ZnO-Based Thin Films for Applications In Chemical Sensing

Zinc oxide (ZnO) is a wide band-gap semiconductor with important applications in electronics, photonics, chemical sensing and catalysis.  Sol-gel chemistry provides a convenient route for low-cost fabrication of ZnO functional coatings with controlled microstructure. Critical textural, chemical and electronic properties can be further manipulated by doping ZnO with minor components. In this work, we applied sol-gel wet-chemical techniques to preparation of ZnO, Al-ZnO (Al:Zn = 1:10) and Cu-ZnO (Cu:Zn = 1:10) thin film functional layers for chemiresistive sensors.

Cu and Al dopants influence the films' surface morphology and their thermally induced chemical and structural evolution. As prepared (room temperature) films exhibit the structure of Layered Basic Zinc Acetate (LBZA), a lamellar ZnO precursor.  When annealed at temperatures through 700 ^oC, the films display similar chemical evolution patterns—characterization by IR spectroscopy reveals that temperatures above 500 ^oC are needed to completely desorb solvents and decompose precursors. X-ray diffraction results show that Cu facilitates c-axis orientation of the annealed film, while Al slows its crystallization. Details of surface morphology, imaged by electron microscopy, also depend on the choice of dopant. 

UV-vis spectra show that the Al dopant slightly widens the band-gap of calcined film, while Cu increases absorption broadly throughout the visible range. Chemiresistive sensors, fabricated by coating thin film functional layers onto interdigitated electrode transducers, were evaluated for their sensitivity to oxygen at operating temperatures through 600 ^oC.  A sensor coated with undoped ZnO displays good sensitivity for O₂ at intermediate temperatures, ~400 ^oC, likely reflecting an optimal balance between surface O₂ coverage and carrier availability.  Cu-ZnO, on the other hand, has a higher base resistance (in nitrogen) and is less sensitive to O₂.