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The catalytic growth of vertically-aligned carbon nanotubes (VA-CNTs) forest usually requires thin catalyst films deposited by multi-step and costly physical vapor deposition techniques. Here, we demonstrate that an efficient catalyst and its supporting layer for VACNT growth can be prepared by using a simple solution of Fe(NO3)3 and Al(NO3)3 deposited on silica in a single step. This process being much simpler and cheaper than existing preparation methods, it can easily be transferred to industry for the low-cost, thin and large-area coating of catalyst for VA-CNT growth. Our study shows that aluminum hydroxides preferentially react with the SiO2 surface while iron hydroxides tend to form oxide or hydroxide nanoparticles, thus allowing preparation of an aluminum-based buffer layer with iron-based nanoparticles at its surface. Optimization of the Fe/Al ratio and salt concentrations yielded catalysts with performances similar to standard Fe/Al2O3 catalysts prepared by physical vapor deposition., The Government of Aragon, and the European Social Fund are gratefully acknowledged. R.A. gratefully acknowledges the project “Construyendo Europa desde Aragon” 2014-2020 (grant number E/26). R.A. gratefully acknowledges the support from the Spanish Ministry of Economy and Competitiveness (MINECO) through project grant MAT2016-79776-P (AEI/FEDER, UE). Part of this work has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No 642742.

On-site and real-time measurement of gas concentrations are crucial for both the understanding and the monitoring of industrial and environmental processes. In recent years, there is an increasing need to develop portable multi-gas analysis tools allowing in situ detection of complex gas mixtures mainly due to safety, process and environmental considerations. A promising approach is based on the integration of the different parts of the analytical system (i.e. pre-concentrator, gas chromatography column, gravimetric sensors) in a silicon die by using standard microelectronic technologies. Each of these devices need to be coated by an appropriate functional layer. Metal Organic Frameworks (MOF), hybrid microporous crystalline materials with tuneable properties, are attractive for this type of application regarding their high specific surface area and chemical stability. However, these materials are usually synthetized via solvothermal techniques, which complicates the growth of continuous thin films and their integration in micro-devices. Recently, vapor phase-based routes were reported for the synthesis of MOF thin films [1-2]. These solvent-free synthesis methods hold great economic and environmental perspectives since they can not only avoid the use of solvents and metallic salts, but also reduce contamination and impurity incorporated into the crystals during the synthesis step. Moreover, vapour-phase process are usually preferred to obtain conformal coatings in the high aspect ratio features of devices and they present a good compatibility with the Si technology. These breakthroughs have paved the way for the use of MOF in micro- and nanotechnologies but much still remains to be done to control the growth and understand the potential and the limits of these growth methods. In this work, pinhole-free and crystalline zeolitic imidazolate framework-8 (ZIF-8) layers were grown on different types of substrates and devices that can be used for gas analysis. Si wafers, Quartz Crystal Microbalance, Si micro-pillars arrays and 3D foam were envisaged. First, ZnO films were deposited by Atomic Layer Deposition (ALD). Then, the formation of ZIF-8 was realized through a vapor-solid reaction with a vaporized linker (2-methylimidazole). Continuous ZIF-8 films were obtained and the impact of the process parameters on the MOF growth and on the material properties was studied. Additionally, the composition, roughness and structure of the films were studied by FTIR, EDX, XPS, XRD and AFM experiments. Finally, the as-synthesized films were thermally activated and the porosity was assessed using ellipsometric-porosimetry. The adsorption properties of the films were also investigated using gravimetric gas sensors. Several gases were tested (methanol, toluene, formaldehyde) in order to determine the benefit of ZIF-8 thin films in sensors. It is demonstrated that the synthesis approach based on an ALD deposition followed by a vapor-solid treatment in appropriate conditions is compatible with different kinds of substrates, which allows the use of ZIF-8 as sensitive layer in devices for gas sensing. [1] I. Stassen et al., Nat. Mat., 15, 304–310 (2016) [2] E. Ahvenniemi et al., Chem. Com., 52, 1139-1142 (2016)

We report here the formation – via a bottom-up approach – of Cu nanoparticles supported on silica substrates as catalysts for the growth of silicon nanowires. The density and the size (3–5 nm) of Cu particles were controlled by exploiting the density and the distribution of surface silanols on both 3D silica nanoparticles and 2D silica thin films. The growth of Si nanowires on such Cu nanoparticles indicates that a critical particle size is needed for the fast growth of silicon nanowires.