Continuous ultrathin UiO-66-NH2 coatings on a polymeric substrate synthesized by a layer-by-layer method: a kind of promising membrane for oil–water separation

Jian Gao ab, Wei Wei a, Yuan Yin a, Meihua Liu a, Chunbai Zheng a, Yifan Zhang *a and Pengyang Deng *a
aCAS Key Laboratory of High-Performance Synthetic Rubber and its Composite Materials, Changchun Institute of Applied Chemistry, Changchun 130022, China. E-mail: yfzhang@ciac.ac.cn; pydeng@ciac.ac.cn
bUniversity of the Chinese Academy of Sciences, Beijing 100039, China

Received 26th November 2019 , Accepted 3rd February 2020

First published on 4th February 2020


In this paper, we successfully controllably synthesize continuous nanothickness MOF coatings (NTMCs) by a layer-by-layer method on a polymeric substrate. The polymeric substrate was pretreated with high energy γ-irradiation to induce a high surface density of living reactive groups, which ensure the formation of continuous surface-integrated NTMCs. SEM, FT-IR spectroscopy and XPS were used to characterize NTMCs. The thickness and morphology were tuned by the LBL cycles, and NTMCs with a thickness of ∼44 nm were obtained. The chemical bonds between the NTMCs and polymeric substrate were confirmed by XPS and EDS. Moreover, the NTMCs exhibit good performance for oil–water separation. We believe that our work will promote the design and precise synthesis of high-performance MOF based membranes for multiple practical applications in future.


Introduction

Thin MOF films/coatings have aroused a lot of attention in the separation field, because they not only retain high porosity, a uniform and tunable pore size and the well-defined pore structure of MOFs, but also exhibit advantages of thin films, such as a larger surface area, flexibility and easy recycling.1–4 In recent years, many different methods for the preparation of thin MOF films have been developed.5–7 For example, Hou and coworkers developed an immersion technique to form a continuous thin MOF layer about less than 1 μm thick on titania-functionalized porous polymeric supports, and the membranes showed molecular sieving behavior close to the theoretical permeability of ZIF-8.8 Li and coworkers reported a gel-vapour deposition methodology for the production of ultra-thin MOF membranes on polymeric hollow fibres which have a thin thickness of ∼17 nm, and the MOF membranes showed one to three orders of magnitude higher gas permeances than conventional membranes.9 Duan and coworkers fabricated MOF nanosheets with a thickness of ∼3.5 nm on different substrates through a chemical bath deposition method, and the integrated MOF electrodes demonstrated superior performances towards the OER, HER and overall water splitting.10 It is proved that thin MOF films exhibit excellent performance in molecular sieving and gas/liquid separation, as well as catalysis.11–14

However, until now, thin MOF films/coatings have faced great challenges during preparation of robust large-size, high-performance film/coatings on various substrates because the continuity and integrity of MOF film/coatings are easily destroyed during processing, leading to the superiority of the MOF materials could not be given full play in real-life application.15–19 The main problem is two-fold: (i) how to covalently graft high surface density arrays of reactive linkers available for unhindered in situ growth of MOF layers covalently integrated with the substrate, and (ii) how to fabricate large-size membranes with isotropic, spatially integrated MOF coatings of desired thickness retaining excellent MOF-substrate adhesion.

In this paper, the problem was resolved through two steps: first, we utilized a high energy γ-irradiation method to pretreat polymeric substrates which induce high surface density arrays of living reactive groups (LRGs); second, we employ a layer-by-layer (LBL) method to synthesize large size continuous nano-thickness MOF coatings (NTMCs) on the pretreated polymeric substrate, and precisely tune the morphology and the coating thickness by changing LBL cycles. NTMCs with a thickness of ∼44 nm were obtained. And the growth of NTMCs follows a “nucleation-growth-growth termination” process. The chemical bonds between the substrate and MOF coatings are confirmed, ensuring the strong adhesion between the NTMCs and substrate. Moreover, the NTMCs have good hydrophobicity and oleophilicity, making them promising separation membranes for treating oily waste water.

Our work will not only be conducive to deepen the understanding of the growth of NTMCs in theory, but also promote the design and precise synthesis of high-performance MOF based membranes for multiple practical applications in future.

Experimental

Preparation of a MAH-modified carboxyl surface by co-irradiation grafting

PP films were washed in acetone and dried at 60 °C in a vacuum oven prior to packing them vertically into a PE bag. The saturated MAH solution (approximately 1.5 g ml−1 in THF, 15.3 mol L−1) was added to the bag with the PP film to completely immerse it. After deaeration of the MAH solution by bubbling nitrogen gas for 4–7 min, the PE bag was sealed, thus forming a custom-made reactor accommodating any size substrate depending on the size of the bag. Subsequently, the “reactor”, i.e. PE bag with PP and MAH solution, was irradiated at room temperature with a Co-60 gamma source at a rate of 45 Gy per min to the total dose of 20 kGy. The MAH-grafted PP was thus obtained.

To form carboxyl groups on substrate surfaces, the MAH-grafted PP was then hydrolyzed using the following procedure: (i) sonication in ethanol and overnight soaking in water to remove the residual MAH, (ii) drying at 60 °C overnight, (iii) immersion in a 1 mM aqueous KOH solution (30 ml) at 50 °C for 5 min, (iv) immersion in a 1 mM HCl solution (40 ml) at room temperature for 10 min, (v) rinsing in deionized water until neutral pH is reached and (vi) drying in a vacuum oven at 60 °C for 12 h. After hydrolysis, PP-g-MAH was obtained.

Preparation of NTMCs by the LBL method

In contrast to the traditional solvothermal method in which MOF films grow through direct crystallization from the ‘mother solution’, the LBL method sequentially incorporates MOF building blocks, i.e., MOF metal clusters and organic ligands, allowing precise control of film formation. UiO-66-NH2 coatings were prepared on PP-g-MAH films (the surface density of grafted LRGs on the PP film was 13.77 nmol cm−2, for measurement details see the ESI) by the LBL method according to the following procedure:

(1) Growing the metal layer: ZrCl4 (0.15 g) was added to DMF (18 ml) and sonicated for approximately 20 min until the salt was completely dissolved. PP-g-MAH films, vertically placed in a Teflon-lined autoclave (diameter of 28 mm and height of 70 mm), were soaked in this solution and processed for 10 h at 120 °C. After synthesis, the samples were ultrasonically washed (3 times) for 10 min in DMF (30 ml) and then washed 3 times for 10 min in ethanol (30 ml). The samples were then dried overnight at 60 °C. The resultant sample, comprising a single layer of Zr clusters grafted onto PP-g-MAH, was denoted as LBL-1.

(2) Growing the organic ligand layer: a solution of the ligand (BDC-NH2, 0.163 g) and DMF (18 ml) was sonicated for 20 min. The LBL-1 samples fabricated in step (1) were soaked in this solution until reaching saturation and placed vertically in a Teflon-lined autoclave, which was kept at 120 °C for 10 h. After synthesis, the samples were ultrasonically washed (3 times) for 10 min in DMF (30 ml) and then washed 3 times for 10 min in ethanol (30 ml). The samples were then dried overnight at 60 °C. Thus, a single layer of ligands was grafted onto the surface of LBL-1, yielding the ‘2nd-step product’ further denoted as LBL-2.

Multi-layered NTMCs were prepared by repeating step (1) and step (2); i.e., after repeating step (1) on LBL-2, the obtained NTMC was denoted as LBL-3, and repetition of step (2) on LBL-3 yielded an NTMC denoted as LBL-4. Further repetition of steps (1) and (2) produced NTMCs denoted in accordance with the same principle, as shown in Fig. 1.


image file: c9nr10049k-f1.tif
Fig. 1 Schematic illustration of the LBL method for the preparation of ultra-thin MOF coatings on γ-irradiation-induced active surfaces.

Characterization

An FT-IR (Vertex 70, Bruker), an XPS (ESCALAB 250, Thermo Fisher Scientific) and an EDS (Oxford X-MaxN 150) were used to confirm the existence of UiO-66-NH2 on substrates, and a SEM (XL-30, FEI) was used to observe the surface morphology of NTMCs. A contact angle goniometer (DSA30, KRUSS) was used to measure the contact angle of the liquid (2 μL) on NTMCs, such as water, silicone oil, soybean oil and petroleum ether.

Results and discussion

The morphology of NTMCs

Fig. 2 presents the 3-D images of sequentially grown UiO-66-NH2@PP-g-MAH, which include the particle size and coating thickness recorded in each consecutive step. The SEM images (at the magnification level used in these analyses) demonstrate no discernible surface features on the surfaces of LBL-1 and LBL-2. For LBL-1, a dense array of secondary building units (SBUs), Zr6O4(OH)4, is located at each surface-grafted –COOH on PP-g-MAH, and for LBL-2, the earlier grown arrays of SBUs are terminated with BDC-NH2 organic ligands attached during this step of the LBL process. The LBL-3 image appears to show a coating whose build-up was not fully completed under the synthesis conditions (e.g., temperature and time) used in this work: discernible nanoclusters scattered across the substrate surface clearly show unfilled gaps between numerous MOF clusters of variable size and surface coverage density. The LBL-4 image shows a smooth, uniform film with MOF particles with a diameter of 32.3 nm densely covering the surface and forming a uniform and continuous coating layer with a thickness of 44 nm. Upon increasing the number of reaction cycles to obtain LBL-6, the particle size increased to 39.2 nm, covering the surface with a uniform, continuous and defect-free coating with a thickness of 57 nm. Upon further increase of the number of reaction cycles to LBL-7 and above, the increases in the particle size and coating thickness became negligible, and an increasing number of MOF aggregates appear on the finished surfaces. When the reaction cycle increased to LBL-13, the particle size and the coating thickness were 39.2 nm and 61 nm, respectively. Compared with the uniform and continuous MOF coatings formed when fewer than 13 reaction cycles are performed (e.g., LBL-12), some cracks appear in the LBL-13 coating. A further increase in the number of reaction cycles did not increase the particle size or the coating thickness; i.e., the particle size and the coating thickness of LBL-19 were 37.1 nm and 61 nm, respectively, and visible defects and cracks appeared on the surface of LBL-19. The continuous NTMC could form at the reaction cycles 4–12.
image file: c9nr10049k-f2.tif
Fig. 2 SEM images of NTMCs prepared by different LBL cycles. The red arrow marked the thickness of NTMCs.

During the LBL growth process of NTMCs, the ‘plateau’ reaches a maximum particle size and the coating thickness is reached under identical conditions: when the particle lateral (xy) size stops increasing, the coating thickness also stops increasing. The coating thickness [H] is always greater than the lateral size [∼diameter; D] of MOF particles, with Δ(HD) = 12–22 nm (considering the measurement error). This result is due to the fact that during LBL growth, MOF particles exhibit the same growth rate in all directions, i.e., along the x-, y- and z-axes. Before the crystallites commence growth along the x- and y-axes, the active sites of COOH-Zr have already been grafted onto the surface, so the length along the z-axis must be greater than those along the x-axis and y-axis when MOF particles grow along the x-, y- and z-axes simultaneously. In other words, the coating thickness must be larger than the particle size at the initial stage, and when the growth of MOF particles in the x- and y-axes stops, further growth of the coating can only be conducted along the relatively unrestricted z-axis.

Growth mechanism

The growth of NTMCs by the LBL method includes three stages: nucleation-growth-growth termination, as shown in Fig. 3. The nucleation occurs in the first two LBL cycles (LBL-1 and LBL-2), in this stage the LRGs capture Zr4+ which is followed by attachment of BDC, forming the crystal nucleus. The growth stage starts when the LBL cycle is larger than 2. During the growth stage, the particle size of the MOF and the coating thickness increase rapidly. Secondary nucleation and growth also occur. The growth termination occurs when the adjacent MOF particles collide with each other. And when the growth of MOF particles along x-and y- directions stops, the growth along the z-axis is terminated simultaneously. The ‘nucleation-growth-growth termination’ process was also confirmed during the synthesis of NTMCs on the NWF surface by the LBL method (for details see ESI section 3.2).
image file: c9nr10049k-f3.tif
Fig. 3 Growth process of NTMCs during the LBL process.

Characterization of NTMCs

FT-IR spectroscopy was used to characterize NTMCs on the polymeric substrate, as shown in Fig. 4a. Compared to PP, the new absorption peaks appear in the spectra of NTMCs at 3500–3000 cm−1, which are attributed to the deformation vibrations of –NH2,20 and the new absorption peaks at 1600–1200 cm−1 are dominated by the skeletal modes of the MOFs.21 The results indicate that UiO-66-NH2 was successfully synthesized on the PP surface. Typical XPS confirms that UiO-66-NH2 was successfully grown on the PP, as shown in Fig. 4b. The unmodified PP was used as a reference. Compared with PP, the content of oxygen on UiO-66-NH2@PP-g-MAH increases from 2.62 to 12.73 (at%), the content of nitrogen on UiO-66-NH2@PP-g-MAH is 0.57 (at%), and the content of zirconium on UiO-66-NH2@PP-g-MAH is 1.09 (at%). The obvious increase of the content of oxygen, nitrogen and zirconium confirmed the existence of UiO-66-NH2 on the surface of the PP substrate.
image file: c9nr10049k-f4.tif
Fig. 4 Characterization of NTMCs: typical FT-IR spectra (a) and typical XPS pattern (b).

Confirmation of chemical bonds

The stability of NTMCs is important for further industrial applications and is seldom reported for MOF coatings/films. The chemical bonds between the substrate and NTMCs and between MOF layers will endow NTMCs prepared by our technology with good stability. To verify the chemical bonds between the substrates and MOF coatings and the chemical bonds between MOF layers, EDS and XPS were utilized to analyse the Zr content on the surface of NTMCs and the chemical bonds between Zr and O during the growth process of NTMCs, as shown in Fig. 5. The content of Zr on LBL-1 increased from 0 to 2.55 wt%, compared to LBL-0 (Fig. 5a), and the XPS spectrum (Fig. 5b) shows a clear peak on the surface carboxyl peak at 531.5 eV, and a shoulder at 530 eV which is attributed to Zr–O–Zr,22 indicating that Zr ions are chemically bonded with surface carboxyl groups. For LBL-2, the content of Zr ions determined by EDS decreased. In the XPS results, the content of carboxyl groups increased, and the Zr shoulder significantly decreased, indicating a decrease in the content of unreacted Zr ions exposed on the surface due to the successful reaction of the organic ligand with the Zr-terminated reactive sites. A similar change in the Zr content was observed when comparing LBL-3 with LBL-4 and LBL-5 with LBL-6, which proves the existence of chemical bonds between MOF layers.
image file: c9nr10049k-f5.tif
Fig. 5 (a) EDS of NTMCs prepared with different numbers of reaction cycles. (b) Narrow-scan XPS spectra of NTMCs prepared by the LBL method in the Zr 3d5 and O 1s regions.

Surface wettability of NTMCs

The NTMCs have good hydrophobicity and oleophilicity, and the contact angle of four kind liquids on NTMCs including water, silicone oil, soybean oil and petroleum ether is shown in Fig. 6. The water contact angle on NTMCs first increases with LBL reaction cycles and then reaches a plateau. And the silicone oil contact angle of NTMCs decreases with LBL reaction cycles first and then reaches a plateau. The variation of the contact angle with LBL reaction cycles could be explained by the effect of surface morphology on surface wettability. The water contact angle on the UiO-66-NH2@NWF-g-MAH surface could reach 138.7° at LBL-4 and the silicone oil contact angle is ∼0°, indicating that the UiO-66-NH2@NWF-g-MAH could be a promising separation membrane for treating oily wastewater.
image file: c9nr10049k-f6.tif
Fig. 6 Contact angle of NTMCs, water contact angle (a) and silicone oil contact angle (b) for NTMCs on PP, the contact angle of different solvents (water, silicone oil, soybean oil and petroleum ether) for (c) LBL-4 on PP and (d) LBL-4 on NWF. The curve line in (a) is fitted.

Permeation of water and oil through NTMCs

To demonstrate the potential of NTMCs in oil–water separation, the permeation of water and oil through NTMCs was studied. NWF was selected as the substrate to prepare NTMCs (UiO-66-NH2@NWF-g-MAH), because the NWF substrate was usually applied in liquid separation due to its abundant micron-size pores. The permeation of water and soybean oil through UiO-66-NH2@NWF-g-MAH is shown in Fig. 7d and e; water could not permeate through UiO-66-NH2@NWF-g-MAH at a pressure of 28.29 mN cm−2; however, soybean oil under the same pressure could quickly permeate through UiO-66-NH2@NWF-g-MAH (for details see Video S1 and S2). The great difference of permeation between water and soybean oil through NTMCs is consistent with the surface wettability of NTMCs discussed above, confirming that NTMCs could be applied in oil–water separation.
image file: c9nr10049k-f7.tif
Fig. 7 The permeation difference of water and soybean oil through NTMCs. (a) Circular UiO-66-NH2@NWF-g-MAH film, and filtration devices before (b) and after assembly (c). (d) The permeation of water through UiO-66-NH2@NWF-g-MAH and (e) the permeation of soybean oil through UiO-66-NH2@NWF-g-MAH.

Conclusions

In conclusion, we successfully in situ synthesized continuous nano-thickness MOF coatings (NTMCs) by the LBL method, which are integrated with irradiation pretreated polymeric substrates. NTMCs with a thickness of ∼44 nm were obtained, and the morphology and the coating thickness can be tuned by the LBL step. The growth of NTMCs by the LBL method follows a ‘nucleation-growth-growth termination’ process. We confirmed that chemical bonds formed between the substrate and NTMCs and between MOF layers; this ensures the good stability of NTMCs. Moreover, the NTMCs show good hydrophobicity and oleophilicity, which could be applied for developing novel high-performance oil–water separation membranes.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank Prof. Xuewu Ge (University of Science and Technology of China) for his help in irradiation treatment. We are also grateful for the financial support from the Chinese National Nature Science Foundation (Project No. 51603201, Project No.51603202, and Project No. 51803208).

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Footnotes

Electronic supplementary information (ESI) available: Materials and methods for substrate treatment, film fabrication, characterization and liquid filtration. See DOI: 10.1039/c9nr10049k
J. G. and W. W. contributed equally to this work.

This journal is © The Royal Society of Chemistry 2020