| 講義 | 学会報告 | 入学希望者 | トピックス | サイトマップ |
論文題目「Nanoparticle Synthesis of Transition Metal Oxynitride by Induction Thermal Plasma」
Chen Han
1. Introduction
Transition metal oxynitrides have several advantages, such as controllabe bandgap and excellent conductivity compared to their corresponding oxides or nitrides. The properties can be tuned by changing chemical composition. They have been used in many applications such as capacitors, electrodes, and photocatalysts.
The conventional methods have limited the development of applications. For example, the urea glass route for synthesizing tantalum oxynitride nanoparticles is time-consuming and difficult to control the oxynitride composition. The utilization of thermal plasma is a suitable method for synthesizing tantalum compounds nanoparticles due to the high melting point of tantalum. The raw material can be vaporized immediately due to the high-temperature property of thermal plasma. High chemical activity and enthalpy properties promote complete chemical reactions. Furthermore, induction thermal plasma is able to avoid impurity generation from electrodes owing to electrodeless discharge. The purposes of this study are to synthesize titanium, chromium, and tantalum oxynitride nanoparticles with controllable composition by induction thermal plasma and to investigate the formation mechanism.
2. Experiment
The plasma is generated at atmospheric pressure with a power supply of
4 MHz at a plate power of 20 kW. The detail of synthesis for tantalum oxynitride
is introduced here.
Metallic tantalum powder with the size of 45 µm is fed into the plasma
torch via the powder feeder by using Ar carrier gas at 3 L/min. The feed
rate is adjusted to 1 g/min. Argon is also used as sheath gas at 60 L/min
and as inner gas at 5 L/min. The limited amount of oxygen is injected as
sheath gas to promote oxidation. Ammonia is injected as counter flow through
the quenching tube. Nitridation is promoted by NHx radicals as nitrogen
source from NH3. The effect of nitrogen amount on the product is investigated
under the condition of Ta:O=1:2.5. The molar ratio of Ta to N is controlled
to be approximately Ta:N=1:7, 1:15, 1:30, and 1:59.
The effect of oxygen amount on the product is also investigated under the condition of Ta:N=1:59. A limited amount of O2 is also injected as sheath gas at various gas flow rates, with the goal of controlling the molar ratio of Ta to O as Ta:O=1:0, 1:1.25, 1:2, 1:2.5, and 1:5. This approach is adopted because oxidation occurs more readily in comparison to the nitridation of tantalum. The same approach is also adopted for synthesis of titanium and chromium oxynitrides to compare with tantalum system.
The crystal structure of the particles is determined by XRD. The chemical bonding states are confirmed by XPS. The element mapping is analysed by STEM-EDS. The morphology and size distribution are performed by TEM. The direct bandgap is measured by UV Vis.
3. Results and discussion
The diffraction peaks correspond to a cubic structure at the molar ratio
of Ta to N as Ta:N=1:15, 1:30, and 1:59. The peaks shift towards larger
diffraction angle from the standard card of TaN. The diffraction peaks
of γ-TaON, and δ-TaON are also observed. This suggests that tantalum oxynitride
is synthesized.
The major product corresponds to Ta5N4 with a hexagonal structure. Small
amounts of Ta2N and TaN are also observed under the condition of no oxygen
addition. The products exhibit to a cubic structure under the conditions
of oxygen addition. The formation of the cubic structure results from the
introduction of oxygen. Tantalum vapor is oxidized to form TaO in the high-temperature
region, resulting in the nucleation of TaO with a cubic structure. The
nanoparticles are formed as a result of condensation with Ta vapor, TaO
vapor, and NHx radicals based on this TaO core. On the other hand, the
formation of Ta5N4 results from the nucleation of tantalum in the high-temperature
region, resulting in incomplete nitridation.
The diffraction peak shifts towards larger diffraction angle from the standard card of TaN when the amount of oxygen increases. This trend suggests more N atoms in cubic structure are replaced by O atoms as increasing the amount of oxygen. This also suggests that tantalum oxynitride with a cubic structure (TaOxN1-x) is synthesized.
The narrow-scan XPS spectra of Ta 4f shows the peaks under the conditions
of Ta:O:N =1:5:59 and 1:2.5:15 can be deconvoluted into four major doublets
(4f7/2 and 4f5/2). The set of four peaks represents Ta-Ta, Ta-N, Ta-N-O,
and Ta-O, respectively. In contrast, the peaks under the conditions of
Ta:O:N=1:2.5:59 can be only deconvoluted into Ta-Ta and Ta-N. An excessive
ammonia leads to excessively intense nitridation and the disappearance
of Ta-N-O and Ta-O bonding under the relatively low oxygen conditions.
The nitride-based oxynitrides with a cubic structure were synthesized in
Ti, Cr, and Ta systems. The oxynitrides can only form under strong nitriding
conditions. The Gibbs free energy change of nitridation at the nucleation
temperature and the relative integrated intensity of the cubic structures
are adopted to evaluate the nitriding effect.. The high content of cubic
structure can be obtained regardless of the addition of oxygen in Ti and
Cr systems. However, the highest Gibbs free energy change under the condition
of no oxygen addition results in the low content of cubic structure. The
content of cubic structure increases significantly after the addition of
oxygen This increase is attributed to the decrease in the Gibbs energy
change at the nucleation temperature due to the decrease in nucleation
temperature.
4. Conclusion
Titanium, chromium, and tantalum oxynitrides with a cubic structure were
synthesized in this study. Among them, tantalum oxynitrides exhibit two
types of different crystal structure. The nitridation reaction is affected
by the amount of oxygen due to the inhibition of tantalum nucleation. Excessively
intense nitridation results in the disappearance of Ta-N-O and Ta-O bonding
under the relatively low oxygen conditions. The product synthesized under
the condition of Ta:O:N=1:2.5:15 has a mean diameter of 18 nm and a direct
bandgap of 2.98 eV.
国際学会
- Han Chen, Kohei Yamashita, Kaiwen Zhang, Manabu Tanaka, and Takayuki Watanabe: Nanoparticle Synthesis of Transition Metal Oxynitride by Induction Thermal Plasma, Proceedings of 25th International Symposium on Plasma Chemistry, POS-7-103 (2023.5.22 Miyako Messe, Kyoto).
- Yirong Wang, Kaiwen Zhang, Han Chen, Motonori Hirose, Manabu Tanaka, and Takayuki Watanabe: Synthesis of Tenary Titanium-Transition Metal Nitrides Nanoparticles by Induction Thermal Plasmas, 16th International Symposium on Advanced Plasma Science and Its Applications for Nitrides and Nanomaterials, 05aC03O (2024.3.5 Nagoya University).
国内学会
- Han Chen, Yirong Wang, Kaiwen Zhang, 廣瀨基規, 田中学, 渡辺隆行: 高周波熱プラズマを用いた高融点金属酸窒化物ナノ粒子の合成, プラズマ・核融合学会九州・沖縄・山口支部第27回支部大会研究発表論文集, p.75-76, D-2 (2023.12.10 KDDI維新ホール).
- 田中学, 陳翰, 張楷文, 王芸融, 廣瀨基規, 渡辺隆行: 高周波熱プラズマによるタンタル酸窒化物ナノ粒子の合成, 第41回プラズマプロセシング研究会, 24a-3 (2024.1.24 東京工業大学).
