Compositional Dependence of the Structure of TiO2:Fe Nanorods

1
Compositional Dependence of the Structure of
TiO2Fe Nanorods
A. Kremenovic, B. Antic, E. S. Bozin, J. Blanusa,
M. Comor, Ph. Colomban, L. Mazzerolles
2
I N T R O D U C T I O N

• TiO2 is a very promising photocatalyst
• exhibits higher activity compared to that of
  other semiconductors
• shows excellent chemical stability
• stability in nano higher for anatase than rutile
• nontoxic
• environmentally friendly ? photocatalytic
  activity against organic waste e.g. herbicides

3

• pure TiO2 vs. TiO2Fe ? absorption significantly
  shifts from UV towards VIS
• Fe3 in TiO2 can reduce the e-h recombination
  rate
• in nanorods charge carriers are free to move
  throughout the length of the crystal ? lower
  probability of e-h recombination

4
C H A R A C T E R I Z A T I O N

• TEM/HRTEM
• morphology
• crystallographic orientation
• XRPD
• Rietveld line broadening analysis
• PDF
• Magnetic measurements SQUID
• Raman spectroscopy

5
TEM/HRTEM

• crystal form and morphology ? f(Fe) ? flower
• dislocations and stacking faults unobserved

6
TEM/HRTEM - morphology

• rutile nanorods grown from a central nucleus

7
TEM/HRTEM - morphology

• nanorods 30-100 nm in length and 4-5 nm width

8
TEM/HRTEM - crystallographic orientation

• nanorods are grown parallel to the c-axis of the
  rutile structure

9
TEM/HRTEM - crystallographic orientation

• longitudinal section

Fourier Transform (similar to a local
microdiffraction)
10

• transverse section - facets corresponding to
  (110) planes ? 110 plane most dense/stable

Fourier Transform (similar to a local
microdiffraction)
11
XRPD

• data collected at 6-ID-D beam-line at Argonne
  National Laboratory
• ? 0.125677 Å.
• Rietveld line broadening analysis PDF

12
(No Transcript)
13
Rietveld refinement rutile anatase Fe -
1.05
Rwp 3
A
14
Rietveld refinement rutile Fe - 0.22
15
WPPF - rutile Fe - 0.22
16

• Difference between Rietveld and WPPF
• preferential orientation of crystallites no ?
  Ritveld check done
• low crystallite statistics no ? nano specimen
• inadequate line broadening model WPPM in plan
• background problem i.e. amorphous like phase PDF
  and Raman

17

• Structure, unit cell and size strain analysis
• needle like size line broadening model
• isotropic strain broadening model

degree of anisotropy
18

• rutile crystal structure in accord with
  literature
• Fe content and distribution unable to refine
• irregular change of unit cell parameters ? no
  preferential direction for Fe incorporation
• large size broadening anisotropy in accord with
  HRTEM
• small strain anisotropy ? no dislocations and
  stacking faults probable point defects inside

19
PDF spherical particles model
20
HRTEM, Rietveld and PDF anatase
Fe/method HRTEM Rietveld PDF PDF
Fe/method HRTEM Rietveld 1.2 - 4.0 Å 1.2 - 20.0 Å
0 n.o. 4.6(7) (143) (63)
0.22 n.o. n.o. (43) (03)
0.47 n.o. n.o. (33) (03)
1.05 n.o. 6.1(6) (113) (53)
anatase in a flower centre ???
21
SQUID magnetic measurements
22

• the origin of ferromagnetism of TiO2Fe still
  remains a controversial
• here always paramagnetic no matter Fe
• no Fe cluster formations

23
Raman spectroscopy
24
(No Transcript)
25

• rutile and anatase confirmed
• no brookite
• more Fe more defects boson peak
• origin of boson peak - Fe (and vacancy)
  distribution break the vibration
• most of defects are point defects

26
Where is anatase?

• HRTEM anatase unobserved
• XRPD anatase observed in Rietveld but better in
  PDF ? low crystallinity
• Raman best observation of anatase
• Low quantity of anatase, c.c. 5
• Anatase in a flower centre ???
• Anatase first to crystallize then rutile ?

27

• Acknowledgements
• U.S. Dep. of Energy - DE-AC02-98CH10886
• MSTRS
• CNRS
• FP6 INCO-026401 WBC
• FP7 REGPOT3 - 204374 TERCE-NIPMSS

28
Recommendation for further synthesis

• Small amount of Fe stabilize rutile crystal
  structure
• Fe concentration does not influence significantly
  on morphology and size of nanorods
• Fe concentration influence on vacancy
  concentration
• Play with synthesis conditions in order to
  obtain pure rutile with small amount of Fe

29
Supplementary material

• XPS ? only Ti4 but Fe3 (dominantly) and Fe2
• Doping with Fe ions has great influence on
  optical characteristics of the host material ?
  shift of the absorption threshold toward VIS
  spectral region.
• No increase of photocatalytic activity after
  doping.
• The induced photoluminescence as well as the
  decrease of photocatalytic activity is probably
  the consequence of the introduction of oxygen
  vacancies through doping procedure.

30

• For higher dopant concentrations also
  recombination of photogenerated charge carriers
  occurs with higher probability.

31
PDF F(Q)