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On-chip growth of patterned ZnO nanorod sensors with PdO decoration for enhancement of hydrogen-sensing performance
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this study, we used a low-temperature hydrothermal technique to fabricate arrays of sensors with ZnO nanorods grown on-chip. The sensors on the glass substrate then were sputter decorated with Pd at thicknesses of 2, 4, and 8 nm and annealed at 650 °C in air for an hour. Scanning electron microscopy, high resolution transmission microscopy, X-ray diffraction, and surface analysis by X-ray photoelectron spectroscopy characterization demonstrated that decoration of homogenous PdO nanoparticles on the surface of ZnO nanorods had been achieved. The sensors were tested against three reducing gases, namely hydrogen, ethanol, and ammonia, at 350, 400, and 450 °C. The ZnO nanorods decorated with PdO particles from the 2 and 4 nm layers showed the highest responses to H2 at 450 and 350 °C, respectively. These samples also generally exhibited better selectivity for hydrogen than for ethanol and ammonia at the same concentrations and at all tested temperatures. However, the ZnO nanorods decorated with PdO particles from the 8 nm layer showed a reverse sensing behaviour compared with the first two. The sensing mechanism behind these phenomena is discussed in the light of the spillover effect of hydrogen in contact with the PdO particles as well as the negative competition of the PdO thin film formed between the sensor electrodes during sputter decoration, Pd-Zn heterojunction that forms at high temperature and thus influences the conductivity of the ZnO nanorods.

Keyword [en]
Hydrogen-sensing at high temperature; On-chip hydrothermal growth; ZnO nanorods; Sputter-decoration; PdO nanoparticles
National Category
Other Materials Engineering
Identifiers
URN: urn:nbn:se:uu:diva-320156OAI: oai:DiVA.org:uu-320156DiVA: diva2:1088834
Available from: 2017-04-16 Created: 2017-04-16 Last updated: 2017-04-24
In thesis
1. Microfabricated Gas Sensors Based on Hydrothermally Grown 1-D ZnO Nanostructures
Open this publication in new window or tab >>Microfabricated Gas Sensors Based on Hydrothermally Grown 1-D ZnO Nanostructures
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, gas sensors based on on-chip hydrothermally grown 1-D zinc oxide (ZnO) nanostructures are presented, to improve the sensitivity, selectivity, and stability of the gas sensors.

Metal-oxide-semiconductor (MOS) gas sensors are well-established tools for the monitoring of air quality indoors and outdoors. In recent years, the use of 1-D metal oxide nanostructures for sensing toxic gases, such as nitrogen dioxide, ammonia, and hydrogen, has gained significant attention. However, low-dimensional nanorod (NR) gas sensors can be enhanced further. Most works synthesize the NRs first and then transfer them onto electrodes to produce gas sensors, thereby resulting in large batch-to-batch difference.

Therefore, in this thesis six studies on 1-D ZnO NR gas sensors were carried out. First, ultrathin secondary ZnO nanowires (NWs) were successfully grown on a silicon substrate. Second, an on-chip hydrothermally grown ZnO NR gas sensor was developed on a glass substrate. Its performance with regard to sensing nitrogen dioxide and three reductive gases, namely, ethanol, hydrogen, and ammonia, was tested. Third, three 1-D ZnO nanostructures, namely, ZnO NRs, dense ZnO NWs, and sparse ZnO NWs, were synthesized and tested toward nitrogen dioxide. Fourth, hydrothermally grown ZnO NRs, chemical vapor deposited ZnO NWs, and thermal deposited ZnO nanoparticles (NPs) were tested toward ethanol. Fifth, the effect of annealing on the sensitivity and stability of ZnO NR gas sensors was examined. Sixth, ZnO NRs were decorated with palladium oxide NPs and tested toward hydrogen at high temperature.

The following conclusions can be drawn from the work in this thesis: 1) ZnO NWs can be obtained by using a precursor at low concentration, temperature of 90 °C, and long reaction time. 2) ZnO NR gas sensors have better selectivity to nitrogen dioxide compared with ethanol, ammonia, and hydrogen. 3) Sparse ZnO NWs are highly sensitive to nitrogen dioxide compared with dense ZnO NWs and ZnO NRs. 4) ZnO NPs have the highest sensitivity to ethanol compared with dense ZnO NWs and ZnO NRs. The sensitivity of the NPs is due to their small grain sizes and large surface areas. 5) ZnO NRs annealed at 600 °C have lower sensitivity toward nitrogen dioxide but higher long-term stability compared with those annealed at 400 °C. 6) When decorated with palladium oxide, both materials form alloy at a temperature higher than 350 °C and decrease the amount of ZnO, which is the sensing material toward hydrogen. Thus, controlling the amount of palladium oxide on ZnO NRs is necessary.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 60 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1513
Keyword
gas sensor, zinc oxide, on-chip, hydrothermal growth, nanorods, nanowires, annealing, palladium oxide, photoluminescence, alloy, sensitivity, selectivity, stability
National Category
Other Materials Engineering
Identifiers
urn:nbn:se:uu:diva-320183 (URN)978-91-554-9908-2 (ISBN)
Public defence
2017-06-09, 2001, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2017-05-16 Created: 2017-04-17 Last updated: 2017-06-07

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