Structure, Morphology, and Electrochemical Properties of Transition Metal Oxide, Hydroxide, and Phosphate Nanomaterials for Energy Storage

Energy storage technologies are crucial for efficient utilization of electricity. Supercapacitors and rechargeable batteries are of currently available energy storage systems. Transition metal oxides, hydroxides, and phosphates are the most intensely investigated electrode materials for supercapacitors and rechargeable batteries due to their high theoretical charge storage capacities resulted from reversible electrochemical reactions. Their insulating nature, however, causes sluggish electron transport kinetics within these electrode materials, hindering them from reaching the theoretical maximum. The conductivity of these transition metal based-electrode materials can be improved through three main approaches; nanostructuring, chemical substitution, and introducing carbon matrices. These approaches often lead to unique electrochemical properties when combined and balanced.

Ethanol-mediated solvothermal synthesis we developed is found to be highly effective for controlling size and morphology of transition metal-based electrode materials for both pseudocapacitors and batteries. The morphology and the degree of crystallinity of nickel hydroxide are systematically changed by adding various amounts glucose to the solvothermal synthesis. Nickel hydroxide produced in this manner exhibited increased pseudocapacitance, which is partially attributed to the increased surface area. Interestingly, this morphology effect on cobalt doped-nickel hydroxide is found to be more effective at low cobalt contents than at high cobalt contents in terms of improving the electrochemical performance.

Moreover, a thin layer of densely packed nickel oxide flakes on carbon paper substrate was successfully prepared via the glucose-assisted solvothermal synthesis, resulting in the improved electrode conductivity. When reduced graphene oxide was used for conductive coating on as-prepared nickel oxide electrode, the electrode conductivity was only slightly improved. This finding reveals that the influence of reduced graphene oxide coating, increasing the electrode conductivity, is not that obvious when the electrode is already highly conductive to begin with.

We were able to successfully control the interlayer spacing and reduce the particle size of layered titanium hydrogeno phosphate material using our ethanol-mediated solvothermal reaction. In layered structure, interlayer spacing is the key parameter for fast ion diffusion kinetics. The nanosized layered structure prepared via our method, however, exhibited high sodium-ion storage capacity regardless of the interlayer spacing, implying that interlayer space may not be the primary factor for sodium-ion diffusion in nanostructured materials, where many interstitials are available for sodium-ion diffusion.

Our ethanol-mediated solvothermal reaction was also effective for synthesis of NaTi2(PO4)3 nanoparticles with uniform size and morphology, well connected by a carbon nanotube network. This composite electrode exhibited high capacity, which is comparable to that in aqueous electrolyte, probably due to the uniform morphology and size where the preferable surface for sodium-ion diffusion is always available in all individual particles.

Fundamental understandings of the relationship between electrode microstructures and electrochemical properties discussed in this dissertation will be important to design high performance energy storage system applications.

A tight-binding analysis of the electronic properties of III-nitride semiconductors

This thesis divides into two distinct parts, both of which are underpinned by the tight-binding model. The first part covers our implementation of the tight-binding model in conjunction with the Berry phase theory of electronic polarisation to probe the atomistic origins of spontaneous polarisation and piezoelectricity as well as attempting to accurately calculate the values and coefficients associated with these phenomena. We first develop an analytic model for the polarisation of a one-dimensional linear chain of atoms. We compare the zincblende and ideal wurtzite structures in terms of effective charges, spontaneous polarisation and piezoelectric coefficients, within a first nearest neighbour tight-binding model. We further compare these to real wurtzite structures and conclude that accurate quantitative results are beyond the scope of this model but qualitative trends can still be described. The second part of this thesis deals with implementing the tight-binding model to investigate the effect of local alloy fluctuations in bulk AlGaN alloys and InGaN quantum wells. We calculate the band gap evolution of Al1_xGaxN across the full composition range and compare it to experiment as well as fitting bowing parameters to the band gap as well as to the conduction band and valence band edges. We also investigate the wavefunction character of the valence band edge to determine the composition at which the optical polarisation switches in Al1_xGaxN alloys. Finally, we examine electron and hole localisation in InGaN quantum wells. We show how the built-in field localises the carriers along the c-axis and how local alloy fluctuations strongly localise the highest hole states in the c-plane, while the electrons remain delocalised in the c-plane. We show how this localisation affects the charge density overlap and also investigate the effect of well width fluctuations on the localisation of the electrons.

Table 2 & Appendix A. Properties of operations injecting CO2 into saline aquifers

The experience from CO2 injection at pilot projects (Frio, Ketzin, Nagaoka, US Regional Partnerships) and existing commercial operations (Sleipner, Snøhvit, In Salah, acid-gas injection) demonstrates that CO2 geological storage in saline aquifers is technologically feasible. Monitoring and verification technologies have been tested and demonstrated to detect and track the CO2 plume in different subsurface geological environments. By the end of 2008, approximately 20 Mt of CO2 had been successfully injected into saline aquifers by existing operations. Currently, the highest injection rate and total storage volume for a single storage operation are approximately 1 Mt CO2/year and 25 Mt, respectively. If carbon capture and storage (CCS) is to be an effective option for decreasing greenhouse gas emissions, commercial-scale storage operations will require orders of magnitude larger storage capacity than accessed by the existing sites. As a result, new demonstration projects will need to develop and test injection strategies that consider multiple injection wells and the optimisation of the usage of storage space. To accelerate large-scale CCS deployment, demonstration projects should be selected that can be readily employed for commercial use; i.e. projects that fully integrate the capture, transport and storage processes at an industrial emissions source.

Testing quasar unification: radiative transfer in clumpy winds

Various unification schemes interpret the complex phenomenology of quasars and luminous active galactic nuclei (AGN) in terms of a simple picture involving a central black hole, an accretion disc and an associated outflow. Here, we continue our tests of this paradigm by comparing quasar spectra to synthetic spectra of biconical disc wind models, produced with our state-of-the-art Monte Carlo radiative transfer code. Previously, we have shown that we could produce synthetic spectra resembling those of observed broad absorption line (BAL) quasars, but only if the X-ray luminosity was limited to 1043 erg s-1. Here, we introduce a simple treatment of clumping, and find that a filling factor of ˜0.01 moderates the ionization state sufficiently for BAL features to form in the rest-frame UV at more realistic X-ray luminosities. Our fiducial model shows good agreement with AGN X-ray properties and the wind produces strong line emission in, e.g., Lyα and C IV 1550 Å at low inclinations. At high inclinations, the spectra possess prominent LoBAL features. Despite these successes, we cannot reproduce all emission lines seen in quasar spectra with the correct equivalent-width ratios, and we find an angular dependence of emission line equivalent width despite the similarities in the observed emission line properties of BAL and non-BAL quasars. Overall, our work suggests that biconical winds can reproduce much of the qualitative behaviour expected from a unified model, but we cannot yet provide quantitative matches with quasar properties at all viewing angles. Whether disc winds can successfully unify quasars is therefore still an open question.

Fabrication, Optical Analysis and Photonic Applications of Certain Metallic Nanostructure-Integrated Dye Doped Polymer Optical Fibers” submitted by Ms. Suneetha Sebastian

Polymer Optical Fibers have occupied historically a place for large core flexible fibers operating in short distances. In addition to their practical passive application in short-haul communication they constitute a potential research field as active devices with organic dopants. Organic dyes are preferred as dopants over organic semiconductors due to their higher optical cross section. Thus organic dyes as gain media in a polymer fiber is used to develop efficient and narrow laser sources with a tunability throughout the visible region or optical amplifier with high gain. Dyes incorporated in fiber form has added advantage over other solid state forms such as films since the pump power required to excite the molecules in the core of the fiber is less thereby utilising the pump power effectively. In 1987, Muto et.al investigated a dye doped step index polymer fiber laser. Afterwards, numerous researches have been carried out in this area demonstrating laser emission from step index, graded index and hollow optical fibers incorporating various dyes. Among various dyes, Rhodamine6G is the most widely and commonly used laser dye for the last four decades. Rhodamine6G has many desirable optical properties which make it preferable over other organic dyes such as Coumarin, Nile Blue, Curcumin etc. The research focus on the implementation of efficient fiber lasers and amplifiers for short fiber distances. Developing efficient plastic lasers with electrical pumping can be a new proposal in this field which demands lowest possible threshold pump energy of the gain medium in the cavity as an important parameter. One way of improving the efficiency of the lasers, through low threshold pump energy, is by modifying the gain of the amplifiers in the resonator/cavity. Success in the field of Radiative Decay Engineering can pave way to this problem. Laser gain media consisting of dye-nanoparticle composites can improve the efficiency by lowering the lasing threshold and enhancing the photostability. The electric field confined near the surface of metal nanoparticles due to Localized Surface Plasmon Resonance can be very effective for the excitation of active centers to impart high optical gain for lasing. Since the Surface Plasmon Resonance of nanoparticles of gold and silver lies in the visible range, it can affect the spectral emission characteristics of organic dyes such as Rhodamine6G through plasmon field generated by the particles. The change in emission of the dye placed near metal nanoparticles depend on plasmon field strength which in turn depends on the type of metal, size of nanoparticle, surface modification of the particle and the wavelength of incident light. Progress in fabrication of different types of nanostructures lead to the advent of nanospheres, nanoalloys, core-shell and nanowires to name a few. The thesis deals with the fabrication and characterisation of polymer optical fibers with various metallic and bimetallic nanostructures incorporated in the gain media for efficient fiber lasers with low threshold and improved photostability.

Investigations on the structural, optical and magnetic properties of nanostructured cerium oxide in pure and doped forms and its polymer nanocomposites

In recent years, nanoscience and nanotechnology has emerged as one of the most important and exciting frontier areas of research interest in almost all fields of science and technology. This technology provides the path of many breakthrough changes in the near future in many areas of advanced technological applications. Nanotechnology is an interdisciplinary area of research and development. The advent of nanotechnology in the modern times and the beginning of its systematic study can be thought of to have begun with a lecture by the famous physicist Richard Feynman. In 1960 he presented a visionary and prophetic lecture at the meeting of the American Physical Society entitled “there is plenty of room at the bottom” where he speculated on the possibility and potential of nanosized materials. Synthesis of nanomaterials and nanostructures are the essential aspects of nanotechnology. Studies on new physical properties and applications of nanomaterials are possible only when materials are made available with desired size, morphology, crystal structure and chemical composition. Cerium oxide (ceria) is one of the important functional materials with high mechanical strength, thermal stability, excellent optical properties, appreciable oxygen ion conductivity and oxygen storage capacity. Ceria finds a variety of applications in mechanical polishing of microelectronic devices, as catalysts for three-way automatic exhaust systems and as additives in ceramics and phosphors. The doped ceria usually has enhanced catalytic and electrical properties, which depend on a series of factors such as the particle size, the structural characteristics, morphology etc. Ceria based solid solutions have been widely identified as promising electrolytes for intermediate temperature solid oxide fuel cells (SOFC). The success of many promising device technologies depends on the suitable powder synthesis techniques. The challenge for introducing new nanopowder synthesis techniques is to preserve high material quality while attaining the desired composition. The method adopted should give reproducible powder properties, high yield and must be time and energy effective. The use of a variety of new materials in many technological applications has been realized through the use of thin films of these materials. Thus the development of any new material will have good application potential if it can be deposited in thin film form with the same properties. The advantageous properties of thin films include the possibility of tailoring the properties according to film thickness, small mass of the materials involved and high surface to volume ratio. The synthesis of polymer nanocomposites is an integral aspect of polymer nanotechnology. By inserting the nanometric inorganic compounds, the properties of polymers can be improved and this has a lot of applications depending upon the inorganic filler material present in the polymer.

Properties of a composite material with mixed imperfect contact conditions

We present an analytical solution of a mixed boundary value problem for an unbounded 2D doubly periodic domain which is a model of a composite material with mixed imperfect interface conditions. We find the effective conductivity of the composite material with mixed imperfect interface conditions, and also give numerical analysis of several of their properties such as temperature and flux.

Optical and Spin Properties of Defect-Bound Excitons in Semiconductors

Thesis (Ph.D.)--University of Washington, 2016-08

Drying kinetics, biochemical and functional properties of products in convective drying of anchovy (Engraulis Anchoita) fillets

The aim of this work was to study the convective drying of anchovy (Engraulis anchoita) fillets and to evaluate the final product characteristics through its biochemical and functional properties. The drying temperatures were of 50, 60 and 70°C, and the fillet samples were dried with the skins down (with air flow one or the two sides) and skins up (with air flow one side). The drying experimental data were analyzed by Henderson–Pabis model, which showed a good fit (R2 > 0.99 and REQM < 0.05). The moisture effective diffusivity values ranged from 4.1 10–10 to 8.6 10–10 m2 s−1 with the skin down and 2.2 10–10 to 5.5 10–10 m2 s−1 with the skin up, and the activation energy values were 32.2 and 38.4 kJ mol−1, respectively. The product characteristics were significantly affected (p < 0.05) by drying operation conditions. The lower change was in drying at 60°C with air flow for two sides of the samples and skin up. In this condition, the product showed solubility 22.3%; in vitro digestibility 87.4%; contents of available lysine and methionine 7.21 and 2.64 g 100 g−1, respectively; TBA value 1.16 mgMDA kg−1; specific antioxidant activity was 1.91 mMDPPH g−1 min−1, and variation total color was 10.72.

Investigation of properties affecting controlled release of macromolecules from PLGA microspheres

Poly(lactide-co-glycolide), or PLGA, microspheres offer a widely-studied biodegradable option for controlled release of therapeutics. An array of fabrication methodologies have been developed to produce these microspheres with the capacity to encapsulate therapeutics of various types; and produce microspheres of a wide range of sizes for different methods of delivery. The encapsulation, stability, and release profiles of therapeutic release based on physical and thermodynamic properties has also been studied and modeled to an extent. Much research has been devoted to tailoring formulations for improved therapeutic encapsulation and stability as well as selective release profiles. Despite the breadth of available research on PLGA microspheres, further analysis of fundamental principles regarding the microsphere degradation, formation, and therapeutic encapsulation is necessary. This work aims to examine additional fundamental principles related to PLGA microsphere formation and degradation from solvent-evaporation of preformed polymer. In particular, mapping the development of the acidic microenvironment inside the microsphere during degradation and erosion is discussed. Also, the effect of macromolecule size and conformation is examined with respect to microsphere diameter and PLGA molecular weight. Lastly, the effects of mechanical shearing and protein exposure to aqueous media during microsphere formation are examined. In an effort to better understand the acidic microenvironment development across the microsphere diameter, pH sensitive dye conjugated to protein that undergoes conformational change at different acidic pH values was encapsulated in PLGA microspheres of diameters ranging from 40 µm to 80 µm, and used in conjunction with fluorescence resonance energy transfer to measure the radial pH change in the microspheres. Qualitative analysis of confocal micrographs was used to correlate fluorescence intensity with pH value, and obtain the radial pH across the center of the microsphere. Therapeutic encapsulation and release from polymeric microspheres is governed by an interconnected variety of factors, including the therapeutic itself. The globular protein bovine serum albumin, and the elongated and significantly smaller enzyme, lysozyme, were encapsulated in PLGA microspheres ranging from 40 µm to 80 µm in diameter. The initial surface morphology upon microsphere formation, release profiles, and microsphere erosion characteristics were explored in an effort to better understand the effect of protein size, conformation, and known PLGA interaction on the formation and degradation of PLGA microspheres and macromolecule release, with respect to PLGA molecular weight and microsphere diameter. In addition to PLGA behavior and macromolecule behavior, the effect of mechanical stresses during fabrication was examined. Two similar solvent extraction techniques were compared for the fabrication of albumin loaded microspheres. In particular, the homogeneity of the microspheres as well as capacity to retain encapsulated albumin were compared. This preliminary study paves the way for a more rigorous treatment of the effect of mechanical forces present in popular microsphere fabrication. Several factors affecting protein release from PLGA microspheres are examined herein. The technique explored for spatial resolution of the pH inside the microsphere proved mildly effective in producing a reliable method of mapping microsphere pH changes. However, notable trends with respect to microsphere size, PLGA molecular weight, and microsphere porosity were observed. Proposed methods of improving spatial resolution of the acidic microenvironment are also provided. With respect to microsphere formation, studies showed that albumin and lysozyme had little effect on the internal homogeneity of the microsphere. Rather, ionic interactions with PLGA played a more significant role in the encapsulation and release of each macromolecule. Studies also showed that higher instances of mechanical stress led to less homogeneous microspheres with lower protein encapsulation. This suggests that perhaps instead of or in addition to modifying the microsphere formation formulation, the fabrication technique itself should be more closely considered in achieving homogeneous microspheres with desired loading.

Characterization of Non-linear Polymer Properties to Predict Process Induced Warpage and Residual Stress of Electronic Packages

Nonlinear thermo-mechanical properties of advanced polymers are crucial to accurate prediction of the process induced warpage and residual stress of electronics packages. The Fiber Bragg grating (FBG) sensor based method is advanced and implemented to determine temperature and time dependent nonlinear properties. The FBG sensor is embedded in the center of the cylindrical specimen, which deforms together with the specimen. The strains of the specimen at different loading conditions are monitored by the FBG sensor. Two main sources of the warpage are considered: curing induced warpage and coefficient of thermal expansion (CTE) mismatch induced warpage. The effective chemical shrinkage and the equilibrium modulus are needed for the curing induced warpage prediction. Considering various polymeric materials used in microelectronic packages, unique curing setups and procedures are developed for elastomers (extremely low modulus, medium viscosity, room temperature curing), underfill materials (medium modulus, low viscosity, high temperature curing), and epoxy molding compound (EMC: high modulus, high viscosity, high temperature pressure curing), most notably, (1) zero-constraint mold for elastomers; (2) a two-stage curing procedure for underfill materials and (3) an air-cylinder based novel setup for EMC. For the CTE mismatch induced warpage, the temperature dependent CTE and the comprehensive viscoelastic properties are measured. The cured cylindrical specimen with a FBG sensor embedded in the center is further used for viscoelastic property measurements. A uni-axial compressive loading is applied to the specimen to measure the time dependent Young’s modulus. The test is repeated from room temperature to the reflow temperature to capture the time-temperature dependent Young’s modulus. A separate high pressure system is developed for the bulk modulus measurement. The time temperature dependent bulk modulus is measured at the same temperatures as the Young’s modulus. The master curve of the Young’s modulus and bulk modulus of the EMC is created and a single set of the shift factors is determined from the time temperature superposition. The supplementary experiments are conducted to verify the validity of the assumptions associated with the linear viscoelasticity. The measured time-temperature dependent properties are further verified by a shadow moiré and Twyman/Green test.

Optical properties of semiconductor quantum wires

This thesis presents theoretical investigations of the sub band structure and optical properties of semiconductor quantum wires. For the subband structure, we employ multiband effective-mass theory and the effective bond-orbital model both of which fully account for the band mixing and material anisotropy. We also treat the structure geometry in detail taking account of such effects as the compositional grading across material interfaces. Based on the subband structure, we calculate optical properties of quantum-wire structures. A recuring theme is the cross-over from one- to ~wo-dimensional behavior in these structures. This complicated behavior procludes the application of simple theoretical models to obtain the electronic structure. In particular, we calculate laser properties of quantum wires grown in V-grooves and find enhanced performance compared with quantum-well lasers. We also investigate optical anisotropy in quantum-wire arrays and propose an electro-optic device based on such structures.

Manufactured Nanomaterials: is there a correlation between toxicological effects and the physicochemical properties?

Toxicological information on nanomaterials (NMs) is of major importance for safety assessment, since they are already used in many consumer products and promise cutting-edge applications in the future. While the number of different NMs increases exponentially, new strategies for risk assessment are needed to cope with the safety issues, keeping pace with innovation. However, recent studies have suggested that even subtle differences in the physicochemical properties of NMs that are closely related may define different nano-bio interactions, thereby determining their toxic potential. Further research in this field is necessary to allow straightforward grouping strategies leading time-effective risk assessment to enable the safe use of the emerging NMs. In this presentation the case study of the in vitro toxicity testing of a set of multi-walled carbon nanotubes (MWCNTs) in two human cell lines from the respiratory tract will be described. Those MWCNT have been previously characterized in detail, and differ in thickness, length, aspect ratio and morphology. This comprehensive toxicological investigation undertaken in parallel with physicochemical characterization in the cellular moiety showed that the same NM did not display a consistent effect in different cell types, and that, within the same class of NM, different toxic effects could be observed. The correlation of the cytotoxic and genotoxic effects characterized in the two cell lines with their physicochemical properties will be presented and the relevance of considering the NMs properties in the biological context will be discussed. Overall, this case study suggests that nanotoxicity of closely related MWCNTs depends not only on their primary physicochemical properties, or combinations of these properties, but also on the cellular system, and its context. Challenges posed to toxicologists, risk assessors and regulators when addressing the safety assessment of NMs will be highlighted.

Physico-chemical properties of sol-gel synthesized titanosilicates for the uptake of radionuclides from aqueous solutions

Harnessing the power of nuclear reactions has brought huge benefits in terms of nuclear energy, medicine and defence as well as risks including the management of nuclear wastes. One of the main issues for radioactive waste management is liquid radioactive waste (LRW). Different methods have been applied to remediate LRW, thereunder ion exchange and adsorption. Comparative studies have demonstrated that Na2Ti2O3SiO4·2H2O titanosilicate sorption materials are the most promising in terms of Cs+ and Sr2+ retention from LRW. Therefore these TiSi materials became the object of this study. The recently developed in Ukraine sol-gel method of synthesizing these materials was chosen among the other reported approaches since it allows obtaining the TiSi materials in the form of particles with size ≥ 4mm. utilizing inexpensive and bulk stable inorganic precursors and yielded the materials with desirable properties by alteration of the comparatively mild synthesis conditions. The main aim of this study was to investigate the physico-chemical properties of sol-gel synthesized titanosilicates for radionuclide uptake from aqueous solutions. The effect of synthesis conditions on the structural and sorption parameters of TiSi xerogels was planned to determine in order to obtain a highly efficient sorption material. The ability of the obtained TiSis to retain Cs+, Sr2+ and other potentially toxic metal cations from the synthetic and real aqueous solutions was intended to assess. To our expectations, abovementioned studies will illustrate the efficiency and profitability of the chosen synthesis approach, synthesis conditions and the obtained materials. X-ray diffraction, low temperature adsorption/desorption surface area analysis, X-ray photoelectron spectroscopy, infrared spectroscopy and scanning electron microscopy with energy dispersive X-ray spectroscopy was used for xerogels characterization. The sorption capability of the synthesized TiSi gels was studied as a function of pH, adsorbent mass, initial concentration of target ion, contact time, temperature, composition and concentration of the background solution. It was found that the applied sol-gel approach yielded materials with a poorly crystalline sodium titanosilicate structure under relatively mild synthesis conditions. The temperature of HTT has the strongest influence on the structure of the materials and consequently was concluded to be the control factor for the preparation of gels with the desired properties. The obtained materials proved to be effective and selective for both Sr2+ and Cs+ decontamination from synthetic and real aqueous solutions like drinking, ground, sea and mine waters, blood plasma and liquid radioactive wastes.

The broad spectrum of bioactive properties of phenolic extracts: a prospective study in three different plants

Natural resources like plants are currently used all over developed and under developed countries of the world as traditional home remedies and are promising agents for drug discovery as they play crucial role in traditional medicine. The use of plants for medicinal purpose usually varies from country to country and region to region because their use depends on the history, culture, philosophy and personal attitudes of the users (Ahmad et al., 2015). The use of plants and plant products as drugs predates the written human history (Hayta et al., 2014). Plants are a very important resource for traditional drugs and around 80% of the population of the planet use plants for the treatment of many diseases and traditional herbal medicine accounts for 30-50% of the total medicinal consumption in China. In North America, Europe and other well-developed regions over 50% of the population have used traditional preparations at least once (Dos Santos Reinaldo et al., 2015). Medicinal plants have been used over years for multiple purposes, and have increasingly attract the interest of researchers in order to evaluate their contribution to health maintenance and disease’s prevention (Murray, 2004). Recently between 50,000 and 70,000 species of plants are known and are being used in the development of modern drugs. Plants were the main therapeutic agents used by humans from the 19th century, and their role in medicine is always topical (Hayta et al., 2014). The studies of medicinal plants are rapidly increasing due to the search for new active molecules, and to improve the production of plants or bioactive molecules for the pharmaceutical industries (Rates, 2001). Several studies have been reported, but numerous active compounds directly responsible for the observed bioactive properties remain unknown, while in other cases the mechanism of action is not fully understood. According to the WHO 25% of all modern medicines including both western and traditional medicine have been extracted from plants, while 75% of new drugs against infective diseases that have arrived between 1981 and 2002 originated from natural sources, it was reported that the world market for herbal medicines stood at over US $60 billion per year and is growing steadily (Bedoya et al., 2009). Traditional medicine has an important economic impact in the 21st century as it is used worldwide, taking advantage on the low cost, accessibility, flexibility and diversity of medicinal plants (Balunas & Kinghorn, 2005).