Comparative investigation of zirconium oxide (ZrO2) nano and microstructures for structural, optical and photocatalytic properties
Graphical abstract
Highlights
► Synthesis of ZrO2 nano and microstructures was achieved by simple combustion method. ► Microwave combustion method was found to be highly facile, efficient and novel. ► The ZrO2 content has enhanced the photocatalytic activity of TiO2.
Introduction
Recently, nanomaterials have sparked a worldwide interest due to their unique physical and chemical properties in comparison with those of their bulky counterparts. Nowadays, many techniques, such as co-precipitation, electrodeposition, sol–gel chemistry and solvothermal route, have been developed to synthesize materials in nanoscale, but these strategies are always high-energy consuming and require rather long reaction times. Conventional combustion method (CCM) is self-sustained by its own exothermic reaction, which is an effective method for the mass production of various metal oxides [1], [2], [3], [4]. However, microwave assisted combustion synthesis of materials has recently gained importance over the conventional heating methods, due to the reason that microwaves interact with the reactants at the molecular level, where this electromagnetic energy is transferred and converted to heat by rapid kinetics through the motion of the molecules [5]. This results in the formation of nanoparticles, early phase formation and different morphologies within a few minutes of time [6], [7]. Zirconium oxide, ZrO2, has been widely investigated due to its potential applications in many fields such as catalysis, restorative dentistry, high temperature ceramics, polymer nanocomposites and sensor, either in pure or its modified state with other metal oxides [8], [9], [10], [11]. It is now well recognized that the mechanical, electrical, chemical, as well as catalytic properties of ZrO2 can be improved by using nanocrystalline instead of conventional micrometer-sized ZrO2 due to the unusual surface phenomena at the nanoscale [12], [13], [14]. Nanocrystalline ZrO2 powders and colloids have been prepared by quite a few methods, including the sol–gel process, combustion, hydrothermal method, microwave irradiation, etc. [15], [16], [17], [18], [19]. However, it is a challenge to find an efficient way to prepare nanocrystalline ZrO2 with particle size in about several nanometers. Thus, we made an attempt to prepare ZrO2 by two methods namely, microwave combustion method (MCM), and conventional combustion method (CCM) for comparative investigation of their properties. In combustion experiments, rapid combustion and crystallization using urea as a fuel could generate various structural defects. It is, therefore interesting to examine the defect energy levels and surface states of ZrO2. Based on the obtained fundamental knowledge, for the first time in the literature, we have prepared ZrO2 nanostructures with well-crystalline structures and high defect concentration using a MCM approach, while simultaneously avoiding additional calcination procedures. From the results, it is found that MCM is an attractive methodology to control the particle size and properties than CCM. The as-prepared ZrO2 nanostructures were investigated for optical properties.
Although, TiO2 is the most widely investigated photo-catalyst, improving its quantum efficiency is still a major challenge. The improvement of photocatalytic activity of TiO2 can be achieved by doping with metals, the addition of electron donors to the reaction system, and the addition of other semiconductor metal oxides in order to reduce the charge carrier recombination [20]. It has been reported that the addition of ZrO2 to TiO2 can increase the surface area and hydroxyl groups in ZrO2–TiO2 mixed catalysts which, in turn, improves the quantum efficiency of TiO2 [21]. Moreover, ZrO2 and ZrO2–TiO2 binary oxide catalysts have been well investigated for their photocatalytic properties [21], [22], [23], [24], [25], [26], [27]. Such advanced ZrO2–TiO2 mixed oxides extend their application through the generation of new catalytic sites due to a strong interaction between them. Therefore, it is highly interesting and desirable to study the photocatalytic activity of ZrO2–TiO2 mixed oxide.
Section snippets
Preparation of ZrO2 by simple microwave combustion method (MCM) and conventional combustion method (CCM)
All the reagents used were of analar grade obtained from Merck, India and were used as received without further purification. The combustion method takes advantage of fast and self-sustaining chemical reactions. This method uses the advantage of the propellant chemistry in making the redox mixtures, in which the metal nitrate acts as an oxidizing reactant and fuel as a reducing reactant. Stoichiometric amounts of zirconyl nitrate and urea as fuel are calculated based upon propellant chemistry.
Structural Investigations
ZrO2 exists mainly in three phases, viz., monoclinic, tetragonal, and cubic. Crystallization of ZrO2 in a particular phase depends upon the temperature conditions employed during the synthesis and the time of reaction. The XRD patterns were recorded twice on two batches of samples for reproducible results. As shown in Fig. 1a and b, the observed diffraction peaks at 2θ = 30.19°, 35.00°, 50.39°, 58.89°, and 62.85° are associated with [1 1 1], [2 0 0], [2 2 0], [3 1 1], and [2 2 2] plane, respectively. These
Conclusion
ZrO2 nano-crystals have been successfully synthesized by simple and rapid MCM and ZrO2 micro-crystals by CCM using urea as the fuel. In comparison with CCM, MCM is not only a quick processes, but also uniformly spreads the heat through the entire bulk of the reaction mixture, which result in the formation of a nanospherical structure with a narrow distribution of particle size. Both MCM and CCM rendered ZrO2 to crystallize rapidly, assisting in the inclusion of crystal defects. However, MCM
Acknowledgment
The authors duly acknowledge the financial support rendered by the University Grants Commission (UGC) (Ref. F. No. 38-118/2009 (SR)), New Delhi.
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