Microwave Synthesis, Characterization, and Photoluminescence Properties of Nanocrystalline Zirconia

We report synthesis of ZrO2 nanoparticles (NPs) using microwave assisted chemical method at 80°C temperature. Synthesized ZrO2 NPs were calcinated at 400°C under air atmosphere and characterized using FTIR, XRD, SEM, TEM, BET, and EDS for their formation, structure, morphology, size, and elemental composition. XRD results revealed the formation of mixed phase monoclinic and tetragonal ZrO2 phases having crystallite size of the order 8.8 nm from most intense XRD peak as obtained using Scherrer formula. Electron microscope analysis shows that the NPs were less than 10 nm and highly uniform in size having spherical morphology. BET surface area of ZrO2 NPs was found to be 65.85 m2/g with corresponding particle size of 16 nm. The band gap of synthesized NPs was found to be 2.49 eV and PL spectra of ZrO2 synthesized NPs showed strong peak at 414 nm, which corresponds to near band edge emission (UV emission) and a relatively weak peak at 475 and 562 nm.


Introduction
ZrO 2 (zirconia) is a material of great technological importance, having good natural color, high strength, high toughness, high chemical stability, excellent corrosion, and chemical and microbial resistance [1,2]. ZrO 2 is a wide band gap Ptype semiconductor that exhibits abundant oxygen vacancies on its surface. The high ion exchange capacity and redox activities make it useful in many catalytic processes as a catalyst [3]. ZrO 2 is also an important dielectric material that is being investigated for potential application as an insulator in transistors in future nanoelectric devices [4]. Garcia et al. [5] have highlighted its potential to replace SiO 2 in advanced metal oxide semiconductor (MOS) devices and in optical applications.
ZrO 2 has three well-defined crystal structures/phases, that is, cubic (c-ZrO 2 ), tetragonal (t-ZrO 2 ), and monoclinic (m-ZrO 2 ), under normal atmosphere and at different temperatures [6,7]. Generally, m-ZrO 2 phase is thermodynamically stable up to 1100 ∘ C, t-ZrO 2 phase exists in the temperature range of 1100-2370 ∘ C, and the cubic phase is found above 2370 ∘ C [8]. There are contradictory reports on the existence of various phases under different conditions of temperatures. Recently, existence of t-ZrO 2 at low temperature has been reported by many researchers [9][10][11]. Ciuparu et al. [9] have reported the initial existence of amorphous phase, and after heating above 750 ∘ C, t-phase formation has been observed, and transition from tetragonal to monoclinic has been observed after calcinations at 1200 ∘ C. Shukla et al. [11] have observed nucleation of t-ZrO 2 phase from the amorphous phase at 400 ∘ C, which becomes stable at 600 ∘ C, and after calcination at 800 ∘ C complete tetragonal-to-monoclinic phase transformation has been reported. ZrO 2 has many interesting characteristics, such as superior mechanical characteristics, increased fracture toughness, hardness, strength, high thermal expansion coefficient, low thermal conductivity, superplastic deformation, phase stability, and good chemical resistance, which have resulted in a variety of industrial and engineering applications that include high durability coating, catalytic agents, medical prosthetics, cutting tools, synthetic jewels, high density grinding media, wear components, bearings, seals, valves, nose cones and 2 The Scientific World Journal tiles for space shuttles and missiles, turbines, and the most demanding aviation engines [12][13][14][15][16][17]. It also has the best performance as ceramic dental material. ZrO 2 has found uses in solid oxide fuel cells [18] and in NO , O 2 gas sensors [19]. The fully stabilized ZrO 2 is also well suited for high temperature energy conversion systems, due to its high oxygen ion transport capabilities and good long-term stability.
In spite of many unique characteristics, it has serious drawback for engineering application because it undergoes phase transformation with a change in volume at high temperatures, whereas, for optical and electronic applications, precise evaluation of its band gap is of significance since wide variation has been reported in its band gap [5,9]. ZrO 2 is usually in the form of m-ZrO 2 at room temperature. Thus, the synthesis and room temperature stabilization methods of nano-t-ZrO 2 are very important and of scientific significance with application prospects.
To prepare nano-ZrO 2 , several methods have been reported that include sol-gel, flame spray, combustion, glycothermal process, hydrothermal processing, and precipitation routes [7,[20][21][22][23][24]. In particular, a chemical solution method, which involves formation of a stable particle by a direct reaction between the atoms or reaction species, provides the most suitable way of synthesizing a sample of imperfection/defect free particles. The sol-gel method is one of the most common methods that have been used to synthesize nano-ZrO 2 . Many of these methods of nano-ZrO 2 preparation use a synthetic template material, which is not economical and relatively more expensive. In the literature, there are few reports on the usage of microwave method for the synthesis of ZrO 2 [10,25,26] which is the novel route of synthesis of metal oxide NPs, being clean, costeffective, energy efficient, eco-friendly, rapid and convenient method of heating, and results in higher yields in shorter reaction times [27]. Here, we report simple, low temperatures of synthesis of nanocrystalline ZrO 2 NPs using microwave assisted method. To the best of our knowledge, the reaction temperature we used is the lowest used in the synthesis of ZrO 2 NPs. The synthesized NPs of ZrO 2 were investigated using FTIR, XRD, SEM-EDS, and TEM analysis.

Synthesis.
All chemicals used in the synthesis have been used as received from chemical suppliers without any further purification and processing. 50 mL of 0.1 M zirconium oxychloride octahydrate solution was prepared and 15 mL 1 N NaOH solution was added dropwise in given solution with continuous stirring, after precipitation precursor solution was kept in microwave oven for 12 min with power 420 W and temperature 80 ∘ C using RAGA'S microwave system. Microwave used for this experiment was of power range of 140 W to 700 W. Obtained precipitate was filtered, washed 2-3 times with DI water (18.2 MΩ⋅cm resistivity) and then dried at temperature 150 ∘ C for several hours. Dried powder was crushed and calcinated in air atmosphere at 400 ∘ C for 2 hours and obtained sample was characterized.

Characterization.
The prepared annealed samples were characterized for their formation, structure, morphology, and elemental composition using Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction analysis (XRD), scanning electron micrograph (SEM), transmission electron micrograph (TEM), energy dispersive spectrometer (EDS), Brunauer, Emmett and Teller (BET), and UV-Vis spectroscopy. FTIR of samples was performed using Shimadzu Affinity-1 FTIR spectrometer. Crystallographic study was carried out using Bruker AXS, Germany (Model D8 Advanced), diffractometer in the scanning range of 20-80 ∘ (2 ) using Cu K radiations of wavelength 1.5406Å. JEOL ASM 6360A scanning electron microscope (SEM) was used to study the morphology and the elemental analysis. Transmission electron microscopy (TEM) of samples was carried out using FEI-Tecnai G 2 20. Measurement of BET surface area was carried out for nitrogen adsorption using a Micromeritics Ins., USA. UV-Vis spectroscopy of samples was done in the range of about 200-800 nm with the help of Ocean Optics HR4000 high-resolution spectrometer. Room temperature photoluminescence properties of NPs were studied using Perkin Elmer LS55 Luminescence Spectrometer in spectral range of 300-800 nm in the wavelength range of 270-320 nm as excitation source.

FTIR Analysis.
In order to ascertain the molecular nature of the synthesized material, the FTIR spectrum of the ZrO 2 sample was taken as shown in Figure 1. The spectrum of ZrO 2 depends on the nature of the material, preparative procedures used, solid-state structure, and so forth. The observed strong FTIR absorption peak at about 470 cm −1 region is due to the Zr-O vibration, which confirm the formation of ZrO 2 structure [28], prominent peak of 1380 cm −1 region corresponds to O-H bonding, peak in the region of 1553 cm −1 may be due to the adsorbed moisture and in the 3425-3495 cm −1 region is attributed to stretching of O-H groups, characteristic of a highly hydrated compound [28].

Crystallographic Analysis.
To confirm the phase formation of ZrO 2 , XRD pattern of the sample was recorded after calcination in air atmosphere and is shown in Figure 2.
The narrow line widths indicate high crystalline nature of the synthesized material. As reported by Shukla et al., [29] scan range of 27-32 degrees contained the strongest lines for monoclinic as well as tetragonal phases. Observed XRD peaks ( Figure 2) [30,31]. Stabilization of t-phase at lower temperature is most likely due to low surface energy of the t-phase relative to m-phase. At 300 ∘ C, zirconia has an amorphous structure and pure ZrO 2 is crystallized at a temperature of about 400 ∘ C [23], but in our case we have observed mixed phase formation. Mixed phase formation of ZrO 2 is a common feature in zirconia synthesis and has been reported by many authors [28]. Tyagi et al. [23] have reported t-ZrO 2 phase formation at 400 ∘ C, and with increase in calcinations temperature they have reported m and t-ZrO 2 . The occurrence of metastable tetragonal phase is attributed to the critical crystallite size effect as reported by Garvie [32]. Garvie experimentally showed the existence of a critical size of ∼ 30 nm, below which the metastable t-phase is stable and above which the m-phase is stable.
The crystallite size of NPs has been calculated by XRD line broadening of peak using Scherrer's formula [33] = Cos , where is 0.9, is wave length of X-ray source (0.1540598 nm), is full width at half maximum in radians, and is Bragg's diffraction angle. Crystallite sizes, calculated using Scherrer's formula (1), are indicated in Figure 2 corresponding to each peak and are the order of 8.8 nm from most intense XRD peak, whereas average size is about 14 nm which is in close agreement with the particle size of ZrO 2 as obtained from BET surface area analysis as discussed.

Elemental and Morphological
Analysis. The synthesis of ZrO 2 NPs was confirmed by recording EDAX spectra and is depicted in Figure 3. Emission peaks, such as OKa, ZrL1, and ZrLa were observed in the EDAX spectrum confirmed the stoichiometry of synthesized NPs. It shows that Zr and O elements are present almost in stiochoimetric ratio.
Morphological investigations of 400 ∘ C air calcinated ZrO 2 sample were carried out using SEM and TEM and are shown in Figures 4 and 5, respectively. The morphological characterization highlighted the importance of nanocrystalline ZrO 2 preparation in maintaining the nanostructured phase. It is clear from Figure 4 that NPs are of uniform size. Observation of Figure 4 shows that ZrO 2 particles are spherical in nature and size of the particles is in the nm regime, but size could not be finely resolved from SEM. For the purpose, TEM of sample has been shown in Figure 5. As can be seen from TEM micrographs of sample, some agglomeration has been observed due to different t-and mphases present in the sample. In spite of agglomeration of the NPs, it can be observed that the sizes of the particles are of the order 10 nm. The TEM results are also supporting the results of other analyses based on XRD and BET surface area calculations. Figure 5(d) shows an electron diffraction pattern indicating well-defined quasicontinuous diffraction rings of the sample that calcined at 400 ∘ C for 2 h. It is noticeable that the (101), (110), and (200) planes are clearly distinguished [34] as observed in XRD patterns.

BET Analysis.
Surface area analysis was done by nitrogen absorption using BET surface analyzer. NPs size has been calculated from surface area assuming NPs to be of spherical shape, using the following equation [35]: where is the density of ZrO 2 particles and BET is the BET surface area for powder sample calcinated at 400 ∘ C; measured BET surface area has been found to be 65.85 m 2 /g. Particle size of ZrO 2 NPs was found to be 16 nm from (2) which is in agreement with the above discussion under the assumption used in the analysis.

Band Gap Analysis.
Reduction in the nanoparticle size can cause change in the optical band gap of metal oxides through the narrowing of the valence and conduction bands [5]. The other important factors that can affect optical band gap are defect centers, mechanical stress, and changes in the crystallinity. Optical absorption spectra of NPs, as shown in Figure 6, have been studied without taking into account the reflection and transmission losses. Tauc's relation of the absorption coefficient ( ) with the photon energy (ℎ]) has been used to determine the band gap energy of sample and is given by The Scientific World Journal where is the absorption coefficient, 0 is the constant, ℎ] is the photon energy, and is the band gap energy of the material. The value of depends on the probability of transition; it takes values as 1/2, 3/2, 2, and 3 for direct allowed, direct forbidden, indirect allowed, and indirect forbidden, respectively. The variation of ( ℎ]) 2 versus ℎ] is linear at the absorption edge which confirms that ZrO 2 is semiconductor with direct band gap. The plot of ( ℎ]) 2 versus ℎ] is shown in Figure 7. Extrapolating the straight-line portion from higher absorption region of the plot ( ℎ]) 2 versus ℎ] to photon energy axis for zero absorption coefficient value gives the (Figure 8), giving a value of about 2.49 eV [36]. In the literature, a wide variation has been reported in the band gap of ZrO 2 which can be attributed to presence of the zirconia phase, defect state, and morphology. of impurities in the materials, as well as exciton fine structure. Figure 8 shows the room temperature photoluminescence spectra of ZrO 2 NPs excited at four different wavelengths, that is, 270, 290, 300, and 320 nm. Representative fluorescence spectrum with an excitation wavelength of 270 nm shown in Figure 8 exhibited three emission PL bands at around 414, 475, and 563 nm, with 414 nm being the prominent peak. Liang et al. [10] have also reported similar results for t-ZrO 2 . As the fluorescence intensity somewhat varied with the excitation wavelength, the fluorescence band position and the band shape stayed nearly the same for these excitation wavelengths. This indicated that the fluorescence involved the same initial and final states in the excitation wavelengths ranging from 270 to 320 nm. It can be accounted for by fast relaxation from the final state arrived at by photo-excitation to those states from which the fluorescence started [37]. The spectrum features a broad fluorescence band cantered at 414 nm. Although the detailed PL mechanism for the nano-ZrO 2 is still under research, we could ascribe the emissions that appear at short wavelength excitation to the near bandedge transitions. The PL peak at 414 nm is attributed to Zr vacancies which is one of the intense peaks considered to be due of band edge emission due to the free-exciton recombination [38]. In the case of the emission at 475 and 563 nm, it should be due to the involvement of mid-gap trap states, such as surface defects and oxygen vacancies. The weak green emission also implies that there are surface defects in ZrO 2 NPs. Large amounts of surface defects exist on the as-synthesized nano-ZrO 2 particles because of their high surface area. The broad band and the substantial red shift of the band maximum compared to the band gap of the bulk material (5.6 eV) strongly indicate that the fluorescence involves extrinsic states. Because the particle size distribution is very narrow, the broad fluorescence band seems to be mostly caused by the small particle size leading to an inhomogeneous broadening from a distribution of surface or defect states.

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Conclusions
ZrO 2 NPs have been synthesized with the help of a simple microwave assisted chemical process at low temperature. The TEM analysis reveals that the size of the spherical NPs is in the range of 8-10 nm. All analyses have consistently shown fairly uniform NPs with superfine size having mixed t-ZrO 2 and m-ZrO 2 phases. Band gap and PL of the synthesized sample were investigated and band gap of the synthesized ZrO 2 sample was found to be 2.48 eV from Tauc's relation from UV-Vis absorption spectroscopy. Room temperature PL study of synthesized sample showed three emission PL bands at around 414, 475, and 563 nm, with 414 nm being the prominent peak when excited at 270 nm.