Nanosheet-templated graphene oxide membranes for fast molecule separation
Zhen Lin1,2, Chuan Hu1, Qinglin Liu1, Qiugen Zhang1,*
1Department of Chemical and Biochemical Engineering, College of Chemistry & Chemical Engineering, Xiamen University, Xiamen 361005, China.
2Department of Applied Physics, School of Science, Aalto University, P.O. Box 15100, FI-00076 Espoo, Finland
*E-mail: qgzhang@xmu.edu.cn (Zhang); Tel: +86-592-2188072; Fax: +86-592-2184822
Abstr act
Intercalated laminar membrane with controllable interlayer spacing (d -spacing) is one of the most effective membranes for fast molecule separation. In this work, we demonstrate a versatile strategy to create nanosheet-templated water channels in laminar graphene oxide (GO) membranes. The 1.2 nm-thick nickel hydroxide nanosheets as sacrificed intercalators provide a chance to control thed -spacing and simultaneously retain hydrophilicity. The resultant membranes have controllable channels and exhibit over 6 times higher water permeance than the unintercalated membrane. The 880 nm-thick nanosheet-templated GO (NST-GO) membrane has accurate d -spacing of about 1.14 nm and shows high water permeance of 120.3 L m−2 h−1 bar−1 and good molecule separation property, reflecting in high rejection for larger dyes (90.1% for erythrosine b (EB)), while low rejection for smaller dyes (58.3% for methylene blue (MB)). Furthermore, this strategy of intercalating and sacrificing nanosheets has higher potential than traditional intercalation in controllingd -spacing of laminar membranes.
Topical Heading: Separations: Materials, Devices and Processes
Key Words: molecule separation; laminar membrane; sacrificed intercalator; nanosheet-templated membrane; d -spacing control
1. Introduction
Molecule separation membrane has attracted increasing attention over the past few decades because of its accuracy and efficiency.1,2 Typically, isoporous membranes have pores with uniform size which are applicable for molecule separation.3,4 There are many methods to prepare the isoporous membranes, such as interference lithography,5 breath figure,6copolymer self-assembly,7 etc. Among them, controllingd -spacing is an effective way to prepare the isoporous membranes with laminar nanochannels, such as GO membranes, MoS2membranes, WS2 membranes, etc.8,9These membranes have alterable d -spacing because the two-dimensional nanosheets can be slid or intercalated.10 Therefore, it is flexible to adjust different d -spacing and make suitable laminar nanochannels of these membranes for the separation of target molecules.
The GO membranes are one of the most common membranes that have laminar structure.11,12 The d -spacing between two GO nanosheets is about 0.9 nm, which is smaller than most of dye molecules but much larger than water molecules.13,14 This means the water molecules can penetrate easily while the dye molecules are rejected effectively. As a result, the GO membranes often have excellent dye rejection. However, the GO membranes also have unsatisfactory permeance because of their narrow water channels resulting from smalld -spacing.15 Therefore, one of the most effective ways to improve the water permeance without compromising rejection is increasing d -spacing but controlling at an appropriate value.
Recently, intercalating nanomaterials into the interlayers of GO nanosheets for d -spacing control has attracted broad attention.16,17 The most nanomaterials used are zero-dimensional spheres. For example, Chen et al. reported a kind of pyrrolidinium-functionalized fullerene intercalated GO membranes.18 The membranes have broadenedd -spacing of about 1.46 nm, which results in the improvement of water permeance from 4 L m−2 h−1bar−1 to 34 L m−2h−1 bar−1. Another zero-dimensional sphere are ions. Chen et al. intercalated ions, such as K+, Na+, Ca2+, Li+ or Mg2+, into GO interlayers to decrease the d -spacing and fabricated the cation-controlled membranes for desalination.19 The KCl-controlled membrane shows the ion rejection of more than 99%. One-dimensional nanowires or nanotubes are sometimes considered as well. For example, Zeng et al. used the modified carbon nanotubes as intercalators to control the d -spacing at about 0.85 nm.20The membranes have the enhanced water permeance of 34.4 L m−2 h−1 bar−1 with over 90% rejection for dyes. Different from zero-dimensional and one-dimensional nanomaterials, two-dimensional nanosheets are more facile as intercalators because of their large surface area and lamellar structure like GO nanosheets.21 However, they are seldom reported because only the thin and uniform nanosheets can be used for intercalation. For example, Liao et al. used ultrathin zwitterionic MXene nanosheets to intercalate between GO nanosheets.22 The resulting membranes have the high water permeance of 110 L m−2 h−1bar−1 and good rejection for molecules whose size is larger than 2 nm.
Although the intercalators are various, almost of them are designed remaining in the membranes. These intercalators occupy water channels, increase filtration resistance, hinder water transmission, and finally cause unnecessary loss of water permeance. Therefore, a novel way to control the d -spacing but prevent remaining of the intercalators in the membranes should be more effective to improve the permeance of GO membranes. Inorganic hydroxide nanomaterials are potential candidates. They not only regulate d -spacing by their shapes but also can be easily removed by acid dissolution.23 That is, thed -spacing of GO membranes can be controlled accurately, and meanwhile the water channels can be even unimpeded than traditional intercalators after sacrificing the inorganic hydroxides. For example, Huang et al. reported nanostrands-channeled GO membranes by sacrificing copper hydroxide nanostrands as intercalators, which increases d -spacing from 0.85 nm to 0.99 nm.24The membranes have the high rejection of 99% for large molecules like tetrakis (1-methylpyridinium-4-yl) porphyrin p-toluenesulfonate (TMPyP) while low rejection of 36% for small molecules like [Fe(CN)6]3-.
The Ni(OH)2 nanosheets, prepared by a facile way in our previous work, show great potential as intercalators.25,26 The nanosheets are desired two-dimensional nanomaterials which have uniform thickness and controllable size determined by aging time. For example, the size of 0.5 hour-aged Ni(OH)2 nanosheets is about 50×50 nm2 with the thickness of about 1.2 nm. Therefore, if the nanosheets are used as intercalators, the d -spacing of GO membranes can be easily controlled at about 1.2 nm that is a proper size for dye separation. In this paper, we demonstrate a facile strategy to construct hydrophilic laminar channels in the GO membranes for fast molecular separation via the templates of ultrathin Ni(OH)2 nanosheets. As shown in Figure 1a, the 1.2 nm-thick Ni(OH)2 nanosheets with abundant positive charges assembled on the surface of negatively-charged GO nanosheets and formed Ni(OH)2@GO composite nanosheets, which are used to fabricate NST-GO membranes via filtering, reducing and template removing in turn. The Ni(OH)2 nanosheets provide a chance to control the d -spacing between GO nanosheets and simultaneously retain their hydrophilicity after reducing. Amazingly, the resulting NST-GO membranes exhibit over 6 times higher water permeance than the unintercalated membrane, showing a great potential in fast molecular separation.