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Nanotechnology Paper
Metal Organic Frameworks
By:
Daniel Johnston
In our Nano technology class we examine a number of different Nano scale
structures such as carbon Nanotubes, silver Nano wires, and the composition and
construction of diodes. But I believe that I know of another beneficial material, Metal
Organic frameworks or MOFs. (I also did extensive research on MOFs in 2012.) MOFs
are a crystalline mix of organic and inorganic chemicals and are truly intriguing. They
can be constructed, modified, and functionalized into custom built chemical lattices or
cages similar to C60 or Bucky Balls, which can be highly porous and react in specially
designed manners. In this way the material can be designed and used in applications
ranging from gas storage and catalysis to electronic components.
Typically a MOF is a highly ordered nanoporous crystalline material with a
structure that is composed of metal cluster centers connected by organic ligands in a
grid or lattice type fashion (similar to a scaffolding setup to paint a building). The
functionality and usage of the MOF can be custom tailored for the application desired.
For instance by changing out the metal cluster from say a cobalt based compound with
a nickel based one, we can cause its reactivity to be altered for different things. And
also by changing out the ligand with say an optically reactive ligand that will change
shape when exposed to UV light or a longer ligand, we can modify the pore size within
the crystal lattice of the MOF, thereby selectively creating the correct pore and active
site for the desired application. The characteristic creation of a MOF is typically by the
use of solvothermal techniques. This procedure is usually completed by mixing the
ligand of desire and the metal organic center of choice within a solvent then placed
inside a pressure vessel and heated to high temperatures for 36+ hours in a
programmable oven. Where in after the solution is removed from the oven and has
returned to room temperature the crystals of the MOFs can be extracted and analyzed
via Powder X-ray diffraction, Infrared spectroscopy, Ultraviolet spectroscopy, and H
Proton NMR.
The practice of using a MOF for gas storage is the topic that originally caught my
interest when deciding on a research project with my professor whom headed my
research. Gas storage is an important topic when energy storage is considered. An
example being either hydrogen or Natural gas powered cars, how can we store enough
gas on board to travel a given distance? One possible solution would be by using
MOFs that have been designed to maximize the storage capacity of a given container of
gas. This sounds counter intuitive that by placing a material inside of a gas cylinder that
we can increase its storage capacity. But this is possible because of the absorbent and
highly porous nature of MOFs. By adding in a specific MOF to a gas container one can
add from 3-10% extra storage capacity. This can occur because with such a huge
surface area there are many open areas within the MOFs’ crystalline structure that can
sequester the gas thereby increasing the overall storage capacity. I believe that with
further research and development on these materials, I can imagine that we could
safely double our gas storage ability via the use of MOFs and their Nano porosity.1
However, the focus of my research was in regards to the MOFs ability of being
used as a catalyst. Typically a good catalyst is one that reacts very fast with a substrate
and brings about the reaction. But as we try to make the catalyst more reactive, the
catalyst, itself, becomes prone to degradation. For Metal based organometallic
1
Mendoza-Cortes, Jose L.; Han, Sang Soo; Goddard Wa, William A.: High H2Uptake in Li-, Na-, K- Metalated Covalent Organic
Frameworks and Metal Organic Frameworks at 298 K. The Journal of Physical Chemistry., 2012, A116, 1621-31 DOI: 10.1021/jp206981d
catalysts, moisture becomes a big problem for the stability of the metal site, especially if
the metal site is designed to bind to the substrate fast. To analyze the stability of these
different structures, I constructed a number of different MOFs and created their
appropriate molecular variations. In particular I synthesized MOFs with various different
metal sites; Such as Cu(II), Zn(II), Ni(II), Cr(III), Ru(II), Co(II). Also, I synthesized the
molecular complexes of the respective metal sites with similar coordination
environments to their corresponding MOF. I then explored immersion testing in
H2O/Non-aqueous medium of both the molecular and MOF form of the different metal
sites. I immersed the chosen chemical in various concentrations of water and nonaqueous solvents for 12+ hours and then removed the molecular complex/MOFs. I then
examined the MOFs via powder x-ray diffraction (PXRD), and the molecular complex
via UV-spectroscopy and H NMR.2
I discovered that the MOF version of the catalyst was indeed many times (>103
times) more structurally stable and catalytically active. Using the information that the
MOF bolstered catalyst is many times more resilient to degradation then the pure
molecular version, one could think of many applications for MOF stabilized compounds,
especially where highly reactive, robust and cheap catalysts are required.
Next with MOFs as electronic components, there has been interest for their
application in 4 main areas, sensors, passive electronics, active electronics, and as
support structures.
2
Das, S.; Johnston, D. E.; Das, S.: Structural Bolstering of Metal Sites as Nodes in Metal Organic Frameworks. CrystEngComm, 2012, 14, 61366139 DOI: 10.1039/C2CE25555C (among the top 10 most highly accessed papers in Aug, 2012)
As a sensor a silicon cantilever would be coated on one side with a MOF
designed to react to a certain condition. When this sensor is exposed to the condition it
would change shape by absorbing the particles or reacting with the condition and
deflect the cantilever which in turn would cause a laser to register the deflection (similar
to an AFM tip in contact mode). These sensors could be custom made to register
different condition changes such as temperature, pressure, radiation, magnetic field,
electric potential, acoustic waves, and chemical presence. This condition sensitivity is
of course based on the chemical composition and design of the MOF used on the
sensor.
As a passive electronic component the MOF would not play a major role in
transport or control of electrons but rather would function as a dielectric in different
components such as transistors. As a component in a transistor the MOF material is
placed between the source, gate, and the drain connections. In this way the MOF
would assist in decreasing the size of transistors, due to a MOF dielectric material being
better suited then the current dielectric materials in regards to electron flow.
As an active electronic component the MOF would serve as a part of the P-Njunction in say a solar cell. In this application the MOF would be made semi conducting
and would be treated in a way that allows the capture and transport of the electron
excitations to the terminals of the solar cell. This application would be a very good for a
MOF because a MOF can be custom made in extremely thin and flexible layers that
could be treated in a number of different solutions or chemicals to functionalize the MOF
for a desired task or wavelength of light.
As a support structure a MOF lattice could be formed and then for example silver
nanowires or carbon nanotubes could be grown within the crystal lattice allowing the
formation of a composite material that would be highly conductive but also structurally
strong. Essentially this use of MOFs would allow for the design and utilization of single
digit Nano scale arrays of Nano wires which could lead to the further shrinking of
electronic components.3
In conclusion I believe that MOFs have a very big future and huge potential in
energy storage and as a viable electronics addition, as well as to science as a whole. I
believe that these materials are versatile and due to their moldable properties, they are
highly customizable and configurable for a range of desired applications. Additionally
with their highly organized self-assembly structure they could be made cheaply and
efficiently, when the desired MOF is formulated. It is in this fashion that I believe that
they are noteworthy and a fantastic addition to Nano scale material science in general.
Also furthermore, some of the crystals are very nice looking as well such as Co-Zif-9
which reminds me of tiny amethyst crystals.
3
Mark D. Allendorf, Adam Schwartzberg, Vitalie Stavila, A. Alec Talin.: A roadmap of Implementing Metal-Organic Frameworks in Electronic
Devices: Challenges and Critical Directions. Chem.. Eur. J., 2011, 17, 11372 – 11388. DOI: 10.1002/chem.201101595
References:
Mendoza-Cortes, Jose L.; Han, Sang Soo; Goddard Wa, William A.: High H2Uptake in Li-, Na-, K- Metalated Covalent Organic
Frameworks and Metal Organic Frameworks at 298 K. The Journal of Physical Chemistry., 2012, A116, 1621-31 DOI: 10.1021/jp206981d
2
Das, S.; Johnston, D. E.; Das, S.: Structural Bolstering of Metal Sites as Nodes in Metal Organic Frameworks. CrystEngComm, 2012,
14, 6136-6139 DOI: 10.1039/C2CE25555C
3Mark
(among the top 10 most highly accessed papers in Aug, 2012)
D. Allendorf, Adam Schwartzberg, Vitalie Stavila, A. Alec Talin.: A roadmap of Implementing Metal-Organic Frameworks in
Electronic Devices: Challenges and Critical Directions. Chem.. Eur. J., 2011, 17, 11372 – 11388. DOI: 10.1002/chem.201101595