Development of Yttrium-Cerium Oxide Gas Sensor for Low ppm Hydrogen Detection ^†

Abstract

The hydrothermal method was successfully used in the current work to fabricate YCeO nanocomposites, a novel hydrogen (H₂) gas sensor. XRD, FE-SEM, and AFM were performed to determine the crystal structure and morphology of as prepared nanocomposite. The cubic structure of space group Fm3m with a density of 6.74 gmcm⁻³, a volume of 157.81 10⁶ pm³, and a crystallite size of 18.66 nm is obtained in the XRD pattern of YCeO in this instance. Furthermore, the granular structure and roughness of the surface can be seen in FE-SEM and AFM studies. Additionally, hydrogen sensing was performed at a temperature of 28 °C with a hydrogen concentration of 20–120 ppm. The sensor response for hydrogen gas at 20 and 120 ppm was measured to be 1.41 and 2.09, respectively. At 20 ppm, we came to know that the recovery times and rapid response were 75.54 s and 40.81 s, respectively. The long-term stability was also checked for 40 days and the change in sensor response at 20 and 40 days was 1.40 and 1.39, respectively. The investigated sensor device also demonstrates the benefits of a straightforward fabrication procedure, a basic structure, and a very affordable hydrogen detection sensor.

1. Introduction

For energy storage and extraction in different manufacturing units, including different industries such as transportation, domestic consumption, and hydrogen gas (H₂) is essential. However, H₂ is hazardous to handle and use at high temperatures since it is combustible, odorless, and colorless [1,2]. It is being considered to use H₂ gas as a potential substance for use in a variety of fields, such as aeronautical engineering, power production, chemical goods, transportation, and medical biomarkers. However, it is invisible to human senses and traditional gas sensors due to issues including high permeability, flammability, intense chemical reactivity, and the absence of odor, taste, and color. It is crucial to detect H₂ gas molecules at normal temperatures. Devices based on semiconducting metal oxide are dependable and long-lasting for H₂ gas detection. Metal oxides consisting of tungsten, tin, molybdenum, niobium, and zinc have been found to be reliable and fast at detecting H₂ gas [3,4,5,6]. In general, all these applications call for specified properties in nanoparticles. The nanoparticles made from cerium oxide have produced significant advances in biological sciences, industry, and other fields. In addition to all these uses, they have been found to be essential for humanity, considerably aiding in the worldwide growth of a number of industries. However, yttrium oxide nanoparticles have a variety of uses for the study of materials. A key building block for the inorganic synthesis of compounds, yttrium oxide has the capacity to generate the red color, which is a characteristic employed in fluorescent illumination. The exploration of innovative CeO₂/Y₂O₃ composites with tiny pores and a cubic structure, which are attractive candidates for a range of uses because of the enormous area of the surface, low noxiousness, and stability; however, they have been the subject of fewer research papers now [7,8,9,10,11]. As a result, we described the fabrication and characteristics of CeO₂/Y₂O₃ nanocomposites produced by the hydrothermal process, a straightforward method to produce nanostructures using water as a solvent at extreme vapor pressures and low temperatures in an autoclave coated in Teflon. Following that, FE-SEM, XRD, and AFM were used to further understand the structural and morphological characteristics of the as-prepared materials. Lastly, hydrogen sensing was performed at low ppm.

2. Materials and Synthesis

High-purity chemical reagents of SRL grade were then employed in the current work without additional purification. For the creation of the combination of nanoparticles, we employ hydrothermal synthesis. Enough 0.1 M cerium (III) nitrate hexahydrate and 0.1 M yttrium (III) nitrate hexahydrate were dissolved for six hours in distilled water to maintain the homogenous solution. In parallel, distilled water was used to completely dissolve 0.5 M of NaOH. At the ambient temperature, NaOH was then progressively added drop by drop until the pH was 10–11. After that, the whole solution was heated for a period of eight hours at 160 °C in a 100 mL Teflon-lined autoclave. The autoclave was then carried out and permitted to cool at ambient temperature; this produced a small amount of precipitate that was light yellow, which was then dried in a 100 °C oven. In order to create powdered nanocomposite YCeO, 550 °C was used for annealing the material, and after that, it was given time to cool to room temperature. This mixture was then combined with the cerium/yttrium nanocomposites.

Device Fabrication and Measurement Setup

The YCeO sensing film was untrue on a glass substrate 1 by using the spin coating method. The glass substrate (dimension 1 $\times$ 1 cm²) was cleaned in distilled water and acetone instead of ultrasonication. Then, the substrate was dried at 60 °C for 20 min to eliminate organic residues. On the other hand, YCeO powder was discrete in dimethyl formamide (DMF), and a standardized solution was found. The homogenous solution was dropped on a glass substrate, spun at 1500 rpm for 50 s, and then dried on a hot plate for 20 min at 50 °C. This process was repeated twice to obtain the obligatory thickness, and lastly, the thin film was annealed at 400 °C for 2 h in a furnace. After that, silver electrodes were dropped on the sensing film. This sensing device was positioned inside the sensing setup and connected to the end of a Keithley electrometer. All the gasses used are measured by gas cylinders.

3. Results and Discussion

Using powder X-ray diffraction (PXRD), a non-destructive analytical technique for differentiating the different crystalline phases of binary metal oxide nanocomposite, the crystalline phase of the fabricated materials was evaluated. The X-ray diffraction pattern’s recorded peaks were all found to be phases of either yttrium oxide or cerium oxide, and all the diffraction peaks are perfectly coincident with the binary metal oxide’s cubic phase. It also exhibits the excellent crystallinity of the fabricated NPs and their independent cubic fluorite phase in the (Fm3m) space group. To determine the crystal and phase structure of the yttrium cerium oxide, PXRD was used; the resulting X-ray pattern is given in Figure 1a, the (420), (331), (400), (222), (311), (220), (200), and (111) miller indices correlate to the cubic structure of YCeO, and distinct 2θ peaks were found at 79.207, 76.827, 69.524, 59.179, 56.427, 47.553, 33.128, and 28.587 [12].

The NPs’ quality and shape were fundamental elements when examining interactions between nanomaterials. Here, FE-SEM was used to analyze the morphological analyses, as shown in Figure 1b. Under various magnifications, the FE-SEM micrograph was acquired to examine the diameter ranging from 10 to 50 nm of the granular form. The combination of yttrium and cerium to form a nanocomposite structure may be observed clearly in the image, or we can say that both nanoparticles typically exhibit the same shape as fused and aggregated nanoparticles. These granular structures have both a porous shape and an impressive, endless surface area. By analyzing these nanocomposites, we may conclude that it is a precise and economical method of synthesizing YCeO. These tiny YCeO granules increase the material’s surface area and porosity, which is particularly valuable for the adsorption of analytes and enhances the performance of gas sensing.

AFM analysis is the best technique for statistically determining the nanometric dimensional surface roughness and for showcasing the surface nanotexture of the deposited film, as shown in Figure 1c. Like what was seen in the SEM investigation, the surface of the as-grown YCeO films has a granular texture. However, in the AFM images, the crystallites seem larger due to tip convolution. The SEM images demonstrate that the boundaries of the microscopic splits between the grains appear less sharp for a similar reason. In contrast to the “hill” area, which has several orientation-specific crystal-like structures, the “valley” zone is rather smooth. The mean roughness was 40.34 nm, while the RMS roughness was 52 nm, and the average roughness was 130.94 nm according to the estimated value.

4. H₂ Gas Sensing

The response of the fabricated sensor based on YCeO was tested in the range of 20–120 ppm H₂ gas, as shown in Figure 2a. Here, we detected an even graph with fewer variations, which suggests that the response enhancement was due to the YCeO-based sensor. The response to H₂ at 28 °C reached its maximum peak; thus, this suggested that the finest operating temperature of 28 °C should be chosen.

According to the sensor response vs. concentration linear fitting curve, as seen in Figure 2b, the sensor response representation increased linearly as the concentration increased. The linearity coefficient (R²) was determined to be 0.99, indicating the accuracy of a curve that fits linearly. The lowest and highest sensor response values for YCeO were determined to be 1.41 and 2.09, respectively. The YCeO sensor’s sensitivity is additionally determined by the slope of the curve between the constant sensor response and concentration. In the region of 20 to 120 ppm, it was discovered that the YCeO sensor’s sensitivity was 0.067 sensor response/ppm.

$$S=\frac{I_g}{I_a}$$

(1)

$$\mathrm{S}=\mathrm{A}{\left[C\right]}^n+\mathrm{B}$$

(2)

where [C] stands for analyte gas concentration and A and B are both constants. Both response and recovery time for the YCeO-based sensors are protracted with the increase in H₂ concentration. The recovery time is the amount of time needed to lessen the gap between the maximum and lowest current, and the response time (tres) is the amount of time required for the relative current change to reach 90% of the steady state value following H₂ injection [13,14,15,16]. As shown in Figure 2c,d, the YCeO sensor demonstrated a decent response at 28 °C and almost linear growth with H₂ concentrations. Further research on the YCeO-based sensor for 20–120 ppm H₂ at 28 °C revealed that the current decreased following injection into 20 ppm H₂ and returned to its previous value upon discharge of H₂ vapors. The results showed that 40.81 s and 75.54 s, respectively, were the lowest response and recovery times. The current of the sensor film likewise increased as concentration grew. Lower H₂ concentrations result in a weaker connection between H₂ molecules and adsorbed oxygen species, whereas higher H₂ concentrations signify a rise in the sensing film’s current. The slope of the linear fitting curve of sensor response vs. concentrations is used to determine sensitivity. The response and recovery times of the YCeO-based sensor for every concentration were estimated by following Equations (3) and (4).

$$I\left(t\right)=I_a\left(e^{\frac{t}{t_{res}}}\right)$$

(3)

$$I\left(t\right)=I_a\left(1-e^{-\frac{t}{t_{res}}}\right)$$

(4)

This recommends that the gas sensor created on the YCeO can comprehend faster detection of H₂ vapor at low concentrations. The high sensor response and rapid response-recovery abilities of a sensor must be recurring due to the use of the large surface area and surface approachability.

Based on the findings of the experiments, the limit of detection (LOD) for the YCeO-based chemo-resistive gas sensor was determined by Equation (5).

$$LOD=\frac{3.3\times \sigma }{m}$$

(5)

Here, m is the slope of the linear fit curve and σ is the standard deviation of the intercept. The LOD of the YCeO-based chemo-resistive gas sensor was found to be 13.24 ppm.

Here, Figure 3a shows the great repeatability of 6 cycles at 20 ppm of H₂, which shows excellent repeatability. Sensor selectivity is the ability of a sensor to detect a particular gas in the presence of other gases. In other words, by likening the effects of various gases (oxidizing or reducing) on the sensor response, the selectivity may be calculated. Figure 3b depicts the YCeO gas selectivity towards several gases at room temperature, including NH₃, H₂S, NO₂, and H₂. Furthermore, the detected currents of NH₃, H₂S, and NO₂ were low and unstable in comparison to the H₂ gas current. In the presence of NH₃, H₂S, and NO₂ gases, it seems that YCeO thin film has the best selectivity towards H₂, which confirms that H₂ gas molecules are more adsorbent on the surface of YCeO than NH₃, H₂S, and NO₂ gases, as seen in Figure 3b. For sensor optimizations, the long-term stability and repeatability of the sensor are critical partners. A gas sensor’s feature life is also essential to its economic development. By measuring the reactions of the YCeO sensor to 20 ppm H₂ at room temperature (28 °C) for 40 days, the sensor’s long-term dependability was investigated, as shown in Figure 3c. The sensor showed extremely little current and response value changes, indicating strong long-term reliability. The sensor indicates that, after 20 and 40 days, the rapid sensor response was 1.40 and 1.39, respectively, as shown in Figure 3d. This result indicates that physisorption, as opposed to chemisorption, dominates the interactivity between the surface of the as-prepared sample and H₂ gas. This long-term stability provides additional support for its likely industrial uses.

The conventional metal oxide semiconductor gas sensors’ detecting mechanism, particularly hydrogen gas sensing by YCeO-based sensors, is discussed. The current modulation of the YCeO layer is influenced by the chemisorption of oxygen molecules and atoms on the YCeO surface. This results in a negative surface charge, forming a depleted region under the surface in the nanocrystalline layer. The surface of the YCeO semiconductor can easily absorb oxygen in the air, forming chemisorbed oxygen ions O²⁻ or O⁻.

When hydrogen is introduced, the surface charge decreases due to the reaction of chemisorbed O₂ species with H₂, resulting in increased sensor current. When loaded with target gas, the sensor chamber reacts by containing oxygen ions that have been adsorbed, reducing the depletion region width and allowing trapped electrons to move back to the conduction region. For hydrogen, the highest sensor response is shown at even ppm values, with the greatest shift in the depletion region. The responsiveness of the sensor is also affected by its operational temperature and targeted concentration of gas. As temperature rises, oxygen ions are more readily chemisorbed, leading to a large change in the depletion region and sensor resistance. With increasing gas concentration, the sensor’s responsiveness is further improved by a significant interaction with adsorbed oxygen ions [17,18].

Furthermore, this suggests that the gas sensor based on the YCeO-based chemo resistive gas sensor can realize faster detection of H₂ at low concentrations. The high response and quick response-recovery abilities of a sensor must be sustained by effective use of the large surface area and surface approachability. In comparison to previously reported work, the YCeO-based chemo-resistive gas sensors showed high sensing performance at room temperature, as mentioned in

Table 1. Comparative study of H₂ Sensors.

Material	Temp. (°C)	Conc. (ppm)	Response Time (s)	Recovery Time (s)	Ref.
SnO2	130	5000	2	10	[19]
In2O3	260	500	1.7	1.5	[20]
Pd-ZnO nanowires	250	100	25	52	[21]
Pd/In2O3	225	0.1	12	-	[22]
YCeO	28	20	1.41	2.09	Present Work

.

Moreover, we suggest that the semiconductor YCeO plays a role as a catalyst for the interaction between H₂ and atomic oxygen. These outcomes indicate that the reported H₂ sensors based on YCeO that were prepared at low temperatures can be operative to achieve superior sensing performance compared to their counterparts.

5. Conclusions

At room temperature (28 °C), the results of a YCeO-based chemo-resistive gas sensor for the detection of H₂ were excellent. XRD research was conducted to determine the YCeO nanocomposite’s structural details. Furthermore, FE-SEM and AFM were performed to determine the morphology of the prepared material. H₂ was revealed at ambient temperatures in the range of 20–120 ppm. At 20 or 120 ppm, the sensor response for H₂ was determined to be 1.41 and 2.09, respectively. At 20 ppm, it was found that the rapid response and recovery times were 40.81 s and 75.54 s, respectively. The long-term stability was also checked for 40 days and the change in sensor response at 20 and 40 days was 1.40 and 1.39, respectively. The successful creation of a nanocomposite between Y₂O₃ and CeO₂ was the cause of the improved sensor response, which led to improved sensing performance. The chemo-resistive gas sensor built on YCeO successfully demonstrated great long-term stability, exceptional sensor responsiveness, quick response/recovery, and selectivity to H₂ in comparison to other gases. This study reveals the YCeO chemo resistive sensor’s exceptional potential as a low-trace H₂ gas detector.