Study of mechanochemical phase transformation of TiO2 by EPR. Effect of phosphate

doi:10.1016/0167-2738(89)90256-7

Solid State Ionics

Volumes 32–33, Part 1, February–March 1989, Pages 461-465

https://doi.org/10.1016/0167-2738(89)90256-7 Get rights and content

Abstract

The evolution of EPR spectra points out that the grinding of TiO₂ generates Ti³⁺ associated to oxygen vacancies in the bulk of anatase. The corresponding EPR signal of Ti³⁺ located in the surface emerges by outgassing the sample at 200–300°C. If the comminution is in progress a migration of Ti³⁺ from the surface to the bulk takes place. The addition of phosphate inhibits the formation of surface Ti³⁺ defects through a mechanism that implies its chemisorption on TiO₂ as a bidentate ligand which hinders the surface ionic mobility and, therefore, strongly inhibits the anatase-rutile conversion. In such a case the polymorphic transformation would take place through direct nucleation around the bulk defects.

References (19)

H.P. Böehm
Adv. Catal.
(1966)
J.C. Conesa et al.
J. Molecular Catal.
(1982)
A.W. Czanderna et al.
Trans. Faraday Soc.
(1958)
Y. Tekiz et al.
C.R. Acad. Sci.
(1965)
T. Kubo et al.
J. Chem. Soc. Japan. Ind. Chem. Soc.
(1963)
K.J.D. Mackenzie
Trans. Br. Ceram. Soc.
(1975)
R.D. Shannon et al.
J. Am. Ceram. Soc.
(1965)
R.D. Shannon et al.
Am. Mineral.
(1964)
S. Hishita et al.
Yogyo Kyokai Shi
(1978)

There are more references available in the full text version of this article.

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Fig. 1 displays the change in the crystallographical properties by milling the starting anatase with varying amount of AB. A typical mechanochemical polymorphic transformation to the rutile polymorph was observed by milling as reported elsewhere [29,30]. Note that the transformation was slightly retarded by increasing AB concentration.
Core-shell structured TiO₂ - graphene-like carbon nanocomposites, TiO₂@C, were obtained by combining co-milling anatase nanoparticles with 10 mass% non-crystalline carbon black and subsequent heating at 500 °C under 5% H₂ – He gas flow. Partial crystallization of carbon into a fractional graphene structure were confirmed by transmission electron microscopy and Raman spectroscopy. Because of the carbon structural change, electric conductivity of the composites, acquired from the impedance spectra of the consolidated tablet, increased an order of magnitude to 56 mS/cm. We attribute the increased conductivity to the development of aromatic rings as a consequence of partial crystallization. The composite particles are protected from sintering by the carbon shell so that the specific surface area remained as high as 40 m²/g even after heating at 750 °C. The charge and discharge capacities evaluated from a half-cell, TiO₂@C//LiPF₆ + ethylene carbonate//Li, exceeded 200 mAh/g and was stable at least up to 50 cycles.
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Therefore, W inhibited the support from anatase to rutile transformation. It has been reported [30,31] in the literatures that the transition proceeds through the nucleation of rutile at the surface of anatase, and that it is strongly influenced by the oxygen vacancy concentration. The nuclei of rutile seem to be formed by oxygen vacancies that diffuse in the bulk [15].
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Introduction of oxygen vacancies and fluorine into TiO <inf>2</inf> nanoparticles by co-milling with PTFE
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No appreciable peak shifts were observed, indicating that fluorine transferred from PTFE to titania was confined at the near surface region, where the crystallinity was severely reduced, as shown in Fig. 4(d). The phase transformation from anatase to rutile induced by milling, which is revealed by the diffractograms is well documented [57,58]. Broad peaks of rutile are also explained conventionally by the small size of the crystallites.
Solid-state processes of introducing oxygen vacancies and transference of fluorine to n-TiO₂ nanoparticles by co-milling with poly(tetrafluoroethylene) (PTFE) powder were examined by diffuse reflectance spectroscopy (DRS) of UV, visual, near- and mid-IR regions, thermal analyses (TG-DTA), energy-dispersive X-ray spectroscopy (EDXS), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The broad absorption peak at around 8800 cm⁻¹ (1140 nm) was attributed to the change in the electronic states, viz. electrons trapped at the oxygen vacancies (Vo) and d–d transitions of titanium ions. Incorporation of fluorine into n-TiO₂ was concentrated at the near surface region and amounted to ca. 40 at% of the total fluorine in PTFE, after co-milling for 3 h, as confirmed by the F1s XPS spectrum. The overall atomic ratio, F/Ti, determined by EDXS was 0.294. By combining these analytical results, a mechanism of the present solid state processes at the boundary between PTFE and n-TiO₂ was proposed. The entire process is triggered by the partial oxidative decomposition of PTFE. This is accompanied by the abstraction of oxygen atoms from the n-TiO₂ lattices. Loss of the oxygen atoms results in the formation of the diverse states of locally distorted coordination units of titania, i.e. TiO_6−nVo_n, located at the near surface region. This leads subsequent partial ligand exchange between F and O, to incorporate fluorine preferentially to the near surface region of n-TiO₂ particles, where local non-crystalline states predominate.
Composite TiO<inf>2</inf>-SnO<inf>2</inf> nanostructured films prepared by spin-coating with high photocatalytic performance
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Anatase crystallizes in a metastable state at temperatures below 600 °C and at high temperatures transforms to the rutile phase, which has better thermal stability [9]. The transition temperature depends on different physical and chemical parameters; pressure [10], stress [11], oxygen deficiency [12], contaminants [13], etc. The choice of the doping type may play a crucial rule in the crystalline structure and then the stable phase of TiO2 thin films.
We have prepared nanoscale composite TiO₂–SnO₂ thin films with different Sn ratios. Sol–gel TiO₂–SnO₂ solutions with different ratios were deposited on quartz substrates by the spin-coating. XRD and AFM experiments showed that smooth and uniform anatase, anatase-rutile as well as rutile structural thin films were formed at 500 °C depending on Sn ratio. The nanocomposite films with Ti:Sn ratio of 3:1 proved an excellent photocatalytic activity in the UV–Vis region.
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Photocatalytic oxidation of phenol was performed over acid pre-treated TiO₂ prepared by a sol–gel method. Several oxoacids were used in the acid pre-treatment (nitric, sulfuric and phosphoric acids). Wide structural and surface characterisation of catalysts was carried out in order to establish a correlation between the oxoanion stability during calcination and the further photocatalytic behaviour of TiO₂. Pre-treatment with sulfuric and phosphoric acids clearly stabilises TiO₂ catalyst against sintering, maintaining anatase phase and relatively high surface area values with respect to untreated TiO₂. On the other hand, nitric pre-treatment seems to favour sintering process. Acid treatment leads to an excess of adsorbed protons (Brönsted acid sites) incorporated to the TiO₂ surface. During calcination, the elimination of these hydroxyl groups would be the responsible for the generation of a highly defective material that readily losses the oxygen ions from their surface. Different stability of adsorbed oxoanions would stabilise the anatase crystal phase as well these oxygen vacancies formed during calcination, preventing at the same time the sintering process. The occurrence of oxygen vacancies could favour the appearance of small rutile phase dispersed onto the surface of an anatase matrix performing the photoinduced electronic process. Best photocatalytic behaviour is found for pre-sulfated TiO₂ samples calcined at such high temperature for which sulfates have been eliminated.

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Study of mechanochemical phase transformation of TiO2 by EPR. Effect of phosphate

Abstract

Adv. Catal.

J. Molecular Catal.

Trans. Faraday Soc.

C.R. Acad. Sci.

J. Chem. Soc. Japan. Ind. Chem. Soc.

Trans. Br. Ceram. Soc.

J. Am. Ceram. Soc.

Am. Mineral.

Yogyo Kyokai Shi

Study of mechanochemical phase transformation of TiO₂ by EPR. Effect of phosphate