J. Phys. Chem. 1996, 100, 4193-4198
4193
Dopant Reduction in p-Type Oxide Films upon Oxygen Absorption N. Uekawa and K. Kaneko* Department of Chemistry, Faculty of Science, Chiba UniVersity, 1-33 Yayoi-cho, Inage-ku, Chiba-shi, 263 Japan ReceiVed: September 21, 1995; In Final Form: NoVember 30, 1995X
The lower valent cations were homogeneously doped in p-type oxide films. The prepared doped oxide systems were Mg-doped R-Cr2O3, Ni-doped R-Cr2O3, and Li-doped NiO. The Mg doping induced a homogeneous mixed valence state of Cr3+ and Cr6+ in the Cr2O3 film, reducing the Mg dopant to the metallic valence state in the Cr2O3 lattice, which is completely different from the powder or single crystalline system. The mixed valence state formation gave rise to a marked chromism. The mixed valence formation and perfect reduction of the lower valent dopants were evidenced by X-ray photoelectron spectroscopy, electrical conductivity change, and optical absorption. Ni-doped R-Cr2O3 and Li-doped NiO showed similar results. The reduction kinetics of the mixed valence state in the p-type oxide lattice with the aid of the electrical conductivity measurement suggested that the rate-determining step is not the diffusion of oxygen.
Introduction Nonstoichiometry is essentially important in transition metal oxides.1,2 The nonstoichiometry changes markedly the electronic structure of the transition metal oxides.3 The nonstoichiometry varies with the surrounding atmosphere, impurities, and morphologies. Thin films and ultrafine particles are not stable due to high surface energies whose composition is deviated from the stoichiometry.4 Therefore, it is plausible that an unstable nonstoichiometric state can be realized in the film of transition metal oxides. The doping of foreign atoms having a different valence controls the nonstoichiometry to create the mixed valence state which indicates a dramatic change in the electrical and optical properties.5-7 However, the doping mechanism in the transition metal oxide is not fully understood due to inhomogeneous distribution of dopants in the solid. The sol-gel method provides a considerably homogeneous mixed valence oxide.8-10 In the preceding paper, the homogeneous mixed valence state of Fe2+ and Fe3+ in an n-type iron oxide with Ti-doping was realized with the sol-gel technique.11,12 In this article, the valence change of dopants in p-type oxides upon oxygen absorption is described. Bulk p-type semiconductive oxides are deficient in the metal atoms or they have excess interstitial oxygen atoms.3 Even the properties of bulk p-type oxides are associated with the surrounding oxygen pressure according to the following defect reaction.
MOx+δx T MOx + (δx/2)O2
(1)
The properties of the thin film of the p-type oxide should be sensitively affected by the oxygen pressure. Both NiO and Cr2O3 are representative p-type oxide semiconductors.13 In particular, M2+-doped R-Cr2O3 and Li-doped NiO are famous systems in which doping can control the electronic property.14,15 Furthermore, for instance, the mixed valence formation of R-Cr2O3 by M2+ ion doping have been described as follows by Kro¨ger-Vink notation so far:3
2MO + 1/2O2 T 2M′Cr + 2h˙ + 3OOx
(2)
Here, M′Cr and h˙ are a divalent dopant at a Cr lattice position and a hole, respectively. OOx is an oxygen ion of the effective X
Abstract published in AdVance ACS Abstracts, February 1, 1996.
0022-3654/96/20100-4193$12.00/0
charge of zero. The dopant metal ion keeps the ionic state in the lattice. According to this equation, the mixed valence state should change with the oxygen atmosphere sensitively. We observed the unusual dopant state completely different from the established mechanism given by eq 2 in p-type oxide film systems. This paper mainly describes mixed valence formation in the R-Cr2O3 films by Mg2+ ion doping and reduction of divalent dopant ions to the metallic state. Other examples are also described in this paper. Experimental Section Preparation of Impurity-Doped p-Type Oxide Film. Mdoped R-Cr2O3 (M ) Mg or Ni) and Li-doped NiO films were prepared by the polymer precursor method which can be expected to induce highly homogeneous dispersion of dopants in R-Cr2O3. Reagent-grade chromium(III) nitrate enneahydrate (Cr(NO3)3‚9H2O), magnesium nitrate (Mg(NO3)2‚6H2O), and nickel nitrate hexahydrate (Ni(NO3)2‚6H2O) were used as starting materials for preparation of M-doped R-Cr2O3 film.16,17 Nickel nitrate and lithium nitrate (LiNO3) were also used for the preparation of Li-doped NiO film. Chromium nitrate (0.005 mol), magnesium nitrate (x mol), citric acid (0.02 mol), and ethylene glycol (0.05 mol) were dissolved in 100 mL of distilled water. The value x was changed in order to regulate the composition of 0-0.2 of the M/Cr atomic ratio. The Li-doped NiO film was prepared in a similar way. Li content was varied up to 0.3 of Li/Ni atomic ratio. The highly viscous polymerized complex was dissolved in 25 mL of methanol as a coating solution. The film was prepared on the Pyrex glass substrate (50 mm × 25 mm × 2 mm) by spin-coating technique at 2000 rpm for 10 s. The spin-coated glass substrate was calcined at 773 K for 5 min. The film thickness was controlled by stacking procedures of the coating film. Characterization. The crystalline structure of the Mg-doped R-Cr2O3 films were investigated by X-ray diffraction (XRD) using Cu KR radiation (45 kV 20 mA) with a Ni filter. The film showed well crystalline R-Cr2O3. The thickness of the oxide film was measured by the multiple reflection interference method. The thickness of the film was ca. 50 nm per coating. Electron absorption spectra of the film in the wavelength range of 400-800 nm were measured with the aid of a UV-vis spectrometer (Hitachi, 200-10). The dc electrical resistance was © 1996 American Chemical Society
4194 J. Phys. Chem., Vol. 100, No. 10, 1996
Uekawa and Kaneko
Figure 1. Change in electrical resistance of R-Cr2O3 film with Mg doping.
measured at 303 K in vacuo after pretreatment at 393 K and 1 mPa for 2 h. The resistance was determined from the current under a constant voltage with a surface conductive type cell on the square area of 12 × 12 mm2 and an electronic picoammeter (Keithley 485). Shimazu ESCA 850 was used for recording XPS spectra. The base pressure was
