A conductometric sensor based on ZnO nanoneedles for the detection of NO2 is described. The material is grown on chip over Pt interdigitated electrodes patterned on alumina substrates without the need of a catalyst layer. The nanostructure growth relies on two different mechanisms (Vapor-Solid and Liquid-Solid) so nanoneedles with few mu m of length and wurtzite structure are obtained. The procedure is optimized on chip, which supposes a significant advantage in the fabrication of nanostructures on sensing devices. The sensor response has been measured under a target gas (NO2) and two interfering pollutants (benzene and formaldehyde). Lower working temperatures than other pure ZnO nanostructures found in the literature have been achieved and limit of detection (LOD) in the order of ppb has been reached. The significant higher response to NO2 with respect to benzene and formaldehyde makes this sensing device suitable for selective NO2 detection indoors.

Tin dioxide nanowires have been grown by thermal oxidation of sputtered thin films by means of a VLS method. A tin sputtered layer catalyzed by gold nanoparticles acts as a material seed for the localized growth of NWs directly on gas sensor devices, avoiding the manipulation and transport of the nanowires to the electrodes. XRD and HRTEM analysis show that the nanowires crystallize in a rutile structure with a [100] preferential growth direction, and are single-crystalline with diameters lower than 50 nm. The response of nanowires to formaldehyde has been compared to thin film based sensors. A sensitivity of 0.10 ppm(-1) is reported, twofold the sensitivity of the thin film, and short response and recovery times are measured (6 times shorter than thin films). The sensing mechanism proposed for the SnO2 NWs under formaldehyde exposure is explained by means of conduction measurements and FT-IR analysis. Oxygen species chemisorbed on the surface of each SnO2 nanowire produce a band bending, which generates a potential barrier (of 0.74 +/- 0.02 eV at 300 degrees C) between the point contact of different nanowires. As evidenced by IR spectroscopy at 300 degrees C, electrons in the conduction band and in monoionized oxygen vacancies (at 0.33 eV below the bottom of the conduction band) are responsible for gas detection.

The objective of the work described in this paper is to develop a device to monitor air quality in indoor environments integrating three conductometric gas sensors based on thin film and nanostructured metal oxide semiconductors (SnO2, NiO and ZnO). The sensors are incorporated into a single robust, reliable and cheap detection platform, which includes air pre-conditioning and electronics. The main aim of the device is to integrate with HVAC (Heat Ventilation and Air Conditioning) in an energy-efficient way whilst maintaining a high air quality standard within the building. Due to the lack of common EU legislation, the target gases and detection limits have been set after reviewing the literature and the recommendations of different agencies in Europe and the US, focusing on indoor Volatile Organic Compounds (VOCs).

The aim of this work is to develop an easy-to-manufacture and highly-sensitive conductometric microsensor for indoor air quality (IAQ) monitoring. The sensing device consists on ZnO nanostructures on Pt interdigitated electrodes and a Pt heater surrounding the sensing layer, fabricated on one side of a 2.5x2.5 mm2 alumina substrate. ZnO nanostructures are grown in-situ over the electrodes, using the Vapour-Solid (VS) approach. The samples were tested under different concentrations of benzene, formaldehyde, carbon monoxide and nitrogen dioxide, showing significant response to low concentrations of the four gases

The aim of this work is to develop an easy-to-manufacture and highly-sensitive conductometric microsensor for indoor air quality (IAQ) monitoring. The sensing material is nanostructured ZnO on Pt electrodes. A Pt heater surrounds the ZnO, so the whole sensor is fabricated on one side of a 2,5x2,5 mm2 alumina substrate. ZnO nanostructures are grown in-situ over the electrodes, using the Vapour-Solid (VS) approach. The samples have been tested under different concentrations of benzene, formaldehyde and nitrogen dioxide, showing significant response to low concentrations of the three gases.