Fe2O3- Modified Cr2O3 thick Films: Ethanol Sensor

Fe₂O₃– Modified Cr₂O₃ thick Films: Ethanol Sensor

Dinesh Nagaraj Suryawanshi¹, Idris G. Pathan²

¹Department of Physics, Rani Laxmibai College, Parola, 425111

²Arts, Science and Commerce College, Navapur, Maharashtra

DOI: https://doi.org/10.51244/IJRSI.2025.120700126

Received: 06 July 2025; Accepted: 13 July 2025; Published: 06 August 2025

ABSTRACT

Thick films of pure Cr₂O₃ were prepared by screen-printing technique. The surfaces of these films were modified by dipping them into 0.01 M aqueous solution of ferric chloride (FeCl₃) for different intervals of time, followed by firing at 550⁰C for 30 min. firing results the oxidation of FeCl₃ additive in to Fe₂O₃. The characterizations and the gas sensing properties of pure and surface activated Cr₂O₃ thick films have been investigated. The sensor gives response to Ethanol gas at 350^OC. The quick response and fast recovery are the main features of this sensor. The effects of microstructure and activators concentration on the gas response, selectivity, response time and recovery reducing gases were studied and discussed.

Keywords: Fe₂O₃-activated Cr₂O₃, thick film, Ethanol sensor, gas response.

INTRODUCTION

Hazardous and toxic gases from auto and industrial exhausts are polluting the environment. Environmental pollution [1-4] is a burning global issue; pollution has raised its ugly head high in the global environment. Therefore, all industries should have an alarm system detecting and warning for dangerous exhaust gas concentration levels. Thus, the need to monitor and control these gases has led to research and development of a wide variety of sensors using different materials and technologies. The Cr₂O₃ is an important refractory material because of its high melting temperature (2300⁰C) and crystallizes in Hexagonal-Rhombic corundum structure showing p-type semi conductivity. It is well known that, semiconducting oxides such as ZnO, Cr₂O₃, SnO₂, BaTiO₃ and Fe₂O₃ [5-16] are sensitive to various polluting gases. It has been studied that Cr₂O₃ [17-20] was used as a gas-sensing element. In fact, pure Cr₂O₃ was reported to have poor gas sensitivity.

Pure ethanol is called as an absolute alcohol. Ethanol is used for beverages, scientific and industrial purposes. Ethanol is hypnotic (sleep producer) (Solomans and Fryhle 2004) gas having toxic nature. Heavy exposure and consumption of alcoholic beverages, particularly by smokes, increase the risk of cancer (Sodi 2002) of the upper respiratory and digestive tracks. Alcoholic cirrhosis leads to leaver cancer. Amongst the women, of breast cancer increase with alcoholic consumption or exposure. Those working on ethanol synthesis have great chances of being victims of respiratory and digestive track cancer. So there is great demand and emerging challenges for monitoring ethanol gas at trace level.

The gas sensing performance of the material can be improved by incorporating few additives into the base material and/or surface activation (Matsushima et al 1989; Xiangfeng et al 2000) of thick films. Although few researchers have conducted studies on ethanol (Takao et al 1989; Miremadi et al 1994) gas sensors, it has not been possible to produce ethanol sensors in sufficient quantities to meet the demand.

The aim of the present work is to be developing the sensor by modifying pure Cr₂O₃ base material, which could be able to detect the C₂H₅OH vapors. Among the various metal oxide additives tested. Fe₂O₃ in Cr₂O₃ is outstanding in promoting the sensing properties of C₂H₅OH in air.

EXPERIMENTAL

Preparation of material powder

AR grade (99.9 % pure) Cr₂O₃ powder was ball milled to ensure sufficiently fine particle size. The fine powder was calcined at 1000⁰C for 4 h, in air and re-ground. Thick films of so obtained powder were prepared by adopting the procedure explained elsewhere [10]. These films were surface modified by dipping them into a 0.01 M aqueous solution of FeCl₃ for different intervals of time followed by firing at 500⁰C for 30 min. The FeCl₃ dispersed on the film surface was oxidized to Fe₂O₃ in firing process. Thus the sensor elements with different mass % of Fe₂O₃ were obtained. Silver contacts were made by vacuum evaporation for electrical measurements.

Preparation of thick films

The thixotropic paste was formulated by mixing the sintered fine powder of pure Cr₂O₃, with solution of ethyl cellulose (a temporary binder) in a mixture of organic solvents such as butyl cellulose, butyal carbitol acetate and terpinol. The ratio Inorganic to organic part was kept as 80:20 in formulating the paste. The paste was then used to prepare thick films of pure Cr₂O₃.

Characterization

The microstructure and chemical composition of the films were analysed using scanning electron microscope ( SEM,JEOL JED 2300) coupled with energy dispersive spectrometer (EDS JEOL 6360 LA). Thickness measurements were carried out using a Taylor-Hobson (Talystep, UK) system. Electrical and gas sensing characteristics were measured using a static gas sensing system.

Details of gas sensing system

The sensing performance of the sensors was examined using ‘static gas sensing system’ reported elsewhere (Patil et al 2006b).

MATERIALS CHARACTERIZATIONS

Thickness measurement                                                                          

Thickness of thick film was measured by using the Taylor-Hobson (Talystep, UK) system. The thickness of the film was observed in the range from 30 to 40 mm. The reproducibility of the film thickness was archived by maintaining the proper rheology and thixotropy of the paste.

Structural properties (X-ray diffraction studies) 

 Figure 1. XRD of (a) unmodified Cr₂O₃ and (b) Fe₂O₃-activated Cr₂O₃ powder (5 min).

The crystalline structures of the films were analysed with X-ray diffractogram (RIGAKU DMAX 2500) using CuK_α radiation with 1.5418 A⁰.

Figure 1(a) depicts the XRD patterns of unmodified (pure) and Fe₂O₃-activated Cr₂O₃ (5 min). The observed peaks 1(a) are matching well with ASTM reported data of pure Cr₂O₃. The material was observed to be microcrystalline in nature. There are no prominent peaks of Fe₂O₃ associated in XRD pattern, due to smaller wt % of Fe₂O₃ in comparison with Cr₂O₃.

Elemental analysis

The quantitative elemental composition of pure and Fe-modified films were analysed using an energy dispersive spectrometer. The mass % of Cr and O in each samples were not as per stoichiometric proportion and all samples were observed to be oxygen deficient. Excess or deficiency of the constituent material particles leads to semiconducting nature of the material.

Microstructure–SEM

(a)                                                       (b)                                                     (c)

Figure 2. Micrographs of (a) Unmodified Cr₂O₃, (b) Fe₂O₃-activated Cr₂O₃ (5 min) and (c) Fe₂O₃-activated Cr₂O₃ (60 min) thick films

Unmodified Cr₂O₃ film in Fig. 2 (a) consists of randomly distributed grains with larger size and shape distribution. Fig. 2 (b) depicts the microstructure of Fe₂O₃-modified film (5 min). This film consists of smaller grains distributed on the larger grains. Smaller grains may be of Fe₂O₃ and larger ones may be of Cr₂O₃. Fig. 2 (c) depicts the microstructure of a Cr₂O₃-modified film for largest time interval (60 min) consists of particles with smaller size and shape associated with the Cr₂O₃ grains. It is observed from Fig. 2 (b) that the grains of Fe₂O₃-Cr₂O₃ are arranged in manner that, the porosity of the film is larger among all. Thus the effective surface area was expected to increase explosively, which enhances the optimum adsorption of oxygen at higher temperatures. The average grain size of highly sensitive film is observed to be 621 nm, calculated by Scherrer’s formula.

ELECTRICAL PROPERTIES OF SENSOR

I-V Characteristics     

Fig. 3 depicts the I-V characteristics of the Fe₂O₃-modified Cr₂O₃ films. It is clear from the symmetrical I-V characteristics that the silver contacts on the film are ohmic in nature.

Figure 3. I-V characteristics of Fe₂O₃-modified Cr₂O₃ films.

Electrical conductivity

Figure 4. Conductivity – temperature profile of Fe₂O₃-modified Cr₂O₃ films.

The semiconducting nature of Fe₂O₃-modified Cr₂O₃ is observed from the measurements of conductivity with temperature. The semiconductivity in Cr₂O₃ may be attributed to oxygen deficiency in it.  The material would then adsorb the oxygen species at higher temperatures (O₂^– à 2O^– à O^2-). The adsorption chemistry of Fe-modified Cr₂O₃ surface would be different from the pure Cr₂O₃ thick film surface.   The Fe₂O₃ misfits on the surface enhance the oxygen adsorption capability of the surface. The Fe₂O₃ misfits distributed evenly on the surface would have made it possible to adsorb the oxygen ions even at low temperatures. From figure 4 it is clear that, the conductivities of Fe-modified films increase with increase in operating temperature. This behavior confirmed the semiconducting nature of modified Cr₂O₃. The increase in the conductivity of Fe₂O₃-modified Cr₂O₃ could be attributed to the charge-carrier generation mechanism resulted from the electronic defects. These generated electrons and the donor level in the energy band gap of Cr₂O₃ will contribute to increase conductivity.

SENSING PERFORMANCE OF THE SENSOR

Gas response, Selectivity, Response and Recovery time

Gas response (S) is defined as the ratio of the change in conductance of the sensor on exposure to the target gas to the original conductance in air. The relation for S is as:

                                                            S = ( G_g – G_a) / G

where, G_a and G_g are the conductance of sensor in air and in a target gas medium, respectively.

Selectivity or specificity is defined as the ability of a sensor to respond to certain gas in the presence of other gases.

The time taken for the sensor to attain 90 % of the maximum increase in conductance on exposure to the target gas is the response time. The time taken by the sensor to get back 90 % of the original conductance is the recovery time.

Figure 5. Variation of gas response of pure and modified (5 min) Cr₂O₃ thin films.

Figure 6.  Variation of different gas responses with operating temperature.

Figure 7 Variation of gas response with dipping (5 min) of Fe₂O₃ in   Cr₂O₃ samples.

Sensing performance of pure Cr₂O₃ thick films

Figure 5 shows the variation of ethanol (1000 ppm) responses of pure and modified Cr₂O₃ thick film. It is observed that pure and modified Cr₂O₃ film having maximum response to be 2.30 and 27.66 at 400⁰C and 300⁰C, respectively.

Sensing performance of Fe₂O₃-modified Cr₂O₃ thick films

a.  Response of sensor to various gases:

The variation of different gas responses of Fe₂O₃-modified Cr₂O₃ ( 5 min) sample with operating temperature is represented in figure 6.It is clear from the figure that the gas responses go on increasing, reach to their respective maxima and decreased further with increase in operating temperature. It is clear from figure that the Fe₂O₃-modified Cr₂O₃ (5 min) sample shows the largest response to ethanol vapours at 300⁰C.

b.  Active region of the sensor:

Figure 8.  Variation of gas response with gas concentration.

Figure 8 depicts the variation of gas response of   Fe₂O₃-modified Cr₂O₃ (5 min)    sample with ethanol vapour concentrations. It is clear from the figure that gas response goes on increasing linearly with gas concentration up to 1000ppm. The rate of increase in gas response was relatively larger up to 1000 ppm.  The monolayer of gas molecules formed on surface could cover the whole surfaces of the film. The excess gas molecules would remain ideal and would not reach surface active sides of the sensor. So, the gas response at higher concentration of the gas is not expected to increase further in large extent. Thus active region of the sensor would be up to 1000 ppm.

c.  Effect of operating temperature:

Figure 6 depicts the variation of gas response to ethanol vapours (1000 ppm) with operating temperature. The largest response Fe₂O₃-modified Cr₂O₃ (5 min) to ethanol was observed to be 27.66 at 300⁰C.The response could be attributed to the adsorption-desorption type of sensing mechanism. The amount of oxygen adsorbed on the surface would depend on the number of Fe2O3 misfits on the Cr2O3 surface and operating temperature.

Figure 9.  Selectivity factor of the sensor for various gases.

d.   Selectivity factor of Fe₂O₃-modified Cr₂O₃ (5 min) for various gases:

It is observed from figure 9 that the Fe₂O₃-modified Cr₂O₃ sensor gives maximum response to ethanol vapours (1000 ppm) at 300⁰C.The sensor showed highest selectivity for ethanol against all other tested gases: H₂, CO₂, Cl₂ and NH₃.

e.  Response and recovery time:

The response and recovery of Fe₂O₃-modified Cr₂O₃ (5 min) of sensor are represented in figure 10. The response was quick (≈ 16 sec) to 1000 ppm of ethanol, while the recovery was fast (≈ 38 sec). The quick response may be due to faster oxidation of gas. The negglible quantity of the surface reaction product and its high volatility explains its quick response and fast recovery to its initial chemical status.

Figure 10. Response and recovery of Fe2O3 -modified Cr2O3 sample.

CONCLUSIONS

From the result obtained, following conclusions can be made for sensing performance of the sensors.

1. Pure Cr₂O₃ showed low response to ethanol vapours.
2. Among all other additives tested, Fe₂O₃ in Cr₂O₃ is outstanding in promoting the ethanol gas sensing mechanism.
3. The sensitivity of TiO2 based sensors was found to be changing with operating temperature.
4. The doping was observed to be an appropriate method to enhance sensitivity and selectivity to LPG and Cl2 gas.
5. By controlling the distribution and amount of activator in the semiconductor oxide, it could be possible to fabricate the sensor with good sensing properties.

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