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In this work, optical sensing performance of tapered multimode fiber tip coated with graphene oxide (GO) nanostructured thin film towards aqueous ethanol with different concentrations is investigated. The tapering process of the optical fiber is done by a glass processing machine. The multimode optical fiber tip is dip-coated with GO and annealed at 70 °C to enhance the binding of the nanomaterials to the silica fiber. FESEM, Raman microscopy and XRD analyses are performed to micro-characterize the GO thin films. The morphology of the GO is observed to be in sheets forms. The reflectance response of the GO coated fiber tip is compared with the uncoated tip. The measurements are taken using a spectrophotometer in the optical wavelength range of 550–720 nm. The reflectance response of the GO coated fiber tip reduced proportionally, upon exposure to ethanol with concentration range of 5–80%. The dynamic response of the developed sensor showed strong reversibility and repeatability when it is exposed to ethanol with concentrations of 5%, 20% and 40% in distilled water. At room temperature, the sensor shows fast response and recovery as low as 19 and 25 s, respectively.

A simple technique to fabricate nanostructured WO3 thin films onto conductomeric transducers has been employed for ethanol sensing application. Initially, pure tungsten (W) thin films were deposited onto the substrate employing RF sputterer and followed by an etching process. Three types of etching agent were used: nitric (HNO3), sulphuric (H2SO4), and phosphoric (H3PO4) acid. It was found that the surface morphology and crystallinity of the WO3 films were heavily dependant to the etchants employed during the fabrication process. The developed sensors were tested towards ethanol vapor of different concentrations (10–200 ppm) at temperatures between room and 450 °C. The sensors showed stable and reproducible response at optimum operating temperatures. High sensor response towards vaporized ethanol as well as fast τres and τrec was observed during the “adsorption” and “desorption” interval. The recorded maximum response for these devices when exposed towards 100 ppm ethanol was measured to be 8 (Ro = 4.6 kΩ), 5.8 (Ro = 22.5 GΩ), and 5 (Ro = 0.29 MΩ) for HNO3, H3PO4, and H2SO4, respectively. The optimum operating temperatures were determined to be 400, 300–380, and 360 °C for the sensors developed using HNO3, H3PO4, and H2SO4, respectively.

The gasochromic response of WO3 nanostructured films coated with a catalytic Pt or Pd layer on different transparent substrates upon exposure to H2 gas was investigated. WO3 nanostructured films with 500 nm thickness were coated with a 25 Å thick Pt or Pd layer. The films were prepared on quartz, glass, indium-doped tin oxide (ITO) and fluorine-doped tin oxide (FTO) conductive glass. The nanostructured WO3 was deposited by RF magnetron sputtering, and the Pt or Pd layer was deposited by DC sputtering. Characterization of the film revealed that the WO3 was deposited as nanoscale grains of varied size depending on the substrate. WO3 grains on quartz and glass were 30–40 nm in size. WO3 grain sizes on ITO and FTO were 40–60 nm and 300–500 nm, respectively. The WO3 films were observed to show strong gasochromic response, with absorbance changes measured in the Vis-NIR (500–1100 nm) range. The cumulative absorbance response towards H2 is the highest and more stable for Pd/WO3 films on quartz, glass and ITO, compared to the FTO substrate. The gasochromic effect was also stronger in Pd/WO3 films compared to Pt/WO3 films.

Pt/nanostructured WO3/SiC Schottky diodes were fabricated and applied for hydrogen gas sensing applications. The nanostructured WO3 films were synthesized from tungsten coated SiC substrates via an acid-etching method using a 1.5 M HNO3 solution for 1 hr, 2 hrs and 3 hrs duration. Scanning electron microscopy of the developed films revealed platelet crystals with thicknesses in the order of 20-60 nm and lengths between 100-700 nm. X-ray diffraction analysis revealed that the rate of oxidation of tungsten increases as the duration of acid-etching increases. The devices were tested towards hydrogen gas balanced in air at different temperatures from 25°C to 200°C. At 200°C, voltage shifts of 0.45 V, 0.93 V and 2.37 V were recorded for devices acid-etched for 1 hr, 2 hrs and 3 hrs duration, respectively upon exposure to 1% hydrogen, under a constant forward bias current of 500 µA.

An oriented graphitic nanostructured carbon film has been employed as a conductometric hydrogen gas sensor. The carbon film was energetically deposited using a filtered cathodic vacuum arc with a -75 V bias applied to a stainless steel grid placed 1cm from the surface of the Si substrate. The substrate was heated to 400°C prior to deposition. Electron microscopy showed evidence that the film consisted largely of vertically oriented graphitic sheets and had a density of 2.06 g/cm3. 76% of the atoms were bonded in sp2 or graphitic configurations. A change in the device resistance of >; 1.5% was exhibited upon exposure to 1 % hydrogen gas (in synthetic, zero humidity air) at 100°C. The time for the sensor resistance to increase by 1.5 % under these conditions was approximately 60 s and the baseline (zero hydrogen exposure) resistance remained constant to within 0.01% during and after the hydrogen exposures.

Pt/anodized TiO2/SiC based metal-oxide-semiconductor (MOS) devices were fabricated and characterized for their sensitivity towards propene (C3H6). Titanium (Ti) thin films were deposited onto the SiC substrates using a filtered cathodic vacuum arc (FCVA) method. Fluoride ions containing neutral electrolyte (0.5 wt% NH4F in ethylene glycol)were used to anodize the Ti films. The anodized films were subsequently annealed at 600 °C for 4 hrs in an oxygen rich environment to obtain TiO2. The current-voltage(I-V) characteristics of the Pt/TiO2/SiC devices were measured in different concentrations of propene. Exposure to the analyte gas caused a change in the Schottky barrier height and hence a lateral shift in the I-V characteristics. The effective change in the barrier height for 1% propene was calculated as 32.8 meV at 620°C. The dynamic response of the sensors was also investigated and a voltage shift of 157 mV was measured at 620°C during exposure to 1% propene.