Structural characterization of titania by X-ray diffraction, photoacoustic, Raman spectroscopy and electron paramagnetic resonance spectroscopy

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 363–370

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Structural characterization of titania by X-ray diffraction, photoacoustic, Raman spectroscopy and electron paramagnetic resonance spectroscopy R.M. Kadam a,⇑, B. Rajeswari a, Arijit Sengupta a, S.N. Achary b, R.J. Kshirsagar c, V. Natarajan a a

Radiochemistry Division, Bhabha Atomic Research Centre, Trombay 400 085, India Chemistry Division, Bhabha Atomic Research Centre, Trombay 400 085, India c High Pressure & Synchrotron Radiation Physics Division, Bhabha Atomic Research Centre, Trombay 400 085, India b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 XRD, PAS, Raman and EPR studies to

EPR spectra of titania mineral at room temperature (A) as it is (containing microcrystals) and (B) recorded after grinding it for 30 min (polycrystalline samples). 55

5A(Mn )

2

7

-3/2 -1/2

Y

9

4

3 1

X

+3/2 +1/2

-3/2 -1/2

Z

Y

+1/2 +3/2

(A)

+1/2 +3/2

characterize titania.  Presence of both rutile (91%) and anatase (9%) phases.  PAS spectrum showed the presence of V4+, Cr3+, Mn4+ and Fe3+ species.  EPR studies revealed the presence of V4+(d1), Cr3+(d3), Mn4+(d3) and Fe3+(d5) at Ti4+ sites.

3+

4+

Cr at Ti site

X

8

5 6

10 (B) 13

11 12 5+

Cr surface

0

1000

2000

3000

4000

5000

6000

H (Gauss)

a r t i c l e

i n f o

Article history: Received 10 April 2014 Received in revised form 12 August 2014 Accepted 23 August 2014 Available online 3 September 2014 Keywords: Rutile Anatase XRD PAS EPR Raman spectroscopy

a b s t r a c t A titania mineral (obtained from East coast, Orissa, India) was investigated by X-ray diffraction (XRD), photoacoustic spectroscopy (PAS), Raman and Electron Paramagnetic Resonance (EPR) studies. XRD studies indicated the presence of rutile (91%) and anatase (9%) phases in the mineral. Raman investigation supported this information. Both rutile and anatase phases have tetragonal structure (rutile: space group P42/mnm, a = 4.5946(1) Å, c = 2.9597(1) Å, V = 62.48(1) (Å)3, Z = 2; anatase: space group I41/amd, 3.7848(2) Å, 9.5098(11) Å, V = 136.22(2) (Å)3, Z = 4). The deconvoluted PAS spectrum showed nine peaks around 335, 370, 415,485, 555, 605, 659, 690,730 and 785 nm and according to the ligand field theory, these peaks were attributed to the presence of V4+, Cr3+, Mn4+ and Fe3+ species. EPR studies revealed the presence of transition metal ions V4+(d1), Cr3+(d3), Mn4+(d3) and Fe3+(d5) at Ti4+ sites. The EPR spectra are characterized by very large crystal filed splitting (D term) and orthorhombic distortion term (E term) for multiple electron system (s > 1) suggesting that the transition metal ions substitute the Ti4+ in the lattice which is situated in distorted octahedral coordination of oxygen. The possible reasons for observation of unusually large D and E term in the EPR spectra of transition metal ions (S = 3/2 and 5/2) are discussed. Ó 2014 Elsevier B.V. All rights reserved.

Introduction ⇑ Corresponding author. E-mail address: [email protected] (R.M. Kadam). http://dx.doi.org/10.1016/j.saa.2014.08.082 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

Titania, titanium dioxide (TiO2) exists in three crystallographic polymorphs namely anatase, brookite and rutile. All the three crys-

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tal structures are made up of distorted TiO6 octahedra connected to each other in different fashion. Rutile adopts a tetragonal structure (space group; P42/mnm, D14 4h) in which two opposing edges of each octahedron are shared to form linear chain along the [0 0 1] direction and the TiO6 chains are linked together via corner connections [1] whereas anatase exists as tetragonal structure (space group; I41/amd, D19 4h) has no corner sharing but has four edges shared per octahedron. The crystal structure of anatase is made up of zigzag chains of the octahedral linked together through edge sharing (Fig. 1). Whereas, Brookite has orthorhombic modification (space group; Pbca, D15 2h) in which the octahedral share three edges and corners and the dominant structural feature is a chain of edge sharing. The distorted TiO6 octahedra are arranged parallel to the c-axis and are cross linked by shared edges [2]. TiO2 currently finds application in pigments [3], cosmetics ultrathin capacitors [4], photovoltaic cells [5,6] and catalysis[7,8] whose properties can be changed significantly by the presence of transition element such as iron, chromium, manganese and vanadium. The applications for TiO2 strongly depend upon the crystal structure, morphology and size of the particles. Each crystalline modification has different physicochemical properties such as density, refractive index and photochemical reactivity. Rutile has the highest density and refractive index among the three phases and therefore has been widely employed in pigments and cosmetic industries. Chemical purity and the crystal size are the main factors which determine the color of the pigment and photocatalytic degradation of paint resin [3]. Since TiO2 is now the most widely used commercial opacifier, it is known that just a few parts per million (1 ppm = 1 lg/g) of the transition metal ion in titanium dioxide pigment profoundly affect the pigment color and photocatalytic degradation of paint resin [9]. In addition to these, TiO2 also possess the photocatalytic activity and hence is used for the destruction of organic pollutants [10] and for disinfection of water [11]. The presence of metal ions in this matrix can move the absorption edge of TiO2 from UV into blue region of the visible spectrum and hence offer potential improvement in the light harvesting ability of TiO2 photocatalyst [12,13] Recent reports describe the use of Colloidal TiO2 particles of both structures loaded with suitable catalyst in light induced water cleavage schemes where the semi-conductor nature of TiO2 plays an important role [14]. Different structural modifications of TiO2 show different physicochemical properties and therefore are used for different applications. Anatase generally shows better performance than its rutile counterpart in photocatalytic applications [15]. The brookite phase is the least studied mainly owing to the difficulties encountered in obtaining its pure form, though it seems to have marked photocatalytic activity (compared to anatase) in the dehydrogenation of 2-propanol [16].

Most of the other experimental methods that have been quite useful, for example, the characterization of transition metal ions present in TiO2 cannot be applied at these low levels (ultra trace levels). EPR spectroscopy is a non-destructive technique used for identification, characterization and quantification of paramagnetic transition metal ions associated with minerals as in the present case, because this technique is highly sensitive and used for detection of paramagnetic impurities to the extent of less than 5 ppm [17–25]. Apart from these, studies using EPR and PAS spectroscopy [26–29] provide information about the chemical environment and bonding properties of the paramagnetic species in minerals, which in turn, influence the physico-chemical properties of the mineral suitable for specific applications. However, the assignments of an EPR signal recorded from a multi mineral system such as soil and clay, to a specific mineral phase or its chemical form is often difficult because of the spectral overlap of different paramagnetic species. An experimental approach based on the changes in EPR signals upon thermal and chemical stability is reported in literature [19] to characterize the chemical environment of the paramagnetic ions with the complex mineral system. In the present study, the XRD and Raman studies were performed to identify the phase purity of the mineral while EPR and PAS studies were used for the identification and characterization of paramagnetic transition metals associated with titania mineral.

Experimental X-ray diffraction measurements The phase purity and crystal structure of the mineral was analyzed by powder XRD studies. The XRD data was recorded in the two theta range of 10–90° on a PANalytical Powder X-ray diffractometer (X’Pert-Pro) using monochromatized Cu Ka radiation, Nifilter, 45 kV voltage, 40 mA. The quantitative phase analyses and refinement of the structural parameters were carried out by Rietveld refinement method using the Fullprof-2000 program (Rodriguez-Carvajal, J., Fullprof: a program for Rietveld Refinement and Profile matching analysis of complex powder diffraction patterns ILL) [30]. Photoacoustic spectroscopy measurements The PAS experiments were performed using an indigenously designed spectrometer consisting of a 250 W tungsten-halogen lamp used as an excitation source, the radiation of which was modulated by variable speed chopper (33 Hz). A monochromator in combination with the appropriate absorption filters was used for wavelength selection and to eliminate higher order effects. The beam leaving the monochromator was directed into a PA cell. The signal was pre-amplified and fed to a lock-in amplifier connected to a computer. The PA signal was normalized by taking the ratio of signal due to sample to that of carbon black, to eliminate the spectral variation of the illumination source [31]. Infra-red and Raman spectroscopy measurements

Fig. 1. Structures of titania (rutile left side and anatase right side).

The IR absorption spectra was obtained in mid infrared (MIR) (4000–400 cm1) region at 4 cm1 resolution using Bruker Vertex 80V FTIR spectrometer. Globar source, KBr beam splitter and DLaTGS (MIR) detector was used for recording the spectra of mineral samples of 1% in KBr pellets. Raman spectrum was recorded on a homemade Raman spectrometer equipped with peltier cooled Charge Coupled Devices (CCD) and 488 nm argon ion laser, with resolution of 1 cm1.

R.M. Kadam et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 363–370

Electron paramagnetic resonance measurements EPR spectra were recorded on a Bruker EMM 1843 spectrometer operating at X-band frequency (9.45 GHz) having 100 kHz frequency modulation and a phase sensitive detection to obtain a first derivative signal. About 100 mg of polycrystalline sample was used to record EPR spectra. Diphenyl picryl hydrazyl (DPPH) was used for calibration of g-values of paramagnetic species. The temperature was varied in the range 100–300 K using a variable temperature accessory, viz., Eurotherm B-VT 2000. The EPR parameters for different samples have been precisely determined from the calculated spectra, obtained from Bruker SIMFONIA program based on perturbation theory (Weber, R.T., WIN-EPR SIMFONIA manual, 1995). In polycrystalline sample, EPR signals has been simulated by generating 9000 random orientations of the magnetic field and by summing the corresponding 9000 absorption signals. The final signal was obtained by performing a convolution (Gaussian or Lorentzian line shape) of each transition line, adding all contributions and calculating the first derivative signal, the line width of each component has been optimized in order to obtain the best accordance with the observed experimental values [32]. Results and discussion X-ray diffraction studies It can be seen from Fig. 2 that the XRD pattern of the mineral consisted of an intense reflection from rutile phase whereas weaker reflections are from anatase phase. It is revealed that nine relatively low intensity peaks at 2h values corresponding to 25.31, 37.08, 37.94, 38.62, 48.02, 53.99, 55.20, 62.8, and 67 have reflections (1 0 1), (1 0 3), (0 0 4), (1 1 2), (2 0 0), (1 0 5), (2 1 1), (1 1 8) and (1 1 6) respectively. These reflections were attributed to the presence of small amounts of anatase phase in the mineral (JCPDS 21-1272/84-1286). Whereas an intense reflections at 2h values corresponding to 27.35, 36.06, 39.31, 41.36, 44.08, 54.33, 56.72, 62.70, 64.24, 69.17 and 69.70 having respective reflections at (1 1 0), (1 0 1), (2 0 0), (1 1 1), (2 1 0), (2 1 1), (2 2 0), (0 2 2), (3 1 0), (3 0 1) and (1 1 2) were attributed to the presence of major amounts of rutile phase (JCPDS 88-1175). Further analyses of the structural parameters of both phases and estimation of their relative fraction were carried out Rietveld refinement of the powder XRD data. Reported structural details of the pure phases of anatase and rutile were used as starting models for the refinement. The background

Intenisty (a. u.)

RMK-RUTILE-r

10

20

30

40

50

60

70

80

90

Two theta (°) Fig. 2. XRD pattern of polycrystalline sample of titania mineral.

365

and peak profile of the powder XRD data were modeled by considering a 5th order polynomial function and a pesudo-voigt profile function. The specimen sample of titania contains a number of transition metal ions in the structure, we attempted to vary the composition from the refinement of occupation of Ti4+ and O2 sites. However, no appreciable deviation from their stoichiometry could be obtained. This can be attributed to their trace levels, for which the XRD remains unaffected. Thus the powder XRD data were refined by considering stoichiometric rutile and anatase type TiO2. The symmetry of unit cell of both rutile and anatase is found to be tetragonal (rutile: space group P42/mnm, a = 4.5946(1) Å, c = 2.9597(1) Å, V = 62.48(1) (Å)3, Z = 2; anatase: space group I41/amd, 3.7848(2) Å, 9.5098(1) Å, V = 136.22(2) (Å)3, Z = 4). The weight fractions of rutile and anatase phases in the mineral were found to be 91% and 9%, respectively. The details of the refined structural parameters are given in Table 1. The final Rietveld refinement plot for the XRD data of the mineral sample is shown in Fig. 1. The structural parameters of TiO2 as obtained by XRD are given in Table 1. The oxygen octahedral units of both rutile and anatase structures are distorted. The rutile form of TiO2 is a tetragonal crystal belonging to the class D4h [33]. In the rutile structure, the octahedral unit has tetragonal distortion, with two long Ti4+–O2 bonds of 1.984 Å parallel to the h0 0 1i crystal axis and four shorter bonds of 1.946 Å are perpendicular to the h0 0 1i direction. The unit cell of rutile has two symmetrically equivalent Ti4+ ions at 2a sites of space group P42/mnm. However, these Ti4+ ions is surrounded by a slightly deformed oxygen octahedron so that the local symmetry of a Ti4+ site is orthorhombic (mmm), nevertheless, the overall symmetry of the structure is tetragonal because the surroundings of the two equivalent Ti4+ ions differ from one another only by rotation of 90° around the c-axis of crystal. The symmetry of the Ti4+ and coordinated oxygen is D2h. Whereas the structure of anatase consists of the Ti4+ ions situated at site of S4 symmetry. The six nearest oxygen lie at the corner of a distorted octahedral in which two Ti–O distances are 1.964 Å and four are 1.937 Å. The two long bonds form the S4 axis. The remaining four oxygen lie on the corners of a puckered square in which the oxygen are alternately 0.413 Å above and below the plane which contains the Ti4+ ion and is normal to the S4 axis. Infra-red and Raman spectroscopy studies Infrared absorption (FTIR) spectrum of TiO2 sample is given in Supplementary information (S1). The two absorption bands at 1635 cm1 and 3400 cm1 indicates the presence of water of hydration in the mineral. The strong absorption in the frequency range 500–800 cm1 corresponds to Ti–O–Ti bonding. Raman spectroscopy is widely used in the investigation of minerals. TiO2 exists as three polymorphs viz; anatase, rutile, and brookite, belonging to different space groups and exhibit their characteristic Raman bands. The vibrational structure of these polymorphs has been discussed [34]. The irreducible representation of the optical modes for anatase, brookite and rutile have 6 (3Eg + 2B1g + A1g), 36 (9A1g + 9B1g + 9B2g + 9B3g) and 4 (A1g + B1g + B2g + Eg) Raman active modes respectively. In the present studies, anatase phase exhibited characteristic Raman peaks at 143.2 (Eg), 193.6 (Eg), 394.5 (B1g), 513.0 (A1g, B1g doublet) and 635.0 (Eg) cm1. Three weak bands at 700, 796 and 320 cm1 are also observed. These bands are either due to disordered-induced or second order scattering or overtone bands [35]. The rutile phase gives typical scattering at 143.2 (B1g), 439.6 (Eg), 612 (A1g), and 834 (B2g) cm1. The spectrum also exhibit weak bands at 232 and 320 cm1 due to disorderedinduced or second order scattering bands [36,37]. Fig. 3 shows the Raman spectrum of the polycrystalline mineral sample. For both phases, the positions and the relative intensities of the

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Table 1 Structural details of titania matrix by XRD analysis. Rutile

Anatase

SG

P42/mnm

I41/amd

Unit cell parameters a (Å) c (Å) V (Å)3 Z Wt fraction (in%)

4.5946(1) 2.9597(1) 62.48(1) 2 91.05(94)

3.7848(2) 9.5098(1) 136.22(2) 4 8.95(26)

Position coordinates and thermal parameters Ti 2a: (0,0,0) B (Å)2 O 4b: (x,x,0) B (Å)2 RB Rp, Rwp, v2 9.14, 12.6, 2.06

200

400

600

796 834

700

513.0

439.6

320

193.6 232

394.5

612.0

Intensity (a. u.)

635.0

143.2

0.0000, 0.0000, 0.0000 1.09(5) 0.6984(4), 0.6984(4), 0.0000 1.58(8) 4.72

800

1000

-1

Raman shift (cm ) Fig. 3. Raman spectrum of titania mineral 100–1100 cm1.

1.3

0.9

785 nm

690nm

1.0

605 nm

1.1

555 nm

370 nm

PA signal Intensity (a.u.)

1.2

415 nm

485 nm

4b: (0,1/4,3/8) 8e: (0,1/4,z)

(0,1/4,3/8) 1.25(22) 0.0000, 0.2500, 0.5839(15) 2.54(41) 6.24

d–d transition. This band is deconvoluted using Gaussian fits. The deconvoluted PAS spectrum showed eight peaks around 370, 415, 485, 555, 605, 659, 690 and 785 nm. The spectral bands associated with electronic transitions at ca 370, 415, 485, 555 and 690 nm were attributed to the electronic transitions of Fe3+ from the 6A1 (6S) ground state to the higher excited states 4Eg (4D); 4T2 (4D), 4Eg, 4A1g (4G), 4T2 (4G) and 4T1 (4G) respectively. According to ligand field theory, these bands correspond to the distorted octahedral symmetry of Fe3+ in the mineral. The transition in case of Cr3+ in MgO is reported at 625 nm. In the present case, the transition at 659 nm is associated with Cr3+ ion (4A2g (F) ? 4T2g (F)). Since Mn4+ is isoelectronic with Cr3+ (S = 3/2), it is plausible to suggest that the two ions substituted for the cationic sites in the same matrix show similar optical behavior. Since the Mn4+ ion has an effective positive charge higher than Cr3+, the absorption band can occur at slightly higher energy that is 600 nm [38–40]. This transition is ascribed to the 4A2g (F) ? 4T1g (F) transition. Whereas V4+ ion (S = 1/2) in descloizite (zinc–copper–lead hydrous vanadite mineral) is reported to exhibit a broad band at 505 nm and series of absorption bands at lower energies that is 685, 700, 740 and 760 nm [17]. In the present studies, apart from the absorption bands discussed above (Table 2), two more bands at ca 780 nm and 685 nm were also observed. The absorption band at 685 nm due to V4+ is partly obscured by the observed transition 6 A1 (6S) ? 4T1 (4G) of Fe3+. These bands were assigned to presence of V4+ in octahedral distorted sites [41,42]. These results are in agreement with theoretical predictions. The band positions, their assignments and the calculated band energies are listed in Table 2.

EPR spectroscopy studies

0.8

659 nm

0.7 0.6 0.5 300

400

500

600

700

800

900

Wavelength (nm) Fig. 4. PAS spectrum of titania mineral in the range 400–800 nm.

observed Raman bands are in good agreement with those reported in literature. PAS studies The PAS of titania recorded at room temperature is shown in Fig. 4. The PAS spectrum of polycrystalline sample of rutile exhibited broad absorption bands in the range 300–900 nm associated with

Figs. 5(F) and 6(A) show the EPR spectra of natural microcrystalline mineral. The spectrum at g ca 2.00 showed a superposition of a weak eight line structure (which are more prominent at 100 K in finely ground sample) and an intense six line hyperfine splitting (HFS) signal that can be attributed to the presence of V(IV) and Mn(IV) respectively. The six lines correspond to the central transition between the electronic states Ms = ±1/2, which is split due to the hyperfine interaction with the nuclear spin (I = 5/2) of 55Mn (100%) natural abundance. The Mn4+ signal was characterized by g = 2.002 ± 0.001 and A = 0.0085 cm1 [40,43,44]. However, when this mineral was ground thoroughly and its EPR spectrum recorded at room temperature, the sextet hyperfine structure due to Mn4+ completely disappeared and the spectrum consisting of a series of signals corresponding to a range of g values ca 8–1.0 became more prominent (Fig. 6B). These resonances are listed in Table 3. Upon lowering the temperature, the signal consisting of eight line hyperfine structure became more prominent in addition to the already existing signals.

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R.M. Kadam et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 363–370 Table 2 Analysis of PAS spectra for titania sample. Transitions For A1 A1 6 A1 6 A1 6 A1 6 6

Fe (S = 5/2) (6S) ? 4E (4D) (6S) ? 4T2 (4D) (6S) ? 4Eg, 4A1g (4G) (6S) ? 4T2 (4G) (6S) ? 4T1 (4G)

For Cr3+, Mn4+ (S = 3/2) A2g (F) ? 4T2g (F) 4 A2g (F) ? 4T1g (F) 4

a b

Band position wavelength (nm)

Energy (observed) (cm1)

370 415 485 555 690

27,025 24,155 20,620 18,015 14,515

659a 605b

15,175 16,665

3+

PAS transition due to Cr3+ ion. PAS transition due to Mn4+ ion.

55

5A(Mn )

(A)

(B) (A)

1

2

9

7

3+

Cr at Ti

4+

site

X

4

3

(E)

Y

-3/2 -1/2

Z

Y

X

+3/2 +1/2

(D)

+1/2 +3/2

-3/2 -1/2

+1/2 +3/2

(C)

8

5 6

10

(B) 13

11

51

A V

12 5+

Cr surface

(F) 0

1000

2000

3000

4000

5000

6000

H (Gauss) Fig. 6. EPR spectra of titania mineral at room temperature (A) as it is (containing microcrystals) and (B) recorded after grinding it for 30 min (polycrystalline samples).

51

7A V

2500

3000

3500

4000

4500

H (GAUSS) Fig. 5. Temperature variation of the EPR spectra of titania mineral in the range 100– 300 K.

Table 3 Analysis of EPR spectra for rutile sample.

4+

EPR spectroscopy of V

The paramagnetic V4+ ion has 3d1 electronic configuration and an electronic spin S = 1/2. The nuclear spin for the 51V isotope (natural abundance 99.5%) is I = 7/2. Therefore, an eight component hyperfine structure is expected from the dipole–dipole interaction between the magnetic moment of the 51V nucleus and the electronic moment of the paramagnetic V4+ ions. An axial spin Hamiltonian, which includes the hyperfine interaction has been used to describe the EPR spectra of V4+.

H ¼ g k bHz Sz þ g ? bðHx Sx þ Hy Sy Þ þ Ak Iz Sz þ A? ðIx Sx þ Iy Sy Þ

ð1Þ

a

Resonance

g

1 2 3 4 5 6 7 8 9 10 11 12 13

8.1820 5.7559 5.6418 4.9689 4.2622 3.3790 2.6513 2.2262 1.9879 1.9328 1.7682 1.6644 1.3722

Unidentified signals.

Species (835 G) (1187 G) (1211 G) (1375 G) (1603 G) (2022 G) (2577 G) (3069 G) (3437 G) (3525 G) (3864 G) (4105 G) (4979 G)

Fe3+ Cr3+ Fe3+ Cr3+ Fe3+ Fe3+ Fe3+ –a V4+ Cr5+ –a Cr3+ Cr3+

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R.M. Kadam et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 363–370

Table 4 The Spin Hamiltonian parameters (EPR) for transition metal ions (d1, V4+) in rutile and anatase matrix, hyperfine coupling constants (A) values are given in gauss. T.M. ion 4+

V V4+ V4+ V4+

gz

gx

gy

giso

Az

Ay

Ax

Aiso

Ref.

1.956 1.956 1.956 1.956

1.912 1.913 1.912 1.9146

1.914 1.913 1.914 1.9146

1.9273 1.9270 1.9273 1.9284

156 156 142 155

49 45 44 44

35 31 31 44

80 77 72.3 81

[45] [46] [47] Present study

where b is the Bohr magneton; Hx, Hy, Hz are the static magnetic fields; Sx, Sy, Sz are the spin operators of the electron, Ix, Iy and Iz are the spin operators of the nucleus; g|| and g\ are the parallel and perpendicular values of the anisotropic g tensor; A|| and A\ are the parallel and the perpendicular hyperfine values of the hyperfine A tensor. The temperature dependence of the EPR spectra of V4+ is shown in Fig. 5. The spin Hamiltonian parameters obtained from the computer simulation of EPR spectra recorded at 100 K; g|| = 1.9560, A|| = 155 G, g\ = 1.9146, A\ = 44 G (giso = 1.9284; Aiso = 81 G). The observed EPR parameters are close to those reported for V(IV) at substitutional sites in TiO2 [45–47]. The Spin Hamiltonian parameters (EPR) for transition metal ions (d1, V4+) in rutile and anatase are listed in Table 4. The fact that g|| > g\ and A|| > A\ suggested that the V4+ ions are associated with an orthorhombically distorted octahedral symmetry where a V4+ ion is located at the center of the rutile unit cell which has, a slightly distorted oxygen octahedral (D2h). Useful information about the environment of the vanadium ion can be obtained by the analysis of the spin Hamiltonian parameters. The empirical approach of Devidson and Che [46] used a plot of giso = [(g|| + 2g\)/3] and Aiso = [(A|| + 2A\)/ 3] to describe the ligand field geometries (i) a, for vanadium ions in a square-pyramidal or axially distorted octahedral symmetry; EPR spectra of both these geometries are characterized by g\ > g|| and A|| > A\ (giso = 1.955–1.980, Aiso = 80–120 G) (ii) b, for V4+ ions in a tetrahedral symmetry, the axial EPR spectra in tetrahedral geometry are characterized by g\ > g|| and A|| > A\ which is smaller than in the previous case and (iii) c, for V4+ ions in a distorted octahedral symmetry. The EPR spectra observed for this kind of environment are characterized by gz > gx > gy and Az > Ax > Ay, (giso = 1.920– 1.950, Aiso = 60–90 G). On the basis of the relatively low values observed for giso and Aiso (giso = 1.9284; Aiso = 81 G) in rutile mineral, we concluded that the magnetically isolated V4+ ions are subjected to a ligand field described by the geometry wherein the vanadium ions are located at the center of the rutile unit cell surrounded by a slightly distorted oxygen octahedron. Such a substitution is also favoured as the ionic radius of Ti4+ and V4+ in a hexa-coordinated octahedral crystal field are 0.74 Å and 0.72 Å respectively. EPR spectroscopy of Cr3+, Fe3+ and Mn2+ It is well known that chromium can exist in two different spin states/oxidation states, that is S = 3/2 (3+) high spin and S = 1/2 (5+) low spin whereas iron and manganese can exist as S = 5/2 high spin, S = 3/2 mid spin and S = 1/2 low spin. All these states correspond to an odd electron system (Kramer ions S = 1/2, 3/2 and 5/2) and hence they are EPR active. In the present case, the transition metal ions V4+, Cr3+, Fe3+ and Mn2+ substitutes Ti4+ site in the lattice as the ionic radii of all these transition metal ions are comparable to that of Ti4+. While considerable amount of work has been reported on the EPR of transition metal ion doped rutile single crystal, very few reports exist which describes the EPR of polycrystalline samples. For example EPR spectra of Mn2+ [43], Cr3+ [48,49] and Fe3+ [50,51] in single crystal rutile are well known but very few reports have the observation of Fe3+, Mn2+, Cr3+ in polycrystalline rutile because of the complex nature of the EPR spectra obtained at the X-band frequency for many electron systems (like S = 5/2 and 3/2) due to the effect of very large zero field splitting (D term) and

orthorhombic distortion term (E term) [52–54]. The reported EPR parameters for these ions are listed in Table 5. Also it may be noted that the diffusion of transition metal ions in different phases of titania namely rutile, anatase [55] and brookite has been extensively studied by EPR spectroscopy in their polycrystalline form [3,56–60]. The EPR spectra of polycrystalline samples of Fe3+ and Cr3+ have been observed and they were identified using the published single crystal data. The analysis of the EPR spectrum in a single crystal of Fe3+/Cr3+ doped rutile showed that the trivalent impurity ions substitutionally replaces Ti4+ ions in the lattice. The theoretical EPR signals were calculated using the spin Hamiltonian

H ¼ bgHS þ D ½S2z  1=3ðSðS þ 1Þ þ E ðS2x  S2y Þ þ ASI

ð2Þ

where H is the applied field, b is the Bohr magneton, Sx, Sy, Sz are the components of spin along three mutually perpendicular crystalline axes x, y and z, D and E are second order crystal field terms with axial and rhombic-structure parameters, S is the total spin of the electron, g is the spectroscopic factor. Based on the crystal structure, the EPR spectra of Cr3+ in anatase is expected to have an axial symmetry (D – 0 and E = 0), whereas in rutile, the cation sites have D2h symmetry which introduces orthorhombic character (D – 0 and E – 0). The reported EPR parameters of Cr3+ in single crystals of TiO2 (rutile) are g = 1.97, D = 5500 G and E = 2700 G [48]. The large value of E/D indicates that Cr3+ is situated in lower symmetry, which probably arises due to substitution of Cr3+ at Ti4+ site thereby invoking charge compensation due to the presence of oxygen vacancies in the vicinity of Ti4+ ion. In the present study, the EPR of Cr3+ in polycrystalline rutile showed five resonances ca 1187 G, 1375 G, 2577 G, 4105 G and 4979 G. These signals are abbreviated as 2, 3, 7, 12 and 13 respectively (Fig. 7B). The resonance values obtained are in close agreement to those reported for Cr3+ doped single crystal rutile TiO2 by Gerritsen et al. [48]. The five observed resonances are also similar to the spectrum reported for Cr3+ doped polycrystalline rutile by Cordischi et al. [40] and by Evans et al. [53]. Therefore, it can be concluded that these five lines are associated with Cr3+ which is substitutionally incorporated into the rutile matrix. The field positions, g values and their assignments are listed in Table 6. An intense peak at g ca 1.9879 was also observed which is attributed to a surface bound Cr5+ species. EPR spectroscopy of Fe3+ The electronic configuration of the free Fe3+ ion is 3d5, 6S5/2 and therefore the ground state is a non-degenerate orbital singlet and for such species, EPR is observed at room temperature. The EPR of Fe3+ in polycrystalline rutile gave four resonances at ca 835 G (1/2 ? 1/2 (Y)), 1211 G (3/2 ? 3/2 (Z)), 2022 G (1/2 ? 1/2 (X)) and 2597 G (3/2 ? 3/2 (X)). These resonances are designated as 1, 4, 6 and 7 respectively. These resonances with their assignments are shown in Fig. 7(A). These resonances are in close agreement to those reported for Fe3+ doped single crystal rutile TiO2 by Lichtenberger and Addison occupying a substitutional site in rutile lattice [51]. The reported EPR parameters of Fe3+ in single crystals of TiO2 (rutile) are g = 2.00, D = 7279 G and 736 G [51]. The spectrum of Fe doped rutile powder has been investigated and the peaks in the powder spectrum corresponding to the Fe3+ ions can

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R.M. Kadam et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 363–370 Table 5 The reported Spin Hamiltonian parameters (EPR) for transition metal ions (d3 and d5) in rutile and anatase matrix. Zero field splitting (D), Orthorhombic term (E) and hyperfine coupling constants (A) values are given in gauss. g value

D

E

A

Ref.

Mn4+ Mn4+ Cr3+ Cr3+a Cr3+a Fe3+ Fe3+ Fe3+ Fe3+a Fe3+a

1.990 1.9919 1.970 1.973 1.970 2.000 1.950 1.95 2.002 2.005

4321 4368 5500 400 6800 7268 7279 6840 4970 309

120 1400 2700 – 1400 790 736 710 1240 1240

76.8 77.9

[43] [44] [48] [52] [49] [50] [51] [65] [55] [55]

18 15 – – – – –

3/2 -3/2

1/2 -1/2

Anatase phase.

1/2 -1/2 3/2 -3/2

a

T.M. ion

Z34 X12

Y12

X34

4 3

1

7

2

3+

Fe at Ti

4+

site

9

8

5 6

10

(A)

Table 7 Resonances observed in the X band EPR spectrum attributed to Fe3+ in tetragonal TiO2 matrix and the assignments of their transitions. Signal

H0

g

Transition

1 4 5 6 7

835a 1211a 1603d 2022d 2577d

8.1820 5.6418 4.2622 3.3790 2.6513

+1/2 +3/2

1/2 3/2

Y12 Z34

Assignments

+1/2 +3/2

1/2 3/2

X12 X34

Line shape of EPR signal: a – absorption, d – derivative; signal5 (H0 = 1603 G g = 4.2622) is due to Fe3+ at surface in a rhombic environment.

transitions. Due to very large zero field splitting 5/2 ? +5/2 transition was not seen in the single crystal and powder samples. Therefore, it can be concluded that these four features in the EPR signal are associated with Fe3+ which is substitutionally incorporated into the rutile matrix. The field positions, g values and their assignments are listed in Table 7. A weak peak at 1602 G was observed which is attributed to the presence of Fe3+surface bound species. The spectral feature in the EPR of Fe3+ in powder spectra has been treated theoretically by Aasa’s report [61] and also by Dowsing and Gibson [62] earlier. The observed resonance peaks corresponding to the low field side of the Fe3+ in rutile is consistent with Dowsing’s proposal that, for the resonance to be detected below 1000 G, the zero field splitting parameter (D term) greater than 1000 G is predicted with E/D to be approximately 0.33.

3+

Fe surface

13

11

Discussion

12 51

7A (V )

(B)

0

1000

2000

3000

4000

5000

6000

H (Gauss) Fig. 7. EPR spectra of polycrystalline sample of titania mineral (A) recorded at room temperature and (B) recorded at 100 K.

Table 6 Resonances observed in the X band EPR spectrum attributed to Cr3+ in tetragonal TiO2 matrix and the assignments of their transitions. Signal

H0

g

Transition

2 3 7 12 13

1187a 1375a 2577d 4105a 4979d

5.7559 4.9689 2.6513 1.6644 1.3722

3/2 +1/2 +1/2 +3/2 3/2

Axis 1/2 +3/2 +3/2 +1/2 1/2

y z x y x

Line shape of EPR signal: a – absorption, d – derivative; signal10 (H0 = 3525 G g = 1.9328) is due to Cr5+ at surface and signal9 (H0 = 3437 G; g = 1.9879) is due to Ti3+ at surface.

be easily identified from the turning points of the Hamiltonian given in Eq. (2). The features 1, 4, 6 and 7 in the EPR spectra corresponds to the principal values of the 1/2 ? +1/2 and 3/2 ? +3/2

Anatase and Rutile are most commonly occurred polymorphic modifications of TiO2 in nature. Anatase can transform to rutile when heated to 700 °C. Impurity ions in the two crystals can occupy the substitutional and interstitial sites. Both sites have the tetragonal symmetry in anatase and rhombic symmetry in rutile. This explains the importance of the present EPR investigations of the defect model (including the occupied sites and defect structure) of transition metal impurities in these crystals. It should noted here that the present study on EPR mainly emphasizes rutile phase of the titania mineral since the concentration of this phase was found to be around 91% as revealed by XRD studies. Characterization of TiO2 mineral was done using XRD, IR, Raman spectroscopy, PAS and EPR spectroscopy. For identification of major constituents XRD and Raman spectroscopy were used where as EPR technique was used to study the local site symmetry around the transition metal ion and its spin states in the mineral and co-ordination around the metal ion. There was no other detectable impurities present in the mineral. Schosseler and Gehring [19] used a combination of CW EPR and pulsed EPR in along with thermal and chemical methods for identification and characterization of V(IV), Fe(III), Mn(IV) and Cr(III) in a multi mineral system. However, in the present studies, temperature dependence of the EPR spectra was used to isolate the contributions arising from different transition metal ions. The EPR signal for V4+ was obtained below 150 K while signals due to Mn2+, Fe3+ and Cr3+ were observed at room temperature. The most important feature observed in the EPR spectra of Cr3+ and Fe3+ is that, they exhibit a large zero field splitting (D) and orthorhombic distortion (E) terms which is a signature for the presence of axial and rhombic distortions respectively. The presence of large rhombic term (E/D  0.3) for Cr(III) and Fe(III) in rutile suggest that the site symmetry around these transition metal ions is very low i.e. these ions are situated in highly distorted environment. The EPR spectra of Fe3+ and Cr3+ ions have been studied extensively in different polymorphic modifications of TiO2 namely anatase and rutile. The following possibilities can occur (i) a regular substitution of Fe3+ and Cr3+ (trivalent transition metal ions) at

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Ti4+ site in which case the EPR spectrum would show a relatively small zero field splitting (ii) presence of trivalent transition metal ions at interstitial sites; however, this would create a strong local charge disorder and would be possible only if the paramagnetic impurities are too large in size to enter the substitutional sites. The Fe3+ and Cr3+ ions being smaller in size than Ti4+, its occupancy at interstitial site is less probable (iii) the substitution of trivalent transition metal ion for Ti4+ giving rise to an oxygen vacancy at the nearest neighbor required for charge compensation. It is reported that in Fe3+ in tetragonal anatase [55] and SrTiO3 lattices [63,64] showed EPR spectra typical of Fe3+ at the substitutional site; i.e.; Ti4+ site with neighboring oxygen ion vacancy. However, as stated earlier, these spectra are characterized by very large zero field splitting and rhombic distortion term (E term). In case of anatase, for regular substitution, the observed zero field splitting at room temperature is 310 G (no E term) whereas for substitution of Fe3+ for Ti4+ with neighboring oxygen ion vacancy, the reported zero field splitting is 4970 G and orthorhombic distortion term is 1240 G. These two large parameters observed in the Fe3+ and Cr3+ in natural TiO2 suggest that the trivalent transition metal ions are situated either in rhombic crystal field or at Ti4+ sites with neighboring oxygen ion vacancy needed for charge compensation. 4. Conclusions A rutile mineral was investigated by XRD, PAS, Raman and EPR studies. XRD studies indicated the presence of rutile (91%) and anatase (9%) phases in the mineral. These evidences were further supported by investigations using Raman spectroscopy. Both rutile and anatase phases has tetragonal structure (rutile: space group P42/mnm, a = 4.5946(1) Å, c = 2.9597(1) Å, V = 62.48(1) (Å)3, Z = 2; anatase: space group I41/amd, 3.7848(2) Å, 9.5098(11) Å, V = 136.22(2) (Å)3, Z = 4). The deconvoluted PAS spectrum showed prominent peaks around 485, 440, 554, 607 and 690 nm which, according to the ligand field theory, were attributed to V4+, Cr3+, Mn4+ and Fe3+ species. EPR studies revealed the presence of transition metal ions Cr3+, Mn4+(d3) and Fe3+(d5) at Ti4+ sites. The occurrence of a large crystal field splitting (D term) and orthorhombic distortion term (E term) suggested that the transition metal ions substitute the Ti4+ in the lattice which is situated in distorted octahedral coordination of oxygens. Acknowledgements Authors wish to thank Dr. A. Goswami, Head, Radiochemistry Division, BARC and Dr. V.K. Jain, Head, Chemistry Division for their keen interest and encouragement to peruse the work. Authors also wish to acknowledge Dr. A.K. Tyagi for useful discussion. Author wish to thank Dr. A.R. Dhobale for recording the photoacoustic spectrum. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2014.08.082. References [1] A.F. Wells, Structural Inorganic Chemistry, fifth ed., Claredon Press, Oxford, 1984. [2] G.A. Tompsett, G.A. Bowmaker, R.P. Cooney, J.B. Metson, K.A. Rogers, J.M. Seakins, J. Raman Spectrosc. 26 (1995) 57–62. [3] A. Amorelli, J.C. Evans, C.C. Rowlands, T.A. Egerton, J. Chem. Soc., Faraday Trans. 1 (83) (1987) 3541–3548. [4] R.J. Gonzalez, R. Zallen, H. Berger, Phys. Rev. B 55 (1997) 7014–7017. [5] A. Kay, M. Gratzel, Sol. Energy Mater. Sol. Cells 44 (1996) 99–117. [6] B. Oregen, M. Gratzel, Nature 353 (1991) 737–740. [7] M.A. Fox, M.T. Dulay, Chem. Rev. 93 (1993) 341–357.

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