Anodic Behavior of Hafnium in Anhydrous Electrodissolution-Coupled Hafnium Alkoxide Synthesis

The electrodissolution-coupled hafnium alkoxide (Hf(OR)4, R is alkyl) synthesis (EHS) system, which has significant environmental and economic advantages over conventional thermal methods, serves as a promising system for green and efficient Hf(OR)4 electro-synthesis. The EHS system is operated based on the simultaneous heterogeneous reactions of hafnium dissolution and ethanol dehydrogenation, as well as the spontaneous solution-based reaction of Hf4+ and OR−. Employing green ethanol and Hf as feedstocks, the anodic hafnium corrosion/dissolution electrochemical behavior of the Et4NCl or Et4NHSO4 based anhydrous system was investigated through electrochemical measurements combined with SEM observations. The results demonstrated that the Et4NCl-based anhydrous ethanol system exhibited an efficient mechanism of passive film pitting corrosion breakdown and metal hafnium dissolution, while the Et4NHSO4-based anhydrous ethanol system reflected the weak corrosion mechanism of the anodic hafnium under the passive film. The polarization resistance of the Et4NCl system was dramatically lower than that of the Et4NHSO4 system, which indicated that the Et4NCl system had superior anodic hafnium corrosion performance compared to the Et4NHSO4 system. Overall, the investigation of the electrochemical behaviors of anodic hafnium corrosion/dissolution provides theoretical guidance for the efficient operation of EHS electrolysis.

Electrodissolution-Coupled Hafnium Alkoxide Synthesis with High Environmental and Economic Benefits

The conventional thermal method of preparing hafnium alkoxides [Hf(OR)₄ , R=alkyl] - excellent precursors for gate-dielectric HfO₂ on semiconductors - is severely hindered by its unsatisfactory environmental and economic burdens. Herein, we propose a promising electrodissolution-coupled Hf(OR)₄ synthesis (EHS) system for green and efficient electrosynthesis of Hf(OR)₄ . The operational principle of the electrically driven system consists of two simultaneous heterogeneous reactions of Hf dissolution and alcohol dehydrogenation, plus a spontaneous solution-based combination reaction. In applying ethanol as solvent and Hf metal as electrodissolution medium, we achieved waste-free production of high-purity hafnium ethoxide [Hf(OEt)₄ ] with an equivalent "a concomitant" reduction in CO₂ emission of 187.33 g CO₂ per kg Hf(OEt)₄ and a high net profit of 30 477 USD per kg Hf(OEt)₄ . This system is very competitive with the thermal process, which unavoidably releases substantial waste and CO₂ for a net profit of 27 700 USD per kg Hf(OEt)₄ . We anticipate that the environmental and economic benefits of the EHS process could pave the way for its practical application.

Determination of the vapour pressure curves and vaporization enthalpies of hafnium alkoxides using thermogravimetric analysis

In order to identify a volatile metallo-organic precursor for the deposition of hafnium oxide (HfO₂) films for atomic layer deposition (ALD) applications, the evaporative properties of hafnium alkoxides (hafnium isopropoxide, hafnium n-propoxide and hafnium n-butoxide) were investigated using thermogravimetric analysis. These hafnium alkoxide samples were synthesized by the electrochemical method and characterized by Fourier transform infrared spectroscopy, nuclear magnetic resonance and inductively coupled plasma analysis techniques. The characterization results indicated that the products were 99.997% high-purity hafnium alkoxides and could meet the requirement of purity considering the usage of making HfO₂ gate oxide by ALD. Synthesized samples were subjected to a simultaneous thermogravimetric-differential thermal analysis unit at 10 K min^-1 in a dry nitrogen atmosphere flowing at 100 ml min^-1. Benzoic acid was used to calculate a calibration constant, which could then be inserted into a modified Langmuir equation to calculate vapour pressure curves for hafnium isopropoxide and hafnium n-propoxide. Detailed vapour pressure data for the HfO₂ precursor hafnium alkoxides were determined. The vapour pressure curve of hafnium isopropoxide was constructed within the first stage, and calculated to be lnp = 31.157 (±0.200)-13130.57 (±56.50)/T. Hafnium n-propoxide and hafnium n-butoxide were simultaneously undergoing evaporation and decomposition, thus making calculations invalid.