Approximately 1 nm-sized artificial tunnels in wrinkled graphene-graphene oxide composite membranes for efficient dye/dye separation and dye desalination
Graphical abstract
Introduction
The water shortage threatens human health and the development of the global economy [1], [2]. Although sewage treatment of wastewater and desalination of salty water can greatly alleviate a challenge, the existing distillation technologies are economically burdened in terms of energy consumption and following solid waste treatment cost [3], [4]. Therefore, low-energy-driven and economical approaches, such as nanofiltration technology for accurate separation of nm-sized pollutants/salts, are emerging, preventing the generation of solid hazardous wastes and generating revenue from the filtered solutes (i.e. industry-graded salts) at the same time. However, it is still difficult to obtain nanofiltration membranes with satisfied selectivity and permeability for realizing the above functions. For example, zeolitic membranes with well-defined channels and high porosity are identified as potential candidates for high-performance nanofiltration membranes [5], which are conducive to the size sieving of solutes and the rapid penetration of solvent. On the other hand, their implementation is limited by their high degrees of fragility and low practical selectivity caused by poor compatibility with the support [6]. Similarly, metal-organic framework (MOF) [7] and covalent-organic framework (COF) [8] membranes also exhibit excellent selectivity and permeability for wastewater in theory due to their high porosity and specific pore structures. However, their separation performances are degraded by grain boundary defects, intercrystalline cracks, and irregular material morphology [9], [10]. In addition, metal oxide-based ceramic membranes, composed of uniform porous layers by stacking particles, are intensively investigated [11], while their inhomogeneous particle sizes result in the large pore size distribution of the membrane that strongly affects the selectivity performance [12].
Furthermore, monolayered two-dimensional (2D) materials like graphene, with precise atomic pores and extremely short mass transportation paths, exhibit excellent nanofiltration performances in terms of selectivity and permeability [13]. Given the significant challenge in manufacturing membrane-sized free-standing 2D lattices with a high degree of robustness, membranes made of multilayered 2D components (e.g. graphene oxide (GO) [14], [15], [16], MoS2 [48] or other functional metal oxide materials [47]) are much more suitable for practical water treatment applications due to their uniform interlayer distance and facile preparation methods. However, a defect-minimum GO-based membrane (GOM) is composed of at least thousands of GO monolayers, resulting in significantly prolonged mass transport routes between adjacent GO layers and therefore poor permeability [17], [18]. Such a challenge can be alleviated by the introduction of artificial nano-sized tunnels into GOMs as the “shortcuts” for mass transport. Several attempts on the intercalation of either soft (e.g. polyacrylonitrile, polypyrrole gel [19], [20]) or hard (e.g. TiO2 [21], [22]) particles have been carried out to construct the artificial nanotunneled structures. However, the amount of intercalated particles has to be limited to prevent the scarification of the membrane rejection performances, leading to the isolation of most nano-tunnels and deteriorating the water permeability performances. In comparison, low-dimensional nanostructures, such as one-dimensional (1D) nanowires, can efficiently generate interconnected nanotunnels, conducive to the unimpeded transmission of water molecules and therefore resulting in a great enhancement of water flux. Liu et al. [23] investigate the possibility of embedding triethanolamine (TEOA) nanowires with diameters of ∼100 nm into GOM, leading to an increased water flux from 3.75 to 16 L m-2h−1 bar−1. On the other hand, the molecule separation efficiency is greatly reduced due to the significantly widened transport channels compared to that of GO interlayer spacing. Other 1D nanostructures, such as carbon nanotubes (diameters of ∼10 nm) and copper hydroxide nanowires (diameters of ∼5 nm), are studied as alternatives. [24], [25] While their nanofiltration performances maintain with the improved water permeation, nevertheless, the further reduction of the artificial tunnel diameters to ∼1 nm is highly desired given the kinetic dimensions of the majority of pollutant molecules (e.g. dye).
We consider that embedding of approximately 1 nm-sized filler into GOM to induce the desired continuous artificial tunnels is challenging, given the difficulty in precise engineering of the sizes of nanofillers [26], [27], [28], [29], [30], the low compatibility of nanofiller towards the GO host, as well as the high complexity of removing nanofillers and the associated contaminations. Instead, we avoid using any 1D nanomaterials and propose a facile method to introduce the ∼1 nm-diameter nanotunnels in GOM from GO layers instead. It has been recognized that the defect-free graphene monolayer exhibits high surface energy and therefore cannot be existed in a free-standing form. [31] Therefore, modification approaches such as the manipulation of surface chemical compositions and the introduction of morphological wrinkling are usually applied to reduce the surface energy. [18], [31] Such wrinkling feature of the modified monolayered graphene, with dimensions as small as ∼1 nm, could be of particular interest in constructing the artificial nanotunnels within the GOM. [32].
In this work, inspired by such a phenomenon in monolayered graphene, we firstly increase the surface energy of GO monolayers by manipulating the surface functional groups through thermal reduction. The monolayers are then wrinkled with sizes of ∼1 nm to maintain the free-standing form, resulting in a three-dimensional (3D) feature. Furthermore, to avoid the random aggregation of reduced GO monolayers, selective edge-carboxylation is implemented to improve the stability of the 3D structure [33] and simultaneously enhance the dispersibility in solvents which is the key success factor to produce defect-minimum GO-based membranes. Such 3D graphene (WG) sheets are then intercalated into the compact GOM to facilitate the formation of dense ∼1 nm-sized artificial tunnels. The tunnel diameter is confirmed by a comparative investigation of rejection performances between sub-nm-sized and >1 nm-sized PEG nanoparticles. WG is specifically investigated through the studies of surface and cross-sectional morphology, surface functional groups, and D-spacings in reference to that of pristine GO to reveal the driven force of the tunnel formation. As a result, the water permeability of the WG/GO composite membrane at the optimized WG:GO ratio of 1:2 is dramatically enhanced to 65.68 L m-2h−1 bar−1 from 1.36 L m-2h−1 bar−1 for pure GOM. The presence of ∼1 nm-sized nanotunnels also enables highly selective dye rejection performances of >1 nm-sized dye molecules (e.g. CR and TB). Correspondingly, compared to GOM, the CV rejection rate of WG/GO composite membrane is increased from 44.63% to 99.65%, while its NaCl and Na2SO4 rejection rates are decreased from 46.0 % and 62.0 % to 14.02% and 27.90%, respectively. Such results make such ∼1 nm artificial nanotunnel-enabled membranes with excellent dye (>1 nm) /dye (<1nm) and dye/salt separation performances.
Section snippets
Materials
GO (oxygen content: ≤40.0%, average lateral dimension: <4 μm, monolayer probability: <90%) was purchased from Sixth Element Materials Technology Co., Ltd (Changzhou, China). Polyethylene glycol (PEG) gel particles with different molecular weights (400, 600 and 1000), dyes including Trypan blue (TB, negatively charged, Mw 960.81), Congo red (CR, negatively charged, Mw 696.66), Rhodamine B (Rh B, neutral, Mw 479.01), Crystal violet (CV, positively charged, Mw 407.99), Methylene blue (MB,
Results and discussion
WG monolayers are realized using a two-step approach. The first step involves the thermal treatment of initial GO monolayers to induce the 3D wrinkles, in which we name the intermediate sample to “heat-treated reduced graphene oxide (HTGO)”. Subsequently, HTGO monolayers are modified with carboxyl groups to be transformed into WG through weak oxidation, which is expected to alleviate the agglomerate behavior of monolayers. The chemical composition and bonding information of GO, HTGO, and WG
Conclusions
We successfully produced approximately 1 nm-sized artificial tunnels within GOMs with selective and efficient separation properties by the intercalation of WG sheets which were produced by the surface modification of GO. With the optimum WG/GO ratio of 1:2, the GOM was modified sufficiently by abundant approximately 1 nm-sized nanotunnels, confirmed by the selective rejection performances of neutrally-charged PEG gel particles with diameters of >1 nm over those of sub-nm sized. Such a unique
Declaration of Competing Interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Prof. Tejraj (Bhavi) Aminabhavi (Editor of the Chemical Engineering Journal): competing research interests, Prof. Volodymyr Tarabara: competing research interests.
Acknowledgments
This work is supported by the National Natural Science Foundation of China (52172155), the scientific and technological projects for Distinguished Young Scholars of Sichuan Province (2020JDJQ0028). We also thank the Fundamental Research Funds for the Central Universities (Grant No. 2682021CX107 and 2682021CX118).
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