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Pulsed Laser-Deposited nickel oxide thin films as electrochromic anodic materials
Applied Surface Science 186 (2002) 490±495
Pulsed Laser-Deposited nickel oxide thin ®lms as electrochromic anodic materials I. Bouessay*, A. Rougier, B. Beaudoin, J.B. Leriche Laboratoire de ReÂactivite et Chimie des Solides, UMR 6007, Universite de Picardie Jules Verne, 80039 Amiens Cedex, France
Abstract Nickel oxide thin ®lms were deposited on SnO2:F-coated glass using pulsed laser deposition. The in¯uence of the oxygen pressure and the substrate temperature on the structure, morphology and electrochromic performances of NiOx thin ®lms were investigated. Whatever the oxygen pressure PO2 10 1 mbar and substrate temperature RT Ts 300 C NiOx ®lms were crystallized and exhibited a non-stoichiometry x < 1, which increased (i.e. x decreased) with decreasing oxygen pressure. At an optimum oxygen pressure of 10 1 mbar and low substrate temperature, the reversible color/bleaching process was mainly associated to the Ni3/Ni2 redox process. The appearance of a broad signal in the cyclic voltammograms (CVs) of ®lms deposited at higher substrate temperature suggested the existence of surface reactions in agreement with ®lm densi®cation. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Nickel oxide; Pulsed laser deposition; Electrochromic
1. Introduction Electrochromic devices are able to change optical properties (transmittance and/or re¯ectance) in a reversible way under the action of applied electrical potential through charge (ions/electrons) insertion/ extraction [1]. In recent years, transition metal oxides have been largely studied as regards to their potential application as electrochromic materials in displays, smart windows, variable re¯ectance mirror [2]. In particular, nickel oxide-based thin ®lms have attracted special interest for suitable using as a counter-electrode in electrochromic devices for their neutral coloration as well as their moderate cost [1]. NiOx thin ®lms have been prepared by several methods: spray pyrolysis [3], anodic deposition [4], *
Corresponding author. Tel.: 33-3-22-82-76-04; fax: 33-3-22-82-75-90. E-mail address: [email protected] (I. Bouessay).
sputtering [5], electrolysis chemical deposition [6], reactive-sputtering [7], sol±gel process [8]. The electrochromic performances largely depend on the synthesis method and conditions of deposition in relation with modi®ed ®lm structure, stoichiometry and morphology. Despite a large number of studies, electrochromic mechanism is still not yet understood. However, it is believed that this phenomenon is related to the redox process Ni3/Ni2 while an extensive discussion remains regarding the type of ion species (H/OH ) transported upon coloration/decoloration and the ®lm composition. To throw some light on the origin of the electrochromic mechanism, we focused our work on the electrochemical characterization of NiOx thin ®lms deposited using pulsed laser deposition (PLD) technique. This technique is well appropriate for small substrates and its rapid growth suitable for fast electrochemical testing. In this paper, the in¯uence of the substrate temperature and oxygen pressure on the
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 7 5 5 - 3
I. Bouessay et al. / Applied Surface Science 186 (2002) 490±495
structure, morphology and electrochromic performances is discussed. 2. Experimental The NiO target, with a density of 72%, was made from cubic commercial NiO. Films were prepared by PLD using a KrF excimer laser beam (Lambda Physik Compex 102, l 248 nm) with a laser ¯uence of 1± 2 J cm 2. The substrates consisted of a 1 2 cm2 SnO2:F (FTO) coated glass substrates pasted with silver paste to ensure a good thermal contact. The base pressure in the chamber was in the order of 3 10 5 mbar. The depositions were performed in oxygen atmosphere from 10 3 to 10 1 mbar range, and in vacuum (of the order of the base pressure). The substrate temperature varied from room temperature (RT) to 300 8C. The duration of deposition lasted from 30 min to 1 h with a repetition rate of 5 Hz (0.17± Ê s 1). The thickness, determined by pro®lome0.39 A try using a Dektak instrument, was estimated in the 150±350 nm range. The target±substrate distance was between 4 to 5 cm so that the substrate was set at the plume extremity. The structure of the ®lms was examined by X-ray diffraction (XRD) using a Scintag diffractometer (y± 2y con®guration and l Cu Ka 1:5418 Ð). The morphology of the ®lms was investigated by scanning electron microscopy (SEM) with a Philips XL 30 ®eld emission gun (FEG) microscope. The optical transmission measurements were obtained using a Varian double beam, UV±Vis±NIR spectrophotometer CARY-SE, between 250 to 2000 nm. The electrochemical properties of the ®lms have been characterized by cyclic voltammetry performed on an Autolab PGSTAT 30 system. An aqueous potassium hydroxide solution (KOH, 0.1 M) was used as liquid electrolyte, a platinum wire as counter-electrode and Hg/HgO as reference electrode.
491
evolves from slightly beige to brownish with decreasing oxygen pressure. 3.1. Structural characterization 3.1.1. SEM analysis SEM micrograph of NiOx thin ®lm deposited at RT on an FTO substrate in 10 1 mbar oxygen pressure is shown in Fig. 1. Films are formed of agglomerates of 50±150 nm with visible porosity. As the substrate temperature increases and pressure decreases, the average size of the agglomerates increases leading to denser ®lms, which however exhibit visible cracks.
3. Results and discussion
3.1.2. XRD Whatever the substrate temperature, as-deposited NiOx PLD thin ®lms are crystallized. Similar conclusion was obtained for ®lms deposited on an amorphous glass substrate. The diffraction pattern of each ®lm was characterized by three peaks whose width and height decrease and increase respectively with increasing substrate temperature. The peaks located at 2y 37 , 438 and 638 were indexed as (1 1 1), (2 0 0) and (2 2 0), respectively, using a cubic NiO structure (S.G.: F m-3m). The values of the lattice parameter acub. deduced from the position of the X-ray peaks are reported in Table 1. The lattice parameter acub. increases from 4.18 to Ê as the oxygen pressure decreases from 4.24 A 10 1 mbar to vacuum, whereas it remains stable with temperature indicating a higher sensibility of the ®lm stoichiometry to oxygen atmosphere. Indeed, the ®lm stoichiometry can be roughly estimated from the acub. Ê [9], for commercial bulk value which is of 4.1762 A NiO. The slightly higher acub. values, observed for NiOx thin ®lms, suggest the presence of oxygen de®ciency x < 1 correlated to a lower oxidation state of nickel ions (<2). One should note a strong variation of acub. in the 10 2 mbar region that will be further related to a change in morphology and a decrease in electrochromic performances as the oxygen pressure decreases.
Whatever the conditions of deposition (substrate temperature and oxygen pressure), the PLD NiOx thin ®lms appear homogeneous in coloration. Except for the ®lms deposited in vacuum which present a metallic aspect, the ®lms are transparent with a coloration that
3.1.3. Transmission electron microscopy (TEM) The TEM picture of NiOx thin ®lm deposited at RT in 10 1 mbar oxygen pressure is shown in Fig. 2a with corresponding selected area electron diffraction (SAED) pattern in Fig. 2b.
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I. Bouessay et al. / Applied Surface Science 186 (2002) 490±495
Fig. 1. SEM micrograph of NiOx thin ®lm prepared at RT in 10
The ®lms exhibit platelets with ®brous texture. The SAED pattern consists of three diffuse broad rings typical of a poorly crystallized sample. An increase in substrate temperature leads to thinner rings with speckles in relation with an increase in crystallinity conÊ ®rmed by XRD. The d values of 2.35, 2.08 and 1.48 A observed from the SAED pattern correspond to the inter-reticular distances of cubic NiO (S.G.: F m-3m). 3.2. Electrochromic properties For PLD NiOx thin ®lms, the electrochemical capacity decreases with decreasing oxygen pressure
1
mbar oxygen pressure.
leading to a degradation of the electrochromic performances. A similar phenomenon was observed for PLD WO3 thin ®lms [10], for which good electrochromic properties were achieved for ®lms deposited in 10 1 mbar oxygen pressure. Therefore, in the following, attention will be focused on the electro-optical characterization of NiOx ®lms deposited in 10 1 mbar oxygen pressure. 3.2.1. Films deposited at RT The ®rst 150 cycles of Pt/Hg/HgO/KOH 0.1 M/ NiOx cells prepared with NiOx ®lms deposited at RT using an oxygen pressure of 10 1 mbar are shown in Fig. 3.
Table 1 Ê ) of NiOx ®lms calculated from the X-ray diffractogram Values of the lattice parameter acub. (A Ts (8C)
PO2 (mbar) 10
RT 100 200 300
1
4.18 4.18 4.18 4.18
5 10 (2) (1) (1) (1)
4.19 4.19 4.18 4.18
(1) (1) (1) (2)
2
10
2
4.23 4.23 4.24 4.23
5 10 (1) (1) (1) (1)
4.23 4.23 4.23 4.22
(1) (1) (1) (2)
3
10
3
4.23 4.24 4.24 4.22
5 10 (1) (2) (1) (1)
4.24 4.22 4.21 4.21
(1) (1) (1) (1)
5
(vacuum)
I. Bouessay et al. / Applied Surface Science 186 (2002) 490±495
493
Fig. 2. (a) TEM picture and (b) corresponding SAED pattern of NiOx thin ®lm deposited at RT in 10 1 mbar oxygen pressure. The interreticular distance d 2d sin y nl corresponding to each diffraction ring can be obtained by the following relation: K Ll Rd, where R is the distance measuring from the center to the diffraction ring, L the diffraction length and l the electronic wavelength. In our microscope, the constant K is 18.5 cm2 (this value is adjusted for each experiment).
Cyclic voltammograms (CVs) are characterized by a set of reversible peaks, located at 0.6 V in oxidation, prior to currents due to oxygen evolution, and at 0.48 V in reduction. Upon early cycling, both peak intensities (i.e. capacities) continuously increase suggesting a modi®cation of the ®lm nature or morphology. For larger number of cycles, the anodic peak slightly shifts to higher potential whereas the position of the cathodic peak remains constant. An excellent
stability was observed up to hundreds of cycles. In the steady state, CVs are comparable to the ones obtained for nickel hydroxide electrode in Ni//Cd batteries. Therefore, one may assume that cycling in KOH leads to an hydroxylation of the ®lms and that the faradic process associated to the nickel oxidation/reduction involves the intercalation/deintercalation of protons. The redox process Ni3/Ni2 is associated to reversible change in coloration from transparent to brown-
494
I. Bouessay et al. / Applied Surface Science 186 (2002) 490±495
Fig. 3. The ®rst 150 CVs of Pt/Hg/HgO/KOH 0.1 M/NiOx cells prepared with NiOx ®lm deposited at RT in 10 1 mbar oxygen pressure. After checking that no capacity was present below 0.2 V, the cycling was performed in the 0.2±0.67 V range at a sweep rate of 10 mV s 1.
ish as illustrated in Fig. 4 by the variation of the optical transmittance. A total contrast ratio Tb/Tc (transmission in the bleached state/transmission in the colored state) of 1.25 is obtained at 550 nm associated with a coloration ef®ciency (CE) of 15 cm2 C 1; (CE is expressed as the ratio between the contrast in the
Fig. 4. Optical transmittance spectra of NiOx thin ®lm deposited at RT in 10 1 mbar oxygen pressure. The transmittance in the colored state was recorded using an oxidizing potential of 0.6 V and in the bleached state a reducing potential of 0.1 V.
bleached and colored state and the charge density (Q), CE 1=Q log Tb =Tc and is conventionally negative for anodic material). 3.2.2. In¯uence of the substrate temperature The evolution of CVs with substrate temperature is presented in Fig. 5. To ease comparison, the current
Fig. 5. In¯uence of substrate temperatures on CVs of Pt/Hg/HgO/KOH 0.1 M/NiOx cells prepared with NiOx ®lm deposited at various substrate temperature in 10 1 mbar oxygen pressure.
I. Bouessay et al. / Applied Surface Science 186 (2002) 490±495
density of CVs at 200 and 300 8C were multiplied by 3.2 and 3.5, respectively. A similar capacity is obtained for the ®lms deposited at RT and 100 8C, whereas the capacity decreases with higher substrate temperature. The appearance at high temperature of a new set of broad peaks in the 0.25±0.45 V region, which may be associated to a pseudo-capacitive behavior, indicates that surface reactions are now being involved. The decrease in capacity may be related to denser ®lms, whereas the electrolyte oxidation is shifted to lower potentials for high substrate temperature underlining a catalytic effect versus oxygen evolution of the ®lms implying a softer oxidation of the ®lms. 4. Conclusion NiOx thin ®lms were prepared in various oxygen atmosphere and substrate temperature using PLD. Films deposited at 10 1 mbar oxygen pressure, which were closest to stoichiometric NiO and exhibited a granular porous structure, showed the best electrochromic performance that remained however ``unsatisfying'' with a contrast of 1.25 and a coloration ef®ciency of 15 cm2 C 1. In the steady state, reached after a period of activation over 50±150 cycles, electrochromic mechanism was associated to a redox process similar as the one found for Ni(OH)2. With increasing substrate temperature, the presence of broad current peaks suggested that new surface reactions occurred. Electrochemical tests are now being
495
performed in various potential windows in order to separate and identify the origin of each reaction being involved in the bulk or at the surface. Acknowledgements The authors wish to thank J.M. Tarascon and C. Marcel for fruitful discussion and ``le conseil ReÂgional de Picardie'' and ``Le Fonds Social EuropeÂen'' for their ®nancial support. References [1] C.G. Granqvist, Handbook of Inorganic Electrochromics Materials, Elsevier, Amsterdam, 1995. [2] P.M.S. Monk, R.J. Mortimer, D.R. Rosseinsky, Electrochromism fundamental and Applications, VCH, New York, 1995. [3] M. Gomez, A. Medina, W. Estrada, Solar Energy Mater. Solar Cells 64 (2000) 297. [4] M. Chigane, M. Ishikawa, H. Inoue, Solar Energy Mater. Solar Cells 64 (2000) 65. [5] A. Azens, L. Kullman, G. Vaivars, H. Nordborg, C.G. Granqvist, Solid State Ionics 113±115 (1998) 449. [6] M. Chigane, M. Ishikawa, J. Chem. Soc. 94 (1998) 3665. [7] X.G. Wang, Y.S. Jang, N.H. Yang, Y.M. Wang, L. Yuan, S.J. Pang, Solar Energy Mater. Solar Cells 63 (2000) 197. [8] P.K. Sharma, M.C.A. Fantini, A. Gorenstein, Solid State Ionics 113±115 (1998) 457. [9] JCPDS, International Center for Diffraction Data No. 750269. [10] A. Rougier, F. Portemer, A. QueÂdeÂ, M. El Marssi, Appl. Surf. Sci. 1±9 (1999) 153.
Pulsed Laser-Deposited nickel oxide thin ®lms as electrochromic anodic materials I. Bouessay*, A. Rougier, B. Beaudoin, J.B. Leriche Laboratoire de ReÂactivite et Chimie des Solides, UMR 6007, Universite de Picardie Jules Verne, 80039 Amiens Cedex, France
Abstract Nickel oxide thin ®lms were deposited on SnO2:F-coated glass using pulsed laser deposition. The in¯uence of the oxygen pressure and the substrate temperature on the structure, morphology and electrochromic performances of NiOx thin ®lms were investigated. Whatever the oxygen pressure PO2 10 1 mbar and substrate temperature RT Ts 300 C NiOx ®lms were crystallized and exhibited a non-stoichiometry x < 1, which increased (i.e. x decreased) with decreasing oxygen pressure. At an optimum oxygen pressure of 10 1 mbar and low substrate temperature, the reversible color/bleaching process was mainly associated to the Ni3/Ni2 redox process. The appearance of a broad signal in the cyclic voltammograms (CVs) of ®lms deposited at higher substrate temperature suggested the existence of surface reactions in agreement with ®lm densi®cation. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Nickel oxide; Pulsed laser deposition; Electrochromic
1. Introduction Electrochromic devices are able to change optical properties (transmittance and/or re¯ectance) in a reversible way under the action of applied electrical potential through charge (ions/electrons) insertion/ extraction [1]. In recent years, transition metal oxides have been largely studied as regards to their potential application as electrochromic materials in displays, smart windows, variable re¯ectance mirror [2]. In particular, nickel oxide-based thin ®lms have attracted special interest for suitable using as a counter-electrode in electrochromic devices for their neutral coloration as well as their moderate cost [1]. NiOx thin ®lms have been prepared by several methods: spray pyrolysis [3], anodic deposition [4], *
Corresponding author. Tel.: 33-3-22-82-76-04; fax: 33-3-22-82-75-90. E-mail address: [email protected] (I. Bouessay).
sputtering [5], electrolysis chemical deposition [6], reactive-sputtering [7], sol±gel process [8]. The electrochromic performances largely depend on the synthesis method and conditions of deposition in relation with modi®ed ®lm structure, stoichiometry and morphology. Despite a large number of studies, electrochromic mechanism is still not yet understood. However, it is believed that this phenomenon is related to the redox process Ni3/Ni2 while an extensive discussion remains regarding the type of ion species (H/OH ) transported upon coloration/decoloration and the ®lm composition. To throw some light on the origin of the electrochromic mechanism, we focused our work on the electrochemical characterization of NiOx thin ®lms deposited using pulsed laser deposition (PLD) technique. This technique is well appropriate for small substrates and its rapid growth suitable for fast electrochemical testing. In this paper, the in¯uence of the substrate temperature and oxygen pressure on the
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 7 5 5 - 3
I. Bouessay et al. / Applied Surface Science 186 (2002) 490±495
structure, morphology and electrochromic performances is discussed. 2. Experimental The NiO target, with a density of 72%, was made from cubic commercial NiO. Films were prepared by PLD using a KrF excimer laser beam (Lambda Physik Compex 102, l 248 nm) with a laser ¯uence of 1± 2 J cm 2. The substrates consisted of a 1 2 cm2 SnO2:F (FTO) coated glass substrates pasted with silver paste to ensure a good thermal contact. The base pressure in the chamber was in the order of 3 10 5 mbar. The depositions were performed in oxygen atmosphere from 10 3 to 10 1 mbar range, and in vacuum (of the order of the base pressure). The substrate temperature varied from room temperature (RT) to 300 8C. The duration of deposition lasted from 30 min to 1 h with a repetition rate of 5 Hz (0.17± Ê s 1). The thickness, determined by pro®lome0.39 A try using a Dektak instrument, was estimated in the 150±350 nm range. The target±substrate distance was between 4 to 5 cm so that the substrate was set at the plume extremity. The structure of the ®lms was examined by X-ray diffraction (XRD) using a Scintag diffractometer (y± 2y con®guration and l Cu Ka 1:5418 Ð). The morphology of the ®lms was investigated by scanning electron microscopy (SEM) with a Philips XL 30 ®eld emission gun (FEG) microscope. The optical transmission measurements were obtained using a Varian double beam, UV±Vis±NIR spectrophotometer CARY-SE, between 250 to 2000 nm. The electrochemical properties of the ®lms have been characterized by cyclic voltammetry performed on an Autolab PGSTAT 30 system. An aqueous potassium hydroxide solution (KOH, 0.1 M) was used as liquid electrolyte, a platinum wire as counter-electrode and Hg/HgO as reference electrode.
491
evolves from slightly beige to brownish with decreasing oxygen pressure. 3.1. Structural characterization 3.1.1. SEM analysis SEM micrograph of NiOx thin ®lm deposited at RT on an FTO substrate in 10 1 mbar oxygen pressure is shown in Fig. 1. Films are formed of agglomerates of 50±150 nm with visible porosity. As the substrate temperature increases and pressure decreases, the average size of the agglomerates increases leading to denser ®lms, which however exhibit visible cracks.
3. Results and discussion
3.1.2. XRD Whatever the substrate temperature, as-deposited NiOx PLD thin ®lms are crystallized. Similar conclusion was obtained for ®lms deposited on an amorphous glass substrate. The diffraction pattern of each ®lm was characterized by three peaks whose width and height decrease and increase respectively with increasing substrate temperature. The peaks located at 2y 37 , 438 and 638 were indexed as (1 1 1), (2 0 0) and (2 2 0), respectively, using a cubic NiO structure (S.G.: F m-3m). The values of the lattice parameter acub. deduced from the position of the X-ray peaks are reported in Table 1. The lattice parameter acub. increases from 4.18 to Ê as the oxygen pressure decreases from 4.24 A 10 1 mbar to vacuum, whereas it remains stable with temperature indicating a higher sensibility of the ®lm stoichiometry to oxygen atmosphere. Indeed, the ®lm stoichiometry can be roughly estimated from the acub. Ê [9], for commercial bulk value which is of 4.1762 A NiO. The slightly higher acub. values, observed for NiOx thin ®lms, suggest the presence of oxygen de®ciency x < 1 correlated to a lower oxidation state of nickel ions (<2). One should note a strong variation of acub. in the 10 2 mbar region that will be further related to a change in morphology and a decrease in electrochromic performances as the oxygen pressure decreases.
Whatever the conditions of deposition (substrate temperature and oxygen pressure), the PLD NiOx thin ®lms appear homogeneous in coloration. Except for the ®lms deposited in vacuum which present a metallic aspect, the ®lms are transparent with a coloration that
3.1.3. Transmission electron microscopy (TEM) The TEM picture of NiOx thin ®lm deposited at RT in 10 1 mbar oxygen pressure is shown in Fig. 2a with corresponding selected area electron diffraction (SAED) pattern in Fig. 2b.
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I. Bouessay et al. / Applied Surface Science 186 (2002) 490±495
Fig. 1. SEM micrograph of NiOx thin ®lm prepared at RT in 10
The ®lms exhibit platelets with ®brous texture. The SAED pattern consists of three diffuse broad rings typical of a poorly crystallized sample. An increase in substrate temperature leads to thinner rings with speckles in relation with an increase in crystallinity conÊ ®rmed by XRD. The d values of 2.35, 2.08 and 1.48 A observed from the SAED pattern correspond to the inter-reticular distances of cubic NiO (S.G.: F m-3m). 3.2. Electrochromic properties For PLD NiOx thin ®lms, the electrochemical capacity decreases with decreasing oxygen pressure
1
mbar oxygen pressure.
leading to a degradation of the electrochromic performances. A similar phenomenon was observed for PLD WO3 thin ®lms [10], for which good electrochromic properties were achieved for ®lms deposited in 10 1 mbar oxygen pressure. Therefore, in the following, attention will be focused on the electro-optical characterization of NiOx ®lms deposited in 10 1 mbar oxygen pressure. 3.2.1. Films deposited at RT The ®rst 150 cycles of Pt/Hg/HgO/KOH 0.1 M/ NiOx cells prepared with NiOx ®lms deposited at RT using an oxygen pressure of 10 1 mbar are shown in Fig. 3.
Table 1 Ê ) of NiOx ®lms calculated from the X-ray diffractogram Values of the lattice parameter acub. (A Ts (8C)
PO2 (mbar) 10
RT 100 200 300
1
4.18 4.18 4.18 4.18
5 10 (2) (1) (1) (1)
4.19 4.19 4.18 4.18
(1) (1) (1) (2)
2
10
2
4.23 4.23 4.24 4.23
5 10 (1) (1) (1) (1)
4.23 4.23 4.23 4.22
(1) (1) (1) (2)
3
10
3
4.23 4.24 4.24 4.22
5 10 (1) (2) (1) (1)
4.24 4.22 4.21 4.21
(1) (1) (1) (1)
5
(vacuum)
I. Bouessay et al. / Applied Surface Science 186 (2002) 490±495
493
Fig. 2. (a) TEM picture and (b) corresponding SAED pattern of NiOx thin ®lm deposited at RT in 10 1 mbar oxygen pressure. The interreticular distance d 2d sin y nl corresponding to each diffraction ring can be obtained by the following relation: K Ll Rd, where R is the distance measuring from the center to the diffraction ring, L the diffraction length and l the electronic wavelength. In our microscope, the constant K is 18.5 cm2 (this value is adjusted for each experiment).
Cyclic voltammograms (CVs) are characterized by a set of reversible peaks, located at 0.6 V in oxidation, prior to currents due to oxygen evolution, and at 0.48 V in reduction. Upon early cycling, both peak intensities (i.e. capacities) continuously increase suggesting a modi®cation of the ®lm nature or morphology. For larger number of cycles, the anodic peak slightly shifts to higher potential whereas the position of the cathodic peak remains constant. An excellent
stability was observed up to hundreds of cycles. In the steady state, CVs are comparable to the ones obtained for nickel hydroxide electrode in Ni//Cd batteries. Therefore, one may assume that cycling in KOH leads to an hydroxylation of the ®lms and that the faradic process associated to the nickel oxidation/reduction involves the intercalation/deintercalation of protons. The redox process Ni3/Ni2 is associated to reversible change in coloration from transparent to brown-
494
I. Bouessay et al. / Applied Surface Science 186 (2002) 490±495
Fig. 3. The ®rst 150 CVs of Pt/Hg/HgO/KOH 0.1 M/NiOx cells prepared with NiOx ®lm deposited at RT in 10 1 mbar oxygen pressure. After checking that no capacity was present below 0.2 V, the cycling was performed in the 0.2±0.67 V range at a sweep rate of 10 mV s 1.
ish as illustrated in Fig. 4 by the variation of the optical transmittance. A total contrast ratio Tb/Tc (transmission in the bleached state/transmission in the colored state) of 1.25 is obtained at 550 nm associated with a coloration ef®ciency (CE) of 15 cm2 C 1; (CE is expressed as the ratio between the contrast in the
Fig. 4. Optical transmittance spectra of NiOx thin ®lm deposited at RT in 10 1 mbar oxygen pressure. The transmittance in the colored state was recorded using an oxidizing potential of 0.6 V and in the bleached state a reducing potential of 0.1 V.
bleached and colored state and the charge density (Q), CE 1=Q log Tb =Tc and is conventionally negative for anodic material). 3.2.2. In¯uence of the substrate temperature The evolution of CVs with substrate temperature is presented in Fig. 5. To ease comparison, the current
Fig. 5. In¯uence of substrate temperatures on CVs of Pt/Hg/HgO/KOH 0.1 M/NiOx cells prepared with NiOx ®lm deposited at various substrate temperature in 10 1 mbar oxygen pressure.
I. Bouessay et al. / Applied Surface Science 186 (2002) 490±495
density of CVs at 200 and 300 8C were multiplied by 3.2 and 3.5, respectively. A similar capacity is obtained for the ®lms deposited at RT and 100 8C, whereas the capacity decreases with higher substrate temperature. The appearance at high temperature of a new set of broad peaks in the 0.25±0.45 V region, which may be associated to a pseudo-capacitive behavior, indicates that surface reactions are now being involved. The decrease in capacity may be related to denser ®lms, whereas the electrolyte oxidation is shifted to lower potentials for high substrate temperature underlining a catalytic effect versus oxygen evolution of the ®lms implying a softer oxidation of the ®lms. 4. Conclusion NiOx thin ®lms were prepared in various oxygen atmosphere and substrate temperature using PLD. Films deposited at 10 1 mbar oxygen pressure, which were closest to stoichiometric NiO and exhibited a granular porous structure, showed the best electrochromic performance that remained however ``unsatisfying'' with a contrast of 1.25 and a coloration ef®ciency of 15 cm2 C 1. In the steady state, reached after a period of activation over 50±150 cycles, electrochromic mechanism was associated to a redox process similar as the one found for Ni(OH)2. With increasing substrate temperature, the presence of broad current peaks suggested that new surface reactions occurred. Electrochemical tests are now being
495
performed in various potential windows in order to separate and identify the origin of each reaction being involved in the bulk or at the surface. Acknowledgements The authors wish to thank J.M. Tarascon and C. Marcel for fruitful discussion and ``le conseil ReÂgional de Picardie'' and ``Le Fonds Social EuropeÂen'' for their ®nancial support. References [1] C.G. Granqvist, Handbook of Inorganic Electrochromics Materials, Elsevier, Amsterdam, 1995. [2] P.M.S. Monk, R.J. Mortimer, D.R. Rosseinsky, Electrochromism fundamental and Applications, VCH, New York, 1995. [3] M. Gomez, A. Medina, W. Estrada, Solar Energy Mater. Solar Cells 64 (2000) 297. [4] M. Chigane, M. Ishikawa, H. Inoue, Solar Energy Mater. Solar Cells 64 (2000) 65. [5] A. Azens, L. Kullman, G. Vaivars, H. Nordborg, C.G. Granqvist, Solid State Ionics 113±115 (1998) 449. [6] M. Chigane, M. Ishikawa, J. Chem. Soc. 94 (1998) 3665. [7] X.G. Wang, Y.S. Jang, N.H. Yang, Y.M. Wang, L. Yuan, S.J. Pang, Solar Energy Mater. Solar Cells 63 (2000) 197. [8] P.K. Sharma, M.C.A. Fantini, A. Gorenstein, Solid State Ionics 113±115 (1998) 457. [9] JCPDS, International Center for Diffraction Data No. 750269. [10] A. Rougier, F. Portemer, A. QueÂdeÂ, M. El Marssi, Appl. Surf. Sci. 1±9 (1999) 153.