Structural, optical, and gas sensing properties of Ce, Nd, and Pr doped ZnS nanostructured thin films prepared by nebulizer spray pyrolysis method

2.1 Preparation of pure and rare-earth (Ce, Pr, Nd)-doped ZnS thin films

Doped ZnS thin films were synthesised using zinc chloride (ZnCl₂), thiourea (CH₄ N ₂S), and the corresponding rare-earth dopant nitrates, cerium(III) nitrate hexahydrate [Ce(NO₃)₃ 6H₂O], praseodymium(III) nitrate hexahydrate [Pr(NO₃)₃ 6H₂O], and neodymium(III) nitrate hexahydrate [Nd(NO₃)₃ 6H₂O]. Hydrochloric acid (HCl) and isopropanol were used as stabilisers and solvents throughout the preparation procedure. The host precursors were made with 0.2 M zinc chloride and 0.2 M thiourea for every doping condition. For all of the rare-earth elements (Ce, Pr, and Nd), the dopant concentration was adjusted at 2 wt percent. Isopropanol and deionised (DI) water were combined in a 3:1 vol ratio to create a mixed solvent with a total volume of 10 mL. In order to achieve a clear, transparent solution, HCl was added dropwise to the solvent mixture while the metal precursors were being gradually dissolved while being constantly stirred. In order to eliminate surface impurities and improve film adhesion, glass substrates were simultaneously thoroughly cleaned with detergent and then repeatedly rinsed with deionised water. A spray pyrolysis system was filled with the prepared precursor solution. A nozzle-to-substrate distance of 25  mm and a carrier gas pressure of 1.5  kg/cm² were used during deposition, while the substrate temperature was kept at 450 °C. Smooth, uniform ZnS thin films doped with Ce, Pr, or Nd were produced as a result of this process. The films were left to naturally cool to room temperature following deposition. For gas sensing studies, silver paste was applied along the edges of the films to serve as electrical contacts for response measurements. The average film thickness, calculated using the gravimetric weight difference method, was found to be ∼650 nm.

2.2 Characterization

The structural, morphological, optical, and gas sensing characteristics of the synthesized pure and rare-earth (Ce, Pr, Nd)-doped ZnS thin films were evaluated in detail. The crystalline structure, phase purity, and grain size of the films were determined through structural analysis using X-ray diffraction (XRD) with a PANalytical X’Pert Pro diffractometer using Cu-Kα radiation (λ = 1.5406 Å). A Apreo 2 field emission scanning electron microscope (FESEM) was used for analyzing the surface morphology, yielding comprehensive data on the particle distribution, surface texture, and film uniformity. The Ce, Pr, and Nd dopants were successfully incorporated into the ZnS matrix, as confirmed by energy dispersive X-ray spectroscopy (EDX). To evaluate the optical absorbance, and determine the band gap energy, optical characterization was carried out in the wavelength range of 300–1,000 nm using a Shimadzu 2600i spectrophotometer. A PerkinElmer LS55 fluorescence spectrophotometer was used for photoluminescence (PL) investigations in order to examine emission properties and transitions related to defects that are impacted by rare-earth doping. Silver electrodes were applied to the film’s edges to enable electrical measurements for the gas sensing analysis, and each sample were exposed to different NH₃ gas concentrations. The potential of rare-earth doped ZnS thin films as efficient NH₃ gas sensors was demonstrated by the evaluation of the sensing performance in terms of sensitivity, response time, recovery time, and repeatability.

3.1 Crystallite studies

The XRD patterns of pure and rare-earth (Ce, Pr, Nd) doped ZnS thin films are shown in Figure 1. According to JCPDS card No. 01-89-2739, it was verified from the figure that all films show diffraction peaks along (101), (006), (103), (108), (109), and (202) plane, which crystallizes in a hexagonal wurtzite ZnS structure with a P6₃ mc space group [27], 28]. The stable peak positions suggests that the RE ions like Pr³⁺, Ce³⁺, and Nd³⁺ have been successfully incorporated into the ZnS lattice. The polycrystalline nature of the prepared thin films was demonstrated by the distinct and sharp diffraction peaks, which exhibit an apparent preferential orientation towards the (006) plane. Additionally, no extra diffraction peaks associated with secondary phases or unreacted dopant precursors were detected, demonstrating the sample’s phase purity. The addition of Ce, Pr, and Nd results in a decrease in full-width at half maximum (FWHM) and an increase in peak intensity, which suggests better crystallinity, but it does not substantially change the position of the (006) peak (∼28.65°).

Figure 1:

XRD patterns of (a) pure ZnS, (b) Ce-doped ZnS, (c) Pr-doped ZnS, and (d) Nd-doped ZnS thin films.

The average crystallite size (D) and microstrain (ε) for all films were calculated using the following formulas as per Eqs. (1) and (2), respectively [29].

where λ represents the wavelength of X-ray radiation and β belongs to the full width at half maximum. Results, which are shown in Table 1, demonstrate that doping with Ce, Pr, and Nd results in appreciable differences in structural parameters when compared to pure ZnS. The ZnS:Pr thin film sample had the largest crystallite size, and a similar trend was confirmed by the Williamson–Hall (W–H) method as shown in the Supplementary Figure S1. When ZnS thin films are doped with RE ions like Ce³⁺, Pr³⁺, and Nd³⁺, their ionic radii, chemical behavior, and effects on crystal growth may all contribute to the increase in crystallite size. In comparison to Zn²⁺ (0.74 Å), these RE³⁺ ions have larger ionic radii (Ce³⁺ ≈ 1.03 Å, Pr³⁺ ≈ 1.013 Å, and Nd³⁺ ≈ 0.983 Å) [30], [31], [32]. Lattice distortion results from their integration into the ZnS lattice, either by interstitial occupation or substitution. This distortion can reduce internal strain and improve atomic mobility at the ideal doping concentration, which will encourage grain growth and produce larger crystallite sizes. The larger ionic radius of Pr³⁺, which causes greater lattice strain, is the primary cause of the larger crystallite size in ZnS:Pr thin films compared to Ce- and Nd-doped films. This strain encourages the growth of larger crystallites by reducing nucleation sites and promoting grain coalescence. Grain growth is further supported by Pr doping, which also increases adatom diffusion and decreases surface energy. In ZnS:Pr films, these combined effects result in larger crystallites [33], 34]. Moreover, the incorporation of Pr³⁺ is anticipated to produce lesser structural defects and dislocations, resulting in a lattice environment that is less strained and more ordered [35]. The formation of larger crystallites is supported by this structural homogeneity. The strain values, inversely related to crystallite size, show a corresponding decrease for Pr³⁺ dopants due to lesser defect density.

Using standard formulas calculated using Bragg’s law and the hexagonal system volume formula, the lattice parameters (a and c) and unit cell volumes were determined [30].

Here, hkl represents the miller indices. Table 1 provides a summary of the obtained lattice parameters and cell volumes. The values of a and c for the undoped ZnS film are determined to be 3.82 Å and 18.62 Å, respectively, and are in good agreement with data that was previously reported [30], 36]. Confirming the retaining of the wurtzite structure, the computed values remain relatively similar to standard ZnS data. The inclusion of larger RE ions first causes lattice expansion, which is followed by structural distortion. These structural parameters indicate slight variations in the lattice constant values by the addition of RE dopants. Overall, the XRD analysis shows that RE doping modifies the structural characteristics of ZnS thin films in a way that is dependent on the doping level and is mostly controlled by the ionic radius and site occupancy of the dopant ions.

3.2 Morphology analysis

FESEM was used to analyze the surface morphology of the Ce, Pr, and Nd-doped ZnS thin films, as shown in Figure 2(a–d). Depending on the RE dopant used, the micrographs showed notable morphological variations. Unevenly distributed and loosely shaped nanorods were visible on the surface of the undoped ZnS thin films while a change in morphology was seen after doping. ZnS:Pr samples formed comparatively denser clusters than Ce and Nd doped ZnS films, which showed aggregated porous nanorod-like structures. Interestingly, the ZnS:Pr films displayed more distinct larger nanorods morphology alongside interconnected porous structures, suggesting improved one-dimensional growth. According to this morphological evolution, nucleation and growth behavisor are significantly influenced through RE doping. Also, it was observed that, the average particle sizes gradually increased with doping. The ZnS:Pr sample had the largest average particle size among the samples due to the anisotropic crystal growth which facilitates the development of extended nanorod structures. The interaction among dopant ions and the ZnS lattice is the main cause of the variation in particle size and shape among the samples. The formation of porous nanostructures is influenced by variables like local growth dynamics, dopant-lattice strain, and ionic radius. Larger Pr ions promote directional growth and particle coalescence, leading to an even more interconnected nanorod network with the largest average particle size, while other dopants, such as Ce and Nd, induce moderate strain and encourage aggregated rod-like formations.

Figure 2:

FESEM images of (a) pure ZnS, (b) Ce-doped ZnS, (c) Pr-doped ZnS, and (d) Nd-doped ZnS thin films.

EDX was employed to confirm the elemental composition of the prepared films. Supplementary Figure S2(a–d) shows the EDX spectrum of the ZnS: Ce, Pr and Nd thin film within the potential range of 0–10 kV, indicating the presence of Zn, S, RE (Ce, Pr and Nd) elements without any detectable impurities.

3.3 Optical studies

The optical properties of ZnS thin films doped with Ce, Pr, and Nd were investigated by recording their transmittance spectra in the wavelength range of 300–900 nm. As depicted in Figure 3a’, all the films exhibit a decrease in transmittance while doping RE ions. It is evident from Figure 3a’ that, the incorporation of Ce, Pr, and Nd into the ZnS lattice causes a lower transmittance value of ∼75 % for the ZnS:Pr (2 %) films compared to that of undoped ZnS. This abrupt drop in transmittance is a characteristic feature commonly observed in semiconducting materials and is indicative of good crystalline quality. While the larger ionic radius of Pr³⁺ causes more lattice distortion and defects, increasing optical absorption, the larger crystallite size in Pr-doped films enhances light scattering. Further absorption is facilitated by the numerous intra-4f electronic transitions that Pr³⁺ ions display in the visible spectrum. Potential Pr dopant agglomeration or segregation may also worsen light blocking and decrease transparency [37]. This reduction in transmittance implies a reduction in the optical band gap (E _g ) as a result of RE doping.

Figure 3:

UV–Vis. Transmittance spectra (a'), Tauc plots of pure ZnS, (b) Ce-doped ZnS, (c) Pr-doped ZnS, and (d) Nd-doped ZnS thin films (b'), the corresponding band edge diagram (c').

Since ZnS is known to possess a direct band gap [37], the band gap values of the films were calculated using Tauc’s relation [38]:

where α is the absorption coefficient, A is a constant, and hν denotes the incident photon energy. The plots of (αhν)² versus photon energy (hν) for all samples are shown in Figure 3b’. The E _g values were determined by extrapolating the linear region of each curve to the energy axis, as indicated by the arrows in figure. The estimated E _g values were found to be 2.89 eV for pure ZnS, and 2.85, 2.74, and 2.82 eV for ZnS:Ce (2 %), ZnS:Pr (2 %), and ZnS:Nd (2 %) thin films, respectively, with the corresponding band diagram shown in Figure 3(c′). These results confirm a reduction in E _g due to doping with Ce, Pr, and Nd, compared with pure ZnS. The E _g value of the undoped ZnS film aligns well with the reported bulk value [39], [40], [41]. A similar trend of band gap reduction upon doping with rare-earth elements in ZnS and ZnO systems has also been reported by other researchers [42], [43], [44]. From the results, the ZnS:Pr (2 %) film showed a reduced band gap (∼2.74 eV compared to 2.89 eV for undoped ZnS) along with a slight increase in absorption, i.e., a decrease in transmittance. This behavior arises from lattice distortions due to the larger ionic radius of Pr³⁺, increased light scattering from larger crystallite size, additional intra-4f transitions, possible dopant agglomeration, and the introduction of localized energy levels near the valence band edge, all of which enhance absorption and narrow the band gap [45], 46]. Thus, the optical analysis confirms the influence of Ce, Pr, and Nd doping on the electronic structure and band gap behavior of ZnS thin films.

3.4 Photoluminescence analysis

Figure 4 presents the photoluminescence (PL) spectra of pure and Ce-, Pr-, and Nd-doped ZnS thin films measured at room temperature to evaluate their luminescence characteristics. All the samples exhibit weak emission peaks in the ultraviolet region, typically between 370 and 420  nm, with notable peaks around 385–425 nm, attributed to excitonic recombination near the band edge [47]. Such emissions have been previously reported and are associated with self-activated luminescent centers due to electron-hole pair recombination [48], 49]. Additionally, a strong emission peak observed in the visible region around 487–489 nm is linked to the dangling sulfur bonds in the ZnS matrix. This visible emission indicates the presence of native defect sites related to sulfur vacancies or dangling bonds, confirming defect-assisted recombination pathways in the films [50]. Among the doped samples, the 2 % ZnS:Pr films exhibited the highest intensity and peak broadness, which could be due to the increased interaction between the host ZnS lattice and RE ions. This enhancement likely arises from the reduced spatial separation between the dopant ions and the host lattice, promoting more efficient energy transfer. Furthermore, the broad visible emissions in doped films may result from the interaction between s–p transitions of ZnS and the 4f or 5d orbitals of RE dopants, leading to partially allowed transitions and red-shifting of emission peaks. The absence of any additional emission bands related to the dopant ions suggests that no secondary impurity phases formed during film deposition, as also confirmed by XRD analysis. Similar PL behaviors and transitions associated with rare-earth dopants in ZnS thin films have been documented in earlier studies [51], 52].

Figure 4:

PL emission spectra of (a) pure ZnS, (b) Ce-doped ZnS, (c) Pr-doped ZnS, and (d) Nd-doped ZnS thin films.

3.5 Gas sensing properties

The gas sensing behavior of Ce-, Pr-, and Nd-doped ZnS thin films toward NH₃ was examined by measuring the change in electrical current upon gas exposure, as shown in Figure 5. For the measurements, the thin films were mounted inside a sealed chamber fitted with a front panel to hold the sensor assembly. The films were connected to two electrical probes, while NH₃ gas was introduced into the chamber through a metallic tube connected to a heater and a thermocouple. NH₃ vapors were generated by heating an aqueous NH₃ solution to the desired temperature and subsequently released into the chamber. The film surfaces, maintained at room temperature, were exposed to these vapors. Once the electrical current reached saturation, the chamber was opened to allow atmospheric air to enter, thereby accelerating the desorption process. The electrical current response was recorded using a Keithley Source Meter. As seen from the graph, the baseline current of the rare-earth-doped films is higher compared to that of the undoped films. The incorporation of RE³⁺ ions into Zn²⁺ lattice sites or interstitial positions induces lattice distortions and generates defect states, which either provide additional charge carriers or facilitate carrier hopping. Furthermore, the formation of localized energy levels near the band edges narrows the effective band gap, thereby enhancing electron excitation. Consequently, rare-earth doping improves the electrical conductivity of ZnS films by increasing carrier concentration and promoting defect-assisted charge transport. To evaluate the gas sensing performance, the NH₃ response of RE (Ce, Pr, and Nd)-doped ZnS films was investigated at room temperature for different concentrations (50, 100, 150, 200, and 250  ppm), with emphasis on key sensing performance metrics. The adsorption-desorption reactions among the gas molecules and the thin film surface are the backbone of the sensing mechanism. As NH₃ concentrations rise during exposure, a noticeable increase in current was seen, suggesting improved gas interaction as a result of the more readily available gas molecules.

Figure 5:

Current versus time plots of (a) pure ZnS, (b) Ce-doped ZnS, (c) Pr-doped ZnS, and (d) Nd-doped ZnS thin films.

The gas response (S) was calculated using the standard relation [53]:

where I _g and I _a represent the current in NH₃ gas and in air, respectively. When rare earth elements like Ce, Pr, and Nd were added, the gas response of pristine ZnS, which had been found to be relatively low, significantly improved. The doped ZnS sensors gas sensitivities gradually increased with each dopant, as shown in Table 2. The 2 % ZnS:Pr sensor exhibited the highest NH₃ gas sensitivity, recording a response of 1,520. This superior performance is linked to its enhanced surface and optical properties. Pr doping improves crystallinity and produces a more uniform nanostructured surface, thereby increasing the available surface area for gas interaction. PL studies further suggest that Pr incorporation facilitates efficient electron–hole recombination at defect sites, particularly sulfur vacancies, enabling stronger interaction with NH₃ molecules. Additionally, the ZnS:Pr films show greater light absorption and narrow bandgap value compared to Ce- and Nd-doped samples, creating more active adsorption sites. Collectively, these factors account for the significantly improved gas sensing behavior of the ZnS:Pr films [54], 55]. The adsorption-desorption kinetics are accelerated and enhanced electronic interactions could be achieved by the decrease in optical bandgap as observed through Pr incorporation. On the other hand, Ce and Nd doped ZnS films showed lower gas response compared to ZnS:Pr films, despite having better sensitivity when compared to pristine ZnS. This discrepancy most likely results from their varying effects on the structural, morphological, and electronic characteristics of the ZnS matrix, which are crucial for gas sensing behavior (Table 3).

3.5.1 Selectivity

A crucial factor in assessing a gas sensor’s performance is its selectivity towards a particular target gas, which establishes the sensor’s capacity to differentiate the target gas from other interfering gases. The selectivity analysis of the ZnS:Pr thin film sensor exposed to a range of gases at a fixed concentration of 50  ppm, including toluene, acetone, methanol, ethanol, isoproponol, and NH₃, is shown in Figure 6. The ZnS:Pr thin film sensor showed strongest response to NH₃, according to the sensing response, whereas the other gases showed noticeably weaker responses. The strong selectivity of the ZnS:Pr sensor for NH₃ detection is confirmed by this noticeable difference, which is a crucial feature for accurate and dependable NH₃ sensing applications. The distinct effect of the Pr ions on the material’s structural and electrical characteristics is responsible for the increased selectivity of ZnS:Pr. By adding localized energy states and increasing the concentration of charge carriers, Pr incorporation alters the electronic band structure of ZnS and promotes stronger charge transfer reactions with NH₃ molecules. Furthermore, Pr doping enhances the sensor’s affinity for NH₃ by encouraging the development of sulphur vacancies along with other defect sites that function as active adsorption centers. Additionally, the adsorption-desorption kinetics may be enhanced by the Pr dopant’s potential to catalyze the adsorption and partial dissociation of NH₃ molecules on the ZnS surface. Improved surface roughness and the interaction that occurs between the ZnS:Pr surface and NH₃ result in better gas diffusion and efficient analyte recognition.

Figure 6:

Selectivity graph of the ZnS thin film gas sensor.

3.5.2 Stability and repeatability

The ZnS:Pr thin film sensor’s long-term stability over a 50-day period under varied NH₃ gas concentrations (50–250 ppm) is shown in Figure 7. High stability and repeatability in sensing performance were indicated by the gas response’s notable consistency over the entire period of measurement at different concentration of NH₃ gas. The incorporation of Pr ions, which stabilizes the ZnS crystal lattice, reduces defect migration, and improves surface morphology, is principally responsible for this long-term stability. Together, these effects lower the possibility of particle agglomeration or structural deterioration, guaranteeing long-term sensor performance.

Figure 7:

Stability study of the ZnS:Pr thin film gas sensor.

3.5.3 Humidity studies

Figure 8 illustrates the response of the ZnS:Pr sensor at various relative humidity (RH) levels of 32 %, 52 %, 73 %, and 81 %. The sensor’s ability to function dependably in humid conditions was demonstrated by the increasing trend in gas response that was seen as humidity level rises. The increased adsorption of water molecules at surface-active sites, such as sulphur vacancies (V_S) and Pr-induced native defect sites, is responsible for the improved sensing behavior in humid environments. During NH₃ detection, these flaws improve surface reactivity and facilitate charge transfer by acting as efficient adsorption sites for gas and moisture molecules. Surface-adsorbed species and native vacancy defects (such as Zn or S vacancies) affect the sensing mechanism in ZnS:Pr, in contrast to oxide-based sensors that mainly depend on oxygen vacancies. Even in the absence of lattice oxygen, Pr doping alters the defect landscape by expanding active sites and facilitating the adsorption of reactive oxygen species (like O₂ ⁻ and O⁻). A detectable sensor response results from the interaction of these adsorbed species with incoming NH₃ molecules, which changes the electrical resistance and carrier concentration of the film. Moreover, higher humidity improves the adsorption of water molecules at these defect-rich areas, changing the surface chemistry and strengthening the interaction with NH₃. The stable and improved sensing behavior seen under various environmental conditions is explained by the synergistic interaction of humidity, surface defects, and dopant-induced electronic states. Overall, the ZnS:Pr thin film sensor exhibits exceptional long-term stability and robust performance over a wide range of humidity levels, making it a viable option for trustworthy NH₃ gas sensing in practical applications.

Figure 8:

Relative humidity study of the ZnS:Pr thin film gas sensor.

3.5.4 Response and recovery times

Two important parameters that affect sensor performance in gas sensing applications are response time (t_s) and recovery time (t_r) shown in Figure 9. The response time (t_s) is the amount of time required for the sensor’s current output to surpass 90 % of its maximum level following exposure to the target gas. In contrast, the recovery time (t_r) is the time it takes for the sensor’s current to decrease to 10 % of its starting value after the gas source is removed [56]. Particularly, the dynamic behavior of Ce-, Pr-, and Nd-doped ZnS thin films exposed to NH₃ has been studied in relation to these two-time constants. Of all the dopants being studied, ZnS:Pr thin films exhibit the fastest sensing behavior with the fastest response and recovery times of 9.7 s and 8.9 s, respectively, when exposed to 250 ppm of NH₃ gas. This implies that the adsorption and desorption of NH₃ molecules onto the film surface is accelerated by the addition of Pr³⁺ ions to the ZnS lattice, which significantly enhances electron transport and surface reactivity. The quantitative values for t_s and t_r that correlate to different dopant types are summarized in Table 2. The other RE dopants showed increase in both t_s and t_r, suggesting that high ionic radius of dopant ions in ZnS matrix might result in defect clustering or agglomeration, which could obstruct gas diffusion and slow down charge transfer processes. Thus, these results suggests that ZnS:Pr thin films have improved sensing capabilities, making them desirable choices for rapid and efficient NH₃ gas detection in practical situations.

Figure 9:

Response and recovery times of (a) pure ZnS, (b) Ce-doped ZnS, (c) Pr-doped ZnS, and (d) Nd-doped ZnS thin films.

3.5.5 Mechanism

In general, crystallinity, porosity, dopant concentration, and surface defects all affect the charge transfer and band bending behavior of semiconductor materials, which in turn affects their sensing response. When exposed to NH₃, the gas reacts with the oxygen ions (O₂ ⁻) that have been adsorbed on the film surface, causing trapped electrons to return to the conduction band (CB) and enhancing electrical conductivity as shown in Figure 10. The following reaction describes this process:

(7) 2 NH 3 + 4 O 2 − → N 2 + 6 H 2 O + 6 e −

where the film’s resistance and depletion width are decreased by the liberated electrons. The structural distortions induced by the RE dopants results in more vacancies and active adsorption sites, are linked to the improved sensing of ZnS:Pr. Additionally, tests of stability and reproducibility after 50 days showed that the ZnS:Pr sensor performed consistently, with very little variation in response or recovery times over several exposure cycles. The sensor is a promising option for real-world NH₃ gas detection applications because of its exceptional long-term stability and robustness.

Figure 10:

Schematic diagram of the ammonia gas sensing mechanism in the ZnS sample.