Page 1 of 22
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Nitrogen Location and Ti-O Bond Distances in Pristine and N-doped TiO2 Anatase Thin Films by X-ray Absorption Studies Madhusmita Sahoo,∗,† A. K. Yadav,‡ S.N Jha,‡ Dibyendu Bhattacharyya,‡ Tom 1
∗,†
Mathews,
‡
†
†
N. K Sahoo, S. Dash., and A.K.Tyagi
Thin Film and Coatings Section, Surface and Nanoscience Division, Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India., and Atomic and Molecular Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai,400085. E-mail:
[email protected];
[email protected] Abstract
2
This paper reports detailed EXAFS analysis of spray pyrolysis synthesized TiO2 thin films to,
3
locate position of nitrogen in N-doped phase pure(anatase)TiO2 , evaluate Ti-O bond distances
4
and the co-ordination number of Ti. The thin films were synthesized at three different substrate
5
temperatures viz. 400, 450 and 500 o C leading to concomitant occurrence of varying nitrogen
6
concentration (N/Ti) of 0.282, 0.237, 0.24 respectively. The co-ordination number extracted
7
from the EXAFS fitting revealed that films synthesized at 450 o C are stoichiometric, whereas
8
those synthesized at 400 and 500 o C are sub-stoichiometric. The intensity of near edge peaks
9
in XAS was employed to delineate the contribution from both symmetry of TiO6 polyhedra
10
and particle size. The axial and equatorial Ti-O bond distances, corresponding to the different
11
∗ To
whom correspondence should be addressed Film and Coatings Section, Surface and Nanoscience Division, Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India. ‡ Atomic and Molecular Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai,400085. † Thin
1 ACS Paragon Plus Environment
The Journal of Physical Chemistry
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
12
nitrogen concentrations, extracted from the EXAFS fitting, showed good agreement with those
13
calculated from DFT calculations quoted in literature. An increase in the axial and equatorial
14
bond distances with respect to pristine TiO2 is observed and is attributed to the dominant effect
15
of nitrogen occupancy in the interstitial position.
16
Keywords :EXAFS, Co-ordination number, Bond length, Interstitial doping, Spray pyrolysis.
17
Introduction
18
Titanium dioxide(TiO2 ) is a wide band gap semiconducting oxide which finds application in the field
19
of photo-catalysis, 1 solar cells, 2 gas sensors, 3 antimicrobial coatings etc. 4 These properties arise
20
mainly from its tunable electronic energy band structure, where the photo-generated conduction band
21
electrons and the valence band holes induce relevant reduction and oxidation reactions respectively.
22
One of the main issues with TiO2 with respect to its photo-activity is its wide band gap which
23
requires UV radiation for generating the conduction band electrons and valence band holes. The
24
most abundant source of photo-energy is solar radiation and UV constitutes only ∼ 5% of it. Hence,
25
to extend the photo-activity of TiO2 into the visible range a lot of effort has been devoted. Among
26
the various methods adopted for the same, N-doping has found to be the most effective, taking
27
into consideration the detrimental effects such as photo-corrosion, generation of electron-hole
28
recombination sites etc. of other methods. 5
29
Nitrogen doping has been proved to enhance the visible light absorption of TiO2 . 6–10 A large
30
number of research articles have been devoted to elucidate the mechanism behind the visible light
31
absorption in N-doped TiO2 . 8 The visible light activity is attributed to either reduction of band gap 11
32
or presence of isolated interband states in the lattice. 12–14 The above mentioned changes in the band
33
structure also depends on the location of nitrogen in the lattice. In case of substitutional nitrogen the
34
band gap is lowered, while interstitial nitrogen introduces interband states. 15 It has been proposed in
35
the literature that, synthesis under oxygen rich and deficient atmospheres may lead to interstitial and
2 ACS Paragon Plus Environment
Page 2 of 22
Page 3 of 22
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
substitutional doping respectively. 15 As, the location of nitrogen is crucial, several analytical tools
36
are employed to exactly pinpoint the location of nitrogen vis-a-vis the TiO2 lattice. The widely used
37
technique is X-Ray photoelectron spectroscopy (XPS). XPS is a surface analytical technique, which
38
reads the information from surface and subsurface level. Hence, to find the nitrogen location in the
39
lattice one needs to do in-situ sputtering followed by XPS analysis, which may lead to sputtering
40
induced artifacts. In addition, the X-ray photo-electron spectrum corresponding to N 1s electron is
41
noisy due to its low nitrogen concentration. Techniques such as electron paramagnetic resonance,
42
which can detect very low concentrations of paramagnetic species, can not discriminate between
43
interstitial and substitutional doping. 15 X-ray Absorption Spectroscopy (XAS) is particularly and
44
routinely employed for the determination of the dopant location in the crystal lattice. Moreover,
45
it also has the advantage of determining the co-ordination sites composed of elements possessing
46
nearly equal X-ray scattering power. Belver et al. and Ceotto et al. have reported XAS studies of
47
both pristine and N-doped TiO2 . 16,17 The XAS investigations by Belver et al. revealed an overall
48
decrease in Ti-O bond length with increase in N-concentration and attributed it to the creation of
49
oxygen vacancies. However, the work does not throw any light on the exact location of nitrogen
50
in the lattice. Ceotto et al have performed extensive Density Functional Theory (DFT) calculation
51
of N-doped TiO2 and evaluated the axial ((Ti-O)ax ) and equatorial ((Ti-O)eq ) bond distances for
52
various substitutional and interstitial doping concentrations. The theoretically calculated values
53
were then compared with the bond lengths obtained from the Ti-K-edge spectra. 17 From the results
54
of their study, they concluded that at low dopant concentrations (0 < nominal molar ratio N/Ti
