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Synthesis of porous Ce-doped titania coating containing CaTiO3 by MAO and its apatite inducing ability
Synthesis of porous Ce-doped titania coating containing CaTiO 3 by MAO and its apatite inducing ability X. Rao, ¡!–[INS][C. L.]–¿C.L.¡!–[/INS]–¿ Chu, Q. Sun PII: DOI: Reference:
S0257-8972(16)30470-4 doi: 10.1016/j.surfcoat.2016.05.077 SCT 21232
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
11 February 2016 27 May 2016 27 May 2016
Please cite this article as: X. Rao, ¡.!.–[.I.N.S.].[.C.L.].–¿.C.L.¡.!.–[./.I.N.S.].–¿. Chu, Q. Sun, Synthesis of porous Ce-doped titania coating containing CaTiO3 by MAO and its apatite inducing ability, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.05.077
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ACCEPTED MANUSCRIPT Synthesis of porous Ce-doped titania coating containing CaTiO3 by MAO and its apatite inducing ability
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X. Rao, C. L. Chu*, Q. Sun
School of Materials Science and Engineering and Jiangsu Key Laboratory for Advanced
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Metallic Materials, Southeast University, Nanjing, 211189, China
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Abstract
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Bioactive porous CaTiO3-contained titania coatings doped with cerium were prepared on CP-Ti surfaces by micro arc oxidation (MAO) process. The surface morphologies,
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chemical composition, wettability and the phase compositions of coatings were investigated
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by scanning electron microscopy (SEM), energy dispersion X-ray spectrometry (EDS),
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contact angle goniometer and X-ray diffraction (XRD), respectively. The results showed the surface phase compositions of the as-synthesized coatings are anatase, rutile TiO2 and
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perovskite CaTiO3 phases, as well as a few amorphous phases. The participation of Ce in the electrolyte during MAO is evidenced to benefit in crystallizing CaTiO3 phase and enhancing the coating surface hydrophilicity. The results of simulated body fluid (SBF) immersion experiments indicate that the hydrolysis of CaTiO3 during immersion could enhance the nucleation and growth of apatite. In comparison, the porous Ce-doped titania coating containing CaTiO3 exhibits better bioactivity as more apatites were observed at the same immersion duration. Keywords: Porous titania coating; CaTiO3; Ce-doped; Micro arc oxidation; Bioactivity
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1. Introduction
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Titanium and its alloys are frequently used in orthopedic implants and bone replacements due to their excellent mechanical strength, high chemical stability, and
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attractive biocompatibility. However, titanium is a bio-inert material that cannot directly bond to bone immediately after implantation [1, 2]. Hence, it is necessary to improve the
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bioactivity of Ti substrate to gain wider acceptance in the biomedical field. Titanate structures, which not only exhibit good biocompatibility but also induce the formation of
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bone-like apatites, are receiving considerable attentions among bioactive materials and that can be coated on artificial joints or dental implants to stimulate new bone growth and
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enhance osseointegration [3-6]. The convential synthesis ways for depositing titanate on surface of implants with complicated structures are chemistry methods e.g. hydrothermal
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treatment [7-10]. But it barely exhibits adequate bond force to the substrate, leading to exfoliation as a result of continuous movement between the bone and the metal implant. Subsequently it even results in implant loosening and infection [11]. Micro-arc oxidation (MAO), as a convenient technique of coating formation, has been used for years. It provides uniform porous surface structure with good adhesive strength to substrate [12],
and has been considered as a potential surface modification method for
tianium in biomedical area. Recently, it is reported MAO could benefits in the formation of titania-based coatings with excellent bioactivity in Ca and P contained electrolytes [13-18].
ACCEPTED MANUSCRIPT Moreover, MAO could also be successfully used for the fabrication of titanate structure such as BaTiO3 [19-22]. Thus, the fabrication of CaTiO3-contained titania coating using MAO
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comes to be an operable approach.
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Another noticeable advantage of applying MAO is that the surface bioactivity of Ti implant could be promoted by incorporating bioactive ions at the same step of
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synthesizing coating. Up to now, various additional elements (e.g. Zn, Ga, and Sr) have been used to enhance the coating biocompatibility [23, 24]. Besides those as reported elements,
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cerium is considered as one of the potential bioactive elements for the enhancement of implant bone healing. Several in vitro and in vivo studies have evidenced the benefits of
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cerium ion on bone metabolism [25, 26]. However, present studies barly focus on doping cerium in the MAO-formed coating and the influence of Ce on the inducing ability of apatite
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formation is still unclear.
With the objective of creating high bioactive coating on titanium surface with good
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adhesion, porous Ce-doped titania coatings containing CaTiO3 were firstly fabricated on Titanium substrate by MAO in this work, then the coating characteristics including micro-morphology, crystalline structure, chemical composition were evaluated by SEM, XRD and EDS, respectively. Thereafter, the bioactivity of the CaTiO3-contained coating and the positive effect of doping cerium were investigated by Simulated Body Fluid (SBF) immersion test.
2. Experimental 2.1 Materials
ACCEPTED MANUSCRIPT Commercially pure titanium (CP-Ti, Grade 4) specimens with dimensions of 5mm×10mm×2mm were used as the substrates in MAO process. The specimens were
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polished with up to #1200 SiC paper, then ultrasonically cleaned in aceton, ethanol and
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2.2 Micro arc oxidation (MAO) on Ti substrate
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deionized water, respectively. All the samples were air dried for MAO treatment.
For the MAO treatment, The WHD-30 type AC/DC MAO system with the above
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pretreated titanium specimens being the anode and the stainless steel sink as the cathode was used in this study. The CaTiO3-contained coating was fabricated using a solution including
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100g/L (CH3COO)2Ca·H2O and 50g/L KOH as the basic electrolyte. 0.5g/L Ce(NO3)3 was added to the electrolyte for doping Ce. The temperature of the electrolyte was kept
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constantly under 30°C using a water circulator. The MAO was performed at a constant voltage, frequency and duty circle of +300V (forward voltage), -40V(negative voltage),
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300Hz and 12% (contribution of positive pulse duration in a single cycle), respectively. The duration of MAO process was varied from 30~300s to investigate surface morphology characteristics.
2.3 Simulated Body Fluid (SBF) immersion test The bioactivity of the CaTiO3-contained coating was compared with that doped with Ce (namely the Ce-CaTiO3-contained coating) using an immersion test in SBF for 3~28d. For each immersion time, three samples from each kind of coating were employed. The SBF
ACCEPTED MANUSCRIPT solution was prepared according to Ref. [27], the ion concentrations (mM) of the solution are 142 Na+, 5 K+, 1.5 Mg2+, 2.5 Ca2+, 147.8 Cl-, 4.2HCO3- , 1HPO42- and 0.5SO42-, and
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buffered at pH 7.4. Each sample was immersed in a plastic vial containing 50 ml of SBF and
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was kept under static conditions inside a biological thermostat at 36.5°C. The SBF was refreshed every two days so that a lack of ions would not inhibit the apatite formation as
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suggested by Kokubo and Takadama [27]. Afterwards, the samples were removed from the
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SBF, washed with distilled water and then air dried for characterization.
2.4 Characterizations
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The MAO-formed samples were ultrasonically washed in acetone for 10 min and deionized water for 10 min, then dried at room temperature. The surface contact angles were
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measured using the liquid drop method on a contact angle goniometer (JC2000B, China). A 10μl droplet of deionized water was put onto the sample surface to measure the contact
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angle. More than three samples were tested to obtain the average values along with the standard deviation. The phase composition of the CaTiO3-contained coating formed by the MAO treatment was evaluated using X-ray diffraction (TF-XRD, RAD IIA, Rigaku, Japan) with a Cu Kα source operated at 40 kV and 25 mA. The surface morphology of coating was observed using field emission scanning electron microscope (SEM, Sirion 2000, FEI Co.) at 20 kV after the surfaces were coated with Pt and the chemical composition of the coating are investigated by energy dispersive X-ray spectroscopy (EDS; DX-4, Philips, Netherlands). After SBF immersion, the examinations were undertaken by XRD, EDS and SEM at the
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3. Results and discussion
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3.1 Influence of duration time on the microstructure of the Ce-CaTiO3-contained coatings
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Fig.1 presents the surface morphologies of the Ce-CaTiO3-contained coatings changes with the MAO duration varying from 30~300s. With the extending duration, the porous
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surfaces become rough as more Ca-contained matters are deposited via the efforts of
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combining electrophoresis and oxidation. Moreover, it is found that the maximum pore size increases with the processing time; however, the density of pores on the surfaces obviously
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decreases. When the duration rises to 300s, many open pores are filled with calcium
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compounds and most of them become invisible as shown in Fig. 1(g). It could be speculated
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from the high magnification images (Figs. 1(b), (d), (f) and (h)) that the decrease of pores on the surfaces is probably due to the gradual coverage of amorphous calcium compounds on
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porous coatings, according to previous results [15]. It has been reported in articles the surfaces with pore structures exhibit many advantages [28-31]. For example, open pore structures (especially interconnected pore structure) lead to long-term implants stability by providing a good biological fixation to bone; the pores at nano-micro scale benefit in the promoting of osteoblast attachment and proliferation; the existence of pores on surface could help in controlling delivery of drugs or cells. Thus, it is better to keep the pores open on the surface of Ti substrate and the results suggest the MAO duration should not be higher than 180s.
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Fig.1 Surface morphology of the Ce-CaTiO3-contained coatings prepared at various
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300s, low magnification; (h) 300s, high magnification.
The cross sectional micrographs of the Ce-CaTiO3-contained coatings fabricated at
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different duration (30~300s) and the coating thickness as a function of processing time are presented in Fig. 2. At first 60s, the coating grows very fast, then a less growth rate is
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observed during 60~300s. Actually, two reaction make effort on the formation of the MAO coating [32]: (1) the MAO treatment that creates the multiple porous layers, and (2) the
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chemical dissolution that etches the as-formed layer. At the beginning, the MAO treatment takes place more than the chemical dissolution, leading a very fast growth rate. As the
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coating thickness increases, more energy is needed to pass and form the plasma micro discharges. The lack of micro discharges population then decrease the MAO activity.
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Subsequently it decreases the growth rate of porous coating; meanwhile, the chemical dissolution acts and decreases the coating thickness. The competition of the two reaction finally results in the lower growth rate with increasing time [33].
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Fig.2 Cross sectional micrographs of the Ce-CaTiO3-contained coatings prepared at various durations: (a) 30s; (b) 60s; (c) 180s; (d) 300s and (e) the coating thickness as a function of processing time.
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The ratios of elements corresponding to CaTiO3 and Ce (O:Ti:Ca:Ce) from EDS are
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listed in Table 1. The Ca and O contents increase with extending MAO duration, indicating
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the increase of calcium compounds on surface, in accordance with SEM results. The relative content of chemical composition in the coatings prepared at different time is very variable.
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According to the chemical formulae of CaTiO3, the atomic ratio of Ti to Ca should be close to 1:1 exactly. However, these ratios are more than that and the content of Ca increases with
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the extending working duration. These results are probably responsible to the following points: (1) the thickness of MAO formed coatings are relative low and thus it results in more
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diffraction from Ti substrate appearing in the EDS; (2) a few content of amorphous phases
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e.g. CaO(TiO2)2 may be included in the as synthesized coating [14].
Table.1 EDS results of the Ce-CaTiO3-contained coatings at various duration time
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Element(at%)
O
Ti
Ca
Ce
Duration(s)
30
32.32
40.63
4.79
0.15
60
40.95
30.86
8.88
0.16
180
47.97
21.74
9.28
0.18
300
39.75
20.97
10.67
0.23
3.2 Influence of Ce doping on the microstructure of the CaTiO3-contained coating
ACCEPTED MANUSCRIPT Fig. 3 shows SEM photographs of the Ce-undoped sample prepared by MAO (+300V (forward voltage), -40V(negative voltage), 300Hz and 12%, oxidized for 180s). The nanorod
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and nanoflake structures containing calcium are observed on the porous surface, especially,
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most of them are found in or/and near the pores. In comparison with the samples prepared at the same condition (Fig. 1(e) and (f) vs Fig. 3(a) and (b)), the addition of Ce causes a drastic
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change of surface as the Ce-doped surface is rougher than Ce-undoped one. It is also noticed the Ce-undoped coating keeps more open pores, which could probably benefit in bone
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ingrowth [34]. While some pores of the Ce-doped coatings disappear due to the growth of calcium compounds on the wall of pore, which could also be favorable to its bioactivity in
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vitro [35] and in vivo [36, 37]. Moreover, the cross sectional micrograph of Fig. 3(c) indicates the similar thickness of coating is obtained at the same condition without adding
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Ce. Theoretically, the addition of cerium ions in electrolyte could result in accelerating velocity of ion exchange and improving transport capacity in unit interval. Thus, at the same
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duration, the participation of cerium ions in the electrolyte benefits in growing calcium compounds on the surface of porous MAO-formed coating. It is also evidenced by the EDS spectra that the intensity of Ca peak is higher in the Ce-doped coating, as shown in Fig. 4. Moreover, the Ce-doped coating exhibits better hydrophilicity (Fig. 4) as the Ce-intercalated titanate owns higher catalytic activity of producing polar groups (e.g. OH-) [38].
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Fig. 3 Surface morphology of porous Ce-undoped titania coating containing CaTiO3: (a) low
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magnification; (b) high magnification; (c) cross section.
Fig. 4 EDS spectra and water contact angle images of two porous titania coatings containing CaTiO3 (processing time: 180s): (a) undoped with Ce; (b) doped with Ce.
Fig. 5 shows the XRD results of the Ce-undoped and Ce-doped coatings on CP-Ti
ACCEPTED MANUSCRIPT prepared by MAO. The XRD peaks are assigned to the crystalline structures of titanium metal, CaTiO3 (perovskite) and titanium oxide (anatase and rutile), however the appearance
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of the noise baseline indicates the exitence of amorphous phases. The presence of titanium
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peaks on the XRD patterns is most likely due to the penetration of the X-rays beyond the oxide layers, in agreement with previous results [39, 40]. TiO2 is generally known to be a
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precursor for synthesizing the titanates by MAO [15]. The passive coating of TiO2 on the Ti substrate is critical for followed sparking. Initially, the dense passive coating of amorphous
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TiOx are formed on Ti substrate in alkaline solution [41], they immediately transforms into TiO2 crystal at higher voltage; then the increased voltage results in TiO2 dielectric to
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breakdown and finally causes to spark at the solid-liquid interface. In the center of sparking zone, the temperature is estimated to be high enough (3000~10,000K [42]) for melting the
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oxide [21] and could benefit in incorporation of Ca2+ and OH− from the ionized electrolyte into the micro arc oxidzed coating. This process thus gains the possibility of creating a
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condition to meet the requirement of the following reactions [15, 35]: (1) (2) As a result of the above reactions, the primary CaTiO3 are formed in the center of sparking zone and cooled rapidly by electrolyte. During this cooling process, amorphous phases are formed, meanwhile some high-temperature phases of CaTiO3 reserve, e.g. perovskite phase. The TiO2 coating mixed with CaTiO3 grows layer by layer when MAO process continues. The sparks that take place on the interface between the oxide and the
ACCEPTED MANUSCRIPT electrolyte generate massive heat, and then the energy are transferred to the bottom of as-formed coating. As a consequence, the bottom layers of the coating are subject to
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heat-treatment, and would be further crystallized. This interprets the prensence of the
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amorphous phases and crystallized phases of CaTiO3 in the as-formed coating in both XRD spectra. Moreover, the existence of TiO2 peaks indicats the improvement of voltage may
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benefit in enhancing the crystallinity of CaTiO3 due to the combination of reaction and thermal annealing [43]. It is also noticed that the higher crystallinity is obtained in the
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Ce-doped coating. During MAO process, the existence of numerous free cerium ions in electrolyte is likey to increase the electrical current that passes the electrochemical cells,
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subsequently higher number or/and larger size of sparks are generated and provide more heat for crystallizing amorphous CaTiO3 phase. However, no crystal phases of Ce related
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compounds are found in Fig. 5(b), which is probably due to the low content of Ce in coating. A similar result is obtained by the authors in Ref. [44, 45]. According to the study of Cai et
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al. [46], it could be resonably speculated that the form of Ce compound in the MAO fabricated coating is CeO2 .
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Fig. 5 XRD spectra of the two porous titania coatings containing CaTiO3 (processing time:
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180s): (a) undoped with Ce; (b) doped with Ce.
3.3 Bioactivity of the CaTiO3/Ce-CaTiO3 contained coatings in SBF
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It is well known that the capacity of biomaterials to form apatite from SBF could reflect their potential for bonding with bone. Fig. 6 indicates the inducing process of the formation of bioactive apatite on the MAO-formed surfaces. Most of the calcium-contained structures disappear, only a few nanoflake structures are still observed in some areas of coating surface within 3 days’ immersion (Fig. 6(a)), implying the dissolution of CaTiO3 on surface into SBF solution. As immersion time increases, the CaTiO3 structures totally dissolve. Instead, spherical-like structures appear and they are centralized at pores and the area nearby pores. This kind of structures covers partial coating surface with some visible pores after 7d immersion in SBF (Fig. 6(b)). With the increase of immersion time up to 14d, spherical-like
ACCEPTED MANUSCRIPT structures continue growing. They cover the whole coating surface as MAO-formed pores could hardly be seen in Fig. 6(c). Then, the surface with spherical-like structures becomes a
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microscale are also observed after 28d immersion (Fig. 6(d)).
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smooth film with denser grain-like structures. Besides, the huge island-like structures at
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Fig. 6 SEM morphology of the samples with porous CaTiO3-contained coating (processing time: 180s) immersed in SBF at 36.5 °C for 3d (a), 7d (b), 14d (c) and 28d (d); in
ACCEPTED MANUSCRIPT comparison, the samples with porous Ce-CaTiO3-contained coating are immersed under
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identical conditions for 7d (e) and 28d (f).
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During the immersion of a MAO-formed porous titania coating in SBF, OH− groups tending to absorb cations are produced by the bioactive TiO2. Then, it leads to the decrease
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of the surface potential described by Pauling's electro-neutrality rule [20], so as to induce Ca-P nucleation because of the negatively charged interface [18]. Thus, Ca2+ and PO43- ions
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concentrations near the surface and the heterogeneous nucleation of apatite are considered as the key factors that affect the formation process of apatite crystals on the oxide coating
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surface. The formation of stoichiometric apatite from the calcium, phosphate and hydroxyl ions can be expressed as follows:
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10Ca 2+ + 6PO4 3－ + 2OH － → Ca10 ( PO4 ) 6 (OH ) 2
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Upon soaking the samples the CaTiO3-contained coating in SBF solution, calcium titanates
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are slightly hydrolyzed to form Ca2+ and OH- ions in suspension as a result of immersion, and then lead to an increase of ions amount at the liquid/solid interface according to the following equations [35]: (4) Consequently, the enrichment of Ca2+ and OH- ions increases the local supersaturation degree with respect to apatite and its ionic activity product according to Equation 3. Besides, the hydrolysis of the calcium titanate could also produce lots of Ti-OH groups as shown in equation 4 [40]. According to literatures [47-49], the Ti-OH groups could trigger the
ACCEPTED MANUSCRIPT absorption of PO43- by incorporating Ca2+ ions in the SBF solution, which is contributed to the electrostatic potential interaction. As a result, this absorption helps the apatite nuclei to
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be fixed on the porous coating surface.
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Moreover, the spatial arrangement of the surface groups plays a very important role in the formation of apatite. It is noticed that the main phase of CaTiO3 formed during MAO
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process is perovskite phase, which has been reported to provide good sites for the epitaxial
benefits in the formation of apatite.
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adsorptions of Ca and P [47, 50, 51]. Thus, the existence of CaTiO3 on the coating surface
In order to compare the effort of adding Ce, the coatings doped with Ce are also
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immersed in SBF for 7d and 28d under the identical conditions, respectively. The results apparently show there are more precipitates formed on the surface in the early immersion
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stage, it is noticed that island-like precipitates appear on the surface when the sample is only immersed for 7d (Fig. 6(e)). As the immersion time is extended to 28d, some micro-cracks
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are observed on as-deposited apatite film (Fig. 6(f)). It can be concluded that the formation of the micro-cracks is due to the enhancement of the coating thickness, and the difference in thermal coefficient of expansion between the substrate and surface layer during the drying process [52]. Fig. 7 shows the XRD patterns of two kinds of porous titania coatings containing CaTiO3 immersed for 28 days in SBF, respectively. The diffraction intensity of the apatite precipitates on the Ce-undoped coating is relative weak compared with that of Ce-doped sample (Fig. 7(a)), suggesting a poor crystallinity of apatite. The presence of stronger apatite
ACCEPTED MANUSCRIPT peaks around 26° and 32° are observed in Fig. 7(b) [40] indicates the Ce-doped coating has better apatite inducing ability than Ce-undoped coating at the same immersion duration.
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However, the widen peaks of apatite phase indicates the crystalline is not very high. In
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addition, the diffraction peaks of titania are observed on both samples’ surface after SBF immersion, while those of calcium titanate phase decline, suggesting a considerable
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dissolution of the calcium titanates into suspension.
As it is known to all, the doped Ce could result in more OH- due to its enhancive
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catalytic activity [53], which would essentially lead to preferable nucleation of apatite. It could be one of the reasons that explain better apatite inducing ability when Ce is doped in
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coating. Moreover, the formation process of apatite crystals on the titanate-contained film is affected by not only the nucleation of apatite, but also the diffusion of Ca2+ ions from the
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inner layer towards the film surface. Thus, the growth process of apatite is also dependent on the wettability between the sample surface and the SBF. As shown in Fig. 4, the WCA of the
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Ce-doped surface is lower than that of the Ce-undoped surface. Thus, the better hydrophilicity of Ce-doped surface could benefit in apatite formation, agreeing to previous speculations.
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Fig. 7 XRD spectra of the two porous titania coatings containing CaTiO3 (processing time:
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180s) immersed in SBF for 28d: (a) undoped with Ce; (b) doped with Ce.
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Figs. 8(a)~(d) shows the EDS spectra of porous titania coatings containing CaTiO3 immersed for 3, 7, 14 and 28 days in SBF, respectively. It is interesting to observe the Ca content on coating surface firstly decreases from 9.61% to 8.58% (3~7d) and then increases to 15.11% with increasing immersion duration (7~28d). Compared with the Ce doped coating immersed for the same duration (7d and 28d). The higher content of P, corresponding to the amount of apatite on surface, is observed on the surface of Ce doped coating at early immersion stage of 7 days (Fig. 8(b) vs Fig. 8(e)), in agreement with SEM results of Fig. 6. According to XRD results of Fig. 7, the Ce doped coating surface exhibits higher P content (Fig. 8(d) vs
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Fig. 8(f)), indicating a better apatite formation at long-term immersion of 28 days.
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Fig. 8 EDS spectra of the samples with porous CaTiO3-contained coating immersed in SBF at 36.5 °C for 3d (a), 7d (b), 14d (c) and 28d (d); in comparison, the samples with porous Ce-CaTiO3-contained coating are immersed under identical conditions for 7d (e) and 28d (f).
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Based on the EDS results, the change of Ca/P ratio from the samples immersed for
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different time is shown in Fig. 9. After the immersion of 3d,7d,14d and 28d, the Ca/P ratio of
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the samples with CaTiO3-contained coating exhibits a drastic decline from 34.32 to 1.50 and is slightly smaller than that of Ca/P ratio of apatite (Ca/P = 1.67 [35]). On one hand, the ratio
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decreases due to the PO43- ions depositing on the coating surface; on the other hand, it could be contributed to that calcium is leached from calcium titanate at 36.5°C as a function of
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immersion time, in accordance with the XRD results shown in Fig. 7. The Ca amount contained in surface layer after early immersion stage of 3~7 days (at%Ca3d = 9.61 and
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at%Ca7d = 8.58) is lower than that of the original MAO formed coating (at%Caoriginal = 11.82), which could attributed to the hydrolysis of CaTiO3 at the beginning of immersion.
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Thereafter, higher amounts of Ca (at%Ca14d = 12.89 and at%Ca28d = 15.11) and P (at%P14d = 3.08 and at%P28d = 10.06) are obtained during the long term immersion of 14~28 days, in
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agreement with our previous speculation that the existence of CaTiO3 benefits in producing OH- and subsequently leads to better apatite formation. In comparison, while the Ce-doped sample are immersed in SBF for 7d and 28d, the Ca/P ratios of sample surface are 1.41 and 1.33, respectively, which are also closed to that of apatite. Moreover, the lower Ca/P ratio is observed when the Ce-doped samples are immersed for 7d (c.f. Fig. 9) due to a faster increase of PO43- on coating surface. It is also noticed that the higher Ca content is seen on Ce doped surface. The result indicates more apatite are formed as the dissolution of CaTiO3 is not as strong as that occurs on Ce undoped
ACCEPTED MANUSCRIPT coating (Ca7d Ce-doped = 9.78 vs Ca7d Ce-undoped = 8.58). Concerning the formation of apatite in SBF (equation 3), it could be reasonably speculated that the existence of Cerium ions could
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benefit in producing OH- during immersion, which is agreed with previous report [53].
Fig. 9 The Ca/P ratios of the samples with porous CaTiO3-contained coating immersed in
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SBF at 36.5 °C for 3d, 7d, 14d and 28d; in comparison, the samples with porous Ce-CaTiO3-contained coating are immersed under identical conditions for 7d and 28d. 4. Conclusions Porous CaTiO3-contained titania coatings doped with Ce for biomedical applications are successfully fabricated through a MAO process in this work. The results reveal that as synthesized porous oxide layers are consisted of not only anatase and rutile TiO2 but also CaTiO3 phase. The hydrolysis of CaTiO3 could increase the local supersaturation degree at the liquid/solid interface during SBF immersion; on the other hand, the lattice configuration of specific crystal plane of perovskite CaTiO3 benefits in epitaxial adsorption of Ca and P
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formation compared to Ce-undoped samples, since the more hydroxyl functionalized surface
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could be formed on the Ce-doped hydrophilic coatings under the identical immersion
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conditions.
Acknowledgements
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This work was jointly supported by the National Natural Science Foundation of China (Grant No. 31570961) and Jiangsu key laboratory for advanced metallic materials
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References
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ACCEPTED MANUSCRIPT Highlights
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Porous Ce-doped titania coatings containing CaTiO3 are fabricated by one-step MAO process. The existence of CaTiO3 leads to good nucleation and growth of apatite. Doping Ce benefits in forming apatite.
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S0257-8972(16)30470-4 doi: 10.1016/j.surfcoat.2016.05.077 SCT 21232
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
11 February 2016 27 May 2016 27 May 2016
Please cite this article as: X. Rao, ¡.!.–[.I.N.S.].[.C.L.].–¿.C.L.¡.!.–[./.I.N.S.].–¿. Chu, Q. Sun, Synthesis of porous Ce-doped titania coating containing CaTiO3 by MAO and its apatite inducing ability, Surface & Coatings Technology (2016), doi: 10.1016/j.surfcoat.2016.05.077
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ACCEPTED MANUSCRIPT Synthesis of porous Ce-doped titania coating containing CaTiO3 by MAO and its apatite inducing ability
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X. Rao, C. L. Chu*, Q. Sun
School of Materials Science and Engineering and Jiangsu Key Laboratory for Advanced
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Metallic Materials, Southeast University, Nanjing, 211189, China
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Abstract
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Bioactive porous CaTiO3-contained titania coatings doped with cerium were prepared on CP-Ti surfaces by micro arc oxidation (MAO) process. The surface morphologies,
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chemical composition, wettability and the phase compositions of coatings were investigated
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by scanning electron microscopy (SEM), energy dispersion X-ray spectrometry (EDS),
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contact angle goniometer and X-ray diffraction (XRD), respectively. The results showed the surface phase compositions of the as-synthesized coatings are anatase, rutile TiO2 and
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perovskite CaTiO3 phases, as well as a few amorphous phases. The participation of Ce in the electrolyte during MAO is evidenced to benefit in crystallizing CaTiO3 phase and enhancing the coating surface hydrophilicity. The results of simulated body fluid (SBF) immersion experiments indicate that the hydrolysis of CaTiO3 during immersion could enhance the nucleation and growth of apatite. In comparison, the porous Ce-doped titania coating containing CaTiO3 exhibits better bioactivity as more apatites were observed at the same immersion duration. Keywords: Porous titania coating; CaTiO3; Ce-doped; Micro arc oxidation; Bioactivity
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1. Introduction
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Titanium and its alloys are frequently used in orthopedic implants and bone replacements due to their excellent mechanical strength, high chemical stability, and
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attractive biocompatibility. However, titanium is a bio-inert material that cannot directly bond to bone immediately after implantation [1, 2]. Hence, it is necessary to improve the
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bioactivity of Ti substrate to gain wider acceptance in the biomedical field. Titanate structures, which not only exhibit good biocompatibility but also induce the formation of
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bone-like apatites, are receiving considerable attentions among bioactive materials and that can be coated on artificial joints or dental implants to stimulate new bone growth and
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enhance osseointegration [3-6]. The convential synthesis ways for depositing titanate on surface of implants with complicated structures are chemistry methods e.g. hydrothermal
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treatment [7-10]. But it barely exhibits adequate bond force to the substrate, leading to exfoliation as a result of continuous movement between the bone and the metal implant. Subsequently it even results in implant loosening and infection [11]. Micro-arc oxidation (MAO), as a convenient technique of coating formation, has been used for years. It provides uniform porous surface structure with good adhesive strength to substrate [12],
and has been considered as a potential surface modification method for
tianium in biomedical area. Recently, it is reported MAO could benefits in the formation of titania-based coatings with excellent bioactivity in Ca and P contained electrolytes [13-18].
ACCEPTED MANUSCRIPT Moreover, MAO could also be successfully used for the fabrication of titanate structure such as BaTiO3 [19-22]. Thus, the fabrication of CaTiO3-contained titania coating using MAO
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comes to be an operable approach.
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Another noticeable advantage of applying MAO is that the surface bioactivity of Ti implant could be promoted by incorporating bioactive ions at the same step of
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synthesizing coating. Up to now, various additional elements (e.g. Zn, Ga, and Sr) have been used to enhance the coating biocompatibility [23, 24]. Besides those as reported elements,
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cerium is considered as one of the potential bioactive elements for the enhancement of implant bone healing. Several in vitro and in vivo studies have evidenced the benefits of
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cerium ion on bone metabolism [25, 26]. However, present studies barly focus on doping cerium in the MAO-formed coating and the influence of Ce on the inducing ability of apatite
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formation is still unclear.
With the objective of creating high bioactive coating on titanium surface with good
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adhesion, porous Ce-doped titania coatings containing CaTiO3 were firstly fabricated on Titanium substrate by MAO in this work, then the coating characteristics including micro-morphology, crystalline structure, chemical composition were evaluated by SEM, XRD and EDS, respectively. Thereafter, the bioactivity of the CaTiO3-contained coating and the positive effect of doping cerium were investigated by Simulated Body Fluid (SBF) immersion test.
2. Experimental 2.1 Materials
ACCEPTED MANUSCRIPT Commercially pure titanium (CP-Ti, Grade 4) specimens with dimensions of 5mm×10mm×2mm were used as the substrates in MAO process. The specimens were
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polished with up to #1200 SiC paper, then ultrasonically cleaned in aceton, ethanol and
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2.2 Micro arc oxidation (MAO) on Ti substrate
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deionized water, respectively. All the samples were air dried for MAO treatment.
For the MAO treatment, The WHD-30 type AC/DC MAO system with the above
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pretreated titanium specimens being the anode and the stainless steel sink as the cathode was used in this study. The CaTiO3-contained coating was fabricated using a solution including
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100g/L (CH3COO)2Ca·H2O and 50g/L KOH as the basic electrolyte. 0.5g/L Ce(NO3)3 was added to the electrolyte for doping Ce. The temperature of the electrolyte was kept
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constantly under 30°C using a water circulator. The MAO was performed at a constant voltage, frequency and duty circle of +300V (forward voltage), -40V(negative voltage),
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300Hz and 12% (contribution of positive pulse duration in a single cycle), respectively. The duration of MAO process was varied from 30~300s to investigate surface morphology characteristics.
2.3 Simulated Body Fluid (SBF) immersion test The bioactivity of the CaTiO3-contained coating was compared with that doped with Ce (namely the Ce-CaTiO3-contained coating) using an immersion test in SBF for 3~28d. For each immersion time, three samples from each kind of coating were employed. The SBF
ACCEPTED MANUSCRIPT solution was prepared according to Ref. [27], the ion concentrations (mM) of the solution are 142 Na+, 5 K+, 1.5 Mg2+, 2.5 Ca2+, 147.8 Cl-, 4.2HCO3- , 1HPO42- and 0.5SO42-, and
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buffered at pH 7.4. Each sample was immersed in a plastic vial containing 50 ml of SBF and
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was kept under static conditions inside a biological thermostat at 36.5°C. The SBF was refreshed every two days so that a lack of ions would not inhibit the apatite formation as
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suggested by Kokubo and Takadama [27]. Afterwards, the samples were removed from the
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SBF, washed with distilled water and then air dried for characterization.
2.4 Characterizations
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The MAO-formed samples were ultrasonically washed in acetone for 10 min and deionized water for 10 min, then dried at room temperature. The surface contact angles were
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measured using the liquid drop method on a contact angle goniometer (JC2000B, China). A 10μl droplet of deionized water was put onto the sample surface to measure the contact
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angle. More than three samples were tested to obtain the average values along with the standard deviation. The phase composition of the CaTiO3-contained coating formed by the MAO treatment was evaluated using X-ray diffraction (TF-XRD, RAD IIA, Rigaku, Japan) with a Cu Kα source operated at 40 kV and 25 mA. The surface morphology of coating was observed using field emission scanning electron microscope (SEM, Sirion 2000, FEI Co.) at 20 kV after the surfaces were coated with Pt and the chemical composition of the coating are investigated by energy dispersive X-ray spectroscopy (EDS; DX-4, Philips, Netherlands). After SBF immersion, the examinations were undertaken by XRD, EDS and SEM at the
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3. Results and discussion
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3.1 Influence of duration time on the microstructure of the Ce-CaTiO3-contained coatings
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Fig.1 presents the surface morphologies of the Ce-CaTiO3-contained coatings changes with the MAO duration varying from 30~300s. With the extending duration, the porous
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surfaces become rough as more Ca-contained matters are deposited via the efforts of
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combining electrophoresis and oxidation. Moreover, it is found that the maximum pore size increases with the processing time; however, the density of pores on the surfaces obviously
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decreases. When the duration rises to 300s, many open pores are filled with calcium
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compounds and most of them become invisible as shown in Fig. 1(g). It could be speculated
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from the high magnification images (Figs. 1(b), (d), (f) and (h)) that the decrease of pores on the surfaces is probably due to the gradual coverage of amorphous calcium compounds on
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porous coatings, according to previous results [15]. It has been reported in articles the surfaces with pore structures exhibit many advantages [28-31]. For example, open pore structures (especially interconnected pore structure) lead to long-term implants stability by providing a good biological fixation to bone; the pores at nano-micro scale benefit in the promoting of osteoblast attachment and proliferation; the existence of pores on surface could help in controlling delivery of drugs or cells. Thus, it is better to keep the pores open on the surface of Ti substrate and the results suggest the MAO duration should not be higher than 180s.
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(a)
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(e)
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Fig.1 Surface morphology of the Ce-CaTiO3-contained coatings prepared at various
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300s, low magnification; (h) 300s, high magnification.
The cross sectional micrographs of the Ce-CaTiO3-contained coatings fabricated at
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different duration (30~300s) and the coating thickness as a function of processing time are presented in Fig. 2. At first 60s, the coating grows very fast, then a less growth rate is
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observed during 60~300s. Actually, two reaction make effort on the formation of the MAO coating [32]: (1) the MAO treatment that creates the multiple porous layers, and (2) the
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chemical dissolution that etches the as-formed layer. At the beginning, the MAO treatment takes place more than the chemical dissolution, leading a very fast growth rate. As the
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coating thickness increases, more energy is needed to pass and form the plasma micro discharges. The lack of micro discharges population then decrease the MAO activity.
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Subsequently it decreases the growth rate of porous coating; meanwhile, the chemical dissolution acts and decreases the coating thickness. The competition of the two reaction finally results in the lower growth rate with increasing time [33].
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(a)
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Fig.2 Cross sectional micrographs of the Ce-CaTiO3-contained coatings prepared at various durations: (a) 30s; (b) 60s; (c) 180s; (d) 300s and (e) the coating thickness as a function of processing time.
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The ratios of elements corresponding to CaTiO3 and Ce (O:Ti:Ca:Ce) from EDS are
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listed in Table 1. The Ca and O contents increase with extending MAO duration, indicating
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the increase of calcium compounds on surface, in accordance with SEM results. The relative content of chemical composition in the coatings prepared at different time is very variable.
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According to the chemical formulae of CaTiO3, the atomic ratio of Ti to Ca should be close to 1:1 exactly. However, these ratios are more than that and the content of Ca increases with
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the extending working duration. These results are probably responsible to the following points: (1) the thickness of MAO formed coatings are relative low and thus it results in more
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diffraction from Ti substrate appearing in the EDS; (2) a few content of amorphous phases
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e.g. CaO(TiO2)2 may be included in the as synthesized coating [14].
Table.1 EDS results of the Ce-CaTiO3-contained coatings at various duration time
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Element(at%)
O
Ti
Ca
Ce
Duration(s)
30
32.32
40.63
4.79
0.15
60
40.95
30.86
8.88
0.16
180
47.97
21.74
9.28
0.18
300
39.75
20.97
10.67
0.23
3.2 Influence of Ce doping on the microstructure of the CaTiO3-contained coating
ACCEPTED MANUSCRIPT Fig. 3 shows SEM photographs of the Ce-undoped sample prepared by MAO (+300V (forward voltage), -40V(negative voltage), 300Hz and 12%, oxidized for 180s). The nanorod
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and nanoflake structures containing calcium are observed on the porous surface, especially,
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most of them are found in or/and near the pores. In comparison with the samples prepared at the same condition (Fig. 1(e) and (f) vs Fig. 3(a) and (b)), the addition of Ce causes a drastic
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change of surface as the Ce-doped surface is rougher than Ce-undoped one. It is also noticed the Ce-undoped coating keeps more open pores, which could probably benefit in bone
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ingrowth [34]. While some pores of the Ce-doped coatings disappear due to the growth of calcium compounds on the wall of pore, which could also be favorable to its bioactivity in
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vitro [35] and in vivo [36, 37]. Moreover, the cross sectional micrograph of Fig. 3(c) indicates the similar thickness of coating is obtained at the same condition without adding
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Ce. Theoretically, the addition of cerium ions in electrolyte could result in accelerating velocity of ion exchange and improving transport capacity in unit interval. Thus, at the same
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duration, the participation of cerium ions in the electrolyte benefits in growing calcium compounds on the surface of porous MAO-formed coating. It is also evidenced by the EDS spectra that the intensity of Ca peak is higher in the Ce-doped coating, as shown in Fig. 4. Moreover, the Ce-doped coating exhibits better hydrophilicity (Fig. 4) as the Ce-intercalated titanate owns higher catalytic activity of producing polar groups (e.g. OH-) [38].
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Fig. 3 Surface morphology of porous Ce-undoped titania coating containing CaTiO3: (a) low
(b)
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magnification; (b) high magnification; (c) cross section.
Fig. 4 EDS spectra and water contact angle images of two porous titania coatings containing CaTiO3 (processing time: 180s): (a) undoped with Ce; (b) doped with Ce.
Fig. 5 shows the XRD results of the Ce-undoped and Ce-doped coatings on CP-Ti
ACCEPTED MANUSCRIPT prepared by MAO. The XRD peaks are assigned to the crystalline structures of titanium metal, CaTiO3 (perovskite) and titanium oxide (anatase and rutile), however the appearance
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of the noise baseline indicates the exitence of amorphous phases. The presence of titanium
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peaks on the XRD patterns is most likely due to the penetration of the X-rays beyond the oxide layers, in agreement with previous results [39, 40]. TiO2 is generally known to be a
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precursor for synthesizing the titanates by MAO [15]. The passive coating of TiO2 on the Ti substrate is critical for followed sparking. Initially, the dense passive coating of amorphous
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TiOx are formed on Ti substrate in alkaline solution [41], they immediately transforms into TiO2 crystal at higher voltage; then the increased voltage results in TiO2 dielectric to
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breakdown and finally causes to spark at the solid-liquid interface. In the center of sparking zone, the temperature is estimated to be high enough (3000~10,000K [42]) for melting the
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oxide [21] and could benefit in incorporation of Ca2+ and OH− from the ionized electrolyte into the micro arc oxidzed coating. This process thus gains the possibility of creating a
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condition to meet the requirement of the following reactions [15, 35]: (1) (2) As a result of the above reactions, the primary CaTiO3 are formed in the center of sparking zone and cooled rapidly by electrolyte. During this cooling process, amorphous phases are formed, meanwhile some high-temperature phases of CaTiO3 reserve, e.g. perovskite phase. The TiO2 coating mixed with CaTiO3 grows layer by layer when MAO process continues. The sparks that take place on the interface between the oxide and the
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heat-treatment, and would be further crystallized. This interprets the prensence of the
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amorphous phases and crystallized phases of CaTiO3 in the as-formed coating in both XRD spectra. Moreover, the existence of TiO2 peaks indicats the improvement of voltage may
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benefit in enhancing the crystallinity of CaTiO3 due to the combination of reaction and thermal annealing [43]. It is also noticed that the higher crystallinity is obtained in the
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Ce-doped coating. During MAO process, the existence of numerous free cerium ions in electrolyte is likey to increase the electrical current that passes the electrochemical cells,
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subsequently higher number or/and larger size of sparks are generated and provide more heat for crystallizing amorphous CaTiO3 phase. However, no crystal phases of Ce related
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compounds are found in Fig. 5(b), which is probably due to the low content of Ce in coating. A similar result is obtained by the authors in Ref. [44, 45]. According to the study of Cai et
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al. [46], it could be resonably speculated that the form of Ce compound in the MAO fabricated coating is CeO2 .
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Fig. 5 XRD spectra of the two porous titania coatings containing CaTiO3 (processing time:
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180s): (a) undoped with Ce; (b) doped with Ce.
3.3 Bioactivity of the CaTiO3/Ce-CaTiO3 contained coatings in SBF
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It is well known that the capacity of biomaterials to form apatite from SBF could reflect their potential for bonding with bone. Fig. 6 indicates the inducing process of the formation of bioactive apatite on the MAO-formed surfaces. Most of the calcium-contained structures disappear, only a few nanoflake structures are still observed in some areas of coating surface within 3 days’ immersion (Fig. 6(a)), implying the dissolution of CaTiO3 on surface into SBF solution. As immersion time increases, the CaTiO3 structures totally dissolve. Instead, spherical-like structures appear and they are centralized at pores and the area nearby pores. This kind of structures covers partial coating surface with some visible pores after 7d immersion in SBF (Fig. 6(b)). With the increase of immersion time up to 14d, spherical-like
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microscale are also observed after 28d immersion (Fig. 6(d)).
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smooth film with denser grain-like structures. Besides, the huge island-like structures at
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Fig. 6 SEM morphology of the samples with porous CaTiO3-contained coating (processing time: 180s) immersed in SBF at 36.5 °C for 3d (a), 7d (b), 14d (c) and 28d (d); in
ACCEPTED MANUSCRIPT comparison, the samples with porous Ce-CaTiO3-contained coating are immersed under
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identical conditions for 7d (e) and 28d (f).
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During the immersion of a MAO-formed porous titania coating in SBF, OH− groups tending to absorb cations are produced by the bioactive TiO2. Then, it leads to the decrease
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of the surface potential described by Pauling's electro-neutrality rule [20], so as to induce Ca-P nucleation because of the negatively charged interface [18]. Thus, Ca2+ and PO43- ions
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concentrations near the surface and the heterogeneous nucleation of apatite are considered as the key factors that affect the formation process of apatite crystals on the oxide coating
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surface. The formation of stoichiometric apatite from the calcium, phosphate and hydroxyl ions can be expressed as follows:
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10Ca 2+ + 6PO4 3－ + 2OH － → Ca10 ( PO4 ) 6 (OH ) 2
(3)
Upon soaking the samples the CaTiO3-contained coating in SBF solution, calcium titanates
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are slightly hydrolyzed to form Ca2+ and OH- ions in suspension as a result of immersion, and then lead to an increase of ions amount at the liquid/solid interface according to the following equations [35]: (4) Consequently, the enrichment of Ca2+ and OH- ions increases the local supersaturation degree with respect to apatite and its ionic activity product according to Equation 3. Besides, the hydrolysis of the calcium titanate could also produce lots of Ti-OH groups as shown in equation 4 [40]. According to literatures [47-49], the Ti-OH groups could trigger the
ACCEPTED MANUSCRIPT absorption of PO43- by incorporating Ca2+ ions in the SBF solution, which is contributed to the electrostatic potential interaction. As a result, this absorption helps the apatite nuclei to
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be fixed on the porous coating surface.
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Moreover, the spatial arrangement of the surface groups plays a very important role in the formation of apatite. It is noticed that the main phase of CaTiO3 formed during MAO
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process is perovskite phase, which has been reported to provide good sites for the epitaxial
benefits in the formation of apatite.
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adsorptions of Ca and P [47, 50, 51]. Thus, the existence of CaTiO3 on the coating surface
In order to compare the effort of adding Ce, the coatings doped with Ce are also
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immersed in SBF for 7d and 28d under the identical conditions, respectively. The results apparently show there are more precipitates formed on the surface in the early immersion
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stage, it is noticed that island-like precipitates appear on the surface when the sample is only immersed for 7d (Fig. 6(e)). As the immersion time is extended to 28d, some micro-cracks
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are observed on as-deposited apatite film (Fig. 6(f)). It can be concluded that the formation of the micro-cracks is due to the enhancement of the coating thickness, and the difference in thermal coefficient of expansion between the substrate and surface layer during the drying process [52]. Fig. 7 shows the XRD patterns of two kinds of porous titania coatings containing CaTiO3 immersed for 28 days in SBF, respectively. The diffraction intensity of the apatite precipitates on the Ce-undoped coating is relative weak compared with that of Ce-doped sample (Fig. 7(a)), suggesting a poor crystallinity of apatite. The presence of stronger apatite
ACCEPTED MANUSCRIPT peaks around 26° and 32° are observed in Fig. 7(b) [40] indicates the Ce-doped coating has better apatite inducing ability than Ce-undoped coating at the same immersion duration.
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However, the widen peaks of apatite phase indicates the crystalline is not very high. In
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addition, the diffraction peaks of titania are observed on both samples’ surface after SBF immersion, while those of calcium titanate phase decline, suggesting a considerable
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dissolution of the calcium titanates into suspension.
As it is known to all, the doped Ce could result in more OH- due to its enhancive
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catalytic activity [53], which would essentially lead to preferable nucleation of apatite. It could be one of the reasons that explain better apatite inducing ability when Ce is doped in
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coating. Moreover, the formation process of apatite crystals on the titanate-contained film is affected by not only the nucleation of apatite, but also the diffusion of Ca2+ ions from the
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inner layer towards the film surface. Thus, the growth process of apatite is also dependent on the wettability between the sample surface and the SBF. As shown in Fig. 4, the WCA of the
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Ce-doped surface is lower than that of the Ce-undoped surface. Thus, the better hydrophilicity of Ce-doped surface could benefit in apatite formation, agreeing to previous speculations.
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Fig. 7 XRD spectra of the two porous titania coatings containing CaTiO3 (processing time:
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180s) immersed in SBF for 28d: (a) undoped with Ce; (b) doped with Ce.
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Figs. 8(a)~(d) shows the EDS spectra of porous titania coatings containing CaTiO3 immersed for 3, 7, 14 and 28 days in SBF, respectively. It is interesting to observe the Ca content on coating surface firstly decreases from 9.61% to 8.58% (3~7d) and then increases to 15.11% with increasing immersion duration (7~28d). Compared with the Ce doped coating immersed for the same duration (7d and 28d). The higher content of P, corresponding to the amount of apatite on surface, is observed on the surface of Ce doped coating at early immersion stage of 7 days (Fig. 8(b) vs Fig. 8(e)), in agreement with SEM results of Fig. 6. According to XRD results of Fig. 7, the Ce doped coating surface exhibits higher P content (Fig. 8(d) vs
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(a)
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(b)
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Fig. 8(f)), indicating a better apatite formation at long-term immersion of 28 days.
(d)
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(c)
(e)
(f)
Fig. 8 EDS spectra of the samples with porous CaTiO3-contained coating immersed in SBF at 36.5 °C for 3d (a), 7d (b), 14d (c) and 28d (d); in comparison, the samples with porous Ce-CaTiO3-contained coating are immersed under identical conditions for 7d (e) and 28d (f).
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Based on the EDS results, the change of Ca/P ratio from the samples immersed for
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different time is shown in Fig. 9. After the immersion of 3d,7d,14d and 28d, the Ca/P ratio of
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the samples with CaTiO3-contained coating exhibits a drastic decline from 34.32 to 1.50 and is slightly smaller than that of Ca/P ratio of apatite (Ca/P = 1.67 [35]). On one hand, the ratio
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decreases due to the PO43- ions depositing on the coating surface; on the other hand, it could be contributed to that calcium is leached from calcium titanate at 36.5°C as a function of
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immersion time, in accordance with the XRD results shown in Fig. 7. The Ca amount contained in surface layer after early immersion stage of 3~7 days (at%Ca3d = 9.61 and
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at%Ca7d = 8.58) is lower than that of the original MAO formed coating (at%Caoriginal = 11.82), which could attributed to the hydrolysis of CaTiO3 at the beginning of immersion.
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Thereafter, higher amounts of Ca (at%Ca14d = 12.89 and at%Ca28d = 15.11) and P (at%P14d = 3.08 and at%P28d = 10.06) are obtained during the long term immersion of 14~28 days, in
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agreement with our previous speculation that the existence of CaTiO3 benefits in producing OH- and subsequently leads to better apatite formation. In comparison, while the Ce-doped sample are immersed in SBF for 7d and 28d, the Ca/P ratios of sample surface are 1.41 and 1.33, respectively, which are also closed to that of apatite. Moreover, the lower Ca/P ratio is observed when the Ce-doped samples are immersed for 7d (c.f. Fig. 9) due to a faster increase of PO43- on coating surface. It is also noticed that the higher Ca content is seen on Ce doped surface. The result indicates more apatite are formed as the dissolution of CaTiO3 is not as strong as that occurs on Ce undoped
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benefit in producing OH- during immersion, which is agreed with previous report [53].
Fig. 9 The Ca/P ratios of the samples with porous CaTiO3-contained coating immersed in
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SBF at 36.5 °C for 3d, 7d, 14d and 28d; in comparison, the samples with porous Ce-CaTiO3-contained coating are immersed under identical conditions for 7d and 28d. 4. Conclusions Porous CaTiO3-contained titania coatings doped with Ce for biomedical applications are successfully fabricated through a MAO process in this work. The results reveal that as synthesized porous oxide layers are consisted of not only anatase and rutile TiO2 but also CaTiO3 phase. The hydrolysis of CaTiO3 could increase the local supersaturation degree at the liquid/solid interface during SBF immersion; on the other hand, the lattice configuration of specific crystal plane of perovskite CaTiO3 benefits in epitaxial adsorption of Ca and P
ACCEPTED MANUSCRIPT from the SBF. The existence of CaTiO3 leads to good nucleation and growth of apatite. Furthermore, the samples with Ce-doped coating possess higher inducing ability of apatite
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formation compared to Ce-undoped samples, since the more hydroxyl functionalized surface
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could be formed on the Ce-doped hydrophilic coatings under the identical immersion
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conditions.
Acknowledgements
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This work was jointly supported by the National Natural Science Foundation of China (Grant No. 31570961) and Jiangsu key laboratory for advanced metallic materials
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ACCEPTED MANUSCRIPT Highlights
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Porous Ce-doped titania coatings containing CaTiO3 are fabricated by one-step MAO process. The existence of CaTiO3 leads to good nucleation and growth of apatite. Doping Ce benefits in forming apatite.
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