Ag Doped ${\rm WO}_{3}$ Nanomaterials as Relative Humidity Sensor

Abstract

This paper reports humidity sensing studies of pure WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Ag doped WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> prepared through soft chemical route. Prepared powders have been given pellet shape by applying pressure of 350 MPa. Pellets have been annealed at temperatures of 400°C-700°C. When exposed to humidity, resistance of the pellets is found to decrease with increase in relative humidity (RH). Sensing element of Ag doped WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> annealed at 700°C shows average sensitivity of 2.14 M Ω/%RH in the 20%-90% RH range. For this sensing element, the average hysteresis in the value of sensitivity is within 1.00%. For the sensing element of Ag doped WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , the repeatability over different cyclic operations is within ±3.00% and ±1.00% of the measured values of sensitivity after four and six months, respectively. X-ray diffraction (XRD) pattern of this sensing element shows formation of Ag:WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> bronze. As calculated from Scherer's formula, crystallite size for the sensing elements of pure WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and Ag doped WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> are in 12-72 nm and 19-73 nm range, respectively. The average grain size as measured from Scanning Electron Microscopy (SEM) micrograph for pure WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> is 125 nm, and 147 nm for Ag doped WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , suggesting agglomeration of the crystallites in the sensing element to form larger grains.