CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 8 2595–2597
Communications Highly Crystalline Chlorapatite Films Prepared by the Evaporation of a Sodium Chloride Flux Katsuya Teshima,*,† Mitsuo Sakurai,† Kunio Yubuta,‡ Yutaka Sonobayashi,§ Takaomi Suzuki,† Toetsu Shishido,‡ Hiroyuki Sugimura,§ and Shuji Oishi† Department of EnVironmental Science and Technology, Faculty of Engineering, Shinshu UniVersity, Nagano 380-8553, Japan, Institute for Materials Research, Tohoku UniVersity, Sendai 980-8577, Japan, and Department of Material Science and Engineering, Kyoto UniVersity, Kyoto 606-8501, Japan ReceiVed May 11, 2007; ReVised Manuscript ReceiVed February 10, 2008
ABSTRACT: Environmentally friendly, high-quality, and transparent-colorless calcium chlorapatite (Ca5Cl(PO4)3) films were easily fabricated for the first time by the flux evaporation of the NaCl flux. The intermediate layer was essential to form the chlorapatite film on the sapphire surface. The film is thought to epitaxially grow on the (0001) sapphire surface because very smooth and well-developed {101j0} faces were observed. Calcium apatites [Ca5X(PO4)3 (X ) OH, Cl, and F)], which are highly biocompatible, are well-known as the main constituent of bones and teeth in the human body, and are also used as artificial biomaterials.1–3 A variety of apatites have been widely used for industrial applications, and their film/substrate (e.g., biomedical implant) composites have also attracted particular attention as highly biocompatible materials.4–7 There have been many studies on the preparation of apatite films by various techniques, such as pulsed laser deposition, rf-sputtering, electrophoretic deposition, electronbeam deposition, sol-gel preparation, and biomimetic process.4–7 There are, however, several disadvantages in these techniques, such as expensive equipment, high total costs, high environmental loads, and poor crystallinity. In our previous studies, crystals of Ca5Cl(PO4)3 (calcium chlorapatite, denoted as chlorapatite) have been grown by the cooling of the NaCl flux.8,9 The forms of grown crystals were a hexagonal prism and needle (whisker) with pyramidal end faces.8,9 Sodium chloride was found to be a suitable flux for growing the chlorapatite crystals. However, no study has been reported on the fabrication of highly crystalline apatite films. Recently, we developed a unique ruby-coating technique10 on the basis of our experiences in growing the hexagonal bipyramidal ruby crystals.11 The ruby films deposited on alumina materials consisted of many crystals having good crystallinity and well-developed faces. The purpose of this study, therefore, is the environmentally friendly synthesis of highly crystalline chlorapatite films by the evaporation of a NaCl flux. Sapphire (R-Al2O3) was selected as a substrate for apatite coatings because it is the second-hardest natural material (next to diamond) and is widely accepted as a biocompatible material known to humankind. Therefore, R-Al2O3 materials are * Corresponding author: E-mail:
[email protected]. † Shinshu University. ‡ Tohoku University. § Kyoto University.
widely used as biomaterials for dental and orthopedic applications. When R-Al2O3 surfaces are covered with an apatite film, their biocompatibility becomes higher and bone reconstruction readily occurs there. Since various properties of single crystal are much higher than those of polycrystal and amorphous materials, high bone induction effects of apatite single crystals can be expected more than the other materials. Furthermore, their stability in vivo is expected to be much higher than that of an amorphous material. Thus, highly crystalline apatite-coated sapphire materials are thought to have potential as biomaterials. Highly crystalline chlorapatite films were simply synthesized by the evaporation of NaCl flux. A stoichiometric mixture of reagent-grade CaHPO4 · 2H2O (0.785 g), CaCO3 (0.228 g), and CaCl2 (0.086 g) powders was used as a solute, and reagentgrade NaCl (59.201 g) was chosen as the flux. A mixture containing solute of 0.15 mol% was prepared. The solubility curve measured in our previous study9 was applied to decide the solute concentration. The mixtures were about 60 g in weight and were put into platinum crucibles with capacities of 150 cm3. The used substrates were single crystal of sapphire (R-Al2O3, (0001)), mirror polished and nonpolished. The (0001) sapphire substrates were ultrasonically cleaned in ethanol before deposition. The substrates (about 4 cm2, 0.54 g), which were parallel to the crucible wall, were hung in the crucibles including the mixture. The lids were loosely fitted, and the crucibles were then placed in an electric furnace. The crucibles were heated to 1100 °C at about 45 °C h-1 and held at this temperature for 70 h. When the holding program was completed, they were allowed to cool down to room temperature in the furnace. The sapphire substrates coated with crystalline films were then separated by dissolving the residual flux in warm water. The deposited films were observed by use of an optical microscope and scanning electron microscope (SEM). They were identified
10.1021/cg070432m CCC: $40.75 2008 American Chemical Society Published on Web 06/19/2008
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Figure 1. Optical micrograph showing typical transparent-colorless chlorapatite film grown from the NaCl flux. Figure 3. XRD patterns (Cu KR) of chlorapatite film. (a) Film in which a well-developed face was laid in parallel with the holder plate; (b) pulverized crystallites; (c) Ca5Cl(PO4)3 ICDD PDF.12
Figure 4. XPS Ca2p, Cl2p, P2p, and O1s spectra obtained from the chlorapatite film.
Figure 2. SEM images (low and high magnification) of (a) and (b) nonpolished sapphire surface, (c) and (d) chlorapatite film surface, and (e) and (f) chlorapatite film cross-section.
by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectrometer (EDS). Highly crystalline chlorapatite film could be deposited on the nonpolished sapphire surface by the NaCl flux evaporation method. In the case of the mirror polished sapphire substrate, no chlorapatite layer was deposited on the surface. The typical chlorapatite film is shown in Figure 1. The deposited chlorapatite film was colorless and transparent. Figure 2a,b shows surface SEM images of the untreated nonpolished sapphire substrtate. Surface and crosssectional SEM images of the chlorapatite film deposited on the nonpolished sapphire substrate are, respectively, shown in Figure 2, panels c,d and e,f. As is clearly indicated in these figures, a relatively densely packed chlorapatite film was formed on the sapphire substrate although they had many apparent boundaries and cracks. The chlorapatite film had much smoother surfaces compared to the nonpolished sapphire surface. The regularity of boundaries and cracks was also observed on the surface. The film thickness estimated from the SEM image was approximately 25 µm. It is
controllable by growth conditions, such as solute concentration, growth rate, and holding temperature. Figure 3 shows XRD profiles of data for (a) the transparent-colorless film, (b) pulverized crystallites, and (c) Ca5Cl(PO4)3 ICDD PDF.12 The XRD pattern of the film deposited in this study indicated good crystallinity, and the pulverized crystalline pattern (Figure 3b) was found to be the same compared to that of Ca5Cl(PO4)3 (Figure 3c). Only the diffraction pattern of the (303j0) plane was detected in the film, as shown in Figure 3a. It was found that the nonpolished sapphire surface was covered with well-developed {101j0} faces of chlorapatite. The elongation directions of the grain boundaries and cracks are thought to be parallel and perpendicular to the c-axis, which is parallel to the sapphire surface. As a result, many boxy chlorapatite crystals are formed, and are clearly observed on the surface. Figure 4 shows XPS spectra obtained from the chlorapatite deposited substrate. The chemical formula of chlorapatite is composed of 5 Ca atoms, 1 Cl atom, 3 P atoms, and 12 O atoms. The characteristic Ca 2p spectrum is ascribable to the columner Ca I (Ca2p1/2) and II (Ca2p3/2). The relative component concentrations (5:0.5:2.7:13.5 for Ca:Cl:P:O) determined from these peak areas are similar to the stoichiometric value for chlorapatite (5:1:3:12 for Ca:Cl:P:O). On the other hand, in the XPS depth profiles, an intermediate layer was found to be formed between the deposited chlorapatite film and the sapphire substrate. The existence of this intermediate layer was also confirmed by SEM observation and EDS analysis. The same layer was observed on the surfaces of the mirror polished sapphire surface although no chlorapatite film was deposited there.
Communications Here, we discuss the growth mechanism of the chlorapatite film on the nonpolished sapphire surface. The nonpolished one has a relatively rough surface (Figure 3b). In the growth of chlorapatite film, the formation of the intermediate layer (or buffer layer), which consists of Ca, O, and Al atoms, is indispensable. The buffer layer was formed by a chemical reaction between the sapphire substrate and the solute in the high-temperature solution. In the case of the nonpolished substrate having a nano- or microtextured rough surface, the reaction rate was much faster than that of the polished substrate having a relatively smooth surface. After the formation of the intermediate layer, chlorapatite nuclei occurred on the layer, and subsequent the highly crystalline film was epitaxially grown by the evaporation of the NaCl flux. When the same experiment was repeated using the mirror polished substrate covered with the buffer layer, the growth of chlorapatite film was observed on the substrate. Until now, the intermediate layer has not been identified completely. In conclusion, environmentally friendly, highly crystalline chlorpatite films were readily grown for the first time by an isothermal technique involving the evaporation of NaCl used as a flux. The chlorapatite films, which were colorless and transparent, had relatively smooth surfaces, although many cracks and grain boundaries occurred in the films. In addition, it was found that the intermediate layer (buffer layer) was essential to form chlorapatite film on the sapphire surface. We may say that the chlorapatite film epitaxially grew on (0001) sapphire substrate because very smooth, well-developed {101j0} faces were observed, and many cracks and boundaries existed relatively regularly.
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Acknowledgment. This research has been supported by a Grantin-Aid for Science Research (19760466) from the Ministry of Education, Culture, Sport, Science and Technology.
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