Microwave-Assisted Synthesis: A New Tool in Green Technology
SREERENJINI C. R. BHAGYALAKSHMI BALAN1, GLADIYA MANI1, AND SURESH MATHEW12'
School of Chemical Sciences (SCSMahatma Gandhi University, Kottayam 696560, Kerala, India
- 2Advanced Molecular Materials and Research Centre (AMMRC), Mahatma Gandhi University, Kottayam 696560, Kerala, India
Science is always advancing ever since from the earlier days and the advancement had been incremental. Chemical synthesis and manufacturing plays a vital role in the progress of science and in the earlier days, we were more focused upon yield and profit than safety and ecological effects of chemical products and processes. However later on we became aware of the detrimental impacts of harsh chemicals and chemical practices on the environment and started to develop more environment and eco-friendly synthesis methods which are direct and efficient. Microwave-assisted synthesis is a new tool in green technology in this regard. Considerably shorter reaction tune, uniform product formation with high yields, superior product purity and material properties etc are the main attractive of microwave assisted synthesis methods compared to conventional chemical synthesis methods. For the past two decades, microwave assisted synthesis finds application in organic and polymer synthesis, material sciences, nanotechnology, and biochemical processes. Herein, we discuss about the history of development of microwave technology, working principles and microwave-assisted synthesis of nanomaterials for various applications like photo catalysis, propellants, and super capacitors etc.
The emergence of nanoscience and nanotechnology along with the introduction of green chemistry has imposed new demands on material synthesis. The 12 principles of green chemistry expect the evolution of efficient synthetic routes for material synthesis with reduced reaction tune, minimal use of toxic chemicals, and less waste production. In this perspective, microwave-based synthetic routes will be a step toward sustainable development. Microwave-assisted synthesis is considered as an eco-friendly faster synthesis route, due to its ability to couple the reactant molecules quickly by raising the reaction temperature. The bottleneck of conventional synthesis methods such as lengthy reaction time, inhomogeneity in shape and size, and slow reaction pace have been well addressed by microwave method.
In microwave-assisted synthesis, polar molecules in the solvent or conducting ions in a solid are forced to align or rotate with the field and to collide each other rapidly resulting in the dissipation of energy in the form of heat. This ensures uniform or homogeneous heating of the precursor material completely rather than surface to bulk heating taking place in conventional synthesis methods. The faster collision of molecules within the reaction vessel results in an accelerated reaction rate which then leads to a shorter reaction time compared to days or hours of processing time in conventional techniques. Experimental studies prove that microwave synthesis increases the reaction rate exponentially compared to conventional techniques due to the use of higher reaction temperature with lessened side reactions and byproducts formations. This results in the formation of products with high yield and purity.
In conventional techniques, the heat transfer is from the walls of the reaction to the solvent medium. This creates a temperature gradient within the sample. While in the microwave method, the radiations excite the precursors inside the reaction vessel, resulting in an even distribution of temperature within the sample rather than creating a temperature gradient. This even heating and lesser solvent requirement urge the researches to adapt to microwave-assisted synthesis. Microwave-assisted synthesis methods offer more reproducibility as they involve uniform heating and better control over reaction parameters than traditional synthesis techniques. The adoption of microwave synthesis helped to attain high yield and low processing cost. This facile method opens up chances in the evolvement of new material phases which could not be obtained by normal synthesis methods.
The interaction of materials with microwave radiations varies since there is a difference in their susceptibilities. Depending on their response, materials can be broadly classified into three categories: (i) materials that are transparent to microwave irradiation, for example, sulfur (ii) materials that absorb microwave radiation, for example, water, and (iii) materials that reflect microwave radiation, for example, metals. In microwave chemistry, the interaction of microwave radiation with the materials is a prime factor and hence microwave radiation-absorbing materials are of the utmost importance.1 Microwave-assisted synthesis is the foremost and most suitable synthesis route for various materials like for the synthesis of nanoparticles, organic molecules, polymers, magnetic particle synthesis, and so on. Microwave synthesis methodology is widely being explored in the field of supercapacitors, photocatalysts, and propellant catalysts. Due to the availability of large microwave reactors or apparatus, it is now easy to scale up the laboratory level experiments to industrial level without altering the reaction parameters within few minutes than tedious conventional methods.
HISTORY AND DEVELOPMENT
Nowadays, microwave-assisted synthesis has received considerable attention among the scientific community. The increasing diversity and availability of microwave equipment has allowed this technology to become more popular and useful. A breakthrough in microwave technology occurred during the World War II by Dr. Percy LeBaron Spencer, who developed the first folly folictioning microwave oven by joining a high- density electromagnetic field generator device to an enclosed metal box.
Later, in 1947 Raytheon developed the first commercial microwave oven “1161 Radarange” with weight nearly 750 pounds. The first kitchen counterpart, domestic oven was introduced by Amana (a division of Raytheon). It was smaller, safer, and more reliable than the previous models, costing under S500. During 1970s there was a massive increase of microwave ovens elsewhere in the world. In 1978, the first microwave laboratory instrument was developed by CEM Cooperation, USA, for analyzing moisture in solids. Up to the middle of 1980s microwave oven was only used for cooking and defrosting frozen food. Since 1983-1985 microwave radiation was used for chemical analysis.1-2 Robert Gedye, of Laurentian University, Canada, George Majetich of University of Georgia, USA and Raymond Giguere of Mercer University, USA published papers relating to microwave synthesis.3
A revolutionary milestone in the history of microwave synthesis happened in 1990s, the first high pressure microwave vessel “HPV 80” was established by Milestone Sri Italy, for complete digestion of materials like oxides, oils, and pharmaceutical compounds. Later in 1992-1996, CEM Corporation developed a more efficient batch system reactor (MDS 200) and a single-mode cavity system (Star 2) for chemical synthesis. During 1997, Prof. H. M. Kingston of Duquesne University, USA culminated an innovative book titled “Microwave-Enhanced Chemistry-Fundamentals, Sample Preparation and Applications,” edited by H. M. Kingston and S. J. Haswell. Since 2000 microwave chemistry emerged as a promising field of study in chemical synthesis. Companies like CEM, Biotage, Anton pan, and Milestone marketed a number of microwave reactors of varying capacities and temperature control that enlarges the applicability and prosperity of microwave-assisted synthesis.
MICROWAVE-ASSISTED SYNTHESIS OF NANOMATERIALS FOR VARIOUS APPLICATIONS
Microwave synthesis has proved to be a rapid and facile method that offers effective heating source in the synthesis of nanoparticles, which gave high- quality products. Microwave not only accelerates the reaction rate but also saves energy and time.5-6 Now, this technique has been widely used for the synthesis of metal oxide nanoparticles, which can be used in multifunctional applications including photocatalysis,7 photoelectrochemical cells,8 sensor devices,9 and dye-sensitized solar cells.10 Several metal oxides such as ТЮ,,11 ZnO,12 MnO,,13 CeO,,14 and Fe,0,15 and their composites have been extensively used as photocatalyst due to their inherent electronic structures.
With the development of photocatalysis, nanosized TrO, was the most used catalyst for photochemical reactions. Several synthetic strategies such as sol-gel,16 hydrothermal synthesis,17 and microwave methods were adopted to synthesize the TiO,-based photocatalyst. Animdha Jena et al. obtained a phase-pure nanocrystalline anatase TiO, by microwave method. They obtained unique mesoporous titania samples with spherical morphology and narrow size distribution of better catalytic activity than the commercially available TiO, samples.18 A microwave-assisted synthesis of TiO, nanoparticles with an average size of 7 nm was reported by G. S. Falk et al. The rapid and homogenous heating of microwave radiation is capable of inducing uniform nanoparticle distribution within few minutes than conventional methods. The photocatalytic activities of the synthesized samples are comparable with TiO, P25.19
Nitrogen-doped TiO, is a very important visible light photocatalyst due to its stability and inexpensiveness. An in situ microwave-assisted synthesis of N-TiO,/graphitic carbon nitride (g-C3N4) composite was prepared by Xiao-jing Wang. The scanning electron microscope (SEM) micrographs of the composite ensure the successful growth of N-TiO, on the lamellar structure of g-C3N4 without aggregation (Fig. 2.1a). It was again confirmed from the dark particles and grey areas of the transmission electron microscope (ТЕМ) image. The particle with dark color can be assigned to be N-TiO, and the grey area was assigned to be g-C3N4 (Fig. 2.1b). During the calcinations the peroxotitanate releases NH3 results in high porous structure and large surface area. The photocatalytic activities of the synthesized samples were evaluated using rhodamine В (Rh B) and methylene blue (MB), the catalytic activity was increased gradually with the content of N-TiO, increasing from 15 to 40 wt%. The composite with 40 wt% shows the best performance.20
Recently, microwave synthesis of ZnO nanoparticles was reported by the group of Chaiyos Chankaew using longan seeds biowaste. The influence of zinc precursor, particle sizes based on irradiation time, and microwave power were studied. The 80 W 30 cycles of microwave irradiation result in pure hexagonal phase of ZnO nanoparticles of 10-100 nm sizes with a specific surface area of 35 m2/g.21 Photocatalytic ammonia production through nitrate reduction was found to be very effective in context of current efforts. Now, Pd-doped TiO, nanoparticles were synthesized through microwave irradiation and they are utilized as an efficient photocatalyst for ammonia reduction. The Pd-TiO, with 2.65 wt% producing a high rate of ammonia, 21.2 (rmol.22 The morphologies and surface conditions of the ZnO microstructures can be controlled by microwave method. Morphology-controlled ZnO microstructures ranging from 1 to 2 pm were fabricated by varying ammonia concentration in a microwave reactor. The fast microwave-assisted synthesis of ZnO effects mesoporous structures with large specific surface area and pore volume. The SEM micrographs of the synthesized samples clearly confirmed the morphology controlled ZnO microstructures formation. The photocatalytic degradation of the samples with Rh В was increased by increasing the specific surface area.23
FIGURE 2.1 (a) SEM images of the 40 wt% N-TiO,/g-C3N4 composite, (b) ТЕМ images
of the 40 wt% N-TiOy'g-CjN, composite.
Source: Reproduced with permission from Ref. . Copyright 2013 American Chemical Society.
FIGURE 2.2 (a) Photocatalytic activities of N-TiO,, g-C3N4, and N-TiO,/g-C3N4
composites on the degradation of (a) Rh В and (b) MB under visible light irradiation. Source: Reproduced with permission from Ref. . Copyright 2013 American Chemical Society.
As a typical narrow band gap semiconductor pliotocatalysts, tungsten oxides (WOx, 2.4-2.8 eV) are very attractive than other metal oxides.24-25 The microwave-assisted solvothermal synthesis of stacked orthorhombic W03.H,0 and urchin-like monoclinic W18049 nanowires were reported by Arpan Kumar Nayak et al. Both the photocatalytic degradation and hydrogen evolution activity of the synthesized samples was evaluated in neutral medium. The large surface area, oxygen vacancies, and fast charge transport properties enhance then catalytic activity.26
Ferric oxides, mainly a-Fe,03 display significant interest in photocatalytic applications. Their morphology and particle size play an important role in determining the activity of a-Fe,0,. A nanosized a-Fe,0, powder was synthesized by microwave-assisted hydrothermal reaction. This approach contributes new synthetic approach for the synthesis of nanosized a-Fe,03 and uniform particles of about 5 nm in size were formed with a surface area of about 173.0 m2g_1. The catalyst exhibited excellent catalytic performance for the oxidation of CO and 2-propanol to COr27 A simple one-step NaCl-assisted microwave solvothermal method was adopted for the synthesis of a-Fe203 monodisperse microspheres. The advantages of this microwave method were to control the size of microspheres by changing the microwave-solvothermal time. The high resolution transmission electron microscope (HR-ТЕМ) micrographs of the synthesized samples show that the average size of the sample was found to be smaller than 5 mil.28
Like iron oxides, bismuth oxides can also be a promising candidate for the degradation of organic species under visible light. Juliana S. Souza et al. reported Au-doped bismuth vanadate nanoflowers through conventional and microwave methods. It was found to be the composites exhibited same physical-chemical properties as those prepared through conventional heating. They establish how microwaves can replace the well-established methods to synthesize inorganic nanomaterials and reduce both energy and time. The composites show excellent catalytic activity to degrade 95% of MB under U-visible light irradiation.29
Currently, metal-organic framework derived (MOF) porous metal oxides are attractive in the field of sensors, catalysis, electrochemical, and photochemical devices. A simple and fast microwave-assisted synthesis of reduced graphene oxide incorporated MOF derived ZnO was reported by GuangZh et al. The photocatalytic degradation of MB using this composite was achieved with 82% efficiency. The enhanced performance of the composite was attributed to high light absorption and the separation of photogenerated electron-hole pairs. It was clearly observed in the photoluminescence (PL) spectrum.50