Titanium dioxide, TiO2, is an important photocatalytic material that exists as two main polymorphs, anatase and rutile. The presence of either or both of these phases impacts on the photocatalytic performance of the material. The present work reviews the anatase to rutile phase transformation. The synthesis and properties of anatase and rutile are examined, followed by a discussion of the thermodynamics of the phase transformation and the factors affecting its observation. A comprehensive analysis of the reported effects of dopants on the anatase to rutile phase transformation and the mechanisms by which these effects are brought about is presented in this review, yielding a plot of the cationic radius versus the valence characterised by a distinct boundary between inhibitors and promoters of the phase transformation. Further, the likely effects of dopant elements, including those for which experimental data are unavailable, on the phase transformation are deduced and presented on the basis of this analysis.

https://link.springer.com/content/pdf/10.1007%2Fs10853-010-5113-0.pdf

Review of the anatase to rutile phase transformation

Pore size determination in modified micro- and mesoporous materials. Pitfalls and limitations in gas adsorption data analysis

The Role of Inorganic Polymer Technology in the Development of ‘Green Concrete’

Understanding the relationship between geopolymer composition, microstructure and mechanical properties

Carbon dioxide equivalent (CO2-e) emissions: A comparison between geopolymer and OPC cement concrete

Mixed oxide compounds of potential usefulness for fibre coatings (hexagonal celsian, BaAl2Si2O8 and lanthanum hexaluminate, LaAl11O18) were prepared by hybrid sol–gel synthesis and their thermal crystallisation was monitored by thermal analysis, X-ray diffraction and multinuclear solid state MAS NMR. Both the gels convert to the crystalline phase below about 1200°C, via amorphous intermediates in which the Al shows an NMR resonance at 36–38 ppm sometimes ascribed to Al in five-fold coordination. Additional information about the structural changes during thermal treatment was provided by 29Si and 137Ba MAS NMR spectroscopy, showing that the feldspar framework of celsian begins to be established by about 500°C but the Ba is still moving into its polyhedral lattice sites about 400°C after the sluggish onset of crystallization. Lanthanum hexaluminate crystallises sharply at 1230°C via γ-Al2O3.

Crystallization of fibre-coating compounds of potential use in fibre-reinforced oxide ceramics

Comment on “Characterization of Spinel Phase Formed in the Kaolin‐Mullite Thermal Sequence”

Imperfectly ordered KAlSiO4 shows a greatly increased uptake of Ni2+ from solution which motivated this study of the state of Ni2+ in Ni-exchanged KAlSiO4 using X-ray photo-electron spectroscopy (XPS), X-ray fluorescence (XRF), powder X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) and solid state nuclear magnetic resonance spectroscopy (NMR). The imperfectly ordered KAlSiO4 was synthesized by solid-state reaction of kaolinite with K2CO3 at 550°C for 24h. The Ni-exchanged KAlSiO4 samples were obtained by treatment in a solution containing 10-3-10-1M NiCl2 buffered with 0.5M NaCl at 60°C. Comparison of the surface and bulk chemical compositions analyzed by XPS and XRF indicated that the Ni2+ sorbed on the KAlSiO4 is largely concentrated at the surface. The XRD pattern of the Ni-exchanged KAlSiO4 showed weak broad peaks corresponding to a phase with a layered structure and a change in the halo pattern of the KAlSiO4. EXAFS analysis confirmed the presence of a Ni-containing layered double hydroxide (LDH; Ni1-yAly(OH)2Xy⋅nH2O) as indicated by the interatomic Ni-Ni distance and Ni coordination number. The 29Si MAS NMR showed shift and peak splitting corresponding to an increase in the degree of polymerization of the SiO4 tetrahedra forming the framework of the tridymite-type structure and the 27Al MAS NMR showed an increase of six coordinated Al in the Ni-exchanged KAlSiO4. It is concluded, from all these results, that the Ni2+ in Ni-exchanged KAlSiO4 forms a Ni-Al LDH-like phase on the surface of the KAlSiO4. This is considered to be the main reason for the higher Ni2+ uptake compared with various other cations adsorbed on imperfectly ordered KAlSiO4.

Characterization of the State of Ni2+ in Ni-Exchanged KAlSiO4 by XPS, XRF, XRD, EXAFS and NMR

The effect of mechanochemical activation (high-energy grinding) on mixtures of natural FeWO4 with two forms of carbon has been investigated by X-ray powder diffraction and Mossbauer spectroscopy. The crystallinity of the mechanically activated mixtures, which have previously been found to exhibit enhanced reactivity as precursors for carbothermal production of tungsten carbide, is shown by X-ray powder diffraction to decrease on grinding, but without detectable formation of new crystalline phases. Mossbauer spectroscopy shows that the grinding process is accompanied by the appearance of an unexpected new singlet resonance, attributed to the formation of dilute Fe C regions. The Mossbauer spectra also show that mechanochemical activation under the present conditions results in progressive oxidation of the Fe2+ in the wolframite to Fe3+; this oxidation is more marked in wolframite ground in the absence of carbon, suggesting that the latter exerts an oxygen-scavenging effect on the system. Similar results are obtained irrespective of whether the carbon source is activated carbon or graphite. The crystallization of iron tungsten carbides on subsequent heating of the ground mixtures in argon suggests that the regions identified by Mossbauer spectroscopy as containing iron–carbon interactions may also be associated with tungsten.

Kenneth J. D. MacKenzie

Papers

Crystallization of fibre-coating compounds of potential use in fibre-reinforced oxide ceramics

Comment on “Characterization of Spinel Phase Formed in the Kaolin‐Mullite Thermal Sequence”

Characterization of the State of Ni2+ in Ni-Exchanged KAlSiO4 by XPS, XRF, XRD, EXAFS and NMR

Mechanochemical activation of mixtures of wolframite (FeWO4) with carbon, studied by 57Fe Mössbauer spectroscopy

Ferroelectric, pyroelectric and dielectric properties of complex perovskite-structured Pb0.84Ba0.16(Zr0.44Ti0.40Fe0.08Nb0.08)O3 ceramics