Synthesis of ZnO, CuO and their Composite Nanostructures ...liu.diva-portal.org/smash/get/diva2:550644/FULLTEXT01.pdf · Linköping Studies in Science and Technology Dissertation,

Linköping Studies in Science and Technology

Dissertation, No. 1467

Synthesis of ZnO, CuO and their Composite Nanostructures

for Optoelectronics, Sensing and Catalytic Applications

Saima Zaman

Physical Electronics and Nanotechnology

Department of Science and Technology

Linköping University, SE-601 74 Norrköping Sweden

Synthesis of ZnO, CuO and their Composite Nanostructures

for Optoelectronics, Sensing and Catalytic Applications

Saima Zaman

ISBN: 978-91-7519-818-7

ISSN: 0345-7524

Copyright©, 2012, Saima Zaman

[email protected]

Linköping University

Department of Science and Technology

SE- 60174 Norrköping

Sweden

Printed by LiU-Tryck, Linköping 2012

Dedicated to:

My Father and Mother

Abstract

i

Abstract

Research on nanomaterials has become increasingly popular because of their

unique physical, chemical, optical and catalytic properties compared to their bulk

counterparts. Therefore, many efforts have been made to synthesize

multidimensional nanostructures for new and efficient nanodevices. Among those

materials, zinc oxide (ZnO), has gained substantial attention owing to many

outstanding properties. ZnO besides its wide bandgap of 3.34 eV exhibits a

relatively large exciton binding energy (60 meV) at room temperature which is

attractive for optoelectronic applications. Likewise, cupric oxide (CuO), having a

narrow band gap of 1.2 eV and a variety of chemo-physical properties that are

attractive in many fields. Moreover, composite nanostructures of these two oxides

(CuO/ZnO) may pave the way for various new applications.

This thesis can be divided into three parts concerning the synthesis,

characterization and applications of ZnO, CuO and their composite nanostructures.

In the first part the synthesis, characterization and the fabrication of ZnO

nanorods based hybrid light emitting diodes (LEDs) are discussed. The low

temperature chemical growth method was used to synthesize ZnO nanorods on

different substrates, specifically on flexible non-crystalline substrates. Hybrid LEDs

based on ZnO nanorods combined with p-type polymers were fabricated at low

temperature to examine the advantage of both materials. A single and blended light

emissive polymers layer was studied for controlling the quality of the emitted white

light.

The second part deals with the synthesis of CuO nanostructures (NSs) which

were then used to fabricate pH sensors and exploit these NSs as a catalyst for

degradation of organic dyes. The fabricated pH sensor exhibited a linear response

and good potential stability. Furthermore, the catalytic properties of petals and

Abstract

ii

flowers like CuO NSs in the degradation of organic dyes were studied. The results

showed that the catalytic reactivity of the CuO is strongly depending on its shape.

In the third part, an attempt to combine the advantages of both ZnO and CuO

NSs was performed by developing a two-step chemical growth method to

synthesize the composite NSs. The synthesized CuO/ZnO composite NSs revealed

an extended light absorption and enhanced defect related visible emission.

Keywords: Zinc Oxide, Copper (II) oxide, Nanostructures, Low temperature

Growth, Light emitting diodes, pH sensor

Acknowledgement

iii

Acknowledgement

This thesis is the end of my journey in obtaining my Ph.D degree. I have not traveled

alone in this journey. This thesis has been kept on track and been seen through to

completion with the support and encouragement of numerous people. At the end I

would like to thank all those people who made this thesis possible and an

unforgettable experience to me. Truly, words are not enough to express my

gratitude to all of them.

First and foremost, I would like to express my overwhelming gratitude to my

supervisor, Prof. Magnus Willander, for giving me the chance to work in his

research group. Thanks for giving me freedom for developing my research ideas, for

showing me different ways to approach a research problem and the need to be

persistent to accomplish any goal and for always being there whenever I need your

help.

I render to pay special thanks to my co-supervisor, associate Prof. Omer Nour for

his keen interest, encouragement and for his support throughout this work.

I owe my sincere thanks to all my co-authors of the included papers, especially

Ahmed Zainelabdin for his extensive discussions and valuable support during this

work.

I would like to thank our research administrator Ann-Christin Norén for her kind

administrative help and cooperation.

I greatly acknowledge the facilities and financial support provided by the ITN/

Linköping University and MUST University AJK Pakistan throughout my Ph.D.

studies.

Acknowledgement

iv

I offer my sincere wishes and thanks to all present and previous members of the

Physical Electronics and Nanotechnology group for their cooperation and moral

support.

My deepest debt of gratitude is to my entire family for their love and support. I

must appreciate my brothers, my sisters and all my in-laws family for their guidance

and encouragement. Most importantly, my father and mother, it was your dream

which made my journey up to this possible. All I can say is, this small space is not

enough to acknowledge your contributions up to this stage of my life. May Allah

bless you all!!!

At the last but not the least, most special thanks I accord to my husband, Faisal for

his unwavering understanding and support during all these years of my Ph.D. He

was always there cheering me up and stood by me through the good and bad times.

Without you nothing of this would have been possible. Thanks for being with me. I

appreciate my beloved kids Musa and Zoha, who have provided me all the joy of

life with their innocent acts and refreshing me with lovely smiles.

List of Publications

v


Papers included in this thesis

I. Deposition of well-aligned ZnO nanorods at 50oC on metal,

semiconducting polymer, and copper oxides substrates and their structural

and optical Properties

A. Zainelabdin, S. Zaman, G. Amin, O. Nur, and M. Willander.

Crystal Growth & Design, 10, 3250 (2010).

II. ZnO nanorods–polymer hybrid white light emitting diode grown on a

disposable paper substrate

G. Amin, S. Zaman, A. Zainelabdin, O. Nur, and M. Willander

Physics Status Solidi RRL 5, 71 (2011).

III. Influence of the polymer concentration on the electroluminescence of ZnO

nanorods/polymer hybrid light emitting diode

S. Zaman, A. Zainelabdin, G. Amin, O. Nur, and M. Willander

Accepted in Journal of Applied Physics (2012).

IV. Effect of the polymer emission on the electroluminescence characteristics

of ZnO nanorods/p-polymer hybrid light emitting diode


Applied Physics A, 104, 1203 (2011).

V. CuO nanoflowers as an electrochemical pH sensor and the effect of the pH

on the growth

S. Zaman, M. H. Asif, A. Zainelabadin, G. Amin, O. Nur, and M. Willander

Journal of Electroanalytical Chemistry 662, 421 (2011).

VI. Efficient catalytic effect of two-dimensional petals and three-dimensional

flowers like CuO nanostructures on the degradation of organic dyes


vi

S. Zaman, A. Zainelabadin, G. Amin, O. Nur and M. Willander

Journal of Physics and Chemistry of Solids 73, 1320 (2012).

VII. Synthesis and characterization of CuO/ZnO composite nanostructures:

precursor’s effects, and their optical properties

A. Zainelabdin, S. Zaman, S. Hussain, O. Nur, and M. Willander

Submitted (2012).

Related papers not included in this thesis

1. Different interfaces to crystalline ZnO nanorods and their applications

M. Willander, M. H. Asif, S. Zaman, A. Zainelabdin, N. Bano, S. M. Al-Hilli,

and O. Nur

Phys. Status Solidi C 6, 2683 (2009).

2. Low-temperature chemical growth of ZnO nanorods with enhanced UV

emission on plastic substrates


Journal of Nanoelectronics & Optoelectronics, 5, 1, (2010).

3. Stable white light electroluminescence from highly flexible polymer/ZnO

nanorods hybrid heterojunction grown at 50oC

A. Zainelabdin, S. Zaman, G. Amin, O. Nur, and M. Willander

Nanoscale Research Letter, 5, 1442 (2010).

4. ZnO-organic hybrid white light emitting diodes grown on flexible plastic

using low temperature aqueous chemical method

N. Bano, S. Zaman, A. Zainelabdin, S. Hussain, I. Hussain, O. Nur, and M.

Willander

Journal of Applied Physics 108, 043103 (2010).


vii

5. Current-transport studies and trap extraction of hydrothermally grown

ZnO nanotubes using gold Schottky diode

G. Amin, I. Hussain, S. Zaman, N. Bano, O. Nur and Magnus Willander

Physics Status Solidi A, 207, 748 (2010).

6. Luminescence from zinc oxide nanostructures and polymers and their

Hybrid Devices

M. Willander, O. Nur, J. R. Sadaf, M. Q. Israr, S. Zaman, A. Zainelabdin, N.

Bano and I. Hussain

Materials 3, 2643 (2010)

7. Influence of pH, precursor concentration, growth time, and temperature on

the morphology of ZnO nanostructures grown by the hydrothermal method

G. Amin, M. H. Asif, A. Zainelabdin, S. Zaman, O. Nur, and M. Willander

Journal of Nanomaterials doi:10.1155/2011/269692 (2011)

8. Zinc Oxide nanorods/polymer hybrid heterojunctions for white light

emitting diodes

M. Willander, O. Nur, S. Zaman, A. Zainelabdin, N. Bano, and I. Hussain

J. Phys. D: Appl. Phys. 44, 224017 (2011).

9. Intrinsic white light emission from zinc oxide nanorods heterojunctions on

large area substrates

M. Willander, O. Nur , S. Zaman, A. Zainelabdin, G. Amin, J. R. Sadaf, M. Q.

Israr, N. Bano, I. Hussain, and N. H. Alvi

Proceedings of SPIE 7940, 79400A (2011)

10. Zinc Oxide and copper oxide nanostructures: fundamentals and

applications

Magnus Willander, Omer Nur, Gul Amin, A. Zainelabdin and S. Zaman

MRS Fall Meeting 2011.


viii

11. Scale-up synthesis of ZnO nanorods for printing inexpensive ZnO/Polymer

white light emitting diode

G. Amin, M. O. Sandberg, A. Zainelabdin, S. Zaman, O. Nur, and M.

Willander

Journal of Materials Science 47, 4726 (2012).

12. Optical and current transport properties of CuO/ZnO nanocorals p-n

heterostructure hydrothermally synthesized at low temperatures.

A. Zainelabdin, S. Zaman, G. Amin, O. Nur, and M. Willander

Applied Physics A, 108, 921 (2012)

13. CuO/ZnO Nanocorals synthesis via hydrothermal technique: growth

mechanism and their application as Humidity Sensor

A. Zainelabdin, G. Amin, S. Zaman, O. Nur, J. Lu, L. Hultman, and M.

Willander

Journal of Material Chemistry 22, 11583 (2012)

14. CuO nanopetals based electrochemical sensor for selective Ag+

measurements

G. Amin, M. H. Asif, A. Zainelabdin, S. Zaman, O. Nur, and M. Willander

Sensor Letters 10, 1 (2012).

15. Recent progress on growth and device development of ZnO and CuO

nanostructures and graphene nanosheets

M. Willander, K. ul Hassan, O. Nur, A. Zainelabdin, S. Zaman and G. Amin

Journal of Materials Chemistry 22, 2337 (2012)

16. Metal Oxide nanostructures and white light emission

A. Zainelabdin, O. Nur, G. Amin, S. Zaman, and M. Willander

Proceedings of SPIE 8263, 82630N (2012)


ix

17. CuO Interpenetrated leaves architecture originated from (101) Twin

Structure

J. Lu, A. Zainelabdin, S. Zaman, G. Amin, O. Nur, M. Willander and L.

Hultman

Submitted

Co

Contribution to the papers included in the thesis

Contribution to the paper I, II & VII

Part of the experimental work and involved in the editing of the manuscripts.

Contribution to paper III, IV, V, VI

Design and performance of the experiments except the TEM and PL measurements,

data analysis and wrote the first version of the manuscript.

Table of Contents

xi

Table of Contents

Abstract ......................................................................................................................................... i

Acknowledgement ................................................................................................................... iii

List of Publications ....................................................................................................................v

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

1.1 Background ........................................................................................................................ 1

1.2 Objective and outline of this thesis ............................................................................... 3

Materials and properties........................................................................................................... 5

2.1 Zinc Oxide .......................................................................................................................... 5

2.2 Copper (II) Oxide .............................................................................................................. 8

2.3 Polymers ............................................................................................................................. 9

2.3.1 Poly(9,9-dioctylfluorene) (PFO) ............................................................................ 10

2.3.2 MEH-PPV .................................................................................................................. 12

Synthesis and characterization of nanomaterials ............................................................ 13

3.1 Synthesis of ZnO ............................................................................................................. 13

3.1.1 Chemical bath deposition growth of ZnO NRs................................................. 14

3.1.2 Characterization of ZnO NRs ................................................................................ 15

3.2 Synthesis of CuO nanostructures ................................................................................ 17

3.2.1. Characterization of CuO nanostructures ........................................................... 18

3.3 Synthesis of CuO/ZnO composite nanostructure ................................................... 21

3.3.1 Characterization of CuO/ZnO composite nanostructure............................... 22

Fabrication and characterization of devices ...................................................................... 27

Table of Contents

xii

4.1 ZnO NRs/polymers hybrid LEDs .............................................................................. 27

4.1.1 Characterization of hybrid LEDs on disposable paper substrates ............... 31

4.1.2 Influence of the polymer concentration on hybrid LEDs ............................... 32

4.1.3 Effect of the polymer emission on the EL of hybrid LEDs ............................. 35

4.2. CuO based electrochemical pH sensor ..................................................................... 39

4.2.1. Characterization of CuO pH sensors ................................................................. 41

4.3. CuO nanostructures as a catalyst ............................................................................... 43

4.3.1 Catalytic performance of CuO nanostructures ................................................. 44

Conclusion and future work ................................................................................................. 47

References .................................................................................................................................. 49

Introduction

1

Chapter 1

Introduction

The little word "nano" with huge potential has been rapidly indicating itself into the world's

map and has an enormous influence in every aspect of science and engineering fields. The

idea of nanotechnology was introduced by Richard Feynman in his talk “There is a plenty of

room at the bottom”, in 1959. Though he never explicitly mentioned "nanotechnology,"

Feynman suggested that it will eventually be possible to precisely manipulate atoms and

molecules. In general, nanotechnology consists of materials with nanoscale dimensions,

remarkable properties, and great potential.

1.1 Background During the last few decades, nanomaterials have been the subject of extensive

interest because of their potential use in a wide range of fields like, optoelectronics,

catalysis and sensing applications etc. The physical and chemical properties of

nanomaterials can differ significantly from their bulk counterpart because of their

small size. In general, nanomaterials comprised novel properties that are typically

not observed in their conventional, bulk counterparts. Nanomaterials have a much

larger surface area to volume ratio than their bulk counterparts, which is one of the

basis of their novel physical and/or chemical properties. Nanomaterials are

classified into one-dimensional (1D), two-dimensional (2D) and three-dimensional

(3D). At present, research on nanomaterials is intensified and is expanding rapidly.

In addition, metal oxide nanomaterials have drawn a particular attention because of

their excellent structural flexibility combined with other attractive properties. These

metal oxides nanostructures not only inherit the fascinating properties from their

bulk form such as piezoelectricity, chemical sensing, and photodetection, but also

possess unique properties associated with their highly anisotropic geometry and

Introduction

2

size confinement [1]. The combinations of the new and the conventional properties

with the unique effects of nanostructures make the investigation of novel metal

oxide nanostructures a very important issue in research and development both from

fundamental and industrial standpoints.

Among the various metal oxides, zinc oxide (ZnO) possessed a considerable

attention due to its unique properties and applications. In particular, ZnO

nanostructures (NSs) are of intense interest since they can be grown by a variety of

methods with different morphologies. Among the different growth methods, the

chemical bath deposition method is low temperature, simple, inexpensive and

environment friendly method. These are all factors which further contribute to the

resurgent attention in ZnO. Specifically, one-dimensional ZnO nanorods (NRs)

amongst other nanostructures are attractive components for manufacturing

nanoscale electronics and photonic devices as well as their biomedical applications

because of their interesting chemical and physical properties [2, 3]. Also ZnO NRs

can easily be grown on a variety of substrates like metal surface, semiconductors,

glass, plastic and disposable paper substrates etc. [4-7]. Furthermore, a direct wide

band gap ~ 3.37 eV and relatively large excitonic binding energy ~ 60 meV of ZnO

along with many radiative deep level defects, makes ZnO attractive for its emission

tendency in blue/ultraviolet and full colour lighting [8, 9]. To utilize theses

properties of ZnO in LEDs application, another p-type material is necessary as ZnO

NRs is unintentionally n-type material. Since mostly polymers are p-type and their

special properties like low cost, low power consumption, flexible and easy

manufacturing all makes polymers a better choice to use with ZnO NRs to fabricate

a flexible device that utilizes the properties of both materials for large area lighting

and display application [10, 11].

On the other hand, natural abundance of copper (II) oxide (CuO) as well as its

low production cost, good electrochemical and catalytic properties makes the copper

Introduction

3

oxide to be one of the best materials for various applications. CuO also has a variety

of nanostructures and can be grown using the low temperature aqueous chemical

method. It is one of the most important catalysts and is widely used in

environmental catalyst.

1.2 Objective and outline of this thesis

The objective of this thesis is to synthesize metal oxide semiconductor

nanostructures and utilize them for light emitting diodes, catalytic and sensing

applications. For the ease of gathering all presented work, the thesis is divided into

parts:

i) Synthesis of ZnO and CuO nanostructures. The morphology, crystal structure

and crystallinity of the nanostructures were monitored by using scanning electron

microscope (SEM), transmission electron microscope (TEM) and x-ray diffraction.

ii) Fabrication of a solution-processable hybrid ZnO NRs/polymer light emitting

diodes (LEDs) on flexible substrates. N-type ZnO NRs grown along the c-axis direction

act as natural waveguide cavities for making the emitted light to travel to the top of

the devices. Along with the properties of the ZnO NRs, choice of the polymer is also

crucial for tuning the emission of fabricated LEDs. The emission colour of the hybrid

LEDs can be tuned by blending different polymers or by changing the polymer

concentrations.

iii) Utilization of the CuO nanostructures to develop a pH sensor and exploit these

NSs as a catalyst to degrade organic dyes. Owing to its good electrochemical activity,

CuO is a promising candidate for sensing applications. Thus, CuO based pH sensor

was developed to check the sensitivity and response over a wide pH range.

Moreover, CuO is known to be a good catalyst and morphology of the CuO affects

the properties of the catalyst in general. Therefore CuO NSs having different

morphology were investigated to boost the degradation of the organic dyes.

Introduction

4

iv) Finally, extend the growth of ZnO and CuO to achieve their composite CuO/ZnO

nanostructure. The growth kinetics of composite NSs was studied and found that it

depends on the nature and the pH value of the nutrient solution. The CuO/ZnO

nanocomposite exhibited a broad and extended light absorption covering the whole

visible range.

The outline of this thesis is as follow: the general introduction and the

objective of the thesis are presented in this chapter. Some basic properties of ZnO,

CuO and polymers are studied in the following chapter. The synthesis of CuO, ZnO

and their composite nanostructures together with characterization is the subject of

chapter 3. Their applications in hybrid ZnO NRs/polymer LED, pH sensor based on

CuO and catalytic effect of the CuO NSs were demonstrated in the chapter 4. The

final chapter contains the conclusion of whole work and possible future prospects.

Materials and properties

5

Chapter 2


ZnO is a wide bandgap material possessing many interesting properties and probably the

richest family of nanostructures. Moreover, CuO is a narrow bandgap material and has been

studied extensively. In this chapter we aim to narrate some properties of ZnO and CuO in a

comprehensive manner, as well as discuss the polymers which are used in this work.

2.1 Zinc Oxide

Zinc oxide (ZnO) is a metal oxide semiconductor with wurtzite structure

under ambient condition. The wurtzite structure has hexagonal unit cell as shown in

figure 2.1. In this crystal structure, two interpenetrating hexagonal-close-pack (hcp)

sublattices are alternatively stacks along the c-axis. One sublattice consists of four Zn

atoms and the other sublattice consists of four Oxygen O atoms in one unit cell;

every atom of one kind is surrounded by four atoms of the other kind and forms a

tetrahedron structure [12].

Figure 2.1. The hexagonal wurtzite structure of ZnO [reproduced from wikipedia].


6

ZnO commonly consists of polar (0001) and non-polar (10-10), (11-20) surfaces. The

surface energy of the polar surface is higher than the non-polar surfaces and

therefore the preferential growth direction of ZnO NR is along the <0001> [2].

Figure 2.2 shows the schematic diagram of a ZnO NR growing along the <0001>

direction or along the c-axis.

Figure2.2. Schematic diagram of ZnO NR showing the growth direction.

The enormous interest of using ZnO in optoelectronic devices is due to its excellent

optical properties. The direct wide band gap of ZnO ~ 3.4eV is suitable for short

wavelength optoelectronic applications, while the high exciton binding energy ~ 60

meV allows efficient excitonic emission at room temperature [13]. Moreover ZnO, in

addition to the ultraviolet (UV) emission, emits covering the whole visible region i.e.

containing green, yellow and red emission peaks [14-16]. The emission in the visible

region is associated with deep level defects. Generally oxygen vacancies (Vo), zinc

vacancies (VZn), zinc interstitials (Zni), and the incorporation of hydroxyl (OH)

groups in the crystal lattice during the growth of ZnO are most common sources of

the defects related emission [17-19]. ZnO naturally exhibits n-type semiconductor

polarity due to native defects such as oxygen vacancies and zinc interstitials. P-type

doping of ZnO is still a challenging problem that is hindering the possibility of a p-n

homojunction ZnO devices. Furthermore, the remarkable properties of ZnO like

being bio-safe, bio-compatible, having high-electron transfer rates and enhanced

Growth

direction

(0001)

(ퟏퟎퟏퟎ)


7

analytical performance are suitable for intra/extra-cellular sensing applications [20,

21]. Some basic physical parameters of ZnO at the room temperature are presented

in the table 2.1.

Table 2.1. Some basic properties of wurtzite ZnO.

Property

Value

Reference

Lattice Parameters

a= b=3.25 Å

c=5.21 Å

[12, 22]

Stable crystal structure wurtzite [12]

Density 5.606 gm/cm3 [23]

Melting point 1975 oC

Dielectric constant 8.66 [24]

Refractive index 2.008 [12]

Energy gap 3.4 eV, direct [12, 25]

Exciton binding energy 60 meV [26]

Effective mass

Electron/Hole

0.24 mo/0.59 mo

[25]

Electron mobility 100-200 cm2/Vs [23]

Hole mobility 5-50 cm2/Vs

Bulk young modulus 111.2±4.7 GPa [27]


8

2.2 Copper (II) Oxide

Copper (II) oxide (CuO) is another metal oxide semiconductor having narrow

bandgap ~ 1.2 eV in bulk. CuO has monoclinic crystal structure as shown in figure

2.3, and belongs to the space group 2/m. The copper atom is coordinated by four

oxygen atom in a square planer configuration [28]. It is intrinsically p-type

semiconductor. CuO draw much attention since the starting growth material is

inexpensive and easy to get, and the methods to prepare these materials are of low

cost [29].

Figure 2.3. The monoclinic crystal structure of CuO [reproduced from wikipedia].

CuO nanostructures (NSs) have stimulated intensive research due to their high

surface area to volume ratio. CuO NSs are a good candidate for sensing owing to its

exceptional electrochemical activity and the possibility of promoting electron

transfer at a low potentials [30]. Due to the photoconductive and photochemical

properties, CuO NSs are also promising materials for the fabrication of solar cells

[31, 32]. CuO based materials are well known with regard to their high temperature


9

superconductivity and the relatively huge magnetoresistance [33, 34]. Additionally,

this compound is well-known for its excellent performance as a sensing material for

hazardous gas detection and as negative electrode in lithium ions batteries [35-37].

CuO is very important from the standpoint of the catalytic usage and the

morphology affects the properties of a catalyst in general [38]. It is therefore

significant to synthesize novel sizes and shapes of the CuO NSs and to further

improve its application as a catalyst. Some of the physical features of CuO are

summarized in table 2.2.

Table 2.2. Some basic properties of CuO at room temperature.

2.3 Polymers Polymers are material that consists of repeated molecular units which are

linked together by covalent bonds. The origin of the world polymer comes from the

Property

Value

Reference

Lattice parameters

a=4.68 Å

b=3.42 Å

c=5.13 Å

[39]

Crystal structure Monoclinic [40]

Density 6.32 g/cm3 [41]

Melting point 1134 oC [41]

Dielectric constant 18.1 [41]

Refractive index 1.4 [41]

Energy gap 1.2 eV, direct [42]

Hole mobility 0.1-10 cm2/Vs [41]


10

Greek language where poly means ´many´ and mer means ´part´. Conjugated

polymers are novel materials that combine the optoelectronic properties of

semiconductors with the mechanical properties and processing advantage of the

plastics. The conjugated polymers are organic macromolecules which consist at least

of one backbone chain of alternating single and double bonds [43]. It is the π

electrons, which mainly determine the electronic and optical properties of the

molecule. In ground state, the π-electrons form the π-band and the highest energy π-

electron level is known as the highest occupied molecular orbital (HOMO). In

excited state, the π-electrons form the π*-band and the lowest energy π*-electron

level is known as the lowest unoccupied molecular orbital (LUMO). The HOMO

resembles the valence band and the LUMO resembles the conduction band in the

inorganic semiconductor concepts [44].

In this work two light emitting polymers were chosen for the

fabrication of the hybrid LEDs.

2.3.1 Poly(9,9-dioctylfluorene) (PFO)

Poly(9,9-dioctylfluorene) (PFO) is the most promising candidate as a blue-

light-emitting polymer because of its highly efficient photoluminescence, good

thermal, chemical stability at elevated temperatures and solubility in organic

solvents. The HOMO and LUMO energy levels of the PFO are in the range of 5.6 eV

– 5.8 eV and 2.12 eV – 2.6 eV, respectively giving ~ 3.2 eV energy bandgap,

respectively [45]. The chemical structure of the PFO is shown in figure 2.4 [46].

Moreover, the polymorphism of the PFO is useful to study the influence of the phase

morphology on the photophysics of it without the need for chemical modification. In

addition to the normal glassy α-phase, a second phase (i.e., the so-called β-phase),

which is formed as a result of the n-alkyl crystallization of the side chains, has been

identified [47, 48]. The polymer chains are considered to be more planar, and with

longer conjugation length in the β-phase than in the α-phase [49]. The typical

absorption spectrum of the α -phase PFO has a strong featureless peak at about 380


11

nm whereas the β-phase has demonstrated a narrow, well-resolved absorption peak

at 437 nm [50]. These α and β-phases have a number of interesting photophysical

properties and have been investigated as systems well suited to the detailed study of

energy transfer processes [51-53]

Figure 2.4. The chemical structure of the PFO.

Aside from the morphology-induced changes (i.e. β-phase formation), the

observation of the distinct photophysical changes due to photo- and/or electro

degradation processes is of significant importance for device fabrication. The

polyfluorene based devices suffer from a degradation of the device under operation,

which is most visible in the formation of a low energy emission band (green

emission) [54]. This degradation behavior turns the blue emission color of PFO into

blue-green emission. The source of this green emission band is a matter of

controversy. This low energy emission band is associated with the fluorenone

defects (or ketone defects) in the form of fluorenone moieties in the polymer

backbone [55, 56]. Such fluorenone defects, leading to the low energy emission band

in polyfluorene, and can be formed during synthesis, or as a result of a photo-(or

electro)-oxidation degradation processes [57].


12

2.3.2 MEH-PPV Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) is

one of the most widely investigated light-emitting polymers within the class of

PPVs. The MEH-PPV has a small energy bandgap of about 2.1 eV as estimated from

the HUMO-LUMO energy difference [58, 59]. Therefore, it is well-known that this

polymer shows orange-red emission when it is processed into electroluminescence

devices. It can easily be dissolved in many organic solvents which allow the MEH-

PPV to be easily processed for thin films [60]. The chemical structure of the MEH-

PPV is shown in figure 2.5 [61]

Figure 2.5. The chemical structure of MEH-PPV.

Synthesis and characterization of nanomaterials

13

Chapter 3


The growth procedure of the ZnO and CuO nanostructures are described in this chapter. The

low temperature chemical bath deposition method was chosen for the growth of ZnO, CuO

and their composite nanostructures. After the growth, diverse characterization techniques

were used to probe the morphology and structural aspects of the as grown nanomaterials. The

techniques used during this work are: scanning electron microscope (SEM), transmission

electron microscope (TEM) and x-ray diffraction analysis. The working principles of these

techniques are appended in [62-64].

3.1 Synthesis of ZnO

In recent years, with the increasing awareness of both environmental safety

and the need for optimal energy utilization, there is a case for the development of

nonhazardous materials. These materials should not only be compatible with human

life but also with other living forms or species. Moreover, processing methods such

as fabrication, manipulation, treatment, reuse, and recycling of waste materials

should be environmentally friendly. In this respect, the hydrothermal technique

occupies a unique place in modern science and technology.

The hydrothermal growth technique is promising for low cost and for scaling

up the synthesis of nanostructures. This technique is not only useful to grow single

material but can also be useful to synthesize nano-hybrid or nano-composite

materials [65, 66]. Moreover, the hydrothermal growth occurs at low temperature

and therefore has great pledge for nanostructures to be synthesized on various

flexible plastic and paper substrates. This low temperature growth also makes it

easier to integrate nanomaterials with organic optoelectronic material. The chemical

bath deposition (CBD) method is an example of the hydrothermal method which

was used for the synthesis of ZnO NRs and CuO NSs presented in this thesis.


14

3.1.1 Chemical bath deposition growth of ZnO NRs

The chemical bath deposition is a two step growth technique for ZnO NRs

growth. Figure 3.1 is the schematic illustration of the CBD growth of ZnO NRs. First

step is to cover the substrate with a seed layer of ZnO nanoparticles which serve as a

nucleation sites for the growth of the NRs. The seed layer is beneficial for the growth

of aligned NRs and it can also control the density of the grown NRs [67]. The seed

layer that is used in our work is prepared as follow: 5 mM of zinc acetate

(Zn(CH3COO)2) and 2 mM of KOH were dissolved in methanol separately and then

under continuous stirring both solutions were mixed.

Prior to the start of the growth of ZnO NRs substrate cleaning is a necessary

step. Acetone, isopropanol and de-ionized (DI) water were separately used under

ultrasonication bath to clean the substrates. Then spin coating of the seed layer on

the substrate (plastic, paper and metal coated glass) at a speed of 3000 rpm for 30

seconds. The thickness of the seed layer is controlled by adjusting the speed of the

spin coater which can control the density and alignment of the ZnO NRs.

The aqueous solution of zinc nitrate (Zn(NO3)2. 6H2O) and

hexamethylenetetramine (HMT) in equimolar amount of 0.1 M was prepared in a

reaction vessel. The pre-seeded substrate was then immersed in the precursor

solution and loaded in a laboratory oven at 50 oC for several hours (h). After the

growth process the samples were soaked in DI water to remove the residuals and

dried with N2 blow. The ZnO NRs density, morphology and aspect ratio can be

controlled by adjusting the reaction parameters like e.g. precursor concentration, pH

value, growth temperature and growth time [68].


15

Figure 3.1. Schematic diagram of the CBD growth of ZnO NRs.

3.1.2 Characterization of ZnO NRs

The morphology of the ZnO NRs was investigated by using SEM which is an

important technique to study the surface morphology at the nanoscale. The SEM

image of the ZnO NRs grown on various substrates e.g. plastic, metals and

semiconductors are demonstrated in figure 3.2. It is seen that ZnO NRs covered the

substrates uniformly and grow perpendicular to the substrates. ZnO NRs with well

identified hexagonal facet are obtained shown in the high magnification image (inset

of figure 3.2). This shows that the ZnO NRs can easily be grown on any amorphous

(PEDOT:PSS) and crystalline substrate morphology at low temperature ~ 50 oC.

Zinc precursor

solution

Holder

Substrates

Seeded substrate ZnO NRs


16

Figure 3.2. Top view field emission SEM micrographs of ZnO NRs grown at 50 oC on

various substrates. (a) On Ag coated flexible plastic foil, (b) on Cu coated flexible plastic foil,

(c) on CuO coated glass, (d) on Cu2O glass substrate, and (e) on PEDOT/PSS coated flexible

plastic foil. Insets are the corresponding high magnification SEM images [4].(f) the cross

sectional image of ZnO NRs.

X-Ray diffraction (XRD) has been performed for the identification of the

crystal structure and growth orientation of the nanostructures. X-rays diffraction

measurement has been performed with a diffractrometer (X´ pert PANalytical)

equipped with Cu KαI (λ=1.5406 nm). Figure 3.3 is the typical XRD pattern of the

ZnO NRs grown on silver (Ag) coated flexible plastic substrate. It shows a very

intense (002) diffraction peak at 34.4 θ (JCPDS 36.1451), indicating the highly

(f)


17

preferential growth direction of ZnO NRs along the c-axis. Some other peaks are

observed which belongs to the Ag and the substrate.

Figure 3.3. XRD pattern of ZnO NRs synthesized on silver (Ag)

coated flexible plastic substrate.

3.2 Synthesis of CuO nanostructures

CuO nanostructures (NSs) were synthesized by using the same CBD method.

Copper nitrate (Cu(NO3)2.H2O) and HMT were used as a precursor for the growth of

CuO NSs. For CuO nanoflowers (NFs) structure, 5 mM of copper nitrate and 1 mM

of HMT was dissolved in DI water. The substrate was immersed in the reaction

vessel which contains the precursor solution. The reaction vessel was loaded in the

laboratory oven at 90 oC for 4 to 5 h. After growth the substrate was dried in air and

used for morphology and structural analysis. Another petal like CuO NSs was

obtained by adding 1 mL of NaOH into the precursor solution and all other growth

conditions were kept the same.

In order to investigate the influence of the pH on the CuO NSs, the pH of the

solution is adjusted by the addition of HCL or HNO3 from pH 4 to 11. Figure 3.5

shows the SEM images of CuO nanostructures grown under different pH values.

30 40 50 60 70 80-100

0

100

200

300

400

500

600

700

800

220 (Ag)

103Sub

102

200 (Ag)

111 (Ag)

101

002

Inte

nsity

(a.u

.)

2 Theta (deg.)

ZnO peaks


18

3.2.1. Characterization of CuO nanostructures

The morphology investigation of the synthesized CuO NSs (flowers and

petals) has been carried out through SEM as shown in figure 3.4. The CuO NFs are

composed of several layers of leaf like structure. These leaves having wider bases

connected at the center part of the flower and diverging outside with very sharp tips

as seen in the inset of figure 3.4a. The average diameter of the NF is about 4-5 µm

while the diameter of the single leaf is 400-500 nm. Figure 3.4b reveals the SEM

image of the CuO nanopetals possess the full coverage on the substrate. A single

CuO nanopetal consists of several interpenetrating sheets like structure.

Figure 3.4. Typical SEM image of the (a) CuO nanoflowers and (b) CuO nanopetal.

The morphology of the nanostructures grown by chemical solution method strongly

depends on the pH of the solution. Therefore the influences of pH on the

morphology of CuO NSs were investigated. Figure 3.5 represents the SEM image of

the CuO NSs obtained under pH range 6.5 to 11. Flower like and petal like

nanostructure were obtained under inherent pH = 6.5 and pH = 7. By increasing the

pH to 8, 9 and 10 the structure remains the same (petals like) but the diameter was

(b) (a)


19

observed to shrink as shown in figure 3.5c-e. But at a pH=11 the tiny petals merge

together to form an aloe vera like structure with very sharp tips (figure 3.5f).

Figure 3.5. SEM of CuO nanostructures grown under different pH solutions at; (a) inherent

pH = 6.5, (b) pH = 7, (c) pH = 8, (d) pH = 9, (e) pH = 10, and (f) pH = 11 [69]


20

Transmission electron microscope (TEM) has become an important

characterization technique due to its high lateral spatial resolution and its capability

to provide both image and diffraction information from a single sample. Therefore,

TEM and high resolution transmission electron microscope (HRTEM) were used to

further analyze the structure of the as grown CuO NSs. TEM image of the CuO NFs

is in figure 3.6a confirming that this structure is composed of several layers of

leaves. The TEM image together with HRTEM and selected area electron diffraction

(SAED) pattern of the CuO petals are shown in figure 3.6b. The main growth

direction of the petal is along the [010] and some interpenetrating sheets like

structure are interlaced on the petals. As evident, the SAED spots are slightly

stretched as an indication that the CuO NSs are composed of nanocrystals that were

assembled via oriented attachment process [70]. The combined analysis of the

HRTEM and the SAED pattern reveals that the CuO petals are single crystalline.

Figure 3.6. TEM image of the CuO (a) flower and (b) petals like structure.

The inset of (b) is the HRTEM and SAED pattern of the CuO petals.

(b) (a)


21

The crystallinity of the as grown CuO NFs was further examined using XRD

as shown in figure 3.7. All the characterization peaks are assigned to the monoclinic

phase of the CuO according to the JCPDS (card no. 051-661). XRD pattern indicating

the high purity of the single phase of CuO and no other impurity peak was

observed.

Figure 3.7. X ray diffraction pattern of CuO NFs at room temperature.

3.3 Synthesis of CuO/ZnO composite nanostructure

The CuO/ZnO nanocomposite was obtained on glass substrate by using the

two-step synthetic strategy of the hydrothermal method. First, the ZnO NRs were

vertically grown on the substrate by using the method described in [4] and these

NRs were acting as a stem for the subsequent growth of the CuO NSs. For the

secondary growth of CuO NSs two procedures were used. Primarily, only copper

nitrate (CNH) was used as a precursor solution, whereas in the second procedure

HMT was used with the CNH. The freshly grown ZnO NRs substrate was immersed

30 40 50 60 70

Inte

nsity

(a.u

.)

2 Theta (deg.)

(110)

(11-1)-(002)

(111)-(200)

(20-2)


22

having face upward in the precursor solutions and heated for 4-5 h at 60 oC. Dense

hierarchical CuO NSs were selectively assembled on the ZnO NRs and producing a

corals-like CuO/ZnO nanocomposite.

3.3.1 Characterization of CuO/ZnO composite nanostructure

SEM analysis of the CuO/ZnO nanocomposite is shown in figure 3.8. When

the CNH was used as a precursor solution for 4 h, densely hierarchical CuO NSs

were selectively assembled on the ZnO NRs producing a coral like CuO/ZnO

nanocomposite. ZnO NRs act as a stem for this nanocorals structure. SEM image

with 40o tilted image of the grown coral like CuO/ZnO NSs are shown in figure

3.8a-b. Alternatively, when the HMT was added in the precursor solution, dense

coverage of the CuO NSs on the ZnO NRs were observed during short growth time

~ 1.5 h, and the CuO NSs have a larger size comparatively as shown in figure 3.8c-d.

The large dimensions of the CuO NSs grown by CNH + HMT solution can be

explained by the relatively rich OH- group that either existed in the solution and/or

generated by the ZnO NRs template. Whereas for those grown in CNH solution they

only tend to have smaller size as a result of the low OH- concentration.


23

Figure 3.8: a) SEM image of CuO/ZnO nanocorals grown using CNH precursor solution

for 4 h, b) 40o tilted view SEM images of CuO/ZnO nanocorals, c) CuO/ZnO

nanostructures grown for short time of 0.5 or 1.5 h using either CNH+HMT or CNH,

respectively, and d) CuO/ZnO NSs grown using CNH+HMT for 1.5 h at 60 oC.

The grown CuO/ZnO NSs were further investigated with HRTEM, and

SAED. Figure 3.9b and c show TEM images of CuO/ZnO NS grown for 0.5 h and 1.5

h by using CNH+HMT and CNH precursor solutions, respectively. The surface and

tip of the ZnO NRs were severely eroded in the CNH solution during the synthesis

step, as clearly seen in figure 3.9c. However, those grown with the CNH+HMT

1 µm

1 µm 100

100

a) b)

c) d)


24

solution demonstrated less etching effect on the ZnO NR as shown in the TEM of

figure 3.9b. This result is in good agreement with the influence of the weak acidic

conditions on the ZnO NRs. Moreover, CuO NSs grown using different precursor

solutions may greatly affect their shape and dimensions which is concluded by

comparing the TEM image of figure 3.9b-c. The SEAD and the HRTEM of CuO NS

grown on ZnO NR are presented in figure 3.9f-g, respectively. These results reveals

that CuO NS is preferentially grown along the (020) plane of the monoclinic

structure of CuO and highly crystalline in nature with straight and parallel lattice

fringes (d). Figures 3.9d-e represents the HRTEM and the SAED of ZnO NR taken

from the part indicated by the cycle. It is evident from figure 3.9d that ZnO NR was

grown along the [0001], and that the lattice spacing between two neighboring fringes

is 0.52 nm. The SEAD pattern shown in figure 3.9e is indexed to the [0001] direction

of the wurtzite-phase ZnO and it demonstrates the single-crystal nature of the ZnO

NR. No obvious epitaxial relationship was found (not shown here) between the CuO

NSs and the ZnO NRs which agrees well with what is previously reported for

ZnO/CuO hierarchical nanotrees array [66].

The crystal structure obtained was investigated by XRD. Figure 3.10 shows

the XRD pattern of CuO/ZnO composite nanostructure. Both ZnO and CuO peaks

appears in the pattern which confirms the presence of both materials. Two peaks at

2θ value of 35.5o and 38.7o belong to the monoclinic CuO while all other peaks are

related to the ZnO wurtzite structure [71].


25

Figure 3.9: TEM images of a) CuO/ZnO NSs grown for 0.5 h using the CNH+HMT

precursors, and b) CuO/ZnO NSs grown for 1.5 h using the CNH solution, c) the HRTEM

of ZnO NR taken from the indicated cycle, d) the SAED pattern taken from the indicated

area of ZnO NR, e) the SAED pattern of CuO nanoleaf taken from the white area in b), and

f) the HRTEM of CuO taken from the indicated part of b).

.

Figure 3.10. XRD pattern of CuO/ZnO composite nanostructures.

30 35 40 45 50 55 60 65

0

10

20

30

40

50

60

70

CuOZnO

ZnO

ZnO

ZnO

Inte

nsity

(a.u

.)

2 Theta (deg)

CuO/ZnOZnO

CuO

Fabrication and characterization of devices

27

Chapter 4


The synthesized ZnO and CuO nanostructures have a great potential to be used in a wide

range of applications. This chapter deals with some applications of these nanostructures. In

particular, ZnO NRs emerge versatile characteristics useful in extensive fields. Therefore,

efforts have been devoted to the fabrication of ZnO NRs hybrid LEDs to achieve full color

emission. Semiconductor parameter analyzer was used for electrical characterization (I-V) of

the fabricated LEDs. The absorption and the photoluminescence (PL) spectra were measured

with UV-VIS-NIR spectrometer and laser source with specific excitation source. PL is

usually employed to investigate the optical properties of nanostructures [72].

Electroluminescence (EL) is very useful opto-electrical technique which helps to understand

the emission characteristics of the devices [73]. In this thesis the EL spectra were examined

using an Andor-Newton CCD. Moreover, CuO nanoflowers were used to fabricate an

electrochemical pH sensor, and the potentiometric response of pH sensor was measure using

Metrohm pH meter. CuO NSs were also used as catalyst to degrade the organic dyes without

using any irradiation source.

4.1 ZnO NRs/polymers hybrid LEDs

The advent of lighting to human started with fire, and from that time the

obsession with lighting took hold and whisked itself through the ages leading to the

light emitting diodes (LEDs). The lighting area is facing a unique technology

breakthrough with the emergence of LEDs. LED based lighting is though a recent

phenomenon, but LED deployment in this field of application now seems inevitable.

Nevertheless, the invention of blue LEDs in early 1980's gave access to white light

combining red, blue and green LEDs [74]. LEDs have advantages over conventional

light sources, such as higher efficiency, longer life, smaller size, and enhanced

controllability, among other characteristics. LEDs have been seen in successful


28

applications, such as displays, traffic signals, automotive parts, backlighting, accent

lighting, as well as in other applications. The multiple benefits of LEDs and the

continuous increase in their performance allied to the decrease of their

manufacturing costs are likely to make them competitive when compared to

fluorescent lamps and tubes. So far, different types of materials are used to fabricate

white light emitting diodes but still much effort is required to improve the

performance.

With the feature of wide bandgap and large excitonic energy, ZnO was

considered as a prospective material for fabricating light emitting diodes. The

development of p-type doping of ZnO is rapidly growing and many research articles

have been reported [75-78]. But, reproducible p-ZnO with high mobility and high

hole concentration is still difficult. Therefore, the fabrication of homojunction ZnO

LEDs is yet not resolved. To circumvent this problem, an alternate approach was to

develop hybrid p-n junction LEDs, which are composed of heterojunction between

n-ZnO and other p-type materials. Variety of p-type materials such as Si [79], GaN

[80, 81], SiC [82, 83] were used to realize hybrid junctions with ZnO. However, these

materials are expensive and limit the choice to produce cheap flexible devices. To

get an advantage of low temperature ZnO growth on flexible plastic and paper

substrates, p-type polymers are a good choice. Polymers are very promising

materials having many advantages like solution processable, flexible, light weighted

and robust. The easy processing of polymers offers the potential for enormous cost-

savings in many applications. They enable the homogeneous illumination of large

area and lightweight displays can be produced on a variety of flexible substrates.

This provides the ability to conform, bend or roll a display into any shape. One of

the key advantages of the conjugated polymer is the tunability of the emissive

colors. Blue light can be down converted into green and red emission by simply

doping or blending the blue emitter with lower bandgap polymers [84]. Thus,

hybrids ZnO/polymer LEDs combine the advantage of both organic and inorganic


29

semiconductors. These hybrids LEDs are superior to all-organic counterparts

because the optically active component is inorganic nanostructures, which usually

has a higher carrier mobility and lifetime than organic materials [85, 86]. The

emissions of hybrid LEDs exhibit white light impression which covers the whole

visible region. Moreover, vertical ZnO NRs are beneficial in hybrid LEDs because

NRs are like natural waveguiding cavities for causing the emitted light to travel to

the top of the device enhancing the light extraction efficiency from the LED surface

[87]. Although no output power data are yet available for ZnO NR/polymer LEDs,

the published internal quantum efficiency (IQE) results obtained from ZnO NRs are

promising. ZnO NRs grown by the low temperature process showed IQE of 28%

while a higher value of 52% for the IQE of the NBE emission was report for ZnO

NRs grown by the metal organic chemical vapour deposition (MOCVD) [88-90].

ZnO NRs are used to fabricate hybrid ZnO NRs/polymer LEDs. Two

nonconventional substrates, disposable paper and flexible plastic are used as a

substrate for the device fabrication. Paper is ubiquitous in everyday life and a truly

low-cost substrate. It has also recently been considered as a potential substrate for

low-cost flexible electronics [91]. In addition to this, paper is also environmentally

friendly, since it is recyclable and made of renewable raw materials. Usually paper is

insulting and it can degrade in water, therefore the pretreatment of the paper

substrate is essential. For this purpose a cyclotene layer (BCB 4022-35 Dow

Chemical’s) was spin coated on the paper substrate, followed by vacuum baking at

110 oC for 1 h. The cyclotene barrier layer reduces the surface roughness and

protects the surface of the paper from the water nutrient solution. Then PEDOT:PSS

(3,4-ethylenedioxythiophene) poly(styrenesulfonate) solution was spin coated on the

substrate at a speed of 2000 rpm followed by baking at 70 oC for 2 h. Next spin

coating of a blue light emitting polymer (PFO) at a speed of 2000 rpm and anneal at

70 oC for 10 minutes was performed. Finally ZnO NRs were grown on the PFO by

using the method mentioned above [4].


30

To fabricate hybrid LEDs on plastic substrate, commercially available

PEDOT:PSS coated plastic was used due to its flexibility, transparency in the visible

region and good electrical properties. Prior to spin coating the polymer, the

substrate was cleaned by using acetone, isopropanol and DI water subsequently.

Two polymers the PFO (blue emitting) and the MEH PPV (red emitting) were used

separately and their blend prepared with the ratio of 9:1 was also used. Some area of

the substrate was covered to serve as a bottom contact. Then spin coat the polymer

at a speed of 2000 rpm and soft bake for 10-15 min. ZnO NRs were grown on the

polymer layer by following the same procedure discussed in previous chapter.

Figure 4.1. Schematic illustration of the device fabrication.

To complete the device, spin coat the polymethyl methacrylate (PMMA) layer

which serve as an insulator to fill the gap between the ZnO NRs. Reactive ion

etching (RIE) was used to etch the PMMA layer to expose the tips of the ZnO NRs.

(f)

(d

(a) (b

(c)

(e)


31

Finally evaporate the aluminum (Al) top contact on ZnO NRs while the bottom

contact on the PEDOT:PSS was achieved by applying silver (Ag) paste. A schematic

diagram of the fabricated LED is shown in figure 4.1.

4.1.1 Characterization of hybrid LEDs on disposable paper substrates

Current-voltage (I-V) characteristics of LEDs give the key information to

judge the performance/quality of the fabricated diodes [92-94]. Figure 4.2a shows I-

V characteristics of the fabricated ZnO-polymer LEDs consisting of

Ag/PEDOT:PSS/PFO/ZnO NRs/Al. The I-V characteristics indicate the rectifying

behavior of the diode. The inset of figure 4.2a shows a photograph of the folded

substrates and no observable changes were noticed by rolling or bending the

substrates, which demonstrates the good adhesion of the PFO/ZnO NRs structure.

The room temperature EL of the hybrid LEDs is shown in figure 4.2b. This EL

reveals a broad spectrum covering the visible spectrum from 420 nm to 720 nm. The

blue peak appeared at 460 nm belongs to the PFO and is attributed to exciton

emission and its vibronic progression from non-interacting single chains [95]. The

broad green emission appearing at 555 nm is the famous green band emission due to

deep level defects in ZnO. This peak has been reported to be in this region,

especially in low temperature grown ZnO nanorods [96]. The PFO has also a defect

emission in this range i.e. around 520 nm due to the keto/fluorenone defects that

might be intrinsic during synthesis or generated during the device fabrication and

operation [97, 98]. The peaks at 640 nm and 710 nm belong to oxygen interfacial

defects [99]. The light emission in the ZnO NRs hybrid LED suggests that it can be

attributed to both the blue emission from PFO and the ZnO defects related

emissions. The interaction between the ZnO NRs surface and the PFO molecule

makes the PFO link up with ZnO and leads to ZnO NRs/PFO interface. The electron

existing at the interface between the conduction band edge of the ZnO NRs and the

lowest unoccupied molecular orbital of the PFO layer slowly drops to the lower state


32

due to their longer life time. As a result, a broad emission ranges from 420 to 780 nm

and gives rise to white light.

The color quality of the hybrid LEDs on paper was investigated with

Commission International de I`Eclairage (CIE). The emitted light has warm white

impression with color rendering index (CRI) of 94 and color coordinate temperature

(CCT) of 3660 K.

Figure 4.2. (a) Typical I-V characteristics of the fabricated device on paper substrate. The

inset shows a digital photograph of the device. (b) The RT EL spectrum of the hybrid ZnO

NRs-polymer LED [100].

4.1.2 Influence of the polymer concentration on hybrid LEDs

After the demonstration of the ZnO-polymer LEDs, a next step is to ensure

the significance of the polymer concentration on the emission performance of the

hybrid LEDs. Figure 4.3a shows the digital photograph of the hybrid LEDs. The I-V

characteristics of the LEDs fabricated by various concentration of the PFO (2 mg/ml

to 20 mg/ml) are shown in figure 4.3b. This figure reveals that the turn-on voltage of

-6 -4 -2 0 2 4 6

0.0

5.0x10-6

1.0x10-5

1.5x10-5

2.0x10-5

2.5x10-5

3.0x10-5

Cur

rent

(A)

Voltage(V)

450 500 550 600 650 700 750

E

L In

tens

ity (a

.u.)

Wavelength (nm)

460

555

710640


33

the devices is concentration dependent. This can be expected due to the fact that by

increasing the thickness of the PFO the diode parasitic resistance will be increased.

The increase of the diode parasitic resistance can be caused by the PFO/ZnO NRs

interface imperfection and/or the excessive contact resistance (series resistance).

Figure 4.3. (a)The photograph of the hybrid LED. (b)The I-V characteristic of the ZnO

NRs/PFO hybrid LEDs for various PFO concentrations.

The room temperature EL spectra demonstrate that the PFO concentration

plays a critical role on the emission spectra of the hybrid LEDs. It was observed that

by increasing the concentration of the PFO in the hybrid LEDs, the EL emission

spectra tend to change markedly as shown in figure 4.4a-b. The changes on the EL

spectra of the PFO/ZnO LEDs can be described as follows. The dominant blue peak

at 452 nm was gradually suppressed with increasing the concentration of the PFO

up to 6 mg/ml. In addition the dominance of the peak at 473 nm started at a

concentration of 8 mg/ml and then continued up to 20 mg/ml. Furthermore, the

intensity of the broad green emission of the PFO/ZnO LEDs is enhanced with

increasing the concentration of the PFO. The intensity drop of the 452 nm peak can

be correlated with the emergence of the β-phase which starts to appear at 6 mg/ml

concentrations. The emission of the β-phase is very close in energy to that of the

vibronic replica of the α-phase (473 nm), and as a result the glassy phase is hindered

-6 -4 -2 0 2 4 6 8 10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

Cur

rent

(mA

)

Voltage (V)

2mg 4mg 6mg 8mg 10mg 20mg

(b)(a)


34

by the formation of the β-phase [101]. The drop in the first peak (452 nm) of the EL

spectra implies either a fast and effective energy transfer from the α-phase to the β-

phase domain [102] or the β-phase may act as low-energy traps for the excitons

because it has a smaller energy gap compared to the α-phase [103]. Another change

which was observed in the EL spectrum is the enhancement of the green peak. This

can be originating from the increased quantity of fluorenone defects that were

introduced at larger concentrations (thickness) or the increased density of ZnO NRs

at higher PFO concentrations, and/or due to both effects. Simultaneously, two

processes involved in the green band emission of the PFO: first energy transfer from

the excitons on the PFO main chain to the fluorenone defect sites. Secondly, trapping

of excitons at the fluorenone defect sites followed by a subsequent radiative

recombination [55]. The fluorenone defects acts as a low-energy trap, and so it is

expected that there will be a competition between the β-phase and the fluorenone

defects for the excitons that were created within the PFO thin film [101].

To confirm our finding that the effect of the injected carriers of the defects

emission from the PFO by the electrochemical oxidation of the polymer chains, we

performed current dependent EL measurements. The results are shown in figure

4.5c. Interestingly at low injection currents (0.3- 0.6 mA) the EL spectrum shows

identical blue peaks as discussed before i.e. (452 nm and 470 nm). At a high injection

current a new blue peak was observed in the EL spectrum which was correlated to

the creation of a new chemical species on the PFO chain. Furthermore, the green

emission peak was also enhanced with increasing the injection current. The current

flowing through the polymer is also believed to play a crucial role in the oxidation of

the polymer. During the electro-oxidation process of the PFO fluorenone defects are

formed which in turns increase the green emission over the blue one.

The corresponding Commission Internationale de I´Eclairage (CIE)

chromaticity coordinates of the spectra at different concentrations are shown in


35

figure 4.4d. The emission color of the LEDs changed from bluish white with color

coordinates (0.259, 0.312) to greenish white (0.309, 0.418) with increasing

concentration of the PFO. While the Correlated Color Temperature is decreases from

11741 K to 6196 K.

Figure 4.4. (a) and (b) electroluminescence spectra of the hybrid LEDs having PFO

concentration from 2mg/ml to 20mg/ml. (c) EL spectra of the ZnO NRs/PFO (2mg/ml) LED

at low and high injections current. (d) Chromaticity diagram (CIE coordinates) for all

fabricated LEDs illustrating the emission color change from bluish to greenish white.

4.1.3 Effect of the polymer emission on the EL of hybrid LEDs

The choice of the polymers with proper concentration is also critical to the

emitted color in ZnO NRs/polymer hybrid LEDs. The emission spectra can be tuned

400 450 500 550 600 650 700 7500

20000

40000

60000

80000 2mg 4mg 6mg 8mg

Inte

nsity

(a.u

)

Wavelength (nm)

(a)

400 450 500 550 600 650 700 7500

20000

40000

60000

80000

Inte

nsity

(a.u

)

Wavelength (nm)

2mg 10mg 20mg

(b)

400 450 500 550 600 650 700 750 800-1x103

0

1x103

2x103

3x103

4x103

5x103

6x103

0.3 mA 5.0 mA

Wavelength (nm)

Inte

nsity

(a.u

)

PFO (2 mg/ml)451

470

465

505540

536(c)

0.0

2.0x103

4.0x103

6.0x103

8.0x103

Inte

nsity

(a.u

)

(d)


36

by selecting proper ratio of the different polymers. Therefore, we demonstrate EL

and colour characteristics of organic-inorganic LEDs made from different p-type

polymer configurations with n-type inorganic ZnO NRs grown at low temperature

on flexible PEDOT:PSS coated plastic substrate.

Three types of LEDs have been fabricated, these were

PEDOT:PSS/PFO/ZnO/Al denoted as LED A; PEDOT:PSS/MEH-PPV/ZnO/Al

denoted as (LED B) and PEDOT:PSS/PFO:MEH-PPV/ZnO/Al denoted as (LED C).

The PFO and MEH-PPV blend was prepared with a molar ratio of 9:1 respectively.

The energy band diagram of the blended LED is shown in figure 4.5a. The band gap

of the PFO and the MEH-PPV are 3.2 eV and 2.1 eV, as estimated from the HOMO-

LUMO energy difference, showing that the MEH-PPV has a smaller band gap

compared to the PFO [58]. Figure 4.5b show the optical absorption spectra of the

PFO, the MEH-PPV, and their blend. The PFO and the MEH-PPV have an

absorption peaks at 385 and 510 nm, respectively. In the blended film the absorption

is mainly due to the PFO and the MEH-PPV is poorly absorbing. Blending with a

more MEH-PPV has been investigated and it showed almost pure MEH-PPV

characteristics. This is why we restricted the investigation to 9:1 for the PFO: MEH-

PPV. The PL spectra of the ZnO NRs, PFO and PFO:MEH-PPV/ZnO (blended/ZnO

) is demonstrated in figure 4.5c. The PFO spectrum shows two well-resolved blue

peaks with a maxima at 430 nm, 457 nm and 490 nm assigned as 0–0, 0–1 and 0–2

interchain singlet transitions [104], together with a featureless broad green band

with maximum at 520 nm originating due to the defects produced during the

fabrication processes of the device [97]. The ZnO NRs exhibits a dominant UV

(ultraviolet) emission peak at 378 nm in the PL spectrum which is attributed to the

near band edge (NBE) emission of ZnO. Visible emission peaks of ZnO NRs at 525

nm and 665 nm are ascribes to deep band defects emission (DBE) which is mostly

observed in low temperature grown samples [17]. The green emission peak (525 nm)

could be attributed to the presence of intrinsic defect emission such as oxygen


37

vacancies or zinc vacancies while the orange-red emission is due to the transition

from Zni to Oi [105, 106].

Figure 4.5. (a) The energy band diagram of LED C formed using the blended polymers. (b)

Absorption spectra of the PFO, the MEH-PPV and 9:1 blend of the PFO: MEH-PPV V

polymers. (c) Photoluminescence spectra of ZnO NRs, PFO and blended polymer/ZnO. (d)

Electroluminescence spectra of the three LEDs[107].

The blended/ZnO NRs PL shows that the PFO blue peaks and ZnO NBE are at the

same positions as we discussed above, while the ZnO NRs DBE peaks strongly

overlapped with MEH-PPV (orange-red) peaks. Because on one hand the ZnO NRs

are densely standing on top of the polymer layer with their length around ∼2 µm.

300 400 500 600 700

0.00

0.04

0.08

0.12

0.16

0.20

Abs

orpt

ion

(a.u

.)

Wavelength (nm)

MEH PPV PFO Blended PFO:MEHPPV

(b)

400 450 500 550 600 650 700 750 8000

500

1000

1500

2000

2500

3000

3500

4000

4500

Inte

nsity

(a.u

)

Wavelength (nm)

LED ALED C LED B

(d)

(a)

400 500 600 700 800-5000

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

Inte

nsity

(a.u

.)

Wavelength (nm)

PFO ZnO Blended/ZnO

430457

378

(c)


38

On the other hand, the PL measurements are very sensitive to the sample surface

and highly suppressing in the bulk contribution; therefore, most of the PL detected

intensity will originate from the top part of the sample (ZnO NRs). The featured

peaks at ∼585 nm and 615 nm can be ascribed to the overlapped emission of the

MEH PPV with the defect emission of ZnO. The emission peak at 585 nm of the

MEH PPV is assigned to single chain exciton transition, whereas the peak at ∼ 615

nm is due to the intrinsic vibronic structure [108].

The room temperature EL of the three LEDs under forward bias is depicted in

figure 4.5d. For LED A (PFO/ZnO), the emission peaks are discussed in the

previous part. LED B (MEH-PPV/ZnO) has an extended emission from 575 nm to

750 nm with a peak at 608 nm, which correspond to the MEH-PPV due to the PPV

backbone that arises from the relaxation of the excited Π-electrons to the ground

state and the shoulder at 640 nm is related to interchain states [109]. The peaks

appeared in the EL spectrum is originating from the MEH-PPV, because the

potential barrier for electrons is 1.2 eV and the mobility of electrons in the ZnO is

higher than that of holes in the MEH-PPV so the recombination primarily occurs in

the MEH-PPV side. For LED C the EL emission covering the entire visible emission,

the blue peak at ~ 455 nm belongs to the PFO, while the green emission is from the

ZnO DBE along with the orange-red emission from the MEH-PPV are completely

intermixed yielding a broad visible emission. It is seen that even at very low

concentration of the MEH-PPV in the blend, the energy transfer is predominant and

the device gives intense emission from the MEH-PPV. The PFO emission is

relatively low but is still high enough to influence the colour quality of the device.

The emission from both polymers is due to the incomplete energy transfer [110].

In figure 4.6 the CIE 1931 color space chromaticity diagram in the (x, y)

coordinate system of the fabricated LEDs are plotted. A bluish white impression can

be seen for LED A while orange-red impression is observed for LED B. The LED C


39

illustrates white light impression which shows the blend ratio of the polymers is

appropriate.

Figure 4.6: The chromaticity coordinates of the three fabricated LEDs.

4.2. CuO based electrochemical pH sensor

Electrochemical sensors are fast, accurate, quantitative and possess sensitive

response as well as being simple and these factors makes them more attractive [111].

Electrochemical sensors respond to electron transfer, electron consumption or

electron generation during a bio/chemical interaction process [112]. They can

generally be categorized as potentiometric, amperometric and conductometric

sensors. In potentiometric sensors the potential change due to the accumulation of

charges on the working electrode is measured relative to a reference electrode when

no current is flowing, whereas in amperometric sensor the current change due to the

electron transfer in the chemical reaction at the electrode at certain applied voltage is

measured [113]. The nanostructured metal oxides with novel functions have opened

new opportunities for electrochemical sensors. Generally, metal oxide

nanostructures have excellent conductivity and catalytic properties, which make

them suitable for acting as electronic wires to enhance electron transfer and


40

improved sensing characteristics [114]. Also, nanostructure with high surface area-

to-volume ratio leads to a short diffusion distance of the analyte toward the

electrode surface, thereby providing a faster response and enhanced analytical

performance.

The pH determination is a strong prerequisite for many biochemical and

biological processes. The pH of a solution, biological and biochemical is vitally

important in order to use the solution effectively. The pH value of blood is an

important index for the human body, even a small change in the pH of the body

fluids can begin a problem in human body. Thus, pH measurements are vitally

important in many applications including medicine. The electrochemical pH sensors

have found use in several other areas, such as industrial hygiene, pollution

measurement and control, hazard monitoring, process control and combustion

control. Moreover, metal oxides have been widely reported to have pH- sensitivity

and therefore develop pH sensors based on these materials [115-119]. Prior to all

these, the production of faster, robust and cost efficient electrochemical sensors have

huge significance for the daily life diagnostics of pH.

Specifically, CuO nanostructures having a large surface area-to-volume ratio,

high surface reaction activity and biocompatibility. CuO is one of the interesting and

appropriate metal oxides for biosensing applications [120-122]. CuO flowers were

grown on gold coated glass substrate by the method mentioned in the previous

chapter. Electrochemical studies are conducted using a two-electrode configurations

consisting of CuO nanoflowers (NFs) as the working electrode and Ag/AgCl/Cl as a

reference electrode. The schematic diagram is shown in figure 4.7. The

electrochemical response was performed using a Metrohm pH meter at room

temperature. The electrochemical potential was measured when the equilibrium

potential was reached and stabilized.


41

Figure 4.7. Schematic diagram of the potentiometric measurement setup of the pH

sensor.

4.2.1. Characterization of CuO pH sensors

The performance of the pH sensor is usually characterized by measuring the

electrochemical potential of the electrode. Figure 4.8a shows the response of the

potential difference (electromotive force emf) of the CuO NFs to the changes in the

pH for a range of 2 to 11. It shows a good linear response and has sensitivity equal to

28mV/decade, and a correlation coefficient r2 value of about 0.953 which is

calculated from near-Nernstian response. The repeatability and stability were

examined to obtain an accurate and reusable pH sensor. It was observed that the

CuO NFs showed stable potentiometric response, excellent reproducibility, and

good sensor stability. The expected sensing mechanism is protonation–

deprotonation [123]. CuO NFs as a pH sensor is based on the activity of the

electrolyte-NFs/interface, in which the H+ specific bonding sites existing at the CuO

surface can hydrogenate after contact with the electrolyte solution. This sites can


42

protonate or deprotonate, leading to surface charge and surface potential that is

dependent on the electrolyte pH [124].

Figure 4.8b shows the output voltage versus time for the fabricated working

electrode at a specific pH. The fabricated electrochemical sensor is repeatable and

stable with time to reach a stable signal of around 25 sec. of detection which shows

relatively fast electrochemical reaction between the CuO NFS and the pH but

slightly lower than the other pH sensors. This may be due to the fact that the

response time is generally affected by the porous properties of the sensing material.

The buffer pH solution needs to accumulate in the liquid in the pores of the CuO

NFs in which the pores increases the response time [118]. Since the CuO NFs sensors

are porous, so the sensor response is slightly slower.

Figure 4.8: (a) Calibration curves of the CuO NFs pH electrode in pH buffer solution, and

(b) Output voltage response as a function of time [69].

2 4 6 8 10 12-200

-150

-100

-50

0

50

100

Vol

tage

(mV

)

pH

calibration curve Linear fit of calibration curve

y= -28x + 153.9

r 2= 0.997

0 100 200 300 400 500 600 700

1.40

1.42

1.44

1.46

1.48

1.50

1.52

1.54

Vol

tage

(V)

Time (sec)

4 ph

a b


43

4.3. CuO nanostructures as a catalyst

With the increasing revolution in science and technology, there was a greater

demand on opting for new chemicals which could be used in various industrial

activities. Organic dyes are one from many new chemicals which could be used in

several industrial processes. Due to their large-scale production and extensive

applications, organic dyes have become an integral part of the industrial

wastewater. In fact, a huge amount of organic dyes are lost in the wastewater during

manufacturing and applications process [125, 126]. These dyes are harmful for

aquatic life and environmental. Various methods have been widely used to handle

the dye removal from wastewaters in order to comply with the environmental

regulations, which are becoming more crucial these days. In recent years,

heterogeneous photocatalysis processes are one of the most efficient methods for

destroying organic pollutants and dyes in aqueous media [127, 128]. Heterogeneous

photocatalysis is a process in which the degradation of organic pollutants is

governed by the combined actions of a semiconductor catalyst, an irradiation source

and an oxidizing agent. One parameter which plays crucial role in the enhancement

of the activity of the catalyst is the irradiation source. Mostly irradiation source

which are commonly used are expensive and pollutant. Therefore, degradation of

the organic dyes without any expensive irradiation source is of special interest,

making the development of suitable technologies for practical applications very

attractive. Hydrogen peroxide (H2O2) is a preferable oxidant because of the

simplicity of handling, the environmentally friendly nature of coproduct (water), the

high oxygen atom efficiency, and the versatility [129]. Moreover, utilization of the

H2O2 in catalytic reaction has great advantage to the environment and industry

because (i) it generates H2O as a sole by-product, (ii) it has a high content of active

oxygen species, and (iii) H2O2 is rather inexpensive [130]. The semiconductor

catalysts which are the core of the whole process are mostly metal oxides that can be

more appealing than the conventional chemical oxidation methods because metal


44

oxides are low cost, nontoxic and capable of extended use without substantial loss of

photocatalytic activity [131-133]. Additionally, with the development of

nanotechnology provide new insight in this field with several nanoscale materials.

Due to their high surface area, nanostructured metal oxides exhibit enhance

reactivity because more reactant should be adsorbed on the surface. Also, previous

studies indicate that this reaction is an apparent structure sensitive process, the

shape and the exposed crystal planes were found to influence the catalyst

performance significantly [134-136]. Moreover, CuO is one of the most important

catalysts among other metal oxides, and is widely used in environmental catalysis. It

has an additional advantage of not requiring any type of irradiation source for its

activation, which can lead to large-scale use of this technology [137].

The catalytic activities of the as grown CuO NSs were investigated using an

aqueous solution of methylene blue (MB) and rhodamine B (RB). The typical

catalytic reactions were carried out at room temperature in a sealed glass beaker

containing 100 mL of an aqueous solution of MB or RB (0.2 g/L). A 20 mL of

hydrogen peroxide (H2O2 30 wt %) and 20 mg of CuO NSs were added to the dye

solution, and the mixture was allowed to react for 30 mints to reach equilibrium.

After a given time interval, 2 mL of solution was withdrawn and the UV-visible

absorption spectrum was measured.

4.3.1 Catalytic performance of CuO nanostructures

Figure 4.9a-f show the degradation of the organic dyes (RB and MB) in the

presence of flowers-like and petals-like CuO NSs, for comparison CuO commercial

powder was also assessed. This demonstrates that the CuO petals are more affective

to degrade both organic dyes as compared to flowers and commercial CuO powder.

This is attributed to the larger specific surface area of the CuO petals which is about

8.4 m2/g as compared to the CuO flowers 5.5 m2/g and commercial CuO powder 1.7

m2/g. The larger the surface area that the catalyst possesses, the more reactants


45

Figure 4.9: (a-d) UV- visible absorption spectral changes of an aqueous solution of RB and

MB in presence of CuO flowers and petals, and H2O2.(e, f) Plots of the concentration ratio of

RB and MB in an aqueous solution against specific time intervals in presence or absence of

CuO .

300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Abs

orba

nce

(a.u

.)

Wavelength (nm)

0 hr 0.5 hr 1 hr 2 hr 3 hr 4 hr 5 hr 6 hr 7 hr 24 hr

CuO petals in MB (d)

0 5 10 15 20 25

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

C/C

0

Time (h)

CuO flower CuO powder CuO petals MB +H2O2

(f)

300 400 500 600 700 800 900-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Abs

orba

nce

(a.u

.)

Wavelength (nm)

0hr 0.5 hr 1 hr 2hr 3hr 4hr 5hr 6hr 7hr 24hr

CuO flowers in MB (c)

0 1 2 3 4 50.0

0.2

0.4

0.6

0.8

1.0

C/C

o

Time (h)

CuO powder RB+H2O2 CuO flower CuO petals

(e)

300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Abs

orba

nce

(a.u

.)

Wavelength (nm)

0 hr 0.5 hr 1 hr 2 hr 3 hr 4 hr 5 hr

CuO flower in RB (a)

300 400 500 600 700 800 9000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Abs

orba

nce

(a.u

.)

Wavelength (nm)

0 hr 0.5 hr 1 hr 2 hr 3 hr 4 hr 5 hr

CuO petals in RB (b)


46

(e.g., O2, OH-, and organic molecules) that should be adsorbed on its surface due to

an increase in the number of active sites, thus leading to higher catalytic activity

[138]. Moreover, the main process of the degradation of the dye is adsorption–

oxidation–desorption mechanism. The MB molecule and H2O2 were first adsorbed

on the surface of the nanostructures, and then free radical species of HO•, HOO•, or

O2•− were produced by the catalytic decomposition of H2O2. Theses free radicals

cause the destructive oxidation of the organic dye.

The fitting of the maximum absorbance plot vs. time suggest that these

catalytic degradations follow the pseudo-first-order reaction with respect to both

dyes [69] (shown in appended paper VI). This indicates that for both dyes the

degradation rates follow the order of CuO petals > CuO flowers > CuO commercial

powder. After demonstration of the catalytic activity of the CuO NSs their

reusability was checked, and it confirms that these NSs are useful for many cycles.

All these results conclude that as prepared CuO NSs could work as an effective and

reusable catalyst at room temperature without using any specific light source.

Conclusion and future work

47

Chapter 5


The present work mainly covers ZnO and CuO nanostructures, study of their low

temperature growth, characterization and applications. This work is mainly divided

into: (i) Synthesis of ZnO NRs and fabricate hybrid LEDs (ii) CuO NSs growth and

applications and (iii) Synthesis and characterization of composite CuO/ZnO NSs.

Well aligned ZnO NRs were synthesized using the wet chemical method at 50 oC on various substrates in paper Ι. We found that lowering the growth temperature

is advantageous for the axial length of the ZnO NRs. The growth of ZnO NRs on

metal, semiconductor and polymer coated substrates is essential for various

applications. The low temperature method does not require any harsh fabrication

process on the target substrates which enables the use of nonconventional substrates

such as plastic, polymer and paper. Utilizing the compatibility of this method with

nonconventional substrates we synthesized ZnO NRs on flexible and disposable

substrates, which are likely to be integrated with future foldable and disposable

electronics. Low cost production is a main attraction of any product and ZnO

nanostructures has much potential to become the choice for cheap devices.

Therefore, we have studied ZnO NRs-based hybrid LEDs in this work. The ZnO

NRs were successfully utilized for white light emitting diodes (LEDs) on flexible

plastic and disposable paper substrates (paper ΙΙ-IV). We investigated that the

polymers with appropriate concentrations is very important to obtain a white light

emission. Moreover, blending different polymers with suitable ratio is another

approach to tune the emission of the hybrid LEDs.

A pH sensor has been carried out with the growth of CuO nanoflowers

(paper V). The fabricated sensor showed extensive advantages of compatible size,


48

low cost and simple fabrication technique. The proposed sensor exhibited a linear

electrochemical response within a wide pH range of 2-11 which shows that the

fabricated sensor has the ability to work over a long range. In addition, good

repeatability and reproducibility were obtained from the fabricated pH sensor.

The catalytic activity of the CuO nanostructures was examined through the

degradation of two organic dyes (paper VΙ). This reveals that the catalytic reactivity

depends on the morphology of the CuO nanostructures. The catalytic performance

suggests that these nanostructures can be efficiently used to degrade organic dyes

without using any irradiation source.

Furthermore, an effort is devoted to synthesize a CuO/ZnO nanocomposite.

For this purpose, a two-step hydrothermal approach was developed in paper VΙΙ to

achieving a hierarchical CuO/ZnO nanocomposite. The formation of the CuO NSs

on the surface of ZnO NRs was investigated using two different nutrient solutions.

The CuO NSs were selectively assembled on the lateral surface of ZnO nanorods

(NRs) upon short growth duration. While the CuO NSs were entirely cover the

whole surface of the ZnO NRs at extended growth durations. The growth kinetics of

the CuO NSs is strongly dependent on the nature and the pH value of the nutrient

solution used in the synthesis.

LEDs based on ZnO nanostructures have potential to become the choice of

the world community due to intrinsic white light and large area. But the efficiency

measurement, interface optimization, surface passivation, and life time are the key

issues for the application of LED devices, and will be the focus of the future work.

Further effort would also be directed to the development of solar cell and transistors

on paper substrates. Moreover, the novel syntheses of composite CuO/ZnO

nanostructures have a potential to fabricate a cheap photovoltaic and sensing

devices. This is also useful for photocatalytic applications and to achieve all these

goals further research is necessary.

References

49

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Synthesis of ZnO, CuO and their Composite Nanostructures ...liu.diva-portal.org/smash/get/diva2:550644/FULLTEXT01.pdf · Linköping Studies in Science and Technology Dissertation,