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Wellcome Dr. Yildirim's Research Website
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Our research addresses structural, magnetic, and transport properties of novel materials with
an eye toward practical applications. This is done by calculating the properties of real materials
using first-principles computational techniques and testing the results by neutron scattering
and other measurements. Systems of particular interest include molecular solids such as the
fullerenes and cubane, nanoporous materials such as metal-organic frameworks for hydrogen storage,
frustrated magnetic systems such as the Kagome lattice and cuprates, novel superconductors
such as iron-pnictides, doped fullerenes and magnesium diboride, and nanomaterials such as
nanotubes and molecular magnets.
RECENT RESEARCH HIGHLIGHTS:
A highly practical route for large-area, single layer graphene from liquid carbon sources such as benzene and methanol
J. Mater. Chem., 2011, 21, 16057-16065
Through a detailed systematic study, we determined the parameters critical for high-quality, single-layer graphene formation and developed a straightforward synthesis that requires no explosive hydrogen or methane gas flow. The synthesis is further simplified by using only a liquid carbon source such as methanol. Of over a dozen liquid carbon sources studied, methanol is found to be unique in that it acts as both a carbon/hydrogen source and an inhibitor to amorphous carbon growth. No deposition of amorphous carbon was observed, regardless of vapor pressure, unlike methane and other hydrocarbons. Finally, we describe a protocol to control graphene growth to a single side or selected location on the copper substrate, which is required for most device applications. Using our novel methods, we have prepared high-quality, single-layer graphene samples at the inch scale that have been thoroughly characterized with Raman spectroscopy, optical transmittance, scanning electron microscopy and sheet resistance measurements. Our method is safe, simple, and economical and will be of value to both fundamental researchers and nanodevice engineers.
Read more:
A highly practical route for large-area, single layer graphene from liquid carbon sources such as benzene and methanol,
Srinivas Gadipelli, Irene Calizo, Jamie Ford, Guangjun Cheng, Angela R. Hight Walker and Taner Yildirim*,
J. Mater. Chem., 2011, 21, 16057-16065
Efficient Carbon Capture in Metal-Organic Frameworks (MOFs)
Energy Environ. Sci., 2011, 4, 2177-2185 DOI: 10.1039/C0EE00700E
Carbon capture is a critical component of the mitigation of CO2 emissions from industrial plants.
Investigations of the application of MOFs to adsorptive carbon capture have focused on their
appreciable storage capacities but fail to address the more pertinent issue of how MOFs
perform under common industrial separation processes that are at the heart of carbon capture.
Typical processes rely on swing adsorption and are limited to relatively low CO2
partial pressures such that the total pore volume and the surface area are under-utilized.
Here, we investigate the performance of a number of MOFs with particular
focus on their behavior at the low pressures commonly used in swing adsorption.
This comparison clearly shows that it is the process that determines which
MOF is optimal rather than there being one best MOF, though MOFs that possess
enhanced binding at open metal sites generally perform better than those
with high surface area. This work will be an important guideline for deciding
the best pair of carbon capture process and MOF material for optimum carbon capture.
Read more:
Carbon capture in metal-organic frameworks-a comparative study,
J. M. Simmons, H. Wu, W. Zhou and Taner Yildirim*,
Energy Environ. Sci., 2011, 4, 2177-2185
DOI: 10.1039/C0EE00700E
Fast and Clean Hydrogen Generation from AB-loaded MOF
Chem. Eur. J. , 20 April 2011, DOI: 10.1002/chem.201100090
Among many hydrogen storage candidate materials, ammonia borane (AB) has been considered
as one of the most promising ones because of its remarkably high hydrogen content
(19.6 wt.%), moderate decomposition temperature and stability. However, the direct
use of pristine AB in practical applications so far is prevented due to its slow
H2 release kinetics below 100 oC and the detrimental volatile byproducts;
ammonia, borazine and diborane.
In this communication, we report that AB intercalated metal-organic frameworks (MOFs)
not only enhance the hydrogen release kinetics but also suppress the
detrimental byproducts which is the most important outstanding issue
for practical fuelcell applications. The observed hydrogen release
kinetics and prevention of byproducts are found to critically
depend on the nature of metal-type and its coordination,
the size of the framework pore structure and the level of AB loading.
The results reported here bring us one step closer to using AB as a
hyrogen storage medium for the polymer electrolyte membrane fuel-cell applications.
Read more:
Nanoconfinement and Catalytic Dehydrogenation of Ammmonia Borane by
Magnesium-Metal-Organic-Framework-74, S. Gadipelli1, J. Ford,
W. Zhou, H. Wu, T. J. Udovic, and Taner Yildirim*,
Chem. Eur. J. 2011, April 20, DOI: 10.1002/chem.201100090
Graphene Oxide Framework Materials Suggested for Hydrogen Storage and Carbon Capture
Angew. Chem. Int. Ed. 2010, 49, 8902-8904
In this communication, we show that one-and-a-half-century-old graphene oxide (GO)
can be easily turned into a potentially useful gas storage material.
GO is a sheet of carbon atoms with many hydroxyl, epoxide and carboxyl surface groups.
In principle, hydrogen can be stored between layers of this lightweight material.
However, the challenge is to separate the layers without filling the space between them.
Here we show that by using the well-known chemistry between diboronic acids and hydroxyl groups,
GO layers can be linked together to form a new layered structure.
Such GOF structures have tunable pore widths, volumes, and binding sites depending
on the linkers chosen, and could exhibit interesting gas sorption properties.
As discussed in the manuscript, ideal GOF structures can adsorb hydrogen up to
6 wt% at 77 K and 1 bar, a value higher than any other porous material known.
Our synthesized GOF materials exhibit 9 kJ/mol and 32 kJ/mol isosteric heat of
adsorption for H2 and CO2, significantly larger than those found in
similar nanoporous materials. The nitrogen BET surface area reaches
a maximum at 470 m2/g. Despite this low surface area,
GOF exhibits 1 wt% H2 uptake at 1 bar. This is much less than what the 'ideal' GOF
structure can hold, suggesting that our initial synthesized
GOF materials could be significantly optimized in the near future.
Read more:
Graphene Oxide Framework Materials: Theoretical Predictions and Experimental Results,
J. W. Burress, S. Gadipelli1, J. Ford, J. M. Simmons, W. Zhou, T. Yildirim*,
Angew. Chem. Int. Ed. 2010, 49, 8902-8904
The Unprecedented Giant Magneto-Elastic Coupling in New Iron-based superconductors
T. Yildirim, Phys. Rev. Lett., 102, 037003 (2009)
From first principles calculations we unravel surprisingly strong interactions between arsenic ions
in iron-pnictides, the strength of which is controlled by the Fe-spin state. Reducing the Fe-magnetic
moment, weakens the Fe-As bonding, and in turn, increases As-As interactions, causing giant
reduction in the c-axis. For
CaFe2As2
system, this reduction is as large as 1.4 Ang. (~13%).
Since the large c-reduction has been recently observed only under high-pressure,
our results suggest that the iron
magnetic moment should be present in Fe-pnictides at all times at ambient pressure.
The giant coupling of the on-site Fe-magnetic moment with the As-As bonding
that we have discovered here may provide a mechanism for the superconductivity.
Read more:
T. Yildirim, Phys. Rev. Lett. 102, 037003 (2009) and
arXiv:0807.3936v2
(PRL's extended version)
Nature and Tunability of H2-Binding in MOFs
W. Zhou and T. Yildirim, J. Phys. Chem. C, 112 , 8132 (2008)
MOF compounds with exposed transition-metal (TM) sites exhibit impressive heats of adsorption of H2,
thus the TM-H2 interaction was believed to be of the "Kubas-type". Our calculation shows that the H2 binding
in Mn4Cl-MOF is not of the expected Kubas-type. Instead, the major contribution to the overall binding
comes from the classical Coulomb interaction, which is not screened due to the open-metal site.
We also show that the orientation of H2 has a surprisingly large effect on the binding potential,
reducing the classical binding energy by almost 30%. Read more...
Strong Dependence of H2-Binding on Metal Ions in MOFs
W. Zhou, H. Wu, and T. Yildirim,
J. Am. Chem. Soc., 130, 15268 (2008),
We conducted a systematic study of the H2 adsorption on a series of isostructural
MOFs, M2(dhtp) (M=Mg, Mn, Co, Ni, Zn). The experimental Qst for H2 of these MOFs range from 8.5 to
12.9 KJ/mol, with increasing Qst in the following order: Zn, Mn, Mg, Co, and Ni.
The H2 binding energies derived from DFT calculations follow the same trend.
We also found a strong correlation between the metal ion radius, the M-H2 distance
and the H2 binding strength, which provides a viable, empirical method
to predict the relative H2 binding strength of different open metals.
Read more...
Metal-Ethylene Complexes for Hydrogen Storage
 In our recent studies
[Phys. Rev. B 76, 085434 (2007),
Phys. Rev. Lett. 97, 226102 (2006)],
we suggest that co-deposition of metals (i.e. Ti/Li) with small organic molecules such
as very cheap ethylene molecule (a ho-hum material that is the building block of the
most common plastic) into nanopores of low-density high surface materials could be
a very promising direction for discovering new materials with better storage properties.
We found to our surprise that the interaction of Ti with the C=C double bond of
ethylene molecule (i.e. C2H4 ) mimics what we found in C60.
Detailed first-principles calculations show that the complex resulting
from attaching a Ti atom to each ends of C2H4 (see figure) will reversibly
adsorb ten H2 molecules. The equivalent material gravimetric capacity of 14%,
if realized in practice, would readily exceed the 2015 DOE system goal.
Another advantage is that, unlike our previously predicted structures
involving fullerenes and nanotubes, the metal-ethylene complexes
have actually been synthesized and actively studied as catalytic
systems for several decades. Their potential for hydrogen storage
was first revealed by our theory and modeling work.
Some preliminary experimental results demonstrating 14 wt% H2 adsorption
at 300K on Ti-C2H4 complexes formed by laser ablation has been recently published in
Phys. Rev. Lett. 100, 105505 (2008).
Press Coverage: physorg.com,
greencarcongress.com,
azonano.com,
treehugger.com,
NikkeiBP_Nozawa.pdf,
cnpublications.net
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