This paper presents the electrochromic (EC) characteristics of nickel oxide (NiO) nanoflakes array prepared by chemical bath deposition (CBD) method with different ammonia concentration. Prepared NiO films have porous structure with 2-dimensionally networked nanoflake arrays. The addition of aqueous ammonia significantly affects the growth of NiO film. Moreover, appropriate amount addition of aqueous ammonia is important in obtaining a balanced EC performance of NiO films between transmittance modulation and cycling durability. Excess concentration of ammonia cause not only a decrease in the thickness of NiO films but also change the growth direction of flakes. XPS results present that high concentration of ammonia could induce the peak shifting toward the low binding energy, which attributed from initial state effect between nickel and oxygen, which is substituted by nitrogen. The best cycling durability at 550 nm was achieved for NiO-25 film with high transmittance modulation (Tbleaching: 86.8% and Tcoloring: 23.4%) and noticeable cycling stability (only 25% decrease after 5000 cycles).
Introduction
As the growing demand for energy increases with world population, innovative technologies for improving energy saving or efficiency with quality of life has been received in enormous interest from many researchers and public [1, 2]. In this point, electrochromic (EC) devices with unique function to change the optical properties (absorbance, transmittance and reflectance) reversibly by an applied voltage are noted as a major technology that can economize energy consumption in architecture by selectively adjusting sunlight flux through windows, which is called to 'smart windows' [3, 4]. There are numerous EC materials that have been widely investigated, such as transition metal oxides, conjugated polymers and mixed composite materials [5, 6]. Among these, nickel oxide (NiO) is a promising candidate for anodic EC material due to its large dynamic range, low operation voltage and outstanding EC efficiency [7, 8]. NiO also has extensive advantages, such as low materials coast, natural abundance and environmentally friendliness [9, 10]. The EC performances of NiO are closely related to their morphologies and sizes [11]. To date, several deposition techniques have been developed for the preparation of porous NiO films, such as hydrothermal [12, 13], electro-deposition process [14], spray pyrolysis [15], sputtering [16, 17] and chemical bath deposition (CBD) [18]. Among them, CBD is a suitable and advantageous technique because of its low-temperature synthetic condition and ease of fabricating the porous structure over the large area [19]. For practical application in smart windows, however, poor cycling durability under repeated redox process is critical limitation of NiO film prepared by CBD [18].
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Up to now, there have been many reports to improve the above issue of NiO films prepared by CBD. Cai et al. reported on the constructed TiO2/NiO core/shell nano rod array prepared by the combination of hydrothermal and CBD methods, resulted in significantly improved EC performance with larger optical modulation and high cycling durability compared with the pure NiO nanoflake films [20]. Similar research was reported by Zhang et al., who fabricated hierarchical SnO2/NiO core/shell nanoflake arrays. Such a composite strategy revealed an outstanding reversibility and cycling durability (> 2200 cycles at 550 nm) than pure NiO nanoflake arrays (< 250 cycles at 550 nm) [21] due to the p-n junction of core shell structure, which can enhance the separation of electron and proton by the electric junction field. Zhang et al. reported another approach to improve the cycling durability of NiO films by Co doping in the CBD process. They also achieved outstanding electrochromism at 1% Co-doped NiO nanoflake arrays with enhanced cycling durability (> 3000 cycles at 550 nm) [22].
However, in spite of the significant improvement of cycling durability of NiO nanoflake arrays by adopting the composite p-n junction or metal doping, our recent study revealed that the fundamental factor against improvement of cycling durability is the synthetic condition according to the exact amount of the compound in the CBD process.
In this study, we report an effect of ammonia concentration on the morphology and their EC properties of NiO nanoflakes array prepared by the CBD method under aqueous solution containing nickel sulfate, potassium persulfate and ammonia water at room temperature. It was found that the optical modulation and cycling durability were significantly dependent on pH value in solution of CBD. To the best of our knowledge, this is the first report of NiO nanoflakes arrays with extremely improved cycling durability (> 5000 cycles at 550 nm) prepared by CBD.
Experimental details
Chemical materials
All solvents and chemicals were of analytical level and used without further purification. ITO-coated glass (10 Ω) was purchased from Omni Science Co., Ltd. Nickel(II)sulfate hexahydrate (NiSO4•6H2O, ≥ 98%), potassium persulfate (K2S2O8, ≥ 99%) and aqueous ammonia (NH4OH, 28-30%) were purchased from Sigma-Aldrich. All aqueous solutions were freshly prepared with deionized water.
Preparation of porous NiO nanoflake
The CBD process was carried out in a 250 ml pyrex beaker at room temperature according to the our previous report [23]. The solution for the CBD was obtained by mixing 100 ml of (0.9 M) nickel(II)sulfate hexahydrate and 150 ml of (0.1 M) potassium persulfate. Then, cleaned indium tin oxide glass (50 mm × 50 mm) which nonconductive side masked with polyimide tape were vertically placed in the above solution under the stirring with 150 rpm. Finally, the reaction started after addition of aqueous ammonia while stirring. The deposition of NiO nanoflakes was carried out for 30 minutes. The CBD reaction mechanism of NiO is as follows [24]:
[Ni(H2O)6-x(NH3)x]2+ + 2OH- → Ni(OH)2 + (6-x)H2O + xNH3 (1)
2Ni(OH)2 + S2O82- → 2NiOOH + 2SO42- + 2H+ (2)
Then the deposited film was washed with deionized water to remove the residuals on the film surface. After removing the tape masks, NiO nanoflakes were calcined at 400 oC for 3 hours. The as-prepared films with different amount of aqueous ammonia (10, 15, 20, 25, 30, 40 ml) were denoted as NiO-10, NiO-15, NiO-20, NiO-25, NiO-30 and NiO-40, respectively.
Characterizations
The morphological information and thickness of films were analyzed by a field-emission scanning electron microscope (FE-SEM, JEOL, JSM-7100 F) with an accelerating voltage of 15 kV. The crystalline information of each film were characterized by an X-ray diffraction (XRD) (Rigaku Ultima IV instrument) with Cu Kα radiation (λ =1.5416 Å). Diffraction patterns were collected for 2 θ values between 10˚ and 80˚ with a 2˚ glancing angle, and scanned at 5˚/min rate. The surface composition and chemical state of the films were evaluated by an X-ray photo electron spectroscopy (XPS) (JEOL, JPS-9010). The transmission spectra of the films in the colored and bleached states were measured with a UV-visible spectrophotometer (USB4000, Ocean Optics, Inc.) in standard cuvettes in the spectral region of 300 to 800 nm. The cyclic voltammetry (CV) and electrochromism tests were performed on a CHI604E (CH Instruments, Inc.) electrochemical workstation and carried out in a three-electrode electrochemical cell containing 0.1 M KOH aqueous solution as the electrolyte. The prepared NiO films acted as the working electrode. Ag/AgCl electrode and platinum wire were used as the reference electrode and counter-electrode, respectively. In the test of kinetics of coloration and bleaching the step voltages for each film were -1.0 and 1.0 V, respectively, and the time interval for characterization was 40 seconds.
Results and discussion
The surface and cross-sectional SEM images of the NiO nanoflake films prepared with different amounts of aqueous ammonia are illustrated. It is well-established that the microstructure of final NiO nanoflakes is drastically affected by the concentration of aqueous ammonia. The NiO-10 film shows a general flat surface that was formed by packing closely stacked nanoflakes and about 70 nm of thickness. However, with increase in the amount of aqueous ammonia until 30 ml, the thickness of the film becomes thicker gradually and 2-dimensionally networked tiny flake array with nearly vertical directions was formed. The highest thickness (735 nm) of NiO nanoflakes was obtained at the NiO-30 film. Interestingly, it was found that a uniform nanoflake array cannot form when the amount of aqueous ammonia injected over 30 ml. For NiO-40 film, 2-dimensionally networked structure became randomly oriented flakes with reduced thickness as 285 nm. Such tendency can be explained by the solubility of Ni in CBD solution according to the different pH [25, 26]. The pH conditions should be well controlled to produce proper Nanostructures that have a direct performance on the performance of devices. The graph for thickness variation of NiO films as a function of pH values. It was found that NiO grown at below 9.0 of pH value led to the formation of NiO nanoflakes with short thicknesses of 300 nm or less. For the films that were prepared with pH value from 9.5 to 10.3, thicker NiO films over 600 nm with clear 2-dimensionally networked structures were obtained. This is somewhat analogous to Ostwald ripening in solids and sols, which lead to the formation of larger particle sizes through the dissolution-precipitation process. However, the pH values at above 10.5 led to the drastic decrease of thickness. According to the dissolution-precipitation mechanism of nickel hydroxide as a function of pH (Boardman et al. [27]), precipitation of Ni(OH)2 begins at pH 7 and reaches a maximum at pH 8.5-10. When the pH is above 10.5, redissolution of precipitated Ni(OH)2 occurs, which may explain the formation of randomly oriented flakes with reduced NiO thickness.
Except for the broad shoulder within 15o-40o corresponding to the amorphous SiO2 from glass substrate, prepared NiO films have three diffraction peaks at 37.2o, 43.3o and 62.8o correspond to (111), (200) and (220) crystal planes of cubic phases (JCPDS 47-1049), respectively. Additionally, no diffraction peaks for other Ni(OH)2 were observed. However, our results show that poor intensity of NiO peaks were obtained because the NiO nanoflakes prepared by CBD method have thinness and porous structure resulting in polycrystalline in nature [28]. For the NiO-10 and NiO-15 films grown at below 9.0 of pH value, no observable peaks can be assigned to NiO, which suggests amorphous matter due to low ammonia concentration in CBD solution, which serve the OH- ions that could make Ni(OH)2. For the NiO-20, NiO-25 and NiO-30 films, the peaks for the cubic phase of the NiO were gradually revealed. However, in the case of the NiO-40 film, a decrease of the intensity of peaks was observed. This agrees well with the above results for microstructure of NiO films.
To understand the detailed composition and oxidation state of each NiO film, XPS measurements were performed. The O 1s peaks of each film mainly includes three components, the peaks located at 529.5, 531.2 and 532.9 eVs correspond to O-Ni2+, O-Ni3+ and adsorbed water bands, respectively [29]. With increasing amounts of aqueous ammonia, shifting of the peaks for O-Ni2+ and O-Ni3+ toward low binding energy were observed in the NiO-30 and NiO-40 films. This may explain that the formation of excess Ni(H2O)6-x(NH3)x at the reaction (1) due to high concentration of aqueous ammonia cause the peak shift toward low binding energy, which attributed from difference in electronegativity between oxygen and nitrogen. These trends are also reflected in the Ni 2p3/2 spectra. The major peaks can be deconvoluted into three peaks at 854.0, 855.9 and 861.2 eV correspond to Ni2+, Ni3+ and shake-up satellite structure, respectively [30]. The Ni3+ mainly derives from high valence nickel oxides, such as Ni2O3, NiOOH because of the effect of persulfate [22]. Therefore, the as-prepared nickel oxide exhibits nonstoichiometric characteristic as NiOx.
In order to investigate the electrochemical characteristic of NiO films as prepared in this study, CV tests were performed. Only one pair of redox peaks appeared for all the films. During the CV curves, the anodic/cathodic peaks are associated with the conversion between NiO and NiOOH due to the insertion/desertion of OH-. As the OH- ions insert and desert, the films are colored and bleached reversibly because of the transformation between Ni3+ and Ni2+ [22]. In the same tenth cycle measurement, NiO-30 films showed a higher peak current and larger curve area. This is because a NiO-30 thin film with the largest thickness consequently provides many active sites in the electrolyte [31].
The color of each film changes from transparent to dark brown. It is clearly seen that the NiO-10 and NiO-40 films show a less transmittance modulation than the other NiO films because those films were thinner in thickness. The transmittance modulation of the NiO-30 film reaches about 72.5 % at 550 nm, while NiO-25 and NiO-20 exhibit about 63.4 % and 57.6 %, respectively. The high transmittance modulation in a NiO-30 film is considered to be due to the structural high thickness of nanoflakes than the other NiO films. Such transmittance modulation trends well agreeing with the above SEM, XRD and CV analysis.
The cycling durability of the NiO films prepared in this study was evaluated by CA measurements and corresponding in situ transmittance at 550 nm.. After being subjected to 3000 cycles, the decay of transmittance modulation for NiO-25 film was about 18.5%. Furthermore, even when subjected to 5000 cycles, the transmittance modulation of NiO-25 film decayed only for 25.2%. These remarkable durability values are much higher than for pure NiO films prepared by similar CBD method reported previously [20-22]. Interestingly, the significant decrease of transmittance modulation after 500 cycles was observed for the NiO-30 films, which has best transmittance modulation, and this decay accelerated until 1200 cycles. Finally the NiO nanoflakes fell off form ITO glass resulting in unmeasurable. This drastic decrease of stability can be considered that weak adhesion between nanoflakes and substrates could occur when excess Ni(H2O)6-x(NH3)x was formed in the CBD solution as described in XPS results. The best cycling durability of the NiO film prepared with appropriate addition of aqueous ammonia can be possibly attributed to the high growth rate of NiO nanoflakes resulted in high transmittance modulation and chemical composition of final NiO films by providing proper amount of OH ions.
Summary
In summary, 2-dimensionally networked NiO nanoflakes films were prepared by the CBD method. The addition of aqueous ammonia significantly affects the growth of NiO films and appropriate additions of amount of aqueous ammonia is important to obtain the balanced EC performance of NiO films between transmittance modulation and cycling durability. The NiO-25 film exhibited high transmittance modulation (Tbleaching: 86.8% and Tcoloring: 23.4%) and noticeable cycling durability (only 25% decrease after 5000 cycles) at 550 nm. This paper presents that the cycling durability of NiO films can be improved by simple aqueous ammonia control without complex composite structure of metal doping, and it can be used to make high quality EC NiO films for smart windows.
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