Int. J. Electrochem. Sci., 17 (2022) Article Number: 220125, doi: 10.20964/2022.01.39
International Journal of

ELECTROCHEMICALSCIENCEwww.electrochemsci.org

Incorporation of Co2+, Cu2+, and Zn2+ ions into nickel oxide thinfilms and their enhanced electrochemical and electrochromic
performances
Kyung Ho Kim1,*, Sho Fujii2, Yoshio Abe1
1

School of Earth, Energy and Environmental Engineering, Kitami Institute of Technology,
165 Koen-cho, Kitami, Hokkaido 090-8507, Japan2
Department of Natural Sciences, National Institute of Technology, Kisarazu College,
2-11-1 Kiyomidai Higashi, Kisarazu, Chiba 292-0041, Japan*
E-mail: khkim@mail.kitmai-it.ac.jp
Received: 2 October 2021 / Accepted: 10 November 2021 / Published: 6 December 2021

Nickel oxide (NiO) thin films incorporating cobalt (Co), copper (Cu), and zinc (Zn) dopants wereprepared using a simple and cost-effective sol-gel spin-coating method. Changing the dopant allowed
the color of the NiO thin films to be tuned. Cracking on the surface of the NiO thin films was effectively
reduced by the incorporation of dopants. The undoped NiO and Co-doped NiO thin films had a relatively
porous nanoparticle structure, whereas the Cu-and Zn-doped NiO thin films had a more compact
nanoparticle structure. Compared with the undoped NiO sample, the cyclic voltammetry (CV) curves ofthe doped NiO thin films exhibited higher integrated current densities. In particular, the CV curves of
the Co-doped NiO thin film showed no symmetric distortion as the scan rate increased, indicating ahighly reversible redox process. At a scan rate of 5 mV/s, the calculated areal specific capacitance was
19.7 mF/cm2, and it showed excellent cycling stability at high scan rate (~88% retention at 100 mV/s
after 2000 cycles). The color of the Co-doped NiO thin film changed from pale pink (bleached state) tobrown (colored state). Co doping demonstrated great potential for improving the electrochemical andelectrochromic performance of NiO thin film, while also offering long-term stability and enhanced
optical modulation.

Keywords: NiO, Thin film, Electrochemical stability, Electrochromic performance

1. INTRODUCTION
Owing to the wide applicability of nanostructured nickel oxide (NiO) thin films, multiple studies
have been conducted to optimize the solution-based preparation methods, which are noted for theirsimplicity, cost-effectiveness, and reliability [1-5]. In addition, doping is recognized as an effective way

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to improve the catalytic and magnetic performance of NiO [3-5]. Ethiraj investigated the characteristics
of Cu-doped NiO nanoparticles synthesized by a wet chemical method as an effective photocatalyst forphenol degradation [3], while Ponnusamy reported that chemically synthesized Co-doped nanoparticles
exhibited ferromagnetic behavior at room temperature [4]. Alternatively, Panigrahi reported that Zndoped NiO nanoparticles synthesized using a wet chemical method have strong potential for
multifunctional applications such as ultraviolet emission devices, random access memory, and data
storage applications [5]. In addition, NiO nanoparticles prepared via the hydrothermal precipitation
method exhibit reliable electrochemical capacitive behavior, thus showing potential as active electrodematerials [6,7]. Moreover, unlike conventional active electrode fabrication methods based onsynthesized nanoparticles, thin films prepared directly on conductive substrates are simpler to fabricate
because no additional processing involving conductive agents such as black carbon or polymer binders
such as polytetrafluorothylene (PTFE) is required [8]. Furthermore, directly prepared NiO thin films canalso be used as electrochromic materials for energy-efficient window applications [9].
In this study, we prepared NiO thin films with various dopants (Co, Cu, Zn) using sol-gel spincoating and compared their morphological, electrochemical, and electrochromic properties with undoped
NiO thin film. Our discussion focuses primarily on the effect of high dopant concentrations on theproperties of NiO thin film. In addition, efficient electrochromic films based on electrochemically stable
NiO offer a solution to the energy crisis and can be used in sustainable applications.

2. EXPERIMENTAL
To prepare the undoped NiO thin film, nickel acetate tetrahydrate (Ni(CH3COO)2·4H2O, 0.5 M)
was dissolved in 2-methoxyethanol (2ME, C3H8O2). After stirring at 60 °C for 1 h, it was aged at room
temperature for 24 h. After spin-coating, the samples were dried at 150 °C for 10 min before being
annealed at 300 °C for 1 h in air. To prepare doped NiO thin films, nickel acetate tetrahydrate(Ni(CH3COO)2·4H2O, 0.4 M) and one of cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O, 0.1 M),
copper acetate monohydrate (Cu(CH3COO)2·H2O, 0.1 M), or zinc acetate dihydrate(Zn(CH3COO)2·2H2O, 0.1 M) were dissolved in 2ME. The spin-coating, drying, and annealing
processes were repeated under the same conditions used to prepare the undoped sample.
The crystallinity of the various thin films was determined using X-ray diffraction (XRD,
D8ADVANCE). Optical transmittance spectra were obtained using ultraviolet-visible spectroscopy(HITACHI, U-2910). The chemical bonding states were identified via Fourier transform infrared
spectroscopy (FTIR, FT/IR-6100) and X-ray photoelectron spectroscopy (XPS, JPS-9200) using a MgK X-ray source. The surface morphologies were analyzed using field emission scanning electron
microscopy (FESEM, JSM-6701). Cyclic voltammetry (CV) measurements were performed using athree-electrode system in which the NiO thin films were prepared on fluorine-doped tin oxide (FTO) as
the working electrode, with Pt as the counter electrode and Ag/AgCl as the reference electrode, in a 1 M
KOH aqueous electrolyte. In-situ optical transmittance spectra were measured using a charge-coupleddevice multichannel detector (USB2000+, Ocean Optics) with halogen and deuterium lamps; air and aquartz-glass cell filled with 1 M KOH were used as references [10].

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3. RESULTS AND DISCUSSIONAs shown by the XRD patterns in Fig.1(a), all samples prepared on glass substrates exhibited
diffraction peaks at 2 positions of 37.3º and 43.3º, which were indexed to the (111) and (200) planesof the cubic phase of NiO (JCPDS No. 04-0835), respectively. The weak intensity and broadness of thediffraction peaks indicate nanocrystalline NiO. No secondary diffraction peaks associated with an oxide
phase (metal or otherwise) were observed. The most likely reason for this is the high solid solubility of
Co, Cu, and Zn in NiO [11-13]. The FTIR spectra (Fig.1(b)) show that the NiO samples all exhibit astrong absorption peak at ~400 cm-1, which corresponds to the Ni-O stretching vibration. A broad andweak absorption peak exists at ~3400 cm-1, which corresponds to stretching hydroxyl groups in thehydrogen-bonded vibration of water [6]. Figure 1(c) shows the optical transmittance spectra of NiO thinfilms prepared on FTO/glass substrates with various dopants. Incorporating different dopants enables
the sample color to be tuned. For example, Zn doping results in the absorption edge shifting slightly
toward shorter wavelengths, while the average transmittance is also higher than that of the other samples.
The potential of highly transparent Zn-doped NiO thin film as hole transport layer in perovskite solar
cells has been highlighted in a previous report [14]. The Co- and Cu-doped NiO samples displayed aslightly reddish color and the undoped NiO sample was light gray.

Figure 1. XRD patterns (a), FTIR spectra (b), and optical transmittance spectra (c) of the NiO thin films
with different dopants.

The XPS profiles in Fig.2(a) show that all samples had characteristics Ni 2p3/2 and Ni 2p1/2 peaks
at ~853.5 and ~871.4 eV, respectively, which is well agreement with other report [15]. Incorporating
dopants led to the emergence of the corresponding 2p peaks for Co, Cu, or Zn [15,16], indicating thepresence of Co2+, Cu2+, or Zn2+ cations in the NiO thin films (Fig.2(b-d)). The O 1s spectra of undoped,
Co-, Cu-, and Zn-doped NiO samples are shown in Fig.2(e-h), respectively. The peaks were fitted using
three different species of oxygen at the binding energies of ~529.3, ~521.2, and ~532.5 eV, which areassociated with metal-bonded oxygen (O1), low-coordination oxygen defects (O2), and physisorbed andchemisorbed oxygen in water on the sample surface (O3), respectively [15]. The O2/O1 ratios of the
doped-NiO samples were slightly higher than that of the undoped NiO sample, implying that doping
benefits the electrochemical performance of NiO thin films [15].

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Figure 2. XPS profiles of the NiO thin films; Ni 2p (a), Co 2p (b), Cu 2p (c), Zn 2p (d), O 1s (e-h); NiO(e), Co-NiO (b,f), Cu-NiO (c,g), Zn-NiO (d,h) thin films.

Figure 3. FESEM images of NiO (a,e), Co- (b,f) , Cu- (c,g), and Zn- (d,h) doped NiO thin films; (a-d)
top images, (e-h) cross-sectional images.

Figure 3 shows the FESEM images of the NiO thin films prepared on FTO/glass substrates. The
cross-sectional images in Fig.3(a-d) show that the thickness of the thin films is approximately 120 nm,
with each film composed of nanoscale particles. This nanoparticle structure could be beneficial forelectrolyte penetration, leading to improved electrochemical performance [2]. As shown in Fig.3(e-h),cracks, indicated in the images by arrows, were observed on the surface of each thin film sample.
However, doping was found to decrease the number of cracks, with Zn proving the most effective dopant
in terms of crack reduction. The surface coverage was determined to be approximately 80, 87, 94, and
97% for undoped and the Co-, Cu-, and Zn-doped NiO samples, respectively. These results might be due

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to the differences in ion radius between Ni2+ (0.069 nm) and the dopants (Co2+ (0.072 nm), Cu2+ (0.073nm), Zn2+ (0.074 nm)) [17,18]. The reduction in crack formation for the doped samples might be causedby the dopant alleviating stress between NiO thin films and FTO.

Figure 4. CV curves of NiO samples with various dopants at a scan rate of 20 mV/s (a), CV curves of
the Co-doped NiO sample at various scan rates (b), current densities of the anodic and cathodic
peaks obtained from the CV curves as a function of the square root of the scan rate (c,d), areal
specific capacitances (e), and CV curves at a scan rate of 100 mV/s with cycling (f-i); NiO (f),Co-NiO (g), Cu-NiO (h), Zn-NiO (i).

The performance of the electrochemical capacitors was evaluated via CV measurements [19],which were conducted between 0.15 and +0.55 V after exposing the thin film samples to ozone. Figure4(a) shows the CV curves of NiO samples with various dopants at a scan rate of 20 mV/s. Similar CV
curve profiles were observed for all samples, which indicates pseudo-capacitance behavior fromFaradaic processes [20]. Each CV curve exhibited a pair of strong anodic and cathodic redox peaks,
which can be attributed primarily to the reversible faradic redox behavior of Ni2+  Ni3+ [20]. Moreover,the potential of the redox peak depends strongly on the dopant. For example, relative to the redox peaksof the undoped NiO sample, those of the Co-doped NiO sample were shifted to the negative potential

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region, which can be attributed to the lower redox potential of Co-based systems relative to their Nibased counterparts [21]. Furthermore, the Cu- and Zn-doped NiO samples both showed a positive shift
in the oxidation peak potential and a negative shift in the reduction peak potential [22,23]. Overall, theCV curves show that the doped NiO samples possess a larger integral area and higher redox current
density compared with the undoped NiO sample.
Figure 4(b) shows the CV curves of the Co-doped NiO sample at various scan rates from 5 to
100 mV/s. As the scan rate increases, the current density increases, and the anodic and cathodic peaks
shift toward the positive and negative potential regions, respectively. The CV curves of the Co-NiO
sample retain their shape as the scan rate increases. As shown in Fig.4(c,d), the current densities of theanodic and cathodic peaks are proportional to the square root of the scan rate, indicating diffusion-limited
redox reactions [21]. Further, the gradients of the Co-doped NiO sample were steeper than those of theother samples, implying an elevated rate of charge transfer and ion diffusion [24].
Figure 4(e) shows the areal specific capacitances calculated from the CV curves. At all scan rates,
the Co-doped NiO sample exhibited higher capacitance values than the others. Capacitance values of
19.7, 19.4, 18.7, 17.3 and 13.8 mF/cm2 were calculated at scan rates of 5, 10, 20, 50, and 100 mV/s,
respectively. The capacitance value is shown to decrease with an increase in the scan rate because of the
reduced rate of electrolyte transport [6,25]. Nevertheless, the Co-doped NiO sample demonstrates animpressive high-rate capability owing to its porous nanoparticle structure (~70% of capacity retention at
100 mV/s), which is a key parameter for satisfying the high power demands of energy storage devices[26]. Significantly, the Co-doped NiO sample outperformed alternative inexpensive transition oxide
electrode material (e.g., copper oxide (CuO), 0.12 mF/cm2) [19].
Figure 4(f-i) shows the CV curves at a scan rate of 100 mV/s with cycling. The electrochemical
cycling stability is critical for practical applications. The CV curves of the Co-doped NiO sample show
no pronounced distortion during cycling, and exhibit excellent cycling stability: ~88% retention over2000 cycles. Although the Cu- and Zn-doped NiO samples also demonstrate high current densities, theyshow relatively poor retention (~56%). A possible explanation for this is the peak shifts resulting fromthe redox reaction, while the compact morphologies of the Cu- and Zn-doped NiO samples mean that
there is insufficient time for the reaction to proceed at higher scan rate [6,26,27]. Compared with theother samples, the Co-doped NiO sample, which has a relatively porous structure, exhibits a promising
electrochemical performance, including cycling durability at a high scan rate.
Figure 5 (a) shows the electrochromic performance of the Co-doped NiO sample at a scan rateof 100 mV/s within a potential window between 0.15 and +0.55 V. Based on the variation of the insitu transmittance at a wavelength of 500 nm, the transmittance change (T) and optical density change(OD) recorded at the 30th (420 s), 1000th (14000 s), and 2000th (28000 s) cycle were ~40% and 0.44,~36% and 0.43, and ~35% and 0.43, respectively. The decrease in transmittance after 2000 cycles was
only ~12%, indicating excellent sustainability compared with other reported NiO film prepared by theprecipitation method (e.g., 61% decrease after 2000 cycles) [28]. The in-situ transmittance spectra of the
Co-doped NiO sample, which were measured for 18 s under an applied potential of 0.15 V and for 18
s at +0.55 V, are shown in Fig.5(b). The corresponding photographs show that the color of the sample
changes from pale pink (bleached) to brown (colored). The optical switching time measured at awavelength of 500 nm was ~4 s for bleached (tb) and ~2 s for colored (tc) states (Fig.5(c)), which is

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comparable to porous nanowall-structured NiO prepared by chemical bath deposition (tb 2.9 s, tc 3.5 s)[29]. These results indicate that the Co-doped NiO sample represents a promising electrochromic thinfilm with excellent cycling durability.

Figure 5. Variations in the in-situ transmittance of Co-doped NiO sample at a scan rate of 100 mV/s
during cycling (a), in-situ transmittance spectra for the colored and bleached states (b), andvariation in the in-situ transmittance at a wavelength of 500 nm with time (c).
4. CONCLUSIONS
We investigated the effects of Co, Cu, and Zn dopants on the optical, morphological,
electrochemical, and electrochromic properties of NiO thin films prepared via sol-gel spin-coating. The
XPS results confirmed that the NiO was doped with Co, Cu, and Zn. The Co- and Cu-doped NiO thinfilms were slightly reddish, whereas the Zn-NiO thin film was highly transparent over the visible
wavelength region. All the NiO thin films were composed of nanoparticles, and exhibited increased
surface coverage compared with the undoped sample to incorporate the Co, Cu, and Zn dopants. In
addition, the CV measurements indicated that each NiO sample exhibited a strong faradaic reaction, withthe symmetric characteristic of the redox peaks suggesting high reversibility. Moreover, the redox peaks
shifted as the composition of the electrode materials changed. The Co-doped NiO sample demonstrated
excellent long-term electrochemical stability, retaining a capacity of ~88% after 2000 cycles,
representing a notable improvement compared with the values of ~70, ~56, and ~56% recorded for the
undoped, Cu-, and Zn-doped NiO, respectively. These results validate the high optical modulation, fast

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optical switching time, and excellent cycling durability of Co-doped NiO compared with undoped NiO.
As such, this simple-to-prepare Co-doped NiO thin film represents an attractive material for applications
requiring high-performance supercapacitors with efficient electrochromic performance.

ACKNOWLEDGEMENT
This study was supported by KNIT Collaborative Research Fund. The authors would like to thankSusumu Tokuda of the Open Facility Center of the Kitami Institute of Technology for his assistance withFESEM characterization and Ayano Yamazaki of Open Facility Division, Global Facility Center,
Creative Research Institution, Hokkaido University for performing XPS measurements.

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