Heterobimetallic mixed metal oxide (HMMO) nanoparticles and their applications t

The chemistry of HMMO (heterobimetallic mixed metal oxide) nanoparticles of the type AB0 3 (perovskite), AB 2 0~ (spinel and antispinel) and AB 2 0 5 (pseudobrookite) has been discussed. Their preparation by the sol-gel route using mixtures of alkoxides, heterobimetallic alkoxides and heterobim~tallic-}l-oxo alkoxides are described. Physical and chemical properties, with special reference to high surface area, morphology, crystal structure, catalysis and destructive adsorption studies, along with some important applications have also been discussed.

Developments in the field of HMMO nanoparticles have produced very significant and interesting results, which are evident from the recent reports published 1 -5 in the literature. HMMO nanoparticles exhibit a wide array of unusual properties and can be considered as new materials that bridge molecular and condensed matter. One of the unusual features is their enhanced surface chemical reactivity towards incoming adsorbates6-8 and chemical warfare agents 9 · 10 . For example, Al 2 0 3 /Mg0 HMMO nanoparticles adsorb 8 S0 2 and CC1 4 , and MgA1 2 0 4 adsorbs and destroys paraxon [diethyl-4-nitrophenol phosphate (DNPP)l 11 more efficiently as compared to commercially prepared and pure MgO and Al 2 0 3 nanoparticles. This enhanced surface reactivity is attributed to the presence of high concentration of edge/corner sites and other defects, which makes HMMO nanoparticles co-ordinatively unsaturated. Furthermore, the presence of two different metal centres in HMMOs increases their efficiency, due to the close proximity of acidic and basic centres.
In short, the following factors are considered to be important for the unusual chemical properties of HMMO nanoparticles : (i) acid-base behaviour, (ii) high surface area (due to the small particle size with dimensions in the range 1-10 nm), (iii) several types of deficiencies in the bulk as well as on the surface. The above stated properties make these HMMO nanoparticles the best candidates for catalysis 12 -18 (both as catalysts and catalytic supports) and destructive adsorption6- 8 . HMMO nanoparticle systems have been given special attention because of their unique size dependent properties, such as optical, electronic, and magnetic, in comparison to normal HMMOs. Semimagnetic semiconductors of nanometer-sized crystallites are expected to be influenced by the quantum confinement of the electronic states and have great potential for a variety of applications due to the unique propeities of quantum dots 3 .
The synthesis of HMMO nanoparticles by the conventional method is based on solid-state reaction 19 , which requires repeated cycles of milling and calcination at high temperature. This results in lack of homogeneity (due to incomplete reaction) and low surface area ( -1 m 2 /g). In contrast to the solid-state method, various other physical and chemical methods are preferred. Physical methods include gas condensation techniques 2 0-22 , spray pyrolysis 23 · 24 , thermochemical decomposition of metal-organic precursors in flame reactors 25 , and some other aerosol processes named according to the energy sources applied to reach the required temperatures during gas-particle conversion. The wet chemical methods include the sol-gel method, reverse micro emulsion technique 2 6-28 and precipitation from solution 29 · 30 . It has been observed that, out of the various physical and chemical methods, the sol-gel method assumes special significance for the preparation of more homogeneous nanosized HMMOs of high purity and surface area.
A survey of the literature shows that sol-gel3 1 -3 7 processing has several advantages over other techniques for synthesizing nanopowders of mixed metal oxide ceramics. These include (i) the production of ultrafine porous powders, (ii) the homogeneity of the product as a result of homogeneous mixing of the starting materials at the molecular level and (iii) the possibility of obtaining the ceramic material in different fonns by controlling the conditions. In this process, a colloidal sol is converted into a gel by ageing. The gel is subsequently calcined by special techniques. giving rise to a crystalline product with homogeneous composition and large surface area.
In view of the solubility of metal alkoxides3 8 -40 and oxo-alkoxides41.42 in organic solvents, these materials are strongly preferred as precursors in sol-gel processes. In heterobimetallic-~-oxo alkoxides, M-0-M' linkage is present, which makes the M-0-M' bond strong and stable as compared to other precursors. Non-cleavage of the M-O-M' bond. even upon hydrolysis followed by dehydration (drying). makes homogeneous oxides of high surface area HMMO nanoparticles. Interestingly, the structural relationship among these precursors, coupled with their versatility towards hydrolysis, led to their increasing use as starting compounds. These compounds arc considered as especially suitable precursors over other precursors such as metal nitrates, acetates, monodispersed metal hydrous oxides, mainly due to the ease of their purification (either by distillation or recrystallization), solubility in organic solvents, and their extremely facile hydrolyzability.
This article describes the synthesis of HMMO nanoparticles (perovskite, spinel and pseudobrookite) using either mixtures of metal alkoxides, heterobimetallic alkoxides or heterobimetallic-J..t-oxo alkoxides as precursors by the sol-gel method. Other methods of the preparation, and composite/doped nanoparticles are not being covered.
The sol-gel route can be considered as a two-step inorganic polymerization. The nature of the inorganic framework depends upon the relative rates of hydrolysis and condensation at different centres. The rate of hydrolysis depends upon the nature of the metal in terms of its electrophilicity and ability to expand its co-ordination number 43 . The hydrolysis rates of transition metal alkoxides (especially in the case of 274 heterobimetallic alkoxides) are very high due to their highly electrophilic nature and ability to expand their coordination number. This sometimes complicates the problem by causing phase segregation and, to overcome this problem, a modified precursor is prepared by reacting the alkoxide with other ligands. This results in a new precursor, which may undergo hydrolysis at a slower rate 44 . Finally, the solvent is removed from the gel. The manner in which the liquid phase is removed from the wet gel determines whether the dried material is a highly porous aerogel or a denser xerogel. Xerogel is formed as the solvent from wet gel evaporates resulting in a collapse of the wet gel structure. In case the network is compliant, the gel deforms due to the capillary forces generated by the liquid phase as it recedes into the body of the gel. Aerogel is actually a nanoscale mesoporous material of low density and high surface area, which results from supercritical drying (SCD) 45 .4 6 or ambient pressure methods upon wet gels.
Structure and morphology of HMMO nanoparticles HMMO nanoparticles are known to have different structure and morphology, depending upon the size and charge of the constituent ions. The nature of these ions determines the final geometry adopted. HMMO nanoparticles prepared by the sol-gel method most often adopt one of the perovskite, spinel and pseudobrookite types of structures.
Perovskite structure (AB0 3 type) HMMOs having the formula AB0 3 are called perovskite, e.g., CaTi0 3 47 , SrTi0 3 48 , BaTi0 3 48 , etc. HMMOs of the type SrTi0 3 and BaTi0 3 have been prepared by reacting both the alkoxides in a 1 : I molar ratio by the sol-gel method, shown in Scheme 1. CaTi0 3 has been prepared 47 from calcium acetate and titanium isopropoxide in l : 1 molar ratio and calcinations of the gel powder in air at temperatures upto 900°.
The XRD pattern (Fig. l) of SrTi0 3 in different solvents shows that it is an amorphous powder in ethanol. Furthennore, comparison of the XRD patterns of HMMO perovskites of the type SrTi0 3 , BaTi0 3 and CaTi0 3 , after calcination at different temperatures, suggests that calcination at 500° under pressure for SrTi0 3 and BaTi0 3 , and at 900° in air for CaTi0 3 , results in the appearance of an intense peak, indicative of high surface/volume ratio. Surface area analyses of these perovskites are given in Tables 1-3, which suggest that HMMO perovskites prepared by the sol-gel process have high surface area as compared to commercially available samples. Transmission electron microscopy (TEM) micrograph 48 of SrTi0 3 (Fig. 2) indicates that it has a crystallite size of 10 nm. when calcined at 500°.    The structure of the perovskite phase was first thought to be cubic, but later con finned as orthorhombic. The truly cubic form is referred to as "ideal perovskite", having a unit cell edge of -4 A containing one AB0 3 . In this phase, the large cation A is surrounded by 12 oxide ions to form cuboctahedral co-ordination, whereas the B cation is surrounded by 6 oxide ions in an octahedral co-ordination.
--- The XRD pattern (Fig. 3a) of MgAl 2 0 4 shows that it is spinel at -500°, but on increasing the temperature, the intensity of the peak increases, which suggests that the size of the particle decreases. Furthermore, the XRD patterns (Fig. 3b)   MgAI 2 0 4 , which shows superimposability, suggesting their spinel structure.
Surface area 51 analyses (Table 4) show that Mg-0-Al, prepared by the oxo bridge method has the highest surface area upto 440m 2 g-1 and 470m 2 g-1 after heat treatment at 500° in vacuum. However, Mg-0-Al, when prepared by the bridged alkoxide method, shows surface areas of only 228 m 2 g-1 and 242 m 2 g-1 after heat treatment. These results reflect the advantages of the method utilizing the oxo-bridged precursors to obtain high surface area homogeneous HMMO nanoparticles. Analogous Zn-0-AI, Fe-0-Al, Co-0-AI and Mn-0-Al surface areas were in the range of 310-340 m 2 g-1 . TEM studies (Fig. 4) of spinels suggest their crystallite sizes.  5 and Table 5) is a good tool for analyzing the co-ordination environment of At 3 + ions, in particular the octahedral to tetrahedral ratio, and their existence. When A 2 + ions occupy one eighth of the tetrahedral holes and B 3 + ions half the octahedral holes, the structure is called "normal" spinel. It is a stable arrangement having tetrahedral arrangement about a divalent cat-  where x is the "inversion parameter" (0 < x < l ).
Pseudobrookite structure (AB 2 0 5 type) HMMOs of this type have the general formula AB 2 0 5 . Pseudobrookite is a rare oxide mineral and provides this type of structure for a variety of minerals and synthetic phases. The mineral name derives from its appearance. Pseudobrookite resembles brookite, one of the titanium dioxide polymorphs. The synthetic phase "Karrooite" MgTi 2 0 5 also has the psuedobrookite structure in which both the metal ions are co-ordinated by six oxygens, e.g., MgTi 2 0 5 53 , FeTi 2 0 5 54 , etc.
All these pseudobrookite phases are metastable at RT. Stability at high temperature has been proposed to result from the mixing of cations between two different cationoxygen octahedra, which are connected by shared edges.

Properties and applications
The properties of HMMO nanoparticles are size dependent. They exhibit unique chemical and physical properties that are remarkably different from those of the corresponding bulk materials. As the particle size decreases, the percentage of atoms residing on the surface mcreases, giving high surface to volume ratios. This allows more reactive centres becoming available for reactions to occur.
A variety of important aspects centre around the unusual chemical properties ofHMMO nanoparticles; some of these are described below : ( 1) As destructive adsorbents : The development of novel methods for the disposal of chlorinated wastes and other toxic gases in the environment, from industries as well as other sources, has become a very urgent task. Recent studies have shown that HMMO n:.111oparticlcs attract significant attention as effective chemisorbents for such toxic gases as well as chlorine and phosphorus containing compounds.
For this purpose. HMMOs play an important role. For example, Al 2 0 3 /Mg0 adsorbs 8 S0 2 , CCI~ and paraxon more efficiently as compared to pure Al 2 0 3 and MgO nanoparticles and commercially available oxides.

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These data indicate that Al 2 0 3 /Mg0 efficiently adsorbs S0 2 in slightly more than one layer. This is probably due to the greater Lewis basicity of the MgO present and large surface area of AI 2 0 3 , which is beneficial for the adsorption of the acid gas.
In the last few decades, pollution by diesel-engine exhausts has become an ever-increasingly serious problem. Soot and NOx are the main components to be removed from diesel exhaust. Among bimetallic spinel type oxides, CuFe 2 0 4 may be useful, which shows intermediate activity and exceptionally high and low selectivity, respectively, to N 2 and N 2 0 formation 56 . The most interesting application of HMMO spinels is for adsorbing paraxon 11 . These particles possess their destructive adsorption ability due to the presence of acidic and basic oxides in one intermingled mixed metal oxide. HMMO nanoparticles. such as MAI 2 0 4 (M =Mg. Ca, Sr. Ba) have been used for this purpose. To compare the relative adsorption efficiencies of various MMOs, 16 IlL of paraxon and I 00 mg of adsorbent were used. The results (Figs. 7 and 8) show that both MgAI 2 0 4 and CaAI 2 0 4 destructively adsorbed all 8 11L, whereas SrAI 2 0 4 and BaAI 2 0 4 were only able to destructively adsorb 6 IlL before their surface becarne saturated. Therefore, HMMO systems of MgAlz04 and CaAI 2 0 4 are better at destructively adsorbing paraxon than SrAI 2 0 4 , BaAI 2 0 4 , pure MgO, CaO and Al 2 0 3 nanoparticles. Another HMMO nanoparticle. SrTi0 3 is photoactive 48 under UV light and is able to decompose organic volatile compounds like acetaldehyde.  (2) In catalysis : Relatively little work has been reported where these nanostructured materials have been explored in classical catalytic processes. Some examples are given below. In CuMn 2 0 4 , the Cu-Mn-0 system is a well-known oxidation catalyst 57 for carbon monoxide oxidation with oxygen at RT. The activity ofCu\1n 2 0 4 decreases at temperatures above 600° due to the crystallization of spinel CuMn 2 0 4 . The high activity of CuMn 2 0 4 is believed to be due to the redox system Cu 2 + + Mn 3 + "---"" cu+ + Mn 4 + and unique adsorption properties of carbon monoxide on Cu 2 +JMn 4 + as well as 0 2 on cu+/Mn 3 +. Similarly, NiMn 2 0 4 and CuCo 2 0 4 also have high activity.
It is clear that surface properties such as the precise surface structures and the role of point defects are crucial to the proper understanding of the catalytic 56 and electrochemical properties. La 2 Ni0 4 has been widely studied for its catalytic properties such as partial oxidation and hypochlorite decomposition. There is continuing interest in La 2 Cu0 4 since it also exhibits catalytic properties. It is used in several chemical reactions 59 -6l including CO oxidation, NO reduction, CH 4 oxidation and as a three-way automobile exhaust catalyst 61 .
Besides these applications, MA1 2 0 4 (Ca, Sr, Ba) 62 have been widely used as hosts for ceramic pigments, practical phosphors and excitation sources for other phosphors and luminous paints.
Recently, Co-Ti pseudobrookite (uncalcined) oxide nanoparticles5 5 were found to be good photocatalysts for the decomposition of acetaldehyde to carbon dioxide.
( 3) In electronics, magnetics and optics : HMMO nanoparticles arc characterized by an ultrafine grain size (<50 nm). These particles are subjects of cunent interest because of their unusual magnetic, optical and electronic properties, which often differ from their bulk properties. The reasons for these are the confinement of electronic and vibrational excitation, quantum size effect and large sur-1/CS-3 face to volume ratio. Perovskite type oxides of the general formula AB0 3 are important in materials science, physics and earth sciences, e.g., for their electrical properties. These are widely used for the preparation of electronic components, electro-optical and photocatalytic materials. For example, SrTi0 3 , BaTi0 3 and related compounds have been extensively used in the preparation of high dielectric constant capacitors, PTC resistors, transducers and fenoelectric memories.
LaGe0 3 and La 2 Ni0 4 are of cunent interest due to their potential applications in solid oxide fuel cells and ceramic membranes for oxygen separation. The LiM 2 0 4 type of spinels is also important. LiTi 2 0 4 was the first oxide superconductor with a transition temperature exceeding 10 K. LiV 2 0 4 showed all the characteristics of a heavy fermion system with an effective mass enhancement of the order of I 00. Similarly, LiMn 2 0 4 a very important positive electrode material for commercial Li-ion batteries.
ZnFe 2 0 4 is well known as an anomalous antifenomagnetic substance. Recent studies have shown that ZnFe 2 0 4 , especially nanometer-sized nanoparticles, with a relatively small band gap, is a potentially useful solar energy material for photoelectric conversion and photochemical hydrogen production from water63. It has the advantage of absorbing visible light, without being sensitive to photoanodic conosion.

Conclusion
The above brief account clearly depicts a versatile fast developing chemistry of HMMO nanoparticles, which is many a time being triggered by the multifaceted demands for advanced oxide-ceramics with novel applications. These HMMOs, prepared by the sol-gel process, are nanoparticles of ultrafine purity and high surface area. This high surface area is responsible for their applications in destructive adsorption, catalysis, and other applications, such as in magnetics, optics and electronics.
Besides the above applications, metal oxide nanoparticles have either been used or tested for their applications in the following fields : ( 1) Decontamination of chemical warfare agents (CWAs): CWAs are generally organophosphorus esters. e.g., nerve agents (also called "Soman") and blister agents (also called "mustard"). Nerve agents can react with the enzyme acetylcholinesterase, which inhibits its control over the central nervous system. Reaction of the CWAs on MgO, CaO or Al 2 0 3 nanoparticles at RT decontaminates 10 them by the cleavage of the P-0 and P-F bonds and make them nontoxic.
(2) Biocidal activity: Some nanomaterials possess biocidal activity 64 · 65 towards spores and negetativie cells, viruses and toxins. For this purpose, CaO and MgO nanoparticles are generally used. MgO nanoparticles adsorb chlorine to form deep yellow adduct MgO.CI 2 nanoparticles. This adduct is capable of oxidizing bacteria. This killing action is not well understood, but may be due to a combination of the abrasive nature of the MgO nanoparticles and oxidizing ability of the surface chlorine.
( 3) Cosmetics : Ti0 2 and ZnO nanoparticles have been used in sunscreen creams for protection against UV irradiation.
(4) Pharmaceuticals: They are also used as drug delivery agents. Nanoparticles have been developed that can safely cross the blood-brain barrier and deliver therapeutic agents to particular parts of the brain. Silica coated semiconductor nanoparticles are readily incorporated into a wide variety of eukaryotic cells. (5) Nanotechnology: It is the science of building machines and materials at the molecular level, where key components are measured in nanometers. This promises medical advances, smarter and lighter materials, more efficient manufacturing, cleaner energy, faster and more efficient electronics and better ways to detect, prevent and treat pollution.
In light of the extensive studies and probable applications of HMMOs, they may prove to be the materials of this century.