Enhanced NH3 Sensing Performance of Mo Cluster-MoS2 Nanocomposite Thin Films via the Sulfurization of Mo6 Cluster Iodides Precursor

The high-performance defect-rich MoS2 dominated by sulfur vacancies as well as Mo-rich environments have been extensively studied in many fields, such as nitrogen reduction reactions, hydrogen evolution reactions, as well as sensing devices for NH3, which are attributed to the under-coordinated Mo atoms playing a significant role as catalytic sites in the defect area. In this study, the Mo cluster-MoS2 composite was creatively synthesized through a one-step sulfurization process via H2/H2S gas flow. The Mo6 cluster iodides (MIs) coated on the fluorine-doped tin oxide (FTO) glass substrate via the electrophoretic deposition method (i.e., MI@FTO) were used as a precursor to form a thin-film nanocomposite. Investigations into the structure, reaction mechanism, and NH3 gas sensing performance were carried out in detail. The results indicated that during the gas flowing, the decomposed Mo6 cluster iodides played the role of template and precursor, forming complicated Mo cluster compounds and eventually producing MoS2. These Mo cluster-MoS2 thin-film nanocomposites were fabricated and applied as gas sensors for the first time. It turns out that after the sulfurization process, the response of MI@FTO for NH3 gas increased three times while showing conversion from p-type to n-type semiconductor, which enhances their possibilities for future device applications.


Introduction
Graphene-like molybdenum disulfide (MoS 2 ), with its well-known two-dimensional (2D) layered "sandwich" structure, has attracted significant attention over the years due to its unique electronic, optical, and chemical properties [1][2][3][4], which can be exploited in transistors [5,6], solar cells [7,8], catalysts [9][10][11], and sensors [12][13][14]. In this "sandwich" structure, one layer of molybdenum atoms is sandwiched between two layers of sulfur atoms. In the domain of catalysis, such MoS 2 three-layer structures have been widely investigated for their catalytic activity, which is strongly dependent on the edge sites [15]. To enhance the catalytic performance, researchers studied the synthesis of defect-rich MoS 2 , in which a sulfur vacancy that created the edge sites plays an important role [16][17][18]. Indeed, at the sulfur vacancy sites, the surrounding under-coordinated Mo atoms introduce gap states that facilitate N 2 absorption or hydrogen bonding, which improves the performance of the nitrogen reduction reaction (NRR) [19], hydrogen evolution reaction (HER) [20,21], as well as sensing [22][23][24].
With the development of defect engineering, researchers have devoted themselves to the synthesis of S vacancies defect-rich MoS 2 by using the hydrothermal route [25,26] and the chemical vapor deposition (CVD) method [27], which can control the sulfurmolybdenum ratio during the reaction to create defects. Alternatively, 2D-MoS 2 decorated with Mo clusters has started to attract some attention as a catalyst for the generation of acetylene via the CO hydrogenation reaction [28]. A higher electrocatalytic NRR activity has been theoretically proposed using Mo x clusters (x = 1~11) supported on MoS 2 [29]. Moreover, Jin et al. demonstrated the in situ fabrication of Mo 6 S 6 -nanowire-terminated edges in the monolayer MoS 2 [30] and developed a mass transport method for the formation of Mo chains induced by Mo clusters along the sulfur vacancy lines, aimed at tuning the defects and, thus, the materials' structure-property relationship in a well-controlled manner [31]. However, only a few studies have been conducted to generate MoS 2 by sulfuring chalcogenide clusters, such as Mo 6 S 2 I 8 , as precursors [32,33].
In this study, we creatively pioneered the efficient synthesis route of Mo cluster-MoS 2 thin-film nanocomposites by a sulfurization process using Mo 6 cluster iodides as the precursors. Mo 6 cluster units are inorganic molecules containing six Mo atoms forming an octahedral skeleton that are connected by direct metal-metal bonds and are bonded with organic or inorganic ligands to stabilize the metal core. The interest in Mo 6 clustersbased compounds started with the discovery of the superconductivity in the Chevrel phases [34]. In recent decades, the optical properties and the chemical activity of Mo 6 clusters derived from their unique electronic structure [35][36][37] have been investigated and developed in many fields [38], such as luminescent [39][40][41], energy [42], and catalysis applications [43]. Among them, Nguyen et al. successfully developed a general process to coat metal-cluster-unit-based thin films, for example, Mo 6 cluster iodides (Cs 2 Mo 6 I 14 , CMI; more specifically, Cs + 2 [{Mo 6 I i 8 }I a 6 ] 2-, where i and a represent the inner and apical iodide ligands, respectively, that surround the Mo 6 octahedra) on a fluorine-doped tin oxide (SnO 2 :F, FTO) glass substrate by the electrophoretic deposition (EPD) method [44]. This EPD method maintained the optoelectronic properties of the initial CMI raw powder, which will significantly promote the fabrication of Mo 6 -cluster-based optoelectronic devices in the future. Moreover, Renaud et al. recently proved that the Mo 6 cluster iodides are a new class of ambipolar materials [45], and the {Mo 6 I i 8 }-based thin films (noted MI@FTO) fabricated via the EPD method have shown a significant application potential as a photoabsorber in an all-solid solar cell in the future. Moreover, Guy et al. used various Mo 6 clusters powders (Cs 2 Mo 6 CI 14 , (TBA) 2 Mo 6 Br 14 , etc.) as precursors to synthesize Mo x N y nanomaterials with different compositions (Mo 2 N and Mo 5 N 6 ) [46]. In contrast to the conventional synthesis using MoO 3 as a precursor, these Mo x N y products were prepared at lower temperatures and showed significant catalytic performances for the water-gas shift reaction.
In a previous study, we successfully produced MoS 2 from Mo 6 Br 12 powder as a precursor via two sulfurization processes [47]. Using the gas flow method, the sulfurization reaction occurred at 250 • C, but the product was a micron-sized amorphous MoS 2 powder. This reaction process still needs to be developed and the reaction mechanism better understood. Therefore, in this study, we decided to use this gas flow method combined with the EPD technology to establish a one-step sulfurization method at low temperature to fabricate a Mo cluster-MoS 2 thin film nanocomposite by using pre-prepared MI@FTO coatings as the precursor. The morphology and the reaction mechanism were investigated in detail. For future device applications, the obtained thin-film samples were carefully evaluated for their NH 3 gas sensing performance. The main results are reported and discussed.

Chemicals
The synthesis path of the Cs 2 Mo 6 I 14 initial powders via the solid-state method [48,49] and the MI@FTO films fabricated by using the EPD method [50] have been well established in our previous studies.
Briefly, Cs 2 Mo 6 I 14 was synthesized by reacting CsI and MoI 2 mixed and heated at 700°C for 4 days in a silica tube sealed under vacuum. For the EPD process, the Cs 2 Mo 6 I 14 raw powder was dissolved in acetone (reagent-grade chemicals, 99.5%) at the concentration of 5 g per liter. The solution was then filtered using a 0.2 µm filter to remove any insoluble impurities. At the same time, the FTO glass substrate (Kenis Corp., Osaka, Japan) was cut into a 25 mm × 10 mm piece and washed in detergent and acetone for 30 min in an ultrasonic bath. Finally, the FTO substrate was used as the anode and immersed in a Cs 2 Mo 6 I 14 cluster solution. The coating process was performed at 10 V for 5 min. Cs + cations and [{Mo 6 I i 8 }X a 6 ] 2− (X = I, OH) were deposited on the cathode and the FTO glass anode, respectively [45]. It should be noted that during the EPD coating process, the [{Mo 6 I i 8 }I a 6 ] 2− cluster molecules will lose two iodide apical ligands, which were replaced by water ligands, as confirmed by the XPS result in a previously published report [45]. Accordingly, the final composition of the MI@FTO film in this study is described as [{Mo 6

Sulfurization Process
All the sulfurization reactions were carried out in a tubular furnace with a flowing H 2 /H 2 S (90%/10%) mixture gas (99.9%, Air Liquid). The MI@FTO sample was initially placed in an aluminum boat, then into the furnace. The reaction chamber was purged with N 2 gas for 30 min before the start of the reaction to ensure removal of the air, and the furnace was always at 1 atmospheric pressure. The H 2 /H 2 S gas was then introduced at the rate of 15 mL/min in the temperature range of 250-450 • C, and the main reaction time was 1 h for all the MI@FTO films with the increasing rate of 5 degrees per minute. The H 2 S acted as the sulfur source for the sulfurization reaction and the H 2 acted as the reducing atmosphere to protect the cluster film from oxidation. An illustration of the EPD and the sulfurization process for the MI@FTO is shown in Figure 1a. According to the reaction temperature, the final products were denoted as MI-250, MI-300, etc. The reaction time was labeled for the specific sample, which is the extended reaction times, e.g., MI-350-3h.

Fabrication and Measurement of Gas Sensor
The fabrication and measurement of the MI@FTO gas sensor are illustrated in Figure 1b. This measurement was conducted by a chemiresistive gas sensor, which consisted of two electrodes connected to the sensing material. The change in resistance or current of the sensor device was measured in order to analyze the concentration of the target gas.
First, a 46 µm Au wire covered the MI@FTO surface, and the covered area was used for the performance measurement. An approximately 50 nm Au layer was deposited on the exposed sample surface by a vacuum evaporator (VG Microtech E6700S, Newhaven, UK) to improve the surface conductivity. A Cu wire was connected from the Au-deposited area to the ampere-meter (Graphtec midi Logger GL240, Graphtec Corporation, Yokohama, Japan) to monitor and record the real-time current. The MI@FTO was placed in an alumina tube with flowing NH 3 gas (99.9%, Sumitomo Seika Chemicals, Tokyo, Japan) for the measurements.

Fabrication and Measurement of Gas Sensor
The fabrication and measurement of the MI@FTO gas sensor are illustrated in Figure  1b. This measurement was conducted by a chemiresistive gas sensor, which consisted of two electrodes connected to the sensing material. The change in resistance or current of the sensor device was measured in order to analyze the concentration of the target gas.
First, a 46 μm Au wire covered the MI@FTO surface, and the covered area was used for the performance measurement. An approximately 50 nm Au layer was deposited on the exposed sample surface by a vacuum evaporator (VG Microtech E6700S, Newhaven, UK) to improve the surface conductivity. A Cu wire was connected from the Au-deposited area to the ampere-meter (Graphtec midi Logger GL240, Graphtec Corporation, Yokohama, Japan) to monitor and record the real-time current. The MI@FTO was placed in an alumina tube with flowing NH3 gas (99.9%, Sumitomo Seika Chemicals, Tokyo, Japan) for the measurements.

Characterization
The structure and morphology of the thin-film samples, before and after sulfurization, were investigated by the grazing-incidence X-ray diffraction technique (GIXRD,

Characterization
The structure and morphology of the thin-film samples, before and after sulfurization, were investigated by the grazing-incidence X-ray diffraction technique (GIXRD, SmartLab, Rigaku Corp., Tokyo, Japan) at room temperature using Cu Kα radiation (λ = 1.5406 Å) at 50 mA and 40 kV power, and the incident beam was set at fixed critical w angles (0.2 • ≤ w ≤ 0.8 • ). The layered structures were analyzed by micro-Raman spectroscopy (HORIBA T64000, Kyoto, Japan) using a 515 nm laser source. The surface morphology and the elemental composition mapping of the thin-film sample were analyzed by field emission scanning electron microscopy (FE-SEM, Hitachi SU4800, Tokyo, Japan) at 10-15 kV coupled with an energy-dispersive X-ray (EDX) analysis device, and high-resolution observations of the powder scratched from the thin-film sample were performed by a high-resolution transmission electron microscope (HRTEM, JEOL 2100F, Tokyo, Japan) equipped with an EDX analysis device. The electron binding energy spectra of the thin-film samples were measured by X-ray photoelectron spectroscopy (XPS, ULVAC Quantum-2000, Chigasaki, Japan) using Al Kα radiation at 15 kV and 50 W and a take-off angle of 45 • . All the binding energies were calibrated versus the C 1s peak of the adventitious carbon at 285 eV.

Structure and Morphology
The GIXRD results in Figure 2a show the patterns of the initial MI@FTO and sulfurized samples. The wide peak observed around 9-12 degrees and the sharp crystal peaks at the higher 2θ angle in the initial MI@FTO pattern can be identified as the amorphous Mo 6 cluster iodide film and SnO 2 :F from the FTO substrate, respectively, which is consistent with our previous studies [45,51]. MoS 2 and SnS are found as the final products during this sulfurization process (see MI-450 pattern in Figure 2a). Indeed, MoS 2 is the final product of the sulfurization reaction starting from the MI@FTO thin-film precursor occurring from 250 • C, and this result is consistent with the powdered Mo 6 Br 12 cluster precursor [47]. The wide peak initially measured around 9-12 degrees, due to the Mo 6 cluster iodides, has shifted to 14 degrees upon increasing the temperature. This indicates that the Mo 6 cluster iodide has entirely disappeared in favor of MoS 2 . The presence of SnS crystals over 350 • C is unexpected. The SnO 2 layer between the Mo 6 cluster iodides and the SiO 2 glass layer plays the role of the Sn source leading to SnS crystals at the end of the process. The pattern of MI-300 shows complex peaks of multiple phases. Considering that SnO 2 may have been involved in the reaction, the product might contain the Mo x S y complex [52], the Sn x S y complexes [53], and also the Sn-Mo cluster complexes [54]. Additionally, the involvement of an O source derived from SnO 2 may also have contributed to the formation of metal oxysulfide complexes [55]. In general, the FTO substrates are used as substrates for the synthesis of tin sulfides using the CVD method because the SnO 2 surface is suitable for the growth of tin sulfides and does not participate in the reaction at such temperatures [56,57]. This fact implies that the excellent catalytic properties of the Mo 6 cluster iodides promote the SnO 2 layer in order to participate in the sulfurization reaction [57].  Figure 3a shows pictures of the initial MI@FTO and sulfurized samples at different temperatures. The initial MI@FTO film is a translucent orange film. During the sulfurization process, the color of the MI@FTO films undergo a change from orange to dark brown (MI-250) to dark gray with a metallic luster (from MI-300 to MI-450), which indicates that the sulfurization reaction occurs beyond 250 °C. The dark gray color of the metallic luster sample must also correspond to the MoS2 and SnS phases. The surface morphology of the initial MI@FTO observed by FE-SEM and illustrated in Figure 3b shows isolated [{Mo6I i 8}X a 6] 2− (X = I, OH) nanoparticles with diameters around 20-50 nm up to 300 °C. As the temperature increases to 350 °C, the nanoparticles are sintered into a continuous porous structure. In contrast to the well-grown SnS crystal, the final product of the reaction, only one wide diffraction pattern present in the XRD patterns was used to confirm the formation of MoS 2 . In the Raman spectrum (see Figure 2b), the MoS 2 layered structure peaks are observed around 375 cm −1 and 400 cm −1 , which represent the in-plane vibration (E 2g 1 ) and out-of-plane vibration (A 1g ), respectively, beyond 300 • C. It is indicated that the Mo 6 cluster iodides, used as a precursor, are gradually sulfurized into the layered MoS 2 structure, which is consistent with the wide peak around 14 degrees corresponding to the MoS 2 (002) surface in the XRD pattern. Figure 3a shows pictures of the initial MI@FTO and sulfurized samples at different temperatures. The initial MI@FTO film is a translucent orange film. During the sulfurization process, the color of the MI@FTO films undergo a change from orange to dark brown (MI-250) to dark gray with a metallic luster (from MI-300 to MI-450), which indicates that the sulfurization reaction occurs beyond 250 • C. The dark gray color of the metallic luster sample must also correspond to the MoS 2 and SnS phases. The surface morphology of the initial MI@FTO observed by FE-SEM and illustrated in Figure 3b shows isolated [{Mo 6 I i 8 }X a 6 ] 2− (X = I, OH) nanoparticles with diameters around 20-50 nm up to 300 • C. As the temperature increases to 350 • C, the nanoparticles are sintered into a continuous porous structure.  Figure 3a shows pictures of the initial MI@FTO and sulfurized samples at different temperatures. The initial MI@FTO film is a translucent orange film. During the sulfurization process, the color of the MI@FTO films undergo a change from orange to dark brown (MI-250) to dark gray with a metallic luster (from MI-300 to MI-450), which indicates that the sulfurization reaction occurs beyond 250 °C. The dark gray color of the metallic luster sample must also correspond to the MoS2 and SnS phases. The surface morphology of the initial MI@FTO observed by FE-SEM and illustrated in Figure 3b shows isolated [{Mo6I i 8}X a 6] 2− (X = I, OH) nanoparticles with diameters around 20-50 nm up to 300 °C. As the temperature increases to 350 °C, the nanoparticles are sintered into a continuous porous structure. At 400 • C, the surface morphology becomes nanoflaked with a surface area around 100 nm 2 . At the same time, the isolated bulk crystal around 500 nm in size is also observed on the surface, as shown in Figure 4a,b. When the temperature increases to 450 • C, the surface becomes bumpy. Isolated crystals and cracks can also be observed on the surface, as shown in Figure 4c,d.
At 400 °C, the surface morphology becomes nanoflaked with a surface are 100 nm 2 . At the same time, the isolated bulk crystal around 500 nm in size is also on the surface, as shown in Figure 4a,b. When the temperature increases to 45 surface becomes bumpy. Isolated crystals and cracks can also be observed on th as shown in Figure 4c,d. These results illustrate that there are two reactions occurring during the sulf of MI@FTO, i.e., one starting at 250 °C, when the Mo6 clusters are sulfurized, l MoS2; one starting at 350 °C, not only when the Mo6 cluster sulfurization reactio but also when the Mo6 clusters catalyze the sulfurization of SnO2 into SnS.

Mechanism of the MI@FTO Sulfurized into MoS2 and SnS crystal Growth Hypoth
In order to clarify the sulfurization mechanism of the Mo6 cluster iodides, th powder samples were scratched from the films for HRTEM observations (see Fig order to exclude interference from the FTO substrate and provide an easy samp ration. It is noteworthy that the EDX analysis in Figure 5a demonstrates that the MI tains Mo, S, O, and I elements, but not the Sn element, which means that SnO2 volved in the reaction below 250 ℃. The high-resolution analysis of the O elem nanoparticle revealed that the Mo is oxidized. In addition to the MoS2 layer stru MoO3 crystal structure is also observed in Figure 5b. Moreover, the electron en spectroscopy (EELS) analysis in Figure 5c revealed that there is a high concen Mo and S elements in the shell of the MI-250 nanoparticles but just a little oxyg versely, the core contains a small amount of S and a high concentration of the M elements. This fact shows that, first, the film-bound water during the EPD proce serves as the O source, suggesting that the initial Mo6 cluster iodide is probably into MoO3. The H2S gas then serves as the S source for the subsequent sulfuri produce MoS2 on the nanoparticle surface.
It has been reported that MoO3 can play the role of a precursor on the FTO s at low temperatures to be sulfurized into MoS2. On the other hand, SnO2 in the F ing is stable and does not participate in the reaction even when the temperature These results illustrate that there are two reactions occurring during the sulfurization of MI@FTO, i.e., one starting at 250 • C, when the Mo 6 clusters are sulfurized, leading to MoS 2 ; one starting at 350 • C, not only when the Mo 6 cluster sulfurization reaction occurs, but also when the Mo 6 clusters catalyze the sulfurization of SnO 2 into SnS.

Mechanism of the MI@FTO Sulfurized into MoS 2 and SnS crystal Growth Hypothesis
In order to clarify the sulfurization mechanism of the Mo 6 cluster iodides, the MI-250 powder samples were scratched from the films for HRTEM observations (see Figure 5) in order to exclude interference from the FTO substrate and provide an easy sample preparation.
It is noteworthy that the EDX analysis in Figure 5a demonstrates that the MI-250 contains Mo, S, O, and I elements, but not the Sn element, which means that SnO 2 is not involved in the reaction below 250°C. The high-resolution analysis of the O element in the nanoparticle revealed that the Mo is oxidized. In addition to the MoS 2 layer structure, the MoO 3 crystal structure is also observed in Figure 5b. Moreover, the electron energy loss spectroscopy (EELS) analysis in Figure 5c revealed that there is a high concentration of Mo and S elements in the shell of the MI-250 nanoparticles but just a little oxygen. Conversely, the core contains a small amount of S and a high concentration of the Mo and O elements. This fact shows that, first, the film-bound water during the EPD process [45,58] serves as the O source, suggesting that the initial Mo 6 cluster iodide is probably oxidized into MoO 3 . The H 2 S gas then serves as the S source for the subsequent sulfurization to produce MoS 2 on the nanoparticle surface.
It has been reported that MoO 3 can play the role of a precursor on the FTO substrates at low temperatures to be sulfurized into MoS 2 . On the other hand, SnO 2 in the FTO coating is stable and does not participate in the reaction even when the temperature increases to 700 • C; thus, the final product, the MoS 2 sulfurized from the MoO 3 precursor, would not contain SnS. [59]. This fact confirms that the MoO 3 used as a mid-product in this study is not the key material that catalyzes the SnO 2 :F layer on the FTO substrate to generate SnS under the H 2 /H 2 S.
Our study is the first report showing that Mo 6 cluster iodides deposited on FTO substrates can generate SnS at 350 • C without external Sn sources, indicating that the Mo 6 cluster iodides have a catalytic role. Although the reaction mechanism is still unclear, we attempted to provide some explanations for the growth of the SnS crystals. Figure 6a shows the FE-SEM observation focusing on the crack of the thin-film samples at different temperatures. The main morphology starting from the Mo 6 cluster iodide nanoparticles is consistent with the observation shown in Figure 3. At high temperatures, cracks inevitably occur in the MI@FTO films, and the appearance of the cracks exposes the SnO 2 layer to the H 2 /H 2 S gas flow and concomitantly to the Mo 6 -cluster-iodide-based layer. Multiple SnS x phases are observed by heating of the blank FTO, free of Mo 6 cluster iodides, starting from 350 • C (see Figure S1). However, the SnS crystals grow well from the exposed SnO 2 surface and are catalyzed by the Mo 6 cluster iodides at MI-350, and MI-400 also shows the bulk SnS crystal around 500 nm. to 700 °C; thus, the final product, the MoS2 sulfurized from the MoO3 precursor, would not contain SnS. [59]. This fact confirms that the MoO3 used as a mid-product in this study is not the key material that catalyzes the SnO2:F layer on the FTO substrate to generate SnS under the H2/H2S. Our study is the first report showing that Mo6 cluster iodides deposited on FTO substrates can generate SnS at 350 °C without external Sn sources, indicating that the Mo6 cluster iodides have a catalytic role. Although the reaction mechanism is still unclear, we attempted to provide some explanations for the growth of the SnS crystals. Figure 6a shows the FE-SEM observation focusing on the crack of the thin-film samples at different temperatures. The main morphology starting from the Mo6 cluster iodide nanoparticles is consistent with the observation shown in Figure 3. At high temperatures, cracks inevitably occur in the MI@FTO films, and the appearance of the cracks exposes the SnO2 layer to the H2/H2S gas flow and concomitantly to the Mo6-cluster-iodide-based layer. Multiple SnSx phases are observed by heating of the blank FTO, free of Mo6 cluster iodides, starting from 350 °C (see Figure S1). However, the SnS crystals grow well from the exposed SnO2 surface and are catalyzed by the Mo6 cluster iodides at MI-350, and MI-400 also shows the bulk SnS crystal around 500 nm.
Based on this result, we extended the sulfurization time to 3 h (i.e., MI-350-3h), and the morphology of MI-350-3h still retains the shape of the nanoparticles with a diameter of about 50 nm, but these particles are sintered together. Figure 6b shows a cross-section Based on this result, we extended the sulfurization time to 3 h (i.e., MI-350-3h), and the morphology of MI-350-3h still retains the shape of the nanoparticles with a diameter of about 50 nm, but these particles are sintered together. Figure 6b shows a cross-section of the MI-350-3h film sample with the presence of Sn that highlights the migration of the Sn in the Mo layer and a very uniform doping. This indicates that the SnO 2 layer reacts with the Mo 6 cluster iodide along the layer, not only on the top surface.
The MI-350-3h powder samples were also scratched from the films for the HRTEM observations (see Figure 7). The layer spacing of the 2D structure is 0.65 nm, which is consistent with MoS 2 . In addition, the lattice spacing of the nanocrystal structure is 0.28 nm, which is consistent with the (111) plane of SnS from the XRD results. The element mapping clearly shows the uniform distribution of the Mo, Sn, and S elements in the sample. The MI-350-3h powder samples were also scratched from the films for the HRTEM observations (see Figure 7). The layer spacing of the 2D structure is 0.65 nm, which is consistent with MoS2. In addition, the lattice spacing of the nanocrystal structure is 0.28 nm, which is consistent with the (111) plane of SnS from the XRD results. The element mapping clearly shows the uniform distribution of the Mo, Sn, and S elements in the sample.
It is noteworthy that the EDX elemental quantitative analysis result shows that the content of the I element in MI-350-3h is I% = 0.07, while in the MI-250 sample, I% = 3.24. The content of the I element is much lower than those of the Mo and S elements in the sulfurized MI@FTO sample as the sulfur replaces the iodine ligand in the Mo6 cluster unit to form the MoxSy cluster. The Mo cluster structure gradually decomposes to form MoS2, which is the final product. The I2 gas generated by the decomposed Mox (1 ≤ x ≤ 6) cluster iodides reacts with H2/H2S and leaves the reaction chamber as a by-product in the gas flow. The Mo6 cluster iodides and I2 gas by-product during decomposition of the Mox cluster (1 ≤ x ≤ 6) may play an important role here to catalyze the SnO2:F layer to the SnS crystal.
The generation of halogen gas by-products during the sulfurization of the Mo6 clusters has also been reported in our previous studies [47]. By using the chemical vapor transport (CVT) method, the Mo6Br12 and sulfur powder were mixed and sealed in a   It is noteworthy that the EDX elemental quantitative analysis result shows that the content of the I element in MI-350-3h is I% = 0.07, while in the MI-250 sample, I% = 3.24. The content of the I element is much lower than those of the Mo and S elements in the sulfurized MI@FTO sample as the sulfur replaces the iodine ligand in the Mo 6 cluster unit to form the Mo x S y cluster. The Mo cluster structure gradually decomposes to form MoS 2 , which is the final product. The I 2 gas generated by the decomposed Mo x (1 ≤ x ≤ 6) cluster iodides reacts with H 2 /H 2 S and leaves the reaction chamber as a by-product in the gas flow. The Mo 6 cluster iodides and I 2 gas by-product during decomposition of the Mo x cluster (1 ≤ x ≤ 6) may play an important role here to catalyze the SnO 2 :F layer to the SnS crystal.
The generation of halogen gas by-products during the sulfurization of the Mo 6 clusters has also been reported in our previous studies [47]. By using the chemical vapor transport (CVT) method, the Mo 6 Br 12 and sulfur powder were mixed and sealed in a quartz vacuum tube for the sulfurization reaction; the Br 2 gas and MoBr 3 by-product are retained in the tube and affects the pressure of the vacuum tube reaction system, which affects the final product, MoS 2 , as a 3D-formed nanosheet or single crystal. Figure 8 shows the XPS results of four thin-film samples, i.e., MI@FTO, MI-250, MI-350, and MI-350-3h. One Mo 3d 5/2 -3d 3/2 doublet peak is identified in the Mo 3d spectrum for the initial MI@FTO with binding energies of 229.3 eV-232.4 eV, suggesting the Mo 4+ states. Moreover, two I 3d 5/2 -3d 3/2 doublet peaks in the I 3d spectrum with a ratio of 2:1 are observed, which is consistent with our earlier research and correspond to the ratio of the inner ligands to apical ligands in the [{Mo 6 [61][62][63][64]. Finally, the Mo 6+ doublet peak must be due to MoO x , which is produced from the MI@FTO and bound water. Two doublet peaks are observed in the MI-250 I 3d spectrum with a ratio of roughly 1:1. The presence of the I 3d peak is not only consistent with the EDX elemental quantitative analysis result of the MI-250 sample (see Figure 5a), but the two doublet peaks representing the inner ligand and apical ligand also suggest that MI-250 contains a minor amount of molybdenum iodide sulfide clusters [65]. The three S 2p 3/2 -2p 1/2 doublet peaks present in the MI-250 S 2p spectrum should all belong to the Mo-S compounds because MI-250 is devoid of Sn. The terminal S 2− in MoS 2 is indicated by a major doublet at the binding energies of 162.1 eV-163.3 eV. On the other hand, a minor doublet is assigned to the apical/bridge S at the slightly higher binding energies of 162.9 eV-164.2 eV, which belong to the Mo x S y phase with the Mo 5+ state. An additional terminal S 2− doublet peak is also recognized as a minor doublet observed at the lower binding energies of 161.2 eV-162.4 eV. These low-binding energies are caused by the effect of H 2 during the synthesis [66].
In the MI-350 and MI-350-3h samples, no Mo 6+ state, representing MoO x , is found in the Mo 3d spectrum. The I 3d peak also disappears, proving that MoO x as an intermediate product is completely sulfurized. The content of the Mo 5+ ions in the product significantly decreases when the sulfurization time increases from 1 to 3 h, demonstrating that the Mo x S y cluster phase gradually produces MoS 2 that is compatible with the Raman results.
Two Sn 3d 3d 5/2 -3d 3/2 doublet peaks are observed in the Sn 3d spectrum of MI-350 and MI-350-3h, suggesting the Sn 4+ and Sn 2+ states. Sn 4+ is associated with the Sn-doping into the MoS 2 complex and the SnO 2 substrate combined with the HRTEM results obtained for MI-350-3h, whereas the Sn 2+ state corresponds to the SnS crystals. In the S 2p spectrum, the doublet peak corresponds to the Sn-S bonds. These results demonstrate that the SnO 2coated layer in the FTO substrate begins to take part in the reaction and synthesizes SnS, which is consistent with the XRD results, in which the Mo 6 clusters demonstrate their outstanding catalytic capabilities.  In the MI-350 and MI-350-3h samples, no Mo 6+ state, representing MoOx, is found in the Mo 3d spectrum. The I 3d peak also disappears, proving that MoOx as an intermediate product is completely sulfurized. The content of the Mo 5+ ions in the product significantly decreases when the sulfurization time increases from 1 to 3 h, demonstrating that the MoxSy cluster phase gradually produces MoS2 that is compatible with the Raman results.
Two Sn 3d 3d5/2-3d3/2 doublet peaks are observed in the Sn 3d spectrum of MI-350 and MI-350-3h, suggesting the Sn 4+ and Sn 2+ states. Sn 4+ is associated with the Sn-doping into the MoS2 complex and the SnO2 substrate combined with the HRTEM results obtained for MI-350-3h, whereas the Sn 2+ state corresponds to the SnS crystals. In the S 2p spectrum, the doublet peak corresponds to the Sn-S bonds. These results demonstrate that the SnO2coated layer in the FTO substrate begins to take part in the reaction and synthesizes SnS, which is consistent with the XRD results, in which the Mo6 clusters demonstrate their outstanding catalytic capabilities.
Based on this result, the sulfurization reaction of the Mo6 cluster iodide thin films can be described as follows: first, the Mo6 clusters acquire bound water during the EPD coating, and the bound water reacts with the Mo6 cluster iodides at 250 °C to form MoOx; subsequently, MoOx is sulfurized into MoS2 by H2S. In this process, the iodide ligand is gradually replaced by sulfur ligands and the Mo6 clusters are gradually decomposed. When the reaction temperature becomes higher or the reaction time becomes longer, the final product is the layered MoS2. Thus, the composition of the MI-250 thin film should be the MoOx-MoxSy cluster-MoS2 composite with doped iodide.
On the other hand, the main component of MI-300 should be a metal sulfide (oxysulfide) complex of Mo and Sn, while the components of MI-350 and MI-350-3h should be SnS-MoS2 nanoparticles. The MI-350-3h thin film has a porous sintered particles morphology. The distribution of the Mo and Sn elements is, thus, uniform, which is obtained at such a low synthesis temperature. Although the reaction mechanism of SnO2 catalyzed by Mo6 cluster iodides to form SnS is not known, it is worth further research.

NH3 Sensor Application
Ligated Mo6-cluster-containing compounds used for sensing activity are currently limited to oxygen sensor [67,68], pH sensing [69], biological sensing [70], and photo-response [58,71]. According to theoretical calculations, such compounds are predicted to have a good selectivity for NH3 in the NRR [29]. Previous studies have also demonstrated Based on this result, the sulfurization reaction of the Mo 6 cluster iodide thin films can be described as follows: first, the Mo 6 clusters acquire bound water during the EPD coating, and the bound water reacts with the Mo 6 cluster iodides at 250 • C to form MoO x ; subsequently, MoO x is sulfurized into MoS 2 by H 2 S. In this process, the iodide ligand is gradually replaced by sulfur ligands and the Mo 6 clusters are gradually decomposed. When the reaction temperature becomes higher or the reaction time becomes longer, the final product is the layered MoS 2 . Thus, the composition of the MI-250 thin film should be the MoO x -Mo x S y cluster-MoS 2 composite with doped iodide.
On the other hand, the main component of MI-300 should be a metal sulfide (oxysulfide) complex of Mo and Sn, while the components of MI-350 and MI-350-3h should be SnS-MoS 2 nanoparticles. The MI-350-3h thin film has a porous sintered particles morphology. The distribution of the Mo and Sn elements is, thus, uniform, which is obtained at such a low synthesis temperature. Although the reaction mechanism of SnO 2 catalyzed by Mo 6 cluster iodides to form SnS is not known, it is worth further research.

NH 3 Sensor Application
Ligated Mo 6 -cluster-containing compounds used for sensing activity are currently limited to oxygen sensor [67,68], pH sensing [69], biological sensing [70], and photoresponse [58,71]. According to theoretical calculations, such compounds are predicted to have a good selectivity for NH 3 in the NRR [29]. Previous studies have also demonstrated that they exhibit a photo-response and can react with NH 3 to form Mo x N y species at 400 • C [46,58,71]. Thus, we inferred that Mo 6 cluster iodides should have a specific adsorption of NH 3 even at room temperature and that this adsorption could cause a potential change when a voltage is applied to the MI@FTO, which was proved to be an ambipolar material (E g = 1.9 eV). In addition, the nanoparticle morphology of the material could also increase the specific surface area and, thus, enhance the adsorption effect. Based on these facts, a simple MI@FTO sensor was designed and fabricated to verify this hypothesis (see Figure 1b).
The MI@FTO and sulfurized sample gas sensors were carefully tested at room temperature, 1 atmosphere, and in shaded light in order to investigate the sensing performance of NH 3 under a N 2 atmosphere for its potential applications in gas detection. The flow rate of the N 2 gas stream was constant at 100 sccm. The flow rate of the NH 3 gas was adjusted to achieve a change in the concentration of NH 3 in the atmosphere. Its real-time current change was detected with the voltage of 0.1 V by flowing NH 3 gas in the alumina tube. The gas sensing response (%) was calculated using the following equation: where I 0 is the base current, and I g refers to the corresponding current with the NH 3 gas exposure. Figure 9a shows the result of the NH 3 sensing performance of the initial MI@FTO and MI-250 samples. The initial MI@FTO with the pure Mo 6 cluster iodide film was, for the first time, used as a gas sensor and demonstrated a significant responsiveness. Gas sensing mechanisms can be interpreted as electron (or hole) transport between a sensing film and the target gas via charge transfer. Depending on whether the gas is oxidizing or reducing (e.g., NH 3 ) and the sensing film is a p-or n-type semiconductor, electrons are withdrawn from or donated to the sensing film, which would cause the resistance of the sensing material to increase or decrease [72][73][74]. The initial MI@FTO shows an increased resistance when exposed to NH 3 gas, while MI-250 shows a decrease in resistance, proving that MI@FTO shows the behavior of a p-type semiconductor, while MI-250 is an n-type conductor (see Figure 9e). This conversion could be the result of a sulfur defect and iodine doping [75][76][77]. It is worth noting that when the sulfurization temperature is above 400 °C, the SnO2 layer in the FTO substrate will be destroyed and mixed into the sample (the HRTEM image of MI-400 is shown in Figure S2). Thus, the samples, which were sulfurized beyond 400 °C, were not made into a gas sensor for measurements. Figure 9c shows the responses of MI@FTO and MI-250 based on different NH3 gas flow rates. As the measurement circuit is at a constant voltage (0.1 V), the response calculated for the current is actually equivalent to the rate of change of the sample's resistance. The response is taken as an absolute value for the comparison; MI-250's response rate The other samples (e.g., MI-300, MI-350-1h, and MI-350-3h) that contain Sn also exhibit the n-type characteristics but with a slight (or no) responsiveness to NH 3 (see Figure 9b), even poorer than the MI@FTO, which may be due to the decomposition of the Mo cluster, thus reducing the activity site. The XRD results show that the composition of MI-300 is an extreme complex of Mo-Sn metal oxysulfur compounds, and the gas detector of MI-300 does not exhibit any selectivity for NH 3 gas. For the MI-350-1h and MI-350-3h samples, XRD results show that the main component of the product is MoS 2 , which indicates that the Mo 6 cluster structure has been largely decomposed and the SnS nanocrystal has been generated together, while the macrostructure of the material changes into a porous sinter. Both MI-350-1h and MI-350-3h show a poorer selectivity for NH 3 compared to the initial MI@FTO, which may be mainly due to the decomposition of the Mo 6 cluster. During the gas sensor measurements, most of the samples, including MI-350-3h, very quickly stabilize their resistance in ambient N 2 , while MI-350-1h continues to reduce its resistance in the ambient N 2 , proving, on the one hand, that it is also an n-type semiconductor, and on the other hand, it has a high capacity for adsorption than selectivity. This result is consistent with the XPS Mo 3d spectrum, which shows that MI-350-1h has more Mo 5+ components compared to MI-350-3h, meaning that it has more active sites.
It is worth noting that when the sulfurization temperature is above 400 • C, the SnO 2 layer in the FTO substrate will be destroyed and mixed into the sample (the HRTEM image of MI-400 is shown in Figure S2). Thus, the samples, which were sulfurized beyond 400 • C, were not made into a gas sensor for measurements. Figure 9c shows the responses of MI@FTO and MI-250 based on different NH 3 gas flow rates. As the measurement circuit is at a constant voltage (0.1 V), the response calculated for the current is actually equivalent to the rate of change of the sample's resistance. The response is taken as an absolute value for the comparison; MI-250's response rate improves 3 times versus that of the MI@FTO (11% to 31%) at an NH 3 rate of 35 sccm. The increase in the response rate should be attributed to the activity sites provided by the Mo cluster [29] and the core-shell structure of MoO 3 -MoS 2 [78], and probably attributed to the p-n heterojunction structure [79] between the Mo 6 cluster and MoS 2 .
Based on the XRD results, the components of MI-250 can be described as a p-type Mo x cluster (x ≤ 6) and sulfurized n-type MoS 2 (and also MoO x ). The XPS I 3d spectrum of MI-250 also shows two clear doublets, proving that the Mo 6 cluster core is still present. Recently, it has been reported that when the Mo 6 cluster iodide is immobilized on p-type graphene (Mo 6 @Graphene), this nanohybrid material exhibits a sensitivity to NO 2 gas but not NH 3 gas when made into the chemiresistive gas sensor. The response of Mo 6 @Graphene to NH 3 is even lower than the response of the initial graphene to NH 3 , because the NH 3 cannot exchange charges, and only few positive charges (hole) are transferred from the p-type graphene layer toward the Mo 6 cluster [80]. These results demonstrate that the sulfurized strategy is effective in enhancing the NH 3 gas response of the Mo 6 cluster iodide, probably due to the p-n heterojunction structure between the Mo 6 cluster and MoS 2 .
After the sulfurization process, the resistivity of the samples shows a good linear relationship at different temperatures (see Figure S3). It means that the sulfurized Mo 6 cluster layer has good ohmic contact with the FTO electrode. Based on that, the resistance change of the gas sensor while it is exposed to the target gas can be reflected due to the negligible contact resistance at the interface. It is worth pointing out that the resistance of MI-250 and MI-300 varies very little with temperature (a small increase with temperature), which indicates that MI-250 has good stability as a gas sensor. In contrast, the resistance of MI-350, MI-350-3h, and MI-400 decreases rapidly as temperature rises, indicating that they have a high potential for future thermosensitive applications. Figure 9d, based on Figure 9a, shows the response time and recovery time of MI@FTO and MI-250 at the NH 3 gas flow rate of 35 sccm. The response time was measured from the resistance point of the sample increasing by 10% to 90% of an end point (the maximum resistance point), while the sample was exposed to the target gas, and the recovery time was measured at the point where the resistance decreased from 90% of an end point to the 10% resistance point after the target gas was turned off. The response and recovery times for the NH 3 detection at 35 sccm by the MI@FTO are 29 s and 198 s, respectively.
In comparison, the MI-250 response and recovery times are 174 s and 254 s, respectively, both significantly lower than the performance of the MI@FTO. MI@FTO's fast response and recovery are attributed to its Mo 6 cluster core [29].
After the gas sensor measurement, the final resistance of all the samples in this 45-min long test does not go back to zero when the NH 3 gas turns off, which may be due to the fact that the sensor surface is not fully cleaned and some NH 3 is still adsorbed. This behavior may be related to operating temperature and gas flow rate.
The sulfurization process starting from the Mo 6 cluster iodides as the precursor leading to the expected MoS 2 product and their NH 3 sensing enhancement is evidence that this novel synthesis strategy is feasible. The transition of the Mo 6 -cluster-iodide-based films from p-type to n-type semiconductors due to sulfurization reactions also opens up new design possibilities for future device applications, such as p-n junctions.

Conclusions
This study established a simple and efficient method for the synthesis of Mo cluster-MoS 2 nanocomposites, which not only demonstrates the feasibility of sulfuring Mo 6 cluster iodides as precursors to generate MoS 2 , but also directly allows the synthesis of nanocomposite thin films in just one step. This study also explored the performance of Mo 6 cluster iodides and Mo cluster-MoS 2 nanocomposites as gas sensors for the first time. It was proved that the Mo 6 cluster iodide gas sensor for NH 3 at room temperature responds as a p-type semiconductor. After the sulfurization process at low temperature (250 • C), the response to NH 3 gas increased by three times while the conversion from the p-type semiconductor to n-type occurred. Our results obtained in this study show the possibilities of Mo 6 clusters and Mo-cluster-based MoS 2 material for future applications.  Data Availability Statement: All data concerning this study are contained in the present manuscript, in previous articles or Supplementary Materials, whose references have been provided.

Conflicts of Interest:
The authors declare no conflict of interest.