The History Of Nanomaterials

Published Date: 02 Nov 2017

Abstract

Dye-sensitized solar cell (DSSC) is a kind of third generation solar cell which normally contains wide band-gap metal oxide working electrode sensitized by dye molecules. This new molecular solar cell technology has a great potential to achieve low-cost investments, easy fabrication, short energy-payback time and good performance under diffuse light conditions, while also have great potential for market due to semi-transparency and multi-colour range possibilities, and the ability to be fabricated on flexible substrates. Based on these years’ work, the main challenges for the future research are, i.e., carrier trapping and recombination loss between photo-injected electrons in the working electrode and electrolyte, the potential drop in the regeneration process at counter electrode, etc. With previous researchers’ breakthrough of using nanostructured working electrode systems such as TiO2 nanoparticles, which has great improved dye absorption amount. With the world record of 12.3 by Grätzel group, the main direction of the working electrode research filed in now to explore this path.

Nanostructured ZnO and TiO2 have some potential benefits over traditional nanoparticle network DSSC devices related to optical, electrical and strain relaxation effects; new charge separation mechanisms high surface area and low cost. So that the charge transport and collection as well as high absorption of dye for improving DSSC performance can be enhanced by employing ordered dimensional ZnO and TiO2 nanostructures such as nanorods, nanotube arrays or other low demission nanostructure. However, DSSCs based on nanorods arrays or other low dimension with power conversion efficiency higher than 12% achieved from the conventional DSSCs have yet to be demonstrated. Due to a number of unresolved questions haven’t been answered, such as stability, low cost reproducible synthesis method, etc.

So this work will focus on developing ZnO-based nanostructured materials for improving the performance of DSSCs by using a low cost and low temperature aqueous solution growth approach, to analyse their compositional, morphology and functional properties using advanced characterization techniques, to get a better understanding of the mechanisms behind nanostructure growth. Then as prepared ZnO nanostructures will be used as template for ZnO/TiO2 hybrid nanostructures which will be used to enhance charge transport and collection as well as chemical stability, and hence, power conversion efficiency.

Acknowledgement

I express my hearty thanks to my supervisor, Dr Richard Fu, for his great supports and guidance during my study. I have enjoyed my research under his supervision. I

shall never forget everything he has supported and provided for me.

I would like to express my deep appreciation to Professor Frank Placido and Dr

Jong Han for being my dissertation committee members.

I would like to thank Dr Nei Robension in Electrical Engineering Department for allowing me to use the facilities in his lab and Dr. Sungjin Kim for helping me to use those facilities.

Thanks to all of my past and present group members and colleagues in Physics

Department, Dr. Seongjin Jang, Savas Delikanli, Gen Long, James Perry, Samanthe Perera,

Dr. Hongwang Zhang, and Dr. Everett Fraser. Also I thank to Dongho Lee in Electrical

Engineering Department and Dr. Jangwon Seo in Chemistry Department for helps on my research.

I would like to thank all the faculty and staff members in Physics Department. I

appreciate Dr. William Condit’s supervision when I was a teaching assistant. Finally, there are no words that can express my gratitude to my parents for unwavering support and love.

Publication lists

Contents

List of Figures

List of Tables

1 Introduction

The wide band-gap semiconductor, such as ZnO and TiO2, has a huge commercial potential for applications in photoelectric, piezoelectric and sensing, due to its easily controlled and simply fabrication process, environment friendly, excellent properties of photoelectric and piezoelectric, and largely abundant resources in the nature and so on. In particular, effort has been made to use these nanostructured semiconductors as solar cell components and catalysts for energy harvesting and environmental protection purposes [1] . ZnO and TiO2 have similar band edge energy (~3.2 eV), but different chemical and physical properties. [2] One key reason for the popularity in research work on this topic is the proper combination of ZnO and TiO2 nanostructures may offer different properties from single phase of ZnO or TiO2, will be an promising efficient route to enhance the performance of devices or catalysts which utilize only one of the two oxides. So study of low dimensional (LD) ZnO and TiO2 materials and how to use them has become a leading edge in nanoscience and nanotechnology and the optoelectronic device application of proper combination of LD ZnO and TiO2 nanostructures becomes one of the major focuses in recent nanoscience research.

However, the controlling of accurate shape and size of LD ZnO and TiO2 nanostructures for DSSC application in an economic way is still a big challenge for the researchers. Furthermore, some daunting challenges must be addressed before the benefits can be realized commercially. These challenges include surface and interface recombination, surface roughness, mechanical and chemical stability, fine morphology and doping control, nanowire array uniformity, and synthetic scalability. Some progress has been made in most of these areas, but much more work is needed, especially that related to controllable producing and chemical stability.

Therefore, get good use of the advantages of ZnO and TiO2 nanostructures and get through the problems for controllable producing and chemical stability. This work will not only focus on developing a cost-effective technique for the preparation of ZnO nanorods and nanowalls and ZnO/TiO2 composites with novel structural and functional properties, but also obtaining some beneficial results in aspects of their optical properties, which builds theoretical and experimental foundation for much better understanding fundamental physics and better performance of DSSC performance

The overall objectives of this project include:

Synthesize reproducible and controllable one-dimensional ZnO nanostructure

Assemble devices and its applications using ZnO nanostructure and the application of dye sensitive solar cell (DSSC);

Electron beam deposition for the modified layer of TiO2 for improving the ZnO nanostructure based DSSC performance

A two-step process for fabrication of a novel ZnO/TiO2 composites nanostructures will be made for further improvement of DSSC performance

The outline of this report is as follow:

Chapter 2 presents a short literature review.

Chapter 3 shows experimental and characterized procedures

Chapter 4 includes experiment work on ZnO nanorods synthesis and testing of nanorods DSSC performance.

Chapter 5 presents experiment work on optimized process for synthesis TiO2 which will be used for making ZnO-TiO2 composites nanostructures to improve DSSC performance.

Finally, conclusions and my future work will be presented in Chapter 6

2. Literature survey

Nowadays, the products of semiconductor could be seen everywhere in the daily life of human being. In 1947, the first semiconductor transistor was made at Bell Lab, USA. Since then, the semiconductor industry has kept growing rapidly. The development process from lab scale III-V compound semiconductors and gallium arsenide (GaAs, 1st generation of semiconductors) to mass production silicon (Si, 2nd generation of semiconductors) only took about 20 year[ [3] ]

Because of limitations of the 1st and 2nd generation semiconductors, the third generation semiconductors (such as SiC, GaN and ZnO, etc.) have been applied and turned into the research focus in the field of semiconductor these days [ [4] ]. Furthermore, this was improved since the 1st nanostructured carbon nanotube was reported by Dr. Iijima[ [5] ], and the excellent and unique properties (electronic, optoelectronic, chemical, etc.) of nanomaterial have attracted interests of many researchers. Large effort has been focused on the synthesis, characterization and device assembling of different semiconductor nanomaterial[ [6] ].

Among various nanostructures, nanowires, nanorods, nanotubes, etc. could become very important and reliable "building blocks" for nanodevices [ [7] ] since these structures can function as miniaturized devices as well as electrical interconnects. Moreover, they could be easily grown on various substrates, even on flexible polymer substrates. Another unique advantage for these nanostructures is the strain in the nanowires, nanorods or nanotubes can be efficiently relieved by elastic relaxation at the free lateral surfaces rather than by conventional plastic relaxation[ [8] ].

2.1 Nanomaterial

For the material called nanomaterial, scale is described by nanometer (nm) which is a measurement of length (1nm=10-9m). For an easy understanding of the scale take a piece of office paper as an example which is about 1ï‚´105nm thick. Generally speaking, nanomaterial means various kinds of nanostructured materials which possess at least one dimension in the nanometer scale. The definitions for typical classification are demonstrated in Figure 2.1: zero dimension (0D) such as semiconducting and ceramic nanoparticles; one dimension (1D) such as nanowires, nanoribbons and nanotubes; two dimension (2D) nanostructures such as thin films. Materials reduced to nanoscale will show very different and interesting properties compared to what they exhibit on bulk scale. Take some normal materials as examples: solids turn into liquids at room temperature (gold); inert element becomes catalysts (platinum); insulators become conductors (silicon) and so on. In addition, nano-crystallites of bulk inorganic solids have been shown to exhibit size dependent properties, such as higher energy gaps, lower melting points, and non-thermodynamic structures[ [9] ],[ [10] ],[ [11] ].The chemical and physical properties of materials change as their size approaches the nanoscale and as the percentage of atoms at the surface of a material becomes significant. But for buck materials larger than 1 micrometer the percentage of atoms at the surface is very small relative to the total number of atoms of the material [12] ]

2D Nanostructure

1D Nanostructure

0D Nanostructure

<100nm

<100nm

<100nm

<100nm

<100nm

Thin films

Nanosheets

Nanorods

Nanowires

Nanobelts

Nanotubes

 

Nanopowders/particles

Quantum dots

 

Fig.2-1 Nanomaterial classifications according to the dimensions [ [13] ]

The big difference of properties between bulk and nanomaterial could be for two reasons. First, nanomaterial has a really large surface area than the same mass of material produced in larger form which could make material more chemically reactive, and affect their strength or electrical properties. Another reason is that the well knows the "small size effect". It said that laws of classical physics give way to quantum effects, provoking optical, electrical and magnetic behaviors different from those of the same material at a larger scale. Due to their small dimensions, nanomaterial have extremely large surface area to volume ration, which makes a large fraction of atoms of the materials to be the surface or interfacial atoms, resulting in more "surface" dependent material properties. Especially when the sizes of nanomaterial are comparable to Debye length, the entire material will be affected by the surface properties of nanomaterial[ [14] ].So this in turn may enhance or modify the properties of the bulk materials. For example, nanoparticles and nanorods enhanced the sensitivity and sensor selectivity. Some of the nanoparticles can be used as vary active catalysts. For some special shapes of nanomaterial which bring the quantum effects. For instance, some nanoparticles can be viewed as a 0D quantum dot while various nanotubes and nanowires can be viewed as quantum wires. Furthermore, the energy band structure and charge carrier density in the materials can be modified quite differently from their bulk count part and in turn will modify the electronic and optical properties of the materials. Take lasers and light emitting diodes (LED) as examples, both of them are obtained very promising properties by using quantum dots and wires. High density information storage using quantum dot devices is also a fast developing area.

Table.2-1 Classification and commercial interest of nanomaterial

Type of nanomaterials

Example of amplications

Metal oxides:

Zinc oxide

Titania

Iron oxide

Alumina Zirconia

Solar cells

UV-A protection

Medicine/ pharmacy

Additives for polymer composites

Sensors

Carbon nanostructures

carbon nanowires

carbonnanotubes(single/multiwall)

Electronic filed emission

Additives for polymer composites(mechanical performance and conductivity)

Batteries

Organic nanoparticles

micronized drugs and chemicals

polymer dispersions

Metals:

Ag

Au

Ni

Catalytic applications

optoelectronics

wound dressings

Furthermore, nanomaterial and its usage are developing very fast and expanded extensively to other fields of interest due to the novel and excellent properties. Such as, nanoparticles are potentially used in catalysts, energy storage, drug deliver and biomedicines. Nanorods can be potentially used in laser, solar cells, and high sensitivity sensors and photonics. Nanostruced films can be sued in displays and high efficiency photovoltaic[ [15] ],[ [16] ],[ [17] ].For get a good general view of nanomaterial, the normal classification and commercial interest are shown in Table 2.1.

2.2. ZnO nanostructure

In this work, we will focus more on the ZnO-based nanostructures. In order to utilize the nanostructured ZnO materials for application, it is required that their morphology, crystalline structure, orientation, and surface modifications are well controllable and reproducible. A wide range of synthesized techniques have been used, such as solution chemical methods[ [18] ]，[ [19] ], [ [20] ], physical vapor deposition[ [21] ], [ [22] ], [ [23] ], metal–organic chemical vapor deposition (MOCVD) [ [24] ], [ [25] ], [ [26] ], molecular beam epitaxy (MBE)[ [27] ], pulsed laser deposition[ [28] ], [ [29] ] and sputtering[ [30] ].

2.2.1 Solution chemical methods

Lots of researchers have reported the controllable growth of highly oriented ZnO nanorods/wires by using aqueous solution. Normally a solution contains zinc ions (Zn2+) and a source of hydroxide ions (OH-) is used. The OH- will react with the Zn2+ to form ZnO. So several zinc salts can be used(i.e. zinc acetate dehydrate[ [31] ] , zinc chloride[ [32] ], etc.) to provide Zn2+ , as the OH- source which could also be generated by ammonia29, ammonium chloride and so on. So this gives more choice which is quite useful to reach reaction demands. The great advantage of aqueous solution method is the reaction condition: low temperature (below 100), cheap chemical agent and easier operation, in addition to the low cost and great potential for scale-up. However, the biggest problem is that lot of defects would produce in the crystal and the dimension is limited in the progress.

There are still a lot of groups are making efforts on improving the process. The longest ZnO nanorods arrays (20-50) can be easily obtained by using multilayer assembly process which has just been reported by C.k. Xu et al in 2012[ [33] ], they used polyethyleneimine(PEI) to hinder only the lateral growth of the nanowires in solution and repeatedly introduced to fresh solution bath in order to obtain long wires.

2.2.2 Vapor transport method

Vapor transport method means the process relay on the carrier gas and vapor, such as physical vapor deposition[ [34] ]，[ [35] ]，[ [36] ]. They have been considering one of the simplest and popular synthesis methods and they have been considered as great method for fabricating various nanostructures. For the growth process, it is critical to obtain Zn and oxygen or oxygen mixture vapor and then react with each other in this process. There are several ways for generating Zn and oxygen vapor. One is directly sublimating power source material (ZnO) at a very high temperature(1400)[ [37] ], and the subsequent deposition of the vapor in a certain temperature region to form desired nanostructures. Another method is using lower temperature, but need to mix ZnO powers with graphite powder as the source materials, which is called carbon-thermal evaporation[ [38] ]. By using this method the evaporation temperature can be decreased from 1400 to 800.

2.2.3Metal–organic chemical vapor deposition (MOCVD)

Metal–organic chemical vapor deposition (MOCVD) achieves growth of material via surface reaction of organic or metal-organic precursor compounds containing the required chemical elements, in which organometallic Zn compound[ [39] ]，[ [40] ]，[ [41] ], take diethy-zinc for example, which is used under appropriate oxygen or N2O flow[ [42] ]. The outstanding of this method is the growth temperature can be as low as 4000.

2.2.4 Pulsed laser deposition (PLD)

Pulsed laser deposition (PLD) is using a laser beam which focused through a vacuum onto the surface of a target material. At sufficiently high flux densities and short pulse durations, the target material is rapidly heated to its evaporation temperature and forms a vapor plume. Unlike thermal evaporation, where the vapor composition is dependent on the vapor pressures of the elements within the source material, laser ablation produces a plume of material with similar stoichiometry to that of the target material. Once the vapor plume has been formed, it is collected onto a cooler substrate that can promote nucleation and growth of nanostructures. But the cost is higher than others methods, due to the high power of the laser beam, the source materials could be sublimated at relatively low temperature.

Table 2-2 advantage and disadvantage of different method

Methods

Advantage

disadvantage

Solution chemical method

low cost, less hazardous, good quality and repeatability

Most of the process need operate manual and need operator knows chemistry

PVD

Could produce various kinds of nanostructured and high efficiency

Easily incorporate catalysts or impurities into nanostructures. less likely to be able to integrate with flexible organic substrates

MOCVD and MBE

High quality array

poor sample uniformity, low efficiency and little choices of substrate

PLD

High efficiency

Less controllability and repeatability compared with other techniques.

Comparatively speaking (as shown in Table 2-2), solution chemical method is one of the ideal synthesis methods for several reasons: low cost, less hazardous, and thus capable of easy scaling control[ [43] ], [ [44] ]. The whole growth process occurs at a relatively low temperature, which is quite good for flexible organic substrates. In addition, there is no need for the use of metal catalysts during the growth. This is very important for avoiding of impurity in order to be integrated with well-developed silicon technologies[ [45] ]. Moreover, there are a variety of parameters that can be used for effectively controlling the structures, morphologies and properties of the samples[ [46] ]，[ [47] ]. All of these advantages demonstrate the solution chemical method as a very versatile and powerful technique for growing 1D ZnO nanostructures. In this research work, we will use solution chemical method to grow ZnO nanostructures.

2.2. ZnO nanomaterial for solar cell applications

1D ZnO nanostructured materials have been found innovative applications in a variety of areas due to their unique and excellent chemical and physical functional properties, including high performance nano-sensors[ [48] ],[ [49] ],[ [50] ], solar cells[ [51] ],[ [52] ],[ [53] ],[ [54] ], piezoelectric generators[ [55] ],[ [56] ],[ [57] ], light-emitting diodes[ [58] ], field emission devices[ [59] ],[ [60] ],[ [61] ] and ultraviolet (UV) lasers[ [62] ],[ [63] ]. In the following, solar cell application using ZnO nanostructures which are closely related to my project will be discussed in details.

Silicon-based conventional solar cells dominate the solar energy industry for decades. Extensive studies of other inorganic and organic materials have been carried out for improving efficiency and reducing the cost per unit energy output[ [64] ]. ZnO nanowire/rod arrays are good candidates for solar cell applications because of three key reasons:

relatively high surface to volume ratio that enables interfacial charge separation and enhances the light absorption,

Fast electron transport along the crystalline nanowires that improves the charge collection efficiency,

Low reflectivity.

ZnO nanowire/rod arrays have been implemented for both conventional p–n junction solar cells and excitonic solar cells (including organic, hybrid organic–inorganic, dye-sensitized, and nanoparticle-sensitized solar cells). In the following, dye-sensitized solar cell (DSSC) based on ZnO nanostructures will be reviewed.

2.3 Dye sensitized solar cells

Since the 1st Dye Sensitized Solar Cells (DSSC) in terms of low cost materials and manufacturing was introduced by Michael Gratzel in 1991[ [65] ]. DSSC has been regarded as promising devices for energy applications because of the advantages of low cost, ease of manufacture and fabricated by low toxicity materials[ [66] ]. Classical structures of DSSC as shown in Fig.2-3.The heart of the DSSC device is the photoelectrode which is based on wide-band gap semiconductor (in the form of mesoporous nanocrystalline films) sensitized by a monolayer of dye molecules. Because the working principle is different form normal p-n junction cell, photoelectrode layer always require a large internal surface area, which ensures sufficient dye loading onto the photoelectrode and thus a good light harvesting efficiency for the solar cell. DSSCs performance in terms of open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and overall efficiency (η) is typically measured under AM 1.5 illuminations.

2.3.1 Metal oxide semiconductors for photoanode

Titanium oxide (TiO2) is the photoanode material of the first DSSC announced by Grätzel which got very good performance in 1991[63]. the improvement of DSSCs aims to enhance light absorption, light scattering, charge transport and improve interfacial charge transfer, while suppression of interfacial recombination is desired. The architecture of TiO2 for photoanode has been optimized to enhance the cell efficiency. The role of metal oxide semiconductor active layer is to provide adhesion surface to adsorb dye and acts a photoelectron acceptor. TiO2 active layer for light absorption is around 10 μm thick, and TiO2 nanoparticles should be small size (~20 nm) to provide large surface to volume ratio for photosensitizer adsorption. To give effective light scattering, 3 μm mesoporous scattering layer is coated on the active layer [ [67] ]. Lastly, an ultrathin TiO2 is coated on the entire structure by TiCl4 chemical bath deposition, which was proposed by Nazeeruddin et al[ [68] ]. The use of TiCl4 treatment modifies mesoporous TiO2 surface roughness which intensifies dye adsorption [ [69] , [70] , [71] ]. Today, lots of efforts have been on the TiO2 photoanode, but due to the disadvantage of TiO2─ (a) very low electron mobility which cause very high combination rate which is bad for efficiency improvement (b) expensive to produce nanostructures for further improvement of dye absorption mount per unit volume. (c) Normal route for synthesis anatase TiO2 is have, usually need post-treatment such as annealing.

So a lot of researchers have focused on using ZnO instead of TiO2 for photoanode. Because of ZnO has similar band gap and conduction band-edge to TiO2 anatase, as shown in Fig. 2.2. Furthermore, the superior electron mobility benefits electron transport, as compared with TiO2. The chemical stability, however, is not comparable to TiO2, since ZnO dissolves in both acidic and alkaline environments. Because of chemical instability in acidic medium, dye adsorption time should be preciously controlled, since common photosensitizers for instance N3 or N719 contain acidic carboxylic anchoring groups [ [72] ]. After long dye soaking time, the dissolution of ZnO takes place at the ZnO/photosensitizer interface, leading to formation of insoluble complexes with Zn2+ and photosensitizer[ [73] ]. The insoluble complexes may cause precipitation on the ZnO surfaces, and affect effective charge injection [ [74] ].

Ultimately the future design of photosensitizer for ZnO-based DSSCs should enable panchromatic light absorption while at the same time TiO2 modified layer will be made for

Fig.2.2 Band positions of several metal oxide semiconductors, ruthenium-based dyes and electrolytes. Notations of VB, CB, Eg, S and S* represent valence band, conduction band-edge, band gap, highest occupied molecular orbitals (HOMO) and lowest unoccupied molecular orbitals (LUMO), respectively[65,67]

improving stability and good dye combination. Detailed discussions regarding ZnO properties and TiO2 properties factors affecting ZnO-based DSSCs will be given in the next report.

2.3.2 Ruthenium-based photosensitizers

Thousands of photosensitizers have been extensively investigated and the future design for photosensitizers mainly focuses on panchromatic light absorption, strong anchoring group to bind the dye on the semiconductor surface, high excited state energy level compared with conduction band-edge, lower energy of the oxidized state than redox level and unfavorable dye aggregation.

Thousands of photosensitizers have been extensively investigated and the future design for photosensitizers mainly focuses on panchromatic light absorption, strong anchoring group to bind the dye on the semiconductor surface, high excited state energy level compared with conduction band-edge, lower energy of the oxidized state than redox level and unfavorable dye aggregation.

Anderson and coworkers first used Ru complexes with carboxylated bipyridine ligands for sensitization of TiO2 single crystals in 1979 [ [75] ], while similar Ru complexes with three carboxylated bipyridine ligands first applied in DSSCs in 1985 having incident photon-to-current conversion efficiency (IPCE) and overall efficiency of 44 % and 1.5 %, respectively [ [76] ]. Until 1991, O’Regan and Grätzel used trinuclear Ru complex[ [77] ], mesoporous TiO2 anode and organic solvent based electrolyte to achieve high efficiency of 7.1-7.9 % [66]. In particular, thiocyanato derivative ruthenium(II) dye (Y = SCN) , designated as N3, exhibits a broad visible light absorption, an elongation of IPCE spectrum to 800 nm, long excited state lifetime and intense adsorption on the semiconductor surface due to four carboxyl groups. (Bu4N)2[Ru(dcbpy)2(NCS)2, named as N719, improves power conversion efficiency. It is found that N3 has absorption maxima at 518 nm and 380nm, and the corresponding extinction coefficients are 1.3 and 1.33×104 M-1cm-1, respectively. While N719 has absorption maxima at 535nm and 395 nm and the extinction coefficients are 1.47 and 1.43×104 M-1cm-1, respectively[66]. A large number of novel dyes have been developed in recent years. Nevertheless, N3 and N719 remain most commonly used. Therefore, in this work N719 was chosen as the main dye for further study used.

2.3.4 Electrolyte redox couples

Electrolyte redox couple in DSSCs structure plays an important role to regenerate oxidized dye molecules. Redox species should possess fast regeneration and appropriate redox level with respect to HOMO level of dye as well as conduction band-edge of TiO2. Iodide/ triiodide () /) redox couple has widely used in DSSCs, since it has fast regeneration with oxidized dye molecules and slow recombination with photo injected electrons at TiO2 interface [ [78] ]. It is highlighted that the open circuit voltage strongly depends on the Fermi level of the semiconductor and redox level of the iodide/ triiodide. Although iodide/ triiodide redox couple benefits the regeneration of oxidized dye, the open circuit voltage is constrained below 800 mV [76] due to energy offset between Fermi level of TiO2 and redox potential, as shown in Fig 2.2. Although other redox couples for example cobalt (II/III) complexes [ [79] ] and bromide/tribromide[ [80] ] have been used in DSSCs as electrolyte, iodide/triiodide redox couple is the most commonly used, and hence it was used in this work.

In addition to the redox couples, additives in electrolyte strongly influence the redox potential, surface dye organization and conduction band-edge shift as well as surface blocking due to surface adsorption. The first additive, 4-tert-butylpyridine (t-BP), was applied in DSSCs by Grätzel and coworkers in 1993[63] and the open circuit voltage is significantly improved from 0.38 V to 0.72 V.

2.3.5 Counter electrodes

The counter electrode does not only complete the external circuit, but also regenerates oxidized form of redox couples by accepting electrons at the counter electrode interface. For example, iodide donates electrons to the oxidized dye molecules to form triiodide, while the triiodide undergoes reduction to return to iodide. Therefore, the counter electrode should possess good electrocatalytic properties to undergo reduction. Platinum counter electrode is chemically stable and exhibits a good charge transfer resistance, as compared with TCO [ [81] ]. In terms of charge transfer resistance, platinum coated counter electrodes immersed in iodide/ triiodide electrolyte have been studied by electrochemical impedance spectroscopy (EIS). Papa Georgiou et al [ [82] , [83] ] have reported that platinum clusters with the particle sizes up to 20 nm are formed by thermal decomposition on TCO at 380 and it exhibits a charge transfer resistance value of 0.07Ωcm 2 with 5 µg/cm2 platinum loadings. In this case, the counter electrode still remains transparent after very low platinum loadings. In addition, 450nm thick sputtered platinum layers on TCO substrate shows 0.05Ωcm2, as compared with 25Ωcm 2 for TCO substrate [ [84] ]. Other counter-electrodes, such as those based on carbon materials (nanotubes, grapheme, polymers, etc.) have also been demonstrated. However, platinum-coated electrodes typically result in high efficiency and hence they were used in this work. Other counter-electrodes, such as those based on carbon materials (nanotubes, graphene, polymers, etc.) have also been demonstrated. However, platinum-coated electrodes typically result in high efficiency and hence they were used in this work.

Fig.2.3 Classical structure and operating principle of DSSC [85] 

2.3.6 Working principle

As shown in Fig.2.3, the working process contains 7 parts: photo-excitation of dye molecules (Fig.2.3①, reaction as shown in E.q 2-1) which made electron transferred to the conduction band of the metal oxide semiconductor (Fig.2.3②, reaction as shown in E.q 2-2,normally the speed of this process is 1010~1012s-1). Then the electron goes through the metal oxide semiconductor (Fig.2.3.③, reaction as shown in E.q 2-3, normally the speed of this process is 100~103s-1 which is strongly depend on the electron mobility of the semiconductor). After the charge transfer, the excited state dye are then reduced by iodide anions I- while the iodide ions are oxidized to (Fig.2.3④, reaction as shown in E.q 2-4, normally the speed of this process is 108s-1).The .ions diffuses towards the counter-electrode, where the chemical reduction to iodide ion takes place (Fig.2.3⑤, reaction as shown in E.q 2-5, exchange speed of the current density is 10-2~10-1A/cm2).The electrons which are collected by the metal oxide semiconductor will go through the outer circuit then reach the counter electron. So an integrated circuit is formed. But in the whole circle there are two negative processes: the electrons which had been injected into the conduction band of the metal oxide will have an effect of recombination with oxide dye (Fig.2.3⑥, reaction as shown in E.q 2-6) or acceptors in electrolyte (Fig.2.3⑦, reaction as shown in E.q 2-7, exchange speed of the current density is 10-11~10-9A/cm2)

Excitation (E.q 2-1)

Injection (E.q 2-2)

Charge transport in semiconductor (E.q 2-3)

Regeneration (E.q 2-4)

Deoxidizing reaction (E.q 2-5)

+ Electron recaptures (dark reaction) (E.q 2-6)

Recombination (dark reaction) (E.q 2-7)

As mentioned above, the DSSC is one complex photocurrent conversion system. The whole process is accomplished by the different component with different functional properties. So obviously, for improving the efficiency of DSSC, there are several ways

Increasing the amount of dye molecular absorbed on the metal oxidation semiconductor surface

Improving the metal oxidation semiconductor properties to get better electron collection efficiency

Decreasing the two negative process which cause recombination. That leads to decrease the efficiency

Making novel dye to harvest more light to increase the DSSC performance

By modifying and improving the electrolyte to decrease recombination and improve DSSC performance

2.4 Summary

To date, replacing the mesoporous nanocrystalline films with 1D ZnO nanowires/rods or nanotubes[ [86] ],[ [87] ] is considered to be an effective way to solve this issue. This is because the ZnO nanostructures can afford a very large surface area and a direct conduction pathway for the photo generated electron. They also exhibit much higher electronic mobility (200-1000 cm2V-1S-1) than TiO2 nanostructures (0.1-4.0 cm2V-1S-1), which would be favorable for rapid electron transport in photoelectrode with reduced recombination loss [ [88] ],[ [89] ],[ [90] ]. Thus lots of efforts have been made for 1D ZnO nanostructure based DSSC investigation, and a brief summary is listed in Table 2-3. Although there are a lot research work in this field, the knowledge about the electrode layer, surface recombination or other surface effects as well as stability for the applications of ZnO nanostructures is still limited.

Table.2-3 Summary of Research on ZnO nanowire/rods DSSC

Photoelectrode

efficiency

Comment

Ref.

Hierarchical ZnO nanoflower

1.9%

The highest in ZnO DSSC

[ [91] ]

Hierarchical branched ZnO nanowires

1.5%

Exhibited an improvement compared to nanowires

[ [92] ]

Hierarchical ZnO nanowire- nanosheet

4.8%

This combination structure shows nearly twice as high as efficiency of nanosheet

[ [93] ]

ZnO/TSPcCu hybrid nanostructure

0.05% to 0.48%

Surface modified by other materials indicated new approach for enhancement efficiency

[ [94] ]

ZnO and TiO2 nanotubes

Increased from 1.2% to 2.1%

More and more modification method was introduced in 1D nanostructures for changing performance

[ [95] ]

Integrated ZnO nanotube

arrays

1.44%

Compare with modified sample, just using ZnO is hard to make a great improved just from structure improvement

[ [96] ]

Clearly, ZnO nanostructure is promising for the applications for DSSC. But the efficiency is quite slow. Even though ZnO has superior electron mobility and various nanostructures to enhance surface to volume ratio, the overall efficiency is still much lower than TiO2. Nowadays, the efficiency of ZnO-based DSSCs is ranging from 0.2 % to 7.5 % [ [97] , [98] ]. Effects of immersion time and dye concentration, formation of Zn2+-dye complexes and long-lived interface-bound charge separated pairs have been proposed to cause low efficiency.

So based on the reviewing previous researchers’ results, aims and objectives of this work had been decided:

Aims

Low temperature aqueous chemical solution growth techniques such as chemical bath deposition (CBD) and hydrothermal routes have been proved as one of the most efficient ways to produce ZnO-based crystal and highly orientated nanostructure arrays and other complex crystal nanostructures. As aforementioned, aqueous solution growth offers advantages of low consumable and facilities cost, uniform growth on large areas, wide range of applicable substrates and environmental friendliness. The best profile is the composition, structure and property of these nanostructures could be possibly modulated through fine control of the precursors in the aqueous solutions, such as reaction solution concentration, solution doping, pH value, temperature, duration time, etc. However, the chemical reactions involved in the growth processes are highly complicated. Little understanding of the crystal growth behaviours, furthermore, still challenges the successful fabrication of nanostructures with prefect orientation, controllable morphology, and applicable properties. This has so far stalled desirable applications in biological, electronic, optical, medical, and energy conversions areas.

In this work, the fabrication process will be focused on growth of ZnO nanostructures in neutral solution, which has not been studied systematically. The solution pH is one of the most important parameters in solution chemistry and crystallization. Lower pH can change the super saturation level, increase charge density and reduce interfacial energy. These will benefit the nucleation and growth of ZnO to occur with a relatively lower rate on substrates. And also provide better opportunities to open a new way to tailor the structural and properties of ZnO, such as desirable density, aspect ratio and diameter or controllable nanostructure morphology evolution.

Systematic study of using ZnO nanostructures as template to produce ZnO/TiO2 composite nanostructure is another task in this work. Purposely built ZnO/TiO2 composite with novel structure may offer an efficient approach to improve the chemical stability of ZnO, which is a shortage of ZnO nanostructures. The coupling effect between ZnO and TiO2 nanostructures may further enhance the performance of DSSC compared with that utilize only one of the two components.

The aims pursued in this work therefore are:

To fabricate nanostructured ZnO and novel composite structures with controllable morphology, orientation and dimension using low cost and temperature aqueous chemical solution growth methods.

To understand the mechanisms behind the nucleation and growth processes and the reaction pathways/kinetics involved in the development of ZnO nanostructures as well as ZnO-TiO2 composite structures;

To characterize the structural, chemical and physical properties of nanostructured ZnO and ZnO-TiO2 using advanced materials characterization techniques;

To increase the performance of DSSCs which are made of the as-grown nanostructured materials. Different geometry of ZnO/TiO2 hybrid nanostructures (super long NRs core shell structure, anabranches, nanowalls, etc.) will be made for getting higher amount absorption of dye per unit area.

Additionally, it is expected that a model of the equivalent circuit used for nanostructured ZnO/TiO2 hybrid DSSC will be developed that will investigate the charge transport and collection in details to improve the understanding of the influence of these factors on the overall cell performance.

Objectives

Growth of ZnO Nanostructures with Controlled Architectures

The ability to control the shape, dimension, morphology, and orientation of nanostructures as well as to fabricate large scale low-dimensional arrays and nanowalls complex architectures on various types of substrates is critical to the development of the next generation smart and functional materials. The first objective of this work is to fabricate ZnO based nanostructures with well-defined structural features by controlling the conditions of the templates/substrates, the type of precursors and the aqueous chemical solutions. Four types of ZnO nanostructures, nanorods (finished in 1st year), nanotubes, nanoflowers and complex nanowalls will be synthesized in this work.

Synthesis of ZnO/TiO2 core-shell structures as well as composite nanostructures

The second objective is the fabrication of ZnO/TiO2 core-shell structures and composite nanostructures using a multiple -steps process. These composite nanostructures will have significantly different physical and chemical properties in comparison with the single structure due to the combination effects and increased surface-to-value ratio. They are expected to exhibit improved performances in DSSC applications due to this method will get over the disadvantages of ZnO/TiO2 and get good usage of advantages.

Structure and Property Characterizations

Structural and functional analyses are the important part of this study and are the key to establish the correlation of processing, composition/structure and property/performance. The nanostructure and property characterization will provide fundamental information on (1) thermodynamics and kinetics of nucleation and growth of ZnO nanostructures as well as ZnO/TiO2 composited nanostructures; (2) design of novel materials with appropriate composition, morphology, texture, orientation and physical properties suitable for DSSCs applications.

DSSCs applications

Firstly, based on the materials and knowledge obtained from the first three stages, ZnO nanostructures grown on large surface areas will be used to assemble DSSC devices. Secondly, ZnO/TiO2 composited nanostructures will be applied in assembling DSSC devices using their distinguished physical and chemical characteristics. Finally, to get better understanding of the nanostructures effect on working electrode, electrochemical impedance spectroscopy (EIS) will be employed for investigating the interfacial charge transfer between working electrode and counter electrode. Further study of the equivalent circuit model for ZnO/TiO2 composited nanostructures will also carried out while compares the data with pure ZnO.

3 Experimental procedures and methodology

3.1 Materials and Reagents

In this work, unless otherwise stated, all chemicals and solvents used were at least reagent grade, purchased from Sigma-Aldrich Company and were used without further purification. Details of the chemicals and solvents are listed in Table 3-1.

Table.3-1 Reagents and solvent used in the work

Reagents

Standard

zinc nitrate hexahydrate

99.0%

ammonium hydroxide

28wt % NH3 in water, 99.99%

hexamethylenetetra-mine

99.0%

deionized water

resistivity higher than 18.2 MΩcm(prepared by UWS chemical department)

Ti3O5

99.99%

LiI

99.9%

Iodine

99.99%

4-tert-butylpyridine

96%

Tetrabutylammonium iodide

99.0%

Acetonitrile

99.9%

3.2 Synthesize procedures

In this section, different synthesis instruments and characterization techniques were introduced for investigation of material structure and optical properties. Hydro-thermal chemical method and e-beam evaporation were used for preparing different nanostructured material. Magnetron sputtering was used to deposit ZnO seed layer.

3.2.1 Synthesis of nanomaterial

(1) Substance cleaning

For the preparation of the nanostructured material grown on the substrate, the requirements of the surface quality are very strict. This is mainly because that any defects (i.e. metal tarnished, organic tarnished, tarnished dust, etc.) on the surface of the substrate will cause bad effects on the whole growth process. Therefore, it will affect the quality and performance of the device. Accordingly, the cleaning of the substrate becomes one of the critical steps of nanomaterial fabrication.

The details of substrate cleaning process as shown below: Firstly, the substances (including glass slides and silica) were placed in a detergent-containing solution and ultrasonically cleaned for 30 minutes, and then the organic solvent of chloroform and acetone was used for ultrasonic cleaning of 30 minutes, respectively. Secondly, the pre-cleaned substances were put in deionized water, ultrasonic cleaning for 30 minutes. Finally all the samples were dried in a small venting chamber and saved in special slice box with a plastic seal bar.

(2) Seed layer and metal layer preparation

All the ZnO seed layers for nanorods growth were deposited using DC magnetron sputter (Nodiko) at a Zn target power of 400W. Gas flow rates for ZnO deposition was Ar/O2 =35sccm/35sccm. The thickness of the ZnO seed layers were 25nm and 50 nm. For the Zn seed layer which was also prepared by using this system at a Zn target power of 400W. Gas for Zn deposition was Ar only with a gas flow rate at 30. The thickness of the Zn seed layer was 50nm.

(3) Thermal aqueous solution method for nanostructured material

As described in the second chapter, the thermal aqueous solution method is one of the commonly used methods for the preparation of nanomaterials. The critical parameter in this method is the temperature of water bath, reaction time and concentration of the solution. In this work, a thermal water bath which contains heater and stirring were used for heating the reaction vessel. For the preparation of the growth process, firstly the substance is placed inside a glass container together with reaction solution and covered with a cap. Before start the growth, the temperature of the water bath was set to a designed temperature, and then the reaction vessel was put into the water bath. After the reaction, samples were removed and placed in deionized water for cleaning, then put in the oven to dry. If multiple steps of growth are needed, the above steps will be repeated.

To grow ZnO NRs using hydro-thermo method, there are generally two ways to use. The first one is alkali solution method. Transparent aqueous solution was prepared by dissolving a calculated amount of Zn(NO3)2·6H2O into 30 mL of deionized water to make solutions varied from 0.02 to 0.08M. Ammonia water was used as a pH adjustment chemical. The amount of the added ammonia water (normally 0.5-3 mL) was dependent on continuing pH detection. Hydrothermal process was carried out by suspending the substances upside down in a reaction vessel filled with the transparent aqueous solution. The reaction vessel was heated to a temperature of 45-90oC by using thermal water bath.

For another method, HMTA solution method which still has a lot of uncurtains in the growth process worth make more efforts on it. In this method, transparent aqueous solutions of 0.1M-0.01M containing Zn(NO3)2·6H2O and HMTA (Zn(NO3)2: HMTA=1:1) was made at room temperature, respectively. The organic chemical was introduced in this method is aiming for producing ZnO nanorods on a big range of pH value. Combination with alkali solution method, nearly all kinds of substances could be used. However, the growth mechanism of ZnO nanostructure in this kind of organic solution is not fully understood yet and the development of a simple and controllable solution method is still required. Therefore, in this work, a series of experiments had been carried out to find the optimized parameters for applications. The key parameters which will affect the growth process include temperature, growth time, Zn2+ concentration, pH and seed layer morphology. To reveal and understand the effects of these parameters, a series of experiments at various conditions were carried out:

The Zn2+ concentrations were adjusted from 0.01 to 0.1 M.

The growth temperatures were controlled from 45 to 95 °C.

The growth durations were selected in the range of 1 to 6 h,

Multiple-steps route was used for long rods.

Fig.3.1 Schematic process of the hydrothermal growth for the fabrication of ZnO nanorods [ [99] ].

In this work, Si (100) wafer, flexible polymer, silica and quartz slices with/without ITO were chosen as the standard substrates. Si substrates were mainly used to investigate the effects of various experimental parameters on the growth of the ZnO NRs. Effects of different substrate on the ZnO NRs growth were then investigated. All the polymer/quartz slices were cleaned by a standard cleaning progress as explained before.

The alkali solution method (details mentioned in Chapter 3) which has been studied for many years. The parameters are very reliable. In this work, we firstly synthesized ZnO NRs based on those reported from the previous researchers, with an SEM image shown in Fig4-1(a) which shows a great agreement with previous researchers work Fig4-1(b)[ [100] ]. However, we did not do much work using this method because it has been well studied for years. The parameters are very reliable. Not necessary to do much work on it for further study. Furthermore, the PH value is too high during the growth process which is not suitable for further modification and shape control.

1um

1um

(a) (b)

Fig.4-1 Nanorods prepared via alkali solution method (a)as prepared nanorods in our group using same parameters with Fig4-1(b); (b)previous researchers’ results[1]

For another method, HMTA solution method which still has a lot of uncurtains in the growth process worth make more efforts on it. In this method, transparent aqueous solutions of 0.1M-0.02M containing Zn(NO3)2·6H2O and HMTA (details as shown in Chapter 3) was made at room temperature, respectively. The organic chemical was introduced in this method is aiming for producing ZnO nanorods on a big range of pH value. Combination with alkali solution method, nearly all kinds of substances could be used. However, the growth mechanism of ZnO nanostructure in this kind of organic solution is not fully understood yet and the development of a simple and controllable solution method is still required. Therefore, in this work, a series of experiments had been carried out to find the optimized parameters for DSSC applications. The key parameters which will affect the growth process include temperature, growth time, Zn2+ concentration and seed layer morphology. To reveal and understand the effects of these, serious of experiments have been carried out.

(4) Thermal evaporation method for modified layer

Thermal evaporation method is a common method for growth of thin films. In this work, we used thermal evaporation method to prepare enhanced nanostructure layer to improve the performance of ZnO nanostructured material. The apparatus used in the experiment is an electron beam evaporation system (SATIS MS LAB 380 series). The system has two electron beam (e-beam) sources and a sweep generator. Various deposition conditions could be adjusted by using the effects of O2 gas flow rate and plasma conditions which would have enhanced effects on the structure and functional properties o