Influence of crystalline phase and defects in the ZrO 2 nanoparticles synthesized by thermal plasma route on its photocatalytic properties Ashok B. Nawale a , Nilesh S. Kanhe a , S.V. Bhoraskar a , V.L. Mathe a, *, A.K. Das b a Department of Physics, University of Pune, Pune 411007, India b Laser and Plasma Technology Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India 1. Introduction Zirconium dioxide is an important material which is widely used in ceramic technology and heterogeneous catalysis [1]. Nanometric ZrO 2 (zirconia) belongs to one of the technologically important class of materials having wide range of applications [2]. Zirconia occurs in three different polymorphic forms, namely; monoclinic (M), tetragonal (T) and cubic (C) phases. The monoclinic phase is thermodynamically stable at temperature T < 1170 8C, which transforms into the tetragonal phase above 1170 8C and into cubic phase at above 2370 8C [3]. Normally these high temperature phases which are technologically important due to their symmetric properties are not stable at room temperature. To stabilize them at room temperature one needs to add some stabilizers such as yttria, calcia or magnesia in zirconia. The observed phase stability largely depends on the techniques used to produce the material and the size of the resulting particles [4]. Considering the vast technological applications of different phases of zirconia, the phase controlled study of zirconia becomes an important aspect to study. The effects of thermal treatment on the crystal structure and morphology of the nanoparticles of zirconia have been studied by different groups [5–9]. Rashad and Baioumy [5] have reported that the co-precipitation route led to the formation of tetragonal ZrO 2 with low crystallinity at 700 8C but the tetragonal phase was transformed to monoclinic one at temperatures ranging between 1000 and 1200 8C. The citrate gel combustion route led to the formation of monoclinic phase without presence of tetragonal phase in the temperature range between 1000 and 1200 8C. In contrast, microemulsion refined precipitation tech- nique led to the formation of tetragonal phase with high crystallinity, compared with the other routes at 700 8C. However the tetragonal phase was transformed into cubic phase at the calcination temperatures of 1000–1200 8C. Ward and Ko [6] reported that depending upon the heat treatment; zirconia aerogel possesses either a completely amorphous, tetragonal or monoclinic form. Santos et al. [9] reported the presence of tetragonal phase in ZrO 2 at a temperature higher than 400 8C which transforms into monoclinic phase above 600 8C. It has also been reported [8] that oxygen vacancies play an important role in deciding the final crystalline phase of the system. As per this report, in case of Yttria Stabilized Zirconia (YSZ) annealed at 1500 8C for 100 h, the amount of monoclinic phase was found to increased with the slower cooling rate; whereas, fast cooling rate led to the formation of cubic phase. The detailed mechanism of oxygen vacancy related phase transformations can also be found in the same literature. Zirconia, due to its n-type semiconducting nature, has been considered as a photocatalyst in the photochemical heterogeneous reactions [10]. Relatively wide band gap value and the high negative value of the conduction band allows its use as a photocatalyst in the production of hydrogen through water decomposition [11,12]. Botta et al. [13] reported that the photocatalytic activity depends on the structural and optical Materials Research Bulletin 47 (2012) 3432–3439 A R T I C L E I N F O Article history: Received 28 July 2011 Received in revised form 11 July 2012 Accepted 18 July 2012 Available online 27 July 2012 Keywords: A. Nanostructures, C. X-ray diffraction, D. Crystal structure, D. Defects, D. Catalytic properties A B S T R A C T The photocatalytic activity of pure ZrO 2 samples, prepared by a thermal plasma route, and characterized by X-ray diffraction technique was tested for the degradation of methylene blue, which is known to be a hazardous dye. Although, all these samples synthesized at different operating pressures of the thermal plasma reactor showed the photocatalytic activity; the sample synthesized at 1.33 bar of operating pressure showed the highest photocatalytic activity. The blue shift in the band gap of monoclinic phase, as observed from the photo-absorption spectroscopy, may be attributed to the enhanced photocatalytic activity. The existence of different defect states and their concentration as, inferred from the photo- absorption measurements were also found to be responsible for the enhanced photocatalytic activity of the as synthesized samples. ß 2012 Elsevier Ltd. All rights reserved. * Corresponding author. Tel.: +91 020 25692678; fax: +91 020 25691684. E-mail address: vlmathe@physics.unipune.ac.in (V.L. Mathe). Contents lists available at SciVerse ScienceDirect Materials Research Bulletin jo u rn al h om ep age: ww w.els evier.c o m/lo c ate/mat res b u 0025-5408/$ – see front matter ß 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2012.07.010