nickel fluffy spheres for fast magnetic separation and efficient removal of organic solvents from water

Materials Letters 254 (2019) 440–443

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Hybrid carbon nanotubes/graphene/nickel fluffy spheres for fast magnetic separation and efficient removal of organic solvents from water Tao Tao a, Guangyao Li b,⇑, Yanli He c, Pengzheng Duan d a

Material Science and Engineering College, Central South University of Forestry and Technology, Changsha 410004, China College of Engineering, Zhejiang A&F University, Hangzhou 311300, China Hebei Agricultural University, Baoding 071001, China d Xilinmen Furniture Co., Ltd., Shaoxing 312001, China b c

a r t i c l e

i n f o

Article history: Received 19 December 2018 Received in revised form 24 May 2019 Accepted 29 June 2019 Available online 1 July 2019 Keywords: Carbon nanotubes/graphene/nickel fluffy spheres Biomaterials Carbon materials Organic solvent removal

a b s t r a c t Carbon nanotubes/graphene/nickel (CNT-G-Ni) fluffy spheres were synthesized using renewable carbon sources. Carbon-encapsulated nickel nanoparticles (CENNs) were prepared by hydrothermal carbonization (HTC) of potato starch and nickel nitrate, and then followed by catalytic graphitization at 900 °C. The prepared GENNs were cracked at 700 °C in biogas, during which graphene shells over GENNs were peeled off and formed graphene nanoplatelets, and simultaneously multiwall carbon nanotubes (MWCNTs) were formed over nickel particles and in-situ integrated into graphene nanoplatelets to obtain CNT-G-Ni fluffy spheres. The prepared 3D nanocomposite was investigated for the removal of organic solvent from water and its adsorption capacity above 110 g/g. Ó 2019 Elsevier B.V. All rights reserved.

1. Introduction Organic solvents are used in the furniture production, especially in varnishing and cleaning of furniture components such as benzene, chlorobenzene, glycol ethers, methylene chloride, acetone, toluene, ethylbenzene, and xylenes [1]. Leaked or spilled organic solvents and oil are impacting on the environment by water, soil, and air contamination [2]. Organic solvents and oil in water may severely affect the marine environment by causing a decline in phytoplankton and poisoning fishes, marine mammals, and seabirds [3]. Various methods have been developed to clean up and recovery organic solvents and oil from water, including bioremediation [4], oil booms usage [5], sorbents usage [6], controlled burning [7], and skimming [8]. Among these methods, adsorption is regarded as an effective and economical technology due to its high efficiency, simple design, and smooth operation. Significant types of adsorbents in use are activated alumina, silica gel, activated carbon, molecular sieve carbon, molecular sieve zeolites and polymeric adsorbents [9]. In the past decade, activated carbons are widely used adsorbents for a broad range of environmental applications due to their high surface area, microporous structure, and ⇑ Corresponding author. E-mail address: [email protected] (G. Li). https://doi.org/10.1016/j.matlet.2019.06.104 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.

remarkable mechanical, thermal and chemical stabilities. Other carbon-based materials, carbon nanotubes (CNTs), graphene, carbon foams, and carbon aerogels are also extensively studied as efficient adsorbents. Among them, graphene and CNTs have shown significant advantages over the traditional synthetic or natural adsorbent materials due to their high adsorption capacity, reusability, and environment-friendly. Graphene, a unique twodimensional (2D) structured material, is recently recognized as excellent potential adsorbents for removal of spilled oil due to its hydrophobic and tailorable interfacial properties [10]. However, ultra-thin graphene films and layered paper-like graphene have low porosity and tend to aggregate in the water, which is not beneficial for adsorption of contaminants [10]. To overcome these limitations, various 3D structural materials have been prepared using graphene as the main components [10]. In this study, an innovative, green and straightforward approach is demonstrated to prepare CNT-G-Ni nanocomposites with renewable carbon. The nanostructures obtained from this study are characterized by SEM, HRTEM, and XRD. The proposed method is simple and economical for the scalable production of highly porous CNT-G-Ni fluffy spheres, which can be used for efficient and cost-effective oil and organic solvent removal and water purification.

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2. Experimental

3. Results and discussion

2.1. Preparation of CENNs

3.1. Characterization

CENNs were synthesized hydrothermally by the reduction of Ni2+ in a potato starch solution. Sixty grams of Ni(NO3)2
6H2O were dissolved in the 2500 mL starch solution (containing 300 g potato starch, the pH value of the solution was 3.0). The mixture was stirred for 30 min, and then transferred into a 3 L Parr reactor. The autoclave was heated up to 180 °C and kept for 3 h. After the reaction completed, a black product was obtained, and washed three times with DI water and dried at 110 °C overnight.

CO2, CH4, CO, and H2 were recorded when treated GENNs in biogas at 700 °C (Fig. 1a). Dry reforming reaction (CH4 + CO2 ? 2CO + 2H2) will occur in biogas at high temperature, since nickel nanoparticles are very active for this reaction. CH4 and CO2 consumed by 1:1 ratio along with the CO and H2 formation over about 470 °C. The extra methane in biogas will be dissociated over Ni nanoparticles (CH4  C⁄ + 2H2) with more carbon deposited between the graphene shell and nickel core, which expand the core in volume to facilitate the cracking of the graphene shell [11]. Then, the graphene shell is peeled off from the nickel particle and formed graphene nanoplatelets. Methane is continuously cracking over the naked nickel particle, CNTs will be formed simultaneously. Fig. 1b shows the XRD patterns of the CENNs, GENNs, and CNTG-Ni samples. The XRD pattern of CENNs (Fig. 1b-1) shows five peaks at 37.3°, 43.3°, 63.0°, 75.6°, and 79.8°, which are well indexed to the (1 1 1), (2 0 0), (2 2 0), (3 1 1), and (2 2 2) planes of NiO. For the GENNs sample (Fig. 1b-2), three strong diffraction peaks at 2h of 44.5°, 51.8°, 76.4° correspond to (1 1 1), (2 0 0), and (2 2 0) planes of Ni. There is a diffraction peak appearing at about 2h = 26.6°, which is indexed to (0 0 2) planes of graphene. From the XRD pattern of the CNT-G-Ni sample (Fig. 1b-3), there are strong diffraction peaks at 2h = 26.1° and 44.2° correspond to MWCNTs besides the nickel diffraction peaks. SEM images of the CENNs, GENNs, and CNT-G-Ni samples are shown in Fig. 2. The agglomeration of CENNs is spherical (Fig. 2a1) with a uniform particle size of 1–3 mm. The morphology of the single CENN agglomerate exhibits a porous structure (Fig. 2a2). The majority CENNs in the agglomeration structure had their diameters less than 10 nm and embedded in bulk carbon (Fig. 2a2). SEM images of GENNs are shown in Fig. 2b1 and b2. The agglomerate size shrank to 0.5–1 mm after catalytic graphitization (Fig. 2b1). There were more pores formed within the CENN agglomerate, and the pore size was varying from 10 to 150 nm for the GENNs. The porous GENN sphere is composed of uniform nanoparticles of 5–10 nm in diameter (Fig. 2b2). SEM images of CNT-G-Ni structure (Fig. 2c1 and c2) revealed a fluffy sphere after flowing biogas through GENNs at 700 °C for 30 min. CNTs are observed growth across the GENN agglomerate spheres and stretched over the surface to form a fluffy structure (Fig. 2c1). The fluffy surface of the sphere is made of CNTs of 50–80 nm in diameter and 2–3 mm in length (Fig. 2c1 and c2).

2.2. Formation of GENNs and CNT-G-Ni nanocomposites Thirty grams of the dried CENNs were put into a tubular reactor. The reactor was first purged with high purity argon for 30 min, followed by heating at 3 °C /min to 900 °C and kept at 900 °C for 1 h. The reactor was cooled down to 30 °C, then switched to 200 mL/ min biogas (60% CH4 and 40% CO2). The reactor was then heated up to 700 °C at 5 °C/min and kept at 700 °C for 30 min. The furnace was cooled down to room temperature at 10 °C/min. Vent gas was analyzed using an on-line residue gas analyzer (RGA, Hidden QGA). 2.3. Characterization X-ray powder diffraction (XRD) patterns of the samples were obtained using a Rigaku Ultima III diffractometer (CuKa radiation with k = 1.5406 Å). The sample morphology was investigated with a JEOL JSM-6500F Scanned Electron Microscope (SEM). Highresolution transmission electron microscopy (HRTEM) analysis of samples was performed on a JEM-2100. 2.4. Adsorption capacity test For adsorption tests, organic solvents and fuels (50 mL) stained with Sudan red 5B was transferred into a 100 mL beaker. The CNTG-Ni fluffy sphere sample (W1) was weighed before immersing in the oil. The adsorbent was then removed after 60 min and allowed to drain for 5 min. The saturated adsorbent was then weighed (W2). The adsorption capacity (Q) was calculated using the weight of the adsorbent before and after adsorption as follows:

Q ¼ ðW2  W1Þ=W1 All adsorption experiments were performed three times, and an average value of adsorption capacity was obtained.

Fig. 1. (a) RGA results of gases released from GENNs heated to 700 °C under biogas, (b) XRD: (1) CENNs, (2) GENNs, and (3) CNT-G-Ni.

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Fig. 2. SEM images of (a) CENNs, (b) GENNs, and (c) CNT/G/Ni fluffy spheres.

The HRTEM images of CENNs, GENNs, and CNT-G-Ni samples are shown in Fig. 3. The HRTEM image (Fig. 3a) of the fresh CENNs shows that nickel oxide nanoparticles with diameters of 3–5 nm are uniformly embedded in bulk carbon of the sphere. The HRTEM image of the GENNs shows the nickel particles are encapsulated in 2–10 layers of graphene (Fig. 3b); the diameter of the nickel core in GENNs is between 5 and 10 nm. HRTEM images (Fig. 3c and d) show that graphene nanoplatelets (Fig. 3c) and MWCNTs and nickel nanoparticles (Fig. 3d) are in the CNT-G-Ni sample. The sizes of graphene nanoplatelets, CNT, and nickel particles are 20–50 nm, 50–80 nm, and 30–50 nm in diameters, respectively. The HRTEM images shown in Fig. 3c and d confirm the formation of the 3D CNT-G-Ni nanostructures. Most nickel nanoparticles of CNT-G-Ni are observed inside the CNTs with diameters of 30–50 nm (Fig. 3d), while nickel particles of GENNs are encapsulated in graphene shells with diameters of 5–10 nm, which means cracking of GENN core–shell structures when treating them in biogas at 700 °C. Nickel nanoparticles separated from graphene shells will merge to large size through the sintering process at high temperature, and graphene nanoplatelets form simultaneously from the cracked graphene shells. XPS spectra of samples were plotted in Fig. S1, very weak Ni2p3/2 peaks for all the samples might be due to nickel particles were encapsulated in carbon, graphene or MWCNTs, respectively.

3.2. Sorption of organic liquids Fig. S2 shows the adsorption capacities (Q) of the CNT-G-Ni fluffy spheres for six different organic solvents and oils (toluene, chloroform, dichlorobenzene, hexane, gasoline, and diesel). In Fig. S2, the adsorption capacity of the CNT-G-Ni 3D composite ranges from 112 to 145 g/g. The Q was in the order of dichlorobenzene > toluene > diesel > chloroform > gasoline > hexane. The adsorption capacity of the hybrid fluffy spheres is better than the bare 3D graphene [10], indicating the important role of fluffy CNTs. More importantly, the absorbed organic solvents and oil can be simply removed by heating at 150 °C. In this way, the hybrid foam can be reused multiple times without significant loss in adsorption capacity.

4. Conclusion An innovative, green, and one-step approach was demonstrated to prepare CNT-G-Ni 3D nanocomposite using renewable carbon sources. CENNs are first synthesized by HTC of potato starch at low temperature; CENNs are further catalytically graphitized to GENNs; finally, GENNs are cracked in biogas at 700 °C. Graphene shells are peeled off from GENNs and formed graphene nanoplate-

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Fig. 3. HRTEM images of (a) CENNs, (b) GENNs, and (c, d) CNT/G/Ni fluffy spheres.

lets; MWCNTs grew and in-situ integrated into the graphene nanoplatelets. The prepared 3D CNT-G-Ni nanocomposites exhibit outstanding adsorption performance for the removal of organic solvents from the water.

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.matlet.2019.06.104. References

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.

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Acknowledgments The authors are grateful for the financial supports from the Key R&D Projects of Zhejiang Province (No. 2018C01136), the Natural Science Foundation of Zhejiang Province, China (Grant No. LQY18C160001).

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