Structural and Optical Properties of Zirconia Nanoparticles by Thermal Treatment Synthesis

Zirconium dioxide nanoparticles with monoclinic blended structure were successfully synthesized by thermal treatment method using zirconium (IV) acetate hydroxide as the metal precursor, polyvinylpyrrolidone as the capping agent, and deionized water as a solvent. The chemicals were mixed and stirred to form a homogeneous solution and hereafter directly underwent calcination to attain the pure nanocrystalline powder, whichwas confirmed by FTIR, EDX, andXRDanalyses.The control over the size and optical properties of nanoparticles was achieved through changing in calcination temperatures from 600 to 900C. The obtained average particle sizes from XRD spectra and TEM images showed that the particle size increased with increasing calcination temperature. The optical properties which were investigated using a UV-Vis spectrophotometer showed a decrease in the band gap energy with increasing calcination temperature due to the enlargement of the particle size. These results prove that, by eliminating drying process (24 h) in the present thermal treatment method, size-controlled zirconia nanoparticles were conveniently manufactured with a reduction of synthesize time and energy consumption, suitable for large-scale fabrication.


Introduction
ZrO 2 (zirconia) is a material of great technological importance, having good natural color, high strength, transformation toughness, high chemical stability, excellent corrosion resisting material, and chemical and microbial resistance [1,2].ZrO 2 is a wide band gap p-type semiconductor that exhibits abundant oxygen vacancies on its surface.The high ion exchange capacity and redox activities make it useful in catalysis [3].ZrO 2 is also an important dielectric material for potential application as an insulator in transistors in future nanoelectric devices [4].Garcia et al. have highlighted its potential to replace SiO 2 in advanced metal oxide semiconductor (MOS) devices and in optical applications [5].
ZrO 2 nanoparticles have found uses in solid oxide fuel cells [6] and in nitrogen oxide, oxygen gas sensors [7].The fully stabilized ZrO 2 nanoparticles are also well suited for high temperature energy conversion systems, attributed to its high oxygen ion transport capabilities and long-term stability.
ZrO 2 is a wide band gap p-type semiconductor that exhibits abundant oxygen vacancies on its surface.
Several techniques are available for producing zirconia nanoparticles, such as sol/gel method [11], vapor phase method [12], pyrolysis [13], spray pyrolysis [14], hydrolysis [15], hydrothermal [16], and microwave plasma [17].However, these methods faced many limitation factors such as complicated procedures, high reaction temperature, long reaction time, toxic reagents and by-products use, and high cost of production, which made it difficult to prepare zirconia nanoparticles on a large-scale production.
Recently, thermal treatment method has been used in syntheses of several nanomaterials including metals ferrite nanoparticles [18][19][20][21], zinc oxide nanoparticles [22], cadmium oxide nanoparticles [23], and thermoluminescence nanomaterials [24].The drying process for 24 h at 80 ∘ C before undergoing calcination is a common feature in the thermal treatment synthesis.However, in the synthesis of ZrO 2 nanoparticles, a solution containing metal precursor and capping mediator is directly submitted to calcination, thus eliminating the drying process and reducing the preparation time and the energy consumption.This modified thermal treatment synthesis looks greener than the earlier thermal treatment methods for fabrication of metal oxide nanoparticles such as nanosized ZrO 2 powder, a necessity for large-scale industrial application.The technique is relatively environmentally friendly as no toxic material discharges into the drainage system.The effect of calcination temperature on the structural, particle size, and optical properties of ZrO 2 nanoparticles is also investigated using various techniques.The PVP solution is made by dissolving 3 g of PVP powder in 100 mL of deionized water at a temperature of 70 ∘ C and magnetically stirred for 2 h.The metal precursor of 0.2 mmol was added to the PVP solution and stirred continuously for another 2 h until a semitransparent solution was obtained with no significant precipitation of materials.The mixture was directly placed in an alumina crucible for calcination at different temperatures ranging from 600 to 900 ∘ C in a retention time of 3 h [25,26], to decompose the polymer and to crystallize the metal oxide nanoparticles.

Characterization
Procedures.Thermal analysis of initial solution dried at 30 ∘ C was investigated by Thermogravimetric Analysis (TGA) and Derivative Thermogravimetry Analysis (DTG) using a Perkin Elmer Thermal Analyzer model TGA7/DTA7 in the presence of N 2 with 10 ∘ C/min heating rate from room temperature to 1000 ∘ C to optimize the heat treatment program.Fourier transform infrared spectroscopy (FTIR) was used to study the chemical composition of samples using Perkin Elmer Spectrum 1650.Energy dispersive Xray (EDX) measurements were performed under a variable pressure scanning electron microscope (VPSEM, LEO 1455) with an Oxford INCA EDX 300 microanalysis attachment.The crystal phase of prepared samples was determined by X-ray diffraction (XRD) technique using Shimadzu-6000 diffractometer utilizing Cu K (0.154 nm) radiation.The morphology and average particle size of the nanocrystalline powder were evaluated using Hitachi H-7100 Transmission Electron Microscope (TEM) operating at accelerating voltage of 100 kV.The average size and size distribution of nanoparticles were determined by Image tool software.The optical reflectance spectra of samples were recorded using the UV-Vis spectrometer (Shimadzu-UV1650PC SHIMADZU), and the band gap energy was evaluated from the reflectance spectra using the Kubelka-Munk function.All the mentioned measuring devices above were calibrated before conducting the measurements where applicable.

The Role of PVP in Synthesis Process.
According to previous studies [27][28][29], the use of PVP in nanoparticles synthesis is very imperative as it plays four crucial roles: control the growth of nanoparticles, limit the agglomeration of nanoparticles, enhance the degree of crystallinity degree of particles, and produce uniform particle size distribution.
In the first step of making initial solution, PVP works as a stabilizer or a mediator for dissolving complex metallic salts through steric and electrostatic stabilization of the amide groups of the pyrrolidone rings and the methylene groups.By dissolving zirconium acetate in PVP solution, strong ionic bonds between the amide group of polymeric chain and metallic ions, Zr 4+ .The uniform immobilization of metallic ions in the cavities of the polymer chains tends to form nanoparticles with uniform distribution.In the calcination step, although the organic matters will be decomposed to gasses such as N 2 , NO, CO, or CO 2 , trace of carbon residual that bonded on the surface of nanoparticles, which takes higher temperatures to decompose as shown by the weight loss in (Figure 1), can protect them from uncontrolled growth and agglomeration [21,28,29].

Thermal Analysis (TGA/DTG
). Thermal analysis of initial solution allows the optimization of the heat treatment program and shows the suitable temperature at which calcination process must take place.The Thermogravimetric Analysis accompanied with its derivative form (TGA-DTG curves) was implemented for the initial solution dried at 30 ∘ C and its thermogram is clearly illustrated in (Figure 1).The TGA curve demonstrates two weight loss steps.The first insignificant weight loss (∼22%) occurred between ∼30 and ∼250 ∘ C, which attributed to the trapped moisture and acetate in the sample, respectively, whereas the second one, the drastic weight loss between 400 and 470 ∘ C, is due to the decomposition of organic residues (i.e., PVP).Starting at the temperature of 471 ∘ C, the remaining material was almost pure ZrO 2 nanoparticles with some left-out carbonaceous product formed as a result of overheated PVP content which is confirmed by FTIR, EDX, and XRD results.The sharp and strong peaks in DTG curve with maxima at ∼62 and 436 ∘ C also validate the minor weight loss of trapped moisture and combustion of PVP, respectively.

Phase Composition Analysis (FTIR).
The FTIR spectra of the samples before and after calcination are depicted in (Figure 2) in the wavenumber ranging from 280 to 4000 cm −1 .The presence of absorption peaks at wavenumbers above 600 cm −1 in the sample before calcination (30 ∘ C) indicates the presence of covalent bonds from organic sources from acetate and PVP.The absorption peaks at wavenumber of 3405, 2943, and 1649 cm −1 are assigned to N-H, C-H, and C=O stretching vibrations, respectively.Further, the absorption peak found at 1425 cm −1 is stemmed from a vibration due to C-H bending in methylene group, while the band at 1278 cm −1 is associated with C-N stretching vibration.Finally, two peaks located at 831 and 635 cm −1 are linked to vibrations occurring due to C-C ring and C-N=O bending [30].The calcination of samples at 600 to 900 ∘ C has led to the relative disappearance of covalent bands of peaks between 4000 and 1000 cm −1 , which can be implicitly referred to as decomposition of PVP used during the preparation of the sample.The remaining peaks from 355 to 285 cm −1 derived from ionic bond conforming the formation of crystalline ZrO 2 nanoparticles [31].  in Figure 3.The host material of ZrO 2 nanoparticles exhibits four elemental peaks; three for zirconium element located at 0.27, 1.85, and 2.17 keV and one for oxygen element located at 0.5 keV.From EDX data, the weight ratio of Zr : O is around 69 : 26.The existence of some low intensity peaks in the spectrum is attributed to the traces of carbon residual in the sample, which is due to the uncompleted decomposition of PVP at 600 ∘ C. The estimated weight ratio of carbon residual in the samples is less than 5% (Figure 1), so that the reaction yield of zirconia nanoparticles can be determined by weighing the products.

Structural Analysis (XRD).
The crystal structure, crystallinity, and crystallite size of synthesized ZrO 2 nanoparticles have been investigated by generating diffraction patterns, XRD, from the crystalline powder samples at ambient temperature in 2 range of 20 ∘ -70 ∘ .The XRD profile of the assynthesized and thermally treated samples is demonstrated in Figure 4.For the as-synthesized sample, (dried initial solution at 30 ∘ C), there is a characteristic noncrystallized phase of PVP in its amorphous structure.However, by applying different calcination temperatures, the crystalline behavior of the sample begins to appear with peaks indexed by ( 111), ( 002), ( 022), ( 202), ( 113), (311), and ( 222) planes (ICDD PDF 79-1768) which are assigned to monoclinic zirconium blended structure as reported in previous works [20,21].With increasing of calcination temperature, the diffraction peaks become sharper and narrower and their intensity notably increases, which is a sign of the significant enhancement in crystallinity of the ZrO 2 nanocrystals that originates from the increment of crystalline planes caused by the particle size enlargement [12].Scherrer's formula was utilized to compute the crystallite sizes from the full width of the half-maximum (FWHM) peak broadening of the (111) peak of the XRD patterns: where  is the average crystallite size,  is the X-ray wavelength, that is, 0.1542 nm,  is the angular line width  of full width of half-maximum (FWHM) intensity, and  is Bragg's angle.Table 1 presents the calculated crystallite sizes corresponding to different calcination temperatures.
Based on the result shown in this table, it is observed that the crystallite size increases allometrically with increasing of calcination temperature, from 6 nm at 600 ∘ C to 28 nm at 900 ∘ C.

Morphology and Size
Distribution.TEM micrographs of semiconducting ZrO 2 nanoparticles at different calcination temperatures of 600, 700, 800, and 900 ∘ C are shown in Figure 5.
From the illustration, a uniform morphology with adequate distribution was achieved.The particle size distribution which is calcination temperature dependent and has growth mechanism lies in the range of ∼7-32 nm.A nucleation process started where Zr 4+ reacts with two O 2+ from air to form ZrO 2 0 molecules, which are distributed uniformly (as discussed in Section 3.  + nanoparticles.The increment of particle size with calcination temperature is supposed to be due to removal of trace of carbon (Figure 1) on the particles surface that forced neighboring particles to agglomerate and creating eventually larger particles.The average particle sizes obtained from TEM images are in a good agreement with the XRD profile results presented in Table 1.

Optical Properties.
The optical band gap energies for calcined samples at different temperatures were determined from reflectance spectra using Kubelka-Munk equation: where ( ∞ ) is the so-called reemission parameter or Kubelka-Munk function, ℎV is the incident photon energy,  ∞ is the diffuse reflectance that is obtained from  ∞ =  sample / standard , and  is a constant depending on the transition probability and the diffuse reflectance  ∞ [14].The values of (( ∞ ) ⋅ ℎ]) 2 versus (ℎV) were plotted for calcined samples at different temperatures as illustrated in Figure 6.Straight lines were drawn to fit the experimental curves and were prolonged to cut off the (ℎV) axis in order to determine the optical band gap values of the ZrO 2 nanoparticles at different calcination temperatures.It was found that the optical band gap has decreased with the elevation of calcination temperature from 4.83 eV at 600 ∘ C to 4.74 eV at 900 ∘ C as shown in Table 2.The decrement of the energy band gap with the increasing calcination temperatures is credited to the growth of the particle and crystallinity improvement, according to the XRD analysis.
It is supposed that, as the particle size increases, the number of atoms that form a particle also increases, consequently rendering the valence and conduction electrons more attractive to the ions core of the particles and hence decreasing the band gap of the particles [22].Perhaps the contribution of phonons has a role in trapping electrons and consequently influences the band gap energy level during optical properties measurement [32].

Conclusions
Zirconia nanoparticles have been successfully synthesized by direct calcination of an aqueous solution containing zirconium acetate as metal precursor and polyvinylpyrrolidone as capping mediator.The present thermal treatment method exclusive of drying process was able to remove the organic compounds efficiently during the calcination process and leave behind the residue of crystalline ZrO 2 nanoparticles.It was also revealed that the calcination temperature is a key factor in controlling the particle size from 7 to 32 nm when calcination temperature increased from 600 to 900 ∘ C, respectively.The band gap energy of ZrO 2 nanoparticles decreased from 4.83 eV at 600 ∘ C to 4.74 eV at 900 ∘ C as a result of increasing particle size.This modified thermal treatment to synthesize ZrO 2 nanoparticles is a very straightforward, costeffective, and environmentally friendly method, which could be exploited for large-scale industrial fabrication.

2
nanoparticles was analyzed by EDX as shown
1) and then immediately undergo agglomeration to form ZrO 2 0 2 or ZrO 2 0  ( > 2) nanoparticles, when the capping PVP polymer gradually decomposed during calcination starting at about 471 ∘ C. Large ZrO 2 0  nanoparticles can agglomerate further with other ZrO 2 0  nanoparticles at higher calcination temperature to form even
Photon energy h (eV)

Figure 6 :
Figure 6: Plot of the square of Kubelka-Munk function versus photon energy for calcined samples at different temperatures.

Table 1 :
The calculated crystallite sizes and particle size according to different calcination temperatures.

Table 2 :
Summary of calcination temperatures and equivalent band gap energies.