FMR and photocatalytic investigations of nNi-TiO2 (n=1%, 5% and 10%) compounds

(1)

FMR and photocatalytic

investigations of nNi-TiO

₂

(n=1%, 5% and 10%) compounds

1,2

2

1

2

2 N. Guskos A. Guskos , S. Glenis , G. Zolnierkiwicz , J. Typek ,

2

3

4

3 P. Berczynski , D. Dolat , B. Grzmil , B. Ohtani and A.W. Morawski

1 Department of Solid State, Faculty of Physics, University of Athens, Panepistimiopolis, Zografou 15 784, Greece

2 Department of Physics, West Pomeranian University of Technology, Al. Piastow 48, 70-311 Szczecin, Poland

3 Department of Chemical and Environmental Engineering, West Pomeranian University of Technology,

Al. Piastow 17, 70-310 Szczecin, Poland

4 Catalysis Research Center, Hokkaido University, Sapporo 001-0021 Japan

Magnetic resonance spectra were registered on BRUKER E500 X-band (9.4 GHz) spectrometer equipped with Oxford helium

flow cryostat enabling measurements in 4-300 K temperature range.

Apparatus

( )

(

)

( )

[

]

( )

(

)

[

]

[

( )

2

(

₀

)

2

]

0 2 0 2 0 2 0 2 0 2 2 0 2 0 0 2 2 2 0 2 0

2 )

(

B B B B B B B B B

H

I

d

+

D

+

D

+

-D

+

D

+

µ

where H is the true resonance field, Ä is the true linewidth connected with relaxation of the Landau-Lifshitz

0 H

type, and ä a true linewidth connected with relaxation of the Bloch-Bloembergen type.

_H

The Callen-lineshape

200 300 400 500 600 700 800 0 20 40 60 80 100

R

e

fle

ct

a

n

ce

(%

)

W avelength (nm ) 1Ni,N-TiO₂/800 5Ni,N-TiO₂/800 10Ni,N-TiO₂/800 Rutile Ni

C

Rutile Rutile TiN Ni 20 30 40 50 60 70 80

2theta (degree)

1Ni,N-TiO

₂

/800

In

te

n

s

it

y

(a

.u

)

5Ni,N-TiO

₂

10Ni,N-TiO

₂ 0 1000 2000 3000 4000 5000 6000 7000 -100000 -50000 0 50000 100000 d c "/ d H [a rb . u n it s ] Magnetic field [G] 4K 8K 16.5K 30.9K 44.3K 60K 75K 90K 120K 150K 180K 210K 240K 270K 290K 290 K 4 K 10% Ni 0 1000 2000 3000 4000 5000 6000 7000 -100000 -80000 -60000 -40000 -20000 0 20000 40000 60000 80000 100000 d c "/ d H [a rb . u n it s ] Magnetic field [G] 4K 8K 16.8K 30.0K 44.1K 60K 75K 90K 120K 150K 180K 210K 240K 270K 290K 290 K 4 K 5% Ni 0 1000 2000 3000 4000 5000 6000 7000 -80000 -60000 -40000 -20000 0 20000 40000 60000 80000 d c "/ d H [a rb . u n it s ] Magnetic field [G] 4K 8K 16.5K 30.4K 44.6K 60K 75K 90K 120K 150K 180K 210K 240K 270K 290K 290 K 4K 4 K _{1% Ni} 0 2000 4000 6000 8000 10000 -50000 -40000 -30000 -20000 -10000 0 10000 20000 30000 40000 50000 60000 70000 5% Ni experiment component 1 component 2 component 3 component 4 fit (1+2+3+4) d c "/ d H [a rb . u n it s ] Magnetic field [G] 4 K 0 1000 2000 3000 4000 5000 6000 7000 -30000 -20000 -10000 0 10000 20000 30000 5% Ni 90 K d c "/ d H [a rb . u n it s ] Magnetic field [G] experiment component 1 component 2 component 3 component 4 fit (1+2+3+4) 0 1000 2000 3000 4000 5000 6000 7000 -100000 -75000 -50000 -25000 0 25000 50000 75000 100000 d c "/ d H [a rb . u n it s ] Magnetic field [G] experiment component 1 component 2 component 3 component 4 fit (1+2+3+4) 290 K 5% Ni 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 0 200 400 600 800 1000 1200 1400 1600 In te g ra te d in te n s it y [a rb . u n it s ] Temperature [K] 20 40 60 80 100 120 140 160 180 200 220 240 260 280 600 800 1000 1200 1400 1600 1800 L in e w id th dB [G ] 20 40 60 80 100 120 140 160 180 200 220 240 260 280 0 500 1000 1500 2000 2500 3000 L in e w id th DB [G ] 20 40 60 80 100 120 140 160 180 200 220 240 260 280 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 10% Ni R e s o n a n c e fi e ld [G ] Component 1 Component 2 Component 3 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 0 50 100 150 200 250 300 350 400 450 500 In te g ra te d in te n s it y [a rb . u n it s ] Temperature [K] 20 40 60 80 100 120 140 160 180 200 220 240 260 280 0 200 400 600 800 1000 1200 1400 L in e w id th dB [G ] ₂₀ ₄₀ ₆₀ ₈₀ ₁₀₀ ₁₂₀ ₁₄₀ ₁₆₀ ₁₈₀ ₂₀₀ ₂₂₀ ₂₄₀ ₂₆₀ ₂₈₀ 0 200 400 600 800 1000 1200 1400 L in e w id th DB [G ] 20 40 60 80 100 120 140 160 180 200 220 240 260 280 1000 2000 3000 4000 5000 6000 5% Ni 10% Ni10% Ni R e s o n a n c e fi e ld [G ] Component 1 Component 2 Component 3 0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 0 50 100 150 200 250 300 350 400 In te g ra te d in te n s it y [a rb . u n it s ] Temperature [K] 20 40 60 80 100 120 140 160 180 200 220 240 260 280 0 200 400 600 800 1000 1200 1400 L in e w id th dB [G ] ₂₀ ₄₀ ₆₀ ₈₀ ₁₀₀ ₁₂₀ ₁₄₀ ₁₆₀ ₁₈₀ ₂₀₀ ₂₂₀ ₂₄₀ ₂₆₀ ₂₈₀ 0 200 400 600 800 1000 1200 L in e w id th DB [G ] 20 40 60 80 100 120 140 160 180 200 220 240 260 280 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 1% Ni R e s o n a n c e fi e ld [G ] Component 1 Component 2 Component 3

Titanium dioxide (TiO ) is well known as chemically stable and harmless

2

photocatalyst. However, ultraviolet-light irradiation is necessary to fulfill its

photocatalytic functions, so TiO photocatalyst holds limitation of application range

2

and effective use of light. In order to improve its photocatalytic efficiency and to

expand the application fields, development of visible-light or natural sunlight

responsive photocatalyst is demanded. This is done by modifying the surface of the

semiconductor and by using transition metals or non-metallic elements doped into

TiO . Nickel is one of transition elements used to modify the titania surface but Ni

2

doped TiO system is sparsely studied, so information of its physical and chemical

2

properties is needed.

Introduction

Water suspension of an industrial grade amorphous titanium dioxide (TiO /A) from

2

sulfate technology supplied by “Chemical Factory Police S.A.” (Poland) was used as a

starting material. Commercial TiO P25 (Evonik, Germany) was used for a comparison

2

purpose. About 20 g of TiO water suspension, containing ca. 35 wt.% of titanium dioxide

2

and ca. 8 wt.% of residual sulfuric acid as related to TiO content, was introduced into a

2

beaker containing aqueous solution of Ni(NO ) and stirred for 48 h. The amount of Ni

3 2

introduced to the beaker was of 1 wt.%, 5 wt.% or 10 wt.% relatively to TiO content. After

2

water evaporation, the samples were dried at 80°C for 24 h in an oven. Subsequently, the

material was calcined for 4 hours at 800°C in NH flow.

3

Sample preparation

The phase composition of Ni,N-co-modified TiO is rather complex . 2

In case of the sample prepared with 1 wt.% of Ni it is difficult to distinguish any other phase than rutile. Thus, we have assumed that if any other crystalline phase

is present in the structure of 1Ni,N-TiO its amount is negligible. In samples 2

5Ni,N-TiO and 10Ni,N-TiO , a metallic nickel was detected. In sample 5Ni,N-2 2

TiO , besides the metallic nickel, also TiN phase appeared after modification 2

process.

This light absorption increase was significantly higher in case of the co-modified samples, which corresponded to their almost black color, whereas the single-modified samples were yellow. The main reasons for this phenomenon are: the presence of nickel oxides, the doping with nitrogen as well as the lattice defects in the photocatalysts structure.

A high photocatalytic activity of 5Ni,N-TiO sample is caused by the following factors: a) 2

rutile form of TiO , which assures better stability and light absorption ability than anatase or 2

amorphous TiO ; b) doped nitrogen in the photocatalyst structure, mainly in the bulk of TiO , 2 2

which allows visible light absorption by narrowing the band-gap of the material; c) the presence

3+

of Ti ions in the bulk of the photocatalyst in amount which increases the visible light absorption,

additionally increasing TiO stability but does not yet serve as hole trap and therefore does not 2

inhibit the photocurrent; d) nickel modification, resulting in significant amounts of nickel on the photocatalysts surface as well as in the bulk; e) the presence of TiN on its surface, which may also serve as an electron trap and increase the charge separation

0 .0 0 0 0 .0 0 5 0 .0 1 0 0 .0 1 5 0 .0 2 0 0 .0 2 5 0 .0 3 0

P 2 5 N-Ti O 2 1 Ni ,N-Ti O 2 5 Ni ,N-Ti O 2 1 0 Ni ,N-Ti O 2

CO 2 ev ol ut io n ra te (µ m ol *m in -1 ) H g l a m p > 4 0 0 n m X e l a m p > 4 5 0 n m