Guin Satya N, Sanyal Dirtha, Biswas Kanishka
New Chemistry Unit , Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR) , Jakkur P.O. , Bangalore 560064 , India . Email:
Variable Energy Cyclotron Centre , 1/AF Bidhannagar , Kolkata 700064 , India.
Chem Sci. 2016 Jan 1;7(1):534-543. doi: 10.1039/c5sc02966j. Epub 2015 Oct 8.
Copper and silver based chalcogenides, chalco-halides, and halides form a unique class of semiconductors, as they display mixed ionic and electronic conduction in their superionic phase. These compounds are composed of softly coupled cationic and anionic substructures, and undergo a transition to a superionic phase displaying changes in the substructure of their mobile ions with varying temperature. Here, we demonstrate a facile, ambient and capping agent free solution based synthesis of AgCuS nanocrystals and their temperature dependent (300-550 K) thermoelectric properties. AgCuS is known to show fascinating p-n-p type conduction switching in its bulk polycrystalline form. Temperature dependent synchrotron powder X-ray diffraction, heat capacity and Raman spectroscopy measurements indicate the observation of two superionic phase transitions, from a room temperature ordered orthorhombic (β) to a partially disordered hexagonal (α) phase at ∼365 K and from the hexagonal (α) to a fully disordered cubic (δ) phase at ∼439 K, in nanocrystalline AgCuS. The size reduction to the nanoscale resulted in a large variation in the thermoelectric properties compared to its bulk counterpart. Temperature dependent Seebeck coefficient measurements indicate that the nanocrystalline AgCuS does not display the p-n-p type conduction switching property like its bulk form, but remains p-type throughout the measured temperature range due to the presence of excess localized Ag vacancies. Nanocrystalline AgCuS exhibits a wider electronic band gap (∼1.2 eV) compared to that of the bulk AgCuS (∼0.9 eV), which is not sufficient to close the band gap to form a semimetallic intermediate state during the orthorhombic to hexagonal superionic phase transition, thus AgCuS nanocrystals do not show conduction type switching properties like their bulk counterpart. The present study demonstrates that ambient solution phase synthesis and size reduction to the nanoscale can tailor the order-disorder phase transition, the band gap and the electronic conduction properties in superionic compounds, which will not only enrich solid-state inorganic chemistry but also open a new avenue to design multifunctional materials.
铜基和银基硫族化合物、硫卤化物及卤化物构成了一类独特的半导体，因为它们在超离子相表现出离子传导和电子传导的混合特性。这些化合物由弱耦合的阳离子和阴离子子结构组成，并随着温度变化经历向超离子相的转变，此时其移动离子的子结构会发生变化。在此，我们展示了一种简便、无需封端剂且在环境条件下基于溶液的合成方法来制备AgCuS纳米晶体，并研究其温度依赖型（300 - 550 K）热电性能。已知块状多晶形式的AgCuS呈现出迷人的p - n - p型导电转变。温度依赖型同步辐射粉末X射线衍射、热容量和拉曼光谱测量表明，在纳米晶AgCuS中观察到了两个超离子相转变，从室温有序正交晶系（β）在约365 K转变为部分无序六方晶系（α）相，以及从六方晶系（α）在约439 K转变为完全无序立方晶系（δ）相。与块状材料相比，尺寸减小到纳米尺度导致热电性能有很大变化。温度依赖型塞贝克系数测量表明，纳米晶AgCuS不像其块状形式那样表现出p - n - p型导电转变特性，而是由于存在过量的局域Ag空位，在整个测量温度范围内都保持p型。与块状AgCuS（约0.9 eV）相比，纳米晶AgCuS表现出更宽的电子带隙（约1.2 eV），这不足以在正交晶系到六方超离子相转变过程中关闭带隙以形成半金属中间态，因此AgCuS纳米晶体不像其块状对应物那样表现出导电类型转变特性。本研究表明，环境溶液相合成以及尺寸减小到纳米尺度可以调整超离子化合物中的有序 - 无序相转变、带隙和电子传导特性，这不仅将丰富固态无机化学，还将为设计多功能材料开辟一条新途径。
