Hydrothermal synthesis is one of the numerous ways of producing ZnO nanoparticles. This research examines the use of this method for the production of ZnO nanorods. The method was found to have the advantage of simplicity and environmental friendliness over other ZnO nanorods production routes. To further enhance the suitability of the method for commercial purposes, synthesis was performed without the use of growth-assistive catalysts. Variation of the nanorods sizes was achieved by autoclaving at different temperatures and for different reaction durations. Other factors varied in the reaction process included initial concentration of the reagents and stirring.
Hydrothermal synthesis shows great promise for large scale economic synthesis of ZnO nanorods. Process conditions for optimal production of the nanorods have also been identified in the report.
CHAPTER ONE INTRODUCTION
In the currently evolving field of nanomaterials, innovations continually exploit the ability of nanoparticles to exhibit characteristics remarkably different from their macro/bulk properties (Antony, 2010). The market for nanoparticles has been projected to grow at a compound average annual growth (CAGR) of about 30% (BCC research, 2007) to well over $1.7 billion by 2012 (Dagliden, 2008).
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With so many uses being discovered for nanoparticles in the energy, cosmetics, catalysis and structural applications sectors, research into cheaper and better means of production can never be more justified (BCC research, 2007, Dagliden, 2008). BCC research (2007) estimates that nanoparticles would fetch close to $ 357.3 million in catalytic applications and about $ 604.2 million in energy related uses by the year 2012.
Zinc Oxide also called Zincite possesses the wurtzite crystal structure (Elliot, 2008). ZnO has turned out to be a very useful component in the cosmetics production sector as it is transparent to white light but traps Ultraviolet (UV) rays (Elen, et al. 2009). This property is largely exploited in the production of sunscreens, anti-aging creams, fillers and paints etc (Li, et al. 1998, TGA, 2010).
However, the beauty industry is not where Zincite nanoparticles are mostly employed. Research by BCC research (2007) and Dagliden (2008), indicate that nanoparticles generally are heavily in use in the electronics industry. Properties of ZnO nanoparticles such as a wide band gap of 3.37 eV at room temperature (Jeong, et al. 2007), and an exciton binding energy of 60meV (Ni, et al. 2005) make it suitable for use in wide and direct band gap semiconductors. In addition, ZnO's wurtzite structure endows it with piezo-electric and pyro-electric properties. These properties are exploited in the production of piezo-electric transducers, gas sensors, acoustic mechanisms, conductive films, solar conversion devices, etc (NI, et al. 2005).
Another advantage of zinc as a source of nanoceramics is that it possesses just two oxidation states and is not easily hydrolyzed (Chittofrati and Matijevic, 1989; IZA, 2011), hence eliminates formation of multiple intermediates during reaction. This proves very significant for commercial endeavours in terms of reduced cost for purification or removal of unwanted by-products and also cost of instrumentation and analysis
Zincite nanocrystals come in different forms and shapes. Commonly reported morphologies include; primastic, bi-pyramidal, dumbbell-shaped, ellipsoidal, nanorods, tetrapods, nanoplates, nanowires, nanowhiskers, nanospherical, microrods, microspheres, star shaped, etc. They could be produced as dispersed primary particles and in other cases intertwined or agglomerated units (Ni, et al. 2005; Kuo, et al. 2005).
Due to the popularity of ZnO nanoparticles, quite a lot of research work has gone into its production process. There are over 20 pathways of synthesizing or fabricating ZnO nanoparticles depending on the desired size and morphology. Examples of such methods include; the sol-gel method, the wet chemical synthesis method, the gas-phase reaction synthesis (Ni, et al. 2005), solvothermal synthesis, spray pyrolysis, thermal decomposition, precipitation, template mediated growth method and hydrothermal synthesis ( Lu and Yeh, 1999; Suchanek and Riman, 2006; Xu, et al. 2002).
Despite the many advantages of ZnO, there are concerns over its use in nanoparticle form, and even though human beings have long interacted with nanosized materials (as they are components of our biosphere), increased activity and consequent usage of nanoparticles in everyday life occurrences poses a new challenge (Buzea, et al. 2007). The big question is how would the human body handle nanoparticles? Not much is known about the effects of nanorods on the human body. Knowledge of how the human cells interact with nano-sized crystals (which are most times smaller than a virus) could be essential in tailoring future safety guidelines and legislation, equipment design, etc (Aruoja, et al. 2008; Ding, 2010, Milne, 2010).
1.2 RATIONALE FOR RESEARCH
Always on Time
Marked to Standard
An understanding of the growth mechanisms of ZnO nanoparticles is vital to having control over morphology and size variation of the nanorods and ensuring reproducibility of results industrially (Nishizawa, et al. 1984).
So far, most of preceding research into hydrothermal synthesis reactions has focused on the use of growth additives/catalyst such as Cetyltrimethylammonium bromide (CTAB), Polyethylene glycol (PEG), Polyvinyl Pyrrolidone (PVP), trisodium citrate, etc to achieve morphology and size variations. Unfortunately, they inadvertently add up to process costs, complexity and the loss of environmental friendliness of the process (Elen, et al. 2009; Kuo, et al. 2005), hence the need to seek for other methods of achieving similar results as have been achieved previously in a greener way.
Manufacturing has now become more bio-inclined, and at present all hands are on deck to refine past and present chemical processes to have a more gentle impact on the environment and also reduce the need for discharges and waste disposal, therefore new methods of synthesis that require the use of materials which are as close to nature as possible are highly coveted to improve recyclability and ensure an environmentally sustainable production processes.
1.3 AIMS OBJECTIVES AND SCOPE OF THE RESEARCH
This research is aimed at the production of nanorods of ZnO of different sizes via hydrothermal synthesis. Methods/procedures for achieving variation of size of the nanorods in the absence of growth coordinating additives would be investigated experimentally, and a characterization study of the product nanocrystals would also be conducted in the project.
1.3.2 SCOPE OF RESEARCH
There are several aspects of nanoparticles production that require further understanding. However, only a limited study can be carried out within the timescale of this proposed project, and it will not be possible to exhaustively examine the subject matter (if that were ever possible). The scope of research under this project will be limited to only the hydrothermal synthesis of nanorods of Zincite (ZnO).
Also, due to the extensive nature of data required for such endeavours, no attempt will be made to provide mathematical correlations to model the production process.
Characterisation of the product particles will be carried out using Scanning and Transmission Electron Microscopy, X-ray Diffraction analysis and Dynamic Light Scattering.
To achieve the stated aims of this research, the following objectives will provide the focal point of this research.
Hydrothermal Production of ZnO nanorods with aspect ratios greater than 2 (i.e. length of nanorod greater than twice its diameter).
Achieve variation of the sizes of the nanorods by changing process conditions.
Achieve reproducibility of experimental conditions and results
Locate optimal process conditions and chemistry for formation of target nanorods
Seek for means of controlling crystal size without the use of growth additives/catalyst
Perform Scanning Electron Microscopy (SEM), Transition Electron Microscopy (TEM), X-ray Diffraction and Dynamic Light Scattering analysis of nanorods produced
Research scale-up potential of hydrothermal synthesis of ZnO nanorods
Produce a well documented report of experimental findings
1.4 LAYOUT OF REPORT
The first chapter of this report provides a general overview of nanoparticles; it provides a context for the investigations to be conducted herein and then covers the scope, limitations and objectives of the research work.
Chapter two covers an extensive literature review into the past and present processes, practices and peculiarities of the hydrothermal synthesis process. Preceding research works are reviewed with a view to identify, trends, thought patterns and key conclusions. Alternative processes to the hydrothermal process are also reviewed.
Chapter three provides information on the basic chemical principles behind hydrothermal synthesis, alternative process routes are considered in terms of reagents and type of precursors. An in-depth description of procedures for synthesis and characterisation is documented.
Chapter four provides an outline of the plan of work, details of activities, targets, time intervals and work schedule. A progress report on research is also included.
A summary of evidence gathered from the literature survey is presented in chapter five along with a summary of the aims and objectives of the research wok.
CHAPTER TWO LITERATURE REVIEW
2.1 Properties of Zinc and Zincite
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Zinc is regarded as a transition metal (II - IV). It appears to be bluish pale gray, zinc is a solid at room temperature and it has a hexagonal crystal lattice. Zinc exhibits metallic bonding and has two know oxidation states (+1) and (+2). Zn as it is usually abbreviated is the 23rd most abundant metal in the earth's crust. It could also be found in very dilute quantities in seawater (IZA, 2011; USGS, 2011).
Zinc occurs naturally as sphalerite (Zn, Fe)S, Zincite (ZnO) and smithsonite (ZnCOÂ3). Locations in the world where zinc is mined commercially include; Canada, USA, Australia, Peru and China (IZA, 2011; USGS, 2011).
Zinc oxide is an amphoteric oxidative product of zinc metal. At room temperature, ZnO is a white powder with a molecular mass of 81.39 AMU and density of 5610 kgm-3 it is considered non-toxic to human beings, but because it possesses anti-microbial properties, it is unsuitable to discharge into aquatic bodies. ZnO nanoparticles have been found to be compatible with both aqueous and organic solvents (American Elements, 2005; IZA, 2011).
ZnO crystals generally are composed of a positive polar plane dominated by Zinc and a negative polar plane dominated by Oxygen (Zhang, et al. 2003). Other properties of ZnO include, wide band gap of 3.37 eV at room temperature, large exciton binding energy of 60meV,( Ling et al. 2001; Ni, et al. 2005). ZnO has a wurtzite crystal structure, P63mc (186) (Ohio, 2011). ZnO is also one of the few nano-oxides that exhibit quantum confinement effects (Meulenkamp, 1998).
Fig 1: ZnO wurtzite structure (Ohio, 2011)
2.2 Morphologies and sizes of ZnO nanoparticles
Different morphologies have been reported by several authors of ZnO nanoparticles from various synthesis methods. Chittofrati and Matijevic, (1989) reported obtaining nanorods from hydrothermal synthesis at 1500C for 2 hours using Zn(NO3)2 and NaOH as reactants with PPC as a growth assistant at a pH of 12.1 and also with KOH under similar process conditions, but at a pH of 13.3. Xu, et al. (2002) synthesized nanorods through the thermal decomposition route from Zinc acetate and oxalic acid with nonyl phenyl ether (9) / (5) and NaCl flux as catalyst. Hu, et al. 2001 produced nanowhiskers by reductive-oxidation of ZnS powder at 13000C. Ling, 2005 reported the growth of nanodendrites by controllable gas vaporization (CGVA) of Zn-Cu alloy in air at 12500C. Haile and Johnson (1988) produced spherical ZnO nanocrystals through aqueous precipitation from separate solutions of ZnSO4 7H2O and ZnCl2 mixed with NH4OH. Kitture, et al 2010 synthesized both tetrapod and spherical nanostructures of ZnO using the citrate gel route. A catalyst free metal-organic vapour-phase epitaxy method was employed by Jeong, et al. 2007 to produce nanorods and other nanoparticles of ZnO. Zhang, et al report a solid state reaction at room temperature between Zinc acetate dehydrate and NaOH catalysed by di-ethanolamine (DEA) to produce ZnO nanorods. Chen and Gao, (2006) synthesized ZnO nanorods arrays via a wet-chemical method that involved thermal decomposition of Zinc acetate Si, ITO and glass substrate. Li, et al. (1998) reported acicular nanoparticles produced through a hydrothermal discharging - gas method. Flower-like and sword like nanostructures of ZnO were reported by Zhang, et al. (2003) after Cetylltrimethylammonium bromide (CTAB) assisted hydrothermal process from Zinc acetate and NaOH. Kuo, et al (2005) hydrothermally synthesized microspheres and hexagonal microrods with sheet and plate-like nanostructures from Zinc nitrate hexahydrate and hexamethylenetetramine (HMT) catalysed by trisodium citrate. Ni, et al. (2005) reported the production of ZnO nanorods from hydrothermal reaction of ZnCl2 and KOH in the presence of CTAB at 1200C. Microwave assisted hydrothermal synthesis was employed by Shojaee, et al. (2009) to produce ZnO nanorods from Zn(NO3)2 6H2O and HMT. Giri, et al. (2009) reported obtaining vertical ZnO nanorods arrays from vapour-transfer and thermal evaporation of ZnO powder mixed with graphite powder. Guo, et al. (2010) reported a green hydrothermal synthesis of ZnO nanoparticles from ZnCO3.3Zn(OH)2 and H2O2 at 1700C. Xu, et al. (2005) produced ZnO nanorods via hydrothermal synthesis at 1820C assisted by CTAB. The hydrothermal synthesis route was employed by Liu and Zeng, (2002) to produce ZnO nanorods smaller that 50nm from Zn(NO3)2 6H2O, NaOH and ethylene diamine. Finally Polsongkram et al. (2008) reported the growth of ZnO nanorods hydrothermally from Zinc nitrate and HMT.
Fig 2: Morphologies reported in literature
Table 1 below summarizes the average sizes of ZnO nanorods only reported in literature
Table 1: size variation of product nanorods reported in literature
Xu, et al. 2002
10 - 60
1 - 3
Tam, et al. 2006
55 - 70
Zhang, et al. 2010
Solid state reaction at room temperature
Jeong, et al. 2007
Catalyst-free metal-organic vapour phase epitaxy
Elen, et al. 2006
Li, et al. 1998
Hydrothermal discharging-gas method
Kuo, et al. 2005
100 - 400
1 - 1.5
Ni, et al. 2005
Guo, et al. 2010
45 - 490
Xu, et al. 2005
40 - 80
Liu and Zeng, 2002
1.5 - 2.0
2.3 Uses of ZnO nanoparticles
The uses of ZnO have increased steadily as more people are able to exploit its uniqueness and other chemical, physical, optical and mechanical properties (Ni, et al. 2005). At present ZnO in its nanoparticle form is commonly used in gas sensors, solar windows, acoustic devices, Light Emitting Diodes (LED), optical waveguides, semiconductors, piezo-electric transducers, UV absorbers, detection devices, etc (Elen, et al. 2009; Giri, et al. 2009; Kuo, et al. 2005; Ni, et al. 2005).
Besides electronics, ZnO nanoparticles are used in the cosmetic industry for sunscreens (TGA, 2010), in the food industry as an additive and in the manufacture of rubber (Brayner, et al. 2010). ZnO has also made a foray into medicine, with suggestions for use as a drug delivery mechanism in treatment of cancer (Zhang, et al. 2010).
2.4 Methods of preparing ZnO nanoparticles
ZnO nanoparticles can be prepared from several methods. Although the hydrothermal route is seems to be gaining prominence above other methods. Methods such as thermal decomposition (Xu, et al. 2002), aqueous precipitation (Haile and Johnson Jnr, 1988), the citrate gel method (Kitture, et al. 2010), solid state reaction at room temperature (Zhang, et al. 2010), hydrothermal discharging-gas (Li, et al. 1998), vapour-transfer and thermal evaporation (Giri, et al. 2009), sol gel method (Meulenkamp, 1998), wet chemical method (Chen and Gao, 2006), reductive-oxidation (Hu, et al. 2001), oxidation (Ling, 2005; Pei, et al. 2009), hydrothermal decomposition (Nishizawa, et al. 1984) have all been reported to synthesis nanoparticles under various process conditions. Some at high temperatures, others needing growth-assisting additives. Almost all of the synthesis routes obtainable in the nanoceramics field could be applied to synthesize ZnO nanoparticles (Dem'yanets et al. 2008).
2.5 Comparison of methods of ZnO nanoparticle synthesis
Almost all types of methods available for synthesis of nanoceramics could be applied to the production ZnO nanoparticles (Dem'yanets, et al. 2008). Most methods require high temperatures for synthesis such as 9100C for thermal decomposition (Xu, et al. 2002), 12500C for oxidation of alloy method (Ling et al. 2005), 13000C for reductive-oxidation synthesis (Hu, et al. 2001), etc. However, methods such as the citrate gel method (Kitture, et al. 2010), wet chemical method (Chen and Gao, 2006) and the hydrothermal method (Kuo, 2005; Nishizawa, 1984; Suchanek, 2008) in contrast occur within the range of 500C - 4000C. Zhang, et al. (2010) reports synthesis at room temperature.
Methods employed in the synthesis of ZnO nanoparticles can be separated into categories on the basis of unifying characteristics shared by some of the methods, e.g. reaction phase, inclusion of catalyst, temperature range, start off point, etc.
Vapour -Liquid-Solid group of methods
Gas phase reaction
Evaporative decomposition of solution
2. Solution-Liquid-Solid group of methods
Wet chemical synthesis
3. Fabrication group of methods
Electron beam lithography (EBL) method
Scanning Tunnelling Microscopy (SCM) method
4. Template- supported growth group of methods
- Silicon substrate
- Glass substrate
- Zinc foil substrate
5. Solid / Gel group of methods
Citrate gel synthesis
Direct Oxidation synthesis
2.6 Hydrothermal synthesis
The phrase 'hydrothermal synthesis' is applicable to a range of processes in material science. Hydrothermal synthesis could mean solvothermal process (no uses of aqueous solvent), green chemistry (usage of supercritical solvents H2O and CO2 to replace organic solvents), glycothermal synthesis, etc (Yoshimura and Byrappa, 2007).
Hydrothermal synthesis on the whole, is a solution-based crystallization process performed under temperatures and pressures that are above ambient conditions in a closed reactor (Yoshimura and Byrappa, 2007).
Reactors employed in hydrothermal synthesis include; general purpose autoclaves, Morey autoclaves, Tuttle-Roy type reactors, etc (Dem'yanets, et al. 2008).
Suchanek (2008) opines that nanocrystals obtained from the hydrothermal synthesis route posses better UV absorption power and purity.
Besides ZnO, the hydrothermal synthesis method could be adapted for the synthesis of a varied number of other oxides such as zeolites, quartz, GaPO4, GaN, langsite and non-oxides such as carbon nanotubes, diamonds and nanocomposites (Dem'yanets, et al. 2008; Suchanek and Riman, 2006).
Advantages of the hydrothermal synthesis route include;
Simplicity of process
Easy reproducibility of results and process conditions
Less observed aggregation of product particles
The process is self purifying
In terms of commercialization,
there is ease of automation of charging, transportation, mixing and recovery of products as a result of liquid phase of materials involved
Fewer precursors and intermediates are involved reducing the cost of process control and instrumentation significantly
Low temperature and pressure operating conditions mean that reactants are always in the liquid phase, thus helping to maintain Stoichiometric balance
There's are capabilities of in-situ measurement of production process
(Dem'yanets, et al. 2008; Suchanek and Riman, 2006; Yoshimura and Byrappa, 2007).
Disadvantages associated with hydrothermal synthesis include; little or no control over reaction kinetics because it is occurs in a covered up reactor. Nanoproducts could also exhibit 1-D functionality (Suchanek and Riman, 2006).
2.6.1 Evolution of hydrothermal synthesis
The roots of the hydrothermal synthesis could be traced back to around 1850 when geologists tried to simulate natural hydrothermal phenomena in the laboratory (Morey, 1952). Sir Roderick Muchinson is credited with the coinage of the word 'hydrothermal' (Yoshimura and Byrappa, 2007).
It was initially thought that hydrothermal synthesis could only be performed under supercritical conditions and this assumption led to the neglect of deeper research into the process, however, recent reports now indicate otherwise with reports of successful hydrothermal synthesis at temperatures and autogenous pressures below 2000C and 1.5 MPa respectively (Suchanek and Riman, 2006).
Major landmarks in the evolution of hydrothermal synthesis include; the successful production of aluminium from bauxite by K. J. Bayer in 1908 and the production of quartz crystals in a Papin's digester by E.T. Schafthual in 1985 (Yoshimura and Byrappa, 2007).
Most of the research reported on hydrothermal synthesis, have included the use of amines such as DEA, HMT, NH3, ETA, CTAB, etc in the form of pH stabilizers, precursor pre-treatment, growth additives or catalyst. However in light of environmental friendliness, the above compounds are not favourites, in the quest for a commercially viable and environmentally benign production route, avoidance of inorganic solvents, growth catalyst, etc is essential due to the environmental load they add to the process (Elen, et al. 2009; Suchanek, 2008).
The future outlook for hydrothermal processes is green chemistry, multi-energy processing technology and designed of purposeful reactors that could incorporate multi-energy processing and kinetic stimulation of reactions (Guo, et al 2010; Yoshimura and Byrappa, 2007)
Table 2: Evolution of hydrothermal synthesis of materials (adapted from Yoshimura and Byrappa, 2007).
EXAMPLE, MATERIALS APPLIED
Sulphate ore, oxide ore
Crystal synthesis, growth
quartz, oxides, sulphides
Fine crystals with controlled composition, size and shape
PZT, ZrO2, PSZ, BaTiOÂ3, hydroxyapatite
Hydroxyapatite, Mg-Sulphite, K-titanate
Crystalline films (thin, thick)
BaTiO3, LiNbO3, ferrite, carbon, LiNiO2
Combination with electro-, photo-, mechano-, electrochemical, etc
Synthesis, alternation, coating, modification
Organic or biomaterials
Hydrolysis, wet-combustion, extraction, polymerization, decomposition, remediation
Synthesis, extraction, reaction
Synthesis, extraction, reaction
Over the years, hydrothermal synthesis equipment have evolved from reactions in barrel of a gun to high temperature, high pressure reactions in a 'bomb' unto modern relatively low temperatures and pressure autoclave reactions (Morey, 1952).
Fig 3: Early hydrothermal synthesis reaction setup (Morey, 1952)
2.6.2 Hybrids of the hydrothermal process
Various hybrids of the hydrothermal synthesis method exist. Hybridization is usually performed to improve reaction kinetics or attenuate one of the key factors influencing the reaction (Suchanek and Riman, 2006). Below is a list of examples of various hybrids of hydrothermal synthesis
Solvothermal synthesis: H2O is substituted with an organic / inorganic solvent
Microwave-Hydrothermal synthesis: Microwave assisted hydrothermal synthesis
Hydrothermal-Electrochemical synthesis: hydrothermal conditions combined with electrochemistry
Hydrothermal-Sonochemical synthesis: Ultrasound assisted hydrothermal synthesis
Hydrothermal-Photochemical synthesis: hydrothermal synthesis combined with optical radiation
Mechanochemical-Hydrothermal synthesis: Mechanochemistry combined with hydrothermal synthesis
Hydrothermal hot pressing: Hydrothermal conditions combined with hot pressing
Green chemistry: usage of supercritical solvents H2O and CO2 to replace organic solvents
(Suchanek and Riman, 2006; Yoshimura and Byrappa, 2007).
Below is a summary of reagents, catalysts employed in various hydrothermal synthesis experiments
Table 3: Summary of pervious experimental research on hydrothermal process / synthesis
Kuo, et al. 2005
Zn(NO3)2.6H2O (Zinc Nitrate Hexahydrate)
Lu and Yeh, 1999
Xu, et al. 2002
Zn(CH3COO)2.2H2O (Zinc Acetate dihydrate)
H2C2O4.2H2O (Oxalic acid)
Nonyl Phenyl ether (9)/(5) and NaCl flux
Nishizawa, et al. 1984
H2O (redistilled water)
Ni, et al. 2005
KOH (Potassium Hydroxide)
Tam, et al. 2006
NHÂ4OH, KOH, LiOH, NaOH, TEA, Ethylenediamine
Elen, et al. 2006
Li, et al. 1998
Zhang, et al. 2003
Zn(CH3COO)2.6H2O, ZnCl2, Zn2SO4, Zn(NO3)2
Polsongkram, et al. 2008
C6H12N14(HMT), NH4OH, H2O
Guo, et al. 2010
Liu, and Zeng 2002
NaOH, C2H5OH, C2H4(NH2)2
2.6.3 Key factors in hydrothermal synthesis
ZnO is a polar crystal which plays a role in the growth mechanism of the nanoparticles (Chen and Gao, 2006). Most importantly, temperature, type of precursor, concentration ratio of reagents, duration of reaction, presence or otherwise of catalyst, pH of solution, pre-treatment of precursor, rate of heating, stirring and type of substrate used as template have been identified as key factors in the hydrothermal synthesis of ZnO nanoparticles (Li, et al. 1998; Meulenkamp, 1998; Elen, et al. 2009). Almost all methods considered seem to be unanimous on the influencing power of temperature on the size and morphology of ZnO nanoparticles Suchanek (2008) notes that autogenous pressures in the autoclave and the reaction time for a hydrothermal synthesis reaction are temperature dependent.
2.6.4 Brief on scale-up of hydrothermal processes
Large scale production efforts on nanoparticles would require a simple, efficient and high yield method which would also be environmentally sustainable (Elen, et al. 2006). These considerations make the hydrothermal route more attractive than other methods of synthesizing ZnO nanoparticles (Yoshimura and Byrappa, 2007).
Companies that have developed commercial procedures for production of ZnO nanoparticles based include Cabot corporation, Sakai Chemical Co., Murata Industries, Ferro Corporation, etc (Suchanek and Riman, 2006).
Although, a substantial amount of authors mention large-scale hydrothermal synthesis in experiment- based research reports, the largest single crystal of ZnO reported so far is between 180 - 200g (Yoshimura and Byrappa, 2007). Nevertheless, the potential for hydrothermal production of ZnO crystals is not in doubt as even nature produces crystals hydrothermally by the tonne and hydrothermal production of quartz crystals occurs also in tonnes (Suchanek and Riman, 2006). Due to its observed advantages, the hydrothermal synthesis method has been tipped to lead the future of nanoparticle production on an industrial basis (Dem'yanets, et al. 2008).
2.7. Toxicity of ZnO nanoparticles
Nanoparticles by virtue of their size portend a toxicological risk much different from the macro sized particles. Nanoparticles tend to be more reactive as a result of increase surface area to volume ratio (Brayner, et al. 2010). They also have an increased potential to bypass protective guards and screens which are effective for larger sized particles. In addition, materials which are inert in macro forms could become reactive on the nanoscale and detection mechanisms designed specifically to exploit bulk characteristic of a material for example spectrum colour of light reflected could become ineffective with nanoparticles of the same material (TGA, 2010).
Brayner, et al. (2010) performed ecotoxicological studies on ZnO nanoparticles and noted that microorganisms reacted to the presence of ZnO nanoparticles leading to cell wall damage and cell death in some samples of Arabena flos-aquae and Euglena gracitis tested.
2.8 Key players and activity hubs of research into ZnO nanoparticle synthesis.
Due to the many advantages derivable from the use of ZnO nanoparticles, interest in its synthesis, properties has been generated practically all over the world. Initially, the United States was at the fore-front of research endeavours (citation), however recent survey indicates that the bulk of activity around hydrothermal synthesis, ZnO nanoparticle production occurs in Asia spearheaded by China. Below is a chart of distribution of research activities worldwide.
Fig 4: Activity hubs for ZnO nanoparticle research and synthesis
2.9 Gaps in existing research into hydrothermal synthesis of ZnO nanoparticles
The field of ZnO nanoparticles production is a well researched area with dozens of papers being published monthly all over the world especially from the activity hubs. However, there is the need for an in-depth research into the commercial production of ZnO nanoparticles via hydrothermal synthesis. At present, several lab scale results exist but none on a kilogram / tonnes based continuous hydrothermal synthesis of ZnO nanoparticles. The largest reported hydrothermal production of ZnO nanoparticles is between 180 - 200g (Hu, et al 2001; Yoshimura and Byrappa, 2007). It is common knowledge that process conditions, reaction kinetics most times change when the scale of production is increased or decreased.
In addition, most of the reactors used for hydrothermal synthesis are general purpose autoclaves (Yoshimura and Byrappa, 2007), to maximize control over the hydrothermal process especially with multi-energy technology; there is the need for new purpose built reactors for precise size and morphology control and variation, as well as reproducibility of reaction conditions and results.
CHAPTER THREE RESEARCH METHODOLOGY
3.1 Chemistry of hydrothermal process
A unifying feature for all processes that can be termed 'hydrothermal' is reaction under pressurized conditions at temperatures above room temperature in a closed vessel. Hydrothermal reactions could involve homogenous or heterogeneous systems (Yoshimura and Byrappa, 2007).
Hydrothermal synthesis involves dissolution of normally insoluble of semi-soluble compounds in water / solvent, at elevated pressures and temperatures. This is followed by crystal nucleation and growth from dissolved growth units (precursors) (Suchanek and Riman, 2006).
Referring to table 3 above, the following reagents have been applied in hydrothermal synthesis reactions, and form possible reagent sources for this research work
Possible sources of zinc [Zn+]: Zn(NO3)2.6H2O (Zinc nitrate hexahydrate); Zn[CH3COO]2.2H2O (Zinc acetate dihydrate); Zn2SO4 (Zinc Sulphate); ZnCO3.3Zn(OH)2 ; ZnCl2 (Zinc Chloride); Zn(NO3)2 (Zinc Nitrate);
Possible sources of [OH]-: H2O (de-ionized water); NaOH (Sodium hydroxide), KOH (Potassium hydroxide), H2O2 (hydrogen peroxide); NH4OH (Ammonium hydroxide); C2H5OH (ethanol); LiOH (Lithium Hydroxide);
In light of the objective of this project to synthesize ZnO nanorods along with environmentally benign side products, ZnCO3.3Zn(OH)2 and ZnCl2 would be chosen as to provide [Zn]+, while NaOH and H2O2 would supply [OH]- ions.
The following equations describe the general reaction pathway for hydrothermal synthesis
3.2 Analytical methods
Transition Electron Microscopy: Transition Electron Microscopy (TEM) developed by Max Knoll and Ernst Ruska in 1931 images objects by focusing accelerated electrons from an electron gun on a test sample via a set of condenser lenses and apertures. TEM exploits the small de Broglie wavelengths of Electrons to capture images and it is also capable of providing knowledge of chemical composition and crystallinity (DoITPoMS, 2007)
Scanning Electron Microscopy: Scanning Electron Microscopy (SEM) employs the usage of electromagnetism instead of lenses in the process of image magnification. SEM was developed in the 1950's, its advantage come in its ability to allow imaging of larger samples with vivid clarity. SEM produces an image from the electrons and x-rays that are backscattered, when the sample is hit by a beam of focused electrons. To enable imaging, non-metallic samples are required to be made conductive and moisture free (Purdue, 2010).
X-ray Diffraction: this is an analytical method based on the decrease in intensity of KÎ± lines of an x-ray as it penetrates a sample. The fractional decrease is said to be proportional to the distanced traversed by the x-ray beam according to the following expression
Where: = fractional decrease inn intensity of x-ray beam; = wavelength of x-ray; Î¼ = linear absorption coefficient; Ï = density of sample; x = distance traversed by x-ray;
An X-ray pattern displayed by each material is unique and can be used to identify the substances present in the sample including level of purity. (MATTER, 2000; MRL, (n.d.))
Dynamic Light Scattering: This is a non-invasive analytical technique that measures the size of molecules or particles based on Raleigh scattering or Mie scattering of Monochromatic light under Brownian motion effects. DLS also called Photon Correlation Spectroscopy (PCS) or Quasi-Elastic Light Scattering (QELS) measures the hydrodynamic diameter / radius of a particle (Brokenhaven, 2011, Malvern, 2011).
3.3 Equipment for use
In the pursuit of the aims of this project, a couple of laboratory equipment would be utilized either for reagent preparation, experimental work or analysis of results. Some the key equipment expected to be used include;
Autoclave (stirred and non-stirred)
Desiccators / oven
Transition Electron Microscope
Scanning Electron Microscope
DLS particle size analyzer
3.4 Proposed procedure for experimentation
Procedures described by Elen, et al. (2009) and Guo, et al. 2010 would be largely applied in this experiment with a few modifications.
After equipment preparation and calibration (if required), the zinc compound powders (ZnCl2 and ZnCO3.3Zn(OH)2 would be dissolved in de-ionized water to form solutions of appropriate concentrations respectively. The [OH]- donor solutions (H2O2 and NaOH) of predetermined molar concentrations would be added using a pipette to the [Zn]2+ solutions under constant stirring.
After the mixing stage, the resultant solutions would be transferred into sample containers and placed batch by batch in an autoclave.
The temperature of the autoclave would be raised by constant rate of heating to temperatures ranging from 1000C to 3000C to and maintained for different periods of time before the heating is stopped and the autoclave cooled to room temperatures.
The products of the hydrothermal reaction would be washed with de-ionized water and ethanol to remove impurities prior to drying in an oven. After drying, the ZnO Nanoproducts would be re-dispersed in
3.5 Design of experiment
During the conduct of the experiment, temperature, duration of reaction and Zn2+ / OH- molar ratios would be varied keeping all other factors constant to achieve variation in size of the ZnO nanorods obtained hydrothermally.
3.6 Statistical analysis
On completion of all experimentation, qualitative and quantitative analysis, statistical correlation of experiment results would be performed to determine best operating conditions and key influencing factors during the hydrothermal synthesis.
CHAPTER FOUR PROJECT PLAN
4.0 WORK PLAN OUTLINE
The project comprises of 5 major activities which would run sometimes concurrently and at other times consecutively through a timeline of 24 weeks. They are:
Review of related literature
Post experiment analysis and research
Research paper based on project
Each of the activities is further broken down into sub-activities. Fig 5 below provides a Gantt chart describing in detail, the activities, duration, of each task embedded in the project.
Interim report: it is expected that an interim report would have been submitted by May 6, 2011
Experimentation: Both hydrothermal synthesis experiments and analytical experiments are expected to have been completed by August 15, 2011
Analysis of results: Analysis of the result obtained from literature survey and experimentation is expected to have been wrapped up by August 31, 2011
Seminar presentation: A seminar based on research findings is billed for between the 7th and 9th September 2011.
Report writing: production of a draft report is scheduled for 2 September, 2011 and a final report for 16 September, 2011
This report marks a second in a series of reports on the research work, the first being a pilot study. For this project, it also heralds preparations for laboratory work. So far, the following activities have been accomplished (for those with a fixed timeline), or are in an advanced state of progress.
- Pilot study of research work
- Literature survey
- Design of experiment
- Interim report
4.3 GANTT CHART
Below is a Gantt chart presentation of the timeline of key events that chart the course of this project.
Fig 5: Gantt chart of project activities
CHAPTER FIVE CONCLUSION
The hydrothermal synthesis method has been proven to be a simple route to production of ZnO nanorods. Hydrothermal synthesis holds promise for large-scale production ventures and it is capable of yielding high purity and stable ZnO nanorods consuming less energy in terms of heating costs, hydrothermal synthesis can also be performed without the use of growth assisting additives and inorganic pH regulators, thus greatly increasing the environmental appeal of the process.
This project covers the production of nanorods of zinc oxide (ZnO) within the size range of 100nm to 400nm under hydrothermal conditions by reacting ZnCl2 and ZnCO3.3Zn(OH)2 with NaOH and H2O2 respectively in the absence of any growth catalyst. Size variation of the nanorods was achieved from variation of temperature, reaction time, rate of heating, method of mixing and initial concentration of reagents.
Analysis of the process and nanoproducts was made with a view to establishing best operating conditions for production of target sizes. The scale-up potential of the process was also examined.
This report represents a review of literature, and methodology of the hydrothermal synthesis of ZnO nanorods. It also provides an insight into the planning and layout of execution of the research project