# Cu Ferrospinel Thin Films for Sub-ppm NO2 Sensing

Sabah Zahaf, Lionel Presmanes, Antoine Barnabé, Thierry Camps, Philippe Menini

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Sabah Zahaf, Lionel Presmanes, Antoine Barnabé, Thierry Camps, Philippe Menini. Cu Ferrospinel Thin Films for Sub-ppm NO2 Sensing. PROCEEDINGS, 2024, 97 (1), pp.168. 10.3390/proceed-ings2024097168. [hal-04796278](https://hal.science/hal-04796278v1)

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Abstract

# Cu Ferrospinel Thin Films for Sub-ppm NO₂ Sensing †

Sabah Zahaf 1,2, Lionel Presmanes 1,* , Antoine Barnabé 1, Thierry Camps 2 and Philippe Menini 2 ID

**Abstract:** The paper reports the synthesis and characterization of a copper ferrospinel thin layer. The thin layer (25 nm) was synthesized by RF sputtering of a pure CuFe₂O₄ target. The material structure and microstructure were characterized using X-ray diffraction and transmission electron microscopy. Simplified test substrates were microfabricated with interdigitated Pt electrodes to investigate electrical properties in a controlled atmosphere. The sensitive layer was stabilized by annealing at 550 °C under air. NO₂ responses at a low concentration (<1 ppm) and 50% relative humidity were measured between 100 and 500 °C. The optimal response was obtained at 250 °C.

**Keywords:** ferrospinel; semiconducting oxide; thin layer; RF sputtering; gas sensors; NO₂ sensing

## 1. Introduction

Nitrogen dioxide (NO₂) is a toxic gas produced by industrial and domestic use, in particular by internal combustion engines used in vehicles. NO₂ is dangerous for human health, with a threshold limit value (TLV) of 0.5 ppm. Many sensing technologies are being developed; among them, metal oxide semiconductor gas sensors are the most widely used for NO₂ detection [1]. Copper ferrospinel is an interesting candidate thanks to its electrical and reactional properties. Indeed, CuFe₂O₄ is used as a catalyst in particular for the removal of NOx emitted from diesel exhaust [2].

## 2. Materials and Methods

Twenty-five nm thin layers were elaborated by RF sputtering at 2 Pa under argon on various substrates and annealed at 550 °C under air. The structural properties were studied by X-ray diffraction (XRD) in grazing incidence (α = 1°), transmission electron microscopy (TEM) and Raman spectroscopy. Microstructure analysis was performed using scanning electron microscopy (SEM). The gas-sensing performance was studied using simplified test substrates with Pt interdigitated electrodes (Figure 1) to measure the resistance variation under clean and polluted air. The test substrate was introduced on a heating chuck in a small chamber in which the atmosphere was well controlled.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Electrical microdevice design used for sensing performance evaluation with 16 Pt electrodes of 100 µm width. The distance between each electrode is 50 µm.</figcaption></figure>

## 3. Discussion

XRD analysis on a thin layer deposited on a substrate without electrodes (Figure 2) reveals a quadratic CuFe₂O₄ pattern (JCPDS card 034-0425) at room temperature. No additional phases were detected. Transmission Electronic Microscopy confirms the XRD results.

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**Citation:** Zahaf, S.; Presmanes, L.; Barnabé, A.; Camps, T.; Menini, P. Cu Ferrospinel Thin Films for Sub-ppm NO₂ Sensing. *Proceedings* **2024**, *97*, 168. https://doi.org/10.3390/proceedings2024097168

Academic Editors: Pietro Siciliano and Luca Francioso

Published: 9 April 2024

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**Copyright:** © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

<footnote n="1">1 CIRIMAT, Université Toulouse 3 Paul Sabatier, Toulouse INP, Université de Toulouse, CNRS, 118 Route de Narbonne, 31062 Toulouse, France; sabah.zahaf@laas.fr (S.Z.); antoine.barnabe@univ-tlse3.fr (A.B.)</footnote>

<footnote n="2">2 Laboratoire d'Analyse et d'Architecture des Systèmes (LAAS), Université de Toulouse, CNRS, UPS, 7 Avenue du Colonel Roche, 31031 Toulouse, France; thierry.camps@laas.fr (T.C.); philippe.menini@laas.fr (P.M.)</footnote>

<footnote>* Correspondence: lionel-presmanes@univ-tlse3.fr</footnote>

<footnote>† Presented at the XXXV EUROSENSORS Conference, Lecce, Italy, 10–13 September 2023.</footnote>
<figure><img src="image_5.png" /><figcaption>**Figure 2.** XRD pattern of copper ferrospinel thin film.</figcaption></figure>

Structural properties were also investigated under temperature since the sensing performances are measured at high temperatures. The literature shows structural distortions when CuFe₂O₄ is heated, due to a cooperative Jahn-Teller effect [3]. High temperature XRD analysis was performed and revealed that elaborated ferrospinel thin film turns into a cubic structure above 350 °C.

The resistance behavior under air and NO₂ at 250 ppb was investigated at different temperatures in isothermal mode. As the NO₂ is an oxidizing gas, the resistance increased, meaning that the material is an n-type semiconductor. The R<sub>NO₂</sub>/R<sub>air</sub> ratio was calculated at each temperature and is represented in Figure 3. The best response was obtained at 250 °C and decreased above that temperature (Figure 3). Furthermore, above 450 °C the response was reversed; the copper ferrospinel film behaved like a p-type semiconductor. Gas-sensing performances seem to depend on structural properties, since the structural transition starts at 350 °C.

<figure><img src="image_6.png" /><figcaption>**Figure 3.** NO₂-sensing performance.</figcaption></figure>
**Author Contributions:** Experimentations and abstract redaction by S.Z.; work supervision by L.P. and P.M.; material characterization support by A.B. and electrical characterization support by T.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received cofounding from ELLONA. This work was also supported by the LAAS-CNRS micro and nanotechnologies platform, a member of the french Renatech network.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data are available in this manuscript.

**Conflicts of Interest:** The authors declare no conflicts of interest.

## References

1. Wetchakun, K.; Samerjai, T.; Tamaekong, N.; Liewhiran, C.; Siriwong, C.; Kruefu, V.; Wisitsoraat, A.; Tuantranont, A.; Phanichphant, S. Semiconducting metal oxides as sensors for environmentally hazardous gases. *Sens. Actuators B Chem.* **2011**, *160*, 580–591. [CrossRef]

2. Shangguan, W.; Teraoka, Y.; Kagawa, S. Promotion effect of potassium on the catalytic property of CuFe₂O₄ for the simultaneous removal of NOx and diesel soot particulate. *Appl. Catal. B Environ.* **1998**, *16*, 149–154. [CrossRef]

3. Balagurov, A.; Bobrikov, I.; Maschenko, M.; Sangaa, D.; Simkin, V. Structural phase transition in CuFe₂O₄ spinel. *Crystallogr. Rep.* **2013**, *58*, 710–717. [CrossRef]

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