Investigation of Ionic Diode Behaviour in Nanoporous Materials

Investigation of ionic diode behaviour in nanoporous materials

Ionic-diodes are devices which show asymmetric transport of ions depending on the direction of the applied bias. Essentially, this means that the current in one bias (positive or negative) is very different vs. the opposite bias. This opens up the possibility for use as sensors or in desalination applications. Recently, it has been experimentally demonstrated that zeolitic imidazolate frameworks (ZIFs) can demonstrate such behaviour [1].

The mechanism for this diodic behaviour is, however, still relatively unknown. It has been postulated to be related to cation/anion size-asymmetry and shown to be influenced by pH. It is the goal of this masters work to perform detailed investigations into the charge-transport mechanism, both within the ZIF material and the surrounding fluid reservoirs, under various operating conditions. In this manner, the suitability of these materials for various desalination applications can be assessed.

The proposed masters work will involve synthesis of ZIF materials in glass (transparent) systems in order to form charge-selective interfaces. After this synthesis, detailed investigation into the ion-transport phenomena will be carried out through direct electrical characterization (conductance), along with characterization of specific ionic species transport through the use of fluorescence-sensitive dyes (fluorescence lifetime imaging microscopy) and other transport mechanisms. In this manner, the direct transport of various ionic species can be tracked throughout the system and a greater understanding behind the charge-transport mechanism obtained.

For further information, please do not hesitate to contact:

Özlem Haval Demirel (o.h.demirel@utwente.nl), Films in Fluids, Meander 322

Jeff Wood (j.a.wood@utwente.nl), Soft Matter, Fluidics and Interfaces, Meander 313

[1] Madrid et al. “Ion flow in a zeolitic imidazolate framework results in ionic diode phenomena.” Chem. Comm. 2016. 52: 2792

Theoretical Investigation of Electroforming MOFs on Copper Hollow Fibers

Theoretical Investigation of Electroforming MOFs on Copper Hollow Fibres

Recently in the Films in Fluids group a novel technique for producing highly controllable hollow fibres using an environmentally friendly ionic-gelation technique has been developed. Through the addition of nanoparticles, metallic oxide or metallic hollow fibres can be easily fabricated. Using this technique, the FiF group has pursued the intriguing idea of utilizing this simple fibre production technique in order to fabricate copper-based metal organic frameworks (MOFs). MOFs are metal-ions which are coordinated to organic ligands, forming network structures (1d, 2d or 3d) that can be porous. MOFs are of potential interest in catalysis, sensing and gas separation and having the capacity to form high specific surface area hollow fibres is an attractive option for making use of the unique MOF properties.

Research has been carried out in fabricating Cu(BTC) using an electrochemical synthesis method. In this approach, a Cu hollow-fibre is used as the working electrode in a cell containing an ethanol-water mixture containing an organic ligand - trimesic acid (BTC). BTC then reacts with copper to form Cu-BTC, a copper-hollow fibre MOF. The underlying mechanism behind this transformation and its enhancement using applied potential has been postulated but not confirmed.


It is the goal of this proposed Masters assignment to develop a theoretical model to understand this system, which we are presently investigating experimentally. This model framework can then be implemented in a 2 or 3d numerical simulation (COMSOL) in order to predict or fit experimental results. For example, what is the role of the electric-field distribution within the system, ion dissociation in the water-ethanol mixture, etc. This will greatly aid in understanding of the potential of this approach for synthesis of MOF hollow-fibres and its potential applicability at larger scales.

For more information, please contact:

Özlem Haval Demirel (o.h.demirel@utwente.nl), Films in Fluids, Meander 322

Jeff Wood (j.a.wood@utwente.nl), Soft Matter, Fluidics and Interfaces, Meander 313

[1] Schäfer et al. Unraveling a two-step oxidation mechanism in electrochemical Cu-MOF synthesis. Chem. Comm. 2016. 52: 4722

LL and GLL flows in patterned capillaries

Two (LL) and Three-Phase (GLL) Flows and Extractions in Patterned Capillaries

The enhancement of mass transfer/extraction efficiency of liquid-liquid (LL) extraction systems using an inert gas phase (GLL) is an interesting phenomena with applicability to a wide number of systems. More recently, work has been done to measure the extraction performance and mechanism by studying the process at smaller length scales in capillaries (mm scale) by performing extractions and measuring the resulting flow patterns by visualization (high-speed camera). At these scales, the interfacial forces cannot be neglected and play an important role in phase stability and the nature of flow through the system; parallel-flows or two- and three-phase slug flows may arise, which have important implications for the mass transfer efficiency in these types of systems. The mechanism behind this enhancement is presently under investigation, requiring detailed measurements at varying flow conditions in order to determine the gas effect on various process parameters. These include interfacial area between liquid phases and the stability of flow regimes for example.


Figure 1: Two-Phase Slug Flow for Water-Octanol

The purpose of the proposed assignment is to investigate the influence of patterned wettability gradients in a capillary on the two- and three-phase flows (LL and GLL) and liquid-liquid extraction in these systems . Switching between hydrophilic and hydrophobic domains in a periodic fashion can lead to phase inversions/mixing patterns which may greatly enhance mass transfer. This will be accomplished through image/video capture using a high-speed camera, with subsequent flow patterns characterized using image analysis. Study of these flow patterns is then highly relevant for quantifying what possible enhancements can be realized through the use of such patterned-capillaries. Corresponding liquid-liquid extractions will be investigated using an established model system (acetic acid-water-octanol) initially, with an emphasis on determining the role of gas and wettability patterning on the resulting extraction efficiencies vs. a two-phase flow system.

If you are interested in this project or wish further details, please contact:

Email: j.a.wood@utwente.nl
Office: Meander 313
Phone Extension: 2961

Email: r.g.h.lammertink@utwente.nl
Office: Meander 314
Phone Extension: 2063

Diffusio phoresis of photocatalytic particles

Diffusio-phoresis of photocatalytic particles under self-generated concentration gradients

The generation of flow within the interfacial structure due to concentration gradients was described quantitatively for the first time by Anderson [1] and extended by Ajdari and Bocquet [2] for solvophobic surfaces. The flow is driven by an osmotic pressure gradient which builds up inside the interfacial layer where the interaction potential between the chemical species and solid spans (Fig. 1). Moreover, if the surface is reactive, the diffusio-osmotic flow could be promoted without any external input, through self-generated concentration gradients. If the solid surface is not immobilized, e.g in a colloidal system, the surface flow will propel small particles.

In this project, the migration of photocatalytic particles (TiO2) under self-generated concentration gradients will be studied systematically in a microreactor where an aqueous solution of an organic contaminant is contacted under continuous flow with a particle suspension containing various concentrations of the same contaminant. When UV light is turned on, the photocatalytic particles decompose the contaminant lowering the concentration inside the colloidal stream. The difference in concentration that is generated via the photocatalytic reaction leads to the migration of particles toward the higher concentration site (Fig 2).

Figure 1: Concept illustration for diffusio-osmosis

Figure 2: A methylene blue (MB) solution is contacted with a TiO2 suspension stream containing the same MB concentration in a 200 µm width channel. The dotted line is used as a guideline to show the spreading of the colloidal stream upon illumination. a. UV off b. UV on

Please do not hesitate to contact Aura Visan for additional information! (a.visan@utwente.nl)

[1] J. L. Anderson, Annu. Rev. Fluid Mech., 1989, 21, 61–99.

[2] A. Ajdari and L. Bocquet, Phys. Rev. Lett., 2006, 96, 1–4.

Interfacial transport for hydrogen peroxide decomposition at pt/au surfaces

Interfacial transport for hydrogen peroxide decomposition at Pt/Au surfaces

Mass transfer is always an issue in heterogeneous catalysis as reactions at liquid-solid interface involve the migration of species to the active catalytic sites. While macroscopic mixing can help in the bulk, the limiting factor lies in the boundary layer transport where concentration depletion or enrichment occurs due to the viscosity related velocity decrease towards the solid wall. The resolution could come from a surface driven flow which would impact globally the conversion capacity of the system.

Fig. 1 H2O2 decomposition on a Pt/Au Janus particle
Fig. 2 Concept illustration for catalytic heterogeneity

In this project a model reaction will be studied, namely the hydrogen peroxide oxidation and reduction on Pt and Au, respectively. The proton production and consumption leads to a charge distribution which translates into a self-generated electric field which in turn enhances electro-migration. The fast kinetics combined with the self-generated electro-osmosis leads to very intense flows with vortex structures. The focus in literature for this type of system is mostly on migration of engineered particles which are propelled by the flow at the surface (Fig 1). The interest here is to observe directly the fluid dynamics and mass transfer at immobilized surfaces. The interfacial transport is studied on length scales comparable with the boundary layer using a microfluidic platform (Fig. 2). Niche analytical techniques such as µPIV and FLIM will make possible the direct visualization of velocity and concentration profiles. Both µPIV and FLIM require complex data processing giving the student the opportunity to acquire valuable skills. The challenge is to develop a 3D particle tracking protocol.

Please do not hesitate to contact Aura Visan for additional information!(a.visan@utwente.nl)

Catalyst design for biomass conversion

Catalyst design for biomass conversion

Catalysis is ever-present in industry, up to 90% of the processes use catalysts. However, this valuable experience covers mostly the oil based feedstock processing. For well-known reasons, there is a global interest for more sustainable resources. The catalytic conversion of biomass is a promising alternative for chemical and fuel production. Especially for chemical synthesis this route is highly promising due to the richness and complexity of the chemical composition of biomass waste. This is a clear distinction with respect to oil composition. While oil consists of hydrocarbons and, hence, is hydrophobic, biomass is high in oxygen content and consequently hydrophilic. Therefore, catalysts will have to fulfil completely different requirements in the case of biomass conversion.

Fig. 1 Microchannel 500 µm wide with catalyst patches
Fig. 2 High temperature, high pressure setup

The current project attempts to investigate key reactions in the valorification of biomass waste involving platform molecules such as levulinic acid to γ-valerolactone and glucose/fructose to hydroxyl-methylfurfural. The starting point is screening different materials, e.g. probe the potential of abundant metal oxides as catalysts instead of more expensive noble metals. The challenges that will be addressed are the stability of catalyst under aqueous conditions, the effect of hydration as well as other surface functionalities. Nevertheless, the main idea is to study the effect of surface heterogeneity on interfacial transport. It starts from the intuitive idea that consecutive reactions would benefit from alternating catalysts in order to consume intermediates that could potentially poison the catalyst and to generate local gradients that would shift the equilibrium of the reaction increasing overall conversion.

In order to design the optimum interface, the implications of solute – catalyst and solvent – catalyst interactions will also be investigated by changing the catalyst wettability (also alternating solvophobic and solvophilic surfaces), as well as probing the concentration profiles for various catalyst arrangements.

In this project, the effect of catalytic surface heterogeneity (Fig. 1) on interfacial transport is studied on length scales comparable with the boundary layer using a microfluidic platform (Fig. 2). The most challenging aspect for this system is the slow reaction rate. In order to provide a significant consumption flux for the reactant, a high catalytic surface area is necessary. Here the difficulty is to pattern porous materials inside microchannels.

Fig. 3: SEM of a sputtered ZrO2 layer
Fig. 4: Levulinic acid to GVL over alternating catalysts

The patterning of the catalytic material using deposition methods compatible with photolithography is straightforward. The important drawback for these deposition methods is the dense material that is obtained. The non-porous layer (Fig. 3) has a low surface area rendering a low catalytic activity to these films. Different approaches will be pursued in order to pattern porous layers inside the microchannels.

The next step after of the fabrication of porous discontinuous layers is alternating two porous materials for tandem reactions (Fig. 4). The effect of catalyst wettability will also be investigated. The catalyst will be hydrophobized by silane surface functionalization.

The oxide porous materials can act as a catalyst directly or serve as a support for metallic nanoparticles. In the later situation, the metal has to be wet impregnated in a second step. Obviously, every fabrication step has to be followed by a thorough characterization procedure. Here, the student will have the opportunity to get acquainted to different characterization techniques from XRD, SEM, EDX, XPS to ellipsometry and TEM.

Please do not hesitate to contact Aura Visan for additional information!

(a.visan@utwente.nl)