AbstractâThe conductometric response of metal oxide nanowires and nanocrystals to oxidizing molecules at room temperature and under UV illumination is ...
1ΣΠ∆ΦΦΕϑΟΗΤ ΠΓ ΥΙΦ 2009 4ΘΒΟϑΤΙ ∃ΠΟΓΦΣΦΟ∆Φ ΠΟ &ΜΦ∆ΥΣΠΟ %ΦΩϑ∆ΦΤ - ∋ΦΧ 11-13, 2009. 4ΒΟΥϑΒΗΠ ΕΦ ∃ΠΝΘΠΤΥΦΜΒ, 4ΘΒϑΟ.
Room temperature conductometric gas sensors based on metal oxide nanowires and nanocrystals Manzanares M.a, Prades J.D.a, Cirera A a, Andreu T.a, Hernández-Ramírez F.a, Jiménez-Rodríguez R.a, Romano-Rodríguez A.a, Morante J.R.a,b a EME/XaRMAE/IN2UB, Departament d’Electrònica, Universitat de Barcelona, SPAIN b Institut de Recerca en Energia de Catalunya (IREC), SPAIN II. EXPERIMENTAL Abstract—The conductometric response of metal oxide nanowires and nanocrystals to oxidizing molecules at room temperature and under UV illumination is studied. We demonstrate that UV illumination induces an electron-hole pair generation as well as a surface oxygen’s desorption leading to a reduced surface that is highly reactive to oxidizing species. Based on these mechanisms, we present room temperature conductometric sensors based on nanoparticles and nanowires. The electrical response of these devices under different illumination conditions is theoretically modeled and predicted. The interfering effect of other gases, such as humidity, is also analyzed.
I. INTRODUCTION Metal oxides (MOXs) are extensively used as active sensing material for transducing chemical information to electrical signals. On one hand, their surface properties enable this functionality. On the other hand, their oxygen vacancies play a key role in the effectiveness of gas-MOX interactions. Among these materials, tin dioxide (SnO2), tungsten trioxide (WO3), indium oxide (In2O3) and zinc oxide (ZnO) are some of the most relevant materials used for gas sensing applications. Nowadays, MOXs are essentially being used as nanocrystals because of their higher surface-to-volume ratio, their improved surface stability and their well-controlled intrinsic properties. As a consequence, the utilization of bundles of nanowires and 3D networks of nanocrystals has been reported as a promising alternative to improve gas sensing response [1], [2]. The use of advanced nanostructured materials reveals and emphasizes some properties and arise important questions about the actual mechanisms that occur at the surface of metal oxides. Here we study two of them. i) The transduction mechanisms of both oxidizing and reducing molecules that are effective at room temperature. ii) The persistence of the photoconductivity (PPC) long after switching off the UV illumination. We demonstrate that UV illumination combined with temperature can be used to reset the surface after gas measurements. We also show that it is possible to use this approach to develop gas sensors that can operate at room temperature. [3]–[8].
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In this work, different MOXs (WO3 and In2O3) were synthesized replicating two different mesoporous structures (KIT-6 and SBA-15), following a procedure described elsewhere [9]. SnO2 nanowires were also used [10]. MOXs based sensors have been prepared in two kinds of substrates: micromechanized silicon hotplates and alumina substrates (screen-printed on the top side with a Pt electrode circuit and on the back side with a heater). SnO2 devices have been prepared by direct CVD growth of the bundles of nanowires whereas mesoporous structure samples have been deposited by micro-dropping or spin coating.. The sensors responses were measured in an experimental setup similar to the one described in [11]. Inside the test chamber of 40 cm3 in volume, a constant flow-through of 0.2 l/min regulated by Bronkhorst mass flow controllers was maintained. In order to expose the in-chamber samples to UV light, the test chamber was equipped with a UV-transparent quartz window (Linkam TS 1500). A xenon lamp (with a maximum in its spectral distribution at 365 nm) and a monochromatic LED (340nm) were used as UV light sources. The measurements were carried out both in dry and humid air (50% of relative humidity (RH)). III. RESULTS AND DISCUSSION Fig. 1 shows the change in the resistance of nano-structured In2O3 under UV illumination and at room temperature (RT). Similar results were obtained with the rest of the here-studied MOXs (ZnO, WO3, SnO2). After the switching off the UV illumination, the resistance increases slightly and evolves towards a new steady value that is far away from the initial one. This corresponds to the persistent photoconductivity situation. It is noteworthy that initial value of the resistance can only be recovered by heating the sensor. This complex phenomena is explained by the electron-hole separation effects caused by the built-in potential near the MOX surface that prevents the recombination of the photogenerated electrons and holes [12] Under UV illumination, these nanomaterials present
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1ΣΠ∆ΦΦΕϑΟΗΤ ΠΓ ΥΙΦ 2009 4ΘΒΟϑΤΙ ∃ΠΟΓΦΣΦΟ∆Φ ΠΟ &ΜΦ∆ΥΣΠΟ %ΦΩϑ∆ΦΤ - ∋ΦΧ 11-13, 2009. 4ΒΟΥϑΒΗΠ ΕΦ ∃ΠΝΘΠΤΥΦΜΒ, 4ΘΒϑΟ. 10
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response towards oxidizing gases, even at room temperature (Fig. 2 and Fig. 3). Moreover, illumination no only enlarges the magnitude of the response but also accelerate its recovery. To explain this behavior, it is necessary to assume that UV light can desorb oxidizing species (NO2 and oxygens) from the MOXs surface [13], [14]. Another encouraging result is found when comparing with measurements made at different operating temperatures. As shown in Fig. 4, the sensitivity, the stability, the response time and the recovery time are much better under UV illumination than at the usual working temperatures. These behaviors are opening a new breath for engineering a new technology of sensor devices working at room temperature and with an energy consumption balance even smaller that the reported for the integrated hotplate based gas sensors. Besides an energetic saving, operating at room temperature allows an improvement in safety when working in
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Fig. 3. Measurements made at RT and RH=50%. (a) Response of two similar WO3 samples with KIT-6 structure to 0.4, 0.7, and 1.0 ppm of NO2 under UV illumination. (b) Response of WO3 sample with SBA-15 structure to 0.2 ppm of NO2 in dark and under UV illumination.
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Fig. 4. SnO2 nanowires based sensor response to 10 ppm of NO2 (a) under UV illumination and (b) at different working temperatures.
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1ΣΠ∆ΦΦΕϑΟΗΤ ΠΓ ΥΙΦ 2009 4ΘΒΟϑΤΙ ∃ΠΟΓΦΣΦΟ∆Φ ΠΟ &ΜΦ∆ΥΣΠΟ %ΦΩϑ∆ΦΤ - ∋ΦΧ 11-13, 2009. 4ΒΟΥϑΒΗΠ ΕΦ ∃ΠΝΘΠΤΥΦΜΒ, 4ΘΒϑΟ. ACKNOWLEDGMENT This work has been funded by the EU [project NANOS4 (MMP4-CT-2003-001528)] and the Spanish Government [project MAGASENS (NAN2004-09380-C04-01)]. SnO2 nanowires were supplied by the CNR-INFM Group within the Nanos4 Consortium framework.
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