Fig. 1. shows the XRD of the thin films before and after annealing at (200–300 and 400)°C for 1 h. The figure indexed peaks such as (210), (301), (311), (202), (212), and (711) are characteristic peaks of the orthorhombic structure of the SnS, according to JCPDS standard card (PDF Card No. 39-0354). The strongest peak is often located around 2θ ≈ 31–32°. It reverts to the (301) or (210) plane depending on the peak intensity and growth direction. This confirms the formation of a polycrystalline SnS phase without distinct secondary phases such as SnS₂ or Sn₂S₃, especially at higher annealing temperatures.

Using the Shearer equation to calculated the size of the crystals (D) [9]: (1) D = k λ β   cos θ where: β (rad) represents the full width at half maximum, θ symbolizes the Bragg angle and k is a Scherrer constant (k = 0.94). Eq. (2) can be used to find the density of the dislocations (δ) [10]:

The microstress (ε) of thin films of tin sulfide was found using the formula [11].

XRD results in Table 1 for SnS films show that grain size does not increase linearly with increasing temperature but rather is influenced by a balance between several physical processes. Initially, as the annealing temperature increases from the unannealed state to moderate temperatures (200–300)°C, there is an increase in the atomic diffusion energy of atoms within the crystal lattice, allowing them to move towards more ordered positions and reducing defects and cracks. This typically leads to an increase in grain size and improved crystallinity due to the merging of smaller grains and a reduction in high-energy grain boundary regions [12]. However, at higher temperatures within this range, recrystallization, or a change in preferred crystallization orientation, may occur, rearranging the grains and forming new, smaller grains with different surface energies. Annealing not only leads to grain growth but also to the reorganization of the structural composition at certain temperatures. Therefore, the crystal size decreases at that temperature, and sulfur atoms are lost at high annealing temperatures (400°C). The competition between recrystallization, atomic diffusion, and surface energy equilibrium forces the fluctuation in grain size with increasing annealing temperature of tin sulfide films [13,14].

Fig. 2 and Fig. 3 (AFM) images of tin sulfide films show that the film surface consists of tightly packed nanoparticles with an irregular distribution, characterized by clear peaks and protrusions separated by depressions, confirming the polycrystalline nature of the film. This is consistent with XRD. This is consistent with the study [13]. The images also show variations in grain size and surface density among the samples. This variation is attributed to the annealing effect, which promotes surface diffusion and recrystallization, leading to the merging of small grains and the formation of larger ones [12]. Furthermore, increasing the temperature leads to a decrease in the number of grain boundaries due to their growth, which in turn reduces dispersion centers and improves electrical conductivity, as reported by [15]. The agglomeration pattern observed in the images is explained by the Volmer–Weber island growth mode, where primary nuclei form and then gradually grow and merge to form a continuous layer—a mechanism common in polycrystalline SnS films [14]. Additionally, increased surface roughness leads to an increase in the effective surface area and adsorption sites, which improves the response of the films in gas sensing applications, consistent with what has been reported in the literature on p-type semiconductor sensors [13].

The transmittance spectra of SnS thin films before and after annealingin Fig. 4 shows very low values in the UV-Vis and visible regions due to strong absorption of high-energy (UV) photons. This is normal because the photon energy is greater than the energy gap (E_g) (~1.3–1.5 eV). This is consistent with previous studies [16]. A gradual increase in transmittance is observed at higher wavelengths (NIR). This behavior is attributed to the decrease in photon energy at longer wavelengths. Furthermore, the decrease in transmittance with increasing annealing temperature is associated with improved crystallinity, increased grain size, and enhanced light scattering at grain boundaries. This is consistent with previous studies [17].

Fig. 5 shows the reflectance of annealed and unannealed SnS films. High values are observed in the ultraviolet region, followed by a decrease in the visible light range due to strong photoabsorption. At longer wavelengths, reflectance increases as the photon energy falls below the bandgap energy. The change in reflectance with annealing temperature is attributed to variations in crystallinity, grain size, and surface shape. The film annealed at 300°C exhibits the lowest reflectance, indicating increased photo absorption, which is consistent with previous studies [18].

The absorption coefficient (α) of tin sulfide films was found by formula [10].

The absorption coefficient was found to be 10⁴ cm⁻¹, as shown in Fig. 6.

Fig. 7 represents Extinction coefficient (K) of tin sulfide films before and after annealing at (200, 300, and 400)°C.

The extinction coefficient, K, is directly related to the material’s ability to absorb light and is given by the equation [10]:

The extinction coefficient in SnS increases near the energy gap due to direct electron transitions. The highest value at 200°C indicates improved optical properties. The decrease in k at high energies indicates the end of the strong absorption band. The curve behavior confirms that annealing directly affects the electronic structure and optical density [17].

The band gap ( E g ) can be determined by equation [18],

The band gap of the films was determined by plotting (αhν)² versus hν from Fig. 8. E g = 1.35 eV was found for the films before annealing, and the E g increased with increasing annealing temp.

This increase in the energy gap can be explained by the fact that annealing improves the crystallinity of the films, which reduces the structural defects that usually cause the appearance of tails in the band structure (Aurbach tails).

Additionally, heat treatment can induce minor changes in the chemical composition of the thin film, particularly the re-evaporation of sulfur at higher temperatures, which can alter the electronic structure and charge carrier concentration. These structural modifications can contribute to band structure rearrangements and a widening of the band gap. This is agreed with the findings of researchers [5,19].

The Fig. 9, Fig. 10, Fig. 11 and Fig. 12 illustrate the change in resistance over time for the films before and after annealing and several different operating temperatures (50, 100 and 150)°C. It was found that resistance decreases when exposed to carbon dioxide gas, and this proves that the SnS films are of the p-type. Carbon dioxide (CO₂) is a gas with weak oxidizing behavior when it reacts with semiconductor surfaces. When tin sulfide films, which often exhibit p-type semiconducting behavior, are exposed to CO₂, adsorption of gas molecules occurs on the film surface, where the gas acts as an electron acceptor. The removal of electrons from the film surface increases the number of holes (positive carriers), the main carriers in p-type materials, resulting in a decrease in the film’s electrical resistance. This phenomenon is attributed to the change in surface charge density and the modification of the depletion region caused by the interaction between the adsorbed gas and the material surface [20].

As can be seen from Fig. 9, Fig. 10, Fig. 11 and Fig. 12, the sensor’s resistance decreases with increasing operating temperature (50–150°C). This is attributed to the thermal excitation of charge carriers and the activation of surface interactions between gas molecules and the adsorbed oxygen species. These processes increase the concentration of charge carriers and reduce the potential barriers at the particle boundaries, leading to increased conductivity and decreased resistance [21].

The sensitivity of SnS films was estimated by using the Formula (7) [22,23].

In which Ra resistance in air, Rg the resistance in the presence of gas. It can be seen increasing sensitivity with increasing operating temp. The interactions between the sensor surface and gas determine the sensitivity regarding the tin sulfide sensor. Generally, increasing the operating temp. produces a more homogeneous surface with uniform roughness, resulting in a larger surface area and a higher possibility of gas reaction. Sensing is widely understood to be a surface process governed mostly by desorption and adsorption [24,25]. The sensitivity of membranes to CO₂ gas increases with increasing annealing temp. due to improved crystallinity and surface structure rearrangement, leading to reduced defects and enhanced charge transfer. Annealing at higher temperatures (400°C) promotes the formation of a more active surface with efficient adsorption sites, increasing the interaction of CO₂ with oxygen adsorbed on the surface and thus raising the S% value. Furthermore, achieving an optimal operating temperature (150°C at 400°C annealing) indicates a balance between the rate of gas adsorption and the rate of de-adsorption, which is essential for obtaining the highest response [26]. In general, improving the crystallinity and increasing surface stability through annealing enhances CO₂ sensitivity performance within a specific temperature range as shown in Table 2.

Fig. 13 shows the change in sensitivity of SnS films before and after annealing at different temperatures, along with changes in operating temperature. We observe that sensitivity generally increases with increasing operating temperature due to the activation of surface interactions between gas molecules and the oxygen adsorbed on the film surface. This leads to increased charge transfer within the material and improved sensitivity [22].