Self-Ordered Nanoporous Nickel Oxide/Fluoride Composite Film with ...

Electrochemical and Solid-State Letters, 13 共8兲 C21-C24 共2010兲

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1099-0062/2010/13共8兲/C21/4/$28.00 © The Electrochemical Society

Self-Ordered Nanoporous Nickel Oxide/Fluoride Composite Film with Strong Electrochromic Contrast Nabeen K. Shrestha,* Min Yang, and Patrik Schmukiz,** Department of Materials Science, WW4-LKO, University of Erlangen-Nuremberg, 91058 Erlangen, Germany Self-ordered nanoporous nickel oxide/fluoride composite layers were grown by anodization of a Ni substrate in 85% H3PO4 solution containing 0.5 M NH4F. This oxide film contained a very high concentration of NiF2 共up to 30–40%兲. The layers show a very good electrochromic performance. In fact, the nanoporous oxide/fluoride films exhibit a nearly twofold stronger optical contrast and much higher cycling stability than the fluoride-free reference Ni oxide films. © 2010 The Electrochemical Society. 关DOI: 10.1149/1.3430656兴 All rights reserved. Manuscript submitted February 5, 2010; revised manuscript received March 25, 2010. Published May 20, 2010.

Electrochromic 共EC兲 materials are of great interest for application in switchable mirrors, electronic display devices, and energy efficient smart windows.1,2 These materials can be reversibly switched between a colored and a bleached state upon electrochemical oxidation or reduction. Among the various EC materials, solidstate inorganics in particular, metal oxide semiconducting materials1-8 such as Ni oxide films have been investigated extensively due to their higher cycling stability compared with organic materials. Ni oxide is also used as an optical active counter electrode to WO3 electrode in a complementary EC device, where the optical modulation increases due to the simultaneous modulation of both electrodes.1,9 The EC performance of a material not only depends on its intrinsic properties but also depends strongly on the morphology of the materials. Generally, the color contrast and the optical switching speed increase drastically if nanostructured, high surface area materials are used. For example, porous layers of WO3 4 or tubular structured layers of TiO2 and other nanomaterials3,5-7,10-13 show significantly enhanced properties compared with compact layers. In the present work, to the best of our knowledge, we report for the first time on the electrochemical formation of a self-organized porous layer of Ni oxide film grown on Ni using a fluoride-containing electrolyte and describe its enhanced EC performance. Experimental Self-ordered nanoporous Ni oxide films were grown on a Ni foil by anodization. For this, a 0.125 mm thick Ni foil 共ChemPUR, Germany兲 was degreased by sonicating in ethanol for 5 min, followed by washing in deionized water and drying the foil in a N2 stream. 1 cm2 of this foil was exposed to the electrolyte by pressing it together with a copper plate back contact against an O-ring opening of an electrochemical cell. Anodization was carried out in an electrolyte containing 0.5 M NH4F in 85% H3PO4 using a threeelectrode electrochemical system with a Pt counter electrode and a Ag/AgCl reference electrode. The applied potential was swept with the rate of 50 mV s−1 from the OCP to the final potential of 6.0 V vs Ag/AgCl, and the final potential was held for 1 h. For reference, a thin Ni oxide film was thermally grown on a Ni substrate 共RefTG-Ni oxide兲 by annealing the substrate at 350°C for 1 h in air.14 Also, a thin porous Ni oxide film was deposited on a Ni foil using a chemical bath approach 共Ref-CB-Ni oxide兲.15 In brief, a Ni substrate was immersed into a chemical bath solution containing 20 mL of 1 M NiSO4, 15 mL of 0.25 M K2S2O8, and 5 mL of NH3·H2O 共25– 28%兲 for 15 min to reach a thickness of approximately 300 nm and then was annealed at 300°C in air for 1 h. The anodized film was characterized using a scanning electron microscope 共SEM兲共Hitachi FE-SEM S4800兲, an X-ray diffracto-

* Electrochemical Society Student Member. ** Electrochemical Society Fellow. z

E-mail: [email protected]

meter 共XRD, Philips X’Pert兲, and an X-ray photoelectron spectrometer 共XPS, PHI 5600兲. Measurements of the EC performance of the films were carried out in an aqueous electrolyte containing 1 M KOH, applying a potential pulsed combined with reflectance measurement of the color contrast response. Reflectance measurements were carried out with a fiber optic illuminator 共tungsten halogen lamp, Ocean Optics兲. Results and Discussion In a preliminary set of experiments, Ni foils were anodized in a concentrated H3PO4 solution containing 0.5 M NH4F at different potentials ranging from 2.0 to 20 V. However, a clearly visible layer was observed only at potentials ranging from 5.0 to 15 V. The experimental results performed at different applied potentials revealed that the most promising crack-free layers are obtained at 6.0 V. Figure 1a shows the top SEM view of the anodized layer at 6.0 V for 1 h. As evident, a self-organized porous structure with a pore diameter of approximately 50 nm can be seen in this case. At lower potentials no self-ordered porous structure was observed, while at higher potentials a cracked spongy layer was obtained 共Fig. 1b兲. Energy dispersive X-ray analysis 共EDX兲 of this anodized film given in Table I showed that it mainly composed of Ni, O, F, and a small amount of P. Results from XPS analysis are in line with the data shown in Table I. Figure 2a shows survey XPS spectra of anodic layers under different annealing conditions. The Ni 2p3/2 XPS peak located approximately at 857 eV reveals a peak character for NiO.16 Similarly, the O 1s XPS peak located at approximately 531 eV is in accordance with NiO being formed by anodization.17 The position of the F 1s XPS peak located at approximately 685 eV is well in accordance with NiF2. EDX and XPS indicate a high concentration of NiF2 to be present in the film.16 In addition, clearly, the peak intensity of F is significantly decreased after annealing at 300°C for 4 h. The XPS depth profile for the different elements 共Fig. 2b兲 shows that the anodic film has a thickness of approximately 300 nm. Fluoride is present throughout the film, and phosphorus is not strongly incorporated. When the anodically grown Ni oxide film was immersed in deionized water for overnight, no significant reduction in the F content could be observed. The presence of NiF2 in the film is additionally confirmed by the XRD pattern shown in Fig. 2c. The peaks relevant for NiO and NiF2 identification are presented enlarged in the inset 共the most significant indication for NiF2 is the peak at 41°兲. Figure 3a shows the cyclic voltammograms of the as prepared film in 1 M KOH for different sweep rates. The anodic peaks correspond to intercalation of OH− ions into the nanostructured film, while the cathodic peaks correspond to deintercalation of the OH− ions. When the film was anodically polarized, it turned immediately to dark while this color completely bleached off during cathodic polarization. A considerable number of investigations reported on the electrochemical mechanism for the coloration/bleaching process in Ni oxide.18-20 The most accepted mechanism is that a structural change from NiO to a hydroxyl/oxide structure takes place in the

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F KLL Ni LMM Ni LMM F 1s Ni LMM

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Ni 3s Ni 3p

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non-annealed P 2s

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F KLL Ni 2p

8

o

annealed 300 C-1h o

annealed 300 C-4h 0 1200 (c)

1000

800

(d)

(e)

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Figure 1. Top view SEM images of self-ordered porous Ni oxide/fluoride composite film grown on a Ni foil by anodization in 85% H3PO4 containing 0.5 NH4F at 共a兲 6.0 and 共b兲 15 V, 共c兲 Ni oxide film deposited on a Ni foil using a chemical bath deposition approach, and 共d兲 thermally grown oxide layer on a Ni foil after annealing in air at 300°C for 共e兲 1 and 共f兲 4 h.

Atomic Concentration (%)

100

1 µm

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0

(b) Ni 2P

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40 20

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first cycle. After this preconditioning step, the reversible color change reaction from transparent to dark brown can be described by the following electrochemical reaction21 Ni共OH兲2 + OH− ↔ NiOOH + H2O + e−

Table I. Species in Ni oxide film for different thermal treatments in air.

Nonannealed

Annealed at 300°C, 1 h

Annealed at 300°C, 4 h

37.56 17.75 42.01 2.68

40.45 30.40 26.57 2.58

37.96 32.52 17.39 2.39

300

400

500

Sputter Depth (nm)

关1兴

NiO or Ni共OH兲2 have an optical bandgap 共Eg兲 of approximately 3.6 eV, while Eg for NiOOH is at ca. 1.5 eV.22 Although the straight line obtained from the plot of peak current vs square root of sweeping rates 共inset of Fig. 3a兲 may suggest that the above electron-transfer reaction involving the coloration/bleaching process is controlled by the diffusion of OH− ions into and out the nanostructured film, the separation of the redox peak potential was increased with increasing scan rate. To produce a stronger color contrast between coloration and bleached states and to study the insertion kinetics, chronoamperometry was carried out by pulsing the potential between ⫺0.3 and 0.8 V. Additionally, to investigate the influence of the F content on the EC performance of the film, the anodized samples were annealed at 300°C for 1 and 4 h to vary the F content in the film. The results shown in Table I reveal that the F content in the film decreases with the annealing time. However, the crystalline structures of the film

Ni O F P

400

80

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Atom %

600

Binding Energy (eV)

(c) DC

A

A

A= Ni substrate B= NiO C=Ni2O3 D=NiF2

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30

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40

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DC D B D B DC D D B B 20

D D D

40

B B B 60

o

annealed 300 C-4h AD o

annealed 300 C-1h A non-annealed Ni substrate 80

2θ

Figure 2. 共Color online兲共a兲 Survey XPS spectra of the Ni oxide/fluoride composite film shown in Fig. 1a with different annealing conditions. 共b兲 Depth profile for the different elements. 共c兲 XRD patterns of the Ni oxide/ fluoride composite film shown in Fig. 1a.

are more or less similar after the thermal treatments, as shown by the XRD patterns in Fig. 2c. Figure 3b and c shows the in situ color contrast between color and bleached states in terms of reflectivity 共⌬R, difference in reflectance兲 after applying potential pulse. The optical photographs are



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0.09

0.03

-0.3 V

0.05 0.04 0.03 0.02 0.01 3

4

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5 6 7 8 9 10 11 1/2 1/2 [Sweep rate] / [mV/s]

10 mV/s 25 mV/s 50 mV/s 100 mV/s

-0.03 -0.06 -0.4

300oC annealed, 1h - NiO non-annealed - NiO

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Ref-CB-Ni oxide o 300 C, 4h-Ni oxide o 300 C, 1h-Ni oxide non- annealed-Ni oxide Ref-TG-Ni oxide

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600

1000

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0.8

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-0.3V Reflectance (a.u.)

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Time (s)

Potential (V) 1.0

Figure 3. 共Color online兲共a兲 CV of the Ni oxide/fluoride composite film shown in Fig. 1a in an aqueous electrolyte of 1 M KOH. Inset figure shows the linear plot of anodic current vs square root of sweeping rate. 关共b兲 and 共c兲兴 In situ reflectance of the Ni oxide/fluoride composite films, shown in Table I, containing different amounts of fluoride species while applying voltage pulses. Inset of 共b兲: Optical photographs of the colored and the bleached state for anodic Ni oxide/fluoride composite thin film. 共d兲 Electrochemical stability of the annealed 共300°C, 1 h兲 Ni oxide/fluoride composite film, shown in Fig. 1a, in terms of in situ reflectance as a dependence of CV cycles.

0.4

800

1000

(d) 0.7

0

20

Time (s)

present in the inset of Fig. 3b. As shown by the above electrochemical Reaction 1, the color developed during the anodic potential pulse is due to the intercalation of OH− into the Ni2+ 共transparent兲 structure and accompanied Ni3+ formation 共colored state兲. This colored Ni3+ state reduces back to the transparent Ni2+ when cathodic potential pulse is applied. Figure 3b clearly shows that this coloring and bleaching process in nonannealed as well as in annealed samples is reversible. However, the color contrast and the kinetics of this process are highly influenced by the F content of the oxide films. As evident in Fig. 3c, all the F-containing samples of the present work exhibit a significantly stronger color contrast than the reference Ni oxide films. The thermally grown Ref-TG-Ni oxide film under the same experimental conditions exhibited nearly no color contrast. Although, at this stage, no clear-cut mechanistic explanation can be given, the present results show that fluorides significantly enhance the EC effect. All three samples containing different contents of F, as shown in Table I, have a similar architecture 共i.e., are porous兲 and a similar crystalline structure 共Fig. 2c兲. Moreover, the layer annealed for 4 h showed a more porous morphology than that layer annealed for 1 h 共see Fig. 1e and f兲. Usually, a more porous structure is expected to enhance the intercalation capacity due to a higher surface area. However, comparing the 4 h annealed and 1 h annealed samples, the more porous one 共4 h兲 but a lower F content showed a lower color contrast. Therefore, this indicates that the enhanced EC effect exhibited by these samples in Fig. 3c can be ascribed mainly to the F content in the samples. In Fig. 3c, the reflectivity 共⌬R兲 values for the samples with an F content of F = 40.01, 26.57, 17.54, and 0.00 atom % are 0.97, 0.90, 0.65, and 0.53 a.u., respectively. That is, the color contrast for the samples with F = 40.01 and 26.57 atom % is 1.8- and 1.7-fold stronger than the reference sample 共Ref-CB-Ni oxide兲 with F = 0.00 atom %. Also, the switching kinetics between colored and bleached states for the samples is strongly influenced by the fluoride content; the respective recovery times to the fully bleached state for 0, 26.57, 42, and 17.39% are 80, 130, 230, and 880 s, respectively. Generally, the EC performance of a material decreases with the number of switching cycles due to slow structural desintegration.7 In the present work, the electrochemical stability of the Ni oxide film, which exhibited the best EC performance 共containing F

40

1380

1400

1420

Cycles

= 26.57 atom % annealed at 300°C for 1 h兲, was characterized in terms of cycling stability using cyclic voltammetry 共CV兲. Figure 3d shows that the reflectivity 共⌬R兲 of the sample up to 60 CV cycles is almost the same 共⌬R = 0.20兲. Thereafter, ⌬R of the sample decreased slightly, and it reached the value of 0.15 at the end of 1400 cycles, indicating the degradation of the EC performance by only 25%. However, the EC performance of the reference sample 共RefCB-Ni oxide兲 with F = 0.00 atom % has been reported to be reduced by 40% already after 300 cycles.15 Conclusion In the present work, a thin film of a self-ordered Ni oxide nanoporous structure was grown on a Ni substrate by anodization of the substrate in a concentrated H3PO4 solution containing fluoride ions. A strong uptake of fluorides into the grown layer was found. XPS and XRD investigations reveal that the fluoride is chemically present in the form of NiF2. These fluoride-containing porous Ni oxide films show a strongly enhanced EC performance in electrochemical intercalation experiments. The porous Ni oxide films containing fluoride show a nearly twofold stronger optical contrast and much higher electrochemical stability than the fluoride-free reference Ni oxide films. Acknowledgments The authors acknowledge the Alexander von Humboldt Foundation 共for supporting N.K.S.兲 and DFG for financial support. Helga Hildebrand and Ulrike Marten-Jahns are acknowledged for the XPS and XRD measurements, respectively. University of Erlangen assisted in meeting the publication costs of this article.

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