Influence of Temperature and Time For Direct Hydroxyapatite Electrodeposition on Superhydrophilic Vertically Aligned Carbon Nanotube Films

Abstract

Synthesis of vertically aligned multiwalled carbon nanotubes films The VACNTs were produced as a thin film, using a microwave plasma chamber (2.45 GHz) [20-22]. The substrates were 10 mm titanium squares, covered by a thin nickel layer (10 nm) deposited by an e-beam evaporator. The nickel layer was pre-treated to promote nanocluster formation, which forms the catalyst for VACNT growth. The pre-treatment was carried out for 5 min in plasma of N 2 /H 2 (10/90sccm), at a substrate temperature around 760°C. After pre-treatment, CH 4 (14 sccm) was inserted into the chamber at a substrate temperature of 760°C for 2 min. The reactor was kept at a pressure of 30 Torr during the whole process. VACNT functionalized by polar groups to obtain superhydrophilicity Functionalization of the VACNT tips by the incorporation of oxygen-containing groups was performed in a pulsed-direct current plasma reactor with an oxygen flow rate of 1 sccm, at a pressure of 85 mTorr,-700 V and with a frequency of 20 kHz [23]. A chemical surface modification was calculated using liquids with different surface tensions and polarities as shown and calculated elsewhere [23]. Hydroxyapatite electrodeposition process on VACNT The electrodeposition of HA crystals on VACNT films was performed using 0.042 molL-1 Ca(NO 3) 2 .4H 2 O + 0.025 molL-1 (NH 4).2HPO 4 electrolytes (P H =4.7). The electrochemical measurements were made using a three-electrode cell coupled to Autolab PGSTAT 302 equipment. Superhydrophilic and as-grown VACNT films were used as working electrode, and the geometric area in contact with electrolytic solution was 0.27cm 2. A platinum coil wire served as auxiliary electrode and Ag/AgCl electrode was used as reference electrode. A cyclic voltammetry study was carried out using the previously described solution in order to determine the parameters for electrodeposition of HA films on superhydrophilic and as-grown VACNT. The cyclic voltammogram was obtained by scanning the potential between 0.5 and-2.5 V at a scan rate of 10 mV s-1. Based on the cyclic voltammetry results, HA films were produced applying a constant potential of-2.0 for different electrodeposition conditions, for periods of 20, 30, and 120 min; and batch temperatures at 23 and 70°C. All these parameters and conditions were used to evaluate the crystal formation and evolution current differences of HA electrodeposition process. Characterization of functionalized VACNT and HA crystals SEM (field emission gun, JEOL JSM-6330F) was used to observe the structure of VACNTs and HA crystal morphologies. The elemental composition of the coating was investigated by energy-dispersive x-ray. The structural analysis of HA crystals was performed by DRX diffractometer (X-Pert Philips) with Cu Ka radiation generated at 40 kV and 50 mA. Raman spectroscopy (Renishaw micro-Raman model 2000 with Ar laser, λ = 514.5 nm) measurements were carried out to analyze the chemical composition of HA coatings. Results and Discussion The electrochemical deposition temperature (hydrothermal processing) was varied to improve the HA content of the coatings. The transient of cathodic current may be divided in two periods. Initially, a rapid increase of cathodic current due to electric double-layer charging was observed. Figure 1a and Figure 1b show the difference between evolutions of the current as a function of deposition time of HA on superhydrophilic VACNT films in 23 and 70°C (bath temperature). NiTi alloys were used as control. A rapid increase of cathodic current due to electric double-layer charging (region 1) was observed (Figure 1a) for all the substrates. However, no cathodic current difference between NiTi and superhydrophilic VACNT was observed when the temperature of the solution was kept in 23°C (approximate value of 0.25 mA). We observed higher current values when the temperature of the solution was kept at 70°C. By comparing superhydrophilic VACNT and NiTi alloys, it was clearly observed a higher current value for superhydrophilic VACNT serving as an electrode (-0.60 mA). During the second period (region 2), the current value slowly decreased to a limit value, indicating that hydroxyl ion generation process due to the reduction of water and dissolved oxygen on nonuniform superhydrophilic VACNT (a better description about this phenomena is follow discussed in equations 1-7). In this stage, bath temperature and superficial area were both essential to determine the Figure 1: (a) Current-time profile during the deposition of HA films on superhydrophilic VACNT films and NiTi alloys in 0.042 molL-1 Ca(NO 3) 2 .4H 2 O + 0.025 molL-1 (NH 4) 2 HPO 4 solution. Applied potential of-2.0 V (vs. Ag/AgCl), and solution temperature kept at 23 and 70 o C. (b) Differences of average current evolution of HA films electrodeposition on superhydrophilic VACNT films and NiTi alloys in solution temperature kept at 23 and 70ºC.