The colouration of titanates by transition-metal ions in view of solar energy applications

doi:10.1016/0022-1902(81)80324-7

Journal of Inorganic and Nuclear Chemistry

Volume 43, Issue 7, 1981, Pages 1499-1503

https://doi.org/10.1016/0022-1902(81)80324-7 Get rights and content

Abstract

The diffuse reflection spectra of a number of transition-metal ions (Cr, Mn, Ni) in several titanates (CaTiO₃, SrTiO₃, MgTiO₃) and zirconates (CaZrO₃, SrZrO₃) have been studied. Next to the crystal-field transitions we observed charge-transfer transitions, especially Cr³⁺ → Ti⁴⁺. These are not observed for the tetravalent transition metal ions. It is argued that they should be observed for the divalent ions. From these results predictions on the occurrence of charge-transfer spectra of the type Mⁿ⁺ → Ti⁴⁺ are made. Finally their implications for the sensitization of titanate semiconductor anodes in photoelectrochemical cells are considered.

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Citation Excerpt :
Consequently, the presence of the absorption band from 505 nm suggests that Zr4+ was substituted for Cr4+. However, there is a steep slope observed on the DRS spectra, in the visible range, whose origin does not seem to be purely related to crystal field effects [33]. The CIEL*a*b coordinates of CaZr1−xCrxO3 samples annealed at 1400 °C for 2 h indicate that the brightness of the samples (L*) decreases with the increase of the chromium content (Fig. 7).
Chromium doping of calcium zirconate, CaZr_1−xCr_xO₃ (x = 0–0.07), leads to red shade pigments. XRD analysis confirmed that CaZr_1−xCr_xO₃ solid solution is the only crystalline phase present in the samples annealed at 1400 °C. The substitution of Zr⁴⁺ by Cr⁴⁺ causes a progressive decrease of the unit cell volume, because Zr⁴⁺ has an ionic radius larger than Cr⁴⁺. The CaZr_1−xCr_xO₃ crystallite size decreases as the substitution degree increases, from 64 nm (x = 0) to 46 nm (x = 0.07). The diffuse reflectance spectra of CaZr_1−xCr_xO₃ pigments show a broad absorption band between 400 and 650 nm. CaZr_1−xCr_xO₃ pigments are located in the red-yellow area of the CIEL*a*b* color space. The proportion of red ranges between 16.7 and 19.0, whilst the proportion of yellow varies between 6.4 and 8.2. The brightness of the pigments decreases as the chromium proportion increases. Testing the CaZr_0.93Cr_0.07O₃ pigment in coloring a semi-transparent enamel showed that the pigment is dissolved by the molten glass. At the same time, Cr⁴⁺ is reduced to Cr³⁺, which is responsible for the green color of the enamel.
Visible light absorption of binuclear TiOCoII charge-transfer unit assembled in mesoporous silica
2007, Microporous and Mesoporous Materials
Grafting of Co^II(NCCH₃)₂Cl₂ onto mesoporous Ti–MCM-41 silica in acetonitrile solution affords binuclear Ti–O–Co^II sites on the pore surface under complete replacement of the precursor ligands by interactions with anchored Ti centers and the silica surface. The Co^II ligand field spectrum signals that the Co centers are anchored on the pore surface in tetrahedral coordination. FT-infrared action spectroscopy using ammonia gas adsorption reveals Co–O–Si bond modes at 831 and 762 cm⁻¹. No Co oxide clusters are observed in the as-synthesized material. The bimetallic moieties feature an absorption extending from the UV into the visible to about 600 nm which is attributed to the Ti^IV–O–Co^II → Ti^III–O–Co^III metal-to-metal charge-transfer (MMCT) transition. The chromophore is absent in MCM-41 containing Ti and Co centers isolated from each other; this material was synthesized by grafting Co^II onto a Ti–MCM-41 sample with the Ti centers protected by a cyclopentadienyl ligand. The result indicates that the appearance of the charge-transfer absorption requires that the metal centers are linked by an oxo bridge, which is additionally supported by XANES spectroscopy. The MMCT chromophore of Ti–O–Co^II units has sufficient oxidation power to serve as visible light electron pump for driving multi-electron transfer catalysts of demanding uphill reactions such as water oxidation.
An overview on semiconductor particulate systems for photoproduction of hydrogen
1998, International Journal of Hydrogen Energy
Hydrogen production from aqueous solutions using semiconductor particles as photocatalysts has been reviewed highlighting the experimental techniques and mechanistic aspects. Several factors which influence the hydrogen production efficiency of semiconductor particles have been critically analysed with specific examples from literature. Photocatalytic efficiencies of individual semiconductors such as, CdS, TiO₂, WO₃ have also been assessed with reference to hydrogen production in the presence and absence of loaded metals, electrondonors/acceptors and hole scavengers. The main focus of this overview is to provide literature-based information on hydrogen production reactions using semiconductor particles in the presence of light energy.
Optical absorption of Ni2+ (d8) and Mn4+ (d3) in some spinel oxides
1992, Solid State Communications
The optical absorption spectra of the spinel series Li[Ni_0.5Mn_1.5-xTi_x]O₄ are investigated for Ti substitutions up to x = 1.35. Absorption bands of octahedral Ni²⁺ (d⁸) and Mn⁴⁺ (d³) are observed simultaneously, and can be attributed unambiguously by comparison with Mn⁴⁺ (d³) absorption bands in LiZn_0.5Mn_1.5O₄ and LiMg_0.5Mn_1.5O₄. For Ni²⁺(d⁸) values of the octahedral ligand field parameter Dq and the Racah parameters B and C are estimated.
Doping effects of transition metal ions on the photosensitization of WO<inf>3</inf> particles
1988, Solar Energy Materials
Diffuse reflection spectra of WO₃ powders doped with first row transition metal ions Ti(III), V(IV), Cr(III), Mn(II), Fe(III), Co(II), Ni(II) and Cu(II) and RuO₂ indicate that the doping has increased the absorbance of the doped semiconductor in the visible region of the solar radiation. The fundamental band gap of all the doped WO₃ is being shifted towards the longer wavelength region. Doping also changes the colour of the WO₃ powder. RuO₂, Cr(III), Mn(II) and Fe(III) are found to be more effective in photosensitization. The spectral observations have been correlated with the crystal field transitions of the metal ions. X-ray diffraction studies of the doped and undoped WO₃ powders proved that there is no modification in the crystal structure of WO₃ due to doping. These studies are found to be helpful in selecting the dopants to sensitize the semiconductor powders.
Incorporation of manganese ions in magnesium titanate. A structural thermogravimetric, optical (reflectance) and electron spin resonance study
1987, Reactivity of Solids
Polycrystalline samples of magnesium titanate containing a few percent (atomic) of manganese oxides, heated in air at 850 ° C, have been studied by X-ray diffraction, electron spin resonance, optical reflectance, chemical and thermogravimetric analysis.
The results show that manganese may be dissolved in the ilmenite structure in a limited amount as Mn²⁺, Mn³⁺ and Mn⁴⁺. The latter species is present only in a very minute amount. By heating in an atmosphere of hydrogen, reduction to Mn²⁺ of manganese ions with an oxidation number higher than 2+ is first observed in solid solution. Segregation of part of the manganese then occurs, probably as the monoxide, MnO.

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Journal of Inorganic and Nuclear Chemistry

Abstract

References (24)

Solar Energy

J. Mater. Sci.

Solar Energy Mater.

J. Inorg. Nucl. Chem.

J. Electrochem. Soc.

Some Aspects of the Luminescence of Solids

Inorganic Electronic Spectroscopy

J. Electrochem. Soc.

Solar Energy Mater.

Chem. Phys. Lett.

Cited by (46)

Chromium-doped calcium zirconate - A potential red shade pigment: Preparation, characterization and testing

Visible light absorption of binuclear TiOCo<sup>II</sup> charge-transfer unit assembled in mesoporous silica

An overview on semiconductor particulate systems for photoproduction of hydrogen

Optical absorption of Ni<sup>2+</sup> (d<sup>8</sup>) and Mn<sup>4+</sup> (d<sup>3</sup>) in some spinel oxides

Doping effects of transition metal ions on the photosensitization of WO<inf>3</inf> particles

Incorporation of manganese ions in magnesium titanate. A structural thermogravimetric, optical (reflectance) and electron spin resonance study