Enhanced phase stability of metastable tetragonal zirconia nanocrystals prepared through novel chemical routes

Abstract

The present work deals with the synthesis and enhancement of metastable tetragonal zirconia nanocrystals through novel chemical routes. Nanocrystalline t-ZrO2 is a technologically significant material that finds extensive use as catalyst or catalyst support, oxygen sensor and structural components. In this work, ZrO2 nanoparticles are synthesized using both reduction and hydrolysis techniques. These techniques are known to result in the production of nanocrystalline materials. Also, promote the stabilization of t-phase of ZrO2 at the nano level at moderate temperature, which is one of the primary objective of this work. For reduction technique, the Zr-salts are reduced by the addition of strong (Sodium borohydride, NaBH4) and weak (Hydrazine hydrate, N2H5OH) reducing agents. However for hydrolysis technique, Zr-salts are hydrolyzed by adding ammonium hydroxide (NH4OH). In this process, geletion and precipitation occurred at low and high pH respectively with the addition of these agents. Here, the main objective is to stabilize the t-ZrO2 through geletion (low pH) and precipitation (high pH) route by using NaBH4 and N2H5OH and compare with the powders synthesized through hydrolysis process by the addition of NH4OH. It was observed that the crystallization temperature of t-ZrO2 for low pH is greater as well as broad range than the high pH ZrO2 samples. Stabilization of metastable t-ZrO2 is observed up to 600 °C for both pH in case of samples reduction by NaBH4. Stabilization of metastable tZrO2 is observed up to 500 °C for low pH in case of samples reduction by N2H5OH. Fully tZrO2 is seen for low pH sample, however mixture of t- and m- ZrO2 are observed for high pH sample up to 500 °C for the case of NH4OH. The as-prepared powders contain lots of water in the form of ZrO(OH)2.xH2O as studied from the IR analysis. From the XRD results it has been concluded that small crystallites (<25nm) stabilizes metastable t-ZrO2 upto moderate temperature. Low pH has better stability on metastable t-ZrO2 compared to high pH. Strong reducing agent (NaBH4) gives better stability on t-ZrO2 upto 600 °C compared to NH4OH. Reduction technique controls the crystallite size and slowly converts to m-ZrO2 as the calcination temperature increases. From the thermal as well as microstructural studies it was concluded that the stabilization of t-ZrO2 is dependent on the initial pH of the precursor, calcination temperature and also on the crystallite size of the ZrO2.