Surface modification and cathodic electrophoretic deposition of ceramic materials and composites using celestine blue dye

Abstract

A new method has been developed for the surface modification of inorganic particles, which allowed their efficient electrostatic dispersion and cathodic electrophoretic deposition (EPD). The approach is based on the use of cationic celestine blue (CB) dye as a charging and dispersing agent. The key advantages of this approach are related to its applicability to different materials and strong adsorption of CB to the inorganic surfaces, which is of critical importance for efficient particle dispersion. Proof-of-concept studies involved the EPD of thin films of various materials, such as TiO₂, MnO₂, Mn₃O₄, BaTiO₃, halloysite nanotubes, zirconia and yttria. The results of the deposition rate measurements, Fourier transform infrared spectroscopy, UV-vis and quartz crystal microbalance studies provided an insight into the mechanism of CB adsorption, which involved the interactions of the OH groups of the catechol ligand of CB and metal atoms on the particle surface. It was demonstrated that CB can be used as an efficient dispersing agent for the nanoparticle synthesis by chemical precipitation methods. The feasibility of EPD of various oxide materials paved the way to the EPD of various composites using CB as a co-dispersant for the individual components. Thin films of individual oxides and composites were investigated by electron microscopy and X-ray diffraction methods. The benefits of cathodic EPD for nanotechnology were demonstrated by the formation of nanostructured MnO₂ films on commercial high surface area current collectors for energy storage in electrochemical supercapacitors. Testing results showed that the method allowed the fabrication of efficient electrodes with high capacitance and excellent capacitance retention at high charge–discharge rates. The new method paves the way for the deposition of other functional materials and composites for advanced applications.

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1. Introduction

The adsorption of organic molecules on inorganic particles is currently under intensive investigation for many applications in nanotechnology. Significant interest has been generated in the use of organic molecules with strong interfacial adhesion as dispersants for the synthesis of inorganic nanoparticles^1–3 and fabrication of thin films by colloidal methods.^4–6 The dispersant adsorption on nanoparticles is of critical importance for the fabrication of stable suspensions. The non-adsorbed dispersant can be considered as an electrolyte,⁷ which according to the DLVO (Derjaguin–Landau–Verwey–Overbeek) theory decreases the thickness of the electrical double layer and promotes particle coagulation. The need in advanced dispersing agents for nanotechnology has generated significant interest in the investigation of new adhesion mechanisms. Recent studies of mussel adhesion to inorganic materials opened a new avenue for further advances in this area.

Fundamental investigations showed^8–11 that strong mussel adhesion to different surfaces in saline water involves mussel protein macromolecules, containing catecholic amino acid, L-3,4-dihydroxyphenylalanine (DOPA). The adhesion mechanism involves metal atoms on the material surface and OH groups of the catechol ligand of DOPA; it is based on bidentate bridging bonding or bidentate chelating bonding. In the investigations of DOPA, adsorbed on titanium dioxide in water, the pull-off forces of >500 pN were reported for a single DOPA molecule.¹¹ DOPA is now considered as an important component of many advanced moisture resistant adhesives.¹¹ The analysis of DOPA adsorption on various substrates has driven the investigations of other molecules from the catechol family. The closest molecular analogue of DOPA is dopamine (DA).

Several investigations indicated that DA adsorption resulted in improved functional properties of inorganic nanoparticles. The DA modified magnetic Fe₃O₄ nanoparticles showed enhanced magnetization and increased superparamagnetic blocking temperature.^12,13 It was found that DA adsorption changed the microstructure of the magnetic dead layer on the nanoparticle surface and promoted magnetic ordering in the surface layer. DA was used as an anchor for the functionalization of Fe₃O₄ nanoparticles using a ‘click’ chemistry method.¹⁴ Especially interesting are the investigations of DA adsorption on TiO₂, which showed significant changes in the structure of the surface layer^15,16 and revealed the enhanced electronic and photovoltaic properties of the TiO₂ nanoparticles.^17–19 The DA sensitized TiO₂ showed improved charge transfer at the organic–inorganic interface. The DA modified TiO₂ nanoparticles and films were investigated for the application in biosensors, photoelectrochemical transducers and optical devices.^16–20 The adsorption of DA resulted in enhanced luminescence properties²¹ of ZnO.

The strong adsorption of DOPA and DA on the surface of inorganic nanoparticles is of special interest for applications in electrophoretic nanotechnology of thin films. Electrophoretic deposition (EPD)^22–25 involves electrophoretic motion of charged particles in a suspension toward an electrode and deposit formation under the influence of an applied electric field. In this method, inorganic particles must be charged^26,27 and well dispersed in a solvent by the adsorbed dispersants.

The use of DOPA as a charging and dispersing agent for inorganic particles presents difficulties attributed to zwitterionic properties of this molecule. Several investigations^28–30 were focused on the use of DA for cathodic EPD of ZnO, TiO₂ and MnO₂ nanoparticles. However, the use of DA for particle dispersion generates some problems, related to the small size of the DA molecule and self-polymerization of DA in the solutions. It is important to note that amino group of DA must be protonated in order to impart a charge to the DA molecules in the solutions. Therefore, in the previous investigations^28–30 DA was used in the form of dopamine hydrochloride (DA–HCl). In this case, competitive adsorption of the protonated DA and free H⁺ ions from the acidic solutions on the particle surface can be expected. In our investigation we demonstrate that such problems can be avoided by using a celestine blue (CB) dye as a new dispersant from the catechol family for cathodic EPD.

The goal of this investigation is the application of CB as a charging and dispersing agent for cathodic EPD of inorganic nanoparticles. Similar to DA, the structure of CB (Fig. 1) includes a catechol ligand, which can provide CB adsorption on inorganic nanoparticles. The relatively large size of the CB molecules, compared to that of DA, is beneficial for the electrosteric dispersion. Our results indicated that particle charging can be achieved by the dissociation of CB in the solutions and adsorption of cationic CB on the particle surface. Therefore, the difficulties related to the protonation of DA molecules or self-polymerization of DA can be avoided. Moreover, we demonstrate that CB can be used for the dispersion and EPD of various functional materials and fabrication of composites.

Fig. 1 (A) Chemical structure of CB and (B and C) suggested adsorption mechanisms, involving metal atoms M on a particle surface: (B) bidentate bridging bonding and (C) bidentate chelating bonding.

2. Experimental procedures

2.1. Materials

Celestine blue (CB) dye, KMnO₄, Mn(NO₃)₂, NH₄OH, TiO₂ (<25 nm nanoparticles, anatase), BaTiO₃ (average particle size ∼1 μm) and halloysite nanotubes HNT were purchased from Aldrich. Previous investigations³¹ showed that HNT have relatively uniform inner and outer diameters in the range of 10–20 and 40–80 nm, respectively. The lengths of the HNT were typically in the range of 0.5–5 μm. The chemical precipitation method for the fabrication of zirconia I and II particles is described in the ESI.†

The procedure for the fabrication of MnO₂ was similar to the one, described in a previous investigation.³² Precipitation of MnO₂ particles was achieved by adding 25 ml isopropanol to aqueous 0.2 M KMnO₄ solution. In this procedure, isopropanol was used as a reducing agent for KMnO₄. The MnO₂ powder, prepared by this method³² contained mainly amorphous phase with a small amount of birnessite (JCPDS file 80-1098), which was converted to the cryptomelane (JCPDS file 44-1386) phase after annealing at 300 °C during 2 h. In contrast to the previous investigation,³² the solution was ultrasonicated during the reaction in order to avoid the agglomeration of MnO₂ particles. The formation of non-agglomerated particles was confirmed by transmission electron microscopy (TEM) investigations. Typical TEM images at different magnifications are shown in Fig. 2. The low magnification image indicated that ultrasonic agitation was beneficial in order to avoid particle agglomeration (Fig. 2A). This is in contrast to the results of the previous investigations,³² which showed agglomeration of the primary particles. The typical size of the primary particles (Fig. 2B) was in the range of 50–100 nm in agreement with the results of the previous investigation.³²

Fig. 2 (A and B) TEM images of MnO₂ particles at different magnifications.

The procedure for the fabrication of Mn₃O₄ nanoparticles involved slow addition of ∼15 ml of NH₄OH solution to 100 ml of aqueous 0.1 M Mn(NO₃)₂ solution at a temperature of 60 °C in order to achieve pH = 11. Then 1 ml of H₂O₂ was added to the mixture and stirring was continued during 1 h at 60 °C. The precipitate was washed and dried at in air. The formation of the crystalline Mn₃O₄ phase (JCPDS file 24-734) was confirmed by X-ray diffraction studies (ESI, Fig. S1†). The typical particle size was about 20–50 nm (Fig. 3).

Fig. 3 (A and B) TEM images of Mn₃O₄ particles at different magnifications.

2.2. Electrophoretic deposition

Stainless steel foils and Ni plaques (Vale Canada) were used as substrates for film deposition by EPD. The distance between the cathodic substrates and platinum counter electrodes was 15 mm. The concentration of the ceramic powders in the suspensions in ethanol was 4 g L⁻¹. The concentration of CB was varied in the range of 0–0.5 g L⁻¹. Before the deposition, the suspensions were ultrasonicated for 30 min to achieve a homogeneous dispersion of the oxide particles. EPD was performed at a deposition voltage of 20 V, the deposition time was varied in the range of 1–8 min. After deposition, the deposits were dried in air for 48 h.

2.3. Characterization

Electrophoretic deposits were removed from the stainless steel substrates for Fourier transform infrared spectroscopy (FTIR), UV-vis and XRD studies. FTIR investigations were performed using a Bio-Rad FTS-40 instrument. The UV-vis spectra were obtained using Cary-50 UV-vis spectrophotometer. The adsorption of CB has been monitored in situ using a quartz crystal microbalance (QCM 922, Princeton Applied Research) controlled by a computer, as described in the ESI.†

X-ray diffraction (XRD) studies were performed using a powder diffractometer (Nicolet I2, monochromatized CuK_α radiation) at a scanning speed of 0.5° min⁻¹. Electron microscopy investigations were performed using a Philips CM12 transmission electron microscope (TEM) and a JEOL JSM-7000F scanning electron microscope (SEM).

Electrochemical studies were performed using a potentiostat (PARSTAT 2273, Princeton Applied Research). Working electrodes with area of 1 cm² were prepared by EPD of MnO₂ on Ni plaque current collectors. The counter electrode was a platinum gauze, and the reference electrode was a standard calomel electrode (SCE). Capacitive behavior and electrochemical impedance of the electrodes were investigated in 0.5 M Na₂SO₄ aqueous solutions. Cyclic voltammetry (CV) studies were performed at scan rates of 2–100 mV s⁻¹. The total capacitance C = Q/ΔV was calculated using half the integrated area of the CV curve to obtain the charge (Q), and subsequently dividing the charge by the width of the potential window (ΔV). The mass-normalized specific capacitance C_m = C/m (m-sample mass) and area-normalized specific capacitance C_s = C/S (S-sample area), calculated from the CV data, were investigated. The alternating current (AC) complex impedance Z* = Z′ − iZ′′ was measured in the frequency range of 10 mHz–100 kHz at the amplitude of the AC signal of 5 mV. The complex AC capacitance C* = C′ − iC′′ was calculated from the impedance data as C′ = Z′′/ω|Z|² and C′′ = Z′/ω|Z|², where ω = 2πf, f – frequency.

3. Results and discussion

The suspensions of MnO₂, Mn₃O₄, TiO₂, zirconia I, HNT and BaTiO₃ particles in ethanol were unstable and showed rapid sedimentation after the ultrasonic agitation. In contrast, the addition of CB allowed the formation of stable suspensions. The suspensions of nanoparticles, such as MnO₂, Mn₃O₄, TiO₂, zirconia I, containing 0.5 g L⁻¹ CB, were stable for more than 7 days. The suspensions of larger particles, such as BaTiO₃ and HNT, containing 0.5 g L⁻¹ CB, were stable during 3–4 days. Cathodic deposits were obtained from all the suspensions of ceramic particles, containing CB, by EPD at a deposition voltage of 20 V.

The influence of charging additives on the electrokinetic behavior of oxide particles is usually analyzed on the basis of zeta-potential measurements. However, the concept of zeta potential has been developed for rigid particles.³³ The use of this concept for the analysis of oxide particles, containing adsorbed large organic molecules, presents difficulties.^33,34 Therefore, in this investigation the influence of CB on the EPD of various oxide particles was analyzed on the basis of the deposition yield measurements.

Fig. 4 shows the influence of CB concentration on the deposition yield for MnO₂, Mn₃O₄, TiO₂ and BaTiO₃ suspensions. Significant increase in the deposition yield was observed (Fig. 4) with increasing CB concentration in the range of 0–0.1 g L⁻¹ and then the deposition yield increased gradually at higher CB concentrations. This indicated that the addition of CB to the suspensions resulted in CB adsorption on the oxide particles. Similar to DOPA, DA and other materials from the catechol family,³⁵ the CB adsorption involves bidentate bridging bonding (Fig. 1B) or bidentate chelating bonding (Fig. 1C). The adsorbed CB provided suspension stability and imparted a positive charge to the particles for cathodic EPD. However, we cannot exclude the possibility that the suspensions also included free, non-adsorbed CB. The high deposition rate obtained at low CB concentration indicated that CB was efficiently adsorbed on the oxide particles and the amount of free CB in the suspensions was very low. As pointed out above, a non-adsorbed dispersing agent is detrimental for the suspension stabilization. The results indicated that CB is a promising charging agent for the EPD of materials. Fig. 4A shows that MnO₂ was deposited cathodically in ethanol without CB. Such deposition was performed immediately after the ultrasonic agitation, because the suspension of MnO₂ in ethanol was unstable. The origin of the natural charge of MnO₂ was described in ref. 29. It is related to strong ability of MnO₂ to adsorb H⁺ from solvents. The adsorption of CB on MnO₂ resulted in increased surface charge, significant increase in suspension stability and increased deposition rate.

Fig. 4 Deposit mass versus CB concentration in 4 g L⁻¹ suspensions of (A) MnO₂, (B) Mn₃O₄, (C) TiO₂ and (D) BaTiO₃ at a deposition voltage of 20 V and deposition time of 5 min.

The deposition yield increased with increasing deposition time at a constant CB concentration and constant voltage (Fig. 5). The decrease in the deposition rate with time is related to the decreasing electric field in the suspension due to the increasing voltage drop in the growing films. Similar results were obtained for other oxide materials, such as HNT and Y₂O₃ (ESI, Fig. S2 and S3†). The amount of the deposited material can be varied by changing of the CB concentration and deposition time.

Fig. 5 Deposit mass versus deposition time for 4 g L⁻¹ suspensions of (A) MnO₂, (B) Mn₃O₄, (C) TiO₂ and (D) BaTiO₃, containing 0.5 g L⁻¹ CB at a deposition voltage of 20 V.

Fig. 6 shows typical SEM images of the films, prepared by EPD. The films of MnO₂, Mn₃O₄ and TiO₂ contained nanoparticles. The BaTiO₃ films contained larger particles in agreement with the particle size data provided by the manufacturer. The difference in surface density and porosity of the films can be attributed to the gas evolution, particle size and packing of the particles. It was found that the increase in CB concentration above 0.5 g L⁻¹ resulted in enhanced gas evolution at the cathodic substrates and increased deposit porosity. Therefore, the amount of CB in the bath composition must be optimized at concentrations below 0.5 g L⁻¹. In order to confirm the CB adsorption on the particle surface, the deposits were removed from the substrates and investigated by FTIR and UV-vis methods. Our experiments showed that CB cannot be deposited from pure CB solutions. We suggest that mutual electrostatic repulsion of the CB molecules at the electrode surface prevented film formation. We found that CB was included in the deposit only when it was adsorbed on the ceramic particles. The non-adsorbed CB remained in the bulk solutions. Therefore, FTIR and UV-vis analyses of the deposits allowed the investigation of the CB adsorption on the particles.

Fig. 6 SEM images of films, prepared from 4 g L⁻¹ suspensions of (A) MnO₂, (B) Mn₃O₄, (C) TiO₂ and (D) BaTiO₃, containing 0.5 g L⁻¹ CB at a deposition voltage of 20 V.

Fig. 7A shows the FTIR spectra of the MnO₂, Mn₃O₄, TiO₂ and BaTiO₃ deposits. The absorptions in the range of 1625–1627 cm⁻¹, 1567–1571 cm⁻¹, 1442–1443 cm⁻¹ and 1390–1400 cm⁻¹ (Fig. 7A(a–d)) were attributed to stretching vibrations of the aromatic ring ν(C–C) and ν(C=C)^35–37 of adsorbed CB. The absorption in the range of 1344–1346 cm⁻¹ were attributed to stretching ν(C–N) and ν(C=N) vibrations³⁸ of CB. The broad absorptions in the UV-vis spectra in the range of 622–625 nm (Fig. 7B(a–d)) were related to adsorbed CB.³⁹ Therefore, the FTIR and UV-vis data confirmed that deposited ceramic particles contained adsorbed CB. The QCM data presented in Fig. 8 provide additional evidence of strong CB adsorption on MnO₂. The mass gain of gold coated quartz resonator, containing a MnO₂ film, was observed after the injection of CB into the solvent. The mass gain increased with time, showing a gradual decrease of the adsorption rate.

Fig. 7 (A) FTIR and (B)UV-vis data for deposits, prepared from 4 g L⁻¹ suspensions of (a) MnO₂, (b) Mn₃O₄, (c) TiO₂ and (d) BaTiO₃, containing 0.5 g L⁻¹ CB at a deposition voltage of 20 V.

Fig. 8 Mass gain, measured using QCM for the MnO₂ film deposited on a gold coated quartz crystal versus time after the injection of CB. Arrow shows injection time.

The possibility of EPD of different materials using CB as a charged dispersing agent paved the way for the fabrication of composites using CB as a co-dispersant for individual components. Fig. 9 shows SEM images and XRD patterns of the composites, prepared from the mixed suspensions, containing TiO₂ and other materials, such as MnO₂, HNT and BaTiO₃. The TiO₂–MnO₂ composite contained nanoparticles of both materials (Fig. 9A), the corresponding XRD pattern (Fig. 9D(a)) showed peaks of TiO₂ and MnO₂ phases. The SEM image of TiO₂–HNT composite (Fig. 9B) showed nanoparticles of TiO₂ and HNT, the corresponding XRD pattern (Fig. 9D(b)) included peaks of both phases. The SEM image of TiO₂–BaTiO₃ composite (Fig. 9C) showed nanoparticles of TiO₂ and larger BaTiO₃ particles, the XRD peaks of both materials were observed in the corresponding XRD pattern (Fig. 9D(c)). Therefore, various composites can be obtained using CB as a co-dispersant for different materials.

Fig. 9 (A–C) SEM images of films and (D) corresponding X-ray diffraction patterns for the films, prepared from mixed suspensions, containing 2 g L⁻¹ TiO₂ and (A), (D(a)) 2 g L⁻¹ MnO₂, (B), (D(b)) 2 g L⁻¹ halloysite and (C), (D(c)) BaTiO₃ at a deposition voltage of 20 V (● – JCPDS file 21-1272, ♦ – JCPDS file 44-1386, ■ – JCPDS file 29-1487,▼ – JCPDS file 5-0626).

The experimental results, discussed above indicated that the use of CB as a charging and dispersing agent allowed EPD of various materials and composites. In this approach, many problems related to the application of EPD for nanotechnology can be addressed. It is known that the application of electric field to the suspensions of nanoparticles or stirring of the suspensions can result in the particle agglomeration and sedimentation.⁷ Previous investigations showed that the fabrication of stable colloidal dispersions of MnO₂ nanoparticles with concentration above 1 mM presents difficulties⁴⁰ due to the high surface energy of the MnO₂ nanoparticles, which promotes their agglomeration and sedimentation. However, significantly higher concentrations of the nanoparticles are necessary for practical applications in EPD and other colloidal methods. Such problems can be successfully addressed by the use of CB. Nanoparticles of TiO₂, MnO₂, Mn₃O₄ (Fig. 4–6) and HNT (Fig. S2†) were successfully deposited by adding CB to the suspensions. Another strategy involves the use of CB during synthesis of nanoparticles. Fig. 10 compares the deposition yield data for zirconia I, prepared without CB and zirconia II prepared using CB as a dispersant during synthesis. Similar to other materials (Fig. 4, 5 and ESI†), zirconia I can be deposited cathodically by adding CB as a dispersant. The deposit mass increased with increasing CB concentration in the suspensions and deposition time (Fig. 10A(a) and B(a)). It was found that zirconia II can be deposited without the addition of CB to the suspensions. The deposition rate was about 0.1 mg cm⁻² min⁻¹. This result indicated that CB adsorbed on zirconia II during synthesis and remained adsorbed after multiple washings of the precipitate with DI water. The addition of CB to the zirconia II suspensions allowed increased deposition yield. Fig. 10 indicates that the deposition yield, obtained from the zirconia II suspensions was significantly higher, compared to that for zirconia I suspensions at the same CB concentrations. The difference can be attributed to the CB, adsorbed on zirconia II during synthesis.

Fig. 10 Deposit mass versus (A) CB concentration in suspensions at a deposition time of 5 min and (B) versus deposition time for CB concentration of 0.5 g L⁻¹ for suspensions of (a) zirconia I and (b) zirconia II at a deposition voltage 20 V.

EPD of oxide nanoparticles can be used for many important applications, utilizing their functional properties. It is expected that similar to DA, the catecholate type of CB bonding can result in the modification of functional properties of nanomaterials. Moreover, CB exhibits interesting properties for application in sensors and photovoltaic devices.^39,41 The use of CB as a dispersant and charging agent offers many advantages, compared to DA. As pointed out above, the disadvantages of DA are related to poor stability of DA solutions and self-polymerization. Moreover, DA has no charge and must be protonated in acidic solutions. The use of acids generates problems related to increasing ion concentration in the suspension, which results in reduced colloidal stability of the suspensions. Moreover, the acids can react with ceramic powders. Such problems are avoided using CB. The CB dissociation in ethanol resulted in the formation of cationic species, which strongly adsorbed on ceramic particles. Moreover, the larger size of CB, compared to that of DA, is beneficial for the electrosteric dispersion. It is suggested that CB can be used for EPD other materials, such as metal nanoparticles or metal containing polymers.

It is important to note that CB allows cathodic EPD of materials, which offers many processing advantages, compared to anodic EPD. It is in this regard that anodic EPD generates problems related to anodic dissolution of non-noble substrates and chemical instability of some materials in anodic conditions. It is important to note that cathodic electrodeposition is widely used for electroplating of metals, alloys, electrosynthesis of various oxides, quantum dots and other materials. Therefore, further development of cathodic EPD is important for the fabrication of composite materials by combined electrochemical methods. In contrast, anodic electrodeposition has limited utility due to the limited number of materials, which can be deposited using anodic techniques. Especially important is the possibility of cathodic EPD of materials on high surface area non-noble substrates for the development of advanced energy storage devices.

As a step in this direction, cathodic EPD was investigated for the deposition of MnO₂ on high surface area Ni plaque substrates for application in electrodes of electrochemical supercapacitors.

Commercial Ni plaque current collectors, used in this investigation, were designed for high power battery applications.^42–44 In the battery technology, cathodic electrosynthesis is used for the electrochemical loading of the porous Ni plaques with Ni(OH)₂ active material. However, the electrosynthesis of MnO₂ for supercapacitor and battery applications is usually performed by the anodic oxidation⁴⁵ of Mn²⁺ salts. In this case, difficulties are attributed to the use of high surface area porous Ni plaques, which show significant dissolution in anodic conditions. This problem can be avoided by the use of cathodic EPD.

Fig. 11A shows CVs at different scan rates for the Ni-plaque based electrode prepared by cathodic EPD of MnO₂ nanoparticles (shown in Fig. 2). The nearly box shapes of the CVs indicated good capacitive behaviour. It is important to note that box shape CVs were obtained at a scan rate as high as 100 mV s⁻¹. The current increased with increasing scan rate, indicating good capacitance retention. It should be mentioned that in other investigations, significant deviations from the ideal box shape CVs and poor capacitance retention were observed at high scan rates.⁴⁶

Fig. 11 (A) CVs at scan rates of (a) 10, (b) 20, (c) 50 and (d) 100 mV s⁻¹, (B) C_s and C_m versus scan rate, (C) C^′_s and (D) C^′′_s versus frequency for 2 mg cm⁻² MnO₂ electrode in 0.5Na₂SO₄ electrolyte.

The MnO₂ electrodes, prepared by cathodic EPD showed (Fig. 11B) specific capacitances of 0.34 and 0.30 F cm⁻² at scan rates of 2 and 100 mV s⁻¹, respectively, and excellent capacitance retention of 88.5% in the scan rate range of 2–100 mV s⁻¹. For comparison, recent advances⁴⁵ in the development of anodic electrodeposition of MnO₂ on 3D porous Ni electrodes showed the highest capacitance of 121.6 mF cm⁻². The higher capacitance achieved in our investigation indicates that cathodic EPD is a promising technique for the fabrication of ES electrodes. It is important to note that the deposition rate of EPD is typically by 1–2 orders of magnitude higher, compared to the deposition rate of the electrosynthesis process.⁷ Fig. 11C and D show frequency dependencies of the components of complex capacitance, calculated from the impedance data. The dependencies showed relaxation type dispersion,⁴⁷ as indicated by the fast reduction in C′ with frequency in the range of 0.7–10 Hz and related maximum in the frequency dependence of C′′ at ∼8 Hz. The good capacitance retention at high scan rates (Fig. 11B) indicated that the electrodes, prepared by cathodic EPD are promising for the development of high power supercapacitors. The good electrochemical performance was achieved due to the use of cathodic EPD and good dispersion of MnO₂ nanoparticles, which allowed good electrolyte access to the MnO₂ surface. It is expected that CB can be efficiently used for the EPD of other functional materials for various applications.

4. Conclusions

CB showed strong adsorption on surfaces of different materials. The adsorption mechanism involved the interactions of the OH groups of the catechol ligand of CB and metal atoms on the particle surface. The adsorbed CB allowed efficient particle dispersion and imparted a positive charge for cathodic EPD. The strong CB adsorption was critical for efficient dispersion, which was achieved at low CB concentrations. This new finding opens a new and promising strategy for EPD of various materials, such as TiO₂, MnO₂, Mn₃O₄, BaTiO₃, halloysite nanotubes, zirconia and yttria, using CB as universal dispersing agent. The results indicated that CB can be used as a co-dispersant for the EPD of various materials and fabrication of composite films. CB can be used as efficient dispersing agent for synthesis of nanoparticles. The method developed in this investigation can be used for the EPD of nanoparticles on high surface area metallic substrates. EPD of MnO₂ nanoparticles on commercial Ni plaques allowed the fabrication of efficient electrodes for electrochemical supercapacitors, which showed high capacitance and excellent capacitance retention at high charge–discharge rates. The method, developed in this investigation can be used for the EPD of various functional nanomaterials and composites for advanced applications.

Acknowledgements

The authors gratefully acknowledge the Natural Sciences and Engineering Research Council of Canada for the financial support and Vale Canada Company for the donation of Ni plaques.

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Footnote

† Electronic supplementary information (ESI) available: QCM studies, fabrication of zirconia, deposition yield data. See DOI: 10.1039/c4ra03938f

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