ABSTRACT

Mesoporous nanostructures of zinc oxide (ZnO) were successfully synthesized by chemical bath deposition (CBD) technique and used to fabricate photoelectrochemical (PEC) solar cells. The synthesis proceeded in an alkaline bath of an aqueous solution of 0.1 M Zn(NO₃)₂.6H₂O at a bath temperature of 353 K and pH of 11.5 on microscope glass slide and stainless steel slide substrates. The ZnO was doped with Al and Cu at varying concentrations from 1-5 at. %. As-deposited films were annealed at 673 K for 2 h.  The synthesized ZnO thin films had thickness in the range of 2.03-10.43 µm. Crystal structure studies revealed that all the ZnO thin films were polycrystalline with hexagonal wurtzite structure and preferential growth in the 002 crystal plane. Crystallite sizes in the range of 8-29 nm were obtained along the 002 crystal plane and the crystallinity of the doped samples was strongly affected by the concentrations of the dopants. Surface morphological studies indicated that the synthesized ZnO thin films had nanoflakes, nanodendrites and nanorods morphologies which confirmed the effects of surfactant, dopants and annealing on ZnO. Optical studies revealed that all the films had low absorbance in the visible region of the solar spectrum with transmittance ranging from 42-90 %.  Energy band gaps of the undoped and doped ZnO were found to decrease from 3.03 eV to 2.70 eV. All the measured optical properties of the ZnO thin films showed strong dependence on the concentration of the dopants. Surface wettability studies confirmed that all the synthesized ZnO thin films were porous (hydrophilic), giving water contact angles in the range of 0^o to 71.3^o. For the first time in literature, this synthesized ZnO thin films were further sensitized with Indigofera arrecta plant dye and also Rhodamine 6G to develop/fabricate dye-sensitized solar cells (DSSCs).  The fabricated PEC solar cells produced short circuit current (I_SC) of 12.34 μA/cm² and open circuit voltage (V_OC) of 388 V for unsensitized undoped ZnO electrode giving power conversion efficiencies (η) of 0.003 and fill factor (FF) of 0.43. Unsensitized aluminum doped zinc oxide (AZO) electrode yielded I_SC in the range of 21 μA/cm² to 29 μA/cm², V_OC in the range of 333 mV to 641 mV, η in the range 0.0037 % to 0.01 % and FF in the range of 0.34 to 0.43. While unsensitized copper doped zinc oxide (CZO) electrodes produced I_SC in the range of 16 μA/cm² to 98 μA/cm² and V_OC in the rage of 774 mV to 796 mV, giving η in the range of 0.0009 % to 0.062 % and FF in the range of 0.06 to 0.63. Further upon dye-sensitization of the ZnO electrodes with Rhodamine 6G and Indigofera arrecta plant dye respectively, the PEC solar cells of undoped ZnO produced I_SC of 0.24 mA/cm² and V_OC of 360mV for Rhodamine 6G and I_SC of 0.29 mA/cm² and V_OC of 595 mV for Indigofera arrecta plant dye.  These produced η of 0.11 % and FF of 0.45 for Rhodamine 6G and η of 0.16 % and FF of 0.44 for Indigofera arrecta plant dye. AZO electrodes produced I_SC in the ranges of 0.3 mA/cm² to 0.4 mA/cm² and V_OC in the ranges 376 mV to 515 mV using Rhodamine 6G and I_SC in the range of 0.84 mA/cm² to 1.35 mA/cm² and V_OC in the range of 596 mV to 664 mV for Indigofera arrecta plant dye. These yielded η in the range of 0.16 % to 0.22 % and FF in the range of 0.40 to 0.49 for Rhodamine 6G and η in the range of 0.29 % to 0.51 % and FF in the range of 0.42 to 0.47 using Indigofera arrecta plant dye. On the other hand, CZO electrodes produced I_SC in the range of 0.49 mA/cm² to 0.97 mA/cm² and V_OC in the range of 355 mV to 473 mV for Rhodamine 6G and I_SC in the range of 0.91 mA/cm² to 6.8 mA/cm² and V_OC in the range of 656 mV to 914 mV using Indigofera arrecta plant dye.  These yielded η in the range of 0.18 % to 0.39 % and FF in the range of 0.39 to 0.46 using Rhodamine 6G and η in the range of 0.40 % to 4.16 % and FF in the range of 0.42 to 0.54 for Indigofera arrecta plant dye. The η of 4.16 % obtained in this work using Indigofera arrecta plant dye is the highest ever obtained using natural dyes from plants as found in the available literature.  Electrochemical impedance spectra of all the PEC solar cells shows impedance variations that agrees with the I_SC and V_OC results for all the cells as stated above.

ACKNOWLEDGEMENT.

When a man undertakes a journey, he has the good will of both God and men. When he returns, he brings with him great thankfulness for the grace and mercy of God and for the good wishes of men. My journey could not have been easily gotten this far if God had not granted it. I have indeed enjoyed special favours from God during the whole period of this work and so I glorify His name.

There are men who are akin to God in affairs of man. Their blessing, advice, assistance and challenging personalities are a source of strength, a sense of direction and dedication to duty and opens up a world of insatiable aspiration for us. Their lives provide us with the inspirational model in the process of struggling for self-actualization.

I am particularly grateful to my supervisors, Professor R.U. Osuji and Dr. F.I. Ezema for their relentless efforts, their great concern for the success of this research work, their tolerating patience and their commitment to academic excellence. They have gone extra miles from supervisor-student relationship to ensure excellence in the work. I owe a lot of appreciation to Professor C.D. Lokhande, coordinator of Thin film Laboratory, Department of Physics, Shivaji University, Klohapur, India and the entire students of the Department for their immense contributions to the success of this work. Professor C.D. Lokhande offered me his laboratory, materials and guidance for the experimental part of this work; above all, their overwhelming hospitality kept me comfortable during my stay in Kolhapur, India.

I am especially indebted to my wife, Mrs. Catherine Tyona and my children, Master Joshua Keghtor, Ms. Happiness Nguvan, Ms. Joy Mimi and Master Caleb Tavershima Tyona and also my Cousin, Ms. Msuur Nyiter. They have endured patiently all the hardships that besiege the family as a result of this work: my consistent absence and financial short fall amongst many others. My gratitude goes to the entire family of Late Evangelist Tyona Kange: Mr. Samuel Kange, Mr. Terkimbi Tyona just to mention few, for their persistent prayer, moral support and otherwise throughout the course of this work.

I would not forget the good relationship I enjoyed with the staff and postgraduate students of the Department of Physics and Astronomy, University of Nigeria, Nsukka. I really appreciate my friends who helped in the course of this work: Mr. James Ezema, Dr. S.B. Jambure, Dr. Ravindra Bulakhe and Dr. Nana Shinde just to mention few.

You have all made this journey a bearable one. God bless you all.

CHAPTER ONE

INTRODUCTION AND THEORETICAL BACKGROUND

1.1.        NANOMATERIALS

1.1.1.     General Introduction to Nanomaterials

Although nanotechnology is a relatively recent development in scientific research, the development of its central concepts happened over a longer period of time. The emergence of nanotechnology in the 1980s was caused by the convergence of experimental advances such as in the invention of the scanning tunneling microscope (STM) in 1981 by Gerd Binnig and Heinrich Rohrer at IBM Zurich Research Laboratory, for which they received the Nobel Prize in Physics in 1986. Harry Kroto, Richard Smalley, and Robert Curl, discovered fullerenes in 1985, who together won the 1996 Nobel Prize in Chemistry (Cembrero et al. 2004).

Around the same time, K. Eric Drexler developed and popularized the concept of nanotechnology and founded the field of molecular nanotechnology. In 1979, Drexler encountered Richard Feynman’s 1959 talk, “There’s Plenty of Room at the Bottom”. The term “nanotechnology”, originally coined by Norio Taniguchi in 1974, was unknowingly appropriated by Drexler in his 1986 book, “Engines of Creation: The Coming Era of Nanotechnology”, which proposed the idea of a nanoscale “assembler” which would be able to build a copy of itself and of other items of arbitrary complexity. He also first published the term “grey goo” to describe what might happen if a hypothetical self-replicating molecular nanotechnology went out of control. Drexler’s vision of nanotechnology is often called “Molecular Nanotechnology” (MNT) or “molecular manufacturing,” and Drexler at one point proposed the term “zettatech” which never became popular (The Institute of Nanotechnology, 2006; Allhoff et al. 2010).

In the early 2000s, the field was subject to growing public awareness and controversy, with prominent debates about both its potential implications, exemplified by the Royal Society’s report on nanotechnology, as well as the feasibility of the applications envisioned by advocates of molecular nanotechnology, which culminated in the public debate between Eric Drexler and Richard Smalley in 2001 and 2003 (Wang et al. 2006). Governments moved to promote and fund research into nanotechnology with programs such as the National Nanotechnology Initiative.

The early 200s also saw the beginnings of commercial applications of nnotechnology, although these were limited to bulk applications of nanomaterials, such as the Silver Nano platform for using silver nanoparticles as an antibacterial agent, nanoparticle-based transparent sunscreens, and carbon nanotubes for stain-resistant textiles (The Institute of Nanotechnology, 2006; Porter et al. 2007).

Nanotechnology is the engineering of functional systems at the molecular scale. This covers both current work and concepts that are more advanced. In its original sense, nanotechnology refers to the projected ability to construct items from the bottom up, using techniques and tools being developed today to make complete, high performance products (Singh et al. 2004; Kostoff et al. 2007).

One nanometer (nm) is one billionth, or 10−9 of a meter. By comparison, typical carbon-carbon bond lengths, or the spacing between these atoms in a molecule, are in the range 0.12–0.15 nm, and a DNA double-helix has a diameter around 2 nm. On the other hand, the smallest cellular life-forms, the bacteria of the genus Mycoplasma, are around 200 nm in length. By convention, nanotechnology is taken as the scale range from 1 to 100 nm following the definition used by the National Nanotechnology Initiative in the US. The lower limit is set by the size of atoms (hydrogen has the smallest atoms, which are approximately a quarter of a nm diameter) since nanotechnology must build its devices from atoms and molecules. The upper limit is more or less arbitrary but is around the size that phenomena not observed in larger structures start to become apparent and can be made use of in the nano device. These new phenomena make nanotechnology distinct from devices which are merely miniaturised versions of an equivalent macroscopic device such as integrated circuits; such devices are on a larger scale and come under the description of microtechnology (Singh et al. 2004; Konenkamp et al. 2002).

Two main approaches are used in nanotechnology, “bottom-up” and “top-down”. In the bottom-up approach, materials and devices are built from molecular components which assemble themselves chemically by principles of molecular recognition. In the top-down approach, nano-objects are constructed from larger entities without atomic-level control (Wang et al. 2006; Porter et al 2007).

Areas of Physics such as nanoelectronics, nanomechanics, nanophotonics and nanoionics have evolved during the last few decades to provide a basic scientific foundation of nanotechnology. A number of physical phenomena become pronounced as the size of the system decreases. These include statistical mechanical effects, as well as quantum mechanical effects, for example the “quantum size effect” where the electronic properties of solids are altered with great reductions in particle size (Cavalcanti et al. 2007). This effect does not come into play by going from macro to micro dimensions. However, quantum effects become dominant when the nanometer size range is reached, typically at distances of 100 nanometers or less, the so called quantum realm. Additionally, a number of physical (mechanical, electrical, optical, etc.) properties change when compared to macroscopic systems. One example is the increase in surface area to volume ratio altering mechanical, thermal and catalytic properties of materials. Diffusion and reactions at nanoscale, nanostructured materials and nanodevices with fast ion transport are generally referred to as nanoionics. Mechanical properties of nanosystems are of interest in the nanomechanics research. The catalytic activity of nanomaterials also opens potential risks in their interaction with biomaterials (Singh et al. 2004; Cavalcanti et al. 2007).

Materials reduced to the nanoscale can show different properties compared to what they exhibit on a macroscale, enabling unique applications. For instance, opaque substances become transparent, example copper; stable materials turn combustible, example aluminum; insoluble materials become soluble, example gold. A material such as gold, which is chemically inert at normal scales, can serve as a potent chemical catalyst at nanoscales. Much of the fascination with nanotechnology stems from these quantum and surface phenomena that matter exhibits at the nanoscale (Cenbrero et al. 2004). Modern synthetic chemistry has reached the point where it is possible to prepare small molecules to almost any structure. These methods are used today to manufacture a wide variety of useful chemicals such as pharmaceuticals or commercial polymers. This ability raises the question of extending this kind of control to the next-larger level, seeking methods to assemble these single molecules into supramolecular assemblies consisting of many molecules arranged in a well defined manner Wang et al. 2006). Nanotechnology is one of the frontier areas of science due to its versatile application in various fields. Nanomaterials find applications in fields such as miniaturization in electronics, catalysis, optics, biological and in the solar energy sector. In addition, nanomaterials yield the next generation computer chips, better insulation materials, tougher and harder nanomaterials cutting tools, elimination of pollutants, high energy batteries, efficient solar cells, high power magnets, high-sensitivity sensors, automobiles with greater fuel efficiency, aerospace components with enhanced performance (Guo et al. 2010).

 

1.1.2.     Why Nanomaterials?

Materials reduced to the nanoscale can show different properties compared to what they exhibit on a macroscale, enabling unique applications (Allhoff et al. 2010). Two principal factors are responsible for such significant difference in properties of nanoscale materials from other materials: increased relative surface area and quantum effects. These factors can affect or enhance reactivity, strength and electrical characteristics (Allhoff et al. 2010).

Increase in surface area to volume ratio:

Materials at nanoscale has relatively larger surface area when compared to the same volume (or mass) of the material produced in a large form. Consider a sphere of radius r, the surface area and volume of the sphere are given by Eq. (1.1) and (1.2) below:

(1.1)

(1.2)

The ratio of the surface area to the volume will yield

(1.3)

Eq. (1.3) indicates an increase in surface to volume ratio with decrease in size of material. Thus, when there is a decrease in radius of a sphere, its surface area to volume area ratio increases (Allhoff et al. 2010; Singh et al. 2004).

Quantum confinement effect:

Contemporary literature on ultradisperse semiconductor distinguishes between the effects arising as a result of increase in surface area and the degree of surface imperfection with decrease in the size of the crystals. Quantum size effects due to radical change in the electronic state of the semiconductor (Fig. 1.1) crystals less than a certain “critical” size, determined in turn by the extent to which the electron-hole pair photogenerated in the semiconductor is delocalized (Allhoff et al. 2010; Singh et al. 2004). These effects also differ in the range of sizes in which they appear. Size effects are observed in semiconductor crystals measuring 10 – 100 nm, whereas quantum size effects are usually characteristic of nanocrystallites measuring less than 10 nm. In the literature, semiconductor nanoparticles in which quantum size effect of one type or another appear, are often called quantum size particles or quantum points in order to

Fig. 1.1. Dependence of quantum-sized semiconductor’s band gap on particle size (Wang, 2004).

emphasize their special electronic structure (Allhoff et al. 2010; Singh et al. 2004). It is necessary to mention the tentative nature of the size in so far as the exact “critical” size after which the appearance of quantum size effects can be expected is largely determined by the chemical nature of the semiconductor and can vary from 0.5 (CuCl) to 46 (PbSe) or more nanometers.

From the positions of quantum mechanics, the “critical” size (the threshold for the appearance of quantum size effects) corresponds to the De Broglie wavelength of the free electron. During the analysis of the interband absorption of the semiconductor nanoparticle, the Bohr radius of the exciton (aB), which can be calculated from the electrophysical constants of the bulk semiconductor, can be used as such a criterion. The data that have accumulated on size effects in semiconductor nanoparticles make it possible to examine them according to the nature of the effect on the properties of the nanocrystals. Restriction of the free motion of the exciton in the bulk of the nanocrystal leads to an increase of its energy   in relation to the volume (bulk) semiconductor  (Guo et al. 2010) The increase in the energy of the exciton as a result of the quantum size effect  can be calculated in the approximation of the effective masses, which is based on assumptions about the parabolic nature of the permitted energy band close to their edges and the invariability of the effective masses of the electron of the conduction band  and the hole of the valence band  in the transition from the bulk to the ultradisperse semiconductors. The approximation gives the following expression for ∆E (Guo et al. 2010):

(1.4)

where ћ is the Plank’s constant,   and   are the effective masses of electron and hole respectively, R is the atomic radius and   is the Rydberg energy.

In the above Eq. (1.4), the first term, which depends on R2, corresponds to the increase in the energy of the exciton as a result of its spatial restriction in the potential box-the semiconductor nanocrystal. The second term determines the energy of coulombic interaction of the electron and the hole in the composition of the exciton and increases with decrease in the size of the nanoparticles. The value of   in the third term is called the Rydberg energy of the exciton and takes account of the correlation between the motion of the electron and the hole. As seen, the last two terms of the above Eq. (1.4) lead to a decrease in the energy of the exciton, which is restricted in the volume of the particle (Guo et al. 2010).

 

1.2.        ZINC OXIDE (ZnO)

1.2.1.     Introduction to Zinc Oxide (ZnO)

There are three one-dimensional (1 D) nanostrauctures that are most actively studied in nanotechnology, these include: carbon nanotubes, Silicon nanowires and ZnO nanowires/nanorods. ZnO is a key technological material. The lack of a centre of symmetry in the wurtzite structure, combined with large electromechanical coupling, results in strong piezoelectric and pyroelectric properties and the consequent use of ZnO in mechanical actuators and piezoelectric sensors. In addition, ZnO is a wide band-gap (3.37 eV) compound semiconductor that is suitable for short wavelength optoelectronic applications. The high exciton binding energy (60meV) in ZnO crystal can ensure efficient excitonic emission at room temperature. Also, room temperature ultraviolet (UV) luminescence has been reported in disordered nanoparticles and thin films. ZnO is transparent to visible light and can be made highly conductive by doping (Wang, 2004).

ZnO is a versatile functional material that has a diverse group of growth morphologies, such as nanocombs, nanorings, nanohelixes/nanosprings, nanobelts, nanowires and nanocages. Its nanostructures have a wide range of high technology applications such as surface acoustic wave filters, photonic crystals, photodetectors, light emitting diodes, photodiodes, gas sensors, optical modulator waveguide, solar cells and varistors (Wang, 2004).

Different nanostructures of ZnO have been reported (Fig. 1.2). ZnO films have also been synthesised by several techniques such as molecular beam epitaxy, RF magnetron sputtering, chemical vapour deposition, physical vapour deposition, spray pyrolysis, electrodedeposition, aqueous chemical method, chemical bath deposition (CBD), successive ionic layer adsorption and reaction (SILAR) and sol-gel spin coating. However, the last three methods present several advantages such as: inexpensive, easy procedure, possibility of large-scale deposition, low temperature growth and direct control over the film morphology (Zhiyong and Jia, 2005; Ezema and Osuji, 2007; Ezugwu et al. 2007).

CBD and SILAR greatly facilitate to grow 1D ZnO nanostructures by the thermal decomposition of hexamethylenetetramine (HMTA) and zinc acetate. A seed layer of ZnO nanoparticle grown on the substrate is used to initiate the growth of the nanostructure on the substrate. HMTA is a highly water soluble, non-ionic tetradentate cylic tertiary amine. Thermal degradation of HMTA releases hydroxyl ions which react with Zn2+ ions to form ZnO as follows:

(CH2)6N4 + 6H2O  6HCHO + 4NH3

 

 

One of the most attracting applications of ZnO is in dye-sensitized solar cells (DSSCs), offering an inexpensive production and relatively high energy conversion efficiency (Choi et al. 2011). Porous titanium dioxide (TiO2) was known to be the commonly used semiconductor anodes with ruthenium complexes as the mostly used sensitizer. However, it is difficult to grow TiO2 anisotropically to obtain hierarchical and ordered structures (Park and Kim, 2008). ZnO has several advantages over TiO2: it can be easily grown into a large number of nanostructures, it exhibits higher electron mobility than TiO2 (200 cm2 V-1s-1 for ZnO as against 155 cm2V-1s-1 for TiO2). Debye Huckel screening length in ZnO is about 4 nm for a carrier concentration of 1018 cm-3 and is very stable against photo corrosion; it has high excitonic stability and it is an

 

Fig 1.2. Different nanostructures of ZnO: (a) nanorings (b) nanobelts (c) nanocombs (d) nanowires (e) nanohelixes (f) nanorods (g) nanoparticles (h) nanoflowers (i) nanotubes (Wang, 2004; Zhiyong and Jia, 2005).

 

environmental friendly material (Zhiyong and Jia, 2005; Choi et al.2011; Suzuki et al. 2006; Kong et al. 2004). These properties make ZnO more desirable in DSSCs application. In the growth process of ZnO nanostructures for DSSCs, it is important to use organic surfactants (structure directing agents) in the CBD and SILAR methods due to their direct effect on morphological and structural properties which consequently affect the performance of dye-sensitized solar cells. The surfactant molecules bind to the different ZnO surfaces and affect crystal growth (Ezema and Osuji, 2007; Ezugwu et al. 2007).

1.2.2.     Why ZnO Nanostructures?

The discovery of carbon nanotubes rapidly stimulated interest in 1D materials with various morphologies such as wires, rods, ring, belts etc, due to their great potential or fundamental studies of the effects of morphology, dimensionality and size on their physical and chemical properties (Zhiyong and Jia, 2005). 1D nanostructures such as nanowires and nanorods are ideal systems for investigating the dependence of the electrical and mechanical properties on size and dimensionality. They are expected to play an important role as both interconnect and functional components in the fabrication of nanoscale electronic and optoelectronic devices. Many unique and fascinating properties have already been proposed or demonstrated for this class of materials, such as superior mechanical toughness, higher luminescence efficiency, enhancement of thermoelectric figure of merit and a lower lasing threshold (Zhiyong and Jia, 2005; Choi et al. 2011).

In the old generations of photoelectrochemeical (PEC) solar cells, photoelectrodes were made from bulky semiconductor materials such as Si, GaAs or CdS. However, these kinds of photoelectrodes when exposed to light they undergo photocorrosion that results in poor stability of the PEC solar cell. The use of sensitized wide bandgap semiconductors such as TiO2, or ZnO resulted in high chemical stability of the cell due to their resistance to photocorrosion (Kostoff et al. 2007; Khalil, 2011). The problem with bulky single or poly-crystalline wide bandgap is the low light to current conversion efficiency mainly due to inadequate adsorption of sensitizer because of limited surface area of the electrode. One approach to enhance light-harvesting efficiency (LHE) and hence the light to current conversion efficiency is to increase surface area (the roughness factor) of the sensitized photoelectrode. Due to the remarkable changes in mechanical, electrical, magnetic, optical and chemical properties of nanostructured materials compared to its phase in bulk structures, it received considerable attention (Wang et al. 2006). Moreover, because the area occupied by one dye molecule is much larger than its optical cross section for light capture, the absorption of light by a monolayer of dye is insubstantial. It has been confirmed that high photovoltaic efficiency cannot be achieved with the use of a flat layer of semiconductor or wide bandgap semiconductor oxide surface but rather by use of nanostructured layer of very high roughness factor (surface area). Therefore, bulky layer of ZnO were replaced with nanoporous ZnO layer as a photoelectrode (Khalil, 2011).

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