**3. Growth and characterisation of CuO thin films**

It is expected that Cu2O thin films can be oxidized by the annealing in air and thus converted into CuO. Therefore, annealing effects of the electrodeposited Cu2O thin films in air were investigated in order to obtain a single phase CuO thin films on Ti substrate. Cu2O thin films on Ti substrates were prepared under the potentiostatic condition of -200 mV Vs SCE for 60 min. in the three electrode electrochemical cell containing 0.1 M sodium acetate and 0.01 M cupric acetate aqueous solution. Temperature of the bath was maintained to 55 C and the electrolyte was continuously stirred using a magnetic stirrer. All the thin films are uniform and having a thickness of about 1 m which was calculated by monitoring the total charge passed during the film deposition through the working electrode (WE).

The bulk structure of the films, which were annealed at different temparatures and durations, can be determined by XRD measurements and Fig. 6 shows the XRD spectra of the films annealed at 150 to 500 C in air, in addition to the as grown Cu2O. Results show that Cu2O structure remains stable even though films are annealed at 300 C, as reported by Siripala *et al*. (1996). Formation of CuO structure can be observed when films are aneealed at

Fig. 6. X-ray diffraction patterns of electrodeposited Cu2O thin films a) as grown and annealed at b) 150 oC, c) 400 oC and d) 500 oC

films are uniform and well adhered to substrate. Garutara *et al*. (Garuthara & Siripala, 2006) carried out the photoluminescence (PL) characterisation for the electrodeposited n-type polycrystalline Cu2O. They showed the existence of the donor energy level of 0.38 eV below the bottom of the conduction band due to the oxygen vacancies and confirmed that the n-type conductivity is due to the oxygen vacancies created in the lattice. Previously reported electrodeposited Cu2O in a various deposition bath, except slightly acidic acetate bath, attribute p-type conductivity due to the Cu vacancies created in the lattice as thermally

It is expected that Cu2O thin films can be oxidized by the annealing in air and thus converted into CuO. Therefore, annealing effects of the electrodeposited Cu2O thin films in air were investigated in order to obtain a single phase CuO thin films on Ti substrate. Cu2O thin films on Ti substrates were prepared under the potentiostatic condition of -200 mV Vs SCE for 60 min. in the three electrode electrochemical cell containing 0.1 M sodium acetate and 0.01 M cupric acetate aqueous solution. Temperature of the bath was maintained to 55 C and the electrolyte was continuously stirred using a magnetic stirrer. All the thin films are uniform and having a thickness of about 1 m which was calculated by monitoring the

total charge passed during the film deposition through the working electrode (WE).

The bulk structure of the films, which were annealed at different temparatures and durations, can be determined by XRD measurements and Fig. 6 shows the XRD spectra of the films annealed at 150 to 500 C in air, in addition to the as grown Cu2O. Results show that Cu2O structure remains stable even though films are annealed at 300 C, as reported by Siripala *et al*. (1996). Formation of CuO structure can be observed when films are aneealed at

28 30 32 34 36 38 40 42 44

2 (deg)

Fig. 6. X-ray diffraction patterns of electrodeposited Cu2O thin films a) as grown and

Ti

CuO

CuO

Cu

O2

Ti

CuO

Cu

O2

Ti

CuO

**3. Growth and characterisation of CuO thin films** 

0.00

annealed at b) 150 oC, c) 400 oC and d) 500 oC

0.02

0.04

Counts (a.u.)

Cu

O2

a) As grown b) 150 <sup>o</sup> C

c) 400 o C

d) 500 <sup>0</sup> C

0.06

0.08

grown films.

400 oC for 15 min. Fig. 6 shows that the intensities of the peaks correspondent to the CuO structure increases while intensities of the peaks correspondent to the Cu2O structure decreases with the increasing of annealing temperature and duration. The reflections from the Cu2O structure disappear when the film is annealed at 500 C for 30 min. in air. It is reveled that the single phase CuO thin films on Ti substrate can be prepared by annealing Cu2O in air.

The surface morphology of the annealing Cu2O thin films is studied with SEMs. Fig. 7 shows SEMs of (a) as grown, and annealed in air at (b) 175 C, (c) 400 C and (d) 500 C. Results reveal that, by increasing the annealing temperature, the size of the cubic shape polycrystalline grain gradually increase up to 200 C, change to the different shape at 400 oC and converted to the monoclinic like shape polycrystalline grain at 500 oC. Cu2O thin films have the cubic-like polycrystalline grains. SEMs clearly show that structural phase transition take place from Cu2O, Cu2O-CuO, CuO as reveal by the XRD patterns. CuO crystallites are in the order of 250 nm.

Fig. 7. Scanning electron micrographs of the electrodeposited semiconductor Cu2O thin films a) as grown and annealed in air at (b) 175 C, (c) 400 C and (d) 500 C

Photosensitivity (Voc and Isc) of the annealed electrodeposited Cu2O thin films in a two electrode PEC cell containing 0.1 M sodium acetate aqueous solution, under white light illumination of 90 W/m2, shows that initial n-type photoconductivity changes to the p-type after annealing 300 oC. Type of the photoconductivity of the Cu2O thin films can be converted from n- to p-type with annealing because of Cu2O structure remain same even if films annealed at 300 oC as revealed by XRD patterns.

Electrodeposited Cu2O Thin Films for Fabrication of CuO/Cu2O Heterojunction 99

current-voltage characteristics of the thin films annealed at (a) 250 C and (b) 300 C. The similar behaviour is observed for the thin films annealed at less than 250 C and annealed at grater than 300 C, respectively, and is reproducible for each film. In Fig. 8(a), the anodic photocurrent increases with increasing the anodic potential. This suggests that the n-type photoconductivity is due to an anodic potential behaviour, and is reproducibly observed for the thin films annealed at < 250 C. This suggests that the n-type photoconductivity is due to the anodic potential barrier formed at the semiconductor/electrolyte interface, as the inset of Fig. 8(a). However, the photocurrent-potential behaviour is completely changed for the film annealed at ≥ 300 C. In Fig. 8(b), the cathodic photocurrent results from the cathodic potential barrier formed at the interface, as shown in the inset. This cathodic photoresponse assures that the electrical conductivity of the electrodeposited Cu2O films can be changed from the n-type to p-type property by annealing in air. Fig. 9 shows the dark and light current-voltage characteristics of the CuO thin film in a PEC cell containing 0.1 M sodium acetate aqueous solution. The cathodic photocurrent is produced in the range from the anodic to cathodic bias potentials, and the cathodic photocurrent increases with increasing the cathodic potential. This suggests that the p-type photoconductivity is due to the cathodic potential barrier forms at the semiconductor/electrolyte interface. It reveals that the

Structural phase transition from Cu2O to CuO with annealing and the quality of thin films can be further investigated using Extended X-ray Absorption Fine Structure (EXAFS) which gives local structure around Cu ions. Fig. 10 shows the X-ray absorption spectra (XAS) in the region of 8800 to 9430 eV near the Cu-K edge for the thin films, annealed at 150, 400, and 500 C by using the florescence detection (FD) method. XAS suggest that the local structures around Cu ions in the annealed Cu2O thin films are remain same when films annealed at less than 300 oC and significantly different when films annealed at

Refinements of a Fourier transformation spectrum |F(R)| obtained from the oscillating EXAFS spectra can be used to study the quality of Cu2O and CuO thin films. Fig. 11, solid circles show the observed |F(R)| of the thin film annealed at 150 C, where the abscissa is a radial distance (R(Å)) from a X-ray absorbing Cu ion to its surrounding cations and anions. Fig. 11, a solid line shows a theoretical |F(R)|. The refinement produces a good fit between the observed and theoretical |F(R)| indicating the local structure around Cu ions of the film is verymuch similar to the ideal Cu2O structure. Fig. 12 is similar refinement for CuO thin film. These results convince that the thin films are high quality single phase Cu2O and CuO structures (free of amorphous phases and impurities). Detail investigation has been reported

It is characterised that single phase Cu2O thin films are converted to two phase Cu2O and CuO composit films with increasing the annealing temperature. Single phase CuO thin films can be obtained by annealing at 500 oC for 30 min in air. Extended X-ray absorption fine structure (EXAFS) near the Cu K edge of the Cu2O thin films (annealed at 150 oC for 15 min.) and CuO thin films (annealed at 500 oC for 30 min.) are confirmed that the films are high quality single phase Cu2O and CuO (free of amophous phases) respectively. Conductivity type of the films strongly depends on the annealing treatment. n-type conductivity of the Cu2O thin films are changed to p-type when the films are annealed at 300 oC. CuO thin films

are photoactive and p-type in a PEC containing 0.1 M sodium acetate.

electrodeposited CuO thin films are p-type semiconductors.

grater than 300 oC.

(Wijesundera et al., 2007).

Fig. 8. Dark and light current voltage characteristics of electrodeposited Cu2O thin film electrodes annealed at (a) 250 C and (b) 300 C. Energy level diagrams for n-type and p-type Cu2O films in the electrolyte are shown in the insets, where the electron, hole, anodic and cathodic potential are denoted by solid and open circles, eA and eC respectively.

The photoactivity of the thin films has been further studied by the dark and light current voltage characteristics in a three electrode electrochemical cell. The counter and the reference electrodes are Pt plate and SCE, respectively. The bias voltage has been applied to the working electrode (Ti/Cu2O) with respect to the SCE. Fig. 8 shows the dark and light

n-Cu-O/Electrolyte

e<sup>A</sup>

EC E ff EV





Photocurrent (A/cm2

)



0

50


0

50

Photocurrent (A/cm2

)

100

150

200

Fig. 8. Dark and light current voltage characteristics of electrodeposited Cu2O thin film electrodes annealed at (a) 250 C and (b) 300 C. Energy level diagrams for n-type and p-type Cu2O films in the electrolyte are shown in the insets, where the electron, hole, anodic and cathodic potential are denoted by solid and open circles, eA and eC respectively.

Light Off


Bias voltage Vs SCE (mV)

Light On

EC

EF Ev

The photoactivity of the thin films has been further studied by the dark and light current voltage characteristics in a three electrode electrochemical cell. The counter and the reference electrodes are Pt plate and SCE, respectively. The bias voltage has been applied to the working electrode (Ti/Cu2O) with respect to the SCE. Fig. 8 shows the dark and light


Bias voltage Vs SCE (mV)

(b)

p-Cu-O/Electrolyte

e<sup>C</sup>

Light Off (a)

Light On

current-voltage characteristics of the thin films annealed at (a) 250 C and (b) 300 C. The similar behaviour is observed for the thin films annealed at less than 250 C and annealed at grater than 300 C, respectively, and is reproducible for each film. In Fig. 8(a), the anodic photocurrent increases with increasing the anodic potential. This suggests that the n-type photoconductivity is due to an anodic potential behaviour, and is reproducibly observed for the thin films annealed at < 250 C. This suggests that the n-type photoconductivity is due to the anodic potential barrier formed at the semiconductor/electrolyte interface, as the inset of Fig. 8(a). However, the photocurrent-potential behaviour is completely changed for the film annealed at ≥ 300 C. In Fig. 8(b), the cathodic photocurrent results from the cathodic potential barrier formed at the interface, as shown in the inset. This cathodic photoresponse assures that the electrical conductivity of the electrodeposited Cu2O films can be changed from the n-type to p-type property by annealing in air. Fig. 9 shows the dark and light current-voltage characteristics of the CuO thin film in a PEC cell containing 0.1 M sodium acetate aqueous solution. The cathodic photocurrent is produced in the range from the anodic to cathodic bias potentials, and the cathodic photocurrent increases with increasing the cathodic potential. This suggests that the p-type photoconductivity is due to the cathodic potential barrier forms at the semiconductor/electrolyte interface. It reveals that the

electrodeposited CuO thin films are p-type semiconductors.

Structural phase transition from Cu2O to CuO with annealing and the quality of thin films can be further investigated using Extended X-ray Absorption Fine Structure (EXAFS) which gives local structure around Cu ions. Fig. 10 shows the X-ray absorption spectra (XAS) in the region of 8800 to 9430 eV near the Cu-K edge for the thin films, annealed at 150, 400, and 500 C by using the florescence detection (FD) method. XAS suggest that the local structures around Cu ions in the annealed Cu2O thin films are remain same when films annealed at less than 300 oC and significantly different when films annealed at grater than 300 oC.

Refinements of a Fourier transformation spectrum |F(R)| obtained from the oscillating EXAFS spectra can be used to study the quality of Cu2O and CuO thin films. Fig. 11, solid circles show the observed |F(R)| of the thin film annealed at 150 C, where the abscissa is a radial distance (R(Å)) from a X-ray absorbing Cu ion to its surrounding cations and anions. Fig. 11, a solid line shows a theoretical |F(R)|. The refinement produces a good fit between the observed and theoretical |F(R)| indicating the local structure around Cu ions of the film is verymuch similar to the ideal Cu2O structure. Fig. 12 is similar refinement for CuO thin film. These results convince that the thin films are high quality single phase Cu2O and CuO structures (free of amorphous phases and impurities). Detail investigation has been reported (Wijesundera et al., 2007).

It is characterised that single phase Cu2O thin films are converted to two phase Cu2O and CuO composit films with increasing the annealing temperature. Single phase CuO thin films can be obtained by annealing at 500 oC for 30 min in air. Extended X-ray absorption fine structure (EXAFS) near the Cu K edge of the Cu2O thin films (annealed at 150 oC for 15 min.) and CuO thin films (annealed at 500 oC for 30 min.) are confirmed that the films are high quality single phase Cu2O and CuO (free of amophous phases) respectively. Conductivity type of the films strongly depends on the annealing treatment. n-type conductivity of the Cu2O thin films are changed to p-type when the films are annealed at 300 oC. CuO thin films are photoactive and p-type in a PEC containing 0.1 M sodium acetate.

Electrodeposited Cu2O Thin Films for Fabrication of CuO/Cu2O Heterojunction 101

 Data Fit Cu2 O

 Data Fit CuO

012345

Fig. 11. Theoretical |F(R)|of the EXAFS spectrum at Cu K-edge obtained by the least squares refinement compared to the observed |F(R)| for the Cu2O thin film annealed at

R(Å)

012345

R(Å)

Fig. 12. Theoretical |F(R)| of the EXAFS spectrum at Cu K-edge obtained by the least squares refinement compared to the observed |F(R)| for the Cu2O thin film annealed at

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

0.00

0.05

0.10

0.15


0.20

0.25

0.30

0.35

150C

500C


Fig. 9. Dark and light current voltage characterisation of CuO thin film in a PEC cell containing 0.1 M sodium acetate aqueous solution.

Fig. 10. X–ray absorption spectra of annealed Cu2O thin at 150, 300, 400 and 500C in air

0 Light off


500 o

C for 30 min

 500 o C

 400 o C

 300 o C

 150 o C

Bias voltage Vs SCE (mV)

8800 8900 9000 9100 9200 9300 9400

E(eV)

Fig. 10. X–ray absorption spectra of annealed Cu2O thin at 150, 300, 400 and 500C in air

Fig. 9. Dark and light current voltage characterisation of CuO thin film in a PEC cell

Light on


0.0

0.5

1.0

1.5

I

F/IO

2.0

2.5

3.0

containing 0.1 M sodium acetate aqueous solution.


Photocurrent (A/cm2

)


Fig. 11. Theoretical |F(R)|of the EXAFS spectrum at Cu K-edge obtained by the least squares refinement compared to the observed |F(R)| for the Cu2O thin film annealed at 150C

Fig. 12. Theoretical |F(R)| of the EXAFS spectrum at Cu K-edge obtained by the least squares refinement compared to the observed |F(R)| for the Cu2O thin film annealed at 500C

Electrodeposited Cu2O Thin Films for Fabrication of CuO/Cu2O Heterojunction 103

grains on the CuO film and Figs. 14(a) to (c) show that the amount of Cu2O increases with increasing the deposition potential. The SEMs reveal that the well covered Cu2O layer can be deposited on Ti/CuO electrode under the potentiostatical condition of -550 mV Vs SCE and above. Grain size of the Cu2O deposited on Ti substrate is in the range of ~ 1-2 m as shown in Fig. 14(a) while it is lower to 1 m when Cu2O deposited on CuO at the deposition potential of -550 mV Vs SCE. The SEM with low magnification of Cu2O deposited at -700 mV Vs SCE clearly shows the existence of Cu on the surface of Cu2O as shown in the

(a) (b)

(c)

Fig. 14. Scanning electron micrograph of Cu2O thin films electrodeposited on Ti/CuO

XRD and SEM reveal that well-covered single phase polycrystalline Cu2O thin film on the Ti*/*CuO electrode can be possible at the deposition potential of −550 mV Vs SCE in an acetate bath. Structural matching of two semiconductors is very essential for fabricating a heterojunction. In general, the cubic-like Cu2O grains and the monoclinic-like CuO grains are not match with each other to make the CuO/Cu2O heterojunction. However, the electrodeposition technique produces the good matching of the Cu2O grains to the monoclinic-like CuO grains. The shape of the grains can be easily changed when the electrodeposition technique is used to grow a semiconductor. The electrodeposition is a very good tool to fabricate the heterojunctions as it does not depend on the grain shape of the material. Further, the SEMs of the Cu2O/CuO heterojunction suggested that the Cu2O polycrystalline grains are grown from the surfaces of the CuO polycrystalline grains and make the good contacts between two thin film layers. For the completion of the device, very

electrode at (a) -250 mV, (b) -400 mV and (c) -550 mV Vs SCE

XRD pattern of the film deposited at -700 mV Vs SCE.
