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Influence of pH on the structural, spectral, optical, morphological and photocatalytic properties of ­ZrO2 nanoparticles synthesized by sol–gel technique Ashwati Dharr1 · A. Arjun1 · T. Raguram1 · K. S. Rajni1  Received: 31 March 2020 / Accepted: 29 July 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020

Abstract In the present work, Z ­ rO2 nanoparticles are synthesized with different pH (3 to 8) by the sol–gel technique. From X-ray diffraction analysis, the synthesized particles show a tetragonal crystal structure, and the average crystallite size is found to vary from 7.35 to 4.7 nm as the pH increases from 3 to 8. From the FTIR analysis, the peak observed at 621 cm−1 is ascribed to Zr–O vibrations in the nano-tetragonal ­ZrO2. The bandgap values are found to increase from 2.0 to 2.9 eV with an increase in pH. From the PL analysis, the observed peaks show a large band and considerable red shift in the band maximum compared to the bandgap of bulk Z ­ rO2. This strongly expresses that the fluorescence involves extrinsic states. From the morphological analysis, the synthesized nanoparticles are found to be spherical in shape and EDS results confirm the presence of Zr and O. The photocatalytic analysis carried out using Alizarin yellow dye, shows that the degradation efficiency is 84.04% for Z ­ rO2 (S3-pH 6). This is attributed to the excess –OH group present on the surface of ­ZrO2.

1 Introduction Zirconium dioxide is an important multifunctional, widebandgap p-type semiconductor that has abundant oxygen vacancies on its surface. Zirconia belongs to the class of photoresist metal oxides. The existing lattice defects create new defect centers by trapping the carriers. The intrinsic defects in powdered Zirconia are anion vacancies [1, 2]. Nanometric ­ZrO2 has unique properties such as good chemical resistance, relatively high fracture toughness, hardness, low thermal conductivity at high temperature, ionic conductivity, and stable photochemical properties [2]. Due to its oxygen ion conductivity and rich oxygen defects on the surface of Zirconia, it finds applications in the fields of ceramics, semiconductors, electro-optical materials, solidstate electrolytes, thermal barrier coatings, gas sensing, corrosion resistance, catalyst or as a catalyst supporter [3, 4]. Zirconia finds direct application in photonics due to its stable chemical properties. Also, it is used as photocatalyst due to its high ion exchange capacity and redox activity * K. S. Rajni [email protected] 1



Department of Sciences, Amrita School of Engineering, Amrita Vishwa Vidyapeetham, Coimbatore, India

to degrade organic pollutants and dyes like rhodamine B, methyl orange, and Alizarin Yellow [5, 6]. ZrO2 has three different crystal phases—monoclinic (m-ZrO2), tetragonal (t-ZrO2) and cubic (c-ZrO2)—which exist at different temperatures under normal atmospheric conditions [7]. m-ZrO 2 is thermodynamically stable at room temperature, t-ZrO 2 exists at higher temperatures (1100 – 2370 °C) but is stabilized at room tem