Tag Archive: Experiment

Sep 25

Mechanochemical route to the synthesis of nanostructured Aluminium nitride

Authors: S. A. Rounaghi, H. Eshghi, S. Scudino, A. Vyalikh, D. E. P. Vanpoucke, W. Gruner,
S. Oswald, A. R. Kiani Rashid, M. Samadi Khoshkhoo, U. Scheler and J. Eckert
Journal: Scientific Reports 6, 33375 (2016)
doi: 10.1038/srep33375
IF(2016): 4.259
export: bibtex
pdf: <Sci.Rep.> (open access)

Abstract

Hexagonal Aluminium nitride (h-AlN) is an important wide-bandgap semiconductor material which is conventionally fabricated by high temperature carbothermal reduction of alumina under toxic ammonia atmosphere. Here we report a simple, low cost and potentially scalable mechanochemical procedure for the green synthesis of nanostructured h-AlN from a powder mixture of Aluminium and melamine precursors. A combination of experimental and theoretical techniques has been employed to provide comprehensive mechanistic insights on the reactivity of melamine, solid state metalorganic interactions and the structural transformation of Al to h-AlN under non-equilibrium ball milling conditions. The results reveal that melamine is adsorbed through the amine groups on the Aluminium surface due to the long-range van der Waals forces. The high energy provided by milling leads to the deammoniation of melamine at the initial stages followed by the polymerization and formation of a carbon nitride network, by the decomposition of the amine groups and, finally, by the subsequent diffusion of nitrogen into the Aluminium structure to form h-AlN

Mar 01

Helium flash: the beginning of a new chapter.

During the past two and a half years, part of being a delocalized physicist has meant for me that I had to work at one end of the country while my girlfriend and son lived at the other. Today this situation drastically changed, as I moved with my FWO-postdoctoral project from my alma mater to the University of Hasselt, where I started in the Wide Band Gap Materials group of Prof. Ken Haenen.

My delocalization will now take the form of Metal-Organic Frameworks on the one side and Diamond based materials on the other. As the sole computational solid state physicist in an otherwise entirely experimental group (and even institute) I seem to have returned to a well known configuration (At Ghent university I was initially the house-theoretician of the SCRiPTS group). Also the idea of performing calculations on diamond brings back memories, since this allotrope of carbon lives two levels above the germanium on which Pt nanowires grow. All-in-all I look forward to an exciting time. But first things first: getting my HPC credentials and data safely transported from the one end of the country to the other.

Nov 16

A Flexible Photoactive Titanium Metal-Organic Framework Based on a [TiIV33-O)(O)2(COO)6] Cluster

Authors: Bart Bueken, Frederik Vermoortele, Danny E. P. Vanpoucke, Helge Reinsch, Chih-Chin Tsou, Pieterjan Valvekens, Trees De Baerdemaeker, Rob Ameloot, Christine E. A. Kirschhock, Veronique Van Speybroeck, James M. Mayer and Dirk De Vos
Journal: Angew. Chem. Int. Ed. 54(47), 13912-13917 (2015)
doi: 10.1002/anie.201505512
IF(2015): 11.705
export: bibtex
pdf: <Angew.Chem.Int.Ed.> 

Abstract

The synthesis of titanium-carboxylate metal-organic frameworks (MOFs) is hampered by the high reactivity of the commonly employed alkoxide precursors. Here, we present an innovative approach to Ti-based MOFs using titanocene dichloride to synthesize COK-69, the first breathing Ti-MOF built up of trans-1,4- cyclohexanedicarboxylate linkers and an unprecedented [TiIV33-O)(O)2(COO)6] cluster. The photoactive properties of COK-69 were investigated in-depth by proton-coupled electron transfer experiments, which revealed that up to one TiIV per cluster can be photoreduced to TiIII, while preserving the structural integrity of the framework. From molecular modeling, the electronic structure of COK-69 was determined and a band gap of 3.77 eV was found.

Nov 05

Understanding intrinsic light absorption properties of UiO-66 frameworks: A combined theoretical and experimental study

Authors: Kevin Hendrickx, Danny E.P. Vanpoucke, Karen Leus, Kurt Lejaeghere,
Andy Van Yperen-De Deyne, Veronique Van Speybroeck, Pascal Van Der
Voort, and Karen Hemelsoet
Journal: Inorg. Chem. 54(22), 10701-10710 (2015)
doi: 10.1021/acs.inorgchem.5b01593
IF(2015): 4.820
export: bibtex
pdf:  <Inorg.Chem.>

Abstract

Linker-functionalization of UiO-66 modifies the optical band gap and thus the color of the MOF.

Linker-functionalization of UiO-66 modifies the optical band gap and thus the color of the MOF.

A combined theoretical and experimental study is performed in order to elucidate the eff ects of linker functional groups on the photoabsorption properties of UiO-66-type materials. This study, in which both mono- and di-functionalized linkers (with X= -OH, -NH2, -SH) are studied, aims to obtain a more complete picture on the choice of functionalization. Static Time-Dependent Density Functional Theory (TD-DFT) calculations combined with Molecular Dynamics simulations are performed on the linkers and compared to experimental UV/VIS spectra, in order to understand the electronic eff ects governing the absorption spectra. Di-substituted linkers show larger shifts compared to mono-substituted variants, making them promising candidates for further study as photocatalysts. Next, the interaction between the linker and the inorganic part of the framework is theoretically investigated using a cluster model. The proposed Ligand-to-Metal-Charge Transfer (LMCT) is theoretically observed and is influenced by the differences in functionalization. Finally, computed electronic properties of the periodic UiO-66 materials reveal that the band gap can be altered by linker functionalization and ranges from 4.0 down to 2.2 eV. Study of the periodic Density of States (DOS) allows to explain the band gap modulations of the framework in terms of a functionalization-induced band in the band gap of the original UiO-66 host.

Sep 12

Fine-tuning the theoretically predicted structure of MIL-47(V) with the aid of powder X-ray diffraction

Authors: Thomas Bogaerts, Louis Vanduyfhuys, Danny E. P. Vanpoucke, Jelle Wieme,
Michel Waroquier, Pascal van der Voort and Veronique van Speybroeck
Journal: Cryst. Eng. Comm. 17(45), 8612-8622 (2015)
doi: 10.1039/c5ce01388g
IF(2015): 3.849
export: bibtex
pdf: <Cryst.Eng.Comm.> 
Graphical Abstract: Which model represents the experimental XRD-spectra best? Ferromagnetic or anti-ferromagnetic chains? With of without offset?
Graphical Abstract: Which model represents the experimental XRD-spectra best? Ferromagnetic or anti-ferromagnetic chains? With of without offset?

Abstract

The structural characterization of complex crystalline materials such as metal organic frameworks can prove a very difficult challenge both for experimentalists as for theoreticians. From theory, the flat potential energy surface of these highly flexible structures often leads to different geometries that are energetically very close to each other. In this work a distinction between various computationally determined structures is made by comparing experimental and theoretically derived X-ray diffractograms which are produced from the materials geometry. The presented approach allows to choose the most appropriate geometry of a MIL-47(V) MOF and even distinguish between different electronic configurations that induce small structural changes. Moreover the techniques presented here are used to verify the applicability of a newly developed force field for this material. The discussed methodology is of significant importance for modelling studies where accurate geometries are crucial, such as mechanical properties and adsorption of guest molecules.

Sep 10

IAP-meeting 2015: poster

Falling ill is always a bummer. It’s even more annoying when you just finished preparing a poster for a conference you intended to attend (in the current case this is the annual IAP meeting). Per doctor’s orders I am not allowed to be patient zero at the above conference, so my poster will end up alone at the site (luckily my nice colleagues will take it along and put it up). Because misery loves company (or it’s just a personal skill to pick the wrong moment) I had also decided to make this poster a bit more interactive through a spartan setup: As little text as possible, only a trail of images through  which I would tell the story of the research…As you can see I was asking for trouble.

Not being able to be there physically, and knowing that most people nowadays own a smart-phone, I came up with the following solution: One of my colleagues will also put up a QR-code, sending the interested reader to this blog-post, where he/she will be able to read the story of the poster. (Questions can be put in the comments, and the full size version of the poster can be reached by clicking on the picture below.)

Abstract

Poster created for the 2015 IAP meeting on september 11<SUP>th</SUP>, 2015 in Hasselt, Belgium.

Poster created for the 2015 IAP meeting on September 11th, 2015 in Hasselt, Belgium.

Metal-Organic Frameworks (MOFs) are a versatile class of crystalline materials showing great promise in a wide range of applications. Recently, light-based applications, with a focus on luminescence and photo-catalysis, have become of interest. Although new luminescent MOFs are readily synthesized, a fundamental understanding of the underlying mechanisms in the electronic structure is often lacking.

First principles, or ab initio simulations of these MOFs can be used both for validating the experimentally proposed atomistic model of the MOF and for elucidating its luminescent behavior. On this poster, two different MOF-topologies are investigated. In the first case, we consider the well-known UiO-66(Zr) MOF. For this MOF, it is known that functionalization of the linkers modifies its luminescent behavior. As our second case, we consider the very recently created/synthesized COK-69(Ti) MOF. This new MOF is both flexible and luminescent, making it of interest for various applications.

The Old: UiO-66(Zr)-X

Atomic Structure

In our work on the UiO-66, we made use of the primitive unit cell, which contains only a single node and six linker molecules. This cell still contains about 120 atoms (in contrast to about 480 atoms for the conventional cubic cell) making it a rather large system from the point of view of ab initio calculations. The relation between this primitive unit cell and the conventional cubic cell is indicated by comparison to the diamond primitive and cubic cell (top left corner).

The functionalized versions of this MOF were created by manually replacing some of the H atoms of the BDC-linker (benzene-1,4-dicarboxylic acid) by the functional group of interest (OH or SH) and then optimizing the entire structure.

Ball-and-stick model of a primitive unit cell of UiO-66.

Ball-and-stick model of a primitive unit cell of UiO-66(Zr). Linker functionalization is indicated on the right. Primitive and conventional unit cells for diamond are given as reference.

Electronic Structure

Electronic band structure and DOS of UiO-66(Zr)-2,5SH

The calculated electronic band structure (left) and density of states (right) of the double SH-functionalized UiO-66(Zr). The conduction band is colored in blue, while the gap states related to the functional groups are colored green. The “old” valence band is colored yellow. This picture is a modified version of the published one.(Ref 1)

Starting from the optimized geometrical structures, the electronic structure is investigated. Taking three high-symmetry lines of the first Brillouin zone, the band structure was generated for all the functionalized MOFs.

The first aspect that drew my attention was the fact that the bottom conduction bands (indicated in blue) remained unchanged while part of the top of the valence band (indicated in green) splits off and moved upward into the band gap. At this point, nomenclature also becomes a bit of a problem. In a doped semi-conductor, the green bands would be called gap states, which would mean that the band gap of the host-material remains unchanged (which is actually also the case here, the distance between the yellow valence band and the blue conduction band is exactly the same for all functionalized UiO-66(Zr) systems we investigated). However, unlike those semiconductors, these gap states are entirely filled, and contain a significant electron occupation (in doped semi-conductors, these states often appear due to ppm doping). Because of this, they take the role of the valence band leading to a measured band gap equal to the distance between the top green bands and the conduction band (blue). So we end up with two band gaps. To have a clear link with experiments on MOFs, we will call the latter the band gap, while we will call the distance between the yellow and blue bands the “super band gap” (super, to indicate that we go beyond the size of the band gap, but it can still be considered a band gap. If that were not the case, we should call it the “supra band gap”).

The discussion of the super band gap can be rather short: it remains unchanged from the value of the unfunctionalized UiO-66(Zr): roughly 4 eV. In contrast, the band gap depends on both the functional group, and the number of functional groups present on each linker. In case of the double SH-functionalized linkers, each functional group leads to a gap state that is being split of from the valence band (cf. two green bands in the right picture).

Orbital character of valence and conduction band.

Orbital character of gap states, and valence and conduction bands for OH functionalized linkers in UiO-66(Zr).

Analysis of the orbital character shows that the splitting of the valence band can be taken quite literal. Where the valence band (or HOMO if you use molecular terminology) of the unfunctionalized UiO-66(Zr) mainly consists of the π-orbital of the BDC linker, this orbital is split upon functionalization. The conduction band orbital (or LUMO) on the other hand is barely modified.

Because LDA and GGA functionals are well-known to underestimate the experimental band gaps (even though the band structure is qualitatively well represented), we have also used a hybrid functional (HSE06, which was developed for solids) to calculate the band gap, and as expected, we find that the qualitative picture of the electron density of states (DOS) is retained, and the resulting calculated band gap is in perfect agreement with the experimentally measured values (experiments performed by Kevin Hendrickx of the Centre for Ordered Materials, Organometallics and Catalysis at Ghent University).

In conclusion, our ab initio calculations have shown us that functionalization of the linkers leads to a splitting of the valence band and the creation of a gap state, and that the band gap can be predicted with great accuracy for these materials.

The New: COK-69(Ti)

Atomic Structure

Ball-and-stick model of the COK-69(Ti) MOF.

Ball-and-stick model of the COK-69(Ti) MOF. A single triangular Ti cluster is shown in more detail.

The COK-69(Ti) MOF is a newly developed MOF by the Center for Surface Chemistry and Catalysis of the university of Leuven. It is one of the few Ti containing MOFs that have already been synthesized. Because of this, the initial model provided was not sufficiently accurate to perform good electronic structure calculations. The weak point of the model was the uncertainty of the actual structure of the triangular Ti-O clusters. The original model (figure a) was not charge balanced. As a result, the electronic structure of this model showed it to be a metal (or a very narrow band gap semiconductor), in clear disagreement with experiment. Charge balance could be obtained in several ways: removal of O atoms, formation of H2O bound to the cluster (e.g. figure c) or the formation of OH groups (e.g. figure b). By investigating different models, we found that the removal of O atoms is highly unfavorable, while the formation of OH groups and a bound H2O molecule are comparable in stability. As a result of the latter observation, it is not unreasonable to assume that under experimental conditions the bound H2O molecule dissociates and lead to the formation of two OH groups, and that this process is also reversed, leading to a constant moving back and forth between the two models.

Models for Ti clusters in the COK-69(Ti) MOF.

Schematic representation of possible triangular Ti clusters for the CO-69(Ti) MOF.

Electronic Structure

Also, the calculated electronic structure for both models is reasonably comparable: similar sized band gaps, and the same character for the valence (mainly O states) and the conduction (mainly Ti states) bands. Making it hard to give preference to one model over the other as being the actual ground state structure of this MOF, without further study.

Irradiated COK-69

More interestingly, we found the cluster with three OH groups (cf. figure d) to be most stable. In such a model, two of the Ti atoms should have an oxidation number of 4, while one has an oxidation number of 3. Looking into the electronic structure of this specific model of the COK-69 shows some amazing features. Firstly, the band gap is much reduced to about the size associated with a semiconductor, and secondly, the states of the Ti3+ atom show a valence to conduction transition of 3.2 eV, which roughly coincides with the blue color obtained for the irradiated COK-69 MOF.

Samples of the COK-69(Ti) MOF.

Two samples of the COK-69(Ti) MOF. The normal COK-69 at the top, and the irradiated COK-69 MOF at the bottom. Figure taken from Ref 2.

Ti3+ centers are known to provide a blue color in other materials, and it is now also shown to be the case for this MOF. In addition, experiments on the irradiated COK-69 MOF also showed that no more than 1/3 of the Ti atoms could be Ti3+, which is also the maximum indicated by our model (one Ti per Ti-cluster).

Another interesting bonus provided by this last model is from the theoretical perspective. Due to the symmetry of the cluster and the strong correlation of the Ti-d states, standard DFT is not able to differentiate between the Ti4+ and Ti3+ atoms. As such, the atomic charge is the same for all. By adding an additional Hubbard U potential on the Ti-d states (the so-called DFT+U approach) it is possible to differentiate between the different Ti oxidation states, as is shown by the nice bifurcation diagram.

Differentiation of Ti species.

Differentiation of Ti species as function of the U value used in a DFT+U approach. Atomic charges are calculated using the Hirshfeld-I partitioning scheme[3]. Figure taken from Ref 2.

In conclusion, our ab initio calculations allowed us to build a more accurate model of the COK-69 MOF and provide a model for the irradiated COK-69 MOF. In case of the latter, the calculated electronic structure can be used to elucidate the blue color of the irradiated COK-69.

References

[1] “Understanding intrinsic light absorption properties of UiO-66 frameworks”, K. Hendrickx, D.E.P. Vanpoucke, K. Leus, et al.  Inorganic Chemistry (in revision)

[2] “A Flexible Photoactive Titanium MOF based on a [TiIV3(µ3-O)O2(COO)6]-Cluster”, B. Beuken, F. Vermoortele, D.E.P. Vanpoucke, et al. Angewandte Chemie (accepted)

[3] D.E.P. Vanpoucke, P. Bultinck, and I. Van Driessche, J. Comput. Chem. 34 405-417 (2013) & J. Comput. Chem. 34 422-427 (2013)

Oct 28

Comment on ‘Europium doping induced symmetry deviation and its impact on the second harmonic generation of doped ZnO nanowires’

Authors: Danny E. P. Vanpoucke
Journal: Nanotechnology 25(45), 458001 (2014)
doi: 10.1088/0957-4484/25/45/458001
IF(2014): 3.821
export: bibtex
pdf: <Nanotechnology>

Abstract

In Dhara et al. 2014 Nanotechnology 25 225202, the authors reported on the synthesis of Eu-doped ZnO nanowires (NWs) and investigated the influence of Eu doping on the second harmonic generation (SHG). Maximum SHG was found to correlate strongly with the structural deformation attributed to Eu3+ doping. In this comment, we show the deformation of interest is due to the presence of Eu2+ dopants, based on both the experimental data presented by Dhara et al. and ab-initio density functional theory calculations.

Oct 28

New Functionalized Metal-Organic Frameworks MIL-47-X (X = -Cl, -Br, -CH3, -CF3, -OH, -OCH3): Synthesis, Characterization and CO2 Adsorption Properties

Authors: Shyam Biswas, Danny E. P. Vanpoucke, Toon Verstraelen, Matthias Vandichel, Sarah Couck, Karen Leus, Ying-Ya Liu, Michel Waroquier, Veronique Van Speybroeck, Joeri F. M. Denayer, and Pascal Van Der Voort
Journal: J. Phys. Chem. C 117(44), 22784-22796 (2013)
doi: 10.1021/jp406835n
IF(2013): 4.835
export: bibtex
pdf: <J.Phys.Chem.C>

Abstract

Six new functionalized vanadium hydroxo terephthalates [VIII(OH)(BDC-X)]·n(guests) (MIL-47(VIII)-X-AS) (BDC = 1,4-benzenedicarboxylate; X = −Cl, −Br, −CH3, −CF3, −OH, −OCH3; AS = as-synthesized) along with the parent MIL-47 were synthesized under rapid microwave-assisted hydrothermal conditions (170 °C, 30 min, 150 W). The unreacted H2BDC-X and/or occluded solvent molecules can be removed by thermal activation under vacuum, leading to the empty-pore forms of the title compounds (MIL-47(VIV)-X). Except pristine MIL-47 (+III oxidation state), the vanadium atoms in all the evacuated functionalized solids stayed in the +IV oxidation state. The phase purity of the compounds was ascertained by X-ray powder diffraction (XRPD), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, Raman, thermogravimetric (TG), and elemental analysis. The structural similarity of the filled and empty-pore forms of the functionalized compounds with the respective forms of parent MIL-47 was verified by cell parameter determination from XRPD data. TGA and temperature-dependent XRPD (TDXRPD) experiments in an air atmosphere indicate high thermal stability in the 330–385 °C range. All the thermally activated compounds exhibit significant microporosity (SBET in the 305–897 m2 g–1 range), as verified by the N2 and CO2 sorption analysis. Among the six functionalized compounds, MIL-47(VIV)-OCH3 shows the highest CO2 uptake, demonstrating the determining role of functional groups on the CO2 sorption behavior. For this compound and pristine MIL-47(VIV), Widom particle insertion simulations were performed based on ab initio calculated crystal structures. The theoretical Henry coefficients show a good agreement with the experimental values, and calculated isosurfaces for the local excess chemical potential indicate the enhanced CO2 affinity is due to two effects: (i) the interaction between the methoxy group and CO2 and (ii) the collapse of the MIL-47(VIV)-OCH3 framework.

Oct 28

Aqueous CSD approach for the growth of novel, lattice-tuned LaxCe1- xOδ epitaxial layers

Authors: Vyshnavi Narayanan, Petra Lommens, Klaartje De Buysser, Danny E.P. Vanpoucke, Ruben Huehne, Leopoldo Molina, Gustaaf Van Tendeloo , Pascal Van Der Voort, Isabel Van Driessche
Journal: J. Mater. Chem. 22, 8476-8483 (2012)
doi: 10.1039/C2JM15752G
IF(2012): 6.101
export: bibtex
pdf: <J.Mater.Chem.>

Abstract

Lanthanum–cerium oxide (LCO) films were deposited on Ni-5%W substrates by chemical solution deposition (CSD) from water-based precursors. LCO films containing different ratios of lanthanum and cerium ions (from CeO2 to La2Ce2O7) were prepared. The composition of the layers was optimized towards the formation of LCO buffer layers, lattice-matched with the superconducting YBa2Cu3Oy layer, useful for the development of coated conductors. Single, crack-free LCO layers with a thickness of up to 140 nm could be obtained in a single deposition step. The crystallinity and microstructure of these lattice-matched LCO layers were studied by X-ray diffraction techniques, RHEED and SEM. We find that only layers with thickness below 100 nm show a crystalline top surface although both thick and thin layers show good biaxial texture in XRD. On the most promising layers, AFM and (S)TEM were performed to further evaluate their morphology. The overall surface roughness varies between 3.9 and 7.5 nm, while the layers appear much more dense than the frequently used La2Zr2O7 (LZO) systems, showing much smaller nanovoids (1–2 nm) than the latter system. Their effective buffer layer action was studied using XPS. The thin LCO layers supported the growth of superconducting YBCO deposited using PLD methods.