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High-Temperature Methane Sensors Based on ZnGa₂O₄:Er Ceramics for Combustion Monitoring

Technologies 2025, 13(7), 286; https://doi.org/10.3390/technologies13070286

by Aleksei V. Almaev^1,2

, Zhakyp T. Karipbayev^3,*

, Askhat B. Kakimov³

, Nikita N. Yakovlev¹

, Olzhas I. Kukenov¹

, Alexandr O. Korchemagin¹

, Gulzhanat A. Akmetova-Abdik^3,*, Kuat K. Kumarbekov³, Amangeldy M. Zhunusbekov³

, Leonid A. Mochalov^4,5

, Ekaterina A. Slapovskaya⁴

, Petr M. Korusenko^6,7

, Aleksandra V. Koroleva⁸, Evgeniy V. Zhizhin⁸

and Anatoli I. Popov^3,9

Reviewer 1: Anonymous

Reviewer 2:

Hao Cui

Reviewer 3: Anonymous

Reviewer 4: Anonymous

Technologies 2025, 13(7), 286; https://doi.org/10.3390/technologies13070286

Submission received: 28 April 2025 / Revised: 15 June 2025 / Accepted: 18 June 2025 / Published: 4 July 2025

(This article belongs to the Section Innovations in Materials Science and Materials Processing)

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

How does the formation of Er₃Ga₅O₁₂ phase specifically enhance CH₄ sensing performance, and is there evidence of catalytic activity?
Were process parameters (e.g., ball-milling duration, annealing temperature) optimized for phase purity and grain growth?
How do the sensing properties of ZGO+Er compare to recent advanced CH₄ sensors (e.g., MOFs, nanoarrays) in high-temperature regimes?
What is the mechanistic origin of humidity interference (RH 30-70%) on CH₄ response, and how might surface hydroxylation affect O₂ adsorption?
Long-term stability data spans only 19 days—what degradation mechanisms could emerge over months/years at 650°C?
Can surface-sensitive characterization (e.g., XPS, EDX mapping) confirm increased active sites or Er-induced dopant effects proposed in the sensing mechanism?

Author Response

Response to the Reviewer

Dear Reviewer,

We are very grateful to you for the careful reading and the very valuable comments on our manuscript. The constructive criticism has helped us to considerably improve our manuscript. Please find below one-by-one response to your comments and questions raised.

How does the formation of Er₃Ga₅O₁₂ phase specifically enhance CH₄ sensing performance, and is there evidence of catalytic activity?

Authors’ response:

We cannot prove the catalytic activity of Er₃Ga₅O₁₂. According to the reviewer's comment, significant changes have been made to the texts of the discussions and conclusions:

Discussion

According to the results of SEM and AFM (Fig. 4 and 5) the addition of Er leads to the formation of a larger active surface, represented by two types of microcrystals with characteristic sizes of 1 – 7 µm and 0.5 – 4 µm, respectively. At the same time, the base resistance of the samples (Fig. 8) (resistance in pure air) decreases only by a factor of 2.0 – 2.6, and the response of the samples to CH₄ increases by a factor of 11.1 (Fig. 9). The gas-sensing, electrical-conductivity and other properties of Er₃Ga₅O₁₂ remain unexplored. Therefore, the proposal of a mechanism for increasing the response to gas based on the formation of a new phase of Er₃Ga₅O₁₂ is not justified. It has been shown that Er₃Ga₅O₁₂ demonstrates unique magnetic [72] and phosphor [73] properties, and is also used as a catalyst support for the Ni catalyst for CH₄ decomposition at T = 650 °C [74]. It is noted that Er₃Ga₅O₁₂ provides the most intense CH₄ decomposition. The temperature of use of the Ni/Er₃Ga₅O₁₂ catalyst coincides with the temperature of the maximum response of ZGO+Er to CH₄. However, the results of our research only allow us to conclude that the increase in the ZnGa₂O₄ response with Er addition is due to an increase in the material's active surface area (Fig. 4 and 5). This, in turn, contributes to an increase in the surface density of adsorption centers for O^2-, which leads to an increase in the response to reducing gases [68]. A similar case was observed when Er was added to SnO₂ [57,75].

…

Conclusions

Pure ZnGa₂O₄ and ZnGa₂O₄ with Er addition ceramic pellets were synthesized. For the first time, the gas sensitivity of ZnGa₂O₄ with Er addition ceramic pellets has been studied. The addition of Er leads to the formation of the Er₃Ga₅O₁₂ phase in the ZnGa₂O₄ host, more importantly to the formation of a larger active surface and allows an 11.1-fold increase in the response of ZnGa₂O₄ to CH₄. The samples were characterized by a wide dynamic range of CH₄ concentrations, 100 – 20000 ppm, weak dependence of gas-sensitive characteristics on relative humidity in the range of 30 – 70%, weak changes of gas-sensitive characteristics under cyclic gas exposure, and stabilization of characteristics for long-term tests. They also exhibited high responses to C₂H₄, C₃H₈, C₄H₁₀, NO₂ and H₂. A possible mechanism of the sensing effect of ZnGa₂O₄ with Er addition was proposed. It was suggested that the increase in gas responses with Er addition is due to the formation of a larger active surface. Therefore, ZnGa₂O₄ with Er addition is very prospective for the development of high-temperature hydrocarbons sensors for systems of monitoring and control of combustion processes and for determination of ideal fuel/air mixture. Our future work will focus on optimizing the gas-sensitive properties of high-temperature hydrocarbons sensors based on ZnGa₂O₄ with Er addition ceramic pellets with the aim of increasing their gas sensitivity, decreasing effect of on relative humidity in the range of 0 – 30% and improving their selectivity under extreme operating conditions.

References:

Singh, G.; Virpal; Singh, R.C. Highly Sensitive Gas Sensor Based on Er-Doped SnO₂ Nanostructures and Its Temperature Dependent Selectivity towards Hydrogen and Ethanol. Sens. Actuators B Chem. 2019, 282, 373–383. https://doi.org/10.1016/j.snb.2018.11.086.

…

Almaev, A.V.; Chernikov, E.V.; Davletkildeev, N.A.; Sokolov, D.V. Oxygen Sensors Based on Gallium Oxide Thin Films with Addition of Chromium. Superlattices Microstruct. 2020, 139, 106392. https://doi.org/10.1016/j.spmi.2020.106392.

…

Cai, Y.; Wilson, M.N.; Beare, J.; Lygouras, C.; Thomas, G.; Yahne, D.R.; Ross, K.M.; Taddei, K.M.; Sala, G.; Dabkowska, H.A.; et al. Crystal Fields and Magnetic Structure of the Ising Antiferromagnet Er₃Ga₅O₁₂. Phys. Rev. B 2019, 100, 184415. https://doi.org/10.1103/PhysRevB.100.184415.
Nogales, E.; García, J.A.; Mendez, B.; Piqueras, J.; Lorenz, K.; Alves, E. Visible and Infrared Luminescence Study of Er Doped β-Ga₂O₃ and Er₃Ga₅O₁₂. J. Phys. D Appl. Phys. 2008, 41, 065406. https://doi.org/10.1088/0022-3727/41/6/065406.
Nakayama, A.; Asai, K.; Nagayasu, Y.; Iwamoto, S.; Yagasaki, E.; Inoue, M. Effect of Support Particle Morphology of Ni Catalysts on Growth of Carbon Nanotubes by Methane Decomposition. J. Jpn. Pet. Inst. 2006, 49, 308–314. https://doi.org/10.1627/jpi.49.308.
Weber, I.T.; Valentini, A.; Probst, L.F.D.; Longo, E.; Leite, E.R. Catalytic Activity of Nanometric Pure and Rare Earth-Doped SnO₂ Samples. Mater. Lett. 2008, 62, 1677–1680. https://doi.org/10.1016/j.matlet.2007.09.058.

Were process parameters (e.g., ball-milling duration, annealing temperature) optimized for phase purity and grain growth?

Authors’ response:

The process parameters were carefully selected and effectively optimized to achieve phase purity and controlled grain growth in the synthesized ceramics. According to the Materials and Methods section, the raw powders of Ga₂O₃ and ZnO (and Er₂O₃ for doped samples) were mixed in stoichiometric proportions, ground and homogenized in an alundum crucible, and then pressed into pellets at 1 MPa. The pellets underwent annealing at a temperature of 1500 °C for 10 hours in air. The use of high-temperature annealing and prolonged dwell time was intentionally chosen to promote complete reaction between the oxides, ensure high phase purity, and facilitate grain growth.

The structural characterization by XRD confirmed the successful formation of the desired ZnGa₂O₄ spinel phase as the primary crystalline phase, with minimal secondary phase content. Notably, the crystallite size increased significantly from 64.8 nm in undoped ZnGa₂O₄ to 106.8 nm in Er-doped samples, indicating enhanced grain growth. The addition of Er not only improved the phase composition (increasing the ZnGa₂O₄ content and suppressing β-Ga₂O₃) but also promoted crystallite enlargement and the development of a well-defined microstructure, as further evidenced by SEM and AFM analyses.

Thus, the chosen process parameters—especially the annealing temperature and duration—were effective in obtaining ceramics with high phase purity and optimized grain size, which are critical for achieving superior functional properties in high-temperature methane sensors

How do the sensing properties of ZGO+Er compare to recent advanced CH₄ sensors (e.g.,MOFs, nanoarrays) in high-temperature regimes?

Authors’ response:

To begin with, it should be noted that the high operating temperatures of the sensors are temperatures not lower than 600 °C [3,4].

The authors are not aware of any publications devoted to the research and development of CH₄ sensors based on ZnGa₂O₄. To date CH₄ sensors based on binary metal-oxide films and nanostructures (SnO₂, ZnO and TiO₂, primary) have been researched and developed [59-64]. The achieved response to CH₄ for ZGO+Er samples exceed the response for most sensors based on binary MOSs presented. However, ZGO+Er samples are characterized by high operating temperature, which on the one hand can lead to an increase in the power consumption of sensors, and on the other hand – the ability of sensors to function at elevated temperatures is of interest for a number of practical applications where majority of the binary MOSs are not able to function reliably. High-temperature CH₄ sensors are of interest for the development of systems based on them for monitoring and control of combustion processes, for determining the ideal fuel/air mixture [82]. With this in mind, it is reasonable to compare the gas-sensitive characteristics of CH₄ sensors based on MOSs capable of functioning in the high-temperature region corresponding to the range of T = 600 – 1000 °C. In addition, high operating temperatures ensure high sensor speed performance [83], low base resistance, and surface regeneration [81]. Table 5 shows a comparison of gas-sensitive characteristics of high-temperature CH₄ sensors based on MOSs. The responses of ZGO+Er samples to equal CH₄ concentrations are significantly higher than those of thin Ga₂O₃ films with addition of 0.1 at% of SnO₂ and thick LaFe_0.95W_0.05O₃ films. In Ref. [6], sensors based on polycrystalline Ga₂O₃ thin films with responses exceeding those of ZGO+Er were presented. However, such high responses were obtained at a higher Т, at which we did not measure the characteristics of ZGO+Er samples due to the manifestation of characteristic drift. In addition, the authors explain the high responses for CH₄ by partial combustion of the gas with the formation of ionized oxygen defects [6].

Table 5. Gas-sensitive characteristics of high-temperature CH₄ sensors based on MOSs.

Material	CH₄ concentration (ppm)	Temperature (°C)	Response (a.u.)	Refs.
Ga₂O₃ with addition of 0.1 at% SnО₂	10⁴	740	10	[7]
Ga₂O₃	5000	740	~80	[6]
LaFe_0.95W_0.05O₃	10⁴	650	~8	[8]
ZGO+Er	100 10⁴	650	2.91 20.74	This work

References:

Liu, Y.; Parisi, J.; Sun, X.; Lei, Y. Solid-state gas sensors for high temperature applications – a review. J. Mater. Chem. A 2014, 2, 9919–9943. https://doi.org/10.1039/c3ta15008a.
Ghosh, A.; Zhang, C.; Shi, S.Q.; Zhang, H. High-Temperature Gas Sensors for Harsh Environment Applications: A Review. Clean – Soil Air Water 2019, 47, 1800491. https://doi.org/10.1002/clen.201800491.

…

Fleischer, M.; Meixner, H. A Selective CH₄ Sensor Using Semiconducting Ga₂O₃ Thin Films Based on Temperature Switching of Multigas Reactions. Sens. Actuators B Chem. 1995, 25, 544–547. https://doi.org/10.1016/0925-4005(95)85118-6.
Frank, J.; Fleischer, M.; Meixner, H.; Feltz, A. Enhancement of Sensitivity and Conductivity of Semiconducting Ga₂O₃ Gas Sensors by Doping with SnO₂. Sens. Actuators B Chem. 1998, 49, 110–114. https://doi.org/10.1016/S0925-4005(98)00094-X.
Moseley, P.T.; Oprea, A.; Merdrignac-Conanec, O.; Kerlau, M.; Bârsan, N.; Weimar, U. Limitations on the Use of Perovskite-Structure Oxides in Gas Sensing as a Result of the Concurrent Operation of Separate Mechanisms. Sens. Actuators B Chem. 2008, 133, 543–546. https://doi.org/10.1016/j.snb.2008.03.019.

…

Jiao, M.Z.; Chen, X.Y.; Hu, K.X.; Qian, D.Y.; Zhao, X.H.; Ding, E.J. Recent Developments of Nanomaterials-Based Conductive Type Methane Sensors. Rare Met. 2021, 40, 1515–1527. https://doi.org/10.1007/s12598-020-01679-9.
Basu, S.; Basu, P.K. Nanocrystalline Metal Oxides for Methane Sensors: Role of Noble Metals. J. Sensors 2009, 861968. https://doi.org/10.1155/2009/861968.
Fu, L.; You, S.; Li, G.; Li, X.; Fan, Z. Application of Semiconductor Metal Oxide in Chemiresistive Methane Gas Sensor: Recent Developments and Future Perspectives. Molecules 2023, 28, 6710. https://doi.org/10.3390/molecules28186710.
Gautam, Y.K.; Sharma, K.; Tyagi, S.; Ambedkar, A.K.; Chaudhary, M.; Pal Singh B. Nanostructured metal oxide semiconductor-based sensors for greenhouse gas detection: progress and challenges. R. Soc. Open Sci. 2021, 8, 201324. http://doi.org/10.1098/rsos.201324.
Chen, L.; Yu, Q.; Pan, C.; Song, Y.; Dong, H.; Xie, X.; Li, Y.; Liu, J.; Wang, D.; Chen, X. Chemiresistive gas sensors based on electrospun semiconductor metal oxides: A review. Talanta 2022, 246, 12352. https://doi.org/10.1016/j.talanta.2022.123527.
Kekana, M.T.M.; Mosuang, T.E.; Ntsendwana, B.; Sikhwivhilu, L.M.; Mahladisa, M.A. Notable synthesis, properties and chemical gas sensing trends on molybdenum disulphides and diselenides two-dimensional nanostructures: A critical review. Chemosphere 2024, 366, 143497. https://doi.org/10.1016/j.chemosphere.2024.143497.

…

Korotcenkov, G.; Cho, B.K. Instability of Metal Oxide-Based Conductometric Gas Sensors and Approaches to Stability Improvement (Short Survey). Sens. Actuators B Chem. 2011, 156, 527–538. https://doi.org/10.1016/j.snb.2011.02.024.
Liu, Y.; Parisi, J.; Sun, X.; Lei, Y. Solid-State Gas Sensors for High Temperature Applications—A Review. J. Mater. Chem. A 2014, 2, 9919–9943. https://doi.org/10.1039/C3TA15008A.
Yakovlev, N.N.; Almaev, A.V.; Nikolaev, V.I.; Kushnarev, B.O.; Pechnikov, A.I.; Stepanov, S.I.; Chikiryaka, A.V.; Timashov, R.B.; Scheglov, M.P.; Butenko, P.N.; et al. Low-Resistivity Gas Sensors Based on the In₂O₃-Ga₂O₃ Mixed Compounds Films. Mater. Today Commun. 2023, 34, 105241. https://doi.org/10.1016/j.mtcomm.2022.105241.

What is the mechanistic origin of humidity interference (RH 30-70%) on CH₄ response, and how might surface hydroxylation affect O₂ adsorption?

Authors’ response:

According to this comment of the reviewer, we changed the text of the manuscript.

3.2. Gas-sensitive properties

Increasing the humidity of the gas-air mixture leads to a decrease in the sample’s response to CH₄ as exhibited in Fig. 12. The response of samples decreases significantly by 46% when the RH increases from 0 to 30% (Fig. 12 (b)). Increasing the RH from 0 to 30% results in an increase in R_g by a factor of about 2.13, while R_air increases by only a factor of 1.14 (Fig. 12 (a)). The decrease in the response to CH₄ with increasing RH is due to the large-scale increase in R_g. It is worth noting that in the range of RH = 30 – 70%, the response to CH₄ is practically independent of humidity.

Figure 12. Effect of humidity on ZGO+Er samples base resistance, resistance in gas mixture of pure dry air + 2000 ppm of CH₄ (a) and response to 2000 ppm CH₄ (b) at T = 650 °C.

…

Discussion

An increase in the base resistance of n-type MOSs is not typical when humidity increases. During chemisorption at high temperatures, water molecules exhibit reducing gas properties and the effect should be reversed [69]. Further research is needed to explain the dependence of R_air on RH. An increase in R_g with an increase in RH can be explained by H₂O and CH₄ molecules competing for adsorption centers S_a. In an atmosphere of moist pure air the following reaction takes place [78]:

H₂O + O^2- + S_a → 2(S_a–OH) + 2e^-.

(7)

According to reaction (7), the semiconductor surface is poisoned by OH-groups, leading to a decrease in the surface density of S_a and N_i. As RH increases, fewer CH₄ molecules are chemisorbed onto the semiconductor surface, while R_g increases and S decreases.

To minimise the impact of humidity on the gas-sensitive properties of the samples, it is sensible to use ultrathin films of materials that can absorb H₂O molecules and are deposited on the surface of the sensitive layer. One such material is SiO₂ [78]. It is worth noting that this material can withstand high-temperature operating conditions, which is important for developing appropriate high-temperature gas sensors [79]. To further reduce the effect of humidity, operating modes involving modulation of the operating temperature and/or exposure to ultraviolet radiation are advisable [80].

…

Conclusions

References:

Gaman, V.I.; Almaev, A.V. Dependences of Characteristics of Sensors Based on Tin Dioxide on the Hydrogen Concentration and Humidity of Gas Mixture. Russ. Phys. J. 2017, 60, 90–100. https://doi.org/10.1007/s11182-017-1046-2.

…

Postica, V.; Lupan, O.; Gapeeva, A.; Hansen, L.; Khaledialidusti, R.; Mishra, A.K.; Drewes, J.; Kersten, H.; Faupel, F.; Adelung, R.; Hansen, S. Improved Long-Term Stability and Reduced Humidity Effect in Gas Sensing: SiO₂ Ultra-Thin Layered ZnO Columnar Films. Adv. Mater. Technol. 2021, 6, 2001137. https://doi.org/10.1002/admt.202001137.
Fleischer, M.; Seth, M.; Kohl, C.-D.; Meixner, H. A selective H₂ sensor implemented using Ga₂O₃ thin-films which are covered with a gas-filtering SiO₂ layer. Sens. Actuators B: Chem. 1996, 36(1–3), 297-302. https://doi.org/10.1016/S0925-4005(97)80085-8.
Wang, Y.; Zhou, Y. Recent Progress on Anti-Humidity Strategies of Chemiresistive Gas Sensors. Materials2022, 15, 8728. https://doi.org/10.3390/ma15248728.

Long-term stability data spans only 19 days—what degradation mechanisms could emergeover months/years at 650°C?

Authors’ response:

According to this comment of the reviewer, we changed the text of the manuscript:

According to refs. [68,69], S ~exp(eφ_s). The mechanisms of the gas-sensitive characteristics drift of MOSs gas sensors are considered in refs. [81]. Operating sensors at high temperatures and under the exposure to CH₄ over a long period can cause structural transformations, changes in microcrystal size, and poisoning of the sample surface. As a result of these processes N_i can change. Other processes, such as phase transformations, the formation of electrically active defects, and bulk diffusion primarily result in a change in N_d. According to our results (Fig. 13), an increase in base resistance and response to CH₄ indicates an increase in the N_i²/N_d over the duration of the tests. We believe that, as the test duration increases further, the characteristics will continue to stabilize, as is typical for MOSs gas sensors [81].

References:

Almaev, A.V.; Chernikov, E.V.; Davletkildeev, N.A.; Sokolov, D.V. Oxygen Sensors Based on Gallium Oxide Thin Films with Addition of Chromium. Superlattices Microstruct. 2020, 139, 106392. https://doi.org/10.1016/j.spmi.2020.106392.
Gaman, V.I.; Almaev, A.V. Dependences of Characteristics of Sensors Based on Tin Dioxide on the Hydrogen Concentration and Humidity of Gas Mixture. Russ. Phys. J. 2017, 60, 90–100. https://doi.org/10.1007/s11182-017-1046-2.

…

Korotcenkov, G.; Cho, B.K. Instability of Metal Oxide-Based Conductometric Gas Sensors and Approaches to Stability Improvement (Short Survey). Sens. Actuators B Chem. 2011, 156, 527–538. https://doi.org/10.1016/j.snb.2011.02.024.

Can surface-sensitive characterization (e.g., XPS, EDX mapping) confirm increased active sites or Er-induced dopant effects proposed in the sensing mechanism?

Authors’ response:

These methods can be used to confirm the proposed mechanism of the sensory effect of samples containing Er. We have added the relevant research results to the text of the manuscript in accordance with the reviewer's comments.

The chemical composition of the studied samples ZGO and ZGO+Er was determined based on a detailed analysis of the photoelectron lines of the corresponding elements from the survey PE spectra (not shown) taking into account the atomic sensitivity factors based on Scofield Cross-sections (Table 2). The data in Table 2 show that both samples contain zinc, gallium, oxygen and carbon. In the case of ZGO+Er, Er is also additionally present in an amount of 0.2 at.%. In general, it can be noted that the chemical composition of the samples is similar, and the presence of a small amount of carbon (about 3.5–3.8 at.%) is apparently associated with its introduction into the composition of the surface layers of the samples at the stage of their synthesis.

Table 2. Chemical composition of the surface region of ZGO and ZGO+Er samples by XPS.

Material	Concentration, at.%
Material	[Zn]	[Ga]	[O]	[Er]	[C]
ZGO	12.1	31.8	52.3	-	3.8
ZGO+Er	11.9	31.5	52.9	0.2	3.5

Now let us proceed to a detailed examination of the core-level (Ga 2p_3/2, Zn 2p_3/2, O 1s, Er 4d) PE spectra of both samples, starting with the Ga 2p_3/2 spectra (Fig. 3). It is clearly seen that the maximum of the Ga 2p_3/2 spectrum in the ZGO sample is located at 1119.6 eV, which is +1.2 eV higher than for the reference sample Ga₂O₃. Such a high-energy shift is due to the fact that gallium is surrounded not only by oxygen, but also by zinc in the ZnGa₂O₄ compound. At the same time, a slight asymmetry of the line is observed on the side of low binding energies, which indicates the presence of a small amount of gallium also in Ga₂O₃. This result agrees well with the XRD data (Fig. 2). When moving from the ZGO sample to the ZGO+Er sample, one can see a subsequent high-energy shift of the 2p_3/2 spectrum by +0.9 eV, as well as the appearance of a clear intense shoulder on the side of low binding energies. Both results are due to the incorporation of erbium into the sample structure. Moreover, the largest amount of gallium in ZGO+Er is in the ZnGa₂O₄ compound, as in the ZGO sample, while some amount is in another compound. Considering that the position of the low-energy component in ZGO+Er is 0.4 eV higher than that of the similar component in the ZGO sample, and the fact that according to XRD data Ga₂O₃ is absent in ZGO+Er, we can conclude that gallium is in a compound with erbium. Combining the data of XRD, EDX, information obtained from the analysis of diffuse reflectance spectra and the results of XPS, it can be assumed that the low-energy component in the spectrum of Ga 2p_3/2 is associated with the Er₃Ga₅O₁₂ compound. It is interesting that similar energy shifts in the Zn 2p_3/2 spectra are observed in the series ZnO → ZGO → ZGO+Er (Fig. 3). However, two important features should be noted: the magnitudes of these shifts are smaller than in the case of Ga 2p_3/2 spectra, and the fact that in both studied samples the 2p_3/2 zinc line is symmetrical. These results indicate, on the one hand, that zinc is in only one chemical state – in ZnGa₂O₄, and on the other hand, that additionally introduced erbium in ZGO+Er has a slightly smaller effect on the charge state of zinc, compared to gallium. When analyzing the O 1s spectra (Fig. 6), only small energy shifts can be seen, since the binding energies of the metal oxides used in this work differ insignificantly. Finally, the analysis of the Er 4d spectra allowed us to establish that erbium is present only in the ZGO+Er sample. Moreover, the shape of the spectrum in the studied sample and in the spectrum of the reference compound Er₂O₃ is close, which indicates the presence of erbium in Er³⁺. At the same time, taking into account the shift of +1.1 eV of the Er 4d spectrum in the ZGO+Er sample relative to the Er₂O₃ spectrum and the conclusions made above when analyzing the Ga 2p_3/2 spectrum, we can be confident that erbium is bonded with gallium and oxygen in the compound Er₃Ga₅O₁₂.

Figure 3. Ga 2p_3/2, Zn 2p_3/2, O 1s, Er 4d spectra of ZGO and ZGO+Er samples as well as the reference compounds Ga₂O₃, ZnO and pure Er₂O₃.

We added EDX mapping to the text of the manuscript and corrected the corresponding text in response to the reviewer's comment:

EDX analysis showed that the second type microcrystals for ZGO+Er samples (Fig. 6) are characterized by high Er content. The concentration of Er in these microcrystals exceeds the concentrations of Ga and Zn. Based on the XRD and EDX results, it can be assumed that microcrystals of the second type correspond to the superstoichiometric Er₃Ga_5-xO_12-y phase.

Figure 6. SEM image (a) and corresponding EDX mapping (b) of the ZGO+Er surface.

In addition, the EDX mapping for each detected element are also shown below. These Figures are presented only in response to the reviewer's comments.

Figure. EDX mapping of the ZGO+Er surface.

Reviewer 2 Report

Comments and Suggestions for Authors

This work entitled “Investigation of Methane Sensing Properties of Er-Ion Doped Zinc Digallate Ceramics” is somewhat interesting and has the potential for publication in this journal. It has some issues that could be addressed for possible acceptance. Here are my comments for revision:

Why the authors directly choose ZnGa2O4 for analyzing the gas sensing performance? Such analysis should firstly be mentioned in the introduction.
Also, the authors should compare its performance with other 2D materials such as TMDs, graphene and so on.
The sensing mechanism of the ZnGa2O4 upon CH4 should be further analyzed using the semiconducting nature. And the sensing response can also use the modulated percentage of the electrical resistance. How to consider this issue?
The authors should highlight the significance of Er-Ion for modification. How about their performance compared with other noble metals such as Pd and Au.
Besides, the introduction is too short and does not contain informative information. Apart from the added analysis of the research purpose, the authors can add and analyze more novel references about TMDs, enrich the readability of this work, such as: https://doi.org/10.1080/00268976.2025.2492391
The authors can study the performance of the pristine ZnGa2O4 monolayer and then highlight the significance of Er-Ion.
The subscript throughout this work should be clearly written and the impact of the humidity on gas sensing should be analyzed.

Author Response

Response to the Reviewer

Dear Reviewer,

Why the authors directly choose ZnGa2O4 for analyzing the gas sensing performance? Such analysis should firstly be mentioned in the introduction.

Authors’ response:

According to this comment of the reviewer, we added corresponding information to the text of the manuscript:

CH₄ is a colorless and odorless gas. It is a major component of natural gas and a potent greenhouse gas [1,2]. Given the widespread use of CH₄ in industry and households as an energy source, there is a need to increase the efficiency of CH₄ combustion and ensure the equipment meets ecological requirements, it is necessary to measure the CH₄ concentration in the exhaust gases of combustion systems. To this end, sensors are required that can withstand extreme operating conditions, including temperatures of at least 600 °C, as well as high pressure and gas flow rates. Most sensor electronics materials, metal oxides and chalcogenides cannot function reliably at temperatures above 600 °C [3-5]. Previously, high-temperature CH₄ sensors based on thin Ga₂O₃ [6,7] and thick LaFe_0.95W_0.05O₃ films [8] were investigated. However, these sensors were not highly sensitive and/or were produced using film deposition methods that are not highly reproducible. There is a need to develop new materials for high-temperature CH₄ sensors that can be produced using relatively inexpensive methods.

Zinc gallate (ZnGa₂O₄), being an ultra-wide bandgap semiconductor with high chemical and thermal stability, is a promising material for high-temperature sensors. It possesses a spinel-type crystal structure [9,10]. The band gap width, E_g, of ZnGa₂O₄ depends significantly on the synthesis conditions, а presence of impurities and subsequent treatments, and is reported within relatively range of 4.1 – 5.2 eV [9-14]. To date, ZnGa₂O₄ has been shown to be of interest in several areas of electronics and photonics as a material for transparent conducting electrodes [15], luminophores films [16,17], solar-blind UV detectors with high speed-performance [18,19], power electronics devices [20,21] and gas sensors [11,22]. The last application field is practically not well-developed due to the absence of cost-effective and high deposition rates methods for ZnGa₂O₄ synthesis as well as for fabrication of structures on its basis with large active surface. It is particularly difficult to obtain ZnGa₂O₄ compounds with the appropriate spinel crystal lattice. The literature analysis summarized in Table 1 shows that despite these obstacles, sensors based on ZnGa₂O₄ capable of functioning in a wide temperature range and demonstrating high responses to nitrogen oxides and volatile hydrocarbon vapors in the operating temperature range of T = 300 – 450 °C have been developed. The very limited number of literature sources does not yet allow us to draw some general conclusions on the advantages and disadvantages of ZnGa₂O₄ as a sensitive material for gas sensors in comparison with the well-developed binary metal-oxide semiconductors (MOSs) SnO₂, ZnO, TiO₂, WO₃, Ga₂O₃ [23-35] or transition metal dichalcogenides (TMDs) [5,36].

The characteristics of ZnGa₂O₄-based gas sensors, including gas response and speed-performance, can be controlled by forming arrays of nanoscale structures, changing the topology of contacts to the semiconductor layer, forming heterostructures, and depositing metal-catalyst nanoclusters on the semiconductor surface [11,12,22,37-41].

A Taguchi-type sensor based on ZnGa₂O₄, obtained by solid state reaction assisted by high-energy ball-milling of a mixture of GaOOH and ZnO powders at room temperature (RT) for 7 hours and with periodic addition of ethanol as a lubricant, was presented in Ref. [12]. A paste was formed from the obtained powder and deposited in a thin layer on a ceramic tube, inside of which a heater was formed. The signal from the sensing layer was picked up by means of gold electrodes formed to the semiconductor. The Taguchi-type sensors with a sensitive SnO₂ layer have been manufactured since the late 1960s and are reliable and stable devices with a low base resistance [42], but still exhibit some disadvantages as discussed in Ref. [43]. The use of inexpensive and easy-to-implement ceramic technologies for the synthesis of ZnGa₂O₄, similar to the one described in Ref. [12], is promising. However, there is practically no published report devoted to the study of gas-sensitive properties of such materials.

Table 1. Comparison of gas-sensitive characteristics of ZnGa₂O₄-based structures

Material	Gas	Gas concentration (ppm)	Temperature (°C)	Response (a.u.)	Refs.
ZnGa₂O₄ thin films	NO	6.25	300	22.21	[11]
ZnGa₂O₄	LPG*	50	340	7.9	[12]
ZnGa₂O₄ nanorods	NO₂	10	300	2.85	[22]
ZnGa₂O₄-core/TiO₂-shell nanorods	NO₂	10	300	8.76	[22]
ZnGa₂O₄ thin films	NO	10	300	11.647	[37]
ZnGa₂O₄	LPG*	1000	25	1.32	[38]
ZnGa₂O₄/Graphene	LPG*	1000	25	1.56	[38]
ZnGa₂O₄ thin films	C₂H₄O	300	450	1450	[39]
ZnGa₂O₄:N thin films	C₂H₄O	300	400	11000	[39]
ZnGa₂O₄-core/ZnO-shell nanowires	NO₂	5	250	12	[40]
ZnGa₂O₄ nanowires	NO₂	5	25 + ultraviolet exposure	2.91	[41]
ZnGa₂O₄ nanowires with Au nanoparticles	NO₂	5	25 + ultraviolet exposure	8.61	[41]

* LPG is the liquid petroleum gas.

Also, the authors should compare its performance with other 2D materials such as TMDs, graphene and so on.

Authors’ response:

The studied samples do not belong to the 2D materials class. The studied samples are ceramic pellets with a large active surface. According to the results of structural studies (Section 3.1), the microrelief features of the samples, pores and crystallites have characteristic dimensions significantly larger than the characteristic thicknesses of 2D materials. In addition, the studied samples showed prospects for the development of high-temperature CH₄ sensors based on them. Most sensor electronics materials, metal and metal chalcogenides are unable to function at temperatures T > 600 °C [1*,2*].

References:

1*. Liu, Y.; Parisi, J.; Sun, X.; Lei, Y. Solid-state gas sensors for high temperature applications – a review. J. Mater. Chem. A 2014, 2, 9919–9943. https://doi.org/10.1039/c3ta15008a.

2*. Ghosh, A.; Zhang, C.; Shi, S.Q.; Zhang, H. High-Temperature Gas Sensors for Harsh Environment Applications: A Review. Clean – Soil Air Water 2019, 47, 1800491. https://doi.org/10.1002/clen.201800491.

The sensing mechanism of the ZnGa2O4 upon CH4 should be further analyzed using the semiconducting nature. And the sensing response can also use the modulated percentage of the electrical resistance. How to consider this issue?

Authors’ response:

The sensing mechanism employed by the authors is applicable to polycrystalline semiconductors comprising large microcrystals. When the Debye length is much smaller than the size of the microcrystals, the main factor limiting electron conductivity is the presence of a potential barrier at the microcrystal boundary.

Besides that, the following ratio was chosen as the response S of the samples to CH₄:

S = R_air/R_g,

(1)

where R_air is the resistance of samples in pure dry air; R_g is the resistance of samples in a gas mixture of pure dry air + CH₄. The response values achieved are very high, so it is more appropriate to express them as a relative value rather than as a percentage!

The sensing mechanism:

For polycrystalline semiconductors with large microcrystals, as in our case, the conditions at the microcrystal boundary influence the charge carrier transport [59]. For such semiconductors, the resistance in air is described by the following formula [68]:

R_air = R₀×exp[(eφ_s/(kT_K)],

(4)

where R₀ is a value determined by geometric and electrophysical parameters of the sample; eφ_s is the energy bands bending at the microcrystal boundary; e is the electron charge; φ_s is the surface potential in air; k is the Boltzmann constant. R₀ depends weakly on changes in the atmospheric composition. eφ_s ~ N_i²/N_d and N_i ~n_ox^l, where N_i is the surface density of chemisorbed oxygen ions; N_d is the donor concentration; n_ox is the oxygen concentration; l < 1 [68,69]. n_ox = const and N_i changes in pure dry air are due to changes in sample temperature and ambient humidity at experimental conditions [68,69]. For binary MOSs, analysis of data [70] obtained by electron paramagnetic resonance spectroscopy and temperature programmed desorption, as well as extensive first-principles analysis [71], indicates that oxygen is chemisorbed in the form of O²^- at high temperatures. Based on these findings, we believe that a similar situation exists for the studied samples. The interaction of CH₄ and O^2- molecules may occur in two stages at T = 650 °C [59,60]:

CH₄ → CH₃ + H,	(5)
CH₃ + H + 4O^2- → CO₂+ 2H₂O + 8e^-.	(6)

At the first stage, dissociative adsorption of CH₄ molecules occurs with the appearance of CH₃ + H fragments on the semiconductor surface, which interact with O^2- ions to form CO₂, H₂O molecules and free electrons, which are returned to the semiconductor. As a result of reaction (6), N_i is reversibly reduced, leading to a decrease in eφ_s and the resistance of the semiconductor. The reaction products of CO₂ and H₂O are desorbed from the semiconductor surface.

References:

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Basu, S.; Basu, P.K. Nanocrystalline Metal Oxides for Methane Sensors: Role of Noble Metals. J. Sensors 2009, 861968. https://doi.org/10.1155/2009/861968.

…

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Gurlo, A. Interplay between O₂ and SnO₂: Oxygen Ionosorption and Spectroscopic Evidence for Adsorbed Oxygen. ChemPhysChem 2006, 7, 2041–2052. https://doi.org/10.1002/cphc.200600292.
Sopiha, K.V.; Malyi, O.I.; Persson, C.; Wu, P. Chemistry of Oxygen Ionosorption on SnO₂ Surfaces. ACS Appl. Mater. Interfaces 2021, 13, 33664–33676. https://doi.org/10.1021/acsami.1c08236.

The authors should highlight the significance of Er-Ion for modification. How about their performance compared withother noble metals such as Pd and Au.

Authors’ response:

The authors of the manuscript, along with many other scientists in this field, are aware that Pd and Au are used as modifiers to optimise the gas-sensitive characteristics of metal oxide semiconductor-based sensors. Several dozen other metal modifiers are also used in various states to enhance response, reduce operating temperatures and base resistance, increase the effect of humidity and improve performance stability. While it is likely that deposition noble metal clusters on the material's surface will increase sensitivity, one should also consider that adding an element to the sensor complicates its production and may cause instability in its characteristics [3*,4*]. The study of the effect of noble metal additives is beyond the scope of this manuscript. The corresponding text of the manuscript had changed.

It is important to note that doping of ZnGa₂O₄ with various impurities is widely used and studied in detail for the development of effective luminescent materials [47-56], however, to the best of our knowledge there is not any publication dealing with gas-sensitive properties of ZnGa₂O₄ with Er additive. It has been shown that doping with high concentrations of Er nanostructured SnO₂ [57] and BiFeO₃ [58] allows for high responses to alcohol and acetone vapors. At the same time, the sensors are characterized by long-term stability of gas-sensitive characteristics. Moreover, the method employed in our study for synthesis of ZnGa₂O₄ is characterized by relative simplicity and low cost.

References:

3*. Korotcenkov, G.; Cho, B.K. Instability of Metal Oxide-Based Conductometric Gas Sensors and Approaches to Stability Improvement (Short Survey). Sens. Actuators B Chem. 2011, 156, 527–538. https://doi.org/10.1016/j.snb.2011.02.024.

4*. Korotcenkov, G.; Cho, B.K. Engineering approaches to improvement of conductometric gas sensor parameters. Part 2: Decrease of dissipated (consumable) power and improvement stability and reliability. Sens. Actuators B Chem. 2014, 198, 316–341. https://doi.org/10.1016/j.snb.2014.03.069.

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Allix, M.; Chenu, S.; Véron, E.; Poumeyrol, T.; Kouadri-Boudjelthia, E.; Alahrache, S. Considerable Improvement of Long-Persistent Luminescence in Germanium and Tin Substituted ZnGa₂O₄. Chem. Mater. 2013, 25, 1600–1606. https://doi.org/10.1021/cm304101n.
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Besides, the introduction is too short and does not contain informative information. Apart from the added analysis of the research purpose, the authors can add and analyze more novel references about TMDs, enrich the readability of this work, such as: https://doi.org/10.1080/00268976.2025.2492391

Authors’ response:

The studied samples do not belong to the 2D materials class. The studied samples are ceramic pellets with a large active surface. We had changed the introduction in order to bring more informative information and had added mentioned publication to reference list.

Table 1. Comparison of gas-sensitive characteristics of ZnGa₂O₄-based structures

Material	Gas	Gas concentration (ppm)	Temperature (°C)	Response (a.u.)	Refs.
ZnGa₂O₄ thin films	NO	6.25	300	22.21	[11]
ZnGa₂O₄	LPG*	50	340	7.9	[12]
ZnGa₂O₄ nanorods	NO₂	10	300	2.85	[22]
ZnGa₂O₄-core/TiO₂-shell nanorods	NO₂	10	300	8.76	[22]
ZnGa₂O₄ thin films	NO	10	300	11.647	[37]
ZnGa₂O₄	LPG*	1000	25	1.32	[38]
ZnGa₂O₄/Graphene	LPG*	1000	25	1.56	[38]
ZnGa₂O₄ thin films	C₂H₄O	300	450	1450	[39]
ZnGa₂O₄:N thin films	C₂H₄O	300	400	11000	[39]
ZnGa₂O₄-core/ZnO-shell nanowires	NO₂	5	250	12	[40]
ZnGa₂O₄ nanowires	NO₂	5	25 + ultraviolet exposure	2.91	[41]
ZnGa₂O₄ nanowires with Au nanoparticles	NO₂	5	25 + ultraviolet exposure	8.61	[41]

* LPG is the liquid petroleum gas.

Actually, Refs. [11,37] report on development and study of sensors based on ZnGa₂O₄ with high sensitivity to NO compared to other gases. These sensors can be considered quasi-selective. The authors proposed a mechanism according to which there is a special type of dangling bonds on the surface of ZnGa₂O₄, which are effective adsorption centers for NO. Their sensors practically did not respond to CO₂, CO and SO₂, but showed a much lower response to NO₂ than to NO. High responses of ZnGa₂O₄-based sensors to NO₂ exposures were reported in Refs. [22,40,41] as well as by utilization of the well-known method of increasing sensitivity and selectivity of MOSs – exposure to ultraviolet radiation [41]. In turn, a few papers have shown high sensitivity of ZnGa₂O₄ to LPG. The responses to LPG are comparable to the responses to NO_x or much higher, which was obtained for sensors based on thin films of ZnGa₂O₄ and ZnGa₂O₄:N at T = 450 °C and 400 °C, respectively [39]. High sensitivity to NO_x is typical of many MOSs with a number of advantages and disadvantages [44,45]. Indeed, there is much less work done on the development of sensors for volatile hydrocarbon vapors, including LPG [46]. The reason for the high sensitivity of ZnGa₂O₄ to LPG is still not clear and is not considered in detail in these papers. The LPG is a mixture of several hydrocarbons like CH₄, C₃H₈, C₄H₁₀, etc. [12,46]. Therefore, it is reasonable to investigate the gas-sensitive properties of ZnGa₂O₄ under exposure to the LPG components separately to gain inside into the sensory mechanisms of ZnGa₂O₄ and to design highly sensitive and selective sensors. In this research, we focus on investigation of the gas-sensitive properties of ZnGa₂O₄ upon exposure to CH₄, which is highly relevant to environment quality monitoring and combustion monitoring systems.

More specifically, our research is devoted to the development and investigation of CH₄ sensors based on ZnGa₂O₄ and ZnGa₂O₄ with Er addition ceramics. It is important to note that doping of ZnGa₂O₄ with various impurities is widely used and studied in detail for the development of effective luminescent materials [47-56], however, to the best of our knowledge there is not any publication dealing with gas-sensitive properties of ZnGa₂O₄ with Er additive. It has been shown that doping with high concentrations of Er nanostructured SnO₂ [57] and BiFeO₃ [58] allows for high responses to alcohol and acetone vapors. At the same time, the sensors are characterized by long-term stability of gas-sensitive characteristics. Moreover, the method employed in our study for synthesis of ZnGa₂O₄ is characterized by relative simplicity and low cost.

Various types of sensor [1,59-63] are being developed and used to detect CH₄, including optical, calorimetric, pyroelectric, electrochemical and chemoresistive sensors. Those based on MOSs and TDMs are of particular interest due to their low production cost, miniaturisation potential, and compatibility with microelectronic technologies and materials [1,59-63]. Many sensors have been developed to date, mainly based on SnO₂, ZnO and TiO₂, including commercial samples. However, research is ongoing to solve several issues, such as reducing the power consumed by sensors, increasing selectivity, stability and speed performance, reducing the effect of ambient humidity and detection limits. At the same time, methods of “green manufacturing” for producing sensory materials are being actively considered. In order to address these issues, the investigation of MOSs and methods for modifying their gas-sensitive properties, as well as the study of heterostructures and nanostructures based on MOSs and other semiconductors, is ongoing. Additionally, new materials such as p-type MOSs, TMDs and various allotropic forms of carbon, along with heterostructures based on these materials, are being employed [1,5,30-35,59-64]. Furthermore, improvements to sensor design are being made. It is worth noting that these studies practically do not consider the development of high-temperature CH₄ sensors.

The authors can study the performance of the pristine ZnGa2O4 monolayer and then highlight the significance of Er-Ion.

Authors’ response:

The samples studied are not monocrystalline and do not consist of a single layer of atoms. This is explained in detail in the results of our research. It is difficult to refer to them as monolayers. Please note that the pristine ZnGa₂O₄ sample has been investigated.

3.1. Structural properties

The XRD analysis (Fig. 2) of the ZnGa₂O₄ (ZGO) sample revealed that the main crystalline phase is zinc digallium oxide (ZnGa₂O₄), identified as a cubic spinel structure belonging to the Fd-3m space group and referenced in ICDD 01-086-0415. This primary phase comprises 80.7% of the material, while the secondary phase, β-Ga₂O₃, accounts for 19.3% and is indexed as ICDD 00-041-1103. The lattice parameters for ZnGa₂O₄ were determined to be a = b = c = 8.3279 Å with a unit cell volume of 577.568 Å³. D_c of ZnGa₂O₄ is 64.8 nm, with an associated ε of 0.066%. In contrast, the secondary β-Ga₂O₃ phase exhibited lattice parameters of a = 12.2373 Å, b = 3.0429 Å, c = 5.8092 Å, and β = 103.83°, with a smaller D_c of 46.4 nm and ε of 0.067%.

In the Er-doped sample (ZGO+Er), the proportion of the primary ZnGa₂O₄ phase increased to 91.8%, with lattice parameters slightly expanded to a = 8.3331 Å and a unit cell volume of 578.66 Å³. D_c of the ZnGa₂O₄ phase in this sample increased to 106.8 nm, while ε was slightly higher at 0.068%. The secondary phase in the ZGO+Er sample, triherbium pentagallium oxide (Er₃Ga₅O₁₂ or erbium gallium garnet), accounted for 8.2% of the composition and is referenced in ICDD 04-007-9142. This phase exhibited a cubic structure with lattice parameter a = 12.258 Å, a unit cell volume of 1841.8 Å³, D_c of 50.5 nm, and ε of 0.072%.

Figure 2. XRD spectra of ZGO and ZGO+Er samples.

The high degree of crystallinity in the ZnGa₂O₄ phase across both samples confirms the successful formation of polycrystalline ZnGa₂O₄ with a cubic spinel structure. The differences in phase composition and crystallite sizes between the pure and Er-doped samples indicate that the incorporation of erbium influences the structural and microstructural properties. The formation of the Er₃Ga₅O₁₂ phase suggests a partial interaction between Er₂O₃ and Ga₂O₃ during high-temperature annealing, as confirmed by the identification of phases using the ICDD database. These results demonstrate that both the ZGO and ZGO+Er samples possess desirable structural qualities, making them suitable for potential applications in optical and electronic technologies.

…

The surface of ZGO samples is relatively smooth and characterized by the presence of pores extending to the surface (Fig. 4 (a)). The pore sizes on the surface of the samples vary from 8 µm to 17 µm. The addition of Er significantly changes the surface morphology of the samples (see Fig. 4 (b)). The SEM image of the ZGO+Er surface clearly distinguishes microcrystals of two types, with different electron densities, and the interfaces between microcrystals of one and different types. Microcrystals of the first type have facets of regular geometric shapes characteristic of the cubic crystal lattice. The size of microcrystals of the first type varies in the range from 1 µm to 7 µm. Microcrystals of the second type, spherical in shape, are predominantly embedded in the structure of the samples and segregate on the surface. The size of microcrystals of the second type varies in the range from 0.5 µm to 4 µm. In addition, for ZGO+Er the density of pores extending to the surface of the samples increases. The size of pores extending to the surface of ZGO+Er samples varies from 1 µm to 7 µm. According to the analysis of AFM images (Fig. 5), the root mean square of surface roughness of the samples increases from 787.8 nm to 895.5 nm with the addition of Er. ZGO+Er samples are characterized by significantly larger active surface. The increased pore density and roughness of the ZGO+Er samples must contribute to the higher CH₄ response by enhancing gas adsorption. Also, on the surface of samples of both types there are small clusters of the order of 10 – 100 nm, probably due to the residues of precursors that did not react during synthesis of the samples.

Figure 4. SEM images of the ZGO (a) and ZGO+Er (b) surfaces.

Figure 5. AFM images of the ZGO (a) and ZGO+Er (b) surfaces.

There are difficulties in determining the concentration of constituent elements in the samples due to the overlap of Ga and Zn lines on the one hand, and the insulating properties of the samples at RT on the other hand. EDX analysis of the ZGO samples did not reveal the presence of extrinsic elements other than Ga, Zn and O, which are homogeneously distributed over the area. EDX analysis showed that the second type microcrystals for ZGO+Er samples (Fig. 6) are characterized by high Er content. The concentration of Er in these microcrystals exceeds the concentrations of Ga and Zn. Based on the XRD and EDX results, it can be assumed that microcrystals of the second type correspond to the superstoichiometric Er₃Ga_5-xO_12-y phase.

Figure 6. SEM image (a) and corresponding EDX mapping (b) of the ZGO+Er surface.

The diffuse reflectance spectrum of the ZGO sample is typical (Fig. 7). Regardless of the chemical composition of the samples, a sharp absorption edge in the ultraviolet region is observed, which corresponds to intrinsic absorption in ZnGa₂O₄. The Kubelka-Munk estimates showed that the E_g of ZGO is 4.29 eV, which is in agreement with the literature data [66]. When Er₂O₃ is added into the ZGO composition, a slight decrease in the reflectance intensity in the absorption edge region is observed. This may be due to the introduction of defects or the state of levels inside the band gap of ZnGa₂O₄ due to the presence of Er³⁺ ions. Additional peaks appear on the reflection spectrum of the ZGO+Er sample in the wavelength range of 400 – 800 nm, due to the appearance of additional energy levels in the band gap of ZnGa₂O₄, as well as partially coinciding with the absorption of the Er₃Ga₅O₁₂ phase. The spectrum typical of the ZGO+Er samples contains a set of narrow peaks characteristic of the internal 4f→f transitions of electrons in Er³⁺ ions, the positions of which are presented in Table 3.

Figure 7. Reflection spectra of ZGO and ZGO+Er samples.

Table 3. Spectral lines of electron transitions in the Er³⁺ ion for ZGO+Er samples.

Transition from ⁴I_15/2	Spectral Interval (nm)
⁴G_11/2	355.0 – 387.0
²H_9/2	407.84
⁴F_{3/2, 5/2}	438.0 – 463.0
⁴F_7/2	476.2 – 506.0
²H_11/2	504.0 – 530.0
⁴S_3/2	538 – 564
⁴F_9/2	630.0 – 683.0
⁴I_9/2	781 – 842

3.2. Gas-sensitive properties

Fig. 8 displays the temperature dependencies of the samples basic resistance in dry air in Arrhenius coordinates, ln(R_air) vs. 10³/T_K, where R_air is chosen as the basic resistance; T_K is the absolute temperature of the sample. At T = 250 °C, R_air is high and reaches ~10¹² Ohm. As the temperature increases, the R_air of the samples decreases according to the exponential law. Heating the samples up to T > 650 °C leads to an irreversible increase in R_air. For this reason, measurements of gas-sensitive properties of the samples at T > 650 °C were not performed. R_air of ZGO samples with addition of Er decreases in 2.0 – 2.6 times in the whole T interval. Regardless of the chemical composition of the samples, two sections can be distinguished on the Arrhenius curves in the interval of T = 250 – 650 °C, the first in the interval of T = 250 – 350 °C, the second – T = 350 – 650 °C. The first section is characterized by the activation energy ΔE₁, the second - by ΔE₂. For ZGO samples ΔE₁ = 0.68 ± 0.08 eV and ΔE₂ = 1.56 ± 0.02 eV. For ZGO+Er samples, ΔE₁ = 0.7 ± 0.1 eV and ΔE₂ = 1.52 ± 0.04 eV. The addition of Er has no effect on the position of the levels associated with ΔE₁ and ΔE₂, which are due to the presence of defects in ZGO, including antisite defects [67].

For ZGO it was possible to register the response to CH₄ in the range of T = 550 – 650 °C (Fig. 9). These samples are characterized by weak responses to CH₄. The maximum response S_max at T = 650 °C and under exposure to 10⁴ ppm of CH₄ is only 1.33 a. u. The addition of Er allows to increase significantly the response of samples to CH₄ and to extend the range of T, at which the samples show sensitivity to gas, to the low temperature region. S_max for ZGO+Er at T = 650 °C and exposure to 10⁴ ppm of CH₄ is 14.77 a. u. At the same time, the base resistivity of the samples with addition of Er decreased only by a factor of 2. Due to the high sensitivity of ZGO+Er samples to CH₄, we will further focus on their gas-sensitive properties.

Figure 8. Dependence of the sample’s basic resistance in ln scale on the inverse absolute temperature.

Figure 9. Temperature dependencies of the ZGO and ZGO+Er responses to 10⁴ ppm of CH₄.

The subscript throughout this work should be clearly written and the impact of the humidity on gas sensing should be analyzed.

Authors’ response:

In response to the reviewer's comments, the authors corrected the subscripts throughout the manuscript. The effect of humidity on the response of the samples to CH₄ was analyzed.

3.2. Gas-sensitive properties

Figure 12. Effect of humidity on ZGO+Er samples base resistance, resistance in gas mixture of pure dry air + 2000 ppm of CH₄ (a) and response to 2000 ppm CH₄ (b) at T = 650 °C.

…

Discussion

H₂O + O^2- + S_a → 2(S_a–OH) + 2e^-.

(7)

…

Conclusions

References:

Gaman, V.I.; Almaev, A.V. Dependences of Characteristics of Sensors Based on Tin Dioxide on the Hydrogen Concentration and Humidity of Gas Mixture. Russ. Phys. J. 2017, 60, 90–100. https://doi.org/10.1007/s11182-017-1046-2.

…

Postica, V.; Lupan, O.; Gapeeva, A.; Hansen, L.; Khaledialidusti, R.; Mishra, A.K.; Drewes, J.; Kersten, H.; Faupel, F.; Adelung, R.; Hansen, S. Improved Long-Term Stability and Reduced Humidity Effect in Gas Sensing: SiO₂ Ultra-Thin Layered ZnO Columnar Films. Adv. Mater. Technol. 2021, 6, 2001137. https://doi.org/10.1002/admt.202001137.
Fleischer, M.; Seth, M.; Kohl, C.-D.; Meixner, H. A selective H₂ sensor implemented using Ga₂O₃ thin-films which are covered with a gas-filtering SiO₂ layer. Sens. Actuators B: Chem. 1996, 36(1–3), 297-302. https://doi.org/10.1016/S0925-4005(97)80085-8.
Wang, Y.; Zhou, Y. Recent Progress on Anti-Humidity Strategies of Chemiresistive Gas Sensors. Materials2022, 15, 8728. https://doi.org/10.3390/ma15248728.

Reviewer 3 Report

Comments and Suggestions for Authors

The synthesis and investigation of the structural and gas-sensing properties of ceramic pellets made from pure and Er-doped ZnGa2O4 were conducted. The Er-doped material is notable for its broad dynamic range for CH4 concentrations (from 100 to 20000 ppm), low sensitivity to humidity variations within the 30-70% relative humidity range, and robust stability under cyclic gas exposure. Furthermore, the samples showed strong responses to NO2 (3.37 a.u.) and H2 (4.77 a.u.) at 100 ppm gas concentration and 650 °C.

THE WHOLE WORK IS INTERESTING

POINTS FOR IMPROVEMENT:

A literature review made by the reviewer by using Google Scholar and authors keywords revealed 84 published works. I do believe there is space to add some recent references
What's the general status in the area of specific gas (CH4) sensors?Are there any alternative materials and methods?
What's the purpose of this reasearch? What's the specific problem solved?
Propose applications for the specific sensors.
Very often methane coexists with other gas hydrocarbons. What's the effect of other hydrocarbon gases on this sensor?
What are the drawbacks of these sensors?
Propose ideas for future work.

Author Response

Response to the Reviewer

Dear Reviewer,

A literature review made by the reviewer by using Google Scholar and authors keywords revealed 84 published works. I do believe there is space to add some recent references

Authors’ response:

Modern published works have been included. The following are just some examples of where recent references have been used.

Introduction

The very limited number of literature sources does not yet allow us to draw some general conclusions on the advantages and disadvantages of ZnGa₂O₄ as a sensitive material for gas sensors in comparison with the well-developed binary metal-oxide semiconductors (MOSs) SnO₂, ZnO, TiO₂, WO₃, Ga₂O₃ [23-35] or transition metal dichalcogenides (TMDs) [5,36].

…

Discussion

To date CH₄ sensors based on binary metal-oxide films and nanostructures (SnO₂, ZnO and TiO₂, primary) have been researched and developed [59-64].

Added references:

Wu, H.; Zhong, S.; Bin, Y.; Jiang, X.; Cui, H. Ni-decorated WS₂-WSe₂ heterostructure as a novel sensing candidate upon C₂H₂ and C₂H₄ in oil-filled transformers: a first-principles investigation. Mol. Phys. 2025, e2492391. https://doi.org/10.1080/00268976.2025.2492391.

…

Pathania, A.; Dhanda, N.; Verma, R.; Sun, A.C.A.; Thakur, P.; Thakur, A. Review—Metal Oxide Chemoresistive Gas Sensing Mechanism, Parameters, and Applications. ECS Sens. Plus 2024, 3, 013401. https://doi.org/10.1149/2754-2726/ad2152.
Bulowski, W.; Knura, R.; Socha, R.P.; Basiura, M.; Skibińska, K.; Wojnicki, M. Thin Film Semiconductor Metal Oxide Oxygen Sensors: Limitations, Challenges, and Future Progress. Electronics 2024, 13, 3409. https://doi.org/10.3390/electronics13173409.
Zhai, H.; Wu, Z.; Fang, Z. Recent Progress of Ga₂O₃-Based Gas Sensors. Ceram. Int. 2022, 48, 24213–24233. https://doi.org/10.1016/j.ceramint.2022.06.066.
Almaev, A.V.; Yakovlev, N.N.; Erzakova, N.N.; Mochalov, L.A.; Kudryashov, M.A.; Kudryashova, Y.P.; Nesov, S.N. Gas Sensitivity of PECVD β-Ga₂O₃ Films with Large Active Surface. Mater. Chem. Phys. 2024, 320, 129430. https://doi.org/10.1016/j.matchemphys.2024.129430.
Almaev, A.V.; Yakovlev, N.N.; Verkholetov, M.G.; Rudakov, G.A.; Litvinova, K.I. High Oxygen Sensitivity of TiO₂ Thin Films Deposited by ALD. Micromachines 2023, 14, 1875. https://doi.org/10.3390/mi14101875.
Almaev, A.; Yakovlev, N.; Kopyev, V.; Nikolaev, V.; Butenko, P.; Deng, J.; Pechnikov, A.; Korusenko, P.; Koroleva, A.; Zhizhin, E. High Sensitivity Low-Temperature Hydrogen Sensors Based on SnO₂/κ(ε)-Ga₂O₃:Sn Heterostructure. Chemosensors 2023, 11, 325. https://doi.org/10.3390/chemosensors11060325.
Dimitrova, Z.; Gogova, D. On the Structure, Stress and Optical Properties of CVD Tungsten Oxide Films. Mater. Res. Bull. 2005, 40, 333–340. https://doi.org/10.1016/j.materresbull.2004.10.017.
Yurchenko, O.; Diehle, P.; Altmann, F.; Schmitt, K.; Wöllenstein, J. Co₃O₄-Based Materials as Potential Catalysts for Methane Detection in Catalytic Gas Sensors. Sensors 2024, 24, 2599. https://doi.org/10.3390/s24082599.
Zhan, H.; Li, H.; Yang, X.; Zhao, R.; Liu, Q.; Duan, Y.; Shen, Z. Pt-functionalized ZnO nanosheets gas sensor for highly sensitive detecting of methane. J Mater Sci: Mater Electron 2024, 35, 1871. https://doi.org/10.1007/s10854-024-13645-7.
Furuta, D.; Sayahi, T.; Li, J.; Wilson, B.; Presto, A.A.; Li, J. Characterization of inexpensive metal oxide sensor performance for trace methane detection. Atmos. Meas. Tech. 2022, 15, 5117–5128. https://doi.org/10.5194/amt-15-5117-2022.
Shaposhnik, A.V.; Moskalev, P.V.; Arefieva, O.A.; Zvyagin, A.A.; Kul, O.V.; Vasiliev, A.A. Selective determination of hydrogen in a mixture with methane using a single metal oxide sensor. Int. J. Hydrogen Energy 2024, 82, 523–530. https://doi.org/10.1016/j.ijhydene.2024.07.379.
Yablokov, M.Y.; Vasiliev, A.A.; Gainutdinov, R.V.; Sokolov, A.V. Determination of Methane Dissolved in Water Using Metal-Oxide Sensors. J. Anal. Chem. 2023, 78, 385–389. https://doi.org/10.1134/S1061934823020156.
Santos, J.P.; Sanchez-Vicente, C.; Sayago, I. Rare Earth Doped Metal Oxide Sensors for the Detection of Methane at Room Temperature. Chem. Eng. Trans. 2024, 112, 109–114. https://doi.org/10.3303/CET24112019.
Mirzaei, A.; Alizadeh, M.; Ansari, H.R.; Moayedi, M.; Kordrostami, Z.; Safaeian, H.; Lee, M.L.; Kim, T.-U.; Kim, J.-Y.; Kim, H.W.; Kim, S.S. Resistive gas sensors for the detection of NH3 gas based on 2D WS₂, WSe₂, MoS₂, and MoSe₂: a review. Nanotechnology 2024, 35, 332002. https://doi.org/10.1088/1361-6528/ad4b22.

…

Jiao, M.Z.; Chen, X.Y.; Hu, K.X.; Qian, D.Y.; Zhao, X.H.; Ding, E.J. Recent Developments of Nanomaterials-Based Conductive Type Methane Sensors. Rare Met. 2021, 40, 1515–1527. https://doi.org/10.1007/s12598-020-01679-9.
Basu, S.; Basu, P.K. Nanocrystalline Metal Oxides for Methane Sensors: Role of Noble Metals. J. Sensors 2009, 861968. https://doi.org/10.1155/2009/861968.
Fu, L.; You, S.; Li, G.; Li, X.; Fan, Z. Application of Semiconductor Metal Oxide in Chemiresistive Methane Gas Sensor: Recent Developments and Future Perspectives. Molecules 2023, 28, 6710. https://doi.org/10.3390/molecules28186710.
Gautam, Y.K.; Sharma, K.; Tyagi, S.; Ambedkar, A.K.; Chaudhary, M.; Pal Singh B. Nanostructured metal oxide semiconductor-based sensors for greenhouse gas detection: progress and challenges. R. Soc. Open Sci. 2021, 8, 201324. http://doi.org/10.1098/rsos.201324.
Chen, L.; Yu, Q.; Pan, C.; Song, Y.; Dong, H.; Xie, X.; Li, Y.; Liu, J.; Wang, D.; Chen, X. Chemiresistive gas sensors based on electrospun semiconductor metal oxides: A review. Talanta 2022, 246, 12352. https://doi.org/10.1016/j.talanta.2022.123527.
Kekana, M.T.M.; Mosuang, T.E.; Ntsendwana, B.; Sikhwivhilu, L.M.; Mahladisa, M.A. Notable synthesis, properties and chemical gas sensing trends on molybdenum disulphides and diselenides two-dimensional nanostructures: A critical review. Chemosphere 2024, 366, 143497. https://doi.org/10.1016/j.chemosphere.2024.143497.

What's the general status in the area of specific gas (CH4) sensors? Are there any alternative materials and methods?

Authors’ response:

References:

Aldhafeeri, T.; Tran, M.K.; Vrolyk, R.; Pope, M.; Fowler, M. A Review of Methane Gas Detection Sensors: Recent Developments and Future Perspectives. Inventions 2020, 5, 28. https://doi.org/10.3390/inventions5030028.

…

Wu, H.; Zhong, S.; Bin, Y.; Jiang, X.; Cui, H. Ni-decorated WS₂-WSe₂ heterostructure as a novel sensing candidate upon C₂H₂ and C₂H₄ in oil-filled transformers: a first-principles investigation. Mol. Phys. 2025, e2492391. https://doi.org/10.1080/00268976.2025.2492391.

…

Yurchenko, O.; Diehle, P.; Altmann, F.; Schmitt, K.; Wöllenstein, J. Co₃O₄-Based Materials as Potential Catalysts for Methane Detection in Catalytic Gas Sensors. Sensors 2024, 24, 2599. https://doi.org/10.3390/s24082599.
Zhan, H.; Li, H.; Yang, X.; Zhao, R.; Liu, Q.; Duan, Y.; Shen, Z. Pt-functionalized ZnO nanosheets gas sensor for highly sensitive detecting of methane. J Mater Sci: Mater Electron 2024, 35, 1871. https://doi.org/10.1007/s10854-024-13645-7.
Furuta, D.; Sayahi, T.; Li, J.; Wilson, B.; Presto, A.A.; Li, J. Characterization of inexpensive metal oxide sensor performance for trace methane detection. Atmos. Meas. Tech. 2022, 15, 5117–5128. https://doi.org/10.5194/amt-15-5117-2022.
Shaposhnik, A.V.; Moskalev, P.V.; Arefieva, O.A.; Zvyagin, A.A.; Kul, O.V.; Vasiliev, A.A. Selective determination of hydrogen in a mixture with methane using a single metal oxide sensor. Int. J. Hydrogen Energy 2024, 82, 523–530. https://doi.org/10.1016/j.ijhydene.2024.07.379.
Yablokov, M.Y.; Vasiliev, A.A.; Gainutdinov, R.V.; Sokolov, A.V. Determination of Methane Dissolved in Water Using Metal-Oxide Sensors. J. Anal. Chem. 2023, 78, 385–389. https://doi.org/10.1134/S1061934823020156.
Santos, J.P.; Sanchez-Vicente, C.; Sayago, I. Rare Earth Doped Metal Oxide Sensors for the Detection of Methane at Room Temperature. Chem. Eng. Trans. 2024, 112, 109–114. https://doi.org/10.3303/CET24112019.

…

Jiao, M.Z.; Chen, X.Y.; Hu, K.X.; Qian, D.Y.; Zhao, X.H.; Ding, E.J. Recent Developments of Nanomaterials-Based Conductive Type Methane Sensors. Rare Met. 2021, 40, 1515–1527. https://doi.org/10.1007/s12598-020-01679-9.
Basu, S.; Basu, P.K. Nanocrystalline Metal Oxides for Methane Sensors: Role of Noble Metals. J. Sensors 2009, 861968. https://doi.org/10.1155/2009/861968.
Fu, L.; You, S.; Li, G.; Li, X.; Fan, Z. Application of Semiconductor Metal Oxide in Chemiresistive Methane Gas Sensor: Recent Developments and Future Perspectives. Molecules 2023, 28, 6710. https://doi.org/10.3390/molecules28186710.
Gautam, Y.K.; Sharma, K.; Tyagi, S.; Ambedkar, A.K.; Chaudhary, M.; Pal Singh B. Nanostructured metal oxide semiconductor-based sensors for greenhouse gas detection: progress and challenges. R. Soc. Open Sci. 2021, 8, 201324. http://doi.org/10.1098/rsos.201324.
Chen, L.; Yu, Q.; Pan, C.; Song, Y.; Dong, H.; Xie, X.; Li, Y.; Liu, J.; Wang, D.; Chen, X. Chemiresistive gas sensors based on electrospun semiconductor metal oxides: A review. Talanta 2022, 246, 12352. https://doi.org/10.1016/j.talanta.2022.123527.
Kekana, M.T.M.; Mosuang, T.E.; Ntsendwana, B.; Sikhwivhilu, L.M.; Mahladisa, M.A. Notable synthesis, properties and chemical gas sensing trends on molybdenum disulphides and diselenides two-dimensional nanostructures: A critical review. Chemosphere 2024, 366, 143497. https://doi.org/10.1016/j.chemosphere.2024.143497.

What's the purpose of this reasearch? What's the specific problem solved?

Authors’ response:

References:

Aldhafeeri, T.; Tran, M.K.; Vrolyk, R.; Pope, M.; Fowler, M. A Review of Methane Gas Detection Sensors: Recent Developments and Future Perspectives. Inventions 2020, 5, 28. https://doi.org/10.3390/inventions5030028.
Gautam, Y.K.; Sharma, K.; Tyagi, S.; Ambedkar, A.K.; Chaudhary, M.; Pal Singh B. Nanostructured metal oxide semiconductor-based sensors for greenhouse gas detection: progress and challenges. R. Soc. Open Sci. 2021, 8, 201324. http://doi.org/10.1098/rsos.201324.
Liu, Y.; Parisi, J.; Sun, X.; Lei, Y. Solid-state gas sensors for high temperature applications – a review. J. Mater. Chem. A 2014, 2, 9919–9943. https://doi.org/10.1039/c3ta15008a.
Ghosh, A.; Zhang, C.; Shi, S.Q.; Zhang, H. High-Temperature Gas Sensors for Harsh Environment Applications: A Review. Clean – Soil Air Water 2019, 47, 1800491. https://doi.org/10.1002/clen.201800491.
Wu, H.; Zhong, S.; Bin, Y.; Jiang, X.; Cui, H. Ni-decorated WS₂-WSe₂ heterostructure as a novel sensing candidate upon C₂H₂ and C₂H₄ in oil-filled transformers: a first-principles investigation. Mol. Phys. 2025, e2492391. https://doi.org/10.1080/00268976.2025.2492391.
Fleischer, M.; Meixner, H. A Selective CH₄ Sensor Using Semiconducting Ga₂O₃ Thin Films Based on Temperature Switching of Multigas Reactions. Sens. Actuators B Chem. 1995, 25, 544–547. https://doi.org/10.1016/0925-4005(95)85118-6.
Frank, J.; Fleischer, M.; Meixner, H.; Feltz, A. Enhancement of Sensitivity and Conductivity of Semiconducting Ga₂O₃ Gas Sensors by Doping with SnO₂. Sens. Actuators B Chem. 1998, 49, 110–114. https://doi.org/10.1016/S0925-4005(98)00094-X.
Moseley, P.T.; Oprea, A.; Merdrignac-Conanec, O.; Kerlau, M.; Bârsan, N.; Weimar, U. Limitations on the Use of Perovskite-Structure Oxides in Gas Sensing as a Result of the Concurrent Operation of Separate Mechanisms. Sens. Actuators B Chem. 2008, 133, 543–546. https://doi.org/10.1016/j.snb.2008.03.019.

Propose applications for the specific sensors.

Authors’ response:

High-temperature CH₄ sensors are of interest for the development of systems based on them for monitoring and control of combustion processes, for determining the ideal fuel/air mixture [82]. With this in mind, it is reasonable to compare the gas-sensitive characteristics of CH₄ sensors based on MOSs capable of functioning in the high-temperature region corresponding to the range of T = 600 – 1000 °C. In addition, high operating temperatures ensure high sensor speed performance [83], low base resistance, and surface regeneration [81].

References:

[81] Korotcenkov, G.; Cho, B.K. Instability of Metal Oxide-Based Conductometric Gas Sensors and Approaches to Stability Improvement (Short Survey). Sens. Actuators B Chem. 2011, 156, 527–538. https://doi.org/10.1016/j.snb.2011.02.024.

[82] Liu, Y.; Parisi, J.; Sun, X.; Lei, Y. Solid-State Gas Sensors for High Temperature Applications—A Review. J. Mater. Chem. A 2014, 2, 9919–9943. https://doi.org/10.1039/C3TA15008A.

[83] Yakovlev, N.N.; Almaev, A.V.; Nikolaev, V.I.; Kushnarev, B.O.; Pechnikov, A.I.; Stepanov, S.I.; Chikiryaka, A.V.; Timashov, R.B.; Scheglov, M.P.; Butenko, P.N.; et al. Low-Resistivity Gas Sensors Based on the In₂O₃-Ga₂O₃ Mixed Compounds Films. Mater. Today Commun. 2023, 34, 105241. https://doi.org/10.1016/j.mtcomm.2022.105241.

Very often methane coexists with other gas hydrocarbons. What's the effect of other hydrocarbon gases on this sensor?

Authors’ response:

The authors conducted additional research and changed Figure 14 and the corresponding text.

The results of the selectivity evaluation for the ZGO+Er samples at T = 650 °C are shown in Fig. 14. The resistance of the samples dropped under exposure to H₂, CO, CO₂, NH₃, and hydrocarbons, while the resistance of the samples increased under exposure to NO₂ and O₂. The ratios of the ZGO+Er samples responses to other gases and CH₄ at same gas concentration of 100 ppm are presented in Table 4. As can be seen, the responses to NO₂, H₂ and hydrocarbons outperform those to CH₄. The samples showed a particularly high response when exposed to hydrocarbons (C₂H₄, C₃H₈ and C₄H₁₀). A noticeable response to NH₃ was also obtained. For the remaining gases CO, CO₂ and O₂ the responses were low. The response to 5 ppm of NO was 1.22 a. u.

Figure 14. Comparison of the ZGO+Er samples responses to different gases at T = 650 °C.

Table 4. Ratios of the ZGO+Er samples responses to 100 ppm CH₄ and other gases.

Gas	Response ratio at 100 ppm
NO₂	1.16
H₂	1.64
CO	0.47
CO₂	0.58
NH₃	0.71
C₂H₄	2.41
C₃H₈	2.75
C₄H₁₀	3.09

What are the drawbacks of these sensors?

Authors’ response:

The main disadvantages of the sensors studied, as well as of many others based on metal oxides, are the lack of absolute and relative selectivity and the significant dependence of gas-sensitive characteristics on humidity within the 0–30% RH range. Our future work will focus on addressing these issues. It is worth noting that the studied samples meet the criteria for high-temperature gas sensors.

Propose ideas for future work.

Authors’ response:

We have added corresponding information to the conclusion in response to this comment.

Our future work will focus on optimizing the gas-sensitive properties of high-temperature hydrocarbons sensors based on ZnGa₂O₄ with Er addition ceramic pellets with the aim of increasing their gas sensitivity, decreasing effect of on relative humidity in the range of 0 – 30% and improving their selectivity under extreme operating conditions.

References:

Postica, V.; Lupan, O.; Gapeeva, A.; Hansen, L.; Khaledialidusti, R.; Mishra, A.K.; Drewes, J.; Kersten, H.; Faupel, F.; Adelung, R.; Hansen, S. Improved Long-Term Stability and Reduced Humidity Effect in Gas Sensing: SiO₂ Ultra-Thin Layered ZnO Columnar Films. Adv. Mater. Technol. 2021, 6, 2001137. https://doi.org/10.1002/admt.202001137.
79. Fleischer, M.; Seth, M.; Kohl, C.-D.; Meixner, H. A selective H₂ sensor implemented using Ga₂O₃ thin-films which are covered with a gas-filtering SiO₂ Sens. Actuators B: Chem. 1996, 36(1–3), 297-302. https://doi.org/10.1016/S0925-4005(97)80085-8.

Reviewer 4 Report

Comments and Suggestions for Authors

The manuscript presents an interesting study on Er-doped ZnGa₂O₄ ceramics for CH₄ sensing, with promising results for high-temperature applications. However, several issues need addressing

The document states that the band gap (Eg) of ZGO is 4.29 eV, citing agreement with literature [46]. However, the introduction (Page 1, Line 33) mentions a range of 4.4–5.2 eV for ZnGa₂O₄. The reported value of 4.29 eV is slightly below this range, which may indicate a discrepancy or require justification (e.g., due to synthesis conditions or measurement method).
The text states that ZGO+Er samples show higher responses to NO₂ and H₂ compared to CH₄ at 100 ppm, but the discussion (Page 12) claims they "do not exhibit exceptional selectivity to CH₄". This is inconsistent with the abstract’s emphasis on CH₄ sensing. The lack of a clear selectivity metric (e.g., response ratios) makes it hard to evaluate selectivity.
The conclusions repeat the results too much. I’d emphasize the novelty (e.g., first study of Er-doped ZnGa₂O₄ for CH₄ sensing) and suggest future work, like lowering the operating temperature. Let’s discuss these at our next meeting.
Humidity Sensitivity: The claim of “weak dependence” on humidity (Page 15) is misleading, as Figure 10 shows a significant response drop from 0–30% RH. Quantify this effect (e.g., percentage decrease in S) and discuss mitigation strategies.
The SEM and AFM results (Figs. 3-4) are strong, but the text (Page 6) jumps between pore sizes and microcrystal types without connecting them to sensing. Maybe add: “The increased pore density and roughness in ZGO+Er enhance gas adsorption, contributing to the higher CH₄ response.”

Author Response

Response to the Reviewer

Dear Reviewer,

The document states that the band gap (Eg) of ZGO is 4.29 eV, citing agreement with literature [46]. However, the introduction (Page 1, Line 33) mentions a range of 4.4–5.2 eV for ZnGa₂O₄. The reported value of 4.29 eV is slightly below this range, which may indicate a discrepancy or require justification (e.g., due to synthesis conditions or measurement method).

Authors’ response:

According to this comment of the reviewer, we changed the text of the manuscript.

The band gap width, E_g, of ZnGa₂O₄ depends significantly on the synthesis conditions, а presence of impurities and subsequent treatments, and is reported within relatively range of 4.1 – 5.2 eV [9-14].

References:

Chikoidze, E.; Sartel, C.; Madaci, I.; Mohamed, H.; Vilar, C.; Ballesteros, B.; Belarre, F.; Corro, E.; Vales-Castro, P.; Sauthier, G.; et al. p-Type Ultrawide-Band-Gap Spinel ZnGa₂O₄: New Perspectives for Energy Electronics. Cryst. Growth Des. 2020, 20, 2535–2546. https://doi.org/10.1021/acs.cgd.9b01669.
Byun, H.J.; Kim, J.U.; Yang, H. Blue, Green, and Red Emission from Undoped and Doped ZnGa₂O₄ Colloidal Nanocrystals. Nanotechnology 2009, 20, 495602. https://doi.org/10.1088/0957-4484/20/49/495602.
Wu, M.R.; Li, W.Z.; Tung, C.Y.; Huang, C.Y.; Chiang, Y.H.; Liu, P.L.; Horng, R.H. NO Gas Sensor Based on ZnGa₂O₄ Epilayer Grown by Metalorganic Chemical Vapor Deposition. Sci. Rep. 2019, 9, 7459. https://doi.org/10.1038/s41598-019-43752-z.
Chen, C.; Li, G.; Liu, Y. Synthesis of ZnGa₂O₄ Assisted by High-Energy Ball Milling and Its Gas-Sensing Characteristics. Powder Technol. 2015, 281, 7–11. https://doi.org/10.1016/j.powtec.2015.04.041.
Mushtaq, U.; Ayoub, I.;·Yagoub, M.Y.A.; Shivaramu, N.J.; Coetsee, E.; Swart, H.C.; Kumar, V. Effect of Li+ monovalent ion on the structural and optical properties of Dy³⁺ doped ZnGa₂O₄ phosphor. Appl. Phys. A 2024, 130, 494. https://doi.org/10.1007/s00339-024-07634-0.
Wang, B.; Wang, H.; Tu, B.; Zheng, K.; Gu, H.; Wang, W.; Fu, Z.; Optical transmission, dispersion, and transition behavior of ZnGa₂O₄ transparent ceramic. J. Am. Ceram. Soc. 2023, 106, 1230–1239. https://doi.org/10.1111/jace.18857.

The text states that ZGO+Er samples show higher responses to NO₂ and H₂ compared to CH₄ at 100 ppm, but the discussion (Page 12) claims they "do not exhibit exceptional selectivity to CH₄". This is inconsistent with the abstract’s emphasis on CH₄ sensing. The lack of a clear selectivity metric (e.g., response ratios) makes it hard to evaluate selectivity.

Authors’ response:

According to this comment of the reviewer, we changed the abstract, corresponding text of the manuscript and added Table 4.

Abstract: The use of CH₄ as an energy source is increasing every day. To increase the efficiency of CH₄ combustion and ensure the equipment meets ecological requirements, it is necessary to measure the CH₄ concentration in the exhaust gases of combustion systems. To this end, sensors are required that can withstand extreme operating conditions, including temperatures of at least 600 °C, as well as high pressure and gas flow rate. ZnGa₂O₄, being an ultra-wide bandgap semiconductor with high chemical and thermal stability, is a promising material for such sensors. The synthesis and investigation of the structural and CH₄ sensing properties of ceramic pellets made from pure and Er-doped ZnGa₂O₄ were conducted. Doping with Er leads to the formation of a secondary Er₃Ga₅O₁₂ phase and an increase in the active surface area. This structural change significantly enhanced the CH₄ response, demonstrating an 11.1-fold improvement at a concentration of 10⁴ ppm. At the optimal response temperature of 650 °C, the Er-doped ZnGa₂O₄ exhibited responses of 2.91 a.u. and 20.74 a.u. to 100 ppm and 10⁴ ppm of CH₄, respectively. The Er-doped material is notable for its broad dynamic range for CH₄ concentrations (from 100 to 20000 ppm), low sensitivity to humidity variations within the 30-70% relative humidity range, and robust stability under cyclic gas exposure. In addition to CH₄, the sensitivity of Er-doped ZnGa₂O₄ to other gases at a temperature of 650 °C was investigated. The samples showed strong responses to C₂H₄, C₃H₈, C₄H₁₀, NO₂ and H₂, which, at gases concentrations of 100 ppm, were higher than the response to CH₄ by a factor of 2.41, 2.75, 3.09, 1.16 and 1.64, respectively. The study proposes a plausible mechanism explaining the sensing effect of Er-doped ZnGa₂O₄ and discusses its potential for developing high-temperature CH₄ sensors for applications such as combustion monitoring systems and determining the ideal fuel/air mixture.

…

3.2. Gas-sensitive properties

Figure 14. Comparison of the ZGO+Er samples responses to different gases at T = 650 °C.

Table 4. Ratios of the ZGO+Er samples responses to 100 ppm CH₄ and other gases.

Gas	Response ratio at 100 ppm
NO₂	1.16
H₂	1.64
CO	0.47
CO₂	0.58
NH₃	0.71
C₂H₄	2.41
C₃H₈	2.75
C₄H₁₀	3.09

The conclusions repeat the results too much. I’d emphasize the novelty (e.g., first study of Er-doped ZnGa₂O₄ for CH₄ sensing) and suggest future work, like lowering the operating temperature. Let’s discuss these at our next meeting.

Authors’ response:

According to this comment of the reviewer, we changed the conclusion.

Humidity Sensitivity: The claim of “weak dependence” on humidity (Page 15) is misleading, as Figure 10 shows a significant response drop from 0–30% RH. Quantify this effect (e.g.,percentage decrease in S) and discuss mitigation strategies.

Authors’ response:

According to this comment of the reviewer, we changed the text of the manuscript:

3.2. Gas-sensitive properties

Figure 12. Effect of humidity on ZGO+Er samples base resistance, resistance in gas mixture of pure dry air + 2000 ppm of CH₄ (a) and response to 2000 ppm CH₄ (b) at T = 650 °C.

…

Discussion

H₂O + O^2- + S_a → 2(S_a–OH) + 2e^-.

(7)

…

Conclusions

References:

Gaman, V.I.; Almaev, A.V. Dependences of Characteristics of Sensors Based on Tin Dioxide on the Hydrogen Concentration and Humidity of Gas Mixture. Russ. Phys. J. 2017, 60, 90–100. https://doi.org/10.1007/s11182-017-1046-2.

…

Postica, V.; Lupan, O.; Gapeeva, A.; Hansen, L.; Khaledialidusti, R.; Mishra, A.K.; Drewes, J.; Kersten, H.; Faupel, F.; Adelung, R.; Hansen, S. Improved Long-Term Stability and Reduced Humidity Effect in Gas Sensing: SiO₂ Ultra-Thin Layered ZnO Columnar Films. Adv. Mater. Technol. 2021, 6, 2001137. https://doi.org/10.1002/admt.202001137.
Fleischer, M.; Seth, M.; Kohl, C.-D.; Meixner, H. A selective H₂ sensor implemented using Ga₂O₃ thin-films which are covered with a gas-filtering SiO₂ layer. Sens. Actuators B: Chem. 1996, 36(1–3), 297-302. https://doi.org/10.1016/S0925-4005(97)80085-8.
Wang, Y.; Zhou, Y. Recent Progress on Anti-Humidity Strategies of Chemiresistive Gas Sensors. Materials2022, 15, 8728. https://doi.org/10.3390/ma15248728.

The SEM and AFM results (Figs. 3-4) are strong, but the text (Page 6) jumps between pore sizes and microcrystal types without connecting them to sensing. Maybe add: “The increased pore density and roughness in ZGO+Er enhance gas adsorption, contributing to the higher CH₄ response.”

Authors’ response:

According to this comment of the reviewer, we changed the text of the manuscript:

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