XAS and GIXRD Study of Co Sites in CoAl₂O₄ Layers Grown by MOCVD

Abstract

The chemical environment of Co sites in CoAl2O4 layers grown by metal−organic chemical vapor deposition has been investigated by X-ray absorption spectroscopy (XAS) and X-ray diffraction (XRD). It is shown that the air- or [O2 + H2O]-annealing at 500 °C of the layers deposited at low temperature induce a partial crystallization with the formation of (Co1−2ηAl2η)(Co2ηAl2(1−η))O4 spinel (η = 0.2−0.27). Nevertheless, slightly more than half of Co remains in an amorphous phase, and XAS data are consistent with the formation of a medium-range ordered Co3O4 phase, especially upon air-annealing. Layers grown at higher temperatures (600−650 °C) exhibit a similar but more complex structure, since the presence of an additional medium-range ordered phase (likely, CoAl2O4) is also revealed. The air-annealing at high temperature (800 °C) generates blue, almost completely crystalline, CoAl2O4 layers. Optical properties of deposited layers are discussed by referring to the outcomes of structural results. In particular, the optical absorption spectrum results negligibly affected by the presence of the amorphous phase, while absorptions present in the 300−500 nm range, responsible for the green layer color and evident in samples annealed in an oxidizing atmosphere or grown at high temperature, are likely caused by the octahedrally coordinated Co ions of the partially inverted spinel (Co1−2ηAl2η)(Co2ηAl2(1−η))O4 phase. Despite the XRD analysis that ultimately demonstrates the presence of octahedrally coordinated Co ions, whose oxidation state in the spinel phase is in majority Co(II), the occurrence of Co(III) species with an octahedral environment cannot be ruled out

Synchrotron radiation-based multi-analytical approach for studying underglaze color: The microstructure of Chinese Qinghua blue decors (Ming dynasty)

“…5a), the spectra exhibit the pre-edge at ~7709 eV and the edge energy at ~7717 eV consistent with bivalent Co (Co 2+ ) [22,37]. In addition, with a white line at ~7726 eV and the energy crests (~7741, ~7758, ~7781 eV), these XANES spectra are congruent with the ones of cobalt aluminate (CoAl2O4) particles in which Co 2+ ions are in tetrahedral coordination [22,37]. Compared to these spectra, the XANES spectra recovered in glassy matrix (Fig.…”

Synthesis, structure and optical properties of CoAl2O4 spinel nanocrystals

“…Among the class of materials, cobalt aluminate (CoAl 2 O 4 ) spinel, known as Thenard's blue, is widely used as catalyst, color filter for automotive lamps or pigment layer on luminescent materials because of its thermal, chemical, photochemical stability and peculiar optical properties [7][8][9][10][11][12]. In recent years, much work has been done on the preparation and the optical properties of CoAl 2 O 4 spinel materials [13][14][15][16][17][18][19].…”

Two-step growth mechanism of supported Co3O4-based sea-urchin like hierarchical nanostructures

“…The small preedge peak in the region 'A' (at about 7710 eV) for Co oxides is related to a dipole allowed transition from the state 1s to mixed p-d orbital and is very low for a centrosymmetric Co site (i.e., octahedral) for which p-d mixing is not allowed. [15]. For the as-deposited particle layer the absorption spectrum (see Figure 5(a)) is similar to the one of Co foil (see in particular the absorption in the 'A' region, significantly higher than for the Co-oxides), indicating that a relevant fraction of Co is in the metallic state.…”

“…Starting from this promising photocatalytic result, we address here a full structural investigation to unveil the complex growth mechanism of these 3D hierarchical nanostructures. The results presented are based on scanning electron microscopy, X-ray diffraction and X-ray absorption spectroscopy, allowing to get a full picture on both the amorphous and crystalline phases involved in the growth process [14,15,16,17]. The investigation of both the full deposited layer and separately of the surface particles that constitute the seed for the urchins formation allowed to shed light on a complex growth process, showing how diffusion mechanisms and chemical effects can be effectively coupled to obtain hierarchical structures of transition metal oxides.…”