Fabrication and Characterization of Novel Luminescent Polymer Coatings Based on Halloysite Nanotubes

Qadeer Alam

Supervisors Prof. Dr. Tuula Pakkanen

Prof. Dr. Mika Suvanto

Abstract

Polymeric nanocomposite materials have been investigated extensively for their mechanical properties, tensile strength and other exciting features like flame retardancy. However, fabrication of luminescent polymer nanocomposite materials is rarely studied. In this investigation, we have established a working protocol for the development of highly luminescent polymer nanocomposite materials which can be used in form of spray coatings. Halloysite nanotubes (HNTs) were impregnated with different organic dyes, Rhodamine B, Rhodamine 6G and Fluorescein. These modified nanoparticles were dispersed uniformly in the polystyrene matrix. Special emphasis was on studying the effect of HNTs on the luminescence properties of dye materials. For this purpose samples of polymer coating solution containing impregnated HNTs and polystyrene were compared with their corresponding dye solutions. These dye solutions and polymer coating solution were having equivalent amount of luminescent material in them. Significant increase in the luminescence intensity of polymer coating solutions containing Rhodamine 6G and Fluorescein was observed when compared with their corresponding dye solutions. HNTs and polymeric matrix provided an environment which enhanced the photoluminescence intensity of polymer coatings.

Contents

1. Introduction ............................................................................................................. 1

1.1 Mechanisms for luminescence ............................................................................. 1

1.2 Types of luminescence ......................................................................................... 2

1.3 Competing Processes with Luminescence ............................................................ 2

1.4 Polymer Light Emitting Devices (PLEDs) ........................................................... 3

1.4.1 Blue light emitting polymers ............................................................................ 3

1.4.2 Blue light emitting homopolymers.................................................................... 3

1.4.3 Blue light emitting copolymers ......................................................................... 4

1.4.4 Polymer blends ................................................................................................. 5

1.5 Upconversion and downconversion ...................................................................... 5

1.5.1 Upconversion ................................................................................................... 5

1.5.2 Downconversion .............................................................................................. 6

1.6 Fluorescent nanocomposites ................................................................................ 7

1.6.1 Coumarin 7 dye nanocomposites ...................................................................... 7

1.6.2 Hybrid organic dye nanocomposties ................................................................. 8

1.6.3 Organic dyes encapsulated in Nickel Phosphate VSB-1 .................................. 10

1.6.4 Epoxy/Clay nanocomposites........................................................................... 12

1.7 Halloysite nanoclay............................................................................................ 13

1.7.1 Applications of halloysite nanoclay ................................................................ 13

1.7.2 Dispersing methods for HNTs ........................................................................ 15

1.7.3 Adsorbing organic dyes by HNTs ................................................................... 16

1.7.4 Loading of HNTs for controlled release .......................................................... 17

2. Aims of Study ....................................................................................................... 19

3. Experimental ......................................................................................................... 20

3.1 Materials ............................................................................................................ 20

3.1.1 Organic dyes .................................................................................................. 20

3.1.2 Hallyosite nanotubes ...................................................................................... 20

3.1.3 Polymer .......................................................................................................... 21

3.2 Methods ............................................................................................................. 21

3.2.1 Scanning Electron Microscopy (SEM)............................................................ 21

3.2.2 Energy Dispersive Spectroscopy (EDS) .......................................................... 21

3.2.3 FT-IR spectroscopy ........................................................................................ 21

3.2.4 UV-Visible spectroscopy ................................................................................ 21

3.2.5 Fluorescence spectroscopy ............................................................................. 21

3.2.6 Transmission Electron Microscopy (TEM) ..................................................... 22

3.3 Sample preparation ............................................................................................ 22

3.3.1 Impregnation of HNTs with organic dyes ....................................................... 22

3.3.2 Washing of impregnated HNTs ...................................................................... 22

3.3.3 Drying of impregnated HNTs ......................................................................... 23

3.3.4 Polymer film casting ...................................................................................... 23

4. Results and discussion ........................................................................................... 24

4.1 Characterization of impregnated HNTs .............................................................. 24

4.2 Characterization of polymer coatings ................................................................. 25

4.3 Transmission Electron Microscopy .................................................................... 27

4.4 Electronic absorption of composites and dye solutions ....................................... 29

4.5 Interactions between organic dyes and HNTs ..................................................... 32

4.6 Photoluminescence study of composites and dye solutions ................................. 34

4.6.1 Amount of Loading in HNTs .......................................................................... 36

4.6.2 PL spectra of composites and control samples ................................................ 38

5. Conclusions ........................................................................................................... 42

6. Bibliography ......................................................................................................... 43

Abbreviations

CB Chlorobenzene

DMF N, N – Dimethylforamide

EDS Energy Dispersive Spectroscopy

EEMs Excitation – Emission Matrices

Fluo Fluorescein

FT-IR Fourier Transform Infrared Spectroscopy

HNTs Halloysite Nanotubes

HR-TEM High Resolution – Transmission Electron Microscopy

LEDs Light Emitting Diodes

OLEDs Organic Light Emitting Diodes

PLEDs Polymer Light Emitting Devices

PPP Poly(p-phenylene)

PS Polystyrene

Rh6G Rhodamine 6G

RhB Rhodamine B

SEM Scanning Electron Microscopy

THF Tetrahydrofuran

Tol Toluene

UV-Vis Ultra Violet – Visible Spectroscopy

VSB-1 Versailles Santa Barbara – 1

1 1. Introduction

Luminescence can be defined as a transformation of electromagnetic radiation into visible light. The fundamental principle of this process is the excitation of the electrons in chromophore. Once electrons are excited, they leave their ground energy level and move up to the excited energy level because of the energy provided to them. Upon returning to the ground energy level the excess amount of energy is released in the form of visible light. This phenomenon of emitting light after the absorption of energy is known as luminescence. Luminescence comprises phosphorescence and fluorescence. Both of these processes are contributing in the emission of visible light. Fluorescence phenomenon was observed for the first time in mineral fluorite. Fluorite mineral emitted the visible light when excited with UV source. This phenomenon is fast, lasting for nano-seconds only. Phosphorescence is the continuous emission of visible light after the excitation. This process is rather long and can last for hours or even days1. In Fig. 1. a general spectrum for excitation and emission of fluorescent dye is given. It can be noted from the Fig. 1 that emission wavelength is having lower energy as compare to the excitation energy. This phenomenon is knows as Stokes shift in which some energy is lost in the form of heat and rest is emitting in the form of visible light.

1.1 Mechanisms for luminescence

I. Charge Transfer Luminescence: In this mechanism transition takes place from the orbital of one ion to the orbital of other ion. This kind of transition can change the nature of bond and charge distribution of the optical center. Example of this is MgWO4, which is used in fluorescent lamps. Emission of visible light occurs when oxygen ion transfer its charge to the empty d-orbitals of tungsten ion. These materials are also known as self-activated as they do not need any activator for initiation.

II. Donor-Acceptor Luminescence: Semi-conductor which are doped with both p-type and n-type impurities exhibits this kind of luminescence. The generation of visible light takes place when transition between neutral donor and neutral acceptor takes place.

III. Long Afterglow Luminescence: When photo-excited ion is trapped inside the lattice, it emits visible light after sometime. This phenomenon is known as long after glow luminescence. SrAl2O4:Eu:Dy is common example of long afterglow luminescence. Long afterglow can last for hours or even days2.

2 1.2 Types of luminescence

Luminescence can be divided into different categories on the basis of stimulus which initiate luminescence in chromophore. Some common types of luminescence include photoluminescence, radioluminescence, cathodeluminescence, electroluminescence, thermosluminescence, chemiluminescence, bioluminescence, triboluminescence and sonoluminescence.

Photoluminescence is the process in which chromophore produce visible light when excited with the help of light. These chromophores can be organic, inorganic or organometallic in nature3. This process is being used in the production of electricity by using luminescent solar cell concentrators4.

In electroluminescence emission of visible light can be observed when voltage is applied. Upon application of voltage holes and electrons are generated within luminescent material and their recombination leads to emission of visible light. This phenomenon is widely used in organic light emitting diodes (OLEDs)5, flat panel displays and solid state lighting6, screen printing7.

Figure. 1 Excitation-Emission spectrum for the fluorescent dye8.

1.3 Competing Processes with Luminescence

I. Radiative Transitions: In chromophore excited electrons while coming back to ground level do not emit visible light. Contrary to it, they transfer their energy radiatively to another ion.

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II. Non-Radiative Transitions: These are losses which occur when energy absorbed by chromophore is released to the crystal lattice. This energy is used by the local vibrations in crystal. In order to increase the efficiency of luminescent material these kind of losses should be minimized.

III. Cross Relaxation: It can take place between two same type lanthanides or between two different elements which have two energy level separated by same difference. Among these two chemical species one compound can be excited by the emission wavelength of other. In this kind of energy losses luminescent materials absorb energy and the electron is transferred to the excited state. While coming back to the ground level energy is released which is picked up by chemical species having the same energy gap for the excitation of electron. This can result in the quenching8.

1.4 Polymer Light Emitting Devices (PLEDs)

Light emitting polymer devices like LEDs make use of material which has strong luminescence properties. These luminescent materials can be made up by using polymers which have chromophores in their polymer backbone. Another method to obtain luminescent materials is by doping inorganic or organic luminescent materials. These materials can be incorporated in the polymer matrix in order to achieve luminescence properties.

1.4.1 Blue light emitting polymers

Blue light emitting polymers have attained special interest of researchers because of their ability to produce good display systems. The generation of blue color light can be achieved by using homopolymer or copolymer depending upon the application. Blue color emission can be obtained from polymers by incorporating different chromophores like phenyl or fluorene or heterocyclic structure such as thiophene, pyridine and furan. Polymer containing these chromophores in their polymer chain can be utilized for the emission of blue light. These polymers are extensively used in the light emitting diodes9.

1.4.2 Blue light emitting homopolymers

i. Poly(p-phenylene) (PPP)

Different derivatives of poly(p-phenylene) have shown photoluminescence spectra in the range of blue region. Precursors of PPP were spin coated on the ITO glass. Desired polymer was obtained by heating ITO glass coated with prepolymers. The photoluminescence spectrum was acquired by exciting the sample at 351 nm and emission

4 spectrum was obtained at 460 nm. PPP are used in different application because of their mechanical and thermal stability10.

ii. Polyfluorene

Polyfuorene was modified to obtain the blue light emission. Two different kinds of blue light emitting polymers were prepared by substituting them with different chromophores. Dihexadecyl was substituted to the main chain of polyfluorene which gave emission at the 410 nm. The other substitution was carried out by using monohexyl to the main chain which resulted in emission wavelength of 385 nm11.

iii. Polythiophene

Different derivatives of polythiophene also exhibit emission because of modification in main chain. Poly(dioctylthiophene) was prepared by oxidizing monomers in presence of FeCl3. The photoluminescence spectrum of this polymer was observed at 470 nm12.

Figure 2. General structure of A) Poly(p-phenylene), B) Polyfluorene and C) Polythiophene

1.4.3 Blue light emitting copolymers

Tuning of emission wavelength can be done by introducing different chromophores in the back bone of polymer. Derivatives of PPP, fluorene and thiophene based copolymers were investigated for their blue light emitting properties 9.

5 1.4.4 Polymer blends

Instead of using single polymer for the generation of visible light, blending of different polymers can be used to achieve good photophysical properties. Efficiency of photoluminescence can be increased by mixing two different polymers. The underlying notion behind this practice is the dilution of light emitting polymer with some photostable polymer. For this purpose polystyrene and PMMA are widely used. Due to the dilution of light emitting polymers photoluminescence increases because the light emitting centers are separated from each other. Thus reducing quenching which occurs when chromophores are present in high concentration. Enhancement in photoluminescence intensity can be obtained by mixing two different light emitting polymers with each other9.

1.5 Upconversion and downconversion

From last few decades, electricity generation by utilizing solar energy has been focus of extensive research. Dye sensitized polymers have been used for producing electricity by using luminescence properties of organic molecules. Devices which use this kind of material are known as the luminescent solar concentrator4.

By using solar energy excitation of the electron from the ground state to the excited state takes place. This excitation corresponds to specific energy gap and only small portion of energy is utilized in this process. The rest of energy goes to transmission losses and black body losses. In transmission losses, the portion of sunlight which is insufficient for the transition is lost. While the other portion which has higher energy than the required energy end up heating the material. One possible solution of this problem is use of different luminescent materials in the form of layers maximizing the use of solar energy. The other novel way to use maximum energy is constructing materials capable of upconversions and downconversion.

1.5.1 Upconversion

The phenomenon of upconversion is attributed to the class of materials which can be excited by using two low energy photons and emits high energy photon. The process is opposite to the normal luminescence process which follows Stoke’s law. Upconversion can take place with two different mechanisms which include simultaneous absorption of two photons and resonant energy transfer.

In first mechanism electron in chromophore absorbs one photon and moves to intermediate excitation level. Here absorption of another photon takes place thus moving electron to higher energy level. Then upon relaxation high energy photon is released 13. Upconversion can be done by using lanthanide doped inorganic compounds and organic

6 sensitizer/acceptor system. These materials are being researched for their use in solar cells to increase efficiency.

Second method is transfer of energy through resonant energy transfer. In this mechanism, two excited ions which are present near to each other interact. This interaction results in transfer of energy from one excited ion to another. By this transfer of energy the electron present in acceptor ion moves to final excited level. Then high energy photon is released. Both mechanisms are shown in Fig. 3 B) and C).

1.5.2 Downconversion

In downconversion, high energy photons are absorbed by the luminescent material and low energy photons with the longer wavelength are released. This is opposite to the upconversion but should not be confused with normal luminescence process which follows Stoke’s law. In downconversion low energy photons are emitted in cascading fashion.

While in normal luminescence process low energy photon is released along with heat14. The main goal of downconversion is to reduce the energy of photons. This kind of materials are being used in fluorescent lamps which are more efficient than lamps which work according to Stoke’s law

15. The mechanism of downconversion is shown in Fig. 3 D).

Figure 3. A) Luminescence following Stoke Downshifting, B) Upconversion by absorption of two simultaneous photons, C) Upconversion through resonant energy transfer and D) Downconversion14.

7 1.6 Fluorescent nanocomposites

The use of small organic molecules as a fluorescent material is quite common in many applications. These applications include light emitting diodes5, luminescent solar concentrators4. Organic dyes are extensively used because of their easy modification which can result in changing the emission wavelength. Higher quantum efficiency is also an important feature while selecting these dye molecules. Apart from these advantages, they also have their limitation which is their low life time and lower resistance to photobleaching 16.

Encapsulation of the organic dye molecules which work as chromophores in inorganic framework is beneficial for the organic compounds. Inorganic environment provides many advantages which are rarely found in the polymer matrix. Inorganic matrix provides the rigid environment for the functional organic chromophores. This kind of matrix is favorable because of low cost and it also increases the photostability of the dyes which cannot be attained in the presence of polymer matrix16.

1.6.1 Coumarin 7 dye nanocomposites

Comparison between pure coumarin 7 dye and coumarin 7 dye entrapped in silica shell was investigated by Kulkarni et al17. Characterization of these dyes was carried out by using EDS and photoluminescence spectra. Photostability of pure coumarin 7 dye and coumarin 7 dye entrapped in silica shell was also studied by irradiating samples with UV light. Synthesis of silica shell particles containing coumarin dye was carried out by modifying Stöber process and the presence of dye inside the particle was confirmed by utilizing spot EDS analysis. The excitation wavelength for the dye entrapped in silica shell and pure coumarin dye was 350 nm. Solid form of dyes were used for the measurement of PL spectra. The emission spectra for both dyes were almost same except the fact that emission spectrum for original dye was sharper as compare to dye entrapped in silica shell. Author explained these differences on the basis of interaction which took place between the silica particle and dye molecules.

Photoluminescence spectra of these dyes were also obtained in ethanol solution. Overlaid PL spectra of solid dyes and dyes solution is provided in Fig. 4. Sample of pure dye solution was excited with 340 nm and the emission spectrum was obtained at 536 nm. While the particles which were present in the silica shell were isolated from the ethanol due to their inorganic matrix. Different excitation spectra was reported for the solution of pure dye and dye entrapped in silica shell. Emission spectrum of solution containing coumarin dye entrapped in silica shell was redshifted by 16 nm when compared with emission spectrum of pure dye solution.

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Photostability of the both forms of dyes was studied. Solid samples of original dye and dye entrapped in silica shell were irradiated with UV lamp of 35 W for 3 hours. Significant decrease in the emission spectrum of original dye was observed after the irradiation. But the dye molecules which were entrapped inside the silica particle were more photostable as compare to the original dye. No significant decrease in the emission spectrum of core shell particles was observed.

Figure 4. Photoluminescence spectra of coumarin 7 dye and coumarin 7 dye silica core shell particles in presence of ethanol17.

1.6.2 Hybrid organic dye nanocomposties

Hybrid nanocomposites containing organic dye have been studied extensively because of their novel application in the field of laser and light emitting diode18,19. Fakis et al., 20

reported the study of dye molecules in the porous silica film. In this study derivative of the fluorene dye was use. The structure of the dye is show in Fig. 5. Core of this dye is made up of fluorene and the phthalimide groups are substituted on edges. The dye was abbreviated as F(PHT)2. Porous silica film was prepared and the diameter of the pores in silica film was 20 nm with the length of 1 μm. SEM analysis was performed to verify the presence of dye in pores of silica film. For the sake of comparison, photoluminescence spectra of the dye solution, film of dye and the composite of porous silica and dye were

9 obtained. All of these samples were prepared in two different solvent in order to study the effect of solvent on photoluminescence. For this purpose aromatic solvent chlorobenzene (CB) and non-aromatic solvent like tetrahyrofuran (THF) was used. Excitation wavelength of 340 nm was used for procurement of photoluminescence spectra of all samples.

Figure 5. Structure of dye20

After the impregnation of the dye molecule in the porous silica the films were washed many times with the solvent to remove the dye molecules present outside the pores. Fig. 6 show emission spectra of the composites made of porous silica and dye, pure film and dye solution in both solvent.

Figure 6. PL spectra of dye in porous silica, dye in solution and dye in films. A) Samples were prepared in THF B) samples were prepared in CB20.

10 It is evident from the spectra that composites materials showed blue shift when compared with the corresponding pure film and solution. According to the author the blue shift was a result of impregnation because in the pores of silica film dye molecules were not interacting with solvent molecules. The dye molecules were behaving independently in the pores as solvent was completely vaporized before acquisition of photoluminescence spectra. In this study the nature of solvent was determining the conformation of dye molecules. Chlorobenzene favors the planar conformation of the dye molecule thus making the penetration of dye molecules into pores easy. Tetrahydrofuran as a non-aromatic solvents twist the conformation of the dye molecules. So choosing a right solvent can facilitate the adsorption into the pores.

1.6.3 Organic dyes encapsulated in Nickel Phosphate VSB-1

Inorganic matrix is not only used for the encapsulation of the chromophores, it can also increase the photoluminescence properties of the organic dyes. Nickel phosphate VSB–1 (Versailles Santa Barbara – 1) is a nanoporous material. This VSB–1 was used by Wang at el., 16 for the impregnation of rhodamine 6G, 8-anilino-1-naphthalenesulfonic acid and 7-diethylamino-4-methyl coumarin. These dyes were represented by author as (1), (2) and (3) respectively.

Encapsulation of these dyes was carried out in VSB–1. For this purpose nanoporous material VSB-1 was evacuated for the duration of 3 hours. Then VSB-1 was mixed with the 8 x 10-5 molar solution of dyes. The mixtures were stirred for 48 hours to facilitate the impregnation of dyes in pores. After stirring, washing was performed to wash the dye molecules present on the surface of VSB-1. To confirm the presence of dyes in the VSB-1 IR and UV spectra was acquired for the pure VSB-1 and VSB-1 impregnated with organic dyes. There was a significant difference in spectra which confirmed the presence of organic dye molecules inside the nanopores of nickel phosphate (VSB-1).

Solid state photoluminescence spectra were obtained from dye molecules encapsulated in the VSB-1 and dye solutions. Solution of Rhodamine 6G was prepared in water and 8-anilino-1-naphthalenesulfonic acid and 7-diethylamino-4-methyl coumarin solutions were prepared in hexane. The emission wavelengths for these solutions were 555 nm, 434 nm and 411 nm, respectively. Red shift was observed in the photoluminescence spectra of dye which were impregnated into VSB-1. In order to compare the intensity of photoluminescence solid state spectra were obtained for the pure dyes and impregnated dyes in the VSB-1. The spectra are shown in Fig. 7. Significant increase in the emission efficiency of dyes was observed when they were present inside nanoporous VSB-1.

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Figure 7. Photoluminescence spectra of dyes before and after encapsulation in VSB-1 obtained from solid samples. (a represent the pure dye and b represent dye in VSB-1)16.

According to this study the increase in emission efficiency (Table. 1) after the encapsulation was 50, 45, and 57 times for dye 1, 2, and 3, respectively. Increase in the efficiency was explained because of the monomer form of dye which is expected to be present in the nanopores of VSB-1. Lesser emission intensity in pure form of dyes was attributed to the quenching of fluorescence intensity. When dye molecules forms aggregate the phenomenon of quenching decreases the intensity of luminescence.

Table 1. PL properties, Relative emission intensity of the dye before and after encapsulation in VSB-116.

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1.6.4 Epoxy/Clay nanocomposites

Effect of different clay materials such as montmorillonite, organically modified montmorillinite and halloysite nanoclay on the photoluminescence properties of epoxy resin (Bisphenol A diglycidyl ether) was reported by Mondradon et al.,21. Bisphenol A diglycidyl ether and 4-nonyl-benzyl amine were used as a matrix and hardener respectively. These epoxy and clay nanocomposites were prepared by varying amount of clay in it. Solution casting method was used for the preparation of these samples. Four samples with clay content of 0, 2, 5 and 15 wt-% were prepared. Clay material was dispersed in chloroform by using sonication for the duration of 30 min. After the sonication process epoxy resin was added to the solution. Homogeneity in mixture was achieved by using continuous stirring for 30 min. This mixture was kept at 60° C for 10 min in order to remove the chloroform from the mixture. Then hardener was added to the mixture and mixed by using sonication for 5 min. Epoxy resin and hardener was used in ratio of 50:50. After mixing, degassing was carried out under vacuum. Once degassing was finished these samples were casted in glass mold. After the duration of 5 days polymer sample were ready for the further investigation.

Characterization of these samples was done by using FT-IR spectroscopy, X-ray diffraction, transmission electron microscopy, UV-Visible spectroscopy, atomic force microscopy and photoluminescence spectroscopy. Agglomeration of the halloysite nanotubes was observed in the sample containing 2 wt-% of the clay. It seems that there was not very extensive hydrogen bonding between the halloysite nanotube and the amine group of resin. Good dispersion can be achieved by using the hardener which facilitates these interactions.

No significant difference was noted in the photoluminescence spectra of samples containing montmorillonite clay. PL spectrum of the epoxy/HNTs composite material was different from PL spectra of pure epoxy. Red shift in the emission band of the epoxy/clay nanocomposite material was observed. During the curing some interactions between nanoclay and epoxy resin was achieved which account for this red shift in the emission spectra.

Quenching in the photoluminescence intensity was noted with the addition of montmorillonite and organically modified montmorillonite. The significant increase in the photoluminescence intensity of composite material was observed when 5 wt-% of the halloysite nanoclay was present in the sample. Other epoxy composite materials containing 2 and 15 wt-% of the nanoclay have shown similar quenching like other clays. According to the author increase in the photoluminescence intensity can be attributed with the presence

13 of epoxy fiber inside the lumen of halloysite nanoclay but no explanation was provided why this interaction was not seen in the samples containing 2 and 15 wt-% of epoxy halloysite nanoclay composites.

1.7 Halloysite nanoclay

Halloysites nanotubes (HNTs) are the center of extensive research because of the certain features which distinct them from other naturally occurring clays like montmorillonite and kaolinite. This includes rich abundance of HNTs in many places around the globe. Natural deposits of HNTs are found in New Zealand22, Belgium23, France24 and China25. HNTs are popular among researchers for their exciting features such as their high length to diameter ratio, nanoscale lumen, comparatively low number of OH on surface26, rigidity, easy dispersion in polymer matrix and their compatibility with biological systems27. Due to these features, HNTs have been used in numerous applications in the field of material chemistry. Chemical formula of HNTs is Al4 Si4 O10 (OH) 8 . 4H2 O. Reported dimensions for HNTs are 1 – 30 nm inner diameter, 30 – 50 nm outer diameter and 100 – 2000 nm in length. The most prominent application of the HNTs is the production of thin walled porcelain and crucible products which are considered as high quality ceramics26.

Figure 8. Structure of Halloysite nanotubes (HNTs)28.

1.7.1 Applications of halloysite nanoclay

Corrosion of metals is a common problem in industry and with household equipment. Research on this subject is very comprehensive. Use of HNTs in corrosion protection is gaining interest because they can be used as a container containing the corrosion inhibitor. These inhibitors are usually toxic materials so their impact on the environment is a serious

14 concern while designing these inhibition mechanisms. The use of corrosion protecting coating containing HNTs doped with anticorrosion material is reported recently29. Corrosion process triggers release of the anticorrosion agent from the HNTs in a controllable fashion. For the protection of copper and transition metals benzotriazole and its derivative can be used. Due to the water soluble nature of benzotriazole it is not possible to use it directly along with passive coatings like paints. The formation of voids in coating is expected upon contact with water. This can significantly hamper the efficiency of protective coatings. To overcome this controlled release of anticorrosion agents is achieved by loading the HNTs and placing the stopper at their ends. Time for the complete release of these anticorrosion materials depends on the nature of stopper placed at the end of HNTs. Encapsulation and controlled release of the substance by the nanoparticles resulted in the tremendous increase in effectiveness of these coatings30.

Increase in thermal stability and fire retardancy was reported when HNTs were added with polypropylene (PP). Melt blending was used for the preparation of HNTs and polypropylene nanocomposites. Decrease in flammability of these nanocomposites was confirmed by heat release rate data which was collected by using cone calorimeter. Decomposition products were trapped inside the HNTs that results in increase in the thermal stability of material. This stability can also be attributed to the hindrance for heat and mass transfer because of the HNTs31. Introduction of the HNTs to the thermoplastic causes enhancement of different properties such as thermal stability, mechanical strength and fire retardancy. HNTs were used in linear low density polyethylene to study the effect on thermal stability and mechanical properties. Significant increase in flame retardance ability of thermoplastic was reported upon addition of HNTs in polymer32.

HNTs are used as a matrix for the different catalyst. They increase life time of the catalysts and hinder degradation of catalyst particles. Encapsulation of the catalyst is efficient technique for easy recovery of the catalyst after the reaction. Their potential as an immobilization matrix is evident according the study carried by Liu et al., 33. In this study silver nanoparticles were chosen because of their huge potential for the catalytic activity. Recovery of the silver nanoparticles from the reaction mixture after the completion of reaction is complicated task. To ensure the reuse of the silver nanoparticle HNTs were used. Silver nanoparticles of 10 nm were immobilized on the clay material. For this immobilization of silver nanoparticles polyol process was used for the reduction of AgNO3 in situ. Then this catalyst was used for the reduction of 4-nitrophenol in alkaline solution in presence of NaBH4.

Nano-templating such as production of polymeric nanowires and nanotubes gained a lot of interest recently. Naturally abundant HNTs clay which is not very expensive is being used

15 for this purpose. Impregnation in the lumen of HNTs with aniline was carried out. The polymerization of nanowires took place inside the lumen HNTs34.

1.7.2 Dispersing methods for HNTs

One of the main challenges while using HNTs in the polymer nanocomposite is homogenous dispersion of clay throughout the matrix. Agglomeration of clay particles is very common problem. This agglomeration is facilitated because of small particle size and high surface area of HNTs. Mechanical stirring or magnetic bar stirring is not sufficient to provide homogenous dispersion of HNTs in epoxies. It has been reported by Deng et al.,35 that good dispersion of clay particles in nanocomposites made up of HNTs/epoxies resins can be obtained. The agglomerated HNTs particle can be separated by using the sheer stress. In order to obtain good homogenous dispersion of HNTs in epoxies matrix use of ball mill is effective route. The more efficient dispersion was obtained when treatment of HNTs was carried out with potassium acetate. This chemical treatment was useful in reducing the size of HNTs. These chemically treated clay nanoparticles were subjected to ball mill which yielded in good dispersion35. This induction of clay particle resulted in increase of mechanical strength of nanocomposites.

Du et al36 reported that impregnation of HNTs with organic compounds which can make hydrogen bond with the species present on the surface of HNTs such as hydroxyl group and Si – O – Si can result in the good dispersion in polypropylene matrix. The author explained this dispersion of inorganic HNTs in organic polymer on the basis of hydrogen bonding self-assembly process. This process has shown homogenous dispersion of HNTs in the polymer matrix resulting in better mechanical properties.

Another approach which was investigated for the dispersion of HNTs in thermoplastic polymer matrix is direct melt blending. This process was relatively easy to perform on large scale due to its simple protocol. Homogenous dispersion of the HNTs can be expected because of the fact that there are lesser hydroxyl group present on the surface as compared to montmorillonite and kaolinite. Uniform dispersion of HNTs by melt mixing was observed in the polymers having high polarity. Melt blending was successfully used for the preparation of the polymer nanocomposites containing HNTs and polyamides. HNTs and polyamide 6 nanocomposite material was prepared by using melt blending. Successful dispersion was obtained without doing any surface modification of the HNTs26.

Good dispersion of the HNTs in the epoxy resin matrix was reported by Liu et al36. Uniform and even distribution is obtained by this method. Diglycidyl ether of bisphenol A in liquid form was used as an epoxy resin and the bisphenol A dicyanate ester as a hardener. HNTs were dispersed in the epoxy resin at 70° C. This mixture was stirred for the

16 60 min to obtain a homogenous mixture. After the mixing of epoxy resin and HNTs degassing of the mixture was done. At this stage hardener was added to the existing mixture. Again the stirring of the mixture of epoxy resin, HNTs and hardener was carried out for 10 min. again degassing of the mixture was performed for the 30 min. Then these mixtures were poured into steel mold having a coating of Teflon. The ratio of epoxy resin and hardener was kept at 100:40. Curing of these samples was carried out. The curing schedule was 150 °C/2 hours + 180 °C/1 hour + 200 °C/2 hours. Once the curing is completed the samples were cooled at room temperature over the duration of 8 hours. The maximum workable loading of the halloysite nanotubes was 12 wt-% for easy processing and casting. At higher loading of HNTs the viscosity of the sample was increased and they were not easy to process.

1.7.3 Adsorbing organic dyes by HNTs

Many organic dyes are harmful for the humans and environment but still they are used at large scale in many industries like in leather, paper, printing and plastic preparation. So, there is a need to find innovative ways to remove these dyes from waste after use. Different clay materials were studied for the removal of these dyes from the waste as they have good adsorption qualities. HNTs have edge over the other type of clays like perlite, dolomite, montmorillonite and zeolite. These clays are used for the adsorbing pollutants because of their inexpensiveness, larger surface area and high adsorption capacity. Adsorption qualities of these clay materials were utilized for the removal of color from effluents. This is a very useful technique because of easy procedure and decolorization of waste water can be carried out by employing different clays27. HNTs have shown their potential as an adsorbent because of their easy availability and nano size structure.

Zhang et al 37 investigated HNTs due to their potential for adsorbing dyes from waste water. They have demonstrated the removal of cationic dye methyl violet from the aqueous solution. Rapid adsorption and high adsorption capacity of the HNTs were reported in this study. High adsorption rate, inexpensiveness of HNTs and easy recovery via calcination method made these HNTs potential candidates.

A similar study was carried out on HNTs for adsorption of cationic dye natural red from the aqueous solution. After the characterization of the HNTs by using XRD, FT-IR, TEM and BET they were used for the adsorption of dye. Increase in adsorption capacity was observed with the increase in initial concentration of dye, initial pH and temperature. Adsorption of natural red dye in HNTs was spontaneous and endothermic process. This study demonstrated that HNTs are suitable candidate for the adsorption of organic dyes38.

17 Adsorption of the methylene blue from aqueous solution was also carried out by using HNTs. Methylene blue is cationic dye which is separated from the solution by using HNTs. The HNTs acted as a nano-adsorbent for the dye molecules. The adsorption was optimized under basic conditions, high starting concentration of dye and low temperature39. It is evident that HNTs have a high affinity for the organic dye adsorption. An optimal condition for the adsorption varies possibly because of the dye structure. The high adsorption capability of HNTs for organic dye is well established.

1.7.4 Loading of HNTs for controlled release

HNTs are being used for the loading and controlled release of drugs. The use of nanocontainers was reported to achieve uniform distribution of drugs40. Active component of drug was encapsulated in order to reach the target for the controlled release of drug. In one such kind of study HNTs were loaded with the gentamicin sulfate. Aqueous solution of the drug and powdered nanoclay was used for the encapsulation process. By mixing both of these components a suspension was made. Mixing of this mixture was carried out by using sonication for 2 hours. After the sonication process vacuum was applied. Vacuuming helped the drug to move inside the lumen of HNTs. For this purpose vacuuming was performed at a pressure of 100 torr for 20 mins. After the vacuuming the mixture was left for another 20 min at atmospheric pressure. This process was repeated for three times to optimize the quantity of drug into the lumen of halloysite nanoclay. To remove the drug which may be present at the surface of these nanoparticles washing was performed. Washing was followed by the centrifugation and to drying. Drying was carried out in the oven for 2 hours at 55° C. By using this procedure the HNTs loading of 10-15 wt% of drug was achieved.

The adsorption of the anti-inflammatory drug 5-amino salicylic acid on HNTs was reported recently. The hallow lumen of the nanoclay particle was used as a carrier of drug. The HNTs used in this study had a diameter of less than 100 nm and length between 500 nm - 1 μm. UV spectroscopy was used for the determination of the drug which was adsorbed on

the halloysite nanoclay. Author has explained the process on the basis of two separate processes. First of all drug was adsorbed on the outer surface of the HNTs. This was rapid adsorption. Then drug adsorbed into the lumen of the HNTs. The adsorption inside the nanoclay was reported to have slow adsorption rate 41.

One of the most common and simple method for the loading of HNTs is depicted in Fig. 9, Filling of the empty lumen can be performed in varieties of ways. The simplest method is use of dry powder of HNTs and mixing it with the concentrated solution of the compound of interest. To achieve a good loading of the substance in the lumen solvent should be chosen carefully. The solvents which can dissolve maximum amount of compound to be

18 loaded into lumen of HNTs are favorable. Solvent should have good wetting properties for HNTs and low viscosity to achieve optimum loading. After the preparation of the mixture of compound and HNTs, stirring is used to disperse the HNTs in the concentrated solution. Then this suspension was placed under vacuum for about 10 – 30 min. Under the vacuum fizzling sound is the indication of the removal of air from the empty lumen of HNTs. After vacuum treatment the mixture is placed under normal atmospheric pressure. The removal of vacuum facilitates the filling of empty lumen by pulling molecules inside the cavity. By repeating this process for 3 – 4 times the efficient loading of nanotube can be achieved. Once the loading is complete washing of the HNTs is carried out which remove the loosely bounded atoms from the HNTs. Washing also helps in removing the molecules attached to the surface of HNTs 42.

Figure 9. Loading of benzotriazole in the lumen of HNTs 43.

Number of factors can influence the loading of material in the lumen of HNTs. One of them is concentration gradient. To achieve good loading of the HNTs solvents which have a low boiling point and high vapor pressure are preferable, for example acetone. To ionize the hydroxyl groups which are present on the walls of HNTs, solvent with high a dielectric constant is suitable. This ionization can stabilize the suspension of the HNTs and it can also increase the loading of the negatively charged particle inside the lumens44.

19 2. Aims of Study

Luminescent polymer surfaces have found exciting applications in luminescent solar concentrators4, organic light emitting diodes5, flat panel displays and solid state lighting6. The objective of this study was to fabricate luminescent polymer surfaces which can be used as coating materials. For this purpose, these coatings should be highly luminescent and resistant to photobleaching. In addition to that, good mechanical strength was also desired. To achieve these features, highly luminescent dyes, HNTs and polystyrene were used. HNTs were expected to provide inorganic environment to organic chromophores and mechanical strength to polymer coatings. Our goal in study was, to;

1. Encapsulate organic chromophore in HNTs. 2. Achieve uniform distribution of HNTs in polymer matrix. 3. Investigate effects of HNTs on photoluminescence of organic dyes. 4. Study interactions between HNTs and organic dyes. 5. Optimize photoluminescence of luminescent polymer coatings.

20 3. Experimental

3.1 Materials

3.1.1 Organic dyes

Three different highly fluorescent organic dyes were chosen for this study. Which includes Rhodamine B (RhB), Rhodamine 6G (Rh6G) and Fluorescein (Fluo) given in Table 2.

Table 2. Properties of Commercially available Luminescent Dyes Organic Dyes CAS-Number Molecular Formula Molecular Weight Supplier

Rhodamine B 81-88-9 C28 H31 Cl N2 O3 479.01 Sigma Aldrich

Rhodamine 6G 989-38-8 C28 H31 Cl N2 O3 479.02 Alfa Aesar

Fluorescein 2321-07-5 C20H12O5 332.31 Sigma Aldrich

Figure 10. Molecular structure of Luminescent organic dyes. A) Rhodamine B B) Rhodamine 6G and C) Fluorescein

3.1.2 Hallyosite nanotubes

Halloysite Nanotubes (HNTs) are aluminosilicates having chemical formula of H4Al2O9Si2. 2H2O, pore volume 1.26-1.34 mL/g in form of nanopowder (CAS number: 1332-58-7) was procured from Sigma Aldrich. HNTs were used without any pretreatment.

21 3.1.3 Polymer

Polystyrene (PS) CAS number: 9003-53-6, Mw of 35000, density 1.06g/mL was obtained from Sigma-Aldrich and used without any further purification.

3.2 Methods

3.2.1 Scanning Electron Microscopy (SEM)

Dispersion characterization of HNTs in the PS polymer matrix was performed by using S4800 FE-SEM, Hitachi. All samples were coated with Au layer of 4nm to assist SEM measurements.

3.2.2 Energy Dispersive Spectroscopy (EDS)

Distribution of HNTs in the polymer matrix was studied by using Thermo Electron EDS attached with S4800 FE-SEM, Hitachi. Point and Shoot method was employed to characterize elemental composition of HNTs.

3.2.3 FT-IR spectroscopy

Infrared Spectroscopy was performed on both impregnated and pure HNTs (undried) in order to study the interactions taking place between fluorescent dye molecules and HNTs. Attenuated Total Reflection Fourier Transform Infrared Spectroscopy (ATR-FTIR) Vertex 60 was employed to study these interactions. FT-IR spectra of organic dyes was obtained by diluting them in solid KBR powder.

3.2.4 UV-Visible spectroscopy

Perkin-Elmer Lambda 900 UV/VIS/NIR spectrometer was used for the measurement of maximum absorption of solid polymer coatings and dye solutions. Maximum absorption of solid polymer coatings was acquired by using integrating sphere.

3.2.5 Fluorescence spectroscopy

Excitation-Emission matrices (EEMs) of polymer coatings having luminescent dye and HNTs were obtained by using bispectrometer. Xenon lamp (Oriel M-66923 housing, Newport corporation, Irvine, California and Osram XBO 450 W bulb, Osram AG,

22 Switzerland) was used as a light source. Czerny–Turner monochromator (DTMc300, Bentham Instruments Ltd, United Kingdom) was employed to direct the light from source. Emission intensity was measured by using spectrograph detector (PMA-12, Hamamatsu Photonics K.K., Japan). In all measurements incidence and detection angles were 0° and 45° respectively.

3.2.6 Transmission Electron Microscopy (TEM)

High Resolution Transmission Electron Microscopy (HR-TEM) was performed by using Jeol, JEM-2100F for the impregnated and empty HNTs in order to investigate the loading content.

3.3 Sample preparation

3.3.1 Impregnation of HNTs with organic dyes

Impregnation of HNTs was performed under the vacuum. First of all, weighed amount of dye was added into the 2.5 ml of N,N-Dimethylformamide (DMF). Homogenous solution of organic dye was prepared by continuous stirring for 5 minutes in single neck round bottom flask with stopcock septum port. Once the solution was formed 2 grams of HNTs were added into the solution and vacuum was applied for the impregnation. This mixture was kept under vacuum for the duration of 7 hours. Then resultant mixture was kept at atmospheric pressure for overnight. In the end impregnated HNTs were kept for drying under vacuum for 48 to 60 hours. HNTs were impregnated with all three dyes by using above-mentioned protocol.

3.3.2 Washing of impregnated HNTs

After the impregnation, HNTs were washed thrice with DMF by employing centrifuging. Washing was performed by employing centrifugation with following parameters.

RPM: 5300 Time: 30 Minutes

After the washing, supernatant was decanted from the mixture and HNTs were left for the complete evaporation of solvent. Supernatant which was decanted after washing was left for drying in fume hood. After drying the DMF completely, amount of washed dye was determined.

23 3.3.3 Drying of impregnated HNTs

After decanting the supernatant, impregnated HNTs were left for drying in the desiccator under vacuum for 24 – 36 hours. Once the drying was complete, these impregnated HNTs were used for the fabrication of polymer coatings.

3.3.4 Polymer film casting

Firstly, 40-wt% PS solution in toluene (Tol) or in N,N-Dimethylformamide (DMF) was prepared by using mild heating around 40°-50° C and constant magnetic stirring. After preparation of the homogenous solution, impregnated HNTs (10-wt% of PS) were added to the solution and mixed with magnetic stirrer for 10 minutes at 40°-50° C. Then this mixture was sonicated for 40 minutes at 40° C. After the sonication, the resultant mixture was casted in the silicon mold and left for drying in the fume hood. Complete drying was achieved after 48-72 hours.

24 4. Results and discussion

4.1 Characterization of impregnated HNTs

After impregnating HNTs with the luminescent dyes (RhB, Rh6G & Fluo) structural characterization was carried out by using SEM. Elemental composition of these impregnated tubes was determined by using EDS. Impregnated HNTs were sprinkled over carbon tape and 4nm thick layer of Au was sputtered on the sample to assist SEM measurements. In Fig. 11 SEM and EDS images for the RhB impregnated HNTs (RhB/HNTs) are presented.

Figure 11. A) SEM Image of RhB Impregnated HNTs, B) EDS image of RhB Impregnated HNTs showing the areas analyzed by point and shoot, C) EDS spectra of Point 1 and D) EDS spectra of Point 2.

In Fig. 11 A) one of the HNTs is present in the tilted position, exposing the end of HNT which is encircled. The interior diameter of this HNT was 51 nm which is in accordance with reported value of 30 – 70 nm45. Elemental analysis of this HNT was performed by using EDS which is shown in Fig.11 C). In this spectra the Si, Al and O peaks corresponds to the composition of HNTs and carbon peak indicates the presence of organic dye RhB in

25 vicinity of HNTs. Likewise, Rh6G and fluorescein impregnated HNTs were analyzed for their morphology and composition.

Figure 12. A) EDS Image of Rh6G/HNTs, B) EDS spectrum of Point 1 of the Rh6G/HNTs, C) EDS Image of Fluo/HNTs, D) EDS spectrum of Point 1 from Fluo/HNTs.

In Fig. 12 EDS spectra of Rh6G and fluo impregnated HNTs are given. This spectra shows carbon peaks which confirms the presence of the organic dyes Rh6G and Fluo. In EDS spectra, the carbon peaks are originating due to the presence of dyes which were impregnated in the HNTs. In Fig.12 D), Au peak is representing the 4nm layer of the Au which was used to facilitate the SEM measurements. However, there may be contribution from the carbon tape used for sample preparation. In both EDS spectra Si, Al and O peaks are confirming the existence of the HNTs in the samples.

4.2 Characterization of polymer coatings

Dispersion of HNTs in the polymer matrix PS was achieved by optimizing the fabrication protocol. Uniform and even dispersion of the HNTs in the polymer matrix is very crucial for the optical properties of coatings. Agglomeration of HNTs will not only result in irregular distribution but it can severely reduce the luminescence intensity because of quenching. Polymer coatings containing RhB, Rh6G and Fluo impregnated HNTs were fabricated and dispersion of HNTs was characterized by using SEM and EDS.

26 Polymer films containing RhB, Rh6G and Fluo impregnated HNTs in PS matrix from now on will be referred to as RhB/HNTs@PS, Rh6G/HNTs@PS and Fluo/HNTs@PS respectively.

Figure 13. A) SEM image of RhB/HNTs@PS showing areas analyzed for elemental analysis, B) and C) EDS spectra of encircled parts.

In Fig. 13 A), bright spots on the surface of polymer film accounts for RhB impregnated HNTs. RhB/HNTs@PS shows well distributed HNTs in the polymer matrix in spite of minor agglomeration of nanoparticles. Encircled areas were characterized with EDS to confirm the presence of HNTs. Intense peak of carbon in the EDS spectra originated because of high C content in the polymer matrix while Si, Al and O peaks confirm the presence of the HNTs. Similarly, Rh6G/HNTs@PS and Fluo/HNTs@PS were also characterized for their morphology and dispersion of HNTs by using SEM and EDS.

Figure 14. SEM Images of Polymer films, A) Rh6G/HNTs@PS and B) Fluo/HNTs@PS

27 SEM images of polymer films containing Rh6G and Fluo impregnated HNTs are shown in Fig.14 A) and B) respectively. In all polymer films HNTs were seen as a bright spots on the polymer surface. These bright spots were characterize with EDS to analyze their elemental composition. Si, Al and O peaks in the EDS spectra confirmed that these bright spots are HNTs.

4.3 Transmission Electron Microscopy

High resolution electron microscopy (HR-TEM) was performed of the empty and loaded HNTs to observe the changes in morphology. Fig. 15 shows the HNTs which were impregnated with Rh6G, the nanochannels in these HNTs are not very visible which suggest the presence of Rh6G in the HNTs. Due to the very low amount of loading it is not possible to observe completely full nanochannel of HNTs.

Figure 15. A) and B) HR-TEM images of the Rh6G impregnated HNTs

HNTs imprenated with the RhB are shown in Fig. 16. Nanochannels of the HNTs are partially filled with the organic dye molecules (Fig. 16 A). The dark patches on HNTs represents RhB which is present inside the nanochannels of HNTs.

28

Figure 16. A) and B) HR-TEM images of the RhB impregnated HNTs

In Fig. 17 empty nano-channels of the HNTs can be seen clearly. These channel are not visible in the impregnated HNTs. Comparison of impregnated HNTs with empty HNTs confirm the presence of luminescent materials in the HNTs.

Figure 17. TEM image of pure HNTs and inset is showing the tubular morphology of HNT46.

29 4.4 Electronic absorption of composites and dye solutions

Maximum absorption of polymer films RhB/HNTs@PS, Rh6G/HNTs@PS and Fluo/HNTs@PS was recorded and compared with their corresponding dye solutions. RhB and Rh6G solutions were prepared in methanol and Fluo was dissolved in water. In Fig.18, UV-Vis spectra for the RhB/HNTs@PS and RhB solution are given. The maximum absorption of the polymer film and RhB solution was recorded at 555 nm and 539 nm, respectively.

Figure 18. A) UV-Visible spectra for RhB/HNTs@PS (λ max = 555 nm) and B) UV-Visible spectra of RhB solution prepared in methanol (λ max = 539 nm) and C) An overlaid spectra of both A) and B)

The red shift in the maximum absorption of the RhB/HNTs@PS indicates that dye is present in the different matrix. Red shift of 16 nm was observed in RhB/HNTs@PS with respect to dye solution. The difference in the wavelength can be attributed to the presence of HNTs and polymer matrix in surrounding of dye molecules. In addition to red shift, broadening of peak was also observed in case of RhB/HNTs@PS. PS and HNTs are providing complex matrix to the dye molecules which is broadening and moving the maximum absorption to the longer wavelength. In order to compare, an overlaid spectra of both RhB/HNTs@PS and RhB solution is given in the Fig. 18 C).

30 Electronic absorption because of PS polymer film was recorded at 290 nm. Reiney et al., 47 have reported maximum absorption of polystyrene at the same wavelength. In addition to that, two bands at 357 and 411 nm were observed (data not shown). These bands can be attributed to the HNTs.

Likewise, spectra of Rh6G/HNTs@PS, Fluo/HNTs@PS, Rh6G/Methanol and Fluo/H2O were also measured to understand the effect of matrix on the maximum absorption. λ max for Rh6G/HNTs@PS and Rh6G dye solution was 515 nm and 519 nm, respectively. Maximum absorption for the Rh6G/HNTs@PS moved to longer wavelength by 4 nm as compared to the Rh6G dye solution. In case of Fluo/HNTs@PS maximum absorption was at 474 nm and for the Fluo dye solution 494 nm. Significant red shift of 20 nm was recorded in the case of polymer films containing Fluo impregnated HNTs. Fig. 19 present the spectra for Rh6G/Methanol and Fluo/H2O solutions and their respective polymer films.

Figure 19: UV-Visible spectra A) Rh6G/HNTs@PS, B) Rh6G Dye solution in Methanol, C) Fluo/HNTs@PS and D) Fluorescein dye solution in distilled water.

In UV-Visible spectra characteristic peaks of RhB/Methanol, Rh6G/Methanol and Fluo/H2O were 539, 515 and 474 nm, respectively. Presence of these characteristic bands in

31 the polymer films prove the existence of dye molecule in the film. In all polymer films maximum absorption moved to the longer wavelength as compared to the dye solution.

Figure 20. UV-Visible spectra of polymer film and respective dye solution, A) Rh6G dye solution and Rh6G/HNTs@PS, B) Fluo dye solution in water and Fluo/HNTs@PS.

The large extended conjugated system of dye molecules were broken by dissolving the dye into solvent. These large conjugated networks were again established by confining the dye into nano channels of HNTs thus moving highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO) closer to each other. By reducing the gap between HOMO and LUMO the maximum absorption of polymer films moves to lower energy causing red shift16 as shown in Fig. 20. The characteristic absorption peaks of all polymer coatings and their respective dye solutions are summarized in the Table 3.

Table 3. Summary of λ max of dye’s solution, their respective polymer films and red shift.

λ max (nm) of Dye

Solutions λ max (nm) of Polymer

Films Red Shift in λ max (nm)

of Polymer Films Rhodamine B 539 555 16

Rhodamine 6G 515 519 4 Fluorescein 474 494 20

32 4.5 Interactions between organic dyes and HNTs

FT-IR spectroscopy was employed to confirm the presence of dye molecules in the HNTs. In addition to that, interactions taking place between dye molecules and HNTs were investigated. IR bands of dyes, pure HNTs and impregnated HNTs were studied and compared with each other.

In Fig. 21 IR spectra of RhB dye, pure HNTs and RhB impregnated HNTs are presented. These spectra were compared with each other to verify the existence of RhB dye molecules in the HNTs. The IR spectrum of pure HNTs (given in Fig. 21 a) is not showing any distinctive bands in the region 1200 to 1800 cm-1. By comparing IR spectra of RhB/HNTs with pure HNTs, new IR bands were noted at 1253, 1386, 1440, 1498 and 1678 cm-1. These IR bands confirm the presence of the RhB dye in the HNTs as they are the characteristic bands belonging to the pure RhB dye as shown in Fig. 21 (c).

Figure 21. IR Spectra (a) Pure HNTs (b) HNTs impregnated with RhB, (b*) Inset of the RhB/HNTs spectra ranging from 1200 to 1800 cm-1 highlighting characteristic bands of pure dye and (c) Pure RhB Dye.

In FT-IR spectrum of pure HNTs, two stretching bands of Al – OH are present at 3625 and 3699 cm-1 48. These bands are not very distinctive in the RhB/HNTs spectrum which indicates the formation of some weak interactions between HNTs and dyes molecules. These interactions are taking place between highly electronegative elements like O and N present in the RhB dye molecules and Al – OH of the halloysite nanotubes.

33 Similarly, the interactions taking place between other dyes, Rh6G and Fluo with HNTs were investigated thoroughly. In Fig. 22 spectra of pure HNTs, Rh6G impregnated HNTs and pure Rh6G are given. IR bands at 1254, 1308, 1385, 1416, 1440, 1500, 1535, 1612 and 1678 cm-1 were observed in the IR spectrum of Rh6G/HNTs given in Fig. 22 b. Spectrum of pure HNTs does not shows any distinctive bands in this region. These are the characteristic bands of pure Rh6G dye (Fig. 22 c). Emergence of these IR bands in the spectrum of Rh6G/HNTs confirms the presence of dye molecule in the HNTs.

Figure 22. IR Spectra (a) Pure HNTs (b) HNTs impregnated with Rh6G, (b*) Inset of the Rh6G/HNTs spectra ranging from 1200 to 1800 cm-1 highlighting characteristic bands of pure Rh6G dye and (c) Pure Rh6G Dye.

Presence of fluorescein in HNTs was studied by using the same approach. For this purpose spectra of pure HNTs (Fig. 23 a) and pure dye (Fig. 23 c) were compared with the Fluo impregnated HNTs (Fig. 23 b). Upon comparing the spectrum of pure HNTs with the Fluo/HNT the presence of new IR bands in impregnated HNTs was realized. These IR bands were present at 1331, 1393, 1464, 1580 and 1648 cm-1. The existence of these IR bands in the Fluo/HNTs confirms the presence of dye in the HNTs as spectrum of pure Fluo dye showed these characteristic peaks. The spectrum of pure Fluo dye is presented in the Fig. 23 c.

34

Figure 23. IR Spectra (a) Pure HNTs (b) HNTs impregnated with Fluorescein, (b*) Inset of the Fluo/HNTs spectra ranging from 1200 to 1800 cm-1 highlighting characteristic bands of pure fluorescein dye and (c) Pure Fluorescein Dye.

Interactions between fluorescein dye and HNTs were also observed. IR spectra of Rh6G/HNTs have shown no bands of hydroxyl group of HNTs at 3625 and 3699 cm-1 that indicates the presence of interactions between hydroxyl groups present inside HNTs lumen and Fluo.

4.6 Photoluminescence study of composites and dye solutions

One of the main objective of this study was to investigate effects of the Halloysite nanotubes on the photoluminescence (PL) of the organic chromophore. For this purpose HNTs were modified with the help of luminescent materials. Samples were prepared by altering the parameters such as amount of loading in HNTs, number of washing, different type of solvents, impregnation time, treatment of HNTs and their dispersion quality. The main objective was optimization of these parameters for the fabrication of highly luminescent polymer surfaces. Photoluminescence intensity of samples was increased by increasing number of washings and impregnation time. Impregnation time was prolonged by using solvents with high boiling point. However, agglomeration of HNTs in the polymer matrix has resulted in quenching of emission intensity.

35

Figure 24. Luminescence intensities of polymer coatings containing HNTs impregnated with solution of different molarities A) RhB/HNTs@PS, B) Rh6G/HNTs@PS, C) Fluo/HNTs@PS and D) Emission Wavelength for the all Dye and HNTs combination.

HNTs were loaded with dye by using solution of three different molarities, 0.05, 0.078 and 0.13 M. After the loading, these HNTs were used for the fabrication of polymer coatings and characterized for their PL intensity. In Fig. 24 the luminescence intensities of polymer coatings containing HNTs loaded with different amount of dye were plotted against the molarity of dye solution used for the impregnation. By using the HNTs impregnated with higher amount of dye in the fabrication of polymer coatings sharp decrease in the luminescence intensity was noted. This trend was noted in case of all dyes, RhB, Rh6G and Fluo which is shown in Fig. 24 A), B) and C) respectively. Highly luminescent samples were the ones which were fabricated by using lowest amount of dye content. These samples were selected for the further investigation.

36 4.6.1 Amount of Loading in HNTs

After selecting the highly luminescent samples, the amount of loading in HNTs was determined. In order to calculate loading of luminescent materials in HNTs, different variation of centrifugation parameters were tested. After extensive experimentation, washing of impregnated HNTs at 5300 rpm for 30 minutes was found effective. Impregnated HNTs were washed three times and supernatant was preserved in beakers of known weight. After evaporating the supernatant amount of washed dye material was calculated. This process was repeated three times and average value was used. The outline of the whole procedure is provided in the Fig. 25.

Figure 25. Outline of the washing procedure which was used for the calculating amount of loaded dyes in HNTs.

Supernatant was left for drying, which resulted in the evaporation of solvent. After evaporating the solvent amount of dye in supernatant was determined. Dye samples from supernatant were investigated by using scanning electron microscopy and energy dispersive spectroscopy to ensure the absence of HNTs. In Fig. 26 EDS image and spectra for the dye collected from supernatant are presented. In the EDS spectra, the peaks for carbon and oxygen originate from dye crystals (shown in Fig. 26 A) collected from the supernatant. However, no peaks for silicon and aluminum was observed which confirms dye collected from supernatant was free of HNTs.

37

Figure 26. A) SEM image of the RhB dye collected from supernatant showing the points which were studied by EDS, B) EDS spectra of Point 1 C) EDS spectra of Point 2 and D) EDS spectra of Point 3

In EDS spectra there were no peaks of Si and Al which implies that the washed dye was free of HNTs. In this manner washed dye was calculated subsequently providing the amount of dye loaded in the HNTs. The amount of dye used for the impregnation and the dye loaded in HNTs is provided in the Table 4. Amount of dye loaded in HNTs was very crucial for this investigation. By measuring the loading content of HNTs, control samples were prepared. These control samples were dye solutions containing equivalent amount of dye used for the fabrication of polymer coatings solution of respective dye.

Table 4. Listing the amount of dye used for impregnation and loaded dye in HNTs

Dye/HNTs Total amount of Dye used

for loading HNTs (mg)

Loaded Dye in

HNTs (mg)

% Amount of

Loaded Dye

RhB/HNTs 61 27 44.9

Rh6G/HNTs 60 22 37.4

Fluo/HNTs 43 17 40

38 4.6.2 PL spectra of composites and control samples

Comparison between PL spectra of control sample and polymer coatings solution was carried out in order to study the effect of HNTs on the PL intensity of dyes. For this purpose PL spectra of polymer coating solution samples were recorded and compared with their control samples. By determining the loading content of HNTs, control samples were prepared. These control samples were dye solutions which contained equivalent amount of dye used in the fabrication of polymer coatings solution of corresponding dye.

Polymer coating solution of RhB was prepared by dissolving PS in toluene and then adding RhB impregnated HNTs to the solution. This sample will be referred to as RhB/HNTs/PS/Tol. PL spectra for this sample was recorded and compared with the control sample which contained RhB dissolved in chloroform (RhB/Chloroform). Both control and RhB/HNTs/PS/Tol samples were composed of equivalent amount of RhB dye. Abbreviations for the polymer coating solution of RhB, Rh6G, Fluo and their respective control samples are provided in the Table 5.

Table 5. Abbreviated names for the samples of Polymer coating solutions and their respective control samples.

Polymer Coating Solutions Control Samples

RhB/HNTs/PS/Tol RhB/Chloroform

Rh6G/HNTs/PS/DMF Rh6G/ Chloroform

Fluo/HNTs/PS/DMF Fluo/DMF

PL spectra showing excitation and emission peaks of the RhB/HNTs/PS/Tol and RhB/Chloroform are given in Fig. 27 A) and B) respectively. Sample of polymer coating solution RhB/HNTs/PS/Tol was showing luminescence at the 604 nm while the control sample RhB/Chloroform was showing excitation peak at 470 nm and emission peak at 590 nm. Emission peak of RhB/HNTs/PS/Tol was shifted to the longer wavelength by 14 nm with respect to the control sample. This red shift in the emission wavelength can be attributed to the presence of matrix and HNTs (which were present) in the RhB/HNTs/PS/Tol sample. The overlaid emission spectra of RhB/HNTs/PS/Tol and RhB/Tol is presented in the Fig. 27 C. It is evident from the spectra that RhB/Tol was highly fluorescent. RhB/HNTs/PS/Tol have shown 7 times lesser luminescence intensity than control sample.

39

Figure 27. PL Spectra A) RhB/HNTs/PS/Tol showing excitation and emission peaks, B) Excitation and emission spectra of RhB/Chloroform used as control sample and C) Overlaid emission spectra of RhB/HNTs/PS/Tol and RhB/chloroform representing difference in emission Intensity.

Polymer coating solution Rh6G/HNTs/PS/DMF and its control sample Rh6G/DMF were used to retrieve PL spectra. Excitation and emission spectra for these samples are given in the Fig. 28 A) and B). Rh6G/HNTs/PS/DMF showed excitation peak at 430 nm and emission peak at 575 nm while Rh6G/Chloroform have shown excitation and emission peak at 558 and 564 nm, respectively. Emission peak of the Rh6G/HNTs/PS/DMF redshifted by the 7 nm with respect to Rh6G/Chloroform.

In Fig. 28 C), overlaid emission spectra of both Rh6G/HNTs/PS/DMF and Rh6G/DMF is provided. The emission intensity of the polymer coating sample which were having Rh6G dye impregnated in the HNTs increased by the factor of 1.5 when compared with the control sample which contained equivalent amount of Rh6G dye in DMF. HNTs not only increased the emission intensity of Rh6G dye but it is also expected to provide stability and longer life time to chromophore due to the inorganic environment.

40

Figure 28. Photoluminescence Spectra of A) Rh6G/HNTs/PS/DMF B) Control sample Rh6G/Chloroform and C) Overlaid spectra of both Rh6G/HNTs/PS/DMF and reference sample Rh6G/Chloroform.

PL spectra of polymer coatings solution composed of fluorescein impregnated HNTs, Fluo/HNTs/PS/DMF (Fig. 29 A) was obtained and compared with its control sample Fluo/DMF (Fig. 29 B). Emission and excitation peaks of Fluo/HNTs/PS/DMF were noted at the 406 and 550 nm, respectively. Increase in the emission intensity by the factor of 8 was observed for Fluo/HNTs/PS/DMF when compared with its control sample. The overlaid spectra of Fluo/HNTs/PS/DMF and Fluo/DMF is given in Fig. 29 C).

HNTs provided the inorganic framework to the organic chromophore and it also enhanced the emission intensity of the Rh6G and Fluo significantly. In case of RhB the emission intensity was quite low when compared with its control sample. To determine the exact phenomenon of these changes further experimentation is needed. But it can speculated that HNTs are reducing the process of quenching by confining the organic chromophore into their nanochannels.

41

Figure 29. PL spectra for A) Polymer coating solution Fluo/HNTs/PS/DMF, B) Control sample Fluo/DMF and C) Overlaid emission spectra of Fluo/HNTs/PS/DMF and its reference sample Fluo/DMF.

Summary of photoluminescence analysis by providing excitation and emission peaks for the polymer coating solution and its control samples is given in Table 6. In case of Fluo/DMF excitation peak was quite broad. Further experimentation is required in order to ascertain the excitation peaks of this sample.

Table 6. Excitation and Emission peaks for the polymer coating and their respective reference samples.

Samples Excitation Peaks (nm) Emission Peaks (nm)

RhB/HNTs/PS/Tol 454 604

RhB/Chloroform 470 590

Rh6G/HNTs/PS/DMF 430 575

Rh6G/ Chloroform 558 568

Fluo/HNTs/PS/DMF 406 556

Fluo/DMF -- 550

42 5. Conclusions

Highly luminescent polymer coatings based on HNTs and organic dyes were fabricated and then optimized to improve their optical properties. A working protocol for the fabrication of these coatings was established and tested. These coatings were characterized for the dispersion of HNTs in polymer matrix, absorption of light, surface morphology and for the interaction between the HNTs and dye materials. Special emphasis was on the investigation of effects of HNTs on the photoluminescence intensity of dye materials. PL intensities of HNTs impregnated with RhB, Rh6G, Fluo were compared with PL intensities of the corresponding dye solution. Enhancement of the PL intensity of Rh6G and Fluo by utilizing HNTs is reported in this work. By incorporating HNTs in these luminescent polymer coatings, lifetime and mechanical strength is expected to increase.

This investigation provides basis for the development of highly luminescent spray coatings based on nano-composite materials. It also demonstrates enhancement in luminescence ability of dye materials by incorporating halloysite nanotubes. However, the exact mechanism of luminescence enhancement process is not clear. Further experimentation can be done in order to unravel this process. This work successfully answer the question related to the effects of HNTs on the luminescence of dye materials. In addition to that, it also presents new challenges like fabrication of coatings which can emit white light and demonstrate high mechanical strength, elongated lifetime and photostability.

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