Novel resistive readout CO2 sensor based on receptor -transducer work function coupling and electron affinity - band bending compensation
Final Report Abstract
The goal of this work was to demonstrate that it is possible to realize a chemoresistive CO2 sensor by combining two different materials in a receptor-transducer manner: the material acting as a CO2 sensitive receptor, transfers its induced work function/electronic affinity change to a band bending change for the second material. The first materials that were chosen as candidates for the receptor, because of the known CO2 strong interaction with basic hydroxyl groups, were metal hydroxides. Among them, nanocystalline neodymium hydroxide (Nd(OH)3) was already synthesised by the group of Markus Niederberger (ETH Zürich), by a non aqueous preparation route. It was used for the preparation of sensing layers - made into a paste, screen printed onto alumina substrates, thermally treated - and then tested at different temperatures above 200°C for CO2 detection; surprisingly this material shows large resistance changes upon CO2 exposure. A later XRD measurement identified neodymium oxycarbonate (Nd2O2CO3) as the actual sensing material. Additional XRD examinations could clarify how the hydroxide transforms to the operational sensing material: if the material is heated to over 200°C it forms an amorphous phase, which under CO2 exposure, during the first CO2 sensor measurement, switched to the crystalline oxycarbonate. This unexpected result indicated a possible novel chemoresitive material (which would directly change resistance when exposed to CO2) without any additional changes. In both cases, receptor or receptor and transducer, the Neodymium oxycarbonate gas sensing mechanism is worth investigating and we have done that by using operando DRIFT spectroscopy, meaning that a sensor is mounted into the test chamber inside of the infrared spectrometer and its electrical resistance (sensor signal) is measured simultaneously measured with the recording of the infrared spectra. The experiments indicated that the sensing is based on the interaction between water vapour and the oxycarbonate, which is influenced by the presence of CO2 in the ambient. The hydroxyl groups formed by the interaction with water vapour are, most probably, donors that determine a decrease of the sensing layer resistance. The exposure to CO2 reduces the amount of surface hydroxyls by substituting them with carbonate groups. The presence of humidity is, though, essential for the sensing but even low levels, almost present in indoor air quality applications, are enough for good sensor signals. One of the crucial issues for gas sensors is the effect of other gases that may be present in the ambient atmosphere (interfering gases). Tests were performed to H2, NO2, CO and O2. The only important interference is provided by CO, the sensing mechanism of which involves the conversion to CO2. It is also worth noting that besides the low "sensor effect" of oxygen, the detection of CO2 does not really depend on the oxygen concentration in the ambient. This fact very much differentiates the oxycarbonate from the "classical" semiconducting metal oxides used for gas sensing (SnO2, WO3, In2O3, etc). It was very interesting to find out if the CO2 sensitivity is related to neodymium, to its class - rare earthsor to something else (also Nd is a relatively expensive metal so a replacement was also economically interesting). Therefore, different other rare earth hydroxides were synthesised by using the same synthesis procedure. Firstly, a Praseodymium hydroxide (Pr(OH)3) was synthesized but it was observed that during the thermal treatments this material converts directly to a metal oxide (Pr9O16) and shows no sensitivity to CO2. Secondly, Lanthanum hydroxide was synthesized, which directly converts to the oxycarbonate already at 250°C; the resulting Lanthanum oxycarbonate shows large resistance changes when exposed to CO2. In fact, the overall sensing properties are a little bit better, at a much lower precursor cost. Summarizing, somehow unexpectedly, we identified two CO2 sensitive chemoresistive materials in the class of rare earths oxycarbonates; we also shown that the sensing is based the interaction between the pre-adsorbed hydroxyl groups and CO;. The main drawback is the very high value of the resistance of the sensing layer, which makes the costs of the associated electronics prohibitively high. This was making us go back to the initial idea, namely use a different material in order to translate the large changes taking place onto the surface of the receptor into a resistance change of the transducer. It is expected that changes in the band bending at the surface of the receptor will not induce changes in the band bending (and in the electrical resistance) of the transducer so it was very important to investigate the work function changes at the receptor surface; if besides band bending there are also large electron affinity changes, we hoped to see those translated into band bending (resistance changes) of the transducer. The experiments revealed that both matenals show a very high change in the work function while exposed to CO2 that can't be associated to the change of resistance/band bending only. The transducer material we chose is SnO2 and it was made by a common wet chemistry technology; it is well-studied in our lab and showing negligible resistance changes when exposed to CO2. Mixtures of 5 and 10% wt. percent of Nd2O2CO3 / La2O2CO3 in SnO2 were prepared; the concentrations were chosen in order to make sure that, on the one hand, there is no percolation through the oxycarbonates and, on the other hand, there is enough receptor material to have an influence on the transducer. The measurement results show that the mixtures experience large resistance changes under CO2 exposure. The values of the sensor resistance are still very large and, on the positive side, the response time is much faster when compared with the pure receptor materials. We think that by that we met the objective of our research because we were able to demonstrate that the changes of the electron affinity of the receptor material are translated into changes of the electrical resistance of the transducer. We were also able to find two novel CO2 receptor materials and we explained the sensing mechanism. To move on from the proof of concept in the direction of a prototype gas sensor, we need to address a couple of issues, some of them related to the basic understanding of the sensing and the others of practical relevance. For the latter, we need to find ways to decrease the sensors' resistance; it is too high to be measured easily/cheaply; the approach is to test different transducer material, especially semiconducting oxides with high intrinsic conductivity. Also here, we will need to optimize the sensing layer morphology and the coupling between the receptor and transducer. Another practical issue is related to the cost of the precursors, because the lanthanum isopropoxide precursors are very expensive, difficult to handle and are not readily available on the market; this asks for finding a different synthesis routes for hydroxides that can be used for conversion a CO2 sensitive oxycarbonate. On the basic understanding side, we need to deepen our understanding of the reaction between the oxycarbonate and CO2 and on the electronic coupling between the receptor and the transducer. We also need to better understand the conduction process in the oxycarbonate.
Publications
- Sensoren im Automobil IV - Expert-Verlag (ISBN-13: 978-3-8169-3066-2)
- CO2 sensing with chemoresistive Nd2O2CO3 sensors - Operando insights. Procedia Chemistry 1,650-653(2009)
A. Haensch, I. Djerj, M. Niederberger, N. Barsan, U. Weimar
- Eurosensors 2009. CO2 sensing with chemoresistive Nd2O2CO3 sensors; Operando insights
- Neodymium Dioxide Carbonate as a Sensing Layer for Chemoresistive CO2 Sensing. Chemistry of Materials 21 , 5375-5381(2009)
I. Djerdj, A. Haensch, D. Koziej, S. Pokhrel, N. Barsan, U. Weimar, M. Niederberger
- Eurosensors 2010. Rare earth oxycarbonates as a new class of materials for chemoresistive CO2 gas sensors
- Rare earth oxycarbonates as a material class for chemoresistive CO2 gas sensors. Procedia Engineering 5,139-142(2010)
A. Haensch, D. Koziej, M. Niederberger, N. Barsan, U. Weimar
