BST thin film; Sol–gel; AFM; Humidity sensor; pH of sol
Polycrystalline (Ba, Sr)TiO3 (BST) thin films were prepared by the sol–gel technique using titanium isopropoxide and 2-ethyl hexanoate precursors for barium and strontium. The pH value of the precursor sol was adjusted by adding ammonium hydroxide. These films were studied at 7.4 and 9.0 pH and at different molarities ranging from 0.15 to 0.30 M. The films were porous and adsorb water vapour when exposed to a humid environment. The glancing angle X-ray diffraction (GAXRD) pattern clearly shows a preferential orientation of crystallites in (110) direction with increase of pH. Atomic force microscopy (AFM) shows that grain size decreases from 67 to 38 nm with the increase of pH from 7.4 to 9.5, while surface roughness decreases from 0.81 to 0.74 nm. We have found that sensitivity, linearity and response time of the humidity sensor utilizing these films can be improved by tailoring the process parameters such as pH and molarity of the precursor sol. The calculated sensitivity is found to be 46 at 0.30 M, which is higher as compared to the values 40 and 15 at 0.15 M and 0.20 M, respectively, for a given 7.4 pH of sol. As we increase the pH of sol, it was observed that sensitivity of the deposited films increases and reaches to the maximum value (approx. 189) at 9.0 pH.