Hafnium oxide (HfO2) is an important ceramic material due to its high dielectric constant (ɛ∼30), high melting point (2758°C) and greater chemical stability . HfO2 and its solid solutions with SiO2 are promising replacements for SiO2 for their potential applications as gate dielectrics . Recently the optical applications of HfO2 are gaining widespread interest. Due to its transparency over a wide range from ultraviolet to mid-infrared, it is used as materials for heat resistant, reflective and protective optical coating [3–5]. HfO2 found promising optical coating applications such as filters, beam splitters, anti-reflection coating, high reflectivity mirrors, etc. [6,7].
Hafnium in bulk can adopt three different crystal structures at ambient temperatures. At room temperature it is stable in monoclinic structure, transforms to tetragonal at about 1720°C and becomes cubic at about 2600°C . Synthesis of advanced ceramics and specialty materials as nanocrystals is one of the major challenges in the development of material processing technology . The advantages of nanocrystalline materials are superior phase homogeneity, sinterability and microstructure leading to unique mechanical, electrical, dielectric, magnetic, optical and catalytic properties . There have been increasing interests in the use of nanoparticles to optical systems because of their enhanced optical properties due to their smaller size . Because of its high chemical stability, high cost and high processing temperatures, hafnium oxide is less studied in the form of nanomaterials than other simple oxides. Recently the synthesis of nanocrystalline hafnia by a sol–gel method was reported [12–14]. The synthesis of nanocrystalline HfO2 by the hydrolysis of hafnium oxychloride in ethanol was reported . The preparation of nanocrystalline HfO2 by ultrasonically assisted hydrothermal treatment has also been reported .
Recently combustion synthesis technique has been reported as an easy, economical and time-saving method to synthesize advanced ceramic powders and functional materials [17–19]. Since solution mixing is generally used in combustion synthesis, it results in relatively ultra-phase homogeneity than in any other techniques. Generally in combustion synthesis, which uses PVA as a complexing agent and urea as fuel, it requires post-annealing or calcination of the precursor to get phase purity. Recently, with the use of citric sphingosine 1-phosphate receptor modulator as complexing agent and ammonia as fuel instead of PVA and urea, we were able to prepare a number of phase pure perovskites and scheelites as nanopowders in a single-step combustion itself, hence avoiding the need of post-annealing or calcination steps . The powder thus obtained shows superior phase homogeneity, purity, improved sinterability, etc. than that of their conventional coarse-grained and micron-sized counterparts. However there are only a few reports available on the synthesis of nanocrystalline single oxides especially group IVB metals such as hafnia, zirconia, niobium, tin, etc., particularly hafnia, which are widely required as optical functional materials in opto-electronic devices. This may be due to the fact that pure hafnia is costly and availability is hard compared to the other group IVB metals. In the present paper, we focus our attention to prepare highly pure hafnia and report the single-step synthesis of hafnium oxide nanoparticles by a modified combustion synthesis technique, its structural characterization, particulate properties, photoluminescent and related properties. The process also explores a value addition in the synthesize of ultrafine nanocrystalline phase pure hafnium oxide from relatively low cost and easily available coarse-grained hafnium chloride powder.
Materials and methods
In the present study the modified auto-igniting combustion technique  was used for the synthesis of nanoparticles of HfO2. In a typical synthesis, aqueous solution containing ions of Hf was prepared by dissolving high purity HfCl4 (99%) in double distilled water (200ml) in a glass beaker. Citric acid (99%) was then added to the solution containing Hf ions. Amount of citric acid was calculated based on total valence of the oxidizing and the reducing agents for maximum release of energy during combustion . Oxidant/fuel ratio of the system was adjusted by adding nitric acid and ammonium hydroxide and the ratio was kept at unity. The solution containing the precursor mixture at a pH of ∼7.0 was heated using a hot plate at ∼250°C in a ventilated fume hood. The solution boils on heating and undergoes dehydration accompanied by foam. The foam then gets ignited by itself and on persistent heating giving voluminous and fluffy product of combustion. The combustion product was subsequently characterized as single-phase nanocrystals of HfO2. A schematic diagram of the modified combustion synthesis technique is shown in Fig. 1.