NiO films

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Current Applied Physics 9 (2009) 67–72 www.elsevier.com/locate/cap www.kps.or.kr

Preparation and characterization of F doped SnO2 films and electrochromic properties of FTO/NiO films K.K. Purushothaman a, M. Dhanashankar b, G. Muralidharan a,* b

a Department of Physics, Gandhigram Rural University, Gandhigram 624 302, India Department of Physics, Kamaraj College of Engineering and Technology, Virudhunagar 626 001, India

Received 31 October 2007; accepted 6 November 2007 Available online 3 December 2007

Abstract The dependence of structural and electrical properties of SnO2 films, prepared using spray pyrolysis technique, on the concentration of fluorine is reported. X-ray diffraction, FTIR and scanning electron microscope (SEM) studies have been performed on SnO2:F (FTO) films coated on glass substrates. Measured values of Hall coefficient and resistivity are reported. The 7.5 m% of F doped film had a resistivity of 15  104 X cm, carrier density of 18.7  1019 cm3 and mobility of 21.86 cm2 V1 S1. The NiO film was coated on an FTO substrate and its electrochromic (EC) behavior was studied and the results are reported and discussed in this paper. Ó 2007 Elsevier B.V. All rights reserved. PACS: 71.20.Nr; 72.10.d; 73.21.Ac; 73.50.h Keywords: Thin films; Spray pyrolysis; FTO; Sol–gel; Electrochromics

1. Introduction SnO2 is most promising material for gas sensors [1], photovoltaic solar energy conversion [2] and electrochromic devices and e-windows [3]. SnO2 behaves like an n-type semiconductor and has a tetragonal structure, similar to the rutile structure with a wide energy gap (Eg = 3.67 eV). The doping of impurities like Zn, F,. . .,etc., affect the SnO2 structure and properties [4,5]. Among various dopants fluorine is preferred because F doped films show a high transparency and good conductivity. The SnO2 films can be prepared by number of deposition techniques such as metal organic chemical vapor deposition, chemical vapor deposition, magnetron sputtering, rf sputtering, spray pyrolysis, sol–gel dip coating, evaporation, pulsed laser deposition and pyrosol method. Among these techniques the spray pyrolysis is well suited for the preparation of pure *

Corresponding author. Tel.: +91 451 2452371; fax: +91 451 245 234466. E-mail address: muralg@rediffmail.com (G. Muralidharan). 1567-1739/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2007.11.010

and doped tin oxide thin films because of its simple and inexpensive experimental arrangement, ease of adding dopants, high growth rate, reproducibility and mass production capability for uniform large area coatings. These are desirable for industrial, solar cell and electrochromic device applications. The FTO films are used as conducting layers in electrochromic devices and they serve also as the passive counter electrode. The electrochromic properties of the devices are characterized by the transparency and resistivity of the conducting layer. Electrochromic materials exhibit a reversible change their optical properties upon charge insertion–extraction induced by an external voltage [6]. This property makes EC materials to be of considerable interest for optical devices of different types, such as elements of information display, light shutter, smart window and variable reflectance mirrors [7–9]. Among various electrochromic materials, nickel oxide is an interesting material owing to its potential use as a battery electrode, high EC efficiency, cyclic reversibility, durability and large dynamic range [10–14]. Nickel oxide is an anodic electrochromic

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material, which is able to change the colour from transparent to a deep brown on the application of an electric potential. The optical transmission and optoelectronic properties have been reported in our earlier paper [15]. In the present work, highly transparent and conducting tin oxide and fluorine doped tin oxide thin films have been prepared from SnCl2  2H2O precursor using spray pyrolysis technique. Their structural and electrical properties have been investigated. A nickel oxide film was coated on an FTO substrate using dip coating method and electrochromic properties of the NiO were studied and are reported in this paper. 2. Experimental details

3. Result and discussion 3.1. Structural properties The crystal structure of the FTO thin films was determined by X-ray diffraction technique. The XRD spectra of the films for different F concentration are presented in Fig. 1. The films exhibit a preferred orientation along (2 0 0) for both doped and undoped SnO2 films. This pattern confirms the formation of SnO2 films in tetragonal

Fig. 1. XRD patterns for (a) 0, (b) 7.5, and (c) 10 m% of F doped SnO2 films.

rutile structure. Fukano and Motohiro [16] reported the films deposited by spray pyrolysis at 325–340 °C to exhibit (2 0 0) orientation. Ramaiah and Raja [17] reported SnO2 films to be formed with (2 0 0) as the preferred orientation. In the present work it is observed that the peak intensity decreases for 10 m% of NH4F indicative of partial amorphous state of the film. The grain size of the film was calculated using Debye–Scherrer formula is in the range of 25–33 nm. The FTIR spectrum for 7.5 m% of F doped film is shown in Fig. 2. The bands appear at 437, 536, 586, and 642 cm1 belong to vibration of SnO2. Banerjee et al. [18] reported the absorption peaks between 400 and 700 cm1, attributable to

100 90

Transmittance (%)

The substrate was placed on the substrate holder inside a tubular furnace. Its temperature was monitored and controlled (to an accuracy of ±1 °C) by a PID temperature controller that uses a Cr–Al thermocouple as the sensing element. Before spraying the material, the substrate was kept at 550 °C. A spray solution containing stannous chloride, methanol and HCl was used. For fluorine doping, NH4F dissolved in double distilled water was added to the starting solution. In the present work F-doping was done to get F concentration up to 10 m%. The substrate temperature of 550 °C, substrate to nozzle distance of 25 cm, flow rate of 8 ml/min and air pressure of 1.5 kg cm2 were maintained for the preparation of the thin films. The NiO film was coated using sol–gel dip coating method at a withdrawal speed of 15 cm/min using Ni(Ac)2  4H2O as a precursor and 2-methoxyethanol as a solvent (0.15 m solution was made). The X-ray diffraction patterns of the films were ˚ ). FTIR specrecorded using Cu Ka radiation (1.540560 A trum was recorded using JASCO 460 PLUS spectrophotometer. The transmission spectra of films were recorded from 190 to 1100 nm using a Perkin–Elmer k-35 UV–vis spectrometer. Hall measurements were carried out using electromagnet capable of producing 20KG at a pole separation of 1 cm. The SEM was recorded using a JEOLJSM-5610LV. The film was subjected to electrochemical ion insertion/extraction in a three electrode cell with Pt as the counter electrode, Ag/AgCl as the reference electrode, 0.1 M KOH as the electrolyte at a scan speed of 25 mV s1. TEXAS instruments CHI 6438 was used to record the cyclicvoltammetry curves.

80 70 60 50 40 30 20 400

500

600

700

800

900

1000

1100

1200

Wavenumber (cm-1)

Fig. 2. FTIR spectrum for 7.5 m% of F doped SnO2 film coated on Si wafer.

K.K. Purushothaman et al. / Current Applied Physics 9 (2009) 67–72

Sn–O and Sn–O–Sn vibrations of SnO2. Kersen and Sundberg [19] reported a band at 625 cm1 corresponding to SnO2 vibrations. The film surfaces were investigated by SEM. From the SEM images the surface is found to be smooth and uniform. The morphology is presented in Fig. 3. The F doped films seem to indicate that the film is made of almost uniform crystallites. 3.2. Electrical properties The free carrier density N and their mobility lH have been measured using Hall measurements. The Hall coefficient RH, carrier density N and mobility lH were calculated using Eq. (1)–(3), respectively.

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trical resistivity. However, at higher concentration of F in the solution, the resistivity increases due to the cancellation of the effect of oxygen vacancies by the substitution of fluorine atoms and/or the accumulation of fluorine atoms at the grain boundaries forming Sn–F bonds reported by Martinez and Acosta [21]. Acosta et al. [24] reported the resistivity decrease up to 8 wt% of fluorine doping. They also reported a five fold increase in resistivity beyond 8 wt% of fluorine. Our results well agree with the previous reports Table 1 with transmission greater then 82%, IR reflectance of 93.57% and with a figure of merit value of

120

25

ð1Þ ð2Þ

lH ¼ RH =q:

ð3Þ

Fig. 4 gives the variation of Hall mobility (lH), carrier density (N) and resistivity (q) of SnO2:F films with various F concentration. It is observed that the value of lH (21.86 cm2 V1 s1) and N (18.7  1019 cm3) are maximum and q (15  104 X cm) is minimum for 7.5 m% of F doping film indicating this to be the optimum concentration for possible applications. Hall measurements indicate the FTO films to be of n-type semiconductors. Our results are compared with the previous reports and the data are presented in Table 1. The increase in carrier density and Hall mobility with increase in fluorine doping can be explained to be due to the enhanced crystallinity of the films. A decrease of the same for 10 m% of F doped film may be ascribed to the lesser crystallinity of the films as indicated by the X-ray diffraction pattern (Fig. 1). The enhanced crystallinity of the films helps to reduce the loss of carriers due to scattering at the grain boundaries as reported by Vasu and Subrahmanyam [23]. As a consequence of an increase in carrier density and Hall mobility, the resistivity of these films decreases up to 7.5 m% of F doping and increases beyond that doping level. This can be attributed to fluorine ions occupying the oxygen sites in the SnO2 lattice creating free electrons giving rise to a decrease in elec-

Hall mobility 20

Carrier density/cm3 Mobility cm2/ V S

RH ¼ ðt=BZ ÞðV H =I x Þ N ¼ 1=RH e

100

Resistivity 80

15 60 10 40 5

Resistivity x 10-4 Ω cm

Carrier density

20

0 0.00

2.50

5.00

7.50

0 10.00

Fm %

Fig. 4. Variation in carrier density N, Hall mobility lH, and resistivity q for different fluorine doping in SnO2 film.

Table 1 Table of resistivity, mobility and carrier density of FTO as reported by other authors and in the present study q  104 X cm

l cm2 V1 s1

N/cm3

Reference

5.7 – 5.43 4 0.189 78 6 15

30 16 7.38 2.2 30.71 6.8 – 21.86

37 30 15.58 9 10.77 11.7 – 18.7

Fukano and Motohiro [16] Ramaiah and Raja [17] Moholkar et al. [20] Martinez and Acosta [21] Mwamburi [22] Vasu and Subramanian [23] Acosta et al [24] Present study

Fig. 3. SEM images for (a) 0 and (b) 7.5 m% of F doped film.

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0.025 (X/h)1 at 550 nm (for 7.5 m% of F doped films) [15] so this is suitable for the applications like conducting layer in electrochromic devices and IR reflecting windows. 3.3. Mean free path and Fermi energy The Fermi energy EF of the FTO films can be calculated using the relation, EF ¼ ðh2 =8m Þð3N =pÞ2=3 ;

ð4Þ

where m* = 0.17m0. The values are reported in the Table 2 and kBT = 0.026 eV, where kB is the Boltzman constant and T absolute temperature. This calculation shows the EF>>kT which indicates the FTO films to be had as degenerate semiconductors. Savarimuthu et al. [25] have developed In2O3 films via spin coating route. They reported the In2O3 films to act as degenerate semiconductors. The Fermi energy increases up to 7.5 m% of F doping and decreases beyond the same concentration. This is in good agreement with resistivity, Hall mobility and carrier density results. The mean free path of the free carrier can be estimated using the relation (5), 1=3

l ¼ ðh=2eÞð3N =pÞ

devices. An electrochromic material is able to change its optical properties when the film is subjected to an external electric field. Multilayered (six layered) NiO films were coated on a FTO substrate and its XRD spectrum Fig. 5 reveals the NiO film to be in the amorphous state. The electrochromic properties are studied using cyclicvoltammetry (CV) technique. Besides electrical and mass transport phenomena, during the CV experiments, optical changes such as colouration and decolouration were observed in NiO films at different stages of the processes. The film was cycled from 0.25 to 0.7 V at a potential sweep rate of 25 mV s1 in 0.1 M KOH solution. At the anodic potential of 0.7 V the film appears to be coloured (brown) and towards cathodic potential of 0.25 V it reaches the bleached state. The colouration and bleaching of the film is associated with the deintercalation/intercalation of OH ions or extraction of H+ ion. The commonly accepted reaction mechanisms for charge extraction/insertion are NiO þ OH $ NiOOH þ e þ

NiðOHÞ2 $ NiOOH þ H þ e

ð6Þ 



ð7Þ

NiðOHÞ2 þ OH $ NiOOH þ H2 O þ e



ð8Þ

ð5Þ

l:

The mean free path for the FTO films is in the range 1.11– 2.54 nm, shown in Table 2, which is considerably shorter than the grain size of 25–33 nm. This means that the grain boundary scattering is not the dominant mechanism in reducing the mobility of the free carriers in these films but there are other scattering mechanisms such as impurity scattering and lattice scattering that affect the electrical conductivity. Vasu and Subrahmanyam [26] in their work on SnO2 films coated using spray pyrolysis in a range of 553–713 K reported the mean free path values of 3.43– 9.19 nm. In the present XRD spectra show that there is an improvement in the crystallinity from 0 to 7.5 m% of F doped films and beyond 7.5 m% of F doped films there is a disorder in the atomic rearrangement of the crystallites. This will tend to increase the Fermi energy and mean free path up to 7.5 m% of F and decreases there after.

Different reactions have been observed for NiO films prepared by different techniques. Nuclear analysis showed that colouration of nickel oxide occurs upon hydrogen extraction [27]. Most of the previous work on nickel oxide support release of proton and electron [28,29]. Anodic and colloidally precipitated hydrated nickel oxide coatings involve hydroxyl reaction [30]. Reaction involving hydroxyl insertion into nickel oxide lattice is suggested for EC effects in sputtered NiO films and NiO films prepared by electrochemical deposition followed by heat treatment [31–33]. The cyclic voltammetry spectrum Fig. 6 exhibits an anodic peak at 0.55 V and a cathodic peak at 0.45 V. The peaks associated with each cycles correspond to the oxidation and reduction process during the electrochemical experiment. The anodic peak current density is 7.48  105 A cm2 and cathodic peak current density is

3.4. Application of FTO films 150

Table 2 Fermi energy and mean free path values for 0–10 m% of F doped films Fluorine (m%)

Fermi energy (eV)

Mean free path (nm)

0 2.5 5 7.5 10

0.133 0.147 0.170 0.198 0.121

1.12 1.75 1.93 2.55 0.54

125

Intensity (arb.units)

3.4.1. Electrochromic properties The FTO films are used as a conducting layer and as a passive counter electrode as well in the electrochromic

100 75 50 25 0 10

20

30

40

50

60

70

2θ

Fig. 5. XRD pattern for NiO film coated by sol–gel dip coating method.

K.K. Purushothaman et al. / Current Applied Physics 9 (2009) 67–72

at a spectral of 550 nm.The photopic contrast ratio (Tbleached/Tcoloured) is 1.90.

2.00E -04

Current (A/cm2)

1.50E -04

4. Conclusion

1.00E -04 5.00E -05 0.00E +00 -5.00E -05 -1.00E -04 -0.4

-0.2

0

0.2

0.4

0.6

0.8

Potential (V)

Fig. 6. Cyclic voltammetric curve for 325 nm thick NiO film coated on FTO substrate.

4.40  105 A cm2. The diffusion coefficient is calculated using the formula ð9Þ

Here n is the number of electrons and Co is the number of active ions involved in the reaction, v the scanning speed, ip the anodic/cathodic peak current and D the diffusion coefficient. The D value is calculated using the relation (9) is 3.03  1013 cm2 s1 for anodic and 1.05  1013 cm2 s1 for cathodic peaks. Korosec and Bukovec [34] reported oxidation and reduction peaks to appear at 0.57 and 0.39 V respectively for sol–gel dip coated films with a change in transmittance of 39%. Nakoka et al. [35] reported anodic peak appearing at 0.62 V corresponding to a cathodic peak at 0.25 V for electrochemically deposited nickel oxide film. A home made electrochromic response cell with 1 M NaOH as a electrolyte and graphite bar as a counter electrode was used for colouration/bleaching process by applying ±2 V. The transmittance spectrum for the coloured and bleached state is shown in Fig. 7. The change in optical density DOD630 nm = log(Tbleached)  log(Tcoloured) = 0.28. The percentage change in transmittance was found to be 38% 100 90

Transmission (%)

80 70 a

50 40 30 20

b

10 0 300

400

500

600

700

800

Transparent conducting films of fluorine doped tin oxide were deposited using a spray pyrolysis method. The nano structured FTO films are prepared at 550 °C with good transparency. The fluorine doping shows an increase in Hall mobility, carrier density and decrease in resistivity up to 7.5 m% of F. NiO film was coated on 7.5 m% of F doped SnO2 film and its electrochromic properties was studied. The observed change in optical density is DOD = 0.28 (at 550 nm). The above studies suggest 7.5 m% of F to be the optimum concentration for developing FTO films for electrochromic and applications requiring a transparent conducting layer. References

ip ¼ 2:72  105  n2=3  D1=2  C o  v1=2 :

60

71

900

1000

1100

Wavelength (nm)

Fig. 7. Transmittance spectra for (a) bleached and (b) coloured state of the FTO/NiO film.

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