水分子嵌入對NaxCoO2•yH2O電子結構的影響

Abstract

我們利用KMnO4溶液除去Na0.68CoO2薄膜部分的鈉離子，並置入潮濕室使水進入NaxCoO2結構中，形成NaxCoO2 • yH2O薄膜。而NaxCoO2 • yH2O粉末的製備，則是將Na0.7CoO2粉末浸入Br2/CH3CN溶液中除去部分鈉離子，並置入潮濕室使水進入NaxCoO2結構中，形成NaxCoO2 • yH2O粉末。我們發現與NaxCoO2的Co K-edge 及O K-edge X光吸收光譜比較起來， NaxCoO2 • yH2O(y = 0.7、1.4)較NaxCoO2 • yH2O(y = 0) 之鈷的價數低，推測是由於水的結構影響了鈷的價數。也就是說，Na0.35CoO2 • yH2O(y = 0.7、1.4)之鈷的價數不是只由鈉的含量來決定的3.65，反之，可能是因H3O+存在或其它因素所造成的3.4 ~ 3.5。因此，我們認為在NaxCoO2 • yH2O中，y = 0、0.7、及1.4的電子結構不同。另外，由O K-edge X光吸收光譜我們也發現與能帶理論計算相同的結果，就是a1g軌域的能帶寬為Na0.35CoO2 > Na0.35CoO2 • 0.7H2O > Na0.35CoO2 •1.4H2O。

NaxCoO2 • yH2O thin films deposited on Al2O3(0001) have been fabricated using the KMnO4(aq) chemical deintercalation method, which leads to partial deintercalation of Na+ ions and intercalation of H2O molecules. Measurements of polarization dependent Co-K edge and O-K edge XAS (X-ray Absorption Spectroscopy) have been performed on NaxCoO2 • yH2O powders and thin films with y = 0.7 and y = 1.4. The spectra reveal the variations of cobalt valence causing NaxCoO2 • yH2O for different y’s. It has recently been pointed out that the valence of Co ions in this material is not +3.65 as naively expected from the Na content x of ~0.35. Instead, the actual valence is +3.4 ~ 3.5 irrespective of x perhaps because of the presence of oxonium ions (H3O+) or other unknown factors. We argue that the electronic structures are different between NaxCoO2 • yH2O and NaxCoO2. We compare our O-K edge XAS with first-principle band structure calculations and find that, for the bandwidth, Na0.35CoO2 > Na0.35CoO2 • 0.7H2O > Na0.35CoO2 •1.4H2O in the a1g band around the Fermi energy .