Synthesis and Thermal Stability of HfO 2 Nanoparticles
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Synthesis and Thermal Stability of HfO2 Nanoparticles Girija S. Chaubey, Yuan Yao, Julien P. A. Makongo, Pranati Sahoo, Pierre F. P. Poudeu and John B. Wiley* Advanced Materials Research Institute and Department of Chemistry, University of New Orleans, New Orleans, LA 70148
ABSTRACT A simple method is reported for the synthesis of monodispersed HfO2 nanoparticles by the ammonia catalyzed hydrolysis and condensation of hafnium (IV) tert-butoxide in the presence of surfactants at room temperature. Transmission electron microscopy shows faceted nanoparticles with an average diameter of 3-4 nm. As-synthesized nanoparticles are amorphous in nature and crystallize upon moderate heat treatment. The HfO2 nanoparticles have a narrow size distribution, large specific surface area and good thermal stability. Specific surface area was about 239 m2/g on as-prepared nanoparticle samples while those annealed at 500 °C have specific surface area of 221 m2/g indicating that there was no significant increase in particle size. This result was further confirmed by TEM images of nanoparticles annealed at 300 °C and 500 °C. X-ray diffraction studies of the crystallized nanoparticles revealed that HfO2 nanoparticles were monoclinic in structure. The synthetic procedure used in this work can be readily modified for large scale production of monodispersed HfO2 nanoparticles.
INTRODUCTION Hafnia is a very important ceramic material due to its large dielectric constant (ε~ 30), high melting point (~ 2758 °C) and great chemical inertness. Hafnia-based oxides are emerging as a leading material to replace silica as gate insulator in field effect transistors [1] and in carbon tube-based non-volatile RAM [2]. They are also widely applied in optical coatings [3], catalysis [4], sensors [5], and heat resistant and highly reflective materials. Performance in such applications can often be further improved by using nanoscale particles. For example, increase in the surface area due to nanosize may enhance catalysis and sensor properties in comparison to their bulk counter-parts. The structure of bulk HfO2 is temperature dependent [6]. HfO2 exists in a monoclinic structure from room temperature to 1700 °C, a tetragonal structure is observed above this temperature, and above 2600 °C HfO2 is cubic. High temperature tetragonal and cubic phases can be stabilized at room temperature by controlling the nanoparticle size below 30 nm [7]. Interest in such ceramic materials involves the inclusion of homogeneous nanoparticles into thermoelectric matrices. This approach has been shown to reduce thermal conductivity of materials and improve their thermoelectric performance [8]. For high temperature applications of nanoparticles, such as nanoinclusions in a high temperature thermoelectric matrix for the reduction of thermal conductivity [8], high thermal and chemical stability of the nanoparticles are desired. Several methods have been developed to synthesize pure and doped hafnia nanoparticles, including solvothermal [9, 10], hydrothermal [11, 12], m
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