Optical and electrochemical properties of Cu-doped NiO films prepared by electrochemical deposition

Applied Surface Science 257 (2011) 3974–3979

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Optical and electrochemical properties of Cu-doped NiO films prepared by electrochemical deposition Lili Zhao, Ge Su ∗ , Wei Liu, Lixin Cao, Jing Wang, Zheng Dong, Meiqin Song Institute of Materials Science and Engineering, Ocean University of China, 238 Songling Road, Qingdao 266100, China

a r t i c l e

i n f o

Article history: Received 6 October 2010 Received in revised form 24 November 2010 Accepted 24 November 2010 Available online 2 December 2010 Keywords: Cu-doped NiO films Cathodic deposition Electrochromic Variation of transmittance

a b s t r a c t Cu-doped nickel oxide (NiO) thin films were prepared by electrochemial deposition (cathodic deposition) technique onto the fluorine doped tin oxide (F: SnO2 ; FTO) coated glass substrates from organic solutions. Effects of Cu content on the morphology, structure, optical and electrochromic properties of NiO films were investigated by means of scanning electron microscope (SEM), X-ray diffraction (XRD), ultraviolet–visible spectrophotometer (UV–vis) and cyclic voltammetry (CV), respectively. SEM images indicated the formation of nanorods after Cu was added. The films were formed with amorphous or short-range ordered NiO grains and a trace of face-centered cubic Nix Cu1−x O confirmed by XRD. The transmittances of both bleached state and colored state were significantly lowered when Cu was added. The NiO films doped with Cu (the molar ratio was 1/8) exhibited the optimum electrochromic behavior with a variation of transmittance (T) up to ∼80% at the wavelength range of 350–600 nm. Cu doping reduces the response time for both the coloring and bleaching states, and the reversibility of the redox reaction was increased as well. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Electrochromic (EC) materials, which can reversibly change their optical properties under an applied electric field, have received a great interest due to their low power consumption, high coloration efficiency (CE), and excellent memory effect under open circuit conditions [1,2]. Among various EC oxides of transition metals, nickel oxide (NiO), as a representative of anode colored materials, is able to change color from transparent in the reduced state to a deep brown black in the oxidized state reversibly when a positive/negative potential is applied [2,3]. Recently, NiO has offered promising candidature for many applications such as lithium ion batteries [4], solar cells, antiferromagentic layer [5], photocatalysts, electrochemical capacitors [6], electrochromic and optical coatings [7]. Furthermore, nickel oxides are being applied in smart windows, reflectance-adjustable car rearview mirrors and high contrast non-emissive information displays due to its excellent chemical stability as well as high electrochromic efficiency, good color neutrality, and fast reaction kinetics, good cyclic reversibility [5,8]. Electrochromism in NiO thin films is rather complicated and there is no single accepted model for the mechanism that controls the coloring/bleaching process [9]. It is generally accepted that the transition from a colored to a bleached state is related to the

∗ Corresponding author. Tel.: +86 532 66781690. E-mail address: [email protected] (G. Su). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.11.160

reversible transformation between Ni(II) and Ni(III) [6]. Recently, aiming at improving the electrochromic properties of NiO films, many researchers begin to investigate doping NiO with metal ions. Avendano et al. doped NiO with a wide variety of additives (Mg, Al, Si, V, Y, Zr, Nb, Ag, or Ta.) to enhance optical properties and durability [10]. Provazi et al. reported that the presence of several percents of additives such as Cd, Co, and Zn in hydrated nickel oxides can strongly improve the electrochemical properties [11]. Ferreira et al. modified the spectral response by doping metallic particles such as Cu, Ag, Au elements and discovered that the dispersion and clustering of metallic particles severely affected the transmittance [12]. The film properties depend on their chemical composition, configuration and structure, which are strongly influenced by deposition techniques such as reactive sputtering [13], thermal decomposition/oxidation [14], sol–gel [15], chemical bath deposition [16], DC magnetron sputtering [9], vacuum evaporation, pulsed laser deposition and sputtering [17], chemical vapor deposition and electrochemical deposition method [8]. Of all the physical and chemical deposition techniques, electrochemical deposition is a quite promising and facile route for industrial applications due to its low-cost, environmental friendly process, and feasibility of room temperature growth on large area [6]. Up to now, Cu–Ni oxide has been studied as catalysts, but research about its electrochromic properties is still rare [18]. In this study, an attempt was made to deposit Cu-doped NiO thin films with excellent electrochromic properties via cathodic deposition technique from N, N-dimethyl formamide (DMF) solutions controlled by potentiostat at room temperature without post-heat

L. Zhao et al. / Applied Surface Science 257 (2011) 3974–3979

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Table 1 Cu content depended Cu/Ni mole ratio in the films, T, OD, EO /ER , iO /iR and response time for coloration/bleaching.

Cu 0 Cu 1 Cu 2 Cu 3

Cu/Ni

T (%)

OD

EO (V)

ER (V)

iO (mA)

iR (mA)

EO –ER (V)

Response time tc /tb (s)

0 0.592 0.689 0.913

72.3 73.3 83.9 50.5

0.721 0.846 1.385 1.239

0.661 0.625 0.613 0.592

−0.262 −0.119 −0.204 −0.249

4.783 4.231 4.241 4.273

−4.894 −4.251 −5.484 −6.464

0.923 0.744 0.817 0.841

5.1/3.0 2.6/2.2 3.4/2.5 4.6/2.8

treatments. Fluorine tin oxide (F: SnO2 , FTO) coated glasses were used as substrates as well as cathodes. The influence of Cu content on the surface morphology, structure, optical and electrochromic properties of the deposited films were investigated in detail. 2. Experimental Cu-doped NiO thin films were prepared on FTO coated glasses by cathodic deposition at room temperature. NiCl2 ·6H2 O (AR) and CuCl2 ·2H2 O (AR) were used as the sources of Ni and Cu, respectively. LiClO4 ·3H2 O (AR) was used as the supporting electrolyte. They were firstly dried in a vacuum oven until the crystallization water was released completely. The plating solutions were prepared by dissolving 0.13 g NiCl2 and 1.064 g LiClO4 in 50 ml distilled N,N-dimethyl formamide (DMF). Different amounts of CuCl2 were added into above solutions with varied Cu contents (notated as Cu 0, Cu 1, Cu 2 and Cu 3 corresponding to Cu/Ni molar concentration ratio 0, 1/12, 1/8 and 1/4, respectively). The solutions were stirred at 50◦ until they were transparent, and then aged for 24 h at room temperature prior to depositing films. The conducting substrates used for the deposition of Cu-doped NiO thin films in this study were fluorine doped tin oxide (FTO) coated glass slices (10 × 30 mm2 in size) with sheet resistance of about 16 /. The substrates were cleaned ultrasonically in acetone, ethanol and then rinsed in high-purity deionized water prior to the films preparation. This treatment was necessary for the formation of uniform and adherent films. A platinum sheet of 10 × 20 mm2 in dimensions was served as the anode. Cu-doped NiO thin films were cathodically deposited at room temperature for 15 min under potentiostatic condition. A dc voltage of 3.5 V was applied during the deposition process. The surface morphology and composition of the Cu-doped NiO thin films were investigated using a cold field emission scanning electron microscope (SEM, JSM-6700F, JEOL Japan) equipped with an energy X-ray microanalysis system (EDX, Oxford INCA). Xray diffraction patterns were recorded by an X-ray diffractometer ˚ (XRD, Bruker D8 ADVANVCE) with Cu K␣ radiation ( = 1.54178 A). Ultraviolet visible spectrophotometer (UV–vis, HitacHi-3010) was employed to characterize the optical absorption and transmission of the films. Electrochemical properties of the films were investigated by cyclic voltammetry (CV) performed on an electrochemical workstation (LK9805Z, China) with a scanning rate of 40 mV/s between −0.8 V and 1.2 V at room temperature. Electrochemical measurements were carried out in 0.5 M KOH with a standard three-electrode configuration consisting of a sample (working electrode), a conventional saturated calomel electrode (SEC) immersed in saturated KCl solution (reference electrode) and a high purity platinum sheet (counter electrode). 3. Results and discussion 3.1. Film synthesis The Cu-doped NiO films were cathodically deposited at 3.5 V from N,N-dimethyl formamide solution consisting of Ni2+ and Cu2+ ions. When the voltage was applied, the two kinds of metallic ions codeposited onto the surface of FTO glasses, the reactions occurred

as follows: Ni2+ + 2e− → Ni;

Cu2+ + 2e− → Cu

The particles of Ni and Cu were so small that they were oxidized into metallic oxides as soon as the ions were reduced. The metallic oxides exhibited electrochromic properties. 3.2. Surface morphology and chemical composition The SEM surface images of Cu-doped NiO thin films with different Cu contents are shown in Fig. 1. The excellent morphological change has been observed due to the different Cu contents in the films. As shown in Fig. 1(a), the homogenous Cu 0 film covers the substrate entirely and exhibits a porous structure in nano-scale. Compared to undoped NiO film, deposits containing Cu as additive produce the structure of nanorods (Fig. 1(b), (c) and (d)). When the Cu content in the film is low (see Fig. 1(b)), only a small number of nanorod structures can be observed, the film is relatively compact. When the content of Cu increases, more nanorods appear in the Cu 2 film (Fig. 1(c)). In Cu 3 film (Fig. 1(d)), an even more uniform surface with large number of nanorods is observed. Some NiO particles aggregate and grow along with the nanorods. From the images it can be inferred that the coelectrodeposition of Ni and Cu oxides generates substances with preferred orientation and the Cu doping reduces the size of NiO particles, thereby, enhances the electrochromic properties of the films. The morphology of the films is largely influenced by the content of additives, while the electrochemical and optical properties strongly depend on the morphology [11]. Table 1 shows the chemical compositions of the films determined by EDX. The atomic percent of Ni increases with the Cu content, indicating that the Cu doping is beneficial to the deposition of Ni. Increasement of the content of Ni oxides is in quite agreement with the formation of nanorods obtained from SEM. 3.3. Structural properties Fig. 2 shows the X-ray diffraction patterns of undoped NiO film (Cu 0) and Cu-doped NiO film (Cu 2). It reveals that both the two samples exhibit six obvious diffraction peaks denoted by star (*), which are assigned to rutile SnO2 originated from the FTO coated glass substrates (JCPDS 46-1088), and the little distortion of lattice is due to the doping of F in SnO2 . The presence of three weak diffraction peaks (denoted by inverted triangle ()) along (1 1 1) and (2 0 0) planes in Cu-doped NiO film can be ascribed to facecentered cubic Nix Cu1−x O (JCPDS 78-0648), which are originated from the nanorods shown in SEM images of Fig. 1. It can be seen that the Cu doping increases the crystallinity of the film, and the amount of nanorods increases with the Cu content in the films. He et al. obtained the similar conclusions in Cux Ni1−x O prepared by sol–gel dip-coating [18]. There are no significant peaks corresponding to nickel oxides in undoped NiO film (Cu 0). But, according to our previous work [19], the chemical composition (state) of nickel oxide is nanocrystalline of NiO with the grain size of 2–7 nm. There were two grain orientations corresponding to the spacing of 0.2341 nm and 0.2075 nm, which are in great agreement with the (1 1 1) and (2 0 0) planes of NiO(JCPDS 44-1159). It implies that the deposited nickel oxides are predominantly amorphous or

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L. Zhao et al. / Applied Surface Science 257 (2011) 3974–3979

Fig. 1. SEM images of Cu-doped NiO thin films with different Cu contents deposited on FTO-coated glass substrates at room temperature for 15 min. (a) Cu 0 film; (b) Cu 1 film; (c) Cu 2 film; (d) Cu 3 film. The scale bars represent 100 nm.

short-range ordered structure because of high deposition speed as revealed by Liu et al. [2]. 3.4. UV–vis optical properties The UV–vis absorption spectra of Cu-doped NiO thin films with different Cu contents are shown in Fig. 3. The wavelength ranges from 300 nm to 900 nm. Because the band gap of NiO is 3.7 eV, the peaks at about 335 nm are attributed to nickel oxides. It can be seen that the intensity of the peaks increases with the Cu content, indicating that the Cu doping increases the crystallintiy of Nix Cu1−x O. Additionally, Fig. 3 clearly shows that the absorption edge of NiO film exhibits an obvious red-shift to the visible region as Cu dop-

ing. This phenomenon was also found and explained as the change of the width of band-gap [18]. Park et al. discovered the similar discipline in Ni1−x Znx O films [20]. According to their explanations, it is attributable to the lattice expansion which causes the energy decrease of the Ni d-like conduction band. Fig. 4 shows the optical transmittance spectra of the bleached and colored state of Cu-doped NiO films with different Cu contents in the wavelength range from 350 nm to 750 nm (bleaching voltage = −2.5 V, coloring voltage = +2.5 V, in 0.5 M KOH solution). The optical data is normalized using FTO coated conducting glass substrate as a reference while measuring the transmittance [6]. It is noted that the electrochromic properties change with the different contents of Cu. The results indicate that the transmission of both the colored state and bleached state decrease as the Cu content

0.8

(200)

(b)

*(200)

(110)

*

(a)

10

(101)

*

20

30

Absorbance / a.u.

Intensity / a.u.

(111 )

40

(211)

*

50

(301) (310)

**

60

70

80

2 /Degree Fig. 2. XRD patterns of Cu-doped thin films with different Cu-doped contents on FTO coated glass substrates. (a) undoped NiO film (Cu 0 film) and (b) Cu-doped NiO film (Cu 2 film).

0.6

Cu 3

0.4

Cu 2 Cu 1

0.2

Cu 0

0.0 300

400

500

600

700

800

900

Wavelength / nm Fig. 3. Absorption spectra of Cu-doped NiO thin films with different Cu contents on FTO coated glass substrates from 300 nm to 900 nm.

L. Zhao et al. / Applied Surface Science 257 (2011) 3974–3979

100

Cu 0

80

Transmittance / %

Transmittance / %

100

60 40 20 0 300

400

500

600

700

Cu 1

80 60 40 20 0 300

800

400

Wavelength / nm 100

Cu 2

80 60 40 20 0 300

400

500

600

500

600

700

800

700

800

Wavelength / nm

Transmittance / %

Transmittance / %

100

3977

700

800

Cu 3

80 60 40 20 0 300

400

Wavelength / nm

500

600

Wavelength / nm

Fig. 4. Transmittance spectra of bleached and colored states of Cu-doped NiO films with different Cu contents in the wavelength ranges from 350 nm to 750 nm. Solid line: bleached state; dash line: colored state.

OD = log

T  b

Tc

where Tb and Tc are the transmittance of the bleached state and colored state, respectively [8]. The great value of OD refers to the better EC properties. Optical density changes obtained from Fig. 4 are given in Table 1 and it is obvious that OD is enhanced by doping Cu especially in the wavelength range of 350–600 nm. The value of OD increases with the Cu content firstly and then decreases after Cu content reaches a critical value. The greatest OD of Cu 2 film can reach 1.385, which is far larger than that of the undoped NiO film.

3.5. Electrochemical properties 3.5.1. Cyclic voltammograms The electrochemical behavior of Cu-doped NiO electrode has been investigated. Fig. 5 shows the cyclic voltammetry (CV) curves of Cu-doped NiO films with different Cu contents in 0.5 M KOH in the first cycle in the scanning potential range of −0.8 V to +1.2 V with a scan rate of 40 mV s−1 . The two current peaks at −0.262 V and 0.661 V correspond to the redox couple Ni(II)/Ni(III). The anodic current is responsible for the change in color of the films from transparent to dark brown. On the negative sweep, the color changed back to transparent. Such a bleaching and coloration process is attributed to the mutual conversion involving the Ni(II)/Ni(III)

15

10

Current / mA

increases, and the influence on the transmittance of bleached state is larger than that of colored state. The maximum transmittance change T (=Tb − Tc , where Tb and Tc are the transmittance of the fully bleached and colored states) of Cu-doped NiO films are shown in Table 1. According to the figures, the value of T increases with the content of Cu at the first, and when the Cu content reaches a critical value (Cu 2 film, the moral ratio is 1/8), the transmittance contrast begins to decrease. When the molar concentration ratio of Cu: Ni is 1/8, the Cu-doped NiO film (Cu 2) exhibits the best electrochromic properties, which indicates that more OH− insert in the Cu-doped NiO film than the undoped NiO film, just like NiO films doped with B2 O3 [3]. The overall transmittance in the colored state for the films is lower than 40% in the visible region (350–750 nm), and the transmittance for the bleached state is more than 80%. This phenomenon is probably caused by a more favorable lattice arrangement, which favors the OH− insertion and diffusion through the lattice. Similar behaviors were found in Nb2 O5 and Li-doped WO3 films thin films [21,22]. The optical density change (OD) can be defined by:

5

0

-5

-1.0

Cu 1 Cu 0 Cu 3 -0.5

Cu 2 0.0

0.5

1.0

1.5

E vs. SEC / V Fig. 5. Cyclic voltammetry (CV) curves of the Cu-doped NiO films in 0.5 M KOH in the first cycle. The scanning potential range is from −0.8 V to +1.2 V with a scan rate of 40 mV s−1 .

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40

Current Density / mA .cm

Current / mA

-2

15

Cu 0 20

Cu 3 0

Cu 1

Cu 1 Cu 2

Cu 2 -20

Cu 0 0

5

10

Cu 3 15

10th cycle 10

5

2000th cycle 0

4000th cycle 1000th cycle

-5 20

Time / sec Fig. 6. Chronoamperometric curves of the Cu-doped NiO films with different Cu contents.

-0.6

-0.3

0.0

0.3

0.6

0.9

1.2

1.5

E vs. SEC / V Fig. 7. The 10th, 1000th, 2000th and 4000th cyclic voltammograms for Cu-doped NiO thin film (Cu 2). Cycling is performed between −0.4 V and 1.0 V at a sweep rate of 40 mV s−1 .

transformation: Ni(OH)2 (bleached) + OH−  NiOOH (colored) + H2 O + e− The anodic and cathodic peaks of the films change as Cu doping and the results relating to the profile of the voltammograms are outlined in Table 1. The Cu doping causes a positive shift of the cathodic peak potential, while the influence on the anodic peak potential is not serious, and the shift becomes unobvious when the content of Cu increases. The reversibility of the electrode reaction can be measured by the difference (EO –ER ) between the oxidation potential (anodic peak potential, EO ) and the reduction potential (cathodic peak potential, ER ) [11]. It can be referred from the results in Table 1 that the Cu doping improves the reversibility of the electrode reaction and Cu 1 films exhibit the most reversible behavior. Furthermore, the Cu 2 and Cu 3 films show much higher current in cathodic peak potential when compared to Cu 0 film. It may suggest that the amount of redox reaction in the two layers is much more than that in Cu 0 film. As shown in Fig. 1, the existence of copper in Cu-doped films reduces the size of nickel oxide particles and probably provides larger active surface area for charge insertion and extraction than Cu 0 film. Therefore, the two films show higher current density. 3.5.2. Response time The chronoamperometric curves of the Cu-doped NiO films with different Cu contents are shown in Fig. 6. To study the response time of coloration/bleaching, a standard three-electrode configuration with 0.5 M KOH as electrolyte is used to study the process by applying ±1.0 V. Response time for coloration (tc ) and bleaching (tb ) is the time required for the anodic/cathodic current (iO /iR ) to achieve a steady state level and the values of tc and tb estimated for all the samples are given in Table 1. The results reveal that the switching time of coloration/bleaching is about 5.1 s/ 3.0 s for the undoped NiO films, while the Cu doped films yield a faster response. It is observed that tc increases with the Cu content in films noticeably while there is no appreciable effect on tb , and the bleaching is faster than coloration, just the same as NiO film prepared by sol–gel [23]. The possible reason for Cu-doped films have faster response time is the surface area: the Cu doping reduces the size of nickel oxide particles and provides more active surface for redox reaction with OH− , which is confirmed by the surface morphology. 3.5.3. Cycle life The electrochemical stability of the film is one of the key parameters to be taken into account when one looks for its applications. Therefore, Cu-doped NiO thin film (Cu 2) is further tested for long

time electrochemical cycling at a scan rate of 40 mV/s within the −0.4 V to 1.0 V as shown in Fig. 7. Both the andoic and cathodic peak areas decreased as the cycle increased, indicating that the mount of charge injected and extracted during the electrochromic reaction was reduced, which was contributed to some substances lost their activity such as NiOOH or Ni(OH)2 . After 4000 cycle, the relative capacity remains over 40%. 4. Conclusions Cu-doped nickel oxides films were prepared by electrochemical deposition method. They exhibit good electrochromic properties. With the increase of the Cu content, the change of transmittance increased firstly and decreased after Cu content reached the critical value. The undoped NiO film has an amorphous or short-range ordered structure. While the Cu-doped films show the formation of nanorods of Nix Cu1−x O, indicating that the Cu doping increases the crystallinity of the films, and the amount of nanorods increases with the Cu content in the films. The film with the Cu/Ni molar ratio of 1:8 has better electrochromic properties compared to the undoped NiO film. A variation of transmittance between the bleached state and the colored state is up to ∼80% in the wavelength range of 350–600 nm. Hence, compared with the undoped NiO film, with the same preparing parameters and testing conditions, the Cu-doped films show better reversibility and faster response time. These good EC properties promote the potential application of the Cu-doped films for EC devices. Acknowledgments The project sponsored by the Scientific Research Foundation for the returned overseas Chinese scholars, State Education Ministry and the Nature Science Foundation of Shandong Province of China (ZR2010EM027). References [1] Y. Makimura, A. Rougier, J.M. Tarascon, Cobalt and tantalum additions for enhanced electrochromic performances of nickel-based oxide thin films grown by pulsed laser deposition, Appl. Surf. Sci. 252 (2006) 4593–4598. [2] H.R. Liu, G.Y. Yan, F. Liu, Y.Y. Zhong, B.X. Feng, Structural, electrochemical and optical properties of NiOx Hy thin films prepared by electrochemical deposition, J. Alloys Compd. 481 (2009) 385–389. [3] X.C. Lou, X.J. Zhao, X. He, Boron doping effects in electrochromic properties of NiO films prepared by sol–gel, Sol. Energy 83 (2009) 2103–2108. [4] M. Vidotti, R.P. Salvador, S.I. Córdoba de Torresi, Synthesis and characterization of stable Co and Cd doped nickel hydroxide nanoparticles for electrochemical applications, Ultrason. Sonochem. 16 (2009) 35–40.

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