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The Effect of Zr-Doping and Crystallite Size on the Mechanical Properties of TiO2 Rutile and Anatase
(2008)
- TiO2 is an important technological material, used as white pigment, as wide band gap semiconductor in electrochemical dye solar cells, for photocatalysis and in photochemical energy-conversion processes. The most abundant phases are rutile, anatase and brookite. In addition, there are a number of metastable low density modifications. The compression behavior of anatase and rutile was studied for TiO2 and Ti0.9Zr0.1O2 starting materials with crystallite size in the micro- and also nanometer range. Quenched samples of rutile, anatase and high pressure polymorphs synthesized at up to 10 GPa adopt 10 mol% ZrO2, Zr-doped TiO2 starting materials therefore have the composition Ti0.9Zr0.1O2. Compression experiments were carried out in the diamond anvil cell and samples were characterized by in-situ X-ray diffraction, X-ray absorption and Raman spectroscopic measurements. A sol-gel route was developed for the synthesis of nanoscale anatase Tix:Zr1-xO2 with x=0.90 and 1.0, which was annealed at 1000°C to microscale rutile. In hydrothermal experiments, nanoscale anatase Ti0.9Zr0.1O2 was used as starting material for the synthesis of microscale Zr-doped anatase. Neither the incorporation of Zr nor the decrease of crystallite size to the nanometer range modifies the bulk modulus of rutile. These results are different from those of anatase, where a decreasse of crystallite size and doping with Zr leads to an increase of the bulk modulus. Second order EoS fits (K0’=4) resulted in a bulk modulus of microscale anatase of K0=178 GPa and K0=179 GPa. The nanoscale counterpart shows much higher values of K0=237 GPa and K0=243 GPa. In this study, it was found that microscale anatase Ti0.90Zr0.10O2 has K0=195 GPa, which is comparable to undoped material. Largest values were found for nanoscale anatase Ti0.90Zr0.10O2 with K0=258 GPa. Zr-doping thus reduces the compressibility of nanoanatase, even though ZrO2 polymorphs are more compressible than the corresponding TiO2 forms. For the Zr-doped nanoanatase, XRD analysis showed a significant change in compression behavior at pressures 4 GPa, suggested as a consequence of deviatoric stresses during experimental compression of the nanoscale material. Computations on supercells with different distances of neighboring Zr-atoms suggested cluster formation of Zr in the (Ti,Zr)O2 anatase. The resulting structural distortions can further augment the change in compression behavior. Zr-doped nanoanatase becomes stiffer upon multiple compression cycles. While the bulk modulus of the first compression was 211 GPa, after the sample was decompressed, the second compression showed a bulk modulus of 249 GPa. We suggest that partial pressure induced amorphization plays an important role for the observed stiffening. Anatase and rutile TiO2 transform to the MI phase upon compression. The transition pressure increases with a decreasing crystallite size from 12 GPa for microscale material to 18 GPa for crystallite size of 12 nm. For anatase, smaller particles transform to an amorphous phase at pressures of 20–24 GPa. Zr-doping does not seem to vary the transformation pressure. Ab-initio all-electron density functional electronic structure simulations on the ground state energetics of the TiO2 phases rutile, anatase, brookite, TiO2II and MI-phase were performed using the projector augmented wave and the linear augmented plane wave methods along with local density approximation (LDA) and two types of generalized gradient approximations (GGA), using the formulations by Perdew, Bunge and Enzerhoff, referred to as PBE, and by Wu and Cohen, reffered to as WC. The zero pressure volumes are predicted smaller by <3% in LDA computations and larger by 8 and 0.4% in PBE and WC computations. The stable structure at 0 GPa is baddeleyite for LDA computations and anatase for GGA computations, contradicting experimental results that determine rutile as the most stable phase. Rutile appears to have the highest energy in LDA computations and intermediate energy in GGA computations.
