Phase (III) 2024

Phase name: Validation of the NO₂ gas detection method.

Phase results:

During this phase, the validation of the NO₂ gas detection method was realized through the testing of the sensors developed during the previous stages of the project, at room temperature (24 °C), for two different NO₂ concentrations, in the presence of possible interferent gases and at different values of the relative humidity (RH).
A new Bl-Gr-based SAW sensor was tested at two NO₂ concentrations (1 ppm and 1.5 ppm) for the validation of the detection method. The phase shifts of the sensor were recorded for both concentrations, obtaining the following values: -0.102 and -0.253, respectively. Using the equation from the calibration curve, the concentration of NO₂ from the detection chamber was calculated, resulting the values of 1.06 ppm and 1.574 ppm, respectively. The relative error for the first value was about 6.7% and for the second unknown tested NO₂ concentration was 4.93%, respectively.
The selectivity of the sensors was also evaluated by detection tests at room temperature, using different possible interferent gases, such as CH₂O, NH₃ and CO. Both sensors showed high phase shift values in the presence of NO₂, in comparison to the values obtained in the presence of other possible interferent gases. The response of the Bl-Gr SAW sensor displayed small phase shifts for the interferent gases, thus emphasizing the high selectivity of the developed sensor for the NO₂ detection at room temperature. The S-Gr SAW sensor showed significant variation in the phase shift response in the presence of the possible interferent gases, especially for CH₂O, exhibiting a lower selectivity towards NO₂ in the presence of the respective gas.
To evaluate the influence of the relative humidity over the gas sensors’ performance, measurements on both sensors at room temperature for the NO₂ concentration of 2 pppm were carried out at different values of RH (between 20% and 80%). The results obtained during the detection tests at different RH values indicated that the bilayer graphene (Bl-Gr) presented a more stable response for NO₂ in variable humidity conditions, compared to the S-doped graphene (S-Gr), thus it can be successfully used as a sensing material for the development of SAW sensors.

Deliverables:

• Report on the NO₂ SAWR sensor’s testing;
• Final scientific and financial report;
• Dissemination report.

Phase (II) 2023

Phase name: Fabrication of the SAWR sensor and its integration with graphene-based nanomaterials. Characterization of SAWR sensors for NO₂ detection.

Phase results:

During this phase, the photolithographic mask for the development of SAWR sensors was fabricated. The technological flow for obtaining surface acoustic wave structures was also developed. The SAW sensors, with a delay line configuration, were manufactured of Cr/Au, using the photolithographic technique. Quartz was used as the piezoelectric substrate for the developed devices. At the end of the technological flow, the wafers were cut into chips and later, encapsulated. The structures were then characterized using the vector network analyzer and the operating frequency of the devices was about 91.55 MHz. The structures were later functionalized using two carbon-based materials: bilayer graphene (Bl-Gr) and sulfur-doped graphene (S-Gr). The deposition(in the detection area of the sensor) of the two sensitive layers was achieved using the electrochemical delamination technique, in the case of the Bl-Gr film and the "drop casting" method, in the case of the S-Gr layer.

Finally, the developed sensors were characterized at room temperature, in a NO2 atmosphere. Similar sensitivity values were recorded for both sensors (0.2979 degrees/ppm for the sensor based on Bl-Gr and 0.2645 degrees/ppm for the sensor functionalized with S-Gr), but the advantage of the one based on S-Gr consisted in a simpler and more efficient functionalization method. From the point of view of the linearity range, the sensor based on bilayer graphene recorded a more extensive linearity range and a higher correlation coefficient (close to 1), which recommends it to be used in the more precise detection of NO₂, at low concentrations (at ppm level).

Deliverables:
• 30 fabricated SAWR devices
• Report on the SAWR devices’ characterization
• 20 NO2-SAWR sensors developed

Phase (I) 2022

Phase name: Materials and methods development

Phase results:

During this stage, graphene deposition and characterization processes were developed. Graphene will be used in subsequent stages as a sensitive layer for NO2 detection. Two types of graphene were chosen: chemically synthesized S-Gr and monolayer Gr, deposited by CVD. Two different graphene deposition on the detection area methods were used: i) drop-casting and ii) transfer by electrochemical delamination. The graphene films were characterized both before and after their deposition on the Au substrate, using SEM microscopy, Raman spectroscopy and EDX techniques. In the case of graphene deposited by CVD, the double transfer was carried out, in order to obtain the bi-layer for the detection area.
Another direction addressed in this research was the design of the SAW resonators. To observe the influences of the distance between the IDT and the reflector and the number of electrodes on the shape of the transfer characteristic of the developed sensors, four different configurations were proposed. The masks were designed using the CleWin software tool.

Deliverables:
• Report on the GR deposition process
• Report on the GR transfer method on the piezoelectric substrate
• Characterization of the 2D materials selected for the NO2 sensor

 

Disemination

- SURFACE ACOUSTIC WAVE SENSORS FOR NO2 DETECTION BASED ON SULFUR-DOPED GRAPHENE, A. M. Baracu, V. Buiculescu, L. A. Dinu, C. Brasoveanu, R. Müller, IMT Bucharest, Romania, CAS 2022, October 2022
- Open Access, Review Article: Livia Alexandra Dinu,Valentin Buiculescu and Angela Mihaela Baracu*, Recent Progress on Nanomaterials for NO₂ Surface Acoustic Wave Sensors, Nanomaterials 2022, 12(12), 2120; https://doi.org/10.3390/nano12122120
- Published: 20 June 2022 (This article belongs to the Special Issue Nanomaterials in Gas Sensors)
- Tineri cercetători din IMT - carieră profesională independentă în cadrul proiectelor Postdoctorale și Tinere echipe în vederea stimulării excelenței științifice în cercetarea româneasca, Market Watch, Nr. 245, Iunie 2022
- Recent Progress on Nanomaterials for NO₂ Surface Acoustic Wave Sensors, Nanomaterials 2022, 12(12), 2120; https://doi.org/10.3390/nano12122120