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Titania and titania–silver nanocomposite coatings grown by electrophoretic deposition from aqueous suspensions
Surface & Coatings Technology 205 (2010) 2562–2571
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Titania and titania–silver nanocomposite coatings grown by electrophoretic deposition from aqueous suspensions M.J. Santillán a,⁎, N.E. Quaranta b, A.R. Boccaccini c,⁎ a b c
FCAI, Universidad Nacional de Cuyo. CONICET, San Rafael, Argentina Universidad Tecnológica Nacional, FR San Nicolás, CIC, San Nicolás, Argentina Institute of Biomaterials, University of Erlangen-Nuremberg, 91058 Erlangen, Germany
a r t i c l e
i n f o
Article history: Received 8 August 2010 Accepted in revised form 1 October 2010 Available online 12 October 2010 Keywords: Electrophoretic deposition Titania coatings Antibacterial coatings Biomedical applications Silver nanoparticles
a b s t r a c t This study focused on the synthesis of titania–silver (TiO2–Ag) nanocomposite coatings with potential enhanced antibacterial properties for applications in orthopedic implants. Ag nanoparticles (4 nm) were grown directly on the surface of commercially available TiO2 nanoparticles (23 nm) by nucleophilic reaction. The electrophoretic deposition (EPD) of TiO2 and TiO2–Ag nanoparticles on titanium substrates using aqueous suspensions was investigated. Best results were achieved by using water and ethanol as co-solvents. The EPD process (voltage: 3 V, deposition time: 90 s) led to TiO2 based coatings on titanium substrates exhibiting homogeneous and uniform microstructure. In vitro bioactivity tests in Kokubo's simulated body fluid (SBF) to evaluate the formation of hydroxyapatite (HA) on the coating surface were performed. The results showed that the extent of HA formation rapidly increased with increasing time in SBF and it decreased as silver amount in the coating increased. XRD, TEM and SEM-EDAX were used to investigate the microstructure of the nanomaterials and coatings. The high bioactivity of the TiO2 based electrophoretic coatings indicates their potential for use as bioactive antibacterial layers in orthopedic implants. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titania (TiO2) films have been extensively studied and applied for their well-known biocompatibility and biocide properties [1–3]. As a biomaterial, titania has potential use in hard tissue replacement and bone tissue engineering applications [4–8]. Titania is normally applied as a coating on metallic substrates in order to improve the integration of orthopedic implants in host bone tissue [4–8]. Infections caused after implantation of orthopedic devices represent major complications in traumatologic surgery [9]. This phenomenon is mainly due to bacterial colonization of implanted materials, through adhesion and accumulation of bacteria on the coating surface and consequently the formation of a biofilm [10]. Therefore, researchers are increasingly focusing on improving the antibacterial property of implants [11]. In recent years, it has become apparent that the use of inorganic antibacterial materials may lead to better results than those achieved with organic antibacterial materials in terms of durability, toxicity and selectivity of action [12]. In this regard, the usefulness of Ag as an antibacterial agent has been known for several years and Ag is currently one of the preferred elements used in antibacterial coatings for several applications [13–15].
⁎ Corresponding authors. E-mail addresses: [email protected] (M.J. Santillán), [email protected] (A.R. Boccaccini). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.10.001
Diverse types of silver-doped materials for antibacterial applications have been developed, including silver-doped ceramics based on calcium phosphate, silica, titania and bioactive-glass [11,13,16–19]. Traditionally TiO2 coatings for biomedical application have been prepared by spray plasma technique [20,21]. However, high processing temperatures are commonly needed for these methods. Consequently the crystalline structure of the coatings is usually rutile and microstructural features are difficult to control. Electrophoretic deposition (EPD) [22,23] is one of the most effective techniques to assemble nanoparticles because it is a relatively simple, versatile and low cost process which allows the use of complex shape components as deposition substrate [22–24]. EPD is a colloidal process wherein ceramic coatings are created directly from a stable colloidal suspension by application of an electric field. Several investigations on TiO2 EPD have been carried out using TiO2 organic suspensions, in particular with acetylacetone or acetone as solvents [25–27]. Only a few studies have been carried out using TiO2 aqueous suspensions [28]. In this research, the synthesis of TiO2–Ag composite nanoparticles and the fabrication of TiO2 and TiO2–Ag coatings on titanium substrates by EPD were investigated. Ag nanoparticles (NPAg) were directly formed and grown on the surface of TiO2 nanoparticles (NPTiO2) from nucleophilic reaction catalyzed by alkalis. The advantage to form NPAg on a supporting titania nanostructure is the resulting homogeneity of the Ag distribution which should enable the release of Ag in a controlled manner [18,29–31]. The production of
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films for biomedical applications requires good control of the deposition conditions because this will affect the microstructure and ultimately the properties of the films. For this reason, EPD was chosen in the present work as a promising technique for deposition of nanostructured bioactive TiO2–Ag coatings. 2. Experimental procedure 2.1. Starting materials TiO2 nanopowder (P25-Degussa, Germany) was used (mean particle size: 23 nm). The crystallographic composition of the TiO2 nanoparticles is a 70/30 mixture of anatase and rutile, respectively, as given by the supplier. Ethanol (Tetrahedrom) was used as co-solvent. The chemical precursors used for synthesis of the Ag–TiO2 nanocomposite were TiO2 nanopowder, silver nitrate (AgNO3, Aldrich, 97%), and ammonium hydroxide (NH4OH, Tetrahedrom).TiO2–Ag nanocomposite, nitric acid (HNO 3 , Tetrahedrom) and 2-propanol (C3H7·OH, Tetrahedrom) were used for investigating washing procedures. Certain amounts of HNO3 or NaOH solutions, both 0.05 M, were added for pH adjustment. 2.2. Synthesis of TiO2–Ag nanoparticles The fabrication of TiO2 nanoparticles doped with silver (TiO2–Ag) was realized by a chemical process catalyzed by alkalis, modifying the route used originally by Kim et al. to synthesize Ag-particles on silica [18]. 50 mL of aqueous suspensions of NPTiO2 with AgNO3 in appropriate proportions were prepared to obtain the TiO2–Ag composite material, with 2 and 10 wt.% of Ag+ with respect to NPTiO2 content (wt.%: referred to the solids percentage by weight). After that, a solution 0.5 M of NH4OH was added to the slurry in order to preserve a relation TiO2: NH4OH = 0.10, 0.25, 0.40 and 0.55. The full system was maintained in reflux at ~60 °C for 6 h to facilitate the nucleation process of NPAg and the adsorption of NPAg on the NPTiO2 surfaces. Subsequently, the solid product was filtered in vacuum to recover the solid material. The product was divided in two parts which were washed in two different ways: a) with distilled water followed by HNO3 (0.05 M) and b) with 2-propanol up to pH 5, to stabilize the NPAg on the NPTiO2 surfaces. Finally, in both cases, the product was washed with distilled water up to pH ~7. 2.3. Suspension development and EPD of TiO2 The optimal composition of TiO2 aqueous suspensions for EPD was obtained after testing the effect of three different surfactants, Tiron [(OH)2C6H2(SO3Na)2], poly-vinyl-butyral (PVB) and poly-methyl cellulose (PMC) and different amounts of ethanol, as a co-solvent. The best deposition by EPD in terms of density and homogeneity of the coatings was achieved using ethanol as co-solvent. For the deposition of TiO2–Ag nanoparticles, a suspension with 6% (v/vo) of ethanol was used. Table 1 lists the composition of the suspensions used for electrophoretic deposition of TiO2 and TiO2–Ag nanoparticles. The quantities of reagents are referred to the solids percentage by weight (wt.%). All EPD experiments were carried out at room temperature and under constant voltage conditions. Before EPD, the suspension was
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ultrasonically stirred during 3 h. The EPD cell consisted of a glass container and two electrodes. Titanium (Ti) sheets, with an area of 1.4 cm2, were used as deposition substrates and a stainless steel sheet with identical area as counter-electrode. The distance between both electrodes was maintained at 2 cm. Both the working electrode and the counter-electrode in the EPD container were connected to a d.c. power supply. The electrodes were cleaned with acetone before each experiment to enhance adhesion of the coating. After EPD the samples were dried in a desiccator at room temperature for 24 h followed by heat treatment at 700 °C (2 °C/min) for 2 h in vacuum. 2.4. In vitro bioactivity tests of TiO2 and TiO2–Ag composite coatings Bioactivity tests were carried out in simulated body fluid (SBF) according to the protocol of Kokubo et al. [32]. The SBF was prepared by dissolving respective amounts of reagent grade chemicals of NaCl, NaHCO3, KCl, K2HPO4·3H2O, MgCl2·6H2O, CaCl2·2H2O, and Na2SO4 into distilled water. The pH of SBF was adjusted to physiological pH (~ 7.25) by HCl (1 M) and tris(hydroxyl–methyl) aminomethane. Coated specimens were soaked in SBF solution and maintained at 36.5 ± 0.5 °C for 1 and 4 weeks to evaluate the growth of hydroxyapatite (HA) layer on the surface coatings. The tests were carried out maintaining a surface–volume ratio (S/V) of 0.005 cm-1. The SBF was refreshed every 3 days. After that, the samples were rinsed in distilled water three times, dried and stored in desiccators for further characterization. 2.5. Characterization techniques A High Resolution Transmission Electron Microscope (HR-TEM, Philips CM200) was used to observe and characterize the shape, size and degree of agglomeration of NPTiO2 and TiO2–Ag nanocomposite particles. Chemical analysis was carried out by TEM-energy dispersive (EDS). X-ray diffraction (XRD) patterns were obtained using a Philips PW 1700 diffractometer equipped with a graphite monochromator and Cu Kα radiation. The patterns were collected in the 20–70° 2θ range with scan steps of 0.02°. Scanning Electron Microscopy (SEM, Philips 515) was employed to evaluate the microstructure, morphology and adhesion of the deposited coatings to the substrates. 2.6. Mechanical properties The values of microhardness and fracture toughness (KIc) were obtained by indentation technique. Vickers microhardness measurements were performed applying a 9.8 N load for 20 s on the surface of the coatings. A total of 5 measurements for each sample was carried out. The fracture toughness was determined by indenting the coating cross-section with a Vickers indenter at a 200 g load for 20 s, with the indenter aligned such that one of its diagonals would be parallel to the substrate surface and using the equation proposed by Anstis et al. [33]. Young's Modulus values were obtained using the Knoop indentation method at a 200 g load for 15 s and employing the expression proposed by Conway [34] relating H/E to the residual length of the minor diagonal of a Knoop indentation. 3. Results and discussion 3.1. Synthesis of Ag doped titania
Table 1 Composition of suspensions used in EPD of TiO2 and TiO2–Ag nanoparticles. Suspension
TiO2 suspension TAG2: TiO2–Ag susp. (2%) TAG10: TiO2–Ag susp. (10%)
Solvent (mL)
Solids (wt.%)
Water
Ethanol
TiO2
TAG
9.4 9.4 9.4
6 6 6
1.03 – –
– 1.07 1.21
The synthesis of silver doped titania nanoparticles (TiO2–Ag) by a wet route has been shown for the first time in this research, modifying the synthesis process reported by Kim et al. [18], developed originally to obtain silver particles supported on SiO2. In the current approach, NPTiO2 in aqueous medium are hydroxylated generating Ti–OH groups. Like other oxide surfaces in aqueous medium, the titania surface is OH terminated, amphoteric, and has a pH dependent surface
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charge. Oxide surfaces can acquire a charge by adsorption or protons dissociation, e.g., for titania [35]: þ
−
ð1Þ
≡ Ti−OH + H2 O↔≡ Ti−OH2 + OH −
≡ Ti−OH + H2 O↔≡ Ti−OH
+ H3 O
þ
ð2Þ
The alkaline condition generates strong nucleophiles which deprotonate hydroxyl ligands; subsequently, these nucleophilic groups can react with electrophilic materials as metal ions. Accordingly, a nanocomposite may be formed by autoreduction between noble metal ions with hydroxilated oxide surfaces. When the Ti–OH groups react with a Lewis basis, as NH4OH, a nucleophilic attack on the –OH groups occurs booting H+ from the coordination sphere. The steric configuration of the NPTiO2 surface's groups allows subsequently the reduction of Ag+ due to its spatial arrangement. In this manner, oxygen is present on the surface with excess negative charge, which can easily react with Ag+ ion, auto reduced through a chain reaction with other –OH groups on the surface. The reduction power will decrease as the amount of –OH groups available on the surface of TiO2 decreases. A similar explanation was given for the case of silica doped with Ag in a process catalyzed by basis [18]. This synthesis process implied different steps, as explained below: a) The first step is the deprotonation of hydroxyl ligand in TiOH from nucleophilic that is attacked by a Lewis basis at hydrated oxide: −
Ti−OH + B : →TiO
ð3Þ
+ BH
b) The second step is an electrophilic attack by Ag+ that is bonded at the nucleophilic TiO- group:
þ
TiO + Ag →TiO−Ag
ð4Þ
c) The third step is the formation of þ
TiOH + Ag →TiO−Ag + H
þ
NPAg
on the NPTiO2 surface: ð5Þ
Fig. 1 shows schematically the process previously described. The stoichiometric ratio of reactants influences the final Ag:TiO2 ratio in the nanocomposite material. As the amount of AgNO3 increases, higher is the NPAg quantity grown on the TiO2 surface. Moreover, it was observed that the size and distribution of NPAg on the TiO2 surface are strongly influenced by the final washing treatment.
To determine the optimized Ag:TiO2 ratio, the molar ratios of AgNO3 and catalyst were controlled. After several experiments, the optimal amount of alkalis (NH4OH) necessary to maintain the stoichiometric ratio of reagents in the final product was determined. It was found that the desired stoichiometric ratio of precursors (TiO2: Ag) is maintained in the final product when the proportion of (TiO2: NH4OH) is 0.25 for 0.01 b [Ag+] b 0.15. The influence of washing conditions on the final product was evaluated qualitatively by TEM observations. Fig. 2 shows TEM micrographs of Ag–TiO2 nanocomposite particles with a TiO2:Ag ratio (in weight) of 90:10 (TAG10) for different washing conditions. Fig. 2.a) shows TAG10 nanocomposite washed only with water, while Fig. 2.b) shows TAG10 nanocomposite washed with HNO3, isopropanol and finally distilled water. It can be seen in Fig. 2.a) that the NPAg size and distribution on the NPTiO2 surface are not uniform. It was difficult to observe individual NPAg formed on the NPTiO2 surfaces, and frequently particle agglomerations were observed, which may induce a decline of the antibacterial properties. However it is possible that some Ag aggregations that did not react with the NPTiO2 surface are dissolved by nitric acid resulting in individual NPAg on NPTiO2 surfaces. In Fig. 2.b) it is observed that NPAg are smaller than in Fig. 2a) and they are distributed uniformly on the surface of NPTiO2. Therefore, it was determined that the washing process of TiO2–Ag composite should be done with HNO3 and isopropanol according to the procedure indicated above. Fig. 3a shows a high magnification TEM micrograph of TiO2–Ag nanoparticles with 10 wt.% Ag (TAG10). The inset shows a HR-TEM micrograph of NPAg where it is possible to observe the different atomic planes, corresponding to Ag2O (d~ 2.35 Å) and Ag. The nanoparticles on NPTiO2 are a mixture of elemental Ag and Ag2O that then dissociate into silver ions in aqueous medium. Both forms of silver will release silver ions providing antimicrobial activity. This ionic form of silver has been shown to be effective against a broad range of bacteria [36]. The diameter distribution of the NPAg was estimated from TEM images. The results are presented in Fig. 3b indicating that the size distribution of NPAg is narrow and it is in the 2–5 nm range. A log normal distribution fit gave m = 4.1 nm, r = 0.8 nm for the mean and the standard deviation of the NPAg diameter, respectively. The EDS analysis on Ag–TiO2 nanoparticles shows peaks of Ti, O and Ag, not detecting the presence of other elements in the spectra (Cu peaks are due to the TEM grid used) (Fig. 4). Table 2 summarizes the stoichiometric relationship of the precursors used and the final composition obtained by semiquantitative EDS analysis. It is possible to conclude that the amount of NPAg grown on the NPTiO2 is closely related to the quantity of precursor used in the synthesis of the composite material, as expected.
Fig. 1. Representative schema of the chemical mechanism for the synthesis and growth of NPAg on the TiO2 nanoparticle surface.
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Fig. 2. TEM micrographs of TiO2–Ag nanocomposite particles obtained by nucleophilic reaction after washing with a) distiller water and b) nitric acid, isopropanol and distiller water.
3.2. Electrophoretic deposition of TiO2 and TiO2–Ag nanoparticles The key parameters influencing the EPD process are: suspension concentration, deposition time and applied voltage. In the present study, these parameters were investigated systematically in wide ranges and their influence on the deposition process was studied to optimize the EPD conditions, as discussed next. 3.2.1. Electrophoretic deposition of NPTiO2 The use of co-solvents may be beneficial to deposit NPTiO2 by EPD from an aqueous system [28]. Adding a short chain alcohol (such as ethanol) to the suspension can prevent water hydrolysis when relatively low voltages are applied [28,37]. Different ways to stabilize suspensions for EPD are known [22,23,38]. In this research, electrostatic stabilization was used to stabilize NPTiO2 suspensions without adding surfactants. In this context, a mixture of ethanol and water as the solvent was used. The effect of the concentration of ethanol ([Et– OH]) on suspension stability and on the quality of the films obtained by EPD was investigated. 3.2.2. Optimization of NPTiO2 suspensions Different amounts of ethanol (2, 4, 6, 10 and 15 v%vo; v0: refereed to solvent volume) were added to an aqueous suspension containing 1.03 wt.% of NPTiO2, in order to explore its effect on the stability of the suspension and consequently on the generation of surface charge on NPTiO2. Oxide surfaces in aqueous suspension will hydrate to form surface hydroxyl groups. The chemical species adsorbed on the surface of the nanoparticles depend on the medium pH. The use of co-solvents has the advantage of facilitating suspension stabilization, enabling also to avoid the occurrence of water hydrolysis at relatively low voltages.
Ethanol readily dissociates according to the equilibrium reaction (Eq. (6)): CH3 −CH2 −OH + H2 O⇔CH3 −CH2 −O− + H3 Oþ H3 Oþ ⇔H2 O + Hþ
ð6Þ
For the present case, protons produced in this reaction are adsorbed onto amphoteric hydroxyl groups present on the surface of NPTiO2. TiO2 coatings were deposited on the negative electrode (cathode) under the EPD conditions employed, confirming the positive charge on NPTiO2 in aqueous suspension. In this investigation it was necessary to sonicate the suspensions for long periods of time (N3 h) in order to achieve a complete dispersion of NPTiO2 and the adsorption of H+ on their surfaces. In previous studies, Lebrette et al. [28,39] used a mixture of water and 10% ethanol with addition of Tiron to stabilize TiO2 suspensions for EPD. Our previous research and other reports in the literature [40,41] indicate that good quality coatings can be obtained by EPD when the amount of additives in suspensions is minimum, because only a reduced portion of them is effectively adsorbed on particles, contributing to stabilize the suspension. The ions that are not adsorbed on suspended particles produce an increase of the suspension's ionic strength and consequently a decrease in the thickness of the electrical double layer of particles occurs, which originates agglomeration of particles and consequently, a deterioration of the quality of the deposited coatings [39]. Therefore, in the present research it was sought to stabilize the suspension by adjusting the pH value and by minimizing the addition of additives. Several series of EPD experiments with different ethanol concentrations in suspension and different applied voltages were carried out, as indicated above.
Fig. 3. a) TEM micrograph of TiO2–Ag nanocomposite, the inset shows a HR-TEM image of the NPAg. b) Particle size distribution of NPAg.
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Fig. 4. EDS spectra of TiO2–Ag nanocomposite for 2 and 10wt.% Ag (TAG2 and TAG10, respectively) (Cu peaks are due to the TEM grid).
Fig. 5 shows a plot of the amount of TiO2 deposited by EPD applying 3 V as function of EPD time for suspensions with different ethanol contents. It was observed that using only water, it was not possible to obtain any TiO2 deposit on Ti substrates. For deposition time (t) of t ≤ 3.5 min, the best deposition rate was achieved using a suspension containing 4 v%vo of ethanol. For higher deposition times, the amount of TiO2 nanoparticles deposited on the substrate decreased. Visual inspection of EPD coatings fabricated with longer EPD times revealed that the coatings were not properly adhered to the substrate and the covering of the substrate was inhomogeneous. For ethanol concentrations ≥6 v%vo, the deposition rate was seen to fall dramatically. One possible explanation for this behavior may be as follows. The increase of ethanol concentration in suspension leads to an increase in the availability of free H+. As a consequence, titania nanoparticles acquire a positive electrical charge and an excess of H+ in suspension develops. When an external voltage is applied, the excess H+ in suspension interferes with the deposition of NPTiO2 on the Ti substrate because their electrophoretic mobility is higher. The presence of high concentrations of ethanol should decrease the mobility of the ceramic particles. This effect can be compared with the situation resulting from the reaction between I2 and acetylacetone when these reagents are used in TiO2 suspensions for EPD, as discussed in the literature [26,27,42]. In general, it is expected that the applied voltage will have a significant influence on the final characteristics of the coatings. The Smoluchowski's equation proposed to determine the electrophoretic mobility of suspended particles during EPD [43,44] is related to the electrophoretic velocity υ (Eq. (7)): υ=
ζ Eε 4πη
Fig. 5. Electrophoretic deposit weight of TiO2 nanoparticles vs. deposition time for aqueous suspensions with different amounts of ethanol (v%v0) (Applied voltage = 3V).
r N H+, will have a relatively low electrophoretic velocity and different effects can be expected in the EPD coatings. Fig. 6 shows that for ethanol concentrations ≤ 6 v%vo, the deposited TiO2 amount increased as deposition time increased. The suspensions with higher [Et–OH] did not present a defined behavior in terms of deposition rate as function of deposition time, showing that, in general, the deposited TiO2 weight decreases significantly. This behavior is consistent with the evolution of the pH of the suspensions as a function of [Et–OH] (Fig. 7). As [Et–OH] increases, pH decreases due to the increase of the amount of free H+, leading to a reduction of deposition rate, which is related to suspensions with higher conductivity for H+. According to the results obtained, it was necessary to compromise between the TiO2 amount deposited on the substrate and the quality of the coatings acquired. A suspension containing [Et–OH] = 4 v%vo was selected as optimum for EPD in the present experiments.
ð7Þ
where ζ is the zeta potential, ε the dielectric constant and η is the viscosity. Considering that the electrophoretic velocity υ is proportional to the quantity QE/rη, where Q is particle charge and r is its radius [44], it is possible to deduce that particles (or composites) with Table 2 Semiquantitative chemical composition of samples TAG2 and TAG10 obtained by EDS. Samples
Ag (wt. %)
Ti (wt. %)
O (wt. %)
TAG2 TAG10
1,91 9,76
73,55 67,67
24,54 22,57
Fig. 6. Dependence of deposit weight of TiO2 nanoparticles on ethanol concentration [Et–OH] at different EPD times.
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a lower quantity of material deposited per unit of EPD time for the same applied voltage is observed. This behavior can be explained if one considers the voltage drop that occurs as a result of the growth of the ceramic coating layer. As the film thickness increases, so does its electrical resistance and therefore the voltage drop increases, leading to a reduction of the amount of TiO2 nanoparticles deposited per unit of time. This phenomenon is observed in the plot of deposited weight vs. applied voltage for different EPD times (Fig. 8).
In order to optimize the remaining EPD parameters, two sets of experiments were carried out. One involved the deposition process at constant time, with applied voltages investigated in the range of 2 to 20 V. In another series of experiments the voltage applied was maintained constant, changing the deposition time from 1 to 4 min. When voltages ≤ 2 V were applied, TiO2 films did not adhere to the substrates. This behavior can be attributed to insufficient electrophoretic mobility of the particles, which cannot overcome electrostatic repulsion forces, in relation to the overlap of electrical double layers. At voltages N4 V release of oxygen and hydrogen from the electrodes due to water electrolysis was observed. Moreover an increase in the generation of these gases as the voltage increased was detected. Fig. 8 shows a plot of the TiO2 amount deposited per unit area as function of deposition time for different applied voltages. It is observed that with higher voltages the deposited TiO2 amount increases; however the evolution of gases at the electrodes interferes with the deposition, producing highly inhomogeneous films, this effect has been frequently discussed in the literature [22,37,38]. This phenomenon may be in fact comparable to the situation where there is an excess of free H+ in suspension that affects also negatively the morphology of the coating (discussed above). Under these conditions,
3.2.3. Optimization of electrophoretic deposition parameters for TiO2–Ag nanoparticles Several experiments were carried out in order to evaluate the optimal composition of suspensions of TiO2–Ag nanoparticles for EPD. The final compositions of suspensions used are shown in Table 1. The electrostatic stabilization of the suspension required ultrasonic agitation for a long period of time (t N 180 min) to promote the dissociation of ethanol and the subsequent adsorption of protons on the TiO2–Ag entities. This is similar to the stabilization process of TiO2 nanoparticles in aqueous suspensions whose physicochemical principles were discussed above. In order to optimize the EPD parameters, several series of EPD tests were carried out applying different constant voltages (in the range: 3– 20 V) and deposition times (30–210 s). The plot showing deposited TiO2–Ag (sample TAG10) weight vs. deposition time at different voltages is presented in Fig. 9. At applied voltages b2 V the deposition rate of the TiO2–Ag nanoparticles is extremely small, this behavior is similar to that observed for EPD of NPTiO2 from aqueous suspensions, discussed in Section 3.2.2. At voltages N2 V, the deposition process is greatly improved but the coatings become more inhomogeneous as voltage increases. These results are in a good agreement with the prediction of the Hamaker equation [45], which predicts an increase in the amount of deposited material with increasing deposition time. For applied voltages N6 V it can be seen that the deposition rate decreases. The explanation for this phenomenon is similar to that given earlier when describing the EPD of NPTiO2 from aqueous suspension. The excessive release of gases at the electrodes causes the detachment of the TiO2–Ag coating from the substrate. However, at 4 V a homogeneous coating is obtained for deposition times in the range: 2 ≤ t ≤ 10 min. Based on these experimental results, the optimal conditions chosen for EPD were therefore: 4 V–6 min for TAG10 suspensions. Fig. 10 shows the EPD rate at conditions of 4 V– 6 min as a function of pH for TAG10 suspension. The highest
Fig. 8. TiO2 deposit weight vs. EPD time for different applied voltages, using an aqueous suspension with 1.03 wt.% of solids and 4 v%v0 of ethanol.
Fig. 9. Deposit weight of TiO2–Ag nanoparticles (TAG10 suspension, Table 1) on Ti substrate vs. EPD time for different applied voltages.
Fig. 7. pH variation of TiO2 aqueous suspensions and electrophoretic deposit weight on Ti substrate as function of ethanol concentration [Et–OH].
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Fig. 10. Deposit weight of TiO2–Ag nanoparticles (TAG10 suspension) vs. pH for EPD at conditions: 4 V, 6 min.
deposition rate which corresponds to this suspension is achieved at pH ~6.8. Under these conditions there is no flocculation of particles and the coatings are homogeneous. The TAG10 aqueous suspension containing Et–OH as co-solvent had a pH ~6.7, changing very little for both compositions studied (Table 1).
3.3. Microstructural characterization Fig. 11 shows the X-ray diffraction (XRD) pattern corresponding to a TAG10 coating before heat treatment. Only reflections corresponding to the TiO2 phase anatase (PDF 021-1272) and rutile (PDF 021-1276) are observed. Additionally, some peaks corresponding to Ag (PDF 004-0783) and Ag2O (PDF 075-1532) are detected. Titania coatings obtained under deposition conditions of 3 V and different deposition times were characterized morphologically and microstructurally by SEM (Fig. 12a–d). A surface view of the EPD coatings obtained at 3 V after sintering at 700 °C for 2 h in vacuum shows that only a small TiO2 amount was deposited on the substrate after 60 s of deposition (Fig. 12a). As deposition time increased, the coating was seen to become more uniform. Meanwhile, cracks in the
coatings become more evident due to the increase in thickness (Fig. 12b–d), as a consequence of the stresses generated between the substrate and the film upon drying [46]. The coatings obtained at 3 V and 90 s present qualitatively the best morphological characteristics (Fig. 12b), where the extent of cracking has been minimized after heat treatment. Two different magnifications of the top view of the coating obtained at optimal deposition conditions (3 V and 90 s) (Fig. 12b) are shown in Fig. 13. The high degree of uniformity of the coating can be confirmed in these micrographs. Microcracks can be observed (e.g. in Fig. 13a), although they are very small compared to those on TiO2 coatings obtained by EPD from acetylacetone suspensions [25,26]. It is known that acetylacetone has low surface tension but evaporates quickly, causing a high number of cracks in the deposits. In the case when water–ethanol mixtures are used as solvent, the evaporation rate is lower and the film density is higher, indicating that a good packing of ceramic particles during EPD is achieved as can be observed in Fig. 13a. The surfaces of electrophoretic TiO2–Ag coatings for two different compositions (TAG2 and TAG10) sintered at 700 °C for 2 h in vacuum are compared in Fig. 14. It is noted that the coating replicates the substrate surface roughness. The coatings have high homogeneity and a negligible number of cracks in both compositions, similarly to TiO2 deposits discussed above. 3.4. Bioactivity tests TiO2 and other biomaterial coatings immersed in SBF for a given period of time become covered by a layer of hydroxyapatite (HA) if they exhibit bioactive behavior. Thus the rate of deposition and microstructure of this HA layer on the surface of coatings are commonly used to evaluate the material bioactivity [32]. This simple acellular bioactivity test in SBF was carried out on the present coatings. Fig. 15 shows SEM micrographs of the surfaces of two groups of samples (0 and 10 wt.% Ag) after 4 weeks of immersion in SBF. It is possible to assess qualitatively that the HA amount formed on the surface of coatings increases as immersion time in SBF increases. The apatite-forming ability was high in both groups at 1 week, nearly the entire surface was covered with an HA layer. The apatite-forming ability differs between samples without and with Ag. SEM micrographs taken after 4 weeks of immersion, this phenomenon was observed, with indication that the TiO2 sample (0 wt.% Ag) is completely covered with HA (Fig. 15a) while the HA layer deposited on the TiO2–Ag sample is less dense and developed to a lesser extent (Fig. 15b). It is known that functionalized surfaces with hydroxyl groups greatly enhance the nucleation and growth of HA [47]. A possible explanation of the present results could be the reduced availability of Ti–OH groups due to the presence of Ti–O–Ag bonds and because of this, deposition of HA is impeded, at least partially. Orthopedic implants are frequently associated with high risk of bacterial infection [48]. For this reason it is important to confer a bacteriologic effect to materials used in implants. Use of TAG coatings could prevent post-surgical infection. In this application, TAG coatings might have major advantages in comparison to the direct use of antibiotics. For example, no resistance of bacteria to Ag + ion has been reported so far and the effect of Ag+ can be continuous and long lasting if the gradual ion release from coatings is achieved [49]. The positive biocidal activity of the present TAG coatings has been confirmed and it will be reported in a future study. 3.5. Mechanical properties
Fig. 11. XRD patterns of electrophoretic TAG10 coating before sintering. Reflections corresponding to rutile, anatase, Ag and Ag2O are shown.
Vickers microhardness values (HV) and Young's modulus (E) were measured on selected sintered samples. Table 3 resumes the results obtained for the TiO2, TAG2 and TAG10 coatings. Vickers microhardness values are between 210 and 219 kg/mm2, while the values for
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Fig. 12. SEM micrographs of the surface of sintered TiO2 coatings. These coatings were obtained by EPD at 3 V and deposition times of a) 60 s, b) 90 s, c) 120 s and d) 180 s.
Young's modulus are between 28 and 30 GPa, indicating very similar values of measured properties for the three types of coatings. The indentation fracture toughness values for the three types of coatings obtained by applying the Anstis equation [33] are very similar, in the range of 0.85–0.87 MPa.m1/2. Nowadays, most implant ceramic coatings are fabricated by thermal spraying [50,51]. The values obtained in this research are lower in terms of microhardness and fracture toughness than those reported by Lima and Marple [52] for TiO2 coatings produced by thermal spraying. The KIc values measured on the present coatings are influenced by the residual porosity and they are within the expected range (KIc b 1 MPa.m1/2). Moreover, comparing our results with those of Moskalewicz et al. [26] for TiO2 coatings on Ti–6Al–7Nb prepared by EPD, it is evident that the present coatings have higher values of hardness and similar values of E. Obviously the different sintering conditions used in diverse studies affect differently the microstructure of the deposits, including
porosity, grain size and crystalline structure, which result in different mechanical properties measured in the different studies. 4. Conclusions TiO2 deposits on Ti substrates were obtained by EPD using an aqueous suspension containing 6 v%vo ethanol and 1.03 wt.% solids. Optimal parameters used for EPD were: constant voltage of 3 V and deposition time of 90 s. They were chosen based on the qualitative assessment of the macroscopic homogeneity of the coatings and of the quantity of the material deposited, considering also the formation of the lowest number of cracks upon drying of the deposits. In order to provide antibacterial properties to the coatings for application in orthopedic implants, a new nanocomposite material (TiO2–Ag) was synthesized through a nucleophilic process, which enables the direct grow of Ag on NPTiO2. Stable suspensions of TiO2–Ag
Fig. 13. SEM micrographs at different magnifications (a,b) of the surface of sintered TiO2 coatings obtained by EPD (3 V–90 s) from water–ethanol aqueous suspensions.
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Fig. 14. SEM micrographs of the surface of TiO2–Ag coatings obtained by EPD (4 V–6 min) and sintered at 700 °C for 2 h in vacuum. TAG2: a) and b) and TAG10: c) and d).
for EPD have been achieved using ethanol and water as co-solvent without dispersants. The results indicate that H+ from ethanol provides electrosteric stabilization and charging of TiO2 and TiO2–Ag promoting EPD process. The feasibility of EPD of TiO2–Ag coatings on titanium substrate was successfully demonstrated. All TiO2 based coatings showed the development of a hydroxyapatite layer on their surface during in vitro bioactivity tests in SBF. The degree of bioactivity was seen to qualitatively decrease as Ag content in TiO2 coatings increases. The mechanical properties of the TiO2 based coatings, such as Young's modulus, microhardness and indentation fracture toughness, were acceptable, e.g. comparable to values reported in the literature. The results of this research indicate that further improvement would be necessary to optimize the microstructure of coatings obtained by EPD, especially a reduction of porosity and an increase of coating KIc are required. Future studies will investigate the biocide activity of those coatings.
Acknowledgments This work was financially supported by CONICET and SeCyT-FCAI (UNCuyo) under PI +D 06/C183 and 06/P10. The authors thank S. Clavijo, F. Membrives (FCAI-UNCuyo, San Rafael, Argentina), M. Prado, C. G. Oliver. M. S. Moreno and the “Characterization of Materials Group” (CAB-IB, Bariloche, Argentina) for assistance and discussions.
Table 3 Vickers microhardness (Hv), Young's modulus (E) and indentation fracture toughness (KIc) of TiO2, TAG2 and TAG10 coatings.
Fig. 15. SEM micrographs of the surface of TiO2 based coatings obtained by EPD (4 V, 6 min) on Ti substrates after 4 weeks of SBF immersion: a) 0 wt.% Ag and b) 10 wt.% Ag (TAG10). The insets are higher magnification images indicating formation of a hydroxyapatite.
Samples
Hv (Kg/mm2)
E (GPa)
KIc (MPa.m1/2)
TiO2 TAG2 TAG10
210 ± 5 208 ± 8 219 ± 3
28 ± 1 29 ± 2 30 ± 2
0,87 ± 0,02 0,86 ± 0,01 0,85 ± 0,01
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Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Titania and titania–silver nanocomposite coatings grown by electrophoretic deposition from aqueous suspensions M.J. Santillán a,⁎, N.E. Quaranta b, A.R. Boccaccini c,⁎ a b c
FCAI, Universidad Nacional de Cuyo. CONICET, San Rafael, Argentina Universidad Tecnológica Nacional, FR San Nicolás, CIC, San Nicolás, Argentina Institute of Biomaterials, University of Erlangen-Nuremberg, 91058 Erlangen, Germany
a r t i c l e
i n f o
Article history: Received 8 August 2010 Accepted in revised form 1 October 2010 Available online 12 October 2010 Keywords: Electrophoretic deposition Titania coatings Antibacterial coatings Biomedical applications Silver nanoparticles
a b s t r a c t This study focused on the synthesis of titania–silver (TiO2–Ag) nanocomposite coatings with potential enhanced antibacterial properties for applications in orthopedic implants. Ag nanoparticles (4 nm) were grown directly on the surface of commercially available TiO2 nanoparticles (23 nm) by nucleophilic reaction. The electrophoretic deposition (EPD) of TiO2 and TiO2–Ag nanoparticles on titanium substrates using aqueous suspensions was investigated. Best results were achieved by using water and ethanol as co-solvents. The EPD process (voltage: 3 V, deposition time: 90 s) led to TiO2 based coatings on titanium substrates exhibiting homogeneous and uniform microstructure. In vitro bioactivity tests in Kokubo's simulated body fluid (SBF) to evaluate the formation of hydroxyapatite (HA) on the coating surface were performed. The results showed that the extent of HA formation rapidly increased with increasing time in SBF and it decreased as silver amount in the coating increased. XRD, TEM and SEM-EDAX were used to investigate the microstructure of the nanomaterials and coatings. The high bioactivity of the TiO2 based electrophoretic coatings indicates their potential for use as bioactive antibacterial layers in orthopedic implants. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Titania (TiO2) films have been extensively studied and applied for their well-known biocompatibility and biocide properties [1–3]. As a biomaterial, titania has potential use in hard tissue replacement and bone tissue engineering applications [4–8]. Titania is normally applied as a coating on metallic substrates in order to improve the integration of orthopedic implants in host bone tissue [4–8]. Infections caused after implantation of orthopedic devices represent major complications in traumatologic surgery [9]. This phenomenon is mainly due to bacterial colonization of implanted materials, through adhesion and accumulation of bacteria on the coating surface and consequently the formation of a biofilm [10]. Therefore, researchers are increasingly focusing on improving the antibacterial property of implants [11]. In recent years, it has become apparent that the use of inorganic antibacterial materials may lead to better results than those achieved with organic antibacterial materials in terms of durability, toxicity and selectivity of action [12]. In this regard, the usefulness of Ag as an antibacterial agent has been known for several years and Ag is currently one of the preferred elements used in antibacterial coatings for several applications [13–15].
⁎ Corresponding authors. E-mail addresses: [email protected] (M.J. Santillán), [email protected] (A.R. Boccaccini). 0257-8972/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2010.10.001
Diverse types of silver-doped materials for antibacterial applications have been developed, including silver-doped ceramics based on calcium phosphate, silica, titania and bioactive-glass [11,13,16–19]. Traditionally TiO2 coatings for biomedical application have been prepared by spray plasma technique [20,21]. However, high processing temperatures are commonly needed for these methods. Consequently the crystalline structure of the coatings is usually rutile and microstructural features are difficult to control. Electrophoretic deposition (EPD) [22,23] is one of the most effective techniques to assemble nanoparticles because it is a relatively simple, versatile and low cost process which allows the use of complex shape components as deposition substrate [22–24]. EPD is a colloidal process wherein ceramic coatings are created directly from a stable colloidal suspension by application of an electric field. Several investigations on TiO2 EPD have been carried out using TiO2 organic suspensions, in particular with acetylacetone or acetone as solvents [25–27]. Only a few studies have been carried out using TiO2 aqueous suspensions [28]. In this research, the synthesis of TiO2–Ag composite nanoparticles and the fabrication of TiO2 and TiO2–Ag coatings on titanium substrates by EPD were investigated. Ag nanoparticles (NPAg) were directly formed and grown on the surface of TiO2 nanoparticles (NPTiO2) from nucleophilic reaction catalyzed by alkalis. The advantage to form NPAg on a supporting titania nanostructure is the resulting homogeneity of the Ag distribution which should enable the release of Ag in a controlled manner [18,29–31]. The production of
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films for biomedical applications requires good control of the deposition conditions because this will affect the microstructure and ultimately the properties of the films. For this reason, EPD was chosen in the present work as a promising technique for deposition of nanostructured bioactive TiO2–Ag coatings. 2. Experimental procedure 2.1. Starting materials TiO2 nanopowder (P25-Degussa, Germany) was used (mean particle size: 23 nm). The crystallographic composition of the TiO2 nanoparticles is a 70/30 mixture of anatase and rutile, respectively, as given by the supplier. Ethanol (Tetrahedrom) was used as co-solvent. The chemical precursors used for synthesis of the Ag–TiO2 nanocomposite were TiO2 nanopowder, silver nitrate (AgNO3, Aldrich, 97%), and ammonium hydroxide (NH4OH, Tetrahedrom).TiO2–Ag nanocomposite, nitric acid (HNO 3 , Tetrahedrom) and 2-propanol (C3H7·OH, Tetrahedrom) were used for investigating washing procedures. Certain amounts of HNO3 or NaOH solutions, both 0.05 M, were added for pH adjustment. 2.2. Synthesis of TiO2–Ag nanoparticles The fabrication of TiO2 nanoparticles doped with silver (TiO2–Ag) was realized by a chemical process catalyzed by alkalis, modifying the route used originally by Kim et al. to synthesize Ag-particles on silica [18]. 50 mL of aqueous suspensions of NPTiO2 with AgNO3 in appropriate proportions were prepared to obtain the TiO2–Ag composite material, with 2 and 10 wt.% of Ag+ with respect to NPTiO2 content (wt.%: referred to the solids percentage by weight). After that, a solution 0.5 M of NH4OH was added to the slurry in order to preserve a relation TiO2: NH4OH = 0.10, 0.25, 0.40 and 0.55. The full system was maintained in reflux at ~60 °C for 6 h to facilitate the nucleation process of NPAg and the adsorption of NPAg on the NPTiO2 surfaces. Subsequently, the solid product was filtered in vacuum to recover the solid material. The product was divided in two parts which were washed in two different ways: a) with distilled water followed by HNO3 (0.05 M) and b) with 2-propanol up to pH 5, to stabilize the NPAg on the NPTiO2 surfaces. Finally, in both cases, the product was washed with distilled water up to pH ~7. 2.3. Suspension development and EPD of TiO2 The optimal composition of TiO2 aqueous suspensions for EPD was obtained after testing the effect of three different surfactants, Tiron [(OH)2C6H2(SO3Na)2], poly-vinyl-butyral (PVB) and poly-methyl cellulose (PMC) and different amounts of ethanol, as a co-solvent. The best deposition by EPD in terms of density and homogeneity of the coatings was achieved using ethanol as co-solvent. For the deposition of TiO2–Ag nanoparticles, a suspension with 6% (v/vo) of ethanol was used. Table 1 lists the composition of the suspensions used for electrophoretic deposition of TiO2 and TiO2–Ag nanoparticles. The quantities of reagents are referred to the solids percentage by weight (wt.%). All EPD experiments were carried out at room temperature and under constant voltage conditions. Before EPD, the suspension was
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ultrasonically stirred during 3 h. The EPD cell consisted of a glass container and two electrodes. Titanium (Ti) sheets, with an area of 1.4 cm2, were used as deposition substrates and a stainless steel sheet with identical area as counter-electrode. The distance between both electrodes was maintained at 2 cm. Both the working electrode and the counter-electrode in the EPD container were connected to a d.c. power supply. The electrodes were cleaned with acetone before each experiment to enhance adhesion of the coating. After EPD the samples were dried in a desiccator at room temperature for 24 h followed by heat treatment at 700 °C (2 °C/min) for 2 h in vacuum. 2.4. In vitro bioactivity tests of TiO2 and TiO2–Ag composite coatings Bioactivity tests were carried out in simulated body fluid (SBF) according to the protocol of Kokubo et al. [32]. The SBF was prepared by dissolving respective amounts of reagent grade chemicals of NaCl, NaHCO3, KCl, K2HPO4·3H2O, MgCl2·6H2O, CaCl2·2H2O, and Na2SO4 into distilled water. The pH of SBF was adjusted to physiological pH (~ 7.25) by HCl (1 M) and tris(hydroxyl–methyl) aminomethane. Coated specimens were soaked in SBF solution and maintained at 36.5 ± 0.5 °C for 1 and 4 weeks to evaluate the growth of hydroxyapatite (HA) layer on the surface coatings. The tests were carried out maintaining a surface–volume ratio (S/V) of 0.005 cm-1. The SBF was refreshed every 3 days. After that, the samples were rinsed in distilled water three times, dried and stored in desiccators for further characterization. 2.5. Characterization techniques A High Resolution Transmission Electron Microscope (HR-TEM, Philips CM200) was used to observe and characterize the shape, size and degree of agglomeration of NPTiO2 and TiO2–Ag nanocomposite particles. Chemical analysis was carried out by TEM-energy dispersive (EDS). X-ray diffraction (XRD) patterns were obtained using a Philips PW 1700 diffractometer equipped with a graphite monochromator and Cu Kα radiation. The patterns were collected in the 20–70° 2θ range with scan steps of 0.02°. Scanning Electron Microscopy (SEM, Philips 515) was employed to evaluate the microstructure, morphology and adhesion of the deposited coatings to the substrates. 2.6. Mechanical properties The values of microhardness and fracture toughness (KIc) were obtained by indentation technique. Vickers microhardness measurements were performed applying a 9.8 N load for 20 s on the surface of the coatings. A total of 5 measurements for each sample was carried out. The fracture toughness was determined by indenting the coating cross-section with a Vickers indenter at a 200 g load for 20 s, with the indenter aligned such that one of its diagonals would be parallel to the substrate surface and using the equation proposed by Anstis et al. [33]. Young's Modulus values were obtained using the Knoop indentation method at a 200 g load for 15 s and employing the expression proposed by Conway [34] relating H/E to the residual length of the minor diagonal of a Knoop indentation. 3. Results and discussion 3.1. Synthesis of Ag doped titania
Table 1 Composition of suspensions used in EPD of TiO2 and TiO2–Ag nanoparticles. Suspension
TiO2 suspension TAG2: TiO2–Ag susp. (2%) TAG10: TiO2–Ag susp. (10%)
Solvent (mL)
Solids (wt.%)
Water
Ethanol
TiO2
TAG
9.4 9.4 9.4
6 6 6
1.03 – –
– 1.07 1.21
The synthesis of silver doped titania nanoparticles (TiO2–Ag) by a wet route has been shown for the first time in this research, modifying the synthesis process reported by Kim et al. [18], developed originally to obtain silver particles supported on SiO2. In the current approach, NPTiO2 in aqueous medium are hydroxylated generating Ti–OH groups. Like other oxide surfaces in aqueous medium, the titania surface is OH terminated, amphoteric, and has a pH dependent surface
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charge. Oxide surfaces can acquire a charge by adsorption or protons dissociation, e.g., for titania [35]: þ
−
ð1Þ
≡ Ti−OH + H2 O↔≡ Ti−OH2 + OH −
≡ Ti−OH + H2 O↔≡ Ti−OH
+ H3 O
þ
ð2Þ
The alkaline condition generates strong nucleophiles which deprotonate hydroxyl ligands; subsequently, these nucleophilic groups can react with electrophilic materials as metal ions. Accordingly, a nanocomposite may be formed by autoreduction between noble metal ions with hydroxilated oxide surfaces. When the Ti–OH groups react with a Lewis basis, as NH4OH, a nucleophilic attack on the –OH groups occurs booting H+ from the coordination sphere. The steric configuration of the NPTiO2 surface's groups allows subsequently the reduction of Ag+ due to its spatial arrangement. In this manner, oxygen is present on the surface with excess negative charge, which can easily react with Ag+ ion, auto reduced through a chain reaction with other –OH groups on the surface. The reduction power will decrease as the amount of –OH groups available on the surface of TiO2 decreases. A similar explanation was given for the case of silica doped with Ag in a process catalyzed by basis [18]. This synthesis process implied different steps, as explained below: a) The first step is the deprotonation of hydroxyl ligand in TiOH from nucleophilic that is attacked by a Lewis basis at hydrated oxide: −
Ti−OH + B : →TiO
ð3Þ
+ BH
b) The second step is an electrophilic attack by Ag+ that is bonded at the nucleophilic TiO- group:
þ
TiO + Ag →TiO−Ag
ð4Þ
c) The third step is the formation of þ
TiOH + Ag →TiO−Ag + H
þ
NPAg
on the NPTiO2 surface: ð5Þ
Fig. 1 shows schematically the process previously described. The stoichiometric ratio of reactants influences the final Ag:TiO2 ratio in the nanocomposite material. As the amount of AgNO3 increases, higher is the NPAg quantity grown on the TiO2 surface. Moreover, it was observed that the size and distribution of NPAg on the TiO2 surface are strongly influenced by the final washing treatment.
To determine the optimized Ag:TiO2 ratio, the molar ratios of AgNO3 and catalyst were controlled. After several experiments, the optimal amount of alkalis (NH4OH) necessary to maintain the stoichiometric ratio of reagents in the final product was determined. It was found that the desired stoichiometric ratio of precursors (TiO2: Ag) is maintained in the final product when the proportion of (TiO2: NH4OH) is 0.25 for 0.01 b [Ag+] b 0.15. The influence of washing conditions on the final product was evaluated qualitatively by TEM observations. Fig. 2 shows TEM micrographs of Ag–TiO2 nanocomposite particles with a TiO2:Ag ratio (in weight) of 90:10 (TAG10) for different washing conditions. Fig. 2.a) shows TAG10 nanocomposite washed only with water, while Fig. 2.b) shows TAG10 nanocomposite washed with HNO3, isopropanol and finally distilled water. It can be seen in Fig. 2.a) that the NPAg size and distribution on the NPTiO2 surface are not uniform. It was difficult to observe individual NPAg formed on the NPTiO2 surfaces, and frequently particle agglomerations were observed, which may induce a decline of the antibacterial properties. However it is possible that some Ag aggregations that did not react with the NPTiO2 surface are dissolved by nitric acid resulting in individual NPAg on NPTiO2 surfaces. In Fig. 2.b) it is observed that NPAg are smaller than in Fig. 2a) and they are distributed uniformly on the surface of NPTiO2. Therefore, it was determined that the washing process of TiO2–Ag composite should be done with HNO3 and isopropanol according to the procedure indicated above. Fig. 3a shows a high magnification TEM micrograph of TiO2–Ag nanoparticles with 10 wt.% Ag (TAG10). The inset shows a HR-TEM micrograph of NPAg where it is possible to observe the different atomic planes, corresponding to Ag2O (d~ 2.35 Å) and Ag. The nanoparticles on NPTiO2 are a mixture of elemental Ag and Ag2O that then dissociate into silver ions in aqueous medium. Both forms of silver will release silver ions providing antimicrobial activity. This ionic form of silver has been shown to be effective against a broad range of bacteria [36]. The diameter distribution of the NPAg was estimated from TEM images. The results are presented in Fig. 3b indicating that the size distribution of NPAg is narrow and it is in the 2–5 nm range. A log normal distribution fit gave m = 4.1 nm, r = 0.8 nm for the mean and the standard deviation of the NPAg diameter, respectively. The EDS analysis on Ag–TiO2 nanoparticles shows peaks of Ti, O and Ag, not detecting the presence of other elements in the spectra (Cu peaks are due to the TEM grid used) (Fig. 4). Table 2 summarizes the stoichiometric relationship of the precursors used and the final composition obtained by semiquantitative EDS analysis. It is possible to conclude that the amount of NPAg grown on the NPTiO2 is closely related to the quantity of precursor used in the synthesis of the composite material, as expected.
Fig. 1. Representative schema of the chemical mechanism for the synthesis and growth of NPAg on the TiO2 nanoparticle surface.
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Fig. 2. TEM micrographs of TiO2–Ag nanocomposite particles obtained by nucleophilic reaction after washing with a) distiller water and b) nitric acid, isopropanol and distiller water.
3.2. Electrophoretic deposition of TiO2 and TiO2–Ag nanoparticles The key parameters influencing the EPD process are: suspension concentration, deposition time and applied voltage. In the present study, these parameters were investigated systematically in wide ranges and their influence on the deposition process was studied to optimize the EPD conditions, as discussed next. 3.2.1. Electrophoretic deposition of NPTiO2 The use of co-solvents may be beneficial to deposit NPTiO2 by EPD from an aqueous system [28]. Adding a short chain alcohol (such as ethanol) to the suspension can prevent water hydrolysis when relatively low voltages are applied [28,37]. Different ways to stabilize suspensions for EPD are known [22,23,38]. In this research, electrostatic stabilization was used to stabilize NPTiO2 suspensions without adding surfactants. In this context, a mixture of ethanol and water as the solvent was used. The effect of the concentration of ethanol ([Et– OH]) on suspension stability and on the quality of the films obtained by EPD was investigated. 3.2.2. Optimization of NPTiO2 suspensions Different amounts of ethanol (2, 4, 6, 10 and 15 v%vo; v0: refereed to solvent volume) were added to an aqueous suspension containing 1.03 wt.% of NPTiO2, in order to explore its effect on the stability of the suspension and consequently on the generation of surface charge on NPTiO2. Oxide surfaces in aqueous suspension will hydrate to form surface hydroxyl groups. The chemical species adsorbed on the surface of the nanoparticles depend on the medium pH. The use of co-solvents has the advantage of facilitating suspension stabilization, enabling also to avoid the occurrence of water hydrolysis at relatively low voltages.
Ethanol readily dissociates according to the equilibrium reaction (Eq. (6)): CH3 −CH2 −OH + H2 O⇔CH3 −CH2 −O− + H3 Oþ H3 Oþ ⇔H2 O + Hþ
ð6Þ
For the present case, protons produced in this reaction are adsorbed onto amphoteric hydroxyl groups present on the surface of NPTiO2. TiO2 coatings were deposited on the negative electrode (cathode) under the EPD conditions employed, confirming the positive charge on NPTiO2 in aqueous suspension. In this investigation it was necessary to sonicate the suspensions for long periods of time (N3 h) in order to achieve a complete dispersion of NPTiO2 and the adsorption of H+ on their surfaces. In previous studies, Lebrette et al. [28,39] used a mixture of water and 10% ethanol with addition of Tiron to stabilize TiO2 suspensions for EPD. Our previous research and other reports in the literature [40,41] indicate that good quality coatings can be obtained by EPD when the amount of additives in suspensions is minimum, because only a reduced portion of them is effectively adsorbed on particles, contributing to stabilize the suspension. The ions that are not adsorbed on suspended particles produce an increase of the suspension's ionic strength and consequently a decrease in the thickness of the electrical double layer of particles occurs, which originates agglomeration of particles and consequently, a deterioration of the quality of the deposited coatings [39]. Therefore, in the present research it was sought to stabilize the suspension by adjusting the pH value and by minimizing the addition of additives. Several series of EPD experiments with different ethanol concentrations in suspension and different applied voltages were carried out, as indicated above.
Fig. 3. a) TEM micrograph of TiO2–Ag nanocomposite, the inset shows a HR-TEM image of the NPAg. b) Particle size distribution of NPAg.
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Fig. 4. EDS spectra of TiO2–Ag nanocomposite for 2 and 10wt.% Ag (TAG2 and TAG10, respectively) (Cu peaks are due to the TEM grid).
Fig. 5 shows a plot of the amount of TiO2 deposited by EPD applying 3 V as function of EPD time for suspensions with different ethanol contents. It was observed that using only water, it was not possible to obtain any TiO2 deposit on Ti substrates. For deposition time (t) of t ≤ 3.5 min, the best deposition rate was achieved using a suspension containing 4 v%vo of ethanol. For higher deposition times, the amount of TiO2 nanoparticles deposited on the substrate decreased. Visual inspection of EPD coatings fabricated with longer EPD times revealed that the coatings were not properly adhered to the substrate and the covering of the substrate was inhomogeneous. For ethanol concentrations ≥6 v%vo, the deposition rate was seen to fall dramatically. One possible explanation for this behavior may be as follows. The increase of ethanol concentration in suspension leads to an increase in the availability of free H+. As a consequence, titania nanoparticles acquire a positive electrical charge and an excess of H+ in suspension develops. When an external voltage is applied, the excess H+ in suspension interferes with the deposition of NPTiO2 on the Ti substrate because their electrophoretic mobility is higher. The presence of high concentrations of ethanol should decrease the mobility of the ceramic particles. This effect can be compared with the situation resulting from the reaction between I2 and acetylacetone when these reagents are used in TiO2 suspensions for EPD, as discussed in the literature [26,27,42]. In general, it is expected that the applied voltage will have a significant influence on the final characteristics of the coatings. The Smoluchowski's equation proposed to determine the electrophoretic mobility of suspended particles during EPD [43,44] is related to the electrophoretic velocity υ (Eq. (7)): υ=
ζ Eε 4πη
Fig. 5. Electrophoretic deposit weight of TiO2 nanoparticles vs. deposition time for aqueous suspensions with different amounts of ethanol (v%v0) (Applied voltage = 3V).
r N H+, will have a relatively low electrophoretic velocity and different effects can be expected in the EPD coatings. Fig. 6 shows that for ethanol concentrations ≤ 6 v%vo, the deposited TiO2 amount increased as deposition time increased. The suspensions with higher [Et–OH] did not present a defined behavior in terms of deposition rate as function of deposition time, showing that, in general, the deposited TiO2 weight decreases significantly. This behavior is consistent with the evolution of the pH of the suspensions as a function of [Et–OH] (Fig. 7). As [Et–OH] increases, pH decreases due to the increase of the amount of free H+, leading to a reduction of deposition rate, which is related to suspensions with higher conductivity for H+. According to the results obtained, it was necessary to compromise between the TiO2 amount deposited on the substrate and the quality of the coatings acquired. A suspension containing [Et–OH] = 4 v%vo was selected as optimum for EPD in the present experiments.
ð7Þ
where ζ is the zeta potential, ε the dielectric constant and η is the viscosity. Considering that the electrophoretic velocity υ is proportional to the quantity QE/rη, where Q is particle charge and r is its radius [44], it is possible to deduce that particles (or composites) with Table 2 Semiquantitative chemical composition of samples TAG2 and TAG10 obtained by EDS. Samples
Ag (wt. %)
Ti (wt. %)
O (wt. %)
TAG2 TAG10
1,91 9,76
73,55 67,67
24,54 22,57
Fig. 6. Dependence of deposit weight of TiO2 nanoparticles on ethanol concentration [Et–OH] at different EPD times.
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a lower quantity of material deposited per unit of EPD time for the same applied voltage is observed. This behavior can be explained if one considers the voltage drop that occurs as a result of the growth of the ceramic coating layer. As the film thickness increases, so does its electrical resistance and therefore the voltage drop increases, leading to a reduction of the amount of TiO2 nanoparticles deposited per unit of time. This phenomenon is observed in the plot of deposited weight vs. applied voltage for different EPD times (Fig. 8).
In order to optimize the remaining EPD parameters, two sets of experiments were carried out. One involved the deposition process at constant time, with applied voltages investigated in the range of 2 to 20 V. In another series of experiments the voltage applied was maintained constant, changing the deposition time from 1 to 4 min. When voltages ≤ 2 V were applied, TiO2 films did not adhere to the substrates. This behavior can be attributed to insufficient electrophoretic mobility of the particles, which cannot overcome electrostatic repulsion forces, in relation to the overlap of electrical double layers. At voltages N4 V release of oxygen and hydrogen from the electrodes due to water electrolysis was observed. Moreover an increase in the generation of these gases as the voltage increased was detected. Fig. 8 shows a plot of the TiO2 amount deposited per unit area as function of deposition time for different applied voltages. It is observed that with higher voltages the deposited TiO2 amount increases; however the evolution of gases at the electrodes interferes with the deposition, producing highly inhomogeneous films, this effect has been frequently discussed in the literature [22,37,38]. This phenomenon may be in fact comparable to the situation where there is an excess of free H+ in suspension that affects also negatively the morphology of the coating (discussed above). Under these conditions,
3.2.3. Optimization of electrophoretic deposition parameters for TiO2–Ag nanoparticles Several experiments were carried out in order to evaluate the optimal composition of suspensions of TiO2–Ag nanoparticles for EPD. The final compositions of suspensions used are shown in Table 1. The electrostatic stabilization of the suspension required ultrasonic agitation for a long period of time (t N 180 min) to promote the dissociation of ethanol and the subsequent adsorption of protons on the TiO2–Ag entities. This is similar to the stabilization process of TiO2 nanoparticles in aqueous suspensions whose physicochemical principles were discussed above. In order to optimize the EPD parameters, several series of EPD tests were carried out applying different constant voltages (in the range: 3– 20 V) and deposition times (30–210 s). The plot showing deposited TiO2–Ag (sample TAG10) weight vs. deposition time at different voltages is presented in Fig. 9. At applied voltages b2 V the deposition rate of the TiO2–Ag nanoparticles is extremely small, this behavior is similar to that observed for EPD of NPTiO2 from aqueous suspensions, discussed in Section 3.2.2. At voltages N2 V, the deposition process is greatly improved but the coatings become more inhomogeneous as voltage increases. These results are in a good agreement with the prediction of the Hamaker equation [45], which predicts an increase in the amount of deposited material with increasing deposition time. For applied voltages N6 V it can be seen that the deposition rate decreases. The explanation for this phenomenon is similar to that given earlier when describing the EPD of NPTiO2 from aqueous suspension. The excessive release of gases at the electrodes causes the detachment of the TiO2–Ag coating from the substrate. However, at 4 V a homogeneous coating is obtained for deposition times in the range: 2 ≤ t ≤ 10 min. Based on these experimental results, the optimal conditions chosen for EPD were therefore: 4 V–6 min for TAG10 suspensions. Fig. 10 shows the EPD rate at conditions of 4 V– 6 min as a function of pH for TAG10 suspension. The highest
Fig. 8. TiO2 deposit weight vs. EPD time for different applied voltages, using an aqueous suspension with 1.03 wt.% of solids and 4 v%v0 of ethanol.
Fig. 9. Deposit weight of TiO2–Ag nanoparticles (TAG10 suspension, Table 1) on Ti substrate vs. EPD time for different applied voltages.
Fig. 7. pH variation of TiO2 aqueous suspensions and electrophoretic deposit weight on Ti substrate as function of ethanol concentration [Et–OH].
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Fig. 10. Deposit weight of TiO2–Ag nanoparticles (TAG10 suspension) vs. pH for EPD at conditions: 4 V, 6 min.
deposition rate which corresponds to this suspension is achieved at pH ~6.8. Under these conditions there is no flocculation of particles and the coatings are homogeneous. The TAG10 aqueous suspension containing Et–OH as co-solvent had a pH ~6.7, changing very little for both compositions studied (Table 1).
3.3. Microstructural characterization Fig. 11 shows the X-ray diffraction (XRD) pattern corresponding to a TAG10 coating before heat treatment. Only reflections corresponding to the TiO2 phase anatase (PDF 021-1272) and rutile (PDF 021-1276) are observed. Additionally, some peaks corresponding to Ag (PDF 004-0783) and Ag2O (PDF 075-1532) are detected. Titania coatings obtained under deposition conditions of 3 V and different deposition times were characterized morphologically and microstructurally by SEM (Fig. 12a–d). A surface view of the EPD coatings obtained at 3 V after sintering at 700 °C for 2 h in vacuum shows that only a small TiO2 amount was deposited on the substrate after 60 s of deposition (Fig. 12a). As deposition time increased, the coating was seen to become more uniform. Meanwhile, cracks in the
coatings become more evident due to the increase in thickness (Fig. 12b–d), as a consequence of the stresses generated between the substrate and the film upon drying [46]. The coatings obtained at 3 V and 90 s present qualitatively the best morphological characteristics (Fig. 12b), where the extent of cracking has been minimized after heat treatment. Two different magnifications of the top view of the coating obtained at optimal deposition conditions (3 V and 90 s) (Fig. 12b) are shown in Fig. 13. The high degree of uniformity of the coating can be confirmed in these micrographs. Microcracks can be observed (e.g. in Fig. 13a), although they are very small compared to those on TiO2 coatings obtained by EPD from acetylacetone suspensions [25,26]. It is known that acetylacetone has low surface tension but evaporates quickly, causing a high number of cracks in the deposits. In the case when water–ethanol mixtures are used as solvent, the evaporation rate is lower and the film density is higher, indicating that a good packing of ceramic particles during EPD is achieved as can be observed in Fig. 13a. The surfaces of electrophoretic TiO2–Ag coatings for two different compositions (TAG2 and TAG10) sintered at 700 °C for 2 h in vacuum are compared in Fig. 14. It is noted that the coating replicates the substrate surface roughness. The coatings have high homogeneity and a negligible number of cracks in both compositions, similarly to TiO2 deposits discussed above. 3.4. Bioactivity tests TiO2 and other biomaterial coatings immersed in SBF for a given period of time become covered by a layer of hydroxyapatite (HA) if they exhibit bioactive behavior. Thus the rate of deposition and microstructure of this HA layer on the surface of coatings are commonly used to evaluate the material bioactivity [32]. This simple acellular bioactivity test in SBF was carried out on the present coatings. Fig. 15 shows SEM micrographs of the surfaces of two groups of samples (0 and 10 wt.% Ag) after 4 weeks of immersion in SBF. It is possible to assess qualitatively that the HA amount formed on the surface of coatings increases as immersion time in SBF increases. The apatite-forming ability was high in both groups at 1 week, nearly the entire surface was covered with an HA layer. The apatite-forming ability differs between samples without and with Ag. SEM micrographs taken after 4 weeks of immersion, this phenomenon was observed, with indication that the TiO2 sample (0 wt.% Ag) is completely covered with HA (Fig. 15a) while the HA layer deposited on the TiO2–Ag sample is less dense and developed to a lesser extent (Fig. 15b). It is known that functionalized surfaces with hydroxyl groups greatly enhance the nucleation and growth of HA [47]. A possible explanation of the present results could be the reduced availability of Ti–OH groups due to the presence of Ti–O–Ag bonds and because of this, deposition of HA is impeded, at least partially. Orthopedic implants are frequently associated with high risk of bacterial infection [48]. For this reason it is important to confer a bacteriologic effect to materials used in implants. Use of TAG coatings could prevent post-surgical infection. In this application, TAG coatings might have major advantages in comparison to the direct use of antibiotics. For example, no resistance of bacteria to Ag + ion has been reported so far and the effect of Ag+ can be continuous and long lasting if the gradual ion release from coatings is achieved [49]. The positive biocidal activity of the present TAG coatings has been confirmed and it will be reported in a future study. 3.5. Mechanical properties
Fig. 11. XRD patterns of electrophoretic TAG10 coating before sintering. Reflections corresponding to rutile, anatase, Ag and Ag2O are shown.
Vickers microhardness values (HV) and Young's modulus (E) were measured on selected sintered samples. Table 3 resumes the results obtained for the TiO2, TAG2 and TAG10 coatings. Vickers microhardness values are between 210 and 219 kg/mm2, while the values for
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Fig. 12. SEM micrographs of the surface of sintered TiO2 coatings. These coatings were obtained by EPD at 3 V and deposition times of a) 60 s, b) 90 s, c) 120 s and d) 180 s.
Young's modulus are between 28 and 30 GPa, indicating very similar values of measured properties for the three types of coatings. The indentation fracture toughness values for the three types of coatings obtained by applying the Anstis equation [33] are very similar, in the range of 0.85–0.87 MPa.m1/2. Nowadays, most implant ceramic coatings are fabricated by thermal spraying [50,51]. The values obtained in this research are lower in terms of microhardness and fracture toughness than those reported by Lima and Marple [52] for TiO2 coatings produced by thermal spraying. The KIc values measured on the present coatings are influenced by the residual porosity and they are within the expected range (KIc b 1 MPa.m1/2). Moreover, comparing our results with those of Moskalewicz et al. [26] for TiO2 coatings on Ti–6Al–7Nb prepared by EPD, it is evident that the present coatings have higher values of hardness and similar values of E. Obviously the different sintering conditions used in diverse studies affect differently the microstructure of the deposits, including
porosity, grain size and crystalline structure, which result in different mechanical properties measured in the different studies. 4. Conclusions TiO2 deposits on Ti substrates were obtained by EPD using an aqueous suspension containing 6 v%vo ethanol and 1.03 wt.% solids. Optimal parameters used for EPD were: constant voltage of 3 V and deposition time of 90 s. They were chosen based on the qualitative assessment of the macroscopic homogeneity of the coatings and of the quantity of the material deposited, considering also the formation of the lowest number of cracks upon drying of the deposits. In order to provide antibacterial properties to the coatings for application in orthopedic implants, a new nanocomposite material (TiO2–Ag) was synthesized through a nucleophilic process, which enables the direct grow of Ag on NPTiO2. Stable suspensions of TiO2–Ag
Fig. 13. SEM micrographs at different magnifications (a,b) of the surface of sintered TiO2 coatings obtained by EPD (3 V–90 s) from water–ethanol aqueous suspensions.
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Fig. 14. SEM micrographs of the surface of TiO2–Ag coatings obtained by EPD (4 V–6 min) and sintered at 700 °C for 2 h in vacuum. TAG2: a) and b) and TAG10: c) and d).
for EPD have been achieved using ethanol and water as co-solvent without dispersants. The results indicate that H+ from ethanol provides electrosteric stabilization and charging of TiO2 and TiO2–Ag promoting EPD process. The feasibility of EPD of TiO2–Ag coatings on titanium substrate was successfully demonstrated. All TiO2 based coatings showed the development of a hydroxyapatite layer on their surface during in vitro bioactivity tests in SBF. The degree of bioactivity was seen to qualitatively decrease as Ag content in TiO2 coatings increases. The mechanical properties of the TiO2 based coatings, such as Young's modulus, microhardness and indentation fracture toughness, were acceptable, e.g. comparable to values reported in the literature. The results of this research indicate that further improvement would be necessary to optimize the microstructure of coatings obtained by EPD, especially a reduction of porosity and an increase of coating KIc are required. Future studies will investigate the biocide activity of those coatings.
Acknowledgments This work was financially supported by CONICET and SeCyT-FCAI (UNCuyo) under PI +D 06/C183 and 06/P10. The authors thank S. Clavijo, F. Membrives (FCAI-UNCuyo, San Rafael, Argentina), M. Prado, C. G. Oliver. M. S. Moreno and the “Characterization of Materials Group” (CAB-IB, Bariloche, Argentina) for assistance and discussions.
Table 3 Vickers microhardness (Hv), Young's modulus (E) and indentation fracture toughness (KIc) of TiO2, TAG2 and TAG10 coatings.
Fig. 15. SEM micrographs of the surface of TiO2 based coatings obtained by EPD (4 V, 6 min) on Ti substrates after 4 weeks of SBF immersion: a) 0 wt.% Ag and b) 10 wt.% Ag (TAG10). The insets are higher magnification images indicating formation of a hydroxyapatite.
Samples
Hv (Kg/mm2)
E (GPa)
KIc (MPa.m1/2)
TiO2 TAG2 TAG10
210 ± 5 208 ± 8 219 ± 3
28 ± 1 29 ± 2 30 ± 2
0,87 ± 0,02 0,86 ± 0,01 0,85 ± 0,01
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