Le Nestour Anne, Gaudon Manuel, Villeneuve Gérard, Daturi Marco, Andriessen Ronn, Demourgues Alain
Institut de Chimie de la Matière Condensée, UPR 9048 CNRS, Université de Bordeaux I, 87 Avenue du Dr. A. Schweitzer, 33608 Pessac Cedex, France.
Inorg Chem. 2007 May 14;46(10):4067-78. doi: 10.1021/ic0624064. Epub 2007 Apr 7.
Zn1-xCuxAl2O4 (0 < or = x < 0.30) compounds have been synthesized by polyesterification using metallic salts and annealing at low temperatures as well as by conventional solid state. XRD-powder data refinements (Rietveld method) have demonstrated that both compound series crystallize in the spinel structure (Fd3m) and exhibit similar inversion rates. This low-temperature route lead to metastable phases with crystallite sizes around 40 nm whereas particle sizes are larger than 1 moicrom in the case of solid-state route. This preparative method largely described in the literature allows stabilizing reduced copper states thanks to the presence of reductive organic species, which are decomposed below T = 700 degrees C. The absorption spectra of the x = 0.15 composition exhibit strong differences depending on the synthesis route. These differences can be explained by the occurrence of Cu2+/Cu+ mixed valencies in compounds prepared by the low-temperature route; 33% of monovalent copper has been identified in the x = 0.15 composition prepared by low-temperature process, whereas the solid-state compound contains only divalent copper. Reductive properties of polyesterification reaction implying citric acid and low annealing temperature (T = 700 degrees C) are mainly responsible of the occurrence of the Cu2+/Cu+ mixed valencies. Actually, the annealing under air at T = 1000 degrees C of divided zinc-copper aluminates prepared at low temperatures (T = 700 degrees C) leads to the oxidation reaction Cu+ --> Cu2+ + e- confirmed by the evolution of magnetic measurements, ESR spectra, and optical absorption properties. Defects such as oxygen vacancies in the anionic network leading to reduction in the cations coordination number could also explain the strong evolution of optical absorption spectra especially around lambda = 700 nm where intervalencies transfer (Cu+/Cu2+) as well as intra-atomic d-d transitions (Cu2+ in a 5-fold coordination) can occur. Finally the occurrence of monovalent and divalent copper at the surface of such divided oxides, probably in tetrahedral sites, has been demonstrated by FTIR spectroscopy using the co-adsorption of CO and NO as probe molecules.
通过使用金属盐的聚酯化反应并在低温下退火以及通过传统的固态法合成了Zn1-xCuxAl2O4（0≤x＜0.30）化合物。XRD粉末数据精修（Rietveld方法）表明，这两个化合物系列均以尖晶石结构（Fd3m）结晶，并表现出相似的反演率。这种低温路线导致形成微晶尺寸约为40nm的亚稳相，而在固态路线的情况下，颗粒尺寸大于1μm。文献中大量描述的这种制备方法由于存在还原有机物种而能够稳定还原态的铜，这些还原有机物种在T = 700℃以下分解。x = 0.15组成的吸收光谱根据合成路线表现出很大差异。这些差异可以通过低温路线制备的化合物中Cu2+/Cu+混合价态的出现来解释；在通过低温工艺制备的x = 0.15组成中，已鉴定出33%的一价铜，而固态化合物仅包含二价铜。聚酯化反应（涉及柠檬酸和低退火温度（T = 700℃））的还原性质是Cu2+/Cu+混合价态出现的主要原因。实际上，在低温（T = 700℃）下制备的分散锌铜铝酸盐在T = 1000℃的空气中退火会导致氧化反应Cu+→Cu2+ + e-，这通过磁性测量、ESR光谱和光吸收性质的变化得到证实。阴离子网络中的氧空位等缺陷导致阳离子配位数减少，这也可以解释光吸收光谱的强烈变化，特别是在λ = 700nm附近，此时会发生价间转移（Cu+/Cu2+）以及原子内d-d跃迁（五配位的Cu2+）。最后，通过使用CO和NO的共吸附作为探针分子的FTIR光谱，已证明在这种分散氧化物的表面可能存在于四面体位置的一价和二价铜。
