FABRICATION AND SURFACE FUNCTIONALIZATION OF POROUS

ORGANIC FRAMEWORKS NANOMEMBRANES

Kamal BatraFinal Year Undergraduate Student

Department of ChemistryIndian Institute of Technology, Kharagpur

Project Guide : Dr. Manuel Tsotsalas, IFG/KIT15 May 2014 to 15 July 2014

2

AIM AND OBJECTIVE

To explore a facile method to develop porous organic frameworks (POF’s) nanomembranes on gold coated silicon/mica substrate.

To Investigate the surface functionalized POFs, end-capped with different chemical reagents using characterization techniques such as Drop Shape Analyser (DSA) & Infrared spectroscopy (IR)

To measure contact angle range on surfaces that can influence the hydrophobicity/hydrophilicity of nanomembranes on substrates

To explore the potential applications of POFs in functional devices such as gas storage, separation, sensing, catalysis etc.

3

WHAT ARE NANOPOROUS MATERIALS ? Consist of a regular inorganic or organic structure,

supporting a periodic and porous system

Nanoporous solids as a subclass of nanomaterials.

Nanoporous materials can be subdivided into 3 categories, set out by IUPAC:

Micro porous materials: 0.2–2 nm Meso porous materials: 2–50 nm Macro porous materials: 50–1000 nm

4

Among the nanoporous solids 3 individual classes can be recognized, such as

Zeolites, crystalline alumina-silicates with framework type structures that form pores with uniform sizes of molecular dimensions by corner sharing TO4 tetrahedral (e.g. SiO4 and AlO4).

Metal Organic Frameworks (MOFs), also known as porous coordination polymers (PCPs), are crystalline compounds consisting of metal ions or clusters coordinated to often rigid organic molecules

Large Pore size for both Zeolite and MOFs come in range of 0.2 to 2 nm

Porous Organic Frameworks (POFs)

?

Fig.2 : Basic MOF structure

Fig.1 : Basic Zeolite structure

5

What are Porous Organic Frameworks (POFs) ?

Porous organic frameworks (POFs) are important subclass of nanoporous materials

They are composed of different organic moieties linked by covalent bonds, resulting in ordered and rigid structures

Exceptional thermal stabilities (to temperatures up to 600°C), and Low Framework Densities.

They exhibit permanent porosity with specific surface areas.

These materials have potential uses in areas such as Gas storage, separation, and catalysis.

Fig.3 : 1D, 2D & 3D structures of POFs

6

Schlenk Line Technique

A Schlenk line (SL) is a valuable device in synthetic organic and inorganic chemistry, particularly for oxygen and moisture sensitive reactions.

The Schlenk line, developed by Wilhelm Schlenk, is comprised of a twin manifold with a number of ports for using nitrogen/vacuum for multiple reactions/manipulations.

Fig.4 : A Schlenk line system at IFG/KIT north campus research laboratory

7

SELF-ASSEMBLED MONOLAYERS (SAMS) Self-assembled monolayers (SAM) of organic molecules are molecular

assemblies formed spontaneously on surfaces by adsorption and are organized into more or less large ordered domains.

Fig.5: Organic molecule attached to substrate forming SAMs

SAM consisting of a head group, tail and functional end group is depicted in Fig.5 Common head groups include thiols, silanes, etc. SAMs are created by the chemisorption of "head groups" onto a substrate from either the vapour or liquid phase followed by a slow organization of "tail groups

8

Only since 1983 the formation of SAMs on metallic surfaces is investigated in a systematic fashion (first study by Nuzzo and Allara);

In this case, the anchor group is realized by the thiol function (HS–) that forms a stable gold thiolate (Au–S–R) when brought into contact with gold surfaces

Preparation of Self-Assembled Monolayers

Fig 6: Generation of SAMs on Gold coated substrate

9

CLICK CHEMISTRY AT ORGANIC SURFACES

"Click Chemistry" is a term that was introduced by K. B. Sharpless in 2001 and is one of the most common and reliable methods to link molecules covalently, and has many applications in a variety of disciplines.

Click Chemistry : Azide-Alkyne Cycloaddition

10Fig.7 Schematic representation of their layer-by-layer synthesis on

functionalized surfaces

For fabrication of the porous organic nanomembranes, an alkyne containing molecule and an azide containing molecule are needed as building blocks and the reaction is catalysed by Tetrakis (acetonitrile) copper (I) hexafluorophosphate. The experimental studies begin with the fabrication of POFs with the help of molecules below by schlenk line technique for device fabrication.

FABRICATION OF POFs

11

SPIN COATING TECHNIQUE

Spin-coating is a process that involves depositing a small puddle of a fluid material on to the center of a substrate and then spinning the substrate at high speed (~3000 rpm).

The thickness of the film also depends on the viscosity and concentration of the solution and the solvent

Fig 8: spin coater unit

After the fabrication of 8 cycles SAMs on gold coated mica substrate, Poly (methyl methacrylate) [PMMA] was fabricated on the top of mica substrate that helps in detaching the nano membrane from the mica substrate. The parameters chosen were 3000 rpm rotation speed and 120 sec for spinning. The post spin-coat samples were also baked at 80 OC for 5 minutes

12

Detachment of Membrane from Mica Substrate

Following flow chart describes the detachment of membrane from substrate :

13

Sr. No.

Chemicals Required Amount

1 Propargyl alcohol 0.00173 ml

2 Propargyl bromide 0.00266 ml

3 Propargyl ether 0.00308 ml

4 Ethylene Ferrocene 0.00630 g

5 Phenyl acetylene 0.00330 ml

6 4- Fluorophenyl acetylene 0.00340 ml

SURFACE FUNCTIONALIZATIONTable 1. Calculated amount of chemical reagents for end capping on the surfaces of HCP substrate for the surface functionalization.

Fig9. Six test tube containing different end caps for surface functionalization

14

CHARACTERIZATIONS TECHNIQUES A number of different experimental methods are available for the

characterization of SAMs

(A) Contact angle measurements

Drop shape analysis (DSA) is an image analysis method for determining the contact angle from the shadow image of a sessile drop and the surface tension or interfacial tension from the shadow image of a pendant drop.

Fig 10: Drop Shape Analyser – DSA100 (left) and drop image for contact angle measurement (right)

15

Measuring the contact angle is the simplest method to characterize organic SAMs generated by the reaction of thiols with gold. A drop of water or some other liquid is applied to the surface and photographed from the side.

The wetting behaviour can be deduced from these contact angles. The surface shown in (a) is hydrophilic and consists of thiols with terminal OH functions. (b) Was taken for a CH3-terminated surface that was produced from a simple alkanethiol. In this case, the organic surface is water repellent (hydrophobic): the drop of water rolls off.

16

(B) Infrared (IR) Spectroscopy As a matter of course, investigating the wettability of organic surfaces only with water or other solvents is not sufficient for a more detailed physico-chemical characterization.

A deeper understanding requires for the determination of the exact chemical composition of the organic ultrathin layers, molecular orientation, and lateral arrangement.

Fig. 11: Bruker Vertex 80 equipped with a NB-MCT detector

17

CHARACTERIZATION RESULTS

Contact Angle measurement of HCP on gold coated substrate before surface functionalization with different chemical reagents

Date Average Contact Angle

26/05/2014 L =95.980 ; R =97.460

27/05/2014 L =101.46 ; R =98.900

Contact Angle Measurements- Substrate N3-(CH2)n-SH in Ethanol without chemical reagents

Date Average Contact Angle

13/06/2014 L =72.62 ; R =72.92

18

S.No Average Contact Angles Chemical Reagents

1. 44.81o Propargyl Alcohol

2. 48.58o Ethylene Ferrocene

3. 57.82o Porpargyl Ether

4. 65.54o Phenyl Acetylene

5. 67.63o Propargyl Bromide

Summary of Contact Angle measurement of [N3-(CH2)n-SH] on gold coated silicon substrate, surface functionalization with different chemical reagents

19

S.No Average Contact Angles Chemical Reagents

1. 73.92o Ethylene Ferrocene

2. 68.18o Phenyl Acetylene

3. 55.93o Propargyl Bromide

4. 54.93o Propargyl Alcohol

5. 48.50o Porpargyl Ether

Summary of Contact Angle measurement of HCP on gold coated mica substrate, surface functionalization with different chemical reagents

Summary of Contact Angle measurement of HCP on gold coated silicon substrate, surface functionalization with different chemical reagents

S.No Average Contact Angles Chemical Reagents

1. 44.20o Ethylene Ferrocene

2. 45.65o Phenyl Acetylene

3. 47.03o Propargyl Alcohol

4. 50.40o Porpargyl Ether

5. 51.21o Propargyl Bromide

20

CONCLUSIONS

In conclusion our results demonstrate the successful surface functionalization of highly stable freestanding nanomembrane via Copper-catalysed azide–alkyne addition. This process to surface functionalize POF nanomembrane represents a versatile strategy to chemically functionalize the nanometer thick membrane with tunable properties. The contact angle experiments clearly demonstrate that the surface functionalization of the membranes has a strong influence on the hydrophobicity of the membranes.

THANK YOU !!