Controlled synthesis of Co 3 O 4 spinel with Co(acac) 3 as precursor{ Patrick Mountapmbeme Kouotou, Zhen-Yu Tian,* Udo Mundloch, Naoufal Bahlawane{ and Katharina Kohse-Ho ¨inghaus Received 25th June 2012, Accepted 1st September 2012 DOI: 10.1039/c2ra21277c Cobalt( III) acetylacetonate (Co(acac) 3 ) was used as a precursor to grow pure Co 3 O 4 with pulsed-spray evaporation chemical vapor deposition (PSE-CVD). The effect of solvent and substrate temperature on the growth kinetics and morphology of the films was investigated. The obtained spinel exhibited good catalytic performance. Among transition metal oxides, Co 3 O 4 exhibits interesting electrical, magnetic, optical and catalytic properties. 1 In the last decade, the catalytic activity of Co 3 O 4 and Co-based spinels has substantially increased the interest towards their technological application. 2 The promising ability of Co 3 O 4 to exchange its lattice oxygen with the atmosphere sustains its high potential application in air pollution control via the oxidative abatement of CO 3 and organic pollutants from exhaust streams. 4 Several methods have been reported previously to obtain Co 3 O 4 thin films, including sol–gel, 5 spray pyrolysis, 6 atomic layer deposition, 7 sputtering 8 and chemical vapor deposition (CVD). 9–11 Among these techniques, pulsed-spray evaporation CVD (PSE- CVD) is considered a promising method to prepare pure films due to its low cost, simplicity, high throughput and easy control of the thickness and morphology of the samples. Particularly, PSE-CVD is suitable for precursors with limited thermal stability and well adapted for systematic synthesis of single-phase spinel or perovskite oxides with tailored composition on various relevant substrates. Most efforts to grow Co 3 O 4 spinel by CVD involved several precursors (Table S1, ESI{). Metal–organic compounds (MOC) used as precursors are either commercially available or can be easily prepared in the laboratory. Of all the MOC, cobalt b-diketonates serve as non-toxic (environmentally friendly), volatile and inexpen- sive precursors. In the family of b-diketonates, cobalt acetylaceto- nates are the easiest to prepare, requiring no special atmosphere or environment. 12 Recent investigations reported the use of cobalt( II) acetylacetonate (Co(acac) 2 ) for the synthesis of Co 3 O 4 . 13,14 However, this precursor shows instability in several solvents, and the selection of alcohol presents a particularly complicated effect on the formed cobalt phase. 13 In contrast to other solvents, alcohols exhibit certain reactivity with cobalt acetylacetonate under deposition conditions. This reactivity could lead to the formation of carbide phases at low temperature in the absence of oxygen. 15 Therefore, an alternative may be needed. The present work is focused on the controlled synthesis of Co 3 O 4 spinel by PSE-CVD using cobalt(III) acetylacetonate (Co(acac) 3 ) as precursor and the investigation of the effect of solvent and substrate temperature (T s ) on the growth kinetics and film morphology. Moreover, the catalytic performance of the sample obtained with tetrahydrofuran (THF) as the solvent was tested in terms of total oxidation of CO and propene (C 3 H 6 ) in a flow reactor. Special attention has been paid to the effect of aging on the stability and the ultraviolet-visible (UV-vis) absorption spectrum of the ethanol (EtOH) solution containing Co(acac) 2 and Co(acac) 3 , respectively. The solutions were stored in a closed atmosphere for several days and the results are shown in Fig. 1. For Co(acac) 2 /EtOH solution (A, C and E), the pink color transforms gradually into dark green, and a broad optical absorption band peaking at 590 nm is observed progressively after three days. However, neither color nor UV-vis spectra are changed for Co(acac) 3 /EtOH solution (B, D and F). The absorption peak is characteristic of an octahedral Co(III) Department of Chemistry, Bielefeld University, Universita ¨tsstraße 25, D-33615 Bielefeld, Germany. E-mail: zhenyu.tian@uni-bielefeld.de; Fax: +49-521 1066027; Tel: +49-521 1062199 { Electronic supplementary information (ESI) available: list of used cobalt precursor, experimental condition, solvent properties, comparison with other results, gas phase evolution of different solvents with and without Co(acac) 3 . See DOI: 10.1039/c2ra21277c { Present address: Nanomaterials Research Unit, SAM Department, Centre de Recherche Public-Gabriel Lippmann, L-4422 Belvaux, Luxembourg. Fig. 1 UV-vis spectra of EtOH solution of Co(acac) 2 (A, C and E) Co(acac) 3 (B, D and F). RSC Advances Dynamic Article Links Cite this: RSC Advances, 2012, 2, 10809–10812 www.rsc.org/advances COMMUNICATION This journal is ß The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 10809–10812 | 10809