Studying two-dimensional zeolites with the tools of surface science: MFI nanosheets on Au(111)
Graphical abstract
Introduction
Zeolites are fascinating materials from the topological point [1] of view and of critical importance in several industrial processes related to separation of molecules, adsorption and catalysis [2], [3]. However, their 3D structures impose certain limitations to interrogate them with atomic precision and their detailed fundamental study using surface science methods has been very limited due to technical reasons [4]. One of the most important burdens is the fact that several of the surface science methods either require or benefit from the material under study being electrically conductive, which zeolites are not. Recent work has shown that crystalline two-dimensional aluminosilicate frameworks consisting of a planar arrangement hexagonal prisms, also known as double six-membered rings (D6R), as secondary building blocks, show similar chemical behavior as zeolites and can be used for model system studies [5]. This structure however does not contain the wide range of 3D pores and channels present in 3D zeolites and while crucial information can be obtained from them and their interaction with molecules, they have important limitations, which have been outlined in a recent perspective article [4].
The recent development of thin zeolite nanosheets [6], [7], [8], [9] containing a full range of 3D pores and channels and their deposition on conductive supports [10] provides now the opportunity to use surface science methods such as X-ray Photoelectron Spectroscopy (XPS), Scanning Tunneling Microscopy (STM) and Infrared Reflection Absorption Spectroscopy (IRRAS) to study these materials with unprecedented atomic level detail [4]. Furthermore, while surface science experiments are often performed under ultra-high vacuum conditions (UHV), the advent of elevated pressure versions of surface science methods in the last few years, allows carrying out these studies at conditions of pressure and temperature more relevant to the practical application of zeolites.
In this paper we explore these new opportunities by using Polarization Modulation IRRAS (PM-IRRAS) and Ambient Pressure XPS (AP-XPS) to study the case of all-Si MFI zeolite (or silicalite) nanosheets deposited on an Au(111) substrate. We chose to use atomically flat gold as a substrate because of its inert nature, to avoid any potential reactions of methanol on more reactive substrates, and the (111) face since it is as well the thermodynamically most stable one. Additionally, Au(111) remains clean upon air exposure. Note that MFI is the framework of ZSM-5, one of the most important zeolites for catalysis applications, including the methanol to gasoline conversion [11] among many others. XPS provides information on the composition of the structure as well as the oxidation states of all the elements in the framework and potentially adsorbates within the pores of the framework. IRRAS on the other hand, provides exquisite information on framework phonon vibrations as well as vibrational modes of molecules adsorbed or trapped within the pores. In order to illustrate the use of this technique, we study the case of methanol adsorption on MFI nanosheets, of importance to applications in catalysis for methanol to hydrocarbon conversion [11], [12] and for separation of alcohols from aqueous solutions [13], [14], [15], [16].
While this work provides a proof-of-concept of the surface science approach by IRRAS and XPS, we see no reason for other surface science methods not to be applied to the study of this and other two-dimensional nanoporous materials and we expect that there will be substantial growth in this direction in the coming years, including the application of scanning probe microscopies to perhaps even atomically resolve the surface of these structures.
Section snippets
Materials and methods
The Au(111) single crystal surface was cleaned with cycles of Ar+ sputtering and annealing at 900 K. The 3 nm thick MFI nanosheets were then deposited onto the Au(111) surface using a Langmuir–Schaefer method described in reference [10]. Studies on the structure and integrity of the MFI nanosheets prepared and deposited following the same procedure used for this work were reported in previous work, using high-resolution TEM and electron diffraction [10]. Further crystallographic investigation on
Results and discussion
Crystalline materials, such as zeolites show phonon framework vibrations detectable within the mid-IR range, i.e. 4000–400 cm−1 [20]. These phonon vibrations are associated to TO stretching, where T is a tetrahedral atom, most commonly Si, as in the case described here. These phonon modes are found below 1300 cm−1 and extend beyond the mid-IR range into the far-IR. Representations of the top and side views of the MFI nanosheet are shown in Fig. 1.
Fig. 2 shows an AFM image of the MFI nanosheets
Conclusions
Recent developments in the preparation of two-dimensional zeolites and their deposition onto well-defined conductive single crystal surfaces opens up the possibility of using the wide variety of tools developed through decades by the surface science community for their detailed study. This will result in much deeper levels of understanding of zeolites as well as their surfaces from the structural and chemical points of view. In this work we do a first approach to this idea by using infrared
Acknowledgements
Research carried out in part at the Center for Functional Nanomaterials and at the CSX-2 beamline of the National Synchrotron Light Source II, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-SC0012704. MFI nanosheet preparation was supported from the Center for Gas Separations Relevant to Clean Energy Technologies, an Energy Frontier Research Center funded by the US Department of Energy, Office of
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