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 , 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.
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  zone axis. These results track well with a previous study by Liu et al. . 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).
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 2p3/2 located at 74.4 eV and a lower intensity sub-peak Al 2p1/2 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 2p1/2 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.
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
Table 1. O and N amounts in the AlN samples
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 (Rg/Ra) 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.
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 tres and recovery time trec 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.
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 . 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.
Material Morphology Temperature / °C H2 concentration / ppm Response (tres / s)/(trec / s) Pt-decorated SiC  Nanowire 600 20000 1.25 (Rg/Ra) 3/45 GaN  Nanorice 500 750 101.5 (Ra/Rg) 22/26 TiO2  Thin film 500 50 50.4 (Ra/Rg) 10/5 GaN  Thin film 310 10000 325 (Ra/Rg) 9/15 SnO2/Y-GZO  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.
Table 2. Comparison of high-temperature hydrogen gas-sensing materials
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 . 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.
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  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 . These findings strongly support our hypothesis, and thus the possible mechanism may also be applicable for these morphologies.
Morphology control of aluminum nitride (AlN) for a novel high-temperature hydrogen sensor
23 March 2020
Revised: 3 July 2020
Accepted: 13 July 2020
Available online: 15 July 2020
Abstract: Hydrogen is a promising renewable energy source for fossil-free transportation and electrical energy generation. However, leaking hydrogen in high-temperature production processes can cause an explosion, which endangers production workers and surrounding areas. To detect leaks early, we used a sensor material based on a wide bandgap aluminum nitride (AlN) that can withstand a high-temperature environment. Three unique AlN morphologies (rod-like, nest-like, and hexagonal plate-like) were synthesized by a direct nitridation method at 1400°C using γ-AlOOH as a precursor. The gas-sensing performance shows that a hexagonal plate-like morphology exhibited p-type sensing behavior and showed good repeatability as well as the highest response (S = 58.7) toward a 750 ppm leak of H2 gas at high temperature (500°C) compared with the rod-like and nest-like morphologies. Furthermore, the hexagonal plate-like morphology showed fast response and recovery times of 40 and 82 s, respectively. The surface facet of the hexagonal morphology of AlN might be energetically favorable for gas adsorption–desorption for enhanced hydrogen detection.