J. Chem. SOC., Faraday Trans. I , 1987, 83 (12), 3541-3548

An Electron Spin Resonance Study of Rutile and Anatase Titanium Dioxide Polycrystalline Powders treated with

Transition-metal Ions Angelo Amorelli, Jeffrey C. Evans* and Christopher C. Rowlands

Department of Chemistry, University College Cardif, P.O. Box 78, Cardif CFI IXL, Wales

Terry A. Egerton Central Laboratories, Tioxide UK Ltd, Portrack Lane, Stockton-on- Tees,

Cleveland TS18 2NQ

Rutile and anatase forms of titanium dioxide polycrystalline powders have been treated with low levels, ca. 200 ppm wt/wt, of Cr3+ and Fe3+ ions. The effects of calcination temperature upon these powders are described. At temperatures below 500 "C, both rutile and anatase forms of doped powder exhibited e.s.r. resonances at g = 1.97 and g = 4.3 ascribed to surface chromium and iron species, respectively. At higher temperature migration of these surface bound ions into the bulk of the rutile powder is observed. In the case of both chromium and iron substitutionally incorporated within the rutile lattice, photochromic activity is observed. Diffusion of ions into the bulk upon calcination is also demonstrated (using e.s.r.) by the corresponding anatase powder, although the results appear to indicate that the process is more difficult for the anatase case. Trapping of transition- metal ions at substitutional sites within the anatase structure is also possible by impregnation of a hydrous precursor pulp of TiO, prior to calcination.

Titanium dioxide (TiO,) pigments are manufactured in either the rutile or anatase structural forms. Chemical purity and crystal size are the main factors which determine the colour of these pigments. The principal contaminants are transition elements, such as iron, chromium and manganese, consequently the study of these transition-metal impurities within TiO, is of interest. The technique of electron spin resonance is well suited for the investigation of low levels of paramagnetic impurities within the TiO, structure, and has in fact been used to investigate both chromium and iron-impregnated TiO,. Gerritsen et al.' and Castner et al.,, respectively, reported the e.s.r. spectra of Cr3+ and Fe3+-doped single crystals of rutile TiO,, whilst Evans et u I . , ~ and Thorp and Eggleston4 studied the corresponding polycrystalline rutile TiO, powders. The work carried out by Evans3 demonstrated the effects of calcination temperature upon Cr3+- impregnated polycrystalline rutile TiO, powders. In the present study we have extended the work to investigate the effect of calcination temperature upon both Cr3+- and Fe3+- doped rutile and anatase polycrystalline TiO, powders.

Experimental Materials Samples of rutile and anatase TiO, pigments were supplied for this study by Tioxide UK Ltd, as was the precursor, hydrous TiO, pulp. The transition-metal ions were obtained from BDH spectrosol solutions for atomic absorption, which contained 1 mg of metal ion per cm3 of solution.

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1View Article Online / Journal Homepage / Table of Contents for this issue

3 542 E.S.R. of Rutile and Anatase

\

20 mT *

Fig. 1. E.s.r. spectra of Fe3+- and Cr3+-treated rutile powder: (a) heated at 110 "C/2 h, (6) heated at 950 "C/2 h. Numbering on spectrum refers to table 1.

Procedure

Batches of TiO, pigment were impregnated to predetermined concentrations of dopant ions, i.e. Cr3+ and Fe3+. Loadings of 200 ppm wt/wt dopant ion to solid were prepared. The method for impregnation involved the addition of 1 cm3 of spectrosol standard solution containing 1 mg of dopant ion, to 5 g of pigment. The mixture was then diluted with distilled water. The resultant slurry was subsequently heated until dry, with vigorous stirring to ensure even distribution of the dopant ion. The product was air dried at 110 "C for 30-60 min and then ground to ensure a homogeneous mixture. All calcinations were performed in a double-tube furnace controlled by a ' West ' temperature controller, employing a Pt/ 13 YO Rh : Pt thermocouple and allowing direct temperature settings. The samples of doped pigment were placed in 10 cm long ceramic boats situated at the centre of the furnace. Calcinations of 2-5 h duration were used in this study.

E.s.r. spectra were obtained on a Varian El09 spectrometer operating in the X-band (9.5 GHz). Higher-derivative spectra were produced by a modulation system using a subharmonic generator. Experimental g values were determined with reference to a standard marker: diphenyl picryl hydrazyl (DPPH), for which g = 2.0036. Relative concentration measurements were performed by integration of the e.s.r. spectra. The process involved the use of a North Star Horizon microcomputer interfaced to the e.s.r. spectrometer via an analogue- to-digital converter.

Results and Discussion Cr3+- and Fe3+-doped Rutile The results of the present study demonstrate that both Cr3+ and Fe3+ ions placed onto the surface of rutile TiO, will migrate into the lattice, when subjected to temperatures in excess of 400 "C. Fig. 1 shows the e.s.r. spectra for rutile doped with both Fe3+ and

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A . Amorelli et al.

100-

90

80

70

.g 60- 5 Y

5 0 - a 2 .2 40- 8 Y

30

20

10

3543

-

-

-

-

-

- \

L Av I I '&.-A I A 1 I I I I I

\ \

0, \

Table 1. X-Band resonances detected in e.s.r. spectrum of 950 "C calcination product : Fe3+/

Cr3+/Ti0,

resonancea species g

1 2 3 4 5b 6 7 8 9b

10 1 1 12

Fe3+ Cr3+ Fe3+ Cr3+ Fe3+ Fe3+ Cr3+ Fe3+ Cri+ Cr3+ Fe3+ Cr3+

8.11 5.64 5.57 5.04 4.3 3.36 2.64 2.6 1.97 1.67 1.52 1.32

a Fig. l(b). * Surface-bound species.

80 -

7 0 -

60 -

50 -

4 0 -

30 -

20

10

-

-

Fig. 2. Effect of calcination temperature on the intensities of e.s.r. resonances assigned to surface bound or substitutionally incorporated ions of (a) chromium and (b) iron. (---) Surface, (-)

substitutional.

Cr3+ ions, and subjected to calcinations at 110 and 950 "C. The spectrum of the sample heated at 110 "C [fig. 1 (a)] shows a strong isotropic resonance at g = 1.97 and a relatively weak, but broad resonance at g = 4.3. The former is attributed in the literature3 to a surface chromium species. The latter signal, at g = 4.3, has been assigned to Fe3+ ions in a rhombic environment,' and in this study we propose that it can also be ascribed to an iron surface species. As the temperature of calcination is increased, e.s.r. resonances associated with substitutionally incorporated ions are observed to develop,

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3544 E.S.R. of Rutile and Anatase

- 20 mT

( b )

- 20 mT

r

(ii)

0-

--s

Fig. 3. E.s.r. spectra of calcined samples of anatase treated with transition metals: (a) Iron (i) 450 "C/2 h, (ii) 750 "C/2 h; (6) chromium (i) 450 "C/2 h, (ii) 750 "C/2 h, (iii) 750 "C/5 h.

S = surface resonance, B = bulk resonance.

whilst the peaks due to surface species at g = 1.97 and 4.3 are found to diminish in intensity. This is demonstrated by the spectrum of the 950 "C calcination products, where several e.s.r. peaks are exhibited from 8&500 mT [fig. 1 (b)]. These resonances are detailed in table 1. The positions of these resonances agree with those previously reported3.' for Cr3+- and Fe3+-impregnated rutile TiO, powders. The effect of temperature on the e.s.r. signals, and consequently the location of the dopant ions, was monitored by integration of resonances assigned to both surface and bulk resonances. Fig. 2 illustrates the effects of temperature on the location of the dopant ions. This behaviour can be explained by the fact that the crystal structure of rutile TiO, possesses open 'channels' along its c - a ~ i s , ~ which enable diffusion to occur into the lattice via this route. Consequently, transition-metal ions present at the surface can migrate into the crystal structure, and occupy cation sites vacated by Ti4+ ions. Carter and Okaya' determined that substitution of Ti4+ ions occur with iron in the Fe3+ state. Cr3+ ions are also accommodated at Ti4+ sites, since its ionic size, like Fe3+, is similar to Ti4+. On the surface, chromium is reported' to be present in three oxidation states Cr3+, Cr6+ and Cr5+. All three species are proposed to be involved in producing the g = 1.97 resonance. Earlier work by van Reijen and Cossee8 reported that the isotropic resonance at g = 1.97 was due to a Cr5+ species in a square-pyramidal symmetry. Evans' provided evidence for Crs+ and Cr3+ ions on the surface, and proposed Crs+-Cr3+-Crs+ clusters, giving an

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A . Amorelli et al. 3545

Table 2. X-Band resonances detected in e.s.r. spectrum of Cr3+ and Fe3+ ions in cation sites of

anatase (9.35 GHz)

ion field/mT transition

Cr3+ 33.9 _ _ ;*; 34.8 _ _ ;*i 30.3 2 2

38.1 i - 3 26.2 i - 3 42.3 2 2

Fe3+ 33.7 - _ ;-;

_ _ 3* -1

- _ 3*--1

average oxidation state of +5. A Cr5+ or pseudo-Cr5+ species would therefore be responsible for the g = 1.97 signal.

For the rutile TiO, powders doped with both Fe3+ and Cr3+ ions, photochromic behaviour was observed, although no difference in the e.s.r. spectra could be detected for the samples, before and after exposure to light. Many hypotheses for the photochromic mechanisms have been put forward for iron-doped r~ t i l e .~ . " Faughan and Kiss'' explained the photochromism of SrTiO, as being due to electron transfer from Fe3+ ions to Mo6+, producing Fe4+ and Mo5+. A similar synergestic mechanism is inferred by the chromium-iron system, since rutile TiO, powders doped with only Cr3+ or Fe3+ did not exhibit the same activity.

Cr3+- and Fe3+-doped Anatase Powders

Portions of Fe3+-impregnated anatase TiO, polycrystalline powder were calcined at various temperatures in the range 45&950 "C. The e.s.r. spectra [fig. 3(a)] of samples calcined below 500 "C exhibit a peak at g = 4.3, and a multiplet centred at g = 1.97 attributed to a chromium impurity,', and no e.s.r. resonances associated with substitutionally incorporated Fe3+ ions are observed. For iron, these resonances would be expected at g = 2 for axial ~ymmetry. '~ The surface bound Fe3+ contribute to the g = 4.3 resonance. The corresponding Cr3+-treated anatase powder, exhibited a strong resonance at g = 1.97, associated with surface chromium. Calcination of the Cr3+ and Fe3+ at higher temperatures, i.e. 750 "C, show that some migration of the surface ions was occurring, although not apparently to the same degree as found for rutile. This effect is shown by the appearance of e.s.r. resonances ascribed to Fe3+ and Cr3+ at substitutional cation sites within anatase [fig. 3 (b)] .

When the anatase is converted to rutile by calcination above 900 "C, the familiar e.s.r. spectra associated with rutile appears, showing substitutional incorporation of Fe3+ and Cr3+ ions, and an additional peak at g = 2.04 assigned to an intrinsic defect. We suggest that this defect is caused by the anatase converting to rutile; this transformation was confirmed by X-ray diffraction.

These results suggest that the structure of anatase is permeable to entry by these ions, although it appears to be more difficult in comparison with the corresponding rutile powder. This is shown in fig. 3(b), where substantial amounts of chromium remain on the surface after calcination at 750 "C for 2 and 5 h, whereas fig. 2 demonstrates that the bulk of the chromium has entered the rutile crystal after calcination at 750 "C for 2 h. This difference may be explained by the fact that the crystal structures of anatase and rutile are dissimilar, consequently the migration path of ions down the open c-axis channels present in rutile may be hindered in anatase.

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3546 E.S.R. of Rutile and Anatase

g = 1.97 t

g = 2.00

Fig. 4. E.s.r. spectra for (a) Cr3+ and (b) Fe3+ in anatase. Impregnated pulp calcined at 800 "C for 2 h to produce anatase modification.

Transition-metal-doped Hydrous TiO, Pulp

Samples of an amorphous TiO, pulp impregnated with Cr3+ or Fe3+ ions and calcined in the range 450-800 "C to produce the anatase modification of TiO,, exhibited e.s.r. resonances (table 2) assigned to Cr3+ or Fe3+ ions located at vacated Ti4+ sites. These ions will lie in a crystalline field with small anisotropy of axial symmetry12 and produce the powder e.s.r. spectra shown in fig. 4.

For the Fe3+/anatase, a strong resonance is observed at g = 2.00, with a broad underlying signal. This resonance is associated with Fe3+ in axial symmetry.13 In the case of Cr3+/anatase, a strong multiplet is observed, as well as weaker satellite resonances. This spectrum has been reported by Ebert,12 and discussed in general by Burns14 for S = species in axial symmetry. The strong resonance at g = 1.97 is caused by a second- order effect of the zero-field splitting on the f * - f transition. The weaker outer resonances are the fine structure due to if-) i, -it-, - $ transitions. The spin-Hamiltonian for Cr3+ ( S = i) in an axial crystalline field is given to first order by:

i%? = gpH.S+D[S;-$!S(S+ 1 ) ] + AZ-S

and the spectrum can be described to second order by

D2 VO

v = v ,+D(3 cos28- 1)+6 cos26(1 -cos2 8 ) - .

Therefore, the splittings of the weaker fine-structure transitions will give 2 0 and 4 0 for crystalline orientations with crystal-field axes nearly parallel or perpendicular to the magnetic field direction and the zero-field splittings of +*-f is proportional to D2/ hv,. From the values measured off the spectrum. D was calculated as being 39.6 mT. To unequivocally confirm that the absorptions [fig. 4(a)] are due to chromium, 53Cr- enriched Cr,03 was used to impregnate the pulp. The resulting spectrum, after calcination to form anatase, exhibited a complex series of resonances centred at g = 1.97 (fig. 5). This was interpreted as being the resultant spectrum of Cr3+ in anatase perturbed by the hyperfine interaction of the 53Cr ( I = :) nuclei. The third-derivative spectrum

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A . Amorelli et al. 3 547

- 20 mT t

300 mT

Fig. 5. 53Cr-enriched Cr3+ in anatase TiO,. Inset contains third-derivative expansion of multiplet.

resolved the it) - f resonance into 10 lines, i.e. eight lines due to 53Cr and two due to 52Cr. The value of the hyperfine splitting is found to be 1.88 mT.

Conclusions It has been demonstrated that Fe3+ and Cr3+ ions placed onto the surface of rutile will migrate into the crystal lattice at high temperatures via open channels in the rutile structure. The corresponding anatase powders also exhibit this behaviour, although the results indicate that the process is more difficult, because of the different crystal structure. The use of 53Cr has confirmed the assignments of the e.s.r. spectra to chromium species.

The presence of both Fe3+ and Cr3+ ions within rutile imparts photochromic activity to the powder by some as yet undetermined synergistic mechanism.

We thank Tioxide UK Ltd for supplying the samples of TiO, powders and pulp. A. A. thanks Tioxide UK Ltd for the provision of a bursary.

References 1 H. J. Gerritsen, S. E. Harrison, H. R. Lewis and J. P. Wittke, Phys. Rev. Lett . , 1959, 24, 153. 2 T. Castner Jr, G. S. Newell, W. C. Holton and C. P. Slichter, J . Chem. Phys., 1960, 32, 668. 3 J . C. Evans, C. P. Relf, C. C. Rowlands, T. A. Egerton and A. J. Pearman, J. Muter. Sci. Lett., 1984,

4 J . S. Thorp and H. S. Eggleston, J . Muter. Sci. Lett., 1985, 4, 1146. 5 H. B. Huntington and G. A. Sullivan, Phys. Rev. Lett., 1965, 17, 177. 6 D. L. Carter and A. Okaya, Phys. Rev., 1960, 118, 1485. 7 J . C. Evans, C. P. Relf, C. C. Rowlands, T. A. Egerton and A. J. Pearman, J . Muter. Sci. Lett., 1985,

8 L. L. van Reijen and P. Cossee, Discuss. Furuduy SOC., 1966, 41, 277. 9 W. Clark and P. Broadhead, J . Phys. C, 1969, 3, 1047.

3, 695.

4, 809.

10 J. S. Thorp and H. S. Eggleston, J . Mater. Sci., 1985, 20, 2369.

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3 548 E.S.R. of Rutile and Anatase

1 1 B. W. Faughan and Z. J. Kiss. Phys. Rev. Lett., 1968, 21, 1331. 12 T. Ebert and J. Scheve, Magnetic Resonance and Relaxation, Proc. XiVth Colloque Amkre, ed. R.

13 R. Aasa, J. Chem. Phys., 1970, 52, 3919. 14 G. Burns, J . Appl. Phys., 1970, 52, 3919.

Blinc (North Holland, Amsterdam, 1967).

Paper 71863; Received 15th May, 1987

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