1. Adelina Stănoiu (former Tomescu), U-1700-038M-8092
2. Cristian Eugen Simion, U-1700-030F-9765
3. Corneliu Ghica, U-1700-029U-5827
4. Ioana Dorina Vlaicu, U-1700-037A-2366
5. Andrei Cristian Kuncser, U-1700-032N-6988
6. Cătălina Gabriela Mihalcea, U-1900-062F-0766
7. Ionel Florinel Mercioniu, U-1700-039E-8823
8. Daniela Ghica, U-1700-035Y-0481
9. Mariana Ștefan, U-1700-034N-5930
10. Ion Viorel Dinu, U-1700-030A-3912
11. Ovidiu Gabriel Florea, U-1700-032D-4435

Stage 1/2022 – Quantitative and qualitative Gd doping level effects.
In the case of chemo-resistive gas sensors, the main requirements are related to sensitivity and selectivity, along with a good stability over time of the material parameters. Therefore, at this stage we proceeded to a material selection based on the synthesis route, the exhaustive morpho-structural evaluations and the preliminary sensing investigations.
Activity 1.1. Synthesis of nanostructured powders: SnO₂, Gd₂O_3, and SnO₂ doped with 1-20 at. % Gd.
A premise of any fundamental study or any commercial application is the reproducibility and control of each step in the technological process. Therefore, the optimization of the material synthesis in accordance with the experiment and the final application, was taken into consideration at this stage. Pure and Gd-doped SnO2 powders, with the general formula Sn1-xGdxO(4-x)/2 (where x = 0.01; 0.05; 0.1; 0.2, corresponding to nominal concentrations of 1, 5, 10, 20 at. %) , and of Gd2O3 were synthesized by two alternative methods, co-precipitation and hydrothermal.
Activity 1.2. Studies on the morphology, grain size, crystal structure, crystallinity phase purity as well as pore structure by microstructural investigations of the obtained nanopowders: SnO₂, Gd₂O_3, and SnO₂ doped with 1-20 at. % Gd.
X-ray diffraction (XRD) was performed with a Bruker D8 Advance X-ray diffractometer. The Rietveld analysis indicates the formation of a SnO2 - Gd2O3 nanocomposite even for the SnO2 sample doped with 5%, considering the presence of Gd2O3 as a secondary phase with poor crystallization. TEM/HRTEM analytical results were obtained with JEOL JEM-2100 and JEOL ARM 200F transmission electron microscopes, equipped with X-ray detectors used for EDS type investigations. The results of the distribution of nanoparticles, the electron diffractogram and the HRTEM images obtained at different magnifications reveal the shape of the nanoparticles, the uniformity of the crystallite sizes and highlight the crystallographic planes. The STEM-EDS maps highlight the presence and distribution of the constituent elements, confirm the presence of Gd in the sample and the uniformity of its distribution. The two morphologies of nanoparticles, quasi-spheres and sticks, provide distinct EDS signals corresponding to tetragonal SnO2 and cubic Gd2O3. The average specific surface areas for the pure SnO2 systems and for the SnO2 systems doped with 1%, 3%, 5%, 10%, 20%Gd were estimated theoretically using the size distributions obtained by TEM. The textural properties were determined with the BET method (Brunauer, Emmett, Teller), using the Micromeritics ASAP 2020 instrument, by measuring the amount of gas (N2) adsorbed/desorbed from the surface of the powders in order to determine the specific surface area and porosity of the materials indicated by TEM/HRTEM as being porous. Electron spin resonance (RES) measurements were made in the X (9.87 GHz) and Q (34.16 GHz) microwave frequency bands. The RES spectra of all measured samples present signals characteristic of Gd3+ ions located in isolated positions in the network and respectively agglomerated. The CO2 detection properties for the sensors obtained based on the studied materials were evaluated under dynamic conditions similar to those in the field. The preliminary selection highlights the hydrothermally prepared Gd2O3 whose nano-sticks present a strong diffraction contrast typical of single-crystal micro-objects, in contrast to the mild contrast of the weakly crystallized Gd2O3 nano-sticks obtained by coprecipitation.

Stage 2/2023 –Reaction path for the interaction with CO₂ under real operating conditions.
At this stage, we set out to analyze the reaction path for the interaction with CO₂ in real operating conditions. The study allows us to see how the detection performance of Sn_1-xGd_xO_(4-x)/2 is influenced by operating temperature, relative air humidity and CO₂ concentration.
A premise of any fundamental study or any commercial application is the technology of making samples, respectively the reproducibility and control of each step in the technological process.
Thus,

• pure SnO₂ powders doped with Gd, with the general formula Sn_1-xGd_xO_(4-x)/2 (where x = 0.01; 0.05; 0.1; 0.2, corresponding to nominal concentrations of 1, 5, 10, 20 at. %), and of Gd₂O₃ were obtained by two alternative synthesis methods: co-precipitation and hydrothermal, as described in Stage 1/2022;
• the obtained powders were transformed into a paste with controlled viscosity by mixing with an organic solvent and deposited by screen-printing in the form of a thick and porous layer on commercial alumina substrates;
• the obtained sensors were evaluated in dynamic conditions similar to those in the field, ensured by a Gas Mixing System fully controlled by the computer, as described in Stage 1/2022.

Activity 2.1. Thick film layer deposition.
The sensitive layers of Sn_1-xGd_xO_(4-x)/2 used as material for CO₂ sensors were prepared in such a way that the share of surface phenomena in the global resistance of the layer was as high as possible, respectively thick and porous layers. The surface, defined as the interface between the volume of the metal oxide semiconductor (MOS) and the ambient atmosphere, plays an important role in the whole class of phenomena for which the interaction between the MOS and the environment is important.
The substrate
For simple operation and exploitation of the sensitive potential of MOS, the geometry of the substrate and the mode of operation were considered. The substrates used to make the sensors are marketed by CeramTec Germany, are made of alumina (Al₂O₃) in planar technology and are equipped with electrodes and a platinum (Pt) heater. The electrode technology determines the mode of operation. In our case, the interdigital Pt electrodes over which the MOS layer is deposited favour the measurement of electrical resistance variations of MOS, a physical parameter sensitive to chemical interactions with gases in the ambient atmosphere. The Pt heater allows modulating the temperature of the MOS layer in order to determine the optimal detection and selectivity, by favoring a certain chemical interaction.
Screen-printing deposition of the MOS layer on the Al₂O₃ substrate – obtaining the sensors
The materials prepared in the form of powders were mixed in equal proportions with 1,2 Propanediol and mortared for 15 minutes in order to obtain a medium viscosity paste, which is transferred onto commercial Al₂O₃ substrates, by the screen-printing method. The obtained sensors are slowly dried at room temperature, then in an oven at 60°C and calcined in an oven, in air flow, gradually increasing the temperature up to 500°C for the complete removal of the organic solvent and achieving the adhesion of the layer to the substrate.
Optical inspection and sensor selection depending on the uniformity of the sensitive layers.
After deposition and heat treatment, the sensitive layers are checked with the Olympus MX50 optical microscope, which allows the selection of those with superior uniformity, without cracks or agglomerations.
Heater calibration
For precise thermal control, the Pt meander-type heater deposited on the planar Al₂O₃ substrate was coated with heat-resistant black paint (up to 800°C) with the known emissivity (ε=0.95). Then, the heater was calibrated with the Lumasense IN-5L Plus optical pyrometer with adjustable emissivity, applying voltage and simultaneously measuring the current through the heater and the temperature of the paint layer. By this procedure, the high-precision calibration curve T(°C) = fP(W) of the heater is obtained.

Activity 2.2. Electrical investigations for preliminary selection with respect to the sensing properties under real operating conditions.
The sensing properties are:
The sensor signal S measures the impact of the stimulus on the sensor response. The higher the value of S, the more significant the change caused by a certain stimulus, so S can be used to evaluate the detection potential of a certain stimulus.
Selectivity (m_ij). Sensors are generally sensitive to several stimuli and for this reason, lack selectivity. Selectivity is therefore a useful measure for evaluating the specificity of the sensor by comparing the effects of various gases on the sensor. The mij selectivity of a sensor allows comparison of the sensor signal to a monitored gas with the sensor signal to another interfering stimulus.
Long-term stability (ζ) of the sensor signal. Besides the ability to sense a stimulus quickly and with high precision, a sensor is also evaluated from the point of view of its stability (time stability of the evaluated parameter) and its reproducibility. In the case of chemical sensors, due to prolonged and repeated heating-cooling cycles and exposure to moisture and various gases, it is well-known and generally accepted that the base resistance (in air) and sensor signal for MOS material varies with time.
Response and recovery time. These two measurements are usually used to measure the "speed" of the sensor. Response time (t_response) measures the minimum time required to measure a stimulus. In general, the transient response time is defined as t_90%, i.e. the time required to reach 90% of the equilibrium value of the sensor response in the presence of the stimulus. The recovery time (t_recovery) represents the time required for the sensor's response to reach the base resistance value, after the removal of the stimulus.
Real operating conditions
The CO₂ detection properties for the sensors obtained based on the studied materials were evaluated under dynamic conditions similar to those in the field (atmospheric pressure, variable relative humidity depending on the climatological conditions, variable temperature, and variable concentration of gases). Field conditions were simulated in the laboratory using the fully computer-controlled Gas Mixing System (SMG). In the temperature range in which the evaluation of the sensing properties of Sn_1-xGd_xO_(4-x)/2 was carried out, water vapour from the ambient atmosphere (%RH) acts as a potentially interfering reducing gas with CO₂. Therefore, we analyzed the influence of %RH on the sensor signal of Sn_1-xGd_xO_(4-x)/2 at the optimal operating temperature and progressive CO₂ concentration. The applicative potential was evaluated based on the relationship between the operating temperature and the electrical power consumed.

Member of the doctoral committee of Drd. Cătălina G. Mihalcea, research assistant in Laboratory 70 - Atomic Structures and Defects in Advanced Materials (LASDAM), INCDFM. Dr. Cătălina G. Mihalcea carries out her activity in the research topic of the group in which she is included, of the CERIC-ERIC and PN-III-P4-PCE-2021-0384 projects. The topic of the doctoral thesis is "Nanostructured materials for gas sensing: correlations between functional, electronic and microstructural properties" and it is carried out under the guidance of the supervisor Prof. C. M. Teodorescu, at the Doctoral School of Physics, University of Bucharest.

ISI papers:

1. Corneliu Ghica, Catalina G. Mihalcea, Cristian E. Simion, Ioana D. Vlaicu, Daniela Ghica, Ion V. Dinu, Ovidiu G. Florea, Adelina Stanoiu*, Influence of relative humidity on CO₂ interaction mechanism for Gd-doped SnO₂ with respect to pure SnO₂ and Gd₂O₃, Sens. Actuators B. Chem. 368, 1 October 2022, 132130, https://doi.org/10.1016/j.snb.2022.132130
2. Andrei C. Kuncser, Ioana D. Vlaicu, Ion V. Dinu, Cristian E. Simion, Alexandra C. Iacoban, Ovidiu G. Florea, Adelina Stanoiu*, The impact of the synthesis temperature on SnO₂ morphology and sensitivity to CO₂ under in-field conditions, Materials Letters 325, 15 October 2022, 132855, https://doi.org/10.1016/j.matlet.2022.132855
3. Adelina Stanoiu, Corneliu Ghica, Catalina Gabriela Mihalcea, Daniela Ghica and Cristian Eugen Simion*, The Role of the Synthesis Routes on the CO-Sensing Mechanism of NiO-Based Gas Sensors, Chemosensors 10 (11), 9 November 2022, 466, https://doi.org/10.3390/chemosensors10110466
4. Cristian E. Simion, Ioana D. Vlaicu, Alexandra C. Iacoban, Catalina G. Mihalcea, Corneliu Ghica and Adelina Stanoiu*, The influence of the synthesis method on Gd₂O₃ morpho-structural properties and sensitivity to CO₂ under in-field conditions, Materials Chemistry and Physics 296, 15 February 2023, 127354, https://doi.org/10.1016/j.matchemphys.2023.127354
5. Cristian.E. Simion, Benjamin Junker, Udo Weimar, Adelina Stanoiu, Nicolae Bârsan, Sensing mechanisms of CO and H₂ with NiO material – DRIFTS investigations, Sens. Actuators B. Chem. 390 , 1 September 2023, 134028, https://doi.org/10.1016/j.snb.2023.134028
6. A. Stanoiu, D. Ghica, C.G. Mihalcea, I.D. Vlaicu, O.G. Florea, S. Bulat, C. Ghica, C.E. Simion*, GHGs detection by tuning the operating temperature of Sn1-xGdxO(4-x)/2, Proceedings of the IEEE, November 2023, pp. 63-66, https://doi.org/10.1109/CAS59036.2023.10303672
7. C. E. Simion, I. V. Dinu, O. G. Florea and A. Stanoiu*, The role of interdigital electrodes on sensing performances with p-type NiO-based gas sensors -link to experiments, Proceedings of the IEEE, November 2023, pp. 11-18, https://doi.org/10.1109/CAS59036.2023.10303715

Papers presented at conferences:

1. Catalina G. Mihalcea*, Corneliu Ghica, Cristian E. Simion, Ioana D. Vlaicu, Daniela Ghica, Ion V. Dinu, Ovidiu G. Florea, Adelina Stanoiu, The structure and morphology of pure SnO₂, Gd-doped SnO₂ and pure Gd₂O₃ nanoparticles for applications in chemoresistive gas sensors, oral presentation at the 19th International conference on Advanced Nanomaterials ANM 2022, July 27-29, University of Aveiro, Aveiro – Portugal
2. Catalina G. Mihalcea*, Corneliu Ghica, Cristian E. Simion, Ioana D. Vlaicu, Daniela Ghica, Ion V. Dinu, Ovidiu G. Florea, Adelina Stanoiu, Structural and morphological properties of Gd-doped SnO₂ nanopowders and their role in the gas sensing mechanism, poster session 2 at the 9th International Conference on Optical, Optoelectronic and Photonic Materials and Applications & 14th Europhysical Conference on Defects in Insulating Materials ICOOPMA-EURODIM 2022, July 3–8, Ghent, Belgium
3. Catalina Gabriela Mihalcea, Corneliu Ghica, Adelina Stanoiu, Cristian Eugen Simion, Ioana Dorina Vlaicu, Alexandra Corina Iacoban, Daniela Ghica, Ionel Florinel Mercioniu, Morphology and structure of SnO₂-based nanomaterials obtained by different synthesis routes for gas sensing applications, poster session at the 2023 Fall Meeting of the European Materials Research Society (E-MRS), September 18-21, Warsaw, Poland
4. A. Stanoiu, D. Ghica, C.G. Mihalcea, I.D. Vlaicu, O.G. Florea, S. Bulat, C. Ghica, C.E. Simion, GHGs detection by tuning the operating temperature of Sn_1-xGd_xO_(4-x)/2, oral presentation at the 2023 International Semiconductor Conference (CAS) ⁴⁶th Edition, 11-13 October, Sinaia, Romania
5. C.E. Simion, I.V. Dinu, O.G. Florea, A. Stanoiu, The role of interdigital electrodes on sensing performances with p-type NiO-based gas sensors – link to experiments, oral presentation at the 2023 International Semiconductor Conference (CAS) ⁴⁶th Edition, 11-13 October, Sinaia, Romania

Papers under publication at ISI journals:

1. Ion V. Dinu, Cristian E. Simion, Nicoleta G. Apostol, Ovidiu G. Florea, Catalina G. Mihalcea and Adelina Stanoiu, Conduction mechanism of Gd₂O₃ induced by CO₂ under in-field conditions, accepted on 24 noiembrie 2023 at Physica E: Low-dimensional Systems and Nanostructures, Reference: PHYSE_115862.

Experimental methods for investigating gas-sensing properties under in-field conditions - NIMP facilities:

https://infim.ro/wp-content/uploads/2022/11/Gas-Mixing-Station-scaled.jpg

Dr. Adelina Stănoiu

adelina.stanoiu@infim.ro

stanoiu.adelina@yahoo.com