Copper Electroless Process Optimization to Modify Boron Doped Diamond at Different Boron Levels

Abstract

Composite of Cu/BDD (boron doped diamond) have the potential to use as solid electrode for application in environmental approaches. Among many Cu coating processes, the electroless plating has emerged in recent years due to its low cost, fast deposition rate, and good filling capability. In addition, its major advantage is its simplicity, where the processes can take place at room temperatures and pressures [1]. In the Cu electroless process, a hard pre-treatment is needed for cleaning the surface and for creating functional groups that can act as anchor points on the BDD samples. The pre-treatments can be obtained using different oxidation techniques. In this study the anodic electrochemical pre-treatment was carried out using 0.5 mol L-1 H2SO4 and a fixed potential of 3 V for 30 min. The present study was undertaken in order to investigate the influence of the BDD doping level in the electroless synthesis as well as the effect of some deposition parameters such as the solution pH and the deposition time, to obtain Cu/BDD composites. This material is very attractive for nitrate electroreduction related to environmental interest. BDD films were grown by hot filament-assisted chemical vapor deposition technique on the titanium substrate, activated by methane and hydrogen gas mixture with a pressure of 40 Torr, at 650ºC, for 24 h. Boron source was obtained by an additional hydrogen line passing through a bubbler containing B2O3 dissolved in methanol with a controlled B/C ratio that led to films with different doping level (5000 and 15000 ppm). Prior to the Cu electroless deposition, the sensitization on the BDD was achieved using a solution of 40 mL L-1 HCl containing 0.04 mol L-1 SnCl2 for 5 min and the activation was made using a solution containing 7x10-4 mol L-1 PdCl2 with 2.5 mL L-1 HCl for 5 min. Ultrasonic vibration was used for both steps. The electroless Cu deposition was carried out for different times (30, 60, 180 and 2400 s) at room temperature. The bath composition was 0.1 mol L-1 CuSO4 + 0.2 mol L-1 KNaC4H4O6 + 17.5 mL L-1 HCHO. The pH level was adjusted to 8, 10 and 12 by the addition of NaOH. The Raman scattering spectra showed the peak close to 1332 cm-1 corresponds to the vibration of a diamond first-order phonon, confirmed the quality of these BDD films. This peak decreased in intensity due to the boron incorporation in diamond films. The effect of boron doping is reflected in the spectral features. There is the appearance of the two bands located at 500 cm-1 and 1220 cm-1 attributed to B-B vibrations and B-C vibrations, respectively. According to Scanning Electron Microscopy images the Cu deposits showed an uniform particle distribution on the BDD surface for solution of pH 12. On the other hand, no Cu deposit was verified at pH 8 and at pH 10 in the range of time deposition from 30 to 180 s. However, for deposition time of 2400 s, the deposit presented low particle density. Similar experimental conditions for electroless Cu deposition on lightly doped diamond showed the same behavior as a function of pH. The Cu deposit rate increased with increasing the deposition time, for both BDD films. Comparing the highly and the lightly BDD electrode, higher deposition rate was observed in the highly doped electrode. This result can be attributed to the better conductivity of this electrode due to its higher boron content. It is noteworthy that the sensitization and activation processes are a crucial step for high quality coatings independently of the BDD doping level. Acknowledgments: Brazilian National Institute of Science and Technology (INCT) for Climate Change funded by CNPq Grant Number 573797/2008-0 e FAPESP Grant Number 2008/57719-9.