Background
Titania polymorphs
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TiO2 II or srilankite, an orthorhombic polymorph of the lead oxide structure
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Cubic fluorite-type polymorph
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Pyrite-type polymorph
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Monoclinic baddeleyite-type polymorph
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Cotunnite-type polymorph
Titania properties
Property | Anatase | Rutile | Reference |
---|---|---|---|
Crystal structure | Tetragonal | Tetragonal | [13] |
Atoms per unit cell (Z) | 4 | 2 | |
Space group | \( l{\frac{4}{a}} \)md | \( P{\frac{{4_{2} }}{m}} \)nm | |
Lattice parameters (nm) | a = 0.3785 | a = 0.4594 | |
c = 0.9514 | c = 0.29589 | ||
Unit cell volume (nm3)a
| 0.1363 | 0.0624 | |
Density (kg m−3) | 3894 | 4250 | |
Calculated indirect band gap | |||
(eV) | 3.23–3.59 | 3.02–3.24 | |
(nm) | 345.4–383.9 | 382.7–410.1 | |
Experimental band gap | |||
(eV) | ~3.2 | ~3.0 | |
(nm) | ~387 | ~413 | |
Refractive index | 2.54, 2.49 | 2.79, 2.903 | |
Solubility in HF | Soluble | Insoluble | [23] |
Solubility in H2O | Insoluble | Insoluble | [13] |
Hardness (Mohs) | 5.5–6 | 6–6.5 | [24] |
Bulk modulus (GPa) | 183 | 206 | [20] |
Titania applications
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Energy
Photocatalytic effect
Formation and analysis of titania phases
Phase formation during synthesis of TiO2
Synthesis method | Mechanism | Phases formed | References | |||
---|---|---|---|---|---|---|
Amorphous | Anatase | Rutile | Anatase + rutile | |||
Room temperature hydrolysis of TiCl4
| Precipitation from room temperature solutions of TiCl4
| ✓ | ||||
Room temperature sol–gel synthesis | Hydrolysis of TiCl4 or an organo-metallic compound | ✓ | ||||
Flame pyrolysis of TiCl4
| Combustion of TiCl4 with oxygen; used in industrial processes | ✓ | ✓ | |||
Solvothermal/hydrothermal | Precipitation of TiO2 from aqueous or organic solution at elevated temperatures | ✓ | ✓ | ✓ | ||
Chemical vapour deposition | Spraying of Ti-bearing solution | ✓ | ✓ | ✓ | ✓ | |
Physical vapour deposition | Deposition of evaporated Ti and its subsequent oxidation | ✓ | ✓ | ✓ | ✓ |
Anatase to rutile transformation
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Particle size
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Particle shape (aspect ratio)
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Surface area
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Atmosphere
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Volume of sample
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Nature of sample container
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Heating rate
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Soaking time
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Impurities (from raw materials and container)
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Measurement technique
Phase differentiation and quantification of anatase/rutile ratio
X-ray diffraction
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Preferred orientation: rutile and/or anatase crystallites may be present in preferred orientation owing to morphological and/or sample preparation effects, which may lead to altered XRD relative peak intensities.
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Lattice distortion: the presence of dissolved dopants and/or impurities, especially if differential solubility occurs, may alter the peak heights and areas, thereby altering the relative intensities of the XRD peaks.
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Degree of crystallinity: the presence of dopants may increase (nucleation) or decrease (lattice distortion/stress) the degree of crystallinity, which would alter the consequent peak intensities, particularly if these dopants are preferentially present in one of the phases.
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Morphology: rutile may form in an acicular morphology, which would alter the XRD peak intensities of these grains relative to typically equiaxed rutile [129].
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Surface nucleation of rutile: enhanced surface nucleation of rutile owing to heat (thermal gradients) and segregation (chemical gradients) effects would increase the XRD peak intensities of this phase; this would be similar to encapsulation.
Laser Raman microspectroscopy
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Minimal or no sample preparation
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Nondestructive
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Local and general phase analyses (≤1 μm beam diameter)
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Mapping capability
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Rapid analyses (~1 min scan)
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No preferred orientation effect
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Greater sensitivity than XRD
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Sensitive to nanoscale phases
Differential solubility
Impedance spectroscopy
Rutile–anatase mixtures
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Time
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Temperature
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Atmosphere
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Intrinsic chemical composition (purity of raw materials)
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Extrinsic contamination (from processing)
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Chemical homogeneity (e.g., segregation)
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Microstructural homogeneity (e.g., grain boundary precipitates)
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Thermal homogeneity during heating (e.g., resulting from sample size and shape)
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Mineralogical phase assemblage
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Particle size distribution
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Agglomerate size distribution
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Grain morphology
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Agglomerate morphology
Thermodynamics and kinetics of the anatase to rutile phase transformation
Stability of the TiO2 phases
Year | Temp. (°C)a
| Fabrication details | Definition | Ref |
---|---|---|---|---|
1961 | 610 | Highly pure powder | Onset temperature observed by XRD after firing for 24 h | [113] |
1965 | (1190) | Powders from three different suppliers | Apices of DTA peaks (data not shown) | [109] |
(1138) | ||||
(1115) | ||||
1968 | 610 | Commercially available reagent grade powder | Onset temperature observed by XRD after extended firing (1–5 days) | [154] |
1995 | 390 | Sol–gel synthesised powder | Appearance of detectable rutile peak by XRD after 1 week | [123] |
1996 | 675 | Sol–gel synthesised powder | Appearance of detectable rutile peak by XRD after 4 min | [1] |
1997 | (787) | Sol–gel synthesised powder | Reported value: Apex of DTA peak | [155] |
720 | Corrected value: Onset of DTA peak | |||
1997 | 465 | 4-6 nm particles prepared through a sol–gel method | Appearance of detectable rutile peak by XRD | [156] |
1999 | (616) | Sol–gel synthesised powder | Reported value: 50% transformation observed by XRD | [157] |
2001 | (680) | Sol–gel synthesised powder | Reported value: 50% transformation observed by XRD | [82] |
600 | Onset temperature from graph | |||
2001 | 600 | Sol–gel synthesised powder | Appearance of detectable rutile peak by XRD | [108] |
2002 | (700) | Sol–gel synthesised powder | 50% transformation observed by XRD | [124] |
600 | Appearance of detectable rutile peak by XRD | |||
2005 | 600–700 | Highly pure nanocrystals synthesised fromTiCl4 Sol–Gel | Appearance of detectable rutile peak by XRD | [99] |
2007 | (900) | Sol–gel synthesised powder | Reported value: apex of broad DTA peak (data not shown) | [158] |
Kinetics of the anatase to rutile phase transformation
Effects of impurities and dopants on the anatase to rutile phase transformation
Dopant effects
Cationic dopants
Phase transformation inhibitors | |
---|---|
Cation | Dopant phases used |
Al | |
Au | HAuCl4 [178] |
B | BCl3 [120] |
Ba | Ba(NO3)2 [157] |
Ca | Ca(NO3) [157] |
Ce | CeO2 [179] |
Dy | Dy2O3 [180] |
Eu | |
Er | |
Fe | FeCl2 [171] |
Ho | Ho2O3 [181] |
La | |
Mn | |
Nb | NbCl5 [185] |
Nd | Nd2O3 [181] |
P | |
Si | |
Sm | |
Sr | Sr(NO3)2 [157] |
Tb | Tb4O7 [181] |
Tm | Tm2O3 [181] |
Y | |
Zr |
Phase transformation promoters | |
---|---|
Cation | Dopant phases used |
Al | AlCl3(g) [187] |
Cd | CdO [165] |
Co | CoO [165] |
Cr | CrCl3 [185] |
Cu | |
Fe | |
Li | LiF [118] |
Mn | |
Na | NaF [165] |
Ni | |
Sb | Sb2O3 [190] |
Sn | |
V | |
Zn | ZnO [165] |
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The most common valence for each cation has been used, without regard to oxidation–reduction effects (except as noted below).
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Inhibition or promotion of the phase transformation is reported on the basis of the effects described in the literature sources given in Table 5.
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In the case of cations for which contradictory effects are reported (Mn and Fe), the most common finding was used.
Anionic dopants
Nitrogen doping
Fluorine doping
Chlorine doping
Valence changes
Predictive analysis
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The potential for valence changes in dopant cations (e.g., Fe3+ → Fe2+).
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The potential for oxygen vacancy formation through reduction (e.g., C- and N-doping).
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The known effects of anionic dopants (e.g., F− and Cl−).
Inhibitors
Promoters
Carbon doping
Importance of doping methods
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Dry mixing: this involves the blending of dry powders of anatase and dopant-bearing phases, such as oxides. Both large particle sizes and inhomogeneous mixing are associated with increased diffusion distances.
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Molecular-level mixing: this method offers the most intimate level of association and involves mixing of a soluble titanium-bearing compound, typically an organometallic, such as titanium isopropoxide, with a soluble dopant-bearing compounds in an organic or aqueous solution. This level of mixing often is obtained through the use of doped sol–gels or co-precipitation.