Sensors were fabricated using a screen-printing technique using WO3 produced by a sol-gel process. Surface functionalization was achieved by using normal and catalyzed SiO2 coatings. The variation of sensor response to ammonia gas was investigated for both undoped and platinum doped WO3 sensor layers. The response to ammonia exhibited significant improvement following surface modification with catalyzed silica. The increase has been attributed to an increase in Schottky barrier height.