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Influence of current density on Al:NiO thin films via electrochemical deposition: Semiconducting and electrochromic properties
Materials Science in Semiconductor Processing 109 (2020) 104958
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Influence of current density on Al:NiO thin films via electrochemical deposition: Semiconducting and electrochromic properties Y.E. Firat * Physics Department, Kamil Ozdag Faculty of Sciences, Karamanoglu Mehmetbey University, Yunus Emre Campus, 70100, Karaman, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords: Nickel oxide Electrochromic Electrodeposition Coloration efficiency Mott-Schottky
The uniform aluminum-doped nickel oxide (Al:NiO) thin films were fabricated on indium tin oxide (ITO) by an efficient route of galvanostatic mode together with the help of conventional electrochemical methods. The cathodic deposition current densities were increased from 4 mA cm 2 to 7 mA cm 2 by a step of 1 mA cm 2 for the growth time of 30 s. The produced films were annealed in air at 400 � C for 1 h. The effect of deposition current on the behaviors of Al:NiO thin films were investigated by a number of methods including scanning electron microscopy (SEM), energy dispersive X-rays analysis (EDX), X-ray diffraction (XRD), Ultraviolet–Visible (UV–Vis) spectroscopy, Mott-Schottky analysis, and electrochemical impedance spectroscopy (EIS). X-ray diffraction analysis confirms the crystallinity of all deposits. SEM studies indicate that the surface morphologies of Al:NiO alter sensitively depending on the applied current density. From chronoamperometric curves of the two stage of the produced films, the fastest response times are found to be tb ¼ 0.85 s and tc ¼ 1.90 s for Al:NiO at an applied current density of 4 mA cm 2. It is found that the average coloration efficiency (η) reaches to a value of 50 cm2/C for Al:NiO fabricated at 5 mA cm 2. Significantly, the electrode displays long-term cyclic durability after 500 cycles in the potential window from 0 to þ0.8 V in 0.1 M potassium hydroxide (KOH) aqueous solution. The concept of Mott-Schottky is considered as approach that confirms the p-type semiconducting behavior of all the Al:NiO films with the order of ~1012 - 1015 cm 3 acceptor density. The results demonstrate promising electrochemical properties to enhance the electrochromic performance of NiO thin films which is required for the growth of outstanding electrode materials in smart glass and open a new insight into further applications of devices.
1. Introduction
are expected to have potential candidate for their outstanding high ef ficiency applied in smart glass and supercapacitors. As an important family of metal oxides, nickel oxides have emerged as alternative anodic p-type materials with its high theoretical capacitance to store the charges, fast transfer/diffusion rate of electrons and electrolyte ions and low cost [3,6,8–10]. However, development of desired electrode mate rials with metal oxides, in particular NiO and WO3, is a major drawback such as limited electrical conductivity [8] and poor electrochemical durability, which restricts their high-performance electrochromic device applications [3,6]. An efficient strategy to improve the performance of electrochromic device can be achieved by the substitution of dopant elements into the electrochromic host material. As it was already verified many report attentions by several re searchers, approaches to enriching the number of free charge-carriers and accessible surface areas in electrochromic devices include substi tution of foreign elements into NiO electrode material, leading greatly to
Electrochromic devices (ECDs) have emerged as an energy saving technology for decades due to appropriate control of solar heat and reducing building energy use. ECDs basically refer to reversible con version of optical property, induced electrochemically, whose macro scopic effect is the variation of color by the applied electrical field (or voltage) [1–7]. As is well known that the layout of an electrochromic device, structurally, consists of several electrodes; glass, transparent electrode, electrochromic layer, electrolyte, ion storage layer, trans parent electrode and glass [1]. To further promote high performance of the device, electrochromic layers that largely affect the properties of the ECDs should be considered. On the basis of the material components, electrochromic materials can be classified into two main groups; organic and transition metal oxide materials. Compared with organic-based materials, metal oxides
* Solar Cell Laboratory, Physics Department, Sciences and Arts Faculty, Uludag University, 16059, Gorukle, Bursa, Turkey. E-mail address: [email protected]. https://doi.org/10.1016/j.mssp.2020.104958 Received 9 November 2019; Received in revised form 16 January 2020; Accepted 20 January 2020 Available online 26 January 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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improvement in electrical conductivity and performances. For instance, Wen et al. have reported the synthesis of Ir–NiO, which exhibits excel lent durability during cycling LiClO4-PC and the lattice constant of the electrode was increased by the adding of Ir element [1]. Zhang et al. demonstrated the electronic conductivity of NiO was enhanced by appropriate doping of Co and, in consequence, the material had higher coloration efficiency of 47.7 cm2 C 1 at a wavelength of 550 nm [6]. Lin et al. synthesized a nitrogen (N) doped NiO electrochromic film by means of radio frequency magnetron sputtering using a ceramic NiO target [7]. They found that the charge reversibility, coloration efficiency and switching times for the N-doped NiO were significantly enhanced compare to pure NiO thin films [7]. Many approaches, including dip coating [9], electrodeposition [11], reactive dc magnetron sputtering [1], chemical bath deposition [6], magnetron sputtering [7,12] and electrodeposition [13] to fabricate the NiO have been successfully achieved to design a high efficiency ECD device. Considering these methods, electrodeposition seems to be effective strategy as it is rela tively simple, low-cost production and facile method for the large-area production of various oxide nanostructures that can regulate proper ties and qualities of the materials by manipulating some deposition conditions [14]. We recently reported that the electrochromic performance of NiO films can be increased by doping of Cu atoms into the films [15]. Based on the above considerations, the goal of this study is to fabricate Al-doped NiO thin films at different current density values onto ITO-coated glass substrate through a facile galvanostatic route of elec trodeposition, followed by annealing and the comprehensive electro chromic performance of the fabricated Al:NiO electrodes are investigated with each other as well.
glass substrate using galvanostatic mode of electrodeposition. In brief, 0.2 M NiSO4⋅6H2O was dissolved in 50 mL deionized water. Then pH of the solution was slowly adjusted to 8 with appropriate amount of KOH. To obtain a 5 mol % doped thin film, an appropriate amount of aluminum chloride (AlCl3) was used as source for Al element. The mixture was then well stirred under continuous sonication until uniform dispersed solution was formed. Fig. 1a shows galvanostatic deposition curves of the Al:NiO at different current densities on ITO-coated glass substrate. One-step galvanostatic electrodeposition was employed at a constant cathodic current density of 4 mA cm 2, 5 mA cm 2, 6 mA cm 2 and 7 mA cm 2 for 30 s, and the films are called sample A, sample B, sample C and sample D, respectively. After deposition, the fabricated films were washed with deionized water and then calcined by electric furnace with the temperature of 400 � C for 1 h in air atmosphere. Then, the films were naturally cooled at room temperature. 2.3. Characterizations The surface morphology and elemental composition of the produced films were observed by scanning electron microscopy (ZEISS Gem iniSEM 300) coupled with an energy dispersive X-rays spectroscopy (EDX). The X-ray diffraction (XRD) pattern of the films was analyzed with an PANalyical Xpert Pro MPD using a Cu Kα radiation source (λ ¼ 1.5418 Å) at the 2θ range of 5 to 85� . The UV-VIS measurements of the films were recorded with a Shimadzu UV-2600 spectrophotometer. 2.4. Electrochemical measurements All electrochemical tests were conducted on a conventional three electrode electrochemical workstation (Gamry Reference 3000 Poten tiostat/Galvanostat). The ITO-coated glass substrate, platinum wire and Ag/AgCl were utilized as working, counter and reference electrode, respectively. In this study, all applied potentials were referred with respect to Ag/AgCl electrode with no correction for Ohmic potential losses. The main electrochemical properties such as cyclic voltammetry (CV), chronoamperometry (CA) tests, electrochemical impedance spec troscopy (EIS) and Mott-Schottky were performed in 0.1 M KOH elec trolyte at room temperature. The colored and bleached response times of the films were evaluated by repeating the CA process switching poten tials of �1.0 V at a 20 s time step interval. The CV tests were carried out in the potential range from 0.0 to þ0.8 V at a scanning rate of 50 mVs 1 for 500 cycles. The electrochemical impedance spectra (EIS) measure ments were carried out by applying an AC amplitude of 5 mV in the frequency range 0.2 Hz–300 kHz. The Nyquist spectra were fitted to an equivalent circuit by Gamry Echem Analyst software. The Mott– Schottky plots of Al:NiO electrodes were applied under dark conditions at an AC potential of 5 mV for 1000 Hz in 0.1 M KOH solution. The
2. Experimental 2.1. Materials Nickel sulfate pentahydrate (NiSO4⋅6H2O, >99.0%), aluminum chloride (AlCl3, >99.0%) and potassium hydroxide (KOH, >99.0%) were purchased from Sigma-Aldrich. All chemicals were commercially purchased and used without further purification. Before the deposition, in order to remove contaminants from the surface, the (ITO)-coated glass (1 � 2 cm2 in size) with a sheet resistance of 0–10 Ω/cm2 was ultrasonically cleaned with deionized water, acetone and isopropyl alcohol for several times. All the reactions were prepared with deionized water (resistivity, 18.2 MΩ cm) as a solvent. 2.2. Synthesis of the Al:NiO thin films Al:NiO thin films were synthesized on indium tin oxide (ITO) coated
Fig. 1. a) Galvanostatic voltage-time transient curves at different current density values, b) X-ray diffraction pattern of the Al:NiO thin films (high magnification of the NiO(200) phase are depicted in the right). 2
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Materials Science in Semiconductor Processing 109 (2020) 104958
applied DC potential was ranged from 0.0 to mV/s.
1.0 V with intervals of 10
the samples, it can be seen that two characteristic peaks at 2θ ¼ 45.6� and 60.4� correspond to the to (200) and (220) crystal planes of facecentered cubic (fcc) NiO structure, respectively (JCDPS No. 47–1049) [8,9,17–19]. Due to the high intensity peaks of ITO, the peaks of NiO located at 45.6� are not seen clearly. For this purpose, the high magni fication of these peaks is given in the right side of Fig. 1b. In view of their intensity and number of peaks, the films have low crystallinity nature of NiO, which leads to the fast response time of the colored and bleached state. These findings are in accordance with the work done by Huang et al. [20]. They reported that high crystalline nature of the electrode gives rise to slow coloration time of ECDs. The different applied current density values in the deposition of Al:NiO nanoparticles don’t have a clear impact on their crystallinity. Moreover, there is no other apparent crystalline phase, which means that the Al is successfully embedded in the NiO crystal structure. This is also supported by the EDX analysis in the following section. It can be concluded that the heat treatment at 400 � C for 1 h in air makes it possible to achieve a crystalline nature of Al: NiO thin films. Fig. 2a–d shows the typical SEM images of Al:NiO thin films with different applied current density depositions. It is seen that the surface of the films turns from rectangular to spherical formations as the current density deposition increases, and there are clearly cracks on the surface of sample A and sample B thin films. Higher crack could be desirable since it contributes easier accessibility of Kþ ions for improving the response time. These observations are supported by the chro noamperometry results. The sample A with the lowest current density deposition has uniform surface in back plane, and composed of the aggregated structures that are oriented in different regions (Fig. 2a). It is clear to see from Fig. 2b that the sample B film displays crystalline rectangular nanorods array with a width of ~ 1 μm. Tiny needle-like crystallites are standing closely to these tops on the ITO surface. The Al:NiO thin film deposited at 6 mA cm 2 is formed by closely packed and spherically shaped nanoparticles with a diameter changing between 375 nm and 568 nm (Fig. 2c). When the deposition current density in creases to 7 mA cm 2, spherical shapes become less densely populated and the diameters of the spheres are in the interval between 193 and 504 nm (Fig. 2d). From the images, all thin films have highly compact on the ITO surface and this good compact leads to easier accessibility of the electron between external circuit and electrochromic electrode [21]. In order to determine the elemental composition of the prepared
3. Results and discussion 3.1. Galvanostatic electrodeposition of Al:NiO thin films To understand the effect of the applied current density in the syn thesis of Al:NiO films on ITO surface from the aqueous electrolyte composed of NiSO4⋅6H2O, AlCl3 and KOH, some galvanostatic experi ments were applied at different cathodic current density values such as the 4, 5, 6 and 7 mA cm 2. Fig. 1a shows the voltage transient curves of the fabricated films for 30 s. As expected, the starting potential increases with the increasing applied current density. Although the total deposi tion time is applied 30 s, the data records interestingly are limited within a certain time. This is most probably due to the low ion transfer speed of the electrolyte. As can be seen from Fig. 1a, V-t transient profiles have the same behavior, and consistent with the literature [16], but voltage variations are different. At the beginning deposition of the films, all of the curves are varied by a sharp voltage increase prior to approach a plateau. This may be correlated with the instantaneous growth and increasing nucleation process of Al:NiO. After the steady state potential (0–12 s), the absolute voltage value begins to increase due to continuous deposition of the layers when the ITO-coated glass substrate starts to be completely saturated. The possible growth mechanisms of NiO take place through: NiSO4 → Ni2þ þ SO24
(1)
SO24 þ H2 O þ 2e →SO23 þ 2OH
(2)
Ni2þ þ 2OH →NiðOHÞ2
(3)
NiðOHÞ2
heat; 400� C
→
(4)
NiO þ H2 O
3.2. Structural and morphological properties Fig. 1b presents the crystalline structure and phase of the Al:NiO thin films in the 2θ range of 5� –85� . The resultant diffraction peaks at 2θ ¼ 21.4� , 30.4� , 35.6� and 50.6� can be assigned to the ITO substrate. For all
Fig. 2. FE-SEM images of fabricated a) sample A, b) sample B, c) sample C and d) sample D. 3
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Materials Science in Semiconductor Processing 109 (2020) 104958
samples, energy dispersive X-ray spectrometry (EDX) measurement were carried out, and the presence of all elements of electrode materials are summarized in Table 1. The signal of Si comes from the substrate. The presence of K in the EDX analysis corresponds to the KOH solution used in the preparation of the Al:NiO thin films for pH adjustment. It is evident that the all the films include all expected elements Al, Ni and O, indicating successful formation of Al:NiO. The atomic percentage of aluminum varies between 0.32 and 4.48 depending on the deposition current density. The reason of Al element variations in the Al:NiO thin films with increasing applied current density values can be interpreted by the characteristic deposition current density of Al, which occurs intensely at 6 mA cm 2. It is concluded that the surface features and elemental constituents of the Al:NiO thin films strongly depend on the deposition current density (Fig. 2; Table 1).
B-substituted NiO films using sol-gel technique, and they found the CE of 30 cm2/C, which is slightly lower than that of the work presented here [24]. 3.4. Electrochromic properties The switching time characteristic of the Al:NiO electrode between colored and bleached state is of great importance for its practical application in ECDs. To further quantify the response times, the chro noamperometry was carried out by setting the voltages between þ1.0 V and 1.0 V for a step of 20s (Fig. 5). Generally, the term ‘‘response time’’ refers to the time required for a current density to reach 90% of its ab solute maximum value within a potential step determined by the chro noamperometry curve. The response times of the sample A, B, C and D are listed in Table 2. As can be seen from Fig. 5, the response times of sample A for bleached (tb) and colored (tc) state are 0.85 and 1.9 s, respectively, which demonstrates a best kinetic reaction among all fabricated films. The calculated response times of the films for bleaching are in the range of 0.85–3.15 s that highly comparable to the work done by Rocha [25] (0.8–3.0 s). It is worth to mention here that the response speeds for the bleached state are much faster than that of colored due to the higher conductivity of the bleached state of the NiO [26]. To explore the electrochemical activity and stability of the Al:NiO, the cyclic voltammetry analysis was performed a potential window of 0.0 to þ0.8 V at a scan rate of 50 mV s 1 in 0.1 M KOH for 500 cycles (Fig. 6). According to the coloration efficiency analysis, the sample A shows the best performance among all films. Therefore, the sample A was selected to examine cyclic stability in the 0.1 M KOH electrolyte. The anodic peak at around þ0.6 V and cathodic peak at around þ0.1 V can be clearly seen on the CV curves of the Al:NiO electrode, which is in accordance with the electron transfer reaction reported in the literature [3]. It is also illustrated in Fig. 6 that the Al:NiO electrode clearly shows large integrated within the CV curves, revealing good rate capability in the solution. The reversible redox mechanism can be written as [3]:
3.3. Optical properties The optical transmittance spectra of the Al:NiO thin films in its bleached and colored states were recorded in the wavelength range of 300–1400 nm (Fig. 3). In this test, bleached state of the Al:NiO films was tuned through applying a potential of 1 V and colored by a voltage of þ1 V. The calculated optical modulation values are listed in Table 2. For the optical modulation, owing to the enhanced electrochemical activity to improve Kþ insertion and extraction, the highest value (71.16% at 550 nm) is observed in the sample A. Compared to the sample A film, the optical modulation of the sample B, C and D is evidently lower at the wavelength of 550 nm probably due to a synergistic effect between the NiO and Al. Such variations may be due to the distinct morphologies and different ratio of Al elements in the film. In order to better understand the electrochromic performance of the ECD material, the coloration efficiency CE (η) is one of the fundamental indicative parameter. The CE (η) represents the change in optical density (ΔOD) per unit charge density (Qin), and can be formulated as following relations [6,22]: CEðηÞ ¼
ðΔODÞλ¼550 nm Qin
ðΔODÞ ¼ log Z
(9)
Tb Tc
(11)
IðtÞ⋅dt t1
where Tb and Tc are the optical transmittances of the Al:NiO thin films in bleached state and the colored state at a certain wavelength, respectively. ΔOD is the change in optical density. Qin corresponds the inserted per unit area within the colored time, obtained from integrating the area under the chronoamperometry curve. The spectral of coloration efficiency (η) as a function of wavelength (λ) is plotted in Fig. 4. It can be seen from this figure, the CE for the electrodeposited sample B reaches to a maximum value of 50 cm2/C at low wavelengths, which reveals the good ion access in EC performance. For sample A, B and D, CE values for ion insertion were found to be 39.89, 36.82 and 37.59 cm2/C at 550 nm, respectively, which is nearly ~2 times greater than that of the sample C (19.05 cm2/C). The obtained result appears to be comparable with those reported in the literature [23]. Lou et al. have prepared Al doped
Al
Ni
O
Si
K
S
C
Sample A Sample B Sample C Sample D
0.32 1.11 4.48 0.52
55.08 66.7 47.97 55.89
28.32 17.07 37.48 32.37
0.03 1.20 0.30 0.06
7.56 5.48 1.79 4.07
5.07 7.72 9.36 3.84
3.63 0.72 3.61 3.26
NiO þ H2 O ↔ NiOOH þ Hþ þ e
(6)
3.5. ElS and Mott-Schottky analysis Electrochemical impedance spectroscopy (EIS) is an informative technique to further understand the electrochemical behavior of elec trode materials for ECDs. Fig. 7a shows the Nyquist plot for the Al:NiO electrode in 0.1 M KOH within the frequency range from 0.2 Hz to 300 kHz. From Fig. 7a, it can be seen that the Nyquist plot for all the films are highly similar. The simulative equivalent circuit diagram is shown in the inset image, which is constituted by a resistance of deposition solution RS, which is in series with the parallel connection of a charge transfer resistance (Rct) and a constant phase element (CPE). As can be seen that the observed impedance behavior of the Al:NiO electrode is composed of a partial semicircle curve in the high and low frequency parts. As we know the diameter of the semicircle corresponded to the charge transfer resistance (Rct) or electrochemical reaction resistance that is related to the interface between the Al:NiO electrode and electrolyte. Moreover, the Rs value can be obtained by the intercept at the real impedance axis (Z0 ) in the high frequency. Given the microscopic roughness of the electrode material, constant phase element (CPE) is used instead of pure capacitor, which can lead to an inhomogeneous distribution in the so lution resistance. Constant phase element (CPE) as a function of fre quency could be given in below [27]:
Table 1 Elemental compositions of the Al-doped NiO films (including at% ratio). Elements → Samples↓
(5)
It is important to comment that the OH ion exchanging in the intercalation/deintercalation process along with the reduction reactions of Ni2þ to Ni3þ/Ni2þ to Ni3þ causes the change in optical properties of the films. From cyclic stability one can understand Al:NiO material has stable and desirable for a good electrode material after 500 cycle.
(10)
t2
Qin ¼
NiOðbleachÞ þ xOH þ xe ↔ NiOOHðdark brownÞ
4
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Materials Science in Semiconductor Processing 109 (2020) 104958
Fig. 3. a) The optical transmittance in the colored and bleached state for a) sample A, b) sample B, c) sample C and d) sample D. Table 2 Electrochromic parameters of fabricated sample A, B, C and D (at the wavelength of 550 nm). Samples
Sample A Sample B Sample C Sample D
Qin (mC) 13.49 25.75 29.98 28.07
ZCPE ¼ 1=½Y0 ðj⋅ωÞα �
Response Time (s) tb
tc
0.85 1.65 2.51 3.15
1.90 3.88 5.19 5.80
Optical modulation
Coloration Efficiency (cm2/C)
71.16 54.26 43.61 62.58
39.89 36.82 19.05 37.59
(12)
Where Y0 is the capacitance when α ¼ 1 (constant) and ω is the angular frequency. The fitted EIS parameters are listed in Table 3. From the goodness of fit degree value of 10 6, well agreement is observed be tween the measured and simulated data. It can be see that the sample B possesses lowest RS and RCT values of 33.47 and 2168 Ω, respectively, which may be due to easy diffusion and movement of electrons/ions on the surface of sample B, thus confirming highest electrochemical con ductivity as compared to the others. The highest charge transfers resis tance (RCT) and the solution resistance values (RS) are found to be 7573 and 325.6 Ω, respectively, for sample D. The calculated EIS parameters are comparable to the data derived from the work previously published by Yang et al. [28] while studying the electrochemical performance of nickel-oxide/carbon sphere hybrids synthesized reflux-hydrothermal technique. They reported that the charge transfer resistance values of the produced films were in the range of 913 and 6223 Ω. These results
Fig. 4. Chronoamperometry curves with potential range from for different Al:NiO thin films.
1.0 V to þ1.0 V
indicate that, the improved electrical conductivity of Al:NiO electrode might be one of the crucial factors contributing to small diffusion limitation. Mott-Schottky analysis is performed to get information about the semiconducting properties of oxide films in contact with electrolytes, and it is commonly used to estimate the carrier density, types of majority carriers and flat band potential of a semiconductor. The capacitance as a function of the electrode potential (V) can be calculated through M S 5
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Materials Science in Semiconductor Processing 109 (2020) 104958
where C is the interfacial capacitance, NA is the number of acceptors for p-type semiconductor, V is the applied voltage, VFB is the flat band po tential, kB is Boltzmann’s constant (1.381 � 10 23 J K 1), T is the ab solute temperature, q is the elementary charge (-e for electron and þe for holes) (1.602 � 10 19 C), ϵ0 and εr is vacuum permittivity (8.85 � 10 14 Fcm 1) and dielectric constant of the deposited films (~12 for NiO), respectively. The Mott-Schottky plots of the deposited Al:NiO films are given in Fig. 7b. The negative slope of the linear part for each line plot confirms the p-type semiconducting behavior of Al:NiO electrodes, a shape that is congruent with the reported in the literature [30]. The acceptor density (NA) can also be conveniently calculated from the slope of the M S plot and the values are determined to be in the range of 1012 - 1015 cm 3 depending on the deposition current density. This significant change in NA demonstrates that doping of Al element causes NiO electrode to be more p-type, probably by the replacement of Ni2þ by Alþ atoms at the electrode surface. Also, the quantitatively analyzes of NA are consistent with the EDX results. Moreover, from Fig. 7b it is possible to determine the value of flat-band potential (VFB), which obtained by the extrapo lation of the linear portion of the curve to 1/C2 ¼ 0 at potential (V) axis. When the applied current density for the synthesis of Al:NiO is raised, a negative shift of VFB is seen as it changes from 0.794 V to 0.976 V (Table 4).
Fig. 5. The variation of coloration efficiency (η) of sample A, B, C and D as a function of wavelength.
4. Conclusions As a consequence, it is reported a simple, eco-friendly and facile method for the direct growth of the Al:NiO thin films on the ITO-coated glass substrates at four different deposition current densities in the range between 4 mA cm 2 and 7 mA cm 2, followed by heat treatment at a temperature of 400 � C in air. The SEM analysis displays that different growth current densities cause a considerable variation of morpholog ical features and elemental compositions of the electrode surface. From XRD analysis of all the samples, it is found that the different ratios of Al Table 3 EIS Parameters obtained from Nyquist Plots of Al:NiO electrodes.
Fig. 6. Cyclic voltammogram in 0.1 M KOH electrolyte at 50 mV s to 500th.
equation as follows [29]: � �� � 1 2 kT ¼ V V FB C2 eε0 εr NA e
1
from 1st
Deposited Films→ Parameters↓
Sample A
Sample B
Sample C
Sample D
RS (ohms) RCT (ohms) Y0 (S‧s2)
187.3 4302 353.4 � 10 6 879.9 � 10 3 2.863 � 10 3
33.47 2168 894.5 � 10 6 860.6 � 10 3 570 � 10
42.79 8281 229.8 � 10 6 916.3 � 10 3 890.9 � 10 6
325.6 7373 321.7 � 10 6 835.6 � 10 3 23.1 � 10
Alpha
(13)
Goodness of Fit
6
Fig. 7. a) Nyquist plots (the inset shows the equivalent circuit model), b) Mott-schottky plots of the Al:NiO thin films. 6
3
Y.E. Firat
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Kim, Direct growth of NiO nanosheets on mesoporous TiN film for energy storage devices, Appl. Surf. Sci. 420 (2017) 849–857, https://doi.org/10.1016/j.apsusc.2017.05.216. [20] Q. Huang, Q. Zhang, Y. Xiao, Y. He, X. Diao, Improved electrochromic performance of NiO-based thin films by lithium and tantalum co-doping, J. Alloys Compd. 747 (2018) 416–422, https://doi.org/10.1016/j.jallcom.2018.02.232. [21] F. Lin, D. Nordlund, T.C. Weng, R.G. Moore, D.T. Gillaspie, A.C. Dillon, R. M. Richards, C. Engtrakul, Hole doping in al-containing nickel oxide materials to improve electrochromic performance, ACS Appl. Mater. Interfaces 5 (2013) 301–309, https://doi.org/10.1021/am302097b. [22] N. Akkurt, S. Pat, S. Elmas, S¸. Korkmaz, Electrochromic properties of UV - colored WO 3 thin film deposited by thermionic vacuum arc, J. Mater. Sci. Mater. Electron. (2019), https://doi.org/10.1007/s10854-019-02642-w. [23] V.V. Kondalkar, P.B. Patil, R.M. Mane, P.S. Patil, S. Choudhury, P.N. Bhosal, Electrochromic performance of nickel oxide thin film: synthesis via electrodeposition technique, Macromol. Symp. 361 (2016) 47–50, https://doi.org/ 10.1002/masy.201400253. [24] X. Lou, X. Zhao, J. Feng, X. Zhou, Electrochromic properties of Al doped Bsubsituted NiO films prepared by sol-gel, Prog. Org. Coating 64 (2009) 300–303, https://doi.org/10.1016/j.porgcoat.2008.09.006. [25] M. Da Rocha, Y. He, X. Diao, A. Rougier, Influence of cycling temperature on the electrochromic properties of WO3//NiO devices built with various thicknesses, Sol. Energy Mater. Sol. Cells 177 (2018) 57–65, https://doi.org/10.1016/j. solmat.2017.05.070. [26] K.S. Ahn, Y.C. Nah, Y.E. Sung, K.Y. Cho, S.S. Shin, J.K. Park, All-solid-state electrochromic device composed of WO3 and Ni(OH)2 with a Ta2O5 protective layer, Appl. Phys. Lett. 81 (2002) 3930–3932, https://doi.org/10.1063/ 1.1522478. [27] A.M. Al-Enizi, A.A. Elzatahry, A.M. Abdullah, A. Vinu, H. Iwai, S.S. Al-Deyab, High electrocatalytic performance of nitrogen-doped carbon nanofiber-supported nickel oxide nanocomposite for methanol oxidation in alkaline medium, Appl. Surf. Sci. 401 (2017) 306–313, https://doi.org/10.1016/j.apsusc.2017.01.038. [28] X. Yang, Y. Zhang, G. Wu, C. Zhu, W. Zou, Y. Gao, J. Tian, Z. Zheng, Nanoelectrical investigation and electrochemical performance of nickel-oxide/carbon sphere hybrids through interface manipulation, J. Colloid Interface Sci. 469 (2016) 287–295, https://doi.org/10.1016/j.jcis.2016.02.031. [29] S. Zhang, R. Shi, Y. Tan, Corrosion behavior of the oxide films modified with zincizing treatment on AISI 1020 steel, J. Alloys Compd. 711 (2017) 155–161, https://doi.org/10.1016/j.jallcom.2017.03.327. [30] Z. Saki, K. Sveinbj€ ornsson, G. Boschloo, N. Taghavinia, The effect of lithium-doping in solution-processed nickel oxide films for perovskite solar cells, ChemPhysChem (2019) 3322–3327, https://doi.org/10.1002/cphc.201900856.
Table 4 Acceptor density and flat-band values of Al:NiO thin films. Parameters → Deposited Films ↓
NA (1/cm3)
Sample A Sample B Sample C Sample D
1.59 1.29 2.09 8.01
� 1012 � 1015 � 1015 � 1014
VFB (Volt) 0.794 0.975 0.976 0.929
Carrier type p p p p
element don’t change the crystal phase of NiO and it is only located. The presence of characteristic peaks of NiO is supported by XRD as well as by EDX. Upon applying potentials of �1 V, the color of Al:NiO electrode switches between dark brown and transparent. Coloration efficiency varies between 19.05 cm2/C and 39.89 cm2/C depending on the in crease of the applied current density in the synthesis of Al:NiO thin films. All of the deposits have p-type semiconducting behavior of Al:NiO thin films, which is verified by Mott-schottky analysis. These kinds of Al:NiO thin films can be expected to be a promising electrode, which may provide a new approach to counter electrode for next-generation and high-performance electrochromic devices. Declaration of competing interests The authors declare the following financial interests/personal re lationships which may be considered as potential competing interests: Acknowledgements � Uni This work was supported by the Research Fund of the Uludag versity, Project number HDP(F)–2017/23. The author thanks to Uludag University for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2020.104958. References [1] R.T. Wen, G.A. Niklasson, C.G. Granqvist, Electrochromic iridium-containing nickel oxide films with excellent electrochemical cycling performance, J. Electrochem. Soc. 163 (2016) E7–E13, https://doi.org/10.1149/2.0591602jes. [2] R.T. Wen, G.A. Niklasson, C.G. Granqvist, Strongly improved electrochemical cycling durability by adding iridium to electrochromic nickel oxide films, ACS Appl. Mater. Interfaces 7 (2015) 9319–9322, https://doi.org/10.1021/ acsami.5b01715. [3] Y.F. Yuan, X.H. Xia, J.B. Wu, Y.B. Chen, J.L. Yang, S.Y. Guo, Enhanced electrochromic properties of ordered porous nickel oxide thin film prepared by selfassembled colloidal crystal template-assisted electrodeposition, Electrochim. Acta 56 (2011) 1208–1212, https://doi.org/10.1016/j.electacta.2010.10.097. � [4] R. Cerc Koro�sec, M. Felicijan, B. Zener, M. Pompe, G. Dra�zi�c, J. Pade�znik Gomil�sek, B. Pihlar, P. Bukovec, The role of thermal analysis in optimization of electrochromic effect of nickel oxide thin films, prepared by the sol-gel method: Part III, Thermochim. Acta 655 (2017) 344–350, https://doi.org/10.1016/j. tca.2017.07.010. [5] J. Shi, L. Lai, P. Zhang, H. Li, Y. Qin, Y. Gao, L. Luo, J. Lu, Aluminum doped nickel oxide thin film with improved electrochromic performance from layered double hydroxides precursor in situ pyrolytic route, J. Solid State Chem. 241 (2016) 1–8, https://doi.org/10.1016/j.jssc.2016.05.032. [6] J.H. Zhang, G.F. Cai, D. Zhou, H. Tang, X.L. Wang, C.D. Gu, J.P. Tu, Co-doped NiO nanoflake array films with enhanced electrochromic properties, J. Mater. Chem. C. 2 (2014) 7013–7021, https://doi.org/10.1039/c4tc01033g. [7] F. Lin, D.T. Gillaspie, A.C. Dillon, R.M. Richards, C. Engtrakul, Nitrogen-doped nickel oxide thin films for enhanced electrochromic applications, Thin Solid Films 527 (2013) 26–30, https://doi.org/10.1016/j.tsf.2012.12.031.
7
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Influence of current density on Al:NiO thin films via electrochemical deposition: Semiconducting and electrochromic properties Y.E. Firat * Physics Department, Kamil Ozdag Faculty of Sciences, Karamanoglu Mehmetbey University, Yunus Emre Campus, 70100, Karaman, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords: Nickel oxide Electrochromic Electrodeposition Coloration efficiency Mott-Schottky
The uniform aluminum-doped nickel oxide (Al:NiO) thin films were fabricated on indium tin oxide (ITO) by an efficient route of galvanostatic mode together with the help of conventional electrochemical methods. The cathodic deposition current densities were increased from 4 mA cm 2 to 7 mA cm 2 by a step of 1 mA cm 2 for the growth time of 30 s. The produced films were annealed in air at 400 � C for 1 h. The effect of deposition current on the behaviors of Al:NiO thin films were investigated by a number of methods including scanning electron microscopy (SEM), energy dispersive X-rays analysis (EDX), X-ray diffraction (XRD), Ultraviolet–Visible (UV–Vis) spectroscopy, Mott-Schottky analysis, and electrochemical impedance spectroscopy (EIS). X-ray diffraction analysis confirms the crystallinity of all deposits. SEM studies indicate that the surface morphologies of Al:NiO alter sensitively depending on the applied current density. From chronoamperometric curves of the two stage of the produced films, the fastest response times are found to be tb ¼ 0.85 s and tc ¼ 1.90 s for Al:NiO at an applied current density of 4 mA cm 2. It is found that the average coloration efficiency (η) reaches to a value of 50 cm2/C for Al:NiO fabricated at 5 mA cm 2. Significantly, the electrode displays long-term cyclic durability after 500 cycles in the potential window from 0 to þ0.8 V in 0.1 M potassium hydroxide (KOH) aqueous solution. The concept of Mott-Schottky is considered as approach that confirms the p-type semiconducting behavior of all the Al:NiO films with the order of ~1012 - 1015 cm 3 acceptor density. The results demonstrate promising electrochemical properties to enhance the electrochromic performance of NiO thin films which is required for the growth of outstanding electrode materials in smart glass and open a new insight into further applications of devices.
1. Introduction
are expected to have potential candidate for their outstanding high ef ficiency applied in smart glass and supercapacitors. As an important family of metal oxides, nickel oxides have emerged as alternative anodic p-type materials with its high theoretical capacitance to store the charges, fast transfer/diffusion rate of electrons and electrolyte ions and low cost [3,6,8–10]. However, development of desired electrode mate rials with metal oxides, in particular NiO and WO3, is a major drawback such as limited electrical conductivity [8] and poor electrochemical durability, which restricts their high-performance electrochromic device applications [3,6]. An efficient strategy to improve the performance of electrochromic device can be achieved by the substitution of dopant elements into the electrochromic host material. As it was already verified many report attentions by several re searchers, approaches to enriching the number of free charge-carriers and accessible surface areas in electrochromic devices include substi tution of foreign elements into NiO electrode material, leading greatly to
Electrochromic devices (ECDs) have emerged as an energy saving technology for decades due to appropriate control of solar heat and reducing building energy use. ECDs basically refer to reversible con version of optical property, induced electrochemically, whose macro scopic effect is the variation of color by the applied electrical field (or voltage) [1–7]. As is well known that the layout of an electrochromic device, structurally, consists of several electrodes; glass, transparent electrode, electrochromic layer, electrolyte, ion storage layer, trans parent electrode and glass [1]. To further promote high performance of the device, electrochromic layers that largely affect the properties of the ECDs should be considered. On the basis of the material components, electrochromic materials can be classified into two main groups; organic and transition metal oxide materials. Compared with organic-based materials, metal oxides
* Solar Cell Laboratory, Physics Department, Sciences and Arts Faculty, Uludag University, 16059, Gorukle, Bursa, Turkey. E-mail address: [email protected]. https://doi.org/10.1016/j.mssp.2020.104958 Received 9 November 2019; Received in revised form 16 January 2020; Accepted 20 January 2020 Available online 26 January 2020 1369-8001/© 2020 Elsevier Ltd. All rights reserved.
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Materials Science in Semiconductor Processing 109 (2020) 104958
improvement in electrical conductivity and performances. For instance, Wen et al. have reported the synthesis of Ir–NiO, which exhibits excel lent durability during cycling LiClO4-PC and the lattice constant of the electrode was increased by the adding of Ir element [1]. Zhang et al. demonstrated the electronic conductivity of NiO was enhanced by appropriate doping of Co and, in consequence, the material had higher coloration efficiency of 47.7 cm2 C 1 at a wavelength of 550 nm [6]. Lin et al. synthesized a nitrogen (N) doped NiO electrochromic film by means of radio frequency magnetron sputtering using a ceramic NiO target [7]. They found that the charge reversibility, coloration efficiency and switching times for the N-doped NiO were significantly enhanced compare to pure NiO thin films [7]. Many approaches, including dip coating [9], electrodeposition [11], reactive dc magnetron sputtering [1], chemical bath deposition [6], magnetron sputtering [7,12] and electrodeposition [13] to fabricate the NiO have been successfully achieved to design a high efficiency ECD device. Considering these methods, electrodeposition seems to be effective strategy as it is rela tively simple, low-cost production and facile method for the large-area production of various oxide nanostructures that can regulate proper ties and qualities of the materials by manipulating some deposition conditions [14]. We recently reported that the electrochromic performance of NiO films can be increased by doping of Cu atoms into the films [15]. Based on the above considerations, the goal of this study is to fabricate Al-doped NiO thin films at different current density values onto ITO-coated glass substrate through a facile galvanostatic route of elec trodeposition, followed by annealing and the comprehensive electro chromic performance of the fabricated Al:NiO electrodes are investigated with each other as well.
glass substrate using galvanostatic mode of electrodeposition. In brief, 0.2 M NiSO4⋅6H2O was dissolved in 50 mL deionized water. Then pH of the solution was slowly adjusted to 8 with appropriate amount of KOH. To obtain a 5 mol % doped thin film, an appropriate amount of aluminum chloride (AlCl3) was used as source for Al element. The mixture was then well stirred under continuous sonication until uniform dispersed solution was formed. Fig. 1a shows galvanostatic deposition curves of the Al:NiO at different current densities on ITO-coated glass substrate. One-step galvanostatic electrodeposition was employed at a constant cathodic current density of 4 mA cm 2, 5 mA cm 2, 6 mA cm 2 and 7 mA cm 2 for 30 s, and the films are called sample A, sample B, sample C and sample D, respectively. After deposition, the fabricated films were washed with deionized water and then calcined by electric furnace with the temperature of 400 � C for 1 h in air atmosphere. Then, the films were naturally cooled at room temperature. 2.3. Characterizations The surface morphology and elemental composition of the produced films were observed by scanning electron microscopy (ZEISS Gem iniSEM 300) coupled with an energy dispersive X-rays spectroscopy (EDX). The X-ray diffraction (XRD) pattern of the films was analyzed with an PANalyical Xpert Pro MPD using a Cu Kα radiation source (λ ¼ 1.5418 Å) at the 2θ range of 5 to 85� . The UV-VIS measurements of the films were recorded with a Shimadzu UV-2600 spectrophotometer. 2.4. Electrochemical measurements All electrochemical tests were conducted on a conventional three electrode electrochemical workstation (Gamry Reference 3000 Poten tiostat/Galvanostat). The ITO-coated glass substrate, platinum wire and Ag/AgCl were utilized as working, counter and reference electrode, respectively. In this study, all applied potentials were referred with respect to Ag/AgCl electrode with no correction for Ohmic potential losses. The main electrochemical properties such as cyclic voltammetry (CV), chronoamperometry (CA) tests, electrochemical impedance spec troscopy (EIS) and Mott-Schottky were performed in 0.1 M KOH elec trolyte at room temperature. The colored and bleached response times of the films were evaluated by repeating the CA process switching poten tials of �1.0 V at a 20 s time step interval. The CV tests were carried out in the potential range from 0.0 to þ0.8 V at a scanning rate of 50 mVs 1 for 500 cycles. The electrochemical impedance spectra (EIS) measure ments were carried out by applying an AC amplitude of 5 mV in the frequency range 0.2 Hz–300 kHz. The Nyquist spectra were fitted to an equivalent circuit by Gamry Echem Analyst software. The Mott– Schottky plots of Al:NiO electrodes were applied under dark conditions at an AC potential of 5 mV for 1000 Hz in 0.1 M KOH solution. The
2. Experimental 2.1. Materials Nickel sulfate pentahydrate (NiSO4⋅6H2O, >99.0%), aluminum chloride (AlCl3, >99.0%) and potassium hydroxide (KOH, >99.0%) were purchased from Sigma-Aldrich. All chemicals were commercially purchased and used without further purification. Before the deposition, in order to remove contaminants from the surface, the (ITO)-coated glass (1 � 2 cm2 in size) with a sheet resistance of 0–10 Ω/cm2 was ultrasonically cleaned with deionized water, acetone and isopropyl alcohol for several times. All the reactions were prepared with deionized water (resistivity, 18.2 MΩ cm) as a solvent. 2.2. Synthesis of the Al:NiO thin films Al:NiO thin films were synthesized on indium tin oxide (ITO) coated
Fig. 1. a) Galvanostatic voltage-time transient curves at different current density values, b) X-ray diffraction pattern of the Al:NiO thin films (high magnification of the NiO(200) phase are depicted in the right). 2
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Materials Science in Semiconductor Processing 109 (2020) 104958
applied DC potential was ranged from 0.0 to mV/s.
1.0 V with intervals of 10
the samples, it can be seen that two characteristic peaks at 2θ ¼ 45.6� and 60.4� correspond to the to (200) and (220) crystal planes of facecentered cubic (fcc) NiO structure, respectively (JCDPS No. 47–1049) [8,9,17–19]. Due to the high intensity peaks of ITO, the peaks of NiO located at 45.6� are not seen clearly. For this purpose, the high magni fication of these peaks is given in the right side of Fig. 1b. In view of their intensity and number of peaks, the films have low crystallinity nature of NiO, which leads to the fast response time of the colored and bleached state. These findings are in accordance with the work done by Huang et al. [20]. They reported that high crystalline nature of the electrode gives rise to slow coloration time of ECDs. The different applied current density values in the deposition of Al:NiO nanoparticles don’t have a clear impact on their crystallinity. Moreover, there is no other apparent crystalline phase, which means that the Al is successfully embedded in the NiO crystal structure. This is also supported by the EDX analysis in the following section. It can be concluded that the heat treatment at 400 � C for 1 h in air makes it possible to achieve a crystalline nature of Al: NiO thin films. Fig. 2a–d shows the typical SEM images of Al:NiO thin films with different applied current density depositions. It is seen that the surface of the films turns from rectangular to spherical formations as the current density deposition increases, and there are clearly cracks on the surface of sample A and sample B thin films. Higher crack could be desirable since it contributes easier accessibility of Kþ ions for improving the response time. These observations are supported by the chro noamperometry results. The sample A with the lowest current density deposition has uniform surface in back plane, and composed of the aggregated structures that are oriented in different regions (Fig. 2a). It is clear to see from Fig. 2b that the sample B film displays crystalline rectangular nanorods array with a width of ~ 1 μm. Tiny needle-like crystallites are standing closely to these tops on the ITO surface. The Al:NiO thin film deposited at 6 mA cm 2 is formed by closely packed and spherically shaped nanoparticles with a diameter changing between 375 nm and 568 nm (Fig. 2c). When the deposition current density in creases to 7 mA cm 2, spherical shapes become less densely populated and the diameters of the spheres are in the interval between 193 and 504 nm (Fig. 2d). From the images, all thin films have highly compact on the ITO surface and this good compact leads to easier accessibility of the electron between external circuit and electrochromic electrode [21]. In order to determine the elemental composition of the prepared
3. Results and discussion 3.1. Galvanostatic electrodeposition of Al:NiO thin films To understand the effect of the applied current density in the syn thesis of Al:NiO films on ITO surface from the aqueous electrolyte composed of NiSO4⋅6H2O, AlCl3 and KOH, some galvanostatic experi ments were applied at different cathodic current density values such as the 4, 5, 6 and 7 mA cm 2. Fig. 1a shows the voltage transient curves of the fabricated films for 30 s. As expected, the starting potential increases with the increasing applied current density. Although the total deposi tion time is applied 30 s, the data records interestingly are limited within a certain time. This is most probably due to the low ion transfer speed of the electrolyte. As can be seen from Fig. 1a, V-t transient profiles have the same behavior, and consistent with the literature [16], but voltage variations are different. At the beginning deposition of the films, all of the curves are varied by a sharp voltage increase prior to approach a plateau. This may be correlated with the instantaneous growth and increasing nucleation process of Al:NiO. After the steady state potential (0–12 s), the absolute voltage value begins to increase due to continuous deposition of the layers when the ITO-coated glass substrate starts to be completely saturated. The possible growth mechanisms of NiO take place through: NiSO4 → Ni2þ þ SO24
(1)
SO24 þ H2 O þ 2e →SO23 þ 2OH
(2)
Ni2þ þ 2OH →NiðOHÞ2
(3)
NiðOHÞ2
heat; 400� C
→
(4)
NiO þ H2 O
3.2. Structural and morphological properties Fig. 1b presents the crystalline structure and phase of the Al:NiO thin films in the 2θ range of 5� –85� . The resultant diffraction peaks at 2θ ¼ 21.4� , 30.4� , 35.6� and 50.6� can be assigned to the ITO substrate. For all
Fig. 2. FE-SEM images of fabricated a) sample A, b) sample B, c) sample C and d) sample D. 3
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Materials Science in Semiconductor Processing 109 (2020) 104958
samples, energy dispersive X-ray spectrometry (EDX) measurement were carried out, and the presence of all elements of electrode materials are summarized in Table 1. The signal of Si comes from the substrate. The presence of K in the EDX analysis corresponds to the KOH solution used in the preparation of the Al:NiO thin films for pH adjustment. It is evident that the all the films include all expected elements Al, Ni and O, indicating successful formation of Al:NiO. The atomic percentage of aluminum varies between 0.32 and 4.48 depending on the deposition current density. The reason of Al element variations in the Al:NiO thin films with increasing applied current density values can be interpreted by the characteristic deposition current density of Al, which occurs intensely at 6 mA cm 2. It is concluded that the surface features and elemental constituents of the Al:NiO thin films strongly depend on the deposition current density (Fig. 2; Table 1).
B-substituted NiO films using sol-gel technique, and they found the CE of 30 cm2/C, which is slightly lower than that of the work presented here [24]. 3.4. Electrochromic properties The switching time characteristic of the Al:NiO electrode between colored and bleached state is of great importance for its practical application in ECDs. To further quantify the response times, the chro noamperometry was carried out by setting the voltages between þ1.0 V and 1.0 V for a step of 20s (Fig. 5). Generally, the term ‘‘response time’’ refers to the time required for a current density to reach 90% of its ab solute maximum value within a potential step determined by the chro noamperometry curve. The response times of the sample A, B, C and D are listed in Table 2. As can be seen from Fig. 5, the response times of sample A for bleached (tb) and colored (tc) state are 0.85 and 1.9 s, respectively, which demonstrates a best kinetic reaction among all fabricated films. The calculated response times of the films for bleaching are in the range of 0.85–3.15 s that highly comparable to the work done by Rocha [25] (0.8–3.0 s). It is worth to mention here that the response speeds for the bleached state are much faster than that of colored due to the higher conductivity of the bleached state of the NiO [26]. To explore the electrochemical activity and stability of the Al:NiO, the cyclic voltammetry analysis was performed a potential window of 0.0 to þ0.8 V at a scan rate of 50 mV s 1 in 0.1 M KOH for 500 cycles (Fig. 6). According to the coloration efficiency analysis, the sample A shows the best performance among all films. Therefore, the sample A was selected to examine cyclic stability in the 0.1 M KOH electrolyte. The anodic peak at around þ0.6 V and cathodic peak at around þ0.1 V can be clearly seen on the CV curves of the Al:NiO electrode, which is in accordance with the electron transfer reaction reported in the literature [3]. It is also illustrated in Fig. 6 that the Al:NiO electrode clearly shows large integrated within the CV curves, revealing good rate capability in the solution. The reversible redox mechanism can be written as [3]:
3.3. Optical properties The optical transmittance spectra of the Al:NiO thin films in its bleached and colored states were recorded in the wavelength range of 300–1400 nm (Fig. 3). In this test, bleached state of the Al:NiO films was tuned through applying a potential of 1 V and colored by a voltage of þ1 V. The calculated optical modulation values are listed in Table 2. For the optical modulation, owing to the enhanced electrochemical activity to improve Kþ insertion and extraction, the highest value (71.16% at 550 nm) is observed in the sample A. Compared to the sample A film, the optical modulation of the sample B, C and D is evidently lower at the wavelength of 550 nm probably due to a synergistic effect between the NiO and Al. Such variations may be due to the distinct morphologies and different ratio of Al elements in the film. In order to better understand the electrochromic performance of the ECD material, the coloration efficiency CE (η) is one of the fundamental indicative parameter. The CE (η) represents the change in optical density (ΔOD) per unit charge density (Qin), and can be formulated as following relations [6,22]: CEðηÞ ¼
ðΔODÞλ¼550 nm Qin
ðΔODÞ ¼ log Z
(9)
Tb Tc
(11)
IðtÞ⋅dt t1
where Tb and Tc are the optical transmittances of the Al:NiO thin films in bleached state and the colored state at a certain wavelength, respectively. ΔOD is the change in optical density. Qin corresponds the inserted per unit area within the colored time, obtained from integrating the area under the chronoamperometry curve. The spectral of coloration efficiency (η) as a function of wavelength (λ) is plotted in Fig. 4. It can be seen from this figure, the CE for the electrodeposited sample B reaches to a maximum value of 50 cm2/C at low wavelengths, which reveals the good ion access in EC performance. For sample A, B and D, CE values for ion insertion were found to be 39.89, 36.82 and 37.59 cm2/C at 550 nm, respectively, which is nearly ~2 times greater than that of the sample C (19.05 cm2/C). The obtained result appears to be comparable with those reported in the literature [23]. Lou et al. have prepared Al doped
Al
Ni
O
Si
K
S
C
Sample A Sample B Sample C Sample D
0.32 1.11 4.48 0.52
55.08 66.7 47.97 55.89
28.32 17.07 37.48 32.37
0.03 1.20 0.30 0.06
7.56 5.48 1.79 4.07
5.07 7.72 9.36 3.84
3.63 0.72 3.61 3.26
NiO þ H2 O ↔ NiOOH þ Hþ þ e
(6)
3.5. ElS and Mott-Schottky analysis Electrochemical impedance spectroscopy (EIS) is an informative technique to further understand the electrochemical behavior of elec trode materials for ECDs. Fig. 7a shows the Nyquist plot for the Al:NiO electrode in 0.1 M KOH within the frequency range from 0.2 Hz to 300 kHz. From Fig. 7a, it can be seen that the Nyquist plot for all the films are highly similar. The simulative equivalent circuit diagram is shown in the inset image, which is constituted by a resistance of deposition solution RS, which is in series with the parallel connection of a charge transfer resistance (Rct) and a constant phase element (CPE). As can be seen that the observed impedance behavior of the Al:NiO electrode is composed of a partial semicircle curve in the high and low frequency parts. As we know the diameter of the semicircle corresponded to the charge transfer resistance (Rct) or electrochemical reaction resistance that is related to the interface between the Al:NiO electrode and electrolyte. Moreover, the Rs value can be obtained by the intercept at the real impedance axis (Z0 ) in the high frequency. Given the microscopic roughness of the electrode material, constant phase element (CPE) is used instead of pure capacitor, which can lead to an inhomogeneous distribution in the so lution resistance. Constant phase element (CPE) as a function of fre quency could be given in below [27]:
Table 1 Elemental compositions of the Al-doped NiO films (including at% ratio). Elements → Samples↓
(5)
It is important to comment that the OH ion exchanging in the intercalation/deintercalation process along with the reduction reactions of Ni2þ to Ni3þ/Ni2þ to Ni3þ causes the change in optical properties of the films. From cyclic stability one can understand Al:NiO material has stable and desirable for a good electrode material after 500 cycle.
(10)
t2
Qin ¼
NiOðbleachÞ þ xOH þ xe ↔ NiOOHðdark brownÞ
4
Y.E. Firat
Materials Science in Semiconductor Processing 109 (2020) 104958
Fig. 3. a) The optical transmittance in the colored and bleached state for a) sample A, b) sample B, c) sample C and d) sample D. Table 2 Electrochromic parameters of fabricated sample A, B, C and D (at the wavelength of 550 nm). Samples
Sample A Sample B Sample C Sample D
Qin (mC) 13.49 25.75 29.98 28.07
ZCPE ¼ 1=½Y0 ðj⋅ωÞα �
Response Time (s) tb
tc
0.85 1.65 2.51 3.15
1.90 3.88 5.19 5.80
Optical modulation
Coloration Efficiency (cm2/C)
71.16 54.26 43.61 62.58
39.89 36.82 19.05 37.59
(12)
Where Y0 is the capacitance when α ¼ 1 (constant) and ω is the angular frequency. The fitted EIS parameters are listed in Table 3. From the goodness of fit degree value of 10 6, well agreement is observed be tween the measured and simulated data. It can be see that the sample B possesses lowest RS and RCT values of 33.47 and 2168 Ω, respectively, which may be due to easy diffusion and movement of electrons/ions on the surface of sample B, thus confirming highest electrochemical con ductivity as compared to the others. The highest charge transfers resis tance (RCT) and the solution resistance values (RS) are found to be 7573 and 325.6 Ω, respectively, for sample D. The calculated EIS parameters are comparable to the data derived from the work previously published by Yang et al. [28] while studying the electrochemical performance of nickel-oxide/carbon sphere hybrids synthesized reflux-hydrothermal technique. They reported that the charge transfer resistance values of the produced films were in the range of 913 and 6223 Ω. These results
Fig. 4. Chronoamperometry curves with potential range from for different Al:NiO thin films.
1.0 V to þ1.0 V
indicate that, the improved electrical conductivity of Al:NiO electrode might be one of the crucial factors contributing to small diffusion limitation. Mott-Schottky analysis is performed to get information about the semiconducting properties of oxide films in contact with electrolytes, and it is commonly used to estimate the carrier density, types of majority carriers and flat band potential of a semiconductor. The capacitance as a function of the electrode potential (V) can be calculated through M S 5
Y.E. Firat
Materials Science in Semiconductor Processing 109 (2020) 104958
where C is the interfacial capacitance, NA is the number of acceptors for p-type semiconductor, V is the applied voltage, VFB is the flat band po tential, kB is Boltzmann’s constant (1.381 � 10 23 J K 1), T is the ab solute temperature, q is the elementary charge (-e for electron and þe for holes) (1.602 � 10 19 C), ϵ0 and εr is vacuum permittivity (8.85 � 10 14 Fcm 1) and dielectric constant of the deposited films (~12 for NiO), respectively. The Mott-Schottky plots of the deposited Al:NiO films are given in Fig. 7b. The negative slope of the linear part for each line plot confirms the p-type semiconducting behavior of Al:NiO electrodes, a shape that is congruent with the reported in the literature [30]. The acceptor density (NA) can also be conveniently calculated from the slope of the M S plot and the values are determined to be in the range of 1012 - 1015 cm 3 depending on the deposition current density. This significant change in NA demonstrates that doping of Al element causes NiO electrode to be more p-type, probably by the replacement of Ni2þ by Alþ atoms at the electrode surface. Also, the quantitatively analyzes of NA are consistent with the EDX results. Moreover, from Fig. 7b it is possible to determine the value of flat-band potential (VFB), which obtained by the extrapo lation of the linear portion of the curve to 1/C2 ¼ 0 at potential (V) axis. When the applied current density for the synthesis of Al:NiO is raised, a negative shift of VFB is seen as it changes from 0.794 V to 0.976 V (Table 4).
Fig. 5. The variation of coloration efficiency (η) of sample A, B, C and D as a function of wavelength.
4. Conclusions As a consequence, it is reported a simple, eco-friendly and facile method for the direct growth of the Al:NiO thin films on the ITO-coated glass substrates at four different deposition current densities in the range between 4 mA cm 2 and 7 mA cm 2, followed by heat treatment at a temperature of 400 � C in air. The SEM analysis displays that different growth current densities cause a considerable variation of morpholog ical features and elemental compositions of the electrode surface. From XRD analysis of all the samples, it is found that the different ratios of Al Table 3 EIS Parameters obtained from Nyquist Plots of Al:NiO electrodes.
Fig. 6. Cyclic voltammogram in 0.1 M KOH electrolyte at 50 mV s to 500th.
equation as follows [29]: � �� � 1 2 kT ¼ V V FB C2 eε0 εr NA e
1
from 1st
Deposited Films→ Parameters↓
Sample A
Sample B
Sample C
Sample D
RS (ohms) RCT (ohms) Y0 (S‧s2)
187.3 4302 353.4 � 10 6 879.9 � 10 3 2.863 � 10 3
33.47 2168 894.5 � 10 6 860.6 � 10 3 570 � 10
42.79 8281 229.8 � 10 6 916.3 � 10 3 890.9 � 10 6
325.6 7373 321.7 � 10 6 835.6 � 10 3 23.1 � 10
Alpha
(13)
Goodness of Fit
6
Fig. 7. a) Nyquist plots (the inset shows the equivalent circuit model), b) Mott-schottky plots of the Al:NiO thin films. 6
3
Y.E. Firat
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Table 4 Acceptor density and flat-band values of Al:NiO thin films. Parameters → Deposited Films ↓
NA (1/cm3)
Sample A Sample B Sample C Sample D
1.59 1.29 2.09 8.01
� 1012 � 1015 � 1015 � 1014
VFB (Volt) 0.794 0.975 0.976 0.929
Carrier type p p p p
element don’t change the crystal phase of NiO and it is only located. The presence of characteristic peaks of NiO is supported by XRD as well as by EDX. Upon applying potentials of �1 V, the color of Al:NiO electrode switches between dark brown and transparent. Coloration efficiency varies between 19.05 cm2/C and 39.89 cm2/C depending on the in crease of the applied current density in the synthesis of Al:NiO thin films. All of the deposits have p-type semiconducting behavior of Al:NiO thin films, which is verified by Mott-schottky analysis. These kinds of Al:NiO thin films can be expected to be a promising electrode, which may provide a new approach to counter electrode for next-generation and high-performance electrochromic devices. Declaration of competing interests The authors declare the following financial interests/personal re lationships which may be considered as potential competing interests: Acknowledgements � Uni This work was supported by the Research Fund of the Uludag versity, Project number HDP(F)–2017/23. The author thanks to Uludag University for financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2020.104958. References [1] R.T. Wen, G.A. Niklasson, C.G. Granqvist, Electrochromic iridium-containing nickel oxide films with excellent electrochemical cycling performance, J. Electrochem. Soc. 163 (2016) E7–E13, https://doi.org/10.1149/2.0591602jes. [2] R.T. Wen, G.A. Niklasson, C.G. Granqvist, Strongly improved electrochemical cycling durability by adding iridium to electrochromic nickel oxide films, ACS Appl. Mater. Interfaces 7 (2015) 9319–9322, https://doi.org/10.1021/ acsami.5b01715. [3] Y.F. Yuan, X.H. Xia, J.B. Wu, Y.B. Chen, J.L. Yang, S.Y. Guo, Enhanced electrochromic properties of ordered porous nickel oxide thin film prepared by selfassembled colloidal crystal template-assisted electrodeposition, Electrochim. Acta 56 (2011) 1208–1212, https://doi.org/10.1016/j.electacta.2010.10.097. � [4] R. Cerc Koro�sec, M. Felicijan, B. Zener, M. Pompe, G. Dra�zi�c, J. Pade�znik Gomil�sek, B. Pihlar, P. Bukovec, The role of thermal analysis in optimization of electrochromic effect of nickel oxide thin films, prepared by the sol-gel method: Part III, Thermochim. Acta 655 (2017) 344–350, https://doi.org/10.1016/j. tca.2017.07.010. [5] J. Shi, L. Lai, P. Zhang, H. Li, Y. Qin, Y. Gao, L. Luo, J. Lu, Aluminum doped nickel oxide thin film with improved electrochromic performance from layered double hydroxides precursor in situ pyrolytic route, J. Solid State Chem. 241 (2016) 1–8, https://doi.org/10.1016/j.jssc.2016.05.032. [6] J.H. Zhang, G.F. Cai, D. Zhou, H. Tang, X.L. Wang, C.D. Gu, J.P. Tu, Co-doped NiO nanoflake array films with enhanced electrochromic properties, J. Mater. Chem. C. 2 (2014) 7013–7021, https://doi.org/10.1039/c4tc01033g. [7] F. Lin, D.T. Gillaspie, A.C. Dillon, R.M. Richards, C. Engtrakul, Nitrogen-doped nickel oxide thin films for enhanced electrochromic applications, Thin Solid Films 527 (2013) 26–30, https://doi.org/10.1016/j.tsf.2012.12.031.
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