Light activated gas nanosensors
- Autores: Oriol González León
- Directores de la Tesis: Eduard Llobet Valero (dir. tes.), X. Vilanova (dir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2018
- Idioma: español
- Tribunal Calificador de la Tesis: Eduardo García Breijo (presid.), Alfonso Jose Romero Nevado (secret.), Mohammed James Anthony Covington (voc.)
- Programa de doctorado: Programa de Doctorado en Tecnologías para Nanosistemas, Bioingeniería y Energía por la Universidad Rovira i Virgili
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX hdl.handle.net
- Resumen
In the last years, the lack of reproducibility, stability and selectivity have been considered as some of the major problems in gas sensing systems based on metal oxide gas sensors. Extracting information from the transient response of metal oxide gas sensors has enabled achieving good results in the improvement of selectivity and also response stability.
The present thesis wants to explore the possibilites of using UV irradiation to improve performance in MOx sensors, in other words, to use an alternative method than temperature, to promote sensor response and baseline recovery. This technique is harnessed by the chemical industry, in which light is used in many photocatalytic reactions that either allow generating new compounds or increase efficiency in standard processes. Oxide materials play often a key role in such photocatalytic reactions.
Using UV light to activate the reactions taking place on the surface of metal oxide gas sensors has been considered by different research groups. According to their research, excitation by means of UV light can affect, in different ways, charge carrier transport across the grain boundaries appearing in polycrystalline metal oxides. Namely, increasing the density of free carriers by photo generation, decreasing the intergrain barrier height, affecting the intergrain states charge and increasing the probability of tunneling through intergrain barriers, since the depletion layer width of adjacent grains is decreased. Moreover, light can change the occupancy of defects by both electrons and holes, this affecting the absorption capacity of the metal oxide surface. On the other hand, illumination can also cause structural defects in the oxide lattice. In this case, these defects disappear when UV light is turned off, but with a long recovery period, that is usually temperature dependent. In other cases, UV light is used exclusively to clean the metal oxide surface, avoiding poisoning. In fact, it was found that using UV light exclusively during the recovery period of room temperature operated In2O3 gas sensors, was more efficient than continuous UV illumination for measuring NO2. UV promotes desorption of NO2 and helps regaining the sensor baseline.
For conducting our experiments, a sensor test chamber was designed and constructed in Teflon. Its inner volume was 24 cm3. The chamber contains sockets to which up to six sensors can be plugged in to be tested. The cover lid houses two UV LEDs so sensors can be operated in ‘temperature mode’ when a constant current is driven through their heating element while the UV diodes are OFF; in ‘UV activation mode’ when heaters are not used and the UV LEDs are ON; and in ‘mixed mode’ when both heating elements and UV LEDs are used simultaneously. The LED to sensing film distance was set to 12 mm, which considering the radiation aperture of the LEDs used (120º), ensured achieving a homogeneous UV irradiation of the sensors. The UV LEDs employed were manufactured by SETI, Sensor Electronic Technology Inc., (model UVTOP320TO39FW) and their maximal emitting optical power was 400 µW at 325 nm. The specifications of the UV-LED indicated that to avoid saturation, the drive current should be kept below 30 mA at an ambient temperature under 55ºC. In our experiments, the drive current was pulsed (diode was on 50% of the time only) and limited to 15 mA. Furthermore, the LED was placed 2 cm away from the heated area of the sensor and kept inserted in a thermally insulating housing. Therefore, the occurrence of saturation effects could be ruled out considering our experimental set-up.
The principal objective of the present thesis was to try combine the use of UV-irradiation and temperature excitation in metal oxides, which we implemented by combining short pulses of UV excitation together with mild heating of the sensing film and analysing the ripple created in film resistance. Both during response and recovery cycles of the sensor towards oxidizing or reducing gases, the UV diode was periodically switched ON and OFF by employing a square driving current signal with a period set to 1 minute, while the sensor was operated at a given constant temperature (e.g., at room temperature, 50ºC or 100ºC). For example, when the sensor is in the presence of an oxidizing gas such as nitrogen dioxide (detection phase) or in pure dry air (cleaning phase), sensor resistance increases while the UV diode is switched OFF and decreases while the UV diode is switched ON, which results in the overall sensor response presenting a ripple.
This ripple appears because UV light reduces the metal oxide, which tends to re-oxidise when UV light is switched OFF. This ripple, which is caused by the effect of surface cleaning (UV-ON) and oxidation (UV-OFF) processes, is considered a transient signal in sensor response and the rate of resistance change both when the UV is ON or OFF have been found to give useful information for determining gas concentration. Especially the rate of resistance change when the light is OFF has been explored in this thesis, since it can be assumed to be closely dependent on the adsorption and reaction processes taking place on the gas-sensitive surface. Evaluating the rate in the period when the light is ON leads to similar information, but the signal has been found to be noisier, especially for tungsten oxide sensors. This rate is computed as the local derivative of the resistance response curve, that is, the derivative when the UV diode is switched OFF. We have found that the signal transients generated from pulsing UV light are dependent on gas concentration. In particular the rate of resistance change defined above shows a sudden increase when the sensor becomes exposed to the rising edge of a gas pulse, and that a quite stable plateau is attained, the value of which is related to gas concentration. The time needed for this plateau to be reached is much faster than the response time of the sensor when operated in isothermal conditions and in the absence of UV light (i.e. standard operation). When the gas is removed from the ambient, the resistance rate shows a sharp decrease as well. When the operating temperature is increased from RT to, for example, 100ºC, the value of the rate stabilizes at higher values, indicating that the adsorption-reaction process is faster. In this thesis we show that pulsed-UV activation has great potential for improving the sensitivity and selectivity in MOx sensors for oxidant and reducing gases, while applying only mild heating or even operating sensors at RT.
This alternative sensor operation approach helps decreasing both response time and power consumption. In figure 3 it can be observed response time comparation under different temperature and UV activations for 10 ppm NH3 detection, we applied to the signals a base line correction and then a normalization, to be able to compare the response time. We can observe that for the same sensor we can obtain a faster response for low temperatures using Pulsed-UV technique.
During the first year we focused our efforts in the synthesis and characterization of a nanostructured In2O3 material in collaboration with Sergio Rosso [36]. We obtained crystalline In2O3 nano- octahedra via a vapour phase transport growth method. This material is the first one we used for applying the pulsed-UV methods developed in this thesis. Sensors were produced by employing a screen-printing technique, because given the high synthesis temperature involved in vapour phase transport, indium oxide octahedral could not be grown directly onto the application substrate. Therefore, the as-synthesized nano-octahedra were mechanically removed from the growth substrate employing a razor blade and mixed in a solution of 1,2-propanediol to form a printable ink. This ink was screen-printed on top of commercially-available (Ceram Tech GmBH), alumina transducer elements, which comprised a pair of Pt interdigitate electrodes (front side) and a Pt heating resistor (back side).
As a conclusion of the present thesis, this pulsed-UV procedure allows the determination of the gas concentration in a faster way using lower temperatures (even at RT), also under humidity conditions and both for oxidising and reducing gases. In the present thesis we have developed a novel way to activate metal oxide gas sensitive layers, combining pulsed-UV light and low temperature heating. Before this thesis, there was no literature available on the implementation of this methodology.
The approach developed in this thesis results in a dramatic enhancement in sensitivity, in significant savings in power consumption, significantly reduced response times and more reliable quantification. This has been achieved by exploiting the dynamics of sensor response under pulsed UV light.
The usefulness of this new approach has been proved employing two different sensing layers: In2O3 and WO3 for detecting oxidising gases (nitrogen dioxide) and reducing gases (ethanol, acetone and ammonia). Results show, that a reduction in the operating temperature of the gas sensors (even RT operation) is achieved. This means savings in power consumption, and improved sensitivity when pulsed UV light excitation is combined to mild heating (up to 100ºC). This reduction in working temperature, would enable using these materials in a wide spectrum of application substrates, including flexible polymeric substrates such as Kapton or polyimide.
In addition, we have proved that a dramatic reduction in sensor response time (at lower operating temperatures) is achieved. This is due to the analysis of response transients employing the rate of resistance change that reaches a steady state value well before sensor resistance reaches the steady state. Furthermore, the recovery time for these transients is even faster than when only temperature excitation is employed.
It was noticed that this technique is useful for, not only increasing sensitivity, but also for improving the selectivity. For instance, when measuring ethanol and acetone at low operating temperature, both reducing gases show opposite behaviour, that is, in the case of acetone, the sensor behaves as a p-type semiconductor, while it behaves as a n-type semiconductor for ethanol.
