# Inkjet Printing as High-Throughput Technique for the Fabrication of NiCo2O4 Films

Authors: Reyna Dianela Bacelis-Martínez; Gerko Oskam; Geonel Rodriguez Gattorno; Miguel Angel Ruiz-Gómez
Journal: Advances in Materials Science and Engineering
 (2017)

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## Abstract

Owing to its distinctive physicochemical properties, nickel-cobalt mixed oxide (NiCo2O4) has become a promising and innovative material for applications in many technological fields. The design of fast and reliable techniques for the deposition of this material is essential in the development of applications. In this work, NiCo2O4 films were successfully prepared by an inkjet printing technique using a suitable ink obtained from metal nitrates in a glycerol-water mixture. In order to deposit well-defined and uniform film patterns, the instrumental parameters such as drop spacing and inkjet voltage have been explored. The pure crystalline bimetallic nickel cobaltite oxide is obtained at 500°C with a homogeneous compositional distribution along the film. The average thickness observed by scanning electron microscopy is around 490 nm, whereas X-ray photoelectron spectroscopy analysis revealed that the film surface presents mixed oxidation states for both metals: Co2+, Co3+, Ni2+, and Ni3+. The electrocatalytic performance of inkjet-printed NiCo2O4 films for the water oxidation reaction is comparable with earlier reports.


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## Body

## 1. Introduction

NiCo2O4 oxide has recently been a motivating topic from the viewpoint of materials science because of its interesting catalytic, electrical, and optical properties. These properties are related to important technological applications such as electrodes for fuel cells [1, 2], electrodes in sodium- and lithium-ion batteries [2–4], optoelectronic [5, 6] and magnetic devices [7, 8], supercapacitors [4, 9–13], flat-panel displays [2, 14], infrared transparent conducting materials [2, 5, 14], glucose sensors [15], and several electrochemical reactions [1, 6, 7, 16–20]. In particular, the production of clean energy from renewable sources as in the electrocatalytic water splitting process is an attractive alternative for an energy economy based on hydrogen [20, 21]. However, for practical applications, it is necessary to develop high performance electrocatalysts by using nonprecious metals for both the anodic oxygen evolution reaction (OER) and the cathodic hydrogen evolution reaction (HER) [17, 19, 22]. Specifically, efficient and stable electrocatalysts are required for the anodic half-reaction due to the multielectron transfer needed for water oxidation, which is important to improve the overall water splitting process [17, 20, 22]. In this regard, NiCo2O4 as a cobaltite spinel has shown excellent electrocatalytic properties, because the mixed oxidation states of its cations favor the adsorption of water and hydroxide ions. Also, it facilitates the permeability and uptake reversibility of oxygen and provides active sites for chemisorption [1, 17, 22]. Recently, NiCo2O4 was reported as an efficient bifunctional material for both the OER and the HER, being regarded as a very active nonprecious metal electrocatalyst [22]. In addition, other significant advantages of NiCo2O4 to be considered are its better electrochemical properties and electrical conductivity compared with pure NiO or Co3O4 oxides [1, 4, 9]. Moreover, its constituent metals are earth-abundant, cheap, and not toxic [4, 10, 22].Many of the above-mentioned applications of NiCo2O4 require its fabrication in thin film form; moreover, taking into consideration a possible scale-up, the surface dimensions of the film is an important issue. In this respect, NiCo2O4 films have been principally prepared by physical deposition processes such as pulsed laser deposition [16, 23–25] and radio frequency magnetron sputtering [5, 8], as well as by means of chemical routes including electrodeposition [6, 14, 15], sol-gel [2], hydrothermal synthesis [26], spray pyrolysis [4], and chemical vapor deposition [27]. In general, if cost of the process and equipment requirements are considered, the solution-based chemical methods are more appropriate than physical routes. As a part of the solution-based chemical approaches, inkjet printing recently has gained a lot of acceptance in many areas of science and engineering as a high-throughput deposition technique for fabrication of thin films and devices [28–33]. This technique is able to overcome several difficulties presented by traditional routes, generating new functionalities and performance. Some important advantages are as follows: (i) negligible materials waste, therefore, it is an environment-friendly and low cost route; (ii) a noncontact method, hence, it minimizes film contamination; and (iii) the possibility of depositing an exact amount of material in a pattern previously designed. In particular, the piezoelectric drop-on-demand inkjet printer ejects the ink only when it is required with generation of droplets within 20–50 μm in size, achieving high placement accuracy of drops favoring the formation of good quality micrometric patterns. Also, these characteristics allow the preparation of continuous films specifically designed with a size of several centimeters.It is known that, in the inkjet printing technique, the quality of the prepared films depends considerably on the physicochemical properties of the ink used during the process as well as several printing parameters that need to be optimized. This paper is focused on the formulation and analysis of a suitable ink used in a drop-on-demand materials printer, where the parameters drop spacing and jetting voltage were evaluated in order to obtain a homogeneous 10 mm × 10 mm NiCo2O4 film. The properties of the films were determined by several physicochemical characterization techniques and the electrocatalytic activity of the most uniform films was tested in the water oxidation reaction.


## 2. Materials and Methods

The ink was prepared dissolving 0.5814 g Ni(NO3)2·6H2O and 1.1621 g Co(NO3)2·6H2O (Sigma Aldrich ≥ 97%) in a mixture of glycerol : deionized water (65 : 35 wt.%) at room temperature. The ink was analyzed by UV-Vis spectroscopy (Agilent Instruments) in the range of 200–800 nm. The rheological properties were determined by means of a Kinexus-Pro rheometer operated at 35°C using ~1 mL of ink; the frequency range was 0.01–100 s−1. The thermogravimetric and differential scanning calorimetric (TGA-DSC) analysis was performed on a TA Instruments-Discovery equipment with a heating rate of 5°C/min within 50–500°C under air flow at 25 mL/min. Before printing, the FTO (fluorine-doped tin oxide) Pilkington TEC 8 Ω/sq. substrates were cleaned using a piranha solution for 10 min and rinsed with deionized water. Inkjet printing was realized using a drop-on-demand Dimatix materials printer DMP-2831 (Fujifilm) and a DMC-11610 piezoelectric cartridge (16 nozzles and 254 μm spacing) filled with 1.5 mL of our formulated ink. A 10 × 10 mm2 continuous square pattern was printed, varying the jetting voltage (25 and 35 V) and drop spacing (15, 20, and 30 μm), maintaining the platen temperature at 40°C. In addition, some demonstrative patterns of micrometric dots and lines were inkjet-printed. All the obtained deposits were heat treated from 30 to 500°C with a ramp of 2°C/min and kept at 200, 300, and 500°C during 30, 30, and 60 min, respectively. The morphology and chemical composition were analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM-EDS) using a JEOL JSM-7600F microscope and X-Max Oxford Instruments detector. The crystalline phase was determined using a Siemens D5000 diffractometer with Cu Kα radiation (λ = 1.5418 Å). X-ray diffraction (XRD) data were collected at room temperature from 30 to 70° with grazing-angle incidence of 1°, step size of 0.02°, step time of 3 s, 34 kV, and 25 mA. Oxidation states and quantification were examined by using X-ray photoelectron spectroscopy (XPS) on a K-Alpha (Thermo Scientific) spectrometer, employing monochromatized Al Kα radiation (1486.6 eV) and double-focusing hemispherical analyzer. The electronic absorption spectra were measured in five zones of the films with an UV-Vis-NIR microspectrophotometer (MSP500RT, Angstrom Sun Technologies) within 400–1000 nm with a 40x objective and spot size of 10 μm; an FTO substrate was used as reference. Electrochemical analysis was carried out using an Autolab PGSTAT302N/FRA2 (Metrohm Autolab) apparatus and a system with the inkjet-printed film as working electrode, Ag/AgCl (3 M) as reference electrode, and a platinum wire as counterelectrode. Linear-scanning voltammetry was performed in 0.1 M KOH solution, within 0–0.7 V and at a scan speed of 5 mV/s.


## 3. Results and Discussion

### 3.1. Ink Physicochemical Properties

The formulated ink is a reddish and homogeneous solution (see inset photograph in Figure1(a)) with excellent stability and without formation of any sediment for three months under regular storage conditions (25°C and light protected). Figure 1(a) depicts the UV-Vis spectrum of the ink, where absorption bands at 229 and 297 nm match with electronic transitions from nonbonding (lone-pair) n orbital to an antibonding π orbital (n-π∗ transitions), typical of nitrate ions. The bands at 394, 468, and 513 nm correspond to d-d transitions of a cobalt aquo-complex ([Co(H2O)6]2+), whereas the bands at 654 and 730 nm are related to a nickel aquo-complex ([Ni(H2O)6]2+). This is relevant since the UV-Vis spectrum allows us to follow the ink stability with time.Figure 1
(a) UV-Vis absorption spectrum of the formulated ink; (b) shear rate versus shear stress; (c) viscosity as function of the shear rate; (d) shear modulus (viscous and elastic components) versus frequency.
(a)
(b)
(c)
(d)Analysis of the rheological properties of the ink was carried out, and, according to the plot of shear stress versus shear rate, Figure1(b), a slight deviation from Newtonian behavior is observed. From Figure 1(c) of the viscosity versus shear rate, a high-frequency viscosity value of the ink of 5.7 mPa·s was estimated; this value is close to the reported value for a glycerol : water (65 : 35 wt.%) mixture [34]. Figure 1(d) presents the plot of shear modulus versus frequency, where the elastic component is larger than the viscous component with increasing frequency, which means that the ink behaves like a solid at higher frequencies. On the other hand, as literature indicates for a glycerol : water (65 : 35 wt.%) mixture, the surface tension of the ink is expected to be around 65 dynes/cm [34]. Considering that physicochemical characteristics of an ink (viscosity and surface tension) play an important role for appropriate jetting during the printing process, it is likely that the glycerol-water formulated ink presents good performance, as the values are close to those required for a drop-on-demand piezoelectric inkjet printer [35]. In addition, the higher boiling point (~290°C) and heat of vaporization (~89 kJ/mol) of glycerol in comparison to water (~100°C and ~40 kJ/mol) suggests a low evaporation rate of the inkjet-printed drops favoring deposition of a homogeneous and continuous film on the substrate.In order to establish the annealing procedure after inkjet printing, a thermal analysis was performed. The thermogravimetric curve shown in Figure2(a) suggests that the ink decomposes in three stages; the first step occurs between 30 and 95°C with a sharp weight loss of 55.7%, and this is due to partial evaporation of the water-glycerol mixture. After water evaporation, the ink has a higher proportion of glycerol, and the second weight loss step of 39.2% is observed from 95 to 255°C. Finally, the third step takes place at 325°C accompanied by a weight loss of 2.8%, which corresponds to decomposition of nitrates present in the ink. Figure 2(b) exhibits the DSC curve, showing two endothermic and one exothermic peaks at 36.1, 150.6, and 260.7°C, respectively. The exothermic event could be related to partial oxidation of Ni2+ and Co2+ as well as a probable transformation of NO to NO2. After 375°C, no events were observed in both TGA and DSC curves. The obtained information from this analysis is important because it allows choosing the most adequate temperatures for drying and calcination in order to obtain a homogeneous film with the desired crystal phase.Figure 2
Thermal analysis curves of the formulated ink: (a) thermogravimetric curve and (b) differential scanning calorimetric curve.
(a)
(b)


### 3.2. Inkjet-Printed NiCo2O4 Films

Besides the ink properties, the printing settings must be optimized for each ink to obtain good quality films. In this work, different values of drop spacing (DS) and jetting voltage (JV) were evaluated. Taking into consideration both a visual and a microscopic inspection, after a thermal treatment at 500°C during 1 hour, the best result was obtained for a inkjet-printed film using DS = 20μm and JV = 35 V. For that reason, this is the representative film that will be further analyzed in this work, referred to hereafter as F-20-35. The DS parameter is related to the number of jetted drops per area and therefore to the pattern resolution; for example, a DS value of 20 μm corresponds to 2,510 drops/mm2 and 1,270 dots per inch (dpi). Thus, if DS is low (e.g., 15 μm), an overspreading of the ink on substrate occurs, provoking that the pattern shape is undefined; whereas if DS is high (e.g., 30 μm), the drops do not interact with each other and do not coalesce to form a continuous film. On the other hand, JV principally controls drop velocity, where increasing JV increases velocity. This parameter should be adjusted depending on the physical properties of the ink. In our formulated ink, a high JV of 35 V (40 V is the upper limit in the printer) was required because of the high value (65 dynes/cm) of the surface tension; the recommended range is 35–70 dynes/cm.A photograph of a representative F-20-35 NiCo2O4 film is presented in Figure 3(a), where a black coating with a well-defined 10 mm × 10 mm square is observed, while SEM imaging and EDS mapping analysis of the surface reveal a continuous coating formed by nanoparticles interconnected to each other with a homogenous elemental distribution.Figure 3
(a) SEM micrograph using secondary-electron mode and elemental chemical mapping of the film surface (upper left inset is a photograph of the deposited sample) and (b) cross-sectional SEM image using the low-angle back-scattered electron mode and EDS line scan.The quantitative EDS results indicate an atomic percentage of 7.94, 16.05, and 76.01 for nickel, cobalt, and oxygen, respectively; therefore, the proportion nickel : cobalt is 1.00 : 2.02 in good agreement with the stoichiometry of NiCo2O4 oxide. Also, the thickness was obtained from a cross-sectional SEM image (see Figure 3(b)), obtaining an average value of 490 nm. In order to confirm the film homogeneity, a linear-scanning EDS analysis of the cross section was done, where nickel, cobalt, and oxygen are homogeneously distributed with the same proportion observed in the surface analysis results. Table 1 lists a summary of some of the typical synthesis methods for fabrication of NiCo2O4 films, their thickness values, and XRD phase purity, as well as their specific applications and properties. As can be seen in Table 1, the thickness of the inkjet-printed F-20-35 NiCo2O4 films is comparable with those obtained previously by both physical and chemical approaches.Table 1
Summary of reports on NiCo2O4 films prepared by different techniques.
Synthesis method
Precursors
Substrate
Thickness
XRD phase
Application/properties
Ref.
Inkjet printing
Ni(NO3)2·6H2O Co(NO3)2·6H2O Glycerol, deionized water
FTO
490 nm
Pure
Water oxidation reaction
This work
RF sputtering
Ni-Co metal alloy targets
Silicon
550–830 nm
Impure
Electrical and magnetic
[8]
Pulsed laser deposition
Polycrystalline NiCo2O4 target
MgAl2O4 (001)
240 nm
Pure
Electrical and magnetic
[16]
Spray pyrolysis
Ni(NO3)2·6H2O Co(NO3)2·6H2O Methanol
Aluminum foil
12µm
Pure
Mesoporosity
[36]
Sol-gel
Co(CH3COO)2·4H2O Ni(CH3COO)2·4H2O 2-Methoxyethanol Ethanolamine
Al2O3 (0001)
900–1000 nm
Impure
Electrical resistivity
[2]
Hydrothermal
Ni(NO3)2·6H2O Co(NO3)2·6H2O Urea, NH4F, deionized water
FTO
1–4.5µm
Pure
Dye sensitized solar cells
[26]
Chemical bath deposition
NiCl2·6H2O CoCl2·6H2O Deionized water KOH (electrolyte)
Stainless steel
2.3–18µm
Pure
Supercapacitors
[37]
Electrodeposition
Ni(NO3)2·6H2O Co(NO3)2·6H2O Deionized water KCl (electrolyte)
Stainless steel and Indium-Tin Oxide (ITO)
4µm
Pure
Methanol fuel cells
[6]
Electrodeposition
Ni(NO3)2·6H2O Co(NO3)2·6H2O Deionized water KCl (electrolyte)
ITO
Not reported
Pure
Glucose sensing
[15]
Spray pyrolysis
NiCl2·6H2O CoCl2·6H2O Deionized water
FTO
Not reported
Pure
Supercapacitors
[4]
Solvothermal
NiCl2·6H2O CoCl2·6H2O Hexamethylenetetramine
FTO
Not reported
Pure
Water oxidation reaction
[17]In order to confirm the micrometric resolution achieved with the inkjet printing technique, Figure4 presents representative optical and SEM images of dots and line patterns of inkjet-printed NiCo2O4 on FTO using the optimized parameters for our ink. The micrographs reveal well-defined dot arrays (Figures 4(a)–4(c)), where the dots are separated at ~230 μm and ~210 μm in the X and Y directions, respectively; at higher magnification, it is observed that the drops have a size ranging within 45–52 μm. On the other hand, a uniform and continuous line pattern (in both X and Y directions) was also successfully obtained; see Figures 4(d)–4(f). The width of well-defined lines was ~190 μm.Figure 4
Optical and SEM micrographs of inkjet-printed NiCo2O4. In (a)–(c), a dot array pattern is presented, and, in (d)–(f), a multiline pattern is depicted.
(a)
(b)
(c)
(d)
(e)
(f)The XRD pattern of the F-20-35 films (see Figure5) exhibits a set of well-defined reflections at 2θ values of 31.2, 36.6, 38.9, 44.7, 59.0, and 65.2°, which match well with the (022), (113), (222), (004), (115), and (044) crystal planes, respectively, of the pure cubic phase NiCo2O4 (JCPDS #20-0781). This result is similar to other reports on the synthesis of single phase NiCo2O4 film using pulsed laser deposition [16], spray pyrolysis [4, 36], hydrothermal route [26], chemical bath deposition [37], electrodeposition [6, 15] and solvothermal route [17], as indicated in Table 1.Figure 5
XRD pattern of a NiCo2O4 film (sample F-20-35) after annealing at 500°C for 1 hour. The reflections marked with asterisk correspond to the fluorine-doped tin oxide (FTO) substrate; the line pattern corresponds to the reference JCPDS # 20-0781.Figure6 shows the XPS analysis of a F-20-35 film, showing that the cobalt 2p spectrum presents peaks at 780 and 796 eV related to the spin orbit splitting, 2p3/2 and 2p1/2, respectively. Since the binding energies of Co3+ (779.4 eV) and Co2+ (780.8 eV) are very close, the presence of satellite peaks is very important in order to determine the oxidation states of cobalt. In this respect, the observed satellites at 785 and 802 eV can be assigned to the presence of Co2+ ions [8]. On the other hand, the nickel 2p spectrum contains peaks associated with 2p3/2 (854 eV) and 2p1/2 (872 eV) as well as their respective satellite peaks at 861 and 880 eV. The main peak at 854 eV is assigned to octahedral Ni2+ ions, whereas the small shoulder at 856 eV has been ascribed to Ni3+ present in NiCo2O4 oxide [2, 6]. The contribution of every metallic cation was determined by a curve-fitting analysis of their respective XPS spectra. According to the quantitative XPS analysis and calculated areas from 2p3/2 peaks of cobalt and nickel, the surface atomic content is 13.9, 15.1, 6.1, and 4.4% for Co2+, Co3+, Ni2+, and Ni3+, respectively. This means that the mixed oxidation states of cobalt and nickel favor the charge neutrality and structural stability of the inkjet-printed NiCo2O4 film, as previously observed for a sol-gel film [2].Figure 6
XPS spectra for (a) cobalt 2p and (b) nickel 2p. The colored solid lines below the spectra are derived from curve-fitting analysis.
(a)
(b)The electronic absorption spectra obtained from 5 different zones along the NiCo2O4 film are presented in Figure 7. The absorption profile is as expected from the black color of the oxide, absorbing in the entire visible region of the electromagnetic spectrum, as previously reported [7]. Also, the slight variations in absorbance suggest a good optical homogeneity of the coating. It is known that generally the color of film is an indication of the optical band gap, which can be calculated from the UV-Vis spectrum considering the absorption onset edge and by extrapolation of a straight line to the abscissa axis. Therefore, based on the extended absorption-edge spectrum of NiCo2O4, the optical band gap of this oxide could be lower than 1.24 eV, because the absorbance value is still approximately 0.7 units at 1000 nm. However, the exact band gap of NiCo2O4 is still controversial: using first-principle calculations, Zhu et al. reported a band gap of 0.2462 eV [10], whereas Shi et al. obtained an indirect band gap of 2.0 eV and a direct gap of 2.1 eV [38]. Dileep et al. utilized high resolution electron energy loss spectroscopy to probe the optical band gaps at the nanoscale in epitaxial thin films with diverse structural order, where the different values were related to indirect, direct, and interband transitions [23].Figure 7
UV-Vis electronic absorption spectra measured in 5 different areas of NiCo2O4 film. The inset drawing indicates the 5 analyzed zones.The film was evaluated in the electrochemical water oxidation reaction, and Figure8 shows the corresponding linear sweep voltammetry curve. It displays a small peak at 0.4 V (versus Ag/AgCl) due to the activation process derived of the oxidation of cobalt ions (Co2+→Co3+→Co4+) as previously established in the literature [17–19]. Then, the current increases at an onset potential of ~0.53 V because the OER proceeds principally via hydroxide ions oxidation (4OH-→O2+2H2O+4e-); a current density of 2.7 mA/cm2 at 0.7 V versus Ag/AgCl (1.67 V versus reversible hydrogen electrode, RHE) was achieved using our inkjet-printed NiCo2O4 film; this value is similar to earlier reports [17, 20]. In particular, the value compares well with the current reported for a NiCo2O4 film (with nanosheets morphology) prepared via a solvothermal route, on the same substrate (FTO) [17]. However, for a more adequate comparison, it is necessary to consider if NiCo2O4 is obtained as powder or film, the size and shape of the nano- or microstructure (which depends directly on the synthesis conditions), type of substrate used, the fabrication of the electrode, and its geometric and electroactive area. Table S1 (see Supplementary Material available online at https://doi.org/10.1155/2017/9647458) shows a summary of reports on the electrocatalytic properties of NiCo2O4 prepared by a variety of methods for using in the OER.Figure 8
Linear sweep voltammetry curve obtained from the analysis of NiCo2O4 inkjet-printed film.Taking these observations into consideration, according to the obtained results and based on the merits of the inkjet printing methodology, this route can be successfully used to scale up very active electrocatalytic materials applied in the clean energy production field.


## 3.1. Ink Physicochemical Properties

The formulated ink is a reddish and homogeneous solution (see inset photograph in Figure1(a)) with excellent stability and without formation of any sediment for three months under regular storage conditions (25°C and light protected). Figure 1(a) depicts the UV-Vis spectrum of the ink, where absorption bands at 229 and 297 nm match with electronic transitions from nonbonding (lone-pair) n orbital to an antibonding π orbital (n-π∗ transitions), typical of nitrate ions. The bands at 394, 468, and 513 nm correspond to d-d transitions of a cobalt aquo-complex ([Co(H2O)6]2+), whereas the bands at 654 and 730 nm are related to a nickel aquo-complex ([Ni(H2O)6]2+). This is relevant since the UV-Vis spectrum allows us to follow the ink stability with time.Figure 1
(a) UV-Vis absorption spectrum of the formulated ink; (b) shear rate versus shear stress; (c) viscosity as function of the shear rate; (d) shear modulus (viscous and elastic components) versus frequency.
(a)
(b)
(c)
(d)Analysis of the rheological properties of the ink was carried out, and, according to the plot of shear stress versus shear rate, Figure1(b), a slight deviation from Newtonian behavior is observed. From Figure 1(c) of the viscosity versus shear rate, a high-frequency viscosity value of the ink of 5.7 mPa·s was estimated; this value is close to the reported value for a glycerol : water (65 : 35 wt.%) mixture [34]. Figure 1(d) presents the plot of shear modulus versus frequency, where the elastic component is larger than the viscous component with increasing frequency, which means that the ink behaves like a solid at higher frequencies. On the other hand, as literature indicates for a glycerol : water (65 : 35 wt.%) mixture, the surface tension of the ink is expected to be around 65 dynes/cm [34]. Considering that physicochemical characteristics of an ink (viscosity and surface tension) play an important role for appropriate jetting during the printing process, it is likely that the glycerol-water formulated ink presents good performance, as the values are close to those required for a drop-on-demand piezoelectric inkjet printer [35]. In addition, the higher boiling point (~290°C) and heat of vaporization (~89 kJ/mol) of glycerol in comparison to water (~100°C and ~40 kJ/mol) suggests a low evaporation rate of the inkjet-printed drops favoring deposition of a homogeneous and continuous film on the substrate.In order to establish the annealing procedure after inkjet printing, a thermal analysis was performed. The thermogravimetric curve shown in Figure2(a) suggests that the ink decomposes in three stages; the first step occurs between 30 and 95°C with a sharp weight loss of 55.7%, and this is due to partial evaporation of the water-glycerol mixture. After water evaporation, the ink has a higher proportion of glycerol, and the second weight loss step of 39.2% is observed from 95 to 255°C. Finally, the third step takes place at 325°C accompanied by a weight loss of 2.8%, which corresponds to decomposition of nitrates present in the ink. Figure 2(b) exhibits the DSC curve, showing two endothermic and one exothermic peaks at 36.1, 150.6, and 260.7°C, respectively. The exothermic event could be related to partial oxidation of Ni2+ and Co2+ as well as a probable transformation of NO to NO2. After 375°C, no events were observed in both TGA and DSC curves. The obtained information from this analysis is important because it allows choosing the most adequate temperatures for drying and calcination in order to obtain a homogeneous film with the desired crystal phase.Figure 2
Thermal analysis curves of the formulated ink: (a) thermogravimetric curve and (b) differential scanning calorimetric curve.
(a)
(b)


## 3.2. Inkjet-Printed NiCo2O4 Films

Besides the ink properties, the printing settings must be optimized for each ink to obtain good quality films. In this work, different values of drop spacing (DS) and jetting voltage (JV) were evaluated. Taking into consideration both a visual and a microscopic inspection, after a thermal treatment at 500°C during 1 hour, the best result was obtained for a inkjet-printed film using DS = 20μm and JV = 35 V. For that reason, this is the representative film that will be further analyzed in this work, referred to hereafter as F-20-35. The DS parameter is related to the number of jetted drops per area and therefore to the pattern resolution; for example, a DS value of 20 μm corresponds to 2,510 drops/mm2 and 1,270 dots per inch (dpi). Thus, if DS is low (e.g., 15 μm), an overspreading of the ink on substrate occurs, provoking that the pattern shape is undefined; whereas if DS is high (e.g., 30 μm), the drops do not interact with each other and do not coalesce to form a continuous film. On the other hand, JV principally controls drop velocity, where increasing JV increases velocity. This parameter should be adjusted depending on the physical properties of the ink. In our formulated ink, a high JV of 35 V (40 V is the upper limit in the printer) was required because of the high value (65 dynes/cm) of the surface tension; the recommended range is 35–70 dynes/cm.A photograph of a representative F-20-35 NiCo2O4 film is presented in Figure 3(a), where a black coating with a well-defined 10 mm × 10 mm square is observed, while SEM imaging and EDS mapping analysis of the surface reveal a continuous coating formed by nanoparticles interconnected to each other with a homogenous elemental distribution.Figure 3
(a) SEM micrograph using secondary-electron mode and elemental chemical mapping of the film surface (upper left inset is a photograph of the deposited sample) and (b) cross-sectional SEM image using the low-angle back-scattered electron mode and EDS line scan.The quantitative EDS results indicate an atomic percentage of 7.94, 16.05, and 76.01 for nickel, cobalt, and oxygen, respectively; therefore, the proportion nickel : cobalt is 1.00 : 2.02 in good agreement with the stoichiometry of NiCo2O4 oxide. Also, the thickness was obtained from a cross-sectional SEM image (see Figure 3(b)), obtaining an average value of 490 nm. In order to confirm the film homogeneity, a linear-scanning EDS analysis of the cross section was done, where nickel, cobalt, and oxygen are homogeneously distributed with the same proportion observed in the surface analysis results. Table 1 lists a summary of some of the typical synthesis methods for fabrication of NiCo2O4 films, their thickness values, and XRD phase purity, as well as their specific applications and properties. As can be seen in Table 1, the thickness of the inkjet-printed F-20-35 NiCo2O4 films is comparable with those obtained previously by both physical and chemical approaches.Table 1
Summary of reports on NiCo2O4 films prepared by different techniques.
Synthesis method
Precursors
Substrate
Thickness
XRD phase
Application/properties
Ref.
Inkjet printing
Ni(NO3)2·6H2O Co(NO3)2·6H2O Glycerol, deionized water
FTO
490 nm
Pure
Water oxidation reaction
This work
RF sputtering
Ni-Co metal alloy targets
Silicon
550–830 nm
Impure
Electrical and magnetic
[8]
Pulsed laser deposition
Polycrystalline NiCo2O4 target
MgAl2O4 (001)
240 nm
Pure
Electrical and magnetic
[16]
Spray pyrolysis
Ni(NO3)2·6H2O Co(NO3)2·6H2O Methanol
Aluminum foil
12µm
Pure
Mesoporosity
[36]
Sol-gel
Co(CH3COO)2·4H2O Ni(CH3COO)2·4H2O 2-Methoxyethanol Ethanolamine
Al2O3 (0001)
900–1000 nm
Impure
Electrical resistivity
[2]
Hydrothermal
Ni(NO3)2·6H2O Co(NO3)2·6H2O Urea, NH4F, deionized water
FTO
1–4.5µm
Pure
Dye sensitized solar cells
[26]
Chemical bath deposition
NiCl2·6H2O CoCl2·6H2O Deionized water KOH (electrolyte)
Stainless steel
2.3–18µm
Pure
Supercapacitors
[37]
Electrodeposition
Ni(NO3)2·6H2O Co(NO3)2·6H2O Deionized water KCl (electrolyte)
Stainless steel and Indium-Tin Oxide (ITO)
4µm
Pure
Methanol fuel cells
[6]
Electrodeposition
Ni(NO3)2·6H2O Co(NO3)2·6H2O Deionized water KCl (electrolyte)
ITO
Not reported
Pure
Glucose sensing
[15]
Spray pyrolysis
NiCl2·6H2O CoCl2·6H2O Deionized water
FTO
Not reported
Pure
Supercapacitors
[4]
Solvothermal
NiCl2·6H2O CoCl2·6H2O Hexamethylenetetramine
FTO
Not reported
Pure
Water oxidation reaction
[17]In order to confirm the micrometric resolution achieved with the inkjet printing technique, Figure4 presents representative optical and SEM images of dots and line patterns of inkjet-printed NiCo2O4 on FTO using the optimized parameters for our ink. The micrographs reveal well-defined dot arrays (Figures 4(a)–4(c)), where the dots are separated at ~230 μm and ~210 μm in the X and Y directions, respectively; at higher magnification, it is observed that the drops have a size ranging within 45–52 μm. On the other hand, a uniform and continuous line pattern (in both X and Y directions) was also successfully obtained; see Figures 4(d)–4(f). The width of well-defined lines was ~190 μm.Figure 4
Optical and SEM micrographs of inkjet-printed NiCo2O4. In (a)–(c), a dot array pattern is presented, and, in (d)–(f), a multiline pattern is depicted.
(a)
(b)
(c)
(d)
(e)
(f)The XRD pattern of the F-20-35 films (see Figure5) exhibits a set of well-defined reflections at 2θ values of 31.2, 36.6, 38.9, 44.7, 59.0, and 65.2°, which match well with the (022), (113), (222), (004), (115), and (044) crystal planes, respectively, of the pure cubic phase NiCo2O4 (JCPDS #20-0781). This result is similar to other reports on the synthesis of single phase NiCo2O4 film using pulsed laser deposition [16], spray pyrolysis [4, 36], hydrothermal route [26], chemical bath deposition [37], electrodeposition [6, 15] and solvothermal route [17], as indicated in Table 1.Figure 5
XRD pattern of a NiCo2O4 film (sample F-20-35) after annealing at 500°C for 1 hour. The reflections marked with asterisk correspond to the fluorine-doped tin oxide (FTO) substrate; the line pattern corresponds to the reference JCPDS # 20-0781.Figure6 shows the XPS analysis of a F-20-35 film, showing that the cobalt 2p spectrum presents peaks at 780 and 796 eV related to the spin orbit splitting, 2p3/2 and 2p1/2, respectively. Since the binding energies of Co3+ (779.4 eV) and Co2+ (780.8 eV) are very close, the presence of satellite peaks is very important in order to determine the oxidation states of cobalt. In this respect, the observed satellites at 785 and 802 eV can be assigned to the presence of Co2+ ions [8]. On the other hand, the nickel 2p spectrum contains peaks associated with 2p3/2 (854 eV) and 2p1/2 (872 eV) as well as their respective satellite peaks at 861 and 880 eV. The main peak at 854 eV is assigned to octahedral Ni2+ ions, whereas the small shoulder at 856 eV has been ascribed to Ni3+ present in NiCo2O4 oxide [2, 6]. The contribution of every metallic cation was determined by a curve-fitting analysis of their respective XPS spectra. According to the quantitative XPS analysis and calculated areas from 2p3/2 peaks of cobalt and nickel, the surface atomic content is 13.9, 15.1, 6.1, and 4.4% for Co2+, Co3+, Ni2+, and Ni3+, respectively. This means that the mixed oxidation states of cobalt and nickel favor the charge neutrality and structural stability of the inkjet-printed NiCo2O4 film, as previously observed for a sol-gel film [2].Figure 6
XPS spectra for (a) cobalt 2p and (b) nickel 2p. The colored solid lines below the spectra are derived from curve-fitting analysis.
(a)
(b)The electronic absorption spectra obtained from 5 different zones along the NiCo2O4 film are presented in Figure 7. The absorption profile is as expected from the black color of the oxide, absorbing in the entire visible region of the electromagnetic spectrum, as previously reported [7]. Also, the slight variations in absorbance suggest a good optical homogeneity of the coating. It is known that generally the color of film is an indication of the optical band gap, which can be calculated from the UV-Vis spectrum considering the absorption onset edge and by extrapolation of a straight line to the abscissa axis. Therefore, based on the extended absorption-edge spectrum of NiCo2O4, the optical band gap of this oxide could be lower than 1.24 eV, because the absorbance value is still approximately 0.7 units at 1000 nm. However, the exact band gap of NiCo2O4 is still controversial: using first-principle calculations, Zhu et al. reported a band gap of 0.2462 eV [10], whereas Shi et al. obtained an indirect band gap of 2.0 eV and a direct gap of 2.1 eV [38]. Dileep et al. utilized high resolution electron energy loss spectroscopy to probe the optical band gaps at the nanoscale in epitaxial thin films with diverse structural order, where the different values were related to indirect, direct, and interband transitions [23].Figure 7
UV-Vis electronic absorption spectra measured in 5 different areas of NiCo2O4 film. The inset drawing indicates the 5 analyzed zones.The film was evaluated in the electrochemical water oxidation reaction, and Figure8 shows the corresponding linear sweep voltammetry curve. It displays a small peak at 0.4 V (versus Ag/AgCl) due to the activation process derived of the oxidation of cobalt ions (Co2+→Co3+→Co4+) as previously established in the literature [17–19]. Then, the current increases at an onset potential of ~0.53 V because the OER proceeds principally via hydroxide ions oxidation (4OH-→O2+2H2O+4e-); a current density of 2.7 mA/cm2 at 0.7 V versus Ag/AgCl (1.67 V versus reversible hydrogen electrode, RHE) was achieved using our inkjet-printed NiCo2O4 film; this value is similar to earlier reports [17, 20]. In particular, the value compares well with the current reported for a NiCo2O4 film (with nanosheets morphology) prepared via a solvothermal route, on the same substrate (FTO) [17]. However, for a more adequate comparison, it is necessary to consider if NiCo2O4 is obtained as powder or film, the size and shape of the nano- or microstructure (which depends directly on the synthesis conditions), type of substrate used, the fabrication of the electrode, and its geometric and electroactive area. Table S1 (see Supplementary Material available online at https://doi.org/10.1155/2017/9647458) shows a summary of reports on the electrocatalytic properties of NiCo2O4 prepared by a variety of methods for using in the OER.Figure 8
Linear sweep voltammetry curve obtained from the analysis of NiCo2O4 inkjet-printed film.Taking these observations into consideration, according to the obtained results and based on the merits of the inkjet printing methodology, this route can be successfully used to scale up very active electrocatalytic materials applied in the clean energy production field.


## 4. Conclusions

Inkjet printing has been efficiently utilized for the preparation of high-quality NiCo2O4 thin films. The physicochemical properties of the ink, as well as the optimized values of drop spacing (20 μm) and jetting voltage (35 V), were used to optimize the uniformity and homogeneity of the fabricated films. XPS results illustrate that the inkjet-printed films have a chemically active redox surface that is composed of a mix of oxidation states, of both cobalt and nickel cations. A current density of 2.7 mA/cm2 at 0.7 V was reached using inkjet-printed NiCo2O4 film in the water oxidation reaction, a value that is comparable with previous reports. Due to the simplicity, low cost, and high performance for preparation of large-area coatings by means of inkjet printing, this is an attractive approach for the fabrication of (photo)electrode films in a variety of solar energy conversion systems.



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*Source: 9647458-2017-07-06.xml*