Figure 2. a-f) SEM images of the prepared MG (a,b,c) and PMG (d,e,f) samples at different magnifications. The inset in (d) shows the digital image of the prepared PMG aerogel that is balanced on a flower.

The N2 adsorption-desorption isotherms and pore size distribution plots are shown in Figure 3. For both the MG (Figure 3(a,b)) and PMG (Figure 3(c,d)) samples, the isothermal lines exhibit the Type IV shape, which indicates the mesoporous structure [53]. The Brunauer-Emmet-Teller (BET) 13

specific surface area of the PMG sample is 98.6 m2/g, which is around 6-fold higher than that of the MG sample (17.2 m2/g). The smaller pore size of PMG relative to that of MG (observed under SEM, Figure 2) can also be evidenced by the pore size distribution plot (Figure 3(b,d)), as determined by the Barrett-Joiner-Halenda (BJH) method. For the PMG sample (Figure 3(d)), the peak positions of the pore size distribution plot can be found at 3.9, 6.5, and 9.6 nm, which is significantly smaller than 19.0, 32.1, 82.7 nm for the MG sample (Figure 3(b)). For more information on the critical parameters obtained from the N2 adsorption-desorption measurement, one can refer to Table S1 in the Electronic Supporting Information (ESI). The larger specific surface area and pore volume of PMG, with the strikingly smaller pore size, can be due to the well-formed porous network structure that consists of highly exfoliated graphene/MoS2 composite sheets. The high degree of exfoliation stems from the PDA-assisted dispersion and stabilization of graphene/MoS2 composite sheets. The oxidative GO tends to react with reductive DA through a redox reaction, resulting in the formation of RGO, and in the meanwhile, DA self-polymerizes into PDA that simultaneously coats on the surface of RGO. We thus speculate that the DA probably prefers to react with GO through a redox reaction when compared with MoS2, resulting in the PDA-coated RGO that would then further bind the formed MoS2 in a fast way due to the fascinating PDA surface, as schematically displayed in Figure 1.

14

Figure 3. a-d) N2 adsorption-desorption isotherm of MG (a) and PMG (c), together with pore size distribution plots of MG (b) and PMG (d).

The elemental mapping images of MG and PMG are presented in Figure S1(a-e) and Figure S1(f-k) in the ESI, respectively. Compared with MG, an additional element (i.e., N) is detected for PMG. The element N unambiguously originates from the PDA composition. The EDX spectra of MG and PMG are also provided in Figure S2 and S3 in the ESI, respectively. Although the distribution of all of the elements is uniform under SEM at the microscale, the aggregation at the nanoscale can be revealed by HRTEM images, as shown in Figure 4. In Figure 4(a), the TEM image of GO shows a large sheet-like structure with curved edges that exist for minimizing the high 15

surface energy of nanosheets. The HRTEM image of GO proves its amorphous nature (Figure 4(b)), which can be further evidenced by the SAED patterns that show faint diffraction rings [54], without noticeable diffraction spots on the rings (Figure 4(c)). These results can also indicate a disordered stacking of GO sheets. Compared with MG (Figure 4(d,e)), PMG exhibits a much uniform surface of the sheet configurations (Figure 4(g,h)). The transparent PMG sheets indicate the vast extent of exfoliation, and no aggregation can be detected in the TEM image. By contrast, the aggregation of the MoS2 on the graphene surface can be found in the TEM image of MG (Figure 4(e)), as marked with a red dashed rectangle. The spacing between the adjacent lattice fringes is also underlined for the agglomerate highlighted in the inset of Figure 4(e), which is calculated to be 0.25 nm that can be assigned to (102) crystallographic facets of MoS2 [36]. The aggregation of MoS2 also results in the emergence of bright diffraction spots in the SAED pattern of MG, while diffraction spots shrink and become scattered along the ring as for PMG due to its polycrystalline nature [55]. The typical crystallographic facets, assigned to the MoS2 hexagonal phase, are marked in the SAED patterns for both MG and PMG. While the MoS2 nanocrystallites are well dispersed on the PG surface of PMG owing to the strong complexation interactions between MoS2 and PDA, the MoS2 tends to aggregate on the RGO surface of MG due to a lack of the interfacial adhesion afforded by PDA. Again, these results further demonstrate the significant role that PDA plays in the generation of a MoS2/graphene composite with strengthened interfacial adhesion. The XRD patterns provided in Figure 5 present the crystal structure of various prepared samples. GO shows a typical XRD band around 10o, indexed to its (001) crystallographic plane. The redox reaction between GO and DA can be verified by the disappearance of the (001) plane of GO, and instead, a new broad diffraction peak at approximately 25o appears, as generated by the RGO 16

(002) plane diffraction. Based on the Bragg’s law, the interplanar spacing (i.e., 0.36 nm) is more significant than that in graphite (0.34 nm), indicating the adequate reaction of DA with GO, forming the RGO sheets with disordered stacking. For both MG and PMG, the typical diffraction bands corresponding to MoS2 can be observed, in agreement with the JCPDS card (PDF#37-1492) of hexagonal MoS2.

Figure 4. a-i) TEM images of the prepared GO (a,b), MG (d,e) and PMG (g,h) samples at different magnifications, together with the SAED patterns of GO (c), MG (f), and PMG (i). The inset in the panel (e) presents the spacing of adjacent crystalline fringes, which are zoomed in from the region marked with the dashed rectangle.

17

Figure 5. XRD patterns of the prepared GO, PG, MG, and PMG samples. The JCPDS Card (No. 37-1492) indexed to hexagonal MoS2 is also shown for comparison.

The XPS spectra of PMG and MG are provided in Figure 6 and 7, respectively. In the XPS spectra in the O1s (Figure 6(a) and Figure 7(a)) and C1s (Figure 6(b) and Figure 7(b)) regions, the oxygen-containing functional groups can be discerned through deconvolution, albeit with much lower contents relative to that of GO as a result of the active reduction reactions (Figure S4 and Figure S5 in the ESI). These functionalities of the PMG sample likely include carbonyl, hydroxyl, and epoxy groups. In comparison with MG, the lowered content of oxygen-containing functional groups can be noted for PMG, which is confirmed by the comparison of their high-resolution C1s 18

spectra shown in Figure S6 in the ESI. Besides, fewer peaks can be resolved for the C1s core-level XPS spectrum of PMG (Figure 6(b)) when compared with that of MG (Figure 7(b)); e.g., the peak, assigned to the carboxyl group, can be resolved for MG but not for PMG, which also indicates a higher degree of reduction of GO in PMG. Such a greater extent of reduction can be attributed to an additional reduction impact from DA, in addition to the hydrothermal reaction-enabled reduction. The existence of the PDA layer in PMG can also be confirmed by the TGA curves shown in Figure S7 in the ESI, and the PDA composition accounts for about 3.2 wt.%, similar to the result reported elsewhere [56]. Such a limited content of PDA is sufficient to offer a fascinating surface for binding MoS2 since the PDA is ready to form a nano-thin film on the RGO surface (Figure S8 in the ESI) [28]. The enhanced thermal stability of MG and PMG relative to that of GO (shown in Figure S7 in the ESI) also consolidates the hydrothermal conversion of GO to RGO. In the high-resolution XPS N1s spectrum of PMG (Figure 6(c)), the peak around 394 eV can be assigned to Mo 3p3/2 [57]. A hump can be seen on the side of the Mo 3p3/2 peak, which can be further deconvoluted into two sub-peaks. One is centered at about 398.5 eV and can be assigned to the –N= group that originates from the pyridine-like structure of PDA, while the other is found at approximately 399.8 eV and can be indexed to the N-H group that arises from the amine groups in the PDA heterocycle [58]. In the high-resolution XPS Mo 3d spectrum of PMG (Figure 6(d)), three peaks can be resolved around 231.1, 227.9, and 225.1 eV, which can be assigned to Mo4+ 3d5/2, Mo4+ 3d3/2, and S 2s, respectively [38]. Compared with the previous report [59], the Mo 3d peaks in the XPS spectrum of PMG shift to lower binding energies, implying effective interfacial binding between MoS2 and PDA-modified RGO [60]. The high-resolution XPS S 2p spectrum of PMG (Figure 6(e)) can be deconvoluted into two sub-peaks at 162.1 and 160.8 eV, which are corresponding to S2- sp1/2 and S2- 2p3/2, respectively. 19

In comparison with the high-resolution Mo 3d and S 2p XPS spectra of PMG (Figure 6(d) and 6(e)), a significant difference can be observed for those of MG (Figure 7(c) and 7(d)). Two more peaks can be deconvoluted at higher binding energies in the Mo 3d spectrum of MG relative to that of PMG, which can be attributed to Mo6+ 3d5/2 and Mo6+ 3d3/2, resulting from the oxidation of Mo4+ during the exposure to air [36, 60]. Furthermore, one more peak assigned to S22- can be resolved for MG relative to that for PMG [60]. These results thereby demonstrate that PMG shows higher stability in comparison with MG owing to the PDA-aided stabilization.

Figure 6. a-e) High-resolution XPS C 1s (a), O 1s (b), N1s (c), Mo 3d (d) and S 2p (e) core-level spectra of the PMG sample.

20

Figure 7. a-d) High-resolution XPS C 1s (a), O 1s (b), Mo 3d (c) and S 2p (d) core-level spectra of the MG sample.

Raman spectra of the typical samples, including GO, MG, and PMG, are provided in Figure 8. In Figure 8(a), the D band and G band can be observed for these carbon-based materials, which corresponds to the K-point phonons of sp3-hybridized carbon atoms and the E2g phonon of sp2-hybridized carbon atoms, respectively [61]. The D band results from the defects and disordered structure, while the G band arises from the intact carbon planes composed of sp2-hybridized carbon atoms [29]. Compared to GO, the D and G band positions are red-shifted for MG and PMG, which indicates the interactions between graphene and MoS2 [62]. For PMG, the even more considerable extent of the redshift in the D band position is due to the PDA coating firmly attached to the RGO 21

surface and the MoS2 nanocrystals anchored on the PG surface [62]. The D-to-G band intensity ratio (ID/IG) can be an indication of the concentration of the structural defects in the MG and PMG sample (Figure 8(b)). Compared to GO, the ID/IG ratio is steeply increased for MG, along with the considerably increased full width at half maximum (FWHM) of the D band (Figure 8(c)) and the slighted decreased FWHM of the G band (Figure 8(d)). These results indicate that the greater extent of structural disorder is introduced in MG, with a more significant number of the sp2-hybridized carbon domains [63, 64], as generated by the hydrothermal-induced reduction and restoration. Interestingly, the ID/IG ratio calculated for PMG is much smaller than that for MG, even slightly smaller than that for GO, together with the smallest FWHM of the D band and largest FWHM of the G band. The introduction of DA in the hydrothermal reaction system can significantly alter the microstructure of PMG, in stark contrast to the counterpart prepared in the absence of DA (Figure 1). The double reduction effects from the hydrothermal reaction and the redox reaction imparted via DA facilitate the vast extent of graphitization of the RGO sheets. The uniform PDA coating on the RGO surface is favorable for the uniform binding of the nano-size MoS2 crystals, as evidenced by the TEM images (Figure 4(g,h)) and the SAED patterns (Figure 4(i)).

22

Figure 8. a-d) Raman spectra of the GO, MG and PMG samples (a), and comparison of ID/IG values (b), FWHM of the Raman D peak (c), and FWHM of the Raman G peak (d) among the GO, MG, and PMG samples.

3.2. Adsorption performance, kinetics, isotherms, and thermodynamics

The adsorption of a water-soluble organic pollutant is presented to demonstrate the potential of the prepared PMG aerogel for environmental remediation, and the results are provided in Figure 9(a-i). The adsorption efficiency between MG and PMG is first compared (Figure 9(a)). During the adsorption process, the adsorption amount of PMG toward methylene green is higher than that of MG at any duration, suggesting that the PMG has an overwhelmingly better adsorption performance for the organic contaminant. In addition to methylene green, methylene blue was also used to confirm this result, and PMG is also obviously superior to MG in the uptake of methylene blue 23

(Table S2 in the ESI). The focus is then brought to the analysis of the adsorption kinetics, isotherms, and thermodynamics of the system with PMG. For the adsorption kinetics, the obtained data were fitted by the commonly-used pseudo-first-order and pseudo-second-order kinetic models, which can be expressed by Eq. 4 and Eq. 5, respectively. 𝑘1

log(𝑞𝑒 ― 𝑞𝑡) = log 𝑞𝑒 ― 2.303𝑡 𝑡 𝑞𝑡

1

(4)

𝑡

(5)

= 𝑘 𝑞2 + 𝑞𝑒 2 𝑒

where qe and qt represent the equilibrium adsorption capacity and adsorption amount at time t, respectively, and k1 and k2 are the pseudo-first-order and pseudo-second-order rate constants, respectively. The adsorption of methylene green onto PMG better follows the pseudo-second-order model (Figure 9(c)) as compared to the pseudo-first-order model (Figure 9(b)); this is evidenced by the improved correlation coefficient value being close to 1 during the fitting under the pseudo-second-order model. From Table S3 in the ESI, it can also be noted that there is a significant difference between the measured (qe,exp) and calculated (qe,cal) equilibrium adsorption amounts, as obtained by fitting with the pseudo-first-order model, while qe,exp is close to qe,cal under the pseudo-second-order model. These results indicate that the pseudo-first-order model is not suitable for the description of the experimental data, whereas they can be well described by the pseudo-second-order model which is based on the assumption that the rate-limiting step is determined by chemical sorption [65]. For the investigation of the adsorption isotherms, the two most commonly-used adsorption isotherm models were adopted for the analysis of the experimental data, namely Langmuir and Freundlich models. The Langmuir model assumes that the dye molecules are adsorbed onto the 24

homogenous surface in the monolayer fashion and that there are no interactions between the adsorbate molecules on the adsorbent surface. The Langmuir model can be expressed as Eq. 6: 𝒒𝒆 =

𝒒𝒎𝒂𝒙𝒌𝑳𝑪𝒆

(6)

𝟏 + 𝒌𝑳𝑪𝒆

A linear form can be described by Eq. 7: 𝑪𝒆

𝑪𝒆

𝟏

(7)

𝒒𝒆 = 𝒒𝒎𝒂𝒙 + 𝒒𝒎𝒂𝒙𝒌𝑳

where Ce represents the equilibrium dye concentration in the solution, qe is the equilibrium adsorption amount, qmax denotes the maximum amount of methylene green adsorbed by the per unit weight of PMG to form a complete monolayer on the surface bound at high Ce, and kL is the Langmuir constant that is related to the affinity of the binding sites. For the evaluation of whether the adsorption reaction is favorable, unfavorable or irreversible, one can refer to Eq. 8: 𝟏

(8)

𝑹𝑳 = 𝟏 + 𝒌𝑳𝑪𝟎

where kL represents the Langmuir constant, and C0 denotes the maximum concentration of the initial dye solutions. The value RL can be used to indicate the favorability of the adsorption reaction, which is presented as follows: RL＞1, unfavorable RL＜1, favorable RL＝1, Linear RL=0, irreversible The Freundlich model is based on the assumption that the adsorption takes place on the inhomogeneous surface in the multilayer fashion, and that there exist interactions between adsorbed dye molecules. The Freundlich model can be expressed by Eq. 9, with the linear form described by 25

Eq. 10: 𝑞𝑒 = 𝑘𝐹𝐶1/𝑛 𝑒

(9) 1

(10)

ln𝑞𝑒 = ln𝑘𝐹 + 𝑛ln𝐶𝑒

where Ce and qe are the dye concentration and adsorption amount under equilibrium, respectively, and kF represents the Freundlich constant that is correlated to the adsorption capacity. The isothermal adsorption results are shown in Figure 9(d-f) and Figure 9(g-i). All the adsorption system at different temperatures can be better fitted by the Langmuir model, indicating the homogenous adsorption of the dye molecules onto the PMG surface with a homogeneous PDA layer and uniformly distributed MoS2 nanocrystallites. The adsorption at higher temperatures yields a larger maximum adsorption capacity (i.e., qmax), as shown in Table S4 in the ESI. The qmax is calculated as 201.2, 227.8, and 235.3 mg/g for PMG at 303, 313, and 323 K, respectively, implying an endothermic adsorption process. The calculated qmax is much larger than that of some adsorbents reported for the uptake of methylene green (Table S5 in the ESI), such as nanoporous cobalt/carbon materials (~72 mg/g) [66], fungus-like MoS2 nanosheets (~63 mg/g) [67], and MoS2 nanosheets decorated with magnetic Fe3O4 nanoparticles (~20 mg/g) [68]. The RL values are also presented in Table S4 in the ESI, and all of them are in the range of 0~1. The adsorption is thus favorable for all the systems under different temperature conditions. Apart from the excellent adsorption capacity of the PMG aerogel adsorbent, it also exhibits satisfactory recycling properties (Figure S9 in the ESI) due to the adequate interfacial adhesion afforded by PDA that stabilizes both the MoS2 and RGO. Additionally, all the 1/n values listed in Table S4 in the ESI are in the range of 0.1~0.5, also revealing the favorable adsorption at all the temperatures. Based on the isothermal adsorption at different temperatures, the thermodynamics of the adsorption of methylene green on the PMG 26

samples can be analyzed. The enthalpy change ∆H and entropy change ∆S can be calculated from the slope and intercept of the linear fitting line (Eq. 11), respectively, and the Gibbs free energy change ∆G can be obtained based on Eq. 12. log

( )= 𝑞𝑒

𝐶𝑒

∆𝑆

∆𝐻

(11)

2.303𝑅 ― 2.303𝑅𝑇

(12)

∆𝐺 = ∆𝐻 ― 𝑇∆𝑆

From Table S6 in the ESI, it can be noted that the ΔG° value is negative for all the adsorption system regardless of the initial dye concentration and reaction temperatures, which indicates that the dye molecules can be spontaneously adsorbed onto the PMG surface. The positive ΔH° value is an indication of the endothermic adsorption process, while the positive ΔS° value implies that the adsorption of the dye molecules causes the homogenous PMG surface to be disordered. It is also worth pointing out that the weakness of the bio-inspired aerogel adsorbent lies in the diffusion barrier (generated by its microchannels) to the dye molecules and its closed micropores that are inaccessible to the dye molecules, lowering its adsorption performance when compared to the powdery adsorbent that can be highly exfoliated and dispersed in the solution where the adsorption takes place. It is believed that the adsorption performance of the bio-inspired composite aerogel can be further improved through structural optimization (e.g., tuning the ratio of MoS2 to RGO).

27

Figure 9. a-i) Investigation of adsorption performance of the prepared samples. (a) Comparison of the adsorption efficiency between MG and PMG. (b,c) Adsorption kinetic analysis of the PMG sample based on the pseudo-first-order (b) and pseudo-second-order (c) kinetic models. (d-i) Adsorption isotherm analysis of the PMG sample based on Langmuir (d-f) and Freundlich (g-i) models under different temperature conditions, including 303 K (d,g), 313 K (e,h) and 323 K (f,i).

28

4. Conclusion

We have developed a facile one-step hydrothermal route to the 3D composite aerogel with the bio-inspired adhesion of graphene and MoS2 based on PDA, without energy-consuming calcination processing. The biomolecule DA plays a pivotal role in the formation of well-constructed composite aerogel with uniformly dispersed MoS2 nanocrystallites on the surface of PDA-modified graphene without aggregation. Without the bio-inspired interfacial adhesion, the aggregation of MoS2 nanocrystallites occurs, leading to substantial decreases in both the specific surface area and total pore volume, and a considerable increase in the average pore size, in comparison with the case with additional bio-inspired adhesion interactions between graphene and MoS2. The presence of PDA can also impede the oxidation of MoS2 when exposure to air. Such a composite aerogel prepared by the bio-inspired one-step process shows an excellent adsorption capacity toward methylene green, together with satisfactory recycling properties, and the adsorption process is spontaneous and endothermic, and well follows the Langmuir isotherm model and pseudo-second-order kinetics.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

29

Acknowledgments

We acknowledge the National Natural Science Foundation of China (51702050, 51803028), the Featured Innovation Project of the Department of Education of Guangdong Province (2017KTSCX188), the Open Research Foundation of Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development (Y807s31001), the Open Research Foundation of Guangdong Provincial Key Laboratory of Industrial Surfactant (GDLS-01-2019), the Open Fund for Key Lab of Guangdong High Property and Functional Macromolecular Materials (20190017), and the Project of Department of Education of Guangdong Province (2017KQNCX214).

Appendix A. Supplementary data

Supplementary data to this article can be found online at https:// doi.org/.

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34

Highlights

1.

Bio-inspired interfacial adhesion between graphene and MoS2 to form an aerogel.

2.

Dopamine-aided uniform dispersion of MoS2 crystallites on the modified graphene.

3.

Enhanced specific surface area and porosity of the aerogel are unravelled.

4.

Improved adsorption capacity and good robustness are demonstrated.

5.

Spontaneous and endothermic adsorption onto the homogenous adsorption sites.

35

Graphical abstract

36

All of the authors have participated in the preparation of the manuscript and contributed scientifically. The specific contribution from each author is provided in the table as follows: Contribution

Author

Conceptualization

Wuqing Zhu, Huawen Hu, and Dongchu Chen

Data curation

Wuqing Zhu, Huawen Hu, and Dongchu Chen

Formal analysis

Huawen Hu, and Dongchu Chen

Funding acquisition

Huawen Hu, Dongchu Chen, and Yinlei Lin

Methodology

Wuqing Zhu, Yinlei Lin, and Huawen Hu

Investigation

Wuqing Zhu, Yinlei Lin, Wanwen Kang, and Haiyan Quan

Project administration

Huawen Hu, Yinlei Lin, and Dongchu Chen

Resources

Yuyuan Zhang, Menglei Chang, and Kun Wang,

Software

Weibin Zhang, Zhiqiang Li, and Hongyang Wei

Supervision

Huawen Hu, and Dongchu Chen

Validation

Huawen Hu, and Dongchu Chen

Writing—original draft

Wuqing Zhu, and Huawen Hu

Writing—review

and Huawen Hu, Dongchu Chen, Min Zhang, and Ting Fan

editing

37

Declaration of Interest Statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

38