Study of Structural and Optical Properties of Zirconium Oxide Nanoparticles

In present study Zirconium dioxide (ZrO2) nanoparticles (NPs) were synthesized by using the chemical method. For the synthesis of desired NPs, oleyl amine (OA) was used as a surfactant material. OA plays a crucial role in inhibiting the aggregation of ZrO2 nanocrystals. Particle surface stabilisation is facilitated by it. The average crystallite size estimated from X-ray diffraction (XRD) using Scherrer equation, to be 6.15nm. UV-vis absorption spectra in the wavelength range of 200-900 nm were obtained; energy band gap obtained approximately 2.52 eV in as prepared ZrO2 NPs. Using FT-IR, the functional group and band structure of ZrO2 were studied.


Introduction
New approaches for the production, research, and manipulation of various nanostructures have been specifically considered due to their exciting features.Furthermore, the prospect of altering the properties of synthesized structures by varying the sizes and shapes of the original NPs merits special consideration.Many physical and chemical method were used for the synthesis of metallic NPs.Among of these methods, chemical method is more suitable for synthesis of metallic NPs due to its low-cost, straightforward, environmentally friendly, and easy to implement.Nanomaterials are fascinating because of their size effects [1].Zirconia is a transition metal that has innumerable unique qualities and advantages over others.The ability to form satirically stable varied coordinate compounds, the capacity to accept lone pairs of electrons, high charges, large size, large surface area high thermal and mechanical stability, spherical electron orbital, acidic-basic characteristics, oxidizing-reducing ability, short ionic radius, and tetravalent properties are some of these properties [2].Due to these features, both basic and application-based research have a wide range of uses and applications.Zirconium has a wide range of uses, such as adsorbents, photo catalysts, coagulants etc.
Many methods, including the sol-gel process [3], precipitation [4], the hydrothermal method [5], microwave irradiation, etc., have been used to create nanocrystalline ZrO2.Zirconia (ZrO2) is a common ceramic oxide with several advantages, including chemical inertness, thermal resistance, high mechanical strength, and a multi properties surface.[1] There are three crystallographic variations exist in zirconia.Crystal modifications of zirconium dioxide: monoclinic (m) at low temperatures and tetragonal (t) and cubic (c) at high temperatures (c).At room temperature, the metastable tetragonal ZrO2 modification (t-ZrO2) is produced during the synthesis of widely dispersed zirconium dioxide, but this modification doesn't become the equilibrium monoclinic modification until annealing at 873-1273 K, which is followed by recrystallization-induced particle growth [6][7][8][9][10].
In present study, Chemical method is used for the fabrication of Zirconia (ZrO2) NPs.In the manufacture of the ZrO2 NPs, Zirconium Oxychloride was employed as the zirconium salt and OA as the surfactant/capping substance.Furthermore, structural, and optical characterizations were carried out to examine the produced sample's characteristics.

Materials and synthesis method
For the synthesises of ZrO2 NPs, Zirconium Oxychloride (ZrOCl2.8H2O)used as a metal precursor, Citric acid (C2O4H2) for pH balance and Oleylamine (C18H37N) were purchased from the Sigma-Aldrich.Distilled water was used as solvent.The highest analytical quality chemicals were all used in the synthesis.The chemical method was used to synthesis nanocrystalline ZrO2.To create ZrO2 NPs, the precursor ZrOCl2.8H2O was first dissolved in distilled water and stirred for five minutes.Further, citric acid (C2O4H2) was added to the mixture followed by surfactant Oleylamine and stirred in the magnetic stirrer to make homogeneous solution.After The total chemical mixture was heated in an oven for five minutes at 70° C. The final step involved transferring the solution to a ceramic crucible and calcined it to 450° C for three hours in the furnace.The outcome was a black substance that is changed into a fine grey powder, when was grinded in the mortar pestle.That grey colour nano powder was used for various characterisation for analysis.

Characterizations
The XRD analysis was performed using a Proto AXRD (X-ray Diffractometer) with a Cu-Kα radiation source (wavelength = 1.5406Å).The sample was scanned in the range of 2θ from 20° to 90°.It was used to record the crystallographic phase of the produced Zirconia nano powder at ambient temperature.UV-Vis's absorption spectra ranging from 200-900 nm wavelength were recorded by UV-2600 in the absorbance and transmittance mode.The Fourier Transform Infrared (FT-IR) Spectrometer, Bruker Alpha was used to measure the FT-IR spectra in the range of 500-4000 cm-1 further analysis was done using Perkin Elmer Spectrum 1 operating in diffuse reflection mode.

XRD
The XRD results were obtained for as prepared sample shown in Figure 1.All diffraction peaks can be seen clearly.The crystallite size was calculated by Debye-Scherrer relation.Which has been shown by the following equation.

𝑫 = 𝒌𝝀 𝜷𝒄𝒐𝒔𝜽
Where, D represents the size of the crystallite in this equation, β represents the full width at half maxima (in radian) of the diffracted X-ray, k is the constant called shape factor that takes the value of 0.9 (for spherical particles), λ indicate the wavelength of the X-ray, and θ is the diffracted Bragg's angle.The peak positions were identified in comparison with the reference diffraction data as JCPDS ref. card no.899069.Four observed peaks with 2θ values of 30.250, 35.070, 50.439, 59.951 degree, corresponds to the (1 1 1) (2 0 0) (2 2 0) and (3 1 1) diffraction peaks of the crystalline Zirconia.As per XRD analysis the crystallite size was observed to be 6.15 nm.From the XRD result, it was predicted that OA act both as a stabilizing and reducing agent during the chemical reaction.

FT-IR Spectroscopy
Further, FT-IR spectroscopy results have been shown in figure [2] in the wavelength ranging from 500-4000 cm -1 .FT-IR spectrum shows three strong absorption peaks at 3843 cm -1 , 3735 cm -1 respectively, these bands indicate the hydrogen bond.Absorption peak observed at the wave number 3611 cm -1 confirms the existence of hydroxyl (O-H) group, characteristics of highly hydrated compound is due to moisture [10].
It has great potential applications in fuel cells as this highly hydrated compound enhance water uptake within membranes in Zirconia.One absorption peak in the region of 2360 cm -1 corresponds to C≡C and C≡N.Another peak in the region of 1695 cm -1 is attributed to the carbonyl (C=C) group and at 1517 cm - 1 is for C=O [11].The region below 1500 cm -1 is known as fingerprint region which is unique and specific for every particle.Absorption peak at 1216 cm -1 confirms the formation of Zr-OH bond.

Optical Studies
To assess the optical response of the synthesized zirconia, the UV-vis absorption spectra in the wavelength range of 200-900 nm were obtained.Figure [3] depicts the absorbance spectra obtained from UV-Vis experiments.The diffuse reflectance spectroscopy was used to determine the direct band gap of ZrO2 NPs.The two peaks were observed at the wavelength of 325 nm and 369 nm in spectrum.Poor absorption caused by the surface defects, impurities etc.The tauc plot can be used to compute the band gap energy from an optical absorption spectrum, assuming that the energy-dependent absorption coefficient can be represented as.(  )(ℎ) = (ℎ −   ) Figure [3] represents graph between the (F(r)(hν)) 2 and Energy (hν).Band gap energy obtained from the plot is approximately 2.52 eV.A band gap of 2.52 eV means it behaves like a semiconductor, conducts electricity.Due to its valence and conduction i.e., excitation states may have small orbital overlap and it becomes easy to excite an electron to higher energy state with less power.From the analysis, it may also be due to Zr +3 ions transition in monoclinic phase of Zirconia.Low band gap also refers to high intrinsic conduction.Band gaps can be affected by defects such as defect concentration, defect type and sometime depends on specific material.In present work, a reduced band gap produced by the existence of oxygen vacancies can lead to defect states inside the band gap and involves extrinsic transitions [12].

Conclusion
Zirconia NPs have been synthesized successfully.The average crystallite size of the Zirconia synthesized was observed 6.15 nm.It was observed Oleylamine as surfactant material controlling the crystallite size for the synthesis of ZrO2 NPs.The energy band gap of ZrO2 NPs is observed 2.52 eV.Peaks in the FT-IR spectrum confirm various absorption peaks and formation of the Zr-OH band and presence of Oleylamine as capping agent.And another absorption peak in the region of 2360 cm -1 corresponds to C≡C and C≡N also indicated the presence of Oleylamine on the surface of NPs.

Figure 2 .
Figure 2. FT-IR spectra of ZrO2 nanoparticles (ℎ) = constant(hν −   ) Where Eg represents energy band gap, α is the absorption coefficient and n is the constant, (hv) is the photon energy.The value of n is influenced by the nature of the electron transition.It can have values of 1/2 and 2 for the direct and indirect permissible transitions, respectively.Since in the powder form, surface of particles is non-homogeneous.The Kubelka-Munk (K-M) relation can be used to analyse the scattered and absorbed light from an inhomogeneous ZrO2 nano powder (ref 805).The equations are as follows.reflectance.S and K are the scattering and absorption coefficients, respectively.F (Rd) is the Kubelka-Munk function.