1.   Introduction

Hydrogen energy has gained significant interest in recent years and is expected to replace hydrocarbon-based fuels to achieve sustainability. Hydrogen can be produced at high temperatures (greater than 400°C) in a solid oxide electrolysis cell (SOECs) through the decomposition of water molecules (H₂O) [1–2]. However, hydrogen leaks may occur during production that can cause explosions that threaten human safety. Moreover, hydrogen is a colorless and odorless gas that makes it difficult to detect using human senses, and therefore a responsive gas sensor is required for rapid hydrogen monitoring [3]. Metal oxides, such as SnO₂, ZnO, and CuO, have been used for hydrogen gas-sensing materials [4]. Nevertheless, their sensing properties diminish at high temperatures due to depression of the surface reaction [5]. The availability of suitable materials that can withstand such a harsh environment is very limited. A recent study showed that group-III nitrides, especially GaN-based sensors, exhibited an excellent response toward the ppm level of H₂ gas and good chemical stability at elevated temperatures [6]. Interestingly, the sensing property of GaN gas has dramatically exceeded its respective oxide (β-Ga₂O₃) performance. Additionally, wide bandgap semiconductors are promising candidates for high-temperature sensor applications due to their resilience to harsh environments (extreme temperatures, acid/basic conditions, high humidity) chemical stability, and mechanical robustness [7–11]. Aluminum nitride (AlN), another group-III nitride with a wide bandgap greater than 6.2 eV, is well known for electronic packaging in high-performance computing due to its high thermally conductive, mechanical, and chemical stability [12]. From this perspective, AlN might be a suitable candidate as a novel high-temperature hydrogen sensor material and AlN with a nanotube morphology has previously shown responsivity to non-volatile organic compounds (VOC) gas [13]. Since gas-sensing properties of materials are dependent on their morphologies and surface features, such as exposed facets and defects [14], AlN with a unique morphology and surface facet might offer more responsivity to hydrogen gas.

This paper reports the utilization of AlN as a novel gas sensor material for hydrogen detection at temperatures greater than 400°C. Various unique morphologies of AlN were obtained by a facile nitridation process from a γ-AlOOH precursor at 1400°C under NH₃ gas. The gas-sensing properties and mechanism toward hydrogen gas at high temperature and the influence of morphology on the gas-sensing response were systematically discussed.

2.   Experimental

Three different morphologies of AlN were prepared, hexagonal plate-like, nest-like, and rod-like. First, γ-AlOOH precursors with various morphologies were synthesized based on our previous research [15]. In typical synthesis of the hexagonal plate-like and the nest-like morphologies of γ-AlOOH, 5 mmol of Al(NO₃)₃·9H₂O was dissolved in 50 mL of distilled water followed by stirring for 15 min. In the case of the hexagonal plate-like preparation, 25 mmol of Na₂CO₃ was introduced to the mixture, while for the nest-like preparation, 1 mmol of cetyltrimethylammonium bromide (CTAB) was added. These solutions were then transferred to a 100 mL teflon-lined autoclave and treated at 200°C for 24 and 12 h for the hexagonal plate-like and the nest-like, respectively. After the hydrothermal treatment, the γ-AlOOH products were collected by vacuum filtration, dried at 60°C for 24 h, and ground with a mortar. Additionally, a commercial rod-like γ-AlOOH (BMI 090314, Mitsubishi Gas Chemical) was used. Finally, 0.2 g of as-prepared γ-AlOOH was mixed with 0.6 mL of N₂H₄·H₂O and nitridated at 1400°C in a continuous flow of NH₃ gas (200 mL/min) for 4 h to obtain the AlN phase.

The crystal phase, microstructure, and chemical state of the samples were characterized by X-Ray diffraction (XRD, Bruker D2 PHASER) with Cu K_α radiation (λ = 0.1542 nm), field-emission scanning electron microscopy (FESEM, Hitachi S-4800), transmission electron microscopy (TEM, JEOL JEM-2010), and X-ray photoelectron spectroscopy (XPS, ULVAC PHI5600). N₂ adsorption measurements (Quantachrome, NOVA 4200e) were used to measure the specific surface area. The fabrication of the gas sensor device was previously reported [16]. In a typical fabrication, interdigitated Au electrodes (5 mm × 2 mm) with comb-type architectures were combined with borosilicate glass as a device support by pasting silver paste at each electrode end. The sensor device was calcined at 400°C for 30 min to increase the device stability and remove unnecessary organic impurities from the silver paste solvent. The slurry containing AlN powder (100 mg/mL) was deposited on the surface of the sensor device until the AlN material covered the whole surface. Prior to the gas-sensing evaluation, the sensor device was dried in a 60°C depressurized oven overnight. The gas-sensing performance was evaluated by a lab-built gas sensor system (Agilent 34970A Data Acquisition). The system measured the electrical resistance of the sensor upon exposure at ambient air (R_a) and target gas (R_g). Each exposure lasted for 25 min. The gas-sensing response was calculated as S = R_g/R_a. The recovery/response time was defined as the time needed to reach 90% of the sensor resistance variation.

3.   Results and discussion

In the synthesis process of AlN, we utilized aluminum hydroxides (γ-AlOOH) with various morphologies prepared via a hydrothermal process as the starting material. In our previous investigation [15], the surface agent and mineralizer were critical factors for controlling the final morphology of the hydrothermal product. Using the preparation of the γ-AlOOH with nest-like morphology as an example, under hydrothermal treatment, CTAB dissociated into a positively-charged cation surfactant (CTA⁺), which then attached to the nuclei of γ-AlOOH by electrostatic interaction and induced anisotropic particle growth, leading to the formation of individual nanorods. Finally, each nanorod assembled at the edge and a hierarchical nest-like structure was formed. The formation mechanism was reasonable considering CTAB is a popular directing agent for nanorods and nanotubes [17–18]. The hexagonal plate-like morphology of γ-AlOOH was governed by Na₂CO₃, acting as a mineralizer to promote the formation of OH⁻. The abundance of OH⁻ species in the hydrothermal condition is believed to be responsible for the formation of platelet structures of metal oxides and hydroxides [19–20].

The γ-AlOOH obtained with nest-like or hexagonal plate-like morphology and as-received γ-AlOOH with rod-like morphology were nitridated under the conditions described above. Fig. 1 shows the XRD patterns of the AlN obtained with three morphologies. Diffraction peaks emerged at 33.2°, 36°, 37.9°, 49.8°, 59.3°, 66.1°, 69.7°, 71.5°, and 72.7° corresponding to the (100), (002), (101), (102), (110), (103), (200), (112), and (201) crystal planes of the wurtzite-type AlN according to the standard (JCPDS No. 25-1133). Without peaks related to impurities, the AlN prepared from the γ-AlOOH precursor possessed excellent purity in addition to the high crystallinity indicated by the high intensity.

Figure  1.  XRD patterns of the obtained AlN with various morphologies.

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SEM and TEM were used for a comprehensive understanding of the morphological features of the obtained AlN nanostructures. Fig. 2 shows the SEM, TEM, and high-resolution TEM images of the rod-like (Figs. 2(a)–2(c)), nest-like (Figs. 2(d)–2(f)), and hexagonal plate-like (Figs. 2(g)–2(i)) morphologies of AlN, with their corresponding selected area diffraction patterns (SAED) in the insets. All of the final AlN morphologies were similar to those of the γ-morphology of the AlOOH starting material, suggesting that the morphology can be retained even if a high-temperature nitridation was utilized. As shown in Figs. 2(a)–2(b), the size of the rod-like AlN was approximately 3 μm in length with a width of 400 nm, and a small porosity was observed. The high-resolution TEM image (Fig. 2(c)) clearly shows the d-lattice spacing of 0.235 nm that is attributed to the (101) plane of wurtzite-type AlN and the inset SAED suggested the AlN with a rod-like morphology possessed a nearly single-crystalline property, although the crystallinity was not high. Meanwhile, Figs. 2(d)–2(f) show the nest-like morphology of AlN after nitridation, which was the smallest particle size (500 nm in width and 1 μm in height) with a hole in the middle of the structure. Furthermore, as shown in Fig. 2(f), d-spacing corresponding to the (101) plane was also observed, similar to that of the rod-like sample. However, a diffuse ring was observed in the SAED pattern of the nest-like sample, suggesting polycrystalline properties. Figs. 2(g)–2(h) confirmed the size of the hexagonal plate-like microstructure was approximately 7 μm. Additionally, the macroporosity was clearly recognized in which the water molecular evaporation during the nitridation process is described as follows γ-AlOOH + NH₃ → AlN + 2H₂O. Specifically, the high-resolution TEM image in Fig. 2(i) shows that the interplanar distance of 0.270 nm corresponds to the (100) plane. The hexagonal plate-like morphology shows a single-crystalline feature as confirmed in the SAED patterns. Furthermore, the SAED patterns are attributed to the (100), (110), and (101) interplanar planes of the [001] zone axis. These results track well with a previous study by Liu et al. [21]. Based on this analysis, the hexagonal plate-like morphology of AlN exhibited an (001) exposed facet, which might be beneficial for better gas-sensing properties [22–23].

Figure  2.  SEM, TEM, and high-resolution TEM images of (a–c) rod-like, (d–f) nest-like, and (g–i) hexagonal-plate morphologies of AlN. Insets of (c), (f), and (i) are the corresponding selected area diffraction patterns (SAEDs).

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The full scan and corresponding individual spectra of Al 2p, N 1s, and O 1s elements of the hexagonal plate-like morphology of AlN are shown in Fig. 3. As shown in Fig. 3(a), the core spectra were deconvoluted using a Gaussian function that was confirmed by the chemicals previously described. The spectra of Al 2p (Fig. 3(b)) was split into a higher intensity sub-peak Al 2p_3/2 located at 74.4 eV and a lower intensity sub-peak Al 2p_1/2 located at 75.9 eV corresponding to Al–N and Al–O chemical bonding [24–25], respectively. The Al–O bonding may indicate the presence of oxygen in the form of amorphous aluminum oxide (Al₂O₃) due to surface oxidation. Fig. 3(c) depicts the core-level spectra of N 1s that was deconvoluted into two sub-peaks. The first sub-peak located at 397.3 eV was attributed to Al–N bonding, which proved the formation of AlN. The second sub-peak at 399.1 eV was well-matched with N–O bonding, which formed the Al–O–N system, usually referred to as an aluminum oxynitride phase. For the O 1s core-level spectra displayed in Fig. 3(d), the sub-peak at 531.1 eV was assigned to the Al–O bond contributed to by amorphous Al₂O₃, which is consistent with the Al 2p_1/2 sub-peaks result. The sub-peak at 533.9 eV may be attributed to the surface hydroxyl group. XPS analysis confirmed the formation of the AlN phase with oxygen impurities in the form of aluminum oxide. We further investigated the oxygen and nitrogen content in AlN with hexagonal plate-like morphology using an oxygen–nitrogen analyzer, and the results are summarized in Table 1.

Figure  3.  (a) Full XPS spectra and corresponding (b) Al 2p, (c) N 1s, and (d) O 1s core spectra of AlN with a hexagonal-plate morphology.

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Table  1.  O and N amounts in the AlN samples

Sample	Nitrogen / wt%	Oxygen / wt%	N/O mass ratio
Rod-like	29.5	5.80	5.08
Nest-like	35.0	4.03	8.68
Hexagonal	33.8	4.04	8.36

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We first evaluated the influence of temperature on the hydrogen sensing response to determine the optimum working temperature at which a sensor showed the highest response to hydrogen gas, which is a critical parameter for practical applications. AlN is sensitive to air and may be oxidized at high temperature. Therefore, the testing temperatures were limited to 400–500°C to prevent oxidation. Moreover, the measurement device capability was up to 500°C. The gas-sensing performance of the AlN samples was compared with Al₂O₃ with a hexagonal plate-like morphology that was synthesized by the calcination of γ-AlOOH at 1400°C for 4 h. The XRD, TEM, and high-resolution TEM images of the obtained Al₂O₃ are displayed in Figs. 4(a), 4(b), and 4(c) respectively. The α-Al₂O₃ phase was the most dominant calcination product. The hexagonal structure has many porosities due to the evaporation of H₂O molecules and a polycrystalline nature as confirmed by the diffuse ring in the SAED pattern. Fig. 5(a) displays the sensitivity of AlN sensors at different temperatures as well as Al₂O₃ with a hexagonal plate-like morphology for comparison. The sensing response of both AlN and Al₂O₃ was greatly influenced by the sensor operating temperature. As the sensor working temperature increased, the sensor response also increased. Temperature is a driving force that promotes the chemisorption of oxygen species on the particle surface and gas sensor response. Among the prepared morphologies, the hexagonal plate-like AlN showed the highest sensing response toward hydrogen gas at all temperatures. For instance, the response (R_g/R_a) was 36.5 at 400°C, 47.8 at 450°C, and 58.7 at 500°C. The optimum working temperature could not be obtained for this material as the response may still increase as the working temperature increases. However, the response may gradually decrease after the oxidation of AlN, as Al₂O₃ did not show any response to hydrogen (Fig. 5(a), green color). Therefore, the maximum operating temperature for AlN is around 500°C. Fig. 5(b) displayed the response/recovery characteristic of AlN with different morphologies and a hexagonal plate-like Al₂O₃ to H₂ gas detection (750 ppm) at 500°C. During exposure to H₂ gas, noticeable resistance changes were detected for all sensors, and this change was reproducible for several testing times, indicating their excellent sensor stability at high temperature, except for the sensor with a nest-like AlN morphology (Fig. 5(b), blue color). The electrical resistance of the nest-like AlN did not recover to the initial state, potentially because adsorbed H₂ was partially trapped in the complex morphology. The sensing response may be because of the oxidized surface of the AlN. However, as the Al₂O₃ phase exhibited no response to hydrogen gas, the observed response of the AlN samples originated in AlN phase. The AlN sample showed p-type semiconductor sensing behavior where the resistance increased upon exposure to a reducing gas, i.e., hydrogen. The origin of the p-type conductivity of AlN itself might be from oxygen incorporation, which was previously demonstrated with evidence of oxygen induced p-type conductivity [26–28]. Additionally, there were noticeable differences in the resistance for each sensor material. The large variation in the base resistance of the AlN sample was believed to be due to its crystallinity and crystalline boundary. For example, AlN with a hexagonal plate-like morphology, which possessed a single-crystalline feature and short crystalline boundary, showed the lowest electrical resistivity, while the nest-like morphology of AlN with a polycrystalline nature exhibited the highest resistivity. A single-crystalline material should exhibit a lower electrical resistivity than that of a polycrystalline material because of fewer grain boundaries, meaning that it has better charge carrier mobility [29–30]. Therefore, the single-crystalline AlN should possess better electrical conductivity than polycrystalline AlN.

Figure  4.  (a) XRD, (b) TEM, and (c) high-resolution TEM images of Al₂O₃ with a hexagonal-plate morphology. Inset of (c) is the corresponding SAED patterns.

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Figure  5.  (a) Response vs. temperature of AlN with different morphologies and Al₂O₃ with hexagonal plate-like morphology to 750 ppm of hydrogen, (b) response/recovery feature of AlN and Al₂O₃ samples, (c) response vs. specific surface area, and (d) response time t_res and recovery time t_rec of hexagonal plate-like morphology of AlN at 500°C. In (b), blue, red, and black lines represented AlN with nest-like, rod-like, and hexagonal plate morphology, respectively, and green line represented Al₂O₃ samples.

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Fig. 5(c) shows the response values of AlN samples were 58.7, 1.6, and 4.7 for the hexagonal plate-like, the rod-like, and the nest-like, respectively. In some cases, a greater specific surface area would enhance the gas-sensing sensitivity [31]. However, in our case, the specific surface area does not have a linear correlation with the sensing response. Therefore, it is suspected that the crystallinity and surface facets may be essential for the enhanced hydrogen gas-sensing properties [23,32]. Evaluation of the response and recovery times of the AlN sensors are the next important parameters. Since the hexagonal plate-like AlN possessed the highest sensitivity and adequate stability, it was used as the representative morphology. The response and recovery times (Fig. 5(d)) of the hexagonal plate-like AlN was 40 and 82 s, respectively, which suggested a rapid response and recovery times for H₂ detection. A comparison with previous works on high-temperature hydrogen gas sensing is shown in Table 2. The AlN samples from this study could detect a relatively low concentration of hydrogen gas with high response value, although the sensing response and recovery times can be improved.

Table  2.  Comparison of high-temperature hydrogen gas-sensing materials

Material	Morphology	Temperature / °C	H2 concentration / ppm	Response	(tres / s)/(trec / s)
Pt-decorated SiC [33]	Nanowire	600	20000	1.25 (Rg/Ra)	3/45
GaN [6]	Nanorice	500	750	101.5 (Ra/Rg)	22/26
TiO2 [34]	Thin film	500	50	50.4 (Ra/Rg)	10/5
GaN [35]	Thin film	310	10000	325 (Ra/Rg)	9/15
SnO2/Y-GZO [36]	Nanocomposite	600	4000	1.04 (Ra/Rg)	210/300
AlN	Hexagonal plate	500	750	58.7 (Rg/Ra)	40/82
Note: Y-GZO represents yttrium-doped gadolinium zirconate.

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The gas-sensing performance showed that the controllable morphology of AlN behaved like a p-type semiconductor in which the resistance increased in target reductive gas. Accordingly, a possible gas-sensing mechanism is proposed to clarify the gas-sensing property of the AlN-based sensor. The hexagonal plate-like morphology was used as the example as it had the best sensing response. As illustrated in Fig. 6, upon heating in air at 500°C, oxygen is adsorbed to the outer surface of the particles, leading to the formation of oxygen species in the form of O²⁻. The ion sorption of oxygen in p-type semiconductor creates hole accumulation layers (HALs), which are known to be highly conductive, at the surface of the particle [37]. When H₂ gas was exposed to the p-type semiconductor-based gas sensor, it will react with O²⁻ and then the released electrons are pushed back into the insulating core, which results from thinner HALs and causes an increase in the sensor resistance.

Figure  6.  Simplified illustration of AlN gas sensing materials towards hydrogen gas.

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Additionally, the reason for the superior hydrogen gas-sensing properties of the hexagonal plate-like morphology of AlN compared with the morphologies was investigated. As previously discussed, the AlN with hexagonal plate-like morphology may have an exposed surface facet of (001), which may also be favorable for hydrogen gas adsorption. Strak and co-workers [38] revealed that the N₂ and H₂ gases were strongly adsorbed to a clean (001) exposed surface plane of hexagonal wurtzite-type AlN. They suggested that N and H adatoms were attached to the Al-terminated surface with very low adsorption energy, indicating their favorability. In another study, upon the introduction of H₂ molecules to the surface of AlN with a (001) plane, the H₂ molecules were adsorbed on top of the Al site, with a binding energy of 0.212 eV/H₂ and a distance of 0.2179 nm [39]. These findings strongly support our hypothesis, and thus the possible mechanism may also be applicable for these morphologies.

4.   Conclusion

High purity AlN was successfully prepared with rod-like, nest-like, and hexagonal plate-like morphologies by nitridation of morphologically-controlled hydroxide base precursors. The hexagonal plate-like AlN morphology exhibited the best gas-sensing properties of 750 ppm of H₂ gas at 500°C compared with the other morphologies, indicated by the highest response (S) of 58.7 and a quick response/recovery time of 40 s/82 s. The improved gas-sensing properties of the hexagonal plate-like morphology of AlN may be attributed to the (001) surface facet of AlN, which was effective for hydrogen gas adsorption. These results suggest that AlN with a unique structure has potential as a novel H₂ gas-sensing material that can be operated at high temperature.

Acknowledgements

This work was financially support by the Japan Society for the Promotion of Science (JSPS) Grant-in-Aid for Scientific Research (KAKENHI) (No. 20H00297 and Innovative Areas No. JP16H06439) and the Cooperative Research Program of Dynamic Alliance for Open Innovations Bridging Human, Environment and Materials in the “Network Joint Research Center for Materials and Devices”.