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Article

Hollow Mesoporous ZnO/ZnCo2O4 Based on Ostwald Ripening for H2S Detection

by
Hongtao Wang
1,2,*,
Yang Liu
1,2,
Yuanchao Xie
1,2,
Jianan Ma
1,2,
Dan Han
1,2 and
Shengbo Sang
1,2,*
1
Shanxi Key Laboratory of Micro Nano Sensors & Artificial Intelligence Perception, College of Integrated Circuits, Taiyuan University of Technology, Taiyuan 030024, China
2
Key Lab of Advanced Transducers and Intelligent Control System of the Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China
*
Authors to whom correspondence should be addressed.
Chemosensors 2025, 13(7), 239; https://doi.org/10.3390/chemosensors13070239
Submission received: 28 May 2025 / Revised: 30 June 2025 / Accepted: 1 July 2025 / Published: 5 July 2025
(This article belongs to the Special Issue Recent Progress in Nano Material-Based Gas Sensors)
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Abstract

Mesoporous ZnO/ZnCo2O4 nanocomposites with excellent gas-sensing performance were synthesized using the Ostwald ripening method. The as-prepared ZnO/ZnCo2O4 comprised aggregated monodisperse nanoparticles, and the nanoparticle size grew with increasing thermal treatment temperature. Increasing the calcination temperature did not significantly change the overall size of the ZnO/ZnCo2O4 nanocomposites, but the pore size and specific surface area were noticeably affected. The gas-sensing results showed that ZnO/ZnCo2O4 composites calcined at 500 °C exhibited the highest response to H2S at 200 °C, with a detection limit of 500 ppb. The ZnO/ZnCo2O4 composites also exhibited remarkable selectivity, response/recovery speed, and stability. Their excellent gas-sensing performance might be attributed to their porous structure, large specific surface area, and the heterogeneous interface between ZnO and ZnCo2O4. This work not only represents a new example of the Ostwald ripening-based formation of inorganic hollow structures in a template-free aqueous solution but also provides a novel and efficient sensing material for the detection of H2S gas.

1. Introduction

Hollow nanostructures with a defined shape, composition and shell/internal structure have attracted considerable attention because of their promising applications in various fields, such as energy storage and conversion, nanoscale chemical reactors, efficient catalysts, and gas sensing [1,2,3]. In particular, hollow nanospheres with porous shells offer greater advantages in gas detection because of their larger specific surface area and superior permeability compared with other conventional nanostructures. The larger specific surface area is not only beneficial to oxygen adsorption but also provides more reactivity sites for redox reactions with test gases. Moreover, the dense and cross-linked pore structure on the surface makes gas diffusion more efficient, which is perfect for the uptake and release of guest molecules [4,5]. Until now, the template method has been the most common strategy to synthesize diverse hollow materials, including mono-dispersive silica, polymer latex spheres, carbon spheres, and even gas bubbles [6,7,8]. Despite its wide applicability, the template method still faces potential incompatibility issues between core and shell materials. In addition, the yield of hollow materials synthesized using template-assisted methods is usually low, and their shells are not intact, which usually leads to poor mechanical performance [9,10]. Therefore, the rational design and controllable synthesis of high-quality anisotropic hollow structures with “one-step” synthetic template-free methods deserve further study.
Hydrogen sulfide (H2S) is a colorless, highly toxic gas which smells like rotten eggs. It is denser than air, highly soluble in water, chemically unstable, and has strong reducing properties. H2S can react with many metal ions to form sulfides, and its sources include volcanic eruptions, natural gas and oil extraction, organic matter decomposition, industrial processes, etc. [11,12]. H2S can acidify the soil, pollute water bodies, endanger plant growth, and cause the death of aquatic organisms in the natural environment. In the human body, low concentrations of H2S can irritate the mucous membranes of the eyes and respiratory tract, while high concentrations can rapidly paralyze the nerves, causing breathing difficulties, coma, and even sudden death. Long-term exposure may also lead to chronic poisoning and neurological damage [13,14,15]. Therefore, the development of high-performance H2S detection equipment is of great significance for environmental protection and human health. Compared with traditional methods such as infrared spectroscopy or gas chromatography–mass spectrometry, metal oxide semiconductor (MOS)-based gas sensors are smaller, cost less, and offer a faster response. In addition, no complex sample pretreatment is required, making these sensors suitable for on-site rapid detection. They are also easy to integrate and can meet the requirements of portable and intelligent monitoring, which is an especially significant advantage in the field of early warning of low-concentration gas. However, their sensitivity and lower limit of detection still need to be improved to meet the requirements of practical applications.
Binary metal oxides have garnered significant research attention for gas-sensing applications, primarily due to their robust material attributes. The engagement of gas molecules with these oxides hinges on the surface atoms of the metal oxides, while the catalytic behavior of such oxides is governed by the relative acidic and basic nature of the atoms residing on their surfaces [16,17]. Compared with binary metal oxides, ternary metal oxides such as ZnCo2O4 show significant advantages as gas-sensitive materials for chemical resistance gas sensors. From the perspective of crystal structure, the unique spinel structure of ZnCo2O4 can provide more surface active sites for gas molecules, enhancing the latter’s physical adsorption and chemical dissociation capabilities. At the level of the electron conduction mechanism, the redox pairs formed by the coexistence of polymetal ions promote the electron transfer process, accelerate the reaction rate, and thereby significantly enhance the gas-sensitive response speed. Furthermore, the lattice distortion produced by the multi-metal system increases the concentration of oxygen vacancies, optimizes the quantity and activity of surface-adsorbed oxygen species, and provides a more favorable chemical environment for gas-sensitive reactions. The synergistic effect of different metal elements can also effectively regulate the band structure of materials, achieving selective adsorption and response to specific gases such as H2S gas [18,19].
In this work, hollow porous ZnO/ZnCo2O4 nanospheres were successfully synthesized using “one-step” synthetic template-free methods. The size of the nanoparticles which make up the nanospheres and the surface hole density were effectively controlled by changing the calcination temperature. The gas-sensing results showed that ZnO/ZnCo2O4 composites calcined at 500 °C exhibited the highest response to H2S gas, and the limit of detection was as low as 500 ppb. In addition, ZnO/ZnCo2O4 composites showed fast response and recovery, as well as excellent selectivity and long-term stability. Their outstanding gas-sensitive performance is attributed to their hollow porous structure, heterogeneous interface, and the synergistic effect between ZnO and ZnCo2O4. Finally, the gas-sensing mechanism of ZnO/ZnCo2O4 composites specific to H2S gas is also discussed in detail.

2. Materials and Methods

2.1. Materials

The zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Aladdin Reagent, Co., Ltd., Shanghai, China, 99%), cobalt nitrate hexahydrate (Co(NO3)2·6H2O, Aladdin Reagent, Co., Ltd., Shanghai, China, 99.9%), ethylene glycol ((CH2OH)2, Beijing Chemical Works, Beijing, China, 99.5%), and ethanol (CH3CH2OH, Beijing Chemical Works, Beijing, China, 99.5%) in this study were all of analytical grade and were used without further purification.

2.2. Synthesis of ZnO/ZnCo2O4 Hollow Microspheres

In a typical synthesis procedure, 2.14 mmol of Co(NO3)2·6H2O was first dissolved in 40 mL of an ethanol–ethylene glycol mixture with a volume ratio of 1:4. Second, 2.00 mmol of Zn(NO3)2·6H2O was added to the above solution under continuous magnetic stirring. After 1 h of stirring, the resulting precursor solution was transferred to a 50 mL Teflon-lined autoclave. Then, the high-pressure reactor was placed in an oven at 180 °C for 12 h. The solid product was collected and washed several times using water and ethanol, following which it was dried in air at 60 °C for 6 h. The final product was a ZnO/ZnCo2O4 precursor. In order to identify the influence of calcination temperature on the size of the nanoparticles which form the microspheres and the surface hole density, the ZnO/ZnCo2O4 precursor was annealed at 400 °C, 500 °C, 600 °C, and 800 °C for 2 h, with a heating rate of 2 °C/min. The final as-prepared samples were recorded as ZnO/ZnCo2O4-400, ZnO/ZnCo2O4-500, ZnO/ZnCo2O4-600, and ZnO/ZnCo2O4-800, respectively.

2.3. Characterization

Thermal analysis was performed using a NETZSCH STA 449F3 simultaneous thermal analyzer (Netzsch, Selb in Bavaria, Germany). Thermogravimetric (TG) and differential scanning calorimetry (DSC) measurements (Linseis, Selb in Bavaria, Germany) were conducted in air at a temperature ranging from 30 to 800 °C, at 10 °C/min. The phase composition of the as-synthesized ZnO/ZnCo2O4 composites was characterized using powder X-ray diffraction (XRD) on a Rigaku D/max-2500 diffractometer (Rigaku, Tokyo Metropolis, Japan) (Cu-Kα radiation, λ = 1.5418 Å). Their morphology was examined via field-emission scanning electron microscopy (FESEM, JEOL JSM-7500F, Jeol, Tokyo Metropolis, Japan, 15 kV), while nanostructure details were analyzed using transmission electron microscopy (TEM) and higher-magnification TEM (HRTEM) on a JEOL JEM-2100 microscope (200 kV). The specific surface area of the ZnO/ZnCo2O4 composites was calculated from N2 adsorption isotherms (Micromeritics Gemini VII 2390, Micromeritics Instrument Corporation, Norcross, GA, USA) via the Brunauer–Emmett–Teller (BET) method (surface area and porosity system).

2.4. Manufacturing and Measurement of Sensing Elements

Gas sensors were fabricated as follows: firstly, the as-prepared ZnO/ZnCo2O4 powders were mixed with ethanol to form a slurry. Secondly, the obtained slurry was brush-coated onto alumina tubes to form a relatively uniform sensitive film (4 mm length, 1.2 mm external diameter, and 0.8 mm internal diameter), which was equipped with paired gold electrodes at both ends for electrical contacts. After drying on a heating platform for 30 min, the alumina tubes were calcined at 300 °C for 2 h. A Ni-Cr alloy coil was then inserted as a heater for temperature regulation. Finally, the above-mentioned alumina tubes were welded onto a hexagonal base for subsequent gas sensitivity tests. The sensing element was a hollow cylinder with a diameter of approximately 0.5 cm and a height of approximately 2 cm, with the Ni-Cr heating wire passing through its interior. The resistance of the heating element was about 38 Ω, and its power consumption was about 0.24 W.
Gas sensitivity was tested in a self-made dynamic gas-mixing gas sensitivity test system; the gas chamber was a closed space with a volume of 5 L made of organic glass. The prepared gas-sensitive elements were placed on the probe platform in the gas chamber to form a circuit. The built-in multimeter was connected to detect resistance changes, and the data was collected through the computer end. When the resistance of the gas-sensitive element reached a stable state in the air atmosphere, the test gas was injected. After the resistance reached equilibrium again in the test gas atmosphere, air was re-injected to complete the full gas-sensitive test cycle. The gases used in the gas sensitivity test, such as H2S, NO2, and CO, were all purchased from Juyang Gas Co., Ltd., Taiyuan, China, which is a manufacturer of standard gases. The gas flow rate of the target gas during the detection process was 200 sccm. The gas chamber was maintained at room temperature, with a humidity of about 30% RH. Furthermore, a dual-fluid humidity generator was used to create atmospheres with different humidity in the gas chamber in order to study the influence of humidity on the gas sensitivity performance. In this work, the gas-sensing response was defined as S, as follows:
S = Ra/Rg or Rg/Ra
In addition, the response and recovery time—two important features used to characterize sensor performance—were defined as the time needed for the sensor to reach 90% of its full-scale response (or the baseline value) when the gas is in (or out).

3. Results

3.1. Characterization of Materials

ZnO/ZnCo2O4 nanospheres were synthesized via Ostwald ripening, a process by which bigger particles form at the expense of smaller ones through dissolution, recrystallization, and complicated molecular dynamics. The possible formation mechanism of ZnO/ZnCo2O4 hollow nanospheres is shown by the schematic diagram in Figure 1a. Zn(NO3)2·6H2O aqueous solution was mixed with Co(NO3)2·6H2O solution to rapidly produce ZnO/ZnCo2O4 nanoparticles at high temperature and pressure. In the early periods of nucleation and growth, smaller ZnO/ZnCo2O4 microcrystals, with greater curvature and surface energy, were produced in the central area. As the reaction went on, bigger ZnO/ZnCo2O4 microcrystals with smaller curvature and surface energy were produced in the outer region. In the Ostwald ripening method, in order to pursue a balance of surface energy, the microcrystals in the central location will gradually dissolve, diffuse, and recrystallize in the outer regions. As a result, an inner cavity is generated during this process.
To establish the optimal calcination temperature, the ZnO/ZnCo2O4 complex precursor’s thermal decomposition pathway was comprehensively characterized through simultaneous thermogravimetric–differential thermal analysis (TG-DTA) under ambient air conditions (Figure 1b). The result demonstrated distinct mass loss from 50 °C to 450 °C. The initial gradual weight reduction of about 5% observed between 50 and 200 °C corresponds to a desorption of physisorbed surface water and solvent molecules (ethanol/ethylene glycol) embedded in the micropores. Subsequently, a significant mass depletion of about 10% occurs within the 200–330 °C regime, primarily ascribed to hydroxyl decomposition nucleates with amorphous Zn-Co oxides. Another significant mass depletion of about 10% occurs within the 330–450 °C regime, mostly attributed to spinel phase transformation and lattice rearrangement in the amorphous Zn-Co oxides. These decomposition processes manifested as three intense exothermic peaks in the DTA curve, centered at approximately 150 °C, 280 °C, and 380 °C, confirming the energetics of molecular dissociation. Building upon these thermal transition signatures, a systematic calcination study spanning 400–800 °C was designed to elucidate the temperature-dependent variations in the crystalline phase formation, microstructural evolution, and specific surface area development of the derived products.
To confirm the phase composition, the XRD results of the ZnO/ZnCo2O4 samples are shown in Figure 1c. All of the diffraction peaks can be unambiguously assigned to ZnCo2O4 (JCPDS No.23-1390) and ZnO (JCPDS No.36-1451) [20]. In addition, no other impurity peaks of Co3O4 appear, indicating that the precursor has been transformed into the ZnO/ZnCo2O4 heterojunction. Moreover, the intensity of the diffraction peaks increases with the rising calcination temperature, indicating crystal growth and an improvement in crystallinity.
The microstructure and morphology of the ZnO/ZnCo2O4 samples calcined at 400 °C to 800 °C characterized via SEM are shown in Figure 1d–g, and higher-magnification SEM images are presented in Figure 2. It is worth noting that the morphologies of ZnO/ZnCo2O4 depended on the temperature of the heat treatment. A significant expansion in the size of the nanoparticles which make up the nanospheres was observed in the products after the thermal decomposition temperature increased; the average nanoparticle diameters of the four final products were around 10 nm, 30 nm, 50 nm, and 100 nm, respectively. Among them, ZnO/ZnCo2O4-400 and ZnO/ZnCo2O4-500 exhibited a morphology of interconnected nanoparticles, forming large open cracks, holes, and interstitial structures on the exterior surfaces. For ZnO/ZnCo2O4-600, the number of cross-linked nanoparticles on the surface decreased because of agglomeration. When the calcination temperature rose to 800 °C, dense surface pore structures no longer existed because of crystal growth. The change in surface density indirectly affects the specific surface area of the material and its amount of active sites, resulting in different performance in practical applications.
The hollow structure of the ZnO/ZnCo2O4-500 nanospheres was confirmed via TEM measurement. Figure 3a reveals distinct electron density variations across individual ZnO/ZnCo2O4 nanospheres, where an electron-transparent core contrasts sharply with a peripheral electron-dense shell. This characteristic intensity difference confirms the hollow architecture of these hierarchical structures. A core diameter of approximately 500 nm and a peripheral shell thickness of 100 nm can be observed in these hollow nanostructures. In addition, the shell of ZnO/ZnCo2O4 is composed of numerous tiny nanocrystals with a diameter of about 25 nm, and some nanoparticles also exist in the inner cavity. Figure 3b shows the HRTEM image of a single nanoparticle from the nanosphere shell. The fringe spacing of 0.468 nm and 0.260 nm agrees well with the spacing of the (111) plane of the cubic spinel ZnCo2O4 and the (002) plane hexagonal ZnO structure, respectively [20]. The chemical compositions of the ZnO/ZnCo2O4-500 hollow nanospheres were verified using EDS, as shown in Figure 3c–e, which clearly indicate that Zn and Co elements are evenly distributed throughout the whole nanosphere. Therefore, based on the analysis of the TEM elemental mapping results, it can be concluded that ZnO and ZnCo2O4 were evenly mixed to form ZnO/ZnCo2O4 nanospheres.
The N2 adsorption–desorption isotherms of the as-prepared ZnO/ZnCo2O4 composites are shown in Figure 4. All ZnO/ZnCo2O4 composites exhibited characteristic Type IV adsorption isotherms with H3-type hysteresis loops, confirming their mesoporous architectures. It can be seen that the specific surface area of the ZnO/ZnCo2O4 composites decreased with the increase in calcination temperature. Generally speaking, a high specific surface area can provide more adsorption active sites for the adsorption of oxygen molecules. The specific surface areas of the four final samples were 32.0, 27.8, 19.2, and 9.4 m2/g, and their respective pore sizes were 16.4 nm, 38.5 nm, 65.6 nm, and 80 nm.

3.2. Gas-Sensing Performance

To investigate the sensing performance of ZnO/ZnCo2O4, gas sensitivity tests were conducted using a self-built gas-sensing system. The responses to 100 ppm of H2S were investigated under different temperatures, ranging from 170 to 300 °C, and the results are shown in Figure 5a. It is obvious that the response first increased with temperature, and the samples all reached the maximum response at an operating temperature of 200 °C. However, when the operating temperature was further increased above 200 °C, the responses decreased, as the excessive energy boosted the desorption of gas molecules. Peak sensor responses occur at an optimal temperature, when chemisorbed ion loading maximizes during adsorption–desorption equilibrium [21]. Furthermore, it is worth noting that the response ZnO/ZnCo2O4-500 to 100 ppm of H2S was much higher than that of ZnO/ZnCo2O4-600 and ZnO/ZnCo2O4-800, which can be attributed to the more close-grained pore structures. The dynamic response curves of the as-prepared sensors to 100–500 ppm of H2S are shown in Figure 5b. For all as-prepared sensors, it can obviously be seen that when H2S was injected into the tested chamber, the responses of the sensors rapidly increased, and the response values varied with the change in concentration. Among them, ZnO/ZnCo2O4-500 exhibited the biggest stepwise distribution of the curves, which indicates that ZnO/ZnCo2O4-500 was most sensitive to H2S. For a more intuitive comparison, the calculated responses of the four sensors are plotted in Figure 5c, with quantitative and clear descriptions. ZnO/ZnCo2O4-500 clearly shows the most obvious growth trend, while the rising tendencies of ZnO/ZnCo2O4-400, ZnO/ZnCo2O4-600, and ZnO/ZnCo2O4-800 tend to be gentle. This is possibly due to the fact that, because of its dense pore structure, ZnO/ZnCo2O4-500 is rich in surface-adsorbed oxygen species when its surface is covered with sufficient target gas molecules, leading to the biggest response growth. Although ZnO/ZnCo2O4-400 has the biggest specific surface area, its amorphous surface inhibits the diffusion of oxygen and test gas molecules, resulting in a worse response.
In addition, the dynamic response curves of ZnO/ZnCo2O4-500 at lower concentration of H2S were also investigated; the results are shown in Figure 5d. It can be seen that the response of ZnO/ZnCo2O4-500 to 1, 5, 10, 20, 30, and 40 ppm of H2S was 1.7, 3.2, 6, 12, 22, and 34, respectively. ZnO/ZnCo2O4-500 still maintained a complete response and recovery process, even in an atmosphere with a lower concentration of H2S gas. The response and recovery time are two important features characterizing the performance of sensors, defined as the time needed for the sensor to reach 90% of its full-scale response (or the baseline value) when the gas is in (or out). As shown in Figure 5e, the response and recovery time to 500 ppb of H2S was 208 and 186 s, respectively.
Since selectivity is another important parameter for assessing the practical applications of gas sensors, the response of ZnO/ZnCo2O4-500 to 500 ppm of various analytes and 20 ppm of H2S at 200 °C is shown in Figure 5f. Compared with other gases, ZnO/ZnCo2O4-500 exhibited the highest response to H2S. It is gratifying to see that the response of the sample to some reducing gases, such as carbon monoxide, hydrogen, and chlorine, was negligible, which is very beneficial for practical applications. The specific recognition ability of ZnO/ZnCo2O4-500 for H2S gas is discussed in detail in Section 3.3 (Gas-Sensing Mechanism).
Figure 6a shows the relationship between the gas-sensing response of ZnO/ZnCo2O4-500 and relative humidity (RH). It can be seen that the response decreased with the increase in relative humidity. The influence of humidity on the response of MOS-based gas sensors shows a complex multi-dimensional correlation, and its mechanism of action involves changes in surface adsorption kinetics, charge carrier regulation, and the physical and chemical properties of the material. In a low-humidity environment, water molecules are adsorbed on the surface of ZnO/ZnCo2O4 composites in the form of a monolayer, indirectly affecting the gas adsorption efficiency by competing for active sites with gas molecules or changing the concentration of surface oxygen species. Under high-humidity conditions, the physical adsorption of water may form a continuous water film, hindering the diffusion and transmission of the target gas to the sensitive layer or changing the surface charge distribution through the proton conduction effect, resulting in the attenuation of the sensor response signal [22,23]. However, even in an atmosphere with 90% RH, ZnO/ZnCo2O4-500 still maintained a relatively objective response, which indicates that it has the potential for practical application. Long-term stability is another important indicator for the practical applications of gas sensors. In order to explore the long-term stability of ZnO/ZnCo2O4-500 for H2S gas, gas sensitivity tests were conducted every three days for one month, as shown in Figure 6b. It can be seen that the response varied in each test, but the variation was very small, which means ZnO/ZnCo2O4-500 has excellent long-term stability.

3.3. Gas-Sensing Mechanism

It is well known that the gas-sensing response depends on the interaction between the adsorbed oxygen ions and the test gases on the surface of the material [24,25,26]. ZnCo2O4 is a typical p-type ternary metal oxide semiconductor, and its charge carrier is the hole. When ZnCo2O4 is exposed to an air atmosphere, at higher temperatures, oxygen molecules will adsorb on the surface and capture electrons from ZnCo2O4 to generate the chemisorbed oxygen species of O2−, O−, and O2− and a hole accumulation layer, leading to a decrease in resistance (Reaction (2) to (5)):
O2(gas) → O2(ads)
2O2(ads) + 2e− → 2O2−
O2(ads) + 2e− → 2O−
O2(ads) + 4e− → 2O2−
When the sensor was exposed to H2S, the adsorbed oxygen species reacted with the H2S gas molecules on the material’s surface (Reaction (6) to (8)), which led to the release of electrons trapped in the ionized oxygen species back into the conduction band and reduced the hole concentration in the shell layer through electron-hole recombination, thereby increasing the measured resistance of the sensor [27,28]:
3O2− + 2H2S → 2H2O + 2SO2 + 3e−
3O− + H2S → H2O + SO2 + 3e−
3O2− + H2S → H2O + SO2 + 6e−
The reactions above analyze the gas sensitivity mechanism from the perspective of redox reactions to H2S on the surface of ZnO/ZnCo2O4 composites. In addition, there may also be gas-sensitive responses caused by changes in the phase composition of ZnO/ZnCo2O4 composites in a H2S gas atmosphere, as shown in Reaction (9):
ZnO + H2S → ZnS
Even at room temperature, the surface of ZnO could undergo a slow gas–solid reaction with H2S to form ZnS films, controlled by surface adsorption and diffusion. In this work, the optimal operating temperature of ZnO/ZnCo2O4 composites was 200 °C. Higher temperatures could accelerate the decomposition of H2S and the combination of S2− and Zn2+, significantly increasing the reaction rate. Since the resistance of ZnS is much greater than that of ZnO, Reaction (9) would also lead to an increase in the resistance of ZnO/ZnCo2O4 composites. If H2S gas was cut off, oxygen would be re-adsorbed onto the surface of ZnO/ZnCo2O4 composites to form negative oxygen ion species, or ZnS would be re-oxidized to ZnO, so the resistance would return to its initial level.
According to the theoretical basis above, the superior response of ZnO/ZnCo2O4-500 to H2S can be attributed to two factors. Firstly, ZnO/ZnCo2O4-500 has hollow porous structures with the biggest specific surface area, which can offer extra active sites for gas absorption and for the redox reaction with H2S. What is more, the thin and permeable shells of hollow structures minimize gaseous diffusion paths, kinetically accelerating both the response and recovery speed.
Secondly, the improved sensing performance was attributed to the synergistic effect between ZnO and ZnCo2O4. Owing to ZnCo2O4’s lower Fermi level, electrons transfer from ZnO to ZnCo2O4, while holes simultaneously migrate in the opposite direction until Fermi level alignment occurs at the interface. During this process, a depletion layer will be formed between ZnO and ZnCo2O4, and smaller carrier concentration can produce a better gas-sensing response. Moreover, the injection of electrons will promote the adsorption of oxygen. When adsorbed oxygen species react with H2S, electrons will be released back to the sensing material, increasing its electrical resistance [29,30]. The changes in band bending at the heterogeneous interface between ZnO and ZnCo2O4 in different atmospheres are shown in Figure 7b–e.
The pronounced selectivity toward H2S over NO2, CO, H2, Cl2, ethanol, methanol, and HCHO originates from synergistic mechanisms at the ZnO/ZnCo2O4 interface. First, H2S undergoes preferential dissociation on Zn2+ Lewis acid sites, forming conductive ZnS layers, which dramatically modulates carrier concentration. Second, the unique redox couple in Co3+/Co2+ facilitates rapid sulfide oxidation, amplifying conductivity changes. Conversely, competing gases exhibit weaker interactions: NO2 adsorption is sterically hindered by the spinel structure, while reducing gases (CO, H2) lack driving forces for sulfidation. Organic vapors experience incomplete oxidation at operating temperatures due to insufficient catalytic activation. This specificity is further enhanced by H2S’s smaller kinetic diameter (3.6 Å) versus that of ethanol (4.5 Å) or formaldehyde (3.8 Å), enabling deeper diffusion into mesopores.

4. Conclusions

In summary, we have demonstrated a facile and scalable synthesis of ZnO/ZnCo2O4 hollow nanospheres using the Ostwald ripening method, controlling both the size of the nanoparticles that make up the nanospheres and the hole density by changing the calcination temperature. When evaluated as a sensing material for the detection of hazardous gases, the ZnO/ZnCo2O4-500 composites exhibited an enhanced gas-sensing performance specific to H2S. In addition, they possessed remarkable selectivity when dealing with other reducing gases, such as CO, H2, SO2, etc. This might be attributed to their hollow porous structure and the heterogeneous interface between the ZnO and ZnCo2O4. This work not only represents a new example of the Ostwald ripening-based formation of inorganic hollow structures in a template-free aqueous solution but also provides a brand new and efficient sensing material for the detection of H2S gas.

Author Contributions

Conceptualization, H.W.; methodology, H.W.; software, Y.X.; validation, Y.X. and Y.L.; formal analysis, Y.X.; investigation, Y.X.; resources, Y.L.; data curation, Y.L.; writing—original draft preparation, H.W. and S.S.; writing—review and editing, H.W. and D.H.; funding acquisition, H.W. and J.M. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the National Natural Science Foundation of China (NO. 62403347, 52205599) and the Youth Science Fund Program of Shanxi Province (202203021212203).

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors on request.

Conflicts of Interest

The authors declare that they have no competing interests.

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Chemosensors 13 00239 g001
Figure 1. (a) Formation process of ZnO/ZnCo2O4 hollow porous nanospheres based on chemical transformation process induced by Ostwald ripening method; (b) TG and DTA curves of as-prepared precursor; (c) XRD results; (d–g) SEM images of ZnO/ZnCo2O4 composites.
Figure 1. (a) Formation process of ZnO/ZnCo2O4 hollow porous nanospheres based on chemical transformation process induced by Ostwald ripening method; (b) TG and DTA curves of as-prepared precursor; (c) XRD results; (d–g) SEM images of ZnO/ZnCo2O4 composites.
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Figure 2. Higher-magnification SEM images of ZnO/ZnCo2O4 composites: (a) ZnO/ZnCo2O4 calcined at 400 °C; (b) ZnO/ZnCo2O4 calcined at 500 °C; (c) ZnO/ZnCo2O4 calcined at 600 °C; (d) ZnO/ZnCo2O4 calcined at 800 °C.
Figure 2. Higher-magnification SEM images of ZnO/ZnCo2O4 composites: (a) ZnO/ZnCo2O4 calcined at 400 °C; (b) ZnO/ZnCo2O4 calcined at 500 °C; (c) ZnO/ZnCo2O4 calcined at 600 °C; (d) ZnO/ZnCo2O4 calcined at 800 °C.
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Figure 3. (a) Low- and (b) high-magnification SEM images of ZnO/ZnCo2O4 calcined at 500 °C and (c–e) the corresponding elemental mapping images.
Figure 3. (a) Low- and (b) high-magnification SEM images of ZnO/ZnCo2O4 calcined at 500 °C and (c–e) the corresponding elemental mapping images.
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Figure 4. N2 adsorption and desorption isotherm and pore size distribution of ZnO/ZnCo2O4 calcined at different temperatures: (a) ZnO/ZnCo2O4 calcined at 400 °C; (b) ZnO/ZnCo2O4 calcined at 500 °C; (c) ZnO/ZnCo2O4 calcined at 600 °C; (d) ZnO/ZnCo2O4 calcined at 800 °C.
Figure 4. N2 adsorption and desorption isotherm and pore size distribution of ZnO/ZnCo2O4 calcined at different temperatures: (a) ZnO/ZnCo2O4 calcined at 400 °C; (b) ZnO/ZnCo2O4 calcined at 500 °C; (c) ZnO/ZnCo2O4 calcined at 600 °C; (d) ZnO/ZnCo2O4 calcined at 800 °C.
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Figure 5. (a) Responses of ZnO/ZnCo2O4 composites to 100 ppm H2S as a function of the operating temperature; (b) dynamic response curves of ZnO/ZnCo2O4 composites to different concentrations of H2S at 200 °C; (c) line graph between the gas-sensitive response of ZnO/ZnCo2O4 composites at 200 °C and H2S concentration; (d) dynamic response curves of ZnO/ZnCo2O4-500 to 1–40 ppm of H2S at 200 °C; (e) response and recovery time of ZnO/ZnCo2O4-500 to 500 ppb of H2S at 200 °C; (f) selectivity at 200 °C.
Figure 5. (a) Responses of ZnO/ZnCo2O4 composites to 100 ppm H2S as a function of the operating temperature; (b) dynamic response curves of ZnO/ZnCo2O4 composites to different concentrations of H2S at 200 °C; (c) line graph between the gas-sensitive response of ZnO/ZnCo2O4 composites at 200 °C and H2S concentration; (d) dynamic response curves of ZnO/ZnCo2O4-500 to 1–40 ppm of H2S at 200 °C; (e) response and recovery time of ZnO/ZnCo2O4-500 to 500 ppb of H2S at 200 °C; (f) selectivity at 200 °C.
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Figure 6. (a) Relationship between gas-sensing response of ZnO/ZnCo2O4-500 to 40 ppm H2S at 200 °C and humidity, ranging from 30% RH to 90% RH; (b) long-term stability of response of ZnO/ZnCo2O4-500 to 40 ppm H2S gas at 200 °C, measured during one month.
Figure 6. (a) Relationship between gas-sensing response of ZnO/ZnCo2O4-500 to 40 ppm H2S at 200 °C and humidity, ranging from 30% RH to 90% RH; (b) long-term stability of response of ZnO/ZnCo2O4-500 to 40 ppm H2S gas at 200 °C, measured during one month.
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Figure 7. (a) Gas-sensitive reaction model of ZnO/ZnCo2O4 composite materials to H2S. Energy band change: (b) before contact; (c) after contact; (d) in air; (e) in H2S.
Figure 7. (a) Gas-sensitive reaction model of ZnO/ZnCo2O4 composite materials to H2S. Energy band change: (b) before contact; (c) after contact; (d) in air; (e) in H2S.
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MDPI and ACS Style

Wang, H.; Liu, Y.; Xie, Y.; Ma, J.; Han, D.; Sang, S. Hollow Mesoporous ZnO/ZnCo2O4 Based on Ostwald Ripening for H2S Detection. Chemosensors 2025, 13, 239. https://doi.org/10.3390/chemosensors13070239

AMA Style

Wang H, Liu Y, Xie Y, Ma J, Han D, Sang S. Hollow Mesoporous ZnO/ZnCo2O4 Based on Ostwald Ripening for H2S Detection. Chemosensors. 2025; 13(7):239. https://doi.org/10.3390/chemosensors13070239

Chicago/Turabian Style

Wang, Hongtao, Yang Liu, Yuanchao Xie, Jianan Ma, Dan Han, and Shengbo Sang. 2025. "Hollow Mesoporous ZnO/ZnCo2O4 Based on Ostwald Ripening for H2S Detection" Chemosensors 13, no. 7: 239. https://doi.org/10.3390/chemosensors13070239

APA Style

Wang, H., Liu, Y., Xie, Y., Ma, J., Han, D., & Sang, S. (2025). Hollow Mesoporous ZnO/ZnCo2O4 Based on Ostwald Ripening for H2S Detection. Chemosensors, 13(7), 239. https://doi.org/10.3390/chemosensors13070239

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