Revistas
Revista:
SENSORS AND ACTUATORS B-CHEMICAL
ISSN:
0925-4005
Año:
2022
Vol.:
361
Págs.:
131694
In this article, CFD simulations results are presented as a key tool to the comprehension of the target gas concentration evolution in a test chamber, at different working conditions. The simulation results are compared with the experimental data, which shows a qualitative good correlation with the evolution of the concentration gradient detected. The experiments were carried out using an aluminum gas test chamber, where a WO3 based conductometric sensor is introduced. The results demonstrate how the response time is dependent on the sensor working conditions. Analyzing the CFD and experimental results, some assumptions for this behavior are proposed. The WO3 sensor needs a Pt heating element, which is heated up to 300 degrees C. As the response is highly temperature-dependent, the temperature distribution on the sensor surface was measured by an IR thermographic camera. The simulation results show that the temperature distribution matches with those obtained experimentally. To validate the model, a mesh and time step convergence study was also implemented.
Revista:
RSC ADVANCES
ISSN:
2046-2069
Año:
2021
Vol.:
11
N°:
54
Págs.:
34144 - 34151
ZnO conductometric gas sensors have been widely studied due to their good sensitivity, cost-efficiency, long stability and simple fabrication. This work is focused on NO2 sensing, which is a toxic and irritating gas. The developed sensor consists of interdigitated electrodes covered by a ZnO sensing layer. ZnO has been grown by means of the aerosol assisted chemical vapor deposition technique and then nanostructured by laser interference lithography with a UV laser. The SEM and XRD results show vertically oriented growth of ZnO grains and a 2D periodic nanopatterning of the material with a period of 800 nm. Nanostructuring lowers the base resistance of the developed sensors and modifies the sensor response to NO2. Maximum sensitivity is obtained at 175 degrees C achieving a change of 600% in sensor resistance for 4 ppm NO(2)versus a 400% change for the non-nanostructured material. However, the most relevant results have been obtained at temperatures below 125 degrees C. While the non-nanostructured material does not respond to NO2 at such low temperatures, nanostructured ZnO allows NO2 sensing even at room temperature. The room temperature sensing capability possibly derives from the increase of both the surface defects and the surface-to-volume ratio. The long stability and the gas sensing under humid conditions have also been tested, showing improvements of sensitivity for the nanostructured sensors.
Revista:
RSC ADVANCES
ISSN:
2046-2069
Año:
2021
Vol.:
11
N°:
30
Págs.:
18493 - 18499
Aerosol-assisted Chemical Vapor Deposition (AACVD) is a thermally activated CVD technique that uses micro-droplets as deposition precursors. An AACVD system with a custom-designed reaction chamber has been implemented to grow ZnO thin films using zinc chloride as a precursor. The present work aims to study the impact of the deposition parameters on the thin film, as well as the microstructure evolution and growth kinetics. Aerosol flow has an effect on the density of nucleation sites and on the grain size. The temperature affects the morphology of the grown ZnO, showing a preferential orientation along the c-axis for 350 degrees C, 375 degrees C and 400 degrees C substrate temperatures. The microstructural evolution and the growth kinetics are also presented. A different evolution behavior has been observed for 350 degrees C, where nucleation site density is the highest at the early stages and it decreases over time in contrast with the cases of 375 degrees C and 400 degrees C, where there is an initial increase and a subsequent decrease. The activation energy of the chemical reaction is 1.06 eV. The optical characterization of the material has been performed through reflection measurements showing a relationship between the spectrum and the ZnO film thickness. The electrical characterization has been done by means of an interdigital capacitor, with which it is possible to measure the grain and grain boundary resistance of the material. Both resistances are of the order of 10(5)-10(6) omega.
Autores:
Parellada-Monreal, L. (Autor de correspondencia); Gherardi, S.; Zonta, G.; et al.
Revista:
SENSORS AND ACTUATORS B-CHEMICAL
ISSN:
0925-4005
Año:
2020
Vol.:
305
Págs.:
127226
In this paper two kind of sensors based on WO3 sputtered by magnetron sputtering and annealed at 600 degrees C have been studied. The first kind was processed by two-dimensional direct laser interfering patterning (DLIP) and the second one without any additional treatment. Morphological and structural characterization have shown a hole structure in a periodic line-pattern for the DLIP-processed sensors and a flat surface for the only-annealed sensors, both with a tetragonal WO3 phase. TOF-SIMS analysis has revealed that the first WO3 layers are reduced for both samples, which could improve sensing performance. Promising response enhancement of DLIP-processed sensors has been observed for low concentrations of NO2 (from 0.5 ppm to 5 ppm) at 200 degrees C, lowering the limit of detection (LOD) to 10 ppb, half of the LOD of the only-annealed sensors (20 ppb). Cross sensitivity to CO and HCHO have been investigated and the sensing mechanisms discussed.
Revista:
APPLIED SURFACE SCIENCE
ISSN:
0169-4332
Año:
2019
Vol.:
476
Págs.:
569 - 575
Femtosecond laser-induced periodic structures (LIPSS) have been processed on ZnO thin film gas sensor devices for nitrogen dioxide (NO2) detection. From the morphology point of view, the nanostructures have been identified as high spatial frequency LIPSS (HSFL) with an average period of 145 nm. Through Raman analysis, a decrease of the typical wurtzite ZnO structure is shown, with a possible increase of defects such as Zn interstitials. The response under NO2 is enhanced if compared with the only-annealed ZnO thin film for concentrations as low as 1 ppm, reaching 1 ppb of detection limit (LOD) for the sensors with LIPSS. The Zn interstitials defects could be the source of the adsorbed NO2 species increasing the sensitivity. Reproducible results have been measured during 11 weeks in a row.
Revista:
APPLIED SURFACE SCIENCE
ISSN:
0169-4332
Año:
2018
Vol.:
441
N°:
31
Págs.:
331 - 340
ZnO thin film sputtered on alumina substrate is processed by Direct Laser Interference Patterning (DLIP). The heat transfer equation has been simulated for interference patterns with a period of 730 nm and two different fluences (85 mJ/cm2 and 165 mJ/cm2). A thermal threshold of 900 K, where crystal modification occurs has been calculated, indicating a lateral and depth processing around 173 nm and 140 nm, respectively. The experimentally reproduced samples have been analyzed from the structural and composition point of view and compared to conventional thermal treatments at three different temperatures (600 ºC, 700 ºC and 800 ºC). Promising properties have been observed for the laser treated samples, such as low influence on the thin film/substrate interface, an improvement of the crystallographic structure, as well as a decrease of the oxygen content from O/Zn = 2.10 to 1.38 for the highest fluence, getting closer to the stoichiometry. The DLIP characteristics could be suitable for the replacement of annealing process in the case of substrates that cannot achieve high temperatures as most of flexible substrates.
Revista:
SENSORS AND ACTUATORS B-CHEMICAL
ISSN:
0925-4005
Año:
2018
Vol.:
255
Págs.:
1244 - 1253
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.
Revista:
ELECTROANALYSIS
ISSN:
1521-4109
Año:
2017
Vol.:
29
N°:
10
Págs.:
2358 - 2634
A potentiometric thin-film sensor to detect CO2 in a wide range (2-100%) has been developed. The system has been fabricated depositing a reference electrode of Pt, a solid electrolyte of YSZ (Yttria-stabilized Zirconia), a sensing phase made of Li2CO3 and a working electrode of Au via Physical Vapor Deposition (PVD). Characterization of the different elements has provided the optimal fabrication parameters and the system response for CO2 concentrations can be measured from 2 to 100% at 450°C. The sensor behaves as a non-Nerstian system and slightly deviates from a linear response with the logarithm of CO2 until the CO2 concentration reaches the 30%. Higher CO2 amounts make the response divert more from the Nernst law but give a stable and reproducible response to CO2 in a wide range of concentrations. Based on these promising results the recovery time, stability, repeatability and selectivity of the sensor have been measured. The performance showed by the thin film sensor proves the feasibility of the use of this system for biogas and natural gas applications owing to its very good consistency at low temperature in a wide concentration range.
Revista:
RSC ADVANCES
ISSN:
2046-2069
Año:
2016
Vol.:
6
N°:
22
Págs.:
18558 - 18566
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.
Revista:
CRYSTENGCOMM
ISSN:
1466-8033
Año:
2015
Vol.:
17
N°:
7
Págs.:
1597 - 1602
Li2CO3 sputtered films of 300 nm have been subjected to physical and electrochemical characterization methods to analyze the influence of annealing treatments at 600 degrees C for 2 h, 6 h, 12 h and 18 h on the microstructure, surface and conductivity. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS) have been used for this purpose. XRD and FT-IR have illustrated the evolution of the microstructure with annealing time. AFM analysis has shown the growth of new Li2CO3 particles that increases with annealing time presenting a maximum diameter of 16.8 mu m without compromising the continuity of the films. EIS measurements have described a fall in the activation energy of the Li2CO3 thin films presenting a minimum around 1.18 eV. The results concerning the activation energy of the films have shown an improvement compared to the results obtained previously for Li2CO3. These results serve to understand and optimize the behaviour of the Li2CO3 thin films in gas sensors, fuel cells or Li+ ion batteries.
Revista:
METTERDAAD MAGAZINE
ISSN:
2212-0114
Año:
2015
Vol.:
14
N°:
Part 3
Págs.:
475 - 485
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).
Revista:
PROCEDIA ENGINEERING
ISSN:
1877-7058
Año:
2015
Vol.:
120
Págs.:
711 - 716
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
Revista:
PROCEDIA ENGINEERING
ISSN:
1877-7058
Año:
2014
Vol.:
87
Págs.:
983 - 986
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.
Revista:
CRYSTENGCOMM
ISSN:
1466-8033
Año:
2014
Vol.:
16
N°:
27
Págs.:
6033 - 6038
The physical and electrochemical characterization of Li2CO3 thin films allows for an improved understanding of their behaviour in electrochemical devices. Firstly, the Li2CO3 deposition process by RF magnetron sputtering was described. Afterwards, X-ray diffraction (XRD), atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS) characterization techniques were employed to study the influence of temperature on the material. This way, the optimal annealing temperature as well as the optimal operating temperature of Li2CO3 were determined. In light of the obtained results, it was concluded that a fabrication annealing temperature of 600 degrees C and an optimal operating temperature of 350 degrees C were set.
Revista:
ACTA MATERIALIA
ISSN:
1359-6454
Año:
2013
Vol.:
61
N°:
4
Págs.:
1146 - 1153
In this work the sensing mechanism of p-type semiconducting NiO thin films under the exposure to formaldehyde is explained. The influence of the sensing layer thickness and annealing treatment on the structural, optical and electrical properties of the samples is studied. The height of the potential barrier is estimated from temperature-stimulated conductance measurements. The potential barrier height is linked to oxygen ionosorption on the semiconductor surface. Furthermore, Fourier transform-IR analysis was carried out in order to determine the chemical reactions that govern the process of gas detection and the temperature range at which they occur. As a result of the study, it is possible to explain how the thickness and annealing treatment affect the sensing mechanism of the samples.
Revista:
APPLIED SURFACE SCIENCE
ISSN:
0169-4332
Año:
2013
Vol.:
276
Págs.:
229 - 235
A study of the influence of annealing temperature on the structural, morphological and optical properties of WO3 thin films is presented. The coatings are deposited by RF reactive magnetron sputtering and characterized by XRD analysis and FESEM. The XRD diagrams of the samples show a phase transition from tetragonal to monoclinic when the annealing temperature is raised from 800 to 900 degrees C. Moreover, the increase of the annealing temperature to 800 degrees C favors the presence of a granular structure on the surface of the film. A decrease in the optical energy band gap (3.65-3.5 eV and 3.5-3.05 eV for direct and indirect transitions respectively) with annealing temperature has been measured employing Tauc's relation. Furthermore, WO3 thin films are processed by laser interference lithography (LIL) and periodic nanostructures are obtained. The processed films are characterized by a hexagonal symmetry with a period of 340 nm and the diameter of the nanostructured holes of 150 nm. These films show improved morphological properties of interest in several applications (gas sensors, photonic crystals, etc.) independent of the annealing temperature. .
Revista:
THIN SOLID FILMS
ISSN:
0040-6090
Año:
2012
Vol.:
520
N°:
14
Págs.:
4792 - 4796
The aim of the present work is to test the sensing behaviour of tin dioxide nanowires, which have been grown directly onto a sensing device. This device consists in an alumina substrate provided with platinum interdigitated microelectrodes and a Pt heater on the reverse side. The nanowire growing process based on a vapour-liquid-solid method consists of three steps: deposition of a tin thin film by DC sputtering, a 5 nm-thick Au layer deposition and an annealing treatment in the presence of oxygen for the growth of the SnO2 nanowires. These samples have been tested under different concentrations of formaldehyde (HCHO), showing a high sensitivity and very short response and recovery times even at low operating temperatures (130 degrees C).
Revista:
SENSORS
ISSN:
1424-8220
Año:
2011
Vol.:
11
N°:
5
Págs.:
5047 - 5057
Carbon dioxide detection is a relevant issue in many fields, and this work focuses on the use of a BaTiO3-CuO sputtered thin film layer in a gas sensor prototype for air quality measurements. For this, a double side sensor was fabricated, with a Pt heater on one side and the sensing layer over the electrodes on the other side. The uniformity of the temperature on the sensing layer was tested and further tests to check its sensing performance were carried out. Humidity influence in the detection was found to be almost negligible within the usual range in air quality measurements and repeatability tests show satisfactory results for air quality control purposes.
Revista:
THIN SOLID FILMS
ISSN:
0040-6090
Año:
2011
Vol.:
520
N°:
3
Págs.:
947 - 952
The aim of the present work is to test the performance of nickel oxide (NiO) thin films to low concentrations of formaldehyde (HCHO). NiO thin films were deposited on alumina substrates by RF reactive magnetron sputtering in a mixed atmosphere of argon and oxygen. A Pt heating resistor was deposited on the reverse side for an accurate control of the operating temperature. Samples were annealed in synthetic air for 4 h at 700 degrees C in order to stabilise their microstructure. Two different thicknesses (150 and 300 nm) were deposited in order to study the influence of this parameter on both the microstructure and sensor response. Both XRD analysis and FEG-SEM images show a smaller grain size for the 150 nm-thick samples. The best operating temperature was established at 340 and 300 degrees C for 150 and 300 nm-thick samples respectively. A higher sensitivity was obtained for the samples of lower thickness for a set of HCHO concentrations ranging from 5 to 20 ppm. Moreover, the repeatability of the experiments was tested for the most sensitive samples.
Revista:
SENSOR LETTERS
ISSN:
1546-198X
Año:
2011
Vol.:
9
N°:
1
Págs.:
64 - 68
The aim of the present work is to test the performance of thin films of NiO obtained by vacuum technology to toxic gases. The first gas considered is NO2 due to the fact that it is toxic to human body and harmful to the environment. NiO thin films were deposited by RF reactive magnetron sputtering on alumina substrates provided with Pt interdigitated microelectrodes and a Pt heater on the reverse side. The samples were annealed at 600 degrees C, 700 degrees C and 800 degrees C in order to compare their response to the selected gas. XRD and energy dispersive spectroscopy (EDS) analysis were carried out to correlate the different electrical responses to the material microstructure. Moreover, morphological characterization of the sensing films was performed by field emission gun-scanning electron microscopy. Two different thicknesses were deposited (150 nm and 300 nm) in order to study the influence of this parameter. The samples were exposed to concentrations between 5 and 100 ppm of NO2. The 150 nm-thick samples at 700 degrees C showed the best responses to low concentrations at 300 degrees C.
Revista:
SENSORS AND ACTUATORS B-CHEMICAL
ISSN:
0925-4005
Año:
2010
Vol.:
149
N°:
2
Págs.:
368 - 372
A photoactivated room temperature BaTiO(3)-CuO thin film for carbon dioxide detection is presented. The response time to carbon dioxide is around 2 min and the sensitivity shows a logarithmic relation between 500 and 5000 ppm of CO(2). The electrical changes are related to the physical behavior of the photoactivated material according to the p-n heterojunction formed between the BaTiO(3) and the CuO. The chemical reaction that allows the sensing process is based on the interaction of the CO(2) and the hydroxyl group's (OH(-)) adsorbed on the p-n heterojunction. The impedance variations measured are related to the charge migration from the surface to the bulk of the material. The photochemical stimulation allows the possibility of sensing at room temperature (25 +/- 1 degrees C).
