Transcript Slide 1

Enhanced Photo-efficiency of Immobilized TiO2 Catalyst
N. Baram1*, D. Starosvetsky1, J. Starosvetsky2, M. Epshtein2, R. Armon2, Y. Ein-Eli1
Department of Materials Engineering1, Environmental and Civil Engineering2,
Technion-Israel Institute of Technology, Haifa 32000, Israel
Introduction
The Principle of Photocatalysis
Different aspects of water treatment are considered the most urgent
topics at the present and will influence our future life and
Photocatalytic oxidation of organic compounds is an advanced method
for removal of impurities from water. Titanium dioxide is close to being
the ideal photocatalyst in several ways: relatively inexpensive,
chemically stable, the light required to activate the catalyst may be
long-wavelength UV such as the natural UV component of the sunlight
and the produced oxidant is powerful with elimination potential of most
types of microorganisms1. The main problem of this process is the low
efficiency due to high electron/hole recombination rate2.
The efficiency of the photocatalysis process depends on the amount of
generated holes, which is typically low, due to the high electron-hole
recombination rate.
The holes concentration may be enhanced by:
1. Increasing the effective surface area of the photocatalyst,
2. Retarding the electron-hole recombination with the use of anodic
bias.
In this work, immobilized nanotubular TiO2 with high surface area was
grown by anodization of Ti in aqueous solution containing fluoride ions
and compared to mesoporous oxide layers. The efficiency and kinetics
of the photoelectrocatalytic devices were studied and compared to
Degussa P-25 powder TiO2 for E.coli bacteria inactivation.
Experimental5
Anodization in aqueous solutions
Under UV illumination electrons and holes are
produced3,4:
Nanotubular TiO2
TiO2  h  e  h


•Electrolyte – 1M Na2SO4 + 0.5%wt NaF
•2hr, constant potential of 20V.
The following reactions occur:


H2O  h  H OH


2
Eg=3.1 eV
E  0.28VSHE
O2  e  O

2
E  2.74VSHE
0
0

O  H  HO2


HO2  e  H  H2O2
schematic diagram
showing the potentials
for various
RedOx processes
occurring on the TiO2
surface at pH 7
Mesoporous TiO2
•Electrolyte – 0.5M H2SO4
•Constant current Density 100 mA/cm2.
•Final potential:
- 110V (HS110V)
- 150V (HS150V)
Microbiology experiments
•2 Petri dishes + control.
•Bacteria – 106 CFU/ml E.Coli in 0.01%
saline without nutrient broth.
Hydroxyl radicals have high oxidation potential:


OH  H  e  H2O
E  2.74VSHE

E  1.78VSHE
•Anodic bias – 0-5V
0

H2O2  2H  2e  4H2O
UV
nm
0
control
Pt
Microbiology Studies
7
Nanotubular TiO2
HS150V TiO2
6
HS110V TiO2
Log [CFU/ml]
5
P25 Powder TiO2
4
3
2
Electrochemical
Characterization
Linear sweep
voltammetry
curves under UV
illumination and
in the dark
1
Top and cross section HRSEM micrographs of TiO2 growth via
anodization in 1M Na2SO4 + 0.5%wt NaF solution
350
HS110V in the dark
HS110V under illumination
HS150V in the dark
HS150V under illumination
Nanotubular TiO2 in the dark
300
250
2
Effect of Photocatalyst
I [A/cm ]
Characterization
TiO2
Nanotubular TiO2 under illumination
200
150
100
50
0
0
20
40
0
60
-50
Time [min]
Kmax
log (nres)
4.99 ± 0.90
4.78 ± 2.51
4.94 ± 1.20
2.75 ± 1.07
1.39 ± 0.12
0.81 ± 0.20
0.37 ± 0.02
0.37 ± 0.02
--0.42 ± 0.3
0.12 ± 0.10
0.1 ± 1.02
Faster elimination rate without deceleration period for the
nanotubular TiO2 – faster than Degussa P-25 powder TiO2
Effect of Anodic Bias
Only Ti!
7
The oxide is Amorphous
5V
4V
3V
2V
1V
0.2V
6
The oxide is crystalline: Anatase
Log [CFU/ml]
5
HS150V
160
Potential [V]
140
Ti
Anatase
Rutile
HS110V
120
4
2
3
4
HS110V TiO2
200
HS150V TiO2
Nanotubular TiO2
Photocurrent:
I Ph  Itot  I dark
150
100
50
0
0
1
2
3
4
Potential [VSCE]
Nanotubular TiO2 possesses the highest photocurrent
3
Summary
2
1
100
0
80
0
2
4
6
8
10 12 14 16 18 20 22 24 26 28 30 32
Time [min]
60
20
20
30
40
50
60
70
80
2
0
20
40
60
80
Time [sec]
Anodization curve of Ti in 0.5M H2SO4 solution. The final
potentials of 110V and 150V for the HS110V and HS150V TiO2,
respectively, are marked on the curve, along with high resolution
SEM micrographs and XRD patterns.
SL [min]
3.82
4.37
4.98
4.14 ± 2.79
6.49 ± 2.19
6.00 ± 2.59
Anodic Bias [V]
5
4
3
2
1
0.2
40
0
1
Potential [VSCE]
2
SL [min]
Photocurrent [A/cm ]
photocatalyst type
nanotubular TiO2
HS150V
HS110V
P-25 Powder TiO2
0
Kmax
2.35
2.42
2.81
1.42 ± 0.37
1.10 ± 0.21
0.62 ± 0.07
• Anodic bias is also capable of reducing
electron/hole pair recombination process i.e.
increasing the efficiency
Faster elimination rate and shorter incubation period
when the applied anodic bias is increased
Disinfection Under Sun Light Irradiation
7
1. Serpone, N., Pelizzetti, E., Photocatalysis Fundamentals and Applications, A. Wiley, USA p.
126-157, 1989.
2. Hoffmann, M.R., Scot, T.M., Wonyong, C.H., Bahnemann, D.W., Chem. Rev., 95, 69-96 (1995).
3. Fujishima. A., Rao, T.N., Tryk, D.A., J. Photochem. & Photobio. C, 1, 1-21, 2000.
4. Sunada, K., Kikuchi, Y., Hashimoto, K., Fujishima, A., Enviro. Sci. &Tech., 32, 5 (1998).
5. Baram, N., Starosvetsky, D., Starosvetsky, J., Epshtein, M., Armon, R., Ein-Eli, Y., Electrochem.
Comm., 9, 1684-1688 (2007).
Log [CFU/ml]
6
References
•Anodic polarization is capable of growing
thick,
crystalline,
nanoporous
and
nanotubular oxide layer with high surface area
•The
combination
of
immobilized,
electrochemically grown titania with an
application of extremely high anodic bias and
UV
illumination,
led
to
a
dramatic
improvement in measured photocurrent and
E. coli elimination
•100% elimination was also achieved under
sun illumination after 15 minutes
5
4
3
2
0
Acknowledgements
Control
UV
UV+TiO2
1
0
2
4
6
8
10
12
14
16
Time [min]
Complete elimination was achieved after 15 min.
This work was supported by “NATAF"
program at the Israeli Ministry of Industry
and Trade, Chief Scientist Office & by
Russell Berrie Nanotechnology Institute.