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Sep 18, 2017 - ORIGINAL ARTICLE. Effect of polyethylenimine addition and washing on stability and electrophoretic deposition of Co3O4 nanoparticles.
Received: 8 April 2017

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Accepted: 18 September 2017

DOI: 10.1111/jace.15244

ORIGINAL ARTICLE

Effect of polyethylenimine addition and washing on stability and electrophoretic deposition of Co3O4 nanoparticles Ali Asghar Sadeghi Ghazvini1 | Ehsan Taheri-Nassaj1

| Babak Raissi2 | Reza Riahifar2 |

Maziar Sahba Yaghmaee2 1 Department of Materials Science and Engineering, Tarbiat Modares University, Tehran, Iran 2

Battery and Sensor Research Group, Materials and Energy Research Center, Karaj, Iran Correspondence Ehsan Taheri-Nassaj, Department of Materials Science and Engineering, Tarbiat Modares University, Tehran, Iran. Email: [email protected]

Abstract In this work, the parameters of cobalt oxide suspension such as conductivity, zeta potential, particle size, stability, and finally the electrophoretic behavior of particles in the absence and presence of polyethylenimine (PEI) in acetone medium were investigated. Also, the effects of washing on the stability and electrophoretic deposition of Co3O4 were studied. Characterization of the obtained layer by optical microscopy revealed that there was no deposition in the suspension without PEI, while a uniform layer was formed in the presence of PEI additive. Scanning electron microscopy (SEM) results confirmed the uniformity of layer obtained in acetone using PEI additive. Moreover, SEM results demonstrated that more porous microstructures were obtained at longer deposition durations. The difference in the porosity of the layers, as indicated by the SEM micrographs, is attributed to increase in the deposition time. KEYWORDS cobalt/cobalt compounds, electrophoretic deposition, nanoparticles

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| INTRODUCTION

Cobalt oxide is an important magnetic semiconductor that exhibits polymorphism and exists in several structures such as monoxide or cobaltous oxide (CoO), cobaltic oxide (Co2O3), and cobaltosic oxide or cobalt cobaltite (Co3O4). Co3O4 is a p-type semiconductor with spinel structure and has the highest number of applications among the mentioned polymorphs due to its unique properties. Co3O4 has a wide range of applications due to its stability as well as its optical, electrical, and magnetic properties.1-6 Co3O4 has been extensively used as catalyst7 and electro-catalyst8 as well as in, Li-batteries,9 gas sensors,10 and supercapacitors.11 In most industrial applications of Co3O4, crack-free thick or thin films with a uniform microstructure is required. To obtain these layers numerous methods such as sputtering,12 sol gel,4,13 electrodepostion,14 spray pyrolysis,15 and chemical vapor deposition13 have been J Am Ceram Soc. 2018;101:553–561.

employed. In addition to these techniques, electrophoretic deposition (EPD)16 has been vastly employed as a simpler and cheaper route that makes it possible to obtain reliable ceramic thick films. EPD is a colloidal process wherein ceramic bodies can be shaped directly by utilizing DC electric field to a stable colloidal suspension. In practice, EPD is a combination of two processes: (i) electrophoresis, which is the motion of charged particles in suspension under the application of an electric field; and (ii) deposition, defined as the coagulation of particles into a dense mass. In order to employ this technique, it is essential to prepare a stable suspension containing charged particles (high zeta-potential) free to move in the suspension. The main parameters concerning the suspension are the particle concentration, size distribution, conductivity, and pH of the solution as well as the viscosity of the medium.17-19 For repeatable EPD results an additive usually should be used as a charging agent. Polyethyleneimine (PEI) is a cationic polyelectrolyte that

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© 2017 The American Ceramic Society

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has been widely used in EPD processes in aqueous and nonaqueous suspensions.20-26 In this study, we investigated the effect of PEI on stability, zeta potential, particle size, and EPD behavior of Co3O4 nanoparticles in acetone. The study of stability via zeta potential requires diluted suspension.27 Since diluted suspensions may have a different behavior compared with primary suspension, we used a typical type of spectrophotometer to study stability of concentrated suspensions. For this purpose, we used a home-made designed turbidimeter to study the stability of suspension with and without PEI. Finally, porous crack-free layers with nanostructured morphologies were obtained.

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| EXPERIMENTAL PROCEDURES

Cobalt oxide (Co3O4) nanopowders with the average primary particle size of 10-30 nm was used as the starting material (US Research Nanomaterial, Inc., Houston, TX, USA). The organic solvent, acetone, was purchased from Merck (Darmstadt, Germany) (Art.1.000.14.2500) and used as-received without any further purification. Two stainless steel substrates with an area of 2 cm2 (2 9 1) and 6 cm2 (3 9 2) were used as similar area pair of anode and cathode (with a deposition area of 1 and 4 cm2, respectively). Suspensions containing between 4 upto10 g/L Co3O4 nanoparticles were prepared with and without polyethylenimine (PEI). Polyethylenimine (Sigma Aldrich, Taufkirchen, Germany 408727, branched Mw ~ 10 000) was diluted in distilled water (1 part PEI and 4 part water) and

SADEGHI GHAZVINI

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then 50 and 100 microliter (lL) of the solution were added to the 50 mL suspension of 4 g/L Co3O4 in acetone. Each suspension was sonicated for 15 min. The EPD was carried out at a constant voltage of 100 V. The anode and cathode electrodes with a fixed distance of 1 and 2 cm were connected to a regulated DC power supply (FANAVARENE SAHAND AZAR, Tabriz, Iran, PSD 2600). The schematic view of experimental set-up is shown in Figure 1. The substrates were washed with distilled water and acetone and then dried in air. We used a multi-step deposition technique for Co3O4 nanoparticles. This technique consists of deposition process followed by drying at ambient temperature several times. For obtaining deposition yield, the electrodes were weighed after every step (after drying in air). In order to evaluate the deposition rate, the process was carried out at different time intervals of 30, 60, 90, and 120 seconds employing a constant applied voltage of 100 V. The washing process of Co3O4 nanoparticles was accomplished using a centrifuge (2634 g) for 15 minutes. After the completion of the washing process, the supernatant was removed and fresh water was added. Every time, the conductivity of the removed supernatant was measured. In round six of washing process, the supernatant was removed and the remaining powder was dried in air. The inductively coupled plasma mass spectrometer analysis (ICP-MS; Agilent 4500, Santa Clara, CA, USA) was used for detecting ions in supernatant. The nanopowders were dried in air. Scanning electron microscope (Te scan MIRA ΙΙ LMU, Brno – Kohoutovice, Czech Republic) was employed to investigate the deposition pattern of the obtained layers. Moreover, quantitative analysis over the deposition microstructure was carried out by

F I G U R E 1 The schematic view of experimental set-up (A) and dimension of electrodes (B) [Color figure can be viewed at wileyonlinelibrary.com]

SADEGHI GHAZVINI

optical microscope (Olympus/BX61, Tokyo, Japan). Both suspensions were characterized by measuring the Zeta potential and hydrodynamic diameter of Co3O4 nanoparticles using a Malvern Zeta sizer (Malvern/Nano ZS, Worcestershire, UK). Hydrodynamic particle size is composed of electric double layer (EDL) thickness and particle size. To study the role of PEI on the stability of the suspensions, we used a developed Turbidimeter for suspensions with and without PEI. Blue LEDs were used for a light source and Cadminum sulfide (CdS) detectors were applied as light detectors. They are placed correspondingly at three different heights in a test tube. Conductivity measurements of the prepared suspensions were carried out by a WTW-inolab (Weilheim, Germany) conductivity meter. The deposited materials were removed from the substrates in order to prepare powder samples for simultaneous thermal analysis (STA). Thermogravimetric analysis (TGA) was carried out in air at a heating rate of 5°C min1 using a thermoanalyser (Pl-STA 1640 England).

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ET AL.

| RESULTS AND DISCUSSIONS | Characterization of nanopowders

The SEM image of Co3O4 particles is shown in Figure 2, which confirms their spherical shape and nano-metric dimensions. The estimated average size of nanoparticles is about 90 nanometer (The calculation was made with Digimizer software).

F I G U R E 2 SEM image of Co3O4 nanoparticle [Color figure can be viewed at wileyonlinelibrary.com]

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| Stability of the suspensions

Regarding the stability tests carrying out by the turbidimeter: If the particles in suspension start to settle down, the light received by the detectors should increase. The intensity of the light received by the detectors is measured in millivolts. A schematic view of the Turbidimeter is shown in Figure 3. According to Equations 1 and 2, we can calculate the absorbance (A) and turbidity (s) by the following relations: 1 I0 s ¼ lnð Þ d Id T¼

2:303 A Id or A ¼  log T ¼  log d I0

(1) (2)

where “T” is transmittance, “d” is the sample thickness, “I0” is the incident light intensity, “Id” is the intensity exiting the sample and “A” is the absorbance.28-30 In the first step, the as-received Co3O4 nanoparticles were dispersed in acetone medium where fast sedimentation was observed. We observed that sedimentation takes place much slower by adding PEI. In addition, we washed Co3O4 six times (6 w Co3O4) by deionized water and dispersed it in acetone solvent with 50 lL of PEI (Figures 4 and 5). We noticed that by washing the nanoparticles can have a possitive effect on the stability of the suspension. A comparison between the curves in Figures 4 and 5 demonstrate that PEI is a proper additive for increasing the stability of nanoparticles in acetone medium. Due to the presence of secondary amino nitrogen groups that can release protons, polyethylenimine (PEI) is a highly positive charge cationic polyelectrolyte.25 As reported by Guo,31 adding PEI in Co3O4 suspension makes N-H bending and stretch peak on FTIR spectrum. Ghashghaie et al.32 reported that the zeta

F I G U R E 3 A schematic view of home-made turbidimeter, where D.1, D.2 and D.3 represent the 90° (front) detector and D.4, D.5, and D.6 represent the 180° (scattering) detector [Color figure can be viewed at wileyonlinelibrary.com]

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T A B L E 1 The conductivity of the removed supernatant water

F I G U R E 4 Stability of Co3O4 nanoparticles in acetone medium (1) without additive, (2) with PEI, (3) washed nanoparticles with PEI (related to D.5 detector) [Color figure can be viewed at wileyonlinelibrary.com]

Cycles of washing

DI-water

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2

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Conductivity of supernatant (mS/cm)

2.9

75.5

29.9

7

5.3

5.4

5.6

compared with the as-received powder condition. The conductivity of the supernatant removed water is represented in Table 1, where the conductivity of the supernatant is shown to have increased from 2.9 to 75.5 lS/cm after one time washing. This result clearly indicates that the ions and contaminants have been transferred from nanoparticles to water medium. After every washing cyle, the supernatant conductivity decrease from 75.5 to 29.9, 7, 5.3, 5.4, and 5.6, respectively. The supernatant conductivity results given in Table 1 reveals that the transfer of ions into water reaches a steady state after the forth washing. ICP-MS was used to evaluate ions (Na+1, Li+1, K+1, and Ca+2) concentration in supernatant after washing Co3O4 nanoparticles. Table 2 shows that after washing, ions leave particle surface and solved in water. It seems that washing the contaminating ions from the surface of nanoparticles results in a stronger interaction between PEI and surface of nanoparticles. Ions on the top layer of particles can be the by-product of the synthesis process of nanoparticles. The Zeta potential results confirms that the addition of PEI caused Co3O4 nanoparticles to acquire higher surface charge (Table 3). Moreover, this additive caused decrease in the particle size from 273.5 to 249.5 nm. Washing the nanoparticles enables to decrease the particles size down to 108.1 nm in the presence of PEI.

3.3 | Electrophoretic deposition of Co3O4 nanoparticles F I G U R E 5 The Curves of absorbance of Co3O4 nanoparticles in acetone medium (1) without additive, (2) with PEI, (3) washed nanoparticles with PEI (related to D.5 detector) [Color figure can be viewed at wileyonlinelibrary.com]

potential of unwashed ZnO particles in acetone is less than its value if particles were washed. They also found that conductivity of the suspension containing unwashed particles is lower than that of the one containing washed ZnO particles. They attributed it to the release of ions from the surface of particles into the liquid. Hatton et al.27 investigated the effect of washing on stabilizing particle size and EPD of ZrO2 in ethanol suspension. They concluded that the zeta-potential, and consequently, the stability of the suspension increased by washing up to three washes, suggesting the removal of the electrolyte. It is suggested that the ionic (Na+1) surface contamination, which is reduced by washing cycles, is the cause of the low zeta-potential and deposition rate of the asreceived powder. Thus, EPD improved remarkably

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| Effect of PEI

Electrophoretic deposition was carried in acetone medium with and without PEI. No deposition was observed in the absence of PEI, while a uniform deposit was formed in 100 V. The current-time curve of this EPD process is illustrated in Figure 6. The decline of current with time can be a sign of the formation of nonconductive cobalt oxide T A B L E 2 Evaluation of the alkaline ions before and after washing measured by (ICP-MS) Ion

Deionized water (DI)

Supernatant of 1 step washing

Supernatant of 2 step washing

Na+1 (ppm)