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Article

Mechanoactivation of Indium Oxide-Based Gas Sensors for Efficiency Enhancement

by
Maria I. Ikim
1,
Varvara A. Demina
1,
Elena Y. Spiridonova
1,
Egor D. Baldin
1,
Olusegun J. Ilegbusi
2,* and
Leonid I. Trakhtenberg
1,3
1
N.N. Semenov Federal Research Center for Chemical Physics RAS, 4 Kosygin Street, Moscow 119991, Russia
2
Department of Mechanical and Aerospace Engineering, University of Central Florida, Orlando, FL 32816, USA
3
Chemical Faculty, Lomonosov Moscow State University, Moscow 119991, Russia
*
Author to whom correspondence should be addressed.
Chemosensors 2026, 14(5), 101; https://doi.org/10.3390/chemosensors14050101
Submission received: 12 March 2026 / Revised: 14 April 2026 / Accepted: 20 April 2026 / Published: 23 April 2026
(This article belongs to the Special Issue Functional Nanomaterial-Based Gas Sensors)
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Abstract

Indium oxide was mechanically activated, and its effect on the operation of semiconductor gas-sensitive devices was evaluated. The structural and morphological characteristics of In2O3 following mechanical activation were examined. The powder treatment produced a defective particle surface structure, enhanced specific surface area, and improved material diffusion properties. Experimental evidence indicates a substantial enhancement in the reactivity of indium oxide with diverse gases, stemming from alterations in grain structure and the formation of novel adsorption sites. The results obtained demonstrate that mechanoactivation is a promising technological tool for the development of energy-efficient sensors.

1. Introduction

Metal oxide-based conductometric gas sensors are widely utilized due to their high sensitivity, low cost, and ease of fabrication. A major drawback hindering their practical implementation is the requirement for elevated operating temperatures, essential for the surface adsorption and desorption reactions of gases. Reducing the operating temperature of the sensor is a critical area of development, as it leads to decreased energy consumption and extended operational lifespan for sensor devices [1].
Sensor materials based on metal oxides are synthesized through various methods, such as spray pyrolysis [2], template [3] and impregnation [4] techniques, sol–gel technology [5], and hydrothermal and solid-state synthesis [6,7]. Mechanical milling is an effective method for reducing particle size, decreasing agglomeration, and homogenizing materials [8,9]. It relies on high-intensity grinding, resulting in particle size reduction, increased specific surface area, and the formation of lattice defects [10].
All of these factors activate surface-active sites, influence the energy band structure during sensor fabrication, and consequently affect the diffusion of the target gas [9,11,12,13]. There are some studies of nanostructured metal oxide materials synthesized using the ball milling technique for use in gas sensors. For example, this method has yielded the optimal formulations of SnO2-α-Fe2O3 to determine the ethanol content, and α-Fe2O3-ZrO2 is promising for low-temperature determination of oxygen in nitrogen [14]. SnO2 powders with the addition of 1 mol% CuO and 3 mol% ZnO obtained by the ball milling method demonstrated high selectivity to CO gas at relatively low temperatures [15]. Nanoparticles of 3 at% La-doped SnO2 prepared by the ball-milling solid chemical reaction method exhibited the highest response to 31.5–50 ppm formaldehyde vapor, with the response and recovery time being 5 and 26 s, respectively [16]. By varying the shredding parameters such as speed, time and solvent used, the structure of ZnO nanoparticles was optimized to effectively detect gases such as dry air/oxygen, argon, nitrogen, hydrogen and methane and atmospheric humidity [17]. An electrochemical sensor based on Fe-TiO2, obtained by high-energy ball grinding, demonstrated the ability to quickly determine paracetamol in pharmaceutical samples [18].
Increasing the ball milling time of Co3O4 (0.5 wt.%) composites with SnO2 led to an increase in pore volume within the 20–35 nm size range. This leads to a significant increase in the response to CO, while such an effective reaction is not observed for H2, which is attributed to the higher diffusion coefficient of hydrogen [19]. For instance, high-energy ball milling of the perovskite LaCoO3 reduces crystallite sizes by a factor of 4 and increases sensitivity to carbon monoxide by a factor of 10 compared to perovskites obtained via the sol–gel method or solid-state reaction, respectively [20]. It is also shown that the oxygen sensitivity of sensors based on SrTiO3 obtained by the high-energy ball milling technique is much higher than commercial sensors [21].
Indium oxide (In2O3) is promising for the detection of various toxic or flammable gases due to its chemical and thermal stability, high concentration of electrons and their mobility [22,23,24]. Ball milling of indium oxide enhances the powder crystallinity and causes agglomeration of both precursor and indium oxide particles [25]. It has been shown that high-energy ball milling of indium oxide powder can influence particle morphology and dispersion, and also produce polymorphic transformations [26,27]. Under ambient conditions, ball milling facilitated the transformation of cubic indium oxide into the hexagonal phase, which is typically observed only at elevated temperatures and pressures [28]. The synthesized mechanoactivated h-In2O3 served as an anode material for lithium-ion batteries, exhibiting an enhanced and stable capacity compared to c-In2O3. It was demonstrated in another study that, after 40 h of mechanical milling, In2O3 became fully ferromagnetic with a hysteresis loop at room temperature. This phenomenon was attributed to intrinsic defects, specifically microstrain, generated during the milling process.
Furthermore, this study found no evidence of iron impurities or secondary phases that could be responsible for the observed ferromagnetism [29]. The reduction in particle size, an increase in defect density and phase transformations of indium oxide are expected to influence its sensing properties. For example, the phase transformations of indium oxide in the In2O3/graphene nanocomposite obtained by the hydrothermal method resulted in a 20-fold increase in response at 5 ppm NO2, achieving ultra-high sensitivity with minimal energy consumption compared to In2O3 [30]. It should be emphasized that the primary method used in recent years for In2O3 synthesis and hydrogen-sensing research has been hydrothermal or its variants (Table 1). The response of hydrothermal oxide obtained from indium to different concentrations of hydrogen has been found to be low. Therefore, various strategies are used to improve its sensory properties, such as creating homo-/heterotransitions, alloying different ions and modifying it with precious metals [30,31,32,33,34,35,36].
The above information shows that the use of mechanoactivation to enhance the sensory properties of indium oxide has not been investigated, although the method has been used to improve optical, electrochemical and magnetic properties [25,28,29]. Therefore, in the present work, the mesoactivation of In2O3 and its influence on gas-sensing properties were investigated for the detection of hydrogen, carbon monoxide, and ammonia. By varying the mechanical grinding time from 5 to 20 min, the structure and morphology of the nanoparticles of indium oxide can be adjusted to produce improved sensory characteristics of the obtained samples.

2. Materials and Methods

Commercial indium oxide from LANHIT (purity: 99.999%) was utilized for sample preparation. The grinding was performed using a high-energy Aronov mill operating at a frequency of 50 Hz and an amplitude of 50 mm. The volume of the grinding vessel was 108 mL, the mass of the hardened steel balls was 272 g, and the oxide mixture loading was 10 g. To prevent the overheating of the mixture and the mill, the process was carried out in single grinding cycles, each lasting 1 min, followed by 10 min breaks. Material samples were sequentially extracted after 5, 10, 15, and 20 min of milling to analyze changes in structural and functional characteristics. The powders of commercial indium oxide and those obtained after grinding for 5, 10, 15, and 20 min were designated as In2O3 (0 min), In2O3 (5 min), In2O3 (10 min), In2O3 (15 min), and In2O3 (20 min), respectively.
The structure of the powders obtained was investigated using X-ray diffraction analysis on a Rigaku SmartLab X-Ray Diffractometer (Rigaku, Tokyo, Japan) with monochromatic Cu Kα radiation with a wavelength of 1.5406 Å. The scanning range for the 2θ angle was 15° to 80° at a speed of 5°/min. The average nanoparticle size was determined from the XRD data according to the Debye–Scherrer formula, i.e., D = 0.9λ/(β·cosθ), where β is the peak full width at half maximum, and θ is the diffraction angle corresponding to the given peak.
The specific surface area was determined by the Brunauer–Emmett–Teller (BET) method at a relative pressure of P/P0 = 0.30 using an AMI-400 TPx (Altamira Instruments, Pittsburgh, PA, USA). Temperature-programmed reduction by hydrogen (H2-TPR) was carried out on the same instrument. Before measurements, the samples were pretreated in an argon stream at 300 °C for 10 min, then cooled to 50 °C. Reduction was carried out in a gas mixture of 10% H2 in Ar at a flow rate of 30 mL/min. The temperature was raised to 500 °C at a heating rate of 15 °C/min. The morphology and structure of the particles were determined by scanning electron microscopy using a Prisma E instrument (Thermo Fisher Scientific, Waltham, MA, USA). The local structure of the samples was investigated by Raman spectroscopy using the Raman microscope–spectrometer SENTERRA (Bruker, Ettlingen, Germany) with excitation at a wavelength of 785 nm.
To form the sensitive layers, the powders were mixed with terpineol, and then the homogeneous paste was applied to commercial polycor plates equipped with a platinum heater and contacts for resistance measurement [37]. Then, the plate with the applied layer was gradually heated to 550 °C to achieve the constant resistance of the resulting film. Three sensors were made for each of the obtained samples and used for the measurements.
The conductivity and sensory characteristics were investigated on a device that we developed [37]. Measurements were performed in the temperature range of 300–550 °C at a relative humidity (RH) of 30%. The temperature control accuracy was 1 °C. Certified ready-made gas mixtures containing 900 ppm H2, CO, and NH3 in air were used to determine sensor characteristics. Clean air or certified gas mixture was pumped at a rate of 200 cm/min through a measuring chamber of approximately 1 cm3 volume equipped with a sensor. The gas flow was regulated by electromagnetic valves. As a result, the filling of the measuring chamber with a mixture of gases and its subsequent air purge took only a fraction of a second, allowing for high-precision kinetics to be tracked when the concentration of analyzed gas in the air changes.
The change in sensor resistance was recorded with the Keithley 34465A multimeter (Keysight, Santa Rosa, CA, USA). The signal from the multimeter was transmitted to a computer, where a special program provided the kinetic curve of change in resistance, reflecting the variation over time during the supply of the gas mixture being analyzed. Also, the response time τresp and the relaxation time τrelax of the sensor are defined as the time needed to achieve 90% change in sensor resistance during the introducion and removal of the analyzed gas, respectively.

3. Results and Discussion

Mechanical action on the material can influence its phase composition, particle size, and the number of crystal lattice defects [9]. Phase and structural changes in the mechanically treated samples were investigated using X-ray diffraction analysis. Figure 1 displays the evolution of XRD spectra for commercial In2O3 (0 min) powder and powders milled for 5, 10, 15, and 20 min.
The diffraction peaks at 2θ ≈ 21.5°, 30.6°, 35.5°, 45.69°, and 51.03° correspond to the (211), (222), (400), (431), and (440) crystallographic planes of the cubic In2O3 structure (JCPDS Card No. 71-2194). This result indicates the presence of the indium oxide crystalline phase in all samples. It has been shown that prolonged milling exceeding 96 h can lead to phase transformations in indium oxide due to plastic deformation and the formation of large defects during the milling process [28,38]. Our results show that, irrespective of the milling time, the absence of any extra peaks confirms that the samples contain neither impurities nor secondary phases. However, milling leads to the broadening of diffraction peaks and a decrease in their intensity (Figure 1). These changes can be attributed to a reduction in both nanoparticle size and strain in the crystal lattice due to defects introduced during the grinding process.
The size of the indium oxide nanoparticles, calculated using the Debye–Scherrer method from the XRD data, was 36 nm for the initial sample (Figure 2). During the first 15 min of milling, there is a rapid decrease in particle size of 50%. The particle size decreases from 18 nm to 16 nm, as the milling time is extended to 20 min, which is not as intense as the result for the 15 min of grinding. Conversely, the specific surface area of the particles steadily increases with grinding time, rising from 9.5 m2/g for the initial indium oxide to 19.4 m2/g after 20 min of grinding. Thus, it can be assumed that, during the grinding of indium oxide, there is initially an active reduction in the size of nanoparticles, accompanied by an increase in specific surface area. Then, with a prolonged grinding time from 15 min to 20 min, an increase in particle porosity occurs, which enhances the specific surface area.
The structural parameters calculated from the XRD data are presented in Table 2. The lattice parameter of the ground samples is higher than that of the commercial indium oxide, a trend observed independently of the processing time (Table 2). The competition between defect accumulation and microstrain, resulting in local lattice expansion, could be responsible for this phenomenon. Mechanical activation has been shown to induce microdistortions in crystallites [9].
Therefore, the Williamson–Hall method was used to separate the contributions of dispersion and microstrain to the broadening of the In2O3 reflections [39]. It is observed that the crystallite size decreases, similar to the estimation by the Debye–Scherrer method, while the lattice strains increase with the extended grinding time (Table 2). The differences in particle sizes measured by the different methods (Figure 2 and Table 2) are attributed to the fact that the Debye–Scherrer method accounts for peak size shrinkage only, while the Holder–Wagner method allows for the separation of size contributions and microshapes [27,40]. A similar behavior was observed in mechanochemically activated ZnO samples [41,42]. Analysis of the diffraction parameters reveals a reduction in crystallite size and an increase in internal microstrains, causing the partial disordering of the In2O3 structure upon mechanical treatment.
Furthermore, the Raman spectroscopy data corroborate the changes observed in the crystalline structure of indium oxide. Figure 3 displays the Raman scattering spectra for indium oxide powders produced under different grinding times. The peaks with the highest intensity are located at 132, 306, 365, 495, and 629 cm−1. All the modes correspond to the cubic bixbyite (c-In2O3) crystal structure, and the absence of new peak formation during grinding is further corroborated by the X-ray diffraction results. However, with prolonged mechanical exposure over 96 h, the Raman spectra exhibit a new peak at 161 cm−1, indicative of the formation of the hexagonal phase of indium oxide [38]. Moreover, after grinding for 5 min, supplementary peaks emerge between 1200 and 2000 cm−1, but these peaks are transient and disappear with prolonged grinding. These peaks are indicative of luminescence originating from the In2O3 (5 min) powder and its subsequent quenching due to concentration effects. The luminescence observed in indium oxide In2O3 (5 min) probably stems from a confluence of structural and electronic factors.
Increasing the grinding time leads to peak broadening and reduced intensity, providing evidence for enhanced disorder within the crystal structure and the creation of defects. Beyond this, a red shift is noted. This shift increases with extended grinding times, reaching 10 cm−1 for the Raman peak near 132 cm−1 after 20 min of milling.
It has been demonstrated that reducing the crystallite size can induce a red shift in Raman peaks within the 1–10 cm−1 range. This phenomenon is attributed to the spatial confinement of phonons and alterations in phonon mode frequencies stemming from sample strains or defects [43,44].
The grinding process in the Aronova mill affects not only the structural but also the morphological characteristics of the indium oxide nanopowder particles. This is corroborated by the scanning electron microscopy data (Figure 4).
The as-received In2O3 powder is therefore highly inhomogeneous. It comprises particles with elongated shapes and sharp facets, 365 ± 100 nm long and with a characteristic aspect ratio of ~4, as well as cubic particles measuring 0.92 ± 0.17 µm (Figure 4a). Furthermore, large, irregularly shaped agglomerates are present, with dimensions of 7.7 ± 1.9 µm. These agglomerates exhibit variations, including those with rough surfaces and others that are denser and possess smooth surfaces.
Mechanoactivation leads to the gradual homogenization of the powder with increasing grinding time. Thus, after 5 min of grinding, three types of particles can be distinguished (Figure 4b). The smallest individual particles become more spherical, and their size decreases to 247 ± 160 nm. The larger particle aggregates also become more rounded in shape, their maximum dimensions decrease to 5 µm, and their surfaces are uniformly rough. Additionally, single particles of the order of 40 µm composed of large particle aggregates are observed.
As the grinding time increases, such particles are encountered less frequently, and they disappear entirely after 20 min of grinding. The number of large aggregates decreases, while individual particles, conversely, are encountered more frequently, with their size being approximately 25 ± 10 nm after 15 min of grinding and remaining unchanged with further increases in grinding time (Figure 4c). Consequently, extending the mechanochemical processing time of indium oxide powder induces morphological and structural alterations. These modifications may influence the quantity of active gas interaction sites, thereby impacting the sensing properties of the resultant powders.
The study of the conductive properties of mechanically activated indium oxide powders shows that the resistance of all investigated samples decreases in the temperature range of 280–500 °C (Figure 5).
The mechanism of charge transport in indium oxide involves electrons that are generated by oxygen vacancies. Elevated temperatures increase the degree of ionization and the mobility of charge carriers, leading to a reduction in resistance, thereby substantiating the electronic nature of conductivity in all specimens.
Grinding for five minutes in an Aronov mill results in a reduction in the sensor film’s resistance relative to the commercial sample (Figure 5).
High-energy ball milling is known to introduce structural disorder, strain, and point defects into metal oxide powders, including In2O3. We hypothesize that mechanochemical activation in the early stages of milling may generate donor-type point defects (possibly oxygen vacancies and/or interstitial defects), which can introduce additional electronic states within the band gap. Such states could increase the free carrier concentration in the conduction band, thereby decreasing the overall resistance of the material. At the same time, prolonged milling reduces the crystallite size (Figure 2 and Table 2), which increases the total area of grain boundaries and impedes charge carrier transport, eventually leading to higher resistance. Notably, Raman spectra recorded with 785 nm excitation reveal a strong luminescence background in the 1000–2250 cm−1 range for the sample milled for 5 min (Figure S1). This finding corresponds to near-infrared emission approximately in the 865–965 nm region (maximum ~900 nm). We tentatively attribute the appearance and subsequent quenching of this luminescence to the formation and further evolution of optically active defects at short milling times. However, concentration quenching or additional structural changes cannot be excluded (see Supplementary Materials).
The electrons are capable of readily moving into the conduction band, thereby augmenting the concentration of free charge carriers and consequently reducing the resistance of the sample. In contrast, an increase in milling time from 10 to 20 min results in a rise in resistance. This is accompanied by an increase in the number of individual particles and a decrease in their size, thereby causing an increase in resistance by impeding charge transport at the intercrystalline boundaries.
The nature of conductivity in the obtained samples determines the specifics of the sensor effects during hydrogen detection. Figure 6a illustrates the temperature dependence of the sensor response observed in films fabricated from mechanoactivated indium oxide nanopowders when detecting 900 ppm of H2.
The resulting profiles exhibit a maximum, denoted as Smax, at defined operating temperatures (Tmax). The emergence of the maximum can be explained by two competing processes: the reaction of adsorbed hydrogen with oxygen centers and the desorption of gas from the sensor surface. Increasing the temperature promotes an increase in the rate of the sensor reaction, but at the same time leads to a decrease in the concentration of adsorbed gas. Notably, the optimal operating temperature (Tmax) for the mechanoactivated samples, irrespective of the milling duration, is approximately 100 °C lower than the commercial counterpart (Figure 6a). The reduction in the operating temperature of the sensitive element is important both for increasing the energy efficiency of resistive gas sensors and for expanding their application scope.
For the interpretation of the results, the catalytic activity of the sample surfaces was studied via temperature-programmed hydrogenation (TPR-H2) (Figure 6b). Increasing the temperature first leads to the reduction in various forms of chemisorbed oxygen, and subsequently, at temperatures above 300 °C, bulk reduction to metallic indium occurs [45]. Figure 6b displays the TPR-H2 profiles within the temperature range of 100–350 °C. The commercial indium oxide powder exhibits a prominent peak centered at approximately 280 °C, along with a less pronounced peak in the region of 160–170 °C.
During powder grinding, the high-temperature peak becomes progressively less intense, while the intensity of the low-temperature peak increases. These peaks correspond to oxidative–regenerative reactions of H2 with the chemorbic oxygen ions on the surface of the samples. It has been shown that peaks of hydrogen recovery on the In2O3 surface with three different morphologies occur at different temperatures [46]. According to the SAM, commercial indium oxide is highly heterogeneous and contains particles with different morphologies. The process of grinding leads to an increase in the population of smaller, smoother, and more defective particles. Thus, it can be assumed that the two-peak appearance of the TPR curve is due to the oxygen rebound associated with the particle surface as well as the oxygen located in less accessible areas, for example, pores and interparticle contacts within agglomerates. For the sample subjected to 20 min of shredding, only one maximum is observed in the TPR profile, which can be explained by the deeper destruction of agglomerates and the presence of more defects. Thus, oxygen is restored from a more homogeneous surface in terms of energy characteristics.
A similar pattern was previously observed during the hydrogen reduction in hydrothermally synthesized In2O3 powder with varying morphologies. Thus, cubic indium oxide particles showed a peak at 250 °C, while smaller and spherical particles displayed a peak at 160 °C [37]. Both the commercial indium oxide and the powder consumed an equal amount of hydrogen (5.4 mmol/g) after 5 min of grinding. In contrast, powders subjected to 10, 15, and 20 min of grinding showed a decrease in H2 consumption, falling within the range of 3.8–4.4 mmol/g. The hydrogen sensory data (Figure 6a) is well correlated with the H2-TPR profiles of the samples (Figure 6b). Using time-resistance dependence, the kinetics of hydrogen reaction with charged oxygen forms was investigated in two temperature ranges of 360–400 °C and 440–480 °C. These ranges correspond to the maximum sensory response temperatures of mechanoactivated and commercial oxides, respectively. The dependence of ln(dR/dt) on 1/T was constructed for each temperature range.
The activation energies of these reactions were calculated using the Arrhenius relation. Thus, in the original powder, the activation energy decreases as the temperature increases from 1.07 ± 0.14 eV (360–400 °C) to 0.48 ± 0.01 eV (440–480 °C). Conversely it increases from 0.95 ± 0.18 eV to 1.71 ± 0.16 eV in the sample obtained with 20 min of shredding. This result implies that it is energetically better to react the original commercial indium oxide with hydrogen at higher temperatures. Notably, on the H2-TPR curves, there is a predominance of the high-temperature peak reaction (Figure 6b). At the same time, the 20 min sample shows the opposite pattern when activation energy is lower at 360–400 °C and low temperature peaks prevail on the H2-TPR curves. The operating temperature of such sensors is also reduced (Figure 6).
The results from the temperature-programmed reduction in mechanically activated indium oxide nanopowders correlate well with their sensor properties for hydrogen detection. Thus, when detecting 900 ppm of hydrogen, sensors based on In2O3 (0 min) and In2O3 (5 min) powders demonstrate a maximum response of approximately 26, but at different operating temperatures. Samples subjected to 10, 15, and 20 min of grinding exhibit a lower maximum response to hydrogen (see Figure 6a). However, it is worth noting that, at temperatures below 450 °C, the response to hydrogen from the mechanically activated samples is nearly twice as large as that of the commercial sample. This implies that, with equivalent sensor power consumption, the mechanically treated indium oxide demonstrates enhanced sensitivity to hydrogen, a significant factor for their practical implementation.
The sensing properties of the sensitive layers based on the mechanically activated indium oxide nanopowders were also investigated for the detection of 900 ppm carbon monoxide and ammonia (Figure 7). It should be noted that grinding has a stronger influence on the temperature of the maximum sensor response to these gases compared to hydrogen.
In the context of carbon monoxide detection, commercial indium oxide exhibits an operating temperature of approximately 500 °C. Similar to the case with hydrogen, mechanoactivation regardless of grinding time leads to a significant reduction in operating temperature by approximately 150 °C (Figure 7a). For ammonia detection, a similar trend is observed, although the temperature reduction is somewhat less, from 550 °C for the commercial sample to 450 °C for the mechanoactivated samples (Figure 7b).
However, the impact of mechanoactivation on the signal amplitude shows a degree of difference. The maximum sensor response to carbon monoxide correlates positively with the mechanochemical treatment time, increasing from 9.5 for the In2O3 (5 min) sample to 13.5 for the In2O3 (20 min) sample. The trend observed during ammonia detection also shows an increase in the maximum sensor response corresponding to an increase in mechanochemical treatment time, from 5 to 20 min. The difference compared to carbon monoxide detection is that, after 15 min of indium oxide treatment, there is no effect of further mechanoactivation on CO detection plateaus, while sensor activity continues to increase with grinding time for ammonia detection.
Thus, mechanoactivation leads to a reduction in the operating temperature for the detection of all investigated gases, regardless of the grinding time. Thus, at a temperature of 380 °C, the sensory response of mechanically activated samples is 1.7–2.3 times higher than that for commercial indium oxide, depending on the gas being analyzed (Figure 7c). These data correlate well with the more than two-fold increase in the specific surface area of the samples upon mechanical stimulation (see Figure 2 and Figure 7c). It can be seen that the sensitivity of all samples to CO and NH3 is much lower than that to H2, which indicates high selectivity towards hydrogen. Thus, the SH2/SCO values for the In2O3 (0 min), In2O3 (5 min), In2O3 (10 min), In2O3 (15 min), and In2O3 (20 min) samples are 2.4, 2.9, 2.2, 1.8, and 1.8, while the SH2/SNH3 values are 3.9, 4.1, 3.4, 2.7, and 2.5, respectively. Notably, the In2O3 (5 min) sample not only exhibits the highest hydrogen response but also superior selectivity. Furthermore, the detection limit of In2O3 (5 min) is 223 ppb, which is lower than that of commercial indium oxide (541 ppb), indicating the higher sensitivity of the mechanically activated sample in detecting hydrogen.
The detection limit was calculated using the formula LOD = 3σ/S, where σ and S are the standard deviations of the sensor resistance and the slope. The corresponding data are presented in Figure S2. The response/recovery characteristics of the In2O3 (5 min) sample were repeated over five cycles using 900 ppm H2 at 360 °C, demonstrating good reproducibility of the results for hydrogen detection. To study long-term stability, the resistance to hydrogen exposure and the response to it were tested every five days for three months. The results are shown in Figure S3. The In2O3 (5 min) sample shows a virtually constant response with a maximum deviation of less than 5% over the course of a month. Thus, mechanical activation not only increases the response, sensitivity, and selectivity of the samples, but also provides excellent reproducibility and stability.
Nevertheless, its influence on the detection efficiency of H2, CO, and NH3 varies. The sensor response magnitude is determined by the abundance and accessibility of defects, which function as the active centers for gas-phase reactions.
As previously demonstrated, the grinding process leads to an increase in the number of defects in indium oxide. However, their accessibility depends on the structural and morphological characteristics of In2O3 and on the properties of the detected gas, particularly on the size and shape of the detected gas molecule. Thus, hydrogen and carbon monoxide molecules are linear, while the ammonia molecule has a trigonal pyramidal shape [47]. The H2 and CO sensor responses are higher than NH3 responses due to the differences in the reactivity of these molecules on the indium oxide surface, although the small kinetic diameter of NH3 (0.26 nm) may facilitate its diffusion to the active sites compared to H2 (0.289 nm) or CO (0.376 nm).
The determining factor in the sensory process is how effectively a gas molecule reacts with the chemisorbed oxygen ions on the surface, as a result of which electrons are returned to the semiconductor’s conductivity zone. The interaction of NH3 with oxygen ions occurs and forms intermediate nitrosin groups or ammonium ions. Such a reaction is less effective in releasing charge carriers than reactions with H2 and CO, which easily enter oxidative–regenerative oxygen reactions. The response/recovery time values also confirm this. Their values for In2O3 (5 min) in hydrogen detection are 0.7c/56c, in carbon monoxide detection are 1.5c/80.1c and in ammonia detection are 1.8c/117c (Figure S4).
The maximum response magnitude is enhanced by increasing the grinding time of indium oxide, when considering the operating temperature of the mechanoactivated samples. This is related to the decrease in their particle size and the increase in their specific surface area and porosity. The only exception is the sensor response to hydrogen of the In2O3 (5 min) sample. As demonstrated earlier, this particular sample is unique in its luminescence and, therefore, possesses additional lattice defects. Yet, hydrogen has the greatest access to these defects because of the molecule’s smallest size, which facilitates its diffusion in indium oxide.

4. Conclusions

Ball milling is an effective technological technique used to change the morphology and crystalline structure of powders. The powder becomes more homogeneous, the particle size decreases, and their surface becomes smoother as a result of grinding the indium oxide. In this process, the crystalline structure of cubic indium oxide is preserved, while the number of defects within it increases.
Studies using Raman spectroscopy have shown that In2O3 samples exhibit luminescence after five minutes of grinding. This phenomenon, attributed to the presence of specific defects, vanishes upon prolonged mechanical treatment. Structural and morphological changes in indium oxide powders affect the sensing properties of sensors based on them. All mechanochemically activated samples exhibit a reduced operating temperature. This reduction is approximately 100 °C for the detection of hydrogen and ammonia and 150 °C for carbon monoxide. This result significantly enhances the energy efficiency of the sensors, which is important for practical applications.
We observe that the effect of mechanical processing duration on the sensor response magnitude differs among the gases investigated. In the case of hydrogen, which is the smallest molecule considered, the peak sensor response is exhibited by the sample subjected to the minimum grinding duration (5 min). This is attributed to the fact that luminescent defects, present only in this sample, are readily accessible for the diffusion of small hydrogen molecules. For the detection of larger molecules such as carbon monoxide or ammonia, samples subjected to longer mechanical processing exhibit the best sensing characteristics, due to the reduced particle size of indium oxide and the formation of accessible crystal structure defects. The experiments performed reveal that methanoactivation improves the performance of indium oxide-based sensors by increasing their response and lowering their operating temperature by 100–150 °C in the detection of H2, CO and NH3. Improvements occur as a result of structural and morphological changes and the formation of additional defects. This method can also be used for other metal oxides as a promising technology to give sensors with new and improved functional properties.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemosensors14050101/s1. Figure S1: Detailed Raman spectra in the 1000–2500 cm−1 for 2-, 5- and 10-min samples (a). Temperature dependence of the resistance of sensitive layers based on commercial In2O3 milled over different time durations (b); Figure S2: Dynamic response and recovery curve of In2O3 (0 min) on exposure to 50–900 ppm of H2. On the insert dependence of the sensor response of samples In2O3 (0 min) and In2O3 (5 min) on H2 concentration (b); Figure S3: The In2O3 (5 min) sensor response cyclic (900 ppm H2, T = 360 °C, 30% humidity). On the insert long-term stability (3 months) of In2O3 (5 min) to 900 ppm H2 at 380 °C; Figure S4: Dynamic response and recovery curve of In2O3 (5 min) on 900 ppm H2 (a), CO (b) and NH3 (c) at temperature 380 °C.

Author Contributions

M.I.I.: conceptualization, investigation, writing—original draft. V.A.D.: investigation, writing original draft. E.Y.S.: methodology, writing original draft. E.D.B.: synthesis, writing original draft. O.J.I.: validation, writing—review and editing. L.I.T.: conceptualization, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research is supported by a subsidy from the Ministry of Science and Higher Education of the Russian Federation for N.N. Semenov Federal Research Centre of Chemical Physics, RAS, within the framework of State Assignment No. 125012200595-8.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article/Supplementary Material. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

References

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Figure 1. XRD spectra of commercial In2O3 powder and milled for different time durations.
Figure 1. XRD spectra of commercial In2O3 powder and milled for different time durations.
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Figure 2. Specific surface area and particle size of commercial In2O3 and milled over different time durations.
Figure 2. Specific surface area and particle size of commercial In2O3 and milled over different time durations.
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Figure 3. Raman scattering spectra for commercial In2O3 milled over different time durations.
Figure 3. Raman scattering spectra for commercial In2O3 milled over different time durations.
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Figure 4. SEM images of commercial indium oxide powder (a) and subjected to mechanical activation for 5 min (b) and 20 min (c).
Figure 4. SEM images of commercial indium oxide powder (a) and subjected to mechanical activation for 5 min (b) and 20 min (c).
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Figure 5. Temperature dependence of the resistance of sensitive layers based on commercial In2O3 milled over different time durations.
Figure 5. Temperature dependence of the resistance of sensitive layers based on commercial In2O3 milled over different time durations.
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Figure 6. Temperature dependence of the sensor response to 900 ppm H2 (a) and TPR-H2 curves of commercial In2O3 milled over different time durations (b). The errors in graph (a) represent the standard deviation of the readings of three sensors.
Figure 6. Temperature dependence of the sensor response to 900 ppm H2 (a) and TPR-H2 curves of commercial In2O3 milled over different time durations (b). The errors in graph (a) represent the standard deviation of the readings of three sensors.
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Figure 7. Temperature dependence of the sensor response to 900 ppm CO (a) and 900 ppm NH3 (b) of commercial In2O3 milled over different time durations. Sensor response to 900 ppm H2, CO, and NH3 at 380 °C versus sample milling time (c). Error bars represent the standard deviation of the three sensor measurements.
Figure 7. Temperature dependence of the sensor response to 900 ppm CO (a) and 900 ppm NH3 (b) of commercial In2O3 milled over different time durations. Sensor response to 900 ppm H2, CO, and NH3 at 380 °C versus sample milling time (c). Error bars represent the standard deviation of the three sensor measurements.
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Table 1. Hydrogen sensory properties of indium oxide obtained by different methods.
Table 1. Hydrogen sensory properties of indium oxide obtained by different methods.
In2O3 Synthesis MethodResponse
(Rg/R0)		H2 Concentration (ppm)Operating Temperature (°C)References
Hydrothermal method1.6221000160[31]
Hydrothermal method1.7200360[32]
Hydrothermal method1.150160[33]
Hydrothermal method10500300[34]
Solvothermal method1.520280[35]
Solvothermal method5.71500340[36]
Table 2. Characteristics of In2O3 powders after milling obtained from XRD data.
Table 2. Characteristics of In2O3 powders after milling obtained from XRD data.
SampleLattice Parameter, ÅCrystallite Size, nm (Hall Method)Strain, %
In2O3 (0 min)10.115951.90.31
In2O3 (5 min)10.1206333.30.32
In2O3 (10 min)10.119125.50.4
In2O3 (15 min)10.117216.00.45
In2O3 (20 min)10.117913.20.47
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Ikim, M.I.; Demina, V.A.; Spiridonova, E.Y.; Baldin, E.D.; Ilegbusi, O.J.; Trakhtenberg, L.I. Mechanoactivation of Indium Oxide-Based Gas Sensors for Efficiency Enhancement. Chemosensors 2026, 14, 101. https://doi.org/10.3390/chemosensors14050101

AMA Style

Ikim MI, Demina VA, Spiridonova EY, Baldin ED, Ilegbusi OJ, Trakhtenberg LI. Mechanoactivation of Indium Oxide-Based Gas Sensors for Efficiency Enhancement. Chemosensors. 2026; 14(5):101. https://doi.org/10.3390/chemosensors14050101

Chicago/Turabian Style

Ikim, Maria I., Varvara A. Demina, Elena Y. Spiridonova, Egor D. Baldin, Olusegun J. Ilegbusi, and Leonid I. Trakhtenberg. 2026. "Mechanoactivation of Indium Oxide-Based Gas Sensors for Efficiency Enhancement" Chemosensors 14, no. 5: 101. https://doi.org/10.3390/chemosensors14050101

APA Style

Ikim, M. I., Demina, V. A., Spiridonova, E. Y., Baldin, E. D., Ilegbusi, O. J., & Trakhtenberg, L. I. (2026). Mechanoactivation of Indium Oxide-Based Gas Sensors for Efficiency Enhancement. Chemosensors, 14(5), 101. https://doi.org/10.3390/chemosensors14050101

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