新穎光催化材料CuFeO_2單晶製備與物理性質之研究

Single Crystal Growth and Physical Properties of CuFeO_2 as a Novel Photocatalytic Material

高孟瑀(M.Y. Kuo) ； 周妍妤(Y.Y. Chou) ； 劉思謙(S.C. Liou) ； 陳威廷(W.T. Chen) ； 周方正(F.C. Chou) ； 殳國俊(G.J. Shu)

《鑛冶：中國鑛冶工程學會會刊》 64卷2期 (2020/06) Pp. 72-80

https://doi.org/10.30069/MM.202006_64(2).07

摘要

人工光合作用（Artificial photosynthesis）是一種利用太陽能激發材料後，將水分子分解產生氫氣與氧氣的一種新穎綠能技術。長期以來研究方向多專注在尋找合適的光催化（Photocatalytic）材料來嘗試提高其轉換效率，但一直沒有重大的突破使得人工光合作用在產業應用上受到限制。近年來Delafossite礦物材料CuFeO_2被發現可以在太陽光的大波長範圍內進行光催化水分解製氫，大幅地提升了轉換效率，使得人工光合作用重新被受到關注。為了對CuFeO_2的特殊材料物理特性做出充分地瞭解，本研究藉助人工合成的高品質CuFeO_2單晶，系統性的透過磁性測量、晶體結構分析與電子能量損失能譜分析（Electron Energy Loss Spectroscopy, EELS），證明出Cu^+和(FeO_2)^-層間存在有很強的自旋－軌域交互作用（Spin-Orbital Coupling, SOC），使得CuFeO_2材料內部具有：（1）偏大於理論值的有效磁偶極矩（μ_(eff)）；（2）在Cu^(+1)上發現金屬態與共價態共存之特徵；與（3）介穩態（metastable）能帶結構的形成。我們認為此SOC機制在光催化過程中扮演極為重要的角色，特別是介穩態能帶的存在延長了電子－電洞再結合時間（recombination time），有利光催化反應產生，提供了開發人工光合作用材料重要的學理依據。

關鍵字

銅鐵礦 ； 單晶 ； 光催化水分解製氫 ； 自旋－軌域交互作用 ； 介穩態能帶

並列摘要

Artificial photosynthesis (AP) is a novel green-energy technology that uses solar energy to excite materials and decompose water for hydrogen and oxygen production. The research of AP has been focused on finding suitable photocatalytic materials and trying to improve the conversion efficiency. However, the progress of AP technology is hampered mostly by the material stability in water and low conversion efficiency of practical use. Lately the mineral of delafossite CuFeO_2 has been demonstrated to be suitable candidate for photocatalytic water splitting and produce hydrogen in a broad wavelength of sunlight, which improves the conversion efficiency significantly. To understand the unique physical properties of CuFeO_2, systematic magnetic, electrical, crystal structural, and electron energy loss spectroscopy (EELS) measurements were employed on high-quality single crystal sample of CuFeO_2 in this study. Experimental results indicate that there is a strong spin-orbital coupling (SOC) between Cu^+ ions and (FeO_2)^- layers. The SOC causes (1) larger effective magnetic moment (μ_(eff)), (2) coexisting metallic and covalent states in Cu^+ ion, and (3) the unexpected formation of a metastable band between valence and conduction bands. We propose that the SOC mechanism plays the key role in the photocatalytic process through prolonged electron-hole recombination time that favors the water splitting reaction.

並列關鍵字

delafossite ； single crystal ； artificial photosynthesis ； spin-orbital coupling ； metastable band structure