Improving the Loading Capacity of MetalâOrganic Framework Thin

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Improving the Loading Capacity of Metal-OrganicFramework Thin Films by Using Optimized Linkers Wei Guo, Mei Qin Zha, Zhengbang Wang, Engelbert Redel, Zhengtao Xu, and Christof Wöll ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08622 • Publication Date (Web): 30 Aug 2016 Downloaded from http://pubs.acs.org on August 31, 2016

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ACS Applied Materials & Interfaces

Improving the Loading Capacity of Metal-Organic-Framework Thin Films by Using Optimized Linkers Wei Guo1,2, Meiqin Zha3, Zhengbang Wang1, Engelbert Redel1, Zhengtao Xu3, Christof Wöll1* 1

Karlsruhe Institute of Technology, Institute of Functional Interfaces (IFG), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

2

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Luoshi Road 122#, Wuhan 430070, PR China

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Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong, PR China

*

E-mail: [email protected]; Fax: +49-(0)721-608-23478; Tel: +49-(0)721-608-23775

Abstract: The large surface area of metal-organic frameworks (MOFs) sparks great interest in storage applications. Whereas the bulk of MOF applications focuses on the incorporation of gases, in this paper we demonstrate that these highly porous frameworks are also well suited for the storage of metal ions. For well-defined, highly oriented surface-anchored MOF thin films, grown on modified gold surfaces by using liquid-phase epitaxy (LPE),also referred to as SURMOFs, we have determined the loading of two different types of MOF-materials with a total of 7 types of metal ions (Zn2+, Ag+, Pd2+, Fe3+, Cd2+, Ni2+, Co2+). Measurements using a quartz-crystal microbalance (QCM) allowed determining loading capacities as well as diffusion constants in a quantitative fashion. The adsorption capacities are observed to be highly ion-specific, with the uptake being largest for Fe3+ and Pd2+-ions, with 6 and 4 metal ions per MOF pore, respectively. By comparing results for SURMOFs fabricated from different types of linkers, we demonstrate that in particular 1

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S-containing functionalities drastically improve the storage capacity of MOFs for metal ions. Key words: MOF, SURMOF, ion uptake, quartz crystal microbalance, optimized linkers

Introduction The porous structure of metal−organic frameworks (MOFs)1, 2 results in large loading capacities for guest molecules. Because this class of material, which is constructed by connecting di- or higher-topic organic linkers with metal or metal-oxo clusters, is rather numerous and very flexible, it provides an ideal platform for a variety of applications in biomedicine,3 for the construction of chemical sensors,4 as well as in gas separation5 and catalysts.6 During the past decade, it has emerged that aside from developing new types of MOF topologies that it is also interesting to fine-tune properties of a given MOF by adding different functionalities to the organic linkers, e.g. amino-,7azide-,8 and thiol-groups.9 By attaching such additional side groups, the performance parameters of MOF materials can be improved substantially, as has been demonstrated for different application fields, e.g., drug delivery,8fabrication ofsensors9, 10 and gas adsorption.11 A very appealing application of MOF is the uptake of guest species, in particular metal ions.12 Generally, directly loading of metal ions into MOF pores is inefficient if the organic linkers used for the construction of the MOFs do not contain any functional side groups. For example, a recent study employing XPS, ICP-OES and IR data revealed that only two La3+ atoms could be loaded into the large pores of HKUST-1 via the impregnation method.13With regard to achieving higher storage capacities, it is therefore mandatory to add additional functional groups to the inner walls of the pores within the molecular frameworks. He and co-workers have reported a novel type of sulfur-containing BDC-linkers (BDC: benzenedicarboxylic acid), which they used to fabricate a variant of MOF-5.14 The sulfur-containing moieties in this modified MOF-5 provide excellent centers to bind noble-metal ions, as evidenced by characteristic color changes of the host material. In the present work, we fabricated a special type of MOF coatings, referred to as surface-mounted metal-organic frameworks (SURMOFs).15 These porous, monolithic and crystalline metal-organic thin layers 2


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were grown on modified gold surfaces using the liquid-phase epitaxy (LPE) technique.16 LPE is quite attractive for depositing MOFs in a layer-by-layer fashion, since it allows for the growth of laterally patterned structures17 as well as multi-heteroepitaxial layers. In this work, we used the LPE technique to fabricate a layer-pillar-type Cu2(atBDC)2(dabco) SURMOF, [atBDC, 2,5-bis(allylthio)terephthalic acid;14 dabco, 1,4-Diazabicyclo[2.2.2]octane] (see Figure 1a and b) on a quartz crystal microbalance (QCM) sensor which allows one to accurately monitor the mass increase during the SURMOF growth and the subsequent loading process. Previous work has demonstrated that QCM is a simple but efficient way to investigate the uptake behavior of guests in SURMOF thin films.18 We have obtained the total mass uptake of the Cu2(atBDC)2(dabco) SURMOF for a series of metal ions (Zn2+, Ag+, Pd2+, Fe3+, Cd2+, Ni2+, Co2+). The largest adsorption capacity was found for Fe3+ and Pd2+ ions. Our results demonstrate that the sulfur and alkene side-chain attached to the BDC ligand leads to an improved adsorption capacity for Fe3+ and Pd2+ uptake. A quantitative analysis of the temporal evolution of the uptake curves also allows to determine the corresponding diffusion coefficients, a comparison of Cu2(atBDC)2(dabco) SURMOF with Cu2(BDC)2(dabco) SURMOF reveals that in parallel to the increase of the loading capacities the presence of the side chains the diffusion coefficients decreases.

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Figure 1(a) Schematic illustration of Cu2(atBDC)2(dabco) SURMOF; (b) The structure formula of atBDC and dabco molecules. The SURMOF samples used in the present work were either grown on modified Au surfaces using the LPE technique (for QCM and IR measurements), or on quartz glass for the UV-Vis measurements. The surface modification of the Au-substrates was carried out by depositing a SAM (self-assembled monolayer) made from 11-mercapto-1-undecanol (MUD, 99%, Aldrich) for the Au substrates.19 For the surface treatment of quartz glass we used O2 plasma cleaning.20 The SURMOF samples used for the uptake analysis were grown inside a QCM system in an alternating fashion (Figure S-1), as described in detail in an earlier publication.19 Gradually, adding metal solution and ligand solution in a sequential fashion leads to a stepwise mass increase of the deposited layer. This layer-by-layer growth of SURMOF on the sensor surface can be monitored directly, see Figure S-5. After completion of the growth, the freshly synthesized MOF thin film can be directly used for further experiments such as monitoring of adsorption/desorption of guest molecules into SURMOFs.21 For the present experiments, 4


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the pumping times were 10 min for the copper acetate solution (1mM) and 20 min for the atBDC/dabco solution (0.2mM/0.1mM). Each LPE step was followed by a rinsing step (5 min) with pure ethanol to remove residual reactants. A total of 25 growth cycles were used for all SURMOFs investigated in this work. From the total uptake as determined by QCM and the known density of this material we obtain SURMOF thicknesses of SURMOFs of 80 nm [Cu2(BDC)2(dabco)] and 64 nm [Cu2(atBDC)2(dabco)], respectively. In addition to SURMOFs grown on QCM substrates, we have also deposited these MOF thin films on regular Au-substrates by a pump system by employing the same parameters as used in the QCM system (reaction temperature, solution concentration, numbers of growth cycles and reaction time), as described in detail in an earlier publication.22 These samples were used for the XRD and IR characterization. Before further processing, we have characterized all SURMOF samples by X-ray diffraction (XRD) (Figure2 and Figure S-2). Further characterization of the Cu2(atBDC)2(dabco) and Cu2(BDC)2(dabco) SURMOFs was carried out using Infrared (IR) spectroscopy (Figure S-3) and UV-Vis spectroscopy (Figure S-4). The presence of a broad and strong band at 1700-1300 cm-1 is assigned to vibrations of COO- of pristine Cu2(BDC)2(dabco) SURMOF (Figure S-3, red). The position of these bands agrees well with those reported previously for pristine Cu2(BDC)2(dabco) SURMOF.23 In contrast to the Cu2(BDC)2(dabco) (Table S-1), a new characteristic vibration at 924 cm-1 (R-CH=CH2) is observed for the Cu2(atBDC)2(dabco) SURMOF sample (Figure S-3, black), which can unambiguously be assigned to the alkene functional groups. UV-Vis result show an absorption increase in the range of 324-424 nm for the Cu2(atBDC)2(dabco) SURMOF relative to the Cu2(BDC)2(dabco) SURMOF, which is explained by the absorption of the thio-ether unit within the additional side chain (Figure S-4). In Figure 2 we present XRD data recorded in an out-of-plane geometry for the pristine, monolithic and oriented SURMOFs. As reported in previous works for this type of SURMOFs grown on MUD-SAM modified Au-substrates,22 the (001) peaks are well-defined and sharp (Figure 2, left red). The relative intensities of the diffraction peaks are in good agreement with simulations assuming perfect Cu2(BDC)2(dabco) material (Figure 2, left blue). The absence of diffraction peaks for other crystallographic directions in the out-of-plane 5



data reveals that the SURMOFs growth proceeds only along the [001] crystallographic direction on MUD-SAM-modified Au substrate. This oriented growth is also evidenced by the in-plane XRD data, which shows the [100] and [200] XRD-peaks, but not the [001] peaks (Figure 2, right red). The positions of the XRD peaks seen for Cu2(BDC)2(dabco) SURMOF is the same as that seen for Cu2(atBDC)2(dabco) SURMOF, demonstrating that the unit cell dimensions of the two different layer-pillar SURMOFs are the same. In-plane

Out-of-plane Cu2(atBDC)2(dabco)

Cu2(atBDC)2(dabco)

Cu2(BDC)2(dabco)

Cu2(BDC)2(dabco)

Simulation for (001) orientation Simulation for powder MOF

Simulation for (001) orientation of in-plane Simulation for powder MOF

(100)

(001)

Normalized Intensity / cps

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(200)

(002)

(110)

(210)

(100) (110) (101)

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2θ /

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Figure 2 X-ray diffraction patterns recorded for Cu2(atBDC)2(dabco) (black), Cu2(BDC)2(dabco) (red), simulation of (001) orientation (blue) and simulation of powder (magenta) of SURMOFs. The quantitative analysis of metal ion loading into the SURMOF was performed using QCM uptake experiments. As shown in Figure 3, a series of metal nitrate compounds [Zn(NO3)2, Ag(NO3), Pd(NO3)2, Fe(NO3)3, Cd(NO3)2, Ni(NO3)2, Co(NO3)2] were investigated. For all ions, the solutions of the same concentrations (0.1 mM) were prepared by dissolving the inorganic compounds in ethanol. The QCM-data (Figure 3) revealed that the maximum uptake capacities for Fe3+ and Pd2+ are 0.41 µg·cm-2 and 0.49 µg·cm-2, respectively (Figure 3, green and blue). From the corresponding thicknesses (64 nm) the corresponding number of ions per pore can be computed and are found to amount to 6 Fe3+ and 4 Pd2+ molecules per pore of Cu2(atBDC)2(dabco) SURMOF. The maximum loading capacities for of Ag+ and Co2+ are much lower (Figure 6


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3, red and wine), they amount to 0.08 µg·cm-2 (0.6 molecules per pore) for Ag+ and 0.03 µg·cm-2 (0.48 molecules per pore) for Co2+. The lowest loading capacities,

Improving the Loading Capacity of MetalâOrganic Framework Thin

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Improving the Loading Capacity of MetalâOrganic Framework Thin