The deposition of metals using the thermal CVD process faces challenging issues regarding the purity and the nucleation kinetics on semiconducting surfaces. The use of adequate deposition chemistry is proposed as an alternative to overcome these limitations. In fact, transition-metal cations, which exhibit enhanced catalytic redox character, are able to induce the dehydrogenation reaction of alcohols under CVD conditions. Taking advantage of this intrinsic reactivity, a new method that enables the deposition of transition metal thin films through a self-catalyzed reaction pathway is developed. Hence, alcohols were used as powerful co-reactants to grow technologically relevant transition metals (i. e. Ni, Fe, Co, Cu, Ag, Pt and Ru) starting from commercially available, robust and cost-effective precursors.

Using this process, the precursors and the selected alcohol are admitted to the reactor as a single liquid feedstock, enabling the growth of device-quality metal thin films as well as a controlled growth of a large variety of alloys.

Since the driving force in the metals deposition is the intrinsic catalytic activity of the transition metal cations towards the alcohols, metal cations which do not possess a strong catalytic redox character, such as Zn, Al and Sn, grow as oxides under the same deposition conditions, which turns out to be an immense advantage. In fact, this process offers a unique chance for the CVD-growth of metal-metal oxide nano-composite thin films with the possibility to tune the composition of the metallic nano-particles and that of the oxide matrix as well. These composites can be considered as a ground for the development of innovative materials with tailored optical, electric and magnetic properties.

References:

- N. Bahlawane, P.A. Premkumar, Z. Tian, X. Hong, F. Qi, K. Kohse-Höinghaus, Chem. Mater. 22, 92-100, 2010

- N. Bahlawane, P.A. Premkumar, F. Reilmann, K. Kohse-Höinghaus, J. Wang, F. Qi, B. Gehl, M. Bäumer, J. Electrochem. Soc. 156(10), D452-D455, 2009

- P.A. Premkumar, A. Turchanin, N. Bahlawane, Chem. Mater. 19, 6206-6211, 2007

- N. Bahlawane, P.A. Premkumar, K. Onwuka, K. Kohse-Höinghaus, G. Reiss, Microelectron. Eng. 84, 2481-2485, 2007

- N. Bahlawane, P.A. Premkumar, K. Onwuka, K. Rott, G. Reiss, K. Kohse-Höinghaus, Surf. Coat. Technol. 201(22-23), 8914-8918, 2007

Plasma-enhanced chemical vapor deposition (PECVD) was used to prepare thin films of cobalt oxide. Cobalt oxide-based (CoO and Co₃O₄) catalysts were chosen for their efficiency during mineralisation of organic pollutants achieved by catalytic ozonation. Several authors have reported that Co₃O₄ spinel is the best active phase for the catalytic ozonation^[1,2], and a very active component for hydrocarbons oxidation and combustion^[3,4]. Moreover, the antibacterial properties of these products are not negligible^[5]. In this paper, we used two types of PECVD processes for the production of cobalt thin films. In the first one, cobalt thin films were deposited using a parallel electrodes RF low-pressure plasma reactor (13.56 MHz, 100 Pa, 200 W) with cobalt carbonyl (Co(CO)₈) as precursor sprayed in gas carrier (argon and oxygen). In the second process, a solution of nitrate salt of cobalt is sprayed into a low pressure plasma discharge (600 Pa, 120 W) to obtain Co_xO_y layers.

The chemical composition and the microstructural evolution of the coating were studied by X-ray photo-electron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). To characterize the morphologies and the thicknesses of the studied films, scanning electron microscopy and EDX were used.

Considering thin films obtained from cobalt carbonyl precursor, analyses confirmed the presence of cobalt oxide (layer of 2 µm) on the surface of the substrate. XRD investigation showed the presence of crystalline phase of Co₃O₄ (crystallite size of about 42 nm) as shown in Figure 1. In the case of coatings produced from a solution of cobalt nitrate salt, a layer of 3 µm and an amorphous form of Co_xO_y oxides on surface was observed, as shown in Figure 2. Catalytic ozonation will be tested for oxidation of para-chlorobenzoic acid (pCBA) used as persistent organic pollutant.

^[1] P.M. Alvarez et Al , Applied Catalysis B: Environmental 72 (2007) 322-330

^[2] Hu, C., Xing, S., Qu, J., He, H., J. Phys. Chem. C 112, (2008),5978–5983

^[3] T.-C. Xiao, S.-F. Ji, H.-T. Wang, K.S. Coleman, M.L.H. Green, J. Mol.Catal., A Chem. 175 (2001) 111.

^[4] M.M. Zwinkels, S.G. Jaras, P.G. Menon, T.A. Griffin, Catal. Rev., Sci.Eng. 35 (1993) 319.

^[5] Y-M. Jeong , J-K. Lee , S-C. Ha, S H. Kim; Thin Solid Films 517 (2009) 2855–2858