IntroductionDifferent 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.

Enhanced Photo-efficiency of Immobilized TiO2 CatalystN. 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

Eg=3.1 eV

Summary•Anodic polarization is capable of growing thick, crystalline, nanoporous and nanotubular oxide layer with high surface area

• Anodic bias is also capable of reducing electron/hole pair recombination process i.e. increasing the efficiency

•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

AcknowledgementsThis work was supported by “NATAF" program at the Israeli Ministry of Industry and Trade, Chief Scientist Office & by Russell Berrie Nanotechnology Institute.

References1. 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).

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.

Characterization

Top and cross section HRSEM micrographs of TiO2 growth via anodization in 1M Na2SO4 + 0.5%wt NaF solution

Only Ti!

The oxide is Amorphous The oxide is crystalline: Anatase

Electrochemical Characterization

Photocurrent:

Ph tot darkI I I

Linear sweep voltammetry curves under UV illumination and in the dark

Experimental5

Anodization in aqueous solutions

Nanotubular TiO2

•Electrolyte – 1M Na2SO4 + 0.5%wt NaF

•2hr, constant potential of 20V.

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.

•Anodic bias – 0-5V

Pt

TiO2

UV nm

controlPt

TiO2

UV nm

Pt

TiO2

UV nm

UV nm

controlcontrol

The Principle of Photocatalysis

Under UV illumination electrons and holes are produced3,4:

The following reactions occur:

Hydroxyl radicals have high oxidation potential:

2H O h H OH 0 2.74 SHEE V

2 2O e O 0 0.28 SHEE V

2 2O H HO

2 2 2HO e H H O

2TiO h e h

2OH H e H O 0 2.74 SHEE V

2 2 22 2 4H O H e H O 0 1.78 SHEE V

schematic diagram showing the potentials for various RedOx processes occurring on the TiO2

surface at pH 7

Microbiology Studies

0 2 4 6 8 10 12 14 160

1

2

3

4

5

6

7

Lo

g [

CF

U/m

l]

Time [min]

Control UV UV+TiO

2

Effect of Photocatalyst

Effect of Anodic Bias

Disinfection Under Sun Light Irradiation

Complete elimination was achieved after 15 min.

Faster elimination rate and shorter incubation period when the applied anodic bias is increased

Faster elimination rate without deceleration period for the nanotubular TiO2 – faster than Degussa P-25 powder TiO2

HS150V

HS110V

20 30 40 50 60 70 80

TiAnataseRutile

2

0 20 40 60 800

20

40

60

80

100

120

140

160

Po

ten

tia

l [V

]

Time [sec]

HS150V

HS110V

20 30 40 50 60 70 80

TiAnataseRutile

2

0 20 40 60 800

20

40

60

80

100

120

140

160

Po

ten

tia

l [V

]

Time [sec]

20 30 40 50 60 70 80

TiAnataseRutile

2

0 20 40 60 800

20

40

60

80

100

120

140

160

Po

ten

tia

l [V

]

Time [sec]

0 20 40 600

1

2

3

4

5

6

7

Lo

g [

CF

U/m

l]

Time [min]

Nanotubular TiO2

HS150V TiO2

HS110V TiO2

P25 Powder TiO2

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 320

1

2

3

4

5

6

7

Lo

g [

CF

U/m

l]

Time [min]

5V 4V 3V 2V 1V 0.2V

0 1 2 3 40

50

100

150

200

Ph

oto

curr

ent

[A

/cm

2 ]

Potential [VSCE

]

HS110V TiO2

HS150V TiO2

Nanotubular TiO2

0 1 2 3 4-50

0

50

100

150

200

250

300

350

I [

A/c

m2 ]

Potential [VSCE

]

HS110V in the dark HS110V under illumination HS150V in the dark HS150V under illumination Nanotubular TiO

2 in the dark

Nanotubular TiO2 under illumination

Nanotubular TiO2 possesses the highest photocurrent

KmaxSL [min]Anodic Bias [V]

2.353.8252.424.3742.814.983

1.42 ± 0.374.14 ± 2.7921.10 ± 0.216.49 ± 2.1910.62 ± 0.076.00 ± 2.590.2

log (nres)KmaxSL [min]photocatalyst type

---1.39 ± 0.124.99 ± 0.90nanotubular TiO2

0.42 ± 0.30.81 ± 0.204.78 ± 2.51HS150V0.12 ± 0.100.37 ± 0.024.94 ± 1.20HS110V0.1 ± 1.020.37 ± 0.022.75 ± 1.07P-25 Powder TiO2