Si Doped Metastable Epsilon-WO3 Nano-Particle Film for Human Breath Acetone Sensing

Acetone is an important marker in the human breath for type 2 diabetes whose concentration is proportional to the blood glucose level. Portable handheld acetone sensor have a strong potential for replacing the existing invasive blood glucose monitoring infrastructure. Metal oxide semiconductors such as n-type WO₃ have remarkable sensing properties for acetone, NO_x, H₂ and O₃. In the crystal structure of WO_3, the WO₃ octahedron has a central W atom surrounded by six oxygen ions at the corners. The slight rotation of these octahedra with respect to each other, as well as unequal bond lengths in the octahedral coordination, causes lattice distortion and reduces the symmetry giving rise to different stable polymorphic phases depending on the temperature (monoclinic ε WO₃ (-40 °C), triclinic δ WO₃ (17 °C), monoclinic γ WO₃ (320 °C), orthorhombic β WO₃ (720 °C), tetragonal α WO₃ (900 °C). In particular, the ε-WO₃ is of interest for acetone sensors because of the existence of a non-vanishing polarization in its octahedra. This spontaneous polarization is responsible for the selective detection of low concentration of high dipole moment analytes such as acetone. Sensor response (S) is a function of the conductivity of the WO₃film on inter-digitated Pt or Au electrodes which is given by the following equation:

S = (C_analyte/C_air)-1

where, C_analyte is the conductivity of the film in presence of testing analyte and C_airis the conductivity of the film in air.

The gas detection occurs because of the reaction on the WO₃surface in presence of these gases causing a change in electron concentration and hence fluctuation in electrical conductivity.

In this work Reactive Spray Deposition Technology (RSDT) has been employed as a single step flame based process for synthesizing 7-15 nm of SiO₂ doped WO₃ particles of 100% ε phase directly on Pt or Au interdigitated electrodes. RSDT is able to synthesize this phase of WO₃ nano particles by (1) Rapidly quenching the particles produced from the film from 700 ˚C to 30 ˚C in 7 s, via an air quench flowing at 10 L/min. (2) Doping WO₃ with 3-5 wt% amorphous SiO₂ to prevent the formation of stable γ WO₃. Quenching the particles as soon as they are formed serves two roles. First it assists in the formation of acentric ε WO₃ by freezing the WO₃ lattice in its thermodynamically unstable state by rapid reduction of temperature from 700 ˚C to 30 ˚C in 7 s. This does not provide enough time for the tungsten atoms to settle down in their stable position. Second it prevents sintering of the particles. Smaller size WO₃ particles tends to have a higher concentration of defects and higher deformation is plausible during crystal growth. Amorphous silica introduces defects in the WO₃lattice and pin the particle surfaces together, thereby reducing the driving force for phase transformation. The influence of crystal structures on sensing activities will be explored in detail.

The structure and morphology of the WO₃ films prepared by RSDT has been probed by XRD, Raman Spectroscopy, scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). The as-prepared WO₃ sensors will be applied for the detection of acetone sensitivities (0.5-5 ppm in simulated breath) in the temperature range of 30-500 ˚C. Characteristics such as sensitivity, selectivity, limit of detection, reproducibility, and stability will be reported. The selectivity for the interfering analytes such as humidity, CO₂, NO₂, H₂ and ammonia will also be investigated. Finally the sensing mechanism will be discussed.