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Cathodic electrolytic deposition of zirconia films
Surface & Coatings Technology 195 (2005) 138 – 146 www.elsevier.com/locate/surfcoat
Cathodic electrolytic deposition of zirconia films X. Pang, I. Zhitomirsky*, M. Niewczas Department of Materials Science and Engineering, McMaster University, 1280 Main Street West Hamilton, Ontario, Canada L8S 4L7 Received 23 January 2004; accepted in revised form 12 August 2004
Abstract Cathodic electrodeposition of composite zirconia-poly(diallyldimethylammonium chloride) (PDDA) films was performed from ZrOCl2 solutions in a mixed methanol–water solvent containing PDDA. The electrodeposition method is based on the electrosynthesis of zirconia and electrophoresis of PDDA. The method enables the room-temperature electrosynthesis of crystalline zirconia nanoparticles in the polymer matrix. The crystallinity and particle size of zirconia are influenced by the solvent and PDDA. The use of PDDA allowed to prevent cracking attributed to drying shrinkage and obtain thick and adherent films. Sintering at 800 8C resulted in the burning out of PDDA and the formation of tetragonal zirconia films. Obtained deposits were studied using scanning and transmission electron microscopy (SEM and TEM, respectively), atomic force microscopy (AFM), thermogravimetric (TG) analysis and X-ray diffraction (XRD) analysis. The mechanism of deposition is discussed. D 2004 Elsevier B.V. All rights reserved. PACS: 82.45.Qr; 81.15.Pq Keywords: Electrophoresis; Electrosynthesis; Zirconia; Composite film; Crystallization; Nanoparticles
1. Introduction Cathodic electrolytic deposition is a relatively new technique in ceramic processing that enables the fabrication of nanostructured films. Several investigations have been undertaken on the electrolytic deposition of zirconia films [1–9]. The high melting point, outstanding thermal and chemical stability, excellent mechanical properties and biocompatibility of ZrO2 make this material important for many applications. The electrolytic deposition of zirconia is an interesting way to apply high-temperature coatings for the oxidation protection of materials at elevated temperatures. This technique has been utilized for the formation of nanostructured protective coatings on metals, graphite and carbon fibers [2,5,8]. The electrodeposition of zirconia is under investigation for the fabrication of advanced films
* Corresponding author. Fax: +1 905 528 9295. E-mail address: [email protected] (I. Zhitomirsky). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.216
for the corrosion protection of metals [4,5,9]. The ability to deposit thin zirconia films on substrates of complex shape offers many important opportunities in the application of electrodeposition for the corrosion protection of biomedical implants [9]. Electrodeposition is being recognized as an effective technique for the fabrication of yttria stabilized zirconia films and the composite films based on zirconium oxide [10–13]. It has been suggested that this method could also be used for the fabrication of solid oxide fuel cells [13]. Common difficulties associated with the electrodeposition of zirconia films via hydroxide precursors [2,5,9] include deposit cracking and low adhesion of the cathodic deposits. Electrolytic zirconium hydroxide films are prone to cracking during drying when the deposit thickness exceeds 0.1–0.2 Am [14]. Such film cracking is attributed to drying shrinkage. The dehydration of the hydroxide precursor during the film firing results in the additional shrinkage and cracking. Film cracking is a common problem of wet chemical deposition methods. The deposition and firing procedures are usually repeated several times to produce a relatively thick film. It is important to note that multilayer electro-
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deposition of zirconia films presents difficulties, related to the formation of highly insulating zirconia layers after the dehydration of the hydroxide precursor. The low adhesion of zirconia films has been attributed to the negative charge of the colloidal zirconium hydroxide particles formed in the high-pH region at the cathode surface [14,15]. Recently, a novel method has emerged for the fabrication of polymer–ceramic composite films. In this method, cathodic electrosynthesis of inorganic particles was combined with the electrophoretic deposition (EPD) of cationic polyelectrolytes. Several electrochemical strategies based on the use of strong, weak and chelating polyelectrolytes are currently under development [14–16]. Poly(diallyldimethylammonium chloride) (PDDA) is a strong cationic polyelectrolyte which maintains a positive charge under basic conditions. It was suggested that the deposit formation is driven by the Coulombic attraction between the two charged species: the cationic PDDA and the negatively charged colloidal particles formed at the electrode surface. The use of cationic PDDA with inherent binding properties enables the formation of adherent and crack-free composite films [14,15]. The thickness of the composite films was in the range of 5–10 Am. An important finding was that the composition of the films could be changed by a variation of the PDDA concentration in the solutions. It was suggested that the novel approach could be utilized for the fabrication of thick zirconia films. The use of PDDA additive as a binder is important to prevent cracks and improve the adhesion of the colloidal zirconia particles formed at the electrode surface. Moreover, in this case, the electrostatic repulsion of the PDDA macromolecules can be reduced. The high positive charge of the PDDA macromolecules can be compensated by the negative charge of the colloidal particles formed at the electrode surface. This research, motivated by the importance of zirconia films for various applications, addressed the possibility of fabricating thick zirconia films. The problem of deposit cracking and low deposit adhesion has been addressed by the use of burnable PDDA additives. The influence of the solvent and PDDA on the electrosynthesis and crystallization behavior of zirconia has been studied.
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deposition experiments, followed by drying at room temperature for 24 h. After drying, the deposits were scraped from the electrodes for thermogravimetric (TG) and X-ray diffraction (XRD) analysis. XRD studies were also performed on zirconia films sintered on the stainlesssteel substrates. The thermoanalyzer (Netzsch STH-409) was operated in air between room temperature and 1200 8C at a heating rate of 5 8C/min. The phase content was determined by XRD analysis with a diffractometer (Nicolet I2) using monochromatized Cu Ka radiation at a scanning speed of 0.58/min. The surface morphology and microstructures of the deposited films were studied using a Philips 515 scanning electron microscope (SEM). The films were also studied using a high-resolution transmission electron microscope (TEM) Jeol 2010 FEG. Working magnifications were in the range of 800,000–1000,000. The surface topography of the prepared films was imaged using atomic force microscopy (AFM) (NanoScopeIIIa, Digital Instruments) in tapping mode.
3. Experimental results Electrodeposition experiments performed from the 5 mM ZrOCl2 solutions containing 0–1.0 g/l PDDA resulted in the formation of cathodic deposits. Fig. 1 shows the deposit weight as a function of the deposition time for the 5 mM ZrOCl2 solutions without additives and containing various amounts of PDDA. In the experiments performed from the pure ZrOCl2 solutions, a deposit spallation was observed at deposition times exceeding 4 min, and no weight gain was obtained at longer deposition duration. The deposit weight– deposition time dependence showed a maximum at a deposition time of 4 min. The deposits obtained exhibited low adhesion.
2. Experimental procedures Commercial purity ZrOCl2S8H2O (Fluka) and PDDA (M w=400,000–500,000; Aldrich) were used as starting materials. Solutions of 5 mM ZrOCl2 containing 0–1.0 g/l PDDA in methanol–water (5 vol.% water) solvent were prepared for the electrodeposition. Cathodic deposits were obtained by galvanostatic method on stainless-steel AISI 301 and Pt foils at current densities ranging from 1 to 5 mA/ cm2. The electrochemical cell for deposition included a cathodic substrate centered between two parallel platinum counterelectrodes. Deposit weights were obtained by weighing the stainless-steel substrates before and after the
Fig. 1. Deposit weight versus deposition time for the deposits prepared from the 5 mM ZrOCl2 solutions without additives (a) and containing 0.5 (b) and 1.0 g/l (c) PDDA on a stainless-steel substrate at a current density of 3 mA/cm2.
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In contrast, continuous increase in the deposit weight was observed for the solutions containing 0.5–1 g/l PDDA. Nearly linear deposit weight–time dependencies were recorded, as shown in Fig. 1. This implies a possibility of controlling the amount of the deposited material by changing the deposition time at a constant current density. The increase in the PDDA concentration in the solutions resulted in a higher deposition rate (Fig. 1). The obtained deposits adhered well to the stainless-steel and Pt substrates. It should be noted that a continuous increase in the deposit weight was observed for a deposition duration of up to 30 min. However, at deposition durations exceeding ~20 min, the deposits obtained were porous. TG data for the deposits prepared from the pure ZrOCl2 solutions exhibited a weight loss that can be attributed to the dehydration of the deposits (Fig. 2). Two distinct steps in the TG curve can be distinguished. A sharp reduction of the sample weight was observed in the range of 20–150 8C. An additional step in the weight loss was recorded in the range of 310–350 8C. At higher temperatures, the weight changed gradually. The total weight loss at 1200 8C was found to be 18.4% of the initial sample weight. TG data for the deposits obtained from the 5 mM ZrOCl2 solutions containing different amounts of PDDA are also shown in Fig. 2. The total weight loss at 1200 8C for the deposits prepared from the solutions containing 0.5 and 1.0 g/l PDDA was found to be 52.3 and 68.0 wt.%, respectively. No appreciable weight change was observed at temperatures exceeding 550 8C. These results, coupled with the TG data for the deposits prepared from the pure ZrOCl2 solutions, indicate the electrochemical codeposition of the PDDA and zirconium species. The weight loss for the deposits prepared from the solutions containing PDDA is related to the burning out of the organic phase and the dehydration of the inorganic phase. The TG data have been used to evaluate the polymer content in the composite deposits. The amount of PDDA was found to be 41.5 and 60.8 wt.% for the deposits prepared from the solutions containing 0.5 and 1.0 g/l
Fig. 2. TG data for the deposits prepared from the 5 mM ZrOCl2 solutions without additives (a) and containing 0.5 (b) and 1.0 g/l (c) PDDA.
Fig. 3. XRD patterns of the deposits prepared from the 5 mM ZrOCl2 solutions without PDDA (a) and containing 0.5 g/l PDDA (b).
PDDA, respectively. This indicates the formation of the composite films containing different amounts of PDDA. The increase in the PDDA concentration in the solutions resulted in an increasing PDDA content in the deposits. XRD patterns for the fresh deposits exhibited very broad peaks (Fig. 3), which could be attributed to the small crystal size of zirconia. However, it is suggested that the deposits also contained an amorphous phase. Fig. 4 shows the high-
Fig. 4. High-resolution TEM images of the deposits prepared from the 5 mM ZrOCl2 solutions containing 1.0 (a) and 0.5 g/l PDDA (b).
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resolution TEM pictures of the deposits prepared from the ZrOCl2 solutions containing 0.5 and 1 g/l PDDA. The pictures show amorphous and crystalline particles distributed in a polymer matrix. For the deposits prepared from solutions containing 1 g/l PDDA, some particles were partially crystallized or amorphous. The TEM investigations show that the deposits prepared from the solutions containing 0.5 g/ l PDDA have a larger concentration of crystalline particles and larger particle size, as compared with the deposits obtained from the solutions containing 1 g/l PDDA. The average diameter of the crystalline particles was about 4–5 nm (Fig. 4b). The structure of the particles has been studied by a nanodiffraction technique. The analysis of the corresponding electron diffraction pattern shown in Fig. 5 indicates the crystallization of hydrous cubic zirconia [17]. However, based on the electron diffraction pattern, it is difficult to distinguish between the cubic and tetragonal zirconia. Fig. 6 shows the AFM images of the composite films on polished stainless-steel substrates obtained from the ZrOCl2 solutions containing 1 g/l PDDA. The root-mean-square (RMS) surface roughness was found to be 3.2 and 4.2 nm for current densities of 3 and 5 mA/cm2, respectively. The relatively high surface roughness of the films prepared by the electrodeposition could be attributed to the gas evolution at the cathode surface during deposition. As a result, a higher surface roughness was observed at a higher current density (Fig. 6).
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Fig. 7 compares SEM images of the as-prepared films for the deposition times of 1, 2 and 5 min. The deposits prepared from the ZrOCl2 solutions without PDDA exhibited cracking, which increased with the increasing deposition time (Fig. 7c). Deposits prepared from ZrOCl2 solutions containing 0.5 g/l PDDA exhibited a crack-free morphology. The deposits containing PDDA adhered well to the substrates. Fig. 8 shows the SEM picture of the deposits sintered at 600 8C. The sintering of the deposits prepared from the pure ZrOCl2 solutions resulted in a bcracked-mudQ appearance [2], as shown in Fig. 8 (b and c). The deposits exhibited low adhesion and could easily be removed from the substrates. On the other hand, the deposits prepared from the solutions containing PDDA and sintered at the same conditions were crack-free and adherent. The deposit weight–deposition time dependencies shown in Fig. 1, coupled with the results of TG analysis and SEM investigations of the sintered films, indicate that, using PDDA as a binder, crack-free and adherent zirconia films of various thicknesses in the range of up to 1 Am could be obtained. Fig. 9 shows the XRD patterns of the deposits prepared from 5 mM the ZrOCl2 solutions containing 0.5 g/l PDDA and annealed at different temperatures. The XRD patterns at 200 and 400 8C indicate that the deposits mainly contained an amorphous phase. At 600 8C, broad peaks were observed, which can be attributed to tetra-
Fig. 5. The selected-area electron nanodiffraction pattern for the deposit shown in Fig. 4b.
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a previous paper [15]. The proposed approach is based on two processes: the electrosynthesis of inorganic particles and the electrophoresis of PDDA. The cathodic electrosynthesis of the zirconium hydroxide particles is achieved by the electrogenerated base method. In this method, the cathodic reduction of water results in high pH conditions at the electrode surface: 2H2 O þ 2e YH2 þ 2OH
ð1Þ
In zirconyl chloride solutions, formation of tetramers [Zr4(OH)8(H2O)16]8+ can be expected [18]. The tetramers show a tendency to release protons and form acidic bulk solutions: ½Zr4 ðOHÞ8 ðH2 OÞ16 8þ Y½ZrðOHÞ2þx Sð4 xÞH2 Oð84xÞþ þ 4xHþ 4
ð2Þ
It is suggested that zirconium species are hydrolyzed by the electrogenerated base to form colloidal particles in the basic conditions at the cathode surface. The following surface reactions occur in aqueous and alcoholic (ROH) suspensions [15,19] at high pH:
Fig. 6. AFM images of the composite films prepared from the 5 mM ZrOCl2+1.0 g/l PDDA solution at current densities of 3 (a) and 5 mA/cm2 (b), with deposition time 3 min.
gonal or cubic zirconia. At this temperature, it is difficult to distinguish between tetragonal and cubic zirconia, owing to the peak broadening. The peaks become sharper at higher temperatures. The XRD pattern at 800 8C displayed peaks of tetragonal zirconia (JCPDS file 17923). Further increase of annealing temperature resulted in the transformation to monoclinic zirconia (JCPDS file 37-1484) at 1000 8C. Fig. 10 shows the XRD patterns of zirconia films of different thickness sintered on stainlesssteel substrates. The XRD patterns exhibit peaks of zirconia and the substrates. Small peaks of Fe2O3 (JCPDS file 33-664) could be attributed to the partial oxidation of the substrates. The relative intensity of the Fe2O3 and substrate peaks decreases with the increasing deposition time and deposit thickness, as shown in Fig. 10.
4. Discussion The mechanism of cathodic electrodeposition of composite zirconium hydroxide-PDDA films has been described in
Zr OH þ OH YZr O þ H2 O
ð3Þ
Zr OH þ RO YZr O þ ROH
ð4Þ
The isoelectric point of hydrous zirconia was reported to be 6.7 [20]. Therefore, it is suggested that the colloidal particles are negatively charged at the cathode surface. The electric field provides electrophoretic motion of cationic PDDA macromolecules towards the cathode. PDDA macromolecules maintain a high positive charge in acidic and basic conditions. The electrophoresis and electrosynthesis result in the accumulation of the PDDA macromolecules and colloidal particles at the electrode surface. The heterocoagulation of the colloidal particles and the PDDA macromolecules at the electrode surface results in the deposit formation [15]. It is important to note that no deposit formation was observed in the electrodeposition experiments performed from the pure aqueous PDDA solutions [14,15]. It is suggested that the strong electrostatic repulsion of the PDDA macromolecules at the electrode surface prevents the deposit formation. On the other hand, low adhesion and low Faradaic efficiencies [1,7,14] were reported for the deposits prepared from pure aqueous ZrOCl2 or ZrO(NO3)2 solutions. The low adhesion of zirconia deposits could be attributed to the electrostatic repulsion of the negatively charged colloidal particles formed at the electrode surface, the particle– electrode electrostatic interactions and other factors, described in Ref. [15]. However, adherent deposits were obtained from the aqueous ZrOCl2 solutions containing PDDA. The attraction of the PDDA macromolecules and colloidal particles could be electrostatic or non-electrostatic,
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Fig. 7. SEM pictures of the deposits on stainless-steel substrates prepared from the pure 5 mM ZrOCl2 solution without additives (a, b and c) and containing 0.5 g/l PDDA (d, e and f); deposition time: 1 (a and d), 2 (b and e) and 5 min (c and f).
or a combination of both. An important strategy to improve the adsorption of the polymer macromolecules on the colloidal particles is based on the use of a poor solvent [15,16]. In this paper, the electrodeposition was performed using a mixed methanol–water solvent. Note that water molecules are necessary for the base generation in reaction (1). A polymer can be adsorbed on the surface of the colloidal particles when its solubility in the dispersion
medium is low. On the other hand, the lower dielectric constant of the mixed methanol–water solvent results in a reduced dissociation of the ionizable groups and a reduced steric and electrostatic repulsion of the PDDA macromolecules. It is in this regard that the thickness of the deposited polyelectrolyte multilayers [21] increased with the increasing ethanol content in the mixed water–ethanol solvent. Therefore, the methanol–water solvent of a lower
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Fig. 8. SEM pictures of the deposits on stainless-steel substrates prepared from the pure 5 mM ZrOCl2 solution without additives (a, b and c) and containing 0.5 g/l PDDA (d, e and f); deposition time: 1 (a and d), 2 (b and e) and 5 min (c and f) after sintering at 600 8C for 1 h.
dielectric constant could improve the codeposition of the PDDA and inorganic nanoparticles. The deposition rate W in the EPD method [22–26] can be described by the following equation: W ¼ ClU =d
ð5Þ
where C is the concentration of the colloidal particles or polymer macromolecules; U=U apU dep, U ap is the applied
voltage and U dep is the voltage drop in the deposit; d is the distance between electrodes; and l is the electrophoretic mobility of the particles or polyelectrolytes. The constant current EPD enables a constant electric field U/d in the solutions and a constant deposition rate [25,26]. On the other hand, Faraday’s law governs the electrosynthesis of the inorganic materials. Therefore, a constant deposition rate of the organic and inorganic phases
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Fig. 9. XRD patterns for the deposits obtained from the 5 mM ZrOCl2 solutions containing 0.5 g/l PDDA after annealing during 1 h at 200 (a), 400 (b), 600 (c), 800 (d), and 1000 8C (e). E: Tetragonal zirconia; .: monoclinic zirconia.
could be expected at a constant current mode of electrodeposition. Indeed, nearly linear deposit weight–time dependencies were obtained in this paper. The increase in the deposition rate with the increasing PDDA concentration in the solutions (Fig. 1) is in agreement with Eq. (5). An important point to be discussed is the influence of the solvent and PDDA on the electrosynthesis of zirconium species. It is known that methanol can remove hydroxyl groups and water from the surface of zirconium hydroxide [27]. Zirconium species can precipitate as zirconium hydroxide or hydrated zirconium oxide [28]. Clearfield [17] prepared monoclinic and cubic hydrous zirconia. A close agreement between the interplanar spacings of the hydrous oxides and those of the calcined products was observed [17]. The cubic hydrous zirconia was found to be stable up to 650 8C. The influence of polyelectrolytes on the chemical precipitation of crystalline inorganic nanoparticles has been reported in several papers [29–31]. The possibility of lowtemperature synthesis of crystalline inorganic phases under the influence of organic phases opens novel opportunities for the fabrication of nanomaterials with advanced functional properties [29–32]. Recent investigations indicate that the phase content and crystallinity of the inorganic nanoparticles prepared by electrosynthesis is influenced by the PDDA additives [32–34]. The cathodic electrodeposition enabled the formation of Fe3O4 nanocrystals in the PDDA matrix. The obtained films exhibited super-paramagnetic properties [32]. The results of our work revealed the possibility of electrosynthesis of crystalline zirconia particles at room temperature in a polymer matrix. It is suggested that the size and crystallinity of the particles are influenced by the
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solvent and PDDA. In the cathodic electrodeposition method, zirconia particles were prepared in situ in the PDDA matrix and separated by the polymer. The possibility of the fabrication of zirconia nanoparticles by chemical precipitation in a polymer matrix has also been reported in the literature [35]. The method enabled the control of the particle size. The electrodeposition experiments reported in Refs. [1,2,7] resulted in amorphous deposits. Crystallization of the deposits was observed at temperatures exceeding 400 8C. In contrast, room temperature electrosynthesis of monoclinic zirconia was reported in Ref. [6], where the crystalline deposits were formed on the self-assembled monolayer (SAM) of pentane-1,5-dithiol on polycrystalline gold substrates. It was shown that the SAM influenced the crystallization behavior of zirconia. Our work suggests that the electrosynthesis of zirconium species in situ, in a PDDA matrix, results in the formation of hydrous zirconia. The following heat treatment causes the deposit dehydration and the formation of tetragonal zirconia phase. The electrosynthesis of crystalline oxides and hydroxides was also reported in other investigations. Habazaki et al. [36] reported the electrosynthesis of orthorhombic WO3SH2O. In their work, the heat treatment resulted in the deposit dehydration and formation of an amorphous phase at 250 8C. The crystallization of monoclinic WO3 was reported at 350 8C. The formation of crystallized tetragonal zirconia from amorphous precursors by electrosynthesis method was reported in other investigations [2,7]. It was shown that the heat treatment of the precursors at 400–450 8C resulted in the formation of tetragonal nanocrystallites. The size of the zirconia particles was ~10 nm. Heat treatment at 500– 600 8C resulted in the increasing particle size and transformation of tetragonal zirconia to stable monoclinic phase. However, our results showed that the zirconia films sintered at 800 8C still exhibited a tetragonal structure (Fig. 9). It is suggested that the absence of tetragonal–monoclinic phase
Fig. 10. XRD patterns for the deposits prepared from the 5 mM ZrOCl2+0.5 g/l PDDA solution at a current density of 3 mA/cm2 after annealing at 600 8C during 1 h; deposition time: 22 (a) and 30 min (b). E: Tetragonal zirconia; .: Fe2O3; and n: substrate.
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transformation in the temperature range of 20–800 8C is important to prevent cracking during the sintering of the nanostructured films prepared by electrosynthesis. However, the kinetics of the transformation to the monoclinic phase is influenced by factors such as temperature, particle size and heat treatment duration [7]. The results of this paper indicate the possibility of electrosynthesis of adherent and thick films. The use of PDDA with inherent binding properties is important to prevent cracking attributed to drying shrinkage. However, the increase of the PDDA concentration in the deposits could result in an increasing sintering shrinkage. Therefore, further experiments will be focused on the optimization of the PDDA concentration in the solutions and deposits.
5. Conclusions The electrosynthesis of zirconia in a PDDA matrix resulted in the formation of crystalline nanoparticles. The particle size and crystallinity are influenced by the polymer and solvent used for the electrosynthesis. The method enabled the formation of thick and adherent films. AFM studies revealed surface roughness, which could be attributed to gas evolution. The use of PDDA with inherent binding properties allowed to reduce cracking attributed to drying shrinkage and to improve the deposit adhesion. The positive charge of the PDDA macromolecules is compensated by the negative charge of the zirconia particles formed at the electrode surface. The amount of the deposited material and the deposit composition could be controlled by a variation of the deposition time and PDDA concentration in the solutions. The method enabled the formation of 1-Amthick films of tetragonal zirconia on stainless-steel substrates. Transformation to monoclinic zirconia was observed at temperatures exceeding 800 8C.
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Cathodic electrolytic deposition of zirconia films X. Pang, I. Zhitomirsky*, M. Niewczas Department of Materials Science and Engineering, McMaster University, 1280 Main Street West Hamilton, Ontario, Canada L8S 4L7 Received 23 January 2004; accepted in revised form 12 August 2004
Abstract Cathodic electrodeposition of composite zirconia-poly(diallyldimethylammonium chloride) (PDDA) films was performed from ZrOCl2 solutions in a mixed methanol–water solvent containing PDDA. The electrodeposition method is based on the electrosynthesis of zirconia and electrophoresis of PDDA. The method enables the room-temperature electrosynthesis of crystalline zirconia nanoparticles in the polymer matrix. The crystallinity and particle size of zirconia are influenced by the solvent and PDDA. The use of PDDA allowed to prevent cracking attributed to drying shrinkage and obtain thick and adherent films. Sintering at 800 8C resulted in the burning out of PDDA and the formation of tetragonal zirconia films. Obtained deposits were studied using scanning and transmission electron microscopy (SEM and TEM, respectively), atomic force microscopy (AFM), thermogravimetric (TG) analysis and X-ray diffraction (XRD) analysis. The mechanism of deposition is discussed. D 2004 Elsevier B.V. All rights reserved. PACS: 82.45.Qr; 81.15.Pq Keywords: Electrophoresis; Electrosynthesis; Zirconia; Composite film; Crystallization; Nanoparticles
1. Introduction Cathodic electrolytic deposition is a relatively new technique in ceramic processing that enables the fabrication of nanostructured films. Several investigations have been undertaken on the electrolytic deposition of zirconia films [1–9]. The high melting point, outstanding thermal and chemical stability, excellent mechanical properties and biocompatibility of ZrO2 make this material important for many applications. The electrolytic deposition of zirconia is an interesting way to apply high-temperature coatings for the oxidation protection of materials at elevated temperatures. This technique has been utilized for the formation of nanostructured protective coatings on metals, graphite and carbon fibers [2,5,8]. The electrodeposition of zirconia is under investigation for the fabrication of advanced films
* Corresponding author. Fax: +1 905 528 9295. E-mail address: [email protected] (I. Zhitomirsky). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.08.216
for the corrosion protection of metals [4,5,9]. The ability to deposit thin zirconia films on substrates of complex shape offers many important opportunities in the application of electrodeposition for the corrosion protection of biomedical implants [9]. Electrodeposition is being recognized as an effective technique for the fabrication of yttria stabilized zirconia films and the composite films based on zirconium oxide [10–13]. It has been suggested that this method could also be used for the fabrication of solid oxide fuel cells [13]. Common difficulties associated with the electrodeposition of zirconia films via hydroxide precursors [2,5,9] include deposit cracking and low adhesion of the cathodic deposits. Electrolytic zirconium hydroxide films are prone to cracking during drying when the deposit thickness exceeds 0.1–0.2 Am [14]. Such film cracking is attributed to drying shrinkage. The dehydration of the hydroxide precursor during the film firing results in the additional shrinkage and cracking. Film cracking is a common problem of wet chemical deposition methods. The deposition and firing procedures are usually repeated several times to produce a relatively thick film. It is important to note that multilayer electro-
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deposition of zirconia films presents difficulties, related to the formation of highly insulating zirconia layers after the dehydration of the hydroxide precursor. The low adhesion of zirconia films has been attributed to the negative charge of the colloidal zirconium hydroxide particles formed in the high-pH region at the cathode surface [14,15]. Recently, a novel method has emerged for the fabrication of polymer–ceramic composite films. In this method, cathodic electrosynthesis of inorganic particles was combined with the electrophoretic deposition (EPD) of cationic polyelectrolytes. Several electrochemical strategies based on the use of strong, weak and chelating polyelectrolytes are currently under development [14–16]. Poly(diallyldimethylammonium chloride) (PDDA) is a strong cationic polyelectrolyte which maintains a positive charge under basic conditions. It was suggested that the deposit formation is driven by the Coulombic attraction between the two charged species: the cationic PDDA and the negatively charged colloidal particles formed at the electrode surface. The use of cationic PDDA with inherent binding properties enables the formation of adherent and crack-free composite films [14,15]. The thickness of the composite films was in the range of 5–10 Am. An important finding was that the composition of the films could be changed by a variation of the PDDA concentration in the solutions. It was suggested that the novel approach could be utilized for the fabrication of thick zirconia films. The use of PDDA additive as a binder is important to prevent cracks and improve the adhesion of the colloidal zirconia particles formed at the electrode surface. Moreover, in this case, the electrostatic repulsion of the PDDA macromolecules can be reduced. The high positive charge of the PDDA macromolecules can be compensated by the negative charge of the colloidal particles formed at the electrode surface. This research, motivated by the importance of zirconia films for various applications, addressed the possibility of fabricating thick zirconia films. The problem of deposit cracking and low deposit adhesion has been addressed by the use of burnable PDDA additives. The influence of the solvent and PDDA on the electrosynthesis and crystallization behavior of zirconia has been studied.
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deposition experiments, followed by drying at room temperature for 24 h. After drying, the deposits were scraped from the electrodes for thermogravimetric (TG) and X-ray diffraction (XRD) analysis. XRD studies were also performed on zirconia films sintered on the stainlesssteel substrates. The thermoanalyzer (Netzsch STH-409) was operated in air between room temperature and 1200 8C at a heating rate of 5 8C/min. The phase content was determined by XRD analysis with a diffractometer (Nicolet I2) using monochromatized Cu Ka radiation at a scanning speed of 0.58/min. The surface morphology and microstructures of the deposited films were studied using a Philips 515 scanning electron microscope (SEM). The films were also studied using a high-resolution transmission electron microscope (TEM) Jeol 2010 FEG. Working magnifications were in the range of 800,000–1000,000. The surface topography of the prepared films was imaged using atomic force microscopy (AFM) (NanoScopeIIIa, Digital Instruments) in tapping mode.
3. Experimental results Electrodeposition experiments performed from the 5 mM ZrOCl2 solutions containing 0–1.0 g/l PDDA resulted in the formation of cathodic deposits. Fig. 1 shows the deposit weight as a function of the deposition time for the 5 mM ZrOCl2 solutions without additives and containing various amounts of PDDA. In the experiments performed from the pure ZrOCl2 solutions, a deposit spallation was observed at deposition times exceeding 4 min, and no weight gain was obtained at longer deposition duration. The deposit weight– deposition time dependence showed a maximum at a deposition time of 4 min. The deposits obtained exhibited low adhesion.
2. Experimental procedures Commercial purity ZrOCl2S8H2O (Fluka) and PDDA (M w=400,000–500,000; Aldrich) were used as starting materials. Solutions of 5 mM ZrOCl2 containing 0–1.0 g/l PDDA in methanol–water (5 vol.% water) solvent were prepared for the electrodeposition. Cathodic deposits were obtained by galvanostatic method on stainless-steel AISI 301 and Pt foils at current densities ranging from 1 to 5 mA/ cm2. The electrochemical cell for deposition included a cathodic substrate centered between two parallel platinum counterelectrodes. Deposit weights were obtained by weighing the stainless-steel substrates before and after the
Fig. 1. Deposit weight versus deposition time for the deposits prepared from the 5 mM ZrOCl2 solutions without additives (a) and containing 0.5 (b) and 1.0 g/l (c) PDDA on a stainless-steel substrate at a current density of 3 mA/cm2.
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In contrast, continuous increase in the deposit weight was observed for the solutions containing 0.5–1 g/l PDDA. Nearly linear deposit weight–time dependencies were recorded, as shown in Fig. 1. This implies a possibility of controlling the amount of the deposited material by changing the deposition time at a constant current density. The increase in the PDDA concentration in the solutions resulted in a higher deposition rate (Fig. 1). The obtained deposits adhered well to the stainless-steel and Pt substrates. It should be noted that a continuous increase in the deposit weight was observed for a deposition duration of up to 30 min. However, at deposition durations exceeding ~20 min, the deposits obtained were porous. TG data for the deposits prepared from the pure ZrOCl2 solutions exhibited a weight loss that can be attributed to the dehydration of the deposits (Fig. 2). Two distinct steps in the TG curve can be distinguished. A sharp reduction of the sample weight was observed in the range of 20–150 8C. An additional step in the weight loss was recorded in the range of 310–350 8C. At higher temperatures, the weight changed gradually. The total weight loss at 1200 8C was found to be 18.4% of the initial sample weight. TG data for the deposits obtained from the 5 mM ZrOCl2 solutions containing different amounts of PDDA are also shown in Fig. 2. The total weight loss at 1200 8C for the deposits prepared from the solutions containing 0.5 and 1.0 g/l PDDA was found to be 52.3 and 68.0 wt.%, respectively. No appreciable weight change was observed at temperatures exceeding 550 8C. These results, coupled with the TG data for the deposits prepared from the pure ZrOCl2 solutions, indicate the electrochemical codeposition of the PDDA and zirconium species. The weight loss for the deposits prepared from the solutions containing PDDA is related to the burning out of the organic phase and the dehydration of the inorganic phase. The TG data have been used to evaluate the polymer content in the composite deposits. The amount of PDDA was found to be 41.5 and 60.8 wt.% for the deposits prepared from the solutions containing 0.5 and 1.0 g/l
Fig. 2. TG data for the deposits prepared from the 5 mM ZrOCl2 solutions without additives (a) and containing 0.5 (b) and 1.0 g/l (c) PDDA.
Fig. 3. XRD patterns of the deposits prepared from the 5 mM ZrOCl2 solutions without PDDA (a) and containing 0.5 g/l PDDA (b).
PDDA, respectively. This indicates the formation of the composite films containing different amounts of PDDA. The increase in the PDDA concentration in the solutions resulted in an increasing PDDA content in the deposits. XRD patterns for the fresh deposits exhibited very broad peaks (Fig. 3), which could be attributed to the small crystal size of zirconia. However, it is suggested that the deposits also contained an amorphous phase. Fig. 4 shows the high-
Fig. 4. High-resolution TEM images of the deposits prepared from the 5 mM ZrOCl2 solutions containing 1.0 (a) and 0.5 g/l PDDA (b).
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resolution TEM pictures of the deposits prepared from the ZrOCl2 solutions containing 0.5 and 1 g/l PDDA. The pictures show amorphous and crystalline particles distributed in a polymer matrix. For the deposits prepared from solutions containing 1 g/l PDDA, some particles were partially crystallized or amorphous. The TEM investigations show that the deposits prepared from the solutions containing 0.5 g/ l PDDA have a larger concentration of crystalline particles and larger particle size, as compared with the deposits obtained from the solutions containing 1 g/l PDDA. The average diameter of the crystalline particles was about 4–5 nm (Fig. 4b). The structure of the particles has been studied by a nanodiffraction technique. The analysis of the corresponding electron diffraction pattern shown in Fig. 5 indicates the crystallization of hydrous cubic zirconia [17]. However, based on the electron diffraction pattern, it is difficult to distinguish between the cubic and tetragonal zirconia. Fig. 6 shows the AFM images of the composite films on polished stainless-steel substrates obtained from the ZrOCl2 solutions containing 1 g/l PDDA. The root-mean-square (RMS) surface roughness was found to be 3.2 and 4.2 nm for current densities of 3 and 5 mA/cm2, respectively. The relatively high surface roughness of the films prepared by the electrodeposition could be attributed to the gas evolution at the cathode surface during deposition. As a result, a higher surface roughness was observed at a higher current density (Fig. 6).
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Fig. 7 compares SEM images of the as-prepared films for the deposition times of 1, 2 and 5 min. The deposits prepared from the ZrOCl2 solutions without PDDA exhibited cracking, which increased with the increasing deposition time (Fig. 7c). Deposits prepared from ZrOCl2 solutions containing 0.5 g/l PDDA exhibited a crack-free morphology. The deposits containing PDDA adhered well to the substrates. Fig. 8 shows the SEM picture of the deposits sintered at 600 8C. The sintering of the deposits prepared from the pure ZrOCl2 solutions resulted in a bcracked-mudQ appearance [2], as shown in Fig. 8 (b and c). The deposits exhibited low adhesion and could easily be removed from the substrates. On the other hand, the deposits prepared from the solutions containing PDDA and sintered at the same conditions were crack-free and adherent. The deposit weight–deposition time dependencies shown in Fig. 1, coupled with the results of TG analysis and SEM investigations of the sintered films, indicate that, using PDDA as a binder, crack-free and adherent zirconia films of various thicknesses in the range of up to 1 Am could be obtained. Fig. 9 shows the XRD patterns of the deposits prepared from 5 mM the ZrOCl2 solutions containing 0.5 g/l PDDA and annealed at different temperatures. The XRD patterns at 200 and 400 8C indicate that the deposits mainly contained an amorphous phase. At 600 8C, broad peaks were observed, which can be attributed to tetra-
Fig. 5. The selected-area electron nanodiffraction pattern for the deposit shown in Fig. 4b.
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a previous paper [15]. The proposed approach is based on two processes: the electrosynthesis of inorganic particles and the electrophoresis of PDDA. The cathodic electrosynthesis of the zirconium hydroxide particles is achieved by the electrogenerated base method. In this method, the cathodic reduction of water results in high pH conditions at the electrode surface: 2H2 O þ 2e YH2 þ 2OH
ð1Þ
In zirconyl chloride solutions, formation of tetramers [Zr4(OH)8(H2O)16]8+ can be expected [18]. The tetramers show a tendency to release protons and form acidic bulk solutions: ½Zr4 ðOHÞ8 ðH2 OÞ16 8þ Y½ZrðOHÞ2þx Sð4 xÞH2 Oð84xÞþ þ 4xHþ 4
ð2Þ
It is suggested that zirconium species are hydrolyzed by the electrogenerated base to form colloidal particles in the basic conditions at the cathode surface. The following surface reactions occur in aqueous and alcoholic (ROH) suspensions [15,19] at high pH:
Fig. 6. AFM images of the composite films prepared from the 5 mM ZrOCl2+1.0 g/l PDDA solution at current densities of 3 (a) and 5 mA/cm2 (b), with deposition time 3 min.
gonal or cubic zirconia. At this temperature, it is difficult to distinguish between tetragonal and cubic zirconia, owing to the peak broadening. The peaks become sharper at higher temperatures. The XRD pattern at 800 8C displayed peaks of tetragonal zirconia (JCPDS file 17923). Further increase of annealing temperature resulted in the transformation to monoclinic zirconia (JCPDS file 37-1484) at 1000 8C. Fig. 10 shows the XRD patterns of zirconia films of different thickness sintered on stainlesssteel substrates. The XRD patterns exhibit peaks of zirconia and the substrates. Small peaks of Fe2O3 (JCPDS file 33-664) could be attributed to the partial oxidation of the substrates. The relative intensity of the Fe2O3 and substrate peaks decreases with the increasing deposition time and deposit thickness, as shown in Fig. 10.
4. Discussion The mechanism of cathodic electrodeposition of composite zirconium hydroxide-PDDA films has been described in
Zr OH þ OH YZr O þ H2 O
ð3Þ
Zr OH þ RO YZr O þ ROH
ð4Þ
The isoelectric point of hydrous zirconia was reported to be 6.7 [20]. Therefore, it is suggested that the colloidal particles are negatively charged at the cathode surface. The electric field provides electrophoretic motion of cationic PDDA macromolecules towards the cathode. PDDA macromolecules maintain a high positive charge in acidic and basic conditions. The electrophoresis and electrosynthesis result in the accumulation of the PDDA macromolecules and colloidal particles at the electrode surface. The heterocoagulation of the colloidal particles and the PDDA macromolecules at the electrode surface results in the deposit formation [15]. It is important to note that no deposit formation was observed in the electrodeposition experiments performed from the pure aqueous PDDA solutions [14,15]. It is suggested that the strong electrostatic repulsion of the PDDA macromolecules at the electrode surface prevents the deposit formation. On the other hand, low adhesion and low Faradaic efficiencies [1,7,14] were reported for the deposits prepared from pure aqueous ZrOCl2 or ZrO(NO3)2 solutions. The low adhesion of zirconia deposits could be attributed to the electrostatic repulsion of the negatively charged colloidal particles formed at the electrode surface, the particle– electrode electrostatic interactions and other factors, described in Ref. [15]. However, adherent deposits were obtained from the aqueous ZrOCl2 solutions containing PDDA. The attraction of the PDDA macromolecules and colloidal particles could be electrostatic or non-electrostatic,
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Fig. 7. SEM pictures of the deposits on stainless-steel substrates prepared from the pure 5 mM ZrOCl2 solution without additives (a, b and c) and containing 0.5 g/l PDDA (d, e and f); deposition time: 1 (a and d), 2 (b and e) and 5 min (c and f).
or a combination of both. An important strategy to improve the adsorption of the polymer macromolecules on the colloidal particles is based on the use of a poor solvent [15,16]. In this paper, the electrodeposition was performed using a mixed methanol–water solvent. Note that water molecules are necessary for the base generation in reaction (1). A polymer can be adsorbed on the surface of the colloidal particles when its solubility in the dispersion
medium is low. On the other hand, the lower dielectric constant of the mixed methanol–water solvent results in a reduced dissociation of the ionizable groups and a reduced steric and electrostatic repulsion of the PDDA macromolecules. It is in this regard that the thickness of the deposited polyelectrolyte multilayers [21] increased with the increasing ethanol content in the mixed water–ethanol solvent. Therefore, the methanol–water solvent of a lower
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Fig. 8. SEM pictures of the deposits on stainless-steel substrates prepared from the pure 5 mM ZrOCl2 solution without additives (a, b and c) and containing 0.5 g/l PDDA (d, e and f); deposition time: 1 (a and d), 2 (b and e) and 5 min (c and f) after sintering at 600 8C for 1 h.
dielectric constant could improve the codeposition of the PDDA and inorganic nanoparticles. The deposition rate W in the EPD method [22–26] can be described by the following equation: W ¼ ClU =d
ð5Þ
where C is the concentration of the colloidal particles or polymer macromolecules; U=U apU dep, U ap is the applied
voltage and U dep is the voltage drop in the deposit; d is the distance between electrodes; and l is the electrophoretic mobility of the particles or polyelectrolytes. The constant current EPD enables a constant electric field U/d in the solutions and a constant deposition rate [25,26]. On the other hand, Faraday’s law governs the electrosynthesis of the inorganic materials. Therefore, a constant deposition rate of the organic and inorganic phases
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Fig. 9. XRD patterns for the deposits obtained from the 5 mM ZrOCl2 solutions containing 0.5 g/l PDDA after annealing during 1 h at 200 (a), 400 (b), 600 (c), 800 (d), and 1000 8C (e). E: Tetragonal zirconia; .: monoclinic zirconia.
could be expected at a constant current mode of electrodeposition. Indeed, nearly linear deposit weight–time dependencies were obtained in this paper. The increase in the deposition rate with the increasing PDDA concentration in the solutions (Fig. 1) is in agreement with Eq. (5). An important point to be discussed is the influence of the solvent and PDDA on the electrosynthesis of zirconium species. It is known that methanol can remove hydroxyl groups and water from the surface of zirconium hydroxide [27]. Zirconium species can precipitate as zirconium hydroxide or hydrated zirconium oxide [28]. Clearfield [17] prepared monoclinic and cubic hydrous zirconia. A close agreement between the interplanar spacings of the hydrous oxides and those of the calcined products was observed [17]. The cubic hydrous zirconia was found to be stable up to 650 8C. The influence of polyelectrolytes on the chemical precipitation of crystalline inorganic nanoparticles has been reported in several papers [29–31]. The possibility of lowtemperature synthesis of crystalline inorganic phases under the influence of organic phases opens novel opportunities for the fabrication of nanomaterials with advanced functional properties [29–32]. Recent investigations indicate that the phase content and crystallinity of the inorganic nanoparticles prepared by electrosynthesis is influenced by the PDDA additives [32–34]. The cathodic electrodeposition enabled the formation of Fe3O4 nanocrystals in the PDDA matrix. The obtained films exhibited super-paramagnetic properties [32]. The results of our work revealed the possibility of electrosynthesis of crystalline zirconia particles at room temperature in a polymer matrix. It is suggested that the size and crystallinity of the particles are influenced by the
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solvent and PDDA. In the cathodic electrodeposition method, zirconia particles were prepared in situ in the PDDA matrix and separated by the polymer. The possibility of the fabrication of zirconia nanoparticles by chemical precipitation in a polymer matrix has also been reported in the literature [35]. The method enabled the control of the particle size. The electrodeposition experiments reported in Refs. [1,2,7] resulted in amorphous deposits. Crystallization of the deposits was observed at temperatures exceeding 400 8C. In contrast, room temperature electrosynthesis of monoclinic zirconia was reported in Ref. [6], where the crystalline deposits were formed on the self-assembled monolayer (SAM) of pentane-1,5-dithiol on polycrystalline gold substrates. It was shown that the SAM influenced the crystallization behavior of zirconia. Our work suggests that the electrosynthesis of zirconium species in situ, in a PDDA matrix, results in the formation of hydrous zirconia. The following heat treatment causes the deposit dehydration and the formation of tetragonal zirconia phase. The electrosynthesis of crystalline oxides and hydroxides was also reported in other investigations. Habazaki et al. [36] reported the electrosynthesis of orthorhombic WO3SH2O. In their work, the heat treatment resulted in the deposit dehydration and formation of an amorphous phase at 250 8C. The crystallization of monoclinic WO3 was reported at 350 8C. The formation of crystallized tetragonal zirconia from amorphous precursors by electrosynthesis method was reported in other investigations [2,7]. It was shown that the heat treatment of the precursors at 400–450 8C resulted in the formation of tetragonal nanocrystallites. The size of the zirconia particles was ~10 nm. Heat treatment at 500– 600 8C resulted in the increasing particle size and transformation of tetragonal zirconia to stable monoclinic phase. However, our results showed that the zirconia films sintered at 800 8C still exhibited a tetragonal structure (Fig. 9). It is suggested that the absence of tetragonal–monoclinic phase
Fig. 10. XRD patterns for the deposits prepared from the 5 mM ZrOCl2+0.5 g/l PDDA solution at a current density of 3 mA/cm2 after annealing at 600 8C during 1 h; deposition time: 22 (a) and 30 min (b). E: Tetragonal zirconia; .: Fe2O3; and n: substrate.
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transformation in the temperature range of 20–800 8C is important to prevent cracking during the sintering of the nanostructured films prepared by electrosynthesis. However, the kinetics of the transformation to the monoclinic phase is influenced by factors such as temperature, particle size and heat treatment duration [7]. The results of this paper indicate the possibility of electrosynthesis of adherent and thick films. The use of PDDA with inherent binding properties is important to prevent cracking attributed to drying shrinkage. However, the increase of the PDDA concentration in the deposits could result in an increasing sintering shrinkage. Therefore, further experiments will be focused on the optimization of the PDDA concentration in the solutions and deposits.
5. Conclusions The electrosynthesis of zirconia in a PDDA matrix resulted in the formation of crystalline nanoparticles. The particle size and crystallinity are influenced by the polymer and solvent used for the electrosynthesis. The method enabled the formation of thick and adherent films. AFM studies revealed surface roughness, which could be attributed to gas evolution. The use of PDDA with inherent binding properties allowed to reduce cracking attributed to drying shrinkage and to improve the deposit adhesion. The positive charge of the PDDA macromolecules is compensated by the negative charge of the zirconia particles formed at the electrode surface. The amount of the deposited material and the deposit composition could be controlled by a variation of the deposition time and PDDA concentration in the solutions. The method enabled the formation of 1-Amthick films of tetragonal zirconia on stainless-steel substrates. Transformation to monoclinic zirconia was observed at temperatures exceeding 800 8C.
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