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Photocatalysis: semiconductor physics

Carlos J. TavaresCenter of Physics, University of Minho, Portugal

ctavares@fisica.uminho.ptwww.fisica.uminho.pt

Where do I come from?

Guimarães

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Guimarães

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Introduction>>

• Photocatalysis is an active, attractive, andgrowing area regarding the removal of organicpollutants from water and air.

• It is based on the photonic activation of ametal oxide (semiconductor) catalyst.

• In order to increase the photocatalyticefficiency of these photocatalysts, it isimportant to enhance their specific surfacearea, crystallinity and the absorption oflight from the solar spectra amongst otherproperties.

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Solar spectrum>>

http://www.marusyosangyo.jp/

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Titanium dioxide is a light-activated catalyst used in the dissociation (cleaning) of organic and inorganic materials.

Photocatalysis mechanism>>

http://photochemistryportal.net/

Bahnemann, D., Photocatalyticwater treatment: solar energyapplications, Solar Energy, 2004, 77, 445–459

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Promotion of an electron to the conduction band, on irradiation by UVlight, results in a ‘hole’ in the valence band – essentially a detriment ofthe electron density that was localized on that orbital, and usuallyassigned a positive charge to symbolize the loss of negative. The holeis powerfully oxidizing – the orbital very much wants to retrieveelectron density just lost after light irradiation. It can retrieve this simplyby the electron in the conduction band recombining with the valenceband – recombination is a sum of radiative (i.e. emission may beobserved) and non-radiative processes. Based on the energy gap law,the fact that rutile energy levels are closer mean that the non-radiativeprocess is more efficient, and hence recombination is more efficient.

Alternative pathways to recombination are possible, and as you canguess, these result in the use of these materials as photocatalysts.The hole has the potential to oxidize water that may be on the surface ofthe material resulting in the formation of hydoxyl radicals. Hydroxylradicals are themselves very powerful oxidisers, and can easilyoxidize any organic species that happens to be nearby, ultimately tocarbon dioxide and water. Meanwhile, upstairs in the conduction band,the electron has no hole to recombine with, since the hole has oxidizedsurface bound water. It quickly looks for an alternative to reduce, andrapidly reduces oxygen to form the superoxide anion. This cansubsequently react with water to form, again, the hydroxyl radical.

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Titanium Dioxide>>

TiO2 holds several advantages:

• High chemical stability

• Excellent functionalization

• Non-toxic

• Cheap to synthesize

However… in most cases, has the disadvantage of requiring UV light to enhance photocatalysis

Ti

O

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oxidation potential of OH• is 2.8V

SC band-gap>>

Michael Grätzel, Nature 414, 338-344 (2001)

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http://photochemistryportal.net/

Band-gap tuning>> anionic doping

Nakamura R, Tanaka T, and Nakato Y., Mechanism for visible light responses in anodic photocurrents at N-doped TiO2 filmelectrodes, J. Phys Chem. B., 2004, 108, 10617 – 10620. (See also Irie, H et al, J. Phys Chem. B., 2003, 107, 5483 – 5486).

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Titania Density of States (DOS)>>

Asahi, et al., Science Vol. 293, 2001

Energy (eV)

TiO2 Band Structure

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Rutile Anatase

R. Sanjines, et al., Journal of Applied Physics 75 (1994) 2945

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By incorporating a small amount ofsilver (1–5%) results in increasedefficiency in photocatalysis.

Silver has a “Fermi level” or electronaccepting region at an energy justbelow the TiO2 conduction band.

Therefore, after light absorption andcharge separation, the electron inthe conduction band can beeffectively trapped by the silver,while the hole oxidizes water andforms hydroxyl radicals, without thethreat of recombination.

band-gap tuning>> cationic doping

>>TiO2 Photocatalytic Nanoparticles synthesis

Titanium precursor

+H2O:2-PrOH

solution

SOL

200°C

2-PrOH washing and sample drying

(60°C, 4h)

TiO2

NanoparticlesN-TiO2

Nanoparticles

Urea

Hydrolysis

Condensation

Thermal treatment2h

Nitrogen doping

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3,0 3,5 4,0 4,50

1

2

3

4

5

6

7

8

[F(R

)h]0

.5 (

arb

. un

its)

h (eV)

TiO2

TiO2 tangent

N-TiO2 (1g)

N-TiO2 (1g) tangent

N-TiO2 (4g)

N-TiO2 (4g) tangent

3,0 3,5 4,0 4,50

1

2

3

4

5

6

7

8

[F(R

)h]0

.5 (

arb

. un

its)

h (eV)

TiO2

TiO2 tangent

TiO2 100ºC

TiO2 100ºC tangent

TiO2 200ºC

TiO2 200ºC tangent

Sample Band Gap (eV)

TiO2 RT 3.45

TiO2 100°C 3.24

TiO2 200°C 3.20

Sample Band Gap (eV)

TiO2 200°C 3.20

N-TiO2-1g 3.12

N-TiO2-4g 2.90

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Photocatalytic activation at

higher wavelengths

Increasing synthesis

temperature

Increasing dopant

(urea) mass

Vis UVVis UV

>>Band-gap measurement

>>Photocatalytic degradation analysis

Greater photocatalytic activity under UV

radiation in the N-doped samples

0 60 120 180 240

0,2

0,4

0,6

0,8

1,0

Co

nce

ntr

atio

n (

C/C

0)

Time (min)

TiO2

N-TiO2 (1g)

N-TiO2 (4g)

Nanoparticles immersed in a Methylene Blue

solution

• UV light (LED 365 nm)

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UV-Vis reactor for photocatalysis experiments>>

0 60 120 180 240 300 360

0,2

0,4

0,6

0,8

1,0

Co

nce

ntr

atio

n (

C/C

0)

Time (min)

TiO2

N-TiO2 (1g)

N-TiO2 (4g)

Mass of doping material significantly influences the

photocatalytic behavior of the TiO2 nanoparticles under

visible light

Nanoparticles immersed in a Methylene Blue solution

• Xenon lamp (400-900 nm)

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>>Photocatalytic degradation analysis

400 450 500 550 600

0

2000

4000

6000

8000

10000

Inte

nsity (

counts

)

Wavelength (nm)

0 min

30 min

60 min

90 min

120 min

180 min

240 min

300 min

360 min

UV light - 365 nm

Photocatalytic generated

radical

Luminescent compound

Coumarin

+

˙OH

7-Hydroxycoumarin

7-hydroxycoumarin

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>>Quantification of *OH radicals

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Crystallinity assessment (TiO2 NPs)>>

20 30 40 50 600

20

40

60

80

100

data

Rietveld simul.

Spacegroup: I41/amdS

Cell Volume (Å^3): 136.4

Crystallite Size (nm): 13.2

Lattice parameters

a (Å): 3.7921

c (Å): 9.4913

Inte

nsi

ty (

arb

.un

its)

2 (º)

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BET Analysis>>

10 20 30 50 100 200 300 500 1000 2000

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40C

um

ula

tive P

ore

Volu

me (

cc/g

)

Pore Radius (angstrom)

NTiO2-pH3

Surface Area = 235.428 m²/g

Pore Volume = 0.382 cc/g

Pore Radius Dv(r) = 33.955 إ

V

dV(logr)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

dV

(lo

gr)

Magnetron sputtering is very versatile tool for thephysical vapour deposition (PVD) of thin films foroptical, semiconductor, wear and corrosion prevention,photovoltaic, self-cleaning, biomedical applications, toname a few.These coatings are performed in high vacuum andexhibit properties unlike the same bulk materials.

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Introduction

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Sputtering chamber>>

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HI-ERDA: level of N-doping>>

0 1000 2000 3000 4000 50000

20

40

60

80

100

Ato

mic

fra

ctio

n /

%

Depth / ×1015

at./cm2

glass

Ti

O

C,H

N

0 50 100 150 200 2500

50

100

150

200

250

Energy (arb. units)

Bra

gg p

eak h

eight

(arb

. unit

s)

1.000

2.034

4.135

8.409

17.10

34.77

70.71

143.8

292.4

594.6

1209

2459

5000

C

N

O

scattered 35

Cl

XPS: detection of substitutionalnitrogen

200 400 600 800 1000 1200

N1s

CKVV

TiLMM

OKVV

O1s

Ti2p

N1s peak400 eV

396 eV

Counts

/ a

.u.

Binding Energy / eV

1.19 at% N

0.72 at% N

C1s

binding energy difference (O1s-Ti2p3/2)= 529.9-457.4=72.5 eV >71.4 eV !!

atomic β-N states

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Ti

O

N

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Enhancement in UV-visible catalysisafter thermal annealment on glass substrates>>

200 300 400 500 600 700 800

0.0

0.5

1.0

1.5

Ab

sorb

ance

/ a

.u.

/ nm

Red 41 dye 1E-5M

0min

30min

60min

120min

180min

240min

300min

360min

0 1 2 3 40,00000

0,00025

0,00050

0,00075

0,00100

0,00125

UV

Vis

k /

min

-1

N2:O

2 flow rate / sccm

ln(C/Co)=-kt

Thank you for your attention

30ctavares@fisica.uminho.pt