ZSM-5 zeolite films on Si substrates grown by in situ seeding and secondary crystal growth and application in an electrochemical hydrocarbon gas sensor Alessandro Zampieri a , Andreas Dubbe b , Wilhelm Schwieger a, * , Abhijeet Avhale a , Ralf Moos b a University of Erlangen-Nuremberg, Institute of Chemical Reaction Engineering, 91058 Erlangen, Germany b University of Bayreuth, Chair of Functional Materials, 95440 Bayreuth, Germany Received 29 May 2007; received in revised form 9 August 2007; accepted 20 August 2007 Available online 25 August 2007 Abstract ZSM-5 zeolite films were grown on Si substrates by a two-step hydrothermal synthesis consisting of in situ seeding and secondary crystal growth. The films were 8–13 lm thick and partly oriented with the c-axis perpendicular to the substrate surface. After ion exchange with sodium ions, one film was applied as solid electrolyte in a potentiometric hydrocarbon gas sensor. A fast and reversible voltage response of the sensor to varying propane concentrations (100 ppm – 10%) was observed in O 2 /CO 2 /N 2 gas mixtures at 723 K. Ó 2007 Elsevier Inc. All rights reserved. Keywords: ZSM-5; Zeolite film; Silicon; Ionic conduction; Hydrocarbon gas sensor 1. Introduction The preparation and characterization of zeolite films on various supports has burgeoned over the past years [1]. Zeolite films on porous supports are applied as catalysts and membranes in chemical engineering [2–6]. Novel appli- cations of zeolites as advanced functional materials for microelectronics, chemical sensors, and other micro devices have been reviewed in [3,7–11]. These devices typically require silicon as substrate [12] and accordingly, techniques for preparation and patterning [4,5,13] of zeolite films on silicon have been developed [13–20]. Zeolite films are generally prepared by means of hydro- thermal in situ growth techniques in liquid synthesis mix- tures [1,3–6,13,17–27]. Attempts to utilize vapour deposition methods led to amorphous films, which could be transformed into zeolite phases by subsequent hydro- thermal treatment [28]. Dip coating [29], spin coating [14–16], and sedimentation [30] from zeolite sols or suspen- sions resulted in porous films of non-interconnected crys- tals. Sintering of zeolite films for further densification is generally not possible. A common hydrothermal synthesis method for zeolite films is the one-step process, where nucleation and film growth occurs in the same process step [17,19,24]. More advantageous are two-step schemes, in which the seeding step is decoupled from the film growth step [13,20–22,27]. After covering the substrate with zeolite nano-crystals in the seeding step, the seed layer can be grown into a dense film of intergrown crystals in successive hydrothermal treatments. The advantage of this prepara- tion pathway is that both steps can be optimised separately. For the growth process, solutions with lower precursor concentrations are used, which favour the growth of the seeds rather then further nucleation. The seeds are com- monly prepared ex situ and attached to the substrate sur- face via adsorption [29] or grafting [22,27]. A variant is the in situ seeding and secondary crystal growth technique, 1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.08.026 * Corresponding author. Tel.: +49 9131 8528910; fax: +49 9131 8527421. E-mail address: wilhelm.schwieger@rzmail.uni-erlangen.de (W. Schwieger). www.elsevier.com/locate/micromeso Available online at www.sciencedirect.com Microporous and Mesoporous Materials 111 (2008) 530–535