Research Article Microwave-Assisted Synthesis of Mixed Metal-Oxide Nanoparticles

Hindawi Publishing Corporation Journal of Nanoparticles Volume 2013, Article ID 737831, 11 pages http://dx.doi.org/10.1155/2013/737831 Research Article Microwave-Assisted Synthesis of Mixed Metal-Oxide Nanoparticles Akrati Verma,1 Reena Dwivedi,1 R. Prasad,1 and K. S. Bartwal2 1 School of Chemical Sciences, Devi Ahilya University, Indore 452001, India 2 Laser Materials Development & Devices Division, RRCAT, Indore 452013, India Correspondence should be addressed to K. S. Bartwal; [email protected] Received 21 January 2013; Accepted 9 February 2013 Academic Editor: Amir Kajbafvala Copyright © 2013 Akrati Verma et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Nanoparticles of mixed metal oxides, ZrO2,ZrTiO4,andZrV2O7 were prepared by microwave-assisted citrate sol-gel and solution combustion method. The prepared nanoparticles were characterized for their structural details using XRD and TEM techniques. The broadening of Raman bands is ascribed to local compositional fluctuations or local positional disordering produced dueto 4+ 4+ random distribution of Zr and Ti between equivalent sites. The XPS spectra confirm the incorporation of Ti in ZrO2 and suggest Zr as well as Ti in +4 oxidation state. Gelation and fast combustion seem to be the reason for smaller particle sizes. ZrV2O7 nanocrystalline material was synthesized by microwave- assisted solution combustion method. Low angle powder XRD measurements confirm the mesoporous nature of the prepared material. The effect of calcination temperature on the phase trans- formation of the materials has been investigated. Among tetragonal, monoclinic, and cubic phases, the monoclinic phase is pre- dominant at higher calcinations temperature. The XPS confirms the incorporation2 ofV O5 in ZrO2 andsuggeststhatZrandV are in the same oxidation state (+4). The average particle sizes for ZrO2,ZrTiO4,andZrV2O7 were found to be in the ranges of 5–10 nm, 2–5 nm, and 10–50 nm, respectively. 1. Introduction Coprecipitation (wet precipitation) is the most widely used method for oxide synthesis. In this method hydroxide Combination of two metal oxides M1OandM2Ocanbe of the metals is precipitated from an aqueous solution of the either a simple mechanical mixing involving weak van der metal salt by titrating it with ammonia solution. The hydrox- Waals forces or a chemical possessing chemical linkages of the ideiswashed,dried,andcalcinedtogetthemetaloxide.Sol- type M1-O-M2. The physicochemical properties of the latter gel method is used to prepare metal oxides by hydrolysis and combination will be entirely different from the simple combi- condensation of metal alkoxides M(OR): nation of individual oxides (mechanical mixture). The degree of dispersion in the chemical rout depends on preparation method and synthetic conditions. Because of this, many dif- M(OR) +HOH → M(OR)−(OH) +ROH (1) ferent synthetic routes for mixed metal oxides have been developed. Some of the popular routes for preparation of 2 mixed oxides are coprecipitation, sol-gel method, condensed The reaction follows an SN mechanism in which the nucle- − + phase combustion, and microwave-assisted solution combus- ophile OH adds to the M and increases its coordination tion method. number in the transition state: H H+ H+ − + − − − O + M O R O + M O R OH M O OH M + ROH (2) H H R 2 Journal of Nanoparticles + The H changes its position to alkoxy group producing a causes the breakdown of the polymer, and a solid amorphous protonated ROH species which is subsequently eliminated, precursor material is finally obtained. On subsequent heating ∘ and metal hydroxide is produced. Production of MgO is between 500 and 900 C,thecationsareoxidizedtoformthe a typical example. The rate of hydrolysis and condensation respective metal oxides. Microwave-assisted MVSCS tech- depends on (1) electronegativity of the metals atom, (2) nique can be used for preparing materials or catalysts, catalyst ability of the metal atom to increase its coordination number, support, fuel cells, capacitors, Li-ion rechargeable batteries, (3) steric hindrance of the alkoxy group, (4) solvents, (5) dye-sensitized solar cells, solid oxide fuel cells (SOFC), and molecular structure of the metal alkoxide, (6) hydrolysis ratio direct methanol fuel cells (DMFCs). ℎ =H2O/Metal, and (7) catalyst. The rate of hydrolysis of The development of zirconia (ZrO2) nanoparticles has 7 Ti(OEt)4 is 10 times faster than that of Si(OEt)4.Thiscanbe attracted much attention due to their multifunctional char- attributed to the ability of Ti to expand its coordination num- acteristics. Nanoparticles have been recognized to have ber from 4 to 6. For the same reason the hydrolysis of Sn(OR)4 potential in the area of photonic applications. It has several is much faster compared to Si(OR)4. applications such as solid oxide fuel cell, biosensors, H2 gas Most advanced ceramics are multicomponent systems storage material, oxygen sensor, catalyst, and catalyst support having two or more types of cations in the lattice. It is there- [2–5]. In addition, zirconia is used as piezoelectric material, fore necessary to prepare gels of high homogeneity in which electrooptic material, and dielectric material [6–8]. It is also cations of various kinds are distributed uniformly at an used as support to disperse various noble and transition met- atomic scale through M-O-M bridges. A major problem in als for distinct catalytic applications. Zirconia is a well-known forming homogeneous multicomponent gel is the unequal solidacidcatalystandann-typesemiconductormaterial. hydrolysis and condensation rates of the metal alkoxides. This ZrO2 is also used as toughening ceramics in thighbone and may result in chemical inhomogeneities, leading to higher oral planting [9]. The existence of metastable tetragonal (t- crystallization temperatures or even undesired crystalline ZrO2) at low temperature has been synthesized by several phases. Several approaches have been attempted to overcome methods. Some of the methodologies such as oxidation of this problem including partial prehydrolysis of less reactive ZrCl2 by molecular oxygen [10], molten hydroxides method precursors, matching of hydrolysis rates by chemical modifi- [11], nonhydrolytic sol-gel reaction between isopropoxide cation with chelating ligands, and synthesis of heterometallic and ZrCl4 [12], and sol-gel template technique [13, 14]are alkoxides. Conventional self-propagating high temperature developedtopreparenanocrystallineZrO2. synthesis (SHS) or condensed phase combustion is associated The traditional preparation of zirconium titanate (ZrTiO4) with difficulties like large particle size of nanomaterials and ceramic is based on solid-state reaction between the TiO2 ∘ ∘ high reaction temperature of about 2000 C. Combustion of and ZrO2 powders at high temperatures (above 1400 C) metal oxides by Mg in the presence of NaCl produces small [15]. In order to improve the functional properties of the particles of metals coated with NaCl. Subsequent treatment ceramic material, heat treatments are generally necessary of the powder with dilute acid removes MgO impurity. Metal which consume high amount of energy. Chemical method canberecoveredbywashingthesamplewithwater. basedonCoprecipitationofthereactiveprecursorswas Microwave-assisted method of oxide synthesis is gaining developed to prepare powders with a high purity and low popularity because of its high rate of reaction, efficient heat treatment cost after reaction16 [ ]. Low temperature synthesis transfer, and environmental friendly nature. In this process of zirconium titanate has been reported by Karakchiev et al. material is directly heated by radiation instead of indirect [1]andDosSantosetal.[17]. Zirconium titanate possessing heating by thermal sources leading to higher temperature a layered wolframite-type structure (ABO4) with space group homogeneity in the reaction mixture. In this process of heat- P2/a finds applications as microwave components. As catalyst ing, microwave radiation interacts with the polar molecules and catalyst supports, ZrTiO4 is employed for a wide variety possessing dipole moment and makes them reorient through of catalytic applications both in liquid and gaseous phases [18, rotation. A large number of molecules try to orient together 19]. Recently it has been reported that the oxide nanoparticles resulting in collision and production of heat. Thus microwave are promising for photonics, and their monodispersion and heating is energy conversion method in which electro- appropriate contact with the surrounding medium are very magnetic radiation is converted into heat energy rather crucial parameters [20, 21]. than heat transfer by convection in conventional heat- Vanadium incorporated zirconia (V2O5-ZrO2)isameso- ing. In microwave-assisted solution combustion synthesis porous material and is very useful catalyst. Nanoporous (MWSCS) an aqueous solution of metal nitrate (oxidant) and materials consist of a regular organic or inorganic framework fuel (urea, citric acid) is subjected to a microwave heating supporting a regular, porous structure. A mesoporous mate- for few minutes to obtain a viscous gel which on drying and rial is a material containing pores with diameters between calcination produces the metal oxide/mixed metal oxide. The 2and50nm.The-phase (cubic) of ZrV2O7 is the stable solution combustion method seems to be modification

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