Two-dimensional niobate nanosheet membranes for water treatment: Effect of nanosheet preparation method on membrane performance
Graphical abstract
Introduction
Two-dimensional (2D) nanosheet membranes, such as graphene-based, are expected to function as size-selective molecular separation membranes, based on their unique atomic thickness with micrometer lateral dimensions. Different types of separation membranes using 2D nanosheets have been studied, including nanosheet composite membranes [1], [2], [3], [4], porous nanosheet membranes [5], [6], [7] and stacked nanosheet membranes [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. Stacked nanosheet membranes have been widely studied because of their separation capability using tunable 2D nanochannels, electrostatic interaction, and adsorption properties of the nanosheets. Stacked nanosheet membranes are formed by assembling nanosheets into ultrathin membranes containing nanochannels between the stacked sheets that allow water to pass through while rejecting unwanted solutes. In addition, stacked nanosheet membranes are relatively easy to fabricate via various processes including vacuum filtration [8], [9], [12], [13], [14], vacuum suction [15], pressure-assisted filtration [16], spin casting [17], the layer-by-layer approach [18], [19], the wet phase inversion method [10] or spray coating [11]. In addition, a wide variety of nanosheet materials and support membranes are available. Most studies to-date have fabricated graphene-based membranes with high flux and separation performance. However, only a narrow range of nanosheet materials, such as graphene oxide (GO) [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], transition metal dichalcogenides [20], [21], [22], [23] and transition metal oxides [24], [25] have been studied for membrane preparation, although many other nanosheet materials have been reported. Transition metal oxide nanosheets offer attractive surface and catalytic properties [26], [27], [28], [29], [30], [31]; therefore, they represent promising materials for potentially high-functioning membranes.
Recently, we developed stacked nanosheet membranes using single molecular sheets of niobate. The membranes were prepared with a bottom-up approach using the hydrothermal method [25]. Results showed that the niobate nanosheet membranes demonstrated high structural stability in water via chemical cross-linking between their sheets and superior nanofiltration performance when compared with graphene oxide membranes fabricated by a similar vacuum filtration method [25]. The most common preparation method of 2D metal oxide nanosheets is a top-down approach using the exfoliation method [26], [27], [28]. Typically, multi-layered bulk compounds can be exfoliated by first intercalating their layers with base molecules, followed by rupturing these layered structures to yield unilaminar colloids. The lateral size of crystallite nanosheets and the chemical agents in the colloidal metal oxide nanosheet solution depend on the preparation methods. To fabricate various kinds of nanosheet membranes with good separation performance and functionality using metal oxide nanosheets, it is important to study the effects of nanosheet preparation method on membrane performance. In this study, stacked nanosheet membranes were fabricated using niobate nanosheets prepared by the hydrothermal and exfoliation methods. Membrane structure and the performance of the stacked niobate nanosheet membranes were evaluated and compared.
Section snippets
Materials
All chemicals used in this study were analytical grade with over 99% purity and were obtained from Wako Pure Chemical Industries, Osaka, Japan unless otherwise stated. All aqueous solutions were prepared with Milli-Q water (>18.2 MΩ cm, Merck Millipore, Billerica, MA, USA).
Preparation of single layer niobate nanosheets by the hydrothermal method (bottom-up approach)
Niobate molecular sheets were synthesized by the hydrothermal method using amine surfactants in ammonia solution [29]. In this experiment, 1.989 g of niobium(V) ethoxide (Nb(OEt)5) (Kojundo Chemical Laboratory Co., Ltd.,
Fabrication of niobate nanosheet membranes using colloidal nanosheets prepared by different methods
Single-molecular layers of niobate nanosheets were prepared by the conventional exfoliation method using TBAOH from layered niobate, and the hydrothermal method in the presence of TEOA. According to XRD patterns (Fig. S1), HNb3O8 is a highly crystalline material consisting of alternating layers of negatively charged NbO6 octahedra and protons [27], [28]. The niobate nanosheet materials synthesized by hydrothermal method show similar diffraction patterns to those of HNb3O8; however, the peaks
Conclusions
Stacked niobate nanosheet membranes were fabricated using niobate nanosheets prepared by hydrothermal or exfoliation methods. It was found that both membrane types had a dense structure and retained high structural stability in water. Both niobate nanosheet membranes act as nanofiltration membranes with excellent rejection performances against anionic dyes and salts. Higher water permeance but lower rejection of neutral molecule (PEG) and anionic species (Na2SO4 and Acid Red 265) was observed
Acknowledgements
This work was supported by JSPS KAKENHI Grant Number JP16K06829 and Kurita Water and Environment Foundation Grant Number 16A078.
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