The Fe_0.2Ni_0.8WO₄ powders were synthesized using a solid-state reaction. High purity Fe₂O₃ (Kojundo Chemical Lab, 99.9%), NiO (Kojundo Chemical Lab, 99.97%), and WO₃ (Kojundo Chemical Lab, 99.9%) powders were mixed in a 0.1: 0.8: 1 mole ratio in an alumina mortar. The mixed powders performed a heat treatment in an electric box furnace at 1,050°C for 12 h. To synthesize the 0.05% of Na doped Fe_0.2Ni_0.8WO₄, we add 0.025% mole ratio of the Na₂CO₃ powder (Kojundo Chemical Lab, 99%), as following the equation 0.025: 0.1: 0.8: 1 = Na₂CO₃: Fe₂O₃: NiO: WO₃.

The hydrothermally synthesized Na-Fe_0.2Ni_0.8WO₄ is used Na₂WO₄.2H₂O, FeCl₂, and NiCl₂.6H₂O precursor and totally dissolved in Di-water. After that, a Teflon container containing the substance was performed hydrothermal synthesis using an autoclave under 180°C for 6 h. The synthesized wet samples were carried out overnight in the dry process in the vacuum oven at 60°C. Sequentially, dried powders were calcinated at 600°C for 1 h.

Morphological measurements were conducted by scanning electron microscopy (SEM, JEOL-7800F, JEOL Ltd.). The crystal structure characterization was performed using X-ray diffraction (XRD, Smart Lab, Rigaku) with Cu Kα radiation. Chemical bonding states were analyzed via X-ray photoelectron spectroscopy (XPS, K-alpha, Thermo Fisher Scientific Co.) Raman spectroscopy (LabRam Aramis, Horiba Jovin Yvon) and FT-IR measurement (Invenio, Brucker) were utilized to confirm the vibration mode of NiWO₄.

A 2-probe electrode configuration was employed for the gas sensing analysis. Fe_0.2Ni_0.8WO₄, Na- Fe_0.2Ni_0.8WO₄, and Hydrothermal synthesized Na-Fe_0.2Ni_0.8WO₄ properties put on the gold electrodes positioned on an alumina substrate with 0.2 mL of ethanol on synthesized powders and physically pressed the powder to fix the powder on the substrate. The gas-sensing performance of the fabricated sensors was evaluated within a custom-built chamber equipped with mass flow controllers, maintaining a fixed flow rate of 500 standard cubic centimeters per minute using air as the carrier gas. The sensors were exposed to target gas concentrations ranging up to 20 ppm for 100 s, followed by a recovery period in the air for 200 s at 30°C. The resistance values in air (R_a) and upon exposure to the target gases (R_g) were recorded, and the sensor response (R_g/R_a) was determined by calculating the ratio of resistance values in ambient air to under target gas exposure conditions. Gas sensing measurements were performed for various gases, including NH₃, H₂S, NO₂, SO₂, Benzene, p-Xylene, HCHO, and Acetone.

The morphological shape of Fe_0.2Ni_0.8WO₄, Na- Fe_0.2Ni_0.8WO₄ (Na-Fe_0.2), and Na-Fe_0.2Ni_0.8WO₄ hydrothermal (Na-Fe_0.2-Hydro) synthesized powders is shown in SEM and EDS results in Figure 1A. EDS results show the evenly mixed elements such as O, Ni, Fe, and W. The surface area measured by Krypton Brunauer-Emmett-Teller (BET) analysis exhibits the 0.093 m²/g of Fe_0.2Ni_0.8WO₄ and 0.238 m²/g of Na- Fe_0.2Ni_0.8WO₄ through the solid-state synthesized method. However, the hydrothermally synthesized Na-Fe_0.2Ni_0.8WO₄ powder shows a surface area more than 67 times larger (15.970 m²/g), which is related to the smaller particle size of Na-Fe_0.2-Hydro sample. The higher surface area of Na-Fe_0.2-Hydro samples is fabricated by the hydrothermal synthesis method, which makes it possible to precise control over compositions through liquid or multiphase reactions. Hydrothermal synthesis is a widely used solution-based method for preparing nanomaterials across a broad temperature range, allowing control over material morphology, including the synthesis of nanoparticles, nanorods, nanotubes, hollow nanospheres, etc., (Darr et al., 2017) A detailed analysis of the cationic ratio using ICP analysis for all samples, as shown in Table 1, indicates an approximately 0.2 Fe and 0.8 Ni ratio with W deficiency. The Na concentration shows 0.014 at% in Na-Fe_0.2-Hydro and 0.059 at% in Na-Fe_0.2 samples, respectively. Considering the higher surface area manifested by using hydrothermally synthesized powder, it commonly shows higher microscopic and induces crystal structure distortion from intrinsic composition, which drives the higher gas absorption site for higher gas sensitivity.

To understand the detail of structure change depending on the N a doping and the nano-sized structure manipulation process, we performed crystal structure analysis for all samples to identify the effect of the Na dopant and increased surface area such as XRD, Raman, and PL spectroscopy. Figure 2A exhibits the powder XRD pattern, compared with the reported NiWO₄ XRD results. All samples exhibit the single phase of XRD patterns without impurities as the NiWO₄ reference (Han et al., 2024; Suh et al., 2024). Thus, the doping elements, such as Fe and Na, are well-dissolved in the NiWO₄ crystal structure. The Raman spectroscopy results in Figure 2B demonstrate the different bonding vibration modes between cation-oxygen anions. The detailed view on the left side of the inset figure describes the crystal structure of Na-Fe_0.2Ni_0.8WO₄. The P2/c space group of the monoclinic wolframite structure NiWO₄ is composed of corner-shared [NiO₆] and [WO₆] octahedral structures (Han et al., 2024; Suh et al., 2024), which have substitutional dopant Fe at the Ni site, and the Na are located in the interstitial free space between each of octahedral structures. Among the diverse vibration modes, two Raman active modes were observed: the highest intensity peaks at 880–892 cm⁻¹ ranged correspond to the symmetric vibrations of W–O bonding for symmetric vibrations, whereas the Raman peaks near 350–362 cm⁻¹ are the symmetric vibrations of [NiO₆] or [FeO₆] octahedral structures. The red-shift when interstitial Na doping on Fe_0.2Ni_0.8WO₄ indicates the lower bonding strength between cations and O via partially located Na in the free space of crystal structure sequentially the fabricated larger surface area also causes a red peak shift compared to the Fe_0.2Ni_0.8WO₄ samples that carried the local lattice structure distortion such as lower crystallinity [NiO₆] and [WO₆] octahedral structures from the higher surface area than intrinsic Fe_0.2Ni_0.8WO₄ powders. Although hydrothermally synthesized MO powders have tended to fabricate the higher crystallographic defects from significantly higher surface area and generate external carriers (called F-center), the Fe_0.2Ni_0.8WO₄, (Na-Fe_0.2), and (Na-Fe_0.2-Hydro) powders exhibit the negligible intensity in the 300–800 nm wavelength range for all samples (as shown in Figure 2C), demonstrating the ignorable oxygen vacancy in the crystal structure, which is matched with the XPS results.

The XPS results in Figures 3A–E show the chemical states of each element in the Na-Fe_0.2-Hydro, Na-Fe_0.2, and Fe_0.2Ni_0.8WO₄ powders. The Na 1s binding energies are 1,071.6 eV for Na-Fe_0.2-Hydro and 1,071.4 eV for Na-Fe_0.2 samples, similar to unionized Na states and lower than the binding energy of Na₂O. Regarding the binding energies of the cations, specifically Fe 2p, Ni 2p, and W 4f, are shown lower binding energies at the maximum intensity of each peak when Na is doped at the interstitial site compared to the intrinsic Fe_0.2Ni_0.8WO₄ and Na-Fe_0.2Ni_0.8WO₄ structure due to the electron doping to Fe_0.2Ni_0.8WO₄ structure via the Na dopant acting as electron donor.

However, the similar binding energies of Na-Fe_0.2-Hydro and Fe_0.2Ni_0.8WO₄ samples or blue shift than the Na-Fe_0.2 powders indicate an offset between the chemical states due to electron donation and surface effects that can tune the cations charge state change, which presents the higher binding energy shifting on cations charge state owing to the larger surface area (Tsunekawa et al., 2000). Additionally, the O 1s results show a symmetrical peak shape near 530 eV, which corresponds to a negligible oxygen vacancy effect in all samples, as measured by the PL results. Thus, the difference in gas sensing performance among Na-Fe_0.2-Hydro, Na-Fe_0.2, and Fe_0.2Ni_0.8WO₄ powders predominantly depends on the charge states of the cations and the crystal structure distortion, which are induced by the doping and the formation of a larger surface area.

The gas sensing performance is illustrated in Figures 4A, B. The gas sensing measurements for analyte gases at 20 ppm were conducted at room temperature using Au electrode coated Al₂O₃ substrate and sample powders are fixed by physically pressed on the substate with few drops of ethanol. The Fe_0.2 sample presented almost none of the gas response under all gases such as NH₃, H₂S, NO₂, SO₂, Benzene, p-Xylene, HCHO, and Acetone, which presented a 10–20 range of gas sensing response and vague gas selectivity. However, both Na-Fe_0.2-Hydro and Na-Fe_0.2 samples exhibit a reasonable gas sensing response except under NO₂ and H₂S, which maintain negligible gas sensing response. The emergence of gas sensing functionality on the Na-Fe_0.2 indicates that the incorporation of Na plays a crucial role in activating the gas adsorption site by carrier doping, which follows the interstitially located alkali metal acting as a carrier donor to matrix materials. On the other hand, the Na-Fe_0.2-Hydro sample demonstrates a remarkable response (R_g/R_a) at room temperature. Among the improved gas sensing response in the Na-Fe_0.2-hydro sample, over 40 gas sensing responses under NH₃, which is more than four times higher than the NH₃ gas sensing response (∼10 R_g/R_a) observed in the Na-Fe_0.2 sample. This substantial improvement in the gas response can be attributed to the synergistic effects of carrier doping and the distorted crystal structure through the hydrothermal synthesis method. The response and recovery times, shown in Figure 2C, indicate that the sensor quickly reaches 90% of its maximum response within 23 s after the start of gas exposure. This rapid response time is essential for real-time monitoring and quick detection of gas leaks. Additionally, the recovery time is 10 s, returning to 10% of its response after the gas exposure is turned off. This quick recovery is crucial for ensuring that the sensor can be reused for continuous monitoring without long delays between measurements.

These results highlight the effectiveness of Na doping and the hydrothermal synthesis method in significantly enhancing the gas sensing performance of Fe_0.2Ni_0.8WO₄. Furthermore, considering the higher gas physisorption behavior on MO shown at room temperature due to the lower activation energy of the chemical reaction between the gas and the MO surface, the weak oxygen vacancy effect on both the solid-state synthesized and hydrothermally synthesized samples suggests that the gas adsorption site is contributed by the cations. This is generated by the modified crystal structure and cation charge states, which are key factors contributing to the improved sensitivity and selectivity toward NH₃ gas sensing. The rapid response and recovery times further demonstrate the potential of these materials for practical applications in real-time gas sensing at room temperature for conventional MO composition.