Документ взят из кэша поисковой машины. Адрес оригинального документа : http://electr003.chem.msu.ru/rus/chteniya/f34_ganzha.pdf Дата изменения: Sun Oct 31 19:21:44 2010 Дата индексирования: Mon Oct 1 21:41:30 2012 Кодировка: Windows-1251

Voronezh State University: Main Building

Oxide nanofilms on silver, copper and their alloys with gold: the kinetics of anodic formation and semiconductor properties
Sergei Ganzha

Voronezh State University, Department of Physical Chemistry, Universitetskaya pl. 1, 394006 Voronezh, Russia e-mail: serganzha@gmail.com


The motivation
The oxide formation strongly depends on the state of the metal/solution interface which is determined by the crystalline structure and chemical composition of the electrode as well as the electrode potential and the solution composition. The role of Ag crystal face in OH- adsorption and initial stages of Ag2O growth was revealed [1, 2]. The oxide formation and some structural properties of the oxides on single crystals (111) and (001) of copper were discussed [3]. The role of chemical irregularity of the electrode surface, modeled by the alloying, remains unexamined. Gold is considered the most suitable metal since I) Ag-Au as well as Cu-Au system is a continuous series of solid solutions and II) gold is thermodynamically stable at the potentials of silver and copper oxide formation.

2

The aim is to reveal the influence of the kinetic features of silver and copper oxide
formation, caused by the crystal face of the electrode and its alloying with gold, and some semiconductor properties of the anodic oxide nanofilms.
[1] Droog JMM, J. Electroanal. Chem. 115 (1980) 225 [2] Doubova LM, Daolio S, Pagura C, De Battisti A, Trasatti S, Russ. J. Electrochem. 38 (2002) 20 [3] Kunze J, Maurice V, Klein LH; Strehblow H-H, Marcus P, Cor. Sci. 46 (2004) 245


Experimental
The objects
Agpoly Cupoly Ag100, Ag 110, Ag111 Ag-Au (XAu = 1; 4; 15 at.%) Cu-Au (XAu = 4; 15 at.%)
Deoxygenated 0.1 M KOH

3

The methods
Linear voltammetry and chronoammetry (IPC-Compact) Photocurrent and photopotential spectroscopy (original equipment with a sensibility of 10 nA and 2 ч 3 V) SEM (JEOL JSM6380LV)

The parameters of light-emitting diodes (LED)
Parameters

LED
LDUV53393 LDUV3333 LSBI3333 NSPB300A NSPE590S NSPG500S HLMP-EL08-VY000 HLMP-ED16UX000

Wave length , nm
385 400 430 470 505 525 592 630

Standard angle of divergence, 0
18 30 15 15 10 15 6 15

Specific light intensity, mWatt cm- 2
1.25 3 1.5 3 3 2 2 3.75


4

SEM- images of Ag2O (L = 120 nm) on Ag, Ag4Au and Ag15Au

Ag

poly

(E = 0.56 V)

Ag15Au (E = 0.70 V)

Ag4Au (E = 0.58 V)


The current efficiency / % of Ag2O formation (nominator) and the film thickness L / nm (denominator)
E = 0.56 V q/
mC cm
-2

5

Ag1Au Ag
poly

Ag4Au

Ag15Au

Ag

100

Ag

110

Ag

111

E=0.57 V E=0.60 V E=0.77 V

2 3 4 5 7

71 2.4 73 3.7 79 5.3 83 6.7 85 10.0

50 1.7 58 2.9 76 5.1 89 7.5 90 10.6

47 1.6 56 2.8 63 4.2 69 5.8 77 9.1

60 2.0 70 3.5 76 5.1 82 6.9 83 9.8

33 1.1 41 2.1 60 4.0 65 5.5 72 8.5

45 1.5 64 3.2 65 4.4 66 5.6 70 8.2

40 1.3 47 2.4 46 3.1 58 4.9 64 7.5


Photocurrent and photopotential - thin films (L < W)
i ph = i - idark e jn (x) т=
x =0

6

- jp ( x)

x =0



Eph is calculated under condition of iph=0

i ph = ef 0 (1 - R

вн еш out о тр ref

) (1

-e

-2 L

)

=i

max ph

(1

-e

-2 L

)
)

2i

max ph

L

Assumptions:

2kT f 0 (1 - R out )L2 ref E ph =- e e N D Dn

e( E - E kT

R

fb

inn вн утр ref отр

1

L/ 2LD<1

- quantum efficiency f - coefficient of holes assimilation at the outer interface in electrochemical reaction 0 - light intensity R - ND Dn Efb - coefficient of reflection from the outer interface coefficient of light absorption - concentration of donor defects - coefficient of electron diffusion - flatband potential
out ref


Photocurrent measurements in chronoammetry of Ag
i = idark +iph
Reference electrode Work electrode Counter electrode
Red

7

Ox

The scheme of photocurrent registration
L - film thickness W - space charge region LD - Debye's length

Chronoammogram of Agpoly at E=0.56 V and photocurrent


The growth of photocurrent with film thickness
i ph = e 0f (1 - R out ) (1 - e ref
-2 L

8

)=i

max ph

(1 - e

-2 L

) 2i

max ph

L

(L < W)

Ag2O on Ag

poly 110

Ag2O on Ag

The dependence of photocurrent (=470 nm, 0=3.561015 photon cm-2 s-1) on Ag2O film thickness


9

Structural and optical parameters of Ag2O at different potentials of film formation on polycrystalline Ag (=470 nm; 0=3.561015 photon cm-2 s-1)
E/V
0.52
-2

0.53 4.2 73 0.90 111 256 2.98 72.5 0.65

0.54 4.3 75 1.08 93 213 4.51 58.9 0.63

0.55 4.0 70 1.10 91 209 4.90 56.5 0.62

0.56 4.4 77 2.30 44 100 22.40 26.4 0.61

i

max ph

/ Acm
out ref

4.4 77 0.90 111 256

f(1- R

)10

4 -1

10-5 / cm -1 / nm W / nm ND10
-15

/ cm

-3

2.83 74.3 0.67

LD / nm L
D

W (Ag2O on Aghkl) >> W (Ag2O on Agpoly) ND (Ag2O on Aghkl) << ND (Ag2O on Agpoly) Hence, more stoichiometric oxide is formed on silver single crystals.


Photocurrent in Ag2O on Ag-Au alloys
The dependence of photocurrent on Ag2O thickness

10

Processing

photon cm-2 s

-1

Optical and structural parameters of Ag2O on Ag-Au alloys

Au

/ at.% 1 4 15

E/V 0.58 0.6 0.77

10-5 / cm0.3 0.1 0.009

1

W / nm 767 2300 25556

ND 10-12 / cm-3 414 49.7 0.66

f(1- R

out ref

)104

LD / nm 194 570 4870

L

D

70 72 72

0.58 0.57 0.44


Photopotential measurements after switching off the polarization
The scheme of photopotential registration E=E+Eph E = Edark + Eph Pt(Pt) Chronopotentiogram of Agpoly in 0.1 M KOH q = 7 mC cm-2, = 470 nm, 0 = 3.561015 photon cm-2 s

11

-1

L - the film thickness W - space charge region LD - Debye's length


12

Photopotential - time dependence (after the polarization switching off)
in Ag2O formed on Agpoly; q = 2(), 3( ), 4(), 5( ), 7( ) mC cm

-2

st st ln [Eph (t ) - Eph ] = ln [Eph (0) - Eph ] - kt

in Ag2O formed on Agpoly and single crystals; q = 5 mC cm-2


Photocurrent spectroscopy

13

(

i ph h

)

2/ m

= C1L2/

m

(

h - E

bg

)

1 and 2 are the constants, the parameter m is equal to 1 or 4 for direct or indirect optical transition respectively
nm)

Ebg (Ag2O on Ag

poly)

= 2.32 eV

The estimation of band gap of Ag2O for direct transitions


14

The parameters of Ag(I) oxide formed at E = 0.56 V on different substrates
Parameter Ag2O|Agpoly / cm-1 W / nm LD / nm L f(1D

Electrode system Ag2O|Aghkl 0.7ч1.4 164ч330 43ч87 0.60ч0.61 75ч79 2.1ч8.3 2.23 2.09 Ag2O|Ag-Au 0.001ч0.1 767ч2300 194ч570 0.57ч0.58 70ч72 0.05ч0.4 2.19 -

2.3 100 26 0.61

R

out ref

)104

77 22.4 2.32 2.09

ND1015 / cm-3

E

bg

direct

/

eV

on iph data on

Eph

data


Electrochemical investigation on Cu and Cu-Au in 0.1 KOH
dE/dt = 1 mV s
-1

15

Cu

Cu4Au

Cu15Au


16

SEM-images of Cu2O and CuO formed on Cu (a) E = -0.19 V (t = 60 min, L = 16 nm) (b) E = 0.10 V (t = 15 min, L = 40 nm)

poly

(b)

The current efficiency / % of the oxide formation on Cu and Cu-Au The electrode The range II (Cu2O formation) The range III (CuO formation) Cu ~100 62 Cu4Au ~100 85 Cu15Au ~100 57


Photocurrent measurements on copper

17

Potential range I:

Potential range II E = - 0.22 V CuOH

CuOH Cu2O

iph = 0

i ph = 2i

max ph

L

Potential range II E = - 0.20 V

Cu + OH

-

CuOH(n-type) + ePotential range II E = - 0.17 V Potential range III E = 0.10 V

?Cu2O(p-type) + ? H2O

Cu2O

CuO

Potential range III

E = 0.40 V iph = 0


Photocurrent measurements on Cu-Au

18

iph = 0

E = -0.1 V (potential range II)

E = 0.1 V (potential range III)


Photocurrent spectroscopy

19

(

i ph h

)

2/ m

= C1L2/

m

(

h - E

bg

)

1 is a constants, the parameter m = 4 for indirect optical transition

Cu2O formation E = - 0.17 V

Cu2O and CuO formation E = 0.10 V


Conclusions

20

The predominant route of Ag(I) oxides anodic formation is not the precipitation from the near-electrode layer, but mainly the direct electrochemical reaction. The thickness of these oxide films does not exceed the space charge region. On Ag and Ag-Au alloys n-type Ag(I) oxide with a prevalence of donor defects is formed. The transition from polycrystalline Ag to single crystals as well as the alloying of silver with gold up to 4 at.% results in a decrease of the band gap of Ag2O and an increase of the stoichiometry. On Cu and Cu-Au alloys the p-type Cu(I) and Cu(II) oxides with a prevalence of acceptor defects are formed. Cu(I) oxide has the band gap of 2.2 eV with the prevalence of indirect optical transitions. At the initial stage of anodic oxidation the oxide layer with n-type conductivity appears. During the thickening the n-type oxide phase transforms into the p-type Cu(I) oxide. Copper is subject to corrosion even in a thoroughly deoxygenated solution with Cu2O formation. The preliminary anodic formation of a thin Cu(I) oxide as well as the alloying of copper with gold (up to 4 and 15 at.%) hampers this process.

Acknowledgements
We are grateful for financial supporting to Russian Foundation for Basic Research (09-03-00554a)


SEM

21

Ag

poly

Ag2O|Ag

110

Ag2O|Ag

111

Ag2O|Ag

poly

Ag2O|Ag

100


Photocurrent spectroscopy
/ cm-1 / ref.un.

22

Ebg = 2.32 eV (direct transitions)

Xang X.Y. Acta Phys.-Chim Sin. - 2003. - V. 19, 3

/ nm

Dependence of coefficient of light absorption on the wave length
in Ag2O on Ag
poly

(E=0.56 V)


Photopotential spectroscopy

23

(

E p h h

)

2/ m

= C2 L4/

m

(

h - E

bg

)

The estimation of band gap in Ag2O for direct transitions


The role of crystallografic orientation

24

E=0.52 V

[110] < [100] < [111]

E=0.56 V


Coulometry
The anodic charge of copper oxidation, the cathodic charge of oxide film reduction and the difference charge of copper corrosion ( = 10 minutes) E, V Qa = Qox , mC
-0.7 -0.5 -0.3 -0.2 -0.17 -0.15 -0.12 -0.05 0

25

0.1

0

0

0.4

1

1.8

1.7

2.6

3.4

5.4

67.8

Qk = Qred , mC

9.3

9.0

9.6

6.6

3.3

5.4

8.1

11

9.8

36.6

Qk - Qa = Qcor , mC

9.3

9.0

9.2

5.6

1.5

3.7

5.5

7.6

4.4

-


Photocurrent and photopotential - the theory
for bulk n-type semiconductor (L > W)
d dx d ( x) dp( x) + p(x) dx dx = -a1e

26

Assumptions:
- x

(0 x W)

N+ N D

D

d dn( x) d ( x) + n( x) = -a 2 e dx dx dx
d 2( x ) e2 p0e =- 2 dx 0 kT
- (x )

- x

(0 x W)
(x )

The volume of semiconductor out of space charge region is quasi-neutral Distribution of n and p in space charge region in equilibrium state satisfies Boltzmann low Recombination during the radiation is negligibly small W - space charge region L - film thickness LD - Debye's length Lp - diffusion length

+ N D - n 0e

- NA

(0 x W) L>W

2 e( x ) 1 ( x - W ) (x ) = =- 2 kT 2L D ( x - L )2 + ( W 2 - L2

)

,

L
d p( x ) =-a1e dx 2

2

-x

+

p(x ) - p L2p

0

(W x L)


Photocurrent and photopotential - thin films (L < W)
with a high level of light absorption (LD 1)
eE ph - kT eN D D n 1 - e e + 2L D F L / 2L D e( E - E kT

27

i ph = ef 0 (1 - R

out ref

) (1

+R e

inn -L ref

) (1

-e

- L

)

(

)

-

fb

)

kT f 0 (1 - R E ph =- ln 1 + e

out ref

) (1

- R inn e ref
n

-L

) (1

-e

-L

)

NDD

L

D

L 2F 2L e D

e( E - E kT

fb

)



- quantum efficiency; 0 - light intensity f - coefficient of holes assimilation in electrochemical reactions at the outer interface ND - concentration of donor defects; Dn - coefficient of electron diffusion - coefficient of light absorption; Efb - flatband potential R
out ref

and R

inn ref

- coefficients of reflection from the outer and inner interfaces L < LD hence (L / 2 LD) < 1 and F (L / 2 LD) L / 2 LD

F(u) - integral of Doson F(u) u at u < 1


28

The dependences of photopotential on film thickness and light intensity

2kT f 0 (1 - R )L E ph =- e e N D Dn
out ref 2

e( E - E kT

fb

)

The photopotential in Ag2O formed on Agpoly and Aghkl at E = 0.56 V


29

The scheme of potentiostatic measurements on Cu and Cu-Au
E/V polarization switching off "fresh" solution

cathodic prepolarization

polarization and photocurrent measurements

photopotential measurements

cathodic reduction

t / min

E = -0.7; -0.5; -0.3; -0.2; -0.15; -0.12; -0.1; -0.05; 0.0; 0.1; 0.2; 0.4 V (s.h.e.) = 10, 15, 20, 25, 30 minutes


Photopotential measurements without any polarization

30

Eph, V

Eph ~ L L ~ t1/2 Eph ~ t
t, s

2

The photopotential on copper surface in deoxygenated 0.1M KOH; = 400 nm, 0 = 7.121015 photon/cm2 s


Photopotential measurements on Cu and Cu-Au after the polarization in the range II ( = 400 nm)

31

Cu4Au (E = -0.16 V)

Cu15Au (E = -0.16 V)


The dependence of the initial photopotential (a) and polarization current (b) on the potential of Cu-electrode

32

The clear-cut correlation between the Eph(0) - t dependence and the voltammogram shows a close interrelation between the kinetics of the oxide formation and the oxide structure predetermining the value of the photopotential.


Flat-band potential of Cu2O and CuO Cu
/F

33

Cu4Au
/F

Cu15Au
/F

E/V

E/V

E/V

Cu Efb(Cu2O) / V Efb(CuO) / V -0.30 0.20

Cu4Au -0.35 0.15

Cu15Au -0.45 -


XPS-study of Ag4Au after Ag2O formation in 0.1 M KOH

3 4

O
Ar (AG62); 2.3 kV; i = 60 mA cm-2
Time of etching, s

We are grateful to professor Leonid Kazanskiy for help in this measurements


Photopotential measurements on Cu and Cu-Au after the polarization in the ranges I and II ( = 400 nm)
Eph / V
E = -0.7 V min
t/s

35

L~

t1/2
2

Eph / V

Eph ~ L Eph ~ t
eq ECu
O/Cu

E = -0.1 V t/s Ecor / V

2

Ecor / V

(rest potential)

min
t/s t/s